Roy A. Periana

Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

Green plants convert CO2 to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO2 and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO2, formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies. When operating at near-ambient conditions, using water as a solvent, a high-turnover iridium catalyst enables a reversible hydrogen storage system that uses carbon dioxide, formate and formic acid. Proton-responsive ligands in the catalyst allow it to be turned on or off by controlling the pH of the solution.

A Mercury-Catalyzed, High-Yield System for the Oxidation of Methane to Methanol

A homogeneous system for the selective, catalytic oxidation of methane to methanol via methyl bisulfate is reported. The net reaction catalyzed by mercuric ions, Hg(II), is the oxidation of methane by concentrated sulfuric acid to produce methyl bisulfate, water, and sulfur dioxide. The reaction is efficient. At a methane conversion of 50 percent, 85 percent selectivity to methyl bisulfate (∼43 percent yield; the major side product is carbon dioxide) was achieved at a molar productivity of 10–7 mole per cubic centimeter per second and Hg(II) turnover frequency of 10–3 per second. Separate hydrolysis of methyl bisulfate and reoxidation of the sulfur dioxide with air provides a potentially practical scheme for the oxidation of methane to methanol with molecular oxygen. The primary steps of the Hg(II)-catalyzed reaction were individually examined and the essential elements of the mechanism were identified. The Hg(II) ion reacts with methane by an electrophilic displacement mechanism to produce an observable species, CH3HgOSO3H, 1. Under the reaction conditions, 1 readily decomposes to CH3OSO3H and the reduced mercurous species, Hg22+ The catalytic cycle is completed by the reoxidation of Hg22+ with H2SO4 to regenerate Hg(II) and byproducts SO2 and H2O. Thallium(III), palladium(II), and the cations of platinum and gold also oxidize methane to methyl bisulfate in sulfuric acid.

Designing Catalysts for Functionalization of Unactivated C–H Bonds Based on the CH Activation Reaction

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.

Acceleration of Nucleophilic CH Activation by Strongly Basic Solvents

(IPI)Ru(II)(OH)(n)(H(2)O)(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.

Main-Group Compounds Selectively Oxidize Mixtures of Methane, Ethane, and Propane to Alcohol Esters

Light Alkanes, Heavy Metals Hydraulic fracturing, or fracking, has rapidly increased the supply of natural gas and has motivated methods to convert its constituents into commodity chemicals. Hashiguchi et al. (p. 1232) have found that lead and thallium salts are both efficient and selective oxidants, not only for methane, but for ethane and propane as well. In trifluoroacetic acid solvent, the alkanes are cleanly oxidized to the trifluoroacetate esters of their respective alcohols and 1,2-diols. Building on earlier discoveries, this work paves the way to developing methods that reduce our dependence on petroleum for industrial feedstocks. Lead and thallium salts are surprisingly selective and efficient oxidants of the constituents of natural gas. Much of the recent research on homogeneous alkane oxidation has focused on the use of transition metal catalysts. Here, we report that the electrophilic main-group cations thallium(III) and lead(IV) stoichiometrically oxidize methane, ethane, and propane, separately or as a one-pot mixture, to corresponding alcohol esters in trifluoroacetic acid solvent. Esters of methanol, ethanol, ethylene glycol, isopropanol, and propylene glycol are obtained with greater than 95% selectivity in concentrations up to 1.48 molar within 3 hours at 180°C. Experiment and theory support a mechanism involving electrophilic carbon-hydrogen bond activation to generate metal alkyl intermediates. We posit that the comparatively high reactivity of these d10 main-group cations relative to transition metals stems from facile alkane coordination at vacant sites, enabled by the overall lability of the ligand sphere and the absence of ligand field stabilization energies in systems with filled d-orbitals.

Regioselective hydrophenylation of olefins catalyzed by an Ir(III) complex

Abstract The novel, anti-Markovnikov, arylation of olefins with benzene to produce straight-chain alkylbenzenes with higher selectivity than branched alkylbenzenes is catalyzed by [Ir(μ-acac-O,O′,C3)(acac-O,O′)(acac-C3)]2 (acac=acetylacetonato), 1 [J. Am. Chem. Soc. 122 (2000) 7414]. The reaction of benzene with propylene gave n-propylbenzene and cumene in 61 and 39% selectivities, respectively. The reaction of benzene and styrene afforded 1,2-diphenylethane in 98% selectivity. Considering the anti-Markovnikov regioselectivity and lack of inhibition by water, we propose that the reaction does not proceed via a Friedel–Crafts, carbocation, mechanism. Complex 1, amongst the various transition metal complexes examined, is the most efficient for catalyzing the anti-Markovnikov olefin arylation. The crystal structure of complex 1 was solved and is consistent with a binuclear Ir(III) structure with three different types of coordinated acac ligands as reported by earlier solution IR and NMR analyses. [Ir(μ-acac-O,O′,C3)(acac-O,O′)Cl]2, 2, was prepared by the reaction of complex 1 with benzoyl chloride, and the crystal structure was also reported.

Novel bis-acac-O,O-Ir(III) catalyst for anti-Markovnikov, hydroarylation of olefins operates by arene CH activation.

A novel, thermally stable, homogeneous Ir catalyst for the anti-Markovnikov, hydroarylation of olefins is shown to operate by arene CH activation via the formation of a bisacac-O,O phenyl-Ir(III) species.

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 Charge Transfer Reveals Electrophilic, Ambiphilic, and Nucleophilic Carbon-Hydrogen Bond Activation

Absolutely localized molecular orbital energy decomposition analysis of C-H activation transition states (TSs), including Pt, Au, Ir, Ru, W, Sc, and Re metal centers, shows an electrophilic, ambiphilic, and nucleophilic charge transfer (CT) continuum irrespective of the bonding paradigm (oxidative addition, sigma-bond metathesis, oxidative hydrogen migration, 1,2-substitution). Pt(II) insertion and Au(III) substitution TSs are highly electrophilic and dominated by C-H bond to metal/ligand orbital stabilization, while Ir-X and Ru-X (X = R, NH(2), OR, or BOR(2)) substitution TSs are ambiphilic in nature. In this ambiphilic activation regime, an increase in one direction of CT typically leads to a decrease in the reverse direction. Comparison of Tp(CO)Ru-OH and Tp(CO)Ru-NH(2) complexes showed no evidence for the classic d(pi)-p(pi) repulsion model. Complexes such as and Cp(CO)(2)W-B(OR)(2), (PNP)Ir(I), Cp(2)ScMe, and (acac-kappaO,kappaO)(2)Re(III)-OH were found to mediate nucleophilic C-H activation, where the CT is dominated by the metal/ligand orbital to C-H antibonding orbital interaction. This CT continuum ultimately affects the metal-alkyl intermediate polarization and possible functionalization reactions. This analysis will impact the design of new activation reactions and stimulate the discovery of more nucleophilic activation complexes.

Selective CH Functionalization of Methane, Ethane, and Propane by a Perfluoroarene Iodine(III) Complex

Direct partial oxidation of methane, ethane, and propane to their respective trifluoroacetate esters is achieved by a homogeneous hypervalent iodine(III) complex in non-superacidic (trifluoroacetic acid) solvent. The reaction is highly selective for ester formation (>99%). In the case of ethane, greater than 0.5 M EtTFA can be achieved. Preliminary kinetic analysis and density functional calculations support a nonradical electrophilic CH activation and iodine alkyl functionalization mechanism.