Critical Role of External Axial Ligands in Chirality Amplification of trans-Cyclohexane-1,2-diamine in Salen Complexes

A series of Mn^IV(salen)(L)₂ complexes bearing different external axial ligands (L = Cl, NO₃, N₃, and OCH₂CF₃) from chiral salen ligands with trans-cyclohexane-1,2-diamine as a chiral scaffold are synthesized, to gain insight into conformational properties of metal salen complexes. X-ray crystal structures show that Mn^IV(salen)(OCH₂CF₃)₂ and Mn^IV(salen)(N₃)₂ adopt a stepped conformation with one of two salicylidene rings pointing upward and the other pointing downward due to the bias from the trans-cyclohexane-1,2-diamine moiety, which is in clear contrast to a relatively planar solid-state conformation for Mn^IV(salen)(Cl)₂. The CH₂Cl₂ solution of Mn^IV(salen)(L)₂ shows circular dichroism of increasing intensity in the order L = Cl < NO₃ ≪ N₃ < OCH₂CF₃, which indicates Mn^IV(salen)(L)₂ adopts a solution conformation of an increasing chiral distortion in this order. Quantum-chemical calculations with a symmetry adapted cluster-configuration interaction method indicate that a stepped conformation exhibits more intense circular dichroism than a planar conformation. The present study clarifies an unexpected new finding that the external axial ligands (L) play a critical role in amplifying the chirality in trans-cyclohexane-1,2-diamine in Mn^IV(salen)(L)₂ to facilitate the formation of a chirally distorted conformation, possibly a stepped conformation.