Elucidation of the mechanism of carboxy ester cleavage promoted by a Cu(II) alkoxide complex of a tripodal ligand (N3OX)

Citation
J. Xia et al., Elucidation of the mechanism of carboxy ester cleavage promoted by a Cu(II) alkoxide complex of a tripodal ligand (N3OX), J CHEM S DA, (14), 2001, pp. 2109-2115
Citations number
42
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Inorganic & Nuclear Chemistry
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS
ISSN journal
1472-7773 → ACNP
Issue
14
Year of publication
2001
Pages
2109 - 2115
Database
ISI
SICI code
1472-7773(2001):14<2109:EOTMOC>2.0.ZU;2-6
Abstract
A solution complexation study by potentiometric titration of the new Cu(ii) complexes of the tripodal ligand 2-[bis(2-aminoethyl)amino]ethanol (L) has revealed that with coordination to Cu(ii), the hydroxyl group of L exhibit s a rather low pK(a) value of 8.5 to give Cu(ii) alkoxide, which is also co nfirmed by solution ES-MS. Release of the second hydrogen atom from the coo rdinated water occurs at pK(a)=10.6 at 25 degreesC. X-Ray diffraction analy sis of [CuL(Cl)](ClO4) revealed that Cu(ii) adopts a trigonal-bipyramidal g eometry, different from all the previously reported N3OX-copper complexes. The CuL-promoted hydrolysis of 4-nitrophenyl acetate (NA) showed a second-o rder rate constant of 0.043 M-1 s(-1) and further study of the hydrolysis m echanism provided evidence for the preference of a transesterification rath erthan a hydrolysis path. Additionally, an ab initio Hartree-Fock study str ongly suggested that the Cu(ii) alkoxide will be generated prior to Cu(ii) hydroxide due to the energy preference for the former, with the energy diff erence estimated to be 55.77 kJ mol(-1), so the Cu(ii) coordinated alcohol will exhibit a lower pK(a) than Cu(ii) coordinated water. Mulliken populati on analysis showed that the HOMO oxygen lone pair energy for Cu(ii) alkoxid e is 0.148 eV higher than that of Cu(ii) hydroxide, according to which the oxygen of Cu(ii) alkoxide is expected to be more nucleophilic.