Synthesis and crystal structure of chalcogenide cluster compound

ARTICLE

Synthesis and crystal structure of chalcogenide cluster compound

Bárbara Tirloni^I;Davi F. Back^I; Robert A. Burrow^I; Gelson N. M. de Oliveira^I; Marcos A. Villetti^II; Ernesto S. Lang^*,I

^IDepartamento de Química, Universidade Federal de Santa Maria, 97105-900, Santa Maria-RS, Brazil

^IIDepartamento de Física, Universidade Federal de Santa Maria, 97105-900, Santa Maria-RS, Brazil

ABSTRACT

Three new cluster compounds were synthesized from Hg(EPh)₂ (E = Se, Te; Ph = phenyl) in organic solvents. Two of these compounds, [Hg₂Cl₂(SePh)₂(PCy₃)₂], (1), and [Hg₂Br₂(SePh)₂(PCy₃)₂], (2), were prepared by reaction of Hg(SePh)₂, HgX₂ (X = Cl, Br) and tricyclohexylphosphine, PCy₃, in dimethylformamide. The reaction of Hg(TePh)₂ with HgBr₂ in tetrahydrofuran using triphenylphosphine or 2,2’-bipyridine as co-ligands gave the polymeric cluster [{Hg₅Br₃(TePh)₇}_n] (3), whose dissolution in dimethylsulfoxide yielded the cluster [Hg₃Br₃(TePh)₃]·2dmso. The influence of different ligands, coordinating solvents and reaction stoichiometries on the formation of the title compounds is also discussed. All complexes were characterized by elemental analysis, thermogravimetric analysis and single crystal X-ray diffractometry.

Keywords: mercury(II) complexes, selenium, tellurium, clusters

RESUMO

Três novos compostos foram preparados a partir de Hg(EPh)₂ (E = Se, Te; Ph = fenila) em solventes orgânicos. Dois destes compostos são clusters moleculares, [Hg₂Cl₂(SePh)₂(PCy₃)₂], (1), e [Hg₂Br₂(SePh)₂(PCy₃)₂], (2), obtidos pela reação de Hg(SePh)₂ com HgX₂ (X = Cl, Br) e tricicloexilfosfina, PCy₃, em dimetilformamida. A reação de Hg(TePh)₂ e HgBr₂ em tetrahidrofurano usando trifenilfosfina ou 2,2’-bipiridina como co-ligantes produz o cluster polimérico [{Hg₅Br₃(TePh)₇}_n], (3), cuja dissolução em dimetilsulfóxido leva à formação do cluster [Hg₃Br₃(TePh)₃]·2dmso. Neste trabalho avalia-se a influência de diferentes ligantes, solventes coordenantes e estequiometrias de reações na formação dos produtos. Todos os compostos foram caracterizados por análise elementar, análise termogravimétrica e tiveram suas estruturas cristalinas determinadas por difratometria de raios X em monocristais.

Introduction

The search for new metalchalcogen-containing building blocks is of great interest for many reasons, principally because the controllable size of the blocks produces tunable optoeletronic properties of the bulk products,^1,2 but also due to other new materials and biological applications.³ This emerging field – super lattices of crystalline chalcogenides– links two traditional, but distinct, areas of research: chalcogenide clusters and porous materials.^4-8

Chalcogenide cluster compounds with mercury are still quite rare, even though these materials possess interesting properties as bulk- or nano-materials. The alloy Cd_1-xHg_xTe, for example, is a well known material in long-wavelength IR detector technologies, and HgTe nanoparticles have been proposed as potential amplifiers in telecommunication devices.^9-12

Some authors have proposed alternative ways to increase the research on metal-chalcogenide clusters. Steigerwald and co-workers^13-16 have demonstrated the utility of trialkylphosphine tellurides (R₃P=Te) as a source of soluble "tellurium(0)" to form numerous metal-telluride polynuclear complexes. Corrigan¹⁷ and Fenske^18-20 have introduced condensation reactions of phosphine-stabilized copper-tellurolate clusters for the generation of higher nuclearity complexes,¹⁷ as well as the use of arylselenolate ligands to stabilize copper-selenide and silver-selenide clusters.^18-20 Mercury selenide and telluride cluster compounds and their conversion have been studied recently by Eichhöfer and Deglmann¹² and previously by other authors,^13-21 in order to extend optical investigation on IIb-VI cluster molecules.

