pH-Specific Crystalline Binary and Ternary MetalâOrganic Framework

Article pubs.acs.org/crystal

pH-Specific Crystalline Binary and Ternary Metal−Organic Framework Materials of Pb(II) with (Di)Tricarboxylate Ligands and N,N′‑Aromatic Chelators. Structure, Architecture-Lattice Dimensionality, and Electronic Spectroscopic Property Correlations Catherine Gabriel,† Paraskevi Karakosta,† Angelos A. Vangelis,† Catherine P. Raptopoulou,‡ Aris Terzis,‡ Vassilis Psycharis,‡ Marko Bertmer,§ Constantin Mateescu,∥ and Athanasios Salifoglou*,† †

Department of Chemical Engineering, Laboratory of Inorganic Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece ‡ Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, Department of Materials Science, NCSR “Demokritos”, Aghia Paraskevi 15310, Attiki, Greece § Faculty of Physics and Earth Sciences, Institute of Experimental Physics II, Leipzig University, Leipzig 04103, Germany ∥ Banat’s University of Agricultural Sciences and Veterinary Medicine from Timisoara, Timisoara 300645, Romania S Supporting Information *

ABSTRACT: Efforts to probe and delineate intricate structure−property relationships key to the development of crystalline Pb(II)-containing metal−organic framework materials led to the design and pH-specific hydrothermal synthetic investigation of binary/ternary Pb(II)-(di)tricarboxylate ligand (succinic, glutaric, tricarballylic acids) systems in the presence of variable-nature aromatic N,N′-chelators bipyridine (bpy)/phenanthroline (phen). The arisen crystalline materials [Pb(phen)(suc)]n (1), [Pb 3(phen)3(glu)3]n·7nH2O (2), [Pb3(tca)2]n (3), and [Pb2(phen)2(tcaH)2]n·nH2O (4) provide evidence for structural correlations linking the nature of ligands with Pb(II) chemistry and the emerging crystallinepolymeric assemblies. Detailed physicochemical characterization (Fourier transform infrared spectroscopy, 13C-,207Pb-cross polarization magic angle spinning NMR, thermogravimetric analysis, luminescence) reveals distinct architecture, lattice dimensionality (2D-3D), and luminescence property correlations and identifies structural and electronic factors interweaving into the design of functional materials.

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INTRODUCTION Metal−organic framework (MOFs) materials have drawn considerable attention over the past few years due to their unique solid state properties, which profoundly influence their structural architecture and potential applications.1−7 In this regard, assembling variable nuclearity MOFs with unique structures and reactivity patterns relies heavily on the nature of the metal ionic center and the adjoining ligand, which upon reaction give rise to the desired materials. Through such carefully designed strategies, a number of MOFs have been synthesized, bearing properties that enable a diverse array of applications, such as the extraction of harmful metals from the environment.8−11 Key in all such cases is the identification and subsequent enlistment of physicochemical factors linked to the basic components (metal and ligand) of an MOF into a synthetic process allowing for structural architecture as well as lattice dimensionality control. Ostensibly, metal−ligand coordination chemistry in such processes emerges as a principal synthetic tool of variable yet distinct functionality MOFs and © XXXX American Chemical Society

one that has been applied to the specific family of materials encompassing lead (Pb(II)), with concomitant use in heterogeneous catalytic and superconducting systems.12−17 That chemistry is often influenced by the presence of the lone pair of electrons on Pb(II), thereby distinguishing its coordination reactivity from that of conventional transition metals.18 Divalent lead compounds sometimes exhibit an ‘‘inert-pair effect” with low as well as high coordination numbers.19 Concurrently, stereochemically active lone-pair Pb(II) compounds are of great interest owing to the manifestation of luminescence;20−22 it is known that this property is also influenced by the nature of the organic anion bound to Pb(II).23,24 However, luminescence in Pb(II) coordination compounds is rarely reported.25,26 There is, therefore, a welldefined inter-relationship of distinct structural and electronic Received: November 5, 2014 Revised: February 13, 2015

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DOI: 10.1021/cg501628d Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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factors bestowed upon Pb(II)-containing metal organic coordination materials that projects the light-emitting properties observed in such materials.27−32 Bearing in mind that (poly)carboxylic acid ligands present a chemically active family of metal ion binders and chelators, leading to a diverse spectrum of coordination polymeric materials in MOFs, employment of such organic substrates was considered a potent contributor to the aforementioned structure-light emitting properties in new materials containing Pb(II). In such an extensive family of organic acids, those bearing α,ω-dicarboxylic acid groups emerge as good candidates in the design of novel metal−organic polymers. The structural features of such acids, their compositional and conformational state, and the projected flexibility in chemical reactivity are consistent with (a) their involvement in thus far studied abiotic as well as biological systems, and (b) the influence they exert in processes and materials arising thereof, in which they participate. Driven by the need to probe in depth the influence of such organic substrates in the development of crystalline Pb(II)-containing materials of unique structural architecture, lattice dimensionality, and spectroscopic signature, a systematic chemical reactivity study of organic acids with Pb(II) was undertaken in our laboratories.33−35 In line with past work in the field by our lab, succinic and glutaric acids were selected as candidates for structurally defined lightemitting materials. Their selection was based on their (a) nature as dicarboxylic acids, (b) coordination binding ability and flexibility upon increasing the length of the spacer methylene chain between the terminal carboxylic groups by one carbon, −OOC(CH2)xCOO−, designated as “sucH2” (x = 2), and “gluH2” (x = 3), respectively, and (c) higher flexibility compared to that in the simplest organic dicarboxylic acid of the specific family, i.e., oxalic acid (x = 0). In a further attempt to increase the number of carboxylic acid terminals in the organic substrate reacting with Pb(II), thereby probing the effects expected to arise as a result of Pb(II) binding, 1,3,5tricarboxylic acid, designated as “tcaH3”, was employed in the investigation of the aforementioned type of chemical reactivity. Tricarballylic acid is structurally congener to its predecessor dicarboxylic glutaric acid, with the central carbon hydrogen replaced by a carboxylic acid moiety. Advancing, however, the investigation into the design of more complex MOFs in the above systems necessitates the use of N-containing aromatic chelators capable of binding Pb(II) centers concurrently with the mentioned carboxylic acids. Such potent chelators include 1,10-phenanthroline (phen) and 2,2′bipyridine (2,2′-bpy), bearing a bulky, rigid (in the case of phen and more flexible in the case of 2,2′-bpy) and hydrophobic structure, potentially capable of influencing ternary crystal architecture and luminescent spectroscopic properties (Scheme 1). Consistent with the ground principles linked to the development of distinctly structured crystalline binary and ternary MOF materials in Pb(II)-organic acid-(aromatic chelator) systems, the systematic synthetic study of the ternary systems of Pb(II)-sucH2-phen and Pb(II)-gluH2-phen was undertaken along with that of Pb(II)-tcaH3-(2,2′-bpy/phen), employing the aliphatic tricarboxylic tricarballylic acid. Complete characterization of the newly isolated materials (a) reveals the contribution of the ligand heteroatom in the formulation of the structural architecture, lattice dimensionality, and physicochemical properties of the isolated MOFs, (b) establishes lattice-dimensionality relationships emerging

Scheme 1. Di- and Tricarboxylic Acids along with Aromatic N,N′-Chelators Employed in This Work

through the chemical reactivity of the systems examined, ultimately affording well-configured crystalline hybrid polymeric MOFs bearing characteristic spectroscopic signatures, and (c) reveals the pronounced luminescence properties of the Pb(II)-containing binary and ternary MOFs, collectively meriting the design and engineering of new materials of specified structure−property functions.

