X-ray structural determination and comparison of Bis-imine Schiff bases with trans- and cis-1,2-cyclohexanediamine backbones

Abstract

Analysis of the X-ray crystal structures of diimines cis-N,N’-bis(pyridine-2-ylmethylene)-1,2-cyclohexanediamine, known compound trans-N,N’-bis(pyridine-2-ylmethylene)-1,2-cyclohexanediamine, and racemic trans-N,N’-bis(phenylmethylene)-1,2-cyclohexanediamine reveals that the pyridyl diimines have greater imine C = N to 2-pyridine ring coplanarity than those found in the phenyl-substitued diimine. For the phenyl diimine, the lack of planarity and loss of conjugation is attributed to N = CH to ortho hydrogen steric repulsion that is relieved in the pyridyl diimines. In the phenyl diimine the closest intramolecular contact is between the N = CH hydrogen atom and an ortho hydrogen on the phenyl ring. The cis- and trans- pyridyl diimine molecules pack together via slightly different types of weak interactions. For the cis complex, the shortest interaction involves the nitrogen atom of the imine and a cyclohexyl hydrogen atom on an adjacent molecule. A ring CH to pyridine centroid interaction leads to the formation of tight dimers in the cis- diimine. For the trans-diimine the shortest interaction also involves the imine nitrogen atom, but with an interaction with a pyridyl hydrogen atom and it does not form dimers. The predominant packing interactions in trans-2 are due to ring stacking which gives rise to a 2D-layered structure.