New Coordination Polymers and Porous Supramolecular Metal

Article pubs.acs.org/crystal

New Coordination Polymers and Porous Supramolecular Metal Organic Network Based on the Trinuclear Triangular Secondary Building Unit [Cu3(μ3-OH)(μ-pz)3]2+ and 4,4′-Bypiridine. 1° Corrado Di Nicola,§ Federica Garau,# Massimo Gazzano,⊥ M. Fátima C. Guedes da Silva,†,‡ Arianna Lanza,# Magda Monari,*,∥ Fabrizio Nestola,& Luciano Pandolfo,*,# Claudio Pettinari,*,§ and Armando J. L. Pombeiro† §

Scuola di Farmacia, University of Camerino,Via S. Agostino, 1, I-62032 Camerino (MC), Italy Dipartimento di Scienze Chimiche, University of Padova, Via Marzolo, 1, I-35131 Padova, Italy ⊥ ISOF-CNR, Via Selmi, 2, I-40126 Bologna, Italy † Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal ‡ Universidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon, Av. do Campo Grande, 376, 1749-024, Lisbon, Portugal ∥ 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: The trinuclear triangular CuII complex [Cu3(μ3-OH)(μ-pz)3(HCOO)2(Hpz)2] (Hpz = pyrazole) reacts with 4,4′-bipyridine (bpy) yielding a two-dimensional (2D) waved sheets, two three-dimensional (3D) coordination polymers (CPs), as well as a hexanuclear CuII cluster, depending on the reagent ratios and reaction conditions. Single crystal X-ray diffraction (XRD) determinations point out that, while CPs crystal structures are not porous, the hexanuclear CuII clusters are packed in the solid state generating a stable porous 3D supramolecular network, where two kinds of perpendicular, hydrophobic channels (ca. 4.83 × 5.86 Å2 and 4.99 × 4.79 Å2, corresponding to the 24.7% of the total crystal volume) are present. In the “as-synthesized” compound, channels of one kind are empty, while the other ones are occupied by guest bpy molecules which can be removed by soaking the crystals in suitable solvents (benzene, toluene, c-hexane) maintaining intact the crystal skeleton. Moreover, two of the above complexes act as catalysts (or catalyst precursors) in the peroxidative oxidation of cyclohexane.

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INTRODUCTION The interest in coordination polymers (CPs)1 is not only due to the intrinsically “beautiful structures” often observed in these compounds, but also to their possible applications in relevant industrial fields2 as catalysis,3 magnetism,4 gas storage,5 etc. Recently, Yaghi and co-workers6 proposed a distinction between metal organic frameworks (MOFs) and CPs, mainly based on the evaluation of bond energies involved in the formation of the polymeric network. In the same article, the authors outlined a relevant issue, that is, that if a polymeric network is positively charged, the presence of anions to balance the charge is often detrimental to the functionality of the network itself. In fact, these, normally small, noncoordinating anions very often occupy pores and channels, thus hampering, to a great extent, the possibility to adsorb other molecules, as they cannot be eliminated and their exchange with other smaller ions often results in the collapse of the entire structure. For this reason, it is more profitable to use, in the building of © XXXX American Chemical Society

CPs, anions having multiple, coordinative ability, as sulfates, phosphates, mono-, di-, and polycarboxylates, azolates, etc. In recent years, we have studied the interactions of some transition metal ions with anionic N-donor (mainly pyrazolates) and monocarboxylates, and synthesized mono-, di-, hexa-, and heptanuclear complexes, as well as CPs showing interesting structural features, sorption−desorption properties, and catalytic activity. Besides zinc(II),7 cadmium(II), and mercury(II)8 carboxylates, we addressed our attention to the interaction of copper(II) carboxylates with pyrazole, showing in several cases how different reaction conditions give rise to largely different compounds. For example, by reacting different copper(II) carboxylates with pyrazole (Hpz), in MeCN, we obtained a one-dimensional (1D) CP based on the repetition of the mononuclear secondary building unit (SBU) [Cu(μ-pz)2], that, Received: January 17, 2012 Revised: March 22, 2012

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dx.doi.org/10.1021/cg300080a | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Scheme 1

simulator.17 The reported m/z values correspond to the most intense signal of the isotopic clusters. 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. Catalytic Activity Studies. To the reaction flask with the catalyst precursor (10 μmol) were added 4 mL of acetonitrile (MeCN), 0.05− 0.3 mmol (typically 0.1 mmol) of HNO3, 1.0 mmol of C6H12, and 2.5−10.0 mmol of H2O2 (30% in H2O), in this order. The reaction mixture was stirred for 6 h at room temperature (ca. 25 °C) and under air at atmospheric pressure. Cycloheptanone (90 μL) was then added as the internal standard, diethyl ether (typically 9.0 mL) to extract the substrate and the organic products of the reaction mixture, and PPh3 (0.5 g) to reduce the organo-hydroperoxides if formed, according to a method developed by Shul’pin.18 The resulting mixture was stirred for a few minutes and then a sample taken from the organic phase was analyzed by GC using a FISONS Instruments GC 8000 series gas chromatograph with a DB WAX fused silica capillary column (30 m × 0.25 mm × 0.25 μm) and the Jasco-Borwin v.1.50 software. The GC analyses of the aqueous phase showed the presence of only traces (less than 0.05%) of oxidation products. Crystallographic Data Collection and Structure Determination. The X-ray intensity data of compounds 2−5 were measured on a Bruker Apex II CCD diffractometer. Cell dimensions and the orientation matrix were initially determined from a least-squares refinement on reflections measured in three sets of 20 exposures, collected in three different ω regions, and eventually refined against all data. A full sphere of reciprocal space was scanned by 0.3° ω steps. The software SMART19 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,19 and an empirical absorption correction was applied using SADABS.20 The structures were solved by direct methods (SIR 97)21 and subsequent Fourier syntheses and refined by full-matrix leastsquares on F2 (SHELXTL)22 using anisotropic thermal parameters for all non-hydrogen atoms. In complex 2b six crystallization water molecules were found for each dodecanuclear complex, whereas for 2a because of the poorer quality of the crystal, the data set was corrected for disordered solvent with the PLATON/SQUEEZE program (see Supporting Information). Finally also in the asymmetric unit of 4 and 5 one crystallization methanol and water molecule were detected, respectively. Moreover in [{Cu3(μ3-OMe)(μ-pz)3(HCOO) 2(μC10H8N2)}2]·2MeOH, 4·2MeOH, the methoxy carbon [C(22)] showed a high thermal motion due to substitutional disorder with a H atom indicating the presence of some [{Cu3 (μ 3-OH)(μpz)3(HCOO)2(μ-C10H8N2)}2]·2MeOH in the crystal, and therefore the refinement of the methoxy carbon was carried out using the PART instruction of the SHELX program, giving occupation factors of 56 and 44% for C(22) and H(111), respectively. The molecular graphics were generated by using the Mercury 2.323 program. Color codes for all molecular graphics are yellow-orange (Cu), blue (N), red (O), gray (C), white (H). Crystal data and details of the data collections for compounds 2b, 3, 4, and 5 are reported in Table 1. In the Supporting

