bipyrazolato-Based Porous Coordination Polymers - ACS Publications

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Enlarging an Isoreticular Family: 3,3′,5,5′-Tetramethyl-4,4′bipyrazolato-Based Porous Coordination Polymers Aurel Tăbăcaru,*,† Claudio Pettinari,† Ivan Timokhin,‡ Fabio Marchetti,‡ Francisco Carrasco-Marín,§ Francisco José Maldonado-Hódar,§ Simona Galli,*,# and Norberto Masciocchi# †

Scuola di Scienze del Farmaco e dei Prodotti della Salute, Università di Camerino, via S. Agostino 1, 62032 Camerino, Italy Scuola di Scienze e Tecnologie, Università di Camerino, via S. Agostino 1, 62032 Camerino, Italy § Departamento de Química Inorgánica, Universidad de Granada, Av. Fuentenueva s/n, 18071, Granada, Spain # Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy ‡

S Supporting Information *

ABSTRACT: The solvothermal reaction between the rigid spacer 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (H2Me4BPZ) and a number of transition metal salts promoted the formation of the coordination polymers [M(Me4BPZ)] (M = Zn, 1; Co, 2; Cd, 3; Cu, 4). Ab initio X-ray powder diffraction analyses revealed the main structural aspects of the four materials. 1 and 2 are representative examples of the so-called isoreticular strategy: isostructural to [M(BPZ)] and [M(BDP)] (H2BPZ = 4,4′bipyrazole; H2BDP = 1,4-bis(1H-pyrazol-4-yl)benzene), they feature three-dimensional (3-D) porous networks containing square-shaped channels. In 3, tetrahedral Cd(II) ions are arranged within homochiral helices reciprocally linked by radial Me4BPZ spacers, overall creating a 3-D nonporous network. Finally, the 3D porous framework of 4 comprises square Cu4 nodes linked to eight neighboring ones by the bridging spacers. Thermogravimetric analyses, coupled to variable-temperature X-ray powder diffraction, demonstrated the remarkable thermal robustness of all the materials, decomposing above 300 °C, and their stability for consecutive heating−cooling cycles. N2 and CO2 adsorption measurements at 77 and 273 K, respectively, were employed to probe the permanent porosity of the materials and to give a coherent picture of their textural properties including specific surface areas, micro- and mesopore volumes, as well as size distributions.

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delivery, and imaging.11 As underlined above, a key aspect of PCPs is the possibility of modulating their functional properties by varying the nature of nodes and spacers. In this respect, the isoreticular approach, originally proposed by Yaghi,12 is definitely a cornerstone: pore size and decoration, hence functionality, may be systematically varied upon modifying the length and the substitution of the spacers, while preserving the framework topology. The versatility of PCPs may be increased even by pre-13 or postsynthetic14 modifications of the ligands. After the pioneering work by Yaghi and co-workers on the species [Zn4O(BDC)3] (BDC = 1,4-benzenedicarboxylate), universally known as MOF-5,2a,b a vast number of polycarboxylato-based PCPs were reported and intensively investigated for their high crystallinity, remarkable specific surface areas, tunable pore size and shape, uptake capacity, and selectivity.15 Despite these promising features, polycarboxylato-based PCPs typically possess a weak point in terms of chemical stability against acidic or basic media and even against moisture,16 making them easily

INTRODUCTION In the last few decades, porous coordination polymers (PCPs)1 have been intensively investigated in many different disciplines, ranging from (inorganic, organic, and solid state) chemistry, to physics, materials science, biology, and even pharmacology. Their success was initially catalyzed by the advantages they possess as hydrogen storage materials2 in terms of regularity, designability, and versatility with respect to conventional, natural and synthetic, inorganic compounds. As a matter of fact, coupling the specific stereochemical requirements of selected metal ions to sizable, shapeable, and functionalizable organic linkers resulted into a vast number of fascinating structural topologies, possessing specific functional properties.3 Nonetheless, it was soon realized that, independently from the nature of the nodes, of the spacers, and of the overall topology, hydrogen storage performances conforming to the roadmap traced by the U.S. Department of Energy4 could be attained only at very low temperatures and rather high pressures.5 It was concomitantly recognized that this huge class of materials could bring remarkable fallouts for a number of other industrially and technologically relevant applications, such as gas and volatile organic compounds storage or separation,6 heterogeneous catalysis,7 luminescence,8 magnetism,9 ferroelectricity,10 drug © 2013 American Chemical Society

Received: April 5, 2013 Revised: May 7, 2013 Published: May 13, 2013 3087

dx.doi.org/10.1021/cg400495w | Cryst. Growth Des. 2013, 13, 3087−3097

Crystal Growth & Design

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hydrolizable and thus not completely adequate to sustain the harsh conditions of industrial applications. Polyazolato-based ligands may be a valid alternative to the carboxylato-based ones. As a matter of fact, a vast number of tetrazolato-, 1,2,3- and 1,2,4-triazolato-, imidazolato- and pyrazolato-based PCPs have been reported.17 In general, the strength of the metal-to-nitrogen bonds within this class of compounds is strictly related to the basicity of the corresponding neutral azole:18 the more basic the latter, the stronger the bond. In this respect, we19 and others17,20 have concurred to demonstrate that, in view of the higher pKa of pyrazole, pyrazolato-containing spacers provide stronger and more stable metal-to-ligand coordinative bonds, conferring to the corresponding PCPs higher thermal robustness, and, in specific occasions, chemical resistance to harsh acidic and basic conditions19c or to boiling solvents.21 During the past decade, the ditopic ligand 3,3′,5,5′tetramethyl-4,4′-bipyrazole (H2Me4BPZ, Scheme 1) has proved

