[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]-[M-3(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 d(33) 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.