Tetracopper assembly complexes comprised of one dimetallic core and two monometallic auxiliaries: Intramolecular electron-transfer relevant to multicopper oxidases

Two tetracopper assembly complexes, comprised of one dimetallic di(3-iminomethylsalicylato)dicopper(II) core and two monometallic copper(II) auxiliaries attached to the imino nitrogens of the dinuclear core through an alkane chain, have been prepared. [Cu-4(L-1)](PF6)(4). 2CH(3)CN . 3H(2)O (1) has di(2-pyridylmethyl)aminecopper(LI) as the monometallic auxiliary, and [Cu-4(L-2)] (ClO4)(4). CH3OH (2) has 1,4,8,11-tetraazacyclotetradecanecopper(II) as the auxiliary. Assembly 1 in acetonitrile shows a two-electron reduction at -0.08 V (vs SCE) followed by a one-electron reduction at -0.42 V. Together with EPR studies for electrolyzed solutions, it is shown that the two monometallic auxiliaries are reduced at -0.08 V, followed by an intramolecular electron transfer from one of the reduced auxiliaries to the dimetallic core and by the second reduction at the resulting monometallic Cu-II center at - 0.42 V: {Cu(II) -Cu-2(I,II) -Cu(II)}/{Cu(I) -Cu-2(II,II)-Cu(I)} --> (Cu(I) -Cu-2(I,II)-Cu(II)}/{Cu(I)-Cu-2-(I,II)-Cu(I)}. The CV of 2 in DMSO shows two couples at -0.68 and -0.99 V attributable to the stepwise reductions: {Cu(II)-Cu-2(II,II)-Cu(II)}/{Cu(II)-Cu-2(I,II)-Cu(II)}/(Cu(I)-Cu-2(I,II)-Cu(II)}. Assembly 1 is reduced with ascorbic acid to the (Cu(I)-Cu-2(I,II)-Cu(I)} species, whereas 2 is not reduced with ascorbic acid. The relevance of the intramolecular electron transfer observed for 1 to multicopper oxidases is discussed.