Article

EXPERIMENTAL SECTION

Materials and Methods. The ditopic spacer 3,3′,5,5′-tetramethyl4,4′-bipyrazole (H2Me4BPZ) was synthesized according to the method already reported in the literature,28 by simple condensation of 3,4diacetyl-hexane-2,5-dione with hydrazine hydrate in water. The white solid thus obtained was further recrystallized from boiling water and characterized by elemental analysis, IR, 1H NMR, ESI-MS, and measurement of the melting point. All the other chemicals and reagents were purchased from Sigma Aldrich Co. and were used as received, without further purification. All the solvents were distilled prior to use. Elemental analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer. IR spectra were recorded from 4000 to 650 cm−1 with a Perkin-Elmer Spectrum 100 instrument by total reflectance on a CdSe crystal. Thermogravimetric analyses (TGA) were carried out in a N2 stream with a Perkin-Elmer STA 6000 simultaneous thermal analyzer with a heating rate of 10 °C/ min. Synthesis of [Zn(Me4BPZ)] (1). H2Me4BPZ (0.19 g, 1 mmol) was dissolved in 20 mL of dimethylformamide. After the complete dissolution of the ligand, Zn(CF3SO3)2 (0.36 g, 1 mmol) was added. The clear solution initially formed was left under stirring into a high pressure glass tube at 130 °C for 24 h. A white precipitate was obtained that was filtered off, washed twice with dimethylformamide, dried in air, and then in vacuo at 100 °C. Yield: 75%. 1 is insoluble in alcohols, DMSO, acetone, CH3CN, chlorinated solvents and water. Elem. Anal. Calc. for C10H12N4Zn (FW = 253.62 g/mol): C, 47.36; H, 4.77; N, 22.09%. Found: C, 47.13; H, 4.82; N, 21.59%. IR (cm−1): 2922(vw) ν(CH3); 1605(vw), 1501(vw) ν(CC + CN); 1475(w); 1435(m); 1370(vw); 1328(m); 1127(m); 1083(vw); 1037(vs); 978(vw); 785(m); 716(w); 685(w). Synthesis of [Co(Me4BPZ)] (2). H2Me4BPZ (0.19 g, 1 mmol) was dissolved in 20 mL of dimethylformamide. After the complete dissolution of the ligand, Co(NO3)2·6H2O (0.29 g, 1 mmol) was added. The purple solution initially formed was left under stirring into a high pressure glass tube at 130 °C for 24 h, until a dark purple precipitate appeared. The latter was filtered off, washed twice with dimethylformamide, dried in air and then in vacuo at 100 °C. Yield: 85%. 2 is insoluble in alcohols, DMSO, acetone, CH3CN, chlorinated solvents, and water. Elem. Anal. Calc. for C10H12CoN4 (FW = 247.17 g/mol): C, 48.60; H, 4.89; N, 22.67%. Found: C, 48.25; H, 4.51; N, 22.19%. IR (cm−1): 2921(vw) ν(CH3); 1567(w), 1498(vw) ν(CC + CN); 1472(w); 1431(m); 1370(w); 1317(m); 1116(m); 1082(vw); 1035(vs); 976(vw); 787(m); 716(w); 684(w). Synthesis of [Cd(Me4BPZ)] (3). H2Me4BPZ (0.19 g, 1 mmol) was dissolved in 20 mL of dimethylformamide. After the complete dissolution of the ligand, Cd(NO3)2·4H2O (0.31 g, 1 mmol) was added. The clear solution initially formed was left under stirring into a high pressure glass tube at 150 °C for 24 h, until a white precipitate formed. The latter was filtered off, washed twice with dimethylformamide, dried in air and then in vacuo at 100 °C. Yield: 89%. 3 is insoluble in alcohols, DMSO, acetone, CH3CN, chlorinated solvents and water. Elem. Anal. Calc. for C10H12CdN4 (FW = 300.64 g/mol): C, 39.95; H, 4.02; N, 18.64%. Found: C, 39.49; H, 3.82; N, 18.23%. IR (cm−1): 2917(vw) ν(CH3); 1497(vw) ν(CC + CN); 1471(w); 1429(m); 1368(w); 1318(m); 1126(m); 1073(vw); 1037(vs); 974(vw); 803(vw); 781(m); 713(w); 685(w). Synthesis of [Cu(Me4BPZ)] (4). H2Me4BPZ (0.19 g, 1 mmol) was dissolved in 20 mL of dimethylformamide. After the complete dissolution of the ligand, Cu(CH3COO)2 (0.18 g, 1 mmol) was added. The green solution initially formed was left under stirring into a high pressure glass tube at 100 °C for 24 h, until a brown precipitate formed. The latter was filtered off, washed twice with dimethylformamide, dried in air and then in vacuo at 100 °C. Yield: 65%. 4 is insoluble in alcohols, DMSO, acetone, CH3CN, chlorinated solvents and water. Elem. Anal. Calc. for C10H12CuN4 (FW = 251.76 g/mol): C, 47.71; H, 4.80; N, 22.25%. Found: C, 39.59; H, 4.01; N, 17.82%. Notably, the observed elemental analysis differs significantly from that calculated for [Cu(Me4BPZ)]. Formulas such as [Cu3(Me4BPZ)2(OH)2] or [Cu(Me4BPZ)]·nH2O, matching the

Scheme 1. 3,3′,5,5′-Tetramethyl-4,4′-bipyrazole (H2Me4BPZ)

its great versatility, especially because the methyl groups induce a reciprocal twist of the two five-membered rings, in some cases even above 50°. This occurrence may impart an additional degree of freedom to the framework, resulting into new supramolecular architectures. H2Me4BPZ has been associated to a wide range of metal ions, both as neutral bidentate ligand, and as dianionic tetradentate spacer, with the isolation of one-, two-, and three-dimensional (1-D,22 2-D,22,23 and 3-D) coordination polymers, mostly with inorganic counteranions.24 The versatility of H2Me4BPZ is also witnessed by the isolation of mixed-ligand coordination polymers, coupling aromatic polycarboxylic acids to either the neutral H2Me4BPZ linker,25a−c or its dianionic counterpart.25d Finally, emerging because of their relevant functional properties, it is worth mentioning a few porous coordination polymers, namely, the supramolecular isomers [M2(Me4BPZ)] (M = Ag, Cu), with exceptional framework flexibility and propensity to accommodate unsaturated hydrocarbons;20c,26a [Cu(H2Me4BPZ)Br]·0.5H2O, with a non-centrosymmetric polar packing, resulting in a strong second harmonic generation response and ferroelectric properties;26b and the PCP featuring Cu(II)Cu(I)15I17 clusters, with interesting paramagnetic and thermochromic properties.26c In the search for thermally robust and functional coordination polymers, and prompted by our long-term experience with polyazolato-based CPs,19,27 we decided to enlarge the isoreticular family [M(L)] (H2L = 4,4′-bipyrazole, 4,4′-bis(pyrazol-4-yl)benzene), exploiting the coordinative potentialities of the H2Me4BPZ spacer toward late transition metal ions. Thus, in the present contribution, we report on the four PCPs isolated by reacting H2Me4BPZ with Zn(II), Co(II), Cd(II), and Cu(II), following exclusively solvothermal routes. Details about the syntheses, the spectroscopic properties, the thermal behavior, the structural features, and the adsorption measurements toward N2 at 77 K and CO2 at 273 K are provided. 3088



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Table 1. Main Crystallochemical Data and Refinement Details for Species 1−4 empirical formula FW, g mol−1 crystal system SPGR, Z a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 ρcalcd, g cm−3 F(000) μ(Cu Kα), cm−1 T, K refinement 2θ range, deg data, parameters Rp, Rwpa RBragga

1

2

3

4

C10H12N4Zn 253.61 tetragonal P4̅2c, 2 8.8118(5) 8.8118(5) 7.3668(5) 90 90 90 572.01(7) 1.47 260 27.57 298(2) 6.5−105.0 4926, 27 0.071, 0,133 0.042

C10H12CoN4 247.15 tetragonal P4̅2c, 2 8.8105(3) 8.8105(3) 7.3427(3) 90 90 90 569.98(2) 1.44 254 115.84 298(2) 8.0−105.0 4851, 27 0.004, 0.006b 0.005b

C10H12CdN4 300.63 hexagonal P6122, 6 9.13264(7) 9.13264(7) 23.0997(2) 90 90 120 1668.51(3) 1.79 296 154.75 298(2) 10.0−105.0 4751, 24 0.059, 0.084 0.054

C10H12CuN4 251.76 cubic Im3m ̅ ,8 13.4617(4) 13.4617(4) 13.4617(4) 90 90 90 2439.5(2) 1.37 258 23.17 298(2) 7.0−105.0 4901, 44 0.039, 0.072 0.030

Rp = ∑i|yi,o − yi,c|/∑i|yi,o|; Rwp = [∑iwi (yi,o − yi,c)2/∑iwi(yi,o)2]1/2; RB = ∑n|In,o − In,c|/∑nIn,o, where yi,o and yi,c are the observed and calculated profile intensities, respectively, while In,o and In,c the observed and calculated intensities. The summations run over i data points or n Bragg independent reflections. Statistical weights wi are normally taken as 1/yi,o. bThe extremely low figures of merit are due to the high contribution of the background to the whole signal, mainly because of the non negligible fluorescence of the Co(II)-containing material. a

data and structure refinement details. The final Rietveld refinements plots are collectively supplied in Figure S1 of the Supporting Information. The not completely satisfactory match between the experimental and the calculated traces of species 4 is due to the presence of conditioned disorder (see below): the latter results into peak shift and anisotropic broadening, which cannot be traced back to specific classes of Bragg reflections. Further investigation along this line is currently going on, yet it is out of the scope of the present contribution. Notably, this high disorder could not be modeled completely; consequently, the five-membered rings do not appear planar and regular as they should be. Fractional atomic coordinates are provided with the Supporting Information as CIF files. Crystallographic data in CIF format have been deposited at the Cambridge Crystallographic Data Center as supplementary publications Nos. 930979−930982. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-335033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk). Variable-Temperature X-ray Powder Diffraction. Variabletemperature X-ray powder diffraction (VT-XRPD) experiments were performed on the as-synthesized species 1−4 to highlight their structural response to temperature variation. To that aim, two different sets of measurements were accomplished: progressive heating up to decomposition was performed in order to assess the thermal robustness and the permanent porosity of the materials; a series of heating−cooling cycles was carried out to evaluate their stability during consecutive series of thermal activation. Both sets of experiments were performed in air in a pertinent low-angle 2θ range, using a custom-made sample heater assembled by Officina Elettrotecnica di Tenno, Ponte Arche, Italy. Powdered polycrystalline batches of 1−4 were ground in an agate mortar and deposited in the hollow of an aluminum sample holder. The thermal behavior was followed up to decomposition heating the samples in situ with steps of 20 °C. The parametric treatment34 of the VT-XRPD data with the Le Bail method allowed to depict the variation of the unit cell parameters as a function of the temperature. The temperature cycles were performed varying the temperature, in situ, between 50 °C (to prevent possible undesired contribution by moisture) and 200 °C. When first reaching 200 °C, a number of XRPD traces were acquired till an asymptote was reached, to ensure the removal of moisture potentially

