Crystal structure of aqua(nitrato-κO)dioxido{2-[3-(pyridin-2-yl-κN)-1H-1,2,4-triazol-5-yl-κN 4]phenolato-κO}uranium(VI) acetonitrile monosolvate monohydrate

The UVI atom exhibits a pentagonal–bipyramidal N2O5 coordination environment. In the complex, the 1,2,4-triazole ligand is coordinated in a tridentate manner.

In the title compound, [U(C 13 H 9 N 4 O)(NO 3 )O 2 (H 2 O)]ÁCH 3 CNÁH 2 O, the U VI atom is seven-coordinated in a distorted pentagonal-bipyramidal N 2 O 5 manner by one tridentate triazole ligand, one monodentate nitrate anion and one water molecule in the equatorial plane and by two uranyl(VI) O atoms in the axial positions. In the crystal, the U VI complex molecule is linked to the water and acetonitrile solvent molecules through N-HÁ Á ÁN, O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, forming a sheet structure parallel to the bc plane. The sheets are further linked by an additional O-HÁ Á ÁO hydrogen bond, forming a threedimensional network.

Chemical context
The synthesis of coordination compounds with N-donor heterocyclic ligands is one of the fastest growing areas of coordination chemistry. 1,2,4-Triazoles and their derivatives can be assigned for such types of ligands. The presence of the 1,2,4-triazole ring in the organic ligand provides an additional site for coordination (Aromí et al., 2011). The presence of additional donor groups in the 3-and 5-positions of the triazole moiety provides a greater number of possibilities for chelation of metal ions, involving tridentate bis-chelate functions.
It should be noted that UO 2 2+ complexes with such types of ligands have rarely been investigated. Thus, only three uranyl complexes with 1,2,4-triazole derivatives have been characterized (Daro et al., 2001;Weng et al., 2012;Raspertova et al., 2012). As part of our continuing study of uranium coordination compounds with nitrogen-donor ligands (Raspertova et al., 2012), we report here the structure of the title compound.

Structural commentary
The coordination polyhedron of the U VI atom in the title complex is a distorted pentagonal bipyramid. It is coordinated in a tridentate manner by the 1,2,4,-triazole ligand together with the water molecule and the monodentate nitrate anion in the equatorial plane. Two oxido ligands are placed in the axial positions (Fig. 1). The U1-O1 bond length [2.206 (3) Å ] is comparable with those reported for related six-membered chelate fragments involving phenolate and N-atom donors (Sopo et al., 2008;Ahmadi et al., 2012). The U-N bond lengths [2.489 (4) and 2.658 (4) Å ] are consistent with the situation in other pyridine-bonded uranium complexes (Amoroso et al., 1996;Gatto et al., 2004). The uranyl group is not exactly linear [O2 U1 O3 = 175.36 (14) ]. Non-linear O U groups are generally found in uranyl complexes with five non-symmetrically bonding equatorial ligands. All nonhydrogen atoms of the organic ligand are coplanar within 0.01 Å . The N1-C7 and C7-N2 bond lengths of the triazole ring are equalized [1.336 (5) Å for both]. This value is longer than a Csp 2 N double bond (1.276 Å ) and shorter than a Csp 2 -N single bond (1.347 Å ) (Orpen et al., 1994). It can be assumed that the structure of the triazole ring is the superposition of two possible resonance structures as shown in Fig. 2.

Figure 3
Packing diagram of the title compound, viewed along the b axis. Intermolecular hydrogen bonds are shown as dashed lines.

Figure 2
Scheme showing two possible resonance structures in the triazole ligand.