High-spin chloro mononuclear Mn^III complexes: A multifrequency high-field EPR study

The isolation, structural characterization, and electronic properties of two six-coordinated chloromanganese (III) complexes, [Mn(terpy)(Cl)₃] (1) and [Mn(Phterpy)(Cl)₃] (2), are reported (terpy=2,2′: 6′2″-terpyridine, Phterpy=4′-phenyl-2,2′:6′, 2″-terpyridine). These complexes complement a series of mononuclear azide and fluoride Mn(III) complexes synthesized with neutral N-tridentate ligands, [Mn(L)(X)₃] (X=F^- or N₃^- and L=terpy or bpea [N,N-bis(2-pyridylmethyl)-ethylamine)], previously described. Similar to these previous complexes, 1 and 2 exhibit a Jahn-Teller distortion of the octahedron, characteristic of a high-spin Mn(III) complex (S=2). The analysis of the crystallographic data shows that, in both cases, the manganese ion lies in the center of a distorted octahedron characterized by an elongation along the tetragonal axis. Their electronic properties were investigated by multifrequency EPR (190-475 GHz) performed in the solid state at different temperatures (5-15 K). This study confirms our previous results and further shows that: i) the sign of D is correlated with the nature of the tetragonal distortion; ii) the magnitude of D is not sensitive to the nature of the anions in our series of rhombic complexes, contrary to the porphyrinic systems; iii) the |E/D| values (0.124 for 1 and 0.085 for 2) are smaller compared to those found for the [Mn(L)(X)₃] complexes (in the range of 0.146 to 0.234); and iv) the E term increases when the ligand-field strength of the equatorial ligands decreases.