Coordination Polymers Based on the Trinuclear Triangular Secondary

Article pubs.acs.org/crystal

Coordination Polymers Based on the Trinuclear Triangular Secondary Building Unit [Cu3(μ3‑OH)(μ-pz)3]2+ (pz = pyrazolate) and Succinate Anion Corrado Di Nicola,§ Enrico Forlin,# Federica Garau,# Massimo Gazzano,† Arianna Lanza,# Magda Monari,*,‡ Fabrizio Nestola,◊ Luciano Pandolfo,*,# Claudio Pettinari,*,§ Alberto Zorzi,# and Federico Zorzi◊ §

Scuola di Farmacia, University of Camerino, Via S. Agostino, 1, I-62032 Camerino (MC), Italy Dipartimento di Scienze Chimiche, University of Padova, Via Marzolo, 1, I-35131 Padova, Italy † ISOF-CNR, Via Selmi, 2, I-40126 Bologna, Italy ‡ Dipartimento di Chimica “G. Ciamician”, University of Bologna, Via Selmi, 2, I-40126 Bologna, Italy ◊ Dipartimento di Geoscienze, University of Padova, Via Gradenigo, 6, I-35131 Padova, Italy #

S Supporting Information *

ABSTRACT: Reaction conditions (solvent, temperature, pressure) and reagents ratios control the formation of different products from the reactions involving CuII, pyrazole (Hpz), and succinate ion (Suc). Three different coordination polymers (CPs) (one of which porous) based on the trinuclear triangular Cu3(μ3OH)(μ-pz)3 secondary building unit (SBU), as well as a 1D CP based on the Cu(Hpz)2 SBU were obtained. Moreover, a 3D supramolecular network, formed through quite strong H-bonding interactions involving the mononuclear Cu(HSuc)2(Hpz)4 complex, was also synthesized when an excess of H2Suc was added.

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INTRODUCTION

In the course of our studies, we have deeply examined the interaction of copper(II) carboxylates with pyrazole in order to obtain CPs with interesting structural features, sorption− desorption properties and catalytic activity. Particularly, by reacting copper(II) carboxylates with pyrazole (Hpz), in MeCN, a 1D CP, formed through the assembly of the mononuclear SBU [Cu(μ-pz)2], was obtained,10 that presents the “porosity without pores” behavior.11 When the reaction of copper(II) carboxylates with Hpz was instead carried out in protic solvents we obtained compounds a-l, (Scheme 1), that are all based on the trinuclear triangular [Cu3(μ3-OH)(μ-pz)3]2+ moiety, with the positive charge neutralized by two coordinated carboxylates. These trinuclear derivatives self-assemble in hexanuclear clusters through carboxylate bridges forming 1- or 2D CPs or supramolecular networks.8a,b,d,f−h The [Cu3(μ3-OH)(μ-pz)3]2+ moiety is quite stable, and remained to a large extent intact after the reaction of [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(Hpz)], b, with HCl and other strong acids.8e,i Moreover, by treating compound a with 4,4′-bypiridine (bpy) it was possible to obtain 2D and 3D CPS,

The interest for coordination polymers (CPs) continues to grow and the researches aimed to obtain new stable compounds, potentially useful in numerous different fields as catalysis,1 magnetism,2 gas storage and capture,3 etc., involve an increasing number of scientists.4 The concept of secondary building unit (SBU), introduced by Yaghi and co-workers in 2001,5 is particularly useful not only for the description of CPs structures, but also for the possibility to design specific SBUs with the aim to forecast the structures of the CPs obtained from them. In this context, an important issue was outlined, that is, it is preferable to employ polytopic coordinating anions (mono-, bi-, and polycarboxylates, azolates, phosphate, etc.) to balance the charges of the SBUs, as the presence of noncoordinating, normally small, anions is often detrimental to the functionality of the network itself.6 A list of SBUs employed to obtain a large number of CPs has been provided in the same article but, among a large plethora of nuclearities, trinuclear triangular SBUs are relatively scarcely represented. Even though trinuclear triangular CuII systems have been known for a long time,7 they have not been systematically used as SBUs, excluding, to the best of our knowledge, our own works8 and two recently appeared papers.9 © XXXX American Chemical Society

Received: August 31, 2012 Revised: November 24, 2012

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dx.doi.org/10.1021/cg3012712 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1

