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 x 10-3 s-1), and complex 2, which is less reactive than 3, displays comparable activities towards both substrates (2.0 x 10-4 s-1 and 5.4 x 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.