Interaction of the Trinuclear Triangular Secondary Building Unit [Cu3

Article pubs.acs.org/crystal

Interaction of the Trinuclear Triangular Secondary Building Unit [Cu3(μ3‑OH)(μ-pz)3]2+ with 4,4′-Bipyridine. Structural Characterizations of New Coordination Polymers and Hexanuclear CuII Clusters. 2°† Corrado Di Nicola,∇ Federica Garau,‡ Massimo Gazzano,§ Arianna Lanza,‡,∥ Magda Monari,*,⊥ Fabrizio Nestola,# Luciano Pandolfo,*,‡ and Claudio Pettinari*,∇ ∇

Scuola di Farmacia, University of Camerino, Via S. Agostino, 1, I-62032 Camerino, Macerata, Italy Dipartimento di Scienze Chimiche, University of Padova, Via Marzolo, 1, I-35131 Padova, Italy § ISOF-CNR, Via Selmi, 2, I-40126 Bologna, Italy ∥ Departement für Chemie und Biochemie, University of Bern, Freiestrasse, 3, CH-3012 Bern, Switzerland ⊥ 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: By reacting 4,4′-bipyridine (bpy) with selected trinuclear triangular CuII complexes, [Cu3(μ3-OH)(μ-pz)3(RCOO)2(LL′)] [pz = pyrazolate anion; R = CH3, CH3CH2, CH2CH, CH2C(CH3); L, L′ = Hpz, H2O, MeOH] in MeOH, the substitution of monotopic ligands by ditopic bpy was observed. Depending on the stoichiometric reaction ratios, different compounds were isolated and structurally characterized. One- and two-dimensional coordination polymers (CPs), as well as two hexanuclear CuII clusters were identified. One of the hexanuclear clusters self-assembles into a supramolecular three-dimensional structure, and its crystal packing shows the presence of two intersecting channels, one of which is almost completely occupied by guest bpy, while in the second one guest water molecules are present. This compound also shows a reversible, thermally induced, single-crystal-to-singlecrystal transition.

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INTRODUCTION Research studies in the field of coordination polymers (CPs), in some cases referred to as metal organic frameworks (MOFs)1 even though the distinction between the two terms is still a debated issue,2,3 are continuously growing4−7 mainly due to their possible applications in relevant industrial fields.8 CPs may indeed find applications as heterogeneous catalysts,9−11 molecular magnets,12 fillers for gas storage tanks,13,14 etc. In this context, it is noteworthy that in the last few years BASF has started the industrial production of some CPs,8,15 which are now commercialized by Sigma-Aldrich under the Basolite [Z1200(ZIF-8), A100(MIL-53(Al)), C300 (HKUST-1), F300(Fe-BTC)] and Basosiv [M050, (Magnesium formate)] denominations.16 In 2001 Yaghi and co-workers17 adopted the concept of secondary building units (SBUs) defined as “molecular complexes and cluster entities” that can be employed to assemble MOFs using polytopic organic linkers and then

provided a list of SBUs that are potentially useful to build MOFs.18 Even though a large plethora of nuclearities are present in this list, trinuclear triangular SBUs are relatively scarcely represented. Trinuclear triangular CuII systems, which play a fundamental role in some catalytic biological oxidation processes,19,20 have been known for a long time;21−23 nevertheless, they have not been systematically employed as SBUs, excluding, to the best of our knowledge, a few papers that appeared in the last few years.24−30 In this context, we succeeded in the synthesis of a series of solid compounds 1a−j, based on the trinuclear triangular moiety [Cu3(μ3-OH)(μ-pz)3]2+ (Scheme 1) that self-assembles in hexanuclear clusters through monatomic carboxylate bridges and also form extended 1D or 2D CPs or supramolecular networks.31−36 Received: November 9, 2014 Revised: December 12, 2014

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For the first part, see Di Nicola et al. Cryst. Growth Des., 2012, 12 (6), pp 2890−2901.39 © XXXX American Chemical Society

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DOI: 10.1021/cg501647r Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design Scheme 1. Reactions of Copper(II) Carboxylates with Pyrazole

The trinuclear [Cu3(μ3-OH)(μ-pz)3]2+ fragment is particularly stable, and it was maintained intact to a large extent even when [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(Hpz)], 1b, was reacted with aqueous HCl37 and other strong acids38 leading to the formation of different CPs. This stability prompted us to start a research project based on the study of the interaction of compounds 1a−j with neutral ditopic nitrogen donor ligands. The first results, obtained by reacting compound 1a with 4,4′bipyridine (bpy) in different reaction conditions, have already been published,39 and here we report the results obtained from the reactions of compounds 1b−f with the same ligand.40

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difference Fourier maps and refined isotropically, were added in calculated positions, included in the final stage of refinement and refined with U(H) = 1.2Ueq(C) or U(H) = 1.5Ueq(C-Me) and allowed to ride on their carrier atoms. In the crystal packings of all compounds, except 6, crystallization solvent molecules are present. Whereas 4 at room temperature crystallizes in the orthorhombic space group Ibam, at 100 K the structural model of 4 (4′) was refined in the orthorhombic space group Pbcn that belongs to the maximal nonisomorphic subgroups of Ibam. X-ray powder diffraction (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. Molecular graphics were generated by using Mercury 3.3.48,49 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 2−7 are reported in Table 1. Syntheses. The reactions were carried out in MeOH both in open vessels at room temperature (r.t.) and in solvothermal conditions. Moreover, different reaction ratios were employed. Reactions at r.t. and 1 Atm. Reactions of [Cu3(μ3-OH)(μpz)3(CH3COO)2(Hpz)], 1b, with 4,4′-Bipyridine. (a). Synthesis of [{Cu3(μ3-OH)(μ-pz)3(CH3COO)2(μ-bpy)}2]·H2O·4MeOH, 2. To a solution of 1b (206 mg, 0.35 mmol) in 70 mL of MeOH, 126 mg of bpy (0.8 mmol) in 8 mL of CH3OH was added. The obtained solution was stirred for some minutes and then allowed to evaporate in the air, yielding well-formed blue crystals, suitable for a single crystal X-ray diffraction (SC-XRD) determination. 2: Yield 217 mg, 80%. Elem. Anal. Calcd for [{Cu3(OH)(C3H3N2)3(CH3COO)2(C10H8N2)}2]·H2O·4MeOH: C, 39.71; H, 4.40; N, 14.82. Found: C, 38.92; H, 4.22; N, 14.62. IR (cm−1): 3109, 3048, 1575, 1490, 1396, 1381, 1280, 1176, 1059, 808, 760, 679, 626. μeff (293 K) = 4.26 μB (calculated for C50H66Cu6N16O15). (b). Synthesis of [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(μ-bpy)2(H2O)]· bpy·4H2O·4MeOH, 3. To a solution of 1b (220 mg, 0.37 mmol) in 80 mL of MeOH, 240 mg of bpy (1.55 mmol) in 20 mL of MeOH was added. The obtained solution was stirred for some minutes, and slow evaporation in the air yielded well-shaped prismatic blue crystals of compound 3. By standing in the air, compound 3 quickly turned into a green powder upon loss of solvent, and thus the crystals were kept in their mother liquors and a SC-XRD determination was carried out on a wet crystal covered by mineral oil. The elemental analysis was instead carried out on the dried sample, 3′, and it fits quite well with the [Cu3(OH)(pz)3(CH3COO)2(bpy)3(H2O)] formulation. Also IR and XRPD determinations were carried out on the dried, green powder 3′. 3′: Elem. Anal. Calcd for [Cu3(OH)(C3H3N2)3(CH3COO)2(C10H8N2)3(H2O)]: C, 50.96; H, 4.18; N, 16.58. Found: C, 50.18; H, 4.29; N, 16.65. IR (cm−1): 3400, 3032, 1591, 1487, 1408, 1382, 1335, 1275, 1219, 1177, 1059, 998, 805, 759, 675, 620. (c). Synthesis of [{Cu3(μ3-OH)(μ-pz)3(CH3COO)(H2O)(μ-bpy)(bpy)2}2(OH)2]·bpy·2H2O, 4. To a solution of 1b (201 mg, 0.34 mmol) in 100 mL of MeOH a large excess of bpy (430 mg, 2.75 mmol) was added under stirring. A little solid was filtered off, and the