even though does not present pores, shows interesting reversible crystal-to-crystal transformations and a vapochromic behavior associated with the sorption/desorption of small molecules.9 On the contrary, when the reaction of copper(II) carboxylates with Hpz was carried out in protic solvents (water or alcohols containing traces of water, and also MeCN containing at least 1% of water10) we obtained compounds 1a−j based on the triangular [Cu3(μ3-OH)(μ-pz)3]2+ moiety, whose positive charge is balanced by two carboxylate ions coordinated to CuII ions (Scheme 1). Moreover, these trinuclear compounds further self-assemble in hexanuclear clusters through carboxylate bridges and/or form extended 1D or two-dimensional (2D) CPs or supramolecular networks based on weak, noncovalent interactions.11 The trinuclear [Cu3(μ3-OH)(μ-pz)3]2+ moiety is quite stable, as evidenced by the fact that it remained to a large extent intact when [Cu3(μ3OH)(μ-pz)3(CH3COO)2(Hpz)], 1b, was reacted with aqueous HCl12 and other strong acids13 yielding a porous coordination polymer (PCP) and other CPs. Even though trinuclear triangular CuII systems have been known for a long time,14 they have not been systematically used as SBUs to synthesize CPs. To the best of our knowledge, excluding our previous articles,11−13 only one paper reporting the formation of CPs based on triangular trinuclear CuII SBUs has recently appeared.15 Here we report the results of the reactions of compound [Cu3(μ3-OH)(μ-pz)3(HCOO)2(Hpz)2]·H2O, 1a, with 4,4′bipyridine (bpy), in different conditions, which yield CPs and a porous, supramolecular metal−organic solid.16 The hydrophobic channels of this latter porous network contain free bpy that can be removed by soaking the solid in suitable solvents, maintaining the structural integrity of the crystal. Moreover, some of the obtained compounds act as catalysts (or catalyst precursors) in the peroxidative oxidation, in mild conditions, of cyclohexane to cyclohexanone and cyclohexanol.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were purchased from Aldrich and used without further purification. All the reactions and manipulations were carried out in air. The syntheses were performed both in ambient and solvothermal conditions. Compound 1a was prepared as previously reported.11b Elemental analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer. Infrared spectra from 4000 to 600 cm−1 were recorded with a Perkin-Elmer Spectrum One Model FTIR spectrometer with ATR mode. Positive electrospray mass spectra were obtained with a Series 1100 MSI detector HP spectrometer, using MeOH as the mobile phase. Solutions for electrospray ionization mass spectrometry (ESIMS) were prepared using reagent grade methanol, water, and/or acetonitrile, and obtained data (m/z and intensities) were compared with those calculated by using the IsoPro isotopic abundance B



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Table 1. Crystal Data and Structure Refinement for 2b·6H2O, 3, 4·2MeOH, and 5·2H2O compound

2b·6H2O

3

4·2MeOHc

5·2H2O

formula fw T (K) λ (Å) 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 θ limits, ° reflections collected unique obs reflections [Fo > 4σ(Fo)] goodness-of-fit-on F2 R1 (F)a, wR2 (F2) [I > 2σ(I)]b largest diff peak and hole, e·Å−3

C54H80Cu12N26O32 2367.92 296(2) 0.71073 monoclinic P21/n 12.4917(8) 25.8228(16) 14.9460(9) 90 106.085(1) 90 4632.4(5) 2 1.698 2.780 2352 0.20 × 0.25 × 0.30 1.58−28.00 38136 10898 [Rint = 0.0321] 1.058 0.0441, 0.1328 −1.112 and 1.373

C90H84Cu6N26O10 2071.07 296(2) 0.71073 orthorhombic Ibam 36.098(3) 12.767(1) 24.031(2) 90 90 90 11074.6(16) 4 1.242 1.192 4232 0.22 × 0.28 × 0.32 1.69−27.00 58111 6100 [Rint = 0.0531] 1.119 0.0524, 0.1431 −0.389 and 0.875

C46H52Cu6N16O12c 1402.28c 296(2) 0.71073 orthorhombic Pbcn 16.330(3) 23.484(4) 14.805(2) 90 90 90 5677.9(15) 4 1.640c 2.279c 2808 0.18 × 0.22 × 0.28 2.21−24.50 27830 4294 [Rint = 0.0910] 1.127 0.0661, 0.1110 −0.563 and 0.669

C52H56Cu6N18O14 1538.39 296(2) 0.71073 monoclinic P21/n 16.429(3) 8.0925(13) 22.732(4) 90 91.292(5) 90 3021.5(9) 2 1.691 2.153 1560 0.15 × 0.20 × 0.25 1.79−25.00 19287 5179 [Rint = 0.0943] 0.885 0.0531, 0.0850 −0.394 and 0.420

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2 where w = 1/[σ2(Fo2) + (aP)2 + bP] where P = (Fo2 + Fc2)/3. cBased on pure 4·2MeOH. a

(pz) 6 (HCOO) 4 ] + . μ eff (295 K) = 4.394 μ B (calculated for C56H84Cu12N26O32). Synthesis of [{[Cu3(μ3-OH)(μ-pz)3(HCOO)2(H2O)][Cu3(μ3-OH)(μpz)3(HCOO)2(H2O)2]}2(μ-C10H8N2)]·6H2O, 2b. The reaction was carried out as in the case of 2a, yielding well-formed blue crystals suitable for a single-crystal XRD determination. The obtaining of 2a or 2b seems due only to fortuitous small differences in the crystallization process, as compound 2b differs from 2a only for the coordination of a water molecule instead of MeOH. Synthesis of [{Cu 3 (μ 3 -OH)(μ-pz) 3 (HCOO)(H 2 O)(μ-C 10 H 8 N 2 )(C10H8N2)2}2(OH)2]·C10H8N2, 3. To a solution of 1a (200 mg, 0.3 mmol) in MeOH (30 mL) a solution of bpy (191 mg, 1.2 mmol) in MeOH (5 mL) was added under stirring. The obtained solution was stirred for 5 min, and then left to evaporate in the air, yielding wellformed green crystals, accompanied by white needles of bpy. Washings with diethyl ether removed bpy and the green crystals of 3 were dried under a moderate vacuum (0.1 mmHg) at r.t.. 3: Yield 322 mg, 95%. M.p.: 125−130 °C (d). Elem. Anal. Calcd for [{Cu3(OH)(pz)3(HCOO)(H2O)(C10H8N2)3}2(OH)2](C10H8N2) C, 52.19; H, 4.09; N, 17.58. Found: C, 51.36; H, 4.03; N, 17.63 IR (KBr, cm−1): 3418, 3239, 3142, 2924, 2852, 2830, 1631, 1601, 1534, 1492, 1414, 1383, 1340, 1324, 1276, 1224, 1180, 1063, 1003, 877, 810, 761, 625, 483. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 157.2 (8) [bpy + H]+, 624.3 (80) [Cu3(OCH3)(pz)3(HCOO)(bpy)]+, 744.5 (20) [Cu3(OCH3)2(pz)2(HCOO)(bpy)2]+, 785.3 (100) [Cu3(OH)(pz)2(HCOO)2(bpy)2(MeCN)]+, 1303.4 (50) [Cu6(OH)5(pz)6(HCOO)(bpy)2(H2O)3+Na]+. μeff (295 K) = 3.019 μB (calculated for C90H84Cu6N26O10). Attempted Recrystallizations of 3. In different experiments, compound 3 was dissolved in MeOH. The blue solutions formed were let to slowly evaporate in the air yielding blue crystals of different compounds. In detail, it was possible to isolate three different compounds, namely, compound 2b (characterized by XRPD), compound [{Cu3(μ3-OMe)(μ-pz)3(HCOO)2(μC 10 H 8 N 2 )} 2 ]·2MeOH, 4, and compound [{Cu 3 (μ 3 -OH)(μpz)3(HCOO)2(μ-C10H8N2)(H2O)}2(μ-C10H8N2)]·2H2O, 5 which were characterized by single-crystal XRD determinations. Compound

