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 (<b>1</b>; 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 (<b>2</b>), switching to the high-spin state while experiencing a profound crystallographic change. Compound <b>2</b> can be reversed to the original material upon reabsorption of acetone. Single crystal X-ray diffraction experiments on the latter system (<b>1</b>′) and on <b>2</b> show that these are reversible single-crystal-to-single-crystal (SCSC) transformations. Likewise, complex <b>2</b> can replace acetone by MeOH and H₂O to form [Fe­(bpp)­(H₂L)]­(ClO₄)₂·1.25MeOH·0.5H₂O (<b>3</b>) through a SCSC process that also implies a switch to the spin state. The <b>3</b>→<b>1</b> 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