Metal complexes of corrole continue to attract the interest of different research groups, since the coordination chemistry of such a macrocycle presents peculiar and intriguing characteristics, such as for example its non-innocent character as a ligand. The Periodic Table of Metallocorrolates has been widely expanded in recent years, including also main group elements. Among them, phosphorus complexes of corrole have been of particular interest, since these compounds can be easily prepared in good yields, are stable, and show unusual optical properties. For these reasons, phosphorus corroles represent a class of interesting compounds to be exploited in the field of material science or for biomedical investigations and the definition of synthetic pathways for their functionalization is an important step to optimize their properties for various applications. We have recently developed different pathway for the selective introduction of different substituents in the peripheral positions of phosphorus triarylcorrole complexes. The characterization of such a complexes showed some peculiar properties, such as for example an unusual ruffled geometry of the corrole core, a conformation that has not been considered possible for such a macrocycle.

Two symmetric ditopic supramolecular templates (1 and 2) each presenting two hydrogen bonding recognition subunits were synthesized. Each such subunit comprises the same donor and acceptor pattern, capable of binding a substrate molecule with complementary hydrogen bonding groups to form a supramolecular complex. Substrate molecules, such as thymine or uracil derivatives, yield 2 : 1 complexes with the acceptors involving two hydrogen bonds to each subunit with ideal orientation for subsequent [2 + 2] dimerization upon photoirradiation. Selective syn photoproduct formation and concomitant suppression of the trans isomer are favored by orientation of the two guest nucleobases within the template cleft. Complementary donor and acceptor hydrogen bonding induced positioning of the two substrates and steric hindrance within the template clefts are responsible for the selective product formation.

As examples of supramolecular devices performing chemical (ionic, molecular) control of binding events and models of related natural systems, two molecular conformational switches are described, which display cation-controlled nanomechanical motion coupled to substrate binding and release. The substrate binding relies on donor/acceptor interactions, provided by intercalation between planar sites located at the extremities of the switching units, whereas cation complexation is responsible for conformational regulation. The terpyridine py-py-py-based receptor is activated toward substrate binding upon complexation of a zinc(II) cation and operates in a two-state process. The replacement of the central pyridine by a 4,6-disubstituted pyridimine as in py-pym-py induces a state reversal and yields a new receptor which binds a substrate in the absence of cation, and releases it when copper(I) is introduced, following a three-step process. These systems represent effector-triggered supramolecular switching devices leading toward multistate nanomechanical chemical systems. These two systems illustrate the use of simple conformational switches in the binding site and allosteric regulation of substrate affinity.

The novel ligand L-1 containing two bispyridyl-naphthyridine type subunits forms a tetranuclear complex, displaying two dimetallic dirhodium sites bridged by a pyrimidine group, and presenting specific electrochemical and spectral properties. (C) 2002 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

Supramolecular polymers are described that are derived from the association of two homoditopic heterocomplementary monomers through sextuple hydrogen-bonding at-rays. They form fibers and a variety of different materials depending on the conditions. The strong affinity of the DAD-DAD (D donor, A = acceptor) hydrogen-bonding sites for double-faced cyanuric acid type wedges drives the supramolecular polymeric assembly in apolar and chlorinated organic solvents. The marked influence of stoichiometry, as well as end-capping and cross-linking agents upon fiber fort-nation is revealed in solution and by electron microscopy (EM). The results further contribute to the development of a supramolecular polymer chemistry that comprizes reversible polymers formed through recognition-controlled noncovalent connections between the molecular components. Such materials are, by nature, dynamic and present adaptive character in view of their ability to respond to external stimuli.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Helically preorganized oligopyridine-dicarboxamide strands are found to undergo dimerization into double helical supramolecular architectures. Dimerization of single helical strands with five or seven pyridine rings has been characterized by NMR and mass spectrometry in various solvent/ temperature conditions. Solution studies and stochastic dynamic simulations consistently show an increasing duplex stability with increasing strand length. The double helical structures of three different dimers was characterized in the solid phase by X-ray diffraction analysis. Both aromatic stacking and hydrogen bonding contribute the double helical arrangement of the oligopyridinedicarboxamide strand. Inter-strand interactions involve extensive face-to-face overlap between aromatic rings, which is not possible in the single helical monomers. Most hydrogen bonds occur within each strand of the duplex and stabilize its helical shape. Some inter-strand hydrogen bonds are found in the crystal structures. Dynamic studies by NMR as well as by molecular modeling computations yield structural and kinetic information on the double helices and on monomer-dimer interconversion. In addition, they reveal the presence of a spring-like extension/compression as well as rotational displacement motions.

