Complexes of terpyridine scaffold as efficient photocatalysts for the degradation of methylene blue pollutant in wastewater effluents

In the present protocol, a series of varyingly substituted terpyridine-based ligands have been designed and synthesized. The synthesized ligands were coordinated with different transition metal cations. Their complexation with Fe(II), Co(II) and Zn(II) led to interesting coordination compounds which for the first time, were evaluated for their photocatalytic potential against the degradation of methylene blue. Almost all the tested complexes 1–9 exhibited fascinating photocatalytic properties. However, noteworthy, photocatalytic results revealed that zinc complexes 1 ([Zn (4′-(p-tolyl)-2,2′:6′,2″-terpyridine)(SO₄)²⁻]), 4 ([Zn (N,N-Dimethyl-4-(2,2′:6′,2″-terpyridin-4′-yl)aniline)(SO₄)²⁻]) & 7 ([Zn (N,N-Diphenyl-4-(2,2′:6′,2″-terpyridin-4′-yl)aniline)(SO₄)²⁻]) have high potential against the degradation of methylene blue followed by iron complexes 3 ([Fe(4′-(p-tolyl)-2,2′:6′,2″-terpyridin)₂](PF₆)₂), 6 ([Fe(4′-(N,N-Dimethyl-4-(2,2′:6′,2″-terpyridin-4′-yl)aniline)₂]2PF₆) & 9 ([Fe(N,N-Diphenyl-4-(2,2′:6′,2″-terpyridin-4′-yl)aniline)₂]2PF₆) and then cobalt complexes 2 (Co(4′-(p-tolyl)-2,2′:6′,2″-terpyridin)₂](PF₆)₂), 5 ([Co(4′-(N,N-Dimethyl-4-(2,2′:6′,2″-terpyridin-4′-yl)aniline)₂]2PF₆) & 8 ([Co(N,N-Diphenyl-4-(2,2′:6′,2″-terpyridin-4′-yl)aniline)₂]2PF₆). Among all the complexes, maximum degradation efficiencies were observed for complex 1 (67.31 %) and complex 7 (65.91 %). The optimized photocatalytic conditions were found to be 10 mg/L of initial concentration of dye solution, 20 mg of catalyst dose, pH = 4 and irradiation time of 40 min. At optimized conditions complex 7 was found to be more efficient photocatalyst with rate constant value of 0.011 min⁻¹ as compared to the rate constant value of 0.009 min⁻¹ for complex 1. Both complexes followed the pseudo-first order kinetics model. The reusability test indicated only 8.62 % and 7.01 % loss of photocatalytic activity for complex 1 and complex 7, respectively, up to four cycles of photodegradation reflecting that these catalysts can be used again and again. This study suggested the remarkable potential of terpyridine metal complexes for the environmental remediation to remove harmful pollutants from aqueous bodies.