14-Electron four-coordinate Ru(II) carbyl complexes and their five-coordinate precursors: Synthesis, double agostic interactions, and reactivity

Article abstract of DOI:10.1021/ja990621w

The structure of five-coordinate Ru(II) complexes RuHCl(CO)(PⁱPr₃)₂,1, RuCl₂(CO)(PⁱPr₃)₂, 2, and Ru(Ph)Cl(CO)(P^tBu₂Me)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 = P^tBu₂Me, 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.