Polynuclear Complexes Containing Ditopic Bispyrazolylmethane

Article pubs.acs.org/crystal

Polynuclear Complexes Containing Ditopic Bispyrazolylmethane Ligands. Influence of Metal Geometry and Supramolecular Interactions M. Carmen Carrión,†,‡ Gema Durá,† Félix A. Jalón,† Blanca R. Manzano,*,† and Ana M. Rodríguez§ †

Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, Facultad de 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: The new ligands bis(pyrazol-1-yl)(pyridine-4yl)methane (bpzm4py) (L1) and bis(3,5-dimethylpyrazol-1yl)(pyridine-4-yl)methane (bpz*m4py) (L2) were synthesized and were made to react with different metallic starting materials. In the case of Pd(II), chloride or allyl trinuclear complexes were synthesized, in which the central palladium is bonded to two ligands through the pyridine moiety. Mononuclear [Pd(allyl)L]X complexes were also isolated. On using other M(II) centers (M = Co, Ni, Zn), which could adopt an octahedral geometry, box-like cyclic dimers formed by the self-assembly of two metal centers and two ligands in a head-to-tail disposition were obtained. All metal ions exhibited a distorted octahedral geometry. A complex of Ag(I) with similar cyclic dimers connected through difluorophosphate anions to generate zigzag chains was also crystallized. The silver center was five-coordinate and the chain interactions gave rise to the formation of sheets. In the solid state, different noncovalent interactions were present in the molecular and supramolecular structures, including hydrogen bonds, π−π stacking and anion−π or CH−π interactions. Examples of possible synergy between some of these interactions were found. Where possible, the solution chemistry was analyzed and correlated with the solid state structure. The existence of polynuclear species in solution was evaluated and the effect of some noncovalent interactions on the NMR resonances was observed.

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π−π stacking,11 and cation−π,12 anion−π,13 and XH−π interactions.14 A judicious selection of the building blocks and how they are connected allows some control over the final structure of the MOMs (metal−organic materials). Since most transition metal connectors are cationic, an anionic source should neutralize the overall charge. These anions exist as free guests, counterions or linkers in the coordination polymers and they usually act as hydrogen bond acceptors through their oxygen or fluorine atoms and may influence the crystal structure of the material obtained. Other factors, such as the solvent or other species that may act as templates, can also affect the structure of the supramolecule. A wide variety of metal atoms in their stable oxidation states have been used in the self-assembly processes and the most common organic linkers have been polycarboxylic aromatic molecules and nitrogenated ligands such as bipyridines or other species containing azaheterocycles.3,4e,15

INTRODUCTION In recent years significant progress has been made in the synthesis and characterization of infinite one-, two-, and threedimensional inorganic/organic hybrid networks formed by the self-assembly of organic and metallic components.1 Among them, MOFs (metal organic frameworks), which are crystalline and porous compounds involving strong metal−ligand interactions,2 are receiving particular attention because of their applications.3 The pores of MOFs are usually occupied by solvent molecules that must be removed for most applications, without structural collapse, to generate a permanent porosity. Applications include gas storage and separation,4 ion exchange,4e,5 catalysis,3,4e,f storage and release of drugs,4f use as semiconductors6 or sensors,7 and also applications related to magnetic2,4f,8 or luminescent properties.4f,6 The key features that influence the architecture of selfassembled species are the building blocks: the metal, which usually has a preferred coordination environment, and the ligand, which not only provides the donor atoms situated at specific positions1a,9 but can also provide potential interaction sites to generate noncovalent interactions, such as hydrogen-bonding,10 © 2012 American Chemical Society

Received: December 20, 2011 Revised: February 2, 2012 Published: February 9, 2012 1952

dx.doi.org/10.1021/cg201677s | Cryst. Growth Des. 2012, 12, 1952−1969

Crystal Growth & Design

Article

A “two-step self-assembly” methodology has also been developed and this involves the synthesis of molecular building blocks that are themselves metal complexes. In this way, a metalloligand is synthesized initially and this compound is subsequently added to another metal ion, which acts as a node in the framework and, consequently, two kinds of metal center could coexist in a framework.4e,j These metalloligands may expand the type of connectors available and offer control over the bond angles and number of coordination sites and could also allow the possibility of generating longer spacers to modify the size of the cavity in the case of MOF derivatives. In recent years, we have synthesized bis(pyrazol-1-yl)methane derivatives that contain a substituent at the methynic carbon.16,17 These types of derivatives may be classified as scorpionates (or heteroscorpionates) if they have the ability to behave as facially coordinating ligands and the chemistry of these systems has been profusely developed.18 However, if the position of the donor atom in the third substituent and the rigidity of the ligand preclude coordination of the three donor atoms to the same metallic center, the ligand will probably behave as a bridge. Ligands of this type were reported by Carrano et al. and these examples involve the use of (4- or 3-carboxyphenyl)bis(pyrazolyl)methane ligands for the synthesis of mononuclear complexes or box-like cyclic dimers of Cu(II),19 Ni(II), and Co(II),20 which lead to supramolecular structures through different weak interactions, mainly hydrogen bonds. Covalently bonded polymeric 1D chains were obtained by including dicarboxylate ligands to bridge the dinuclear units.21 Dinuclear species or chain-like structures have also been obtained with other pyrazolyl-containing ligands and Ag(I),22 Cu(II),23 or Au(I).24 Other examples of poly(pyrazolyl)methane ligands that act as bridges have been described and include systems containing pyrazolyl-pyridine “arms”25 or the ditopic phenylene- or methylenelinked tris- or bis(pyrazolyl)methane ligands described by Reger et al. that led to discrete molecules of Pt(II), Re(I),26 or Ru(II)27 and also to bi- or trinuclear or 1D coordination polymers of Ag(I).28 In the work reported here, we targeted the synthesis of bis(pyrazol-1-yl)methane ligands with a 4-pyridyl substituent in the central carbon with the aim of obtaining all-nitrogen donor ligands for use as connectors between metallic centers or to obtain metalloligands that could form supramolecular structures by self-assembly. The ligands obtained and the numbering are represented in Chart 1.

that it has recently been demonstrated that in different systems there is interplay between ion−π and π−π, CH−π or hydrogenbonding interactions, which can lead to cooperative effects,29,30 possible examples of this synergy were sought in this study. A comparison between the solid state and solution structure was also undertaken.

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EXPERIMENTAL SECTION

General Comments. The synthesis of the ligands and palladium complexes was carried out under a nitrogen atmosphere using standard Schlenk techniques, while the rest of the reactions were carried out in the air. Solvents were freshly distilled from the appropriate drying agents and degassed before use. Elemental analyses were performed with a Thermo Quest Flash EA1112 microanalyzer. IR spectra were recorded on microcrystalline solids with an ATR system on IRPRESTIGE-21 Shimadzu (4000−600 cm−1) spectrophotometer. Mass spectrometry measurements were carried out on a matrix assisted laser desorption ionization-time-of-flight (MALDI-TOF) Applied Biosystems Voyager DE STR system or with a Q-q-TOF hybrid analyzer with an electrospray ionization source (ESI-TOF) QStar Elite Applied Biosystems spectrometer. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were made with an ATDTG SETARAM apparatus with a 92−16.18 graphite oven and CS32 controller. The analyses were made, without applying initial vacuum and with a heating rate of 5 °C/min, under an air flux in a platinum crucible.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 1H−13C g-HMQC and g-HMBC spectra, the standard VARIAN pulse sequences were used (VNMR 6.1 C software). The spectra were acquired using 7996 (1H) and 25133.5 Hz (13C) spectrum widths; 16 transients of 2048 data points were collected for each of the 256 increments. 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, and b refer to singlet, doublet, and broad, respectively. The carbon resonances are singlets. UV−visible spectra were recorded on an Uvikon-XS spectrofluorimeter. The starting materials bis(pyrazol-1-yl)ketone (bpzCO) and bis(3,5-dimethylpyrazol-1-yl)ketone (bpz*CO),17c PdCl2(PhCN)231 and [Pd(η3-C4H7)(μ-Cl)]232 were prepared according to literature procedures. The metallic salts CoCl2, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, AgBF4, and AgPF6 were purchased from Fluka or Aldrich and were used without further purification. X-ray Structure Determination for L2, 2·2Me2CO, 3, 4·Me2CO· 0.5H2O, 7·DMF, 9·2DMF, 10·3DMF, 11, and 13·0.5THF. 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 SAINT33 and an absorption correction was performed with the program SADABS.34 Far all structures, a successful solution by the direct methods provided most nonhydrogen atoms from the E-map.35 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. Complex 3 shows disorder for the central allyl group, one pyrazol and the BF4− counterions and some restraints are used (DELU, SIMU and FLAT). In the crystal structure of compound 7, two DMF coordinated molecules are disordered over two positions and are modeled as a 50:50 isotropic mixture. Compounds 3, 7, and 11 crystallize with some DMF molecules of solvent heavily disordered. Compound 13 crystallize with some acetone molecules disordered. A significant amount

Chart 1. Ligands L1 and L2, Numbering and Abbreviations

Other objectives of this work were to analyze the effect on the final structure of the type of metal and the anion used by considering their potential coordination ability and the possibility of hydrogen bond formation. We were also interested in evaluating the influence of the noncovalent interactions on the shape of the molecules and the crystalline structure. Considering 1953



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H3+H5-py), 8.09 (s, 1H, Hα), 8.64 (bs, 2H, H2+H6-py) ppm. 13C{1H} NMR (acetone-d6, 100 MHz, 25 °C): δ = 10.48 (Me5-pz), 14.06 (Me3pz), 20.97 (Me-allyl), 59.76 (CH2-allyl), 68.13 (Cα), 107.6 (C4-pz), 120.9 (C3+C5-py), 150.48 (C4-py), 153.60 (C2+C6-py) ppm. 19F NMR (acetone-d6, 211 MHz, 25 °C) δ = −151.8 ppm. IR (ATR) ν/cm−1: 1557 ν(CN); 1047, 1032 ν(BF4−); 1393, 864 (2-Me-allyl). λmax (MeOH, ε): 222 (33700 M−1 cm−1); 270 (5000 M−1 cm−1). MS (MALDI-TOF+,SA): m/z (assign., rel. int. %): 442 [Pd(C4H7)(bpz*m4py)+, 100]. [Zn(bpz*m4py)(DMF)(NO3)]2(NO3), 12. Zn(NO3)2 (10.0 mg, 0.03 mmol) and L2 (10.0 mg, 0.03 mmol) were dissolved in DMF (1 mL) at room temperature. The mixture was stirred for 5 minutes. By slow diffusion of diethyl ether in gas phase into the DMF solution, compound 12 was obtained as a crystalline colorless product that was dried under vacuum. Yield: 11.7 mg, 36%. Anal. Calcd for C32H38N14O12Zn2·2C3H7NO·H2O: C, 41.28, H, 4.92; N, 20.27. Found: C, 41.37; H, 4.81; N, 20.40. 1H NMR (methanol-d4, 500 MHz, 25 °C): the resonances of the ligand are broad. Those of the box-like dimer are the following: δ 2.17−2.20 (bs, 24H, Me3+Me5-pz), 6.35−6.48 (bs, 8 H, H3+H5-py and H4-pz), 8.05 (bs, 2H, Hα), 8.25 (bs, 4H, H2+H6-py) ppm. IR (ATR) ν/cm−1: 1654 ν (CN), 1460, 1309, 1037 (η2- NO3−); 1423, 698 (NO3−). λ max (MeOH, ε): 261 (4253 M−1cm −1); 289 (2310 M−1cm−1). MS (MALDI-TOF+, SA): m/z (assign., rel. int. %): 501 [Zn2(bpz*m4py)+ + 5H2O, 100], 281 [bpz*m4py, 13].

of time was invested in identifying and refining the disordered molecules. Bond length restraints were applied to model the molecules but the resulting isotropic displacement coefficients suggested the molecules were mobile. Option squeeze of program PLATON36 was used to correct the diffraction data for diffuse scattering effects. Note that all derived results in the corresponding tables are based on the known contents. X-ray crystallographic information files (CIF) are available for compounds L2, 2·2Me2CO, 3, 4·Me2CO·0.5H2O, 7·DMF, 9·2DMF, 10·3DMF, 11, and 13·0.5THF. Syntheses of the New Derivatives. The synthesis of all the new derivatives described in this paper is included in the Supporting Information. In the following paragraphs, the synthesis of some representative compounds with NMR data relevant for the Results and Discussion section is included. bpz*m4py, L2. bpz*CO (2 g, 9.16 mmol) and 4-pyridinecarboxaldehyde (0.87 mL, 9.16 mmol) were mixed in toluene (30 mL). After refluxing for 72 h, the yellowish solution was evaporated, and the residue was washed with pentane (3 × 10 mL), obtaining a white solid of bpz*m4py. Yield: 2 g, 78%. Anal. Calcd for C16H19N5: C, 68.30; H, 6.81; N, 24.89. Found: C, 68.54; H, 6.73; N, 25.10. 1H NMR (acetoned6, 500 MHz, 25 °C): δ = 2.12 (s, 6H, Me3-pz), 2.22 (s, 6H, Me5-pz), 5.91 (s, 2H, H4-pz), 6.96 (d, J = 4.3 Hz, 2H, H3 + H5-py), 7.77 (s, 1H, Hα), 8.58 (d, J = 4.3 Hz, 2H, H2 + H6-py) ppm. 13C{1H} NMR (acetone-d6, 125 MHz, 25 °C): δ = 11.16 (Me5-pz), 13.16 (Me3-pz), 72.73 (Cα), 106.72 (C4-pz), 122.19 (C3+C5-py), 141.40 (C5-pz), 145.92 (C4-py), 148.19 (C3-pz), 150.08 (C2+C6-py) ppm. IR (ATR) ν/cm−1: 1595 ν(CN). λmax (MeOH, ε): 219 (13970 M−1cm−1); 261 (2526 M−1cm−1). Single crystals suitable for X-ray analysis were obtained by slow evaporation of a toluene solution of L2. [Pd3(η3-C4H7)3(bpz*m4py)2](BF4)3, 4. [PdCl(η3-C4H7)]2 (21.0 mg, 0.05 mmol) and AgBF4 (21.0 mg, 0.10 mmol) were dissolved in acetone (5 mL), and the mixture, protected from the light, was stirred for 2 h at room temperature. After this time, it was filtered and the solution added to an acetone (5 mL) solution of L2 (20.0 mg, 0.07 mmol). The reaction mixture was stirred at room temperature for 1 h. The solvent was evaporated to dryness and the white product dried under vacuum. Yield: 37.0 mg, 75%. Anal. Calcd. for C44H59B3N10F12Pd3·0.5C3H6O: C, 41.30; H, 4.80; N, 10.26. Found: C, 40.95; H, 4.93; N, 10.16. 1H NMR (acetone-d6, 400 MHz, 25 °C): δ = 1.36 (s, 6H, Me-allylext), 2.24 (s, 3H, Me-allylcent), 2.33 (s, 12H, Me3-pz), 2.65 (s, 12H, Me5-pz), 3.19 (bs, 4H, Hanti-allylext), 3.28 (bs, 2H, Hanti-allylcent), 3.91 (bs, 2H, Hsyn-allylcent), 4.06 (bs, 4H, Hsynallylext), 6.37 (s, 4H, H4-pz), 6.76 (d, J = 4.9 Hz, 4H, H3+ H5-py), 8.00 (s, 2H, Hα), 8.67 (d, J = 4.9 Hz, 4H, H2+ H6-py) ppm. 13C{1H} NMR (acetone-d6, 100 MHz, 25 °C): δ = 10.21 (Me5-pz), 13.70 (Me 3-pz), 20.70 (Me-allyl ext), 22.63 (Me-allylcent), 60.02 (CH2-allylext), 60.95 (CH2-allylcent), 67.59 (Cα), 107.64 (C4-pz), 123.58 (C3+C5-py), 132.42 (C-allylext), 135.94 (C-allylcent), 145.24 (C5-pz), 148.09 (C4py), 152.61 (C2+C6-py), 154.07 (C3-pz) ppm. 19F NMR (acetone-d6, 211 MHz, 25 °C) δ = −150.9 ppm. IR (ATR) ν/cm−1: 1558 ν(C N); 1049, 1035 ν(BF4−); 1392, 867 (2-Me-allyl). λmax (MeOH, ε): 222 (79480 M−1cm−1); 285 (7840 M−1cm−1). MS (MALDI-TOF+,SA): m/z (assign., rel. int. %): 442 [Pd(C4H7)(bpz*m4py)+, 100]. Colorless crystals of 4·Me2CO·0.5H2O, suitable for X-ray diffraction, were obtained by slow diffusion of diethyl ether in gas phase into an acetone solution of complex 4. [Pd3(η3-C4H7)(bpz*m4py)](BF4), 6. [PdCl(η3-C4H7)]2 (25.0 mg, 0.06 mmol) and AgBF4 (25.0 mg, 0.12 mmol) were dissolved in acetone (5 mL), and the mixture, protected from the light, was stirred for 2 h at room temperature. After this time, it was filtered and the solution added to an acetone (5 mL) solution of L2 (70.0 mg, 0.25 mmol). The reaction mixture was stirred at room temperature for 1 h. The pale solution was evaporated under vacuum and the residue was washed with ether (3 × 4 mL) obtaining the complex as a white powder. Yield: 41.0 mg, 61%. Anal. Calcd for C20H26BN5F4Pd· 0.5H2O: C, 44.60; H, 5.05; N, 13.00. Found: C, 44.47; H, 4.55; N, 12.63. 1H NMR (acetone-d6, 400 MHz, 25 °C): δ = 1.37 (bs, 3H, Me-allyl), 2.35 (s, 6H, Me3-pz), 2.72 (s, 6H, Me5-pz), 3.12 (bs, 2H, Hanti-allyl), 4.02 (bs, 2H, Hsyn-allyl), 6.41 (s, 2H, H4-pz), 6.65 (bs, 2H,

