Tuning the Electrochemical and Photophysical Properties of Osmium-Based Photoredox Catalysts

Abstract

The use of low-energy deep-red (DR) and near-infrared (NIR) light to excite chromophores enables catalysis to ensue across barriers such as materials and tissues. Herein, we report the detailed photophysical characterization of a library of OsII polypyridyl photosensitizers that absorb low-energy light. By tuning ligand scaffold and electron density, we access a range of synthetically useful excited state energies and redox potentials.1 Introduction1.1 Scope1.2 Measuring Ground-State Redox Potentials1.3 Measuring Photophysical Properties1.4 Synthesis of Osmium Complexes2 Properties of Osmium Complexes2.1 Redox Potentials of Os(L)2-Type Complexes2.2 Redox Potentials of Os(L)3-Type Complexes2.3 UV/Vis Absorption and Emission Spectroscopy3 Conclusions

Orange Light-Driven C(sp2)–C(sp3) Cross-Coupling via Spin-Forbidden Ir(III) Metallaphotoredox Catalysis

“…10,43,44 High-throughput experimentation (HTE) revealed the optimal conditions for alkyl BF 3 K arylation using Ir-4 with 1.2 equiv. of p-methoxybenzyl (PMB) BF 3 K, furnishing the desired product (19) Electron-poor (19)(20)(21)23) to moderately electron-rich (24) aryl bromides are well-tolerated, as are sterically hindered (22) substrates. Free phenols (26) are competent coupling partners, exclusively forming the C−C product with no C−O byproduct observed (Scheme 4).…”

“…In 2019, we along with Congreve and Campos demonstrated the synthetic utility of low-energy photoredox catalysis by generating orange light in situ with triplet-fusion upconversion . This seminal work was followed by Gianetti’s report of a red light-absorbing organic photocatalyst and our development of Os­(II) polypyridyl complexes that undergo spin-forbidden excitation (SFE) under red light irradiation to directly access the catalytically relevant triplet (T 1 ) excited state. , Circumventing the energetically costly process of intersystem crossing (ISC) has enabled complementary low-energy methods to visible light Ru/Ir photoredox catalysis (Scheme B). − However, the redox demands of nickel-assisted C­(sp 2 )–C­(sp 3 ) cross-coupling require up to +1.20 eV for PC* to oxidize a sacrificial reductant or nucleophile and −1.20 eV for PC •– (vs SCE in MeCN) to regenerate the Ni active species following reductive elimination of the product. , Thus, the current low-energy PCs are incompatible with more challenging synthetic transformations, as they do not possess this broad redox window.…”

Improved transition metal photosensitizers to drive advances in photocatalysis

“…In these studies, a series of osmium(II) complexes with variable polypyridyl analogues were synthesized. 17,41 Most complexes exhibit direct triplet absorption in the deep red or NIR region (600-800 nm). Through these studies, the photophysical properties of osmium complexes could be enhanced by using strong s-donor ligands through the destabilization of the HOMO.…”

Applications of red light photoredox catalysis in organic synthesis

“…5). 23 Though the λ max of Os-based PCs lie mostly in the purple or blue light regions, they also show moderate absorptions in red light or even NIR regions. After the pioneering work of polypyridyl Os complexes for NIR photoredox catalysis was introduced by the Rovis group in 2020, 6 c several intriguing red light-induced Os complex-catalyzed reactions were also reported.…”