Elmer STA 6000 using nitrogen as carrier gas (30 mL/min) and an heating rate of 5°/min. Crystallographic Data Collection and Structure Determination for 2, 3, 4, and 5. The X-ray intensity data of compounds 2− 5 were measured on a Bruker Apex II CCD diffractometer. Cell dimensions and the orientation matrix were initially determined from a least-squares refinement on reflections measured in three sets of 20 exposures, collected in three different ω regions, and eventually refined against all data. A full sphere of reciprocal space was scanned by 0.3° ω steps. In all cases, X-ray diffraction data were collected at room temperature using Mo Kα radiation. The software SMART15 was used for collecting frames of data, indexing reflections and determination of lattice parameters. The collected frames were then processed for integration by the SAINT program,15 and an empirical absorption correction was applied using SADABS.16 The structures were solved by direct methods (SIR 2004)17 and subsequent Fourier syntheses and refined by full-matrix least-squares on F2 (SHELXL97)18 attributing anisotropic thermal parameters to all the non-hydrogen atoms in 2, 4, and 5. In 3, the uncoordinated solvent molecules were refined with isotropic atomic displacement parameters. The μ3-OH hydrogens were located in the Fourier map and refined isotropically [U(H) = 1.2Ueq(O)]. The pyrazolate and methylene hydrogens bound to C atoms were placed in calculated positions and refined with isotropic thermal parameters U(H) = 1.2Ueq(C), and allowed to ride on their carrier carbons. The unit cell of compound 3 contains several solvent molecules (water or methanol), which are easily released and therefore the crystal was covered with paraffin oil to prevent fast degradation in air during the data collection. However the crystal showed some decay under the X-ray beam, and the data set was not of very high quality. In addition in 3 positional disorder over two sites was observed for the μ3-OH group and a constrained refinement of the site occupation factors of μ3-OH was applied leading to a value of 0.68 and 0.32 for the major and minor isomer, respectively. Three coordination water molecules were located in the difference maps, one of them being partially occupied. In subsequent difference maps, two further sites were found to be partially occupied by uncoordinated crystallization water molecules. The water hydrogen atoms could not be located, though they are probably involved in hydrogen bonds. Since the framework of 3 contains three equivalent large solvent accessible voids (980−985 Å3) and the electron density peaks were not unequivocally interpretable, the SQUEEZE routine of PLATON19 was applied to calculate the solvent disorder area and remove its contribution to the overall intensity data. Presumably additional solvent molecules (MeOH or H2O) occupy the framework cavities of 3. XRPD investigations were carried out by means of a PANalytical X′Pert Pro diffractometer equipped with an X′Celerator fast detector and using Cu Kα radiation transmitted through 0.04 rad Soller slit and 0.5° divergence slit. The powders were mounted on a silicon zerobackground sample holder (0.3 mm deep, 16 mm diameter). X-ray diffraction data were collected between 3° and 70° 2θ with a step size of 0.017° and 60 s counting time. The cell parameters of compound 1

as well as a hexanuclear cluster that self-assembles into a porous supramolecular network.8j Finally, we recently succeeded in the syntheses of stable isomorphous 2D CPs, based on the trinuclear triangular SBU [Cu3(μ3-OH)(μ-pz)3]2+, through the reaction of copper(II) fumarate or 2-methylfumarate with Hpz.12 Here, we report the syntheses, carried out in different reaction conditions, of three CPs based on the trinuclear triangular Cu3(μ3-OH)(μ-pz)3 SBUs joined through the flexible succinate (Suc) dianion. For two of them, the formation of 3D CP networks has been ascertained by single-crystal (SC) XRD, and a porous structure was observed in one of them. Moreover, this porous CP undergoes an irreversible crystal-to-crystal process which transforms it into the nonporous one, by loss of guest solvents and coordinated water molecules under moderate vacuum, at rt. Finally, we obtained and structurally characterized two mononuclear copper(II) pyrazole [Cu(Hpz)x (x = 2, 4)] succinate complexes, one of which self-assembles forming a 1D CP. The different synthetic procedures employed are reported in the Experimental Section and in the Supporting Information.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were purchased from Aldrich and used without further purification. Copper succinate was prepared according to literature method.13 All the reactions and manipulations were carried out in air. The syntheses were performed both in ambient [1 atm, room temperature (rt)] and solvothermal conditions. Elemental analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer. Infrared spectra from 4000 to 600 cm−1 were recorded with a Perkin-Elmer Spectrum One Model FTIR spectrometer with ATR mode. Positive electrospray mass spectra were obtained with a Series 1100 MSI detector HP spectrometer, using MeOH as mobile phase. Solutions for electrospray ionization mass spectrometry (ESI-MS) were prepared using reagent grade methanol, water and/or acetonitrile, and obtained data (m/z and intensities) were compared with those calculated by using the IsoPro isotopic abundance simulator.14 The reported m/z values correspond to the most intense signal of the isotopic clusters. Reflectance solid-state UV/vis spectra were recorded on a Varian Cary 5E spectrophotometer, equipped with a device for reflectance measurements. The magnetic susceptibilities were measured at room temperature (20−28 °C) by the Gouy method with a Sherwood Scientific magnetic balance MSB-Auto, using HgCo(NCS)4 as calibrant and corrected for diamagnetism with the appropriate Pascal constants. The magnetic moments (in μB) were calculated from the equation μeff = 2.84(XmcorrT)1/2. TGA measurements were carried out on compounds 1 and 2 with a Simultaneous Thermal Analyzer PerkinB



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Table 1. Crystal Data and Structure Refinement for Compounds 2−5 compound

2

3

4

5

formula FW crystal symmetry space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) cell volume (Å3) Z Dc (Mg m−3) μ(Mo Kα) (mm−1) F(000) crystal size (mm) θ range (deg) reflns collected unique reflns (Rint) obsd reflns [I > 2σ(I)]/params R1(F), R2(F2) [I > 2σ(I)] GOF on F2 largest diff. peak and hole (e Å−3)

