13C NMR spectroscopic determination of ligand donor strengths using N-Heterocyclic carbene complexes of palladium(II)

Abstract

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 = l,3-diisopropylbenzimidazolin-2ylidene; 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 heterobis(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. © 2009 American Chemical Society.