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RESULTS AND DISCUSSION Synthesis and General Characterization of the Ligands. The ligands used in this work, bpzm4py (L1) and bpz*m4py (L2), were synthesized in good yield by following a green methodology developed in our research group for the synthesis of substituted bis(pyrazol-1-yl)methane ligands. This approach involves the use of solid triphosgene instead of the previously reported and more volatile phosgene.17c The intermediate ketone derivatives bis(pyrazol-1-yl)ketone (bpzCO) or bis(3,5-dimethylpyrazol-1-yl)ketone (bpz*CO) were reacted with pyridine-4-carboxaldehyde in refluxing toluene (Scheme 1). Scheme 1. Synthesis of L1 and L2

The 1H NMR spectra of these ligands show that both pyrazolyl groups are equivalent and NOE effects, observed between Hα and H3/5 of the pyridine ring and also with H5/Me5 of the pyrazolyl rings, confirm the formation of the new compounds. The structure of ligand L2 was determined by X-ray diffraction (see solid state characterization). Synthesis and General Characterization of the New Metallic Derivatives. The ligands L1 and L2 can be schematically represented as shown in Chart 2. If we consider the possible products that can be formed by reaction of these ligands and a square-planar metal center with two accessible coordination sites, some coordination alternatives are represented in Chart 2 for different M:L stoichiometries. In structures I, II, and III, which involve chelate coordination through the pyrazole rings (I) or monodentate coordination via the pyridinic nitrogen (II, III), some ligand donor atoms remain 1954



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Chart 2. Schematic Representation of Ligands L1 and L2 and Some Hypothetical Species That Could Be Formed by Reaction with Square-Planar Metallic Centers

Zn(II), and Ag(I) derivatives were performed as described below. The reaction of ligands L1 and L2 with PdCl2(PhCN)2 was initially performed with Pd:L ratios of 1:1 and 1:2 to assess the possibility of obtaining mononuclear complexes of type I or II, respectively. Very insoluble derivatives were obtained and elemental analysis data were not consistent with the expected stoichiometries. The 1H NMR spectra had to be recorded in DMSO-d6 and a complex set of broad signals was obtained, possibly due to interchange processes between different species that could even involve the solvent. As a result, the isolation of crystals was targeted and those obtained had the stoichiometry Pd/L = 3:2. The same crystals were obtained on reacting the starting materials in a 3:2 ratio. On the basis of steric requirements, the most reasonable structure for the new derivatives 1 and 2 is IV, in which the central palladium atom has a trans disposition (see Scheme 2). Considering the formation of these products, we decided to analyze the possible formation of a structure of type V, in which an ancillary ligand forces a cis disposition in the central palladium center, as well as the possibility of forming structures of type I or III. Thus, the reactions of ligands L1 or L2 with the allyl-solvate derivative generated in situ, that is, [Pd(η3-C4H7)(acetone)2]BF4, were performed with different Pd/L ratios (Scheme 2). On employing a 1:1 ratio the main products obtained, after purification of the corresponding solid, were the trinuclear derivatives 3 and 4 (from ligands L1 and L2, respectively) with a structure of type V. These compounds were better obtained using a Pd:L ratio of 3:2. To obtain the mononuclear derivatives 5 and 6, with a structure of type I, an excess of ligand (1:2 ratio) was necessary. Species of type III (with two ligands) were not detected. The structures of 2, 3, and 4 were determined by X-ray diffraction (see below), and their trinuclear nature was confirmed. Reaction of the ligands L1 and L2 with other M(II) centers (M = Co, Ni, Zn) (M/L = 1:1) with at least three accessible coordination sites gave box-like cyclic dimers with an M/L ratio of 1:1, reflecting a structure of type VI (Chart 3). The derivatives obtained (7−12) are shown in Scheme 3. In the case of complexes 7, 9, 10, and 11 the structures were determined by X-ray diffraction (see below) and the ancillary ligands are those included in Scheme 3. Anions are present to achieve electroneutrality and, in some cases, one is coordinated to each metal center. It is interesting to note that in the case of 7 a [CoCl4]2− complex per dimer is present as counteranion, a fact that explains the intense blue color of the complex. Complex 8 is

that are not coordinated to the metal and the formation of supramolecular species through coordination to other centers could be possible. In fact, it must be stated that, unless the coordinaton to another metallic center takes place, the adoption of structures II and III, in which the chelate system remains uncoordinated, is not very probable. Only four of the several hundreds of structures reported to contain a bis(pyrazolyl) moiety and a third donor atom in a group bonded to the central carbon atom exhibited one uncoordinated pyrazolyl fragment.16a,37 In contrast, in IV and V the M:L ratio of 3:2 means that the number of metal coordination sites and donor atoms of the ligands are the same and thus all donor atoms are coordinated. When an anti or syn orientation of the two ligands is possible in the complex (e.g., II−V) the anti orientation, which would involve a lower steric requirement, is shown in the figure. It is interesting to note that in structure IV, in which the metal centers are in two different environments, the two external centers have the same structure as in I and the central metal center is similar to that in II. The same applies for V, which has similarities with I and III. If the reaction of ligands L1 and L2 is carried out with metals with at least three accessible coordination sites, for example, with an octahedral or tetrahedral geometry, the most plausible derivatives that could be obtained are those shown in Chart 3 Chart 3. Possible Species That Could Be Formed by Reaction of Ligands L1 or L2 with Octahedral Metallic Centers

(VI and VII), that is, dimeric or polymeric species. An interesting case of the formation of both type of derivatives in silver complexes with bis(pyrazolyl)methane ligands with a thioether function has been reported by Marchiò.22c In Chart 3 octahedral metallic centers have been drawn and species VII has the three donor atoms in a facial disposition, although a trans orientation of the pyridine fragment would also be possible with respect to one of the pyrazole rings. For both structures, the existence of bridging groups in the remaining positions could give rise to the formation of species of higher nuclearity. To assess the behavior of the ligands against metal centers of different geometry, the reactions with Pd(II), Co(II), Ni(II), 1955



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Scheme 2. Synthesis of the Palladium Complexes 1−6

Scheme 3. Synthesis and Structures of Complexes 7−12

Scheme 4. Synthesis and Structure of Complex 13

also blue and the presence of tetrahedral cobalt centers is also proposed. Elemental analyses were performed on crystals that were subjected to vacuum for about twelve hours and it was found that the amount of solvent was lower than that in the crystals used for X-ray diffraction. The lower solvent content in these products was also confirmed by 1H NMR spectroscopy. In the case of complex 10, thermogravimetric analysis was performed on crystals not previously submitted to vacuum. In the temperature range 30 to 110 °C a mass loss of 16% was observed and this corresponds to 3 DMF molecules (per dimer) assigned to the crystallization solvent. From 110 to 200 °C, another mass loss was observed up to a value of 28%. This second loss should correspond to the two coordinated DMF molecules per dimer. Endothermic peaks were observed for each mass loss. The reaction of L1 with silver(I) was also investigated. This metallic center does not have stereochemical rigidity and this could lead to new types of structures. When L1 was reacted with AgPF6, crystals of 13 with the stoichiometry [Ag(PF2O2)(L1)]·1/2THF were obtained. The hydrolysis of hexafluorophosphate to give difluorophosphate is not uncommon, especially in the presence of silver(I) centers,38 and this process can take

place with the water present in the AgPF6 solid used in the reaction. We were unable to obtain crystals of the hexafluorophosphate derivative. Complex 13 consists of box-like cyclic dimers similar to those described previously, but in this case they are connected through double difluorophosphate bridges to give zigzag chains (see Scheme 4 and below for the solid state structure). 1956



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Table 1. Crystal Data and Structure Refinement for L2, 2·2Me2CO, 3, 4·Me2CO·0.5H2O, and 7·DMF L2 empirical formula fw temp (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) vol (Å3) Z density (calcd) (g/cm3) abs coeff (mm−1) F(000) cryst size (mm3) index ranges

reflns collected independent reflns Data/restraints/params GOFc on F2 final R indices [I > 2σ(I)] largest diff. peak and hole, e·Å−3

2·2Me2CO

3

C16H19N5 281.36 230(2) 0.71073 monoclinic P21/n 16.083(5) 9.929(3) 19.757(6)

C38H50Cl6N10O2Pd3 1210.78 230(2) 0.71073 monoclinic P21/n 9.1235(9) 15.526(2) 17.465(2)

C36H43B3F12N10Pd3 1195.43 230(2) 0.71073 monoclinic P21/c 11.949(3) 26.138(6) 16.170(3)

99.919(6)

99.508(3)

102.88(1)

3107.5(16) 8 1.203 0.076 1200 0.33 × 0.27 × 0.12 −19 ≤ h ≤ 19 −11 ≤ k ≤ 11 −23 ≤ l ≤ 23 16 747 5363 [R(int) = 0.1239] 5363/0/388 0.840 R1a = 0.0700 wR2b = 0.1615 0.187 and −0.197



4·Me2CO·0.5H2O

7·DMF

C47H66B3F12N10O1.50Pd3 1374.73 100(2) 0.71073 triclinic P1̅ 10.0566(4) 14.3393(5) 20.4904(5) 79.761(2) 89.847(2) 73.098(2) 2778.38(16) 2 1.643 1.047 1382 0.21 × 0.17 × 0.08 −12 ≤ h ≤ 12 −17 ≤ k ≤ 17 0 ≤ l ≤ 25 10 922 10922 [R(int) = 0.0000] 10922/2/713 1.054 R1a = 0.0331 wR2b = 0.0683 1.144 and −0.556

