Synthesis and Structural Characterizations of New Coordination

Article pubs.acs.org/crystal

Synthesis and Structural Characterizations of New Coordination Polymers Generated by the Interaction Between the Trinuclear Triangular SBU [Cu3(μ3‑OH)(μ-pz)3]2+ and 4,4′-Bipyridine. 3° Francesca Condello,† Federica Garau,‡ Arianna Lanza,‡,§ Magda Monari,*,∥ Fabrizio Nestola,⊥ Luciano Pandolfo,*,‡ and Claudio Pettinari*,† †

Scuola di Farmacia, Università di Camerino, Via S. Agostino, 1, I-62032 Camerino (MC), Italy Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo, 1, I-35131 Padova, Italy § Departement für Chemie und Biochemie, Universität Bern, Freiestrasse, 3, CH-3012 Bern, Switzerland ∥ Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi, 2, I-40126 Bologna, Italy ⊥ Dipartimento di Geoscienze, Università di Padova, Via Gradenigo, 6, I-35131 Padova, Italy

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‡

S Supporting Information *

ABSTRACT: The reactions of 4,4′-bipyridine with selected trinuclear triangular copper(II) complexes, [Cu3(μ3-OH)(μ-pz)3(RCOO)2Lx], [pz = pyrazolate anion, R = CH3(CH2)n (2 ≤ n ≤ 5); L = H2O, MeOH, EtOH] yielded a series of 1D coordination polymers (CPs) based on the repetition of [Cu3(μ3-OH)(μ-pz)3] secondary building units joined by bipyridine. The CPs were characterized by conventional analytical methods (elemental analyses, ESI-MS, IR spectra) and single crystal XRD determinations. An unprecedented 1D CP, generated through the bipyridine bridging hexanuclear copper clusters moieties, two 1D CPs presenting structural analogies, and two monodimensional tapes having almost exactly superimposable structures, were obtained. In one case, the crystal packing makes evident the presence of small, not-connected pores, accounting for ca. 6% of free cell volume.

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INTRODUCTION

These trinuclear compounds self-assemble in hexanuclear clusters through monatomic carboxylate bridges and form 1D or 2D CPs or supramolecular networks.28−31 Copper(II) trinuclear triangular assemblies capped by OH− or O2− ions are well-known, but they were normally obtained thanks to the deprotonation promoted by the presence of hexogenous bases,32−37 while in our case the deprotonation of water and Hpz is favored by the basicity of the carboxylates, as witnessed, as an example, by the failure of the reaction when copper(II) trifluoroacetate was employed.28 Anyhow, the trinuclear [Cu3(μ3-OH)(μ-pz)3]2+ cluster is quite stable (it remained in a good extent intact even when compound 1b was reacted with aqueous HCl38 and other strong acids,39 generating in some cases porous derivatives); thus, we started a research project aiming to synthesize unprecedented CPs by connecting the trinuclear clusters present in compounds 1a−j with rigid or flexible ditopic nitrogen ligands. First results, obtained by reacting compounds 1a40 and 1b−f41 with 4,4′bipyridine (bpy) in different reaction conditions, have been published. Here we report the synthesis and structural

Coordination polymers (CPs) continue to be the object of numerous studies1−20 mainly due to their possible applications in relevant industrial fields as catalysis,1,2,18 magnetisms,19 and gas storage3,20 as witnessed by the fact that BASF21,22 has produced in the last years some CPs, which are now commercialized by Sigma-Aldrich as Basolite A100, C300, F300, Z1200, and Basosiv M050.23 In the course of our studies on the interaction of transition metal ions with dinitrogen ligands and monocarboxylates ions, we synthesized mono and polynuclear complexes as well as CPs possessing some interesting features, as porosity and sorption− desorption properties. Our attention has been mainly focused on the reactions of copper(II) carboxylates with pyrazole (Hpz) carried out in very mild conditions, observing a different behavior depending on the solvent employed. In MeCN it was possible to isolate quantitatively a 1D CP based on the Cu(pz)2 secondary building unit (SBU),24−26 which adsorbs−desorbs reversibly small molecules (water, ammonia) evidencing the “porosity without pores” behavior.27 When the reactions were instead carried out in protic solvents, compounds 1a−j, based on the trinuclear triangular fragment [Cu3(μ3-OH)(μ-pz)3]2+, whose charge is balanced by two carboxylates coordinated to CuII ions, always formed (Scheme 1). © XXXX American Chemical Society

Received: May 12, 2015 Revised: August 1, 2015

A

DOI: 10.1021/acs.cgd.5b00661 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Scheme 1. Reactions of Copper(II) Carboxylates with Pyrazole


Table 1. Crystal Data and Structure Refinement for Compounds 2−6

a

compound

2

3

4

5

6

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) T (K) θ limits (deg) Refl. collected unique refl. (Rint) GooF on F2 R1(F)a, wR2(F2)b largest diff. peak and hole (e Å−3)

C28H34Cu3N8O5 753.26 triclinic P1̅ 8.2121(11) 11.6294(15) 17.864(2) 86.450(2) 80.388(2) 83.515(2) 1669.7(4) 2 1.500 1.941 772 0.30 × 0.25 × 0.22 296 1.76−27.00 14046 7163 (0.0538) 1.008 0.0628, 0.1458 0.950 and −0.849



C31H42Cu3N8O6 813.35 triclinic P1̅ 12.486(8) 12.753(8) 13.882(9) 62.927(6) 65.953(7) 70.069(7) 1764(2) 2 1.531 1.845 838 0.27 × 0.20 × 0.10 296 1.73−24.71 14019 5940 (0.0809) 1.045 0.0937, 0.2506 1.625 and −1.772

C28H40Cu3N7O5 745.29 triclinic P1̅ 10.970(7) 12.495(8) 13.287(8) 86.549(6) 73.839(6) 68.911(5) 1630.5(18) 2 1.518 1.985 768 0.30 × 0.20 × 0.10 296 1.60−24.75 11673 5494 (0.1127) 1.011 0.1003, 0,2416 1.314 and −1.067

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP], and P = (Fo2 + Fc2)/3.

characterization of five new 1D CPs synthesized in the reaction of 1g−j with bpy.

