Vanadium clusters

Iseda M, Nishio T, Han S Y, Yoshida H, Terasaki A and Kondow T 1997 Electronic structure of vanadium cluster anions as studied by photoeieotron spectroscopy J. Chem. Phys. 106 2182... 

Gronbeck H and Rosen A 1997 Geometric and electronic properties of small vanadium clusters a density functional study J. Chem. Phys. 107 10 620... 

Wu H, Desai S R and Wang L S 1996 Evolution of the electronic structure of small vanadium clusters from molecular to bulk-like Phys. Rev. Lett. 77 2436... 

Liu F, Khanna S N and Jena P 1991 Magnetism in small vanadium clusters Phys. Rev. B 43 8179... 

Both vanadium and niobium metals form dihydrides only at high pressures(27), and numerous phases with hydrogen compositions less than one(28). Experiments were performed to saturate vanadium clusters with deuterium. Figure 4 is a plot of the number of deuteri urn molecules found in the products. The solid straight lines are for D V ratios of 1 and 2. The corresponding curved dashed lines include corrections for some bulk atoms(2c). The best fit to the data including only surface atoms indicate a stoichiometry of 1.5. It is likely that this high surface stoichiometry is an indication of bulk i ncorporation of deuterium. 

Figure 3. Logarithm of the reactivities of niobium and vanadium clusters. The Exxon data (circles) are scaled relative to the Argonne Fe o reactivity, and the Rice data (crosses) are normalized to Nbg.

Open M4 units are also quite rare. In the selenobromides M4Se16Br2 (M=Nb, Ta) there are tetranuclear units M4(p-Se2)r,, which result from condensation of three M2( r-Se2)2 clusters. The M-M distances within them are about 3.1 A.52 Single crystal structure determination of V5S8 revealed the presence of rhombic vanadium clusters in the structure with hinge V-V bond of 2.88 A, peripheral bonds of 3.04 A and strong intercluster bonding (V-V 2.92 A).53... 

Under anhydrous conditions, tetranuclear vanadium clusters such as [V404 (0CH2)3CCH3 3(0C2H5)3] can be generated in alcoholic solutions with l,l,l-tris(hydroxymethyl)ethane. The coordination does vary somewhat with the alcohol utilized in the preparation, but all retain the tetranuclear structure. Each vanadium nucleus in the complex has octahedral coordination. The clusters are not stable to hydrolysis and decompose in chloroform solution in the presence of just a few equivalents of water [5],... 

SCHEME 10.2 The cofactor for nitrogenase activity, an iron-sulfur-vanadium cluster. 

Thus, a number of systems of the catalytic and noncatalytic reduction of dinitrogen to hydrazin and ammonia and the successful synthesis of model iron- and iron-molibdenum (vanadium) clusters have been reported. These investigations have formed a basis for subsequent progress in mimicking the nitrogenase reaction. 

Last but not least, mention should be made of the use of redox active organometallic clusters for the formation of CT complex. An early example was reported by Eisenberg, Miller and co-workers who exploited the trinuclear niobium cluster [Nb3(/i-Cl)6(>7 -C6Me6)3] and prepared the [TCNQ] salt, upon oxidation to the dicationic form. This material was found to be paramagnetic and to display a relatively modest conductivity (urt = 0.001 Scm , semiconductor) [75]. Finally, the pentanuclear trigonal bipyramidal vanadium cluster [(MeCp)5V5(/r -S)6] was shown to undergo CT complex formation with TCNQ affording [(MeCp)5V5(/i -S)6] [(TCNQ)2] [76]. This material, which was structurally characterized, is an n-type semiconductor with a band gap of 0.25 eV. 

Saturation studies of vanadium clusters containing up to 30 atoms suggest... 

Polynuclear aggregates of octahedral V(III) (cf, S = 1) are also a potential source of SMMs. The fact that the number of known V(III) clusters is small is due to its air-sensitivity, which adds some difficulty to their preparation and handUng. Among the relatively few paramagnetic vanadium clusters reported [188-190], only one structural type has proved to be a SMM. 

The three catalysts with different vanadium loadings gave different activities and selectivities and the catalyst with 6.4% by weight of V2O5 was the most active and selective. A similar trend has been also obtained in the runs performed at 400°C. These results demonstrate the sensitivity of ODH of propane to the structure of the vanadium clusters, but it seems that other factors must also be considered, such as the acidity of these clusters and the effect of titanium that, as we have demonstrated, can be amorphous, in this case, and not necessarily present as a homogeneous surface but simply surrounding vanadium oxide sites. 

A more powerful experimental technique to probe the electronic structure of transition-metal clusters is size-selected anion photoelectron spectroscopy (PES) [70. 71. 72. 73. 74. 75 and 76]. In PES experiments, a size-selected anion cluster is photodetached by a fixed wavelength photon and the kinetic energies of the photoemitted electrons are measured. PES experiments provide direct measure of the electron affinity and electronic energy levels of neutral clusters. This technique has been used to study many types of clusters over a large cluster size range and can probe how the electronic structures of transition-metal clusters evolve from molecular to bulk [77. 78. 79, 80 and M] Research has focused on the 3d transition-metal clusters, for which there have also been many theoretical studies [82, M, M, 86, M and 89]. It is found that the electronic structure of the small transition-metal clusters is molecular in nature, with discrete electronic states. However, the electronic structure of the transition-metal clusters approaches that of the bulk rapidly. Figure Cl. 14 shows that the electronic structure of vanadium clusters with 65 atoms is already very similar to that of bulk vanadium [90]. Other 3d transition-metal clusters also show bulk-like electronic structures in similar size range [78]. 

G. A. Ozin and C. G. Francis, J. Mol Struct., 59, 55 (1980). The same experiments can be earned out at 290 K using a meta-substituted pentaphenyl ether (Santovac-5) manufactured by Monsanto. The spectroscopic properties of the products up to a vanadium cluster nuclearity, n - 3 (microsolution) or n = 2 (macroscale), are identical to those of the DC510 supported vanadium. 

Fig.22 STM images of ordered metal cluster arrays grown on Al203/Ni3Al(l 11). a Vanadium clusters grown at 550 K [184], b Pd clusters grown at RT [186], and c Fe clusters grown on the secondary Pd/Al203/Ni3Al(l 11) template [77]...

Figure 24. A view of the structure of Kn Vi50(P04)i8 parallel to the crystallographic a axis (a). The nonanuclear vanadium cluster embedded in the structure (b).

Figure 53. (a) The pentanuclear vanadium cluster embedded in the structure of [Mc2NH2]K4... 

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