Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Cyclic (Alkenyl)(Amino)Carbene (SMeCAenAC): Introducing a Member to the Cyclic (Alkyl)(Amino)Carbenes Family Featuring a Narrow Energy Gap

Herein, we report the carbene-like activity of a nonisolable, highly ambiphilic cyclic (alkenyl)(amino)carbene (SMeCAenAC, 3), which is stabilized as [(SMeCAenAC)(H)N(SiMe3)2] (4). This protected form (4) is stable in air and moisture. Compound 4 can be used as a carbene source through deamination upon heating to 130-140 °C. Moreover, density functional theory (DFT) calculations indicate that SMeCAenAC has the smallest singlet-triplet gap (37.05 kcal/mol) and a narrow highest occupied molecule orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap (3.92 eV) among the cyclic (alkyl)(amino)carbenes (CAACs). The precursor of carbene (3) can be synthesized on a multigram scale with a good yield. Moreover, the SMeCAenAC-coordinated copper complex showed excellent efficiency in the catalytic addition of phenols to electron-deficient olefins. This study also highlights that [SMeCAenAC-H]OTf can be used for metal-free catalysis, a property uniquely characteristic of an ambiphilic carbene, even though the formation of free SMeCAenAC (3) was not achieved.

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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Ambient Temperature Isolation of a Monatomic Boron(0) Complex

The first bottleable example of a neutral Group 13 atom bound only by neutral donor ligands (L) has been fully characterized by spectroscopic methods and its structure determined by a single-crystal X-ray diffraction study. A two-coordinate paramagnetic L2B0 complex can readily be accessed through a facile reduction reaction and is stabilized by π-accepting cyclic (alkyl)(amino)carbene (CAAC) ligands. Further reduction of (CAAC)2B leads to the isolation of a stable diamagnetic boride anion. In turn, oxidation leads to the putative formation of a transient two-coordinate cationic borylene, which has been trapped to form a stable boron(I) complex. Density functional theory calculations support the formulation of (CAAC)2B as a boron(0) complex stabilized by strong multiple bonding.

Carbene-Functionalized Bulky-Cyclopentadiene Rings

A series of significantly bulky mono- and di-substituted cyclic alkyl-amino carbene (cAAC)- functionalized cyclopentadiene ring (Cp) compounds were synthesized. The functionalization of the Cp ring with cAAC ligands makes them significantly bulkier, while retaining their ligation properties. These compounds display interesting fluorescence properties. In these compounds, intra-molecular charge transfer is observed from electron-rich carbene to electron-deficient cyclopentadiene unit. These high yielding compounds have been characterized by X-ray single-crystal diffraction and their emission properties have been studied. Rotational conformers (via C-C bond rotation) play a pivotal role with possible different extent of intramolecular charge-transfer (ICT) from carbene to cyclopentadiene ring. Variable temperature-dependent NMR studies were performed along with NOSY, COSY and different 2D NMR techniques to estimate the energy barriers and 1,5-Hydrogen shift.

Exploring Stibanyl Ligand for Accessing Arsinidene and Arsaketene Adducts, and Phosphaketene

The salt metathesis reaction involving a diamine-based antimony chloride precursor with sodium arsaethynolate in the presence of PMe3 leads to the formation of stibanyl-functionalized PMe3-arsinidene (2). Detailed analyses through single-crystal X-ray diffraction and density functional theory of 2 confirm the presence of covalent Sb-As bonds and reveal its polarized nature with a multiple-bond character. In contrast to the formation of complex 2, substituting PMe3 with xylyl isocyanide or 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene (IiPr) produces an isocyanide-arsinidene adduct (3) and an IiPr-arsaketene complex (4), respectively. Furthermore, the related reactions of precursor 1 with sodium phosphaethynolate yield both a stibanyl-phosphaketene (5) and a stibanyl-functionalized IiPr-phosphaketene adduct (6).

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.

