Synthesis and Study of Palladium(II) and Platinum(II) Complexes Supported by a Common “Wingtip” N-Heterocyclic Carbene

Two annulated imidazolium salts, 2-(phenyl)imidazo[1,5-a]pyridin-4-ylium hexafluorophosphate 1⋅H(PF₆) and 1-methyl-2-(phenyl)imidazo[1,5-a]pyridin-4-ylium hexafluorophosphate 2⋅H(PF₆), were synthesized via formylative cyclization of the corresponding Schiff bases followed by anion metathesis with KPF₆. Independently treating 1⋅H(PF₆) or 2⋅H(PF₆) with silver oxide and then palladium chloride in acetonitrile led to the formation of the complexes [Pd(1)₂Cl(CH₃CN)]PF₆ (1 a) and [Pd(2)₂Cl(CH₃CN)]PF₆ (2 a), respectively. Likewise, [Pt(1)₂Cl(CH₃CN)]PF₆ (1 b) and [Pt(2)₂Cl(CH₃CN)]PF₆ (2 b) were synthesized using similar transmetallation chemistry. The complexes were characterized using various spectroscopic techniques and the solid state structures of 1-H(PF₆) as well as 2 a were elucidated using X-ray diffraction analyses. A series of DFT calculations were also performed to gain further insight into the respective structures of the complexes. Complexes 1 a and 2 a were found to facilitate Suzuki coupling reactions under relatively mild conditions.