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Atom economical and safe epoxidations of a variety of fatty acid methyl esters (FAMEs, including methyl oleate, methyl linolenate, methyl linoleate, and methyl soyate) under near-isothermal conditions are demonstrated using a tungsten-based Venturello catalyst and H2O2 as an oxidant under solvent-free conditions. By systematically optimizing the substrate/catalyst ratio, stirring rate, H2O2 addition rate, and external cooling rate, safe operation with minimal temperature rise was demonstrated in a semibatch reactor for each of the substrates with total selectivity toward the epoxides at complete conversion. In the case of methyl oleate epoxidation, which produces only one epoxide as product, intrinsic conversion/selectivity data obtained under near-isothermal (±0.1 °C) conditions were regressed with suitable kinetic and reactor models to obtain an activation energy of 85 ± 6.8 kJ/mol. These results provide valuable guidance for the rational design, scaleup, and safe operation of FAME epoxidation reactors.

2,5-Furandicarboxylic acid (FDCA) is a valuable nonphthalate biomass-based plastic precursor with the potential to replace terephthalic acid (TPA) in a variety of polymer applications. In this work, the Co/Mn/Br-catalyzed semicontinuous oxidation of 5-hydroxymethylfurfural (HMF) to FDCA has been demonstrated at temperatures lower than those employed in the traditional Mid-Century (MC) process. As HMF is more susceptible to side reactions (e.g., the overoxidation to CO and CO2), lower temperatures compared to the MC process are typically used to prevent substrate burning. However, lower temperatures afford a much reduced FDCA yield compared to that of TPA in p-xylene oxidation. Therefore, optimization of other operating variables such as catalyst composition, water concentration in the acetic acid solvent, and pressure is essential to maximize FDCA yield. Using such optimization, we show that the FDCA yield can be enhanced to 90% at a 1/0.015/0.5 molar ratio of Co, Mn, and Br, 7% (v/v) water, 30 bar (CO2/O...

We have shown that liquid CO2 dissolves O3 in higher concentrations compared to conventional solvents and is therefore an effective medium for the ozonolysis of unsaturated compounds. The Beer–Lambert law extinction coefficient for ozone in liquid CO2 is determined to be 3.2 ± 0.3 M–1 cm–1. Further, the Henry’s law coefficient for ozone dissolution in dense phase CO2 is determined to be on the order of 0.25 mol L–1 bar–1 in the temperature range −10 to +4 °C. Rapid ozonolysis of methyl oleate in liquid CO2 has been successfully demonstrated with the 1,2,4-trioxolanes (secondary ozonides) being the major stable product, as confirmed by NMR analysis. The product distribution is consistent with a lack of solvent cage effects due to the low polarity of liquid CO2. In addition to the inertness of liquid CO2 to ozone attack, the dominant presence of dense CO2 in the vapor phase promotes inherent process safety.

We demonstrate a continuous reactor for performing the ozonolysis of fatty acid methyl esters (FAMEs) using liquid CO2 as solvent. The fast reaction kinetics allows the use of small-volume reactors to completely convert the FAMEs, forming secondary ozonides as the primary products. The short residence times also help maximize the yields of the secondary ozonides by minimizing over-oxidation and the formation of oligomeric products. The liquid CO2 medium promotes safe reactor operation by providing an essential fraction of overall reactor cooling and by diluting the vapor phase organics. We also demonstrate a continuous stirred reactor for the safe thermal decomposition of the secondary ozonides to their corresponding acids and aldehydes. Using a lumped kinetic model for the thermal decomposition of the ozonolysis products, we estimate activation energy values of 108.6 +/- 0.6 kJ mol(-1) for the decomposition of secondary ozonides and 122 +/- 3 kJ mol(-1) for the decomposition of the undesired oligomeric species. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 2819-2826, 2017

Electronic structure calculations have been used to investigate possible gas- phase decomposition pathways of α-hydroxyalkyl hydroperoxides (HHPs), an important source of tropospheric hydrogen peroxide and carbonyl compounds. The uncatalyzed as well as water- and acid-catalyzed decomposition of multiple HHPs have been examined at the M06-2X/aug- cc-pVTZ level of theory. The calculations indicate that, compared to an uncatalyzed or water- catalyzed reaction, the free-energy barrier of an acid-catalyzed decomposition leading to an aldehyde or ketone and hydrogen peroxide is dramatically lowered. The calculations also find a direct correlation between the catalytic effect of an acid and the distance separating its hydrogen acceptor and donor sites. Interestingly, the catalytic effect of an acid on the HHP decomposition resulting in the formation of carboxylic acid and water is relatively much smaller. Moreover, since the free-energy barrier of the acid-catalyzed aldehyde- or ketone- forming decomposition is ∼25% lower than that required to break the O−OH linkage of the HHP leading to the formation of hydroxyl radical, these results suggest that HHP decomposition is likely not an important source of tropospheric hydroxyl radical. Finally, transition state theory estimates indicate that the effective rate constants for the acid-catalyzed aldehyde- or ketone-forming HHP decomposition pathways are 2− 3 orders of magnitude faster than those for the water-catalyzed reaction, indicating that an acid-catalyzed HHP decomposition is kinetically favored as well.

Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecular reaction of the Criegee intermediate (CI) with carbon monoxide (CO) that is proposed to be important in both atmospheric and industrial chemistry. A new mechanism is suggested in which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting in the formation of a carbonyl compound (aldehyde or ketone depending upon the CI) and carbon dioxide. Fourteen different CIs, including ones resulting from biogenic ozonolysis, are considered. Consistent with previous reports for other CI bimolecular reactions, the anti conformers are found to react faster than the syn conformers. However, this can be attributed to steric effects and not hyperconjugation as generally invoked. The oxidation reaction is slow, with barrier heights between 6.3 and 14.7 kcal/mol and estimated reaction rate constants 6-12 orders-of-magnitude smaller than previously reported literature estimates. The reaction is thus expected to be unimportant in the context of tropospheric oxidation chemistry. However, the reaction mechanism suggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds.

