2.1 General synthesis

High-temperature syntheses are usually carried out in sealed silica tubes, from powdered elements (Re, S, Se) and sometimes, metal halides. Halogen is also introduced for instance as ReCl₅ or liquid bromine, the latter being introduced under argon and frozen before sealing the mixture in a silica tube. Stoichiometric amounts are used, with the exception of bromine, for which an excess of 10–20% is generally used. Reaction temperatures are typically in the 700–950 °C range for a reaction time of 24–48 h; very long times (up to 10–15 days) are sometime used in order to increase the size of crystals.

Under these conditions, numerous original compounds have been reported. As an example, a quaternary diagram, relative to the Cs–Re–S–Br system, is displayed on Fig. 1, using a pseudo-ternary representation, where ternary compounds lie onto the axes, and quaternary ones inside the triangle.

2.2 Predictable approach of expected phases

Expected phases can be predicted using together the following rules:

• • rhenium is in the oxidation state + III;
• • the elementary building block is the unit Re₆L₁₄ or more precisely Re₆Lⁱ₈L^a₆, where Lⁱ can be a mixture of halogen and chalcogen, very often with a statistical disorder, but with an overall composition such as (X_xY_1–x)ⁱ₈ where x is an integer, while L^a is preferentially a halogen ligand. It is to be noted that, due to the often-encountered statistical distribution on inner ligands, it is advisable to choose systems where halogen and chalcogen ligands have very different diffusion factors in order to be able to accurately derive the stoichiometry from structural determination;
• • the cluster units can be either isolated or interconnected via two types of bridges, following a nomenclature early established by H. Schäfer [3]: i) simple inter-cluster bridges L^a–a where the bridging atom is in apical position for the two adjacent clusters (Fig. 2a) and is systematically a halogen (with the unique exception of the ternary chalcogenides described below, where there is obviously no choice) ; ii) double inter-cluster bridges L^a–i, meaning that the bridging atom is in apical position for a cluster and in inner position for the adjacent one (Fig. 2b) ; indeed, each bridging atom will be counted for one in the environment of the cluster (in order to maintain the Re₆Lⁱ₈L^a₆ unit formula), and counted for 1/2 for establishing the chemical formula of the compound; a third bridging type, L^i–i, has been reported in the crystallochemistry of molybdenum clusters, but do not seems to form in the case of rhenium;
• • the VEC (Valence Electron Count), defined as the number of electrons available to form metal–metal bonds, is fixed to 24, implying twelve order-one bonds; the VEC is easily calculated, taking into account seven valence electrons for each rhenium atom in the d⁴ state, then deducing the number of electrons transferred to the ligands, in an ionic model, and finally adding the electrons transferred from the eventual additional element to get a counter-cation; this ‘magic number’, corresponding to fulfilling the metal–metal bonding states of the Re₆L₁₄ molecular orbital diagram, is respected in most cases, although a VEC of 23 has been reported in limited examples, in which the cluster has been oxidized [2,4];
• • if an anionic charge is carried by the unit, it has then of course to be balanced by counter-cations.

The results are illustrated in Table 1, in the example of rhenium thio-bromides. Going from the left to the right along a row corresponds to replacing two bromines by one sulfur ligand, without charge change, but implies a decrease by unity of the total number of available ligands, and then the formation of an inter-clusters bridge. In contrast, from the top to bottom is the successive addition of one cation (univalent in this example), and then of one bromine, implying the breakage of an inter-cluster bridge.

Note that, for a given stoichiometry, it is possible within the above assumptions, to derive several developed formulas: as it can be seen in Table 1, it depends on the distribution of the ligands (inner, apical and a–a or i–a bridging). Some factors such as steric hindrance or preferential inner or apical site occupancy could favor a given architecture.

