Noncovalent Interactions of Heteroboranes

This chapter deals with noncovalent interactions between heteroboranes and their various organic or biomolecular partners. At first, the physical essence of noncovalent interactions in general is discussed. Focusing then on boron clusters, their contacts are discussed based on the unusual electron distribution within the boron hydride cages and especially around the heteroatoms (i.e. non-boron atoms incorporated in the cluster framework) or the substituents replacing the terminal hydrogens. The bare (i.e. not linked to hydrogen) heteroatoms within the cage bear prevailingly a partial positive charge. This results in an opposite direction of the compound dipole moments (as proved experimentally), contrary to what would be expected from the electronegativity concept. The anisotropic distribution of the electron density around the heteroatoms gives rise to the so-called σ-holes, regions of positive electrostatic potential (ESP). This can be viewed as a driving force for noncovalent interactions with, e.g. organic aromatics or Lewis bases. Examples of σ-hole bonding of heteroboranes incorporated in the cluster cages are chalcogen or pnictogen bonding. Heteroatoms as exo-substituents can also be centers of σ-hole bonding, in this case halogen bonding. An important characteristics of the σ-hole bonding is that it can be tuned, e.g. by other exo-substituents or by the point of attachment to the cage – whether the exo-halogens are bonded to boron or carbon atoms within a particular cluster. Apart from the σ-hole bonding, the hydride character of the terminal hydrogens of the heteroboranes is responsible for forming unique dihydrogen H ⋯ H bonds, which provides the essence of heteroborane noncovalent interactions with various organic, inorganic and molecules of biological interest. The influence of various molecular shapes on the strength of these nonclassical contacts is discussed. Likewise, the choices of optimal mathematical models and computational protocols to study the crucial energy terms contributing to these interactions are reviewed.