Binding of Gold(III) from Solutions by the [Ag6{S2CN(CH2)6}6] Cluster: Synthesis, Thermal Behavior, and Self-Organization of the Supramolecular Structure of the Double Complex [Au{S2CN(CH2)6}2]2[AgCl2]Cl·2CHCl3 (Role of Secondary Au⋅⋅⋅Cl, Ag⋅⋅⋅S, and Cl⋅⋅⋅Cl Interactions)

The capability of silver(I) cyclo-hexamethylenedithiocarbamate to concentrate gold(III) from solutions characterized by a high level of salinity (5.15 M NaCl) into the solid phase has been established. The double chloroform-solvated Au(III)–Ag(I) complex [Au{S2CN(CH2)6}2]2[AgCl2]Cl·2CHCl3 (I) was preparatively isolated as an individual form of binding of [AuCl4]– anions. The composition of the ionic structural units of compound I indicates that gold(III) binding from a solution to the solid phase is accompanied by the complete redistribution of the HmDtc ligands between the coordination spheres of Ag(I) and Au(III). Complex I characterized by IR spectroscopy, simultaneous thermal analysis, and X-ray structure analysis (CIF file CCDC no. 2051654) exhibits the supramolecular structure containing two oppositely charged pseudo-polymeric subsystems. Complex cations [Au{S2CN(CH2)6}2]+ and anions [AgCl2]– (in a ratio of 2 : 1) form a complicatedly organized cation-anionic pseudo-polymeric ribbon ({[Au(HmDtc)2]⋅⋅⋅[AgCl2]⋅⋅⋅[Au(HmDtc)2]}+)n due to secondary interactions Ag⋅⋅⋅S (3.2613 Å) and Au⋅⋅⋅Cl (3.2765 Å). The pseudo-polymeric ribbon consists of two rows of cations and a row of anions. The outer-sphere chloride ions combine the solvate chloroform molecules by two equivalent hydrogen bonds Cl⋅⋅⋅H–C yielding anion-molecular triads [Cl3CH⋅⋅⋅Cl⋅⋅⋅HCCl3]–, which are involved in the formation of the supramolecular ribbon due to the secondary Cl⋅⋅⋅Cl interactions (3.4058 Å) between the nonequivalent chlorine atoms of the nearest solvate molecules. The study of the thermal behavior of complex I makes it possible to determine the character of thermolysis and conditions for the quantitative regeneration of bound gold.


INTRODUCTION
Silver(I) dithiocarbamate (Dtc) complexes are convenient precursors for the preparation of both acanthite (α-Ag 2 S) films and nanopowders [1][2][3][4] and Janus nanoparticles Ag 2 S/Ag 0 (including a combination of the metal and metal sulfide) [5,6], which are of practical interest for microelectronics. Since Ag 2 S is characterized by low toxicity and high stability in biological media, it is promising to use it in medicine for manufacturing biocompatible nanoprobes [7,8]. It is noteworthy that a low toxicity of silver against cells of mammals compared to other metals is combined in a unique manner with a high toxicity against microor-ganisms [9]. In addition, we have previously found the capability of silver(I) dialkyldithiocarbamates to bind gold(III) from solutions into the solid phase, forming pseudo-polymeric heterometallic Au(III)-Ag(I) compounds of the ionic type with the complicatedly organized supramolecular structures [10,11]. Compounds of this type can be of interest as precursors of gold nanoparticles and thin films, which are promising for practical use in diverse areas [12][13][14][15], and of Janus particles of the Ag 2 S/Au 0 type [16].
Continuing the above research, in this work we studied the reaction of the hexanuclear silver(I) cyclohexamethylenedithiocarbamate (HmDtc) cluster, whose structure was determined earlier [17], with AuCl 3 in a 5.15 M solution of NaCl. Bis(N,N-cyclohexamethylenedithiocarbamato-S,S ')gold (III) dichloroargentate chloride solvated by chloroform of the composition [Au{S 2 CN(CH 2 ) 6 } 2 ] 2 [AgCl 2 ]Cl·2CHCl 3 (I) was preparatively isolated from the reaction products as the form of binding gold(III). The structural organization, thermal behavior, and spectral characteristics of compound I were studied by single-crystal X-ray diffraction (XRD) analysis, simultaneous thermal analysis (STA), and IR spectroscopy. EXPERIMENTAL Sodium N,N-cyclo-hexamethylenedithiocarbamate Na{S 2 CN(CH 2 ) 6 }·2H 2 O was synthesized by the reaction of hexamethyleneimine (Aldrich) with carbon sulfide (Merck) in an alkaline medium [18]. The initial silver(I) complex [Ag 6 {S 2 CN(CH 2 ) 6 } 6 ] was synthesized using a described procedure [17].
