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 (HL(PNP) = 2,6-bis((bis-tert-butylphosphaneyl)-methyl)pyridin-4-ol). The cobalt(II) complexes [(L(PNP))Co(II)Cl], 1, and [(L(PNP))Co(II)(MeCN)](+), 2(+), with the deprotonated ligand and [(HL(PNP))Co(II)(MeCN)(2)](2+), 2H(2+), with the protonated ligand, were synthesized and characterized. 2H(2+) has a pK(a) of 18 ± 1, and the reduction of 2H(2+) appears at -1.08 V vs. FeCp(2)(+|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) H(2), 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 H(2) 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 (k(2) of 10(4) s(-1) M(-1) for the reaction of 2H(+) with TEMPO(•)) and H(2) loss can be easily outcompeted.