SYNTHESIS AND CRYSTAL STRUCTURE OF BIS(CITRATO)GERMANATE AND STANNATE WITH TRIS(PHENANTHROLINE)NICKEL(II) CATION

The new complexes [Ni(phen)3][Ge(HCit)2]·2H2O (1), [Ni(phen)3][Sn(HCit)2]·3H2O (2) (where phen is 1,10-phenanthroline, H4Cit is citric acid) were synthesized. The identity, composition, and thermal stability of the complexes were established by elemental analysis, thermogravimetry, and IR spectroscopy. According to the data of X-ray diffraction, the bis(citrate)germanate/bis(citrate)stannate [Ge/Sn(HCit)2] 2is the anion, while [Ni(phen)3] 2+ is the cation in the studied complexes. The coordination polyhedrons of Ge, Sn and Ni atoms are octahedral and are formed by three pairs of oxygen atoms of different types of two HCit -3 ligands or by three 1,10-phenanthroline molecules.


Introduction
Obtaining and determining of the structure of coordination compounds formed with biometals and hydroxycarbonic acids is of great interest nowadays because of the creation of new medicines on their basis. Citric acid is the direct participant of a cycle of three carbonic acids (the Krebs cycle) and is present in blood plasma [1]; germanium lactate-citrate possesses radioprotective properties [2].
This work is dedicated to the development of new synthesis methods and study of structure of new heterometal-different-ligand complexes of germanium(IV) or tin(IV) and nickel(II) with citric acid and 1,10-phenanthroline as promising biologically active compounds.

Methods
Elemental analysis for germanium, tin and nickel was performed using inductively coupled plasma atomic emission spectroscopy with an Optima 2000 DV instrument (PerkinElmer). Analysis for C, H, and N was performed using a CE-440 Elemental Analyzer.
Thermogravimetric analysis (TGA) was carried out using a Q-1500D with a heating rate of 10°C/min on air in the temperature range 20-1000°С.
The IR spectra in the range of 4000-400 cm -1 were recorded as potassium bromide pellets on a Frontier spectrometer (PerkinElmer). The absorption bands were attributed according to the reference data for citric acid and our earlier obtained germanium(IV) coordination compounds with hydroxycarboxylic acids [3,4,6,7].
X-ray crystallography. Crystal data for structures 1 and 2 were measured on a Xcalibur-3 diffractometer (graphite monochromated MoKα radiation, CCD detector, ω-scanning). The structures were solved by the direct method using the SHELXTL package [9]. Full-matrix leastsquares refinement against F 2 in anisotropic approximation was used for non-hydrogen atoms. Positions of hydrogen atoms were determined from electron density difference maps and refined by -riding‖ model with U iso = nU eq of the carrier atom (n= 1.5 for hydroxyl groups and n= 1.2 for other hydrogen atoms). CCDC 1854034 and 1854035 contains the supplementary crystallographic data for 1 and 2, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Full use of the CCDC package was also made for searching in the CSD Database.

