Dinuclear Zn II Complexes of Polydentate Polyamines as Minimalist Models of Hydrolytic Reactions

The synthesis of the novel macrocycle 2,6,9,12,16‐pentaaza[17](2,9)(1,10)phenanthrolinophane (L3) is reported. Speciation studies on the systems Zn II −L3 and Zn II −L2 (L2 =2,6,10,13,17,21‐hexaaza[22]metacyclophane) performed in aqueous solution show the formation of mono‐ and dinuclear Zn II complexes. In the two systems, the dinuclear complexes readily hydroxylate, with the hydroxo species being the main ones in solution at relatively low pH values. This feature makes these complexes promising hydrolytic agents for carboxy and phosphate esters. The hydrolytic ability of the L1−L3 dinuclear complexes toward the carboxy and phosphate ester bond was tested by addition of p ‐nitrophenyl acetate (NA) and bis( p ‐nitrophenyl)phosphate (BNP). While in the case of NA the cleavage takes place through a simple bimolecular mechanism and the hydrolysis rate depends on the nucleophilicity of the Zn II −OH functions, in the case of BNP the hydrolytic mechanism involves substrate interaction with the metal ions and nucleophilic attack of a Zn II ‐bound hydroxide ion at the phosphorus atom. The L3 complex gives rise to the highest hydrolysis rate constant { k = 62 × 10 −5 M −1 ·s −1 vs. k = 8 × 10 −5 and 4 × 10 −5 M −1 ·s −1 for [Zn 2 L2(OH) 2 ] 2+ and [Zn 2 L1(OH) 2 ] 2+ , respectively}. This may be related to a stronger interaction with the substrate due to the synergetic role in BNP binding played by the phenanthroline unit, which can give π‐stacking and hydrophobic interactions with the aromatic units of the substrate, and by the two Zn II ions, which can act cooperatively in BNP coordination. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)