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Hochstapler: Die Sulfate diprotonierter substituierter Cyclo[8]pyrrole bilden zusammen mit elektronarmen Acceptorverbindungen diskotische Flüssigkristalle. Die kolumnaren Mesophasen (siehe Bild; rot: Trinitrobenzol, grün: Cyclo[8]pyrrol-Einheiten, gelb: Sulfationen, grau: von den Substituenten ausgefüllter Raum) werden durch Elektronendonor-Elektronenacceptor-Wechselwirkungen stabilisiert.

It stacks up nicely: Exposure of dihydrogen sulfate salts of appropriately substituted cyclo[8]pyrroles to electron-deficient acceptor molecules gives discotic liquid crystals. The columnar mesophase structures are stabilized by electron-donor/electron-acceptor interactions (see picture; red: trinitrobenzene, green: cyclo[8]pyrrole cores, yellow: sulfate ions, gray: volume occupied by the substituents).

A number of substituted zinc(II) porphycenes and porphyrins have been synthesized as potentially mesogenic materials. One of the resulting porphycenes, bearing eight decyloxy chains, exhibits two mesophases, a transient lamellar phase (Lam) and a highly ordered lamello-columnar phase (L_Col), with remarkably different structural characteristics. The same zinc(II) porphycene also forms an electron donor–acceptor (EDA) complex with tetracyanoquinodimethane (TCNQ), generating a hexagonal columnar mesophase (Col_h) that is thermally stable up to ca. 200 °C. The EDA interaction between porphycene and TCNQ has been probed using electronic and vibrational spectroscopy. A mixture of zinc(II) porphyrins, isomeric with the above porphycene complex, forms a rectangular columnar mesophase (Col_r).

New cationic anion receptors, based on the use of pyrrole-substituted bipyridine and coordinated to transition metals, are described. Specifically, polypyridine–ruthenium and –rhodium cores have been functionalized to generate an anion binding site. The design was chosen to probe the influence of the pyrrole-to-pyrrole separation on anion-binding affinities and selectivities; this distance is greater in the new systems of this report (receptors 1 and 2) relative to that present in related dipyrrolyl quinoxaline based receptors 3 and 4. Solution-phase anion-binding studies, carried out by means of ¹H NMR spectroscopic titrations in [D₆]DMSO and isothermal titration calorimetry (ITC) in DMSO, reveal that 1 and 2 bind most simple anions with substantially higher affinity than either 3 or 4. In the case of chloride anion, structural studies, carried out by means of single-crystal X-ray diffraction analyses, are consistent with the solution-phase results and reveal that receptors 1 and 2 are both able to stabilize complexes with this halide anion in the solid state.

Seeing is believing: Treatment of a tetrathiafulvalene-functionalized calix[4]pyrrole 1 with chloride anions in CH₂Cl₂ produces bowl-like receptor 1⋅Cl⁻, which is able to encapsulate C₆₀ in a 2:1 barrel-like manner. The color of C₆₀⊂(1⋅Cl⁻)₂ (green in CH₂Cl₂) differs dramatically from its constituents (yellow and magenta for 1 and C₆₀, respectively), thus allowing its formation to be monitored visually.

Sehen heißt wissen: Das Tetrathiafulvalen-funktionalisierte Calix[4]pyrrol 1 bildet mit Chloridionen in CH₂Cl₂ den schalenförmigen Rezeptor 1⋅Cl⁻, der C₆₀ in einem 2:1-Komplex einschließt. C₆₀⊂(1⋅Cl⁻)₂ unterscheidet sich durch seine Farbe (grün in CH₂Cl₂) von den Konstituenten 1 (gelb) und C₆₀ (rotviolett), sodass die Komplexierung visuell verfolgt werden kann.

Ditopic cation–anion binding is revealed by solid-state studies of the anion-recognition behavior of the calix[4]pyrrole unit. Anion binding causes an electron-rich conelike cavity to be formed that can then bind large polarizable cations. Furthermore, complexation of calix[4]pyrrole with cesium carbonate results in dimerization, in which a single anion bridges two calix[4]pyrroles (see the crystal structure of the Cs₂CO₃ complex).

Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained.

A series of novel calixpyrrole-like macrocycles, calix[n]bis(pyrrol-2-yl)benzene (calix[n]BPBs, n=2–4) 9 a–11 a, have been synthesized by means of the TFA-catalyzed condensation reaction of bis(pyrrol-2-yl)benzene 8 a with acetone. Calix[2]BPB 9 a represents an expanded version of calix[4]pyrrole in which two of the four meso bridges are replaced by benzene rings. By contrast, systems 10 a and 11 a, which bear great considerable to calixbipyrroles 2 and 3, represent higher homologues of the basic calix[n]BPB motif. Solution-phase anion binding studies, carried out by means of ¹H NMR spectroscopic titrations in [D₂]dichloromethane and isothermal titration calorimetry (ITC) in 1,2-dichloroethane, reveal that 9 a binds typical small anions with substantially higher affinities than 1, even though the same number of hydrogen bonding donor groups are found in both compounds. The basic building block for 9 a, benzene dipyrrole 8 a, also displays a higher affinity for anions than the building block for 1, dimethyldipyrromethane 16. Structural studies, carried out by single-crystal X-ray diffraction analyses, are consistent with the solution-phase results and reveal that 9 a is able to stabilize complexes with chloride and nitrate in the solid state. Structures of the PF₆⁻ and NO₃⁻ complexes of 10 a were also solved as were those of the acetone adduct of 9 a and the ethyl acetate adduct of 11 a.

Der Gast „macht“ den Wirt: Makrocyclische 2,6-Diamidopyridin-Bipyrrol-Rezeptoren entstehen kombinatorisch selektiert in Gegenwart eines anionischen Gastmoleküls. Diese stimulusgetriebene Reaktion ähnelt jüngsten Strategien zum Aufbau von Makrocyclen durch dynamische kombinatorische Prozesse. Das Bild zeigt als Beispiel die Wechselwirkung von Bipyrrol-Einheiten mit einem Sulfat-Ion.

Doppelt wirksam: Das Erkennen des Chloridions und der H⁺/Cl⁻-Ionentransport („symport“) durch die Membran könnten die biologische Wirkung von Prodigiosin gegen Tumoren erklären, wie aus Modellstudien unter Beteiligung pyrrolbasierter Anionenrezeptoren folgt, die als Mimetika der Schlüsselelemente dieses natürlichen Pigments entworfen wurden (siehe Bild).

Ditop gebundene Kationen und Anionen enthüllten Festkörperstudien zur Anionenerkennung durch die Calix[4]pyrrol-Einheit. Durch Bindung eines Anions bildet sich ein elektronenreicher kegelförmiger Hohlraum, der dann große polarisierbare Kationen aufnehmen kann. Die Komplexierung von Calix[4]pyrrol mit Caesiumcarbonat bewirkt eine Dimerisierung, wobei ein einzelnes Anion zwei Calix[4]pyrrole verbrückt (siehe Struktur des Cs₂CO₃-Komplexes).

Four up, four down: Reductive alkylation of cyclo[8]pyrrole and its derivatives gives rise to the corresponding reduced per-N-alkylated products, a reaction unknown in the case of porphyrins. These bowl-like molecules are arranged with four substituents up and four substituents down in a well-defined alternating fashion, as observed from their crystal structures (see picture; blue N).

Vier rauf, vier runter: Die reduktive Alkylierung von Cyclo[8]pyrrol und seinen Derivaten liefert die entsprechenden reduzierten per-N-alkylierten Produkte, eine Reaktion, die von Porphyrinen nicht bekannt ist. In diesen schalenförmigen Molekülen sind die acht Substituenten abwechselnd nach oben und nach unten gerichtet, wie die Kristallstrukturanalyse ergab (siehe Bild).

