page_title: Boron porphyrins
section_title: Geometry
hierarchical_section_title: Boron porphyrins / Geometry
caption_attribution_description: English: Chemical structure of Boron porphyrin:johbrbr
context_page_description: Boron porphyrins are a variety of porphyrin, a common macrocycle used for photosensitization and metal trapping applications, that incorporate boron. The central four nitrogen atoms in a porphyrin macrocycle form a unique molecular pocket which is known to accommodate transition metals of various sizes and oxidation states. Due to the diversity of binding modes available to porphyrin, there is a growing interest in introducing other elements into this pocket.
  Boron in particular has been shown to prefer binding to porphyrin in a 2:1 stoichiometry, primarily due to its small atomic radius, but the Group XIII element will bind in a 1:1 ratio with corrole, a macromolecule with a structure similar to porphyrin but with a smaller N₄ pocket.
  Boron porphyrins are of interest because of the unique geometric environment to which both boron and porphyrin are subjected upon B-N₍₎ bond formation. These new geometric motifs lead to novel reactivity, one of the most surprising examples being sterically-induced reductive coupling. Possible applications for boron porphyrins include BNCT delivery agents and OLED devices.
context_section_description: 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 N₄ 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 N₄ 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 N₄ 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 N₄ 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.
caption: Corrole binding modes. Left to right: dipyrromethene (A), dipyrromethene (B), bipyrrole (C)[9]
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