Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform

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Spektroskopische Charakterisierung eines reaktiven [Cu2(μ‐OH)2]2+ Intermediates in Cu/TEMPO‐katalysierten aeroben Alkoholoxidationen

CuI/TEMPO-Katalysatorsysteme (TEMPO=2,2,6,6-Tetramethylpiperidinoxyl) sind vielseitige Katalysatoren für aerobe Alkoholoxidationsreaktionen zur selektiven Synthese von Aldehyden. Jedoch sind mehrere Aspekte des Reaktionsmechanismus noch nicht aufgeklärt, was hauptsächlich daran liegt, dass bisher keine reaktiven Intermediate identifiziert werden konnten. Wir zeigen hier die Synthese und Charakterisierung eines dinuklearen Komplexes [L12Cu2]2+ (1), der in Gegenwart von TEMPO die katalytische 4 H+/4 e− Reduktion von O2 zu Wasser an die Oxidation von benzylischen und aliphatischen Alkoholen koppeln kann. Die Mechanismen der katalytischen O2-Reduktion und der Alkoholoxidation wurden sowohl durch spektroskopische Detektion der reaktiven Intermediate in der Gas- und der kondensierten Phase als auch durch kinetische Studien an jedem Reaktionsschritt in den Katalysezyklen aufgeklärt. Die intermediären Bis(μ-oxido)dikupfer(III)- (2) und Bis(μ-hydroxido)dikupfer(II)-Spezies (3) wurden als wichtige Reaktanden in beiden Reaktionen identifiziert. Die vorliegende Studie ermöglicht tiefgehende mechanistische Einblicke in die aerobe Alkoholoxidation, die eine wertvolle Grundlage bieten um übergangsmetallkatalysierte Reaktionen mit redoxaktiven Cokatalysatoren besser zu verstehen.Peer Reviewe

Story of two CO2+ cations

The classical understanding of charge transfer between a dication and its neutral counterpart would predict the formation of two singly charged species with statistical energy distribution. For the reaction of CO2 ++ with CO2 it would be thus expected that in a collision event, both emerging singly charged ions CO2 + "forget" about their electronic origin and will be indistinguishable. We have, however, found that this simple picture does not work and upon the collision both generated singly charged ions "remember" their origin and carry off a different internal-energy content. Accordingly, the cations show very different abundance of their subsequent fragmentation. It will be shown that the unsymmetrical energy distribution between the reactants is a consequence of the formation of an elusive C2O4 ++ intermediat

Flavinium Catalysed Photooxidation: Detection and Characterization of Elusive Peroxyflavinium Intermediates

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Transforming hemithioindigo from a two-way to a one-way molecular photoswitch by isolation in the gas phase

Hemithioindigo compounds are attractive two-way molecular photoswitches combining stilbene and thioindigo parts connected by a C–C double bond. In solution, these photoswitches have been well studied. This study presents the investigation of a hemithioindigo derivative in the gas phase. Visible absorption spectra, measured by standard (visPD) and helium-tagging visible photodissociation (He-visPD) techniques were used to unravel absorption characteristics at the level of isolated molecules at 3 Kelvin. Comparison between the Z and E isomers shows a quite distinctive behavior upon visible light absorption. The Z isomer readily undergoes Z → E conversion in the gas phase, as evidenced by the changes in the helium-tagging infrared photodissociation (He-IRPD) spectra. Surprisingly, visible light excitation of the E isomer does not lead to efficient E → Z isomerization unlike in solution. Instead, the ions relax back to their ground state. Influencing the microenvironment of the E isomer by complexation with the highly polar betaine zwitterion resulted in absorption changes, albeit without activating the photoswitching process. Hence, isolation in the gas phase transforms hemithioindigo into a one-way molecular photoswitch. Furthermore, the combination of He-visPD and IRPD spectroscopies proved to be an excellent method for studying photochemical processes such as the double-bond isomerization in the gas phase

Why can a gold salt react as a base?

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Cold and ultracold reactive collisions between FeO+ and H2

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Hydrogen Bonding Effect on the Oxygen Binding and Activa-tion in Cobalt(III)-peroxo Complexes

Cobalt(III)peroxo complexes serve as model metal complexes mediating oxygen activation. We report a systematic study of the effect of the hydrogen bonding on the oxygen binding and on the O-O bond activation within the cobalt(III)-peroxo complexes. To this end, we prepared a series of tris(pyridin-2-ylmethyl)amine based cobalt(III)peroxo complexes having either none, one, two or three amino groups in the secondary coordination sphere. The hydrogen bonding between the amino group(s) and the peroxo ligand was investigated within the isolated complexes in the gas phase using helium tag-ging IR photodissociation spectroscopy, energy-resolved collision induced dissociation experiments and density func-tional theory. The results show that the hydrogen bonding stabilizes the cobalt(III)-peroxo core, but the effect is on the order of units of kcal mol-1. Introduction of the first amino group to the secondary coordination sphere has the largest stabilization effect; more amino groups do not change the results significantly. The amino group can transfer a hydrogen atom to the peroxo ligands which results in the O-O bond cleavage. This process is thermodynamically favored over the O2 elimination, but entropically disfavored

Chemoselectivity in the Oxidation of Cycloalkenes with a Non-Heme Iron(IV)-Oxo-Chloride Complex: Epoxidation vs. Hydroxylation Selectivity

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Aliphatic and Aromatic C-H Bond Oxidation by High-Valent Manganese(IV)-Hydroxo Species

The strong C-H bond activation of hydrocarbons is a difficult reaction in environmental and biological chemistry. Herein, a high-valent manganese(IV)-hydroxo complex, [MnIV(CHDAP-O)(OH)]2+ (2), was synthesized and character-ized by various physicochemical measurements, such as ultra-violet-visible (UV-vis), electrospray ionization-mass spectrome-try (ESI-MS), electron paramagnetic resonance (EPR), and helium-tagging infrared photodissociation (IRPD) methods. The one-electron reduction potential (Ered) of 2 was determined to be 0.93 V vs SCE by redox titration. 2 is formed via a transient green species assigned to a manganese(IV)-bis(hydroxo) complex, [MnIV(CHDAP)(OH)2]2+ (2 '), which performs intramolecular aliphatic C-H bond activation. The kinetic isotope effect (KIE) value of 4.8 in the intramolecular oxidation was observed, which indicates that the C-H bond activation occurs via rate-determining hydrogen atom abstraction. Further, complex 2 can activate the C-H bonds of aromatic compounds, anthracene and its derivatives, under mild conditions. The KIE value of 1.0 was obtained in the oxidation of anthracene. The rate constant (ket) of electron transfer (ET) from N,N '-dimethylaniline derivatives to 2 is fitted by Marcus theory of electron transfer to afford the reorganization energy of ET (lambda = 1.59 eV). The driving force dependence of log ket for oxidation of anthracene derivatives by 2 is well evaluated by Marcus theory of electron transfer. Detailed kinetic studies, including the KIE value and Marcus theory of outer-sphere electron transfer, imply that the mechanism of aromatic C-H bond hydroxylation by 2 proceeds via the rate-determining electron-transfer pathway