Transition metal fluorocarbon complexes

Nuclei other than hydrogen The point dipolar method to estimate distances is difficult to apply for atoms carrying appreciable spin density. This applies to the centre atom of a radical or a transition metal ion complex, but often also to atoms or ligands other than hydrogen. This is for instance the case for in the fluorocarbon anions discussed in Chapter 5. Theoretically computed values are in these cases often sufficiently accurate to support or reject an assignment. In favourable cases modern DFT or ab initio methods can even be of predictive value to reproduce experimental spectra as exemplified in Chapter 5. 

In contrast to their hydrocarbon analogues, many unsaturated fluorocarbons react with anionic transition metal nucleophiles to yield products arising from net displacement of fluoride ion. This synthetic approach has been successful in producing a host of fluorovinyl and fluoroaryl complexes (95-110). 

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 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]. 

Chemical consequences of fluorocarbon phosphines The coordinating properties of pentamethylcyclopentaarsine Trifluorophosphine complexes of transition metals... 

Although most soluble homogeneous catalysts could be made fluorous-soluble by attaching fluorous ponytails to the catalyst core in appropriate size and number [9], transition metal complexes have mostly been converted to fluorous-soluble through ligand modification [10]. The most effective fluorocarbon moieties are linear or branched perfluoroalkyl chains with high carbon number that may con-... 

Functionalization of highly fluorinated molecules at transition metal centers is limited not only by the difficulty of G-F bond activation, but also by the well-established observations that metal-fluoroalkyl and metal-fluoroaryl bonds are stronger and much more kinetically inert than the corresponding metal-alkyl and metal-aryl bonds. Thus, migration, alkene metathesis, GO insertion, alkene insertion, a- and / -elimination of fluorinated substrates are all very challenging to achieve. The reactions described in this chapter form the reaction repertoire for the transition metal complexes interacting with fluorocarbons, a very different set from those for hydrocarbons. 

For a discussion of these and related fluorocarbon complexes of transition metals the reader is referred to the chapter by Treichel and Stone in Volume I of this series. 

An additional interesting physical property associated with perfluorinated phosphine transition metal complexes is fluorocarbon solubility. In contrast to the generally poor solubility of complexes with hydrocarbon ligand substituents in fluorocarbon media, (dfepe)Cr(CO)4 (68) and ((CF3)3P)xFe(CO)5 x (46) are moderately soluble in perfiuoroalkane solvents. While cw-(dfepe)2RuH2 (81) is only sparingly soluble in most polar and nonpolar solvents, it is substantially more... 

In concluding this survey of fluorocarbon tt complexes of transition metals it is of interest to me ntion two compounds which, although they do not have... 

All these arguments for the stability of the M—Rf complexes are speculative and they neglect considerations which may be significant, such as the relative free energies of the fluorocarbon and hydrocarbon radicals. It should be noted that generally very little is known about the importance of factors which affect the stability of transition metal complexes and that a very small change in bond energy may have a marked effect on thermal stability. 

---