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(μ-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(μ-ditz) (2) and [PdBr­(DPPP)]2(μ-ditz)­Br2 (3), respectively. Bromido substitution of all-cis-2 gave tetra-acetato complex all-cis-[Pd­(CH3COO)2(PPh3)]2(μ-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.