Exocyclic Coordination of Thiamacrocycles Leading to cis- and trans-Palladium(II) Complexes and a Tripalladium(II) Complex Incorporating Acetimidic Anhydride

Preferential formation of cis- or trans-palladium­(II) complexes controlled via the exocyclic binding sites embedded in dithiamacrocycles (L¹ = −S­(CH₂)₂S–; L² = −S­(CH₂)₂O­(CH₂)₂S−) is reported. From the reaction with K₂PdCl₄, the shorter sulfur-to-sulfur separation in L¹ preferentially leads to the formation of cis-[Pd­(L¹)­Cl₂] (1), while L², incorporating a larger sulfur-to-sulfur separation, coordinates in a trans fashion to form a cyclic dimer, trans-[Pd₂(L²)₂Cl₄] (2). The observed results illustrate the possibility for the controlled formation of cis/trans square-planar complexes through binding-site design. When palladium­(II) acetate was substituted for K₂PdCl₄ in the above reaction, L¹ gave no product, while L² resulted in the formation of a unique tripalladium­(II) complex, [Pd₃(L²)­(CH₃C­(N)­OC­(N)­CH₃)­(CH₃COO)₄] (3), in which three Pd^II atoms are linked by acetimidic anhydride, CH₃C­(N)­OC­(N)­CH₃, derived from the acetonitrile solvent employed. In the ¹H NMR spectrum for 3, specific methylene signals for methylene protons adjacent to S donors exhibit large complexation-induced splitting of the geminal proton signals into axial and equatorial proton peaks, thus indicating magnetically nonequivalent geminal protons that reflect the restricted conformation of the metallabicycle.