1D−3D Metal−Organic Lattice Assemblies through Chemical

DOI: 10.1021/cg100907p

1D-3D Metal-Organic Lattice Assemblies through Chemical Reactivity and Metal-Assisted Ligand Transformations in Ternary Pb(II)-Phenanthroline-(Hydroxy)dicarboxylic Acid Systems )

2011, Vol. 11 382–395

C. Gabriel,† C. P. Raptopoulou,‡ V. Psycharis,‡ A. Terzis,‡ M. Zervou,§ C. Mateescu, and A. Salifoglou*,† †

)

Department of Chemical Engineering, Laboratory of Inorganic Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece, ‡Institute of Materials Science, NCSR “Demokritos”, Aghia Paraskevi 15310, Attiki, Greece, §Laboratory of Molecular Analysis, Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, Athens 11635, Greece, and Banat University of Agricultural Sciences and Veterinary Medicine, Timisoara 1900, Romania

Received July 8, 2010; Revised Manuscript Received November 28, 2010

ABSTRACT: Variable-pH hydrothermal reactions of Pb(II) with dicarboxylic acids in the presence of 1,10-phenanthroline led to new solid-state compounds [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (1), [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2), [Pb2(C12H8N2)(C4H2O4)2]n (3), and [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4). All compounds were characterized by elemental analysis, FT-IR, CP-MAS NMR, and single crystal X-ray diffraction. The collective chemical reactivity in the ternary Pb(II)phenanthroline-dicarboxylate systems unravels seldom seen metal-assisted ligand malate/maleate to fumarate transformations, which in the presence of Pb(II) and 1,10-phenanthroline contribute to the assembly of 2D (2) and 3D lattice networks (3 and 4). All distinct assemblies in 1-4 reveal interwoven crystal lattice connections, reflecting unique physicochemical properties.

Introduction Lead has long since been used in a range of applications, including roofing, fuel additives, batteries, and solder, and it is often present as a contaminant in the environment. As a heavy toxic metal, it is commonly found in critical life cycles due to its widespread use in numerous industrial applications.1 Potential molecular mechanisms of Pb(II) toxicity may involve several different types of proteins.2 Therefore, good knowledge of Pb(II) coordination properties, including aspects such as the lone pair of electrons, coordination number, and coordination geometry, is crucial in understanding Pb(II) toxicity at the molecular level.3 On the other hand, in contrast to transition metals, the main group metal ion Pb(II) exhibits unique coordination preferences and electronic properties rarely observed in the rest of the periodic table, presenting unique opportunities for the assembly and synthesis of novel structures with new and interesting lattice features and physicochemical properties.4 As a result of the pluripotent chemical reactivity of Pb(II), its coordination chemistry and lattice architecture properties of arising materials are a subject of ongoing research worldwide.5 Coordination polymers constitute one of the most important classes of organic-inorganic hybrid materials, which have attracted great research interest not only because of their intriguing variety of architectures and topologies but also because of their fascinating potential applications in functional solid materials, ion exchange, catalysis, and the development of optical, electronic, and magnetic devices.6-9 Consequently, a variety of coordination polymers of variable M(II)-L composition have been constructed by careful selection of the metal ions and multidentate bridging ligands, L. Some organic N- or O-donors such as 1,10-phenanthroline (phen) and mono-, di-, and polycarboxylate ligands are often *Author to whom correspondence should be addressed. Tel: þ30-2310996-179. Fax: þ30-2310-996-196. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 12/31/2010

chosen to assemble various complexes.10-12 Most of such efforts have focused on coordination polymers containing group d10 transition metal ions and lanthanide ions, not only because of their propensity to yield novel structures but also because of their good photoluminescent properties.13 To this end, while many Pb(II)-carboxylates present in macrocyclic,14,15 encapsulating,16 or bulky organometallic17,18 ligand-containing compounds have been reported in the literature,19 a limited number of binary and ternary Pb(II)-dicarboxylates containing low molecular mass aliphatic organic acid ligands are known to (a) be involved in metal-assisted ligand transformation and (b) exhibit distinct chemical reactivity linked to unique crystal lattice assembly.20 Encouraged by successful efforts in our lab to employ hydrothermal synthesis for the assembly of crystalline materials containing Pb(II), we pursued the synthesis, isolation, and crystallization of new binary and ternary compounds of Pb(II) with variable nature O,N-containing aromatic and carboxylic acid ligands. Hence, we herein report research efforts on ternary Pb(II)-malic, maleic, and fumaric acid systems containing phen, in which employment of well-designed hydrothermal synthesis led in a unique way to the formation, isolation, and crystallization of four new compounds containing Pb(II)-fumarate and phen. In this regard, several types of 1D, 2D, or 3D supramolecular architectures, assembled from Pb(II) centers with dicarboxylates and phen, arose with discrete structural features, reflecting a wide and interwoven diversity of uniquely defined solid-state lattices. Experimental Section Materials and Methods. All experiments were carried out under aerobic conditions. Nanopure quality water was used for all synthetic reactions. Pb(NO3)2, Pb(CH3COO)2 3 3H2O, maleic acid, and sodium hydroxide were purchased from Fluka. Fumaric acid was r 2010 American Chemical Society

Article supplied by Merck. Malic acid and 1,10-phenanthroline (phen) were supplied by Aldrich. Physical Measurements. FT-infrared spectra were recorded on a Thermo, Nicolet IR 200 FT-infrared spectrometer. A ThermoFinnigan Flash EA 1112 CHNS elemental analyzer was used for the simultaneous determination of carbon, hydrogen, and nitrogen (%). The analyzer operation is based on the dynamic flash combustion of the sample (at 1800 °C) followed by reduction, trapping, complete GC separation, and detection of the products. The instrument (a) is fully automated and is controlled by a PC via the Eager 300 dedicated software and (b) is capable of handling solid, liquid, or gaseous substances. Solid-State NMR Spectroscopy. Solid state CP-MAS 13C NMR spectra were obtained on a Varian 400 MHz spectrometer operating at 100.53 MHz. In each case, a sufficient sample quantity was placed in a 3.2 mm rotor. A double resonance HX probe was used. The spinning rate was set at 12 kHz. The RAMP-CP pulse sequence of the VnmrJ library was applied, whereby the 13C spin-lock amplitude is varied linearly during cross-polarization (CP), while the 1H spin-lock amplitude is kept constant. RAMP-CP eliminates the Hartmann-Hahn matching profile dependence from the MAS spinning rate and optimizes signal intensity.21 The adamantane (C10H16) CH group was used as an external reference (38.54 ppm) to report the chemical shifts of 13C resonance peaks. Preparation of [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (1). Method A. A quantity of Pb(NO3)2 (0.20 g, 0.60 mmol) was placed in a flask and dissolved in 10 mL of H2O. Subsequently, malic acid (0.16 g, 1.2 mmol) was added slowly and under continuous stirring. Then, phen (0.11 g, 0.55 mmol) was added under stirring. Finally, a solution of sodium hydroxide was added slowly to adjust the pH to a final value of ∼8.5. The resulting solution was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflon-lined stainless steel reactor and heated to 160 °C for 85 h. Subsequently, the reactor was allowed to cool to room temperature. At the bottom of the Teflonlined stainless steel reactor appeared yellow crystals. The resulting crystals (0.16 g, yield 40%) were collected by filtration, washed with water, and air-dried. Anal. Calcd for 1, [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (C52H34O10N10Pb2, MW 1373.30): C, 45.0; H, 2.4; N, 10.0. Found: C, 44.7; H, 2.2; N, 9.9. Method B. A quantity of Pb(NO3)2 (0.33 g, 1.0 mmol) was placed in a flask and dissolved in 13 mL of H2O. Subsequently, maleic acid (0.23 g, 2.0 mmol) was added slowly and under continuous stirring. Then, phen (0.20 g, 1.0 mmol) was added under stirring. Finally, a solution of sodium hydroxide was added slowly to adjust the pH to a final value of ∼9. The resulting solution was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflonlined stainless steel reactor and heated to 160 °C for 85 h. Subsequently, the reactor was allowed to cool to room temperature. At the bottom of the Teflon-lined stainless steel reactor appeared yellow crystals. The resulting crystals (0.24 g, yield ∼58%) were collected by filtration, washed with water, and air-dried. Anal. Calcd for 1, [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (C52H34O10N10Pb2, MW 1373.30): C, 45.0; H, 2.4; N, 10.0. Found: C, 44.8; H, 2.3; N, 9.8. Preparation of Complex [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2). A quantity of Pb(CH3COO)2 3 3H2O (0.20 g, 0.54 mmol) was placed in a flask and dissolved in 10 mL of H2O. Subsequently, maleic acid (0.12 g, 0.93 mmol) was added slowly and under continuous stirring. Then, phen (0.11 g, 0.54 mmol) was added. Finally, a solution of sodium hydroxide was added slowly to adjust the pH to a final value of ∼9. The resulting solution was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflon-lined stainless steel reactor and heated to 160 °C for 85 h. Thereafter, the reactor was allowed to cool to room temperature. The resulting solution was left for slow evaporation, and after a day, white-brown crystals (dendrites) appeared (compound 2). The resulting crystals (80 mg, yield 22%) were collected by filtration, washed with water, and air-dried. The remainder of the solution was left for slow evaporation, and another type of crystals (rods) precipitated out. These crystals were different from the first ones and were found to belong to compound 4 (vide infra) (0.10 g, yield 34%). Anal. Calcd for 2, [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2) (C53H46O13N8Pb2, MW 1417.38): C, 44.8; H, 3.3; N, 7.9. Found: C, 44.5; H, 3.5; N, 7.7.

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Preparation of [Pb2(C12H8N2)(C4H2O4)2]n (3). Method A. A quantity of Pb(CH3COO)2 3 3H2O (0.20 g, 0.54 mmol) was placed in a flask and dissolved in 10 mL of H2O. Subsequently, fumaric acid (0.12 g, 0.93 mmol) was added slowly and under continuous stirring. Then, phen (0.11 g, 0.54 mmol) was added. The final pH value of the resulting solution was 3.5. The resulting solution was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflon-lined stainless steel reactor and heated to 160 °C for 85 h. Subsequently, the reactor was allowed to cool to room temperature. At the bottom of the Teflon-lined stainless steel reactor appeared yellow-brown crystals. The resulting crystals (0.10 g, yield 46%) were collected by filtration, washed with water, and air-dried. Anal. Calcd for 3, [Pb2(C12H8N2)(C4H2O4)2]n (3) (C20H12O8N2Pb2, MW 822.7): C, 29.2; H, 1.4; N, 3.4. Found: C, 28.9; H, 1.5; N, 3.4. Method B. A quantity of Pb(CH3COO)2 3 3H2O (0.20 g, 0.54 mmol) was placed in a flask and dissolved in 10 mL of H2O. Subsequently, malic acid (0.14 g, 0.93 mmol) was added slowly and under continuous stirring. Then, phen (0.11 g, 0.54 mmol) was added under stirring. The final pH value of the solution was 4.5. The resulting reaction mixture was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflon-lined stainless steel reactor and heated to 170 °C for 96 h. Subsequently, the reactor was allowed to cool to room temperature. At the bottom of the Teflon-lined stainless steel reactor appeared yellow crystals. The resulting crystals (0.15 g, yield 50%) were collected by filtration, washed with water, and air-dried. Anal. Calcd for 3, [Pb2(C12H8N2)(C4H2O4)2]n (3) (C20H12O8N2Pb2, MW 822.7): C, 29.2; H, 1.4; N, 3.4. Found: C, 29.0; H, 1.3; N, 3.3. Method C. A quantity of Pb(NO3)2 (0.20 g, 0.60 mmol) was placed in a flask and dissolved in 10 mL of H2O. Subsequently, fumaric acid (0.14 g, 1.0 mmol) was added slowly and under continuous stirring. Then, phen (0.12 g, 0.60 mmol) was added followed by the addition of NaOH solution. The final pH value of the resulting solution was 4.5. The resulting solution was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflon-lined stainless steel reactor and heated to 160 °C for 85 h. Subsequently, the reactor was allowed to cool to room temperature. At the bottom of the Teflon-lined stainless steel reactor appeared yellow-brown crystals. The resulting crystals (0.11 g, yield 54%) were collected by filtration, washed with water, and air-dried. Anal. Calcd for 3, [Pb2(C12H8N2)(C4H2O4)2]n (3) (C20H12O8N2Pb2, MW 822.7): C, 29.2; H, 1.4; N, 3.4. Found: C, 28.7; H, 1.7; N, 3.6. Preparation of [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4). A quantity of Pb(CH3COO)2 3 3H2O (0.20 g, 0.54 mmol) was placed in a flask and dissolved in 10 mL of H2O. Subsequently, maleic acid (0.12 g, 0.93 mmol) was added slowly and under continuous stirring. Then, phen (0.11 g, 0.54 mmol) was added under stirring. Finally, a solution of sodium hydroxide was added slowly to adjust the pH to a final value of ∼9. The resulting reaction mixture was left to stir for a half hour at room temperature. Then, it was placed in a 23 mL Teflonlined stainless steel reactor and heated to 160 °C for 85 h. Subsequently, the reactor was allowed to cool to room temperature. The resulting solution was left for slow evaporation, and one week later, white-brown crystals appeared. The resulting crystals (0.15 g, yield 51%) were collected by filtration, washed with water, and air-dried. Anal. Calcd for 4, [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4) (C16H14O6N2Pb, MW 537.48): C, 35.7; H, 2.6; N, 5.2. Found: C, 35.9; H, 2.8; N, 5.0. X-ray Crystal Structure Determination. X-ray quality crystals of compounds 1, 2, 3, and 4 were grown from aqueous solutions subjected to hydrothermal reactivity conditions. A single crystal, with dimensions 0.15 0.17 0.85 mm3 (1) and 0.10 0.20 0.50 mm3 (2) was mounted on a glass fiber. Diffraction measurements were made on a Crystal Logic dual-goniometer diffractometer, using graphite monochromated Mo KR radiation (λ = 0.71073 A˚). Unit cell dimensions were determined and refined by using the angular settings of 25 automatically centered reflections in the range 11 < 2θ < 23°. Intensity data were measured by using θ-2θ scans. Throughout data collection, three standard reflections were monitored every 97 reflections, and they showed less than 3% variation and no decay. Lorentz, polarization, and j-scan absorption corrections were applied by using Crystal Logic software. A single crystal, with dimensions 0.07 0.10 0.25 mm3 (3) and 0.16 0.18 0.38 mm3 (4) was taken directly from the mother liquor and immediately

