SUPPLEMENTARY INFORMATION

doi:1.138/nature1743 Supplementary Methods and Discussion 1. Synthesis and routine analysis of DUT-49 Cu(NO 3 ) 2 3H 2 O (Sigma Aldrich, 99.5%), N-methyl-2-pyrrolidone (NMP) (AppliChem, 99%), and ethanol (anhyd.) (VWR Prolabo) were used for the synthesis of DUT-49 (C 4 H 2 N 2 O 8 Cu 2 ). The ligand 9,9'-([1,1'-biphenyl]-4,4'- diyl)bis(9h-carbazole-3,6-dicarboxylic acid) H 4 BBCDC was synthesized following a procedure previously reported by our group. The synthetic protocol and analytic data can be obtained from reference 1. To obtain crystallites small enough for the in situ PXRD investigations a slightly different synthetic protocol was used to synthesize DUT-49: H 4 BBCDC (2 g, 3.3 mmol) was dissolved in 36 ml of NMP at 8 C in a 5 ml Schott bottle. Cu(NO 3 ) 2 3H 2 O (1.53 g, 6.29 mmol) was added to the hot solution and dissolved by sonication at 6 C. The resulting solution was heated at 8 C for 24 h. The green precipitate was washed with fresh NMP several times until the solvent appeared colorless. The NMP was replaced by ethanol during multiple washing cycles and the material was dried using a protocol involving supercritical CO 2. DUT-49 powder suspended in ethanol was placed in glass filter frits in a Jumbo Critical Point Dryer 132J AB (SPI Supplies) which was subsequently filled with liquid CO 2 (99.995% purity) at 15 C and 5 bar. To ensure a complete substitution of ethanol by CO 2, the liquid in the autoclave was exchanged with fresh CO 2 18 times over a period of 5 days using a valve at the bottom. The temperature and pressure was then raised beyond the supercritical point of CO 2 to 35 C and 1 bar and kept until the temperature and pressure was constant. The supercritical CO 2 was steadily released over 3 hours and the dry powder was transferred and stored in an argon filled glove box. To ensure a complete removal of ethanol (especially from the open metal sites of the Cu-paddlewheels) additional activation at 5 C in a Schlenk-tube under dynamic vacuum of 1-3 mbar for 24 hours was performed. A dark blue to violet powder with a yield of 53 % (1.28 g) was obtained and stored under inert gas. The compound turns to a light blue color upon contact to ethanol or water indicating solvation. For routine analysis the synthesized and activated DUT-49 was filled in capillaries with.5 mm inner diameter and powder X-ray diffraction (PXRD) patterns were collected in transmission geometry with a STOE STADI P diffractometer operated at 4 kv and 3 ma with monochromated Cu-Kα 1 (λ =.1545 nm) radiation, a scan speed of 3 s/step and a step size of.1 2Ѳ. Thermogravimetric analysis (TGA) was carried out in air atmosphere using a NETZSCH STA 49 thermal analyzer at a heating rate of 5 K min -1 (Figure S1b). Elemental analysis (C, H, N) was performed with a HEKATECH EA 3 Euro Vector CHNS analyzer (C 4 H 2 N 2 O 8 Cu 2 : Calculated: C: 61.3%, H: 2.57%, N: 3.57%; Found: C: 59.48 %, H: 2.51%, N: 3.72%). Phase purity was also confirmed by PXRD (Figure S1a). Scanning electron microscopy (SEM) images of DUT-49 before and after adsorption of methane at 111 K as well as after exposure to ambient humid atmosphere for 2 h were taken with secondary electrons in a HITACHI SU82 microscope using 1. kv acceleration voltage and 1.8 mm working distance. WWW.NATURE.COM/NATURE 1

a) DUT-49 after ads. n-butane at 298 K DUT-49 activated DUT-49 as made DUT-49 op theor. b) 12 11 1 DUT-49 activated before adsorption experiment DUT-49 after ads. - des. of n-butane at 298 K Normalized intensity (a. u.) Weight loss (%) 9 8 7 6 5 4 3 2 5 1 15 2 25 3 35 4 2 degree 1 2 3 4 5 6 T ( C) Figure S1. a) PXRD patterns of DUT-49: Theoretical DUT-49op (calculated from the crystal structure) in red; as made material in orange, activated material in green, and after n-butane adsorption at 298 K in blue; b) TGA of DUT- 49 performed in air before and after n-butane adsorption at 298 K. Recyclability of the observed adsorption behavior and phase transitions was investigated and proven by in situ PXRD experiments (Figure S31) under specific conditions during adsorption and desorption. However, if the gas molecules were completely removed from the sample by evacuation at p < 1-3 mbar and the sample was warmed up to ambient temperature and exposed to nitrogen from the adsorption instrument after automated analysis, a change in sample color from dark blue to light green along with an amorphization of the material was observed. A similar color change was observed after all performed adsorption-desorption cycles that showed a NGA transition using normal desorption routines. In contrast, the routine described in 2.2 allows reversible cycling of the isotherm and NGA-step while preserving DUT-49op even after complete guest removal. Powder of DUT-49 handled under Ar-atmosphere before and after adsorption of methane at 111 K was investigated by SEM (Figure S2 and Figure S3). a) b) Figure S2. SEM images of DUT-49 before adsorption experiment. Scale bars correspond to a) 1 µm and b) 5 µm. WWW.NATURE.COM/NATURE 2

a) b) Figure S3. SEM images of DUT-49 after adsorption-desorption cycle kept under inert atmosphere. Scale bars correspond to a) 1 µm and b) 5 µm. a) b) Figure S4. SEM images of DUT-49 after adsorption-desorption cycle kept under ambient humid atmosphere for 2 h. Scale bars correspond to a) 1 µm and b) 5 µm. No drastic change of the average crystal size as well as outer appearance of the cubic shaped crystals in the size range of 1-5 µm (Figure S2 and Figure S3) could be detected. However, after the crystals were exposed to humidity the crystals undergo a drastic change in texture (Figure S4) indicating a decomposition of the framework. 2. Volumetric adsorption experiments Volumetric adsorption experiments were carried out on a BELSORP-max and gases with high purity were used (N 2 : 99.999%, CH 4 : 99.995%, n-butane: 99.95, He: 99.999 %). For volumetric adsorption experiments the measuring routine of BELSORP-max was used. Targeted relative pressures in the range of.1 1 kpa were defined and limits of excess and allowance amount were set to 1 and 2 cm 3 g -1, respectively. If not mentioned differently, equilibration conditions for each point were: 1% pressure change within 35 s. The dead volume was routinely determined using Helium. Values for the adsorbed amount of gas in the framework are all given at standard temperature and pressure (STP). Information about the measuring routine and algorithm were extracted from the BELSORP-max Instruction Manual Ver. 1.3.3. 2.1 Volumetric adsorption of nitrogen at 77 K For adsorption experiments with N 2 at 77 K the sample cell was cooled with liquid nitrogen in a Dewar vessel. WWW.NATURE.COM/NATURE 3

8 N 2 adsorbed (mmol g -1 ) 7 6 5 4 3 2 1 Ads. N 2 at 77 K before n-butane ads. Des. N 2 at 77 K before n-butane ads. Ads. N 2 at 77 K after n-butane ads. Des. N 2 at 77 K after n-butane ads. 1 2 3 4 5 6 7 8 9 1 p (kpa) Figure S5. N 2 adsorption isotherm at 77 K of DUT-49 before (green) and after (blue) adsorption-desorption cycle of n-butane at 298 K (see isotherm of butane adsorption Figure S1). Similar to the PXRD and TGA the N 2 adsorption of DUT-49 at 77 K is strongly altered after an adsorption-desorption cycle of n-butane at 298 K. A decrease in uptake of 8 % and type I shape of the isotherm confirm pore shrinkage and decrease in pore volume (Figure S5). 2.2 Volumetric adsorption of methane For methane adsorption experiments a closed cycle He-cryostat was used to set the adsorption temperature in the range of 91 121 K. The cryostat DE-22AG was operated by a temperature controller LS-336 (Lake Shore) and the heat produced by the cryostat is removed from the system by a helium compressor ARS-2HW. The sample was placed in a Cu-cell and connected to the BELSORP-max adsorption instrument with a Cu-capillary. To avoid condensation of moisture, the cell was isolated from the atmosphere with a casket. The space between the cell and the casket was evacuated (p = 1-4 mbar). DUT-49 samples were transferred into the measuring cells in a glove-box under Aratmosphere (H 2 O, O 2 <.1 ppm). Before each adsorption experiment the sample was treated at 15 C in vacuum for 3 h. After the measurement the samples were transferred back into the glove-box. Two consecutive adsorption-desorption experiments on one DUT-49 sample were carried out to investigate recyclability and the presence of a transformation from DUT-49cp to DUT-49op at very low adsorption pressure during desorption (Figure S6). WWW.NATURE.COM/NATURE 4

a) 7 65 6 55 b) 3 25 Methane adsorbed (mmol g -1 ) 5 45 4 35 3 25 2 Methane adsorbed (mmol g -1 ) 2 15 15 1 1 5 1 2 3 4 5 6 7 8 9 1 p (kpa) 5.1 1 1 p (kpa) Ads. at 111 K cycle 1 Des. at 111 K cycle 1 Ads. at 111 K cycle 2 Des. at 111 K cycle 2 Figure S6. a) Cycling of methane sorption isotherms at 111 K cycle 1: methane adsorption-desorption at 111 K, cycle 2: subsequent evacuation in dynamic vacuum (p < 1-4 mbar) at 111 K for 48 h followed by adsorption-desorption at 111 K. From previous adsorption and PXRD experiments it was shown that guest free DUT-49cp becomes unstable at room temperature (Figure S1) so the removal of methane was performed at the adsorption temperature of 111 K. After the first adsorption-desorption cycle the sample was treated in dynamic vacuum with p<1-3 Pa at 111 K for 48 h to remove all methane molecules from the framework. The second adsorption cycle shows an altered adsorption behavior especially at pressures < 1 kpa suggesting that the textural properties of the sample differs from the first cycle. The observed hysteresis from 1-15 kpa compared to the first cycle suggests an even more complex structural transformation. In addition, the adsorption in the pressure range from.1 1 kpa of the second cycle suggests that no transformation of DUT-49cp to DUT-49op occurred. Due to limitation of synchrotron beam time no in situ PXRD investigations of this time consuming experiment could be performed to investigate the occurring structural transitions in more detail. However, the reproducibility of adsorption isotherms (including the NGA-step) could be achieved, if the desorption of methane was performed at higher temperature. WWW.NATURE.COM/NATURE 5

a) b) Methane adsorbed (mmol g -1 ) 7 Ads. at 111 K cycle 1 65 Ads. at 111 K cycle 2 6 Ads. at 111 K cycle 3 55 Ads. at 111 K cycle 4 5 Ads. at 111 K cycle 5 45 4 35 3 25 2 15 Methane adsorbed (mmol g -1 ) 35 3 25 2 15 1 1 5 5 1 2 3 4 5 6 7 8 9 1 p (kpa) 2 4 6 8 1121416182 p (kpa) Figure S7. a) Cyclic methane adsorption isotherms at 111 K. Cycle 1-5: methane adsorption at 111 K followed by warming the sample to 298 K (1 K min -1 ) at constant methane pressure of 1 kpa; b) magnified pressure region of the isotherms below 2 kpa. Due to the op cp op transformation during adsorption, the DUT-49op phase is present at high p/p, with the pores completely filled with methane. In the previous experiment (Figure S6) DUT-49op could not be restored by removal of the guest at low temperature. Because no NGA transition or hysteresis (and thus no structural transition) is observed for methane adsorption at 298 K, removal of methane from CH 4 @DUT-49op can be achieved by increasing the temperature from 111 K to 298 K at constant methane pressure of 1 kpa (this should prevent a transformation back to DUT-49cp) and subsequent removal of the methane in dynamic vacuum for 12 h. Following this procedure, the NGA-step could be reproduced in 5 successive adsorption cycles (Figure S7). A loss of uptake at saturation pressure can be associated to partial decomposition of the sample during the recycling. This decomposition can originate from a partial transformation of DUT-49op to DUT-49cp since DUT-49cp is present at 1 KPa during adsorption of methane at 121 K (Figure S3). PXRD and adsorption experiments show that the highly strained DUT-49cp upon thermal desorption transforms into an ill-defined non-porous amorphous solid. WWW.NATURE.COM/NATURE 6

a) b) 7 65 6 Ads. at 111 K cycle 1 Ads. at 111 K cycle 2 4 35 Methane adsorbed (mmol g -1 ) 55 5 45 4 35 3 25 2 15 1 5 1 2 3 4 5 6 7 8 9 1 p (kpa) Methane adsorbed (mmol g -1 ) 3 25 2 15 1 5 5 1 15 2 25 p (kpa) Figure S8. Cyclic methane adsorption isotherms at 111 K with dosing volume of 15-3 cm 3 g -1 and equilibration conditions of <1% pressure change within 3 s. Cycle 1-2: methane adsorption at 111 K followed by warming the sample to 298 K (3 K min -1 ) starting at a methane pressure of 7 kpa (at 111 K) to 12 kpa (at 298 K); subsequent removal of the methane in dynamic vacuum for 12 h at 298 K b) magnified pressure region below 2 kpa. A faster temperature increase (3 K min -1 ) at lower starting pressure (7 kpa) seems to induce a complete decomposition of the sample and loss of porosity supporting the findings mentioned above. WWW.NATURE.COM/NATURE 7

a) b) 7 65 6 Ads. at 111 K cycle 1 Ads. at 111 K cycle 2 Des. at 111 K cycle 2 4 35 Methane adsorbed (mmol g -1 ) 55 5 45 4 35 3 25 2 Methane adsorbed (mmol g -1 ) 3 25 2 15 1 15 1 5 5 2 4 6 8 1 p (kpa) 5 1 15 2 25 p (kpa) Figure S9. Cyclic methane adsorption isotherms at 111 K with dosing volume of 25-4 cm 3 g -1 and equilibration conditions of <1% pressure change within 3 s. Cycle 1-2: methane adsorption at 111 K followed by warming the sample to 2 K (2 K min -1 ) in a sealed sample cell. After 1 min the pressure was slowly released to 1 KPa and the sample was warmed up to 298 K; subsequent removal of the methane in dynamic vacuum for 12 h at 298 K b) magnified pressure region of the isotherms below 2 kpa. To avoid the phase transition upon warming, the sample cell was loaded with methane at 111 K close to the saturation pressure (1 kpa) and sealed by closing a valve connected to the cell. The cell was then heated up to 2 K at 2 K min -1 and the pressure was slowly released to ambient pressure followed by warming to 298 K and removal of the methane in vacuum. The second cycle shows almost a superimposed adsorption branch at low pressures (Figure S9) and only little loss of porosity indicating that an optimization of the procedure can result in a complete recyclability of the NGA transition in DUT-49. The differences observed for the NGA step in the cycled isotherms are further explained in 3.3.2. 2.3 Volumetric adsorption of n-butane n-butane adsorption experiments in the temperature range of 273-38 K were performed in a standard BELSORP glass cell with glass rod and a thermostat (JULABO M25, temperature accuracy.1 C) was used to control the adsorption temperature. To avoid contact with atmospheric moisture, DUT-49 samples were transferred into the measuring cells in a glove-box under Ar atmosphere (H 2 O, O 2 <.1 ppm) and treated at 15 C in vacuum (1-3 mbar) for 12 h prior to the adsorption experiment. n-butane adsorption isotherms measured at 288, 298, and 38 K support WWW.NATURE.COM/NATURE 8

the increased NGA-step observed during adsorption of methane at increased temperatures. However no NGA-step but a large hysteresis was observed at 273 K. The isotherm recorded at 38 K shows larger uptake after the transition compared to the isotherm at 298 K suggesting a different structural transition to a more porous structure, compared to DUT-49cp also confirmed by a two-step hysteresis in desorption. Unfortunately, no in situ PXRD studies were carried out at this adsorption temperature so far. a) b) 3 3 n-butane adsorbed (mmol g -1 ) 25 2 15 1 5 12 1 8 n-butane adsorbed (mmol g -1 ) 25 2 15 1 5.1.15..2.4.6.8 1. p/p 1 1 p (kpa) Ads. at 273 K Ads. at 288 K Ads. at 298 K Ads. at 38 K Des. at 273 K Des. at 288 K Des. at 298 K Des. at 38 K Figure S1. n-butane adsorption isotherms at various temperatures plotted against relative pressure (a) and against absolute pressure (b). 2.4 Visual observation of the volumetric adsorption of n-butane at 298 K To monitor the macroscopic effects of NGA on the bulk material a capillary with a diameter of 1.5 mm was sealed at the bottom and filled with 12.1 mg DUT-49 which was compacted to obtain a 5 cm high compact bed of DUT-49. The filled capillary was placed in a regular glass adsorption cell. Before measurement with n-butane, the sample was treated in vacuum at 15 C for 8 h. No thermostat was used in order to better visualize the sample, but the temperature was monitored with a thermometer during the measurement showing only slight temperature fluctuation between 298 and 299 K. A camera was installed to monitor the sample during adsorption of n-butane (see supplementary video 1). WWW.NATURE.COM/NATURE 9

a) n-butane adsorbed (mmol g -1 ) 12 11 1 9 8 7 6 5 4 3 2 1 2 3 b) Ads. n-butane at 298-299 K equilibration 2 s Ads. n-butane at 298 K equilibration 4, - 11, s Des. n-butane at 298 K equilibration 4, - 11, s 1 1 2 3 4 5 6 7 8 9 1 p (kpa) 1 2 3 Figure S11. a) n-butane adsorption isotherms at 298 K. Equilibration time of 4, 11, s per point (blue) an isotherm measured in parallel to film the sample at 298 299 K and equilibrium conditions: 1 % of pressure change within 2 s (red). b) Snapshots at selected points of the recorded video during the adsorption. Comparison of the isotherms with varying equilibrium conditions shows only minor differences. Besides the movement of the sample bed during NGA transition, a color change from blue to dark blue-greenish, and to almost black was observed (Fig. S11). 3. Pressure profiles and kinetics during volumetric adsorption experiments 3.1. Description of the performed volumetric adsorption experiments In volumetric adsorption experiments, the amount of adsorbed gas in a porous material is determined by measuring the pressure change of the gas in an accurately known volume (dead volume of sample cell, V d ) and manifold or reference volume (V r ) under isothermal conditions. V d represents the empty space inside the sample cell. WWW.NATURE.COM/NATURE 1

Vacuum a) b) EV1 Vacuum EV1 EV2 Manifold PS1 EV2 Manifold PS1 EV3 EV3 Sample cell Gas supply PS2 Gas supply PS2 T T Cryostat Sample cell Thermostat EV: Electromagnetic valve T: Temperature control thermostat PS: Pressure sensor Scheme S1. Representation of the BELSORP-max a) connected to a thermostat, b) connected to a cryostat system. Because sample amount and texture change for each measurement V d is determined before each adsorption experiment following an automated procedure. At constant temperature Helium is dosed from the manifold (V r ) with an initial pressure p1 i to the exhausted sample cell while the equilibrated pressures p1 e and p2 e are monitored by sensor PS1 and PS2 connected to V r and V d, respectively (see Scheme S1). The calibrated V r value is instrument specific and given by the manufacturer (in the conducted investigations the V r is 27.69756 cm 3 for adsorption of methane and 24.4649 cm 3 for adsorption of n-butane). V d can then be calculated following formula (1): (1) During the volumetric adsorption experiments performed in this work each point in the adsorption isotherm is measured by the following routine implemented in the BEL-measuring software. To simplify the process and calculations the assumption of an ideal gas as adsorptive is made while in the conducted experiments a non-ideality correction was performed: 1) Adjustment and equilibration of dosing pressure in manifold p1 i monitored by PS1 controlled by EV1 and EV2 2) If the dosing pressure is reached, the valve EV3 opens for 1 s connecting the sample cell to the manifold (characteristic drop of p1 i (PS1) and increase of p2 i (PS2)) 3) Equilibration of pressures in sample cell and manifold yield p2 e (PS2) and p1 e (PS1), respectively From these values the dosed Volume V1 for the n-th adsorption step is first calculated using formula (2) where m s represents the sample mass and T the temperature of the manifold: 1.. (2) WWW.NATURE.COM/NATURE 11

