Structural variations in self-assembled coordination polymers constructed by some carboxylate bridged di-copper(II) nodes and hexamethylenetetramine spacers

Abstract

Four new copper(II)-hexamethylenetetramine (hmt) complexes [Cu₃(2-nbz)₆(μ₂-hmt)]_n (1), [Cu₃(pa)₆(μ₃-hmt)]_n (2), [Cu₃(4-nbz)₆(μ₃-hmt)]_n (3) and [Cu(3-nbz)₂(μ₄-hmt)•H₂O]_n (4) with three isomeric nitrobenzoates, [2-nbz = 2-nitrobenzoate, 3-nbz = 3-nitrobenzoate, 4-nbz = 4-nitrobenzoate] and phenylacetate (pa) have been synthesized and structurally characterized by X-ray crystallography. Their identities have also been established by elemental analysis and IR studies. In all four complexes, the respective [Cu₂(carboxylato)₄] unit with four bridging carboxylate ligands in the familiar η₁:η₁:μ coordination mode acts as a linear node whereas the spacer hmt coordinates in different ways which result in the observed structural variations. Complex 1 is a 1D zigzag polymer where hmt acts as angular spacer. In complexes 2 and 3, μ3-hmt is pyramidal and the resulting structures are 2D polymers with fused hexagonal rings. Complex 4 is a very rare example of a 3D diamondoid polymer with μ₄-hmt acting as a tetrahedral spacer. Various weak forces, i.e. lone pair–π, π–π and CH–π interactions, play a key role in stabilizing the observed structures.