Mesoionic Carbene Research Articles

Axially chiral bis-1,2,3-Triazol-4-ylidene–Ag(I)-MIC and, bis-Au(I)-MIC complexes of (R)-BINOL and (-)-Menthol scaffold: Synthesis, structure, and characterizations

Herein, we report the novel axially chiral bis-Ag(I)-MIC and, bis-Au(I)-MIC complexes bearing axially chiral bis-1,2,3-triazolium-derived mesoionic carbene (tz-MIC) ligands were synthesized. The enantiopure R-BINOL was employed as a basic unit to synthesize a axially chiral bis-1,2,3-triazolium-derived mesoionic carbene (tz-MIC) ligands (1–2)a. In particular, the axially chiral bis-1,2,3-triazolium-derived mesoionic carbene (tz-MIC) ligands (1–2)a, were obtained from the reaction of corresponding bis-1,2,3-triazole ligand precursor with methyl and ethyl iodide in 82−90% yields. Novel axially chiral bis-Ag(I)-MIC complexes (1 − 2)b, were prepared by the treatment of corresponding axially chiral bis-1,2,3-triazolium-derived mesoionic carbene (tz-MIC) iodide salts, (1 − 2)a, with Ag2O via in-situ deprotonation method in 69−86% yields. Novel axially chiral bis-Au(I)-MIC complex-2c was synthesized from their respective novel axially chiral bis-Ag(I)-MIC complex-2b, using transmetallation reaction with (SMe2)AuCl in 70% yield. All these novel axially chiral bis-Ag(I)-MIC and bis-Au(I)-MIC complexes were isolated for the first time and structurally characterized by 1H NMR and 13C{1H}-NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, elemental analysis, specific optical rotation and, single crystal X-ray crystallography.

Reactivity of an Unprotected Mesoionic N-Heterocyclic Olefin

We report a novel mesoionic N-heterocyclic olefin (mNHO) derived from a 1,2,3-triazolium salt. With the carbenic position unprotected, the mNHO can tautomerize into the mesoionic carbene (MIC) form...

Chiral tricyclic triazolooxazine derived mesoionic carbene (MIC)-Pd(II) complexes of cyclohexene oxide scaffold: Synthesis, structure, and characterizations

We herein report the well-defined two novel chiral palladium(II) complexes 1(b−c), of the chiral N-fused tricyclic triazolooxazine derived mesoionic carbene (tz-MIC) ligand 1a. The chiral tricyclic PEPPSI type complex (pyridine enhanced pre-catalyst preparation stabilization and initiation) namely, trans-[tz-MIC)-PdI2(pyridine)] (1b), and the chiral tricyclic bis(tz-MIC)-palladium complex namely, cis-[tz-MIC)2-PdCl2] (1c). The chiral tricyclic trans-[tz-MIC)-PdI2(pyridine)] PEPPSI type complex (pyridine enhanced pre-catalyst preparation stabilization and initiation) (1b) was directly obtained from their respective chiral tricyclic triazolooxazinium iodide salt (1a), by treatment with PdCl2 and K2CO3 in pyridine in 77% yield. The chiral tricyclic triazolooxazinium iodide salt (1a), was first converted to its in-situ silver analogue by reaction with Ag2O and then subsequently upon treatment with (COD)PdCl2 to produce the chiral tricyclic (tz-MIC)2PdCl2 type palladium complex (1c) in 84% yield. All these palladium complexes were isolated for the first time and structurally characterized by 1H NMR and 13C{1H}-NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray crystallography.

Synthesis, stability, and reactivity of mesoionic carbene iridium dihydride complexes

Pyridyl-triazolylidene ligands with variable donor properties were used as tunable ligands at a dihydride iridium (III) center. The straightforward synthesis of this type of ligand allows for an easy incorporation of electron-donating substituents in different positions of the pyridine ring or different functional groups such as esters, alkoxy, or aliphatic chains on the C4 position of the triazole heterocycle. The stability of these hydride metal systems allowed these complexes to be used as models for studying the influence of the ligand modifications on hydride reactivity. Spectroscopic analysis provided unambiguous structural assignment of the dihydride system. Modulation of the electronic properties of the wingtip substituents did not appreciably alter the reactivity of the hydrides. Reactivity studies using acids with a wide range of pK a values indicated a correlation between hydride reactivity and acidity and showed exclusive reactivity towards the less shielded hydride trans to the carbene carbon rather than the more shielded hydride trans to the pyridine ring, suggesting that the trans effect is more relevant in these reactions than the NMR spectroscopically deduced hydridic character.

