Search

Although diradicals should exhibit a rather small reaction barrier as compared to closed-shell species for activating kinetically inert molecules, the activation and functionalization of carbon dioxide with stable main-group diradicals remain virtually unexplored. In this work, we present a thorough study on CO 2 activation, reversible capture, and (de)oxygenation mediated by stable Group 14 singlet diradicals (i.e., diradicaloids) [(ADC)E] 2 (E = Si, Ge, Sn) based on an anionic dicarbene (ADC) framework (ADC = PhC{N(Dipp)C} 2 ; Dipp = 2,6- i Pr 2 C 6 H 3 ). [(ADC)E] 2 readily undergo [4 + 2]-cycloadditions with CO 2 to result in barrelene-type bis-metallylenes [(ADC)E] 2 (OC═O). The CO 2 addition is reversible for E = Ge; thus, CO 2 detaches under vacuum or at an elevated temperature and regenerates [(ADC)Ge] 2 . [(ADC)Sn] 2 (OC═O) is isolable but deoxygenates additional CO 2 to form [(ADC)Sn] 2 (O 2 CO) and CO. [(ADC)Si] 2 (OC═O) is extremely reactive and could not be isolated or detected as it spontaneously reacts further with CO 2 to yield elusive monomeric Si(IV) oxides [(ADC)Si(O)] 2 (CO n ) or carbonates [(ADC)Si(CO 3 )] 2 (CO n ) ( n = 1 or 2) via the (de)oxygenation of CO 2 . The molecular structures of all isolated compounds have been established by X-ray diffraction, and a mechanistic insight of their formation has been suggested by DFT calculations.

Diradicals based on a meta-quinodimethane (m-QDM) scaffold generally have a triplet ground state and are rather scarce. Herein, m-QDM-based non-Kekulé diradicals [3,3'-(NHC) 2 BP] (3-NHC) (NHC = SIPr = C{N(Dipp)CH 2 } 2 ; IPr = C{N(Dipp)CH} 2 , Me-IPr = C{N(Dipp)CMe} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ; BP = 1,1'-C 6 H 4 C 6 H 4 ) featuring N-heterocyclic carbene (NHC) pendants are reported as crystalline solids. The EPR spectra of 3-NHC show both allowed (Δm s = 1) and forbidden (Δm s = 2; 'half-field') transitions characteristic for triplet diradicals. Variable temperature EPR studies however reveal a singlet ground state for 3-SIPr. Consistent with the EPR spectra, calculations predict a remarkably small singlet-triplet energy gap (ΔE ST ≤ 0.26 kcal/mol) for the 3-NHC compounds. The calculated singlet diradical character for the ground states of the 3-NHC compounds amounts to ~99 %., (© 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbH.)

The isolation of silicon analogues of phenyl anions such as (C 6 H 5 ) - and (C 6 H 4 ) 2- is challenging owing to their extremely high reactivity associated with their silylene character and weak C-Si π-interaction. Herein, we report the first annulated 1,4-disilabenzene-1,4-diide compound [(ADC)Si] 2 ( 5 ) based on anionic dicarbene (ADC) scaffolds (ADC = PhC{N(Dipp)C} 2 ; Dipp = 2,6- i Pr 2 C 6 H 3 ) as a green-yellow crystalline solid. Compound 5 is prepared by KC 8 reduction of the Si(IV) chloride [(ADC)SiCl 3 ] 2 ( 3 ) or the cyclic bis-chlorosilylene [(ADC)SiCl] 2 ( 4 ), which are also prepared for the first time. 5 is a neutral molecule, and each of the two-coordinated Si(I) atoms has a lone pair and an unpaired electron. Experimental and theoretical data indicate delocalization of the silicon unpaired electrons, resulting in a 6π-electron C 4 Si 2 ring in 5 . The diradical character ( y ) for 5 amounts to 15%. At room temperature, 5 readily reacts with dihydrogen (H 2 ) to form the elusive bis-hydridosilylenes [(ADC)SiH] 2 ( Z )- 6 and ( E )- 6 . The [4 + 2]-cycloaddition of 5 and PhC≡CPh in yielding the barrelene-type bis-silylene [(ADC)SiCPh] 2 ( 7 ) emphasizes the diradical reactivity of 5 . With elemental sulfur, 5 results in the S 2 - and S 3 -bridged silathione derivatives [(ADC)Si(S)] 2 (μ-S 2 ) ( 8a ) and [(ADC)Si(S)] 2 (μ-S 3 ) ( 8b ). Moreover, the treatment of 5 with Fe 2 (CO) 9 affords the Fe(0) complex [(ADC)Si(Fe(CO) 4 )] 2 (μ-CO) ( 9 ), in which each silicon atom serves as a two-electron σ-donor ligand and shares one electron with the bridging CO unit to form two Si-C bonds. The molecular structures of all compounds have been established by X-ray diffraction, and representative compounds have been analyzed by quantum chemical calculations.

N-Heterocyclic carbene (NHC) analogues of Wittig hydrocarbon, [(NHC)(Stil)(NHC)] (3a-c) (NHC = SIPr (1a) = C[N(Dipp)CH 2 ] 2 , Dipp = 2,6-iPr 2 C 6 H 3 ; IPr (1b) = C[N(Dipp)CH] 2 ; Me-IPr (1c) = C[N(Dipp)CMe] 2 and Stil = C 6 H 4 CHCHC 6 H 4 ) 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 KC 8 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(C 6 F 5 ) 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., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Carbocyclic aluminium halides [(ADC)AlX 2 ] 2 (2-X) (X=F, Cl, and I) based on an anionic dicarbene (ADC=PhC{N(Dipp)C} 2 , Dipp = 2,6-iPr 2 C 6 H 3 ) framework are prepared as crystalline solids by dehydrohalogenations of the alane [(ADC)AlH 2 ] 2 (1). KC 8 reduction of 2-I affords the peri-annulated Al(III) compound [(ADC H )AlH] 2 (4) (ADC H= PhC{N(Dipp)C 2 (Dipp H )N}, Dipp H =2-iPr,6-(Me 2 C)C 6 H 3 )) as a colorless crystalline solid in 76 % yield. The formation of 4 suggests intramolecular insertion of the putative bis-aluminylene species [(ADC)Al] 2 (3) into the methine C-H bond of HCMe 2 group. Calculations predict singlet ground state for 3, while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol. Compounds 2-F, 2-Cl, 2-I, and 4 have been characterized by NMR spectroscopy and their solid-state molecular structures have been established by single crystal X-ray diffraction., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Herein, reactivity studies of a cyclic bis-hydridostannylene [(ADC)SnH] 2 (1-H 2 ) (ADC=PhC{(NDipp)C} 2 ; Dipp=2,6-iPr 2 C 6 H 3 ) with various unsaturated organic substrates are reported. Reactions of terminal alkynes (RC≡CH) with 1-H 2 afford mixed acetylide-vinyl-functionalized bis-stannylenes via dehydrogenation and hydrostannylation. Treatment of 1-H 2 with PhC≡CCH 3 gives a unique distannabarrelene via dehydrogenative C (sp3) -H stannylation and hydrostannylation of the C≡CCH 3 moiety. 1-H 2 undergoes dehydrogenative [2+2]-cycloaddition reactions with diphenylacetylene, azobenzene, acetone, benzophenone, and benzaldehyde to form the 1,4-distannabarrelene derivatives. The elimination of H 2 in these reactions suggests the masked-diradical property of 1-H 2 . In fact, these [2+2]-cycloaddition products are also accessible on treatments of the Sn(I) diradicaloid [(ADC)Sn] 2 (1) with appropriate reagents. All compounds have been characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. Moreover, the catalytic activity of 1-H 2 has been shown for the hydroboration of unsaturated substrates., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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-iPr 2 C 6 H 3 ; Ar = Ph, DMP (4-Me 2 NC 6 H 4 ), Bp (4-PhC 6 H 4 )) to yield (E)-ethene-1,2-bis(olate) (i.e. - O-C=C-O - = CO en ) bridged mesoionic carbene (iMIC) lithium compounds CO en -[(iMIC)Li] 2 (CO en -[iMIC] 2 = [ArC{(Dipp)NC} 2 (CO)] 2 ) in quantitative yields. CO en -[(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 CuCl 2 ), and AgCl to afford the dinuclear compounds CO on -[(iMIC)E] 2 (E = Se, CuCl, AgCl) featuring a 1,2-dione bridged neutral bis-iMIC (i.e. CO on -[iMIC] 2 = [ArC{(Dipp)NC} 2 (C=O)] 2 ). CO en -[(iMIC)Li] 2 undergo redox-neutral salt metathesis reactions with LiAlH 4 and (Et 2 O) 2 BeBr 2 and afford CO en -[(iMIC)AlH 2 ] 2 and CO en -[(iMIC)BeBr] 2 , in which the dianionic CO en -moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X-ray diffraction. Stereoelectronic properties of CO on -[iMIC] 2 are quantified by experimental and theoretical methods., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Decarbonylation of a cyclic bis-phosphaethynolatostannylene [(ADC)Sn(PCO)] 2 based on an anionic dicarbene framework (ADC = PhC{N(Dipp)C} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ) under UV light results in the formation of a Sn 2 P 2 cluster compound [(ADC)SnP] 2 as a green crystalline solid. The electronic structure of [(ADC)SnP] 2 is analyzed by quantum-chemical calculations. At room temperature, [(ADC)SnP] 2 reversibly binds with CO 2 and forms [(ADC) 2 {SnOC(O)P}SnP]. [(ADC)SnP] 2 enables catalytic hydroboration of CO 2 and reacts with elemental selenium and Fe 2 (CO) 9 to afford [(ADC) 2 {Sn(Se)P 2 }SnSe] and [(ADC)Sn{Fe(CO) 4 }P] 2 , respectively. All compounds are characterized by multinuclear NMR spectroscopy and their solid-state molecular structures are determined by single-crystal X-ray diffraction., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Herein, ring-opening of mesoionic carbenes (iMICs) (iMIC=[ArC{N(Dipp)} 2 C(SiMe 3 )C:) (Dipp=2,6-iPr 2 C 6 H 3 , Ar=Ph, 4-Me 2 NC 6 H 4 or 4-PhC 6 H 4 ) based on an 1,3-imidazole scaffold to yield N-ethynylformimidamide (eFIM) derivatives as crystalline solids (eFIM={(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe 3 ) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t 1/2= 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., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Cyclic organoalane compounds [(ADC Ar )AlH 2 ] 2 (ADC Ar = ArC{(DippN)C} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ; Ar = Ph or 4-PhC 6 H 4 (Bp)) based on anionic dicarbene (ADC) frameworks have been reported as crystalline solids. Treatments of Li(ADC Ar ) with LiAlH 4 at room temperature afford [(ADC Ar )AlH 2 ] 2 with the concomitant release of LiH. Compounds [(ADC Ar )AlH 2 ] 2 are stable crystalline solids and are freely soluble in common organic solvents. They are annulated tricyclic compounds with an almost planar central C 4 Al 2 -core embedded between two peripheral 1,3-imidazole (C 3 N 2 ) rings. At room temperature, [(ADC Ph )AlH 2 ] 2 readily reacts with CO 2 to form two- and four-fold hydroalumination products [(ADC Ph )AlH(OCHO)] 2 and [(ADC Ph )Al(OCHO) 2 ] 2 , respectively. Further hydroalumination reactivity of [(ADC Ph )AlH 2 ] 2 has been shown with isocyanate (RNCO) and isothiocyanate (RNCS) species (R=alkyl or aryl group). All compounds have been characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The first 1,4-distibabenzene-1,4-diide compound [(ADC)Sb] 2 (5) based on an anionic dicarbene (ADC) (ADC=PhC{N(Dipp)C} 2 , Dipp=2,6-iPr 2 C 6 H 3 ) is reported as a bordeaux-red solid. Compound 5, featuring a central six-membered C 4 Sb 2 ring with formally Sb I atoms may be regarded as a base-stabilized cyclic bis-stibinidene in which each of the Sb atoms bears two lone-pairs of electrons. 5 undergoes 2 e-oxidation with Ph 3 C[B(C 6 F 5 ) 4 ] to afford [(ADC)Sb] 2 [B(C 6 F 5 ) 4 ] 2 (6) as a brick-red solid. Each of the Sb atoms of 6 has an unpaired electron and a lone-pair. The broken-symmetry open-shell singlet diradical solution for (6) 2+ is calculated to be 2.13 kcal mol -1 more stable than the closed-shell singlet. The diradical character of (6) 2+ according to SS-CASSCF (state-specific complete active space self-consistent field) and UHF (unrestricted Hartree-Fock) methods amounts to 36 % and 39 %, respectively. Treatments of 6 with (PhE) 2 yield [(ADC)Sb(EPh)] 2 [B(C 6 F 5 ) 4 ] 2 (7-E) (E=S or Se). Reaction of 5 with (cod)Mo(CO) 4 affords [(ADC)Sb] 2 Mo(CO) 4 (8)., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Herein, the first stable anions K[SIPr Bp ] (4 a-K) and K[IPr Bp ] (4 b-K) (SIPr Bp =BpC{N(Dipp)CH 2 } 2 , IPr Bp =BpC{N(Dipp)CH} 2 ; Bp=4-PhC 6 H 4 ; Dipp=2,6-iPr 2 C 6 H 3 ) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a-K and 4 b-K are prepared by KC 8 reduction of the neutral radicals [SIPr Bp ] (3 a) and [IPr Bp ] (3 b), respectively. The radicals 3 a and 3 b as well as [Me-IPr Bp ] 3 c (Me-IPr Bp =BpC{N(Dipp)CMe} 2 ) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPr Bp )Br (2 a), (IPr Bp )Br (2 b), and (Me-IPr Bp )Br (2 c) with KC 8 . The cyclic voltammograms of 2 a-2 c exhibit two one-electron reversible redox processes in -0.5 to -2.5 V region that correspond to the radicals 3 a-3 c and the anions (4 a-4 c) - . Computational calculations suggest a closed-shell singlet ground state for (4 a-4 c) - with the singlet-triplet energy gap of 17-24 kcal mol -1 ., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C 2 As 2 -diradicaloid {(IPr)CAs} 2 (6) (IPr = C{N(Dipp)CH} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ). Treatment of (IPr)CH 2 (1) with AsCl 3 affords the Lewis adduct {(IPr)CH 2 }AsCl 3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl 2 (3) and {(IPr)CAsCl} 2 (5 a) or [{(IPr)CAs} 2 Cl]OTf (5 b). Reduction of 5 a (or 5 b) with magnesium turnings gives 6 as a red crystalline solid in 90% yield. Compound 6 featuring a planar C 2 As 2 ring is diamagnetic and exhibits well resolved NMR signals. DFT calculations reveal a singlet ground state for 6 with a small singlet-triplet energy gap of 8.7 kcal mol -1 . The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe) 2 and Fe 2 (CO) 9 give rise to {(IPr)CAs(SePh)} 2 (7) and {(IPr)CAs} 2 Fe(CO) 4 (8), respectively., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Mesoionic dithiolates [(MIDt Ar )Li(LiBr) 2 (THF) 3 ] (MIDt Ar ={SC(NDipp)} 2 CAr; Dipp=2,6-iPr 2 C 6 H 3 ; Ar=Ph 3 a, 3-MeC 6 H 4 (3-Tol) 3 b, 4-Me 2 NC 6 H 4 (DMP) 3 c) and [(MIDt Ph )Li(THF) 2 ] (4) are readily accessible (in≥90 % yields) as crystalline solids on treatments of anionic dicarbenes Li(ADC Ar ) (2 a-c) (ADC Ar ={C(NDipp) 2 } 2 CAr) with elemental sulfur. 3 a-c and 4 are monoanionic ditopic ligands with both the sulfur atoms formally negatively charged, while the 1,3-imidazole unit bears a formal positive charge. Treatment of 4 with (L)GeCl 2 (L=1,4-dioxane) affords the germylene (MIDt Ph )GeCl (5) featuring a three-coordinated Ge atom. 5 reacts with (L)GeCl 2 to give the Ge-Ge catenation product (MIDt Ph )GeGeCl 3 (6). KC 8 reduction of 5 yields the homoleptic germylene (MIDt Ph ) 2 Ge (7). Compounds 3 a-c and 4-7 have been characterized by spectroscopic studies and single-crystal X-ray diffraction. The electronic structures of 4-7 have been analyzed by DFT calculations., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Phosphino-functionalized mesoionic olefins (p-MIOs), (iMIC)CHR (iMIC = PhC{N(Dipp)} 2 C(PPh 2 )C, Dipp = 2,6-iPr 2 C 6 H 3 ; R = H 4a or Ph 4b), derived from a 1,3-imidazole-based mesoionic carbene (iMIC) are reported. The p-MIOs 4a (blue) and 4b (green) are crystalline solids and have been characterized by NMR spectroscopy and X-ray diffraction as well as DFT calculations. The calculated proton affinities of 4a (291.3 kcal mol -1 ) and 4b (288.1 kcal mol -1 ) support their remarkable basicity that exceeds that of classical NHCs. Treatment of 4a with (Me 2 S)AuCl (and AgOTf) affords the dinuclear gold compounds [(4a)Au](X) 2 (5-X) (X = Cl or OTf) with a short Au⋯Au distance of 3.130(2) Å for 5-Cl. The reaction of 4a with the main-group reagent PhBCl 2 yields the boronium cation [(4a)BPhCl]Cl (6).

