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

Crystalline [FeL₂ ][BF₄ ]₂ ⋅Me₂ 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 (ΔT1/2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P2₁ /c, Z=4) to phase 2 (P2₁ , 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 [FeL₂ ][ClO₄ ]₂ ⋅Me₂ CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P2₁ /c, Z=8) without a change in spin state.