Synthesis, Structural Diversity, and Properties of Cd Metal–Organic Frameworks Based on 2-(5-Bromo-pyridin-3-yl)-1H-imidazole-4,5-dicarboxylate and N-Heterocyclic Ancillary Ligands

Abstract

Five 1–3D cadmium(II) coordination polymers, namely, [Cd(HL)(DMF)]n (1), [Cd4(HL)4(H2O)6]n (2), [Cd(HL)(phen)]n (3), [Cd2(HL)2(bpy)]n (4), and {[Cd3(HL)3(pbim)2]·H2O}n (5), have been synthesized and fully characterized [H3L = 2-(5-bromo-pyridin-3-yl)-1H-imidazole-4,5-dicarboxylic acid, DMF = N,N-dimethylformamide, phen = 1,10-phenanthroline, bpy = 4,4′-bipyridine, and pbim = 1,1′-(5-methyl-1,3-phenylene)bis(1H-imidazole)]. In those complexes, the doubly deprotonated H3L ligands and CdII ions display versatile coordination modes to construct various structures with interesting topologies. Complex 1 is a 2D helical structure with (4·82) topology built up from 3-connected (HL)2– and CdII nodes. Complex 2 containing (HL)2–-bridged tetranuclear CdII subunits is a 3D helical structure where both (HL)2– and CdII adopt three coordination modes to form a rare 3-connected network with (5·8·12)2(5·12·16)2(5·82)(82·12) topology. Substituting the smaller terminal ligands DMF and water with the larger terminal ligand phen, (HL)2– ligands only use their imidazoledicarboxylate groups to bis-chelate CdII into a chain structure of 3. Complex 4 is a (3,4)-connected 3D network with (4·82)(4·82·103) topology built up from the (HL)2–-bridged (4,82) mesolayer observed in 1 being further linked by rodlike bpy bridges replacing terminal ligand DMF. Complex 5 consists of (HL)2–- and pbim-bridged macrocycle chains and is a (3,4)-connected 2D novel network with (3·4·8)2(3·4·5·82·9)2(32·82·92) topology. As expected, the strongly bis-chelating coordination mode μ-kN,O:kN′,O′ of the imidazoledicarboxylate of (HL)2– absolutely dominates the assemblies with CdII in those complexes. Intriguingly, the additional ligands, such as smaller terminal ligands water and DMF, larger planar terminal ligand phen, as well as rodlike and V-shaped bridges bpy and pbim, exert obvious influence on the coordination modes of ligands (HL)2– and the resulting architectures. Clearly, larger terminal and bridging ligands phen and pbim could limit the coordination of the pyridyl of (HL)2– through steric hindrance. Meanwhile, the solid-state photoluminescence of those compounds at room temperature was also investigated, and the results indicate that their emissions are significantly influenced by the additional ligands incorporating into the networks.