Synthesis and electron paramagnetic resonance studies of seven coordinated Mn(II) complexes with tridentate N-donor ligands

Abstract

The reactions of Mn(NO3)2·4H2O with the heterocyclic aromatic ligands 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz), 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz) and 2,6-bis(pyrazol-1-yl)pyridine (bppy) in MeOH gave the cationic complexes [Mn(L)(NO3)(MeOH)2](NO3) (L = tppz (1), tptz (3), bppy (5)) and the neutral complexes [Mn(L)(NO3)2(S)] (L = tppz, S = H2O (2); L = tptz, S = MeOH (4); L = bppy, S = MeOH (6)), all of which contain the MnII ion in the rare seven-coordinate pentagonal bipyramidal geometry (the crystal structure of 6 contains also a MnII complex showing the rare eight-coordinate triangular dodecahedron geometry). The reactions of Mn(ClO4)2·6H2O with tptz and bppy in MeCN gave [Mn(tptz)(H2O)2(MeCN)](ClO4)2 (7) and [Mn(bppy)2](ClO4)2 (8), in which the MnII ion exhibits six-coordination. Electron Paramagnetic Resonance spectroscopy at X- and Q-band on powder samples was used in order to determine the zero field splitting (ZFS) parameters for 1–4 and 7, 8. All complexes exhibit ZFS parameter, |D| in the range 0.08–0.11 cm−1. The experimentally determined values are in agreement with theoretical calculations which reveal the contribution of each mechanism responsible for the ZFS interaction. The present results add further evidence that for seven-coordinated MnII complexes comprising N/O ligands ZFS is comparable to this found in six-coordinated MnII complexes.