Siamese-twin porphyrin is a
pyrazole-containing expanded
porphyrin incorporating two
porphyrin-like binding pockets. The macrocycle, however, does not possess an aromatic π system but rather two separated conjugation pathways that are isolated by the
pyrazole junctions. Mono- and bimetallic complexes of the
Siamese-twin porphyrin are known. This work addresses in detail the electronic consequences that monometalation (with PdII) has on the electronic properties of the nonmetalated binding pocket by studying the solid-state structure,
acid/base, and electrochemical properties of the monopalladium twin-
porphyrin complex. Specifically, metalation leads to a switch of the protonation sites of the free-base pocket. The unusual location of the
protons at adjacent pyrrolic
nitrogen atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy. The one-electron oxidation and reduction events are both
ligand-centered, as derived by spectroelectrochemical and electron paramagnetic resonance measurements, but are located on different halves of the molecule. Single-electron oxidation (-0.32 V vs Fc/Fc+) generated an organic radical centered on the
metal-coordinating side of the
ligand, while single-electron reduction (-1.59 V vs Fc/Fc+) led to the formation of an organic radical on the free-base side of the macrocycle. Density functional theory calculations corroborated the redox chemistry observed. The possibility of selectively preparing the monometallic complexes carrying two distinct redox sites-a
metal-containing oxidation site and a
metal-free reduction site-further expands the potential of
Siamese-twin porphyrins to serve as an adjustable platform for multielectron redox processes in chemical catalysis or molecular electronics applications.