cis–trans Isomerisation of CpRe(CO)₂(H)(Ar_F) (Ar_F= C₆F_nH_5−n; n = 0–5) is the rate determining step in C–H activation of fluoroarenes: a DFT study

Abstract

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...

Photochemical CCl bond activation of trichloroarenes by the rhenium complex (η5-C5Me5)Re(CO)3. X-ray structure of trans-(η5-C5Me5)Re(CO)2(2-methoxy-4,5-dichlorophenyl)chloride

“…Since the activation occurs at the chlorine ortho with respect to the OMe, then two possibilities for the coordination of the unsaturated fragment [(h 5 -C 5 Me 5 )Re(CO) 2 ] to 2,4,5-trichloroanisole can be envisaged: coordination through the CCl Ä/C(OMe) or CCl Ä/ CH bonds of the molecule. The former can be discarded by considering both experimental results and recent theoretical studies which have demonstrated that the same rhenium fragment coordinate to the less substituted double bond of partially fluorinated [21] and chlorinated [20] benzenes. In the second intermediate, the fragment should coordinate the CCl Ä/CH bond at the 2,3-position of the arene.…”

“…1) reveals that the activation occurs in the position ortho with respect to the OMe group. Taking into account the model proposed for the C Ã/H and C Ã/F bonds activation of arenes or fluorinated arenes by transition metal complexes [19], and also the experimental evidence for the coordination of chlorobenzenes to rhenium in the complexes (h 5 -C 5 Me 5 )Re(CO) 2 (5,6-h 2 -1,2,4-C 6 H 3 Cl 3 ) and (h 5 -C 5 Me 5 )Re(CO) 2 (5,6-h 2 -1,2,3,4-C 6 H 2 Cl 4 ) [20], we have also considered an intermediate with the chloroarene coordinated in an h 2 -fashion, prior to the insertion of the metal fragment into the C Ã/Cl bond. Since the activation occurs at the chlorine ortho with respect to the OMe, then two possibilities for the coordination of the unsaturated fragment [(h 5 -C 5 Me 5 )Re(CO) 2 ] to 2,4,5-trichloroanisole can be envisaged: coordination through the CCl Ä/C(OMe) or CCl Ä/ CH bonds of the molecule.…”

Selectivity of C–H Activation and Competition between C–H and C–F Bond Activation at Fluorocarbons

“…Thus, fluorobenzene and 1,2-difluorobenzene lead to isolable complexes coordinated via either one or two fluorines, respectively with d 0 Cp 2 Ti IV , 21 The binding energy to a given fluorobenzene, as calculated in a d 6 cyclopentadienyl rhenium complex with DFT(B3LYP) method, decreases from  2 -HC=CH bond to  2 -HC=CF, and to  2 -FC=CF (discussed in more detail in section 4.2.2.3). 16,88 The preference for coordinating the least fluorinated double bond of fluoroarenes is not general. Ni(PEt 3 ) 2 forms an  2 -complex with C 6 F 5 H which could be detected by NMR spectroscopy, but was fluxional even at the lowest temperatures measured.…”

“…The reactions of niobium imido complexes ( -diketiminate)Nb(=N t Bu)( 6 -C 6 H 6 ) with partially fluorinated benzenes Ar-F yield C-F activation products (-diketiminate)Nb(=N t Bu)(Ar)F. 158 Similarly, TpW(NO)( The mechanism of the C-H activation reaction to form products of type C has been probed by DFT(B3PW91) calculations. 88,100 The Re( 2 -arene) complex is converted to a -C-H complex before oxidative addition occurs to yield a cis product that then isomerizes to the final trans-isomer.…”

Unexpected Intermediates and Products in the C−F Bond Activation of Tetrafluorobenzenes with a Bis(triethylphosphine)Nickel Synthon: Direct Evidence of a Rapid and Reversible C−H Bond Activation by Ni(0)

“…However, the selective activation of C-F bonds in the presence of C-H bonds generates additional issues, because many second and third row late transition metal complexes that are capable of the oxidative addition of C-F bonds display lower activation barriers for C-H bond oxidative addition. 4,5 Thus, despite the efficacy of these heavier metals in the activation of perfluorinated aromatic substrates, they are of no utility with more synthetically useful substrates such as penta-, tetra-, tri-and difluorobenzene. Although this difficulty has been bypassed in a few cases by utilizing metal complexes that react with aromatic substrates via electron transfer or nucleophilic mechanisms, 6 these complexes produce activation products which cannot be conveniently incorporated into catalytic cycles.…”