A Proton-Responsive Annulated Mesoionic Carbene (MIC) Scaffold on Ir Complex for Proton/Hydride Shuttle: An Experimental and Computational Investigation on Reductive Amination of Aldehyde

A Cp*Ir(III) complex (1) bearing a proton-responsive hydroxy unit on an annulated imidazo[1,2-a][1,8]naphthyridine based mesoionic carbene scaffold was synthesized by two different synthetic routes. The molecular structure of 1 revealed an anionic lactam form of the ligand. The acid–base equilibrium between the lactam-lactim tautomers on the ligand scaffold was examined by 1H NMR and UV–vis spectra. The pKa of the appendage −OH group in the lactim form of 1 was estimated to assess the proton transfer property of the catalyst. The catalytic efficacy of 1 for reductive amination of aldehyde was evaluated by utilizing three different hydrogen sources: molecular H2, iPrOH/KOtBu combination, and HCOOH/Et3N (5:2) azeotropic mixture. The HCOOH/Et3N (5:2) azeotropic mixture protocol was found to be the best among the three different hydrogenation methods. Catalyst 1 hydrogenates imines chemoselectively over carbonyls under the reaction conditions. A range of aldehydes was reductively aminated to the corresponding secondary amines using the HCOOH/Et3N (5:2) azeotropic mixture. Further, catalyst 1 showed high efficiency for the reduction of a wide variety of N-heterocyclic imine derivatives. The lactam-lactim tautomerization of the ligand system is proposed for direct hydrogenation, whereas only the lactam form operates in the strongly basic medium (iPrOH/KOtBu). Under HCOOH/Et3N (5:2) conditions, the lactam scaffold is not protonated; rather, an outer-sphere hydride transfer from formate to the Ir is proposed, which is supported by 1H NMR and DFT calculations. Finally, ligand-promoted hydride transfer from metal-hydride to the protonated imine affords the corresponding amine. A close agreement between the experimentally estimated and computed thermodynamic/kinetic parameters gives credence to the metal-ligand cooperative mechanism for the imine hydrogenation reaction using the HCOOH/Et3N (5:2) azeotropic mixture.

Electronic Asymmetry of an Annelated Pyridyl–Mesoionic Carbene Scaffold: Application in Pd(II)-Catalyzed Wacker-Type Oxidation of Olefins

The two donor modules of an annelated pyridyl–mesoionic carbene ligand (aPmic) have different σ- and π-bonding characteristics leading to its electronic asymmetry. A Pd(II) complex 1 featuring aPmi...

An Annelated Mesoionic Carbene (MIC) Based Ru(II) Catalyst for Chemo- and Stereoselective Semihydrogenation of Internal and Terminal Alkynes

The catalytic utility of [RuL1(CO)2I2] (1), containing an annelated π-conjugated imidazo-naphthyridine-based mesoionic carbene (MIC) ligand (L1), is evaluated for E-selective alkyne semihydrogenation. The precatalyst 1, in combination with 2 equiv of AgBArF, semihydrogenates a broad range of internal alkynes with molecular hydrogen (5 bar) in water. (E)-Alkenes are accessed in high yields, and a number of reducible functional groups are tolerated. A chelate MIC ligand and two cis carbonyls provide a well-defined platform at the Ru center for hydrogenation and isomerization. The loss of two iodides and the presence of two carbonyls render the Ru center electron deficient and thus the formation of metal vinylidenes with terminal alkynes is avoided. This is leveraged for the semihydrogenation of terminal alkynes by the same catalytic system in isopropyl alcohol. Reaction profile, isomerization, kinetic, and DFT studies reveal initial alkyne hydrogenation to a (Z)-alkene, which further isomerizes to an (E)-alkene via metal-catalyzed Z → E isomerization.

