Thioether-Functionalized N-Heterocyclic Carbenes: Mono- and Bis-(S,CNHC) Palladium Complexes, Catalytic C-C Coupling, and Characterization of a Unique Ag4I4 (S,CNHC)2 Planar Cluster

We report a one-step synthesis for new N-aryl-N'-thioether imidazolium salts that are precursors to (S,C-NHC) ligands in N-heterocyclic carbene (NHC) complexes. The crystal structure of the N-(2,6-diisopropylphenyl)-N'-ethyl-(ethyl)-sulfide imidazolium hexafluorophosphate 8 center dot HPF6 was determined by X-ray diffraction and revealed H-bonding interactions between the PF6 anion and, in particular, the imidazolium 2-H proton. The corresponding Ag(I) NHC complexes 1 center dot AgX-8 center dot AgX were synthesized and fully characterized. An unprecedented planar, centrosymmetric cluster, [Ag-4(mu(3)-I)(2)(mu(2)-I)(2)(mu(2)-S,C-NHC)(2)] [5 center dot(Agl)(2)](2), was obtained in which two functional carbene ligands bridge two edges of a silver rectangle. With the N-alkyl-N'-thioether ligands, [PdCl2(S,C-NHC)(2)] complexes were prepared by two different routes: the usual transmetalation reaction involving the Ag(I) NHC reagent or a stepwise sequence involving deprotonation of the imidazolium function in zwitterionic intermediates where the thioether function is bound to the Pd(II) center. The crystal structures of two representative complexes, 10 and 12, with an ethyl- or a phenyl-thioether function, respectively, coordinated to the Pd(II) center, have been determined by X-ray diffraction and confirmed their mononuclear structure. In the neutral complexes trans-[PdCl2(C-NHC)(2)] (17-20) the thioether group is not coordinated to the metal center, as also confirmed by the crystal structure determination by X-ray diffraction of the bis-NHC Pd(II) dichloro complex 18, which also established the trans-anti arrangement of the ligands. The First examples of bis-chelated dicationic palladium(II) complexes with thioether-functionalized NHCs, [Pd(S,C-NHC)(2)][BF4](2) (21-24), are reported, which were selectively obtained with a cis arrangement of the ligands. The Pd(II) complexes 9-24 were evaluated in Suzuki-Miyaura cross-coupling reactions under various conditions, and a higher catalytic activity was observed for the complexes in which the sulfur atom is coordinated to the metal center.