Ernst-G. Jäger
University of Jena
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Featured researches published by Ernst-G. Jäger.
Inorganica Chimica Acta | 1997
Ernst-G. Jäger; Kerstin Schuhmann; Herlmar Görls
Abstract Chiral nickel(II) and copper(II) Schiff base complexes with [N2O2] coordination derived from (1R.2R)- and (1S.2S)-1,2-diamino- cyclohexane were prepared and characterized by 1H, 13C NMR, IR and mass spectroscopy and polarimetric measurement in the case of the ligands. Cyclovoltammetric and spectrophotometric measurements revealed that the redox behavior and the Lewis acidity are influenced strongly by the substituents of the ligand. Substituents with high electron-withdrawing effect stabilize the lower oxidation state and promote the addition of axial ligands. The new nickel complex NiIc fits in a diagram which presents the correlation between the reduction potentials and lg β2 values of a variety of known nickel(II) Schiff base complexes with different substituents Rt, R2 and bridges X. No significant differences 1g β2 with (R)-(+)-1-phenylethylamine were observed for RRNiIc and SSNiIc. Ligand SS2HIa (C20H30N2O6) crystallized in the orthorhombic system (P212121) with unit cell dimensions a = 11.035(2), b = 16.126(3), c = 24.676(5) A , and Z = 8; R = 0.045 and wR2 = 0.115. The nickel complex RRNiIb (C18H14O2N2Ni · 0.5CH2Cl2) cyrstallized in the orthorhombic system (P21212) with unit cell dimensions a = 18.404(1), b = 18.657(1), c = 5.621(1) A , and Z = 4; r = 0.044 and wR 2 = 0.0108 . The structures were solved by direct methods and were refined by full-matrix least-squares procedures.
Angewandte Chemie | 2000
Rainer Wegner; Michael Gottschaldt; Helmer Görls; Ernst-G. Jäger; Dieter Klemm
Remarkably high magnetic coupling and O(2)-activation ability analogous to that of catechol oxidase are characteristics of the first structurally defined, low-symmetry oligonuclear copper complexes of tridentate beta-oxoenamine ligands based on amino carbohydrates (the structure of a bis(acetylbutenonylaminoglucosidato)bis(µ-acetato)tricopper(II) complex is shown).
Inorganica Chimica Acta | 2002
Birgit Weber; Helmar Görls; Manfred Rudolph; Ernst-G. Jäger
Abstract Several new nitrosyliron complexes FeL NO (L 2− : macrocyclic [N 4 ] coordinated or open-chain [N 2 O 2 ] coordinated Schiff base ligand) were synthesised and compared with respect to their chemical behaviour (reactivity with dioxygen; redox potentials), their T -dependent magnetic susceptibility and their molecule structure. These properties reflect clearly the electronic influence of the equatorial chelate ligand (if present, also of an additional axial ligand) on the electron density at the central atom and, as a consequence, on the NO-binding mode. The macrocyclic complexes with the strongest equatorial donor ligands give pentacoordinated low spin ( S =1/2) derivatives, best to interpret as [ Fe III L + NO − ]—similar as the halides or pseudo-halides FeL X . These complexes show no tendency to add additional axial ligands and are insensitive to air in non-coordinating solvents. In presence of a donor ligand D (pyridine, MeOH) the reaction with dioxygen results in an octahedral nitro derivative FeL ( NO 2 )× D that can also be prepared by reaction of the iron(III) complex [ FeL × 2D ] + with sodium nitrite. This could be proved especially for the most electron-rich complex Fe1 NO which can also be oxidised electrochemically in MeCN to give [ Fe1 NO ] + and [ Fe1 NO ] 2+ in two reversible steps. The [N 2 O 2 ] coordinated complexes with weaker donor ligands in the equatorial plane show a more differentiated behaviour depending on electronic effects of the peripheral ligand substituents. Octahedral adducts FeL NO × D could be isolated from several of such complexes under anaerobic conditions. Their reaction with dioxygen gives the μ-oxo derivative ( FeL ) 2 O as the main product. Decreasing electron density at the central atom results in a trend towards decreasing transition temperature between a low-temperature S =1/2 and a high-temperature S =3/2 state. The atom distances within the first coordination sphere of the complex Fe5 NO × MeOH with the most electron withdrawing substitution of the equatorial ligand are significantly longer than those of the other complexes and show a surprising similarity to those of octahedral high-spin di-adducts FeL × 2D of the iron(II) complexes. This fact suggests an interpretation as [ Fe II L NO × MeOH ] with a coupling of the iron(II) S =4/2 state with the unpaired electron of the NO radical. The electronic influence of the equatorial ligands is also reflected in a significant correlation between the angle FeNO (varying from 140 to 167°) and the NO stretching frequencies (varying from 1629 to 1812 cm −1 ).
Chemical Physics Letters | 2000
Bernd Müller; Guido Leibeling; Ernst-G. Jäger
Abstract The magnetic characterization of two different magnetic chain compounds containing the same Schiff base ligand L but different axially coordinated ligands is reported: The spin crossover complex [Fe(L)(HIm)1.80] and the magnetic compound [Fe(L)(MeOH)1.83]. (L=diethyl (E,E)-2,2′-[1,2-phenylenebis(iminomethylidyne)]-bis[3-oxobutanoate], MeOH=methanol, HIm=imidazole.) The compound [Fe(L)(HIm)1.80] displays a cooperative spin crossover phenomenon (high-spin↔low-spin transition) centered at T1/2=330 K, and the high-spin compound [Fe(L)(MeOH)1.83] shows a weak spontaneous magnetization below Tc=9.5 K. Both complexes are chemically connected via strong hydrogen bonds which influences the magnetic properties of both compounds.
Journal of Molecular Catalysis A-chemical | 2003
Rainer Wegner; Michael Gottschaldt; Wolfgang Poppitz; Ernst-G. Jäger; Dieter Klemm
Abstract Recently, we reported the structure and properties of several copper(II) complexes with aminocarbohydrate-based ligands. Four of these complexes are capable of catalyzing the oxidation of 3,5-di- t butyl -catechol (dtbc) to the corresponding quinone. The present work contains new compounds of this ligand series of which most form different structure types than the previously described. Investigations on the influence of possible inhibitors like kojic and cinnamic acid, as well as simple ligands like chloride ions and of the pH-value on the catalytic reaction are investigated.
Inorganica Chimica Acta | 1999
Indira Käpplinger; Heike Keutel; Ernst-G. Jäger
Abstract The orientation of axial ligand planes in fifteen octahedral homo- and (in one case) hetero-ligand diadducts of the porphyrin-like macrocyclic iron(II)/(III) complexes Fe 1a , Fe 1b , Fe 2 and Fe 3 have been characterised by X-ray structure analysis. In contrast to iron porphyrins, the equatorial Fe–N distances are shorter than those of the axial Fe–N bonds. The bond lengths within the first coordination sphere are nearly independent of the oxidation step of the central atom. The relative orientation of the axial ligand planes is discussed with respect to the oxidation step of iron, the symmetry of the ring system and—especially for derivatives of the tetraaza[14]annulene Fe 3— to the saddle-shaped distortion of the macrocyclic plane. The reactivity of the axial coordination sites has been evaluated by equilibrium studies of the consecutive ligand substitution in the octahedral diadducts. The quotient K 1 / K 2 for the individual steps of ligand exchange, which represents a quantitative measure of ‘push–pull’ effects of different axial ligands in mixed adducts, is particularly high for the combination of P(OEt) 3 with pyridine bases as trans ligands.
