Combined Experimental and Computational Studies on the Nature of Aromatic C−H Activation by Octahedral Ruthenium(II) Complexes: Evidence for σ-Bond Metathesis from Hammett Studies

Abstract

Octahedral ruthenium complexes of the type TpRu(L)(NCMe)R [Tp ) hydridotris(pyrazolyl)borate; R ) alkyl or aryl; L ) CO or PMe 3 ] have been shown previously to initiate the C-H activation of aromatic substrates. In order to probe the nature of the C-H activation step, reaction rates have been theoretically obtained for the conversion of TpRu(L)(η 2 -C,C-C 6 H 5 X)Me to TpRu(L)(p-C 6 H 4 X) and CH 4 where X is varied among Br, Cl, CN, F, H, NH 2 , NO 2 , and OMe. A linear Hammett correlation is calculated with a positive F value of 2.6 for L ) CO and 3.2 for L ) PMe 3 . Calculated kinetic data for the aromatic C-H activations indicate that an electrophilic aromatic substitution mechanism is unlikely. While experiments cannot fully replicate the entire range of calculated Hammett plots, reactivity trends are consistent with the calculations that suggest activation barriers to overall metal-mediated arene C-H bond cleavage are reduced by the presence of electron-withdrawing groups in the position para to the site of activation. Previous mechanistic studies, as well as the structure and imaginary vibrational modes of the present transition states, validate that the C-H activation for this family of TpRu complexes occurs through a σ-bond metathesis-type pathway.

Mechanisms of C–H bond activation: rich synergy between computation and experiment

“…Indeed Hall refers to a 'spectrum of mechanisms for metal-mediated hydrogen transfer' and other workers have emphasized this in their work. 16, 23 This continuum is already apparent in the range of Ru ◊ ◊ ◊ H distances reported by Gunnoe and Cundari. In addition, a common feature of these SBM processes is that C-H activation involves a degree of heterolytic character and therefore proton transfer.…”

“…This is apparently consistent with the ability of the more electron-releasing PMe 3 ligand to stabilize 'oxidative character' in the transition state. 22, 23 However, steric effects can dominate in these systems in particular, the combination of ligands present is important. Thus when L = CO computed barriers are relatively insensitive to changes in the accepting group, R (R = Ph: DG ‡ = 15.5 kcal mol -1 R = CH 2 CH 2 Ph: DG ‡ = 13.5 kcal mol -1 ), but activation barriers increase significantly in the more sterically encumbered PMe 3 analogues (R = Ph: DG ‡ = 17.1 kcal mol -1 ; R = CH 2 CH 2 Ph: DG ‡ = 20.0 kcal mol -1 ).…”

Aromatic C−H Activation and Catalytic Hydrophenylation of Ethylene by TpRu{P(OCH2)3CEt}(NCMe)Ph

“…25,28,43,44 The calculated free energy of TS2 is the highest species along the overall potential energy surface. Therefore, the calculations suggest that benzene C-H activation is the ratedetermining step in the overall catalytic cycle using complex 4.…”

Self-control tames the coupling of reactive radicals

“…41 In contrast, a negligible reaction constant is observed for stoichiometric C-H activation via σ-bond metathesis (σBM) at Sc, 42 whilst Ru-mediated C-H activation via σBM is predicted to have a positive reaction constant (ρ = 2.6 to 3.2). 43 The significance of the reaction constant for Au-catalyzed arylation is discussed in later sections.…”

“…However, it fails to account for the accelerating effect of silane ortho-methylation, and provides no explanation for the selectivity between the silane and the arene (i.e., chemoselectivity; vide infra). As an alternative, scenario (b) fails to account for both the accelerating effects of ortho-methylation and the reaction constant (a negligible or positive ρ value is expected for σBM), [42][43] although invoking a reaction manifold fundamentally different to that operating for C-H auration offers a clear basis for chemoselectivity. Finally, rate-limiting formation of a WI, scenario (c), accounts not only for the accelerating effect of ortho-methylation, but also, in the limit of an early TS, 48 for the absence of KIEs, the insensitivity to the nature of alkyl substituents and the magnitude/direction of the reaction constant.…”