Steric Effects on Uranyl
Complexation: Synthetic, Structural, and Theoretical Studies of Carbamoyl
Pyrazole Compounds of the Uranyl(VI) Ion
- Publication date
- Publisher
Abstract
New bifunctional pyrazole based ligands of the type [C<sub>3</sub>HR<sub>2</sub>N<sub>2</sub>CONR′] (where R = H or CH<sub>3</sub>; R′ = CH<sub>3</sub>, C<sub>2</sub>H<sub>5</sub>,
or <sup>i</sup>C<sub>3</sub>H<sub>7</sub>) were prepared and characterized.
The coordination chemistry of these ligands with uranyl nitrate and
uranyl bis(dibenzoyl methanate) was studied with infrared (IR), <sup>1</sup>H NMR, electrospray-mass spectrometry (ES-MS), elemental analysis,
and single crystal X-ray diffraction methods. The structure of compound
[UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>(C<sub>3</sub>H<sub>3</sub>N<sub>2</sub>CON{C<sub>2</sub>H<sub>5</sub>}<sub>2</sub>)] (<b>2</b>) 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 [UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>(C<sub>5</sub>H<sub>7</sub>N<sub>2</sub>CON {C<sub>2</sub>H<sub>5</sub>}<sub>2</sub>)<sub>2</sub>], (<b>5</b>) 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 [UO<sub>2</sub>(DBM)<sub>2</sub>C<sub>3</sub>H<sub>3</sub>N<sub>2</sub>CON{C<sub>2</sub>H<sub>5</sub>}<sub>2</sub>] (<b>8</b>) (where DBM = C<sub>6</sub>H<sub>5</sub>COCHCOC<sub>6</sub>H<sub>5</sub>) 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 <b>2</b> and <b>8</b> 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
