Mechanistic Studies of the Proton-Coupled Electron Transfer Reactivity of a Cobalt Complex with a Proton-Responsive...

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 (HL^PNP = 2,6-bis((bis-tert-butylphosphaneyl)-methyl)pyridin-4-ol). The cobalt(II) complexes [(L^PNP)Co^IICl], 1, and [(L^PNP)Co^II(MeCN)]⁺, 2⁺, with the deprotonated ligand and [(HL^PNP)Co^II(MeCN)₂]²⁺, 2H²⁺, with the protonated ligand, were synthesized and characterized. 2H²⁺ has a pK_a 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 [(HL^PNP)Co^I(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.