From the equilibrated pressures p2 e the change in Volume V2 caused by the adsorption can be calculated following formula (3): 2.. (3) The change in adsorption ΔV in the n-th point can be derived from the changes of V1 and V2 (4): 1 2 (4) And the adsorbed volume V(n) in the n-th adsorption step correlating to the equilibrated pressure p2 e (n) can be written as: 1 (5) For normal porous materials the initial gas pressure in the sample cell (p2 i ) decreases steadily after gas dosing until equilibrium is reached due to the adsorption. During NGA however, p2 i increases spontaneously without additional dosing of gas or temperature change. Once the critical pressure required for the structural transition is reached, the NGA produces an increase of p2 e due to the gas desorption from the framework. Because the transition happens in a sealed measuring cell at constant temperature, external factors can be ruled out as trigger for NGA. As a result, the NGA-transition can be directly monitored by the pressure sensor PS2 (Scheme S1) connected to the cell. It is difficult to distinguish from the obtained pressure profile the process of adsorption caused by dosing from the pressure increase caused by NGA, because they are superimposed. In most cases p Cell does not reach a plateau before the sudden pressure burst, caused by NGA, is observed. The pressure increase induced by NGA results not only in a drop of uptake and negative step in the isotherm, but also in an artificial shift on the p-axis, marked as Δp. Since Δn(NGA) is obtained from the isotherm, this information correlates to the equilibrated pressure, but does not directly reflect the pressure evolution during NGA. For this reason the pressure Δp(NGA) was extracted from the pressure profile during the NGA transition. This pressure corresponds to the difference between the lowest pressure after dosing and the highest pressure reached after NGA. If Δp(NGA) is divided by the sample mass, the specific p(nga) value can be obtained for a certain adsorptive and adsorption temperature. However, to determine Δn(NGA) and Δp(NGA) very precisely, a much higher resolution of the isotherm as well as a high level of temperature stability would be required. WWW.NATURE.COM/NATURE 12

3.2. Kinetic and pressure profiles of the performed experiments a) p cell (kpa) 4. 3.5 3. 2.5 2. 1.5 3. 2.5 2. 1.5 p(nga) = 1.6 kpa 54 541 542 Pressure sample cell Pressure manifold p(nga) p 4. 3.5 3. 2.5 2. 1.5 p manifold (kpa) b) Methane adsorbed (mmol g -1 ) 28.5 28. 27.5 27. 26.5 p = 1.61 kpa Ads. at 91 K n = -1.37 mmol g -1 1. 5275 53 5325 535 5375 54 5425 545 1. 26. 1. 1.5 2. 2.5 3. 3.5 4. t (s) p (kpa) Figure S12. a) NGA kinetic profile during methane adsorption at 91 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. a) b) p cell (kpa) 7. 6.5 6. 5.5 5. 4.5 4. 6. 5.6 5.2 4.8 4.4 49 495 41 Pressure sample cell Pressure manifold p(nga) = 1.41 kpa 4. 4 45 41 415 t (s) p(nga) p 7. 6.5 6. 5.5 5. 4.5 p manifold (kpa) Methane adsorbed (mmol g -1 ) 32 3 28 26 24 Ads. at 11 K p = 1.95 kpa n = -6.29 mmol g -1 4 5 6 7 p (kpa) Figure S13. a) NGA kinetic profile during methane adsorption at 11 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. WWW.NATURE.COM/NATURE 13

a) b) p cell (kpa) 18 17 16 15 14 17 16 15 14 13 1853 1854 p(nga) = 2.27 kpa 13 13 Pressure sample cell Pressure manifold 12 12 17 175 18 185 19 195 2 t (s) p(nga) p 18 17 16 15 14 p manifold (kpa) Methane adsorbed (mmol g -1 ) 33 32 31 3 29 28 27 26 25 24 23 22 p = 3.7 kpa 21 12 13 14 15 16 17 18 p (kpa) Ads. at 111 K n = -8.62 mmol g -1 Figure S14 a) NGA kinetic profile during methane adsorption at 111 K with magnified inset (equilibrium criteria: pressure change <1% during 72 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. a) b) p cell (kpa) 4 38 36 34 32 38 36 34 p(nga) = 4.52 kpa 32 37 371 372 Pressure sample cell Pressure manifold p(nga) p 4 38 36 34 32 p manifold (kpa) Methane adsorbed (mmol g -1 ) 36 34 32 3 28 26 24 p = 6.57 kpa Ads. at 11 K n = -9.87 mmol g -1 3 3 22 36 364 368 372 376 t (s) 3 32 34 36 38 4 p (kpa) Figure S15. a) NGA kinetic profile during methane adsorption at 121 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. WWW.NATURE.COM/NATURE 14

a) b) p cell (kpa) 14 13 12 11 12.4 12. 11.6 11.2 215 2175 22 Pressure sample cell Pressure manifold p 14 13 12 11 p manifold (kpa) n-butane adsorbed (mmol g -1 ) 9.4 9.2 9. 8.8 p = 1.2 kpa Ads. at 273 K n =.31 mmol g -1 225 25 275 21 2125 215 2175 22 2225 t (s) 8.6 11 12 13 14 p (kpa) Figure S16. a) NGA kinetic profile during n-butane adsorption at 273 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. a) p cell (kpa) 26 25 24 23 24 23 26 262 p(nga) =.64 kpa p(nga) p 26 25 24 23 p manifold (kpa) b) n-butane adsorbed (mmol g -1 ) 9.3 9.2 9.1 9. 8.9 8.8 8.7 p = 1.62 kpa Ads. at 288 K n = -.35 mmol g -1 22 Pressure sample cell Pressure manifold 22 8.6 25 252 254 256 258 26 262 264 266 t (s) 22 23 24 25 26 p (kpa) Figure S17. a) NGA kinetic profile during n-butane adsorption at 288 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. WWW.NATURE.COM/NATURE 15

a) p cell (kpa) 44 42 4 38 36 34 32 3 28 38 36 34 32 p(nga) = 2.58 kpa 377 378 379 p(nga) p 44 42 4 38 36 34 32 3 28 p manifold (kpa) n-butane adsorbed (mmol g -1 ) b) 1 9 8 p = 8.11 kpa Ads. at 298 K n = -1.44 mmol g -1 26 24 Pressure sample cell Pressure manifold 34 36 38 4 26 24 7 24 28 32 36 4 44 t (s) p (kpa) Figure S18. a) NGA kinetic profile during n-butane adsorption at 298 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 )); b) Corresponding adsorption isotherm region. a) b) 1. Ads. at 298 K 36 36 p cell (kpa) 34 32 3 34 p(nga) = 1.46 kpa 656 66 664 p(nga) p 34 32 3 p manifold (kpa) n-butane adsorbed (mmol g -1 ) 9.5 9. 8.5 p = 4.72 kpa n = -.73 mmol g -1 28 Pressure sample cell Pressure manifold 5 55 6 65 7 28 8. 28 29 3 31 32 33 34 35 36 37 t (s) p (kpa) Figure S19. a) NGA kinetic profile during n-butane adsorption at 298 K with magnified inset (equilibrium criteria: pressure change <1% during 4, - 11, s, dosing volume 1-2 cm 3 g -1 ); b) Corresponding adsorption isotherm region. WWW.NATURE.COM/NATURE 16

