NEUTRAL COMPLEXES OF SILVER HALIDES AND
TETRAMETHYLTHIURAM MONOSULFIDE

Luis I. Victoriano

Facultad de Ciencias Químicas, Universidad de Concepción, P.O. Box 160-C, Concepcion,
Chile. lvictori@udec.cl
(Received: May 24, 2000 - Accepted: July 31, 2000)

SUMMARY

The preparation and characterization of 1:1 adducts of silver halides AgX (X=Cl, Br, I) with N,N,N’,N’-Tetramethylthiuram Monosulfide is reported. The products are characterized by elemental analyses, magnetic measurements and IR-Raman spectroscopy. The data are interpreted in terms of a halogen-dibridged structure of D_2h symmetry.

KEYWORDS: Silver, Sulfur ligands, Coordination compounds.

RESUMEN

Se informa sobre la preparación y caracterización de aductos 1:1 entre haluros de plata AgX (X=Cl, Br, I) y N,N,N’,N’-Tetrametiltiurano Monosulfuro. Los productos se caracterizan mediante análisis elemental, medidas magnéticas y espectroscopía IR-Raman. Los datos se interpretan en base a estructuras con dos halógenos puente, de simetría D_2h.

PALABRAS CLAVES: Plata, Ligandos azufrados, Compuestos de coordinación.

INTRODUCTION

N,N,N’,N’-Tetraalkylthiuram disulfides are the semi-esters of N,N-dialkyldithiocarbamate anions. Previous publications have offered a general background as to the most important applications of these molecules.⁽¹⁾ These species are related by the two-electron redox process:

R₂NC(S)S-SC(S)NR₂ + 2 e- = 2 R₂NC(S)S^-

The chemistry of the ligands above and varios copper(0), (I) and (II) species has proved rich and occasionally unexpected. Thus the disulfides and copper metal afford Cu(II) dithiocarbamates, while copper(I) or (II) halides yield respectively Cu(II) or (III) dithiocarbamates. The monosulfides, available through S-abstraction from the disulfides may lead to Cu(I) adducts of considerable structural variety in the solid state.^{(2, 3, 4, 5)} A logic extension of these studies includes silver(O) and (I) metal species.

Based on the reactivity and stability of its coordination complexes, the silver(I) ion has been classified as a class B or "soft" acid, where P- and S-donors are preferred over N- and P- congeners. However, many systems involving silver(I) and S-donors feature cluster or oligomeric arrays of considerable complexity.⁽⁶⁾ With a view to extending the list of possible structural motifs so far known, the study of the coordination chemistry of silver halides with thiuram sulfides has now been undertaken.

The coordinating properties of silver halides have been only scarcely studied. In this manner, complexes with sulfur donors have been reported and unambiguously characterized for only AgL₂Cl systems (L=Thiourea⁽⁷⁾, Dimethylformamide⁽⁸⁾). Our hopes of expanding this meager series by structurally characterizing a number of thiuram monosulfide complexes with silver halides have been frustrated by the solubility properties of the materials isolated.

EXPERIMENTAL

General Preparative Methods, Spectroscopy and Analyses: The ligand was purchased from Fluka and purified as described previously⁽⁹⁾. Silver halides were obtained from aqueous solutions of silver nitrate and sodium halides in the appropriate mole ratios. Solvents were used with no previous purification. Microanalytical services were available at the University of Concepción. IR and Raman spectra were obtained by techniques already described⁽¹⁰⁾.

Preparation of the Complexes: The 1:1 adducts of silver halides and the ligand were prepared by a general method which consisted of dissolving 2.0 mmol. (0.416 g) of the ligand in 5.0 mL of acetonitrile and adding 1.0 mmol. of the halide with continuous stirring. After 6 to 18 hours of reaction, depending on the grain size of the halide, the white-gray appearance of the halide had changed to a dull (X=Cl) to lemon-yellow (X=I) and the amorphous solids were filtered off, washed with fresh acetonitrile (3 mL) followed by diethyl ether (5 mL), dried by suction and then pumped off. Yields were essentially quantitative, indicating that little complex was left in solution. Further purification was impossible, due to the insolubility of the products. Spectroscopic characterization was therefore carried out on these crude materials.

Chlorobis(N,N-dimethylthiocarbamoyl)sulfidosilver(I)
AgMe₄tmsCl: pale yellow solid, m.p: 128ºC, Anal:Calcd. for C₆H₁₂N₂S₃AgCl (%) C, 20.5; H, 3.4; Cl, 10.1. Found: C, 20.3; H, 3.2; Cl, 10.3. IR data are reported in Tables 1 and 2

Bromobis(N,N-dimethylthiocarbamoyl)sulfidosilver(I)
AgMe₄tmsBr: lemon yellow solid, m.p: 148ºC, Anal:Calcd. for C₆H₁₂N₂S₃AgBr (%) C, 18.2; H, 3.1; Br, 20.2. Found: C, 18.3; H, 3.2; Br, 20.6. For IR data seeTables 1 and 2

