Pancakes under Pressure: A Case Study on Isostructural Dithia- and

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Pancakes under Pressure: A Case Study on Isostructural Dithia- and Diselenadiazolyl Radical Dimers Wenjun Yong,† Kristina Lekin,# Robert P. C. Bauer,⊥ John S. Tse,⊥ Serge Desgreniers,‡ Richard A. Secco,† Naohisa Hirao,∥ and Richard T. Oakley*,# †

Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada ⊥ Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada ‡ Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada ∥ Materials Science Division, Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198, Japan

Inorg. Chem. Downloaded from pubs.acs.org by MACQUARIE UNIV on 02/26/19. For personal use only.

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S Supporting Information *

ABSTRACT: The isostructural dimers of the 1,4-phenylenebridged bis-1,2,3,5-dithia- and bis-1,2,3,5-diselenadiazolyl diradicals 1,4-S/Se are small band gap semiconductors. The response of their molecular and solid state electronic structures to pressure has been explored over the range 0− 10 GPa. The crystal structures, which consist of cofacially aligned (pancake) π-dimers packed into herringbone arrays, experience a continuous, near-isotropic compression. While the intramolecular covalent E−E (E = S/Se) bonds remain relatively unchanged with pressurization, the intradimer E···E separations are significantly shortened. Molecular and band electronic structure calculations using density functional theory methods indicate that compression of the π-dimers leads to a widening of the gap ΔE between the highest occupied and lowest unoccupied molecular orbitals of the dimer, an effect that offsets the expected decrease in the valence-to-conduction band gap Eg occasioned by pressure-induced spreading of the valence and conduction bands. Consistent with the predicted consequences of this competition between intra- and interdimer interactions, variable temperature high pressure conductivity measurements reveal at best an order-of-magnitude increase in conductivity with pressure for the two compounds over the pressure range 0− 10 GPa. While a small reduction in the thermal activation energy Eact with increasing pressure is observed, extrapolation of the rate of decrease suggests a projected onset of metallization (Eact ≈ 0) in excess of 20 GPa.

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INTRODUCTION It is well-known that changes in the physical properties of molecular solids can be induced by the application of relatively mild physical pressure (P < 10 GPa).1 Isotropic and anisotropic magnetic exchange interactions in metal complexes, for example, are particularly sensitive to small pressureinduced structural perturbations,2 as are exchange interactions and hence ordering temperatures in organic radical-based magnets.3 Improvements in the charge transport properties of radical conductors are also possible, leading to materials displaying bad metal4 and even Fermi liquid behavior.5 Metallization of closed-shell organics under pressure has proven more of a challenge,6 as a result of the generally large highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO−LUMO) separation ΔE coupled with weak intermolecular interactions, which lead to narrow bandwidths WVB and WCB for the valence and conduction bands and hence a consequently large band gap Eg (Figure 1a). The incorporation of heavy heteroatoms (S, Se, Te) to increase bandwidths and hence close the band gap has been explored, but attempts to generate a metallic state by the © XXXX American Chemical Society

Figure 1. Evolution of the HOMO−LUMO gap ΔE of a (a) closedshell molecule and (b) a cofacial radical π-dimer into the optical band gap Eg between the valence/conduction bands of the corresponding molecular solid.

application of pressure has met with limited success. For example, while enhancement of S-,7 Se-,8 and Te-based9 polyacenes has been observed, formation of a metallic state by Received: January 15, 2019

A

DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry closure of the valence-conduction band gap Eg usually requires pressures above 20 GPa, approaching the region where covalent bonds begin to rupture.10 Various molecular design strategies to reduce ΔE and hence Eg have been pursued, the most notable involving the use of internal salts11 and chargeseparated zwitterions,12,13 where the resulting small ΔE can be fine-tuned by substituent and heavy atom effects. With favorable packing motifs, semiconductors capable of metallization at pressures well below 10 GPa have been made.14 Small band gap semiconductors can also be prepared by using the closed-shell dimers of π-radicals as the molecular building blocks.15−17 These systems take advantage of the tendency of many planar π-radicals to associate cofacially in the solid state to produce a motif now popularized as a “pancake” π-bond.18 While dispersive effects may play an important role in the binding of these cofacial dimers, the interannular interactions are well described to a first approximation in terms of the weak π-overlap between two radical SOMOs, which gives rise to a small HOMO−LUMO gap ΔE for the dimer (Figure 1b). This small intradimer HOMO−LUMO splitting, when coupled with strong interdimer interactions in the solid state and consequently large solid state bandwidths WVB and WCB can afford a small band gap Eg.19 The structures of the π-dimers of many organic radicals, including various classes of thiazyls,20 have been explored; 1,2,3,5-dithiadiazolyl (DTDA) radicals and their seleniumbased variants diselenadiazolyls (DSDA) have received particular attention.21 While the latter bind weakly in solution,22 association can be suppressed in the solid state by the use of bulky ligands. 23 In most cases, however, dimerization is observed, the favored (but not exclusive) mode involving a pancake-type interaction between the two radical SOMOs. A range of solid state geometries for the resulting dimers is known, the most common (Figure 2a,b)

behavior, and preliminary high pressure (HP) measurements (P < 2 GPa) on the selenium-based materials indicated a small improvement in conductivity. Despite the apparently favorable initial response of the conductivity of these radical dimers to pressure, the lingering question has remained regarding the detailed effect of a pressure-induced compression of the dimers. Would the pressure-driven increase in bandwidth WVB and WCB be sufficient to close the band gap Eg or would the increasing intradimer interaction ΔE work against the interdimer interactions and favor a larger Eg value? To address this possibility, we have undertaken an experimental and theoretical analysis of the effect of pressure on the crystal and electronic structure of the matched pair of diradicals 1,4-S/Se. The results, derived from variable temperature HP conductivity and structural measurements combined with density functional theory band calculations, allow the conclusion that while bandwidths are indeed improved for both compounds by the application of pressure, the HOMO−LUMO separation is widened. The net result is that the overall response to pressure of the band gap Eg, and hence conductivity, is limited.

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RESULTS AND DISCUSSION Synthesis. The preparation of the two diradicals 1,4-S/Se followed the known procedure (Scheme 1) involving the

Scheme 1

conversion of 1,4-dicyanobenzene into the corresponding bispersilylated amidine,28 followed by condensation of the latter with sulfur monochloride or selenium dichloride to afford the bisdithia- and bisdiselenadiazolylium dichlorides [1,4-S/Se][Cl]2.17 Subsequent reduction of the latter with triphenylantimony liberates the corresponding diradicals 1,4-S/Se, and fractional sublimation in vacuo yields crystals of the dimers suitable for HP crystallographic and conductivity measurements. High Pressure Crystallography. Early single crystal X-ray diffraction studies on 1,4-S and 1,4-Se established that they are isostructural, belonging to the monoclinic space group P21/n.17 Unit cell parameters are listed in Table S1; a drawing of the herringbone packing pattern of 1,4-S dimers is shown in Figure 3, along with a summary of intramolecular metrics of both compounds at ambient pressure. In both cases the dimers consist of pairs of cofacially aligned diradicals associated at each end by a nominal 4-center 2-electron bond.29 Given the simplicity of the small, centrosymmetric unit cell and the thermal stability of the dimers, which remain locked into an essentially diamagnetic state up to 450 K,17 these materials represent ideal candidates for the variable temperature conductivity measurements described below. We have performed HP crystallographic measurements on both 1,4-S and 1,4-Se using synchrotron radiation and

Figure 2. (a) Superimposed and (b) herringbone π-stacked arrays of dimers of 1,2,3−5-DTDA (E = S) and DSDA (E = Se) radicals. Bifunctional 1,3- and 1,4-phenylene bridged radicals (c) 1,3-S/Se and (d) 1,4-S/Se.

corresponding to superimposed (Peierls distorted) 24 πstacks15,16,25 and herringbone arrays.17,26 From an electronic perspective, structures comprised of superimposed π-stacks are highly one-dimensional (1D), whereas the herringbone pattern is more conducive to lateral (2D) interactions. Early attempts to improve the dimensionality and hence electronic performance of these materials led to the development of bifunctional radicals,27 including the 1,3- and 1,4-phenylene-bridged materials 1,3-S/Se15a and 1,4-S/Se17 (Figure 2). In the solid state these dimers adopt π-stacked and herringbone packing patterns, respectively. Conductivity measurements on the Sebased variants confirmed small band gap semiconductive B

DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Herringbone packing of pancake π-dimers of 1,4-S, with (mean) intramolecular E−E bond distances, intradimer E···E (red) and interdimer E···E′ (blue) contacts in 1,4-E (E = S, Se) (from ref 17).

Figure 5. Contraction in the unit cell dimensions of (a) 1,4-S and (b) 1,4-Se over the pressure range 0−10 GPa.

of the ambient pressure single crystal solutions, that is, a single diradical unit; in the crystal structure, the full dimer is generated by inversion. While this approach worked well, affording solutions with good Rietveld refinements, the resulting contractions in interplanar S···S and Se···Se distances did not track smoothly with changes in pressure. To resolve this issue, we selected a subset of the initial solutions as starting points for a series of density functional theory (DFT)-based geometry optimizations. Using the experimentally derived lattice parameters, optimized crystal coordinates over the range 0−10 GPa were calculated using the recently developed strongly constrained and appropriately normed (SCAN) metageneralized gradient approximation for short- and intermediate-range interactions with the long-range van der Waals interaction from rVV10, the revised Vydrov-van Voorhis nonlocal correlation functional.30 This combination has shown to predict excellent geometries, binding energies, and band gaps for a large variety of materials, in particular, for layered compounds.31 Details of two of the resulting optimized crystal geometries are shown in Figure 6. These calculated

diamond anvil cell (DAC) techniques. Powder diffraction data were collected at room temperature as a function of increasing pressure up to 15 GPa, with helium as the pressure transmitting medium. As may be seen in Figure 4, which

Figure 4. X-ray powder diffraction patterns (λ = 0.412650 Å) for (a) 1,4-S and (b) 1,4-Se, recorded at regular pressure intervals from 0 to 13 GPa.

provides “waterfall” plots of the diffraction patterns for the two compounds, the retention of resolution and the uniform evolution of the positions of the diffraction peaks with increasing pressure indicate that there is neither a phase change nor sample degradation as a result of compression. The data sets from 0 to 13 GPa were indexed in the P21/n space group, and variations in the resulting unit cell dimensions for both compounds as a function of pressure are shown in Figure 5. All three axes display similar linear compressibilities, with the response of the a- and c-axes being slightly greater than the b-axis, the direction perpendicular to the herringbone stacks. After indexing, the structures were solved and refined in DASH using rigid-body models based on the asymmetric unit

Figure 6. DFT Calculated intra- and intermolecular distances (in Å) for 1,4-S and 1,4-Se.

geometries were then used as starting points for rigid body Rietveld refinements. Representative observed and calculated powder patterns for 1,4-S/Se are provided in Figures S1 and S2; refinement indices are provided in Table S1. The resulting set of DFT optimized crystal structures provides a reliable frame of reference for assessing structural changes in the two dimers as a function of pressure. Figure 7 illustrates the changes in (i) the mean intramolecular E−E C

DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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over the pressure range 0−10 GPa, with σRT reaching a value near 10−4 S cm−1 at 8 GPa. The behavior of 1,4-S is more unusual, with σRT at first rising from near 10−7 S cm−1 at 0 GPa, then falling slightly near 3 GPa before returning to increase slowly, approaching 10−6 S cm−1 at 12 GPa. The pressure dependence of Eact for 1,4-Se is consistent with its conductivity profile, affording a slow decrease from 0.4 eV at 0 GPa to 0.25 eV at 10 GPa. The variation in Eact for 1,4-S is more bimodal, like its conductivity dependence, with Eact rising to near 0.5 eV at 6 GPa, near the minimum in σRT, before falling to near 0.4 eV near 12 GPa. Interpretation of these results is provided below. Electronic Structure Calculations. The most notable feature of the HP conductivity measurements is the absence of a significant change in the values of σRT and Eact with increasing pressure. While a small increase in σRT and concomitant decrease in E act is observed for 1,4-Se, extrapolation of the results suggests that pressure-induced metallization would require pressures well in excess of 20 GPa. To probe more deeply the reasons for this poor response for both 1,4-S/Se, in particular the possibility of a “trade-off” between the effects of pressure on (i) intradimer hopping interactions t1, which increase the HOMO−LUMO gap ΔE, and (ii) interdimer hopping interactions t2, which increase valence/conduction bandwidths WVB/WCB and lead to closure of the band gap Eg (Figure 9),32 we carried out a series of DFT calculations at both the molecular and solid state level, using the DFT optimized molecular and solid state geometries.