a) 44 44 b) 1. Ads. at 298 K p cell (kpa) 42 4 38 36 34 32 p(nga) = 2.58 kpa 39 38 762 768 774 78 p(nga) p 42 4 38 36 34 32 p manifold (kpa) n-butane adsorbed (mmol g -1 ) 9.5 9. 8.5 p = 6.93 kpa n = -.62 mmol g -1 3 Pressure sample cell Pressure manifold 72 74 76 78 8 t (s) 3 8. 3 32 34 36 38 4 42 44 p (kpa) Figure S2. a) NGA kinetic profile during n-butane adsorption in a capillary recorded by a camera (see movie 1) at 298-299 K with magnified inset (equilibrium criteria: pressure change <1% during 2 s, dosing volume 1-2 cm 3 g - 1 ), b) corresponding adsorption isotherm region. a) 42 4 42 b) 1.5 Ads. at 298 K p cell (kpa) 4 38 36 34 32 38 36 34 32 p(nga) = 6.48 kpa 284 285 286 287 p(nga) p 4 38 36 34 32 p manifold (kpa) n-butane adsorbed (mmol g -1 ) 1. 9.5 9. 8.5 p = 7.63 kpa n = -1.31 g -1 Pressure sample cell Pressure manifold 3 27 275 28 285 29 t (s) 3 8. 3 32 34 36 38 4 42 p (kpa) Figure S21. a) NGA kinetic profile with increase sample amount during n-butane adsorption at 298 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 1-2 cm 3 g -1 ); b) Corresponding adsorption isotherm region. WWW.NATURE.COM/NATURE 17

Table S1. Data obtained from time dependent pressure profiles and adsorption isotherms Figure Gas T (K) Δn mmol g -1 Δp kpa Δp(NGA) (kpa) Δp(NGA) (kpa g -1 ) # Figure S12 methane 91 1.37 1.61 1.6 15.9 Figure S13 methane 11 6.29 1.95 1.41 364.3 Figure S14 methane 111 8.62 3.7 2.27 493.4 Figure S15 methane 121 9.87 6.57 4.52 43.4 Figure S16 n-butane 273 no NGA no NGA no NGA no NGA Figure S17 n-butane 288.35 1.62.64 67.3 Figure S18 n-butane 298 1.44 8.11 2.58 28 Figure S19 n-butane 298.73 4.72 1.46 158.6 Figure S2 n-butane 298.62 6.93 2.58 213.2 Figure S21 n-butane 298 1.31 7.63 6.48 243.3 Figure S24 n-butane 298 2.17* n. a. n. a. n. a. Figure S25 n-butane 298 2.13* n. a. n. a. n. a. * extracted from gravimetric analysis; # V sample cell is 28.3 cm 3 for methane adsorption, 18.1 cm 3-21.2 cm 3 for adsorption of n-butane and 22.9 cm 3 for the filming experiment (Fig. S21). In addition to the extracted values in Table S1, the pressure profiles also provide information about the kinetics of the transitions. For methane, a much steeper profile is observed compared to n-butane. For given reasons a quantitative analysis of rate constants is not possible from the data obtained. However, qualitative differences among the methane adsorption experiments at different temperatures can be observed. A peak similar to the usual dosing can be observed during NGA at 91 K (2 K below the boiling point of methane) which differs from the isotherms collected at temperatures of 111 K or higher. 3.3. Impact of NGA and structural transitions on volumetric adsorption measurements Common inaccuracies in volumetric adsorption experiments originate from insufficient equilibration time. However, this is not the case in this work as demonstrated in Fig S12 - Fig S21. The counterintuitive NGA in DUT-49 as well as the related structural transition may pose challenges in volumetric adsorption measurements due to changes in the dead volume V d and in some instance the dosing pressure p1 i (Figure S22 - S 23). 3.3.1. Changes in dead volume associated to structural transitions during adsorption The dead volume V d determined at the beginning of the adsorption experiment is used for the calculation of each adsorption step assuming that it is constant during the experiment. Structural transitions such as the DUT-49op to DUT-49cp transition during adsorption can change the dead volume and are not taken into account for the calculation of the adsorbed volume (formula (1) - (5)). To estimate the influence of the skeletal density change on the determined methane uptake, the He densities (void/dead volumes) for DUT-49op and DUT-49cp were calculated from the crystal structures by SOLV procedure of WWW.NATURE.COM/NATURE 18

PLATON software. The calculated density of 1.97 g cm -3 for DUT-49op matches well with the skeletal density measured using Helium as test gas (1.92 g cm -3 ). The closed phase DUT-49cp has a slightly lower calculated density of 1.87 g cm -3. The experimental measurement of the density of DUT-49cp using Helium is not accessible, because the phase is not stable in the guest-free form. From the calculated densities, the change in dead volume V d caused by network transformation is.3 cm 3 g -1 (DUT- 49). Assuming this deviation as a source of error in the calculation of the adsorbed volume (formula (3)) an error of 3.2x1-4 mmol g -1 (.73 cm 3 (STP) g -1 ) at p/p =.1 and 111 K (methane density.12 mol l -1 ) is estimated. Thus the error caused by changes in dead volume is four orders of magnitude smaller as compared to the observed Δn during NGA. 3.3.2. Impacts of NGA on the dosing mechanism of volumetric adsorption experiments Effects on the isotherm are also expected if the valve EV3 is still open during the pressure amplification of NGA because gas is then pressured back into the manifold upon dosing. This backpressure can be monitored by a sudden increase of p1 i during the 1 s dosing period (Fig. S22 - S 23). a) b) 22 22 2 p(nga) 22 32 Ads. at 111 K p = 6.74 kpa p cell (kpa) 2 18 16 18 16 14 27 2715 273 p(nga) 2 18 16 p manifold (kpa) Methane adsorbed (mmol g -1 ) 3 28 26 24 n = -6.76 mmol g -1 14 Pressure sample cell Pressure manifold 14 22 2694 2696 2698 27 272 12 14 16 18 2 22 24 t (s) p (kpa) Figure S 22. a) NGA kinetic profile during methane adsorption at 111 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 25-4 cm 3 g -1 ); b) Corresponding adsorption isotherm region. WWW.NATURE.COM/NATURE 19

