Electrochemical studies of the MI/II and MII/III (M = Ni, Cu) couples in mono-to tetranuclear complexes with oxamato/oxamidato ligands

Abstract The oligo-oxamates oxamide- N,N′- bis(o-phenylene oxamic acid ethyl ester (= L 1 H 4 Et 2 , 5 ), oxamide- N,N′- bis(4,5-dimethyl-o-phenylene oxamic acid ethyl ester (= L 2 H 4 Et 2 Me 4 , 6 ) and oxamide- N,N′ -bis(o-phenylene- N 1 -methyloxalamide (= L 3 H 6 Me 2 , 7 ) were used as precursor for the synthesis of the binuclear complexes [ n Bu 4 N] 2 [Cu 2 (L 1 )] ( 8 ), [ n Bu 4 N] 2 [Cu 2 (L 2 Me 4 )] ( 9 ), [ n Bu 4 N] 2 [Cu 2 (L 3 Me 2 )] ( 10 ), [ n Bu 4 N] 2 [Ni 2 (L 1 )] ( 11 ) and [ n Bu 4 N] 2 [Ni 2 (L 3 Me 2 )] ( 12 ), the trinuclear complexes [Cu 3 (L 1 )(pmdta)] ( 13 ) and [Cu 3 (L 2 Me 4 )(pmdta)] ( 14 ) as well as the tetranuclear complexes [Cu 4 (L 1 )(pmdta) 2 ](NO 3 ) 2 ] ( 15 ), [Cu 4 (L 2 Me 4 )(pmdta) 2 ](NO 3 ) 2 ] ( 16 ) and [Cu 4 (L 3 Me 2 )(pmdta) 2 ](NO 3 ) 2 ] ( 17 ), (pmdta =  N,N,N′,N´´,N´´- pentamethyldiethylenetriamine). The redox properties of the multinuclear complexes 8 ‒ 17 were studied by cyclic voltammetry and compared comprehensively to the ones of related mononuclear bis(oxamato), (oxamato)(oxamidato) and bis(oxamidato) complexes. The studies established further the crucial relationship between the molecular structure and the reversibility of individual redox processes and prove that unlike Ni II -containing multinuclear complexes Cu II ions can be reversibly oxidized when delivered in CuN 3 O and CuN 4 coordination units. On the other hand, all here reported binuclear Ni II -complexes can be reversibly reduced. Furthermore, reversible electrochemical reduction of the terminal {Cu(pmdta)} 2+ fragments within 13 ‒ 17 and of [Cu(pmdta)(NO 3 ) 2 ] is demonstrated, providing means as new reducing agents or electron storage material.