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14-Electron Four-Coordinate Ru(II) Carbyl Complexes and Their Five-Coordinate Precursors:  Synthesis, Double Agostic Interactions, and Reactivity

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journal contribution
posted on 21.08.1999, 00:00 by Dejian Huang, William E. Streib, John C. Bollinger, Kenneth G. Caulton, Rainer F. Winter, Thomas Scheiring
The structure of five-coordinate Ru(II) complexes RuHCl(CO)(PiPr3)2, 1, RuCl2(CO)(PiPr3)2, 2, and Ru(Ph)Cl(CO)(PtBu2Me)2, 12, are reported. All three of these complexes have square-based pyramid geometry with the strongest σ-donor ligand trans to the vacant site. These 16-electron complexes do not show bona fide agostic interactions. This is attributed to the strong trans influence ligand (H, CO, and Ph) and π-donation of the Cl, which is further supported by the fact that two agostic interactions are present in the Cl- removal product of 12, i.e., the four-coordinate [RuPh(CO)L2]BAr‘4 (L = PtBu2Me, Ar‘ = 3,5-C6H3(CF3)2), 16. Structural comparison of 16 and 12 reveals that removal of Cl- does not change the remaining ligand arrangements but creates two low-lying LUMOs for agostic interactions, which persist in solution as evidenced by IR spectroscopy. Reactions of 16 with E−H (E = B, C(sp)) bonds cleave the Ru−Ph bond and form Ru−E/H bonds by different mechanisms. The reaction with catecholborane gives [RuH(CO)L2]BAr‘4, which further reacts with catecholborane to give [Ru(BR2)(CO)L2]BAr‘4. However, the reaction with Me3SiCCH undergoes a multistep transformation to give a PhCCSiMe3- and Me3SiCCH-coupled product, the mechanism of which is discussed. Reaction of RuCl2(CO)L2 with 1 equiv MeLi affords RuMeCl(CO)L2, 5, which further reacts with MeLi forming RuMe2(CO)L2, 7. Variable-temperature 13C{1H} NMR spectra reveal the two methyls in 7 are inequivalent and exchange by overcoming an energy barrier of 6.8 kcal/mol at −30 °C. The chloride of 5 can be removed to give [RuMe(CO)L2]BAr‘4.