Synthesis and reactivity of an osmium(iii) aminoguanidine complex.

The biological activities of aminoguanidine (GNH) 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(NH)(NHNH)}(L)(CN)] (OsGNH, HL = 2-(2-hydroxyphenyl)benzoxazole). The antioxidant properties of OsGNH have been investigated by reactions with various oxidants, including O, HO, m-chloroperbenzoic acid (m-CPBA) and Ce(iv). Various osmium products are produced, which depend on the type of oxidant used. OsGNH is readily oxidized by O or HO under ambient conditions to afford an osmium(iii) formamidine complex, [Os(NHC[double bond, length as m-dash]NH)(L)(CN)] (OsFA, FA = formamidine). With m-CPBA, the nitrosyl complex, mer-[Os(NO)(L)(CN)] (OsNO), is formed instead. On the other hand, the nitrido complex mer-[Os(N)(L)(CN)] (OsN) is produced when the one-electron oxidant (NH)[Ce(NO)] (Ce(iv)) is employed. The molecular structures of OsGNH and OsFA have been determined by X-ray crystallography. The oxidation of OsGNH to OsFA by O or HO 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.