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.

可负载外来分子的多孔材料在基础与应用研究中至关重要。然而，能够通过内部晶格重组结合客体分子的无孔材料极为罕见——这类材料的结构完整性通常无法抵御客体交换过程。本研究中报道的新型杂配低自旋铁(II)配合物[Fe(bpp)(H₂L)](ClO₄)₂·1.5C₃H₆O（化合物1；其中bpp为2,6-双(吡唑-3-基)吡啶（2,6-bis(pyrazol-3-yl)pyridine），H₂L为2,6-双(5-(2-甲氧基苯基)吡唑-3-基)吡啶（2,6-bis(5-(2-methoxyphenyl)pyrazol-3-yl)pyridine））以紧凑离散的无孔材料形式结晶，可负载丙酮溶剂分子。该体系可脱除三分之一的这类溶剂分子，得到[Fe(bpp)(H₂L)](ClO₄)₂·C₃H₆O（化合物2），同时转变为高自旋态，并伴随显著的晶体学结构变化。将化合物2重新吸附丙酮后，可恢复至原始的化合物1结构。对原始体系（1'）与化合物2开展的单晶X射线衍射实验证实，二者之间存在可逆的单晶到单晶（Single-crystal-to-single-crystal, SCSC）转变。类似地，化合物2可通过SCSC过程将丙酮替换为甲醇（MeOH）与水，得到[Fe(bpp)(H₂L)](ClO₄)₂·1.25MeOH·0.5H₂O（化合物3），该过程同样伴随自旋态转变。通过重新吸附丙酮实现化合物3向化合物1的转变的过程同样得到了验证。除了在室温下发生自旋转换外，这一系列SCSC转变还会引发宏观的颜色变化，可通过肉眼直接观测。因此，这类可逆的化学交换过程可在其发生的温度下被便捷感知——这与现有少数自旋基无孔传感器的情况不同，这类传感器的过程监测与检测机制之间存在较大的温度差。