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EXPERIMENTAL SECTION

Materials and Methods. All experiments were performed in the air. The reagents used were obtained from commercial sources (Fluka and Aldrich) and used as received. Elemental analysis for the determination of carbon, hydrogen, and nitrogen (%) was performed on a Thermo-Finnigan Flash EA 1112 CHNS elemental analyzer. Fourier transform-infrared (FT-IR) spectra were recorded on a Thermo, Nicolet IR 200 FT-infrared spectrometer. Thermogravimetric (TGA) experiments were run on a TA Instruments thermal analyzer, model Q600, system. Luminescence measurements in the solid state were carried out on a Hitachi F7000 spectrophotometer. Solid state 13 C cross polarization magic angle spinning (CPMAS) NMR spectra were recorded on a Varian 400 MHz spectrometer operating at 100.53 MHz for 1 and 2, using a double resonance HX probe with a 3.2 mm rotor.36 For 3 and 4, a Bruker Avance 750 spectrometer was used. The 207 Pb NMR spectra for 1−4 were obtained either on a Bruker Avance 400 or an Avance 750 spectrometer, at frequencies of 83.67 and 156.71 MHz, respectively.37−39 Detailed information about the physical measurements is given in the Supporting Information. Synthesis. [Pb(phen)(suc)]n (1). A mixture of Pb(CH3COO)2· 3H2O (0.20 g, 0.54 mmol), sucH2 (0.13 g, 1.08 mmol), and phen (0.10 g, 0.54 mmol) was placed in a flask in the sequence specified and dissolved in 10 mL of H2O. The final pH value was ∼5. The arising reaction mixture was allowed to stir at room temperature for 1/2 h. Then, it was placed in a Teflon-lined stainless steel reactor (23 mL) and heated to 160 °C for 3-1/2 days. Upon cooling of the reactor to room temperature, crystals appeared at the bottom. They were isolated by filtration, water-washed, and dried in the air. Yield 0.11 g (∼41%). Anal. calcd for 1, [Pb(phen)(suc)]n (1) (C16H12O4N2Pb, Mr 503.47): %C 38.13, H 2.38, N 5.56. Found: %C 37.97, H 2.28, N 5.42. IR (KBr, cm−1) for 1: vas(COO−): 1564−1537 cm−1, vs(COO−): 1384 cm−1. [Pb3(phen)3(glu)3]n·7nH2O (2). A mixture of Pb(CH3COO)· 0.3H2O (0.37 g, 1.0 mmol), gluH2 (0.26 g 2.0 mmol), and phen (0.20 g, 1.0 mmol) was placed in a flask in the sequence specified and dissolved in 10 mL of H2O. The final pH value was ∼5. The arising reaction mixture was allowed to stir at room temperature for 1/2 h and then placed in a Teflon-lined stainless steel reactor (23 mL) and heated to 160 °C for 3-1/2 days. The reactor was subsequently allowed to cool down to room temperature. The recovered solution was allowed to slowly evaporate and a few days later colorless crystals appeared. Yield 0.15 g (27%). Anal. Calcd for 2, [Pb3(phen)3(glu)3]n· 7nH2O (2) (C51H56N6O19Pb3, Mr 1678.60): % C 36.45, H 3.36, N 5.00. Found: % C 36.99, H 3.26, N 4.92. IR (KBr, cm−1) for 2: vas(COO−): 1550 cm−1, vs(COO−): 1424−1397 cm−1. B



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Table 1. Summary of Crystal, Intensity Collection, and Refinement Data for [Pb(phen)(suc)]n (1), [Pb3(phen)3(glu)3]n·7nH2O (2), [Pb3(tca)2]n (3), and [Pb2(phen)2(tcaH)2]n·nH2O (4) formula formula mass T, K wavelength, λ (Å) space group a (Å) b (Å) c (Å) α, deg β, deg γ, deg V, (Å3) Z Dcalcd (Mg m−3) abs coeff (μ), mm−1 observed reflections I > 2σ(I) R indicesa a

1

2

3

4

C16H12N2O4Pb 503.47 293 Mo Kα 0.71073 C2/c 9.955(10) 21.030(4) 7.925(9) 90.00 119.47(2) 90.00 1444 (2) 4 2.315 11.703 1270 R = 0.0286 Rw = 0.0719

C51H56N6O19Pb3 1678.60 180 Mo Kα 0.71073 P21 10.3030(2) 25.2045(4) 10.3907(1) 90.00 92.932(1) 90.00 2694.75(7) 2 2.058 9.430 9158 R = 0.0329 Rw = 0.0808

C12H10O12Pb3 967.77 160 Cu Kα 1.54178 C2/c 19.7130(2) 8.1510(1) 9.7928(1) 90.00 92.525(1) 90.00 1571.99(3) 4 4.089 62.121 1121 R = 0.0526 Rw = 0.1231

C36H30N4O13Pb2 1141.02 160 Cu Kα 1.54178 P1̅ 7.9254(1) 10.7452(1) 11.3467(2) 115.145(1) 98.272(1) 92.346(1) 859.98(2) 1 2.203 19.477 2602 R = 0.0418 Rw = 0.1000

R = {(∑∥Fo| − |Fc∥)/(∑(|Fo|))}, Rw = {(∑[w(F2o − F2c )2])/(∑[w(F2o)2])}1/2.

[Pb3(tca)2]n (3). A mixture of Pb(CH3COO)2·3H2O (0.37 g, 1.0 mmol), tcaH3 (0.34 g, 2.0 mmol), and 2,2′-bpy (0.16 g, 1.0 mmol) was placed in a flask in the sequence specified and dissolved in 10 mL of H2O. The pH of the reaction mixture was adjusted to ∼5 with aqueous NaOH. The arising solution was allowed to stir at room temperature for 1/2 h. Then, it was placed in a Teflon-lined stainless steel reactor (23 mL) and heated to 160 °C for 3-1/2 days. The reactor was subsequently allowed to cool down to room temperature. The recovered solution was allowed to slowly evaporate, and a few days later colorless crystals appeared. Yield 0.14 g (45%). Anal. calcd. for 3, [Pb3(tca)2]n (3) (C12H10O12Pb3, Mr 967.77): %C 14.89, H 1.04. Found: %C 14.62, H 0.99. IR (KBr, cm−1) for 3: vas(COO−): 1519 cm−1, vs(COO−): 1379 cm−1. [Pb2(phen)2(tcaH)2]n·nH2O (4). A mixture of Pb(CH3COO)2·3H2O (0.37 g, 1.0 mmol), tcaH3 (0.34 g, 2.0 mmol), and phen (0.20 g, 1.0 mmol) was placed in a flask in the sequence specified and dissolved in 10 mL of H2O. The final pH value was ∼5. The reaction mixture was stirred for 1/2 h. Then, it was placed in a Teflon-lined stainless steel reactor (23 mL) and heated to 160 °C for 3-1/2 days. Subsequently, the reactor was allowed to cool down to room temperature. The recovered solution was allowed to slowly evaporate and a few days later colorless crystals emerged. Yield 0.17 g (30%). Anal. calcd for 4, [Pb2(phen)2(tcaH)2]n·nH2O (4) (C36H30N4O13Pb2, Mr 1141.02): % C 37.86, H 2.62, N 4.90. Found: % C 37.47, H 2.51, N 5.00. IR (KBr, cm−1) for 4: vas(COO−): 1559−1517 cm−1, vs(COO−): 1405 cm−1. X-ray Crystal Structure Determination. X-ray quality crystals of compounds 1−4 with dimensions 0.25 × 0.30 × 0.55 mm (1), 0.18 × 0.30 × 0.71 mm (2), 0.08 × 0.09 × 0.11 mm (3), and 0.17 × 0.41 × 0.50 mm (4) were mounted on glass fibers. Data collection for 1 took place at room temperature. The crystals of 2−4 were picked from the mother liquor and cooled to −93 °C (2) and −113 °C (3, 4) in a nitrogen-cold stream on the diffractometer goniometer. Diffraction measurements for 1 were made on a Crystal Logic Dual Goniometer diffractometer (upgraded by Crystal Logic) using graphite monochromated Mo Kα radiation. Diffraction measurements for 2−4 were made on a Rigaku R-AXIS SPIDER Image Plate diffractometer using graphite monochromated Mo Kα (2) or Cu Kα (3, 4) radiation.40 Crystallographic details are provided in Table 1. The structures of 1−4 were solved as previously reported (Supporting Information).41−43

In the case of 1, Pb(CH3COO)2, succinic acid, and phen (experimental molar ratio 1:2:1) reacted in water at pH ≈ 5 (Reaction 1).

Through an analogous reaction, Pb(CH3COO)2, glutaric acid, and phen (experimental molar ratio 1:2:1) reacted in water at pH ≈ 5, thereby affording compound 2 (Reaction 2):

A ternary system reaction of Pb(CH3COO)2 with tcaH3, 2,2′bpy (experimental molar ratio 1:2:1) and sodium hydroxide in water at pH ≈ 5 led to the isolation of 3. The aromatic chelator was absent from the lattice in 3 (vide infra) (Reaction 3):

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RESULTS AND DISCUSSION Synthesis. The synthetic strategy for compounds 1−4 involved hydrothermal synthesis and subsequent crystallization. C



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Table 2. Bond Lengths [Å] and Angles [deg] for [Pb(phen)(suc)]n (1)a Distances (Å) Pb−N(1) Pb−N(1′) Pb−O(1) Pb−O(1′) N(1′)−Pb−N(1) N(1)−Pb−O(1) N(1′)−Pb−O(1) N(1)−Pb−O(1′) N(1′)−Pb−O(1′) N(1)−Pb−O(2‴) N(1)−Pb−O(2″) N(1′)−Pb−O(2″) O(1″)−Pb−O(1) O(1″)−Pb−O(1′) O(1″)−Pb−O(1‴) O(1‴)−Pb−O(1) O(1‴)−Pb−O(1′) O(1‴)−Pb−N(1′)

2.535(4) Pb−O(2″) 2.535(4) Pb−O(2‴) 2.670(5) Pb−O(1″) 2.670(5) Pb−O(1‴) Angles (°) 65.5(2) 78.6(2) 140.6(2) 140.6(2) 78.6(2) 74.4(1) 76.2(1) 74.4 (1) 91.1(2) 74.2(2) 136.5(2) 74.2(2) 91.1(2) 122.6(2)

N(1′)−Pb−O(2‴) O(1)−Pb−O(1′) O(2″)−Pb−O(2‴) O(2″)−Pb−O(1′) O(1′)−Pb−O(2‴) O(1)−Pb−O(2″) O(1)−Pb−O(2‴) O(1″)−Pb−O(2″) O(1″)−Pb−O(2‴) O(1″)−Pb−N(1) O(1″)−Pb−N(1′) O(1‴)−Pb−O(2″) O(1‴)−Pb−O(2‴) O(1‴)−Pb−N(1)

2.646(5) 2.646(5) 2.882(7) 2.882(7) 76.2(1) 140.1(2) 144.8(2) 110.2(2) 82.0(2) 82.0(2) 110.2(2) 46.4(2) 155.9(2) 122.6(1) 94.8(2) 155.9(2) 46.4(2) 94.8(1)

a Symmetry operations: (′) −x, y, 0.5 − z; (″) x, −y, 0.5 + z; (‴) −x, −y, −z.

