(Hetero)bimetallic and Tetranuclear Complexes of Pincer-Bridged N-Heterocyclic Carbene Ligands

Site-selective mono-palladation of potentially pentadentate dipropyl-pyridine-2,6-dicarboxamide bridged diNHC proligands 1(Bn/Me) can be achieved by using different bases, affording complexes of the type trans-[PdCl2(diNHC)] (3(Bn/Me)) and [PdCl(N',N,N')]BF4 (4(Bn/Me)), respectively, as metallo-proligands capable of binding additional metals. The 2nd palladation of both metalloligands using Pd(OAc)(2), however, gave the same tetrapalladium complex 5, in which the ligand coordinates in a mu,(KCN3)-C-4,KC mode due to the entropically favorable tetradentate encapsulation of one Pd-II ion, leaving a monodentate NHC coordination to the 2nd palladium center. Single deprotonation of the diazolium complex 4(Bn) affords the benzimidazolium complex 7 as the key intermediate in the formation of the tetrapalladium complex 5. Metallo-NHC precursor 7 also provides easy access to the hetero-bimetallic palladium/gold complex 8.