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

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.