Symmetrical and Non‐symmetrical Pd (II) Pincer Complexes Bearing Mesoionic N‐heterocyclic Thiones: Synthesis, Characterizations and Catalytic Properties

In contrast to well‐established symmetrical pincer complexes, non‐symmetrical metal pincers with the loss of C2v symmetry are much less studied. In this work, mesoionic NHTs (NHTs = N‐heterocyclic thiones), which can be viewed as the sulfur adducts of mesoionic carbenes, are incorporated into pincer complexes for the first time. Two symmetrical and non‐symmetrical phenylene‐bridged bis (NHT) compounds 3a/3b were synthesized as proligands via a “cycloaddition‐alkylation‐thionation” reaction sequence. In a case to access bis (NHT) compound 3c, N‐dealkylation reactions occurred. The carbene NMR signals of NHTs are only partially correlated to the π‐accepting abilities of carbenes, which is different from Bertrand's carbene‐phosphinidene system. The structural analysis of 3a/3b indicates that they possess C‐S partial double bonds. 3a and 3b served as the precursors to access two aryl anion‐linked [SCS/S′] pincer complexes 6a/6b. An external base proved to be essential for this cyclopalladation process. The catalytic properties of 6a/6b in the cycloisomerizations of alkynoic acids have been examined. Finally, non‐symmetrical complex 6a shows superior catalytic performance in such transformations contrasting to its symmetrical counterpart 6b.

A Diiron Hydrogenase Mimic Featuring a 1,2,3-Triazolylidene.

A novel complex featuring a mesoionic carbene [Fe₂(CO)5(trz)(μ-pdt)] (1) (trz = 1-phenyl-l,3-methyl,4-butyl-1,2,3-triazol-5-ylidene), was synthesized and spectroscopically and structurally characterized. The reductive behaviour of this compound in the presence and in the absence of acid (CH₃CO₂H) was examined by cyclic voltammetry (CV) that revealed the lack of efficient activity towards proton reduction.

O-Functionalised NHC Ligands for Efficient Nickel-catalysed C-O Hydrosilylation.

A series of C,O-bidentate chelating mesoionic carbene nickel(ii) complexes [Ni(NHC^PhO)₂] (NHC = imidazolylidene or triazolylidene) were applied for hydrosilylation of carbonyl groups. The catalytic system is selective towards aldehyde reduction and tolerant to electron-donating and -withdrawing group substituents. Stoichiometric experiments in the presence of different silanes lends support to a metal-ligand cooperative activation of the Si-H bond. Catalytic performance of the nickel complexes is dependent on the triazolylidene substituents. Butyl-substituted triazolylidene ligands impart turnover numbers up to 7,400 and turnover frequencies of almost 30,000 h-1, identifying this complex as one of the best-performing nickel catalysts for hydrosilylation and demonstrating the outstanding potential of O-functionalised NHC ligands in combination with first-row transition metals.

Synthesis and Photophysical Properties of T-Shaped Coinage-Metal Complexes.

The photophysical properties of a series of T‐shaped coinage d 10 metal complexes, supported by a bis(mesoionic carbene)carbazolide (CNC) pincer ligand, are explored. The series includes a rare new example of a tridentate T‐shaped AgI complex. Post‐complexation modification of the AuI complex provides access to a linear cationic AuI complex following ligand alkylation, or the first example of a cationic square planar AuIII−F complex from electrophilic attack on the metal centre. Emissions ranging from blue (CuI) to orange (AgI) are obtained, with variable contributions of thermally‐dependent fluorescence and phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutral T‐shaped AuI, cationic linear AuI and square planar cationic AuIII). The higher quantum yield and longer decay lifetime of the linear gold(I) complex are indicative of increased phosphorescence contribution.

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO2 Reduction.

Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three ReI complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The ReI compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO2 to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive ReI complexes and for electrocatalytic CO2 reduction.

Quantifying the Electronic and Steric Properties of 1,3-Imidazole-Based Mesoionic Carbenes (iMICs)

1,3-Imidazole-based mesoionic carbenes (iMICs) are a very promising class of carbon-donor ligands in synthesis and catalysis. However, a systematic study of the quantification of stereoelectronic properties of iMICs is lacking. This is most likely due to the absence of rational synthetic routes to suitable iMIC compounds. Herein, we report the synthesis of the appropriate phosphinidene (iMICAr)PPh (iMICAr = :CCH(NDipp)2CAr; Ar = Ph (5a), DMP (5b); DMP = 4-Me2NC6H4; Dipp = 2,6-iPr2C6H3), selenium (iMICAr)Se (Ar = Ph (6a), DMP (6b)), and nickel carbonyl compounds (iMICAr)Ni(CO)3 (Ar = Ph (7a), DMP (7b)). The π-accepting property of iMICAr (Ar = Ph (2a), DMP (2b)) has been evaluated by 31P and 77Se NMR spectroscopic analyses of 5 and 6, respectively. The overall donor property of 2 has been assessed by determining the Tolman electronic parameter (TEP) of 7. Studies suggest that iMICAr (2) are stronger σ donors and weaker π acceptors compared to classical N-heterocyclic carbenes (NHCs). The steric profile of iMICAr (2) has been obtained by calculating the percentage buried volume (%Vbur) for (iMICAr)CuX (Ar = Ph, X = I (8a); Ar = DMP, X = Br (8b)).

Mesoionic Carbene-Iridium Complex Catalyzed Ortho-Selective Hydrogen Isotope Exchange of Anilines with High Functional Group Tolerance.

Mesoionic carbene-iridium complex 1a has been investigated in the hydrogen isotope exchange (HIE) reaction of anilines. By employing 1 mol % of 1a as catalyst, anilines were selectively deuterated at the ortho-position with high deuteration levels. High ortho-selectivity was observed for anilines with various competing directing groups, which is in contrast with catalytic results of Kerr's catalysts.

Chiral Tricyclic 1,2,3-Triazol-3-Ylidene–Pd(II) Complexes of Cyclohexene Oxide Scaffold: Synthesis, Structure, and Characterizations

We herein report the two novel well-defined palladium(II) complexes 1(b−c), of the chiral N-fused tricyclic 1,2,3-triazolium-derived mesoionic carbenes (MIC) ligand 1a. The chiral tricyclic PEPPSI type complex namely, trans-{2-methyl-(5aS,9aS)-5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazin-3-ylidene}PdI2(pyridine)} (1b), and the chiral tricyclic bis(tz-MIC)-palladium complex namely, {2-methyl-(5aS,9aS)-5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazinium-3-ylidene}2PdCl2} (1c). The chiral tricyclic (tz-MIC)PdI2(pyridine) type PEPPSI complex (1b) was obtained by direct reaction of the corresponding chiral tricyclic 1,2,3-triazolium-derived mesoionic carbenes (MIC) ligand 1a, by treatment with PdCl2 in the presence of K2CO3 as a base in pyridine in 87% yield. (Scheme 1). The chiral tricyclic 1,2,3-triazolium iodide salt (1a) was converted to its in-situ silver analogue by reaction with Ag2O and then subsequently upon treatment with (COD)PdCl2 to produce the chiral tricyclic (tz-MIC)2PdCl2 type palladium complex (1c). All these palladium complexes were isolated for the first time and structurally characterized by 1H NMR and 13C{1H}-NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray crystallography.

A crystalline C5-protonated 1,3-imidazol-4-ylidene

The first C5-protonated 1,3-imidazol-based mesoionic carbene iMICBp (2) (iMICBp = :C{CH(NDipp)2C(Bp)}; Dipp = 2,6-iPr2C6H3; Bp = 4-PhC6H4) has been reported as a crystalline solid. Spectroscopic, X-ray diffraction, and computational studies clearly support the carbenic nature of 2, which has been further corroborated by its reactions with Ni(CO)4, (Me2S)AuCl, white phosphorus, and CO2.

