title: Symmetry aspects of H2 splitting by five-coordinate d6 ruthenium amides, and calculations on acetophenone hydrogenation, ruthenium alkoxide formation, and subsequent hydrogenolysis in a model trans-Ru(H)2(diamine)(diphosphine) system.
abstract: The potential energy surface (PES) of H(2) addition to the Ru═N bond of a model five-coordinate ruthenium amide (Ru═N), leading to an octahedral trans-Ru(H)(2)(diamine)(diphosphine) (HRu-NH) and subsequent acetophenone hydrogenation, is studied using M06 density functional theory methods. A qualitative molecular orbital analysis reveals that H(2) addition to the ground state of Ru═N (which has a distorted trigonal-bipyramidal geometry) fits the criterion of a symmetry-forbidden reaction. A transition state (TS) for H(2) heterolytic splitting by Ru═N corresponds to the reaction taking place on an excited state of the Ru═N having a square-pyramidal geometry and gives ΔG°(⧧) = 19.5 kcal/mol. The reaction between HRu-NH and acetophenone proceeds by a localized hydride-transfer TS with ΔG°(⧧) = 11.5 kcal/mol. This TS leads to an ion pair between a square-pyramidal d(6) ruthenium amino cation and the alkoxide and is uphill from the separated reactants by 3.5 kcal/mol. Subsequent abstraction of the amino proton by the alkoxide within the ion pair is barrierless, but it also lacks any thermodynamic driving force. In contrast, reorientation of the alkoxide within the ion pair to form an octahedral ruthenium alkoxide is calculated to be exoergic by 7.1 kcal/mol. These features of the PES suggest that the known rapid production of ruthenium alkoxides when stoichiometric amounts of acetophenone and HRu-NH are reacted at low temperatures proceeds by a simple direct route following hydride transfer. For the simplified model complex, ruthenium alkoxide is calculated to be the thermodynamic product of the hydrogenation reaction (exoergic by 3.6 kcal/mol). A TS for H(2) heterolytic splitting across the Ru-alkoxide bond is calculated to have ΔG°(⧧) (16.0 kcal/mol), slightly smaller than that of H(2) addition to the five-coordinate Ru═N.
authors.affiliation: Department of Chemistry, American University of Beirut, Beirut, Lebanon. fh19@aub.edu.lb
authors.forename: Faraj
authors.initials: F
authors.lastname: Hasanayn
authors.forename: Robert H
authors.initials: RH
authors.lastname: Morris