Finally, the bottom-left part of the Table is empty because it would correspond to cationic units. A few examples of the latter have been reported in the case of Nb₆ and Ta₆ clusters [5] but are, up to now, unknown in the case of rhenium, with simple ligands: only the dications [Re₆Te₈(TeBr₂)₆]Br₂ [6] and its iodine homologue [7] have been reported; they are obtained under medium temperature conditions (400–450 °C), starting from a precursor containing already the trinuclear or hexanuclear rhenium cluster (Re₃Br₉ and Re₆Te₁₅, respectively).

2.3 Ternary compounds

A number of alkaline or alkaline-earth sulfides or selenides of rhenium, based on the Re₆Y₁₄ unit have been obtained in the W. Bronger group. Depending on the stoichiometry and synthesis conditions, most of the reported compounds present apical–apical bridges of sulfide S^a–a or disulfide (S–S)^a–a type extending along one to three directions [1,8,9]. An exception is the Cs₁₀Re₆S₁₄ compound, based on isolated cluster anion, with a very important negative charge [10].

Ternary rhenium chalcohalides based on octahedral Re₆ clusters have been reported and are described in detail elsewhere [1]. They correspond to the general formula Re₆X_4+xY_10–2x (integer 0 ≤ x ≤ 4). Then the first term, for instance Re₆S₄Br₁₀ corresponds to isolated units, while the three following ones illustrate a progressive unit condensation along one, two and three directions of the space. The final step is illustrated for instance by Re₆S₈Br₂, that is built from one pair of sulfide inner-apical bridges and two apical bromine bridges leading to the 3-D Re₆[Sⁱ₆ S^i–a_2/2] S^a–i_2/2Br^a–a_4/2, while the chloro-seleno homologue Re₆Se₈Cl₂ exhibits a strongly 2-D structure with two selenide inner-apical double bridges and terminal apical chlorines, and is written as Re₆[Seⁱ₄ Se^i–a_4/2] Se^a–i_4/2Cl^a₂.

2.4 Quaternary compounds

They obey of course to the same rules as the ternary chalcohalides, but, due to the charges equilibrium, the number of combinations is even larger, leading to a very rich crystallochemistry.

2.4.1 Discrete units

They lie on the diagonal of Table 1, which corresponds to the richest possible alkaline formulations. Of course, all of them exhibit six apical bromine ligands.

All relevant examples of compounds are known, although there are some unexpected results:

• • a non integer compound, Cs_0.4Re₆S_4.4Br_9.6, is isostructural [11] with the monoclinic potassium thiochloride KRe₆S₅Cl₉, while CsRe₆S₅Br₉ presents, in contrast, a trigonal structure extending up to Cs₂Re₆S₆Br₈ along a solid solution;
• • a second non-integer phase, Cs_2.5Re₆S_6.5Br_7.5, has been detected [11] but not fully characterized;
• • the compound at the right bottom end of the Table, corresponding to the highest anionic charge, Cs₄Re₆S₈Br₆, was unknown up to very recently. In contrast, Cs₅Re₆S₈Br₇ is easily synthesized and could be written as a double salt Cs₄Re₆S₈Br₆, CsBr in accordance with the above nomenclature. However, from the structural determination [12] the latter is better described as a salt of a tetravalent complex cation (Cs₅Br)⁴⁺(Re₆S₈Br₆)^4– (see Fig. 3a). The ‘true’ tetra-anionic unit based Cs₄Re₆S₈Br₆ has been recently obtained as a secondary phase in the preparation of Cs₅Re₆S₈Br₇ and structurally characterized [13]; the arrangement of cesium cations is very different, as they form a very regular cuboctahedron (Fig. 3b) surrounding the tetra-anionic unit.

