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

Abstract

A di- N-functionalized 14-membered tetraaza macrocycle, [H 4L 3](ClO 4) 2 (L 3 = 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 (L 2). The copper(II) complexes [CuL 2](ClO 4) 2 and [Cu(H 2L 3)](ClO 4) 2 were prepared and characterized. The complex [Cu(H 2L 3)] 2+ readily reacts with methanol to yield [CuL 4] 2+ (L 4 = 1,8-bis(2-carbomethoxyethyl)-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane). The N-CH 2CH 2COOH groups of [Cu(H 2L 3)](ClO 4) 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, [CuL 4](ClO 4) 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 [CuL 2](ClO 4) 2 is rapidly decomposed in 0.1 M NaOH solution even at room temperature. On the other hand, [Cu(H 2L 3)](ClO 4) 2 and [CuL 4](ClO 4) 2 are quite inert against decomposition under similar basic conditions. In acidic or basic aqueous solutions, [CuL 4] 2+ is hydrolyzed to [Cu(H 2L 3)] 2+ or [CuL 3].