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

Abstract

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/μ₂-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.