Enhancing superexchange through frontier orbital engineering in a van der Waals metal-organic magnet

Van der Waals metal-organic magnets (vdW MOMs) offer the possibility of realis- ing a wide-range of magnetic functionalities not possible in inorganic vdW magnets. In particular, they offer the opportunity to enhance magnetic interactions using organic functionalisation to control ligand energy levels directly. We report a new vdW MOM, CrCl2(btd) (btd = 2,1,3-benzothiadiazole), assembled from a high energy early transi- tion metal and a redox-active ligand, which leads to a small charge-transfer gap. We solve the structure using a combination of powder neutron diffraction and single crystal electron diffraction, showing that this material contains Cr2+btd0. Magnetometry, neutron diffraction and inelastic neutron scattering allow us to quantitatively determine the magnetic properties of this vdW magnet, showing it is a collinear rectangular antiferromagnetwithsmalleasyaxisanisotropy(JCl =−15.31(5)K,Jbtd =−6.96(10)K, D = −2.0(2) K). Critically, we find that the superexchange through the redox-active btd is significantly stronger than in the non-redox-active CrCl2(pym) (pym = pyrimidine) analogue (Jpym = 1.2(2)K), demonstrating that exchange can be enhanced without the need for electron transfer onto ligands. This work provides a route towards vdW MOMs with strong magnetic superexchange.