[Ni(phen) 3 ][Sn(HCit) 2 ]·3H 2 O (2)
It was prepared by dissolving citric acid (0.01 mol, 4.2 g) in water (20 mL), further, this solution was brought to boiling, SnCl 4 (0.005 mol, 0.625 mL) was added, heated for 10 min and cooled. After addition of tin tetrachloride to the acid solution, pH was brought to 1 by adding ammonium hydroxide. This was done because the complexation does not take place in a highly acidic medium, while further increase in the pH results in hydrolysis.
In the second step, the solution of Ni(CH 3 COO) 2 ·4H 2 O in water (5 mL) was added, the mixture was stirred and the complex [Ni(H 2 O) 6 ][Sn(HCit) 2 ]·4H 2 O [3] crystallized at room temperature. The blue precipitate was filtered off on a Schott glass filter, washed with cold water, and dried at room temperature (20°С).
In the third step, a suspension of [Ni(H 2 O) 6 ][Sn(HCit) 2 ]·4H 2 O (0.0523 g, 0.5 mmol) and 1,10-phenanthroline (0.27 g, 1.5 mmol) was stirred in 20 mL of warm distilled water. On the next day, a red crystalline solid precipitated, from which single crystals were collected mechanically and analyzed by X-ray crystallography.
Elemental composition, based on singlecrystal data for C 48
The thermal decomposition of 1 starts with an endotherm peak within the temperature range of 80-140°C, which is due to the elimination to the gas phase of two molecules of crystallized water. During further heating within the temperature range from 290 to 360°C, complex 1 eliminates one molecule of phenanthroline and two СО 2 molecules from the malate ligands.
The first endothermic peak on the thermogravimetric curve of 2 is due to the elimination of three water molecules to the gas phase. The second endothermic peak corresponds to the loss of two СО 2 molecules; complex 2 eliminates one molecule of phenanthroline (the third endothermic peak) within the temperature range 320-360°C.
Finally, complexes 1 and 2 undergo oxidative thermal degradation and combustion of the organic part of their molecules. According to the calculated weight loss on the thermogravimetric curve of complexes, the extensive thermal decomposition (1000°С) of 1 gives nickel(II) metagermanate while in the case of 2, nickel(II) stannate is formed.
The IR spectra of compounds 1 and 2 were found to exhibit similar sets of absorption bands, which were compared with the data obtained for other different-metal and different-ligand bis(citrato)germanates and bis(citrate)stannates [3,4,6,7]. The presence of a free carboxyl group -COOH in the molecules of the obtained complexes is indicated by the ν(C=O) band at 1720 cm -1 in their IR spectra [11]. In comparison with the IR spectrum of citric acid, the spectra of complexes 1 and 2 show characteristic ν as (COO -) and ν s (COO -) bands, alkoxide type ν(C-O) bands at 1080 cm -1 , and Ge-O, Sn-O stretching bands. The absorption band at 1340 cm -1 can be assigned to the ν(CN) heterocycle, the bands at 1588, 1518 cm -1 -to the ν(C-C) phenanthroline ring vibrations. There are found deformation vibrations δ(C-H) of the aromatic rings: planar vibrations at 1041 cm -1 ; non-planar vibrations at region 987-905 cm -1 [12].
The [Ni(phen) 3 ][Ge/Sn(HCit) 2 ]·nH 2 O 1 and 2 were found to be complex salts that exist as hydrates in crystal phase. The bis(citrate)germanate [Ge(HCit) 2 ] 2in 1 and bis(citrate)stannate [Sn(HCit) 2 ] 2in 2 carries out the role of dianion while nickel [Ni(phen) 3 ] 2+ complexes are cations in both compounds. There are two parts of cations in the independent part of the unit cell in both structures. This is caused by the fact that Ni atoms are located in a special position on the 2-fold axis. Such a structure of compounds 1 and 2 is very similar to the previously described complexes [6].
Ni atoms are coordinated by three molecules of phenanthroline in cations of the structures 1 and 2. The location of Ni atoms in a special position causes the fact that only 1.5 phenanthroline molecules are symmetrically independent (Figure 1(b)). One of the 1,10-phenanthroline molecules is disordered over two positions with equal populations due to symmetrical position relatively the 2-fold axis in the cation B of structure 1 (Figure 1(с)).     (Table 1).
In the crystal phase anions, cations, and water molecules of compounds 1 and 2 form alternate layers, which are parallel to the bc crystallographic plane (Figure 2). The layers can be divided into three types: 1) layers containing only anions; 2) layers containing cations of type A; 3) layers consisting of cations of type B. The anions within the layer are bound by the O-H···O intermolecular hydrogen bonds (as shown in Table 3).

Conclusions
Complexes of germanium(IV) and nickel(II) with citric acid and 1,10-phenanthroline were synthesized.