methane Ligands: Effect of the Anions and Ligand Substitut - American

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Metal Supramolecular Frameworks with Silver and Ditopic Bis(pyrazolyl)methane Ligands: Effect of the Anions and Ligand Substitution Gema Durá,† M. Carmen Carrión,†,‡ Félix A. Jalón,† Ana M. Rodríguez,§ and Blanca R. Manzano*,† †

Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, Facultad de Ciencias y Tecnologías Químicas-IRICA, Avda. C. J. Cela, 10, 13071 Ciudad Real, Spain ‡ Fundación PCYTA, Paseo de la Innovación, 1, Edificio Emprendedores, 02006 Albacete, Spain § Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, Escuela Técnica Superior de Ingenieros Industriales, Avda. C. J. Cela, 3, 13071 Ciudad Real, Spain S Supporting Information *

ABSTRACT: A study of the self-assembly of different silver salts and the potentially ditopic ligands bis(pyrazol-1-yl)(pyridine-4-yl)methane (L1) and bis(3,5-dimethylpyrazol-1-yl)(pyridine-4-yl)methane (L2) was undertaken and species’ with discrete or polymeric structures were obtained. Reactions with L1 always yielded derivatives that had a box-like cyclic dimer structure with two metal centers and two ligands. However, on using ligand L2, the resulting structure strongly depended on the coordination ability of the anion. On using anions that have coordinating ability (X = NO3−, CF3SO3−, PF2O2−), a similar box-like structure was formed. However, the use of anions with lower coordinating ability (X = PF6−, ClO4−) led to coordination polymers with a helical disposition. Spontaneous resolution was observed in the two compounds, and only one enantiomer was present in the crystals studied. Solid-state circular dichroism spectroscopy was applied to bunches of crystals, and the results indicate that growth of single colonies of homochiral crystals starting from single nucleation points may take place. When the helical complexes were recrystallized, box-like cyclic dimers were obtained. The presence or absence of methyl groups on the pyrazolyl rings had a strong influence on the supramolecular structure. In solution, the box-like dimers containing L1 were more stable than those of L2.

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INTRODUCTION The construction and characterization of new supramolecular architectures formed by the coordination-driven self-assembly of organic and metallic components,1 including polymers and metal organic frameworks (MOFs), is an area of intense current interest in inorganic crystal engineering.2 The high level of interest arises not only because of the fascinating structures of these materials but also for their multiple applications in gas storage and separation,3 ion exchange,3e,4 catalysis,3e,f,5 storage and release of drugs,3f,6 use as microreactors,7 semiconductors, or as materials with metallic conductivity8 or sensors9 and also applications related to biological,10 magnetic,3f,11 or luminescence properties.3f,8 The main factors that influence the final structure of selfassembled species are the building blocks: the metal ions, which have a preference for a particular coordination geometry, and the ligand, which provides the donor atoms in the required positions for coordination1a,12 and can also provide potential interaction sites to generate noncovalent interactions, such as hydrogenbonding,13 π−π stacking,14 and cation−π,15 anion−π,16 and XH−π interactions.17 In an effort to achieve the self-assembly process, a wide variety of metal atoms have been used in their stable oxidation states in conjunction with organic linkers, the © XXXX American Chemical Society

most common of which are polycarboxylic aromatic molecules and nitrogenated ligands such as bipyridines or other species that contain azaheterocycles.3e,5,18 Although the formation of certain bonds between organic components and metal centers is foreseeable, it is still difficult to design specific metal−organic materials (MOM) with predictable structures and the presence of multiple weak intermolecular interactions that manifest themselves in a cumulative and complementary manner makes it impossible to predict the overall higher-dimensional packing structures. Insights into the selfassembly process can be obtained by carrying out a systematic study into a series of similar derivatives assembled from a specific type of ligand with the same metal center and carrying out subtle modifications such as changing the substitution pattern of the ligand, the anion, the pH, or the crystallization solvent.18c,19 The coordination chemistry of poly(pyrazolyl)borate or -methane ligands has been widely explored since the pioneering works of Trofimenko.20,21 Different examples of bis(pyrazol1-yl)methane derivatives that contain a substituent at the central Received: April 1, 2014 Revised: May 16, 2014

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dx.doi.org/10.1021/cg5004484 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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carbon have been described by us22 and others.20,21 This family of derivatives can be considered as scorpionates (or heteroscorpionates) if they have the ability to behave as facially coordinating ligands and the chemistry of these systems has been developed to a significant extent.20,21 However, if the third substituent of the ligand is not able to coordinate to the same metal center, the ligand will probably behave as a bridge, and examples of poly(pyrazolyl)methane ligands that act as bridges with different backbones have been described.23,24 In a previous study on the ligands L1 and L2 (Chart 1),25 which were also used

Chart 2. Structures That Have Been Obtained Using ThreeCoordinated L1 or L2 and Metals of Different Geometry

Chart 1. Ligands L1 and L2 and Numbering

in this work, we described complexes in which the ligands were coordinated to two metallic centers, and the final structure mainly depended on the metal geometry. On employing squareplanar ions such as Pd(II), with two positions occupied by ancillary ligands, the preferential formation of trinuclear species was observed with both ligands. However, the use of metal centers that were able to give an octahedral geometry (M = Co(II), Ni(II), Zn(II)) led to the formation of box-like cyclic dimers through the self-assembly of two metal centers and two ligands in a head-to-tail disposition (Chart 2), in a similar way to the complexes reported by Carrano with (4- or 3carboxyphenyl)bis(pyrazolyl)methane ligands26 and by other authors.27 Important differences were not found between the two ligands. Considering the stereochemical flexibility of the silver centers28 and the lability of the Ag−N bonds,29 we envisaged that the reaction of L1 or L2 with silver salts could open new possibilities for different structures and that the weak noncovalent interactions could have a strong influence on the molecular structure and the organization of the supramolecular network.19b,30 Preliminary studies were performed with AgBF4.31 In this case, a strong effect of the type of ligand was found. Thus, the reaction with L1 led to a box-like cyclic dimer with tricoordinated silver ions (complex I), whereas with L2 coordination polymers were obtained with a structure that depended on the crystallization conditions: a zigzag polymer (tricoordinated silver centers, Chart 2, complex IV) and a homochiral helix (di- and tetracoordinated silver ions, complex III). With AgPF2O2 and L1 the box-like cyclic dimer was also formed, but in this case, the dimers were connected by double difluorophosphato bridges to give a monodimensional polymer with pentacoordinated silver centers25 (Chart 2, complex II). The versatility found in the resulting silver complexes and the apparently strong effect of the type of ligand led us to analyze more deeply the self-assembly of ligands L1 and L2 and different silver salts, with the aim of performing a systematic study of the influence on the molecular and supramolecular structure of different factors such as the substitution (Me) on the pyrazolyl rings, the anion and the crystallization conditions. Specific questions that we addressed were would it be possible to obtain a silver complex with L1 that had a structure other than the box-like cyclic dimer or, alternatively, a silver complex of L2 with a boxlike structure?

The anion can influence the crystal structure in two main ways: (i) its potential coordination ability and (ii) the possibility of hydrogen bond formation through its oxygen or fluorine atoms, or other weak interactions. In this sense, we introduced anions that do not tend to coordinate easily, like PF6−32 or BF4−,33 ones that have a weak coordinating ability, e.g., ClO4−,34,35 and others with a higher tendency to coordination, such as CF3SO3−,19f,36 PF2O2−,25,37 and mainly NO3−.38 Examples of anion control in silver coordination derivatives have been described in the literature.18c,19b Very frequently, the effect is ascribed to the coordinating ability of the anion19f,39 although in others the differences are due to other factors including, for example, their size,40 shape,41 or even the solubility of the silver salt in the specific solvent.42 It was envisaged that the study described here could allow correlations to be established between supramolecular interactions and the structural characteristics of complexes, an area of knowledge that is important for the rational design and construction of metal−organic supramolecular complexes and in crystal engineering.

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EXPERIMENTAL SECTION

General Comments. All the syntheses described in this article were carried out in an air atmosphere. Solvents were dried from the

