Trifluoromethyl-containing transition metal complexes

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. 

The first successful syntheses utilizing trifluoromethyl iodide in transition metal chemistry were reported by Stone and his students. Stone reasoned that if CF3I would not react with transition metal anions to form trifluoromethyl derivatives [see Eq. (3)] then perhaps compounds containing perfluoroalkyl substituents could be generated by the oxidative addition of perfluoroalkyl halides to low valent transition metal substrates (9,10). The first reported trifluoromethyl-substi-tuted transition metal complex prepared by this route is shown in Eq. (4) (41). 

The development of new types of reagents for the formation of perfluoroalkyl derivatives of metals is of interest since, as discussed below (Section II), the types of trifluoromethyl complexes which could be prepared until quite recently have been severely limited by the synthetic methods that have been historically available. Virtually all of the trifluoromethyl-containing transition metal species prepared prior to the 1980s arose from only two types of reactions thermal decarbonylations of trifluoroacetyl complexes or oxidative additions of CF3I. 

Each of these synthetic methods is limited to low valent metal complexes in each case at most two CF3 groups have been substituted onto a metal ion. In Section III some of the contemporary approaches that have been designed to curtail the enforced reliance upon only two precursors are indicated and some of the early results from these preliminary studies into the chemistry of trifluoromethyl-containing transition metal compounds are presented. 

Immediately upon their isolation, trifluoromethyl-containing derivatives of (low valent) transition metals were universally found to be much more robust, both thermally and oxidatively, than their methylated counterparts. Many perfluoromethyl complexes, for example, have been found to be unreactive in air whereas the corresponding methyl derivatives are air sensitive. Under anaerobic conditions a number of trifluoromethylated species decompose at temperatures approximately 100°C higher than the analogous methylated compounds. 

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