Metallomicelle copper complexes

Tagaki et al. [24] and Fomasier et al. [25] reported another type of metallomicelle attached with a metal-bound alkoxide nucleophile. Tagaki s zinc(II) and copper(II) complexes (with possible structures 6a and b) promoted the hydrolysis of 4-nitrophenyl picolinate in a comicellar system with hexadecyl trimethy-lammonium bromide. However, no detailed mechanistic study was reported. Scrimin s zinc(II) and copper(II) complexes (proposed structures 7a and b) also promoted the hydrolysis of 4-nitrophenyl picolinate. A postulated mechanism for the catalytic activity of 7 is shown in Figure 4. An aggregate of 7 more effectively... 

This tendency of metalloaggregates to modulate the coordination to transition metal ions appears quite general. For instance, very recently we have observed [49] that copper(II) metallomicelles made of the ligand shown in Figure 11 shift the coordination equilibrium toward the five-coordinate complex even at very low pH, while the water-soluble ligand is only tetracoordinated with the second imidazole in its protonated form. Clearly, the cationic metalloaggregate drives the deprotonation of the second... 

The modulation of the coordination to the transition metal has not necessarily positive implications on the reactivity. For instance, we observed [50] that the copper(II) complex (8) of tetramethyl-l,2-diaminoethane catalyzes the hydrolysis of the phosphoric acid triester PNPDPP via an electrophilic mechanism which involves the pseudointramolecular attack of deprotonated water, as illustrated in (9). The electrophilic mechanism contribution to the hydrolytic process totally disappears in micellar aggregates made of the amphiphilic complex (10). Clearly, micellization does not allow the P O group of the substrate to interact with the metal ion. This could be a result of steric constraint of the substrate when bound to the micelle and/or the formation of binuclear dihydroxy complexes, like (11), in the aggregate. So, in spite of the quite large rate accelerations observed [51] in the cleavage of PNPDPP in metallomicelles made of the amphiphilic complex (10), the second-order rate constant [allowing for the difference in pXa of the H2O molecules bound to copper(II) in micelles and monomers] is higher for (8) than for (10) (k > 250). 

Most of the studies on the effects of metallomicelles on the rate of hydrolysis of esters involve so-called activated esters in which nucleophilic attack is the rate-determining step. The effects of copper-containing metallomicelles (formed from both copper(II)-hydrophobic ligand complex as well as from hydrophobic ligand [L = Af,Af,M-trimethyl-Ai -tetradecylethylenediamine] containing free ions) on the rate of hydrolysis of amides 19 and 20 as well as activated esters 21, 22, and 23 have been stndied at pH 7.0 and 31°C. The apparent rate enhancements (kj i) of Cn +(L) metallomicelles on the rate of hydrolysis of 19 to 23, 2-nitrophenyl acetate (2-NPA), and 4-nitrophenyl acetate (4-NPA) under various reaction conditions are summarized in Table 6.4." The actual rate enhancements due to metallomicelles, Cu +(L), under various reaction conditions are not possible to estimate because of the lack of pseudo-first-order rate constants (kobs) for hydrolysis of 19 to 23, 2-NPA, and 4-NPA in the presence of Cu -A7V,A 7V -tetramethylethylenediamine complex. However, the values of k,, for 19 and 20 are almost same at 0.002-M Cu +(L) and 0.002 M Cu +(L) + 0.001-M Triton. The presence of 0.001-Af CTABr comicelles has no effect on k, for 20 but decreases kj i from 46 to 29 for 19 (Table 6.4), which may be attributed to larger cationie mixed micellar affinity of anionic 19 than that of neutral 20. 

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