N-Aryl-9,10-phenanthreneimines as scaffolds for exploring non-covalent interactions: a structural and computational study

A series of 10-[(4-halo-2,6-diisopropylphenyl)imino]phenanthren-9-ones and derivatives of the phenanthrene-9,10-dione ligand have been synthesised and structurally characterised to explore two types of noncovalent interactions, namely the influence of the steric bulk upon the resulting C–H···π and π-stacking interactions and halogen bonding. Selected noncovalent interactions have additionally been analysed by DFT and AIM techniques. No halogen bonding has been observed in these systems, but X lone pair···π, C–H···O=C and C–H···π interactions are the prevalent ones in the halogenated systems. Removal of the steric bulk in N-(2,4,6-trimethylphenyl)-9,10-iminophenanthrenequinone affords different noncovalent interactions, but the C–H···O=C hydrogen bonds are observed. Surprisingly, in N-(2,6-dimethylphenyl)-9,10-iminophenanthrenequinone and N-(phenyl)-9,10-iminophenanthrenequinone these C–H···O=C hydrogen bonds are not observed. However, they are observed in the related 2,6-di-tert-butylphenanthrene-9,10-dione. The π-interactions in dimers extracted from the crystal structures have been analysed by DFT and AIM. Spectroscopic investigations are also presented and these show only small perturbations to the O=C–C=N fragment