Targeted Ferromagnetic Coupling in a Trinuclear Copper(II) Complex:  Analysis of the <i>S</i><sub>t</sub> = <sup>3</sup>/<sub>2</sub> Spin Ground State

The trinuclear Cu<sup>II</sup> complex [(talen)Cu<sup>II</sup><sub>3</sub>] (<b>1</b>) using the new triplesalen ligand H<sub>6</sub>talen has been synthesized and structurally characterized. The three Cu<sup>II</sup> ions are bridged in a <i>m</i>-phenylene linkage by the phloroglucinol backbone of the ligand. This <i>m</i>-phenylene bridging mode results in ferromagnetic couplings with an <i>S</i><sub>t</sub> = <sup>3</sup>/<sub>2</sub> spin ground state, which has been analyzed by means of EPR spectroscopy and DFT calculations. The EPR spectrum exhibits an unprecedented pattern of 10 hyperfine lines due to the coupling of three Cu<sup>II</sup> ions (<i>I</i> = <sup>3</sup>/<sub>2</sub>). Resonances around <i>g</i> = 4 in both perpendicular and parallel mode EPR spectra demonstrate a zero-field splitting of <i>D</i> ∼ 74 × 10<sup>-4</sup> cm<sup>-1</sup> arising from anisotropic/antisymmetric exchange interactions. The DFT calculations show an alteration in the sign of the spin densities of the central benzene ring corroborating the spin-polarization mechanism as origin for the ferromagnetic coupling.