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cooled to -93 °C. Diffraction measurements were made on a Rigaku R-AXIS SPIDER Image Plate diffractometer using graphite monochromated Mo KR radiation (λ = 0.71073 A˚) for 3 and Cu KR radiation (λ = 1.54178 A˚) for 4. Data collection (ω-scans) and processing (cell refinement, data reduction, and empirical absorption correction) were carried out using the CrystalClear program package.22 Crystallographic details are given in Table 1. The structures of compounds 1-4 were solved by direct methods using SHELXS-97,23 and they were refined by full-matrix leastsquares techniques on F2 with SHELXL-97.24 All non-H atoms were refined anisotropically. Hydrogen atoms were either located by difference maps and were refined isotropically or introduced at calculated positions as riding on bonded atoms. Plots of all structures were drawn using the Diamond 3.1 crystallographic package.25

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leading to the formation of compound 3 is shown below (Reactions 4-6):

Results and Discussion Synthesis. The hydrothermal synthesis of compound 1 was expediently pursued through a facile reaction between lead nitrate and malic or maleic acid in aqueous solutions. The pH at which the reaction was carried out was 8.5-9. The adjustment of pH was made by addition of phen and aqueous sodium hydroxide. The stoichiometric reaction leading to the formation of the title compound is shown below (Reactions 1 and 2):

Compound 4 was the product of the aqueous reaction between Pb(CH3COO)2 and maleic acid, at pH ∼ 9, in the presence of phen and sodium hydroxide. The stoichiometric reaction leading to the isolation of compound 4 is shown below (Reaction 7):

In a similar reaction, Pb(CH3COO)2 and maleic acid reacted in water at pH ∼ 9, with phen and aqueous sodium hydroxide, and led to the isolation of crystalline compound 2. Due to the complexity of the reaction and its products, a nonstoichiometric reaction leading to the formation of 2 is depicted below (Reaction 3):

Compound 3 was the product of the aqueous reaction between Pb(CH3COO)2 or Pb(NO3)2 and fumaric acid, at pH 3.5 (method A) and 4.5 (method C), in the presence of phen (methods A and C) and NaOH (method C). In an alternative way, 3 was obtained through the aqueous reaction between Pb(CH3COO)2 and malic acid at pH 4.5 in the presence of phen (method B). The stoichiometric reaction

The derived Pb(II)-fumarate materials were easily retrieved in pure crystalline form through hydrothermal synthesis. Elemental analysis of the isolated colorless crystalline products projected the molecular formulations [Pb2(C12H8N2)4(C4H2O4)](NO3)2, [Pb 2(C 12 H8 N2 )4 (CO 3 )(C4H 2O 4)] 3 6H 2O, [Pb2(C 12 H8 N2 )(C4H2O4)2], and [Pb(C12H8N2)(C4H2O4)] 3 2H2O, reflected in 1-4, respectively. Further spectroscopic evaluation of the crystalline products by FT-IR confirmed the presence of fumarate bound to Pb(II), thus being in line with the proposed molecular formulations. Finally, X-ray crystallography confirmed the analytical and spectroscopic results by rendering the lattice molecular formulation of the crystalline products in all four cases. Variable reaction conditions leading to new binary and ternary Pb(II) complex species of fumarate, maleate, and malate are currently being investigated. Compounds 1-4 are insoluble in water and are stable in the crystalline form in air at room temperature for long periods of time. Description of Structures. The structure of complex 1 consists of centrosymmetric dinuclear cations [Pb2(phen)4(fum)]2þ (phen = 1,10-phenanthroline, fum = fumarate)

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Table 1. Summary of Crystal, Intensity Collection, and Refinement Data for [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (1), [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2), [Pb2(C12H8N2)(C4H2O4)2]n (3), and [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4) formula formula weight T, K wavelength, λ (A˚) space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (Mg m-3) abs coeff (μ), mm-1 2θmax (deg) no. of reflections collected/unique/used (ΔF)max/(ΔF)min (e/A˚3) GOF R/Rw (for all data) R indices a a

b

1

2

3

4

C52H34O10N10Pb2 1373.30 298 0.71073 triclinic P1 9.026(3) 11.459(3) 11.950(4) 96.75(1) 109.01(1) 100.34(1) 1128.8(6) 1 2.020 7.525 49.58 4124/3854 [R(int) = 0.0105]/3854 1.005/-1.152 1.121 0.0257/0.0579 R = 0.0225 Rw = 0.0561b

C53H46O13N8Pb2 1417.38 298 0.71073 triclinic P1 10.286(5) 14.865(7) 18.283(8) 91.54(2) 104.92(1) 109.76(2) 2522(2) 2 1.866 6.742 49.54 8909/8538 [R(int) = 0.0189]/8538 1.965/-1.761 1.064 0.0524/0.1137 R = 0.0406 Rw = 0.1050b

C20H12O8N2Pb2 822.7 180 0.71073 monoclinic P2/c 8.1380(3) 13.5949(6) 8.7768(4) 90.00 107.551(1) 90.00 925.82(7) 2 2.951 18.219 51.98 15569/1822 [R(int) = 0.0464]/1822 1.119 /-1.069 1.137 0.0222/0.0419 R = 0.0192 Rw = 0.0408b

C16H14N2O6Pb 537.48 180 1.54178 triclinic P1 7.7098(1) 9.5232(2) 11.5987(2) 85.407(1) 81.678(1) 71.184(1) 797.14(2) 2 2.239 20.929 130 13805/2485 [R(int) = 0.0665]/2485 1.618 /-2.085 1.104 0.0351/0.0856 R = 0.0347 Rw = 0.0853b

R values are based on F values; Rw values are based on F2. sP ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P jjFo j - jFc jj ½wðFo2 - Fc2 Þ2 P , Rw ¼ R ¼ P ðjFo jÞ ½wðFo2 Þ2

[(1):3598, (2):7101, (3):1651, (4):2448 refs I > 2σ(I)]. Table 2. Bond Lengths (A˚) for [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (1), [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2), [Pb2(C12H8N2)(C4H2O4)2]n (3), and [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4)

Figure 1. Partially labeled plot of the cation in [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (1) with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity. Primed atoms are generated by symmetry operation: (0 ) = 1 - x, -y, 1 - z.

