‘Chimie douce’ synthesis, crystal and electronic structures of the novel Hg~4.2Mo15X19 (X = S, Se) compounds containing Hg3, Mo6 and Mo9 clusters

Diala Salloum; Patrick Gougeon; Michel Potel; Régis Gautier

doi:10.1016/j.crci.2005.02.027

‘Chimie douce’ synthesis, crystal and electronic structures of the novel Hg_~4.2Mo₁₅X₁₉ (X = S, Se) compounds containing Hg₃, Mo₆ and Mo₉ clusters

Diala Salloum ¹ ; Patrick Gougeon ¹ ; Michel Potel ¹ ; Régis Gautier ²

¹ Laboratoire de chimie du solide et inorganique moléculaire, UMR CNRS 6511, université Rennes-1, Institut de chimie de Rennes, avenue du Général-Leclerc, 35042 Rennes cedex, France
² Laboratoire de physicochimie, UMR CNRS 6511, École nationale supérieure de chimie de Rennes, Institut de chimie de Rennes, campus de Beaulieu, 35700 Rennes, France

Comptes Rendus. Chimie, Volume 8 (2005) no. 11-12, pp. 1743-1749.

Résumés

Anglais
Français

The new compounds Hg_~4.2Mo₁₅X₁₉ (X = S, Se) were obtained by inserting mercury into the metastable Mo₁₅X₁₉ compounds at low temperature. Their crystal structures, determined from single-crystal X-ray diffraction data, show that the molybdenum–chalcogenide network is maintained through the synthesis. It consists of an equal mixture of Mo₆X₈X₆ and Mo₉X₁₁X₆ cluster units interconnected through Mo–X bonds as in the parent compounds. In both compounds, the mercury is present either as Hg²⁺ cations or forming linear clusters Hg₃²⁺. It is the first time that the latter cluster is observed in a chalcogen environment. .

Les nouveaux composés Hg_~4.2Mo₁₅X₁₉ (X = S, Se) ont été obtenus par insertion de mercure à basse température dans les binaires métastables Mo₁₅X₁₉. Leurs structures cristallines, déterminées par diffraction des rayons X sur monocristal, montrent que le réseau molybdène–chalcogène est maintenu durant la synthèse. Il est constitué de motifs Mo₆X₈X₆ et Mo₉X₁₁X₆ en proportion égale et interconnectés entre eux par des liaisons Mo–X, comme dans les composés parents. Dans les deux composés, le mercure se présente, soit sous la forme cationique Hg²⁺, soit formant des clusters linéaires Hg₃²⁺. C'est la première fois qu'un tel cluster est observé dans un environnement de chalcogènes. .

Métadonnées

Reçu le : 2004-08-31
Accepté le : 2005-01-18
Publié le : 2005-06-13

DOI : 10.1016/j.crci.2005.02.027

Keywords: Soft chemistry, Molybdenum, Mercury, Cluster, Chalcogenide compounds, X-ray crystal structures
Mots clés : Chimie douce, Molybdène, Mercure, Cluster, Chalcogénures, Structures cristallines par rayon X

Affiliations des auteurs :

Diala Salloum ¹ ; Patrick Gougeon ¹ ; Michel Potel ¹ ; Régis Gautier ²

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     author = {Diala Salloum and Patrick Gougeon and Michel Potel and R\'egis Gautier},
     title = {{\textquoteleft}Chimie douce{\textquoteright} synthesis, crystal and electronic structures of the novel {Hg\protect\textsubscript{~4.2}Mo\protect\textsubscript{15}X\protect\textsubscript{19}} {(X\hspace{0.25em}=\hspace{0.25em}S,} {Se)} compounds containing {Hg\protect\textsubscript{3},} {Mo\protect\textsubscript{6}} and {Mo\protect\textsubscript{9}} clusters},
     journal = {Comptes Rendus. Chimie},
     pages = {1743--1749},
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%A Patrick Gougeon
%A Michel Potel
%A Régis Gautier
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Diala Salloum; Patrick Gougeon; Michel Potel; Régis Gautier. ‘Chimie douce’ synthesis, crystal and electronic structures of the novel Hg_~4.2Mo₁₅X₁₉ (X = S, Se) compounds containing Hg₃, Mo₆ and Mo₉ clusters. Comptes Rendus. Chimie, Volume 8 (2005) no. 11-12, pp. 1743-1749. doi : 10.1016/j.crci.2005.02.027. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2005.02.027/

