Mechanistic basis for tuning iridium hydride photochemistry from H₂ evolution to hydride transfer hydrodechlorination

Abstract

<p>The innate H₂ evolution photochemistry of an iridium hydride complex is diverted towards photochemical hydrodechlorination of dichloromethane with high selectivity and quantum yield.</p>

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

“…Consequently, photoredox catalysis until now largely relied on single electron transfer, but the possibility of photo-HAT could open completely new perspectives. Our new insights complement recent photophysical and -chemical studies of iridium hydride [59][60][61][62][63][64][65][66][67] and related complexes. 94,95 In particular, recent work established that the hydrodechlorination of CH 2 Cl 2 proceeds via excited-state "self-quenching" producing a pair of [Ir IV -H] 2+ and [Ir II -H] 0 species, which then undergoes (sequential) hydride transfer and HAT to form two equivalents of CHCl 3 from two molecules of CH 2 Cl 2 solvent.…”

“…The hydride complex is formed in situ from a robust Ir(III) precursor complex, and HAT occurs to stable olen substrates and does not require highly reactive radicals as reaction partners. Thus, alongside the abovementioned very recent study by Miller, Dempsey, and coworkers, 64 our work opens a conceptually new avenue to photoredox chemistry, exploiting visible-light excitation of a simple metal complex for inducing H-atom rather than electron transfer.…”

“…This work illustrates a new type of photo-reactivity for an archetypal metal hydride complex, complementing prior studies on photoacid and "self-quenching" behavior of this well-known class of compounds. [59][60][61][62][63][64][65][66][67] From a photochemical perspective, the discovery of photo-HAT from the [Cp*Ir(phen)(H)] + complex could represent an important breakthrough, because the vast majority of previously explored metal-based photosensitizers operate on the basis of photoinduced electron transfer. Consequently, photoredox catalysis until now largely relied on single electron transfer, but the possibility of photo-HAT could open completely new perspectives.…”

“…67 By contrast, in neat CH 2 Cl 2 the "selfquenching" products (comprised of one oxidized iridium hydride complex and one reduced iridium hydride complex) undergo sequential hydride transfer and HAT, enabling the hydrodechlorination of two equivalents of CH 2 Cl 2 to CH 3 Cl. 64 In the present work, [Cp*Ir(phen)(H)] + (phen ¼ 1,10-phenanthroline) was formed in situ via photo-irradiation of [Cp*Ir(phen)Cl] + in CH 3 CN in presence of excess triethylamine (TEA), which served as a combined electron and proton source. Continued irradiation then promoted [Cp*Ir(phen)(H)] + to its (relatively long-lived) 3 MLCT state.…”

“…61,65,67 Herein we demonstrate that photo-HAT represents an additional reaction pathway for iridium hydride complexes, complementing recent work demonstrating sequential hydride and HAT transfer. 64 to separate base and oxidant molecules. 22 Iridium cyclopentadienyl hydride complexes are a particularly well investigated class of compounds, 59 which have been shown to function as photoacids 60 (Fig.…”