ConspectusMolecules and materials with easily tunable electronic structures and properties are at the forefront of contemporary research. π-Conjugation is fundamental in organic chemistry and plays a key role in the design of molecular materials. In this Account, we showcase the applicability of N-heterocyclic vinyl (NHV) substituents based on classical N-heterocyclic carbenes (NHCs) for tuning the structure, properties, and stability of main-group species (E) via π-conjugation and/or π-donation.NHVs such as [(NHC)═CR] (R = H or aryl) are monoanionic ligands formally derived by the deprotonation of N-heterocyclic olefins (NHOs), (NHC)═CHR. Further deprotonation of [(NHC)═CR] (R = H) is viable, giving rise to N-heterocyclic vinylidene (NHVD) species such as (NHC)═C. NHVs and NHVDs feature a highly polarizable exocyclic C NHC ═C bond because of the presence of adjacent π-donor nitrogen atoms. The nature of the NHC, in particular the π-acceptor property, has a direct consequence on the polarity of the C NHC ═C bond and hence on the magnitude of π-conjugation in the derived molecules. Thus, the electronic structure, especially the energy and shape of frontier molecular orbitals, HOMO and LUMO, of derived species can be fine-tuned by a judicious choice of the carbene unit. For instance, the HOMO of classical diphosphenes (RP═PR) (R = alkyl or aryl) is invariably the phosphorus lone-pair orbital, while the P═P π-bond is HOMO - 1 or HOMO - 2. In strong contrast, the HOMO of divinyldiphosphenes (R = NHV) is mainly the P═P π-bond. This is owing to the π-conjugation, resulting in the lowering of the LUMO and raising of the HOMO energy. They have a remarkably small HOMO-LUMO energy gap (4.15-4.50 eV) and readily undergo 1e-oxidations, giving rise to stable radical cations and dications.By employing a similar approach, one can access divinyldiarsenes and the corresponding radical cations and dications as crystalline solids. The use of divinyldiphosphenes and divinyldiarsenes as promising ligands in the stabilization of metalloradicals has been shown. By a logical selection of singlet carbenes, stable 2-phosha-1,3-butadiene and 2-arsa-1,3-butadiene compounds, as well as related radical cations and dications, can be prepared as crystalline solids.The relevance of NHV ligands as potent π-donors has been demonstrated for the stabilization of elusive electrophilic phosphinidene and arsinidene complexes {(NHV)E}Fe(CO) 4 (E = P or As). Moreover, stable singlet diradicaloid [(NHC)CP] 2 and p -quinodimethane derivatives [(NHC)CP 2 ] 2 based on an NHVD framework are accessible as stable solids.In this Account, a special emphasis is given to the contributions from this laboratory. The author hopes that this Account will serve as a useful reference guide for researchers interested in studying and applying NHV and NHVD scaffolds in modern molecular chemistry and materials sciences.

