Hysteretic Spin Crossover with High Transition Temperatures in Two Cobalt(II) Complexes.

Cooperative spin crossover transitions with thermal hysteresis loops are rarely observed in cobalt(II) complexes. Herein, two new mononuclear cobalt(II) complexes with hysteretic spin crossover at relatively high temperatures (from 320 to 400 K), namely, [Co(terpy-CH2OH)2]·X2 (terpy-CH2OH = 4'-(hydroxymethyl)-2,2';6',2″-terpyridine, X = SCN-(1) and SeCN- (2)), have been synthesized and characterized structurally and magnetically. Both compounds are mononuclear CoII complexes with two chelating terpy-CH2OH ligands. Magnetic measurements revealed the existence of the hysteretic SCO transitions for both complexes. For compound 1, a one-step transition with T1/2↑= 334.5 K was observed upon heating, while a two-step transition is observed upon cooling with T1/2↓(1) = 329.3 K and T1/2↓(2) = 324.1 K (at a temperature sweep rate of 5 K/min). As for compound 2, a hysteresis loop with a width of 5 K (T1/2↓ = 391.6 K and T1/2↑ = 396.6 K, at a sweep rate of 5 K/min) can be observed. Thanks to the absence of the crystallized lattice solvents, their single crystals are stable enough at high temperatures for the structure determination at both spin states, which reveals that the hysteretic SCO transitions in both complexes originate from the crystallographic phase transitions involving a thermally induced order-disorder transition of the dangling -CH2OH groups in the ligand. This work shows that the modification of the terpy ligand has an important effect on the magnetic properties of the resulting cobalt(II) complexes.

Two three-dimensional [MoIII(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 [MoIII(CN)7]4- unit with MnII 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 Pi, 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 MnII 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 MnII-[MoIII(CN)7]4- magnets with high magnetic ordering temperatures.