Bicyclic (alkyl)(amino)carbenes (BICAACs): synthesis, characteristics, and applications

Carbenes in general and isolable NHCs (N-heterocyclic carbenes) in particular have been useful ligands in recent years. The emergence of CAACs [cyclic(alkyl)(amino)carbenes], BICAACs [bicyclic(alkyl)(amino)carbenes], and many other carbenes has marked revolutionary milestones in this field. These carbenes possess an intriguing blend of highly electrophilic and nucleophilic characteristics, owing to their remarkably narrow HOMO-LUMO energy gap. The isolation and characterization of these carbenes hold significance not only due to their fascinating electronic properties but have demonstrated their prowess across various domains, including isolation of transition metal complexes, medicinal applications, catalysis, and radical stabilization. While the chemistry of 5-membered NHCs and CAACs has been extensively explored, the investigation of BICAACs has just begun. This review covers the synthesis, characterization, and reactivity of BICAACs and outlines the diverse applications of BICAACs in organometallic chemistry, metal-free catalysis, and main-group chemistry.

A Discrete Trialane with a Near-Linear Al3 Axis

The presence of inherent electronic unsaturation in aluminum predominantly results in the formation of aluminum clusters, with very few examples of compounds containing discrete chains of aluminum atoms in existence. In this work, we present the successful synthesis and structural authentication of a highly unusual trialane species with a near-linear chain of three Al atoms, alongside a carbene-stabilized aluminyl anion ([LAlR2]-), an alternative product produced by varying the reaction conditions. Quantum-chemical calculations have been applied to elucidate the electronic structure and bonding of these novel compounds. Additionally, we successfully trapped a reaction intermediate using an alkyne, suggesting the intermediacy of a base-stabilized monomeric alumylene (LRAl:), which is also investigated through computational methods.

A Strongly Ambiphilic Ferrocene-Based Cyclic (Alkyl)(amino)carbene - Specific Decomposition to an Enamine by a 1,2-Phenyl Shift

The recently described crystalline cyclic (alkyl)(amino)carbene with a 1,1'-ferrocenylene (fc) backbone fc(CPh2-C-NMes) (A, Mes=mesityl) is highly reactive due to its particularly pronounced ambiphilicity and is thermally not stable in solution due to an intramolecular insertion of the divalent carbon atom into a methyl C-H bond of the Mes substituent. The closely related congener fc(CPh2-C-N-p-C6H4-tBu) (1) cannot undergo such an insertion reaction. Nevertheless, 1 is too short-lived for isolation due to a rapid 1,2-shift of a phenyl group, furnishing the isomeric cyclic enamine fc[C(Ph)=C(Ph)-N-p-C6H4-tBu] (1') in a specific decomposition process unprecedented for CAACs. Trapping of 1 was possible with carbon monoxide, elemental selenium and with [CuBr(SMe2)], respectively affording the aminoketene 1=C=O, the selenoamide 1=Se and the homoleptic CuI complex [Cu(1)2][CuBr2]. 1 is an even stronger ambiphile than A according to NMR spectroscopic data. Similar to A, 1 does not react with H2, because the experimentally observed intramolecular process is kinetically more favourable according to DFT results.

Synthesis and structures of molecular beryllium Grignard analogues featuring terminal and bridging pseudohalides

The carbene-stabilised beryllium Grignards [(CAAC)BeBrR] (R = CAACH 1a, Dur 1b; CAAC/H = 1-(2,6-diisopropylphenyl)-2,2,4,4-tetramethylpyrrolidin-2-yl/idene; Dur = 2,3,5,6-tetramethylphenyl) undergo salt metathesis with various pseudohalide salt precursors. Whereas with [NaNCS] the thiocyanato Grignards [(CAAC)Be(NCS)R] (R = CAACH 2a, Dur 2b) are obtained selectively, salt metatheses with [Na(OCP)(dioxane)2.3] and [K(OCN)] are fraught with side reactions, in particular scrambling of both neutral and anionic ligands, leading to complex product mixtures, from which the first examples of beryllium phosphaethynolate Grignards [(thf)2(CAACH)Be(OCP)] (3) and [(CAAC)Be(OCP)R] (R = CAACH 4a, Dur 4b), as well as the isocyanate-bridged hexamer [(CAAC)BrBe(1,3-μ-OCN)]6 (7) were determined as the main products. The complexity of possible side reactions is seen in complex 5, a byproduct of the salt metathesis of 1b with [Na(OCP)(dioxane)2.3], which hints at radical redox processes, OCP homocoupling, OCP coupling with CAAC, as well as OCP insertion into the Be-R bond. Finally, the unstable, tetrameric cyano-bridged beryllium Grignard [(thf)(CAACH)Be(1,2-μ-CN)] (8) was obtained by salt metathesis of 1a with [Na/KSeCN] alongside one equiv. CAACSe. The new complexes were characterised by heteronuclear NMR and IR spectroscopy, as well X-ray crystallography.

