Electrostatic Work Causes Unexpected Reactivity in Ionic Photoredox Catalysts in Low Dielectric Constant Solvents

We show that in low dielectric constant (ε_<i>r</i>) solvents, the prototypical cationic photoredox catalyst [Ir(III)(dFCF₃ppy)₂-(5,5'-dCF₃bpy)]⁺ is capable of oxidizing its counterion in an unexpected photoinduced electron transfer (PET) process. Photoinduced oxidation of the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (abbv. [BAr₄^F]^-) anion leads to its irreversible decomposition and a buildup of the neutral Ir(III)(dFCF₃ppy)₃-(5,5'-dCF₃ bpy^·-) (abbv. [Ir(dCF₃^·-)]⁰) species. The rate constant of the PET reaction, <i>k</i>_<i>rxn</i>, between the two oppositely charged ions was determined by monitoring the growth of absorption features associated with the singly reduced product molecule, [Ir(dCF₃^·-)]⁰, in various solvents with a range of ε_<i>r</i>. The PET reaction between the ions of [Ir(dCF₃) - BAr₄^F] is predicted to be nonspontaneous (Δ<i>G</i>_PET ≥ 0) in high ε_<i>r</i> solvents, such as acetonitrile, and we observe that <i>k</i>_<i>rxn</i> ≃ 0 under these circumstances. However, <i>k</i>_<i>rxn</i> increases as ε_<i>r</i> decreases. We attribute this change in spontaneity to the electrostatic work described by the Born (Δ<i>G</i>_<i>S</i>) and Coulomb (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>W</mml:mi></mml:math>) correction terms to the change in Gibbs free energy of a PET (Δ<i>G</i>_PET). The electrostatic work associated with these often-neglected corrections can be utilized to design novel and surprising photoredox chemistry. Our facile preparation of [Ir(dCF₃^·-)]⁰ is one example of a general rule: ion-paired reactants can result in energetic neutral products that chemically store photon energy without an associated Coulomb binding between them.