EXPERIMENTAL SECTION

Materials and Methods. All the reactions and manipulations were carried out in air. Some reactions were performed both in ambient and solvothermal conditions. Trinuclear triangular CuII derivatives 1b−f were prepared as previously reported.31,32,35 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 FTIR spectrometer with ATR mode. Positive electrospray mass spectra (ESI-MS) were obtained with a Series 1100 MSI detector HP spectrometer, using MeOH as mobile phase. Solutions for ESI-MS were prepared using reagent grade methanol, water, and/or acetonitrile, and obtained intensities and m/z values (peaks containing copper(II) ions are identified with the most intense signal of the isotopic clusters) were compared with those calculated by using the IsoPro isotopic abundance simulator.41 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. Crystallographic Data Collection and Structure Determination. The X-ray intensity data of compounds 2−7 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. The software SMART42 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,42 and an empirical absorption correction was applied using SADABS.43 The structures were solved by direct methods (SIR 9744 or SIR 200445) and subsequent Fourier syntheses and refined by fullmatrix least-squares on F2 (SHELXTL46 or SHELXL9747) using anisotropic thermal parameters for all non-hydrogen atoms. All the hydrogen atoms except the hydroxy ones, which were located in B


Article

Crystal Growth & Design Table 1. Crystal Data and Structure Refinement for Compounds 2−7 compound formula FW crystal symmetry space group a, Å b, Å c, Å α, ° β, ° γ, ° cell volume, Å3 Z Dc, Mg m−3 μ(Mo Kα), mm−1 F(000) crystal size, mm T, K θ limits, ° refln collected unique refln (Rint) GOF on F2 R1(F)a, wR2(F2)b largest diff. peak and hole, e Å−3 compound

2

formula FW crystal symmetry space group a, Å b, Å c, Å α, ° β, ° γ, ° cell volume, Å3 Z Dc, Mg m−3 μ(Mo Kα), mm−1 F(000) crystal size, mm T, K θ limits, ° refln collected unique refln (Rint) GOF on F2 R1(F)a, wR2(F2)b largest diff peak and hole, e Å−3 a

3

C46H48Cu6N16O10·4MeOH· H2O 1512.43 triclinic P1̅ 12.637(2) 13.239(2) 20.110(3) 74.822(1) 76.150(2) 86.264(2) 3152.7(9) 2 1.593 2.062 1544 0.30 × 0.30 × 0.27 100 2.14−27.00 61948 13667 (0.0395) 1.144 0.0569, 0.1080 1.240 and −1.076

4

C33H34Cu3N10O6·C10H8N2· 4MeOH·4H2O 1213.74 monoclinic C2/c 21.3303(10) 13.9282(7) 18.8191(8) 90 105.898(2) 90 5377.2(4) 4 1.499 1.251 2524 0.27 × 0.25 × 0.15 100 1.94−24.70 51733 4507 (0.0588) 1.054 0.0452, 0.1077 0.870 and −0.331

C82H78Cu6N24O8·C10H8N2· (OH)2·2H2O 2135.16 orthorhombic Ibam 35.272(4) 13.7758(14) 23.686(2) 90 90 90 11509(2) 4 1.232 1.150 4384 0.25 × 0.20 × 0.20 296 1.72−24.80 49274 4998 (0.1063) 1.068 0.0730, 0.1821 0.493 and −0.448

4′ C82H78Cu6N24O8·C10H8N2· (OH)2·4H2O 2171.19 orthorhombic Pbcn 35.4340(11) 13.4460(4) 23.5802(8) 90 90 90 11234.7(6) 4 1.284 1.181 4364 0.30 × 0.25 × 0.20 100 1.73−25.0 71433 9782 (0.0850) 1.068 0.0912, 0.2299 1.572 and −2.162

5

6

7

C50H56Cu6N16O10·2H2O 1458.38 triclinic P1̅ 11.062(2) 12.636(3) 12.816(3) 86.064(2) 71.413(2) 66.729(2) 1556.5(6) 1 1.556 2.082 742 0.27 × 0.25 × 0.15 296 1.68−25.06 14901 5491 (0.0913) 0.951 0.0482, 0.1087 0.569 and −0.508

C25H26Cu3N8O6 725.16 triclinic P1̅ 11.174(4) 12.452(4) 13.109(8) 113.890(5) 94.844(6) 113.569(4) 1460.9(11) 2 1.649 2.217 734.0 0.25 × 0.20 × 0.05 296 2.03−24.00 5881 3959 (0.0438) 0.979 0.0667, 0.1531 0.916 and −1.303

C38H36Cu3N12O5·2H2O 967.44 monoclinic P21/n 12.705(6) 17.071(8) 19.391(10) 90 98.032(4) 90 4164(4) 4 1.543 1.581 1964 0.30 × 0.10 × 0.05 296 1.81−24.00 45706 6140 (0.1005) 1.040 0.1282, 0.2165 0.973 and −0.882

R1 = ∑∥Fo| − |Fc|/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP] where P = (Fo2 + 2Fc2)/3. Reaction of [Cu3(μ3-OH)(μ-pz)3(C2H5COO)2(H2O)], 1c, with 4,4′Bipyridine. Synthesis of [{Cu3(μ3-OH)(μ-pz)3(C2H5COO)2(μ-bpy)}2]· 2H2O, 5. To a solution of 1c (252 mg, 0.44 mmol) in 30 mL of MeOH, a solution of bpy (136 mg, 0.9 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and then was left to evaporate in the air yielding well-formed blue crystals, suitable for a SC-XRD determination. 5: Yield 140 mg, 48%. Elem. Anal. Calcd for [{Cu3(OH)(C3H3N2)3(C2H5COO)2(C10H8N2)}2]·2H2O: C, 41.18; H, 4.15; N, 15.37. Found: C, 41.17; H, 4.11; N, 15.50. IR (cm−1): 3405, 3230, 3103, 2964, 2933, 1603, 1575, 1495, 1464, 1420, 1398, 1379, 1284,

blue solution was left to evaporate almost to dryness in the air yielding a mixture of green crystals of 4 and some white needles of bpy. The green solid was rapidly washed with few milliliters of cold toluene to dissolve bpy crystals and dried under a vacuum. 4: Yield 280 mg, 77%. Elem. Anal. Calcd for [{Cu3(OH)(C 3 H 3 N 2 ) 3 (CH 3 COO)(H 2 O)(C 10 H 8 N 2 )(C 10 H 8 N 2 ) 2 } 2 (OH) 2 ]· (C10H8N2)·2H2O: C, 51.75; H, 4.34; N, 17.06. Found: C, 50.95; H, 4.53; N, 16.87. IR (cm−1): 3365, 3303, 3046, 1597, 1536, 1490, 1410, 1385, 1341, 1219, 1178,1061, 1003, 809, 758, 624. μeff (293 K) = 4.26 μB (calculated for C92H92Cu6N26O12). C


