Heterobimetallic Pd-Ru Catalysts for the Synthesis of Functionalized Polynorbornenes via Reductive Heck/ROMP Reactions

Heterobimetallic complexes offer greater versatility for applications in homogeneous catalysis than their monometallic counterparts. To take advantage of this synergy, three [Pd(N,N,S-TSC)(4-NH₂Py)] complexes (mono-Pd^Me, mono-Pd^Me,Me and mono-Pd^Ph) were synthesized from [PdCl₂(MeCN)₂] and appropriate thiosemicarbazide (TSC) ligand precursors, followed by the addition of 4-aminopyridine (4-NH₂Py). The reaction of these mono-Pd^L complexes with [RuCl₂(η⁶-p-cymene)]₂ then afforded three heterobimetallic complexes with the generic formula [{RuCl₂(p-cymene)}-μ-{(4-NH₂Py)Pd (TSC)}] (Ru-Pd^Me, Ru-Pd^Me,Me and Ru-Pd^Ph). All these novel compounds were characterized by elemental analysis, spectroscopic techniques, cyclic voltammetry, and density functional theory (DFT) calculations. The molecular structure of mono-Pd^Me was determined by X-ray diffraction analysis. The Ru-Pd^L complexes were evaluated as bifunctional catalysts in the reductive Heck coupling of norbornadiene (NBD) with iodobenzene and the ring-opening metathesis polymerization (ROMP) of phenylnorbornene (Ph-NBE). They were very active for the reductive Heck reaction, reaching yields higher than 90% when a [Pd]/[NBD]/[PhI] ratio of 1/418/1344 was employed at 120°C. Upon activation with ethyl diazoacetate (EDA), they were also able to polymerize Ph-NBE with yields around 70% using a [Ph-NBE]/[Ru-Pd^L] ratio of 3000 at 50°C for 30 min. The Ru-Pd^Me,Me complex showed the highest activity in the ROMP of Ph-NBE and was successfully applied for mediating the synthesis of poly (Ph-NBE) from NBD via tandem catalysis.