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 5b 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 °C.