2025/03/07 by Máire Griffin, Eli Zysman‐Colman · 1 voice
Chemistry · Chemical Engineering · #N-Heterocyclic Carbenes in Organic and Inorganic Chemistry #Catalytic Cross-Coupling Reactions #Ammonia Synthesis and Nitrogen Reduction
paper · pdf · doi:10.26434/chemrxiv-2025-t373h
openalex publication_date 2025/03/07 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/15
Iridium(III) photocatalysts of the type Ir(CN)3 and [Ir(CN)2(NN)]+ (where CN and NN 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 CN 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.