Vanadium cluster compounds

Niobium and tantalum provide no counterpart to the cationic chemistry of vanadium in the -t-3 and -t-2 oxidation states. Instead, they form a series of cluster compounds based... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

It is possible to use ancillary ligands in addition to phosphonic acids in building up nanosized cluster compounds of late transition metal ions. Thus, the reaction of CuCl2 with tert-butylphosphonic acid in the presence of 3,5-dimethylpyrazole affords a dodecanuclear copper phosphonate with an interesting cage structure,3 Similarly, large vanadium phosphonate clusters with up to 18 vanadium atoms have been assembled from phosphonic acids.35... 

Characterization of all cluster compounds has been carried out primarily by single crystal x-ray structure determination. In some cases the exact structural details are ambiguous, for example cluster charge can be difficult to determine due to i) possible mixed valence of the vanadium centres, ii) difficulty in location of hydroxy protons and iii) the possibility that waters of crystallization may infact be hydroxonium ions. In some structures boron hydroxy protons are well located and imply that all trigonal B-O termini around these clusters are protonated, since the reaction pH (7-9) is similar in most cases. 

The complexes derived from simple monodentate carboxylates are not stable, and only one mononuclear complex with a sterically hindered carboxylate has been successfully isolated, [VCKCbC Bujs] (13).103 Attempts to prepare carboxylates in organic solvents often led to the formation of oligomeric cluster compounds with a variety of interesting structural types.103 In aqueous solutions, these vanadium complexes appear to contain six-coordinate vanadium.60,104 Even carbonate has been found to form a weak vanadiumtV) complex in solution.105... 

Modern hydroprocessing is carried out with the use of catalysts. The typical catalysts for this process are oxides and sulfides of nickel, cobalt, molybdenum, and vanadium. Recently, new catalysts for hydrotreatment have been developed based on metal cluster compounds. 

Fig. 7. Cluster compounds modeling the vanadium site in the M cluster of vanadium-nitrogenase. N represents the pyrazolyl moiety dmf = dimethylformamide.

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

The most unusual geometries among these types of clusters are clearly the multiple-decker sandwich structures observed for vanadium ion-benzene clusters Vn(C6H6)+ [142, 151-157]. One interesting finding is that early transition metals all appear to form sandwich compounds with benzene but for the late metals the benzene adducts coat a central metal cluster [158]. 

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