Double C-H bond activation of an NHCN-methyl group on triruthenium and triosmium carbonyl clusters: A DFT mechanistic study

摘要

The mechanisms of two recently reported thermal transformations of NHC-triruthenium and -triosmium cluster complexes, which involve the unusual oxidative addition of two C-H bonds of an NHC N-methyl group, have been investigated by density functional theory calculations. The transformations of [M-3(Me(2)Im)(CO)(11)] (Me2IM = 1,3-dimethylimidazol-2-ylidene; M = Ru (1a), Os (1b)) into the ligand-capped dihydrido derivatives [M-3(mu-H)(2)(mu(3)-kappa(2)-MeImCH)(CO)(9)] (M = Ru (3a), Os (3b)) are mechanistically very similar, but they differ in the energy barriers of key steps. For both metal systems (M = Ru, Os), the first step is a ligand rearrangement that moves the Me(2)Im ligand from an equatorial (in 1a and 1b) to an axial coordination position. Both C-H activation steps are oxidative addition processes, and each one is preceded by a CO elimination step that provides a coordinatively unsaturated intermediate. The first C-H oxidative addition occurs via a transition state that implies an unusual interaction of an N-methyl hydrogen atom with two metal atoms simultaneously. This transition state directly leads to an intermediate that contains an edge-bridging hydride and an edge-bridging MeImCH(2) ligand, i.e., [M-3(mu-H)(mu-kappa(2)- MeImCH(2))(CO)(10)] (M = Ru (2a), Os (2b)). The second C-H oxidative addition takes place via an interaction of the unbridged metal atom with a CH2 hydrogen atom of the bridging MeImCH(2) ligand. This gives a face-capping MeImCH ligand and a terminal hydride that subsequently rearranges to an edge-bridging position to give 3a or 3b. The activation barriers of both CO elimination steps are higher for the osmium system than for the ruthenium system. For both metal systems, the slowest step is the first CO elimination.