Molybdenum cluster species

Trinuclear clusters play an important role in the chemistry of molybdenum and tungsten. Deep red, isostructural clusters containing both [M03O4F9]3- (148, 149) and [W304F9]5 (149-151) are later additions to this family of simplest types of cluster species. Their basic structures conform to the fii-type of trinuclear electron-poor transition-metal clusters where the metals are in a distorted octahedral environment. 

Molybdenum Clusters of the Type Mo Cl L. The chemistry described here was initiated with the notion that it should be possible to observe addition of molecular species X-Y across multiple metal-metal bonds, for example as in the process... 

Enzymes containing molybdenum are of two types (1) Nitrogenases, which are required for converting atmospheric nitrogen to nitrogen compounds (NH3, for example) nitrogenases contain a characteristic polymetal atom cluster species called the iron-molybdenum cofactor, FeMoco (Section 17-E-10). (2) The other Mo enzymes, all of which have some variant of a characteristic molybdenum cofactor, Moco. 

The lower nuclearity clusters [Mo5Cl13]2 were isolated from MoCls in AlCl3/KCl/BiCl3/Bi melts that led to the hexanuclear species (17). This is the only intermediate isolated from reaction mixture that produces the Mo6C18 4+ core. Solution chemistry has yielded Mo2, Mo3, and Mo4 halide species from mononuclear molybdenum complexes, suggesting that a nucleation process similar to that proposed for the tungsten systems may take place during the formation of hexanuclear molybdenum clusters (18-22). 

During the past several years it has been shown that both molybdenum and tungsten in oxidation states of +4 or thereabouts, have marked predilection to form trinuclear cluster species. For the tungsten cluster species, there are three different structural types with respect to the ligand arrangements (Figure 24 a, b, c) which are based on an equilateral triangle of M—bonded atoms. Structure (a) contains the [W3(jU3-X)(jU-Y)3] nucleus, where X = 0, Cl,... 

In accordance with what it was indicated in previous works [10,12] during the calcination of solids obtained from adsorption of high AHM concentration solutions on y-alumina, the molybdenum polymeric species decomposes, thus leading to MoOj clusters. This crystalline compound has been observed by X-ray diffraction. The same fact can be expected from the results of samples prepared with AHM and PA solutions (P/Mo=0.4). Actually, M0O3 was detected in samples corresponding to the second plateau. However, the presence of MoO was not observed in samples prepared from FK solutions. It can therefore be said that, in this case, after calcination, the molybdenum would be as Mo(t) which interacts strongly with the alumina, and as Mo(o) probably better dispersed than in the previous systems. 

In the high-valence cluster species, metal atoms forming the metal network appear to have positive intermediate oxidation states. However, metal oxidation states in cluster are always lower than those characteristic for the same metals in classical mononuclear complexes. The ligands associated to this class of cluster are normally good cr-donors that according to the classification of Pearson would have intermediate hardness. Among these compounds the most frequent are the halides, specially chlorides and bromides. An example of this cluster class is the molybdenum species [Mo6Xs] shown in Fig. 2.1c. 

A different interpretation of the role played by the different molybdenum oxide species deposited on the surface of silica was proposed for the oxidation of CH4 into HCHO. Thus, Suzuki et al. [125] concluded that well-dispersed molybdenum oxide clusters on the silica surface are the active species for the reaction. By contrast, Ozkan and coworkers [126,127] proposed that the Mo=0 sites present in the M0O3 crystals are catalytically active while Mo-O-Mo bonds accelerate the deep oxidation of methane. However, the reactivity of supported molybdenum oxide differs from bulk M0O3, and no direct extrapolation can be made [103]. 

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-0, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained vinually intact after ion exchange. 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

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