G. Ferraudi

Complexes of macrocycles with pendant arms as models for biological molecules

Abstract Recent literature results on inorganic models related to metalloenzymes which transport electrons or activate small molecules are discussed in this review. The molecular systems are essentially restricted to macrocyclic or polydentate ligands preferentially containing nitrogen atoms and pendant arms. The discussion is mainly concerned with aspects such as spectroscopic, electrochemical and chemical characterization of the systems and electro- or photo-activation of small molecules.

Carbon dioxide activation by aza-macrocyclic complexes

Abstract Experimental observations of photocatalysis and electroinduced-activation of CO 2 mediated by coordination compounds with aza-macrocyclic ligands are discussed in this review. The production of fuels from the reduction of CO 2 catalyzed by coordination complexes of simple tetraaza-macrocyclilis, porphyrin, phthalocyanine and biphenanthrolinic hexaazacyclophanes are described. The use of this macrocyclic chemistry, i.e. for simple models, in the development of multipurpose supramolecular functions is emphasized.

PHOTOCHEMISTRY OF SOME THREE-MEMBERED HETEROCYCLES

Abstract The solid-state photolysis of oxiranes and aziridines produces highly colored ylide intermediates. The stability of these intermediates is dependent on a combination of electronic and steric factors as well as the solid-state constraints of the environment. Flash photolysis studies indicate that two different ylides can be produced in solution at room temperature. These intermediates appear to be identical to those formed consecutively in the solid state or low-temperature glasses. By the use of certain gas-solid reactions, it is possible to control the lifetime of the intermediate so that it is destroyed immediately or continues to exist almost indefinitely.

A flash photolytic investigation of low energy homolytic processes in methylcobalamin.

Die Co-Methyl-o-Bindung in Methylcobalamin wird bereits durch Bestrahlung bei niedriger Energie gespalten.

Electrocatalytic reduction of CO2 by aza-macrocyclic complexes of Ni(II), Co(II), and Cu(II). Theoretical contribution to probable mechanisms

Abstract The electrocatalyzed reduction of carbon dioxide was investigated with cobalt(II), nickel(II) and copper(II) complexes of hexaaza-macrocycle ligands. The macrocyclic ligands were obtained via the condensation of phenanthroline or bipyridine. The X-ray structure of a triflate derivative was determined and it showed the less common “CuN4O2” centrosymmetric octahedral configuration with apically elongated CuO bonds. A preference for a neutral, mononuclear, octahedral configuration is exhibited by the planar macrocycle with an extended π-electron system. Its structure is characterized by shorter CuN bonds than those of Cu complexes with saturated macrocycles. The reduction of the complexes has been studied by cyclic voltammetry and UV–Vis spectroscopy. The results show that the metal center in its lower oxidation state, i.e. M(I), is the active site. The metal center is oxidized to M(II) in the presence of CO2 with the concomitant formation of an adduct between the metal center and CO2. A further one electron reduction regenerates the catalyst and produces carbon monoxide and/or formic acid. A theoretical model is proposed for a simple mechanism that involves initial coordination of CO2 to the metal center.

Oxygenation of Cobalt Porphyrinates: Coordination or Oxidation?

The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHIm)], is reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-N(Im) = 2.145(3) and (Co-N(p))(av) = 1.979(3) A. This five-coordinate complex, in the presence of dioxygen and excess 2-methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of cobalt(II), and the formation of the neutral cobalt(III) complex [Co(alpha,alpha,beta,beta-TpivPP)(2-MeHIm)(2-MeIm(-)]. Two distinct examples of this complex have been structurally characterized, and both have structural parameters consistent with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-N(Im) distances ranging from 1.952 to 1.972 A, but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2-MeHIm)(O(2))] is an intermediate state for the oxidation reaction and that the atropisomerization process is photocatalyzed. A reaction route is proposed based on the spectroscopic studies.

Synthesis, Photophysical, Photochemical, and Computational Studies of Coumarin-Labeled Nicotinamide Derivatives

The syntheses and photophysical/photochemical properties of two amide-tethered coumarin-labeled nicotinamides are described. Photochemical studies of 6-bromo-7-hydroxycoumarin-4-ylmethylnicotinamide (BHC-nicotinamide) revealed an unexpected solvent effect. This result is rationalized by computational studies of the different protonation states using TD-DFT with the M06L/6-311+G** method with implicit and explicit solvation models. Molecular orbital energies responsible for the λ(max) excitation show that the functionalization of the coumarin ring results in a strong red-shift from 330 to 370 nm when the pH of solution is increased from 3.06 to 8.07. From this MO analysis, a model for solvent interactions has been proposed. The BHC-nicotinamide proved to be photochemically stable, which is also interpreted in terms of NBO calculations. The results provide a set of principles for the rational design of either photostable labeling reagents or photolabile cage compounds.

Electrochemical reduction of carbon dioxide by hexa-azamacrocyclic complexes

Diethenodypyrido-hexa-aza-tetradecine complexes act as a tetradentate ligand, and its complexes of Co(II), Ni(I1) and Cu(II), are essentially planar. This work shows electrochemical, radiolytic and spectroscopic studies of electron transfer processes and solution equilibria. The presence of carbon dioxide notoriously change the optical properties in solution. Some pulse radiolysis studies for the reduction process of these complexes in carbon dioxide medium are also shown. In addition, electrochemical studies were performed in carbon dioxide atmosphere in order to obtain preliminary information about electrocatalytical reductions.

Synthetic N-substituted metal aza-macrocyclic complexes: Properties and applications

Recent contributions to the chemistry of macrocyclic metal complexes and ligands are reviewed. The compounds reviewed have the following structural features: aza-macrocycles having 1,10-phenanthroline and 2,2′-bipyridine groups and pendants arms; highly aromatic aza-macrocycles with phthalocyanine and porphyrin units incorporating pendant arms; aliphatic aza-macrocycles with pendant arms and/or polynucleating aza-macrocycles. Several of their chemical properties are critically reviewed but the emphasis is on structural and synthetic details.

Photochemical properties of tin(IV) phthalocyanines: a comparison between the dichloro and superphthalocyanines

Abstract The photochemical behavior of Tin(IV) dichlorophthalocyaninate, Sn(pc)Cl 2 , and tin(IV) super phthalocyanine, Sn(pc) 2 , has been investigated by flash photolysis. Irradiations, λ exc ∼ 640 nm, of Sn(pc)Cl 2 generate the lowest lying triplet state, ( 3 ππ * )Sn(pc)Cl 2 , with a half lifetime, τ ∼ 40 μ s . Quenching of this excited state with paradinitrobenzene produces a Sn(IV) phthalocyanine radical which isomerizes to a Sn(III) complex in a μ s time domain. The photochemistry of Sn(pc) 2 can be described in terms of an unreactive 3 ππ * and a reactive CT excited state. Dissociation reactions form Sn II (pc) and Sn IV (pc)(solvent) 2 . The regeneration of the superphthalocyanine takes place through a Sn(III) intermediate.