Cluster units

Lower formal oxidation states are stabilized, however, by M-M bonding in ternary chalcogenides such as M MeQn, M4M6Q13 (M = alkali metal M = Re, Tc Q = S, Se) and the recently reported M gMeS. Their structures are all based on the face-capped, octahedral MeXg cluster unit found in Chevrel phases (p. 1018) and in the dihalides of Mo and W... 

Fig. 5.1 [(Zr6Z)XTg] cluster unit in reduced zirconium halides.

Fig. 5.19 View of the triclinic structure of the mixed-halide A [(Zr6Z)(CI,l)i8] phase (here with Z = Fe), which consists of molecular cluster units (thermal ellipsoids with 50% probability).

Examples of recently reported oxychlorides with one- or quasi-one-dimensional frameworks include A2Ti2Nb6Cli405 and Cs2Ti4Nbi5Cli80i5 in which the octahedral cluster unit has 5 or 6 oxygen ligands, respectively. 

Fig. 6.4 (a) Fragment of the structure of Cs2Ti4Nb6Clis06 (light and dark) polyhedra represent (Ti(l)Cl402) and (Ti(2)Cl6) octahedra, respectively), (b) Cluster unit in... 

Cs2Ti4Nb6Clis06. (c) A tetrameric propellerlike (Ti4CI]206) unit in Cs2Ti4Nb6Chg06 built from four (TiLj) octahedra that share edges, (d) Cluster unit in PbLusNbeChsOe. 

The structural features of most niobium oxychlorides known to-date are summarized in Table 6.1. The use of a combination of chloride and oxide hgands leads to compounds with unique structure types [41], characterized by a remarkable variety of cluster frameworks, ranging from discrete cluster units to chains, layers, and three-dimensional nets, some topologies of which are unprecedented in compounds containing octahedral clusters. Most of the niobium oxychlorides known to date have anisotropic structures (the exceptions are Cs2LuNb,5Cli70 and PbLusNbsClisOg). 

Fig. 6.9 Differentiation of niobium atoms and outer chloride ligands in the cluster unit of Ti2Nb6Cli404 and [Tl5(Ti2Cl9)][(NbeCI,204)3(Ti3Cl4)2]. The numbers in the boxes are electrostatic charges calculated by the DFT method.

Fig. 6.12 (a) A perspective of AxNb6Cli202 structure in the a direction, (b) A view of the cluster framework in AxNb6CI,202 emphasizing the difference in intercluster linkages in the a direction and within the be plane, (c) Cluster unit in AxNb5Cli202-... 

Geometry optimization was performed on the discrete cluster unit to eliminate the effects of crystal packing and interactions with titanium ions. The calculations were carried out in local spin density approximation using a SPARTAN 5.0.3 package (Wavefanction, Inc., Irvine, CA 92612 USA). 

The exploratory solid-state synthetic work of John Corbett has illustrated the diversity, beauty and richness of this chemistry with a large variety of new phases and structures [1-3]. John Corbett was also the pioneer who recognized the potential of these cluster polymers in the development of a versatile solution chemistry [4]. Once the cluster unit has been identified in the solid state, the excision of this motif appears as the most rational method for accessing these cluster complexes in solution [5]. 

The coordination of redox-active ligands such as 1,2-bis-dithiolates, to the M03Q7 cluster unit, results in oxidation-active complexes in sharp contrast with the electrochemical behavior found for the [Mo3S7Br6] di-anion for which no oxidation process is observed by cyclic voltammetry in acetonitrile within the allowed solvent window [38]. The oxidation potentials are easily accessible and this property can be used to obtain a new family of single-component molecular conductors as will be presented in the next section. Upon reduction, [M03S7 (dithiolate)3] type-11 complexes transform into [Mo3S4(dithiolate)3] type-I dianions, as represented in Eq. (7). 

The potential of the cluster units described here to participate in intermolecular chalcogen-chalcogen interactions combined with the easy modification of their outer coordination sphere with ligands of different nature, i.e., redox active, hydrogen donors, bi-functional, etc., make these systems useful blocks for the construction of supramolecular materials with multi-physical properties. 

Cluster unit with an interstitial atom in compounds such as ZrgCCl14 and ThgFeBr15... 

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