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

The first boron complexes of porphycenes, structural isomers of porphyrin, are reported. They are synthesized in good yields by reacting the free-base porphycene ligands with BF₃·Et₂O through a microwave-assisted method. Depending on the substituent group of porphycenes, two different coordination structures, mono- and diboron porphycenes, are obtained simultaneously. The single crystal structures and DFT calculations suggest that the boron atom of the monoboron porphycene is favorably coordinated on the dipyrroethene site, and the regioisomer of diboron porphycene is of cisoid stereochemistry, which is more stable than transoid. We also investigate the protonation behavior of boron porphycene complexes. Diboron porphycene does not undergo protonation, whereas monoboron porphycene undergoes protonation at the nonboron coordinating pyrroline site, resulting in a red shift in both absorption and emission spectra. Protonation and deprotonation of monoboron porphycene can be reversibly triggered using acids and bases.