Figure 7. (a, b) Calculated contraction over the pressure range 0−10 GPa in covalent E−E bonds, intradimer E···E pancake π-bonds, and interdimer E···E′ contacts in 1,4-S and 1,4-Se.

bond lengths, (ii) the mean intradimer separation E···E (d1, d2), and (iii) the mean interdimer separation E···E′ (d3, d4) along the herringbone spines of 1,4-S and 1,4-Se over the pressure range 0−10 GPa. The first result reflects the response of the internal covalent E−E bonds to pressure. Their length remains relatively constant, contracting by just 0.02/0.04 Å (1−2%) for E = S/Se, thereby confirming that the use of a rigid-body approximation represents a reasonable model for describing the molecular structure over this pressure range. The intradimer E···E bonds, however, are significantly more responsive, contracting by 0.22/0.21 Å (∼7%) for E = S/Se, from which we conclude that the “pancake” π-bonds of cofacially aligned DTDA/DSDA radical dimers are more compressible than conventional covalent bonds, and the resulting structures cannot be treated within a rigid-body approximation. Finally, the somewhat larger contraction in the interdimer E···E′ separations (0.49/0.41 Å for E = S/Se) is consistent with the expected ease of compression of intermolecular contacts. High Pressure Conductivity. To quantify and extend the earlier conductivity measurements on 1,4-Se, we explored the response of the room-temperature conductivity σRT and thermal activation energy Eact of both 1,4-S and 1,4-Se to applied pressure using multianvil press (MAP) techniques. For the Se-based compound, the conductivity rises with increasing pressure, as shown in Figure 8, but the response is sluggish, affording only an order of magnitude increase in conductivity

Figure 9. (a) HOMO−LUMO pairings of a model DTDA (R = H) radical SOMO and the resulting energy gap ΔE of a pancake π-dimer. (b) Idealized valence/conduction band gap Eg of herringbone arrays of π-dimers as a function of intra-and interdimer hopping integrals t1 and t2; α is the Coulomb parameter for a single radical SOMO.

As a first step, we performed single point DFT calculations at the B3LYP/6-31G(d,p) level to assess the energetic separation of the bonding and antibonding combinations of the two SOMOs of a cofacially linked pair of DTDA (DSDA) radicals (Figure 9), using molecular geometries extracted from the pressure-dependent solid state optimizations of 1,4-S and 1,4-Se described above. These take into account the small pressure-induced contraction in the covalent bonds as well as the more substantial compression of the intradimer contacts. The results, plotted in Figure 10a in terms of the eigenvalues of the Kohn−Sham frontier orbitals of the dimers as a function of pressure, show a widening of the HOMO−LUMO gap ΔE by

Figure 8. Pressure dependence of (a) the room-temperature conductivity σRT and (b) the thermal activation energy Eact of the pancake dimers of 1,4-S and 1,4-Se. D

DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

bands appears to be entirely offset by the widening of the HOMO−LUMO gap, so that at this level of theory, plots of the calculated direct band gap Eg (Figure 10b) show a slight increase for 1,4-Se rather than a decrease. However, in the case of 1,4-S there is a slight turnover near 7 GPa, as observed experimentally for the Eact value.

