New ionic half-metallocenes of early lanthanides

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Abstract

We present in this study a new and one-step method allowing the preparation of an unprecedented family of stable half-lanthanidocenes. X-ray analysis shows that the isolated compounds all display the same ionic Ln–Mg bimetallic structure consisting of two anionic (CpR)Ln(BH4)3 species and one cationic Mg(THF)6 (CpR = C5Me5, Ln = Nd, 1a, Ln = La, 1b; CpR = C5H5, Ln = Nd, 2a; CpR = C5H2Ph3, Ln = Nd, 3a). Such complexes display high stability with respect to disproportionation in solution. Combined with dialkylmagnesium, neodymium complexes provide powerful catalysts for stereospecific isoprene polymerisation.

Introduction

Monocyclopentadienyl compounds of the rare earths represent a family of complexes that interest for catalysis has spectacularly increased since very recently [1], [2]. The field of polymerisation is particularly most concerned, with the elaboration of unique controlled structure specialty polymeric materials [3], [4]. However, the potential of such compounds – also called half-sandwiches or half-metallocenes – in catalysis is still little explored compared to metallocenes, mainly because their synthesis remains a challenge [5], due to the occurrence of comproportionation reactions [6], [7]. This has been observed in particular in the early series, even in the presence of one bulky cyclopentadienyl ligand [8], [9]. Thus, the half-lanthanidocenes chemistry remains mainly limited to the smaller elements [6], [10].

Recently however, taking advantage of the modularity of hapticity of the BH4 group, several monocyclopentadienyl complexes could be isolated by ionic metathesis (Scheme 1) [11], [12], [13].

The more sophisticated σ-bond metathesis requires highly reactive homoleptic LnR3 precursors (R = alkyl [6], [14], allyl [15], phenyl [16], or amido group [17]), but this method advantageously allows the formation of the expected product in one step, with a minimal experimental work-up, as the formation of inorganic salts is avoided. However, heating may be necessary to achieve the metathesis reaction, and the possible formation of hazardous by-products requires extreme attention [18]. Furthermore, ligand scrambling may be not completely excluded using this strategy [5], [16].

In the present paper, we propose an alternative method for the preparation of half-sandwiches of lanthanides in high yield, which conjugates the advantages of σ-bond metathesis, and the use of standard starting materials, the trisborohydrides Ln(BH4)3(THF)3. Isolated complexes all exhibit the general ionic formula [(CpR)Ln(BH4)3]2[Mg(THF)6] (CpR is a cyclopentadienyl ligand). Polymerisation experiments show that all complexes are efficient precatalysts for isoprene polymerisation, affording highly trans-regular polymer in a controlled and living manner.

Section snippets

General scheme

Ln(BH4)3(THF)3 are common, easy to handle, and stable lanthanide compounds [19]. We first expected that they could be valuable starting materials for σ-bond metathesis since the [Ln]-BH4 moiety is known to display the chemical behavior of an hydride in some cases [20]. However, we observed by 1H NMR that no reaction occurred between C5Me5H and Nd(BH4)3(THF)3 even after a prolonged period. On the other hand, when the same reaction was carried out in the presence of one half-equivalent of BEM (n

X-ray studies

1a, 1b, 2a and 3a are all trinuclear ionic compounds comprising two anionic half-neodymocene trisborohydride [(CpR)Nd(BH4)3] moieties and one cationic hexa-THF magnesium [Mg(THF)6]2+ adduct (Fig. 1, Fig. 2, Fig. 3) that alternate in the unit cell without particular cation–anion interaction [21], [22]. This ionic trinuclear structure is comparable to that of [Mg(THF)6][Nd(allyl)4]2(2THF), obtained by ionic metathesis, with discrete [Mg(THF)6]2+ cation and allyl neodymate anions [27].

The

Stability of early half-lanthanidocenes

As far as we know, a borohydride anion [(CpR)Ln(BH4)3] was observed just at once, in the ionic complex {[Sm(BH4)2(THF)n]+[(C5Me4nPr)Sm(BH4)3]}. This compound was obtained from the reaction of Sm(BH4)3(THF)3 with a half equivalent of K(C5Me4nPr) (Eq. (1)) [30] and crystals were isolated with n = 5 THF per metal:Sm(BH4)3(THF)3+1/2KC5Me4Prn2.THF1.toluene1/2{[Sm(BH4)2(THF)3]+[(C5Me4Prn)Sm(BH4)3]}

Monocyclopentadienyl of the larger lanthanides (from La to Sm) are known to easily undergo ligands

Isoprene polymerisation

We showed recently that combining a lanthanide borohydride with a dialkylmagnesium is a valuable strategy to prepare, in situ, efficient lanthanide-based catalysts that are also highly trans-stereospecific towards isoprene polymerization [4], [25]. Similarly, the addition of a stoichiometric amount of BEM (1:1) to ionic neodymium complexes 1a, 2a, and 3a gave rise in each case to a very valuable initiator (Table 1, runs 1–3). All these catalysts afford a highly trans-regular polymer (>95%

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    Presented at the Sixth International Conference on f-elements, Wrocław, Poland, 4–9 September 2006.

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