Exploiting click-chemistry: backbone post-functionalisation of homoleptic gold(I) 1,2,3-triazole-5-ylidene complexes

The synthesis of a homoleptic azide-functionalised Au(I) bis-1,2,3-triazole-5-ylidene complex is reported, starting from a backbone-modified 1,2,3-triazolium salt ligand precursor. The incorporated azide handle allows for a straightforward modification of the complex according to click-chemistry protocols without impacting the steric shielding around the metal center, demonstrating the superiority of the presented triazole ligand framework over imidazole based systems. Employing the SPAAC and the CuAAC reactions, post-modification of the complex is facilitated with two model substrates, while retaining very high antiproliferative activity (nanomolar range IC50 values) in A2780 and MCF-7 human cancer cells.

Anion-cation synergistic metal-free catalytic oxidative homocoupling of benzylamines by triazolium iodide salts

Triazolium iodide salts are excellent catalysts for the selective oxidative coupling of benzylamines to yield imines. This metal-free reaction proceeds in quantitative spectroscopic yields when run in refluxing 1,2-dichlorobenzene and open to the air. No catalytic activity was observed with related triazolium tetrafluoroborate salts. Variation of catalyst and reaction atmosphere provides mechanistic insights, and revealed dioxygen as the terminal oxidant and the iodine/iodide couple as key redox component in the catalytic dehydrogenation pathway. While molecular iodine is competent as a catalyst in its own right, the triazolium cation triples the reaction rate and reaches turnover frequencies up to 30 h-1, presumably through beneficial interactions of the electron-poor azolium π system and I2, which facilitate the electron transfer from the substrate to iodine and concomitant formation of I-. This acceleration is specific for triazolium cations and represents a hybrid anion/cation catalytic process as a simple and straightforward route towards imine products, with economic advantages over previously reported metal-based catalytic systems.

Attempted synthesis of a meta-metalated calix[4]arene

An evidence for the formation of a rare meta-metalated inherently chiral calix[4]arene is described. Our strategy involved using a mesoionic carbene to direct C–H activation, but proved to form an unexpectedly unstable intermediate that was identified through high-resolution mass spectrometry. On route to our target, a new optimized method to mononitrocalix[4]arenes was developed, including optimized and high yielding transformations to azide and 1,2,3-triazole derivatives which may have application in other areas of research.

Carbohydrate-functionalized N-heterocyclic carbene Ru(ii) complexes: synthesis, characterization and catalytic transfer hydrogenation activity

Triazolylidene NHCs decorated with a carbohydrate wingtip group were complexed to a ruthenium(ii) center. Deprotection of the carbohydrate in the metal complex affords a carbohydrate–NHC hybrid system for use as a transfer hydrogenation catalyst.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp*(C^N)L]ⁿ⁺ (Cp* = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H₂) or etherification (elimination of H₂O).

An Fe3O4@P4VP@FeCl3 core–shell heterogeneous catalyst for aerobic oxidation of alcohols and benzylic oxidation reaction

A novel Fe₃O₄@P4VP@FeCl₃ core–shell catalyst has been developed through coordination interaction between P4VP and FeCl₃, which was utilized in selective oxidation of alcohols using molecular oxygen as the oxidant.

Synthesis, Characterization, and Relative Study on the Catalytic Activity of Zinc Oxide Nanoparticles Doped MnCO3, –MnO2, and –Mn2O3 Nanocomposites for Aerial Oxidation of Alcohols

Zinc oxide nanoparticles doped manganese carbonate catalysts [X% ZnOx–MnCO3] (where X = 0–7) were prepared via a facile and straightforward coprecipitation procedure, which upon different calcination treatments yields different manganese oxides, that is, [X% ZnOx–MnO2] and [X% ZnOx–Mn2O3]. A comparative catalytic study was conducted to evaluate the catalytic efficiency between carbonates and oxides for the selective oxidation of secondary alcohols to corresponding ketones using molecular oxygen as a green oxidizing agent without using any additives or bases. The prepared catalysts were characterized by different techniques such as SEM, EDX, XRD, TEM, TGA, BET, and FTIR spectroscopy. The 1% ZnOx–MnCO3 calcined at 300°C exhibited the best catalytic performance and possessed highest surface area, suggesting that the calcination temperature and surface area play a significant role in the alcohol oxidation. The 1% ZnOx–MnCO3 catalyst exhibited superior catalytic performance and selectivity in the aerial oxidation of 1-phenylethanol, where 100% alcohol conversion and more than 99% product selectivity were obtained in only 5 min with superior specific activity (48 mmol·g−1·h−1) and 390.6 turnover frequency (TOF). The specific activity obtained is the highest so far (to the best of our knowledge) compared to the catalysts already reported in the literatures used for the oxidation of 1-phenylethanol. It was found that ZnOx nanoparticles play an essential role in enhancing the catalytic efficiency for the selective oxidation of alcohols. The scope of the oxidation process is extended to different types of alcohols. A variety of primary, benzylic, aliphatic, allylic, and heteroaromatic alcohols were selectively oxidized into their corresponding carbonyls with 100% convertibility without overoxidation to the carboxylic acids under base-free conditions.