Version originale du texte intégral

1 Introduction

Inorganic compounds containing low-valent molybdenum are generally characterized by metal clusters of diverse sizes and geometries. In reduced molybdenum chalcogenides, the Mo clusters can accept a variable number of electrons (often called metallic electrons or ME) available for Mo–Mo bondings. The ME count, which govern the physical properties, can be modified through topotactic reduction–oxidation reactions at low temperatures. This is particularly well exemplified by the silver deinsertion from Ag_3.6Mo₉Se₁₁ that yield the isomorphous metastable compound o-Mo₉Se₁₁ [1]. The latter phase is a superconductor with 32 ME per Mo₉ cluster while the parent ternary compound is a semiconductor and have 35.6 ME per Mo₉. Previous works have shown that the indium molybdenum cluster chalcogenides such as InMo₆S₈, In_3.7Mo₁₅S₁₉, In_3.3Mo₁₅Se₁₉, In₂Mo₁₅Se₁₉ or In₂Mo₆Se₆ are also good candidates for topotactic reduction–oxidation reactions at low temperatures [2–6]. Indeed, the indium can be removed easily from the latter ternary compounds by oxidation with I₂ or HCl gas to give the corresponding metastable binary compounds. From the binaries thus obtained, new ternary phases can synthesized by electrochemical reactions or by low temperature diffusion of metals having low melting point [3]. By using the latter technique we could prepare the new mercury molybdenum chalcogenides Hg_~4.2Mo₁₅X₁₉ (X = S, Se), the syntheses and crystal and electronic structures of which are described in this paper.

2 Experimental section

2.1 Syntheses

The compounds Hg_~4.2Mo₁₅X₁₉ (X = S, Se) were obtained in three steps. The first one was the synthesis of the solid-state compounds In_3.7Mo₁₅S₁₉ and In_3.3Mo₁₅Se₁₉ then the obtaining of the binaries Mo₁₅X₁₉ (X = S, Se) by ‘chimie douce’ and, finally the synthesis of the title compounds by inserting mercury into the Mo₁₅X₁₉ compounds at low temperature. All these steps were performed on single-crystals.

2.1.1 In_3.7Mo₁₅S₁₉ and In_3.3Mo₁₅Se₁₉

Starting materials used for the synthesis of In_3.7Mo₁₅S₁₉ were MoS₂, In₂S₃ and Mo, all in powder form. For In_3.3Mo₁₅Se₁₉, the initial products were MoSe₂, InSe and Mo. Before use, Mo powder was reduced under H₂ flowing gas at 1000 °C during 10 h in order to eliminate any trace of oxygen. The molybdenum disulfide and diselenide were prepared by the reaction of sulfur and selenium, respectively, with H₂ reduced Mo in a ratio 2:1 in an evacuated (ca. 10^–2 Pa Ar residual pressure) and flame-baked silica ampoule, heated at 800 °C during 2 days. The indium sesquisulfide was obtained by heating at 800 °C stoichiometric amounts of indium metal and sulfur powder in an evacuated sealed silica ampoule. InSe was synthesized from indium and selenium shots heated at 450 °C in an evacuated sealed silica ampoule. All starting reagents were found monophasic on the basis of their powder X-ray diffraction diagram recorded with an Inel curve sensitive position detector CPS 120 using Cu Kα₁ radiation. In order to avoid any contamination by oxygen and moisture, the starting reagents were mixed, ground together in a mortar and then cold-pressed in a purified argon-filled glove box. The pellets (ca. 3 g) of the starting reagents were then loaded in molybdenum crucibles (depth: 2.5 cm; diam.: 1.5 cm), which were previously cleaned by heating at about 1500 °C in a high frequency furnace for 15 min under a dynamic vacuum of about 10^–3 Pa and then sealed under a low argon pressure (300 h Pa) using an arc welding system. For In_3.7Mo₁₅S₁₉, the molybdenum crucible was heated at a rate of 50 °C h^–1 up to 1120 °C and held there for 48 h, then cooled at 100 °C h^–1 to 1000 °C and finally cooled down to room temperature in the furnace. For the selenide, the temperature of reaction was 1300 °C. The resulting products were black and air-stable and were found as pure phases on the basis of their X-ray powder diffraction diagram.

