Exploring the Versatility of N-Pyrazole, P-Phosphinite Hybrid Ligands

Article pubs.acs.org/crystal

Exploring the Versatility of N‑Pyrazole, P‑Phosphinite Hybrid Ligands against Pd(II). From Monomers and Dimers to One-Dimensional Chain, Two-Dimensional Layer Polymers and Three-Dimensional Networks Sergio Muñoz,† Miguel Guerrero,† Joseps Ros,† Teodor Parella,‡ Merce Font-Bardia,§ and Josefina Pons*,† †

Departament de Química, Unitat de Química Inorgànica, Universitat Autònoma de Barcelona, 08193-Bellaterra, Barcelona, Spain Departament de Química i Servei de RMN, Universitat Autònoma de Barcelona, 08193-Bellaterra, Barcelona, Spain § Cristal·lografia, Mineralogia i Depòsits Minerals i Unitat de Difracció de RX, Centres Científics i Tecnològics, Universitat de Barcelona, Martí i Franquès s/n, 08028-Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: The versatility of N-pyrazole, P-phosphinite hybrid ligands [2-(3,5dimethyl-1H-pyrazol-1-yl)ethyldiphenylphosphinite (L1), 3-(3,5-dimethyl-1H-pyrazol-1-yl)propyldiphenylphosphinite (L2), and 2-(3,5-diphenyl-1H-pyrazol-1-yl)ethyldiphenylphosphinite (L3)] has been studied toward Pd(II). The reactions lead to a mixture of three complexes in solution for each ligand ([PdCl2(Lx)], [PdCl2(Lx)2] and [PdCl2(Lx)]2) obtaining monomers (where L acts as mono-/ bidentated agents) and dimers where the two L bridge the two metallic atoms in a head-to-tail arrangement. Complexes have been isolated and fully characterized by analytical and spectroscopic methods. X-ray crystal structures of some of the complexes as well as of other species [PdCl2L3]2 (only detected in the solid state) are presented. Furthermore, these complexes were prepared to examine the effects of the L structure on the topology and interpenetration form (one-dimensional chain, twodimensional layer polymers and three-dimensional networks). Additionally, experimental and simulated NMR studies and a battery of modifications in the synthesis of the complexes (relation ratio of the starting products, order of addition, solvents and temperature) permit a better understanding of the role of ligands in the selfassembly process.

1. INTRODUCTION The design and use of coordinated compounds in the selfassembly of supramolecular architectures have received considerable attention over the past several decades;1 they are at the forefront of modern materials chemistry due to the interest in crystalline materials engineering tailored to industially relevant hygrogen storage, catalysis, or molecular recognition.2 Hybrid ligands have garnered an interesting field of chemistry for their intriguing structures and potential applications. The combination of hard and soft bases in an organic framework and its reactivity against metallic atoms provide complexes with hemilabile properties. The deep studies of these systems permit improvement of the developing of the theory of the coordination bond and give valuable information to other scientific fields such as bioinorganic chemistry, pharmacology or nanocience.3 One of the extended lines of studies during the last decades is the synthesis, characterization and applications of hybrid ligands with a pyrazole group.4 Our group has contributed in this field developing new hybrid ligands that contain a pyrazole group and other functionalized © XXXX American Chemical Society

groups (alcohol-, ether-, thioether-, amino-, and phosphine-) and studying its reactivity against transition metals.5 The P-phosphinite group has attracted some interest in the last few years, corroborated by the increase of the publications related to this field. The capacity of the P-phosphinite group to modulate the electronic and steric properties in front of Pphosphines has given more attention in the study of its reactivity against transition and its applications in catalysis.6 In the literature, only 11 metallic structures are described where the metallic center is connected to an N-pyrazole and Pphosphinite functional groups, having the two functionalized groups belonging to the same ligand and chelating the metallic center7 and the rest of the structures correspond to metallic complexes where the functional groups are not connected.8 Our group has synthesized and characterized four new Npyrazole, P-phosphinite hybrid ligands and studied their reactivity with RuII and RhI contributing with five new Received: September 29, 2012 Revised: October 22, 2012

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dx.doi.org/10.1021/cg3014333 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Synthesis of Pd(II) Complexes (a, b and c) with L Ligands

structures.9 In order to continue with these studies, we have focused our interest on the study of the reactivity of these Npyrazole, P-phosphinite hybrid ligands toward Pd(II) and study the process of self-assembly which leads different 1D-, 2D-, and 3D-supramolecular frameworks thanks to noncovalent interaction.