a) p cell (kpa) 28 26 24 22 2 18 16 14 12 22 2 18 16 14 12 29825 2985 29875 Pressure sample cell Pressure manifold 2978 298 2982 2984 2986 2988 28 26 24 22 2 18 16 14 12 p manifold (kpa) b) Methane adsorbed (mmol g -1 ) 28 27 26 25 24 23 22 21 2 Ads. at 111 K p = 8.36 kpa n = -5.9 mmol g -1 1 12 14 16 18 2 22 24 t (s) p (kpa) Figure S 23. a) NGA kinetic profile during methane adsorption at 111 K with magnified inset (equilibrium criteria: pressure change <1% during 36 s, dosing volume 25-4 cm 3 g -1 ); b) Corresponding adsorption isotherm region. This phenomenon occurred in measurements using a high initial dosing volume of 25-4 cm 3 g -1 causing a prompt NGA-transition compared to a delayed NGA at lower dosing volumes. For the cycling experiments in Fig. S9 two profiles for this backpressure could be monitored. A drop of p1 i is detected following a rapid increase in pressure and two steps are observed for p2 i (Fig. S22), while in Fig. S23 no step is detected at all indicating that both, the dosing and NGA happen in parallel. In both measurements the equilibrated pressures p1 e and p2 e are on the same level after dosing indicating that the NGA happened during the 1 s dosing where manifold and cell are connected and a free exchange of gas is possible. These observations differ strongly from the behavior observed for measurements with lower dosing volumes (1-2 cm 3 g -1 ) (Fig S12 -Fig S21) where p2 e is always below the level of p1 e after the NGA. Thus, in order to separate dosing and NGA, small dosing volumes are preferred. Due to the backpressure into the dosing volume, both, the calculation of V1 and V2 are indirectly affected. Because p1 e is artificially increased, the calculated V1 is smaller than the actual one (the calculated dosing volume is artificially decreased). In contrast p2 e is artificially decreased which decreases V2. As a result the ΔV value is increased leading to an exaggerated value of Δn(NGA). Due to the cumulative calculation of the isotherm this error impacts all following adsorption points and can be an explanation for the slightly lower total uptake in the isotherms in Fig. S9. A numerical correction of this artefact is difficult. However, small dosing volumes and small pressure steps in the adsorption measurement can clearly be used to avoid this artefact as shown above. WWW.NATURE.COM/NATURE 2

4. Gravimetric adsorption in parallel to differential scanning calorimetry (DSC) In contrast to the volumetric adsorption experiments, NGA and the pressure amplification does not impact gravimetric measurements. The experiments were performed on a Setaram TG-DSC 111 equipped with a Tian-Calvet microcalorimeter which allows measuring the gravimetric uptake (by electronic microbalance) in parallel to differential scanning calorimetry (DSC). It was used to investigate the adsorption behavior of n-butane at 298 K. The measurements were performed at a constant flow of 8 ml min -1 of a defined mixture of n-butane and N 2. The sample was placed in a tubular hydrophobic filter paper in a Pt-basket and attached to the balance (Scheme S2). Prior to the measurement the sample was activated in a pure N 2 stream (8 ml min -1 ) at 15 C for 8 h. For each point in the adsorption isotherm a defined n-butane/n 2 ratio was adjusted using MFC1 and MFC2 at a constant flow of 8 ml min - 1. Each step was equilibrated for 15 6 min until a constant mass and DSC signal was reached. The DSC signal was obtained by referencing the sample heat flow to an empty Pt-filter paper basket. See Scheme S2: MFC 1 FD Gas supply n-butane MFC 2 Gas supply N 2 TCHFT TCHFT Thermostate T Exhaust line FC FD: Flow detector MFC: Mass flow controller T: Temperature control thermostat TCHFT: Thermocouple-carrying heat-flux transducer of Scheme S2. Schematic representation of the Setaram TG-DSC 111 device. The NGA transition can be clearly observed in the gravimetric setup, supporting the results obtained from volumetric experiments. In two independent measurements similar Δm values and DSC signals for NGA were obtained (Figure S24, Figure S25). Equilibration times of 1 h also support the evidence of long lived metastable states. 21 WWW.NATURE.COM/NATURE 21

a) b) mass (mg) 3.4 3.2 3. 2.8 2.6 2.4 Mass Heat flow 275 2775 28 2825 285 2875 29 t (s) m = -.41 mg exothermal.5. -.5-1. -1.5 Heat flow (mw) n-butane adsorbed (mg g -1 ).6.5.4.3.2.1. m(nga) = -.126 mg g -1 1 2 3 4 5 6 Volume-% n-butane Ads. at 298 K Des. at 298 K Figure S24. a) Section of mass change and DSC signal obtained during NGA by n-butane adsorption at 298 K (equilibration time of 36 s for each point); b) n-butane adsorption/desorption isotherms measured gravimetrically at 298 K. a) -.5 b) mass (mg) 1.8 1.7 1.6 1.5 1.4 1.3 1.2 23.1 V-% n-butane m(nga) = -.41 mg exothermal Mass Heat flow 27 272 274 276 -.6 -.7 -.8 -.9-1. -1.1-1.2 Heat flow (mw) n-butane adsorbed (g g -1 ).5.4.3.2.1. m(nga) = -.124 mg g -1 Ads. at 298 K Des. at 298 K 1 2 3 4 5 6 7 t (s) Volume-% n-butane Figure S25. a) Section of mass change and DSC signal obtained during NGA by n-butane adsorption at 298 K (equilibration time of 9 s for each point); b) n-butane adsorption/desorption isotherms measured gravimetrically at 298 K. The release of n-butane during NGA causes a decrease of the adsorbed mass corresponding to the pressure increase in the cell during volumetric experiments. The DSC signal is rather complex showing a large exothermal signal followed 22 WWW.NATURE.COM/NATURE 22

by an endothermal peak which seems to be a fingerprint for this transition involving exothermic (increased adsorption enthalpy in DUT-49cp) and endothermic contributions (gas release, phase transition, see also Figure S44). 23 WWW.NATURE.COM/NATURE 23

5. Temperature dependent PXRD experiments Temperature dependent PXRD experiments were performed in a closed chamber under dynamic vacuum of 1-4 mbar in the temperature range of 1 725 K with steps of 25 K on a PANALYTICAL X PERT PRO with λ =.1545 nm in Bragg-Brentano-geometry. The temperature was raised with 5 K min -1 starting at 1 K. For each temperature three PXRD patterns were collected under isothermal conditions in the 2Ѳ range from 2.5 up to 22, with a step size of.28 and 4 s per step. a) 7 b) 7 (111) (2) (22) Intensity (a. u.) 46,4 6 6 4,675 34,95 5 5 29,225 T (K) 4 T (K) 4 23,5 3 3 17,775 12,5 2 2 6,325 1 1 6 2 4 6 8 1 12 14 16 18 2 22 3. 3.5 4. 4.5 5. 5.5 6. c) 2 (degree) 275 K (after 65 K) 65 K 275 K 1 K d) 46.75 46.5 2 (degree) 12, 1,5 Intensity normalized a.u. Lattice parameter, a ( Å) 46.25 46. 45.75 45.5 99, 97,5 96, 94,5 Unit cell volume, V ( Å3 ) 2.5 3. 3.5 4. 4.5 5. 5.5 6. 2 (degree) 45.25 1 2 3 4 5 6 7 T (K) 93, Figure S26. a) Contour plot of temperature depending PXRD; b) Enlarged section of temperature depending PXRD, c) selected PXRD patterns on the reversibility of the observed negative thermal expansion d) Temperature dependent evolution of lattice constant a and cell volume V in DUT-49op. 24 WWW.NATURE.COM/NATURE 24

