text: One of the major differences between p-block-element-centered porphyrins and transition-metal-centered porphyrins is the far smaller size of the interstitial atom, especially in the case of the first-row p-block. Other than protons, the next smallest atom known to bind to the central N pocket is lithium. The first two isolated lithium porphyrin complexes each reported a 2:1 metal to base ratio, and XRD suggested both lithium atoms reside out of the porphyrin plane. 
  Boron has a covalent radius of 85 pm, significantly smaller than lithium's 133 pm. This suggests the porphyrin pocket is more likely to accommodate two boron atoms rather than one. Indeed, each boron porphyrin synthesized thus far has adopted a ratio of 2:1, with a range of orientations relative to the N plane. The boron atoms can exist in the same plane as the porphyrin (both with and without additional out-of-plane B-X bonds), or out of N plane in either a cisoid or transoid geometry.
  This coordination motif is interesting because it introduces both boron and porphyrin to geometries they do not regularly adopt. Porphyrin readily binds to transition metals, which are capable of octahedral or square planar geometries. Boron, without available d-orbitals, typically adopts a trigonal planar or tetrahedral local bonding environment. Diboryl porphyrins,  on the other hand, find boron in a pseudo-tetrahedral local environment and introduce a tetragonal distortion to the porphyrin, as can be seen in the DFT image above.
  Corroles are distinct from porphyrins in that they contain one less methine to bridge between pyrrole units, creating a lower-symmetry compound and a smaller N pocket. For boron chemistry, this slightly smaller core allows for the possibility of binding to a single boron, whereas the porphyrin pocket has thus far always bound two. For such monoboryl corroles, DFT studies have suggested the boron preferentially binds to the dipyrromethene (A) site shown here, in which stability is attained by maximizing both BX—HN hydrogen bonding and BH—HN dihydrogen bonding, in addition to minimizing steric crowding.
  The Brothers group has shown the stereochemical implications of comparing diboryl porphyrin with diboryl corrole: porphyrin prefers transoid orientation of the diboryl unit, whereas corrole prefers the cisoid orientation.<nowiki/>