Complex formation between NADH model compounds and metalloporphyrins

Abstract

Various NADH model compounds, 1-(X-benzyl)-1,4-dihydronicotinamide (X-BNAH: X = 4-MeO, 4-Me, H, 4-Cl₂, and 2,4-Cl₂) and 10-methyl-9,10-dihydroacridine (AcrH₂), form complexes with metalloporphyrins (FeTPPClO₄, FeTPPCl, MnTPPClO₄, and ZnTPP; TPP: tetraphenylporphyrin) in dichloromethane or chloroform. For the BNAH–ZnTPP system, the stoicheiometry of the complex formation is 1:1 with the formation constant K = 50 dm³ mol^–1 in dichloromethane at 298 K. In the BNAH–MTPPClO₄ system (M = Fe and Mn), both 1:1 and 2:1 complexes are formed depending on the ratio of BNAH to MTPPClO₄. The formation constant becomes larger as the donor ability of NADH model compounds increases. Both the five-co-ordinate FeTPP(BNAH)⁺ and six-co-ordinate FeTPP(BNAH)₂⁺ complexes are high-spin (S= 5/2) species. The complex formation between the reduced metalloporphyrin (FeTPP and MnTPP) with BNAH was also investigated by the technique of cyclic voltammetry, which revealed that the reduced iron porphyrin FeTPP also forms a bis-co-ordination complex with BNAH, but the MnTPP forms only a mono-ligand adduct with BNAH. NADH model compounds are shown to act as two-electron donors in the electron-transfer reactions with FeTPPClO₄ in the presence of oxygen in acetonitrile, when the overall rates are determined by the rates of electron transfer from X-BNAH to FeTPP⁺ together with the competition between the back electron transfer from FeTPP to X-BNAH⁺˙ and the deprotonation of BNAH⁺˙.