Carbene complexes. Part 21. Synthesis and characterisation of bis(carbene)molybdenum(II) complexes and dimetal(0) complexes of the Group 6 elements containing novel bridging bis(carbene) ligands; X-ray structures of [Mo(CO)2(LEt)2(OSO2CF3)2][LEt=CN(Et)(CH2)2NEt] and [W(CO)5{C(OEt)CH2C6H4CH2C(OEt)-o}W(CO)5]

Abstract

Treatment of the bis(carbene)tetracarbonylmolybdenum (0) complex cis-[Mo(CO)₄(L^Et)₂][L^Et= [graphic omitted]Et] with two equivalents of silver trifluoromethanesulphonate in tetrahydrofuran (thf) readily yields the molybdenum(II) complex [Mo(CO)₂(L^Et)₂(OSO₂CF₃)₂](1). Reaction of the complexes [M(CO)₆](M = Cr or W) with Mg[(CH₂)₂C₆H₄-o](thf)(M = Cr) or o-C₆H₄(CH₂MgCl)₂(M = W) in thf affords the crystalline yellow µ-bis(carbene)-dimetal(0) complex [{M(CO)₅[C([graphic omitted]g(thf)_n][M = Cr, n= 3 (2a); M = W, n= 4 (2b)], which when extracted into water and treated with [Et₃O][BF₄] furnishes the appropriate orange µ-bis(carbene) bimetallic complex [[graphic omitted](OEt)CH₂C₆H₄CH₂[graphic omitted](CO)₅][M = Cr (3a) or W (3b)]. Complex (3a) with an excess of PEt₃ gives the red-orange complex [{[graphic omitted](OEt)CH₂C₆H₄CH₂[graphic omitted](CO)₄(PEt₃)}]. Reaction of [M(CO)₆] with o-C₆H₄[CH(SiMe₃)Li(tmen)]₂[tmen = Me₂N(CH₂)₂NMe₂] yields [[graphic omitted]OLi(OEt₂)(tmen)]CH(SiMe₃)C₆H₄CH SiMe₃)[graphic omitted](CO)₅]. In the crystalline bis(carbene)-molybdenum(II) complex (1), the Mo atom resides at the centre of a severely distorted octahedron [C_carb–Mo–C_carb 134.9(2)°], with the two five-membered L^Et rings arranged so as to be approximately parallel, and mean lengths Mo–C_carb 2.154(5), Mo–CO 1.961(6), and Mo–O 2.177(4)Å. In the crystalline ditungsten(0) complex (3b), each W is in an octahedral environment and selected mean parameters include W–C_carb 2.155(12), W–CO 2.03(4), and C_carb–O 1.317(14)Å.