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Synthesis and Properties of η6-Silabenzene−M(CO)3 Complexes (M = Cr, Mo)

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posted on 05.12.2005, 00:00 by Akihiro Shinohara, Nobuhiro Takeda, Takahiro Sasamori, Takeshi Matsumoto, Norihiro Tokitoh
Synthesis of the first neutral η6-silabenzene complexes [M(η6-C5H5SiTbt)(CO)3] (M = Cr (2), Mo (3); Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) was achieved by the ligand exchange reactions of [M(CH3CN)3(CO)3] (M = Cr, Mo) with the kinetically stabilized silabenzene 1. X-ray crystallographic analysis of 2 revealed that the η6-coordinated silabenzene rings of the complexes have almost planar geometries with delocalized π-electron structures. The structures of η6-silabenzene complexes 2 and 3 were fully characterized by NMR, UV/vis, and IR spectroscopic analysis. All the 1H, 13C, and 29Si NMR chemical shifts of the SiC5H5 rings of 2 and 3 were shifted to upper field relative to those for silabenzene 1, due to the π-coordination of the silabenzene ring to the metal center. The carbonyl stretching frequencies of 2 and 3 were observed in a region of lower wavenumber as compared to those of the corresponding group 6 metal complexes of benzene. In the UV/vis spectra of 2 and 3, the absorption maxima were slightly red-shifted compared to those for the corresponding benzene complexes. The formation of η6-silabenzene complexes 2 and 3 is very important from the viewpoints of the novel chemical reactivity of silaaromatics. Silabenzene complexes 2 and 3 were thermally stable under an inert atmosphere but air and moisture sensitive. These compounds underwent ready addition reactions with water exclusively at their 1,2-positions to give the corresponding hydroxysilane 8a. The high regioselectivity observed for the addition reactions to 2 and 3 is in a sharp contrast to the competitive 1,2- and 1,4-addition reactions of free silabenzene 1 with water.