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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. The software SMART43 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,43 and an empirical absorption correction was applied using SADABS.44 The structures were solved by direct methods (SIR 97)45 and subsequent Fourier syntheses and refined by full-matrix least-squares on F2 (SHELXTL)46 using anisotropic thermal parameters for all non-hydrogen atoms. The structure of 4, collected at 115 K, was solved in the centric space group P1̅ revealing disorder problems in the two different hexanoate chains that are also very close to inversion centers. The hexanoate ligand bridging two copper atoms in 4 shows splitting over two sites in the terminal part of the chain starting from C28 (C28B) that was refined with final occupation factors of 0.59 and 0.41, respectively, whereas the chelating hexanoate shows large ADPs from the second C atom having the sixth carbon lying on one inversion center. This artifact generating not real intermolecular contacts is caused by the inversion center that makes impossible to rationalize the disorder of this chain. The aromatic, methylene, and methyl hydrogen atoms were placed in calculated positions and refined with isotropic thermal parameters

EXPERIMENTAL SECTION

Materials and Methods. All the reactions and manipulations were carried out in air. Compounds 1g−j were prepared as previously reported.29,31 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 PerkinElmer Spectrum One Model FTIR spectrometer with ATR mode. Positive electrospray mass spectra (ESI-MS) were obtained with a Series 1100 MSI detector HP spectrometer, using methanol as mobile phase. Solutions for ESI-MS were prepared using reagent grade methanol, water, and/or acetonitrile, and obtained data (masses and intensities) were compared with those calculated by using the IsoPro isotopic abundance simulator.42 Peaks containing copper(II) ions are identified with the most intense signal of the isotopic clusters. The magnetic susceptibilities were measured at room temperature (20−28 °C) 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. Crystallographic Data Collection and Structure Determination. The X-ray intensity data were measured on a Bruker Apex II B




Article

Reaction of [Cu3(μ3-OH)(μ-pz)3(Me(CH2)4COO)2(EtOH)], 1i, with 4,4′-Bipyridine. Synthesis of [Cu3(μ3-OMe)(μ-pz)3(Me(CH2)4COO)2(μC10H8N2)], 4. To a solution of 1i (107 mg, 0.16 mmol) in 20 mL of MeOH, a solution of bpy (196 mg, 1.26 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and then let to evaporate in the air, yielding blue rhombic platelets, suitable for a SCXRD determination. Compound 4. Yield 112 mg, 89%. Elem. Anal. Calcd for [Cu3(μ3OMe)(μ-pz)3(C5H11COO)2(C10H8N2)]: C, 47.49; H, 5.23; N, 13.84. Found: C, 47.38; H, 5.32; N, 13.83. IR (KBr, cm−1): 3094, 2955, 2930, 2838, 2813, 1606, 1568, 1534, 1490, 1418, 1396, 1379, 1273, 1221, 1177, 1062, 987, 844, 817, 757, 745. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 157.1 (83) [bpy + H]+; 408.2 (100) [Cu3(O) (pz)3]+; 454.0 (63) [Cu3(OMe)2(pz)3]+; 458.0 (83) [Cu 3 (OH) 2 (pz) 3 (MeOH)] + ; 538.0 (42) [Cu 3 (OMe) (pz)3(C5H11COO)]+; 566.1 (47) [Cu3(OMe) (pz)3(OH)(H2O)7]+, 574.2 (41) [Cu3(OMe) (pz)3(C5H11COO)(H2O)2]+, 622.2 (30) [Cu3(OMe) (pz)2(C5H11COO)2(H2O)2]+, 686.2 (28) [Cu3(OMe) (pz) 3 (C 5 H11 COO) 2 (MeOH) + H] + , 742.3 (33) [Cu 3 (OMe) (pz)3(C6H11COO) (MeOH)2(H2O)8]+, 831.2 (24) [Cu3(OMe) (pz)3(C5H11COO)2(MeOH)2(H2O)4(MeCN) + H]+. μeff (295 K) = 3.968 μB (calculated for C64H84Cu6N16O10). μeff (295 K) = 2.814 μB (calculated for C32H42Cu3N8O5). Synthesis of [Cu 3 (μ 3 -OH)(μ-pz) 3 (Me(CH 2 ) 4 COO) 2 (H 2 O)(μC10H8N2)], 5. To a solution of 1i (230 mg, 0.33 mmol) in 20 mL of MeOH, a solution of bpy (105 mg, 0.67 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and then let to evaporate in the air, yielding a mixture of crystals of 4 and 5. The latter were manually separated from the mixture, and one crystal was employed for a SCXRD determination. Reaction of [Cu3(μ3-OH)(μ-pz)3(Me(CH2)5COO)2(EtOH)], 1j, with 4,4′-Bipyridine. Synthesis of [Cu3(μ3-OH)(μ-pz)3(Me(CH2)5COO)2(μC10H8N2)], 6. To a solution of 1j (252 mg, 0.4 mmol) in 30 mL of MeOH, a solution of bpy (110 mg, 0.7 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and then allowed to slowly evaporate in the air, yielding well-formed blue crystals, suitable for a SCXRD determination. Compound 6. Yield 238 mg, 80%. Elem. Anal. Calcd for [Cu3(μ3OH)(μ-pz)3(CH3(CH2)5COO)2·(C10H8N2)]: C, 48.14; H, 5.39; N, 13.61. Found: C, 47.06; H, 5.69; N, 13.21. IR (KBr, cm−1): 3441, 3284, 2955, 2929, 2858, 1571, 1538, 1492, 1399, 1382, 1280, 1178, 1108, 1060, 836, 812, 759, 750, 626, 487. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 588.4 (12) [Cu 3 (OH) (pz) 3 (C 6 H 13 COO) (MeOH)(H 2 O)] + , 620.4 (18) [Cu 3(OH) (pz)3 (C 6H13COO) (MeOH) 2(H2 O)] +, 650.4 (10) 682.5 (8) [Cu3(pz)3(C6H13COO)2]+, [Cu 3 (pz) 3 (C 6 H 13 COO) 2 (MeOH)] + , 713.3 (100) [Cu 3 (OH) (pz)4(Hpz) (MeOH)4(MeCN)]+, 1161.7 (28) [Cu 6 (OH) 3 (pz) 6 (C 6 H 13 COO) 2 (MeOH)(H2 O) 2 ] + , 1197.7 (85) [Cu 6 (OH) 3 (pz) 6 (C 6 H 13 COO) 2 (MeOH)(H2 O) 4 ] + , 1233.8 (55) [Cu 6 (OH) 3 (pz) 6 (C 6 H 13 COO) 2 (MeOH)(H2 O) 6 ] + , 1295.8 (18) [Cu6(OH)2(pz)6(C6H13COO)3(H2O)5]+. μeff (295 K) = 2.256 μB (calculated for C28H40Cu3N7O5).