and nitrate counterions in the crystal lattice (Figure 1). Compound 1 crystallizes in the triclinic space group P1 with one molecule in the unit cell. The asymmetric unit contains one Pb(II) ion, two phen ligands, one-half of the fumarato(-2) ligand, and one nitrate counterion; the dinuclear structure of 1 is generated through an inversion center in the middle of the CdC bond of the fumarato(-2) ligand. The two Pb(II) ions in the dinuclear cationic assembly are bridged through the fumarato(-2) ligand (Pb 3 3 3 Pb = 9.58(6) A˚); each of the carboxylato groups is coordinated to a Pb(II) ion in a symmetric bidentate chelating mode (Pb-O(1) = 2.538(3) A˚, Pb-O(2) = 2.624(3) A˚). Two phen molecules contribute to coordination number six around each metal ion (Table 2). The two phen molecules bound to each Pb(II) center form an angle of 86.8(1)° between them. The distance between two parallel phen ligands on the symmetrically related Pb(II) centers is 9.95(6) A˚; these phen ligands make an angle of 88.6°

Pb-N(21) Pb-O(1) Pb-N(1)

2.521(3) 2.538(3) 2.589(4)

2 Pb(1)-O(1) Pb(1)-O(3) Pb(1)-N(1) Pb(1)-N(2) Pb(1)-O(14) Pb(1)-N(12) Pb(1)-N(11) Pb(1)-O(13)

1

Pb-N(22) Pb-O(2) Pb-N(2)

2.621(3) 2.624(3) 2.701(4)

2.419(5) 2.625(6) 2.635(7) 2.661(6) 2.816(7) 2.820(6) 2.845(7) 2.868(7)

Pb(2)-O(1) Pb(2)-O(2) Pb(2)-N(32) Pb(2)-O(12) Pb(2)-N(31) Pb(2)-O(11) Pb(2)-N(22) Pb(2)-N(21)

2.451(5) 2.509(6) 2.636(6) 2.687(8) 2.696(7) 2.872(7) 2.890(6) 2.896(7)

3 Pb(1)-N(1) Pb(1)-N(10 ) Pb(1)-O(11) Pb(1)-O(1100 ) Pb(1)-O(21) Pb(1)-O(210 ) Pb(2)-O(120 )

2.538(4) 2.538(4) 2.556(3) 2.556(3) 2.684(3) 2.684(3) 2.439(3)

Pb(2)-O(12*) Pb(2)-O(21) Pb(2)-O(21000 ) Pb(2)-O(2200 ) Pb(2)-O(22**) Pb(2)-O(22) Pb(2)-O(22000 )

2.439(3) 2.589(3) 2.589(3) 2.801(4) 2.801(4) 2.853(3) 2.853(3)

4 Pb(1)-O(21) Pb(1)-O(31) Pb(1)-N(1)

2.379(5) 2.454(5) 2.562(7)

Pb(1)-N(2) Pb(1)-O(22) Pb(1)-O(220 )

2.643(6) 2.656(5) 2.768(6)

with the best mean plane defined by the fumarato atoms. The displacement of the second phen ligand from the corresponding ligand bound to the symmetrically related Pb(II) center is 1.86(6) A˚; these phen ligands make an angle of 4.7° with the best mean plane defined by the fumarato atoms. The structure is in line with the structure recently presented in the literature.27a

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Figure 2. (a) Partially labeled plot of a very small fragment of the structure of [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2) with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity. Primed atoms are generated by symmetry operation: (0 ) = x, 1 þ y, z. (b) Structure presentation of 2 showing π-π interactions between phen ligands in the arising 2D lattice. (c) Plot of the 2D structure of 2 due to hydrogen bonding interactions (dashed lines). Color code: Pb, purple; O, red; N, blue; C, gray. Carbon atoms of the phen molecules and all hydrogen atoms have been omitted for clarity.

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Crystal Growth & Design, Vol. 11, No. 2, 2011 Table 3. Hydrogen Bonds in the Structures of 2 and 4

interaction

D3 3 3A H3 3 3A (A˚) (A˚)

D-H 3 3 3 A (deg)

symmetry operation

O1w-H1wA 3 3 3 O4w O1w-H1wB 3 3 3 O14 O2w-H2wA 3 3 3 O1w O2w-H2wB 3 3 3 O11 O3w-H3wA 3 3 3 O2w O3w-H3wB 3 3 3 O13 O4w-H4wB 3 3 3 O3 O5w-H5wB 3 3 3 O12 O6w-H6wA 3 3 3 O2

2.781 2.801 2.829 2.852 2.797 2.828 2.782 2.778 2.765

2 1.892 2.006 2.080 2.251 1.994 2.018 1.980 2.193 2.057

151.3 174.0 1597 166.7 165.0 174.0 160.9 129.1 143.8

x, y, z 1 - x, 1 - y, 1 - z x, y, z 1 - x, 2 - y, 1 - z x, y, z -x, 1 - y, 1 - z -x, 1 - y, 1 - z -x, 2 - y, 1 - z 1 þ x, y, z

O1w-H1wA 3 3 3 O21 O1w-H1wB 3 3 3 O32 O2w-H2wA 3 3 3 O31 O2w-H2wB 3 3 3 O1w

2.927 2.872 2.949 2.926

4 2.194 2.062 2.128 2.353

148.0 166.5 175.6 127.3

1 þ x, y, z x, y, z 1 - x, -y, -z x, y, z

The molecular lattice structure of complex 2 reveals a 1D coordination polymer. Compound 2 crystallizes in the triclinic space group P1 with two Pb(II) ions, four phen ligands, one fumarato(-2) ligand, one carbonato(-2) ligand, and six lattice water molecules in the asymmetric unit. The polymeric chain is generated through translation along the b-axis. The repeating unit can be described as generated by dinuclear [Pb2(phen)4(fum)]2þ cations, in much the same way as those in 1, and bridging carbonato(-2) ligands (Figure 2a). Each of the Pb(II) ions in the dinuclear cationic assembly is coordinated to two phen molecules and a chelating carboxylato moiety of the fumarato(-2) ligand. Thus, each of the Pb(II) ions exhibits an N4O4 coordination environment (Table 2). The Pb 3 3 3 Pb interatomic distance through the bridging dicarboxylato ligand is 10.130 A˚. One of the carboxylato groups of the bridging fumarato(-2) ligand is coordinated to Pb(1) in a symmetric chelating mode (Pb(1)-O(13) = 2.868(7) A˚, Pb(1)-O(14) = 2.816(7) A˚), while the second one is coordinated to Pb(2) in an asymmetric chelating mode (Pb(2)-O(11) = 2.872(7) A˚, Pb(2)O(12) = 2.687(8) A˚ ). The dinuclear units are further bridged through a carbonato(-2) ligand, which adopts the μ2-κ2O:κO0 :κO00 coordination mode (Pb 3 3 3 Pb = 4.824 A˚). The chains of 2 extend parallel to the crystallographic b-axis. There are two phen molecules coordinated to Pb(1) and Pb(2) of the dinuclear cationic assembly. The corresponding phen ligands are almost parallel to each other and participate in weak π-π stacking interactions, which further contribute to the stability of the compound (average distances and angles between their best mean planes are 3.510 A˚, 3.7° and 3.585 A˚, 5.8° for the phen molecules defined by N1,N2/N21,N22 and N11,N12/N31,N32, respectively) (Figure 2b). The angle between the planes defined by the two phen ligands bound to the same Pb(II) center is 37.3° and 41.4° for the two metal centers. In the lattice structure of 2, there is an extensive network of hydrogen bonds between the solvate water molecules and the carboxylato oxygen atoms, which result in the formation of a 2D lattice network extending parallel to the crystallographic ab plane (Figure 2c, Table 3). The molecular structure of complex 3 reveals a 2D coordination polymer. Compound 3 crystallizes in the monoclinic space group P2/c. The asymmetric unit contains two Pb(II) ions, each sitting on a 2-fold axis of symmetry, one-half phen molecule, also sitting on the 2-fold axis of symmetry passing through the Pb(1) ion and also crossing the central aromatic ring, and two half fumarato(-2) ligands, each sitting on

