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

A series of heterometallic formate framework templated by amines were solvothermally prepared. They feature the formula of [AI][CrMII(HCO2)6] (AI = NH4H2OI and M = Mn for 1, AI = CH3NH3I and M = Fe for 2, AI = CH3NH2CH3I and M = Co for 3, AI = CH3NH3I 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 CrIII (t2g3eg) and MII (t2g3eg2 for MnII, t2g4eg2 for FeII, t2g5eg2 for CoII, t2g6eg2 for NiII) 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 CrIII and MnII ions, compound 1 displays a large magnetocaloric effect bearing the maximum of magnetic entropy change (-Δ Smmax) 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 Smmax = 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 CrIII and MII ions. Our work provides a successful instance to modulate the strength of FO exchange via analyzing the orthogonal magnetic orbitals of heterometallic ions.