Definition of magneto-structural correlations for the MnII ion

The electronic properties of the high spin mononuclear MnII complexes [Mn(tpa)(NCS)2] (1) (tpa=tris-2-picolylamine), [Mn(tBu3-terpy)2](PF6)2 (2) (tBu3-terpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine) and [Mn(terpy)2](I)2 (3) (terpy=2,2':6',2''-terpyridine) with an N6 coordination sphere have been determined by multifrequency EPR spectroscopy. The X-ray structures of 1.CH3CN and 2.C4H10 O.0.5 C2H5OH.0.5 CH3OH reveal that the MnII ion lies at the center of a distorted octahedron. The D-values of 1-3 all fall in the narrow range of 0.041 to 0.105 cm(-1). The comparison of the results reported here and those found in the literature is consistent with the following observation: the D value is sensitive to the coordination number (6 or 5) of the MnII ion as long as the coordination sphere involves only nitrogen and/or oxygen based ligands. This magneto-structural correlation has been analyzed in this work though DFT model calculations. The zero-field splitting (zfs) parameters of 1-3 have been calculated and are in reasonable agreement with the experimental values. Hypothetical simplified models [Mn(NH3)x(OH2)y]2+ (x+y=5 or 6 and [Mn(NH3)5X]+ (X=OH, Cl)) have been constructed to investigate the origin of the zfs. This investigation reveals i) that D is sensitive to the coordination number (5 or 6) of the MnII ion, ii) for the five coordinate systems the major contribution to D is the spin-orbit coupling part, iii) for the six coordinate systems the major contribution to D is the spin-spin interaction and iv) the deprotonation of a water ligand leads to an increase of D, consistent with the relative ligand fields of OH(-) versus H2O.