Experimental and Computational Studies of the Macrocyclic Effect of an Auxiliary Ligand on Electron and Proton Transfers Within Ternary Copper(II)-Histidine Complexes

The dissociation of [CuII(L)His]-2+ complexes [L = diethylenetriamine (dien) or 1,4,7-triazacyclononane (9-aneN3)] bears a strong resemblance to the previously reported behavior of [CuII(L)GGH]-2+ complexes. We have used low energy collision-induced dissociation experiments and density functional theory (DFT) calculations at the B3LYP/6-31+G(d) level to study the macrocyclic effect of the auxiliary ligands on the formation of His-+ from prototypical [CuII(L)His]-2+ systems. DFT revealed that the relative energy barriers of the same electron transfer (ET) dissociation pathways of [CuII(9-aneN3)His]-2+ and [CuII(dien)His]-2+ are very similar, with the ET reactions of [CuII(9-aneN3)His]-2+ leading to the generation of two distinct His-+ species; in contrast, the proton transfer (PT) dissociation pathways of [CuII(9-aneN3)His]-2+ and [CuII(dien)His]-2+ differ considerably. The PT reactions of [CuII(9-aneN3)His]-2+ are associated with substantially higher barriers (>13 kcal/mol) than those of [CuII(dien)His]-2+. Thus, the sterically encumbered auxiliary 9-aneN3 ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto non-observable histidine radical cations.