The Organic Secondary Building Unit: Strong Intermolecular π Interactions Define Topology in MIT-25, a Mesoporous MOF with Proton-Replete Channels

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1 Supporting Information The Organic Secondary Building Unit: Strong Intermolecular π Interactions Define Topology in MIT-25, a Mesoporous MOF with Proton-Replete Channels Sarah S. Park, 1 Christopher H. Hendon, 1 Alistair J. Fielding 2, Aron Walsh 3,4, Michael O Keeffe 5 and Mircea Dincă 1 * 1 Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States 2 School of Chemistry and the Photon Science Institute, The University of Manchester, Manchester, M13 9PL, United Kingdom 3 Department of Materials, Imperial College London, London, SW7 2AZ, United Kingdom 4 Department of Materials Science and Engineering, Yonsei University, Seoul, South Korea 5 School of Molecular Sciences, Arizona State University, Tempe, Arizona, 85287, United States mdinca@mit.edu S1

2 Table of Contents Page Materials... S3 Synthesis of MIT S3 Synthesis of MIT S3 X-ray diffraction studies... S3 Gas sorption measurements... S4 Physical measurements... S4 Computational details - Geometrical optimization of proton for MIT S5 - DFT determination of the nominal stoichiometry and pore volume of MIT S6 - Electronic structure of the free ligands oligomeric species... S6 Figure S1. Model coordination environments around the Mg 2+ center... S5 Figure S2. The ssp network found in MIT S7 Figure S3. Pore size determination with electrostatic potential and geometric method... S8 Figure S4. Thermogravimetric analysis plots for MIT S9 Figure S5. BET surface area analysis of MIT S10 Table S1. BET parameters and pressure ranges used for calculation of BET surface area of MIT-25.. S10 Figure S6. Barrett-Joyner-Halenda (BJH) pore size distribution curve for MIT S11 Figure S7. PXRD patterns for MIT-25 and MIT S12 Figure S8. Multifrequency continuous-wave EPR spectra of MIT S13 Figure S9. Structural and topology representation of MIT S14 Figure S10. The electrostatic potential projected across the electron density maps for the H 4 TTFTB and H 4 TBAPy neutral and 1e - oxidized species... S15 Figure S11. Thermal ellipsoid plots for MIT-25 and MIT S16 Table S2. Selected bond distances for MIT S16 Table S3. X-ray experimental details for MIT S17 Table S4. X-ray experimental details for MIT S18 References... S19 S2