2.1.2 Mo₁₅S₁₉ and Mo₁₅Se₁₉

Single-crystals of the binary compounds were obtained from oxidation of single-crystals of In_3.7Mo₁₅S₁₉ and In_3.3Mo₁₅Se₁₉ by iodine in silica tube sealed under vacuum.

The end of the tube containing the crystals of the indium compounds and an excess of iodine was placed in a furnace with about 3 cm of the other end sticking out of the furnace, at about room temperature. The furnace was then heated at 300 °C for 96 h. At the end of the reaction, crystals of InI₃ and I₂ were obtained at the cool end of the tube.

2.1.3 Hg_~4.2Mo₁₅X₁₉ (X = S, Se)

The mercury ternary phases were prepared by diffusion of mercury into crystals of Mo₁₅X₁₉ in silica tube sealed under vacuum at 400 °C during 96 h. In both cases, an excess of mercury was used.

2.2 Single-crystal X-ray studies

The X-ray diffraction data were collected on an Nonius Kappa CCD diffractometer using graphite-monochromated Mo–Kα radiation (λ = 0.71073 Å). The COLLECT program package [7] was employed to establish the angular scan conditions (ϕ and ω scans) used in the data collections. The data sets were processed using EvalCCD [8] for the integration procedure. For both crystals, an absorption correction was applied using the description of the crystal faces and the analytical method described by de Meulenaar and Tompa [9]. The structures were refined using SHELXL-97 [10]. The positions of the Mo and S atoms in In_3.7Mo₁₅S₁₉ [2] were used in the first stage of the refinement. The Hg positions were revealed by subsequent difference Fourier syntheses. Refinement of the occupancy factor of the Hg3 atoms led to the following stoichiometries: Hg_4.145(6)Mo₁₅S₁₉ and Hg_4.278(6)Mo₁₅Se₁₉. Relevant crystallographic data are listed in Table 1, and selected bond distances are given in Table 2.

Formula	Hg_4.145Mo₁₅S₁₉	Hg_4.278Mo₁₅Se₁₉
Formula weight (g mol^–1)	2879.90	3796.86
Space group	P6₃/m	P6₃/m
a (Å)	9.5368(1)	9.8856(2)
c (Å)	18.5673(3)	19.2059(3)
Z	2	2
V (Å³)	1462.46(3)	1625.44(5)
ρ_calcd (g cm^–3)	6.473	7.745
T (°C)	20	20
λ (Å)	0.71073	0.71073
μ (mm^–1)	29.190	46.636
R₁^a	0.0426	0.0493
wR₂^b	0.1027	0.1112

^a R₁ = Σ∣∣F_o∣–∣F_c∣∣/Σ∣F_o∣.

^b wR₂ = {Σ[w(F²_o−F²_c)²]/Σ[w(F²_o)²]}^1/2.

