Self-assembly directed by NH-O hydrogen bonding: new molecular assemblies derived from N,N-tetramethyl-1,2-diaminoethane and malonic or fumaric acid incorporating 'proton transferred' hydrogen bonds

Co-crystallisation of N,N-tetramethyl-1,2-diaminoethane (TMEN) and malonic or fumaric acid from water in a 1:1 ratio results in crystalline products whose X-ray structures show that they contain hydrogen-bonded motifs in self-assembled arrays. The stoichiometry of the malonate-containing assembly is 1:2, [TMENH2(hydrogen malonate)2], while that of the fumarate-containing assembly is 1:1:1 (TMENH2, fumarate, fumaric acid). In each structure, protons from individual carboxylic acids have been transferred onto the nitrogens of N,N-tetramethyl-1,2-diaminoethane (TMEN) to yield diammonium cations; the ammonium protons hydrogen bond to carboxylate oxygens of hydrogen malonate in one case or to fumarate anions in the other. The hydrogen malonate-containing structure has layers of aligned diprotonated TMEN separated by layers of hydrogen malonate anions. Successive hydrogen malonate ions are hydrogen bound in an 'alternate’ manner between adjacent protonated TMEN layers. Each [TMENH2(hydrogen malonate)2] moiety describes an approximate 'Z’ configuration which are in turn stacked at an oblique angle; adjacent ‘columns of Zs’ are partially interlaced to form the overall array. The fumarate-containing structure consists of parallel ribbons of alternating, hydrogen bonded, fumarate dianions and fumaric acid molecules. These form layers alternating with layers of diammonium ions which lie essentially in the xy plane. The diammonium ions form parallel staggered head-to-tail chains along the y axis and so run at an angle across the fumarate-fumaric acid ribbons