In an analogous reaction, Pb(CH3COO)2 reacted with tcaH3 and phen in water (experimental molar ratio 1:2:1) at pH ≈ 5, ultimately leading to the isolation of 4 (Reaction 4):

The derived binary and ternary PbII-suc, PbII-glu, and PbII-tcaH materials were recovered in crystalline form. Elemental analysis suggested the formulation [Pb(phen)(suc)] (1), [Pb3(phen)3(glu)3]·7H2O (2), [Pb3(tca)2] (3), and [Pb2(phen)2(tcaH)2]·H2O (4), respectively. FT-IR spectroscopy and X-ray crystallography confirmed the molecular formulation of the four compounds. Extensive chemical reactivity of the ternary systems of Pb(II) with suc, glu, and tca/tcaH, under variable synthetic conditions, in anticipation of new materials, is still under investigation in our lab. The new materials 1−4 are very stable in the crystalline form in the air. They are insoluble in water or any other solvent. Description of Crystallographic Structures. Compound 1 crystallizes in the monoclinic space group C2/c. The solid state structure of 1 reveals a 2D coordination polymer parallel to the (010) plane. The mononuclear repeating unit is comprised of one Pb(II) center, one suc(−2) ligand, and one phen ligand (Figure 1A). Bond distances and angles for 1 are given in Table 2. The Pb(II) center resides on a 2-fold axis of symmetry passing through the metal ion and the middle of the N(1)··· N(1′) (′: −x, y, 0.5 − z) interatomic vector of the phen ligand. The suc(−2) ligand exhibits conformational disorder, and the two methylene atoms of the −CH2−CH2− group occupy two positions, i.e., C(8)−C(9) and C(8*)−C(9*), each with half

Figure 1. (A) Partially labeled plot of the Pb coordination sphere in 1. Symmetry operations: (′) −x, y, 0.5 − z; (″) x, −y, 0.5 + z; (‴) −x, −y, −z, (*) −1 − x, y, −0.5 − z. (B) Part of the 2D network structure of 1. (C) 3D supramolecular structure of 1 due to π−π interactions (yellow lines) between the phen molecules (see text for details). Color code: Pb, purple; O, red; N, blue; C, white.

occupancy exemplified through a 2-fold axis symmetry (*: −1 − x, y, 0.5 − z) in Figure 1A. An additional 2-fold axis passing through the center of the C(8)−C(9*) (*: −1 − x, y, 0.5 − z) bond of the succinato ligand relates the two configurational disordered sites of the ligand (Figure 1A). The Pb(II) ion is bound to six carboxylato oxygen atoms from four different suc(−2) ligands and two nitrogen atoms from one phen molecule (Figure 1A). As a result, the coordination number of Pb(II) is eight. One oxygen atom of the two carboxylato groups at both ends (O1 atoms in Figure 1B) of suc(−2) ligands bridges two neighboring Pb(II) ions, resulting in the formation of polymeric inorganic chains parallel to the c axis, based on edge sharing PbO8 polyhedra, with the closest Pb···Pb‴ D



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Scheme 2. Coordination Modes of the suc(−2), glu(−2) and tca(−3), tca(−2) ligands in 1−4

(−x, −y, −z) distance being equal to 4.430(1) Å. In addition, both oxygen atoms at the two carboxylato ends of the suc(−2) ligand are coordinated in a chelating fashion to two opposite lying Pb(II) ions along the a-axis (O1, O2, and O1*, O2* in Figure 1B). Thus, each carboxylato group exhibits a μ2-κ2O:κO′ coordination mode, and consequently each succinato ligand presents an overall μ4-κ2O:κ2O′:κO″:κO‴ coordination mode employing its carboxylato units to bridge four symmetry related Pb(II) ions (Scheme 2, Figure 1B), two of them residing on the same inorganic chain along the c axis and the other two on opposite chains along the a axis. The polyhedral “inorganic” chains are linked through the succinato ligands in the direction of the a-axis, thereby forming 2D networks extending parallel to the ac plane. The observed Pb···Pb⁗ (1 + x, y, z) interatomic distances along the a axis are 9.955(1) Å, i.e., the length of the a-axis. Within the 2D network, the neighboring μ4-κ2O:κ2O′:κO″:κO‴ succinato ligands promote the formation of 14-membered macrometallocycles, (Pb−O−C−C−C−C−O)2. The aromatic rings from phen ligands, which hang above and below the layers described above and belong to neighboring layers, interact through weak π−π interactions, thereby contributing to the formation of a 3D network (Figure 1C). The phen mean planes in different 2D layers are parallel to each other with an intercentroid distance of 3.548 Å. Compound 2 crystallizes in the monoclinic space group P21, revealing a lattice of a 1D coordination polymer, with the

solvent water molecules being hydrogen bonded to the polymeric units. Bond distances and angles for 2 are provided in Table 3. The repeating unit consists of three Pb(II) metal ions, three phen, and three glutarato glu(−2) ligands. Pb(1) and Pb(2) exhibit a coordination number eight, arisen by six carboxylato oxygen atoms from four different glu(−2) ligands and two nitrogen atoms from one phen ligand (Figure 2A). Pb(3) exhibits a coordination number seven, formulated by five carboxylato oxygen atoms from three different glu(−2) ligands and two nitrogen atoms from one phen molecule (Figure 2A). The three Pb metal ions form trinuclear units with the phen molecules at terminal positions and the glu(−2) ligands at bridging positions through carboxylato oxygen atoms. The polyhedron around Pb(1) shares the O(2)···O(12) and O(1)··· O(24′) edges with the polyhedra surrounding the Pb(2) and Pb(3) ions, respectively. The trinuclear assembly is completed through the corner shared (position of atom O(22)) by the polyhedra around Pb(2) and Pb(3) ions. The Pb(1) and Pb(2) ions are linked through O(2) and O(12) atoms at an interatomic distance of 4.169 Å and form an almost planar [Pb2O2] unit with a torsion angle of ∼9°. The Pb(1) and Pb(3) centers are linked through O(1) and O(24′), forming a planar [Pb2O2] unit with a torsion angle of 1.5° and an interatomic distance of 4.037 Å between the metal ions. Pb(2) and Pb(3) are linked only through O(22) and exhibit a larger interatomic distance, 5.437 Å. The glutarato ligands, in addition to their E



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Table 3. Bond Lengths [Å] and Angles [deg] for [Pb3(phen)3(glu)3]n·7nH2O (2)a Distances (Å) Pb(1)−O(4′) Pb(1)−N(2) Pb(1)−O(3′) Pb(1)−O(1) Pb(1)−N(1) Pb(1)−O(2) Pb(1)−O(24′) Pb(1)−O(12) N(1)−Pb(1)−N(2) N(1)−Pb(1)−O(1) N(1)−Pb(1)−O(2) N(1)−Pb(1)−O(12) N(1)−Pb(1)−O(3′) N(1)−Pb(1)−O(4′) N(1)−Pb(1)−O(24′) N(2)−Pb(1)−O(1) N(2)−Pb(1)−O(2) N(2)−Pb(1)−O(12) N(2)−Pb(1)−O(3′) N(2)−Pb(1)−O(4′) N(2)−Pb(1)−O(24′) O(1)−Pb(1)−O(2) O(1)−Pb(1)−O(12) O(1)−Pb(1)−O(3′) O(1)−Pb(1)−O(4′) O(1)−Pb(1)−O(24′) O(2)−Pb(1)−O(12) O(2)−Pb(1)−O(3′) O(2)−Pb(1)−O(4′) O(2)−Pb(1)−O(24′) O(12)−Pb(1)−O(3′) O(12)−Pb(1)−O(4′) O(12)−Pb(1)−O(24′) O(3′)−Pb(1)−O(4′) O(3′)−Pb(1)−O(24′) O(4′)−Pb(1)−O(24′) a

2.515(6) 2.542(7) 2.585(6) 2.592(6) 2.611(7) 2.788(6) 2.918(6) 3.006(6) 64.4(2) 131.9(2) 93.8(2) 72.8(2) 123.3(2) 78.8(2) 136.5(2) 78.8(2) 80.7(2) 133.6(2) 79.0(2) 77.4(2) 152.3(2) 48.6(2) 121.7(2) 74.5(2) 123.4(2) 89.6(2) 88.1(1) 122.2(2) 157.9(2) 110.3(2) 143.3(2) 111.8(2) 73.8(2) 50.9(2) 73.7(2) 88.7(2)

