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.