Studies on the Effect of Positive and Negative Charges on the ⁷⁷Se NMR Shifts of Selenones and Selenenyls of N-heterocyclic Carbenes of Imidazolium-4,5-dicarboxylates

1,3-Imidazole-4,5-dicarboxylic acid derivatives were investigated with respect to the properties of the underlying N-heterocyclic carbenes. Their selenium adducts were prepared as diesters, monoesters, and dianionic dicarboxylates. Se-methylation yielded selenenyls (cationic selenoethers). ⁷⁷Se NMR shifts of selenium adducts are considered a measure of the electronic properties of the underlying N-heterocyclic carbene. Since they are very sensitive to external (solvent, pH, temperature, reference reagent, reference method) and internal parameters (substituent effects, steric effects, nonclassical H-bonding, anisotropy), difficulties can arise in reliably interpreting these values. Our model compounds allow the systematic investigation of the influence of charges on the ⁷⁷Se NMR shifts since both structural changes and changes of the measurement conditions have been minimized. Depending on the charge, the ⁷⁷Se NMR shifts cover the range from 42.1 ppm (dianionic dicarboxylate) via 65.0 ppm (monoanionic dicarboxylate) and 92.5 ppm (neutral diester) to 153.8 ppm (cationic selenoether) in DMSO-<i>d</i>₆. The signals also show considerable solvent and pH dependence, which was investigated with a selection of NMR solvents (MeOD, CDCl₃, CD₂Cl₂, CD₃COOD, CD₃CN, acetone-d₆, THF-<i>d</i>₈, pyridine-d₅, toluene-d₈, DMSO-<i>d</i>₆) and pH values in D₂O. Linear relationships were found between the HOMO and LUMO energies and the ⁷⁷Se NMR shifts, respectively.