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The Different Supramolecular Arrangements of the Triangular [Cu3(µ3-OH)(µ-pz)3]2+ (pz ) Pyrazolate) Secondary Building Units. Synthesis of a Coordination Polymer with Permanent Hexagonal Channels

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 676-685

Maurizio Casarin,§ Augusto Cingolani,† Corrado Di Nicola,† Daniele Falcomer,# Magda Monari,*,‡ Luciano Pandolfo,*,§ and Claudio Pettinari*,† Department of Chemical Sciences, UniVersity of PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy, Department of Chemical Sciences, Via S. Agostino 1, UniVersity of Camerino, I-62032 Camerino (MC), Italy, Scientific and Technologic Department, UniVersity of Verona, Ca’Vignal, Strada Le Grazie 15, I-37143 Verona, Italy, and Department of Chemistry “G. Ciamician”, UniVersity of Bologna, Via Selmi 2, I-40126 Bologna, Italy ReceiVed July 27, 2006; ReVised Manuscript ReceiVed December 4, 2006

ABSTRACT: By reaction of the trinuclear triangular copper(II) complex [Cu3(µ3-OH)(µ-pz)3(MeCOO)2(Hpz)] (Hpz ) pyrazole), 1b, with aqueous HCl, four different crystalline species were formed and recovered through fractional crystallization. In order, the hexanuclear dicationic [{Cu3(µ3-OH)(µ-pz)3(Hpz)2}2(µ-MeCOO)2](Cl)2‚2H2O, 2, the mononuclear [CuCl2(Hpz)4], 3, the heptanuclear neutral [{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}2{CuCl2(Hpz)2}], 4, and the hexanuclear neutral [{Cu3(µ3-OH)(µ-pz)3(Cl)(Hpz)3}2(µ-Cl)2]‚H2O, 5, complexes were obtained. Compounds 2, 4, and 5 all maintain the [Cu3(µ3-OH)(µ-pz)3]2+ core; nevertheless, they exhibit relevant differences in their molecular structures and supramolecular arrangements. In compound 2 a hexanuclear cluster, based on two monodentate acetate groups bridging two [Cu3(µ3-OH)(µ-pz)3(Hpz)2] units and clearly reminiscent of the structure of 1b, was observed, while the sequential replacement of the acetates by chloride ions generated 4 and 5. Although these two compounds were formed according to the same stoichiometry, they are characterized by very different molecular and supramolecular structures. The hexanuclear species 5 arranged, through hydrogen bonds, into a 3D, nonporous metal-organic framework (MOF), while the heptanuclear species 4 self-assembled through Cu-Cl bridges, giving rise to a 3D MOF having permanent hexagonal, star-shaped, parallel channels. The internal free diameter of these channels is about 4 Å, leading to a free space corresponding to ca. 9% of the total crystal volume. Introduction The obtainment of molecular-based metal-organic frameworks (MOFs) having porous structures is an attractive goal with numerous potential applications in adsorption, gas storage, molecular recognition, heterogeneous catalysis, etc.1 Despite the fact that some strategies concerning rational syntheses of MOFs mimicking zeolites have been proposed,2 serendipity still plays a relevant role in the preparation of molecular-based tubular coordination complexes. In particular, reaction conditions such as solvent and temperature, besides electronic and structural properties of the reagents, are of paramount importance whenever MOFs are obtained through the self-assembly of secondary building units (SBUs). Furthermore, the nature of the involved metal(s) is of some value, being that it is relatively easier to predict the supramolecular behavior of coordinatively “stiff” ions, such as Cu(I) or Pt(II) compared with that of coordinatively “elastic” species, such as Cu(II). In this context, we have recently reported that trinuclear triangular copper(II) species [Cu3(µ3-OH)(µ-pz)3(RCOO)2LxL′y], 1a-d (R ) H (a), Me (b), Et (c), Pr (d); L, L′ ) H2O, MeOH, EtOH, Hpz; Hpz ) pyrazole), with the same [Cu3(µ3-OH)(µpz)3]2+ core, are formed in one-step one-pot syntheses, when* To whom correspondence should be addressed. E-mail addresses: [email protected] (M.M.); [email protected] (L.P.); [email protected] (C.P.). 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.). § University of Padova. † University of Camerino. # University of Verona. ‡ University of Bologna.

ever copper(II) carboxylates are mixed with Hpz in water or alcohols.3 This appears to be a general behavior, as also witnessed by preliminary outcomes of the reactions of other copper(II) carboxylates with Hpz,4 thus establishing a general rational procedure for the preparation of such trinuclear clusters. On the other hand, these SBUs (having different R, L, and L′) give rise, through spontaneous self-assembling processes, to different 1D or 2D MOFs. Particularly, 1a forms a 1D MOF through syn-anti bridging formate ions, whereas in the other three cases two asymmetrically bridging monodentate carboxylates give rise to hexanuclear species, which for R ) Me or Et (1b,c), further self-assemble in more complex ways.3b Even the role played by the solvent deserves to be stressed, as evidenced by the formation of a completely different 1D MOF, [{Cu(µpz)2}n], when the reaction between Cu(RCOO)2 and Hpz was carried out in MeCN, independently of R.5 In Chart 1, the structures of 1a-d trinuclear SBUs and a fragment of the 1D MOF [{Cu(µ-pz)2}n] are schematized. Since the [Cu3(µ3-OH)(µ-pz)3]2+ core of 1a-d is a relatively stable fragment,3a we have tried to break off the supramolecular assembly of these compounds, maintaining unchanged the triangular moiety, aiming at different rearrangements of the “free” SBUs with formation of different MOFs that would be independent from the carboxylate ions. Here we report the first results of this study. Experimental Section Materials and Methods. All chemicals were purchased from Aldrich and used without further purification. All reactions and crystallization were carried out in the air. Compound 1b was synthesized as previously reported.3a Elemental analyses (C, H, N) were performed with a Fisons

