Symmetry breakings in a metal organic framework with a confined guest

The MOF $\left[{\text{Fe(tvp)}}_2{\left(\text{NCS}\right)}_2\right]·2\text{BzCHO}$ is demonstrated to undergo a complex sequence of phase transitions and spin-crossover behavior of its constitutive ${\text{Fe}}^{\text{II}}$ ions upon adsorption of benzaldehyde guest molecules. Our study, combining Raman and synchrotron x-ray diffraction measurements on a single crystal, reveals that the conversion from the pure high-spin to the pure low-spin phases implies a rich sequence of intermediate phases, with symmetry breaking forming at least three different space groups. These different symmetries involve spin-state ordering, ligand ordering, and guest ordering, interpreted by combining magnetic susceptibility, Raman measurements, density functional theory, and force field molecular dynamics calculations.

Figure 1

(a) Molecular model used for DFT calculations (red: Fe, blue: N, gray: C, yellow: S). (b) ${\chi }_{M}T$ product vs temperature of the empty host system upon cooling. Inset: Representation of the host structure showing the 3D interpenetrated MOF framework of the $\left[{\text{Fe(tvp)}}_2{\left(\text{NCS}\right)}_2\right]·2\text{BzCHO}$. The 2BzCHO guest is omitted for clarity. (c) ${\chi }_{M}T$ product vs temperature of this host/guest system upon warming (in gray) and upon cooling (in black). Right red axis indicates the corresponding $x_{\text{HS}}$ ratio, assuming a residual paramagnetic contribution equal to 0.025 ${\mathrm{cm}}^3$ K ${\mathrm{mol}}^{-1}$ [16].

Figure 2

The diffraction image of the (a*, b*) plane in three different phases: (a) Phase I, measured at $T=250$ K. (b) Phase II, measured at $T=210$ K with the appearance of $h+k$ odd Bragg peaks associated with the loss of a gliding plane. The unit cell of these high symmetry phases is shown in blue. (c) Phase III, measured at $T=195$ K: the superstructure Bragg peaks are described with the new unit cell shown in red, ${\mathbf{a}}^{*\prime }=({\mathbf{a}}^{*}-{\mathbf{b}}^{*})/4,{{\mathbf{b}}^{*}}^{\prime }=({\mathbf{a}}^{*}+{\mathbf{b}}^{*})/2$. (d) Schematic representation of the reciprocal cells showing the axis changes between phases II and III.

Figure 3

(a) Temperature evolution of Bragg peaks appearing in phases II below $T_{c1}$ and III below $T_{c2}$. In black circles, Bragg peaks appearing in phase II due to the loss of glide planes (averaged over 16 Bragg peaks, normalized), and in black triangles, superstructure Bragg peaks revealing the cell multiplication in phase III (averaged over 309 peaks). (b) and (c) Temperature evolution (on cooling) of unit cell volume (c) and the cell parameters (b) in the unit cell basis from the high temperature tetragonal phase. $T_{c1}$ and $T_{c2}$ indicated as red vertical lines are extracted from intensities evolutions shown in (a). Dashed vertical black line indicates the anomaly correlated with the one observed in magnetic measurements.

Figure 4

Schematic representation of the host structures in the low symmetry phases. The guest molecules are omitted for clarity. (a) Phase II: View in the (a, b) plane. (b) Phase II: View in the (b, c), showing the two interpenetrated lattice (in gray and magenta), displaced in opposite direction from the high symmetry position (1/4 and 3/4 along c). (c) Phase III: View in the (a, b) plane showing the large 4 by 2 unit cell (the high symmetry tetragonal cell is shown in light gray for reference). Independent Fe sites are marked with different symbols. (d) Phase III: Possible spin state ordering compatible with the symmetry depicted in (c) and the 1/4 high-spin fraction on the plateau at $T=175$ K.

Figure 5

(a) Calculated Raman spectra within the frequency 950–1250 ${\mathrm{cm}}^{-1}$ in the high-spin (orange) and low-spin state (blue). (b) Measured Raman spectra in the corresponding frequency range at $T=270$ K and $T=100$ K.

Figure 6

(a) Temperature evolution of Raman spectra measured between 1000–1045 ${\mathrm{cm}}^{-1}$. (b) Temperature evolution of the related peak amplitudes. (c) Temperature evolution of the frequency of three Raman peaks, extracted from a fit of pattern (a) with a sum of Gaussian peaks; in (b) and (c) the vertical red lines points $T_{c1}=212$ K and $T_{c2}=201$ K as extracted from the x-ray diffraction study. (d) Schematic representation of the most intense calculated Raman mode in the corresponding wave number range. The vectors indicate the direction of the force momentum (red: Fe, blue: N, gray: C, hydrogen atoms are omitted for clarity).

Figure 7

(a) Temperature evolution of Raman spectra measured between 1195–1220 ${\mathrm{cm}}^{-1}$. (b) Temperature evolution of the frequency of the peak (extracted from a fit with a Gaussian function) showing a softening around the phase transition, and (c) of the corresponding peak width ($\sigma$); in (b) and (c) the vertical red lines points $T_{c1}=212$ K and $T_{c2}=201$ K as extracted from an x-ray diffraction study. (d) Schematic representation of the calculated Raman mode at 1205 ${\mathrm{cm}}^{-1}$ in the HS state. The vectors indicate the direction of the force momentum (red: Fe, blue: N, gray: C, hydrogen atoms are omitted for clarity).

Figure 8

(a) Radial distribution functions between the center of mass of the guest molecules. (b) Angular distribution of the angle between $u_1$ and $u_2$ for both LS and HS forms (normalized to the value at 180 deg).

Figure 9

(a) Radial distribution functions between the oxygen atom of the benzaldehyde and the hydrogen atom bound to the carbon atom of the MOF-SCO material. (b) Illustration of characteristic $\mathrm{CH}\cdots \mathrm{O}$ of the first peak of RDF, showing one representative configuration of the host/guest interactions implying hydrogen bonds.

Figure 10

(a) Radial distribution functions between the oxygen atom of one guest molecule of BzCHO and the hydrogen atom of another guest molecule. (b) Illustration of characteristic $\text{O}\cdots \text{H}$ of the first peak of the RDF, showing one over many configurations of the arrangement of the guest dimers along the channel direction (red: O, gray: C, white: H). (c) High temperature structure as solved based at $T=250$ K on x-ray synchrotron data revealing the absence of apparent ordering of the guest molecules. The gold peaks show residual electronic density in the middle of the channels, indicating the possible position of the disordered guest molecules.