Trinuclear complexes of palladium(ii) with chalcogenated N-heterocyclic carbenes: catalysis of selective nitrile–primary amide interconversion and Sonogashira coupling

The interconversion of nitriles to amides and Sonogashira coupling were catalyzed with Pd(ii) complexes (0.5–2 mol%).

Palladium(ii)-Acetylacetonato Complexes with Mesoionic Carbenes: Synthesis, Structures and Their Application in the Suzuki-Miyaura Cross Coupling Reaction

A series of novel palladium(ii) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(ii) acetylacetonate is described herein. All complexes were characterized by ¹H- and ¹³C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(ii) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.

Synthesis and catalytic applications of 1,2,3-triazolylidene gold(i) complexes in silver-free oxazoline syntheses and C-H bond activation

A series of novel 1,2,3-triazolylidene gold(i) chloride complexes have been synthesised and fully characterised. Silver-free methodologies for chloride ion abstraction of these complexes were evaluated for their potential as Au-based catalyst precursors. Using simple potassium salts or MeOTf as chloride scavengers produced metal complexes that catalyse both the regioselective synthesis of oxazolines and the C-H activation of benzene or styrene for carbene transfer from ethyl diazoacetate. These results indicate that Ag-free activation of 1,2,3-triazolylidene gold(i) chloride complexes is feasible for the generation of catalytically active Au triazolylidene species. However, silver-mediated activation imparts substantially higher catalytic activity in oxazoline synthesis.

RuII, IrIII and OsII mesoionic carbene complexes: efficient catalysts for transfer hydrogenation of selected functionalities

Pyridine- and pyrimidine-appended triazolylidene donors are used as ligands for the transfer hydrogenation of a series of functionalities.

Ru(II), Os(II), and Ir(III) complexes with chelating pyridyl-mesoionic carbene ligands: structural characterization and applications in transfer hydrogenation catalysis

Chelating ligands with one pyridine donor and one mesoionic carbene donor are fast establishing themselves as privileged ligands in homogeneous catalysis. The synthesis of several new Ir(III)-Cp*- and Os(II)-Cym complexes (Cp* = pentamethylcyclopentadienyl, Cym = p-cymene=4-isopropyl-toluene) derived from chelating pyridyltriazolylidenes where the additional pyridine donor was incorporated via the azide part of the triazole is presented. Furthermore, different 4-substituted phenylacetylene building blocks have been used to introduce electronic fine-tuning in the ligands. The ligands thus can be generally described as 4-(4-R-phenyl)-3-methyl-1-(pyridin-2-yl)-1H-1,2,3-triazol-5-ylidene (with R being H (L(1)), Me (L(2)), OMe (L(3)), CN (L(4)), CF3 (L(5)), Br (L(6)) or NO2 (L(7))). The corresponding complexes (Ir-1 to Ir-7 and Os-1 to Os-7) were characterized by standard spectroscopic methods, and the expected three-legged, piano-stool type coordination was unambiguously confirmed by X-ray diffraction analysis of selected compounds. Together with Ru(II) analogues previously reported by us, a total of 21 complexes were tested as (pre)catalysts for the transfer hydrogenation of carbonyl groups, showing a remarkable reactivity even at very low catalyst loadings. The electronic effects of the ligands as well as different substrates were investigated. Some mechanistic elucidations are also presented.

A novel iron(iii)-based heterogeneous catalyst for aqueous oxidation of alcohols using molecular oxygen

The first example of an iron(iii)-based heterogeneous catalyst for alcohol oxidation reactions in water employing molecular oxygen has been reported. Interestingly, the immobilized catalyst shows superior activity over its homogeneous counterpart.

Ruthenium hydroxycyclopentadienyl N-heterocyclic carbene complexes as transfer hydrogenation catalysts

Novel Ru–NHC complexes bearing cyclopentadienones/hydroxycyclopentadienyls are active and selective in transfer hydrogenation exploiting the cooperation of both classes of ligands.

