A Pd(II)-hydroxyporphycene: synthesis, characterization, and photoinduced proton-coupled electron transfer

• Published in: Research on chemical intermediates 2013-01, Vol.39 (1), p.161-176
• Main Authors: Okawara, Toru, Abe, Masaaki, Shimakoshi, Hisashi, Hisaeda, Yoshio
• Format: Article
• Language: English
• Subjects: Catalysis; Chemistry; Chemistry and Materials Science; Inorganic Chemistry; Physical Chemistry
• ISSN: 0922-6168; 1568-5675
• DOI: 10.1007/s11164-012-0640-8

A Pd(II) complex, Pd(TPrPc-OH) ( 1 , TPrPc-OH = 9-hydroxy-2,7,12,17-tetrapropylporphycenato dianion), has been synthesized and characterized. 1 H NMR spectroscopy revealed that compound 1 exists as its enol form in solution. The H atom of the hydroxy group in 1 was exchanged with deuterium on addition of ethanol- d 6 . UV–visible spectra showed a red shift of the Q band of 1 in THF compared with that of the acetoxy derivative Pd(TPrPc-OAc) ( 2 , TPrPc-OAc = 9-acetoxy-2,7,12,17-tetrapropylporphycenato dianion). The p K a value of the hydroxy group in 1 was determined, by means of a UV–visible titration experiment, to be 10.56. A cyclic voltammogram of 1 in a mixture of THF and Britton–Robinson buffered aqueous solution revealed one-electron and one-proton coupled transfer in the oxidation process in the pH range from 2.7 to 10.5, which was identified by pH-varying experiments and the Pourbaix diagram. Transient absorption spectroscopy revealed that an electron-transfer reaction occurred from the triplet excited-state of 1 to 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ) upon pulse laser irradiation at 532 nm. Such an intermolecular photoinduced electron-transfer reaction was not observed between the Ni analog, Ni(TPrPc-OH), and DQ. The reaction rate constant, k q , was indicative of a kinetic isotope effect with k q(H) / k q(D)  = 1.7, supporting the belief that the exited-state electron transfer from 1 to DQ is accompanied by proton transfer.