Giant anisotropic thermal expansion of copper-cyanido flat layers with flexible copper nodes

Abstract

Flat layers are usually considered as structurally rigid motifs in two-dimensional (2D) materials. In this work, we demonstrate that a flat honeycomb-layer composed of distorted tri-coordinate copper ions bridged with cyanidos in (tetraethylammonium)Cu₂(CN)₃ exhibits high in-plane flexibility. This resulted in an extremely large anisotropic 2D-thermal expansion.

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Two-dimensional (2D) materials have attracted attention due to their functionalities coupled with large anisotropy of the layer between the in-plane and stacking directions. The flexibility of each individual layer, coupled with the change of chemical and physical properties, is a key structural feature for exploring functionalities such as adsorption,^1,2 magnetism,^3,4 and electronic properties.^5,6 Thus, unveiling the relationship between a flexible structure and a response to stimuli for layers is of high significance not only for deepening fundamental insights on 2D-materials but also for the development of future devices utilizing thin films^7,8 and membranes.^9,10

The motif of layer structures, such as flat and undulation, significantly influences the anisotropic structural transformation. For example, undulating layers in uniaxial^11,12 and bi-axial^13–16 directions are highly flexible due to the anisotropic and isotropic structural transformations, respectively, involving layer-expansion/contraction (Fig. 1). By contrast, flat layers tend generally to be rigid and less anisotropic in their in-plane directions.^17,18 These relationships between layer-motif and flexibility have been demonstrated by various 2D-materials such as graphene,^19–21 covalent-organic frameworks^22–24 and coordination polymers (CPs)/metal–organic frameworks.^25–27

Fig. 1 Schematics displaying the relationships between layer-motifs and structural flexibility. Flexible layers with (a) uniaxial and (b) bi-axial undulation. (c) A conventional flat and rigid layer. (d) Flat but flexible distorted layer, demonstrated in this work. Arrows indicate directions of layer-deformation.

2D-CPs are modern materials that exhibit the designability of layer structures constructed by metal ions connected with ligands via coordination bonds. These relatively dynamic connections provide characteristic structural flexibility, giving rise to various thermal expansion (TE) behavior. Our group has reported about composition-dependent TE of several cyanido-bridged 2D-CPs such as Mn(solv.)[Pd(CN)₄]·xsolv. (solv. = H₂O, and MeOH),²⁸ [Mn(salen)]₂[M(CN)₄]·xH₂O (M = Pt, PtI₂, and MnN),²⁹ and (PPh₄)Cu₂(CN)₃ (PPh₄Cu; PPh₄⁺ = tetraphenylphosphonium),³⁰ proving the structure-flexibility relationship of flexible-undulating and rigid-flat layers. Notably, flat honeycomb layers of PPh₄Cu were very rigid with zero area thermal expansion involving a double distortion relaxation mechanism involving a coupling of structural changes of the PPh₄⁺ and [Cu₂(CN)₃]⁻ counterparts (Table 1). Such strongly coupled behaviour between the honeycomb layers and sandwiched cations motivated us to explore other counter-cations to bring out the unusual flexibility of the flat layers.³¹

Table 1 CTEs of TEACu and PPh₄Cu³⁰ (* indicates stacking directions)

CTE (MK^−1)	a	b	c	V
TEACu	−34.7(4)	70.9(9)	132(20)*	168.2(28)
PPh4Cu	5.5(3)	86(3)*	−5.3(3)	86.2(14)

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Herein, we report that a flat honeycomb layer of (tetraethylammonium)[Cu₂(CN)₃] (TEACu)³² is highly flexible, giving rise to giant anisotropic in-plane thermal expansion whose coefficients are −34.7 MK⁻¹ (M = 10⁶) and 70.9 MK⁻¹. Metal node distortion in tricoordinate Cu(I) accounts for the resultant unusual layer deformation, which was also investigated by theoretical and spectroscopy methods.