U(H) = 1.2 Ueq(C) or U(H) = 1.5 Ueq(C) (methyl H), respectively, and allowed to ride on their carrier carbons, whereas the hydroxy H atoms in 3, 5, and 6 were located in the Fourier map and refined isotropically [U(H) = 1.2 Ueq(O). Large residual electron density is observed in 4 and 5 in the vicinity of the hexyl chains and in 6 close to one copper atom. XRPD diffraction 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. Molecular graphics were generated by using Mercury 3.5.147,48 program. Color codes for all molecular graphics: orange (Cu), blue (N), red (O), gray (C), white (H). Crystal data and details of data collections for compounds 2−6 are reported in Table 1. Syntheses. Reaction of [Cu3(μ3-OH)(μ-pz)3(Me(CH2)2COO)2(MeOH)(H2O)], 1g, with 4,4′-Bipyridine. Synthesis of [Cu3(μ3-OMe)(μ-pz)3(Me(CH2)2COO)2(μ-C10H8N2)], 2. To a solution of 1g (251 mg, 0.4 mmol) in 30 mL of MeOH, a solution of bpy (133 mg, 0.9 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and then allowed to slowly evaporate in the air, yielding blue well-formed crystals, suitable for a single crystal (SC) XRD determination. Compound 2. Yield 245 mg, 83%. Elem. Anal. Calcd for [Cu3(μ3OCH3)(μ-pz)3(C3H7COO)2(C10H8N2)]: C, 44.65; H, 4.55; N, 14.88. Found: C, 44.22; H, 4.46; N, 14.81. IR (KBr, cm−1): 3144, 3097, 3045, 3018, 2960, 2930, 2900, 2872, 2813, 1610, 1570, 1540, 1491, 1421, 1405, 1380, 1341, 1273, 1257, 1221, 1179, 1074, 1063, 1013, 990, 967, 891, 880, 845, 816, 757, 749, 640, 625. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 510.2 (12) [Cu3(OCH3) (pz)3(C3H7COO)]+, 598.5 (20) [Cu3(OCH3) (pz)3(C3H7COO)2 + H]+, 620.2 (15) [Cu3(OCH3) (pz)3(C3H7COO)2 + Na]+, 634.3 (15) [Cu 3 (OCH 3 ) (pz) 3 (C 3 H 7 COO) 2 (H 2 O) 2 + H] + , 666.3 (78) [Cu3(OCH3) (pz)3(C3H7COO) (bpy)]+, 686.4 (100) [Cu3(OCH3) (pz)3(C3H7COO) (MeOH)(H2O)8]+, 711.3 (22) [Cu3(OCH3) (pz)3(C3H7COO)2(MeCN)(H2O)4 + H]+, 722.4 (22) [Cu3(OCH3) (pz)3(C3H7COO) (MeOH)(H2O)10]+, 747.3 (10) [Cu3(OCH3) (pz) 3 (C 3 H 7 COO) 2 (MeCN)(H 2 O) 6 + H] + , 1071.4 (90) [Cu 6 (OH) 3 (pz) 7 (C 3 H 7 COO) (MeOH) 2 (H 2 O)] + , 1107.4 (20) [Cu6(OCH3)2(pz)6(C3H7COO)3]+, 1127.5 (15) [Cu 6 (OCH 3 ) 2 (pz) 5 (C 3 H 7 COO) 4 ] + . μ eff (295 K) = 2.432 μ B (calculated for C28H34Cu3N8O5). Reaction of [Cu3(μ3-OH)(μ-pz)3(Me(CH2)3COO)2(H2O)], 1h, with 4, 4′ - B i py r id i n e . S y n t h e s i s of [ Cu 3 ( μ 3 - O H ) ( μ - p z ) 3 (M e(CH2)3COO)2(MeOH)(μ-C10H8N2)], 3. To a solution of 1h (249 mg, 0.4 mmol) in 30 mL of MeOH, a solution of bpy (251 mg, 1.6 mmol) in 10 mL of MeOH was added. The obtained solution was stirred for 5 min and then allowed to slowly evaporate in the air, yielding blue wellformed crystals, suitable for a SCXRD determination. Compound 3. Yield 230 mg, 72%. Elem. Anal. Calcd for [{Cu3(μ3OH)(μ-pz)3(C4H9COO)2(C10H8N2) (MeOH)}]: C, 45.08; H, 5.04; N, 14.02. Found: C, 44.45; H, 4.71; N, 13.96. IR (KBr, cm−1): 3377, 3222, 3131, 3104, 3063, 2954, 2931, 2871, 1615,1598, 1562, 1533, 1488, 1451, 1415, 1379, 1345, 1319, 1292, 1277, 1220, 1177, 1165, 1098, 1059, 998, 930, 879, 858, 813, 760, 746, 662. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 157.1 (18) [bpy + H]+, 524.3 (8) [Cu3(OCH3) (pz)3(C4H9COO)]+, 560.3 (20) [Cu3(OH) (pz)3(C4H9COO) (MeOH)(H2O)]+, 592.3 (40) [Cu 3 (OH) (pz) 3 (C 4 H 9 COO) (MeOH) 2 (H 2 O)] + , 626.3 (15) [Cu 3 (pz) 3 (C 4 H 9 COO) 2 (MeOH)] + , 648.2 (8) [Cu 3 (OH) (pz) 3 (C 4 H 9 COO) 2 (H 2 O) 2 + H] + , 662.5 (15) [Cu 3 (OH) (pz)3(C4H9COO)2(MeOH)(H2O) + H]+, 680.3 (50) [Cu3(OH) (pz)3(C4H9COO)2(MeOH)(H2O)2 + H]+, 685.3 (100) [Cu3(OH) (pz)3(C4H9COO)2(MeOH) (MeCN) + H]+, 714.5 (70) [Cu3(OH) (pz)3(C4H9COO) (MeOH)3(H2O)6]+, 750.5 (20) [Cu3(pz)3(C4H9COO)2(bpy)]+, 800.5 (8) [Cu3(OH) (pz) 3 (C 4 H 9 COO) 2 (bpy) (MeOH) + H] + , 1079.4 (20) [Cu6(OH)4(pz)6(C4H9COO)(H2O)7]+, 1113.6 (60) [Cu 6 (OH) 4 (pz) 6 (C 4 H 9 COO) (MeOH) 5 ] + , 1149.6 (65) [Cu 6(OH)4(pz)6(C4H9COO) (MeOH)5(H2O)2]+, 1185.6 (15) [Cu6(OH)4(pz)6(C4H9COO) (MeOH)5(H2O)4]+. μeff (295 K) = 2.380 μB (calculated for C30H40Cu3N8O6).

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RESULTS AND DISCUSSION

In the last years we succeeded in the easy synthesis of the trinuclear triangular CuII cluster [Cu3(μ3-OH)(μ-pz)3]2+, having the charge balanced by two monocarboxylates that, acting in a di- or tritopic fashion, generated various hexanuclear clusters and 1- or 2D CPs.28−31 As said in the Introduction, we reacted some of the above-mentioned compounds with bpy obtaining different CPs and hexanuclear derivatives,40,41 while here we report the results of the reactions of compounds 1g−j with bpy, leading to the formation of five new CPs, 2−6. SCXRD structural determinations carried out on compounds 2−6 confirmed that the trinuclear triangular structure was maintained even though, in some cases, minor modifications with respect to the structures of starting compounds occurred. C



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As an example, the reaction of compound 1g with bpy in MeOH yielded the trinuclear species [Cu3(μ3-OMe)(μpz)3(Me(CH2)2COO)2(μ-C10H8N2)], 2 (Figure 1), where


Figure 2. Hexanuclear SBU (in green) generating a 1D CP in compound 2.

The magnetic susceptibility at r.t. of 2 (2.432 μB, calculated for the trinuclear assembly) is lower than that expected for three independent CuII ions, thus indicating some kind of antiferromagnetic exchange coupling, analogously to what was found in numerous strictly related derivatives.28−41 While classical H-bonds are not present, weak intermolecular nonclassical H-bonds involving two H atoms of one bpy and one oxygen of the chelating butyrate belonging to adjacent chains [C11···O5i 3.32(1) Å, C11−H11···O5i 173°, C13···O5i 3.21(1) Å, C13−H13···O5i 174°, symmetry code: (i) −x, −y, 2 − z] likely contribute to define the crystal packing of 2, where butyrate chains and methoxy groups pertaining to different, parallel, CPs interdigitate each other (Figure S1). Incidentally, this crystal packing generates a series of small, inaccessible 0D pores, accounting for ca. 6% of free volume, as calculated by PLATON/SQUEEZE program51 with a probe radius of 1 Å (see Figures S2). The reaction of 1h with bpy generated the species [Cu3(μ3OH)(μ-pz)3(Me(CH2)3COO)2(MeOH)(μ-C10H8N2)], 3 (Figure 3), displaying a structure almost identical to that of the strictly related derivative, [Cu 3 (μ 3 -OH)(μ-pz) 3 (CH 2  CHCOO)2(H2O)(μ-C10H8N2)], A.41