	Hg_4.145Mo₁₅S₁₉	Hg_4.278Mo₁₅Se₁₉
Mo1–Mo1 (×2)	2.6910(4)	2.6871(14)
Mo1–Mo1 (×2)	2.7182(5)	2.7100(8)
Mo2–Mo2 (×2)	2.6819(4)	2.6714(12)
Mo2–Mo3	2.7123(4)	2.7200(8)
Mo2–Mo3	2.7493(5)	2.7674(9)
Mo3–Mo3 (×2)	2.7484(7)	2.7591(13)
Mo1–Mo2	3.2739(5)	3.4866(9)
Mo1–X1	2.4455(13)	2.5667(11)
Mo1–X1	2.4466(10)	2.5816(10)
Mo1–X1	2.4856(15)	2.6107(14)
Mo1–X2	2.4925(9)	2.6510(12)
Mo1–X4	2.4331(15)	2.5440(12)
Mo2–X1	2.5014(9)	2.6640(12)
Mo2–X2	2.4769(13)	2.6102(10)
Mo2–X2	2.4374(15)	2.5736(10)
Mo2–X3	2.5760(6)	2.6665(10)
Mo2–X5	2.4127(13)	2.5338(11)
Mo3–X2 (×2)	2.4442(11)	2.5930(8)
Mo3–X3	2.4518(13)	2.574(2)
Mo3–X3	2.4570(12)	2.574(2)
Hg1–X1 (×3)	3.3218(7)	3.4211(10)
Hg1–X2 (×3)	3.2035(12)	3.2485(10)
Hg1–X5	2.5618(17)	2.6974(13)
Hg2–X3 (×3)	3.5129(15)	3.6247(9)
Hg2–X2 (×6)	3.7953(11)	3.8857(9)
Hg3–X2 (×2)	2.7369(11)	2.7124(11)
Hg3–X3	2.4276(13)	2.561(2)
Hg3–X4 (×2)	2.7403(14)	2.9577(14)
Hg1–Hg2 (×2)	2.6356(3)	2.6441(6)

2.3 Theoretical calculations

Calculations have been carried out within the extended Hückel formalism [11,12] with the program YAeHMOP [13]. The exponents (ξ) and the valence shell ionization potentials (H_ii in eV) were (respectively): 1.817, –20.0 for S 3 s; 1.817, –13.3 for S 3p; 1.956, –8.34 for Mo 5 s; 1.921, –5.24 for Mo 5p; 2.649, –13.68 for Hg 6 s; 2.631, –8.47 for Hg 6p. H_ii values for Mo 4d and Hg 5d were set equal to –10.50 and –17.50, respectively. Linear combination of two Slater-type orbitals of exponents ζ₁ = 4.542 and ζ₂ = 1.901 with the weighting coefficients c₁ = c₂ = 0.5898, and ζ₁ = 6.436 and ζ₂ = 3.032 with the weighting coefficients c₁ = 0.6438 and c₂ = 0.5215 were used to represent the Mo 4d and Hg 5d atomic orbitals, respectively. The density of states (DOS) and Crystal Orbital Overlap Populations (COOP) were obtained using a set of 18 k points.

2.4 Magnetic measurements

The magnetic susceptibility was measured using a SQUID magnetometer (MPMS-XL, Quantum Design).