observed EAs, must be discarded on the basis of the absence of O−H stretching bands in the IR spectrum of 4, and of the structure determination. Moreover, different preparations of 4 result in slightly different EAs, which can be satisfactorily interpreted by adopting a formula of the kind [Cu(Me4BPZ)]·nCuO, i.e., by admitting the presence of small amounts of CuO as byproduct, undetected by X-ray powder diffraction. IR (cm−1): 2913(w) ν(CH3); 1496(vw) ν(CC + CN); 1481(w); 1439(m); 1369(w); 1337(m); 1158(m); 1073(vw); 1050(vs); 875(vw); 799(vw); 706(w); 691(w). X-ray Powder Diffraction Crystal Structures Determination. Powdered, polycrystalline batches of compounds 1−4 were gently ground in an agate mortar; then, they were deposited in the hollow of an aluminum-framed, zero-background sample holder. Diffraction data were collected at room temperature on a Bruker AXS D8 Advance diffractometer, equipped with Ni-filtered Cu Kα radiation (λ = 1.5418 Å), a Lynxeye linear position-sensitive detector, and the following optics: primary beam Soller slits (2.3°), fixed divergence slit (0.5°), and receiving slit (8 mm). The generator was set at 40 kV and 40 mA. A visual inspection of the preliminary acquisitions revealed that 1 and 2 are isomorphous. To carry out the structure determinations, overnight scans were performed in the 2θ range of 5−105°, with steps of 0.02°. A standard peak search below 30° was followed by indexing through the singular value decomposition method29 implemented in TOPAS-R,30 which allowed the determination of the lattice parameters. Systematic absences and the above-mentioned isomorphism permitted individuation of the most probable space groups. Prior to structure solution, Le Bail refinements were carried out to confirm unit cells and space groups. Preliminary structural models were determined ab initio by the simulated annealing approach implemented in TOPAS-R. An idealized rigid model was used to describe the crystallographically independent portion of the ligand but in the case of 4;31 the torsion angles around the C−C single bonds were let refine. Structure refinements were carried out by means of the Rietveld method32 with TOPAS-R. The peak shapes were described with the fundamental parameters approach.33 In the case of 1 and 2, the anisotropic broadening of the peaks was modeled by means of spherical harmonics. The background was modeled by a Chebyshev polynomial function. The thermal effect was simulated by using a single isotropic parameter for the metal ions, augmented by 2.0 Å2 for lighter atoms. Table 1 contains the most relevant crystallochemical 3089



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Figure 1. Representation of the crystal structure of species 1, [Zn(Me4BPZ)]: (a) the square-shaped channels running along the crystallographic axis c; (b) the 1-D chains of ligand-bridged, collinear metal ions, possessing a tetrahedral coordination sphere. Zinc, yellow; nitrogen, blue; carbon, gray; hydrogen, light gray. The crystal structure of species 2, the Co(II) homologous, is, at the drawing level, undistinguishable. Significant bond distances (Å) and angles (°) for 1: Zn−N 1.9016(6); bridged Zn···Zn 3.6834(3), 8.8118(6); N−Zn−N 105.07(2), 111.72(1). Significant bond distances (Å) and angles (°) for 2: Co−N 1.8951(6); bridged Co···Co 3.6714(1), 8.8105(3); N−Co−N 105.41(2), 111.54(1).

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present. When comparing the TGA and VT-XRPD results, the reader must be aware that the thermocouple of the VT-XRPD setup is not in direct contact with the sample, this determining a slight difference in the temperature at which the same event is detected by the two techniques. The TGA temperatures have to be considered more reliable. Gas Adsorption Measurements. Specific surface areas and pore texture were determined by N2 and CO2 adsorption at 77 and 273 K, respectively. Adsorption isotherms were measured by the volumetric method with a Quantachrome 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 (10−6 mbar) before adsorption experiments. The two adopted probes are complementary and allow analysis of different ranges of porosity.35 Taking into account the higher adsorption temperature and lower relative pressure used in the case of CO2, adsorption takes place only into the narrowest micropores. On the contrary, according to Gurvitch’s rule, the volume of nitrogen adsorbed at a relative pressure of 0.95, V0.95, is a measure of both the micro- and mesopore volumes. Different parameters, including pore size distribution (PSD), can be obtained by fitting the isotherms with a number of models based on specific requirements, the validity of which was analyzed and compared since the earliest studies.36 The BET specific surface areas were determined by fitting the lowpressure zone of the N2 adsorption isotherms by the Brunauer− Emmett−Teller (BET) equation and considering the molecular area of N2 at 77 K to be 0.162 nm2.37 The Dubinin−Radushkevich (DR) equation38 was applied to both N2- and CO2-adsorption isotherms to estimate the micropore volume, W0. The mesopore volume, Vmeso, was obtained from the difference between V0.95 and W0(N2). Also mesopore size distributions can be obtained by applying different methods to the N2 adsorption isotherms,39 such as the classical Barrett−Joyner−Halenda (BJH) method, and the more recent non-local density functional theory (NLDFT), each one being only a different approach to obtain the same piece of information, whose advantages and drawbacks are continuously a matter of discussion.40 Thus, the use complementary techniques should be appropriate to determine a satisfactory PSD.

RESULTS AND DISCUSSION Synthesis and Spectroscopic Characterization of Species 1−4. Several screening reactions were carried out modulating conditions and parameters (solvent, cosolvent, temperature, time, concentration), in order to identify the best path toward each compound. As a result, the isolation of the homoleptic coordination polymers [M(Me4BPZ)] (M = Zn, 1; Co, 2; Cd, 3; Cu, 4) requires the implementation of solvothermal reactions. As a matter of fact, species 1−4 can be obtained in high-pressure glass tubes at high temperatures (in the range 100−150 °C), under autogenous pressure, by reacting the proper MX2 salt (X− = CF3SO3−, 1; NO3−, 2 and 3; CH3COO−, 4) and H2Me4BPZ in dimethylformamide for 24 h. To isolate compound 4, containing a redox-active metal ion, the temperature of 100 °C was carefully selected in order to avoid the reduction of Cu(II) to Cu(I). In all of the cases, ligand deprotonation does not require an external deprotonating agent, as it was previously experienced with the compounds [M(BPZ)] (M = Zn, Co, Cd; H2BPZ = 4,4′-bipyrazole).19b All the isolated species precipitate in high yields as air- and moisture-stable polycrystalline powders, insoluble in water and in the most common organic solvents. This observation suggests the polymeric nature of their crystal structures. To monitor the success of the reactions, above all at the stage of preliminary screening, and to characterize the isolated products, IR spectroscopy was routinely applied. For example, only by setting the temperature in the range of 100−150 °C the IR spectra did not show either the N−H stretching band of the ligand, or the characteristic bands of the counterions, in favor of the formation of the target [M(Me4BPZ)] species. Indeed, the IR spectra of 1−4 (Figure S2) are consistent with the proposed formulations and present quite similar absorption bands. The absence of N−H stretching bands indicates complete deprotonation of the organic spacer along with the formation 3090



