Synthesis and complexes of the sexidentate macrocycle 15-methyl-1,4,7,10,13-pentaazacyclohexadecan-15-amine

The potentially sexidentate polyamine macrocycle 15-methyl-1,4,7,10,13-pentaazacyclohexadecan-15-amine (1) was prepared via a copper(II)-templated route from 3,6,9-triazaundecan-1,11-diamine, formaldehyde and nitroethane which first formed the copper(II) complex of the macrocycle 15-methyl-15-nitro-1,4,7,10,13-pentaazacyclohexadecane (2), reduced subsequently with zinc and aqueous acid to yield 1. The hexaamine 1, with five secondary amine groups in the macrocyclic ring and one pendant primary amine group, forms inert sexidentate octahedral complexes with cobalt(III), chromium(III) and iron(III). An X-ray structure of [Co(1)](ClO4)3 defines the distorted octahedron of the complex cation and shows it is a symmetrical isomer with all nitrogens bound and the central aza group trans to the pendant primary amine group. The [M(1)]3+ ions are all stable indefinitely in aqueous solution and exhibit spectra consistent with MN6 d3 (Cr), low-spin d5 (Fe) and low-spin d6 (Co) electronic ground states. For each complex, a reversible M(III/II) redox couple is observed.