C39H57Cl6Co3N15O5 1205.49 230(2) 0.71073 triclinic P1̅ 12.1430(5) 12.9480(5) 21.0620(8) 88.990(2) 81.930(2) 81.516(2) 3242.8(2) 2 1.235 1.051 1238 0.21 × 0.15 × 0.09 −12 ≤ h ≤ 14 −15 ≤ k ≤ 15 −19 ≤ l ≤ 25 21 007 11286 [R(int) = 0.0341] 11286/0/617 0.831 R1a = 0.0487 wR2b = 0.1408 0.453 and −0.361

R1 = Σ||Fo| − |Fc|/Σ|Fo|. bwR2 = {Σw(Fo2 − Fc2)2/Σw(Fo2)2}1/2. cGOF = {Σ [w((Fo2 − Fc2)2)/(n-p)}1/2, where n = number of reflections and p = total number of parameters refined. a

Solid State Structure of L2. The spatial group is P21/n. Symmetry elements are not present in the molecular structure and, as expected, the pyrazole nitrogens are in an anti disposition to avoid repulsion between the electron pairs (see Figure 1). Two independent molecules are observed, which are connected through two weak hydrogen bonds formed between Hα of one molecule and the free nitrogen of one pyrazole ring of the other molecule (dC27−N2 = 3.37 Å, dC11−N7 = 3.33 Å). These pyrazole rings are also involved in a CH−π interaction between one proton of the Me3 group and the pyridine ring of the other molecule (dH4A−Ct = 3.08 Å, dH20C−Ct = 2.95 Å, Ct = centroid) (see Figure 1). These pairs of molecules interact to form chains through weak CH−π interactions involving the pyrazole rings that do not participate in the formation of the aforementioned hydrogen bonds (dH7−Ct = 2.94 Å), and these chains run in a parallel disposition through new CH−π interactions (dH5C−Ct = 3.06 Å) to complete the supramolecular structure. Solid State Structure of the Complex [Pd 3Cl 6(bpz*m4py)2]·2Me2CO, 2·2Me2CO. Complex 2 is trinuclear and presents an inversion center in the central Pd2 atom, a situation that makes the two halves of the molecule identical (Figure 2). The Pd2 atom is coordinated to two chloride ligands and two nitrogens of the pyridine rings from the bpz*m4py ligands, in a trans disposition, with a perfect square planar geometry (τ4 = 0).41 The Pd1 centers, which are on both sides of the molecule, adopt a slightly distorted square planar

The IR spectra of all complexes showed bands corresponding to the stretching frequencies of the CN bonds typical of the pyrazole and pyridine rings, as well as for the 2-Me-allyl group and the BF4− (complexes 3-6) or difluorohosphate38d (complex 13, ν(P−F) and ν(P−O)) counteranions. The ν(B−F) of the BF4− group bands were split, indicating a possible decrease in the symmetry of the anions. Bands typical of free nitrate were observed for 9−12. The presence of coordinated nitrate groups was deduced for complexes 10, 11, and 12. Splitting of the ν(NO) band that is of E′ symmetry in the free anion was observed (two bands at around 1460 and 1295 cm−1 were detected) and, in addition, the band of A1′ symmetry is now active and appears at around 1030 cm−1.39 In complexes 10 and 11, the coordinated nitrate is bidentate, and we, therefore, propose the same situation for 12. As we have previously found for other complexes that contain similar ligands,40 for a pair of derivatives with a specific metallic fragment the derivative that contains ligand L2 (with methylated pyrazolyl groups) is the most soluble. Solid-State Characterization. The crystallographic information for L2, 2·2Me2CO, 3, 4·Me2CO·0.5H2O, 7·DMF, 9·2DMF, 10·3DMF, 11, and 13·0.5THF is given in Tables 1 and 2. Tables with a selection of 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. 1957



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Table 2. Crystal Data and Structure Refinement for 9·2DMF, 10·3DMF, 11, and 13·0.5THF 9·2DMF empirical formula fw temp (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) vol (Å3) Z density (calcd) (g/cm3) abs coeff (mm−1) F(000) cryst size (mm3) index ranges

reflns collected independent reflns data/restraints/params GOFc on F2 final R indices [I > 2σ(I)] largest diff. peak and hole, e·Å−3

C48H78N22Ni2O20 1400.74 230(2) 0.71073 monoclinic P21/c 21.996(1) 12.9390(6) 25.184(1) 113.445(3) 6575.6(6) 4 1.415 0.658 2944 0.18 × 0.17 × 0.08 −27 ≤ h ≤ 27 −15 ≤ k ≤ 15 −31 ≤ l ≤ 31 44 241 13 332 [R(int) = 0.0952] 13332/0/845 0.932 R1a = 0.0664 wR2b = 0.1556 0.957 and −0.450

10·3DMF C47H73N19Ni2O17 1293.62 230(2) 0.71073 triclinic P1̅ 10.133(1) 13.8385(6) 13.9286(6) 115.960(4) 95.722(5) 103.508(5) 1661.4(2) 1 1.293 0.641 680 0.19 × 0.16 × 0.12 −13 ≤ h ≤ 12 −18 ≤ k ≤ 18 −18 ≤ l ≤ 18 16 858 7912 [R(int) = 0.0347] 7912/0/416 1.101 R1a = 0.0689 wR2b = 0.2109 1.420 and −0.464

11

13·0.5THF

C30H36N16O14Zn2 975.49 230(2) 0.71073 monoclinic C2/c 11.8548(6) 22.389(1) 19.3240(9)

C26H26Ag2F4N10O4.50P2 904.25 230(2) 0.71073 monoclinic C2/c 24.466(2) 10.4265(7) 17.483(1)

103.487(3)

91.497(5)



R1 = Σ||Fo| − |Fc|/Σ|Fo|. bwR2 = {Σw(Fo2 − Fc2)2/Σw(Fo2)2}1/2. cGOF = {Σ [w((Fo2 − Fc2)2)/(n − p)}1/2, where n = number of reflections and p = total number of parameters refined. a

Figure 1. Two molecules of the ligand bpz*m4py, L2, connected by hydrogen bonds (red) and CH−π interactions (purple).

Figure 2. X-ray structure of the complex [Pd3Cl6(bpz*m4py)2]· 2Me2CO, 2·2Me2CO. The pyrazolyl rings of other molecules are indicated to show the CH−π interaction (purple). Hydrogen bonds (red) and lone-pair−π interactions (brown) are also shown.

geometry (τ4 = 0.06). These palladium atoms are coordinated to two chloride atoms and the two pyrazole rings of the ligand. The Pd−N distances are in the range 2.02−2.04 Å. The ligand bpz*m4py forms a six-membered metallacycle by coordination to the Pd1 atoms and this ring adopts the typical boat conformation with the pyridine ring in the axial position, a common arrangement for these types of ligands with one substituent in the central carbon.16,17a,c,42 The dihedral angle PdNN/N4(pz)

is 140.69°, the bite angle of the chelate ligand is 85.28° and the dihedral angle formed by the pyrazole rings is 107.44°. These parameters, which are referred to as the structural parameters of the ligand throughout this paper, were found to be structurally related for the same type of derivative and a concomitant increase or reduction in these three values was found.16b,17a,c,43 (A table in the Supporting Information contains all of these parameters for the derivatives described in the paper whose 1958



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structures were determined by X-ray diffraction). Comparison of the data for complex 2 with those of other dichloropalladium derivatives that contain bis(3,5-dimethylpyrazol-1-yl)methane ligands bearing one substituent in the central carbon44 shows that the values are similar except for the case of the pz−pz angle, which is smaller for 2 (reported values are about 117°). The two pyridine rings are nearly coplanar. Chlorine atoms are not in the plane of the pyridine rings and the dihedral angle between the coordination plane and the plane of the pyridine rings is 42.87°. The acetone molecules that crystallize in the structure (two per trinuclear complex) establish different interactions with the molecules; namely a lone pair−π interaction with a pyridine ring, several hydrogen bonds and a CH−π interaction with a pyrazole ring (Figure 2). Molecules of the trinuclear complex interact on both sides through weak hydrogen bonds between Cl1 of one molecule and H4 of the pyrazole ring (H2) of a neighboring molecule (dC2−Cl1 = 3.56 Å), thus leading to one-dimensional ribbons running along the a-axis (see Figure 3). This structure can also

Figure 4. Formation of the 3D supramolecular structure for complex 2. Projection along a axis. Pd···HC interactions are represented in green and hydrogen bonds in red. Acetone molecules have been omitted for clarity.