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2-fold axes of symmetry passing through the middle of the CdC bond. As shown in Figure 3a, Pb(1) ions are bridged through fumarato ligands forming chains parallel to the main diagonal of the (010) plain ([101] crystallographic direction). Pb(2) ions are also bridged through fumarato ligands along the same crystallographic direction. Pb(1), in addition to four carboxylato oxygen atoms from four different fumarato(-2) ligands, is also bound to two nitrogen atoms of the phen molecule. As a result, the coordination number around Pb(1) is six. The repeating unit along the chains containing the Pb(1) ions is Pb-fumarate-phen {type-A chains: [Pb-fumaratephen]n}. Pb(2) exhibits a coordination number eight, formulated by carboxylato oxygen atoms from six different fumarato(-2) ligands. The repeating unit along the chains containing the Pb(2) ions is Pb-fumarate {type-B chains: [Pb-fumarate]n}. Thus, Pb(1) and Pb(2) present a N2O4 and O8 coordination environment, respectively (Table 2). The two fumarato(-2) ligands adopt different coordination modes (Scheme 1) upon binding to the two Pb(II) metal ion centers; specifically, one adopts a μ4-κO:κO0 :κO00 :κO000 coordination mode, utilizing its two carboxylato moieties to bridge two Pb(1) and two Pb(2) ions in a monodentate fashion (Pb(1)-O(11) = 2.556(3) A˚, Pb(2)-O(12) = 2.439(3) A˚), thereby promoting linkage of type-A and type-B chains (Figure 3a). The second one adopts a μ6-κ2O:κ2O0 :κ2O00 :κ2O000 coordination mode, utilizing its two carboxylato moieties to bridge two Pb(1) and four Pb(2) ions in a chelating bis(bridging) asymmetric fashion (Pb(1)-O(21) = 2.684(3) A˚, Pb(2)-O(21) = 2.589(3) A˚, Pb(2)-O(2200 ) = 2.801(4) A˚, and Pb(2)-O(22) = 2.853(3) A˚). O(21) and O(22) are also linking type-A and type-B chains (Figure 3a). Figure 3b is a view of the structure along the [-1, 0, -1] crystallographic direction and shows the arrangement of type-A and type-B chains within the 2D layer formed by them. Symmetrically equivalent chains along the normal direction are related through translational symmetry and along the horizontal direction through centers of symmetry. As a result of the fumarato coordination, 14-membered macrometallocycles are formed. Analogous coordination modes of fumarate ligand were first shown in the case of [Pb(fum)]n.27b It is worth pointing out that the two adjacently located fumarate ligands binding abutting Pb(II) ion centers are distinct in the conformation that they adopt. To this end, the two carboxylate anchors in one of the two ligands define planes the dihedral angle of which is 18.3o, whereas the corresponding carboxylate group planes of the other fumarate ligand exhibit a dihedral angle of 74.9o. Ostensibly, there is a pronounced difference between the two fumarate ligands, which upon binding to the Pb(II) centers twist their terminal carboxylates so as to fulfill the coordination requirements of the Pb(II) ions in a distinct fashion. In the lattice of 3, two Pb(2) atoms are linked by two O(22) anchors from fumarate ligands, thereby forming planar four-membered rings Pb(2)-O(22)-Pb(2)-O(22) (Pb(2) 3 3 3 Pb(2**) = 4.700(1) A˚ (**) = 1 - x, -y, -z). The Pb2O2 units are arranged in a zigzag fashion inside the 2D layer (Figure 3c). The closest Pb 3 3 3 Pb interatomic distances are Pb(1) 3 3 3 Pb(2) = 3.300(3) A˚ (& = -1 þ x, y, z) through the μ4κO:κO0 :κO00 :κO000 fumarato(-2) ligand and Pb(1) 3 3 3 Pb(2) = 4.418(1) A˚ through the μ6-κ2O:κ2O0 :κ2O00 :κ2O000 fumarato(-2) ligand. The so-formed 2D network extends along the crystallographic ac plane (Figure 3c). The presence of weak π-π interactions between the aromatic rings of the phen molecules contributes to the stability of the compound through the formation of an overall 3D lattice structure (Figure 3d).

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Gabriel et al. Scheme 1. Coordination Modes of the Fumarato(-2) Ligand in Compounds 1-4

Figure 3. (a) Partially labeled plot of a very small fragment of the 2D structure of [Pb2(C12H8N2)(C4H2O4)2]n (3) with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity. Type-A and type-B chains described in the text are indicated by arrows. Primed atoms are generated by symmetry operations: (0 ) = -x, y, 0.5 - z; (00 ) x, -y, 0.5 þ z; (000 ) = 1 - x, y, 0.5 - z; (*) = 1 þ x, y, z; (**) = 1 - x, -y, -z; (***) = -1 þ x, y, -1 þ z; (#) = -1 - x, y, -0.5 - z; (##) = x, y, -1 þ z; (%) = -x, y, -0.5 - z; (&) = -1 þ x, y, z; (@) = -x, -y, -z. (b) Arrangement of type-A and type-B chains within the 2D layer. The view is along the chain direction. (c) Plot of the 2D network of 3 extending along the bc plane. (d) Structure presentation of 3 showing π-π interactions between phen ligands in the arising 3D lattice.

