Structural flexibility and selective guest accommodation in two Cu^II metal-organic coordination frameworks

Abstract

Two metal–organic coordination frameworks of Cu^II, [Na₂Cu(2,4‐pyrdc)(H₂O)(μ‐OH₂)₂]_n (1) and {[Cu(2,5‐pyrdc) (NH₃)](2H₂O)}_n (2) (2,4‐pyrdc = pyridine‐2,4‐dicarboxylate; 2,5‐pyrdc = pyridine‐2,5‐dicarboxylate), were synthesized and structurally characterized. The structure of compound 1 was known and shows that Cu(2,4‐pyrdc)₂(H₂O) functions as a metalloligand and is linked to two different Na^I atoms to form a 3D heterometallic Cu^II–Na^I framework. The single crystals of compound 2 were obtained from aqueous ammoniacal solution and crystallize in the triclinic (P$\bar {1}$equation image) crystal system. Compound 2 is a 2D sheet consisting of two different Cu^II 1D chains bridged by a 2,5‐pyrdc ligand. Stacking of the 2D sheets results in a 3D supramolecular host with 1D water‐filled channels. Both frameworks are highly thermally stable and exhibit reversible structural transformation upon removal of the metal‐bound water and NH₃ molecules for 1 and 2, respectively. Sorption studies reveal that desolvated frameworks 1′ and 2′ both behave nonporous to N₂. However, 1′ exhibits structural transformation and hysteretic stepwise sorption of H₂O and MeOH molecules, but THF and C₆H₆ molecules are not adsorbed. Similarly, H₂O and MeOH molecules are easily adsorbed in 2′, but THF and C₆H₆ molecules are not. Such high selectivity in 1′ and 2′ was correlated to the smaller pore aperture and specific host–guest interaction conferred by the unsaturated Lewis acidic sites on the pore surfaces and the Lewis basic adsorbates. Low‐temperature magnetic study of 2 revealed that theCuII atoms are antiferromagnetically coupled, with J = –1.45 cm^-1 and g = 2.01.