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

A series of Mn<sup>IV</sup>(salen)(L)<sub>2</sub> complexes bearing different external axial ligands (L = Cl, NO<sub>3</sub>, N<sub>3</sub>, and OCH<sub>2</sub>CF<sub>3</sub>) from chiral salen ligands with <i>trans</i>-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<sup>IV</sup>(salen)(OCH<sub>2</sub>CF<sub>3</sub>)<sub>2</sub> and Mn<sup>IV</sup>(salen)(N<sub>3</sub>)<sub>2</sub> adopt a stepped conformation with one of two salicylidene rings pointing upward and the other pointing downward due to the bias from the <i>trans</i>-cyclohexane-1,2-diamine moiety, which is in clear contrast to a relatively planar solid-state conformation for Mn<sup>IV</sup>(salen)(Cl)<sub>2</sub>. The CH<sub>2</sub>Cl<sub>2</sub> solution of Mn<sup>IV</sup>(salen)(L)<sub>2</sub> shows circular dichroism of increasing intensity in the order L = Cl < NO<sub>3</sub> ≪ N<sub>3</sub> < OCH<sub>2</sub>CF<sub>3</sub>, which indicates Mn<sup>IV</sup>(salen)(L)<sub>2</sub> 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 <i>trans</i>-cyclohexane-1,2-diamine in Mn<sup>IV</sup>(salen)(L)<sub>2</sub> to facilitate the formation of a chirally distorted conformation, possibly a stepped conformation.