Emanuel Vogel

Perspektiven der Cycloheptatrien-Norcaradien-Valenztautomerie

EINLEITUNG Es ist mir eine besondere Freude, auf diesem Symposium das Wort ergreifen zu dürfen, haben doch die Untersuchungen, über die ich hier berichten möchte, vor nunmehr genau zehn Jahren im Karlsruher Organisch-Chemischen Institut ihren Ausgang genommen. Unser Interesse an Valenzisomerisierungen konzentrierte sich damals auf die Cyclohepta· trien-Norcaradien-Valenztautomerie, ein Problem, dem sich schon um die Jahrhundertwende R. Willstätterl gegenübergestellt sah, und das trotz intensiver neuerer Bearbeitung2 noch immer einer befriedigenden Lösung harrt.

Figure Eight Cyclooctapyrroles: Enantiomeric Separation and Determination of the Absolute Configuration of a Binuclear Metal Complex.

The hypothesized racemate separation of cyclooctapyrroles with chiral figure eight conformations-and of metal complexes derived from these ligands-has been realized. The cyclooctapyrrole 1 (as hexadecaethyl derivative), which according to NMR analysis exhibits restricted mobility, and its binuclear palladium and copper complexes could be separated into stable enantiomers by preparative chromatography on a chiral phase. In the case of the palladium complex and the free ligand 1 it was also possible to determine the absolute configuration of the enantiomers.

Electrochemical and Spectral Characterization of Iron Corroles in High and Low Oxidation States: First Structural Characterization of an Iron(IV) Tetrapyrrole π Cation Radical

The electrochemistry and spectroscopic properties of three iron corroles were examined in benzonitrile, dichloromethane, and pyridine containing 0.1 M tetra-n-butylammonium perchlorate or tetra-n-ethylammonium hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)FeIV(C6H5), (OEC)FeIVCl, and (OEC)FeIII(py), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. Each iron(IV) corrole undergoes two one-electron reductions and two or three one-electron oxidations depending upon the solvent. Under the same solution conditions, the iron(III) corrole undergoes a single one-electron reduction and one or two one-electron oxidations. Each singly oxidized and singly reduced product was characterized by UV-vis and/or EPR spectroscopy. The data indicate a conversion of (OEC)FeIV(C6H5) and (OEC)FeIVCl to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole π cation radicals upon a one-electron oxidation. The metal center in [(OEC)FeIII(C6H5)]- is low spin (S = 1/2) as compared to electrogenerated [(OEC)FeIIICl]-, which contains an intermediate-spin (S = 3/2) iron(III). (OEC)FeIII(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)FeIII(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) π cation radical. Singly oxidized [(OEC)FeIV(C6H5)]•+ is the first iron(IV) tetrapyrrole π cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P1̄ with a = 10.783(3) Å, b = 13.826(3) Å, c = 14.151(3) Å, α = 78.95(2)°, β = 89.59(2)°, and γ = 72.98(2)° at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F o2 yields R1 = 0.0864 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) Å. The formulation of the electron distribution in this compound was confirmed by Mössbauer, X-ray crystallographic, and magnetic susceptibility data as well as by EPR spectroscopy, which gives evidence for strong antiferromagnetic coupling between the iron(IV) center and the singly oxidized corrole macrocycle.

Proton tunnelling in porphycene seeded in a supersonic jet

Abstract Porphycene, a constitutional isomer of porphyrin, was investigated in a supersonic jet expansion using laser-induced fluorescence and hole burning techniques. The lines recorded in the fluorescence excitation spectrum were split into doublets. No splitting was detected if one or both inner hydrogen atoms were replaced by deuterium. The doublet structure also disappeared upon formation of a complex between porphycene and water or alcohol. These findings indicate that the origin of the doublet structure in bare porphycene is the tunneling splitting due to the exchange of two inner protons between the nitrogen atoms. From the spectral response to deuteration, and to a change in cooling conditions, it is deduced that the tunneling splitting is higher in the ground state than in the first excited singlet state.

15N NMR study of proton localization and proton transfer thermodynamics and kinetics in polycrystalline porphycene

