A practical, inexpensive, green chemical process for degrading environmental pollutants is greatly needed, especially for persistent chlorinated pollutants. Here we describe the activation of hydrogen peroxide by tetraamidomacrocylic ligand (TAML) iron catalysts, to destroy the priority pollutants pentachlorophenol (PCP) and 2,4,6-trichlorophenol (TCP). In water, in minutes, under ambient conditions of temperature and pressure, PCP and TCP are completely destroyed at catalyst:substrate ratios of 1:715 and 1:2000, respectively. The fate of about 90% of the carbon and about 99% of the chlorine has been determined in each case. Neither dioxins nor any other toxic compounds are detectable products, and the catalysts themselves show low toxicity.

Exceptionally high peroxidase-like and catalase-like activities of iron(III)-TAML activators of H 2O 2 ( 1: Tetra-Amidato-Macrocyclic-Ligand Fe (III) complexes [ F e{1,2-X 2C 6H 2-4,5-( NCOCMe 2 NCO) 2CR 2}(OH 2)] (-)) are reported from pH 6-12.4 and 25-45 degrees C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [Ru (II)( o-C 6H 4py)(phen) 2]PF 6 ( 2) or "ruthenium dye", occurs via the equation [ Ru II ] + 1/2 H 2 O 2 + H +-->(Fe III - TAML) [ Ru III ] + H 2 O, following a simple rate law rate = k obs (per)[ 1][H 2O 2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H 2 O 2 -->(Fe III - TAML) H 2 O + 1/2 O 2) obeys a similar rate law: rate = k obs (cat)[ 1][H 2O 2]). The rate constants, k obs (per) and k obs (cat), are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H 2O 2 in this pH range. Complexes 1 exist as axial diaqua species [FeL(H 2O) 2] (-) ( 1 aqua) which are deprotonated to afford [FeL(OH)(H 2O)] (2-) ( 1 OH) at pH 9-10. The pathways 1 aqua + H 2O 2 ( k 1), 1 OH + H 2O 2 ( k 2), and 1 OH + HO 2 (-) ( k 4) afford one or more oxidized Fe-TAML species that further rapidly oxidize the dye (peroxidase-like activity) or a second H 2O 2 molecule (catalase-like activity). This mechanism is supported by the observations that (i) the catalase-like activity of 1 is controllably retarded by addition of reducing agents into solution and (ii) second order kinetics in H 2O 2 has been observed when the rate of O 2 evolution was monitored in the presence of added reducing agents. The performances of the 1 complexes in catalyzing H 2O 2 oxidations are shown to compare favorably with the peroxidases further establishing Fe (III)-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.

The reaction between an Fe(III) complex and O(2) to afford a stable catalytically active diiron(IV)-mu-oxo compound is described. Phosphonium salts of orange five-coordinated Fe(III)-TAML complexes with an axial aqua ligand ([PPh(4)]1-H(2)O, tetraamidato macrocyclic Fe(III) species derived from 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11-benzotetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone) react rapidly with O(2) in CH(2)Cl(2) or other weakly coordinating solvents to produce black mu-oxo-bridged diiron(IV) complexes, 2, in high yields. Complexes 2 have been characterized by X-ray crystallography (2 cases), microanalytical data, mass spectrometry, UV/Vis, Mossbauer, and (1)H NMR spectroscopies. Mossbauer data show that the diamagnetic Fe-O-Fe unit contains antiferromagnetically coupled S = 1 Fe(IV) sites; diamagnetic (1)H NMR spectra are observed. The oxidation of PPh(3) to OPPh(3) by 2 was confirmed by UV/Vis and GC-MS. Labeling experiments with (18)O(2) and H(2)(18)O established that the bridging oxygen atom of 2 derives from O(2). Complexes 2 catalyze the selective oxidation of benzylic alcohols into the corresponding aldehydes and bleach rapidly organic dyes, such as Orange II in MeCN-H(2)O mixtures; reactivity evidence suggests that free radical autoxidation is not involved. This work highlights a promising development for the advancement of green oxidation technology, as O(2) is an abundant, clean, and inexpensive oxidizing agent.

Enzymes that activate dioxygen or hydrogen peroxide exhibit remarkable catalytic activity and selectivity, but oxidative and hydrolytic fragility limit their technological applicability. Their catalytic activity-pH profiles are usually bell-shaped, resulting from either reversible or irreversible activity loss at extreme pH. 1 Their mere existence challenges chemists to design low-molecular, protein-free catalysts 2 that are at least as active, while being more robust in aggressively oxidizing acidic and basic environments. Recently, we introduced a new class of oxidatively robust "green" 3 catalysts for H 2 O 2 oxidation of a wide spectrum of substrates 4 including facile polychlorophenol mineralization. 5 The Fe III centers of the nontoxic catalysts 5 are coordinated to tetra-amido macrocyclic ligands giving TAML oxidant activators, Figure 1.When extracted from the protein environments of horseradish peroxidase (HRP) or cytochrome P450, the catalytic properties of the Fe III porphyrin cofactors are inadequate. In contrast, Fe III -TAML activators are exceptionally active; the second-order rate constants for the rate-limiting activation of H 2 O 2 are as high as 10 4 M -1 s -1 at 25°C and optimal pH. 6 TAML catalysts are effective in nanomolar to low micromolar concentrations in water where they can attain turnover frequencies in the thousands per minute. 5 Despite this high activity, the design protocol applied to developing Fe III -TAML activators (R ) alkyl) led, inter alia, to their protection from rapid oxidative degradation. However, as with HRP, they rapidly lose activity at pH < 4 by demetalation, although HRP's Fe IIIporphyrin is more resistant to acidic conditions. 7 Inactivation mechanisms for small molecule TAML activators should be much easier to understand than those of enzymes, and such understanding should facilitate the design of more stable catalysts. We illustrate this argument using structural, spectral, kinetic, and mechanistic studies of Fe III -TAML activators. Stabilization against H + -induced demetalation has been achieved of 10 5 -fold and 10 11 -fold in mildly and strongly acidic solutions, respectively.Iron(III)-TAML activators are synthesized either in chloro or in aqua forms (Figure 1). 8 X-ray structures demonstrate 5-coordination (5-CN) with long Fe-Cl (1a,e) and Fe-OH 2 (2a) bonds (Figure 1 and Supporting Information). 9 5-CN is typical of HRP iron. 1b If the same geometry holds in solution, the open axial site could contribute to both high catalytic activity of Fe III -TAMLs toward H 2 O 2 and the facile proteolysis. The speciation of Fe III -TAMLs in water was studied by UV/vis and EPR. Principal conclusions derived from consistent results (Supporting Information) are summarized below and shown in Scheme 1. (i) The chloro ligands of 1 undergo rapid hydrolysis as expected for species with long FeCl bonds; 10 equilibrium data, K Cl (Table 1), indicate insignificant Cl -coordination for [Cl -] e 0.5 M. (ii) Cl -hydrolysis affords aqua species accounting for the similarity of 1...