[Ru(xantsil)(CO)(η 6 -toluene)]: Synthon for a Highly Unsaturated Ruthenium(II) Complex through Facile Dissociation of the Toluene Ligand [xantsil ) (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)] Masaaki Okazaki, † Nobukazu Yamahira, Jim Josephus Gabrillo Minglana, ‡ Takashi Komuro, Hiroshi Ogino, § and Hiromi Tobita* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan ReceiVed NoVember 28, 2007 The reactions of [Ru(xantsil)(CO)(η 6 -C 6 H 5 CH 3 )] [1a, xantsil ) (9,9-dimethylxanthene-4,5-diyl)bis- (dimethylsilyl))] with some electron-donating molecules were reported. When 1a was dissolved in benzene, benzene replaced the η 6 -toluene ligand easily at room temperature to give [Ru(xantsil)(CO)(η 6 -C 6 H 6 )] (1b). The η 6 -toluene ligand was also substituted by sterically less demanding two-electron donors smoothly at room temperature to afford [Ru(xantsil)(CO)L 3 ] (L ) CH 3 CN (2), t BuNC (3), and PMe 3 (4)). The X-ray diffraction studies revealed that they take a typical octahedral geometry, in which the xantsil ligand is coordinated to the Ru(II) center in κ 2 (Si,Si) fashion. Reactions of 1a with sterically demanding phosphines gave [Ru(xantsil)(CO)(PR 3 )] (R ) i Pr (5) and Cy (6)). According to the X-ray diffraction study, complex 6 takes a square-pyramidal geometry, in which the xantsil ligand is bound to the Ru(II) center in κ 3 (Si,Si,O) fashion and one of the silyl groups occupies the apical position. The coordinatively unsaturated Ru(II) center is slightly stabilized by the agostic interaction by the C-H bonds in PCy 3 [Ru ··· H: 2.89 and 3.06 Å]. The highly coordinatively unsaturated nature in 6 was indicated by the reaction with carbon monoxide molecules to give [Ru(xantsil)(CO) 3 (PCy 3 )] (7) at room temperature. The typical octahedral geometry with the κ 2 (Si,Si)-xantsil ligand was established by the X-ray diffraction study. Introduction Highly coordinatively unsaturated transition-metal complexes possessing 14 valence electrons are recognized as key inter- mediates in the homogeneous catalytic or stoichiometric reac- tions. In most of the reactions, the intermediates derived from oxidative addition or binding of substrates should leave adequate coordination vacancies, allowing further transformation reac- tions. 1 The η 6 -arene-coordinated complex could be a promising candidate for such a species through the dissociation of the arene ligand. Thus, the exchange of arene molecules between free and bound states has been an important class of reactions in organometallic and surface chemistry. The arene exchange has been mechanistically investigated for late-transition-metal com- plexes. 2 Several possible mechanisms have been reported so far. One mechanism involves an associative process as shown in Scheme 1. An initial rearrangement of the arene from η 6 - to η 4 -coordination occurs to give B. The incoming arene coordi- nates with B in η 2 -fashion to give C. Intramolecular rearrange- ment between two arene molecules, dissociation of the η 2 -arene molecule, and rearrangement of the resultant arene ligand from η 4 to η 6 give F. Taking into account the strong σ-donor and the trans-influencing ability of the silyl ligands, 3 they are expected to exhibit an exceptionally strong trans effect. Therefore, silyl ancillary ligands would accelerate the dissocia- tion of the η 6 -arene ligand to generate highly coordinatively unsaturated intermediates. Chelate formation often stabilizes the complex, and a series of (phosphinoalkyl)silyl ligands were demonstrated to work as supporting ligands and enabled reactivity studies on their complexes. 4 Recently, we have developed a new type of bis(silyl) chelating ligand, (9,9- * Corresponding author. E-mail: tobita@mail.tains.tohoku.ac.jp. † International Research Centre for Elements Science, Institute for Chemical Research, Kyoto University, Uji Kyoto, 611-0011, Japan. ‡ Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101, Philippines. § The Open University of Japan, Chiba, 261-8586, Japan. (1) Crabtree, R. H. In ActiVation and Functionalization of Methane; Davies, J. A. Ed.; VCH: New York, 1990; p 69. (2) (a) Muetterties, E. L.; Bleeke, J. R.; Sievert, A. C. J. Organomet. Chem. 1979, 178, 197. (b) Traylor, T. G.; Stewart, K. J.; Goldberg, M. J. J. Am. Chem.Soc. 1984, 106, 4445. (c) White, C.; Thompson, S. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1977, 1654. (d) Green, M. L. H.; Joyner, D. S.; Wallis, J. M. J. Chem. Soc., Dalton Trans. 1987, 2823. (3) (a) Chatt, J.; Eaborn, C.; Ibekwe, S. Chem. Commun. 1966, 700. (b) Chatt, J.; Eaborn, C.; Ibekwe, S. D.; Kapoor, P. N. J. Chem. Soc. A 1970, 1343. (c) Bentham, J. E.; Cradock, S.; Ebsworth, E. A. 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