Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry1,2,3. These mechanochemical mechanisms have been studied extensively in one-dimensional polymers under tensile stress4,5,6,7,8,9 using ring-opening10 and reorganization11, polymer unzipping6,12 and disulfide reduction13,14 as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation15. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components—a compressible (‘soft’) mechanophore and incompressible (‘hard’) ligands. In these ‘molecular anvils’, isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal–organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility16,17,18,19, in which bending of bond angles or shearing of adjacent chains activates the metal–chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.
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We thank C. Beavers, J. Yan and S. Teat from the Advanced Light Source for help with XRD measurements, and C. Park and D. Popov from the Advanced Photon Source for XAS measurements. This work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, under contracts DE-AC02-76SF00515 and DE-FG02-06ER46262. D.P. acknowledges support from Hong Kong Research Grants Council (project number ECS-26305017), the National Natural Science Foundation of China (project number 11774072) and the Alfred P. Sloan Foundation through the Deep Carbon Observatory. D.S.-I. acknowledges support from PAPIIT IA203116/27 and CONACYT FC-2015-2/829. This research used resources of the Advanced Light Source, which is a US Department of Energy (DOE) Office of Science User Facility under contract DE-AC02-05CH11231. This research also used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract DE-AC02-06CH11357. Portions of this work were performed at the Stanford Nano Shared Facilities, supported by the National Science Foundation under award ECCS-1542152. The computational work used resources at the Stanford Research Computing Center, the Research Computing Center at the University of Chicago, and the Deep Carbon Observatory computer cluster.
The authors declare no competing financial interests.
Reviewer Information Nature thanks D. Braga, S. James and L. Yan for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, Relative motions (red arrows) of the rigid ligands under hydrostatic pressure anisotropically deform the mechanophore, leading to reactivity. b, Motion of the ligands is blocked by their steric repulsion, protecting the mechanophore from deformation.
a, Cu-S-M9. b, Ag-S-Ada. c, Ag-S-mDia. Each panel depicts the asymmetric unit (left) and the unit cell (right). Atoms are represented by their thermal ellipsoids at the 50% probability level. Copper, silver, sulfur, carbon and boron are denoted by red, green, yellow, grey and pink respectively. Hydrogen atoms are omitted for clarity. The toluene molecules (depicted in grey) reside in the interstitial spaces between Cu-S-M9 molecules with a 1:1 molar ratio.
a, TEM image of uncompressed Cu-S-M9. b, TEM image of Cu-S-M9 after compression to 8 GPa. Scale bars in a and b, 100 nm. c, Zoom-in view of b. Scale bar, 10 nm. No inorganic lattice structures are identified in these samples. d, Representative EDS of unreacted Cu-S-M9. Both Cu and S peaks can be seen. The asterisks mark Ni peaks originating from the TEM grid. The star marks the position of the Si K-edge, an impurity probably introduced during TEM sample preparation.
Red and blue represent uncompressed Cu-S-M9 and sample after compression to 12 GPa, respectively. Discrete dots and solid continuous lines denote experimental data and fitting, respectively. a, The S 2p spectra are fitted to two Voigt peaks representing 2p1/2 and 2p3/2 peaks (dashed lines). The peak position of the uncompressed sample, 162 eV, is characteristic of metal thiolates. The upshift of the binding energy to 163 eV after compression can be attributed to the oxidation of sulfur to form, for example, disulfides. b, The Cu 2p spectra consist of two peaks, namely 2p1/2 at about 936 eV and 2p3/2 at about 954 eV. The absence of satellite peaks excludes the formation of Cu(ii) species. Moreover, the 2p3/2 peak of the uncompressed sample is best fitted by two Voigt peaks (the shoulder peak is denoted by the dashed line and marked by the arrow), characteristic of Cu(i). The 2p3/2 peak of the compressed sample is best fitted with a single Voigt peak, consistent with Cu(0). These features support the conclusion that copper is in the +1 valence state in the pristine sample, and reduced to the zero valence state upon compression to 12 GPa.
Peaks are registered to β-Cu2S (chalcocite, PDF Number 00-026-1116). The pyrolysis product of Cu-S-Ada (red) is more crystallized than that of Cu-S-M9 (blue); however, the two strongest peaks, (110) at 20.75° (d = 1.96 Å) and (103) at 21.85° (d = 1.87 Å) can be clearly seen in both samples. Pyrolysis was carried out at 400 °C in sealed quartz tubes under vacuum. XRD was recorded with a MoKα source (λ = 0.7107 Å).
The dashed line shows ambient XRD calculated from the single-crystal structure. The solid lines are in situ XRD measured at 0.5 GPa (black), 6.1 GPa (red) and back to 0.8 GPa (blue). λ = 0.6199 Å.
Extended Data Figure 7 Experimental and computed unit cell volumes and key structural features of Cu-S-M9 at high pressure.
a, Unit cell volumes determined by experiment and computed by DFT using different exchange-correlation functionals. b–d, S1–S2 distance (b), S1–Cu1–S2 bond angle (c) and average Cu1–S1/Cu1–S2 bond length (d) computed using PBE (red) and vdW-DF (blue) functionals. The unit cell volumes computed by PBE, vdW-DF and vdW-DF2 agree well with experimental data, with less than 5% deviation. The vdW-DF-cx functional gave consistently smaller unit cell volume, probably owing to overestimation of the dispersion interaction. Furthermore, PBE and vdW-DF show the same trends in the key structural changes, that is, decreasing S1–S2 distance and S1–Cu1–S2 bond angle, as well as increasing Cu–S bond length beyond 6 GPa. These results show that the van der Waals interactions do not substantially affect the computed high-pressure structures in our systems.
a, Structure of the Cu-S-M9 molecule. b–d, Changes in S–S distances (b), M9–M9 distances (c) and S–Cu–S bond angles (d) as functions of pressure. All data are extracted from DFT computations.
a, At ambient pressure; b, at 12 GPa. The isosurfaces depict the probability density times the sign of the HOMO wavefunction ψ. Red and blue represent isovalues of ±2 × 10−4 per cubic bohr, respectively. The dashed lines in the 12 GPa structure mark the nodal planes across Cu–S bonds.
a, b, Cu-S-Ada before (a) and after (b) 20 GPa compression. Scale bars in a and b, 1 μm. c, Ag-S-Ada after 20 GPa compression. Scale bar, 20 nm. Inset, Fourier transform pattern of c.
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Yan, H., Yang, F., Pan, D. et al. Sterically controlled mechanochemistry under hydrostatic pressure. Nature 554, 505–510 (2018). https://doi.org/10.1038/nature25765
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