Researchers at the University of Kansas, Center for Environmentally Beneficial Catalysis (CEBC) recently reported a spray process concept as a greener alternative to the conventional Mid-Century (MC) process to produce high-purity terephthalic acid (TPA) [

Kinetic investigations of the liquid phase oxidation of p-xylene (pX) to terephthalic acid (TPA) with Co/Mn/Br catalyst were performed in a stirred 50 mL Parr reactor at 200 °C and 15 bar pressure under conditions wherein product precipitation is avoided. The oxidant (O2) was introduced by sparging into the liquid phase at constant gas-phase O2 partial pressure. Apparent kinetic rate constants, estimated by regressing experimental conversion data to a pseudo-first order lumped kinetic model, are at least an order of magnitude greater than those reported in the literature for similar catalytic reactions. We attribute this difference to the presence of gas–solid and liquid–solid mass transfer resistances in the previous studies wherein the TPA product precipitates as it forms, trapping intermediate products and slowing down their oxidation rates. Our results also indicate that it is not possible to completely eliminate the gas–liquid mass transfer limitations associated with the fast intermediate oxidation ...

A spray reactor in which the liquid phase containing dissolved p-xylene (pX) and the catalyst is dispersed as fine droplets by a nozzle into a continuous vapor phase containing the oxidant (O2) is shown to produce high-purity terephthalic acid (TPA) with less than 25 ppm 4-carboxybenzaldehyde (4-CBA) in the solid TPA product. In sharp contrast, the solid TPA product obtained from a conventional stirred reactor similar to the configuration used in the conventional Mid-Century (MC) process contains nearly 1000 ppm 4-CBA even though the reactor is operated at similar pressure and temperature (15 bar and 200 °C) but with the gas phase dispersed into the liquid phase. The dramatic improvement in TPA product quality during spray reactor operation is attributed to two main factors: the alleviation of interphase gas–liquid mass transfer limitations that facilitates more complete oxidation of the pX and the intermediate oxidation products to TPA, and reduced backmixing that enhances the oxidation rates. Theoretical calculations show that the time constants for O2 diffusion in typical spray droplets (50 μm diameter) is 1–2 orders of magnitude lower than the kinetic rate constant confirming complete O2 penetration and saturation of the droplets. Further, the usage of CO2 as an inert gas and the dominance of acetic acid (>50 mol.%) in the vapor phase under reaction conditions create an environment that falls outside of the flammability envelope. Mathematical modeling of the stirred reactor using MC process conditions accurately predicts the steady state temperatures observed in industrial reactors. The model also clearly divulges that the cooling provided by partial evaporation of the acetic acid solvent is vital to maintain stable steady state operation. By eliminating the need for a hydrogenation step, the spray reactor has the potential to not only reduce capital and operating costs but also provide environmental benefits.

Until a decade ago, the industrial technologies for producing propylene oxide from propylene were predominantly based on variations of the venerable chlorohydrin and organic hydroperoxide processes. Within the past decade, highly selective H2O2-based propylene epoxidation technologies have been developed by Dow-BASF (HPPO process) and the University of Kansas Center for Environmentally Beneficial Catalysis (CEBC-PO process). We present comparative economic and environmental impact analyses based on plant scale simulations of the processes for an assumed 200,000 tonnes/yr of PO production capacity and employing relevant process data from the literature. The predicted capital costs for the CEBC-PO process ($228 million) and HPPO process ($275 million) are lower than the conventional PO/TBA process ($372 million). The PO production costs via the conventional PO/TBA and HPPO processes are 150.4¢/lb PO (profit 87.9¢/lb, assuming a market value of 41¢/lb for the TBA co-product and 42¢/lb for the enriched propan...

An ethylene epoxidation process concept that employs methyltrioxorhenium (MTO) as catalyst and H2O2 as oxidant and selectively produces EO (with no CO2 as byproduct) has been demonstrated recently by researchers at the University of Kansas Center for Environmentally Beneficial Catalysis (CEBC). On the basis of a plant-scale simulation of the CEBC process using Aspen HYSYS, preliminary economic and environmental assessments of the process are performed, both of which are benchmarked against the conventional silver-catalyzed ethylene epoxidation process. The capital costs for both processes lie within prediction uncertainty. The EO production cost for the conventional process is estimated to be 58 ¢/lb EO. The CEBC process has the potential to be competitive with the conventional process if the MTO catalyst remains active, selective and stable for at least one year at a leaching rate of approximately 0.11 lb MTO/h (or 0.7 ppm Re in the reactor effluent). While CO2 emissions as byproduct are eliminated in th...

Recently a homogeneous liquid-phase ethylene oxide (EO) process with nearly total EO selectivity, catalyzed by methyltrioxorhenium with H2O2 as an oxidant, was reported. Fundamental mass transfer and kinetic studies of this reaction are reported in the present work. Volumetric expansion studies revealed that the liquid reaction phase (methanol + H2O2/H2O) is expanded by up to 12% by compressed ethylene in the 20–40°C range and up to 50 bars. This represents an increase in ethylene solubility by approximately one-order of magnitude, attributed to the unique exploitation of near-critical ethylene (Pc = 50.76 bar; Tc = 9.5°C). Interphase mass-transfer coefficients for ethylene dissolution into the liquid phase were obtained experimentally. Operating at conditions that enhanced the ethylene solubility and eliminated interphase mass-transfer limitations maximized the EO productivity (1.61–4.97 g EO/h/g cat), rendering it comparable to the conventional process. Intrinsic kinetic parameters, estimated from fixed-time semibatch reactor studies, disclosed the moderate activation energy (57 ± 2 kJ/mol). © 2012 American Institute of Chemical Engineers AIChE J, 59: 180–187, 2013

The complex [Mn(IV)(Me2EBC)(OH)2](PF6)2, in which Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, is a remarkably selective H2O2 oxidation catalyst that has been shown to be useful in removing stains from fabrics without affecting their colors. Mn(IV) is the highest oxidation state detected and the dihydroxo complex forms a peroxyhydroxy derivative that is responsible for catalytic oxidations. Study of the diethyl homolog of this catalyst has revealed surprising differences in chemical behavior. Oxidation of this new manganese complex, Mn(Et2EBC)Cl2, using aqueous H2O2, at −30°C following removal of chloride ion, yields [Mn(Et2EBC)(OH)2](PF6)2. Above 0°C, H2O2 oxidation of Mn(Et2EBC)Cl2 oxidizes the ethyl substituents. X-ray structure determinations of Et2EBC complexes with Mn(II), Mn(III), and Mn(IV) are reported. The complex [Mn(Et2EBC)(OH)2](PF6)2 displays a surprisingly mild oxidizing potential of +0.556 V for the Mn4+/Mn3+ couple; however, its hydrogen abstraction ability for selec...

Among industrial chemical processes, ethylene oxide manufacture emits the largest amount of CO 2 (∼2–3 million tons/yr), as byproduct from the burning of both the ethylene (feed) and ethylene oxide. Further, the conventional silver-based catalytic process presents safety challenges due to the formation of explosive ethylene oxide/O 2 mixtures in the gas phase. By judicious choice of the catalyst (methyltrioxorhenium), oxidant (H 2 O 2 ) and reaction medium (methanol/water), a homogeneous liquid phase catalytic system has been demonstrated that eliminates CO 2 formation while producing ethylene oxide at >90% selectivity at near-ambient temperatures. Given its high volatility, the ethylene oxide is easily recovered from the reaction phase by distillation. The vicinity of the gaseous ethylene feed to its critical temperature (9 °C) is exploited to significantly increase its solubility in the liquid reaction phase by facile compression beyond the critical pressure of ethylene (∼50 bar). Since H 2 O 2 is stable at typical reaction temperatures (40 °C or less), potentially explosive ethylene oxide/O 2 mixtures are avoided in the gas phase. In addition to the potential of arresting the carbon footprint of a large-scale industrial process, the demonstrated process concept shows how gas-expanded liquids can be generally exploited in homogeneous catalysis to enhance productivity.

Using oxygen-18 labeled probes, the deactivation of the catalyst methyltrioxorhenium in oxidation reactions using hydrogen peroxide has been shown to result from the nucleophilic attack of HOO− on the parent catalyst rather than by HO− attack on intermediate A, opening possible routes to enhanced catalyst durability.

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of nature's choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).

This work correlates the basicity of various axial ligands (AL), including triethylamine, methylimidazole, pyridine, acetonitrile or water, with the efficacy of catalytic oxidation of 2,6-di-tert-butylphenol by Co(salen)(AL) in CO2-expanded methylene chloride. Further, Co(salen)(AL)O2 abstracts hydrogen atoms from substrates of high to intermediate reactivity (BDECH ≤~90 kcal/mol).

Two ultra rigid, o-xylylene cross-bridged macrobicyclic ligands, 1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (H2XBC), and 13,19-dimethyl-1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (Me2XBC), have been synthesized and the manganese complexes have been synthesized and characterized, including an X-ray structure determination. Mn(Me2XBC)Cl2 displays a relatively high redox potential for the Mn2+/Mn3+ couple (+0.947V vs SHE, measured in CH3CN), suggesting that the manganese(III) complex may be capable of hydrogen abstraction from moderately active substrates. Direct reaction of the freshly synthesized manganese(III) complex, [Mn(Me2XBC)Cl2]PF6, with 1,4-cyclohexadiene confirmed its hydrogen abstracting ability. The manganese(II)/Me2XBC complex is activated by dioxygen in buffered basic aqueous solutions and catalyzes hydrogen abstraction from selected substrates. A possible mechanism for this manganese complex catalyzed dioxygen activation and hydrogen abstraction is proposed.

A novel biphasic process concept for the synthesis of propylene oxide (PO) from propylene is presented, using the long known catalyst, methyl trioxorhenium and aqueous hydrogen peroxide as the oxidant. Propylene is fed as gas, which is transported to the liquid phase containing the oxidant, catalyst and methanol as a cosolvent that improves propylene solubility. The selective oxidation produces PO which is distilled easily from the liquid phase taking advantage of the relatively low normal boiling point of PO ( ∼ 34 ∘ C ) compared to those of methanol and water. The process satisfies the sustainability principles of waste minimization, use of benign reagents and process intensification at mild conditions. The process produces PO in yields exceeding 98%. It operates at essentially ambient temperature ( 20 – 35 ∘ C ) and moderate pressures ( ∼ 20 bar ) , using easily recyclable, low hazard aqueous methanol, as solvent. The catalyst, methyl trioxorhenium, is robust under the operating conditions. A key aspect of the innovation is the use of nitrogen gas pressure to enhance propylene availability in the liquid phase increasing its conversion from ∼ 80 % (without N 2 ) to complete (with ∼ 20 bar N 2 ) in a few hours. These findings pave the way for catalyst durability and recycle studies, aimed at demonstrating a continuous process that is economically and environmentally sustainable compared to existing processes.