3 Results and discussion

3.1 Crystal and electronic structures

A view of the crystal structure of Hg_4.15Mo₁₅S₁₉ is shown in Fig. 1 as example. The molybdenum-chalcogen frameworks of the Hg_~4.2Mo₁₅X₁₉ (X = S, Se) compounds are similar to those of the parent compounds In_3.7Mo₁₅S₁₉ and In_3.3Mo₁₅Se₁₉. They consist of an equal mixture of ${Mo}_{6} X_{8}^{i} X_{6}^{a}$ and ${Mo}_{9} X_{11}^{i} X_{6}^{a}$ (X = S, Se) cluster units linked through interunit Mo–X bonds (Fig. 2). The first unit is similar to that encountered in the ternary molybdenum chalcogenides M_xMo₆X₈ (M = Na, K, Ca, Sr, Ba, Sn, Pb, rare earth metal, 3d element; X = S, Se or Te), known as Chevrel phases. It corresponds to an Mo₆ octahedron surrounded by 8 face-capping inner $X_{}^{i}$ (6 X1 and 2 X4) and 6 apical $X_{}^{a}$ (X2) ligands. The Mo₉ core of the second unit results from the face sharing of 2 octahedral Mo₆ clusters. The Mo₉ cluster is surrounded by 11 $X_{}^{i}$ (6 X2, 3 X3 and 2 X5) atoms capping the faces of the bioctahedron and 6 apical $X_{}^{a}$ (X1) ligands above the ending Mo atoms. In the sulfide compound, the Mo–Mo distances within the Mo₆ clusters are 2.6910(4) Å (2.6871(14) in the selenide) for the intra-triangle distances (distances within the Mo₃ triangles formed by the Mo atoms related through the threefold axis) and 2.7182(5) Å (2.7100(8) in the selenide) for the inter-triangle distances. The Mo–Mo distances within the Mo₉ clusters are 2.6819(4) and 2.7484(7) Å (2.6714(12) and 2.7591(13) Å in the selenide) for the intra-triangle distances between the Mo2 and Mo3 atoms, respectively, and 2.7123(4) and 2.7493(5) Å (2.7200(8) and 2.7674(9) Å in the selenide) for those between the Mo₃ triangles. The ${Mo}_{6} X_{8}^{i} X_{6}^{a}$ and ${Mo}_{9} X_{11}^{i} X_{6}^{a}$ units are centered at 2b and 2c positions and have the point-group symmetry $\overline{3}$ and 3/m, respectively. The chalcogen atoms bridge either one (X1, X2, X4 and X5) or two (X3) Mo triangular faces of the clusters. Moreover the chalcogens X1 and X2 are linked to a Mo atom of a neighboring cluster. The Mo–X bond distances range from 2.4167(11) to 2.5032(14) Å within the Mo₆X₈ unit and from 2.3908(8) to 2.6262(11) Å within the Mo₉X₁₁. Each ${Mo}_{9} X_{11}^{i} X_{6}^{a}$ unit is interconnected to 6 ${Mo}_{6} X_{8}^{i} X_{6}^{a}$ units (and vice-versa) via interunit Mo1–X2 bonds (respectively, Mo2–X1) (Fig. 2) to form the three-dimensional Mo–X framework, the connectivity formula of which is ${Mo}_{9} X_{5}^{i} X_{6/2}^{i-a} X_{6/2}^{a-i}$ ${Mo}_{6} X_{2}^{i} X_{6/2}^{i-a} X_{6/2}^{a-i}$ . It results from this arrangement that the shortest intercluster Mo1–Mo2 distance between the Mo₆ and Mo₉ clusters is 3.2739(5) Å in the sulfide and 3.4866(9) Å in the selenide, indicating only weak metal–metal interaction.

Fig. 1
View of the crystal structure of Hg_4.145Mo₁₅S₁₉, (97% probability ellipsoids).

In both compounds, the mercury is present either as Hg²⁺ cations (ie Hg3) or forming linear trinuclear clusters Hg₃²⁺units (i-.e. Hg1 and Hg2) (Fig. 3)_.. The Hg²⁺ cations occupy the sites in which the In³⁺ cations are located in the indium parent compounds (Fig. 3). The latter position corresponds to a triangular group of distorted octahedral cavities around the threefold axis, which are formed by two Mo₆X₈ and three Mo₉X₁₁ units. These cavities are only partially occupied with 1.145(6) and 1.278(6) Hg²⁺ cations per triangular group for the sulfide and selenide compounds, respectively. The Hg3–S distances range from 2.4276(13) to 2.7403(14) Å and, the Hg3–Se ones from 2.561(2) to 2.9577(14) Å.

Fig. 3
Views of the In and Hg sites in In_3.7Mo₁₅S₁₉, (left) and Hg_~4.2Mo₁₅X₁₉ (right).

