An Incomplete Spin Transition Associated with a Z′=1→Z′=24 Crystallographic Symmetry Breaking

Crystalline [FeL (2)][BF(4)](2) ⋅Me(2)CO (L=N‐[2,6‐di{pyrazol‐1‐yl}pyrid‐4‐yl]acetamide) is high‐spin at room temperature, and undergoes an abrupt, hysteretic spin‐crossover at T [Formula: see text] =137 K (ΔT [Formula: see text] =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P2(1)/c, Z=4) to phase 2 (P2(1), Z=48). The cations associate into chains in the crystal through weak intermolecular π⋅⋅⋅π interactions. Phase 2 contains a mixture of high‐spin and low‐spin molecules, which are grouped into triads along these chains. The perchlorate salt [FeL (2)][ClO(4)](2) ⋅Me(2)CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P2(1)/c, Z=8) without a change in spin state.