Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Crystalline [Fe(bppSMe)(2)][BF4](2) (1; bppSMe=4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) undergoes an abrupt spin-crossover (SCO) event at 265 +/- 5K. The crystals also undergo a separate phase transition near 205K, involving a contraction of the unit-cell a axis to one-third of its original value (high-temperature phase1; Pbcn, Z=12; low-temperature phase2; Pbcn, Z=4). The SCO-active phase1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase1 between 140-300K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)(2)][BF4](2) (2; bppBr=4-bromo-2,6-di(pyrazol-1-yl)pyridine) and [Fe(bppI)(2)][BF4](2) (3; bppI=4-iodo-2,6-di(pyrazol-1-yl)-pyridine) exhibit more gradual SCO near room temperature, and adopt phase2 in both spin states. Comparison of 1-3 reveals that the more cooperative spin transition in 1, and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light-induced excited-spin-state trapping (LIESST) effect with T(LIESST=70-80K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.