Tetrahedral Pegs in Square Holes: Stereochemistry of Diboron Porphyrazines and Phthalocyanines

Abstract

The first examples of diboron complexes of the tetrapyrroles octaethylporphyrazine (OEPz) and 2,9,16,23‐tetra‐t‐butyl‐phthalocyanine (Pc) are reported, counterpoints to the better known monoboron tripyrroles, subporphyrazine and subphthalocyanine. Two stereochemical possibilities are observed, with cisoid‐B2OF2(OEPz), both cisoid‐B2OPh2(OEPz) and transoid‐B2OPh2(OEPz), transoid‐B2OF2(Pc) and cisoid‐B2OPh2(Pc) having been isolated and characterised, including structure determinations for the OEPz complexes. This variation in stereochemistry, which can be extended to include the previously reported transoid‐B2OF2(porphyrin), cisoid‐[B2OF2(corrole)]−, and both transoid‐ and cisoid‐B2OF2(calixphyrin), prompted a wider DFT study to elucidate the factors influencing the stereochemical preferences. This shows that the cisoid/transoid preference is correlated to the ease with which the macrocycle accommodates a rectangularly distorted N4 cavity.

Rectangular Holes in Porphyrin Isomers Act As Mono- and Binucleating Ligands: Stereochemistry of Mono- and Diboron Porphycenes and Their Protonation Behaviors

“…This value is very close to the previously calculated result (24.0 kcal mol −1 ) by Brothers and co-workers. 10 These results are highly consistent with the experimentally obtained crystal structures of boron porphycenes (Figure 2).…”

“…Both cisoid and transoid examples of B 2 OF 2 complexes of calixphyrins have been reported . Recently, boron complexes of phthalocyanine ( transoid -B 2 OF 2 ) and porphyrazine ( cisoid -B 2 OF 2 ) were reported by Brothers and co-workers . On the basis of the experiment data and a DFT study, they further elucidated the factor influencing the cisoid / transoid preference of diboron tetrapyrrole macrocycle complexes, which should be correlated to the ease with which the macrocycles accommodated a rectangularly distorted N 4 cavity.…”

“…9 Recently, boron complexes of phthalocyanine (transoid-B 2 OF 2 ) and porphyrazine (cisoid-B 2 OF 2 ) were reported by Brothers and co-workers. 10 On the basis of the experiment data and a DFT study, they further elucidated the factor influencing the cisoid/ transoid preference of diboron tetrapyrrole macrocycle complexes, which should be correlated to the ease with which the macrocycles accommodated a rectangularly distorted N 4 cavity. Besides the diboron porphyrinoids, a monoboron complex can be obtained in the case of a special corrole that exhibited a smaller N 4 cavity (Figure 1a).…”

“…This result further proved the conclusion that the ability of the macrocycle to accommodate a rectangularly distorted N 4 cavity correlates with stereochemical preference. 10 In addition, the protonation for monoboron porphycenes that have one pyrroline site by the addition of trifluoroacetic acid (TFA) was performed. These complexes are without precedence in the chemistry of porphycene, although the fact that protonation behaviors was observed for monoboron calixphyrins.…”

“…Boron, the first element of group 13, has attracted great interest in the field of coordination chemistry of porphyrinoids, owing to its unique properties and a smaller covalent radius compared with the coordination space of porphyrinoids . The boron porphyrinoids, to date, were successfully prepared using porphyrins, corroles, , calixphyrins, phthalocyanines, or porphyrazines as ligands when reacting with a boron source in the presence of bases. These reported boron porphyrinoids have been verified to showcase the unexpected macrocycles as binucleating ligands in which two boron atoms are coordinated by a single porphyrinoid ligand, with each boron bonded to two adjacent nitrogen atoms.…”