The Dimerization and Oligomerization of Alkenes Catalyzed with Transition Metal Complexes: Catalytic Systems and Reaction Mechanisms

Abstract

Dimers and oligomers of alkenes represent a category of compounds that are in great demand in diverse industrial sectors. Among the developing synthetic methods, the catalysis of alkene dimerization and oligomerization using transition metal salts and complexes is of undoubted interest for practical applications. This approach demonstrates substantial potential, offering not only elevated reaction rates but also precise control over the chemo-, regio-, and stereoselectivity of the reactions. In this review, we discuss the data on catalytic systems for alkene dimerization and oligomerization. Our focus lies in the analysis of how the activity and chemoselectivity of these catalytic systems are influenced by various factors, such as the nature of the transition metal, the ligand environment, the activator, and the substrate structure. Notably, this review particularly discusses reaction mechanisms, encompassing metal complex activation, structural and dynamic features, and the reactivity of hydride intermediates, which serve as potential catalytically active centers in alkene dimerization and oligomerization.

1. Introduction

The dimers and oligomers of alkenes belong to a broad class of compounds that are in high demand across various industrial sectors. Typically, they are used as comonomers in ethylene polymerization and serve as raw materials for the production of adhesives, surfactants, flavors, synthetic fuel additives, and more [1,2,3,4,5,6]. Alkene oligomers are synthesized catalytically using various methods, including heterogeneous acid catalysis with the use of phosphoric acid on silica, ion exchange resins, silica-aluminas, zeolites, etc., where the classical mechanism involving carbocations (carbenium pathway) is realized [6,7,8]. Another way to synthesize oligomers involves transition metal catalysis (Zr, Ti, Hf, Ni, Co, Fe, V, and Ta) in which the Cossee–Arlman mechanism is implemented [9]. Metal-catalyzed processes, such as the oligo- and polymerization of ethylene on chromium catalysts (Philips), the preparation of linear α-olefins via ethylene oligomerization on a nickel catalyst (SHOP = shell higher olefin process), the oligomerization of light alkenes into C4–C10 olefins using AlphaButol, AlphaHexol, Dimersol, and Difasol process technologies, and others, were developed to produce olefin oligomers [10,11,12,13,14]. The oligomerization of alkenes (1-butene and 1-hexene) synthesized from renewable plant raw materials to obtain jet and diesel fuels is also attracting the attention of the scientific community [6,15].

Among the developing methods with significant potential for advancement and practical implementation is the catalysis of alkene dimerization and oligomerization using Ti subgroup metal salts and complexes, which provide high reaction rates and effective control of their chemo-, regio-, and stereoselectivity. The classical heterogeneous Ziegler–Natta catalysis is widely used in the production of polyethylene and polypropylene [16,17,18,19,20,21]. The discovery of metallocenes [22], along with organoaluminum [23,24,25] and boron activators [22,26,27], enabled the process to be transferred from a heterogeneous medium to a homogeneous one, which provided undoubted positive effects—such as an increase in the activity and the possibility to effectively control stereoselectivity—and allowed for a detailed study to be conducted on the reaction mechanisms. Homogeneous systems effectively catalyze alkene di-, oligo-, and polymerization [1,28,29,30], as well as the hydro-, carbo-, and cyclometalation of olefins and acetylenes [31,32,33,34,35], which can not only be considered as the methods of multiple bond functionalization but also as the initial stages of chain growth in the oligo- and polymerization processes.

Information concerning the catalytic alkene oligomerization and the types of products it produces can be seen in Scheme 1. The nature of the active reaction centers determines the structural types of the resulting oligomers and the regio- and stereoselectivity of a substrate’s insertion. In the processes presented in Scheme 1, hydride, alkyl, or alkene intermediates act as active reaction centers, which, in the early stages, facilitate alkene hydro-, carbo-, or cyclometalation, respectively. Chain termination occurs through the elimination of the oligomeric product, generating metal hydrides, or through the transfer of the growing chain to an organometallic cocatalyst or alkene. Metal hydrides, therefore, can serve as dominant reaction centers in these catalytic systems.

Recent reviews on catalytic ethylene, propylene, and higher olefin oligomerization discussed the chemical methodologies, possible reaction mechanisms, techniques used to study the structure and physicochemical properties of oligomers, the practical implementation of these processes in the industry, and the potential applications for the resulting products [1,3,36,37,38,39,40,41,42].

This review paper considers catalytic systems based on metal complexes for the synthesis of alkene dimers and oligomers in the context of the dependence of the activity and chemoselectivity of catalytic systems on the nature of the transition metal, ligand environment, activator, and substrate structure. This review pays particular attention to the reaction mechanisms, including the metal complex activation, the structural and dynamic features, and the reactivity of hydride intermediates, as potential catalytically active centers in alkene dimerization and oligomerization.

2. Catalytic Synthesis of Terminal Alkene Dimers and Oligomers

In the literature, significant attention is given to Ti subgroup metal complexes, the use of which, in homogeneous catalytic systems, ensures alkene dimerization, oligomerization, and polymerization with high yields and high chemo- and stereoselectivity [1,5,40]. The selective transformation of α-olefins (propene (1a), 1-butene (1b), 1-hexene (1c), 1-octene (1d), and 3-methyl-1-butene (1e)) into vinylidene dimers (2a–e) under the action of a catalytic system consisting of Cp₂ZrCl₂ (3) or Cp₂ZrMe₂ (4) and aluminoxane, obtained in situ through the reaction of AlR₃ (R = Me, Et, and Buⁱ) with CuSO₄·5H₂O, was reported in one of the first works on this topic (Scheme 2) [43]. Dimeric products were obtained with a selectivity of up to 96% during the reaction performed at 40–70 °C for 0.5–2 h with the following reagent ratio: [Zr]:[Al]:[1-alkene] = 1:(8–100):(600–4670). The highest alkene conversion rate and selectivity towards the dimerization were achieved in the reaction, which was catalyzed with Cp₂ZrCl₂ and methylaluminoxane (MAO).

A dimeric product (2d) was obtained with a yield of 59% in the reaction of 1-octene with AlMe₃ in the presence of Cp₂ZrCl₂ (3) in 1,2-dichloroethane at 22 °C for 12 h (Scheme 3) [44]. It was assumed that the initial stage of the reaction was the alkene carbometalation and the formation of metal alkyl 7, which hydrometalates 1-octene through state 8. As a result, 2-methyl-1-octene and the hydrometalation product, n-OctML_n, are accumulated in the mixture. The latter reacts with 1-octene via carbometalation to provide 2-(n-hexyl)-1-decyl metal that hydrometalates 1-octene to form n-OctML_n and dimer 2d (Scheme 3).

Furthermore, terminal alkenes 1b, 1c, and 1e–h were dimerized in the presence of the catalytic system Cp₂ZrCl₂-MAO at an Al/Zr ratio of 1:1 and a room temperature of 25 °C for 24 h with the product yield of 80–90% (Scheme 4) [45,46]. 3-Methyl-1-butene (1e) provided a mixture of 2-methyl-2-butene (9e, 77%), 2-methyl-1-butene (10e, 17%), and 5-methyl-2-(methylethyl)-1-hexene (2e, 3%). The reaction of o-diallylbenzene (1i) with Cp₂ZrCl₂ and MAO at an Al/Zr ratio of 4:1 for 3 days provided a cyclic product, methylenecycloheptane 11i, with a 70% yield.

The mechanism proposed in [45,46] for the dimerization reaction implies the insertion of 1-alkene into a Zr-H bond of zirconocene hydride 14 to obtain Zr-alkyl complex 15, which then carbometalates the second alkene molecule, producing alkyl derivative 16. Subsequent β-H elimination in alkyl complex 16 provides dimer 2 and hydride complex 14 (Scheme 4). It is noted that the presence of chlorine in a catalytic system is an important factor for the dimerization reaction. In confirmation, a higher oligomer formation was seen in the presence of Cp₂ZrMe₂ (4) as a catalyst and MAO (without Cl atoms) was given. The chlorine atom probably ensures the fast β-H elimination, but not the growth of an alkyl chain [46].

A dimeric hydride complex, [(2,4,7-Me₃-Ind)₂Y(μ-H)]₂ (17) (Scheme 5), appeared to be an effective catalyst for the homodimerization of various α-olefins [47]. The reaction was performed in benzene at 80–100 °C for 2–24 h, and there was a 20–50-fold molar excess of α-olefins. The head-to-tail dimerization was observed in the case of 1-hexene (1c) and 3-methyl-1-butene (1e) with a selectivity of >98%.

The reaction runs through sequential 1,2-insertion followed by β-H elimination (Scheme 6) [47]. The homodimerization of trimethylvinylsilane (1i) and styrene (1k) occurred, forming head-to-head products. The reactions presumably proceed through an initial 1,2-insertion into the Y-H bond, followed by a subsequent 2,1-coordination and β-H abstraction. Olefins 1l–o, containing heteroatoms, 3,3-dimethyl-1-butene (1j), and allylbenzene (1h), did not undergo homodimerization under the reaction conditions. In the reaction with 1h, C-H activation arose, resulting in the formation of a catalytically inactive allyl complex, Ind’₂Y(η³-CH₂CHCHPh). The reaction of 17 with 1l–o provided stable alkyl complexes that deactivated the catalyst.

Complex 17 also showed activity in the styrene codimerization with alkenes (H₂C=CHR) that produced trans-1-phenyl-4-alkylbut-1-enes (20) with more than an 88% yield at 80–100 °C (Scheme 5). The reaction probably occurred through the 1,2-coordination of α-olefin into the Y-H bond, followed by a subsequent 2,1-insertion of styrene into the Y-C bond of the alkyl intermediate and β-H elimination (Scheme 6). Heteroatom-containing olefins 1l–o readily formed head-to-head codimers with styrene. However, these substrates exhibited lower reactivity, and the accompanying formation of the styrene homodimer was observed.

Complexes of various structures were subsequently tested in the reactions to find selective catalysts for alkene dimerization and oligomerization. For example, 1-pentene (1f) was transformed into oligomers in the presence of catalytic systems based on bis-cyclopentadienyl complexes 3, 22, and 23 and MAO in a ratio ([Zr]:[MAO]:[substrate] = 1:1000:30,000) at 60 °C for 24 h in toluene (Scheme 7) [48]. Oligomeric products with low molecular weights were obtained: dimers (25%), trimers (18%), and tetramers (14%). The use of catalysts with ansa-indenyl ligands (EBI)ZrCl₂ (24) and (SBI)ZrCl₂ (25) led to the formation of isotactic poly(1-pentene) (M_N = 1700–4400 g mol⁻¹, PDI = 4.75–6.41). Further studies on the catalytic properties of complexes 3 and 22–28 at a reagent ratio ([metallocene]:[MAO]:[1-alkene] = 1:(1000–8000):30,000) at 20–150 °C demonstrated the dependence of the reaction chemoselectivity upon the metallocene structure [49]. The reaction of 1-pentene, catalyzed with complexes 22 and 23 and MAO ([Zr]:[MAO] = 1:1000), resulted in an atactic polypentene, whereas ansa-indenyl complexes 24–26 provided an isotactic polymer with stereoselectivities of 0.91, 0.45, and 0.64 (mmmm), respectively. The polymer with the highest molecular weight (M_W = 149,000, PDI = 1.85–2.08) was obtained by using catalyst 26. The reaction, which was catalyzed with cyclopentadienyl complexes 3, 27, and 28, under these conditions, afforded the oligomeric products with 2–4 units. In this case, the highest conversion (50%) was achieved in the presence of bimetallic complex 27, whereas the yields of dimers, trimers, and tetramers were 10, 20, and 20%, respectively. An increase in the amount of MAO to 6000 eq. in the case of complex 3 caused an increase to 80% in the alkene conversion, and the yields of dimers, trimers, and tetramers increased to 15, 30, and 35%, respectively (Scheme 7) [49].

Branched α-olefins were regioselectively dimerized at 20 °C in toluene for 3–142 h upon the action of Cp₂MCl₂ (M = Ti (29), Zr (3), and Hf (22)) or Me₂SiCp₂ZrCl₂ (30) and MAO at the following ratio: [M]:[MAO] = 1:581 (Scheme 8) [50]. Complex 30 with Me₂Si-bonded cyclopentadienyl ligands showed the highest activity and regioselectivity, providing dimers 2q–t with yields of up to 100%. 3-Methyl-1-butene (1e) and 3-methyl-1-pentene (1p) provided dimers with yields of 11 and 19%, as well as oligomeric products 21e and 21p, correspondingly.

