Structural dimorphism of N-(3-hydroxysalicylidene)-2,4,6-trimethylaniline

The title compound N-(3-hydroxysalicylidene)-2,4,6-trimethylaniline exhibits dimorphism. The structure of (I) is orthorhombic Pbca with a = 22.222(3), b = 16.200(2), c = 7.596(2) Å, V = 2735(1) ų, and Z = 8. The structure of (II) is monoclinic P2₁/c with a = 11.734(1), b = 15.734(1), c = 7.432(2) Å, = 100.97(1)°, V = 1347.0(5)ų, and Z = 4. The two structures are analyzed in relation with the possibility of an equilibrium between two tautomeric phenol and quinone forms by rapid hydrogen transfer. The molecules associated by intermolecular hydrogen bonds form centrosymmetric dimers. The compounds are thermochromic. The results are compared with the photochromic analog N-(hydroxysalicylidene)-2,4,6-trimethylaniline (compound III) and with the thermochromic one N-(5-hydroxysalicylidene)-2,4,6-trimethylaniline (compound IV). It is concluded that the 3-hydroxyl substituent of the salicylidene moiety enables the intermolecular hydrogen transfer from oxygen to nitrogen.