The dissociation of [Cu
II(L)His]
•2+ complexes [L=diethylenetriamine (dien) or 1,4,7-triazacyclononane (9-aneN
3)] bears a strong resemblance to the previously reported behavior of [Cu
II(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 [Cu
II(L)His]
•2+ systems. DFT revealed that the relative energy barriers of the same electron-transfer (ET) dissociation pathways of [Cu
II(9-aneN
3)His]
•2+ and [Cu
II(dien)His]
•2+ are very similar, with the ET reactions of [Cu
II(9-aneN
3)His]
•2+ leading to the generation of two distinct His
•+ species; in contrast, the proton transfer (PT) dissociation pathways of [Cu
II(9-aneN
3)His]
•2+ and [Cu
II(dien)His]
•2+ differ considerably. The PT reactions of [Cu
II(9-aneN
3)His]
•2+ are associated with substantially higher barriers (>13 kcal/mol) than those of [Cu
II(dien)His]
•2+. Thus, the sterically encumbered auxiliary 9-aneN
3 ligand facilitates ET reactions while moderating PT reactions, allowing the formation of hitherto nonobservable histidine
radical cations.
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