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appropriate drying agents before use. Elemental analyses were performed with a Thermo Quest Flash EA1112 microanalyzer. IR spectra were recorded on solids with an ATR system on a Shimadzu IRPRESTIGE-21 (4000−600 cm−1) spectrophotometer. Mass spectrometry measurements in methanol solution were carried out on a matrix-assisted laser desorption ionization-time-of-flight (MALDITOF, 2,5-dihydroxybenzoic acid, DHB, matrix) Applied Biosystems Voyager DE STR system. The FAB+ mass spectrometry measurements (methanol solution, m-nitrobenzyl alcohol, NBA, matrix) were made with a Thermo MAT95XP mass spectrometer with magnetic sector. Powder X-ray diffraction patterns were collected on Bruker D8 Advance (LynxEye Detector) and Philips X’Pert MPD diffractometers, with Cu−Kα radiation employed in both cases. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out on an ATDTG SETARAM apparatus with a 92−16.18 graphite oven and CS32 controller. The analyses were performed without applying an initial vacuum and at a heating rate of 5 °C/min under an air flux in a platinum crucible. Crystal pictures were obtained with an Olympus SZX7 stereoscopic microscope. 1H, 13C{1H}, 31P{1H}, and 19F{1H} spectra were recorded on Varian Unity 300, Varian Gemini 400, and Inova 500 spectrometers. Chemical shifts (ppm) are relative to tetramethylsilane (1H, 13C NMR), H3PO4/85% (31P NMR), and CFCl3 (19F NMR). Coupling constants (J) are in Hertz. The NOE difference spectra were recorded with a 5000 Hz spectrum width, an acquisition time of 3.27 s, a pulse width of 90°, a relaxation delay of 4 s, an irradiation power of 5−10 dB, and a number of scans of 240. For variable temperature spectra, the probe temperature (±1 K) was controlled by a standard unit calibrated with a methanol reference. In the NMR data, s, d, t, sept, m, and b refer to singlet, doublet, triplet, septuplet, multiplet, and broad signal, respectively. The carbon resonances are singlets. The starting materials bis(pyrazol-1-yl)ketone (bpzCO) and bis(3,5dimethylpyrazol-1-yl)ketone (bpz*CO)22e and the ligands L1 and L2 25 were prepared according to literature procedures. Metallic salts AgBF4, AgPF6, AgNO3, AgOTf, and AgClO4 were purchased from Aldrich or Panreac and were used without further purification. Ag(I) compounds should be stored with the exclusion of light in order to avoid reduction to Ag(0). AgPF2O2 was obtained by hydrolysis of AgPF6 in dichloromethane solution. Then, 0.5 g of AgPF6 (2.0 mmol) were solved in 10 mL of nondried dichloromethane, and the solution was stirred for 3 days at room temperature, protected from light. Afterward, the solid was filtrated and dried under vacuum. A white solid was obtained (0.26 g, 63%). Synthesis Safety Note. Transition metal perchlorates should be handled with caution as they are hazardous and explosive upon heating. Such problems were not encountered in the present study. Circular Dichroism (CD) Spectroscopy. The CD spectra were recorded at room temperature (22 °C) with a Jasco J-810 spectropolarimeter in the solid-state. KBr pellets were prepared by grinding dried KBr with a suitable amount of the crystalline sample (bunches of crystals) with an approximate mass ratio of sample/KBr = 1:100 and subsequent pressing. The measurements were made in a nitrogen atmosphere with a N2 flux of 5L/min with a scan rate of 100 nm/min. Five scans were made for each sample. X-ray Structure Determination for 3, 4·0.5THF, 5−7, 8· 0.25CH2Cl2·0.25THF, 9·0.25THF, 10, and 11. For all compounds, the crystal evaluation and data collection were performed on a Bruker X8 APEX II CCD area detector diffractometer using graphite monochromated Mo−Kα radiation (λ = 0.71073 Å, sealed X-ray tube). Data were integrated using SAINT,43 and an absorption correction was performed with the program SADABS.44 For all structures, a successful solution by direct methods provided most non-hydrogen atoms from the E-map.45 The remaining non-hydrogen atoms were located in an alternating series of least-squares cycles and difference Fourier maps. All non-hydrogen atoms were refined with anisotropic displacement coefficients unless specified otherwise. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. The compounds 5 and 9 show disordered solvent of crystallization. After elaborate attempts to include discrete

solvent molecular entities in the refinement, it was found advantageous to squeeze the solvents, and the final refinement was performed with the modification of the structure factors for the electron densities of the remaining disordered solvent regions.46 For compound 7, only crystals of low quality (Rint = 0.226) could be grown. We made several attempts to obtain better quality data for this structure. However, because of disorder and poor crystal quality, the R2 value is high but we believe that the structural characterization is valid. The compounds 4, 7, 8, 9, 10, and 11 show disorder in the counteranions and some restraints were used. For 4 and 10, the bond lengths were restrained to be the same using DFIX and SADI instructions, and the atomic displacement parameters were restrained using DELU and SIMU instructions in SHELXL fullmatrix least-squares refinement. In general, all the atoms involved in the disorder have been refined with isotropic displacement coefficients.The Flack parameter of complex 8 deviates from the ideal value of 0, and thus, this structure has been refined with TWIN instruction as a racemic twin reaching a value of 0.28(7) for the BASF parameter. Synthesis of the New Derivatives. [Ag(L1)]2(PF6)2, 1. L1 (40 mg, 0.18 mmol) was dissolved in methanol (2 mL), and the solution was added to a methanolic (2 mL) solution of AgPF6 (45 mg, 0.18 mmol). The mixture was protected from light and was stirred for 30 min at room temperature. The resulting white solid was filtered off and dried under vacuum. Yield: 51 mg, 60%. Anal. Calcd for C24H22Ag2F12N10P2· 0.5CH3OH: C, 30.27; H, 2.49; N, 14.41. Found: C, 30.74; H, 2.80; N, 14.32. 1H NMR (methanol-d4, 400 MHz, 25 °C), δ: 6.63 (t, J = 2.1 Hz, 4H, H4-pz), 6.74 (d, J = 5.9 Hz, 4H, H3+H5-py), 7.89 (bs, 4H, H3-pz), 8.24 (d, J = 5.9 Hz, 4H, H2+H6-py), 8.30 (d, J = 2.3 Hz, 4H, H5-pz), 8.36 (s, 2H, Hα) ppm. 13C{1H} NMR (methanol-d4, 100 MHz, 25 °C), δ: 106.68 (C4-pz), 122.53 (C3 + C5-py), 133.51 (C5-pz), 143.25 (C3-pz), 146.53 (C4-py), 151.15 (C2 + C6-py) ppm. 19F{1H} NMR (methanol-d4, 158.25 MHz, 25 °C): −74.76 (d, J = 706.1 Hz) ppm. 31P{1H} NMR (methanol-d4, 127.2 MHz, 25 °C): −144.62 (sept, J = 720.0 Hz) ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 812 [Ag2(L1)2X+, 4.82%], 557 [Ag(L1)2+, 19.52%], 332 [Ag(L1)+, 100%], 226 [(L1)+, 99.50%]. IR (ATR) ν/cm−1: 1516 ν(CN); 829 ν(PF6−). [Ag(L1)]2(ClO4)2, 2. L1 (20 mg, 0.09 mmol) was dissolved in methanol (2 mL), and it was added to a methanolic (2 mL) solution of AgClO4 (18 mg, 0.09 mmol). The mixture was protected from light and was stirred for 30 min at room temperature. The resulting white solid was filtered off and dried under vacuum. Yield: 14 mg, 36%. Anal. Calcd for C24H22Ag2Cl2N10O8: C, 33.32; H, 2.56; N, 16.19. Found: C, 33.69; H, 2.12; N, 16.61. 1H NMR (methanol-d4, 400 MHz, 25 °C), δ: 6.48 (t, J = 2.3 Hz, 4H, H4-pz), 6.69 (d, J = 5.3 Hz, 4H, H3 + H5-py), 7.68 (d, J = 1.4 Hz, 4H, H3-pz), 8.09 (d, J = 2.9 Hz, 4H, H5-pz), 8.19 (s, 2H, Hα), 8.29 (d, J = 5.3 Hz, 4H, H2 + H6-py) ppm. Because of its low solubility, the 13 C NMR has not been possible to register. MS (FAB+, 3-NBA): m/z (assign., rel. int. %): 765 [Ag2(L1)2ClO4+, 43.42%], 665 [Ag2(L1)2+, 18.72%], 557 [Ag(L1)2+, 100%]. IR (ATR) ν/cm−1: 1508 ν(CN); 1103, 1066, 621 (ClO4−). [Ag(L1)(NO3)]2, 3. The product was obtained by crystallization in two phases: the lower phase was a solution of L1 (20 mg, 0.09 mmol) in 2 mL of THF, and the upper phase consisted of a solution of AgNO3 (16 mg, 0.09 mmol) in methanol (2 mL). The phases were left to diffuse protected from light for several days at room temperature. After this time, colorless crystals, suitable for X-ray diffraction, of the product were obtained. Yield: 5 mg, 14%. Anal. Calcd for C24H22Ag2N12O6: C, 36.48; H, 2.81; N, 21.27. Found: C, 36.77; H, 2.75; N, 21.01. 1H NMR (methanol-d4, 300 MHz, 25 °C), δ: 6.57 (m, 4H, H4-pz), 6.79 (d, J = 4.7 Hz, 4H, H3 + H5-py), 7.75 (s, 4H, H3-pz), 8.18 (s, 4H, H5-pz), 8.41 (d, J = 4.7 Hz, 4H, H2 + H6-py) ppm. 13C{1H} NMR (methanol-d4, 75 MHz, 25 °C): 106.87 (C4-pz), 122.44 (C3 + C5-py), 133.96 (C5-pz), 142.21 (C3-pz), 150.47 (C2 + C6-py) ppm. MS (FAB+, 3-NBA): m/z (assign., rel. int. %): 728 [Ag2(L1)2NO3+, 25.06%], 666 [Ag2(L1)2 + H+, 10.20%], 557 [Ag(L1)2+, 31.39%], 332 [Ag(L1)+, 100%]. IR (ATR) ν/cm−1: 1508 ν(CN); 1421, 1298, 1037 (NO3−). [Ag(L1)(CF3SO3)]2, 4. L1 (47 mg, 0.21 mmol) was dissolved in acetone (2 mL) and added to an acetone (3 mL) solution of AgCF3SO3 (55 mg, 0.21 mmol). The mixture was protected from light and was stirred for 10 min at room temperature. The resulting white solid was filtered off and dried under vacuum. Yield: 75 mg, 74%. Anal. Calcd For C