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Figure 2. Representation of the crystal structure of species 3, [Cd(Me4BPZ)], viewed down [100] (a) and [001] (b). The helical motif described by the metal centers, folding about a 61 axis, is highlighted by the orange, dashed lines along the horizontal axis c in (a). Cadmium, yellow; nitrogen, blue; carbon, gray; hydrogen, light gray. The apparent triangular holes in (b) are occupied by hydrogen atoms (removed for the sake of clarity). Significant bond distances (Å) and angles (°): Cd−N 2.201(3), 2.229(3); bridged Cd···Cd 3.9759(5), 9.133(2); N−Cd−N 97.4(1), 102.74(2), 114.4(2), 119.60(6).

mutually linked by the spacers along a and b, with the consequent formation of a 3-D framework possessing 1-D, square-shaped channels running along c (Figure 1a). The vertices of the channels are occupied by tetrahedral MN4 nodes, while their walls are defined by the Me4BPZ spacers, the methyl substituents pointing inward. Notably, the ligand is severely twisted around the C−C exocyclic bond, the M−N−N−M torsion angle being higher than −70° both in 1 and in 2. This apparently high value should not surprise: actually, as witnessed by a search within the Cambridge Structural Database for pyrazolato-based ligands bridging transition metal ions (ca. 1500 cases), while the distribution of the values of the M−N− N−M torsion angle is centered about 0°, a standard deviation of 14.3° is observed, with minimum and maximum values of ca. ± 85°. Large deviations from planarity are expected for sterically hindered 3,5-substituted pyrazolates within chains of pyrazolato-bridged metal ions, as in the case of 1 and 2. In the case of the [M(BPZ)] derivatives,19b the ligands are not functionalized by the methyl groups, thus being less sterically hindered and less prone to twist. The steric hindrance of the substituents in positions 3,5 is also the reason for the inexistence of simple [M(dimethylpyrazolato)2] polymers, the [M(pyrazolato)2] ones being on the contrary very common.

of metal-bridging bipyrazolato dianions, eventually confirmed by the structural analyses (see following sections). The absorption bands detected in the range 1501−1496 cm−1 are due to the so-called “breathing” of the pyrazolato rings; their intensity is significantly reduced with respect to those of the free ligand, reasonably as a consequence of coordination. Structural Analysis of Species 1 and 2. All the crystal and molecular structures discussed hereafter were retrieved from state-of-the-art powder diffraction methods applied to laboratory data. The main structural details are described in the following, while the most relevant geometrical parameters are reported in the pertinent figure captions. [Zn(Me4BPZ)], 1, and [Co(Me4BPZ)], 2, are isostructural to [M(BPZ)]19b and [M(BDP)]19f (M = Zn, Co; H2BDP = 4,4′-bis(pyrazol-4-yl)benzene): although maintaining the same topology, due to the presence of the methyl groups on the ligand they crystallize in the tetragonal space group P4̅2c, possessing lower symmetry than that of [M(BPZ)] and [Zn(BDP)], P42/mmc. Thus, 1 and 2 constitute novel members of the isoreticular series12 [M(L)] (L = BPZ, BDP, Me4BPZ). As [M(BPZ)] and [M(BDP)] (M = Zn, Co), 1 and 2 are characterized by 3-D porous networks: 1-D chains of ligandbridged, collinear metal ions (Figure 1b) run along the crystallographic axis c, with a pitch of c/2. The chains are 3091



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Figure 3. (a) Portion of the crystal packing of species 4 along [001], adopting an ordered model in the space group I4/m: the unreasonable disposition of the ligands, with the methyl groups hitting each other, can be clearly appreciated. (b, c) Two (metrically cubic) arbitrarily periodic structures in which a coherent rotation is applied to the ligands departing from the Cu4 node. This sequence should help the reader to shift gradually from the ordered to the correct disordered model. Copper, yellow; nitrogen, blue; carbon, gray. Significant bond distances (Å) and angles (°): Cu− N 2.14(1); bridged Cu···Cu 3.288(4), 9.172(3); N−Cu−N 89.66(3), 171.1(5).

isomorphous to [Cu(BPZ)], which crystallizes in the orthorhombic space group Imma and possesses a 3-D network with 1-D rhombic channels.19b As a matter of fact, 4 crystallizes in the cubic space group Im3m ̅ and features a 3-D framework in which conditioned disorder is present. An ordered, yet physically not sound, model, easier to describe and visualize, can be obtained by decreasing the crystal symmetry down to the tetragonal subgroup I4/m (Figure 3a). In such a model, square-shaped Cu4 nodes, lying in the ab plane, can be identified. The edges of the squares are bridged by eight pyrazolato moieties, each metal center thus possessing a square planar stereochemistry. A similar tetranuclear node had been already found in [Mn(DMF) 6 ] 3 [(Mn 4 Cl) 3 (BTT) 8 (H2O)12]2·Solv,44 which, nonetheless, is not affected by the disorder detailed below. The eight spacers departing from each node connect it to eight surrounding ones, overall creating a 3D polymer. In this ordered model, the ligands are flat and show D2h symmetry, with the obvious consequence that the methyl groups bump into each other. The steric hindrance of the substituents requires twisted spacers: the five-membered rings must thus twist about the exocyclic C−C bond (Figure 3b,c). The actual twist inevitably modifies the orientation of the Cu4 squares: rather than being all parallel to the ab plane, neighboring squares are reciprocally rotated by 120° along [111] (D2 symmetry), with no rigorous crystal periodicity. A conditioned disorder is actually present, which spreads the 120° reciprocal disposition of the squares throughout the crystal, avoiding the physically unsound one at 0°. As a result of the disorder, in Im3̅m each Cu4 square appears randomly oriented in three different directions normal to each other, overall describing an octahedron (Figure S6). At the macroscopic level, the disorder is reflected by the unique features characterizing the XRPD trace of 4 (Figure S1), namely, peak shift and anisotropic broadening, which cannot be traced back to specific classes of Bragg reflections. Further investigation in this respect is currently going on, yet it is out of the scope of the present contribution. Adopting one of the twisted crystal structures in I4/m, a void volume of 40% can be estimated. Thermal Behavior of Species 1−4. The thermal behavior of species 1−4 was assessed by coupling thermogravimetric

