Mellitate-based coordination polymers with a recurring motif: controlling dimensionality with secondary ligands

Hydrothermal reactions of benzenehexacarboxylic acid, also known as mellitic acid, and transition metal cations in the presence of N-donor ancillary ligands afford five novel coordination polymers with the general formula of [M-2(H(2)mel)(L)(H2O)(6)]center dot nH(2)O, where M = Co, L = 2,2'-bpy (1); M = Co, L = 4,4'-bpy (2a); M = Ni, L = 4,4'-bpy (2b); M = Co, L = tpt (3a); M = Ni, L = tpt (3b). The structures were determined by single-crystal X-ray diffraction. Powder X-ray diffraction (PXRD), elemental analysis, IR spectroscopy, and magnetic measurements were also carried out for some of the compounds. All three structures contain the same one-dimensional secondary building units (1D-SBUs), metal-mellitate double chains made of two types of octahedrally-coordinated metal atoms. The 1D-SBUs are stacked on top of each other and held together by very short hydrogen bonds to generate 2D-layers that are also identical in all three structures and can be viewed as two-dimensional tertiary building units (2D-TBUs). The three structures differ in dimensionality due to the different connectivities provided by the different nitrogen-based co-ligands. The chains in 1 are terminated by the chelating 2,2'-bpy and are thus isolated. They are interconnected by the 4,4'-bpy linker to form two 3D interpenetrating frameworks in 2. The same chains in 3 are interlinked by the bent tri-dentate tpt ligands to form two-dimensional structures. Since all compounds contain the same building unit, only the magnetic properties of one compound, 2a, were measured and analyzed.