Synthesis and catalytic alcohol oxidation and ketone transfer hydrogenation activity of donor-functionalized mesoionic triazolylidene ruthenium(II) complexes

We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that comprise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/cymene spectator ligands performed better than the solvento ruthenium complexes. The donor functionality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequencies as high as 1400 h(-1) for cyclohexanone transfer hydrogenation. The role of the donor group is discussed in mechanistic terms.

Cyclometalated mono- and dinuclear Ir(III) complexes with "click"-derived triazoles and mesoionic carbenes

Orthometalation at Ir(III) centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C-H activation sites, we present here mono- and dinuclear Ir(III) complexes with "click"-derived 1,2,3-triazole and 1,2,3-triazol-5-ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir(III) centers. Structural characterization of the complexes reveal a piano stool-type of coordination around the metal centers with the "click"-derived ligands bound either with C^N or C^C donor sets to the Ir(III) centers. Furthermore, whereas bond localization is observed within the 1,2,3-triazole ligands, a more delocalized situation is found in their 1,2,3-triazol-5-ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer-pure, dinuclear cyclometalated Ir(III) complexes with poly-mesoionic-carbene ligands.

Computational and experimental study on the electrocatalytic reduction of CO2 to CO by a new mononuclear ruthenium(ii) complex

A new mononuclear ruthenium(ii) complex, trans-[Ru(dmb)2(Cl)(EtOH)](PF6) (dmb = 4,4'-dimethyl-2,2'-bipyridine), has been prepared and characterized by elemental analysis, spectroscopic techniques and single crystal X-ray structure determination. The complex was studied as a precatalyst for the electrocatalytic reduction of CO2 to CO in an acetonitrile solution by cyclic voltammetry (CV). The catalytic mechanism was investigated by means of quantum chemical calculations to gain deeper insight into the process of CO2 reduction. The results suggest that the reaction proceeds in six steps initiating by the two sequential 1ē reductions at the dmb ligands followed by CO2 addition to give a metallocarboxylate intermediate. This intermediate undergoes further reduction and loses a CO molecule. The results reported in this paper are of great significance in providing theoretical insight into a class of electrocatalysts for reduction of CO2 to CO.

Sterically driven synthesis of ruthenium and ruthenium–silver N-heterocyclic carbene complexes

A sterically driven synthetic route from non-bulky silver NHC to novel Ru(NHC) complexes and from bulky Ag(NHC) to unprecedented heterobimetallic Ru–Ag(NHC) complexes is presented.

1,2,3-Triazolylidene ruthenium(ii)(η6-arene) complexes: synthesis, metallation and reactivity

Three bis(1,2,3-triazolylidene) silver(i) complexes were reacted with [RuCl₂(p-cymene)]₂ and the ruthenium complexes [RCH₂N₂(NMe)C₂Ph]RuCl₂(p-cymene) (R = C₆H₂Me₃4a1, C₆H₂iPr₃4b1, Ph 4c1) were isolated as major products with the minor C(sp²)–H activated products [RCH₂N₂(NMe)C₂C₆H₄]RuCl(p-cymene) (R = C₆H₂Me₃4a2, C₆H₂iPr₃4b2, Ph 4c2) and [(C₆H₄)CH₂N₂(NMe)C₂Ph]RuCl(p-cymene) (4c3).

Gold(I) and Palladium(II) Complexes of 1,3,4-Trisubstituted 1,2,3-Triazol-5-ylidene "Click" Carbenes: Systematic Study of the Electronic and Steric Influence on Catalytic Activity

The synthesis of a small family of six electronically and sterically modified 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene gold(I) chloride complexes is described. Additionally, the corresponding trans-[PdBr2(iPr2-bimy)(1,3,4-trisubstituted 1,2,3-triazol-5-ylidene)] complexes are also generated and used to examine the donor strength of the 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene ligands. All compounds have been characterized by 1H and 13C NMR and IR spectroscopy, high-resolution electrospray mass spectrometry (HR-ESI-MS), and elemental analysis. The molecular structures of four of the gold(I) and four of the palladium(II) complexes were determined using X-ray crystallography. Finally, it is demonstrated that these 1,2,3-triazol-5-ylidene gold(I) chloride complexes (Au(trz)Cl) are able to catalyze the cycloisomerization of 1,6-enynes, in high yield and regioselectivity, as well as the intermolecular direct etherification of allylic alcohols. Exploiting the Au(trz)Cl precatalysts allowed the etherification of allylic alcohols to be carried out under milder conditions, with better yield and regioselectivity than selected commercially available gold(I) catalysts.