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

Abstract

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.