Coordination polymers based on trinuclear and mononuclear copper-pyrazolate building moieties connected by fumarate or 2-methylfumarate ions

Journal of Organometallic Chemistry 714 (2012) 74e80

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Coordination polymers based on trinuclear and mononuclear copper-pyrazolate building moieties connected by fumarate or 2-methylfumarate ions Corrado Di Nicola a, Enrico Forlin b, Federica Garau b, Arianna Lanza b, Marta Maria Natile c, Fabrizio Nestola d, Luciano Pandolfo b, *, Claudio Pettinari a, * a

Scuola del Farmaco e dei Prodotti della Salute, University of Camerino, Via S. Agostino, 1, I-63032 Camerino (MC), Italy Dip. di Scienze Chimiche, University of Padova, Via Marzolo, 1, I-35131 Padova, Italy CNR e ISTM. Dip. di Scienze Chimiche, University of Padova, Via Marzolo, 1, I-35131 Padova, Italy d Dip. di Geoscienze, University of Padova, Via Gradenigo, 6, I-35131 Padova, Italy b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 February 2012 Received in revised form 16 March 2012 Accepted 19 March 2012

By reacting water solutions of pyrazole (Hpz) with copper(II) fumarate or 2-methylfumarate in solvothermal conditions two almost identical trinuclear triangular complexes, [Cu3(m3-OH)(mpz)3{R(COO)2}(Hpz)], [R ¼ CH]CH, C(CH3)]CH], have been obtained. The two carboxylate groups of both fumarate and 2-methylfumarate show different arrangements, one acting as monotopic ligand while the other one has a ditopic behavior. Thus, in both cases, wavy 2D isomorphous Coordination Polymers (CPs), that further self-assemble through strong H-bonds into two 3D networks, have been obtained. When copper(II) 2-methylfumarate was reacted with pyrazole at r.t. the mononuclear [Cu(Hpz)2(H2O)(2-methylfumarate)$H2O] complex was, in addition, obtained. In this case the bridging dianions produce instead monodimensional CPs that self-assemble into supramolecular tapes mediated by H-bond interactions, involving coordinated and crystallization water molecules. The mononuclear and trinuclear species were preliminary tested as catalysts for the oxidation of styrene by H2O2. Ó 2012 Elsevier B.V. All rights reserved.

Dedicated to the memory of our friend and colleague Professor Klaus Josef Müller. Keywords: Coordination polymers Copper(II) complexes Pyrazole Bicarboxylates Isomorphous structures

1. Introduction The official history of Coordination Polymers (CPs) begins about 30 years ago when Robson and Hoskins [1] proposed that “a new and potentially extensive class of solid polymeric materials with unprecedented and possibly useful properties may be afforded by linking centers with either a tetrahedral or an octahedral array of valences with rod-like connecting units”. Since the early 1990s, the number of studies on these materials (sometimes referred to as Metal Organic Frameworks (MOFs) [2]) having polymeric structures based on metal ions and organic polytopic ligands, has increased greatly, as they are promising materials for applications in gas storage, gas purification, anion exchange, heterogeneous catalysis, etc. [3]. In 2001 Yaghi and co-workers introduced the relevant concept of Secondary Building Units (SBUs) [4] as “molecular complexes and cluster entities” that can be employed as fundamental bricks joined by polytopic organic linkers to build MOFs. A large list of SBUs, useful in the synthesis of MOFs, has been recently * Corresponding authors. E-mail address: [email protected] (C. Pettinari). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2012.03.026

provided by the same research group [2]. Even though trinuclear triangular systems are long time known [5], this specific kind of SBUs is scarcely used in the obtaining of MOFs. Particularly, excluding, to the best of our knowledge, one paper recently appeared [6], trinuclear triangular CuII moieties have not been systematically used to obtain MOFs. In this context, in the recent years we have started a research project concerning the study of the interactions of transition metal ions with pyrazolates and monocarboxylates, synthesizing mono-, and polynuclear complexes as well as CPs. Besides zinc(II), cadmium(II) and mercury(II) carboxylates [7], we have been mainly interested in the study of the interaction of copper(II) carboxylates with pyrazole (Hpz). More specifically, by reacting at r.t. CuII carboxylates with Hpz in protic solvents (H2O, alcohols) solid compounds were obtained, all based on the trinuclear triangular fragment [Cu3(m3-OH)(m-pz)3]2þ, its charge being balanced by two carboxylate ions coordinated to CuII (Chart 1) [8]. These trinuclear compounds self-assemble into hexanuclear clusters through monoatomic carboxylate bridges and form extended 1D or 2D CPs or supramolecular networks [9]. Taking into account that the trinuclear triangular [Cu3(m3OH)(m-pz)3]2þ is quite stable [10], we planned to obtain CPs based

C. Di Nicola et al. / Journal of Organometallic Chemistry 714 (2012) 74e80

on the same SBU by using bicarboxylate ions instead of monocarboxylates. Here we report that, by reacting in water copper(II) fumarate, 1, and copper(II) 2-methylfumarate, 2, with Hpz, in different reaction conditions, three CPs were obtained. Particularly, the two isomorphous trinuclear triangular derivatives [Cu3(m3-OH)(m-pz)3(Fum)(Hpz)], 3, (Fum ¼ fumarate dianion) and [Cu3(m3-OH)(m-pz)3(MeFum)(Hpz)], 4, (MeFum ¼ 2methylfumarate or mesaconate dianion) and the mononuclear complex [Cu(MeFum)(Hpz)2(H2O)]$H2O, 5, self-assemble in the solid state generating 2D and 1D CPs. 2+ N Cu

H

N N

Cu

abundance simulator [13]. The reported m/z values correspond to the most intense signal of the isotopic clusters. The magnetic susceptibilities of 4 and 50 (vide infra) were measured at room temperature (20e28  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 mB) were calculated from the corr TÞ1=2 . equation meff ¼ 2:84ðXm 2.2. General procedure for the catalytic oxidation of styrene by H2O2 Styrene (1.0 mmol, 104 mg) and 3 (0.05 mmol, 30.3 mg) were dissolved in acetone (2.0 mL). Aqueous hydrogen peroxide [306 mg, 35% (w/w), 3.0 mmol] was then added as one portion and the reaction mixture was stirred under nitrogen at ambient temperature for 24 h. Then a small portion (20 mL) of the resultant solution was diluted to 1 mL and analyzed by GCeMS. Control experiments without either the catalyst or H2O2 were performed under identical conditions. Identical procedures were employed for compounds 4 and 5.

N Cu

O

75

N N

Chart 1.

2.3. Crystallographic data collection and structure determination

2. Experimental section 2.1. Materials and methods All the reactions and manipulations were carried out in air. The syntheses were performed both in ambient and solvothermal conditions. Compounds 1 [11] and 2 [12] were prepared as previously reported. Elemental analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNSeO elemental analyzer. Infrared spectra from 4000 to 250 cm1 were recorded with a Perkin Elmer Spectrum 100 FT-IR spectrophotometer. The positive electrospray mass spectrum (ESI-MS) of compound 5 was obtained with a Series 1100 MSI detector HP spectrometer, using MeOH as mobile phase. The solution for ESI-MS was prepared using reagent grade methanol and water, and the obtained data (m/z and intensities) were compared with those calculated by using the IsoPro isotopic

X-ray diffraction data of compounds 3e5 were collected at Dipartimento di Geoscienze, University of Padova, using a STOE STADI IV four-circles diffractometer, equipped with a Sapphire 1 CCD detector from Oxford Diffraction and a graphite monochromatized Mo Ka radiation source, operating at 50 kV and 40 mA. For all crystals, 1380 frames were collected by scanning a full sphere of reciprocal space by 1 steps in u. Unit cell parameters were determined using CrysalisRED [14], and numerical absorption correction was performed using X-RED and X-SHAPE [15]. The WinGX package [16] was used for the subsequent steps. In detail, the crystal structures were solved by direct methods using Sir2004 [17], and refined with SHELXL97 [18]. PLATON [19] was used for analyzing and validating the structures. Molecular graphics were generated by using Mercury 2.4.5 program [20]. Crystal data and details of data collections for compounds 3e5 are reported in Table 1.

