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

Crystalline [Fe L 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 T1 / 2 =137 K (Δ T1 / 2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 ( P 2 1 / c , Z= 4) to phase 2 ( P 2 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 [Fe L 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 ( P 2 1 / c , Z= 8) without a change in spin state.