Article

Crystal Growth & Design

Figure 1. Asymmetric unit of compound 2 with partial atom labeling scheme. 3104, 3024, 1565, 1545, 1490, 1425, 1413, 1382, 1340, 1279, 1180, 1063, 813, 765, 748, 678, 627. μeff (293 K) = 3.23 μB (calculated for C38H40Cu3N12O7). Reaction of [Cu3(μ3-OH)(μ-pz)3(CH2C(CH3)COO)2], 1f, with 4,4′-Bipyridine. Synthesis of [Cu3(μ3-OH)(μ-pz)3(CH2C(CH3)COO)2(bpy)(Hpz)(H2O)], 8. To a solution of 1f (100 mg, 0.17 mmol) in 50 mL of MeOH a solution of bpy (55 mg, 0.35 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and then allowed to evaporate in the air, yielding a blue microcrystalline solid. From mother liquors, a mixture of small blue crystals and violet thin needles formed. 8: Yield 71 mg, 58%. Elem. Anal. Calcd for Cu3(OH)(C3H3N2)3(CH2C(CH3)COO)2(C10H8N2)(C3H4N2)(H2O): C, 43.87; H, 4.17; N, 17.05. Found: C, 44.61; H, 4.02; N, 16.40. IR (cm−1): 3404, 3107, 2921, 1640, 1609, 1561, 1539, 1492, 1451, 1417, 1400, 1381, 1364, 1275, 1234, 1219, 1181, 1073, 1061, 1000, 938, 832, 816, 758, 747, 640, 626. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 157.1 (22) [bpy+H]+, 544.2 (15) [Cu3(OH)(pz) 3 (CH 2 C(CH 3 )COO)(MeOH)(H 2 O)] + , 576.3 (22) [Cu 3 (OH)(pz) 3 (CH 2 C(CH 3 )COO)(MeOH) 2 (H 2 O)] + , 630.2 (10) [Cu 3 (OH)(pz) 3 (CH 2 C(CH3 )COO)(MeOH) 2 (H 2 O) 4 ] + , 664.3 (75) [Cu3(OCH3)(pz)3(CH2C(CH3)COO)(bpy)]+, 669.2 (90) [Cu3(OH)(pz) 3(CH2C(CH3)COO)2(H2O)5+H]+, 682.4 (100) [Cu3(OH)(pz)3(CH2C(CH3)COO)(bpy)(MeOH)]+, 718.4 (15) [Cu3(OH)(pz)3(CH2C(CH3)COO)(bpy)(MeOH)(H2O)2]+, 1027.4 (25) [Cu 6(OH)3 (pz)7 (CH2C(CH3)COO)(MeCN)] +, 1063.3 (90) [Cu 6 (OH) 3 (pz) 7 (CH 2 C(CH 3 )COO)(MeCN)(H2O)2]+, 1101.4 (75) [Cu6(OH)3(pz)7(CH2C(CH3)COO)(MeCN)(H 2 O) 4 ] + . μ eff (291 K) = 3.10 μ B (calculated for C30H34Cu3N10O6). Solvothermal Reactions. The solvothermal reactions were carried out in 23 mL Teflon-lined Parr reactors by using MeOH as solvent, according to the procedures indicated below. Reactions of [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(Hpz)], 1b, with 4,4′Bipyridine. Different reagent ratios were employed. The Parr reactors were charged with 50 mg of 1b (0.085 mmol) dissolved in ca. 10 mL of MeOH, and bpy enough to reach different reagent ratios (bpy/1b = 1:1, 2:1, 4:1 and 8:1). The reactors were maintained at 70 °C for 24 h and then allowed to reach r.t.. From the reaction in a 1:1 ratio only the starting material was isolated, while with the 2:1 ratio a mixture of blue, very small, unidentified crystals and a dark blue powder was obtained. By increasing the bpy/1b ratio compounds 2 and 3 were obtained, accompanied by brown powders, and in lower yields with respect to the reactions carried out in the air at r.t. Reactions of [Cu3(μ3-OH)(μ-pz)3(C2H5COO)2(H2O)], 1c, with 4,4′Bipyridine. The reactions were carried out by charging the Parr reactors with 100 mg of 1c (0.175 mmol) dissolved in ca. 10 mL of MeOH, and bpy enough to reach bpy/1c = 1:1, 2:1, 4:1, and 8:1 molar

1252, 1221, 1176, 1060, 886, 817, 767, 750, 645, 626. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 532.3 (15) [Cu3(OH)(pz)3(C2H5COO)(H2O)(MeOH)]+, 564.3 (30) [Cu 3 (OH)(pz) 3 (C 2 H 5 COO)(H 2 O)(MeOH) 2 ] + , 606.3 (10) [Cu3(OH)(pz)3(C2H5COO)2(H2O)(MeOH) + H]+, 652.3 (56) [Cu3(OMe)(pz)3(C2H5COO)(bpy)]+, 655.0 (53) [Cu3(OH)(pz)3(C2H5COO)(MeCN)(MeOH)3(H2O)2]+, 657.3 (100) [Cu3(OH)(pz)2(C2H5COO)2(MeOH)4(MeCN)]+, 659.2 (74) [Cu3(OH)(pz)3(C2H5COO)(MeCN)(MeOH)2(H2O)4]+, 1029.3 (80) [Cu6(OH)3(pz)7(C2H5COO)(H2O)3]+, 1065.3 (77) [Cu6(OH)3(pz)7(C2H5COO)(H2O)5]+. μeff (295 K) = 3.32 μB (calculated for C50H60Cu6N16O12). Reaction of [Cu3(μ3-OH)(μ-pz)3(CH2CHCOO)2(H2O)2(Hpz)], 1d, with 4,4′-Bipyridine. (a). Synthesis of [{Cu3(μ3-OH)(μ-pz)3(CH2 CHCOO)2(μ-bpy)(H2O)}2], 6. To a solution of 1d (100 mg, 0.15 mmol) in 15 mL of MeOH, a solution of bpy (48 mg, 0.3 mmol) in 5 mL of MeOH was added. The obtained solution was stirred for 5 min and slow evaporation in the air yielded well-formed blue crystals, suitable for an SC-XRD determination. 6: Yield 65 mg, 60%. Elem. Anal. Calcd for [Cu3(OH)(C3H3N2)3(CH2CHCOO)2(C10H8N2)(H2O)]: C, 41.41; H, 3.61; N, 15.45. Found: C, 41.17; H, 3.53; N, 15.15. IR (cm−1): 3397, 3237, 3099, 2927, 2853, 1638, 1620, 1604, 1563, 1647, 1491, 1426, 1413, 1382, 1364, 1338, 1279, 1220, 1181, 1163, 1062, 993, 964, 941, 895, 833, 815, 766, 748, 681, 629. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 157.1 (20) [bpy+H]+, 530.2 (18) [Cu3(OH)(pz)3 (CH2CHCOO)(MeOH)(H 2O)]+, 552.3 (18) [Cu3(OH)(pz)3(CH2CHCOO)2+H]+, 562.3 (37) [Cu3(OH)(pz)3(CH2CHCOO)(MeOH)2(H2O)]+, 650.3 (75) [Cu3(OCH3)(pz)3(CH2 CHCOO)(bpy)]+, 654.3 (70) [Cu 3(OH)(pz) 3 (CH2  CHCOO)(bpy)(H2 O)] + , 655.3 (100) [Cu 3 (OH)(pz) 3(CH2  CHCOO)2(MeOH)(H2O)4+H]+, 1023.3 (70) [Cu6(OH)4(pz)5(CH 2 CHCOO) 2 (MeOH) 3] +, 1059.3 (72) [Cu 6(OH) 3 (pz) 6 (CH2CHCOO)2(MeOH)2(H2O)]+, 1383.5 (15) [Cu6(OH)2(pz)6(CH2CHCOO)3(bpy)2(MeCN)]+. μeff (291 K) =3.10 μB (calculated for [C25H26Cu3N8O6]). Compound 6 was also obtained when [Cu3(μ3-OH)(μ-pz)3(CH2 CHCOO)2(MeOH)], 1e, was reacted in MeOH with 4,4′-bipyridine in 1:2 molar ratio. (b). Synthesis of [Cu3(μ3-OH)(μ-pz)3(CH2CHCOO)2(Hpz)(μbpy)2]·2H2O, 7. To a solution of 1d (204 mg, 0.32 mmol) in 30 mL of MeOH, a solution of bpy (400 mg, 2.55 mmol) in 10 mL of MeOH was added. The obtained solution was stirred for 5 min and then allowed to evaporate in the air, yielding blue-green crystals, suitable for a SC-XRD determination. 7: Yield 211 mg, 70%. Elem. Anal. Calcd for [Cu3(OH)(C3H3N2)3(CH2CHCOO)2(C3H4N2)(C10H8N2)2]·2H2O: C, 47.18; H, 4.17; N, 17.37. Found: C, 48.85; H, 4.07; N, 16.85. IR (cm−1): 3381, 3237, D


