Self-assembled molecular complexes and coordination polymers of Cd^II, hexamine and monocarboxylates: structural analysis and theoretical studies of supramolecular interactions

Abstract

Four new cadmium (II) complexes [Cd₂(bz)₄(H₂O)₄(μ ₂-hmt)]•Hbz•H₂O (1), [Cd₃(bz)₆(H₂O)₆(μ ₂-hmt) ₂]•6H₂O (2), [Cd(pa) ₂(H₂O)(μ ₂-hmt)]_n (3) and {[Cd₃(ac)₆(H₂O)₃(μ₃-hmt) ₂]•6H₂O}_n (4) with hexamine (hmt) and monocarboxylate ions, benzoate (bz), phenylacetate (pa) or acetate (ac) have been synthesized and characterized structurally. Structure determinations reveal that 1 is dinuclear, 2 is trinuclear, 3 is a one-dimensional (1D) infinite chain and 4 is a two-dimensional (2D) polymer with fused hexagonal rings consisting of Cd^II and hmt. All the Cd^II atoms in the four complexes (except one Cd^II in 2) possess seven-coordinate pentagonal bipyramidal geometry with the various chelating bidentate carboxylate groups in equatorial sites. One of the Cd^II ions in 2, a complex that contains two monodentate carboxylates is in a distorted octahedral environment. The bridging mode of hmt is μ ₂- in complexes 1−3 but is μ₃- in complex 4. In all complexes, there are significant numbers of H-bonds, C−H/π and π−π interactions which play crucial roles in forming the supramolecular networks. The importance of the noncovalent interactions in terms of energies and geometries has been analyzed using high level ab initio calculations. The effect of the cadmium coordinated to hmt on the energetic features of the C−H/π interaction is analyzed. Finally, the interplay between C−H/π and π−π interactions observed in the crystal structure of 3 is also studied.