Pushing Electrons—Which Carbene Ligand for Which Application?

Recent advances in the use of N-heterocyclic carbene adducts of N, P, C elements as supporting ligands in organometallic chemistry

With the rapid development of N-heterocyclic carbene (NHC) coordination chemistry, the potential of NHCs as strong electron-donating ligands has been fully explored. At the same time, an increasing number of p-block element NHC adducts, due to their possession of lone pairs or polarized carbon-element π-bonds, have gradually become important ligands of significant interest in coordination chemistry. Among these, N-heterocyclic imine (NHI), N-heterocyclic phosphinidene (NHCP) and N-heterocyclic olefin (NHO) adducts, formed by linking the 2-position of the N-heterocycle with nitrogen, phosphorus, and carbon elements, respectively, have been widely applied in both main-group and transition metal chemistry. The efficient stabilization of positive charges by the N-heterocycle allows the exocyclic atoms to accumulate negative charges, thereby significantly enhancing the basicity and nucleophilicity of the exocyclic elements. The improvement in nucleophilicity has facilitated the successful synthesis of numerous complexes with unique element-metal bonds. Additionally, by modifying the N-substituents, the kinetic stabilization requirements of highly reactive species can be fulfilled. This review aims to summarize the latest research progress on NHIs, NHCPs, and NHOs in main-group and transition metal compounds. It is important to emphasize that this review focuses on the commonly used five-membered N-heterocyclic carbene adducts, including imidazolin-2-ylidenes, imidazolidin-2-ylidenes, and benzimidazolin-2-ylidenes.

Multi-site reduction of hexachlorophosphazene to low-valent PN heterocycles and extension to the reduction of poly-chlorophosphazene

Facile one and two site reduction of hexachlorophosphazene using cyclic (alkyl)(amino)carbene substituents are shown to yield P-CAACMe-cyclo-(PNP(Cl)2NP(Cl)2N) 1 and P,P'-bis-CAACMe-cyclo-(PNPNP(Cl)2N) 2 (CAACMe = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene), respectively. Compound 1 is characterized by its predominantly phosphorus-centered HOMO, which results in typical phosphine-type nucleophilic and reductive reactivity; however, the resultant compounds of such reactions feature properties distinct from their classical phosphine analogues due to the CAAC-centered LUMO, which acts as an acceptor for both intramolecular interactions and photophysical excitations. In contrast, compound 2 exhibits π-conjugation spanning the endocyclic PNP moiety and the two CAACMe substituents, despite its non-planar structure. Treatment of 2 with [Cp*RuCl]4 results in the electrophilic displacement of one of the CAACMe moieties by two Cp*RuCl fragments to yield the spirocyclic compound 3. Preliminary results show that the methodology used to reduce hexachlorophosphazene to 1 can be directly transposed to the regiospecific reduction of poly-chlorophosphazene, to yield poly-1, a fundamentally new class of inorganic polymer that possesses a phosphorus center with chemically active lone pairs in the main chain.

Capture of an In Situ Formed Distanna-S-heterocyclic Carbene

The planar, non-twisted distannene Sn2(TIPS)4 (1, TIPS = SiiPr3) reacts with CS2 to form a tetrathiaethylene derivative via the dimerization of two S-heterocyclic carbenes (SHCs). These intermediary-formed SHCs can be transferred to palladium as ligands or captured with B(C6F5)3 (BCF) and, furthermore, facilitate a new pathway for formation of the stannaethene SHC═Sn(TIPS)2 by the reaction of 1 with (PPh3)2Pd-CS2. In addition to the characterization of the new complexes, theoretical calculations of the frontier orbitals were performed, which indicate a high π-acceptor character of the SHC.

A tridentate and dianionic N-heterocyclic olefin (NHO) with two unsymmetrically-tethered aryloxide sidearms on bismuth: a fortuitous followed by systematic discovery

Herein, we report our previously developed bifunctional imidaozlium-phenol LH2Br ([HO-4,6-tBu2-C6H2-2-CH2{CH(NCH = CHNAr)}]Br; Ar = Dipp = 2,6-iPr2-C6H3), leading to three distinct outcomes with bismuth by simply altering the relative ratios of the reacting components. Treating LH2Br and Bi(HMDS)3 (HMDS = N(SiMe3)2) in a 1 : 1 ratio gives the NHC-Bi complex [(L)BiBr(HMDS)] (1). In contrast, LH2Br, Bi(HMDS)3, and KHMDS in a 2 : 1 : 1 ratio serendipitously result in an unprecedented NHO-Bi complex [(DippNHO2ArO)BiBr] (2), featuring a novel dianoinic and tridentate NHO ligand with two unsymmetric aryloxide sidearms. A tri-acidic imidazolium salt L'H3I ([HO-4,6-tBu2-C6H2-2-CH2{C(HO-4,6-tBu2-C6H2-2-CH2)(NCH = CHNMe)}]I) is synthesized independently as a potential precursor to such an NHO framework, and the corresponding NHO-BiI complex [(MeNHO2ArO)Bi(μ-I)]2 (32) is made as proof of concept. Lastly, treating LH2Br and Bi(HMDS)3 in a 2 : 1 ratio also unexpectedly leads to an 'abnormal' NHC-Bi complex [(aL)BiBr2(DippImd)] (4; DippImd = Dipp-imidazole). The multi-component and essentially multi-step reactions leading to 2 and 4 are challenging to fully elucidate mechanistically. Still, control experiments indicate 1 as a possible intermediate in both cases. Based on these results and prior insights into LH2Br and an intermediate LH, plausible routes for both 2 and 4 are hypothesized. DFT calculations are also performed to analyze the bonding in 2 and 32 and to justify an NHC to aNHC isomerization towards the formation of 4.

