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

Abstract

We show that in low dielectric constant (ε_r) 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, k_rxn, 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_r. The PET reaction between the ions of [Ir(dCF₃) – BAr₄^F] is predicted to be nonspontaneous (ΔG_PET ≥ 0) in high ε_r solvents, such as acetonitrile, and we observe that k_rxn ≃ 0 under these circumstances. However, k_rxn increases as ε_r decreases. We attribute this change in spontaneity to the electrostatic work described by the Born (ΔG_S) and Coulomb ($W$) correction terms to the change in Gibbs free energy of a PET (ΔG_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.