A Mechanistic Perspective on Plastically Flexible Coordination Polymers

Abstract

Mechanical flexibility in single crystals of covalently bound materials is a fascinating and poorly understood phenomenon. We present here the first example of a plastically flexible one‐dimensional (1D) coordination polymer. The compound [Zn(μ‐Cl)2(3,5‐dichloropyridine)2]n is flexible over two crystallographic faces. Remarkably, the single crystal remains intact when bent to 180°. A combination of microscopy, diffraction, and spectroscopic studies have been used to probe the structural response of the crystal lattice to mechanical bending. Deformation of the covalent polymer chains does not appear to be responsible for the observed macroscopic bending. Instead, our results suggest that mechanical bending occurs by displacement of the coordination polymer chains. Based on experimental and theoretical evidence, we propose a new model for mechanical flexibility in 1D coordination polymers. Moreover, our calculations propose a cause of the different mechanical properties of this compound and a structurally similar elastic material.

Reversibility in a Plastically Flexible Coordination Polymer Crystal: A High-Pressure Study

“…Single crystals of (1) deform plastically across two crystallographic faces when exposed to three-point bending (Figure 1c-d). 9 The reported vibrational spectra indicated that no structural changes occur within the CP chains during bending. Instead, the high mosaicity which resulted from bending was ascribed to inter-CP chain distortions.…”

“…No significant changes are observed in the electronic structure of the covalent Zn-Cl-Zn chain upon compression, ESI S3.4. Single crystals of (1) exhibit unmistakable plasticity during three-point bending Figure 1c-d. 9 In stark contrast, the HP response of (1) is fully reversible, even following compression to 9.34 GPa. During decompression the crystallographic unit cell relaxes along the same pressure-volume (p-V) path with no detectable hysteresis, Figure 2a-b.…”

“…Models have been proposed to explain the mechanical flexibility of molecular and metal organic crystals. [9][10][11] Conceptually, these models are derived from analysis of the intermolecular interactions which are present in the solid state. Plastically flexible crystals typically comprise low energy shear planes, whereas elastically flexible materials often contain relatively weak isotropic intermolecular interactions.…”

“…6,10,12 Notable exceptions include the recently discovered class of flexible single crystal coordination polymers (CP). 9,13 The mechanisms of flexible materials must be better understood before such materials can be selectively designed for targeted, practical applications as advanced functional materials.…”

“…As a model system we studied the HP response of a plastically bendable 1D coordination polymer (CP) [Zn(μ-Cl)2(3,5-dichloropyridine)2]n (1), Figure 1. Isostructural to other mechanically flexible CPs, 9,13 (1) crystallizes in the tetragonal space group 4 ̅ 2, with covalent CP chains aligned along the crystallographic c-axis. Single crystals of (1) deform plastically across two crystallographic faces when exposed to three-point bending (Figure 1c-d).…”