Metal–Ligand Cooperation in Cp*Ir-Pyridylpyrrole Complexes: Rational Design and Catalytic Activity in Formic Acid Dehydrogenation and CO₂ Hydrogenation under Ambient Conditions

Abstract

Interconversion between CO 2 + H 2 and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO 2 hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)] n+ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H 2 O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H 2 O catalyzes FA dehydrogenation at 90 °C with a TOF max of 45 900 h −1 . Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO 2 hydrogenation in the presence of base to formate under atmospheric pressure (CO 2 /H 2 = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h −1 in aqueous solution and with a TOF value of 29 h −1 in a methanol/H 2 O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal−ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H−H bond cleavage in CO 2 hydrogenation.

Cyclic Amide-Anchored NHC-Based Cp*Ir Catalysts for Bidirectional Hydrogenation–Dehydrogenation with CO2/HCO2H Couple

“…Consequently, the formation of the iridium hydride species (Ir–H) from the iridium-formato intermediate (Ir–OCHO) via β-hydride elimination and CO 2 release, rather than the H 2 elimination by the reaction of the Ir–H intermediate with proton (H + or H 2 O) would govern the rate of the reaction of the present Ir - 1 -catalyzed reaction. The same trend has been reported by the groups of Himeda and Xiao, where generation of Ir–H was the rate-limiting step for the dehydrogenation of formic acid. ,, …”

“…With these couples, not only an effective carbon-neutral H 2 -based fuel system but also the carbon capture and utilization (CCU) protocol can be implemented at a time. − Obviously, to materialize such systems, among other factors, developing efficient catalysts for bidirectional CO 2 (or HCO 3 – ) hydrogenation and HCO 2 H (or HCO 2 – ) dehydrogenation is equally important. In the domain of homogeneous transition metal-based catalysis, although a great advancement has already been achieved for the individual hydrogenation − and dehydrogenation − reactions with two different catalysts, there are only a limited number of reports in the literature describing both the processes using a single catalyst. − Among these, some catalysts are relatively more active in hydrogenation than dehydrogenation, , while some others are the opposite. ,,, Single bidirectional transition metal catalysts, which are highly efficient in both hydrogenation and dehydrogenation, are so far designed with phosphine-based (PNP pincer) or N , N -donor-based ligands (Figure B).…”

Single-Site Iridium Picolinamide Catalyst Immobilized onto Silica for the Hydrogenation of CO2 and the Dehydrogenation of Formic Acid

“…This induction period might be due to the possible formation of poorly active (or inactive), out-of-cycle, dinuclear species. 101 As discussed above, most of the Ir sites in Ir_PicaSi_SiO 2 are well separated (≈1 Ir every 10 nm 2 ), making the associative process unlikely. After 6 h, the activity of Ir_PicaSi_SiO 2 decreased to 231 h –1 , whereas that of 3 decreased down to 418 h –1 .…”

“…30 min, which is absent in that of Ir_PicaSi_SiO 2 (Figure b). This induction period might be due to the possible formation of poorly active (or inactive), out-of-cycle, dinuclear species . As discussed above, most of the Ir sites in Ir_PicaSi_SiO 2 are well separated (≈1 Ir every 10 nm 2 ), making the associative process unlikely.…”

A ligand design strategy to enhance catalyst stability for efficient formic acid dehydrogenation

“…32,33 Hydrogen generated from aqueous FA could avoid the aforementioned problems. Those iridium-based complexes were found to be particularly efficient in this field, 34,35 including [Cp*Ir(DHBP) (H 2 O)] (DHBP = 4,4′-dihydroxy-2,2′-bipyridine; TOF = 14 000 h −1 ), 36 N^C cyclometallated iridium(III) complexes based on 2-aryl imidazoline ligands (TOF = 147 000 h −1 ), 37 [Cp*Ir(2,2′-bi-2-imidazoline)Cl]Cl (TOF = 487 500 h −1 ), 38 Ir complexes with picolinamide-based ligands [39][40][41] and pyridylidene-amine ligands containing a chelating phenolate (TOF = 280 000 h −1 ), 42 cis-mer-[IrH 2 Cl(mtppms) 3 ] (mtppms = monosulfonated triphenylphosphine) (TOF = 298 000 h −1 ), 43 Ir complexes with diphenylethylenediamine derivatives (TOF = 11 000 h −1 ) 44 etc. Through the investigation of the structureactivity relationship of these catalysts, researchers found that electron-donating moieties on the ligands, 36,[45][46][47][48] utilization of N-heterocyclic rings with higher electron density 49,50 and pendent-group effects [51][52][53] could improve the reaction rate for FADH.…”