Journal of the American Chemical Society, Vol.131, No.34, 12394-12405, 2009
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, N-3, 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)(N-3)(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 << N-3 < 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.