Spin Crossover under Extreme Conditions: A Structural Approach

Abstract

Structural studies of several iron complexes that show spin crossover (SCO) properties are presented and intrinsic links between the structure and properties have been established for these systems. X-ray diffraction experiments of samples under extremes of temperature and pressure and after irradiation with laser light have been performed to characterise the structure of these various compounds.

The incomplete thermal SCO observed in the dinuclear species [<Fe(bbp)(NCS)2>2 4,4’- bipy]•2MeOH has been investigated, revealing a complex series of crystallographic phase transitions that result in a material with a mixture of high spin (HS) and low spin (LS) states that displays long-range order at low temperature. The meta-stable high spin (HS*) state that arises as a result of the Light Induced Excited Spin State Trapping (LIESST) effect has also been structurally characterised at 30 K. Further single crystal X-ray diffraction experiments at high pressure have shown that it is possible to achieve a complete spin transition to a fully LS state, which is not thermally accessible. Another species that shows crystallographic ordering of mixed spin states is an FeIII complex with a hexadentate Schiff base ligand, which has been studied using both single crystal and powder diffraction techniques.

The structure of the photo-induced HS* state of [Fe(bapbpy) NCS)2] has been determined at 30 K from diffraction data which comprise contributions from both the HS* state and the residual LS component of the crystal, which is non-merrohedrally twinned.

The structure of [Fe(picen)(NCS)2] has been determined at 30 K after a range of cooling speeds, showing that cooling of the sample results in partial population of the HS* state. The photo-induced HS* state, and that induced by thermal trapping are shown to be structurally similar.

A gradual spin transition has been identified purely from structural analysis in an FeII complex with a hexadentate N6-donating ligand. The gradual nature of the spin transition may be attributed to the poor cooperativiy observed between iron centres.

This study shows how structural features can be used to gain insights into some of the more unusual aspects of the SCO phenomenon such as stepped spin transitions, ordered mixed spin intermediate phases, trapping of meta-stable states as a result of light irradiation or temperature and pressure-induced spin transitions.