Größer ist besser: Untersuchungen zur Anionen-Bindung bescheinigen dem bipyrrolhaltigen makrocyclischen Calix[3]bipyrrol 2 eine deutlich bessere Fähigkeit zur Bindung großer Halogenidionen (z. B. Br⁻) als dem kleineren pyrrolhaltigen Calix[4]pyrrol 1. Die Komplexierung wurde mittels ¹H-NMR-Spektroskopie, isothermer Titrationskalorimetrie und Röntgenstrukturanalyse untersucht. Auch gelang die Synthese des noch größeren Calix[4]bipyrrols.

Bigger is better: Preliminary anion-binding studies reveal that bipyrrole-containing macrocycle calix[3]bipyrrole 2 binds large halide anions (for example, Br⁻) with affinities that are substantially enhanced relative to those of the smaller pyrrole-containing calix[4]pyrrole 1. The complexation has been studied by ¹H NMR spectroscopy, isothermal titration calorimetry, and X-ray crystallography. In addition, the synthesis of the even larger calix[4]bipyrrole was achieved.

Expanded porphyrins are synthetic analogues of the porphyrins, and differ from these and other naturally occurring tetrapyrrolic macrocycles by containing a larger central core with a minimum of 17 atoms, while retaining the extended conjugation features that are a hallmark of these quintessential biological pigments. The result of core expansion is to produce systems with novel spectral and electronic features, interesting and, often unprecedented, cation-coordination properties, and, in many cases, an ability to bind anions in certain protonation states. Also adding to the appeal of expanded porphyrins is their central role in addressing issues of aromaticity. In many cases, they also display structural features, such as decidedly nonplanar “figure-eight” motifs, that have no antecedents in the chemistry of porphyrins or related macrocyclic compounds. In this Review, the various synthetic approaches now being employed to produce expanded porphyrins as well as their various applications-related aspects are discussed.

Expandierte Porphyrine sind synthetische Analoga der Porphyrine und unterscheiden sich von diesen und anderen natürlichen makrocyclischen Tetrapyrrolen durch einen größeren zentralen Ring mit mindestens 17 Atomen, wobei die ausgedehnte Konjugation erhalten bleibt, die die biologischen Pigmente kennzeichnet. Durch die Erweiterung des zentralen Rings entstehen Verbindungen mit neuen spektralen und elektronischen Merkmalen, ungewöhnlichen Komplexbildungseigenschaften und in vielen Fällen der Eigenschaft, in bestimmten Protonierungszuständen Anionen zu binden. Zum Interesse an den expandierten Porphyrinen trägt auch ihre Schlüsselrolle bei der Klärung von Fragen zur Aromatizität bei. In vielen Fällen weisen sie in der Chemie der Porphyrine einmalige Strukturmerkmale auf, z. B. die nichtplanaren Figure-Eight-Konformationen. Dieser Aufsatz beschreibt schwerpunktmäßig die heute verwendeten Syntheseverfahren für expandierte Porphyrine und geht daneben auf spezielle Anwendungsaspekte ein.

The synthesis, spectroscopic properties, and computational analysis of an imidazole-based analogue of porphycene are described. The macrocycle, given the trivial name “imidacene”, was prepared by reductive coupling of a diformyl-substituted 2,2′-biimidazole using low-valent titanium, followed by treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Imidacene displays a porphyrin-like electronic structure, as judged by its ¹H NMR, ¹³C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 π-electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high-level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 π-electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 π-electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 π- and 22 π-electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 π-electron five-membered rings. Our computational analysis predicts that routine ¹H NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.

