1,10-Phenanthrolin-1-ium nitrate–aquabis(4-hydroxybenzoato-κ2 O,O′)(nitrato-κ2 O,O′)(1,10-phenanthroline-κ2 N,N′)erbium(III)–1,10-phenanthroline–water (1/1/0.5/2)

In the title compound, C12H9N2 +·NO3 −·[Er(C7H5O3)2(NO3)(C12H8N2)(H2O)]·0.5C12H8N2·2H2O, the water-molecule-coordinated ErIII ion is chelated by one 1,10-phenanthroline (phen) ligand, two 4-hydroxybenzoate anions and one nitrate anion in a monocapped square-antiprismatic coordination geometry. The uncoordinating phen molecule is approximately parallel to the 1,10-phenanthrolin-1-ium (Hphen) anion [dihedral angle = 3.3 (4)°]. The centroid–centroid distance of 3.801 (5) Å between pyridine rings suggests the existence of π–π stacking. The crystal structure contains an extensive network of classical O—H⋯O and N—H⋯O and weak C—H⋯O hydrogen bonds. C—H⋯π interactions between phen and 4-hydroxybenzoate is also present in the crystal structure. In the crystal, the uncoordinating phen is equally disordered over two sites about an inversion center.


Comment
The coordination chemistry of erbium (III) with N and O donor ligands has been investigated in the past decade and numbers of erbium (III) complexes with different donor ligands have been synthesized and studied by X-ray crystallography (Liu et al., 2010;Neelgund et al., 2007;Liu et al., 2007). The title compound was recently obtained from the reaction of erbium nitrate, sodium benzoate and phen in an methanol-water mixture, and its crystal structure is reported here. Since no 4-hydrobenzoic acid ligand is present in the starting reaction mixture, it may be derived from the benzoic acid via in situ substitution (Xiong et al., 2001) under hydrothermal condition.
The Er III ion is nine-coordinated by two N atoms of a phen ligand, four carboxylate O atoms of two 4-hydroxybenzoate anions, two O atoms of nitrate anion and one O atom of a water molecule. The resulting coordination geometry is a monocapped square antiprismatic coordination (Table 1 and Fig. 1).

Experimental
Erbium trinitrate solution was prepared by dissolving Er(NO 3 ) 3 .6H 2 O (0.4631 g, 1.00 mmole) at room temperature with stirring. The ligand solution was prepared by dissolving benzoic acid (0.4889 g, 4 mmole) and 1,10-phenanthroline (4 mmole) in 20 ml methanol at room temperature. The pH of the ligand solution was adjusted to about 6 with 2 N NaOH.
The Er solution was added drop wise and slowly to the ligand solution. The reaction mixture was stirred for 2 h at room temperature. Pink crystals were obtained at room temperature over a period 3 months.

Refinement
Position C82, N6, C86, N7, C87 and C88 of the phen ring split into two different atoms with 50% occupancies for each, respectively. H atoms bonded to O and N atoms were placed in calculated positions and refined with the distances constrains of O-H = 0.82, N-H = 0.86 Å, and U iso (H)= 1.2U eq (N) and 1.5U eq (O). Other H atoms were positioned geometrically with C-H = 0.93 Å and refined using a riding model with U iso (H) = 1.2U eq (C). Fig. 1. View of the title compound with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level [symmetry code: (i) 2 -x, -y, 1 -z].

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and tor-

sion angles
Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted Rfactors wR and all goodnesses of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.