Selected Recent Publications

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.

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*

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.

Isolation of a Ge(I) Diradicaloid and Dihydrogen Splitting

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

J. Am. Chem. Soc. 2021, n/a, doi: 10.1021/jacs.0c11828.

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, M. van Gastel, A. Hinz, R. S. Ghadwal* 

      Chem. Commun. 2019, 55, 10408–10411.

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.

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.

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.

Anorganische Chemie und Strukturchemie (ACS)

Fakultät für Chemie, Universität Bielefeld

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