N-Heterocyclic Carbenes: A Benchmark Study on their Singlet-Triplet Energy Gap as a Critical Molecular Descriptor

N-heterocyclic carbenes (NHCs) are used extensively in modern chemistry and materials science. The in-depth understanding of their electronic structure and their metal complexes remains an important topic of research and of experimental and theoretical interest. Herein, the adiabatic singlet-triplet gap as a superior, quantifiable critical descriptor, sensitive to the nature and the structural diversity of the NHCs, for a successful rationalization of experimental observations and computationally extracted trends is established. The choice is supported by a benchmark study on the electronic structures of NHCs, using high-level ab initio methods, that is, complete active space self-consistent field, n-electron valence second-order perturbation theory, multireference configuration interaction + singles + doubles, and domain-based local pair natural orbital-coupled cluster method with single-, double-, and perturbative triple excitations along with density functional theory methods such as BP86, M06, and M06-L, B3LYP, PBE0, TPSSh, CAM-B3LYP, and B2PLYP. In contrast to the adiabatic singlet-triplet (S-T) gap preferred as descriptor, the highest occupied molecular orbital-lowest unoccupied molecular orbital gap or the S-T vertical gap that has been used in the past occasionally leads to controversial results; some of these are critically discussed below. Extrapolation of these ideas to a group of copper-NHC complexes is also described.

Recent advances in heavier group 15 (P, As, Sb, Bi) radical chemistry - frameworks, small molecule reactivity, and catalysis

Main group radical chemistry has been a targeted research area for several decades. With growing examples of phosphorus radicals, even heavier pnictogen radicals including arsenic, antimony, and bismuth have also become important targets. A diverse framework of group 15 radicals has been reported in the 21st century and is covered herein. Reactivity and applications of selected radicals and future directions for this field are discussed.

Synthesis of a Transient Cationic Phosphaborene and its Trapping by Facile Aliphatic C-H Bond Activation

The synthesis of a transient cationic phosphaborene [(Mes*)P=B(CAAC)]+ (Mes*=2,4,6,-tri-tert-butylphenyl, CAAC=cyclic alkylamino carbene) by halide abstraction from the B-brominated analogue is reported. This species was found to undergo rapid and selective intramolecular aliphatic C-H bond activation to yield a phosphinoborenium cation, which undergoes facile deprotonation to give a cyclic base-stabilized phosphaborene. Computational investigation of the mechanism of C-H activation indicates a boron-centred activation route with an exceptionally low barrier of 8 kJ mol-1, followed by a nearly barrierless hydride migration from boron to phosphorus. This underscores the enhanced reactivity of phosphaborenes compared to their lighter iminoborane analogues and indicates that steric stabilization alone is likely insufficient to isolate persistent cationic phosphaborenes, necessitating the use of alternative design strategies.

Harnessing Multi-Center-2-Electron Bonds for Carbene Metal-Hydride Nanocluster Catalysis

N-Heterocyclic carbene (NHC) ligands possess the ability to stabilize metal-based nanomaterials for a broad range of applications. With respect to metal-hydride nanomaterials, however, carbenes are rare, which is surprising if one considers the importance of metal-hydride bonds across the chemical sciences. In this study, we introduce a bottom-up approach that leverages preexisting metal-metal m-center-n-electron (mc-ne) bonds to access a highly stable cyclic(alkyl)amino carbene (CAAC) copper-hydride nanocluster, [(CAAC)6Cu14H12][OTf]2 with superior stability compared to Stryker's reagent, a popular commercial phosphine-based copper hydride catalyst. Density functional theory (DFT) calculations reveal that the enhanced stability stems from hydride-to-ligand backbonding with the π-accepting carbene. This new cluster emerges as an efficient and selective copper-hydride pre-catalyst, thereby providing a bench-stable alternative for catalytic applications.

π-Lewis Base Activation of Carbonyls and Hexafluorobenzene

We report hitherto elusive side-on η2-bonded palladium(0) carbonyl (anthraquinone, benzaldehyde) and arene (benzene, hexafluorobenzene) palladium(0) complexes and present the catalytic hydrodefluorination of hexafluorobenzene by cyclohexene. The comparison with respective cyclohexene, pyridine and tetrahydrofuran complexes reveals that the experimental ligand binding strengths follow the order THF Collapse

Key Words

• Catalysis
• Chemical Bonding
• Intermediates
• Organometallic Chemistry
• Spectroscopy

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MESH Headings Collapse

Grants

• 948185 H2020 European Research Council
• 256/506-1 Deutsche Forschungsgemeinschaft
• 256/582-1 Deutsche Forschungsgemeinschaft
• 440719683 Deutsche Forschungsgemeinschaft
• 2008-390540038 Deutsche Forschungsgemeinschaft

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Affiliation(s)

Aditesh Mondal Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany
Kevin Breitwieser Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany
Sergi Danés Departament de Química, Institut de Química Computacional I Catàlisi, Universitat de Girona, c/m. Aurelia Capmany 69, 17003, Girona, Spain
Annette Grünwald Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany Inorganic and General Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, D-91058, Erlangen, Germany
Frank W Heinemann Inorganic and General Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, D-91058, Erlangen, Germany
Bernd Morgenstern Solid State Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany
Frank Müller Experimental Physics and Center for Biophysics, Saarland University, Campus E2.9, D-66123, Saarbrücken, Germany
Michael Haumann Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195, Berlin, Germany
Maximilian Schütze Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany
Dustin Kass Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany
Kallol Ray Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany
Dominik Munz Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany

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Correlation Between NMR Coupling Constants and σ-Donating Properties of N-Heterocyclic Carbenes and Their Derivatives

Is the 1JC-H coupling constant for protonated carbene a relevant measure of its σ-donation ability? This paper addresses this question by comparing calculated 1JC-H values with various experimental and theoretical approaches across a broad spectrum of carbene compounds. We examined Arduengo-type NHCs based on the 2-imidazolylidene scaffold and many other derivatives with modified frameworks, including carbenes with extended, saturated, or conjugated rings, reduced heteroatom stabilization, alternative heteroatoms, permanently charged carbenes, acyclic carbenes, amidocarbenes, and cyclic amino(alkyl/aryl) carbenes, carbodicarbenes and carbodiphosphoranes. Our findings reveal a nuanced relationship between different parameters associated with σ-interaction, such as 1JC-H, Huynh electronic parameter (HEP), σ-donation from ETS-NOCV, and lone pair energy. Notably, the best correlation was observed between 1JC-H and the ETS-NOCV method, particularly for mono- and diaminocarbenes, highlighting the utility of 1JC-H in comparing σ-donation among structurally similar carbene types. However, the use of 1JC-H as a universal measure across all carbene classes appears limited, especially when considering carbenes with significantly different structural frameworks. While HEP is less effective for carbenes with diverse structural backbones, our study suggests that 1JC-H has potential across a broader range of systems. Additionally, the analysis demonstrates that lone pair energy reflects basicity rather than σ-donor ability.

