Catalytic Properties of Luminescent Tris-Homoleptic Cyclometalated Iridium(III) Complexes in the Oxidation of Morin in Visible Light

Rachael Njogu; Lydia Njenga; David Kariuki; Amir Yusuf; Ola Wendt

Rachael Njogu Department of Chemistry, University of Nairobi
Lydia Njenga Department of Chemistry, University of Nairobi
David Kariuki Department of Chemistry, University of Nairobi
Amir Yusuf Department of Chemistry, University of Nairobi
Ola Wendt Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Sweden

Keywords: Morin, Catalytic Oxidation, Photoredox, Tris-Homoleptic Ir(III) complexes

Abstract

The photo-oxidation of Morin, 2′,3,4′,5,7-pentahydroxyflavone by six luminescent homoleptic tris-cyclometalated iridium(III) complexes was investigated with the aim of evaluating the catalytic properties of the complexes. The Iridium complexes were synthesized using 2-(1-naphthyl)-pyridine (npy) ligand and its derivatives such as npy-OMe, npy-CF3 and npy-Me to form Tris-homoleptic cyclometalated complexes; Ir(npy)₃, Ir(npy-OMe)₃, Ir(npy-CF₃)₃, Ir(npy-Me)₃, Ir(Me-npy-Me)₃, and Ir(Me-npy)₃ with substitution at para position relative to nitrogen. The ligand substitution positions were found to influence the excited state lifetimes, where the complexes exhibited long lifetimes, τ, 1.4 – 3.6 μs, ensuring time for substrates to react before relaxation to ground state. All the six complexes displayed reversible or pseudo reversible redox processes with ground state oxidation potential range of 0.57 to 0.93V compared to Standard Calomel Electrode in CH₂Cl₂. The complexes degraded morin with rate constants k_obs between 0.023-0.036 s -1 within a timescale of 12 minutes. The Ir(npy-Me)₃ complex was found to have a high degradation with a rate constant of k_obs = 0.036 s ^-1. Degradation reactions using all the six Iridium (III) complexes photoredox catalysts were found to follow first order kinetics and ca. 10-fold faster compared to similar oxidative reactions.