Source of catalysis of dephosphorylation of p-nitrophenyldiphenylphosphate by metallomicelles

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 C-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 C-12 (or longer) chain complexes 1b,c and 2b,c form metallomicelles. The apparent pK, 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 la-c have two such positions, complexes la-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.