The mean planes of the phen molecules belonging to different 2D networks are parallel to each other, and the average distance between the mean planes is 3.308 A˚. The molecular lattice structure of complex 4 reveals a 2D coordination polymer. Compound 4 crystallizes in the triclinic space group P1. The asymmetric unit contains one Pb(II) ion, one phen ligand, two half fumarato(-2) ligands, each sitting on an inversion center in the middle of the CdC bond, and two lattice water molecules (Figure 4a). Each Pb(II) metal ion is six-coordinate and bound to the two nitrogen atoms of the phen ligand and to four carboxylato oxygen atoms from three fumarato(-2) ligands (Figure 4a, Table 2). There are two fumarato ligands, each of which resides on the crystallographic inversion center, exhibiting different coordination modes (Scheme 1). One ligand adopts a μ2-κO:κO0 coordination mode, utilizing its two carboxylato groups to bridge two symmetry related Pb(1) ions, through translation along the [-1 1 0] crystallographic direction, in a monodentate fashion (Pb(1)-O(31) = 2.454(5) A˚). The second one adopts a μ4-κ2O:κ2O0 :κO00 :κO000 coordination mode, utilizing its two carboxylato moieties to bridge four symmetry related Pb(1) ions in a chelating bridging asymmetric fashion (Pb(1)-O(21) = 2.379(5) A˚, Pb(1)-O(22) = 2.656(5) A˚, and Pb(1)-O(220 ) = 2.768(6) A˚). Two Pb(1) atoms are linked by two O(22) atoms, forming planar fourmembered rings Pb(1)-O(22)-Pb(10 )-O(220 ). The closest Pb 3 3 3 Pb interatomic distances are Pb(1) 3 3 3 Pb(10 ) = 4.368(1) A˚ (0 = -x, 1 - y, -z) and Pb(1) 3 3 3 Pb(1*) = 7.710(1) A˚ (* = -1 þ x, y, z) through the bridging μ4-κ2O:κ2O0 :κO00 :κO000 fumarato ligand. The second fumarato ligand bridges the two Pb(1) 3 3 3 Pb(1*) ions along the a-axis. Pb(1) ions form a 2D network extending parallel to the crystallographic ab plane (Figure 4b). The 2D network of 4 can be alternatively described as generated by dinuclear units containing a Pb2O2

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Figure 4. (a) Partially labeled plot of a very small fragment of the 2D structure of [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4) with thermal ellipsoids at the 30% probability level. H atoms have been omitted for clarity. Primed atoms are generated by symmetry operations: (0 ) = -x, 1 - y, -z; (00 ) = 1 - x, -y, -z; (000 ) = -1 - x, 1 - y, -z; (*) = -1 þ x, y, z. (b) Plot of the 2D network of 4 extending parallel to the ac plane.

core, which is very common in Pb(II)-O compounds. On each Pb ion center of the Pb2O2 core, a phen ligand is attached, thereby forming a Pb2O2(phen)2 unit. These units are linked through the first fumarato ligand along [-1 1 0] and through the second fumarato ligand along the a axis crystallographic direction, thus forming a 2D network, as clearly seen in Figure 5a. The dinuclear units are linked through the two different fumarato(-2) ligands and form 28-membered macrometallocycles. Above and below these cavities, the water solvate molecules are hung via hydrogen bonding interactions (Figure 5a, Table 3). The presence of weak π-π interactions between the aromatic rings of the phen molecules contributes to the stability of the 2D network through formation of an overall 3D lattice structure (Figure 5b). The mean planes of the phen molecules belonging to different 2D networks are parallel to each other, and the average distance between the mean planes is 3.403 A˚. The Pb-O bond distances in 1, 2, 3, and 4 are quite similar and analogous to those observed in other complexes, such as [Pb(NNO)(FA)0.5] (2.483(8)-2.586(8) A˚)26 (NNO = nicotinic acid N-oxide, FA = fumaric acid), {[Pb2(fum)2(H2O)4] 3

2H2O}n (2.517(5)-2.858(5) A˚),27b [Pb(fum)]n (2.399(5)-2.811 (5) A˚),27b [Pb(C4H4O4)] (2.44(1)-2.91(1) A˚),28 Pb2(phen)4(C4H4O4)(NO3)2 (2.490(3)-2.638(3) A˚),29 and [Pb(C6H6O7)]n (2.397(7)-2.847(1) A˚).30 The Pb-N bond distances in 1, 2, 3, and 4 are also similar to the corresponding distances in other complexes, such as Pb2(phen)4(C4H4O4)(NO3)2 (2.508(3)2.715(3) A˚).29 FT-IR Spectroscopy. The FT-infrared spectra of 1-4 in KBr revealed the presence of vibrationally active carboxylate groups. Antisymmetric as well as symmetric vibrations for the carboxylate groups of the coordinated fumarato(-2) ligands were present in all cases of the above compounds. Specifically, antisymmetric stretching vibrations νas(COO-) for the carboxylate carbonyls appeared in the range 15531516 cm-1 for 1, 1565-1470 cm-1 for 2, 1540 cm-1 for 3, and 1573-1526 cm-1 for 4. Symmetric vibrations νs(COO-) for the same groups appeared in the range 1422-1320 cm-1 for 1, 1422-1360 cm-1 for 2, 1381 cm-1 for 3, and 14011361 cm-1 for 4. The frequencies of the observed carbonyl vibrations were shifted to lower values in comparison to the corresponding vibrations in free fumaric acid, indicating changes

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Figure 5. Plot of the 2D network of 4 showing the 28-membered macrometallocycle in 4 and hydrogen bonding interactions (dashed lines): (a) looking down the c-axis and a-axis. Color code as in Figure 2. Carbon atoms of the phen molecules and all hydrogen atoms have been omitted for clarity. (b) Structure presentation of 4 showing π-π interactions between phen ligands in the arising 3D lattice.

in the vibrational status of the fumarate ligand upon binding to the Pb(II) ion. Confirmation of this contention came from the X-ray crystal structures of 1, 2, 3, and 4. NMR Spectroscopy. Solid state CP-MAS 13C NMR spectra were obtained on a Varian 400 MHz spectrometer operating at 100.53 MHz. In each case, a sufficient sample quantity was placed in a 3.20 mm rotor. The spectra of complexes 1-4 were recorded with 1000 scans, using a 90° pulse width of 5.80 μs for 1 and 4 and 5.15 μs for 3, a contact pulse of 3.70 ms for 1 and 4 and 2.00 ms for 3, and a recycle delay of 20 s for 1, 8 s for 3, and 10 s for 4. The 13C CP-MAS-NMR spectra of complexes 1-4 are in good agreement with the coordination of fumaric acid and phen around Pb(II) ion. The spectrum of 1 exhibits a pattern of resonances in the middle field and a peak in the low field area (Figure 6). The resonances in the middle field appear around 121.6-146.4 ppm and suggest coordination of the phen ligand around Pb(II). The resonance for the CdC double bond carbons of the fumarate ligand appears also in the same range, and thus, it is difficult to discern. Finally, in the low field region, where the carbonyl carbon resonances are expected to appear, there is a resonance at 167.8 ppm corresponding to the fumarate carboxylate group bound to the Pb(II) ion.

In the spectrum of 3, a pattern of resonances appears in the middle field and two resonances in the low field region (Figure 7). The resonances in the middle field appear around 120.3-149. 6 ppm and are congruent with the coordination of the phen ligand around Pb(II). The resonance for the CdC double bond carbons of the fumarate ligand appears also in the same range, and thus, it is difficult to discern. Finally, in the low field region, where the carbonyl carbon resonances are expected to appear, there are two resonances in the range 168.8-173.4 ppm, attributed to the fumarate carboxylate groups bound to the Pb(II) ion. The fact that there are two resonances in the low field region likely suggests the distinctly different mode through which the two carboxylate groups are coordinated to the Pb(II) ions in the lattice. The spectrum of 4 exhibits a pattern of resonances in the middle field and two resonances in the low field area (Figure 8). The resonances in the middle field show up around 124.3-149. 7 ppm and are in line with the introduction of the phen ligand in the coordination sphere of Pb(II). The resonance for the CdC double bond carbons of the fumarate ligand appears also in the same range, and thus, it is difficult to discern. Finally, in the low field region, where the carbonyl carbon resonances are expected to appear, there are two

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with the structural composition of all compounds investigated and have been confirmed by X-ray crystallography. Discussion

Figure 6. CP-MAS state.

