Kinetic Analysis of the Formation and Decay of a Non-Heme Ferric Hydroperoxide Species Susceptible to O–O Bond Homolysis

The formation of a ferric hydroperoxide species from [Fe­(bbpc)­(MeCN)2]2+ (bbpc = N,N′-dibenzyl-N,N′-bis­(2-pyridylmethyl)-1,2-cyclohexanediamine) and its subsequent decomposition were analyzed kinetically. The rate of decay is not strongly influenced by the presence of either water or substrate, suggesting that the ferric hydroperoxide degrades through O–O bond homolysis and is not the relevant metal-based oxidant in the observed catalysis of C–H activation. The rate law corresponding to the complex’s formation from O2 is consistent with the intermediacy of a mononuclear ferric superoxo species

Computational Examination of the Mechanism of Alkene Epoxidation Catalyzed by Gallium(III) Complexes with N‑Donor Ligands

The ability of gallium­(III) complexes to catalyze the epoxidation of alkenes by peracetic acid has been examined with density functional theory calculations. According to the calculations, the chloride anions of the precatalyst [Ga­(phen)₂Cl₂]⁺ (phen = 1,10-phenanthroline) can be displaced by either acetic or peracetic acid through dissociative ligand exchange pathways; both acetic and peracetic acid deprotonate upon binding to the formally tricationic metal center. Because of the high basicity of peracetate relative to that of chloride, only the acetate for chloride exchange occurs spontaneously, providing a rationale for the preponderance of gallium acetate adducts observed in the reaction mixtures. With respect to the mechanism of olefin epoxidation, the computational results suggest that the peracetic acid is most efficiently activated for redox activity when it binds to the metal center in a κ² fashion, with the carbonyl oxygen atom serving as the second point of attachment. The phen ligands’ coordination to the gallium is essential for the catalysis, and the lowest energy pathways for alkene oxidation proceed through hexacoordinate Ga­(III) species with four Ga–N bonds. A natural bond order analysis confirms the electrophilic nature of the metal-containing oxidant

Kinetic Analysis of the Formation and Decay of a Non-Heme Ferric Hydroperoxide Species Susceptible to O–O Bond Homolysis

The formation of a ferric hydroperoxide species from [Fe­(bbpc)­(MeCN)₂]²⁺ (bbpc = <i><i>N</i>,<i>N</i></i>′-dibenzyl-<i><i>N,N</i></i>′-bis­(2-pyridylmethyl)-1,2-cyclohexanediamine) and its subsequent decomposition were analyzed kinetically. The rate of decay is not strongly influenced by the presence of either water or substrate, suggesting that the ferric hydroperoxide degrades through O–O bond homolysis and is not the relevant metal-based oxidant in the observed catalysis of C–H activation. The rate law corresponding to the complex’s formation from O₂ is consistent with the intermediacy of a mononuclear ferric superoxo species

6-Methylpyridyl for Pyridyl Substitution Tunes the Properties of Fluorescent Zinc Sensors of the Zinpyr Family

To prepare fluorescent zinc sensors with binding affinities lower than that of the parent 9-(o-carboxyphenyl)-2,7-dichloro-4,5-bis(bis(2-pyridylmethyl)methylaminomethyl)-6-hydroxy-3-xanthenone (ZP1), dimethylated and tetramethylated derivatives were synthesized having either two or four of the pyridyl subunits methylated at the 6-position. Like the parent ZP1, both Me2ZP1 and Me4ZP1 exhibit increased fluorescence in the presence of Zn2+. The integrated emission of Me2ZP1 increases 4-fold in the presence of excess zinc, whereas Me4ZP1 displays 2.5-fold enhanced fluorescence for Zn2+. Methylating the 6-positions of the pyridyl rings raises the dissociation constant of the sensors and lowers the pKa values associated with the tertiary amine ligands in a systematic manner. The properties of the dimethylated Me2ZP1 dye resemble those of ZP1, but the tetramethylated Me4ZP1 differs greatly from ZP1 in terms of its brightness, affinity toward Zn2+, exchange kinetics, and metal sensitivity. Both Me2ZP1 and Me4ZP1 can enter HeLa cells and signal the presence of Zn2+. Staining caused by both dyes is punctate, with localization patterns resembling that observed for ZP1

