Crystal Engineering of Noncentrosymmetric Structures Based on 2-Amino-5-nitropyridine and <i>n</i>-Chloroacetic Acid Assemblies

Abstract

New noncentrosymmetric crystals including the 2-amino-5-nitropyridinium and 2-amino-5-nitropyridine chromophores have been designed following a strategy first applied to the 2-amino-5-nitropyridinium dihydrogen phosphate. If a counteranion able to self-aggregate in chains or layers as (H2PO4-)n is associated to the 2-amino-5-nitropyridinium cation, a noncentrosymmetric structure based on herringbone motifs is systematically formed. In the same way it has been observed that a bulky group grafted on the amine radical of the 2-amino-5-nitropyridine induces a noncentrosymmetric structure containing herringbone motifs of molecular entities. The structures of the 2-amino-5-nitropyridinium dichloroacetate ionic crystal and 2-amino-5-nitropyridine/chloroacetic acid cocrystal having respectively the P212121 and Cc symmetries confirm the validity of this strategy. The packings of these assemblies are discussed in terms of steric hindrance and hydrogen bonds. Powder samples of these materials exhibit second harmonic generation signals comparable to that of 3-methyl-4-nitropyridine N-oxide under Nd3+:YAG laser (1.06 μm) illumination.