C13H14Cu3N6O5 524.92 monoclinic P21/n 9.7487(10) 11.0061(15) 15.1631(18) 90 94.360(11) 90 1622.2(3) 4 2.149 3.94 1044 0.2 × 0.1 × 0.08 3.0−26.0 31760 3183 (0.070) 2819/247 0.045, 0.087 1.06 0.76 and −0.89

C13H14Cu3N6O5·3.5H2O 587.98 trigonal R3̅ 26.625(7) 26.625(7) 20.024(7) 90 90 120 12293(6) 18 1.430 2.36 5328 0.25 × 0.25 × 0.28 1.4−24.5 7721 4163 (0.089) 1529/269 0.066, 0.161 0.952 0.72 and −0.34

C10H12CuN4O4 315.78 monoclinic P21/c 4.8630(7) 13.578(2) 9.3241(13) 90 95.563(2) 90 612.76(15) 2 1.711 1.80 322 0.2 × 0.1 × 0.1 2.7−27.0 4960 1334 (0.015) 1193/95 0.023, 0.073 1.06 0.26 and −0.35

C20H26CuN8O8 570.03 tetragonal I41/a 22.4290(8) 22.4290(8) 10.2923(7) 90 90 90 5177.6(4) 8 1.463 0.90 2360 0.6 × 0.35 × 0.33 3.4−26.0 52126 2540 (0.048) 1502/173 0.032, 0.044 1.04 0.66 and −0.24

were obtained through the elaboration of its powder diffractogram, by using Expo 2011,20 and HighScore Plus21 software. The molecular graphics were generated by using Mercury 3.0.1 program.22 Color codes for all molecular graphics: yellow-orange (Cu), blue (N), red (O), gray (C), white (H). Crystal data and details of the data collections for compounds 2−5 are reported in Table 1. Syntheses. Synthesis of [Cu3(μ3-OH)(μ-pz)3(Suc)(Hpz)], 1. To a suspension of 1.047 g of Cu(Suc)·2H2O (4.9 mmol) in 45 mL of H2O a solution of 0.69 g of Hpz (10.1 mmol) in 15 mL of H2O was added under stirring, obtaining a blue solution and a microcrystalline solid 1 that was filtered, washed with water and dried in the air. A further crop of microcrystalline 1, for a comprehensive yield of 410 mg (42%), was obtained by evaporation of mother liquors, followed by well-formed crystals of 4 (vide inf ra). Elem. Anal. Calcd for Cu3(OH)(pz)3(Suc)(Hpz): C 32.41, H 3.06, N 18.90. Found C 32.34, H 2.96, N 19.00. IR (KBr cm−1): 3428, 3143, 3088, 3054, 2978, 2875, 2821, 1733, 1703, 1622, 1579s, 1506, 1490, 1430, 1422, 1384, 1370, 1344, 1330, 1272, 1233, 1184, 1177, 1171, 1161, 1073, 1062, 1023, 966, 947, 917, 895, 854, 790, 778, 743, 675, 626, 621, 615, 486. λmax/nm (solid-state reflectance): 611. μeff (297 K) 2.41 μB Compound 1 was also obtained through different synthetic procedures, as microcrystalline powders, in lower yields (see Supporting Information). Synthesis of [Cu3(μ3-OH)(μ-pz)3(Suc)], 2. A solution of Hpz (277 mg, 4.1 mmol) in H2O (30 mL) was added to a suspension of Cu(Suc)·2H2O (438 mg, 2.0 mmol) in H2O (75 mL). The mixture was sealed in a glass vial and maintained at 115 °C for 4 h and then cooled at rt, yielding 246 mg (69%) of crystals of 2 suitable for a SCXRD determination. The phase purity of 2 was confirmed by comparison of its XRPD diffractogram with the calculated one (see Supporting Information Figure S1) m.p.: 245−248 °C (d). Elem. Anal. Calcd for Cu3(OH)(pz)3(Suc): C, 29.74: H, 2.69; N, 16.01. Found: C, 29.83; H, 2.64; N, 15.98. IR (KBr, cm−1): 3408s, 3120w, 3100w, 1600s, 1553s, 1496m, 1488s, 1428s, 1414s, 1385s, 1298s, 1280m, 1223m, 1185m, 1168m, 1071s, 1055s, 997w, 971w, 900w, 888w, 778s, 769s, 645m, 630s, 587w, 530w, 474w, 440w, 370s, 360s. μeff (297 K) = 2.32 μB

Compound 2 was also obtained through different synthetic procedures, in lower yields and/or as microcrystalline powders (see Supporting Information). Synthesis of [Cu3(μ3-OH)(μ-pz)3(Suc)(H2O)2.5]·xH2O·yMeOH, 3. To the solution resulting from the neutralization of H2Suc (23.7 mg, 0.2 mmol in 6.5 mL of MeOH) with 0.4 mL of NaOH 1 M in water, 26.7 mg of Hpz (0.41 mmol) were added, followed by 5 mL of a 4 × 10−2 M solution of CuSO4·5H2O in MeOH. A pale blue solid formed immediately and the suspension was sealed in a glass vial and maintained at 85 °C for 3 h. The vial was then cooled at r.t., yielding a mixture of an unidentified gray powder and well-formed blue crystals. Some of the latter were manually separated from the powder and identified as 3 through a SC-XRD determination. Because of the presence of the gray powder it was impossible to perform other characterizations, as well as to obtain the correct yield of 3. In a distinct synthetic procedure, compound [Cu3(μ3-OH)(μpz)3(Suc)(H2O)(MeOH)]·H2O, 3a, was obtained (see Discussion). Synthesis of [Cu(Suc)(Hpz)2], 4. Purple crystals of 4 (674 mg, 44%,) suitable for a SC-XRD determination, were obtained by slow evaporation in the air of the mother liquors of the synthesis of 1 in approximately a week. The phase purity of 4 was confirmed by comparison of its XRPD diffractogram with the calculated one (see Supporting Information Figure S2). Elem. Anal. Calcd for Cu(Suc)(Hpz)2: C 38.04, H 3.83, N 17.74. Found C 38.13, H 3.85, N 17.96. IR (KBr, cm−1): 3255, 3144, 3124, 2927, 2519, 2329, 1596s, 1521, 1467, 1428, 1382s, 1351, 1277, 1252, 1215s, 1164, 1111, 1079, 1050, 956, 909, 875, 816, 782, 682, 638, 911, 563. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 69.2 (90) [Hpz + H]+, 141.1 (68) [H2Succ + Na]+, 199.0 (37) [CuI(Hpz)2]+, 316.0 (100) [Cu(Succ)(Hpz)2 + H]+, 576.0 (35) [Cu 3 (OH)(pz) 3 ((HSuc)(MeOH)(H 2 O)] + , 625.9 (6) [Cu3(OH)(pz)3(Hpz)(HSuc)(MeOH)]+. μeff (297 K) = 1.35 μB Synthesis of [Cu(HSuc)2(Hpz)4], 5. A mixture of 294 mg of H2Suc (2.49 mmol) and 687 mg of Hpz (9.95 mmol) was added under stirring to 533 mg of Cu(Suc)·2H2O (2.47 mmol) suspended in 40 mL of H2O, obtaining in a few minutes a blue solution. By fractional crystallization through evaporation in the air a small crop of microcrystalline compound 4 (43 mg, 5%), identified through elemental analysis and IR spectrum, was first obtained, followed by 576 mg (yield 41%) of blue crystals of 5, suitable for a SC-XRD C



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Scheme 2

determination. The phase purity of 5 was confirmed by comparison of its XRPD diffractogram with the calculated one (see Supporting Information Figure S3). Elem. Anal. Calcd for Cu(HSuc)2(Hpz)4: C 42.14, H 4.60, N 19.66. Found C 42.02, H 4.78, N 17.77. IR (KBr, cm−1): 3282s, 3135m, 1702s, 1683s, 1580s, 1556s, 1523m, 1465m, 1426s, 1411sh, 1366s, 1308m, 1249s, 1220s, 1186s, 1165s, 1121s, 1065s, 1051s, 996w, 963w, 945m, 911w, 897w, 875m, 847m, 781s, 761s, 690w, 657m, 610s, 597m, 430m, 295m. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 69.2 (82) [Hpz + H]+, 141.1 (26) [H2Succ + Na]+, 199.1 (100) [CuI(Hpz)2]+, 316.0 (70) [Cu(HSucc)(Hpz)2]+. μeff (297 K) = 1.65 μB. Compound 5 was also obtained through different synthetic procedures, in lower yields and/or as microcrystalline powders (see Supporting Information).

When the reaction between copper succinate and Hpz in water was carried out in hydrothermal conditions it was possible to isolate well-formed crystals of the [Cu3(μ3-OH)(μpz)3(Suc)] species (2) suitable for a SC-XRD characterization. In Figure 1 the structure of 2 is reported, showing that two

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RESULTS AND DISCUSSION In the course of our studies on pyrazolate-based CuII CPs, we recently reported that 1D and 2D CPs, based on the trinuclear triangular Cu3(μ3-OH)(μ-pz)3 moiety, were obtained through the interaction of CuII ions and Hpz, with rigid fumarate or 2methylfumarate anions.12 Here we report the completely different results obtained when the flexible succinate anion was instead employed. These results, obtained by carrying out the reactions in different conditions, are synthetically summarized in Scheme 2, while the specific reaction conditions can be found in the Experimental Section and in the Supporting Information. The reaction at rt of copper succinate in water with Hpz in 1:2 molar ratio resulted in the isolation of the microcrystalline, highly insoluble species [Cu3(μ3-OH)(μ-pz)3(Suc)(Hpz)], 1. Compound 1 was characterized on the bases of the elemental analysis, the value of its magnetic susceptibility, falling in the range found for structurally established trinuclear triangular CuII systems,8,9,12 and from the solid-state reflectance visible spectrum showing a maximum at 611 nm, almost identical to the values found for other trinuclear triangular CuII species.8a,b,f Although a full structural characterization was not possible, we were able to define from the XRPD pattern (Supporting Information Figure S4) the probable unit cell parameters of 1, that resulted to be monoclinic, with a = 18.1846(7) Å, b = 11.6018(4) Å, c = 10.1516(3) Å; α = 90°, β = 104.843(2)°, γ = 90°; cell volume =2070.252 Å3, Z = 4. The reliability of such unit cell data is confirmed by the excellent agreement of the refined profile with the experimental one (see difference plot in Supporting Information Figure S4). Moreover, the volume is consistent with a structural model containing 4 formula units per unit cell, considering a volume of approximately 16 Å3 per each non-hydrogen atom, which is a typical value. A TGA measurement (Supporting Information Figure S5) shows that 1 is quite stable up to 200 °C when it decomposes losing, both Hpz and succinate ion in the range 200−235 °C (calcd 31%, found 31%), in agreement to the proposed formulation. At 250 °C, the decomposition of the remaining ligands starts, and at ∼315 °C, only Cu is present (calcd 32%, found 32%).

