Pack more into the box: How to fit two metal ions in one porphyrin. Porphyrins’ default coordination mode allows the binding of a single metal cation in the central NNNN cavity. In order to allow the accommodation of two metal ions inside a standard porphyrin perimeter, they are introduced in place of two inner NH groups, yielding a 21,23-dimetallaporphyrin. The 18-π-electron aromatic circuit is maintained