Synthesis, Structure, and Bonding Nature of Ethynediyl-Bridged Bis(silylene) Dinuclear Complexes of Tungsten and Molybdenum

Reactions of Ph2HSiCCSiHPh2 with 2 equiv of the labile complexes Cp*(CO)2M(NCMe)Me (1a, M  W; 2, M  Mo; Cp*  η5-C5Me5) gave the novel CC-bridged dinuclear complexes Cp*(CO)2M(SiPh2)(μ-CC)(SiPh2)M(CO)2Cp* (5, M  W; 6, M  Mo), whose molecular structures were determined by X-ray crystallography. The CC bridge interacts with both the metal and silylene centers of two Cp*(CO)2M(SiPh2) fragments to form two M–Si–C three-membered-ring skeletons which are linked nearly perpendicularly to each other. The W–Si bond distances of 5 are comparable to those of typical base-stabilized tungsten silylene complexes. The C–C bond distance is much longer than a typical CC triple-bond distance and is similar to a typical CC double-bond distance. The bonding nature and electronic structure of 5 were disclosed by a DFT study of the model complex Cp(CO)2W(SiH2)(μ-CC)(SiH2)W(CO)2Cp (5M; Cp  η5-C5H5). This study demonstrates that 5M is an ethynediyl-bridged bis(silylene) dinuclear tungsten complex which contains various charge transfer (CT) interactions between the tungsten (W), silylene (SiH2), and ethynediyl (CC), as follows. (1) CTs occur from the lone pairs (φCClp) and π orbital (φCCπ) of the ethynediyl to the unoccupied d orbital (dWunoc) of the W and from the occupied d orbital (dWocc) of the W to the π* orbital (φCCπ*) of the ethynediyl. (2) CTs occur from the lone pair orbital (φSilp) of the silylene to dWunoc and from dWocc to the empty p orbital (φSip) of the silylene. (3) CT occurs from φCCπ to φSip, which leads to considerably strong Si–C bonding interactions and a considerably large elongation of the C–C distance. The mixing of φCCπ into φCCπ* induces π orbital polarization of the CC moiety in one plane and a reverse π orbital polarization in the perpendicular plane. These polarizations in addition to the CT from dWocc to φCCπ* also participate in the C–C bond weakening of the ethynediyl. Reaction of 1a with 1 equiv of Ph2HSiCCSiHPh2 afforded a mixture of the mononuclear acetylide–silylene complex Cp*(CO)2W(CCSiHPh2)(SiPh2) (7) and dinuclear complex 5. Addition of 1a to the mixture resulted in the conversion of 7 to 5, indicating the intermediacy of 7 in the formation of 5 in the 1:2 reaction of the bis(silyl)acetylene and 1a. A similar 1:1 reaction using molybdenum complex 2 strongly suggests the formation of an equilibrium mixture of the acetylide–silylene complex Cp*(CO)2Mo(CCSiHPh2)(SiPh2) (8) and silapropargyl/alkynylsilyl complex Cp*(CO)2Mo(η3-Ph2SiCCSiHPh2) (9) in addition to dinuclear complex 6. Mononuclear complexes 8 and 9 were converted to 6 upon reaction with 2. The fluxional behavior of dinuclear complexes 5 and 6 in solution is also described.