Article pubs.acs.org/crystal
The Lead Coordination Polymers Containing Pyrazine-2,3Dicarboxylic Acid: Rapid Structural Transformations and Cation Exchange Fendi Y. Wardana,† Seik-Weng Ng,†,‡ and Arief C. Wibowo*,† †
Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, 50603, Malaysia Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203 Jeddah, Saudi Arabia
‡
S Supporting Information *
ABSTRACT: A new Pb-based coordination polymer synthesized with pyrazine-2,3-dicarboxylic acid (H2pzdc) is reported along with its structural characterizations. The compound, Pb2Cl2(Hpzdc)2(H2O)2 (1), is synthesized using a simple, aqueous-based dissolution−slow evaporation method with optimized conditions. It crystallizes in the C2/ m space group (a = 11.561(1) Å, b = 8.4327(7) Å, c = 11.792(1) Å, β = 126.3070(8)°), with a 1D coordination polymer consisting of Pb2Cl2 dimers with monoprotonated dicarboxylic acids located between the chains. Soaking 1 in an aqueous solution of Hg(II) or Co(II) at room temperature resulted in a structural transformation into a newly formed 2D or 3D structure, respectively. The 2D compound, Pb(pzdc)(H2O) (2), crystallizes in the Pbca space group (a = 9.2624(9) Å, b = 12.0268(9) Å, c = 13.9430(8) Å), whereas the rapidly formed 3D compound, Pb2.67Co0.33(pzdc)3(H2O) (3), crystallizes in the P21/n space group (a = 10.9665(7) Å, b = 8.1909(4) Å, c = 25.0238(1) Å, β = 91.593(5)°). A structure similar to 3 without any dopant, Pb3(pzdc)3(H2O) (3a), is also synthesized. A rapid, room-temperature cation exchange is also observed when soaking 1 into an aqueous solution of Cu(II), giving a known structure. Systematic studies on the syntheses and characterizations of 1 and its remarkably rapid structural transformations and cation exchange at room temperature are presented.
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INTRODUCTION Metal−organic coordination polymers (MOCPs) are typically synthesized hydro- or solvothermally using aromatic dicarboxylic acid based ligands and transition elements, in particular, post transition elements, for potential use in gas storage,1−5 magnetic,1,2,6,7 and luminescent applications.1,2,8−10 Convenience and relatively faster results have been the appealing factors for many researchers to use a hydro- or solvothermal synthetic method rather than the dissolution−slow evaporation technique, as reflected in the number of publications. Pyrazine-2,3-dicarboxylic acid (H2pzdc) ligand, whether by itself or combined with a coligand, has been one of the most frequently used aromatic dicarboxylic acid ligands to synthesize diverse structures of transition-metal-based MOCPs or metal− organic frameworks, MOFs. Several notable compounds include [Cu2(pzdc)2(L)] (L = pyrazine, 4,4′-bipyridine, N-(4pyridyl)isonicotinamide, or 1,2-di(4-pyridyl)glycol),11,12 Cubased MOFs potential for gas adsorption application. Zhang and co-workers have reported heterometallic coordination polymers of Gd2Ag6(pzdc)6(H2O)9·8H2O and LnAg(pzdc)2(H2O)2·2H2O (Ln = Tb, Dy, Ho, and Er), in which Dy- and Er-containing compounds exhibit magnetic properties.13 Other silver-based pzdc compounds have also been recently published.14 To our surprise, only two reports have been found on Pb-containing pzdc coordination polymers so far,15,16 despite the fact that main group elements such as bismuth and © XXXX American Chemical Society
