Structural variation of transition metal coordination polymers based on bent carboxylate and flexible spacer ligand: polymorphism, gas adsorption and SC-SC transmetallation

Reaction of the bent dicarboxylate ligand H₂OBA (H₂OBA = 4,4′-oxybisbenzoic acid) and the flexible linker 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L₁), under diverse reaction conditions, forms two polymorphic Co(II) coordination polymers (CPs): {[Co(OBA)(L₁)]·DMF}_n (1), as a three dimensional (3D) framework with a pcu alpha-Po primitive cubic topology, and {[Co(OBA)(L₁)]·DMF}_n, (2), as a two dimensional (2D) structure with a 6-c uninodal net topology. Gas adsorption measurements of the desolvated Co(II) CPs show negligible uptake of all gases in 1, while 2 exhibits moderate uptake of CO₂, with good selectivity over N₂ and CH₄. With Zn(II), reaction of H₂OBA and L₁ produces a different 2D CP, {[Zn_0.5(OBA)_0.5(L₁)_0.5]}_n (3). Finally, three isostructural Cd(II) CPs, {[Cd(OBA)(L₁)]·DMF}_n (4), {[Cd(OBA)(L₁)]·DEF}_n (5), and {[Cd(OBA)(L₁)]·DMA}_n (6) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide), that differ only in the lattice solvent molecules and show 2D structural arrangements are prepared. Interestingly, CP 4 undergoes single-crystal to single-crystal (SC-SC) transmetallation reaction at room temperature, yielding isostructural {[Cu(OBA)(L₁)]·DMF}_n (7) that cannot be synthesized independently. Moreover, the luminescence properties of compounds 1, 2, 3, and 4 have been studied in the solid state at room temperature. All the complexes are characterized by elemental analysis, IR, TGA, PXRD and single crystal X-ray diffraction.