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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.

A 1,2-dibromodigermene having Bbt groups (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl) was synthesized by the reaction of BbtLi with GeBr2·dioxane. X-ray crystallographic analysis revealed that it retains the Ge&dbd;Ge double bond in the solid state, while the equilibrium between the 1,2-dibromodigermene and the corresponding bromogermylenes in solution was evidenced by the UV−vis spectroscopic studies and the chemical reactions. The reactions of the 1,2-dibromodigermene with elemental sulfur in toluene resulted in the formation of a cis-dithiadigermetane derivative, indicating the generation of a transient germathiocarbonyl bromide derivative via the sulfurization reaction of the Bbt-substituted bromogermylene generated in the equilibrium state.

Treatment of dichloroferrocenylphosphine with two molar amounts of a lithium phosphide bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) group afforded the corresponding 1,3-dihydro-2-ferrocenyltriphosphane [1; (TbtHP)2PFc, Fc = ferrocenyl] as a mixture of three diastereomers in 73% yield. In sharp contrast to the previously reported 1,3-dihydrotriphosphanes [(RHP)2PR, R = Ph, t-Bu], 1 was quite stable toward air and moisture either in the solid state or in solution at ambient temperature. The structural characterization of 1 was achieved by NMR spectra and X-ray crystallographic analysis. In the ³¹P{¹H} NMR spectrum of the mixture of three diastereomers of 1, the characteristic two A2B and one ABX system were observed as signals assignable to two meso and one dl isomer, respectively. The X-ray crystallographic analysis for a single crystal obtained from the diastereomer mixture of 1 revealed its molecular structure, having P−P bond lengths of 2.2304(12) and 2.2322(12) Å and a P−P−P bond angle of 96.17(5)°, although the configuration could not be determined. Thermolysis of 1 in toluene led to the quantitative formation of TbtPH2 (2) and (E)-TbtP&dbd;PFc (3), as judged by the ¹H and ³¹P NMR spectra. Kinetic studies indicated that the thermolysis of 1 is a first-order reaction including a unimolecular dissociative process, which was reasonablely supported by theoretical calculations.

Thermal reactions of a stable germacyclopropabenzene, Tbt(Dip)GeC6H4 (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Dip = 2,6-diisopropylphenyl), with elemental chalcogens in C6D6 at 130 °C resulted in the formation of novel Ge-containing four-membered heterocycles, 2H-benzo[c][1,2]chalcogenagermete derivatives. This is the first example of a ring expansion reaction of cyclopropabenzene derivatives using elemental chalcogen, while several types of ring-opening reactions have been explored for cyclopropabenzene so far. In addition, the 2H-benzo[c][1,2]telluragermete is the first stable compound as a tellurete derivative. Structures of the chalcogenagermetes obtained here were elucidated by X-ray crystallographic analysis, and the experimentally obtained structural parameters were supported by theoretical calculations. The skeletons of chalcogenagermetes have very little strain in their four-membered rings annelated with a benzene ring.

Reactions of stable silylene-isocyanide complexes Tbt(Mes)SiCNAr (3a, Ar = Mes*; 3b, Ar = Tbt; 3c, Ar = Tip; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl, Mes* = 2,4,6-tri-tert-butylphenyl, Tip = 2,4,6-triisopropylphenyl) with boron compounds were investigated. Reactions of 3a,b with BH3·THF afforded the first stable silylborane-isocyanide complexes, Tbt(Mes)SiHBH2·CNAr (4a,b). Silylene complex 3a reacted with BH3·PPh3 as well to give the corresponding silylborane-phosphine complex Tbt(Mes)SiHBH2·PPh3 (12). In addition, silylene 2 thermally generated from the corresponding disilene, Tbt(Mes)Si&dbd;Si(Mes)Tbt (1), gave 12 when reacted with BH3·PPh3. These results strongly suggested that the reactions of 3 with BH3·THF and BH3·PPh3 proceeded via insertion of a silylene, Tbt(Mes)Si:  (2), into the B−H bond rather than the nucleophilic attack of the silicon atom of 3 toward the boron atom. Thermal dissociation of 4 and 12 into a base-free silylborane, Tbt(Mes)SiHBH2 (9), was evidenced by the base-exchange reactions. Furthermore, silylene 2 was found to insert into boron−halogen bonds as well as boron−hydrogen bonds. The novel reactivity of silylene 2 was applied to the syntheses of a variety of silylborane derivatives such as base-free silylboranes, silylborane-isocyanide complexes, and silylborates.

The treatment of an overcrowded diaryldilithiogermane, Tbt(Dip)GeLi2 (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Dip = 2,6-diisopropylphenyl), generated by exhaustive reduction of the corresponding dibromogermane Tbt(Dip)GeBr2, with 1,2-dibromobenzene resulted in the isolation of the first stable germacyclopropabenzene, which was fully characterized by ¹H and ¹³C NMR spectra, FAB-MS, and X-ray structural analysis. As for the ring deformation of cyclopropabenzene and its heavier group 14 element analogues, the experimental results are in good agreement with those obtained by theoretical calculations.