lead coordinate well to alkoxide and/or carboxylic acids and can exhibit either stereochemically active or inactive lone pairs.17−37 Pb(II), with its large ionic radii, can adopt a wide range of coordination numbers (2−10)34,36,38−40 and has the ability to form multidimensional inorganic−organic hybrid structures through the formation of Pb-X-Pb (X = O, N, Cl, S),41,42 exhibiting potential in a variety of applications such as photoluminescence,33,36,37,43 birefringence,21,44 and selective gas adsorption.45 In recent years, there has been considerable interest in the dynamics of MOCP in solvent medium, often accompanied by structural transformations due to the dissolution−recrystallization process for applications such as physical and chemical adsorption, ion exchange, sensor technology, drug delivery, and many more.46−73 The structural transformation capability shall enhance with the use of a Pb(II) cation due to its flexibility in adopting various donor atoms and coordination numbers as well as geometry (hemidirected or holodirected).74 In fact, Vittal and co-workers as well as Morsali et. al, each has reported Pb-based MOCPs that undergo structural transformations in a solvent medium.54,75 Enriching the Pb-based MOCP structural transformation library, herein, we report a new Pb-containing Received: September 27, 2015 Revised: October 12, 2015
A
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
a
B
1149/1/75 1.107 Robs = 0.0256, wRobs = 0.0677 Rall = 0.0262, wRall = 0.0679 1.333 and −3.197
monoclinic C2/m a = 11.561(1) Å, α = 90.00° b = 8.4327(7) Å, β = 126.3070(8)° c = 11.792(1) Å, γ = 90.00° 926.33(1) 2 3.067 18.504 776 0.40 × 0.30 × 0.20 2.14−27.56 −14 ≤ h ≤ 13, −10 ≤ k ≤ 9, −15 ≤ l ≤ 15 4501 1149 [Rint = 0.0459] 99.8
C12H10Cl2N4O10Pb2 855.52 295(2)
R = ∑∥Fo| − |Fc∥/∑∥Fo∥. wR2 = {∑[w(F2o − F2c )2]/∑[w(F2o)2]}1/2.
volume (A3) Z density (calculated) (g/cm3) absorption coefficient ( mm−1) F(000) crystal size (mm3) θ range for data collection (deg) index ranges reflections collected independent reflections completeness to θ = 27.56° (%) refinement method data/restraints/parameters goodness-of-fit final R indices [I > 2σ(I)] R indices [all data] largest diff. peak and hole (e·Å−3)
empirical formula formula weight temperature (K) wavelength crystal system space group unit cell dimensions
compound 1
Table 1. Crystal Data and Structure Refinement Details for 1, 2, and 3a
0.71073 Å orthorhombic Pbca a = 9.2624(9) Å, α = 90.00° b = 12.0268(9) Å, β = 90.00° c = 13.9430(8) Å, γ = 90.00° 1553.2(2) 8 3.347 21.722 1408 0.20 × 0.05 × 0.02 2.92−27.56 −12 ≤ h ≤ 7, −15 ≤ k ≤ 13, −18 ≤ l ≤ 12 5329 1790 [Rint = 0.0538] 99.9 full-matrix least-squares on F2 1790/78/127 1.036 Robs = 0.0351, wRobs = 0.0738 Rall = 0.0542, wRall = 0.0847 2.560 and −2.150
C6H4N2O5Pb 391.30 100(2)
compound 2
5194/228/373 1.166 Robs = 0.0548, wRobs = 0.1002 Rall = 0.0756, wRall = 0.1073 3.684 and −1.741
monoclinic P21/n a = 10.9665(7) Å, α = 90° b = 8.1909(4) Å, β = 91.593(5)° c = 25.0238(1) Å, γ = 90° 2246.9(2) 4 3.218 20.264 1959 0.08 × 0.04 × 0.02 2.97−27.59 −14 ≤ h ≤ 10, −10 ≤ k ≤ 7, −32 ≤ l ≤ 32 13 586 5194 [Rint = 0.0578] 99.6
C18H8Co0.33N6O13Pb2.67 1088.47 100(2)
compound 3
Crystal Growth & Design Article
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Table 2. Representative Bond Lengths (Å) and Bond Angles (deg) of Compound 1a Pb(1)−Cl(1) Pb(1)−Cl(1) Pb(1)−O(1) Pb(1)−O(1w) Pb(1)−N(1) O(1)−Pb(1)−O(1) O(1)−Pb(1)−O(1w) O(1)−Pb(1)−Cl(1) O(1w)−Pb(1)−Cl(1) O(1)−Pb(1)−N(1) a