Steroids | 2002
Rainer Wegner; Manuela Dubs; Helmar Görls; Christian Robl; Bruno Schönecker; Ernst-G. Jäger
Copper is next to iron the most important element in the biological transport, storage and in redox reactions of dioxygen. A bioanalogous activation of dioxygen with copper complexes is used for catalytical epoxidation, allylic hydroxylation and oxidative coupling of aromatic substrates, for example. With stereochemical information in form of chiral ligands, enantioselective reactions may be possible. Another aspect of interest on copper catalyzed reactions with dioxygen is that the exact mechanism and biological function of some enzymes (especially catechol oxidase) is yet not fully clear. For studies mimicking the copper-containing catechol oxidase appropriate chiral steroid ligands with defined stereochemistry and conformation have been synthesized. The four diastereomeric 16,17-aminoalcohols of the 3-methoxy-estra-1,3,5(10)-triene series have been condensed with salicylic aldehyde and different beta-ketoenols to the chiral ligand types 1-5. These compounds with different steric and electronic properties and different arrangements of the neighboring hydroxy and nitrogen functions were reacted with copper(II) acetate to copper complexes. The structure of these complexes will be discussed. The bioanalogous oxidation of 3,5-di-tbutyl-catechol (dtbc) to the corresponding quinone was catalyzed by most of the complexes, indicating their ability to activate dioxygen. The trans configurations c and d showed an activity one magnitude higher than the cis configurations a and b. Comparing compounds with the same diastereomeric configuration, the main influence was that of the peripheral R(1-3) substituents at the beta-ketoenaminic group which are useful for the fine-tuning of the properties of the copper atoms like redox potential and Lewis acidity.
ChemInform | 2000
Ernst-G. Jäger
Transition metals play an essential role as active sites of many enzymes. The complex catalytic performance of these biocatalysts should present a continuing challenge to chemists far into the third Millennium. Many attempts have been made in the past decades of the last century to develop new catalysts based on coordination compounds which mimic natural models. Initial success and the first technical applications involved particularly porphyrins and complexes of salicylaldimines. In this paper, we give an overview of chelate complexes of tetradentate Schiff base ligands with either a macrocyclic [N4]2- or an open chain [N2O2]2- donor set derived from aliphatic 3-oxoaldehydes and diamines. These complexes represent a link between those of porphyrin type and those of salicylaldimine type and prove to have many properties and reactions in common with them. The high variability of the complexes’ ligands with regard to the ring size, the extent of the π-electron system, and electronic as well as steric effects of peripheral substituents allow a broad variation of those properties decisive in catalytic performance; such as redox potentials, reactivity of axial coordination sites, and the spin state of the central atom. The redox couples NiII/I as well as the equilibrium constants for the addition of axial ligands to the planar nickel(II) or the penta-coordinated organo-cobalt(III) complexes reflect the high sensitivity of the central atom to electronic effects from equatorial ligands. Some of the macrocyclic nickel complexes are good electrocatalysts for the reduction of carbon dioxide. Most of the discussion focuses on iron complexes, especially their reactivity with different axial ligands, the “push-pull” effects in adducts with mixed axial ligands and some special structural features. First observations of the catalysis of hydroquinone oxidation by an oxidase-like four-electron reduction of dioxygen show that — besides redox potentials, axial reactivity and spin state — the formation of oligonuclear units, stabilized by H-bridges between peripheral oxo-groups and/or axial ligands, seem to play an essential role in catalytic performance. H-bridges are obviously also responsible for the formation of “molecule based magnets” with specific solid-state structures and cooperative magnetic properties.
Molecular Crystals and Liquid Crystals | 1999
Bernd Müller; Guido Leibeling; Ernst-G. Jäger
Abstract The high spin iron(II) chelate complex Fe1×2MeOH as well as its 1:1 derivative with TCNE have been investigated with respect to cooperative magnetic properties. Fe1×2MeOH represents a magnetic chain compound, wherein an intermolecular interaction via hydrogen bonds between the CH-groups of axially coordinated methanol and the ester groups located on the ligand periphery is observed. The weak spontaneous magnetization below approximately 10 K indicates cooperative magnetic behavior. The reaction of the iron(II) chelate complex with the electron acceptor TCNE leads to a 1:1 charge-transfer complex. The spontaneous magnetization occurs here below 10 K and is significantly higher compared to the magnetic iron(II) chelate chain compound.
European Journal of Inorganic Chemistry | 1998
Kerstin Schuhmann; Ernst-G. Jäger
The syntheses of new chiral 1,2-diphenylethylene-bridged nickel(II) and methylcobalt(III) Schiff base complexes are described. The equilibrium constants for the axial addition of the chiral bases (R)- and (S)-1-phenylethylamine to the enantiomerically pure metal complexes have been determined by means of spectrophotometric titrations. Comparison of the constants for the cyclohexanediyl-bridged complexes (type 1) with those for the more bulky 1,2-diphenylethylene-bridged complexes (type 2) reveals that chiral recognition is only observed in the case of type-2 complexes.