Direct Arylation Mediated by Palladium Complexes with Rigid Phosphine-Functionalized N-Heterocyclic Carbenes

New phosphine-functionalized N-heterocyclic carbene ligand precursors ((LH)-H-1 center dot I and (LH)-H-2 center dot I) were prepared. The former compound contains a PPh2 moiety, whereas the latter one has a more electron-donating PCy2 group. A reaction of (LH)-H-1 center dot center dot center dot I with PdCl2 in the presence of base afforded the complex (PdLClI)-Cl-1. An interconversion process between the two structural isomers of (PdLClI)-Cl-1 was observed by variable-temperature NMR. The coalescence temperature (T-c) is at 368 K with the free energy of activation (Delta G*) found to be 68.9 kJ mol(-1). A similar reaction of (LH)-H-2 center dot I with PdCl2 produced a mixture of (PdLClI)-Cl-2, (PdLClI2)-Cl-2, and (PdLClI2.)-Cl-2 Unlike (PdLClI)-Cl-1, (PdLClI)-Cl-2 undergoes a much faster exchange between the two structural isomers. Pure PdLI2 (L = L-1 and L-2) can be obtained by adding NaI in the reaction mixture to facilitate ligand exchange. [PdL(CH3C N)(2)](BF4)(2) were also prepared straightforwardly by salt metathesis reactions. In total, seven new palladium complexes of L were structurally characterized, and in each of these structures an anagostic interaction exists between the palladium center and one of the diastereotopic methylene protons. The interaction also exists in solution, as evidenced by the downfield NMR shift of the proton. The new complexes PdLI2 and [PdL(CH3C N)(2)](BF4)(2) show mediocre activities in benchmark Suzuki coupling reactions under conventional and microwave heating. They are, however, highly promising in mediating direct arylation of phenyl halides and diphenylacetylene, affording 9-benzylidene-9H-fluorene in good yields.