Carbodicarbene-Stibenium Ion-Mediated Functionalization of C(sp3)-H and C(sp)-H Bonds

Main-group element-mediated C-H activation remains experimentally challenging and the development of clear concepts and design principles has been limited by the increased reactivity of relevant complexes, especially for the heavier elements. Herein, we report that the stibenium ion [(pyCDC)Sb][NTf2]3 (1) (pyCDC=bis-pyridyl carbodicarbene; NTf2=bis(trifluoromethanesulfonyl)imide) reacts with acetonitrile in the presence of the base 2,6-di-tert-butylpyridine to enable C(sp3)-H bond breaking to generate the stiba-methylene nitrile complex [(pyCDC)Sb(CH2CN)][NTf2]2 (2). Kinetic analyses were performed to elucidate the rate dependence for all the substrates involved in the reaction. Computational studies suggest that C-H activation proceeds via a mechanism in which acetonitrile first coordinates to the Sb center through the nitrogen atom in a κ1 fashion, thereby weakening the C-H bond which can then be deprotonated by base in solution. Further, we show that 1 reacts with terminal alkynes in the presence of 2,6-di-tert-butylpyridine to enable C(sp)-H bond breaking to form stiba-alkynyl adducts of the type [(pyCDC)Sb(CCR)][NTf2]2 (3 a-f). Compound 1 shows excellent specificity for the activation of the terminal C(sp)-H bond even across alkynes with diverse functionality. The resulting stiba-methylene nitrile and stiba-alkynyl adducts react with elemental iodine (I2) to produce iodoacetonitrile and iodoalkynes, while regenerating an Sb trication.

Synthesis and electrochemical properties of molybdenum nitrido complexes supported by redox-active NHC and MIC ligands

We report the synthesis of a series of molybdenum nitrido complexes supported by bis-phenolate N-heterocyclic and mesoionic carbenes (NHC & MIC). The reaction between MoN(OtBu)3 and the corresponding azolium salts [H3L1]Cl and [H3L2]Cl (with L1 = bis-phenolate triazolylidene and L2 = bis-phenolate benzimidazolylidene) gives clean access to the corresponding NHC/MIC complexes 1-Cl and 2-Cl. Electrochemical investigations of these complexes showed that they can be reversibly reduced at potentials of -1.13 and -1.01 V vs. Fc/[Fc]+ and the reduced complexes [1-Cl]- and [2-Cl]- can be cleanly isolated after chemical reduction with one equivalent of decamethylcobaltocene. Exchange of the halide atoms is furthermore reported to give a series of nitrido complexes supported by tert-butanolate (1-OtBu and 2-OtBu), perfluoro-tert-butanolate (1-OtBuF9 and 2-OtBuF9), tritylate (1-OCPh3 and 2-OCPh3), mesitolate (1-OMes and 2-OMes), thio-tert-butanolate (1-StBu), thiotritylate (1-SCPh3 and 2-SCPh3) and thiomesitolate complexes (1-SMes). The electrochemical properties of all complexes were evaluated and compared. All isolated complexes were characterized by multinuclear and multidimensional NMR spectroscopy and (if applicable) by EPR spectroscopy. Furthermore, the reactivity of 1-Cl and 2-Cl in the presence of protons and decamethylcobaltocene was investigated, which shows facile extrusion of ammonia, yielding diamagnetic bis-molybdenum(III) complexes 3 and 4.

IPr*F - Highly Hindered, Fluorinated N-Heterocyclic Carbenes

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon-fluorine bonds. Simultaneously, sterically-hindered N-heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition-metal-catalysis. Herein, we report the first class of sterically-hindered, fluorinated N-heterocyclic carbenes. These ligands feature variable fluorine substitution at the N-aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one-pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon-fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers.

A Lead(II) Substituted Triplet Carbene

Reaction of the pincer-type ligand L3 supported complex [L3PbBr][BArF24] (1) with Li[(C(═N2)TMS)] furnishes [L3Pb(C(═N2)TMS)][BArF24] (2). Diazo-compound 2 eliminates dinitrogen upon irradiation affording formal plumba-alkyne 3, which persists in cold fluoroarene solutions. Variable temperature UV/Vis and NMR spectroscopies in combination with quantum-chemical calculations identify 3 as a metal-substituted triplet carbene. In-crystallo irradiation of [L3Pb(C(═N2)TMS)(tol)][BArF24] (2·tol) provides a snapshot of intermolecular C-H bond insertion with toluene (4).

A bis-Phenolate Carbene-Supported bis-μ-Oxo Iron(IV/IV) Complex with a [FeIV(μ-O)2FeIV] Diamond Core Derived from Dioxygen Activation

The diiron(II) complex, [(OCO)Fe(MeCN)]2 (1, MeCN = acetonitrile), supported by the bis-phenolate carbene pincer ligand, 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)benzimidazolin-2-ylidene (OCO), was synthesized and characterized by single-crystal X-ray diffraction, 1H nuclear magnetic resonance, infrared (IR) vibrational, ultraviolet/visible/near-infrared (UV/vis/NIR) electronic absorption, 57Fe Mössbauer, X-band electron paramagnetic resonance (EPR) and SQUID magnetization measurements. Complex 1 activates dioxygen to yield the diferric, μ-oxo-bridged complex [(OCO)Fe(py)(μ-O)Fe(O(C═O)O)(py)] (2) that was isolated and fully characterized. In 2, one of the iron-carbene bonds was oxidized to give a urea motif, resulting in an O(CNHC═O)O binding site, while the other Fe(OCO) unit remained unchanged. When the reaction is performed at -80 °C, an intensively colored, purple intermediate is observed (INT, λmax = 570 nm; ε = 5600 mol L-1 cm-1). INT acts as a sluggish oxidant, reacting only with easily oxidizable substrates, such as PPh3 or 2-phenylpropionic aldehyde (2-PPA). The identity of INT can be best described as a dinuclear complex containing a closed diamond core motif [(OCO)FeIV(μ-O)2FeIV(OCO)]. This proposal is based on extensive spectroscopic [UV/vis/NIR electronic absorption, 57Fe Mössbauer, X-band EPR, resonance Raman (rRaman), X-ray absorption, and nuclear resonance vibrational (NRVS)] and computational studies. The conversion of the diiron(II) complex 1 to the oxo diiron(IV) intermediate INT is reminiscent of the O2 activation process in soluble methane monooxygenases (sMMO). Most importantly, the low reactivity of INT supports the consensus that the [FeIV(μ-O)2FeIV] diamond core in sMMO is kinetically inert and needs to open up to terminal FeIV═O cores to react with the strong C-H bonds of methane.

