Cooperative High‐Temperature Spin Crossover Accompanied by a Highly Anisotropic Structural Distortion

Spin transitions are a spectacular example of molecular switching that can provoke extreme electronic and structural reorganizations in coordination compounds. A new 3D cyanoheterometallic framework, [Fe(pz){Au(CN) 2 } 2 ], has been synthesized in which a highly cooperative spin crossover has been observed at 367 and 349 K in heating and cooling modes, respectively. Mössbauer spectroscopy revealed a complete transition between the diamagnetic and paramagnetic states of the iron centres. The low‐spin‐to‐high‐spin transition induced a drastic structural distortion involving a large one‐directional expansion (ca. 10.6 %) and contraction (ca. 9.6 %) of the lattice. Negative thermal expansion along the c axis was detected below and above the transition temperature.