Studies on the Effect of Positive and Negative Charges on the Se 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). Se 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 Se NMR shifts since both structural changes and changes of the measurement conditions have been minimized. Depending on the charge, the Se 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-. The signals also show considerable solvent and pH dependence, which was investigated with a selection of NMR solvents (MeOD, CDCl, CDCl, CDCOOD, CDCN, acetone-d, THF-, pyridine-d, toluene-d, DMSO-) and pH values in DO. Linear relationships were found between the HOMO and LUMO energies and the Se NMR shifts, respectively.