10.1021/cg060501h CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007

Honeycomb Channels in a Self-Assembled Cu(II) MOF Chart 1

Instruments 1108 CHNS-O elemental analyzer. A Perkin-Elmer 1100 B atomic absorption spectrometer has been used for quantitative determination of copper. IR spectra were recorded from 4000 to 100 cm-1 with a Perkin-Elmer system 2000 FT-IR instrument. The electrical conductances of methanol solutions of 2, 3, and 4 were measured with a Crison CDTM 522 conductimeter at room temperature. Positive electrospray mass spectra were obtained with a Series 1100 MSI detector HP spectrometer, using MeOH as mobile phase. Solutions for electrospray ionization mass spectrometry (ESI-MS) were prepared using reagent grade methanol, and obtained data (masses and intensities) were compared with those calculated by using the IsoPro isotopic abundance simulator.6 Peaks containing copper(II) ions are identified as the centers of 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 1/2 equation µeff ) 2.84(Xcorr m T) . Reflectance solid-state and solution UV-vis spectra were recorded on a Varian Cary 5E spectrophotometer, equipped with a device for reflectance measurements. Computational Details. Density functional (DF) calculations have been carried out by using the Amsterdam Density Functional (ADF) 2005 package.7 Optimized geometries have been evaluated by employing generalized gradient (GGA) corrections self-consistently included through the Becke-Perdew (BP) formula.8 A triple-ζ Slater-type basis set was used for (i) Cu atoms, (ii) all the atoms directly bonded to them, and (iii) the hydroxyl and Hpz hydrogen atoms, whereas a double-ζ basis set was adopted for the remaining atoms of the complex. The inner cores of Cu (1s2s2p), O (1s), C (1s), and N (1s) atoms have been kept frozen throughout the calculations. All the numerical experiments have been carried out by including spin-polarization effects. Syntheses. A suspension of 1.731 g of 1b (2.9 mmol) in 70 mL of water was treated under vigorous stirring with 100 mL of aqueous 0.1 M HCl. The mixture was stirred for about 2 h, and a little remaining solid was filtered off, obtaining a clear blue solution that was concentrated under vacuum until about 40 mL and then let to slowly evaporate in the air. In about 1 month, several fractions of crystals formed and were separately collected. After elimination of those containing mixtures of compounds, four different fractions were obtained whose purity was assessed through comparison of their XRPD spectra with the simulated ones obtained from X-ray single-crystal data (Vide infra). In the order of their obtainment, the four fractions were identified as [{Cu3(µ3-OH)(µ-pz)3(Hpz)2}2(µ-MeCOO)2](Cl)2‚2H2O, 2 (112 mg, 0,085 mmol, 5.9% yield); [CuCl2(Hpz)4], 3 (450 mg, 1.11 mmol, 12.8% yield); [{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2}2][CuCl2(Hpz)2], 4 (210 mg, 0.14 mmol, 11.3% yield); and [{Cu3(µ3-OH)(µ-pz)3(Cl)(µ-Cl)(Hpz)3}2‚H2O], 5 (25 mg, 0.018 mmol, 1.2% yield).

Crystal Growth & Design, Vol. 7, No. 4, 2007 677 Mother liquors were taken to dryness, resulting in a green-blue solid, which we were unable to purify further. In the collected vapors, acetic acid and Hpz were detected. [{Cu3(µ3-OH)(µ-pz)3(Hpz)2}2(µ-MeCOO)2](Cl)2‚2H2O, 2: Mp 257259 °C. Anal. (%) Calcd for C34H42Cl2Cu6N20O6: C, 31.05; H, 3.53; Cu, 28.99; N, 21.30. Found: C, 31.66; H, 3.10; Cu, 28.8; N, 21.74. IR (Nujol, cm-1): 3400 br, 3340 br (NH), 3154 m, 3113 m, 2127 br, 1616 sh, 1573 s, 1489 m, 1480 m, 1400 s, 509 m, 488 m, 459 w, 419 br, 400 w, 353 s, 326 m, 313 w, 303 w, 273 w, 254 w, 234 m, 225 m, 213 m, 208 m. µeff (295 K): 4.45 µB. ESI-MS (+) (MeOH) (higher peaks, relative abundance %): 69 (80) [pzH2]+, 199 (25) [Cu(pz)(Hpz)]+, 258 (100) [Cu(MeCOO)(Hpz)2]+, 476 (25) [Cu3(pz)4(OH)]+; 512 (25) [Cu3(pz)3Cl(OH)(Hpz)]+; 526 (15) [Cu3(pz)5]+; 562 (28) [Cu3(pz)4Cl(Hpz)]+; 594 (58) [Cu3(pz)5(Hpz)]+, 1012 (20) [Cu6(MeCOO)3(pz)6(OH)2(H2O)]+. Λm (CH3OH, 1.7 × 10-3 M) ) 53 Ω-1 cm2 mol-1. [CuCl2(Hpz)4], 3: Mp 233-235 °C. Anal. (%) Calcd for C12H16CuCl2N8: C, 35.43; H, 3.96; Cu, 15.62; N, 27.55. Found: C, 35.40; H, 4.20; Cu, 15.0; N, 27.98. IR (Nujol, cm-1): 3320 br, 3241 br, 3209 br (NH), 3111 m, 1630 br, 1521 m, 1511 m, 600 s, 398 w, 384 w, 375 w, 352 w, 325 w, 302 sh, 287 s br, 274 s br, 265 sh, 224 m, 205 m. µeff (295 K): 1.88 µB. ESI-MS (+) (MeCN) (higher peaks, relative abundance %): 302 (100) [Cu(Hpz)3Cl]+. Λm (CH3OH, 1.7 × 10-3 M) ) 63 Ω-1 cm2 mol-1. [{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}2{CuCl2(Hpz)2}], 4: Mp 260-262 °C. Anal. (%) Calcd for C36H48Cl6Cu7N24O4: C, 28.18; H, 3.14; Cu, 28.91; N, 21.96. Found: C, 28.36; H. 3.12; Cu, 29.0; N, 21.96%. IR (Nujol, cm-1): 3400 br, 329 4br (NH + OH), 3137 m (NH), 1640 br, 1524 w, 764 m, 754 m, 741 m, 624 s, 596 s, 452 br, 367 m, 353 m, 317 w, 298 w br, 283 w br, 243 m. µeff (295 K): 4.068 µΒ. ESI-MS (+) (MeOH) (higher peaks, relative abundance %): 69 (95) [pzH2]+; 199 (90) [Cu(pz)(Hpz)]+; 258 (100) [Cu(MeCOO)(Hpz)2]+; 474 (65) [Cu3(pz)4(OH)]+; 512 (70) [Cu3(pz)3Cl(OH)(Hpz)]+; 526 (50) [Cu3(pz)5]+; 562 (75) [Cu3(pz)4Cl(Hpz)]+; 594 (30) [Cu3(pz)5(Hpz)]+; Λm (CH3OH, 1.7 × 10-3 M) ) 42 Ω-1 cm2 mol-1. [{Cu3(µ3-OH)(µ-pz)3(Cl)(Hpz)3}2(µ-Cl)2]‚H2O, 5: Mp 268-270 °C. Anal. (%) Calcd for C36H46Cl4Cu6N24O3: C, 31.20; H, 3.34; Cu, 27.51; N, 24.25%. Found: C, 31.33; H. 3.12; Cu, 29.0; N, 24.36%. IR (Nujol, cm-1): 3400 sh, 3326 br, 3232 s br, 3113 m, 1624 w br, 1523 w, 1513 w, 776 m, 756 m, 593 w, 574 w, 519 w, 449 br, 420 br, 353 s, 82 w, 227 m, 224 m. ESI-MS (+) (MeOH) (higher peaks, relative abundance %): 69 (100) [pzH2]+; 199 (80) [Cu(pz)(Hpz)]+; 474 (55) [Cu3(pz)4(OH)]+; 512 (90) [Cu3(pz)3Cl(OH)(Hpz)]+; 526 (55) [Cu3(pz)5]+; 562 (92) [Cu3(pz)4Cl(Hpz)]+; 594 (58) [Cu3(pz)5(Hpz)]+. X-ray Crystallography. Single-crystal X-ray intensity data were measured on a Bruker SMART 2000 CCD area detector diffractometer for 2 and 4 and on a Bruker SMART Apex II CCD area detector diffractometer for 5. 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. For all crystals, a full sphere of reciprocal space was scanned by 0.3° ω steps. The software SMART9 was used for collecting frames of data, indexing reflections, and determining lattice parameters. The collected frames were then processed for integration by the SAINT program,9 and an empirical absorption correction was applied using SADABS.10 The structures were solved by direct methods (SIR 97)11 and subsequent Fourier syntheses and refined by full-matrix least-squares on F2 (SHELXTL),12 using anisotropic thermal parameters for all non-hydrogen atoms. All hydrogen atoms, except the pyrazole NH and hydroxy hydrogens, which were located in the Fourier map and refined isotropically, were added in calculated positions, included in the final stage of refinement with isotropic thermal parameters, U(H) ) 1.2Ueq(C) [U(H) ) 1.5Ueq(C-Me)], and allowed to ride on their carrier carbons. Two disordered crystallization water molecules were found in the unit cell of 2, whereas in 5 one water molecule is statistically distributed over two symmetry-related locations between the two [Cu3(µ3-OH)(µ-pz)3(Cl)(Hpz)3]+ units doubly bridged by two chlorides (Vide infra). Crystal data and details of data collection for compounds 2, 4, and 5 are reported in Table 1. X-ray powder diffraction investigations were carried out by means of a PANalytical X’Pert Pro diffractometer equipped with an X’Celerator fast detector and using Cu KR radiation.