Figure 1. Capped-stick representation of 2 with partial atom-labeling scheme evidencing the 12-membered metallacycle formed through two syn-syn succinate moieties bridging two symmetry equivalent Cu3(μ3OH)(μ-pz)3 moieties (symmetry code i: −x + 1, −y, −z; ii −x + 2, −y, −z).

Cu3(μ3-OH)(μ-pz)3 clusters are joined together through the C(10)O(2)O(3) succinate moieties acting in the bridging syn− syn fashion [O(2)−Cu(1) 1.992(4), O(3)−Cu(2i) 1.964(3) Å, (symmetry code i: −x + 1, −y, −z)]. Thus generated 12membered metallacycle, which presents an inversion center placed midway of the O(1)···O(1i) segment, is structurally analogous to other metallacycles previously found in the structures of compounds c,8b g,8g and h8g (see Scheme 1). Both Cu(2) and Cu(3) exhibit a square planar coordination, while Cu(1) adopts a square pyramidal geometry. The capping μ3−OH is placed out of the plane defined by the three copper ions [0.445(4) Å] (Supporting Information Figure S6), a feature normally found in analogous derivatives.7,8a,b,d−j,9 The other relevant geometrical parameters of the [Cu3(μ3-OH)(μpz)3] moiety are in the range normally found for these structures and are reported in Supporting Information Table S1. The C(13)O(4)O(5) succinate carboxylic group is engaged in the formation of a coordination polymer, acting as a syn−anti bridge between Cu(3) of another hexanuclear unit [O(5)− Cu(3ii) 1.994(4) Å, symmetry code ii: −x + 2, −y, −z]23 and Cu(1) of a third hexanuclear unit [O(4)−Cu(1iii) 2.296(5) Å, symmetry code iii: −x + 3/2, y − 1/2, −z + 1/2] (see also Supporting Information Figure S7). Incidentally, these latter interactions are responsible of the formation of another 12membered metallacycle, as shown in Figure 2. Because of the multiplicity of these latter interactions a nonporous 3D network, sketched in Figure 3, is obtained. Compound 2 has been obtained also by using different synthetic procedures (see Supporting Information). As an example, microcrystalline crops of 2 were obtained by refluxing a water suspension of copper succinate with Hpz in 1:2 molar ratio (yield ∼72%) or by treating at rt a water solution of sodium succinate with Hpz (1:2 molar ratio), followed by the D



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Figure 2. Partial capped-stick representation of the polymeric assembly of 2. Ball-and-stick representations evidence, from left to right, the syn−syn 12-membered metallacycle, the succinate connection and the syn−anti 12-membered metallacycle. For sake of clarity the H atoms and the pyrazolate C atoms are omitted.

Figure 4. Capped-stick representation of the asymmetric unit of 3 with partial atom labeling scheme. Crystallization solvent molecules are omitted.

0.68 and 0.32, respectively, is 0.444(14) Å out of the plane defined by the three copper ions (see Supporting Information Figure S9), analogously to what found in this type of derivatives.7,8a,b,d−j,9 The other geometrical parameters of the [Cu3(μ3-OH)(μ-pz)3] moiety, reported in Supporting Information Table S2, are quite normal. It is to note that the three water molecules are very weakly coordinated to Cu, a relevant feature for the stability of 3 (vide inf ra).25 The carboxylate C(10)O(2)O(3) acts also as a bridge in a syn−anti fashion with Cu(2) of another trinuclear unit [O(3)−Cu(2i) 2.012(9) Å, symmetry code i: x − y + 1, x + 1, −z + 1] and, through this kind of connection, six asymmetric trinuclear units selfassemble generating the macrocycle shown in Figure 5.

Figure 3. Crystal packing of 2. For sake of clarity the H atoms and the pyrazolate C atoms are omitted.

