Decoupled spin crossover and structural phase transition in a molecular iron(II) complex

Laurence J. Kershaw Cook (1) , Helena Shepherd (2) , Tim P. Comyn (3) , Chérif Baldé (4, 5) , Oscar Cespedes (6) , Guillaume Chastanet (4) , Malcolm A Halcrow (1)

1 School of Chemistry [Leeds]
2 Department of Chemistry
3 Institute for Materials Research
4 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux
5 Sciences and Technology Department
6 School of Physics and Astronomy [Leeds]

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±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit-cell a axis to one-third of its original value (high-temperature phase 1; Pbcn, Z=12; low-temperature phase 2; Pbcn, Z=4). The SCO-active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140-300 K, 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 phase 2 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=70-80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.

Mots clés: crystal engineering; iron; N‐ligands; photomagnetic properties; spin crossover

Domaines: Chimie de coordination

Laurence J. Kershaw Cook, Helena Shepherd, Tim P. Comyn, Chérif Baldé, Oscar Cespedes, et al.. Decoupled spin crossover and structural phase transition in a molecular iron(II) complex. Chemistry - A European Journal, 2015, 21 (12), pp.4805-4816. ⟨10.1002/chem.201406307⟩. ⟨hal-01128451⟩