Reconceptualizing the IrIII Role in Metallaphotoredox Catalysis: From Strong Photooxidant to Potent Energy Donor

Dual IrIII/LnNiII metallaphotoredox catalyzed C(sp3)-C(sp2) cross-coupling reactions are widely assumed to proceed by photoinduced single electron transfer steps due to the highly oxidizing IrIII* excited state (IrIII = [Ir(dF(CF3)ppy)2(dtbbpy)]+[PF6]-; dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; Ln = dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). Using time-resolved absorption and emission spectroscopy, we reveal that energy transfer between IrIII* and various LnNiII precatalysts and intermediates with kq >= 108 M-1 s-1 also drives catalysis. Specifically, the excited states of LnNiII dihalide precatalysts/organometallic intermediates accessible by energy transfer appear to drive bond homolysis, halogen radical elimination, and reductive elimination reactions that facilitate formation of cross-coupled products. Energy transfer dynamics consequently circumvent the need for photoinduced electron transfer, thereby extending substrate scopes to coupling partners that cannot be oxidized by IrIII*. Within a cross-electrophile coupling model reaction between 4-bromobenzotrifluoride and bromocyclohexane, energy transfer activates the LnNiII precatalyst at early reaction times before nucleophilic reductants are present. In the absence of IrIII, direct excitation of LnNiII(Br)2 also activates the precatalyst to form a LnNiII(Br)(Aryl) intermediate. To compare energy transfer and electron transfer kinetics, we determined rate constants for reductive quenching by Br- (kSET = 4.1 x 108 M-1 s-1) and for the subsequent electron transfer from reduced IrIII*- to LnNiII(Br)2 (kSET = 4.1 x 107 M-1 s-1) using Stern-Volmer analysis and pulse radiolysis, respectively. Energy transfer rate constants are competitive with the electron transfer rate constants and energy transfer is a parallel pathway within metallaphotoredox catalysis. Exploiting the energy transfer mechanism, we demonstrate highly selective cross-electrophile coupling between 4-chlorobenzotrifluoride and bromocyclohexane to form exclusively cross-coupled product. With alkyl-trifluoroborate nucleophiles that do not reductively quench IrIII* emission, transmetalation with LnNiII(Br/Cl)(Aryl) followed by energy transfer also drives excited state reductive elimination to form C(sp3)-C(sp2) cross-coupled product. Similarly, energy transfer rather than NiII oxidation drives C(sp2)-OR reductive elimination, despite the strongly oxidizing ability of IrIII*. In total, these reactions demonstrate energy transfer processes from IrIII* to LnNiII in metallaphotoredox catalysis that can unlock alternative reactive pathways.