title: 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.
abstract: 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→XHand 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.
authors.affiliation: Fukui Institute for Fundamental Chemistry, Kyoto University , Takano-Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan.
authors.forename: Yue
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Fukui Institute for Fundamental Chemistry, Kyoto University , Takano-Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan.
authors.forename: Shigeyoshi
authors.identifier: http://orcid.org/0000-0002-1783-3282
authors.initials: S
authors.lastname: Sakaki