Redox Behavior and Reactions of Cobalt Porphycene Derivatives

Tetrapyrrole compounds are very important for the biological system, and these are pigments of life.  Heme protein has an iron porphyrin, and chlorophyll contains a magnesium porphyrin derivative.  And vitamin B₁₂ has a cobalt corrin.  These tetrapyrrole compounds have various functions.¹  Inspired by the significance of the porphyrins, a new research direction has emerged that is devoted to the preparation and study of non-porphyrin tetrapyrrolic macrocycles.  In this symposium, we will present photochemical and electrochemical reactions of porphycene derivative, which is a porpyrin isomer.

We prepared various brominated porphycenes.²  The singlet oxygen production ability via energy transfer from the triplet state of the porphycene was controlled by the number of bromine.  And then, the porphycene having an ionic liquid tag was synthesized and the photo-oxidation reaction in ionic liquid was investigated.³

Tetrapropyl porphycene (H₂TPrPc) was synthesized according to the reported procedure.⁴  A hydrogenated porphycene was obtained by a hydrogenation of H₂TPrPc in the presence of zinc and hydrochloric acid.⁵  Cobalt complexes (cobalt porphycene and cobalt dihydroporphycene) were synthesized by the reaction with [Co^II(acac)₂] as a metal source.  Redox properties were examined by cyclic voltammetry and controlled potential electrolysis.  UV-vis spectra of controlled-potential electrolysis revealed that spectra of cobalt porphycene and cobalt dihydroporphycene were quite different in the reduced conditions.  These data indicate that the cobalt porphycene was reduced to form a ligand reduced anionradical, but cobalt dihydroporphycene was reduced to form Co(I) species.  It suggests the ligand hydrogenation can change the redox behavior.

It is known that cobalt porphyrin can form a Co-C bond by the oxidative addition with Co(I) species and an alkyl halide.  Cobalt porphycene is not reduced to form the corresponding Co(I) species unlike cobalt porphyrin.  We found that Co-C bond formation is mediated by the reduced ligand of cobalt porphycene.  This new reaction, “redox-active ligand-mediated Co-C bond formation of porphycene”, will be reported.

Reference

(1) Hisaeda, Y.; Tahara, K.; Shimakoshi, H.; Masuko, T. Pure Appl. Chem., 2013, 85, 1415.

(2) Shimakoshi, H.; Baba, T.; Iseki, Y.; Aritome, I.; Endo, A.; Adachi, C.; Hisaeda, Y. Chem. Commun., 2008, 2882.

(3) Shimakoshi, H.; Sasaki, K.; Iseki, Y.; Hisaeda Y. J. Porphyrins Phthalocyanines, 2012, 16, 530.

(4) Vogel, E.; Balci, M.; Pramod, K.; Koch, P.; Lex, J.; Ermer, O. Angew. Chem. Int. Ed. Engl., 1987, 26, 928.

(5) Okawara, T.; Hashimoto, K.; Abe, M.; Shimakoshi, H.; Hisaeda, Y. Chem. Commun., 2012, 48, 5413.