Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex

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.