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diffraction, were obtained by careful diffusion of an acetone solution of L2 into a methanol solution of AgPF2O2. {[Ag2(L2)2](PF6)2}n, 8. L2 (30 mg, 0.10 mmol) was dissolved in acetone (2 mL) and added to an acetone (3 mL) solution of AgPF6 (27 mg, 0.10 mmol). The mixture was protected from light and was stirred for 10 min at room temperature. The resulting white solid was filtered off and dried under vacuum. Yield: 49 mg, 86%. Anal. Calcd for C32H38Ag2F12N10P2·Me2CO: C, 37.32; H, 3.94; N, 12.43. Found: C, 37.64; H, 3.81; N, 11.68. 1H NMR (methanol-d4, 500 MHz, 25 °C), δ: 2.33 (bs, 12H, Me3-pz), 2.56 (s, 12H, Me5-pz), 6.20 (s, 4H, H4-pz), 6.70 (d, J = 5.1 Hz, 4H, H3 + H5-py), 7.98 (s, 2H, Hα), 8.25 (bs, 4H, H2 + H6-py) ppm. 13C{1H} NMR (methanol-d4, 125 MHz, 25 °C): 9.92 (Me5-pz), 12.97 (Me3-pz), 67.54 (Cα), 106.45 (C4-pz), 122.97 (C3 + C5-py), 143.92 (C5-pz), 151.66 (C2 + C6-py) ppm. 19F{1H} NMR (methanol-d4, 158.25 MHz, 25 °C): −76.04 (d, J = 710 Hz) ppm. 31 1 P{ H} NMR (methanol-d4, 127.2 MHz, 25 °C): −142.90 (sept, J = 710 Hz) ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 671 [Ag(L2)2+, 19.34%], 497 [Ag2(L2)+, 79.75%], 388 [Ag(L2)+, 51.66%], 282 [(L2)+, 100%]. IR (ATR) ν/cm−1: 1560 ν(CN); 835 ν(PF6−). Crystals of 8·0.25CH2Cl2·0.25THF, suitable for X-ray diffraction, were obtained by careful diffusion of a dichloromethane solution of L2 into an acetone solution of AgPF6, using THF in the medium phase to make the diffusion slower. Crystals of [Ag(L2)(C4H10O)]2(PF6)2 (10), suitable for X-ray diffraction, were obtained by slow diffusion of diethyl ether in gas phase into a methanol solution of complex 8. {[Ag2(L2)2](ClO4)2}n, 9. Ligand L2 (60 mg, 0.20 mmol) was dissolved in acetone (2 mL) and added to a methanolic (3 mL) solution of AgClO4 (42 mg, 0.20 mmol). The mixture was protected from light and was stirred for 10 min at room temperature. The solution was evaporated under vacuum, and the residue was washed with diethyl ether (2 × 5 mL), obtaining a white solid that was dried under vacuum. Yield: 55 mg, 57%. Anal. Calcd for C32H38Ag2Cl2N10O8: C, 39.33; H, 3.92; N, 14.33. Found: C, 39.44; H, 3.76; N, 13.89. 1H NMR (methanol-d4, 300 MHz, 25 °C), δ: 2.35 (bs, 12H, Me3-pz), 2.58 (s, 12H, Me5-pz), 6.22 (s, 4H, H4-pz), 6.70 (bs, 4H, H3 + H5-py), 8.00 (s, 2H, Hα), 8.20 (bs, 4H, H2 + H6-py) ppm. 13C{1H} NMR (methanol-d4, 75 MHz, 25 °C): 9.90 (Me5-pz), 12.88 (Me3-pz), 67.60 (Cα), 106.60 (C4-pz), 123.1 (C3 + C5-py), 143.90 (C5-pz), 151.80 (C2 + C6-py) ppm. MS (MALDITOF+, BP): m/z (assign., rel. int. %): 671 [Ag(L2)2+, 43.54%], 489 [Ag(L2)(ClO4)+, 20.96%], 388 [Ag(L2)+, 100%]. IR (ATR) ν/cm−1: 1556 ν(CN); 1097, 1064, 621 (ClO4−). Crystals of 9, suitable for X-ray diffraction, were obtained by careful diffusion of a dichloromethane solution of L2 into an acetone solution of AgClO4, using THF in the medium phase to make the diffusion slower. A 1H NMR spectrum (methanol-d4) of the crystals revealed the presence of 0.33 molecules of dichloromethane and 0.5 molecules of THF per ligand molecule. [Ag(L2)]2(ClO4)2 (11). Crystals of 11 suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether in gas phase into a methanolic solution of complex 9. Calcd for C32H38Ag2Cl2N10O8: C, 39.33; H, 3.92; N, 14.33. Found: C, 39.00; H, 4.02; N, 14.25. 1H NMR (methanol-d4, 500 MHz, 25 °C), δ: 2.35 (bs, 12H, Me3-pz), 2.58 (s, 12H, Me5-pz), 6.22 (s, 4H, H4-pz), 6.71 (d, J = 5.1 Hz, 4H, H3 + H5-py), 8.00 (s, 2H, Hα), 8.20 (bs, 4H, H2 + H6-py) ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 877 [Ag2(L2)2(ClO4)+, 10.23%], 497 [Ag2(L2)+, 16.57%], 670 [Ag(L2)2(ClO4)+, 31.13%], 388 [Ag(L2)+, 100%]. IR (ATR) ν/cm−1: 1558 ν(CN); 621, 1033, 1097 ν(ClO4−).

C26H22Ag2F6N10O6S2: C, 32.38; H, 2.30; N, 14.52; S, 6.65. Found: C, 32.48; H, 2.28; N, 14.21; S, 6.68. 1H NMR (methanol-d4, 500 MHz, 25 °C), δ: 6.62 (t, J = 2.4 Hz, 4H, H4-pz), 6.73 (d, J = 4.6 Hz, 4H, H3 + H5-py), 7.89 (d, J = 2.0 Hz, 4H, H3-pz), 8.21 (d, J = 4.6 Hz, 4H, H2 + H6py), 8.30 (d, J = 2.4 Hz, 4H, H5-pz), 8.39 (s, 2H, Hα) ppm. 13C{1H} NMR (methanol-d4, 125 MHz, 25 °C): 73.76 (Cα), 106.94 (C4-pz), 122.82 (C3 + C5-py), 133.88 (C5-pz), 143.55 (C3-pz), 146.94 (C4-py), 151.54 (C2 + C6-py) ppm. 19F{1H} NMR (methanol-d4, 158.25 MHz, 25 °C): −81.42 ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 815 [Ag2(L1)2(CF3SO3)+, 4.82%], 590 [Ag2(L1)(CF3SO3)+, 18.11%], 557 [Ag(L1)2+, 27.37%], 332 [Ag(L1)+, 92.34%], 226 [(L1)+, 100%]. IR (ATR) ν/cm−1: 1516 ν(CN); 632, 1145, 1220, 1294 (CF3SO3−). Crystals of 4·0.5THF, suitable for X-ray diffraction, were obtained by careful diffusion of an acetone solution of L1 into a THF solution of AgCF3SO3. [Ag(L2)(NO3)]2, 5. L2 (30 mg, 0.10 mmol) was dissolved in acetone (2 mL) and added to a methanol (3 mL) solution of AgNO3 (18 mg, 0.10 mmol). The mixture was protected from light and was stirred for 10 min at room temperature. The solution was evaporated under vacuum, and the residue was washed with diethyl ether (2 × 5 mL), obtaining a white solid. Yield: 12 mg, 26%. Anal. Calcd for C32H38Ag2N12O6: C, 42.57; H, 4.25; N, 18.63. Found: C, 42.48; H, 4.19; N, 18.03. 1H NMR (methanol-d4, 300 MHz, 25 °C), δ: 2.29 (bs, 12H, Me3-pz), 2.56 (s, 12H, Me5-pz), 6.18 (s, 4H, H4-pz), 6.69 (bs, 4H, H3 + H5-py), 7.97 (s, 2H, Hα), 8.30 (bs, 4H, H2 + H6-py) ppm. 13C{1H} NMR (methanol-d4, 75 MHz, 25 °C): 9.82 (Me5-pz), 13.06 (b, Me3-pz), 67.55 (Cα), 106.47 (C4-pz), 122.91 (C3 + C5-py), 143.95 (C5-pz), 151.71 (C2 + C6-py) ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 669 [Ag(L2)2+, 86.99%], 388 [Ag(L2)+, 100%], 282 [(L2)+, 85.43%]. IR (ATR) ν/cm−1: 1556 ν(CN); 1417, 1298, 1033 (NO3−). Crystals of 5, suitable for X-ray diffraction, were obtained by slow diffusion of diethyl ether in gas phase into a methanol solution of the complex. [Ag(L2)(CF3SO3)]2, 6. L2 (30 mg, 0.10 mmol) was dissolved in acetone (2 mL) and added to an acetone (3 mL) solution of AgCF3SO3 (27 mg, 0.10 mmol). The mixture was protected from light and was stirred for 10 min at room temperature. The resulting white solid was filtered off and dried under vacuum. Yield: 43 mg, 80%. Anal. Calcd for C34H38Ag2F6N10O6S2: C, 37.53; H, 3.42; N, 12.57; S, 5.88. Found: C, 37.97; H, 3.56; N, 13.01; S, 5.96. 1H NMR (methanol-d4, 300 MHz, 25 °C), δ: 2.33 (bs, 12H, Me3-pz), 2.56 (s, 12H, Me5-pz), 6.21 (s, 4H, H4-pz), 6.70 (bs, 4H, H3 + H5-py), 8.00 (s, 2H, Hα), 8.21 (bs, 4H, H2 + H6-py) ppm. 13C{1H} NMR (methanol-d4, 75 MHz, 25 °C): 9.94 (Me5pz), 13.00 (Me3-pz), 67.57 (Cα), 106.48 (C4-pz), 123.16 (C3 + C5-py), 143.90 (C5-pz), 151.45 (C2 + C6-py) ppm. 19F{1H} NMR (metanol-d4, 158.25 MHz, 25 °C): −81.22 ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 497 [Ag2(L2)+, 17.69%], 388 [Ag(L2)+, 76.05%], 282 [(L2)+, 100%]. IR (ATR) ν/cm−1: 1562 ν(CN); 636, 1163, 1228, 1280 (CF3SO3−). Crystals of 6, suitable for X-ray diffraction, were obtained by slow diffusion of diethyl ether in gas phase into a methanol solution of the complex. [Ag(L2)(PF2O2)]2, 7. L2 (30 mg, 0.10 mmol) was dissolved in acetone (2 mL) and added to a methanolic (3 mL) solution of AgPF2O2 (22 mg, 0.10 mmol). The mixture was protected from light and was stirred for 10 min at room temperature. The solution was evaporated under vacuum, and the residue was washed with diethyl ether (2 × 5 mL), obtaining a white solid. Yield: 37 mg, 71%. Anal. Calcd for C32H38Ag2F4N10O4P2: C, 37.52; H, 3.74; N, 13.67. Found: C, 37.14; H, 3.65; N, 13.07. 1H NMR (methanol-d4, 400 MHz, 25 °C), δ: 2.35 (bs, 12H, Me3-pz), 2.58 (s, 12H, Me5-pz), 6.22 (s, 4H, H4-pz), 6.70 (d, J = 5.8 Hz, 4H, H3 + H5-py), 8.00 (s, 2H, Hα), 8.20 (bs, 4H, H2 + H6-py) ppm. 13C{1H} NMR (methanol-d4, 75 MHz, 25 °C), δ: 9.68 (Me5-pz), 12.81 (Me3-pz), 67.28 (Cα), 106.21 (C4-pz), 122.8 (C3 + C5-py), 143.70 (C5-pz), 151.43 (C2 + C6-py) ppm. 19F{1H} NMR (methanol-d4, 158.25 MHz, 25 °C): −84.96 (d, J = 957.5 Hz) ppm. 31P{1H} NMR (methanol-d4, 127.2 MHz, 25 °C): −15.40 (t, J = 953.2 Hz) ppm. MS (MALDI-TOF+, BP): m/z (assign., rel. int. %): 671 [Ag(L2)2+, 11.55%], 497 [Ag2(L2)+, 9.26%], 388 [Ag(L2)+, 100%], 282 [(L2)+, 46,24%]. IR (ATR) ν/cm−1: 1558 ν(CN); 1298, 1033, 844 ν(PF2O2−). Crystals of 7, suitable for X-ray