The cover picture shows the supramolecular tetramer that exists as a repeating motif in crystals of the crown ether quinoxaline derivative whose synthesis is shown on the left-hand side of the figure. As illustrated on the right-hand side of the graphic, a water molecule, held in place by hydrogen bonds, is bound between each of the two layers that make up the tetramer. These layers are, in turn, made up of two molecules of crown ether functionalized quinoxaline. For more on the self-assembly properties of this and related quinoxaline species, as well as a discussion of their anion and cation binding properties see the article by J. L. Sessler et al. on p. 3768 ff. We thank Mr. Gheorghe (“Dan”) Pantos for his help in generating this cover illustration.

The synthesis of four crown-substituted dipyrrolylquinoxalines 1−4 is reported. The key step in the synthesis is reaction of a 1,2-diaminobenzocrown with 1,2-bis(1H-pyrrol-2-yl)ethanedione (20). A single crystal X-ray diffraction analysis of the 18-crown-6-dipyrrolylquinoxaline 1 revealed that this molecule forms a tetramer centered around a single molecule of water, with no fewer than 10 hydrogen bonds holding the supramolecular structure together. In the case of the 15-crown-5-dipyrrolylquinoxaline 2, however, X-ray diffraction analysis revealed that this species exists as a dimeric pair in the solid state, with the NH protons of one pyrrole lying within hydrogen-bonding distance of two oxygen atoms on an adjacent crown ether. A second single crystal structure of 2 was solved; it demonstrated that this system is able to coordinate a potassium cation, thereby forming an intermolecular sandwich complex, at least in the solid state. In [D₆]acetone solution receptor 2 and congeners 1, 3 and 4 were also found to complex sodium and potassium cations within the crown diethyl ether binding sites, in a 1:1 manner as judged by ¹H NMR spectroscopic analyses. Although systems 1−4 appear to bind fluoride anion as well as cations, the inability to obtain quantitative binding affinities of 1−4 with fluoride anion rendered the evaluation of the systems for cooperative binding impossible. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Das Titelbild zeigt die einfache Einstufensynthese von Cyclo[8]pyrrolen, einer neuen Klasse aromatischer Heteroannulene. Ausgehend von konventionellen Bipyrrol-Vorstufen lassen sich diese porphyrinartigen Syteme in einer neu entwickelten zweiphasigen oxidativen Kupplung unter Verwendung von wässrigem FeCl₃ als Oxidans in Ausbeuten von über 70% erhalten. Formal leiten sich die Cyclo[8]pyrrole von Porphyrin durch Ersatz eines verbrückenden meso-Kohlenstoffatoms durch eine Pyrroleinheit ab. Sie unterscheiden sich spektroskopisch von den normalen Tetrapyrrol-Makrocyclen durch eine charakteristische rotverschobene Q-Bande im Absorptionsspektrum. Im protonierten Zustand können Cyclo[8]pyrrole leicht Anionen binden, wie die im Hintergrund gezeigte Struktur des Sulfatsalzes im Kristall unterstreicht. Mehr über diese neuartigen Moleküle erfahren Sie in der Zuschrift von J. L. Sessler et al. auf Seite 1480 ff.

In einem einzigen Schritt erhält man ausgehend von Bipyrrol-Vorstufen in über 70 % Ausbeute Cyclo[8]pyrrole. Diese verkörpern eine neue Klasse von Porphyrin-Analoga und weisen die charakteristische scheibenartige Struktur der Porphyrine auf. Aus der Struktur eines solchen Cyclo[8]pyrrols im Kristall (siehe Bild) geht hervor, dass der Makrocyclus sich wie ein zweifach protonierter Rezeptor verhält, der insgesamt acht Wasserstoffbrücken zu einem Sulfation im Zentrum ausrichtet.