addition of copper sulfate (yield ∼45%). In both cases the obtained compounds were identified through elemental analyses, IR and XRPD comparison with an authentic sample. As compounds 1 and 2 apparently differ by one Hpz molecule, a TGA measurement (Supporting Information Figure S8) was performed also on compound 2 evidencing a decomposition pattern strictly related to that shown by 1. Compound 2 is a little bit more thermally stable than 1 and its decomposition starts around 235 °C losing the succinate ion in the range 235− 250 °C (calcd 22%, found 22%). At 250 °C, analogously to 1, the decomposition of the remaining ligands starts and, at ∼315 °C, only Cu is present (calcd 36%, found 35%). Interestingly, when the second procedure to obtain 2 (Na2Suc + 2Hpz + CuSO4) was performed in MeOH/water and in solvothermal conditions, well-formed blue crystals of compound [Cu3(μ3-OH)(μ-pz)3(Suc)(H2O) 2.5]·x(H2O)·y(MeOH), 3, were instead obtained, accompanied by small quantities of unidentified gray powders. A SC-XRD determination was performed on one blue crystal and in Figure 4 is reported the asymmetric unit of 3. The succinate carboxylate C(10)O(2)O(3) is bonded to Cu(1) [Cu(1)−O(2) 1.985(7) Å],24 while C(13)O(4)O(5) is responsible, with other strong interactions, of the formation of a 3D CP (vide inf ra). The three copper ions form an almost equilateral triangle [Cu(1)···Cu(2) 3.352(2), Cu(2)···Cu(3) 3.363(2), Cu(1)···Cu(3) 3.356(2) Å] and also the O(1A)−Cu distances are very similar [Cu(1)−O(1A) 2.005(10), Cu(2)− O(1A) 1.971(10), Cu(3)−O(1A) 1.990(9) Å]. The capping μ3-OH, disordered over two sites with occupation factors of

Figure 5. View down the crystallographic c axis of a macrocyclic rosette obtained through the self-assembling of six asymmetric units of 3 (see text). For sake of clarity the H atoms and the pyrazolate C atoms, as well as crystallization solvent molecules are omitted.

This rosette can be seen as a sort of tertiary building unit which is repeated in the coordination polymer formation. Actually, each of the six C(13)O(4)O(5) succinate moieties chelates Cu(3) of a trinuclear unit belonging to another rosette [O(4)−Cu(3ii) 1.977(7), O(5)−Cu(3ii) 2.664(8) Å, symmetry code ii: y − 2/3, −x + y + 2/3, −z + 2/3] (see Supporting Information Figure S10) generating, according to an arbitrary description, a 2D CP partly shown in Figure 6. In spite of the seemingly large pores visible in each rosette (Figure 5 and 6), when the hydrogen atoms are included it becomes evident that the pores are very small, (see Supporting Information Figure S11 for a space-filling representation). Moreover, due to the offset self-assembling of the layers (vide inf ra) also these small voids are, in any case, not available. E



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15 disordered methanol molecules (or even more water molecules) in each channel per unit cell. Even though no MeOH molecule could clearly be located in the crystal structure, because of the poor data quality, we believe that guest MeOH molecules permeate the pores and that its easy evaporation at room conditions is the cause for the rapid collapse of the framework. On the other hand, when a sample of 3 is dried under vacuum (0.1 mmHg) at rt, the resulting compound has an elemental analysis corresponding to the formulation [Cu3(μ3-OH)(μ-pz)3(Suc)], thus indicating that, besides MeOH, also weakly coordinated water molecules were eliminated by drying 3 under vacuum. Quite surprisingly, the XRPD diffractogram of this latter compound (a quite pure crystalline phase) is identical to the calculated one of 2, (Figure 9) indicating that a crystal-to-

Figure 6. Partial view down the crystallographic c axis of a layer of 3 formed through the connection of seven macrocyclic rosettes (see text). For sake of clarity the H atoms and the pyrazolate C atoms, as well as crystallization solvent molecules are omitted.

These layers, having a thickness of about 18 Å (see Supporting Information Figure S12), further self-assemble through other C(13)O(4)O(5)−Cu(3) interactions forming the 3D, CP shown in Figure 7.

Figure 9. Calculated and experimental XRPD diffractograms of 3 as prepared (blue), 2 (red), and 3 dried under vacuum (black).

crystal transformation from 3 to 2 occurred in the solid state. We have checked the reversibility of this transformation by solvothermally treating at 85 °C a suspension of 2 in a water/ methanol solution for 18 h, with no apparent result. This behavior evidence that: (i) the presence of solvents in the pores is of paramount importance not only for the stability of 3 but also for its formation, likely playing a templating role in the self-assembling which generates the peculiar porous crystal packing; (ii) compound 3, treated under vacuum at rt, undergoes a rearrangement of strong bonds, transforming easily in 2 through a solid-state crystal-to-crystal process, while the trinuclear moiety maintains intact its structure; (iii) compound 2 is evidently the most stable (isolated) trinuclear copper pyrazolate succinate species.

Figure 7. Partial view down the crystallographic a axis of three layers of 3, where each layer is drawn in different colors. For sake of clarity the H atoms and the pyrazolate C atoms, as well as crystallization solvent molecules are omitted.

