LAUSR

The biological activities of aminoguanidine (GNH2) and its derivatives have been extensively studied due to their properties as radical scavengers and antioxidants. Some of their biological activities may result from their binding to various metals present in biological systems. However, the reactivity of coordinated aminoguanidines has not been investigated. We report herein the synthesis, structure and reactivity of a novel osmium(iii) complex bearing the parent aminoguanidine, mer-[Os{NHC(NH2)(NHNH2)}(L)(CN)3]- (OsGNH2, HL = 2-(2-hydroxyphenyl)benzoxazole). The antioxidant properties of OsGNH2 have been investigated by reactions with various oxidants, including O2, H2O2, m-chloroperbenzoic acid (m-CPBA) and Ce(iv). Various osmium products are produced, which depend on the type of oxidant used. OsGNH2 is readily oxidized by O2 or H2O2 under ambient conditions to afford an osmium(iii) formamidine complex, [OsIII(NH2C[double bond, length as m-dash]NH)(L)(CN)3]- (OsFA, FA = formamidine). With m-CPBA, the nitrosyl complex, mer-[Os(NO)(L)(CN)3]- (OsNO), is formed instead. On the other hand, the nitrido complex mer-[Os(N)(L)(CN)3]- (OsN) is produced when the one-electron oxidant (NH4)2[CeIV(NO3)6] (Ce(iv)) is employed. The molecular structures of OsGNH2 and OsFA have been determined by X-ray crystallography. The oxidation of OsGNH2 to OsFA by O2 or H2O2 is proposed to go through initial dehydrogenation to give a diazoamidine intermediate. In the oxidation by m-CPBA and Ce(iv), it is proposed that the initially formed OsFA is further oxidized to OsNO and OsN, respectively, via osmium(iii) hydrogen cyanamido and osmium(iv) cyanoimido intermediates.