Copper(II) bromide, nitrate and perchlorate complexes with sterically demanding N-(6-methylpyridin-2-yl) acetamide ligands

Functionalized acid amides are widely used in biology, medicine, environmental chemistry and many other areas. Among them, pyridine-substituted amides, in particular N-(pyridin-2-yl) acetamide and its derivatives, play an important role due to their excellent chelating properties. The donor properties of these ligands can be effectively modified by introducing electron-donating substituents (e.g. alkyl groups) into the heterocycle. On the other hand, substituents in the alpha-positionof the pyridine ring can create steric hindrance, which significantly influences the coordination number and geometry. To achieve a better understanding of these effects, copper(II) complexes with sterically demanding N-(6-methylpyridin-2-yl) acetamide ligands (L) and monoanions of different size, shape and coordination ability have been chosen as model compounds. The crystal structures of three new compounds, bromidobis[N-(6-methylpyridin-2-yl-kappa N) acetamide-kappa O] copper(II) bromide, [CuBr(C8H10N2O)] Br, (I), aquabis[N-(6-methylpyridin-2-yl-kappa N) acetamide-kappa O] copper(II) dinitrate, [Cu(C8H10N2O)-( H2O)](NO3)(2), (II), and aquabis[N-(6-methylpyridin-2-yl-kappa N) acetamide-kappa O]-copper( II) bis(perchlorate), [Cu(C8H10N2O)(H2O)](ClO4)(2), (III), have been determined by single-crystal X-ray diffraction analysis. It has been shown that the presence of the 6-methyl group results in either a distorted square-pyramidal or a distorted trigonal-bipyramidal coordination geometry around the Cu-II centres instead of the typical octahedral geometry observed when the methyl substituent is absent or occupies any other position on the pyridine ring. Moreover, due to the steric hindrance provided by the L ligands, only the bromide ligand, the smallest of the series, enters into the first coordination sphere of the Cu II ion in (I). In (II) and (III), the vacant coordination site of the Cu II ion is occupied by a water molecule, while the nitrate and perchlorate anions are not involved in coordination to the metal centre. The structures of (I)-(III) are characterized by the presence of one-dimensional infinite chains formed by hydrogen bonds of the types N-H center dot center dot center dot Br [in (I)], N-H center dot center dot center dot O and O-H center dot center dot center dot O [in (II) and (III)] between the amide groups of the L ligands, the coordinated water molecules and the uncoordinated anions. The hydrogenbonded chains are further interconnected through pi-pi stacking interactions between the pyridine rings of the L ligands, with approximate interplanar separations of 3.5-3.6 angstrom.