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

A series of NHC-containing [(CN)-N-boolean AND]- or [(CC)-C-boolean AND']-type palladacyclic complexes of the general formula [PdBr(Pr-i(2)-bimy)((LX)-X-boolean AND)] (5-8, 11, 12, Pr-i(2)-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 [(CN)-N-boolean AND]-type palladacycles 5-8 were prepared from acetato-bridged dipalladium complexes [Pd(mu-CH3COO)((CN)-N-boolean AND)](2) (1-4) and Pr-i(2)-bimy center dot H(+)Br(-)as precursors. In the case of the [(CC)-C-boolean AND']-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr2(Pr-i(2)-bimy)(trz)] (8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the Pr-i(2)-bimy probe were first prepared followed by acetate-assisted cyclopalladations. The C-13(carbene) NMR signals of the Pr-i(2)-bimy ligands in all complexes (i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or [(LX)-X-boolean AND]-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 [(LX)-X-boolean AND]-type chelators studied in this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.