Stereochemistry of Hexacoordinated Zn(II), Cu(II), Ni(II), and Co(II) Complexes with Iminodiacetamide Ligands

Metal complexes of iminodiacetamide (imda) ligands and metal ions Zn­(II), Cu­(II), Ni­(II), and Co­(II) were prepared using eight imda ligands (L1–L8) substituted with groups of different steric and electronic properties on the central amine N atom (H atom, methyl, isopropyl, and benzyl) and the para position of the phenyl rings (nitro and dimethylamino). The effect of these substituents on the stoichiometry (ML and ML 2 ), geometry, and stereochemistry (mer, trans-fac, cis-fac) of the complexes was studied in the solid state, in solution, and by density functional theory calculations. Single-crystal and powder X-ray diffraction, thermogravimetry, and IR spectroscopy showed that in the solid state imda ligands preferentially form trans-fac ML 2 complexes, with the exception of the cis-fac complex 7 Zn . NMR spectroscopy of diamagnetic Zn­(II) and paramagnetic Co­(II) complexes revealed the formation of both ML and ML 2 complexes in solution, which was also confirmed by UV–vis titrations. Variable-temperature NMR was used to study the effect of the substituent on the central amine N atom on the Zn–N bond strength and nitrogen inversion. The relative stabilities of the isomers were rationalized by computations and the optimized structures used for geometry analysis.