Chromium , complexes

These complexes undergo two separate oxidations, both in acetonitrile (MeCN) [26, 27] and Af,W-dimethylformamide (DMF) [28] solution. As shown in Table 7-2, 

Only in the case of the complex in which the ferrocene and the benzenechromiumtricarbonyl fragments are separated from each other by an aliphatic bridge 

R= -CH2C(H)(0H)- precluding electronic interaction, are both oxidation steps (the first on the ferrocene center and the second on the chromium center) chemically reversible. It should be noted that removal of both of the two electrons becomes more difficult on complexation of the two metal fragments. 

As an example, Fig.7-7 illustrates the X-ray structure of one member of this series, (jy -C5H5)Fe[j -C5H4-(SiMe2)2-(J/ -C6H5)Cr(CO)3]. The cyclopentadienyl rings of the ferrocenyl unit assume an eclipsed configuration. 

It can be seen that, on complexation, either the ferrocene or the chromium oxidations become slightly more difficult, but the difference in redox potential decreases with increasing number of SiMe2 units. This suggests that they tend to prevent interactions between the two metal centers. 

The commonest oxidation states of chromium in its complexes are III and II (electronic configuration d3 and d4, respectively), even if Cr(I)-d5 complexes are known. 

The first three steps have been tentatively assigned to the sequence Cr(III)/Cr(II)/Cr(I)/Cr(0), while the fourth has been assigned as ligand centred. However, in judging the remarkable redox ability of these complexes one must bear in mind the comments made previously concerning the reciprocal redox ability of the metal ion and the polypyridine ligands. 

It is clear that the Cr(III)/Cr(II) reduction becomes progressively more difficult on passing from terpyridine to bipyridine to phenantroline. 

In the absence of crystallographic data one cannot discuss in detail the structural variations triggered by these reduction processes, but their electrochemical reversibility, or quasireversibility, suggests that there are not significant structural rearrangements. 

Cr-N distance = 2.03 A average N-Cr-N angle = 78.7°. (b) Cyclic voltam-mogram recorded at a dropping mercury electrode in a MeCN solution of [Cr(terpy)2][C104]3. Scan rate 0.1 Vs 1 

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Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

After penetration of the hide by the chromium the pH is raised to about 3.5—4.0. At this higher pH a change occurs in the chromium complexes as the basicity of the chromium increases and binding to the protein becomes possible. Chromium binds firmly to the protein forming a cross-link species, and as the pH increases the hydrogen is removed from the complex forming a stable stmcture. 

Sihcone products dominate the pressure-sensitive adhesive release paper market, but other materials such as Quilon (E.I. du Pont de Nemours Co., Inc.), a Werner-type chromium complex, stearato chromic chloride [12768-56-8] are also used. Various base papers are used, including polyethylene-coated kraft as well as polymer substrates such as polyethylene or polyester film. Sihcone coatings that cross-link to form a film and also bond to the cellulose are used in various forms, such as solvent and solventless dispersions and emulsions. Technical requirements for the coated papers include good release, no contamination of the adhesive being protected, no blocking in roUs, good solvent holdout with respect to adhesives appHed from solvent, and good thermal and dimensional stabiUty (see Silicon COMPOUNDS, silicones). 

Chromium complexes of long-chain fatty acids are exceUent water repeUents which are also used for their food-release properties in certain packaging appHcations. The presence of chromium has raised environmental concerns, despite the fact that the metal is in the trivalent rather than in the highly toxic hexavalent state. This material is available as Qudon (DuPont). 

Premetallized Dyes. Although discovered in 1912, the 1 1 chromium complexes known as Palatine Fast (BASF) and Neolan (Ciba) dyes had httie practical use as wool dyes until 1920 when it was found that a strongly acidic dyebath (pH ca 2.0) (51) was requited to obtain satisfactory dyeing and acceptable fastness properties. Dyes of this type exemplified by Neolan Blue 2G [6370-12-3] (57) (Cl Acid Blue 158A Cl 15050) are stiU in use despite the damage to the wool caused by the strong acid in the dyebath. 

The use of acetoacetaniUdes and 2,4-dihydroxyquinolines as coupling components is demonstrated in the 1 1 chromium complexes Neolan YeUow... 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Methylthiophene is metallated in the 5-position whereas 3-methoxy-, 3-methylthio-, 3-carboxy- and 3-bromo-thiophenes are metallated in the 2-position (80TL5051). Lithiation of tricarbonyl(i7 -N-protected indole)chromium complexes occurs initially at C-2. If this position is trimethylsilylated, subsequent lithiation is at C-7 with minor amounts at C-4 (81CC1260). Tricarbonyl(Tj -l-triisopropylsilylindole)chromium(0) is selectively lithiated at C-4 by n-butyllithium-TMEDA. This offers an attractive intermediate for the preparation of 4-substituted indoles by reaction with electrophiles and deprotection by irradiation (82CC467). 

Hydroquinone synthesis (regiospecific) from alkynes and carbonyl carbene chromium complexes... 

Chromium complexes of fluoroaromatics undergo fluorine replacement more readily and in high yield [24] (equation 14). 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Chromium lignosulfonates are the biggest contributions to heavy metals in drilling fluids. Although studies have shown minimal environmental impact, substitutes exist that can result in lower chromium levels in muds. The less used chromium lignites (trivalent chromium complexes) are similar in character and performance with less chromium. Nonchromium substitutes are effective in many situations. Typical total chromium levels in muds are 100-1000 mg/1. 

Chromium(IH) forms many complexes, among them those with the following ligands. Give the formula and charge of each chromium complex ion described below. 

S)-Tricarbonyl(2-methoxyacetophenone)chromium is a starting material which provides remarkable substrate-induced stereoselectivity. Thus, its conversion into a boron enolate and subsequent addition to aldehydes delivers the chromium complexes 7 and 8 with diastereomeric ratios of 92 8 to 95 559. 

In a rather different approach optically active chromium complexes of 2,3-dihydro-1 H-in-denone are used as chiral enolate precursors. These chiral complexes react with 3-buten-2-one in benzene using l,5-diazabicyclo[4.3.0]non-5-ene as the base. The diastereomeric ratio of the product is the same irrespectively of whether the exo- or the Noisomer of the chromium... 

Fig. la. Structure of the complex bis( -butoxysilane-diyl)tetracarbonyliron(O) x HMPA 4. b. Molecular structure of the chromium complex bis( -butoxy-silanediyl)pentacarbonylchromium(O) x HMPA 9. c. Van der Waals surface of 9... 

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