Synthesis, structure, and fluxional behavior of donor-stabilized bis(silylene)tungsten complexes

Donor-stabilized bis(silylene)tungsten complexes CpW(CO)(2){(SiMe2)... Do ...(SiMe2)} (Cp = eta-C5H5; Do = NEt2 (1), OMe (2)) were synthesized by photolysis of a C6D6 solution containing CpW(CO)(3)Me and HSiMe(2)SiMe(2)Do. The X-ray crystal structures of 1 and 2 revealed that the W-Si bonds (2.502(2) and 2.501(2) Angstrom for 1; 2.490(3) Angstrom for 2) are significantly shorter than those of structurally similar silyltungsten complexes, while the Si-N bonds (1.930(6) and 1.920(7) Angstrom) in 1 and the Si-O bonds (1.792(7) A) in 2 are much longer than usual Si-N and Si-O single bonds. These structural data indicate that the W-Si bonds bear partial double bond character, whereas the Si-N and Si-O bonds are regarded as a hybrid of covalent bonding and dative bonding. The unsaturated nature of the tungsten-silicon bonds is also indicated by the significant downfield shift of the Si-29 NMR signals as well as large coupling constants between Si-29 and W-183 (1 delta = 62.1 ppm, (1)J(W-Si) = 91.5 Hz; 2 delta = 99.3 ppm, (1)J(W-Si) = 99.3 Hz) compared to those of structurally similar silyltungsten complexes. The tungsten complexes 1 and 2 showed fluxional behavior due to silylene-methyl group exchange and, in the case of 1, N-Et group exchange. A mechanism containing the generation of a base-free silyl(silylene)tungsten complex as the key intermediate is proposed for the fluxional process.