Carbodiphosphorane-Activated Distibene and Dibismuthene Dications

Low-valent antimony and bismuth have emerged as novel platforms for achieving reversible small-molecule activation at main-group metals. Although various examples of oxidative addition reactions at monomeric Sb(I) and Bi(I) have been reported, the chemistry of the heavy group 15 Sb(I)═Sb(I)/Bi(I)═Bi(I) double bonds toward small molecules remains largely unexplored. In this study, we present a straightforward synthesis of distibene and dibismuthene dications coordinated with a neutral carbodiphosphorane (CDP) ligand. The nonbonding interactions between the occupied p-orbital at the CDP ligand and the π-bonding orbital of the Sb═Sb/Bi═Bi bonds yield compounds with exceptionally small HOMO-LUMO gaps. In addition, the reduction of steric hindrance compared to known neutral derivatives stabilized with bulky aryl groups allows for better accessibility of the double bonds. This high reactivity is demonstrated in the oxidative addition of distibene to diphenyldisulfide as well as in [2+2] cycloadditions to alkynes. Additionally, the Sb═Sb bond reversibly adds to 2,3-dimethylbutadiene in a [4+2] cycloaddition reaction.

From d8 to d1: Iron(0) and Iron(I) Complexes Complete the Series of Eight Fe Oxidation States within the TIMMNMes Ligand Framework

Reduction of the ferrous precursor [(TIMMNMes)Fe(Cl)]+ (1) (TIMMNMes = tris-[(3-mesitylimidazol-2-ylidene)methyl]amine) to the low-valent iron(0) complex [(TIMMNMes)Fe(CO)3] (2) is presented, where the tris(N-heterocyclic carbene) (NHC) ligand framework remains intact, yet the coordination mode changed from 3-fold to 2-fold coordination of the carbene arms. Further, the corresponding iron(I) complexes [(TIMMNMes)Fe(L)]+ (L = free site, η1-N2, CO, py) (3) are synthesized and fully characterized. Complexes 1-3 demonstrate the notable steric and electronic flexibility of the TIMMNMes ligand framework by variation of the Fe-N anchor and Fe-carbene distances and the variable size of the axial cavity occupation. This is further underpinned by the oxidation of 3-N2 in a reaction with benzophenone to yield the corresponding, charge-separated iron(II) radical complex [(TIMMNMes)Fe(OCPh2)]+ (4). We found rather surprising similarities in the reactivity behavior when going to low- or high-valent oxidation states of the central iron ion. This is demonstrated by the closely related reactivity of 3-N2, where H atom abstraction with TEMPO triggers the formation of the metallacycle [(TIMMNMes*)Fe(py)]+ (5), and the reactivity of the highly unstable Fe(VII) nitride complex [(TIMMNMes)Fe(N)(F)]3+ to give the metallacyclic Fe(V) imido complex [(TIMMNMesN)Fe(NMes)(MeCN)]3+ (6) upon warming. Thus, the employed tris(carbene) chelate is not only capable of stabilizing the superoxidized Fe(VI) and Fe(VII) nitrides but equally supports the iron center in its low oxidation states 0 and +1. Isolation and characterization of these zero- and monovalent iron complexes demonstrate the extraordinary capability of the tris(carbene) chelate TIMMN to support iron in eight different oxidation states within the very same ligand platform.

Pentacyclic fused diborepinium ions with carbene- and carbone-mediated deep-blue to red emission

Designing molecules that can undergo late-stage modifications resulting in specific optical properties is useful for developing structure-function trends in materials, which ultimately advance optoelectronic applications. Herein, we report a series of fused diborepinium ions stabilized by carbene and carbone ligands (diamino-N-heterocyclic carbenes, cyclic(alkyl)(amino) carbenes, carbodicarbenes, and carbodiphosphoranes), including a detailed bonding analysis. These are the first structurally confirmed examples of diborepin dications and we detail how distortions in the core of the pentacyclic fused system impact aromaticity, stability, and their light-emitting properties. Using the same fused diborepin scaffold, coordinating ligands were used to dramatically shift the emission profile, which exhibit colors ranging from blue to red (358-643 nm). Notably, these diborepinium ions access expanded regions of the visible spectrum compared to known examples of borepins, with quantum yields up to 60%. Carbones were determined to be superior stabilizing ligands, resulting in improved stability in the solution and solid states. Density functional theory was used to provide insight into the bonding as well as the specific transitions that result in the observed photophysical properties.

Tapping into the coordinative potential of a C-functional 1,4-diphosphabarrelene using two sets of complementary ligand centres

7,8-Dihydro-1,4-diphosphabarrelene diselones and bis(NHCs) were synthesised and employed as multitopic P,Se and P,C ligands in coordination chemistry, benefitting from a unique bent, P-bridged topology, thus being promising new building blocks.

Genuine carbene versus carbene-like reactivity

Singlet carbenes are not always isolable and often even elude direct detection. When they escape observation, their formation can sometimes be evidenced by in situ trapping experiments. However, is carbene-like reactivity genuine evidence of carbene formation? Herein, using the first example of a spectroscopically characterized cyclic (amino)(aryl)carbene (CAArC), we cast doubt on the most common carbene trapping reactions as sufficient proof of carbene formation.