Table 1 Crystal data and structure refinement for compounds 3e5. Compound

3

4

5

Formula FW Crystal symmetry Space group a, Å b, Å c, Å a,  b,  g,  Cell volume, Å3 Z Dc, Mg m3 m(Mo Ka), mm1 F (000) Crystal size, mm q limits,  Refl collected Unique refl (Rint) GooF on F2 R1(F)a, wR2(F2)b Largest diff. peak and hole, e Å3 P P a R1 ¼ jjFoj  jFcj/ jFoj. P P b wR2 ¼ [ w(F2o  F2c )2/ w(F2o)2]1/2, where w ¼ 1/[s2(F2o)

C16H16Cu3N8O5 590.99 Monoclinic P21/c 10.1122 (8) 11.7268 (6) 18.1481 (17) 90 107.034 (7) 90 2057.7 (3) 4 1.908 3.12 1180 0.30  0.15  0.05 3.2e28.0 42,037 4501 (0.085) 1.02 0.047, 0.081 0.48 and 0.45

C17H18Cu3N8O5 605.01 Monoclinic P21/c 10.1315 (19) 11.856 (3) 19.097 (4) 90 109.464 (14) 90 2162.8 (8) 4 1.858 2.97 1212 0.30  0.10  0.08 2.7e28.0 44,073 4720 (0.12) 0.99 0.034, 0.129 0.36 and 0.45

C11H14CuN4O5$H2O 363.82 Triclinic P1 8.3901 (6) 9.3943 (6) 11.4284 (11) 108.329 (8) 99.316 (7) 112.138 (7) 750.69 (10) 2 1.610 1.49 374 0.40  0.17  0.09 3.3e27.8 15,753 3172 (0.047) 1.00 0.042, 0.091 0.68 and 0.41

þ (aP)2 þ bP], where P ¼ (F2o þ 2F2c )/3.

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Table 2 Selected bond lengths (Å) and angles ( ) for 3 and 4. 3 Cu(1)eN(6) Cu(1)eN(1) Cu(1)eO(2) Cu(1)eO(1) Cu(1)eO(3) Cu(2)eN(2) Cu(2)eN(3) Cu(2)eN(7) Cu(2)eO(1) Cu(3)eN(4) Cu(3)eN(5) Cu(3)eO(1) N(1)eN(2) N(6)eCu(1)eO(3) N(1)eCu(1)eO(2) N(1)eCu(1)eN(6) N(2)eCu(2)eN(3) N(3)eCu(2)eN(7) N(2)eCu(2)eN(7)

4 1.939 1.943 1.969 1.975 2.740 1.956 1.937 2.005 2.011 1.942 1.943 1.994 1.367

92.69 90.75 174.48 164.60 93.04 92.05

3

(4) (4) (3) (3) (4) (4) (4) (4) (3) (4) (4) (3) (5)

1.929 1.934 1.976 1.997 2.790 1.985 1.961 1.988 1.993 1.945 1.911 1.988 1.359

(9) (10) (7) (7) (11) (8) (9) (10) (8) (9) (9) (7) (12)

N(3)eN(4) N(5)eN(6) N(7)eN(8) O(2)eC(13) O(3)eC(13) O(4)eC(16) O(5)eC(16) C(14)eC(15) Cu(2)eO(4I) Cu(3)eO(5II) Cu(3)eO(4II) C(15)eC(17)

(14) (14) (17) (16) (15) (16)

94.5 91.7 172.6 168.2 92.8 91.5

(4) (4) (5) (4) (4) (4)

N(4)eCu(3)eO(5II) N(5)eCu(3)eO(5II) N(4)eCu(3)eN(5) Cu(1)eO(1)eCu(3) Cu(1)eO(1)eCu2 Cu(2)eO(1)eCu(3)

4 1.367 1.356 1.349 1.293 1.215 1.251 1.257 1.307 2.361 1.971 2.798

90.84 93.53 172.30 114.61 117.93 116.39

(5) (5) (5) (5) (5) (5) (5) (6) (3) (3) (3)

1.339 1.388 1.339 1.298 1.206 1.200 1.332 1.323 2.348 1.957 2.948 1.540

(12) (11) (13) (14) (13) (14) (13) (14) (9) (8) (8) (17)

(15) (14) (17) (14) (15) (15)

92.0 92.1 174.0 114.2 117.7 116.5

(4) (4) (5) (4) (4) (4)

Symmetry codes: (I) ex þ 1, y e 1/2, ez þ 1/2; (II) x þ 1, ey þ 3/2, z þ 1/2.

