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.