Reactions of palladium and gold complexes with zinc-thiolate chelates using electrospray mass spectrometry and X-ray diffraction: molecular identification of [Pd(bme-dach)], [Au(bme-dach]+ and [ZnCl(bme-dach)]2Pd

Abstract

The reaction between the complexes [MCl(L)]Cl_x (L = 2,2′,2′′-terpyridine, terpy and dien, diethylenetriamine; M = Pd, x = 1; M = Au, x = 2) and [Zn(bme-dach)]₂, an N₂S₂-Zn-thiolate bridged dimer used to mimic zinc finger protein sites, was studied by Electrospray Ionisation Mass Spectrometry and the structures of some of the products confirmed by X-ray crystallography. All reactions investigated in this work gave heteronuclear (Zn-thiolate)-metal products, the predominant species being the trinuclear dithiolate-bridged aggregate {[Zn(bme-dach)]₂M}ⁿ⁺ (M = Pd, Au). X-Ray diffraction studies verified the molecular structure of [{ZnCl(bme-dach)}₂Pd], and further confirmed that the zinc within the [Zn(bme-dach)]₂ unit was retained within the N₂S₂ binding site. The Zn-bound thiolates form stable thiolate bridges to Pd²⁺ in a stair-step shape, held together by a planar PdS₄ center. In addition, both zinc atoms maintained penta-coordinate coordination with apical chloride ligands rather than the more commonly observed tetrahedral geometry. Further, [Pd(bme-dach)] was directly synthesized for X-ray structural characterization of the metal exchanged product observed in mass spectrometry experiments. In the case of Au compounds, the reactions were very fast and the products were similar for both [AuCl(L)]Cl₂ (L = terpy and dien) starting materials. In addition to the multimetallic Zn,Au,Zn aggregate formation, the predominant species from the reaction between [Zn(bme-dach)]₂ and both Au compounds was the [Au(bme-dach]⁺ cation observable via ESI-MS, suggesting Zn/Au metal exchange immediately after mixing the compounds. The direct synthesis of [Au(bme-dach)]BPh₄ confirmed the molecular structure of this species through X-ray crystallography. The reactivity profile of Pd²⁺ and Au³⁺ species is compared with previous studies using the isostructural Pt compounds and the biological relevance of the results discussed.