Occurrence of a rare 4(9).6(6) structural topology, chirality, and weak ferromagnetism in the [NH4][MII(HCOO)3] (M = Mn, Co, Ni) frameworks

We report the synthesis, crystal structures, thermal, IR, UV-vis, and magnetic properties of a series of divalent transition metal formates, NH4]M(HCOO)3], where M = divalent Mn, Co, or Ni. They crystallize in the hexagonal chiral space group P6(3)22. The structure consists of octahedral metal centers connected by the anti-anti formate ligands, and the ammonium cations sit in the channels. The chiral structure is a framework with the rarely observed 49.66 topology, and the chirality is derived from the handedness imposed by the formate ligands around the metals and the presence of units with only one handedness. The thermal properties are characterized by a decomposition at ca. 200 degrees C. The three compounds exhibit an antiferromagnetic ground state at 8.4, 9.8, and 29.5 K for Mn, Co, and Ni, respectively. The last two display a weak spontaneous magnetization due to a small canting of the moments below the critical temperature, and the Co compound shows a further transition at lower temperatures. The isothermal magnetizations at 2 K show spin-flop fields of 600 Oe (Mn), 14 kOe (Co), and above 50 kOe (Ni) and a small hysteresis with a remnant magnetization of 25 cm3 G mol(-1) (Co) and 50 cm3 G mol(-1) (Ni) and coercive field of 400 Oe (Co) and 830 Oe (Ni).