Computational studies on selective aromatic C-F bond activation at rhodium and ruthenium

Abstract

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.