2.739(2) 3.053(2) 2.458(4) 2.695(6) 2.846(6) 67.87(18) 74.57(15) 82.11(11) 151.78(13) 126.33(12)
O(1)−Pb(1)−N(1) O(1w)−Pb(1)−N(1) Cl(1)−Pb(1)−N(1) N(1)−Pb(1)−N(1) O(1)−Pb(1)−Cl(1) O(1w)−Pb(1)−Cl(1) Cl(1)−Pb(1)−Cl(1) N(1)−Pb(1)−Cl(1) Pb(1)−Cl(1)−Pb(1) C(1)−O(1)−Pb(1)
58.51(12) 92.76(8) 88.33(8) 173.59(17) 142.23(10) 126.37(13) 81.85(6) 87.06(8) 98.15(6) 133.1(3)
Symmetry transformations used to generate equivalent atoms: (1) −x + 1, −y, −z + 1; (2) x, −y, z; (3) x, −y + 1, z. using different solvents or mixture of solvents, were done in a similar fashion. A quantitative analysis was performed by SEM-EDS on some selected compounds. Structural Transformation of 1 into 2. 1 (0.03 g, 0.035 mmol) was immersed into an ∼7 mL aqueous solution of Hg(NO3)2·H2O (0.066 mmol). The colorless ribbon single crystals, Pb(pzdc)(H2O) (2), were obtained along with polycrystalline powder after a minimum of 2 weeks of soaking. Structural Transformation of 1 into 3. A similar soaking procedure was adopted to obtain 3, but using Co(NO3)2·6H2O or CoCl2·6H2O (0.066 mmol) instead. The colorless plate single crystals, Pb2.67Co0.33(pzdc)3(H2O) (3), were obtained within 2 h of soaking. Structural Transformation of 1 into 3a. To obtain 3a, a similar soaking procedure was also adopted, but using L-(+)-tartaric acid (0.066 mmol) instead. The colorless plate single crystals, Pb3(pzdc)3(H2O) (3a), were obtained along with polycrystalline powder after a minimum of 2 weeks of soaking. Cation Exchange of 1. A soaking procedure similar to that above was adopted to obtain a cation-exchanged product, but using Cu(NO3)2·3H2O or CuCl2·2H2O (0.066 mmol) instead. The bluish green plate single crystals were obtained within 2 h of soaking. X-ray Crystallography. X-ray intensity data from a colorless ribbon crystal (1) were measured at 295(2) K using a Bruker SMART APEX diffractometer (Mo Kα radiation, λ = 0.71073 Å). A colorless ribbon crystal (2) and colorless plate crystals (3 and 3a) were measured at 100(2) K using an Agilent Supernova diffractometer (Mo Kα radiation, λ = 0.71073 Å). Direct methods structure solution, difference Fourier calculations, and full-matrix least-squares refinement against F2 were performed with SHELXTL.76 Powder X-ray Diffraction. Ground mixtures of polycrystalline powders and/or single crystals of the samples were used to collect powder X-ray diffraction patterns using a PanAlytical X’Pert Pro Powder Diffractometer (Cu Kα radiation λ = 1.5418 Å) over the 2θ range of 5−50°, with a step size of 0.02° and a scan speed of 0.25°/ min. The measured patterns were compared to the simulated diffraction patterns using the respective single crystal data.
pzdc, Pb2Cl2(Hpzdc)2(H2O)2 (1), featuring a 1D dimer with monoprotonated dicarboxylic acid. The said compound is synthesized using a simple, aqueous-based dissolution−slow evaporation method unlike the prevailing hydro- or solvothermal method. The 1D compound undergoes structural transformations into 2D and 3D coordination polymers, as well as cation exchange by simple soaking in aqueous solutions of different divalent cations or anion at room temperature. Some of the transformations occur remarkably fast, forming good quality single crystals just within 2 h of soaking. A systematic study on the syntheses of 1, including the role of auxiliary Ndonor ligands during reaction and crystallization, its structure along with its structural transformations as well as cation exchange is described.