2.4. Syntheses 2.4.1. Synthesis of [Cu3(m3-OH)(m-pz)3(Fum)(Hpz)], 3 Pyrazole (Hpz) (75 mg, 1.10 mmol) was added to a suspension of 48 mg (0.27 mmol) of copper(II) fumarate dihydrate, Cu(Fum)$ 2H2O, in 12 mL of H2O. The mixture was heated to 115  C in solvothermal conditions during 8 h. Blue crystals, suitable for a singlecrystal XRD determination, were obtained, which were manually separated from a little quantity of unreacted 1. 3: Yield: 20 mg, 37.6%. IR (KBr, cm1): 3434, 3105, 1616, 1576, 1490, 1400, 1383, 1342, 1269, 1183, 1172, 1064, 987, 789, 774, 753, 688, 621. 2.4.2. Synthesis of [Cu3(m3-OH)(m-pz)3(MeFum)(Hpz)], 4 The synthesis of 4 was carried out by following a procedure analogous to that of 3. Hpz (526 mg, 7.73 mmol), dissolved in 30 mL of H2O, was added to a suspension of 439 mg (1.93 mmol) of copper(II) 2-methylfumarate dihydrate Cu(MeFum)$2H2O in 75 mL of H2O. The mixture was heated to 115  C in solvothermal conditions during 3 h. Blue needle-shaped crystals, suitable for a single-crystal XRD determination were obtained. 4: Yield: 220 mg, 56.6%. Elem. Anal. Calcd for [Cu3(OH)(pz)3(MeFum)(Hpz)]: C, 33.75; H, 3.00; N, 18.52. Found: C, 33.71; H, 2.95; N, 18.74. IR (KBr, cm1): 3423, 3151, 3110, 1654, 1620, 1612, 1581, 1559, 1510, 1490, 1431, 1424, 1395, 1381, 1367, 1336, 1289, 1274, 1182, 1175, 1070, 1061, 940, 808, 763, 685, 624, 614. meff (295 K) ¼ 2.23 BM [calculated for Cu3(m3-OH)(m-Pz)3(MeFum)(HPz)]. 2.4.3. Synthesis of [Cu(MeFum)(Hpz)2(H2O)]$(H2O), 5 A suspension of 509 mg (2.24 mmol) of Cu(MeFum)$2H2O in 120 mL of H2O, was added to a solution containing 611 mg (8.97 mmol) of Hpz in 80 mL of H2O. The mixture was stirred for ca. 30 min, obtaining a clear blue solution. On standing, a microcrystalline precipitate of compound 4 (132 mg, 0.22 mmol, 29.5%) was obtained as first fraction, then blue, well-formed crystals of 5 were obtained upon slow evaporation of the mother liquors. 5: Yield: 355 mg, 48.4%; ESI-MS (þ) (MeOH, MeCN) (higher peaks, m/z; relative abundance, %): 69.2 (22) [Hpz þ H]þ, 264.0 (18) [Cu(MeFum)(H2O)4 þ H]þ, 328.0 (100) [Cu(MeFum)(Hpz)2 þ H]þ, þ H]þ, 587.0 (1.5) 519.0 (6) [Cu2(MeFum)2(Hpz)2 þ [Cu2(MeFum)2(Hpz)3 þ H] . On standing in the air compound 5 looses water easily, becoming pale violet. The resulting compound, 50 , was

