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

A series of MnIV(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 MnIV(salen)(OCH2CF3)2 and MnIV(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 MnIV(salen)(Cl)2. The CH2Cl2 solution of MnIV(salen)(L)2 shows circular dichroism of increasing intensity in the order L = Cl < NO3 ≪ N3 < OCH2CF3, which indicates MnIV(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 MnIV(salen)(L)2 to facilitate the formation of a chirally distorted conformation, possibly a stepped conformation.