Selective Oxidation of H₂ and CO by NiIr Catalyst in Aqueous Solution: A DFT Mechanistic Study

Abstract

One of the challenges in utilizing hydrogen gas (H 2 ) as a sustainable fossil fuel alternative is the inhibition of H 2 oxidation by carbon monoxide (CO), which is involved in the industrial production of H 2 sources. To solve this problem, a catalyst that selectively oxidizes either CO or H 2 or one that co-oxidizes H 2 and CO is needed. Recently, a NiIr catalyst [Ni II Cl(X)Ir III Cl(η 5 -C 5 Me 5 )], (X = N,N′dimethyl-3,7-diazanonane-1,9-dithiolate), which efficiently and selectively oxidizes either H 2 or CO depending on the pH, has been developed (Angew. Chem. Int. Ed. 2017, 56, 9723−9726). In the present work, density functional theory (DFT) calculations are employed to elucidate the pH-dependent reaction mechanisms of H 2 and CO oxidation catalyzed by this NiIr catalyst. During H 2 oxidation, our calculations suggest that dihydrogen binds to the Ir center and generates an Ir(III)−dihydrogen complex, followed by subsequent isomerization to an Ir(V)−dihydride species. Then, a proton is abstracted by a buffer base, CH 3 COO − , resulting in the formation of a hydride complex. The catalytic cycle completes with electron transfer from the hydride complex to a protonated 2,6-dichlorobenzeneindophenol (DCIP) and a proton transfer from the oxidized hydride complex to a buffer base. The CO oxidation mechanism involves three distinct steps, i.e., (1) formation of a metal carbonyl complex, (2) formation of a metallocarboxylic acid, and (3) conversion of the metallocarboxylic acid to a hydride complex. The formation of the metallocarboxylic acid involves nucleophilic attack of OH − to the carbonyl-C followed by a large structural change with concomitant cleavage of the Ir−S bond and rotation of the COOH group along the NiIr axis. During the conversion of the metallocarboxylic acid to the hydride complex, intramolecular proton transfer followed by removal of CO 2 leads to the formation of the hydride complexes. In addition, the barrier heights for the binding of small molecules (H 2 , OH − , H 2 O, and CO) to Ir were calculated, and the results indicated that dissociation from Ir is a faster process than the binding of H 2 O and H 2 . These calculations indicate that H 2 oxidation is inhibited by CO and OH − and thus prefers acidic conditions. In contrast, the CO oxidation reactions occur more favorably under basic conditions, as the formation of the metallocarboxylic acid involves OH − attack to a carbonyl-C and the binding of OH − to Ni largely stabilizes the triplet spin state of the complex. Taken together, these calculations provide a rationale for the experimentally observed pH-dependent, selective oxidations of H 2 and CO.

Multiwavelets applied to metal–ligand interactions: Energies free from basis set errors

“…We have build our test set to include ligands of various size, many of which are common incoming ligands in metal-catalyzed reactions (such as H2, CO, alkenes, methanol). 3,4,5,6,7,8,9,10 Several features are observed from our computed results (Figure 2, see also SI, Figure S1 to S4). Firstly, the magnitude of the BSSE is largest for DZ basis sets, with an average value of 9.92 kcal/mol for the 26 kcal/mol).…”

“…1,2 Many of the studied reactions involve metal complexes that throughout the course of the reaction bind or lose a ligand, for example, there may be incoming substrates such as alkenes or hydrogen (H2), or leaving ligands such as solvent or product molecules (Scheme 1). 3,4,5,6,7,8,9,10 The difficulty of contemporary DFT functionals to accurately compute metal-ligand interactions has been highlighted in the literature. 11,12,13,14,15,16,17,18,19,20,21,22 Many of the reported studies focus on the performance of different DFT functionals and the importance of including dispersion corrections in the computed energies.…”

H2activation by hydrogenase-inspired NiFe catalyst using frustrated Lewis pair: effect of buffer and halide ion in the heterolytic H–H bond cleavage

“…The coordination of Cl − or OH − stabilises the NiIr complex depending on the spin multiplicity, as subsequently confirmed by DFT calculation. 51 The coordination of OH − is unlikely because the H 2 activation reaction by the present NiFe complex was conducted at an almost neutral pH. Also, the dissociated Cl − ligand rebinding in complex 1 is less likely due to the diluteness.…”

Multiwavelets Applied to Metal-Ligand Interactions: Energies Free from Basis Set Errors

“…1,2 Many of the studied reactions involve metal complexes that throughout the course of the reaction bind or lose a ligand, for example, there may be incoming substrates such as alkenes or hydrogen (H2), or leaving ligands such as solvent or product molecules (Scheme 1). 3,4,5,6,7,8,9,10 The difficulty of contemporary DFT functionals to accurately compute metal-ligand interactions has been highlighted in the literature. 11,12,13,14,15,16,17,18,19,20,21,22 Many of the reported studies focus on the performance of different DFT functionals and the importance of including dispersion corrections in the computed energies.…”