Heterocyclic Salt Synthesis
and Rational Properties Tailoring



Air Force Research Laboratory, 10 East Saturn Blvd.,
Edwards AFB, CA 93524-7680, USA

Abstract: Chemical structure determines the inherent properties displayed by
a given compound, and these properties, in turn, produce a specifc performance
behavior. Rationally designing chemical structure to predictably modify compound
properties, such that performance behavior can be tailored in a controlled manner,
defnes the objective of a pertinent synthesis effort. Achieving this objective by
introducing structural alterations in a neutral covalent compound offers only one
approach for resultant properties modifcation. Heterocyclic salts signifcantly
enhance the fexibility for achieving properties modifcation via three strategic
approaches: (1) compositionally pairing various cation structural classes with
a number of anion structural classes, (2) systematically altering the structure of
the cation; and, (3) systematically altering the structure of the anion. To illustrate
this premise, four general synthesis methods to synthesize heterocyclic salts,
including several new binary heterocyclium icosahedral closo-borane and closo-
carborane salts, frst are outlined. Secondly, properties modifcation approaches of
neutral covalent compounds are then compared with those approaches available
for various heterocyclic salts. Lastly, a key example, using three unsaturated
bridged heterocyclium di-cation salts, demonstrates how rational structure design,
and its effect on resultant predictable properties modifcation, produces tailored
performance behavior to reach the thermochemical initiation threshold needed for
combustion. This is achieved with predictable properties modifcations that increase
salt energy content, or that accelerate the reaction kinetics of the thermochemical
initiation process.

Keywords: binary heterocyclic salts, heterocyclic salt synthesis, ionic
liquids, bridged heterocyclic di-cation salts, rational structure design, predictive
properties modification, tailored performance behavior, thermochemical