Pyrazolato-Bridged Dinuclear Complexes of Ruthenium(II) and Rhodium(III) with N‑Heterocyclic Carbene Ligands: Synthesis, Characterization, and Electrochemical Properties

Pyrazolato-bridged dinuclear complexes of ruthenium and rhodium were synthesized from N-heterocyclic carbene (NHC) precursors, 3,5-bis­[(methylimidazolium-1-yl)­methyl]-1H-pyrazole bis­(hexafluorophosphate), and the metal precursors [Ru­(p-cymene)­Cl2]2 and [Rh­(η5-C5Me5)­Cl2]2. Depending on the reaction conditions, dinuclear bis­(imidazolium) complexes or the corresponding bis­(NHC) complexes were formed. These complexes were characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The metal–metal distances are in the range 3.85–3.92 Å. Accordingly, a metal–metal bond can be excluded in all cases. The electronic properties were examined by cyclic voltammetry (CV) to detect possible electronic coupling between the metal centers. In the case of the imidazolium complexes irreversible processes are observed in CV, indicating decomposition. The Ru–bis­(NHC) complexcoordinatively saturated with six acetonitrile molecules instead of p-cymene ligandsshows three reversible redox processes. Density functional theory (DFT) calculations were used to verify the processes during CV. The Rh–bis­(NHC) complex decomposes through irreversible reductions.