Given our interest in the organochalcogenide chemistry, we have investigated the reactions of M(EPh)₂ with MX₂ (M = Hg, Cd; E = Se, Te; X = Cl, Br, I) in coordinating solvents.^22,23 We have already reported results about the influence of ligands, coordinating solvents and reaction stoichiometry on the size and stereochemistry of different cluster compounds.^24-30 In a recent communication we described the reactions of Hg(TePh)₂ with M'-salts (M' = Ag^I, Co^II, Ni^II), stabilized by phosphines (PPh₃ or P(CH₃)₂Ph) and pyridine.^31-33 These papers discuss new synthetic routes to achieve either binary as ternary cluster compounds.

Finally, we have found out that a critical step to develop new clusters under controlled conditions is the adequate understanding of the processes which lead to the stabilization and isolation of species like PhEHgX (E = Se, Te; X = Cl, Br). In order to demonstrate these procedures and to extend the applicability of this methodology to further examples, we describe the preparation and the structural characterization of the two cluster molecules [Hg₂Cl₂(SePh)₂(PCy₃)₂], (1), and [Hg₂Br₂(SePh)₂(PCy₃)₂], (2), as well as of the polymeric cluster [{Hg₅Br₃(TePh)₇}_n] (3).

Experimental

Materials and measurements

HgCl₂ and HgBr₂ and other analytical grade reagents and solvents were obtained commercially (Aldrich or Sigma) and used without further purification. Elemental analyses (CHN) were attained with a Perkin-Elmer 2400 analyzer. A Bruker CCD X8 APEX II diffractometer was used for the X-ray structure analyses. The equipment was operated using graphite monochromator and Mo-K_α radiation (λ = 0.71073 Å).

Crystal structure determinations

Colorless crystals of the compounds 1, 2 and 3 were obtained from their solutions at room temperature. The structures were solved with SHELXS by direct methods.³⁴ All non-hydrogen atoms were refined with anisotropic displacement parameters with SHELXL.³⁴ Hydrogen atoms were placed at their theoretical ideal positions. More detailed information about the structure determinations is given in Table 1.

Thermogravimetric analyses

Thermogravimetric measurements were carried out on a Shimadzu DTG-60 thermogravimetric analyzer. The experiments were performed in the temperature range from 25 to 700 ºC with a heating rate of 10 ºC min^-1 for each sample. The average sample size was 10 mg and the nitrogen flow-rate was 50 mL min^-1.

Preparation of [Hg₂Cl₂(SePh)₂(PCy₃)₂], (1)

To a solution of 0.051 g (0.1 mmol) of Hg(SePh)₂ in 5 mL of dmf (dimethylformamide), 0.027g (0.1 mmol) of HgCl₂ was added and the mixture was stirred for 10 min. Tricyclohexylphosphine (0.056g, 0.2 mmol) was also added and after 4 h stirring under Ar atmosphere, a light yellow solution was obtained. After 5 days, colorless crystals suitable for X-ray analysis had formed. Yield: 0.106g, 79% based on Hg(SePh)₂. Colorless crystalline substance. Elemental analysis for compound 1 (Found: C, 42.79; H, 5.59. Calc. for C₄₈H₇₆Cl₂Hg₂P₂Se₂: C, 42.86; H, 5.70%).

Preparation of [Hg₂Br₂(SePh)₂(PCy₃)₂], (2)

To a solution of 0.051g (0.1 mmol) of Hg(SePh)₂ in 5 mL of dmf, 0.036g (0.1 mmol) of HgBr₂ was added and the mixture was stirred for 10 min. Thereafter 0.056 g (0.2 mmol) of tricyclohexylphosphine was also added and, after 4 h stirring under Ar atmosphere, a light yellow solution was obtained. After 7 days colorless crystals suitable for X-ray analysis were formed. Yield: 0.117g, 82% based on Hg(SePh)₂. Colorless crystalline substance. Elemental analysis for compound 2 (Found: C, 40.27; H, 5.16. Calc. for C₄₈H₇₆Br₂Hg₂P₂Se₂: C, 40.20; H, 5.34%).

Preparation of [{Hg₅Br₃(TePh)₇}_n], (3)

A mixture of 0.213 g (0.35 mmol) of Hg(TePh)₂, 0.054 g (0.15 mmol) of HgBr₂ and 0.078 g (0.3 mmol) of PPh₃ or 2,2’-bipyridine (0.032 g, 0.2 mmol), was suspended in 15 mL tetrahydrofuran and stirred for 1 h at 65 ºC. During the reaction the yellow suspension turned orange. The solid was removed by filtration, and after 7 days the solution yielded colorless crystals suitable for X-ray analysis. Yield: 0.109 g, 41% based on Hg(TePh)₂. Colorless crystalline substance. Elemental analysis for compound 3 (Found: C, 19.02; H, 1.35. Calc. for C₄₂H₃₅Br₃Hg₅Te₇: C, 18.87; H, 1.31%).

