Crystal structures, dielectric, and CO₂-adsorption properties of one-dimensional [Cu(II)₂(adamantane-1-carboxylate)₄(pyrazine)]∞ coordination polymers with polar ligands

Cu(II) binuclear coordination polymers of [Cu(II)(ADCOO)₄(pz)]∞ (1), [Cu(II)(ADCOO)₄(F-pz)]∞ (2), and [Cu(II)(Cl-ADCOO)₄(pz)]∞ (3) were prepared, where ADCOO, Cl-ADCOO, pz, and F-pz are adamantane-1- carboxylate, 3-chloroadamantane-1-carboxylate, pyrazine, and 2-fluoropyrazine, respectively. The Cu(II) binuclear units of [Cu(II)(ADCOO)₄] and [Cu(II)(Cl-ADCOO)₄] were formed by the four bridging ligands of ADCOO and Cl-ADCOO to the two Cu(II) ions, respectively, which were further connected by the axial bidentate ligands of pz or F-pz, to form one-dimensional (1D) coordination polymers. Although the 1D coordination polymers were observed in the typical structures of crystals 1, 2, and 3, the packing structure of polymer 3 was different from those of crystals 1 and 2. Three-dimensional (3D) interdigitated polymer arrays were observed in 1 and 2 as cubic crystal systems with the I-43d space group, whereas the two-dimensional array of 1D polymers of 3 was the same as that of the [Cu(II)(C₆H₅COO)₄(pz)]∞ crystal. The crystal structure of 2 was not affected by the introduction of a polar F group into the axial pz ligand, whereas the introduction of the Cl-substituent into the ADCOO ligand significantly modified the packing structure of the coordination polymers. The polar ligands of F-pz in 2 and Cl-ADCOO in 3 were static at 110 K according to X-ray crystal structural analyses. The absolute magnitude of the dielectric constants was decreased in the order of 3, 2, and 1, which is consistent with the dipole moments of the ligands. No CO₂ gas adsorption-desorption processes were observed in crystals 1, 2, and 3 at 195 K. The 3D interdigitated polymer arrangements in 1 and 2, and the strong dipole- dipole interaction between Cl-ADCOO ligands, were not responsible for the structural changes in the CO₂ gas adsorption-desorption.