on an Esquire 3000 ion trap mass spectrometer from Bruker Daltonics. The MS/ESI(+) experiments was done on CH2Cl2/CH3CN (1:5) mix solvents. Matrix assisted laser desorption/ionization (MALDI) timeof-flight (TOF) mass spectrometry were carried out by the staff of the Institut de Biotecnologia i Medicina of the Universitat Autònoma de Barcelona on a positive ion mode on a Bruker-Daltonics Ultraflex time-of-flight instrument. Ion acceleration was set to 25 kV. All mass spectra were externally calibrated using a standard peptide mixture. Lyophilized samples were resuspended in CH3CN/CHCl3 or THF and mixed with 2,5-dihydroxybenzoic acid (DHB) solution or 1,8dihidroxyanthracene-9(10H)-one (dithranol) (1:1). DHB and dithranol solutions were prepared by mixing 15 mg of matrix with 1 mL of CH3CN/CHCl3 in THF and 0.5 μL of the mixture were applied on a ground steel plate. The precursor complex [PdCl2(CH3CN)2] was prepared by using previously published procedures.10 The ligands 2-(3,5-dimethyl-1Hpyrazol-1-yl)ethyldiphenylphosphinite (L1), 3-(3,5-dimethyl-1H-pyrazol-1-yl)propyldiphenylphosphinite (L2), and 2-(3,5-diphenyl-1Hpyrazol-1-yl)ethyldiphenylphosphinite (L3) were prepared as described in the literature.9a,c 2.2. X-ray Crystal Structures. Suitable crystals for X-ray diffraction of 1a and 2a were obtained by slow evaporation of a CH3CN solution of the corresponding compounds during one week. Complexes 3c·2H2O·CH3CN and 3d·CH3CN were obtained from the mixture of L3 with [PdCl2(CH3CN)2] without purification. Compound 3c·CH3CN·2H2O was obtained by slow diffusion in CH2Cl2/ pentane in 1:10 proportion during six months and 3d·CH3CN was obtained by slow evaporation of a CHCl3 solution in one week. Prismatic crystals were selected and mounted on a Enraf-Nonius CAD4 four-circle diffractometer for 1a and on a MAR345 diffractometer with an image plate detector for 2a, 3c·CH3CN·2H2O and 3d·CH3CN. Unit-cell parameters were determined from automatic centering of 25 reflections (12 < θ < 21°) for 1a, 2969, 1363, and 1006 reflections (3 < θ < 31°) were determined for 2a, 3c·CH3CN·2H2O and 3d·CH3CN, respectively. All structures were refined by the leastsquares method. Intensities were collected with graphite monochromatized Mo Kα radiation. 5848 (1a), 10882 (2a), 22297 (3c·CH3CN·2H2O) and 32696 (3d·CH3CN) reflections were measured. 4890 (1a), 5071 (2a), 6900 (3c·CH3CN·2H2O) and 13525 (3d·CH3CN) reflections were assumed as observed applying the condition I > 2σ(I). Lorentz-polarization but no absorption corrections were made on 2a, 3c and 3d. For 1a the structure was solved by Patterson synthesis and for 2a, 3c·CH3CN·2H2O and 3d·CH3CN by direct methods using the SHELXS-97 computer program and refined by full-matrix least-squares method with the SHELXL-97 computer program.11 The function minimized for all the structures was Σ w||FO|2 − |FC|2|2 where w = [σ2(I) + (aP)2 + bP]−1 [a = 0.0811 and b = 0.1736 (1a); a = 0.0729 and b = 0.7472 (2a); a = 0.0151 and b = 30.5156 (3c·CH3CN·2H2O);