As a rule, the application of other transition metal complexes changes the regioselectivity of a reaction. For example, pyridine bis(imine) iron complexes 31a,b, upon activation with MAO, MMAO, or AlR₃ (R = Et, Buⁱ)-B(C₆F₅)₃ (Al/Fe = 70–480), demonstrate the ability to dimerize various α-olefins, such as 1b, 1c, 1u, and 1x (Scheme 9) [51]. This results in the formation of a mixture of linear olefin dimers 18b, 18c, 18u, and 18x with internal double bonds (63–80%) and monomethyl-branched dimers 35b, 35c, 35u, and 35x (18–36%). Additionally, trisubstituted vinylidene (2-alkylalkenes) or α-olefinic products were detected in trace amounts. High alkene conversion up to 76% was achieved in the presence of 31a–c and 31e at 30–50 °C. The sterically less hindered complex 31d provided monomethyl-branched dimers 35b, 35c, 35u, and 35x.

The reaction mechanism consists of consecutive stages of the 1,2-insertion of the initial olefin into the Fe-H bond, followed by a 2,1-insertion of the second olefin (Scheme 10). Subsequent β-H elimination leads to the formation of linear dimers. Successive 2,1-insertions of alkenes and β-H eliminations produce Me-substituted dimers.

Pyridine bis(imine) cobalt catalysts 32a–d, when activated with MMAO (Al/Co = 200–500), dimerized α-olefins with lower productivity compared to similar iron systems (TON for 1-butene: 42,000 vs. 147,000 for Co and Fe, respectively) (Scheme 11) [52]. The main products were linear dimers (>97%) and butene isomers in an 18b/iso-1b ratio of 0.47–0.7. In the dimerization of propylene, linear hexenes, nonenes, and dodecenes were obtained with turnovers exceeding 200,000 moles of propylene/mole Co (17,000 g oligomer/g Co complex). Complexes 32a and 32b, in combination with MMAO or EtAlCl₂, induced the isomerization of 1-hexene.

The main stages of the stepwise reaction mechanism include a consecutive 1,2-insertion of olefin and a 2,1-insertion into Co-Alkyl, followed by chain termination to provide alkenes with internal and terminal double bonds (Scheme 12) [52].

Nevertheless, mixed ethylene Co complex 32e catalyzed the transformation of terminal alkenes into vinylidene dimers of a head-to-tail type with yields of 66–80% in the presence of an organoboron activator, HBArF, at a [Co]:[B]:[1-alkene] ratio of 1:0.81:670 in contrast to the post-metallocene catalysts 32a–d (Scheme 13) [53]. Moreover, linear terminal alkene 1w formed in the reaction with a yield of up to 14%, presumably due to the isomerization processes in intermediate alkyl Co complexes.

α-Olefins 1c, 1d, 1f, 1g, and 1x undergo tail-to-tail dimerization under the action of a catalytic system, WCl₆/R′NH₂/R″₃N/EtAlCl₂, obtained in situ at a [W]:[R′NH₂]:[R″₃N]:[EtAlCl₂]:[1-alkene] molar ratio of 1:(1–4):(0–4):12:(834–5000) to provide predominantly methyl-branched products (33c, 33d, 33f, 33g, and 33x) (Scheme 14) [54]. Alkene conversion at a level of 80% and high selectivity towards the dimerization were achieved (>99%) due to the optimal choice of a chlorine-containing organoaluminum activator (EtAlCl₂) and a solvent, PhCl. This effect on the reaction initiation was attributed to the generation of bimetallic catalytically active centers with a W-Cl-Al bridge.

Upon a detailed analysis of the reaction products using the example of 1-pentene dimers, it was demonstrated that fractions of linear C10 products (constituting only 0.1% of the dimer fraction) contain trans-5-decene, cis-4-decene, dienes, and decane (Scheme 15) [54]. The authors proposed a Cossee-type mechanism [9], noting that the initial insertion of an alkene occurs equally as 1,2- and 2,1-, followed by a subsequent regioselective alkene 1,2-coordination. Therefore, the dominant structures appear to be B and C, which provide the main reaction products.

Low-molecular-weight oligomeric products, including 1-hexene dimers, were synthesized with high yields (73–97%) and selectivity (≥98%) in the presence of Zr and Hf post-metallocene complexes with amino-bis(phenolate) [ONNO] ligands and a neutral activator, B(C₆F₅)₃, at 65–85 °C for 4 h and the following reagent ratio: [metallocene]:[B]:[1-hexene] = 1:1.1:100 (Scheme 16) [55]. The highest activity in the oligomerization was achieved in the presence of Zr catalysts 34a–c; in this case, the molecular weights of the products corresponded to a typical Flory–Schulz distribution [56]. Hafnium catalysts 34d–f showed lower activities in contrast to the zirconium analogs; however, they showed greater selectivity in dimerization. In addition, the molecular weight distribution of the products obtained in the presence of hafnium catalysts did not follow the Schultz–Flory distribution. The high selectivity in the formation of vinylidene dimers was explained by the prevalence of 1,2-alkene insertions into catalytically active centers, both in primary M-H and secondary M-Alkyl species. It was also noted that the chain termination rate for these systems exceeds the rate of chain propagation. In the case of regioerror, i.e., alkene 2,1-insertions, conversely, the chain propagation prevails because the elimination is practically impossible; therefore, chain termination via β-H elimination will occur when the 1,2-incorporation of an alkene takes place. The authors explain that deviations from the Schultz–Flory distribution are caused by the presence of two or more conformations of hafnium active centers, which have different activities towards alkene (the assumption was made from the ¹H NMR spectra of the initial complexes depending on temperature). For zirconium analogs, it seems that either one isomer is characteristic, or the exchange between conformations is very fast (the energy barrier is small), so it does not significantly affect the distribution of oligomers.

A highly regioselective method for the oligomerization of 1-hexene and 1-octene was developed at relatively low catalyst loadings (0.0019–0.0075 mol%) using Zr complexes 35a and 35b with [OSSO]-type aryl-substituted bis(phenolate) ligands and modified methylaluminoxane (dMMAO) (Scheme 17) [57]. The catalytic system predominantly produced dimers with terminal vinylidene groups (74–91%) and trimers (8–11%) at 25–40 °C for 1 h with the following reagent ratio: [Zr]:[Al]:[1-alkene] = 1:(100–300):(13,350–53,500). The TOF values were adjusted by changing the structure of an aryl substituent R₁ at the ortho position of a phenolate moiety of the [OSSO] ligand and the number of dMMAO equivalents used. The highest TOF value (up to 11,100 h⁻¹) was observed for phenyl-substituted precatalyst 35a. The authors explained the low alkene conversion (10–77%) in the presence of 35a and 35b with the deactivation of Zr-H active species during oligomerization.

Bis-phenolate titanium complexes 36a–c, activated by B(C₆F₅)₃ ([Ti]/[B] = 1), catalyzed the transformation of 1-hexene into vinylene oligomers with high yield (up to 97%) and selectivity (99%) (Scheme 18) [58]. Zirconium (36d) and hafnium analogs (36e) showed significantly lower activity (yield of up to 22%), but better selectivity towards vinylidene oligomers (up to 95%). This dependence of regioselectivity on the nature of the transition metal was confirmed in an experiment with ¹³C-labeled hexene: the cross-linking of an alkene in the case of a Zr catalyst occurs as successive stages of a 1,2-insertion of an olefin into M-H species, a 1,2-insertion of an alkene into M-Alkyl, and β-H elimination. In the case of Ti, the stages of 1,2-olefin insertion into M-H, 2,1-alkene insertion into M-Alkyl, and β-H elimination occur. The rate of 2,1-olefin insertion is affected by solvation, an increase in the bulkiness of the ligand and the growing chain, as well as temperature. Thus, low temperatures down to −80 °C in the case of 36a led to the following ratio: [vinylene]/[vinylidene] = 52/48.

Dimers and oligomers of terminal alkenes were synthesized in catalytic systems based on various zirconocenes (3 and 37–52) with cyclopentadienyl ligands; indenyl ligands; fluorenyl ligands, including ansa-complexes; and heterocenes, which were activated in several steps by AlBuⁱ₃, Et₂AlCl, and methylaluminoxane (Scheme 19) [5,40,59,60,61,62]. Cyclopentadienyl complexes, including Cp₂ZrCl₂ (3), (Me₂C)₂Cp₂ZrCl₂ (37), (Me₂Si)₂Cp₂ZrCl₂ (38), and OSiMe₂Cp₂ZrCl₂ (39), at low Al_MAO/Zr ratios (1–10), catalyzed the regio- and chemoselective formation of head-to-tail α-olefin dimers with yields of 82–94% and 100% alkene conversion [40,61]. The oligomers of α-olefins (1-hexene, 1-octene, and 1-decene) were obtained in the reactions, catalyzed by zirconocenes 40, 41, 42, and 48 and organoaluminum cocatalysts at the following ratio: [Zr]:[AlBuⁱ₃]:[MAO]:[1-alkene] = 1:20:10:2000 [40,59,60,62]. A yield of 1-hexene dimer decreased to 40–52% and a yield of oligomers increased to 55–57% under the same conditions in the presence of the CpIndZr₂Cl₂ complex (44) [59,60]. Higher 1-hexene oligomers with Mw = 3900 Da were produced by the Ind₂ZrCl₂ complex (45) [59]. The TOF values were (1–2.4)·10⁵ h⁻¹ when 46 and 47 were used in the oligomerization of alkenes 1c, 1d, 1u, and 1x [40].

To explain the catalytic action of the systems, the mechanisms presented in Scheme 20, Scheme 21 and Scheme 22 were suggested. For example, Zr,Al complex A stabilized by the ClMAO^– anion [63] formed in the reaction of Cp₂ZrCl₂ with AlBuⁱ₃ and MAO was proposed as a catalytically active center for the alkene dimerization reaction (Scheme 20) [59]. An excess of OAC (MAO or AlBuⁱ₃) increases the amount of dihydride complex B. Catalytically active center A coordinates an alkene molecule at the Zr–H bond to form alkyl complex A1, the further alkylation of which provides intermediate A2. Chlorine atoms in complex A2 promote the process of β-H transfer to the Zr atom, rather than the coordination of the third and subsequent substrate molecules (chain growth). An alkene dimer and catalytically active center A are formed after β-H transfer to a Zr atom. Intermediate B is electrophilic and seems to be sterically accessible for α-olefin oligomerization. An increase in selectivity of the reaction observed in the dimerization after the treatment of a reaction mixture with Et₂AlCl is probably due to the conversion of B to A (Scheme 20). The selectivity of the dimerization reaction of α-olefins, therefore, depends mainly on the ratio of catalytically active sites A and B.

The initial stages of the propene dimerization and oligomerization with the participation of Zr,Al–complexes were simulated at the DFT M-06X/DGDZVP level of theory to confirm the proposed mechanism [62]. The profiles of propene oligomerization reactions catalyzed by [Cp₂ZrH]⁺ cation I-0 and cationic bimetallic complexes [Cp₂Zr(µ-H)(µ-X)AlR₂]⁺ (X = H, Cl, and Me; R = Me and Buⁱ) I-0X were constructed (Scheme 21). Further, activation energies were calculated for the two reaction pathways: the formation of a vinylidene propene dimer via TS-4 and the chain growth via TS-5 (Scheme 22).

A difference between the mechanisms for traditional mononuclear [Cp₂Zr-alkyl]⁺ and binuclear [Cp₂Zr-alkyl(R₂AlX)]⁺ species is shown (Scheme 22). Without R₂AlX coordination, oligomerization is the favored reaction route. When X = H, highly stable β-agostic complexes I-2X-bo form, so the reactions slow down. If X = Cl, the main direction of the action is the formation of vinylidene dimers. The transition states of β-H elimination TS-4X (X = H and Cl) show a Zr-Al concerted effect. If X = Me, then there is no significant promotion in the β-H elimination process in TS-4 [62].

It was found that the use of molecular hydrogen at a low MAO concentration leads to the results being not typical for Ziegler–Natta processes [62]. The dimerization accelerates, and the selectivity of the reaction in this pathway increases without the formation of hydrogenolysis products in the presence of hydrogen. The DFT simulation showed that the I-2H-bo complex can react with H₂ without breaking an H-H bond but with a loss of β-agostic coordination. Molecular hydrogen, therefore, acts as an additional activator for the I-2H-bo hydride complex, which is probably an active and selective catalyst of a dimerization reaction.

Zr heterocene complexes 48 and 49a–f modified with AlBuⁱ₃ and MMAO-12 were studied in the reaction of 1-decene oligomerization in molecular hydrogen at a [Zr]:[Al]:[MAO]:[1-alkene] ratio of 1:10:75:50,000 at 80–100 °C (Scheme 13) [64]. The conversion of 1-decene reached 99% in the presence of 49d at 80 °C for 4 h, and the formation of low-viscosity oligomers was observed. As the temperature rises to 100 °C, the content of 1-decene dimer increases to 28%. Nevertheless, heterocene 49f was shown to be the most effective catalyst for the synthesis of low-viscosity 1-decene oligomers among the studied complexes. Moreover, the catalytic system based on complex 49f and an activator, (PhHNMe₂)[B(C₆F₅)₄], enabled us to achieve a maximum yield (63 wt%) of the most valuable trimer–tetramer fractions of alkene oligomers at a [Zr]:[Al]:[B]:[1-alkene] ratio of reagents of 1:150:1.5:200,000 in 1 atm H₂ at 80–110 °C [64].

Unsymmetrical complexes 50a–c, 51, and 52 in the presence of AlBuⁱ₃, (PhHNMe₂)[B(C₆F₅)₄], and H₂ (1 bar) at a [Zr]:[Al]:[B]:[1-alkene] ratio of 1:100:1.5:100,000 at 100 °C catalyzed the formation of light 1-decene oligomers with an alkene conversion rate of 86–99% [65]. A gradual decrease in the reaction temperature as 1-decene was shown to reduce the content of dimers (down to 10%) and increase the proportion of oligomers (up to 84%) in the reaction products.

The authors of [64] presented a mechanism for the activation of zirconocene complexes by isobutylalanes, arylboranes, and MAO, as depicted in Scheme 23. They noticed that the classical mechanism implies the participation of active catalytic species, such as alkyl zirconocene cations (L₂Zr-R⁺ (L₂ = η⁵-ligands)) stabilized by [B(C₆F₅)₄]^–, [B(C₆F₅)₃R]^–, or XMAO^– counterions (X = Cl and Me) (Scheme 23A). The reaction between L₂ZrCl₂ and AlBuⁱ₃ produces zirconocene alkyl chloride, L₂Zr(Cl)Buⁱ. An excess of AlBuⁱ₃ or HAlBuⁱ₂ provides various neutral hydride Zr,Al complexes D and E (Scheme 23B). A cationic hydride bimetallic complex F is generated in the presence of perfluoroarylboranes (Scheme 23B). Under the action of excess ClAlBuⁱ₂, cation F transforms into dichloride Zr,Al-complex G, which can also be formed by the reaction between L₂ZrCl₂ and R₂Al⁺. Complex G was isolated and characterized by NMR and an X-ray diffraction analysis [64]. Cationic hydride complex F belongs to the category of dormant states as well as species [L₂Zr-(μ-Me)₂-AlMe₂]⁺ (H). Alkenyl hydride Zr-(μ-H)-Al complex I formed in the presence of excess AlBuⁱ₃ is considered to be potentially active towards α-olefins (Scheme 23B) [64]. However, a complex similar to I was shown to be inactive in alkene polymerization [66,67]. Moreover, the reaction mechanism involving metallocenes and AlBuⁱ₃ should take into account the participation of a cationic species, “AlBuⁱ₂⁺”, formed as a result of the reaction of OAC with a boron activator.

Further, when considering the possible stages of alkene oligomerization (which are coordination, chain growth, and termination, involving the β-H transfer and β-H elimination stages), the authors noted [65] that in the case of heterocenes, the processes of β-H elimination apparently prevail at the final stage of the reaction, when most of the monomer is consumed, leading to the accumulation of C20 dimers in the products (Scheme 24). The β-H elimination can be facilitated by the coordination of R’₂AlCl at the Zr center. The competing processes of chain propagation and termination are influenced by both steric and electronic factors of the η⁵-ligand. It is noted that electron-donating alkyl substituents in the ligand of the complex lead to a decrease in the electrophilicity of the Zr atom and, consequently, to a decrease in catalytic activity, for example, in the case of 49f vs. 50a and 50b. Nevertheless, the lower electrophilicity of Zr (49f) or steric hindrances (for example, in the case of 51 or 52) of the ligand does not promote β-H transfer, which provides higher yields of C30+ or C50+ oligomers [65].

The alkene oligomers were obtained in the reaction catalyzed by ansa-Ph₂Si(Cp)(9-Flu)ZrCl₂ (53) in the presence of MAO at an Al/Zr of 200 and a temperature of 60 °C (Scheme 25) [68]. This system showed the activity to be 131–155 kg mol_Zr⁻¹ h⁻¹ (PDI = 2.06–2.25). The oligomers constituted a mixture of regioisomeric products with a terminal vinylidene (21) and internal double bonds (54) according to the ¹H and ¹³C NMR spectra. Oligomers with vinylene R′CH=CHR″ and vinylidene CH₂=CHR′R″ groups were the major products. Oligomers containing internal disubstituted vinylene groups were formed through 2,1-insertion and β-H elimination or 2,1-insertion and rearrangement, followed by β-H elimination. An NMR analysis of the intensities of the double bond signals and saturated end groups showed the preferential chain transfer to the cocatalyst.

Zirconocenes (3, 25, 56–59) and methylaluminoxane catalyzed terminal alkene transformation into dimers 2c, 2d, and 2u with yields of up to 89% at a [Zr]:[MAO]:[1-alkene] reactant ratio of 1:(50–300):(289–2600) and a temperature of 70–100 °C for 3–4 h (Scheme 26) [69]. Catalysts 25 and 59 exhibited the highest activity in the oligomerization reaction. Complex 59 demonstrated superior selectivity towards dimer formation. Dimers 2c, 2d, and 2u were converted into tetramers (55c, 55d, and 55u) under the action of a TiCl₄-Et₂AlCl system.

The authors proposed a mechanism for metallocene-catalyzed dimerization based on a structural analysis of alkene dimers (Scheme 27) [69]. The formation of unsaturated (structures A–C) and saturated products (structure D) occurred due to the β-H elimination at cationic metal alkyl centers and chain transfer to a non-transition metal atom (Al), respectively [69]. The vinylidene group (-C=CH₂) (structure C) was generated via a 1,2-coordination of an alkene with a [Cp₂ZrH]⁺ cation and the subsequent β-H elimination of the product. Alkene 1,2-coordination, cation rearrangement, and β-H elimination produce structures A and B with trisubstituted vinyl groups (-C=C(CH₃)-).

1-Decene was transformed into oligomers under the action of post-metallocene complexes [M{2,2′-(OC₆H₂-4,6-^tBu₂)₂NHC₂H₄NH}(OⁱPr)₂] (60a–c) (M = Ti (a), Zr (b), and Hf (c)) and the (Ph₃C)[B(C₆F₅)₄] activator at a [B]:[M] ratio of 1:(0.25–1.5) at 80–120 °C [70] (Scheme 28). The activity of the catalytic system was quantified as 362–484 g_oligomer mmol_cat⁻¹ h⁻¹. The resulting oligomers were characterized by the tacticity values (mm + rr) of 88.5% (Ti), 87.3% (Zr), and 86.8% (Hf); the molecular weight (M_N = 445–608 g mol⁻¹); and the PDI value (1.13–1.30). The resulting oligomers differed in structure and contained vinylidene fragments (CH₂=CRR′ (21u, δ_H 4.7–4.8 ppm)), vinyl fragments (CH₂=CHR (54u, δ_H 4.9 and 5.6 ppm)), trisubstituted vinylene groups (RCH=CR′R″ (54u, δ_H 5.2 ppm)), and disubstituted vinylene groups (RCH=CHR′ (54u; δ_H 5.3–5.5 ppm)).

The monomer consumption, the number of active sites, and the number of unsaturated end groups during the oligomerization reaction were evaluated for each catalytic system in the course of the study of the kinetics of 1-decene oligomerization reaction catalyzed by 60a–c (Scheme 28) [70]. An initiation rate constant (k_i) in the presence of complex 60b appeared to be higher than those of 60a and 60c (Scheme 28). The k_i value was inversely related to the molecular weight of an oligomeric product. A catalyst with a high k_i, when the number of active centers is high, leads to low-molecular-weight oligomers. The Ti-based catalytic system exhibited a higher chain propagation rate compared to those of the Zr- and Hf-based systems. Moreover, the reaction initiation stage was found to be slower in comparison to the chain propagation. A decrease in the chain growth constants k_p in the Ti > Hf > Zr series was probably due to the electronic nature of metal centers. The rate of formation of a vinylidene product did not depend on the concentration of 1-decene, whereas the rate of formation of a product with an internal double bond was of the first order relative to the monomer concentration. The k_vinylidene and k_vinylene values were calculated from the initiation rate constants, k_i, where k_vinylene > k_vinylidene by a factor of 2–10. The degree of catalyst involvement in the reaction was 40–60%. The misinsertion stage was slower than the propagation stage for all studied catalysts. The chain termination process runs via the chain β-H transfer to a monomer and the β-H elimination reaction (Scheme 28) [70].

A study of the activity and chemoselectivity of η⁵-metal complexes 3, 22–24, 29, 30, 37, 45, 58, and 61–64 in the presence of various OACs (HAlBuⁱ₂, ClAlMe₂, ClAlEt₂, ClAlBuⁱ₂, AlMe₃, AlEt₃, and AlBuⁱ₃) and activators (MMAO-12, (Ph₃C)[B(C₆F₅)₄], and B(C₆F₅)₃) in alkene dimerization and oligomerization showed that either HAlBuⁱ₂ or AlBuⁱ₃ at certain ratios ensure the selectivity of the reaction towards dimerization in comparison with AlMe₃ or AlEt₃ (Scheme 29) [71]. Moreover, Cp₂ZrCl₂-(AlBuⁱ₃ or HAlBuⁱ₂) or [Cp₂ZrH₂]₂-ClAlR₂ (R = Me, Et, Buⁱ) systems produced predominantly head-to-tail dimers (2c,d,h,k,u,z) in the presence of MMAO-12 or B(C₆F₅)₃ activators at the [Zr]:[Al]:[MMAO-12]:[1-alkene] ratio of 1:3:30:(50–1000) or the [Zr]:[Al]:[B]:[alkene] ratio of 4:16:1:1000, correspondingly, at 20–60 °C for 5–180 min in toluene with a yield of up to 98% (2c, 98%; 2d, 91%; 2u, 87%; 2z, 95%; 2h, 61%; 2k, 58%) (Scheme 29) [71,72].

The use of chlorinated solvents (CH₂Cl₂ and CHCl₃) in the Cp₂ZrY₂-YAlBuⁱ₂ (Y = H, Cl) systems’ activator (MMAO-12, (Ph₃C)[B(C₆F₅)₄]) accelerated the reaction and increased the yield of dimeric products [73]. Under these conditions, the dimers obtained in the first minutes were substrates for the subsequent dimerization and formation of tetramer 55 with yields of up to 79%. Adding an ionic-type cocatalyst, (Ph₃C)[B(C₆F₅)₄], to either the Cp₂ZrCl₂-HAlBuⁱ₂ or [Cp₂ZrH₂]₂-ClAlBuⁱ₂ catalytic systems typically resulted in the formation of oligomeric products [72]. Replacing the transition metal atom from Zr to Ti or Hf under the same conditions led to a decrease in activity and selectivity towards dimers [73].

A study on the influence of the ligand structure on the activity and chemoselectivity of the system L₂ZrCl₂-HAlBuⁱ₂-MMAO-12 revealed that dimerization occurs with the participation of Zr complexes with sterically unhindered ligands (L = Cp, ansa-Me₂CCp₂, ansa-(Me₂C)₂Cp₂, and ansa-Me₂SiCp₂) [74]. Zirconocenes with bulky cyclopentadienyl (L = C₅Me₅ and rac-H₄C₂[THInd]₂) or electron-withdrawing indenyl (L = Ind, Me₂CInd₂, H₄C₂[Ind]₂ and BIPh(Ind)₂) substituents in the presence of HAlBuⁱ₂ and MMAO-12 or (Ph₃C)[B(C₆F₅)₄] activators predominantly yielded 1-hexene oligomers, which is consistent with the data in Ref. [60]. The assessment of the stereoselectivity of the reaction using ¹³C NMR spectroscopy showed a dependence of this parameter on the π-ligand environment of the metal and the type of activator [74]. Catalysts with indenyl ligands 45, 62, and 24 were found to be the most stereoselective, demonstrating isotacticity levels of 67%, 93%, and 71%, respectively. An oligomer with an isotacticity level of 67% was obtained under the action of complex 45 in the presence of MMAO-12, whereas (Ph₃C)[B(C₆F₅)₄] led to an atactic product. The opposite situation was observed for complex 62 with ansa-bridged ligands: the highest stereoselectivity was achieved in the presence of (Ph₃C)[B(C₆F₅)₄].

These facts indicate that a cocatalyst has a significant influence on the stereoregulation process during the alkene coordination through catalytically active centers. As a result, the data on the structure and reactivity of possible intermediates [71,72,74,75], the high selectivity of a reaction towards the dimerization, and completely different rates of oligomerization and dimerization processes allow us to propose a mechanism (Scheme 30). The mechanism implies the involvement of bis-zirconium hydride structures as precursors of dimerization reaction active sites. At the first stage of the reaction, the hydrometalation of alkenes proceeds with the participation of one of the zirconium centers. The introduction of the second alkene molecule, the carbometalation stage, and the β-H elimination stage can also proceed in concert with the involvement of two zirconium atoms. Finally, the dimerization product (2) and the starting bis-zirconium complex are formed. Examples of such bimetallic catalysis are known for the polymerization of alkenes in the presence of subgroup 4 metal complexes [76], as well as ethylene tetramerization reactions on chromium catalysts [77,78,79].

Thus, the literature provides extensive information on the dimerization and oligomerization of alkenes under the action of homogeneous catalytic systems based on metallocenes and post-metallocenes. Typically, these works emphasize the key roles of metal hydride intermediates as active species. Therefore, the study of the structure and reactivity of hydride complexes of transition metals is a relevant task to develop models of reaction mechanisms.

3. Structure of Catalytically Active Centers

3.1. Reactions of Organoaluminum Compounds with Activators and Metal Complexes

Many researchers noted that the formation of catalytically active centers for the oligo- and polymerization of alkenes is preceded by the interaction of the activator with organoaluminum compounds. For example, in the reaction of AlR₃ (R = Me, Et, and Buⁱ) with a B-containing activator upon heating and different Al/B ratios, the formation of a mixture of AlR_3–x(C₆F₅)_x derivatives was found (Scheme 31 and Scheme 32) [80]. The NMR monitoring of the reaction of AlMe₃ with (Ph₃C)[B(C₆F₅)₄] in d₈-toluene at a temperature of 60 °C for 4.5 h showed that MeCPh₃ (δ_H 0.74 ppm) and BMe₃ (δ_B 86.8 ppm) were formed. It is assumed that the interaction between AlMe₃ and (Ph₃C)[B(C₆F₅)₄] first gives the intermediate [AlMe₂]⁺[B(C₆F₅)₄]^–, which immediately decomposes to AlMe₂(C₆F₅) and B(C₆F₅)₃ (Scheme 31). The transformation of [B(C₆F₅)₄]^- is started due to the generation of the highly electrophilic “[AlR₂]⁺” cation. Over time, the replacement of the Me group in the OAC molecule by C₆F₅ occurs to provide the final products, Al(C₆F₅)₃ and BMe₃. Moreover, neutral B(C₆F₅)₃ also participates in ligand exchange with AlMe₃. The organoaluminum products of intermolecular exchange differed in the values of the ¹⁹F NMR chemical shifts δ_F presented in Scheme 31. The further interaction of Al(C₆F₅)₃ with Cp₂ZrMe₂ at −60 °C in CD₂Cl₂ provided [Cp₂ZrMe(µ-Me)Al(C₆F₅)₃] (65). In the ¹H NMR spectrum of compound 65, singlet signals of protons were observed: a Cp ring at δ_H 6.44 ppm and Zr-Me and Zr-Me-Al groups at δ_H 0.51 and −0.26 ppm, respectively. The reaction of Cp₂ZrMe₂ with AlMe₂(C₆F₅) or Al(C₆F₅)₃ (at a Zr:Al ratio of 1:1) in d₈-toluene at room temperature provided a yellow complex, [Cp₂ZrMe(C₆F₅)] (66), and the ¹H NMR spectrum exhibited characteristic signals of both Cp and Me groups at δ_H 5.66 and 0.31 ppm, correspondingly [80].

The reaction of AlBuⁱ₃ with (Ph₃C)[B(C₆F₅)₄] was accompanied by the elimination of isobutene, Ph₃CH, and the generation of anunstable ionic pair, [AlBuⁱ₂]⁺[B(C₆F₅)₄]^–, which also decomposed to AlBuⁱ_3–x(C₆F₅)_x and BBuⁱ_x(C₆F₅)_3–x (Scheme 32) [80].

A similar reaction of AlBuⁱ₃ with an activator, (PhNHMe₂)[B(C₆F₅)₄], produced AlBuⁱ_3–x(C₆F₅)_x, isobutane, and PhNMe₂, which was assumed to proceed through the formation of an ionic pair, [AlBuⁱ₂]⁺[B(C₆F₅)₄]^–, according to Scheme 33 [67]. Then, the ionic pair, [AlBuⁱ₂]⁺[B(C₆F₅)₄]^–, transformed into AlBuⁱ_3−x(C₆F₅)_x and BBuⁱ_x(C₆F₅)_3−x. The reaction of AlBuⁱ₃ with an activator, apparently, yields the “[AlBuⁱ₂]⁺” species, which further reacts with excess AlBuⁱ₃, producing HAlBuⁱ₂ and [Buⁱ₂AlCH₂CMe₂]⁺. The latter, upon losing isobutylene, regenerates the “[AlBuⁱ₂]⁺” cation.

Accumulating in the system, the [AlBuⁱ₂]⁺ cation removes a chlorine atom or a β-H from the Ph₂C(CpFlu)ZrClBuⁱ complex to provide [Ph₂C(CpFlu)ZrBuⁱ]⁺ (Scheme 34). Subsequently, upon reacting with an excess of AlBuⁱ₃, it yields a binuclear complex, [Ph₂C(CpFlu)ZrBuⁱ·AlBuⁱ₃]⁺ (67), which further provides metallocycle [Ph₂C(CpFlu)Zr(µ-H)(µ-C₄H₇)AlBuⁱ₂]⁺ (68) as a result of isobutane elimination [67].

In the course of NMR monitoring of the reaction of B(C₆F₅)₃ with AlEt₃ in CD₂Cl₂, Al(C₆F₅)_3–nEt_n monomers and Al₂(C₆F₅)_6–nEt_n dimers were observed [81]. Depending on the ratio of B(C₆F₅)₃ and OAC, the Al(C₆F₅)₃ → Al(C₆F₅)₂Et ↔ Al₂(C₆F₅)₄Et₂ → Al₂(C₆F₅)₃Et₃ → Al₂(C₆F₅)₂Et₄ → Al₂(C₆F₅)Et₅ compounds formed, which were clearly distinguished in the ¹⁹F NMR spectra by the signal of the p-F substituent in the benzene ring. For example, at a [B(C₆F₅)₃]:[AlEt₃] ratio of 1:9, the Al₂(C₆F₅)₂Et₄ (Al⁴*) and Al₂(C₆F₅)Et₅ (Al⁵*) dimers, together with a monomer, Al(C₆F₅)₂Et, were identified (Scheme 35).

Higher organoaluminum compounds AlR₃ (R = i-Bu and n-C₆H₁₃) were also capable of participating in an exchange reaction with B(C₆F₅)₃ [81]. The starting arylborane and its anion, [B(C₆F₅)₃R]^–, were in equilibrium at an equimolar ratio (Scheme 36). A large excess of AlR₃ shifts the equilibrium towards the exchange products—BR₃ and Al(C₆F₅)R₂ (R = Buⁱ and n-C₆H₁₃). The signals of B(C₆F₅)₃ (B), B(C₆F₅)₂(Buⁱ) (B*), and [B(C₆F₅)₃(Buⁱ)]^– (B*^–) were detected in the ¹⁹F NMR spectrum of the reaction mixture of AlBuⁱ₃ with B(C₆F₅)₃. There was almost no alkyl exchange with B(C₆F₅)₃ in the case of higher trialkylalane Al(n-C₈H₁₇)₃ [81].

The MAO cocatalyst also exchanges with OACs similar to B-containing activators. An NMR study, for example, showed that Buⁱ₂Al(µ-Me)₂AlBuⁱ₂ dimers and (AlMe_(1+2x−y)Buⁱ_yO(_1−x))_n clusters, whose methyl and methylene protons gave the signals at δ_H 1.12 and 0.34 ppm, respectively, formed in a system, (SBI)ZrCl₂-MAO-AlBuⁱ₃ [82]. These clusters (types I and II) contain aluminum centers with higher Lewis acidity levels compared to the corresponding clusters in the original MAO, judging by the EPR signals observed in these solutions upon the addition of TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl). The clusters were characterized by hyperfine structure constants, a_Al = 1.0 ± 0.1 (I) and 1.9 ± 0.1 (II) G. When AlBuⁱ₃ was added to MAO, the constant of type II Al centers increased (a_Al = 4.0–4.5 G). As a result, ion pairs of the [(SBI)ZrMe]⁺[Me-(MAO-TIBA)]^– type were detected in the catalytic system ((SBI)ZrCl₂-MAO-AlBuⁱ₃).

Further, it was shown that the addition of AlBuⁱ₃ to the solutions containing methylaluminoxane (MAO) and (SBI)ZrCl₂ provides Al₂(µ-Me)₂Me(_4−x)Buⁱ_x dimers (Scheme 37) [83]. The broadened signals in the ¹H NMR spectra at δ_H 0.35, 1.10, and 2.00 ppm were assigned to Buⁱ groups bound to MAO clusters. It was assumed that Buⁱ-MAO led to the transformation of a [(SBI)Zr(μ-Me)₂AlMe₂]⁺ cationic adduct into [(SBI)Zr(μ-Me)₂AlMeBuⁱ]⁺ and [(SBI)Zr(μ-Me)₂AlBu₂ⁱ]⁺. These compounds were unstable and subsequently transformed into zirconocene hydrides with isobutene elimination. MAO and AlBuⁱ₃, therefore, are exchanged actively by alkyl groups to form dialuminum derivatives and mixed aluminoxanes (Scheme 37).

An NMR study of a reaction mixture (SBI)ZrCl₂-MAO-HAlBuⁱ₂ showed that MAO exchange with HAlBuⁱ₂ provided H-substituted aluminoxane (Equation (1)) [84]. In the ¹H NMR spectrum of the mixture, a signal of an H atom at δ_H 3.75 ppm was observed at relatively high ratios of [HAlBuⁱ₂]:[Zr] > 20, alongside the signals of the Ind ligands at δ_H 5.54 and 6.51 ppm. The signal was attributed to alkylaluminum dimers with a hydride bridge, R₂Al(µ-R)(µ-H)AlR₂ (R = Me or Buⁱ). The signals of Buⁱ groups of Al₂(µ-Me)₂Me_4–xBuⁱ_x mixed dimers were observed at δ_H 1.86 ppm at low [HAlBuⁱ₂]:[Zr] ratios (<20). Two broadened signals at δ_H 3.60 and 4.10 ppm were assigned to hydride derivatives of MAO (H-MAO):

HAlBuⁱ₂ + Me-MAO → ½ Al₂(µ-Me)₂Buⁱ₄ + H-MAO

(1)

The activation of transition metal complexes in the L_nMCl₂-AlBuⁱ₃-[PhNMe₂H][B(C₆F₅)₄] or (Ph₃C)[B(C₆F₅)₄] systems occurs through the “AlBuⁱ₂⁺” cation, which is generated by the rapid reaction between borate salts and AlBuⁱ₃ (Scheme 38) [85]. An excess of AlBuⁱ₃ in the system alkylates the [L_nM-Cl]⁺ cation and provides [L_nMBuⁱ]⁺[B(C₆F₅)₄]^– species. Moreover, the reaction of AlBuⁱ₃ with NMe₂Ph in the presence of (Ph₃C)[B(C₆F₅)₄] produces an ionic compound, {[Buⁱ₂(PhNMe₂)Al]₂(µ-H)}⁺[B(C₆F₅)₄]^– (69a), which can act as an activator of the catalytic olefin polymerization reaction [85]. The ¹H NMR spectrum of OAC 69a exhibits singlet signals of hydride bridging atoms of the Al-H-Al bond at δ_H 2.86 ppm. The reaction mechanism involves the removal of the Cl atom from the initial metallocene by the [AlBuⁱ₂(NMe₂Ph)]⁺ cation and the subsequent Cl-H exchange between [L_nM-Cl]⁺ and the resulting HAl(NMe₂Ph)Buⁱ₂ to provide catalytically active hydride centers, such as [L_nM-H]⁺ (Scheme 38). As a result, a new activator, {[Buⁱ₂(PhNMe₂)Al]₂(µ-H)}⁺[B(C₆F₅)₄]⁻ (69a), has been proposed, which ensures the high catalytic activity of the entire system in olefin polymerization reactions [85].