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SUMMARY AND CONCLUSIONS The combination of a small HOMO−LUMO gap and herringbone packing of the pancake π-dimers of 1,4-phenylene-bridged 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolyl radicals 1,4-S/Se affords small band gap semiconductive behavior. However, the use of physical pressure to enhance the conductivity of these materials is of limited value, as the pressure-driven widening of the HOMO−LUMO gap of the πdimer is sufficient to offset the effect of band broadening: “what is gained on the swings is lost on the roundabouts”. It remains to be seen whether a similar argument applies to structures based on superimposed pancake π-dimers, as found for the analogous 1,3-phenylene-bridged materials. In the meantime, the present study demonstrates that compression of pancake π-dimers in general is relatively facile, giving rise to a significant increase in the HOMO−LUMO splitting. As a result, pressure-driven metallization is less likely than for single closed-shell molecules, where internal covalent bonds, and the associated HOMO/LUMO energies, are less sensitive to the effects of pressure. The present behavior also differs from that of hypervalent σ-dimers of bis-selenathiazolyls, which, in some cases,34 undergo metallization near 5 GPa as a result of closure of the HOMO−LUMO gap driven by a pressure-induced molecular distortion.

Figure 10. (a) Calculated spreading of the HOMO and LUMO eigenvalues of model DTDA and DSDA π-dimers (R = H) over the pressure range P = 0−10 GPa. (b) Calculated direct band gap of 1,4-S and 1,4-Se over the pressure range 0−10 GPa.

0.62 eV for E = S and 0.56 eV for E = Se occasioned by an increase in intradimer hopping (overlap). The second step in assessing the relative importance of intraand interdimer interactions involved a series of band structure calculations based on the optimized crystal geometries of both 1,4-S and 1,4-Se as a function of pressure. These were performed with the Vienna ab initio simulation package33 using a plane wave basis set and projected augmented potentials to replace the atom core electrons. As demonstrated by the band dispersion diagrams (Figure 11) for the two

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EXPERIMENTAL SECTION

General Methods and Procedures. Samples of 1,4-S and 1,4-Se suitable for high pressure crystallographic and conductivity measurements were prepared as previously described17 and purified by dynamic fractional vacuum sublimation in an ATS series 3210 threezone tube furnace, mounted horizontally and linked to a series 1400 temperature control system. Crystallography. High pressure X-ray diffraction experiments on 1,4-S and 1,4-Se were performed at BL10XU, SPring-8, using synchrotron radiation (λ = 0.412650 Å) and powdered samples mounted in a diamond anvil cell, with helium as the pressure transmitting medium. Data sets were collected over the pressure range 0−15 GPa at ambient temperature (293 K) and as a function of increasing pressure, and the data from 0 to 12 GPa indexed for the monoclinic space group P21/n with DICVOL,35 as provided in DASH 3.36.36 Unit cell data as a function of pressure are shown in Figure 5. Initial structural solutions were developed using simulated annealing methods and a rigid body restriction using a model based on the ambient pressure single crystal coordinates. These were then used as starting points for constrained-cell optimizations of the atomic coordinates, as described below. Selected optimizations (at 4.1 and 7.6 GPa) were then refined by Rietveld methods37 using the GSAS program.38 Atomic positions were not refined and, as a result, standard deviations for atomic coordinates are not available; isotropic thermal parameters are taken from DASH refinements. Final Rietveld indices Rp and Rwp are listed in Table S1. Calculated and experimental PXRD patterns are illustrated in Figures S1 and S2. Multi-Anvil Press Conductivity Measurements. High pressure conductivity experiments on 1,4-S and 1,4-Se were performed in a 3000-ton multianvil press using a Cr2O3-doped MgO octahedron as the pressure transmitting medium.39 The pressure was generated by three electric oil pumps, transmitted through a split-cylinder module to six steel anvils, then to eight tungsten carbide (WC) cubes with 32 mm edge length, and finally through the eight truncated corners of

Figure 11. DFT band structures of 1,4-S at (a) 0 GPa and (b) 7.6 GPa and of 1,4-Se at (c) P = 0 GPa and (d) 7.6 GPa.

compounds at ambient pressure (0 GPa), the electronic structures are relatively two-dimensional, with significant crystal orbital dispersion from Z → Γ and Y → A but little along A → B. As expected, spreading of both the valence and conduction bands is slightly greater in the Se-compound, the estimated band gap for which (Eg = 0.8 eV) is in good agreement with the observed thermal activation energy Eact = 0.40 eV, assuming Eg ≈ 2 Eact. As illustrated by the dispersion plots at 10 GPa, bandwidths increase at elevated pressure, particularly for the valence band. However, the resulting band gap contracts very little. To the contrary, the spreading of the E

DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry these cubes to the octahedral pressure medium. The force−pressure relationships for the 18/11 (octahedral-edge-length (mm)/truncatededge-length (mm)) cell configurations adopted in these experiments were determined from prior calibrations of the applied hydraulic load against pressures of structure transformations in standards at room temperature (Bi I ↔ II at 2.55 GPa, Bi III ↔ V at 7.7 GPa, Sn I ↔ II at 9.4 GPa, and Pb I ↔ II at 13.4 GPa). The pressure cell was modified to include a cylindrical heater made from a rhenium (Re) foil of 0.05 mm thickness and a type C thermocouple with its junction placed in contact with the outside wall of the Re heater. Powder samples were densely packed in a boron nitride (σBN = 10−11 S cm−1) cup with Pt disk electrodes in direct contact with the samples at both ends. Four wire AC (Solartron 1260 impedance analyzer) resistance measurements were made at a frequency of 1 kHz. A series of resistance measurements was performed at pressures up to 10 GPa and temperatures ranging from 298 to 400 K to derive Arrhenius activation energies. In each case, the pressure was first increased to the target value, and then resistance measurements were made at fixed temperature intervals of 10 K on heating/cooling at constant pressure. The contiguous cylinder-shaped sample was extracted from the recovered pressure cell, and the sample geometry was measured to convert resistance to conductivity. The effects of lead resistance have been applied to the data. For convenience, the term σRT refers to the conductivity at 298 K. Electronic Structure Calculations. Rigid-cell DFT-based geometry optimizations of the solid state structures of 1,4-S and 1,4-Se over the pressure range 0−10 GPa were performed starting from the experimentally derived lattice parameters, using the recently developed strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation for short- and intermediaterange interactions with the long-range van der Waals interaction from rVV10, the revised Vydrov-van Voorhis nonlocal correlation functional.30 All calculated crystal structures (Tables S2 and S3) were fully optimized with a force tolerance smaller than 0.002 eV/atom. Molecular HOMO−LUMO gaps as a function of pressure were calculated with atomic coordinates from the geometry optimizations, using with the Gaussian 09 suite of programs;40 the B3LYP functional and a polarized, split-valence basis set with double-ζ (6-31G(d,p)) functions were employed. All solid state calculations were performed with the Vienna Ab initio simulation package41 using a plane wave basis set and projected augmented potentials to replace the atom core electrons. A plane wave energy cutoff of 450 eV was used throughout. A definition of the k-point path used in the band structure calculations is provided in Table S4.

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Richard A. Secco: 0000-0001-5029-659X Richard T. Oakley: 0000-0002-7185-2580 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council of Canada (NSERCC) for financial support. We also thank the Japan Synchrotron Radiation Research Institute (JASRI) for beam time, the Canada Foundation for Innovation for funding to R.A.S. for a 3000 ton multianvil press, and the Government of Canada for a Tier I Canada Research Chair to J.S.T.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00142. Details of crystal and PXRD refinements, and DFT calculations (PDF) Accession Codes

