Computational studies on selective aromatic C-F bond activation at rhodium and ruthenium
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Density functional theory (DFT) calculations have been carried out to study the selective C–F bond activation of fluoroaromatics at rhodium and ruthenium complexes. The C–F activation reaction of C6F5H with [Rh(SiR3)(PMe3)3] (R3 = Me2Ph, Ph3) to give [Rh(4-C6F4H)(PMe3)3] and FSiR3, has been studied computationally. Using a model system, [Rh(SiMe3)(PMe3)3], calculations show that the lowest energy process occurs via initial phosphine dissociation and subsequent C–F oxidative addition to give trans-[Rh(4-C6F4H)(F)(SiMe3)(PMe3)2], with computed free energies of activation (∆G‡) of +13.2 kcal/mol and +12.4 kcal/mol, respectively. Reductive elimination and phosphine association to give the final products [Rh(4-C6F4H)(PMe3)3] and FSiMe3 are found to be facile. In addition, calculations show that C–F activation at trans- [Rh(SiMe3)(PMe3)2] is more accessible kinetically and thermodynamically than C–H activation (∆∆G‡ = 2.9 kcal/mol, ∆∆G = 51.3 kcal/mol). DFT calculations have been used to model the reaction of C5NF5 at the 2-position with [Rh(X)(PEt3)3] (X = Si(OEt)3, Bpin, where Bpin = pinacolate = –OCMe2CMe2O–). C–F activation at the computational models [Rh(X)(PMe3)3] (X = Si(OMe)3 and Bpin) shows that the lowest pathways proceed via novel silyl- and boryl-assisted C–F activation in which short RhN contacts are computed in the transition states. These occur via modest barriers (∆G‡ = +26.1 kcal/mol and +20.1 kcal/mol, respectively, relative to the two separated reactants) and also account for the experimental selectivity. The hydrodefluorination (HDF) reaction of C6F5H at [Ru(H)2(CO)(NHC)(PR3)2] (NHC = SIMes, SIPr, IMes, IPr; R = Ph) to give 1,2,3,4-C6F4H2, has been investigated. Calculations on small (NHC = IMe, R = H) and full systems (NHC = IMes, R = Ph) have allowed a novel class of reaction mechanism to be defined involving a nucleophilic attack of one hydride ligand at C6F5H. The most accessible pathway has a computed transition state energy of +20.1 kcal/mol in THF (PCM, approach). In addition, calculations reveal that the use of a more sterically encumbered full model system is essential to explain the unusual ortho-regioselectivity observed experimentally.