Excision Versus Synthesis of Molybdenum Chalcogenide Clusters 

The 'Chevrel Phases', M_xMo₆Q₈ (Q = S, Se, Te) came to the forefront of solid-state chemistry in the 1970s and 1980s. These molybdenum chalcogenides attracted intense interest because of their diverse physical and chemical properties, such as high-critical field (Hc2) superconductivity, magnetic ordering, fast ion conductivity, and catalytic activity for hydrosulfurization. The metal-metal bonded Mo₆Q₈ clusters found in the Chevrel-phase solids are tightly cross-linked by Mo-Q bonds to form a three-dimensional extended framework. In fact, the Mo₆Q₈-cluster compounds are the first in a series of compounds with the general composition M_n-2Mo_3nQ_3n+2 (M = alkali metal, Q = S, Se, Te, n = 2, 3, 4, 5,12 and ¥). The last members of the family (n = ¥) are the polymeric M^IMo₃Q₃ (M = Na, K, Rb, Cs, Tl, Q = S, Se, Te) in which extended chains are built up of trans-face-sharing Mo₆Q₆ octahedra (or a stacking of staggered Mo₃Q₃ monomers). In phases with finite n, oligomeric units exist in which the stacking of Mo₃Se₃ units is truncated by capping with a single Q atom at each end (e.g. Mo₁₈Se₂₀ = Se(Mo₃Se₃)₆Se).

We and others are interested in isolating and derivatizing discrete cluster subunits of these solids, so that these electroactive clusters can be studies as isolated species and perhaps exploited in the preparation of either novel conducting materials or liquid crystals.

 

Preparative work aimed at the formation of Mo₆Se₈ clusters has involved the assembly from smaller cluster fragments, Mo₃Q₄, or the Q/Cl exchange starting with a [Mo₆Cl₈]⁴⁺ cluster. Fedorov and coworkers recently discovered that the binary Chevrel phase compound Mo₆Se₈ serves as a precursor to discrete [Mo₆Se₈(CN)₆]^7- and [Mo₆Se₈(CN)₆]^6- cluster ions when reacted with molten KCN. Though the reactivity and ligand field strength of cyanide makes it an attractive nucleophile for such a ³cluster excision² process, it seemed remarkable that the Mo₆Se₈ clusters might be extricated intact from the tightly crosslinked Chevrel phase structure.

 

These reports suggest the potential use of cyanides as a reaction media for excising or synthesizing chalcogenide clusters. Molten cyanides may serve as a medium for excision or synthesis of other, larger molybdenum chalcogenide clusters. One focus of this research is the use of metal cyanides as a medium for isolating new molybdenum chalcogenide cluster species. Specifically:

1. Can M_n-2Mo_3nSe_3n+2 solids be used as precursors for ³excision² reactions with the aim of obtaining oligomeric clusters in these solids as discrete species?
2. What is the nature of Mo_nQ_m-based cluster products, redox chemistry and reactivity? Many of such metal-metal bonded fragments exhibit variable metal-centered-electron (MCE) counts in precursor solids, thus the range of accessible oxidation states in their molecular analog is of interest.
3. What ligand substitutions can we execute with Mo_nQ_m-based clusters? Might the higher members of the Mo_3nSe_3n+2 series exhibit interesting liquid crystalline behavior?

The reaction of molten KCN with Mo₆Se₈ under anaerobic conditions, yields the new linear chain compound, K₆[Mo₆Se₈(CN)₄(CN)_2/2]. This one-dimensional chain compound, K₆[Mo₆Se₈(CN)₄(CN)_2/2] crystallizes in a body-centered tetragonal structure (I4/m, a = 11.50585(4) Å, c = 9.5177(4) Å), shown in Figure 3. Each of the four molybdenum atoms in the basal-plane of each [Mo₆Se₈]^- cluster are bound to a terminal CN^- ligand, whereas each apical molybdenum atom is bound to a bridging cyanide that links adjacent [Mo₆Se₈]^- clusters to form linear chains, [Mo₆Se₈(CN)₄(CN)_2/2^6-]_n, that propagate up the c-axis.

 

Theoretical treatments for [Mo₆Se₈]^- core indicate that there are 21 cluster bonding electrons and an e_g¹ electron configuration (²E_g state) is expected. In the observed tetragonal environment (nominally D_4h), the e_g (O_h) orbital set will split such that the a_1g(D_4h) descendent will be the SOMO (singly occupied MO) and the b_1g(D_4h) orbital will be unoccupied. The EPR spectrum (T = 10K) is characteristic of an axial system; averaging of g|| (1.9822(1)) and g^ (2.4425(1)) gives g-value of 2.289(1) (Figure 4c). Over the temperature range 1.8 ­ 400K, magnetic susceptibility data are well described by the Curie-Weiss relation, c = c₀ + C/(T + q); c₀ = 1.216 ´ 10^-5 emu/mol, C = 0.497 emu·K/mol, and q = 2.74 K. Magnetic ordering was not observed at any temperature. The magnetic moment obtained from the Curie constant C is 1.99 B.M. (m_eff = 2.828 C^1/2). The magnetic data shows that the unpaired cluster electrons are uncoupled by the bridging cyanide ligands.

 

Mo₆Se₈ need not produce products containing Mo₆Se₈ clusters. Indeed, a reaction done at lower temperature yielded a cubane (Mo₄Se₄) containing K₇NaMo₄Se₄(CN)₁₂·20H₂O. Interestingly, a reaction of Cs₂Mo₁₂Se₁₄ with the molten mixture of NaCN/CsCl produced a single crystal of Cs₇NaMo₄Se₄(CN)₁₂ which was isostructural with the potassium derivative and gave the same cubane structure. Mo₄Se₄(CN)₁₂-containing compounds are not new, but have been prepared by other synthetic routes. However, the [Mo₄Se₄(CN)₁₂]^8- species that we have obtained are the most reduced of such cubanes to date. Under conditions so far examined, only Mo₆Se₈ and the cubane type clusters have been found.