Stepwise selective reaction of two N-cyanoethyl groups attached to C-racemic tetraaza macrocyclic nickel(II) and copper(II) complexes in aqueous solutions

Stepwise hydrolysis of two N-(CH2)2CN groups attached to [Ni(C-racemic-L2)(OAc)]+ and [Cu(C-racemic-L2)]2+ (L2 = 1,8-bis(N-cyanoethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) has been investigated. The reaction of [Ni(C-meso-L2)]2+ has also been examined. Interestingly, [Ni(C-racemic-L2)(OAc)]+ is readily hydrolyzed to [Ni(C-racemic-L3)]2+ bearing one N-(CH2)2CONH2 and one N-(CH2)2CN pendant arms at pH ≤6, whereas [Cu(C-racemic-L2)]2+ and [Ni(C-meso-L2)]2+ are quite inert against hydrolysis under similar acidic conditions. Although [Cu(C-racemic-L2)]2+ is hydrolyzed to [Cu(C-racemic-L3)]2+ at pH 9, [Ni(C-meso-L2)]2+ readily undergoes C–N bond cleavage to yield [Ni(C-meso-L1)]2+ (L1 = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) in basic aqueous solutions. The hetero-functionalized complex [Ni(C-racemic-L3)]2+ undergoes hydrolysis and C–N bond cleavage at pH 9 and 13, respectively; both [Ni(C-racemic-L4)]2+ bearing two N-(CH2)2CONH2 pendant arms and [Ni(C-racemic-L5)]2+ bearing one N-(CH2)2CONH2 group can be prepared selectively by controlling pH of the solution. However, [Cu(C-racemic-L3)]2+ readily undergoes C–N bond cleavage to produce [Cu(C-racemic-L5)]2+ even at pH 9. Crystal structure of [Ni(C-racemic-L3)]2+ shows that the complex has severely distorted trigonal bipyramidal coordination geometry. Electronic absorption spectra of [Cu(C-racemic-L3)]2+, [Ni(C-racemic-L5)]2+, and [Cu(C-racemic-L5)]2+ indicate that they also have trigonal bipyramidal coordination geometry.