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 [H 2 Os 3 (CO) 10 ] and [Ru 3 (CO) 10 L 2 ] (L = CO, MeCN) with 2-ethynylpyridine has been investigated. Treatment of [H 2 Os 3 (CO) 10 ] with excess 2-ethynylpyridine affords [HOs 3 (CO) 10 (μ-C 5 H 4 NCH=CH)] (1), [HOs 3 (CO) 9 (μ 3 -C 5 H 4 NC[double bond, length as m-dash]CH 2 )] (2), [HOs 3 (CO) 9 (μ 3 -C 5 H 4 NC[double bond, length as m-dash]CCO 2 )] (3), and [HOs 3 (CO) 10 (μ-CH[double bond, length as m-dash]CHC 5 H 4 N)] (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 3 (CO) 12 ] and 2-ethynylpyridine is C-C bond coupling of the alkyne moiety to furnish the triruthenium clusters [Ru 3 (CO) 7 (μ-CO){μ 3 -C 5 H 4 NC[double bond, length as m-dash]CHC(C 5 H 4 N)[double bond, length as m-dash]CH}] (5) and [Ru 3 (CO) 7 (μ-CO){μ 3 -C 5 H 4 NCCHC(C 5 H 4 N)CHCHC(C 5 H 4 N)}] (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 via 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).