Cyclic (Amino)(aryl)carbenes (CAArCs) as Strong σ‐Donating and π‐Accepting Ligands for Transition Metals

Ultra-Long Lived Luminescent Triplet Excited States in Cyclic (Alkyl)(amino)carbene Complexes of Zn(II) Halides

The high element abundance and d¹⁰ electron configuration make Zn^II‐based compounds attractive candidates for the development of novel photoactive molecules. Although a large library of purely fluorescent compounds exists, emission involving triplet excited states is a rare phenomenon for zinc complexes. We have investigated the photophysical and ‐chemical properties of a series of dimeric and monomeric Zn^II halide complexes bearing a cyclic (alkyl)(amino)carbene (cAAC) as chromophore unit. Specifically, [(cAAC)XZn(μ‐X)₂ZnX(cAAC)] (X=Cl (1), Br (2), I (3)) and [ZnX₂(cAAC)(NCMe)] (X=Br (4), I (5)) were isolated and fully characterized, showing intense visible light photoluminescence under UV irradiation at 297 K and fast photo‐induced transformation. At 77 K, the compounds exhibit improved stability allowing to record ultra‐long lifetimes in the millisecond regime. DFT/MRCI calculations confirm that the emission stems from ³XCT/LE_cAAC states and indicate the phototransformation to be related to asymmetric distortion of the complexes by cAAC ligand rotation. This study enhances our understanding of the excited state properties for future development and application of new classes of Zn^II phosphorescent complexes.

Redox-Switchable Complexes Based on Nanographene-NHCs

A series of rhodium and iridium complexes with a N‐heterocyclic carbene (NHC) ligand decorated with a perylene‐diimide‐pyrene moiety are described. Electrochemical studies reveal that the complexes can undergo two successive one‐electron reduction events, associated to the reduction of the PDI moiety attached to the NHC ligand. The reduction of the ligand produces a significant increase on its electron‐donating character, as observed from the infrared spectroelectrochemical studies. The rhodium complex was tested in the [3+2] cycloaddition of diphenylcyclopropenone and methylphenylacetylene, where it displayed a redox‐switchable behavior. The neutral complex showed moderate activity, which was suppressed when the catalyst was reduced.

Realizing 1,1‐Dehydration of Secondary Alcohols to Carbenes: Pyrrolidin‐2‐ols as a Source of Cyclic (Alkyl)(Amino)Carbenes

Herein we report secondary pyrrolidin‐2‐ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC‐Cu^I‐complexes and cyclic thiones when reacted with Cu^I‐salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)‐center. This result demonstrates a convenient and facile access to CAAC‐based Cu^I‐salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes—realizing after 185 years Dumas’ dream who tried to prepare the parent carbene (CH₂) by 1,1‐dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the O−H bond to the carbon(II)‐center. This emphasizes the ability of carbon‐compounds to mimic the reactivity of transition‐metal complexes: reversible oxidative addition and reductive elimination of the O−H bond to/from the C(II)/C(IV)‐centre.

Recent advances in the domain of Cyclic (alkyl)(amino) carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

Indolizy Carbene Ligand. Evaluation of Electronic Properties and Applications in Asymmetric Gold(I) Catalysis

We report herein a new family of carbene ligands based on an indolizine-ylidene (Indolizy) moiety. The corresponding gold(I) complexes are easily obtained from the gold(I)-promoted cyclization of allenylpyridine precursors. Evaluation of the electronic properties by experimental methods and also by DFT calculations confirms strong σ-donating and π-accepting properties of these ligands. Cationization of the gold(I) complexes generates catalytic species that trigger diverse reactions of (poly)unsaturated precursors. When armed with a methylene phosphine oxide moiety on the stereogenic center adjacent to the nitrogen atom, the corresponding bifunctional carbene ligands give rise to highly enantioselective heterocyclizations. DFT calculations brought some rationalization and highlighted the critical roles played by the phosphine oxide group and the tosylate anion in the asymmetric cyclization of γ-allenols.

Theoretical Assessment of Dinitrogen Fixation on Carbon Atom

Dinitrogen activation in non-metallic systems has received considerable attention in recent years. Herein, we report the theoretical feasibility of N₂ fixation using aminocarbenes (L) or their anionic derivatives. The molecular descriptors of L and anionic L^- , which affect the interaction of L and anionic L^- with N₂ , were identified through multiple linear regression analysis. Additionally, the electron flow during C-N bond formation was confirmed by performing intrinsic reaction coordination calculations with intrinsic bond orbital analysis for the reaction of anionic L^- with N₂ .

Recent Representative Advances on the Synthesis and Reactivity of N-Heterocyclic-Carbene-Supported Zinc Complexes

The present account reviews the most recent noteworthy developments on the synthesis, structure and catalytic applications of Zn-NHC species, a class of complexes that have attracted attention over the past five to ten years due to their enhanced robustness and hydrolytic stability versus classical Zn organometallics. In particular, thanks to NHC stabilization, access to unprecedented Zn species were recently achieved, including two-coordinate Zn(II) organocations and thermally stable molecularly well-defined Zn hydride species, opening the way to effective Zn-mediated hydro-silylation/-boration catalysis of various unsaturated substrates under mild conditions. The potential of NHC-Zn species for the stabilization of unprecedented Zn species and use in various catalytic applications is only emerging and the vast array of readily available NHC structures should promote future developments of the field.

