Solvent-mediated assembly of chiral/achiral hydrophilic Ca(ii)-tetrafluoroterephthalate coordination frameworks: 3D chiral water aggregation, structural transformation and selective CO2 adsorption

Abstract

The spontaneous self-assembly reactions of 2,3,5,6-tetrafluorobenzenedicarboxylic acid (H₂BDC-F₄) with Ca(NO₃)₂·4H₂O in different solvents resulted in the generation of hydrophilic 3D chiral and achiral Ca(II)–organic frameworks {[Ca₄(BDC-F₄)₄(H₂O)₄]·4H₂O}_n (1) and [Ca(BDC-F₄)(MeOH)₂]_n (2), respectively. Complex 1 exhibits higher water solubility compared to 2, and further dissolution of 1 in water results in a new achiral crystal, [Ca(BDC-F₄)(H₂O)₄]_n (3). All complexes have been characterized by elemental analysis, IR spectroscopy, and single-crystal and powder X-ray diffraction techniques. Complex 1 crystallizes in the tetragonal P4₁2₁2 chiral space group and features a rare binodal 5-connected network, directed by a unique 3D chiral (10,3)-a water aggregation consisting of two distinct helical arrays. Complex 2 crystallizes in the space group C2/c and has a binodal 4-connected pts net. Complex 3 crystallizes in the space group P2₁/m and presents a 2D layer framework containing the unprecedented polymeric [Ca^II–H₂O]_n chain based on triple μ₂-O aqua bridges. The structural discrepancies illustrate the solvent-induced effect on the construction and structural transformation of chiral and achiral coordination frameworks. Moreover, gas (N₂ and CO₂) adsorption studies show that the dehydrated framework 1a exhibits excellent selective CO₂ gas uptake at 195 K.