Effect of an additional functional group on the structure and properties of copper(II) and nickel(II) o-iminoquinone complexes

Nickel(II) and copper(II) bis-o-iminosemiquinone complexes were synthesized based on 4,6-di-tert-butyl-N-(2-methoxyphenyl)-o-aminophenol (Ni(imSQOMe)2 (1) and Cu(imSQOMe)2 (2)), and methyl 2-(3,5-di-tert-butyl-2-hydroxyphenylamino)benzoate (Ni(imSQC(O)OMe)2 (3) and Cu(imSQC(O)OMe)2 (4)). The molecular structures of these complexes were established by X-ray diffraction. The metal atoms in complexes 1, 3, and 4 are in square-planar coordination, whereas the square-planar coordination of the copper atom in complex 2 is substantially distorted, as evidenced by the angle between the planes of the o-iminosemiquinone ligands (22.7°). Complexes 1 and 3 are diamagnetic. The absence of line broadening in the 1H NMR spectra of these compounds at 293 K attests to the singlet spin state. The temperature dependences of the effective magnetic moment for complexes 2 and 4 are indicative of their doublet ground spin state (S = 1/2) and the thermally populated quartet state (S = 3/2). The presence of an unpaired electron on the metal center in the ground spin state for compounds 2 and 4 was confirmed by EPR spectroscopy.