Structural variations in self-assembled coordination complexes of hexamethylenetetramine, zinc(II) and carboxylates (RCOO−, R = –CH3/−C6H5): Encapsulation of the water hexamer in benzoate assembly

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Two new zinc(II) complexes [Zn2(ac)3(hmt)2(OH)]·H2O (1) and [{Zn5(Obz)8(H2O)2(μ3-OH)2}(μ2-hmt){Zn2(Obz)4}·6H2O]n (2) have been synthesized using two different carboxylate salts (acetate (ac) for 1 and benzoate (Obz) for 2) and hexamethylenetetramine (hmt). Both the complexes have been structurally characterized by X-ray crystallography. Their identities have also been established by elemental analysis and IR spectral studies. Complex 1 is a dimer in which two zinc atoms contain equivalent four-coordinate tetrahedral environments. On the other hand, complex 2 is a 1D zig-zag coordination polymer containing alternative zinc pentamers and zinc dimers separated by bridging μ2-hmts. The centrosymmetric pentamer unit contains three independent zinc atoms, one being four-coordinate with a tetrahedral environment and other two being six-coordinate with octahedral geometries whereas the centrosymmetric dimer unit contains two five-coordinate Zn atoms with square pyramidal geometries. H-bonding interactions play a key role in stabilizing the observed structures. In complex 1, the water molecule is held within the dimer and in 2, a rare water hexamer is encapsulated in the supramolecular assembly by hydrogen bonds. The DFT calculations reveal that inter- and outer-cluster H-bond stabilization energies of water hexamer are −29.99 and −15.93 kcal/mol respectively. Lower stabilization energy from the ideal hexamer of chair conformer is attributed to the deformation of H-atoms as well as longer hydrogen bonded O⋯O distances. Additionally, the extra stability due to outer-cluster H-bonds is responsible for significant deformation from ideal chair conformer of hexamer cluster.