Distant protonated pyridine groups in water-soluble iron porphyrin electrocatalysts promote selective oxygen reduction to water.

Fe(III)-meso-tetra(pyridyl)porphyrins are electrocatalysts for the reduction of dioxygen in aqueous acidic solution. The 2-pyridyl derivatives, both the triflate and chloride salts, are more selective for the desired 4e(-) reduction than the isomeric 4-pyridyl complexes. The inward-pointing pyridinium groups influence proton delivery despite their distance from the iron centre.

Pyrazole analogues of porphyrins and oxophlorins.

A series of porphyrin analogues with pyrazole rings replacing one of the usual pyrrole subunits have been synthesized. This was accomplished by reacting 1-phenyl, 1-methyl and 1-ethyl pyrazole-1,3-dicarbaldehydes with a tripyrrane in the presence of TFA, followed by an oxidation step. The initially formed phlorin product was sufficiently stable for the N-phenyl system to be isolated and characterized, although the related N-alkyl phlorin analogues were less stable. Attempts to dehydrogenate the intermediary phlorins with DDQ resulted in decomposition, but the N-alkylphlorins could be oxidized with 0.2% aqueous ferric chloride solutions. Although the phenyl-substituted phlorin could not be oxidized under these conditions, it did afford the pyrazoloporphyrin upon treatment with silver acetate under acidic conditions. Oxidations with silver acetate also afforded oxophlorin analogues where the oxo-linkage was selectively formed at the 5-position. The pyrazole-containing porphyrin analogues are cross-conjugated and exhibit only a small degree of diatropic character. The internal CH resonances were observed between 5.27 and 5.87 ppm, while the external meso-protons fell into a range of 6.84-7.88 ppm. The borderline overall aromatic character was attributed to dipolar resonance contributors. Protonation considerably increased the diatropicity and the diprotonated dications formed from these porphyrin analogues gave the internal CH resonance at upfield values of 2.65-3.20 ppm. The aromatic character was enhanced by the presence of an electron-donating alkyl substituent on the nitrogen compared to the phenyl-substituted species. The pyrazoloporphyrins reacted with nickel(II) acetate in DMF, or palladium(II) acetate in acetonitrile, to give the corresponding organometallic derivatives. The metal complexes showed increased diatropic character but protonation afforded nonaromatic cations. The oxophlorin analogues were also nonaromatic in the free base and protonated forms. This work extends our understanding of carbaporphyrinoid systems and provides the first detailed studies on pyrazole-containing porphyrin analogues.

Synthesis and application of pyridine-based ambipolar hosts: control of charge balance in organic light-emitting devices by chemical structure modification.

We studied the influence of a pyridine moiety versus a phenyl moiety when introduced in the molecular design of an ambipolar host. These pyridine-based host materials for organic light-emitting diodes (OLEDs) were synthesized in three to five steps from commercially available starting materials. The isomeric hosts have similar HOMO/LUMO energies; however, data from OLEDs fabricated using the above host materials demonstrate that small structural modification of the host results in significant changes in its carrier-transporting characteristics.

Adding to the confusion! Synthesis and metalation of pyrazole analogues of the porphyrins.

N-substituted pyrazole dialdehydes are shown to react with a tripyrrane under '3 + 1' conditions to give aza-analogues of the N-confused porphyrins; these novel porphyrinoids show borderline aromatic properties and readily afford organometallic derivatives with Ni(OAc)2 and Pd(OAc)2.

Synthesis and reactivity of N-methyl and N-phenyl meso-unsubstituted N-confused porphyrins.

Condensations of 1-methyl and 1-phenyl-2,4-pyrroledicarbaldehydes with a tripyrrane in TFA-dichloromethane, followed by oxidation with aqueous FeCl(3), gave novel cross-conjugated meso-unsubstituted N-confused porphyrins (NCPs; 12). These porphyrin analogues showed significant diatropic ring currents that were enhanced upon protonation. Reactions with nickel(II) acetate in refluxing DMF, or palladium(II) acetate in acetonitrile, gave good yields of the corresponding nickel(II) or palladium(II) organometallic derivatives 18 and 19. These complexes were stable and the proton NMR spectra showed slightly increased downfield shifts to the external protons. Addition of TFA resulted in C-protonation at the internal carbon to give aromatic cations that showed the inner CH resonance between -2.5 and -4.0 ppm. The nickel(II) cations 20a and 20b slowly underwent demetalation but the related palladium cations 20c and 20d were quite robust and showed no loss of palladium after 1 week at room temperature. Reaction of NCPs 12 with silver(I) acetate gave silver(III) derivatives 21a and 21b where an oxidation had occurred at C-3 to afford a lactam unit. The silver complexes showed strong diatropic ring currents and porphyrin-like UV-vis spectra with a Soret band near 430 nm. N-Methyl NCP 21a also reacted with gold(III) acetate to give the gold(III) NCP 21c, albeit in low yield, and this species showed similar spectroscopic properties to silver(III) NCP 21a. Syntheses of N-phenyl NCP 12b were accompanied by the formation of the 3-oxo derivative 15b, and the related N-methyl product 16a could also be obtained when the reaction mixtures were oxidized with silver(I) acetate under acidic conditions. The proton NMR spectra for these aromatic NCPs in CDCl(3) show the internal CH shifted upfield to near -6.5 ppm, while the external meso-protons are strongly deshielded giving 4 singlets between 9 and 10 ppm. This study demonstrates that meso-unsubstituted NCPs have unusual reactivity and unique spectroscopic properties, and these results complement and extend the work on the much better known meso-tetraaryl NCPs.