The group of Xinyi Wang from the School of Chemistry and Chemical Engineering in Nanjing University has achieved new progress in the field of single-chain magnets. The new result was recently published in the Angew. Chem. Int. Ed. under the title “Enhanced Single-Chain Magnet Behavior via Anisotropic Exchange in a Cyano-Bridged MoIII-MnII Chain” (Angew. Chem. Int. Ed.,2020, DOI: 10.1002/anie.202001706). The first author of this work is the Ph. D student Le Shi.
Since the first observation of the slow magnetic relaxation in a one-dimensional coordination polymer, single-chain magnets (SCMs) present another important breakthrough in the field of molecular magnetism after the discovery of single-molecule magnets (SMMs). Different from the SMMs, the energy barriers (U) of SCMs depend not only on the magnetic anisotropy of the magnetic ions but also on the magnitude of intrachain magnetic exchange. Hence, it should be easier to increase the U value of the SCMs by enhancing the magnetic coupling between the spin carriers. In this vein, [Mo(CN)7]4- unit is a very attractive starting material due to the strong magnetic anisotropy of the MoIII center. The strong anisotropic MoIII-CN-MnII exchange interaction was theoretically predicted to be promising to prepare the high performance SCMs.
Figure 1 Crystal structures (a) and magnetic properties of SCM (b-d)
Based on the previously achievements in the [Mo(CN)7]4- bridged SMMs (J. Am. Chem. Soc.,2013, 135, 13302-13305; J. Am. Chem. Soc.,2017, 139, 11714-11717), Wang’s group successfully synthesized a new one-dimensional MnII-[Mo(CN)7]4- SCM. Single crystal diffraction revealed that the chain can be described as a 1D assembly of the Mn2Mo SMMs connected by the equatorial CN groups (Figure 1a). Both the dc and ac magnetic susceptibilities data unambiguously confirm that the chain behave as a SCM with a high relaxation barrier of 178 K. Open magnetic hysteresis loops are observed below 5.2 K, with a coercive field of 1.5 T at 2 K.
The SCM behavior mainly originated from the anisotropic magnetic exchange since the low spin MoIII (S = 1/2) does not have zero-field splitting and the D value of the MnII centers is expected to be very small, which is significantly different from the most of the reported SCMs. The {Mn2Mo}n SCM together with previously reported Mn2Mo SMMs further emphasize the importance of the anisotropic magnetic exchange to the construction of molecular magnetic materials of high magnetic anisotropy and provides a new strategy for high performance molecular nanomagnets.
This work was conducted in collaboration with Prof. Kim R Dunbar of Texas A &M University. Dr. Yue Zhao and Dr. Tian-Wei Wang from the State Key Laboratory of Coordination Chemistry provided supports in the crystal structure refinement and magnetism measurement. The work also supported by the National Key R&D Program of China and NSFC.