Synthesis and structure of bimetallic complexes with σ,π-bridged monocarbene ligands

The Fischer method was employed to synthesize bimetallic monocarbene complexes from lithiated [Cr(SC 4 H 4 )(CO) 3 ] and metal hexacarbonyls. In addition to the formation of [M{C[η 5 -C 4 H 3 SCr(CO) 3 ]OEt}(CO) 5 ] (M = Cr 1 or W 2) the complexes [M{C[η 5 -C 4 H 3 SCr(CO) 3 ]O(CH 2 ) 4 OEt}(CO) 5 ] (M = Cr 3 or W 4), were also obtained. The formation of 3 and 4 is ascribed to the activation of tetrahydrofuran (thf) by a σ,π-co-ordinated acyl metalate, which triggers the cleavage of the thf ring and leads to its inclusion into the alkoxy substituent of the resulting carbene moiety. In polar solvents the Cr(CO) 3 fragments are displaced to give known pentacarbonyl[ethoxy(thienyl)carbene] complexes and the new mononuclear carbene complexes [M{C(C 4 H 3 S)O(CH 2 ) 4 OEt }(CO) 5 ] (M = Cr 5 or W 6). The analogous benzo[b]thienyl carbene complex [Cr{C[η 6 -C 8 H 5 SCr(CO) 3 ]OEt}(CO) 5 ] 7 was prepared similarly, but did not afford the thf-inserted product and the corresponding conversion into the mononuclear carbene complex [Cr{C(C 8 H 5 S)OEt}(CO) 5 ] 8, was much slower.