Reaction of Copper (II) Chloroacetate with Pyrazole. Synthesis of a

Article pubs.acs.org/crystal

Reaction of Copper(II) Chloroacetate with Pyrazole. Synthesis of a One-Dimensional Coordination Polymer and Unexpected Dehydrochlorination Reaction Silvia Carlotto,† Maurizio Casarin,*,† Arianna Lanza,*,‡ Fabrizio Nestola,§ Luciano Pandolfo,*,† Claudio Pettinari,∥ and Rebecca Scatena† †

Dipartimento di Scienze Chimiche , Università di Padova, Via Marzolo, 1, I-35131 Padova, Italy Departement für Chemie und Biochemie, Universität Bern, Freiestrasse, 3, CH-3012 Bern, Switzerland § Dipartimento di Geoscienze, Università di Padova, Via Gradenigo, 6, I-35131 Padova, Italy ∥ Scuola di Farmacia, Università di Camerino, Via S. Agostino, 1, I-62032 Camerino, Macerata, Italy ‡

S Supporting Information *

ABSTRACT: The reaction of copper(II) chloroacetate (1d) with pyrazole (Hpz) mainly yielded the mononuclear compound [Cu(μ-ClCH2COO)2(Hpz)2] (2m), which selfassembled generating a one-dimensional coordination polymer. Moreover, the concomitant isolation of the tetranuclear [{Cu2(μ-pz)(μ-OCH2COO)(Hpz)(MeOH)}2(μClCH2COO)2] (3t) and hexanuclear [{Cu3(μ3-OH)(μ-pz)3(Hpz)2}2(μ-ClCH2COO)2](Cl)2 (4h) species evidenced the occurrence of a peculiar, previously unreported, dehydrochlorination reaction and the formation of the trinuclear triangular moiety [Cu3(μ3-OH)(μ-pz)3]. Theoretical calculations based on density functional theory including solvation effects indicate a possible pathway for the formation of 3t. Interestingly, besides the energy minimum corresponding to 3t, a further relative energy minimum is found for a species which can be considered a possible reaction intermediate.

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INTRODUCTION The obtaining of new coordination polymers (CPs) continues to be pursued by numerous research groups1−14 not only because these derivatives often show topologically interesting structures but also for their possible applications as storage media15,16 or heterogeneous catalysts.17−19 Incidentally, it is noteworthy that BASF has been producing some CPs for some years20,21 (marketed by Sigma-Aldrich under the names of Basolite A100, C300, F300, Z1200, and Basosiv M05022) and that high throughput strategies to obtain interesting CPs (i.e., HKUST-1, UiO-66, NOTT-400) have been recently proposed.23 Recently, both metal-azolate systems24−33 and carboxylate ions, having different bridging metal coordination capabilities, have emerged as promising building blocks for the synthesis of CPs. In the course of our studies on the interaction of CuII with dinitrogen ligands and monocarboxylate ions, we have synthesized mono- and polynuclear complexes as well as CPs evidencing some interesting features, such as porosity, sorption−desorption properties, and catalytic activity. In more detail, the reactions of copper(II) carboxylates with pyrazole (Hpz) carried out in MeCN quantitatively yielded [Cu(pz)2],34−36 an one-dimensional (1D) CP characterized by the so-called “porosity without pore” behavior.37 At variance to © XXXX American Chemical Society

that, the same reactions carried out in protic solvents generated compounds based on the trinuclear triangular fragment [Cu3(μ3-OH)(μ-pz)3]2+ whose charge is balanced by two carboxylates coordinated to CuII ions (Scheme 1). These trinuclear moieties self-assemble through carboxylate bridges into 1D or two-dimensional (2D) CPs as well as hexanuclear clusters and supramolecular networks.38−41 In the synthesis of the above-mentioned trinuclear assemblies, a relevant role seems to be played by the basicity of carboxylate ions, which favors the deprotonation of pyrazole and water to produce pyrazolate and hydroxyl ions. This hypothesis is suggested by some experimental data; for instance, the reactions of CuCl2 or Cu(NO3)2 with Hpz lead to trinuclear triangular species only if an exogenous base such as KOH or NaOH is added;42 moreover, the reaction of copper(II) trifluoroacetate with Hpz yielded only the mononuclear [Cu(CF3COO)2(Hpz)2] species, being the weak trifluoroacetate anion unable to efficiently deprotonate water and Hpz.38 Received: September 21, 2015 Revised: October 20, 2015

