Activation of Carbon−Hydrogen Bonds via 1,2-Addition across M−X (X = OH or NH₂) Bonds of d⁶Transition Metals as a Potential Key Step in Hydrocarbon Functionalization:  A Computational Study

Abstract

Recent reports of 1,2-addition of C-H bonds across Ru-X (X ) amido, hydroxo) bonds of TpRu-(PMe3)X fragments {Tp ) hydridotris(pyrazolyl)borate} suggest opportunities for the development of new catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations, computational examinations of the efficacy of model d 6 transition metal complexes of the form [(Tab)M-(PH3)2X] q (Tab ) tris-azo-borate; X ) OH, NH2; q ) -1 to +2; M ) Tc I , Re I , Ru II , Co III , Ir III , Ni IV , Pt IV ) for the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition states were located and found to have relatively little metal-hydrogen interaction. The C-H activation process is best described as a metal-mediated proton transfer in which the metal center and ligand X function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/ functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d 6 electron configuration {especially the TabCo(PH3)2(OH) + complex and related Co(III) systems} are predicted to be the most promising for further catalyst investigation.

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

“…53 The first step in the C-H activation was loss of phosphine to provide a formally 16-electron fivecoordinate species which may be stabilised by p-donation by X. In this study h 2 -benzene adducts were only observed for the dicationic Ni or Pt complexes (X = OH, NH 2 ) and the monocationic [Ir-OH] complex.…”

“…7 Calculated metric data for C-H activation of benzene by [(Tab)Ru(PH 3 )(X)] where X = Me, NH 2 , OH. 53 Cundari and Gunnoe conclude that this type of C-H bond activation is inherently more facile when the receiving atom is anionic and heteroatomic than hydrocarbyl and the mechanism should be viewed as an internal proton transfer. They also note that the complexes with more electrophilic metals give earlier transition states (shorter C-H distances, longer X-H distances).…”

“…57 In contrast, for the model system [(Tab)Ir(PH 3 )X(C 6 H 6 )] the difference in DG ‡ is 11.3 kcal mol -1 between X = OH and NH 2 with the barrier being lower for OH. 53 …”

Catalytic Oxy‐Functionalization of Methane and Other Hydrocarbons: Fundamental Advancements and New Strategies

“…[5] Cundari, Gunnoe, et al performed an atoms in molecules (AIM) analysis, which locates critical points of electron density, for the RuÀX (X= OH, NH 2 ) benzene CÀH activation transition state. [113] The analysis indicates four bond-critical points (MÀX, XÀH, CÀH, and MÀC) as well as a ring-critical point implying a four-membered ring transition state. Most notably the AIM analysis did not indicate a bond-critical point between the metal and hydrogen in the active site of the transition state.…”

Activation of sp3 Carbon–Hydrogen Bonds by a Ruthenium(II) Complex and Subsequent Metal‐Mediated CC and CN Bond Formation

“…[22] Consistent with these models, previous computational studies suggested that the basicity of X in Scheme 5 greatly impacts the extent to which Ru interacts with the activated proton in the transition state. [20] For example, we have recently determined that when the ligand receiving the activated hydrogen atom possesses a lone pair (e.g. hydroxo or amido ligands), the calculated Ru À H distance in the transition state is increased relative to when X = Me.…”