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EXPERIMENTAL SECTION
All reagents are commercially available and used without further purification. PbO (99.999%), pyrazine-2,3-dicarboxylic acid (97%), piperazine (99%), Cu(NO3)2·3H2O (99%), CuCl2·2H2O (≥99.0%), Co(NO3)2·6H2O (≥98%), CoCl2·6H2O (≥98.0%), Hg(NO3)2·H2O (≥98.0%), and L-(+)-tartaric acid (99.5%) were purchased from Sigma-Aldrich. Elemental analyses were performed using a PerkinElmer CHNS/O 2400 Series II. Quantitative analyses were performed using a Hitachi SU8220 scanning electron microscopeenergy dispersive spectrometry (SEM-EDS). Attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectra were recorded on a PerkinElmer FTIR-Spectrum 400. Synthesis of Pb2Cl2(Hpzdc)2(H2O)2 (1). A mixture of PbO (0.089 g, 0.4 mmol), H2pzdc (0.067 g, 0.4 mmol), and piperazine (0.0215 g, 0.25 mmol) in DI water (20 mL) was heated and stirred at 60 °C. Subsequently, 4−6 drops of HCl was added into the mixture, resulting in a clear solution. The stirring and heating were continued for 20 min. Large, colorless, ribbon crystals were obtained in a quantitative yield after 2−3 days of slow evaporation. 1 can also be obtained by mixing an aqueous, clear solution of PbO (with addition of 4−6 drops of HCl) and an aqueous, clear solution of H2pzdc and piperazine, followed by slow evaporation. The final products were washed thoroughly with fresh DI water, isolated by vacuum filtration, and dried at room temperature. Other synthetic combinations of Pb(II) precursors with or without other auxiliary N-donor ligands were done in a similar fashion. A quantitative analysis of the crystals obtained was performed by CHN analysis. Anal. Calcd (Found) for C12H10Cl2N4O10Pb2: C, 16.85 (16.76); H, 1.18 (1.15); N, 6.55 (6.44). Soaking 1 in a Solution of Divalent Cation Precursor at Room Temperature. 1 (0.03 g, 0.035 mmol) was immersed into an ∼7 mL aqueous solution of either nitrate and/or chloride precursor of M(II) (M = Hg, Co, or Cu (0.066 mmol)) or into an ∼7 mL aqueous solution of L-(+)-tartaric acid (0.066 mmol). The dissolution− recrystallization progress was examined in situ under an optical microscope as a function of soaking time at room temperature. The obtained products (2, 3, 3a, and cation-exchanged) were then washed thoroughly with fresh DI water and dried at room temperature prior to further characterizations. Other representative soaking conditions,
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RESULTS AND DISCUSSION Syntheses of 1. Relatively air-stable compound 1 with quantitative yield giving the largest single crystals was identified by the reaction of aqueous-based H2pzdc with PbO in the presence of piperazine under dissolution−slow evaporation with optimized conditions. The structure was determined by single crystal X-ray diffraction and CHN analysis, and its single phase purity was examined by powder X-ray diffraction. Prior to this discovery, we attempted several synthetic conditions, which involved combinations of the following variables, such as (a) Pb precursor (PbO, Pb(NO3)2, or PbCl2) and (b) with or without the presence of an auxiliary N-donor ligand (e.g., piperazine, pyrazole, or ethylenediamine). The phase of 1 was already formed even without the presence of an auxiliary N-donor ligand. The impurity peaks in the PXRD pattern (Figure S1) disappear after adding one of the auxiliary N-donor ligands C
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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(shown in this case is piperazine). The best synthetic condition that produced quantitative yield with the largest single crystals was the condition mentioned above. From such exploratory works, we found that piperazine was the best crystallizing agent for 1. A hydrothermal method using the same precursors was also attempted, and it, however, only produced PbCl2 needle crystals. Besides HCl, dissolution of PbO precursor can also be done using other HX (X = Br and I) and as such, exploration of new compounds resulting from analogue reactions is ongoing and will be reported in our subsequent communications. Structural Description of 1. Relevant crystallographic data for the single crystal structure refinement for 1 is found in Table 1. Table 2 shows representative bond distances and angles of 1. Figure S2A depicts the atom labels of 1, which consists of one lead atom site, one bridging chlorine atom site, one monoprotonated ligand (Hpzdc−), and one coordinated water. The oxygen and the nitrogen atoms of the Hpzdc− ligand as well as the independent chlorine bridging ligand and the oxygen of water complete the coordination sphere around the lead cation, forming a slightly distorted pentagonal bipyramid, PbCl2N2O3 (Figure S2B). Such distorted polyhedra have also been observed in other lead-based coordination compounds.18,77−81 The Pb−N bond length is 2.846(6) Å, Pb− O fall into the range of 2.458(4)−2.695(6) Å, and Pb−Cl fall into the range of 2.739(2)−3.053(2) Å, typically observed in Pb-based coordination polymers.18,77−81 The structure is built around centrosymmetric Pb2Cl2 dimers, connected by Hpzdc− ligands to form a chain (A) extended along the b-axis (Figure 1). The overall structure is
Figure 2. 1D coordination polymer of 1 showing A and A′ chains related by a glide plane along the b-axis with highlights on the weak hydrogen bonds (dashed green bonds).
