MECHANOCHEMICAL PREPARATION OF HYDROGEN BONDED ADDUCTS BETWEEN THE DIAMINE 1,4-DIAZABICYCLO[2.2.2]OCTANE AND DICARBOXYLIC ACIDS OF VARIABLE CHAIN LENGTH: A DIFFRACTION AND SOLID-STATE NMR STUDY

Mechanical mixing of solid dicarboxylic acids of variable chain length HOOC(CH2),COOH (n = 1-7) with solid 1,4-diazabicyclo[2.2.2] octane generates the corresponding salts or cocrystals of the formula [N(CH2CH2)(3)N]-H-[OOC(CH2)(n)COOH] (n=1-7). Preparation of the same systems from solution has been instrumental for a full characterization of the mechanochemical products by means of single-crystal and powder-diffraction X-ray analyses, as well as by solid-state NMR. The acid-base adducts, whether involving proton transfer from the COOH group to the N-acceptor, that is having ((-)OH)-H-...-N(+) interactions, or the formation of neutral O-(HN)-N-... hydrogen bonds, show a melting point alternation phenomenon analogous to that shown by the neutral carboxylic acids. The carbon chemical shift tensors of the COOH group obtained from the side-band intensity of low speed spinning NMR spectra provide a reliable criterion for assigning the protonation state of the adducts.