High Triplet Energy Iridium(III) NHC Complexes as Photocatalysts

Iridium(III) photocatalysts of the type Ir(C^N)3 and [Ir(C^N)2(N^N)]+ (where C^N and N^N rep-resent cyclometalating and ancillary ligands, like 2-phenylpyridinato and 2,2’-bipyridine, re-spectively) have seen widespread use over the past two decades. One of the most popular is fac-Ir(ppy)3, a strongly photoreducing photocatalyst (E*ox = -1.75 V vs. SCE in MeCN) that pos-sesses a reasonably high triplet energy (ET = 2.54 eV in MeCN). Despite its popularity, there has been relatively little exploration of other homoleptic neutral iridium(III) complexes as photo-catalysts. Replacement of the pyridyl moiety of the C^N ligands with more strongly -donating N-heterocyclic carbene (NHC) groups affords complexes with much higher bandgaps and ET, and significantly cathodically shifted ground-state redox potentials. In this study, mer- and fac¬¬-Ir(pmi)3 (where pmi represents 1-phenyl-3-methylimidazolin-2-ylidene-C,C2¬) were investigated as photocatalysts. These isomeric complexes have exceptionally high ET = 3.28 and 3.30 eV, re-spectively, and are very strongly reducing photocatalysts (E*ox = -2.72 and -2.67 V vs. SCE re-spectively). Both complexes consistently outperformed fac¬¬-Ir(ppy)3 across a range of photore-dox, energy transfer, and metallaphotoredox transformations. Additionally, Ir(pmi)3 exhibited significantly improved photostability compared to fac-Ir(ppy)3. This study highlights Ir(pmi)3 as an easy to synthesize, powerful, and versatile photocatalyst that should be a welcome addi-tion into the toolbox of photocatalysts for the synthetic organic chemist.