Needle-shaped colourless single crystals of TEACu were prepared by slow evaporation of a solution of Cu(NO₃)₂·3H₂O, NaCN, and TEACl in 6 M NH₃aq. (Fig. S1, ESI†). TEACu crystallizes in the space group of Ibam. Anionic honeycomb-layers are composed of Cu^I ions bridged with cyanido ligands. These layers are flat within the ab-planes, but are constituted of highly distorted hexagonal units resulting in the distortion of the angles in the tri-coordinated geometry of the Cu^I centers (Fig. 2a and b). The angles of Cu–Cu–Cu of the hexagons are 102.7°, 109.5°, and 147.8° at 100 K. As a result, each hexagon unit exhibited a clear elongation along either the (110)- or the (1−10)-direction. This anisotropic distortion of the layers was induced by the presence of TEA cations in the interlayer spaces (Fig. 2c). Uniquely, two ethyl substituents of TEA fit diagonally into a hexagon, giving rise to the anisotropic distortion of the corresponding hexagon along the a-axis. The IR spectra of TEACu displayed a stretching peak of a cyanido group at 2117 cm⁻¹ (Fig. S2, ESI†). TGA demonstrated that TEACu is stable up to 600 K (Fig. S3, ESI†).

Fig. 2 Crystal structure of TEACu in the (a) stacking and (b) in-plane directions. Colour code: Orange; Cu, Light blue; N, Gray; C. Cyanido-bridges between Cu centers were disordered in the layer, and are presented as C atoms. (c) The packing structure of TEACu where red and blue coloured parts indicate upper and lower layers, respectively. (d) Relative cell length of TEACu. Black, red, and blue show the a-, b-, and c-axis, respectively. (e) Temperature variations of the angles of Cu–Cu–Cu and (f) the distances of the diagonal lines within a Cu(CN)₆ hexagon. (g) The schematic image of the deformation of a Cu₆(CN)₆ hexagon.

We found that the flat layer of TEACu is extremely flexible, as confirmed by its TE behavior at 100–400 K. Variable temperature (VT)-single crystal X-ray diffraction results of TEACu provided the coefficient of TE (CTE) of α_a = −34.7(4) MK⁻¹, α_b = 70.9(9) MK⁻¹, α_c = 132(20) MK⁻¹ (M = 10⁶) (Fig. 2d and Table 1). These large CTE values are comparable with those of highly flexible undulating layered CPs.^33,34 Moreover, it is much more characteristic than anisotropic TE of other 2D-flat layered materials with rigid layers and same-signed in-plane TE (i.e. both axes in the plane show positive or negative TE behavior). The highly anisotropic in-plane TE behavior of TEACu is due to the structural transformation in which the stretched layer changes to its ideal geometry. Thus, the node distortion around the Cu^I centers is responsible for the transformation of the honeycomb layers. More specifically, the angles of Cu–Cu–Cu showed large changes to become closer to 120°, which is shown in a regular hexagonal lattice, as the temperature increased (Fig. 2e). The geometry-change of the hexagon in the layer is also shown by diagonal lengths between Cu ions of the corresponding hexagon (Fig. 2f). These large decreases along the a axis and increases along the b axis are in accordance with the corresponding negative and positive TEs, respectively (Fig. 2g and Fig. S4, ESI†). Note that VT-IR and VT-Raman spectra displayed a noticeable shift for the stretching peaks of cyanido toward low energy regions (Fig. S5 and S6, ESI†). The luminescent properties of TEACu based on its copper-cyanido charge transfer were previously investigated at 77 K and room temperature, with an intensity maximum showing a slight shift from 380 to 371 nm.³² These spectra changes reflected the local structural transformations involving the node distortion during the TE behavior.

The characteristic anisotropic deformation of flat layers involves the flexibility of tricoordinate Cu^I nodes, supported by quantum chemical calculation. The optimized structure of a local [Cu(CN)₃]²⁻ part indicated C/N–Cu–C/N angles closer to 120° than TEACu (Fig. S7, ESI†). Therefore, it was demonstrated that a crystal packing effect on the coordination geometry of Cu nodes is present via the formation of the infinite honeycomb layers and corresponding anionic-layer stacking with cations. Uniquely, the HOMO of [Cu(CN)₃]²⁻ is composed of the hybridization of the 3d_x²–y² and 4p_y orbitals (Cu) and σ_CN orbital (cyanido), suggesting that the overlap of these orbitals by decreasing the Cu node distortion results in the stabilization of the corresponding structure (Fig. S8 and S9, ESI†). Further calculation of a [Cu(CN)₃]²⁻ moiety at 400 K demonstrated that its geometry changed to become close to an ideal Y-shape, which is less stable than that at 100 K by 0.31 eV. These results are in accordance with the flexible tricoordinate Cu^I node arising from the degree of freedom of the low-coordination on metal ions. Note that a similar role of flexible metal nodes on anisotropic TE has also been reported in other 3D metal-cyanidos,^35,36 metal-sulfocyanidos, and metal-selenocyanide compounds.³⁷