Article

Crystal Growth & Design ratios. The reactors were maintained at 70 °C for 40 h in the cases of 1:1 and 4:1 ratios and 24 h in the other cases, and then left to reach r.t. In all cases, small quantities of brown powders were filtered off, and from mother liquors dark blue crystals were isolated in low yield and identified from elemental analysis, IR, and XRD unit cell parameters as compound 5.

to the possible π−π interaction between the pyridinic and the pyrazolate rings defined by N15 and N3−N4 nitrogens, respectively, (see Figure S2 of the Supporting Information). Moreover, the pyridinic rings of both bpy molecules are not coplanar, forming dihedral angles of ca. 15 and 19°.50 Anyhow, due to the coordination of two bpy molecules to the same Cu1 ion, the hexanuclear units further self-assemble forming 2D fish-net sheets, where nanometric holes (ca. 20 × 10 Å2) are present (see Figure 3).

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RESULTS AND DISCUSSION In the present work we report the results of the reactions of compounds 1b−f with bpy. The isolation of seven new derivatives (2−8) was achieved, and compounds 2−7 were characterized by SC-XRD analysis, while the characterization of 8 was obtained through other physicochemical determinations. By reacting 1b with bpy (1:2 molar ratio) in MeOH at r.t., compound [{Cu 3 (μ 3 -OH)(μ-pz) 3 (CH 3 COO) 2 (μ-bpy)} 2 ]· H2O·4MeOH, 2, was obtained. A SC-XRD determination revealed that the asymmetric unit of 2 consists of two, very similar, independent trinuclear triangular CuII clusters joined by one bpy molecule (Figure 1). All Cu−N(pyrazolate) bond lengths fall in the range 1.940− 1.979(5) Å, whereas the Cu−OH distances are between 1.982 and 2.006(3) Å, and the two μ3-oxygens lie 0.486(3) and 0.473(4) Å, respectively, out of the Cu3 planes. All these values are in the ranges normally found in analogous compounds21−39 and, together with the other most relevant bond distances and angles, are reported in Table S1 of the Supporting Information. The asymmetric unit can be viewed as an SBU generating the polymeric network of 2. Actually, the double positive charge of each trinuclear cluster is neutralized by two acetate ions, one of them chelating asymmetrically Cu3 and Cu6, respectively, while the second one is responsible of a part of the polymeric development of 2. In detail, O3 bridges Cu2 and Cu2i (pertaining to another SBU) in a monatomic asymmetric way [Cu2−O3 2.315(4), Cu2i−O3 1.976(4) Å, symmetry code: (i) −x, −y, −z + 1], and, analogously, O7 bridges Cu5 and Cu5ii [O7−Cu5 1.969(4), O7−Cu5ii 2.284(4) Å, symmetry code: (ii) −x + 3, −y + 1, −z]. Thus, two distinct inversion centers are placed midway along the vectors joining Cu2 to Cu2i and Cu5 to Cu5ii. A fragment of the 1D CP thus formed is sketched in Figure 2.

Figure 3. Compound 2. Flat fish-net 2D CP obtained through bpy connecting 1D CPs. H atoms and crystallization solvent molecules have been omitted.

In spite of the holes’ large dimensions, neither interpenetration nor catenation is observed, contrarily to what was previously found in a strictly related derivative.39 Nevertheless, since the crystal packing of 2 consists of a series of offset parallel sheets, the access to these large holes is hampered, as shown in Figure 4. Moreover, in 2 crystallization water and methanol molecules (Figure S3 of the Supporting Information) are placed between (and connected through more or less strong H-bonds) adjacent sheets that are at a mean distance of ca. 6.75 Å. When 1b was reacted in MeOH with bpy in a 1:4 molar ratio, compound [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(μ-bpy)2(H2O)]·bpy·4H2O·4MeOH, 3, was obtained. Its molecular structure, reported in Figure 5, consists of the usual trinuclear Cu3(μ3-OH)(μ-pz)3 moiety, to which are coordinated one water molecule [Cu2−O1W 2.024(4) Å], two symmetry equivalent acetate ions [symmetry code: (i) −x + 1, y, −z + 0.5], bonded to the Cu1 ions in the monodentate mode [Cu1− O2 2.028(2) Å]51 and two symmetry equivalent bpy molecules equally bonded to Cu1 [Cu1−N4 2.329(3) Å]. H2O, MeOH, and bpy crystallization molecules are also present. The three Cu ions form an almost equilateral triangle, and the nonbonding Cu···Cu distances as well as the μ3-O−Cu bonds are in the ranges normally found in analogous compounds.21−39 The molecule possesses a crystallographic 2-fold axis passing through O1W, Cu2, O1, N2, C2 and in the middle of N3−N3i [symmetry code: (i) −x + 1, y, −z + 0.5] bond. Furthermore, the presence of elongated thermal ellipsoids of O1 and Cu2, together with the apparent coplanarity of O1 with the Cu3 plane, indicates that Cu2 and O1 are disordered in two positions with populations of 0.5/0.5 by symmetry reasons. As a consequence, the Cu2−N5 distance [2.587(3) Å] is the average of the two actual asymmetric Cu2− N5 distances [Cu2−N5ii 2.321(3) and Cu2−N5iii 2.862(3) Å, symmetry operation: (ii) −x + 0.5, −y + 0.5, −z; (iii) x + 0.5, −y + 0.5, z + 0.5], which are found if the disorder of the Cu2 atom is not taken into account (Figure S4, Supporting

Figure 2. Compound 2. Partial view of the 1D CP formed through carboxylate bridges. The blue and red dashed boxes show the two distinct hexanuclear moieties formed through the two monatomic asymmetric bridging of O3 and O7, respectively. H atoms and crystallization solvent molecules have been omitted.