A

DOI: 10.1021/acs.cgd.5b01365 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1

Table 1. Crystal Data and Structure Refinement for Compounds 1−4

a

compound

1d

2m

3t

4h

formula FW, g mol−1 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, mm3 θ limits, ° refl. collected unique refl. (Rint) GOF on F2 R1(F),a wR2(F2)b F2 > 2σ(F2) R1(F),a wR2(F2)b all data largest diff. peak and hole, e Å−3

C8H17Cl4Cu2O12.5 582.09 triclinic P1̅ 11.3022(3) 13.1824(4) 15.4119(4) 65.155(3) 76.631(2) 88.938(2) 2019.18(11) 4 1.915 2.690 1164 0.349 × 0.224 × 0.101 1.70932.094 55602 13028 (0.0398) 1.062 0.0343, 0.0854 0.0542, 0.0939 0.747 and −0.511

C10H12Cl2CuN4O4 386.69 triclinic P1̅ 5.0783(2) 7.5176(4) 9.9479(4) 73.150(4) 83.485(4) 77.841(4) 354.76(3) 1 1.810 1.936 195 0.257 × 0.117 × 0.044 2.14232.020 8910 2279 (0.0277) 1.048 0.0272, 0.0635 0.0314, 0.0659 0.325 and −0.317

C22H30Cl2Cu4N8O12 923.64 monoclinic P21/n 8.49653(9) 16.08972(15) 12.88024(14) 90 109.0254(12) 90 1664.63(3) 2 1.843 2.751 928 0.463 × 0.154 × 0.124 2.09731.043 36780 5161 (0.0230) 1.022 0.0296, 0.0889 0.0338, 0.0909 1.167 and −1.141

C34H40Cl4Cu6N20O6 1347.90 triclinic P1̅ 8.8168(9) 8.9397(9) 16.8959(16) 78.816(8) 78.074(8) 68.557(9) 1202.5(2) 1 1.861 2.894 674 0.144 × 0.101 × 0.077 2.46925.998 17358 4713 (0.0711) 1.104 0.0770, 0.2122 0.1046, 0.2324 1.762 and −0.734

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. equipped with a Dectris Pilatus 200 K detector. A Mo Kα X-ray microsource working at 50 kV and 0.8 mA (40 W) with mirror focusing optics (beam size 110 μm) was used. The sample−detector distance was 68 mm. The diffracted intensities were acquired up to 55° in 2θ, using 1° ω scans with exposure times ranging from 20 to 60 s, recording a full sphere of reciprocal space. The program CrysAlisPro43 was used for data collection, unit cell and space group determination, and data reduction. Numerical absorption correction based on the crystal shape was applied with the SCALE3 ABS PACK within CrysAlisPro. The structures were solved by direct methods with SIR9244 and completed and refined with SHELXL-201445 using the WinGX46 interface. H atoms were placed in geometrically calculated positions, and all non-hydrogen atoms were refined with anisotropic thermal displacement parameters. Crystal data and relevant details of structure refinement for compounds 1−4 are reported in Table 1. All over the work, in the labeling of isolated compounds we have adopted, besides the 1−4 digits, the symbols m, d, t, and h to indicate the mono-, di-, tetra-, and hexanuclear metal assemblies, respectively. Complete information about the crystal structure and molecular geometry is available in CIF format as Supporting Information. Molecular graphics were generated with the program Mercury 3.5.1.47,48 Color codes for all molecular graphics are yellow-orange (Cu), blue (N), red (O), gray (C), white (H).

To look into the formation mechanisms of trinuclear triangular copper derivatives, we reacted Hpz with copper(II) chloroacetate, hereafter Cu(Clac)2. The most abundant product was a 1D CP based on the mononuclear Cu(μ-Clac)2(Hpz)2 moiety, but the obtaining of a hexanuclear and a tetranuclear derivative evidenced the formation also of trinuclear species and the occurrence of a previously unreported dehydrochlorination reaction.

