Photokatalytische Bildung von Wasserstoff aus Thiolen in Gegenwart von Vitamin B12-Modellverbindungen mit Azid als photochemischem Opferliganden

Photocatalytic Hydrogen Formation from Thioles in the Presence of Vitamin-B₁₂ Model Complexes with Azide as Photochemical Sacrificial Ligand The photolysis of [N₃Co(chelat)B] complexes ( 1–3 ) (chelat = dimethylglyoxime, dmg; N,N′-o-phynylenebis(salicylidenimine), salphen; N,N′-ethylene-bis(salicylidenimine), salen; B = pyridine) leads by homolytic cleavage of the Co–N₃ bond to both coordinatively unsaturated cobalt(II) chelates [Co(chelat)B] and N₃ ligand radicals that undergo fast decay to dinitrogen. The photolysis of the cobalt (III) complexes 1–3 in the presence of thiophenole and other thioles proceeds catalytically and yields the corresponding disulphides and dihydrogen. The mechanism of this photocatalytic generation of dihydrogen is due to the catalytic activity of the coordinatively unsaturated cobalt(II) species formed photochemically. A photocatalytic cycle is proposed describing the generation of hydrogen. Possible photochemical and thermal steps of that cycle are discussed.     

Photochemische Untersuchungen von Vitamin B12-Modellverbindungen mit Azid bzw. Thiolat als Axialliganden – Precursorverbindungen für koordinativ ungesättigte Cobalt(II)-Komplexe

Photochemistry of Azido and Thiolato Vitamin-B₁₂ Model Complexes as Precursor Compounds for Coordinatively Unsaturated Cobalt(II) Complexes The photolysis of [LCo(chelat)B] complexes ( 1–3 ) (L = azide, N; thiolate, RS⁻; chelat = dimethylglyoxime, dmg; N,N′-o-phenylene-bis(salicylidenimine), salphen; N,N′-ethylene-bis(salicylidenimine), salen; B = pyridine, imidazole, triphenylphosphine) leads upon the homolytic cleavage of the Co L bond to both coordinatively unsaturated reactive cobalt(II) chelates [Co(chelat)B] and ligand radicals L^•. The efficiency of these photochemical redox reactions is described in relation to the structure of the cobalt(III) chelates, the wavelength of irradiation, the light-intensity as well as the solvents and substrates used during the photochemical experiments. Further, sensitization experiments using [Ru(bipy)₃]Cl₂ as sensitizer are described and the redox potentials of the investigated complexes are discussed.     

Fast multilevel implementation of recursive spectral bisection for partitioning unstructured problems

If problems involving unstructured meshes are to be solved efficiently on distributed-memory parallel computers, the meshes must be partitioned and distributed across processors in a way that balances the computational load and minimizes communication. The recursive spectral bisection method (RSB) has been shown to be very effective for such partitioning problems compared to alternative methods, but RSB in its simplest form is expensive. Here a multilevel version of RSB is introduced that attains about an order-of-magnitude improvement in run time on typical examples.     

Acylierung von Heterocyclen mit Kohlensäurederivaten. I. Kinetik und Mechanismus der Reaktion von 2-Aminobenzimidazolen mit Arylcyanaten

Acylation of Heterocycles with Carbonic Acid Derivatives. I. Kinetics and Mechanism of the Reaction of 2-Aminobenzimidazoles with Aryl Cyanates The second order rate constants for the reaction of 2-amino-benzimidazoles (2-ABI) with aryl cyanates forming 2-amino-benzimidazole aryl ester imide 3 have been determined in dependence on substituent effects by u. v. measurements. The results are interpreted by a six-membered cyclic transition state in which the electrophilic attack of the cyanate on the endocyclic N atom is catalyzed by an H bridge interaction of the exocyclic amino group of 2-ABI with the OCN group.