Structure and Stereodynamics of Aryldiimino Derivatives

Aromatic diimino derivatives, having two NCPh₂ moieties bonded at the 1,8 positions of the anthraquinone, anthracene, biphenylene, and naphthalene rings, have been investigated by variable- temperature NMR spectroscopy, X-ray diffraction, and DFT calculations. In all the compounds, the imino substituents are essentially orthogonal to the aromatic plane and the phenyl groups bonded to the CN carbons are diastereotopic (i.e., cis or trans to the aromatic N-substituent of the NC moiety). The barriers for the cis/trans interconversion of the phenyl groups by planar nitrogen inversion have been determined. In the cases of anthracene and anthraquinone derivatives, the presence of syn and anti conformers was detected, and their interconversion barriers, due to the Ar−N rotation, were measured. At very low temperature, restricted rotations of the phenyl groups, displaying barriers in the range 4.8−6.9 kcal mol⁻¹, were also observed for all compounds: the barriers for the rotation of the cis were found to be larger than for the trans phenyl groups.