DFT Mechanistic Study on Alkene Hydrogenation Catalysis of Iron Metallaboratrane: Characteristic Features of Iron Species

The variable coordination geometries, multiple spin states, and high density of states of first row transition metals offer a new frontier in the catalytic chemistry. A DFT study has been performed in order to unveil these characteristic features of iron metallaboratrane complex in alkene hydrogenation. A detailed spin-state analysis reveals there exist two minimum energy crossing points in the formation of (TPB)(mu-H)Fe(H) 3(triplet) from (TPB)Fe(N-2) 1(triplet). In the catalytic cycle, 3(triplet) at triplet state plays a role of active species, and the hydrogenation at triplet state is more favorable than that at singlet state. The dissociation of phosphine arm of TPB ligand from Fe center occurs easily in the triplet state, because the antibonding d sigma is singly occupied in 3(triplet). However, a nondissociative pathway without any phosphine ligand dissociation is not likely to occur. The product is formed via the sigma-bond metathesis between a dihydrogen molecule and a Fe-styryl moiety. The usual direct reductive elimination involving bridging hydride (mu-H) is very difficult because the mu-H is strongly bonded with Fe and B atoms.