Ligand Dependence of Magnetic Dimensionality in Chromium(IV) Complexes: Layered vs Three-Dimensional Antiferromagnets

Abstract

X-ray diffraction and thermomagnetic measurements were made on two chromium(IV) complexes, Cr(dien)(O2)2·H2O and Cr(NH3)3(O2)2, with the view of understanding how the ligands might change the magnetic dimensionality in such compounds. The magnetic susceptibility (χ(T)) of Cr(dien)(O2)2·H2O was analyzed according to the quadratic Heisenberg model, H = −2J∑i,j S(i)·S(j) − gβH∑i Sz(i), which yielded J010 = −2.71 K, J100 = −2.86 K, J001 = −2.88 K, g010 = 1.92, g001 = 1.96, and g100 = 1.82, where the indices coincide with alignment of the crystal axes relative to the magnetic field. The heat capacity (Cp) and entropy data support this analysis and show it to be a layered, two-dimensional antiferromagnet. In contrast, χ(T) and Cp(T) measurements on Cr(NH3)3(O2)2 showed it to be a classical three-dimensional antiferromagnet with sharp λ-type transitions at 8.80 and 8.46 K, respectively. Only 38% of the magnetic entropy is removed above TN = 8.46 K, implying that nominal short-range interactions occur, but the system is dominated by three-dimensional magnetic exchange. The data reveal how the ligand as well as lattice hydration affect the spin dimensionality of Cr(IV) compounds.