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

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 (AcrH), to a Cr(iii)-superoxo complex, [(TMC)Cr(O)], in the presence of HOTf in MeCN at 233 K. The acid-promoted hydride transfer from AcrH to [(TMC)Cr(O)] occurs via a proton-coupled hydrogen atom transfer from AcrH to [(TMC)Cr(O)] to produce a radical cation (AcrH˙) with an inverse deuterium isotope effect (KIE) of 0.93(5). AcrH˙ decayed via a proton transfer from AcrH˙ to AcrH with a KIE of 2.0(1), followed by the reaction of 10-methylacridinyl radical (AcrH˙) with [(TMC)Cr(HO)] to produce a 10-methylacridinium ion (AcrH) and [(TMC)Cr]. 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.