N,N'-DIPHENYL-1,4(5)-DIMETHOXYANTHRAQUINONE DIIMINES - BUTTERFLY INVERSION OF ANTHRAQUINONE DIIMINES

Model compounds for polyaromatic quinone imines were synthesized and characterized by variable-temperature H-1 NMR spectroscopy and X-ray single crystal structure determination. N,N'-Diphenyl-1,4(5)-dimethoxyanthraquinone 9,10-diimines were synthesized by condensing 2 equiv of aniline with 1,5-dimethoxyanthraquinone (15DMAQ) and 1,4-dimethoxyanthraquinone (14DMAQ), respectively, in the presence of titanium tetrachloride and 1,4-diazabicyclo[2.2.2]octane (DABCO). The X-ray single crystal structure determination of the diimine of 15DMAQ is reported. The condensation of aniline with anthraquinones results in a buckling of the anthraquinone ring system, giving it a ''butterfly'' conformation while the diimine of 15DMAQ has an anti configuration with the N-phenyl rings lying above the methoxy substituents. The diimine of 14DMAQ has a syn configuration, again with these N-phenyl rings lying above the methoxy substituents. The symmetrical structural butterfly conformations of the diimines allowed for the observation of a dynamic butterfly inversion by variable-temperature H-1 NMR spectroscopy. The energy barriers for the butterfly inversion of N-phenyl 15DMAQ and 14DMAQ diimines are 9.5 kcal/mol. The implications of the crystal structure and molecular dynamics for the polymeric structures are discussed.