2. EXPERIMENTAL SECTION 2.1. General Details. All reactions were performed with the use of a vacuum line and Schlenk techniques. All reagents were commercial grade and were used without further purification except triethylamine which was purified by distillation in KOH. All solvents were dried and distilled by standard methods. The elementals analysis (C, H, N) were carried out by the staff of the Chemical Analyses Service of the Universitat Autònoma de Barcelona on a Carlo Erba CHNS EA-1108 instrument separated by chromatographic column and thermoconductivity detector, and by the staff of the Microanalysis Centre of Universidad Complutense de Madrid on a LECO CHNS-932 with three selective detectors, one for each element: infrared solid state for C and H and thermoconductivity detector for N. Conductivity measurements were performed at room temperature in 10−3 M CH3CN solutions, employing a CyberScan CON 500 (Eutech instrument) conductimeter. IR spectra were run on a Perkin-Elmer FT spectrophotometer series 2000 cm−1 as KBr pellets. The 1H, 1 H{31P}, 13C{1H} and 31P{1H}, 2D 1H−13C HSQC, 2D COSY and 2D NOESY NMR spectra were recorded on a Bruker 250 MHz spectrometer and 500 MHz AVANCE spectrometer in CD3CN solutions. 1H, 1H{31P} and 13C{1H} NMR chemical shift (δ) were determined relative to internal TMS. 31P{1H} NMR chemical shift (δ) were determined relative to external 85% H3PO4. NMR diffusion experiments were carried out at 298 K on a 500 MHz AVANCE spectrometer equipped with a 5 mm TCI cryoprobe. PFG-NMR experiments were performed using the compensated BPLED pulse sequence24 in order to avoid unwanted convection effects, using a diffusion time of 150 ms and a LED delay of 5 ms. For each experiment, sine-shaped pulsed-field gradients with a duration of 1.5 ms followed by a recovery delay of 100 μs were incremented from 2% to 95% of the maximum strength in 16 equally spaced steps. Diffusion coefficients were obtained by measuring the slope in the following linear relationship: ln(Ag/Ao) = −γ2g2δ2(4Δ−δ)D; where Ag and Ao are the signal intensities in the presence and absence of pulsed field gradient (PFG), respectively, γ is the gyromagnetic ratio (rad s g−1), g is the strength of the diffusion gradients (G cm−1), D is the diffusion coefficient of the observed spins (m2 s−1), δ is the length of the diffusion gradient (s), and Δ is the time separation between the leading edges of the two diffusion pulsed gradients (s). The positive ionization spectra MS/ESI(+) were carried out by the staff of the Chemical Analysis Service of the Universitat Autònoma de Barcelona B



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a = 0.0361 and b = 4.6675 (3d·CH3CN)]. For all structures P = (|FO|2 + 2 |FC|2)/3. All H atoms for 1a were located from a difference synthesis and refined with an overall isotropic temperature factor. All H atoms of 2a, 25 H atoms of 3c·CH3CN·2H2O and 50 H atoms of 3d·CH3CN were computed and refined, using a riding model, with an isotropic temperature factor equal to 1.2 times the equivalent temperature factor of the atom which are linked. The parameters refined and other details concerning the refinement of the crystal structures are gathered in Tables S2 and S3 (Supporting Information).

values in acetonitrile for three complexes are in agreement with the presence of nonelectrolyte compounds (9−16 Ω−1 cm2 mol−1).12 The IR spectra of complexes 1a−3a show bands between 1587 and 1554, 1484−1436 and 1030−1028 cm−1 corresponding to ν(CC/CN)ar, δ(CC/CN)ar and ν(P−O−C), respectively, indicating the presence of the coordinated ligand.13 The NMR studies in CD3CN solution give information about the coordination and disposition of the ligands toward PdII. The position of the signals (δ = 102−107 ppm) in the 31P{1H} NMR spectra indicates that the phosphorus atoms are connected to PdII (free ligands δ = 116.7−114.3 ppm),9a,c and the values of chemical shift agree with similar complexes described in the literature.14 The 1H and 13C{1H} NMR do not give important differences between the spectra of free ligand and the complexes, in the methyl and phenyl region. Otherwise, the signal of the H/Cpyrazole appears at high δ with respect to free ligands (1H NMR δ = 6.88−5.74 ppm, 13C {1H} NMR δ = 109.5−108.0 ppm; free ligands 1H NMR δ = 6.50−5.60 ppm, 13C {1H} NMR δ = 106.6−103.9 ppm).9a,c Furthermore, the NMR spectra were studied in detail (1H{31P}, COSY, HSQC and NOESY NMR experiments) to make the assignment of the signals of the chain N-(CH2)n-OP (n = 2−3). Furthermore, simulation of 1H NMR spectra of 1a and 3a has been done with g NMR15 to assign the chemical shifts and calculate the coupling constants (Supporting Information, Figure S1, Table S1). 3.3. Compound 3b. To optimize the yield of 3b some modifications in the reaction of L3 with [PdCl2(CH3CN)2] have been done. The reaction of the L3 ligand with PdII in CH2Cl2 in a 2:1 M/L ratio leads a yellow product, which corresponds to 3b (10%). The elemental analysis for compound 3b is consistent with the formula [PdCl2(L3)2]. The positive ionization spectrum MS-ESI (+) gives a peak with a value of m/z 1037 (100%) attributable to [PdCl(L3)2]+. Molar conductivity of 10−3 M solutions in acetonitrile shows a value attributable to nonelectrolyte compound (12 Ω−1 cm2 mol−1).12 The IR spectrum of 3b shows the signals of ν(CC/ CN)ar at 1550, δ(CC/CN)ar at 1482, 1437 and ν(P− O−C) at 1026 cm−1.13 The value of the chemical shift in the 31 1 P{ H} NMR spectrum, δ = 105.0 ppm, indicates that the phosphorus atoms are connected to Pd(II) and corresponds to the two phosphorus atoms, as the complexes described below and other compounds described in the literature.14 The 1H and 13 C{1H} NMR do not give important differences between the spectra of free ligand and the complex. The signals in the 1H NMR spectrum of the chain N-(CH2)2-OP appear at δ = 4.04 ppm, integrating four protons. 3.4. Crystalline Structures. 3.4.1. Crystal Structures of 1a and 2a. Yellow crystals for X-ray diffraction experiments of complexes 1a and 2a were obtained by slow crystallization in CH3CN solution of each compound. The crystal structures of compounds 1a and 2a consist of discrete neutral monomeric units (Figures 1 and 2; Table 2). The metallic center is connected to the ligand via κ2-NP building a metallocycle ring of seven (1a) and eight (2a) members, and finish their coordination with two chlorine atoms in a cis disposition (chelated ligand effect). The palladium atoms adopt distorted square-planar geometry, shown by the values of distances between metallic center and the main plane N1−P−Cl1−Cl2 [0.005 Å (1a), 0.031 Å (2a)]. The values of Cl1−Pd−Cl2 [90.88(7)° (1a), 91.08(5)° (2a)] and N1−Pd−P [90.00(9)° (1a), 93.84(9)° (2a)] value