Pb(2)−N(12) Pb(2)−O(11) Pb(2)−O(13″) Pb(2)−O(12) Pb(2)−O(14″) Pb(2)−N(11) Pb(2)−O(2) Pb(2)−O(22) Angles (°) N(11)−Pb(2)−N(12) N(11)−Pb(2)−O(2) N(11)−Pb(2)−O(11) N(11)−Pb(2)−O(12) N(11)−Pb(2)−O(22) N(11)−Pb(2)−O(13″) N(11)−Pb(2)−O(14″) N(12)−Pb(2)−O(2) N(12)−Pb(2)−O(11) N(12)−Pb(2)−O(12) N(12)−Pb(2)−O(22) N(12)−Pb(2)−O(13″) N(12)−Pb(2)−O(14″) O(2)−Pb(2)−O(11) O(2)−Pb(2)−O(12) O(2)−Pb(2)−O(22) O(2)−Pb(2)−O(13″) O(2)−Pb(2)−O(14″) O(11)−Pb(2)−O(12) O(11)−Pb(2)−O(22) O(11)−Pb(2)−O(13″) O(11)−Pb(2)−O(14″) O(12)−Pb(2)−O(22) O(12)−Pb(2)−O(13″) O(12)−Pb(2)−O(14″) O(22)−Pb(2)−O(13″) O(22)−Pb(2)−O(14″) O(13″)−Pb(2)−O(14″)

2.538(6) 2.547(6) 2.559(6) 2.608(5) 2.614(6) 2.669(7) 3.063(6) 3.215(5) 63.4(2) 72.3(2) 126.5(2) 85.5(2) 136.5(2) 125.0(2) 83.7(2) 135.0(2) 79.4(2) 81.7(2) 155.3(2) 77.1(2) 77.6(2) 124.4(2) 87.1(2) 64.7(1) 140.2(2) 106.4(2) 50.5(2) 76.2(1) 77.1(2) 125.7(2) 85.5(2) 126.2(2) 159.2(2) 94.0(2) 114.4(2) 50.1(2)

Pb(3)−O(24′) Pb(3)−N(21) Pb(3)−O(22) Pb(3)−N(22) Pb(3)−O(23′) Pb(3)−O(21) Pb(3)−O(1)

N(21)−Pb(3)−N(22) N(21)−Pb(3)−O(21) N(21)−Pb(3)−O(22) N(21)−Pb(3)−O(1) N(21)−Pb(3)−O(23′) N(21)−Pb(3)−O(24′) N(22)−Pb(3)−O(21) N(22)−Pb(3)−O(22) N(22)−Pb(3)−O(1) N(22)−Pb(3)−O(23′) N(22)−Pb(3)−O(24′) O(21)−Pb(3)−O(22) O(21)−Pb(3)−O(1) O(21)−Pb(3)−O(23′) O(21)−Pb(3)−O(24′) O(22)−Pb(3)−O(1) O(22)−Pb(3)−O(23′) O(22)−Pb(3)−O(24′) O(1)−Pb(3)−O(23′) O(1)−Pb(3)−O(24′) O(23′)−Pb(3)−O(24′)

2.496(6) 2.500(7) 2.512(6) 2.592(8) 2.611(6) 2.648(6) 3.166(5)

65.0(2) 78.1(2) 76.3(2) 147.5(2) 72.9(2) 130.8(2) 80.3(2) 123.1(2) 141.4(2) 81.7(2) 129.1(2) 51.1(2) 87.7(1) 150.3(2) 130.8(2) 72.0(1) 125.3(2) 80.6(2) 120.6(1) 85.9(1) 51.2(2)

Symmetry operations: (′) x, y, 1 + z; (″) x, y, −1 + z.

turquoise, orange, and magenta; the π−π interactions between the phen molecules are shown as olive green lines. The phen molecules coordinated to Pb(3) interact through π−π interactions with the phen molecules coordinated to Pb(1) forming an angle of 4.9° with an intercentroid distance of 4.67 Å. Similarly, the phen molecules coordinated to Pb(3) interact through π−π interactions with the phen molecules coordinated to Pb(2) forming an angle of 3.3° with an intercentroid distance of 4.92 Å. The lattice water molecules O3W are hydrogenbonded to O4 and O14 atoms (Table 1S) of the glutarato ligands, thereby forming columns. The so-formed lattice contains channels along the c-axis filled by the rest of the lattice water molecules, which participate in an extensive network of hydrogen bonding interactions. The latter promote the formation of chains (Table 1S, Figure 2C) between (a) the lattice and (b) the water and the carboxylato oxygen atoms of glutarato ligands from neighboring columns, thus further contributing to the stability of the 3D network. The channel architecture in the structure of 2 is shown in Figure 2D. Compound 3 crystallizes in the monoclinic space group C2/c. Bond distances and angles for 3 are given in (Table 4).

bridging role, through the oxygen atoms at one end of the ligands, resulting in the formation of trinuclear Pb(II) ion units, also link neighboring trinuclear Pb(II) units along the c axis through the carboxylato oxygen atoms lying at both ends of the ligands (Figure 2A). This results in the formation of columns of stacked trinuclear Pb(II) units parallel to the c axis, with a repeating formula unit of [Pb3(phen)3(glu)3]. The three glutarato ligands exhibit three different coordination modes (Scheme 2, Figure 2A). The glutarato ligand defined by O(1)··· O(4) exhibits a μ4-κ2O:κ2O′:κO″:κO‴ coordination mode by bridging Pb(1), Pb(2), Pb(3) on one side and Pb(1″) on the other side (Figure 2A). The glutarato ligand defined by O(11)···O(14) atoms exhibits the second coordination mode, μ3-κ2O:κO′:κO″:κO″, by bridging Pb(1) and Pb(2) on one side and Pb(2′) on the other side (Figure 2A). Finally the glutarato ligand defined by O(21)···O(24) atoms exhibits a μ4-κ2O:κ2O′:κO″:κO‴ coordination mode by bridging Pb(2), Pb(3) on one side and Pb(1′) and Pb(3″) on the other side (Figure 2A). In the lattice structure of 2, weak π−π interactions between the phen aromatic rings link the chains into a 3D network. In Figure 2B, three different chains are shown in F



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Figure 2. (A) Partially labeled plot of a small fragment of columns formed in 2. Symmetry operations: (′) x, y, 1 + z; (″) x, y, −1 + z. Color code: Pb, purple; O, red; N, blue; C, white. (B) Side view of three different columns, shown in different colors, interacting through π−π contacts in 2 (see text for details). The sites of hydrogen bonded O3w water molecules are also indicated. (C) Side view of a small part of the 3D network structure of 2 where π−π interactions (olive green lines) between the aromatic rings of the phen molecules, sites of O3w and chains of water solvated molecules are also shown (see text for details). (D) Top view of the 3D network structure of 2 showing the channels formed by the columnar architecture along the c-axis filled by lattice water molecules (indigo blue), the sites of O3w oxygens and the π−π interactions. The hydrogen bonds between the lattice water molecules and the carboxylato oxygen atoms of the ligands forming the columns are not shown for clarity.

ions, centrosymmetrically related, are bridged through two μ3-O(5), two μ2-O(2), and two μ2-O(3) carboxylato atoms and form tetranuclear units with a “butterfly” configuration (Figure 3B). The closest interatomic distances within the “butterfly” units are Pb(1)···Pb(1″) (″ −x, −y, −z) = 4.948 Å, Pb(1)···Pb(2′) (′ −x, 1 − y, −z) = Pb(1″)···Pb(2***) (*** x, −1 + y, z) = 4.519 Å, Pb(1″)···Pb(2′) = Pb(1)···Pb(2***) = 4.271 Å, and Pb(2′)···Pb(2***) = 7.268 Å. Finally, each polyhedron formed around the 10 coordinate Pb(1) ions shares edges with two neighboring polyhedra of the same type, which form around Pb(1) ions strewn along the c axis (Figure 3B). In addition, each polyhedron around Pb(1) ions shares edges with four of the polyhedra forming around Pb(2) ions. Adjacent “butterfly” units, forming edge-sharing tetranuclear

The asymmetric unit cell contains half of one metal ion residing on a 2-fold axis of symmetry, Pb(1); one metal ion in a general position, Pb(2); and one tca ligand. Therefore, the molecular structure of 3 is based on [Pb3(tca)2] repeating units and reveals a 3D coordination polymer. Pb(1) is 10-coordinate, with the coordination sphere being configured by 10 carboxylato oxygen atoms from six different tca(−3) ligands (Figure 3A). Pb(2) is bound to six carboxylato oxygen atoms from four different tca(−3) ligands. Thus, the coordination number of Pb(2) is six (Figure 3A,B). The tca ligand exhibits one coordination mode, μ7-κ3O:κ2O′:κ2O″:κ2O‴:κO⁗:κO⁗′ (Scheme 2), and bridges three Pb(1) and four Pb(2) ions. The polymer structure of 3 is based on a 3D Pb−O “inorganic” skeleton, which is built as follows: two Pb(1) and two Pb(2) G



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Table 4. Bond Lengths [Å] and Angles [deg] for [Pb3(tca)2]n (3)a Distances (Å) Pb(1)−O(6) Pb(1)−O(6‴) Pb(1)−O(3′) Pb(1)−O(3‴″) Pb(1)−O(5″) Pb(1)−O(5⁗) Pb(1)−O(2″) Pb(1)−O(2⁗)

