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, C(15)H(20)NO(+)·C(4)H(3)O(4) (−), (I), and C(9)H(15)INO(+)·C(4)H(3)O(4) (−), (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 0.072 Å, 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.