Mechanistic borderline between one-step hydrogen transfer and sequential transfers of electron and proton in reactions of NADH analogues with triplet excited states of tetrazines and Ru(bpy)₃²⁺*

Efficient energy transfer from Ru(bpy)₃²⁺* (bpy = 2,2′-bipyridine, * denotes the excited state) to 3,6-disubstituted tetrazines [R₂Tz: R = Ph (Ph₂Tz), 2-chlorophenyl [(ClPh)₂Tz], 2-pyridyl (Py₂Tz)] occurs to yield the triplet excited states of tetrazines (³R ₂Tz*), which have longer lifetimes and higher oxidizing ability as compared with those of Ru(bpy)₃²⁺*. The dynamics of hydrogen-transfer reactions from NADH (dihydronicotinamide adenine dinucleotide) analogues has been examined in detail using ³R ₂Tz* by laser flash photolysis measurements. Whether formal hydrogen transfer from NADH analogues to ³R₂Tz* proceeds via a one-step process or sequential electron and proton transfer processes is changed by a subtle difference in the electron donor ability and the deprotonation reactivity of the radical cations of NADH analogues as well as the electron-acceptor ability of ³R₂Tz* and the protonation reactivity of R₂Tz^.-. In the case of ³Ph₂Tz*, which is a weaker electron acceptor than the other tetrazine derivatives [(ClPh)₂Tz; Py₂Tz], direct one-step hydrogen transfer occurs from 10-methyl-9,10-dihydroacridine (AcrH₂) to ³Ph₂Tz* without formation of the radical cation (AcrH₂^.+). The rate constant of the direct hydrogen transfer from AcrH₂ to ³Ph ₂Tz* is larger than that expected from the Gibbs energy relation for the rate constants of electron transfer from various electron donors to ³Ph₂Tz*, exhibiting the primary deuterium kinetic isotope effect. On the other hand, hydrogen transfer from 9-isopropyl-10-methyl-9,10-dihydroacridine (AcrHPrⁱ) and 1-benzyl-1,4-dihydronicotinamide (BNAH) to ³R₂Tz* occurs via sequential electron and proton transfer processes, when both the radical cations and deprotonated radicals of NADH analogues are detected by the laser flash photolysis measurements.