170°) and may be considered as a special type of hydrogen bond. The nature of the anagostic interactions in d8 square planar complexes was addressed theoretically and the conclusions were that the interaction is mainly electrostatic in nature (partial covalence has been found only for the shortest distance in the range) and there is no evidence for the involvement of dz2 orbitals but rather dxz/yz in the interaction.46 The data for complex 2 (dPd2···H7 = 2.79 Å and Pd−H−C angle of 147.80°) unambiguously indicate that a double anagostic interaction is present in each central palladium atom which, if these interactions are considered, may be regarded as pseudo-octahedral. Different examples of anagostic interactions have been described in palladium(II) chemistry that involve the approach of one,47 two48 or even more49 CH groups in a direction approximately perpendicular to the coordination plane. However, these interactions usually take place with CH groups that belong to ligands coordinated to the metal center. Our case is a rare example of intermolecular anagostic interactions. It is possible that these interactions exist in known structures but they have not been found in cases where the 3D structure has not been analyzed. Solid State Characterization of the Complexes [Pd3(η3-C4H 7)3(bpzm4py)2](BF4)3 (3) and [Pd3(η 3-C4H 7) 3(bpz*m4py)2](BF4)3·Me2CO·0.5H2O (4·Me2CO·0.5H2O). There are no symmetry elements in these complexes and therefore the three palladium atoms and the two ligands of the molecules are different. In the case of 3, some disorder exists for the BF4− anions, the central allyl group and the pyrazolyl ring that contain N12. Thus, the parameters or interactions involving these groups will not be detailed and neither the crystalline structure. All palladium atoms have a pseudo square planar geometry, coordinated in all cases to a η3-allyl group and to the nitrogen of the pyridine ring of both ligands (L1 or L2 for 3 and 4, respectively) in the case of the central atom (Pd3) or to two pyrazolyl rings of a specific ligand (terminal Pd atoms) (see Figure 5 for complex 4). The metallacycles formed with these atoms exhibit the usual boat conformation, with the pyridine ring occupying the axial position, as is usual in these types of

Figure 3. Formation of the supramolecular ladder for complex 2 through hydrogen bonds (red). Acetone molecules have been omitted for clarity.

be considered as a supramolecular ladder in which the rungs are the “PdCl2(pyridine)2” units, the nodes are constituted by the terminal palladium fragments and the sides of the ladder are formed through the aforementioned hydrogen bonds. In this ladder, however, the rungs are not perpendicular to the sides. Molecules of a specific ladder interact with four adjacent ladders through two types of interaction: a hydrogen bond between Cl2 and Me3pz (dCl2−C4 = 3.47 Å) and a Pd···H4Cpz contact (see below) involving the central Pd atom (see Figure 4, in which the ladders are represented perpendicular to the drawing plane, bc plane). The angle formed between two adjacent ladders is 65.8°. In this way, the supramolecular three-dimensional framework of 2 is formed. The Pd···HC interactions warrant specific comment. This type of M···HC interaction can be described as agostic or anagostic, with structural and spectroscopic differences45 between them (some confusion exists in the literature concerning this differentiation). The former term implies shorter M···H distances (1.8−2.3 Å) and small M−H−C angles (90−140°), while the latter are characterized by relatively long M···H distances (2.3−2.9 Å) and large M−H−C bond angles (∼110− 1959



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Figure 6. Superposition plot of complexes 3 (pink) and 4 (blue). Pd3 atoms and Cα of one ligand, (C7 for 3 and C11 for 4), were used to fit the two molecules. Palladium atoms are represented as balls and hydrogen bonds are in red.

Figure 5. X-ray structure of the cation of complex 4, [Pd3(η3-C4H7)3(bpz*m4py)2]3+ including also one molecule of H2O, one molecule of acetone and one BF4− anion. Intramolecular interactions are shown: CH−π (purple), lone-pair−π (brown), anion−π (blue), and Pd···O contact (green).

The orientation of the allyl groups in the terminal fragments is the same as that found in similar derivatives,16 that is, with the allylic methyl group pointing toward the α substituent (pyridine in this case). The dihedral angles between the PdNN plane and that of the allyl group range from 110.4 to 119.1° in such a way that the C(central) to CH3 vector is pointing away from the metal. In addition, in these fragments the allyl methyl groups deviate more markedly from the allylic plane (0.31−0.49 Å) than the central allyl methyl group (0.28 Å). This deviation situates these methyl groups closer to the pyridine ring. An interesting aspect of complex 4 concerns the noncovalent interactions. One pyridine ring exhibits, on one side, an anion− π interaction involving a BF4− anion (dF3−Ct = 3.61 Å) along with a CH−π interaction, on the other side, with the participation of an allylic methyl group (dH20A−Ct = 3.01 Å). The other pyridine ring has a similar arrangement but differs in that the interaction is not with an atom of an anion but with the oxygen of a neutral molecule, the crystallization acetone; in other words there is a lone-pair−π interaction (dO1−Ct = 3.60 Å) and a CH−π interaction (dH44C−Ct = 3.11 Å) (see Figure 5). The CH−π interactions may be the origin of the quoted values for the dihedral angle between the allyl and the coordination planes and the deviation of the allyl methyl groups in the terminal fragments. In the context of these interactions, we reported a theoretical study that demonstrated the synergy of anion−π and CH−π interactions in triazine rings and this also included examples in which the two interactions were present on both sides of this heterocycle.30 Complex 4 can be considered as another example that reflects this synergy in this case in a pyridine ring. In the case of complex 3, the CH−π interaction with the participation of the allylic methyl groups is also present according with the strong deviation of these groups from the allylic plane. There are also in both complexes CH−π interactions involving the H3/H5 hydrogen atoms of the pyridine rings and the nearest pyrazolyl groups. The H−Ct distances and the C−H−Ct angles are in the range 2.7−3.1 Å and 111−122°, respectively.

ligands.16,17a,c,42 Although these complexes are cationic, the Pd−N distances are longer than those found in complex 2 (in the range 2.07−2.12 Å). This is due to the higher trans influence of the allyl carbons in comparison to the chloride ligands. In contrast to 2, in these cases, as expected, the cis coordination of the two nitrogenated ligands in the central metallic atom is forced by the presence of the η3-allyl group. The dihedral angles of the pyridine rings with the Pd(3)NN plane are between 44.66° and 67.49°. A high steric hindrance would exist if the two pyridine rings were coplanar. In the case of 4, the central palladium atom exhibits an interaction with the oxygen from the crystallization water that is approximately perpendicular to the coordination plane and with the opposite orientation to that of the allyl methyl group (Pd3···O2 = 3.005(1) Å). The relative orientation of the three palladium fragments is different in the two allyl complexes, as shown by the superposition of the two molecules (see Figure 6). In complex 3, the two terminal groups are situated in an approximate syn disposition whereas in 4 the arrangement can be considered as anti. Several possible factors could be responsible for this difference. However, it is noteworthy that in 4 one BF4− anion is situated between these two terminal metallic fragments and this participates in several hydrogen bonds involving, among other fragments, three methyl groups of the pyrazolyl rings. These groups are not present in 3. If the structural parameters of the nitrogenated ligands are taken into account, it can be concluded that relatively similar values are obtained for the two ligands of the complex in 3 while in the case of 4 there are significant differences. Values between 149.0 and 157.6° were found for the PdNN/N4 angle and the bite angle is in the range 86.7−89.8°; in both cases this angle is comparable to those found in palladium complexes with similar ligands.16b,43a As far as the dihedral angle between pyrazole rings is concerned, values of 114.87° and 138.21° were found for 4 and 135.34° for 3. The reported pz−pz angles are around 124−125°.16 1960



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Figure 7. Formation of the chains of cations in a head-to-tail disposition for complex 4 through double CH−π interactions (purple).

Figure 8. Structure of the box-like cyclic dimers of complexes 7 (a), 9 (b), 10 (c), 11(d), and 13 (e). Hydrogen atoms have been omitted for clarity.

In the case of 4, the packing of the molecules in the 3D structure mainly involves the formation of hydrogen bonds between the three types of BF4− anions and different protons of

the N-donor and allyl ligands as well as the acetone molecules and each particular F atom may participate in two or more hydrogen bonds. The F3 atom that is involved in the 1961



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is in the range 86.5−89° for complexes that contain ligand L1 but is higher for derivative 10 (92.1°), which contains ligand L2 with methylated pyrazolyl groups. The dihedral angle between the two pyrazolyl rings is in the range 137−143°, which is higher than that found for the palladium complexes. However, the most important difference between the derivatives with square-planar or octahedral metallic centers is the MNN/N4 angle, which is in the range 141−154° in the former complexes and 173−178° in the latter, which can be considered as exhibiting metallacycles with a half-chair disposition. The metal is very close to the N4 plane (distances between 0.01 and 0.17 Å while for complexes 2 and 4 this is in the range 0.64−0.95 Å). These differences could be due to the steric requirements of the ligand situated cis to the two pyrazolyl rings in the octahedral complexes. In the case of the five-coordinate silver derivative, the bite angle is the lowest of all the derivatives described here (80.63°) and the other two parameters are more similar to those of the palladium complexes (pz−pz angle =121.03°; MNN/N4 angle = 156.48°). There are several intramolecular noncovalent interactions in these box-like dimers that are common for the five derivatives and may have an influence on their stability and shape. The two pyridine rings are parallel and exhibit a π−π stacking interaction (dCt···Ct = 3.32−3.55 Å for the octahedral complexes and 3.92 Å for 13). This interaction should favor the orientation found for these heterocycles. Besides, all the H atoms in this ring are involved in weak interactions: H2 and H6 form hydrogen bonds with oxygen (chloride or fluoride in the case of 7 and 13, respectively) atoms of the ancillary ligands (X−C distances of 2.92−3.56 Å with one case of 3.90 Å) and H3 and H5 give rise to CH−π interactions with the pyrazole rings of the same ligand (dH−Ct = 2.60−3.00 Å). In some cases, the other side of the pyrazole rings exhibits anion−π (range O−Ct = 2.76− 2.88 Å) interactions with nitrate anions. Some of these interactions are reflected in Figure 9, which corresponds to derivative 9. See Supporting Information for more detailed data.

