Colossal kinetic isotope effects in proton-coupled electron transfer

The kinetics of reduction of benzoquinone (Q) to hydroquinone (H(2)Q) by the Os(IV) hydraziclo (trans-[Os-IV(tpy)(Cl)(2)(N(H)N(CH2)(4)O)]-PF6 = [1]PF6, tpy = 2,2':6',2-terpyridine), sulfilimido (trans-[Os-IV(tpy)(CO2(NS(H)-4-C6H4Me)]PF6 = [2]PF6), and phosphoraniminato (trans-[Os-IV(Tp)(Cl)(2)(NP(H)(Et)(2))] = [3], Tp- = tris(pyrazolyl)-borate) complexes have been studied in 1:1 (vol/vol) CH3CN/H2O and CH3CN/D2O (1-0 M in NH4PF6/KNO3 at 25.0 +/- 0.1degreesC). The reactions are first order in both IQ] and Os([V) complex and occur by parallel pH-independent (k(1)) and pH-dependent (k(2)) pathways that can be separated by pH-dependent measurements. Saturation kinetics are observed for the acid-independent pathway, consistent with formation of a H-bonded intermediate (K-A) followed by a redox step (k(red)). For the pH-independent pathway, k(1)(H2O)/k(1)(D2O) kinetic isotope effects are 455 +/- 8 for [1(+)], 198 +/- 6 for [2(+)], and 178 +/- 5 for [3]. These results provide an example of colossal kinetic isotope effects for proton-coupled electron transfer reactions involving nitrogen, sulfur, and phosphorus as proton-donor atoms.