Cu2+, Zn2+, and Ni2+ Complexes of C2‑Symmetric Pseudopeptides with an Aromatic Central Spacer

Two new tetradentate C2-symmetric pseudopeptidic ligands derived from Val and Phe containing two amino and two amido groups and a central o-substituted aromatic spacer have been prepared. Their complexes with Cu2+, Zn2+, and Ni2+ have been studied by potentiometry, UV–vis spectrophotometry, FT-IR, and ESI-MS. The presence of the aromatic spacer provides Cu2+ complexes with stability constants several orders of magnitude higher than those observed for related ligands containing aliphatic central spacers. Besides, the formation of [MH–2L] complex species is favored. Crystal structures for the corresponding Cu2+ and Ni2+ have been obtained, revealing the metal atom in an essentially square-planar geometry, although, in several instances, the oxygen atom of an amide carbonyl of a second complex species can act as a fifth coordination site. In the case of Zn2+, the only crystal structure obtained displays a square-pyramidal arrangement of the metal center. Finally, preliminary experiments show the catalytic activity of some of these complexes, in particular, Zn2+ complexes, for epoxide ring-opening with using aniline as the nucleophile in a ligand accelerated process.