Search

The design of homogeneous catalysts based on Ir and other late transition metal complexes using O-donor ligands like acetylacetonate (aca), tropolone, aryloxides, catechols, hydroxyaetophenone to stabilize the complexes to the reaction conditions needed for generating functionalized products is discussed. The outcome of the studies helps to utilize, design new, more effective, thermally protic-and oxidant-stable O-donor catalysts for hydrocarbon conversion.

Two organometallic systems([Ir(Mu-acac-O)(acac-C(super 3)](sub 2) and (Tp)Ru(CO)(Ph)-(NCCH3) is discovered that catalyze hydroarylation of unactivated olefins. Hybridization is dependent on the amount of back-bonding to the olefin, these relationships translate into correlations between the amount of back-bonding and activation energy/thermodynamics.

This review summarizes the deliberations of a workshop colloquium on the 'opportunities for catalysis research in carbon management'. Research progress within the context of greenhouse gas/carbon issues in the areas of hydrogen generation and utilization, carbon dioxide activation, catalysis research to further our search for new and more efficient catalytic processes.

The ability of (IPI)Ru(II)[(OH).sub.n][([H.sub.2]O).sub.m] where IPI is the NNN-pincer ligand, 2,6-diimidizoylpyridine to catalyze H/D exchange between hydrocarbons and strongly basic solvents at higher rates than in the case of the solvent alone is described. The evidence indicated that the reaction proceeding by base modulated nucleophilic CH activation could be increased by solvent basicity.

The Catalytica catalyst is successful because the methanol is protected as methyl bisulfate, which is less reactive than methanol toward the catalyst. The analysis has shown a limiting performance for systems that is operated by this type of protection that is well above the Catalytica system.

Several studies are conducted to analyze the localized molecular orbital energy decomposition of the various C-H activation transition states (TSs). The results prove that these discussed metal centers exhibit an electrophilic, ambiphilic and nucleophilic charge transfer in the bond activation.

Se[O.sub.2] has catalyzed the facile oxy-functionalization of [(CO).sub.5]Re(I)-[Me.sup.5-] with I[O.sub.4.sup.-] to generate methanol. Density functional theory calculations have shown that catalysis has involved methyl group transfer from Re to the electrophilic Se center followed by oxidation and subsequent reductive functionalization of the resulting C[H.sub.3]Se(VI) species.

The density functional theory (DFT) is used for studying the mechanisms for the interactions between methylrhenium trioxide (MTO) and [H.sub.2][O.sub.2]/NaOH in THF. The studies have shown that methanol is formed through Baeyer-Villiger type mechanism and another oxygen insertion transition state with a low relative barrier is isolated.

The combined experimental and theoretical evidence is presented for a facile Re-R to Re-OR bond conversion with non-peroxo YOs that proceeds through a low-energy, Baeyer-Villiger (BV)-type, electrophilic O-atom insertion. The results might point to a facile pathway for heteroatom functionalization of M-R intermediates of more electron-rich metals via a BV-type pathway with electrophilic, heteroatom-atom donors, such as YO.

The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)2(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst is discussed. Electron-withdrawing substituents on the acac ligands are good modifications for catalysts with higher activity.

A study was conducted to show that O-donor ligands could facilitate the C-H activation reaction as well as selective functionalization of the M-R intermediate for a picolinate platinum system. The results indicate that a well-defined, late metal, N,O-ligated complex, which is competent for arene C-H activation, exhibits thermal and protic stability and is an efficient catalyst for H/D exchange.

The air, protic, and thermally stable O-donor alkoxo complexes, (acac-O,O)2Ir(OMe)(L), 2-L, L=Py and CH3OH, react in stoichiometric CH activation reactions with benzene to generate the corresponding phenyl complex with co-generation of methanol in high yield. This is the first example of such an intermolecular CH activation reaction with a metal-alkoxo complex.

The well-defined, late-metal, O-ligated complexes are demonstrated, which are competent for alkane C-H activation. The O-ligated complexes exhibit high thermal and protic stability, and are efficient catalysts for H/D exchange reactions with alkanes.

Research is presented into anti-Markovnikov arylation catalyzed by a binuclear iridium complex.

Methylrhenium trioxide (MTO) has the rare ability to stoichiometrically generate methanol at room temperature with an external oxidant (H2O2) under basic conditions. In order to use this transformation as a model for nonredox oxidative C-O coupling, the mechanisms have been elucidated using density functional theory (DFT). Our studies show several possible reaction pathways to form methanol, with the lowest net barrier (ΔH‡) being 23.3 kcal mol-1. The rate-determining step is a direct ‘Baeyer-Villiger’ type concerted oxygen insertion into MTO, forming methoxyrhenium trioxide. The key to the low-energy transition state is the donation of electron density, first, from HOO(-) to the -CH3 group (making -CH3 more nucleophilic and HOO- more electrophilic) and, second, from the Re-C bond to both the forming Re-O and breaking O-O bonds, simultaneously (thus forming the Re-O bond as the Re-C bond is broken). In turn, the ability of MTO to undergo these transfers can be traced to the electrophilic nature of the metal center and to the absence of accessible d-orbitals. If accessible d-orbitals are present, they would most likely donate the required electron density instead of the M-CH3 moiety, and this bond would thus not be broken. It is possible that other metal centers with similar qualities, such as PtIV or IrV, could be competent for the same type of chemistry. [ABSTRACT FROM AUTHOR]

