Structural variations in self-assembled coordination complexes of Zn(II) with hexamethylenetetramine and isomeric 2-, 3- and 4-nitrobenzoates

Three new zinc(II)-hexamethylenetetramine (hmt) complexes [Zn2(4-nbz)4(μ2-hmt)(OH2)(hmt)] (1), [Zn2(2-nbz)4(μ2-hmt)2]n (2) and [Zn3(3-nbz)4(μ2-hmt)(μ2-OH)(μ3-OH)]n (3) with three isomeric nitrobenzoate, [4-nbz = 4-nitrobenzoate, 2-nbz = 2-nitrobenzoate and 3-nbz = 3-nitrobenzoate] have been synthesized and structurally characterized by X-ray crystallography. Their identities have also been established by elemental analysis; IR, NMR, UV–Vis and mass spectral studies. 1 is a dinuclear complex formed by bridging hmt with μ2 coordinating mode. The geometry around the Zn centers in 1 is distorted tetrahedral. Paddle-wheel centrosymmetric Zn2(2-nbz)4 units of complex 2 are interconnected by μ2-hmt forming a one-dimensional chain with square-pyramidal geometries around the Zn centers. Compound 3 contains a μ2/μ3-hydroxido and μ2-hmt bridged 1D chain. In this complex, varied geometries around the Zn centers are observed viz. tetrahedral, square pyramidal and trigonal bipyramidal. Various weak forces, i.e. lone pair–π, π–π and CH–π interactions, play a key role in stabilizing the observed structures for complexes 1, 2 and 3. This series of complexes demonstrates that although the nitro group does not coordinate to the metal center, its presence at the 2-, 3- or 4-position of the phenyl ring has a striking effect on the dimensionality as well as the structure of the resulted coordination polymers, probably due to the participation of the nitro group in l.p.⋯π and/or C–H⋯π interactions.

Graphical abstractSynthesized and structurally characterization of three new zinc(II)-hexamethylenetetramine (hmt) complexes with isomeric nitrobenzoates reveal that one of them is a dinuclear, another one is the first paddlewheel complex of Zn(II) and hmt and the third one is the first example of a complex of any metal ion in which both hydroxido and hmt act as bridging ligands.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Self-assembled coordination complexes of Zn(II) with hexamine. ► Rare example of network containing both hydroxido and hexamine. ► First paddlewheel shaped polymeric complex of hexamine and Zn(II).