The
Siamese-twin porphyrin (2H4) is a unique
pyrazole-expanded
porphyrin providing two adjacent cavities each offering an {N4} binding motif. It was previously found to form stable dicopper(II) or dinickel(II) complexes where both
metal ions are nested in a
porphyrin-like environment. This work addresses the rich redox chemistry of the dicopper complex 2Cu2 that originates from the redox synergy of two proximate
metal ions in combination with the potentially non-innocent expanded
porphyrin ligand. Complementing previous X-ray crystallographic and SQUID data for solid material, the electronic structure of parent 2Cu2 in
solution was now investigated by MCD and EPR spectroscopy. This allowed the assignment of UV-vis absorptions and confirmed the drastic twist of the molecule with ferromagnetically coupled
copper(II)
ions. 2Cu2 was found to exhibit multiple redox events in the potential range from -2.4 to +1.7 V versus Fc/Fc(+), and singly oxidized [2Cu2](+) as well as doubly oxidized [2Cu2](2+) were characterized in detail by various analytical and spectroscopic methods. [2Cu2](+) was found by EPR spectroscopy and DFT calculations to have an S = 1/2 ground state, while [2Cu2](2+) is diamagnetic. Single crystal X-ray crystallography of [2Cu2(
acetone)2](BF4)2 revealed that the 2Cu2 core is structurally invariant upon two-fold oxidation, while XAS measurements at the Cu K-edge for 2Cu2 and [2Cu2(
acetone)2](BF4)2 showed that the
copper ions remain in the +2 oxidation state throughout. The combined experimental and computational evidence identified the
Siamese-twin porphyrin as a multi-electron redox-active
ligand with hidden non-innocence. Each
ligand subunit upon oxidation forms a
ligand-centered radical, though the spin vanishes because of covalency and strong antiferromagnetic coupling between the
ligand radical and the proximate
metal ion. Complexes of the
Siamese-twin porphyrin may thus serve as a valuable bioinspired platform that combines both
metal-
ligand and two-
metal-ion cooperativities for use in multi-electron processes.