New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5 a) and [CrIIIBr2.3Cl0.7(CNC)] (5 b), where CNC=3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5 a and 5 b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5 a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH)=3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

Redox- and Charge-State Dependent Trends in 5, 6, and 7-Membered Boron Heterocycles: A Neutral Ligand Coordination Chemistry Approach to Boracyclic Cations, Anions, and Radicals

ConspectusBoron heterocycles represent an important subset of heteroatom-incorporated rings, attracting attention from organic, inorganic, and materials chemists. The empty pz orbital at the boron center makes them stand out as quintessential Lewis acidic molecules, also serving as a means to modulate electronic structure and photophysical properties in a facile manner. As boracycles are ripe for extensive functionalization, they are used in catalysis, chemical biology, materials science, and continue to be explored as chemical synthons for conjugated materials and reagents. Neutral boron(III)-incorporated polycyclic molecules are some of the most studied types of boracycles, and understanding their redox transformations is important for applications relying on electron transfer and charge transport. While relevant redox species can often be electrochemically observed, it remains challenging to isolate and characterize boracycles where the boron center and/or polycyclic skeleton have been chemically reduced.We describe our recent work isolating 5-, 6-, and 7-membered boracyclic radicals, anions, and cations, focusing on stabilization strategies, ligand-mediated bonding situations, and reactivity. We present a versatile neutral ligand coordination chemistry approach that permits the transformation of boracycles from potent electrophiles to powerful nucleophilic heterocycles that facilitate diverse electron transfer and bond activation chemistry. Although there are a wide range of suitable stabilizing ligands, we have employed both diamino-N-heterocyclic carbenes (NHCs) and cyclic(alkyl)(amino) carbenes (CAACs), which led to boracycles with tunable electronic structures and aromaticity trends. We highlight successful isolation of borafluorene radicals and demonstrate their reversible redox behavior, undergoing oxidation to the cation or reduction to the anion. The borafluorene anion is a chemical synthon that has been used to prepare boryl main-group and transition-metal bonds, luminescent oxabora-spirocycles, borafluorenate-crown ethers, and CO-releasing molecules via carbon dioxide activation. We expanded to 6-membered boracycles and characterized neutral bis(NHC-supported 9-boraphenanthrene)s and the corresponding bis(CAAC-stabilized 9-boraphenanthrene) biradical. We detail the interconvertible multiredox states of boraphenalene, where the boraphenalenyl radical, anion, and cation mimic the charge-states of the all-hydrocarbon analogue. Reactivity studies of the boraphenalenyl anion displayed unusual nucleophilic reactivity at multiple sites on the periphery of the boraphenalenyl tricyclic scaffold. Reduced borepins, 7-membered boron containing heterocycles, have also been isolated. We used a stepwise one-pot synthesis combining the halo-borepin precursor, CAAC, and KC8 to afford the monomeric borepin radicals and anions. The π-system was extended to contain two borepin rings fused in a pentacyclic scaffold, which permitted isolation of diborepin biradicals and a diborepin containing a dibora-quinone core.Our goal is to provide a guide explaining the current structure-function trends and isolation strategies for redox-active boron-incorporated polycyclic molecules to initiate the rational design and use of these types of compounds across a vast chemical space.

N-Heterocyclic Carbene Analogues of Wittig Hydrocarbon

N-Heterocyclic carbene (NHC) analogues of Wittig hydrocarbon, [(NHC)(Stil)(NHC)] (3a-c) (NHC = SIPr (1a) = C[N(Dipp)CH2]2, Dipp = 2,6-iPr2C6H3; IPr (1b) = C[N(Dipp)CH]2; Me-IPr (1c) = C[N(Dipp)CMe]2 and Stil = C6H4CHCHC6H4) have been reported as crystalline solids. 3a-c are prepared by two-electron reductions of the corresponding bis-1,3-imidazoli(ni)um bromides [(NHC)(Stil)NHC)](Br)2 (2a-c) with KC8 in >94 % yields. 2a-c are accessible by the nickel catalyzed direct C-C coupling of NHCs (1a-c) with (E)-4,4'-dibromostilbene. One-electron oxidation of 3a,b yields the corresponding radical cations [(NHC)(Stil)NHC)]B(C6F5)4 4a,b. All compounds have been characterized by UV-Vis/NMR/EPR spectroscopy as well as 2a, 3a, and 3b by single crystal X-ray diffraction. The electronic structures of representative systems have been analyzed by quantum chemical calculations.

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Boosting the π-Acceptor Property of Mesoionic Carbenes by Carbonylation with Carbon Monoxide

We report the room temperature dimerization of carbon monoxide mediated by C4/C5-vicinal anionic dicarbenes Li(ADC) (ADC = ArC{(Dipp)NC}2 ; Dipp = 2,6-iPr2 C6 H3 ; Ar = Ph, DMP (4-Me2 NC6 H4 ), Bp (4-PhC6 H4 )) to yield (E)-ethene-1,2-bis(olate) (i.e. - O-C=C-O- = COen ) bridged mesoionic carbene (iMIC) lithium compounds COen -[(iMIC)Li]2 (COen -[iMIC]2 = [ArC{(Dipp)NC}2 (CO)]2 ) in quantitative yields. COen -[(iMIC)Li]2 are highly colored stable solids, exhibit a strikingly small HOMO-LUMO energy gap, and readily undergo 2e-oxidations with selenium, CuCl (or CuCl2 ), and AgCl to afford the dinuclear compounds COon -[(iMIC)E]2 (E = Se, CuCl, AgCl) featuring a 1,2-dione bridged neutral bis-iMIC (i.e. COon -[iMIC]2 = [ArC{(Dipp)NC}2 (C=O)]2 ). COen -[(iMIC)Li]2 undergo redox-neutral salt metathesis reactions with LiAlH4 and (Et2 O)2 BeBr2 and afford COen -[(iMIC)AlH2 ]2 and COen -[(iMIC)BeBr]2 , in which the dianionic COen -moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X-ray diffraction. Stereoelectronic properties of COon -[iMIC]2 are quantified by experimental and theoretical methods.

