Oxo‐ and Imidovanadium Complexes Incorporating Methylene‐ and Dimethyleneoxa‐Bridged Calix[3]‐ and ‐[4]arenes: Synthesis, Structures and Ethylene Polymerisation Catalysis
Authors: Redshaw Carl , Rowan Michael A. , Warford Lee , Homden Damien M. , Arbaoui Abdessamad , Elsegood Mark R. J. , Dale Sophie H. , Yamato Takehiko , Casas Carol Pérez , Matsui Shigekazu , Matsuura Sadahiko 
Published: November 2006

Reaction of [V(X)(OR)3] (X=O, Np‐tolyl; R=Et, nPr or tBu) with p‐tert‐butylhexahomotrioxacalix[3]areneH3, LH3, affords the air‐stable complexes [{V(X)L}n] (X=O, n=1 (1); X=Np‐tolyl, n=2 (2)). Alternatively, 1 is readily available either from interaction of [V(mes)3THF] with LH3, and subsequent oxidation with O2 or upon reaction of LLi3 with [VOCl3]. Reaction of [V(Np‐tolyl)(OtBu)3] with 1,3‐dimethylether‐p‐tert‐butylcalix[4]areneH2, Cax(OMe)2(OH)2, afforded [{VO(OtBu)}2(μ‐O)Cax(OMe)2(O)2] ⋅2 MeCN (4⋅2 MeCN), in which two vanadium atoms are bound to just one calix[4]arene ligand; the n‐propoxide analogue of 4, namely [{VO(OnPr)}2(μ‐O)Cax(OMe)2(O)2]⋅1.5 MeCN (5⋅1.5 MeCN), has also been isolated from a similar reaction using [V(O)(OnPr)3]. Reaction of [VOCl3], LiOtBu, (Me3Si)2O and Cax(OMe)2(OH)2 gave [{VO(OtBu)Cax(OMe)2(O)2}2Li4O2]⋅8 MeCN (6⋅8 MeCN), in which an Li4O4 cube (two of the oxygen atoms are derived from the calixarene ligands) is sandwiched between two Cax(OMe)2(O)2. The reaction between [V(Np‐tolyl)(OtBu)3] and Cax(OMe)2(OH)2, afforded [V(Np‐tolyl)(OtBu)2Cax(OMe)2(O)(OH)]⋅5 MeCN (7⋅5 MeCN), in which two tert‐butoxide groups remain bound to the tetrahedral vanadium atom, which itself is bound to the calix[4]arene through only one phenolic oxygen atom. Reaction of p‐tert‐butylcalix[4]areneH4, Cax(OH)4 and [V(Np‐tolyl)(OnPr)3] led to loss of the imido group and formation of the dimeric complex [{VCax(O)4(NCMe)}2]⋅6 MeCN (8⋅6 MeCN). Monomeric vanadyl oxo‐ and imidocalix[4]arene complexes [V(X)Cax(O)3(OMe)(NCMe)] (X=O (11), Np‐tolyl (12)) were obtained by the reaction of the methylether‐p‐tert‐butylcalix[4]areneH3, Cax(OMe)(OH)3, and [V(X)(OR)3] (R=Et or nPr). Vanadyl calix[4]arene fragments can be linked by the reaction of 2,6‐bis(bromomethyl)pyridine with Cax(OH)4 and subsequent treatment with [VOCl3] to afford the complex [{VOCax(O)4}2(μ‐2,6‐(CH2)2C5H3N)]⋅4 MeCN (13⋅4 MeCN). The compounds 1–13 have been structurally characterised by single‐crystal X‐ray diffraction. Upon activation with methylaluminoxane, these complexes displayed poor activities, however, the use of dimethylaluminium chloride and the reactivator ethyltrichloroacetate generates highly active, thermally stable catalysts for the conversion of ethylene to, at 25 °C, ultra‐high‐molecular‐weight (> 5 500 000), linear polyethylene, whilst at higher temperature (80 °C), the molecular weight of the polyethylene drops to about 450 000. Using 1 and 2 at 25 °C for ethylene/propylene co‐polymerisation (50:50 feed) leads to ultra‐high‐molecular‐weight (> 2 900 000) polymer with about 14.5 mol % propylene incorporation. The catalytic systems employing the methyleneoxa‐bridged complexes 1 and 2 are an order of magnitude more active than the bimetallic complexes 5 and 13, which, in turn, are an order of magnitude more active than pro‐catalysts 8, 11 and 12. These differences in activity are discussed in terms of the structures of each class of complex.