Dinuclear Triazole-Derived Janus-Type N-Heterocyclic Carbene Complexes of Palladium: Syntheses, Isomerizations, and Catalytic Studies toward Direct C5-Arylation of Imidazoles

The dipalladium triazolidine-diylidene complex all-trans-[PdBr2(CH3CN)](2)(mu-ditz) (1) (ditz = 1,2,4-trimethyltriazolidine-3,5-diylidene) was synthesized via in situ deprotonation of the precursor salt with a basic metal precursor. Ligand replacements of all-trans-1 with monodentate or chelating phosphines afforded the dicarbene-bridged complexes all-cis-[PdBr2-(PPh3)](2)(mu-ditz) (2) and [PdBr(DPPP)](2)(mu-ditz)Br-2 (3), respectively. Bromido substitution of all-cis-2 gave tetra-acetato complex all-cis-[Pd(CH3COO)(2)(PPh3)](2)(mu-ditz) (4) with retention of the configuration as the predominant product. In addition, monopalladium triazolin-5-ylidene complexes trans-[PdBr2(CH3CN)(tazy)] (6, tazy =1,4-dimethyltriazolin-5-ylidene), cis-[PdBr2(PPh3)(tazy)] (7), [PdBr(DPPP)(tazy)]Br (8), and cis-[Pd(CH3COO)(2)(PPh3)(tazy)] (9) were also synthesized as the respective mononuclear equivalents for comparison. A comparative catalytic study revealed the general superiority of dinuclear complexes 1-4 over their respective mononuclear counterparts 6-9 in the direct C5-arylation reaction of 1-methylimidazoles. Overall, mixed dicarbene/diphosphine complex 3 showed the best catalytic performance.