Synthesis and hydrolysis of cis-octahedral nickel(II) complexes with C-racemic tetraaza macrocycles bearing two imidate ester pendant arms

Abstract

Two di-N-functionalized macrocyclic complexes, [Ni(C-racemic-L3)]2+ bearing two NCH2CH2C(NH)OCH3 pendant arms and [Ni(C-racemic-L8)]2+ bearing two NCH2C(NH)OCH3 pendant arms, have been prepared by the reaction of methanol with [Ni(C-racemic-L2)(OAc)]+ (L2=1,8-bis(N-cyanoethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) or [Ni(C-racemic-L7)(OAc)]+ (L7=1,8-bis(N-cyanomethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Both complexes have somewhat distorted cis-octahedral coordination geometry. Their hydrolysis has been investigated under various experimental conditions. The complex [Ni(C-racemic-L3)]2+ is hydrolyzed to [Ni(C-racemic-L4)]2+ bearing two NCH2CH2COOCH3 groups in acidic aqueous solutions (pH ⩽6), but to [Ni(C-racemic-L5)]2+ bearing two N(CH2)2CONH2 pendant arms in basic aqueous solutions (pH ⩾9). Very interestingly, two N(CH2)2C(NH)OMe groups in [Ni(C-racemic-L3)]2+ undergo different reactions at pH 7 to yield [Ni(C-racemic-L6)]2+ bearing one N(CH2)2CONH2 and one N(CH2)2CN pendant arms. Although the NCH2C(NH)OCH3 groups in [Ni(C-racemic-L8)]2+ undergo hydrolysis to form [Ni(C-racemic-L9)]2+ bearing two NCH2CONH2 pendant arms in basic aqueous solutions (pH ⩾9), they are quite inert against hydrolysis in neutral and acidic aqueous solutions (pH ⩽7).