Information crystal data and details of the data collections for 3 after soaking in benzene (3b), toluene (3t), and cyclohexane (3ch) are reported. Powder X-ray diffraction (PXRD) 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. Syntheses. Reactions at r.t. and 1 atm. The reactions were carried out by using MeOH as solvent at room temperature (r.t.) and atmospheric pressure, in open vessels. Reactions of [Cu3(μ3-OH)(μ-pz)3(HCOO)2(Hpz)2]·(H2O), 1a, with 4,4′-Bipyridine. Synthesis of [{[Cu3(μ3-OH)(μ-pz)3(HCOO)2(H2O)][Cu3(μ3-OH)(μ-pz)3(HCOO)2(H2O)(MeOH)]}2(μ-C10H8N2)]·6H2O, 2a. To a solution of 1a (251 mg, 0.4 mmol) in MeOH (30 mL) a solution of bpy (64 mg, 0.4 mmol) in MeOH (5 mL) was added under stirring. The obtained solution was stirred for 5 min, and then left to slowly evaporate in the air, yielding blue crystals. 2a: Yield 150 mg, 66%. M.p.: 145−148 °C (d). Elem. Anal. Calcd for [{[Cu3(OH)(pz)3(HCOO)2(H2O)]-[Cu3(OH)(pz)3(HCOO)2(H2O)(MeOH)]·3H2O]}2(C10H8N2)]: C, 28.07; H, 3.53; N, 15.19. Found: C, 28.35; H, 3.36; N, 15.02. IR (KBr, cm−1): 3583, 3353, 3214, 3133, 2928, 2837, 1562, 1489, 1417, 1380, 1347, 1276, 1222, 1178, 1058, 1011, 967, 923, 882, 817, 751, 668. ESI-MS (+) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 69.2 (22) [Hpz + H]+, 157.1 (55) [bpy + H]+, 199.0 (15) [Cu(pz)(Hpz)]+, 243.9 (25) [Cu(Hpz)2(HCOO)]+, 258.0 (100) [Cu(Hpz)(HCOO)(MeOH) 2 (H 2 O)] + , 287.0 (10) [Cu(pz)(bpy)] + , 453.9 (18) [Cu3(OH)(pz)3(HCOO)]+, 467.9 (37) [Cu3(OH)(pz)2(HCOO)2(H2O)2]+ or [Cu3(OCH3)(pz)3(HCOO)]+, 475.9 (60) [Cu3(OH)2(pz)3(MeOH)(H2O)]+, 503.8 (15) [Cu3(OH)(pz) 3 (HCOO)(MeOH)(H 2 O)] + , 517.8 (15) [Cu 3 (OCH 3 )(pz)3(HCOO)(MeOH)(H2O)]+, 525.8 (28) [Cu 3 (OH) 2 (pz) 3 (MeOH) 2 (H 2 O) 2 ] + , 535.9 (30) [Cu 3 (OH)(pz)3(HCOO)(MeOH)2(H2O)]+ or [Cu3(OCH3)(pz)3(HCOO)2+Na]+, 549.8 (50) [Cu3(OH)(pz) 2 (HCOO) 2 (MeOH) 2 (H 2 O) 3 ] + , 563.8 (37) [Cu 3 (OH)(pz)3(HCOO)2(MeOH)2 + H]+, 577.8 (25) [Cu 3 (OH)(pz) 3 (HCOO)(MeCN) 3 ] + , 980.6 (18) [Cu 6 (OH)C



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Figure 1. Molecular structure of 2b (left). Asymmetric unit of 2b with partial atom numbering scheme (right). For sake of clarity in the second panel H atoms and crystallization water molecules are omitted. 3 was reobtained only when the crystallization was carried out in the presence of an excess of bpy. Solvothermal Reactions. The reactions were carried out in 23 mL Teflon lined Parr reactors, by using MeOH as solvent. Reactions of [Cu3(μ3-OH)(μ-pz)3(HCOO)2(Hpz)2]·(H2O), 1a, with 4,4′-Bipyridine in 1:1, 1:4, and 1:8 ratios. The reactions were carried out by charging the Parr reactors with 50 mg of 1a (0.076 mmol) dissolved in ca. 10 mL of MeOH, and bpy enough to reach the 1a/bpy = 1:1 and 1:4 molar ratios, while 100 mg of 1a and 204 mg of bpy were employed for the 1:8 ratio. The reactors were maintained at 70 °C for 30 h (60 h for 1:8 ratio) under autogenous pressure and then let to reach spontaneously r.t. In all cases light blue solutions with a large quantity of beige to brown powders (which were eliminated) were recovered. Mother liquors were let to go to dryness in the air obtaining in all cases few green crystals of 3, characterized through XRD determination by comparison of the unit cell parameters, together with a large amount of white crystals of bpy. Compound 3 Soaking Experiments. Samples of 100 mg of 3 were suspended in ca. 5 mL of benzene, toluene, or cyclohexane. The suspensions were left undisturbed for 12 h and then the liquid phases were separated. Solvents were removed under vacuum, yielding a white solid that was identified as bpy. Fresh solvent was added to the crystals and this procedure was repeated five times. At the end the solids, whose crystalline habits were apparently unchanged, were kept under the appropriate solvents and analyzed through single crystal XRD determinations.