In this work, the Co(II)-catalyzed oxidations of toluene and p-toluic acid have been carried out under mild conditions. A synergistic effect was found by using zirconium as a cocatalyst and ketones as promoters. Carbon dioxide, an alternative diluent gas to nitrogen, has a strong activation effect on the oxidation, especially on shortening the induction period. Under certain conditions, the reaction did not work at all with N2/O2 but proceeded effectively with CO2/O2, affording high yields of benzoic acid.

A novel monomeric tetravalent manganese complex with the cross-bridged cyclam ligand 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (Me2EBC), [Mn(IV)(Me2EBC)(OH)2](PF6)2, was synthesized by oxidation of Mn(II)(Me2EBC)Cl2 with H2O2 in the presence of NH4PF6)in aqueous solution. The X-ray crystal structure determination of this manganese(IV) compound revealed that it contains two rare terminal hydroxo ligands. EPR studies in dry acetonitrile at 77 K show two broad resonances at g = 1.96 and 3.41, indicating that the manganese(IV) exists as a high-spin d3 species. Resonance Raman (rR) spectra of this manganese(IV) species reveal that the dihydroxy moiety, Mn(IV)(OH)2, is also the dominant species in aqueous solution (pH7). pH titration provides two pK(a) values, 6.86(4) and 10.0(1), associated with stepwise removal of the last two oxygen-bound protons from [Mn(IV)(Me2EBC)(OH)2](2+). The cyclic voltammetry of this manganese(IV) complex in dry acetonitrile at 298 K demonstrates two reversible redox processes at +0.756 and -0.696 V (versus SHE) for the Mn4+/Mn3+ and Mn3+/Mn2+ couples, respectively. This manganese(IV) complex is relatively stable in weak acidic aqueous solution but easily degrades in basic solution to manganese(III) derivatives with an 88 +/- 1% yield.

Olefin epoxidations are a class of reactions appropriate for the investigation of oxygenation processes in general. Here, we report the catalytic epoxidation of various olefins with a novel, cross-bridged cyclam manganese complex, Mn(Me2EBC)Cl2 (Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), using hydrogen peroxide as the terminal oxidant, in acetone/water (ratio 4:1) as the solvent medium. Catalytic epoxidation studies with this system have disclosed reactions that proceed by a nonradical pathway other than the expected oxygen-rebound mechanism that is characteristic of high-valent, late-transition-metal catalysts. Direct treatment of olefins with freshly synthesized [Mn(IV)(Me2EBC)(OH)2](PF6)2 (pKa = 6.86) in either neutral or basic solution confirms earlier observations that neither the oxo-Mn(IV) nor oxo-Mn(V) species is responsible for olefin epoxidization in this case. Catalytic epoxidation experiments using the 18O labels in an acetone/water (H2(18)O) solvent demonstrate that no 18O from water (H2(18)O) is incorporated into epoxide products even though oxygen exchange was observed between the Mn(IV) species and H2(18)O, which leads to the conclusion that oxygen transfer does not proceed by the well-known oxygen-rebound mechanism. Experiments using labeled dioxygen, (18)O2, and hydrogen peroxide, H2(18)O2, confirm that an oxygen atom is transferred directly from the H2(18)O2 oxidant to the olefin substrate in the predominant pathway. The hydrogen peroxide adduct of this high-oxidation-state manganese complex, Mn(IV)(Me2EBC)(O)(OOH)+, was detected by mass spectra in aqueous solutions prepared from Mn(II)(Me2EBC)Cl2 and excess hydrogen peroxide. A Lewis acid pathway, in which oxygen is transferred to the olefin from that adduct, Mn(IV)(Me2EBC)(O)(OOH)+, is proposed for epoxidation reactions mediated by this novel, non-heme manganese complex. A minor radical pathway is also apparent in these systems.

The siderophore-based extraction of iron from the soil by bacteria is proposed as a model for a new separation methodology labeled the soil poultice, a molecular device that would selectively retrieve the complex of a targeted metal ion. In this first feasibility study we describe the synthesis and characterization of molecularly imprinted polymers based on noncovalent interactions, and their application in the specific recognition of macrocyclic metal complexes. The imprinting by salts of {N, N′, N′′, N′′′-tetra(2-cyanoethyl)cyclam}nickel(II) and {N, N′, N′′, N′′′-tetra(2-carbamoylethyl)cyclam}nickel(II) involves multiple hydrogen-bonding interactions when combined during copolymerization with the functional monomer acrylamide or with the crosslinking monomer N,N′-ethylenebisacrylamide. Good rebinding capacity for the imprinting metal complex was observed in both acetonitrile and water. Parallel experiments using the vinylsulfonate salt of either nickel(II) complex adds electrostatic interactions, and us...