3. RESULTS AND DISCUSSION 3.1. Presentation of Complexes. The reaction of L1−L3 against [PdCl2(CH3CN)2] in dry CH2Cl2, at room temperature over 12 h led to a mixture of three complexes in solution for each ligand: [PdCl2(Lx)], [PdCl2(Lx)2] and [PdCl2(Lx)]2 [x = 1 (1a−1c); x = 2 (2a−2c); x = 3 (3a−3c)] (Scheme 1, Table 1). All the details concerning the synthesis and characterization of all the compounds are in the Supporting Information. Table 1. Proportions and Yields of the Obtained Complexes of Ligands L1−L3 against [PdCl2(CH3CN)2] ligand L

1

L2

L3

a b

complex 1a 1b 1c 2a 2b 2c 3a 3b 3c

1

[PdCl2L ] [PdCl2(L1)2] [PdCl2(L1)]2 [PdCl2L2] [PdCl2(L2)2] [PdCl2(L2)]2 [PdCl2L3] [PdCl2(L3)2] [PdCl2(L3)]2

proportiona

yield (%)b

80 10 10 60 20 20 50 30 20

63

36

21 10 10

Calculated integrating pz-CH signals of the mixtures (1H NMR). Calculated from isolated compounds

Complexes 1a−3a and 1b−3b are monomeric, where the metallic centers are coordinated via κ2-N,P to the ligand (complexes a) or are coordinated to two ligands via κ1-P (complexes b). The complexes 1c−3c are dimers where the two ligands bridge the two metallic atoms in a head-to-tail arrangement. The relation ratio of the complexes 1a−1c, 2a− 2c and 3a−3c in solution depend on the ligand: 80% (1a), 10% (1b), 10% (1c); 60% (2a), 20% (2b), 20% (2c) and 50% (3a), 30% (3b), 20% (3c) (The ratio between 1a−1c, 2a−2c and 3a−3c are obtained integrating pz-CH signals in the 1H NMR spectra). The compounds 1a, 2a, 3a, 3b and 3c have been isolated and fully characterized by elemental analysis, mass spectrometry [MALDI-TOF or ESI(+)], conductivity measurements, IR and NMR spectrocopies (see Experimental Section). The structure of complexes 1a, 2a and 3c are confirmed by single-crystal X-ray diffraction. Compounds 1b, 2b, 1c and 2c have been identified by similarity of their 31P{1H}and 1H NMR spectra against isolated compounds. 3.2. Compounds 1a−3a. The reaction of the appropriate ligand (L1−L3) with [PdCl2(CH3CN)2] in CH2Cl2 solution for 12 h at room temperature and a later recrystallization with dry diethyl ether led to [PdCl2(Lx)] complexes [x = 1 (1a); x = 2 (2a); x = 3 (3a) {1a (yellow, 63%), 2a (yellow, 36%) and 3a (orange, 21%)}]. The elemental analyses of complexes 1a−3a are consistent with the formula [PdCl2(Lx)]. The mass spectra (MALDI-TOF) of the three compounds give peaks with m/z values of 467 (100%) (1a), 481 (100%) (2a) and 591 (100%) (3a), attributable to [PdCl(Lx)]+ (L = L1−L3). Conductivity C