pz) 3 (HCOO) 2 (H 2 O)][Cu 3 (μ 3 -OH)(μ-pz) 3 (HCOO) 2 (H2O)2]}2(μ-C10H8N2)]·6H2O, 2b, was isolated. The unique difference between 2a and 2b resides in the presence of a coordinated MeOH molecule in 2a instead of a water molecule (2b), which can be associated with fortuitous, slightly different, crystallization conditions. On the other hand, XRD determinations evidenced that the two compounds are isomorphous, having an almost identical molecular structure and very similar cell parameters. Moreover, they self-assemble forming almost completely identical three-dimensional (3D) CPs, and, for the sake of simplicity, we will describe only the structural features of 2b, which afforded better quality crystals. Crystal data and structure refinement, most relevant structural data (bonds and angles), and images of the structure of 2a are reported in Tables S1 and S2 and in Figure S1, respectively (Supporting Information). The asymmetric unit of compound 2b consists of two almost identical hexanuclear moieties joined by a bpy molecule [Cu(2)−N(7) 2.061(4) Å] (Figure 1). An inversion center is placed in the middle of the central C−C bpy bond. The two trinuclear moieties forming each hexanuclear unit through a syn-anti formate bridge, to some extent reminiscent of the supramolecular structure of 1a,11b are very similar, but not identical (see for example the differences in Cu−Npz and nonbonding Cu···Cu distances, reported in Tables S2 and S3). Moreover, the two trinuclear fragments also differ from each other by the coordination around copper ions. In one case only one water molecule is very weakly coordinated, almost symmetrically, to two copper ions [O(2w)−Cu(6) 2.750(5), O(2w)−Cu(5) 2.734(5) Å], while two water molecules are coordinated to two Cu ions of the other trinuclear fragment [O(7)−Cu(2) 2.312(4), O(1w)−Cu(3) 2.514(4) Å]. All CuII ions have a square pyramidal coordination geometry. In detail, in one trinuclear moiety the water molecule occupies the axial position both for Cu(6) and Cu(5) (see above), while one formate oxygen [O(11′)] of another hexanuclear unit is the axial ligand of Cu(4) [Cu(4)−O(11′) 2.548(3) Å, symmetry code: (I) x + 0.5, −y + 0.5, z + 0.5]. In the second trinuclear fragment, the axial ligands are water molecules for Cu(3) and Cu(2) and the formate O(3) oxygen for Cu(1) [Cu(1)−O(3) 2.311(3) Å]. Other relevant structural features are the distances of μ3-O from the corresponding Cu3 plane, [0.568(3) and 0.531(3) Å for O(2) and O(1) respectively] that fall in the range of values normally observed in the [Cu3(μ3-OH)(μpz)3]2+ unit. The other geometrical features of the hexanuclear moieties and bridging bpy are quite normal for this kind of compound,11−15 and most relevant bond distances and angles are reported in Table S3.

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RESULTS AND DISCUSSION Recently, we started a research project aiming to use compounds 1a−j as sources of trinuclear triangular SBUs to be joined to each other through neutral ditopic ligands. We now report the first results obtained by reacting compound 1a with bpy in different reaction conditions (which determine the reaction pathway toward obtaining different compounds) and the description of the structural motifs present in the obtained compounds. In detail, when 1a and bpy were reacted in MeOH at r.t. in a 1:1 molar ratio, compound [{[Cu3(μ3-OH)(μpz)3(HCOO)2(H2O)][Cu3(μ3-OH)(μ-pz)3(HCOO)2(H2O)(MeOH)]}2(μ-C10H8N2)]·6H2O, 2a, was obtained in good yield. ESI-MS determinations (see Experimental Section) confirmed the stability of the trinuclear Cu3(μ3-OH)(μ-pz)3 moiety whose characteristic isotopical pattern was found in the spectra.24 The magnetic susceptibility value, approximately corresponding to four unpaired electrons for the [{[Cu3(μ3OH)(μ-pz)3(HCOO)2(H2O)][Cu3(μ3-OH)(μpz)3(HCOO)2(H2O)(MeOH)]}2(μ-C10H8N2)]·6H2O formulation, suggests the presence of some antiferromagnetic coupling in the trinuclear arrangements, analogously to that found in strictly related compounds.11−15 In a distinct synthetic experiment, a slightly different compound, [{[Cu3(μ3-OH)(μD



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Figure 2. Top: schematic representation of 2b; the labels indicate the connection sites among one asymmetric unit and the other ones (see text). H atoms as well as coordinated and crystallization water molecules are omitted. Bottom, left: view down the crystallographic b axis of a 1D tape obtained by connecting Cu(6) with Cu(1′) through the syn-anti bridge O(10)−C(26)−O(11) (green dots). Bottom, right: arbitrary view a 2D sheet generated through Cu(4)−O(11′)−Cu(1′) connections among the tapes (green dots). H atoms, pyrazolate C atoms as well as coordinated and crystallization water molecules have been omitted.

Figure 3. Compound 2b: view down the crystallographic a axis of a waved sheet (left) and of the 3D CP formed through the Cu(6)−O(10)− C(26)−O(11)−Cu(1′) connections (right). H atoms, pyrazolate C atoms as well as coordinated and crystallization water molecules have been omitted.

Some noncovalent interactions are present in 2b. Particularly, quite strong “intramolecular” hydrogen bonds are formed between capping μ3-O(1)−H(1) and the noncoordinating O(4) of a formate ion [O(1)···O(4) 2.661(5) Å, O(1)−H(1)− O(4) 156(2)°] and between the water molecule coordinated to Cu(6) and Cu(5) and O(9) of another formate ion [O(2w)···O(9) 2.827(8) Å, O(2w)−H(22w)−O(9) 145°]. Hydrogen bonds are, at least partly, responsible also for the presence of crystallization water molecules, that interact with the dodecanuclear cluster. One molecule of crystallization water behaves as an H-acceptor toward the coordinated water bridging Cu(5) and Cu(6) [O(2w)···O(3w) 2.87(1) Å, O(2w)−H(2w)−O(3w) 154°], while the water molecule coordinated to Cu(3) acts as a weak H-donor toward another crystallization water molecule [O(1w)···O(5w) 3.16 Å, O(1w)−H(1w)−O(5w) 160°]. This latter water molecule is likely further involved into a hydrogen bond with a third

crystallization water molecule [O(5w)···O(4w) 2.89(2) Å] (H atoms of these two water molecules were not located). These supramolecular interactions are shown in Figure S2. The self-assembling of the dodecanuclear SBUs of 2b generates a 3D CP. The quite complicated structure of this CP can be described taking into account that, besides bpy, two other different kinds of connections are present (Figure 2, top). The first one involves Cu(6) and Cu(1″) of another dodecanuclear unit, through the syn-anti bridging O(10)− C(26)−O(11) formate skeleton [Cu(6)−O(10) 1.993(3), Cu(1″)−O(11) 2.014(3) Å], generating 1D infinite tapes running parallel to the crystallographic a axis (Figure 2, bottom left). The second connection involves Cu(4) with Cu(1′) of a third dodecanuclear unit through the bridging O(11′) formate oxygen. In this arbitrary description, the latter connection is responsible for the formation of 2D CPs obtained through the junction of different 1D tapes (Figure 2, bottom right). The E



