Methyl-palladium(II) complexes of pyridine-bridged bis(nucleophilic heterocyclic carbene) ligands: Substituent effects on structure, stability, and catalytic performance

A series of discrete, mononuclear palladium(II)–methyl complexes, together with several palladium(II)–chloro analogues, of pyridine-functionalised bis-NHC ligands have been prepared via ligand transmetallation from the silver(I)-NHC complexes. The reported complexes comprise examples with both the methylene-bridged 2,6-bis[(3-R-imidazolin-2-yliden-1-yl)methyl]pyridine (RC∧N∧C; R = Mes, dipp, tBu) and planar 2,6-bis(3-R-imidazolin-2-yliden-1-yl)pyridine (RCNC; R = Mes, dipp) ligands and, when combined with the previously reported MeC∧N∧C/MeCNC examples, cover a broad spectrum of ligand substituent steric and electronic properties, including the bulky Mes and dipp groups frequently used in catalytic applications. The palladium(II) complexes have been characterised by a variety of methods, including single crystal X-ray crystallography, with the shielding of the Pd–Me groups in the proton NMR spectra of some of the N-aryl substituted examples correlated with the proximity of the aryl rings to the methyl group in the solid state structures. The [PdMe(RC∧N∧C/RCNC)]+ complexes undergo thermal degradation via reductive methyl-NHC coupling to give 2-methyl-3-R-imidazolium-1-yl species with relative stabilities in the order of [PdMe(MesC∧N∧C)]BF4 > [PdMe(MeC∧N∧C)]BF4 ≈ [PdMe(MesCNC)]BF4 > [PdMe(MeCNC)]BF4 > [PdMe(tBuC∧N∧C)]BF4 ≫ [PdMe(tBuCNC)]BF4 (not isolable). A comparison of the activity of the complexes as precatalysts in a model Heck coupling reaction shows greatest activity in those species bearing bulkier N-substituents, with complexes bearing RC∧N∧C ligands generally more efficient precatalysts than those bearing RCNC ligands.