Ferromagnetically-coupled, triangular, [Bu4N]2[CuII3(μ3-Br)2(μ-4-O2N-pz)3Br3] complex revisited: The effect of coordinated halides on spin relaxation properties

Abstract

The magnetic properties of two trinuclear clusters of the type [CuII3(μ3-X)2X3] with X = Cl (1) and Br (2) are compared. Both clusters exhibit ferromagnetic interactions among the three Cu(II) ions yielding an S = 3/2 ground state. Although the clusters have similar structural features, namely a roughly trigonal bipyramidal geometry defined by the X ligands, they exhibit remarkably different anisotropic magnetic properties. This difference is reflected in the effective ZFS of the ground state: D3/2 = +0.088 cm−1 for 1 versus +1.038 cm−1 for 2 (both values at T ≤ 10 K), which values were determined by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy in combination with dc magnetic susceptibility measurements; density functional theory (DFT) calculations are in agreement with the experimental results. The anisotropic parameters for 2, are more than ten-fold larger than those for 1, reflecting significant differences in the mixing of halogen (bromine vs. chloride) character into the Cu-3d orbitals. These differences have important consequences on the magnetic relaxation properties of the clusters, as measured by ac magnetic susceptibility. For 1, relaxation pathways within the S = 3/2 state, wherein the MS levels are close in energy, are negligible in the presence of a small external magnetic field, so that much slower relaxation via only the S = 1/2 excited states is operative. For cluster 2, in contrast, ZFS of the S = 3/2 ground state is relatively large and positive, which provides sufficient pathways for relaxation within the ground state and thus no slow relaxation is observed. The present results demonstrate that structurally similar clusters can have very different relaxation properties and the effect of seemingly “innocent” ligands, (i.e., halido ligands) on relaxation properties can be profound.