Abstract
Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)3 groups attached to the CN - ligands stabilize the metal-based t2g-like orbitals by ∼0.8 eV, leading to high 3MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion.