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66. Diradical Character Enhancement by Spacing: N‐Heterocyclic Carbene Analogues of Müller's Hydrocarbon

       Rottschäfer, D.; Busch, J.; Neumann, B.; Stammler, H.-G.; van Gastel, M.; Kishi, R.; Nakano, M.; Ghadwal,* R. S.   

       Chem. Eur. J. 2018, 9, 16537–16542.


Radical Taming by NHCs. Crystalline N-heterocyclic carbene (NHC) analogues of highly reactive Müller’s hydrocarbon (A), [(NHC)2(C6H4)3](BNHC) are reported (NHC = C{NArCH2}2, SIPr; C{NAr(CH)2}2, IPr; C{NAr}2(CMe)2, Me-IPr and Ar = 2,6-iPr2C6H3). They feature a singlet ground state with small singlet-triplet energy gaps (ΔES-T= ~ 7.4 kcal/mol) along with an intermediate diradical character (y = 65%).

65. Kekulé Diradicaloids Derived from a Classical N-Heterocyclic Carbene

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

       Chem. Sci. 2018, 9, 4970–4976.











Kekulé Diradicaloids: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-iPr2C6H3)}2CH2CH2) by means of nickel catalysis gives rise to 1,3-imidazolinium salts [(SIPr)(C6H4)(SIPr)](I)2 (2) and [(SIPr)(C6H4)2(SIPr)](Br)2 (3) as off-white solids. Two-electron reduction of 2 and 3 with KC8 cleanly yields Kekulé diradicaloid compounds [(SIPr)(C6H4)(SIPr)] (4) and [(SIPr)(C6H4)2(SIPr)] (5), respectively, as crystalline solids. Structural parameters and DFT as well as CASSCF calculations suggest the close-shell singlet ground state for 4 and 5. Calculations reveal a very low singlet-triplet energy gap ∆ES-T for 5 (10.7 kcal/mol), while ∆ES-T for 4 (29.1 kcal/mol) is rather large.

63. N-Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons

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

      Angew. Chem. Int. Ed.  2018, 57,  5838–5842.

      Angew. Chem. 2018, 130, 5940-5944.








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.

64. Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes 

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

      Angew. Chem. Int. Ed.  2018, 57,  4765–4768.

      Angew. Chem. 2018, 130, 4855–4858.




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.

62. The Viability of C5-Protonated- and C4,C5-Ditopic Carbanionic- Abnormal NHCs: A New Dimension in NHC Chemistry

       Rottschäfer, D.;  Ebeler, F.; Strothmann, T., Neumann, B.; Stammler, H.-G.; Mix, A.; Ghadwal,* R. S.

      Chem. Eur. J.  2018, 24,  37163720.










Happy with “H”... Deprotonation of the C2-arylated 1,3-imidazolium salt 3-X affords the C5-protonated aNHC, (aIPrPh) 4. Double deprotonation of 3-X yields C4,C5-ditopic carbanionic aNHC, 13. The facile accessibility of 4 and 13 not only enables an easy access to challenging aNHC-compounds and phosphorus ligands but also provides a unique platform for the design of new hybrid ligands and heterobimetallic complexes.

61. Silylene-Functionalized N-Heterocyclic Carbene (Si-NHC)

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

      Chem. Eur. J.  2018, 24, 380387.

















Congeners hand-in-hand...The synthesis, structure, and reactivity of the first silylene-functionalized N-heterocyclic carbene (Si-NHC), {(L1)Si}(L2), (3) (L1 = PhC(NtBu)2, L2 = CCH(CNAr)2C:, Ar = 2,6-diisopropylphenyl) are reported. The ditopic carbanionic-NHC (dc-NHC), [Li(L2)] (1) reacts with the monochlorosilylene (L1)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 {(L1)Si(E)}L2(E) (E = S, 4; Se, 5; and Te, 6). Compounds 46 feature a highly polarized Siδ+Eδ- bond with a formal charge of −0.72 (4), −0.64 (5), and −0.50 e (6) on the E atom. The WBIs of 1.64 (4), 1.66 (5), and 1.63 (6) for the SiE bond however indicate a moderate pπ ‒ pπ interaction between Si and E atoms. Compound 3 functions as a reducing agent and induces disproportionation of Si2Cl6 to yield the silyl-functionalized-NHC {(L1)SiCl2}(L2) (7). Treatment of 3 with HSiCl3 leads to the formation of the silyl-functionalized imidazolium salt [{(L1)SiCl2}(L2H)]Cl (8). The silylene moiety of 3 readily forms the iridium-silylene complex [{(L2)(L1)Si}Ir(cod)Cl] (9) featuring a free NHC ligand on treatment with [Ir(cod)Cl]2.

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