Synthesis and Characterization of Cu-Containing Chiral Metal Halides and Role of Halogenation of the Organic Ligand

We report herein novel chiral copper metal chlorides, namely, (R/S-ClMBA)2CuCl4 [ClMBA = 4-(chloromethyl)benzylammonium], together with the corresponding racemic compound. The two chiral samples are composed of isolated CuCl4 tetrahedra crystallizing in the noncentrosymmetric C2 space group and present a significant absorption in the visible region of the spectrum with a band gap around 2.50 eV. The racemic compound, on the other hand, gives origin to a different structural topology characterized by the presence of a quite unusual five coordination for Cu(II), with Cu2Cl6- dimers forming infinite chains along the b axis and presents, in addition, a strong reduction of the band gap (around 1.76 eV). The two chiral samples display a circular dichroism response with anisotropy factor values of around 0.02. The actual novel samples are investigated in comparison to the MBA-based (MBA = methylbenzylammonium) system in order to unveil the effect of benzene ring halogenation on the structural and chiroptical properties, indicating that the presence of chlorine leads to a doubling of the anisotropy factor.