Reactivity of [Os3(μ-H)2(CO)10] with N-Heterocyclic Carbenes: A Combined Experimental and DFT Computational Study

• Published in: Organometallics 2010-09, Vol.29 (17), p.3828-3836
• Main Authors: Cabeza, Javier A, del Río, Ignacio, Fernández-Colinas, José M, Pérez-Carreño, Enrique, Sánchez-Vega, M. Gabriela, Vázquez-García, Digna
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om100679a

The unsaturated triosmium dihydrido cluster [Os3(μ-H)2(CO)10] (1) reacts at room temperature with N-heterocyclic carbenes (NHCs) of the 1,3-disubstituted imidazol-2-ylidene type (R1R2Im). While the addition product [Os3H(μ-H)(NHC)(CO)10] has been obtained only for NHC = Mes2Im, CO-substitution derivatives of the type [Os3(μ-H)2(NHC)(CO)9] have been obtained for NHC = Me2Im, MePhIm, Ph2Im, and Mes2Im. Small amounts of the hydroxo-bridged derivatives [Os3(μ-H)(μ-OH)(NHC)(CO)9] (NHC = Me2Im, MePhIm) are also formed in wet solvents. DFT mechanistic studies on the reactions of compound 1 with the small Me2Im and the very bulky Mes2Im 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(μ-H)(NHC)(CO)10]) and ends with the elimination of a CO ligand (in three steps from [Os3H(μ-H)(NHC)(CO)10]) to give the final substitution product [Os3(μ-H)(μ-OH)(NHC)(CO)9]. The relative energy of each stationary point depends upon the NHC ligand used, those of the Mes2Im system being higher than those of the Me2Im system. This computational study has also explained why the addition intermediate [Os3H(μ-H)(NHC)(CO)10] has been isolated only for NHC = Mes2Im.