The void volumes within the channels decrease with the trend [Zn(BDP)] > [Zn(BPZ)] > [Zn(Me4BPZ)] (65%,41 42%, and 15% respectively): as expected within an isoreticular series, both shortening of the ligands and substituents introduction diminish the channels aperture. Inter alia, the nature and disposition of either the clathrated solvent or the substituents are responsible for the actual symmetry of the framework. Indeed, the two tetragonal space groups P42/mmc and P42̅ c are compatible with undistorted networks, featuring square channels either void (as in [M(BPZ)] and [M(Me4BPZ)], M = Zn, Co) or containing disordered solvent (as in [Zn(BDP)]). In the presence of partial ordering of the solvent or of functional groups, other symmetries may be realized: the deformation of the tetragonal framework and of the tetrahedral stereochemistry of the metal ions may indeed result into orthorhombic (P21212, as in [Co(BDP)]20b or Cccm in [Zn(BDP)-X]19a) or monoclinic (P21/c, as in another form of [Co(BDP)]42) space groups. Structural Analysis of Species 3. Though containing a metal center which can adopt a tetrahedral stereochemistry, the Cd(II) derivative 3 is not isomorphous to 1 and 2, yet to [Cd(BPZ)].19b Like the latter, 3 crystallizes in the rather rare hexagonal space group P6122.43 The crystal structure of 3 is characterized by homochiral helices of ligand-bridged Cd(II) ions, winding up along a 61 (or 65, in the enantiomorphic crystals) screw axis parallel to the crystallographic axis c, with a pitch equal to c (Figure 2a). The Me4 BPZ spacers depart from each helix and connect neighboring ones approximately along a and b (Figure 2b), overall creating a 3-D network, with CdN4 tetrahedral nodes. Reasonably due to the vicinity of the helices and to the stereochemistry adopted by the metal centers, a significant twist of the ligands is observed: the torsion angle between the two pyrazolato rings amounts to about 53°, definitely higher than in [Cd(BPZ)], reasonably due to the non-negligible steric hindrance of the methyl groups (see also the comments in the previous section). Notably, no void volume is present, at variance with the value of 50% estimated for [Cd(BPZ)]. Structural Analysis of Species 4. Surprisingly, coupling the Me4BPZ ligand with Cu(II) does not result into a material 3092



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Figure 4. TGA traces of species 1 (red), 2 (purple), 3 (blue), and 4 (green).

Figure 5. (a) Plot of the X-ray powder diffraction patterns measured on 1 as a function of the temperature heating, with steps of 20 °C, up to decomposition. The permanent porosity and rigidity of species 1 can be appreciated. (b) Percentage variation of the unit cell parameters (pT) of 1 as a function of the temperature. The values at 30 °C (p30) have been taken as the references. a, green circles; c, red triangles; V, blue rhombi. (c) Plot of the X-ray powder diffraction patterns measured on 1 during heating−cooling cycles within the range 50−200 °C. Heating step, red; cooling step, blue.

analyses (TGA), under a nitrogen flux, to variable-temperature X-ray powder diffraction (VT-XRPD) experiments, carried out in air. More in detail, two sets of VT-XRPD measurements were performed: the materials were first monitored under progressive heating up to decomposition, in order to substantiate their thermal robustness and their permanent

porosity. Then, a series of consecutive heating−cooling cycles was carried out in the range 50−200 °C, to assess their stability along a series of thermal activations. The acquired TG traces are collectively gathered in Figure 4. As a representative example, Figure 5 provides the results of the VT-XRPD experiments and of the consequent data treatment for species 1. 3093



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Figure 6. N2- (left) and CO2- (right) adsorption isotherms measured at 77 and 273 K, respectively. 1 (green cyrcles), 2 (blue rhombi), 3 (red triangles), 4 (black squares). The empty symbols denote the adsorption branches.

Table 2. Textural Parameters of Activated Species 1−4 Derived from the N2- and CO2-Adsorption Isotherms at 77 and 273 K, Respectively N2 2

[Zn(Me4BPZ)], 1 [Co(Me4BPZ)], 2 [Cd(Me4BPZ)], 3 [Cu(Me4BPZ)], 4

2

3

CO2 3

3

3

3

SBET (m /g)

SDFT (m /g)

VDFT (cm /g)

W0 (cm /g)

V0.95 (cm /g)

Vmeso (cm /g)

W0 (cm /g)

SDFT (m2/g)