As a continuation of this research, a new cocatalyst {[(RPhNMe₂)AlBuⁱ₂]₂(µ-H)}⁺[B(C₆F₅)₄]⁻ 69b (R = C₁₆H₃₃) was synthesized based on prototype 69a (Scheme 38). In contrast to its counterpart (69a), cocatalyst 69b is highly soluble in aliphatic hydrocarbons [86]. The broadened singlet signal of protons of the Al-H-Al bridge was observed at δ_H 2.91 ppm in the ¹H NMR spectrum of structure 69b. The addition of activator 69b to the rac-Me₂Si(2,6-(CH₃)2-4-Ph-1-Ind)₂ZrCl₂ catalyst in the presence of AlBuⁱ₃ enabled the copolymerization of ethylene and 1-hexene at P_C2H4 = 12 bar and 100 °C with a productivity of (0.42–0.61)·10⁶ kg mol_cat⁻¹ h⁻¹.

Thus, organoboron or aluminum activators undergo exchange reactions with aluminum alkyls or aluminum hydrides, leading to the formation of reactive species involved in the generation of catalytically active centers that initiate alkene oligo- and polymerization.

3.2. Structure and Reactivity of Zr,Al–Hydride and Zr,B–Hydride Complexes

The complexes of M,Al or M,B with hydride obtained in the reactions of metallocenes with organoboron or aluminum compounds attracted great attention due to their ability to function as highly active reagents or catalytically active centers of various reactions. Numerous research teams, therefore, synthesized and structurally identified the metallocene hydrides using spectral (NMR and IR) and X-ray diffraction methods.

For the first time, the synthesis of Zr,B–hydride (70,71) and Zr,Al–hydride complexes (72,73) was carried out by the reaction of Cp₂ZrCl₂ with LiBH₄ [87,88] or LiAlH₄ [89], respectively, at room temperature (Scheme 39). The ¹H NMR spectrum of complexes 74 and 75 exhibited a signal of Cp rings at δ_H 5.70 ppm and a quartet signal of four protons of the tetrahydroborate group at δ_H −0.20 ppm [88]. The IR spectra of complexes 76 and 77 contained absorption bands at 1425 cm⁻¹ (Zr-H-Zr bond) and two bands at 1790 and 1700 cm⁻¹ (AlH₄ bond).

The reaction of [Cp₂ZrH₂]₂ (61) with AlR₃ provided complexes 74a–c (Scheme 40) [90,91,92], whose ¹H NMR spectra exhibited triplet signals of bridging hydride atoms at δ_H −1.23–−0.92 ppm (Zr-H-Al bond), and −2.92–−2.74 ppm (Zr-H-Zr bond). Complexes 75b and 75c were obtained by the reaction of 61 with dialkylchloroalanes (Scheme 40) [93]. Complexes 75b and 75c were characterized by a broadened triplet at δ_H −2.68–−2.57 ppm (Zr-H-Zr) and a broadened singlet at δ_H −1.73–−1.52 ppm (Zr-H-Al).

Trihydride Zr,Al complexes 76 and 77 were observed in the reaction of Cp₂ZrCl₂ (3) with 3 eq. of HAlBuⁱ₂ (Scheme 41) [94]. The ¹H NMR spectrum of complex 77 exhibited signals corresponding to Zr-H-Al bridging hydride atoms at δ_H −0.28 and −2.03 ppm. Complex 76 displayed doublet and triplet signals for protons at δ_H −2.03 ppm (Zr-H-Al) and −0.90 ppm (Zr-H), respectively. Structure 78, similar to complex 76, was formed upon the interaction of Cp₂ZrMe₂ and AlBuⁱ₃ [95]. The ¹H NMR spectra of the complex also featured broad singlet signals for bridging hydride atoms at δ_H −2.23 and −1.75 ppm, and a triplet signal for the Zr-H bond at δ_H −1.22 ppm.

Hydride complexes [Cp₂Zr(H)(µ-H)₂AlH₂(L)] (L = C₇H₁₃N, NMe₃) 79a and 79b were obtained in a 50% yield in the Cp₂ZrCl₂-LiAlH₄ system in THF at 0 °C after the addition of H₃GaL (Scheme 42) [96]. Complexes 79a and 79b can be also synthesized by the reaction of Cp₂ZrH₂ with [H₃AlNR₃]. The IR spectrum of complex 79a showed absorption bands at 1736 and 1544 cm⁻¹ (Zr-H), and for complex 79b, bands were observed at 1768 and 1556 cm⁻¹.

The reaction of (Me₃SiCp)₂ZrCl₂ (80) with 2 eq. of LiAlH₄ in ether at room temperature yielded hydride complexes 81a and 81b (Scheme 43) [97]. The broadened signals of bridging hydrides of a Zr-H-Al bond at δ_H 2.06 ppm and broadened signals of terminal hydrides at δ_H −2.57 and −2.05 ppm were detected in the ¹H NMR spectrum of compound 81a. According to X-ray diffraction data, compound 81 crystallizes from the solution as two structures, 81a and 81b. Similar structures were also obtained for Ti (III) [98,99,100].

The reaction of Cp₂ZrMe₂ (4) with (Mes*AlH)₂ (Mes* = C₆H₂-2,4,6-Bu^t₃) led to the Cp₂Zr(H)(μ²-H)₂Al(Me)Mes* complex (82), whereas the Cp₂(H)Zr(μ²-H)₂Al(H)Mes* complex (83) was formed as a result of an interaction between Cp₂Zr(Cl)H and [Mes*AlH₃Li(THF)₂]₂ (Scheme 44) [101]. The structures of 82 and 83 were elucidated using an XRD analysis, NMR, and IR spectroscopy (Scheme 44). The ¹H NMR spectrum of complex 82 showed a doublet and singlet at δ_H −1.83 and −2.63 ppm (Zr-H-Al), a singlet at δ_H 0.02 (Al-Me), as well as a doublet of doublets at δ_H 2.52 ppm (Zr-H_t, ²J_HH = 9.0 and 5.7 Hz). Complex 83 was characterized by the broadened singlets of bridging hydrogen atoms at δ_H −1.99 and −2.81 ppm and a doublet of doublets at δ_H 2.57 ppm (²J_HH = 6.6 Hz, Zr-H_t).

A trinuclear heterometallic complex, Cp₂Zr(X′)(µ-H)₂Al(X)(µ-H)₂TiCp₂ (X, X′ = Cl, H, BH₄) (84) (Scheme 45), was synthesized by the reaction of Cp₂ZrCl₂ and ½ (Cp₂TiCl)₂ with LiBH₄ and LiAlH₄ in toluene at 0 °C [102,103,104]. Complex 84, upon heating to 40 °C, transformed into 85, in which metal atoms were bound by hydride bridges, forming a six-membered ring, Zr₂AlH₃. Complex 85 proved to be unstable, and its decomposition at 40 °C over 2–3 h provided red needle-like crystals of compound 85. For complex 85, the ¹H NMR spectrum in d₈-THF at room temperature exhibited a broad singlet at δ_H −2.0 ppm (w_1/2 ≈ 200 Hz, Al-H-Zr) and a narrow singlet at δ_H −7.96 ppm (Zr-H-Zr). The addition of a catalytic amount of CoBr₂ increased the yield of complex 85 to 25% [103].

Complex [Cp₂ZrH(µ-H)₂]₃Al (86), similar to structure 81a, was obtained by the reaction of Cp₂ZrCl₂ with LiBH₄ and LiAlH₄ in the presence of 5 mol% Cp₂TiCl₂ at 0 °C in THF (Scheme 46) [102]. The structure of complex 86 was determined using the X-ray diffraction method.

The reaction of Cp₂ZrCl₂ (3) with an excess of HAlBuⁱ₂ (in a 1:3 ratio) in d₆-benzene at 25 °C was accompanied by the formation of tetranuclear trihydride complex 87a. The complex is characterized by triplet signals at δ_H −0.89 ppm (Zr-H, J = 7.4 Hz) and doublet signals at δ_H −2.06 ppm (Zr-H-Al, J = 6.8 Hz) (Scheme 47) [105]. The replacement of zirconocene dichloride with dihydride [Cp₂ZrH₂]₂ (61) in the reaction with HAlBuⁱ₂ (at a ratio of [Zr]:[Al] = 1:1) at −75 °C provided initially intermediate 86a, which, after reacting with ClAlBuⁱ₂, transformed into complex 87a (Scheme 47). The ¹H NMR spectrum of compound 86a exhibited broadened signals of Zr-H hydride atoms at δ_H −2.11 and −3.05 ppm. In the case of an excess of the OAC (more than 3 equiv.), structure 88a was formed at −75 °C to 0 °C. For this compound, the ¹H NMR spectra displayed signals: a doublet at δ_H −2.32 ppm (J = 16.5 Hz, Zr-H), a triplet at δ_H −1.46 ppm (J = 15.5 Hz, Zr-H), and a signal at δ_H −3.18 ppm (Al-H-Al) with an integral intensity ratio of 2:1:2. Upon raising the temperature to 25 °C, the proton signals of complex 88a broadened, indicating its propensity for exchange reactions.

The formation of the L₂Zr(µ-H)₃(AlBuⁱ₂)₃(µ-Cl)₂ (87b–d) structures is characteristic for L₂ZrCl₂ complexes containing unbound cyclopentadienyl ligands (L = Buⁿ-C₅H₄ (b), 1,2-Me₂-C₅H₃ (c), or Me₃Si-C₅H₄ (d)) (Scheme 48). The doublet signals of the hydride atoms of Zr-H bonds at δ_H −2.09–−1.13 ppm and triplet signals at δ_H −1.31–−0.20 ppm were detected in the ¹H NMR spectra of compounds 87b–d. In this case, the signals of the protons of a Zr-H bond of compounds 87b–d were shifted downfield by 0.42–0.90 ppm compared to 87a due to a change in the electron density in the substituted cyclopentadienyl ligands [105].

The reactions of the ansa-complexes, including (SBI)ZrCl₂, (EBI)ZrCl₂, (EBTHI)ZrCl₂, Me₂CCp₂ZrCl₂, Me₂SiCp₂ZrCl₂, Me₂Si(2,4-Me₂-Cp)₂ZrCl₂, (Me₂Si)₂Cp₂ZrCl₂, and (Me₂Si)₂(3,5-Prⁱ₂-Cp)₂ZrCl₂, with 2–5 equiv. of HAlBuⁱ₂ in toluene or benzene at room temperature provided hydride complexes 89e–l, whose ¹H NMR spectra showed the broadened signals of hydride atoms of the [Zr-H]₂ fragment at δ_H −1.75–−0.80 ppm (Scheme 48) [105].

The bulky tert-butyl groups in the ansa-complexes, rac-Me₂Si(2-Me₃Si-4-Me₃C-Cp)₂ZrCl₂ and meso-Me₂Si(3-Me₃C-Cp)₂ZrCl₂, led to the formation of trihydride intermediates 90m and 90n. The signals of three nonequivalent protons were observed in the ¹H NMR spectrum of compound 90m at δ_H −1.56, −0.60 (d, J = 8.2 Hz, Zr-H-Al), and 2.68 ppm (dd, J = 5.5 and 9.4 Hz, Zr-H). The signals at δ_H −2.17 (d, J = 5 Hz, Zr-H-Al), −0.21 (d, J = 10.2 Hz, Zr-H-Al), and 3.31 ppm (dd, J = 5.1 and 9.9 Hz, Zr-H) were detected in the ¹H NMR spectrum of compound 90n (Scheme 48) [105].

The interaction of (SBI)ZrCl(µ-H)₂AlBuⁱ₂ (89e) with an excess of AlMe₃ (at a [Zr]:[AlMe₃] ratio of 1:128) provided complex (SBI)ZrCl(µ-H)₂AlMe₂ (91e) (Scheme 49) [105]. Upon the addition of AlMe₃, the signal of a Zr-H bond proton in the ¹H NMR spectrum shifted from δ_H 1.22 to 1.65 ppm but did not completely disappear. Therefore, it was concluded that the resulting compound (91e) is presumably an adduct containing AlMe₃ coordinated to the terminal Cl atom in (SBI)ZrCl(µ-H)₂AlMe₂, rather than the desired product of exchanging the Me group for a chlorine atom.

