Crystal structure and void analysis of tris(2-amino-1-methylbenzimidazolium) hexakis(nitrato-κ2 O,O′)lanthanate(III)

The title hybrid lanthanum complex comprises an icosahedrally arranged La(NO3)6]3– anion that is linked to the organic C8H10N3 + cations through N—H⋯O and C—H⋯O interactions.


Chemical context
Layered lanthanide complexes in the solid state or in solution often represent an one-dimensional transition-metal selfassembly (Chen et al., 2017), frequently incorporated within functional groups from various ligand systems. These complexes not only provide excellent opportunities to widen the research scope of rare-earth compounds, but also feature a novel nuclear secondary building unit (SBU), forming porous and intrinsically electrically conductive structures (Skorupskii & Dincȃ , 2020). Although lanthanide ions have characteristic electronic configurations with their complexes being ideal candidates for new crystal structures and potential applications in superconductivity, magnetism, optics, electronics and catalysis (Eliseeva & Bü nzli, 2010;Woodruff et al., 2013), lanthanide complexes, especially polynuclear clusters, are not well understood (Barry et al., 2016). Some reasons for this are the uncontrollable polynuclear arrangement of lanthanide complexes and the nature of lanthanide ions, with their high coordination numbers, kinetic instabilities, uncertain preferred stereochemistry, and the variable nature of their coordination spheres.

Structural commentary
The La III atom in (1) (Fig. 1) is twelve-coordinate by O atoms of the nitrato ligands with La-O bond lengths varying between 2.612 (2) and 2.707 (2) Å ( Table 1). The nitrato ligands in the resulting [La(NO 3 ) 6 ] 3anion surround the La III atom in a highly distorted icosahedral environment. Bond lengths and angles in the [La(NO 3 ) 6 ] 3anion show no significant deviations from those of other structures where the La III atom is coordinated by nitrate anions and/or water molecules (Drew et al., 1998;Fowkes & Harrison, 2006;Skelton et al., 2019;Polyzou et al., 2012;Bezzubov et al., 2017).
In the unit-cell of (1), each pair of La III atoms nearly lie on each of the crystallographic glide planes [with deviations from the mean planes of 0.00 (7)-0.02 (1) Å ]. The intersection between the La III atoms lying on neighboring glide planes at distances of 12.676 and 14.212 Å , respectively, passes through the center of inversion of the unit-cell.

Supramolecular features
In the crystal structure of (1) the nitrate groups coordinate bidentately to the La III atom. The corresponding La-O-N-O planes are close to coplanar, i.e. deviate slightly from 180 .

Void analysis
Molecular surfaces can be used to quite accurately define the size and shape of a molecule, and to visualize the space belonging to a molecule in a crystal. To check whether the title compound is densely packed or not, a void-space analysis was performed. Based on isosurfaces of the procrystal electron density and electron-density mapping (Fig. 5), we have used the conventional approach of mapping void space by rolling a probe sphere of variable radius over a fused-sphere representation to locate and visualize the void space in a crystalline material, as well as readily compute surface areas and void volumes (Spackman et al., 2021;Turner et al., 2011). Fig. 6 shows the unit-cell packing for the title complex with a 0.002 a.u. void surface, and a volume of 388.80 Å 3 per unit cell. This result indicates that voids occupy 10.7% of the space and, hence, the molecules can be considered as densely packed in the crystal of (1).

Database survey
The structure of the molecular [La(NO 3 ) 6 ] 3anion was first reported by Drew et al. (1998) View of the crystal structure of (1) along [010], showing N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds drawn as blue dotted lines.

Figure 5
The electron density map of (1) in a view along [001].
Structural Database (CSD, version 5.42, update of September 2021; Groom et al., 2016) revealed that there are six other reports of this moiety. One was obtained from the synthesis of a dinuclear Ni II /La III complex containing the rare-earth metal in separate ions (Polyzou et al., 2012), the second in research into materials with luminescent properties for developing new drugs (Esteban-Parra et al., 2020), the third is a lanthanum/ peptide heterometallic complex with interesting optical properties (Bezzubov et al., 2017), the forth was studied during synthesis and theoretical calculations at the DFT level of di-La complexes with a pendant-armed macrocycle (Ferná ndez-Ferná ndez et al., 2006), the fifth is a heteronuclear nitrato lanthanide complex with interesting magnetic properties (Thatipamula et al., 2019), and the sixth is a pyridine imidazolium lanthanum complex (Skelton et al., 2019). The crystal structure of the last compound comprises the anionic unit as ideal [La(NO 3 ) 6 ] 3-, i.e. oppositely faced nitrate moieties lie coplanar to the La III atom, forming a paddle-wheel-shaped structure. The latter is one of the most closely related structures to (1), with the main difference being the number of cations.

Synthesis and crystallization
10 ml of an ethanol solution of La(NO 3 ) 3 Á6H 2 O (216.8 mg, 0.0005 mmol) was stirred at room temperature for 1 h. Then a 10 ml ethanol solution of 2-amino-1-methylbenzimidazole (220.5 mg, 0.0015 mmol) was gradually added dropwise to the stirring mixture over 50 min at 303 K. Immediately after this, the mixture was heated in a reflux condenser at boiling temperature for 30 min. The solution was filtered and allowed to cool. The obtained yellowish single crystalline product was washed several times in pure acetone and allowed to air-dry at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were positioned geometrically with C-H = 0.93-0.96 Å and refined using a riding model with Uiso(H) = 1.5U eq (C) for methyl groups and 1.2U eq (C) for the other groups. Aromatic/amide hydrogen atoms were refined in a similar manner.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq