Matthias Schwalbe

Hangman Corroles: Efficient Synthesis and Oxygen Reaction Chemistry

The construction of a new class of compounds--the hangman corroles--is provided efficiently by the modification of macrocyclic forming reactions from bilanes. Hangman cobalt corroles are furnished in good yields from a one-pot condensation of dipyrromethane with the aldehyde of a xanthene spacer followed by metal insertion using microwave irradiation. In high oxidation states, X-band EPR spectra and DFT calculations of cobalt corrole axially ligated by chloride are consistent with the description of a Co(III) center residing in the one-electron oxidized corrole macrocycle. These high oxidation states are likely accessed in the activation of O-O bonds. Along these lines, we show that the proton-donating group of the hangman platform works in concert with the redox properties of the corrole to enhance the catalytic activity of O-O bond activation. The hangman corroles show enhanced activity for the selective reduction of oxygen to water as compared to their unmodified counterparts. The oxygen adduct, prior to oxygen reduction, is characterized by EPR and absorption spectroscopy.

Xanthene-modified and hangman iron corroles.

Iron corroles modified with a xanthene scaffold are delivered from easily available starting materials in abbreviated reaction times. These new iron corroles have been spectroscopically examined with particular emphasis on defining the oxidation state of the metal center. Investigation of their electronic structure using (57)Fe Mössbauer spectroscopy in conjunction with density functional theory (DFT) calculations reveals the non-innocence of the corrole ligand. Although these iron corroles contain a formal Fe(IV) center, the deprotonated corrole macrocycle ligand is one electron oxidized. The electronic ground state of these complexes is best described as an intermediate spin S = 3/2 Fe(III) site strongly antiferromagnetically coupled to the S = 1/2 of the monoradical dianion corrole [Fe(III)Cl-corrole(+•)]. We show here that iron corroles as well as xanthene-modified and hangman xanthene iron corroles are redox active and catalyze the disproportionation of hydrogen peroxide via the catalase reaction, and that this activity scales with the oxidation potential. The meso position of corrole macrocycle is susceptible toward nucleophilic attack during catalase turnover. The reactivity of peroxide within the hangman cleft reported here adds to the emerging theme that corroles are good at catalyzing two-electron activation of the oxygen-oxygen bond in a variety of substrates.

High-valent metal-oxo intermediates in energy demanding processes: from dioxygen reduction to water splitting

Four-electron reduction of dioxygen to water and splitting of water to dioxygen are extremely important processes in the context of attaining clean renewable energy sources. High-valent metal-oxo cores are proposed as reactive intermediates in these vital processes, although they have only been isolated in extremely rare cases in the biological systems thereby making the mechanism ambiguous. Recent biomimetic studies have, however, aided in our understanding of the fundamental reactivity of the high-valent metal-oxo species in various reactions relevant to energy conversion. All these studies are summarized in the present review.

Efficient Synthesis of Hangman Porphyrins

A two-step synthetic method has been designed to furnish hangman porphyrins in good yields from easily available starting materials. The use of the microwave irradiation technique has been found to be valuable for delivering the carboxylic acid hanging group in a much simplified and less time-consuming basic ester hydrolysis (4 h vs 7 days under harsh acidic conditions). The new route facilitates the synthesis of various hangman porphyrins that previously had limited or no access.

Photochemical CO2 Reduction Catalyzed by Phenanthroline Extended Tetramesityl Porphyrin Complexes Linked with a Rhenium(I) Tricarbonyl Unit

A series of heterodinuclear complexes (M-1-Re) based on a phenanthroline (phen) extended tetramesityl porphyrin ligand (H2-1) has been prepared. The phen moiety of this ligand selectively coordinates a Re(I) tricarbonyl chloride unit, whereas the metal in the porphyrin moiety has been varied: namely, Cu, Pd, Zn, Co, or Fe was used. These dinuclear complexes were fully characterized by standard analytical methods. Additionally, a crystal structure of Cu-1-Re·5.5(C7H8)·0.5(C6H6) could be obtained, and extended time-resolved emission lifetime measurements were conducted. Furthermore, their ability to catalyze the photochemical reduction of CO2 to CO was investigated. Light-driven CO2 reduction experiments were performed in dimethylformamide (DMF) using triethylamine (TEA) as the sacrificial electron donor. The TONs (turnover numbers) of CO were determined and revealed a surprising catalytic activity that is obviously independent from the redox activity of the porphyrin metal. We have recently shown that the parent M-1 compounds are active photocatalysts, but the catalytic activity was dependent on the redox activity of the porphyrin metal. In the case of the new heterodinuclear complexes M-1-Re reported in this study, the catalytic active center seems to be the Re(I) moiety and not the porphyrin. Surprisingly, Zn-1-Re proved to be the most active compound in this series showing a TONCO of 13 after 24 h of illumination using a >375 nm cutoff filter while all other compounds showed minimal activity under this condition.

Terpyridine-porphyrin hetero-Pacman compounds.

The two different coordination spheres afforded by Pacman architectures offer cooperativity derived from two different metal centers. A modular strategy is developed to produce a hetero-Pacman scaffold featuring a porphyrin and terpyridine for metal-ion binding. A double Suzuki reaction was employed to first attach a terpyridine moiety to a xanthene backbone and then attach a porphyrin. The new hetero-Pacman scaffold has been characterized and all building blocks have been isolated and structurally characterized. The principle objective to incorporate different metal centers was confirmed by isolating a trinuclear complex comprising two porphyrinic units and a bis(terpyridine)-iron unit. The compounds described herein expand the Pacman scaffold concept by allowing for the incorporation of a terpyridine-metal complex proximate to a porphyrin-cofactor active site for small-molecule activation.

Soft Scorpionate Anions as Platforms for Novel Heterocycles

Soft scorpionates have thus far been seen mainly as a family of ligands. Their chemistry is extended here to the production of novel cationic macrocycles using dihaloalkanes. By replacing the dihaloalkanes with mild oxidising agents (NO(+), I2) we obtain two unique polycyclic heterocycles. The mechanism which leads to the formation of these polycyclic heterocycles is investigated using ab initio DFT calculations.

Electrocatalytic Carbon Dioxide Reduction by Using Cationic Pentamethylcyclopentadienyl–Iridium Complexes with Unsymmetrically Substituted Bipyridine Ligands

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

Photocatalytic Generation of Hydrogen Using Dinuclear π-Extended Porphyrin-Platinum Compounds

A series of heterodinuclear complexes, M-1-PtX2 with M=H2 , Zn, Cu or Co, X=Cl or I, has been synthesized, and first results on their photocatalytic activity in visible light driven proton reduction are presented. The compounds are based on a phenanthroline extended meso-tetramesityl-porphyrin bridging ligand (H2 -1) incorporating different metal centers in the porphyrin moiety, which functions as a photosensitizer unit. The well-known catalytically active PtX2 fragment resides in the phenanthroline coordination pocket. The synthesis was optimized, compounds were fully characterized and a solid-state structure could be obtained for selected complexes. Photocatalytic studies in acetonitrile/water mixtures using triethylamine as sacrificial electron donor showed that the activity of the complexes depends strongly on the metal center in the porphyrin moiety as well as the halogen ions bound at the platinum(II) center.

Metal-oxo-mediated O-O bond formation reactions in chemistry and biology

Abstract O-O bond formation is one of the key reactions that ensure life on earth. Dioxygen is produced in photosystem II, as well as in chlorite dismutase. The reaction mechanisms occurring in the enzyme active sites are controversially discussed – although their structures have been resolved with less unambiguity. Artificial molecular catalysts have been developed in the last years to obtain vital insights into the O-O bond formation step. This review put together the scarce literature on the topic that helped in understanding the key steps in the O-O bond formation reactions mediated by high-valent oxo complexes of the first-row transition metals.