Colossal kinetic isotope effects in proton-coupled electron transfer

AUTOR(ES)

Huynh, My Hang V.

FONTE

National Academy of Sciences

RESUMO

The kinetics of reduction of benzoquinone (Q) to hydroquinone (H2Q) by the Os(IV) hydrazido (trans-[OsIV(tpy)(Cl)2(N(H)N(CH2)4O)]-PF6 = [1]PF6, tpy = 2,2′:6′,2″-terpyridine), sulfilimido (trans-[OsIV-(tpy)(Cl)2(NS(H)-4-C6H4Me)]PF6 = [2]PF6), and phosphoraniminato (trans-[OsIV(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 ± 0.1°C). The reactions are first order in both [Q] and Os(IV) complex and occur by parallel pH-independent (k1) and pH-dependent (k2) 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 (KA) followed by a redox step (kred). For the pH-independent pathway, k1(H2O)/k1(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.

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