Synthesis and properties of boron porphyrinoids

Boron being a small atom, its coordination chemistry with different porphyrinoids is very unique and unusual compared to coordination chemistry of porphyrinoids with transition metals and other main group elements. One such unique feature is the coordination of more than one boron with the porphyrinoid ligand. The mono- and diboron(III) complexes of porphyrinoids showed distinct coordination variations and exhibited exciting photophysical and redox properties. In this review, we present an overview of the synthesis, structure, and spectral properties of various boron complexes of porphyrinoids that include porphyrins, porphycenes, N-confused and N-fused porphyrins, corroles, triphyrins, expanded porphyrins, and calixphyrins. We described a comparison-based aforementioned boron porphyrinoids addressing their structural varieties, unexpected chemistry associated at both the boron centre, and the porphyrinoid ligand, and possible reasoning behind their unusual coordination chemistry. Interestingly, the usage of the boron porphyrinoids for practical applications has been limited till now. Only the B(III) complexes of 25-oxasmaragdyrins have been explored for ion sensing and photothermal therapy of cancer, whereas some BF2 complexes of calix[16]phyrin, and N-confused calix[8]phyrin have been used for light-harvesting properties, which were also briefly discussed in this review. Thus, we anticipate that the present review article would encourage the porphyrin fraternity to design the new examples of boron porphyrinoids and explore their unrealized potential in modern-day research fields such as developing photocatalysts for organic transformations and solar energy conversion or nano-assemblies for theragnostic agents in clinical applications.