The central role of the metal ion for photoactivity: Zn– vs. Ni–Mabiq

Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and efficient earth-abundant photoredox catalysts remains a challenge. We here present the photochemical and photophysical properties of a Ni–Mabiq catalyst ([Ni^II(Mabiq)]OTf (1); Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N₆)—and of a Zn-containing analogue ([Zn^II(Mabiq)OTf] (2))—using steady state and time resolved optical spectroscopy, time-dependent density functional theory (TDDFT) calculations, and reactivity studies. The Ni and Zn complexes exhibit similar absorption spectra, but markedly different photochemical properties. These differences arise because the excited states of 2 are ligand-localized, whereas metal-centered states account for the photoactivity of 1. The distinct properties of the Ni and Zn complexes are manifest in their behavior in the photo-driven aza-Henry reaction and oxidative coupling of methoxybenzylamine.

The development of earth-abundant photoredox catalysts remains a challenge. Studies of Ni- and Zn-Mabiq complexes demonstrate how the coordinating metal ion influences the photochemistry, photodynamics and reactivity of photocatalysts.     

Differential determination of chromium(VI) and total chromium in natural waters using flow injection on-line separation and preconcentration electrothermal atomic absorption spectrometry.

A rapid, sensitive and selective method for the differential determination of CrIII and CrVI in natural waters is described. Chromium(vi) can be determined directly by flow injection on-line sorbent extraction preconcentration coupled with electrothermal atomic absorption spectrometry using sodium diethyldithiocarbamate as the complexing agent and C18 bonded silica reversed-phase sorbent as the column material. Total Cr can be determined after oxidation of CrIII to CrVI by potassium peroxydisulfate. Chromium(III) can be calculated by difference. The optimum conditions for sorbent extraction of CrVI and oxidation of CrIII to CrVI are evaluated. A 12-fold enhancement in sensitivity compared with direct introduction of 40 microliters samples was achieved after preconcentration for 60 s, giving detection limits of 16 ng l-1 for CrVI and 18 ng l-1 for total Cr (based on 3 sigma). Results obtained for sea-water and river water reference materials were all within the certified range for total Cr with a precision of better than 10% relative standard deviation in the range 100-200 ng l-1. The selectivity of the determination of CrVI was evaluated by analysing spiked reference materials in the presence of CrIII, resulting in quantitative recovery of CrVI.     

Correlations of substituent parameters with the carbonyl stretching force constant in arenetricarbonylchromium complexes

Correlations are established between selected substituent parameters (σ_I,σ_P, σ_R^P,σ_P^O,σ_R^O) and the carbonyl stretching force constant, k(CO), for 28 mono and poly-substituted tricarbonylchromium-complexed arene compounds. On the basis of the statistical results it is concluded that the overall electronic substituent effect transmitted to the carbonyl groups involves both mesomeric and inductive mechanisms. Within the restricted domain, including substituent group and benzene ring, transmission proceeds largely by resonance, with a minor inductive (through-bond and field) effect operative in the same domain. Further transmission from the substituted arene ring to the chromium atom predominantly involves an inductive mechanism. This result, in support of existing literature data, suggests appreciable participation of the ring carbon σ framework in the metal—ring bond formation.     

A Quick Method for the Microanalytical Evaluation of the Dissolving Power of Supercritical Gases

A new method of microextraction with supercritical gases involving direct coupling with thin-layer chromatography enables the solubility behavior of a large number of natural products to be investigated in pressure gradients of up to 400 bar. The results are summarized in general rules that enable the extractability to be predicted on the basis of the chemical structure. The substances that can be satisfactorily extracted in supercritical carbon dioxide and nitrous oxide are preferably lipophilic. The extracting capacity of the two gases was compared using alkaloids. In the case of polar compounds such as sugars and amino acids the solubility is so low even at pressures of 2500 bar (less than 10 μg/Nl) that application is not practicable. Quantitative tests of the solubility behavior of model substances in supercritical gases provide a basis for the technical application of the method.     

10 years after rio-concepts on the contribution of chemistry to a sustainable development

The principles of the United Nations Conference on Environment and Development (UNCED), held in June 1992 in Rio de Janeiro, and Agenda 21, the comprehensive plan of action for the 21st century, adopted 10 years ago by more than 170 governments, address the pressing problems of today and also aim at preparing the world for the challenges of this century. The conservation and management of resources for development are the main focus of interest, to which the sciences will have to make a considerable contribution. Natural, economic, and social sciences will have to be integrated in order to achieve this aim. In their future programs, the associations of the chemical industries in Europe, Japan, and the USA have explicitly accepted their obligation to foster a sustainable development.In this review we investigate innovations in chemistry exemplarily for such a development with regard to their ecological, economical, and social dimensions from an integrated and interdisciplinary perspective. Since base chemicals are produced in large quantities and important product lines are synthesized from them, their resource-saving production is especially important for a sustainable development. This concept has been shown, amongst others, by the example of the syntheses of propylene oxide and adipic acid. In the long run, renewable resources that are catalytically processed could replace fossil raw materials. Separation methods existing today must be improved considerably to lower material and energy consumption. Chemistry might become the pioneer of an innovative energy technique.The design of chemical products should make possible a sustainable processing and recycling and should prevent their bio-accumulation. Methods and criteria to assess their contribution to a sustainable development are necessary. The time taken to introduce the new more sustainable processes and products has to be diminished by linking their development with operational innovation management and with efficient environmental-political control procedures.     

