Azolium-Porphyrin Electrosynthesis

Abstract

Electrochemical oxidation of Zn(II) 2,7,12,17-tetra-tert-butylporphyrin in the presence of a series of azole derivatives (1-methylimidazole, 1-vinyl-1H-imidazole, 2-(1H-imidazol-1-yl)pyridine, 1-methylbenzimidazole, 1-methyl-1H-1,2,4-triazole, and benzothiazole) affords the corresponding meso-substituted azolium-porphyrins in very mild conditions and good yields. It was found that these nucleophiles were strongly ligated to the zinc(II) azolium-porphyrin complexes. Thus a demetalation/remetalation procedure was performed to recover the non-azole-coordinated zinc(II) complexes. X-ray crystallographic structures of three azolium-porphyrins were solved. Cyclic voltammetry analyses provided insight into the electron-withdrawing effect of the azolium substituents.

Keywords: Anodic nucleophilic substitution; Azole nucleophiles; Azolium; Electrochemistry; Electrosynthesis; Porphyrinoids.

Scheme 1

Suggested mechanism for the S_NA reaction with a porphyrin and a nucleophile (Nu=pyridine,[ ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ ] triphenylphosphine,[ ⁹ , ¹² ] or azoles (this work)).

Scheme 2

Electrochemical functionalization of Zn‐1 with azole nucleophiles a‐‐f.

Figure 1

Cyclic voltammograms of Zn‐1 and Zn‐6⁺ in 0.1 M TEABF₄/CH₃CN, WE: Pt, Ø=2 mm, c=10⁻³ M, ν=100 mV s⁻¹.

Figure 2

¹H NMR spectra of Zn‐2⁺ , Zn‐3⁺ , Zn‐4⁺ , Zn‐5⁺ , and f‐(Zn‐7⁺ ) (DMSO‐d₆, 500 MHz, 400 MHz for Zn‐6⁺ , 298 K).

Figure 3

X‐ray crystallographic structure of Zn‐4⁺ , Zn‐5⁺ and Zn‐6⁺ . BF₄ ⁻ anion and solvents were omitted for clarity (see ESI for full characterization). Thermal ellipsoids are scaled to the 50 % probability level.