Analogues of Zinpyr-1 Provide Insight into the Mechanism of Zinc Sensing

Three compounds structurally related to the fluorescent zinc sensor Zinpyr-1 (ZP1) have been synthesized and characterized. In each of these ZinAlkylPyr (ZAP) analogues, an alkyl group (methyl, benzyl) replaces one of the metal-binding picolyl moieties in ZP1. The methyl-for-picolyl substitutions in ZAP1 and ZAP2 have a negligible effect on the optical spectrum of the fluorophore but elevate the quantum yields (Φ = 0.82 (ZAP1), 0.74 (ZAP2)) to values near that of Zn2+-saturated ZP1 (Φ = 0.92). The benzyl-for-picolyl substitution in ZAP3 similarly enhances the quantum yield (Φ = 0.52) relative to that of metal-free ZP1 (Φ = 0.38). As previously observed for methylated ZP1 sensors, methylation of the 6-position of the pyridyl ring diminishes the emission by lowering both the molar extinction coefficient and the quantum yield. Although these new ZAP compounds cannot detect Zn2+ fluorimetrically at neutral pH, complexation of Zn2+ does occur, as evidenced by sizable changes in the optical spectra. The ZAP1−3 probes can detect Zn2+ fluorimetrically at pH 9, indicating that proton-induced background emission obscures any Zn2+-induced fluorescence at pH 7. The tertiary amine groups in ZAP1−3 are less basic than those in ZP1, which implies that the additional pyridine rings are responsible for the emissive response to Zn2+ at pH 7.0

Hydrogen Atom Abstraction by a Mononuclear Ferric Hydroxide Complex: Insights into the Reactivity of Lipoxygenase

The lipoxygenase mimic [FeIII(PY5)(OH)](CF3SO3)2 is synthesized from the reaction of [FeII(PY5)(MeCN)](CF3SO3)2 with iodosobenzene, with low-temperature studies suggesting the possible intermediacy of an Fe(IV) oxo species. The Fe(III)−OH complex is isolated and identified by a combination of solution and solid-state methods, including EPR and IR spectroscopy. [FeIII(PY5)(OH)]2+ reacts with weak X−H bonds in a manner consistent with hydrogen-atom abstraction. The composition of this complex allows meaningful comparisons to be made with previously reported Mn(III)−OH and Fe(III)−OMe lipoxygenase mimics. The bond dissociation energy (BDE) of the O−H bond formed upon reduction to [FeII(PY5)(H2O)]2+ is estimated to be 80 kcal mol-1, 2 kcal mol-1 lower than that in the structurally analogous [MnII(PY5)(H2O)]2+ complex, supporting the generally accepted idea that Mn(III) is the thermodynamically superior oxidant at parity of coordination sphere. The identity of the metal has a large influence on the entropy of activation for the reaction with 9,10-dihydroanthracene; [MnIII(PY5)(OH)]2+ has a 10 eu more negative ΔS⧧ value than either [FeIII(PY5)(OH)]2+ or [FeIII(PY5)(OMe)]2+, presumably because of the increased structural reorganization that occurs upon reduction to [MnII(PY5)(H2O)]2+. The greater enthalpic driving force for the reduction of Mn(III) correlates with [MnIII(PY5)(OH)]2+ reacting more quickly than [FeIII(PY5)(OH)]2+. Curiously, [FeIII(PY5)(OMe)]2+ reacts with substrates only about twice as fast as [FeIII(PY5)(OH)]2+, despite a 4 kcal mol-1 greater enthalpic driving force for the methoxide complex

A Homogeneous Gallium(III) Compound Selectively Catalyzes the Epoxidation of Alkenes

