Chemically Induced Breathing of Flexible Porphyrinic Covalent Cages

The
synthesis of two flexible bis-porphyrin cages 3 and 4, incorporating respectively Zn­(II) porphyrins
and free-base porphyrins, is reported. In both cages, the four covalent
linkers that bind the two porphyrins are functionalized with two 1,2,3-triazolyl
ligands. These cages were characterized by NMR and HRMS, and for cage 3 incorporating 1,4-diazabicyclo[2.2.2]­octane (DABCO), an
X-ray crystallographic structure was obtained. Chemically induced
conformational changes are studied and compared to those of two related
cages with longer flexible linkers. Binding of four silver­(I) ions
to the peripheral ligands opens the flattened structures in solution
and locks the two porphyrins in a face-to-face disposition. Addition
of an excess of acid fully expands the cages due to electrostatic
repulsion between the positively charged sites. These two reversible
processes allow for a chemically induced breathing of the flexible
structures.