Crystallographic evidence of nitrate-π interactions involving the electron-deficient 1,3,5-triazine ring

The reaction of Zn(NO3)(2)center dot 6H(2)O or Cu(NO3)(2)center dot 3H(2)O with the star-shaped ligand 2,4,6-tris(di-2-picolylamino)[1,3,5]-triazine (dipicatriz) in acetonitrile results in the formation of the mono- or trinuclear coordination compounds [ Zn( dipicatriz)(NO3)(2)] (1), [Zn-3(dipicatriz)(NO3)(6)](CH3CN)(3) (2), and [Cu-3(dipicatriz)(NO3)(2)(H2O)(6)](NO3)(4) (3), depending on the metal-to-ligand ratios used during the crystallization process. Their crystal structures exhibit unique supramolecular interactions. Compounds 1 and 2 show anion-pi interactions between coordinated nitrate ions and the s-triazine ring. Compound 3 exhibits remarkable interactions between two noncoordinated nitrate anions and the two faces of the electron-deficient heteroaromatic ring, corroborating earlier theoretical investigations in this area. New theoretical investigations have been carried out on nitrate-pi interactions, taking into account the particular position of the anion toward the aromatic ring observed in the crystal structures.