Synthesis of complex I was carried out using the reaction of freshly precipitated [Ag 6 {S 2 CN(CH 2 ) 6 } 6 ] with AuCl 3 in a 5.15 M solution of NaCl (which is close to the saturation conditions at 20°С). Taking into account the determined composition and structure of complex I, we can present the heterogeneous reaction of gold(III) binding from a solution to the solid phase with the formation of intermediate substance Ia as follows: A solution (10 mL) of Na[AuCl 4 ] containing gold(III) (34.9 mg, 0.177 mmol) was poured to freshly precipitated [Ag 6 {S 2 CN(CH 2 ) 6 } 6 ] (100 mg, 0.059 mmol), and the mixture was stirred at 60°С for 1 h. The greenish-yellow color of the precipitate changed rapidly to red-orange during the first minutes of its interaction with the solution, and further the precipitate turned saturated yellow-orange with the simultaneous decoloring of the solution of gold(III). The degree of binding of gold from solutions to the solid phase was 99.89%, indicating the formation of new compounds in the solid phase. (The residual content of gold in the solution after extraction was determined on an atomic absorption spectrometer Hitachi, model 180−50, class 1). The obtained precipitate of compound Ia was filtered off, washed with water, and dried on a filter. The single crystals of compound I suitable for XRD were obtained by the dissolution of the precipitate in an acetone-chloroform (1 : 1) mixture followed by the slow evaporation of the solvents at room temperature. The IR spectra of the synthesized substances pressed in KBr pellets were recorded at room temperature on an FSM-1201 interferential FT-IR spectrometer in a range of 400-4000 cm -1 (FSpec software, version 4.0.0.2 for Windows, OOO Monitoring, Russia).
XRD was carried out for a prismatic single crystal of compound I on a Bruker-Nonius X8 Apex CCD diffractometer (MoK α radiation, λ = 0.71073 Å, graphite monochromator) at 150(2) K. The data were collected using a standard procedure: ϕ and ω scan modes of narrow frames. An absorption correction was applied empirically using the SADABS program [19]. The structure was determined by a direct method and refined by least squares (for F 2 ) in the full-matrix anisotropic approximation of non-hydrogen atoms using the SHELXL-2018/3 program [20]. The positions of the hydrogen atoms in the HmDtc ligands and solvate CHCl 3 molecules were geometrically calculated and included into refinement by the riding model. The main crystallographic data and structure refinement results for compound I are presented in Table 1. Selected bond lengths and angles are given in Table 2. The geometric parameters of the C-H⋅⋅⋅Cl hydrogen bonds and Cl⋅⋅⋅Cl secondary interactions are listed in Table 3.