Cryoreduction of the [FeO2]6 (n = 6 is the number of electrons in 3d orbitals on Fe and pi* orbitals on O2) dioxygen-bound ferroheme through irradiation at 77 K generates an [FeO2]7 reduced oxy-heme. Numerous investigations have examined [FeO2]7 centers that have been characterized as peroxo-ferric centers, denoted [FeO2]per7, in which a ferriheme binds a dianionic peroxo-ligand. The generation of such an intermediate can be understood heuristically if the [FeO2]6 parent is viewed as a superoxo-ferric center and the injected electron localizes on the O-O moiety. We here report EPR/ENDOR experiments which show quite different properties for the [FeO2]7 centers produced by cryoreduction of monomeric oxy-hemoglobin (oxy-GMH3) from Glycera dibranchiata, which is unlike mammalian "globins" in having a leucine in place of the distal histidine; of frozen aprotic solutions of oxy-ferrous octaethyl porphyrin; and of the oxy-ferrous complex of the heme model, cyclidene. These [FeO2]7 centers are characterized as "superoxo-ferrous" centers ([FeO2]sup7), with nearly unit spin density localized on a superoxo moiety which is end-on coordinated to a low-spin ferrous ion. This assignment is based on their g tensors and 17O hyperfine couplings, which are characteristic of the superoxide ion coordinated to a diamagnetic metal ion, and on the absence of detectable ENDOR signals either from the in-plane 14N ligands or from an exchangeable H-bond proton. Such a center would arise if the electron that adds to the [FeO2]6 superoxo-ferric parent localizes on the Fe ion, to make a superoxo-ferrous moiety. Upon annealing to T150 K, the [FeO2]sup7 species converts to peroxo/hydroperoxo-ferric ([FeO2H]7) intermediates. These experiments suggest that the primary reduction product is [FeO2]sup7 and that the internal redox transition to [FeO2]per7/[FeO2H]7 states is driven at least in part by H-bonding/proton donation by the environment.

New copper(II) and zinc(II) complexes of cross-bridged tetraazamacrocyclic ligands derived from 1,4,8,11-tetraazacyclotetradecane (cyclam), 1,4,7,10-tetraazacyclotridecane ([13]aneN4), and 1,4,7,10-tetraazacyclododecane (cyclen) have been synthesized. The X-ray crystal structures of the two cyclen-derived complexes have been determined and show both metal ions to be in distorted octahedral environments with the ligand in a cis -folded conformation. The bridged-ligand complexes are remarkably stable kinetically under harsh acidic conditions, as seen from their dissociation reactions. The electrochemical and spectroscopic properties of the Cu 2 complexes have also been investigated. # 2003 Elsevier Science B.V. All rights reserved.

The Mn2+ and Fe2+ complexes of the cross-bridged tetraazamacrocyclic ligands, 4,11-dibenzyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), 4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2), 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (3), and 1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (4) provide new compounds of these elements for fundamental studies and applications. These unsubstituted and benzyl substituted derivatives were prepared for comparison of their structures and properties with the known catalytically active dimethyl cross-bridged ligand complexes, which are especially notable for their exceptional kinetic stabilities and redox activity. The X-ray crystal structures of five complexes demonstrate that the ligands enforce a distorted octahedral geometry on the metals with two cis sites occupied by labile ligands. The Fe2+ complexes of the unsubstitued ligands form μ-oxo dimers upon exposure to air, which have also been structurally characterized. Cyclic voltammetry of the monomeric complexes shows reversible redox processes for the M3+/M2+ couples, which are sensitive to solvent, ring size, and ring substitution.

The high spin Co2+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2), 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3), and racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4) have been synthesized and characterized by X-ray crystallography. The Co(III) complexes of 1-3 were also prepared by the chemical oxidation of the Co(II) complexes. The X-ray crystal structures of Co(1)Cl-2, Co(3)Cl-2, and [Co(3)Cl-2]PF6 demonstrate that the ligands enforce a distorted octahedral geometry on Co(II) and Co(III) with two cis sites occupied by chloro ligands. In contrast, the Co(II) complex of 4 is five-coordinate with trigonal bipyramidal geometry. The methyl groups substituted on the carbon atoms of ligand 4 define a shallow cavity, allowing only one chloride ligand to bind to the chelated metal ion. This difference in coordination geometry causes Co(4)Cl+ to be much more difficult to oxidize (E-1/2 = 1.176 V vs. SHE) than the octahedral Co(II) complexes of 1-3 (E-1/2 from -0.157 to 0.173 V vs. SHE). The Co(III) complexes of 1-3 have three absorbances in their electronic spectra, while typical cis-Co(III)N4X2 complexes have only two. This additional splitting of energy states is attributed to the increased distortion from octahedral resulting from the short ethylene cross-bridge on the macrobicyclic ligands. (C) 2002 Elsevier Science B.V. All rights reserved.

The threading of more or less linear axle molecules through macrocyclic molecules, a fundamental process relating to the formation of interlocked molecular structures, has been investigated through the study in acetone of the equilibrium constants for the formation of pseudorotaxanes by NMR methods. The 30 new axle molecules have in common a secondary ammonium group, present as the thiocyanate salt, and an anthracen-9-ylmethyl group, but are rendered unique by the second amine substituent. All rotaxanes involve the well known polyether macrocycle, benzo[24]crown-8. The constants for the binding of axles having linear groups ranging from 2 to 18 carbon atoms show little variation in binding constant but are divided into two groups by their equilibration rates. Those with less than five methylene groups react rapidly on the NMR timescale, whereas those having more than five methylene groups are slow. Branching inhibits binding, but the effect decreases as the branch is moved away from the amine. Phenyl groups weaken binding when close to the amine but strengthen binding when more remote. Some functional groups decrease pseudorotaxane stability (alcohol functions), whereas others increase binding (carboxylic acid groups).

The tautomerization of Criegee intermediates via a 1,4 β-hydrogen atom transfer to yield a vinyl hydroperoxide has been examined in the absence and presence of carboxylic acids. Electronic structure calculations indicate that the organic acids catalyze the tautomerization reaction to such an extent that it becomes a barrierless process. In contrast, water produces only a nominal catalytic effect. Since organic acids are present in parts-per-billion concentrations in the troposphere, the present results suggest that the acid-catalyzed tautomerization, which can also result in formation of hydroxyl radicals, may be a significant pathway for Criegee intermediates.

