Vanadium, borates

The general procedure for the preparation of vanadium borates consists in heating a concentrated H2O solution of boric acid and vanadium oxide in an autoclave at 170 °C for several days [143]. Two different vanadium borate clusters 105 and 106 are obtained, one with two polyborate chains coordinated to a contorted vanadium oxide ring (105) and another one with a macrocyclic Bi8036(0H)6 ring (106). The latter ring is composed of six B306(0H) units and has a chair-like conformation (Fig. 27) [143]. 

Fig. 27. In vanadium borate 106 the macrocyclic borate ring (left) is sandwiched by two smaller (VO)60i2 rings (right) of triangular form via six axial B-( 3-0)-V bonds to form a cage-like cluster anion...

A vanadium borate of uncertain composition is obtained by fusing a mixture of vanadium pentoxidc and boric anhydride. It forms green or yellowish-green crystals which, however, undergo hydrolysis readily.2 A vanadyl borate is also obtained by the action of borax on vanadyl sulphate solution,3 and a complex vanadium borotungstate has been prepared. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Complexes of the titanium and vanadium groups are searee but mainly follow the routine eoordination modes. Sometimes for the dihydrobis(pyrazol-l-yl) borates, the role of M - - - H—B agostie interaetions is signifieant. 

The isoelectronic relationship has been applied to the vanadium complex with hydrotris(pyrazolyl)borato ligand, which possesses the same electronic features as the Cp ligand thus the combination of complex VCl2(NAr)Tp (158) (Ar = 2,6-diisopropylphenyl, Tp = hydrotris(3,5-dimethylpyrazolyl) borate) with MAO was reported to catalyze the polymerization of ethylene (at atmospheric pressure) and propylene (at 7 bar), giving polyethylene (14kg/mol-h, Mw = 47000, Mw/Mn = 3.0) and polypropylene (1 kg/mol h, Mw = 3800, Mw/Mn = 2.0), respectively [256]. 

The Tp hgands (i.e., tris(pyrazolyl)borates or scorpionates [13]) are close analogs to the Cp hgands (i.e., -cyclopentadienyl), and in connection with the chemistry of the Tp Cr complexes mentioned above, a recent example of O2-binding from vanadium chemistry is of interest, even though it is formally not organometalhc chemistry Reaction of the V(IV) complex... 

Holmes, S. and CJ. Carrano. 1991. Models for the binding site in bromoperoxidase Mononuclear vanadium(V) phenolate complexes of the hydridotris(3,5-dimethylpyra-zolyl)borate ligand. Inorg. Chem. 30 1231-1235. 

Oxidation of individual hydrocarbons may be exemplified by the work of Medvedev (232) who has been studying these reactions for a number of years and has more recently investigated the effect of oxygen on polymerization reactions. Neutral phosphates of aluminum, tin or iron, tin borate, etc., were found to be suitable for the conversion of methane to formaldehyde at 500°C. A practical aspect of this research is represented by the homogeneous oxidation of petroleum stocks in the presence of naphthenates which was developed by Petrov into an industrial process to supply fatty acids for the soap making and grease making industry (298,299). Kreshkov oxidized methane to formaldehyde in the presence of chlorine and steam over chlorides of copper or barium or over vanadium pentoxide on carbon (179), but his yields were low. 

Stabilizing the vanadium with auxiliary ligands makes it possible to form complexes with various functionalities including the alkoxy functionality. Such classes of ligands include pyrazolyl borates (15)102 and Schiff bases, and will be described in the appropriate multidentate ligand section. 

The first evidence for activation of exogenous 02 on a vanadium(TV) center was obtained recently.154 The reaction of vanadium(IV) hydroxo tris(3,5-diisopropylpyrazol-l-yl)borate with 02 resulted in the corresponding peroxo complex, which was structurally characterized and found to belong to the class of compounds illustrated in (22).154... 

A cyanohydridoborato vanadium(II) complex, tra s-[V(NCBH3)2(thf)4], has been synthesized and structurally characterized (212).858 Additional oligomeric complexes have been prepared complexes that have been characterized include trinuclear vanadium complexes,859-862 dinuclear V-Fe complexes,863 and the first tetranuclear V11 species [V4(/x3-Cl)2(M-Cl)2(M-CF3C02)2(thf)6].864 The addition of tris(pyrazolyl)borate ligand to the tetranuclear species results in the dissociation of the complex to form a neutral dimeric species [V(tpb)-(/x-Cl)(/x-CF3CO2)2V(th03] (213) in which one of the vanadium centers is facially coordinated by tpb.864... 

This synthesis has been adapted to make the natural (5 -isomer of 114, reducing the initially formed aldehyde to (S)-114 with baker s yeast. [This same synthesis has been adapted to make (S)-callosobruchic acid (115), see below]. Julia s synthesis of a-geraniol (73) was also extended to make 114 by hydro-boration. Hydroboration of 73 with diisopinocampheylborane (made from (- )-a-pinene [(-)-116]) gave only a small ee however. A synthesis of ( )-114 started with the reaction of 2-methylpropiolactone and the ethylene acetal of 3-oxobutylmagnesium bromide. The methyl ester of the acid 117 thus prepared was chain-lengthened by reaction with acetylene and rearrangement with a vanadium catalyst of the ynol thereby obtained. The aldehyde 118 was then reduced with lithium aluminum hydride to 114. ... 

Top row the three components of the Yandulov-Schrock cycle for catalytic dinitrogen reduction. Left the catalyst with the substrate Nj coordinated to Mo (Ar refers to the aryl substituent drawn in detail for one of the nitrogens). Centre the reductant bis(pentamethylcyclopentadienyl)chromium, CpjCr. Right the proton source 2,6-lutidinium borate. Bottom row vanadium complexes with intermediates of nitrogen reduction activated dinitrogen or diazenido(2—) (46a), imide (46b) and ammonia (46c). 

New scorpionate oxovanadium complexes Syntheses and crystal structures of a series of vanadium(V) complexes with mixed ligands of poly(pyrazolyl)borates and substituted acetylacetonates i i n)9y N O R R = Me, Et 82... 

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