Syntheses, structures and fluorescent properties of two novel coordination polymers in the U–Cu–H3pdc system

Abstract

Two novel coordination polymers, UO₂(C₅H₂N₂O₄)(H₂O) (1) and (UO₂)Cu(C₅H₂N₂O₄)₂(H₂O)₂ (2), have been prepared by the hydrothermal reaction of uranyl nitrate hexahydrate [(UO₂(NO₃)₂·6H₂O], 3,5-pyrazoledicarboxylic acid (H₃pdc) and copper(II) nitrate hemipentahydrate (Cu(NO₃)₂·2.5H₂O) and characterized by single-crystal X-ray diffraction, thermogravimetric analyses (TGA) and fluorescence spectroscopy. Compound 1 (monoclinic, P2₁/c, a = 6.9556(6) Å, b = 11.302(1) Å, c = 10.5288(9) Å, β = 90.057(2)° and Z = 4) consists of a two-dimensional sheet containing uranyl hexagonal bipyramids. Compound 2 (triclinic, P-1, a = 5.1014(7) Å, b = 7.6067(11) Å, c = 10.2910(15) Å, α = 72.380(3)°, β = 86.796(3)°, γ = 84.447(3)° and Z = 1) consists of two-dimensional sheets. Both structures contain the linear UO₂²⁺ moiety and have extended networks built up from the H₃pdc ligand. Compound 1 exhibits the characteristic UO₂²⁺ emission spectra when it is excited at the ligand or uranium excitation wavelength. With the addition of the copper metal center in compound 2, the uranium emission is absent regardless of the excitation wavelength.