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RESULTS AND DISCUSSION Synthesis and General Characterization of the New Metallic Derivatives. The synthetic route for the silver complexes is shown in Scheme 1. As stated, derivatives I−IV have been described previously and their structures were determined by X-ray diffraction.25,31 These compounds are included in the scheme for the sake of comparison. The reaction of L1 with different silver salts, regardless of the anion used, resulted in the formation of dinuclear complexes with a box-like structure with two silver centers and two ligands in a head-to-tail disposition (complexes 1−4). Anions were present to achieve electroneutrality, D



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Scheme 1. Silver Complexes Obtained with L1 and L2 Ligands

Chart 2) (complexes 8 and 9, formation of crystals in dichloromethane/THF/acetone). Finally, a different 1D polymer (zigzag, structure type D) with the ligands in a head-to-tail disposition and all silver centers tricoordinated was obtained when complex III was dissolved in methanol and diethyl ether was allowed to diffuse into the solution (complex IV, Chart 2). On applying the same crystallization method (methanol and diethyl ether) to 8 or 9, new complexes 10 and 11 were obtained, and these have a box-like structure with diethyl ether (10) or perchlorate (11) coordinated to fulfill the fourth position of the silver tetrahedral environment. It is noteworthy the strong effect of the coordinating ability of the anions and the crystallization solvents in the structure of complexes with L2. With dichloromethane/THF/acetone and anions of low coordination ability, helical polymers are always formed. However, when methanol is present, box-like cyclic dimers are obtained except in the case of the BF4− anion where a zigzag polymer is formed. In the cyclic dimers, either anion (NO3−, CF3SO3−, PF2O2−, and ClO4−) or diethyl ether (PF6− complex) coordination takes place. The different behavior of the BF4− helix (III) in the recrystallization process of the helices in methanol/diethyl ether may be due to the higher solubility in methanol found for III with respect to 8 and 9. This fact could be due to a higher ability of the BF4− anion to solvate by formation of hydrogen bonds with the methanol molecules. This could hinder the precipitation of the dimer, and thus, the formation of a polymer that should be more insoluble takes place.

and when coordinating anions were used, they were bonded to the metal centers. For the BF4 derivative I, two Ag−F interactions were observed rather than bonding. In II the box-like dimers were bonded through double difluorophosphate bridges to form polymeric chains (see Chart 2). The same type of boxlike dimers with coordinated anions were obtained on using L2 and the coordinating anions NO3−, CF3SO3−, and PF2O2− (complexes 5−7, crystals obtained from acetone/methanol). However, on using L2 and an anion with a weak coordinating ability (BF4−, PF6−, and ClO4−), the structure obtained was a 1D polymer with a helical structure, with the ligands in a headto-head disposition and thus with di- and tetra-coordinated alternating silver centers (i.e., a structure like that of complex III,

Table 1. Crystal Data and Structure Refinement for 3, 4·0.5THF, and 5 empirical formula formula weight temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (calculated) (g/cm3) absorption coefficient (mm−1) F(000) crystal size (mm3) index ranges

reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2c final R indices [I > 2σ(I)]a,b largest diff. peak and hole (e·Å−3)

3

4·0.5THF

5

C24H22Ag2N12O6 790.28 240(2) 0.710 73 monoclinic P21/c 17.0914(9) 10.5090(5) 16.5836(8) 90.0 110.746(1) 90.0 2785.5(2) 4 1.884 1.470 1568 0.19 × 0.15 × 0.12 −24 ≤ h ≤24 −14 ≤ k ≤11 −23 ≤ l ≤22 20477 8445 [R(int) = 0.0490] 8445/0/397 0.719 R1 = 0.0348 wR2 = 0.0516 0.537 and −0.624

C28H26Ag2F6N10O6.50S2 1000.45 230(2) 0.710 73 monoclinic P21/c 11.580(6) 10.831(6) 17.532(9) 90.0 100.360(8) 90.0 2163.1(19) 2 1.536 1.076 992 0.26 × 0.17 × 0.11 −13 ≤ h ≤13 −12 ≤ k ≤12 −20 ≤ l ≤15 13979 3781 [R(int) = 0.0822] 3781/48/311 0.984 R1 = 0.0768 wR2 = 0.2175 1.157 and −0.488

C32H38Ag2N12O6 902.48 230(2) 0.710 73 monoclinic P21/c 8.8841(1) 18.3185(2) 12.7543(2) 90.0 99.527(1) 90.0 2047.05(5) 2 1.464 1.010 912 0.19 × 0.15 × 0.07 −12 ≤ h ≤12 −26 ≤ k ≤24 −18 ≤ l ≤18 21141 6245 [R(int) = 0.0541] 6245/0/239 0.926 R1 = 0.0449 wR2 = 0.0927 0.646 and −0.660

R1 = Σ∥F0| − |Fc|/Σ|F0|. bwR2 = {Σw(F02 − Fc2)2/Σw(F02)2}1/2. cGOF = {Σ[w((F02 − Fc2)2)/(n − p)}1/2, where n = number of reflections and p = total number of parameters refined. a

E



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Table 2. Crystal Data and Structure Refinement for 6, 7, and 8·0.25CH2Cl2·0.25THF empirical formula formula weight temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (calculated) (g/cm3) absorption coefficient (mm−1) F(000) crystal size (mm3) index ranges

reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2c final R indices [I > 2σ(I)]a,b absolute structure parameter largest diff. peak and hole (e·Å−3)

R1 = Σ∥F0| − |Fc|/Σ|F0|. wR2 = {Σw(F0 − p = total number of parameters refined. a

b

2

6

7

8·0.25CH2Cl2·0.25THF

C34H38Ag2F6N10O6S2 1076.60 173(2) 0.710 73 monoclinic P21/c 8.692(1) 24.973(3) 9.372(1) 90.0 103.396(2) 90.0 1979.1(4) 2 1.807 1.182 1080 0.22 × 0.11 × 0.10 −10 ≤ h ≤10 −29 ≤ k ≤29 −11 ≤ l ≤11 18881 3476 [R(int) = 0.0650] 3475/0/275 0.990 R1 = 0.0382 wR2 = 0.0911

C32H38Ag2F4N10O4P2 980.40 230(2) 0.710 73 monoclinic P21/c 8.674(7) 24.68(2) 9.388(8) 90.0 104.689(9) 90.0 1944(3) 2 1.675 1.159 984 0.12 × 0.09 × 0.06 −10 ≤ h ≤10 −29 ≤ k ≤29 −11 ≤ l ≤10 12373 3385 [R(int) = 0.2259] 3385/0/252 0.872 R1 = 0.0805 wR2 = 0.1775

1.020 and −0.759

0.781 and −0.692

C33.25H40.5Ag2N10P2F12Cl0.5O0.25 1107.66 293(2) 0.710 73 orthorhombic P212121 14.5160(10) 14.9160(11) 23.2960(16) 90.0 90.0 90.0 5044.1(6) 4 1.459 0.944 2210 0.18 × 0.12 × 0.08 −17 ≤ h ≤17 −17 ≤ k ≤17 −27 ≤ l ≤15 26630 8832 [R(int) = 0.1046] 8832/12/529 0.959 R1 = 0.0819 wR2 = 0.2013 0.28(7) 1.045 and −0.533

Fc2)2/Σw(F02)2}1/2. cGOF

= {Σ[w((F02 − Fc2)2)/(n − p)}1/2, where n = number of reflections and

The IR spectra of all complexes showed bands corresponding to the stretching frequencies of the (CN + CC) bonds typical of the pyrazole and pyridine rings, as well as the bands due to the different anions.47,48 The presence of coordinated nitrate groups was deduced for complexes 3 and 5. Splitting of the ν(NO) band, which has an E′ symmetry in the free anion, was observed (two bands at around 1415 and 1298 cm−1 were detected), and in addition, the band due to A1′ symmetry is now active and appears at around 1035 cm−1. The difference between the two higher bands indicates a monodentate coordination.49 As far as the triflate group is concerned, it has been proposed that the appearance of a band at around 1300 cm−1 indicates coordination of the anion to a metal center.50 Bands at 1294 and 1280 cm−1 were found for 4 and 6, respectively, due to the coordination of triflate. Elemental analysis data are consistent with the molecular formulas proposed for the complexes. Solid State Characterization. The molecular and crystalline structures of complexes 3, 4·0.5THF, 5−7, 8·0.25CH2Cl2· 0.25THF, 9·0.25THF, 10, and 11 were determined by X-ray diffraction. The crystallographic information is reflected in Tables 1−3, and the data for bond lengths and angles are gathered in the Supporting Information. A complete set of parameters for the noncovalent interactions described below is also given in the Supporting Information. X-ray Structures of Complexes 3, 4·0.5THF, 5, 6, 7, 10, and 11. The molecular structures of all of these complexes are

quite similar and consist of rectangular box-like cyclic dimers formed by the self-assembly of two metal centers and two ligands in a head-to-tail disposition (L1 in 3 and 4 and L2 in 5−7, 10, and 11, see Figure 1). The structures of these molecular cages will be explained together. With the exception of 3, an inversion center exists, and this means that only one-half of the structure is unique. In all complexes the metal ions exhibit a rather distorted tetrahedral geometry by coordination of three nitrogens (two from the pyrazolyl rings of one ligand and the third from the pyridine ring of the other ligand) and one monodentate anion. In the case of complex 10, which contains a noncoordinating anion, PF6−, the fourth position of the tetrahedron is occupied by a diethyl ether molecule. The τ4 value51 (a value of 0 or 1 is expected for regular square-planar or tetrahedral geometries, respectively) varies between 0.67 and 0.83, and on comparing pairs of complexes that contain the same anion, the complex with L2 is always more distorted, probably due to steric hindrance. It is noteworthy that although most of the anions (NO3−, OTf−, and PF2O2−) could behave as bridges,38b,c,19b they do not connect the units. This situation is in contrast to that found in our previously reported complex25 with L1 and the difluorophosphate anion (II). In compound 7 the difluorophosphate anion does not act as bridge, probably due to the steric hindrance created by the methyl groups of the pyrazole rings. The Ag−N distances follow the expected trend considering the basicity of the heterocycles (py > pz* > pz, pz* = 3,5-dimethylpyrazole). The ranges are as follows: dAg−Npy = 2.215−2.287 Å; F