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Table 1. Crystal Data and Experimental Details for [{Cu3(µ3-OH)(µ-pz)3(Hpz)2}2(µ-MeCOO)2](Cl)2‚2H2O, 2, [{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}2{CuCl2(Hpz)2}], 4, and [{Cu3(µ3-OH)(µ-pz)3(Cl)(Hpz)3}2(µ-Cl)2]‚H2O, 5 compound formula FW T, K λ, Å crystal symmetry space group a, Å b, Å c, Å R, deg β, deg γ, deg cell volume, Å3 Z Dc, Mg m-3 µ(Mo KR), mm-1 F(000) crystal size, mm3 θ limits, deg reflns collected unique obsd reflns [Fo > 4σ(Fo)] GOF on F2 R1(F)a, wR2(F2)b largest diff. peak and hole, e Å-3 a

2 C34H42Cl2Cu6N20O6 ‚2H2O 1315.05 293(2) 0.710 73 triclinic P1h (No. 2) 8.889(2) 8.910(2) 16.624(4) 79.859(6) 76.747(6) 66.467(6) 1169.7(5) 1 1.867 2.865 662 0.12 × 0.15 × 0.23 2.53-24.99 6970 ((h, (k, (l) 3755 [R(int) ) 0.0612] 1.052 0.0883, 0.2052 1.750 and -1.442

4 C36H48Cl6Cu7N24O4 1538.46 100(2) 0.710 73 trigonal R3h (No. 148) 32.743(2) 32.743(2) 13.8177(7) 90 90 120 12829(1) 9 1.834 2.907 7101 0.22 × 0.25 × 0.30 2.98-26.98 46 236 ((h, (k, (l) 6211 [R(int) ) 0.1020] 1.020 0.0592, 0.1633 2.035 and -1.116

5 C36H44Cl4Cu6N24O2‚H2O 1386.01 293(2) 0.710 73 tetragonal P4h21m (No. 113) 16.8267(6) 16.8267(6) 8.9583(6) 90 90 90 2536.4(2) 2 1.815 2.747 1392 0.20 × 0.23 × 0.30 1.71-28.71 28 804 ((h, (k, (l) 3300 [R(int) ) 0.0338] 1.069 0.0273, 0.0749 1.298 and -0.822

R1 ) ∑||Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2, where w ) 1/[σ2(Fo2) + (aP)2 + bP] where P ) (Fo2 + 2Fc2)/3.

Molecular graphics were generated using Mercury13 and Schakal14 software. Color codes for all molecular graphics are as follows: yellow (Cu), red (O), blue (N), green (Cl), gray (C), white (H).

Results and Discussion It is possible that the diverse molecular and supramolecular assemblies of compounds 1a-d, mainly determined by different carboxylates, which bridge the triangular moieties in different ways,3 influence their reactivity. For this reason, we started our study with one compound, 1b, and performed on it density functional calculations with the aim to find the possible target(s) of an electrophilic attack. With regard to this it is to be pointed out that, despite compound 1b being indicated as having a trinuclear formulation, [Cu3(µ3-OH)(µ-pz)3(MeCOO)2(Hpz)], it is actually better described as the hexanuclear species [{Cu3(µ3-OH)(µ-pz)3(µ-MeCOO)(Hpz)}2 (µ-MeCOO)2], where two trinuclear units are connected through two monodentate acetate asymmetric bridges.3 Accordingly, ADF calculations have been performed on this hexanuclear cluster, assuming spin state S ) 1, which was found to be more stable than the S ) 0 one by ca. 5 kcal/mol. As far as the possible targets of an electrophilic attack are concerned, Figure 1a, where a partial 3D view15 of the negative electrostatic potential is displayed, ultimately testifies that the preferential sites of such an attack are (i) the acetate groups bridging the hexanuclear units and (ii) the “free” O atom of the acetate group bridging two trinuclear moieties. A partial sketch of the supramolecular arrangement of 1b3b is shown in Figure 1b where the targets of the electrophilic attacks of H+, with reference to the central hexanuclear unit, are evidenced. On this basis, we planned the breaking of the supramolecular assembly of 1b by allowing it to react with a strong acid. By treatment of 1b with an aqueous solution of HCl (1b/ HCl ) ca. 1:3), the initial suspension changed into a blue solution, which by slow evaporation in the air led to four different crystal fractions. The resulting compounds contain Cu(II), MeCOO-, Cl-, pz-, Hpz, and H2O in variable amounts and are characterized by different molecular structures and supramolecular arrangements. In detail, in order of crystallization, we recovered (i) blue platelets of the hexanuclear derivative

