Planar Tetracoordinated Silicon (ptSi): Room-Temperature Stable Compounds Containing Anti-van’t Hoff/Le Bel Silicon

While a variety of compounds containing planar tetracoordinated carbon (ptC), the so-called anti-van’t Hoff/Le Bel carbon, are known experimentally, stable systems containing planar tetracoordinated silicon (ptSi) are barely known. As part of our studies on the application of stereoelectronically well-defined transition-metal fragments to stabilize silicon in unprecedented bonding modes, we report herein the synthesis and full characterization of a series of thermally stable complexes of the general formula [Tp′(CO)2MSiC­(R1)­C­(R2)­M­(CO)2Tp′] (M = Mo, W; R1 = R2 = Me or R1 = H, R2 = SiMe3, Ph; Tp′ = κ3-N,N′,N″-hydridotris­(3,5-dimethylpyrazolyl)­borate), which incorporate a ptSi atom in addition to two ptC atoms. The complexes were obtained by reacting the metallasilylidyne complexes [Tp′(CO)2MSi–M­(CO)2(PMe3)­Tp′] with alkynes R1CCR2 and were comprehensively analyzed by experimental studies and quantum chemical calculations. The analyses revealed that the ptSi atom is embedded in a tricyclic trapezoidal core featuring one internal SiC2 and two outer M–Si–C three-membered rings, which are fused via two Si–C bonds. The structural peculiarities evoked by the presence of an anti-van’t Hoff/Le Bel ptSi center, such as the short M–Si bonds, a nearly linear M–Si–M spine, long M–C bonds, and the presence of two planar tetracoordinated carbon atoms were elucidated by a detailed analysis of the electronic structure, suggesting that one factor for the stabilization of the ptSi geometry is the aromaticity of the central SiC2 ring having two delocalized π electrons. Remarkably, the results further indicate the existence of both anti-van’t Hoff/Le Bel carbon and silicon centers next to each other in the isolated complexes.