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Table 3. Crystal Data and Structure Refinement for 9·0.25THF, 10, and 11 empirical formula formula weight temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (calculated) (g/cm3) absorption coefficient (mm−1) F(000) crystal size (mm3) index ranges

reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2c final R indices [I > 2σ(I)]a,b absolute structure parameter largest diff. peak and hole (e·Å−3)

R1 = Σ∥F0| − |Fc|/Σ|F0|. wR2 = {Σw(F0 − total number of parameters refined. a

b

2

9·0.25THF

10

11

C33H40Ag2N10Cl2O8.25 995.39 293(2) 0.710 73 orthorhombic P212121 14.1019(8) 14.7080(7) 23.1792(10) 90.0 90.0 90.0 4807.6(4) 4 1.375 0.977 2008 0.15 × 0.10 × 0.04 −17 ≤ h ≤20 −20 ≤ k ≤21 −33 ≤ l ≤14 33347 14785 [R(int) = 0.0782] 14785/52/488 0.791 R1 = 0.0636 wR2 = 0.1483 0.00(3) 0.667 and −0.402

C40H58Ag2F12N10O2P2 1216.64 230(2) 0.710 73 monoclinic P21/n 13.631(5) 14.877(5) 14.093(5) 90.0 118.414(4) 90.0 2513.7(14) 2 1.607 0.932 1232 0.23 × 0.17 × 0.11 −16 ≤ h ≤16 −17 ≤ k ≤17 −16 ≤ l ≤16 16289 4432 [R(int) = 0.1312] 4432/116/320 0.902 R1 = 0.0761 wR2 = 0.1968

C32H38Ag2Cl2N10O8 977.36 290(2) 0.710 73 monoclinic P21/c 8.726(2) 24.848(4) 9.261(2) 90.0 106.22(2) 90.0 1928.0(16) 2 1.684 1.216 984 0.21 × 0.12 × 0.06 −10 ≤ h ≤10 −19 ≤ k ≤29 −11 ≤ l ≤11 12655 3351 [R(int) = 0.0374] 3351/0/244 1.029 R1 = 0.0542 wR2 = 0.1458

0.763 and −0.665

0.644 and −1.100

Fc2)2/Σw(F02)2}1/2. cGOF

= {Σ[w((F0 − 2

Fc2)2)/(n

− p)} , where n = number of reflections and p = 1/2

reported for similar complexes,25,31 the two pyridine rings are parallel or almost parallel (maximum angle α = 3.5° for complex 3) and exhibit a π−π stacking interaction (dCt···Ct = 3.70−3.91 Å, Ct = centroid) that should be favored by the head-to-tail disposition of the two rings14a,19b (see Figure 2 as an example). In Figure 2 and others in which noncovalent interactions are drawn, the atoms involved are represented as balls. Other weak intramolecular interactions are also present: (i) H3 and H5 of the pyridine ring give rise to CH−π interactions with the pyrazole heterocycles of the same ligand (dH−Ct = 2.83−3.11 Å); (ii) different H atoms of the pyridine ring form hydrogen bonds with oxygen in the nitrate complexes 3 and 5, while in the triflate derivatives 4 and 6 the interaction is with oxygen or fluorine atoms52 of the anion (X−H distances of 2.32−2.97 Å and X−C distances of 3.27−3.85 Å); (iii) in some derivatives, hydrogen bonds or anion−π interactions are also observed that involve other regions of the dimer. See Supporting Information for more detailed data. It is noteworthy that the box-like dimers obtained in this work with tetrahedral silver centers are similar to those described previously with octahedral or penta-coordinated metal atoms and that they exhibit similar noncovalent interactions, thus reflecting the high stability of this type of structure. In the following paragraphs a short description of the crystalline structures of these complexes with box-like cyclic dimers is given including the supramolecular interactions. In any case, it must not be forgotten that besides the noncovalent interactions,

dAg−Npz* = 2.299−2.372 Å; and dAg−Npz = 2.300−2.426 Å. In all cases, the two Ag−N(pyrazole) bonds are different in length. The Ag−O bonds are in the range 2.308−2.516 Å, and the shortest distance is that involving the PF2O2− anion. There is a difference between complexes with L1 or L2, and this concerns the value of the O−Ag−N(py) (θ) angle, which is smaller in those complexes that contain methylated pyrazolyl groups, possibly due to the steric repulsion between the methyl groups and the anions coordinated to the metal center (see Figure 1 and Table 4). A better comparison can be made for complexes that contain the same anion. The case of 10 is special because in this compound a diethyl ether molecule is coordinated instead of an anion. As far as the parameters of the nitrogenated ligands are concerned, the bite angle is in the range 77.7−83.6°. The dihedral angle between the two pyrazolyl rings is in the range 106.8− 124.5° and is generally lower for the complexes that contain L2. In terms of the dihedral angle formed by the MN(pz)N(pz) plane and that formed by the four pyrazolyl nitrogen atoms (MNN/N4 angle), which is between 121.7 and 157.0°, more consistent values are found for complexes with ligand L1 (147.1− 156.5°). Comparisons of the parameters of the nitrogenated ligands for the complexes described in this article and others reported previously are discussed below. There are several intramolecular noncovalent interactions in these cages that are common to all seven derivatives and may have an influence on their stability and shape. As previously G



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Figure 1. Structure of the box-like cyclic dimers of complexes 3 (a), 4 (b), 5 (c), 6 (d), 7 (e), 10 (f), and 11 (g).

Coulombic attractions between anions and cations will always be present.53 As there are common characteristics for complexes that contain the same ligand, complexes with ligand L1 will be considered first and then those that contain ligand L2 will be discussed afterward. Supramolecular Structure of Complexes [Ag(L1)(NO3)]2, 3, and [Ag(L1)(OTf)]2·0.5 THF, 4·0.5THF. Interactions between the box-like dimers give rise to the formation of chains that extend along the b axis (see Figure 3 for the chain formed in 3). In these complexes, two anions, one from each box, form a double bridge involving hydrogen bonds. In the case of 3 there is also an Ag−O interaction (dAg1−O6 = 3.00 Å). If we consider that a rhomboid can be defined with the two silver atoms and the two methinic carbon atoms as the vertexes, the rhomboids are not situated perpendicular to the direction of propagation of the chain but are clearly inclined. Besides, the interaction between cages takes place in the region of the long side of the rhomboid. Sheets (ab and bc plane in 3 and 4, respectively) are formed by the interaction of the chains through hydrogen bonds and π−π stacking interactions between pyrazolyl and pyridine rings. The anions are situated in the outer regions of these sheets. It is noteworthy that the existence of these π−π stacking interactions involving both pyridine rings of the dimer leads to the formation of small columns of pz−py− py−pz stacked rings (see Figure 4a and Chart 3). Similar columns were also formed in the two previously described25,31 silver complexes, I and II, that contain L1 and exhibit box-like cyclic dimer structures. This situation reflects the strong tendency to form the π−π stacking interaction, and it requires the appropriate orientation of the dimers.

Table 4. Values for the O−Ag−N(py) (θ) Angle (deg) for Different Complexes with a Box-Like Cyclic Structure complexes with L1 (anion) 3

(NO3−)

4 (OTf−)

a

θ 120.1 (Ag1) 126.3 (Ag2) 116.3

complexes with L2 (anion)

θ

(NO3−)

92.8

6 (OTf −) 7 (PF2O2−) 10a (PF6−) 11 (ClO4−)

104.5 102.0 115.4 84.1

5

A molecule of diethyl ether is coordinated to each silver center.

Figure 2. Noncovalent interactions present in the box-like cyclic structure of complex 5. Hydrogen bonds (blue), π−π stacking (red), and CH−π interactions (orange) are shown. H



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Figure 3. Complex 3. Chains of dimers that extend along the b axis formed by Ag−O interactions (gray) and hydrogen bonds (blue).

