New 1: 1 and 2:1 salts in the ' dl -norvaline-maleic acid' system as an example of assembling various crystal structures from similar supramolecular building blocks

Molecular salts and cocrystals of amino acids have potential applications as molecular materials with nonlinear optical, ferroelectric, piezoelectric, and other various target physical properties. The wide choice of amino acids and coformers makes it possible to design various crystal structures. The amino acid-maleic acid system provides a perfect example of a rich variety of crystal structures with different stoichiometries, symmetries and packing motifs built from the molecular building blocks, which are either exactly the same, or differ merely by protonation or as optical isomers. The present paper reports the crystal structures of two new salts of the dl-norvaline-maleic acid system with 1:1 and 2:1 stoichiometries, namely dl-norvalinium hydrogen maleate, C₅H₁₂NO₂ ⁺·C₄H₃O₄ ^-, (I), and dl-norvalinium hydrogen maleate-dl-norvaline, C₅H₁₂NO₂ ⁺·C₄H₃O₄ ^-·C₅H₁₁NO₂, (II). These are the first examples of molecular salts of dl-norvaline with an organic anion. The crystal structure of (I) has the same C₂ ²(12) structure-forming motif which is common for hydrogen maleates of amino acids. The structure of (II) has dimeric cations. Of special interest is that the single crystals of (I) which are originally formed on crystallization from aqueous solution transform into single crystals of (II) if stored in the mother liquor for several hours.The crystals of two new salts in the dl-norvaline-maleic acid system provide examples of different molecular packing of the molecular building blocks, which are either the same, or differ merely by protonation.