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 Mo-2(NN)(2) (1) {NN = -kappa(2)-CH[N(2,6-iPr(2)C(6)H(3))](2)} in the H-H sigma-bond cleavage was investigated. DFT calculations disclosed that the H-H sigma-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 (Delta G degrees(double dagger) = 9.0 kcal/mol) and acceptable activation energies (Delta G degrees(double dagger) = 22.5 kcal/mol), respectively, suggesting that the Mo-Mo quintuple bond can be applied to various s-bond cleavages. In these sigma-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 (d delta-MO) of 1 is at lower energy and the LUMO + 2 (d delta*-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 sigma-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 d delta-type MOs of the Mo-Mo quintuple bond in the transition state. Such a polarized electronic structure enhances the bonding overlap between the d delta-MO of the Mo-Mo bond and the sigma*-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 d delta-type MOs is one of the advantages of the metal-metal multiple bond, because such polarization is impossible in the mononuclear metal complex.