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the functionalized groups in the same ligand chelating the metallic center. Eleven more structures have been reported with a similar core, useful to compare the parameters of structures described.17 3.4.2. Crystal Structures of 3c and 3d. Two types of monocrystals have been obtained from the mixture of compounds of the reactivity of L3 with [PdCl2(CH3CN)2]. A CH2Cl2 solution of the mixture of compounds was evaporated and added slowly to pentane solvent in a 1:10 proportion, and slow evaporation of the solution during six months led to monocrystals of 3c·CH3CN·2H2O (Figure 3).

Figure 1. ORTEP drawing of 1a showing all non-hydrogen atoms and the atom-numbering scheme; 50% probability amplitude displacement ellipsoid shown.

Figure 2. ORTEP drawing of 2a showing all non-hydrogen atoms and the atom-numbering scheme; 50% probability amplitude displacement ellipsoid shown.

Figure 3. ORTEP drawing of 3c·CH3CN·2H2O showing all nonhydrogen atoms. Some atom numbers and solvents have been omitted for clarity; 50% probability amplitude displacement ellipsoid shown.

angles are in agreement with data found in the literature.12,16 The bond distances Pd−N1 [2.051(3) Å (1a), 2.112(3) Å (2a)], Pd−P [2.2262(16) Å (1a), 2.2321(11) Å (2a)], Pd−Cl (trans-P) [2.3767(17) Å (1a), 2.3798(12) Å (2a)], Pd−Cl (cisP) [2.2891(15) Å (1a)] are also in agreement with the values described in the literature: Pd−N [2.026−2.141 Å], Pd−P [2.162−2.238 Å], Pd−Cl (trans-P) [2.351−2.401 Å] and Pd− Cl (cis-P) [2.275−2.301 Å]. However, the distance Pd−Cl (cisP) [2.3621(15) Å (2a)] is slightly larger than the values found in the literature. The distance Pd−Cl (trans-P) is longer than Pd−Cl (cis-P) probably due to the trans effect of the phosphorus atom.12,16 As far as we know, five structures of PdII have been published with the [NP-phosphinite (P(OR1)R22 where R1 is an alkylic chain and R2 are -C6H5, Cl] core and

Also, evaporation to dryness of a CH2Cl2 solution of the mixture of compounds, and recrystallization of the solid in CHCl3 and slow evaporation during one week, led to a new compound not detected in solution: 3d·CH3CN (Figure 4). The molecular structures of the dimeric complexes consist of two ligands that bridge the two PdII atoms forming a 14membered ring in a head-to-tail arrangement (3c) or a head-tohead arrangement (3d). In complex 3c·CH3CN·2H2O, the two metallic centers have a [N, P-phosphinite, Cl2] core and the chlorine atoms are in a -trans disposition. However, in complex 3d·CH3CN one PdII has a [N2, Cl2] core with the chorine atoms in a -trans disposition and the other PdII atom has the

Table 2. Selected Bond Lengths (Å) and Angles (°) for 1a, 2a, 3c·CH3CN·2H2O, and 3d·CH3CN 1a Pd−N1 Pd−P Pd−Cl1 Pd−Cl2 P−Pd−N1 P−Pd−Cl1 Cl2−Pd−Cl1 N1−Pd−Cl2

2a 2.051(3) 2.2262(16) 2.2891(15) 2.3767(17) 90.00(9) 90.05(6) 90.88(7) 89.16(9)

Pd−N1 Pd−P Pd−Cl2 Pd−Cl1 P−Pd−N1 P−Pd−Cl2 Cl2−Pd−Cl1 N1−Pd−Cl1

3c 2.112(3) 2.2321(11) 2.3798(12) 2.3621(15) 93.84(9) 87.73(5) 91.08(5) 88.05(9)

Pd−N1 Pd−P Pd−Cl1 Pd−Cl2 P−Pd−Cl1 P−Pd−Cl2 P−Pd−N1 N1−Pd−Cl1 D

3d 2.136(4) 2.2187(17) 2.3003(16) 2.2881(18) 91.52(6) 88.15(6) 175.45(11) 91.68(12)

Pd1−N1 Pd1−N4 Pd1−Cl1 Pd1−Cl2 Pd2−P1 N4−Pd1−N1 Cl2−Pd1−Cl1 P1−Pd2−P2

2.041(4) 2.023(4) 2.3178(18) 2.3098(17) 2.2290(15) 178.40(16) 172.48(5) 92.26(5)