We demonstrate that a simple gallium­(III) complex, [Ga­(phen)₂Cl₂]Cl (phen = 1,10-phenanthroline), can serve as a homogeneous catalyst for the epoxidation of alkenes. The olefin epoxidations proceed relatively quickly at mild temperatures and, under optimum conditions, are highly selective for the epoxide product

C–H Oxidation by H₂O₂ and O₂ Catalyzed by a Non-Heme Iron Complex with a Sterically Encumbered Tetradentate N‑Donor Ligand

The compound <i>N</i>,<i>N′</i>-dineopentyl-<i>N</i>,<i>N</i>′-bis­(2-pyridylmethyl)-1,2-ethanediamine (dnbpn) and its ferrous complex [Fe­(dnbpn)­(OTf)₂] were synthesized. The Fe­(II) complex was used to catalyze the oxidation of hydrocarbons by H₂O₂ and O₂. Although the catalyzed alkane oxidation by H₂O₂ displays a higher preference for secondary over tertiary carbons than those associated with most previously reported nonheme iron catalysts, the catalytic activity is markedly inferior. In addition to directing the catalyzed oxidation toward the less sterically congested C–H bonds of the substrates, the neopentyl groups destabilize the metal-based oxidants generated from H₂O₂ and the Fe­(II) complex. The presence of benzylic substrates with weak C–H bonds stabilizes an intermediate which we have tentatively assigned as a high-spin ferric hydroperoxide species. The oxidant generated from O₂ reacts with allylic and benzylic C–H bonds in the absence of a sacrificial reductant; less substrate dehydrogenation is observed than with related previously described systems that use O₂ as a terminal oxidant

Catalysis of Alkene Epoxidation by a Series of Gallium(III) Complexes with Neutral N‑Donor Ligands

Six gallium­(III) complexes with N-donor ligands were synthesized to study the mechanism of Ga^III-catalyzed olefin epoxidation. These include 2:1 ligand/metal complexes with the bidentate ligands ethylenediamine, 5-nitro-1,10-phenanthroline, and 5-amino-1,10-phenanthroline, as well as 1:1 ligand/metal complexes with the tetradentate <i>N</i>,<i>N</i>′-bis­(2-pyridylmethyl)-1,2-ethanediamine, the potentially pentadentate <i>N</i>,<i>N</i>,<i>N</i>′-tris­(2-pyridylmethyl)-1,2-ethanediamine, and the potentially hexadentate <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis­(2-pyridylmethyl)-1,2-ethanediamine. In solution, each of the three pyridylamine ligands appears to coordinate to the Ga^III through four donor atoms. The six complexes were tested for their ability to catalyze the epoxidation of alkenes by peracetic acid. Although the complexes with relatively electron-poor phenanthroline derivatives display faster initial reactivity, the gallium­(III) complexes with the polydentate pyridylamine ligands appear to be more robust, with less noticeable decreases in their catalytic activity over time. The more highly chelating trispicen and tpen are associated with markedly decreased activity

Steric Modifications Tune the Regioselectivity of the Alkane Oxidation Catalyzed by Non-Heme Iron Complexes

Iron complexes with the tetradentate N-donor ligand N,N′-di­(phenylmethyl)-N,N′-bis­(2-pyridinylmethyl)-1,2-cyclohexanediamine (bbpc) are reported. Despite the benzyl groups present on the amines, the iron compounds catalyze the oxygenation of cyclohexane to an extent similar to those employing less sterically encumbered ligands. The catalytic activity is strongly dependent on the counterion, with the highest activity and the strongest preference for alkane hydroxylation correlating to the most weakly coordinating anion, SbF6–. The selectivity for the alcohol product over the ketone is amplified when acetic acid is present as an additive. When hydrocarbon substrates with both secondary and tertiary carbons are oxidized by H2O2, the catalyst directs oxidation toward the secondary carbons to a greater degree than other previously reported iron-containing homogeneous catalysts