Octahedral Co(III) salen complexes: the role of peripheral ligand electronics on axial ligand release upon reduction

A series of octahedral Co-III salen complexes (where salen represents a N2O2 bis-Schiff-base bis-phenolate framework) were prepared with axial imidazole ligating groups. When using 1-methylimidazole (1-MeIm) axial ligands, the Co-III/Co-II reduction potential could be altered by 220 mV via variation of the electron-donating ability of the para-ring substituents (R = H (1), OMe (2), tBu (3), Br (4), NO2 (5), and CF3 (6)). In addition, the irreversibility of the reduction process suggested substantial geometrical changes and axial ligand exchange upon reduction to the more labile Co-II oxidation state. Installing an imidazole-coumarin conjugate as the axial ligands resulted in fluorescence quenching when bound to the Co-III centre (R = H (7), OMe (8), and CF3 (9)). The redox properties and fluorescence increase upon ligand release for 7-9 were studied under reducing conditions and in the presence of excess competing ligand (1-MeIm). It was determined that the Lewis acidity of the Co-III centre was the dominant factor in controlling axial ligand exchange for this series of complexes.