Reversible color changes of chiral double salts of d-(+)-camphoric diammonium and transition-metal sulfates via gain and loss of crystalline and coordination water molecules

Six novel chiral double salts (1–6) composed of d-(+)-camphoric diammonium (LH2) and transition-metal sulfates, formulated as {[M(SO4)2(H2O)4](LH2)}·2H2O [M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)], have been prepared and fully characterized including single-crystal X-ray diffraction, Fourier transform infrared (FT-IR) and vibrational circular dichroism (VCD) spectral investigations. Co(II), Ni(II) and Cu(II) compounds 2–4 exhibit reversible color changes between 20 and 140 °C via gain and loss of two crystalline and four coordination water molecules, which can be proved by their thermogravimetric (TG) and powder X-ray diffraction (PXRD) analyses. To the best of our knowledge, this is the first report on chiral double salts showing chromotropism properties.