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.