This particular self-assembly of the layers generates in the crystal packing of 3, three intersecting channels, having the same ellipsoidal shape and comprehensively accounting for a 24% solvent accessible void space as calculated by the PLATON/SQUEEZE program19 with a probe radius of 1.2 Å (Figure 8 and Supporting Information Figure S13). The squeezed electron density corresponds to ∼277−279 electrons in each channel, which would roughly correspond to

Figure 8. Space-filling representations of the crystal packing of 3 evidencing the presence of three intersecting channels. F



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Incidentally, also compound 3a (see Supporting Information) underwent an identical transformation to compound 2 in the same conditions of 3. On the other hand the CPs based on trinuclear triangular copper(II) clusters are not the exclusive products obtained. Actually, by slow evaporation in the air of the mother liquors of the synthesis of 1, a CP based on the mononuclear SBU [Cu(Suc)(Hpz)2], 4, crystallized in good yield. The crystal structure of 4, reported in Figure 10, shows that the copper ion sits on a crystallographic inversion center and

Figure 12. Supramolecular assembly of two 1D CPs of 4. Intra- and intermolecular H-bonds are indicated as red and blue dashed lines, respectively.

copper clusters. In ESI-MS conditions, oligomeric species may be formed26 thus the presence of a trinuclear cluster may be due to some aggregation phenomena in the gas-phase. Nevertheless, in this particular case it is to be emphasized that the trinuclear species 1 can be also obtained by treating 4 with NaOH, (see Supporting Information). Thus, the signals at m/z 576.0 and 625.9, corresponding to the [Cu3(OH)(pz)3(Suc)(MeOH)(H2O) + H]+ and [Cu3(OH)(pz)3(Hpz)(Suc)(MeOH) + H]+ species, respectively, may be a reliable indication of the relatively easy transformation of 4 into some specific trinuclear triangular CuII species in the gas-phase.27 After the isolation of compound 4 from mother liquors of compound 1, small quantities of a different mononuclear species, [Cu(HSuc)2(Hpz)4], 5, crystallized. The unexpected presence of the monodeprotonated succinate framework is probably due to the formation of H2Suc during the synthesis of the trinuclear moieties generating compound 1 (see Scheme 2). To test this hypothesis we treated a suspension of copper(II) succinate in water with Hpz and H2Suc (1:2:1 molar ratios) obtaining a deep blue solution. Slow evaporation in the air yielded first a small crop of 4 followed by the isolation in fairly good yield of well-formed crystals of [Cu(HSuc)2(Hpz)4], 5. In Figure 13 is reported the molecular structure of 5 showing the coordination of two monocarboxylates to the CuII ion. The

Figure 10. Capped-stick representation of 4, with partial labeling scheme. Only the symmetry independent atoms are labeled.

exhibits a square planar geometry, being coordinated by N(1) and O(1) of two symmetry equivalent pyrazole and carboxylate moieties, respectively [Cu(1)−N(1) 1.9775(13), Cu(1)−O(1) 1.9417(10) Å]. The other geometrical parameters are in the range normally found for this kind of compounds and are reported in Supporting Information Table S3. The coordination of the two equivalent carboxylate groups generates 1D CPs running parallel to the crystallographic c axis (Figure 11).

Figure 11. View down the crystallographic a axis of one 1D CP formed by succinate dianions bridging Cu(Hpz)2 moieties.

Finally, N(2)H(2N) groups are involved into weak intramolecular H-bonds with O(1) of the same SBU [N(2)···O(1) 2.834(2) Å, N(2)−H(2N)···O(1) 119(2)°] and into intermolecular H-bonds with O(2) of a carboxylate belonging to a neighbor CP [N(2)···O(2i) 2.766(2) Å, N(2)−H(2N)···O(2i) 138(2)°, symmetry code i x + 1, y, z] (Figure 12). These intraand intermolecular H-bonds reasonably reinforce the molecular structure of 4 and support its supramolecular assembly, respectively. In Supporting Information Figure S15 three different spacefilling representations of the crystal packing of 4 are reported. The ESI-MS spectrum of 4 (Supporting Information Figure S16) is in agreement to its mononuclear structure, showing the most intense signal at m/z 316.0, that corresponds to protonated 4. On the other hand, it is noteworthy the presence of an important signal at 576.0 and a weak one at 625.9 m/z, both having an isotopic pattern corresponding to trinuclear

Figure 13. Capped-stick representation of 5 with partial atom labeling scheme. Only the symmetry independent atoms are labeled.

coordination geometry of Cu(1) (which is located on an inversion center) is octahedral [Cu(1)−O(1) 2.4459(15), Cu(1)−N(1) 2.0145(16), Cu(1)−N(3) 2.0030(17) Å] and is likely sustained by two quite strong intramolecular H-bonds [N(2)···O(2) 2.688(2) Å, N(2)-H(2N)···O(2) 173° and N(4)···O(3) 2.939(3) Å, N(4)-H(4N)···O(3) 149°] (See Supporting Information Figure S17). Other geometrical parameters are in the range normally found for this kind of compounds and are reported in Supporting Information Table S4. Noteworthy, O(2) is also involved into a strong intermolecular H-bond with the carboxylic acid moiety of the G



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Figure 14. Compound 5. Capped stick representation viewed down the crystallographic c axis of a single H-bond network (left) and of a supramolecular sheet (right). The space-filling representation in green puts in evidence one small, square-section, channel (see text).

succinate of another mononuclear unit [O(4i)···O(2) 2.550(2) Å, O(4i) −(H4Ai)···O(2) 176(2)°, symmetry code i: y + 3/4, −x + 3/4, z − 1/4]. This interaction generates a 3D supramolecular network (Figure 13 left) where small cavities are present, (Figure 14 right and Suppporting Information Figure S18). A TGA measurement (Supporting Information Figure S19) shows an almost continuous weight decrease, in the 120−220 °C range, which is consistent with the loss of four Hpz and one H2Suc molecules (calcd 68.5%, found 70%). Thus remaining Cu(Suc), further decomposes yielding Cu, at ∼400 °C (calcd 11%, found 11%)

