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

Abstract

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 ¹³C_carbene 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.