Enhanced Hydrogen-Transfer Catalytic Activity of Iridium N-Heterocyclic Carbenes by Covalent Attachment on Carbon Nanotubes

Abstract

Oxidized multiwall carbon nanotubes (CNT) were covalently modified with appropriate hydroxylending imidazolium salts using their carboxylic acid groups. Characterization of the imidazoliummodified samples through typical solid characterization techniques, such as TGA or XPS, allows for the determination of 16 wt.% in CNT-1 and 31 wt.% in CNT-2 as the amount of the imidazolic fragments in the carbon nanotubes. The imidazolium-functionalized materials were used to prepare nanohybrid materials containing iridium N-heterocyclic carbene (NHC) type organometallic complexes with efficiencies as high as 95 %. The nanotube-supported iridium-NHC materials were active in the heterogeneous iridium-catalyzed hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source. The iridium hybrid materials are more efficient than related homogeneous catalysts based on acetoxy-functionalized Ir-NHC complexes with initial TOFs up to 5550 h -1 . A good recyclability of the catalysts, without any loss of activity, and stability on air was observed.

The kinetics and mechanism of the organo-iridium-catalysed enantioselective reduction of imines

“…This lack of racemisation is not surprising as formate is a better hydride donor than the amine and the acidic nature of the medium means the amine is in its protonated form and unlikely to readily bind to the iridium catalyst. In dichloromethane the ee decreases to below zero, suggesting increased selectivity for the other enantiomer of the product amines (13) and (14) as the reaction progresses, rather than racemisation causing the decrease in ee. Finally, the large excess of formic acid and absence of a hydrogen acceptor indicates that the resting state of the catalyst is likely to be the 18-electron iridium hydride, (3), rather than the 16-electron species, (15), required for dehydrogenation.…”

“…The zero-order rate of formation of the (S)-enantiomer changes proportionally with catalyst concentration (Fig.4) The catalytic rate constants for the transfer hydrogenation of imines (11) or (12) to chiral amines (13) or (14), respectively, catalysed by the iridium complex 3 in acetonitrile and dichloromethane are given in Table 1. The dimethoxy imine (12) is at least 10-fold more basic than the unsubstituted imine (11) 32 and yet there is less than a twofold difference in reactivity for the formation of both enantiomers ( Table 1).…”

“…2,3,4 The first homogeneous catalytic systems appeared in the late 1960s and were based on iridium compounds 5,6 which were followed by the introduction of the versatile pentamethylcyclopentadienyl anion iridium and rhodium (Cp*Ir and Cp*Rh) catalyst precursors for these transformations 7,8,9,10 and their variants. 11,12,13 Although the highly selective asymmetric reduction of alkenes and ketones has been accomplished using chiral rhodium and ruthenium catalysts, 14 the hydrogenation of imines using similar catalysts has proved much less successful. 15 Furthermore, most enantioselectivities obtained are moderate and require a high catalyst loading.…”

Coordination chemistry on carbon surfaces

“…Alvarez and Jimenez also reported the attachment of an iridium-NHC complex on CNT via an ester bond although in two steps: (i) reaction of COCl-modified CNT with hydroxyl-ending imidazolium salts and (ii) in situ carbene-complex formation with [Ir(-OMe)(cod)] 2 as metal precursor able to deprotonate the imidazolium salts (Table 20, entry 7) [356].…”

One-dimensional nitrogen-containing carbon nanostructures

“…Oxidized MWCNTs were covalently modified with appropriate hydroxyl-ending imidazolium salts using their carboxylic acid groups [398]. MWCNTs were recently functionalized by histidine, an imidazole-based amino acid [399], based on the reported procedure for covalent modification of MWCNTs with imidazolium-based ionic liquids by [400].…”