Hidden Non-Innocence in an Expanded Porphyrin: Electronic Structure of the Siamese-Twin Porphyrin’s Dicopper Complex in Different Oxidation States

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.