A constant shift of the reflections to higher 2Ѳ values was observed without appearance of additional peaks indicating a negative thermal expansion (NTE) of the unit cell without a change in symmetry or phase in DUT-49. A peak broadening was observed at temperatures higher than 65 K corresponding to the decomposition of the sample. The latter is in good agreement with the thermogravimetric analysis (Figure S1). To further analyze the NTE, the PXRD patterns in the temperature range 1 65 K were subjected to Pawley refinement using Reflex module in Material Studio 5.. 2 The cell parameter and Volume was plotted as a function of temperature showing a linear behavior in the temperature range of 1 65 K. Using formula (6) the linear thermal expansion coefficient (TEC) was calculated:, (6) With -19.4(1) x 1-6 K -1 the thermal expansion coefficient α l in DUT-49 is larger in magnitude than those observed for MOF-5 ( 14.5(1) x 1-6 K -1 ), 3 HKUST-1(-4.1(1) x 1-6 K -1 ) 4 and MOF-14 (-13 x 1-6 K -1 ). 5 Due to the cubic symmetry of DUT-49op the calculated linear TEC corresponds to a volumetric TEC of α V =-55.4(1) x 1-6 K -1. At 65 K the sample was cooled to 275 K and again heated to 65 K to confirm the reversibility of the negative thermal expansion (Figure S26). The sample was then further heated to the decomposition temperature of around 725 K as detected by the loss of crystallinity. 6. In situ structure analysis of methane adsorption mechanism via PXRD PXRD experiments in parallel to adsorption were performed at BESSY II light source, KMC-2 beamline of Helmholtz-Zentrum Berlin für Materialien und Energie using the recently established experimental setup. 6 The diffraction experiments were performed in transmission geometry using a sample holder with a thickness of 2 mm. The monochromatic radiation with energy of 848 ev (λ = 1.546 Å) was used for all experiments. The diffraction images were measured using 2θ scan mode and Vantec 2 area detector system (Bruker) in the range of 2 5 2θ. A synchrotron beam with dimensions of.5 x.5 mm was used for the experiments. Corundum powder with a crystallite size of 5 µm was used as an external standard. The image frames were integrated using Datasqueeze 2.2.9 software 7 and processed using Fityk.9.8 program 8. The indexing of the PXRD patterns as well as Pawley refinement was performed using Reflex module in Materials Studio 5. 2. In order to correlate the adsorption behavior of methane at temperatures between 91 and 121 K to proposed structural changes, in situ experiments with methane at 91, 111, and 121 K were performed. In addition, in situ adsorption experiments with n-butane at 298 K were performed to investigate the impact of different guest molecules. For each experiment an adsorption isotherm was measured and PXRD patterns are recorded after equilibration at selected points of the isotherm. 6.1. In situ structure analysis of methane adsorption at 91 K Volumetric methane physisorption experiments at 91 K (p = 12.1 kpa) showed small NGA, but large hysteresis. The shape of the isotherm obtained during in situ experiments matches well the isotherm measured in the laboratory, providing perfect crystallographic insight in the structural transitions (Figure S27). 25 WWW.NATURE.COM/NATURE 25

a) p (kpa) b) Methane adsorbed (mmol g -1 ) c) Lattice parameter, a ( Å) 8 7 6 5 4 3 2 1..2.4.6.8 1. 47 46 45 44 43 42 41 4 39 38 37 36 35 2 4 6 8 1 12 15 15 11 12 14 13 Pre-ads. 9 11 1 14 9 1 12 13 1 8 ads ads 3 4 3 15 2 2 14 13 1 1 6 5 p/p 8 34 5 6 7 2 4..1.2.3.4.5.6.7 p/p 34 7 1..1.2.3 14 12 1 8 6 4 2 Methane adsorbed (molecules per unit cell) Normalized intensity (a.u.) 2 4 6 8 1 12 14 16 18 15 14 13 12 11 1 9 8 7 6 5 4 3 2 1 Pre-ads. 2 4 6 8 1 12 14 16 18 2 degree (15) p/p.1 des. at 91 K - DUT-49cp (14) p/p.4 des. at 91 K - DUT-49cp (13) p/p.9 des. at 91 K - DUT-49ip + DUT-49cp (12) p/p.12 des. at 91 K - DUT-49ip (11) p/p.13 des. at 91 K - DUT-49ip (1) p/p.18 des. at 91 K - DUT-49ip (9) p/p.22 des. at 91 K - DUT-49op (8) p/p.31 des. at 91 K - DUT-49op (7) p/p.65 des at 91 K - DUT-49op (6) p/p.49 ads. at 91 K - DUT-49op (5) p/p.46 ads. at 91 K - DUT-49op (4) p/p.38 ads. at 91 K - DUT-49cp + DUT-49op (3) p/p.37 ads. at 91 K - DUT-49cp + DUT-49op (2) p/p.33 ads. at 91 K - DUT-49cp (1) p/p.25 ads. at 91 K - DUT-49cp (Pre-ads.) evac. at 91 K Figure S27. a) Adsorption isotherm of methane at 91 K; b) PXRD patterns obtained at the corresponding adsorption points. c) Lattice parameter evolution. Color code: DUT-49op before measurement (black), DUT-49op (green), DUT- 49cp (red), DUT-49ip (blue), Phase mixture (orange, purple). The PXRD patterns obtained from the evacuated material at 91 K match the pattern calculated from the crystal structure of DUT-49op. At p/p =.6 the isotherm reaches a plateau and two PXRD patterns measured at p/p =.25 and.33 show a shift of peaks and formation of new peaks, indicating a structural transition. In the pressure region between p/p =.37 and.39 a phase mixture exists. 26 WWW.NATURE.COM/NATURE 26

Indexing of all PXRD patterns results in cubic unit cells. The analysis of the systematic absences suggests F23 and Pa3 extinction classes for DUT-49op and DUT-49cp, respectively. The desorption branch of the isotherm shows an additional step in the relative pressure area.15.25 that could indicate the existence of an additional intermediate phase DUT-49ip. PXRD patterns, measured in this region correspond to lattice constants slightly smaller in comparison to DUT-49op but show additional peaks that indicate a change in symmetry of the crystal structure. Obviously even minor structural changes during desorption lead to symmetry reduction of the original F-centered cell, possibly caused by the guest since no such phase transition could be observed during temperature dependent PXRD analysis. The PXRD patterns obtained in this pressure range and at lower pressure can be attributed to Pa3 extinction classes. The evolution of the lattice parameter with the pressure is shown in Figure S27 and Table S2. Table S2. Experimental data on in situ PXRD measured during the methane adsorption/desorption at 91 K. PXRD pattern No p/p Extinction class Lattice parameter, a (Å) Unit cell volume (Å 3 ) Pre-ads - F23 46.371(1) 9971.2(1) 1.245 (ads) Pa3 36.299(1) 47828.2(1) 2.326 (ads) Pa3 36.344(1) 476.3(1) 3.366 (ads) Pa3 F23 36.346(1) 46.384(1) 4814.2(1) 99561.9(1) 4.381 (ads) Pa3 F23 36.364(1) 46.542(1) 4885.6(1) 1817.3(1) 5.456 (ads) F23 46.4(1) 99897.3(1) 6.49 (ads) F23 46.463(1) 134.8(1) 7.65 (des) F23 46.492(1) 1492.7(1) 8.312 (des) F23 46.417(1) 17.2(1) 9.223 (des) F23 46.441(1) 1162.4(1) 1.181 (des) Pa3 45.625(1) 94974.8(1) 11.135 (des) Pa3 45.17(1) 91776.6(1) 12.124 (des) Pa3 44.24(1) 85323.5(1) 13.87 (des) Pa3 36.254(1) 4765.3(1) 14.39 (des) Pa3 36.17(1) 4732.1(1) 15.9 (des) Pa3 36.95(1) 4726.3(1) 6.2. In situ structure analysis of methane adsorption at 111 K Volumetric adsorption experiments show a characteristic temperature dependence of the NGA transition. While Δn(NGA) during methane adsorption at 91 K is only 1.37 mmol g -1, values up to 8.6 mmol g -1 could be reached at 111 K, the boiling point of methane, and 9.87 mmol g -1 at 121 K. In order to study the structural transitions occurring during methane adsorption at 111 K parallelized adsorption/diffraction experiments were performed. Both, the obtained adsorption isotherm as well as PXRD patterns suggests the same phase transition observed at 91 K. 27 WWW.NATURE.COM/NATURE 27