which resulted in the relatively sharp O−H stretch. As a comparison, a fully free O-H group of carboxylic acid in MOCP was observed as a sharp peak at 3600 cm−1.82 Powder X-ray diffraction was used to confirm the purity of 1 (ground single crystals). While elemental analysis can provide feedback concerning the compositional content of the sample, powder X-ray diffraction can provide, in addition, phase purity information and, for example, reveal the presence or absence of polymorphs. The PXRD patterns of the product obtained are shown in Figure 3, showing pure phase of 1 and that no additional polymorph is present in the sample used for further soaking study.
Figure 3. Powder X-ray diffraction patterns of simulated (A) and observed (B) patterns of 1. Figure 1. (A) Pb2Cl2 dimers of compound 1 connected by Hpzdc− forming a chain along the b-axis. (B) View along the b-axis showing 1D coordination polymer.
From 1D to 2D Coordination Polymer. Dynamics of MOCP in a solvent medium has attracted considerable interest in recent years for applications such as physical and chemical adsorption, ion exchange, sensor technology, drug delivery, and many more.46−73 Knowing that 1 has weakly hydrogen-bonded hydroxyl groups (from the monoprotonated dicarboxylic acid) within the chains, we then soaked 1 in several aqueous or ethanol or water/ethanol solutions of different divalent cation or anion precursors at room temperature, to investigate its dynamics in solvent medium. Under in situ observation using an optical microscope, overall, we found that soaking 1 in aqueous solutions of divalent cation or anion precursors exhibited the best properties in terms of visual changes, yielding single crystal products through the dissolution−recrystallization process with a measured pH in the range of 2−3. Similar products were also observed, regardless of the type of cation precursors (nitrate vs chloride) used during soaking experiments, and details of this are represented by soaking 1 in a Co(II) precursor system (from 1D to 3D transformation).
composed of repetitions of AA′ chains, in which A′ relates to A by a glide plane along the b-axis (Figure 2). This figure also shows the monoprotonated dicarboxylic acid with Lewis base oxygen atoms (both O(2) atoms of 2, 3-dicarboxylic acid) located within the chains. There are weak hydrogen bonds, with a bond distance of ∼2.66 Å, between O(1w) atoms of coordinated water and H(2) atoms (note on the disorder) of the monoprotonated dicarboxylic acid, as shown in dashed green bonds (Figure 2). The presence of Lewis base in 1 is confirmed by attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectra from the observation of a relatively sharp peak around 3470 and 3550 cm−1, indicating the existence of weakly hydrogen-bonded hydroxyl groups (Figure S3), confirming the observation in the crystal structure. Such a long hydrogen bond reduces the proton exchange, D
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Table 3. Representative Bond Lengths (Å) and Bond Angles (deg) of Compound 2a Pb(1)−O(1) Pb(1)−O(3) Pb(1)−O(3) Pb(1)−O(4) Pb(1)−O(1w) Pb(1)−N(1) Pb(1)−N(2) O(3)−Pb(1)−O(1w) O(3)−Pb(1)−N(2) O(1w)−Pb(1)−N(2) O(3)−Pb(1)−N(1) O(1w)−Pb(1)−N(1) N(2)−Pb(1)−N(1) O(3)−Pb(1)−O(1)
2.442(6) 2.263(6) 3.198(6) 2.662(6) 2.268(7) 2.398(6) 2.326(6) 137.5(2) 112.3(2) 91.2(2) 95.3(2) 87.9(2) 138.6(2) 98.0(2)
O(1w)−Pb(1)−O(1) N(2)−Pb(1)−O(1) N(1)−Pb(1)−O(1) O(3)−Pb(1)−O(4) O(1w)−Pb(1)−O(4) N(2)−Pb(1)−O(4) N(1)−Pb(1)−O(4) O(1)−Pb(1)−O(4) O(3)−Pb(1)−O(3) O(1w)−Pb(1)−O(3) N(2)−Pb(1)−O(3) N(1)−Pb(1)−O(3) O(1)−Pb(1)−O(3) O(4)−Pb(1)−O(3)
122.2(2) 77.0(2) 68.8(2) 53.1(2) 84.5(2) 129.4(2) 91.8(2) 144.64(19) 62.6(2) 103.97(19) 61.9(2) 157.1(2) 117.95(17) 70.37(16)
a Symmetry transformations used to generate equivalent atoms: (1) −x + 1, y − 1/2, −z + 1/2; (2) x, −y + 1/2, z + 1/2; (3) −x + 1, y + 1/2, −z + 1/2; (4) x, −y + 1/2, z − 1/2.
