Synthesis, redox properties, and EPR spectroscopy of manganese(III) complexes of the ligand N,N-bis(2-hydroxybenzyl)-N′-2-hydroxybenzylidene-1,2-diaminoethane: Formation of mononuclear, dinuclear, and even higher nuclearity complexes

The synthesis and characterization of the title trisphenolate ligand are described. From its reaction with manganese(III) three complexes were isolated. The crystal structures revealed one pentacoordinate monomer and two similar dimers with different solvents of crystallization. In the dimers the metal ions are hexacoordinate and connected through bridging of two phenolates. A combination of electrochemistry and EPR spectroscopy showed that, in acetonitrile, the isolated batches were all identical and mainly monomeric, indicating that the mononuclear complex is in equilibrium with the dimer and perhaps also with complexes of higher nuclearity, as suggested by the detection of both the trimer and the tetramer by electrospray ionization mass spectrometry (ESI-MS). The successful use of the monomer batch as an epoxidation catalyst indicated that a high-valent manganese-oxo species can be formed, although it is probably short-lived. This is also suggested by EPR studies of the species formed by electrochemical oxidation of the complex. Upon one-electron oxidation, a manganese(IV) species was formed, which was at least partly converted to another species containing a phenoxy radical.