Coordination chemistry of sulfur in biological systems attracts a lot of attention (Allegra et al., Reference Allegra, Amodeo and Colombatto2002). One of the reasons is that the redox equilibrium between organic disulfides R–SS–R and mercaptides RS- can be shifted by complexation of the organic molecules by metal ions (Eremin et al., Reference Eremin, Antonov and Panina2009). For instance, the cysteine moieties were involved in mercury ion binding in the regulatory protein MerR (Wilson et al., Reference Wilson, Leang, Morby, Hobman and Brown2000), MerP (Steele and Opella, Reference Steele and Opella1997). Numerous works were devoted to investigations of cysteine activity with metals in biological systems (Shaw et al., Reference Shaw, Stillman and Suzuki1991). In contrast, the chemistry of cysteamine HS(CH2)2NH2 (Cystea), containing a similar sulfur moiety, has not been studied deeply enough, despite the fact that this compound was used in many medications. One of the significant cysteamine application is the use as an effective radioprotective agent (Mark et al., Reference Mark, Othmer, Overberger and Seaborg1982). In medicine cysteamine is also used for the treatment of cystinosis disease. It activity is reduced by binding an excess of intracellular cysteine into soluble cysteine–cysteamine complex which then is removed from the body (Markello et al., Reference Markello, Bernardini and Gahl1993). It is well known that there is the redox equilibrium between cysteamine and cystamine NH2(CH2)2SS(CH2)2NH2 (Cysta) in the body. In cells, these compounds coexist simultaneously and, consequently, their metabolisms are closely related, as well as biological functions. Mutual transformation “Cystea ↔Cysta” could be influenced by metal ions, solution composition, and acidity.
The breaking S–S bond in cystamine with the formation of cysteamine was occurred at synthesis [Ni(Cystea)2]Cl2 and [Cu(Cystea)2]Cl2 (Carrillo et al., Reference Carrillo, Gouzerh and Jeannin1989). The phenomenon was also observed at the formation of [Hg(Cystea)2]Cl2 at 4 °C (Kim et al., Reference Kim, Parkin, Bharara and Atwood2002). The salts of Pd(II) and Pt(II) give cysteamine complexes both in strong acidic and alkaline media (Foye and Kaewchansilp, Reference Foye and Kaewchansilp1979). There are a number of cases where S–S bond in cystamine is left intact when using organic solvents. For example, the PbX 2 (where X = Br, I) interacts with cystamindi-ium dihydrochloride in CH3CN giving CystaH2[PbX 4] (Louvain et al., Reference Louvain, Bi and Mercier2007). The CystaH2[HgCl4] was obtained under neutral conditions in a nitrogen stream (Bharara et al., Reference Bharara, Bui, Parkin and Atwood2005a). There are other ionic compounds containing cystamindi-ium cation, for example, with bismuth CystaH2 [Bi I5], (CystaH2)2I3[Bi I6] (Bi et al., Reference Bi, Louvain, Mercier, Luc and Sahraoui2007, Reference Bi, Louvain, Mercier, Luc, Rau, Kajzar and Sahraoui2008) and with vanadium (CystaH2)2 H2 [V10O28] * 4H2O, (CystaH2)5 H4 [V15O42]2 * 10H2O (Pavani et al., Reference Pavani, Upreti and Ramanan2006). As a rule, cystamindi-ium cations occupy crystalline space by individual particles linking with an anionic part by the Coulomb forces and hydrogen bonds. The anionic part can be fulfilled by the local anions or polyanions and infinite ribbon-like or planar polymers. It is interesting to note that plumbate and bismuthate of cystamindi-ium exhibit nonlinear optical properties (the generation of the second harmonic) and can be interesting from the material science point of view (Louvain et al., Reference Louvain, Bi and Mercier2007). It is assumed that the NLO property is because of acentric geometry of cystamindi-ium cation. Dihedral angle C–S–S–C in the ion at approximately 90° excludes symmetry center in the cation. In the case of copper(I), there are known two compounds CystaH2 [Cu2Br4] and CystaH2 [Cu3Br5], and the first one has two crystal modifications (Louvain et al., Reference Louvain, Mercier and Kurmoo2008). Cu(I) exhibits the specific interaction with cystamindi-ium. It is coordinated by two sulfur atoms of cystamindi-ium cation. As a result copper atom reconstructs its linear coordination into more saturated tetrahedral one.
