The use of Kitaigorodskiiʹs Aufbau principle in the solid-state study of crystalline borane compounds. A preliminary account

Very many molecular structures of polyhedral borane compounds have been established by solid-state crystallographic analysis, but their intermolecular interactions, and the factors that dictate their supramolecular arrangements in the crystal lattice, are essentially unexamined. Kitaigorodskiiʹs Aufbau principle (KAP) forms the basis of a technique for such an examination and simplifies the visualisation of molecular packing by breaking it down into a series of symmetry-generated steps, of which each permits even the weakest intermolecular interactions to be readily identified. KAP is based on close-packing principles, and constructs the crystal of a molecular compound in a three-step process: first, the molecular units pack to form one-dimensional chain substructures, these then pack together to yield two-dimensional sheet substructures; these, in turn, stack on top of one another to yield the observed three-dimensional structure. The method is exemplified by consideration of the molecular arrangements in the ‘wrap-around’ encapsulation of 4,4′-bipyridyl with [6,9-(4,4′-bipyridyl)2-arachno-B10H12] in their 1:1 co-crystal, the complex three-dimensional dihydrogen-bonding network in the crystal structure of H3BNH3, solvent channels in the structure of the co-crystal of n-hexane with [C5H5NMe][1-(SMe)-10-(SMe2)-closo-B10H8], and systematic variations within the crystal structures of [6,9-(NC5H4-4-R)2-arachno-B10H12], where R is Me, Et and n-Pr. It is concluded in this preliminary account that KAP provides a simple yet rigorous tool for studying a whole range of both gross and subtle molecular packing effects in the solid-state.