anion−π interaction also participates in the hydrogen bonding network. The same applies to the oxygen atom of the acetone molecule. A weak double CH−π interaction involving the pyrazole rings coordinated to Pd2 and the allyl group of the Pd1 is also observed (dH20C−Ct = 3.04 Å, dH19A−Ct = 3.14 Å) and this gives rise to the formation of chains, as shown in Figure 7. X-ray Structures of Complexes 7·DMF, 9·2DMF, 10·3DMF, 11, and 13·0.5THF. The molecular structures of complexes 7·DMF, 9·2DMF, 10·3DMF and 11 are quite similar and consist of box-like cyclic dimers formed by the self-assembly of two metal centers and two ligands in a head-to-tail disposition (L1 in 7, 9 and 11 and L2 in 10, see Figure 8a−d). In the case of 13·0.5THF also similar dinuclear species are formed involving L1 (Figure 8e) but in this case they are connected through double PF2O2− bridges to give zigzag chains (see below).The structures of these molecular box-like dimers will be explained together. In the case of 7, two types of units that are very similar exist with Co(1) and Co(2) atoms. Except in the case of 9, an inversion center exists and this means that only one-half of the structure is unique. In all complexes except 13, the metal ions exhibit a distorted octahedral geometry with the three nitrogens (two from the pyrazolyl rings of one ligand and the third from the pyridine ring of the second ligand) in a facial disposition. The other three positions are completed with different ligands: a chloride and two DMF molecules in the case of 7 (one DMF trans to py), three DMF molecules for 9 and a bidentate nitrate and a DMF molecule for 10 and 11. In the case of 11 one oxygen of the nitrate group is trans to the pyridine ring while in 10 this position is occupied by the DMF molecule, possibly because steric repulsion would exist between this molecule and the methyl groups of the pyrazole rings if they were in a relative cis disposition (Figure 8). The counteranions are [CoCl4]2− in the case of 7 and nitrate for the rest of the derivatives. This octahedral geometry is in contrast to that reported by Carrano et al. for similar species formed with the (4-carboxyphenyl)bis(3,5-dimethylpyrazolyl)methane ligand and the M(II) centers Zn,20 Cu,21 Co, and Ni,20,21 where lower coordination numbers were found. In the case of 13, the silver centers are five-coordinate, being bonded to two difluorophosphate groups and to three nitrogen atoms of two ligands that are arranged in a similar disposition to the previous cyclic dimers [see Figure 8e]. The ideal values for τ5 for a trigonal bipyramid or a square planar pyramid are 1 and 0, respectively. The value found for the silver center in 13 is 0.33, reflecting a rather distorted square planar pyramidal geometry.50 The M−N distances differ from one metal to the other (in the range 2.14−2.17 Å for Co, 2.04−2.07 Å for Ni and Zn and 2.27−2.42 Å for Ag). In general, there are no significant differences between the bonds with pyrazole or pyridine rings for a given metal except for 13 where shorter bonds are found with pyridine, the more basic heterocycle. In all cases, the two M−N(pyrazole) bonds are different in length. The Ag−O bonds are in the range 2.55−2.68 Å while for the derivatives with octahedral metallic centers, the M−O distances are shorter. They are in the range 2.03−2.16 Å with the exception of one Zn−O(nitrate) bond in the case of 11 (2.35 Å), a complex in which an asymmetric nitrate is present. The nitrate group of 10 is bonded in a symmetric way. The bite angle of the coordinate nitrate group is 60.41° in the case of 10 but is smaller in 11 (56.71°). As far as the nitrogenated ligands are concerned, in the case of the derivatives with octahedral metallic centers the bite angle

Figure 9. Noncovalent interactions present in the dinuclear unit of complex 9. Hydrogen bonds (red), π−π stacking (black), anion−π (blue) and CH−π (purple) interactions are shown. The ancillary ligands and their interactions have been omitted for clarity. 1962



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Figure 10. Complex 7. Chains arranged along the b-axis formed through the interaction of the box-like cyclic dimers of Co(1) (a) or Co(2) (b). In (a) the direct hydrogen bonds have been omitted. The red circle in b represents the region where the apolar methyl groups are gathered.

The lower value of the MNN/N4 angle found for the silver derivative in comparison with the octahedral derivatives has consequences for two parameters of the box-like dimers: (i) the metal−metal distances are in the range 7.1−7.6 Å for the octahedral derivatives while the value for 13 is 6.48 Å and (ii) the angle β of the π−π stacking interaction (i.e., the angle formed by the H−Ct and the H−plane lines) is 25.6° for 13 but is in the range 6.1−10.7° for the octahedral derivatives. This implies that the pyridine rings are offset to a greater extent in the silver complex. In the following paragraphs a description of the crystal structures of the different derivatives will be given separately. To facilitate the discussion and concerning the box-like cyclic dimers we will mention the long and short side of the box referring to the lines M−pyridine−M and M−Cα, respectively. Complex [CoCl(bpzm4py)(DMF) 2 ] 2 (CoCl 4 )·DMF, 7·DMF. The dimers that contain Co(1) atoms are arranged along the b-axis and this contacts the short side of the box through weak and double hydrogen bonds of the type A−HC/ CH−A, where A = O1A of a DMF molecule and CH = C5H5 (position 4 of a pyrazole ring). These interactions are complemented by hydrogen bonds established between the two dimers with a [CoCl4]2− ion that acts as a bridge connecting them. The dimers that contain Co(2) are also arranged along the b-axis, in this case facing the long side of the box and with an interaction taking place only through hydrogen bonds with a [CoCl4]2− ion that also acts as a bridge (all [CoCl4]2− ions are equivalent) (Figure 10). The apolar methyl groups of one DMF molecule (cis to pyridine) of each dimer are situated in the same region of space, probably due to hydrophobic contacts (see circle in Figure 10b). The interaction of the two types of chains to give the supramolecular structure takes place through direct hydrogen bonds (MeDMF with Cl) or through the participation of [CoCl4]2− or noncoordinated DMF molecules that act as bridges (see Figure 11).

Figure 11. X-ray structure of 7·DMF along the a-axis. The different chains are highlighted.

Complex [Ni(bpzm4py)(DMF)3]2(NO3)4·2DMF, 9·2DMF. The supramolecular three-dimensional framework of 9 consists of chains of dimers that extend along the b-axis in a very similar fashion to that found for the Co(2) dimers of complex 7. The interaction between two consecutive dimers takes place through hydrogen bonds with nitrate groups or with aggregates of nitrate/noncoordinated DMF molecules that act as bridges (Figure 12). In this case the apolar groups of coordinated DMF molecules are also arranged nearby in space but in this case the arrangement involves the two DMF molecules that are cis to the pyridine group. See circle in Figure 12. The aforementioned chains give rise to sheets in the bc plane with different hydrogen bonds involving nitrate and noncoordinated DMF molecules. The formation of hydrogen bonds with nitrate and crystallization solvent molecules also allows the interaction of the sheets to generate the 3D structure. Complex [Ni(NO3)(bpz*m4py)(DMF)]2(NO3)2·3DMF, 10·3DMF. The supramolecular framework of 10, in which 1963



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bonding interaction involving the noncoordinated nitrate groups (all these anions are equivalent) that act as bridges. It is interesting to note that the pyridine heterocycles in this complex exhibit interactions on both sides of the ring, π−π stacking on one side and anion−π interactions on the other, a fact that could reflect the synergy of these interactions, as described previously.29f,51 The three-dimensional framework is formed by the interactions between different chains through hydrogen bonds involving nitrate and DMF groups. Thus, when viewed along the a-axis and considering the position of the metallic centers, a honeycomb disposition is found in which channels containing the DMF molecules are formed (see Figure 14). Complex [Zn(NO3)(bpzm4py)(DMF)]2(NO3)2, 11. In this derivative the dimers are also aligned through the longer side of the box to form a chain that extends along the [101] direction. However, in this case, the consecutive dimers are not perfectly parallel and, in fact, two types of alternating orientations are present (see Figure 15). The dihedral angle between the pyridine rings of two consecutive dimers is 21.61°. The contact of two consecutive units takes place through three types of interaction. In the center of the chain, one of the noncoordinated nitrate anions (containing O11) establishes two anion−π interactions with two pyridine rings, one for each dimer (dO11−Ct = 3.38). In this way, this derivative can also be considered as another example of synergy between π−π stacking and anion− π interactions.29f,51 The other oxygen atoms of this nitrate form hydrogen bonds with the Hα atom of both dimers. In the section of the chain in which the dimers are closest or farthest apart, hydrogen bonds are established either directly between the two units or with nitrate groups acting as bridges. As in other previous examples, the nitrate and DMF molecules give rise to a complex system of hydrogen bonds to form the supramolecular three-dimensional framework. If the structure is viewed in the (101) plane, a honeycomb disposition of the chains is observed with channels that extend along the [101] direction in a similar way to that found in complex 10. If we consider the supramolecular structure of the octahedral complexes that contain box-like cyclic dimers, it can be concluded that the most common mode of interaction between dimers to form chains arises from the face of the long side of the box and involves interactions through direct hydrogen bonds between the dimers or with the intermediacy of nitrate or free DMF molecules as bridges. In some cases, anion−π interactions between nitrate anions and the pyridine rings were also found. The 3D structure is formed through a complex system of hydrogen bonds. In two derivatives a honeycomb disposition of the metallic centers was found.

