Acid-promoted hydride transfer from an NADH analogue to a Cr(iii)–superoxo complex via a proton-coupled hydrogen atom transfer

Abstract

The sequential transfer of an electron, a proton and an electron in a hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) and its analogues has never been separated well. In addition, the effect of acids on hydride transfer from an NADH analogue to a metal–superoxo species has yet to be reported. We report herein the first example of an acid-promoted hydride transfer from an NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH₂), to a Cr(III)–superoxo complex, [(TMC)Cr^III(O₂)]²⁺, in the presence of HOTf in MeCN at 233 K. The acid-promoted hydride transfer from AcrH₂ to [(TMC)Cr^III(O₂)]²⁺ occurs via a proton-coupled hydrogen atom transfer from AcrH₂ to [(TMC)Cr^III(O₂)]²⁺ to produce a radical cation (AcrH₂˙⁺) with an inverse deuterium isotope effect (KIE) of 0.93(5). AcrH₂˙⁺ decayed via a proton transfer from AcrH₂˙⁺ to AcrH₂ with a KIE of 2.0(1), followed by the reaction of 10-methylacridinyl radical (AcrH˙) with [(TMC)Cr^III(H₂O₂)]³⁺ to produce a 10-methylacridinium ion (AcrH⁺) and [(TMC)Cr^III]³⁺. This work provides valuable insights into the mechanism of hydride transfer of NADH analogues by metal–superoxo intermediates, such as the switchover of the reaction mechanism from a one-step to a separated multi-step pathway in the presence of an acid.