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Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex
Author(s) -
Kershaw Cook Laurence J.,
Shepherd Helena J.,
Comyn Tim P.,
Baldé Chérif,
Cespedes Oscar,
Chastanet Guillaume,
Halcrow Malcolm A.
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201406307
Subject(s) - crystallography , spin transition , chemistry , spin crossover , pyridine , steric effects , phase transition , excited state , phase (matter) , stereochemistry , condensed matter physics , medicinal chemistry , physics , atomic physics , organic chemistry
Crystalline [Fe(bppSMe) 2 ][BF 4 ] 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 ][BF 4 ] 2 ( 2 ; bppBr=4‐bromo‐2,6‐di(pyrazol‐1‐yl)pyridine) and [Fe(bppI) 2 ][BF 4 ] 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.