(CAAC)CuCl: A Competent Precatalyst for Carbonyl and Ester Hydrosilylation

Cu-catalyzed carbonyl hydrosilylation involves a ligated "[(L)CuH]" as the active catalyst, where the ligand L has a crucial role toward the stability, stereoselectivity, and enhancement of the hydridicity. Strongly σ-donating N-heterocyclic carbenes (NHCs), their ring-expanded form, and an abnormal NHC as ligands have yielded robust and efficient Cu catalysts. However, cyclic(alkyl)(amino)carbenes (CAACs), despite being stronger σ-donors than NHCs and already having a salient Cu(I) chemistry, are yet to be reported as a similar ligand platform for this purpose. We establish here the familiar [(Me2CAAC)CuCl] as a powerful precatalyst in this regard. Additionally, it also catalyzes the more challenging ester hydrosilylation, which is a rare feat for a Cu catalyst. Apart from the stronger σ-donating ability, the more steric "openness" of CAACs than bulky NHCs also seems to be advantageous. To corroborate, three new (CAAC)CuCl complexes [(ArCH2,MeCAAC)CuCl] (Ar = Ph, 1-naphthyl, and 1-prenyl) are devised, where the effective steric around the copper is practically unaltered from the case of [(Me2CAAC)CuCl]. All three are equally active in carbonyl and ester hydrosilylation as [(Me2CAAC)CuCl]. Computation suggests the carbonyl insertion into a "(CAAC)Cu-H" as the rate-limiting step. To elucidate the involvement of a "(CAAC)CuH", "(PhCH2,MeCAAC)CuH" is generated in situ and is trapped as its BH3 adduct (PhCH2,MeCAAC)CuBH4.

A carbene-stabilized diphosphorus: a triple-bonded diphosphorus (P[triple bond, length as m-dash]P) and a bis(phosphinidene) (P-P) transfer agent

The reaction of the N,N-diisopropyl bromoiminium salt with excess sodium phosphaethynolate (NaPCO) affords a diphospha-urea 2. Under blue light irradiation (450 nm), carbon monoxide is liberated affording the bis(carbene)P2 adduct 3. Photolysis of a benzene solution of 3 at 365 nm gives rise to the carbene dimer, namely the 1,2-bis(diisopropylamino)ethylene as a cis/trans mixture, along with white and red phosphorus. Under the same experimental conditions, but in the presence of excess 2,3-dimethyl-1,3-butadiene, the classical double Diels-Alder adduct of the triple-bonded diphosphorus P[triple bond, length as m-dash]P was obtained along with the bis(phospholene) formally resulting from a double [4 + 1] reaction of the diene with the bis(phosphinidene) form of P2. A stepwise carbene-carbene exchange reaction also occurs between the monosubstituted aminocarbene of 3 and a cyclic (alkyl)(amino)carbene, possibly involving the transient formation of a diphosphorus analogue of a diazo compound.

Streamlined synthetic assembly of α-chiral CAAC ligands and catalytic performance of their copper and ruthenium complexes

The unique electronic and steric parameters of chiral cyclic alkyl amino carbene (CAAC) ligands render them appealing steering ligands for enantioselective transition-metal catalyzed transformations. Due to the lack of efficient synthetic strategies to access particularly attractive α-chiral CAACs assessment and exploitation of their full synthetic potential remain difficult. Herein, we report a streamlined strategy to assemble a library of diastereo- and enantiomerically pure CAAC ligands featuring the notoriously difficult to access α-quaternary stereogenic centers. A tailored Julia-Kocienski olefination reagent allows the Claisen-rearrangement to be leveraged as an expedient route to form the synthetically pivotal racemic α-chiral methallyl aldehydes. Subsequent condensation with chiral amines and further cyclization provided a library of diastereomeric mixtures of the targeted ligand precursors. The CAAC salts as well as their corresponding metal complexes are conveniently separable by standard silica gel flash chromatography closing a long-standing accessibility gap in chiral CAAC ligands with proximal α-chirality. The rapid availability of both diastereomers enables testing of the relevance and synergistic effects of two chiral centers on the ligand in catalytic applications. A broad range of metal complexes with copper, gold, rhodium and ruthenium were obtained and structurally analyzed. The catalytic performances of the corresponding chiral CAAC copper and ruthenium complexes were assessed in enantioselective conjugate borylations and asymmetric ring closing metathesis, displaying selectivities of up 95 : 5 er.