CCDC 1891060−1891063 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

(1) (a) Grochala, W.; Hoffmann, R.; Feng, J.; Ashcroft, N. W. The Chemical Imagination at Work in Very Tight Places. Angew. Chem., Int. Ed. 2007, 46, 3620. (b) MacMillan, P. F. Chemistry at high pressure. Chem. Soc. Rev. 2006, 35, 855. (2) (a) Woodall, C. H.; Craig, G. A.; Prescimone, A.; Misek, M.; Cano, J.; Faus, J.; Probert, M. R.; Parsons, S.; Moggach, S.; MartínezLillo, J.; Murrie, M.; Kamenev, K. V.; Brechin, E. K. Pressure induced enhancement of the magnetic ordering temperature in rhenium (IV) monomers. Nat. Commun. 2016, 7, 13870. (b) Craig, G. A.; Sarkar, A.; Woodall, C. H.; Hay, M. A.; Marriott, K. E.R.; Kamenev, K. V.; Moggach, S. A.; Brechin, E. K.; Parsons, S.; Rajaraman, G.; Murrie, M. Probing the origin of the giant magnetic anisotropy in trigonal bipyramidal Ni(II) under high pressure. Chem. Sci. 2018, 9, 1551. (c) Parois, P.; Moggach, S. A.; Sanchez-Benitez, J.; Kamenev, K. V.; Lennie, A. R.; Warren, J. E.; Brechin, E. K.; Parsons, S.; Murrie, M. Pressure-induced Jahn-Teller switching in a Mn12 nanomagnet. Chem. Commun. 2010, 46, 1881. (d) Prescimone, A.; Morien, C.; Allan, D.; Schlueter, J. A.; Tozer, S. W.; Manson, J. L.; Parsons, S.; Brechin, E. K.; Hill, S. Pressure-Driven Orbital Reorientations and CoordinationSphere Reconstructions in [CuF2(H2O)2(pyz)]. Angew. Chem., Int. Ed. 2012, 51, 7490. (3) (a) Mito, M.; Komorida, Y.; Tsuruda, H.; Tse, J. S.; Desgreniers, S.; Ohishi, Y.; Leitch, A. A.; Cvrkalj, K.; Robertson, C. M.; Oakley, R. T. Heavy Atom Ferromagnets under Pressure: Structural Changes and the Magnetic Response. J. Am. Chem. Soc. 2009, 131, 16012. (b) Thomson, R. I.; Pask, C. M.; Lloyd, G. O.; Mito, M.; Rawson, J. M. Pressure-Induced Enhancement of Magnetic-Ordering Temperature in an Organic Radical to 70 K: A Magnetostructural Correlation. Chem. - Eur. J. 2012, 18, 8629. (c) Lekin, K.; Ogata, K.; Maclean, A.; Mailman, A.; Winter, S. M.; Assoud, A.; Mito, M.; Tse, J. S.; Desgreniers, S.; Hirao, N.; Dube, P. A.; Oakley, R. T. Pushing TC to 27.5 K in a heavy atom radical ferromagnet. Chem. Commun. 2016, 52, 13877. (d) Winter, S. M.; Hill, S.; Oakley, R. T. Magnetic Ordering and Anisotropy in Heavy Atom Radicals. J. Am. Chem. Soc. 2015, 137, 3720. (e) Thirunavukkuarasu, K.; Winter, S. M.; Beedle, C. C.; Kovalev, A. E.; Oakley, R. T.; Hill, S. Pressure dependence of the exchange anisotropy in an organic ferromagnet. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 014412. (f) Irie, K.; Shibayama, K.; Mito, M.; Takagi, S.; Ishizuka, M.; Lekin, K.; Oakley, R. T. High-pressure DC magnetic measurements on a bisdiselenazolyl radical ferromagnet using a vibrating-coil SQUID magnetometer. Phys. Rev. B: Condens. Matter Mater. Phys. 2019, 99, 014417. (4) (a) Leitch, A. A.; Lekin, K.; Winter, S. M.; Downie, L. E.; Tsuruda, H.; Tse, J. S.; Mito, M.; Desgreniers, S.; Dube, P. A.; Zhang, S.; Liu, Q.; Jin, C.; Ohishi, Y.; Oakley, R. T. From Magnets to Metals: The Response of Tetragonal Bisdiselenazolyl Radicals to Pressure. J. Am. Chem. Soc. 2011, 133, 6051. (b) Mailman, A.; Winter, S. M.; Yu, X.; Robertson, C. M.; Yong, W.; Tse, J. S.; Secco, R. A.; Liu, Z.; Dube, P. A.; Howard, J. A. K.; Oakley, R. T. Crossing the Insulator-to-Metal Barrier with a Thiazyl Radical Conductor. J. Am. Chem. Soc. 2012, 134, 9886.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wenjun Yong: 0000-0002-2114-6100 John S. Tse: 0000-0001-8389-7615 F

DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00142 Inorg. Chem. XXXX, XXX, XXX−XXX