Stereochemistry and Electrochemistry of Cobalt(II) and Cobalt(III) Complexes Containing Optically Active Tetradentate Schiff Base Ligands

Cobalt(II) complexes containing tetradentate Schiff base ligands with phenyl substituents, [Co(Schiff base)], have been prepared and the electrochemical properties are reported. The crystal structure of [Co{7-Phsal-(rac)-stien}], where the Schiff base ligand was derived from 2-hydroxybenzophenone and (rac)-1,2-diphenylethylenediamine, has been determined by X-ray structure analysis. Crystal data: monoclinic, P21/n, a = 13.956(2), b = 14.703(2), c = 17.808(3) Å, β = 112.21(1)°, V = 3383.0(9) Å3, Z = 4, and R = 0.052 and Rw = 0.039 for 3976 unique reflections with I > 3σ(I). The two phenyl groups in the N–N chelate moiety are in the axial positions and block the apical sites. In this complex, the redox potential of the Co(III)/Co(II) couple is 0.20 V vs. Ag/Ag+ in acetonitrile and becomes more positive by ca. 300 mV than that for [Co(salen)]. This large positive shift is attributed to the steric effect of the two axially disposed phenyl groups. The redox potentials of the analogous cobalt(II) Schiff base complexes, where (meso)-1,2-diphenylethylenediamine and (R)-1-phenylethylenediamine were used as a diamine, are also explained in terms of the steric effect of the phenyl substituents. The corresponding cobalt(III) Schiff base complexes with two additional axial ligands, [Co(Schiff base)(L)2]ClO4 (L = imidazole (im), 1-methylimidazole (Meim)), have been prepared. The steric interaction between the phenyl groups on the N–N chelate moiety and the axial ligands is discussed based on the X-ray structure, the circular dichroism (CD) spectra, and the 1H NMR spectra. The crystal structure determination of [Co{sal-(meso)-stien}(Meim)2]ClO4 has been performed. Crystal data: monoclinic, P21/c, a = 10.789(2), b = 20.512(3), c = 15.330(2) Å, β = 99.88(1)°, V = 3342.3(8) Å3, Z = 4, and R = 0.051 and Rw = 0.049 for 3414 unique reflections with I > 3σ(I). In order to study the steric effect of the phenyl substituents, the [Co(Schiff base)(L)2]ClO4 complexes have been prepared using (R)-1,2-propanediamine as a diamine. The different behavior shows that the interaction between the phenyl groups is sensitive to the orientation.