Figure 4. (A) Puckered layer of compound 2 as viewed along the c-axis. (B) View along the a-axis showing layered structure of 2.
Figure 5. Representative optical images of soaking study of 1 in an aqueous solution of Co(II) precursor as a function of soaking time (t): (A) t = 5 min, (B) t = 2 h showing the appearance of single crystals of product of 3.
water (Figure S5). The coordination around the lead atom, which forms a slightly distorted monocapped trigonal prism, PbN2O5, is completed by nitrogen and oxygen from the pzdc ligand as well as oxygen from the coordinated water. The Pb− N bond lengths fall into the range of 2.326(6)−2.398(6) Å, and Pb−O fall into the range of 2.263(6)−3.198(6) Å, commonly observed in Pb-based coordination polymers.18,77−81 Relevant crystallographic data for the single crystal structure refinement for 2 are found in Table 1. Table 3 summarizes the selective interatomic distances and angles of 2. The structure is built from the [Pb2(pzdc)4(H2O)2]4− dimer (Figure S6), in which both Pb polyhedra are edge-shared and
Soaking in other solutions did not yield any single crystal product. Soaking 1 in an aqueous solution of Hg(II) nitrate at room temperature resulted in a mixture of polycrystalline powder and ribbon single crystals after a minimum of 2 weeks of soaking. Upon single crystal and powder X-ray diffraction examinations, new compound Pb(pzdc)(H2O) (2) was found, confirming the occurrence of structural transformation from the 1D structure of 1 into the 2D structure of 2. Its PXRD patterns are shown in Figure S4, showing a nearly phase-pure 2. In compound 2, there is only one lead atom site, one pzdc ligand with uncoordinated oxygen, O(2), and one coordinated E
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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and good quality single crystals for single crystal X-ray diffraction. The structural refinement on one of the Pb sites, Pb(1), indicates the presence of Co(II) dopant in 3, giving an overall Pb:Co ratio of 0.89:0.11, confirmed by SEM-EDS elemental mapping (Figure S7). A structure similar to 3 without any dopant, Pb3(pzdc)3(H2O) (3a), was synthesized by soaking 1 into an aqueous solution of L-(+)-tartaric acid. A nearly phase-pure product in the form of polycrystalline powder and colorless plate crystals was obtained after a minimum of 2 weeks of soaking and was characterized by single crystal and powder X-ray diffraction (Figure S8). As 3 and 3a are similar, their structural description is represented by 3. Figure S9A depicts the atom labels of 3, which consist of three pzdc ligands, one coordinated water, and three lead atom sites in which Pb(1) is partially (1/3) occupied by Co(II). The oxygen and the nitrogen atoms of the pzdc ligand, as well as the oxygen of water in the cases of Pb(1) and Co(1), complete the coordination sphere around the cations, except for Pb(3) solely surrounded by oxygen atoms from pzdc ligands. Both Pb(1) and Co(1) have six coordination number, Pb-/Co-N2O4, with one of the axes occupied by the stereochemically active lone pair of the lead atom in the case of Pb(1), giving a more distorted octahedron compared to that of Co(1) (Figure S9B). Such a small distortion in the Co(II) octahedra has also been observed in other cobalt coordination compounds.83−89 A slightly distorted monocapped trigonal prism, PbN2O5, is observed for Pb(2) with a slightly longer Pb−N bond shown as a dashed line. Pb(3) forms a slightly distorted tricapped trigonal prism, PbO9 (Figure S9B). The Pb−N bond lengths fall into the range of 2.570(1)−2.960(1) Å and Pb−O fall into the range of 2.350(1)−2.880(1) Å, typical for lead coordination polymers.18,77−81 Relevant crystallographic data for the single crystal structure refinement for 3 are found in Table 1. Selected interatomic distances and angles of 3 are given in Table S1. The fractional coordinates, the displacement parameters (Ueq), and occupancies of all atoms with estimated standard deviations are given in Table S2 for 3. Each Pb(3) polyhedron is edge-sharing through dioxo bridging to form a chain with a zigzag attachment of Pb(2)