2.4.2 Bridged units

The number and arrangement of ligands shared by adjacent units strongly influences the dimensionality of the structure of the compounds. Indeed, two new low dimensional phases have been recently synthesized and structurally characterized, completing the data of Table 1:

• • CsRe₆S₈Br₃ [14] has for developed formula Cs{[Re₆Sⁱ₆ S^i–a_2/2]S^a–i_2/2Br^a–a_2/2Br^a₂}, meaning a polymeric structure where the clusters are linked by both a pair of double sulfur bridges in one direction and a pair of apical bromine bridges in a second one (Fig. 4). This gives a strong 2-D character to this structure, isostructural with the previously reported seleno-iodide CsRe₆Se₈I₃ [15];
• • Cs₂Re₆S₈Br₄ [16], with the structural formula Cs₂{[Re₆Sⁱ₆ S^i–a_2/2]S^a–i_2/2Br^a₄}, forms infinite chains where the clusters are linked by a pair of double sulfur bridges leading to a 1-D framework (Fig. 5). It turns to be isostructural with Cs₂Re₆Se₈Br₄ [15] and Rb₂Re₆S₈Br₄ [11];

3.1 Solubility of rhenium cluster compounds

The compounds obtained by solid state chemistry are expected to be usable as precursors for a chemistry carried out in solution. Of course, compounds exhibiting a structure where the units are connected by bridges are not soluble, due to the presence of a rigid framework unless they are submitted to so-called [15,19] ‘excision’ reactions: the latter expression means that inter-cluster bridges are broken by a chemical reaction, for instance with alkaline halide, and then the pending ligand sites are filled by the imported halogen ion. In fact, it is usually more efficient to synthesize directly the resulting compound by solid state reaction.

In contrast, ionic compounds based on isolated units present in most cases significant solubility in common polar solvents. Examples are given in Table 2, which shows the lack, up to now, of any systematic. A significant example is illustrated by the comparison of the behavior of Cs₅Re₆S₈Br₇ (Cs₄Re₆S₈Br₆, CsBr or more likely (Cs₅Br)⁴⁺(Re₆S₈Br₆)^4–) and Cs₄Re₆S₈Br₆: in fact (Cs₅Br)⁴⁺(Re₆S₈Br₆)^4– is readily soluble in water, while the latter is not (this difference allowed to isolate it from the reaction mixture), although both compounds are based on the same tetravalent anionic unit (Re₆S₈Br₆)^4–. Even more unexpected is the result of the recrystallization of water solutions of Cs₅Re₆S₈Br₇, that gives again the starting compound. In that case, it could have been expected that the complex cation (Cs₅Br)⁴⁺ would have been broken in solution, and then that Cs₄Re₆S₈Br₆ would have precipitated, the additional CsBr remaining in the solution.

Early experience has shown that substituting the inorganic cation by a tetra-alkylammonium salt, using a simple cation metathesis reaction, increases to a large extent the solubility of the cluster materials in common organic solvents [18].

3.2 Exchange of cationic partner

This route is illustrated by many examples, involving either solvated or complexed inorganic cations, or the use of organic or organometallic partners. Due to the large size of the latter in comparison to the starting inorganic cation, very different stackings are evidenced for the final hybrid compounds.

3.2.1 Solvated counter-cation

An example is the structure of Ca(DMSO)₆Re₆S₆Br₈ [20] obtained in MeCN from the cesium or, more efficiently, the tetrabutylammonium salt of the dianion [Re₆S₆Br₈]^2–, in the presence of CaBr₂ which was solubilized by the addition of DMSO complexant. The structure is built from rows of [Re₆S₆Br₈]^2– units and [Ca(DMSO)₆]²⁺ cations where Ca is octahedrally coordinated by the oxygen atoms of the solvate (Fig. 6). Due to the 1:1 stoichiometry and the analogous overall size of the two partners, the stacking is a giant homothetic analogue of the CsCl type-structure.

3.2.2 Complex counter cation

An illustration is given by the synthesis of the crown-ether derivative Cs₂(18-crown-6)₃ Re₆S₆Br₈ obtained in MeCN by direct complexation (Fig. 7) [20]. Because Cs is too large to be accommodated in the cavity of the crown-ether, a 2:3 ‘club-sandwich’ is formed, that presents a rare example of non-rotational disordered crown-ether. Due to the local environment, the central crown-ether molecule has the usual symmetry, while the outer ones, standing in a strongly asymmetric environment, just between the cluster anion and the Cs cation, exhibit a strongly distorted conformation (all the oxygen atoms being pushed on the same side of the mean plane of the molecule), while all the distances and angles maintain their usual values. Again, the similar size of the two partners leads to a homothetic CsCl stacking, in which the inter-cluster distance (center to center) reaches 12 Å, to be compared to the 9.32 Å separation in the Cs₂Re₆S₆Br₈ precursor.

