Attachment of a Hydrogen‐Bonding Carboxylate Side Chain to an [FeFe]‐Hydrogenase Model Complex: Influence on the Catalytic Mechanism

Azapropanedithiolate (adt)‐bridged model complexes of [FeFe]‐hydrogenase bearing a carboxylic acid functionality have been designed with the aim of decreasing the potential for reduction of protons to hydrogen. Protonation of the bisphosphine complexes 4 – 6 has been studied by in situ IR and NMR spectroscopy, which revealed that protonation with triflic acid most likely takes place first at the N‐bridge for complex 4 but at the FeFe bond for complexes 5 and 6 . Using an excess of acid, the diprotonated species could also be observed, but none of the protonated species was sufficiently stable to be isolated in a pure state. Electrochemical studies have provided an insight into the catalytic mechanisms under strongly acidic conditions, and have also shown that complexes 3 and 6 are electro‐active in aqueous solution even in the absence of acid, presumably due to hydrogen bonding. Hydrogen evolution, driven by visible light, has been observed for three‐component systems consisting of [Ru(bpy) 3 ] 2+ , complex 1 , 2 , or 3 , and ascorbic acid in CH 3 CN/D 2 O solution by on‐line mass spectrometry.