Electrochemistry of Porphyrazines with Exocyclic Pended Pyridyl and N-Methylpyridyl Rings

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
K. M. Kadish, L. Cong (University of Houston), F. Sciscione, E. Viola, M. P. Donzello, and C. Ercolani (Università di Roma Sapienza)
Two new series of porphyrazines with exocyclic pended 2-pyridyl and 2-N-methylpyridyl rings were investigated as to their electrochemical properties. The structures of the investigated compounds are shown in Chart 1a and represented in one case as [Py8PzM], where M = 2H, MgII(H2O), CoII, CuII or ZnII and in the other as [(2-Mepy)8PzM]8+, where M = MgII(H2O) or ZnII. The redox behavior of these new compounds is then compared to that of earlier synthesized porphyrazines with p-extended systems which are represented as [Py8PyzPzM] (Chart 1b), and [Py8QxPzM] where Qx = quinoxaline. Each porphyrazine undergoes multiple one-electron reductions in DMSO, benzonitrile, pyridine or CH2Cl2 while the two octacationic porphyrazines, [(2-Mepy)8PzM]8+, also exhibit 2 or 3 one electron reductions in water containing 0.1 M NaOAc or 0.1 M KCl as supporting electrolyte. The potentials for each redox reaction were measured by cyclic voltammetry and thin-layer UV-visible spectroelectrochemistry was used to characterize some of the redox reactions. Special emphasis was placed on the cobalt(II) derivative [Py8PzCoII]+, which is oxdized in a single step to give [Py8PzCoIII] or reduced in up to five one-electron transfer steps, with the exact potentials and site of electron transfer depending upon the solvent. Pyridine was shown to strongly coordinate to the metal center of [Py8PzCoII] and [Py8PzCoIII] and binding constants were calculated using standard equations. Finally, comparisons are made between redox behavior of the three types of compounds in Chart 1, [Py8QxPzM] and the well-characterized phthalocyanines, [PcM], having the same central metal ions.

Chart 1. Structures for (a) newly investigated porphyrazines and (b) related porphyrazines with p-extended structures.(In the figure : 1) change formula: [(2-Mepy)8PzM]8+ instead of [(CH3-Py)8PzM] ; 2) change the order of metal centers and put the oxidation state: M = 2H, MgII(H2O), CoII, CuII, ZnII instead of M = Mg(H2O), Cu, Zn, 2H, Co ; 3) put M = MgII(H2O), ZnII instead of M = Mg(H2O), Zn)