Département de chimie moléculaire

Potentiometric and spectrophotometric titrns. indicate that aq. solns. that contain equimolar amts. of Cu2+ and tren contain the HCuL3+, CuL2+ and CuL(OH)+ species and that their relative concns. depend on the pH of the soln. The stability consts. and the UV/Vis and EPR spectra of the three species have been detd. The position of the absorption max. clearly corresponds to a trigonal bipyramidal (tbp) geometry for CuL2+, whereas for HCuL3+ and CuL(OH)+ there are also bands that could correspond to square pyramidal (sp) complexes, but the EPR spectra indicate that only HCuL3+ can be considered to be sp. When any of these species is mixed with an excess of acid, an intermediate is formed within the mixing time of the stopped-flow technique. This intermediate undergoes complete decompn. in a second slower step. Interestingly, the spectrum of this intermediate is typical of sp geometry. Kinetic studies on complex formation in general indicate that complexation occurs in a single step, although under certain conditions an addnl. step has been obsd. that probably corresponds to the conversion of CuL2+ to HCuL3+, and the spectral changes indicate that the process involves structural reorganization from tbp to sp geometry. DFT and TD-DFT calcns. have been carried out for the three stable species, as well as for species in a higher protonation state. The results indicate that CuL2+ exists as a species with tetradentate tren and tbp geometry, although a wide range of distortions between the ideal tbp geometry and a geometry closer to sp is possible with a very modest energy cost. The energy change assocd. with hydrolysis of one of the Cu-N bonds to give a species with tridentate tren was found to be slightly higher than that previously found for a related ligand, which contains a substituent at one of the terminal amino groups. For CuL(OH)+, the calcns. suggest that an equil. exists between species with essentially the same energy but different geometries, each one of the species is closer to one of the ideal tbp and sp limits. For HCuL3+, the relevance of the sp geometry was confirmed by the calcns. [on SciFinder(R)]