Synthesis and Applications of Tris-homoleptic Cyclometalated Iridium (Iii) Complexes

Abstract

Tris-cyclometalated homoleptic iridium(III) complexes have been shown to harbor immense potential as functional materials constituents and actively play a critical role in the field of photocatalysis and photosensitization. Herein, this study reports the synthesis, characterization, photophysical and electrochemical properties of iridium(III) complexes of 2-(1-naphthyl)-pyridine (npy) and 1-phenyl isoquinoline (piq) ligands synthesized via Suzuki coupling protocol with subsequent cyclometalation to generate; Ir(npy)3, C-L1; Ir(npy-OMe)3, C-L2; Ir(npy-CF3)3, C-L3; Ir(npy-Me2)3, C-L4; Ir(npy-Me)3, C-L5; Ir(npy-Me)3, C-L6; Ir(piq-F)3, C-L7; Ir(piq-CF3)3, C-L8; Ir(piq-t-Bu)3, C-L9 and Ir(piq-Me)3, C-L10. They were characterized using 1H, 13C, 19F, 2D COSY and HSQC, single crystal XRD, HRMS and elemental (CHN) analyses. Their photophysical and electrochemical properties were also probed through UV-VIS, PL, τ, Φ and CV. In the results, a systematic investigation of ligand substitution patterns showed an influence on the lifetime of the excited state, with slight changes in the absorption and emission spectral features. Specifically, the emission lifetime of a complex of a npy ligand substituted with a strongly electron-withdrawing trifluoromethyl group was longer than that of the corresponding complex with the electronically non-perturbed ligand (3.7 μs versus 1.5 μs). Electronically complementary ligands and complexes with orthogonal configurations showed slightly shorter excited state lifetimes compared with unsubstituted npy (1.4–3.0 μs). All complexes displayed reversible or quasi-reversible redox-couple processes, with the complex of the trifluoromethylated ligand showing the highest ground-state oxidation potential Eox1/2 [IrIII/IrIV = 0.95 V vs. SCE in CH2Cl2]. This study showed that these complexes can be used as efficient photoredox catalysts, as demonstrated by the six of the complexes (C-L1-C-L6) in their application in a regioselective methoxytrifluoromethylation in which the npy complexes showed equal or better performance compared with the archetypical photoredox catalyst tris 2-phenyl pyridine iridium(III). Their second photoredox application was in photobleaching of morin, as model reaction. C-L1 to C-L6 were found to catalyse the degradation of morin in 12 minutes under ambient temperatures in O2 environment at pH 10 with degradation rate constants, Kobs 0.023 s-1 to 0.036 s-1. C-L4, exhibited the highest Kobs = 0.036 s-1. The photoredox catalytic degradation reaction of Morin dye was found to follow first order kinetics. The third photoredox application was on photooxidation of alcohols; benzyl alcohol and 4-methoxy benzene alcohol to corresponding aldehydes. C-L1 had higher yields of 30% and 58% for both reactions under study as compared to the rest of the complexes. In all cases selectivity was > vi 99% for the product. Complexes C-L1 to C-L6 selectively hydrogenated furfural to furfuryl alcohol with C-L1 giving the highest 21% yield. In conclusion, the chemistry of tris homoleptic cyclometalated Ir(III) complexes bearing variants of 2-(naphathalen-1-yl) pyridine and 1-phenyl isoquinoline ligands and their various applications has been reported. The complexes, C-L1-C-L6, were successful in photoredox catalysis and selective hydrogenation of furfural.