Based Coordination Polymers and MetalâOrganic Frameworks

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Article 3

CH-tagged Bis(pyrazolato)-based CPs and MOFs: An Experimental and Theoretical Insight Nello Mosca, Rebecca Vismara, José A. Fernandes, Silvia Casassa, Konstantin V. Domasevitch, Esther Bailon-Garcia, Francisco J. Maldonado-Hodar, Claudio Pettinari, and SIMONA GALLI Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Crystal Growth & Design

CH3-tagged Bis(pyrazolato)-based CPs and MOFs: An Experimental and Theoretical Insight

Nello Mosca,a Rebecca Vismara,b José A. Fernandes,b Silvia Casassa,c Konstantin V. Domasevitch,d Esther Bailón-García,e Francisco J. Maldonado-Hódar,e Claudio Pettinari,a Simona Gallib,f,*

a

Scuola del Farmaco e dei Prodotti della Salute, Università di Camerino, Via S. Agostino 1, 62032 Camerino, Italy. b

Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy.

c

Dipartimento di Chimica, Università di Torino and Nanostructured Interfaces and Surfaces Center of Excellence, Via P. Giuria 5, 10125 Torino, Italy. d

National Taras Shevchenko University of Kyiv, Volodimirska Str. 64, 01033 Kyiv, Ukraine.

e

Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Avda de Fuentenueva s/n, 18071 Granada, Spain. f

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, Via Giusti 9, 50121 Firenze, Italy.

ACS Paragon Plus Environment

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ABSTRACT The novel coordination polymers M(Me2BPZ) (M = Co, Zn; H2Me2BPZ = 3,3’-dimethyl-1H,1’H4,4’-bipyrazole), M(H2Me2BPZ)(CH3COO)2(H2O)2 (M = Co, Ni), and Cu(H2Me2BPZ)(Cl)2 were isolated along conventional or solvothermal routes. Their crystal structure was unveiled by powder X-ray diffraction (PXRD), while their thermal stability was assessed by coupling TGA to variabletemperature PXRD. The textural properties of the M(Me2BPZ) (M = Co, Zn) compounds, featuring 3-D open frameworks with 1-D channels, were assessed by N2 and CO2 adsorption at 77 and 273 K, respectively, and compared to those of the non-methylated isostructural counterparts M(BPZ) (M = Co, Zn; H2BPZ = 4,4’-bipyrazole). The positive effect of the methyl groups in CO2 adsorption, suggested by the adsorption energy trend [Eads(M(Me2BPZ)) > Eads(M(BPZ))] and substantiated by theoretical calculations at the B3LYP-D3 level coupled to topological analyses, is counterbalanced by the higher steric hindrance of Me2BPZ2- vs. BPZ2-, finally reducing the amount of gas adsorbed by the M(Me2BPZ) couple vs. the M(BPZ) one.

KEYWORDS Metal-organic framework, poly(pyrazolato)-based ligand, powder X-ray diffraction, N2 adsorption, CO2 adsorption, theoretical calculations, topological analysis


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1. INTRODUCTION The vast fields of coordination polymers (CPs)1 and metal-organic frameworks (MOFs) 2,3,4,5 have been uncessantly developing due to their potentiality in a wide range of key functional applications, spanning from luminescence to magnetism, heterogeneous catalysis, sensing, gas storage, gas or liquid separation, drug delivery and imaging – to mention only a few. CPs and MOFs are hybrid organic-inorganic compounds built up by metal ions or metal-containing clusters connected by poly(topic) ligands within polymeric networks the topology, crystal structure and functional behavior of which depend on the stereochemical requirements of the metal ion, as well as the hapticity, size, and functionalization of the ligand. Poly(azolate)-based spacers have gained increasing attention

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as an alternative to

poly(carboxylate)-based ones in the design and preparation of thermally stable, functional CPs and MOFs. In this context, we7,8,9,10,11 and others12,13,14,15 have contributed to the growth of the classes of poly(pyrazolate)-based CPs and MOFs. After reporting on the M(BPZ)16 (M = Co, Zn; H2BPZ = 4,4’-bipyrazole, Scheme 1) and M(Me4BPZ)17 (M = Co, Zn; H2Me4BPZ = 3,3’,5,5’-tetramethyl4,4’-bipyrazole, Scheme 1) derivatives, we have focused the attention on the ligand 3,3’-dimethyl1H,1’H-4,4’-bipyrazole (H2Me2BPZ, Scheme 1). Hereafter, we report on the synthesis and investigation of the structural aspects and thermal behavior

of

the

coordination

polymers

M(Me2BPZ)

(M

=

Co,

Zn),

M(H2Me2BPZ)(CH3COO)2(H2O)2 (M = Co, Ni), and Cu(H2Me2BPZ)(Cl)2. Capture of carbon dioxide from anthropogenic emissions is one of the main challenges of current research at the academic and industrial level, as this greenhouse gas contributes more than 60% to global warming. CO2 capture and storage from exhausted gases by nanoporous adsorbents, such as MOFs,18,19,20 have been receiving remarkable consideration. In the light of this, the textural properties of the M(Me2BPZ) (M = Co, Zn) derivatives, featuring 3-D open frameworks, were probed by N2 and CO2 adsorption at 77 and 273 K, respectively, and compared to those of the non-methylated isostructural counterparts M(BPZ)16 (M = Co, Zn), to shed light on the role of the –CH3 tags in the ACS Paragon Plus Environment

