Interaction between maleic acid and N-R-furfurylamines: Crystal structure of 2-methyl-N-[(5-phenylfuran-2-yl)methyl]propan-2-aminium (2Z)-3-carboxyacrylate and N-[(5-iodofuran-2-yl)methyl]-2-methylpropan-2-aminium (2Z)-3-carboxyprop-2-enoate

The title molecular salts, C15H20NO+·C4H3O4 -, (I), and C9H15INO+·C4H3O4-, (II), have very similar molecular geometries for both cation and anion. The anions of both (I) and (II) are practically planar (r.m.s. deviations = 0.062 and 0.072Å, respectively) and adopt a rare symmetrical geometry with the hydroxy H atom approximately equidistant from the two O atoms. In their crystals, the cations and anions in both (I) and (II) form tight ionic pairs via strong N-H⋯O hydrogen bonds, with a roughly perpendicular disposition of the anion to the furan ring of the cation. This ion-pair conformation appears to correlate with the lack of reactivity of these salts in [4 + 2] cycloaddition reactions. In the extended structures of (I) and (II), the ion pairs form hydrogen-bonded chains propagating along [010] and [001], respectively, via N-H⋯O hydrogen bonds.