Molecular strands composed of alternating 2,6-diaminopyridine and 2,6-pyridinedicarbonyl units have been designed to self-organize into single stranded helical structures upon forming intramolecular hydrogen bonds. Pentameric strands 11, 12, and 14, heptameric strands 1 and 20, and undecameric strand 15 have been synthesized using stepwise convergent strategies. Single helical conformations have been characterized in the solid state by single crystal X-ray diffraction analysis for four of these compounds. Helices from pentameric strands 12 and 14 extend over one turn, and helices from heptameric 20 and undecameric 15 species extend to one and a half and two and a half turns, respectively. Intramolecular hydrogen bonds are responsible for the strong bending of the strands. 1H NMR shifts both in polar and nonpolar organic solvents indicate intramolecular overlap between the peripheral aromatic groups. Thus, helical conformations also predominate in solution. Molecular stochastic dynamic simulations of strand folding starting from a high energy extended linear conformer show a rapid (600 ps at 300 K) conversion into a stable helical conformation.

Synthetic single-helical conformations are quite common, but the formation of double helices based on recognition between the two constituent strands is relatively rare. Known examples include duplex formation through base-pair-specific hydrogen bonding and stacking, as found in nucleic acids and their analogues, and polypeptides composed of amino acids with alternating L and D configurations 1 , 2 . Some synthetic polymers 3 and self-assembled fibres 4 have double-helical winding induced by van der Waals interactions. A third mode of non-covalent interaction, coordination of organic ligands to metal ions 5 , 6 , 7 , can give rise to double, triple and quadruple helices, although in this case the assembly is driven by the coordination geometry of the metal and the structure of the ligands, rather than by direct inter-strand complementarity. Here we describe a family of oligomeric molecules with bent conformations, which exhibit dynamic exchange between single and double molecular helices in solution, through spiral sliding of the synthetic oligomer strands. The bent conformations leading to the helical shape of the molecules result from intramolecular hydrogen bonding within 2′-pyridyl-2-pyridinecarboxamide units 8 , 9 , 10 , 11 , 12 , with extensive intermolecular aromatic stacking stabilizing the double-stranded helices that form through dimerization.

A reduced pyrrole subunit directs electrophilic functionalizations of dihydroporphyrins to the antipodal pyrrole ring by confining the chromophore 18-π-electron delocalization pathway to its N(22)H-N(24)H tautomer. The 2,3-disubstituents inhibit oxidation, this being exemplified by the synthesis of perbrominated dodecasubstituted metallochlorins. Regiospecific nitration (using N2O4) of metal-free chlorins provides access to Michael acceptors such as 12-nitro-2,3-disubstituted chlorins which are used in the preparation of highly functionalized tetraaryl-bacteriochlorins by conjugate addition of carbon-centered nucleophiles.

The title dibromoporphyrins 45 and hexabromoporphyrins 1112 are prepared from H2(2-NO2TPP) 1 and Cu(2-NO2TPP) 8, respectively. The β-nitro group confines the 18-π-annulene system of a tetraphenylporphyrin to its N22H-N24H aromatic delocalization pathway which induces the localization of an antipodal double bond on the porphyrin periphery and enhances its susceptibility to electrophilic attack. Dibromination of H2-NO2TPP) 1 occurs regioselectively affording the 12,13-dibromo-2- nitroporphyrin 2 which, upon Michael addition of NaBH4 and re-aromatization of the resulting nitrochlorin 3, provides an entry to 2,3-dibromoTPP 45 as well as an improved route to 2,3- dicyanoporphyrins 67. Perbromination of Cu(2-NO2TPP) 8 and denitration of 9 gave, after demetalation, 2,3,7,8,12,13-hexabromoTPP 12. Both 4 and 12 are structurally characterized by X-ray crystallography.

The condensation of the two oligoheterocyclic aldehydes 8 and 16 with the bis-hydrazine 17 gives the bis-hydrazones 1 and 2. These molecular strands are shown to adopt helical conformations of 1.5 and 2.5 turns, respectively. The helical shape of 1 has been confirmed and structurally characterized by X-ray crystallography. The results indicate that the pyrimidine-hydrazone unit is a satisfatory helicity codon, so that the facile hydrazone formation provides an efficient procedure for generating helical structures. This greatly widens the scope of the methodology based on designed heterocyclic sequences for enforcing helicity in molecular strands, and opens interesting routes towards a variety of derived structures.

The Rothemund condensation reaction of pyrrole and aldehydes is an extensively used route to meso-tetraarylporphyrins, but simple modifications of the reaction conditions allow the formation of different macrocycles other than the expected porphyrin. In the presence of an excess of pyrrole, this modified Rothemund approach leads to the synthesis of meso-triaryl-substituted corroles. This methodology allows the preparation of a wide range of substituted corroles starting from commercially available products. Higher yields have been obtained in the case of benzaldehydes bearing electron-withdrawing substituents, while the reaction fails in the presence of 2,6-disubstituted benzaldehydes. Although if not isolated, some experimental evidences indicate that the linear 5,10,15-triphenylbilane 4 is the precursor of the final corrole ring. Reaction of 5,10,15-triphenylcorrole 2 with an excess of NBS leads to the complete bromination of the macrocycle. Spectroscopic characterization seems to indicate the formation of the porphodimethene-like structure 5, where the macrocyclic aromatic conjugation is interrupted at the 10 position. Metalation of this compound with cobalt acetate and PPhB affords the corresponding complex. The X-ray crystal structure of triphenylphosphine [2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(4-nitrophenyl)corrolato]cobalt(III) 8 confirms the ability of corrole ring to retain an almost planar conformation when fully substituted at the peripheral positions.

Ligand 3 has been shown to self-assemble under coordination of copper(II) cations in a 1:1 ratio in acetonitrile to give equilibrating mixtures of a [2 x 2] grid-type tetranuclear structure 1 and a hexanuclear achitecture of hexagonal shape 2. The latter was confirmed by determination of the crystal structure which further indicated that 2 contained acetonitrile molecules and hydroxo groups bound to the copper(II) centers, which are therefore five-coordinate. The structures assigned to 1 and 2 were further supported by the spectral (mass, UV/Vis) data. The self-assembly process is strongly dependent on the conditions of the medium. An increase in concentration in acetronitrile increases the relative amount of hexamer 2, which appears to be the favored entity at the highest concentrations that can be reached before precipitation occurs. On the other hand, in nitromethane only the tetranuclear complex 1 was detected by mass spectrometry. Replacement of nitromethane by acetonitrile and vice versa indicated the reversible switching between a solution containing either 1 alone or an equilibrium mixture of 1 and 2, respectively. In conclusion, the system described presents several remarkable features: 1) self-assembly with substrate binding, 2) dynamic combinatorial structure generation, and 3) environment-induced structural switching amounting in effect to a process of adaptive self-assembly.

The H-1 NMR spectra of the 5,10,15,20-tetraalkylporphyrins 2-4 reveal a large downfield shift of the signal for the NH protons and a reduction in the activation energy for NH tautomerism as the alkyl substituents become larger; these unusual changes can be rationalized in terms of greatly enhanced intramolecular hydrogen bonding of the NH protons as the bulkier substituents distort the macrocycle from planarity and contract the porphyrin core.

Eine dimere, auch in Lösung stabile Struktur, die auf Wasserstoffbrückenbindungen beruht, charakterisiert das Chloridsalz des abgebildeten expandierten Corrols. Die Synthese, die Röntgenstrukturanalyse sowie die NMR- und UV/Vis-Absorptionsspektren der neuen Spezies bestätigen die Strukturzuordnung sowohl in Lösung als auch im festen Zustand.

Selective formation of trans-nitrochlorins 16-19, cyclopropylchlorins 14, 15, and 20-23, or functionalized trans-chlorins 5-13 by reaction of 2-nitro-5,10,15,20-tetraphenylporphyrin 1 with "active" methylene compounds such as malonates or malononitrile in the presence of base has been achieved. Reaction control is accomplished via sequential Michael additions, followed by intramolecular nucleophilic displacement of a secondary nitro group. Steric as well as thermodynamic effects have been found to govern the selectivity of product formation. Ambient temperature or bulky carbanion substituents lead to nitrochlorins and/or cyclopropylchlorins. Increased reaction temperatures, combined with sterically less encumbered carbanion substituents, favor the formation of disubstituted trans-chlorins. Nucleophilic ring-opening reactions of cyclopropyl-derivative 14 afford disubstituted trans-chlorin products 5 and 25 and provide additional mechanistic evidence for the intermediacy of the cyclopropylchlorin. Use of porphyrins with modified meso-phenyl positions illustrates the generality of this methodology and allows a novel method for the preparation of a wide range of reduced porphyrins, which may find application in fields such as the photodynamic therapy (PDT) of cancer.

Syntheses of novel 15-substituted-oxophlorins via the MacDonald condensation of diformyl-dipyrroketones and 5-substituted-dipyrromethanes are described. The electronic and steric features of the 15-substituent enable facile control over the oxidation potential of the oxophlorins. Introduction of an electron-withdrawing group efficiently minimizes the formation of oxophlorin pi-radicals. Stabilization of neutral pi radicals is promoted by hyperconjugation with a 15-tert-butyl group. A sterically induced stabilization of a novel non-aromatic tautomer of oxophlorin, the so-called "iso-oxophlorin" is demonstrated. These species exist also as 15-iso-oxophlorins upon complexation to divalent metals. Radical formation, enhanced by mild oxidants such as K3FeCN6, yielded pure oligomers and stereospecific supramolecular arrays by radical dimerizations taking place at the 10- and 10'-positions, (C) 1999 Elsevier Science Ltd. All rights reserved.