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novel supramolecular architectures with reversible functional properties. Among these noncovalent interactions, hydrogen bond and π···π stacking interactions are the driving forces used for the creation of supramolecular frameworks.21 In compound 1a, through the coordinated interactions, one PdII, one quelated L1 ligand, and two chloride anions are assembled together to generate a complex unit (Figure 1); interestingly, this unit is further linked with 3 equivalent units (Figure 5a) through hydrogen bond bridges (RC−H···Cl =

Figure 4. ORTEP drawing of 3d·CH3CN showing all non-hydrogen atoms. Some atom numbers and solvents have been omitted for clarity; 50% probability amplitude displacement ellipsoid shown.

[(P-phosphinite)2, Cl2] core with the chlorine atoms in a -cis disposition. In complex 3c·CH3CN·2H2O, the two PdII have a slightly distorted square-planar geometry observed by the values of distances between Pd and the main plane N1−P−Cl1−Cl2 [0.055 Å], and the angles N1−Pd−Cl1, N1−Pd−Cl2, P−Pd− Cl1 and P−Pd−Cl2 [91.68(12)°, 88.52(12)°, 91.52(6)° and 88.15(6)°], respectively. All the values are in agreement with the values found in the literature.18 The bond distances Pd−N1 [2.136(4) Å], Pd−Cl1 [2.3003(16) Å] and Pd−Cl2 [2.2881(18) Å] are also in agreement with the values described in the literature: Pd−N [2.078−2.136 Å] and Pd−Cl [2.282− 2.338 Å]. Otherwise, the distance Pd−P [2.2187(17) Å] is slightly shorter than the values described in the literature [2.221−2.259 Å]. The Pd···Pd separation is 5.395 Å, a normal value in front of the literature [4.656−9.023 Å].16,19 In the literature there are no reports of crystalline structures of dimers with a [N-pyrazole, P-phosphinite, Cl2] core, but 14 are reported crystalline structures with [N-pyrazole, Pphosphine, Cl2] core,16,20 7 of them useful to compare the data of 3c·CH3CN·2H2O.16 In complex 3d·CH3CN, the two PdII centers are not symmetrical and they have a different core structure. The geometry of the two palladium atoms is distorted square-planar. The distances between the Pd1 and the plane N1−N4−Cl1− Cl2 and the Pd2 and the plane P1−P2−Cl3−Cl4 are 0.068 Å and 0.027 Å, respectively. The dihedral angle between the two planes is 85.05° and the Pd···Pd separation is 7.185(2) Å, in agreement with the values described in the literature.12,14,18 The bond distances Pd1−N1 and Pd1−N4 [2.041(4) Å and 2.023(4) Å], Pd1−Cl1, Pd1−Cl2, Pd2−Cl3 and Pd2−Cl4 [2.3178(18) Å, 2.3098(17) Å, 2.3551(17) Å and 2.3544(17) Å] and Pd−P1 [2.2290(15)] are in agreement with the values described in the literature but Pd−P2 [2.2781(14) Å] is slightly larger [2.181−2.258 Å].12,14,18 3.5. Supramolecular Structures. In the process of assembly, the molecule structural units utilize single/cooperative noncovalent interactions as the driving forces to construct

Figure 5. (a) 3D supramolecular structure stabilized by hydrogen bond interactions between adjacent 1a units. (b) π−π intermolecular interaction between two adjacent phenyl rings in 1a.

2.753(3) Å, 2.810(3) Å; °C−H···Cl = 160.67(2)°). Furthermore, complex 1a shows weak intermolecular interaction π−π stacking (Figure 5b). The angle between the two planes formed by C14−C19 of two assymmetric 1 − x, −y, −z molecules is 0° confirming a perfect face-to-face interaction, and the distance between their centroids [3.878(3) Å] is very similar to values found in the literature [3.0−4.6 Å]. However, only C15 and C16 build a parallel displaced π−π stacking interaction. The distance between the pair of carbons is 3.348(2) Å and is also according with the values obtained in the literature [3.3−3.8 Å].22 All the intermolecular interactions together finally form a three-dimensional (3D) supramolecular network. Contrary to what we have found in 1a, complex unit of 2a, through cooperative intermolecular interactions (RC−H···Cl = 2.607(4) Å, 2.807(5) Å; °C−H···Cl = 150.40(2)°) is able to generate a bidimensional (2D) supramolecular layer along the a axis (Figure 6a). Moreover, it is interesting to find that all the methyl groups are located on the external parts of the layer generating important hydrophobic interactions (Figure 6b). Among the noncovalent interactions, hydrophobic interactions, usually existing among alkyl chains of biological macromolecules, are difficult to be observed from a crystallographic perspective.23 In the dimeric structure of 3c, the palladium(II) atoms are linked leading undulating monodimensional (1D) chains (Figure 7a) with intermolecular Pd···Pd distances of 10.370(2) Ǻ . In this case, C−H···Cl is formed between the E



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Figure 6. (a) 2D supramolecular layer stabilized by hydrogen bond interactions between adjacent 2a units. (b) b axis view of the 2D supramolecular layer of 2a.