1,4-Diarsinine-1,4-diide compound [(ADC Ph )As] 2 (5) (ADC Ph ={C(DippN)} 2 CPh, Dipp=2,6-iPr 2 C 6 H 3 ) with a planar C 4 As 2 ring fused between two 1,3-imidazole scaffolds has been isolated as a red crystalline solid. Compound 5, formally comprising an 8π-electron C 4 As 2 ring, is antiaromatic and undergoes 2e-oxidation with AgOTf to form the 6π-electron aromatic system [(ADC Ph )As] 2 (OTf) 2 (6)., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3-E)GaCl 4 [(3-E) .+ = [{(IPr)C(Ph)E} 2 Fe(CO) 3 ] .+ , E = P or As; IPr = C{(NDipp)CH} 2 , Dipp = 2,6-iPr 2 C 6 H 3 ] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E} 2 (1-E) with Fe 2 (CO) 9 affords [{(IPr)C(Ph)E} 2 Fe(CO) 3 ] (2-E), in which 1-E binds to the Fe atom in an allylic (η 3 -EEC vinyl ) fashion and functions as a 4e donor ligand. Complexes 2-E undergo 1e oxidation with GaCl 3 to yield (3-E)GaCl 4 . Spin density analysis revealed that the unpaired electron in (3-E) .+ is mainly located on the Fe (52-64 %) and vinylic C (30-36 %) atoms. Further 1e oxidation of (3-E)GaCl 4 leads to unprecedented η 3 -EEC vinyl to η 3 -EC vinyl C Ph coordination shuttling to form the dications (4-E)(GaCl 4 ) 2 ., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The first Sn I diradical [(ADC Ph )Sn] 2 (4) based on an anionic dicarbene (ADC Ph ={CN(Dipp)} 2 CPh; Dipp=2,6-iPr 2 C 6 H 3 ) scaffold has been isolated as a green crystalline solid by KC 8 reduction of the corresponding bis-chlorostannylene [(ADC Ph )SnCl] 2 (3). The six-membered C 4 Sn 2 -ring of 4 containing six π-electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4-distannabenzene derivative. DFT calculations suggest an open-shell singlet (OS) ground state of 4 with a remarkably small singlet-triplet energy gap (ΔE OS-T =4.4 kcal mol -1 ), which is consistent with CASSCF (ΔE S-T =6.6 kcal mol -1 and diradical character y=37 %) calculations. The diradical 4 splits H 2 at room temperature to yield the bis-hydridostannylene [(ADC Ph )SnH] 2 (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Herein, we report the first 1,4-diphosphinine-1,4-diide compound [(ADC Ph )P] 2 (5-Ph) (ADC Ph =PhC{(NDipp)C} 2 ; Dipp=2,6-iPr 2 C 6 H 3 ) derived from an anionic dicarbene (ADC Ph ) as a red crystalline solid. Compound 5-Ph containing a 16π-electron planar fused-tricyclic ring system was obtained by the 4e reduction of [(ADC Ph )PCl 2 ] 2 (4-Ph) with Mg (or KC 8 ) in a quantitative yield. Experimental and computational results imply that the central 8π-electrons C 4 P 2 ring of 5-Ph, which is fused between two 6π-electrons C 3 N 2 aromatic rings, is antiaromatic. Thus, each of the phosphorus atoms of 5-Ph has two electron-lone-pairs, one in a p-type orbital is in conjugation with the C=C bonds of the C 4 P 2 ring, while the second resides in a σ-symmetric orbital. This can be shown with the gold complex [(ADC Ph )P(AuCl) 2 ] 2 (6-Ph) obtained by reacting 5-Ph with (Me 2 S)AuCl. A mixture of 5-Ph and 4-Ph undergoes comproportionation in the presence of MgCl 2 to form the intermediate oxidation state compound [(ADC Ar )P] 2 (MgCl 4 ) (7-Ph), which is an aromatic species., (© 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Intramolecular 1,2-Dipp migration of seven mesoionic carbenes (iMIC Ar ) 2 a-g (iMIC Ar =ArC{N(Dipp)} 2 CHC; Ar=aryl; Dipp=2,6-iPr 2 C 6 H 3 ) under nickel catalysis to give 1,3-imidazoles (IMD Ar ) 3 a-g (IMD Ar =ArC{N(Dipp)CHC(Dipp)N}) has been reported. The formation of 3 indicates the cleavage of an N-C Dipp bond and the subsequent formation of a C-C Dipp bond in 2, which is unprecedented in NHC chemistry. The use of 3 in accessing super-iMICs (5) (S-iMIC=ArC{N(Dipp)N(Me)C(Dipp)}C) has been shown with selenium (6), gold (7), and palladium (8) compounds. The quantification of the stereoelectronic properties reveals the superior σ-donor strength of 5 compared to that of classical NHCs. Remarkably, the percentage buried volume of 5 (%V bur =45) is the largest known amongst thus far reported iMICs. Catalytic studies show a remarkable activity of 5, which is consistent with their auspicious stereoelectronic features., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The cyclic Ge(I) compound [(ADC Ph )Ge] 2 ( 4 ) (ADC Ph = {CN(Dipp)} 2 CPh, Dipp = 2,6- i Pr 2 C 6 H 3 ) containing a 6π-electron C 4 Ge 2 framework has been isolated as a red crystalline solid. CASSCF calculations reveal a closed-shell singlet ground state for 4 with a considerable diradical character ( y = 34%). Thus, the diradicaloid 4 readily splits dihydrogen at room temperature to yield the elusive bis-hydridogermylene [(ADC Ph )GeH] 2 ( 5 ).

Phosphinidene complexes of the general formula RPM(CO) n (R = an alkyl or aryl group; M = a transition metal) are electrophilic and thermally unstable. Thus, the isolation of these elusive species for structural elucidations remains a challenge. Herein, we report the first terminal phosphinidene complexes [{(NHC)C(Ph)}P]Fe(CO) 4 [NHC = IPr = C{(NDipp)CH} 2 for 3 ; Me-IPr = C{(NDipp)CMe} 2 for 4 ; Dipp = 2,6- i Pr 2 C 6 H 3 ; NHC = N-heterocyclic carbene] as red crystalline solids containing a π-donor N-heterocyclic vinyl (NHV) substituent at the phosphorus atom. Calculations reveal donor-acceptor type bonding between phosphorus and iron atoms in 3 and 4 . The P → Fe donation represents ∼70% of the orbital interaction, whereas the Fe → P π-back-donation corresponds to ∼15% of the orbital interaction. The phosphorus atoms in 3 and 4 carry charges of +0.65e and +0.64e, respectively, indicating the electrophilic character of the phosphinidene {(NHC)C(Ph)}P moiety. Accordingly, 3 reacts with an NHC nucleophile (IMe 4 ) to yield the Lewis adduct [{(NHC)C(Ph)}P(IMe 4 )]Fe(CO) 4 ( 5 ) [IMe 4 = C(NMeCMe) 2 ]. The coordination of an electron-rich NHC (IMe 4 ) to the phosphorus atom in 5 precludes the π-electron density transfer from the NHV to the phosphorus atom. Thus, the C IPr -C vinyl and C vinyl -P bonds of 5 become shorter and longer, respectively, compared to those of 3 .

