Synthetic investigation of competing magnetic interactions in 2D metal-chloranilate radical frameworks

The discovery of emergent materials lies at the intersection of chemistry and condensed matter physics. Synthetic chemistry offers a pathway to create materials with the desired physical and electronic structures that support fundamentally new properties. Metal-organic frameworks are a promising platform for bottom-up chemical design of new materials, owing to their inherent chemical predictability and tunability relative to traditional solid-state materials. Herein, we describe the synthesis and magnetic characterization of a new 2,5-dihydroxy-1,4-benzoquinone based material, (NMe 2 H 2 ) 3.5 Ga 2 (C 6 O 4 Cl 2 ) 3 ( 1 ), which features radical-based electronic spins on the sites of a kagomé lattice, a geometric lattice known to engender exotic electronic properties. Vibrational and electronic spectroscopies, in combination with magnetic susceptibility measurements, revealed 1 exhibits mixed valency between the radical-bearing trianionic and diamagnetic tetraanionic oxidation states of the ligand. This unpaired electron density on the ligand forms a partially occupied kagomé lattice where approximately 85% of the lattice sites are occupied with an S = ½ spin. We found that gallium mediates ferromagnetic coupling between ligand spins, creating a ferromagnetic kagomé lattice. By modulation of the interlayer spacing via post-synthetic cation metathesis of 1 to (NMe 4 ) 3.5 Ga 2 (C 6 O 4 Cl 2 ) 3 ( 2 ) and (NEt 4 ) 2 (NMe 4 ) 1.5 Ga 2 (C 6 O 4 Cl 2 ) 3 ( 3 ), we determined the nature of the magnetic coupling between neighboring planes is antiferromagnetic. Additionally, we determined the role of the metal in mediating this magnetic coupling by comparison of 2 with the In 3+ analogue, (NMe 4 ) 3.5 In 2 (C 6 O 4 Cl 2 ) 3 ( 4 ), and we found that Ga 3+ supports stronger superexchange coupling between ligand-based spins than In 3+ . The combination of intraplanar ferromagnetic coupling and interplanar antiferromagnetic coupling exchange interactions suggests these are promising materials to host topological phenomena. 2D metal-organic frameworks provide insight into kagomé spin physics.