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into account that 1 and 2 differ for a Hpz molecule, we are confident in the presence of some structural similarities between these two compounds. Particularly interesting is the transformation of 3 (and 3a too) (a first generation porous CPs, according to Kitagawa)28 to 2 in very mild conditions (vacuum, rt), which was qualitatively partly observed also at ambient pressure. While it is conceivable that guest molecules located in the pores of 3 (3a) evaporate easily, thus destroying the lattice structure, it is evident that a modest vacuum (0.1 mmHg) is able to eliminate also weakly bonded (see Table S3 and S6) coordinated water or MeOH molecules. On the other hand, we believe that the elimination of weakly coordinated molecules, releasing three coordination sites on Cu ions, is the key for the further transformation in compound 2, for which important structural rearrangements, involving Cu−O(carboxylates) bonds, are needed. Because of the irreversibility of the process, we believe that 3 is in an energetically metastable situation with respect to 2. The stoichiometries of the reactions reported in Scheme 2, account also for the formation of compounds 4 and 5. Actually, compound 4 was obtained from mother liquors of the synthesis of compound 1 where, due to the formation of H2Suc, the environment becomes more acidic, thus favoring neutral pyrazole and water with respect to pyrazolate and OH− anions, respectively. Contemporaneously, the Hpz/Cu ratio in mother liquors increased, leading to the formation of 4. As far as the formation of 5 is concerned, the increased acidity of the environment favors also the formation of the HSuc− species, that produces small quantities of 5 as a second crystalline derivative from mother liquors of the synthesis of compounds 1 and 2 (see Supporting Information). This feature has been validated by the formation of 5, in good yield, when an excess of H2Suc was added to the reaction pot. Work is in progress to test the interaction of other flexible carboxylates, actually, glutarate, adipate and pimelate, with CuII and Hpz, in different reaction conditions, with the aim to further test the easy formation of the trinuclear triangular cluster Cu3(μ3-OH)(μ-pz)3 and to find the conditions to obtain porous and insoluble CPs, more stable than 3 with respect to the loss of guest molecules.

CONCLUSIONS

In this work, we have further confirmed the ability of CuII ions to self-assemble, easily, in stable Cu3(μ3-OH)(μ-pz)3 trinuclear triangular fragments when reacted, in suitable conditions, with Hpz. In particular, the formation of trinuclear species has been favored by a Hpz/Cu molar ratio not very high [experimentally around 2, against a theoretical value in the range 1.33−1 (see Scheme 2)] and by a someway basic environment. If bicarboxylate ions are present, the trinuclear triangular units further self-assemble to generate CPs, while other reaction conditions (different choices of reagents, temperature or solvents) may generate different CPs through specific coordination ways of bicarboxylate ions. Moreover, the formation of crystals suitable for a SC-XRD determination is often a serendipitous issue, even though it seems that solvothermal conditions favor this event. Anyway, while for compounds 2 and 3 it was possible to obtain a XRD determination, the trinuclear structure of compound 1 has been inferred on the basis of the elemental analysis and of the analogy of the solid-state visible spectrum, which shows a pattern identical to those of other structurally validated trinuclear triangular derivatives.8f Due to the insolubility of 1 it was impossible to obtain also an ESI-MS spectrum, possibly showing the isotopic pattern typical of trinuclear CuII fragments.8 On the other hand, the insolubility of 1, as well as that of 2 and 3, strongly suggests the polymeric nature of all these compounds. The TGA results for 1 and 2, showing the loss of Hpz and Suc (compound 1) and Suc (compound 2) in almost the same temperature range (see Supporting Information Figures S5 and S8, respectively) are in complete agreement to the proposed formulations. On the other hand, the XRPD pattern (Supporting Information Figure S4) confirms that compound 1 is a new, highly pure crystalline phase and, its calculated (from XRPD diffractogram) cell volume, is in agreement with a structural model containing four formula units per unit cell, considering the typical volume value of ∼16 Å3 per each non-hydrogen atom. On these bases, taking

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic files for 2, 3, 3a, 4, and 5 in CIF format, Tables S1−S6, and Figures S1−S19. This information is available free of charge via the Internet at http://pubs.acs.org. H



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AUTHOR INFORMATION

Corresponding Author

*Phone: +39 051 2099559 (M.M.); +39 049 8275157 (L.P.); +39 0737 402234 (C.P.). Fax: +39 051 2099456 (M.M.); +39 049 8275161 (L.P.); +39 0737 637345 (C.P.). E-mail: magda. [email protected] (M.M.); [email protected] (L.P.); [email protected] (C.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Project PRAT 2009 “Design, synthesis and characterization of coordination polymers by assembling of oligo-nuclear metal systems and polytopic ligands” of the University of Padova. M.M. wishes to thank the University of Bologna for financial support.

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DEDICATION Dedicated to the memory of our friend and colleague, Professor Klaus Josef Müller. REFERENCES

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Coordination Polymers Based on the Trinuclear Triangular Secondary

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