In the present paper, the reaction of cystamindi-ium dihydrochloride CystaH2Cl2 with CuCl2 in a strongly acidic medium (10 M HCl) has been studied. It was interesting to clear up the S–S bond behavior and coordination mode of Cu(II). The resulting substance CystaH2[CuCl4] was studied by thermal analysis and infrared (IR) spectroscopy. As the substance was available in the form of fine powder, the crystal structure determination was carried out using X-ray powder diffraction technique.
I. EXPERIMENTAL
A. Synthesis
For CystaH2[CuCl4] synthesis 2 ml concentrated HCl (10 M) was added to 0.20 g CystaH2Cl2, and the dry salt CuCl2 in the ratio Cu(II):CystaH2 more than 2:1 was dissolved in the resulting solution. After 3 days the yellow precipitate crystallized from the solution. It was filtered, washed with acetone, and left to dry in air at room temperature. Yield was 60 − 70%. The product can be decomposed in water.
B. Thermal-, HCN- analysis, and IR spectra registration
Thermal analysis was done on NETZSСH409 in air, heating rate was 10 °C min−1, temperature range was 23–700 °C, and the sample weight was 10–15 mg. IR spectra were recorded in the 400–4000 cm−1 region as KBr (0.1%) pellets on FTIR Nicolet 6700 IR spectrophotometer. IR spectrum (KBr, ν, sm−1): 3104, 2947, 2611, 2406, 1564, 1479, 1462, 1444, 1407, 1379, 1320, 1257, 1246, 1228, 1126, 1087, 1055, 1021, 935, 917, 868, 802, 781, 734, 632, 461. The chemical analysis was carried out with the C,H,N-analyzer: EURO EA Elemental Analyzer. Chemical composition, found: С 13.01%, H 4.32%, N 7.36%, S 17.56%, Cu 17.06%; calculated: С 13.36%, H 3.92%, N 7.79%, S 17.83%, Cu 17.67%.
C. X-ray powder diffraction study
Сrystal structure determination CystaH2[CuCl4] was carried out using X-ray powder diffraction data. Experimental data were obtained on X'Pert PRO diffractometer (PANalytical) with a PIXcel detector, equipped with a graphite monochromator. CuKα radiation was applied. The sample was prepared in a cuvette of 25 mm in diameter by means of direct loading. The unit-cell parameters were defined using the programs (Visser, Reference Visser1969; Kirik et al., Reference Kirik, Borisov and Fedorov1981). The space group followed from regular reflection absence. The structural models were determined in the direct space applying the “simulated annealing” approach (Solovyov and Kirik, Reference Solovyov and Kirik1993) with the program FOX (Favre-Nicolin and Černý, Reference Favre-Nicolin and Černý2002). The square planar anionic complex [CuCl4]2− with the regular geometry and cystamindi-ium ion with the standard configuration were the basic molecular particles for the structure modeling. Actually the structure determination consisted in finding an optimal positions and orientations of the molecular particles in the space of a unit cell. The most optimal structural model was refined by the full-profile technique (Rietveld method) using the program FullProf (Rodriguez-Carvajal, Reference Rodriguez-Carvajal2009). Rigid and soft constraints were impose on refined atomic coordinates (Kirik, Reference Kirik1985) using the weight coefficients and taking into account the average values of corresponding distances and angles (Allen, Reference Allen2002). At the final refinement step hydrogen atoms were rigidly attached to the respective carbons (Siemens, 1989). The resulting structural data have been deposited on CSD # 986013.
II. RESULTS AND DISCUSSION
Redox equilibrium in the “Cysta-Cystea” system, occurring in living organisms, currently is not thoroughly studied, from the point of view of solvent type, the acidity of the medium, the influence of different type of metal ions, the solution composition in whole, and temperature (Foye and Kaewchansilp, Reference Foye and Kaewchansilp1979; Mark et al., Reference Mark, Othmer, Overberger and Seaborg1982; Carrillo et al., Reference Carrillo, Gouzerh and Jeannin1989; Shaw et al., Reference Shaw, Stillman and Suzuki1991; Markello et al., Reference Markello, Bernardini and Gahl1993; Steele and Opella, Reference Steele and Opella1997; Wilson et al., Reference Wilson, Leang, Morby, Hobman and Brown2000; Allegra et al., Reference Allegra, Amodeo and Colombatto2002; Kim et al., Reference Kim, Parkin, Bharara and Atwood2002; Bharara et al., Reference Bharara, Bui, Parkin and Atwood2005a; Louvain et al., Reference Louvain, Bi and Mercier2007; Eremin et al., Reference Eremin, Antonov and Panina2009). In most cases, the reaction condition changing leads to results far from expectations that makes impossible consistent description the set of transformations in “Cysta-Cystea”. Nevertheless, the study of Cysta and Cystea behavior under controlled conditions, the analysis of mutual transformations may provide necessary information for complete understanding of its chemical activity. The result of interaction of Cu(II) salt with dihydrochloride of cystamindi-ium in aqueous solution with an excess of hydrochloric acid according to the equation:
$${\rm Cysta}{{\rm H}_ 2}{\rm C}{{\rm l}_ 2}+{\rm CuC}{{\rm l}_ 2} \to {\rm Cysta}{{\rm H}_ 2}{[ {\rm CuC}{{\rm l}_ 4}] _{{\rm solid}}}$$
is investigated in this paper. Excess of hydrochloric acid inhibits [CuCl4]2− anion oxidation because of its low stability in neutral aqueous solution. CystaH2 2+ cations do not decompose in these conditions, resulting in the formation of the final product CystaH2[CuCl4]. Previously, such complex compounds containing CystaH2 2+ ion were prepared exclusively in organic media, particularly from ethanol solutions (Carrillo et al., Reference Carrillo, Gouzerh and Jeannin1989). Thus, the correspondence between the acidity of the medium and solvent plays an essential role in the formation of the final product in this chemical transformation. The variation one of the parameters may irreversibly affect the entire interaction of starting reagents.
According to the thermal analysis data the salt CystaH2[CuCl4] is stable in air up to 200 °C (Figure 1). Thermogravimetric (TG) and differential thermal analysis (DTA) curves show a multistage process of decomposition. The endothermic effect at 219 °C on the DTA curve (Figure 1) is accompanied by weight loss of 11.4(1)%. It allows suppose, that the salt transforms, with cystamine breaking on S–S bond and the release of one HCl molecule gives complex [CuCl3(NH2CH2CH2S)](NH3CH2CH2S) at the first stage. At higher temperature this process is overlapped with a significant substance disintegration including sublimation of the organic component with an additional weight loss of 51.2(1)%. At 310 °C the copper dichloride CuCl2 is formed. In the range higher 300 up to 700 °C the process is described by the sloping graph of the mass loss and intense heat release. This is because of the gradual oxidation of CuCl2 by oxygen, which is accompanied by intensive heat release. Moreover, the DTA curve indicates a multi-stage oxidation process. CuCl2 turns to CuClxO y and then at 545 °C – to CuO with the weight loss 19.8%. The stages of decomposition are presented in Scheme 1.
Figure 1. TG and DTA curves for CystaH2[CuCl4] salt.
Scheme 1. Stages of CystaH2[CuCl4] decomposition in air.
The IR spectra of CystaH2[CuCl4] salt and cystamine (Figure 2) do not contain significant differences thanks to the similar chemical condition of disulfide groups. The presence of S–S bonding in the structure of cystamine was confirmed by X-ray diffraction (XRD) (Kennard, Reference Kennard1965). Observed shifts of some bands are explained the different arrangement of hydrogen bonds. Slight shift of the bands at the 2900–3200 cm−1 [ν(N–H)] and 1560–1600 cm−1, assigned to the deformation vibrations [ν(C–N)], indicates a lack of metal atom coordination by –NH3 + groups through nitrogen atoms. This is also confirmed by the presence of bands at 1447–1462 and 1578–1593 cm−1, referred, to the symmetric deformation and degenerated deformation vibrations of –NH3 + groups respectively. Reducing the length of C–S bond in CystaH2[CuCl4] leads to decreasing frequency of the stretched vibration up to 632 cm−1 compared with the frequency [ν(C–S)] of the free ligand (768 cm−1) (Bharara et al., Reference Bharara, Bui, Parkin and Atwood2005a). For lack of other bands in the region 600–700 and 500–600 cm−1 can be explained by the absence of the Cu–S or Cu–N bonds (Foye and Kaewchansilp, Reference Foye and Kaewchansilp1979). The low-frequency oscillations at 461 cm−1 are because of chemical metal–halogen bonds (Cu–Cl) (Sheludyakova & Basova, Reference Sheludyakova and Basova2002). Thus, on the basis of IR spectroscopic data it can be suggested that CystaH2 2+ cation presents in CystaH2[CuCl4] and there are no coordination of cupper atoms by CystaH2 2+.
