Electronic Structure Study of Rhombic Mn^II Complexes with Hexadentate N₄O₂ Chelate Ligands

Herein, we investigate the structural and electronic properties of nine mononuclear Mn^II complexes with Schiff base ligands synthesized from R-salicylaldehyde and 1,4-bis(3-aminopropyl)piperazine which readily adopt a distorted trigonal prismatic geometry, where the R groups are 3-NO₂ (1), 3-NO₂-5-OMe (2), 3-NO₂-5-Cl (3), 3-Br-5-NO₂ (4), 3-OMe-5-NO₂ (5), 5-NO₂ (6), 3,5-diCl (7), 3,5-diBr (8), and 3,5-diI (9). We utilize Continuous Shape Measure to quantify the degree of geometric distortion relative to other structures published in the Cambridge Structural Database (CSD). X-band EPR spectroscopy indicated atypical anisotropy and higher-field and frequency measurements were used to quantify the zero-field splitting (ZFS) parameters. Simulations of the spectra revealed axial and rhombic ZFS parameters, D = +0.200(2) cm^-1 and |E| = 0.067(2) cm^-1, for a frozen solution of 1. Despite the trigonal geometry of the Mn^II coordination environment, EPR data indicate a rhombic electronic structure with an E/D ratio at the rhombic limit of 0.33. This is ascribed to the different identity of the coordinating atoms (two nitrogens and one oxygen) associated with the triangular faces of the trigonal prism, i.e., even though the coordination geometry is close to trigonal, the ligand field symmetry is not.