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EXPERIMENTAL SECTION

Materials and Methods. All the reactions and manipulations were carried out in air. 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 Jasco FT/IR4100 spectrometer with ATR mode. The magnetic susceptibilities were measured at room temperature (19−28 °C) with a Sherwood Scientific magnetic balance MSB-Auto, using HgCo(NCS)4 as calibrant and corrected for diamagnetism with the appropriate Pascal constants. The magnetic moments (in μB) were calculated from the equation μeff = 2.84(XmcorrT)1/2. Crystallographic Data Collection and Structure Determination. The X-ray intensity data were measured in the air and at room temperature on an Agilent Technologies SuperNova diffractometer, B



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Syntheses. Synthesis of [Cu(Clac)2]. To a solution of chloroacetic acid (28.64 g, 0.303 mol) in 200 mL of water, 12.86 g of Cu(OH)2· CuCO3 (58.2 mmol) was added in small portions under stirring, yielding a dark blue solution that was stirred for 30 min, concentrated at ca. 60 mL under a vacuum, and allowed to evaporate in the air. After 10 days, a green-blue microcrystalline solid was obtained, which was filtered, washed with 20 mL of cold water, and dried under a vacuum. Yield: 14.85 g, 51%. Elem. Anal. %. Calcd for Cu(Clac)2: C = 19.18; H = 1.61. Found: C = 19.46; H = 1.61. IR (ATR, cm−1) 3006, 2964, 1610, 1543, 1423, 1394, 1258, 922, 791, 701. μeff (292 K) = 1.895 μB (calculated for C4H4Cl2CuO4). Recrystallization from water yielded well formed blue-green crystals of [{Cu(μ-Clac)2(H2O)}2]·2.5H2O, 1d, which were dried in the air and employed for a single crystal (SC) XRD determination. Compound 1d: Elem. Anal. %. Calcd for [{Cu(Clac)2(H2O)}2]· 2.5H2O: C = 16.51; H = 2.94. Found: C = 16.68; H = 2.80. IR (ATR, cm−1) 3391, 3018, 2968, 1634, 1581(sh), 1542, 1417, 1396, 1265, 932, 789, 699. μeff (292 K) = 2.761 μB (calculated for C8H17Cl4Cu2O12.5). Reaction of [Cu(Clac)2] + Hpz in MeOH. Synthesis of [Cu(μClac)2 (Hpz)2]n, 2m, [{Cu2 (μ-pz)(μ-OCH 2COO)(Hpz)(MeOH)}2 (μClac)2], 3t, and [{Cu3(μ3-OH)(μ-pz)3(Hpz)2}2(μ-Clac)2](Cl)2, 4h. To a solution of 1.38 g of [Cu(Clac)2] (5.52 mmol) in 50 mL of MeOH, a solution of Hpz (618 mg, 9.1 mmol) in 4 mL of MeOH was added under stirring. The resulting deep blue solution was allowed to evaporate in the air yielding pale-blue platelets of 2m, suitable for a SCXRD determination, that were washed with few drops of MeOH and dried in the air. From mother liquors of 2m a mixture of 2m, lightblue (3t) and dark-blue (4h) crystals was obtained. Some crystals of compound 3t and 4h were manually separated and employed for SCXRD determinations (3t). In the case of 3t, it was possible to obtain also the elemental analysis and IR spectrum. Compound 2m. Yield: 1.054 g, 60% (with respect to Hpz). Elem. Anal. % Calcd for Cu(ClCH2COO)2(Hpz)2: C = 31.06; H = 3.13; N = 14.49. Found: C = 30.65, H = 3.09, N = 13.91. IR (ATR, cm−1): 3145, 3118, 2985, 2945, 1574, 1553, 1404, 1364, 1253, 1169, 1152, 1071, 1050, 956, 931, 916, 886, 853, 784, 687, 617. μeff (293 K) = 2.277 μB (calculated for C10H12Cl2CuN4O4). Compound 3t. Elem. Anal. % Calcd for Cu2(pz)(OCH2COO)(Hpz)(MeOH)(Clac): C = 28.61; H = 3.27; N = 12.13. Found: C = 29.12, H = 3.42, N = 12.13. IR (ATR, cm−1): 3145, 3117, 3053, 2945, 1572, 1402, 1363, 1252, 1168, 1151, 1070, 954, 930, 915, 882, 848, 763, 684, 615, 571. Computational Details. Density functional theory (DFT) calculations have been carried out by using the 2014 release of the ADF package.49 Both the electronic and structural properties of the investigated molecules have been obtained by using the hybrid Becke3−Lee−Yang−Parr (B3LYP) exchange correlation functional, i.e., by combining a standard Vosko-Wilk-Nussar (VWN) generalized gradient50 with a part (20%) of the exact Hartree−Fock exchange.51 All electrons double-ζ with a polarization function (DZP) Slater-type orbitals have been adopted for typical elements, while an all electrons triple-ζ with a polarization function (TZP) basis set has been employed for Cu atoms. Numerical experiments pertaining to the asymmetric unit of 3t as well to the whole tetranuclear species and to a possible reaction intermediate have been run by including spinpolarization effects and by assuming the presence of either an antiferromagnetic (S = 0, number of unpaired electron = 0) or a ferromagnetic (S = 1, number of unpaired electron = 2) coupling. The role played by the solvent (methanol) has been investigated by adopting the COnductor-like Screening MOdel (COSMO) and by assuming the following parameters (radius = 2.53 Å, dielectric constant = 32.6).52−54 Starting geometrical parameters of the asymmetric unit of 3t corresponded to the ones obtained from crystallographic data; they were further optimized without any symmetry constraints and by including (COSMO) or excluding (gas-phase) the solvent effects. Further numerical experiments have been performed by adding a dispersion correction.55

