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Selected Recent Publications

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


R. S. Ghadwal*

Mesoionic carbenes (iMICs) and anionic dicarbenes (ADCs) are readily accessible by the deprotonation of C2-arylated 1,3-imidazolium  salts,  which are prepared by the direct  C2-arylation  of classical  N-heterocyclic carbenes  (NHCs).  The implications of iMICs as potent σ-donor ligands for organometallic catalysis and ADCs as unique building blocks to access conceptually new main-group heterocycles with an annulated C4E2-ring have been showcased.


Isolation of an Annulated 1,4-Distibabenzene Diradicaloid


H. Steffenfauseweh, D. Rottschäfer, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, D. W. Szczepanik , R. S. Ghadwal*

The first 1,4-distibinine-1,4-diide compound [(ADC)Sb]2 (5) based on an anionic dicarbene (ADC) (ADC = PhC{N(Dipp)C}2, Dipp = 2,6-iPr2C6H3) is reported as a bordeaux-red solid. Compound 5, featuring a central six-membered C4Sb2 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 2e-oxidation with Ph3C[B(C6F5)4] to afford [(ADC)Sb]2­[B(C6F5)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 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 39% and 36%, respectively. Treatments of 6 with (PhE)2 yield [(ADC)Sb(EPh)]2[B(C6F5)4]2 (7-E) (E = S or Se). Reaction of 5 with (cod)Mo(CO)4 affords [(ADC)Sb]2­Mo(CO)4 (8).

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Crystalline Anions Based on Classical N-Heterocyclic Carbenes


A. Merschel, D. Rottschäfer, B. Neumann, H.-G. Stammler, M. Ringenberg, M. van Gastel, T. I. Demirer, D. M. Andrada, R. S. Ghadwal*

Herein, the first stable anions K[SIPrBp] (4a-K) and K[IPrBp] (4b-K) (SIPrBp = BpC{N(Dipp)CH2}2, IPrBp = BpC{N(Dipp)CH}2; Bp = 4-PhC6H4; Dipp = 2,6-iPr2C6H3) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4a-K and 4b-K are prepared by KC8 reduction of the neutral radicals [SIPrBp] (3a) and [IPrBp] (3b), respectively. The radicals 3a and 3b as well as [Me-IPrBp] 3c (Me-IPrBp = BpC{N(Dipp)CMe}2) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPrBp)Br (2a), (IPrBp)Br (2b), and (Me-IPrBp)Br (2c) with KC8. The cyclic voltammograms (CVs) of 2a2c exhibit two one-electron reversible redox processes in –0.5 to –2.1 V region that correspond to the radicals 3a3b and the anions (4a4c)–. Computational calculations suggest a closed-shell singlet ground state for (4a4c)– with the singlet-triplet energy gap of 19-21 kcal/mol.

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Isolation of an Arsenic Diradicaloid with a Cyclic C2As2-Core


H. Steffenfauseweh, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, D. M. Andrada, R. S. Ghadwal*

The C2As2-cyclic diradicaloid 6 is isolated as a red crystalline solid via 2e-reduction of 5a. Calculations suggest a singlet ground state for 6, which is 0.34 kcal/mol higher than the broken-symmetry open-shell singlet solution. The HOMO of 6 is located at the arsenic atoms and is trans-annular antibonding, while the LUMO is trans-annular bonding and spans over the C2As2C2-framework. Reactivity studies of 6are shown with (PhSe)2 and Fe2(CO)9.


D. Rottschäfer, T. Glodde, B. Neumann, H.-G. Stammler, D. M. Andrada, R. S. Ghadwal*

1,4-Diarsinine-1,4-diide compound [(ADCPh)As]2 (5) (ADCPh = {C(DippN)}2CPh, Dipp = 2,6-iPr2C6H3) with a planar C4As2-ring fused between two 1,3-imidazole scaffolds has been isolated as a red crystalline solid. Compound 5, formally comprising an 8π-electron C4As2-ring, is antiaromatic and undergoes 2e-oxidation with AgOTf to form the 6π-electron aromatic system [(ADCPh)As]2(OTf)2 (6).


An Open‐Shell Singlet Sn(I) Diradical and H2 Splitting


M. K. Sharma, D. Rottschäfer, T. Glodde, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Angew. Chem. Int. Ed. 2021, 60, 64146418.

Angewandte Chemie 2021, 133,  64856489.

The first Sn(I) diradical [(ADCPh)Sn]2 (4) based on an anionic dicarbene (ADCPh = {CN(Dipp)}2CPh, Dipp = 2,6-iPr2C6H3) scaffold has been isolated as a green crystalline solid by KC8 reduction of the corresponding bis-chlorostannylene [(ADCPh)SnCl]2 (3). The six-membered C4Sn2-ring of 4 containing 6π-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 (ΔEOS-T = 4.4 kcal/mol), which is consistent with CASSCF (ΔES-T = 6.6 kcal/mol and diradical character y = 37%) calculations. The diradical 4 splits H2 at room temperature to yield the bis-hydridostannylene [(ADCPh)SnH]2 (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf.


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


D. Rottschäfer, T. Glodde, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

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 (P4), and CO2.


Isolation of a Ge(I) Diradicaloid and Dihydrogen Splitting


M. K. Sharma, F. Ebeler, T. Glodde, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

The cyclic Ge(I) compound [(ADCPh)Ge]2 (4) (ADCPh = {CN(Dipp)}2CPh, Dipp = 2,6-iPr2C6H3) containing a 6π-electron C4Ge2 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 [(ADCPh)GeH]2 (5).

