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

Abstract

The mixed donor N-heterocyclic carbene (NHC)/thione ligand precursors [1-(3-R-2<i>H</i>-imidazol-1-yl-2-thione)­methyl-3-R-2<i>H</i>-imidazol-2-ium]­X, [H<b>CS</b>^<b>R</b>]­X (R = methyl, benzyl; X = Br, I), have been utilized to prepare a range of silver and palladium complexes. The coordination of <b>CS</b>^<b>R</b> to silver­(I) salts has been explored, providing dimeric complexes of the type [AgX­(<b>CS</b>^<b>R</b>)]₂ (where R = methyl, benzyl; X = Br, I). Structural characterization of [AgX­(<b>CS</b>^<b>Bn</b>)]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 <b>CS</b>^<b>R</b> ligands have additionally been prepared either directly, utilizing [Pd­(OAc)₂] as precursor, or via transmetalation strategies. The dicationic complexes [Pd­(<b>CS</b>^<b>R</b>)₂]­[X]₂ and neutral complexes [PdX₂(<b>CS</b>^<b>R</b>)] (where R = methyl, benzyl; X = Br, I, PF₆) have been synthesized and fully characterized. The <b>CS</b>^<b>R</b> 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(<b>CS</b>^<b>Me</b>)­(L₂)]­[X]₂ and [PdX­(<b>CS</b>^<b>Me</b>)­(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