Transition metal complexes trifluoromethyl compounds

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). 

One important property of CF3-transition metal complexes became apparent almost immediately when all of the low-valent, late transition metal trifluoromethylated compounds then known were found to be significantly more thermally and oxidatively stable than the analogous methylated species. Tetracarbonyl(trifluoromethyl)cobalt(I), for example, was isolated by distillation at 91°C, whereas the hydrocarbyl Co(CO)4(CH3) decomposes at subambient temperatures (72). Additionally, while the reverse of the decarbonylation reaction, CO insertion, is commonly observed in methylated transition metal species, these reactions are essentially unknown for trifluoromethyl metal complexes (13). Prior to 1980, evidence for CO insertion into an M—CF3 bond had been reported in only one case. That reaction employed the photolysis of Mn(C05)(CF3) in an argon matrix at 17 K, and the identity of the product was not determined (14). The clear implication of the above results is that MCF3 metal—carbon bonds are significantly less reactive and thus presumably stronger than MCH3 metal—carbon bonds. 

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. 

Since 1959, a number of other trifluoromethyl transition metal derivatives have been formed from thermal decarbonylations of trifluoroacetyl complexes. Examples of compounds which have been formed by trifluoromethyl migration are listed in Table I. 

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. 

To date, the only trifluoromethyl migrations that have been observed in compounds of the elements that are located in the third row of the transition metals occur in the two Ir(I) complexes which, as noted in Table I, retain the CO linkage after alkyl migration, in CF3CORe(CO)5 (38), and in the platinum(II) species for which a temperature of 210°C was required to initiate the decarbonylation. While the conditions of the last reaction clearly indicate that CF3-Pt bonds are very stable, many if not most organometallic species cannot survive this type of treatment, and the application of the technique to the heavier transition metals is therefore expected to be quite limited. 

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