J. Am. Chem. Soc. 137, 1056–1059 (2015)

Subporphyrins are the smaller cousins of porphyrin macrocycles. They consist of three pyrrole moieties — rather than four as in their better-known counterparts — linked by methine carbons and adopt a bowl-shaped structure that typically features a boron atom, with an axial ligand, in its central cavity. The axial ligand can be exchanged, and although methoxide, hydroxide, fluoride and phenyl-substituted boron-based subporphyrins are known, preparing hydrides has proven difficult. Now, in a collaboration between Yonsei University and Kyoto University, Dongho Kim, Atsuhiro Osuka and co-workers have synthesized subporphyrinato boron(III) hydrides by reduction of their methoxide counterparts.

Credit: © 2015 ACS

Using a Lewis acid was key to the successful synthesis — other reducing agents proved either non- or too reactive — and suggested a mechanism in which the acid facilitates the exchange of the methoxy group. Three derivatives were produced whose methine carbons were decorated with phenyl (pictured), methoxyphenyl and bromophenyl pendant groups, respectively. X-ray crystallography of all three subporphyrins showed that they all adopt similar bowl shapes, pointing to an sp3 hybridized central boron that bears an axial hydride ligand. Characterization by mass spectrometry, 1H- and 11B-NMR spectroscopy, and infrared spectroscopy of the phenyl-decorated compound also indicated the presence of a boron hydride moiety. Furthermore, its absorption and fluorescence emissions, and redox potentials, were consistent with those expected for the hydride compound as determined from comparisons with other subporphyrins and calculations using density functional theory. The infrared stretching frequencies and crystallographic bond distances pointed to a slightly weaker B–H bond than that of ammonia borane.

The chemical reactivity of the phenyl-decorated subporphyrin also concurs with that of a hydridic compound. It produced hydrogen gas on reaction with water and hydrochloric acid, and was able to reduce an aromatic aldehyde and an aromatic imine in the presence of a Lewis acid catalyst. Those hydroboration reactions likely involved a borenium cation, and produced subporphyrin derivatives featuring axial B–O and B–N groups, respectively. AP