Solid-state to solution helicity inversion of pseudotetrahedral chiral copper(II) complexes with 2,4-dihalo-salicylaldiminate ligands

The enantiopure Schiff bases (R or S)-N-1-( phenyl)ethyl-2,4-X-1,X2(-)salicylaldimine (X-1, X-2 = Cl, Br, I) coordinate to copper(II) and provide pseudotetrahedral bis[(S or R)-N-1-(phenyl)ethyl-(2,4-X-1,X-2-salicylaldiminato.2N,O)]-./.-Cu(II) (kappa N-2,O]-Delta/Lambda-Cu(II)(Delta/Lambda-Cu-R o Delta/Lambda-Cu-S). An induced Lambda and Delta-chirality at-metal centre has been launched along the C-2-axis of the molecule. Steric constraints brought by halogen substituents on the coordinating salicylal ring provide diastereoselectively Lambda-Cu-R or Delta-Cu-S as major and Delta Cu-R or Delta-Cu-S as minor diastereomers at solid-state, as evidenced by X-ray crystal structures and PXRD analyses. These results reveal inversion of induced chirality at-metal in comparison to the similar complexes without halogen substituents on the salicylal ring. Electronic circular dichroism (ECD) spectra show mirror-image relationships, confirming enantiomeric excess of the R or S-ligated complexes in solution. Comparisons of experimental and simulated ECD spectra suggest diastereomeric excess of Delta-Cu-R or Delta-Cu-S in solution which correspond to an inversion from the found solid-state Delta-Cu-R or Delta-Cu-S as major diastereomers. In addition, the optimized gas-phase structures also reveal Delta-Cu-R or Delta-Cu-S as slightly more stable than Delta-Cu-R or Delta-Cu-S. Thus, solid-state versus solution (or gas-phase) studies also indicate an interconversion of induced chirality (helicity inversion) at-metal from Delta-Cu-R or Delta-Cu-S to Delta-Cu-R or Delta-Cu-S. Thermal stability increases with the molecular weight of the complexes following Cl g(x,y) > 2.0 and values for g(z)/A(z) >= 135 cm indicate pseudotetrahedral geometry for the complexes