Figure 1. (a) A partial 3D map of the negative electrostatic potential of 1b. The indicated isopotential surfaces correspond to -0.05 au. (b) Part of the supramolecular structure of 1b evidencing the oxygen atom targets of the H+ attacks.

[{Cu3(µ3-OH)(µ-pz)3(Hpz)2}2(µ-MeCOO)2](Cl)2‚2H2O, 2, (ii) dark blue blocks of the mononuclear compound [CuCl2(Hpz)4], 3, (iii) emerald green elongated hexagonal platelets of the heptanuclear species [{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}2{CuCl2(Hpz)2}], 4, and (iv) a small amount of green-blue needles of the hexanuclear derivative [{Cu3(µ3-OH)(µ-pz)3(Cl)-

Honeycomb Channels in a Self-Assembled Cu(II) MOF

Crystal Growth & Design, Vol. 7, No. 4, 2007 679 Scheme 1a

a Reactions conditions: compound 1b partially dissolved in water; aqueous 0.1 M HCl added with stirring (HCl/1b = 3). Required stoichiometric HCl/1b ratios: (i) 8:3; (ii) 6:1; (iii) 10:3; (iv) 10:3.

Figure 2. Molecular structure of 2. Crystallization water molecules are omitted for clarity. Symmetry code: (I) -x + 1, -y, -z + 1. Hydrogen bonds and weak Cu-Cl interactions are indicated as dashed lines.

(Hpz)3}2(µ-Cl)2]‚H2O, 5, in a comprehensive isolated yield of about 31%, based on starting copper (see Experimental Section and Scheme 1). Apart from the mononuclear complex 3,16 whose formation is clearly due to an extensive decomposition of 1b, the presence of the other polynuclear compounds confirms the relatively high stability of the [Cu(µ3-OH)(µ-pz)3]2+ trinuclear triangular moiety. An X-ray single-crystal structure determination carried out on compound 2, the first isolated crystalline product, shows that in the asymmetric unit a trinuclear [Cu3(µ3-OH)(µ-pz)3(Hpz)2(MeCOO)](Cl) moiety is present. The molecular structure of 2 can be better described as a hexanuclear complex (Figure 2) formed by two trinuclear moieties joined through two monodentate asymmetrically bridging acetate groups. An inversion center sitting in the middle of the Cu2O2 diamond generates the second trinuclear unit. This arrangement is similar to that found in 1b3b and suggests that 2 is, very likely, the first product of the HCl attack.

As a matter of fact, the analysis of Figure 1 suggests that compound 2 may be formed through an electrophilic attack of HCl that might break off the supramolecular arrangement of 1b, releasing acetic acid, without affecting the integrity of the hexanuclear units. In 2, four Hpz molecules (two of which derived from partial decomposition of 1b) are terminally coordinated to the four Cu(2) and Cu(3) ions of the resulting doubly charged hexanuclear cation, while the charge is balanced by two chloride ions. Each chloride is placed at the same distance from Cu(2) and Cu(3) [Cu(2)-Cl 2.977(4) Å, Cu(3)Cl 3.029(4) Å] between two Hpz and weakly interacts with both H-N through symmetrical hydrogen bonds (Cl‚‚‚N(8) ) 3.19(1) Å, Cl‚‚‚H(81)-N(8) ) 143°). In each trinuclear moiety, the chemically equivalent Cu(2) and Cu(3) ions adopt a squareplanar geometry, being coordinated by two nitrogens of two different pz, one nitrogen from the terminal Hpz, and the hydroxy oxygen. Taking into account the symmetrically interacting Cl- counterion, both Cu(2) and Cu(3) adopt a squarepyramidal coordination with the chloride occupying the common apical position. It is therefore difficult to classify the chloride as a “true” counterion or a weakly coordinated ligand placed in the axial position of a square pyramid. The value of the molar conductivity in MeOH solution and the extended fragmentation observed in the ESI mass spectrum (see Experimental Section) suggest that compound 2 likely exists in solution as an ionic species or that partial dissociation occurs, likely upon breaking of the Cu-Cl or Cu-N bond or both. On the other hand, similar values of Λm and extended fragmentation in ESI mass spectra have been observed also for neutral compounds 3 and 4 (see Experimental Section). The third copper atom of the trinuclear unit, Cu(1), shows a square-pyramidal geometry, with the base defined by the nitrogens belonging to two different pz units [N(1) and N(6)], µ3-O(1)H(1), and the bridging acetate oxygen, O(2) [Cu(1)O(2) 1.962(7) Å]. The axial position is occupied by O(2′) [Cu(1)-O(2′) 2.422(9) Å] which, in turn, is bonded to the second trinuclear moiety. An accurate inspection of the bridging carboxylates connecting the two trinuclear units, further reinforces the hypothesis that compound 2 is the first product of

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Table 2. Geometrical Parameters Related to the Carboxylate Monodentate Asymmetric Bridges of Compounds 2, 1b, and 1d

2, R ) R′ ) Me 1b,3a R ) R′ ) Me 1d,3b R ) R′ ) Pr

C1-O2 (Å)

C1-O3 (Å)

Cu1-O2 (Å)

Cu1′-O2 (Å)

O3‚‚‚O1′ (Å)

O3‚‚‚H1′-O1′ (deg)

1.25 (1) 1.272(5) 1.279(4)

1.21(1) 1.224(5) 1.250(5)

1.962(7) 1.966(2) 1.986(3)

2.422(9) 2.377(3) 2.399(3)

2.61(1) 2.677(4) 2.763(4)

