E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S

S C I E N T I F I C H I G H L I G H T S

1 0 6 H I G H L I G H T S 2 0 2 1 I

Unlocking the potential of molecule- based magnets above room temperature

The thoughtful synthesis of high-performance molecule-based magnets is an ongoing challenge for the scientific community. This work develops a post-synthetic modification strategy, which allows to increase the exchange interactions between the metallic and organic radical spins of pre-formed coordination networks and results in magnets with unprecedented ordering temperatures and coercivity.

Conventional magnets exhibit several drawbacks due to their purely inorganic origin, such as high energy consumption at fabrication level (e.g., SmCo and AlNiCo) and limited availability of key constituents (e.g., NdFeB and SmCo). While many of these drawbacks could, in principle, be solved by using metal-organic magnets, the low operating temperatures of these materials (usually < 77 K) and the lack of generalised synthetic strategies have precluded their use in real-world applications.

With this in mind, this work illustrates the post-synthetic modification of a couple of pre-formed two-dimensional (2D) coordination networks, CrIII(pyz)2Cl2 (pyz = pyrazine, ferrimagnet below 55 K) and CrII(pyz)2(OSO2CH3)2 (antiferromagnet below 10 K; OSO2CH3− = methylsulfonate anion) [1,2]. The reduction of the organic pyrazine moieties by lithium 1,2 dihydroacenaphthylenide (Li+[C12H10 −]) leads to an increase in the number of spins and to strong magnetic interactions between the constituents, resulting in magnets that operate up to 515 K (Figure 85). Apart from exceeding the previous record working temperature observed for metal-organic magnets by more than 100 K, these new materials display unprecedented hard magnet properties at room temperature (RT), with coercive fields comparable to those of inorganic magnets.

In order to gain insights into these new systems, X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and powder X-ray diffraction (PXRD) experiments were performed at the ID12 and BM01 beamlines. X-ray absorption near-edge structure (XANES) spectra collected at the Cr K-edge for the RT magnets and reference compounds revealed the presence of square planar {CrN4} environments in the RT magnets (Figure 86a), implying the reduction of Cr(III) ions into Cr(II) in the case of CrIII(pyz)2Cl2 and the de-coordination of the axial ligands (Cl- and -OSO2CH3) from the Cr ions. Such findings were further supported by Cr K-edge extended X-ray absorption fine structure (EXAFS) spectra collected for the chlorine-based RT magnet and, more specifically, by the absence of the characteristic signature of the Cr-Cl bond that appears at R ~ 1.9 Å in the parent compound (Figure 86b).

While structure determination was not possible in the case of the methylsulfonate-based RT magnet due to the co-precipitation of the LiOSO2CH3 salt, synchrotron X-ray techniques, together with several spectroscopic, thermogravimetric and analytical techniques, revealed that the chlorine-based RT magnet was best described with the Li0.7[CrII(pyz)2]Cl0.7 · x(THF) (0.25 ≤ x ≤ 1) chemical formula, and enabled a good structural model that fit with the experimental PXRD data to be found. Thus, Li0.7[CrII(pyz)2]Cl0.7 · x(THF) consists of quadratic 2D layers composed of CrII metal ions and radical pyrazine ligands. In addition, 0.7 Li+, 0.7 Cl- and x THF molecules per formula unit are located between the layers (Figure 85). Interestingly, the unusually strong coercive field of this compound can be modulated from 5300 Oe (dlayers = 8.5 Å, x = 1) to 7500 Oe (dlayers = 7.2 Å, x = 0.25) by the amount of THF molecules in the structure, or in other words, by the distance between the layers. Cr K-edge XMCD measurements further support the remarkable magnetic properties of these systems.

Fig. 85: Scheme illustrating the post-synthetic reduction of the

pre-formed CrIII(pyz)2Cl2 pyrazine- based coordination network. Colour code: C, grey; N, blue;

CrIII, dark green; CrII, dark purple; Cl, light green; Li, purple.