3.2.3 Organic cations

Examples of cluster anions assembled with tetra-alkyl ammonium cations are known for long time [18]. A further illustration is the recently obtained structure [20] of (n-Bu₄N)₂[Re₆S₆Br₈], displayed in Fig. 8. As stated above the interest in these hybrids is an increased solubility in organic solvents, opening the way to specific inorganic–organic assemblies. Relevant examples are the hybrids early synthesized with analogues of tetrathiafulvalene by an electro-crystallization route [18].

3.2.4 Organometallic compounds

The chemistry of transition metal organometallic complexes, dominated by the classical 18-electron rule, is interesting to study with complexes with 17 electrons, possibly leading to the discovery of new electron transfer reactions and the access to new physical properties. However, experience shows that small anions, like PF₆^–, BF₄^– or even BPh₄^– commonly used in organometallic chemistry often lead to crystals of quite poor quality. For this reason, we have explored the association of organometallic cations and large-sized anionic metal clusters. In this type of synthesis, the metallic cluster plays the role of a ‘crystallization improver’ and helps for the structural determination by X-ray diffraction due to well organized heavy atoms. Very recent illustrations of this strategy are the synthesis and the structure determination of two organometallic–inorganic hybrids:

• – (i) [Cp*(dppe)Fe–NCCH₃]₂[Re₆S₆Br₈] has been prepared by simple ionic metathesis from Cs₂ Re₆S₆Br₈ and [Cp*(dppe)Fe–NCCH₃]Cl in MeCN and elimination of the formed CsCl precipitate [21]. The structure is shown in Fig. 9. The two entities keep their own individuality and interact by coulombic forces.
• – (ii) The [FeCp₂]₂[Re₆S₆Br₈]2(OC(CH₃)₂) ferricinium salt has been prepared in order to replace, for the same reasons as above, the standard [FeCp₂][PF₆] for the oxidation of organometallic complexes. It is again obtained by a simple metathesis in MeCN followed by a recrystallization from a pentane–acetone mixture [22]. This reaction was possible because the redox potentials of the two entities are such that no electrochemical reaction can occur between them. The structure (Fig. 10) can be described as a succession of planes occupied by the anions and strongly corrugated ladders built from the ferricinium ions.

3.3 Exchange of apical ligands

Another important field of research is the use of the above solid state compounds as cluster precursors in order to prepare Re₆Lⁱ₈(CN)^a₆ units in which the apical ligands are replaced by ambidentate cyanide groups ([23] and references therein).The first example of a rhenium cluster with apical CN ligands was KCs₃Re₆S₈(CN)₆ obtained by reaction at medium temperatures (500 °C) of the cluster-based Re₆Te₁₅ with KSCN [24]. Subsequently an extensive study of the cyano compounds, including especially octahedral rhenium clusters has been carried out in the group of V.E. Fedorov, as reviewed in [23].

The use of the longer and symmetric N₃ chain instead of the asymmetric CN group should allow access to relevant M₆Lⁱ₈(N₃)^a₆ building blocks that should greatly influence the structure and the properties of the resulting compounds. The interaction of the above, water soluble, Cs₄Re₆S₈Br₆,CsBr solid state compound with aqueous solutions of sodium azide has been investigated, giving access to the new cluster compound Cs₄Re₆S₈(N₃)₆·H₂O. The latter constitutes the first example of a Re₆ compound containing N₃ apical ligands [25]. Its structure consists in three crystallographic independent azide (N₃)^– anions that are linked to the cluster in such a way that the value of Re–N–N angle is close to 120°. This value is obviously related to the sp² hybridization of the nitrogen atoms of the azide chain. In the resulting Re₆Sⁱ₈(N₃)^a₆ unit, four of these ligands are ordered, while the two other ones are randomly distributed on two positions with roughly equal occupancies (Fig. 11a). According to a schematic description, the stacking of the structure (Fig. 11b) can be described as a succession of layers containing one half of the Cs⁺ countercations and the water molecules for the first one and the other half of the Cs⁺ cations and the [Re₆Sⁱ₈(N₃)^a₆]^4– units for the second.

