Cyclen transition metal complexes as biomimetic catalysts, phosphate anion sensors and building-blocks in supramolecular assemblies

摘要

This PhD thesis is a study of cyclen transition metal complexes as biomimetic catalysts, phosphate anion sensors and building-blocks in supramolecular assemblies. We have synthesised new Cu(II) and Ni(II) complexes of macrocyclic ligands and compared their hydrolytic properties in aqueous solution towards 4-nitrophenyl acetate and bis(4-nitrophenyl)phosphate with that of identical Zn(II) complexes. Unlike the Zn(II) complexes, the Cu(II) and Ni(II) complexes do not promote the hydrolysis of the substrates, as they do not fulfil one of the two conditions needed for an artificial metallohydrolase: the Cu(II) complexes do not possess two cis-oriented coordination sites on the metal ion for binding the substrate and a water molecule, while Ni(II) is not a strong Lewis acid and does not facilitate deprotonation of the coordinated water to generate the hydroxide nucleophile. Two dinuclear Zn(II) complexes showed the formation of a very stable 1:1 complex with pyrophosphate, with a slow exchange taking place between free and bound anion. Indicator displacement measurements with pyrocatechol violet and coumarin methylsulphonate in buffered solutions (pH 7.4, 25 °C) indicated that the receptor can distinguish phosphate anion derivatives from other anionic species, with binding affinities proportional to the number of negative charges on the anion. The binding affinity can be detected with the naked eye. We report here the synthesis of a new derivative of Cyclen with a tetraazole moiety directly bound to the azamacrocycle and its Zn(II), Ni(II) and Cu(II) complexes. X-ray structure analysis of the Ni(II) compound shows the formation of a stable dimer by coordination of each of the two tetraazole substituents to the neighboring metal cation. Potentiometric titrations of the metal complexes indicate a possible conversion of the monomer to the dimeric structure in solution and show the pKa of the NH-atom on the tetraazole substituent to be between 4.03 and 5.3 depending on the metal ion coordinated by cyclen.