Description: This article discusses the activation of carbon-hydrogen bonds. 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 d6 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 ...

Description: This article discusses conversions of Ruthenium(III) alkyl complexes. Abstract: Single-electron oxidation of the Ru(II) complexes TpRu(L)(L')(R) (L = CO, L' = NCMe, and R = CH3 or CH2CH2Ph; L = L' = PMe3 and R = CH3) with AgOTf leads to alkyl elimination reactions that produce TpRu(L)(L')(OTf) and organic products that likely result from Ru-Calkyl bond homolysis. Density functional calculations on TpRu(CO)(NCMe)(Me) and its Ru(III) cation indicate that the Ru-CH3 homolytic bond dissocation enthalpy is substantially reduced (48.6 to 23.2 kcal/mol) upon oxidation.

Description: This article discusses reactions of TpRu(CO)(NCMe)(Me) (Tp = Hydridotris(pyrazolyl)borate) with heteroaromatic substrates. The Ru(II) complex TpRu(CO)(NCMe)(Me) (Tp = hydridotris(pyrazolyl)borate) initiates carbon-hydrogen bond activation at the 2-position of furan and thiophene to produce methane and TpRu(CO)(NCMe)(aryl) (aryl = 2-furyl or 2-thienyl). Solid-state structures have been determined for TpRu(CO)(NCMe)(2-thienyl) and [TpRu(CO)(μ-C,S-thienyl)]2. The complex TpRu(CO)(NCMe)(2-furyl) serves as a catalyst for the formation of 2-ethylfuran from ethylene and furan. DFT calculations of the C-H activation of furan by {(Tab)Ru-(CO)(Me)} (Tab = tris(azo)borate) indicate that the C-H activation sequence does not proceed through a Ru(IV) oxidative addition intermediate.

Description: This article discusses combined experimental and computational studies on the nature of aromatic C-H activation by octahedral ruthenium(II) complexes. Abstract: Octahedral ruthenium complexes of the type TpRu(L)(NCMe)R [Tp = hydridotris(pyrazolyl)borate; R = alkyl or aryl; L = CO or PMe3] 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)(ƞ2-C,C-C6H5X)Me to TpRu(L)(ρ-C6H4X) and CH4 where X is varied among Br, Cl, CN, F, H, NH2, NO2, and OMe. A linear Hammett correlation is calculated with a positive ρ value of 2.6 for L = CO and 3.2 for L = PMe3. 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 ...

Description: This article discusses octahedral [TpRu(PMe3)2OR]n+ Complexes (Tp = hydridotris(pyrazolyl)borate. The Ru(II) complexes TpRu(PMe3)2OR (R = H or Ph) react with excess phenylacetylene at elevated temperatures to produce the phenylacetylide complex TpRu(PMe3)2(C≡CPh). Kinetic studies indicate that the reaction of TpRu(PMe3)2OH and phenylacetylene likely proceeds through a pathway that involves TpRu(PMe3)2OTf as a catalyst. The reaction of TpRu(PMe3)2OH with 1,4-cyclohexadiene at elevated temperature forms benzene and TpRu(PMe3)2H, while TpRu(PMe3)2OPh does not react with 1,4-cyclohexadiene even after 20 days at 80 ˚C. The paramagnetic Ru(III) complex [TpRu(PMe3)2OH][OTf] is formed upon single-electron oxidation of TpRu(PMe3)2OH with AgOTf. Reactivity studies suggest that [TpRu(PMe3)2OH][OTf] initiates reactions, including hydrogen atom abstraction, with C-H bonds that have bond dissociation energy < 80 kcal/mol. Experimentally, the O-H bond strength of the Ru(II) cation [TpRu(PMe3)2(OH2)][OTf] is estimated to be between 82 and 85 kcal/mol, while computational studies yield a BDE of 84 kcal/mol, which are in reasonable agreement with the observed reactivity of [TpRu(PMe3)2OH]+.