Electron transfer and supramolecular studies of amphiphilic nickel(II) pendant-arm polyazamacrocycles

Three nickel(II) pentaazamacrocycles bearing pendant alkyl tails have been synthesized, and the crystal structure of one (bearing an octyl tail) is reported. The redox potentials of the complexes, for oxidation of the nickel(II) centre, is 0.72 V in all cases, indicating that the pendant alkyl tails have no effect on the redox site. The three compounds have been characterized by IR, CHN and ESMS. The kinetics of oxidation of the complexes by aqua(5,5,7, 12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-l-acetato)nickel(III), [Ni(hmca)(OH2)]2+ have been studied. Only minimal differences in the rates for the three complexes are observed indicating that the tail length has only minimal effect.

The complexes have been studied with regard to their ability to form supramolecular complexes with α- and β-cyclodextrin. Results from ESMS and NMR show evidence of the formation of supramolecular complexes but cannot accurately quantify an association constant. The kinetics of oxidation of the complexes by the above oxidant in the presence of α- and β-cyclodextrin indicate that the rate is slowed by the presence of the cyclodextrins. A derived rate expression allows for association constants to be estimated from the oxidation kinetics.

The complexes are found to have unusual solvation properties in THF - water mixtures. The solubility of the complexes in various THF - water solution compositions have been studied.

The synthesis of a number of larger macrocycles (fifteen- and sixteen-membered) were attempted with little success. However, an attempt to synthesise a 16-membered macrocycle by a template method resulted in the formation of large crystals of trans-(bis-acetonitrilo)bis-(1,3-diaminopropane)nickel(II) perchlorate, [Ni(dap)2(MeCN)2](ClO4) 2,which has been characterized cystallographically. The complex cation has two diaminopropane chelating ligands in a square planar arrangement about the nickel(II) ion, with two axially-coordinated acetonitrile ligands.