Small molecule activation by sila/germa boryne species

The inability of p-block elements to participate in π-backbonding restricts them from activating small molecules like CO, H2 , and so forth. However, the development of the main group metallomimetics became a new pathway, where the main-group elements like boron can bind and activate small molecules like CO and H2 . The concept of the frustrated Lewis pair, Boron-Boron multiple bonds, and borylene are previously illustrated. Some of these reported classes of boron species can mimic the jobs of the metal complexes. Hence, we have theoretically studied the binding of CO/N2 molecules at B-center of elusive species like sila/germa boryne stabilized by donor base ligands (cAAC)BE(Me)(L), where E  Si, L  cAACMe , NHCMe , PMe3 , E  Ge, L  cAACMe and (NHCMe )BE(Me)(cAACMe )). The substitutional analogues of (cAACR )BSiR1 (cAAC) and E  P, L  cAACMe ) have been studied by density functional theory (DFT), natural bond orbital, QTAIM calculations and energy decomposition analysis (EDA) coupled with natural orbital for chemical valence (NOCV) analyses. The computed bond dissociation energy and inner stability analyses by the EDA-NOCV method showed that the CO molecule can bind at the B-center of the above-mentioned species due to stronger σ-donor ability while binding of N2 has been theoretically predicted to be weak. The energy barrier for the CO binding is estimated to be 13-14 kcal/mol by transition state calculation. The change of partial triple bond character to single bond nature of the BSi bond and the bending of CBSi bond angle of sila-boryne species are the reason for the activation energy. Our study reveals the ability of such species to bind and activate the CO molecule to mimic the transition metal-containing complexes. We have additionally shown that binding of Fe(CO)4 and Ni(CO)3 is feasible at Si-center after binding of CO at the B-center.

An isolable, chelating bis[cyclic (alkyl)(amino)carbene] stabilizes a strongly bent, dicoordinate Ni(0) complex

Chelating ligands have had a tremendous impact in coordination chemistry and catalysis. Notwithstanding their success as strongly σ-donating and π-accepting ligands, to date no chelating bis[cyclic (alkyl)(amino)carbenes] have been reported. Herein, we describe a chelating, C2-symmetric bis[cyclic (alkyl)(amino)carbene] ligand, which was isolated as a racemic mixture. The isolation and structural characterization of its isostructural, pseudotetrahedral complexes with iron, cobalt, nickel, and zinc dihalides featuring eight-membered metallacycles demonstrates the binding ability of the bis(carbene). Reduction of the nickel(II) dibromide with potassium graphite produces a dicoordinate nickel(0) complex that features one of the narrowest angles measured in any unsupported dicoordinate transition metal complexes.

Cyclic (Alkyl)(Amino)Carbene-Iminoboryl Compounds with Three Formal Oxidation States

BN/CC isosterism is an effective strategy to build hybrid functional molecules with unique properties. In contrast to the alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) that feature only one reversible reduction wave, the isoelectronic cationic CAAC-iminoboryl adducts could be singly and doubly reduced smoothly. Both the resultant neutral radical and anionic azaborataallenes bear NBC-mixed allenic structures. The former radical has a high spin-density of 0.55e at CCAAC carbon, yet exhibits formal boron-centered radical reactivity. The latter azaborataallenes feature the nucleophilic CCAAC center and polar N(δ-)═B(δ+)═C(δ-) unit, and readily undergo nucleophilic substitution, isocyanide insertion, dipolar addition and cycloaddition reactions etc. The N-substituents have been shown to have a significant influence on the solid-state structure, thermal stability, and reactivity of azaborataallenes. This work showcases the allenic BN-unsaturated species as versatile building blocks in organic synthesis.