2.363(6) 2.363(6) 2.779(8) 2.779(8) 2.872(6) 2.872(6) 2.899(7) 2.899(7)

Pb(1)−O(5) Pb(1)−O(5‴) Pb(2)−O(4) Pb(2)−O(2‴″) Pb(2)−O(1‴″) Pb(2)−O(5*) Pb(2)−O(3) Pb(2)−O(4**)

3.091(7) 3.091(7) 2.379(6) 2.403(7) 2.517(6) 2.518(7) 2.697(7) 2.834(8)

Angles (o) O(5)−Pb(1)−O(5‴) O(5)−Pb(1)−O(5″) O(5)−Pb(1)−O(5⁗) O(5)−Pb(1)−O(6) O(5)−Pb(1)−O(6‴) O(5)−Pb(1)−O(O2″) O(5)−Pb(1)−O(O2⁗) O(5)−Pb(1)−O(O3′) O(5)−Pb(1)−O(O3‴″) O(5‴)−Pb(1)−O(5″) O(5‴)−Pb(1)−O(5⁗) O(5‴)−Pb(1)−O(6) O(5‴)−Pb(1)−O(6‴) O(5‴)−Pb(1)−O(2″) O(5‴)−Pb(1)−O(2⁗) O(5‴)−Pb(1)−O(3′) O(5‴)−Pb(1)−O(3‴″) O(5″)−Pb(1)−O(5⁗) O(5″)−Pb(1)−O(6) O(5″)−Pb(1)−O(6‴) O(5″)−Pb(1)−O(2″) O(5″)−Pb(1)−O(2⁗) O(5″)−Pb(1)−O(3′) O(5″)−Pb(1)−O(3‴″) O(5⁗)−Pb(1)−O(6) O(5⁗)−Pb(1)−O(6‴) O(5⁗)−Pb(1)−O(2″) O(5⁗)−Pb(1)−O(2⁗) O(5⁗)−Pb(1)−O(3′) O(5⁗)−Pb(1)−O(3‴″)

162.6(2) 67.9(2) 120.0(2) 45.8(2) 117.5(2) 132.0(2) 61.8(2) 103.0(2) 70.5(2) 120.0(2) 67.9(2) 117.5(2) 45.8(2) 61.8(2) 132.0(2) 70.5(2) 103.0(2) 131.5(2) 82.8(2) 139.7(2) 66.1(2) 82.5(2) 64.1(2) 137.0(2) 139.7(2) 82.8(2) 82.5(2) 66.1(2) 137.0(2) 66.1(2)

O(6)−Pb(1)−O(6‴) O(6)−Pb(1)−O(2″) O(6)−Pb(1)−O(2⁗) O(6)−Pb(1)−O(3′) O(6)−Pb(1)−O(3‴″) O(6‴)−Pb(1)−O(2″) O(6‴)−Pb(1)−O(2⁗) O(6‴)−Pb(1)−O(3′) O(6‴)−Pb(1)−O(3‴″) O(2″)−Pb(1)−O(2⁗) O(2″)−Pb(1)−O(3′) O(2″)−Pb(1)−O(3‴″) O(2⁗)−Pb(1)-O(3′) O(2⁗)−Pb(1)-O(3‴″) O(3′)−Pb(1)−O(3‴″) O(4)−Pb(2)−O(2‴″) O(4)−Pb(2)−O(1‴″) O(4)−Pb(2)−O(5*) O(4)−Pb(2)−O(3) O(4)−Pb(2)−O(4**) O(2‴″)−Pb(2)−O(1‴″) O(2‴″)−Pb(2)−O(5*) O(2‴″)−Pb(2)−O(3) O(2‴″)−Pb(2)−O(4**) O(1‴″)−Pb(2)−O(5*) O(1‴″)−Pb(2)−O(3) O(1‴″)−Pb(2)−O(4**) O(5*)−Pb(2)−O(3) O(5*)−Pb(2)−O(4**) O(3)−Pb(2)−O(4**)

78.2(2) 136.8(2) 106.2(2) 71.4(2) 76.2(2) 106.2(2) 136.8(2) 76.2(2) 71.4(2) 98.9(2) 68.3(2) 146.7(2) 146.7(2) 68.3(2) 138.0(2) 74.0(2) 80.3(2) 112.6(2) 51.5(2) 63.7(2) 53.4(2) 77.5(2) 91.6(2) 114.8(2) 123.3(2) 127.9(2) 72.7(2) 70.2(2) 163.7(2) 97.7(2)

a Symmetry operations: (′) −x, 1 − y, −z; (″) −x, −y, −z; (‴) −x, y, −0.5 − z; (⁗) x, −y, −0.5 + z; (‴″) x, 1 − y, −0.5 + z; (*) x, 1 + y, z; (**) −0.5 − x, 1.5 − y, −z.

array of chains of ’butterfly’ units linked through the [Pb2O2] groups. Compound 4 crystallizes in the triclinic space group P1̅. The lattice in 4 reveals a 1D coordination polymer. The asymmetric unit contains one Pb(II), one phen, and one tcaH(−2) ligand. Moreover, one lattice water molecule per asymmetric unit is distributed over two crystallographic positions with half occupancy. The repeating unit of the 1D coordination polymer contains a centrosymmetric dinuclear assembly [Pb2(phen)2(tcaH)2]. Each Pb(II) ion is seven-coordinate, bound to the two phen nitrogen atoms and to five carboxylato oxygen atoms from three tcaH(−2) ligands (Figure 4A, Table 5). The tcaH(−2) ligands bridge three Pb(II) ions exhibiting a μ3-κ2O:κO′:κO″:κO‴ coordination mode (Scheme 2). The three carboxylato groups of the tca ligands exhibit three different coordination modes: that defined by O(21)/O(22), which bind Pb(1) in a monodentate fashion, that defined by O(23″)/O(24″) acting as a chelating monatomic bridge

entities, form chains along the c-axis. The orientation of contiguous “butterfly” units, lying along the c axis, changes alternately from the [1̅10] to the [110] direction (Figure 3C, top inclined view). At both ends of each “butterfly” unit, Pb(2) ions from neighboring chains are linked and form [Pb2O2] groups through the μ2-O(4) carboxylato atoms (Figure 3B,C). The Pb(2***)···Pb(2#) (# −0.5 − x, 1.5 − y, −z) interatomic distance within the [Pb2O2] units is 5.483 Å. As it is clearly seen in Figure 3C (top view of the structure along the c axis), neighboring chains sharing μ2-O(4) carboxylato atoms, resulting in the formation of [Pb2O2] groups, built a 3D crystal structure architecture. Tca(−3) ligands link “butterfly” units, lying in neighboring chains, through the O3, O4 carboxylato atoms, and within the same chain through the O1, O2 and O5, O6 pairs of carboxylato atoms (Figure 3A,D). A small part of the 3D framework structure of 3 looking down the c-axis is given in Figure 3E, where the coordination of tca(−3) ligands with Pb(II) ions results in the formation of a hexagonal H



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Figure 3. (A) Partially labeled plot of a very small fragment of the structure in 3 showing the coordination environment of the Pb ions and the coordination mode of the tca(−3) ligand. (B) A small part of the Pb−O “inorganic” skeleton in 3 (see text for details). (C) An inclined view (top) and a view along the c-axis, of a small part of the 3D structure of 3. The dotted lines and arrows indicate the change in orientation of successive “butterfly” units from the [1̅10] to the [110] direction. (D) Coordination of tca ligands with respect to arrangement of “butterfly” units. (E) A small part of the 3D framework structure of 3, looking down the c-axis. Symmetry operations: (′) −x, 1 − y, −z; (″) −x, −y, −z; (‴) −x, y, −0.5 − z; (⁗) x, −y, −0.5 + z; (⁗′) x, 1 − y, −0.5 + z; (*) x, 1 + y, z; (**) −0.5 − x, 1.5 − y, −z; (***) x, −1 + y, z; (****) x, 1 − y, 0.5 + z; (#) −0.5 − x, 0.5 − y, −z; (!) −x, −1 + y, −0.5 + z; (!!) x, 1 − y, −0.5 + z. Color code: Pb(1), purple; Pb(2), pink; O, red; C, white.

dinuclear repeating unit of the 1D coordination polymer and formation of [Pb2O2] units, with the closest interatomic distance being Pb(1)···Pb(1‴) (−x, −y, −z) = 4.607 Å. The dinuclear repeating units are linked through the other two

between two Pb(1) ions, and that defined by O(25′)/O(26′) remaining protonated and weakly coordinated to Pb(1) through the hydroxy group, Pb(1)−O(25′) = 3.151(5) Å. The monatomic bridging of O(23′) promotes formation of the I



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Figure 4. (A) Partially labeled plot of the dinuclear repeating unit in 4. Symmetry operations: (′): −1 + x, y, z; (″): 1 − x, −y, −z; (‴): −x, −y, −z. (B) A small part of the 1D structure of 4. (C) A small part of the supramolecular 2D network in 4 due to π−π interactions (yellow lines) between the phen molecules (see text for details). Color code: Pb, purple; O, red; N, blue; C, white.

