Induced chirality-at-metal and diastereoselectivity at Delta/Delta-configured distorted square-planar copper complexes by enantiopure Schiff base ligands: combined circular dichroism, DFT and X-ray structural studies

Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar) ethyl-2-oxo-1-naphthaldiminato-kappa N-2,O, diastereoselectively yield Delta/Delta-chiral four-coordinated, non-planar Cu((NO)-O-boolean AND)(2) complexes [Ar = C6H5 R/S-L1, m-C6H4OMe R-L2, p-C6H4OMe R/S-L3, and p-C6H4Br R/S-L4]. Two N,O-chelate ligands coordinate to the copper(II) atom in distorted square-planar mode, and induce metal-centered Delta/Delta-chirality at the copper atom in the C-2-symmetric complexes. In the solid state, the R-L1 (or R-L4) ligand chirality diastereoselectively induces a Delta-Cu configuration in Delta-Cu-R-L1 (or Delta-Cu-R-L4), the S-L1 ligand a Delta-Cu configuration in Delta-Cu-S-L1, forming enantiopure crystals upon crystallization. Conversely, the R-L2 ligand combines both Delta/Delta-Cu-R-L2 as a diastereomeric pair in the crystals. In solution, electronic circular dichroism (CD) spectra show full or partial diastereoselectivity towards Delta-Cu for R ligands and towards Delta-Cu for S ligands. The electronic CD spectra measured on all complexes obtained from R ligands (or S ligands), e.g. Cu-R-L1, Cu-R-L2, Cu-R-L3, and Cu-R-L4 (or Cu-S-L1, Cu-S-L3, and Cu-S-L4), show consistent spectral features. TDDFT calculations of the electronic CD spectra for the diastereomers Delta-Cu-R-L1 and.-Cu-R-L1 suggest that the CD spectra are largely dominated by the configuration at the metal center (Delta vs. Delta). The experimental CD spectrum of Cu-R-L1 agrees well with the one calculated for the Delta-Cu-R-L1 configuration. Cyclic voltammetry of Cu-R-L1 reveals a quasi-reversible redox wave corresponding to one-electron transfer for the [(CuL2)-L-II](0)/[(CuL2)-L-I](-1) couple in acetonitrile. DSC analyses for the complexes show an exothermic peak between 377 and 478 K (Delta H = -12 to -43 kJ mol(-1)), corresponding to a phase transformation from distorted square-planar/tetrahedral to regular tetrahedral geometry on heating.