Quantum Spin‐1/2 Dimers in a Low‐Dimensional Tetrabromocuprate Magnet

This work describes a homometallic spin‐1/2 tetrabromocuprate adopting a bilayer structure. Magnetic‐susceptibility measurements show a broad maximum centred near 70 K, with fits to this data using a Heisenberg model consistent with strong antiferromagnetic coupling between neighbouring copper atoms in different layers of the bilayer. There are further weak intralayer ferromagnetic interactions between copper cations in neighbouring dimers. First‐principles calculations are consistent with this, but suggest there is only significant magnetic coupling within one direction of a layer; this would suggest the presence of a spin ladder within the bilayer with antiferromagnetic rung and weaker ferromagnetic rail couplings. Only one way: (3,4‐lutH)2CuBr4 (3,4‐lut=3,4‐lutidine) is a homometallic spin‐1/2 tetrabromocuprate that adopts a bilayer structure with strong antiferromagnetic coupling between dimers across the bilayer and weaker intralayer ferromagnetic interactions. First‐principles calculations suggest there is only significant magnetic coupling within one direction of a layer; this is consistent with a spin‐ladder motif with antiferromagnetic rung and weaker ferromagnetic rail couplings.