Chemistry of structurally developed macrocycles. Part 2. Synthesis and complexing properties of N,N′,N″,N‴-tetra(3-aminopropyl)-1,4,8,11-tetra-azacyclotetradecane with nickel(II)

Abstract

Reduction of N,N′,N″,N‴-tetra(2-cyanoethyl)-1,4,8,11-tetra-azacyclotetradecane with sodium and ethanol in toluene generates the octa-amine N,N′,N″,N‴-tetra(3-aminopropyl)-1,4,8,11-tetraazacyclotetradecane in high yield. Complexation of this ligand with nickel(II) under neutral conditions leads to high-spin five-co-ordinate species in which the co-ordination occurs via the four tertiary amine nitrogen atoms and one of the pendant primary amine nitrogen atoms, irrespective of the nature of the anion. Further nickel may also be co-ordinated by the other three primary amine nitrogen atoms, leading to polymeric products. Selective protonation of the four pendant amine groups may be accomplished reversibly in ethanol without destroying the complex. The protonated primary amine groups serve to stabilise the co-ordination of weak donor anions such as ClO₄^–, BF₄^–, and HSO₄^– at the axial co-ordination site, leading to a variety of five-co-ordinate complexes. The spin state of the nickel is dependent on the ligand field strength of the axial group, with the crossover from low spin to high spin observed on changing from bromide to chloride. The direction of the crossover, in terms of axial ligand field strength, indicates a square-pyramidal, rather than a trigonal-bipyramidal structure for these compounds.