3 Materials Mg(NO 3 ) 2 6H 2 O (J. T. Baker), ethanol (EtOH, ACS grade, J. T. Baker), N,N-dimethylformamide (DMF, 99.9%, Omnisolv), 1,4-dioxane (Sigma-Aldrich, 99.0%) were obtained from commercial sources and used as received. Dry tetrahydrofuran (THF) were obtained by degassing with a vigorous flow of Ar for 45 min and then passing the solvent through two alumina columns in a Glass Contour Solvent System. Tetrathiafulvalene tetrabenzoic acid (H 4 TTFTB) and 1,3,6,8-tetrakis(p-benzoic acid) pyrene (H 4 TBAPy) were prepared according to literature procedures. 1, 2 Synthesis of [Mg 2 H 6 (H 3 O)(C 34 H 16 O 8 S 4 ) 3 ] 1.5(DMF) (H 2 O) (MIT-25) Mg(NO 3 ) 2 6H 2 O (143 mg, mmol) was dissolved in 1:1 water/ethanol (12 ml). H 4 TTFTB (103 mg, mmol) was dissolved in 3:1 DMF/ethanol (8.4 ml). The magnesium nitrate solution was slowly added to the H 4 TTFTB solution in a 20 ml scintillation vial. The reaction mixture was heated at 75 C for 72 hours, affording red needle single crystals (75 % yield). The crystals were collected by filtration and washed with DMF (20 ml x 3) and EtOH (20 ml x 3). Elemental analysis calcd. for [Mg 2 H 6 (H 3 O) (C 34 H 16 O 8 S 4 ) 3 ] 7(DMF) 9(H 2 O) (C 123 H 124 Mg 2 N 7 O 41 S 12 ): C, 52.96; H, 4.48; N, Found: C, 52.98; H, 4.37; N, The crystals were soaked in 15 ml of dry THF for 2 days, during which the solvent was refreshed 2 times. Elemental analysis calcd. for [Mg 2 H 6 (H 3 O) (C 34 H 16 O 8 S 4 ) 3 ] 1.5(DMF) (H 2 O) (C H 69.5 Mg 2 N 1.5 O 27.5 S 12 ): C, 57.02; H, 3.12; N, Found: C, 57.12; H, 3.45; N, The crystals were evacuated at 200 C as determined by thermogravimetric analysis (Figure S2) for 24 hours at 4 mtorr. Elemental analysis calcd. for Mg 2 H 6 (H 3 O)(C 34 H 16 O 8 S 4 ) 3 (C 102 H 57 Mg 2 O 25 S 12 ): C, 57.90; H, Found: C, 57.41; H, 3.09; N, <0.02. Synthesis of [Mg 3 (TBAPy) 2 (µ 2 -OH 2 ) 2 (H 2 O) 2 (H 3 O) 2 ] 6.5(DMF) (H 2 O) 0.5(C 4 H 8 O 2 ) (MIT-26) Mg(NO 3 ) 2 6H 2 O (3.8 mg, mmol) and H 4 TBAPy (5.1 mg, mmol) were dissolved in DMF (1.25 ml) in a 5 ml vial and 1:1 dioxane /H 2 O (1.25 ml) and 5 µl concentrated HCl were added respectively. The reaction mixture was heated at 120 C for 72 hours and cooled to 25 o C with cooling rate 5 o C/min, affording yellow block crystals. The crystals were collected by filtration and washed with 2:1:1 DMF/dioxane/H 2 O. Elemental analysis calcd. for [Mg 3 (H 3 O) 2 (TBAPy) 2 (µ 2 -OH 2 ) 2 (H 2 O) 2 ] 6.5(DMF) (H 2 O) 0.5(C 4 H 8 O 2 ) (C H Mg 3 N 6.5 O 30.5 ): C, 63.31; H, 5.31; N, Found: C, 63.22; H, 5.06; N, X-ray diffraction studies Low-temperature data were collected on a Siemens three-circle diffractometer coupled to a Bruker-APEX CCD detector with graphite-monochromated Mo Kα radiation (λ = Å), performing φ- and ω- scans. The structure was solved by direct methods SHELXS and refined against F 2 on all data by fullmatrix least squares with SHELXL All non-hydrogen atoms were refined anisotropically. Aromatic hydrogen atoms and hydrogen atoms from DMF were included in the model at geometrically calculated positions and refined using a riding model. Bridging carboxylic hydrogen atoms were not resolved from diffraction studies, and were later solved with DFT calculations (Detailed discussion is in computational section). Hydrogen atoms from bridging and coordinating water on Mg 2+ in MIT-26 were resolved with electron density peaks. Residual electron density contributions in MIT-26 were removed by the SQUEEZE 4 in PLATON 5. Details of the data quality and a summary of the residual values of the S3

4 refinements for MIT-25 and MIT-26 are included in Table S3 and Table S4, respectively. Gas sorption measurements A Micromeritics ASAP 2020 Surface Area and Porosity Analyzer was used to measure the nitrogen adsorption isotherms. An oven-dried sample tube equipped with a TranSeal (Micromeritics) was evacuated and tared. The sample was transferred to the sample tube and heated to 200 C under a vacuum of 4 mtorr, until the outgas rate was less than 2 mtorr/minute. The evacuated sample tube was weighed again and the sample mass was determined by subtracting the mass of the previously tared tube. The N 2 isotherm was measured using a liquid nitrogen bath (77 K). Ultra-high purity grade (99.999% purity) N 2 and He, oil-free valves and gas regulators were used for all the free space correction and measurement. Physical measurements Continuous-wave (CW) electron paramagnetic resonance (EPR) spectra were recorded at 9.4 GHz on a Bruker Micro spectrometer with a Super-high-Q rectangular cavity and at 34 GHz on a Bruker EMX spectrometer with an ER 5106 QT resonator. All operating conditions are stated on the figure legends. g values were calibrated against Bruker strong pitch (2.0028). Powder X-ray diffraction (PXRD) patterns were recorded on Bruker D8 Discover diffractometer using Nickel-filtered Cu-Kα radiation (λ = Å) with accelerating voltage and current of 40 kv and 40 ma, respectively. Samples for PXRD were prepared by placing a thin layer of the appropriate material on a zero-background silicon crystal plate. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 Thermogravimetric Analyzer at a heating rate of 1.0 C/min under a nitrogen gas flow of 90 ml/min on a platinum pan. Diffuse reflectance UV-Vis-NIR spectra of desolvated MIT-25 were collected between ev on a Varian Cary 5000i UV-Vis-NIR spectrometer equipped with a DiffusIR diffuse reflectance accessory (Pike Technologies) and referenced to BaSO 4. S4