Porphyrin (P), porphycene (Pc), corrphycene (Cn), and hemiporphycene (Hpc) represent a series of well defined “4-N in” constitutional porphyrin isomers. These isomers, in the form of their octaethyl derivatives, represent a congruent set of porphyrinoids whose properties can be compared. In this study we report how variations in electronic structure and nitrogen-core size in the free-base forms of these four systems are reflected in the properties of their corresponding metal complexes. Specifically, the effects that these differences have on the axial ligation properties of the Zn^II, Mg^II, Ni^II, and Co^II complexes of P, Pc, Cn, and Hpc in toluene using pyridine as the axial ligand are detailed. Also reported are the relative stabilities of these complexes under acidic conditions. It is shown that for the zinc, magnesium, and cobalt complexes, there are distinct differences in the ability to maintain four-, five-, or six-coordinate geometries in the presence of similar concentrations of pyridine. By contrast, no apparent differences in axial ligand binding affinity are seen for the four nickel complexes. Little difference in stability was likewise seen when these same complexes were subject to acid-mediated demetallation, with all four falling into stability class II, according to the accepted porphyrin stability ranking system. High stabilities were also seen in the case of the cobalt complexes, with the Pc and Cn complexes being of stability class III and the P and Hpc derivatives falling into stability class II. The Zn^II and Mg^II complexes were all far less stable than the corresponding Ni^II and Co^II complexes. In this case, semiquantitative analyses of the rate of acid-induced decomposition revealed the following stability sequence P>Cn>Hpc>Pc for both the Zn^II and Mg^II complexes. Single-crystal X-ray diffraction structures were solved for the Zn^II, Mg^II, and Ni^II complexes of the octaethyl derivatives of Hpc, Cn, and Pc as well as a Co^II octamethylcorrphycene and are reported as part of this study. These solid-state structures confirm four-coordinate species for the Ni^II complexes, four- and five-coordinate species for the Mg^II and Zn^II complexes, and a six-coordinate species for the lone Co^II complex.

The synthesis and structural characterization of hybrid heterocalix[4]arene analogues containing pyrrole, benzene, methoxy-substituted benzene, and pyridine subunits is described. Macrocycles 1 and 2, examples of calix[2]benzene[2]pyrrole and calix[1]benzene[3]pyrrole systems, respectively, are synthesized by the condensation of pyrrole and an appropriate phenylbis(carbinol). Macrocycles 3 and 7, examples of calix[2]benzene[1]pyridine[1]pyrrole and calix[1]pyridine[3]pyrrole, respectively, are synthesized by the use of a carbene-based pyrrole-to-pyridine ring-expansion procedure. Single-crystal X-ray analysis reveals that compounds 1 a, 1 b, and 2 b adopt 1,3-alternate conformations in the solid state, whereas compounds 3 and 7 display structures that are best described as “flattened partial cones” in terms of their conformation. (Series a refers to pure benzene-derived systems, whereas series b indicates macrocycles containing 5-methoxyphenyl subunits). In the solid state, the methoxy-functionalized macrocycles 1 b and 2 b, and the chloropyridine-containing macrocycle 7 exist as dimers. In the case of 1 a and 7, these compounds interact with neutral solvent in the solid state. The conformations of the macrocycles in solution were explored by temperature-dependent proton NMR and NOESY spectral analysis. At 188 K, macrocycles 1 a and 2 a adopt flattened 1,3-alternate conformations, whereas macrocycles 3 and 7 exist in the form of flattened partial-cone conformations. Standard proton NMR titration analyses were carried out in the case of macrocycles 1 a and 2 a, and reveal that at least the second of these systems is capable of binding fluoride and chloride anions in CD₂Cl₂ solution at room temperature (K_a=571 and 17 M⁻¹ in the case of 2 a and F⁻ and Cl⁻, respectively).

