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)]center dot 0.33H(2)O} (n) (CrPhen) and {[CaCr2(bpy)(2)(C2O4)(4)]center dot 0.83H(2)O} (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, , and , have been obtained. Both chromium complexes have a small negative (easy axis) magnetic anisotropy K. CrPhen complex is biaxial, i.e., besides axial , there is also significant rhombic zero-field splitting (ZFS) parameter, . 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.