Figure 4. (a) Sheet of complex 3 in the ab plane. π−π interactions are shown as red lines. (b) View of two sheets of 4 along the b axis with THF molecules in spacefill.

formation of sheets that extend along the ac plane (see Figure 7, in which four sheets are shown). The sheets are stacked along the b axis (with a displacement of c/2, Figure 7) through some hydrogen bonds and a double CH···π interaction involving one methyl of one sheet and two pyrazolyl rings of the adjacent sheet (see inset of Figure 7a). In this way hydrophobic regions are generated where the pyrazolyl rings are joined. These regions extend along the ac plane (see Figure 7b and Chart 3). It is noteworthy that intermolecular π−π stacking interactions are not observed in these complexes with L2. Structure of Complex [Ag(L2)(C4H10O)]2(PF6)2, 10. As stated above, this derivative contains L2 and a noncoordinating anion, with one diethyl ether molecule coordinated to each silver center. In the crystalline structure the dimers are connected by the long side of the rhomboid through double bridges involving hydrogen bonds to give a chain that extends along the b axis (Figure 8a). The anions, PF6−, connect the ether molecule of one box and the nitrogenated ligand of the other. Other hydrogen bonds give rise to the formation of the 3D structure, but π−π stacking interactions are not observed. It is observed that the region of the dimers where the pyrazolyl rings are located (“short side of the rhomboid”) is facing the same region of another dimer situated in the direction of the c axis. In this way, hydrophobic regions are created in the crystal (see Figure 8b and Chart 3 for a schematic drawing). If we consider the supramolecular chains formed in complexes with box-like dimers, some common characteristics can be

In 3 the sheets are stacked along the c axis through the formation of hydrogen bonds between sheets. In 4 the tetrahydrofuran crystallization molecules are intercalated between the sheets (see Figure 4b) and interact through the formation of hydrogen bonds on both sides of the sheet. Structure of Complex [Ag(L2)(NO3)]2, 5. In this derivative, the structure is best described as consisting of sheets that extend along the bc plane, where each dimer is connected to another six through hydrogen bonds and CH−π interactions (Figure 5). In the direction of the c axis hydrophobic contacts between the methylated pyrazolyl groups of different dimers are observed. The sheets are connected along the a axis. In this direction two kinds of interactions are observed: hydrogen bonds and hydrophobic contacts. The existence of intermolecular π−π stacking interactions was not detected. Structures of Complexes [Ag(L2)(OTf)]2, 6, [Ag(L2)(PF2O2)]2, 7, and [Ag(L2)(ClO4)]2, 11. The supramolecular structures of the three derivatives are analogous. In a similar way to 3 and 4, these complexes can be described by considering the formation of supramolecular chains. In these complexes the chains are formed by the interaction of parallel dimers through the long side of the rhomboid. Hydrogen bonds are established at the upper and lower edges of the chains (see Figure 6 for the chain of 6 as an example) that extend along the [101] direction. The hydrogen bonds involve noncoordinated oxygen atoms of the anions and hydrogen atoms of the nitrogenated ligands. The contact of the chains through hydrogen bonds gives rise to the I



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Chart 3. Schematic Drawing of the Structures of Complexes I, II, 3, 4, 6, 7, 10, and 11, All of Which Are Composed of Box-Like Cyclic Dimersa

a

A = anion. S = diethyl ether. Blue, red and green lines represent hydrogen bonds, π−π stacking interactions, and hydrophobic contacts, respectively.

differences are observed when comparing complexes that contain either ligand L1 or L2. In derivatives with L1 (I, 3, 4, and complex II, which contains L1 and shares similar characteristics to the other L1 complexes but with Ag−O bonds instead of noncovalent interactions to connect the dimers) the dimers are not parallel but form an angle with the noncovalent interactions between the boxes located approximately in the center of the chain (see Figure 3 and Chart 3). However, in the complexes with L2 (6, 7, 10, and 11) the boxes are arranged in a parallel fashion and the interactions are established at the two edges of the chain (Figures 6 and 8a and Chart 3). This disposition seems to be related to the value of the O−Ag−N(py) (θ) angle, which, as stated previously, is smaller in the complexes that contain methylated pyrazolyl groups (see Table 4). Another important difference between the two groups, which is also related to the relative disposition of the boxes, is that in the L1 derivatives the inclination of the boxes allows the observed π−π interaction (pz−py−py−pz column) to be established. Interestingly, the π−π interaction is never observed in derivatives with L2. It is possible that the presence of the methyl groups in L2 will make the stacking more difficult. Instead, the presence of these methyl groups make the pyrazolyl rings more hydrophobic. Indeed, hydrophobic contacts are observed in all of the complexes with this ligand, and the relative disposition of the boxes tends to maximize the contact between the pyrazolyl rings in the hydrophobic regions (Chart 3). In the case of 5, which also contains L2, hydrophobic contacts were also observed. The organization of hydrophobic and hydrophilic regions with respect to each other has been previously described in the literature.54 The disposition of the dimers in complexes with L2 reflects the fact that the long and short sides of the box are polar and apolar regions, respectively.

Figure 5. Complex 5. Sheets in the bc plane, formed by interactions between each dimer box and another six boxes.

observed (with the exception of complex 5), namely: (i) the interaction between dimers to form chains arises from the face of the long side of the rhomboid (see Chart 3) and (ii) this interaction involves the participation of the coordinated anions that give rise to the formation of hydrogen bonds and, in some cases, of Ag−X (X = O, F) interactions. However, clear J



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Figure 6. Complex 6. Formation of a chain that extends along the [101] direction. Hydrogen bonds are shown in blue. Hydrogen atoms have been omitted for clarity.

Figure 7. Complex 6. (a) Drawing of four sheets in two different colors packed along the b axis, formed by the stacking of three chains each. The inset shows the double CH−π interaction between sheets. (b) Drawing of three sheets along the a axis. In both figures the hydrophobic regions created in the ac plane can be observed. See the green rectangles as examples.

X-ray Structures of Complexes {[Ag 2(L 2 ) 2](PF 6) 2 · 0.25THF·0.25CH 2 Cl 2 } n , (8·0.25THF·0.25CH 2 Cl 2 ), and {[Ag2(L2)2](ClO4)2·0.25THF}n, (9·0.25THF). The molecular structures of these two complexes are quite similar and consist of helical coordination polymers in which spontaneous resolution has taken place. The space group of both complexes is P212121 and the right-handed (P) is the only isomer present in the case of 9 in the crystals studied by X-ray diffraction. In the case of 8, the measured crystal is really a racemic twin where the major isomer is the left-handed (M) with a BASF parameter of 0.28. The helices are composed of asymmetric units of formula [Ag2(L2)2]X2 and contain two different silver centers with coordination numbers of 4 and 2 (Figure 9a) and a head-to-head disposition of the ligands. The tetracoordinated silver atom has a seesaw environment (τ4 = 0.62−0.63)51 by chelating coordination of two ligands through the pyrazolyl rings to give the usual six-membered metallacycle. As the four pyrazolyl rings are symmetrically different, strictly speaking these silver ions are chiral. The other silver center is bonded to two pyridine rings of

two different ligands with an almost linear geometry (NAgN angle of 177.6°). This silver atom also exhibits two interactions with fluorine (8) or oxygen (9) atoms of the anions in an approximate trans disposition to give a distorted square-planar environment. As one would expect, the M−N distances are shorter with pyridine, which is the more basic heterocycle (dAg−Npy = 2.121− 2.143 Å and dAg−Npz = 2.215−2.465 Å). Interestingly, in the two examples the Ag−N(pz) bonds that point to the inside of the helix are shorter than those that point toward the outside region. As far as the parameters of the nitrogenated ligands are concerned, the bite angle is in the range 82.76−85.45°. The dihedral angle between the two pyrazolyl rings varies between 108.15 and 124.92°, which is in the usual range for this kind of ligand. The MNN/N4 angle is rather high, in the range 165.68−173.83°. A comparative analysis of these parameters for the different derivatives is discussed below. The helices are generated around a crystallographic 21 screw axis, with each coil containing four silver ions and four ligands (Figure 9a). The pitches are 14.52 and 14.10 Å for 8 and 9, K



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Figure 8. Complex 10. (a) Chain that extends along the b axis with hydrogen bonding interactions shown in blue. (b) Two chains situated in the bc plane.

The contiguous helices are interdigitated55 (see Figure 11), and one helix transmits the chirality to the adjacent helices. In this way, the single crystal contains the same homochiral helix enantiomer. The helices interact through hydrogen bonds involving the anions. Furthermore, the methyl groups of the pyrazolyl rings are gathered in the same region of space and show hydrophobic contacts (see the rectangle highlighted in Figure 11). In this way, sheets parallel to the ab plane are formed with an interstrand distance of 14.92 Å for 8 and 14.71 Å for 9. The sheets are packed along the c direction (perpendicular to the sheets) through the formation of hydrogen bonds with the intermediacy of the anions and the THF molecules (Figure 12). There is a mutual offset of the sheets of b/2 for both derivatives, and this gives rise to an ABAB··· disposition of the sheets. This situation means that each polymer is surrounded by another six polymers in an elongated hexagonal disposition. The polar and apolar regions of the crystal can be seen in Figure 12. The former correspond to the region of contact between sheets where the anions and THF molecules are located, and the latter is situated in the contact between helices of a single sheet where the pyrazolyl rings are gathered. In order to obtain more information about the spontaneous resolution process, solid-state circular dichroism (CD) spectroscopy (in a KBr pellet) was used to characterize crystals of 9. Although one monocrystal was used for the X-ray structure determination of the helical complexes 9, 8, and III, these compounds crystallize as bunches of small crystals. Bunches of 9

respectively. The helical polymers extend along the a axis and generate chiral channels that host solvent molecules (dichloromethane for 8; in the case of 9, although electronic density was detected in the channel, the high disorder prevented refinement of the solvent molecules). In both cases, there are THF molecules outside the channels. The volume of the channels is 283 and 284 Å3 for 8 and 9, respectively, and this represents 9.7% and 10.5% of the unit cell. The channel has a rectangular shape. The corners of this rectangle are the Cα atoms of the nitrogenated ligands, and the sides are alternately occupied by the di- and tetracoordinated silver centers. The dimensions of the rectangle are 6.7 and 9.5 Å for 8 and 6.8 and 9.6 Å for 9. The anions are not hosted within the channels but are situated on the surface of the helices on both sides of the dicoordinated silver ions, and they participate in several hydrogen bonds. In fact, the helix may be stabilized in 8 by an intrastrand system of hydrogen bonds and the Ag−F interaction because one PF6− anion connects ligands of two consecutive turns of the helix (see Figures 9c and 10). A similar system is observed for 9 involving the ClO4− anion. Hydrogen bonds are also formed involving the anions and the THF and dichloromethane molecules in the case of 8. Different intramolecular CH−π contacts are also observed in the two complexes, including one between a pyridine ring and an H atom from a methyl group of a different ligand that is bonded to the same metallic center. This interaction could also contribute to the stabilization of the helix (see Supporting Information for more details). L



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Figure 9. Helical structure of 8 that extends along the a axis (a). Section of the helix with the dichloromethane molecules situated inside the channel in spacefill (b) and (c) side view of the helix. In panel c noncovalent interactions are shown. Hydrogen bonds are shown in blue, CH−π interactions in orange, and Ag−F contacts in gray. Hydrogen atoms and solvent molecules, except dichloromethane in panel a, have been omitted for clarity.

