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

We present the synthesis, structural characterization, and photophysical properties of dinuclear PdII and PtII–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 PtII congener was also prepared by transmetalation from previously published AgI pillarplex Ag8L2(PF6)4. NMR spectroscopy (1H, 13C, 195Pt) combined with SC-XRD studies elucidated the structure of the PdII and PtII 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 NiII complex Ni2L­(PF6)2, the heavier metal homologues adopt a bent, saddle-shaped structure. As observed for structurally similar PtII 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 PtII 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.