Concatenating Structural Constraint Effects at Tin for the Sequential Generation, Stabilization, and Transfer of Acyclic Aminocarbenes

Structural constraint approaches have been employed toward different ends in recent years, from augmenting the nucleophilicity in pyramidalized low-valent p-block compounds to enhancing the Lewis acidities at planarized tetravalent p-block elements. While previous studies exploited these effects separately, this work introduces a strategy to concatenate structural constraint approaches at individual stages of a reaction sequence in a row to unlock a synthetic path unattainable by conventional methodologies. The boosted nucleophilicity resulting from the constrained tetracoordinated calix[4]pyrrolato stannate(II) dianion enables the reductive formation of sterically unprotected acyclic aminocarbenes. These amino carbenes are stabilized at the concomitantly formed square-planar stannane(IV) as air-stable adducts. Transfer of the carbenes onto copper(I) by cooperativity of the calix[4]pyrrole ligand finalizes this protocol to hitherto unreported yet prototypical carbene complexes. Detailed spectroscopic and quantum theoretical analyses establish the synergy of structural constraints and element-ligand cooperation as the linchpin to this reaction path and its selectivity.

Tricoordinate Beryllium Radicals and Their Reactivity

The one-electron reduction of [(CAAC)Be(Dur)Br] (CAAC = cyclic alkyl(amino)carbene, Dur = 2,3,5,6-tetramethylphenyl = duryl) with lithium sand in diethyl ether yields the first neutral, tricoordinate, and moderately stable beryllium radical, [(CAAC)(Et2O)BeDur]• (2-Et2O), which undergoes a facile second one-electron reduction concomitant with the insertion of the beryllium center into the endocyclic C-NCAAC bond and a cyclopropane-forming C-H bond activation of an adjacent methyl group. In situ generation of 2-Et2O and addition of PMe3 yield the stable analogue, [(CAAC)(Me3P)BeDur]• (2-PMe3), which serves as a platform for PMe3-ligand exchange with stronger donors, generating the radicals [(CAAC)LBeDur]• (2-L, L = isocyanides, pyridines, and N-heterocyclic carbenes). X-ray structural analyses show trigonal-planar beryllium centers and strong π backbonding from the metal to the CAAC ligand. The EPR signals of all six isolated [(CAAC)LBeDur]• radicals display significant, albeit small, hyperfine coupling to the 9Be nucleus. DFT calculations show that the spin density is mostly delocalized over the CAAC π framework and, where present, the isocyanide CN moiety, with only a small proportion (3-6%) on the beryllium center. 2-PMe3 proved thermally unstable at 80 °C, first undergoing radical hydrogen abstraction with the solvent, followed by insertion of beryllium into the endocyclic C-NCAAC bond and PMe3 transfer to the former carbene carbon atom. The reactions with diphenyl disulfide and phenyl azide occur at the beryllium center and yield the corresponding Be(II) phenyl sulfide and amino complexes, respectively, the latter concomitant with radical transfer and hydrogen abstraction by the beryllium-bound nitrogen center.

BICAAC-Derived Covalent and Cationic Ir(I) Complexes: Application of Ir(BICAAC)Cl(COD) Complexes as Catalysts for Transfer Hydrogenation and Hydrosilylation Reactions

The ambiphilic bicyclic (alkyl)(amino)carbenes (Me/iPrBICAAC) upon reaction with [IrCl(COD)]2 smoothly afford mononuclear Ir(I) complexes that have been spectroscopically and structurally characterized. These complexes exhibit good catalytic activity for transfer hydrogenation (TH) of 4-chlorobenzaldehyde using isopropyl alcohol (iPrOH), with turnover frequency values ranging between 6269 and 8093 h-1. Choosing the covalent complex Ir(MeBICAAC)Cl(COD) as a catalyst, a wide array of carbonyls and imines functionalized with electron-withdrawing and electron-donating substituents have been surveyed and afforded their reduced products in moderate-to-good yields. No detachment of the BICAAC unit from the Ir center was observed upon prolonged heating of Ir(MeBICAAC)Cl(COD) in toluene-d8 or isopropyl alcohol-d8, which evidenced good thermal stability of the catalyst. Complex Ir(MeBICAAC)Cl(COD) was also found to be catalytically active for the hydrosilylation of a variety of aldehydes using triethylsilane (Et3SiH).

