Three-Way Crystal-to-Crystal Reversible Transformation and Controlled Spin Switching by a Nonporous Molecular Material

Porous materials capable of hosting
external molecules are paramount
in basic and applied research. Nonporous materials able to incorporate
molecules via internal lattice reorganization are however extremely
rare since their structural integrity usually does not resist the
guest exchange processes. The novel heteroleptic low-spin Fe­(II) complex
[Fe­(bpp)­(H2L)]­(ClO4)2·1.5C3H6O (1; bpp = 2,6-bis­(pyrazol-3-yl)­pyridine,
H2L = 2,6-bis­(5-(2-methoxyphenyl)­pyrazol-3-yl)­pyridine)
crystallizes as a compact discrete, nonporous material hosting solvate
molecules of acetone. The system is able to extrude one-third of these
molecules to lead to [Fe­(bpp)­(H2L)]­(ClO4)2·C3H6O (2), switching
to the high-spin state while experiencing a profound crystallographic
change. Compound 2 can be reversed to the original material
upon reabsorption of acetone. Single crystal X-ray diffraction experiments
on the latter system (1′) and on 2 show that these are reversible single-crystal-to-single-crystal
(SCSC) transformations. Likewise, complex 2 can replace
acetone by MeOH and H2O to form [Fe­(bpp)­(H2L)]­(ClO4)2·1.25MeOH·0.5H2O (3) through a SCSC process that also implies a switch to the
spin state. The 3→1 transformation
through acetone reabsorption is also demonstrated. Besides the spin
switching at room temperature, this series of SCSC transformations
causes macroscopic changes in color that can be followed by the naked
eye. The reversible exchanges of chemicals are therefore easily sensed
at the temperature at which these occur, contrary to what is the case
for most of the few existing nonporous spin-based sensors, which feature
a large temperature gap between the process monitored and the mechanism
of detection.