Synthesis and coordination chemistry of sterically hindered cobalt(II) β-ketoiminate complexes

Abstract

The reactions of cobalt(II) chloride with α,β-unsaturated-β-ketoamines, CH3C(O)CHC(NHR)CH3, where R = -phenyl, HLPh; R = -mesityl, HLmes; R = -2,6-diisopropylphenyl, HLdipp, and sodium hydride in THF solvent under anaerobic conditions affords brown or orange crystals of Co(Lph)2, Co(Lmes)2 and Co(Ldipp)2, respectively. In THF/acetonitrile solvent mixture, the complexation reactions are not successful as unreactive salts containing networks of acetonitrile-coordinated sodium cations and the respective anions are formed. One of them, the sodium salt of HLdipp, is isolated and characterized as [Na(Ldipp)(CH3CN)]4. Single-crystal X-ray diffraction data for the cobalt(II) complexes reveal that the metal centers assume distorted tetrahedral geometries. The degree of distortion increases with the size of the substituents present in the phenyl groups: Co(Lph)2 < Co(Lmes)2 < Co(Ldipp)2. Crystallographic data measured at 296, 200, 150 and 100 K for Co(Ldipp)2 reveal that it exists as two geometrical forms, namely, distorted tetrahedral and distorted square-planar. The data also indicate that the pseudo-tetrahedral to distorted square-planar transition is favored at lower temperatures. Electrochemical data measured in THF and DCM solvents reveal the likely presence of Co(II)-solvent interaction in Co(Lph)2 and Co(Lmes)2 and no such interaction in Co(Ldipp)2. Magnetic susceptibility data for Co(Ldipp)2 in the 4–300 K temperature region reveal spin-crossover at ca. 130 K. Qualitatively, the magnetic and crystallographic data complement each other with respect to the presence of the high spin pseudo-tetrahedral and low spin pseudo-square planar geometries.