C–P\nBond Cleavage Through Hydrogenation in\nRuthenium Complexes Supported by P,N Ligands

The reactivity of ruthenium hydride complexes supported\nby 2-((di-<i>tert</i>-butylphosphaneyl)methyl)pyridine, <b>L1</b> and\n2-(di-<i>tert</i>-butylphosphaneyl)pyridine, <b>L2</b>, was explored. The reaction of {Ru(COD)Cl<sub>2</sub>}<sub><i>x</i></sub> with <b>L1</b> in the presence of base and\n10 bar of H<sub>2</sub> gave the expected complex [Ru(L1)<sub>2</sub>(H)Cl], <b>1</b>, while the same reaction with <b>L2</b> gave [Ru(L2)(P(H)<sup><i>t</i></sup>Bu<sub>2</sub>)(H)Cl], <b>2</b>, that results from the cleavage of a C–P bond. We\nwere able to establish that under the reaction conditions the first\nspecies formed is [Ru(L2)<sub>2</sub>(H)Cl], <b>3</b>, and that\nthis species decomposes to give complex <b>2</b> and is in equilibrium\nwith [Ru(L<sub>2</sub>)<sub>2</sub>Cl<sub>2</sub>], <b>4</b>. The proposed mechanism obtained by DFT has the protonation of the\ncarbon as the highest energy step (38.9 kcal/mol), consistent with\na slow reaction. Preliminary studies reveal that complex <b>2</b> is a very active catalyst in the hydrogenation of benzaldehyde (TONs\nup to 44,000).