3.4 Exchange of both cations and ligands

The M₆Lⁱ₈(CN)^a₆ units mentioned above were subsequently involved in the preparation of compounds with various dimensionalities by recrystallization with 3d transition metal cations (M′ = Mn, Fe, Co, ...) through the formation of Re–CN–M′–NC–Re linkages [23]. The coordination chemistry of octahedral rhenium cluster chalcocyanide complexes is in a stage of rapid development. Interaction of these anions with aqua-cations of different transition metals [M′(H₂O)₆]ⁿ⁺ results in a partial or total replacement of their coordination water molecules by nitrogen atoms of the more nucleophilic CN ligands. As a rule such interaction leads to 3-D polymeric structures where four or all six CN ligands of the cluster anions coordinate to cations via Re–CN–M′ bridges, leading, in the latter case, to giant homothetic structures of Prussian blue [23].

The interaction of [Re₆Y₈(CN)₆]^4– anions (Y = S, Se, Te) with transition metal cations in the presence of a chelating ligand (ethylenediamine) reduces the available number of coordination sites of the transition metal ion and afforded a series of new compounds with lower dimensionality [26]:

• – (i) [{Mn(H₂O)(en)₂}{Mn(en)₂}Re₆Te₈(CN)₆]·3 H₂O is built from a polymeric ‘zigzag’ chain where a {Mn(en)₂}complex coordinated by two trans nitrogen atoms belonging to cis apical cyano ligands of the cluster unit links the latter, while the {Mn(H₂O)(en)₂} complex coordinates a third CN ion of each cluster unit (Fig. 12). The chains are neutral and connected to each other by hydrogen bonds through solvate water molecules;
• – (ii) [Ni(NH₃)₂(en)₂]₂[{Ni(NH₃)₄}Re₆Se₈(CN)₆]–Cl₂·2 H₂O and its telluride homologue are isostructural, differing only by an additional water molecule in the latter. They are built from a strictly linear dianionic chain ···N≡C–[Re₆Se₈(CN)₄]–C≡N–[Ni(NH₃)₄]–N≡C–[Re₆Se₈(CN)₄]–C≡N··· of cluster units and nickel tetra-amino complexes (Fig. 13), compensated by two counter-cations Ni(NH₃)₂(en)₂;
• – (iii) (NH₄)₂[{Ni(en)₂}₃{Re₆Te₈(CN)₆}₂]·6 H₂O is built from wavy layers ···{Ni(en)₂}₃{Re₆Te₈(CN)₆}₂··· where each cluster is coordinated by three nickel diamine complexes while each of the latter bridges two clusters. Because these wavy layers are packed exactly one under another, they form channels extending along the c-axis of the trigonal unit-cell, with diameter varying from 4 to 7.2 Å (Fig. 14), where are hosted the ammonium counter-cations and water molecules (not shown for clarity);
• – (iv) finally, (Et₄N)₂[Cu(NH₃)(en)₂]₂[{Cu(en)₂}{Re₆Te₈(CN)₆}₂]·2 H₂O illustrates the synthesis of the large molecular anion [{Cu(en)₂}{Re₆Te₈(CN)₆}₂]^6–, formed by the dimerization of cluster units via a copper ethylene diamine complex bridge, as shown in Fig. 15. Formally, the formation of these dimers can be described as resulting from the cleavage of the above linear infinite chains, may be in relation with the insertion of the large tetaethylammonium counter cation.