Fluorocarbon-metal bonds

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

Factors affecting the stability of transition-metal bonds to carbon are of continued interest and fluorocarbon transition-metal derivatives are especially interesting [115-117] because of their generally enhanced stability, relative to hydrocarbon analogues. Factors... 

Flash-photolysis studies of the hexacarbonyls Cr(CO)g, Mo(CO)g and W(CO)g are, like previous years, still abundant Nayak and Burkey have found that there are low quantum yields for Cr(CO)g substitution in fluorocarbon solvents - which provides yet more evidence that metal-fluorocarbon interactions are very weak There have been estimates made of solvent-metal bond strengths in (Solvent)M(CO)5 complexes (Solvent = Benzene [M = Mo, W] and Tetrachloromethane [M = Cr]) and the photolysis of W(CO)g in the presence of hex-l-ene has been reported Flash-photolysis studies of the photochemical reactions of silanes with Cr(CO)g have been published Using time-resolved infrared spectroscopy RIR), Turner and co-workers have captured the IR spectrum of the MLCT excited state of W(CO)5(4-cyanopyridine) which rapidly decays to W(CO)5. The excited state of W(CO)5(4-cyanopyridine) is relatively long lived, which makes the experiment possible. This reporter will be interested to see how this technique develops. 

There appear to be limitations on the extent to which reactions between transition metal hydrides and fluoroolefins can be used to make o-bonded fluorocarbon-metal compounds. Unsuccessful attempts have been made to add manganese pentacarbonyl hydride to 1,1-difluoroethylene, 1-chloro-... 

In concluding this section concerned with the preparation of <7-bonded fluorocarbon-metal compounds it is pertinent to mention two reactions which might have given the desired compounds but which in practice do not. 

Frequently, fluorocarbon-metal compounds are known where the comparable alkyl or aryl derivatives are not. To some extent this may be because no serious attempts have been made to prepare the particular alkyl or aryl metal compounds, thus making them unavailable for comparison piuq>oses. However, in many instances either unsuccessful attempts at synthesis have been made, or the hydrocarbon derivative is known but is thermally much less stable than the fluorocarbon analog. At the time of writing much less common are situations where a o--bonded alkyl transition metal group is thermally as robust as the analogous o-bonded fluorocarbon-transition metal group. 

Recently several compounds have been described in which transition metals are bonded to carbon atoms which are in turn joined to a CF3 group. These substances are obtained from reactions between the acetylene hexafluorobut-2-yne and certain metal carbonyls and related compounds. They are obviously closely related to the other fluorocarbon-metal compounds described earlier. 

ABS plastic, a polymer consisting of polybutadiene spheroids is dispersed in a continuous phase of poly(styrene—acrylonitrile). The chromic acid attacks the polybutadiene at a much higher rate than the continuous phase. This gives an excellent microroughened surface with superior metal-to-plastic bond strength. A typical recommended formulation consists of 20 vol % sulfuric acid, 420 g/L chromic acid, and 0.1—1.0% of a fluorocarbon wetting agent. The plastic is treated with this formulation for 6—10 min at 60—65°C. 

One concrete measure of the amount of knowledge which is currently available about fluorocarbon-transition metal complexes is that until recently the entire field could be comprehensively surveyed in one very short chapter in a larger review (9-12). Within the past few years, however, the volume of work has grown to the extent that this type of treatment is no longer possible. Therefore the present discussion narrowly focuses upon only one aspect of fluorocarbon chemistry the synthesis of compounds that contain trifluoromethyl groups bonded to transition metals. 

In summary, several fluorocarbon polymer films are compared for their adhesion to metals. A strong adhesion, as measured by the peel strength, is obtained for Ti and Cr bonding to the polymers with a high concentration of carbon atoms with three fluorine neighbors. 

The design of transition metal complexes capable of C—F bond activation for the functionalization of fluorocarbons has attracted attention recently. It has been known for several years that oxidative addition of an aromatic C—F bond takes place at tungsten(O) to yield stable tungsten(II) metallacycles, the cleaved carbon and fluorine atoms both finishing up bound to the metal centre (Eqn. (2)) [34-36]. 

Peroxidic cure systems are applicable only to fluorocarbon elastomers with cure sites that can generate new stable bonds. Although peroxide-cured fluorocarbon elastomers have inferior heat resistance and compression set, compared with bisphenol cured types they develop excellent physical properties with little or no postcuring. Peroxide cured fluoroelastomers also provide superior resistance to steam, acids, and other aqueous solvents because they do not require metal oxide activators used in bisphenol cure systems. Their difficult processing was an obstacle to their wider use for years, but recent improvements in chemistry and polymerization are offering more opportunities for this class of elastomers [42]. 

The fact that exchange occurs to produce, in each case, the fluorocarbon derivative is quite consistent with the general observation that exchange generally proceeds to give a product where the metal is bonded preferentially to the most electronegative group [33]. 

For a wider discussion of fluorocarbon-transition-metal derivatives, and aspects such as their bonding, the reader is referred to other sources [115-117]. 

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