Carbodiimide and Isocyanate Hydroboration by a Cyclic Carbodiphosphorane Catalyst

We report hydroboration of carbodiimide and isocyanate substrates catalyzed by a cyclic carbodiphosphorane catalyst. The cyclic carbodiphosphorane outperformed the other Lewis basic carbon species tested, including other zerovalent carbon compounds, phosphorus ylides, an N-heterocyclic carbene, and an N-heterocyclic olefin. Hydroborations of seven carbodiimides and nine isocyanates were performed at room temperature to form N-boryl formamidine and N-boryl formamide products. Intermolecular competition experiments demonstrated the selective hydroboration of alkyl isocyanates over carbodiimide and ketone substrates. DFT calculations support a proposed mechanism involving activation of pinacolborane by the carbodiphosphorane catalyst, followed by hydride transfer and B-N bond formation.

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides

Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H+ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to py aziridines at 80 °C but unprecedentedly result N-pico CAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy , as revealed by computational analysis, plays a crucial role in this unusual 1,3-H+ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both N-pico CAAC and py aziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3 SiH. Notably, the py aziridine-Rh(I) is superior than the N-pico CAAC-Rh(I) catalyst.

Four- and three-coordinate planar iron(II) complexes supported by bulky organosilyl ligands

The ligand exchange reaction of (THF)2Fe[Si(SiMe3)3]2 with 2 equivalents of an N-heterocyclic carbene (NHC) led to the formation of a square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands. Conversely, the introduction of a cis-coordinate bidentate organosilyl ligand instead of -Si(SiMe3)3 resulted in the formation of a square planar iron(II) complex supported by a cis-coordinate bidentate organosilyl ligand. A three-coordinate planar iron(II) bis(silyl) complex was also synthesized using a cis-coordinate bidentate organosilyl ligand and a cyclic (alkyl)(amino)carbene auxiliary ligand. Investigation of the catalytic performance of these complexes in the hydrosilylation of acetophenone revealed that the square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands exhibits superior reactivity relative to its tetrahedral precursor.

Terminal methylene phosphonium ions: precursors for transient monosubstituted phosphinocarbenes

Isolable singlet carbenes are among the most important tools in chemistry, but generally require the interaction of two substituents with the electron deficient carbon atom. We herein report a synthetic approach to monosubstituted phosphinocarbenes via deprotonation of hitherto unknown diprotic terminal methylene phosphonium ions. Two methylene phosphonium salts bearing bulky N-heterocyclic imine substituents at the phosphorus atom were isolated and fully characterized. Deprotonation studies indicate the formation of transient monosubstituted carbenes that undergo intermolecular cycloadditions or intramolecular Buchner ring expansion to afford a cycloheptatriene derivative. The reaction mechanism of the latter transformation was elucidated using DFT calculations, which reveal the ambiphilic nature of the phosphinocarbene enabling the insertion into the aromatic C-C bond. Additional computational studies on the role of substituent effects are presented.

Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity

The use of 1,3,4,5-tetramethylimidazol-2-ylidene (IMe) to coordinate with diatomic B₂ species afforded a tetrakis(N-heterocyclic carbene)-diboron(0) [(IMe)₂B-B(IMe)₂] (2). The singly bonded B₂ moiety therein possesses a valence electronic configuration 1σ_g²1π_u²1π_g*² with four vacant molecular orbitals (1σ_u*, 2σ_g, 1π_u', 1π_g'*) coordinated with IMe. Its unprecedented electronic structure is analogous to the energetically unfavorable planar hydrazine with a D_2h symmetry. The two highly reactive π_g* antibonding electrons enable double single-electron-transfer (SET) reactivity in small-molecule activation. Compound 2 underwent a double SET reduction with CO₂ to form two carbon dioxide radical anions CO₂^•-, which then reduced pyridine to yield a carboxylated pyridine reductive coupling dianion [O₂CNC₅(H)₅-C₅(H)₅NCO₂]^2- and converted compound 2 to the tetrakis(N-heterocyclic carbene)-diborene dication [(IMe)₂B═B(IMe)₂]²⁺ (3²⁺). This is a remarkable transition-metal-free SET reduction of CO₂ without ultraviolet/visible (UV/vis) light conditions.