13


13


13

C NMR spectrum of complex 1 in the solid



resonances in the 175.7-181.9 ppm range, attributed to the fumarate carboxylate groups bound to the Pb(II) ion. The fact that there are two resonances in the low field region very likely reflects the different mode through which the two carboxylate groups are coordinated to the Pb(II) ions. All of the resonances lie at lower fields compared to the free fumaric acid or phen. The aforementioned observations are consistent

Diversity in Ternary Pb(II) System Chemical Reactivity. In an attempt to delineate the diverse chemical reactivity of Pb(II) toward dicarboxylic acids and unravel details of that metal ion’s ability to assemble variably configured solid-state lattices with unique structural and physicochemical properties, the aqueous synthetic chemistry of lead, Pb(II), was investigated in ternary systems encompassing three different dicarboxylic acids and phen. The investigation showed that even though three different dicarboxylic acids were used in the synthetic processes, the isolated compounds contained the same ligand, i.e. fumarate. One of the four compounds isolated was a cationic dinuclear Pb(II) assembly with nitrate being the counterion (i.e., 1), whereas the other three species were polymeric compounds (i.e., 2-4) of discrete complexes acting as repeating units. In the case of compound 1, it appears that Pb(II) facilitated the conversion of malic acid, an R-hydroxycarboxylic acid, to fumaric acid, through dehydration, with the final complex having the structure [Pb2(C12H8N2)4(C4H2O4)](NO3)2 (1). In the case of compound 2, the presence of Pb(II) in the reaction mixture contributed to the transformation of maleic acid, a cis-isomer, into fumaric acid, the trans-isomer, of the same ligand, leading to the formation of [Pb2(C12H8N2)4(CO3)(C4H2O4)]n 3 6nH2O (2). In the case of compound 3, the presence of Pb(II) in the reaction mixture had no effect on the employed dicarboxylic acid. To this end, fumaric acid stayed intact and coordinated Pb(II) in a discrete fashion, leading to the ultimately isolated polymeric compound [Pb2(C12H8N2)(C4H2O4)2]n (3). Finally, in the case of compound 4, in the presence of Pb(II), maleic acid turned into fumaric acid, the trans-isomer, of the same ligand, which in turn bound Pb(II), thereby giving rise to the polymeric lattice in [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4). It is worth pointing out that, under hydrothermal conditions, the absence of Pb(II) from reaction mixtures of the same composition as the ones that led to the isolation of compounds 1-4 did not result in the transformation of all employed dicarboxylic acids to fumaric acid. In all four cases, a 1:1 molar ratio of Pb(II)/phen was employed in the synthetic reaction mixtures. However, this molar stoichiometry is reflected only in the case of compound 4, whereas the 1:2 and 2:1 stoichiometries are found in the case of compounds 1, 2, and 3, respectively. As mentioned previously, 1 contains the [Pb2(phen)4(fum)]2þ unit with NO3- counterions, and it can be looked at as a dinuclear complex, whereas 2 contains the [Pb2(C12H8N2)4(CO3)(C4H2O4)] unit, resulting in the growth of the lattice in one direction and the formation of a 1D coordination polymer. The presence of carbonate in the structure of 2 signifies (a) the importance of that inorganic anion acting as a factor in dictating structural arrangements in the assembly of the lattice of 2, and (b) the resilience of the dinuclear unit in 1, affording ample room in its molecular space to accommodate carbonate and turn into 2. The source of carbonate has not been unequivocally determined, yet its presence in 2 coincides with the employment of Pb(CH3COO)2 in the reaction mixture, leading to the synthesis of 2. At this juncture, it is worth emphasizing the fact that use of Pb(NO3)2 reacting with phen and maleic acid under basic conditions leads

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to the formation of 1, whereas use of Pb(CH3COO)2 under the same composition, stoichiometry, and reaction conditions leads reproducibly to the assembly of a polymeric lattice comprised of the carbonate-bridged ternary Pb-phen-fum dinuclear units encountered in 1. In this sense, carbonate acts as a “glue”, bridging the two Pb(II) centers and, in so doing, gives rise to the 1D polymeric lattice in 2. The other two complexes are 2D coordination polymers. The third compound [Pb2(C12H8N2)(C4H2O4)2]n (3) is a polymer with two different Pb(II) centers, which exhibit different coordination environments, i.e. N2O4 and O8 for Pb(1) and Pb(2), respectively. The fourth compound [Pb(C12H8N2)(C4H2O4)]n 3 2nH2O (4) is a hydrate polymer and contains a Pb(II) ion with an N2O4 coordination geometry. The coordination versatility of the fumarate ligand is reflected in the five different coordination modes observed in compounds 1-4 (Scheme 1). The fumarate ligands behave as μ2 ligands in 1, 2, and 4, μ4 ligands in 3 and 4, and μ6 ligands in 3. Its two carboxylato moieties behave as (a) symmetric or asymmetric chelating agents, (b) monodentate or syn,syn bridging agents, and (b) bis-bridging chelating or bridging chelating agents, demonstrating their ability to bind two or more metal ions. Consequently, the versatile coordination modes exhibited by fumarate promote polynuclear metal cluster formation and give rise to multidimensional network topologies. It remains to be rationalized through further research how the variably employed reaction conditions lead to distinctly defined lattices containing the same fumarate ligand (vide infra). Pb(II)-Dicarboxylate Coordination Diversity. It has been suggested that the coordination geometry of PbOn polyhedra in Pb(II) compounds can be described as holo- and hemidirected.31-34 It has been found through ab initio molecular orbital calculations on Pb(II) complexes, in the gas-phase, that a hemidirected geometry emerges if the employed ligands are hard, the ligand coordination number is low, and attractive interactions prevail between the ligands.34,35 On the basis of the aforementioned grounds, the six coordinate Pb(II) centers in 1 exhibit a hemidirected geometry. For the PbO8 polyhedra in 2, a holodirected geometry is exhibited. The six-coordinate Pb(1) centers and the eight-coordinate Pb(2) metal ions in 3 exhibit hemi- and holodirected geometries, respectively. Finally, the six-coordinate Pb(1) centers in 4 exhibit a hemidirected geometry. Lattice Formation through Metal-Assisted Ligand Transformations. Novel compounds with in situ formed ligands are of great interest in synthetic coordination chemistry and organic chemistry due to (a) their potential for discoveries of new organic synthetic reactions and (b) deep insight that they offer for understanding organic synthetic mechanisms.36,37 Even though some such complexes were prepared by traditional synthesis methods,38 the hydro(solvo)thermal method has demonstrated in this particular case (and others) an increasing success in revealing alternative pathways to the synthesis of binary and ternary Pb(II) complexes, reflecting in situ ligand syntheses. To this end, the employed synthetic approaches not only provide a powerful synthetic method for organic ligands that cannot normally be obtained by traditional methods but also represent a potential new direction for novel metal coordination compounds.39-45 Examples involving in situ ligand syntheses under hydrothermal conditions include hydrolyses of R,β-diketone,38 nitrile and ester groups,39 hydroxylation,40,41 carbon-carbon bond formation through reductive coupling or oxidative coupling, formation of triazoles