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Nickel Catalyzed Intramolecular 1,2‐Aryl Migration of Mesoionic Carbenes (iMICs)


A. Merschel, T. Glodde, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Intramolecular 1,2-Dipp migration of seven mesoionic carbenes (iMICAr) 2a-g (iMICAr = ArC{N(Dipp)}2CHC; Ar = aryl; Dipp = 2,6-iPr2C6H3) under nickel catalysis to give 1,3-imidazoles (IMDAr) 3a-g (IMDAr = ArC{N(Dipp)CHC(Dipp)N}) has been reported. The formation of 3 indicates the cleavage of an N‒CDipp bond and the subsequent formation of a C‒CDipp 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 (%Vbur = 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.


Isolation of Singlet Carbenes Derived 2-Phospha-1,3-butadienes and their Sequential One-electron Oxidation to Radical Cations and Dications


M. K. Sharma, S. Blomeyer, T. Glodde, B. Neumann, H.-G. Stammler, A. Hinz, M. van Gastel, R. S. Ghadwal*

A synthetic strategy for the 2-phospha-1,3-butadiene derivatives [{(IPr)C(Ph)}P(cAACMe)] (3a) and [{(IPr)C(Ph)}P(cAACCy)] (3b) (IPr = C{(NDipp)CH}2, Dipp = 2,6-iPr2C6H3; cAACMe = C{(NDipp)CMe2CH2CMe2}; cAACCy = C{(NDipp)CMe2CH2C(Cy)}, Cy = cyclohexyl) containing a C=C‒P=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 GaCl3 to afford radical cations [{(IPr)C(Ph)}P(cAACR)]GaCl4 (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 spectrum of compounds 4a and 4b each exhibits a doublet due to coupling with the 31P nucleus. Further one-electron removal from the radical cations 4a and 4b is also feasible with GaCl3, affording the dications [{(IPr)C(Ph)}P(cAACR)](GaCl4)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.

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Crystalline Divinyldiarsene Radical Cations and Dications

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M. K. Sharma, S. Blomeyer,  B. Neumann, H.-G. Stammler, A. Hinz, M. van Gastel, R. S. Ghadwal*

One-by-one electron oxidation of diarsenes [As2] featuring very efficient π-donor N-heterocyclic vinyl substituents with GaCl3 leads to the formation of radical cations [As2] and dications [As2] as crystalline solids. Experimental and computational studies revealed the delocalization of unpaired electron over the π-conjugated CAs2C framework.

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Direct Functionalization of White Phosphorus with Anionic Dicarbenes and Mesoionic Carbenes: Facile Access to 1,2,3-Triphosphol-2-ides


D. Rottschäfer, S. Blomeyer,  B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Unprecedented [1+3] fragmentation of white phosphorus (P4) and thus the capturing of the P3+ fragment with anionic dicarbenes (ADCs) has been shown to afford the 1,2,3-triphosphol-2-ides I in 93-98% yield. The mesoionic heterocycles I feature 6π-electron C2P3 and C3N2 aromatic systems and serve as two-electron σ-donor ligands. 


Diphosphene radical cations and dications with a π-conjugated C2P2C2-framework


M. K. Sharma, D. Rottschäfer, S. Blomeyer,  B. Neumann, H.-G. Stammler, A. Hinz, M. van Gastel, R. S. Ghadwal*

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


N-Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons


D. Rottschäfer, N. K. T. Ho, B. Neumann, H.-G. Stammler, M. van Gastel, D. M. Andrada, R. S. Ghadwal*

Coupling to cope with: Stable NHC-analogues of Thiele′s and Chichibabin′s hydrocarbons [(IPr)(C6H4)(IPr)] (4) and [(IPr)(C6H4)2(IPr)] (5) (IPr = C{N(2,6-iPr2C6H3)}2CHCH) are reported. Double carbenylation of 1,4-Br2C6H4 and 4,4′-Br2(C6H4)2 with IPr (1) under nickel catalysis gave [(IPr)(C6H4)(IPr)](Br)2 (2) and [(IPr)(C6H4)2(IPr)](Br)2 (3), which on reduction with KC8 afforded 4 and 5 as crystalline solids, respectively. Experimental and computational studies support semi-quinoidal nature of 5 with a small singlet-triplet energy gap ∆ES-T of 10.7 kcal/mol, whereas 4 features more quinoidal character with a rather large ∆ES-T of 25.6 kcal/mol. In view of low ∆ES-T, 4 and 5 may be described as biradicaloids. Moreover, 5 has a considerable (41%) diradical character.

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Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes


D. Rottschäfer, B. Neumann, H.-G. Stammler, M. van Gastel, D. M. Andrada, R. S. Ghadwal*

Crystalline radicals (IPrAr)• (5) and (SIPrAr)• (6) derived from classical N-heterocyclic carbenes (NHCs), (IPr = :C{N(2,6-iPr2C6H3)}2CHCH and SIPr = :C{N(2,6- iPr2C6H3)}2CH2CH2) are readily accessible by one electron reduction of the corresponding C2-arylated 1,3-imidazoli(ni)um cations 3 and 4. Cyclic voltammetry, EPR, and X-ray diffraction studies as well as DFT calculations emphasize the key role of C2-substituent in the stability of NHC-derived radicals.

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