Crystalline 1,4-distannabarrelene compounds [(ADC Ar ) 3 Sn 2 ]SnCl 3 (3-Ar) (ADC Ar ={ArC(NDipp) 2 CC}; Dipp=2,6-iPr 2 C 6 H 3 , Ar=Ph or DMP; DMP=4-Me 2 NC 6 H 4 ) derived from anionic dicarbenes Li(ADC Ar ) (2-Ar) (Ar=Ph or DMP) have been reported. The cationic moiety of 3-Ar features a barrelene framework with three coordinated Sn II atoms at the 1,4-positions, whereas the anionic unit SnCl 3 is formally derived from SnCl 2 and chloride ion. The all carbon substituted bis-stannylenes 3-Ar have been characterized by NMR spectroscopy and X-ray diffraction. DFT calculations reveal that the HOMO of 3-Ph (ϵ=-6.40 eV) is mainly the lone-pair orbital at the Sn II atoms of the barrelene unit. 3-Ar readily react with sulfur and selenium to afford the mixed-valence Sn II /Sn IV compounds [(ADC Ar ) 3 SnSn(E)](SnCl 6 ) 0.5 (E=S 4-Ar, Ar=Ph or DMP; E=Se 5-Ph)., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

2-Arsa-1,3-butadienes (L)As(cAACR) (L = PhC[double bond, length as m-dash]C{(NDipp)CH}2, Dipp = 2,6-iPr2C6H3; cAACR = C{(NDipp)CMe2CH2C(R)}, R = Me22a, R = cyclohexyl (Cy) 2b) and the corresponding radical cations [(L)As(cAACR)]GaCl4 (R = Me23a, Cy 3b) and dications [(L)As(cAACR)](GaCl4)2 (R = Me 4a, Cy 4b) featuring a C[double bond, length as m-dash]C-As[double bond, length as m-dash]C π-conjugated framework are reported.

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

A synthetic strategy for the 2-phospha-1,3-butadiene derivatives [{(IPr)C(Ph)}P(cAAC Me )] ( 3a ) and [{(IPr)C(Ph)}P(cAAC Cy )] ( 3b ) (IPr = C{(NDipp)CH} 2 , Dipp = 2,6-iPr 2 C 6 H 3 ; cAAC Me = C{(NDipp)CMe 2 CH 2 CMe 2 }; cAAC Cy = C{(NDipp)CMe 2 CH 2 C(Cy)}, Cy = cyclohexyl) containing a C[double bond, length as m-dash]C-P[double bond, length as m-dash]C framework has been established. Compounds 3a and 3b have a remarkably small HOMO-LUMO energy gap ( 3a : 5.09; 3b : 5.05 eV) with a very high-lying HOMO (-4.95 eV for each). Consequently, 3a and 3b readily undergo one-electron oxidation with the mild oxidizing agent GaCl 3 to afford radical cations [{(IPr)C(Ph)}P(cAAC R )]GaCl 4 (R = Me 4a , Cy 4b ) as crystalline solids. The main UV-vis absorption band for 4a and 4b is red-shifted with respect to that of 3a and 3b , which is associated with the SOMO related transitions. The EPR spectra of compounds 4a and 4b each exhibit a doublet due to coupling of the unpaired electron with the 31 P nucleus. Further one-electron removal from the radical cations 4a and 4b is also feasible with GaCl 3 , affording the dications [{(IPr)C(Ph)}P(cAAC R )](GaCl 4 ) 2 (R = Me 5a , Cy 5b ) as yellow crystals. The molecular structures of compounds 3-5 have been determined by X-ray diffraction and analyzed by DFT calculations., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The first terminal arsinidene complexes [{(NHC)C(Ph)}As]Fe(CO)4 (NHC = IPr = C{(NDipp)CH}25, Me-IPr = C{(NDipp)CMe}26; Dipp = 2,6-iPr2C6H3) with a two-coordinated arsenic atom have been reported as crystalline solids. Calculations suggest that 5 and 6 are electrophilic, which has been demonstrated by the reaction of 5 with an NHC nucleophile (IMe4), affording the Lewis adduct [{(IPr)C(Ph)}As(IMe4)]Fe(CO)4 (7) (IMe4 = C(NMeCMe)2).

The divinyldiarsene radical cations [{(NHC)C(Ph)}As] 2 (GaCl 4 ) (NHC=IPr: C{(NDipp)CH} 2 3; SIPr: C{(NDipp)CH 2 } 2 4; Dipp=2,6-iPr 2 C 6 H 3 ) and dications [{(NHC)C(Ph)}As] 2 (GaCl 4 ) 2 (NHC=IPr 5; SIPr 6) are readily accessible as crystalline solids on sequential one-electron oxidation of the corresponding divinyldiarsenes [{(NHC)C(Ph)}As] 2 (NHC=IPr 1; SIPr 2) with GaCl 3 . Compounds 3-6 have been characterized by X-ray diffraction, cyclic voltammetry, EPR/NMR spectroscopy, and UV/vis absorption spectroscopy as well as DFT calculations. The sequential removal of one electron from the HOMO, that is mainly the As-As π-bond, of 1 and 2 leads to successive elongation of the As=As bond and contraction of the C-As bonds from 1/2→3/4→5/6. The UV/vis spectrum of 3 and 4 each exhibits a strong absorption in the visible region associated with SOMO-related transitions. The EPR spectrum of 3 and 4 each shows a broadened septet owing to coupling of the unpaired electron with two 75 As (I=3/2) nuclei., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

A series of unique C 2 P 3 -ring compounds [(ADC Ar )P 3 ] (ADC Ar = ArC{(DippN)C} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ; Ar = Ph 4a , 3-MeC 6 H 4 4b , 4-MeC 6 H 4 4c , and 4-Me 2 NC 6 H 4 4d ) are readily accessible in an almost quantitative yield by the direct functionalization of white phosphorus (P 4 ) with appropriate anionic dicarbenes [Li(ADC Ar )]. The formation of 1,2,3-triphosphol-2-ides ( 4a-4d ) suggests unprecedented [3 + 1] fragmentation of P 4 into P 3 + and P - . The P 3 + cation is trapped by the (ADC Ar ) - to give 4 , while the putative P - anion reacts with additional P 4 to yield the Li 3 P 7 species, a useful reagent in the synthesis of organophosphorus compounds. Remarkably, the P 4 fragmentation is also viable with the related mesoionic carbenes (iMICs Ar ) (iMIC Ar = ArC{(DippN) 2 CCH}, i stands for imidazole-based) giving rise to 4 . DFT calculations reveal that both the C 3 N 2 and C 2 P 3 -rings of 4 are 6π-electron aromatic systems. The natural bonding orbital (NBO) analyses indicate that compounds 4 are mesoionic species featuring a negatively polarized C 2 P 3 -ring. The HOMO-3 of 4 is mainly the lone-pair at the central phosphorus atom that undergoes σ-bond formation with a variety of metal-electrophiles to yield complexes [{(ADC Ar )P 3 }M(CO) n ] (M = Fe, n = 4, Ar = Ph 5a or 4-Me-C 6 H 4 5b ; M = Mo, n = 5, Ar = Ph 6 ; M = W, n = 5, Ar = 4-Me 2 NC 6 H 4 7 )., (This journal is © The Royal Society of Chemistry 2019.)