characterized by elemental analysis, magnetic susceptibility and XRPD determination. 50 : Elem. Anal. Calcd for [Cu(MeFum)(Hpz)2]: C, 40.31; H, 3.69; N, 17.09. Found: C, 40.39; H, 3.43; N, 17.18. IR (KBr, cm1): 3325, 3153, 3137, 1604, 1584, 1522, 1465, 1422, 1385, 1337, 1283, 1250, 1135, 1109, 1066, 1046, 912, 805, 770, 702, 607. meff (297 K) ¼ 1.77 BM [calculated for Cu(MeFum)(Hpz)2]. 3. Discussion In the last years we succeeded in the syntheses of a series of CPs based on the triangular [Cu3(m3-OH)(m-pz)3]2þ SBUs which are connected to each other through monocarboxylate anions [8]. We have employed here the same synthetic protocol for the reaction of Hpz with bicarboxylate copper(II) salts, actually fumarate (1) and 2methylfumarate (2). These anions, having a quite rigid similar skeleton, differ only for one methyl, but this little difference seems to have a relevant impact in the obtaining of the corresponding transition metal carboxylates, at least as crystalline material. Actually, from a check on the CCDC database,165 crystal structures containing both fumarate and at least one transition metal ion can be retrieved (53 in the case of copper), while only two Zn [21] and two Cd [22] 2methylfumarate derivatives have been found, and copper complexes are not present. Thus, it was quite unexpected that by reacting 1 and 2 with Hpz in hydrothermal conditions, we obtained well-formed crystals of Cu3(m3-OH)(m-pz)3(Fum)(Hpz)], 3, and [Cu3(m3-OH)(mpz)3(MeFum)(Hpz)], 4, that resulted to be isomorphous. Single-crystal XRD determinations revealed that in both compounds the molecular unit consists of the trinuclear triangular CuII cluster [Cu3(m3-OH)(m-pz)3]2þ, to which a bicarboxylate and a Hpz molecule are coordinated (Fig. 1). Here we describe the molecular structure and the assembly into a 2D CP of compound 3 while most relevant structural features of 4 are reported as Supplementary material. The Cu3N6 nine-membered ring of 3 is formed through the connection of three CuII ions with three pirazolate anions and the capping of a m3-OH ion. The Cu/Cu distances [Cu(1)/Cu(2) 3.4157(8), Cu(2)/Cu(3) 3.4039(8), Cu(3)/Cu(1) 3.3400(8) Å] are in the range found in analogous compounds, as well as the bonding m3-OeCu distances [Cu(1)eO(1) 1.975(3), Cu(2)eO(1) 2.011(3), Cu(3)eO(1) 1.994(3) Å] and m3-O distance from the Cu3 plane [0.388(4) Å] [8,10]. All the three copper ions are pentacoordinated; besides the coordination of m3-O and pyrazolate nitrogens (see

C. Di Nicola et al. / Journal of Organometallic Chemistry 714 (2012) 74e80

77

Fig. 1. Molecular structure of 3 (left) and 4 (right) with partial atom numbering scheme (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

Table 2) one carboxylate moiety asymmetrically chelates Cu(1) [Cu(1)eO(2) 1.969(3), Cu(1)eO(3) 2.740(4) Å], and a Hpz molecule is bonded to Cu(2) [Cu(2)eN(7) 2.005(4) Å]. The coordination scheme is completed by the interactions of Cu(2) and Cu(3) with the carboxylate moieties of two different trinuclear units, being Cu(3) asymmetrically chelated by a carboxylate pertaining to a second trinuclear unit [Cu(3)eO(5I) 1.971(3), Cu(3)eO(4I) 2.798(3) Å], while Cu(2) is bonded to O(4II) of a third trinuclear unit [Cu(2)eO(4II) 2.361(3) Å]. The other geometrical features of 3 are quite normal for this kind of compounds [5,6,8,10], and the most relevant bond distances and angles can be found in Table 2, where the corresponding values of compound 4 are also reported for comparison. In this context, it is to be pointed out that in compound 3 Cu(3) appears to be weakly chelated by carboxylate O(4II)e C(16II)eO(5II), while in the case of 4 Cu(3) is “only” mono00 coordinated by O(5II). Really, in 4 the Cu(3)eO(5II ) [1.957(8) Å] and II Cu(3)/O(4 ) [2.948(8) Å] distances are very close to the corresponding ones of 3, thus the coordination to Cu(3) is very similar in both cases, as evidenced in Fig. 2 where the structures of 3 and 4 are superimposed each other. Compounds 3 and 4, through the above mentioned coordination of Cu(2) and Cu(3) to the carboxylates of adjacent trinuclear units,

