P roton-coupled electron transfer (PCET) reactions are at the heart of important biological processes, including photosynthesis and respiration (1-4). These reactions avoid high-energy intermediates through the concerted transfer of an electron and a proton. PCET is distinguished from H-atom transfer (HAT) in that the transferring electron and proton come from different sites in the reducing agent rather than from a single chemical bond (5-10). Nuclear quantum effects often play an important role, as evidenced experimentally by the kinetic isotope effect (KIE), the ratio of rate constants for the hydrogen and deuterium forms of the reducing agent. Large k H ͞k D KIEs have been observed in other reactions, including H-abstraction by methyl radicals in low-temperature glasses (11), the photoenolization of ortho-methyl aryl ketones in rigid media (12), and in certain enzymatic reactions (13-15). For PCET reactions, values of up to 30 have been observed in solution (16) and Ͼ70 on an oxide surface (17).In earlier work, we described a PCET reaction with a k 1 (H 2 O)͞k 1 (D 2 O) KIE of 175 at room temperature based on the reduction of benzoquinone (Q) by an osmium complex containing a phosphorus-hydrogen bond (18). We report here values of 198 and 455 for PCET reactions from sulfur and nitrogenhydrogen bonds based on an extended analysis of reactions reported earlier in preliminary accounts. 3.0 ϫ 10 Ϫ4 M to 8.0 ϫ 10 Ϫ2 M. The temperature of solutions during the kinetic studies was maintained to within Ϯ0.1°C with use of Lauda RM6 and Thermo Neslab RTE 7 circulating water baths and monitored with an Omega HH-51 thermocouple probe. All rate constants cited in this work are reported as the averages of at least three or more independent experiments.Cyclic voltammetric experiments were measured with the use of Princeton Applied Research (PAR) models 263A and 273 potentiostats, and bulk electrolyses were performed with a PAR model 173 potentiostat͞galvanostat. All measurements were conducted in a three-compartment cell in 1:1 (vol͞vol) CH 3 CN͞ H 2 O with 1.0 M NH 4 PF 6 as the supporting electrolyte. A glassy carbon working electrode was used for aqueous measurements. All potentials are referenced to the saturated sodium chloride͞ calomel electrode (SSCE, 0.236 V vs. the normal hydrogen electrode) at room temperature and are uncorrected for junction potentials. In all cases, the auxiliary electrode was a platinum wire. The solution in the working compartment was deoxygenated by N 2 or argon bubbling. All redox couple potentials are the average of four independent measurements and are Ϯ2 mV.
Results and DiscussionThe kinetics of the reactions shown in Eqs. 1-3 were studied in 1:1 (vol͞vol) CH 3 CN͞H 2 O mixtures at 25.0 Ϯ 0.1°C in 1.0 M NH 4 PF 6 ͞HPF 6 mixtures. The details of the reaction in Eq. 2 with phosphorus as the proton-donor atom were reported in ref. 18, and preliminary accounts with S and N were reported in refs. 19 and 20. The structures of the Os(V) nitrogen-based electronproton acceptor and Os(IV) sulfur-ba...