Iridium-based hydride transfer catalysts: from hydrogen storage to fine chemicals

Abstract

Selective hydrogen transfer remains a central research focus in catalysis: hydrogenation and dehydrogenation have central roles, both historical and contemporary, in all aspects of fuel, agricultural, pharmaceutical, and fine chemical synthesis. Our lab has been involved in this area by designing homogeneous catalysts for dehydrogenation and hydrogen transfer that fill needs ranging from on-demand hydrogen storage to fine chemical synthesis. A keen eye toward mechanism has enabled us to develop systems with excellent selectivity and longevity and demonstrate these in a diversity of high-value applications. Here we describe recent work from our lab in these areas that are linked by a central mechanistic trichotomy of catalyst initiation pathways that lead highly analogous precursors to a diversity of useful applications.

Highly Efficient Iridium‐Catalyzed Production of Hydrogen and Lactate from Glycerol: Rapid Hydrogen Evolution by Bimetallic Iridium Catalysts

“…A generally accepted glycerol dehydrogenation mechanism involving iridium catalysts is presented in Scheme 2. [ 28,29 ] The reaction begins with the dissociation of cyclooctadiene of catalysts 1 , 1' , 2 , and 2' , and CO of catalysts 3 , and 3' , followed by the coordination of an iridium complex with deprotonated glycerol to afford I . The dissociation of CO might not readily occur, resulting in no gas generation for three hours.…”

Conformational twisting of a formate-bridged diiridium complex enables catalytic formic acid dehydrogenation

“…This work is explained elsewhere. 19,20 Further, we are stunned that, despite extensive investigation of highly analogous complexes over many years, 34 precatalyst 1a was not identified for its reactivity with FA prior to our work. Thus, we set about to track the initiation of a family of complexes 1 to see if these convert to 4 or 5 under our FA dehydrogenation conditions and compare the results of these synthetic studies with reaction kinetics data for the dehydrogenation process.…”

“…Scheme 2 illustrates our working mechanistic model for initiation of our FA dehydrogenation catalyst, 1a . 19,20,22 Precatalyst 1a 21 is first transformed to 4a through the intermediacy of several species, including (allyl)iridium hydride complex 2a , 21 that are not observed under catalytic conditions. Iridium dimer 4a is one of a family of iridium species of the same iridium hydride topology that are known to occur in iridium-catalyzed hydrogenations.…”

“…The first is an (allyl)iridium hydride ( 2a ). 19,21,22 The second is a Cs-symmetric hydride-bridged dimer ( 4a ) that is analogous to many related structures characterized by Pfaltz. 23 In the presence of carboxylic acids, we find that 4a converts easily and selectively to structurally-novel iridium complex 5a , a binuclear, formate-bridged iridium species that is present at the beginning and end of our catalytic reaction.…”

“…1,2 Ongoing research on FA dehydrogenation has identified many outstanding catalytic systems, both heterogeneous 3–8 and homogeneous. 9–13 Our group is interested in catalytic hydride manipulation, 14–19 and we have recently reported the first homogeneous iridium catalytic system for the dehydrogenation of neat FA, 20 which presents a unique challenge due to the corrosive nature of the neat liquid. Catalytic systems based on iridium complex 1a have shown a useful average turnover frequency (TOF ca.…”