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adsorption of CO2. Ab initio periodic calculations at the B3LYP-D3 level were carried out to complete the insight obtained by CO2 adsorption.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 3,3’-Dimethyl-1H,1H’-4,4’-bipyrazole (H2Me2BPZ) and 4,4’bipyrazole (H2BPZ) were synthesized following the synthetic methodologies previously published by Sharko et al.21 and Boldog et al.,22 respectively. All of the chemicals and reagents employed were purchased from commercial suppliers and used as received without further purification. All of the solvents were distilled prior to use. IR spectra were recorded as neat from 4000 to 600 cm-1 with a PerkinElmer Spectrum One System instrument. Elemental analyses (C, H, and N) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer. Thermogravimetric analyses (TGA) were carried out under a N2 flow with a Perkin Pyris 1 thermal analyzer with heating rates in the range 5−10 °C/min. Powder X-ray diffraction (PXRD) qualitative analyses were carried out with a Bruker D8 Advance diffractometer (see Section 2.9 for the instrument specifics), acquiring data at room temperature in the 3–35° 2θ range, with steps of 0.02°, and time per step of 1 s. The nature and purity of all the batches employed for the functional characterization were assessed by elemental analysis, IR spectroscopy and PXRD. 2.2. Synthesis of Co(Me2BPZ)·0.9DMF (Co-Me2BPZ·S). H2Me2BPZ (0.0324 g, 0.200 mmol) was dissolved in dimethylformamide (DMF) (8 mL). Then, Co(CH3COO)2·4H2O (0.0498 g, 0.200 mmol) was added. The mixture was left under stirring in a high-pressure glass tube at 120 °C for 24 h, until a violet precipitate appeared. The precipitate was filtered off, washed with hot acetone (2 × 10 mL), and dried under vacuum. Yield: 86%. Co-Me2BPZ·S is insoluble in dimethyl sulfoxide (DMSO), alcohols, acetone, acetonitrile (CH3CN), chlorinated solvents and water. Elem. anal. Calc. for C8H8CoN4·0.9(C3H7NO) (fw = 284.89 g/mol): C, 44.97; H, 5.04; N, 24.01%. Found: C, 44.28; H, 4.68; N, 24.31%. IR (neat, cm-1): 2920 (vw) [ν(C-Haliphatic)], 1674 (vs) [ν(C=O)], 1506 (m) [ν(C=C + C=N)], 1443 (m), 1383 (w), 1345 (s), 1276 (m), 1097 (vs), 1060 (w), 994 (w), 969 (s), ACS Paragon Plus Environment

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824 (m). Co-Me2BPZ·S may be isolated also by using Co(NO3)2·6H2O in either solvothermal conditions, or methanol at 100 °C for 1 day in the presence of aq. KOH (0.4 mmol). 2.3. Synthesis of Zn(Me2BPZ)·0.5DMF (Zn-Me2BPZ·S). H2Me2BPZ (0.0324 g, 0.200 mmol) was dissolved in water (8 mL). Then, Zn(CH3COO)2·2H2O (0.0438 g, 0.200 mmol) was added. The mixture was left under stirring at room temperature for 24 h. A white precipitate was obtained, filtered off, washed with hot DMF (2 × 10 mL), and dried under vacuum. Yield: 81%. ZnMe2BPZ·S is insoluble in DMSO, alcohols, acetone, CH3CN, chlorinated solvents and water. Elem. anal. Calc. for C8H8N4Zn·0.5(C3H7NO) (fw = 262.11 g/mol): C, 43.53; H, 4.42; N, 24.05%. Found: C, 43.57; H, 4.53; N, 23.56%. IR (neat, cm-1): 2924 (vw) [ν(C-Haliphatic)], 1675 (vs) [ν(C=O)], 1506 (m) [ν(C=C + C=N)], 1447 (m), 1382 (w), 1351 (s), 1108 (s), 1052 (w), 997 (w), 979 (s), 826 (m). Zn-Me2BPZ may be isolated in the form of microcrystalline powders also by using other ZnX2 salts (X = NO3- or ClO4-). 2.4. Synthesis of Co(H2Me2BPZ)(CH3COO)2(H2O)2 (Co-H2Me2BPZ). H2Me2BPZ (0.0324 g, 0.200 mmol) was dissolved in methanol (CH3OH) (10 mL). Then, Co(CH3COO)2·4H2O (0.0498 g, 0.200 mmol) was added. The mixture was left under stirring at room temperature for 24 h. A violet precipitate was obtained, filtered off, washed with hot methanol (2 × 10 mL), and dried under vacuum. Yield: 67%. Co-H2Me2BPZ is insoluble in DMSO, CH3CN, alcohols, acetone, chlorinated solvents and water. Elem. anal. Calc. for C12H20CoN4O6 (fw = 375.22 g/mol): C, 38.41; H, 5.37; N, 14.93%. Found: C, 38.68; H, 5.22; N, 15.64%. IR (neat, cm-1): 3336-3200 (br) [ν(O−H) + (N−H)], 2621 (m) [ν(C-H)], 1564 (s) [νasym(C=O)], 1398 (vs) [νsym(C=O)], 1339 (m), 1298 (w), 1114 (s), 1049 (vw), 1017 (m), 955 (vs), 927 (s), 853 (vw). 2.5. Synthesis of Ni(H2Me2BPZ)(CH3COO)2(H2O)2 (Ni-H2Me2BPZ). H2Me2BPZ (0.0324 g, 0.200 mmol) was dissolved in CH3OH (10 mL). Then Ni(CH3COO)2·4H2O (0.0496 g, 0.200 mmol) was added. The mixture was left under stirring at room temperature for 24 h. A cyan precipitate was obtained, filtered off, washed with hot methanol (2 × 10 mL), and dried under vacuum. Yield: 55%. Ni-H2Me2BPZ is insoluble in DMSO, water, alcohols, chlorinated solvents, acetone, and CH3CN. ACS Paragon Plus Environment

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Elem. anal. Calc. for C12H20N4NiO6 (fw = 374.99 g/mol): C, 38.43; H, 5.38; N, 14.94%. Found: C, 38.36; H, 5.31; N, 14.66%. IR (neat, cm-1): 3384-3200 (br) [ν(O−H) + (N−H)], 2617 (m) [ν(C-H)], 1564 (s) [νasym(C=O)], 1403 (vs) [νsym(C=O)], 1340 (m), 1305 (w), 1114 (s), 1042 (vw), 1018 (m), 958 (vs), 928 (s), 852 (vw). 2.6. Synthesis of Cu(H2Me2BPZ)(Cl)2 (Cu-H2Me2BPZ). H2Me2BPZ (0.0324 g, 0.200 mmol) was dissolved in DMF (8 mL). Then, Cu(Cl)2·2H2O (0.0341 g, 0.200 mmol) was added. The mixture was left under stirring in a high-pressure glass tube at 120 °C for 24 h. A green precipitate was formed, filtered off, washed with hot acetone (2 × 10 mL), and dried under vacuum. While washing, the color of the precipitate turned from cyan to green, reasonably due to a change of the stereochemistry of the metal center. 23 Yield: 48%. Cu-H2Me2BPZ is insoluble in chlorinated solvents, DMSO, alcohols, water, acetone, and CH3CN. Elem. anal. Calc. for C8H10Cl2CuN4 (fw = 296.63 g/mol): C, 32.39; H, 3.40; N, 18.89%. Found: C, 32.83; H, 3.36; N, 18.38%. IR (neat, cm-1): 3275 (s) [ν(C−Haromatic)], 2933 (v) [ν(C-Haliphatic)], 1666 (m), 1532-1506 (s) [ν(C=C + C=N)], 1456 (m), 1415 (m), 1315 (m), 1278 (vs), 1188 (m), 1112 (vs), 1002 (s), 961 (vs), 857 (vs). 2.7. Synthesis of Co(BPZ) (Co-BPZ). Co-BPZ was synthesized following a previously published synthetic methodology,16 namely: H2BPZ (0.0268 g, 0.200 mmol) was dissolved in DMF (15 mL). Then, Co(CH3COO)2·4H2O (0.0498 g, 0.200 mmol) was added. The mixture was left under stirring in a high-pressure glass tube at 120 °C for 24 h, until a violet precipitate appeared. The precipitate was filtered off, washed with dichloromethane (2 × 10 mL), and dried under vacuum. Yield: 76%. Elem. anal. Calc. for C6H4CoN4 (fw = 191.05 g/mol): C, 37.22; H, 2.11; N, 29.32%. Found: C, 37.31; H, 2.20; N, 29.45%. IR (neat, cm-1): 3098 (vw) [ν(C-Haromatic)], 1515 (m) [ν(C=C + C=N)], 1434 (m), 1384 (s), 1293 (w), 1258 (s), 1161 (s), 1159 (m), 1042 (s), 1010 (m), 914 (s), 826 (s). 2.8. Synthesis of Zn(BPZ) (Zn-BPZ). Zn-BPZ was synthesized following a previously published synthetic path,16 namely: H2BPZ (0.0268 g, 0.200 mmol) was dissolved in CH3OH (30 mL). Then, sodium methoxide (0.0216 g, 0.4 mmol) was added. After ½ h warming at 45 °C with ACS Paragon Plus Environment

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concomitant stirring, Zn(CH3COO)2·2H2O (0.0438 g, 0.200 mmol) was added. The mixture was left under stirring at room temperature for 24 h. A white precipitate was obtained, filtered off, washed with hot CH3OH (2 × 10 mL), and dried under vacuum. Yield: 72%. Elem. anal. Calc. for C6H4N4Zn (fw = 197.51 g/mol): C, 36.49; H, 2.04; N, 28.37%. Found: C, 36.33; H, 2.21; N, 27.84%. IR (neat, cm-1): 3094 (vw) [ν(C-Haromatic)], 1519 (m) [ν(C=C + C=N)], 1447 (m), 1386 (s), 1291 (w), 1265 (s), 1166 (b,s), 1054 (s), 1016 (m), 917 (s), 829 (s). 2.9. Powder X-ray Diffraction Structural Characterization. Microcrystalline powders of CoMe2BPZ·S, Zn-Me2BPZ·S, Co-H2Me2BPZ, Ni-H2Me2BPZ, and Cu-H2Me2BPZ were deposited in the hollow of a silicon zero-background plate 0.2 mm deep (supplied by Assing Srl, Monterotondo, Italy). Data acquisitions were performed on a vertical-scan Bruker AXS D8 Advance θ:θ diffractometer, equipped with a Bruker Lynxeye linear position-sensitive detector, a Cu X-ray tube (λ = 1.5418 Å), primary beam Soller slits (2.5°), divergence slit (1 mm), antiscatter slit (8 mm), a filter of nickel in the diffracted beam. The generator was set at 40 kV and 40 mA. After preliminary acquisitions for fingerprinting analysis, typically performed in the 3–35° 2θ range, diffraction datasets for structure determination were collected up to 105° 2θ, with steps of 0.02°, with an overall scan time of approximately 12 h. A standard peak search, followed by profile fitting, enabled us to estimate the low-to-medium-angle peak maximum positions which, through the Singular Value Decomposition algorithm24 implemented in TOPAS-R,25 provided approximate unit cell parameters for all the compounds. Space groups were assigned based on the observed systematic absences. For Co-Me2BPZ·S and Zn-Me2BPZ·S, indexing suggested a unit cell of tetragonal metrics (Co-Me2BPZ·S: a = 8.91 Å, c = 7.45 Å, V = 591.9 Å3; Zn-Me2BPZ·S: a = 8.92 Å, c = 7.44 Å, V = 591.5 Å3;), as in the case of the M-BPZ16 (M = Co, Zn) analogues. No structure solution was found in the tetragonal P space groups compatible with the systematic absences, nor in the isomorphic orthorhombic subgroup Cccm. Only decreasing the symmetry down to monoclinic C2/c granted us a successful structure determination. Structure solutions were performed with TOPAS-R by a combined Monte Carlo/Simulated Annealing approach using a rigid body to ACS Paragon Plus Environment

