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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 sp-hybridized meso positions and the interruption of the aromatic system gives rise to specific cavities and often roofor 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 in the core. Similarly, specific conformations and binding motifs can be accessed in porphyrin dications depending on the type, number, regiochemistry and steric bulk of substituents in the respective free base porphyrin. Here we give a brief overview of the different approaches and illustrate their use with regard to the design of ionpair receptors for small molecules and anions and the formation of potentially functional supramolecular complexes.

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