Spin-crossover in a homoleptic cobalt(II) complex containing a redox-active NNO ligand

We report the preparation of a divalent cobalt complex containing the ligand 1-(2-pyridylazo)-2-phenanthrol (papl). The complex is neutral and homoleptic containg two equivalents of the anion of papl. Variable temperature magnetic susceptibility measurements indicate a gradual spin-crossover between the S = 1/2 and S = 3/2 states of the complex. This is corroborated by single crystal X-ray diffraction experiments at 147 and 325 K, which reveal differences in the coordinate bond distances that are consistent with spin-crossover observed in other cobalt(II) complexes. The complex is redox-active and features intense, ligand-centered visible absorptions bands, which are completely changed in alcohol solvents (methanol, ethanol) and indicate oxidation of the complex to cobalt(III) in these polar, protic solvents.