169 164 162

Table 3. Selected Bonding and Nonbonding Distances (Å) and Angles (deg) for 2 Cu(1)-O(1) Cu(2)-O(1) Cu(3)-O(1) Cu(1)-N(1) Cu(1)-N(6) Cu(2)-N(2) Cu(2)-N(3) Cu(2)-N(7) Cu(3)-N(4) Cu(1)-O(2)

1.970(7) 1.988(7) 1.962(7) 1.925(9) 1.953(10) 1.935(8) 1.935(10) 1.988(10) 1.946(10) 1.962(7)

Cu(3)-N(5) Cu(3)-N(9) Cu(1)-O(2′)a Cu(2)-Cl Cu(3)-Cl Cu(1)‚‚‚Cu(2) Cu(2)‚‚‚Cu(3) Cu(1)‚‚‚Cu(3) Cu(1)‚‚‚Cu(1′)a

1.950(3) 2.006(11) 2.422(9) 2.977(4) 3.029(4) 3.354(2) 3.194(2) 3.348(2) 3.421(3)

Cu(1)-O(1)-Cu(2) Cu(1)-O(1)-Cu(3) Cu(2)-O(1)-Cu(3) N(1)-Cu(1)-N(6) Cu(1)-O(2)-Cu(1′)a

115.9(4) 116.8(4) 107.9(3) 163.0(4) 102.1(3)

N(2)-Cu(2)-N(3) N(2)-Cu(2)-N(7) N(4)-Cu(3)-N(5) N(4)-Cu(3)-N(9)

170.8(4) 93.1(4) 169.5(4) 90.7(4)

a

Symmetry code: (I) -x + 1, -y, -z + 1.

Figure 3. Molecular structure of 4. Symmetry code: (I) -x + 1, -y, -z + 2. Relatively weak Cu-Cl interactions are indicated as dashed lines.

the HCl attack. In Table 2 are reported some geometrical parameters concerning the monatomic asymmetric bridges of carboxylates found in compounds 2, 1b, and 1d,17 witnessing the structural analogies present in these compounds. Particularly, besides the very close values of Cu-O and C-O bond distances, it should be stressed that in compound 2 the noncoordinating acetate oxygen [O(3)] is involved in a strong intramolecular hydrogen bond with the µ3-OH hydrogen of the other trinuclear fragment, analogous to what is observed in 1b and 1d. Consistently, the hydrogen bond parameters are almost identical.3a Figures showing the structural analogies among compounds 2, 1b, and 1d are supplied as Supporting Information. The other structural features of 2, reported in Table 3, are in the range found in other trinuclear triangular Cu(II) clusters3 and do not deserve further comment. Its crystal packing shows the presence of two crystallization water molecules in the unit cell and evidences that each hexanuclear unit is isolated from the other ones and not involved in any significant interaction,

even with crystallization water. Moreover, a space-filling view down the crystallographic a-axis reveals the existence of very narrow channels presenting an irregular section (see Supporting Information). The solid-state IR spectrum of 2 is in agreement with the above-reported findings, because it exhibits asymmetric and symmetric ν(COO) stretchings at 1573 and 1400 cm-1, respectively, corresponding to a ∆ value close to those previously reported for compounds having bridging carboxylate ligands.18 It should be noted that almost identical ν(COO) values were found in the IR spectrum of 1d (1570 and 1403 cm-1)3b further reinforcing the similarity between the two structures. Moreover, a number of signals between 500 and 400 cm-1, attributable to Cu-O stretching modes, are present, while no bands assignable to Cu-Cl have been observed, as expected for an ionic or a very weak covalent interaction. The presence of coordinated Hpz and water molecules is confirmed by O-H and N-H stretching signals in the region 3400-3200 cm-1 accompanied by rocking (800 cm-1) and twisting and wagging modes (550500 cm-1).19 Finally, a broad absorption at ca. 2100 cm-1 is present. This band disappears when the carboxylate is removed from 2, and we can tentatively assign it to the O-H‚‚‚O strong hydrogen bond between the µ3-OH of a trinuclear core and the noncoordinating acetate oxygen of the other trinuclear fragment. The room-temperature magnetic susceptibility value of 2 corresponds to 2.225 µB for each trinuclear unit, a lower value than that expected for three independent copper(II) ions, indicating some kind of exchange coupling, analogous to what was previously observed for compound 1b.3a ESI MS spectra carried out on compound 2, as well as those of 4 and 5 (see Experimental Section) dissolved in MeOH and H2O, confirm the existence, at least partial, of the trinuclear cores in these solvents and, in some cases, also the formation of stable hexanuclear islands. However, it should be noted that under the experimental condition, 2 (as well as 4 and 5) fragments extensively, yielding a number of possible building blocks for the formation of different aggregates. The second crop of isolated crystals has been identified as the known mononuclear compound 3, as shown by elemental analysis and comparison of the elementary unit cell parameters, which are identical to those already reported.16 In the IR spectrum, a strong broad absorption at ca. 287 cm-1 could be ascribed to the Cu-Cl stretching.20 Room-temperature magnetic susceptibility of 3 (1.88 µB) is in the range expected for a mononuclear copper(II) molecule. The ESI MS spectrum is also consistent with a mononuclear species. With continuing fractional crystallization, it was possible to isolate emerald green elongated hexagonal platelets of 4, which have been investigated by an X-ray single-crystal determination. The molecular structure of 4 consists of two trinuclear units of [Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)] weakly “bridged” by the copper atom of a [CuCl2(Hpz)2] fragment, which sits on a crystallographic center of symmetry. The unprecedented heptanuclear Cu(II) cluster thus formed is shown in Figure 3.