Fig. 2. Superimposition of the molecular structures of 3 (green) and 4 (red) evidencing the structural analogies between the two compounds (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

also generate almost identical CPs. According to an arbitrary description scheme, the supramolecular assembly of 3 is due to the interaction of the fumarate dianion acting as a bridge between Cu(1) [chelated by O(2) and O(3)] and Cu(3III) [chelated by O(4) and O(5)] of two trinuclear units [symmetry code III: 1 þ x, 1.5  y, 1/2 þ z], thus generating 1D waved CPs, one of which is shown in Fig. 3 left. Moreover, the coordination to Cu(2) of the O(4II) atom pertaining to a third trinuclear unit (Fig. 3 right) interconnects the 1D CPs generating a 2D CP [symmetry code II: x þ 1, y þ 3/2, z þ 1/2], to which stability contribute also weak H-bonds involving m3-O(1)H of each trinuclear unit and O(4II) [O(1)/O(4II) 2.767(5) Å, O(1)e H(1O)/O(4II) 125(1) ]. In Fig. 4 are shown views of one of these infinite layers that have a thickness of ca. 2.8 Å. Finally, these waved layers are connected each other through a series of quite strong H-bonds, involving the pyrazole NH group of one layer and the O(2) pertaining to adjacent layers [NA(8)/OB(2) 2.736(5) Å NA(8)eHA(8N)/OB(2) 162 , A and B indicating two different layers]. In Figure S1 (Supplementary material) is reported a view of the crystal lattice of 3, while in Figures S2eS4 the most relevant structural features of the isomorphous compound 4 are shown. As reported in the Experimental section compound 4 was obtained in good yield from the solvothermal reaction of copper 2methylfumarate with Hpz in water. Whereas, when the reaction was carried out at r.t., 4 was obtained in a lower yield (ca. 30%), as first microcrystalline product, while from the slow evaporation of the mother liquors in the air, compound [Cu(MeFum)(Hpz)2(H2O)]$ H2O, 5, crystallized in good yield [23]. Contrarily to 3 and 4, 5 is soluble in protic solvents and it was possible to obtain its ESI mass spectrum (see Experimental section). Interestingly, besides the most intense signal at m/z ¼ 328.0, corresponding to [Cu(MeFum)(Hpz)2 þ H]þ, a weak signal at 519.0, attributed to [Cu2(MeFum)2(Hpz)2 þ H]þ, evidences the presence of dinuclear arrangements in the gas phase. ESI-MS data cannot be considered an indisputable proof of the existence of specific polynuclear aggregates in the solid state [24], nevertheless, in the case of 5, they may suggest the presence of a polymeric structure that is

Fig. 3. Compound 3. One waved 1D CP (left) and connections generating the 2D CPs (right) (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

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C. Di Nicola et al. / Journal of Organometallic Chemistry 714 (2012) 74e80

Fig. 4. Compound 3. (left) View down the crystallographic a axis of a 2D layer formed through the connections of four 1D CPs. (right) Arbitrary view of the same 2D CP evidencing the thickness of the layer and the connections among four 1D CPs. In both the figures the four 1D CPs are represented with different colors. For clarity hydrogen atoms and coordinated Hpz have been removed (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

partly disrupted in solution. This feature was confirmed by a singlecrystal XRD determination. The XRD determination revealed that 5 consists of a 1D CP obtained through the self-assembling of mononuclear SBUs where a copper(II) ion coordinates two pyrazoles [Cu(1)eN(1) 1.983(3), Cu(1)eN(3) 1.982(3) Å], one water molecule [Cu(1)e O(1w) 2.532(3) Å], a 2-methylfumarate oxygen [Cu(1)eO(1) 1.943(2) Å] and the O(3) oxygen pertaining to a different mononuclear unit [Cu(1)eO(3I) 1.943(2) Å, symmetry code I: x, y þ 1, z] (Fig. 5). Furthermore, intramolecular H-bonds involve the coordinated water molecule with the carboxylate oxygen O(2) [O(1w)/ O(2) 2.731(3) Å, O(1w)eH(1wA)/O(2) 148(4) ] and with a crystallization water molecule, also present in the asymmetric unit of 5 [O(2w)/O(1w) 2.933(4) Å, O(2w)eH(2wA)/O(1w) 127(4) ], which, in turn, interacts also with O(3) oxygen coordinated to Cu(1) [O(2w)/O(3) 2.767(4) Å, O(2w)eH(2wB)/O(3) 161(4) ]. The coordination of O(3I) to Cu(1) is responsible of the formation of the above mentioned 1D CP, running parallel to the crystallographic b axis (Fig. 5b). Moreover, due to a series of Hbonds, a particular supramolecular assembly of these monodimensional CPs is obtained. Actually, a sort of tape is obtained through the self-assembling of two linear CPs faced each other, thanks to corresponding H-bonds between coordinated water molecules pertaining to a chain and the 2-methylfumarate oxygens O(2) of the second one [O(1w)/O(2II) 2.881(4) Å, O(1w)e

