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

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO(2)CF(3)) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc(3+). The observed second-order rate constant (k(obs)) increases with increasing Sc(3+) concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc(3+)-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2+/-0.2), irrespective of Sc(3+) concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP(2)(-) = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine (AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc(3+). In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on Sc(3+)] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k(et) on Sc(3+)] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(*)(-) and Sc(3+) at the low and high concentrations of Sc(3+), 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 Ph(2)Tz(*)(-). The 1:2 complex formation results in the saturated kinetic dependence of k(obs) on Sc(3+)] for the Sc(3+)-promoted hydride transfer, which proceeds via Sc(3+)-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(*)(+) to the 1:1 Ph(2)Tz(*)(-)-Sc(3+) complex and the subsequent facile electron transfer from AcrH(*) to Ph(2)TzH(*). The effects of counteranions on the Sc(3+)-promoted electron transfer and hydride transfer reactions are also reported.