Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light‐Emitting Diode

Abstract

We have explored alkyl substitution of the ligands as a means to improve the performance of the title complexes in photoredox catalytic systems that produce synthetically useable amounts of hydrated electrons through photon pooling. Despite generating a super-reductant, these electron sources only consume the bioavailable ascorbate and are driven by a green light-emitting diode (LED). The substitutions influence the catalyst activity through the interplay of the quenching parameters, the recombination rate of the reduced catalyst OER and the ascorbyl radical across the micelle-water interface, and the quantum yield of OER photoionization. Laser flash photolysis yields comprehensive information on all these processes and allows quantitative predictions of the activity observed in LED kinetics, but the latter method provides the only access to the catalyst stability under illumination on the timescale of the syntheses. The homoleptic complex with dimethylbipyridine ligands emerges as the optimum that combines an activity twice as high with an undiminished stability in relation to the parent compound. With this complex, we have effected dehalogenations of alkyl and aryl chlorides and fluorides, hydrogenations of carbon-carbon double bonds, and self- as well as cross-coupling reactions. All the substrates employed are impervious to ordinary photoredox catalysts but present no problems to the hydrated electron as a super-reductant. A particularly attractive application is selective deuteration with high isotopic purity, which is achieved simply by using heavy water as the solvent.

First Micelle‐Free Photoredox Catalytic Access to Hydrated Electrons for Syntheses and Remediations with a Visible LED or even Sunlight

“…We calibrated it by varying the laser intensity and extrapolating a fitted saturation curve, which is very reliable in this case because the highest available intensity already achieves near‐complete conversion of GS into MLCT. For calibrating the OER spectrum of Figure b, we quenched MLCT with 4‐methoxy phenolate, the radical of which can be observed in isolation and with good sensitivity at the isosbestic point between GS and OER …”

“…This is circumvented by capturing with a substrate and detecting the accumulating product, the concentration of which equals the integral of the generation rate weighted with the capturing efficiency (which is not a constant but a weakly decreasing function of time owing to the depletion of substrate and HUr 2− , which both compete for ). Chloroacetate ClAc is ideally suited as a substrate for this purpose: it rapidly (rate constant, 2×10 9 M −1 s −1 under our conditions) scavenges in a dissociative electron transfer, yet is inert towards less powerful reductants such as OER and cannot intercept the extremely short‐lived excited OER before the electron is ejected by autoionization of that state; the product Cl − can be conveniently monitored with a chloride‐sensitive electrode (SI‐1.3); and the accompanying carboxymethyl radicals do not poison the catalyst because the sacrificial donor renders them innocuous, yielding as by‐products the same radical species that are afforded by the MLCT quenching anyway.…”

“…Attempting to emulate Nature's example, we have recently presented the first catalytic systems that afford synthetically useable amounts of hydrated electrons with a green light‐emitting diode (LED) . The combination of its extreme standard potential ( E °, −2.9 V) and its surprisingly long unquenched life (1–2 μs) makes a valuable relay in reductive photoredox catalysis.…”

“…The combination of its extreme standard potential ( E °, −2.9 V) and its surprisingly long unquenched life (1–2 μs) makes a valuable relay in reductive photoredox catalysis. In even stronger contrast, does not stop short of breaking nonactivated aliphatic carbon–fluorine bonds, but our generation method only involves water as the solvent (though so far together with the surfactant SDS), visible light at safe intensities, and a naturally occurring antioxidant as the sole consumable.…”

“…In our previous reports, we achieved this by micellar compartmentalization. Enclosing the catalyst in an anionic SDS micelle greatly lowers both the rate constant k q (or the associated yield‐determining Stern–Volmer constant K SV ) of MLCT quenching and k rec provided that the sacrificial donor is a dianion.…”