13C NMR Spectroscopic Determination of Ligand Donor Strengths Using N-Heterocyclic Carbene Complexes of Palladium(II)

The electronic parameters of 25 Werner-type and organometallic ligands have been experimentally determined and ranked on a unprecedented unified 13C NMR scale using safe and easily obtainable complexes of the type trans-[PdBr2(iPr2-bimy)L]n− (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = ligand in question) as spectroscopic probes. The methodology is based on the sensitivity of the constant iPr2-bimy carbene signal to the donor strengths of the varying co-ligands, which even allows detection of backbone and substituent effects more accurately than previous carbonyl-based systems. For the evaluation of N-heterocyclic carbenes (NHCs), a one-pot approach to novel hetero-bis(carbene) complexes bearing two different NHCs is introduced. Furthermore, the first complex of a strongly donating indazolin-3-ylidene ligand is presented. The molecular structures of 10 complex probes have been characterized by single-crystal X-ray diffraction analyses.