Lanthanide Texaphyrins as Photocatalysts.

Here, we report the use of gadolinium(III)-, lutetium(III)-, and lanthanum(III)-texaphyrins as bioinspired photocatalysts that promote a novel approach to the degradation of curcumin, a 1,3-diketo-containing natural product. Complexation of curcumin to the lanthanide centers of the texaphyrins yields stable species that display limited reactivity in the dark or under anaerobic conditions. However, upon exposure to mWatt intensity light (pocket flashlight) or simply under standard laboratory illumination in the presence of atmospheric oxygen, substrate oxidation occurs readily to generate curcumin-derived cleavage products. These latter species were identified on the basis of spectroscopic and mass spectrometric analyses. The mild nature of the activation conditions serves to highlight a potential new role for photoactive lanthanide complexes.

Self-Assembled Pyridine-Dipyrrolate Cages.

An inherently nonlinear pyridine dipyrrolate ligand, namely 2,6-bis(3,4-diethyl-5-carboxy-1H-pyrrol-2yl)pyridine (compound 1), is able to distinguish between different zinc(II) cation sources, namely Zn(acac)2 and Zn(OAc)2, respectively. This differentiation is manifest both in terms of the observed fluorescent behavior in mixed organic media and the reaction chemistry. Treatment of 1 with Zn(acac)2 gives rise to a cage dimer, cage-1, wherein two molecules of compound 1 act as double bridging units to connect two individual cage subunits. As inferred from X-ray crystallographic studies, this cage system consists of discrete zinc dimers with hydroxide bridges that, with the assistance of bound DMF solvent molecules, serve to fix the geometry and orientation of the pyridine dipyrrolate building blocks. When a different zinc source, Zn(OAc)2, is used to carry out an ostensibly similar complexation reaction with compound 1, an acetate-bridged 1D abacus-like cage polymer is obtained as inferred from X-ray diffraction analysis. This extended solid state structure, cage-2, contains individual zinc dimer cage submits and appears stabilized by solvent molecules (DMF) and the counteranion (acetate). Rod-like assemblies are also observed by DLS and SEM. This construct, in contrast to cage-1, proved fluorescent in mixed organic media. The structure of the ligand itself (i.e., in the absence of Zn(II)) was confirmed by X-ray crystallographic analysis and was found to assemble into a supramolecular polymer. Conversion to a dimer form was seen upon the addition of TBAOAc. On the basis of the metric parameters, the structures seen in the solid state are stabilized via hydrogen bonding interactions involving solvent molecules.

Quantitative self-assembly of a purely organic three-dimensional catenane in water.

Self-assembly by means of coordinative bond formation has opened up opportunities for the high-yield synthesis of molecules with complex topologies. However, the preparation of purely covalent molecular architectures in aqueous media has remained a challenging task. Here, we present the preparation of a three-dimensional catenane through a self-assembly process that relies on the formation of dynamic hydrazone linkages in an acidic aqueous medium. The quantitative synthesis process and the mechanically interlocked structure of the resulting catenane were established by NMR spectroscopy, mass spectrometry, X-ray crystallography and HPLC studies. In addition, the labile hydrazone linkages of the individual [2]catenane components may be 'locked' by increasing the pH of the solution, yielding a relatively kinetically stable molecule. The present study thus details a simple approach to the creation and control of complex molecular architectures under reaction conditions that mimic biological milieux.

Expanded Porphyrin-Anion Supramolecular Assemblies: Environmentally Responsive Sensors for Organic Solvents and Anions.

Porphyrins have been used frequently to construct supramolecular assemblies. In contrast, noncovalent ensembles derived from expanded porphyrins, larger congeners of naturally occurring tetrapyrrole macrocycles, are all but unknown. Here we report a series of expanded porphyrin-anion supramolecular assemblies. These systems display unique environmentally responsive behavior. Addition of polar organic solvents or common anions to the ensembles leads to either a visible color change, a change in the fluorescence emission features, or differences in solubility. The actual response, which could be followed easily by the naked eye, was found to depend on the specifics of the assembly, as well as the choice of analyte. Using the ensembles of this study, it proved possible to differentiate between common solvents, such as diethyl ether, THF, ethyl acetate, acetone, alcohol, acetonitrile, DMF, and DMSO, identify complex solvent systems, as well as distinguish between the fluoride, chloride, bromide, nitrate, and sulfate anions.

