Electrochemical and Spectroscopic Characterization of Aluminium(III)-para-methyl-meso-tetraphenylporphyrin Complexes Containing Substituted Salicylates as Axial Ligands

A series of aluminium(III)-p-methyl-meso-tetraphenylporphyrin (p-CH3TPP-Al(III)) containing axially coordinated salicylate anion [p-CH3TPP-Al-X)], where X？=？salicylate (SA), 4-chlorosalicylate (4-CSA), 5-chlorosalicylate (5-CSA), 5-flourosalicylate (5-FSA), 4-aminosalicylate (4-ASA), 5-aminosalicylate (5-ASA), 5-nitrosalicylate (5-NSA), and 5-sulfosalicylate (5-SSA), have been synthesized and characterized by various spectroscopic techniques including ultraviolet-visible (UV-vis), infrared (IR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, 13C NMR, and elemental analysis. A detailed study of electrochemistry of all the synthesized compounds has been done to compare their oxidation and reduction mechanisms and to explain the effect of axial coordination on their redox properties. 1. Introduction The field of the macrocyclic chemistry of transition metals is developing very rapidly because of its applications [1] and importance in the area of coordination chemistry [2]. Among the various macrocyclic systems studied, metalloporphyrins constitute an important class. Metalloporphyrins and their complexes with various ligands, which play an important role in biological processes and exhibit catalytic properties, are continuing to remain the subject of systematic experimental and theoretical studies [3, 4]. The axial ligation on the metal center is a synthetic way to synthesize large multiporphyrinic systems [5–10]. Actually much attention has been paid to porphyrin containing zinc(II), magnesium(II), tin(IV), and rhodium(III) as metal centers [11, 12]. Although aluminium(III) porphyrins are well known for their catalytic properties and their rich photo- and electrochemistry [13–15], their use as molecular building blocks for the design of porphyrin arrays remains limited. The synthesis of porphyrin arrays including aluminium(III) porphyrins is mostly limited to the use of phenolates [16] and carboxylates [17] as the axial ligand. Surprisingly, there is no example of aluminium(III) porphyrins axially bonded to salicylate anion. Here, we report the synthesis and characterization of aluminium(III)-p-methyl-meso-tetraphenylporphyrins (p-CH3TPP-Al-X) complexes axially bonded to substituted salicylate anions and study their electrochemistry. 2. Materials and Methods Pyrrole (Fluka, Switzerland) was distilled over KOH pellets under reduced pressure before use. Benzaldehyde was procured from Aldrich, USA, and aluminium oxide (basic) was purchased from Fluka, Switzerland. Anhydrous sodium sulphate (Na2SO4) was procured from Ranbaxy Labs. Ltd.