Magnetism of penta-coordinate Co(II) complexes with field-induced single-molecule magnet properties. Effects of distortion from C2v symmetry.

Abstract

Molecular magnetic materials, especially those showing single-molecule magnet (SMM) behaviors, have potential applications in, e.g., spintronics and high-density information storage. Penta-coordination in high-spin Co^II complexes leads to magnetic anisotropy and such complexes have been pursued in the search for SMMs. High-spin [Co(terpy)X₂] [X = Cl (Co-Cl), Br (Co-Br), I (Co-I); terpy = 2,2':6',2″-terpyridine] have been studied to determine their structural, magnetic, and spectroscopic properties. Advanced spectroscopic techniques [high-frequency and -field electron paramagnetic resonance (HFEPR), far-IR magneto-spectroscopy (FIRMS), and inelastic neutron scattering (INS)] and DC magnetic susceptibility measurements have been used to probe magnetic transitions in the three complexes. The structure of Co-Cl is significantly deviated from C_2v symmetry, leading to orbital contributions and the largest magnetic anisotropy in Co-X. The structures of Co-Br and Co-I are only slightly diverged from C_2v symmetry with quenched orbital contributions to their magnetic properties. Ab initio ligand field theory (AILFT) calculations help understand differences in magnetic properties between Co-Cl and Co-Br/Co-I. Data from alternating current (AC) susceptibility suggest that spin relaxation is primarily through the direct process at low temperatures. Spin-phonon couplings near magnetic transitions have been observed in FIRMS, contributing to spin relaxation. The current work provides a detailed study of the magnetic properties of the three five-coordinate Co^II complexes as field-induced SMMs.