Moreover, we found that all edge lengths between Cu ions of TEACu decreased as the temperature increases (Fig. 3, and Fig. S10, ESI†). Such counterintuitive shrinkage of the cyanido-bridge has been demonstrated by several cyanido-bridged negative thermal expansion materials such as Prussian Blue analogs.^38–40 Low energy transverse vibration of the cyanido was established to interpret unusual contraction of the cyanido-bridging between metal ions. In this sense, the vibrational calculations of the [Cu(CN)₃]²⁻ moiety suggested that this part also exhibits similar vibration below 100 cm⁻¹ (Table S2 and Fig. S11, ESI†). The single crystal X-ray analyses also corroborated that cyanidos exhibited large atomic displacement parameters in the vertical direction from the layers (Fig. S12 and S13, ESI†). Importantly, the resultant decreases of all edge lengths of the Cu₆(CN)₆ hexagon in TEACu are largely different from its analog of PPh₄Cu for which only two edges of the hexagon shrank while the other four expanded (Fig. 3).³⁰ Thus, these different TE behaviors of the anionic honeycomb layers demonstrate that cation species significantly influenced structural changes of the layers via intermolecular interaction. Comparing the closest distances between the cyanidos and hydrogens of the cations between TEACu and PPh₄Cu, TEACu exhibits weaker interaction than PPh₄Cu: the distances are 2.843 Å for TEACu and 2.779 Å for PPh₄Cu (both values are the shortest lengths at 250 K). These results indicate that the PPh₄ cations suppress the transverse vibration of four cyanidos within PPh₄Cu, and thereby intuitive expansion of the corresponding cyanidos occurs. A similar pinning effect for the transverse vibration on negative TE has been demonstrated by PBA via accommodation of lattice waters^41–43 and ScF₆via Li⁺ doping.⁴⁴

Fig. 3 (a) The distance of Cu–Cu in Cu₆(CN)₆ hexagons of TEACu (red) and PPh₄Cu (black). The hexagons of (b) TEACu and (c) PPh₄Cu. Circles, triangles, and crosses are consistent with the length shown in (a).

In several 2D-materials, the unique properties of molecular species have been derived by sandwiching them between layers.^45–49 In this regard, we further analysed the conformation changes of TEA in the interlayer spaces of TEACu. Uniquely, TEA deformed to shrink within the layers along the large interlayer expansion (Fig. S14a and b, ESI†). This result indicates that the TEA appears to expand between layers at low temperatures. This cation deformation in TEACu follows the opposite trend to PPh₄Cu where PPh₄ expanded in the stacking direction via the coupling with the interlayer expansion (Fig. S14c, ESI†). These characteristic structural changes of spherical cations in the interlayer spaces demonstrate that the sandwiching system composed of [Cu₂(CN)₃]⁻ also works as a platform bringing out the unprecedented molecular motion linked with the layer flexibilities.

In conclusion, we demonstrated the extreme flexibility of flat honeycomb layers of TEACuvia its giant anisotropic in-plane thermal expansion. This unusual structural property was underpinned by distorted tricoordinate metal nodes. Thus, this work showed that even in conventionally rigid structural motifs, the material design incorporating node-distortion into the coordination network leads to the enhancement of structural flexibility and control of structural properties.

This work was supported by the Grant-in-Aid for Transformative Research Areas (A) “Supra-ceramics” (JSPS KAKENHI grant number JP22H05144 and JP22H05146). This work was also supported by the JSPS KAKENHI grant number JP24K01457, JP21K18936, JP21H01905 and JP22K19052. This work was partially supported by the Cooperative Research Program of ‘Network Joint Research Centre for Materials and Devices’. The computations in this work were performed using the computer facilities at the Research Institute for Information Technology, Kyushu University, at the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo, and at Cyberscience Center, Tohoku University.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including preparation and characterization of materials, crystal parameters, IR and Raman spectra, TG curve, and calculation results. CCDC 2337198–2337204. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc01232a

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