Product Protection, the Key to Developing High Performance Methane Selective Oxidation Catalysts

Selective, direct conversion of methane to methanol might seem an impossible task since the C−H bond energy of methane is 105 kcal mol^(−1) compared to the C−H bond energy for methanol of 94. We show here that the Catalytica catalyst is successful because the methanol is protected as methyl bisulfate, which is substantially less reactive than methanol toward the catalyst. This analysis suggests a limiting performance for systems that operate by this type of protection that is well above the Catalytica system

Transition state energy decomposition study of acetate-assisted and internal electrophilic substitution C−H bond activation by (acac-O,O)_2Ir(X) complexes (X = CH_3COO, OH)

Chelate-assisted and internal electrophilic substitution type transition states were studied using a DFT-based energy decomposition method. Interaction energies for benzene and methane C−H bond activation by (acac-O,O)_2Ir(X) complexes (X = CH_3COO and OH) were evaluated using the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). A ratio of ~1.5:1 for forward to reverse charge-transfer between (acac-O,O)_2Ir(X) and benzene or methane transition state fragments confirms “ambiphilic” bonding, the result of an interplay between the electrophilic iridium center and the internal base component. This analysis also revealed that polarization effects account for a significant amount of transition state stabilization. The energy penalty to deform reactants into their transition state geometry, distortion energy, was also used to understand the large activation energy difference between six-membered and four-membered acetate-assisted transition states and help explain why these complexes do not activate the methane C−H bond

Iridium complexes bearing a PNP ligand, favoring facile C(sp^3)–H bond cleavage

Hydrogen iodide is lost upon reaction of PNP with IrI_3, where PNP = 2,6-bis-(di-t-butylphosphinomethyl)pyridine to give crystallographically characterized Ir(PNP)*(I)_2, which reacts with H_2 to give Ir(PNP)(H)(I)_2. Ir(PNP)(Cl)_3 is relatively inert towards the intramolecular C–H activation of the tert-butyl's of the PNP ligand

Ligand Lone-Pair Influence on Hydrocarbon C-H Activation: A Computational Perspective

Mid to late transition metal complexes that break hydrocarbon C-H bonds by transferring the hydrogen to a heteroatom ligand while forming a metal-alkyl bond offer a promising strategy for C-H activation. Here we report a density functional (B3LYP, M06, and X3LYP) analysis of cis-(acac)_2MX and TpM(L)X (M=Ir, Ru, Os, and Rh; acac=acetylacetonate, Tp=tris(pyrazolyl)-borate; X=CH_3, OH, OMe, NH_2, and NMe_2) systems for methane C-H bond activation reaction kinetics and thermodynamics.We address the importance of whether a ligand lone pair provides an intrinsic kinetic advantage through possible electronic d_π-p_π repulsions for M-OR and M-NR_2 systems versus M-CH_3 systems. This involves understanding the energetic impact of the X ligand group on ligand loss, C-H bond coordination, and C-H bond cleavage steps as well as understanding how the nucleophilicity of the ligand X group, the electrophilicity of the transition metal center, and cis-ligand stabilization effect influence each of these steps.We also explore how spectator ligands and second- versus third-row transition metal centers impact the energetics of each of these C-H activation steps

Acceleration of Nucleophilic CH Activation by Strongly Basic Solvents

(IPI)Ru(II)(OH)_n(H_2O)_m, 2, where IPI is the NNN-pincer ligand, 2,6-diimidizoylpyridine, is shown to catalyze H/D exchange between hydrocarbons and strongly basic solvents at higher rates than in the case of the solvent alone. Significantly, catalysis by 2 is accelerated rather than inhibited by increasing solvent basicity. The evidence is consistent with the reaction proceeding by base modulated nucleophilic CH activation

Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands

Trends in methane activation have been explored for rhenium-based catalysts in conjunction with bidentate oxygenated ligands of the form (L_1)(L_2)Re(OH)(OH_2) [L_1, L_2 = acac, catechol, glycol]. When placed in acidic media, the equilibrium for this reference catalyst shifts to the protonated forms (L_1)(L_2)Re(OH_2)(OH_2) in almost all cases. In all cases the activation of the reference complex proceeds through a concerted metathesis type transition state, and only one of the 13 reference complexes proceeds with methane activation through a barrier of less than 35 kcal mol-1. Study of the identity complexes (L_1 = L_2) revealed that protonation of the ligand oxygens is unfavorable for acac and catechol, but favorable for glycol; however in only one case is the barrier for methane activation improved by this route. Electron density on the central rhenium is the best predictor for the magnitude of the methane activation barrier; namely, increased electron density (obtained by considering lower oxidation states) on the metal leads to lower barriers. Lower oxidation states form weaker Re−O bonds, which increase lability of the leaving groups and decrease the barrier to proton transfer from methane

Hydrovinylation of Olefins Catalyzed by an Iridium Complex via CH Activation

Olefin dimerizations are typically proposed to proceed via a Cossee−Arlman type migratory mechanism involving relatively electron-rich metal hydrides. We provide experimental evidence and theoretical calculations that show, in contrast, relatively electron-poor O-donor Ir complexes can catalyze the dimerization of olefins via a mechanism that involves olefin CH bond activation and insertion into a metal−vinyl intermediate

Functionalization of Rhenium Aryl Bonds by O-Atom Transfer

Aryltrioxorhenium (ArReO_3) has been demonstrated to show rapid oxy-functionalization upon reaction with O-atom donors, YO, to selectively generate the corresponding phenols in near quantitative yields. (18)^O-Labeling experiments show that the oxygen in the products is exclusively from YO. DFT studies reveal a 10.7 kcal/mol barrier (Ar = Ph) for oxy-functionalization with H_2O_2 via a Baeyer-Villiger type mechanism involving nudeophilic attack of the aryl group on an electrophilic oxygen of YO coordinated to rhenium