The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)[SUB2](L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated ΔH(OK)[SUP‡] = 35.1 kcal/mol and ΔG(298 K)[SUP‡] = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C-H activation via a novel mechanism, oxidative hydrogen migration. The olef in insertion is found to be rate determining, with a calculated ΔH(0 K)[SUP‡] = 27.0 kcal/mol and ΔG(298 K)[SUP‡] = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C-H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate lr[SUPv], with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of β-hydride products is due to kinetic factors, although β-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. [ABSTRACT FROM AUTHOR]

The article presents information on CH activation with an O-Donor iridium-methoxo complex. The CH activation of hydrocarbons to generate M-R intermediates has been the focus of significant effort, given the potential for developing new generations of single-site oxidation catalysts that operate at low temperatures with high yields and selectivities. However, while there are now many well-established CH activation systems, relatively few have been successfully incorporated into catalytic cycles to generate oxy-functionalized products such as alcohols.

Sb(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H 2 , and carbon oligomerization. In contrast to this general assumption, our density functional theory (DFT) calculations revealed an accessible barrier for C-H activation between methane and Sb(V) in sulfuric acid that could potentially outcompete superacid protonolysis. This prompted us to experimentally examine this reaction in sulfuric acid with oleum, which has never been reported because of presumed superacid reactivity. Reaction of methane at 180 °C for 3 h resulted in very high yields of methyl bisulfate without significant overoxidation. Our DFT calculations show that a C-H activation and Sb-Me bond functionalization mechanism to give methyl bisulfate outcompetes methane protonolysis and many other possible reaction mechanisms, such as electron transfer, proton-coupled electron transfer, and hydride abstraction. Our DFT calculations also explain experimental hydrogen-deuterium exchange studies and the absence of methane carbo-functionalization/oligomerization products. Overall, this work demonstrates that in very strong Brønsted acid solvent, Sb(V) can induce innersphere reaction mechanisms akin to transition metals and outcompete superacid reactivity.

The complex [Ru(Mebimpy)(4,4'-((HO)2OPCH2)2bpy)(OH2)](2+) surface bound to tin-doped indium oxide mesoporous nanoparticle film electrodes (nanoITO-Ru(II)(OH2)(2+)) is an electrocatalyst for the selective oxidation of methylrhenium trioxide (MTO) to methanol in acidic aqueous solution. Oxidative activation of the catalyst to nanoITO-Ru(IV)(OH)(3+) induces oxidation of MTO. The reaction is first order in MTO with rate saturation observed at [MTO] > 12 mM with a limiting rate constant of k = 25 s(-1). Methanol is formed selectively in 87% Faradaic yield in controlled potential electrolyses at 1.3 V vs NHE. At higher potentials, oxidation of MTO by nanoITO-Ru(V)(O)(3+) leads to multiple electrolysis products. The results of an electrochemical kinetics study point to a mechanism in which surface oxidation to nanoITO-Ru(IV)(OH)(3+) is followed by direct insertion into the rhenium-methyl bond of MTO with a detectable intermediate.

In an effort to augment or displace petroleum as a source of liquid fuels and chemicals, researchers are seeking lower cost technologies that convert natural gas (largely methane) to products such as methanol. Current methane to methanol technologies based on highly optimized, indirect, high-temperature chemistry (>800 °C) are prohibitively expensive. A new generation of catalysts is needed to rapidly convert methane and O(2) (ideally as air) directly to methanol (or other liquid hydrocarbons) at lower temperatures (~250 °C) and with high selectivity. Our approach is based on the reaction between CH bonds of hydrocarbons (RH) and transition metal complexes, L(n)M-X, to generate activated L(n)M-R intermediates while avoiding the formation of free radicals or carbocations. We have focused on the incorporation of this reaction into catalytic cycles by integrating the activation of the CH bond with the functionalization of L(n)M-R to generate the desired product and regenerate the L(n)M-X complex. To avoid free-radical reactions possible with the direct use of O(2), our approach is based on the use of air-recyclable oxidants. In addition, the solvent serves several roles including protection of the product, generation of highly active catalysts, and in some cases, as the air-regenerable oxidant. We postulate that there could be three distinct classes of catalyst/oxidant/solvent systems. The established electrophilic class combines electron-poor catalysts in acidic solvents that conceptually react by net removal of electrons from the bonding orbitals of the CH bond. The solvent protects the CH(3)OH by conversion to more electron-poor [CH(3)OH(2)](+) or the ester and also increases the electrophilicity of the catalyst by ligand protonation. The nucleophilic class matches electron-rich catalysts with basic solvents and conceptually reacts by net donation of electrons to the antibonding orbitals of the CH bond. In this case, the solvent could protect the CH(3)OH by deprotonation to the more electron-rich [CH(3)O](-) and increases the nucleophilicity of the catalysts by ligand deprotonation. The third grouping involves ambiphilic catalysts that can conceptually react with both the HOMO and LUMO of the CH bond and would typically involve neutral reaction solvents. We call this continuum base- or acid-modulated (BAM) catalysis. In this Account, we describe our efforts to design catalysts following these general principles. We have had the most success with designing electrophilic systems, but unfortunately, the essential role of the acidic solvent also led to catalyst inhibition by CH(3)OH above ~1 M. The ambiphilic catalysts reduced this product inhibition but were too slow and inefficient. To date, we have designed new base-assisted CH activation and L(n)M-R fuctionalization reactions and are working to integrate these into a complete, working catalytic cycle. Although we have yet to design a system that could supplant commercial processes, continued exploration of the BAM catalysis continuum may lead to new systems that will succeed in addressing this valuable goal.