title: Isolation of a C5-deprotonated imidazolium, a crystalline "abnormal" N-heterocyclic carbene.
abstract: The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [N-heterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC.
authors.affiliation: University of California Riverside-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, University of California Riverside, Riverside, CA 92521-0403, USA.
authors.forename: Eugenia
authors.initials: E
authors.lastname: Aldeco-Perez
authors.forename: Amos J
authors.initials: AJ
authors.lastname: Rosenthal
authors.forename: Bruno
authors.initials: B
authors.lastname: Donnadieu
authors.forename: Pattiyil
authors.initials: P
authors.lastname: Parameswaran
authors.forename: Gernot
authors.initials: G
authors.lastname: Frenking
authors.forename: Guy
authors.initials: G
authors.lastname: Bertrand
mesh_terms: D055598:Chemical Phenomena; D003460:Crystallization; D006571:Heterocyclic Compounds / Q000138:chemical synthesis* / Q000737:chemistry / Q000302:isolation & purification*; D007093:Imidazoles / Q000138:chemical synthesis* / Q000737:chemistry / Q000302:isolation & purification*; D007536:Isomerism; D015394:Molecular Structure; D011522:Protons; D013696:Temperature; D013816:Thermodynamics
chemical_list: D006571:Heterocyclic Compounds; D007093:Imidazoles; D011522:Protons