Figure 10. (a) Two helices of complex 8. (b) The inset shows the noncovalent interactions involving the PF6− anion that may stabilize the helix and connect two helices. Hydrogen bonds are shown in blue and the Ag−F contact in gray.

were separated manually from one another, and these crystals were used for the CD studies. The CD spectra of two different bunches are shown in Figure 13. It can be seen that the curves go in opposite directions, and this implies56 that one of enantiomers is preferentially present in the bunches and that locally one enantiomer is formed in excess in each bunch. This finding suggests that the growth of single colonies of homochiral crystals starting form single nucleation points may take place.

Photographs of one and two bunches of 9 are shown in Figure 14, and it can be seen that the bunches growth independently from each other. Spontaneous resolution of molecular compounds upon crystallization to give crystals that can be optically active is a relatively rare phenomenon, but it is of great importance in a number of research areas57 and is of interest because of its involvement in the origin of homochirality. In the case of helical coordination M



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Figure 13. Solid-state CD spectra of two bunches of crystals of 9.

Comparison of Some Structural Parameters of L1 and L in the Complexes Formed. Considering that a relatively large number of structures containing ligands L1 and L2 have been elucidated by X-ray diffraction studies in our current and previous work,25,31 it was decided to perform a comparative analysis of some structural parameters of the ligands in an effort to identify specific trends. The values of the bite angle, the pz−pz and MNN/N4 angles of the metallacycle are gathered in Table 5 for the different complexes, which are classified according to their structure. In addition to the silver complexes, other box-like dimers with octahedral metal centers and palladium(II) trinuclear derivatives have been included. Other parameters related to the py−py π−π stacking interaction of the box-like dimers are also included. Comparison of complexes with different coordination geometries or structures shows that the smallest pz−pz, MNN/N4, and bite angles are observed for the box-like silver dimers, which usually contain tetrahedral metal centers. The angle MNN/N4 is higher for the octahedral derivatives, with values in the range 173.3° to 178.8°. In these cases, the metallacycle should be considered as a half chair rather than as a boat and the metal atoms are very close to the N4 plane. The formation of this structure is 2

Figure 11. Complex 8. Two helices (in different colors) that form a sheet in the ab plane. The green rectangle shows one hydrophobic region.

polymers,58 chiral predetermination is usually achieved by using enantiopure ligands.59 The use of achiral components usually gives racemates of P and M helices,39c,60 and spontaneous resolution without the influence of additional elements of chirality, which yields homochiral helical conglomerates, is not frequent.56,61 Spontaneous chiral resolution requires the efficient transfer of stereochemical information between neighboring homochiral helices.62 One situation that is of great importance for biological systems is that identified in our examples, and it is based on noncovalent supramolecular interactions (hydrogen bonds, π−π interactions, etc.).

Figure 12. View of three sheets of complex 8 along the a axis. The blue and horizontal rectangle on one side and the green and vertical rectangle on the other show examples of polar or apolar regions in the solid, respectively. The atoms of the anions and solvents are represented as balls. N



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Figure 14. Complex 9. Photographs of (a) one bunch of crystals and (b) two bunches of crystals.

Table 5. Structural Parameters for L1 and L2 in Different Complexes Intramolecular π−π stackinge complex [Ag(L1)]2(BF4)2, Ia [Ag(L1)(PF2O2)]n, IIc

structure

M geometry

bite angle (deg)

pz−pz angle (deg)

NMN/N4 angle (deg)

81.0 80.6

112.2 121.0

147.1 156.5

112.8 124.5 118.8 106.8 107.2 108.9 117.4 100.7 137.0 142.9 143.6 139.1 141.4 143.5 118.3 124.9 115.6 108.2 114.5 121.7 106.1

154.7 155.6 151.4 121.7 138.7 143.3 157.0 145.8 176.2 177.1 177.7 178.8 173.3 175.6 172.9 173.8 165.7 173.8 171.7 168.1 156.3

130.3 107.4 126.5

157.5 140.7 151.9

[Ag(L1)(NO3)]2, 3b

box-like dimer tricoordinatedd box-like square planar (polymer) pyramidal box-like dimer tetrahedral

[Ag(L1)(OTf)]2, 4b [Ag(L2)(NO3)]2, 5b [Ag(L2)(OTf)]2, 6b [Ag(L2)(PF2O2)]2, 7b [Ag(L2)(OEt2)]2(PF6)2, 10b [Ag(L2)(ClO4)]2, 11b [Co(L1)Cl(DMF)2)]2[CoCl4]2c

box-like dimer box-like dimer box-like dimer box-like dimer box-like dimer box-like dimer box-like dimer

[Ni(L1)(DMF)3)]2[NO3]4c

box-like dimer octahedral

[Zn(L1)(NO3)(DMF)]2[NO3]2c [Ni(L2)(DMF)(NO3)2]2(NO3)2c {[Ag(L2)](BF4)}n, IIIa

box-like dimer octahedral box-like dimer octahedral helical tetrahedral polymer (+ linear)

{[Ag(L2)](PF6)}n, 8b

helical polymer

tetrahedral (+ linear)

{[Ag(L2)](ClO4)}n, 9b

helical polymer

tetrahedral (+ linear)

{[Ag(L2)](BF4)}n, IVa

zig-zag polymer trinuclear trinuclear trinuclear

t-shaped

80.0 81.1 80.5 83.1 77.7 79.7 83.6 80.6 86.6 86.5 89.2 89.0 89.1 92.1 85.1 84.9 83.3 85.5 84.6 82.8 84.5

square-planar square-planar square-planar

88.9 85.3 88.2

[Pd3(η3-C4H7)3(L1)2](BF4)3c [Pd3Cl6(L2)2]c [Pd3(η3-C4H7)3(L2)2](BF4)3c

tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral octahedral

At···At (Å)

β (deg)

3.85(0) 3.92(0)

C8−C12 = 3.48 C10−C11 = 3.61

26.6 25.6

3.78 (3.5) 3.90(0) 3.79(0) 3.71(0) 3.70(0) 3.89(0) 3.85(0) 3.55(0) 3.50(0) 3.45 (1.22)

C22−C11 = 3.38 N5−C11 = 3.58 C15−C16 = 3.43 C15−C16 = 3.34 C14−C15 = 3.38 N5−C12 = 3.51 C14−C15 = 3.48 C10−N5 = 3.54 C40−N12 = 3.50 N15−C31 = 3.42

3.46(0) 3.32(0)

C10−N5 = 3.45 C14−N5 = 3.31

23.7 25.4 26.3 26.9 26.8 24.8 27.4 27.8 10.1 6.0 6.5 6.6 10.9 8.90

Ct···Ct (Å) (α (deg))

Ref 31 bThis work. cRef 25. dTwo Ag−F interactions are also observed. eCt = centroid; α = dihedral angle formed by the two rings; At···At = distance between the two closest atoms from the two rings; β = angle formed by the Ct−Ct and Ct−plane lines.

a

probably due to steric hindrance between the pyrazolyl rings and the substituent cis to them. If we consider these trends, the value of the MNN/N4 angle for the helical polymers with tetrahedral silver ions is unusually high (171.0°). A closer analysis of the structure leads to the conclusion that in the helical structure, a lower value for this angle would lead to steric hindrance between the pyrazolic methyl groups of one ligand and the pyridine ring of the following ligand in the polymer. Comparison of complexes of the same type, but containing L1 or L2, shows that greater variability is observed in the different parameters for derivatives with L2, whereas the values for L1

derivatives are more consistent. This trend could indicate a higher influence of the steric repulsions in L2 complexes. Furthermore, in cases where sufficient data are available for comparison, a small value for the pz−pz angle is found for derivatives with L2, probably to avoid steric hindrance between the methyl groups and the coordinated anion (or diethyl ether for 10). The geometry of the metal centers and the value of the MNN/ N4 angle influence the parameters of the intramolecular pyridine−pyridine π−π stacking interaction in the box-like dimers. In the octahedral complexes, the pyridine rings are offset to a smaller extent (β = 6−11° for the octahedral derivatives25 and 23.7−27.8° for the silver complexes), and also, the Ct−Ct O