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DISCUSSION During the synthesis of copper(II) chloroacetate, the recrystallization of Cu(Clac)2 from water yielded well formed blue-green crystals of [{Cu(μ-Clac)2(H2O)}2]·2.5H2O (1d) whose previously unreported structure was determined through an SCXRD experiment. Four independent {Cu(μ-Clac)2(H2O)}2 units and 10 crystallization water molecules are present in the cell (see Figure S1 of the Supporting Information), while in Figure 1 a single, classical, paddle-

Figure 1. Classical paddle-wheel structure of hydrated copper(II) chloroacetate, 1d, with partial atom labeling scheme.

wheel unit capped by two water molecules and two crystallization water molecule are shown. Copper−oxygen bond lengths [Cu−OCOO from 1.9556(15) to 1.9788(16) Å and Cu−Owater from 2.105(3) to 2.141(3) Å] are in the range normally found for these compounds and, together with the other geometrical parameters, can be found in the CIF files supplied as Supporting Information. Crystallization water molecules are involved in strong Hbonds56 that likely contribute to favor their insertion into a quite intricate supramolecular network. As an example, as shown in Figure 2, one coordinated H2O acts as a H-bond

Figure 2. Example of H-bonds in compound 1d.

donor toward a crystallization water molecule [O10···O5 2.797(3) Å, O10H10B···O5 176(2)°] and a carboxylate oxygen of a second paddle wheel [O10···O31 2.950(3) Å, O10−H10A···O31 171(2)°], while another crystallization water molecule is likely involved as a H-bond donor (hydrogen atoms of crystallization water molecules were not located in the Fourier-difference map) into a H-bond with O13 carboxylate oxygen [O1···O13 2.914(5) Å]. In this way, crystallization water molecules self-assemble forming six-membered cycles (in chair conformation) which are C



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thus suggesting that the combined effects undergone by O2 (the relatively weak coordination to Cu1i and the H-bond) can be compared to the strong coordination of O1 to Cu1 (see Figure 5).

connected to each other through {Cu(Clac)2(H2O)}2 moieties, as shown in Figure S2. On the other hand, it is to be pointed out that by drying 1d under a vacuum at r.t., not only crystallization but even coordinated water molecules are removed, as indicated by the elemental analysis of Cu(Clac)2 that probably has a 1D polymeric structure, analogous to numerous other, well-known copper(II) monocarboxylates.57 The reaction of Cu(Clac)2 with Hpz in MeOH yielded as the first, most abundant, product the 1D polymeric species [Cu(μClac)2(Hpz)2]n, 2m, whose structure is shown in Figure 3.

Figure 5. H-bond (light-blue dashed line), responsible for the electronic delocalization of the carboxylate fragment in compound 2m.

No other relevant supramolecular interactions are present, and the crystal packing generates small, not connected, pores accounting for ca. 2% of free volume, as calculated by Mercury 3.5.1 program47,48 with a probe radius of 1 Å (see Figure S3). All the other geometrical parameters of 2m are quite normal and can be found in the CIF file supplied as Supporting Information. Slow evaporation of mother liquors of the synthesis of 2m yielded a mixture of crystals where, besides 2m, it was possible to detect the pale blue 3t and the dark blue 4h species, that were characterized through SCXRD determinations carried out on manually isolated crystals. The formula and the structure of 3t, [{Cu2(μ-pz)(μOCH2COO)(Hpz)(MeOH)}2(μ-ClCH2COO)2], are worthy to be deeply examined. In Figure 6 is shown the asymmetric

Figure 3. Molecular structure of compound 2m with partial atom labeling scheme.

