Ionic Boron Clusters as Superchaotropic Anions

Abstract

Boron clusters have become key units in various research fields, ranging from material science to medicine [1,2]. Cage‐like boron clusters (Figure 1.5.1) can be classified as closo‐, nido‐, arachno‐, hypho‐, and so on based on the completeness of the polyhedron, with the closo‐ cluster being closed. With one or two missing vertices, boron clusters are named nido‐ or arachno‐, respectively. They have characteristic three‐dimensional pol‐ yhedral geometries, with delocalized electron‐deficient structures [3–6]. Boron clusters, in their neutral and ionic forms, have shown a unique stability and low toxicity [3,4,6,7]. The ability of the 10B isotope to emit α particles after absorbing neutrons makes them ideal for pharmaceutical and medical applications, in particular for boron neutron cap‐ ture therapy (BNCT) [8–10]. Boron clusters interact with biomolecules, including biomembranes and proteins. This offers more possibilities in medicinal use than just BNCT. In this chapter, we sum‐ marize the current research on the noncovalent interactions of common boron clusters, in particular ionic ones, with supramolecular macrocycles, lipid bilayers, and proteins. We draw conclusions for drug design, and point out areas of future research. Polyhedral boron clusters can be divided into neutral and anionic ones. Neutral clus‐ ters include o‐, p‐, and m‐carboranes (C2B10H12). These isomers are highly hydrophobic. Closo‐dodecaborates (B12X12) are water‐soluble dianionic clusters with icosahedral structure, and they are nontoxic anions [11]. Shortly after their discovery, they were introduced as potential BNCT agents [12,13]. The decahydro‐closo‐decaborate anion (B10X10) is another member of the BnHn clusters; it is water‐soluble as sodium salt. Metalla bisdicarbollides, a different class of anionic boron clusters, are sandwiches of two [C2B9H11] (biscarbollide) clusters with a metal ion in the center (e.g., cobalta bis‐ dicarbollide anions); these clusters have recently emerged in medicinal chemistry as HIV protease inhibitors [14–16]. Ionic Boron Clusters as Superchaotropic Anions: Implications for Drug Design