Iron, Ruthenium, and Osmium Complexes Supported by the Bis(silyl) Chelate Ligand (9,9-Dimethylxanthene-4,5-diyl)bis(dimethylsilyl): Synthesis, Characterization, and Reactivity

The bis(silyl)-type bidentate ligand precursors xantsil-H2 (1a) and 2,7-di-t-butylxantsil-H2 (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 [M3(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 [Ru3(xantsil)(μ-H)2(CO)10] (5) was confirmed and independently synthesized by the reaction of [Ru3(CO)10(CH3CN)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)(η6-toluene)] (6). Dissociation of the three carbonyl ligands would be enhanced by the severe steric repulsion between the SiMe2 moiety and the three fac-carbonyl ligands, high trans effect of silyl groups, and precoordination of the xanthene oxygen atom.