The synthesis and characterization of the crystalline diphosphene radical cations [{(NHC)C(Ph)}P]2(GaCl4) (NHC = IPr = C{(NDipp)CH}23, SIPr = C{(NDipp)CH2}24; Dipp = 2,6-iPr2C6H3) and dications [{(NHC)C(Ph)}P]2(GaCl4)2 (NHC = IPr 5, SIPr 6) featuring a π-conjugated C2P2C2-framework has been reported.

The first divinyldiarsenes [{(NHC)C(Ph)}As] 2 (NHC=IPr 3 a, SIPr 3 b; IPr=C{(NAr)CH} 2 ; SIPr=C{(NAr)CH 2 } 2 ; Ar=2,6-iPr 2 C 6 H 3 ) are reported. Compounds 3 a and 3 b were prepared by the reduction of corresponding chlorides {(NHC)C(Ph)}AsCl 2 (NHC=IPr 2 a, SIPr 2 b) with Mg. Calculations revealed a small HOMO-LUMO energy gap of 3.86 (3 a) and 4.24 eV (3 b). Treatment of 3 a with (Me 2 S)AuCl led to the cleavage of the As=As bond to restore 2 a, which is expected to proceed via the diarsane [{(IPr)C(Ph)}AsCl] 2 (4). Remarkably, 4 as well as 2 a can be selectively accessed on treatment of 3 a with an appropriate amount of C 2 Cl 6 . Moreover, 3 a readily reacts with PhEEPh (E=Se or Te) at room temperature to give {(IPr)C(Ph)}As(EPh) 2 (E=Se 5 a; Te 5 b), revealing the cleavage of As=As and E-E bonds and the formation of As-E bonds. Such highly selective stepwise oxidation (3 a→4→2 a) and bond metathesis (3 a→5 a,b) reactions are unprecedented in main-group chemistry., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The olefinic C-H bond functionalization of (NHC)CHPh (NHC=IPr=C{(NAr)CH} 2 1; SIPr=C{(NAr)CH 2 } 2 2; Ar=2,6-iPr 2 C 6 H 3 ), derived from classical N-heterocyclic carbenes (NHCs), with PCl 3 affords the dichlorovinylphosphanes {(NHC)C(Ph)}PCl 2 (NHC=IPr 3, SIPr 4). Two-electron reduction of 3 and 4 with magnesium leads to the formation of the divinyldiphosphenes [{(NHC)C(Ph)}P] 2 (NHC=IPr 5, SIPr 6) as crystalline solids. Unlike literature-known diphosphenes, which are mostly yellow or orange, 5 is a green whereas 6 is a purple solid. Although the P=P bond lengths of 5 (2.062(1)) and 6 (2.055(1) Å) are comparable to those of the known diphosphenes (2.02-2.08 Å), the C-P bond lengths of 5 (1.785(1)) and 6 (1.797(1) Å) are, however, considerably shorter than a C sp 2 -P single bond length (1.85 Å), indicating a considerable π-conjugation between C=C and P=P moieties. The HOMO-LUMO energy gap for 5 (4.15) and 6 (4.52 eV) is strikingly small and thus the narrowest among the diphosphenes (>4.93 eV) reported as yet. Consequently, 5 readily undergoes P=P bond cleavage at room temperature on treatment with sulfur to form the unique dithiophosphorane {(IPr)C(Ph)}P(S) 2 7. Interestingly, reaction of 5 with selenium gives the selenadiphosphirane [{(IPr)C(Ph)}P] 2 Se 8 with an intact P-P bond., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Para-quinodimethane (pQDM) is a fundamental structural component in many π-conjugated organic molecules and materials. The incorporation of phosphorus atom into π-conjugated frameworks offers unique opportunities for controlling the properties of derived species. A phosphorus analogue of p-quinodimethane (pQDM), (IPrC) 2 P 4 [5, IPr=C{N(Ar)CH 2 } 2 ; Ar=2,6-iPr 2 C 6 H 3 ] featuring a planar P 4 ring, was readily accessible by KC 8 -reduction of (IPrC)(PCl 2 ) 2 (2). Base-mediated C-H functionalization of IPrCH 2 (1) with PCl 3 afforded 2. The formation of 5 was expected to occur through a dimerization of the transient 3H-diphosphirene (IPrC)P 2 (4), which was theoretically suggested to have an intermediate diradical character. Compound 5 underwent photo-induced ring-contraction reaction to form the singlet diradicaloid (IPrCP) 2 VI and white phosphorus (P 4 ). The formation of and VI and P 4 suggested the formal diphosphorus (P 2 ) elimination from 5. Indeed, photolysis of a mixture of 1,3-cyclohexadiene (CHD) and 5 led to the formation of P 2 -entrapped product (CHD) 2 P 2 (6). The compound 5 represents the first organophosphorus species that functions as a P 2 source., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two-fold C-C cross-coupling of N-heterocyclic carbenes [NHCs; SIPr=C(NArCH 2 ) 2 , 1; IPr=C(NArCH) 2 , 2; Me-IPr=C(NArCMe) 2 , 3; Ar=2,6-iPr 2 C 6 H 3 ] with 4,4''-diiodo-p-terphenyl under Ni catalysis furnished [(SIPr)(C 6 H 4 ) 3 (SIPr)](I) 2 (4), [(IPr)(C 6 H 4 ) 3 (IPr)](I) 2 (5), and [(Me-IPr)(C 6 H 4 ) 3 (Me-IPr)](I) 2 (6). Two-electron reduction of 4-6 with KC 8 readily afforded NHC analogues of Müller's hydrocarbon (MH), [(SIPr)(C 6 H 4 ) 3 (SIPr)] (7), [(IPr)(C 6 H 4 ) 3 (IPr)] (8), and [(Me-IPr)(C 6 H 4 ) 3 (Me-IPr)] (9), respectively, as highly colored crystalline solids. Quantum chemical calculations suggested that the singlet ground state for 7-9 possesses a vertical singlet-triplet energy gap ΔE S-T of -7.24 to -7.60 kcal mol -1 , which is significantly lower compared to that of the NHC analogues of Thiele's (TH) and Chichibabin's (CH) (18-38 kcal mol -1 ) hydrocarbons. Importantly, the calculated diradical character (y) of 7-9 (y≈0.6) is considerably higher compared to that of the related TH and CH (y=0.1-0.4), suggesting the open-shell singlet character of 7-9., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Stable N-heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C 6 H 4 )(IPr)] and [(IPr)(C 6 H 4 ) 2 (IPr)] (4 and 5, respectively; IPr=C{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH), are reported. In a nickel-catalyzed double carbenylation of 1,4-Br 2 C 6 H 4 and 4,4'-Br 2 (C 6 H 4 ) 2 with IPr (1), [(IPr)(C 6 H 4 )(IPr)](Br) 2 (2) and [(IPr)(C 6 H 4 ) 2 (IPr)](Br) 2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC 8 . Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet-triplet energy gap ΔE S-T of 10.7 kcal mol -1 , whereas 4 features more quinoidal character with a rather large ΔE S-T of 25.6 kcal mol -1 . In view of the low ΔE S-T , 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The direct double carbenylation of 1,4-diiodobenzene and 4,4'-dibromobiphenyl with a classical N-heterocyclic carbene, SIPr ( 1 ) (SIPr = :C{ N (2,6-iPr 2 C 6 H 3 )} 2 CH 2 CH 2 ), by means of nickel catalysis gives rise to 1,3-imidazolinium salts [(SIPr)(C 6 H 4 )(SIPr)](I) 2 ( 2 ) and [(SIPr)(C 6 H 4 ) 2 (SIPr)](Br) 2 ( 3 ) as off-white solids. Two-electron reduction of 2 and 3 with KC 8 cleanly yields Kekulé diradicaloid compounds [(SIPr)(C 6 H 4 )(SIPr)] ( 4 ) and [(SIPr)(C 6 H 4 ) 2 (SIPr)] ( 5 ), respectively, as crystalline solids. Structural parameters and DFT as well as CASSCF calculations suggest the closed-shell singlet ground state for 4 and 5 . Calculations reveal a very low singlet-triplet energy gap Δ E S-T for 5 (10.7 kcal mol -1 ), while Δ E S-T for 4 (29.1 kcal mol -1 ) is rather large.