5 Computational details - Geometrical optimization of proton for MIT-25 Computations were performed within Kohn-Sham density functional theory (DFT) construct as implemented in the Vienna ab initio simulation package (VASP), a plane-wave basis set code (with projected augmented wave (PAW) scalar-relativistic pseudopotentials). Beginning with the experimentally collected crystal structure, protons were manually placed between the free carboxylates, using VESTA, and subsequent crystal and electronic structure optimizations were performed with Γ- point-only sampling of the first Brillouin zone. The cell volume was held fixed at the T = 150 K value determined from XRD, and was sufficient for the purpose of optimizing the proton positions. Furthermore, we reached the limit of our computational power due to the size of the crystallographic unit cell. A 500 ev plane-wave cut-off and PBEsol functional was employed. Relaxed convergence criterion was required (0.05 ev per atom), because of the freedom and oscillations in the O-H-O direction. The resultant proton positions were found to sit shared between the free carboxylic acids. To support this proton-shared structure, a representative cluster, [MgH 3 (OAc) 6 ] -, was constructed and optimized using the parameters presented for the oligomeric π-stacked TTFTB and TBAPy calculations. Using FHI-aims, a DFT code with a numerical basis, the system was geometrically optimized using the PBE functional and a sufficiently large basis set (i.e. tight ) to include polarization and diffuse functions on light elements. However, sufficient convergence was not reached in the first model system (Figure S1a) due to O-O repulsion. The same model was then constructed with three additional bridging protons (i.e. [Mg(OAc) 3 H 3 ] 1-, Figure S1b), and was geometrically optimized. This structure reached a stable minimum, Figure S1c, with the bridging protons acting to stablize the dangling oxides. Electrostatic potential maps of this O-H-O environment, shown in Figure S1d, depict the stabilizing effect installed by the H +. Figure S1. Model coordination environments were constructed to elucidate the proton locality around the Mg 2+ center. [Mg(OAc) 3 ] 4- and [Mg(OAc) 3 H 3 ] 1- were constructed and geometrically optimized. The former, depicted in (a), did not converge within reasonable convergence criteria, which the latter, (b), did converge to a minimum energy state, (c). This indicated that the bridging protons were integral to the coordination environment and overall structural stability. Electronically, the proton serves as a charge S5

6 shield, as shown by the electrostatic potential map in (d). - DFT determination of the nominal stoichiometry and pore volume of MIT-25 Using the electrostatic potential as computed by the PBEsol method discussed above, the pore diameter was computed by examination of the electrostatic potential map presented in Figure S3a. The aperture was deemed vacuous if the potential had a variance less than 1E-4, which yielded pore aperture of 25 Å and 7 Å for the large and small pores, respectively. This is in stark contrast to the typical method for quantifying pore aperture that uses a somewhat arbitrary and chemically devoid method for the quantification of pore volume (Figure S3c and d). The geometric method overestimates the large pore aperture (which is perfectly hexagonal), and underestimates the small pore aperture, which is significantly distorted. Despite our improved estimation of pore volume, we elected to present the geometric pore apertures in the manuscript, but contend that both are valid. From elemental analysis and EPR, it was clear that there was some radical character on the ligand. In the instance where the ligands were closed shell, the structure was charge compensated by two H 3 O + per formula unit; the H 3 O + positions were delocalized crystallographically and were therefore not included in the model. The same electrostatic potential calculation provides insight into the location of these two H 3 O +. Regions of high potential, or hot spots as shown in Figure S3a and b, are regions where positive charge would be expected to accumulate. Thus, we assign the small pore to be filled with charge-balancing hydronium. - Electronic structure of the free ligands oligomeric species Both ground state and 1e - oxidized monomers, dimers and trimmers were computed using FHI-aims with initial structures obtains from the PBE functional, and electron energies obtained from MP2 single point calculations with and a sufficiently large basis set (i.e. tight ) to include polarization and diffuse functions on light elements. The dimer and trimer species were constructed beginning with the crystallographically obtained geometries and allowed to relax both with constrained carboxylate carbons and full degrees of geometric freedom. The lowest energy structures were taken for the energetic comparisons as presented in Table 1. S6

7 Figure S2. One independent ssp network found in the interpenetrated MIT-25. The connectivity observed in the ssp net is similar to the nbo net (shown simplified). S7

8 Figure S3. The electrostatic potential method of alignment (a and b) uses the tailing of electrostatic potential to plateaus and can be applied through any lattice plane of the material. As a result, the electrostatic potential method provides a chemical accurate estimate of pore aperture, compared to conventional geometric methods (c and d), which defines the pore size by the length between two opposite edges and vertex. S8

9 Figure S4. Thermogravimetric analysis plot for MIT-25. S9

10 Figure S5. BET surface analysis for MIT-25: (a) N 2 isotherm at 77 K, (b) V(1 P/P 0 ) vs. P/P 0 plot for determination of the upper limit of P/P 0 range, which is indicated by the black dashed line. (c) Selection of the P/P 0 range. Selected points are shown by the solid circles (). Table S1. BET parameters and pressure ranges used for the calculation of the BET surface area of MIT- 25. BET surface area 830 m 2 /g P/P 0 range C V m cm 3 /g STP P/P 0 (@ V m ) /( CC+1) The BET pressure range was selected based on the reported criteria. 6, 7 S10

