Bimetallic Platinum Group Complexes of a Macrocyclic Pyrazolate/NHC Hybrid Ligand

We present the synthesis, structural characterization, and photophysical properties of dinuclear Pd^II and Pt^II–NHC complexes Pd₂L­(PF₆)₂ and Pt₂L­(PF₆)₂ based on a macrocyclic calix[4]­imidazolylidene[2]­pyrazolate ligand obtained by in situ deprotonation of the tetraimidazolium salt H₆L­(PF₆)₄. The Pt^II congener was also prepared by transmetalation from previously published Ag^I pillarplex Ag₈L₂(PF₆)₄. NMR spectroscopy (¹H, ¹³C, ¹⁹⁵Pt) combined with SC-XRD studies elucidated the structure of the Pd^II and Pt^II complexes in the solid state and in solution. The d⁸ metal ions of both congeners are coordinated in a slightly distorted square-planar arrangement. Similar to the previously reported Ni^II complex Ni₂L­(PF₆)₂, the heavier metal homologues adopt a bent, saddle-shaped structure. As observed for structurally similar Pt^II complexes in solution, bimetallic Pt₂L­(PF₆)₂ showed photoluminescence in the blue region. In the solid state, emission was observed at a similar energy with unusually short lifetimes compared to other monometallic Pt^II complexes. DFT and TDDFT studies shed light on the nature of the most bathochromic transitions, suggesting a significant pyrazolate- and NHC-centered π–π* character.