Preparation of Complexes Bearing N-Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2-Halogenoindole Derivatives

CAAC precursors 2-chloro-3,3-dimethylindole 1 and 2-chloro-1-ethyl-3,3-dimethylindolium tetrafluoroborate 2BF₄ have been prepared and oxidatively added to [M(PPh₃ )₄ ] (M=Pd, Pt). Salt 2BF₄ reacts with [Pd(PPh₃ )₄ ] in toluene at 25 °C over 4 days to yield complex cis-[3]BF₄ featuring an N-ethyl substituted CAAC, two cis-arranged phosphines and a chloro ligand. Compound trans-[3]BF₄ was obtained from the same reaction at 80 °C over 1 day. Salt 2BF₄ reacts with [Pt(PPh₃ )₄ ] to give cis-[4]BF₄ . The neutral indole derivative 1 adds oxidatively to [Pt(PPh₃ )₄ ] to give trans-[5] featuring a CAAC ligand with an unsubstituted ring-nitrogen atom. This nitrogen atom has been protonated with py⋅HBF₄ to give trans-[6]BF₄ bearing a protic CAAC ligand. The Pd^II complex trans-[7]BF₄ bearing a protic CAAC ligand was obtained in a one-pot reaction from 1 and [Pd(PPh₃ )₄ ] in the presence of py⋅HBF₄ .

Cyclic (Amino)(aryl)carbenes Enter the Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting CuI Compounds

A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries have been investigated in great detail with regard to their structural, electronic, and photophysical properties. Comparison of [CuX(CAArC)] (X = Br (1), Cbz (2), acac (3), Ph₂acac (4), Cp (5), and Cp* (6)) with known Cu^I complexes bearing cyclic (amino)(alkyl), monoamido, or diamido carbenes (CAAC, MAC, or DAC, respectively) as chromophore ligands reveals that the expanded π-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THF with high absorption coefficients of 5-15 × 10³ M^-1 cm^-1 for 1-6. Furthermore, 1-5 show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with λ^em_max = 621-784 nm. Linear [Cu(Cbz)(^DippCAArC)] (2) has been found to be an exceptional deep red (λ_max = 621 nm, ϕ = 0.32, τ_av = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant k_r of ca. 9 × 10⁵ s^-1, exceeding those of commercially employed Ir^III- or Pt^II-based emitters. Time-resolved transient absorption and fluorescence upconversion experiments complemented by quantum chemical calculations employing Kohn-Sham density functional theory and multireference configuration interaction methods as well as temperature-dependent steady-state and time-resolved luminescence studies provide a detailed picture of the excited-state dynamics of 2. To demonstrate the potential applicability of this new class of low-energy emitters in future photonic applications, such as nonclassical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2, showing distinct antibunching as required for single-photon emitters.

Intramolecular Ring-Expansion Reaction (RER) and Intermolecular Coordination of In Situ Generated Cyclic (Amino)(aryl)carbenes (cAArCs)

Cyclic (amino)(aryl)carbenes (cAArCs) based on the isoindoline core were successfully generated in situ by α-elimination of 3-alkoxyisoindolines at high temperatures or by deprotonation of isoindol-2-ium chlorides with sodium or copper(I) acetates at low temperatures. 3-Alkoxy-isoindolines 2 a,b-OR (R=Me, Et, iPr) have been prepared in high yields by the addition of a solution of 2-aryl-1,1-diphenylisoindol-2-ium triflate (1 a,b-OTf; a: aryl=Dipp=2,6-diisopropylphenyl; b: Mesityl-, Mes=2,4,6-trimethylphenyl) to the corresponding alcohol (ROH) with NEt3 at room temperature. Furthermore, the reaction of 2 a,b-OMe in diethyl ether with a tenfold excess of hydrochloric acid led to the isolation of the isoindol-2-ium chlorides 1 a,b-Cl in high yields. The thermally generated cAArC reacts with sulfur to form the thioamide 3 a. Without any additional trapping reagent, in situ generation of 1,1-diphenylisoidolin-3-ylidenes does not lead to the isolation of these compounds, but to the reaction products of the insertion of the carbene carbon atom into an ortho C-H bond of a phenyl substituent, followed by ring-expansion reaction; namely, anthracene derivatives 9-N(H)aryl-10-Ph-C14 H8 4 a,b (a: Dipp; b: Mes). These compounds are conveniently synthesized by deprotonation of the isoindol-2-ium chlorides with sodium acetate in high yields. Deprotonation of 1 a-Cl with copper(I) acetate at low temperatures afforded a mixture of 4 a and the corresponding cAArC copper(I) chloride 5 a, and allowed the isolation and structural characterization of the first example of a cAArC copper complex of general formula [(cAArC)CuCl].

