Electrochemical and spectroelectrochemical behavior of N-methyl-phenothiazinyl meso-substituted porphyrins and their complexes with Ni(II), Cu(II) and Pd(II)

• SWV were recorded for MPtzxPh4−xPorph (x = 0, 1, 2, 4; M = Ni2+, Cu2+, Pd2+).
• Ptz and Porph contribute distinctly to the overall redox response.
• Ptz and M2+ exert opposite effects on the redox activity of the Porph ring.
• Structure–redox activity correlation obeys Hammett’s free energy relationship.
• The spectral activity of the investigated compounds is decided by the Porph moiety.

The electrochemical and spectroelectrochemical properties of 11 new N-methyl phenothiazinyl meso-substituted porphyrins, H2PtzxPh4−xPorph (where Ptz stands for N-methyl phenothiazinyl, Ph for phenyl and Porph for porphyrin; x = 0, 1, 2 or 4), and their complexes with Ni(II), Cu(II) and Pd(II) were investigated. The half wave potentials of the redox processes yielding the anion, dianion or π-cation and dication radicals were estimated from square-wave voltammetry measurements, performed in CH2Cl2 containing 0.1 M TBAPF6 as supporting electrolyte. Two opposite effects decide the electrochemical behavior of the metal complexes corresponding to the Ptz-substituted porphyrins: (i) the electron attracting ability of the Ptz substituent and (ii) the decrease of the electronic affinity of the porphyrin macrocycle induced by the presence of the metallic ion, i.e. Ni(II), Cu(II) or Pd(II). The electrochemical data were rationalized by using the Hammett’s linear free energy relationship. The spectral data, obtained from in situ spectroelectrochemical measurements, confirmed the characteristic behavior of the porphyrin derivatives and were found in good agreement with the electrochemical results.

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