Cl atom and the CH3 group of the cocrystallized acetonitrile (RC−H···Cl = 3.211(5) Å). Moreover, neighboring chains are encored in crossed disposition (112°) to form a 2D supramolecular layer (Figure 7b). Finally, the dimeric compound 3d presents intermolecular Pd···Pd distances smaller (8.639(3) Ǻ ) than in 3c (Figure 8a) also forming monodimensional chains thanks to the C−H···Cl intermolecular interactions (RC−H···Cl = 2.949(2) Ǻ ). The final disposition of 3d is a full 3D network self-assembled (Figure 8b). It is important to mention that in any structure the oxygen moieties do not play any role in the self-assembly process, probably due its bond to P atom. 3.6. Elucidation of the Dimeric Compound in Solution, 3c or 3d. As we have introduced before, in the reactivity of L3 with [PdCl2(CH3CN)2] two types of dimeric compounds have been resolved by X-ray diffraction (3c and 3d); otherwise, in solution only one dimer is observed (3c). To elucidate which species is present in solution a deep characterization was done by positive ionization spectra MSESI(+) and NMR techniques. First, the MS-ESI(+) spectra of the solution mixture of compounds of the reaction of L3 with [PdCl2(CH3CN)2] gives a representative peak with m/z value of 1217 (42%) attributable to [Pd2Cl3(L3)2]+ (Supporting Information, Figure S2). The isotopic distribution is according to the dimeric species; however, it cannot be distinguish if this peak corresponds to 3c or 3d. Interestingly, NMR studies give important information. In the 31P{1H} NMR spectrum of the dimer in acetonitrile solution appears a singlet at δ = 116.1 ppm, and in the 1H NMR spectrum, the protons of the chain N−CH2CH2-OP appears as two triplets at δ = 4.80 (4H) ppm and at δ = 4.20 (4H) ppm. Compound 3c has a c2/c space group in the solid state, so the two ligands L3 are symmetrical. If the distribution

Figure 7. (a) 1D supramolecular chain stabilized by hydrogen bond interactions between adjacent 3c units. (b) c axis view of the monodimensional chain of dimer 3c.

of the ligands around the metallic center is maintained in solution, probably the 31P{1H} NMR spectrum of the complex should show a single peak integrating two phosphorus, and in the 1H NMR spectrum the protons H6a/b and H8a/b should show two triplets according to the presence of the chain N− CH2CH2-OP. However, 3d has a P(−1) space group in the solid state, so probably the solution 31P{1H} NMR spectrum should show two doublets, one for each phosphorus atom according to the asymmetry of the dimer and 1H NMR should show eight signals, one for each proton of the two chains N− CH2CH2-O. Furthermore, PFG-NMR measurements24 in a CD3CN solution of the mixture of compounds at 298 K corroborates this hypothesis. The dimer observed in solution has a D value of 9.12 ± 0.09 × 10−10 m2 s−1, with a hydrodynamic radius (RH) of 7.9 ± 0.60 Å. The RH of the dimer is in agreement with the radius of the crystalline structure of 3c (R = 7.83 × 10−10 Å) and is larger than the radii of the crystalline structure of 3d (R = 7.12 × 10−10 Å). Also, the complex 3a presents a diffusion coefficient value of 1.15 ± 0.09 × 10−9 m2 s−1 that is equivalent to a RH of 6.3 ± 0.60 Å in concordance with similar complexes described in the literature.19 The theoretical relationship between monomer and dimer derivatives should be about 1.26 assuming a similar spherical shape for both structures.25 Our experimental results with 3c (1.25) agree with these predictions; we strongly suggest dimer species detected in solution is 3c, whereas any trace of the alternative 3d complex was observed. Also, in the literature, few examples of dimer complexes are found. In all cases, the F



