Zn(II) Coordination by Open-chain Polyamine Ligands Containing Dipyridine Units. Highly Active Dinuclear Zn(II) Complexes in Phosphate Ester Hydrolysis

The synthesis and characterization of two new open-chain ligands, L1 and L2, containing two 2,2'-dipyridine units connected respectively by a pentaethylenhexamine and a pentapropylenhexamine chain, is reported. Zn(II) com- plexation has been studied by means of potentiometric and UV-vis spectrophotometric measurements in aqueous solution. Both ligands form stable mono- and dinuclear complexes, the metal ions being coordinated by a single dipyridine unit and by the adjacent amine groups. The ability of the binuclear Zn(II) complexes as nucleophilic agents in hydrolytic processes has been tested by using bis(p-nitrophenyl) phosphate (BNPP) as substrate. Both complexes display high rate enhance- ments in BNPP cleavage in aqueous solution. The study outlines that different mechanisms are operative in BNPP cleav- age in the presence of the L1 and L2 complexes.