Synthesis, Structure, and Fluxional Behavior of Donor-Stabilized Bis(silylene)tungsten Complexes

Donor-stabilized bis(silylene)tungsten complexes CpW(CO) 2 {(SiMe 2 )···Do···(SiMe 2 )} (Cp = η-C 5 H 5; Do = NEt 2 ( 1 ), OMe ( 2 )) were synthesized by photolysis of a C 6 D 6 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) Å for 1; 2.490(3) Å for 2 ) are significantly shorter than those of structurally similar silyltungsten complexes, while the Si−N bonds (1.930(6) and 1.920(7) Å) in 1 and the Si−O bonds (1.792(7) Å) 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 29 Si NMR signals as well as large coupling constants between 29 Si and 183 W ( 1 δ = 62.1 ppm, 1 J W - Si = 91.5 Hz; 2 δ = 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.