The thermal behavior of compound I was studied by the STA method including the parallel detection of thermogravimetry (TG) and differential scanning calorimetry (DSC) curves. The study was carried out on RESULTS AND DISCUSSION The strong interaction of vibrations of the C-H, C-C, and C-N bonds in the HmDtc ligands predetermines a complicated IR spectral pattern of complex I and substance Ia. Therefore, when analyzing the spectrum, we additionally used the IR spectral data for the initial [Ag 6 [23], and chloroform СНCl 3 . 1 The IR spectra of compounds I/Ia exhibit absorption bands at 2926/2920 and 2844/ 2849 cm -1 due to stretching vibrations ν as (СН 2 ) and ν s (СН 2 ), respectively. The medium-intensity absorption band at 1459/1473 cm -1 corresponds to the bending (scissor) vibrations δ s of the -СН 2 -groups. The high-intensity band at 1162/1160 cm -1 corresponds to the stretching vibrations of the N-CH 2 bonds. The close positions of the corresponding absorption bands in the IR spectra of sodium hexamethylenedithiocar- RUSSIAN (4) Au(1)-S(2) 2.3333(10) N(2)-C (14) 1.482 (4) Au (1) (5) Au(1)-S(4) 2.3442(10) C(3)-C(4) 1.522 (6) Au (1)···Cl (1) 3.2765(9) C(4)-C(5) 1.530(5) (3) Au (1) [17]) and Na-{S 2 CN(CH 2 ) 6 }·2H 2 O (1485 cm -1 [21]). Thus, all absorption bands discussed lie between the frequency ranges of the ordinary ν(C-N) (1350-1250 cm -1 ) and double ν(C=N) (1690-1640 cm -1 ) bonds, indicating a substantial contribution of double bonding to the formally ordinary N-C(S)S bonds [25]. However, according to the data obtained, much shorter N-C(S)S bonds than those in the initial silver(I) complex (1.335 Å [17]) are expected for complex I. The presence of the solvate chloroform molecules in the structure of complex I is indicated by absorptions observed in two ranges of the IR spectrum (∼3000 and ∼800-600 cm -1 ): (a) the band at 2885 cm -1 corresponding to the ν(С-Н) stretching vibrations (it should be mentioned that the usually relatively weak absorption band of the methine group in the spectrum of compound I exhibits a high intensity, which is caused by a significant polarity of the С-Н bond and the presence of a dipole moment in the CHCl 3 molecule) and (b) two (1 : 1) closely lying high-intensity ν as (C-Cl) bands at 750 and 743 cm -1 and the medium-intensity ν s (C-Cl) band at 657 cm -1 .
In the complex gold(III) cation, the coordination of two nonequivalent HmDtc ligands, close to the S,S '-isobidentate mode (the Au-S bond lengths range from 2.3333 to 2.3465 Å), results in the formation of the bicyclic system [CS 2 AuS 2 C] containing two four-membered metallocycles with the common metal atom. However, the Au-S-S-C and S-Au-C-S torsion angles (Table 2) indicate that the atoms are nearly coplanar in one of the discussed [AuS 2 C] metallocycles, whereas the second cycle is characterized by some tetrahedral distortion due to which the diagonal S(1)-Au-S(4) angle somewhat deviates from 180°. The geometry of the [AuS 4 ] chromophore is close to the planar-tetragonal one, indicating the inner-orbital dsp 2 -hybrid state of the gold atom. As expected from the IR spectral data, the N-C(S)S bonds in complex I (1.294 and 1.306 Å) are significantly shorter than those in the initial silver(I) cluster [17]. The S 2 C-NC 2 structural fragments in the HmDtc ligands are nearly planar, which is indicated by the values of the corresponding torsion angles close to 0° or 180° (Table 2). A specific feature of the [Au{S 2 CN(CH 2 ) 6 } 2 ] + cation (Fig. 2а) is the spatial localization of the seven-membered cyclic fragments -N(CH 2 ) 6 at one side from the medium plane of the central bicyclic fragment (cis orientation). In spite of differences in the geometric characteristics of the -N(CH 2 ) 6 seven-membered heterocyclic fragments of the nonequivalent HmDtc ligands (see the corresponding torsion angles in Table 2), both can be approximated by the "twist-chair" conformations [26][27][28].
Cl⋅⋅⋅Cl occur between the adjacent triads, the Cl (3) and Cl(4) atoms of each CHCl 3 molecule are involved in the Cl⋅⋅⋅Cl interactions (Fig. 5, Table 3), and the Cl(4)⋅⋅⋅Cl(3) a , Cl(4) b ⋅⋅⋅Cl(3) c distance is 3.4058 Å, which is shorter by 0.1 Å than the doubled van der Waals radius of the chlorine atom (3.50 Å [30]). A set of the discussed interactions results in the formation of the second, anion-molecular pseudo-polymeric ribbon of the ([Cl 3 C-H⋅⋅⋅Cl⋅⋅⋅H-CCl 3 ] -) n composition. Thus, the supramolecular structure of compound I includes two oppositely charged pseudo-polymeric subsystems. Two main types of secondary interhalogen interactions differed in the geometry of the corresponding structural fragment are considered [31][32][33]. The first type is characterized by an approximate equality of the RUSSIAN   The interhalogen interactions involving pairs of different halogen atoms (heterointeraction) should also be mentioned [34,35].
The thermal behavior of complex I was studied by the STA method in an argon atmosphere with the simultaneous detection of the TG and DSC curves (Fig. 6). The compound is thermally stable up tõ 89°С. The TG curve shows stepwise mass loss indicating that the thermal destruction process is multistage (Fig. 6a). The first step (~89-144°С) of a 15.45% mass loss corresponds to the complete desolvation of complex I (calcd. 15.46%). The inflection point at 118°С divides the discussed step into two unequal regions (slightly sloping and steeply descending), indicating a jump in an increase in the mass loss rate, which can be due to the melting of the sample. In the discussed temperature range, the DSC curve (Fig. 6b) includes two endoeffects with extremes at 109.2 and 129.7°С. The first extreme was attributed to sample melting (the extrapolated m.p. 98.0°С), and the second extreme is caused by the liberation of the solvate CHCl 3 molecules (extrapolated temperature of the process 122.3°С).
The region of the TG curve in a range of ~144-172°С is characterized by the stabilization of the sample weight followed by reaching the steeply descending thermolysis stage of the nonsolvated form of the com-plex (~172-311°С). The endoeffect at 194.2°С corresponds to the very beginning of the new step of mass loss on the DSC curve and was assigned to melting of the nonsolvated form of complex I (the extrapolated m.p. 190.0°С; for the independent determination in a capillary, melting was established in the range of 190.0-192.0°С). The total mass loss in the discussed region of the TG curve with an inflection point at 236.0°С is 42.86%, indicating a complicated character of thermolysis of the studied compound: simultaneously at the cationic and anionic moieties, which is accompanied by the reduction of gold to the elemental state and formation of AgCl (calcd. 47.45%). The corresponding fragment of the DSC curve exhibits two endoeffects at 216.5 and 263.1°С caused by the sequential thermal transformations of the substance (extrapolated temperatures of the processes are 207.8 and 261.8°С).
The final region of the TG curve (~311-1000°С) shows the smooth mass loss (9.31%) due to the desorption of volatile thermolysis products (7.01%) and reduction of elemental silver from AgCl (2.30%). At the end of thermolysis at 1100°С, the residual weight equal to 32.36% is well consistent with the calculated value for reduced gold and silver (32.50%). When the crucible was opened, fine light yellow balls representing a gold-silver alloy were observed on the bottom (Fig. 6c). The endoeffect with an extreme at 1043.9°С caused by the melting of the reduced metals (the extrapolated m.p. 1040.2°С) is detected in the high-temperature range of the DSC curve. The last value falls on the range between the melting points of gold and silver (1064.18 and 961.78°С, respectively [36]). An analysis of the state diagram of the Au-Ag binary metallic system [37] makes it possible to conclude more certainly that the above presented extrap- Cl (2) Cl (4) Cl (3) olated m.p. corresponds to an alloy with the atomic ratio of the components Au : Ag = 2 : 1.
Thus, the efficient capability of binding gold(III) to the solid phase from solutions with a high level of salinity (5.15 M solution of NaCl) was found for hexanuclear silver(I) cyclo-hexamethylenedithiocarbamate. The double Au(III)-Ag(I) solvated complex [Au{S 2 CN(CH 2 ) 6 } 2 ] 2 [AgCl 2 ]Cl·2CHCl 3 (I) was crystallized from an acetone-chloroform (1 : 1) solution of the heterogeneous reaction products as a form of binding [AuCl 4 ] -. According to the XRD data, the synthesized compound is characterized by the complicated supramolecular structure containing two oppositely charged pseudo-polymeric subsystems ({[Au(HmDtc) 2 ] 2 [AgCl 2 ]} + ) n and ([Cl 3 C-H⋅⋅⋅Cl⋅⋅⋅H-CCl 3 ] -) n . The secondary interactions (Ag⋅⋅⋅S, Au⋅⋅⋅Cl, and Cl⋅⋅⋅Cl) and hydrogen bonds C-H⋅⋅⋅Cl play the determining role in the formation of these subsystems. The study of the thermal behavior of complex I made it possible to establish the character of thermolysis and conditions for the quantitative regeneration of the metals.

ACKNOWLEDGMENTS
Elemental analysis was carried out on a Carlo Erba EA 1108 elemental analyzer at the Center for Collective Use "Physical Methods of Investigation" of Kurnakov Institute