Using high-resolution solid-state 15N cross-polarization magic angle spinning NMR techniques, the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline 15N-labeled porphycene were studied. Whereas at room temperature only a single 15N resonance is observed, indicating an equivalence of all nitrogen atoms arising from a quasi-degenerate fast proton transfer, four signals are observed at low temperatures, exhibiting temperature-dependent line positions. Their analysis is consistent with the presence of either (i) two different molecules A and B in the asymmetric unit, each of which is subject to a quasi-degenerate correlated double proton transfer, or (ii) a single molecule exhibiting all four possible near-degenerate tautomeric states, two trans- and two cis-tautomers, interconverting by fast single proton transfers. The average rate constants of the proton transfer processes are found to be in the nanosecond time-scale. These constants were obtained between 228 and 355K by analysis of the longitudinal 9.12 MHz 15N T1 relaxation times, which exhibit a minimum around 280 K. The relaxation analysis was performed in terms of a quasi-degenerate two-state proton transfer process which modulates the heteronuclear 1H–15N dipole–dipole interaction. From the value of T1 in the minimum, the crystallographic NN distance of 2.63 A and the hydrogen bond correlation for N—H···N hydrogen bonded systems, the two N···H distances of 1.10 and 1.60 A were obtained, i.e. a hydrogen bond angle of 152°, which are significantly different from the corresponding values of 1.03 and 2.28 A and 116° found for porphyrin. The analysis of the temperature dependence of the rate constants indicates tunneling as a major reaction pathway, involving a barrier of about 32 kJ mol−1. The finding of a larger NH distance and a smaller barrier for proton transfer as compared with porphyrin is rationalized in terms of the stronger intramolecular hydrogen bonds in porphycene. A strong coupling between these bonds would indicate that the proton tautomerism in porphycene corresponds to a correlated double proton transfer, in contrast to the stepwise transfer in porphyrin. Finally, a relation between the intrinsic 15N chemical shifts of porphyrinoids and the N···H distance was found, which might be useful for estimating geometries of porphyrinoids. Copyright

Redox properties of porphycenes and metalloporphycenes as compared with porphyrins

An electrochemical analysis of a series of porphycenes and metalloporphycenes in tetrahydrofuran, N,N-dimethylformamide, and dichloromethane is reported. This analysis combined the use of polarographic, steady-state voltammetric, cyclic voltammetric, and spectroelectrochemical methods. The free-base porphycenes studied were porphycene (H 2 Pc), 2,3,6,7,12,13,16,17-octaethylporphycene (H 2 OEPc), 2,7,12,17-tetra-n-propylporphycene (H 2 TPrPc), and its tetrabromo derivative, respectively

Propellanes—XXXVI : Reactions of bridged [10]annulenes with 4-substituted-1,2,4-triazoline-3,5-diones

Abstract 1,6-Methano-, 1,6-oxa-, 1,6-imino- and 1,6-methylimino[10]annulene as well as several derivatives of the first-named compound react with 4-substituted-1,2,4-triazoline 3,5 diones to give mono - and/or bis -adducts. Attack apparently occurs from the side anti - to the bridging atom. Mass spectral results are reported for certain mixed di-adducts.

The photophysical properties of porphycene incorporated in small unilamellar lipid vesicles

Abstract Porphycene (a porphyrin isomer) has been incorporated into small unilamellar vesicles (diameter about 52 nm), and its first excited singlet and triplet states have been investigated under these conditions. Below a local concentration of about 4 mM in the vesicle, the photophysical properties of porphycene are as in organic solvents. Above this critical local concentration a second shorter-lived component appears in the fluorescence and triplet decays, and the fluorescence yield and lifetime and population of the longer-lived triplet are lower. Also, the rate constant of triplet quenching by oxygen is somewhat higher, and the yields of porphycene- and 2,7,12,17-tetra-n-propylporphycene-sensitized singlet molecular oxygen formation in the vesicles drop as well. Furthermore, the absorbance and fluorescence yield change with temperature when the porphycene concentration is greater than the critical limit: at just below the phase transition temperature of the lipid the two values rise steeply from low plateaus to plateaus about twice as high upon warming. The inherently less sensitive Beer—Lambert absorption plot curves downwards starting at a somewhat higher value than the critical local concentration (10 mM). The findings are discussed on the basis of (i) aggregation of porphycene at high local concentrations and (ii) localization at two distinct sites within the vesicle.

ISOPORPHYCENE : THE FOURTH CONSTITUTIONAL ISOMER OF PORPHYRIN WITH AN N4 CORE : OCCURRENCE OF E/Z ISOMERISM

Liberation of the ligand from the nickel complex 1 obtained by template synthesis yielded isoporphycene (as the octaethyl derivative 2), the first constitutional isomer of porphyrin with an N(4) core for which E/Z isomerism is involved: Compound 2 is present as the E isomer, which is in rapid, presumably acid-catalyzed equilibrium with a small amount (2 %) of the Z isomer. The remaining unknown constitutional isomers of porphyrin are considerably higher in energy than the already rather labile isoporphycene, so that the latter should mark the border of existence for this type of structural variant of porphyrin.

Isocyanogen as a pyrolysis product of N-cyano-2,3-diphenyl-cyclopropeneimine

Abstract Isocyanogen, CNCN, has been produced by pyrolysis of N-cyano-2,3-diphenyl-cyclopropeneimine. The molecule was identified by high-resolution infrared and millimeter wave spectroscopy.