Interfacial water with special electron donor properties: Effect of water-surfactant interaction in confined reversed micellar environments and its influence on the coordination chemistry of a copper complex

In this work, we have investigated the behavior of the square-planar mixed-ligand transition-metal complex N,N,N',N'-tetramethylethylenediamine copper (II) acetylacetonate tetraphenylborate, [Cu(acac)(tmen)][B(C6H5)(4)], in anionic benzene/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles (RMs) as well as in cationic benzene/benzyl-n-hexadecyl dimethylammonium chloride (BHDC) RMs with and without water, using absorption spectroscopy. In the absence of water, W-0 = [H2O]/[surfactant] = 0, our results show that the Cu complex structures are not the same in both reversed micellar media. In the BHDC RMs, due to the Cl- counterion present in the system, the Cu complex structure that exists at the interface is the neutral [Cu(acac)(tmen)(Cl)]. In contrast, in AOT RMs the surfactant sulfonate anion cannot act as a ligand and therefore the complex structure is square planar [Cu(acac)(tmen)](+). When water is added to both reversed micellar systems, different situations are observed. At low water content, the water sequestrated by BHDC RMs is unable to act as electron donor because the oxygen nonbonding electron pairs are completely involved in the cationic BHDC polar head group solvation through an ion-dipole interaction. On the contrary, in AOT RMs the results suggest that since the water molecules solvate the sulfonate group through hydrogen bonding interactions, the bulk hydrogen bond network is destroyed at the interface and therefore, the nonbonding electron pairs are more available to interact with the Cu complex. Thus, the electron donor ability of water in AOT RMs is enhanced in comparison with the BHDC RMs and bulk water, giving a remarkable electron donor character to the AOT reversed micellar interfacial water. (C) 2010 Elsevier Inc. All rights reserved.