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Scheme 2. Flexible Coordination of N,P-Hybrid Ligands

compound 3b is the main product. If the solvent is changed from dry CH2Cl2 to dry CH3CN, 1:1 M/L and room temperature (assay 3), compound 3a can be detected (main product) and 3b but not 3c. If a solution of the three compounds is refluxed in dry CH3CN during three days (assay 4), only compound 3a is detected, while compounds 3b and 3c decompose to [PdCl(Ph2PO)2H]226 and [Ph2POx(OH)y].27 Finally, in assay 5, a CH3CN solution of [PdCl2(CH3CN)2] is added to a solution of compound 3b in CH3CN in 1:1 M/L proportion leading to a solution containing compounds 3a and 3c. Variable temperature 1H NMR studies have been done (T = 298−338 K). No equilibrium between complexes has been observed or the equilibrium is much faster than the NMR technique to be detected. These assays permit elucidation of some information about the relation of the compounds in solution. The formation and proportion in solution of compounds 3a−3c and the ratio between them in solution are dependent on the proportion of the starting reagents (metal/ligand) and the solvent used in the synthesis. Also, assay 5 permits better understanding of the mechanism involved in the transformation between the compounds. Probably, as it can be seen in Scheme 3, compound 3b in the presence of [PdCl2(CH3CN)2] can produce a vacant coordination around PdII (species 1). If this vacancy is occupied by the ligand connected to the metallic center, complex 3a is generated, but if two species 1 connect it complex 3c could be obtained. This theory could explain the formation of compound 3d, although only indirect evidence is obtained.

Figure 8. (a) 1D supramolecular chain stabilized by hydrogen bond interactions between adjacent 3d units. (b) 3D supramolecular view of the self-assembled network of dimer 3d.

dimers present the same structure of 3c; with this experimental evidence we could believe the dimer we are able to detect in solution is 3c. 3.7. Versatility Coordination of N-Pirazole, P-Phosphinite Hybrid Ligands against PdII. The reaction of Npyrazole, P-phosphinite ligands against PdII, leads to compounds a, b and c in solution. The diversity of compounds shows the high versatility of N,P-hybrid ligands to connect to the metallic center with different coordination modes and producing different types of complexes. In order to understand the factors that affect the formation of the different compounds, some synthetic conditions have been modified to extract valuable information: the relation ratio of the starting reagents (metal/ligand), order of addition, solvents and temperature used in the synthesis. The assays presented in Scheme 2 correspond to modifications in the synthesis of compounds 3a−3c. The first reaction shows the results of a mixture 1:1 M/ L in dry CH2Cl2 at room temperature (the condition to obtain 3a). In solution compounds 3a, 3b and 3c are detected, although the compound 3a is the main product. In the second reaction, the relation ratio 1:2 M/L is used in dry CH2Cl2 and room temperature (the conditions to obtain 3b). In this assay, compounds 3a and 3b but not 3c are detected; in this case,

4. CONCLUSIONS The versatility of the family of N-pyrazole, P-phosphinite hybrid ligands L1−L3 has been studied using PdII as the metallic center. The reactions lead to a mixture of three complexes for each ligand ([PdCl2(L)], [PdCl2(L)2] and [PdCl2(L)]2) obtaining monomers (where L acts as mono-/bidentated agents) and dimers where the two L bridge the two metallic atoms in a head-to-tail arrangement. The formation of all the compounds in solution show the versatility of N-pyrazole, Pphosphinite ligands to connect to the palladium center. By controlling the relation ratio of the starting reagents (metal:ligand), order of addition, solvents, temperature and solvents crystallization, it is possible to isolate and fully characterize most of them. X-ray crystalline studies allow us to examine the effects of the L structure on the topology and interpenetration form (onedimensional chain, two-dimensional, layer polymers and threedimensional networks). Additionally, experimental and simulated NMR studies and a battery of modifications in the G



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Scheme 3. Possible Mechanism in the Reaction 3b + [PdCl2(CH3CN)2]

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synthesis of the complexes permit better understanding of the role of this family of ligands in the self-assembly process.

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ASSOCIATED CONTENT

S Supporting Information *

The parameters refined and other details concerning the refinement of the crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC reference number 885585 for 1a, 885586 for 2a, 885203 for 3c·CH3CN·2H2O and 885587 for 3d·CH3CN. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ. UK: fax: + 44 1223336033; e-mail: [email protected] or www.htpp://ccdc.cam.ac.uk.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 34-93 581 31 01; e-mail: Josefi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS In memory of Prof. Xavier Solans i Huguet. Support by the Spanish Ministerio de Educacion y Cultura (Projects CTQ2007-63913 BQU and CTQ2009-08328) is gratefully acknowledged.

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REFERENCES

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Exploring the Versatility of N-Pyrazole, P-Phosphinite Hybrid Ligands

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