N-Heterocyclic Carbene Derived 3-Azabutadiene as a π-Base in Classic and Frustrated Lewis Pair Chemistry

N-Heterocyclic carbene (NHC) derived 3-azabutadienes 1 and 2 have been prepared by a single-step reaction of the corresponding NHC with cyclohexyl isocyanide. Compound 1 features π-basic, delocalized nucleophilic sites over the 3-azabutadiene moiety, therefore allowing for coordinating with small Lewis acids, such as AlCl₃ , GaCl₃ , and Me₂ SAuCl, to form diverse classic Lewis adducts 3-5. Combination of 1 with B(C₆ F₅ )₃ or [Ph₃ C][B(C₆ F₅ )₄ ] resulted in single-electron transfer and the obtained radical cation was detected by EPR. In addition, a frustrated Lewis pair comprised of the π-basic 1 and BPh₃ effects the splitting of the O-H bond of phenol and the N-H bond of imidazole to give 7 and 8, respectively. An intrinsic bond orbital (IBO) analysis of the pathway leading to 8 showcases the transformation of the delocalized π-electrons of 1 to a newly formed C-H localized σ-bond.

Planar-Chiral Ferrocene-Based N-Heterocyclic Carbene Ligands

In the last few decades, N-heterocyclic carbene (NHC) ligands, characterized by attractive features, have increased their presence in transition-metal catalysis. However, chiral NHC ligands still require further design enhancement to realize far more exciting synthetic applications. This review documents advances in chiral NHC ligands, focusing on planar-chiral ferrocene-based NHCs. As the basic shape of most classes of NHC rings is planar, it seems rational to employ a planar-chiral element in the design of chiral NHC ligands.

Coumaraz-2-on-4-ylidene: Ambiphilic N-Heterocyclic Carbenes with a Tunable Electronic Structure

Herein, a coumaraz-2-on-4-ylidene (1) as a new example of an ambiphilic N-heterocyclic carbene, having electronic properties that can be fine-tuned, is reported. The N-carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the ⁷⁷ Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine-tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main-group and transition-metal chemistry.

Cobaltocenylidene: A Mesoionic Metalloceno Carbene, Stabilized in a Gold(III) Complex

Oxidative addition of cobaltoceniumdiazonium bis(hexafluoridophosphate) with (pseudo)halide aurates gave gold(III) complexes containing zwitterionic cobaltoceniumide as a ligand. Its selenium derivative, cobaltoceniumselenolate, was obtained by an electrophilic aromatic substitution reaction of iodocobaltocenium iodide with Na2 Se. Spectroscopic and structural data in combination with DFT calculations showed that this cobaltocenylidene species is a mesoionic carbene quite different from common N-heterocyclic carbenes. Its ligand properties (TEP, singlet-triplet gap, nucleophilicity, π-acidity, Brønsted basicity) are in part comparable to those of cyclic (amino)(alkyl/aryl)carbenes. Electrochemistry data showed that the mesoionic cobaltoceniumides are more electron-rich than their parent ferrocenes. The reversible reduction of the tricyanido gold complex appears 50 mV negative of the cobaltocenium/cobaltocene couple, whereas that of the selenide derivative is shifted cathodically by 550 mV.

Nitrogen-Based Lewis Acids: Synthesis and Reactivity of a Cyclic (Alkyl)(Amino)Nitrenium Cation

A room-temperature-stable crystalline cyclic (alkyl)(amino)nitrenium cation 2 features cationic nitrogen atom with a smaller HOMO-LUMO gap compared to that of a 1,2,3-triazolium 5 (an N-heterocyclic nitrenium cation). The low-lying LUMO of 2 results in an enhanced electrophilicity, which allowed for the formation of Lewis adducts with neutral Lewis bases, such as Me₃ P, nBu₃ P, and IiPr. The N-based Lewis acid 2 also forms an FLP with tBu₃ P but subsequently reacts with (PrS)₂ to cleave the S-S bond. Both experimental and theoretical results suggest that the Lewis acidity of 2 is stronger than its N₃ analogues.

Coordination Behavior of a Planar Chiral Cyclic (Amino)(Ferrocenyl)Carbene Ligand in Iridium Complexes

Iridium complexes bearing a new type of chiral carbene ligand, cyclic (amino)(ferrocenyl)carbene (CAFeC), were prepared. The complexes, which are quite stable in air and moisture, were fully characterized by NMR spectroscopy and X-ray diffraction analysis. The NMR spectra of the dicarbonyl complex [IrCl(CO)₂ (CAFeC)] showed signals for two isomers in solution, which were assigned to diastereomeric rotamers arising from the rotation of the chiral CAFeC ligand around the axes of the Ir-C_carbene bond. IR spectroscopy of the dicarbonyl complex revealed that the donor strength of CAFeC is much higher than those of classic N-heterocyclic carbenes (NHCs) and comparable to those of cyclic amino alkyl carbenes (CAACs).