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. [Cu4(L1)](PF6)4·2CH3CN·3H2O (1) has di(2-pyridylmethyl)aminecopper(II) as the monometallic auxiliary, and [Cu4(L2)](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 CuII center at −0.42 V: {Cu(II)−Cu2(II,II)−Cu(II)}/{Cu(I)−Cu2(II,II)−Cu(I)} → {Cu(I)−Cu2(I,II)−Cu(II)}/{Cu(I)−Cu2(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)−Cu2(II,II)−Cu(II)}/{Cu(II)−Cu2(I,II)−Cu(II)}/{Cu(I)−Cu2(I,II)−Cu(II)}. Assembly 1 is reduced with ascorbic acid to the {Cu(I)−Cu2(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.