Donor Strength Determination of Benzoxazolin-2-ylidene, Benzobisoxazolin-2-ylidene, and Their Isocyanide Precursors by 13C NMR Spectroscopy of Their PdII and AuI Complexes

A series of mono- and dinuclear palladium­(II) and gold­(I) complexes containing the iPr2-bimy reporter ligand and monodentate or Janus-type β-functionalized aryl (di)­isocyanides have been prepared. Cyclization of the isocyanide ligands afforded complexes of monodentate or Janus-type N,O-heterocyclic carbenes (NOHCs) via a template-directed process. Using the iPr2-bimy 13Ccarbene signals in these complexes, the electron-donating ability of NOHCs and their isocyanide precursors could be evaluated and compared for the first time. Overall, NOHCs were found to be rather weakly donating carbenes. Spectroscopic comparison of mono- with dinuclear complexes indicated that electronic communication between two metal centers occurs only across the Janus-type diNOHC, but not for the diisocyanide ligand. Support for this claim was also found in the solid-state molecular structures, which revealed coplanarity of all ligands only in the diNOHC complex as a prerequisite for π-electron delocalization.