Thiacalix[4]arene-Based Three-Dimensional Coordination Polymers Incorporating Neutral Bridging Coligands

Abstract

Employing a solvothermal procedure and use of p-tert-butyl-thiacalix[4]arene tetraacetic acid (H4TCTA) and the nitrate salts of cobalt(II) and cadmium(II) have enabled isolation of four three-dimensional metal–organic frameworks (1–4) in the presence of three different types of neutral bridging coligands. In one case, 3, a mixed metal ion system (Co2+/Na+) was obtained. First, the reaction of H4TCTA with Co(NO3)2·6H2O together with the linear bridging coligand BPY (4,4′-bipyridine) afforded a mixture of two 3D frameworks of types [Co2(TCTA)(BPY)]n (1) and [Co2(TCTA)(BPY)2(H2O)4]n (2). Compound 1 features a paddle-wheel unit incorporating Co2+ pairs, Co2(carboxylate)4, involving four different TCTA units in two dimensions and BPY pillaring in the third. In 2, 2D sheets consisting of BPY–Co repeating chains stacked in an A-B-A-B pattern are pillared by TCTA to yield the 3D motif. In the second reaction system, the use of H4TCTA with Co(NO3)2·6H2O in the presence of the bent bridging coligand BPPA (N,N′-bis(pyridine-4-yl)-isophthalamine) and NaOH afforded {[Co3Na2(TCTA)2(BPPA)2(H2O)2]·4DMF·4H2O}n (3). In 3, TCTA yields a honeycomb-shaped 2D sheet via Co–O(carboxylate) bonds; these sheets are linked by Co–O(carboxylate) and Co–(BPPA)–Co bridging bonds to afford the 3D motif. In this case, the Na atom locates inside the calix cavity in a distorted square planar coordination environment and is stabilized by η3-type cation···π interactions. In the third system, the use of H4TCTA with Cd(NO3)2·6H2O in the presence of the flexible bridging coligand diaminohexane (DAH) gave {[Cd2(TCTA)(DAH)(DMF)2]·2DMF·2H2O}n (4), which features an unusual PtS-like 3D structure. The results confirm that the nature of the extended 3D structures is influenced by the bridging coordination behavior of the coligands. In all these structures, the TCTA ligands adopt a 1,3-alternate conformation. Thermal gravimetric analysis performed on the present MOFs revealed that three of these compounds have high thermal stability (∼300 °C).