Designing a σ0π2 singlet ground state carbene from dicationic carbones

Carbenes with a σ0π2 singlet ground state are rare and little is known about their chemistry. Here, we study the potential formation of such carbenes by removal of two electrons from carbones/donor-substituted allenes. The desired electron configuration becomes favorable in the case of bis-diiminium substitution (CAAC motif).

Application of phosphorus-bridged rigid, bent bis(NHCs) as dipodal ligands in main group and transition metal chemistry

Phosphorus-bridged rigid, bent bis(N-heterocyclic) carbenes have not been reported, so far, despite having structural features that could make them interesting ligands in coordination and main group element chemistry. In previous reports, we had demonstrated that tuning of σ3- and σ4-phosphorus environments in planarised bis(NHCs) affects electronic properties and can provide additional coordination sites. Herein, we report on first examples of synthesis and conversion of 1,4-diphosphabarrelene-related compounds into rigid bent, doubly P-bridged bis(NHCs). The formation of main group element adducts with substrates from group 13, 14 and 15 illustrates opportunities to access novel scaffolds and to create nonplanar branching points. DFT calculations reveal the new bis(NHCs) to be good candidates as novel soft/hard ligands with up to four coordination sites. The synthesis of a dinuclear Fe(CO)4 complex is demonstrated. The thermal retro-[4 + 2] cycloaddition was theoretically and experimentally explored for a variety of ionic and zwitterionic 1,4-diphosphabarrelenes, and the generation and trapping of a dinuclear Fe(0) bis(NHC) complex with a tricyclic 1σ2,4 σ2-diphosphinine scaffold is presented.

Ring-Opening of 1,3-Imidazole Based Mesoionic Carbenes (iMICs) and Ring-Closing Clicks: Facile Access to iMIC-Compounds

Herein, ring-opening of mesoionic carbenes (iMICs) (iMIC=[ArC{N(Dipp)}2 C(SiMe3 )C:) (Dipp=2,6-iPr2 C6 H3 , Ar=Ph, 4-Me2 NC6 H4 or 4-PhC6 H4 ) based on an 1,3-imidazole scaffold to yield N-ethynylformimidamide (eFIM) derivatives as crystalline solids (eFIM={(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe3 ) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t1/2= 3 h for Ar=Ph). eFIMs are excellent surrogates of iMICs, which generally have a limited shelf-life, and readily undergo ring-closing click reactions with a variety of main-group as well as transition metal Lewis acids to form hitherto challenging iMIC-compounds in good to excellent yields. In addition to the relevance of eFIMs in the synthesis of iMIC-compounds, quantification of the stereoelectronic properties of a representative iMIC (Ar=Ph) by experimental and theoretical methods suggests remarkably σ-donor property and steric profile of these new ligand sets.

Thermal Stability and Electronic Properties of N-Heterocyclic Carbene-Protected Au13 Nanocluster and Phosphine-Protected Analogues

Despite significant advances in manufacturing atomically precise gold nanoclusters protected by various ligands, there is a limited understanding of the thermal stability dynamics and electronic properties of ligand effects. We conducted ab initio molecular dynamics (AIMD) simulations on the well-characterized [Au13(NHCMe)9Cl3]2+ nanocluster and its counterpart [Au13(PMe3)9Cl3]2+ cluster to evaluate the thermal stability induced by N-heterocyclic carbene (NHC) and phosphine ligands. The result shows that under vacuum conditions, [Au13(PMe3)9Cl3]2+ is more stable than [Au13(NHCMe)9Cl3]2+, and both lead to metal nucleation decomposition, breaking into the Au12 fragment and L-Au-Cl (L = NHCMe or PMe3) complexes eventually. The optical and electronic properties of these two clusters change significantly due to ligand alteration. Furthermore, we have designed a novel [Au13(NHCMe)(PMe3)8Cl3]2+ cluster coprotected by NHC and phosphine ligands, displaying higher thermal stability than the homoligand protected [Au13(NHCMe)9Cl3]2+ and [Au13(PMe3)9Cl3]2+. Our hypothetical species are an interesting model for nanostructured materials, facilitating the experimental exploration of cluster synthesis and catalytic applications.

A complete series of N-heterocyclic tetrylenes (Si-Pb) with a 1,1'-ferrocenediyl backbone enabled by 1,3,2-diazaborolyl N-substituents

The use of bulky 1,3,2-diazaborolyl N-substituents has allowed the synthesis of the complete series of ferrocene-based N-heterocyclic tetrylenes fc[(N{B})2E] (fc = 1,1'-ferrocenediyl, {B} = (HCNC6H3-2,6-iPr2)2B, E = Si-Pb). The silylene fc[(N{B})2Si] is inert towards NH3, CO2 or N2O under ambient conditions and thus significantly less reactive than the N-aryl homologue fc[(NC6H3-2,6-iPr2)2Si]. In accord with its higher reactivity, computational results indicate a more pronounced ambiphilicity of fc[(NC6H3-2,6-iPr2)2Si]. Our computational investigation of the model compound fc[(NBMe2)2Si] suggests that silylenes of this type may be superior to fc[(NC6H3-2,6-iPr2)2Si] in terms of ambiphilicity.

Dimerization and ring-opening in bis(diisopropylamino)cyclopropenylidene (BAC) mediated by [U(NR2)3(CCPh)] (R = SiMe3)

Addition of 2 equiv. of bis(diisopropylamino)cyclopropenylidene (BAC) to [U(NR2)3(CCPh)] (1, R = SiMe3), in Et2O, results in formation of [cyclo-N(iPr)C(Me)2CH(NiPr2)C{CHC3(NiPr2)2}][U(NR2)2(N(SiMe3)SiMe2CH2)(CCPh)] (2) in moderate isolated yield. Complex 2 is the result of coupling and protonation of two BAC molecules, where complex 1 contributes the required proton. It was characterized by NMR spectroscopy and X-ray crystallography and represents a new mode of reactivity of the cyclopropenylidene fragment.

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

1,3-Imidazole-Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N-Heterocyclic Carbenes

Classical N-heterocyclic carbenes (NHCs) featuring the carbene center at the C2-position of 1,3-imidazole framework (i.e. C2-carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ-donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so-called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ-donors than C2-carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5-anionic dicarbenes (ADCs), also based on an 1,3-imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main-group heterocycles, radicals, molecular catalysts, ligands sets, and more.

