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

Crystalline [FeL2][BF4](2)center dot Me2CO (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-1/2 = 137 K (Delta T-1/2 = 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 pi center dot center dot center dot pi 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)center dot Me2CO also adopts phase1 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.