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 realizing a wide range of magnetic functionalities not possible in inorganic vdW magnets. In particular, their modularity allows for specific control over ligand-mediated superexchange and hence their ground states. We report a new vdW MOM, CrCl2(btd) (btd = 2,1,3-benzothiadiazole), assembled from a high-energy early transition 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 measurements allow us to quantitatively determine the magnetic properties of this vdW magnet, showing it is a collinear rectangular antiferromagnet with small easy-axis anisotropy (J Cl = -15.31(5) K, J btd = -6.96(10) K, D = -2.0(2) K). Critically, we find that the superexchange through btd, with a low-lying LUMO, is significantly stronger than in the CrCl2(pym) (pym = pyrimidine) analogue (J pym = +1.2(2) K), demonstrating that exchange can be enhanced by potentially redox-active ligands without actual electron-transfer onto ligands. This work provides a route toward vdW antiferromagnetic MOMs with strong magnetic superexchange.