Reactivity of [Os<sub>3</sub>(μ-H)<sub>2</sub>(CO)<sub>10</sub>] with N-Heterocyclic Carbenes: A Combined Experimental and DFT Computational Study
作者:Javier A. Cabeza、Ignacio del Río、José M. Fernández-Colinas、Enrique Pérez-Carreño、M. Gabriela Sánchez-Vega、Digna Vázquez-García
DOI:10.1021/om100679a
日期:2010.9.13
The unsaturated triosmium dihydrido cluster [Os-3(mu-H)(2)(CO)(10)] (1) reacts at room temperature with N-heterocyclic carbenes (NHCs) of the 1,3-disubstituted imidazol-2-ylidene type (R(1)R(2)Im). While the addition product [Os3H(mu-H)(NHC)(CO)(10)] has been obtained only for NHC = Mes(2)lm, CO-substitution derivatives of the type [Os-3(mu-H)(2)(NHC)(CO)(9)] have been obtained for NHC = Me(2)Im, MePhIm, Ph(2)Im, and Mes(2)Im. Small amounts of the hydroxo-bridged derivatives [Os-3(mu-H)-(mu-OH)(NHC)(CO)(9)] (NHC = Me(2)Im, MePhIm) are also formed in wet solvents. DFT mechanistic studies on the reactions of compound 1 with the small Me(2)Im and the very bulky Mes(2)Im have revealed that the substitution reactions follow an associative reaction pathway that starts with the addition of the NHC to compound 1 (in one elementary step that gives [Os3H(mu-H)(NHC)(CO)(10)]) and ends with the elimination of a CO ligand (in three steps from [Os3H(mu-H)(NHC)(CO)(10)]) to give the final substitution product [Os-3(mu-H)(mu-OH)(NHC)(CO)(9)]. The relative energy of each stationary point depends upon the NHC ligand used, those of the Mes(2)Im system being higher than those of the Me(2)Im system. This computational study has also explained why the addition intermediate [Os3H(mu-H)-(NHC)(CO)(10)] has been isolated only for NHC = Mes(2)Im.