Steric Effects on Uranyl Complexation: Synthetic, Structural, and Theoretical Studies of Carbamoyl Pyrazole Compounds of the Uranyl(VI) Ion

New bifunctional pyrazole based ligands of the type [C3HR2N2CONR′] (where R = H or CH3; R′ = CH3, C2H5,
or iC3H7) were prepared and characterized.
The coordination chemistry of these ligands with uranyl nitrate and
uranyl bis­(dibenzoyl methanate) was studied with infrared (IR), 1H NMR, electrospray-mass spectrometry (ES-MS), elemental analysis,
and single crystal X-ray diffraction methods. The structure of compound
[UO2(NO3)2(C3H3N2CON­{C2H5}2)] (2) shows that the uranium­(VI) ion is surrounded by one nitrogen
atom and seven oxygen atoms in a hexagonal bipyramidal geometry with
the ligand acting as a bidentate chelating ligand and bonds through
both the carbamoyl oxygen and pyrazolyl nitrogen atoms. In the structure
of [UO2(NO3)2(H2O)2(C5H7N2CON {C2H5}2)2], (5) the pyrazole
ligand acts as a second sphere ligand and hydrogen bonds to the water
molecules through carbamoyl oxygen and pyrazolyl nitrogen atoms. The
structure of [UO2(DBM)2C3H3N2CON­{C2H5}2] (8) (where DBM = C6H5COCHCOC6H5) shows that the pyrazole ligand acts as a monodentate
ligand and bonds through the carbamoyl oxygen to the uranyl group.
The ES-MS spectra of 2 and 8 show that the
ligand is similarly bonded to the metal ion in solution. Ab initio
quantum chemical studies show that the steric effect plays the key
role in complexation behavior.