A Donor-Acceptor Cyclopropane by Intramolecular C(sp3)-H Activation at a Cyclic(alkyl)(amino)carbene Center and Its Fascinating Ring-Opening Chemistry

Virtually irreversible intramolecular C-H activations are deleterious for aza-carbenes. A picolyl-tethered cyclic(alkyl)(amino)carbene (CAAC) isomerizes into a donor-acceptor cyclopropane in this manner but restores the CAAC status by retro-C-H activation in the presence of trapping agents like Se or CuCl. The same DA cyclopropane is readily hydrolyzed to a pyrrolidin-2-ol that acts as another ^picoCAAC precursor by undergoing 1,1-dehydration in the presence of Se or CuCl. The chemistry is distinct from the N-heterocyclic carbene analogue throughout.

Carbene Radicals in Transition-Metal-Catalyzed Reactions

Discovered as organometallic curiosities in the 1970s, carbene radicals have become a staple in modern-day homogeneous catalysis. Carbene radicals exhibit nucleophilic radical-type reactivity orthogonal to classical electrophilic diamagnetic Fischer carbenes. Their successful catalytic application has led to the synthesis of a myriad of carbo- and heterocycles, ranging from simple cyclopropanes to more challenging eight-membered rings. The field has matured to employ densely functionalized chiral porphyrin-based platforms that exhibit high enantio-, regio-, and stereoselectivity. Thus far the focus has largely been on cobalt-based systems, but interest has been growing for the past few years to expand the application of carbene radicals to other transition metals. This Perspective covers the advances made since 2011 and gives an overview on the coordination chemistry, reactivity, and catalytic application of carbene radical species using transition metal complexes and catalysts.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

An Air-Stable "Masked" Bis(imino)carbene: A Carbon-Based Dual Ambiphile

Carbenes, once considered laboratory curiosities, now serve as powerful tools in the chemical and material sciences. To date, all stable singlet carbenes are single-site ambiphiles. Here we describe the synthesis of a carbene which is a carbon-based dual ambiphile (both single-site and dual-site). The key is to employ imino substituents derived from a cyclic (alkyl)(amino)carbene (CAAC), which imparts a 1,3-dipolar character to the carbene. Its dual ambiphilic nature is consistent with the ability to activate simple organic molecules in both 1,1- and 1,3-fashion. Furthermore, its 1,3-ambiphilicity facilitates an unprecedented reversible intramolecular dearomative [3 + 2] cycloaddition with a proximal arene substituent, giving the carbene the ability to "mask" itself as an air-stable cycloadduct. We perceive that the concept of dual ambiphilicity opens a new dimension for future carbene chemistry, expanding the repertoire of applications beyond that known for classical single-site ambiphilic carbenes.

Access to a Labile Monomeric Magnesium Radical by Ball-Milling

In order to isolate a monometallic Mg radical, the precursor (Am)MgI⋅(CAAC) (1) was prepared (Am=tBuC(N-DIPP)2 , DIPP=2,6-diisopropylphenyl, CAAC=cyclic (alkyl)(amino)carbene). Reduction of a solution of 1 in toluene with the reducing agent K/KI led to formation of a deep purple complex that rapidly decomposed. Ball-milling of 1 with K/KI gave the low-valent MgI complex (Am)Mg⋅(CAAC) (2) which after rapid extraction with pentane and crystallization was isolated in 15 % yield. Although a benzene solution of 2 decomposes rapidly to give Mg(Am)2 (3) and unidentified products, the radical is stable in the solid state. Its crystal structure shows planar trigonal coordination at Mg. The extremely short Mg-C distance of 2.056(2) Å indicates strong Mg-CAAC bonding. Calculations and EPR measurements show that most of the spin density is in a π* orbital located at the C-N bond in CAAC, leading to significant C-N bond elongation. This is supported by calculated NPA charges in 2: Mg +1.73, CAAC -0.82. Similar metal-to-CAAC charge transfer was calculated for M0 (CAAC)2 and [MI (CAAC)2 + ] (M=Be, Mg, Ca) complexes in which the metal charges range from +1.50 to +1.70. Although the spin density of the radical is mainly located at the CAAC ligand, complex 2 reacts as a low-valent MgI complex: reaction with a I2 solution in toluene gave (Am)MgI⋅(CAAC) (1) as the major product.