Quinquedentate co-ordination of amino-substituted tetraazacycloalkanes to cobalt(III). Part 3. Synthesis of an unsymmetric ligand and crystal structure of its cis-chlorocobalt(III) complex

Abstract

The unsymmetric pendant-arm macrocycle 9-methyl-9-nitro-1,4,7,11-tetraazacyclotetradecane (L¹) was prepared by a copper(II)-directed condensation of 3,6-diazanonane-1,9-diamine with nitroethane and formaldehyde. Reduction of the copper(II) complex of L¹ with zinc in aqueous acid produced the new potentially quinquedentate polyamine 9-methyl-1,4,7,11-tetraazacyclotetradecan-9-amine (L²). Reaction of L² in water with cobalt(II) ion and air, followed by aqueous hydrochloric acid and equilibration with activated charcoal, yielded exclusively cis-[CoL²(Cl)][ClO₄]₂, which crystallized in the monoclinic space group P2₁/c, with a= 12.745(5), b= 11.438(5), c= 14.414(8) and β= 104.80(3)°. The pendant primary amine and two adjacent secondary amines necessarily occupy an octahedral face, with the chloro ligand cis to the primary amine, and secondary amines adopt RRSS stereochemistries. The average Co–N distance is 1.96₀Å, and the Co–Cl distance is 2.252(1)Å. Stereoretentive base hydrolysis (k_OH 9100 dm³ mol^–1 s^–1) and other structural and physical properties are compared with those of the symmetric analogue 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine.