Mono-oxo molybdenum(vi) and tungsten(vi) complexes bearing chelating aryloxides: synthesis, structure and ring opening polymerization of cyclic esters

Abstract

The mono-oxo aryloxide complexes [M(O)(L¹)₂] (M = Mo (1·hexane), W(2·2MeCN)) have been prepared from [Mo(O)(Cl)₄] or [W(O)(Ot-Bu)₄] and two equivalents of the di-phenol 2,2′-ethylidenebis(4,6-di-tert-butylphenol) L¹H₂, respectively. Use of in situ generated [Mo(O)(Ot-Bu)₄] with two equivalents of L¹H₂ also led to the isolation of 1·2MeCN. In the presence of adventitious oxygen, attempts to generate in situ [Mo(O)(Ot-Bu)₄] and reaction with one equivalent of L¹H₂ afforded the bi-metallic complex [Mo(O)(L¹)(μ-O)Li(THF)(MeCN)]₂·2MeCN (3·2MeCN). Use of the tetra-phenol α,α,α′,α′-tetrakis(3,5-di-tert-butyl-2-hydroxyphenyl)-p-xyleneH₄ (L²H₄) with [Mo(O)(Oi-Pr)₄] led to the isolation of {[Mo(O)]L²}₂ (4), whilst the analogous tungsten complex {[W(O)]L²}₂ (5) was isolated from the reaction of L²H₄ with [W(O)(Ot-Bu)₄]. Similar reaction of p-tert-butylcalix[4]areneH₄ (L³H₄) with [Mo(O)(Oi-Pr)₄] afforded ([Mo(O)L³(NCMe)]·3MeCN (6). Modifications of known routes were employed to access complexes [W(Cl)₂L³]·3.5MeCN (7·3.5MeCN) and [W(O)L³(NCMe)] (8), whilst use of [WO(Ot-Bu)₄] with L³H₄ unexpectedly afforded [W(Ot-Bu)₂L³]·MeCN (9·MeCN). The molecular crystal structures for 1–9 are reported, and the ability of these complexes to act as catalysts for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and ω-pentadecalactone (ω-PDL) has been investigated. The molybdenum complexes 1 and 4 were the best performers for ε-Cl and δ-VL, but all complexes exhibited poor control and were also inactive toward the ROP of PDL.