Results and Discussion

Syntheses of the compounds

The reaction of Hg(EPh)₂ (E = Se, Te) with HgX₂ (X = Cl, Br) and PR₃ (R = alkyl, aryl) in coordinating solvents leads, according to Scheme 1, to the formation of [Hg₂Cl₂(SePh)₂(PCy₃)₂], (1), [Hg₂Br₂(SePh)₂(PCy₃)₂], (2), and [{Hg₅Br₃(TePh)₇}_n], (3). Experimental evidence based on our previous results (see compounds 4-17 in Scheme 1) allows us to propose that Hg(EPh)₂ exists according to the redistribution equilibrium shown in equation 1.^24-30

For the syntheses of 1 and 2 through one-pot procedures, the initial reaction represented in equation 1, with the formation of the intermediary [HgX(EPh)], must be quantitative.

Stabilization of the intermediary [HgX(EPh)] in presence of a coordinative solvent (L₁) and a potential ligand (L₂) could generate a new species according to equation 2, which would further condense to form the final products with loss of L₁ to give the correct Hg/X/EPh/L₂ stoichiometry.

In the case of the formation of 1 and 2, the strongly polar dimethylformamide solvent and tricyclohexylphosphine (a strong Lewis σ base and good Lewis π-acid) will both coordinate to the Hg center in the intermediary [HgX(SePh)], A, to form a species like [HgX(SePh)(dmf)(PCy₃)], B. Stoichiometrically, the final reaction for the formation of the solvated monomeric species [HgX(EPh)(dmf)(PCy₃)] should be:

Condensation of the proposed solvated Hg species [HgX(SePh)(dmf)(PCy₃)] with concomitant loss of the solvent will produce the dimeric clusters 1 and 2. Experimentally, we have proven that if an additional equivalent of tricyclohexylphosphine is used, the condensation process can be blocked and only [HgX₂(PCy₃)₂] is isolated and identified.

For the formation of 3 in a one-pot procedure, the final reaction follows the stoichiometric ratio presented in equation 4.

In this case, the less polar solvent thf, together with triphenylphosphine (or 2,2’-bipyridine), should form the intermediary B. In the presence of [HgBr(TePh)], both ligands L₁ and L₂ in B are displaced to give the more stable cluster [{Hg₅Br₃(TePh)₇}_n]. The new polymeric cluster 3 is formed with the displacement of L₁ and L₂, because the higher number of Hg-Te and Hg-Br bonds is preferred to the weaker Hg-P or Hg-N bonds of the displaced co-ligands. If more polar solvents like dimethylsulfoxide (dmso) or pyridine (py) in similar conditions are used, the compounds [Hg₃Br₃(TePh)₃]·2dmso, (6), and [Hg₆Br₄(TePh)₈(py)₂], (13), are obtained.²⁵ When the cluster 3 was re-dissolved in dmso, only the cluster 6 could be isolated. These results suggest that a dynamic process involving the coordination and loss of the solvent is present.

Crystal structure of the cluster compounds

The X-ray crystal data and the experimental conditions for the analyses of the clusters [Hg₂Cl₂(SePh)₂(PCy₃)₂], (1), [Hg₂Br₂(SePh)₂(PCy₃)₂], (2), and [Hg₅Br₃(TePh)₇], (3), are given in Table 1. Tables 2 and 3 present selected bond distances and angles for the title compounds. Figures 1, 2 and 3 show the ORTEP drawing of compounds 1, 2 and 3, respectively. Figure 4 displays the polymeric assembly of 3 along the crystallographic axis a.

Figures 1 and 2 show the molecular structures of two asymmetric units of 1 and 2 linked through halogen bridges. Both compounds crystallize in the monoclinic space group P2₁/n where the center of each molecule is located over an inversion center of this space group. In each single molecule of 1 and 2 the Hg²⁺ ions present a distorted tetrahedral configuration and they are linked through asymmetric [µ-X]^– bridges (X = Cl (1), Br (2)). The distorted tetrahedral coordination of the Hg²⁺ ions is accomplished by one PCy₃ and one PhTe^- ligand.

