A new lanthanum(III) complex containing acetylacetone and 1H-imidazole

The title complex is coordinated by two acetylacetonate, one 1H-imidazole, one nitrate and one water ligand. The molecular plane of the imidazole ligand is almost parallel to that of the nitrate anion.

In the title complex, diaqua(1H-imidazole-N 3 )(nitrato-2 O,O 0 )bis(4-oxopent-2-en-2-olato-2 O,O 0 )lanthanum(III), [La(C 5 H 7 O 2 ) 2 (NO 3 )(C 3 H 4 N 2 )(H 2 O) 2 ], the La atom is coordinated by eight O atoms of two acetylacetonate (acac) anions acting as bidentate ligands, two water molecule as monodentate ligands, one nitrate anions as a bidentate ligand and one N atom of an imidazolate (ImH) molecule as a monodentate ligand. Thus, the coordination number of the La atom is nine in a monocapped square antiprismatic polyhedron. There are three types of intermolecular hydrogen bonds between ligands, the first involving nitrate-water OÁ Á ÁH-O interactions running along the [001] direction, the second involving acac-water OÁ Á ÁH-O interactions along the [010] direction and the third involving an Im-nitrate N-HÁ Á ÁO interaction along the [100] direction (five interactions of this type). Thus, an overall one-dimensional network structure is generated. The molecular plane of an ImH molecule is almost parallel to that of a nitrate ligand, making an angle of only 6.04 (12) . Interestingly, the ImH plane is nearly perpendicular to the planes of two neighbouring acac ligands.

Chemical context
Carboxylic acid-based linkers are often used in metal-organic complexes involving rare earth elements because they can easily build a framework structure due to the oxophilic nature of lanthanide ions. Recently, some imidazole-based metal organic complexes were reported to form such framework structures (Zurawski et al., 2011). A remarkable feature of imidazole-based compounds is the ability to form porous networks, such as zeolitic imidazolate frameworks (ZIFs) (Zurawski et al., 2012;Mü ller-Buschbaum et al., 2015), which show a good performance for gas adsorption with feasible chemical and thermal stability. For example, ZIF-8 and ZIF-11 have a remarkable chemical resistance to boiling alkaline water and organic solvents, and high thermal stability up to 823 K (Park et al., 2006;Zhong et al., 2014). Another interesting feature of these complexes is that they exhibit luminescence based on f-f transitions of lanthanides assisted by the ligand antenna effect . The complexes of rare earth atoms with -diketonates have been investigated widely because of their simple use as organic ligands ISSN 2056-9890 (Binnemans, 2005). These ligands can give an increase in luminescence efficiency and intensity, Eu(acac) 3 (acac is acetylacetonate) being one such complex (Kuz'mina & Eliseeva, 2006). In addition, Tb(acac) 3 is used as an active light-emitting layer in the first LED based on lanthanide complexes (Kido et al., 1990). From the viewpoint of high luminescence efficiency, the luminescence based on the f-d transition of Ce 3+ is quite promising due to its allowed electronic transition. However, the emission of Ce 3+ in metalorganic complexes have been reported only occasionally, for example, in [Ce(triRNTB) 2 ](CF 3 SO 3 ) 3 [NTB = N-substituted tris(N-alkylbenzimidazol-2-ylmethyl)amine] and 1 3 [Ce(Im) 3 -(ImH)]ÁImH (Zheng et al., 2007;Meyer et al., 2015). One of the reasons for this is the difficulty of retaining a certain distance between Ce 3+ ions in order to avoid luminescence quenching caused by energy transfer between Ce 3+ ions.
[Ce(triRNTB) 2 ](CF 3 SO 3 ) 3 shows a blue emission accompanied by neighbouring CeÁ Á ÁCe distance of about 17$18 Å . NTB is a bulky ligand so that it can keep the neighbouring central ions far away. Also, it may be important for the emission of Ce 3+ to construct a structure of isolated entities rather than a framework structure, which does not necessarily guarantee a sufficient long metal-metal distance. During the investigation of the synthesis of lanthanide complexes for Ce 3+ emission using functional ligands, like imidazole with the antenna effect, as well as -diketone derivatives, we have synthesized a novel lanthanum complex, although the cerium derivative has not been synthesized yet. This study reports structural data on a newly synthesized lanthanum complex comprising functional ligands of imidazole and acetylacetone.

Structural commentary
The title complex crystallizes in the monoclinic space group P2 1 /c, with one formula unit of [La(C 5 H 7 O 2 ) 2 (NO 3 )(C 3 H 4 N 2 )-(H 2 O) 2 ]. Each molecule is isolated individually, i.e. the structure is not a framework structure. The central La atom is coordinated by eight O atoms from two acac anions, two water molecules, one nitrate anion and one N atom from one Im ligand (Fig. 1). Thus, the La atom has a monocapped square antiprismatic coordination. The La-O bond lengths can be classified into three categories; the first concerns interactions with a bidentate acac molecule, the second those with a nitrate ion behaving as a bidentate ligand and the third those with a water molecule. All the distances are quite comparable with the corresponding distances reported for acac complexes (Phillips et al., 1968;Antsyshkina et al., 1997;Fukuda et al., 2002) and for nitrate complexes (Al-Karaghouli & Wood, 1972;Frechette et al., 1992;Fukuda et al., 2002). An Im ligand coordinates to the central La atom as a monodentate ligand. The La-N distance is comparable with that of 1 Acta Cryst. (2017). E73, 1739-1742 research communications Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
View of the molecular structure of the title complex, with displacement ellipsoids for non-H atoms drawn at the 50% probability level.

Supramolecular features
The discrete complexes are linked by five kinds of hydrogen bonds (Table 1). There are two types of hydrogen bond chains that lie nearly within the ac plane; the first type are the chains parallel to [100] by centrosymmetric pairs of intermolecular OÁ Á ÁH-N hydrogen bonds between the O atom of a nitrate anion and the H atom of an ImH ligand, and the other type are the chains parallel to [001], formed also by centrosymmetric pairs of intermolecular OÁ Á ÁH-O hydrogen bonds between the O atom of a nitrate anion and the H atom of a water molecule (O12W) (Fig. 2a). It is notable, as shown in Fig. 2(b), that these hydrogen bonds are both almost parallel to the ac plane. This arises from the fact that the angle difference between the molecular planes of the nitrate and ImH molecules is only 6.04 (12) . Along the [010] direction, there are three types of hydrogen-bond chains, all of which are the hydrogen bond between the O atom of the acac anion and the H atom of water molecule (Fig. 3). All the ligands coordinating to the central La atom are involved in hydrogen bonding with neighbouring complexes. In this way, all molecules are connected by hydrogen bonds running in every axis direction, leading to a three-dimensional supramolecular network structure. Furthermore, it should be mentionned that the molecular plane of each ImH ligand is almost perpendic-ular to the molecular planes of the two neighbouring acac anions, making angles of 84.68 (11) and 85.27 (11) , respectively.

Synthesis and crystallization
Colourless plate-like crystals were obtained by slow evaporation from a methanol solution of La(NO 3 ) 3 Á6H 2 O, acetylacetone and 1H-imidazole (1:5:5 molar ratio). The products were filtered off and dried at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C atoms were positioned geometrically after each cycle in idealized locations and refined as riding on their parent C atoms, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C    pically refined without any distance restraint and with restraints of U iso (H) = 1.5U eq (O).

Diaqua(1H-imidazole-κN 3 )(nitrato-κ 2 O,O′)bis(4-oxopent-2-en-2-olato-κ 2 O,O′)lanthanum(III)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.