Synthesis, solid-state molecular structure and solution dynamics of new alkoxy stannylene-transition metal complexes

Abstract

Reactions of the dimeric alkoxy stannylene [Sn(OtBu)2]2 (1) with the THF-stabilised group 6 pentacarbonyl fragments [M(CO)5(THF)] (M = Cr, W) yield the trinuclear complexes [{(OC)5M}{η1-Sn2(OtBu)2(μ-OtBu)2}] [M = Cr (2a), W (2b)] and tetranuclear complexes [{(OC)5M}{(OC)5M′}{μ,η1:1-Sn2(OtBu)2(μ-OtBu)2}] [M = Cr, M′ = W (3a); M = M′ = Cr (3b), W (3c)]. The solid-state structures of 2a, 2b, 3a and 3b have been determined by X-ray crystallography: the tri- and tetranuclear complexes are isotypic and crystallise in the monoclinic space group P21/n and P21/m, respectively. The dimeric structure of the starting molecule is retained in the complexes. Steric and electronic effects in the metal complexes can be separately assigned by correlating systematic changes in the solid-state structures with solution NMR spectra of the complexes. In the 1H, 13C{1H} and 119Sn{1H} temperature-variable NMR spectra, strong drifts of the chemical shifts are observed. The 1H and 13C NMR spectra for the trinuclear complexes reveal two consecutive exchange mechanisms, which involve the bridging and terminal OtBu substituents. The free activation enthalpies could be determined by lineshape analysis for 2a (low-temperature process) and 2b (low- and high-temperature processes). The low-temperature exchange most probably occurs via an associative mechanism.