[Ba₄Cl] Cations Directed Perovskite-like Polar Metal Formate Frameworks {[Ba₄Cl][M₃(HCO₂)₁₃]}_n (M = Mn, Co, and Mg): Microwave-Assisted Synthesis, Structures, and Properties

Novel 3D metal formate frameworks {[Ba₄Cl]­[M₃(HCO₂)₁₃]}_n (M = Mn for 1, Co for 2, and Mg for 3) were successfully assembled via microwave-assisted synthesis. The complexes are rare coordination polymers crystallized at space group P4cc with the polar point group C_4v. In the structure, the M^II ions are bridged by two types of anti-anti formate in forming a 3D pcu framework, and additional formates coordinate to the unsaturated sites of the M^II ions in the framework, giving an anionic M–formate net. Ba₄Cl clusters take the cavities of the net as charge balance, in which the chloride ion deviates from the center of the barium ions. The asymmetric Ba₄Cl structure is transmitted throughout the crystal resulting in polar structure, which is further confirmed by nonlinear optical and piezoelectric test. Nonlinear optical activity tests of 1 and 3 show SHG signals 0.32 and 0.28 times that of KDP, while 2 has a piezoelectric coefficient d₃₃ of 6.8 pC/N along polar axis. Magnetic studies reveal antiferromagnetic coupling between M^II ions in 1 and 2. Spin canting was found only in 2 with anisotropic Co^II ions, and 2 is a canted antiferromagnetically with T_N = 5 K. Further field-induced spin flop was also found in 2 with a critical field 0.9 T.