Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 1. Tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine: A New Macrocycle with Remarkable Electron-Deficient Properties

A new pyrazinoporphyrazine macrocycle carrying externally appended pyridine rings, tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine (hydrated), [Py8TPyzPzH2]·2H2O, was prepared in high yield by direct cyclotetramerization of the precursor, 2,3-dicyano-5,6-di(2-pyridyl)-1,4-pyrazine, [(CN)2Py2Pyz], in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The single-crystal X-ray structure of [(CN)2Py2Pyz] shows a noncoplanar positioning of the pyrazine and pyridine rings in the two slightly different independent molecular units present in the crystal. UV−vis spectra of [Py8TPyzPzH2] were measured in two nondonor solvents (CHCl3, CH2Cl2), a slightly basic solvent (pyridine), and an acidic solvent (CH3COOH). In all cases, the spectral changes are consistent with the occurrence of molecular aggregation and colloidal dispersions which break up with time to give clear solutions containing exclusively the monomeric form of the macrocycle, either neutral [Py8TPyzPzH2] (in CHCl3, CH2Cl2, and CH3COOH) or dianionic [Py8TPyzPz]2- (in pyridine). A spectrally monitored titration of [Py8TPyzPzH2] in CH2Cl2 with TBA(OH) shows the loss of two protons from the macrocyclic core and quantitative conversion of [Py8TPyzPzH2] to [Py8TPyzPz]2-. Cyclic voltammetry and thin-layer spectroelectrochemical measurements show that [Py8TPyzPzH2] is present in CH2Cl2 while [Py8TPyzPz]2- is present in pyridine, but both forms of the compound exhibit identical electrochemical behavior, consistent with a conversion of the dianion to the neutral porphyrazine in pyridine prior to electroreduction via four reversible one-electron transfer steps. No oxidations of the macrocycle are observed in either solvent containing 0.1 M tetrabuthylammonium perchlorate (TBAP). A comparison of the electrochemical behavior for [Py8TPyzPzH2] with what is reported for related phthalocyanine and porphyrazine analogues highlights the remarkable electron-accepting properties of the presently investigated free-base macrocycle.