Density functional theory calculations predict that the gas-phase decomposition of carbonic acid, a high-energy, 1,3-hydrogen atom transfer reaction, can be catalyzed by a monocarboxylic acid or a dicarboxylic acid, including carbonic acid itself. Carboxylic acids are found to be more effective catalysts than water. Among the carboxylic acids, the monocarboxylic acids outperform the dicarboxylic ones wherein the presence of an intramolecular hydrogen bond hampers the hydrogen transfer. Further, the calculations reveal a direct correlation between the catalytic activity of a monocarboxylic acid and its pKa, in contrast to prior assumptions about carboxylic-acid-catalyzed hydrogen-transfer reactions. The catalytic efficacy of a dicarboxylic acid, on the other hand, is significantly affected by the strength of an intramolecular hydrogen bond. Transition-state theory estimates indicate that effective rate constants for the acid-catalyzed decomposition are four orders-of-magnitude larger than those for the water-catalyzed reaction. These results offer new insights into the determinants of general acid catalysis with potentially broad implications.

Environmentally benign oxidations with dense CO2 (either near-critical, ncCO2, or supercritical carbon dioxide, scCO2) as solvent media have been receiving increased attention during the last decade. This paper reviews catalytic oxidations in dense CO2 with emphasis on reported homogeneous systems in scCO2, most of which involve transition metal catalysts and dioxygen or organic peroxides as oxidant. Based on recent work in our laboratory, we offer some perspective and provide examples to demonstrate that scCO2 can be adapted to a broader range of homogeneous oxidations including those that utilize CH3ReO3 as catalyst and t-BuOOH as terminal oxidant, and the oxidation of substituted phenols by dioxygen using Co(salen) complex as catalyst. The advantages of using scCO2 include the total replacement of organic solvents with environmentally benign CO2, the complete miscibility of the oxidants such as O2 in scCO2 eliminating interphase transport limitations, and the resistance of CO2 to oxidation. However, the scCO2-based oxidation has limitations including low reaction rates, inadequate solubilities of a number of transition metal catalysts in CO2 necessitating high process pressures on the order of hundreds of bars, and the lack of pressure-tunability of the dielectric constant of the reaction medium. We present a new process developed in our laboratory in which the conventional solvent medium is only partially replaced by dense CO2. We term this a CO2-expanded solvent medium, which offers several advantages as follows: solvent replacement with dense CO2 by up to 80 mol%, representing a substantial reduction in solvent usage; maintenance of the solubilities of the catalyst and substrate in the reaction mixture while enhancing the miscibility of dioxygen therein; lower process pressures on the order of tens of bars; and pressure-tunable dielectric constants making it possible to realize an optimum reaction medium between scCO2 and neat solvent limits. We distinguish CO2-expanded phases from the traditional concept of a ‘co-solvent’ for a CO2 based system in the following way. To produce a CO2 expanded organic solvent medium, we start with the organic solvent and increase its volume by the addition of CO2, whereas relatively small amounts of ‘co-solvents’ have traditionally been added to dense CO2 phases to improve solubilities of certain compounds. We present examples that show enhanced oxidation rates compared to either neat organic solvent or scCO2 for organic substrates (alkenes and phenols) in CO2-expanded media using dioxygen and metal complexes of both Schiff base and porphyrin ligands. Further, the selectivity toward desired products (alkenes to epoxides; phenols to quinones) is also improved over either neat solvents or scCO2. The CO2-expanded solvents thus offer excellent potential for exploitation in catalytic oxidations.

The crystal structures are reported of trans-dioxocyclam dihydrate, C(10)H(20)N(4)O(2).2H(2)O, a structural isomer of the well known cis-dioxocyclam, and of its novel Ni complex, (1,4,8,11-tetraazacyclotetradecane-2,9-dionato-kappa(4)N)nickel(II) dihydrate, [Ni(C(10)H(18)N(4)O(2))].2H(2)O, the first example of a transition metal complex of this ligand. Both molecules lie on crystallographic centres of inversion. The free ligand has two of its N atoms turned outwards from the ring and hydrogen bonded to water molecules. A major conformational change takes place in the complex in which the ligand binds in a trans tetradentate fashion, as suggested by the electronic spectrum. The nickel(II) ion is low spin, although the electronic spectrum of the complex in water indicates an equilibrium mixture of low-spin and high-spin species. The irreversible electrochemical oxidation of [NiL(1)] (L(1) is deprotonated trans-dioxocyclam, C(10)H(18)N(4)O(2)) in water occurs at a potential of 0.964 V [versus SHE (standard hydrogen electrode)], which is very similar to that for the Ni-cis-dioxocyclam complex.

The novel high spin Ni2+ complexes of the topologically constrained tetraazamacrocycles (1–4) [4,11-dimethyl-1,4,8,11 - tetraazabicyclo[6.6.2]hexadecane (1); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3); racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4)] show striking properties. Potentiometric titrations of the ligands 2 and 4 revealed them to be proton sponges, as reported earlier for 1 [1]. Ligand 3 is less basic, losing its last proton with a pK = 11.3(2). Despite high proton affinities, complexation reactions in the absence of protons successfully yielded Ni2+ complexes in all cases. The X-ray crystal structures of Ni(1)(acac)+, Ni(3)(acac)+ and Ni(1)(OH2)2 2+ demonstrate that the ligands enforce a distorted octahedral geometry on Ni2+ with two cis sites occupied by other ligands. Magnetic measurements and electronic spectroscopy on the corresponding Ni(L)Cl2 (L = 1–3) complexes r...