are related by inversion symmetry. The dimers are further twodimensionally interconnected by a pzdc ligand to form puckered layers (Figure 4A,B), featuring uncoordinated O(2) of the pzdc ligand. From 1D to 3D Coordination Polymer. Compound 1 showed to lose some crystallinity within 5 min of soaking in the Co(II) precursor (Figure 5A). New, plate crystals started to grow on the surface of 1 within 30 min, and the growth appeared to cease within 2 h of soaking (Figure 5B). Upon single crystal X-ray diffraction examination, it was confirmed that 1 underwent structural transformation from a 1D structure into a 3D structure, forming a new compound, Pb2.67Co0.33(pzdc)3(H2O) (3). From in situ visual observation, a dissolution−recrystallization appeared to be the mechanism of the structural transformation as reported by others.54,58,59,63,64,75,81 The PXRD pattern of the soaked sample confirmed that the yield was quantitative and that there was no additional polymorph observed (Figure 6). Further, similar
Figure 6. Powder X-ray diffraction patterns of simulated (A) and observed patterns of 3 using cobalt(II) nitrate (B) and cobalt(II) chloride (C) precursors.
PXRD patterns were observed for both precursors (hydrated Co(NO3)2 and CoCl2) used, which strongly indicate the formation of 3 regardless of the types of precursor used for soaking experiments (Figure 6). Remarkably, the process of structural transformation occurred just within 2 h of soaking to yield relatively large
Figure 7. (A) Chain cluster that builds compound 3. (B) 3D coordination polymer of compound 3 (view along the b-axis). F
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
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Compound 1 showed a loss of crystallinity within 5 min of soaking. As 1 started to slowly dissolve, some bluish green platelike crystals formed freely in solution and on 1 (Figure 8), in which, upon careful single crystal and powder X-ray diffraction examinations (Figures S11 and S12), it was confirmed that both crystals are of similar structure to that of known, easily decomposed, [Cu(pzdc)(H2O)2]x·2xH2O.90 The cation exchange was further confirmed by SEM-EDS elemental mapping on the bulk sample (Figure S13). From the visual observations, we confirm that the cation exchange process adopts a dissolution−exchange−crystallization mechanism similar to reports by Hou and co-workers.58,59,63,64,81 What is remarkable in our work is that the cation exchange process occurs rapidly and single crystals of product of a large enough size with X-ray diffraction quality are obtained within 2 h of soaking. From 1D to 2D, to 3D, to Cation Exchange. Soaking 1 in aqueous solutions of several divalent cations or anion resulted in structural transformations from 1D to 2D, to 3D structures, including cation exchange. The structural change is accompanied by eliminating dichloride bridging of the Pb dimers (Figure 9); as a result, some changes in the pzdc ligand’s coordination modes are observed. In 1, Pb(II) ions are N,Ochelated by the pzdc ligand and the other oxygen atoms of the dicarboxylates are monoprotonated (Figure S14). Besides similar N,O-chelation to Pb(II), 2 also has O,O-chelation, oxygen bridging, as well as uncoordinated oxygen atoms (Figure S14). In 3, a variety of coordination modes are observed for the pzdc ligands, i.e., monodentate, N,O-chelation (regular and weakly coordinate), O,O-chelation, and oxygen bridging, as well as uncoordinated nitrogen atoms (Figure S15). The dioxo bridging exists in a chain of edge-shared Pb(3) polyhedra as well as between Pb(3) and Pb(2) polyhedra (Figure 9). There is no, however, dioxo bridging between Pb(3) and Pb(1) polyhedra (Figure 9), causing the latter to possess only six coordination number, which, in turn, makes Pb(1) available for Co(II) doping, as observed in 3. Further, the almost spontaneous formation of 3 (2 h of soaking) in comparison with 3a (minimum of 2 weeks of soaking) may indicate that the presence of Co(II) dopant greatly favors the formation and crystallization of 3.
