N-heterocyclic carbenes (NHCs) constitute an important class of σ-donor ligands. They are widely used in different fields of chemistry, e.g., in homogenous catalysis or the construction of organometallic materials. NHCs can also be incorporated into macrocyclic ligands, which often possess interesting and useful structural and photophysical properties. Most of the previously described macrocyclic NHC complexes are homogenous complexes in which the metal is only coordinated by the NHCs. Heterogenous complexes with different donor units could offer more options for fine-tuning the complex properties.
Alexander Pöthig and Thomas Pickl, Technical University of Munich, Garching, Germany, and colleagues have synthesized bimetallic, macrocyclic platinum and palladium complexes with ligands that contain four NHC and two pyrazolate units each (Pd complex pictured). The palladium complex was synthesized by reacting Pd(OAc)2, H6L(PF6)4 (L = the free form of the ligand), and sodium acetate at 80°C in acetonitrile. For synthesis of the platinum complex, PtCl2(cod) (cod = 1,5-cyclooctadiene) was used as a precursor. The complexes Pd2L(PF6)2 and Pt2L(PF6)2 were isolated in 18 % and 11 % yield, respectively. The platinum complex could also be synthesized via the transmetalation of Ag8L2(PF6)4.
Both complexes were analyzed using NMR spectroscopy, elemental analysis, and X-ray diffraction, which showed a slightly distorted square-planar coordination sphere for the metals and a saddle shape for the macrocycle (pictured). For both complexes, the team observed absorption in the UV range but no absorption in the visible range, which indicates a lack of π-conjugation over the macrocyclic ligand. The platinum complex additionally showed photoluminescence in the blue region, while no photoluminescence was observed for the palladium complex.
- Bimetallic Platinum Group Complexes of a Macrocyclic Pyrazolate/NHC Hybrid Ligand,
Thomas Pickl, Alexander Pöthig,