The cover picture shows the simple, one-step synthesis of cyclo[8]pyrroles, a new class of aromatic heteroannulene. Using a newly developed biphasic oxidative coupling procedure that employs aqueous FeCl₃ as the oxidant, these novel porphyrin-like systems may be obtained in yields exceeding 70% starting from well-known bipyrrolic precursors. While formally derived from porphyrin as the result of substituting a pyrrole for each of the original bridging meso-carbon atoms, the cyclo[8]pyrroles differ from these quintessential tetrapyrrolic macroycles by virtue of Q-like absorption bands that are red-shifted into the near IR. Cyclo[8]pyrroles also show a propensity for protonation and anion binding that is underscored by the X-ray structure of the sulfate salt that makes up the background for this illustration. More on these exciting new molecules can be found in the communication by J. L. Sessler et al. on pp. 1422–1425.

Easily obtained in a single step from bipyrrolic precursors in yields over 70 %, cyclo[8]pyrroles represent a new class of porphyrin analogues that display the classic disklike structure of porphyrins. The X-ray crystal structure of one of the cyclo[8]pyrroles obtained (see picture) shows that the macrocycle acts as a formal diprotonated receptor and forms eight hydrogen bonds to a sulfate ion bound within its core.

The cover picture shows a comparison of one full rotation for a biimidazole helix and B-form DNA. The back drop is inset with biimidazole molecules used for this investigation. The figures were generated by Vladimir Guelev and Chris Fowler at the University of Texas at Austin from the Povray and Molmol programs. The structures and synthesis of self-assembled 2,2′-biimidazole helices, which are bound together by pairs of N-H⋅⋅⋅N hydrogen bonds, are described in detail by J. L. Sessler et al. on p. 721ff.

2,2′-Biimidazoles were synthesized by palladium(0)-catalyzed coupling of 2-iodoimidazoles bearing an alkyl and an ester group at the 4- and 5-positions, respectively. The products were found to be fluorescent and moderately soluble in organic solvents. Three biimidazoles were subjected to single crystal X-ray diffraction analysis. In all three instances, adjacent molecules were found to be bound together in the solid state by pairs of N-H⋅⋅⋅N hydrogen bonds, forming twisted ribbon-like columns which resemble double helices. The amount of helical twist observed between neighboring biimidazole subunits in these helices varies with the identity of the alkyl and ester groups; in two cases it is approximately 60°, whereas in the third it is about 90°. Mass spectra of six different biimidazoles display ions with masses corresponding to dimers; this indicates that these compounds retain some affinity for each other in the gas phase. The three most soluble biimidazoles also show mass spectrometric peaks ascribable to trimers and tetramers. The solution-phase aggregation tendencies of these latter three compounds were studied by vapor pressure osmometry. In each case, the apparent molecular weight in 1,2-dichloroethane solution is higher than would be expected for free monomers.

The chemistry of porphyrin analogues, fully or partially conjugated macrocyclic systems that contain pyrroles but are not naturally occurring substances, is discussed from an historic perspective. Key developments that led to the emergence of this area as a separate discipline within the generalized porphyrin lexicon are highlighted with a view to allowing the reader to understand better the current state of the art in the field. Copyright © 2000 John Wiley & Sons, Ltd.

Nonplanarity and conformational flexibility of alkyl- and phenyl-substituted rosarin (see diagram) determine its photophysical properties. Both neutral and protonated forms are nonfluorescent. Upon excitation, rapid deactivation occurs by internal conversion to the ground state. Six molecular orbitals have to be taken into account to explain the absorption spectral pattern.

Stable even in dimethyl sulfoxide, dimer I does not dissociate to monomers 1 in organic media. The unique base-pairing interaction involving four hydrogen bonds in the readily available compound 1 was characterized by NMR spectroscopy.

Nicht einmal in Dimethylsulfoxid und schon gar nicht in anderen organischen Medien zerfällt das Dimer I in die Monomere 1. Das Wasserstoffbrückennetz der relativ leicht zu synthetisierenden Verbindung 1 wurde NMR-spektroskopisch charakterisiert.

Anew type of solid support for HPLC separation of nucleotides, oligonucleotides, N-protected amino acids, and perfluorinated biphenyls is revealed in the form of two distinct calix[4]pyrrole-modified silica gels (below).

