Intramolecular C-H...C_(carbene) Hydrogen Bonds and Competing Interactions in Monoprotonated Tripodal Carbenes

摘要

The anionic tripodal N-heterocyclic carbene (C_3N_2H_3)_3BH~- first prepared by Fehlhammer,together with three neutral variants,(C_3N_2H_3)_3CH,(C_3N_2H_3)_3P,and (C_3N_2H_3)_3SiH,have been studied using quantum chemical methods.Isodesmic reactions are used to deduce that the phosphine-bridgehead species in particular has a large-resonance stabilization energy.All the podands undergo substantial conformational change on excitation to the lowest triplet electronic state,with effective localization of the excitation on one of the heterocyclic rings,dearomatizing it.On monoprotonation of the ground states,three of these species display intramolecular C-H...C_(carbene) hydrogen bonding: The nature and strength of these interactions is explored using model (intermolecularly hydrogen-bonded) complexes,isodesmic reactions,and GIAO calculations of chemical shifts.One surprising result is that C-H...C_(carhene_ hydrogen bonds involving ethenic hydrogens can be almost as strong as those involving the imidazolium proton (first identified by Arduengo).The case of the monoprotonated carbon bridgehead species is in particular intriguing.It is stabilized by a competitive C_(carbene)...N interaction of sufficient strength to override the C-H...C_(carhene) bonding motif observed in the other structures.