Magnetic anisotropy of Cr(III) ions in polymeric oxalate complexes as revealed by HF-ESR spectroscopy

Two new Cr(III) paramagnetic polymeric complexes: {; ; [CaCr2(phen)2(C2O4)4]·0.33H2O}; ; n (CrPhen) and {; ; [CaCr2(bpy)2(C2O4)4]·0.83H2O}; ; n (CrBpy), where phen = 1, 10-phenanthroline and bpy = 2, 2′-bipyridine, have been studied by high field-high frequency electron spin resonance (HF-ESR) spectroscopy. From the oriented powder spectra, recorded in the frequency range 83−332 GHz in magnetic fields up to 16 T, the spin-Hamiltonian parameters, g, D and E, have been obtained. Both chromium complexes have a small negative (easy axis) magnetic anisotropy |D| < 1 K. CrPhen complex is biaxial, i.e. besides axial D, there is also significant rhombic zero-field splitting (ZFS) parameter, E. Contrary to CrPhen, CrBpy complex is uniaxial, but described by two sets of axial ZFS parameters. Despite very similar crystallographic structures of both complexes, HF-ESR spectroscopy resolved the existence of two magnetically inequivalent Cr(III)ions in CrBpy complex. The results fully support the earlier X-ray, SQUID magnetization and powder X-band ESR studies