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPr Ar )Br (Ar=Ph, 3 a; 4-DMP, 3 b; 4-DMP=4-Me 2 NC 6 H 4 ) and (SIPr Ar )I (Ar=Ph, 4 a; 4-Tol, 4 b) derived from classical NHCs (IPr=:C{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH, 1; SIPr=:C{N(2,6-iPr 2 C 6 H 3 )} 2 CH 2 CH 2 , 2) gave radicals [(IPr Ar )] . (Ar=Ph, 5 a; 4-DMP, 5 b) and [(SIPr Ar )] . (Ar=Ph, 6 a; 4-Tol, 6 b). Each of 5 a,b and 6 a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid-state characterization of NHC-derived carbon-centered radicals 6 a,b by single-crystal X-ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The first C5-protonated abnormal N-heterocyclic carbene (aNHC), PhC{N(2,6-iPr 2 C 6 H 3 )} 2 CHC: (4) is readily accessible by C4-deprotonation of [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH]X (3 a-X) (Ar=Ph, X=Br or I) with a base. The aNHC 4 is stable at 298 K in [D 8 ]THF solution and has been spectroscopically characterized. The facile availability of 4 enables the synthesis of a series of main-group compounds as well as transition-metal complexes featuring a new phosphorus-aNHC hybrid ligand. Double deprotonation of [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH]X (Ar=Ph, 3 a-X (X=Br or I); 4-Tol, 3 b-Br; 4-DMP, 3 c-Br; Tol=MeC 6 H 4 , DMP=Me 2 NC 6 H 4 ) with nBuLi yields the C4,C5-ditopic carbanionic aNHCs, [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 (C:) 2 ]Li(THF) n (Ar=Ph, 13 a; 4-Tol, 13 b; 4-DMP, 13 c), which on treatment with Ph 2 PCl affords cationic vicinal bisphosphine derivatives [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 {C(PPh 2 } 2 ]X (Ar=Ph, 14 a-X, X=Br or I; 4-Tol, 14 b-Br; 4-DMP, 14 c-Br)., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis, structure, and reactivity of the first silylene-functionalized N-heterocyclic carbene (Si-NHC), {(L 1 )Si}(L 2 ), (3) (L 1 =PhC(NtBu) 2 , L 2 =CCH(CNAr) 2 C:, Ar=2,6-diisopropylphenyl) are reported. The ditopic carbanionic-NHC (dc-NHC), [Li(L 2 )] ∞ (1) reacts with the monochlorosilylene (L 1 )SiCl (2) to afford 3. The HOMO of 3 is the silylene lone-pair orbital, whereas the HOMO-1 is located at the carbene carbon atom. Both Si II and C II functionalities of 3 undergo reactions with chalcogens to give heavier ketone derivatives {(L 1 )Si(E)}L 2 (E) (E=S, 4; Se, 5; and Te, 6). Compounds 4-6 feature a highly polarized Si δ+ -E δ- bond with a formal charge of -0.72 (4), -0.64 (5), and -0.50 e (6) on the E atom. The Wiberg bond indices of 1.64 (4), 1.66 (5), and 1.63 (6) for the Si-E bond however indicate a moderate p π -p π interaction between Si and E atoms. Compound 3 functions as a reducing agent and induces disproportionation of Si 2 Cl 6 to yield the silyl-functionalized NHC, {(L 1 )SiCl 2 }(L 2 ) (7). Treatment of 3 with HSiCl 3 leads to the formation of the silyl-functionalized imidazolium salt [{(L 1 )SiCl 2 }(L 2 H)]Cl (8). The silylene moiety of 3 readily forms the iridium-silylene complex [{(L 2 )(L 1 )Si}Ir(cod)Cl] (9) featuring a free NHC ligand on treatment with [Ir(cod)(μ-Cl)] 2 ., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A highly efficient nickel catalysed method for the direct C2-arylation of N-heterocyclic carbenes (NHCs) is reported. This protocol enables a facile access to C2-arylated imidazolium salts (NHC-Ar)X (X = Cl, Br, I or OTf). Experimental and theoretical studies suggest the viability of a Ni(i)/Ni(ii) catalytic pathway in which the dinuclear Ni(i) species [(NHC)NiX] 2 plays a key role.