The monomeric cluster, [Hg₅Br₃(TePh)₇], of the compound 3, generated by the application of the symmetry of the orthorhombic space group Pnma on the asymmetric unit Hg₃Br₃(TePh)₄ (see Table 3), is displayed in Figure 3. The monomeric units assemble by connections made by four [µ₂-Te(Ph)]^- bridges, where the Teⁱ, Teⁱⁱ, Teⁱⁱⁱ and Te^iv atoms are the connectors into polymeric chains, which extend along the crystallographic axis a, as shown in Figure 4. This compound is very similar in structure to [Hg₅Cl₃(SePh)₇], (4), presented in Scheme 1.²⁴

Thermogravimetric analyses

Thermogravimetric analyses of compounds 1 and 2 in a dynamic nitrogen flow show that the thermal decomposition in the range 25-700 ºC occurs in one step. The mass losses of 100% occur at 270 and 315 ºC to 1 and 2, respectively, suggesting a cleavage of the cluster and sublimation of the resulting fragments. The thermogravimetric analysis of 3 displays a four-step process probably due to stepwise loss of the ligands. The observed mass loss in the first step of 20% at 192 ºC agrees with the theoretical weight loss of two (Ph)₂Te units (21%). Mass losses of 47% at 269 ºC (second step) and 24% at 390 ºC (third step) were also observed, which suggest the cleavage of three PhTeHgBr moieties (theoretical value 54%) and sublimation of two HgTe units (theoretical value 24%), respectively. In the fourth step, a mass loss of 6% at 520 ºC occurs. After the four-step process, 4% of the residue has remained.

Conclusions

The experimental evidence suggests that Hg(EPh)₂ (E = Se, Te) is a valuable precursor for the preparation of small cluster molecules and nanoclusters (see Scheme 1) and represents, in principle, an alternative for many reagents used with these purposes. Since these methods in genera can also be applied to the synthons of the general formula M(ER)_n , for which the examples with M = Mg, Bi, Ga, In, Sn, Sb, Zn, Cd, Hg, Au, Yb, Zr, Hf; E = S, Se, Te; R = aryl, alkyl, have been reported,²² we predict that a systematic methodology for the synthesis of binary and ternary clusters should be possible.^24,32

The differentiated occurrence of small clusters achieving dimeric (1 and 2) and one-dimensional (3), polymeric chains linked through chalcogen and Hg bridges can be associated, among other factors, to a substantial increase of the reactivity, and this is often caused by significant increase in temperature. There are, however, other variables that must be considered as a whole in the growth process of clusters and nanoparticles: stoichiometric proportions of the reagents, solvent, time of reaction, temperature, and the presence (or not) of secondary ligands, seem all to be factors which can affect the growth of the compounds, i.e., whether small or bulk, polycyclic clusters will be the final products.

We have also observed a close similarity in the growth of binary/ternary ME/MM’E nanoclusters with some processes described in the growth of nanoparticles in general.^35,36 The nucleation (growth of the particles) and the well known ‘‘Ostwald ripening’’ (or maturation) seem to be,³⁷ in our case, also dependent on the concentration of the reagents and of the time. We have also noted that all the factors which are able to promote the efficient dissociation of the reagents (solvent polarity, for example), together with a resourceful nucleation (enough time of the particles in solution) should lead to the formation of clusters of higher nuclearity. Presently, we are improving experimental attempts to control the dissociation/association of particles (fragments) in solution, before their crystallization.

Supplementary Information

CCDC 745061, 745062 and 745553 contain the supplementary crystallographic data for 1, 2 and 3, 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: [phone]; or e-mail: [e-mail].

TGA measurements data for 1, 2 and 3 are available free of charge at http://jbcs.sbq.org.br, as PDF file.

Acknowledgments

This work was supported with funds from MCT/CNPq 15/2007 - Universal (Proc. No. 472509/2007-1). The author (B. T.) thanks CNPq for a scholarship.

Received: September 12, 2009

Web Release Date: March 22, 2010

Supplementary Information

Thermogravimetrical measurements

Thermogravimetric measurements were carried out on a Shimadzu DTG-60 device. The experiments were performed in the temperature range from 25 to 700 ºC at heating rates of 10 ºC min^-1 on each sample. The average sample size was 10 mg and the nitrogen flow-rate was 50 mL min^-1.

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Publication Dates

• Publication in this collection
  30 July 2010
• Date of issue
  2010

History

• Accepted
  22 Mar 2010
• Received
  12 Sept 2009