p (kpa) 2 4 6 8 1 2 4 6 8 1 12 14 16 18 14 7 12 11 13 26 25 16 151413 1 12 6 24 17 11 23 5 18 1 22 9 21 9 8 2 4 19 7 19 35 18 6 3 3 17 2 3 2 5 2 16 25 2 21-26 8 4 15 4 5 6 7 25 2 6 3 14 1 aa 21-24 13 1 15 2 ads 12 1 1 11..1.2.3 1..2.4.6.8 1. 9 p/p 8 7 (26) p/p 6.1 des. at 111 K - DUT-49cp 5 (25) p/p.2 des. at 111 K - DUT-49cp 4 (24) p/p.5 des. at 111 K - DUT-49cp 3 (23) (22) (21) p/p.7 des. at 111 K - DUT-49cp p/p.9 des. at 111 K - DUT-49cp p/p.13 des. at 111 K - DUT-49cp 2 1 Pre-ads. (2) p/p.15 des. at 111 K - DUT-49ip + DUT-49cp 2 4 6 8 1 12 14 16 18 (19) p/p.15 des. at 111 K - DUT-49ip + DUT-49cp 2 degree (18) p/p.16 des. at 111 K - DUT-49ip (17) p/p.18 des. at 111 K - DUT-49ip (8) p/p.41 ads. at 111 K - DUT-49op + DUT-49cp (16) p/p.19 des. at 111 K - DUT-49ip (7) p/p.28 ads. at 111 K - DUT-49cp (15) p/p.24 des. at 111 K - DUT-49ip (6) p/p.2 ads. at 111 K - DUT-49cp (14) p/p.29 des. at 111 K - DUT-49op (5) p/p.16 ads. at 111 K - DUT-49cp (13) p/p.34 des. at 111 K - DUT-49op (4) p/p.11 ads. at 111 K - DUT-49cp (12) p/p.39 des. at 111 K - DUT-49op (3) p/p.9 ads. at 111 K - DUT-49op (11) p/p.45 des. at 111 K - DUT-49op (2) p/p.7 ads. at 111 K - DUT-49op (1) p/p.96 ads. at 111 K - DUT-49op (1) p/p.4 ads. at 111 K - DUT-49op (9) p/p.55 ads. at 111 K - DUT-49op + DUT-49cp (Pre-ads.) evac. at 111 K a) b) Methane adsorbed (mmol g -1 ) Figure S28. In situ PXRD in parallel to adsorption. a) Adsorption Isotherm of methane at 111 K. b) PXRD pattern obtained at the corresponding adsorption points. Color code: DUT-49op before measurement (black), DUT-49op (green), DUT-49cp (red), DUT-49ip (blue), Phase mixture (orange, purple). Methane adsorbed (molecules per unit cell) Normalized intensity (a.u.) 28 WWW.NATURE.COM/NATURE 28

16 14 DUT-49op (p/p =.96) DUT-49cp (p/p =.29 ads) DUT-49op (evac.) Absolute intensity (counts) 12 1 8 6 4 2 2 4 6 8 1 12 14 2 deg. Figure S29. Selected PXRD patterns during the adsorption of methane at 111 K in absolute intensity scale (y-shift for the red pattern is 1 counts, y-shift for the blue pattern is 12 counts). 29 WWW.NATURE.COM/NATURE 29

Upon adsorption, DUT-49op undergoes a transition at p/p =.11 to DUT-49cp. The contracted structure is reopened at p/p >.5 to form DUT-49op. During desorption, two plateaus in the range of p/p = 1.35 and p/p =.35.2, were observed corresponding to the DUT-49op to DUT-49ip transition. The steep desorption step at p/p <.2 is accompanied by the transition to DUT-49cp which is retained at low relative pressures of.1. Interestingly, PXRD patterns obtained in this range confirm a further minor continuous contraction of the DUT-49cp structure which is reflected in the change of the unit cell volume from 47453.6(1) Å 3 (p/p =.148) to 46144.7(7) Å 3 (p/p =.8) (Table S3 and Figure S28). Table S3. Crystallographic data on in situ PXRD measured during the methane adsorption at 111 K PXRD pattern No p/p Extinction class Lattice parameter, Unit cell a (Å) volume (Å 3 ) Pre-ads evacuated F23 46.427(1) 171.8(1) 1.41 F23 46.423(1) 146.(1) 2.7 F23 46.458(1) 1272.4(1) 3.9 F23 46.456(1) 1259.5(1) 4.111 Pa3 36.161(1) 47284.8(1) 5.165 Pa3 36.17(1) 4732.1(1) 6.25 Pa3 36.163 (1) 47292.6(1) 7.28 Pa3 36.16(1) 47282.(1) 8.45 Pa3 36.229(1) 47552.(1) 9.552 Pa3 36.3(1) 47832.1(1) 1.965 F23 46.371(1) 9971.2(1) 11.445 F23 46.43(1) 191.2(1) 12.395 F23 46.412(1) 99974.9(1) 13.344 F23 46.394(1) 99858.6(1) 14.299 F23 46.411(1) 99968.4(1) 15.246 Pa3 45.542(1) 94457.5(1) 16.199 Pa3 45.324(1) 9317.5(1) 17.184 Pa3 45.257(1) 92695.2(1) 18.168 Pa3 44.11(1) 85771.9(1) 19.158 Pa3 43.866(1) 8448.1(1) 2.148 Pa3 36.24(1) 47453.6(1) 21.132 Pa3 36.148(1) 47233.8(1) 22.91 Pa3 36.134(1) 47178.9(1) 23.71 Pa3 36.87(1) 46995.1(1) 24.51 Pa3 36.75(1) 46948.2(1) 25.21 Pa3 35.977(1) 46566.6(1) 26.8 Pa3 35.868(1) 46144.7(7) 3 WWW.NATURE.COM/NATURE 3