The copper ion, which lies in an inversion center, displays an octahedral coordination geometry due to two symmetry equivalent pyrazole molecules [Cu1N1 1.9967(12) Å, symmetry code: −x, − y, −z] and two symmetry equivalent bridging syn-anti chloroacetate ions [Cu1O1 2.0037(10), Cu1−O2i 2.4640(12) Å, symmetry code: x − 1, y, z]. Thanks to the ditopic behavior of chloroacetate ions, the mononuclear [Cu(μ-Clac)2(Hpz)2] fragment acts as a secondary building unit (SBU) generating 1D coordination polymers running parallel to the crystallographic a axis. The same ditopic behavior is responsible for the formation of eight-membered metallacycles and for the presence of another inversion center thus located midway in the segment joining the symmetry equivalent Cu ions [symmetry code: 1 + x, y, z].

Figure 6. Asymmetric unit of 3t with partial atom labeling scheme.

unit of 3t, a dinuclear species, where some relevant features are present. First of all, Cu1 and Cu2 are almost symmetrically bridged by O1 [Cu1O1 1.933(2), Cu2O1 1.961(1) Å] and a pyrazolate ion [Cu1N2 1.936(2), Cu2N1 1.964(2) Å], in an arrangement which is partly reminiscent of the trinuclear triangular structure [Cu3(μ3-OH)(μ-pz)3] (see Scheme 1). Another relevant point is related to the C7C8O2 fragment, bridging O1 and Cu1 [C7O1 1.414(3), O2Cu1 1.936(2) Å]. Because of the synthetic procedure employed (the reaction of copper chloroacetate with Hpz in MeOH), it is evident that the fragment CH2COO can come only from the dechlorination of a chloroacetate ion. This is in agreement with a possible dehydrochlorination mechanism entailing the

Figure 4. 1D coordination polymer formed through the self-assembly of [Cu(Clac)2(Hpz)2] SBUs. H atoms are omitted for clarity.

The formation of the eight-membered rings is likely sustained by strong H-bonds56 involving NH groups and O2 [N2···O2 2.6970(18) Å, N2H2N···O2 158°]. Moreover, these strong H-bonds are likely responsible also for the good electronic delocalization in the O1C4O2 carboxylate moiety. Actually, even though the Cu1O1 bond is shorter than Cu1iO2, the C4O1 and C4O2 distances are almost identical [1.2564(17) and 1.2503(18) Å, respectively] D



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interaction of a coordinated ClCH2COO− ion with an OH group originally bridging Cu1 and Cu2, with elimination of HCl and formation of the C7O1 bond. This elimination mechanism (Scheme 2) implies the existence of the species a which can be conceived as an