Figure 1. Asymmetric unit of compound 2 with partial atom labeling scheme.

the μ3-OH group, originally present in 1g, has been replaced by the μ3-OCH3 moiety, likely coming from the deprotonation of MeOH, the reaction solvent. The μ3-OH replacement, evidenced also by ESI-MS results (see Experimental Section), is further confirmed by the IR spectrum of 2 with the lack of any signal attributable to OH stretching. Another relevant feature of compound 2, evidenced by the SCXRD determination, is the absence of the hexanuclear cluster formed by a centrosymmetric pair of trinuclear moieties doubly bridged by oxygens belonging to butyrate groups. These bridges, which are instead present in the starting compound 1g,29 have been likely broken by the coordination of bpy. Crystallographic inversion centers are present in the midpoint of the C−C bond connecting the pyridyl rings of bpy. SCXRD data evidence that the nine-membered Cu3N6 ring of 2 is not planar, the three copper ions forming a scalene triangle. The distance of capping O1 from the plane determined by the Cu3 assembly (ca. 0.63 Å) as well as the bond distances between O1 and CuII ions are quite normal for this kind of compounds.28−41 The two butyrate ions are coordinated to copper(II) ions in different ways. One of them, placed on the opposite site of the nine-membered ring with respect to the μ3-OMe group, coordinates to Cu1 and Cu2 in a slightly asymmetric bridging syn−syn fashion [Cu1−O3 2.183(5), Cu2−O2 2.152(5) Å], while the second one coordinates to Cu3 in a strongly asymmetric chelating way [Cu3−O4 1.961(5), Cu3−O5 2.632(6) Å]. The coordination to CuII ions is completed by N7 and N8 bpy nitrogens and N1−N6 pyrazolate nitrogens. Both Cu1 and Cu2 thus exhibit a square pyramidal pentacoordination (τ549 = 0.24 and 0.04, respectively) with the butyrate oxygens in the axial positions, while Cu3 adopts a square planar coordination to which is to be added the relatively weak chelating coordination of O5. The other structural parameters of 2 are quite normal and most relevant bond distances and angles are reported in Table S1. The existence of two different butyrate coordination modes is evident also in the IR spectrum, where two different absorption bands have been found for both symmetric and asymmetric CO stretching absorptions, one couple being typical of a chelating bidentate carboxylate.50 The ditopic behavior of bpy molecules is responsible for the formation of hexanuclear moieties that further develops in zigzag 1D CPs, one of which is shown in Figure 2.

Figure 3. Molecular structure of 3 with partial atom labeling scheme.

Incidentally, this structural determination let us to confirm the presence of the trinuclear assembly in compound 1h, a feature that was previously proposed only on the bases of other physicochemical determinations.31 Analogously to A, compound 3 self-assembles into a hexanuclear cluster since O4 oxygen belonging to one valerate anion asymmetrically double bridges the Cu1···Cu1i ions of two symmetry equivalent trinuclear moieties (see Figure S3) thus generating an inversion center located midway the Cu1···Cu1i vector [Cu1−O4 2.002(4), Cu1−O4i 2.339(4) Å, symmetry code: (i) −x + 1, −y, −z + 1]. The Cu3N6 nine-membered ring is far from the planarity and the three copper ions form an almost isosceles triangle. The O1−Cu bond lengths, as well as D




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the distance of O1 from the plane determined by the three CuII ions (ca. 0.52 Å) fall in the range normally found in this kind of compounds.28−41 The two valerate ions coordinate to copper(II) ions in different ways, being one of them engaged in the above-mentioned monatomic bridge forming the hexanuclear cluster, while the other one coordinates in a monodentate way to Cu3. The pyrazolate nitrogens N1−N6, the N7 and N8 nitrogens belonging to the ditopic bpy molecule, and methanol O6 complete the coordination spheres of CuII ions. Being that Cu1 is involved in the valerate bridge generating the hexanuclear assembly, it displays a square pyramidal coordination geometry (τ5 = 0.21), analogously to Cu2 and Cu3 (τ5 = 0.20 and 0.10, respectively) with O6 and N8i in the apical positions, respectively. Noteworthy, the coordination of N8i to Cu3 is weaker than that of N7 to Cu2, as witnessed not only by different bond lengths [Cu2−N7 2.037(5), Cu3−N8i 2.649(5) Å], but by the angle of ca. 148° formed by the Cu3−N8i bond with the pyridinic ring (see Figure S4), a feature that was also found in compound A.41 The other geometrical parameters of 3 are quite normal and a list of most relevant bond distances and angles is reported in Table S2. In each half of the hexanuclear moiety of 3 are present two quite strong “intramolecular” H-bonds, the first one involving the hydroxyl of coordinated methanol and the valerate O3 of the same trinuclear fragment [O6···O3 2.670(8) Å, O6−H60··· O3 161(7)°], while the second one involves μ3-OH and the uncoordinated valerate oxygen O5 [O1···O5 2.686(5) Å, O1− H111···O5 155(7)°] likely reinforcing the double bridge of the hexanuclear assembly (Figure S5). Moreover, being the hexanuclear assemblies joined each other through the ditopic behavior of each bpy molecule, a ladder-like 1D CP is thus generated. In Figure 4 it is also possible to see the relatively large, almost elliptic 30-membered macrocycles, whose dimensions

Figure 5. Asymmetric unit of compound 4 with partial atom labeling scheme. H atoms have been omitted for clarity.

