Spectroscopic demonstration of a large antisymmetric exchange contribution to the spin-frustrated ground state of a D₃ symmetric hydroxy-bridged trinuclear Cu(II) complex: Ground-to-excited state superexchange pathways

The magnetic and electronic properties of a spin-frustrated ground state of an antiferromagnetically coupled 3-fold symmetric trinuclear copper complex (TrisOH) is investigated using a combination of variable-temperature variable-field magnetic circular dichroism (VTVH MCD) and powder/single-crystal EPR. Direct evidence for a low-lying excited S = 1/2 state from the zero-field split ground ²E state is provided by the nonlinear dependence of the MCD intensity on 1/Tand the nesting of the VTVH MCD isotherms. A consistent zero-field splitting (Δ) value of ∼65 cm^-1 is obtained from both approaches. In addition, the strong angular dependence of the single-crystal EPR spectrum, with effective g-values from 2.32 down to an unprecedented 1.2, requires in-state spin-orbit coupling of the ²E state via antisymmetric exchange. The observable EPR intensities also require lowering of the symmetry of the trimer structure, likely reflecting a magnetic Jahn-Teller effect. Thus, the A of the ground 2E state is shown to be governed by the competing effects of antisymmetric exchange (G = 36.0 ± 0.8 cm^-1) and symmetry lowering (δ = 17.5 ± 5.0 cm ^-1). G and δ have opposite effects on the spin distribution over the three metal sites where the former tends to delocalize and the latter tends to localize the spin of the S_tot = 1/2 ground state on one metal center. The combined effects lead to partial delocalization, reflected by the observed EPR parallel hyperfine splitting of 74 × 10^-4 cm^-1. The origin of the large G value derives from the efficient superexchange pathway available between the ground d_x2-y2 and excited d_xy orbitals of adjacent Cu sites, via strong σ-type bonds with the in-plane p-orbitals of the bridging hydroxy ligands. This study provides significant insight into the orbital origin of the spin Hamiltonian parameters of a spin-frustrated ground state of a trigonal copper cluster.