A palladacyclic N-heterocyclic carbene system used to probe the donating abilities of monoanionic chelators

A series of NHC-containing [C^N]- or [C^C']-type palladacyclic complexes of the general formula [PdBr(iPr2-bimy)(L^X)] (5-8, 11, 12, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been synthesized and fully characterized. Using these complexes, the donating abilities of monoanionic chelators were probed for the first time. The [C^N]-type palladacycles 5-8 were prepared from acetato-bridged dipalladium complexes [Pd(μ-CH3COO)(C^N)]2 (1-4) and iPr2-bimy·H+Br-as precursors. In the case of the [C^C']-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr2(iPr2-bimy)(trz)] (8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the iPr2-bimy probe were first prepared followed by acetate-assisted cyclopalladations. The 13Ccarbene NMR signals of the iPr2-bimy ligands in all complexes (i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or [L^X]-type chelators with the exception of the Bzpy ligand (Bzpy = 2-(2-pyridinylmethyl)phenyl-C,N). This has been attributed to its larger bite angle, the resulting varied coordination geometry and the lack of electronic delocalization between the two donor units. The donicities of [L^X]-type chelators studied in this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.