Compound 4 displays structural features and connectivity very similar to those of compound 2. Also in this case the capping μ3-OH has been replaced by the μ3-OMe group, as confirmed by the absence of any IR signal due to OH vibrations and the hexanuclear cluster, formed by a centrosymmetric pair of trinuclear moieties doubly bridged by two hexanoates oxygens, is absent. The Cu-OMe bond lengths, as well as the distance of O1 from the plane determined by the three CuII ions (ca. 0.655 Å) are in the ranges normally found in this kind of compounds.28−41 The hexanoate anions that exhibit disorder in the terminal part of the chain coordinate to CuII ions in different ways. One of them, which is placed on the opposite site of the nine-membered ring with respect to the μ3-OMe group, is coordinated to Cu1 and Cu2 in a slightly asymmetric bridging syn−syn fashion [Cu1−O3 2.175(3), Cu2−O2 2.154(3) Å], while the second one chelates Cu3 in a strongly asymmetric way [Cu3−O4 1.962(3), Cu3−O5 2.624(4) Å]. The IR spectrum of 4, in the region where carboxylate vibrations are generally found, is analogous to that of 2, confirming the presence of two different hexanoate groups coordinated to the Cu centers in both asymmetric bridging and chelating fashion.50 The coordination to copper ions is completed by N13 and N16 pertaining to the two ditopic bpy molecules bonded to Cu2 and Cu3, respectively, and by N1−N6 pyrazolates nitrogens. Thus, Cu1 and Cu2 both exhibit a square pyramidal pentacoordination, (τ5 = 0.24 and 0.06, respectively) being the hexanoate oxygens in the axial positions, while, analogously to 2, Cu3 adopts a square planar coordination to which is to be added the relatively weak chelating coordination of O3. The other geometrical parameters of 4 are quite normal, and most relevant bond distances and angles, including those pertaining to the other, very similar, trinuclear fragment, are reported in Table S3. The magnetic susceptibility of compound 4 is 2.814 μB at 295 K, calculated for the trinuclear C32H42Cu3N8O5 formulation. This value, which is largely smaller than that corresponding to three independent CuII ions, may be due to antiferromagnetic coupling within each [Cu3(μ3-OMe)(μ-pz)3] unit and between bpy bridged units. Anyhow, the ditopic bpy linkers connect the trinuclear SBUs generating zigzag, parallel, 1D CPs, one of which is shown in Figure 6. Analogously to 2, parallel chains of 4 do not interact with each other, and the crystal packing (Figure S7) is likely dictated by dispersion forces only. The structural characterization of 4 allows us to authenticate the trinuclear structure of compound 1i that was previously

Figure 4. Compound 3. (left) Ball-and-stick representation down the crystallographic b axis of the ladder-like 1D CP evidencing the 30membered macrocycles. H atoms have been omitted for clarity. (right) Space-filling representation showing the presence of small pores.

(axes ca. 13 × 8.5 Å) are almost identical to those of the analogous assembly present in A. However, the crystal packing in this case is slightly different. In fact, parallel ladders of 3 pack offset (see Figures S6), thus making inaccessible the small channels, which are instead present in A.41 Finally, the magnetic susceptibility value of 3 (2.380 μB), very near to the value found for compound 2, is also in this case in good agreement with the values reported for other trinuclear triangular CuII complexes28−41 and indicates that antiferromagnetic interactions are present. The reaction of the trinuclear triangular hexanoate derivative, 1i, with bpy led to two different products, [Cu3(μ3-OMe)(μpz) 3 (Me(CH 2 ) 4 COO) 2 (μ-C 10 H 8 N 2 )], 4, and [{Cu 3 (μ 3 OH)(μ-pz)3(Me(CH2)4COO)2(H2O)(μ-C10H8N2)}2], 5, depending on the bpy/1i reaction ratios (see Experimental Section). The structure of compound 4, obtained by using the higher bpy/1i ratio, is shown in Figure 5. E



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to Cu2 [Cu2−N7 2.032(8), Cu3ii-N8 2.53(1) Å, symmetry code (ii) x, −1 + y, 1 + z], and the angle formed by the Cu3− N8 bond with the pyridinic ring (ca. 153°) is quite small with respect to that formed by Cu2−N7 (ca. 172°) (Figure S9). Cu1 adopts a square pyramidal pentacoordination (τ5 = 0.16), with hexanoate O2i pertaining to the symmetry related trinuclear cluster in the apical position. The same coordination geometry is shown by Cu2 and Cu3 (τ5 = 0.19 and 0.15, respectively), being their apical positions defined by O6 water oxygen and N7 bpy nitrogen, respectively. Finally, the rings of bpy molecules are not coplanar and the corresponding planes form a dihedral angle of ca. 35°, a feature already found in similar derivatives.40,41 The other geometrical parameters of 5 are quite normal, and a list of most relevant bond distances and angles can be found in Table S4. In each half of the hexanuclear cluster 5, two relatively strong H-bonds are present, the first one “intramolecular”, involving coordinated water and the O5 oxygen pertaining to the hexanoate of the same trinuclear fragment [O6···O5 2.71(2) Å, O6−H66w···O5 168(6)°], while the second one involves the μ3-OH and the uncoordinated hexanoate oxygen O3 pertaining to the other trinuclear unit [O1···O3i 2.65(1) Å, O1−H111··· O3i 158(2)°], thus reinforcing the hexanuclear assembly (Figure S10). Ditopic bpy joins the hexanuclear cluster above-described thus generating ladder-like CPs (Figure 8) very similar to those of 3 and A.41


Figure 6. Capped stick representation of one 1D CP of 4. H atoms have been omitted for clarity.

proposed on the base of other physicochemical determinations.31 However, the structure of 4 does not rule out the presence in compound 1i of a centrosymmetric hexanuclear assembly formed through two monatomic hexanoate oxygens bridging two trinuclear units. Actually, this specific feature is indirectly confirmed by the structure of [Cu3(μ3-OH)(μpz) 3 (Me(CH 2 ) 4 COO) 2 (H 2 O)(μ-C 10 H 8 N 2 )], 5, obtained when a lower bpy/1i reaction ratio was employed (see Experimental Section). More specifically, compound 4 was synthesized as a unique product only when an excess of bpy (8:1) was employed, while, by using lower reaction ratios, 4 was always accompanied by crystals of 5, which were manually separated and employed for a SCXRD determination. Figure 7 shows the molecular structure of 5, which is very similar to those of compound 3 and of the previously reported

Figure 8. Compound 5. Space-filling representation: view down the a crystallographic axis of the ladder-like 1D CP, evidencing the presence of small pores.

Also in this case the CPs are formed through the junction of almost elliptic macrocycles, thus determining holes having approximate dimensions of 13 × 9 Å. However, even though the crystal packing of 5 is different from those of 3 and A41 [as an example, water molecules are involved into “intermolecular” H-bonds with O3 pertaining to adjacent tapes (see Figure S11)], parallel tapes are offset; thus, the access to the holes is hampered, and also compound 5 results as not porous (Figures S12). By reacting 1j with bpy, compound [Cu3(μ3-OH)(μpz)3(CH3(CH2)5COO)2(μ-C10H8N2)], 6, shown in Figure 9, was isolated. The Cu3N6 ring is not planar, and the three copper ions form an almost equilateral triangle. The Cu−O1 bond lengths, as well as O1 distance from the plane described by CuII ions (ca. 0.39 Å) are quite normal for this assembly.28−41 One heptanoate ion is engaged through O2 in the monatomic bridge forming a hexanuclear cluster (vide inf ra), while the second one chelates Cu3 in a strongly asymmetric way [Cu3− O4 1.936(14), Cu3−O5 2.624(20)]. This is confirmed by the presence of different signals due to the asymmetric and


compound A.41 Also in this case, the O2 hexanoate oxygen asymmetrically bridges two trinuclear moieties [O2−Cu1 1.977(7), O2−Cu1i 2.356(8) Å, symmetry code (i) −x, −y + 2, −z + 2] thus generating a hexanuclear cluster with an inversion center midway the Cu1···Cu1i vector (see Figure S8). The Cu3N6 nine-membered ring largely deviates from the planarity and the three copper ions form a scalene triangle. The Cu-OH bond distances, as well as the distance of O1 from the plane described by copper ions (ca. 0.52 Å) fall in the ranges normal for these compounds.28−41 The two hexanoate ions show different coordination behaviors, one of them being engaged in the monatomic bridge forming the hexanuclear cluster, while the other one coordinates in a monodentate way to Cu3. The N1−N6 pyrazolate nitrogens, N7 and N8 pertaining to bridging bpy, and a water molecule bonded to Cu2 complete the coordination spheres of CuII ions. The coordination of N8 to Cu3 is weaker with respect to that of N7 F



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Figure 11. Two parallel 1D CPs in the crystal packing of 6. Different colors indicate different CPs.



for which we were previously unable to obtain a XRD structural determination.31 The r.t. magnetic susceptibility of compound 6 (2.256 μB) is close to the values found for 2 and 3 and analogous compounds28−41 and confirms, also in this case, the presence of some kind of antiferromagnetic coupling.

symmetric CO stretching vibrations found in the region 1600−1300 cm−1 that are in accordance with the values expected for chelating and bridging unidentate carboxylates.50 One molecule of bpy is coordinated to Cu1 with N7 and, acting as a ditopic ligand, generates the 1D CPs that will be described below. The coordination scheme is completed by the interactions of N1−N6 pyrazolate nitrogens with CuII ions that adopt different coordination geometries. Cu1 (τ5 = 0.16) displays a square pyramidal geometry, whereas Cu2 shows an almost perfect square planar coordination. The coordination geometry of Cu3 can also be considered as square planar, slightly modified by the weak chelation of O5. The other geometrical parameters of 6 are quite normal, and most relevant bond distances and angles are reported in Table S5. As said above, compound 6 self-assembles forming a hexanuclear cluster generated by two trinuclear moieties asymmetrically doubly bridged by the O4 heptanoate oxygens, [O2−Cu1 1.959(9) Å, O2−Cu1i 2.304(10) Å, symmetry code (i) −x + 1, −y + 2, −z + 1] with an inversion center located midway the Cu1···Cu1i vector (Figure 10, left). However, an alternative representation can be used, taking into account that another inversion center is located midway the C26−C26ii bond of the ditopic bpy molecule joining Cu2 ions pertaining to two symmetry equivalent trinuclear units [symmetry code (ii) −x, −y + 1, −z + 1] (Figure 10, right). The alternate connections of these two different hexanuclear clusters generate parallel 1D CPs shown in Figure 11. A strong “intermolecular” H-bond involves the μ3-OH and the uncoordinated heptanoate oxygen O3 pertaining to a symmetry related trinuclear unit, [O1···O3iii 2.654(2) Å, O1−H111···O3iii 166(2)°, symmetry code: (iii) 1 − x, 2 − y, 1 − z] thus reinforcing the hexanuclear assembly (Figure S13). In any case, thus-formed parallel CPs do not interact with each other, and the crystal packing of 6 is probably dictated by dispersion forces only. Also in this case the aliphatic chains of adjacent 1D CPs interdigitate as shown in Figure S14. Finally, the determination of the structure of 6 made it possible, also in this case, to confirm the presence of the trinuclear [Cu3(μ3-OH)(μ-pz)3] moiety also in compound 1j,

