DFT methods are used to quantify the relationship between M-C and H-C bond energies; for MLn = Re(eta5-C5H5)(CO)2H and fluorinated aryl ligands, theoretical and experimental investigations of ortho-fluorine substitution indicate a much larger increase in the M-C than in the H-C bond energy, so stabilising C-H activation products.

Density functional calculations have been used to examine the reaction of {CpRe(CO) 2 } with fluorobenzenes C 6 F n H 6Ϫn (n = 0-5). Two classes of product have been observed experimentally (using Cp or Cp*): (a) coordination of the arene in an η 2 fashion and (b) C-H activation to form a hydrido-aryl complex. Increasing the number of fluorines on the arene ring was shown to favour C-H activation. The thermodynamic and kinetic (reaction path) aspects of these transformations have been examined with DFT (B3PW91) calculations. For a given arene, the rhenium moiety is shown to exhibit the following order of thermodynamic preference for coordination: HC᎐ ᎐ CH site > HC᎐ ᎐ CF site > FC᎐ ᎐ CF site. Binding energies to the different arenes do not follow a clear trend and span ca. 20 kJ mol Ϫ1 . The Re-C bond energies in CpRe(CO)(H)(C 6 F n H 5Ϫn ) span 55 kJ mol Ϫ1 . Calculated structural parameters agree with the crystal structure of coordination of C 6 H 6 and C 6 F 6 . Likewise the binding energy of C 6 H 6 is in good agreement with experimental data. The calculated free energy difference between CpRe(CO) 2 (η 2 -C 6 F n H 6Ϫn ) and CpRe(CO) 2 (H)(C 6 F n H 5Ϫn ) shows that preference for the hydrido-aryl complex is determined principally by the bond dissociation energy of the C-H bond of the free arene. The binding energy to the η 2 -arene appears to be only a secondary factor. Three families of complexes are apparent. If there is no F on the carbon ortho to the Re-C bond that is formed, the η 2 -arene complex is energetically preferred. If there is one F at the ortho position, the energies of the products are similar. In the case of two ortho F substituents, the product of oxidative addition is significantly favoured. In agreement with the calculations, experimental evidence shows that benzene only coordinates to Cp*Re(CO) 2 , 1,4-C 6 F 2 H 4 gives a mixture of products and 1,3-C 6 F 2 H 4 gives only the hydrido-aryl complex. The arene with the stronger C-H bond is the one which gives more oxidative addition product because the Re-C bond energy increases with F substitution (and in particular with ortho F) more than twice as fast as the C-H bond dissociation energy. The reaction path for the overall transformation has been determined. The σ C-H complex is identified as an intermediate on the pathway for the oxidative addition. The initial product of oxidative addition is the cis hydridoaryl isomer which subsequently isomerizes to the trans isomer. The rate determining step has been found to be the cis-trans isomerisation process and not the oxidation addition step. The cis-trans isomerisation proceeds via an unconventional concerted motion of H and the two COs. The variation of the Re-C bond energy is the dominant factor in determining the changes in the energy barrier between the different fluoroarenes, resulting in strong correlation between the thermodynamics and kinetics of reaction. The activation barriers are therefore also grouped in three families (0 F ortho, 1 F ortho, 2 F ortho). † Based on the presentati...

