A Chromium(III)‐Superoxo Complex as a Three‐Electron Oxidant with a Large Tunneling Effect in Multi‐Electron Oxidation of NADH Analogues

Abstract

AbstractMetal‐superoxo species are involved in a variety of enzymatic oxidation reactions, and multi‐electron oxidation of substrates is frequently observed in those enzymatic reactions. A CrIII‐superoxo complex, [CrIII(O2)(TMC)(Cl)]+ (1; TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane), is described that acts as a novel three‐electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a CrIV‐oxo complex, [CrIV(O)(TMC)(Cl)]+ (2), is formed by a heterolytic O−O bond cleavage of a putative CrII‐hydroperoxo complex, [CrII(OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1. The comparison of the reactivity of NADH analogues with 1 and p‐chloranil (Cl4Q) indicates that oxidation of NADH analogues by 1 proceeds by proton‐coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.