Synthesis, Covalency Sequence, and Crystal Features of Pentagonal Uranyl Acylpyrazolone Complexes along with DFT...

Three uranyl acylpyrazolone complexes [UO₂(PCBPMP)₂(CH₃CH₂OH)] (complex I), [UO₂(PCBMCPMP)₂(CH₃CH₂OH)] (complex II), and [UO₂(PCBPTMP)₂(CH₃CH₂OH)] (complex III) were synthesized from σ-donating acypyrazolone ligands to analyze their sequence of covalent characteristics, reactivity, and redox properties (PCBPMP: p-chlorobenzoyl 1-phenyl 3-methyl 5-pyrazolone; PCBMCPMP: p-chlorobenzoyl 1-(m-chlorophenyl) 3-methyl 5-pyrazolone; PCBPTMP: p-chlorobenzoyl 1-(p-tolyl) 3-methyl 5-pyrazolone). An examination of the structure, pentagonal bipyramidal geometry, and composition of these complexes was conducted mainly through their single-crystal X-ray diffraction (XRD) data, ¹H nuclear magnetic resonance (NMR) δ-values, plots of thermogravimetric-differential thermal analysis (TG-DTA), significant Fourier transform infrared (FTIR) vibrations, gravimetric estimation, and molar conductivity values. The covalency order was found to be complex II > III > I, which mainly depends on values of stretching frequencies, average bond lengths of axial uranyl bonds, values of average bond lengths on the pentagonal equatorial plane, solvent coordination on the fifth site of a pentagonal plane, and the type of aryl group on the nitrogen of the pyrazolone ring. This was confirmed by FTIR spectroscopy and single-crystal spectral characterization. To verify experimental results by comparison with theoretical results, density functional theory (DFT) calculations were carried out, which further gives evidence for the covalency order through theoretical frequencies and the gap of highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energies. Theoretical bond properties were also examined by the identification of global index parameters. Intermolecular noncovalent surface interactions were studied by the Hirshfeld surface analysis. The irreversible redox behavior of uranyl species was identified through electrochemical cyclic voltammetry-differential pulse voltammetry (CV-DPV) plot analysis.