Photoluminescent calcium azolium carboxylates with diversified calcium coordination geometry and thermal stability

Abstract

Despite the popularity and versatility of transition-metal–azolium carboxylate coordination polymers, there are very few examples of group 2 complexes supported by azolium carboxylate ligands in the literature, and there are none featuring luminescent calcium azolium carboxylates. New ionic calcium coordination networks, {[Ca₂(L¹)₂(H₂O)₄](Br)₄·6H₂O}_∞ (1), {[(L³)₂Ca(H₂O)₂]₂(Br)₂}_∞ (3), {[(L⁴)₂Ca(H₂O)₂]₂(Br)₂}_∞ (4), and {[(L⁵)₂Ca₃(Na)(H₂O)₉(Cl)](Br)₆·2H₂O}_∞ (5) along with binuclear {[Ca₂(L²)₂(H₂O)₉](Br)₄·4H₂O} (2), and trinuclear {[(L⁶)₂Ca₃(H₂O)₉](Br)₆} (6) were isolated from the reaction between the corresponding azolium carboxylates and calcium carbonate in aqueous solution. 1–6 were characterized by FT-IR, NMR, TGA, UV-vis, fluorescence and single crystal X-ray diffraction techniques. Interestingly, the first tetra-cationic binuclear calcium 2 was isolated using L²H₂Br₂ and hexa-cationic trinuclear calcium 6 was isolated using L⁶H₃Br₃. The 3D coordination polymers 1 and 4 were derived with the help of L¹H₂Br₂ and L⁴H₂Br₂, respectively, through Br⋯H hydrogen bonding. The 3D MOF 3 with rhomboidal channels was constructed using L³H₂Br₂, where the channel size is about 4.8 × 2.9 nm. 5 was isolated as a rare 1D coordination polymer. The choice of azolium carboxylates in these solids not only changes the topology of the network but also affects the chemistry exhibited by the network. Calcium azolium carboxylate assemblies 1–4 and 6 exhibit interesting solid-state photoluminescence properties, driven by azolium carboxylate ligands. Variation of the bridging chromophore produced significant effects on the fluorescence properties. 1–4 and 6 represent the first examples of luminescent calcium azolium carboxylate complexes. As can be seen in the six metal–organic assemblies presented in this report, a combination of carboxylate groups and steric hindrance affects the topology and physical properties of the resultant solids.