Vanadium complexes with benzenes

High sensitivity characterizes the methods based on ion-associates formed by anionic complexes of V(V) with basic dyes. The Brilliant Green method has been described above [33]. The vanadium complex with PAR associated with Crystal Violet is extracted into a mixture of benzene with MIBK (3-f2) (e = l.l lO ) [51]. In a proposed flotation-spectrophotometric method, the V complex with 3,5-dinitropyrocatechol, associated with Rhodamine B, is separated by shaking the solution with cyclohexane the separated compound is washed and dissolved in acetone (e = 2.1-10 at 555 nm) [52], A similar sensitivity is achieved in the method using 5,7-dichloro-oxine and Rhodamine 6G [53]. Another flotation-spectrophotometric method for determining V has been based on 3,5-dinitrosalicylic acid and Rhodamine B [54]. 

Hu and coworker developed the silica, titania, ceria, and y-alumina supported vanadium catalyst, which shown unique reactivity for the direct amination of toluene with hydroxylamine hydrochloride, over 50 % total yield of toluidines was obtained on the y-alumina supported vanadium catalyst operated at optimal conditions [87]. They also investigated a sodium metavanadate catalyzed one-step amination of benzene to aniline with hydroxylamine [88], The reaction became more efficient in the presence of oxygen. A free-radical mechanism was proposed based on the results of EPR, NMR, and UV-Vis characterization. Moreover, the catalytic activity of a series of vanadium complexes with N,0- or 0,0-ligands for the liquid-phase direct amination of benzene with hydroxylamine to aniline has been investigated [89]. [VO(OAc>2] was proved to be the most active catalyst for the amination because of its relatively greater electrophilicity and smaller steric hindrance of ligand. 

Some work has been performed with bis (arene)vanadium and bis(heteroarene) vanadium complexes [7-10]. As indicated for the selected complexes shown in Table 1, replacement of benzene by phos-phabenzene and arsabenzene lowers the reduction potential. This counterintuitive result has been explained in terms of a greater positive charge on the metal... 

Thiosalicylic acid forms a green 2 1 complex with V02+ in aqueous ethanol442 (log K1 = 10.24, log K2 = 8.39). Addition of pyridine causes a more than 10-fold increase in the molar extinction coefficient and a yellow shift affording a chloroform extractable complex which can be used for the analysis of vanadium. The thermodynamic parameters for the 2 1 complex of furfurylmercaptan and V02+ are (35 °Q log K, 8.18 AH° = - 8.18 kcal mol"1, AG°, - 23.31 kcal mol"1 AS°, + 49.12 e.u 443 Stepwise formation constants of the V02+ complex of p-(mercaptoacetamide)chloro-benzene have been determined.444... 

The use of a heterogeneous system under nitrogen consisting of a stirred suspension of silica gel with adsorbed ferric-catechol complex in benzene treated with 35% hydrogen peroxide has been reported to result after 2.5 hours in a 60% yield of phenol (ref. 14). The formation of phenol in 56% yield resulted from a mixture of benzene and vanadium(V) catalyst in acetonitrile under nitrogen when reacted for 2 hours at ambient temperature (ref. 15). More recent studies have involved the conversion of benzene in trifluoromethanesulphonic acid to phenol by the electroreduction of dioxygen (ref. 16) and from generation... 

Akin to group 4 metals, various NHC bidentate or tridentate ligands have been used to stabilize group 5 metals as shown in Scheme 14.24 [49,67]. Vanadium complex 44 could be cleanly oxidized (by 4-methylmorpholine Al-oxide) to the corresponding paramagnetic V (IV) oxo complex LV(=0)Cl2 [66,67]. Like the vanadium (V) complex 39, species 45 was found to be air-stable whether in the solid state or in benzene solution. The vanadium (III) complex 46 resulted from the reaction of V(NMe2)4 with the corresponding imidazolium salt and thus proceeded with a reduction to V(III). 

Ge, H., Leng, Y, Zhou, C., et al. (2008). Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Phase Transfer Catalysts Cyclodextrins Complexes with Vanadium-Substituted Heteropoly Acids, Catal. Lett., 124, pp. 324—329. 

In this process,an air-benzene mixture (3-wt. % benzene) is fed to a tubular reactor (see Fig. 2-4). The tubes are loaded with a vanadium complex catalyst on extruded cylinders or spheres of alumina. Molten salt bath is used to maintain the temperature of 355-375°C. The recovery system is quite similar to the SD process. Very high yields and long catalyst life have been claimed. 

Lu YF, Zhu LF, Liu QY, Guo B, Hu XK, Hu CW (2009) Direct amination of benzene to aniline with several typical vanadium complexes. Chin Chem Lett 20 238-240... 

Benzene-Based Catalyst Technology. The catalyst used for the conversion of ben2ene to maleic anhydride consists of supported vanadium oxide [11099-11-9]. The support is an inert oxide such as kieselguhr, alumina [1344-28-17, or sUica, and is of low surface area (142). Supports with higher surface area adversely affect conversion of benzene to maleic anhydride. The conversion of benzene to maleic anhydride is a less complex oxidation than the conversion of butane, so higher catalyst selectivities are obtained. The vanadium oxide on the surface of the support is often modified with molybdenum oxides. There is approximately 70% vanadium oxide and 30% molybdenum oxide [11098-99-0] in the active phase for these fixed-bed catalysts (143). The molybdenum oxide is thought to form either a soUd solution or compound oxide with the vanadium oxide and result in a more active catalyst (142). 

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

Another route to bis( -arene)vanadium(0) compounds is the cocondensation of arenes with vaporized vanadium metal (see Metal Vapor Synthesis of Transition Metal Compounds) On treatment with 1,3-cyclohexadiene and butyllithium, 15-electron vanadocene (5) is converted to 16-electron ( -benzene)( -cyclopentadienyl)vanadium(l) (6) (Scheme 3). Use of potassium naphthalenide affords the corresponding naphthalene complex. 

Several CT complexes of metal bis(arene) compounds containing, among others, the familiar organic acceptor TCNQ have been reported [71]. None of these materials has been shown to display physical properties superior to those of the metallocene systems. One very important exception is related to the reaction of bis(benzene) vanadium (which is isoelectronic with [MnCpf ]) with TCNE, which affords a material with unprecedented properties [72]. The amorphous material obtained displays bulk ferromagnetism at room temperature and its even exceeds the decomposition temperature of the sample of about 350 K. The compound is no longer an arene complex as it has an empirical composition corresponding to V(TCNE)2 1/2(CH2C12). 

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