Constructing stable uranyl-copper(II) bimetallic organic frameworks using rigid thiophenedicarboxylic acid

Abstract

By introducing Cu(II) ions as additional connecting nodes to construct stable heterometallic uranyl-organic frameworks (UOFs) with thiophene-2,5-dicarboxylic acid (H2TDC) and N-donor auxiliary ligands, three new uranyl complexes, namely, [Cu(phen)(UO2)(TDC)2] (1), [Cu(phen-NO2)2(UO2)(TDC)2]·3H2O (2), and [Cu(phen-NH2)(H2O)2(UO2)(TDC)2]·4H2O (3) (phen = 1,10-phenanthroline) have been successfully synthesized by hydrothermal reaction. Complex 1 exhibits a rare three-dimensional (3D) network structure of cds topology connected by UO2, Cu(phen), and TDC units. Both complexes 2 and 3 feature similar one-dimensional chain structures based on UO2, TDC, Cu(Phen-NO2)2 and UO2, TDC, Cu(phen-NH2)(H2O)2 respectively. The N-donor auxiliary ligands play a key role in determining their structural dimensionalities and bandgaps formation. Among the three uranyl complexes, phen with a smaller size than its derivatives, phen-NO2 and phen-NH2, was found to be favorable for constructing 3D UOFs. Moreover, complexes with both phen-NO2 and phen-NH2 showed varying degrees of redshift on optical spectra. The UV–Visible diffuse-reflectance spectra, analyzed with Tauc’s fitting in considering Urbach tailing, revealed an indirect bandgap of 2.94 eV, an indirect bandgap of 2.67 eV, and a direct bandgap of 2.45 eV for 1, 2 and 3, respectively. Moreover, thermogravimetric analyses indicate that 3D anhydrous uranyl complex 1 possessed a robust structure and excellent thermal stability above 250 ℃.