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and tetrazoles by cycloaddition,43,44 replacement of carboxylates by sulfonic groups,46 and alkylation.47 The fact that malic and maleic acids, which were initially employed in the investigation of the aqueous chemistry of Pb(II), turned into fumaric acid is not a frequent phenomenon in the synthetic coordination chemistry of metal ions (Scheme 2). Two cases have been reported in the literature, where such chemical reactivity is observed. The first one was in the case of Cd(II), where the metal ion interacted with maleic acid and the finally arising lattice contained the compound with the molecular formula [Cd(fum)(phen)].48 The second example was in the case of V(IV), where the starting ligand was malic acid and the final lattice contained the compound with the molecular formula {[V(phen)3](fum)}(OH)2 3 10H2O.48 A plausible reason for the observation is associated with the metal-ligand reactivity, with the interconversion taking place in a highly basic environment. Another reason may have to do with the nature of the metal, in view of the fact that when other metals, such as Zn(II), Co(II), and Fe(II), were employed in a highly basic environment, the same phenomenon was not observed.48 On the basis of the in situ metal-assisted ligand transformation reactions that occurred during the syntheses of compounds 1-4, and those reported for other metal coordination compounds in the literature, the following observations can be made (Scheme 3): (a) both maleic acid and fumaric acid can in situ hydrate to form malic acid under weakly basic conditions in the presence of Zn(II)/Co(II)/ Fe(II);48 (b) maleic acid can in situ transform into its transisomer fumaric acid in the presence of Cd(II) under weakly basic conditions and under basic conditions in the presence of Pb(II) (for compounds 2 and 4); (c) malic acid can in situ undergo intramolecular dehydration to form fumaric acid in the presence of V(IV) under acidic conditions and in the presence of Pb(II) under basic conditions (compounds 1 and 3). The inter-relation of all these in situ reactions is summarized in Scheme 3, from which specific observations can be pondered over: (a) A potential mechanism of ligand isomerization could be explained by the fact that malate undergoes dehydration to form maleate, followed by intramolecular isomerization of maleate to form fumarate. No product, however, was observed when the employed metal ion reacted with malic or maleic acid directly. (b) In situ syntheses of organic ligands favor formation of metal coordination compounds. (c) And the metal ion plays an important role in the in situ ligand transformation. In-depth studies targeting the details of the above observations and reflecting sound conclusions on the diverse chemical reactivity observed for Pb(II) are a matter of ongoing investigation in our lab. Ostensibly, the nature of the metal ion affects strongly the in situ ligand synthetic process and generates favorable conditions under which in situ ligand synthesis is further linked to the formation of new binary and ternary Pb(II) lattices. Metal-Organic Framework 1D-3D Lattices. The construction of discrete and polymeric metal-organic complexes is currently attracting considerable attention worldwide, in view of their interesting structural topologies and properties.8,49 In this regard, much progress has been made on the design and synthesis of novel coordination frameworks (involving carboxylate as well as phosphonate ligands) and the relationships between their structures and properties.50,51 Generally, two different types of interactions (covalent bonds and noncovalent intermolecular forces) can be used to construct

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Scheme 2. Metal-Assisted Ligand Transformation and Complex Formation in 1-4

Scheme 3. Schematic Representation of Known in situ Metal-Carboxylic Acid Reactions Involving Fumaric Acid

directing the assembly of supramolecular architectures. Also, conjugated π-systems that strongly influence the physical properties of final products are of considerable interest in coordination compounds.53 Therefore, the design of versatile organic ligands, which are capable of coordinating a metal ion, while concurrently providing a π-conjugated system necessary for the assembly of extended network lattices through π-π interactions, is quite desirable. To date, phen has been widely used to construct supramolecular architectures, owing to its excellent coordinating ability and extended conjugated system that can easily promote π-π interactions. In this regard, specific types of 1D, 2D, or 3D supramolecular architectures assembled from Pb(II) centers with dicarboxylates and phen were successfully isolated. The employment of phen in binary and ternary Pb(II) aqueous systems in the work described herein proves the above contention and exemplifies the ground principles upon which rest synthetic efforts in our lab to construct solid-state lattices possessing (a) unique structural features riding on metal-assisted ligand transformation and concomitant complex formation, and (b) physicochemical properties. Conclusions

diverse supramolecular architectures. To date, considerable research effort has focused on controlling motifs of metal-organic complexes through coordination bonds, whereas relatively less attention has been given to noncovalent π-π interactions.52 π-π interactions can be one of the most powerful forms of noncovalent intermolecular interactions,

pH-specific hydrothermal synthetic reactions of malic, phen, maleic, and fumaric acids with Pb(II) salts afforded four new metal-organic framework species 1-4. Compound 1 is a dimer of Pb(II), fumarate, and phen, whereas complex 2 is a 1D coordination polymer based on the dinuclear unit of 1 linked through bridging carbonato(-2) ligands. The other two compounds are 2D polymers based on dinuclear and

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mononuclear repeating units extending to three-dimensional assemblies through π-π interactions. The investigated chemical reactivity of Pb(II) at the ternary level toward the three (hydroxy)carboxylic acids revealed a metal-linked transformation process turning malic and maleic acids into fumaric acid. The nature of the metal, Pb(II), may be a significant reason for which the starting ligands transformed finally into fumaric acid, bound to the metal center, and promoted formation and assembly of lattices in 1-4. All compounds (a) display unique crystal lattice composition, (b) support the idea of metal-assisted transformation of the initial ligand to its final metal-bound form, (c) exhibit structural features reflecting the significance of π-π interactions in assembling 3D lattice architectures, and (d) exemplify the diversity of the lattice structure, composition, and properties originating in similar binary and ternary aqueous systems of the same metal ion (i.e., Pb(II)), reacting under specific conditions. As a consequence of the present work, further (a) perusal targeting the rationalization of the diverse chemical reactivity observed for Pb(II) in the presence of organic (hydroxyl)carboxylic acids, leading to metal-assisted ligand transformation and synthesis, (b) in-depth understanding of the factors governing metal-linked (poly)carboxylic acid promotion of lattice formation through metal complexation, and (c) research on rational design and construction of binary and ternary Pb(II) π-π containing lattices, displaying unique supramolecular structures as well as physicochemical properties (e.g., luminescence), are currently underway in our laboratory.

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Acknowledgment. This work was supported by a “PENED” grant and by PEP Attikis 2000-2006 Program ATT-28 from the General Secretariat of Research and Technology, Greece. Supporting Information Available: X-ray crystallographic information files (CIF) and detailed tables of bond distances and angles for compounds 1-4. This information is available free of charge via the Internet at http://pubs.acs.org.

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