connected through the relatively weak interactions of Cu1 with O1 belonging to the symmetry equivalent unit [Cu1O1i 2.5504(16) Å] and the Cu1iO1 symmetric one. As a consequence, both Cu1 and Cu2 display slightly distorted square pyramidal coordination geometries (τ565 values 0.2 and 0.25, respectively). This tetranuclear assembly is likely further reinforced by two strong H-bonds56 involving the pyrazole NH group and O2 of the symmetry related dinuclear unit [N4···O2i 2.911(3) Å, N4H4N···O2i 132.2°] as shown in Figure S4. Other “intermolecular” H-bonds involve instead methanol OH and O3 of symmetry related tetranuclear moieties [O6···O3ii 2.727(2) Å, O6−H6M···O3ii 166(3)°, symmetry code: (ii) −x + 1, −y, −z + 2], thus generating a 1D supramolecular assembly, running parallel to the crystallographic a axis, shown in Figure S5. All the other geometrical parameters can be found in the CIF file supplied as Supporting Information. From mother liquors of 2m, besides 2m itself and 3t, darkblue crystals also formed, which were manually separated and identified, only through a SCXRD determination, as the hexanuclear species [{Cu 3 (μ 3 -OH)(μ-pz) 3 (Hpz) 2 } 2 (μClCH2COO)2](Cl)2, 4h. This compound (Figure 8) exhibits a structure very similar to that of [{Cu3(μ3-OH)(μ-pz)3(Hpz)2}2(μ-CH3COO)2](Cl)2·2H2O, A, previously obtained by reacting [{Cu3(μ3-OH)(μ-pz)3(CH3COO)2(Hpz)}2] with HCl.66 Besides the same space group (P1)̅ , compounds 4h and A have almost identical cell parameters and corresponding bond lengths and angles. Analogously to A, whose structure is also reported in Figure 8 for comparison, compound 4h can be described as formed by two symmetry equivalent trinuclear triangular moieties, {Cu3(μ3-OH)(μ-pz)3(Hpz)2} asymmetrically joined by two O2 carboxylates oxygens belonging to two symmetry equivalent chloroacetate ions, thus generating an inversion center located midway the segment Cu1···Cu1i [symmetry code:(i) −x, −y, −z + 1]. The plane defined by the Cu1O2Cu1iCu2i ring is almost perpendicular (89.87°) to the two parallel planes defined by the three copper ions pertaining to each trinuclear unit. Two symmetry equivalent chloride ions (Cl2) provide the electroneutrality to the hexanuclear system. In each trinuclear triangular {Cu3(μ3-OH)(μ-pz)3(Hpz)2} fragment, the distance of capping μ3-O1 from the plane defined by the Cu3 system [0.505(6) Å] as well as the Cu−O1 bond distances [Cu1O1 1.976(6), Cu2O1 1.962(8), Cu3O1 1.975(7) Å] fall in the range normally found for analogous compounds.38−42,58,66,67 Both Cu2 and Cu3 adopt a square planar coordination geometry (τ468 = 0.13 and 0.15, respectively) determined besides O1, by N2, N3, N4, and N5 pyrazolate [Cu2N2 1.948(7), Cu2N3 1.954(8), Cu3N4 1.952(8), Cu3N5 1.961(9) Å] and by N7 and N9 pyrazole nitrogens [Cu2N7 2.00(1), Cu3N9 2.0145(9) Å] even though weak coordinative interactions with Cl2 [Cu2···Cl2 3.015(3), Cu3···Cl2 3.011(3) Å] are likely present, so that it is questionable if Cl2 is to be considered “truly” ionic or weakly coordinated, a feature analogous to that already found in compound A. Cu1 displays instead a square-pyramidal coordination (τ5 = 0.02) due to O1, N1, and N6 [Cu1N1 1.95(1), Cu1N6 1.958(8) Å] pyrazolate nitrogens, and O2 and O2i [Cu1O2 1.980(6), Cu1O2i 2.569(6) Å] carboxylate oxygens. Also in this case a weak coordinative interaction with Cl1 belonging to chloroacetate ion is present [Cu1···Cl1 2.955(3) Å] and, therefore, it could be possible to designate the geometry coordination

Scheme 2

intermediate stage in the formation of a possible trinuclear triangular [Cu3(μ3-OH)(μ-pz)3] fragment, which is likely arrested by the dehydrochlorination process. This mechanism does not conflict with the known reactivity of μ3-OH copper(II) trinuclear triangular species. Actually, Angaridis et al.58 reported that the hydrogen of μ3-OH in the trinuclear triangular CuII species [Cu3(μ3-OH)(μ-pz)3Cl3]− has an acid character and can be removed by treatment with strong bases yielding the [Cu3(μ3-O)(μ-pz)3Cl3]2− species. On the other hand, the elimination of the carbon bonded chlorine atom appears a quite peculiar process since, to the best of our knowledge, dehydrochlorination reactions are reported to involve H and Cl bonded to vicinal carbon atoms, thus generating a double bond, as in thermal autocatalytic dehydrochlorination of PVC,59 and in base promoted60 or Pd0 catalyzed61 formation of alkenes. Scattered ring-formation reactions, through 1,3 and 1,5 dehydrochlorination processes are also reported,62−64 but no one appears to be related to our results. Coming back to the description of the molecular structure of 3t, one molecule of MeOH and one of Hpz bonded to Cu2 [Cu2O6 2.273(2), Cu2N3 2.005(2) Å], besides a chloroacetate ion bonded to Cu1 [Cu1O4 1.965(2) Å], complete the coordination scheme of the asymmetric unit. On the other hand, compound 3t is more correctly described as a tetranuclear species generated through the coupling of two symmetry equivalent units [symmetry code: (i) −x + 2, −y, −z + 2] (Figure 7). In detail, two chloroacetate ions bridge in an almost symmetric syn-syn fashion Cu1 and Cu2 belonging to two symmetry equivalent dinuclear moieties [Cu1O4 1.9655(15), Cu2iO5 1.9810(16) Å], which are further

Figure 7. Tetranuclear assembly of 3t. H atoms are omitted for clarity. E



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Figure 8. Molecular structures of compounds 4h (left) and A (right).

