Transition metal complexes vanadium

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Since then nearly 600 metal-H2 complexes, representing every transition metal from vanadium to platinum, have been isolated or observed... 

The recent interest in five coordination1 has led to an intensive study of a number of transition-metal complexes which appear from their stoichiometry to contain a five-coordinate metal atom. Whereas most of this effort has been focused on the later transition elements, certain key complexes of titanium, vanadium,... 

In this way, the indenyl tethered NHC complexes of yttrium, lutetiiim and scandium were synthesised [135] each still bearing two potentially reactive trimethylsilylmethyl substituents. The potential as single component or cationic (after activation with MAO) catalysts for the polymerisation of olefins was pointed out by the authors, but no studies have as yet been reported. The same can of course be said about the corresponding early transition metal complexes of titanium, zirconium, vanadium and chromium [134,136] where the synthesis of the proligands and the transition metal complexes were reported, but no catalytic studies performed. 

A much more efficient method is the one-pot synthesis of epoxy alcohols by photooxygenation of alkenes in the presence of transition metal complexes of titanium, vanadium or molybdenum. This method has been described for a variety of alkenes84 91. 

It has been reported that several transition metal complexes catalyze the hetero-Diels-Alder reaction between a variety of aldehydes, in particular benzaldehyde, and Danishefsky s diene (Sch. 52). With the [CpRu(CHIRAPHOS)] complex the ee is modest (25 %) (entry 1) [192]. The chiral complex VO(HFBC)2 performs better in this reaction (entry 2) [193]. In experiments directed towards the synthesis of anthra-cyclones, this complex was used in cycloadditions between anthraquinone aldehydes with silyloxy dienes. One example is shown in Sch. 53 [194]. Compared with the chiral aluminum catalyst developed earlier by Yamamoto and co-workers [195], the vanadium catalyst results in lower enantioselectivity but has advantages such as ease of preparation, high solubility, stability towards air and moisture, and selective binding to an aldehyde carbonyl oxygen in the presence of others Lewis-basic coordination sites on the substrate. 

Acetonitrile is a convenient solvent in which to study, by pulse radiolysis, the one-electron reduction of transition metal complexes that are not stable in water or hy-droxylic media. For example, the tantalum compound [Ta2Cl6(4-methylpyridine)4] has been shown to be reduced by CH3CN with A = 1.2 x 10 dm mol s [24], The absorption spectrum of the product shows the characteristic features of d-d transitions and it was suggested that the added electron is delocalized over the double-bonded Ta=Ta moiety. Another example is the radiolytic reduction of the vanadium(III) complex [VCl3(y-pic)3], where y-pic is 4-methylpyridine, to the derivative via the V complex [25]. It was shown by pulse radiolysis that the electron adduct of the complex decayed in a first-order process with k= 1.3 x 10 s which is thought to involve loss of Cl". The intermediate V complex had a... 

This has been one of the useful methods for the preparation of transition metal complexes. Organoantimony and -bismuth anions have also been utilized for the preparation. Several transition metal bonded compounds with vanadium and copper as the transition metal are shown in equations 145 and 146 . ... 

Homogeneous catalysis takes place at transition metal complexes dissolved in the reaction medium. For the oligomerization or polymerization of olefins, such transition metal complexes are provided by some vanadium compounds and the metallocenes, both activated with soluble organoaluminum compounds. The polymers can be dissolved (solution process) or precipitated (slurry process). 

A photochemical approach to cyclopropylmethyl transition-metal complexes was used in the addition of carbonylhydrido(phosphane)vanadium complexes to methylenecyclopropane giving product 3. ... 

The [3-diketiminate ligand with ferf-butyl substituents on the backbone (L Bu, Pr2) gives especially extreme hindrance in late transition metal complexes. Studies on the vanadium chemistry of bulky (3-diketiminates showed that this ligand did not coordinate to vanadium or titanium.1 Only under the right conditions can this ligand be coordinated to Ti(III), albeit in low yields.2... 

Stereoselective oxycarborative addition is also achieved in cycloaddition and cyclooligomeriza-tion reactions. Thus, hetero-Diels-Alder reactions of dienes and aldehydes are not only catalyzed by main group Lewis acids, but also by transition metal complexes 10°. Tris[3-(heptafluoropropyl-hydroxymethylene)-( + )-camphorato]europium [( + )-Eu(hfc)3] and similar vanadium complexes have been used as the chiral catalyst in [4 + 2] cycloadditions of various achiral and chiral dienes to aldehydes63 67-101. With achiral silyloxydienes only moderate asymmetric inductions are observed, however, with chirally modified dienes, high double diastereoselectivities are achieved. Thus, the reaction of benzaldehyde with 3-terf-butyldimethylsilyloxy-l-(/-8-phenvl-menthoxy)-l.3-butadiene (1) gives (2/ .6/ )-4-wf-bntyldimethylsilyloxy-5,6-dihydro-6-phenyl-2-[(17 ,3/ ,45 )-8-phenylmenthoxy]-2f/-pyran (2) in 95% yield with a diasteieoselectivity of 25 1 ss. After crystallization and hydrolysis with trifluoroacetic acid, optically pure (2/ )-2,3-di-hydro-2-phenyl-4-(4//)-pyranone (3) is obtained in 87% yield. 

Hypothesis for the atomic structure of the a and P states are obviously closely related to the suggested mechanism for the formation of these two phases. There appears, however, to be general acceptance of a description of the a phase in terms of a single-site adsorption complex bound to the surface via the carbon atom with a similar mechanism to that used to describe M—C O bonding in the binary carbonyls. Recently, good correlation between the shift in the infrared absorption bands measured by Blyholder and Allen 206) on the first-row transition metals from vanadium to copper has been found by them using a... 

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