Chiral distortion in a MnIV(salen)(N3)2 derived from Jacobsen's catalyst as a possible conformation model for its enantioselective reactions

The Mn(IV)(salen)(N(3))(2) complex (3) from Jacobsen's catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting Mn(III)(salen)(N(3))(CH(3)OH) complex (2) are determined in order to investigate the conformation of the high-valent Mn(IV)(salen) molecule in comparison with that of Mn(III)(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N(3) ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH(3)CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV-vis, electron paramagnetic resonance, (1)H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry.