Fluorocarbon-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 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]. 

Dyotropic Rearrangements and Related cr-o- Exchange Processes, 16, 33 Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Electron-Transfer Reactions of Mononuclear Organotransition Metal Complexes, 23, I Electron-Transfer Reactions of Polynuclear Organotransition Metal Complexes, 24, 87 Fast Exchange Reactions of Group 1, 11, and 111 Organometallic Compounds, 8, 167 Fischer-Tropsch Reaction, 17, 61 Fluorocarbon Derivatives of Metals, 1, 143... 

Either fusion with alkali metals or reaction with aUcali-metal complexes with aromatic hydrocarbons will break down most fluorocarbon systems, due to the high electron affinities of these systems. Such reactions form the basis of some methods of elemental analysis [13], the fluorine being estimated as hydrogen fluoride after ion exchange. Surface defluorination of PTFE occurs with alkali metals and using other techniques [14]. Per-fluorocycloalkanes give aromatic compounds by passage over hot iron and this provides a potential route to a variety of perfluoroaromatic systems (Chapter 9, Section IB). 

The reactions observed for the dimer complex adsorbed in a Nafion film coated on an ITO electrode at different pH by in situ absorption spectral measurements are summarized as shown in Fig. 11. At higher positive potentials and at potentiostatic conditions, a band at around 450 nm was observed indicating the formation of H20-Ru "-Ru -OH2 at acidic conditions and formation of H20-Ru" "-Ru -OH at basic conditions in addition to the absorbance at 655 nm. This shows that during the catalytic water oxidation process, the diaquo dimer complex exists as an intermediate. In a Nafion polymer membrane, the metal complex is isolated and experiences a micro-heterogeneous environment imposed by hydrophobic fluorocarbon moiety and... 

The Ru-red or Ru-brown complex adsorbed in the Nafion membrane is probably present in a microheterogeneous environment imposed by hydro-phobic cluster made of fluorocarbon moiety and by hydrophilic cluster made of sulfonate ions, and the Ru-red or Ru-brown is electrostatically held by the sulfonate ions. In a polymer membrane, the metal complex molecules would be isolated and the microheterogeneous environment would alter the complex-solvent interaction. Such effects are well characterized for macromolecular metal complexes . Since Ru-red and Ru-brown water oxidation catalysts are strong oxidants in their higher oxidation states, they would attack organic ligands of the... 

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-... 

Mitchell, C.M. Stone, F.G.A. Reaction of low-valent metal complexes with fluorocarbons. XXII. Tertiary phosphine gold complexes. J. Chem. Soc., Dalton. 1972, 102-107. 

Above, the parameters necessary to solubilize important nitrogen ligands in fluorocarbon media have been established. In a concomitant manner, important metal complexes that are needed as precatalysts for many classical catalytic reactions require special attention with regard to fluorocarbon solubility, simply because of the polar and/or ionic nature of these complexes. Thus, we found in our experience that, in many cases, the counteranion also needed fluoro ponytails to ensure fluorocarbon solubility, even if the metal ion was coordinated to a fluorous-soluble ligand such as 1. Therefore, we [4, 5], and Pozzi et al. [6], have addressed this critical... 

In another example, the reaction of a Cu(I) complex, [CuCl], with ligand Rfg-TACN 1, provided a fully fluorocarbon-soluble complex 13 (fully characterized) [8b], without appended fluoroponytails on the Cu(I) metal ion [Eq. (2)], isolated from tri-fluoromethylbenzene. This appears to be a general phenomenon with Cu(I) complexes and fluorous ligands, and apparently is predicated on their hydrophobic properties that engender their solubility in hydrophobic solvents, such as fluorocarbons. 

Another attractive line of application of macromolecular complexes as catalysts is the so-called fluorous catalysis, whose idea was proposed in the early 1990s [163-168]. The main idea of such catalysis is that a catalyst is soluble (immobilized) in the perfluorocarbon phase, whereas the product is soluble in an organic solvent. If a suitable solvent for substrates, e.g. toluene or benzene, is used, then the system at sufficiently high temperature is homogeneous, and on cooling, there is phase separation. As a result, the solution of the metal complex in fluorocarbon is readily separable from the reaction products and can be reused. 

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. 

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... 

Perfluorohydrocarbon-metal complexes are also more stable to reaction with oxygen than thdr hydrocarbon analogues. Other differences between M—Rf and M— R, complexes are the greater resistance of the former to cleavage reactions with acids, and the greater volatility of the fluorocarbon complexes. 

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