Figure 2. (a) IR spectra of CystaH2[CuCl4]. (b) IR spectra of cystamine NH2(CH2)2SS(CH2)2NH2.
Crystal structure of CystaH2[CuCl4] was determined by the X-ray powder diffraction technique. The modeling was performed taking into account the structures of the starting molecular fragments CystaH2 2+ и [CuCl4]2− (Figure 3) followed from the synthesis and chemical analyses. It was assumed that the complex anion [CuCl4]2− has a planar square structure typical for Cu(II).
Figure 3. (Color online) [CuCl4]2− and CystaH2 2+ ions of the investigated compound CystaH2[CuCl4]
Initially, the structure solution was obtained in the space group P21/a. After structure refinement this solution provided a good agreement between the calculated and experimental X-ray diffraction powder patterns (R p = 5.04%, R wp = 7.51%, R exp = 4.81%, S = R wp/R exp = 1.56.). However, there were several unexplained reflexes about 1% of relative intensity, which were considered as impurities (Figure 4). It would be assumed that the center of symmetry in the space group P21/a connects two fragment of cysteamine type (NH3CH2CH2S–) giving cystamindi-ium ion CystaH2 2+. However, as it has been mentioned above, the main conformation of the cystamine molecule has no center of symmetry because of the specific configuration of the electronic system of (S–S) bond. This circumstance induced two hypotheses for testing. The first was that cystamindi-ium ion changed its conformation. The support for the hypothesis might be the fact of the specific interaction of cystamine with Cu(I) (Louvain et al., Reference Louvain, Mercier and Kurmoo2008). The second was that the center symmetry was absent. The modeling performed in the group P21 helped to solve the dilemma. It turned out that the reflexes forbidden in space group P21/a supplied a good explanation of “impurity” reflexes on the experimental X-ray powder pattern. This led to the unambiguous choice of space group P21 (Figure 4). Only one possible orientation of cystamindi-ium ion was applied for the refinement. It allowed perceptible improving in X-ray powder diffraction fitting. The model with disordered ions was not applied because of the level of fitting was close to the statistical limit of reliability factors. Final crystallographic data and refinement characteristics are presented in Table I, and the atomic coordinates are listed in Table II. Figure 4 shows the diffraction data for CystaH2[CuCl4], including experimental and calculated X-ray powder diffraction patterns in comparison, the difference curve and the reflex positions.
Figure 4. (Color online) – XRD patterns for CystaH2[CuCl4]: the experimental (dots) and calculated (solid line), the difference (solid line), position of calculated reflections in bottom. In the insertion, the part of the XRD pattern presented where the reflections forbidden in space group P21/a.
Table I. Crystallographic parameters and characteristics of X-ray powder diffraction structure determination for CystaH2[CuCl4].
Table II. (a) Atomic coordinates of CystaH2[CuCl4].