Table 5. Bond Lengths [Å] and Angles [deg] for [Pb2(phen)2(tca)2]n·nH2O (4)a Distances (Å) Pb(1)−O(24″) Pb(1)−O(21) Pb(1)−O(23″) Pb(1)−O(23′)

2.337(5) 2.481(6) 2.779(8) 2.910(1)

Pb(1)−N(2) Pb(1)−N(1) Pb(1)−O(25′)

2.485(7) 2.546(5) 3.151(5)

Angles (°) O(24″)−Pb(1)−O(21) O(24″)−Pb(1)−O(23″) O(24″)−Pb(1)−O(23′) O(24″)−Pb(1)−O(25′) O(24″)−Pb(1)−N(1) O(24″)−Pb(1)−N(2) O(21)−Pb(1)−O(23″) O(21)−Pb(1)−O(23′) O(21)−Pb(1)−O(25′) O(21)−Pb(1)−N(1) O(21)−Pb(1)−N(2) a

79.4(2) 49.7(2) 121.3(2) 150.5(2) 79.6(2) 80.7(2) 109.2(2) 132.5(2) 76.6(2) 141.6(2) 79.1(2)

O(23″)−Pb(1)−O(23′) O(23″)−Pb(1)−O(25′) O(23″)−Pb(1)−N(1) O(23″)−Pb(1)−N(2) O(23′)−Pb(1)−O(25′) O(23′)−Pb(1)−N(1) O(23′)−Pb(1)−N(2) O(25′)−Pb(1)−N(1) O(25′)−Pb(1)−N(2) N(1)−Pb(1)−N(2)

71.9(2) 124.6(2) 80.0(2) 124.4(2) 66.7(2) 85.9(2) 141.4(2) 129.8(2) 110.9(2) 66.0(2)

Symmetry operations: (′) −1 + x, y, z; (″) 1 − x, −y, −z. J



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carboxylato groups of tcaH(−2) and form chains extending parallel to the a-axis (Figure 4B). The Pb···Pb interatomic distances are equal to the length of the a-axis, i.e., 7.925 Å. The specific coordination of tcaH(−2) ligands promotes formation of 16 macrometallocycles, (Pb−O−C−C−C−C−C−O)2, within the 1D chains. The protonated carboxylato group of tcaH(−2) participates in intrachain hydrogen bonding interactions of the type O(25)−H(25O)···O(22) (1 + x, y, z) with dimensions O(25)···O(22) = 2.551 Å, H(25O)···O(22) = 1.822 Å, O(25)−H(25O)···O(22) = 133.9°. The phen ligands in neighboring chains are parallel to each other and participate in weak π−π stacking interactions, thereby contributing to the stability of a 2D network parallel to the ac plane (Figure 4C). The mean planes of the phen molecules belonging to different 1D chains are parallel to each other with an intercentroid distance of 3.432 Å. The Pb−O bond distances in 1−4 resemble those in other Pb(II) compounds, such as [Pb(suc)] (2.44(1)−2.91(1) Å),44 Pb(suc) (2.540(5)−2.670(5) Å),45 Pb2(phen)4(C4H4O4)(NO 3 ) 2 (2.490(3)−2.638(3) Å), 46 [Pb(suc)(H2bbp)] (2.502(4)−2.923(1) Å), 4 7 [Pb(3-pdip)(glu)]·H 2 O (2.397(4)−2.651(4) Å), 48,49 Pb(C 5 H 6 O 4 ) (2.399(1)− 2.853(1)) Å),45 Pb4(oda)3(NO3)·H2O (2.464(13)−2.890(13) Å),50 [Pb(oda)(H2O)] (2.490(3)−2.91(2) Å),51 [Pb3(oda)3]n (2.458(5)−2.862(5) Å),33 [Pb(phen)(oda)]n (2.635(4)− 2.747(6) Å),33 [Pb(tda)]n (2.404(8)−2.841(7) Å),33 [Pb(phen)(tda)]n (2.313(2)−2.785(3) Å),33 [Pb2(phen)4(fum)](NO3)2 (2.538(3)−2.624(3) Å),34 [Pb2(phen)4(CO3)(fum)]·6H2O (2.451(5)−2.872(7) Å),34 [Pb2(phen)(fum)2] (2.439(3)− 2.853(3) Å),34 [Pb(phen)(fum)]·2H2O (2.454(5)−2.656(5) Å),34 [Pb(phen)(Heida)]·4H2O (2.454(6)−2.699(6) Å),35 [Pb3(NO3)(Dpot)]n (2.379(6)−2.880(6) Å),35 [Pb(nic)(fum)0.5] (2.483(8)− 2.586(8) Å),52 (nic = nicotinic acid N-oxide), {[Pb2(fum)2(H2O)4]· 2H2O}n (2.517(5)−2.858(5) Å),53 [Pb(fum)]n (2.399(5)− 2.811(5) Å),53 and [Pb(cit)]n (2.397(7)−2.847(1) Å).54 NMR Spectroscopy. The 13C spectrum of 1 (Figure 5A) reveals a resonance at 38.0 ppm, attributed to the −CH2 groups, and resonances in the range 182.0−186.0 ppm assigned to the −COO− carbons of the bound succinato ligand. Several resonances between 123.0 and 155.0 ppm belong to the phen bound to Pb(II). Compound 2 spectrum (Figure 5B) exhibits two resonances at 24.0 and 41.0 ppm (−CH2 groups) and one resonance at 184.0 ppm (−COO− carbons) for the glutarato ligand bound to Pb(II). Furthermore, there are several signals between 127.0 and 152.0 ppm, assigned to phen bound to Pb(II). Compound 3 spectrum (Figure 5C) exhibits signals at 42.0 and 48.0 ppm (−CH2 groups) and at 182.0 and 185.0 ppm (−COO− carbons) for the tca linker. All resonances are shifted to slightly higher values compared to the free ligand. Finally, Figure 5D exhibits the spectrum of 4, with signals at 38.0 and 42.0 ppm (−CH2 groups) and 173.0, 177.0, and 185.0 ppm (−COOH, −COO− carbons) for the tca ligand. The signals between 122.0 and 149.0 ppm represent carbons of the phen ligand. For the tcaH(−2) signals, the methylene signals are shifted to higher values compared to the free ligand, whereas the carboxylato signals spread over a larger range in comparison to the free ligand. The above observations are consistent with X-ray crystallographic results. The 207Pb nucleus has a spin of 1/2. This property has been previously employed to peruse the structural features of numerous Pb(II) compounds.55−59 In this regard, Pb(II) NMR signals exhibit strong chemical shift anisotropies (CSA), leading to the emergence of spinning sidebands in MAS experiments. Figures 6−9 represent the 207Pb NMR

Figure 5. 13C CPMAS NMR spectrum of 1 (A), 2 (B), 3 (C), and 4 (D) (stars (*) indicate spinning side bands).

spectra of 1−4, respectively. The spectrum of 1 shows a single site. The isotropic chemical shift is δiso = −1852 ppm, and the anisotropy has a width of Δδ = −104 kHz with an asymmetry η = 0.23. For 2, three different sites were identified with parameters of δiso = −1522 ppm, Δδ = −104 kHz, η = 0.19 for site 1, δiso = −295 ppm, Δδ = −98 kHz, η = 0.43 for site 2, and δiso = −903 ppm, Δδ = 22 kHz, η = 0.62 for site 3. The numbering of sites is the same as that derived from the crystal structure analysis. The two sites in compound 3 had parameters of δiso = −1355 ppm, Δδ = −89.4 kHz, δ = 0.64 for site 1, and δiso = −492 ppm, Δδ = −127.7 kHz, η = 0.33 for site 2. Finally, for compound 4, a single site with parameters δiso = −495 ppm, Δδ = −125.1 kHz, η = 0.22 is observed. For all four K



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Figure 6. 207Pb MAS NMR spectrum of 1 (blue) and simulated spectrum (red).

Figure 9. 207Pb MAS NMR spectrum of 4 (blue) and simulated spectrum (red).

In 2, there are two steps, where lattice water molecules are removed, and three sequential processes in the range 170−540 °C, reflecting decomposition of the organic part of the material. No clear plateaus are reached between processes, thereby suggesting instability of the derived intermediates and further decomposition. [Pb3(C12H8N2)3 (C5H6O4 )3 ]n · 7nH 2O + 57nO2 → 3nPbO + 51nCO2 + 3n N2 + 28nH 2O

Mass loss calculations are in line with PbO being the final product for 1 and 2. Decomposition reaches completion at 400 °C for 1 and 570 for 2 °C, with the total mass loss standing at 90.0% (calc 88.5%) and 62.2% (calc 60.2%), respectively. Compound 3 retains its stability up to 270 °C, whereas 4 maintains stability up to 240 °C (Figure 10C,D). Beyond that point, there is a single process step in the range 270−364 °C for 3, reflecting decomposition of the organic part of the title compound.

207

Figure 7. Pb MAS NMR spectrum of 2 (blue) and simulated spectrum (red). The individual lineshapes for the three sites are also given in green (Pb2), purple (Pb1), and bluish-green (Pb3).

