A comparative structural study of alkane- α,ω-diphosphonic acids

Abstract

Abstract In a comparative structural study, the solid state structures within the homologous series of alkane-α,ω-diphosphonic acids, H2O3P–(CH2) n –PO3H2 with n = 6–12, have been characterised by powder and single crystal X-ray diffraction. The crystal structures of the odd-numbered diphosphonic acids were found to be homotypic. For the even-numbered diphosphonic acids – including the two already known polymorphs of butane-1,4-diphosphonic acid – two different types are found. Basically, all alkane-α,ω-diphosphonic acids exhibit pillared-layered structures with their terminal groups forming two-dimensional hydrogen bonded networks which are covalently bridged by alkylene chains. Structural differences occur within the hydrogen bonding systems as well as in the arrangement of the alkylene chains. Based on the existence of different structure types of the alkane-α,ω-diphosphonic acids, the progression of their melting points can be explained.