Multidirectional Solvent-Induced Structural Transformation in Designing a Series of Polycatenated Cobalt(II) Coordination Polymers: Impact on Carbon Dioxide and Hydrogen Uptake

Coordination polymers with external stimuli-responsive structural transformation acquired paramount importance in the advanced material research field due to their eye-catching application to deal with the existing challenging issue, and Co(II) metal complex with d⁷ electronic configuration is a renowned candidate for kinetic accountability and has the potentiality of structural transformation. Bearing these factors in mind, here, a Co(II) congener of a previously reported high hydrogen-adsorbing Cu(II)-based coordination polymer (CP), {[Cu(4-bpe)(2-ntp)]}_n [where 2-ntp^2- = 2-nitroterephthalate and 4-bpe = 1,2-bis-(4-pyridyl)ethane], has been synthesized to study the metal change impact on hydrogen adsorption and solvent-induced structural transformation with their impact on hydrogen uptake. This modified framework has a 2D + 2D → 3D inclined polycatenated framework as comparable to our previously published Cu(II) framework. Here, on the variation of different solvents, the labile Co(II)-containing framework exhibits a structural change through single-crystal to single-crystal (SC-SC) structural transformation and results in three new framework structures. All four frameworks are structurally characterized by elemental analysis, IR, PXRD, TGA, and single-crystal X-ray diffraction. The desolvated parent framework with exposed metal centers exhibits excellent results of H₂ adsorption of 1.3 wt % (145 cc/g) at 77 K and pressure of 1 bar with structural sustainability and CO₂ uptake of 130 cc/g at 195 K and 1 bar. For the other three solvent-mediated structural derivatives, H₂ and CO₂ adsorption have been studied, and the results are correlated with their structure.

Structural characterization of sodium and potassium 3-nitrohydrogenphthalate coordination polymers

The synthesis, crystal structures and properties of two alkali metal 3-nitrohydrogenphthalates obtained by a 1:2 reaction of M2CO3 (M = K or Na) with 3-nitrophthalic acid (LH2) are reported. In the anhydrous potassium coordination polymer [K(LH)] (LH = 2-carboxy-3-nitrobenzoate) 1, the K+ cation is bonded to nine oxygen atoms from six symmetry related (LH)– ligands resulting in a distorted {KO9} coordination polyhedron. Five of the six oxygen atoms including a nitro oxygen atom of the crystallographically unique 2-carboxy-3-nitrobenzoate are involved in metal binding. The μ6-bridging mode of (LH)– places the K+ cations into the layers of the two-dimensional (2D) coordination polymer. Each {KO9} polyhedron in 1 shares edges with two other polyhedra along the b and c axes. A low temperature structure redetermination of [Na(L#H)(H2O)3]·H2O (L#H = 2-carboxy-6-nitrobenzoate) 2 has revealed that the (L#H)− anion is bonded to the Na+ cation in a monodentate fashion via the carbonyl oxygen atom of the –COOH group and two of the three unique aqua ligands exhibit a bridging bidentate mode stabilizing a chain polymer. The structure of compound 2 thus consists of chains of edge-sharing {NaO6} octahedra. Thermal decomposition of 1 or 2 results in the formation of metal carbonate residues.