Synthesis, Characterization, and Structures of Mononuclear and Phenoxo-/Acetato-Bridged Trinuclear Zinc(II) Complexes from Salan Ligands: Study of their Hydrolytic Activity

Synthesis of one mononuclear [ZnL1] (1) and two trinuclear phenoxo-/acetato-bridged zinc(II) complexes [Zn3Cl2(L2)2] (2) and [Zn3(OAc)2(L1)2] (3) containing tetradentate diaminobis(phenolato) “salan” ligands are reported. The ligand precursors and their corresponding zinc complexes are characterized by elemental analysis, NMR, IR, and UV–vis spectroscopy. Molecular structures of complexes 2 and 3 are solved by single-crystal X-ray crystallography, while the fate of the complexes in solution is assessed by ESI-MS spectrometry analysis. All complexes (1–3) were studied for their phosphoesterase activity by using para-nitrophenyl phosphate (PNPP) and bis(2,4–dinitrophenyl) phosphate (BNPP) as substrates. While complex 1 has no activity for these substrates, complex 3 effectively hydrolyses PNPP (1.6 × 10−3 s−1), and complex 2, which is less reactive than 3, displays comparable activities towards both substrates (2.0 × 10−4 s−1 and 5.4 × 10−4 s−1, respectively). The importance of evaluating the speciation of Zn-species present for the understanding of the mechanism of the phosphoesterase reaction is discussed.