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Optical pKa Control in a Bifunctional Iridium Complex
journal contribution
posted on 2019-01-10, 20:17 authored by Ivan Demianets, Jonathan R. Hunt, Jahan M. Dawlaty, Travis J. WilliamsThere are few ways
to switch a catalyst’s reactivity on
or off, or change its selectivity, with external radiation; many of
these involve photochemical activation of a catalyst. In the case
of homogeneous late-transition-metal catalysts, the metal complex
itself is frequently the chromophore involved in such reactivity switching.
We show here a base-pendant ligand–metal bifunctional scaffold
wherein a photobase, a compound that becomes more basic in the excited
state (pKa < pKa*), is used to switch the proton acceptor ability on an active
site of the complex. The system differs from those with metal-centered
chromophores, because the photobase operates independently of the
metal. While excellent progress has been made in photoacid chemistry,
neither a photoacid nor a photobase has been designed into the structure
of a transition-metal catalyst where the metal is not part of the
chromophore. We find that quinoline is an efficient photobase that
preserves its unique properties in the close proximity of an iridium
center: the efficacy of the photobase (9.3 < pKa* < 12.4) in the iridium complex is unhindered relative
to the free quinoline. We apply this notion to successful photodriven
deprotonation of an aliphatic alcohol, thus showing the first case
of metal-orthogonal optical pKa control
in a transition-metal complex.