We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal-organic perovskite ABX(3), [C(NH(2))(3)][M(II)(HCOO)(3)], in which A = C(NH(2))(3) is guanidinium, B = M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO(-). The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2(1). In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti-anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)(3)](-) frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn-Teller effect of Cu(2+) results in a distorted anionic Cu-formate framework that can be regarded as Cu-formate chains through short basal Cu-O bonds linked by the long axial Cu-O bonds. These materials show higher thermal stability than other metal-organic perovskite series of [AmineH][M(HCOO)(3)] templated by the organic monoammonium cations (AmineH(+)) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.

We report here a new class of ammonium metal-formate frameworks of [NH 2 NH 3 ][M(HCOO) 3 ] (M = Mn 2+ , Zn 2+ , Co 2+ and Mg 2+ ) incorporating hydrazinium as the cationic template and component.The perovskite Mn and Zn members possess anionic 4 12•6 3 metal-formate frameworks with cubic cavities occupied by the NH 2 NH 3 + cations, while the Co and Mg members have chiral 4 9 •6 6 metal-formate frameworks, with chiral hexagonal channels accommodating NH 2 NH 3 + cations. On heating, the Mn and Zn members undergo phase transitions around 350 K. The structures change from low temperature (LT) polar phases in Pna2 1 to high temperature (HT) non-polar phases in Pnma, due to the thermally activated librational movement of the NH 2 end of the NH 2 NH 3 + in the cavity and significant framework regulation.The Co and Mg members in LT belong to non-polar P2 1 2 1 2 1 , are probably antiferroelectric, and they show phase transitions at 380 K (Co) and 348 K (Mg), and the structures change to polar HT phases in P6 3 , triggered by the order-disorder transition of the cation from one unique orientation in LT to three of trigonally-disorder state in HT. Accompanying the phase transitions, which are ferro-to para-electric for Mn and Zn members while antiferro-to ferro-electric for Co and Mg, prominent anisotropic thermal expansions including negative ones, and dielectric anomalies, are observed. The spontaneous polarization values are estimated at 3.58 (Mn, 110 K), 3.48 (Zn, 110 K), 2.61 (Co, 405 K) and 3.44 (Mg, 400 K) μC cm −2 , respectively, based on the positive and negative charge separations in the polar structures. The structureproperty relevance is established based on the order-disorder transitions of NH 2 NH 3 + and the conformity and adaptability of the metal-formate frameworks to match such order-disorder alternations. The Mn and Co members show spin-canted antiferromagnetic long-range-ordering, with Néel temperatures of 7.9 K and 13.9 K, respectively. Therefore, the two members show coexistence of electric and magnetic orderings in the low temperature region, and they are possible molecule-based multiferroics.

A systematic study has been carried out on the 3d divalent metal formate 3D magnetic frameworks templated by protonated amines, and the achievements have revealed that metal formate frameworks are very malleable, and their structures depend on the size, shape, charge, and hydrogen bonding geometries of the templating cations. Six kinds of metal formate frameworks have been created. They are chiral frameworks with a (4(9) . 6(6)) topology, perovskite ones with a (4(12) . 6(3)) topology, bi-nodal frameworks of (4(12) . 6(3))(4(9) . 6(6))(n) (n = 1, 2, 3) topologies, and porous diamond frameworks with 6(6) topology. These materials display promising and abundant magnetic, dielectric, porous, and optical properties and the possible combination of them. Therefore, they are of great interest for the study of molecule-based materials. It has been demonstrated that formate, being the smallest and simplest carboxylate, cheap and with low toxicity, thus more biocompatible and environmentally friendly, and having been more or less ignored, will find an important role in the construction of molecule-based materials and provide new materials with interesting properties.