Figure 12. Complex 9. Interaction of the boxes through hydrogen bonds to form chains that extend along the b-axis. The red circle represents the region where the apolar methyl groups are gathered.

the nitrate anions play an important role, can also be viewed by considering the formation of chains with the aforementioned dimers connected along the a-axis through the long sides of the box, as found in 9 and dimers of Co2 in 7. However, in this case the interaction between contiguous dimers involves more than hydrogen bonds. As it can be seen in Figure 13, a double

Figure 13. Complex 10. Interaction of the box-like dimers through anion−π interactions (blue) and hydrogen bonds (red) to give chains that extend along the a-axis.

head-to-tail anion−π interaction involving pyridine rings and the terminal oxygen atoms (O3) of the coordinated nitrate groups is established (dO3−Ct = 3.60 Å). These oxygen atoms also participate in the formation of hydrogen bonds with different H atoms of the N-donor ligand. The interaction between the dimers is complemented with a more peripheral hydrogen-

Figure 14. View along the a-axis of the 3D supramolecular structure for complex 10. (a) Dimers and anions in black with the crystallization DMF molecules in gray. (b) Framework with space-filling representation of the dimers and anions. 1964



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Figure 15. Complex 11. Hydrogen bonds (red) and anion−π interactions (blue) between box-like dimers to form a chain that extends the [101] direction. The interactions are only shown for two dimers.

Complex [Ag(μ2-κ2-O,O-PF2O2)(L1)]2·0.5THF, 13·0.5THF. The box-like dimers of 13 are bonded through double difluorophosphate bridges to generate zigzag chains that extend along the b-axis. These chains are connected through noncovalent interactions to give rise to sheets in the bc plane (see Figure 16).

Figure 17. THF molecules, in spacefill, situated between the sheets in complex 13·0.5THF.

some cases 1H, 19F{1H}, 31P{1H} and 13C{1H} NMR spectra were recorded in order to obtain information about the structures of the new complexes in solution. In the mass spectra of derivatives 1 and 2 the highest mass peak corresponds to trinuclear species, more specifically to the cations [Pd3Cl4L2]+ for 1 and [Pd3Cl5L2]+ for 2. This indicates that the complexes remain trinuclear in solution, at least to some extent. Peaks corresponding to dinuclear or mononuclear species were also observed. In the case of the allyl complexes 3 and 4, polynuclear species were not detected and the peaks of higher intensity were those corresponding to [Pd(Me-allyl)L]+. The absence of polynuclear species, in contrast to the situation found for the chloride derivatives, may be due to the higher trans influence of the allyl groups than the chloride ligands, a characteristic that makes the Pd−N bonds weaker in complexes 3 and 4. Peaks corresponding to the same mononuclear species were observed for 5 and 6. In complexes 7−12, at least one of the complexes synthesized with each metal exhibits peaks corresponding to a dinuclear species. For the cobalt derivatives, for example, the peaks [Co2Cl2(L1)2]2+ for 7 and [Co2(py-CH2-pz*)2]+ for 8 are observed. For the Zn complexes, only fragments containing one metallic atom are observed for 11 while for 12 the species [Zn2(L2) + 5H2O]+ gives rise to the most intense peak. In the case of 13, peaks corresponding to the box-like cyclic dimer with one or two difluorophosphate groups are observed. For all the peaks described, the calculated isotope pattern was in agreement with the formulation.

Figure 16. Four chains of complex 13 that form a sheet in the bc plane. Hydrogen bonds (red), π−π stacking interactions (black) are shown in a region of the plane.

These interactions are of two types: (i) Hydrogen bonds that are present in the four corners of the boxes. In two corners the oxygen or fluorine atoms of the difluorophosphate groups act as hydrogen acceptors and in the other two the CHα group and other CH groups of the pyrazolyl rings act as hydrogen donors. (ii) A π−π stacking interaction involving a pyridine and a pyrazolyl ring of dimers of adjacent chains. Considering that both pyridine rings of a particular unit are involved in this interaction, a kind of short column with π−π stacking involving four rings is formed (see Figure 16). The crystallization THF molecules are situated between the sheets as shown in Figure 17. When considering the different noncovalent interactions found in the complexes described in this paper it is noteworthy that several possible examples of synergy between some of these interactions have been found: more specifically, between anion−π interactions and π−π stacking and also between hydrogen bonds and anion−π interactions where the π ring acts as hydrogen donor. In the crystalline structures of the different species described here the “pyrazolyl embrace” described by Reger28a was not found. This phenomenon involves a concerted set of intermolecular CH−π and π−π stacking interactions that associate the four pyrazolyl rings from two nearby bpzm sites. Solution Chemistry. ESI-TOF or MALDI-TOF mass spectra (see Experimental Section part for details), UV−vis and, in 1965



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fragments (1.59 for 3 and 1.36 ppm for 4) whereas the signals of the central allyl group appear in the usual range for these groups: 2.23 and 2.24 ppm, for 3 and 4, respectively. This difference must be due to the effect of the ring current anisotropy of the pyridine ring if the allyl group has the orientation shown in Chart 4 (CH−π interaction), as found in the solid state. This

The UV−vis spectra of ligands L1 and L2 and those of complexes 1−12 were recorded at room temperature in methanol at concentrations in the range 10−4 to 10−5 M. L1 and L2 exhibit absorption bands at around 220 and 260 nm and these are assigned to π−π* Intra-Ligand Charge Transfer (ILCT) transitions corresponding to the pyrazole and to the pyridine moieties, respectively. The molar extinction coefficient (ε) is around five times higher for the former absorption. These regions fit with the maximum absorption bands in the spectra of N-methypyrazole52 or N-methyl-3,5-dimethylpyrazole53 and 4-methylpyridine.54 For ligand L1 an additional absorption at 308 nm (very low intensity) is observed but this does not appear for ligand L2. However, this absorption is present not only for several complexes with ligand L1 but also for complexes with ligand L2. The ILCT(py) absorption with small energy changes appears in the absorption spectra. However, the corresponding ILCT(pz) absorption is not observed in the spectra of the complexes. Similar behavior has been ascribed in other cases to the coordination of the ligands in solution.55,13h The red-shift of the ILCT(py) absorption in Pd complexes 1−6 is significant, as is the appearance of two bands at 334 and 466 nm for complex 2, which could be assigned to metalto-ligand charge transfer (MLCT) absorptions. These bands reflect the coordination of the ligand in the Pd complexes in solution. In the Co(II) complexes 7 and 8 and in the Ni complex 9, bands at around 310 nm can be assigned to the ILCT absorptions56 but MLCT absorptions were not observed. In contrast, in the Ni(II) complex 10 an additional band at 388 nm can be assigned to an MLCT absorption.57 Solvent dependence of the energy of this MLCT band is observed as this band is blue-shifted by 9 nm in MeCN. This fact can be considered to be indicative of ligand coordination.58 Additionally, in the Co(II) complexes 7 and 8 a weak absorption is observed at around 520 nm due to d-d transitions of a d7 high spin complex. Similarly, in the Ni(II) complexes these weak absorptions appear in the region 550−630 nm, with two bands evident in the case of complex 9. In the Zn(II) complexes 11 and 12, although the ILCT band at around 300 nm can be observed, an MLCT band does not appear in the spectra. The NMR spectra provide information in the cases of complexes 3−6, 11, 12, and 13. As mentioned previously, the chloride derivatives 1 and 2 are very insoluble in the most common solvents and the 1H NMR spectra were registered in dmso-d6, giving rise to complex spectra with broad resonances. Broad and multiple resonances were also observed in the 1H NMR spectra of the paramagnetic complexes 7−10. The signal assignment was made considering bibliographic information and taking into account the data of 1H−1H COSY, 1H−13C g-HMQC, g-HMBC, and NOE difference spectra. The 1H NMR spectra (acetone-d6) of the trinuclear allylic derivatives 3 and 4 are very informative. The presence of a unique and symmetric nitrogenated ligand along with two types of symmetric η3-allylic groups in a 1:2 ratio was deduced. The same conclusion was drawn from the 13C{1H} NMR data. The ratio of the two allyl groups is consistent with the presence of the trinuclear species in solution. In mononuclear 2-Meallyl-palladium complexes with α-substituted bis(pyrazolyl)methane ligands one usually observes evidence for the two isomers that differ in the orientation of the allyl group, albeit in different ratios.16 In 3 and 4, the spectra reflect that only one orientation of the allyl groups is present in each terminal palladium fragment. In relation with this orientation, it is worth noting the low chemical shift of the methyl group of the terminal allyl