The Hg₃²⁺ cluster, which is for the first time observed in a chalcogen environment, resides in the two large cavities that are occupied by the monovalent indium cations related through the mirror plane in In_3.7Mo₁₅S₁₉ and In_3.3Mo₁₅Se₁₉. The Hg₃²⁺ cluster is linear and centrosymmetrical with Hg1–Hg2 distances of 2.6356(3) Å in the sulfide and 2.6441(6) Å in the selenide (Fig. 4). The latter distances are somewhat longer that those observed in the other inorganic compounds such as Hg₃(AsF₆)₂ [14], Hg₃(AlCl₄)₂ [15], Hg₃(NbF₅)₂(SO₄)₂ [16] and Hg₃(TaF₅)₂(SO₄)₂ [17] containing quasi- or linear Hg₃²⁺ groups in which the Hg–Hg distances range from 2.551 to 2.562 Å. The coordination environment about the mercury atoms is also different. The environment of the terminal mercury atoms Hg1 is similar to that of the monovalent indium cations and thus consists of seven chalcogen atoms. The Hg1–X distances range from 2.5618(17) to 3.3218(7) Å in the sulfide and, from 2.6974(13) to 3.421(1) Å in the selenide. The shortest Hg1–X5 bond is collinear with the Hg–Hg bonds and is, most likely, covalent in character. The central mercury atoms Hg2 is surrounded by nine chalcogen atoms forming a tricapped trigonal prism. The distances between the Hg2 mercury and the capping chalcogen atoms are 3.5129(15) Å and 3.6247(9) Å for the sulfide and selenide, respectively, while the other distances between Hg2 and the chalcogens forming the trigonal prism are 3.7953(11) and 3.8857(9), respectively. Hg1–Hg2 distances are consistent with the presence of metal–metal bonding in the Hg₃ unit. The expected metallic electron (ME) count for linear Hg₃–X₂ units with two metal–metal bonds is 34 [17]. Such a ME count induces a + 2 charge on the Hg₃ cluster. This oxidation state is also the one observed for the Hg₃ clusters present in the inorganic compounds mentioned above. Considering Hg²⁺ and Hg₃²⁺ cations in the title compound, the total ME count of Mo₆ and Mo₉ clusters is equal to 56.4. Previous theoretical studies have shown that optimal ME counts, i.e. complete filling of Mo–Mo bonding and non-bonding levels separated from Mo–Mo antibonding levels by a significant energy gap, are equal to 24 and 36 for such Mo₆ and Mo₉ clusters, respectively [18,19]. Assuming weak interaction between the clusters in the solid-state structure, band structure of the title compound showing a significant energy gap for the ME count of 24 + 36 = 60 is expected. Total and Mo₉ projected density of states (DOS) sketched in Fig. 5 were computed for the model compound Hg₄Mo₁₅S₁₉ where only one of the three Hg3 equivalent crystallographic position is occupied. The Fermi level for the ME count of 56 (that corresponds to the presence of four Hg atoms in the unit cell) cuts a narrow peak of DOS mainly centered on d orbitals of the Mo₆ cluster. Metallic properties can be envisioned for the title compound. This was confirmed by magnetic susceptibility measurements that were carried out on batches of single-crystals of about 100 mg in an applied field of 20 Oe using a dc SQUID magnetometer. Indeed, Hg_4.145Mo₁₅S₁₉ and Hg_4.278Mo₁₅Se₁₉ show a diamagnetic shielding signal below 4 K indicating a superconducting transition. Finally, it should be mentioned that a band gap of 0.20 eV separating overall Mo–Mo bonding and non-bonding bands from Mo–Mo antibonding bands is reached for the ME count of 60. Such an electron count would render the material semiconducting through suitable doping. Indeed, Mo–Mo and Mo–S overlap populations hardly change overall upon additions of electrons. Analyses of the COOP (not shown here) indicate that the intercluster Mo1–Mo2 and intra Mo₉ cluster Mo3–Mo3 should slightly lengthen, as well as Mo–S distances, whereas other Mo–Mo bonds of Mo₆ and Mo₉ units should slightly shorten when electrons are added.

Fig. 4
The Hg₃ cluster with its chalcogen environment.

Fig. 5
EHTB calculations for Hg₄Mo₁₅S₁₉: total density of states (plain) and Mo₉ contribution (dotted).

4 Supplementary material

Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: +49 7247 808 666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository number CSD-414276 and CSD-414277.

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Octahedral rhenium cluster chemistry: from high-temperature syntheses to the elaboration of new inorganic/molecular hybrid compounds via solution route

Guillaume Pilet; André Perrin

C. R. Chim (2005)

New molecular architectures built from metallic nanoclusters

Christiane Perrin; André Perrin; Dieter Fenske