DFT Study on the Mechanism of Hydrogen Storage Based on the Formate-Bicarbonate Equilibrium Catalyzed by an Ir-NHC Complex: An Elusive Intramolecular C–H Activation

Introducing Ion Mobility Mass Spectrometry to Identify Site-Selective C-H Bond Activation in N-Heterocyclic Carbene Metal Complexes

The activation of C-H bonds in a selective manner still constitutes a major challenge from a synthetic point of view; thus, it remains an active area of fundamental and applied research. Herein, we introduce ion mobility spectrometry mass spectrometry-based (IM-MS) approaches to uncover site-selective C-H bond activation in a series of metal complexes of general formula [(NHC)LMCl]⁺ (NHC = N-heterocyclic carbene; L = pentamethylcyclopentadiene (Cp*) or p-cymene; M = Pd, Ru, and Ir). The C-H bond activation at the N-bound groups of the NHC ligand is promoted upon collision induced dissociation (CID). The identification of the resulting [(NHC-H)LM]⁺ isomers relies on the distinctive topology that such cyclometalated isomers adopt upon site-selective C-H bond activation. Such topological differences can be reliably evidenced as different mobility peaks in their respective CID-IM mass spectra. Alternative isomers are also identified via dehydrogenation at the Cp*/p-cymene (L) ligands to afford [(NHC)(L-H)M]⁺. The fragmentation of the ion mobility-resolved peaks is also investigated by CID-IM-CID. It enables the assignment of mobility peaks to the specific isomers formed from C(sp²)-H or C(sp³)-H bond activation and distinguishes them from the Cp*/p-cymene (L) dehydrogenation isomers. The conformational change of the NHC ligands upon C-H bond activation, concomitant with cyclometalation, is also discussed on the basis of the estimated collision cross section (CCS). A unique conformation change of the pyrene-tagged NHC members is identified that involves the reorientation of the NHC ring accompanied by a folding of the pyrene moiety.

Immobilization of an Iridium(I)-NHC-Phosphine Catalyst for Hydrogenation Reactions under Batch and Flow Conditions

Na2[Ir(cod)(emim)(mtppts)] (1) with high catalytic activity in various organic- and aqueous-phase hydrogenation reactions was immobilized on several types of commercially available ion-exchange supports. The resulting heterogeneous catalyst was investigated in batch reactions and in an H-Cube flow reactor in the hydrogenation of phenylacetylene, diphenylacetylene, 1-hexyne, and benzylideneacetone. Under proper conditions, the catalyst was highly selective in the hydrogenation of alkynes to alkenes, and demonstrated excellent selectivity in C=C over C=O hydrogenation; furthermore, it displayed remarkable stability. Activity of 1 in hydrogenation of levulinic acid to γ-valerolactone was also assessed.

Formate-Bicarbonate Cycle as a Vehicle for Hydrogen and Energy Storage

In recent years, hydrogen has been considered a promising energy carrier for a sustainable energy economy in the future. An easy solution for the safer storage of hydrogen is challenging and efficient methods are still being explored in this direction. Despite having some progress in this area, no cost-effective and easily applicable solutions that fulfill the requirements of industry are yet to be claimed. Currently, the storage of hydrogen is largely limited to high-pressure compression and liquefaction or in the form of metal hydrides. Formic acid is a good source of hydrogen that also generates CO₂ along with hydrogen on decomposition. Moreover, the hydrogenation of CO₂ is thermodynamically unfavorable and requires high energy input. Alkali metal formates are alternative mild and noncorrosive sources of hydrogen. On decomposition, these metal formates release hydrogen and generate bicarbonates. The generated bicarbonates can be catalytically charged back to alkali formates under optimized hydrogen pressure. Hence, the formate-bicarbonate-based systems being carbon neutral at ambient condition has certain advantages over formic acid. The formate-bicarbonate cycle can be considered as a vehicle for hydrogen and energy storage. The whole process is carbon-neutral, reversible, and sustainable. This Review emphasizes the various catalytic systems employed for reversible formate-bicarbonate conversion. Moreover, a mechanistic investigation, the effect of temperature, pH, kinetics of reversible formate-bicarbonate conversion, and new insights in the field are also discussed in detail.