Photochemical Formic Acid Dehydrogenation by Iridium Complexes: Understanding Mechanism and Overcoming Deactivation

Abstract

The mechanism of photochemical formic acid dehydrogenation catalyzed by [Cp*Ir­(bpy)­(Cl)]+ (1, bpy = 2,2′-bipyridine) and [Cp*Ir­(bpy-OMe)­(Cl)]+ (1-OMe, bpy-OMe = 4,4′-dimethoxy-2,2′-bipyridine) is examined. The catalysts operate with good turnover frequency (TOF) across an unusually wide pH range. Above pH 7, the evolved gas is >95% pure H2 (along with traces of CO2 but no detectable CO). Light-triggered H2 release from a metal hydride intermediate is found to be the turnover-limiting step, based on the observed first-order dependence on catalyst concentration, saturation behavior in formate concentration, and direct in situ observation of a metal hydride resting state during turnover. Deactivation pathways are identified, including ligand loss and aggregate formation, precipitation of insoluble forms of the catalyst, and deprotonation of the iridium hydride intermediate. Guided by mechanistic insights, improved catalytic activity (initial TOF exceeding 50 h–1), stability (>500 turnovers at nearly 5 atm), and selectivity (>95% H2 gas) are achieved.

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

“…Photocatalytic H 2 evolution was rst reported in aqueous conditions, [37][38][39][40][41] with subsequent research demonstrating photoelectrochemical H 2 production and photocatalytic formic acid dehydrogenation. 42,43 In acetonitrile solvent, photolysis of [Cp*Ir(bpy)H] + (either alone or with added acids) also produces H 2 in high yield. 44 Complex [Cp*Ir(bpy)H] + undergoes H 2 evolution via a mechanism that is thus far unique amongst molecular hydrides.…”

Photocatalytic Dehydrogenation of Formic Acid on CdS Nanorods through Ni and Co Redox Mediation under Mild Conditions

“…The generated excitons (positive holes and electrons) can provide necessary energy to activate organic molecules for their conversion to products (FA does not absorb visible light). In spite of the recent advances in heterogeneous/homogeneous catalysis for the release of hydrogen from FA, examples of photocatalysis are scarce and show substantially lower activities . Only recently, CdS has been identified as a potential candidate material for photodecomposition of FA‐to‐H 2 owing to its low cost, suitable valence and conduction band positions, and high visible light activity .…”

Cyclometalated Iridium(III) Complexes with Ligand Effects on the Catalytic C–H Bond Activation of Toluene

“…Until now, the use of the Ir III complex for CHA has rarely been reported , . In other fields, Ir III complexes with bidentate ligands, including carbon–nitrogen (C ^ N), nitrogen–oxygen (N ^ O), and nitrogen–nitrogen (N ^ N) ligands, have been widely applied in organic light‐emitting diodes (OLEDs),, biolabeling,, dye‐sensitized solar cells (DSCs),, and the photoreaction of water;, most of the bidentate ligands wereeasily prepared and have a stable bond to the metal center. Also, the ligands have flexible character and can be modified.…”