VO , complex

Figure 13 Catalytic cycle of VO complexes in four-electron reduction of 02.

Delivery of oxygen from OH through a VO complex is particularly effective when the OH group is pseudoaxial and the f-Bu group ensures this. 

Attempts to model the structure of the V-dependent bromoperoxidase enzyme have resnlted in the characterization by X-ray crystallography of a great number of VO + complexes having N,N,N,N-donor atoms. The imidazole group is thought to mimic the protein enviromnent and complexes such as [VO(SALIMH)(acac)(MeOH)], [VO(SALIMH)SAL], and [VO(SALIMH)2(EtOH)], have been prepared " (see Vanadium in Biology). 

The potential of a silver electrode during the course of the titration of silver nitrate with potassium c an-ide is shown in Fig. 78 the first marked change of potential occurs when one equivalent of cyanide has been added to one of silver, so that the whole of the silver cyanide is precipitated, and the second, when two equivalents of cyanide have been added, corresponds to the complete formation of the Ag(CN)2 ion. It will be seen that the changes of potential occur very sharply in each case this means that the silver cyanide is very slightly soluble and that, t+vo complex ion is very stable. 

The observations on CaMo04 support, therefore, the assignment presented above for the VO complex. 

The point to be stressed here is the decay time of the yellow emission of the VO4 —Bi complex. Note that it is considerably shorter (two orders of magnitude) than the decay time of the isolated VO complex. An obvious explanation for this is the presence of the heavy Bi ion with its strong spin-orbit coupling. The hole of the excited state spends a considerable amount of its time on the Bi ion and as a consequence the spin-selection rule is strongly relaxed. 

The characteristics of ESR spectra for d VO complexes are similar to the ESR spectra for d Cu (where the single-hole formalism is equated with a single unpaired electron) except that each principal axis is split into eight hyperfine lines by the vanadyl nucleus (/ = 7/2). Comparison of g values and hyperfine coupling constants with values for complexes of known structure provides information on the donor atoms coordinated to in an experimental system. The intensity of aquated... 

Polarized single-crystal electronic spectral data and ab initio MO calculations support the assignment of the lower energy absorption in VO complexes to an... 

The major contrast with the Group 4 metals is the stability of VO + complexes which are the most important and the most widely studied of the vanadium(IV) complexes, and are the usual products of the hydrolysis of other vanadium(IV) complexes. behaves as a... 

The racemic substrates depicted in the following diagram form diastereomeric 7t-allylpalladium complexes which resemble the aryl substituted wc.vo-complexes (see meso-n-A 11 y l pa 11 ad i u m Complexes). Each substrate is regioselectivcly attacked (e.g., with ratios of 43 6 and 1 50, respectively, for entry 1) to give the alkylated constitutional isomers in mostly high enantiomeric excess by using the ferrocenyl phosphine 22 32. 

Here it is reduced to or [VO] + by the polyphenolic blood pigment tunichrome. (Note the structural relationship between tunichrome and l-DOPA, 26.5.) Storage of vanadium must involve the formation of or [VO] + complexes, but the nature of these species is not known. 

The particular stability and large number of vanadyl complexes is reflected in the number of ESR studies on these systems. Most VO + complexes have the square pyramid structure and may frequently add on Lewis bases to the sixth coordination position. The ion has been substituted in various alums (d8I) and the spectrum attributed to [V0(H20)4]2+. Substitution in (NH4)2Zn(S04)2 6H20 yielded (100, 448) s.h.f.s. attributable to protons from four equatorial water ligands. After subtraction of the dipolar contribution, was 2.6 gauss. Bernal... 

V0(MNT)2] and the binuclear (116) [Mo2(TDT)5] have also been prepared. The former (42) is much more like a typical VO + complex, showing the dominance of the V=0 system. A simpler method of preparing MNT complexes has been reported (42) which involves direct reaction of a metal salt with sodium dithiocyanoformate and eliminates the troublesome synthesis of NagMNT. 

Styryl-functionalized vanadyl(lV) salen covalently anchored onto mercapto-modifled ACs and SWCNTs showed high catalytic activity in the cyanosilylation of benzaldehyde with substrate conversion of 83 and 93%, respectively [91]. The SWCNTs were shown to be more suitable support for the VO complex relative to the high-surface-area AC, because the latter support exhibited some adventitious activity. The asymmetric version of the reaction was also performed using the chiral vanadyl complex the SWCNTs also behaved as a better support, as the %ee was 66%, whereas for AC the %ee was 48% [91]. The VO(IV) complex immobilized onto SWCNTs was also tested in the catalytic cyanosilylation of hexanal and 4-fluorobenzaldehyde with high substrate conversions 97 and 96%, respectively. 

ESR spectroscopy is a particularly useful technique as it provides information on the stereochemistry, ligand type and degree of covalency of V complexes. The fundamental ESR properties of VO " complexes have been reviewed. " Petrakis and co-workers have shown that... 

In an early publication, Zelentsov reported the preparation of a complex with subnormal magnetic properties = 0.87 BM) with the SB (101). The compound was prepared from dibenzoylmethane and o-aminophenol. The VO " complex with (102) has also been shown to be involved in antiferromagnetic exchange with 7 = —139cm Revenko and Gerbeleu prepared several complexes of (103) derived from Hsal and thiosemicarbazide which... 

Other weak outer-sphere forces, such as electrostatic interactions [43] and steric contributions [44], between the substrate and the VO-complexes [VO(l)] and [VO(3)] were also shown to contribute to the mode of chiral binding in the asymmetric adducts. In the specific case of [VO(3)], removal of the bulky inner ferf-butyl groups from the 3,3 positions was not found to moderate the electronic properties of the VO centre (revealed via EPR) or its interactions with the surrounding ligand " N nuclei (revealed via HYSCORE), but was found to reverse the stereoselectivity of epoxide binding [44]. Whilst homochiral... 

The widest columnar mesophase temperature ranges were obtained for the bis-[l,3-di-(substi-tuted-phenyl)-/3-diketonate] metal complexes bearing ten and twelve chains ((55) R = H or OC H2 +i). The ten-chain copper, palladium, and oxovanadium(IV) complexes ((55) M = Cu, Pd, VO R = H, = 6, 8, 10, 12, 14) were all mesomorphic and the enantiotropic mesophases were identified by optical texture and variable-temperature X-ray diffraction as columnar phases (Table 34). The copper and palladium complexes displayed a Coh phase for short chain length ( = 6, 8 for M = Cu = 6, 8, 10 for M = Pd), which transformed to a Coin phase as the chain length was increased. Surprisingly, no direct Cok-to-Colh phase transition was observed within the same compound, but weakly first-order Cok-to-Cok and Colh-to-Colh phase transitions were found for compounds with intermediate chain lengths. In contrast, the vanadyl complexes exhibited only one Coh mesophase. Infrared studies indicated that the VO complexes possessed a linear V=0—V=0 linear polymeric chain structure in the crystal phase, while no... 

The other magnetic property of interest for VO + complexes is the susceptibility. The theoretical expression for the magnetic susceptibility is of the form ... 

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