around Cu1 as strongly distorted octahedral. A strong Hbond56 [O1···O3i 2.636(9) Å, O1H111···O3i 162°] likely reinforces the hexanuclear assembly, while Cl2 is involved into two nonclassical H-bonds with pyrazole N8H8N [N8···Cl2 3.217(9) Å, N8H8N···Cl2 144°] and N10H10N [N10···Cl2 3.254(10) Å, N10H10N···Cl2 141°] groups. Incidentally, in each trinuclear triangular unit both coordinated pyrazole and chloride ion lie on the opposite side with respect to μ3-OH group, likely due to steric reasons. The other geometric parameters are in the range normally found for this kind of compound38−42,58,66,67 and can be found in the CIF files supplied as Supporting Information. The obtaining of compound 4h supports the dehydrochlorination reaction mechanism proposed for the formation of compound 3t. In detail, the chloride ions which are present in the structure of 4h necessarily come from HCl formed in the dehydrochlorination reaction proposed in Scheme 2 that generates compound 3t. Thus formed HCl makes an electrophilic attack to a preformed hexanuclear species, possibly [{Cu3(μ3-OH)(μ-pz)3(μ-ClCH2COO)2(Hpz)x}2], b, releasing chloroacetic acid and maintaining the integrity of the hexanuclear unit (4h), analogously to what happened in the formation of compound A.66 Thus, with the aim to get further insights into the possible reaction mechanism leading to 3t, theoretical DFT calculations have been performed. Numerical experiments carried out on the asymmetric unit of 3t (see Figure S6 and Table S1) without the inclusion of solvent effects generate optimized geometries whose matching with crystallographic data is rather poor. More specifically, both S = 0 and S = 1 calculations clearly indicate the presence of a direct Cu1O5 interaction (the internuclear distance is 2.78 Å in both cases), which has no experimental confirmation [the Cu1O5 distance is 3.345(2) Å]. Moreover, the lengthening of the Cu2O6 bond distance [from 2.273(2) Å in the X-ray structure to 2.87 Å in the S = 0 one] is so large that the S = 0 optimized Cu2 coordination is no longer 4-fold (see Figure S6b). Minor differences characterize the arrangement of the nitrogen based ligands. The agreement between theory and experiment is definitely improved through the introduction of solvent effects (see Figure S7 and Table S2). Specifically, both copper atoms have a 4-fold coordination, and the Cu1O5 distance is much more similar to the experimental one. As far as the number of unpaired electrons is concerned, the antiferromagnetic coupling is slightly more stable (∼2 kcal/mol) than the ferromagnetic one. Such a tiny difference is consistent with the optimized Cu1···Cu2 distance (3.41 Å), which nicely matches the experimental one [3.3565(4) Å]. The agreement between calculated and experimental structures is clearly evidenced also by the overlay representation reported in Figure 9.

Figure 9. Overlaying capped stick representations of the experimental and calculated (yellow color) asymmetric unit of 3t with partial labeling scheme. The calculated structure was obtained by including solvent (methanol) effects and assuming an antiferromagnetic coupling (S = 0).

Theoretical results so far considered have been obtained by limiting our attention to the dinuclear asymmetric unit of 3t; on the other hand, X-ray crystallographic data are consistent with a tetranuclear structure. Two further series of numerical experiments (S = 0, with the inclusion of solvent effects, with or without the inclusion of a dispersion correction) on the tetranuclear 3t have been then carried out to test the appropriateness of the adopted model (see Figure S8 and Table S3). The optimized structures thus obtained are negligibly affected by the inclusion of dispersion corrections; moreover, geometrical parameters pertaining to the tetranuclear species (see Figure S8b) are very similar to those reported in Figure S7b. On this basis, the forthcoming discussion has been limited to the asymmetric unit of 3t. The capability of the adopted theoretical approach to provide information about the electronic and structural properties of 3t prompted us to exploit ADF to look for a possible intermediate in the dehydrochlorination reaction depicted in Scheme 2. The following three points have been initially considered: (i) chloride ions are formed along the reaction; (ii) 3t includes a dechlorinated species having a five member ring possibly obtained from a chloroacetate ligand; (iii) the peculiar coordination of the hydroxyl group which is present in unsymmetric trinuclear triangular Cu(II) pyrazolate complexes.38−42,58,66,67 Accordingly, we assumed as a possible reaction intermediate the species reported in Scheme 2 and carried out a series of numerical experiments to optimize its structure by assuming S = 0 and including solvent effects (Table S4 and Figure 10). Incidentally, a ferromagnetic intermediate has been also considered (Table S4 and Figure S9), but, similar to the 3t, the S = 0 intermediate results “more” stable by 0.2 kcal/mol. F