Phosphorus derivatives of mesoionic carbenes: synthesis and characterization of triazaphosphole-5-ylidene → BF3 adducts

Trimethylsilyl-substituted triazaphospholes were synthesized by a [3+2] cycloaddition reaction between organic azides and (CH3)3Si-CP. In an attempt to isolate their N-alkylated products, the formation of BF3 adducts of unprecedented triazaphosphol-5-ylidenes was found. The nature of the carboncarbene-boron bond was investigated within the DFT framework, revealing a strong donation of electrons from the carbene carbon atom to the boron atom combined with weak back-bonding.

Evidence for Carbene Intermediates in Isocyanide Homologation by Aluminium(I)

The C-C bond formation between C1 molecules plays an important role in chemistry as manifested by the Fischer-Tropsch (FT) process. Serving as models for the FT process, we report here the reactions between a neutral AlI complex (Me NacNac)Al (1, Me NacNac=HC[(CMe)(NDipp)]2 , Dipp=2,6-diisopropylphenyl) and various isocyanides. The step-by-step coupling mechanism was studied in detail by low-temperature NMR monitoring, isotopic labeling, as well as quantum chemical calculations. Three different products were isolated in reaction of 1 with the sterically encumbered 2,6-bis(benzhydryl)-4-Me-phenyl isocyanide (BhpNC). These products substantiate carbene intermediates. The reaction between 1 and adamantyl isocyanide (AdNC) generated a trimerization product, and a corresponding carbene intermediate could be trapped in the form of a molybdenum(0) complex. Tri-, tetra-, and even pentamerization products were isolated with the sterically less congested phenyl and p-methoxyphenyl isocyanides (PhNC and PMPNC) with concurrent construction of quinoline or indole heterocycles. Overall, this study provides evidence for carbene intermediates in FT-type chemistry of aluminium(I) and isocyanides.

[IPr#-PEPPSI]: A Well-Defined, Highly Hindered and Broadly Applicable Pd(II)-NHC (NHC = N-Heterocyclic Carbene) Precatalyst for Cross-Coupling Reactions

In this Special Issue, "Featured Papers in Organometallic Chemistry", we report on the synthesis and characterization of [IPr#-PEPPSI], a new, well-defined, highly hindered Pd(II)-NHC precatalyst for cross-coupling reactions. This catalyst was commercialized in collaboration with MilliporeSigma, Burlington, ON, Canada (no. 925489) to provide academic and industrial researchers with broad access to reaction screening and optimization. The broad activity of [IPr#-PEPPSI] in cross-coupling reactions in a range of bond activations with C-N, C-O, C-Cl, C-Br, C-S and C-H cleavage is presented. A comprehensive evaluation of the steric and electronic properties is provided. Easy access to the [IPr#-PEPPSI] class of precatalysts based on modular pyridine ligands, together with the steric impact of the IPr# peralkylation framework, will facilitate the implementation of well-defined, air- and moisture-stable Pd(II)-NHC precatalysts in chemistry research.

Gold complexes with remote-substituted amino N-heterocyclic carbenes

The 4-RN-1,3-Ar₂-imidazolium salt, R = iPr, tBu, Ar = Mes, Dipp, Mes = mesityl, Dipp = 2,6-bis-diisopropyl-phenyl was metalated by Au^I at the C2-, C5- and 4-RN positions depending on the reactants and conditions employed; a rare direct rearrangement of a Au^I aminide to an abnormal imidazol-5-ylidene Au^I complex was also observed and based on a DFT study it may involve TfO^- facilitated H⁺ transfer.

(CAAC)Pd(py) Catalysts Disproportionate to Pd(CAAC)2

Palladium complexes with one N-heterocyclic carbene (NHC) and a pyridine ancillary ligand are powerful cross-coupling precatalysts. Herein, we report such complexes with a cyclic (alkyl)(amino)carbene (CAAC) ligand replacing the NHC. We find that the alleged reduced form, (CAAC)Pd(py), disproportionates to the (CAAC)2Pd0 complex and palladium nanoparticles. This notwithstanding, they are potent catalysts in the Buchwald-Hartwig amination with aryl chlorides under mild conditions (60 °C). In the presence of dioxygen, these complexes catalyze the formation of diazenes from anilines. The catalytic activities of the NHC- and CAAC-supported palladium(0) and palladium(II) complexes are similar in the cross-coupling reaction, yet the CAAC complexes are superior for diazene formation.

Chalcogen-substituted carbenes: a density functional study of structure, stability, and donor ability

Chalcogen-substituted carbenes are examined computationally using density functional theory. Several approaches are used to assess the stability and reactivity of chalcogenazol-2-ylidene carbenes (NEHCs; E = O, S, Se, Te). The known unsaturated species 1,3-dimethylimidazol-2-ylidene is studied at the same level of theory as the NEHC molecules, as a reference. Electronic structures, stability towards dimerization, and ligand properties are discussed. The results highlight the NEHCs as potentially valuable ancillary ligands for stabilizing low-valent metals or paramagnetic main group molecules. A simple, effective computational method for evaluating σ donor ability and π acidity of carbenes is presented.

ItOct (ItOctyl) - pushing the limits of ItBu: highly hindered electron-rich N-aliphatic N-heterocyclic carbenes

ItBu (ItBu = 1,3-di-tert-butylimidazol-2-ylidene) represents the most important and most versatile N-alkyl N-heterocyclic carbene available in organic synthesis and catalysis. Herein, we report the synthesis, structural characterization and catalytic activity of ItOct (ItOctyl), C₂-symmetric, higher homologues of ItBu. The new ligand class, including saturated imidazolin-2-ylidene analogues has been commercialized in collaboration with MilliporeSigma: ItOct, 929 298; SItOct, 929 492 to enable broad access of the academic and industrial researchers within the field of organic and inorganic synthesis. We demonstrate that replacement of the t-Bu side chain with t-Oct results in the highest steric volume of N-alkyl N-heterocyclic carbenes reported to date, while retaining the electronic properties inherent to N-aliphatic ligands, such as extremely strong σ-donation crucial to the reactivity of N-alkyl N-heterocyclic carbenes. An efficient large-scale synthesis of imidazolium ItOct and imidazolinium SItOct carbene precursors is presented. Coordination chemistry to Au(i), Cu(i), Ag(i) and Pd(ii) as well as beneficial effects on catalysis using Au(i), Cu(i), Ag(i) and Pd(ii) complexes are described. Considering the tremendous importance of ItBu in catalysis, synthesis and metal stabilization, we anticipate that the new class of ItOct ligands will find wide application in pushing the boundaries of new and existing approaches in organic and inorganic synthesis.