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Å] and to the ditopic bpy [Cu(1)−N(4) 2.216(3) Å], joining the two trinuclear units. Interestingly, the monocoordinated bpy forms an interplanar twist angle between the amine’s aromatic rings of ca. 21° that is similar to the twist angles found in the solid state of bpy for one of the two independent molecules,25 whereas those of ditopic bpy, joining the two trinuclear moieties, are almost coplanar. Incidentally, a π−π interaction between the two ditopic bpy molecules (almost parallel and faced each other at the average graphitic distance of ca. 3.76 Å) is possible. The Cu(2) ions exhibit a square pyramidal coordination geometry with the μ3-OH, pyrazolate nitrogens [Cu(2)−N(2) 1.952(3) Å] and a monodentate formate ion [Cu(2)−O(2) 2.004(4) Å] in equatorial positions, while the coordinated water molecule [Cu(2)−O(4) 2.482(5) Å] occupies the axial site. The other geometrical features of the hexanuclear moiety are quite normal and most relevant bond distances and angles are reported in Table S4. Some noncovalent interactions contribute to the formation of the peculiar crystal packing of 3 (vide infra). Actually, each OH− ion acts as a bridge between two hexanuclear units, being involved into two quite strong hydrogen bonds with capping μ3-OH [O(1)···O(5) 2.711(6) Å, O(1)−H(111)−O(5) 164(1) °] and with the coordinated water molecule of another hexanuclear unit [O(5)···O(4″) 2.861(7) Å, O(5)−H(5a)− O(4″) 178°, symmetry code: (II) −x + 0.5, y + 0.5, −z], as shown in Figure 5.

apparent porosity evidenced in the 1D tapes (Figure 2, bottom left) really does not exist, as it is canceled by the formation of the 2D CP (Figure 2, bottom right). Thus, the obtained sheets are undulated (Figure 3, left) and are further interconnected through the above-described Cu(6)−O(10)−C(26)−O(11)−Cu(1′) connection, leading to the formation of a 3D CP (Figure 3, right). Finally, some “intermolecular” hydrogen bonds, reported in the Figure S3, further contribute to the stability of the 3D CP. The reaction of 1a with an excess of bpy in MeOH afforded the completely different compound 3. In particular, after the evaporation nearly to dryness of the solvent, a mixture of green crystals and bpy was obtained. Upon removal of unreacted bpy by washing with diethyl ether, a single-crystal XRD determination was carried out on one of the remaining green crystals. The molecular structure of [{Cu 3(μ 3-OH)(μpz) 3 (HCOO)(H 2 O)(μ-C 10 H 8 N 2 )(C 10 H 8 N 2 ) 2 } 2 (OH) 2 ]·C10H8N2, 3, is shown in Figure 4. Compound 3 consists of a

Figure 4. Molecular structure of 3 with partial atom numbering scheme.

hexanuclear system generated by the coupling of two identical trinuclear triangular Cu3(μ3-OH)(μ-pz)3 moieties, bridged by two bpy molecules. Moreover, a formate anion was removed from each trinuclear moiety of 1a and substituted by a noncoordinated OH− ion, likely coming from atmospheric water or moisture in the organic solvent. Two bpy molecules, acting as monotopic ligands, and a water molecule complete the coordination of the CuII ions in each trinuclear fragment. Finally, one crystallization bpy molecule per each hexanuclear moiety is also present. The Cu3N6 nine-membered rings are almost planar. The three copper ions define an isosceles triangle [Cu(2)···Cu(1) 3.404(1), Cu(1)···Cu(1′) 3.365(1) Å, symmetry code: (I) x, y, −z] and the distances between μ3-OH and Cu ions [Cu(2)− O(1) 1.993(4), Cu(1)−O(1) 2.023(2) Å] are quite normal for this kind of compounds.11−15 The capping oxygen is out of the Cu3 plane of 0.466(3) Å, a value comparable to those found in analogous derivatives.11−15 In each trinuclear unit, the two symmetry equivalent Cu(1) ions have a TBP coordination geometry due, besides the μ3-OH and pyrazolate coordination [Cu(1)−N(1) 1.965(4), Cu(1)−N(3) 1.955(3) Å], to a bpy molecule acting as a monotopic ligand [Cu(1)−N(5) 2.094(3)

Figure 5. H-bonds (dotted red lines) connecting two hexanuclear units of 3. The crystallization bpy molecules have been omitted.

A further contribution to the crystal packing of 3 is provided by quite strong hydrogen bonds which involve the coordinated water molecules and uncoordinated N(6) nitrogens of the bpy acting as monodentate ligand belonging to neighbor hexanuclear units [O(4)···N(6′″) 2.856(4) Å, O(4)−H(4A)− N(6′″) 172°, symmetry code: (III) −x + 0.5, −y + 0.5, −z + 0.5], as evidenced in Figure 6. These noncovalent interactions lead to the formation of the peculiar 3D structure shown in Figure 7. Actually, the crystal packing of 3 shows the existence of two kinds of perpendicular, hydrophobic channels, running parallel to the crystallographic b and c axes and having almost rectangular sections of ca. 4.83 × 5.86 Å2 and 4.99 × 4.79 Å2, respectively. In the channels running parallel to the b axis guest bpy molecules are hosted, while the channels parallel to c are empty (Figures 7 and S4). A calculation using the SQUEEZE routine of PLATON revealed that the potential total solvent accessible volume is 2736.6 Å3 corresponding to the 24.7% of the unit cell volume. F



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solvent molecules remain in the crystal lattice, at the intersections of the two types of channels. Nevertheless, further evidence of the importance of guest bpy molecules in the formation and stability of its porous network is evidenced by the results obtained on attempting the recrystallization of 3 in MeOH. Actually, when the green crystals of 3 were dissolved in MeOH afforded blue solutions from which, in separate experiments, different compounds (2b, 4, and 5, see Experimental Section) crystallized. Unfortunately, we were unable to determine what leads to the formation of each compound.26 Nevertheless, we report the structural characterization of compounds 4 and 5, while the structure of 2b was determined by comparison of its PXRD diffractogram with that of an authentic sample. In Figure 9 the asymmetric unit of [{Cu3(μ3-OMe)(μpz)3(HCOO)2(μ-C10H8N2)}2]·2MeOH, 4, is shown, where are evident features present also in 3, first of all the hexanuclear system formed through the coupling of two trinuclear triangular moieties bridged by two bpy molecules. Moreover, a crystallographic inversion center is located between the two trinuclear units midway along the vector joining the μ3-oxygens and, also in this case, the Cu3N6 ninemembered rings are almost exactly planar. The Cu···Cu distances [Cu(2)···Cu(1) 3.412(1), Cu(2)···Cu(3) 3.383(1), Cu(3)···Cu(1) 3.360(1) Å] fall in the range normally found in analogous compounds, as well as the bonding μ3-O−Cu distances [Cu(1)−O(1) 2.047(4), Cu(2)−O(1) 2.034(4), Cu(3)−O(1) 1.976(4) Å].11−15 Analogously to 3, the two ditopic bpy molecules are parallel and faced each other at an average distance of ca. 3.61 Å, possibly indicating a π−π interaction between them. On the other hand, some relevant differences with 3 are evident. While in each trinuclear fragment of 3 the double positive charge is balanced by one formate and one hydroxyl anion, in the trinuclear moiety of 4 the presence of two formate ions suggests that the second one is necessarily generated by the decomposition of one-half of compound 3. One of the formate ions chelates Cu(3) [Cu(3)−O(4) 1.976(4), Cu(3)−O(5) 2.750(6) Å], while the second one coordinates to Cu(1) in a monodentate way [Cu(1)−O(2) 2.015(4) Å]. Moreover, the capping group μ3-OH is substituted by a μ3-OMe fragment, likely coming from the deprotonation of MeOH used as solvent.27 The other geometrical features of the hexanuclear moiety are quite normal for this kind of compound,11−15 and the most relevant bond distances and angles are reported in Table S5. Compound 4 self-assembles forming a very peculiar CP. Actually, Cu(1) is joined to Cu(2′) of another hexanuclear unit through formate syn-anti bridges [Cu(1)−O(2) 2.015(4), Cu(2′)−O(3) 2.088(4) Å, symmetry code. (I) −x + 0.5, −y

Figure 6. View down the crystallographic b axis of 3 evidencing Hbonds (red arrows) involving coordinated water molecules and unbound bpy nitrogens.