Iodobis(N,N-dimethylthiocarbamoyl)sulfidosilver(I)
AgMe₄tmsI: bright yellow solid, m.p: 153ºC, Anal:Calcd. for C_6/H₁₂N₂S₃AgI (%) C, 16.3; H, 2.7; I, 28.6.. Found: C, 16.0; H, 2.4; I, 28.8. See Tables 1 and 2 for IR data

RESULTS AND DISCUSSION

Preparative: Solutions of tetramethylthiuram monosulfide in acetonitrile react with silver halides over a period of several hours at room temperature to yield the insoluble 1:1 yellow adducts. Spectroscopic evidence as discussed below, indicate that these are true complexes and not mixtures of the starting materials. The adducts are insoluble in all common organic solvents with the exception of pyridine and dimethylsulfoxide. Presumably drastic changes which involve the displacement of thiuram monosulfide occur in these solvents. All three halide complexes (Cl, Br, I) exhibit a 1:1-electrolyte behavior when they are disssolved in the manner just described. Attempts to determine molecular weights by osmometry in the case of the chloro-derivative are also consistent with a pattern of dissociation and ligand displacement similar to:

AgLX + n S = AgS_n⁺ + X- + L

No efforts have been made to ascertain further the nature of the materials in these solutions.

When the preparative work just described was attempted using thiuram monosulfides bearing the higher homologues in the alkyl series (Et, n-Pr, i-Pr), no visible reaction was apparent at room temperature over time periods of several days. This observation is surprising, considering the prolific coordination chemistry found in the case of copper(I) halides, but may well be related to a higher lattice energy related to the silver salts, as reflected by the values of their solubility products in aqueous solution.

Solid state characterization: Circumstantial evidence in favor of adduct formulations comes from the observation that the materials isolated through the reaction described above do not show the photosensitivity commonly associated with silver halides. All complexes in the series prepared are diamagnetic and thus consistent with a closed shell electron configuration involving silver(I) species. The remainder of the structural characterization is based on IR and Raman (Ra) spectroscopy.

Table 1 shows selected IR and Ra bands observed for the free ligand and its silver complexes in the region 4000- 600 cm^-1. The features observed correspond to vibrations of the ligand modified by coordination to the metal. The small but distinct shift in many of these bands with a change in the nature of the halogen supports the proposal that the complexes under discussion cannot be ionic in the solid.

The vibrational spectra of the ligand is normally interpreted on the basis of three distinct modes⁽¹¹⁾: i) The band located near 1500 cm^-1 (thiureide band) due to the C-N stretching mode which has an important double bond character. This band also has important contributions from S-C=S bending modes. ii) The band near 1000 cm^-1 attributed to a C=S stretch and iii) a band in the 880-840 cm-1 region, which is related to the C-S stretching mode associated with the thioether-like linkage.

Band i) is shifted some 10 to 20 cm^-1 in the complexes with respect to the free ligand. This shift is in the same order of magnitude as the one found in the copper(I) complexes⁽¹⁰⁾ and is absolutely in agreement with complexation of the ligand to the metal centers. As pointed out above, the increase in the magnitude of the shift in progressing through the series I>Br>Cl follows the electron-withdrawing power of the ancillary ligand and hints at a non-ionic structure, were minor shifts would have been observed.

Free ligand as well as complexes show bands in the region 990-1010 cm^-1 and these can be assigned to C=S stretching modes. The doublet aspect of this absorption defies explanation so far, as a single band should be expected⁽¹²). A doublet is also observed in the region 840-860 cm^-1, which can be assigned to a C-S stretch.

Table 2 summarizes the spectroscopic data below 500 cm^-1 for the complexes under study. Assignments are based on previous work⁽¹⁰⁾. The silver-halogen stretching bands appear at frequencies which are somewhat lower than typical terminal values and are believed to arise from bridged structures. A working model is thus considered, based on a halogen-dibridged dimeric formulation of D_2h symmetry, incorporating two tetrahedral Ag centers.

Two Raman-active (A_g+B_1g) and two IR-active (B_2u + B_3u) Ag-S stretching bands, as well as two Raman-active (A_g + B_2g) in addition to two IR-active (B_1u + B_3u) Ag-X modes are expected according to this model. The data in Table 2 is in good agreement with the structure proposed. However, an alternative structure based on oligomeric chains of halogen-bridged ...Ag(S₂)X- units is also consistent with the vibrational spectra and cannot be summarily discarded. It is interesting to contrast the present results with those obtained for the analogous copper(I) complexes. For the latter compounds, a similar halogen-dibridged structure was anticipated on the basis of vibrational spectroscopy for the Me-substituted derivatives, while the evidence for the materials in the Et series pointed at monomeric tricoordinate formulations⁽¹⁰⁾. Single crystal x-ray diffraction studies fully confirmed these expectations^{(3, 4ª)}.

ACKNOWLEDGEMENTS

Support from Universidad de Concepción (DIUC 9721081) and CONICYT (FONDECYT 1990494) is gratefully acknowledged.

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