Magnetooptical and Structural Investigations of Five Dimeric Cobalt(II) Complexes Mimicking Metalloenzyme Active Sites

Four novel cobalt­(II) complexes mimicking metalloenzyme active sites, novel C₁₄H₂₂Cl₁₂Co₂O₁₃·2C₃H₈O (<b>1</b>), C₂₈H₃₆Cl₂₄Co₄O₂₈·4C₄H₈O₂ (<b>2</b>), C₁₆H₂₂Cl₁₂Co₂O₁₃·C₂HCl₃O₂ (<b>3</b>), C₁₆H₂₂Cl₁₂Co₂O₁₃ (<b>4</b>), and one known C₄₀H₇₈Cl₈Co₂O₁₇ (<b>5</b>) are composed of the same core of two high-spin cobalt­(II) centers triply bridged by water and two trichloroacetato (<b>1</b>–<b>4</b>) or two pivalate (<b>5</b>) groups but differ in terminal ligands. The crystal structures of new compounds <b>1</b>–<b>4</b> belong to the space groups <i>P</i>1̅, <i>P</i>2₁/<i>c</i>, <i>P</i>1̅, and <i>Pbcn</i>, respectively. All five investigated complexes contain Co atoms in distorted octahedral coordination. The complexes were characterized by magnetic susceptibility and magnetization measurements and by variable-temperature variable-field magnetic circular dichroism spectroscopy. Experimental data were analyzed in the frame of the theoretical model, which includes an unquenched orbital moment of the Co^II ions. All investigated compounds are antiferromagnetically coupled with exchange constants in the range −1.5 to −2.1 cm^–1. However, there is a significant difference between the crystal-field-splitting parameters