Chart 4. NOEs Observed for Complex 4 (Indicated with Arrows)

orientation has been found to be more stable in allyl derivatives with similar ligands.16 The NOEs observed between the Me3 hydrogens of complex 4 and the Hsyn and Hanti protons of the allyl group also confirm the allyl orientation (see Chart 4). An NOE effect is also observed between Hα and H5/Me5 of the pyrazolyl rings, indicating that the pyridine moiety is situated in the axial position of the metallacycle (see Chart 4). Comparison of the signals of the nitrogenated ligands with those of the free ligands shows deshielding of the pyrazole resonances, which is consistent with coordination to the metal center. However, in the case of the pyridine signals only a very small deshielding is observed for the H2/H6 protons but shielding is evident for the H3/H5 hydrogen atoms (shift of about 0.5 ppm), an effect that must be due to the influence of the π cloud of the pyrazole rings and is consistent with the CH−π interaction detected in the solid state (see Figure 5). If the structures of derivatives 3 and 4 are considered and the asymmetry introduced by the central allyl group is taken into account, the following groups should appear as different in the 1H and 13C{1H} NMR spectra: (i) the two halves of the terminal allyl groups; (ii) the two pyrazolyl rings of a ligand, and (iii) the two halves of the pyridine rings. However, at room temperature all these groups as observed as symmetric. A process of apparent allyl rotation of the central allyl group would create this symmetry. However, this process has only been observed for similar systems above room temperature,16,59 and we propose that this is not operating in our case. If the pyridine rings exhibit unrestricted rotation, the H2/H6 and H3/H5 pyridine protons would be isochronous and we propose that as the explanation for point iii but points i and ii should be yet operating. The equivalence observed in the allyl and pyrazolyl groups could be due to the fact that they are situated far away from the central allyl group. In any case, a variable temperature 1H NMR experiment was carried out for the two derivatives (down to −80 °C). The only splitting was observed in the case of 4. For this derivative, two resonances for the Hsyn of the terminal groups were detected from −40 °C and below. This may be due to the fact that these protons are situated nearer 1966



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ing resonance of the Hα proton was observed with the expected relative integration and this remained unchanged with time. However, at 0 °C the intensity of this signal started to decrease in such a way that after 42 min, the intensity of the resonance was 37% of the initial signal. We propose that deuteration of this position takes place and, as a consequence, the deuterium coupled signal (JCD = 22 Hz) was detected in the corresponding 13C{1H} NMR spectrum. In the case of the silver derivative 13, we also observed the progressive disappearance of the Hα resonance at room temperature in methanol-d4. We consider that this selective deuteration is an interesting process and we will investigate this phenomenon in future work.

the central group than the others and at low temperature, with less thermal motion, they are observed as different. The 1H and 13C NMR spectra of the mononuclear complexes 5 and 6 reflect the presence of a unique isomer with a ligand containing two equivalent pyrazolyl groups thus supporting the coordination of the two pyrazolyl rings to the palladium atom. The 1H NMR resonances of the allyl fragment are similar to the terminal allyl groups of 3 and 4 indicating a similar orientation of this group with respect the N-donor ligand. The 1H NMR spectrum of 11 was recorded in methanol-d4 because the complex was not soluble in acetone and in DMSO total ligand decoordination would be expected. The room temperature spectrum reflects the presence of the corresponding free ligand, indicating that a dissociation process occurs in solution. The Hα resonance was not observed, a fact that will be explained below. To analyze the possible presence of the box-like structures at low temperature, variable temperature 1 H NMR spectra were registered in methanol-d4. When the temperature was decreased, a new species was observed at −50 °C and this was more clearly observed at −90 °C. A 1H− 1 H COSY experiment was performed in order to assign the new signals. A deshielding of 0.4−0.6 ppm was found for the pyrazolyl resonances with respect to those in the free ligand − an observation in accordance with coordination of these rings to a metal center. Interestingly, shielding is found for the pyridine signals. In the case of H2/H6(py) resonance the shift to higher field was 0.45 ppm but the marked change (0.95 ppm) in the case of H3/H5(py) is noteworthy. We propose that the decrease in the temperature slows down the decoordination process and that the new species detected at −90 °C is the box-like dimer, in which the π−π stacking between the two pyridine rings and the CH−π interaction of the H3/H5 atoms with the pyrazolyl rings observed in the solid state are maintained in solution and are responsible for the observed anomalous pyridine chemical shifts. Considering that the box-like unit has two ligands, the amount of box-like dimer detected at −90 °C is around 20%. In the case of 12 at room temperature, besides broad signals of the free ligand, a new set of ligand resonances was detected in the 1H NMR spectrum in methanol-d4. The shift to higher or lower field was in the same sense as observed for the box-like unit of complex 11 (albeit with lower absolute values) and thus we propose that the resonances are due to the dimer unit that is detected in this complex at room temperature. The interchange process with the ligand must occur at a lower rate than in the case of 11. The relative amount of box-like unit detected at this temperature is 38%. This difference between the two Zn derivatives is in accordance with the data from the mass spectra. In the silver complex 13, the 1H and 13C NMR spectra reflect the presence of a unique and symmetric nitrogenated ligand with chemical shifts different from those of the free ligand. A similar spectrum is observed at low temperature (−80 °C). In the proton resonances a deshielding or shielding is observed for the pyrazolyl or pyridyl protons, respectively. This is similar to that found for the box-like units in the Zn derivatives, a fact that points to the existence of the dimer unit in solution and with an influence of the π−π stacking and the CH−π interaction in the pyridyl signals. The expected resonances for the PF2O2− group are observed in the corresponding 19F and 31P NMR spectra.38a,c As stated previously, in the case of 11 a resonance for Hα was not observed. This signal was observed for 12 and, as a result, we monitored by 1H NMR a freshly prepared methanol-d4 solution of 11 at −50 °C. At this temperature the correspond-

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CONCLUSIONS Two new all nitrogen donor ditopic ligands that can act as tridentate systems, bis(pyrazol-1-yl)(pyridine-4-yl)methane (L1) and bis(3,5-dimethylpyrazol-1-yl)(pyridine-4-yl)methane (L2), have been synthesized. The reaction of these ligands with metallic centers yields different derivatives whose structure depends mainly on the metal geometry. With square-planar ions such as Pd(II) the preferential formation of trinuclear species is observed with the central atom bonded to two pyridine rings and the terminal centers interacting with the two pyrazolyl rings in a chelate fashion. The other two positions are completed with two chloride ligands or one allyl ligand. In the case of the allyl derivatives the use of an excess of the nitrogenated ligands allows the formation of mononuclear complexes. Neither of these reactions leads to the formation of species with an uncoordinated bis(pyrazolyl) chelate group. When the reaction of L1 or L2 is performed with metal centers that are able to give an octahedral geometry (M = Co(II), Ni(II), Zn(II)), box-like cyclic dimers are formed by self-assembly of two metal centers and two ligands in a head-totail disposition. All metal ions exhibited a distorted octahedral geometry with the three nitrogens, two from the pyrazolyl rings of one ligand, and the third from the pyridine ring of the other, in a facial disposition. DMF molecules or nitrate anions complete the coordination sphere of the metal. In the solid state, the dimers exhibit different noncovalent interactions including an internal π−π stacking between the two pyridine rings. The dinuclear units are aligned to form chains that in the majority of cases face the long side of the box and interact through hydrogen bonds and, in some cases, also through anion−π interactions involving pyridine rings. Relatively similar box-like units are formed in a fivecoordinate silver complex, but in this case they are bonded through double difluorophosphate bridges to give chains. Noncovalent interactions between the chains give rise to the formation of sheets. The structural characteristics of the nitrogenated ligands in the palladium and silver complexes are relatively similar and differ from those of the derivatives with octahedral metallic centers. The different noncovalent interactions found in the molecular structures apparently influence the shape of the molecules. In some derivatives possible examples of synergy between noncovalent interactions have been found: for example, between anion−π interactions and π−π stacking in pyridine rings and also between hydrogen bonds and anion−π interactions where the π ring acts as a hydrogen donor. In the study of the solution chemistry, evidence for polynuclear fragments was deduced from the mass spectra and NMR information. A clear effect of the noncovalent interactions on the NMR chemical shifts of some resonances was observed. 1967



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The Zn and Ag complexes with L1 also exhibit an unexpected selective deuteration in position α of the nitrogenated ligand in methanol-d4 solution. The mononuclear allyl-Pd derivatives have noncoordinated nitrogen atoms and the metal centers in the complexes with box-like cyclic units contain ligands that can be easily displaced. Thus, these derivatives could be used as building units to form MOF structures. This aspect will also be studied in future work.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format. ORTEP representations of the structures. Description of the synthesis and characterization of the new derivatives. Tables with structural parameters of the ligands and data for noncovalent interactions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MICINN of Spain (CTQ2011-24434, FEDER Funds) and the Junta de Comunidades de Castilla-La Mancha-FEDER Funds (PCI080054 and PEII11-0214). We thank the MEC of Spain for a FPU grant (GD) and INCRECYT program (contract to MCC).

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DEDICATION This paper is dedicated to the memory of our wonderful colleague, Prof. Purificación Escribano, who dedicated her life to chemistry and to defend the rights of women.

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REFERENCES

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