Mo–Mo Quintuple Bond is Highly Reactive in H–H, C–H, and O–H σ‑Bond Cleavages Because of the Polarized Electronic Structure in Transition State

The recently reported high reactivity of the Mo–Mo quintuple bond of Mo2(N∧N)2 (1) {N∧N = μ-κ2-CH­[N­(2,6-iPr2C6H3)]2} in the H–H σ-bond cleavage was investigated. DFT calculations disclosed that the H–H σ-bond cleavage by 1 occurs with nearly no barrier to afford the cis-dihydride species followed by cis–trans isomerization to form the trans-dihydride product, which is consistent with the experimental result. The O–H and C–H bond cleavages by 1 were computationally predicted to occur with moderate (ΔG°⧧ = 9.0 kcal/mol) and acceptable activation energies (ΔG°⧧ = 22.5 kcal/mol), respectively, suggesting that the Mo–Mo quintuple bond can be applied to various σ-bond cleavages. In these σ-bond cleavage reactions, the charge-transfer (CTMo→XH) from the Mo–Mo quintuple bond to the X–H (X = H, C, or O) bond and that (CTXH→Mo) from the X–H bond to the Mo–Mo bond play crucial roles. Though the HOMO (dδ-MO) of 1 is at lower energy and the LUMO + 2 (dδ*-MO) of 1 is at higher energy than those of RhCl­(PMe3)2 (LUMO and LUMO + 1 of 1 are not frontier MO), the H–H σ-bond cleavage by 1 more easily occurs than that by the Rh complex. Hence, the frontier MO energies are not the reason for the high reactivity of 1. The high reactivity of 1 arises from the polarization of dδ-type MOs of the Mo–Mo quintuple bond in the transition state. Such a polarized electronic structure enhances the bonding overlap between the dδ-MO of the Mo–Mo bond and the σ*-antibonding MO of the X–H bond to facilitate the CTMo→XH and reduce the exchange repulsion between the Mo–Mo bond and the X–H bond. This polarized electronic structure of the transition state is similar to that of a frustrated Lewis pair. The easy polarization of the dδ-type MOs is one of the advantages of the metal–metal multiple bond, because such polarization is impossible in the mononuclear metal complex.