Tunable Ferromagnetic Strength in Niccolite Structural Heterometallic Formate Framework Based on Orthogonal Magnetic...

A series of heterometallic formate framework templated by amines were solvothermally prepared. They feature the formula of [A^I][CrM^II(HCO₂)₆] (A^I = NH₄H₂O^I and M = Mn for 1, A^I = CH₃NH₃^I and M = Fe for 2, A^I = CH₃NH₂CH₃^I and M = Co for 3, A^I = CH₃NH₃^I and M = Ni for 4). The title compounds exhibit isostructural niccolite architectures with differences only in the host metal ions and guest amines. Tunable ferromagnetic (FO) strength was realized in the resulting framework under the guidance of orthogonal magnetic orbital analysis of Cr^III (t_2g³e_g) and M^II (t_2g³e_g² for Mn^II, t_2g⁴e_g² for Fe^II, t_2g⁵e_g² for Co^II, t_2g⁶e_g² for Ni^II) ions. The magnetic ordering temperatures derived from the experimental magnetic measurements for 1-4 are lower than 2, 10.3, 7.6, and 22.0 K, respectively. Notably, thanks to the weak FO coupling between Cr^III and Mn^II ions, compound 1 displays a large magnetocaloric effect bearing the maximum of magnetic entropy change (-Δ S_m^max) up to 43.9 J kg^-1 K^-1 with Δ H = 7 T and T = 3.5 K, larger than most reported transition metal-based complexes and commercial gadolinium gallium garnet (Gd₃Ga₅O₁₂) (-Δ S_m^max = 38.4 J kg^-1 K^-1 with Δ H = 7 T). From 1, 2/3, to 4, an enhancement of the magnetic ordering temperature is observable due to the increasing strength of FO interactions between Cr^III and M^II ions. Our work provides a successful instance to modulate the strength of FO exchange via analyzing the orthogonal magnetic orbitals of heterometallic ions.