Switchable Valence States in Dinuclear Cobalt Complexes: The Role of Halogenated Catecholates and Counterions

We present the synthesis and comprehensive characterization of a series of eight dinuclear cobalt complexes, C1-C8, with the general formula [Co₂(L)-(X₄cat)₂]²⁺(A)_{1- 2}, wherein X = Br or Cl, A = SO₄ ^2-, ClO₄ ^-, PF^6-, or B-(Ph)₄ ^-, and L denotes a redox inactive bis-tetradentate bridging ligand. Single-crystal X-ray diffraction at 120 K confirms a low-spin Co-(III) configuration in all compounds. SQUID magnetometry also shows that the complexes remain diamagnetic below room temperature. However, the complexes bearing sulfate anions, C1 and C5, exhibit a distinct thermally induced valence tautomeric transition above room temperature, marked by an increase in magnetic moment. The temperature of this transition is strongly influenced by the electronic properties of the catecholate (cat) ligands, with electron-deficient tetrahalogenated catecholates stabilizing the low-spin state. In addition, counterions and solvent molecules are found to modulate intermolecular interactions in the solid state. Comparative cyclic voltammetry with previously reported ditert-butyl catecholate (dbucat) complex C9 highlights the influence of ligand electronics on redox potentials, with electron-deficient catecholates shifting redox processes to higher potentials. These results highlight the tunability of cobalt valence tautomerism and redox behavior through strategic ligand and counterion selection.