Reactions of triosmium and triruthenium clusters with 2-ethynylpyridine: new modes for alkyne C-C bond coupling and C-H bond activation.

The reaction of the trimetallic clusters [HOs(CO)] and [Ru(CO)L] (L = CO, MeCN) with 2-ethynylpyridine has been investigated. Treatment of [HOs(CO)] with excess 2-ethynylpyridine affords [HOs(CO)(μ-CHNCH=CH)] (1), [HOs(CO)(μ-CHNC[double bond, length as m-dash]CH)] (2), [HOs(CO)(μ-CHNC[double bond, length as m-dash]CCO)] (3), and [HOs(CO)(μ-CH[double bond, length as m-dash]CHCHN)] (4) formed through either the direct addition of the Os-H bond across the C[triple bond, length as m-dash]C bond or acetylenic C-H bond activation of the 2-ethynylpyridine substrate. In contrast, the dominant pathway for the reaction between [Ru(CO)] and 2-ethynylpyridine is C-C bond coupling of the alkyne moiety to furnish the triruthenium clusters [Ru(CO)(μ-CO){μ-CHNC[double bond, length as m-dash]CHC(CHN)[double bond, length as m-dash]CH}] (5) and [Ru(CO)(μ-CO){μ-CHNCCHC(CHN)CHCHC(CHN)}] (6). Cluster 5 contains a metalated 2-pyridyl-substituted diene while 6 exhibits a metalated 2-pyridyl-substituted triene moiety. The functionalized pyridyl ligands in 5 and 6 derive the formal C-C bond coupling of two and three 2-ethynylpyridine molecules, respectively, and 5 and 6 provide evidence for facile alkyne insertion at ruthenium clusters. The solid-state structures of 1-3, 5, and 6 have been determined by single-crystal X-ray diffraction analyses, and the bonding in the product clusters has been investigated by DFT. In the case of 1, the computational results reveal a rare thermodynamic preference for a terminal hydride ligand as opposed to a hydride-bridged Os-Os bond (3c,2e Os-Os-H bond).