396 318 88 376

570 518 118 525

0.20 0.15 0.09 0.21

0.15 0.12 0.04 0.15

0.59 0.22 0.10 0.47

0.44 0.10 0.06 0.32

0.05 0.06 0.03 0.08

90 94 87 202

experiment witnesses that 2 does not experience phase transformations before decomposition: its framework, hence its permanent porosity, is not influenced by temperature increasing (Figure S3a). Similar to 1, and plausibly for the same reasons, in the range 30−310 °C a and c undergo negligible modifications, provoking an overall negative thermal expansion of just 0.4% (Figure S3b). Remarkably, species 3 possesses an outstanding robustness: its decomposition starts about 500 °C, 5% weight loss being observed at 545 °C. This is in line with what is observed in the case of [Cd(BDMPX)] (H2BDMPX = 1,4-bis((3,5-dimethyl1H-pyrazol-4-yl)methyl)benzene).19d Its framework does not collapse upon temperature increase (Figure S4a) and shows a moderate, thermally induced flexibility (Figure S4b): in the temperature range 30−470 °C, while a (and b) increases only 0.1%, the increase of c is six times higher, with a concomitant unit cell volume expansion of 1%. The increase of c can be explained by noting that the Cd(II)-based helices mimic a spring, which augments its pitch when stimulated by temperature, reasonably as a consequence of a relaxation of the twist of the ligands around the exocyclic C−C vector. On the contrary, the steric hindrance of the methyl substituents does not allow a significant variation of a, at variance with what is observed with the unsubstituted BPZ ligand in [Cd(BPZ)].19b Finally, compound 4 is stable up to about 310 °C, the temperature at which a progressive decomposition begins;45 5% weight loss is observed at 350 °C. The lowest stability of 4 can be explained in view of the possible metal reduction of Cu(II)based pyrazole complexes promoted by heating, which has been already observed19d,46 and attributed to the partial oxidation of pyrazole.47 The framework of 4 is preserved upon temperature increase (Figure S5a), and it is definitely rigid (Figure S5b), the unit cell volume undergoing almost no modification in the temperature range 30−210 °C. On the whole, the thermal characterization allowed retrieval of a coherent picture of species 1−4, demonstrating that they

The results obtained with species 2−4 are reported in the Supporting Information as Figures S3−S5. Generally speaking, 1−4 possess a remarkable thermal stability, decomposing at temperatures above 300 °C, thus confirming the key role of polyazolato-based spacers in the formation of strong metal-to-ligand bonds, imparting solidity to the whole material. As expected, 1 and 3, containing redox-inert metal ions, show the highest thermal robustness. Moreover, all the materials are resistant to successive heating−cooling cycles, preserving their crystal structure and, when present, their porosity (Figures 5c, S3c, S4c, S5c). As demonstrated by its TG trace, heating 1 does not provoke weight losses up to ca. 430 °C,45 temperature at which a progressive decomposition begins; 5% weight loss is observed at 500 °C. This is in line with the previous observations on the analogous species [Zn(BPZ)]19b and [Zn(BDP)],19f stable in air up to 450 and 400 °C, respectively. The VT-XRPD measurement confirmed the high thermal stability of 1, and disclosed its permanent porosity: increasing the temperature does not affect its structural features, and its network remains intact up to decomposition (Figure 5a). Data treatment by means of a parametric Le Bail refinement in the range 30−430 °C revealed that a and c undergo negligible and opposite variations, the former slightly decreasing, the latter increasing: the overall result is a modest negative thermal expansion of the unit cell, the volume shrinking about 0.1% (Figure 5b). The high rigidity of the framework is not unexpected: c, the cell axis along which the metal chains run cannot undergo a large variation, as it was the case, e.g., with [M(BDP)]19f (M = Ni, Zn). Similarly, a (and b) cannot experience a significant shrinking, as the closure of the channels is somehow blocked by the methyl groups that point inside. As anticipated above, the Co(II)-containing species 2 possesses a somewhat lower thermal stability than 1, its decomposition starting around 350 °C without previous weight losses; 5% weight loss is observed at 400 °C. The VT-XRPD 3094



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than the kinetic diameters of N2 (3.64 Å) and CO2 (3.30 Å).51 Therefore, some flexibility of the framework during gas sorption (even at 77 K) must be invoked. The pore size distributions (PSDs), estimated by the NLDFT method applied to the N2-adsorption isotherms, are shown in Figure 7. Clearly bimodal PSDs are obtained, showing

(i) are thermally stable; (ii) possess permanent porosity (but in the case of the Cd(II) derivative); (iii) maintain their thermal stability, together with their framework topology, along consecutive heating−cooling series. The assessment of their permanent porosity rules out that the adsorption performances are due exclusively to mesoporosity deriving from interparticle voids. In addition, on the basis of their stability toward repeated heating−cooling sequences, it might be proposed that also their adsorption performances are preserved along cycles of thermal activation followed by adsorption. Gas Adsorption Measurements and Textural Properties. The N2- and CO2-adsorption isotherms of species 1−4 are depicted in Figure 6, while the main parameters obtained therefrom are summarized in Table 2. A preliminary analysis of the shape of the N2-adsorption isotherms indicates that all of them are a combination of types I and II, corresponding to micro/mesoporous materials. The N2-adsorption capacity of the samples increases along the sequence 3 < 2 < 4 < 1. A significant adsorption takes place in all cases at very low P/P0, corresponding to the filling of micropores. With increasing P/ P0, adsorption occurs in larger pores, as manifested by the less steep slope of the isotherms in a large interval of relative pressures. A second slope change is observed at high P/P0 for species 1, 2, and 4, but not in the case of 3. For compound 1, a small hysteresis cycle is also observed, denoting the presence of mesopores. Fitting the N2 isotherms allowed us to obtain BET and DFT specific surface areas in the ranges of 88−396 and 118−570 m2/g, respectively (SBET and SDFT, in Table 2). Notably, the SDFT values retrieved from the CO2-adsorption isotherms are significantly smaller; a similar trend is also observed for the micropore volumes (W0), derived by the DR equation.48 These occurrences suggest that, at 77 K, N2 adsorption takes preferentially place in larger micropores and in the mesopores. The total pore, micropore, and mesopore volumes (V0.95, W0(N2), and Vmeso in Table 2) were retrieved from the N2 adsorption isotherms. V0.95 increases with the trend 3 < 2 < 4 < 1, being about six times greater for 1 than for 3, while W0(N2) is quite similar for 1, 2, and 4. Finally, Vmeso, obtained as the difference between V0.95 and W0(N2), increases along the trend 3 < 2 < 4 < 1. The large difference in the latter values, ranging from 0.06 to 0.44 cm3/g, should not surprise: being nonstructural (in the crystallographic sense), they are a manifestation of the graininess and packing of morphologically distinct materials, highly dependent on sample preparation, precipitation, and postisolation processing. Noteworthy, in the case of 1, the micropore volume detected by CO2 (W0(CO2)) corresponds only to the 5% of the whole porosity probed by N2 (V0.95), which could confer an important molecular sieve effect to this material. Moreover, in the case of 4, W0(CO2) and SDFT are significantly higher than for 1 and 2. This seems to suggest a specific affinity of species 4 toward CO2, similar to that already observed by us in the narrow-pore gismondine-like [Cu(Fpymo)] framework (F-pymo = 5-fluoro-pyrimidinolate).49 The void volumes calculated from the crystal structures of 1, 2, and 4, 0.10, 0.11, and 0.29 cm3/g, respectively, are comparable to the corresponding values of W0(N2), suggesting that the intrinsic porosity of the materials is composed of micropores, while mesoporosity is due to interparticle voids. Worthy of note, while the cavities in 4 are 0.5450 nm wide, the void volume in the 1-D channels of 1 and 2 can be approximately described as a hourglass-shaped “cylinder”, with a neck only 0.3350 nm wide, equal or slightly smaller