[Pb3(C6H5O6 )2 ]n + 10nO2 → 3nPbO + 12nCO2 + 5nH 2O

In the case of compound 4, there is one step between 240 and 280 °C, where the lattice water molecules are removed, and two sequential processes in the range 280−450 °C, projecting decomposition of the organic part of the material. The aforementioned processes involve no clear plateaus, proposing instability and further decomposition of the derived products. [Pb2 (C12H8N2)2 (C6H6O6 )2 ]n ·nH 2O + 38nO2 → Figure 8. 207Pb MAS NMR spectrum of 3 (blue) and simulated spectrum (red). The individual line shapes for the two sites are also given in green (Pb2) and purple (Pb1).

2nPbO + 36nCO2 + 2n N2 + 15nH 2O

Mass loss calculations are in line with PbO being the final decomposition product for 3 and 4. Completion of the decomposition process occurs at 364 °C for 3 and 450 for 4 °C, with the total mass loss being 32.6% (calc 31.9%) and 53.8% (calc 52.2%), respectively. Luminescence. Photoluminescence properties of MOFs bearing metals with a d10 configuration and their association with functional materials applications have been studied in the past.60−62 In this regard, the solid-state spectra of 1−4 along with those of suc(−2), glu(−2), tcaH(−2/−3) and phen ligands were studied at room temperature (Figure 11). The spectral data suggest that none of the free ligands, sucH2, gluH2, or tcaH3, possess any luminescence. The strongest emission

compounds, the number of detected sites is in agreement with the X-ray results. Thermal Studies. The thermal behavior of 1−4 was examined by TGA under an oxygen atmosphere. Compounds 1 and 2 are thermally stable up to 157 and 95 °C, respectively (Figure 10A,B). Beyond that point, sequential processes in the range 157−400 °C for 1 reflect decomposition of the organic part of the material. 2[Pb(C12H8N2)(C4H4O4 )]n + 35nO2 → 2nPbO + 32nCO2 + 2n N2 + 12nH 2O L



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Figure 10. TGA diagrams of 1 (A), 2 (B), 3 (C), and 4 (D).

Figure 11. Solid-state luminescence excitation spectra of 1−4 at room temperature.

an intense band at 559 nm (λex 357 nm). In 3 and 4, also, a strong emission at 472 nm (λex = 315 nm) for 3 and 560 nm (λex = 350 nm) for 4 is observed. The observed features in the

band for free phen appears at 417 nm (λex 368 nm) and could be assigned to π*−π transitions. The emission spectrum of 1 exhibits a strong peak at 562 nm (λex 360 nm), with 2 displaying M



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cases, however, spectroscopic and structural characterization involving elemental analysis, FT-IR, TGA, and 13C-, 207PbCPMAS NMR were quite revealing of the factors contributing to the nature of the lattice, its formation, composition, and structural architecture (vide infra). Structural Speciation and Lattice Dimensionality. In view of the nature of 1−4 derived under the aforementioned conditions, the structural speciation observed in these systems reflects specific architecture and lattice dimensionality attributes in the respective lattices. Key to delineating such a correlation appears to be the (a) nature of the binary and ternary system of the same ligand reacting with Pb(II), and then (b) linkage between the variable nature ligands (succinic and glutaric) and that of their tricarboxylic congener acid seeking Pb(II) binding. In this sense, in the binary system of Pb(II) and succinic acid, the fully deprotonated ligand binds Pb(II) in a number of different modes; thus, the final architecture reveals a 3D44 network with the coordination number of Pb(II) being eight. When the system turns ternary, Pb(II)-succinic acid-phen, the ultimately derived architecture reflects a 2D coordination polymer, and the phen layers interact through weak π−π interactions, thereby contributing to the formation of a 3D lattice network. In the case of glutaric acid, the same trends are observed; when a binary system is employed, a 3D lattice network emerges (PbO7),49 and when the system turns ternary, Pb(II)-gluH2-phen, the observed architecture is 1D, which ultimately turns 3D through hydrogen bonds involving the water molecules and phen layer π−π interactions. The structures of the two systems, Pb(II)-sucH2-phen and Pb(II)-gluH2-phen exhibit several similarities and differences: (a) the lattice in 1 is 2D, whereas that in 2 is 1D. Both of them, however, ultimately develop a 3D architecture through phen interjection, (b) in 1, there is only one eight coordinate Pb(II) ion, whereas in 2 there are three different Pb(II) ions; two of them are eight coordinate (Pb1 and Pb2), with the third one being seven coordinate (Pb3), and (c) regardless of the increasing chain length (by one carbon) between the two carboxylic acid moieties in sucH2 and gluH2 acid, the ultimately achieved lattice dimensionality is 3D and associated with a molecular stoichiometry of Pb(II)/ligand/(phen) = 1:1:(1) in the repeating unit structures of both 1 and 2. Whether that trend continues or changes occur with increasing carbon chain length in the dicarboxylato ligands bound to Pb(II) remains to be seen. When the derivative congener tricarboxylic acid to glutaric acid is used, namely, tricarballylic acid (tca), the results appear to be analogous to those in the dicarboxylic acids. When the material is binary, Pb(II)-tricarballylic acid (3), the observed lattice dimensionality is 3D, and the Pb(II) ion is 10 coordinate with the tricarballylic acid fully deprotonated. When the system turns ternary, Pb(II)-tricarballylic-phen, then lattice dimensionality in the emerging material 4 is reduced to 1D, with the Pb(II) ion being seven coordinate and the tricarballylic acid being doubly deprotonated. Here, too, the 1D lattice structure turns into a higher dimensionality 2D network, as a result of hydrogen bonding and weak phen layer π−π interactions. Coordination Geometry, Number and 207Pb-NMR Correlations. Hemidirected as well as holo-directed stereochemical characterization of PbOn polyhedral assemblies has been previously established for numerous Pb(II)-containing materials, bearing Pb(II) centers with variable coordination numbers (2−10), reflecting equally diverse coordination spheres. Thus, identification of one coordination geometry or the other for Pb(II) centers in a well-defined material provides

three compounds could be assigned to ligand-to-metal-chargetransfer (LMCT) processes.63−66 The fact that the free organic acid ligands do not exhibit any luminescence at room temperature suggests that all compounds could be considered as potential luminescent materials.

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DISCUSSION Transition from Binary to Ternary Pb(II)-di(tri)carboxylic Acid Systems. The rational design of novel crystalline polymeric MOF materials has attracted intense interest owing to their merit as functional materials in catalysis, molecular recognition, separation, and nonlinear optics. It is a great challenge, however, to predict the nature of the ultimately derived materials structures and their photoproperties through control of specific factors influencing the chemistry and associated lattice formation. Thus, systematic research on this topic is essential to understanding the roles of potential structural and electronic factors ultimately affecting formation of crystalline polymeric frameworks. Several such factors, including the coordination environment of metal centers, structural features of the polydentate ligands (i.e., O, N, S binding anchors), solvents, templates, counterions, noncovalent interactions (i.e., aromatic−aromatic interactions) that stabilize certain large and/or polymeric assembly architectures, and so on, may distinctly contribute to the network structure of an anticipated compound. Taking into consideration the aforementioned factors exemplified through structurally congener dicarboxylic succinic and glutaric as well as derivative tricarboxylic tricarballylic acids, pH-specific investigation of their chemical reactivity in the presence of Pb(II) led to two new ternary compounds of Pb(II) with dicarboxylic acids and phen, with the two dicarboxylates differing in the number of spacer −CH2− groups in their alkyl chains. The analogous tricarballylic acid ternary system of Pb(II) incorporating phen provided an equally informative view of the architecture of the coordination polymeric materials isolated and crystallized under the experimental conditions. At the specific pH employed in the reaction systems studied, molecular stoichiometry turned out to be a suitable guide leading to the ternary materials isolated in 1 and 2. In an analogous fashion, the ternary system involving the tricarboxylic derivative of glutaric acid led to a ternary crystalline 4. Quite different was the case of the ternary system of Pb(II)-tricarballylic acid in the presence of 2,2′-bpy instead of phen, thereby leading to the isolation of a binary material 3 rather than a ternary one in 4. In all systems studied, there is a consistent 1:1:1 molar stoichiometry of Pb(II)/ ligand/phen present in the ternary materials 1, 2, 4, whereas a 3:2 Pb(II)/ligand stoichiometry is observed in the ternary system leading to the binary material 3. Thus, the nature of the aromatic chelator has a significant influence on the nature of the material isolated. Concurrently, the structural flexibility and bulk of the two employed N,N′-aromatic chelators in the ternary systems studied influences distinctly differentiated binary and ternary materials within the same Pb(II)-tricarboxylic acid system. The state of deprotonation was clear in the case of the dicarboxylic acids (doubly deprotonated) binding Pb(II), with the tricarboxylic acid discretely using both its doubly and triply deprotonated forms upon Pb(II) coordination. Along the same lines, distinct coordination modes around the central metal ions were adopted by the three ligands, (a) exemplifying their potential denticity upon Pb(II) binding, and (b) formulating the Pb(II) coordination sphere and geometry through their (O,O) and (O,N) anchoring terminals in all materials derived. In all N