In addition, Cu1 is coordinated also by N15 pertaining to the second bpy molecule of the asymmetric unit [Cu1−N15 2.348(5) Å], while N16 pertaining to another SBU coordinates Cu4 [Cu4−N16iii 2.391(5) Å, symmetry code: (iii) x − 1, y − 1, z]. Noteworthy, the coordination of N15 and N16iii to Cu1 and Cu4, respectively, is weaker than that of Cu1 to N13 [Cu1−N13 2.020(4) Å] and Cu4 to N14 [Cu4−N14 2.012(4) Å] as evidenced also by the angle formed, as an example, by the Cu1−N15 bond with the pyridinic ring of ca. 152° (see Figure S1 of the Supporting Information). This feature may be related E


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Figure 4. Space filling crystal packing of 2. Different colors indicate different, parallel sheets. Crystallization solvent molecules have been omitted. Moreover, in 2 crystallization water and methanol molecules (Figure S3 of the Supporting Information) are placed between (and connected through more or less strong H-bonds) adjacent sheets that are at a mean distance of ca. 6.75 Å.

Figure 5. Molecular structure of 3, with partial atom labeling scheme.

Figure 6. Compound 3. View of two hexanuclear clusters described in the text. Arrows indicate monotopic bpy molecules. Crystallization MeOH, H2O, and bpy molecules have been omitted.

Information). From these values, it is clear that Cu2−N5iii is a very weak interaction, meaning that the expected tape-like 1D CP is not formed, and the connection of two different bpy molecules to each trinuclear triangular moiety produces only hexanuclear “clusters” (Figure 6). The other geometrical features of the trinuclear triangular fragment are quite normal,

and most relevant bond distances and angles are reported in Table S2, Supporting Information. The Cu1 ions adopt a square pyramidal coordination with the μ3-OH, O2 acetate oxygen, N1 and N3 pyrazolate nitrogens occupying the square base and N4 in apical position. The coordination of Cu2 is to be considered square pyramidal too, being the seemingly F


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Crystal Growth & Design octahedral geometry due to the above-discussed average between two different Cu2−N5ii and Cu2−N5iii distances. Bpy molecules (monotopic and bridging ones) are, on average, parallel each other, but, even though their distance is ca. 3.56 Å, the pyridyl rings forming each bpy molecule (and consequently those faced each other) are twisted by ca. 14° (see Figure S5, Supporting Information), thus weakening the possible π−π interactions. The molecular structure of 3 includes numerous crystallization molecules interacting among them and with the trinuclear units through a series of quite strong “intramolecular” and “intermolecular” H-bonds (Figures S6 and S7, Supporting Information). All these noncovalent interactions are strong enough52 to contribute, likely in a significant amount, to the crystal packing of 3, shown in Figure 7.

Figure 8. Molecular structure of 4 with partial atom labeling scheme. The red arrow indicates the crystallization molecule of bpy (see text).

coordination of the CuII ions of each trinuclear fragment. The electroneutrality is given, for each trinuclear unit, by a noncoordinated OH− ion, likely coming from adventitious atmospheric water, substituting one of the two acetate anions originally present in 1b. Finally, for each hexanuclear moiety crystallization molecules of water (two) and bpy (one) are also present. In each trinuclear fragment, the Cu3N6 ring is almost planar, and the three copper ions form an isosceles triangle [Cu1···Cu2 3.395(1), Cu1···Cu1i 3.387(2) Å, symmetry code: (i) x, y, −z + 1]. The μ3-OH−Cu bond distances, as well as the distance between the capping oxygen and the plane defined by the Cu3 ions [0.470(6) Å] are in the ranges normally found in analogous compounds.21−39 The two symmetry related Cu1 ions exhibit a TBP coordination geometry defined by two axial pyrazolate nitrogens (N5 and N6), while μ3-O1, N2 of a bpy molecule acting as a monotopic ligand and N1 pertaining to the ditopic bpy define the tbp triangular plane. The Cu2 ion shows instead a square pyramidal geometry due to the coordination of μ3-O1, the two symmetry equivalent N4 pyrazolate nitrogens [Cu2−N4 1.940(7) Å], the monodentate acetate ion [Cu2−O2 1.997(7) Å].53 Finally, one water molecule in the axial position completes the coordination geometry around the Cu2 ion. The planes containing the pyridyl rings of monotopic bpy molecules form a dihedral angle of ca. 31°,50 while the rings of the ditopic bpy molecules joining the two trinuclear moieties are almost exactly coplanar. Moreover, the two ditopic bpy molecules are parallel and face each other at an average graphitic distance of ca. 3.74 Å, possibly indicating a π−π interaction between them. The other geometrical features of the hexanuclear moiety are quite normal, and the most relevant bond distances and angles are reported in Table S3 of the Supporting Information. Analogously to A,39 a series of H-bonds involving μ3-OH, nonbonded OH− ion, coordinated and crystallization water molecules (see Figure S9 of the Supporting Information) very likely contribute to the peculiar supramolecular self-assembly of 4 in the solid state, which generates two intersecting perpendicular channels running parallel to the crystallographic b and c axes, and having sections of ca. 4.63 × 5.89 Å2 and 5.85 × 5.25 Å2, respectively (see Figure 9), accounting for a 24% solvent accessible void space as calculated by the PLATON/ SQUEEZE program.54 Analogously to compound A, the channels parallel to b are occupied by guest bpy molecules,

Figure 7. View down the crystallographic c axis of the crystal packing of 3. Crystallization molecules are highlighted in green (water and MeOH) and magenta (bpy).

On the other hand, as reported in the Experimental Section, blue crystals of 3 rapidly became a green, microcrystalline powder, 3′, by standing in air. The elemental analysis of 3′ was consistent with the [Cu 3 (OH)(pz) 3 (CH 3 COO) 2 (bpy)2 (H2O)]·bpy formulation, corresponding to the loss of crystallization water and methanol molecules from 3. In the XRPD diffractogram of 3′ (Figure S8 of the Supporting Information), the presence of more than one crystal phase is observed as a consequence of the loss of volatile guests molecules, likely causing the collapse of the ordered structure of 3. The exposure of 3′ to methanol and water vapors produced modifications in the XRPD diffractogram (Figure S8, 3″) evidencing the probable, partial, reformation of 3. Moreover, drying of the system 3″ in air generated further modifications in the XRPD diffractogram (Figure S8, 3‴). Taking into account all these features, it is clear that 3 can undergo MeOH and H2O sorption−desorption processes but these are not completely reversible. The reaction of 1b with bpy, in 1:8 molar ratio, carried out in MeOH at r.t. yielded green crystals of [{Cu3(μ3-OH)(μpz)3(CH3COO)(H2O)(μ-bpy)(bpy)2}2(OH)2]·bpy·2H2O, 4. Its molecular structure (see Figure 8) at room temperature is isomorphous with that of compound [{Cu3(μ3-OH)(μ-pz)3(HCOO)(H2O)(μ-bpy)(bpy)2}2(OH)2]·bpy, A, previously obtained through an analogous synthetic procedure.39 Compound 4 consists of a hexanuclear system formed through the coupling of two trinuclear triangular Cu3(μ3OH)(μ-pz)3 moieties doubly bridged by two bpy molecules. Two other bpy molecules, acting in a monotopic way, a monodentate acetate ion, and one water molecule complete the G


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Figure 9. View down the crystallographic b (left) and c (right) axes of the crystal packing of 4. Guest bpy molecules are highlighted in green spacefilling representation. Attempts to remove guest bpy from the channels running parallel to the crystallographic b axis by using moderate heating and vacuum, resulted in the collapse of the structure, while, when 4 was soaked in toluene or cyclohexane, guest bpy was partially exchanged analogously to what happened with compound A.39

Figure 10. View down the crystallographic b axis of 4 (left) and 4′ (right).

while, contrarily to A, in those parallel to c guest water molecules are present. Attempts to remove guest bpy from the channels running parallel to the crystallographic b axis by using moderate heating and vacuum, resulted in the collapse of the structure, while, when 4 was soaked in toluene or cyclohexane, guest bpy was partially exchanged analogously to what happened with compound A.39 A SC-XRD collection of compound 4 carried out at 100 K evidenced a reversible single-crystal-to-single-crystal (SC-toSC) transformation dependent on the temperature. Actually, a significant decrease in symmetry (from Ibam to Pbcn space group) is observed passing from 296 K (4) to 100 K (4′). While the connections among the atoms remain unchanged (the most relevant bond distances are reported in Table S4 of the Supporting Information), the overall geometry of 4′ is evidently twisted with respect to that of 4 (Figure 10), resulting for 4′ in an angle of ca. 170° formed by Cu−N5 bond with the pyridinic ring, while in the case of 4 the corresponding value(s) are ca. 177° (Figure S10, Supporting Information) This lower symmetry of 4′ is a direct consequence of the shortening of the b and c axis and slight elongation of the a axis observed for the

unit cell at 100 K and influences the whole crystal packing of 4′ shown in Figure S11, Supporting Information. The reaction of 1c with bpy in MeOH at r.t. yielded wellformed blue crystals of [{Cu3(μ3-OH)(μ-pz)3(C2H5COO)2(μbpy)}2]·2H2O, 5. Interestingly, the structure of 5, apart from the crystallization molecules, seems almost identical to that of the above-reported compound 2, but in this case the asymmetric unit consists of a single trinuclear triangular fragment (see Figure 11). The Cu···Cu distances, as well as the μ3-O−Cu bond length and the distance of the capping μ3-O1 from the Cu3 plane [ca. 0.480 Å] are in the ranges normally found in analogous compounds.21−39 Cu1 shows a square pyramidal coordination geometry with the bpy nitrogen N7 in axial position and μ3OH, the bpy nitrogen N8 and the pyrazolates N6 and N1 nitrogens lying in the square plane. An almost regular square pyramidal coordination geometry is exhibited also by Cu2, having the basal plane defined by μ3-OH, the propionate oxygen O2 and the pyrazolates nitrogens N3 and N2, while O2i of the symmetry equivalent trinuclear fragment [symmetry code: (i) −x + 2, −y + 2, −z] is in axial position. Also Cu3 shows a largely distorted square pyramidal coordination H


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analogous compounds,21−39 is very likely reinforced by the presence of strong H-bonds involving O1 and O3i [O1···O3i 2.696(5) Å, O1−H111···O3i 172(6)°] (see Figure S12 of the Supporting Information). The supramolecular assembly of 5 is also very similar to that found in 2. Actually, due to the coordination of two bpy molecules to the same Cu1 ion, the hexanuclear units selfassemble forming 2D sheets (Figure 13), where nanometric holes (ca. 20 × 10 Å2) are evident. Excluding one probable H-bond involving the crystallization water molecule and one oxygen of the chelating propionate ion [O1W···O4 2.81(1) Å] (water H atoms were not localized), no other relevant supramolecular interaction is present in 5. Neither interpenetration nor catenation are observed, and the crystal lattice is constituted by parallel sheets (mean distance ca. 6.01 Å), which are not connected to each other and likely interact through dispersion forces only. Nevertheless, the parallel sheets being offset, access to the large holes is hampered, analogously to compound 2 (Figure S13, Supporting Information). The reactions of the two trinuclear acrylate derivatives 1d and 1e with bpy in a 1:2 ratio, carried out in MeOH at r.t., generate in both cases deep-blue crystals of the trinuclear triangular species [Cu3(OH)(pz)3(CH2CHCOO)2(bpy)(H2O)], 6, sketched in Figure 14.

Figure 11. Asymmetric unit of compound 5 with partial atom labeling scheme.

geometry defined by the pyrazolate nitrogens N4 and N5, μ3OH and O4 in the basal plane, while O5 is in axial position. All the other geometrical parameters are quite normal, and most relevant bond distances and angles are reported in Table S5 of the Supporting Information. In Figure 12 is shown the

Figure 14. Asymmetric unit of compound 6 with partial atom labeling scheme.

Figure 12. Hexanuclear assembly of compound 5. H atoms have been omitted.

The Cu···Cu and μ3−O-Cu distances, as well as the distance of the capping μ3-oxygen from the Cu3 plane [0.556(7) Å] are all in the ranges normally found in analogous compounds.21−39 All three Cu ions exhibit a distorted square pyramidal

hexanuclear assembly formed by two trinuclear units joined through O2 of the propionate anions acting in a monatomic bridging asymmetric way. This assembly, often found in other

Figure 13. Capped-stick (left) (H atoms have been omitted) and space-filling (right) representation of a 2D sheet of 5. I


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Figure 15. Hexanuclear assembly of compound 6.

Figure 16. A monodimensional tape-like 1D CP formed by 6 through the self-assembly of 30-membered macrocycles (one of them is shown with a green ball−stick representation on the left). On the right, a green arrow indicates the small solvent accessible voids.

coordination geometry. In particular, besides the μ3-OH and the N7 bpy nitrogen, Cu2 is coordinated by the pyrazolate nitrogens N2 and N3 and water, the latter being in axial position. Cu1 has the square vertexes occupied by μ3-OH, O2 acrylate, and pyrazolate nitrogens N1 and N6, while bpy N8ii, pertaining to a symmetrically related trinuclear unit [Cu1−N8ii 2.55(1) Å, symmetry code: (ii) x, y − 1, z − 1], is in axial position. Noteworthy, this latter interaction is weaker than that of bpy nitrogen N7 with Cu2, as evidenced not only by the different bond lengths but also by the fact that the Cu1−N8ii bond forms an angle of ca. 145° with the pyridinic ring (see Figure S14, Supporting Information). In Cu3, the axial position is occupied by the acrylate oxygen O4i pertaining to the symmetry related trinuclear moiety [Cu3−O4i 2.406(8) Å, symmetry code: (i) −x + 2, −y + 1, −z + 1], while μ3-OH, O4 acrylate oxygen, and N4 and N5 pyrazolate nitrogens define the square base. Finally, due to the presence of a crystallographic inversion center the two trinuclear clusters are bridged by two acrylate ions [O4 and O4i] joining Cu3 and Cu3i in a monatomic asymmetric way, generating the hexanuclear assembly shown in Figure 15. Coordinated water is involved, as H-donor, into a quite strong “intramolecular” H-bond with the acrylate oxygen O3, while another strong H-bond between μ3-OH and acrylate O5 of the trinuclear moiety [O1···O5i 2.720(9) Å, O1−H111···O5i 166°] likely contributes to stabilize the hexanuclear cluster (see Figure S15, Supporting Information). The other geometrical features are quite normal, and most relevant bond distances and angles are reported in Table S6, Supporting Information. The above-mentioned coordination of N8ii to Cu1 generates almost elliptic 30-membered macrocycles (axes ca. 13 × 9 Å),

which self-assemble in tape-shaped 1D CPs (Figure 16, left). Noteworthy, despite their quite large dimensions, the presence, within each macrocycle, of four pyrazolate rings drastically reduces the solvent accessible space (Figure 16, right), to ca. 3%, as calculated by the PLATON/SQUEEZE program.54 Finally, the thus obtained parallel tapes generate the crystal packing shown in Figure S16, Supporting Information. When compound 1d was reacted with a large excess of bpy, blue-green crystals of [Cu3(μ3-OH)(μ-pz)3(CH2CHCOO)2(Hpz)(bpy)2]·2H2O, 7, were isolated. From Figure 17 it is clear that the most relevant difference with respect to compound 6 is

Figure 17. Molecular structure of 7 with partial atom labeling scheme. J


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Figure 18. 1D CP formed by 7. Black dashed lines indicate possible “intramolecular” H-bonds involving crystallization water molecules, while red dashed lines indicate H-bonds between monotopic bpy nitrogen and coordinated pyrazole NH group (see text).

Table 2. Most Relevant Signals in the ESI-MS Spectra of Compounds 5, 6, and 8 compound

signala

R.A.b

assignments

5

564.3 652.3 655.0 657.3 659.2 1029.3 1065.3 650.3 654.3 655.3 1023.3 1059.3 664.3 669.2 682.4 1063.3 1101.4

30 56 53 100 74 80 77 75 70 100 70 72 75 90 100 90 75

[Cu3(OH)(pz)3(C2H5COO)(H2O)(MeOH)2]+ [Cu3(OMe)(pz)3(C2H5COO)(bpy)]+ [Cu3(OH)(pz)3(C2H5COO)(MeCN)(MeOH)3(H2O)2]+ [Cu3(OH)(pz)2(C2H5COO)2(MeCN)(MeOH)4]+ [Cu3(OH)(pz)3(C2H5COO)(MeCN)(MeOH)2(H2O)4]+ [Cu6(OH)3(pz)7(C2H5COO)(H2O)3]+ [Cu6(OH)3(pz)7(C2H5COO)(H2O)5]+ [Cu3(OMe)(pz)3(CH2CHCOO)(bpy)]+ [Cu3(OH)(pz)3(CH2CHCOO)(bpy)(H2O)]+ [Cu3(OH)(pz)3(CH2CHCOO)2(MeOH)(H2O)4+H]+ [Cu6(OH)4(pz)5(CH2CHCOO)2(MeOH)3]+ [Cu6(OH)3(pz)6(CH2CHCOO)2(MeOH)2(H2O)]+ [Cu3(OMe)(pz)3(CH2C(CH3)COO)(bpy)]+ [Cu3(OH)(pz)3(CH2C(CH3)COO)2(H2O)5+H]+ [Cu3(OH)(pz)3(CH2C(CH3)COO)(bpy)(MeOH)]+ [Cu6(OH)3(pz)7(CH2C(CH3)COO)(MeCN)(H2O)2]+ [Cu6(OH)3(pz)7(CH2C(CH3)COO)(MeCN)(H2O)4]+

6

8

a Values corresponding to the higher signals of the isotopic clusters. All isotopic clusters fit satisfactorily with calculated ones.41 bRelative Abundance of the higher signal of the isotopic cluster.

ligand of Cu3, and the square plane is defined by μ3-OH, the pyrazole nitrogen N11, and pyrazolate nitrogens N4 and N5. All the other geometrical features are quite normal, and most relevant bond distances and angles are reported in Table S7, Supporting Information. The above indicated ditopic behavior of a bpy molecule, bridging Cu2 and Cu3 pertaining to two trinuclear units generate 1D CPs, one of which is shown in Figure 18. Noteworthy, even though the second bpy molecule is oriented almost parallel to the first one, N8 does not coordinate to Cu3 of the corresponding trinuclear unit, [N8···Cu3ii 3.07(2) Å]. Instead, a H-bond between N8 and N−H of the neutral pyrazole coordinated to Cu3i [N12i···N8 2.97(2) Å, N12iH12Hi···N8 150°] likely contributes to the overall stability of the CPs. Finally, crystallization water molecules likely form two “intramolecular” H-bonds (water H atoms were not located) with O2 and O5. Interestingly, one of the two water molecules (O2W) is probably involved also in the formation of the overall crystal packing of 7. Actually, it may act as an “interchain” bridge between O5 and O3 pertaining to two distinct 1D CPs

the lack of the hexanuclear assembly and the presence of two molecules of bpy per trinuclear unit, features likely connected to the high bpy/1d ratio used in the synthesis. One of the bpy molecules, coordinated to Cu1 [Cu1−N7 2.36(1) Å] acts as a monotopic ligand, while the other one bridges Cu2 and Cu3 of another trinuclear moiety [Cu2−N9 2.44(1), N10−Cu3i 2.45(1) Å, symmetry code: (i) x − 0.5, −y + 1.5, z + 0.5]. Both acrylate ions coordinate in a monodentate mode to Cu1 [Cu1−O2 2.04(1) Å]55 and to Cu2 [Cu2−O4 1.99(1) Å], respectively. Finally, N11 of a pyrazole molecule is coordinated to Cu3. The μ3-O−Cu bond lengths and the distance of the capping μ3-oxygen from the Cu3 plane [ca. 0.268 Å] fall in the ranges normally found in analogous compounds,21−39 and all the three copper ions exhibit a square pyramidal coordination geometry. Cu1 is coordinated by μ3-OH, the acrylate oxygen O2, pyrazolate nitrogens N1 and N6, and N7 pertaining to the monotopic bpy, the latter being in axial position. Cu2 has the N9 bpy nitrogen as the axial ligand, while the square base vertexes are defined by μ3-OH, O14, and pyrazolate nitrogens N2 and N3. The N10ii nitrogen pertaining to another trinuclear unit [symmetry code: (ii) x + 0.5, −y + 1.5, z − 0.5] is the axial K


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Crystal Growth & Design Table 3. Structural Features of Compounds 2−7 comp 2 3 4 4′ 5 6 7

dimensionality 2D 0D 0D 0D 2D 1D 1D

CP hexanuclear clusters hexanuclear clusters hexanuclear clusters CP ladder like CP tape like CP

porosity

connectors

carboxylates coordination

inaccessible large holes (ca. 20 × 10 Å) not present two kinds of perpendicular channels: void 24% two kinds of perpendicular channels: void 24% inaccessible large holes (ca. 20 × 10 Å) small channels: void 3% not present

bpy, one acetate bpy bpy bpy bpy, one propionate bpy, one acrylate bpy

asymmetric chelating, monatomic bridging monodentate monodentate monodentate asymmetric chelating, monatomic bridging monodentate, monatomic bridging monodentate

[O2W···O3iiì 2.84(3), symmetry code: (iii) −x, −y + 1, −z], thus forming the 2D supramolecular network parallel to the a− c crystallographic plane evidenced in Figure S17, Supporting Information, where the crystal packing of 7, generated by a series of the above-mentioned parallel planes, is also shown. The reaction of 1f with bpy in MeOH at r.t., 1f/bpy = 1:2, yielded the blue microcrystalline compound 8 in good yield. Unfortunately, we were unable to obtain crystals suitable for a SC-XRD determination, and the characterization of 8 was inferred, analogously to other cases,34 through analytical and spectroscopic data. In particular, on the bases of elemental analyses and ESI-MS data, we suggest that also in compound 8 the trinuclear [Cu3(μ3-OH)(μ-pz)3] core is present. Even though the ESI-MS results cannot be employed to prove the presence of specific aggregates in the solid state, nevertheless they suggest the possible existence of the corresponding assemblies in the condensed phase56−61 and can be used to support structural hypotheses. In Table 2 the ESI-MS data of compound 8 are reported, as well as those of compounds 5 and 6 whose molecular structures were established through SCXRD determinations. Almost all the most intense signals correspond to assemblies containing the trinuclear cluster Cu3(OH)(pz)3. Moreover, other strong signals correspond to hexanuclear clusters, suggesting that also this particular aggregation may be present in 8 as well as in 5 and 6. In some cases quite intense signals are due to species where the OH group is exchanged by the MeO group, likely coming from the solvent employed in the measurements.62 IR characterizations of 2−8 (see Experimental Section) are in agreement to the above-reported structural features. On the basis of the Δ criterion [Δ = difference between νasym(COO) and νsym(COO)], concerning carboxylate coordination,63 the signals found at 1591, 1487 cm−1 for compound 3, 1597, 1490 cm−1 for compound 4, and 1565, 1490 cm−1 for compound 7 can be assigned to the CO stretching vibrations of the carboxylates acting, these cases, as monodentate asymmetric ligands. In the case of 2 and 5 the multiplicity of absorptions found between 1605 and 1396 cm−1 with Δ values ranging from 190 to 70 cm−1 suggests the presence of two different coordination modes of the carboxylate ion: monatomic asymmetric bridging and asymmetric chelating mode. The compounds 6 and 8 show analogous signals in the range 1640− 1400 cm−1, and for this reason we hypothesize for both species a monodentate asymmetric coordination as established, in the first case, through SC-XRD. The magnetic susceptibility experiments collected at room temperature for complexes 2, 4, and 5 show values for the hexanuclear species falling in the range 4.26−3.32 μB, lower than those expected for six independent CuII ions, thus indicating some kind of exchange coupling, analogously to that found in strictly related derivatives.22,25,31−39,64−73 For the compounds 6−8 the magnetic susceptibilities data, calculated

for trinuclear species, fall in the range 3.10−3.23 MB, also in this case the values being significantly lower than those expected for three independent CuII ions, likely due to the same exchange coupling.

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CONCLUSIONS In this work we have confirmed the stability of the trinuclear triangular [Cu3(μ3-OH)(μ-pz)3] moiety and the possibility to employ this building block in order to obtain new 1D or 2D CPs as well as hexanuclear clusters, by exchanging monotopic neutral ligands with ditopic bridging bpy. Whereas the coordinative flexibility of CuII makes it difficult to forecast the structures of the reaction products, it is worth noting that a careful choice of reaction conditions (solvent and reactants molar ratios) makes it possible to achieve a large panel of new products through simple reactions, as witnessed by the synthesis of compounds 2, 3, and 4 by starting from 1b or 6 and 7 from 1d. Guest ligands and/or solvent molecules trapped in the crystal lattices of almost all our products can be, in some cases, eliminated or exchanged. For example, in compound 3, [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(μ-bpy)2(H2O)]·bpy·4H2O· 4MeOH, not completely reversible MeOH and H2O sorption− desorption processes occur, while in compound 4 guest bpy molecules can be partly exchanged by soaking with toluene or cyclohexane. Although the different carboxylates bonded to starting trinuclear moieties lead to different products, as synthetically summarized in Table 3, the similarity between [{Cu3(μ3OH)(μ-pz)3(CH3COO)2(μ-bpy)}2]·H2O·4CH3OH, 2, and [{Cu3(μ3-OH)(μ-pz)3(C2H5COO)2(μ-bpy)}2]·2H2O, 5, suggests that, at least in these cases, analogous self-assembly processes, not depending on the length of the carboxylate chains, operate. H-bonds play a relevant role in the stability of compounds 2−7. Besides intermolecular interactions involving crystallization molecules in compounds 2, 3, and 5, also “intramolecular” plus “intermolecular” H-bonds can contribute to the overall stability of the CPs as in compound 4. In this case the selfassembly of the hexanuclear clusters to build the porous crystal lattice of 4 is very likely driven by quite strong H-bonds involving (i) noncoordinated OH− ion, acting as a bridge between μ3-OH and carboxylate oxygen pertaining to another hexanuclear unit and (ii) coordinated water acting as a bridge between bpy nitrogen atoms pertaining to two other distinct hexanuclear clusters. Even though these peculiar interactions are present also in the structure of 4 collected at 100 K (4′), nevertheless a relevant structural feature is revealed by this determination. Actually, compound 4, evidencing a thermally induced reversible SC-to-SC behavior with deformation of the structure passing from 296 to 100 K and vice versa, is an interesting example of “soft porous crystal”.74,75 Interestingly, L


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such a behavior is not related, as often happens, to the elimination or substitution of guest molecules, but to a less common “flexible” response to an external stimulus of the “rigid” ligand bpy coordinated to Cu. H-bonds are important also in the case of [Cu3(μ3-OH)(μpz)3(CH2CHCOO)2(Hpz)(μ-bpy)2]·2H2O, 7, where the interaction between N−H of the pyrazole coordinated to one copper ion of the trinuclear triangular moieties and N of one bpy molecule plays a role comparable to that of bridging bpy. Finally, even though crystallization molecules are not present in compound 6, “intramolecular” H-bonds between μ3-OH and carboxylate oxygen very likely favor the formation of the hexanuclear clusters.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

X-ray crystallographic file for compounds 2, 3, 4, 4′, 5, 6, and 7 in CIF format, Tables S1−S7, and Figures S1−S17. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Authors

*(M.M.) Phone: +39 051 2099559; fax: +39 051 2099456; email: [email protected]. *(C.P.) Phone: +39 0737 402234; fax: +39 0737 637345; email: [email protected]. *(L.P.) Phone: +39 049 8275157; fax: +39 049 8275161; 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.

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REFERENCES

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N


Interaction of the Trinuclear Triangular Secondary Building Unit [Cu3

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