From Divalent to Pentavalent Iron Imido Complexes and an Fe(V) Nitride via N-C Bond Cleavage

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N-anchored tris-NHC chelating ligand tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMN^Mes) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMN^Mes)Fe^I(N₂)]⁺ (1) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMN^Mes)Fe(NAd)]^0,1+,2+,3+ (2^Ad-5^Ad) in oxidation states II to V. The Fe(V) imide [(TIMMN^Mes)Fe(NAd)]³⁺ (5^Ad) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMN^Mes)Fe^V(N)]²⁺ (6) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMN^Mes)Fe^IV(N)]⁺ (7) with an ethyl group using the triethyloxonium salt Et₃OPF₆. This gives access to the analogous series of ethyl imides [(TIMMN^Mes)Fe(NEt)]^0,1+,2+,3+ (2^Et-5^Et), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as ⁵⁷Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.

A 1,2,3-triazole-derived pincer-type mesoionic carbene complex of iron(II): carbonyl elimination and hydrosilylation of aromatic aldehydes via the concerted reaction with hydrosilane and a base

Iron complexes bearing 1,2,3-triazol-5-ylidene were synthesized and applied to the reaction with hydrosilane and homogeneous catalytic hydrosilylation of aromatic ketones and aldehydes. Addition of a free carbene to a solution of Fe(CO)₄Br₂ yielded an octahedral, diamagnetic and cationic iron(II) complex [Fe(1,2,3-triazolylidene)(CO)₂Br]⁺. Pyrolysis of the dicarbonyl complex eliminated the two CO ligands to form a paramagnetic four-coordinate complex. A theoretical study using DFT calculations indicated that the spin state changed from singlet to quintet during ligand elimination. Investigations of the successful hydrosilylation of acetophenone and benzaldehyde derivatives using MIC-iron(II) bromide suggested the importance of the base for efficient conversion in the catalytic process. The bromide-to-hydride exchange reaction, transmetallation, of MIC-iron(II) bromide in the presence of KO^tBu and HSi(OEt)₃ which could occur in the initial process of hydrosilylation was proposed, and supported by a theoretical study.

Activation of Ge-H and Sn-H Bonds with N-Heterocyclic Carbenes and a Cyclic (Alkyl)(amino)carbene

A study of the reactivity of several N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene (cAACMe ) with the group 14 hydrides GeH2 Mes2 and SnH2 Me2 (Me=CH3 , Mes=1,3,5-(CH3 )3 C6 H2 ) is presented. The reaction of GeH2 Mes2 with cAACMe led to the insertion of cAACMe into one Ge-H bond to give cAACMe H-GeHMes2 (1). If 1,3,4,5-tetramethyl-imidazolin-2-ylidene (Me2 ImMe ) was used as the carbene, NHC-mediated dehydrogenative coupling occurred, which led to the NHC-stabilized germylene Me2 ImMe ⋅GeMes2 (2). The reaction of SnH2 Me2 with cAACMe also afforded the insertion product cAACMe H-SnHMe2 (3), and reaction of two equivalents Me2 ImMe with SnH2 Me2 gave the NHC-stabilized stannylene Me2 ImMe ⋅SnMe2 (4). If the sterically more demanding NHCs Me2 ImMe , 1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene (iPr2 ImMe ) and 1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene (Dipp2 Im) were employed, selective formation of cyclic oligomers (SnMe2 )n (5; n=5-8) in high yield was observed. These cyclic oligomers were also obtained from the controlled decomposition of cAACMe H-SnHMe2 (3).

Well-Defined, Air- and Moisture-Stable Palladium-Imidazo[1,5-a]pyridin-3-ylidene Complexes: A Versatile Catalyst Platform for Cross-Coupling Reactions by L-Shaped NHC Ligands

We describe the development of [(NHC)Pd(cinnamyl)Cl] complexes of ImPy (ImPy = imidazo[1,5-a]pyridin-3-ylidene) as a versatile class of precatalysts for cross-coupling reactions. These precatalysts feature fast activation to monoligated Pd(0) with 1:1 Pd to ligand ratio in a rigid imidazo[1,5-a]pyridin-3-ylidene template. Steric matching of the C5-substituent and N2-wingtip in the catalytic pocket of the catalyst framework led to the discovery of ImPyMesDipp as a highly reactive imidazo[1,5-a]pyridin-3-ylidene ligand for Pd-catalyzed cross-coupling of nitroarenes by challenging C-NO2 activation. Kinetic studies demonstrate fast activation and high reactivity of this class of well-defined ImPy-Pd catalysts. Structural studies provide full characteristics of this new class of imidazo[1,5-a]pyridin-3-ylidene ligands. Computational studies establish electronic properties of sterically-restricted imidazo[1,5-a]pyridin-3-ylidene ligands. Finally, a scalable synthesis of C5-substituted imidazo[1,5-a]pyridin-3-ylidene ligands through Ni-catalyzed Kumada cross-coupling is disclosed. The method obviates chromatographic purification at any of the steps, resulting in a facile and modular access to ImPy ligands. We anticipate that well-defined [Pd-ImPy] complexes will find broad utility in organic synthesis and catalysis for activation of unreactive bonds.

Gauging Radical Stabilization with Carbenes

Carbenes, including N-heterocyclic carbene (NHC) ligands, are used extensively to stabilize open-shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical-stabilizing C-donor ligands. With the large variety of C-donor ligands experimentally investigated and their electronic properties established, we report herein their radical-stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π-donation to- and π-donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π-donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group- and transition metal carbene complexes, and the quantification of redox non-innocence.