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CONCLUSIONS First of all, the structural characterization of compounds 3−6 allowed us to confirm the presence of the trinuclear triangular assembly in compounds 1h−j, whose structures were previously proposed only on the base of ESI-MS, magnetic susceptibility, IR and UV/vis spectroscopy, and 1H NMR measurements.31 In addition, we have confirmed that bpy is a ditopic linker suitable to bridge [Cu3(μ3-OH)(μ-pz)3] building blocks, yielding 1D and 2D CPs, depending on the reaction conditions and reactant molar ratios. Moreover, it has been evidenced the relevant stability of the trinuclear structure [Cu3(μ3-OH)(μ-pz)3], even though minor modifications have been in some cases observed in the products of the reaction of bpy with compounds 1. Particularly, the presence of the capping μ3-OMe group in compounds 2 and 4, indicate the relatively easy exchange of μ3-OH with μ3-OMe group, coming from MeOH, employed as solvent, in the presence of basic bpy. Incidentally, the signals due to species containing a MeO group, observed in the ESI spectra of compounds 2 and 4, further reinforce the assessment that ESI-MS is a valuable tool to attribute the trinuclear triangular structure to compounds 2−4 and 6.52−57 Actually, the absence of signals related to the MeO group in ESI spectra of compounds 3 and 6 indicates that MeO groups are pre-existent to the ES ionization of 2 and 4 (in MeOH as solvent) and points out the robustness, in these conditions, of the trinuclear triangular assembly containing the MeO capping moiety. As for the structures of compounds 2−6 are concerned, some features are worth to be pointed out. First of all, both the structure and the 1D polymeric assembly of compound 6, where the hexanuclear fragments are joined through a single ditopic bpy molecule, have not been reported to date and are different from those of all compounds until now isolated by

Figure 10. Compound 6. Hexanuclear clusters obtained through heptanoate (left) and bpy (right) bridges. H atoms have been omitted for clarity. G



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Figure 12. Comparison of the molecular structures (left) and polymeric development of compounds 3 (violet), 5 (green), and A (yellow). H atoms have been omitted for clarity.

Figure 13. Comparison of the molecular structures (left) and polymeric development (right) of compounds 2 (violet) and 4 (yellow). H atoms have been omitted for clarity.

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reacting bpy with trinuclear triangular CuII clusters.40,41 However, the examination of the structures of compounds 2−5 points out that some structural analogies are present in different compounds, evidencing that the obtained results are to some extent independent from the RCOO employed. Actually, the structures of compounds 3, [Cu3(μ3-OH)(μ-pz)3(Me(CH2)3COO)2(MeOH)(μ-C10H8N2)], and 5, [Cu3(μ3-OH)(μpz)3(Me(CH2)4COO)2(H2O)(μ-C10H8N2)], are very similar and almost exactly superimposable to the structure of the parent compound [Cu3(μ3-OH)(μ-pz)3(CH2 CHCOO)2(H2O)(μ-C10H8N2)], A.41 Moreover, also the polymeric developments of 3, 5, and A are practically identical (Figure 12). Interestingly, also compounds 2, [Cu3(μ3-OMe)(μ-pz)3(Me(CH 2 ) 2 COO) 2 (μ-C 10 H 8 N 2 )], and 4, [Cu 3 (μ 3 -OMe)(μpz)3(Me(CH2)4COO)2(μ-C10H8N2)], evidence the same molecular structure and polymeric development, which are completely different from those of 3, 5, and A (Figure 13). In conclusion, by exploiting the trinuclear triangular fragment [Cu3(μ3-OH)(μ-pz)3] as SBU, it has been possible to build a large number of different CPs, which in some cases present interesting structural analogies. Our group is currently developing this research field by examining the use of other neutral di- or polytopic nitrogen ligands, having both rigid or flexible skeletons. Moreover, our interest is also addressed to the use of di- and polycarboxylates instead of monocarboxylate ions, by reacting copper(II) bicarboxylates with Hpz58−60 or by displacing monocarboxylate ions from preformed trinuclear triangular systems and exchanging them with bi- or tricarboxylates.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00661.

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Tables S1−S5 and Figures S1−S14 (PDF) X-ray crystallographic file for compounds 2−6 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*(M.M.) Phone: +39 051 2099559. Fax: +39 051 2099456. Email: [email protected]. *(L.P.) Phone: +39 049 8275157. Fax: +39 049 8275050. Email: [email protected]. *(C.P.) Phone: +39 0737 402234. Fax: +39 0737 637345. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the PRIN-2010-BNZ3F2 Research Project DESCARTES of the Italian Ministry of the University and Research. M.M. wishes to thank the University of Bologna for financial support. Crystallographic data have been deposited at the Cambridge Crystallographic Data Center, CCDC 1400300−1400304 for 2−6. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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