Anionic N-Heterocyclic Carbenes from Mesoionic Imidazolium-4-pyrrolides: The Influence of Substituents, Solvents, and Charge on their ⁷⁷Se NMR Chemical Shifts

Mesoionic compounds are the starting material for the synthesis of unique anionic N-heterocyclic carbenes. Herein, mesoionic imidazolium pyrrolides synthesized from pyrrole-2-carbaldehyde via various <i>N</i>-alkyl-4-pyrroyl-imidazoles are described. These were converted into nine new 4-(pyrrol-2-yl)-substituted imidazolium salts and transformed into the mesoionic title compounds using an anion exchange resin. The DFT-calculated (B3LYP/6-311++G**) CREF values indicate a great potential for the formation of anionic N-heterocyclic carbenes by deprotonation, which were generated and reacted with selenium to obtain selenoureas. The ⁷⁷Se NMR shifts investigated under systematic variation of conditions are dependent on the substitution pattern (Δδ_Se = 133 ppm) and the steric demand of the substituents. Solvent dependencies of the ⁷⁷Se NMR shifts were investigated applying toluene-<i>d</i>₈, THF-<i>d</i>₈, CDCl₃, CD₂Cl₂, pyridine-<i>d</i>₅, acetone-<i>d</i>₆, DMSO-<i>d</i>₆, CD₃CN, AcOD, and MeOD. The influences of the referencing method on the ⁷⁷Se shifts using external or internal Me₂Se or Ph₂Se₂ and solvent can add up to Δδ_Se = ca. 80 ppm. In addition, we observed a temperature dependence of both the selenoureas and the reference reagent Ph₂Se₂ as well as a ⁷⁷Se shift difference of the analyte caused by interaction with internally added Ph₂Se₂. The negative charge of deprotonated selenoureas shifts the values by an additional -20 ppm.