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

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 PdPd (5), PdAg and AgAg (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.