and Pb(1) polyhedra, showing 21 symmetry along the chain, through edge- (dioxo bridging) and corner-sharing, respectively (Figure S10). The overall 3D structure of 3 is built from these chain clusters interconnected in three dimensions by pzdc ligands (Figure 7). Rapid Cation Exchange. To investigate the cation exchange property, we soaked 1 in several aqueous or ethanol or water/ethanol solutions of different Cu(II) precursors at room temperature. We found that soaking 1 in a simple aqueous solution of hydrated CuCl2 or Cu(NO3)2 exhibited the best cation exchange process, yielding single crystals of product, as evidenced by optical microscope images taken as a function of soaking time (Figure 8). Other soaking conditions did not yield any single crystals of cation-exchanged product.
Figure 8. Representative optical images of cation exchange study of 1 in an aqueous solution of Cu(NO3)2·3H2O as a function of soaking time (t): (A) loss of crystallinity in 1 after t = 5 min, (B) appearance of single crystals of product within t = 2 h, (C) zoom-in image of (B) within the red circle.
Figure 9. Structural transformations of 1 into 2 and 3, including cation exchange, which involve replacement of dichloride bridging, by simple soaking into an aqueous solution of the respective divalent cation at room temperature. G
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design Comparing the structure of 1 to that of the cation-exchanged, already reported product, [Cu(pzdc)(H2O)2]x·2xH2O, we can see a similar coordination mode of the pzdc ligand to the metal centers, creating a 1D coordination polymer in both structures (Figure 9). The main difference is that the dichlorine bridging in 1 is replaced by another coordinated water to complete six coordination number of Cu(II) octahedra, CuN2O4, observed in the cation-exchanged product (Figure 9). Such similarity in the pzdc coordinations maybe responsible for the almost spontaneous cation exchange reaction observed. As pH of all of the aqueous solutions of 1 soaked in different divalent cations or anion are very close in proximity (in the range of 2−3), and yet structural changes are still observed from 1D to 2D, to 3D (with or without dopant), including cation exchange, this may infer that the presence of each divalent cation or anion during soaking could possibly act as templating agent to form its respective structure: Hg(II) to form 2, Co(II) to form 3, L-(+)-tartaric acid to form 3a, and Cu(II) to undergo cation exchange. Soaking 1 into aqueous solutions of other divalent cations, such as Ni, Zn, and Cd, as well as cations of other charges, to further investigate and/or confirm the templating function of the cations used, including the investigations on their potential applications, are underway in our laboratory and will be reported in subsequent communication.
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REFERENCES
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CONCLUSION In summary, we have successfully synthesized a novel lead 1D coordination polymer based on the H2pzdc ligand using a simple dissolution−slow evaporation method. We confirm that the phase of 1 was already formed without the presence of any auxiliary N-donor ligand. Adding piperazine, however, produces the largest single crystals of 1 with quantitative yield. Compound 1 contains Pb2Cl2 dimers connected by Hpzdc− ligands into a 1D coordination polymer. Soaking 1 into aqueous solutions of divalent cations or anion at room temperature resulted in various structural transformations from 1D to 2D, to 3D (with or without dopant), as well as a cation exchange product; some occurred almost spontaneously (only within 2 h of soaking), producing good quality single crystals. Although the exact mechanism of these structural transformations in solutions is presently unclear, the presented results certainly enrich the structural dynamics of Pb-based MOCPs. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01392.
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ACKNOWLEDGMENTS
Financial support for this research was provided by HIR Chancellory Grant UM.C/625/1/HIR/211 from the University of Malaya. A.C.W thanks Profs. E. R. T. Tiekink and Hapipah M. Ali for their generous initial support.
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Article
Further details are given in Tables S1 and S2 and Figures S1−S15 (PDF) Crystallographic information for 1 (CIF) Crystallographic information for 2 (CIF) Crystallographic information for 3 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. H
DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
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DOI: 10.1021/acs.cgd.5b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX
