New MnII Complexes with an N/O Coordination Sphere from TripodalN-Centered Ligands − Characterization from Solid State to Solution and Reaction with Superoxide in Non-Aqueous and Aqueous Media

The crystal structure of a dimeric bis(µ-carboxylato)MnIIMnII complex (2) with a tetradentate N-centered tripodal ligand, N,N-bis[(1-methylimidazol-2-yl)methyl]glycinate, is presented. The carboxylates are bridging monodentate. This complex shows some striking differences to the previously published parent compound 1 obtained with the closely related ligand N-[(1-methylimidazol-2-yl)methyl]-N-(2-pyridylmethyl)glycinate. The carboxylato bridges in both structures are shown to be disrupted in solution. The reactivities in solution of compounds 1 and 2 toward superoxide were studied. In anhydrous DMSO solutions, a pathway involving a bis(µ-oxo)MnIIIMnIV complex was identified. In water solutions, IC50 values were measured by the Fridovich test (1.7 ± 0.2 µM for 2 and 2.0 ± 0.2 µM for 1), which showed these complexes to be good superoxide scavengers. Association constants for the complexes were estimated by means of a method based on IC50 measurements of the complexes under several conditions (alone and in the presence of excess ligand). They were found to be consistent with literature data.