Versatile Rh- and Ir-Based Catalysts for CO₂Hydrogenation, Formic Acid Dehydrogenation, and Transfer Hydrogenation of Quinolines

Abstract

Considering the interest in processes related to hydrogen storage such as CO2 hydrogenation and formic acid (FA) decomposition, we have synthesized a set of Ir, Rh, or Ru complexes to be tested as versatile precatalysts in these reactions. In relation with the formation of H2 from FA, the possible applicability of these complexes in the transfer hydrogenation (TH) of challenging substrates as quinoline derivatives using FA/formate as hydrogen donor has also been addressed. Bearing in mind the importance of secondary coordination sphere interactions, N,N′ ligands containing NH2 groups, coordinated or not to the metal center, were used. The general formula of the new complexes are [(p-cymene)­RuCl­(N,N′)]­X, X = Cl–, BF4 – and [Cp*MCl­(N,N′)]­Cl, M = Rh, Ir, where the N,N′ ligands are 8-aminoquinoline (HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline (L3). Some complexes are not active or catalyze only one process. However, the complexes [Cp*MCl­(HL1)]Cl with M = Rh, Ir are versatile catalysts that are active in hydrogenation of quinolines, FA decomposition, and also in CO2 hydrogenation with the iridium derivative being more active and robust. The CO2 hydrogenation takes place in mild conditions using only 5 bar of pressure of each gas (CO2 and H2). The behavior of some precatalysts in D2O and after the addition of 9 equiv of HCO2Na (pseudocatalytic conditions) has been studied in detail and mechanisms for the FA decomposition and the hydrogenation of CO2 have been proposed. For the Ru, Ir, or Rh complexes with ligand HL1, the amido species with the deprotonated ligand are observed. In the case of ruthenium, the formate complex is also detected. For the iridium derivative, both the amido intermediate and the hydrido species have been observed. This hydrido complex undergoes a process of umpolung D+↔ Ir–D. All in all, the results of this work reflect the active role of −NH2 in the transfer of H+.

Recent Developments in the Modelling of Heterogeneous Catalysts for CO2 Conversion to Chemicals

“…The most commonly used catalysts for the conversion of CO 2 into formic acid are precious metals Rh, [159,160] Ru, [161] Ir, [160,162] Pd [163] as they contain homogeneous ligands, such as phosphine ligands, [164] pincer ligands, [164] N-heterocyclic carbene ligands, [164] proton-responsive ligands. [165,166] However, the synthesis of these homogenous catalysts is complicated along with high energy consumption.…”

“…The first databases with experimental adsorption energies reported for the purposes of benchmarking DFT have been proposed by Hammer et al [188] which included 5 chemisorption energies for CO adsorption and Wellendorff et al [189] that [156] À 1.11 [157] -0.078 [149] À 1.109 [158] -100 À 1.47 [149] --À 1.45 [159] --À 1.08 [156] --À 0.92 [151,160] --110…”

“…The most commonly used catalysts for the conversion of CO 2 into formic acid are precious metals Rh, Ru, Ir, Pd as they contain homogeneous ligands, such as phosphine ligands, pincer ligands, N‐heterocyclic carbene ligands, proton‐responsive ligands …”

Copper-Organic Framework-Derived Porous Nanorods for Chemoselective Hydrogenation of Quinoline Compounds at an Aqueous/Oil Interface

“…13,14 Unfortunately, even with high conversion and selectivity, the difficult-toisolate catalysts, ligands, and additives in mixed systems will limit their industrial applications to a large scale. 15,16 In recent years, heterogeneous catalysis, especially the catalysts loaded with precious metals, has attracted more and more attention, such as heterogeneous ones based on Au, Pd, Pt, Ru, Ir, and Rh. In the quinoline hydrogenation, the N-heterocyclic ring is easily coordinated with the metal nanoparticles, which makes the catalyst difficult to be recycled.…”

A Water/Toluene Biphasic Medium Improves Yields and Deuterium Incorporation into Alcohols in the Transfer Hydrogenation of Aldehydes

“…We tested other halide compounds that could behave like 1, but our efforts so far have been unsuccessful. For example, neutral [58] or cationic [59] [Cp*IrCl (N N')] n + (n = 0, 1) species are able of generating hydride species in the aqueous reaction medium, but these species do not exchange deuterium for D 2 O or do so at an insufficient rate to behave as 1. In principle, according to the proposal of Himeda and our own experience, [53][54][55] the generation of the metal-D group must be fast enough to compete kinetically with the TH process to allow a high level of deuterium incorporation.…”