Article

isolated hexanuclear species (b) is likely to be formed in lower yield. This compound reacts with HCl released through the peculiar dehydroalogenation reaction of a third, not isolated, species (a), leading to the ionic hexanuclear dichloride derivative 4h. Moreover, the dehydrochlorination reaction generates a dinuclear moiety, which self-assembles into the tetranuclear species 3t. Numerical experiments carried out to look for a possible intermediate in the dehydrochlorination reaction emphasized the role played by the Cl2C7C8 O2 fragment in the proposed mechanism allowing us to foresee a significant modification of its geometry, taking as a reference the final product. In conclusion, we have confirmed that the basicity of the carboxylate ion is the key to address the reaction of Cu(RCOO)2 with Hpz toward the formation of mononuclear derivatives or trinuclear triangular assemblies. Actually, the formation of the mononuclear species 2m as major product was expected, due to the relatively low pKa value of chloroacetic acid (2.8769). On the other hand, the obtaining, even though in lower yield, of 4h shows that chloroacetate ion is likely in a borderline position to discriminate between the formation of mono- or trinuclear species. Moreover, the isolation of 3t gives not only the key to justify the presence of chloride ions in 4h, but also suggests a possible mechanism for a peculiar dehydrochlorination reaction. Finally, it is to point out that the dinuclear species a can be conceived as a step in the formation of trinuclear triangular species by starting from Cu(RCOO)2 and Hpz, an issue which is worthy to be further deeply studied.

Figure 10. Optimized stable intermediate for the reaction mechanism shown in Scheme 2. The calculated structure was obtained by including solvent (methanol) effects and assuming an antiferromagnetic coupling (S = 0).

Several peculiar features characterize the intermediate structure reported in Figure 10: (i) the coordination of both the pyrazolate ion, bridging almost symmetrically Cu1 and Cu2, and the pyrazole ligand are maintained in 3t; (ii) the Cu1−O1 and Cu2−O1 optimized bond distances are very similar (1.91 and 1.92 Å, respectively), while they are slightly different in 3t [1.933(2) and 1.961(1) Å, respectively], most probably as a consequence of the presence of the H1 atom, directly involved in the dehydrochlorination mechanism; (iii) the Cu1O2 bond distance (1.95 Å) is slightly larger than in 3t [1.936(2) Å]; nevertheless, the most striking variation concerns the Cl2 C7O3O2 fragment, which lies almost perpendicular to the Cu1O1Cu2H1 plane with a Cl2H1 internuclear distance of 2.836 Å. As such, the leading role played by the Cl2C7O3O2 fragment in the dehydrochlorination mechanism allows us to foresee significant modification of its geometry (the intermediate is less stable of 3 plus an HCl molecule by 3 kcal/mol).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01365. Crystallographic data (CCDC 1425411−1425414) can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. X-ray crystallographic files for compounds 1−4 in CIF format (CIF1, CIF2, CIF3, CIF4) Tables S1−S4 and Figures S1−S9 (PDF)

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CONCLUSIONS The reaction of copper(II) chloroacetate with Hpz in MeOH yielded three different compounds summarized in Figure 11. The reaction mainly proceeds with the formation of a mononuclear species, 2m, that, acting as a SBU, self-assembles generating a series of parallel 1D CPs. At the same time, a not

Figure 11. Summary of the reactions observed in the interaction of copper(II) chloroacetate with Hpz in MeOH. G



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AUTHOR INFORMATION

Corresponding Authors

*(M.C.) Phone: +39 049 8275164; fax: +39 049 8275050; email: [email protected]. *(A.L.) Phone: +41 (0)31 6314273; fax: +41 (0)31 6314272; e-mail: [email protected]. *(L.P.) Phone: +39 049 8275157; fax: +39 049 8275050; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the PRIN-2010-BNZ3F2 Research Project DESCARTES of the Italian Ministry of the University and Research and by the University of Padova PRAT CPDA134272/13, project S3MArTA. The Computational Chemistry Community (C3P) of the University of Padova is kindly acknowledged.

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Reaction of Copper (II) Chloroacetate with Pyrazole. Synthesis of a

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