Synthesis and coordination behaviors of 14-membered tetraaza macrocyclic copper(II) complexes bearing two N-propionic acid or N-propionic methyl ester groups

A di-N-functionalized 14-membered tetraaza macrocycle, [H4L3](ClO4)2 (L3 = 1,8-bis(2-carboxyethyl)-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane), has been synthesized by acid hydrolysis of 1,8-bis(2-cyanoethyl)-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L2). The copper(II) complexes [CuL2](ClO4)2 and [Cu(H2L3)](ClO4)2 were prepared and characterized. The complex [Cu(H2L3)]2+ readily reacts with methanol to yield [CuL4]2+ (L4 = 1,8-bis(2-carbomethoxyethyl)-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane). The N-CH2CH2COOH groups of [Cu(H2L3)](ClO4)2 are not coordinated to the metal ion in the solid state but are involved in coordination in various non-aqueous solvents or in aqueous solutions of pH ⩽ 1.0. Interestingly, [CuL4](ClO4)2 exists as two stable structural isomers, 1 (the pendant ester groups are not involved in coordination) and 2 (one of the two ester groups is coordinated to the metal ion), in the solid state; the two isomers can be prepared selectively by controlling ionic strength of a methanol solution of the complex. Crystal structures and coordination behaviors of the two isomers are described. The di-N-cyanoethylated macrocyclic complex [CuL2](ClO4)2 is rapidly decomposed in 0.1 M NaOH solution even at room temperature. On the other hand, [Cu(H2L3)](ClO4)2 and [CuL4](ClO4)2 are quite inert against decomposition under similar basic conditions. In acidic or basic aqueous solutions, [CuL4]2+ is hydrolyzed to [Cu(H2L3)]2+ or [CuL3].