Photoinduced electron transfer in a supramolecular triad produced by porphyrin anion-induced electron transfer from tetrathiafulvalene calix[4]pyrrole to Li(+)@C60.

Binding of a porphyrin carboxylate anion () to tetrathiafulvalene calix[4]pyrrole (TTF-C4P) results in electron transfer from TTF-C4P to Li(+)@C60 to produce the charge-separated state (1/TTF-C4P˙(+)/Li(+)@C60˙(-)) in benzonitrile. Upon photoexcitation of , photoinduced electron transfer from the triplet excited state of to TTF-C4P˙(+) occurs to produce the higher energy charge-separated state (˙(+)/TTF-C4P/Li(+)@C60˙(-)), which decays to the ground state with a lifetime of 4.8 µs.

Synthesis and anti-cancer activities of a water soluble gold(III) porphyrin.

Gold(III) compounds continue to be explored for their potential utility as anticancer agents. A recognized limitation is the reactivity of gold(III), which is typically reduced to the more labile gold(I) state under physiological conditions. The use of porphyrins can overcome this problem. However, to date the stabilization provided by the use a strongly chelating porphyrin is offset by the poor solubility of the resulting complex in aqueous media. In this work, we describe the synthesis and in vitro anti-cancer activity of a gold(III)porphyrin complex with relatively good aqueous solubility. As judged from standard antiproliferation assays, this complex displays an IC50 of 9 µM for the A2780 human ovarian cancer cell line. This is a higher level of potency than displayed by two related control systems.

Synthesis, structural characterization, aromatic characteristics, and metalation of neo-confused porphyrins, a newly discovered class of porphyrin isomers.

Neo-confused porphyrins represent a unique family of porphyrin isomers that retain overall aromatic characteristics by virtue of a 17-atom 18π electron delocalization pathway. These porphyrin analogues have a pyrrolic subunit linked in a 1,3-fashion so that a nitrogen atom is directly connected to a meso-bridging carbon. Pyrrole-3-carbaldehydes were shown to react with sodium hydride and 5-acetoxymethylpyrrole-2-carbaldehydes in DMF to give the crucial neo-confused dipyrrolic dialdehyde intermediates. MacDonald "2 + 2" condensation of the dialdehydes with a dipyrrylmethane afforded a dihydroporphyrinoid, and subsequent oxidation with 0.2% aqueous ferric chloride generated a series of fully conjugated neo-confused porphyrins. Unusual dihydroporphyrin byproducts were also identified. Reaction of neo-confused porphyrins with nickel(II) or palladium(II) acetate in refluxing acetonitrile gave excellent yields of the corresponding organometallic derivatives. Proton NMR spectroscopy demonstrates that the diatropic character of this system is diminished compared to regular porphyrins, although neo-confused porphyrins retain porphyrin-like UV-vis spectra. Protonation led to the sequential formation of mono- and dicationic species. Proton NMR spectra for the dications showed the presence of enhanced diamagnetic ring currents.

Preparation of azulene-derived fulvenedialdehydes and their application to the synthesis of stable adj-dicarbaporphyrinoids.

A "2 + 2" strategy for synthesizing adj-dicarbaporphyrinoid systems has been developed. In a model study, an azulenylmethylpyrrole dialdehyde was condensed with a dipyrrylmethane in the presence of HCl, followed by oxidation with ferric chloride, to give a modest yield of an azuliporphyrin. Fulvene aldehydes were prepared by reacting an indene-derived enamine with azulene aldehydes in the presence of Bu(2)BOTf, and azulene dialdehydes similarly reacted to give fulvene dialdehydes. The dialdehydes were condensed with dipyrrylmethanes in TFA/dichloromethane to afford good to excellent yields of dicarbaporphyrinoids with adjacent indene and azulene subunits. These 22-carbaazuliporphyrins exhibited significant diatropic character, and this property was magnified upon protonation. These characteristics are attributed to tropylium-containing resonance contributors that possess 18π electron delocalization pathways. Protonation studies demonstrated that C-protonation readily occurred at the interior indene carbon, but deuterium exchange also occurred at the internal azulene CH as well as at the meso-positions with TFA-d. Reaction of a carbaazuliporphyrin with silver(I) acetate in methanol or ethanol solutions also gave unusual nonaromatic dialkoxy derivatives.