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| - Spectroscopic Demonstration of a Large AntisymmetricExchange Contribution to the Spin-Frustrated Ground State ofa D3 Symmetric Hydroxy-Bridged Trinuclear Cu(II) Complex: Ground-to-Excited State Superexchange Pathways
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| - The magnetic and electronic properties of a spin-frustrated ground state of an antiferromagneticallycoupled 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. Directevidence for a low-lying excited S = 1/2 state from the zero-field split ground 2E state is provided by thenonlinear dependence of the MCD intensity on 1/T and the nesting of the VTVH MCD isotherms. A consistentzero-field splitting (Δ) value of ∼65 cm-1 is obtained from both approaches. In addition, the strong angulardependence of the single-crystal EPR spectrum, with effective g-values from 2.32 down to an unprecedented1.2, requires in-state spin−orbit coupling of the 2E state via antisymmetric exchange. The observable EPRintensities also require lowering of the symmetry of the trimer structure, likely reflecting a magnetic Jahn−Teller effect. Thus, the Δ of the ground 2E state is shown to be governed by the competing effects ofantisymmetric exchange (G = 36.0 ± 0.8 cm-1) and symmetry lowering (δ = 17.5 ± 5.0 cm-1). G and δhave opposite effects on the spin distribution over the three metal sites where the former tends to delocalizeand the latter tends to localize the spin of the Stot = 1/2 ground state on one metal center. The combinedeffects 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 availablebetween the ground dx2-y2 and excited dxy orbitals of adjacent Cu sites, via strong σ-type bonds with thein-plane p-orbitals of the bridging hydroxy ligands. This study provides significant insight into the orbitalorigin of the spin Hamiltonian parameters of a spin-frustrated ground state of a trigonal copper cluster.
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