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

Abstract

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.