Self-Assembly of Metal-Organic Coordination Polymers Constructed from a Bent Dicarboxylate Ligand: Diversity of Coordination Modes, Structures, and Gas Adsorption

We have synthesized five new metal-organic coordination polymers incorporating the bent ligand H₂hfipbb [4,4′-(hexafluoroisopropylidene)bis(benzoic acid)] with different transition metal ions and co-ligands via solvothermal reactions to give [Zn₂(hfipbb)₂(py)₂]·DMF (1), [Zn₂(hfipbb)₂(4,4′-bipy)(H₂O)] (2), [Zn₂(hfipbb)₂(bpdab)]·2DMF (3), [Cd₂(hfipbb)₂(DMF)₂]·2DMF (4), and [Co(hfipbb)(dpp)]·MeOH (5) (py = pyridine, 4,4′-bipy = 4,4′-bipyridine, bpdab = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, dpp = 1,3-di(4-pyridyl)propane). Compound 1 displays a 2-fold 2D→2D parallel interpenetrated layer network with one-dimensional (1D) helical channels, while 3 exhibits a three-dimensional pillared helical-layer open framework of α-Po topology based upon binuclear paddlewheel units. In compounds 2 and 5, binuclear motifs with double carboxylate bridges are linked by hfipbb²⁻ ligands into a 1D ribbon, which are further assembled into two-dimensional non-interpenetrated (4,4) layers via bipyridyl co-ligands. However, the different bridging modes of hfipbb²⁻ ligands and the different disposition of the coordinated co-ligands around metal ions result in subtle differences in the final architecture. Thus, 2 is based on a binuclear cluster node, double-stranded hfipbb²⁻ linkers, and single-stranded 4,4′-bipy linkers, while 5 is based on a binuclear cluster node and hfipbb²⁻ and dpp linkers which are both double-stranded. Among these compounds, the Cd(II) complex 4 is possibly the most interesting because it represents a rare example in which metal centers are linked by carboxylate groups into infinite chains further joined together by hfipbb²⁻ spacers to form a 2D network with tubular helical channels. All these coordination polymers exhibit low solvent-accessible volumes. Both 3 and 4 retain structural integrity and permanent microporosity upon evacuation of guest molecules, with hydrogen uptakes of 0.57 and 0.78 wt %, respectively, at 20 bar and 77 K.