The crystal structure of CystaH2[CuCl4] as the projection on (bc)-plane is shown in Figure 5, the main interatomic distances and angles are given in Table III. The structure has a layered type (Figure 5). Inorganic layers are built from [CuCl4]2− anions, which alternate with organic layers from protonated cystamine CystaH2 2+ cations. The obtained crystal structure data confirm the presence of a disulfide bond S–S in the CystaH2[CuCl4] with a distance equal to 2.05(2) Å, indicating a lack of coordination CystaH2 2+ ion to Cu(II). Localization CystaH2 2+ cations between the complex anions [CuCl4]2−, as well as coordination of NH3-groups to the corresponding nucleophiles Cl− lead to the formation of hydrogen bonds. Each NH3-group forms three hydrogen bonds with two chlorine atoms belonging to the same complex [CuCl4]2−. The lengths of the hydrogen bonds are given in Table IV. Since each organic cation contains two amino groups on the opposite sides, this leads to the performing a retaining effect between the two anion layers (Figure 6). Compactness of the anionic layer is achieved by parquet package, which allows the occurrence of short contacts between the anionic copper complexes [CuCl4]2−. Chlorine atoms of neighboring anions are oriented along the axis of the square plane complex [CuCl4]2−at a distance of 2.80(1) and 3.05(2) Å completing it to a distorted octahedron, [CuCl6]2−, which is more typical for copper crystallochemistry (Figure 6). The lack of coordination between the metal and sulfur atoms was detected on the example of CystaH2[HgCl4] and described in detail in (Bharara et al., Reference Bharara, Parkin and Atwood2005b). Saving S–S bonding was observed with the coordination centers as Pb, Bi, and V atoms (Pavani et al., Reference Pavani, Upreti and Ramanan2006; Bi et al., Reference Bi, Louvain, Mercier, Luc and Sahraoui2007; Louvain et al., Reference Louvain, Bi and Mercier2007). The coordination sphere around the central atoms in such compounds is formed by oxygen or halogen atoms. Some regularity in these structures can be formulated. The formation of anionic and cationic blocks and their spatial segregation are occurred because of the possibility of additional coordination. In CystaH2[CuCl4], anionic tetrachloride copper complexes form the layers, in which the copper plane coordination is completed to octahedral by chlorine atoms from neighboring complexes. Similar complexes may be localized on the symmetric positions (on symmetry elements), which is the characteristic for the compounds of Cu and Hg, or independently of the cell volume for Pb and V (Pavani et al., Reference Pavani, Upreti and Ramanan2006; Louvain et al., Reference Louvain, Bi and Mercier2007). Secondly, there is an additional energy gain because of the formation of hydrogen bonds, leading to the stabilization of a particular structural adjustment. Note that in most of the known compounds wherein there is no coordination between the sulfur and metal atoms, there are twice as much hydrogen bonding between counterions, than in the presence of such coordination. In most cases in compounds with coordination of the metal atom to sulfur, two sulfur atoms involved into the coordination. It leads to the formation of stable organometallic five- or six-membered rings. Typically, these fragments are formed in the less symmetric structures, when the additional coordination causes increased stabilization of the compound.
Figure 5. (Color online) The arrangement of the molecules in the unit cell of CystaH2[CuCl4] as the projection on the bc plane.
Figure 6. (Color online) The hydrogen bonding (dotted red lines) and the short contacts (dotted green lines) in the unit cell of CystaH2[CuCl4] in the projection to the (ab)-plane.
Table III. The most important interatomic distances and angles for CystaH2[CuCl4].
Table IV. The lengths of hydrogen bonds A–H···B in the structure of [CuCl4][(NH3CH2CH2S−)2].
In conclusion, the interaction of copper(II) dichloride with cystamindi-ium dihydrochloride investigated in the strong acid medium. The reaction results in the formation of the complex compound [NH3(CH2)2SS(CH2)2NH3][CuCl4], which crystallizes in the form of fine polycrystalline powder. According to IR spectroscopy and X-ray powder diffraction analyses, a disulfide bond (S–S) presents in the compound. Complex salt [NH3(CH2)2SS(CH2)2NH3][CuCl4] is stable in air up to 200 °C, and then decomposes in several steps under heating with changing the coordination sphere of copper. The compound has ionic structure with packing of the [CuCl4]2− and CystaH2 2+ ions in the form of layers in the cell. Additional retention between cationic and anionic layers occurs through the formation of hydrogen bonding between the NH3-groups and chlorine atoms of the [CuCl4]2− complex. An additional stabilization factor is the parquet such as anion parking, which provides additional coordination for copper atoms transforming square-planar to a distorted octahedral coordination.
ACKNOWLEDGEMENTS
The authors are grateful to the reviewer for valuable comments, which allowed significantly improve the results. The study was conducted according to the SFU state assignment of RF Ministry of Science and Education in 2014, with financial support ICDD (Grant-in-Aid #10-93).
SUPPLEMENTARY MATERIAL
The supplementary material for this article can be found at http://www.journals.cambridge.org/PDJ