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describe the crystallographically independent portion of the ligand, letting the position of its center of mass and its orientation (when allowed by symmetry) to be refined. The rigid body was built up through the Z-matrix formalism, assigning average values to bond distances and angles.26 For CoMe2BPZ·S and Zn-Me2BPZ·S a rigid body was used also to model DMF which, in both cases, was found disordered within the channels. In all the cases, a model with positional disorder affecting the methyl groups on the positions 3 and 5 of the penta-atomic ring was adopted during the structure refinement, carried out with the Rietveld method: The introduction of disorder affecting the methyl groups was found beneficial for the refinement (as witnessed by the lowering of Rwp) in the case of Cu-H2Me2BPZ. For the other compounds, a 100% occupancy factor for one of the (3 or 5) positions was eventually adopted. The background was modeled by a polynomial function of the Chebyshev type, while peak profiles were described through the Fundamental Parameters Approach.27 A common, refined isotropic thermal factor (Biso) was attributed to all atoms, except to the metal and chloride ions, to which the isotropic thermal factor Biso(M, Cl) = Biso−2.0 (Å2) was assigned. A spherical harmonics description of the anisotropic peak broadening was necessary for all the compounds. In the case of Co-Me2BPZ·S and Zn-Me2BPZ·S even the use of spherical harmonics was not completely satisfactory to model the strong anisotropy of the peak shapes, which could be due to paracrystallinity.28,29 A correction for preferred orientation was applied, adopting the March-Dollase model, 30 , 31 for Ni-H2Me2BPZ along the [001] direction. The final Rietveld refinement plots are shown in Figures S1 and S2 of the Electronic Supplementary Information. The pertinent CIF files are supplied as Electronic Supplementary Information. Crystallographic data for Co-Me2BPZ·S: C8H8CoN4·0.5DMF,

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fw = 255.65 g mol−1,

monoclinic, C2/c, a = 12.581(2) Å, b = 12.632(1) Å, c = 7.4473(5) Å, β = 90.081(9)°, V = 1183.5(2) Å3, Z = 8, Z’ = 4, ρ = 1.441 g cm−3, F(000) = 524, RBragg = 0.010, Rp = 0.016 and Rwp = 0.021, for 4826 data and 44 parameters in the 8.5-105° (2θ) range. CCDC No. 1526565. Crystallographic data for Zn-Me2BPZ·S: C8H8N4Zn·0.3DMF,32 fw = 247.48 g mol−1, monoclinic, C2/c, a = 12.608(3) Å, b = 12.615(3) Å, c = 7.4412(4) Å, β = 89.805(9)°, V = 1183.5(4) ACS Paragon Plus Environment

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Å3, Z = 8, Z’ = 4, ρ = 1.266 g cm−3, F(000) = 507.2, RBragg = 0.043, Rp = 0.067 and Rwp = 0.085, for 4826 data and 37 parameters in the 8.5-105° (2θ) range. CCDC No. 1526569. Crystallographic data for Co-H2Me2BPZ: C12H18CoN4O6, fw = 375.25 g mol−1, monoclinic, C2/m, a = 11.7372(4) Å, b = 7.40881(11) Å, c = 10.0535(2) Å, β = 111.991(2)°, V = 810.63(4) Å3, Z = 8, Z’ = 2, ρ = 1.529 g cm−3, F(000) = 386, RBragg = 0.015, Rp = 0.023 and Rwp = 0.035, for 4851 data and 49 parameters in the 8-105° (2θ) range. CCDC No. 1526564. Crystallographic data for Ni-H2Me2BPZ: C12H18NiN4O6, fw = 372.99 g mol−1, monoclinic, C2/m, a = 11.6615(3) Å, b = 7.3777(2) Å, c = 9.9810(4) Å, β = 111.775(3)°, V = 797.44(5) Å3, Z = 8, Z’ = 2, ρ = 1.553 g cm−3, F(000) = 388, RBragg = 0.034, Rp = 0.026 and Rwp = 0.042, for 4851 data and 43 parameters in the 8-105° (2θ) range. CCDC No. 1526568. Crystallographic data for Cu-H2Me2BPZ: C8H10Cl2CuN4, fw = 296.65 g mol−1, triclinic, P-1, a = 3.8821(2) Å, b = 7.1648(3) Å, c = 10.0461(5) Å, α = 112.758(2)°, β = 98.113(5)°, γ = 87.622(5)°, V = 255.07(2) Å3, Z = 2, Z’ = 1, ρ = 1.931 g cm−3, F(000) = 149, RBragg = 0.025, Rp = 0.023 and Rwp = 0.034, for 4851 data and 35 parameters in the 8.5-105° (2θ) range. CCDC No. 1526566. 2.10. Variable-temperature Powder X-ray Diffraction. To complement the thermogravimetric analyses, the thermal behavior of Co-Me2BPZ·S, Zn-Me2BPZ·S, Co-H2Me2BPZ, Ni-H2Me2BPZ, and Cu-H2Me2BPZ was investigated in situ and operando by variable-temperature powder X-ray diffraction.33 As a general procedure, using a custom-made sample heater (Officina Elettrotecnica di Tenno, Ponte Arche, Italy), 20-mg samples of as-synthesized compounds were heated in air from 30 °C up to decomposition, with steps of 20 °C; a PXRD pattern was acquired at each step, covering a sensible low-to-medium-angle 2θ range. Treating the data acquired before loss of crystallinity by means of Le Bail parametric refinements enabled us to disclose the behavior of the unit cell parameters as a function of the temperature. 2.11. Textural Characterization. Specific surface area and pore texture of the M-BPZ and MMe2BPZ (M = Co, Zn) MOFs were estimated by N2 and CO2 adsorption at 77 and 273 K, respectively. Adsorption isotherms were measured by the volumetric method with a Quantachrome ACS Paragon Plus Environment

