Vanadyl calix[6]arene complexes: synthesis, structural studies and ethylene homo-(co-)polymerization capability

Abstract

Treatment of p-tert-butylcalix[6]areneH₆ (L⁶H₆) with in situ [LiVO(Ot-Bu)₄] afforded, after work-up, the dark green complex [Li(MeCN)₄][V₂(O)₂Li(MeCN)(L⁶H₂)₂]·8MeCN (1·8MeCN). On one occasion, the reaction led to the formation of a mixture of products, the bulk of which differing from 1 only in the amount of solvate, viz.2·9.67MeCN. The second minor, yellow product has the formula {[(VO₂)₂(L⁶H₂)(Li(MeCN)₂)₂]·2MeCN}_n (3·2MeCN), and comprises a 1D polymeric structure with links through the L⁶H₂ ligand and Li₂O₂ units. When the reverse order of addition was employed such that lithium tert-butoxide (7.5 equivalents) was added to L⁶H₆, and subsequently treated with VOCl₃ (2 equiv.), the complex {[VO(THF)][VO(μ-O)]₂Li(THF)(Et₂O)][L⁶]}·2Et₂O·0.5THF (4·2Et₂O·0.5THF), which contains a trinuclear motif possessing a central, octahedral vanadyl centre linked via oxo bridges to two tetrahedral (C_3v) vanadyl centres, was isolated. The calix[6]arene in 4 is severely twisted and adopts a ‘down, down, down, down, out, out’ conformation. Use of excess lithium tert-butoxide led to a complex very similar to 4, differing only in the solvent of crystallization, namely 5·Et₂O·2THF. The ability of 1 and 5 to act as pre-catalysts for ethylene polymerization in the presence of a variety of co-catalysts and under various conditions has been investigated. Co-polymerization of ethylene with propylene and with 1-hexene have also been conducted; results are compared versus VO(OEt)Cl₂.