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

2017-03-14T13:36:49Z (GMT) by Yue Chen Shigeyoshi Sakaki
The recently reported high reactivity of the Mo–Mo quintuple bond of Mo2(NN)2 (1) {NN = 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.