Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

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.