Syntheses and Structures of 2-Acetylpyridine-( 2-amino-benzoylhydrazone ) and its Dioxouranium ( VI ) Complex

A síntese e a caracterização da 2-acetilpiridina-(2-aminobenzoilhidrazona) (HL) e do seu primeiro complexo com dióxido de urânio são descritas neste trabalho. A análise estrutural da hidrazona, HL, por difração de raios X do monocristal, mostra que sua estrutura molecular é quase planar no estado sólido, apresentando ligações de hidrogênio intermoleculares. O complexo [UO 2 (L)(dmf) 2 ](NO 3 ) é formado a partir de uma suspensão em refluxo de HL com nitrato de uranila em metanol, na presença de pequena quantidade de trietilamina. A recristalização em dmf resulta na obtenção de cristais adequados para a difração de raios X. A análise por difração de raios X do complexo mostra uma geometria de coordenação bipiramidal de base pentagonal para o urânio(VI), formada pelos oxigênios da uranila nas posições apicais, uma molécula da hidrazona monodesprotonada e dois átomos de oxigênio das moléculas de dimetilformamida nas posições equatoriais.


Introduction
Schiff base complexes of uranium have aroused interest on account of their stability, high coordination numbers and usefulness in selective chemical separations.Uranyl complexes of ligands with O and N donors have been extensively reviewed and quite a few complexes of hydrazone Schiff bases have been reported. 1,2As part of our systematic studies on the coordination chemistry of the early actinide elements, we recently published a report on the structural chemistry of thiosemicarbazonato complexes of uranium(VI). 3he formation of complexes with moderate stability is observed with acetylpyridine thiosemicarbazone.The presence of the pyridine nitrogen atom is essential for the formation of chelate complexes.This additional donor site is obviously essential to establish a bond between the relatively 'hard' UO 2 2+ ions and 'soft' donor sites such as the thione sulphur atoms in thiosemicarbazones.More stable complexes are expected with 'harder' donor atoms such as oxygen, as provided by semicarbazones or hydrazones.A number of studies have been undertaken with both ligand systems and the formation of chelates with various compositions has been suggested on the basis of spectroscopic data. 4Some derivatives of semicarbazones or hydrazones can be used for the spectrophotometric determination of metal ions including UO 2 2+ . 5In view of the structural interest involved in the coordination chemistry of uranium with Schiff base ligands, the synthesis and characterization of the Lewis base 2-acetylpyridine-(2-aminobenzoylhydrazone) and the first dioxouranium complex with this hydrazone are reported in this paper.

Experimental
Caution! Uranium is a toxic metal and both 235 U and 238 U are radioactive isotopes.Good laboratory practices should be employed when working with any uranic material.UO 2 (NO 3 ) 2 •6H 2 O was purchased from Merck.(NBu 4 ) 2 [UO 2 Cl 4 ] was prepared following a standard procedure from uranyl nitrate, aqueous HCl and tetrabutylammonium chloride.Infrared spectra were recorded for KBr pellets on a Shimadzu FTIR spectrophotometer between 4000 and 400 cm -1 .A Jeol FX400 NMR spectrometer was used to record the 1 H NMR spectra.

X-ray structure determinations
The X-ray diffraction data were collected on a Bruker Kappa APEX II single crystal diffractometer with Mo K α radiation (0.71073 Å).SADABS was used to scale the data and perform the multi-scan absorption correction.The structures were solved by direct methods using SHELXS-97. 6Subsequent Fourier-difference map analyses yielded the positions of the non-hydrogen atoms.Refinement was performed using SHELXL-97. 7Hydrogen atoms were placed in idealized positions and treated with the 'riding model' option of SHELXL97.Crystal data and more details of data collections and refinements are presented in Table 1.Selected bond lengths and bond angles are given in Table 2.

Results and Discussion
The synthesis of 2-acetylpyridine-(2-aminobenzoylhydrazone), HL 1  Selected bond lengths and bond angles are given in Table 2.
The results of spectroscopic and X-ray diffraction studies of HL 1 indicate the protonation of the N(3) atom and the formation of intramolecular hydrogen bonds N(3)-H(3) … N(1) and N(4)-H(4B) … O(1) (Table 3).Additionally, the intermolecular hydrogen bond N(4)-H(4A) … N(2) i is observed in the crystal structure (symmetry operation: i x+½, -y+½ -z+1).The molecules are packed in layers parallel to the ac plane and are held together essentially by van der Waals interactions.UO 2 (NO 3 ) 2 •6H 2 O or (NBu 4 ) 2 [UO 2 Cl 4 ] react with 2-acetylpyridine-(2-aminobenzoyl-hydrazone) to form an orange precipitate.The composition of the product is solvent dependent due to solvent complexation at the free coordination sites trans to the hydrazone.The crystal structure determination confirms that two molecules of dimethylformamide bind to the uranium center, producing [UO 2 (L 1 )(dmf) 2 ](NO 3 ) when the reaction is performed in dmf and the product is crystallized from that solvent.The hydrazone binds in its mono-deprotonated form.An representation of the molecular structure is illustrated in Figure 2 and selected bond lengths and angles are given in Table 2.The coordination sphere of the metal center can be described as a distorted pentagonal bipyramid with the oxo ligands in the axial positions and with a planar equatorial coordination environment (maximum deviation from the mean least-square plane formed by the atoms U(1), N(1), N(2), O(1), O(2) and O(3): r.m.s.0.0466 Å).The X-ray structural analysis confirms the spectroscopic conclusion that the NH 2 group of the hydrazone is not involved in coordination to the metal center.The infrared spectrum of the complex shows a band at 2800 cm -1 , which suggests hydrogen bonds in the solid state and could be assigned to the NH 2 group.The presence of the UO 2 2+ unit is confirmed by a strong band at 894.9 cm -1 , and the Table 2. Selected bond lengths (Å) and bond angles (°) for HL 1 and [UO 2 (L 1 )(dmf) 2 ](NO 3 ) HL 1 [UO 2 (L 1 )(dmf) 2 ](NO 3 )  The hydrazone ligand is almost planar, as a consequence of the constraints imposed by the formation of the pentagonal basal plane.This suggests considerable steric strain for the uranium complex under study.Principal distortions are due to the small bite angles of the hydrazone, which result in bond angles between 62.83(1) and 78.63(1)° for neighboring atoms in the equatorial plane.Deprotonation of the N(2) atom, which occurs during the formation of the metal complex, results in a shift of the double bonds within the backbone of the ligand, increasing the electron density at the corresponding oxygen and nitrogen atoms and their ability to act as donors to metal ions.A shift of the π-system of HL 1 from C(6) to C(8) and a delocalization of the electron density of the C(8)-O(1) double bond over the N(3)-C(8)-O(1) amide system is observed.
The hydrazone unit is deprotonated in the uranium complexes studied in this paper.This is in contrast to the behavior in some lanthanide complexes where no or only partial deprotonation of the NH groups was observed. 9There are intramolecular hydrogen bonds between N(4)-H(4B) … N(3) and intermolecular hydrogen bonds between N(4)-H(4A) … O(5) ii , Table 3 (symmetry operation ii -x+1, y+1, -z+½).
The present study shows that acetylpyridine benzoylhydrazones are well suitable to form stable complexes with dioxouranium(VI) units.The successful synthesis of the 2-acetylpyridine-(2-aminobenzoylhydrazone) and its uranium complex may constitute a starting point to the development of new ligand systems which can perfectly fit the coordination requirements of lanthanide and actinide ions.

Table 1 .
X-ray structure data collection and refinement parameters