11 Figure S6. Barrett-Joyner-Halenda (BJH) 8 pore size distribution curve for MIT-25. S11

12 Figure S7. PXRD patterns for (a) MIT-25 and (b) MIT-26. S12

13 Figure S8. Multifrequency continuous-wave EPR spectra of MIT-25 at 298 K. (a) 9 GHz spectrum recorded at 0.1 mt modulation amplitude, 100 khz, 1 mw power, 1 scan, 1024 points. (b) 34 GHz spectrum recorded at 0.1 mt modulation amplitude, 100 khz, 0.04 mw power, 8 scans, 1024 points. S13

14 Figure S9. Structural and topology representation of MIT-26: The H 4 TBAPy ligand and Mg 2+ octahedral coordination environment may be represented by the red shape and blue polyhedron, respectively (a). Twelve Mg 2+ atoms are linked by a TBAPy dimer (b,f). This connectivity yields two similar pores, in the a direction (c,g)view down through the a axis (c). MIT-26 has I 0 O 2 connectivity (d,h). The inorganic SBU is composed of three octahedral Mg 2+ : each terminal metal is bridged to the central Mg 2+ by two bidentate carboxylates and one water molecule (e). Blue, red, white and gray spheres represent Mg, O, H and C atoms, respectively. All H atoms in (g) and aromatic H atoms in (f) and (h) were omitted for clarity. S14

15 Figure S10. The electrostatic potential projected across the electron density maps for the H4TTFTB neutral and 1e- oxidized species (a and b, respectively) and the H4TBAPy neutral and 1e- oxidized species (c and d, respectively). Positive potential indicates regions of positive charge, e.g. in the oxidized species the sea foam color indicates the locality of the hole. S15

16 Figure S11. Thermal ellipsoid plot of (a) MIT-25 and (b) MIT-26. Table S2. Selected bond distances for MIT-25. MIT-25 TTF TCNQ 9 Neutral TTF 10 C(1) C(4) (5) (4) (3) C(1) S(1) (4) (3) (2) C(1) S(2) (4) (3) (2) C(4) S(3) (4) C(4) S(4) (4) C(7) C(7) (1) C(7) S(5) (4) C(7) S(6) (4) S16

17 Table S3. X-ray experimental details for MIT-25. Identification code Mg-TTFTB Empirical formula C 306 H 144 Mg 6 O 72 S 36 Formula weight Temperature 100 K Wavelength Å Crystal system Hexagonal Space group R -3 Unit cell dimensions a = (10) Å b = (10) Å c = (17) Å α = 90 º β = 90 º γ = 120 º Volume 37804(13) Å 3 Z 3 Density (calculated) mg/m 3 Absorption coefficient mm -1 F(000) 9612 Crystal size 0.06 x 0.01 x 0.01 mm 3 θ range for data collection to Index ranges -88<=h<=90, -76<=k<=90, -14<=l<=11 Reflections collected Independent reflections [R int = ] Completeness to theta = % Absorption correction Multi-scan Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters / 3509 / 750 Goodness-of-fit a on F b R c wr a GOF = (Σ w(f 2 o F 2 c ) 2 /(n p)) 1/2 where n is the number of data and p is the number of parameters refined. b R 1 = Σ F o F c / Σ F o. c wr 2 = (Σ(w(F o 2 F c 2 ) 2 )/ Σ (w(f o 2 ) 2 )) 1/2. S17

18 Table S4. X-ray experimental details for MIT-26. Identification code Mg-TBAPy Empirical formula C H Mg 0.75 N 0.50 O 5.50 Formula weight Temperature 100 K Wavelength Å Crystal system Triclinic Space group P -1 Unit cell dimensions a = (8) Å b = (11) Å c = (12) Å α = (16) º β = (15) º γ = (14) º Volume (3) Å 3 Z 4 Density (calculated) mg/m 3 Absorption coefficient mm -1 F(000) 856 Crystal size x x mm 3 θ range for data collection to Index ranges -16<=h<=16, -21<=k<=21, -23<=l<=23 Reflections collected Independent reflections [R int = ] Completeness to theta = % Absorption correction Multi-scan Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters / 1016 / 598 Goodness-of-fit a on F b R c wr a GOF = (Σ w(f 2 o F 2 c ) 2 /(n p)) 1/2 where n is the number of data and p is the number of parameters refined. b R 1 = Σ F o F c / Σ F o. c wr 2 = (Σ(w(F o 2 F c 2 ) 2 )/ Σ (w(f o 2 ) 2 )) 1/2. S18

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