Zn(II) Coordination to Polyamine Macrocycles Containing Dipyridine Units. New Insights into the Activity of Dinuclear Zn(II) Complexes in Phosphate Ester Hydrolysis

Zn(II) binding by the dipyridine-containing macrocycles L1-L3 has been analyzed by means of potentiometric
measurements in aqueous solutions. These ligands contain one (L1, L2) or two (L3) 2,2‘-dipyridine units as an
integral part of a polyamine macrocyclic framework having different dimensions and numbers of nitrogen donors.
Depending on the number of donors, L1-L3 can form stable mono- and/or dinuclear Zn(II) complexes in a wide pH
range. Facile deprotonation of Zn(II)-coordinated water molecules gives mono- and dihydroxo-complexes from neutral
to alkaline pH values. The ability of these complexes as nucleophilic agents in hydrolytic processes has been
tested by using bis(p-nitrophenyl) phosphate (BNPP) as a substrate. In the dinuclear complexes the two metals
play a cooperative role in BNPP cleavage. In the case of the L2 dinuclear complex [Zn2L2(OH)2]2+, the two metals
act cooperatively through a hydrolytic process involving a bridging interaction of the substrate with the two Zn(II)
ions and a simultaneous nucleophilic attack of a Zn−OH function at phosphorus; in the case of the dizinc complex
with the largest macrocycle L3, only the monohydroxo complex [Zn2L3(OH)]3+ promotes BNPP hydrolysis. BNPP
interacts with a single metal, while the hydroxide anion may operate a nucleophilic attack. Both complexes display
high rate enhancements in BNPP cleavage with respect to previously reported dizinc complexes, due to hydrophobic
and π-stacking interactions between the nitrophenyl groups of BNPP and the dipyridine units of the complexes.