Palladium complexes with an annellated mesoionic carbene (MIC) ligand: catalytic sequential Sonogashira coupling/cyclization reaction for one-pot synthesis of benzofuran, indole, isocoumarin and isoquinolone derivatives.

Two Pd(ii) complexes (1 and 2) featuring a fused π-conjugated imidazo[1,2-a][1,8]naphthyridine-based mesoionic carbene ligand have been synthesized and structurally characterized. Both complexes effectively catalyze the one-pot synthesis of benzofuran starting from phenylacetylene and 2-iodophenol under mild conditions. Complex 1 is found to be an excellent catalyst for the straightforward access to a library of benzofuran, indole, isocoumarin and isoquinolone derivatives by the reaction of terminal alkynes with 2-iodo derivates of phenol, N-methyl aniline, benzoic acid and N-methyl benzamide, respectively. The general utility of the catalytic method is demonstrated using a variety of diversely substituted terminal alkynes and the corresponding desired products are obtained in good to excellent yields. On the basis of control experiments, a two-cycle mechanism is proposed which involves the Sonogashira coupling of 2-iodo derivatives with alkynes and the subsequent cyclization of the corresponding 2-alkynyl compounds.

Copper(I) complexes bearing mesoionic carbene ligands: influencing the activity in catalytic halo-click reactions.

Metal complexes of mesoionic carbene (MIC) ligands are known to catalyze a variety of chemical transformations. In this contribution, we report on the synthesis of a dicationic dicopper(i) complex containing a di-MIC ligand. Two routes are presented for the synthesis of the dicopper complex: Ag-mediated transmetalation and direct deprotonation. For the Ag-mediated transmetalation route, the detection and isolation of several Ag-containing intermediates that are relevant for the final formation of the aforementioned dicopper complex are reported. We then investigate a series of copper(i) complexes based on MIC ligands as precatalysts for the azide halo-alkyne (Click) cycloaddition reaction. In a comparative study, three different halide-containing (I, Br, Cl) substrates have been investigated with different catalysts to survey the behaviour for mono/di-copper-MIC complexes as well as neutral, mono- and di-cationic complexes. The cationic complexes proved to have superior activities compared to the neutral species. These are the first reports on the use of Cu-MIC complexes as precatalysts for the halo-Click reaction.

Tuning electronic structure through halide modulation of mesoionic carbene cobalt complexes.

The first examples of Co(ii) mesoionic carbene complexes (CoX2DippMIC2; X = Cl-, Br-, I-) demonstrate a new electronic perturbation on tetrahedral Co(ii) complexes. Using absorption spectroscopy and magnetometry, the consequences of the MIC's strong σ-donating/minimal π-accepting nature are analyzed and shown to be further tunable by the nature of the coordinated halide.

Synthesis and characterisation of Pd(ii) and Au(i) complexes with mesoionic carbene ligands bearing phosphinoferrocene substituents and isomeric carbene moieites.

While numerous functional ligands combining phosphine and imidazole-2-ylidene (or imidazolin-2-ylidene) donor moieties have already been reported, the chemistry of the corresponding functional mesoionic carbenes (MIC) derived from 1,2,3-triazoles remains nearly untapped. This contribution describes the synthesis of two isomeric series of triazolium salts bearing 1'-(diphenylphosphino)ferrocenyl substituents by [3 + 2] cycloaddition of a P-protected phosphinoferrocene alkyne with azides, or alternatively of a P-protected phosphinoferrocene azide with terminal alkynes, and by subsequent methylation. These salts were used to synthesize structurally unique Pd(ii) complexes featuring a P,C-chelating triazolylidene carbene ligands and Au(i)-MIC complexes with free phosphine groups. The latter were further utilised to prepare Pd(ii)Au(i) heterometallic complexes containing bridging ferrocene phosphino-carbenes as structurally flexible, donor-unsymmetric metalloligands. In addition, the reactivity of the newly prepared, P-protected phosphinoferrocene alkyne Ph2PfcC[triple bond, length as m-dash]CH·BH3 (fc = ferrocene-1,1'-diyl) was investigated, and representatives from all reported compound classes were structurally characterised.