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

Highly Active Single-Atom Nanozymes with High-Loading Iridium for Sensitive Detection of Pesticides

Single-atom nanozymes (SAzymes) are novel mimic-enzyme materials with atomically doped active sites. They play a pivotal role in the field of nanozymes because of their excellent catalytic activities, high utilization efficiency of the metal atoms, and simple model of active sites. Herein, the peroxidase (POD)-like SAzymes with high-loading iridium (Ir) (5.31%) on graphene oxide (GO) nanosheets [Ir(III)/GO] were prepared through a coordination reaction between the Ir(III) complex and the oxygen-containing groups in GO. The preparation strategy avoids nitrogen doping and pyrolysis procedures which are the usually used strategies to improve the GO-based enzyme mimic activity. Ascribed to the highly active Ir atoms, Ir(III)/GO SAzymes demonstrate outstanding POD-like activity without the oxidase-like activity. In advantage of the excellent POD-like activity, a simple and sensitive colorimetric pesticide detection platform is established. The developed sensing platform offers an excellent "switch-on" pirimicarb (PIB) detection in the linear range of 10-300 nM with a limit of detection (LOD) of 2.81 nM. Moreover, the detection platform was fabricated into a portable test kit, which is composed of a test swab and sample processing tube. In the aid of a color-reading APP, the test kit can detect PIB with the LOD of 3.31 nM. It is astonishing to get this excellent detection sensitivity just using the simple colorimetric strategy. This work not only provides a novel strategy to synthesize Ir-based SAzymes but also exhibits the super capability of Ir(III)/GO in the biosensing field.

Selective Oxidation of Glycerol via Acceptorless Dehydrogenation Driven by Ir(I)-NHC Catalysts

Iridium(I) compounds featuring bridge-functionalized bis-NHC ligands (NHC = N-heterocyclic carbene), [Ir(cod)(bis-NHC)] and [Ir(CO)₂(bis-NHC)], have been prepared from the appropriate carboxylate- or hydroxy-functionalized bis-imidazolium salts. The related complexes [Ir(cod)(NHC)₂]⁺ and [IrCl(cod)(NHC)(cod)] have been synthesized from a 3-hydroxypropyl functionalized imidazolium salt. These complexes have been shown to be robust catalysts in the oxidative dehydrogenation of glycerol to lactate (LA) with dihydrogen release. High activity and selectivity to LA were achieved in an open system under low catalyst loadings using KOH as a base. The hydroxy-functionalized bis-NHC catalysts are much more active than both the carboxylate-functionalized ones and the unbridged bis-NHC iridium(I) catalyst with hydroxyalkyl-functionalized NHC ligands. In general, carbonyl complexes are more active than the related 1,5-cyclooctadiene ones. The catalyst [Ir(CO)₂{(MeImCH₂)₂CHOH}]Br exhibits the highest productivity affording TONs to LA up to 15,000 at very low catalyst loadings.

Recent Development in the Synthesis and Catalytic Application of Iridacycles

Cyclometallated complexes are well-known and have found many applications. This article provides a short review on the progress made in the synthesis and application to catalysis of cyclometallated half-sandwich Cp*Ir(III) complexes (Cp*: pentamethylcyclopentadienyl) since 2017. Covered in the review are iridacycles featuring conventional C,N chelates and less common metallocene and carbene-derived C,N and C,C ligands. This is followed by an overview of the studies of their applications in catalysis ranging from asymmetric hydrogenation, transfer hydrogenation, hydrosilylation to dehydrogenation.

Prototype Atomically Dispersed Supported Metal Catalysts: Iridium and Platinum

When metal nanoparticles on supports are made smaller and smaller-to the limit of atomic dispersion-they become cationic and take on new catalytic properties that are only recently being discovered. The synthesis of these materials is reviewed, including their structure characterization-especially by atomic-resolution electron microscopy and X-ray absorption and infrared spectroscopies-and relationships between structure and catalyst performance, for reactions including hydrogenations, oxidations, and the water gas shift. Structure determination is challenging because of the intrinsic nonuniformity of the support surfaces-and therefore the structures on them-but fundamental understanding has advanced rapidly, benefiting from nearly uniform catalysts consisting of metals on well-defined-crystalline-supports and their characterization by spectroscopy and microscopy. Recent advances in atomic-resolution electron microscopy have spurred the field, providing stunning images and deep insights into structure. The iridium catalysts have typically been made from organoiridium precursors, opening the way to understanding and control of the metal-support bonding and ligands on the metal, including catalytic reaction intermediates. Platinum catalysts are usually made with less precision, from salt precursors, but they catalyze a wider array of reactions than the iridium, typically being stable at higher temperatures and seemingly offering rich prospect for discovery of new catalysts.