Honeycomb Channels in a Self-Assembled Cu(II) MOF

Crystal Growth & Design, Vol. 7, No. 4, 2007 681

Table 4. Structural Data of the Octahedral Assemblies of Indicated Compounds compound [CuCl2(Hpz)2(Cl)2] fragment in 4 [{CuCl2(pyridine)2}n]24 [{CuCl2(4-bromopyrazole)2}n]25

Cu-ClA (short) (Å) 2.322(1) 2.299(2) 2.342(2)

Cu-ClB (long) (Å) 3.017(1) 3.026(2) 2.950(3)

Even though DFT results (see Figure 1) suggest that the monodentate bridging acetate of 1b also can be protonated and then released as acetic acid, it is very difficult to propose a rational pathway leading to the formation of 4. In any case, the trinuclear [Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)] moieties can be considered as generated by the [Cu3(µ3-OH)(µ-pz)3]2+ SBU fragment and two Hpz, one H2O, and two Cl- coordinated to it. Both chloride ions are bonded to the same copper, Cu(2), [Cu(2)-Cl(1) 2.450(2) Å, Cu(2)-Cl(2) 2.529(4) Å], which adopts a rather distorted square-pyramidal coordination with the pz nitrogens [N(2) and N(3)], µ3-OH, and Cl(1) in equatorial sites and Cl(2) in axial position. Also Cu(3) has a distorted square-pyramidal geometry, a water molecule being in the axial position [Cu(3)-O(2w) 2.444(8) Å]. The third copper atom [Cu(1)] has a square-planar environment, being coordinated by a terminal Hpz besides the µ3-OH oxygen and the pz nitrogens. In addition to that, Cu(1) weakly interacts with a chlorine atom of the central CuCl2(Hpz)2 unit [Cu(1)‚‚‚Cl(3) 3.110(2) Å]. Therefore, the coordination of the central mononuclear squareplanar CuCl2(Hpz)2 unit, in which a copper atom is sitting on the inversion center of the heptanuclear assembly joining together the two trinuclear units, is more complex. Actually, two chlorine atoms [Cl(1), one for each trinuclear moiety] establish weak interactions with the inner Cu(4) [Cu(4)‚‚‚Cl(1) 3.017(1) Å], which thus achieves a distorted octahedral coordination.21 Interestingly, in order to favor the interactions between each trinuclear unit and the mononuclear central moiety, the bridging pyrazolate ligands of each [Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)] unit are oriented slightly out of the Cu3 plane and opposite to CuCl2(Hpz)2. Such interaction is further favored by the distortion of the square-planar environment at Cu(2). The large value of the Cl(1)-Cu(2)-Cl(2) angle and the concomitant contraction of Cl(1)-Cu(2)-O(1) [101.4(2)° and 146.5(2)°, respectively] are presumably due to a Cl(1) arrangement suitable to favor its interaction with Cu(4). The relevance of this interaction is indirectly confirmed by the lack of a free, structurally characterized square-planar [CuCl2(Hpz)2] complex. Interestingly, despite the fact that a compound having a stoichiometry corresponding to CuCl2(Hpz)2 has been reported by Reedijk et al.,22 its X-ray molecular structure has not been determined, while it was proposed to form a 1D MOF23 analogous to that found in [{CuCl2(pyridine)2}n],24 where a distorted octahedral geometry is determined by two pyridine nitrogens and four chloride ions sharing the edges. Looking at Table 4, where the values of some geometrical parameters observed in the Cu(4) coordination octahedron of 4 are compared with the corresponding data for [{CuCl2(pyridine)2}n]24 and the strictly related [{CuCl2(4-bromopyrazole)2}n],25 the analogy among the three solid-state structures is clearly shown (selected distances and angles for 4 are reported in Table 5). Particularly interesting are the supramolecular coordinative interactions involving Cl(1) and Cu(1′′) of two nearby heptanuclear units [Cl(1)-Cu(1′′) ) 2.941(2) Å; symmetry code (II) y + 1/3, -x + y + 2/3, -z + 5/3]. As a matter of fact, due to these interactions, six trinuclear units connect together forming 24-membered macrocycles, {Cl(1)-Cu(2)-O(1)-Cu(1)}6, which through the intermediacy of the mononuclear CuCl2(Hpz)2 units produce 2D MOFs sheets, where star-shaped holes are evident (Figure 4a). Each intermolecular interaction between Cl(1) and

Cu-N (Å) 1.964(5) 2.004(5) 1.991(6)

N-Cu-ClA (deg) 90.0(2) 90.4(1) 89.8(2)

N-Cu-ClB (deg) 87.6(2) 90.4(1) 89.8(2)

Table 5. Selected Bonding and Nonbonding Distances (Å) and Angles (deg) for 4 Cu(1)-O(1) Cu(2)-O(1) Cu(3)-O(1) Cu(1)-N(1) Cu(1)-N(6) Cu(2)-N(2) Cu(2)-N(3) Cu(2)-Cl(1) Cu(1)-Cl(1′)a Cu(3)-N(4)

1.982(4) 2.037(4) 2.019(4) 1.952(5) 1.957(5) 1.917(5) 1.918(6) 2.450(2) 2.941(2) 1.924(6)

Cu(3)-N(5) Cu(3)-N(7) Cu(3)-O(2w) Cu(2)-Cl(2) Cu(4)-Cl(3) Cu(4)-N(11) Cu(1)‚‚‚Cu(2) Cu(2)‚‚‚Cu(3) Cu(1)‚‚‚Cu(3) Cu(4)‚‚‚Cl(1)

1.942(5) 2.059(7) 2.444(8) 2.529(4) 2.322(1) 1.964(5) 3.319(1) 3.338(1) 3.317(1) 3.017(1)

Cu(1)-O(1)-Cu(2) Cu(1)-O(1)-Cu(3) Cu(2)-O(1)-Cu(3) N(1)-Cu(1)-N(6) N(2)-Cu(2)-Cl(1) Cl(1)-Cu(2)-N(3)

111.3(2) 112.0(2) 110.8(2) 175.8(2) 91.7(2) 91.5(2)

N(2)-Cu(2)-N(3) O(1)-Cu(2)-Cl(1) O(1)-Cu(2)-Cl(2) O(1)-Cu(2)-N(2) Cl(1)-Cu(2)-Cl(2) N(3)-Cu(2)-O(1)

176.6(2) 146.5(2) 112.1(2) 89.0(2) 101.4(2) 89.1(2)

a

Symmetry code: (I) -x + 1, -y, -z + 2.

Figure 4. Supramolecular assembly of 4. (a) View down the crystallographic c-axis showing three hexagonal star-shaped macropores connected through the mononuclear CuCl2(Hpz)2 moieties. The 24membered macrocycle (right) and Cl(1)-Cu(1′′) and Cl(2)‚‚‚H(1′′′)O(1′′′) intermolecular interactions (center) are highlighted by space filling representation. (b) View down the crystallographic a-axis. The alignment of two series of three pores generates two parallel channels, running along the crystallographic c-axis.

