GHADWAL RESEARCH GROUP
Molecular Inorganic Chemistry and Catalysis
advancing low-valent main-group chemistry for catalysis and materials
Selected Recent Publications
Boosting the π-Acceptor Property of Mesoionic Carbenes by Carbonylation with Carbon Monoxide
A. Merschel, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, R. S. Ghadwal*
Direct room temperature dimerization of carbon monoxide by anionic dicarbenes Li(ADC) has been reported to quantitatively yield (E)-ethene-1,2-bis(olate) bridged mesoionic carbene (iMIC) lithium compounds COen-[(iMIC)Li]2. They undergo 2e-oxidation to afford 1,2-dione bridged bis iMIC, COon-(iMIC)2 compounds while redox neutral salt metatheses yield COen-[(iMIC)E]2 containing compounds E = main group element).
Annulated 1,4-Disilabenzene-1,4-diide and Dihydrogen Splitting
F. Ebeler, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, D. W. Szczepanik, R. S. Ghadwal*
The isolation of silicon analogues of phenyl anions such as (C6H5)– and (C6H4)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-iPr2C6H3) as a green-yellow crystalline solid. Compound 5 is prepared by KC8 reduction of the Si(IV) chloride [(ADC)SiCl3]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 C4Si2 ring in 5. The diradical character (y) for 5 amounts to 15%. At room temperature, 5 readily reacts with dihydrogen (H2) to form 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 S2- and S3-bridged silathione derivatives [(ADC)Si(S)]2(μ-S2) (8a) and [(ADC)Si(S)]2(μ-S3) (8b). Moreover, treatment of 5 with Fe2CO9 affords the Fe(0) complex [(ADC)2Si2(µ2-CO)Fe2(CO)8] (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.
Isolation of an Anionic Dicarbene Embedded Sn2P2 Cluster and Reversible CO2 Uptake
F. Ebeler, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, R. S. Ghadwal*
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-iPr2C6H3) under UV light results in the formation of a Sn2P2 cluster compound [(ADC)SnP]2 as a green crystalline solid. The electronic structure of [(ADC)SnP]2 has been analyzed by quantum-chemical calculations. At room temperature, [(ADC)SnP]2 reversibly binds with CO2 and forms [(ADC)2{SnOC(O)P}SnP]. [(ADC)SnP]2 enables catalytic hydroboration of CO2 and reacts with elemental selenium and Fe2(CO)9 to afford [(ADC)2{Sn(Se)P}SnSe] and [(ADC)Sn{Fe(CO)4}P]2, respectively. All compounds have been characterized by multinuclear NMR spectroscopy and their solid-state molecular structures have been determined by single-crystal X-ray diffraction.
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).
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 2a–2c exhibit two one-electron reversible redox processes in –0.5 to –2.1 V region that correspond to the radicals 3a–3b and the anions (4a–4c)–. Computational calculations suggest a closed-shell singlet ground state for (4a–4c)– with the singlet-triplet energy gap of 19-21 kcal/mol.
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, 6414–6418.
Angewandte Chemie 2021, 133, 6485–6489.
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).
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.
Crystalline Divinyldiarsene Radical Cations and Dications
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.
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) (NHC = IPr = C{(NDipp)CH}2, SIPr = C{(NDipp)CH2}2; Dipp = 2,6-iPr2C6H3) and dications [{(NHC)C(Ph)}P]2(GaCl4)2 (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.
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.