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Autosorb 1 instrument. Batches of ca. 50 mg of as-synthesized materials were introduced into preweighed analysis tubes and outgassed overnight at 110 °C under high vacuum (nominally 10−6 mbar) before running the adsorption experiments. The two probes we adopted are complementary and allow analyzing different ranges of porosity:34 CO2 adsorption is used to assess the narrower microporosity (mean micropore width L0 < 0.7 nm), where N2 adsorption can be kinetically restricted.35 On the contrary, according to the Gurvitch rule 36 the volume of N2 adsorbed at the relative pressure of 0.95 (W0.95) is a measure of both the micro- and meso-pore volumes, provided that there are no constrictions at the micropore entrance. Hence, W0.95 is considered as the total pore volume, and the volume of N2 adsorbed within the relative pressure range of 0.40–0.95 is considered as the mesopore volume. 37 BJH 38 and NL-DFT 39 ,40 are the most commonly applied methods to determine pore size distribution (PSD). In the present work, NL-DFT was employed to determine the PSD of M-BPZ and M-Me2BPZ, as it was suggested to provide accurate results in MOFs textural characterization. 41 ,42 The specific surface areas were obtained through the BET method by fitting the N2-adsorption data at low relative pressures (P/P0 < 0.35).43 The DubininRaduskevich equation44,45 (Equation 1) was used to fit both N2 and CO2 isotherms:   A n W = W0 exp  −     β Eads 

  

(1)

where W is the amount of gas absorbed at the relative pressure P/P0; W0 is the limiting amount filling the micropores; A is the differential molar work given by A = RT lnP0/P; β is the affinity coefficient (0.33 and 0.35 for N2 and CO2, respectively); Eads is the characteristic adsorption energy. N2 and CO2 molar volumes were taken as 34.65 and 43.01 cm3/mol, respectively. Once Eads was known, the mean micropore width, L0, was obtained by applying the Stoeckli equation46 (Equation 2):

L0 (nm ) =

Eads

10.8 kJmol −1 − 11.4

(

)


(2)

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which is valid for Eads values between 42 and 20 kJ/mol, corresponding to pore widths between 0.35 and 1.3 nm. 2.12. Theoretical Calculations. The Zn-BPZ 47 and M-Me2BPZ MOFs (M = Co, Zn) were characterized by means of ab initio calculations performed with the quantum-mechanics code CRYSTAL14, 48 which solves the Schrödinger equation for periodic systems in a basis set of localized atomic orbitals. Neglecting the clathrated solvent, the Becke-Lee-Yang-Parr49,50 (B3LYP) hybrid functional was adopted to perform the full geometry optimization of the crystal structure and calculate charge density. In the subsequent study of CO2 diffusion into the channels, for a reliable estimate of the dispersive contributions to the binding energy the D3 a posteriori correction, originally proposed by Grimme 51 and implemented in the CRYSTAL code 52 (B3LYP-D3), was exploited. Atoms were described with all electrons basis sets of 6-21* quality53 and the counterpoise method54 was used to correct the binding energy for the basis set superposition error.55 The PackMonkhorst/Gilat shrinking factors for the k-point sampling of the reciprocal space were set to 4, corresponding to 24 points at which the Hamiltonian matrix was diagonalized. The accuracy of the integral calculations was increased by setting the five tolerances to 7, 7, 7, 10 and 20. The adsorption energy, Eads, was calculated at the B3LYP-D3 level according to the formula:

 E(nCO @ MOF) − E(MOF) − n × E(CO )   2 2 E ads =       n 

(3)

where E(nCO2@MOF), E(MOF), and E(CO2) are the total energies of the MOFs with and without CO2, and of carbon dioxide in the gas phase, respectively, and n corresponds to the number of CO2 molecules per cell.56 Standard enthalpies, H(T), at 0 and 298 K were calculated as follows:

H(T) = E + EZPE + ET + P ×V

(4)

by adding at the pressure per volume term the zero-point (EZPE) and thermal (ET) energy contributions arising from the analysis of the nuclear motion. The heat of adsorption, Hads(T), was then estimated with the same formula reported in Equation 3 by substituting E(nCO2@MOF), E(MOF), E(CO2) with H(nCO2@MOF), H(MOF), H(CO2), respectively. Complete phonon ACS Paragon Plus Environment

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frequencies, at the k = 0 Γ point, were computed within the harmonic approximation by diagonalizing the mass-weighted Hessian matrix,