Interestingly, even though guest bpy does not interact with the host structure, it seems to be the key factor not only for the synthesis of 3 (see Experimental Section, where an excess of bpy is used), likely acting as a templating molecule, but also for its stability. As a matter of fact, attempts to remove guest bpy molecules by using a vacuum (10−3 mmHg) and moderate heating (40 °C) resulted in the collapse of the structure, as evidenced by PXRD determinations (see Figure S5). On the contrary, by soaking of the crystals of 3 in some organic solvents (where 3 is insoluble) guest bpy is completely or almost completely removed. Actually, crystals of 3 after repeatedly soaking in benzene, toluene, and cyclohexane maintained intact their crystalline habit, while, by taking to dryness the soaking solutions, bpy was obtained. The crystals coming from benzene, toluene, and cyclohexane soakings (hereafter 3b, 3t, and 3ch, respectively) were investigated by single-crystal XRD studies at 100 K in order to evaluate if the replacement of bpy with the soaking solvent(s) had taken place. The resulting unit cell parameters of the three crystals are almost identical to those of the as-synthesized 3; guest bpy molecules are completely or partially removed, while some disordered solvent molecules are present at the intersection of the two different types of channels. This is particularly evident for 3ch in which the elimination of bpy is partial (bpy occupation factor ca. 47%). Crystal data and details of data collections for compounds 3b, 3t, and 3ch, reported in Tables S7 and S8, show that the crystal structure remains unchanged even if “in-the-pores” bpy is removed through the soaking process. In Figure 8 are shown the crystal packings of compound 3 as well as those of 3t and 3ch, evidencing that the soaking process may eliminate guest bpy, while disordered, not exactly quantified,

Figure 7. Space filling packing diagram of 3 view down the crystallographic b (left) and c (right) axis. Guest bpy molecules, occupying the channels parallel to b, are shown in green and ball-and-stick representation. G



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Figure 8. View down the crystallographic b (up) and c (down) axes of the ball-and-stick packing diagrams of compounds 3 (left), 3ch (center), and 3t (right). The organometallic framework is represented in blue color, guest bpy molecules are shown in red, and disordered solvent molecules are in yellow, evidencing the different positions in the channels of the solvent molecules with respect to guest bpy. In 3ch almost half of the free bpy molecules (occupation factor 47%) are still present together with the disordered cyclohexane molecule.

Figure 9. Molecular structure of compound 4 (left). Asymmetric unit with the independent half molecule of 4 and partial atom labeling scheme (right). For sake of clarity in the second panel H atoms are omitted.

+ 0.5, z + 0.5], thus generating flat 2D CPs where nanometric holes (ca. 20 × 10 Å2) are present (see Figure 10). On the other hand, due to the large dimensions of the holes, interpenetration occurs. As shown in Figure 11 (left), a hexanuclear unit accommodates into one hole and selfassembles with other hexanuclear units, generating an identical 2D CP lying in a plane that forms a dihedral angle of ca. 70° with the previous one (Figure 11, center). This behavior is repeated in the whole crystal thus giving rise to a 3D CP (Figure 11, right) through interpenetration of 2D CPs. In Figure S6 is reported a partial view of 4, down the crystallographic c axis. In another attempt of recrystallization of 3 in MeOH, the compound [{Cu 3 (μ 3 -OH)(μ-pz) 3 (HCOO) 2 (C 10 H 8 N 2 )(H2O)}2(μ-C10H8N2)]·2H2O, 5, was obtained. The molecular structure, (Figure 12) shows that also in this case an extensive rearrangement of 3 took place.

Figure 10. Compound 4. A flat 2D CP with one nanometric hole (see text) shown with space fill representation.

Compound 5 consists of two trinuclear units joined together by a single ditopic bpy ligand [Cu(1)−N(7) 2.056(6) Å] lying H



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Figure 11. Schematic representation of the formation of the interpenetrated 3D CP of 4 (see text for explanation). For sake of clarity H atoms, MeOH molecules, and pyrazolate carbon atoms have been omitted. The orientation is the same of Figure 10.

Figure 12. Molecular structure of 5 (left). Asymmetric unit with the independent half molecule of 5 and partial atom labeling scheme (right).

Figure 13. Compound 5. (Left) One macrocycle (green color) formed through the connection of four SBUs. (Right) One 2D waved CP formed through the connection of the macrocycles. For sake of clarity H atoms and crystallization water molecules have been omitted.

ligand [Cu(2)−N(8) 2.021(6) Å], while Cu(3) is coordinated to one oxygen of a formate ion pertaining to another hexanuclear unit [Cu(3)−O(4′) 2.016(5) Å]. One formate ion [O(5)−C(26)−O(4)] acts as a bridge in a syn-anti fashion connecting Cu(2) and Cu(3) of different units. Thus, four hexanuclear SBUs generate macrocycles (Figure 13, left), which further self-assemble in a series of waved sheets, one of which is sketched in Figure 13 (right). These waved sheets pack parallel each other with no relevant free space between them. Some noncovalent interactions (three “intramolecular” and one “intermolecular” hydrogen bonds) present in compound 5 are shown in Figure S7. Reactions in Solvothermal Conditions. Compound 1a was reacted with bpy also in solvothermal conditions (MeOH, 70 °C) by using different reagent ratios (bpy/1a from 1:1 to 8:1), extensive decomposition processes being observed (see Experimental Section). In all the cases, from remaining mother liquors few green crystals of 3 were obtained.

on an inversion center located at the midpoint of the C(12)− C(12′) bond [symmetry code: (I) −x + 2, −y + 2, −z] between the two pyridyl units of bridging bpy. The Cu3N6 ninemembered ring of each symmetry equivalent trinuclear moiety is far from planarity. The three independent Cu···Cu distances form a scalene triangle [Cu(2)···Cu(1) 3.235(1), Cu(2)···Cu(3) 3.327(1), Cu(3)···Cu(1) 3.404(1) Å] with values falling in the range typical for this kind of compound.11−15 Quite normal are also the bonding μ3-O−Cu distances [Cu(1)−O(1) 2.020(4), Cu(2)−O(1) 1.975(4), Cu(3)−O(1) 1.981(4) Å] as well as the μ3-O distance from the Cu3 plane [0.535(4) Å].11−15 The most relevant bond distances and angles of the hexanuclear moiety are reported in Table S6. All the CuII ions have a square pyramidal geometry, with O(2), O(5), and O(6) in the axial positions of Cu(1), Cu(2), and Cu(3) respectively [Cu(1)−O(2) 2.435(6), Cu(2)−O(5) 2.401(5), Cu(3)−O(6) 2.325(5) Å]. The coordination around Cu(2) is completed by a bpy molecule acting as a monotopic I



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Catalytic Peroxidative Oxidation of Cyclohexane. Compounds [{[Cu 3 (μ 3 -OH)(μ-pz) 3 (HCOO) 2 (H 2 O)][Cu3(μ3-OH)(μ-pz)3(HCOO)2(H2O)2]}2(μC10H8N2)]·6H2O, 2b, and [{Cu3(μ3-OH)(μ-pz)3(HCOO)(OH)(μ-C10H8N2)(C10H8N2)2}2]·C10H8N2, 3, have been tested as catalysts (or catalyst precursors) in the peroxidative oxidation of cyclohexane. The interest in this reaction arises from the fact that copper is a fundamental element in biological systems where, namely, it is present in numerous enzymes that selectively catalyze various oxidation reactions28 including those performed by the still poorly characterized particulate methane monooxygenase (pMMO), a multicopper enzyme present in methanotrophs, that catalyzes alkane hydroxylation and alkene epoxidation.28e,h,i In the last years, we have shown that di-, tri-, tetra-, and polynuclear copper(II) complexes29,30 as well as the trinuclear triangular copper(II) derivatives 1a−j,11c−e can act as remarkably active and selective catalysts (or catalyst precursors) for the peroxidative oxidation of cycloalkanes to the corresponding alcohols and ketones. In this context, particularly interesting is the oxidation of cyclohexane to cyclohexanol and cyclohexanone, under mild conditions, (Scheme 2) due to the relevance of the latter products as intermediates of the industry of polyamides-6 and nylon-6-6.31

S11 and Figure S10). However, an enhancement of the overall TON is detected, reaching a maximum of 109 mol of oxidation products per mol of catalyst for the smallest tested amount of catalyst 3 (0.63 μmol, that is, for the catalyst/cyclohexane molar ratio of 6.3 × 10−4). As observed in other systems containing the trinuclear triangular {Cu3(μ3-OH)(μ-pz)3} core,11d,e the cyclohexane oxidation appears to proceed mostly via radical mechanisms involving both carbon- and oxygen-centered radicals32 on account of the decrease of the catalytic activity when the experiments were carried out in the presence of either a carbon or an oxygen radical trap (CBrCl3 or Ph2NH, respectively). The process conceivably involves11c−e,28,29 the formation of intermediate peroxo- and oxo-radical complexes, and of cyclohexyl hydroperoxide which undergoes decomposition33 to the final organic products, cyclohexanol and cyclohexanone.

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CONCLUSIONS In conclusion, the stability of the trinuclear triangular [Cu3(μ3OH)(μ-pz)3] moiety and the possibility to use it in obtaining 2D and 3D CPs is confirmed. Four novel CPs (2a,b, 4, and 5) and a hexanuclear complex forming a porous 3D supramolecular assembly (3) have been obtained from compound 1a by exchanging monotopic neutral ligands with bpy. While the coordinative flexibility of CuII makes difficult to forecast the structures of the reaction products, on the other hand the use of this particular SBU makes possible achieving a large panel of new products through simple reactions. Actually, the quite stable 3D CP 2 was obtained by using low bpy/1a reaction ratios, while a large excess of bpy led to the formation of the hexanuclear complex 3, a porous material self-assembled simply through relatively weak noncovalent interactions. In this compound two perpendicular hydrophobic channels are present and guest bpy molecules are hosted in one of them, while the other one is empty. This particularly interesting crystal structure seems to be largely dependent on the presence of guest bpy. In fact, the attempt to remove guest bpy under a vacuum and dissolution in protic solvents demolishes the supramolecular porous assembly. Moreover, attempts to recrystallize 3 in MeOH in the absence of an excess of bpy lead also to a partial decomposition of the trinuclear triangular structure, with formation of a 3D CP, obtained through interpenetration of flat porous sheets, (4) and of a waved 2D CP (5). On the other hand, 3 is instead quite stable in several aprotic solvents (where it is insoluble), and by soaking it in these solvents the guest bpy molecules can be removed and the porous crystal structure remains intact. Solvent molecules do not occupy the “holes” left by bpy, but are present in the crystal lattice at the intersection of the perpendicular channels, as a few, disordered molecules. These facts suggest the possibility that 3, in suitable conditions, may act as a sorbent species for gases and other organic or inorganic molecules, a potentially interesting feature from catalytic and environmental points of view. Work is in progress in our laboratories to examine these possibilities. Finally, compounds 2 and 3 are valuable catalyst precursors for the r.t. peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone by aqueous H2O2. Analogously to what we previously observed with other trinuclear triangular CuII derivatives,11c−e,29a,b the catalytic activity and the selectivity depend on the amount of nitric acid, oxidant, and catalyst employed.

Scheme 2

Catalytic tests were carried out on compounds 2b and 3, by performing the reactions in MeCN/H2O medium in the presence of aq. H2O2, at room temperature. They are active under these mild conditions, the activity and selectivity depending on a variety of factors (acidity of the medium, amount of reagents and of the catalyst, etc.) which can be varied toward optimization, as we have observed for other catalytic systems employed in the alkane functionalization.29b,30 Maximum product yields and turnover numbers (TONs) of 33% and 109 mol of products/mol of catalyst precursor were achieved (Tables S9−S11). Effect of the Amount of Nitric Acid. The addition of HNO3 promotes the catalytic activity with a behavior that is somehow similar to that previously observed with other polynuclear CuII derivatives.11c−e,29 Nevertheless, a too high excess of acid is not recommended since for the HNO3/catalyst molar ratio above ca. 20 no further increase of the catalytic activity is observed (Table S9 and Figure S8). Effect of the Amount of Hydrogen Peroxide. The amount of hydrogen peroxide also has a pronounced effect on the yield of the reaction; the obtained data are summarized in Table S10 and in Figure S9. The increase of the n(H2O2)/ n(substrate) ratio up to 5 or 6 (for catalysts 2b or 3, respectively) results, in all the cases, in an enhancement of the total yield, while a further rise of that ratio leads to a yield drop. No oxidation products were detected in the absence of hydrogen peroxide, indicating that molecular oxygen itself is not sufficient to oxidize the cyclohexane to the corresponding alcohol and ketone. Effect of the Catalyst Amount. Decreasing the amount of catalyst leads to a general decrease of the reaction yields (Table J



