Juan C. Hernández

Stability and kinetics of the acid-promoted decomposition of Cu(II) complexes with hexaazacyclophanes: kinetic studies as a probe to detect changes in the coordination mode of the macrocycles

The synthesis, protonation and Cu(II) coordination features of the novel azacyclophane type receptors 2,6,10,13,17,21-hexaza[22]-(2,6)-pyridinophane (L2), 2,6,9,12,15,19-hexaza[20]-(2,6)-pyridinophane (L5) and 2,6,9,12,15,19-hexaza[20]metacyclophane (L6) are presented. The protonation and Cu(II) constants are analysed and compared with the previously reported open-chain polyamines 4,8,11,15-tetrazaoctadecane-1,18-diamine (L1) and 4,7,10,13-tetraazahexadecane-1,16-diamine (L4) and of the cyclophane 2,6,10,13,17,21-hexaaza[22]paracyclophane (L3). All the systems form mono- and dinuclear complexes whose stability and pH range of existence depend on the type of hydrocarbon chains and molecular topology. The effects of the cyclic or open-chain nature and of the presence of the pyridine rings on the protonation and formation of mono- and dinuclear complexes are discussed. Stopped-flow kinetic measurements on the acid-promoted decomposition of the Cu(II) complexes have been carried out for the different systems. With respect to the decomposition of the dinuclear complexes, because the size of the macrocycles forces both metal ions to be close to each other, the release of the first ion occurs within the mixing time of the stopped-flow except for the dinuclear complexes of L2. However, the most interesting kinetic result is the observation of different kinetics of decomposition for the different mononuclear complexes formed by a given ligand. This effect is especially evident for L3 and L6 and indicates a change in the coordination mode of the ligand for the different mononuclear species. Therefore the Cu(II) ion performs a slippage motion through the macrocyclic cavity driven by pH changes. The stopped-flow experiments are an excellent tool to detect these slippage processes that may be present for the complexes with other macrocycles.

Hydrogen-ion driven molecular motions in Cu2+-complexes of a ditopic phenanthrolinophane ligand.

One of the first kinetic evaluations of a metal ion interchange between the two coordination sites of a ditopic macrocycle is presented.

TEM Investigation of the Synthesis of Rh/CePrO x Catalysts

It is well established that pure praseodymium oxide is sensitive to leaching of Pr species during treatments with slightly acidic aqueous solutions [3]. The usual routines to prepare supported noble metal phases include, as a first step, the impregnation of the supports with aqueous solutions of some precursor of the corresponding metal. These solutions are usually highly acidic. In the case of Ce-Pr mixed oxides impregnation with such solutions could induce severe modifications of the oxide nanostructure and, at the same time, modify the final properties of the supported metal phase.

Surface and interface structure of ceria supported ruthenium

Noble metals highly dispersed on cerium mixed oxide supports Ce(M)O2−x (M=Zr,Tb) are currently being investigated, not only due to their relationship with the so called Three-Ways Catalysts (TWC) but also because of their potential applications in other catalytic processes related to the hydrogen production or the elimination of organic pollutants in residual waters from different industrial processes [1]. Among these supported noble metals catalysts, Ru containing materials seems to be more stable while at the same time very active and selective in comparison with other supported noble metals [2].