Electrostatic Work Causes Unexpected Reactivity in Ionic Photoredox Catalysts in Low Dielectric Constant Solvents
Justin L. Ratkovec, Justin Earley, Max Kudisch, William P. Kopcha, Eve Yuanwei Xu, Robert R. Knowles, Garry Rumbles, Obadiah G. Reid
Abstract
High Resolution Image Download MS PowerPoint Slide We show that in low dielectric constant (ε r ) solvents, the prototypical cationic photoredox catalyst [Ir(III)(dFCF 3 ppy) 2 -(5,5′-dCF 3 bpy)] + 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. [BAr 4 F ] − ) anion leads to its irreversible decomposition and a buildup of the neutral Ir(III)(dFCF 3 ppy) 3 -(5,5′-dCF 3 bpy ·– ) (abbv. [Ir(dCF 3 ·- )] 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(dCF 3 ·– )] 0, in various solvents with a range of ε r . The PET reaction between the ions of [Ir(dCF 3 ) – BAr 4 F ] is predicted to be nonspontaneous (Δ G 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 (Δ G S ) and Coulomb ( W ) correction terms to the change in Gibbs free energy of a PET (Δ G 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(dCF 3 ·– )] 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.