Silver and Palladium Complexes Containing Ditopic N‑Heterocyclic Carbene–Thione Ligands

The mixed donor N-heterocyclic carbene (NHC)/thione ligand
precursors [1-(3-R-2H-imidazol-1-yl-2-thione)­methyl-3-R-2H-imidazol-2-ium]­X, [HCSR]­X (R = methyl, benzyl; X = Br, I), have been utilized to prepare
a range of silver and palladium complexes. The coordination of CSR to silver­(I) salts has been explored,
providing dimeric complexes of the type [AgX­(CSR)]2 (where R = methyl, benzyl; X = Br, I).
Structural characterization of [AgX­(CSBn)]2 revealed a bidentate coordination mode for the
mixed donor ligand and dinuclear structures where the silver centers
are bridged by two bromido centers. Palladium complexes bearing one
or two CSR ligands have additionally
been prepared either directly, utilizing [Pd­(OAc)2] as
precursor, or via transmetalation strategies. The dicationic complexes
[Pd­(CSR)2]­[X]2 and neutral complexes [PdX2(CSR)] (where R = methyl, benzyl; X = Br, I, PF6) have been synthesized and fully characterized. The CSR ligand in the aforementioned complexes
does not undergo transformation of the NHC unit to a urea function,
which had been found to occur in the previously reported copper complexes.
Palladium complexes containing both NHC/thione and bis-phosphine ligands
were also prepared. Complexes of the type [Pd­(CSMe)­(L2)]­[X]2 and [PdX­(CSMe)­(L2)]­[X] (where L2 = dppe, dppp; X = Br, OAc, I, PF6) were obtained.
The presence of the bis-phosphine appears to destabilize the coordination
of the NHC/thione ligand and as a consequence leads to its elimination
from the complex.