A mechanistic dichotomy in scandium ion-promoted hydride transfer of an NADH analogue: delicate balance between one-step hydride-transfer and electron-transfer pathways.

The rate constant (kH) of hydride transfer from an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH2), to 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) increases with increasing Sc(3+) concentration ([Sc(3+)]) to reach a constant value, when all TolSQ molecules form the TolSQ-Sc(3+) complex. When AcrH2 is replaced by the dideuterated compound (AcrD2), however, the rate constant (kD) increases linearly with an increase in ([Sc(3+)]) without exhibiting a saturation behavior. In such a case, the primary kinetic deuterium isotope effect (kH/kD) decreases with increasing ([Sc(3+)]). On the other hand, the rate constant of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium [Ir(ppy)3]to TolSQ also increases linearly with increasing ([Sc(3+)]) at high concentrations of Sc(3+) due to formation of a 1:2 complex between TolSQ*- and Sc(3+), [TolSQ*--(Sc(3+)2], which was detected by ESR. The significant difference with regard to dependence of the rate constant of hydride transfer on ([Sc(3+)]) between AcrH2 and AcrD2 in comparison with that of Sc3+-promoted electron transfer indicates that the reaction pathway is changed from one-step hydride transfer from AcrH2 to the TolSQ-Sc3+ complex to Sc3+-promoted electron transfer from AcrD2 to the TolSQ-Sc3+ complex, followed by proton and electron transfer. Such a change between two reaction pathways, which are employed simultaneously, is also observed by simple changes of temperature and concentration of Sc3+.