Vanadium complexes cyanides

A full account of the crystal structure determination of potassium heptacyano-vanadate(m) dihydrate has appeared.387 Apart from clearing up the uncertainty surrounding the nature of the vanadium(m) cyanide complex, this study has established the V(CN)7- ion as one of the very few examples of a seven-co-ordinate complex containing only simple unidentate ligands. The choice of this co-ordination number has been rationalized on the basis of the nine-orbital rule. The two electrons of the paramagnetic V(CN)4 ion (pef = 2.8 BM) occupy two separate orbitals,... 

The chromous salts, derived from the oxide CrO, arc analogous to the salts of divalent vanadium, manganese, and iron. This is seen in the isomorphism of the sulphates of the type R" SOj-THgO. The stability of such salts increases in the order of the atomic number of the metal. The chief basic oxide of chromium is the sesquioxidc CraO, which is closely allied to ferric oxide, and, like the latter, resembles aluminium oxide. The hydroxide, Cr(OH)3, with bases yields chromites analogous to, but less stable than, the aluminates. Chromic sulphate enters into the formation of alums. The chromic salts are very stable, but in the trivaJent condition the metal shows a marked tendency to form complex ions, both anions and cations thus it resembles iron in producing complex cyanides, whilst it also yields compounds similar to the cobaltamines. 

Vanadium forms a number of complex cyanides and sulfocyanides such... 

Figure 9.1 Possible transition state for the addition of a cyanide ion to an aldehyde using the oligomeric vanadium complex proposed by North.

Vanadium(II) reacted with SCN- and diphenylguanidine (L) to form a ternary complex with a V SCN L ratio of 1 2 2.176 Other ternary complexes like phenylguanidine iron(II) cyanide are being exploited in titrimetric determination of cyanide ions, for example. 

Vanadium(m) forms more stable complexes with NCSe- ion in acetone and methyl cyanide than in DMF.383 Spectroscopic detection of the 1 3 and 1 2 complexes in acetone, the 1 2 complex in methyl cyanide, and the 1 1 complex in all three solvents has been achieved. Stepwise stability constants of the complexes are reported as are their electronic spectral parameters. V(antipyrene)3(NCSe)3 and V(diantipyrrylmethane)(NCSe)3 have been isolated from ethanol solutions. 

Since the vanadium center of vanadocenes is electron-deficient, various vanadium derivatives are formed by the attack with PR3, CO, or unsaturated tt-donating hgands/ This reaction provides a useful synthetic method for the synthesis of jj -alkene complexes as described later. Thus formed Cp 2V(CO) (22) is converted to the cyano derivative (23, see Cyanide Complexes of the Transition Metals) on treatment with the isocyanide (Scheme 12). ( -C5Me5)2V is incubated with O2 at -78 °C to yield [(/u.-)] -C5Me503)V(0)]2 via oxygen insertion into the carbon-vanadium bond." ... 

Other tetradentate, tripodal ligands have been employed successfully to stabilise substrates of VNase and intermediates along the reduction pathway of N2. Examples with NSj, and PSj, ligands are provided in Figure 4.30. The complexes 47 contain vanadium in the oxidation states +III (neutral ligands such as hydrazine and isonitrile), -flV [monoanionic ligands such as methyldiazenido(l-) and cyanide(l—)] and... 

Vanadium(III) complexes, 473 adenine, 475 alcohols, 478 amides, 474, 480 amines, 474 amino acids, 484 ammonia, 474 aqua, 477 arsines, 476 azide, 475 bipyridyl, 475 bromides, 483 carboxylates, 479 catecholates, 478 chlorides, 482 complexones, 485 cyanides, 474,476 (Wethyl sulfoxide, 480 dioxygen, 478 dithiocarbamates, 481 dithiolates, 481 dithiophosphinates, 481 ethers, 478... 

The 1,2-addition of a cyanide ion to a carbonyl compound to form a cyanohydrin is a fundamental carbon-carbon bond-forming reaction in organic chemistry, and has frequently been at the forefront of advances in chemical transformations. In 2000, Belokon and North developed a catalytic system based on a vanadium-salen complex (Scheme 9.1). The synthesis of vanadium(iv) complex 1 was accomplished by refluxing a mixture of the corresponding Schiff base with vanadium(iv) sulfate and pyridine in ethanol under an argon atmosphere. A very low catalyst loading of 0.1 mol% was employed to convert aromatic and aliphatic aldehydes to cyanohydrin silyl ethers 3 with enantioselectivities of 68-95% after 24 h. Further investigations... 

In 2006, Khan reported the dimeric vanadium(v)-salen complex S,S,S,S)-5 (Figure 9.2), which could catalyse the asymmetric addition of a cyanide ion to aldehydes using potassium cyanide with acetic anhydride. The reaction of aromatic aldehydes as substrates with 1-5 mol% of complex 5 afforded adducts 4 in excellent yields (90-99%) and high enantioselectivities (88-95%). In addition, the catalyst was able to be precipitated and recycled by adding -hexane to the postcatalytic reaction mixture. In this way, catalyst 5 was readily recovered and reused four times. 

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