57

and the host-guest interactions were

characterized by exploiting the potentiality of Bader topological analysis of the electron density,58,59 as implemented in the TOPOND program60 incorporated in CRYSTAL14.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Preliminary Characterization. The metal-organic frameworks MMe2BPZ·S (M = Zn and Co; S = solvent) can be isolated in good yields, as microcrystalline powders, along both conventional and solvothermal synthetic routes. Zn-Me2BPZ·S can be isolated by using Zn(CH3COO)2·2H2O as the source of zinc(II) ions in the presence of water, at room temperature. Alternatively, it can be prepared in DMF, in solvothermal conditions, by employing either Zn(CH3COO)2·2H2O, or other ZnX2 salts (X = NO3- or ClO4-). Co-Me2BPZ·S can be prepared in DMF, in solvothermal conditions, by using Co(CH3COO)2·4H2O or Co(NO3)2·6H2O. Alternatively, it can be obtained by gentle warming of Co(NO3)2·6H2O and the ligand in methanol in presence of a base. Worthy of note, due to the higher solubility of H2Me2BPZ vs. H2BPZ in DMF imparted by the methyl groups, a lower amount of solvent is necessary to isolate the methylated MOFs (see the Experimental Section). With reference to the batches isolated in DMF, IR spectroscopy purports the presence of clathrated solvent: the bands centered at 1673 and 1674 cm−1 in the spectra of Zn-Me2BPZ·S and Co-Me2BPZ·S, respectively, can be ascribed to the C=O stretching of DMF.61 The absence of N-H stretching bands indicates the formation of Me2BPZ2dianions. In the absence of a base along a conventional route, reacting M(II) acetates (M = Co, Ni) with H2Me2BPZ

in

methanol,

at

room

temperature,

yielded

the

compounds

M(H2Me2BPZ)(CH3COO)2(H2O)2 (M-H2Me2BPZ). IR spectroscopy suggests the presence of neutral ligands and water (N-H and O-H stretching bands in the range 3384-3336 cm−1 and 26172620 cm−1, respectively). Moreover, the two strong bands peaked at 1564 cm−1 and 1398−1403 ACS Paragon Plus Environment

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cm−1 correspond to the asymmetric (νa) and symmetric (νs) stretching modes of the acetate anion, respectively. Their difference, ∆ν = νa − νs = 166-161 cm−1, likely indicates a bridging coordination mode.62,63 The solvothermal reaction between Cu(II) chloride and H2Me2BPZ led to the formation of Cu(H2Me2BPZ)(Cl)2 (Cu-H2Me2BPZ), for which the presence of H2Me2BPZ is again suggested by IR spectroscopy. For the sake of completeness, it is important to report that the attempts carried out to obtain M(Me2BPZ) (M = Cu and Ni) MOFs adopting other experimental conditions did not led to the expected result. 3.2. Crystal Structure Analysis. 3.2.1. Crystal and Molecular Structure of Co-Me2BPZ·S and Zn-Me2BPZ·S. Co-Me2BPZ·S and Zn-Me2BPZ·S crystallize in the monoclinic space group C2/c and are isostructural. The asymmetric unit contains one M(II) ion (M = Co, Zn) and one Me2BPZ2dianion, both in special positions. Co-Me2BPZ·S and Zn-Me2BPZ·S are isoreticular64 to the two couples of MOFs M-BPZ16 (M = Co, Zn; H2BPZ = 4,4’-bipyrazole), crystallizing in the tetragonal space group P42/mmc, and M-Me4BPZ17 (M = Co, Zn; H2Me4BPZ = 3,3’,5,5’-tetramethyl-4,4’bipyrazole), crystallizing in the tetragonal space group P-42c. Hence, Co-Me2BPZ·S and ZnMe2BPZ·S feature a (4,4)-connected 3-D porous network of PtS topology. Tetrahedral M(II) ions, coordinated by four nitrogen atoms of four Me2BPZ2- ligands, [Co-N distances 2.004(7) and 2.149(7) Å, N-Co-N angles in the range 96.6(4)° to 120.9(7)°; Zn-N distances 2.042(4) and 2.212(4) Å, N-Zn-N angles in the range 88.0(3)° to 112.05(8)°] act as nodes, while N,N’N’’,N’’’exo-tetradentate ligands act as spacers (Figure 1). 1-D channels running parallel to the [001] direction are present. The walls of the channels are decorated by the ligands, which keep the nodes, occupying the vertices, at a distance of 8.914(9) Å (M = Co) or 8.918(7) Å (M = Zn). The channels have a rhombic shape (diagonals of 12.581(2) and 12.632(1) Å for M = Co, 12.608(3) and 12.615(3) Å for M = Zn) and account, at ambient conditions, for an empty volume of ~38%.65 As expected, the empty volume and channel size decrease with the trend M-Me4BPZ < M-Me2BPZ < ACS Paragon Plus Environment

