New iron(II) and manganese(II) complexes of two ultra-rigid, cross-bridged tetraazamacrocycles for catalysis and biomimicry

The high-spin dichloro Mn2+ and Fe2+ complexes of 4,11 dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]-hexadecane (1) and 4.10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2) provide durable new compounds of these elements for important fundamental studies and applications. The compounds are especially noteable for their exceptional kinetic stabilities and redox activity. The X-ray crystal structures of all four complexes demonstrate that the ligands enforce a distorted octahedral geometry on the metals with two cis sites occupied by labile chloride ligands. Magnetic measurements reveal that all are high spin with typical magnetic moments. Cyclic voltammetry of the complexes shows reversible redox processes at +0.110 and +0.038 V (versus SHE) for the Fe3+/Fe2+ couples of Fe(1)Cl-2 and Fe(2)Cl-2, respectively, while the Mn3+/Mn2+ and Mn4+/Mn3+ couples were observed at +0.585 and +1.343 V, and +0.466 and +1.232 V for the complexes Mn(1)Cl-2 and Mn(2)Cl-2, respectively. Mn2+(1) was found to react with H2O2 and other oxidizing agents to produce the Mn4+(1) complex. The catalytic efficacy of Mn4+(1) in aqueous solution has been assessed in the epoxidation reaction of carbamazepine and hydrogen abstraction reaction with 1,4-cyclohexadiene The complex has been found to be a selective catalyst, exhibiting moderate catalytic activity in oxygen transfer, but significantly more effective catalytic activity in hydrogen abstraction reactions.