Cu(1′′) is strengthened by a corresponding hydrogen bond involving Cl(2) and the hydroxy hydrogen µ3-O(1′′)H(1′′) of the same units [O(1′′′)‚‚‚Cl(2) ) 2.966(6) Å, O(1′′′)-H(1′′′)‚ ‚‚Cl(2) ) 161°; symmetry code (III) x - y + 1/3, x - 1/3, -z + 5/ ], thus increasing the overall stability of this particular 3 assembly. Moreover, the holes of adjacent sheets match exactly, thus aligning and forming permanent hexagonal parallel channels running along the crystallographic c-axis (Figure 4b). In order to look into the stabilization induced by the Cl(1)Cu(1′′) interaction and the Cl(2)‚‚‚H(1′′′)-O(1′′′) hydrogen

682 Crystal Growth & Design, Vol. 7, No. 4, 2007

Figure 5. (a) View of two heptanuclear units of 4 interacting to generate a part of the 24-membered macrocycle. (b) The same assembly as in panel a from which two trinuclear units (one for each heptanuclear unit) have been removed. In both panels, the Cl, Cu, O, and H atoms involved in the interactions are evidenced by ball-and-stick representation. The H-bond is indicated as a dashed blue line.

bond, we performed a numerical experiment on a model system constituted by two suitable heptanuclear units for a total of 250 atoms ({[{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}2][CuCl2(Hpz)2]}2, Figure 5a). Despite the use of the thermal population of lowlying empty orbitals, we were unable to get converged results. We then reduced the size of the model systems by removing the trinuclear moieties not directly involved in the formation of a specific 24-membered macrocycle, considering the {[{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}][CuCl2(Hpz)2]}2 assembly (see Figure 5b). Thus obtained theoretical outcomes indicate that the overall energy gain on passing from two isolated fragments, each of them including one trinuclear moiety and the mononuclear fragment ([{Cu3(µ3-OH)(µ-pz)3(Cl)2(Hpz)2(H2O)}][CuCl2(Hpz)2] × 2), to the interacting model system reported in Figure 5b amounts to ca. 40 kcal/mol. Such a result is extremely interesting because it points out that interactions leading to the formation of the 24-membered macrocycle are far from being weak. In Figure 6a, a space-filling packing diagram of the 3D MOF, evidencing the honeycomb porous structure, is shown. Even though the hydrogens belonging to coordinated water (not determined) are not represented, the free section of the channels is clearly evident. Figure 6b highlights that the inner walls of these channels are mainly made up by pyrazolate ions bridging Cu(2) and Cu(3), Cl(2) chloride ions, and coordinated water molecules, while copper ions seem to be not accessible from the interior of the channels. Providing that neither water nor other molecules or ions are present in these channels, their dimensions are worth stressing. The inner diameter of each hexagonal pore has been evaluated

Casarin et al.

Figure 6. (a) Space-filling packing diagram of 4 down the crystallographic c-axis, showing the free section of pores. Only hydrogens belonging to coordinated water (not determined) are omitted. (b) Partial space-filling view of one hole of 4 highlighting the inner wall of the hexagonal channels.

taking into account the van der Waals radii of involved atoms.26 Particularly, the shorter distance [O(2w)-C(6) ) 7.65 Å] shown in Figure 6b corresponds to an effective free pore opening of about 4.2 Å. This value agrees well with the total free volume of 1183 Å3 per cell (ca. 9% of the crystal volume) determined by using the PLATON routine.27 UV-vis spectra of compound 4, both in solid state and MeOH solution (Figure 7), show features very similar to those previously observed in the spectra of 1a-d.3 In particular, besides two shoulders at about 557 and 716 nm, the reflectance solid-state spectrum shows two maxima at 608 and 647 nm. In MeOH solution, the solid-state features mediated into a broad band centered at about 668 nm. It is worth mentioning that even though compounds 1a-d and 4 have almost superimposable UV-vis spectra, the latter is emerald green (as its MeOH solution), while compounds 1a-d and their MeOH solutions are deep blue. The room-temperature magnetic susceptibility value, 4.06 µB, calculated for the heptanuclear formulation of 4 is much smaller than that expected for seven independent copper(II) moieties, thus indicating the existence of some kind of exchange coupling, and deserves a deeper study. Finally, from the reaction of 1b with aqueous HCl, it was possible to obtain also a little crop of the green-blue derivative 5, whose structure was determined by an X-ray study. The molecular structure (Figure 8) shows that, in the solid state, 5 is a neutral hexanuclear compound made up of two trinuclear

Honeycomb Channels in a Self-Assembled Cu(II) MOF

Crystal Growth & Design, Vol. 7, No. 4, 2007 683 Table 6. Selected Bonding and Nonbonding Distances (Å) and Angles (deg) for 5 Cu(1)-O(1) Cu(2)-O(1) Cu(1)-N(1) Cu(1)-N(3) Cu(1)-N(4) Cu(2)-N(2)

2.008(2) 2.030(3) 1.958(2) 1.950(2) 2.018(2) 1.946(2)

Cu(2)-N(6) Cu(2)-Cl(2) Cu(1)-Cl(1) Cu(1)‚‚‚Cl(2) Cu(1)‚‚‚Cu(1′)a Cu(1)‚‚‚Cu(2)

2.016(3) 2.687(1) 2.709(5) 3.430(1) 3.384(6) 3.297(5)

Cu(1)-O(1)-Cu(2) Cu(1)-O(1)-Cu(1′) Cl(2)-Cu(2)-O(1) N(1)-Cu(1)-N(3) N(1)-Cu(1)-N(4)

109.48(8) 114.9(1) 82.86(8) 172.5(1) 89.7(1)

N(4)-Cu(1)-Cl(1) N(1)-Cu(1)-Cl(1) N(2)-Cu(2)-N(6) Cu(1)-Cl(1)-Cu(1′′)b

89.64(7) 94.11(7) 91.24(7) 132.90(4)

a Symmetry code: (I) -y + 0.5, -x + 0.5, z. b Symmetry code: (II) y + 0.5, x - 0.5, z.

Figure 7. Reflectance solid-state (blue) and MeOH solution (red) UVvis spectra of 4.

Figure 9. Ball-and-stick view of 5 evidencing the ADF-optimized geometry of crystallization water.

