Neutral and cationic complexes of the rare earth metals : synthesis, characterisation and polymerisation catalysis

摘要

The most common starting materials for the access to cationic complexes of the rare-earth metals are the homoleptic tris(trimethylsilylmethyl) precursors [Ln(CH2SiMe3)3(thf)2]. The principal objective of this work has been to substitute the well-known CH2SiMe3 group for other fragments, and study the accessibility of the related cationic species. In Chapter B.1, homoleptic tri(allyl)s generate half-sandwich species by reaction with HC5Me4SiMe3, whilst monocationic bis(allyl) species are produced by protonolysis with [NEt3H][BPh4]. They instantaneously decompose via C–H activation in pyridine. Lewis acids such as [Al(CH2SiMe3)3] or [BPh3] are suitable reagents for the abstraction of an allyl moiety from tri(allyl) parents, yielding the corresponding ion-pairs in which the anion features a linear allyl group. Reaction of the neutral half-sandwich complexes with one equivalent of acid produces the corresponding monocationic allyl compounds, which are rare examples of cationic monocyclopentadienyl alkyl complexes of the rare-earth metals. In a similar reaction, the bis(allyl) monocations are converted into dicationic mono(allyl) species, which are stable in pyridine without decomposition for over one day. All cationic species described in this chapter are active catalysts for the polymerisation of 1,3-butadiene. In Chapter B.2, aluminate precursors of the type [Ln(AlMe4)3] are utilised to generate the half-sandwich bis(aluminate) neutral species by reaction with HC5Me4SiMe3. Reaction of these neutral precursors with [NEt3H][BPh4] in thf induces the cleavage of both aluminate moieties as well as protonolysis of one of them, and affords the first structurally authenticated charge-separated cationic half-sandwich methyl derivatives. Abstraction of the remaining methyl group by an additional equivalent of acid yields dicationic cyclopentadienyl complexes. The known reaction of [Ln(AlMe4)3] with two equivalents of acid generates dicationic methyl species. This procedure can be applied to lanthanum and samarium precursors. The resulting methyl dications adopt in the solid state a distorded capped octahedral geometry. The half-sandwich bis(aluminate) scandium species is obtained by salt elimination, but not in pure form. Splitting of the two aluminate moieties in this compounds results in the formation of a dimeric monocyclopentadienyl di(methyl) complex. Investigations on the suitability of the borohydride group BH4 for the access to cationic species are presented in Chapter B.3. Thus, direct protonolysis of the neutral tris(borohydride) parents with one equivalent of [NEt3H][BPh4] yields the corresponding monocationic bis(borohydride) species. The charge separated ion pairs adopt a regular trigonal bipyramidal geometry in the solid state, with trans-arranged borohydride moieties. The cations are slightly more active than their neutral parents for the ring-opening polymerisation of epsilon-caprolactone. In Chapter B.4, the synthesis of the series of alkyl complexes featuring a dianionic constrained geometry ligand is reported. The alkyl complexes are converted into the corresponding dimeric hydride species by reaction with PhSiH3 or H2. Hydrosilylation of 1-decene catalysed by the in situ generated hydride complexes selectively gives 1,2-insertion into the Ln–H bond, whilst mixtures of the Markovnikov and anti-Markovnikov products are obtained in the case of styrene. The structural characterisation of the series of Group 13 tri(alkyl) [E(CH2SiMe3)3]n is presented in Chapter B.5. The boron and gallium homologues are monomeric, whilst the aluminium and indium derivatives form dimers in the solid state. The access to aluminium tri(allyl) as thf-adduct is reported, and the formation of the unprecedent monocationic bis(allyl) species deriving therefrom is described.