Morphologie de Réseaux Polymères Interpénétrés préparés en deux étapes

Résumé Un nouveau modèle thermodynamique prédisant la taille des domaines des réseaux polymères interpénétrés préparés en deux étapes est décrite. Les paramètres influents sont la tension interfaciale et la densité de réticulation des deux polymères, ainsi que la composition. A l'aide de la microscopie électronique à balayage, la théorie a été vérifiée pour le système poly(acrylate den-butyle)/polystyrène.

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A series of sequential interpenetrating polymer networks, IPN's, of poly(n-butyl acrylate) and polystyrene were prepared. A new equation predicting the domain size in IPN's is summarized and shown to yield agreement with the morphologies observed via scanning electron microscopy. The experimental variables required to determine the domain size include the volume fraction and crosslink level of each polymer, the interfacial tension and the temperature.

     

Schwingungsspektren und Kraftkonstanten von W(OCH3)6, Mo(OCH3)6 und [Sb(CH3)4][Sb(OCH3)6]

Vibrational Spectra and Force Constants of W(OCH₃)₆, Mo(OCH₃)₆, and [Sb(CH₃)₄][Sb(OCH₃)₆] The infrared and Raman spectra of the monomeric hexamethoxides of Tungsten and Molybdenum and of the ionic compound [Me₄Sb]⁺[Sb(OMe)₆]^? (prepared from [Sb(OMe)₅]₂ and Me₄SbOMe; Me = CH₃) are recorded and interpreted on the basis of C_3i symmetry. The force fields of W(OMe)₆ and [Sb(OMe)₆]^? are calculated using the same basis set of force constants. Both W? O- and Sb? O- stretching force constants are identical (2.56 N/cm), however the other parts of the valence force field are markedly different.     

Metallorganische diazoalkane : XVI. Synthese von silyldiazoalkanen Me3Si(LnM)CN2

Silyldiazoalkanes Me₃Si(L_nM)CN₂ (L_nM = Me₃Si, Me₃Ge, Me₃Sn, Me₃Pb; Me₃As, Me₃Sb, Me₃Bi) have been synthesized by three different routes: (a) reactions of the Me₃SiCHN₂ with metal amides L_nMNR¹R² of Group IVB and VB elements, using Me₃SnCl as catalyst; (b) reactions of the in situ prepared organolithium compound Me₃SiC(Li)N₂ with organometallic chlorides Me₃MCl (M = Si, Ge); (c) tincarbon bond cleavage reaction of (Me₃Sn)₂CN₂ with Me₃SiN₃, affording Me₃SnN₃, traces of bis(trimethylsilyl)diazomethane (Me₃Si)CN₂, trimethylsilyl(trimethylstannyl)diazomethane Me₃Si(Me₃Sn)CN₂ and bis(trimethylsilyl)aminoisocyanide (Me₃Si)₂NNC as the major reaction products. IR and NMR data (¹H, ¹³C, ²⁹Si, ¹¹⁹Sn, ²⁰⁷Pb) of the new heterometal-diazoalkanes are reported and discussed in comparison to relevant compounds of the organometallic diazoalkane series.     

Heterocyclen aus α-nitroolefinen. XII . 3-Acetyl-4,5-dihydro-5-(methylenamino)furane und 3-acylpyrrole aus α-nitroolefinen und β-diketonen

From α-nitroolefines 1 and β-diketones 2 3-acetyl4,5-dihydro-5-(methy1eneamino)furans 5 are prepared. Furans 5 react in acid medium to yield 1-unsubstituted 3-acylpyrroles 6 , catalytic hydrogenation yields 1-substituted 3-acylpyrroles 9 . The ¹H-nmr and ¹³C-nmr investigations prove the constitutions.     

Insights into the Heck reaction with PCP pincer palladium(II) complexes

[reaction: see text] The Heck reaction of phenyl halides with styrene using a series of related PCP pincer palladium(II) complexes was studied in order to evaluate the effect of ligand structure and electronics on the catalytic activity and to investigate the nature of the catalyst species. We suggest these pincer complexes are precatalysts for highly active forms of metallic palladium. This conclusion is based on kinetic studies (induction periods, sigmoidal kinetics), Hg drop tests, quantitative poisoning experiments, and NMR studies.