Self-assembled coordination polymers of V-shaped bis(pyridyl)thiadiazole dependent upon the spacer length and flexibility of aliphatic dicarboxylate ligands

Abstract

An investigation into 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (bpt) and cupric or cadmium complexes with a large variety of aliphatic α,ω-dicarboxylic acids HOOC-X-COOH (X = (CH₂)_n, n = 0–4 corresponding to oxalic acid (H₂ox), malonic acid (H₂mal), succinic acid (H₂suc), glutaric acid (H₂glu) and adipic acid (H₂adi); CHCH, maleic acid (H₂male) and fumaric acid (H₂fum)) is performed. Ten novel metal–organic architectures, [Cu₂(bpt)(ox)₂]_n·2.92nH₂O (1), [Cu(bpt)(mal)(H₂O)]_n·2nH₂O (2), [Cu(bpt)(glu)]_n (3), [Cu(bpt)(adi)]_n (4), [Cu₂(bpt)₂(fum)₂]_n·3nH₂O (5), [Cd(bpt)(mal)(H₂O)]_n (6), [Cd(bpt)(suc)]_n (7), [Cd(bpt)(glu)]_n·nH₂O (8), [Cd₂(bpt)₂(male)₂]_n·2nH₂O (9), and [Cd(bpt)(fum)(H₂O)]_n·nH₂O (10) were successfully synthesized through varying the organic acid linkers. The molecule 1 is a 3D pillared-layer framework with the open channels encapsulating infinite 1D water chains. Complex 2 shows the contorted 2D layer, which is extended to a 2-fold interpenetrated 3D supramolecular net by antidromic R²₄(8) hydrogen-bonding pattern. Compound 3 is a grid-like layer motif with the terminal bpt ligands as pendants. The MOFs 4 and 5 present two-fold interpenetrating primitive cubic (α-Po) coordination networks. Interestingly, discrete tetrameric water clusters exist in the micropores of the MOF 5. For the cadmium complex 6, six-coordinated metal centers link malonate and bpt to form interdigitated layers through edge-to-face π–π stacking interactions. Coordination polymer 7 can be considered as six-connected self-penetrated topological 3D network, in which the right- and left-handed helices are alternately arranged into a wavy Cd/succinate layer. Flexible glutarate with cadmium atoms produces a 1D ribbon, and is further linked by bpt ligands to produce the 2D double-layered motif 8. Incorporating with rigid butenedioic acid, the reaction of seven-coordinated cadmium and bpt leads to two distinct 2D (4,4) layered frameworks 9 and 10, with a one-dimensional Cd–O chain observed in the former framework. A systematic structural comparison of these ten complexes indicates that the final polymeric structures could be tuned through the introduction of the different backbone length and flexibility of the aliphatic dicarboxylates. In addition, host–guest chemistry of MOFs 1 and 5, and the luminescent properties of cadmium compounds were discussed.