Electrostatic Work Causes Unexpected Reactivity in Ionic Photoredox Catalysts in Low Dielectric Constant Solvents.
Journal Article
Overview
abstract
We show that in low dielectric constant (εr) 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. [BAr4F]-) 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, krxn, 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 εr. The PET reaction between the ions of [Ir(dCF3) - BAr4F] is predicted to be nonspontaneous (ΔGPET ≥ 0) in high εr solvents, such as acetonitrile, and we observe that krxn ≃ 0 under these circumstances. However, krxn increases as εr decreases. We attribute this change in spontaneity to the electrostatic work described by the Born (ΔGS) and Coulomb (W) correction terms to the change in Gibbs free energy of a PET (ΔGPET). 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.
