Scandium ion-promoted reduction of heterocyclic N = N double bond. Hydride transfer vs electron transfer

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH₂) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph₂Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)₃ (OTf = OSO₂-CF₃) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc³⁺. The observed second-order rate constant (k_obs) increases with increasing Sc³⁺ concentration to approach a limited value. When AcrH₂ is replaced by the dideuterated compound (AcrD₂), the rate of Sc³⁺-promoted hydride transfer exhibits the same primary kinetic isotope effect (k_I-/k_D = 5.24±0.2), irrespective of Sc³⁺ concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP^2-= tetraphenylporphyrin dianion) and 10, 10′-dimethyl-9,9′-biacridine [(AcrH)₂] to Ph₂Tz, whereas no electron transfer from CoTPP or (AcrH)₂ to Ph₂Tz occurs in the absence of Sc³⁺. In each case, the observed second-order rate constant of electron transfer (k_et) shows a first-order dependence on [Sc³⁺] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k_et on [Sc₃₊] is ascribed to formation of 1:1 and 1:2 complexes between Ph₂Tz^.- and Sc³⁺ at the low and high concentrations of Sc³⁺, respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph₂Tz^.-. The 1:2 complex formation results in the saturated kinetic dependence of k_obs on [Sc³⁺] for the Sc³⁺-promoted hydride transfer, which proceeds via Sc³⁺-promoted electron transfer from AcrH₂ to Ph₂Tz, followed by proton transfer from AcrH₂^.+ to the 1:1 Ph₂Tz^.- - Sc³⁺ complex and the subsequent facile electron transfer from AcrH^. to Ph₂-TzH^.. The effects of counteranions on the Sc³⁺-promoted electron transfer and hydride transfer reactions are also reported.