Synthesis of a new potentially sexidentate pendant-arm macrocyclic polyamino acid and co-ordination to cobalt(III)

Abstract

From reaction of bis(ethane-1,2-diamine)copper(II) with formaldehyde and diethyl malonate then formaldehyde and nitroethane in turn the pendant-arm macrocyclic complex ion (hydrogen 13-methyl-13-nitro-1,4,8,11-tetraazacyclotetradecane-6-carboxylate)copper(II) was prepared. Reduction with zinc in aqueous acid yields the free pendant-arm β-polyamino acid macrocycle hydrogen 13-amino-13-methyl-1,4,8,11-tetraazacyclotetradecane-6-carboxylate (L3) as the hydrochloride salt. Co-ordination to cobalt(III) was achieved following reaction with cobalt(II) ion and air, yielding the chloro(hydrogen 13-amino-13-methyl-1,4,8,11-tetraazacyclotetradecane-6-carboxylate)cobalt(III) ion, as well as its aqua analogue. The chlorocobalt(III) complex crystallized as the perchlorate salt in the space group Pbca, a= 10.256(4), b= 12.689(5), c= 33.08(2)Å, Z= 8, with the co-ordinated chloride in a cis disposition relative to the pendant amine, the four secondary amines co-ordinated in a folded geometry with uncommon RRRR stereochemistry, and the pendant carboxylic acid group not co-ordinated. Equilibration of the reaction mixture from this synthesis at pH 8 over activated carbon yielded almost exclusively the complex with the ligand bound as a sexidentate ligand where the pendant amine and carboxylate groups occupy trans sites. This complex crystallized as the perchlorate salt in the space group P21/n with a= 10.939(2), b= 13.355(2), c= 14.596(2)Å, β= 102.17(1)° and Z= 4. Metal–donor distances [Co-O 1.899(2), Co–N(pendant) 1.956(2), average Co–N(secondary) 1.951Å] are at the short end of the range for saturated polydentate amino acid ligands, presumably influenced by the sterically efficient ligand.