Figure 8. Molecular structure of 5. The molecule is crystallographically independent only by 1/4 because it sits around a site of C2V symmetry. The water molecule [O(1w)] is statistically distributed over two symmetry-related locations. Symmetry codes : (I) -y + 0.5, -x + 0.5, z; (II) y + 0.5, x - 0.5, z; (III) 1 - x, -y, z. Intramolecular hydrogen bonds are indicated as dashed lines.

units linked together via two chloride bridges and possessing two crystallographic mirror planes passing through the center of the molecule, which possesses a crystallographic C2V symmetry. The [Cu3(µ3-OH)(µ-pz)3Cl(Hpz)2]+ units are connected to each other through two symmetric chloride bridges [Cu(1)Cl(1) 2.709(5) Å, Table 6]. In the trinuclear units, the three copper atoms adopt square-pyramidal geometries and are coordinated either to a terminal chloride, as in the case of Cu(2) [Cu(2)-Cl(2) 2.687(1) Å], or to one bridging chloride [Cu(1) and Cu(1′)] with the chlorine atoms occupying axial sites in both cases but with the terminal Cl(2) pointing in the opposite direction with respect to the two bridging Cl(1) and Cl(1′). The coordination spheres of Cu(1), Cu(1′), and Cu(2) are completed by two equatorial pyrazolate nitrogens, one iminic nitrogen from Hpz, and the bridging µ3-OH oxygen. Each hexanuclear unit contains also one molecule of crystallization water statistically disordered over two sites (50%), which is involved in two hydrogen bonds with both µ3-O(1)H capping groups [O(1)‚‚‚O(1w) ) 2.868(5) Å, O1-H(111)‚‚‚O(1w) ) 137°]. In such an arrangement, the water molecule acts as a

bridge between the two trinuclear units, thus increasing the stability of the overall hexanuclear assembly. The internuclear distance between O(1w) and the nearest Cl(1) atom [2.985(7) Å] suggests also the possible presence of a further hydrogen bond involving the crystallization water and the bridging chlorine. Since the water hydrogen atoms (hereafter indicated as Hy and Hx) were not located by X-ray measurements, their positions were optimized through ADF calculations. The main features of the geometry optimization are shown in Figure 9, inspection of which testifies that the water molecule is tilted toward the nearest bridging Cl(1) thus maximizing the interaction with Hy [Hy‚‚‚Cl(1) ) 2.02 Å; O(1w)-Hy‚‚‚Cl(1) ) 163°]. As a whole, the obtained results agree with the hypothesis that crystallization water is concomitantly involved in a hydrogen bond with a single Cl(1) atom and with both µ3-OH groups. The crystal structure determination evidences that the two Cu3 planes of 5 are not parallel to each other, but the interplanar distances range from 4.968(1) Å [Cu(1)-Cu(1′′), Cu(1′)-Cu(1′′′) symmetry codes as in Figure 8] to 7.506(1) Å for the unbridged Cu(2)-Cu(2′′) separation. This hexanuclear arrangement is very similar to the one found in [Cu3(µ3-OH)(µ-pz)3(µ4-NO3)(Hpz)3]2(µ-NO3)2, A,28 in which two bridging nitrates link together two trinuclear [Cu3(µ3-OH)(µ-pz)3(µ4-NO3)(Hpz)3]+ units.29 Finally, the crystal packing of 5 shows that Cl(2) [and Cl(2′)] weakly interacts, through hydrogen bonds, with two H(7)N(7) moieties of adjacent hexanuclear units [N(7iv)‚‚‚Cl(2) ) 3.228(3) Å, N(7iv)-H(7iv)‚‚‚Cl(2) ) 137°, symmetry code (IV) y, -x, -z + 1], giving rise to a 3D, nonporous MOF network (see Supporting Information). Conclusions This study has confirmed the relative stability of the [Cu3(µ3-OH)(µ-pz)3]2+ moiety, which stood up to the attack of HCl in relatively high concentrations, while acetate ions of 1b are more or less easily displaced. As a consequence, the supramo-

684 Crystal Growth & Design, Vol. 7, No. 4, 2007

lecular arrangement typical of 1b is destroyed, but the [Cu3(µ3-OH)(µ-pz)3]2+ fragments are able to attain other supramolecular assemblies, thus opening the way to the formation of new and interesting MOFs based on this triangular SBU. On the other hand, the interaction of HCl with compound 1b gives rise to a quite complicated series of reactions. While it is very likely that compound 2, still containing two bridging acetate ions, is the first stable product of the reaction (as also suggested by DFT calculations), the mechanism(s) and step(s) leading to compounds 3-5 are still unknown. On the other hand, the formation of 4, a MOF having permanent, completely solvent-free hexagonal channels, through the self-assembly of the stable [Cu3(µ3-OH)(µ-pz)3]2+ moiety, is an important outcome. Noteworthy, as again confirmed by DFT calculations, these channels are quite stable, and according to Kitagawa,30 compound 4 shows a “permanent porosity without any guest molecules in the pores”, thus being a second generation porous coordination compound.31 On the other hand, since sorption and desorption experiments have not yet been performed, 4 should be better considered, in agreement with Barbour’s recommendations,32 as a “potentially porous” compound, which might be the subject of interesting applications in adsorption, gas storage, heterogeneous catalysis, etc. The channels, with an effective free diameter a little bit larger than 4 Å,33 are not occupied by ions or solvent and can accommodate small molecules without deformation or loss of their structural integrity. Incidentally, the free diameter of the pores of 4 compares well with the dimensions of 1D channels (4.0 × 4.0 Å) found in a recently reported microporous MOF that is able to perform gas-chromatographic separation of alkanes on the basis of their steric characteristics, the channels being accessible to linear chain but not to branched alkanes.34 Moreover, the channels of 4 have dimensions comparable also to those of a Cu(II)-based MOF that is able to adsorb (and release) a relatively high amount of molecular hydrogen.35 In the case of compound 4, whose adsorption characteristics are currently under examination, it is also worth recalling that the inner wall of the channels, made up by pyrazolates, chloride ions, and coordinated water, very likely presents a basic character with the potential to preferentially trap acidic species. Preliminary results36 indicate that compound 4 is obtained even in the reaction of 1a with HCl. This fact draws attention to the importance of a deeper study of the stability and reactivity of these trinuclear triangular-shaped molecules and their supramolecular assemblies. Particularly, it will be of interest to determine the reaction conditions leading to larger amounts of 4, ascertaining also whether the isolated yields of different compounds reflect a true difference in stability or are simply due to serendipity. Acknowledgment. The Universities of Bologna, Camerino, and Padova are gratefully acknowledged for financial support. Supporting Information Available: X-ray crystallographic files for 2, 4, and 5 in CIF format, figures showing the crystal packing of compounds 2 and 5, and figures evidencing structural analogies of compounds 2, 1b, and 1d. This material is available free of charge via the Internet at http://pubs.acs.org.

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Honeycomb Channels in a Self-Assembled Cu(II) MOF (30) (31) (32) (33)

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