Two three-dimensional [Mo^III(CN)₇]^4--based magnets showing new topologies and ferrimagnetic ordering below 80 K

The rational design and synthesis of heptacyanomolybdate-based magnets remain a challenge due to the complexity of this system. Here, we reported the crystal structures and magnetic properties of two three-dimensional (3D) frameworks prepared from the self-assembly of the [Mo^III(CN)₇]^4- unit with Mn^II ions in the presence of different amide ligands, namely Mn₂(DMF)(H₂O)₂[Mo(CN)₇]·H₂O·CH₃OH (1) and Mn₂(DEF)(H₂O)[Mo(CN)₇] (2) (DMF = N,N'-dimethylformamide and DEF = N,N'-diethylformamide). Single-crystal structure determinations showed that compound 1 crystallizes in the triclinic space group Pī, while 2 crystallizes in the monoclinic space group P2₁/n. The difference in the structures of 1 and 2 is the coordination mode of the amide molecules: while the DMF molecules in 1 are only terminal ligands, the DEF molecules in 2 act as bridging ligands between two Mn^II centers. Although their space groups and local coordination environments of the metal centers are of some difference, both compounds have similar extended 3D frameworks where the spin centers are bridged by both the CN^- and μ₂-O bridges. They both have a three-nodal 4, 4, 7-connecting topological net with the vertex symbol of {4³·5³}{4⁴·5²}{4⁷·5⁴·6⁶·7⁴} for 1, and {4³·5³}{4⁴·5²}{4⁷·5⁴·6⁷·7³} for 2, respectively. Magnetic measurements revealed that both 1 and 2 exhibit ferrimagnetic ordering below 80 K together with another anomaly at about 45 K probably owing to spin reorientation. Besides, spin frustration and non-linear alignment of the magnetic moments are also possible due to competitive antiferromagnetic interactions between the spin carriers. These compounds expanded the family of Mn^II-[Mo^III(CN)₇]^4- magnets with high magnetic ordering temperatures.