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

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 = C -H to ortho hydrogen steric repulsion that is relieved in the pyridyl diimines. In the phenyl diimine the closest intramolecular contact is between the N = C -H 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 C -H 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. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The analysis of X-ray crystal structures reveals that pyridyl diimines exhibit greater coplanarity between the imine C = N and the 2-pyridine ring compared to phenyl-substituted diimines. The lack of planarity in phenyl diimines is attributed to steric repulsion between the N = C -H and ortho hydrogen atoms, which is relieved in pyridyl diimines. Additionally, the packing interactions differ slightly between cis and trans pyridyl diimines.