H(1wB)/O(2II) 161(4) , symmetry code II: x þ 1, y þ 1, z þ 1] and to another series of H-bonds involving N(4)H(4) and O(2wIII) [N(4)/O(2wIII) 2.777(4) Å, N(4)e H(4)/O(2wIII) 152 , symmetry code III: x þ 1, y þ 2, z þ 1] (see Fig. 6 left). Moreover, a view down the crystallographic b axis (Fig. 6 right) evidences that these couples of monodimensional CPs further interact through H-bonds that N(2)H(2) groups form with the 2-methylfumarate oxygens O(4) of adjacent CPs [N(2)$$$ O(4IV)2.826(4) Å N(2)eH(2)/O(4IV) 144 , symmetry code IV: x þ 1, y þ 1, z]. The other geometrical features are quite normal and are reported on Table 3. This self-assembly generates a particular structure, where crystallization and coordinated water occupy channels running parallel to the crystallographic b axis (Fig. 7) and are reasonably responsible of the particular behavior of compound 5 described below. Actually, compound 5 becomes pale violet (compound 50 ) upon drying in the air. The elemental analysis of 50 shows that both crystallization and coordinated water molecules have been loosed, while an XRPD diffractogram shows that compound 50 is crystalline, but with a pattern completely different from the one calculated for compound 5 (Fig. 8). Moreover, even though 50 is maintained in a wet atmosphere at r.t. for 24 h the diffractogram does not change, thus indicating that, at least in these conditions, the rehydration process is not achieved.

Fig. 5. Compound 5. (left) Molecular structure with partial atom numbering scheme and indication of intramolecular H-bonds (dashed lines). (right) The 1D CP formed through bridging 2-methylfumarate (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

Fig. 6. Compound 5. (left) A supramolecular tape formed by the self-assembling of two 1D CPs through H-bonds involving coordinated and crystallization water molecules. (right) Self-assembling of two of the above mentioned tapes through NH/O H-bonds (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

C. Di Nicola et al. / Journal of Organometallic Chemistry 714 (2012) 74e80 Table 3 Selected bond lengths (Å) and angles ( ) for 5. Cu(1)eO(3I) Cu(1)eO(1) Cu(1)eN(3) Cu(1)eN(1) Cu(1)eO(1w) O(3)eC(10) O(3I)eCu(1)eO(1) O(3I)eCu(1)eN(3) O(1)eCu(1)eN(3) O(1)eCu(1)eO(1w) O(3I)eCu(1)eO(1w)

1.943 1.943 1.982 1.983 2.532 1.289

(2) (2) (3) (3) (3) (4)

O(1)eC(7) O(2)eC(7) O(4)eC(10) C(8)eC(11) C(8)eC(9)

171.58 89.95 90.29 102.52 85.88

(11) (10) (10) (9) (9)

O(3)eCu(1)eN(1) O(1)eCu(1)eN(1) N(3)eCu(1)eN(1) N(3)eCu(1)eO(1w) N(1)eCu(1)eO(1w)

1.268 1.233 1.240 1.497 1.321

(4) (4) (4) (5) (4)

89.62 89.68 176.90 91.59 91.44

(10) (10) (12) (11) (11)

Symmetry codes: (I) x, y þ 1, z; (II) ex þ 1, ey þ 1, ez þ 1; (III) ex þ 1, ey þ 2, ez þ 1; (IV) ex þ 1, ey þ 1, ez.

