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

9 6 H I G H L I G H T S 2 0 2 2 I

Chemical tuning of coordination solids: from antiferromagnetic insulators to highly conducting systems

The ability of molecule-based materials to be insulators, semiconductors or even metals while exhibiting remarkable magnetic properties makes them ideal candidates for spintronics applications. With the help of X-ray absorption spectroscopy, this work reveals the key parameters governing the interplay between magnetism and conductivity in order to control and tune the electronic properties.

In molecule-based materials, the electronic synergy between metal (M) ions and organic linkers is essential to chemically engineer desired conductive and magnetic properties and ultimately, to design the next-generation of spintronic devices. Recent research has focused on MCl2(pyz)2 layered materials (pyz = pyrazine; Figure 86) [1-3]. In the chromium analogue, CrCl2(pyz)2, the reducing power of the CrII precursor leads to an electron transfer between the metal centre and the organic ligand, giving rise to a coordination solid incorporating CrIII and a singly reduced pyrazine scaffold [1].

In this coordination solid, the strong electronic interactions between the metal ions and the pyrazine ligands, together with good orbital overlap and electronic delocalisation, lead to a ferrimagnetic order below 55 K, and the electrical conductivity displays a semiconducting behaviour with a room temperature (RT) value of 0.03 S/cm. Interestingly, when the Cr ion is replaced by V, the ferrimagnetic semiconductor becomes an antiferromagnetic insulator, and a metallic conductor when Ti is chosen. In order to better to better understand these radical changes in the physical properties, X-ray absorption spectroscopy (XAS) experiments at the V, Ti and Cl K-edges were performed at beamline ID12. Considering that the first inflection point of the rising metal K-edge can be used to assess the oxidation state of transition metal ions, it was concluded that the metal ions are in different oxidation states in these isostructural analogues: +2 for vanadium and +3 for titanium (Figures 87a and 87b). Thus, the redox process

involving the oxidation of MII to MIII and the concomitant reduction of one pyrazine is disabled in VCl2(pyz)2 but not in TiCl2(pyz)2 and CrCl2(pyz)2 [1] due to the different local redox potentials of the chosen metal ion. These findings are further supported by XAS measurements at the Cl K-edge, in which the decrease in the oxidation state of the metal ions leads to a decrease in the M Cl bond covalency and therefore, to less intense pre-edge features (black arrow in Figure 87c).

Regarding the physical properties, the pyrazine ligands mediate unprecedently strong antiferromagnetic super- exchange interactions between adjacent S = 3/2 VII spins in VCl2(pyz)2, which result in an antiferromagnetic ground state with a high ordering temperature ( ≈ 120 K). The electrons of the VII ions remain localised on the metal centres, leading to an insulating behaviour. On the other hand, due to strong Ti-pyrazine radical covalency, TiCl2(pyz)2 exhibits Pauli paramagnetism and the highest RT electrical conductivity (5.3 S/cm) observed for any coordination solid based on octahedrally coordinated metal ions. Despite the apparent semiconducting behaviour upon lowering the temperature, the combined and consistent analysis of the electrical conductivity, the large and positive magnetoresistance, the specific heat, the magnetisation data and DFT calculations demonstrate the presence of a correlated Fermi liquid state in this metal- organic material. Even though the electrical conductivity measured on powdered samples is dominated by the contribution of grain boundaries, these complementary theoretical and experimental techniques unambiguously prove the existence of metallic conductivity in TiCl2(pyz)2, providing a general methodology that can be extended to other conducting coordination solids.

In conclusion, this work demonstrates how the choice of the metal ion in a series of isostructural coordination materials allows precise control of their physical properties. The ability to chemically tune the ground state of MCl2(pyz)2 from a ferrimagnetic semiconductor (M = Cr) to an antiferromagnetic insulator (M = V) or a strongly correlated Fermi liquid (M = Ti) represents a promising pathway to design new generations of metallic and even superconducting metal-organic materials.

Fig. 86: Structure of MCl2(pyz)2 (M = Cr, V, Ti) shown (a) perpendicular and (b) parallel to the two-dimensional lattice. Colour code: M, dark green; Cl, light green; C, dark grey; N, blue; H, omitted for clarity.