N-H Hydrogen Bonding in Porphyrins - from Conformational Design to Supramolecular Chemistry

Typically porphyrin N-H units are not involved in intermolecular hydrogen bonds as they are ‘shielded’ by the macrocycle system. However, two possibilities exist to utilize the pyrrole nitrogen atoms in such bonds. A) Porpho(di)methenes and phlorins (calixphyrin-type systems) contain sp³-hybridized meso positions and the interruption of the aromatic system gives rise to specific cavities and often roof- or bowl-shaped conformations making the core nitrogen and N–H units more accessible for coordination. B) Likewise, saddle-distorted nonplanar porphyrins are very basic and can bind anions and neutral molecules via N-H hydrogen bonds. Similarly, specific conformations and binding motifs can be accessed in porphyrin dications (N22,N24-dihydroporphyrins) depending on the type, number, regiochemistry and steric bulk of substituents in the respective free base porphyrin. The different approaches will be compared and analyzed with regard to the design of ion-pair receptors for small molecules and anions and the formation of functional supramolecular complexes.