Building-up Remarkably Stable Magnesium Porphyrin Polymers Self-Assembled via Bidentate Axial Ligands: Synthesis, Structure, Surface Morphology, and Effect of Bridging Ligands

A series of supramolecular architectures of magnesium
tetranitrooctaethylporphyrins
mediated by several bidentate axial ligands have been synthesized
in excellent yields and structurally characterized. Six conjugated
axial ligand with increasing chain lengths have been utilized in the
present investigations in which the Mg···Mg nonbonding
distance between successive ions also increases from 0.73 to 2.70
nm in the series. To the best of our knowledge, this is the first
report where stable metallo-porphyrin polymers with such long spacers
have been synthesized in one pot so easily. Linear one-dimensional
(1D) polymeric chains were observed in the X-ray structure of the
six-coordinated complexes in which porphyrin units are aligned parallel
to each other to have so-called “shish kebab” like architectures
to maintain offset-stacked overlap. However, after an optimum Mg···Mg
nonbonding distance, these 1D chain do not continue, rather they form
five-coordinated porphyrin dimers with “wheel-and-axle”
like architectures which are then self-aggregated by π–π
interactions in a perpendicular manner to fill space created by large
bridging ligands more effectively which consequently results in spherical
structures. The structures of the molecules in solution and their
surface patterns on highly ordered pyrolytic graphite (HOPG) have
also been investigated.