An interesting example of a noncovalently linked organometallic superstructure (shown right) is obtained from the self-assembly of tetrapyrrole-substituted ferrocene. As a result of the self-associating properties of the pyrrole–ester subunits, this system forms ribbon-like infinite chains in the solid state and small but well-characterized oligomers in organic solvents. Selective oxidation of the ferrocene moiety provides a cationic ferrocenium species amenable to electrospray mass spectrometric analysis.

A novel combination of molecules, sapphyrin (an expanded porphyrin) and lasalocid (a natural polyether ionophore), are covalently linked to produce conjugates (such as 1) capable of acting as efficient and selective carriers for aromatic α-amino acids, as demonstrated by U-tube and W-tube model membrane transport studies.

The synthesis of amide-based symmetrical rotaxanes 2a, b bearing porphyrin blocking groups is firstly described. These compounds can be obtained by a one-step one-pot reaction only if the macromonocyclic host template complementary to the threading guest is found. The symmetry of the axle is removed by introducing an unsymmetrical central part, leading to the formation of rotaxane 2c. Different blocking groups were used to obtain the non-symmetrical rotaxane 3a. The bis-Zn²⁺ complex 10a of the rotaxane 2a was isolated. A nonionic supramolecular complex 7 was assumed as threaded intermediate, which was blocked by reaction with the stopper components. The amide-type rotaxane system thus proved to be variable with respect to all its parts including stopper components of different reactivity, bulkiness and supramolecular functionality.

The diprotonated, bishydroperchlorate forms of three isomeric β-octaalkyl-substituted tetrapyrrolic macro-cycles, namely, etioporphyrin II (1), etio porphycene (2), and etiocorrphycene (3), have been characterized both in chloroform solution, by UV/visible spectroscopy and ¹H and proton-correlated 2D ¹⁵N NMR methods, and in the solid state, by single-crystal X-ray diffraction analyses. In the solid state, in marked contradistinction to what is observed for the corresponding free-base forms, the macrocyclic portion of these salts were found to be distorted significantly from planarity with the two perchlorate counteranions being held above and below the average N₄ plane by N–H … O hydrogen bonds in all three cases. In solution, ¹H and proton-correlated 2D ¹⁵N NMR experiments reveal molecular ions of relatively high symmetry (D_2h, D_2h, and C_2v in the case of 1·(HClO₄)₂, 2·(HClO₄)₂, and 3·(HClO₄)₂, respectively) as would be anticipated on the basis of the solid-state results. These same NMR analyses, while revealing slight differences between the three salts in the NH and meso¹H NMR spectral regions, also serve to confirm the generalized congeneric nature of 1·(HClO₄)₂, 2·(HClO₄)₂, and 3·(HClO₄)₂ and support the assignment of the latter two species as being porphyrin-like salts. UV/vis analyses further support this conclusion; in all three instances, strong Soret- and Q-like transitions are observed in dichloromethane that are both distinct from each other (λ_max=404, 549, 570, 593; 388, 409, 599, 666; and 419, 559, 604 for 1·(HClO₄)₂, 2·(HClO₄)₂, and 3·(HClO₄)₂, respectively) and from those of the corresponding free-base forms (λ_max=396, 496, 530, 565, 619; 382, 570, 617, 657; and 410, 509, 539, 574, 628 for 1, 2, and 3 respectively). Protonation experiments were carried out by exposing dichloromethane solutions of the isomers to aqueous perchlorate/perchloric acid solutions of differing pH. These studies reveal that while porphycene 2 adds two protons readily and concurrently, becoming 50% diprotonated when exposed to perchlorate/perchloric solutions with a pH of around 3.6, porphyrin 1 and corrphycene 3 are protonated in a stepwise manner; they become 50% monoprotonated when exposed to perchlorate/perchloric solutions of pH≈3.7 and 3.9, respectively, and diprotonated at pH≤0.8 and 1.3, respectively.