Source of catalysis of dephosphorylation of p-nitrophenyldiphenylphosphate by metallomicelles

Abstract

A series of 2-(dimethylaminomethyl)-6-(alkylaminomethyl)pyridines of various lipophilicities have been synthesized: their complexes with Cu^II ions (2a–c: R = CH₃, C₁₂H₂₅, C₁₆H₃₃) catalyse cleavage of p-nitrophenyldiphenylphosphate (PNPDPP). The rate constants were compared with those obtained with Cu^II complexes made of ligands N-alkyl-N,N′,N′-trimethyl-1,2-diaminoethane (1a–c: R = CH₃, C₁₄H₂₉, C₁₆H₃₃). When the alkyl group is a C₁₂(or longer) chain complexes 1b,c and 2b,c form metallomicelles. The apparent pK_a of Cu^II-coordinated water is ca. 8 for 2a,b, and ca. 6 and ca. 7 for 1a and b, respectively. The aim of the work was to establish the source of rate acceleration in metallomicelles. Because the pyridine-based complexes 2a–c have only one free position for strong coordination to water, while complexes 1a–c have two such positions, complexes 1a–c can potentially activate both the nucleophile and the substrate. The results indicate that: (a) the nucleophilicity of the L–Cu^II–OH species is similar in metallomicelles and monomeric complexes; (b) free OH^– is a slightly better nucleophile than Cu^II-bound OH^– in all complexes but 1a; (c) electrophilic assistance is only present in the monomeric complex 1a and vanishes in metallomicelles.