Figure 7. Pore size distribution determined for 1 (green curve), 2 (blue curve), 3 (red curve), and 4 (black curve) by applying the NLDFT to the N2 adsorption isotherms.

one maximum in the micropore range (ca. 1 nm), and a second one in the range of the narrowest mesopores (from 2 to 5 nm). The PSDs derived from the CO2-adsorption isotherm (Figure S7) are bimodal too, yet they appear too structured to be confidently interpretable. While no species isomorphous to 4 is available for a comparison, comparative analyses can be carried out in the case of species 1−3. As expected from the values of the void volumes, the specific surface areas decrease with the trends [Zn(BDP)]19f > [Zn(BPZ)]19b > [Zn(Me4BPZ)], [Co(BPZ)]19b > [Co(Me4BPZ)],52 and [Cd(BPZ)]19b > [Cd(Me4BPZ)], confirming once more the role of both the length and the steric hindrance of similar ligands within isoreticular series.12 Significantly, at variance with [Cd(BPZ)], the best performer of the series [M(BPZ)],19b [Cd(Me4BPZ)] shows the lowest specific surface area: as a matter of fact, the moderate flexibility observed upon thermal activation cannot be responsible for a significant increase of the void volume with respect to the as-synthesized phase (above all when compared to the 50% void volume of [Cd(BPZ)]). These occurrences definitely indicate that the four methyl groups on the 4,4′bipyrazolyl moiety decrease the accessibility of the pores, which is not counterbalanced by guest-stimulated pore-opening, as observed for example in the case of the pro-porous materials [M(BDMPX)] (M = Zn, Co).19d

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CONCLUSIONS With the aim of enlarging the isoreticular family [M(L)] (H2L = 4,4′-bipyrazole, H2BPZ; 4,4′-bis(pyrazol-4-yl)benzene, H2BDP), in this contribution we have reported on the syntheses, the crystal structures, the thermal behavior, and the adsorption properties of the coordination polymers [M(Me4BPZ)] (M = Zn, 1; Co, 2; Cd, 3; Cu, 4), isolated by coupling bivalent transition metal ions to the spacer 3,3′,5,5′tetramethyl-4,4′-bipyrazole (H2Me4BPZ). Ab-initio structural analyses disclosed the main structural features of the four 3-D frameworks. Notably, being isomorphous to [M(BPZ)] and [M(BDP)], 1 and 2 conform to the so-called isoreticular strategy. TG analyses, juxtaposed to VT-XRPD, demonstrated that all of the four materials (i) are thermally stable, decomposing in air above 350 °C; (ii) possess permanent 3095



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porosity (but the Cd(II) derivative); (iii) preserve thermal stability and framework topology along consecutive heating− cooling series. The latter observation leads to suggest that even their adsorption performances are maintained along cycles of thermal activation followed by adsorption. N2 and CO2 adsorption measurements at 77 and 273 K, respectively, were employed to probe the permanent porosity of the materials and to give a coherent picture of their textural properties. BET specific surface areas in the range 88−396 m2/g were estimated. Micro- and mesopore volumes and size distributions were calculated and compared with the pertinent features derived from the crystal structures, evidencing that the intrinsic porosity of the materials is essentially due to micropores of rather small size.

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ASSOCIATED CONTENT

S Supporting Information *

Final Rietveld refinement plots for species 1−4. IR spectra for compounds 1−4. Results of the VT-XRPD measurements for derivatives 2−4. Pore size distributions derived for 1−4 from the CO2 adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(A.T.) Tel: +39-0737-402234. Fax: +39-0737-402457. E-mail: [email protected]. (S.G.) Tel. +39-031-2386627. Fax +39-031-2386630. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Fondazione Cariplo (Project No. 2011-0289), University of Camerino (grants to A.T. and I.T.), and the Italian Ministry of Instruction, University and Research (Project PRIN Development of Energy-targeted Self-assembled supramolecular systems: a Convergent Approach through Resonant information Transfer between Experiments and Simulations) for funding.

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(45) The small drift observed at low temperature is possibly related to moisture or to a negligible percentage of organic contaminant. (46) Ehlert, M. K.; Rettig, S. J.; Storr, A.; Thompson, R. C.; Trotter, J. Can. J. Chem. 1990, 68, 1444−1449. (47) Schofield, K.; Grimmet, M. R.; Keene, B. R. T. In Heteroaromatic Nitrogen Compounds: The Azoles; Cambridge University Press: Cambridge, 1976; p 154. (48) It is worth to note that, while DFT assesses the micro- and mesoporosity and provides a pore size distribution, the DR equation is specific for adsorption into micropores and provides only a value of the mean pore width (L0). (49) Navarro, J. A. R.; Barea, E.; Rodríguez-Diéguez, A.; Salas, J. M.; Ania, C. O.; Parra, J. B.; Masciocchi, N.; Galli, S.; Sironi, A. J. Am. Chem. Soc. 2008, 130, 3978−3984. (50) van der Waals corrected value. (51) Baker, R. Membrane Technology and Applications; John Wiley and Sons: New York, 2012; Table A15. (52) A comparison with [Co(BDP)] is not completely feasible, due to its unique adsorption behavior, with five steps in the low relative pressure region of the N2 adsorption isotherm, with a Langmuir specific surface area of 2670 m2/g.20b

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