79

and could be assigned to a 2-methylfumarate dianion coordinated in the monodentate or in an unsymmetrical chelating bidentate fashion. Among the catalysts having potential for use in selective oxidations, copper takes a special position owing to its versatility. For this reason compounds 3e5 were preliminarily investigated as catalysts toward styrene oxidation by H2O2 (see Experimental section for details). All reactions were run with 5.0 mol % of Cu catalyst/substrate ratio. The preliminary tests show that compounds 3e5 are almost inactive in the reaction conditions used. When the catalytic experiments were carried out in methanol with a strong excess of H2O2, the two regional isomers 2-methoxy1-phenyl-ethanol and 2-methoxy-2-phenyl-ethanol, likely arising from the oxirane ring-opening by MeOH, and benzaldehyde are formed in 2e3% yield, as confirmed by gas-chromatographyemass spectrometry. When the same experiments were carried out in acetone, benzaldehyde (15e16% yield) was the unique product formed. In this solvent higher amount of Cu catalyst improves the styrene conversion only slightly [26]. 4. Conclusions

Fig. 7. Crystal packing of 5 observed down the crystallographic a axis. Water molecules are shown in space-fill representation (crystallization water in green) (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

The IR spectra of 3 and 4 show at least two similar sets of strong bands corresponding to nas(COO) (ca. 1620 and 1580 cm1) and ns(COO) (ca. 1490 and 1400 cm1). The Dn values [nas(COO)  ns(COO)] (from 180 to 80 cm1) are in accordance with both monodentate and chelating bidentate coordination of carboxylates [25]. The IR spectrum of 50 (compound 5 easily looses water on standing in the air at r.t.) is characterized by a similar profile: two set of absorptions due to carboxylates, are actually present, the Dn being ca. 120 cm1

The reactions of copper(II) monocarboxylates with pyrazole in protic solvents easily generate the trinuclear triangular dication [Cu3(m3-OH)(m-pz)3]2þ whose charge is balanced by two monocarboxylate ions [8]. The driving force of this reaction, besides the evident stability of the trinuclear moiety, is the basicity of monocarboxylate anions which is relevant for the deprotonation of pyrazole and water to generate pyrazolate and hydroxo anions, respectively [8a]. The facile preparation of the trinuclear triangular derivatives 3 and 4, from the analogous reaction in water of copper(II) fumarate and 2-methylfumarate, extends for the first time this behavior to copper bicarboxylates and substantiates the absolute generality of this kind of reaction, thus opening the way to the use of a large series of bi- and tricarboxylates as cheap starting material for the simple obtaining of CPs based on the above mentioned trinuclear triangular SBU. On the other hand, due to the complete absence of any copper(II) 2-methylfumarate derivatives in the CCDC database [in addition, just four transition metal 2methylfumarate complexes (2 Zn and 2 Cd) were retrieved] the obtaining of the first two structurally authenticated crystalline derivatives of copper(II) 2-methylfumarate (compounds 4 and 5) was quite unexpected. Moreover, the isomorphism of compounds 3 and 4 suggests that the steric hindrance of the methyl with respect to the hydrogen is not the fundamental factor for the lack of crystal structures of transition metal 2-methylfumarate derivatives in the CCDC database. Further studies with differently substituted fumarates are needed to determine the most relevant factor(s) directing the course of the synthetic reactions and, likely most relevant, of the crystallization processes. Finally, even though copper is a particularly versatile element for catalytic purposes, as observed also for numerous other trinuclear triangular derivatives [8a,cee], preliminary catalytic experiments showed that our compounds are not very active species toward styrene oxidation by dihydrogen peroxide, the formation of benzaldehyde in acetone being however extremely selective. Acknowledgments

Fig. 8. XPRD patterns of 5 (gray) and 50 (black) (For interpretation of the references to colour in figure legends, the reader is refered to the web version of this article.).

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 University of Camerino.

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Appendix A. Supplementary material CCDC 871272 (3), 871273 (4), and 871274 (5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

[9]

[10]

Appendix B. Supplementary material Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.jorganchem.2012.03.026.

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