A Bifunctional Iridium Catalyst Modified for Persistent Hydrogen Generation from Formic Acid: Understanding Deactivation via Cyclometalation of a 1,2-Diphenylethylenediamine Motif

Thermal degradation of a bifunctional Ir complex with a 1,2-diphenylethylenediamine (DPEN) framework was investigated, which is relevant to catalyst deactivation in the acceptorless dehydrogenation of formic acid. The well-defined hydridoiridium complex 1b, derived from N-triflyl-1,2-diphenylethylenediamine (TfDPEN), proved to be solely transformed at the reflux temperature of 1,2-dimethoxyethane (DME) into two iridacycles (2 and 3) via C-H bond cleavage at the ortho carbon atoms of the phenyl substituents on the diamine backbone. These products were successfully isolated and characterized by NMR, elemental analysis, and X-ray crystallography. The iridacycle formation was significantly enhanced in the presence of water, possibly due to facile deprotonative orthometalation via a hydroxidoiridium intermediate. To prevent the deactivation process caused by the cyclometalation of the DPEN moiety, a hydridoiridium complex (5b) without phenyl substituents was synthesized from N-triflylethylenediamine (TfEN). The modified complex 513 showed a pronounced ability to catalyze hydrogen evolution from formic acid in a 1/1 mixed solvent of water and DME even in the absence of base additives. The initial rate was maintained for a longer time relative to 1b, and thus formic acid was mostly converted within 80 min under the conditions of a HCOOH/5b ratio of 15900 at 60 degrees C.