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.

能够承载外部分子的多孔材料在基础与应用研究中至关重要。然而，可通过内部晶格重组容纳分子的无孔材料却极为罕见，因为其结构完整性通常无法抵御客体交换过程。本研究中的新型杂配位低自旋铁(II)配合物[Fe(bpp)(H₂L)](ClO₄)₂·1.5C₃H₆O（化合物1；其中bpp=2,6-双(吡唑-3-基)吡啶，H₂L=2,6-双(5-(2-甲氧基苯基)吡唑-3-基)吡啶）以紧凑离散的无孔材料形式结晶，可承载丙酮溶剂化分子。该体系可脱除三分之一的此类溶剂分子，得到[Fe(bpp)(H₂L)](ClO₄)₂·C₃H₆O（化合物2），同时伴随显著的晶体学变化并转变为高自旋态。化合物2可通过重新吸附丙酮恢复至原始材料。对体系1'与化合物2开展的单晶X射线衍射实验表明，二者为可逆的单晶-单晶（single-crystal-to-single-crystal, SCSC）转变过程。类似地，化合物2可通过单晶-单晶（single-crystal-to-single-crystal, SCSC）过程以甲醇（MeOH）与水（H₂O）替换丙酮，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。通过重新吸附丙酮亦可实现3→1的转变。除室温下的自旋切换外，该系列SCSC转变还会引发宏观颜色变化，可通过肉眼直接观测。因此，此类可逆化学交换可在其发生的温度下被便捷感知，这与现有的少数自旋基无孔传感器不同——这类传感器的监测过程与检测机制之间存在较大的温度间隙。