Flexibility transition and guest-driven reconstruction in a ferroelastic metal-organic framework†Electronic supplementary information (ESI) available: Atomic coordinates and lattice parameter data. CCDC 1016797. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ce01572jClick here for additional data file

Abstract

The metal-organic framework copper(i) tricyanomethanide, Cu(tcm), undergoes a ferroelastic transition on cooling below T_f = 240 K. Thermal expansion measurements reveal an order-of-magnitude variation in framework flexibility across T_f. The low-temperature phase α-Cu(tcm) exhibits colossal positive and negative thermal expansion that is the strongest ever reported for a framework material. On exposure to acetonitrile, Cu(tcm) undergoes a reconstructive solid-phase transition to acetonitrilocopper(i) tricyanomethanide. This transition can be reversed by heating under vacuum. Infrared spectroscopy measurements are sensitive to the phase change, suggesting that Cu(tcm) may find application in solid-phase acetonitrile sensing.

Fig. 1. The dominant low-energy deformation mode of ‘wine-rack’ framework materials involves ‘breathing’ via changes in the framework angle θ. For the ideal square geometry in (a) this deformation is forbidden by the presence of fourfold rotation axes. Consequently the change in angle from square (a) to rectangular (b) geometries represents a symmetry-breaking ferroelastic phase transition, for which the deviation of the angle θ from 90° is related to the order parameter ε = (b − a)/(a + b). Breathing of the lower-symmetry rectangular geometry is allowed by the remaining symmetry elements (such as the diads shown here), such that the progression from geometries (b) to (c), for example, is continuous in all respects.

Fig. 2. Powder X-ray diffraction data measured using Cu Kα radiation (black lines), with reflection positions indicated by vertical tick marks. (a) β-Cu(tcm), measured at 298 K; Rietveld fit (red lines), and difference (data – fit, blue lines). (b) α-Cu(tcm), measured at 20 K. Impurity peaks from the low-temperature sample environment are indicated by asterisks; both Pawley (red lines) and Rietveld (blue lines) fit and difference curves are shown, with the latter shifted vertically by 0.5 and 1.5 units. Pawley fits were used to extract thermal lattice parameter variations. The quality of the Rietveld fit is low as a result of severe anisotropic peak broadening and the large number of coupled positional degrees of freedom, but is sufficient to demonstrate the displacive nature of the phase transition.

Fig. 3. (a) Representation of the crystal structure of β-Cu(tcm), viewed with the tetragonal c axis oriented vertically. Thermal ellipsoids are represented at 50% probability. (b) Topological representation of the same framework, where Cu atoms are shown as large spheres, tcm moieties in stick representation, and the unit cell in black. The three mutually-interpenetrating (10,3)-b frameworks are coloured differently. The inset shows the chemical structure of the tricyanomethanide anion. (c) The same framework, viewed slightly away from c, illustrating its square wine-rack geometry with θ = 90°. (d) The low-temperature α-Cu(tcm) phase—shown here in terms of the structural model determined using powder X-ray diffraction data collected at 20 K—has the same network connectivity, but the loss of tetragonal symmetry allows θ to deviate from 90°. The Fdd2 unit cell, shown here in black, has twice the volume of the I4₁md cell of β-Cu(tcm).

Fig. 4. (a) Thermal variation in the X-ray powder diffraction pattern of Cu(tcm), shown here over a 2θ range that illustrates the large splitting of certain peaks at T_f = 240 K. (b) The corresponding thermal variation in lattice parameters. The parameters a and b are given by green and red circles, respectively, for the orthorhombic phase and black circles for the tetragonal phase; the c parameter is given by blue circles for both phases. Uncertainties are smaller than the symbols. The black line represents the mean value (a + b) for α-Cu(tcm). (c) The approximately-linear thermal variation of the c lattice parameter, as shown in (b) but given here on an expanded scale. (d) Thermal variation of the spontaneous strain as defined in eqn (2); the corresponding fit of form is shown as a solid line. The strain observed for the “conventional” ferroelastic transition of As₂O₅ is shown in red, where the temperature scale has been shifted by 345 K in order to match transition temperatures.

Fig. 5. (a) Thermogravimetric behaviour of Cu(tcm) and [Cu(MeCN)(tcm)]. The mass loss observed near 400 K for the latter corresponds to volatilisation of one molar equivalent of MeCN. Both TGA curves have been normalised to the mass measured at 420 K (b). High-frequency region of the infrared absorption spectra of Cu(tcm) and [Cu(MeCN)(tcm)] showing the emergence of an additional band in the latter, attributed to the nitrile C–N stretch. Colour scheme as in (a).

Fig. 6. (a) Representation of the crystal structure of [Cu(MeCN)(tcm)], viewed away from a, as determined using single-crystal X-ray diffraction measurements carried out at 100 K. Thermal ellipsoids are drawn at 50% probability level. Panels (b) and (c) show topological representations of, respectively, this same framework and that of [Ag(MeCN)(tcm)] as reported in ref. 42. Both structures are assembled from honeycomb (6,3) nets of connected Cu/Ag cations and tcm^- anions. In (b) these nets stack such that the MeCN ligands of one net interdigitate the hexagonal channels of the next. In (c) pairs of adjacent nets interweave, with the MeCN groups occupying the space between adjacent honeycomb pairs. The corresponding unit cells for both systems are shown in black; the Cu/Ag and tcm components of two adjacent honeycomb nets (but not the coordinated MeCN groups) are shown in space-filling representation.

Fig. 7. (Top) Powder X-ray diffraction patterns of (a) as-prepared Cu(tcm), (b) the same sample after exposure to MeCN, and (c) after subsequent heating at 200 °C for 3 h. (Bottom) Calculated X-ray diffraction patterns of (d) [Cu(MeCN)(tcm)] and (e) Cu(tcm).