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M-BPZ (Table 1): worthy of note, the size of the channels is affected in a more incisive manner upon passing from M-Me2BPZ to M-Me4BPZ, than from M-BPZ to M-Me2BPZ: In the case of the M-BPZ and M-Me2BPZ compounds, the ligand is planar; at variance, in M-Me4BPZ, the steric hindrance of the methyl groups forces the torsion angle about the C-C exocyclic bond to be higher than 70°. As the empty volume decreases by only 4% on passing from M-BPZ to M-Me2BPZ and by 26% on passing from M-BPZ to M-Me4BPZ, for the functional characterization we compared the performance of the M-Me2BPZ MOFs only to that of the M-BPZ MOFs. Incidentally, in both Co-Me2BPZ·S and Zn-Me2BPZ·S the methyl groups are ordered in one of the two possible positions (3 or 5) of the pyrazolate rings. The reader is addressed to Section 2.10 for experimental details. 3.2.2. Crystal and Molecular Structure of Co-H2Me2BPZ and Ni-H2Me2BPZ. Co-H2Me2BPZ and Ni-H2Me2BPZ crystallize in the monoclinic space group C2/m and are isostructural. In the crystal structure, the metal center lays on a 2/m symmetry element, both the ligands on mirror planes, and the water molecule on a two-fold axis, this making the asymmetric unit one fourth of the formula unit. The metal ions show an octahedral coordination sphere of the type trans-MN2O4 defined by the nitrogen atoms of two crystallographically equivalent H2Me2BPZ ligands, one oxygen atom of two equivalent κ1O-acetate anions, and the oxygen atoms of two equivalent water molecules occupying the apical positions (Figure 2a) [Co-Owater 2.051(8) Å, Co-Oacetate 2.132(5) Å, Co-N 2.126(6) Å; Ni-Owater 2.02(1) Å, Ni-Oacetate 2.02(6) Å, Ni-N 2.100(2) Å]. N,N’-exo-bidentate H2Me2BPZ ligands bridge adjacent metal centers along 1-D linear chains running parallel to the [001] direction (the bridged M···M distance is equal to the length of the c-axis) (Figure 2a). The N1-H hydrogen atom of H2Me2BPZ is involved in an intramolecular hydrogen bond with the uncoordinated oxygen atom (O11) of an adjacent acetate ligand [Figure 2b; N1 and O11i: N···O 2.595(9) Å, N-H···O 176.7(8)° for Co-H2Me2BPZ; N···O 2.595(8) Å, N-H···O 168.8(7)° for NiH2Me2BPZ; symmetry code: i) -x, -y, -z]. O11 also behaves as a hydrogen bond acceptor towards two crystallographically equivalent water molecules of neighboring metal chains [O1W and O11ii/iii: ACS Paragon Plus Environment

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O···O 2.802(7) Å, O-H···O 171.3(2)° for Co-H2Me2BPZ; O···O 2.893(9) Å, O-H···O 169.3(2)° for Ni-H2Me2BPZ; symmetry codes: ii) -½+x, ½-y, z; iii) ½-x, ½-y, z]: a graph set motif of the type R24(8)66 can be envisaged (highlighted in green in Figure 2b). These features are in accordance with the bridging coordination mode suggested by IR spectroscopy for the acetate anion (see Section 3.1).

Overall,

Co-H2Me2BPZ

and

Ni-H2Me2BPZ

can

be

classified

as

3-D

hybrid

coordination/hydrogen bonded polymers. At ambient conditions, no empty volume is present.65 3.2.3. Crystal and Molecular Structure of Cu-H2Me2BPZ. Cu-H2Me2BPZ crystallizes in the triclinic space group P-1. The asymmetric unit comprises one Cu(II) metal center and one H2Me2BPZ ligand, both laying on crystallographic inversion centers, and one chloride anion, on a general position. The metal centers show a square planar geometry of the type trans-CuCl2N2 (Figure 3a) [Cu-Cl 2.309(5) Å, Cu-N 2.084(3) Å, N-Cu-Cl 90.0(3)°]. The ligand bridges two metal centres 9.8270(5) Å apart, forming a 1-D coordination polymer (Figure 3b) running parallel to the [011] direction of the unit cell. The polymer is almost planar, with the heavy atoms laying in the (1 -1 1) plane [highest deviation from the (1 -1 1) plane 2.3 Å for Cl1]. The N-H groups of the ligand are involved in hydrogen bonds with the chlorine atoms coordinated to the same metal centre (Figure 3b) [N1···Cl1 2.968(5) Å, N1-H···Cl 120.4(5)°]. Finally, along the [100] direction adjacent polymeric chains interact through π-π stacking between the pyrazole rings [Figure 3a; distance between centroids 3.8821(2) Å]. At ambient conditions, no empty volume is present.65 3.3. Thermal Behavior. 3.3.1. Thermal Behavior of Co-Me2BPZ·S and Zn-Me2BPZ·S. According to their TGA traces (Figure 4), the two compounds M-Me2BPZ·S (M = Co, Zn) are stable, under N2, up to ~340 °C and 460 °C, respectively. Loss of clathrated solvent is denounced, below 200 ºC, by weight losses of ~24% and ~23%, corresponding to ~1 and ~0.9 mol32 of DMF per formula unit (f.u.) for Co-Me2BPZ·S and Zn-Me2BPZ·S, respectively (calculated losses 25.0 and 24.4%, respectively). As assessed by in situ and operando VT-PXRD (Figures 5a and 5b), solvent loss is not accompanied by loss of crystallinity. As highlighted by a Le Bail parametric treatment of the data (Figures 5c and 5d), solvent loss brings about a unit cell volume shrinkage in ACS Paragon Plus Environment

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both compounds. However, the variation in volume has a different trend in the two cases. ZnMe2BPZ shows a step-like decrease of ~0.8% between 130 and 210 ºC, mainly related with the shrinkage of the c-axis. On the other hand, Co-Me2BPZ shows a smooth decrease of ~-1.3% in the temperature range 30-330 ºC, related with the decrease of the b- and c-axes. Above 350 ºC,33 both compounds start suffering from loss of crystallinity. 3.3.2. Thermal Behavior of Co-H2Me2BPZ and Ni-H2Me2BPZ. According to their TGA traces (Figure 4), the two compounds M-H2Me2BPZ are stable, under N2, only up to ~125 °C (M = Co) and 70 °C (M = Ni). The ~30% weight loss observed in the TGA trace of Co-H2Me2BPZ in the temperature range 125-175 °C is in agreement with the release of two molecules of acetic acid per f.u. (theoretical loss 32%). The weight loss of ~10% observed in the range 200-400 °C can be assigned to the loss of two molecules of water per f.u. (theoretical loss 9.6%). Ni-H2Me2BPZ undergoes three consecutive mass losses, namely: 10% (70-150 °C), 20% (150-250 °C) and 10% (250-400 °C), overall corresponding to the loss of two molecules of acetic acid and two molecules of water per f.u. (theoretical loss 41.6%). VT-PXRD (Figures S3a and S3b of the ESI) confirms the limited thermal robustness of the two materials and highlights that no phase change takes place before the first weight loss, which is accompanied by loss of crystallinity (observed starting from 150 °C33 for both compounds). A Le Bail parametric treatment of the data before loss of crystallinity enabled us to highlight that while the a- and c-axes undergo negligible variations in the temperature range 30-150 °C (Figures S3c and S3d of the ESI), the b-axis is affected by a slightly higher variation, increasing by 0.8%, with a concomitant increase of the unit cell volume amounting to 0.8% (M = Co) or 0.9% (M = Ni) in the same temperature range. The slight increase of the b-axis is reasonably promoted by a weakening of the inter-chain hydrogen bond interactions existing, at room temperature, along the [010] direction (Figure 2b). At least in the conditions essayed for the VT-PXRD experiments, loss of coordinated acetic acid and water molecules does not bring about the formation of microcrystalline homoleptic compounds of formula M(Me2BPZ), as observed in the past for Zn(CH3COO)(pz)(Hpz).67 ACS Paragon Plus Environment

