Hybrid Organic–Inorganic Crystals of [Methylhydrazinium]M^IICl₃ (M^II = Co, Ni, Mn)

We report the synthesis and investigation of the physicochemical properties of novel one-dimensional hybrid organic–inorganic chlorides templated by the methylhydrazinium (MHy+) cation, MHyMIICl3 with MII = Mn, Co, and Ni. All crystals exhibit a rare [MIICl5N] coordination sphere. They undergo second-order klassengleiche structural phase transitions at 242 K (Mn), 227 K (Co), and at 223 K (Ni) upon cooling from the high-temperature P21/m to the low-temperature P21/n phase. Dielectric studies showed that the transformation has a weak dielectric response. X-ray diffraction data together with Raman and IR spectroscopy showed that the mechanism of phase transitions involves the changes of the metal-chloride framework, rearrangement of hydrogen bonds, and the different confinement of organic cations in the crystal voids. We show that the ionic radius (and electronegativity) of the metal ions correlates with structural factors, temperature of phase transition, dielectric response, hydrogen bond strength, and tolerance factor. The combination of the MHy+ cation with the small chloride linker does not allow achieving the archetypical perovskite-like architecture. Magnetic measurements revealed that MHyCoCl3 and MHyNiCl3 order antiferromagnetically below the same Néel temperature of about 3.7 K. The noticeable interplay of antiferromagnetic and ferromagnetic correlations leads in both systems to metamagnetic phase transitions in H of about 1 kOe. MHyMnCl3 showed only smeared antiferromagnetic ordering at about 5.0 K, with no trace of ferromagnetic correlations up to the highest field studied. Optical studies showed that all crystals are wide-bandgap materials with bandgaps of 5.34 (Mn), 3.96 (Ni), and 3.64 eV (Co). Moreover, MHyMnCl3 (MHyNiCl3) exhibited red (yellowish-green to orange) photoluminescence under the 450 (375) nm excitation with the activation energies of 299 (49) meV.