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Table 6. Structural Data Range of (O,N,S)-Ligand Bound Pb(II) Coordination in Correlation with NMR Isotropic Chemical Shifts coordination number/isotropic chemical shifts (ppm) coordinating ligand atoms

type of Pb(II) coordination

[Pb(phen)(suc)]n (1) [Pb3(phen)3(glu)3]·7nH2O (2)

6 O, 2 N 6/5 O, 2 N

[Pb3(tca)2]n (3)

10/6 O

compound

[Pb2(phen)2(tcaH)2]·nH2O (4) [Pb3(oda)3]n

5 O, 2 N 7/9 O

[Pb(phen)(oda)]n [Pb(tda)]n [Pb(phen)(tda)]n [Pb(Heida)]n·nH2O [Pb(phen)(heida)]·4H2O [Pb2(HArg)3(H2O)(NO3)7]·3H2O

6 O, 2 N 6 O, 1 S 4 O, 2 N 6 O, 1N 3 O, 3 N 10 O

[Pb(OH2)2(Val)(Ile)(NO3)2]

8O

Pb-eight Pb1-eight Pb2-eight Pb3-seven Pb1-ten Pb2-six Pb-seven Pb1-seven Pb2-seven Pb3-nine Pb-eight Pb-seven Pb-six Pb-seven Pb-six Pb1-ten Pb2-ten Pb-eight

[Pb(leu)(NO3)] [Pb(Hile)2(NO3)(H2O)2]NO3 [Pb(Hasp)(NO3)] [Pb2(Hval)5](ClO4)4·2H2O

7 7 7 7

O O O O

Pb-seven Pb-seven Pb-seven Pb-seven

[Pb(OH2)(Ile)2][NO3]·H2O [Pb(OH2)2(Val)2(NO3)]NO3

7O 6O

Pb-seven Pb-six

5

6

7

8

9

−1852 Pb1: −1522 Pb2: −295

ref

10

this work this work

Pb3: −903 Pb1: −1355 this work Pb2: −492 −495 Pb1: −2232 Pb2: − 2330

this work 33 Pb3: −2873 −1970

−1924.7 −1925 −1572.7 −1424

(−1950) − (−1790) −929 −1774 −2439 (−1390) − (−1755) −1766 −1707

33 33 33 35 35 Pb1: −1285 69 Pb2: −2511 69 69 69 69 69 69 69

specific regions of the spectra, consistent with the presence of variable coordination number (5−10) Pb(II) ions in investigated materials (Table 6).33−35,68−71 In this work, the derived 207 Pb data correspond to coordination numbers, six [Pb(2) in 3], seven [4 and Pb(3) in 2], eight [1, Pb1 and Pb2 in 2], and 10 [Pb1 in 3], thereby ear-tagging the detected nucleus on specific lattice structures in Pb(II)-containing MOFs. It should be emphasized, however, that due to yet unknown factors the intertwined influence of which dictates the structural assembly of the Pb(II)-containing materials, no clear 207Pb shift ranges exist in the respective spectra that relate to specific Pb(II) coordination numbers. It appears that the coordinating atoms can modulate shifts in one or the other direction. In this regard, the anisotropy and asymmetry values might further document the attempted assignments. Nevertheless, for the four studied samples, the shift values fit quite well with literature data. More work on the issue is underway in our laboratories. Structural Differences between Di- and Tricarboxylic Group-Containing Lattice Structures and Luminescence Properties. The electronic optical properties of 1−4 could be reflected upon through classification of the ternary title materials in two categories. In the first category involving the dicarboxylic acids, it appears that there is a very small difference in the excitation and emission wavelengths for both 1 and 2 (which is 3 nm), (a) despite the changes occurring in the length of organic acids by one carbon, and (b) denoting the same molecular stoichiometry Pb(II)/ligand/phen of 1:1:1 in the derived structures and ultimate lattice dimensionality (3D). The transition from the dicarboxylic to the tricarboxylic derivative of glutaric acid shows that there is (a) a clear differentiation in the excitation and emission wavelengths for both the

a structural signature that could be correlated with spectroscopic properties exemplifying potential applications.13,67 In this regard, the eight coordinate Pb(II) centers in 1 exhibit a holodirected geometry. In 2, the eight-coordinate Pb1 and Pb2 centers exhibit holodirected geometries, with the sevencoordinate Pb3 centers displaying a hemidirected geometry. For the 10 coordinate Pb1 centers in 3, a holodirected geometry is observed, while the six coordinate Pb2 centers exhibit a hemidirected geometry. Finally, the seven-coordinate Pb(II) centers in 4, exhibit a hemidirected geometry. Collectively, it appears that in all materials 1−4, (a) Pb(II) centers exhibiting coordination numbers higher than seven are associated with a holodirected geometrical arrangements of the bound ligands in their coordination sphere, whereas those exhibiting coordination numbers 6−7 are associated with hemidirected geometries, (b) when one type of coordination geometry and number is observed the holo- vs hemidirected is unique, whereas when two types of coordination geometry are concurrently present both types of holo- and hemidirected geometry are exhibited, and (c) in the binary system 3, both types of holo- and hemidirected geometries are observed and are associated with the highest and lowest coordination numbers observed in 1−4. The so-derived coordination geometry assignments are in line with the spectroscopic and X-ray structural data on all materials studied. These properties, in turn, are correlated with the NMR spectroscopic signatures of the materials studied, (a) extending correlations with the 13C and 207Pb signals in the NMR spectra, and (b) rendering the coordination geometries observed for the Pb(II) centers structural architecture markers. Past work in the field has revealed various 207Pb NMR spectroscopic patterns linked to O



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binary (3) and ternary (4) materials, (b) a smaller difference between the excitation and emission wavelengths in the binary material 3 than in the ternary material 4 and the corresponding ones belonging to the dicarboxylic acid family (1, 2). That difference increases in 4 and reaches the same level as that in 1 and 2. So the ternary materials 1, 2, 4 exhibit a comparable difference between the excitation and emission wavelengths. (c) a differentiation of the excitation and emission wavelengths in the binary material 3 (and blue-shifted) from those in 4 and the ones corresponding to the dicarboxylic acid compounds 1 and 2. All three ternary materials exhibit luminescence features at very similar energies. (d) a discrete change in structure and molecular Pb(II)/tricarballylic/phen stoichiometry associated with the luminescence behavior in 3 and 4 for a gradually reduced ultimate lattice dimensionality (from 3D to 2D), and (e) a red shift of the excitation and emission wavelengths in the ternary tricarballylic acid system compared to the corresponding binary system. Overall, (a) the patterns observed and changes occurring in the luminescence behavior of 1−4 are specific and sensitive to changes in the number of carboxylic acid moieties present in the ligand bound to Pb(II) in the ternary systems studied, and (b) for the same ligand, binary and ternary species exhibit distinctly differentiated luminescence, with the excitation and emission energies reflecting the identity of the binary vs ternary species.

electronic properties, which could be used in the synthesis of functional optical materials.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystal crystallographic files, in CIF format, (CCDC 1027238 (1), 1027239 (2), 1027240 (3), and 1027241 (4)), and listings of positional and thermal parameters and H-bond distances and angles for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +30-2310-996-179. Fax: +30-2310-996-196. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support “IKY Fellowships of Excellence for Postgraduate studies in Greece − Siemens Program” is gratefully acknowledged. The DFG (German Research Foundation) and the Experimental Physics Institutes of Leipzig University are acknowledged for their support of the Avance 750 MHz NMR spectrometer.

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CONCLUSIONS A systematic pH-specific synthetic study of binary and ternary systems of Pb(II)/di(tri)carboxylic acid/(phen) afforded two families of hybrid metal oxide framework materials of polymeric nature. Succinic and glutaric acids, reflecting abutting dicarboxylic acids differing by just one carbon atom in their methylene chain, linking the carboxylic acid moieties, provided welldefined materials 1 and 2, exemplifying their unique nature and structural integrity through a variety of crystallographic and spectroscopic techniques; both 1 and 2 bear the same molecular Pb(II)/di(tri)carboxylic acid/(phen) 1:1:1 stoichiometry in mononuclear and trinuclear assembly repeating units in their polymeric structure. The branched congener of glutaric acid, i.e., tricarballylic acid, provided discrete binary and ternary materials 3 and 4, denoting the importance of the aromatic N,N′-aromatic chelator in the synthetic process. The nature of the assembly in each polymeric material denotes the unique architecture amply reflected and imprinted upon the physicochemical profile of the title compounds. Structure architecture-lattice dimensionality, structural speciationNMR and luminescence correlations characterize the electronic profiles of all materials studied and project the contribution of the factors influencing architecture, lattice dimensionality (2D-3D), and spectroscopic properties. Albeit early, it appears that specific properties linked to the nature of dicarboxylic vs tricarboxylic acid ligand (spacer methylene chain) and promoting variably configured assemblies in 1−4 influence luminescence for the same or similar structural components in the repeating units of the polymeric materials. Outstanding differences emerge upon going from dicarboxylic to tricarboxylic acid ligands seeking binding to Pb(II), thereby formulating the structural and electronic profile of the materials at the binary and ternary level (interjection of phen, but not 2,2′-bpy). The collective physicochemical properties denote the contribution of specific factors in the design of binary and/or ternary metal oxide framework materials in the case of Pb(II) bearing well-defined

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