Article 19 1 F{ H}, 31P{1H}, and 13C{1H} NMR spectra were recorded where possible in order to obtain information about the structures of the new complexes in solution. In the mass spectra of complexes 3, 4, and I,31 which have the box-like dimer structure and contain L1, the peak corresponding to the fragment [Ag2L2(X)]+ is always observed, a fact that might indicate that the dimeric species is maintained in solution, at least to some extent. This peak is also observed in the cases of 1 and 2, and this shows that these two complexes could exhibit the boxlike dimer entity. In complex II, which is a polymer formed by dimers, the same peak is also observed. However, this peak is not present for the dimeric species with L2 (except in the case of 11) and the highest mass peaks obtained correspond to [Ag2L]+ (6 and 7) or [AgL2]+ (5), reflecting a possible lower stability in solution of the box-like species with the methylated ligand. Similar peaks were observed for the helical polymers 8 ([Ag2L]+) and 9 ([AgL2]+), both of which contain L2. Thus, some correlations when comparing the behavior of complexes with L1 or L2 can be inferred. In any case, for these labile silver complexes the dissociation and subsequent combination of the fragments could also be a possibility. For all of the peaks described here (see experimental section for details), the calculated isotope pattern was consistent with the formulation. The 1H NMR and 13C{1H} NMR spectra of all the complexes were recorded in methanol-d4 because they were not soluble in other common solvents, such as acetone, and in DMSO, total decoordination of the ligand was observed (the 13C{1H} NMR spectrum of 2 could not be obtained due to low solubility). The room temperature spectra show the presence of a unique and symmetric nitrogenated ligand, and the chemical shifts of the signals are different from those of the free ligand. The Hα and Cα resonances were not observed in some cases, and this fact will be explained below. As we reported previously25 for other complexes with L1, in all of the complexes described in this article (with L1 or L2) a deshielding of 0.2−0.4 ppm was found for the pyrazolyl resonances with respect to those in the free ligand, while shielding of 0.2−0.4 ppm is found for the pyridine signals. The chemical shifts of the resonances of the different complexes with L2 are very similar, and in fact, they are identical when the anion is the same regardless of whether the solid structure was a helix or a box-like dimer. In addition, the signals for Me3-pz and H2/6-py, which are the nearest protons to the coordinating positions, are broad for all the complexes that contain L2. A variable temperature NMR study was carried out on complex 6, and it was observed that on decreasing the temperature the resonances became sharper before becoming totally sharp at −30 °C. The existence of one set of signals is indicative of either the formation of one single species in solution or the presence of rapid equilibration between exchanging species. Considering that broad resonances are observed for complexes with L2 with a broadening that changes with temperature and the lability of the Ag−N bonds,19b,29 we propose that for these complexes a rapid equilibrium between species takes place in solution involving Ag−N bond breaking processes that are slowed down at low temperature. The question then arises as to the nature and structure of the main species in solution. While the deshielding found for the pyrazolyl resonances is in accordance with coordination of these rings to a metal center, the shielding of the pyridine signals is contrary to the normal expectation after coordination. The most probable reason for such conflicting shifts is that the major species in solution are the box-like cyclic dimers with π−π stacking between the two pyridine rings, as

Chart 4. Schematic Representation of the Box-Like Cyclic Dimers Formed Either with Silver or with Octahedral Metallic Ionsa

The dashed red lines represent the π−π stacking between the pyridine rings.

a

distances are shorter for these complexes, which have metallacycles with a half-chair disposition (see Chart 4). It can be concluded from the above analysis that, as expected, the substituted bis(pyrazolyl)methane ligands under investigation are rather flexible and that the steric hindrance between them and other parts of the molecule markedly affect their disposition. The parameters of a specific ligand are influenced not only by the metal geometry and the other ligands in the coordination sphere but also by those ligands situated far from the molecule, especially in the case of the helical compounds. Powder XRD and Thermogravimetric Analysis. Because of the inherent interest of homochiral polymers we decided to register powder XRD diffractograms of 9 and also to analyze its chemical and thermal stability. Comparison of the simulated powder XRD pattern obtained with the Mercury program63 and the experimental diffraction pattern found for a real sample (see Supporting Information) showed that the monocrystal corresponded to the crude product, which was usually obtained as a microcrystalline solid. The stability of 9 in acetone and in the absence of solvent was also assessed. Crystals of 9 were submerged in acetone for a period of 24 h, and after filtration, the presence of acetone and the absence of the solvents previously present (dichloromethane, THF) was verified by 1H NMR spectroscopy. The crystals were subsequently placed under vacuum for 24 h, and the absence of all solvents was confirmed by 1H NMR spectroscopy. Furthermore, it was verified by powder XRD diffraction that the crystals retained structural order. However, when the compound was heated under vacuum at 110 °C overnight, part of the crystallinity was lost (see Supporting Information). Thermogravimetric analysis of 9 indicated that all lattice solvent molecules could be removed by around 200 °C (10% mass loss and 11% calculated from the integration of the 1H NMR spectrum) and that decomposition occurred at about 260 °C when a large mass loss occurred at the same time as an exothermic peak appeared (see Supporting Information). We were interested in evaluating whether a change in the ligand could affect the thermal stability of the complexes. With this aim in mind, a comparative thermogravimetric analysis was performed for two complexes, 4 and 6, which have the same structure (box-like dimer) and contain the same anion (OTf−) and ligand L1 or L2, respectively. Compound 4 contained THF as the crystallization solvent and the loss of this solvent was observed between 160 and 200 °C (about 6% of the mass loss and 4.5% theoretically). A new mass loss took place from 240 °C on, and this should correspond to decomposition. The curves are very similar for both derivatives, and this demonstrates the negligible effect of the type of ligand. Solution Chemistry. MALDI-TOF and FAB+ mass spectra (methanol, see experimental section for details) and 1H, P



Article

locally one enantiomer was formed in excess, or even exclusively, in each bunch. This finding indicates that the growth of single colonies of homochiral crystals starting from single nucleation points may take place. Besides, it has been verified that the monocrystals studied by X-ray diffraction corresponded to the crude of the product and that crystals retain structural order after the removal of the solvents. The structural parameters of the nitrogenated ligands are influenced by steric requirements and depend mainly on the metal geometry and on the ligands in the coordination sphere and also on the ligands situated far from the molecule. The parameters are more consistent for the L1 derivatives. Different noncovalent interactions such as hydrogen bonds and π−π and CH−π interactions are observed in the different structures. The box-like dimers exhibit a set of common intramolecular interactions, including an internal π−π stacking between the two pyridine rings, which may contribute to their high stability. The supramolecular interactions are clearly influenced by the presence of methyl groups on the pyrazolyl rings. Thus, in all complexes with L1, a short pz−py−py−pz column joined by π−π interactions is formed, but this structure is not observed in the L2 compounds. Instead, regions of hydrophobic contacts where the pyrazolic methyl groups are gathered are observed in all compounds with this ligand. This difference is due to the increase in the apolar character of the pyrazolyl rings due to the presence of the methyl substituents. In solution, the presence of an internal π−π stacking interaction was deduced from the 1H NMR chemical shifts. It was also deduced that the box-like dimers containing L1 were more stable than those with L2. Thus, both in solid-state and solution, the preference of L1 derivatives for the box-like dimer structure is clearly observed. The complexes with L1 also underwent a selective deuteration in the position α to the nitrogenated ligand in methanol-d4 solution, but this deuteration was not observed for complexes that contain L2. This finding is currently being studied in more depth. We consider it important to note that the synthetic methods described herein involve one-pot procedures and use ligands that are easy to obtain. The set of results obtained allows conclusions to be drawn on the clear influence of the pyrazolyl substitution, the coordinating ability of the anions, and the crystallization conditions. These factors influence not only the molecular but also the supramolecular structure.

found in the solid state, with protons on each ring shielded by the neighboring ring. This situation results in an upfield shift due to the ring current effect. Such upfield shifts have been widely used to investigate aromatic π−π stacking in solution.19f,64 and they have also been observed in similar box-like dimers previously described with L1 or L2.25 Interestingly, derivatives with these ligands where the π−π stacking interaction is not present do not exhibit this highfield shift of the pyridine resonances.25 The broadening of the resonances found in complexes with L2 but not in those containing L1 could reflect an easier breaking of the Ag−N bonds when the methylated ligands are used. This is consistent with the mass spectra, in which the full fragment of the box was not usually found. As stated above, the signal due to Hα was not observed in the corresponding 1H NMR spectra or the integration was lower than for one proton. We previously observed deuteration of the Hα of the ligand for Zn(II) and Ag(I) complexes that contained L1 and had a box-like dimer structure.25,31 This situation was confirmed for all of the complexes containing L1 described in this work, but it has never been found in complexes that contain L2. This finding may be related with the acidity of this proton,65 and it will be studied in more detail in future work. The expected resonances for PF6− (complexes 1 and 8) and PF2O2− groups37,66 (complex 7) were observed in the corresponding 19F and 31P NMR spectra.

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CONCLUSIONS The self-assembly of the ditopic ligands bis(pyrazol-1-yl)(pyridine-4-yl)methane (L1) and bis(3,5-dimethylpyrazol-1-yl)(pyridine-4-yl)methane (L2) with a series of silver salts yielded different derivatives. The structures of these compounds mainly depended on the pyrazolyl substitution and the coordinating ability of the anions but also on the crystallization conditions. Box-like cyclic dimers and two types of coordination polymers (helical and zigzag) were formed. In all of the complexes, the three nitrogen atoms of the ligands were coordinated to two different silver centers with the two pyrazolyl rings bonded in a chelate fashion. When ligand L1 was used, box-like cyclic dimers were always formed by self-assembly of two tetrahedral metal centers and two ligands in a head-to-tail disposition. Different behavior was found on using ligand L2. In this case, the solid state structure mainly depended on the coordination ability of the anion. The use of anions that have coordinating ability (NO3−, CF3SO3−, and PF2O2−) led to the formation of similar box-like cyclic dimers. However, when anions of lower coordinating ability (PF6−, ClO4−, and also with BF4−, previously reported) were used, helical coordination polymers with alternating di- and tetracoordinated silver centers and spontaneous resolution were obtained. Recrystallization of the helical polymers from methanol/ ether led to a transformation into either a box-like cyclic dimer with solvent (diethyl ether, PF6− derivative) or anion (ClO4−) coordination or a zigzag polymer (BF4− derivative, previously reported). These results show the high tendency to form the boxlike cyclic dimers in the case of L1 complexes, whereas with L2 the formation of this structure only occurs when a group with a high enough coordinating ability is present (anion or solvent). The coordinating ability of ClO4− seems to be in the forefront, and the two structures described above were obtained depending on the crystallization conditions. Solid-state circular dichroism spectroscopy applied to bunches of crystals of the helical polymers showed that one or other of the enantiomers was preferentially present in the bunches and that

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format. ORTEP representations of the structures. Tables with a selection of bond distances and angles, and data for noncovalent interactions. Powder X-ray difractograms. Thermogravimetric and DTA analysis of some compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(B.R.M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. Q



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Article

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ACKNOWLEDGMENTS This work was supported by the MICINN of Spain (CTQ201124434, FEDER Funds). We thank the INCRECYT program (contract to M.C.C.) and the MEC of Spain for an FPU grant (to G.D.).

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