Multilayer graphene functionalized through thermal 1,3-dipolar cycloadditions with imino esters: a versatile platform for supported ligands in catalysis

Multilayer graphene (MLG), obtained by mild sonication of graphite in NMP or pyridine, was fully characterized via atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoemission spectroscopy (XPS). Then, it was functionalized via 1,3-dipolar cycloaddition with azomethine ylides generated by thermal 1,2-prototropy of various imino esters. The resulting MLG, containing substituted proline-based amine functions, was characterized by XPS and it showed high nitrogen loading, ranging from 0.6 to 4.2 at% depending on the imino ester used. Among these functionalized MLGs a probe sample was subjected to ester hydrolysis and used as a heterogeneous N,O-chelating ligand to coordinate iridium atomic centers. This supported complex was also characterized by XPS and its catalytic activity was tested in the hydrogen transfer reduction of acetophenone, obtaining up to 85% yield. Furthermore, this catalyst could be recycled up to four times.

A leap forward in iridium-NHC catalysis: new horizons and mechanistic insights

This review summarises the most recent advances in Ir-NHC catalysis while revisiting all the classical reactions in which this type of catalyst has proved to be active. The influence of the ligand system and, in particular, the impact of the NHC ligand on the activity and selectivity of the reaction have been analysed, accompanied by an examination of the great variety of catalytic cycles hitherto reported. The reaction mechanisms so far proposed are described and commented on for each individual process. Moreover, some general considerations that attempt to explain the influence of the NHC from a mechanistic viewpoint are presented at the end of the review. The first sections are dedicated to the most widely explored reactions that use Ir-NHCs, i.e., hydrogenation and transfer hydrogenation, for which a general overview that tries to compile all the Ir-NHC catalysts hitherto reported for these processes is provided. The next sections deal with hydrogen borrowing, hydrosilylation, water splitting, dehydrogenation (of alcohols, alkanes, aminoboranes and formic acid), hydrogen isotope exchange (HIE), signal amplification by reversible exchange and C-H bond functionalisation (silylation and borylation). The last section compiles a series of reactions somewhat less explored for Ir-NHC catalysts that include the hydroalkynylation of imines, hydroamination, diboration of olefins, hydrolysis and methanolysis of silanes, arylation of aldehydes with boronic acids, addition of aroyl chlorides to alkynes, visible light driven reactions, isomerisation of alkenes, asymmetric intramolecular allylic amination and reactions that employ heterometallic catalysts containing at least one Ir-NHC unit.

Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity

A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*M^IIICl{(MeIm)₂CHCOO}] and [M^I(diene){(MeIm)₂CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)₂CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κ²C,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*M^III complexes, whereas in the M^I(diene) complexes it is at the flagpole position of the metallacycle. The complexes [Rh^I(diene){(MeIm)₂CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, bowsprit and flagpole, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of ∼17 kcal·mol^-1 was determined by two-dimensional exchange spectroscopy NMR measurements for [Rh^I(cod){(MeIm)₂CHCOO}]. Reaction of zwitterionic Cp*M^III complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*M^IIICl{(MeIm)₂CHCOOMe}]⁺ or [Cp*M^IIICl{(MeIm)₂CHCOOH}]⁺ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [M^I(diene){(MeIm)₂CHCOOMe}]⁺ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic M^I(diene) complexes with methyl triflate. In contrast, reaction of [Ir^I(cod){(MeIm)₂CHCOO}] with HBF₄·Et₂O (Et = ethyl), CH₃OTf, CH₃I, or I₂ gives cationic iridium(III) octahedral complexes [Ir^IIIX(cod){(MeIm)₂CHCOO}]⁺ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κ²C,C' to κ³C,C',O coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions.

Polymeric Ruthenium Porphyrin-Functionalized Carbon Nanotubes and Graphene for Levulinic Ester Transformations into γ-Valerolactone and Pyrrolidone Derivatives