Four-membered biradicaloid compounds containing a N 2 E 2 (E=main group element) framework have been thoroughly investigated; however, the synthesis of stable analogues with a C 2 P 2 skeleton remains a challenge. Base-mediated double C-H functionalization of IPr=CH 2 (1) (IPr=C[N(2,6-iPr 2 -C 6 H 3 )CH] 2 ) with PCl 3 affords [{(IPr)CP} 2 Cl]Cl (2) as a royal blue solid. Treatment of 2 with KC 8 yields the stable phosphorus biradicaloid [(IPr)CP] 2 (3) featuring a four-membered C 2 P 2 ring. Compound 3 is diamagnetic and shows sharp and temperature-independent NMR resonances, revealing its singlet biradicaloid nature. The stability of 3 is attributed to the σ- and π-electron-donating property of the N-heterocyclic vinylidene [(IPr)C] group., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis and characterization of the N-heterocyclic carbene (NHC) stabilized dichlorosilylene Group 6 metal complexes {(IPr)SiCl 2 }W(CO) 5 (3-W), {(IPr)SiCl 2 } 2 Cr(CO) 4 (4-Cr), and {(IPr)SiCl 2 } 2 W(CO) 4 (4-W) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) are reported. Treatment of 3-W with CsOH in the presence of IPr leads to the formation of an abnormal-NHC (aNHC) metal complex (aIPr H )W(CO) 5 (6-W) (aIPr H = 1,3-bis(2,6-diisopropylphenyl)imidazol-4-ylidene), unveiling an unprecedented normal-to-abnormal transformation route of an NHC. DFT calculations support the proposed mechanism that involves CsOH mediated deprotonation of the IPr-backbone of 3-W to yield a ditopic carbanionic-NHC (dcNHC) complex 5a-W. Subsequent 1,4-migration of the W(CO) 5 moiety and hydrolysis of the unmasked SiCl 2 rationalize the formation of 6-W. The desired H 2 O molecule is generated in the initial step on deprotonation of IPr with CsOH. In contrast to the literature precedents, the calculations indicate that the abnormal complex 6-W is 13.5 kcal mol -1 thermodynamically higher in energy than the normal counterpart (IPr)W(CO) 5 (8-W). Interestingly, as the aNHC-compounds reported so far are more stable than their normal counterparts, this finding showcases an opposite trend. Moreover, reaction pathways to the synthesized and related complexes have been investigated by DFT calculations.

Deprotonation of [IPr Ph ]I (1) with Co{N(SiMe 3 ) 2 } 2 readily affords the abnormal N-heterocyclic carbene (aNHC) complex (aIPr Ph ) 2 CoI 2 (2) (aIPr Ph = 1,3-bis(2,6-iPr 2 C 6 H 3 )-2-phenyl-imidazol-4-ylidene). Treatment of 1 with NaHBEt 3 yields (aIPr Ph )BEt 3 (3) that serves as an aNHC-transfer agent and yields (aIPr Ph )Co{N(SiMe 3 ) 2 } 2 (4) on reaction with Co{N(SiMe 3 ) 2 } 2 .

N-Heterocyclic carbenes (NHCs) are an important class of compounds that are indeed regarded as most versatile carbon-donor ligands in transition metal and organometallic catalysis. In addition, NHCs are also capable of stabilizing a variety of highly reactive main group compounds with intriguing properties. The enormous success of NHCs prompted the investigation of other carbon-based neutral ligands with additional structural and electronic features. Reactivity studies of NHCs and their complexes unveiled routes to new carbon donor ligands. Such NHC-derived ligands can be grouped into three categories: (i) N-heterocyclic olefins (the donor site is extended by an intervening carbon atom), (ii) ditopic carbanionic-NHCs (an additional coordination site is generated by the deprotonation of an NHC), and (iii) abnormal-NHCs (the carbene center of an NHC is moved to different positions). This article summarizes the recent advances in NHC chemistry of compounds featuring the aforementioned ligands, which are exclusively derived by the functionalization of NHCs, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) in particular.

Brønsted acid HNTf2 (Tf = SO2CF3) mediated dehydrogenative hydride abstraction from (L(1))BH3 () and (L(2))BH3 () (L(1) = IPrCH2 = 1,3-(2,6-di-isopropylphenyl)imidazol-2-methylidene (); L(2) = SIPrCH2 = 1,3-(2,6-di-isopropylphenyl)imidazolidin-2-methylidiene ()) affords thermally stable hydride bridged mono-cationic hydrido boron compounds [{(L(1))BH2}2(μ-H)](NTf2) () and [{(L(2))BH2}2(μ-H)](NTf2) (). Furthermore, hydride abstraction yields di-cationic hydrido boron compounds [{(L(1))BH}2(μ-H)2](NTf2)2 () and [{(L(2))BH}2(μ-H)2](NTf2)2 (). Unique cationic boron compounds with CH2BH2(μ-H)BH2CH2 ( and ) and CH2BH(μ-H)2BHCH2 ( and ) moieties feature a 3c-2e bond and have been fully characterized. Interesting electronic and structural features of compounds are analysed using spectroscopic, crystallographic, and computational methods.

Blocking the C2 position of an imidazole-derived classical N-heterocyclic carbene (NHC) with an aryl group is an essential strategy to establish a route to mesoionic carbenes (MICs), which coordinate to the metal via the C4 (or C5) carbon atom. An efficient catalytic route to MIC precursors by direct arylation of an NHC is reported. Treatment of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) with an aryl iodide (RC6H4I) in the presence of 0.5 mol % of [Pd2(dba)3] (dba = dibenzylideneacetone) precatalyst affords the C2-arylated imidazolium salts {IPr(C6H4R)}I (R = H, 4-Me, 2-Me, 4-OMe, 4-COOMe) in excellent (up to 92%) yields. Treatment of {IPr(C6H5)}I with CuI and KN(SiMe3)2 exclusively affords the MIC-copper complex [(IPrPh)CuI]., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Functionalization of the C4 carbon of an imidazol-derived N-heterocyclic carbene (NHC) may allow fine-tuning of the electronic and steric properties of the C2 carbene center. A facile route to silyl-functionalized di-N-heterocyclic carbenes (Di-NHCs) is described. Treatment of the polymeric lithiated NHC, {Li(IPrH)}n (1) (Li(IPrH) = {(N-2,6-iPr2C6H3)2CHCLi}C:) with a dichlorosilane affords monomeric silyl-functionalized Di-NHCs, R2Si(IPrH)2 (R = Ph, 2; Me, 3). Interestingly, silyl-functionalized mono-NHC, Ph2(Cl)Si(IPrH) (4) with a pendant chloro-substituent can also be exclusively isolated maintaining the reactants 1 and Ph2SiCl2 ratio. NHCs 2 and 4 readily form copper complexes, Ph2Si{(IPrH)CuCl}2 (5) and Ph2(Cl)Si{(IPrH)CuCl} (6), on reaction with CuCl. Straightforward conversion of an NHC to a Di-NHC (2 or 3) via C4 functionalization is reported for the first time. Molecular structures of 2, 4, 5 and 6 have been established by single crystal X-ray diffraction studies.

N-heterocyclic olefins (NHOs), IPrCH2 (1) and SIPrCH2 (2) (IPrCH2 = {N(2,6-iPr2C6H3)CH}2CCH2 and SIPrCH2 = {N(2,6-iPr2C6H3)CH2}2CCH2), react with HSiCl3 and afford IPrCH(SiHCl2) (3) and SIPrCH(SiHCl2) (4), respectively. Compounds 3 and 4 have been isolated in almost quantitative yield. Interestingly, treatment of the silylene IPr·SiCl2 with 1 also affords 3, where silylene insertion into a C–H bond is observed. Computational analysis shows a high energy barrier for silylene insertion, therefore a protonation–deprotonation mechanism is more likely.