ic6b03103_si_002.txt (88.89 kB)
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
dataset
posted on 2017-03-14, 13:36 authored by Yue Chen, Shigeyoshi SakakiThe
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.