Further, it was established that in the reaction of L₂ZrCl₂ with HAlBuⁱ₂ (at a Zr:Al ratio of 1:3), the L₂Zr(μ-H)₃(AlBuⁱ₂)₂(μ-Cl) (76a–c, 76f, 76h, and 76i) and Cp₂Zr(μ-H)₃(AlBuⁱ₂)₃(μ-Cl)(μ-H) (93a and 93b) complexes are predominantly formed (Scheme 50) [93,106]. Using EXSY spectroscopy, the exchange between the hydride atoms of complexes 76 and 93 and oligomers [HAlBuⁱ₂]_n was demonstrated. It was assumed that the exchange can proceed through the dissociation of Zr,Al–hydride complexes with the elimination of a HAlBuⁱ₂ monomer.

The dependence of the structure of the intermediates on the nature of the ligand in the initial zirconocene was also demonstrated (Scheme 50) [106]. Metallocenes with bulky η⁵-ligands provided structures 92c–f with a terminal Zr-H bond. ¹H NMR spectra of complexes with sterically hindered ligands, for example, 92c, recorded at 220 K, exhibited signals of bridging hydrogen atoms at δ_H −1.27 ppm and δ_H −0.66 ppm (d, J = 9.6 Hz) (Zr-H-Al), and the signal of the terminal hydride atom at δ_H 4.38 ppm (dd, J = 9.6 Hz, 4.0 Hz). Moreover, the EXSY spectra showed cross-peaks between the [HAlBuⁱ₂]_n hydride signals and a downfield broadened signal at 6.85 ppm, which was attributed to the hydride atoms of the free (Cme₅)₂ZrHCl. A similar pattern in the NMR spectra was observed for sterically hindered complexes 92d–f: bridging hydride atoms of Zr-H-Al fragments resonated in the upfield region at δ_H −1.07–−0.33 ppm, while the signals of a terminal Zr-H bond were shifted to the downfield δ_H 2.05–3.51 ppm. The systems based on zirconocene dichlorides with sterically hindered ligands, which provided intermediates 92c, 92d, and 92f with an open Zr-H bond, appeared to be the most active in alkene hydroalumination reaction.

In the reaction of Cp₂ZrCl₂ with AlBuⁱ₃ (1:5), alkyl chloride complex 94 was detected, which then transformed into complexes 95 and 76, undergoing intermolecular exchange via intermediate 90 (Scheme 51) [106]. The structure of complex 95 was identified based on the observation of three upfield doublets of magnetically non-equivalent hydride atoms of the bridging Zr-H-Al bonds in a 1:1:1 ratio, at δ_H −1.15, −1.83, and −2.48 ppm in the ¹H NMR spectrum recorded at low temperature (230 K). It is noted that the probable reason for the high reactivity of the Cp₂ZrCl₂-AlBuⁱ₃ catalytic system towards alkenes is the absence of fast exchange between hydride clusters, leading to an increase in the lifetime of intermediates with a free Zr-H bond and an absence of opportunity for the formation of larger clusters like complexes 87 and 93.

Indenyl hydride complexes 76f, 76h, and 76i obtained by the reaction of the corresponding zirconocenes with an excess of HAlBuⁱ₂ also contained a [L₂ZrH₃] moiety (Scheme 50). The hydride atoms were in fast exchange with [HAlBuⁱ₂]_n oligomers; therefore, the signals of hydride atoms in the ¹H NMR spectra were significantly broadened at room temperature. As the temperature decreased below 280 K, the exchange slowed down, and the multiplet signals of Zr-H-Al hydrides in the ranges of δ_H −1.55–−1.00 ppm and δ_H 0.62–1.06 ppm were detected in the ¹H NMR spectra of compounds 76f, 76h, and 76i (Scheme 50) [106].

The studies on the catalytic activity of L₂ZrCl₂-XAlBuⁱ₂ systems (L = Cp, CpMe, Ind, C₅Me₅; L₂ = rac-Me₂C(2-Me-4-Bu^t-Cp)₂; meso-Me₂C(2-Me-4-Bu^t-Cp)₂; rac-Me₂C(3-Bu^t-Cp)₂; rac-Me₂C(Ind)₂; rac-Me₂Si(Ind)₂ (SBI); rac-C₂H₄(Ind)₂ (EBI)); and X = H, Cl, and Buⁱ) in the alkene hydroalumination showed that the maximum effect is achieved when complexes with more bulky cyclopentadienyl ligands are used in combination with HAlBuⁱ₂. The catalysts with less bulky ligands are the most active in the reaction of alkenes with AlBuⁱ₃ or ClAlBuⁱ₂. Indenyl zirconium complexes provide a significant decrease in the yield of hydroalumination products, regardless of the structure of OACs. This dependence of the activity of a catalytic system on the nature of OAC and the structure of a ligand in zirconocene is due to the structural and dynamic features of bimetallic hydride intermediates formed in these systems [106].

Complexes 96a–c, together with the intermediates 75a–c and 76a–c (Scheme 52), were observed in the [Cp₂ZrH₂]₂-ClAlR₂ systems (R = Me (a), Et (b), and Buⁱ (c)) [71,74,93]. The spectral pattern for complexes 96a–c differed significantly from those of 75a–c and 76a–c. The ¹H NMR spectra of complexes 96a–c exhibited distinct triplet upfield signals at δ_H −6.64–−6.35 ppm (J = 17.0–17.6 Hz) assigned to a hydride atom of the Zr-H-Zr bond. This signal in the COSY HH spectrum correlated with a doublet at −1.39–−1.18 ppm (J = 17.0–17.6 Hz), with a ratio of integral intensities, 1(Zr-H-Zr):2(Zr-H-Al):20(Cp), which indicated the presence of the [(L₂Zr)₂H₃] moiety in the molecule. Complex 96c, along with 75c and 76c, was also detected in minor amounts in the reaction of Cp₂ZrCl₂ with HAlBuⁱ₂ at a low OAC content (Scheme 52).

For the Cp₂HfCl₂ (22), (CpMe)₂ZrCl₂ (97), Me₂CCp₂ZrCl₂ (58), (Me₂C)₂Cp₂ZrCl₂ (37), Ind₂ZrCl₂ (45), and Me₂CInd₂ZrCl₂ (62) complexes, the reaction with HAlBuⁱ₂ also resulted in the formation of structures 98–105c at a [Al]/[Zr] ratio of 3–8 [74,75]. Intermediates 106–108c were observed at low AOC contents in the system ([Al]/[Zr] = 2–3).

Recently, it has been demonstrated that the reaction of ansa-zirconocene (EBI)ZrCl₂ with an excess of AlBuⁱ₃ (in a 1:12 ratio) in d₆-benzene at 25 °C for 10 min provided a mixture of complexes: (EBI)ZrBuⁱCl (109) (95%) and (EBI)Zr(µ-Cl)(µ-CH₂CH₂)AlBuⁱ₂ (110) (5%) (Scheme 53) [107]. Complexes 109 and 110 transformed into hydride intermediates (EBI)Zr(μ-H)(μ-CH₂CH₂)AlBuⁱ₂ (111) and (EBI)ZrH(μ-H)₂AlBuⁱ₂ (112) after 3 h of the reaction. In the ¹H NMR spectrum of complex 111, the broadened singlet signal of a proton of a Zr-H-Al fragment at δ_H −2.62 ppm, multiplet signals of protons of the Zr(µ-CH₂CH₂)AlBuⁱ₂ bridge at δ_H −2.12, −1.58, 0.14, and 1.17 ppm correlated with the signals in the ¹H-¹³C HSQC spectra at δ_C 4.7 ppm (Al-CH₂) and 53.4 ppm (Zr-CH₂), were observed (Scheme 46). Complex 112 was characterized by the signals of hydrides at δ_H −1.77 (d, J_HH = 6.3 Hz) and −1.44 ppm that correlated with a proton signal at δ_H −0.22 ppm in COSY HH spectra. The hydride complexes (EBI)ZrH(μ-H)₂[μ-H(AlBuⁱ₂)₂] (113) and (EBI)ZrH(μ-H)₂[μ-Cl(AlBuⁱ₂)₂] (114) were detected after 16 h of the experiment. Complexes 111–114 were the major products even after 40 h of reaction (Scheme 53).

3.3. Influence of Al- and B-Containing Activators on Structure and Reactivity of Metallocene Hydrides

Aluminum- and boron-containing activators have significant effects on the structure and reactivity of intermediates formed in metallocene systems. For example, the hydride complexes [Cp′₂ZrH]⁺[MeB(C₆F₅)₃]⁻ (115a,b) and [Cp′₂ZrH]⁺[HB(C₆F₅)₃]⁻ (116a) were observed in the reaction of Cp′₂Zr(CH₃)₂ or Cp’₂ZrH₂ (Cp′ = η⁵-Me₅C₅ (a) and η⁵-Bu^t₂C₅H₃) (b)) with B(C₆F₅)₃ at −78 °C in the presence of H₂ (1 atm) (Scheme 54) [27,108]. In the ¹H NMR spectra of complexes 115a and 116a, the hydride atoms of a Zr-H bond resonated at δ_H 7.70 and 8.18 ppm, respectively. The broadened singlet signals at δ_H 0.10 ppm (B-CH₃, complex 115a) and δ_H 3.98 ppm (B-H, complex 116a) were also detected in the ¹H NMR spectrum. Compounds 115a and 116a turned out to be active homogeneous catalysts for the polymerization of ethylene (3.2·10⁶ g_PE mol_Zr⁻¹ h⁻¹ atm⁻¹, M_W = 4.34·10⁵) and propylene (3.2·10⁵ g_PP mol_Zr⁻¹ h⁻¹, M_W = 3900).

Zr borohydride complexes 117 and 118 were synthesized by the reaction of alkyl zirconocenes with HB(C₆F₅)₂ (Scheme 55) [109,110]. The formation of complex 117 in the reaction of Cp₂ZrMe₂ with HB(C₆F₅)₂ was monitored by NMR spectroscopy through the evolution of CH₄ (δ_H 0.16 ppm) and the appearance of a brick-red precipitate at the bottom of a tube (Scheme 55). In the ¹H NMR spectra of complex 117 in hexane, signals of Cp-ring protons were observed at δ_H 5.23 ppm, signals of CH₂ fragment hydrogen atoms were observed at δ_H 2.29 ppm, and broadened signals of Zr-H-B bridging hydrides were observed at δ_H −2.05 ppm in a ratio of 10:2:2. In the ¹³C NMR spectrum of compound 117, signals of Cp rings and the CH₂ group were detected at δ_C 111.11 ppm and δ_C 0.5 ppm (¹J_C-H = 120 Hz), respectively. The fluorine atoms of a B(C₆F₅)₂ group resonated at δ_F −132.4, −157.2, and −163.4 ppm in the ¹⁹F NMR spectra. The ¹¹B NMR spectrum exhibited a signal at δ_B 0.00 ppm (¹J_H-B = 135 Hz), which is typical of a four-coordinated boron atom. The structure of complex 117 was also confirmed by X-ray diffraction. The replacement of an aliphatic solvent with toluene and an increase in the amount of HB(C₆F₅)₂ to two equivalents led to the formation of complex 118. In the ¹H NMR spectra, the signals of Cp rings and a Zr-H-B hydride atom were shifted to the upfield to δ_H 5.42 and 0.38 ppm, respectively, compared to structure 117. A triplet signal also appeared at δ_B −12.9 ppm (¹J_H-B = 64 Hz) in the ¹¹B NMR spectra. The ¹⁹F NMR spectrum of compound 118 (δ_F −133.0, −156.8, and −163.4 ppm) remained almost unchanged compared to that of complex 117. It turned out that complex 118 was inactive in ethylene polymerization.

The Cp*(η⁵-η¹-C₅Me₄CH₂)ZrR 119a–c compounds (R = Cl, CH₃, and C₆H₅) in reaction with highly electrophilic boranes, HB(C₆F₅)₂ and B(C₆F₅)₃, provided the following hydride cationic complexes: Cp*(η⁵-η¹-C₅Me₄CH₂B(C₆F₅)₂(µ-H)ZrR (120a: R = Cl with 74% yield; 120b: R = C₆H₅, 62% yield) and Cp*[η⁵-C₅Me₄CH₂B(C₆F₅)₃]ZrH (123b, 77% yield) (Scheme 56) [111]. The ¹H NMR spectrum of 120a showed the doublet and the doublet of doublet signals of a CH₂ group at δ_H 3.11 and 2.88 ppm, respectively, as well as broadened signals of a Zr-H-B fragment at δ_H 0.5 ppm. For hydride complex 123b, obtained from Cp*(η⁵-η¹-C₅Me₄CH₂)ZrPh (119b) and B(C₆F₅)₃ through a series of stages, as depicted in Scheme 56, the presence of hydrogen atom signals of CH₂B moiety in the ¹H NMR spectra at δ_H 2.66 and 3.13 ppm is characteristic. Upon increasing the temperature to 50 °C, the Cp*(η⁵-η¹-C₅Me₄CH₂)ZrPh compound (119b) in the NMR tube converted to product 122b, which then transformed into complex 123b after hydrogen bubbling. Compounds 120a, 121a, 122b, and 123b proved to be active catalysts in the ethylene polymerization reaction.

