Unexpected Interactions between Alkyl Straps and Pyridine Ligands in Sulfur-Strapped Porphyrin Nanorings

Strapped or “basket-handle” porphyrins have been
investigated previously as hemoglobin mimics and catalysts. The facial
selectivity of their interactions with axial ligands is a sensitive
test for noncovalent bonding. Here the binding of pyridyl ligands
to zinc porphyrins with thioester-linked alkyl straps is investigated
in solution by NMR spectroscopy and UV–vis titration, and in
the solid state by X-ray crystallography. We expected that coordination
of the axial ligand would occur on the less hindered face of the porphyrin,
away from the strap. Surprisingly, attractive interactions between
the strap and the ligand direct axial coordination to the strapped
face of the porphyrin, except when the strap is short and tight. The
strapped porphyrins were incorporated into π-conjugated cyclic
porphyrin hexamers using template-directed synthesis. The strap and
the sulfur substituents are located either inside or outside the porphyrin
nanoring, depending on the length of the strap. Six-porphyrin nanorings
with outwardly pointing sulfur anchors were prepared for exploring
quantum interference effects in single-molecule charge transport.