Scorpiand Pyrazole Azacyclophanes: Synthesis, Protonation Behavior, and Cu2+ Coordination Chemistry

We report the synthesis, protonation, and Cu2+ coordination properties of a family of scorpiand ligands based on a 3,6,9-triaza-1(3,5)-1H-pyrazolacyclodecaphane scaffold bearing aminoethyl, pyridyl, or quinolyl side arms. Potentiometric, UV-vis, and NMR studies reveal three protonation steps for L1 and four for the pyridyl- and quinolyl-substituted ligands, owing to the additional protonation of the heteroaromatic donor. Cu2+ coordination leads predominantly to mononuclear complexes, whereas L1 also forms binuclear and trinuclear species. The relatively low stability constants of the [CuL]2+ complexes, compared with related pyridinacyclophanes, indicate coordination by a limited number of nitrogen donors. Single-crystal X-ray diffraction of [Cu(HL1)Br2]Br·H2O confirms a tridentate coordination mode involving the side-chain amine, one secondary amine, and the tertiary amine of the macrocycle, with the pyrazole ring adopting a noncoordinating exo orientation. Spectroscopic studies show that protonation controls the side-chain dynamics of the free ligands, whereas Cu2+ binding locks the ligand into a more rigid conformation. The resulting coordination environment affords highly efficient superoxide dismutase mimics, with catalytic activities among the highest reported for related systems.