The bridged tetradentate ligand 5,8-dimethyl-1,5,8,12-tetraazabicyclo[10.3.2]heptadecane (L 1) has been prepared, and its complexes with manganese(II) and iron(II) synthesized and characterized. The crystal structure of MnL 1(NCS)2 has been determined; the Mn is octahedral with cis-NCS ligands, and the asymmetric unit contains two independent molecules differing slightly in the Mn—N—C angles. The oxidation of the complexes by hydrogen peroxide is also discussed.

This review discusses the synthesis of mechanically interlocked molecules where templates orient the reactants to produce permanent structures as the result of new linkages. An introduction outlines the concepts and opportunities of the field, paying special attention to the components of chemical templates. Next, the chemical template types most successfully applied to the synthesis of new interlocked molecular structures, metal ion templates, hydrogen bonded templates, cyclodextrin templates, and π-donor π-acceptor templates, are described. The progress each template type has made towards the goal of true polymeric interlocked structures is noted. The conclusion summarizes the current state of the field and points out new directions that appear ripe for future exploration.

The high-spin dichloro Mn2+ and Fe2+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1) and 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2) provide durable new compounds of these elements for important fundamental studies and applications. The compounds are especially noteable for their exceptional kinetic stabilities and redox activity. The X-ray crystal structures of all four complexes demonstrate that the ligands enforce a distorted octahedral geometry on the metals with two cis sites occupied by labile chloride ligands. Magnetic measurements reveal that all are high spin with typical magnetic moments. Cyclic voltammetry of the complexes shows reversible redox processes at +0.110 and +0.038 V (versus SHE) for the Fe3+/Fe2+ couples of Fe(1)Cl2 and Fe(2)Cl2, respectively, while the Mn3+/Mn2+ and Mn4+/Mn3+ couples were observed at +0.585 and +1.343 V, and +0.466 and +1.232 V for the complexes Mn(1)Cl2 and Mn(2)Cl2, respectively. Mn2+(1) was found to react with H2O2 and o...

The novel bicyclic pentadentate ligand 5-methyl-1,5,9,24,25-pentaazapentacyclo[7.7.7.5.5]pentacosane11,13,15,18(25),20,22-hexene (L1) has been synthesized. Because of its cross-bridged topology it exhibits a relatively rigid preorganized conformation especially appropriate to complex formation, as shown by the crystal structure of the monoprotonated ligand salt, HL1ClH2O [orthorhombic, P212121, a = 9.4405(5) A, b = 13.3617(5) A, c = 16.710(1) A]. The complexes of L1 with both iron(II) and manganese(II) have been characterized, including the crystal structures of [FeL1CH3CN][FeCL4] and [MnL1Cl][PF6] [monoclinic, P21/n, a = 10.0460(5) A, b = 19.237(9) A, c = 15.6254(8) A, beta = 95.97(2)degrees and a = 7.745(2) A, b = 22.786(4) A, c = 14.639(4) A, beta = 105.074(10)degrees respectively]. The manganese complex is high spin with mueff = 5.96 and theta = 2.5 +/- 0.8 cm(-1), indicating weak ferromagnetic interactions. The reactions of the complexes with tert-butyl hydroperoxide and hydrogen peroxide have been shown by ESR spectroscopy to produce the tert-butyl peroxyl and hydroperoxyl radicals, as evidenced by their spin adducts with the spin traps N,N-dimethyl-1-pyrroline-N-oxide and N-tert-butyl-phenyl-nitrone.

N,N′−Diphenylamidines and the silver(I) complexes of their deprotonated anions have been synthesized. Previously uncharacterized tetrameric structural motifs were produced by the inclusion of alkyl substituents at the amidinate carbon. The addition of a 2-methoxy functional group to the phenyl ring resulted in a cationic silver trimer in which hydrogen bonding links silver(I)-bound water molecules to the methoxy substituents. The thermal stabilities of the tetrameric species vary with alkyl chain length. The new complexes are: tetrakis(N,N′-diphenylpropamidinato) tetra silver(I), 1, tetrakis(N,N′-diphenylbutamidinato) tetrasilver(I) 2, tetrakis(N,N′-diphenylpentamidinato) tetra silver(I) 3, (N,N′-diphenyloctamidinato)silver(I) 4, (tetrakis(N,N′-di(4-n-butyl)phenylpropamidinato)tetrasilver(I), 5, bis(N,N′-di(2-methoxy)phenylacetamidinato)diaquatrisilver(I) nitrate 6 and tetrakis(N,N′-di(4-methoxy)phenylacetamidinato) tetrasilver(I), 7. Compounds 1, 5, 6 and 7 were structurally characterized by X-ray methods.

The parallel syntheses of two new cross-bridged tetraazamacrocyclic complexes whose ligands are derived from 1,4,8,11-tetraazacyclotetradecane (cyclam = 14N4) and rac-1,4,8,11-tetraaza-5,5,7,12,12,14-hexamethylcyclotetradecane (tetB = 14N4Me(6)) have been characterized through the crystal structure determination of every stepwise intermediate ligand in the multistep ligand syntheses. These structures show that although the final ligand skeletons are nearly identical, the immediate precursors differ greatly because of the six additional methyl groups of the 14N4Me(6) macrocycle. The inversion from one diastereomer to another of the tetracycle derived from rac-14N4Me(6) has been chemically induced through the successive addition of methyl groups to the reactive tertiary nitrogens, and the novel heterocycles produced have been crystallographically characterized with one showing a conformation not previously known for these systems. The structures of the two copper(II) complexes have significant geometrical differences, and accordingly, their electrochemical and spectroscopic properties are compared. The complexes exhibit remarkable kinetic stability under harsh conditions.

The title complexes, [1,5,9-tris(2-pyridylmethyl)-1,5,9-triazacyclododecane-κ 3 N]iron(II) tetrachloroferrate, [Fe(C 27 H 36 N 6 )][FeCl 4 ], and [1,5,9-tris(2-pyridylmethyl)-1,5,9-triazacyclododecane-κ 3 N]copper(III)bis(hexafluorophosphate) acetonitrile hemisolvate, [Cu(C 27 H 36 N 6 )](PF 6 ) 2 .0.5CH 3 CN, differ significantly in their conformations. The iron complex is near-octahedral, with Fe-N distances of 2.257 (5) (ring) and 2.226 (5) A (pyridine), and fused chelate rings predominately in the boat form. The copper complex is tetragonally distorted, with four shorter and two longer Cu-N bonds [2.049 (6)-2.199 (7) and 2.280 (6)-2.473 (7) A, respectively]; its chelate rings are disordered in boat, chair and skew forms.

The kinetics and mechanism of the catalatic decomposition of hydrogen peroxide by the iron(II) complexes of the macrocyclic ligands 8-methyl-1,4-bis(2-pyridylmethyl)-1,4,8-triazacycloundecane (L1) and 1-methyl-5,9-bis(2-pyridylmethyl)-1,5,9-triazacyclododecane (L2) have been investigated at neutral pH (in phosphate buffer). The oxidation of two equivalents of iron(II) to iron(III) by one equivalent of hydrogen peroxide was observed by spectrophotometric titration. The catalysed decomposition of hydrogen peroxide by iron(III) complex was observed and studied. During the catalytic process a reactive intermediate was scavenged by 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS), but not by bromide ion, indicating that the strongly oxidative intermediate is not the hydroxyl radical, and supporting the alternative possibility, a high valent iron–oxo species. Stopped-flow and steady-state kinetic experiments provide further evidence for the detailed mechanism suggested.

The design and synthesis of a new family of tridentate Schiff base ligands for use in octahedral molecular templates has been successfully demonstrated. As anticipated, two molecules of the ligand, 2,6-pyridinedicarboxaldhydebis(p-hydroxyphenylimine), coordinate equatorially to six-coordinate octahedral metal ions to give orthogonally oriented molecular turns around the anchoring metal ion. This new template offers the advantages that (1) syntheses of the ligand and its complexes are straightforward, giving high yields in simple, one-pot reactions, and (2) structural variations are easily accomplished. X-ray structural analysis has shown that the ligand will only function as a turn when its spatial organization is controlled by coordination to a metal ion. Upon chelation, the phenolic groups are directed across and beyond the metal ion center, and, unlike earlier ligands of this general type which lacked reactive moieties, substitution reactions may be carried out at the free phenolic groups of the octahedral complex. These reactions remain to be optimized in order to achieve the long-term goal of proving their value as synthons for interlocked molecules.

On exposure to oxygen, ferrous heme is thought to autoxidize via three distinct mechanisms: (i) dissociation of protonated superoxide from oxyheme; (ii) reaction between a noncoordinated oxygen molecule and pentacoordinate deoxyheme, and (iii) reaction between a noncoordinated oxygen molecule and an intermediate having water coordinated to the ferrous heme iron. The formation of a hexacoordinate aquomet (H2O.Fe3+) species has been proposed to drive mechanism (iii); consequently, heme proteins with a pentacoordinate met (Fe3+) form might be expected to lack this pathway. We have measured the dependence of autoxidation rate on oxygen concentration for Rhizobium meliloti FixL and Aplysia kurodai myoglobin, which have pentacoordinate met forms. For both proteins, the bell shape of this dependence shows that they autoxidize primarily by mechanism (iii), indicating that a hexacoordinate aquomet species is not required for this mechanism. A novel presentation of the oxygen dependence of autoxidation rates that uses heme saturation, rather than oxygen concentration, more clearly reveals the relative contributions of autoxidation pathways.

The kinetics of O 2 binding to a vacant coordination site on the cobalt(II) ion have been determined, revealing a radical-like character for the reaction. Reversible oxygenation of Co(II) cyclidenes (C4, C5, C6, C8, C12-bridged and unbridged) was studied by a cryogenic stopped-flow method. In the presence of axial base, kinetic parameters are insensitive to the nature of the solvent, and negative entropies of activation suggest that dissociation of a solvent molecule is not the rate-determining step for the dioxygen binding process. This is in contrast to the behavior of previously studied Co(II) complexes. A very low activation energy (1–4 kcal mol −1 ), typical of diffusion controlled processes, was found for dioxygen binding. The binding rate constants for the highest affinity complexes (10 8 M −1 s −1 ) are comparable to the values for natural dioxygen carriers. The size of the lacuna primarily affects the dioxygen binding rates, while the axial bases influence the dioxygen dissociation rates.

Examples of interesting ligands previously requiring lengthy synthetic procedures have been prepared by one-step routes, opening the way to more extensive studies of their complexes and to possible applications. New dicompartmental ligands bearing picolyl pendant arms on the amine nitrogen donors have been synthesized, via the Mannich condensation, from 5-substituted salicylaldehydes, formaldehyde, and N,N‘-bis(2-pyridylmethyl)-1,2-diaminoethane. The protonated acyclic ligand salt, two mononuclear complexes of a macrocyclic ligand with a second compartment featuring a Schiff base, and one of the decomposition products resulting from a retro-Mannich reaction have been structurally characterized. The ligand salt (L1b) has an extended conformation with the ethylenediamine fragment displaying the trans configuration, very different from that of the corresponding closed-site macrocyclic complexes NiH2(L2b)2+ and ZnH2(L2b)2+. The mononuclear macrocyclic complex NiH2(L2b)2+ has a much smaller ligand twist than t...