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3.3.3. Thermal Behavior of Cu-H2Me2BPZ. According to its TGA trace (Figure 4) CuH2Me2BPZ decomposes starting from ~280 °C. VT-PXRD (Figure S4a in the ESI) confirms this evidence also in air and highlights that no phase change takes place before loss of crystallinity is observed (~330 °C).33 A Le Bail parametric treatment of the data reveals (Figure S4b in the ESI) that the unit cell axis undergoing the highest variation is the a-axis (increasing by 2.2% in the temperature range 30-330 °C), this occurrence suggesting a weakening of the π-π interactions along the [100] direction (Figure 3a). Due to thermal expansion, the unit cell volume increases by 2.3%. 3.4. Textural characterization. 3.4.1. N2 adsorption. The N2 adsorption isotherms measured for the MOFs M-BPZ and M-Me2BPZ at 77 K are depicted in Figures 6a and 6b, while the results obtained from their analysis by applying the DR and NL-DFT methods are collected in Table 2. All of the N2 adsorption isotherms are a mixture of type I and type II,68 corresponding to microporous and macroporous/non porous solids, respectively. Micropore filling occurs in all cases at very low relative pressure. In the case of Zn-Me2BPZ and Co-Me2BPZ, but mainly for the latter, the isotherms show a small knee at P/P0 < 0.1, denoting heterogeneity of the micropores (i.e. a wide pore size distribution). After the knee, the isotherms increase linearly and smoothly up to P/P0 ~0.8. Although this slope can be associated to the presence of mesoporosity, the absence of a significant hysteresis loop during desorption witnesses the absence of a significant porosity in the mesoporous range. With the exception of Zn-BPZ, the slope of the isotherms increases abruptly above P/P0 > 0.8; this occurrence is due to the presence of larger mesopores and macropores, possibly interparticle voids, and multilayer adsorption processes. The pore size distribution (PSD) obtained by the NL-DFT method (Figures 6c and 6d) shows in all cases a very sharp maximum in the micropore region, followed by an irregular distribution of larger pores; a second maximum is detected in the range of large mesopores with diameter of ~30 nm, which can be associated, as just commented, to inter-particle voids. Worthy of note, there is a good agreement (Table 2) among the values of the parameters provided by the different calculation methods (BET, DR or DFT) for a given MOF: Similar values of specific surface areas (SSAs) are obtained through the BET or DR equations, as ACS Paragon Plus Environment

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well similar micropore volumes (W0) through the DR or DFT analysis; however, a clear tendency is not observed on passing from the M-BPZ to the M-Me2BPZ series: Indeed, W0 increases, and consequently the surface area values, for the Co-containing MOFs but decreases in the case of the Zn-based MOFs. On the contrary, the mean micropore width (L0, Table 2) accessible to N2 in MBPZ is ~0.5 nm, while in the case of M-Me2BPZ a wider microporosity (L0 ~0.8 nm) is detected. These features are in very good agreement with those retrieved from the crystal structures (Table 1): for Zn-BPZ, the micropore width L0 = 0.52 nm perfectly fits the dimension of the 1-D channels (0.53×0.53 nm2). Similar conclusions can be drawn for Co-Me2BPZ and Zn-Me2BPZ: the two MOFs show 1-D channels 0.76×0.58 and 0.72×0.58 nm2 wide, respectively (Table 1), to be compared to an average micropore size of 0.79 and 0.77 nm, respectively. These results clearly highlight that, at 77 K, N2 reaches all the available microporosity of the MOFs under investigation and that no breathing or swelling effects are at work. Incidentally, either the shape of the N2adsorption isotherms or the specific surface area values of the M-BPZ MOFs are consistent with those previously found.16 3.4.2. CO2 adsorption. To ascertain the role of the methyl groups, adsorption of the quadrupolar molecule CO2 was investigated at 273 K for M-BPZ and M-Me2BPZ. The pertinent adsorption isotherms are collected in Figure 7, while the results obtained from their analysis are provided in Table 3. Incidentally, as these MOFs are eminently microporous, CO2 adsorption helps in complementing the study of their microporosity. 69 It is noteworthy that, in all of the cases, the micropore volume obtained from the N2-adsorption isotherms (W0(N2)) is smaller than that obtained from CO2-adsorption (W0(CO2)) (Tables 2 and 3, respectively). Typically, while W0(N2) > W0(CO2) when N2 condensation occurs upon filling large micropores and mesopores, W0(N2) < W0(CO2) when the accessibility of N2 into ultramicropores (

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