High-throughput Synthesis and Screening of Iridium(III) Photocatalysts for the Fast and Chemoselective Dehalogenation of Aryl Bromides

Abstract

A high-throughput optical screening method for the photocatalytic activity of a structurally diverse library of 1152 cationic iridium­(III) complexes ([Ir­(C^N)2(N^N)]+), corresponding to all combinations of 48 cyclometalating (C^N) and 24 ancillary (N^N) ligands, was developed. This rapid assay utilizes the colorimetric changes of a high contrast indicator dye, coumarin 6, to monitor the photo-induced electron transfer from a sacrificial amine donor to the metal complex excited state. The resulting [Ir­(C^N)2(N^N)]0 can then reduce an aryl bromide to form the highly reactive aryl radical intermediate. The rate of this reaction is dictated by the molecular structure of both coordinating ligands. Relative reaction rate constants determined via this method correlated closely with 19F NMR measurements obtained using a fluorinated substrate. A simple model that expresses the rate constant as a product of a single ″strength″ parameter assigned to each of the 72 ligands can well account for the 1152 measured rate constants. The best performing complexes exhibit much higher reactivity than the benchmark photocatalysts commonly used in photoredox transformations. The catalysts were also successfully tested for their chemoselectivity. The developed screening methodology can enable generation of the large data sets needed to use modern data science to extract structure–activity relationships.

High-Throughput Screening and Automated Data-Driven Analysis of the Triplet Photophysical Properties of Structurally Diverse, Heteroleptic Iridium(III) Complexes

“…Second and third row d 6 transition metals, specifically Ru­(II), Re­(I), Os­(II), and Ir­(III), form photoactive coordination complexes suitable for varied applications including light emitting diodes, oxygen sensing, organic photocatalysis, bioimaging, photodynamic therapy, and solar fuel generation. − Heteroleptic [Ir­(C^N) 2 (N^N)] + complexes (where C^N is a cyclometalating 2-phenylpyridyl ligand and N^N is a 1,2-diimine ancillary ligand, Figure A) are popular scaffolds due to their modular synthesis, enhanced photostability, and long-lived triplet excited state lifetimes arising from strong spin–orbital effects of Ir­(III). − Large ligand-field splitting, caused by the high quantum number of the central Ir d -electrons and the strongly σ-donating carbanions of the cyclometalating ligands, significantly destabilizes metal-centered (MC) antibonding orbitals compared to other 4 d and 5 d metal ion complexes . The substantial increase in these e g *-like orbitals inhibits thermal population of the deactivating 3 MC excited state and allows the large variability in emission color of reported Ir­(III) complexes, spanning the visible spectrum (Figure A).…”

“…Despite the considerable ligand diversity in reported Ir­(III) complexes, a lack of standardized analytical procedures (e.g., spectrometer response, concentration, solvent) prevents the ready comparison of literature values or the creation of large databases to support quantum chemical or machine learning-based prediction efforts. We have previously demonstrated the effectiveness of combinatorial synthesis to generate novel heteroleptic Ir­(III) phosphors. , Here, we combine this approach with high-throughput, parallel synthesis techniques to measure the steady-state emission spectra and excited-state lifetime of an unprecedented library of 1440 structurally diverse complexes. Automated analysis of the experimental emission spectra and lifetime provides a framework for assigning the nature of the triplet excited state.…”

“…We have previously demonstrated the effectiveness of combinatorial synthesis to generate novel heteroleptic Ir(III) phosphors. 10,43 Here, we combine this approach with high-throughput, parallel synthesis techniques to measure the steady-state emission spectra and excited-state lifetime of an unprecedented library of 1440 structurally diverse complexes. Automated analysis of the experimental emission spectra and lifetime provides a framework for assigning the nature of the triplet excited state.…”

Phosphorescent Trinuclear Pt–Ir–Pt Complexes: Insights into the Photophysical and Electrochemical Properties and Interaction with Guanine Nucleobase

“…Initially,5 '-GMP (1 10 À3 m)w as titrated against complexes 3 (2 mL, 1 10 À5 m)a nd 6 (2 mL, 1 10 À5 m;s ee Figure S9) to gain an understanding of the nature of the binding. The emission intensity reached saturation level at 4:1[ 5 '-GMP]/ [3]a nd 2:1[ 5 '-GMP]/ [6]. Thus, complex 3 binds to 5'-GMPi na 1:4f ashion, or one Pt II can accommodate two guaniner esidues, whereas only two 5'-GMP molecules bind to complex 6.…”

“…Yield: 20.2 mg (55 %); 1 HNMR (500 MHz, CDCl 3 ): d = 9.30 (s, 2H), 8.53 (d, J = 4.7 Hz, 2H), 7.88 (d, J = 8.1 Hz, 2H), 7.78 (d, J = 5.5 Hz, 2H), 7.72 (t, J = 7.3 Hz, 6H), 7.65 (t, J = 6.9 Hz, 6H), 7.60 (d, J = 5.6 Hz, 2H), 7.46 (d, J = 5.6 Hz, 2H), 7.18 (t, J = 6.1 Hz, 4H), 7.03-6.97 (m, 6H), 6.87 (t, J = 7.4 Hz, 2H), 6.23 (d, J = 7.5 Hz, 2H), 4.15(s, 4H), 3.98 ppm (s, 8H); 13 CNMR (500 MHz, CDCl 3 ): d = 166. 98, 156.27, 154.92, 149.04, 147.85, 146.94, 142.56, 137.60, 137.08, 130.75, 129.86, 127.06, 124.82, 123.80, 122.45, 122.08, 121.66, 118.65, 115.46, 59.48, 58.38 [Ir(ppy) 2 {2,2'-bpy-4,4'-(CH 2 -dpa-Pt) 2 }]Cl 3 (6): cis-[PtCl 2 (DMSO) 2 ] (7.51 mg, 0.0178 mmol) was stirred for 12 hi nd ichloromethane with complex 5 (10 mg, 0.0089 mmol) in the dark. The resulting orange precipitate was filtered and washed with dichloromethane and diethyl ether.T he orange solid was dried in vacuo.…”