Chromium complexes bonding

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). 

Nickel and palladium react with a number of olefins other than ethylene, to afford a wide range of binary complexes. With styrene (11), Ni atoms react at 77 K to form tris(styrene)Ni(0), a red-brown solid that decomposes at -20 °C. The ability of nickel atoms to coordinate three olefins with a bulky phenyl substituent illustrates that the steric and electronic effects (54,141) responsible for the stability of a tris (planar) coordination are not sufficiently great to preclude formation of a tris complex rather than a bis (olefin) species as the highest-stoichiometry complex. In contrast to the nickel-atom reaction, chromium atoms react (11) with styrene, to form both polystyrene and an intractable material in which chromium is bonded to polystyrene. It would be interesting to ascertain whether such a polymeric material might have any catal3dic activity, in view of the current interest in polymer-sup-ported catalysts (51). 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

The complexes Cr(CO)3L, with L = phenanthrene, naphthalene, or anthracene, are more active for diene hydrogenation than with L = substituted benzenes (see also Section VIII), and this is attributed to an easier displacement of the arene by the diene substrate, the phenanthrene type being asymmetrically bonded, having two longer and more readily cleaved chromium-carbon bonds (198, 199). 

Uemura and coworkers utilized an arene-chromium complex ligand to form the C—N bond on a pyridine ring [146]. In the presence of 3 mol% of the monophosphine(dicarbonyl)chromium... 

Apparently, there is not much advantage in using bond enthalpy contributions to discuss bonding energetics in a series of similar complexes. As already stated, we could have selected any value for Z)//,°(Cr-CO) + DH (Cr-CO) + Z)//j (Cr-CO) and then derived chromium-arene bond dissociation enthalpies in Cr(CO)3(arene) compounds, all based on the same anchor. The trend would not be affected by our choice. Nevertheless, besides emphasizing that the absolute values so obtained should not be regarded as bond dissociation enthalpies, the bond enthalpy contribution concept attempts to consider a pertinent issue in molecular energetics the transferability of bond enthalpies. 

According to Widdowson, [(methoxymethoxy)benzene]tricarbonylchromium (448) was deprotonated with enantiotopos differentiation by n-BuLi/(—)-sparteine (11), and the lithium intermediate 449 was trapped by various electrophiles to give the products 451 with ee values up to 97% (equation 122) . Surprisingly, opposite enantiomers are formed when stoichiometric or excess amounts of base are applied. The authors presume that in the dilithium intermediate 450 the C—Li bond (in the rear) has a higher reactivity than the other one (pointed to the front). The deprotonation procedure was also applied to a couple of 1,4-disubstituted chromium complexes . 

Wood, R. M., Aboud, K. A., Palenik, R. C., and Palenik, G. J. (2000). Bond valence sums in coordination chemistry. Calculation of the oxidation state of chromium complexes containing only Cr-O bonds and a redetermination of the crystal structure of potassium tetra(peroxo)chromate(V). Inorg. Chem. 

The kinetics of aquation of a number of azidochromium(III) complexes have been investigated.303,655 Compared with other acidochromium(III) complexes, the chromium-azide bonds in these species seem remarkably stable to thermal substitution. Hence in the base hydrolysis of [CrN3(NH3)s]2+ a pathway involving initial loss of NH3 concurs with the usual base hydrolysis pathway involving loss of Nj. The aquation of azidochromium(III) complexes is H+-assisted with protonation of the azido ligand accounting for the enhanced reactivity. 

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