Influence of Hydrogen-Bonding Interactions on Nuclearity and Structure of Palladium Tiara-like Complexes

The role of intramolecular hydrogen-bonding interactions upon the nuclearity of palladium tiara-like complexes is reported herein. The synthesis of three palladium tiaras is described with three related thiolate ligands that vary in their hydrogen-bonding capability, amide vs ester for N-acetylcysteamine (tiara 1) vs 2-mercaptoethyl acetate (tiara 2) or ethyl thioglycolate (tiara 3), and in the relative position of the ester group, 2-mercaptoethyl acetate (2) or ethyl thioglycolate (3). Mass spectrometry indicates that, in the absence of protic solvents, N-acetylcysteamine reacts to form exclusively a six-membered tiara, [Pd(SCH₂CH₂NHCOCH₃)₂]₆, 1, whereas the ester containing analogues form both six- and eight-membered tiaras. Single-crystal X-ray diffraction studies indicate the significance of intramolecular N-H···O hydrogen bonds in determining the nuclearity of the amide-containing tiara 1. NMR studies indicate that 1 is not in equilibrium with larger tiaras in solution, and that the smaller size of the aggregate inhibits the fluxional behavior of the pendant thiolate ligands, typically observed for larger tiaras. Electrochemical investigations of 1 reveal reductive processes that exhibit an increase in current upon addition of acid, along with the formation of palladium nanoparticles.