Treatment of Ru(xantsil)(CO)(η6-toluene) (1) [xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)] with PCy3 led to the formation of Ru(xantsil)(CO)(PCy3) (3), in which the xantsil ligand is bound to the ruthenium center in κ3(Si,Si,O) fashion. The highly coordinatively unsaturated nature of 3 is indicated by the reaction with CO to give Ru(xantsil)(CO)3(PCy3) (5).

Ruthenium complexes bearing a xanthene-based bis­(silyl) chelate ligand, “xantsil” ((9,9-dimethylxanthene-4,5-diyl)­bis­(dimethylsilyl)), Ru­{κ3(Si,O,Si)-xantsil}­(CO)­(PR3) (1-Cy: R = Cy (cyclohexyl), 1-Cyp: R = Cyp (cyclopentyl)), were found to catalyze the reactions of internal arylalkynes with tertiary silanes (1–1.3 equiv) at a moderate temperature (room temperature to 70 °C) to give (E)-alkenes having an ortho-silylated aryl group, i.e., (E)-R1C­(H)C­(H)­(C6H3-o-SiR3 3-p-R2). These catalytic reactions involve a unique ortho-selective C–H silylation of an aryl group in arylalkynes accompanied by hydrogenation of their C–C triple bonds (ortho-C–H silylation/hydrogenation). Importantly, in these reactions, the alkynyl moiety of arylalkynes serves as both a directing group and a hydrogen acceptor. The substrate scope of this ortho-C–H silylation/hydrogenation was explored by use of eight combinations of arylalkynes and tertiary silanes. In cases using bulky substrates, the catalytic performance of 1-Cyp with a relatively less bulky phosphine ligand (PCyp3) was shown to be superior to that of the PCy3 analogue 1-Cy.

A new type of catalytic hydrosilylation of arylalkynes was induced by a 16-electron ruthenium bis(silyl) phosphine complex, resulting in ortho-silylation of the aryl group as well as a hydrogenation of the alkyne CC bond to give an (E)-form of alkene selectively. On the other hand, the same reaction using a related bis(silyl) complex having an η6-toluene ligand instead of the phosphine ligand as a catalyst led to a normal hydrosilylation reaction to afford silylalkene.

Ru{κ2(Si,Si)-xantsil}(CO)(η6-C6H5CH3) (1) was found to be a catalyst for oligomerization–deoligomerization of HSiMe2SiMe3 to give H(SiMe2)nMe (n = 1–8 at 90 °C for 2 days). Treatment of 1 with HSiMe2SiMe2OR (R = Me, t-Bu) led to quantitative formation of Ru{κ3(O,Si,Si)-xantsil}(CO)(H){(SiMe2•••O(R)•••SiMe2)} (R = Me (2a), t-Bu (2b)), which also worked as a catalyst for oligomerization–deoligomerization of HSiMe2SiMe3. Based on these experimental results, a mechanism involving silyl(silylene) intermediates was proposed for the oligomerization–deoligomerization of HSiMe2SiMe3. Complex 2a reacted with MeOH in toluene-d8 to give Ru{κ2(Si,Si)-xantsil}(CO)(η6-toluene-d8) and Me2Si(OMe)2 with evolution of H2. Under a CO atmosphere, 2a was smoothly converted to its CO adduct Ru{κ2(Si,Si)-xantsil}(CO)2(H){(SiMe2•••O(Me)•••SiMe2)} (3).Key words: silylene complex, ruthenium, polysilane, dehydrogenative coupling, oligomerization.

The bis(silyl)-type bidentate ligand precursors xantsil-H 2 (1a) and 2,7-di-t-butylxantsil-H 2 (1b) possessing the xanthene backbone were prepared by dilithiation of the 4,5-positions of 9,9-dimethylxanthene or 2,7-di-t-butyl-9,9-dimethylxanthene using n-BuLi in the presence of tetramethylethylenediamine (TMEDA) followed by treatment with chlorodimethylsilane. According to X-ray diffraction analysis of 1b, the xanthene core is close to planar as observed in the dihedral angle of 6.2(2)°between two least-square planes of two aromatic rings in xanthene. UV irradiation of [Fe(CO) 5 ] and 1a in dichloromethane provided cis-[Fe(xantsil)(CO) 4 ] (2), while thermal reactions of [M 3 (CO) 12 ] (M ) Ru and Os) and 1a provided cis-[M(xantsil)(CO) 4 ] (M ) Ru (3) and Os (4)). In the course of the synthetic study on 3, formation of [Ru 3 (xantsil)(µ-H) 2 (CO) 10 ] (5) was confirmed and independently synthesized by the reaction of [Ru 3 (CO) 10 (CH 3 CN) 2 ] with 1a. Thermolysis of 5 and 1a at 120 °C for 13 min afforded 3, indicating its intermediacy to 3. Refluxing the toluene solution of 3 for 3 h resulted in the replacement of three carbonyl ligands with toluene to give [Ru(xantsil)(CO)(η 6 -toluene)] (6). Dissociation of the three carbonyl ligands would be enhanced by the severe steric repulsion between the SiMe 2 moiety and the three fac-carbonyl ligands, high trans effect of silyl groups, and precoordination of the xanthene oxygen atom.