CAS: Pincer Ligand Base Metal Chromophores for Selective Free Radical Reactions

In this project, funded by the Chemical Structure, Dynamics & Mechanism B Program of the Chemistry Division, Jake Soper of the School of Chemistry and Biochemistry at the Georgia Institute of Technology is developing new transition metal catalysts capable of using visible light to convert aryl carbon–hydrogen bonds into carbon–(trifluoro)alkyl bonds, which are important components of many agrochemicals and pharmaceuticals. The goals of this research are to design new earth-abundant metal complexes that can replace photocatalysts based on precious metals and to elaborate new ways to selectively target individual carbon–hydrogen bonds for radical activation and functionalization.

By replacing rare and toxic metal catalysts and stoichiometric, often halogen-containing, oxidants with naturally abundant alternatives, the project forms the basis for new "green" routes to pharmaceutical and materials that protect human health and the environment. In pursuit of these goals, the project also affords opportunities for high school STEM educator development, as well as outreach activities that increase public engagement and science literacy among groups that are traditionally underrepresented in chemistry.

Efforts to elaborate photoredox-active chromophores based on first row transition metals are challenged by short charge-transfer excited-state lifetimes that preclude bimetallic reactivity. Research in this project proposes to circumvent this issue by combining the chromophore and organometallic reaction center in single coordination complex, opening new avenues to impart selectivity in photoredox catalysis cycles that occur via kinetically facile radical steps.

Central to this strategy is the capacity of new pincer complexes to support photolytic cobalt bond activation leading to extrusion of carbon based radicals, which are active for alkylation, fluoroalkylation, and arylation of (hetero)arenes. Substrate directing groups are proposed to shepherd these radicals to specific aryl carbon hydrogen bonds, giving atypical regioselectivity in photochemical Minisci-type homolytic aromatic substitution.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.