New ionic half-metallocenes of early lanthanides☆
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:
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.