Hydrogen Bonding and Polymorphism of Amino Alcohol Salts with Quinaldinate: Structural Study

Three amino alcohols, 3-amino-1-propanol (abbreviated as 3a1pOH), 2-amino-1-butanol (2a1bOH), and 2-amino-2-methyl-1-propanol (2a2m1pOH), were reacted with quinoline-2-carboxylic acid, known as quinaldinic acid. This combination yielded three salts, (3a1pOHH)quin (1, 3a1pOHH⁺ = protonated 3-amino-1-propanol, quin⁻ = anion of quinaldinic acid), (2a1bOHH)quin (2, 2a1bOHH⁺ = protonated 2-amino-1-butanol), and (2a2m1pOHH)quin (3, 2a2m1pOHH⁺ = protonated 2-amino-2-methyl-1-propanol). The 2-amino-1-butanol and 2-amino-2-methyl-1-propanol systems produced two polymorphs each, labeled 2a/2b and 3a/3b, respectively. The compounds were characterized by X-ray structure analysis on single-crystal. The crystal structures of all consisted of protonated amino alcohols with NH₃⁺ moiety and quinaldinate anions with carboxylate moiety. The used amino alcohols contained one OH and one NH₂ functional group, both prone to participate in hydrogen bonding. Therefore, similar connectivity patterns were expected. This proved to be true to some extent as all structures contained the NH₃⁺∙∙∙⁻OOC heterosynthon. Nevertheless, different hydrogen bonding and π∙∙∙π stacking interactions were observed, leading to distinct connectivity motifs. The largest difference in hydrogen bonding occurred between polymorphs 3a and 3b, as they had only one heterosynton in common.