UV irradiation of [(η 5 -C 5 Me 5 )Re(CO) 2 N 2 ] in neat 1,4-C 6 H 4 F 2 generates a mixture of the C-H activation product trans-[(η 5 -C 5 Me 5 )Re(CO) 2 (2,5-C 6 H 3 F 2 )H] 1a and the η 2 -arene complex [(η 5 -C 5 Me 5 )Re(CO) 2 (2,3-η 2 -1,4-C 6 H 4 F 2 )] 2a, identified on the basis of their IR and NMR spectra. Reaction of the mixture with CHBr 3 allowed the isolation of trans-[(η 5 -C 5 Me 5 )Re(CO) 2 (2,5-C 6 H 3 F 2 )Br] 1a-Br; subsequent reaction with LiBEt 3 H followed by HCl at low temperature provided an independent route to 1a free of 2a. Nevertheless, complex 1a converts to 2a above 213 K. NOESY/EXSY spectroscopy of mixtures of 1a and 2a at low temperature shows that a conformer of 2a is populated at low temperature in which the hydrogen atoms on the coordinated carbons point towards the η 5 -C 5 Me 5 ring. Rapid exchange occurs between the hydrogen atoms on the coordinated carbons in 2a and those on the uncoordinated carbons via an unusual [1,4]-metallotropic shift with ∆H ‡ = 59 ± 8 kJ mol Ϫ1 and ∆S ‡ ≈ 0. There is no evidence from NMR spectroscopy for intermolecular exchange between 1a and 2a or between these complexes and free 1,4-C 6 H 4 F 2 . It is postulated that photolysis initially generates 2a which is converted to 1a in a secondary photochemical step. The ratio of 1a to 2a is controlled by the photochemical conditions and the thermal conversion of 1a to 2a. Thermal reaction of a mixture of 1a and 2a yields two dimers, [(η 5 -C 5 Me 5 ) 2 Re 2 (CO) 5 )] and [{(η 5 -C 5 Me 5 )Re(CO) 2 } 2 (µ-2,3-η 2 -4,5-η 2 -1,4-C 6 H 4 F 2 )] 3. Complex 3 was isolated in low yield and shown to contain the first example of coordination to a CH᎐ ᎐ CF bond as well as a CH᎐ ᎐ CH bond. DFT calculations were carried out onTwo minima were located for each of 2b and 4b corresponding to the two conformations of the arene ring with respect to the cyclopentadienyl group. The most stable complex was 2b, followed by 1b and then by 4b, successfully reproducing the stability of 2a compared to 1a and the absence of experimental evidence for 4a. Theoretical investigations of the [1,4]-metallotropic shift show that it occurs via three sequential [1,2]-shifts. The rate determining step is predicted to be the shift across the C-F moiety. In the transition states, the rhenium atom has a short bond to a single carbon atom and extended bonds to two neighbouring carbon atoms. Scheme 1 UV irradiation products of [(η 5 -C 5 R 5 )Re(CO) 2 (N 2 )] (R = Me, H) in 1,4-C 6 H 4 F 2 .

The UV irradiation of [Re(η 5 -C 5 Me 5 )(CO) 3 ] in the presence of C 6 F 6 effected intermolecular C᎐F and intramolecular C-H activation generating [Re(η 6 -C 5 Me 4 CH 2 )(CO) 2 (C 6 F 5 )] 1a in two isomeric forms. In the major isomer the CH 2 group lies trans to the C 6 F 5 group both in solution and in the crystal. In the minor isomer the CH 2 lies cis to the C 6 F 5 group. A similar reaction with C 6 F 5 CF 3 generates [Re(η 6 -C 5 Me 4 CH 2 )(CO) 2 (C 6 F 4 CF 3 )] 1b in four isomeric forms. In the major form the CF 3 group is in the 4 position and the CH 2 group lies trans to the C 6 F 4 CF 3 group. The other three isomers are formed by rotation of the η 6 -C 5 Me 4 CH 2 ligand as above, by placing the CF 3 at the 3 position, and by a combination of the two. Complex 1a reacted with PMe 3 to form the zwitterionic complex [Re(η 5 -C 5 Me 4 CH 2 PMe 3 )(CO) 2 (C 6 F 5 )] and with MeO Ϫ to form the anion [Re(η 5 -C 5 Me 4 CH 2 OMe)(CO) 2 (C 6 F 5 )] Ϫ , isolable as the NEt 4 ϩ salt. The reaction of 1a with HX (X = Cl or Br) generated cis-[Re(η 5 -C 5 Me 5 )(CO) 2 (C 6 F 5 )X] initially. More prolonged reaction led to the trans isomers. On reaction with HI, only the trans isomer was formed. Reaction of 1a with HBF 4 in Et 2 O in the presence of MeCN led to formation of the salt [Re(ηThe halogens Cl 2 , Br 2 and I 2 reacted to form (halogenomethyl)tetramethylcyclopentadienyl complexes trans-[Re(η 5 -C 5 Me 4 CH 2 X)(CO) 2 (C 6 F 5 )X] (X = Cl, Br or I). The bromo complex has been characterized crystallographically.

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