Crystal structure of bis[cis-diaquabis(phenanthroline)cobalt(II)] bis(citrato)germanate(IV) dinitrate

The molecular and crystal structure of the [Co(H2O)2(phen)2]2[Ge(HCit)2](NO3)2 (H4Cit is citric acid, phen is 1,10-phenanthroline) compound was studied using X-ray diffraction analysis.

The asymmetric unit of the title compound, [Co(C 12 H 8 N 2 ) 2 (H 2 O) 2 ] 2 [Ge-(C 6 H 5 O 7 ) 2 ](NO 3 ) 2 , features two complex [(C 12 H 8 N 2 ) 2 (H 2 O) 2 Co] 2+ cations, two NO 3 À anions as well as one centrosymmetric [(C 6 H 5 O 7 ) 2 Ge] 2À anion. Two HCit ligands (Cit = citrate, C 6 H 4 O 7 ) each coordinate via three different oxygen atoms (hydroxylate, -carboxylate, -carboxylate) to the Ge atom, forming a slightly distorted octahedron. The coordination polyhedron of the Co atom is also octahedral, formed by coordination of four nitrogen atoms from two phenanthroline molecules and two water oxygen atoms. In the crystal, the cations and anions are linked by hydrogen bonds and form layers parallel to the bc plane. The structure exhibits disorder of the NO 3 À anion [disorder ratio 0.688 (9) to 0.312 (9)]. There are also highly disordered solvent molecules (presumably water and/or ethanol) in the crystal structure; explicit refinement of these molecules was not possible, and the content of the voids was instead taken into account using reverse Fourier transform methods [SQUEEZE procedure in PLATON;Spek (2015). Acta Cryst. C71, [9][10][11][12][13][14][15][16][17][18]. The given chemical formula and other crystal data do not take into account the unknown solvent molecule(s).

Chemical context
Citric acid (H 4 Cit) is an essential component of the Krebs cycle and a universal intermediate in plant and animal metabolism. Its biocompatibility, hydrophilicity and general safety make citric acid a common component in foodstuffs, beverages, pharmaceuticals, cosmetics, etc (Nangare et al., 2021). Recently, Varbanets and co-workers have reported that germanium coordination compounds with citric acid combined with a second metal and ligand, such as Co and 1,10phenanthroline (phen), show high antihypoxic, cerebroprotective properties and have an activation effect on enzymes (Lukianchuk et al., 2019;Gudzenko et al., 2019a,b). Complex compounds have been obtained through reactions in the system GeO 2 -H 4 Cit-CoX 2 -phen-C 2 H 5 OH-H 2 O (X = Cl, CH 3 COO). The authors reported that the anion of the cobalt salt (chloride and acetate) affects the composition and structure of the complex and results in the formation of cationanionic compounds such as [Co(phen) 3 ][Ge(HCit) 2 ]Á2H 2 O (Seifullina et al., 2017a)  combines with only two molecules of 1,10-phenanthroline and the oxygen atoms of two coordinated water molecules to complete the octahedral metal coordination. In this compound, the third carboxylic group of the citric acid is deprotonated, which leads to a change of the charge of the anion and of the molar Co:Ge ratio, while the coordination polyhedron of the germanium atom remains the same: distorted octahedral, formed by six oxygen atoms of three types of oxygen atoms from two tridentate chelating citrate ligands.
In the present work, we report the synthesis and structural analysis of a new complex, [Co(H 2 O) 2 (phen) 2 ] 2 [Ge(HCit) 2 -(NO 3 ) 2 ], which was synthesized by changing the anion of the initial cobalt(II) salt to nitrate. This study is important for establishing the effect that the anion of the 3d metal salt has on the composition and structure of heterometal bis(citrato)germanates with 1,10-phenanthroline, as well as for the creation of new bioactive compounds.

Structural commentary
The title compound is a salt (Fig. 1), with a complex Co-based cation and two types of anions -the complex anion Ge(HCit) 2 and nitrate. The Ge atom occupies a special position on an inversion centre [the coordinates are (0.5, 1.0, 0.5)] so only half of the complex anion is located in the asymmetric unit. The charge of the two [Co(H 2 O) 2 (phen) 2 ] 2+ cations are compensated by one Ge complex dianion and two nitrate anions.

Figure 1
The molecular structure of [Co( intermolecular O-HÁ Á ÁO hydrogen bonds (Table 2); these supramolecular clusters form layers parallel to the bc plane ( Fig. 2). Voids with a volume of 149 Å 3 containing 49 electrons were found between adjacent layers. The content appears to be a combination of water and ethanol solvent molecules with more than twofold disorder. Refinement of these molecules was not possible, and the content of the voids was instead taken into account using reverse Fourier transform methods (SQUEEZE procedure; Spek, 2015).
During the study of the thermal stability of the synthesized complex (Q-1500D PerkinElmer), it was established that its decomposition starts with an endothermic peak in the range of 393-423 K (peak 413 K). The corresponding weight loss of 1.5% indicates that the complex includes molecules of solvation. Therefore, crystals were dried at 423 K for 30 min to remove solvate molecules prior to the yield calculation and for elemental analysis.
Analysis  The IR spectrum of the complex contains absorption bands for (C O), as (COO À ) and s (COO À ), which indicate the presence of non-equivalent coordinated and free carboxyl groups in the complex. A (C-O) absorption band at 1089 cm À1 evidences that the alcoholic OH groups of the citrate ligands are deprotonated and involved in coordination. The presence of Ge-O stretching vibrations suggests that the carboxylate and hydroxyl groups are bonded to germanium. Absorption bands assigned to the (C-N) heterocycle, the (C-C) phenanthroline ring vibrations and deformation research communications  (15) 166 (6) Symmetry codes: (ii) x þ 1; y þ 1; z; (iii) Àx þ 1; Ày þ 1; Àz þ 1.
vibrations (C-H) of the aromatic rings are also found in the IR spectrum. The compound contains coordinated water molecules, as indicated by the H 2 O deformation vibrations at 1613 cm À1 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3 The structure exhibits disorder of the NO 3 À anion. All N-O bond distances were restrained to be similar to each other (within a standard deviation of 0.02 Å ) and the distance between oxygen atoms O10B and O11B was restrained to a target value of 2.200 (4) Å . U ij values of nitrate atoms closer to each other than 2 Å were restrained to be similar to each other (within a standard deviation of 0.02 Å 2 ). Subject to these conditions, the disorder ratio refined to 0.688 (9):0.312 (9).
There are also highly disordered solvent molecules (presumably water and/or ethanol) in the crystal structure; explicit refinement of these molecules was not possible, and the content of the voids was instead taken into account using reverse Fourier transform methods (SQUEEZE;Spek, 2015) as implemented in the program PLATON (Spek, 2020). The voids with a volume of 149 Å 3 contain 49 electrons.

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.