Binuclear hydride complexes [Cp′₄Zr₂H₃][B(C₆F₄R)₄] 124a and 124b (R = F (a) and SiPrⁱ₃ (b)) were obtained by the reaction of [Cp′₂ZrH₂]₂ with a solution of (Ph₃C)[B(C₆F₄R)₄] in d₈-toluene (Scheme 57) [112]. Two signals of bridging hydrides H¹ and H² at δ_H −2.02 and −2.66 ppm and terminal H³ protons at δ_H 4.55 ppm were detected in the ¹H NMR spectrum of complex 124a at −78 °C. All three signals coalesced at −30 °C due to a fast hydride exchange. The [Cp′₂ZrH₂]₂-(Ph₃C)[B(C₆F₄R)₄] system turned out to be much more active in the homopolymerization of isobutene and the isobutene–isoprene copolymerization compared to the system based on Cp′₂ZrMe₂. Complex 124b, also obtained in the reaction of [Cp′₂ZrH₂]₂ with (Ph₃C)[B(C₆F₄SiPrⁱ₃)₄] in a [Zr]:[B] ratio of 1:1, transformed into pale yellow-green crystals of compound 125b over several days at 5 °C. The structure of the complex was confirmed by X-ray crystallography [112].

Using NMR spectroscopy, it was demonstrated that in the reaction of Ph₂C(CpFlu)ZrCl₂ with AlBuⁱ₃ in the presence of (PhNMe₂H)[B(C₆F₅)₄] with a [Zr]:[Al]:[B] reactant ratio of 1:(10–100):1 in d₆-benzene at 60 °C, the isobutyl derivative, [Ph₂C(CpFlu)ZrBuⁱ·AlBuⁱ₃]⁺, transforms into the allyl hydrido complex, [Ph₂C(CpFlu)Zr(μ-H)(μ-C₄H₇)AlBuⁱ₂][B(C₆F₅)₄] (68) (Scheme 34) [67]. The diastereotopic protons of a Zr–CH₂ bond resonated at δ_H 2.87 and −1.66 ppm, and the hydrogen atoms of an Al-CH₂ fragment resonated at δ_H 2.28 ppm and around 1 ppm in the ¹H NMR spectrum. The signals correlated with resonance lines at δ_C 90.5 ppm (ZrCH₂, ¹J_C-H = 157.5 Hz) and 47.7 ppm (AlCH₂, ¹J_C-H = 129.4 Hz) in the ¹³C NMR spectrum; this indicates the non-symmetric bonding of the allyl moiety. The signal at δ_C 163.4 ppm was attributed to the quarternary methallyl C atom. The hydride atom of a Zr-H-Al bridge resonated at δ_H −2.78 ppm in the ¹H NMR spectrum of compound 68. In the ¹⁹F spectrum of the compound, signals for the [B(C₆F₅)₄]⁻ anion were present as a broad singlet at δ_F −131.8 ppm (o-F), a triplet at δ_F −162.4 ppm (J = 20.4 Hz, p-F), and a multiplet at δ_F −166.2 ppm (m-F), which designated the lack of coordination of the anion with the cation.

The [(SBI)Zr(µ-Cl)₂Zr(SBI)][B(C₆F₅)₄]₂ (126) and [(SBI)Zr(µ-H)(µ-C₄H₇)AlBuⁱ₂][B(C₆F₅)₄] (127) complexes were identified in the reaction of (SBI)ZrX₂ (X = Cl and Me) with AlBuⁱ₃ in the presence of (Ph₃C)[B(C₆F₅)₄] (Scheme 58) [66]. Initially, upon the interaction of (SBI)ZrCl₂ and AlBuⁱ₃ at [Zr]:[Al] = 1:(5–10) with the addition of 1 eq. of Ph₃C[B(C₆F₅)₄] over 5–15 min, ionic dimeric structure 126 occurred, which was characterized using X-ray crystallography (Scheme 58). Complex 127 was formed through several stages at a reagent ratio of (SBI)ZrCl₂ and AlBuⁱ₃ [Zr]:[Al] of 1: ≥20 at room temperature. In the ¹H NMR spectrum of structure 127, singlet proton signals were observed at δ_H 3.03 (Zr-CHH) and −1.73 ppm (Zr-CHH), at δ_H 2.51 (Al-CHH) and 0.22 ppm (Al-CHH), as well as at δ_H −3.35 ppm (Zr-H-Al). The resonance lines of the C atoms of Zr-CH₂ and Al–CH₂ were located at δ_C 86.9 and 53.8 ppm, respectively. A chemical shift of the CH₂ = CMe moiety equal to 166.6 ppm was characteristic of a methallylic structure. It is noted that in the presence of excess AlBuⁱ₃, species 126 serves as precursor for propylene polymerization active sites, whereas species 127 is a thermodynamic sink of the catalytic system.

Zr,B-hydride complexes 128 and 129 were obtained in the reaction of rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium difluoride with HAlBuⁱ₂ and B(C₆F₅)₃ (Scheme 59) [113,114]. Compounds 128 and 129 were identified using X-ray crystallography and NMR spectroscopy. It has been demonstrated that a catalytic system based on metallocene fluorides and AlBuⁱ₃ provides hydride-containing Zr complexes, exhibiting excellent activity in the polymerization of ethylene and propylene.

The Me₂C(Cp)IndMMe(µ-H)B(C₆F₅)₃ (M = Zr and Hf) 130a, 130b, 131 hydride intermediates were observed in the reaction of Me₂C(Cp)IndMMe₂ dialkyl complexes with B(C₆F₅)₃ (Scheme 60) [115]. Two isomeric structures, 130a and 130b, along with an oligomerization product, were detected in the case of a Zr–borohydride complex, Me₂C(Cp)IndZrMe(µ-H)B(C₆F₅)₃, obtained at a [Zr]:[B] reagent ratio of 1:1.2 at 25 °C in d₈-toluene with the addition of 10 eq. of propylene. In the ¹H NMR spectrum of major isomer 130a, signals corresponding to the protons of the Zr-Me bond were observed at δ_H −1.10 ppm (septet, ³J_H-F = 2.2 Hz). The following ¹⁹F NMR signals were typical for isomer 130a: a broadened doublet at δ_F −131.0 (J = 18.3 Hz, o-F), a triplet at δ_F −155.0 (J = 21.4 Hz, p-F), and a multiplet at δ_F −162.0 ppm (m-F). The ¹H NMR spectrum of minor isomer 130b exhibited the signals of protons of a Zr-Me bond at δ_H 0.27 ppm. The ¹⁹F NMR spectrum of compound 130b showed minor differences compared to 130a: a broadened doublet at δ_F −132.2 (J = 18.3 Hz, o-F), a triplet at δ_F −156.5 (J = 21.4 Hz, p-F), and a multiplet at δ_F −162.8 ppm (m-F).

The ansa-hafnocene hydride complex 131 was characterized by X-ray diffraction and NMR spectroscopy as well (Scheme 60) [115]. In the ¹H NMR spectrum, a signal corresponding to the Hf-Me bond was observed at δ_H −1.10 ppm (septet, ³J_H-F = 2.2 Hz), and signals of the Hf-H-B fragment were found at δ_H 0.44 ppm, identified through correlation in the ¹H−¹¹B spectra. The following signals of the HB(C₆F₅)₃ group of compound 131 were detected in the ¹⁹F NMR spectra: a doublet at δ_F −130.7 (J = 24.4 Hz, o-F), a triplet at δ_F −155.0 (J = 21.4 Hz, p-F), and a multiplet at δ_F −162.2 ppm (m-F). Intermediates 130a, 130b, and 131 were found to be relatively inert towards propene and were in an inactive “dormant” state.

Zr,Al–hydride L₂ZrH₃AlH₂ complexes (132a–c) (L = CpMe₅ (a), BuⁿCp (b), and Me₃SiCp (c)) formed the metallocene di- or polynuclear ion pairs with HB(C₆F₅)₃⁻ (134a–c) upon the activation with B(C₆F₅)₃ at −50 °C in a 1:1 mixture of d₅-bromobenzene and d₈-toluene (Scheme 61) [116]. The dinuclear structure of the ion pair 134a was confirmed by the presence of two distinct signals of a C₅Me₅ ligand in a 1:1 ratio and broadened doublets of bridging hydrides of Zr-H^1,2-Al bonds at δ_H −2.94 and −2.13 ppm, as well as terminal protons of the Al-H³ bonds at δ_H 4.16 ppm in the ¹H NMR spectra. The broadened signals of the Al-H⁴-Al moiety were also observed at δ_H 0.4 ppm. The ¹⁹F NMR spectrum showed the characteristic signals of an HB(C₆F₅)₃⁻ anion at δ_F −133.0, −163.2, and −166.3 ppm (134a). Complex 134a transformed into L₂ZrH(µ-H)₂B(C₆F₅)₂ (133a) and (C₆F₅)AlH₂ as a result of thermal decomposition (Scheme 61). The same products were generated in the reaction of 132a with B(C₆F₅)₃ in toluene. A singlet signal at δ_H 6.64 ppm and quartet signals of bridge hydrides at δ_H −0.73 ppm (J_B-H = 75 Hz, Zr-H¹) were observed in the ¹H NMR spectrum of structure 133a. It has been shown that the two bridge hydride atoms are in rapid exchange between the central and side positions, but neither of them exchanges with a terminal hydride of a Zr-H² bond. A broadened doublet at δ_F −130.3 (o-F), a triplet at δ_F −157.6 (p-F), and a multiplet at δ_F −163.6 ppm (m-F), which are characteristic of a tetrahedral fragment H₂B(C₆F₅)₂^– coordinated with Zr, were detected in the ¹⁹F NMR spectrum. The obtained hydride complex 133a exhibited moderate activity in the ethylene polymerization reaction (activity—4 · 10³ g_PE mol⁻¹ h⁻¹ at 25 °C and 2.7 atm). However, the catalyst formed as a result of complex 132a activation with B(C₆F₅)₃ proved to be 1000 times more active than 133a. Complex 132c also resulted in the generation of a more active catalyst at elevated temperatures upon activation with B(C₆F₅)₃. The authors explained this by the higher thermal stability of the particles associated with the bridging anion, HB(C₆F₅)₃⁻ [116].

The interaction of L’HfCl₂ hafnocenes (L = (SBI) (a), Me₂C(C₅H₄)(Flu) (b), Ph₂C(C₅H₄)(Flu) (c), and C₂H₄(Flu)(5,6-C₃H₆-2-MeInd) (d)) with AlBuⁱ₃/(Ph₃C)[B(C₆F₅)₄] provided cationic intermediates, [LHf(μ-H)₂AlBuⁱ₂]⁺ or [LHf(μ-H)₂Al(H)Buⁱ]⁺ (135a–d), which showed greater activity in the alkene polymerization than the heterobinuclear methyl-bridged intermediates, [LHf(μ-Me)₂Al(μ-Me)₂][MeMAO] (136a–d) and [LHf(μ-Me)₂Al(μ-Me)₂][B(C₆F₅)₄] (137a–d) (Scheme 62) [117]. Complex (SBI)HfCl₂, in the reaction with AlBuⁱ₃ and (Ph₃C)[B(C₆F₅)₄] at a [Hf]:[Al]:[B] ratio of 1:(10–50):1.1, gave rise to the viscous product 135a, which settled at the bottom of the NMR tube. The ¹H NMR spectrum of compound 135a showed two signals of hydride atoms at δ_H −1.13 ppm (d, ²J_HH = 5 Hz, H²) and δ_H 1.40 ppm (t, ²J_HH = 5 Hz, H¹), which were correlated in the COSY HH spectra. Complexes 135b and 135c were unstable at 2–5 °C. Hydride complex 135d, obtained at a [Hf]:[Al]:[B] reagent ratio of 1:(40–100):1, was characterized by a signal of an H¹ proton at δ_H −2.11 ppm (d, J_HH = 6 Hz) and a signal of hydrogen atoms H² at δ_H −4.00 ppm (dd, J_HH = 10 Hz and 3.7 Hz). The ¹⁹F NMR spectrum of the complex exhibited the signals of B(C₆F₅)₄^– groups at δ_F −132.5, −163.0, and −166.5 ppm [117].

The bimetallic Zr,Al-trihydride cations [L₂M(μ-H)₃(Al(Buⁱ₂)₂]⁺ (138a–j and 139d) were obtained in L₂MCl₂-HAlBuⁱ₂-(Ph₃C)[B(C₆F₅)₄] catalytic systems (L = EBI, EBTHI, SBI, Cp, Me₂SiCp₂, Me₄C₂Cp₂, (Me₂Si)₂Cp₂, CpBuⁿ, CpSiMe₃, and 1,2-Me₂Cp; M = Zr and Hf) [118]. In the ¹H NMR spectra of compounds 138a–j, doublet and triplet signals of three hydride atoms of a ZrH₃ moiety in the ratio of intensities of 2:1 were observed in the upfield region (Scheme 63). The structures of complexes 138c and 138g were confirmed by X-ray diffraction.

Complex [(SBI)Zr(μ-H)₃(AlBuⁱ₂)₂]⁺ (138c) generated in the (SBI)ZrCl₂–HAlBuⁱ₂-(Ph₃C)[B(C₆F₅)₄] system at a [Zr]:[Al]:[B]:[propylene] ratio of 1:20:1:20 in d₈-toluene at −30°C has been shown to polymerize propylene, yielding an isotactic product with 97mmmm% and PDI = 1.90 (Scheme 64) [119]. Polypropylene was also obtained in the presence of a [(SBI)Zr-(μ-Me)₂AlMe₂]⁺ cation (141) formed in the reaction of complex 140 with (AlMe₃)₂. The polymer contained terminal isopropyl groups originated from the chain termination through its transfer to aluminum. After the complete consumption of aluminum hydride, the [(SBI)Zr(μ-Me)₂AlR₂]⁺ complex with a dimethyl bridge (141) became the sole intermediate in these reaction systems. In the reaction of (SBI)ZrCl₂ with 20 eq. of HAlMe₂ and 1 eq. of (Ph₃C)[B(C₆F₅)₄] in d₈-toluene, the [(SBI)Zr(μ-H)₃(AlMe₂)₂]⁺ intermediate (140) was identified. The cation, [(SBI)Zr(μ-H)₃(AlBuⁱ₂)₂]⁺ (138c), formed under the action of HAlBuⁱ₂ catalyzed the polymerization of propylene, and its analog, [(SBI)Zr(μ-H)₃(AlMe₂)₂]⁺ (140), formed in the presence of HAlMe₂, showed the activity in propene hydroalumination, transforming during this reaction into the [(SBI)Zr(μ-Me)₂AlR₂]⁺ cation (141), which also catalyzed the polymerization of propene.

The study of the Cp₂ZrMe₂-AlMe₃-(Ph₃C)[B(C₆F₅)₄] system (fluorobenzene as a solvent) with the ESI-MS method showed that the main product was the [Cp₂Zr(μ-Me)₂AlMe₂]⁺[B(C₆F₅)₄]⁻ complex (142) (Scheme 65) [120]. When 1-hexene is added to the system, complexes 145 and 146, and the allylic structures [Cp₂Zr(η³-C₆H₁₀)(C₆H₁₂)_nH]⁺ (147a,b) are formed, and the [Cp₂Zr(µ-H)₂AlMe₂]⁺ compound (143) with a mass (m/z) of 279, accumulated as an alkene, is consumed. The formation of dimethylalane hydride structures 144 and 146 is a catalyst deactivation process because a monomer is consumed slowly in the presence of these complexes compared to the starting reaction rates.

There are not much data in the literature on the structures of metallocene hydrides obtained as a result of an interaction with MAO in comparison with hydride complexes activated by B-containing compounds.

It was shown, for example, that aluminum hydride complexes (RCp)₂ZrH₃AlH₂ 132b and 132c (R = Buⁿ (b), Me₃Si (c), Scheme 61) activated by MAO possess higher activity in an ethylene polymerization reaction (132b: 15.8·10⁶ g_oligomer mol_cat⁻¹ h⁻¹ and 132c: 58.1·10⁶ g_oligomer mol_cat⁻¹ h⁻¹) than the corresponding dichloride complexes, BuⁿCp₂ZrCl₂ (40) and (Me₃SiCp)₂ZrCl₂ (80) (40: 11.8·10⁶ g_oligomer mol_cat⁻¹ h⁻¹ and 80: 43.6·10⁶ g_oligomer mol_cat⁻¹ h⁻¹) [116]. The molecular weight of the polymer decreased significantly when using a SiO₂-supported or leached catalyst compared to the corresponding soluble catalyst under the same conditions [121,122]. Polyethylene with M_W = 149,500 was obtained in the presence of complex 132b and SiO₂ modified with MAO (at [Zr]:[Al] = 1:2600, the activity was 5.16 · 10⁶ g_PE mol⁻¹ h⁻¹). The activity of zirconocene 40 in the polymerization reaction on a MAO-SiO₂ carrier was 2.15·10⁶ g_PE mol⁻¹ h⁻¹ (M_W= 229,500). The reactivity of complex 132c ((Me₃SiCp)₂ZrH₃AlH₂) activated by MAO and supported on SiO₂ with the addition of molecular H₂ increased by 25% during ethylene polymerization. Nevertheless, there was a significant decrease in the molecular weight of the product from M_N = 63,700 and M_W = 175,000 to M_N = 691 and M_W = 1930 with the introduction of H₂, which was used as a chain transfer agent in the reactions of ethylene polymerization and copolymerization of ethylene with 1-hexene [121].

Polyethylene was synthesized in the presence of the BuⁿCp₂ZrH₃AlH₂/MAO/KCl system that showed activity at a level of 4.07 · 10⁶ g_PE mol⁻¹h⁻¹ (M_W = 16,950) [123].

The neutral dihydride complexes, (SBI)ZrH₂·{nAlR₂X} (148), were found as a result of the interaction between (SBI)ZrCl₂ (25) and MAO ([Al]MAO/[Zr] = 600) both in the solution and on the surface of SiO₂ in the presence of diisobutylaluminum hydride or triisobutylaluminum (Scheme 66) [84]. The ¹H NMR spectrum of the systems based on (SBI)ZrCl₂, Al₂Me₆, HAlBuⁱ₂, and MAO contained the broadened singlet signals of a Zr–H bond at δ_H −1.39 (SBI)ZrH₂·{2AlMe₃}), −1.54 (SBI)ZrH₂·{2AlMe₂Cl}), and −1.95 ppm (SBI)ZrH₂·{x(AlMeO)_n}) (148). The observed complexes appeared to be inactive in olefin polymerization.

The addition of MMAO-12 to the Cp₂ZrH₂-ClAlR₂ system (R = Me (a), Et (b), and Buⁱ (c)) in a [Zr]:[ClAlR₂]:[MMAO-12] ratio of 1:(1.5–3):(3–8), containing equilibrium mixture of complexes 75a–c, 76a–c, and 96a–c (Scheme 52), led to the appearance of adduct 96a–c with MMAO-12 with the separation of the reaction mixture into light and heavy fractions (Scheme 67) [71,72,73,74,75]. The triplet signals of protons of a Zr-H-Zr bond at δ_H −6.56–−6.44 ppm and doublet signals of a Zr-H-Al fragment at δ_H from −1.74 to −1.28 ppm were observed in the ¹H NMR spectrum of the light fraction in the case of the 96a–c·MAO adduct. The ¹H NMR spectrum of the heavy adduct, 96a–c·MAO, exhibited the broadened signals of hydride atoms in the ranges of δ_H −7.10–−6.54 ppm (Zr-H-Zr) and δ_H −1.44–−1.22 ppm (Zr-H-Al). When (Ph₃C)[B(C₆F₅)₄] was added to the [Cp₂ZrH₂]₂-ClAlEt₂ system (at a ratio of 1:(3–4):(0.15–0.5)), additional upfield triplet and doublet signals at δ_H −6.87 ppm and −1.72 ppm, respectively, appeared in the ¹H NMR spectrum, which were assigned to the 96b·R_nAl(C₆F₅)_3−n adduct [72]. Similar adducts were found in the Cp₂ZrCl₂-HAlR₂-MMAO-12 catalytic systems ((Ph₃C)[B(C₆F₅)₄]) [71,72,73].

Analogous MMAO-12 associations ((106–108c)·MAO, 151c·MAO, 152c·MAO) were observed in the reactions of L₂ZrCl₂ (22, 37, 45, 58, 97) with HAlBuⁱ₂ and MMAO-12 (Scheme 67) [74,75]. Moreover, complexes, probably being a cationic type, [Cp₂ZrH]⁺ (149, 150), whose proton signals were located at δ_H −6.6–−0.1 ppm in the ¹H NMR spectra, formed in the L₂ZrCl₂-HAlBuⁱ₂-(Ph₃C)[B(C₆F₅)₄] catalytic systems (L = Cp, Ind) at a [Zr]:[Al]:[B] ratio of 1:(5–8):0.5 (Scheme 67) [71,72,74].

An NMR study on the activity of the Zr,Al–hydride intermediates towards an alkene (Scheme 67) showed that hydride complexes 76a–c and 98c–102c reacted first to provide hydrometalation product 153. Intermediates with the [(L₂Zr)₂H₃] moiety associated with MAO or R_nAl(C₆F₅)_3−n provided dimer 2. The addition of an alkene to the systems with hydride species of a cationic type, [L₂ZrH]⁺ (149c or 150c), led to the formation of oligomer 21 at a high rate [71,72,74,75]. As a result, studies on the metallocene–OAC–activator systems (MMAO-12, (Ph₃C)[B(C₆F₅)₄]), disclosed the generation of various hydride clusters, including bis-zirconium hydride intermediates of the [(L₂Zr)₂H₃] type, which were the precursors of active centers that initiated an alkene dimerization, whereas cationic species [L₂ZrH]⁺ ensured the formation of the oligomeric products.

4. Conclusions

Dimerization and oligomerization reactions are widely used to convert light olefins resulting from various processes (thermal and catalytic cracking, Fischer–Tropsch synthesis, etc.) into higher olefins that are demanded in various industrial fields. The dimerization and oligomerization of α-olefins are carried out by heterogeneous acid catalysis, which is mainly used for the production of fuels, and by transition metal catalytic systems, utilized primarily for the production of high value-added products.

An analysis of the data in the literature shows that much attention is commonly paid to the consideration of the catalytic properties of transition metal complexes of various structures and systems to search for the most active catalysts for alkene dimerization and oligomerization. Here, the focus is shifting towards post-metallocene complexes and heterocenes, where varying the transition metal atom and ligand structure allows a significant influence to be exerted on the activity of catalytic systems, the regioselectivity of alkene insertion, and the molecular weight distribution of reaction products. The design of new activators with predictable effects in contrast to stochastic-structured aluminoxanes is highly relevant, as well as the search of heterogeneous carriers for catalytic systems, aiming to implement new processes on an industrial scale. Another interesting direction is the variation of monomer structures (including the use of polar monomers) to obtain products with unique physical and physicochemical properties for the development of new materials.

Metal hydrides can act as active centers of the considered catalytic systems. The metal-H bond exhibits remarkable activity, contributing to a diverse array of catalytic applications. This includes the reduction in unsaturated compounds; the di-, oligo-, and polymerization of alkenes with varied structures; as well as the functionalization of olefins and acetylenes through hydrometalation. Despite the large amount of information on the structures of hydride intermediates generated in transition metal complex activator systems, the mechanisms of their actions in the discussed processes remain open questions.

Therefore, the investigation of the mechanisms of alkene dimerization and oligomerization reactions is crucial for a more targeted exploration of novel, efficient catalysts and activators. In this field, significant attention will still be given to metallocene systems as the most convenient models for studying reaction mechanisms. Priority lies in comprehending the structure and dynamics of active centers, a factor that is significantly influenced by the metal’s nature, ligand, and cocatalyst structure. The σ- and π-ligand environments of the transition metal play pivotal roles in determining the lifespan of specific active sites, which are necessary for successful alkene insertion, chain propagation, and termination. Consequently, future research demands a comprehensive approach encompassing the exploration of catalytic system properties and the experimental and theoretical analysis of structural and dynamic features of hydride intermediates. This holistic approach aims to develop robust models for reaction mechanisms and predict the properties of new promising catalytic systems.