Positional Isomerism Governs Coordination, Structure, and Function in Mn(II) and Cu(II) Pyclen–Oxinate Complexes

Abstract

Positional isomerism represents a promising yet underutilized approach for tuning the structure and properties of metal complexes. Herein, two pyclen-based hexadentate ligands incorporating an 8-hydroxyquinolinate (8-HQ) moiety, 3-PCOX and 6-PCOX, were synthesized and their coordination chemistry with essential metal ions was investigated, focusing on Mn(II) and Cu(II). The position of the 8-HQ arm strongly influences metal ion affinity and coordination geometry. The 3-PCOX ligand forms highly stable Mn(II) complexes (log KMnL = 18.09(6)), whereas 6-PCOX exhibits enhanced Cu(II) selectivity (log KCuL = 28.87(11)). Structural studies combining DFT calculations, EPR spectroscopy, and 17O NMR revealed that 3-PCOX stabilizes a pentagonal bipyramidal Mn(II) complex containing one coordinated water molecule, while 6-PCOX forms a distorted octahedral complex lacking inner-sphere hydration. Relaxometric and 17O NMR studies confirmed the presence of a coordinated water molecule in [Mn(3-PCOX)(H2O)]+ and a moderately fast water exchange rate (k298ex = 1.22 × 108 s–1). Both Mn(II) complexes displayed remarkable superoxide dismutase-like activity, whereas the Cu(II) analogues were largely inactive. Kinetic studies showed predominantly acid-assisted dissociation, with [Mn(3-PCOX)(H2O)]+ exhibiting superior kinetic inertness. These results establish positional isomerism as an effective strategy for controlling hydration, reactivity, and function in Mn(II) and Cu(II) complexes.