Aromaticity in 3D – Putting boron clusters at work

From the beginnings of boron cluster chemistry, it was recognized that their three-dimensional cage is characterized by delocalization of electrons. This observation led to the classification of boron clusters as 3D aromatic compounds [1]. Aromaticity in polyhedral boranes and carboranes has been extensively studied theoretically and experimentally [2,3] and the evidence for this type of aromaticity is supported by magnetic properties and their high stability that tends to increase with increasing cluster size. The species with 6 and 12 vertexes show special stability. Indeed, for example, icosahedral dodecaborate dianion is sometimes labeled “superaromatic” since it is often considered as a 3D inorganic benzene analogue. This property extends to open-cage clusters and metallacarborane, a class of boron clusters containing also metal atoms, such as cobalt, iron, chromium and others in various combinations. 3D Aromaticity strongly reflects in the reactivity of carborane heterorganic derivatives, where substituents effect is transmitted through the skeleton in a manner still not at all predictable but reminiscent of two-dimensional organic arene chemistry. Full exploitation of the boron clusters potential in various technological fields [4,5] requires availability of convenient and versatile methods for boron clusters functionalization with various moieties, bearing groups as hydroxyl, amine, azide, triple bond, chelating groups, etc. and/or alkyl chains of various length. It could be necessary also that different functionalization coexist in the same cluster structure to have hetero functionalized, tailor made boron cluster modules. Herein, we describe a progress in our efforts to develop new methodologies for boron cluster functionalization and their possible applications. In particular, current research and potential technological use of heterorganic derivatives of cobalt(III) bis(1,2-dicarbollide) anion, dodecaborate dianion and closo-ortho-carborane are presented. References [1] Russell N. Grimes. Carboranes, 2nd ed., 2011, Elsevier Inc [2] Z. Chen, R.B. King, Chem. Rev., 2005, 105, 3613−3642 [3] R.B. King, Chem. Rev., 2001, 101, 1119–1152 [4] J.F. Valliant et al., Coordination Chemistry Reviews, 2002, 232, 173-230 [5] B.P. Dash et al., New J. Chem., 2011, 35, 1955–1972 [6] J. Poater et al., Chem. Eur. J., 2016, 22, 7437-7443