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) hydrazido (trans-[Os(IV)(tpy)(Cl)(2)(N(H)N(CH(2))(4)O)]-PF(6) = [1]PF(6), tpy = 2,2':6',2"-terpyridine), sulfilimido (trans-[Os(IV)-(tpy)(Cl)(2)(NS(H)-4-C(6)H(4)Me)]PF(6) = [2]PF(6)), 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) CH(3)CN/H(2)O and CH(3)CN/D(2)O (1.0 M in NH(4)PF(6)/KNO(3) at 25.0 +/- 0.1 degrees C). The reactions are first order in both [Q] and Os(IV) 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)(H(2)O)/k(1)(D(2)O) 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.