Polymeric ruthenium porphyrin-functionalized carbon nanotubes (Ru-PP/CNTs) were prepared by the metallation of polymeric porphyrin-functionalized carbon nanotubes (PP/CNTs) with Ru₃(CO)₁₂, whereas PP/CNTs were obtained by the condensation of terephthaldehyde and pyrrole in the presence of CNTs. The Ru-PP/CNTs have a thin layer of highly cross-linked polymeric ruthenium porphyrin coating over the CNT surface via strong π-π stacking interactions, thus showing a bilayered structure with an amorphous polymeric outer surface and an internal CNT core. Polymeric ruthenium porphyrin-functionalized reduced graphene oxide (Ru-PP/RGO) was prepared with a synthetic procedure similar to Ru-PP/CNTs, with RGO as the internal core. Both Ru-PP/CNTs and Ru-PP/RGO showed excellent catalytic performance toward hydrogenation of biomass-related ethyl levulinate (EL) to γ-valerolactone (GVL) with Ru-centered porphyrin units as the catalytic active species. Under optimized reaction conditions, a GVL yield higher than 99% with a complete conversion of EL was observed over both Ru-PP/CNTs and Ru-PP/RGO. In addition to GVL preparation, the versatile Ru-PP/CNTs can efficiently promote reductive amination of EL with various amines for the synthesis of pyrrolidone derivatives, with the corresponding yields ranging from 96.3 to 88.7%. Moreover, the composite materials of both Ru-PP/CNTs and Ru-PP/RGO behave as heterogeneous catalysts in the reaction system and can be easily reused.

Selective cobalt nanoparticles for catalytic transfer hydrogenation of N-heteroarenes

Nitrogen modified cobalt nanoparticles are easily prepared from melamine or melamine resins. The resulting catalysts show excellent selectivity for transfer hydrogenation of N-heteroarenes.

Nitrogen modified cobalt catalysts supported on carbon were prepared by pyrolysis of the mixture generated from cobalt(ii) acetate in aqueous solution of melamine or waste melamine resins, which are widely used as industrial polymers. The obtained nanostructured materials catalyze the transfer hydrogenation of N-heteroarenes with formic acid in the absence of base. The optimal Co/Melamine-2@C-700 catalyst exhibits high activity and selectivity for the dehydrogenation of formic acid into molecular hydrogen and carbon dioxide and allows for the reduction of diverse N-heteroarenes including substrates featuring sensitive functional groups.

Base free transfer hydrogenation using a covalent triazine framework based catalyst

Isomerisation of allylic alcohols to saturated ketones can be efficiently catalysed by a heterogeneous molecular system resulting from Ir^IIICp* anchoring to a covalent triazine framework.

Enhancing the hydrogen transfer catalytic activity of hybrid carbon nanotube-based NHC–iridium catalysts by increasing the oxidation degree of the nanosupport

Oxidation of the support increase the HT catalytic activity.

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

The enantiomeric excess (ee) for the organo-iridium catalysed reduction of imines decreases during the reaction because the rate of formation of the (R)-product amine follows first-order kinetics whereas that for the (S)-enantiomer is zero-order.

N-Heterocyclic olefins as ancillary ligands in catalysis: a study of their behaviour in transfer hydrogenation reactions

Ir(i) complexes featuring an N-heterocyclic olefin ligand (NHO) have been tested in the transfer hydrogenation reaction; this representing the first example of the use of NHOs as ancillary ligands in catalysis.

Magnetic NiO nanoparticles confined within open ends MWCNTs: a novel and highly active catalyst for hydrogenation and synthesis of imines

The novel catalyst is highly active for hydrogenation and synthesis of imines.

Catalytic hydrodechlorination of benzyl chloride promoted by Rh-N-heterocyclic carbene catalysts

The rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2 )3 Si(OiPr)3 )] [COD=cyclooctadiene; R=2,6-diisopropylphenyl (1 a); n-butyl (1 b)] are effective catalyst precursors for the homogeneous hydrodechlorination of benzyl chloride using HSiEt3 as hydrogen source. This reaction is selective to the formation of toluene. However, in presence of a stoichiometric amount of potassium tert-butoxide (KtBuO) the formation of mixtures containing toluene together with 17-19 mol % of the C--C homocoupling product, namely PhCH2 CH2 Ph, is observed. A mechanism proposal based on experimental insights and theoretical calculations at the DFT level that allows explanation of the experimental findings is included. Moreover, the heterogeneous catalytic system based on catalyst 1 a supported on MCM-41 has been demonstrated to be effective for the solvent-free hydrodechlorination of benzyl chloride using HSiEt3 and HSiMe(OSiMe3 )2 .

The advantages of covalently attaching organometallic catalysts to a carbon black support: recyclable Rh(i) complexes that deliver enhanced conversion and product selectivity

A bidentate Rh(i) coordination complex was covalently anchored to a carbon black support. The resultant hybrid catalyst was found to be active for both hydroamination and dihydroalkoxylation reactions and was readily recycled.

Carbon nanotube catalysts: recent advances in synthesis, characterization and applications

Carbon nanotubes are promising materials for various applications.