STRUCTURES AND TAUTOMERIC INTERCONVERSIONS OF ANTHRAQUINONE IMINE DERIVATIVES

Substituted 1-hydroxy-9,10-anthraquinone-9-imines have been found to e xhibit tautomeric interconversions between the 9,10- and 1,10-quinonoi d forms in the solid state as well as in solution. Single-crystal X-ra y crystallography was used to study the structures of 4-(N-acetyl-p-to lylamino)-9-amino-1, 10-anthracenedione and -1-phenylamino-10-mesityli mino-9(10H)-anthracenone at ambient and low temperatures. The former c ompound gave crystals belonging to the monoclinic space group P2(1)/c and, at 295 K, a = 9.684(2), b = 16.371(3), c = 12.097(2) Angstrom, be ta = 110.41(1)degrees, V = 1797 Angstrom(3), Z = 4, R(F) = 0.042, and GOF = 0.863. The latter compound gave crystals belonging to the monocl inic space group C2/c and, at 295 K, a = 38.158(8), b = 6.322(2), c = 19.325(4) Angstrom, beta = 101.78(2)degrees, V = 4564 Angstrom 3, Z = 8, R(F) = 0.052, and GOF = 0.981, while at 150 K, a = 37.852(6), b = 6 .245(1), c = 19.235(4) Angstrom, beta = 102.81(1)degrees, V = 4434 Ang strom 3, R(F) = 0.047, and GOF = 1.006. Comparison of the geometric pa rameters of the two compounds illustrates the considerable zwitterioni c nature of the first, a distinctive feature of which is a very short bond N(9)-C(9) whose length is essentially equal to that of a double b ond. Molecular modeling studies using AMI semiempirical calculations w ere shown to be capable of satisfactorily reproducing the energetics o f the tautomeric equilibrium once the medium effect was taken into acc ount.