Palladium(ii) and palladium(ii)–silver(i) complexes with N-heterocyclic carbene and zwitterionic thiolate mixed ligands: synthesis, structural characterization and catalytic properties

Abstract

The integration of a geometrically rigid Pd(II), a coordinatively monotonous N-heterocyclic carbene 1,3-dimethylimidazoline-2-ylidene (IMe), and a flexible zwitterionic thiolate 4-(trimethylammonio)benzenethiolate (Tab) affords a class of Pd–IMe–Tab complexes with various nuclearities, namely, trans-[Pd(IMe)₂(Tab)₂](OTf)₂ (2, mononuclear), cis-[Pd(IMe)₂(Tab)₂](OTf)(Cl) (3a, mononuclear), cis-[Pd(IMe)₂(Tab)₂](PF₆)₂·MeCN (3b·MeCN, mononuclear), [Pd₂(IMe)₄(Tab)₂](PF₆)₄·2MeCN (4·2MeCN, dinuclear) and [Pd₄(IMe)₄(Tab)₆](OTf)₆(Cl)₂ (5, tetranuclear). Further presence of Ag(I) in the assembly provides a heterometallic octanuclear cluster of [Pd₄Ag₄(IMe)₈(Tab)₁₀](PF₆)₁₂ (6). Compounds 2–6 are formed by the reaction of trans-Pd(IMe)₂Cl₂ (1) with various additional reagents via different reaction pathways. These compounds are characterized by means of FT-IR, ¹H and ¹³C NMR, ESI-MS, elemental analysis and X-ray crystallography. Notably, the skeleton of compound 5 features a [Pd₄S₄] parallelogram wherein each of the four Pd(II) centers bisects the edge defined by the S atoms. The main skeleton of compound 6 is an oval-shaped Pd₄Ag₄S₁₀ unit, featuring an edge-fused norbornane-like (Pd₂Ag₄S₆) framework appended by two additional PdS₂ motifs at the polar positions. Compounds 5 and 6 also feature Pd⋯Pd (5), Pd⋯Ag and Ag⋯Ag (6) interactions. Compound 5 as a representative example is highly effective at catalyzing Suzuki–Miyaura couplings in water, highlighting the potential of applying these types of homo- and heterometallic clusters as catalysts for organic transformations in environmentally benign media.