Functionalization of the [closo-1-CB9H10]- Anion for the Construction of New Classes of Liquid Crystals

The [closo-1-CB9H10](-) anion is a member of an extensive family of s-aromatic closo-boranes that possess impressive stability and functionalization characteristics. In contrast to its bigger, more extensively studied brother, the [closo-1-CB11H12](-) anion, convenient access to the [closo-1-CB9H10](-) anion has only been recently established, and researchers have only begun to develop and understand its fundamental chemistry. The geometrical and electronic properties of the [closo-1-CB9H10](-) anion make it an attractive structural element of novel classes of either zwitterionic or ionic liquid crystals suitable for electro-optical and ion transport applications, respectively. Such materials require a 1,10-difunctionalized [closo-1-CB9H10](-) anion that permits for the formation of molecules of elongated shape. The covalent attachment of an onium fragment or the use of a counterion compensates for the negative charge. This Account highlights the progress made in the advancement and understanding of the fundamental chemistry of the [closo-1-CB9H10](-) anion. We also describe the development of 1,10-difunctionalized derivatives as key intermediates in the preparation of new classes of liquid crystalline materials. We obtained the first isomerically pure 1,10-difunctionalized derivative of the [closo-1-CB9H10]- anion, iodo acid [closo-1-CB9H8-1-COOH-10-I](-), from decaborane through the Brellochs reaction. Functional group transformation of the C(1)-carboxyl group led to a 1-amino derivative and, subsequently, to a synthetically valuable 1-dinitrogen derivative. The latter exhibits reactivity typical for PhN2+ and undergoes diazocoupling and Gomberg-Bachmann arylation reactions. The B(10)-iodine participated in Negishi alkylation and Buchwald-Hartwig amination reactions, leading to 10-hexyl and 10-amino carboxylic acids, respectively. We converted the 10-amino carboxylic acid to a 10-dinitrogen acid [closo-1-CB9H8-1-COOH-10-N-2], which proved to be synthetically valuable in the preparation of 10-pyridinium and 10-sulfonium zwitterionic acids and their liquid crystalline esters. We investigated several intermediates using structural, spectroscopic, and kinetic methods.