Mechanistic Studies of the Proton-Coupled Electron Transfer Reactivity of a Cobalt Complex with a Proton-Responsive PNP Pincer-Type Ligand

Herein, we investigate
the proton-coupled electron transfer (PCET)
reactivity of a cobalt(I) complex with a proton-responsive pyridin-4-ol
PNP pincer-type ligand (HLPNP = 2,6-bis((bis-tert-butylphosphaneyl)-methyl)pyridin-4-ol). The cobalt(II) complexes
[(LPNP)CoIICl], 1, and [(LPNP)CoII(MeCN)]+, 2+, with the deprotonated ligand and [(HLPNP)CoII(MeCN)2]2+, 2H2+, with the protonated ligand, were synthesized
and characterized. 2H2+ has a
pKa of 18 ± 1, and the reduction
of 2H2+ appears at −1.08
V vs. FeCp2+|0 in MeCN. This leads to a bond
dissociation free energy (BDFE) of the OH bond in [(HLPNP)CoI(MeCN)]+, 2H+, of 52 kcal mol–1, which is supported by
DFT calculations. The solution BDFE of 2H+ equals the BDFE of 1/2 H2, and indeed, 2H+ slowly loses
dihydrogen. Kinetic analysis revealed a first-order rate law in 2H+ with a reaction rate constant k of 3.2 × 10–4 s–1 at 25 °C and a positive activation entropy ΔS‡ of 9.4 ± 0.6 cal (ΔH‡ = 24.3 ± 0.2 kcal mol–1) for H2 loss. Based on these kinetic results, H/D labeling
studies, and DFT calculations, a unimolecular mechanism is proposed.
However, H atom transfer from 2H+ to acceptors such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl or
2,4,6-tert-butylphenoxide is very fast (k2 of 104 s–1 M–1 for the reaction of 2H+ with
TEMPO•) and H2 loss can be easily outcompeted.