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

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-tertbutylphenol) L1 H2, respectively. Use of in situ generated [Mo(O)(Ot-Bu)4] with two equivalents of L1 H2 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 L1 H2 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 (L2 H4) 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 L2 H4 with [W(O)(Ot-Bu)4]. Similar reaction of p-tert-butylcalix[4]areneH4 (L3 H4) 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 L3 H4 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.