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(8) Masciocchi, N.; Galli, S.; Alberti, E.; Sironi, A.; Di Nicola, C.; Pettinari, C.; Pandolfo, L. Inorg. Chem. 2006, 45, 9064−9074. (9) (a) Cingolani, A.; Galli, S.; Masciocchi, N.; Pandolfo, L.; Pettinari, C.; Sironi, A. J. Am. Chem. Soc. 2005, 127, 6144−6145. (b) Bencini, A.; Casarin, M.; Forrer, D.; Franco, L.; Garau, F.; Masciocchi, N.; Pandolfo, L.; Pettinari, C.; Ruzzi, M.; Vittadini, A. Inorg. Chem. 2009, 48, 4044−4051. (10) Pandolfo, L. unpublished results. (11) (a) Casarin, M.; Corvaja, C.; Di Nicola, C.; Falcomer, D.; Franco, L.; Monari, M.; Pandolfo, L.; Pettinari, C.; Piccinelli, F.; Tagliatesta, P. Inorg. Chem. 2004, 43, 5865−5876. (b) Casarin, M.; Corvaja, C.; Di Nicola, C.; Falcomer, D.; Franco, L.; Monari, M.; Pandolfo, L.; Pettinari, C.; Piccinelli, F. Inorg. Chem. 2005, 44, 6265− 6276. (c) Di Nicola, C.; Karabach, Y. Yu.; Kirillov, A. M.; Monari, M.; Pandolfo, L.; Pettinari, C.; Pombeiro, A. J. L. Inorg. Chem. 2007, 46, 221−230. (d) Di Nicola, C.; Garau, F.; Karabach, Y. Yu.; Martins, L. M. D. R. S.; Monari, M.; Pandolfo, L.; Pettinari, C.; Pombeiro, A. J. L. Eur. J. Inorg. Chem. 2009, 666−676. (e) Contaldi, S.; Di Nicola, C.; Garau, F.; Karabach, Y. Yu.; Martins, L. M. D. R. S.; Monari, M.; Pandolfo, L.; Pettinari, C.; Pombeiro, A. J. L. Dalton Trans. 2009, 4928−4941. (f) Pettinari, C.; Masciocchi, N.; Pandolfo, L.; Pucci, D. Chem.Eur. J. 2010, 16, 1106−1123. (12) Casarin, M.; Cingolani, A.; Di Nicola, C.; Falcomer, D.; Monari, M.; Pandolfo, L.; Pettinari, C. Cryst. Growth Des. 2007, 4, 676−685. (13) Di Nicola, C.; Garau, F.; Gazzano, M.; Monari, M.; Pandolfo, L.; Pettinari, C.; Pettinari, R. Cryst. Growth Des. 2010, 10, 3120−3131. (14) (a) For a comprehensive review on trinuclear triangular systems see: Zangrando, E.; Casanova, M.; Alessio, E. Chem. Rev. 2008, 108, 5979−5013. See also (b) Rivera-Carrillo, M.; Chakraborty, I.; Mezei, G.; Webster, R. D.; Raptis, R. G. Inorg. Chem. 2008, 47, 7644−7650. (c) Zueva, E. M.; Petrova, M. M.; Herchel, R.; Trávníček, Z.; Raptis, R. G.; Mathivathanan, L.; McGrady, J. E. Dalton Trans. 2009, 5924− 5932. (15) Rivera-Carrillo, M.; Chakraborty, I.; Raptis, R. G. Cryst. Growth Des. 2010, 10, 2606−2616. (16) Manuscripts reporting the results obtained in the reactions of compounds 1b−j with bpy are currently in preparation. (17) Senko, M. W. IsoPro Isotopic Abundance Simulator, v. 2.1: National High Magnetic Field Laboratory, Los Alamos National Laboratory: Los Alamos, NM, 1994. (18) (a) Shul’pin, G. B. J. Mol. Catal. A: Chem. 2002, 189, 39−66. (b) Shul’pin, G. B. C. R. Chim. 2003, 6, 163−178. (19) SMART & SAINT Software Reference Manuals, version 5.051 (Windows NT Version); Bruker Analytical X-ray Instruments Inc.: Madison, WI, 1998. (20) Sheldrick, G. M. SADABS, Program for Empirical Absorption Correction; University of Göttingen: Germany, 1996. (21) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (22) Sheldrick, G. M. SHELXTLplus (Windows NT Version) Structure Determination Package, Version 5.1; Bruker Analytical X-ray Instruments Inc.: Madison, WI, USA, 1998. (23) Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.; Taylor, R.; Towler, M.; van de Streek, J. J. Appl. Crystallogr. 2006, 39, 453−457. (24) Even though in ESI-MS conditions aggregation phenomena, leading to oligomeric species, may occur, the presence of trinuclear clusters in ESI spectra offers valuable indications to suggest the structure of the examined compounds. See for example: (a) Fujita, M.; Ibukuro, F.; Hagihara, H.; Ogura, K. Nature 1994, 367, 720−723. (b) Colton, R.; D’Agostino, A.; Traeger, J. C. Mass Spectrom. Rev. 1995, 14, 79−106. (c) Hop, C. E. C. A.; Bakhtiar, R. J. Chem. Educ. 1996, 73, A162−A169. (d) Favaro, S.; Pandolfo, L.; Traldi, P. Rapid Commun. Mass Spectrom. 1997, 11, 1859−1866. (e) Bonnington, L. S.; Coll, R. K.; Gray, E. J.; Flett, J. I.; Henderson, W. Inorg. Chim. Acta 1999, 290, 213−221. (f) Cingolani, A.; Effendy; Pellei, M.; Pettinari, C.; Santini, C.; Skelton, B. W.; White, A. H. Inorg. Chem. 2002, 41, 6633−6645.

ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files for 2a, 2b, 3, 3b, 3t, 3ch, 4, and 5 in CIF format; figures showing: (i) the molecular structure and the 1D tape of 2a, (ii) intra- and intermolecular hydrogen bonds of 2b, (iii) packing diagram of 3, (iv) X-ray diffraction plots of compound 3 (as prepared and after treatment under a vacuum), (v) crystal packing of 4, (vi) intra- and intermolecular hydrogen bonds of 5, (vii) the effect of the amounts of nitric acid, hydrogen peroxide, and catalyst on the peroxidative oxidation of cyclohexane; tables reporting: (i) selected bond lengths and angles for 2a, 2b, 3, 4, and 5, (ii) the crystal data and structure refinement for 2a, 3b, 3t, and 3ch, (iii) the effect of the amounts of nitric acid, hydrogen peroxide, and catalyst on the peroxidative oxidation of cyclohexane. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Project PRAT 2009 “Design, synthesis and characterization of coordination polymers by assembling of oligo-nuclear metal systems and polytopic ligands” of the University of Padova, and by the Foundation for Science and Technology (FCT), Portugal (Projects PTDC/QUI-QUI/102150/2008 and PEst-OE/QUI/ UI0100/2011). F.G. acknowledges a grant from the “Aldo Gini” Foundation to support a research period at Lisbon. C.P. and C.D.N. thank the University of Camerino for a grant to C.D.N. and M. M. acknowledges the University of Bologna for financial support.

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

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REFERENCES

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