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Ligand-Induced Variations in Symmetry and Structural Dimensionality of Lead Oxide Carboxylates Elaine E. Liu, Calvin Gang, Matthias Zeller, Douglas H Fabini, and Catherine M. Oertel Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01558 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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Crystal Growth & Design

Ligand-Induced Variations in Symmetry and Structural Dimensionality of Lead Oxide Carboxylates Elaine E. Liu,† Calvin Gang,† Matthias Zeller,‡ Douglas H. Fabini,¥ Catherine M. Oertel†,* †Department of Chemistry and Biochemistry, Oberlin College, 119 Woodland St., Oberlin, OH 44074 ‡Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907 ¥ Materials Research Laboratory and Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106 Abstract: Four new lead oxide carboxylates have been prepared through solvothermal reactions between lead oxide and substituted benzoic acids. In these extended inorganic hybrids, edgesharing Pb4O tetrahedra form 1-D inorganic substructures that are further coordinated by the carboxylate ligands. In two isostructural phases, Pb2O(C7H4(CH3)O2)2·0.25(H2O) and Pb2O(C7H4BrO2)2·0.23(C2H5OH), distorted Pb4O tetrahedra produce helical Pb2O2+ chains. The 1-D chains in these structures are coordinated by p-toluate and 4-bromobenzoate ligands, respectively. Right- and left-handed helices are related through inversion, and the structures crystallize in the centrosymmetric I41/a space group. In Pb2O(C7H4O3), linear Pb2O2+ chains are linked into 2-D layers by salicylate ligands, resulting in the achiral, polar space group Fdd2. In Pb3O2(C7H3FO3)·0.71(H2O), linear Pb3O22+ double chains are linked into layers by 4-fluorosalicylate ligands. The compound crystallizes in the chiral, non-centrosymmetric space group P21. For each of the ligands in the new structures as well as in four previously reported structures, a ratio of the non-coordinating to coordinating volumes has been calculated. This ligand volume ratio shows a clear inverse correlation with the overall dimensionality of the lead oxide carboxylates. The analysis encompasses 1- to 3-D structures and ligands of differing denticities. Structures were determined through single crystal X-ray diffraction and the new compounds further characterized via powder X-ray diffraction, infrared Ratios of non-coordinating to coordinating volumes for carboxylate ligands and designation of overall structural dimensionalities of resulting extended hybrid structures. Values shown are based on crystallographically determined atomic coordinates from the current work and from four previously reported structures. spectroscopy, thermogravimetric analysis, and elemental analysis. *Catherine M. Oertel Department of Chemistry and Biochemistry 119 Woodland St. Oberlin College Oberlin, OH 44074 (440) 775-8989 1 ACS Paragon Plus Environment


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fax: (440) 775-6682 [email protected]

2 ACS Paragon Plus Environment

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Ligand-Induced Variations in Symmetry and Structural Dimensionality of Lead Oxide Carboxylates Elaine E. Liu,† Calvin Gang,† Matthias Zeller,‡ Douglas H. Fabini,¥ Catherine M. Oertel†,*

†Department

of Chemistry and Biochemistry, Oberlin College, 119 Woodland St., Oberlin, OH 44074

‡Department

¥

of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907

Materials Research Laboratory and Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106

Abstract: Four new lead oxide carboxylates have been prepared through solvothermal reactions between lead oxide and substituted benzoic acids. In these extended inorganic hybrids, edgesharing Pb4O tetrahedra form 1-D inorganic substructures that are further coordinated by the carboxylate ligands. In two isostructural phases, Pb2O(C7H4(CH3)O2)2·0.25(H2O) and Pb2O(C7H4BrO2)2·0.23(C2H5OH), distorted Pb4O tetrahedra produce helical Pb2O2+ chains. The 1-D chains in these structures are coordinated by p-toluate and 4-bromobenzoate ligands, respectively. Right- and left-handed helices are related through inversion, and the structures crystallize in the centrosymmetric I41/a space group. In Pb2O(C7H4O3), linear Pb2O2+ chains are linked into 2-D layers by salicylate ligands, resulting in the achiral, polar space group Fdd2. In Pb3O2(C7H3FO3)·0.71(H2O), linear Pb3O22+ double chains are linked into layers by 4-fluorosalicylate



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ligands. The compound crystallizes in the chiral, non-centrosymmetric space group P21. For each of the ligands in the new structures as well as in four previously reported structures, a ratio of the non-coordinating to coordinating volumes has been calculated. This ligand volume ratio shows a clear inverse correlation with the overall dimensionality of the lead oxide carboxylates. The analysis encompasses 1- to 3-D structures and ligands of differing denticities. Structures were determined through single crystal X-ray diffraction and the new compounds further characterized via powder X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and elemental analysis.

Introduction

Extended inorganic hybrids are inorganic-organic hybrid materials that include extended inorganic bonding in one or more dimensions.1 In contrast to coordination polymers and metalorganic frameworks in which metal centers are separated by organic ligands, extended inorganic hybrids contain metal atoms bridged by single atoms, leading to infinite M-X-M arrays. The bridging atom, X, is frequently a terminal atom in an oxygen-donating organic ligand or an oxide or halide anion.2-9 Further coordination by organic ligands completes the metal coordination spheres and may also impart additional dimensions of connectivity. The properties of extended inorganic hybrids are influenced not only by their overall dimensionality but by the contributions of inorganic and organic linkages to the total dimensionality. The inorganic substructures resemble inorganic compounds such as binary oxides and can exhibit properties including magnetic ordering, electrical conductivity, and second harmonic generation.2,10,11 Distinct types of bonding in different crystallographic directions are also reflected in anisotropy of mechanical properties such as hardness and elastic modulus.12,13 Several variables determine the dimensional contributions within extended inorganic hybrids. Careful studies of the influences of synthetic conditions including pH, solvent composition, and 4 ACS Paragon Plus Environment

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reaction time and temperature have been reported for the cobalt succinate and magnesium-1,3-5benzenetricarboxylate systems.14-16 For the related class of metal-organic frameworks, the denticities of ligands and the angles between coordinating groups for multitopic ligands have been related to the topologies of the resulting compounds.17-23 However, another distinguishing aspect of shape even for ligands of the same denticity is the balance between coordinating and noncoordinating volumes. This parameter has not yet been considered in analyzing the structures of hybrid materials. Lead oxide carboxylates are extended inorganic hybrids in which edge-sharing Pb4O tetrahedra make up infinite one-dimensional inorganic substructures. In the five examples previously reported, the extent of condensation of the tetrahedra varies to produce Pb4O32+ slabs, Pb3O22+ double chains, and both linear and helical Pb2O2+ single chains.24-26 These inorganic components are further coordinated by carboxylate ligands to form structures with overall dimensionalities ranging from 1- to 3-D. The identity of the carboxylic ligand has been seen to change the connectivity and topology of the inorganic substructure as well as the overall symmetry, but the key ligand properties in influencing the structure have not been identified. In order to probe the role of ligand properties in the connectivities and symmetries of lead oxide carboxylates, compounds have been prepared using functionalized benzoate ligands. We report four new structures: lead oxide p-toluate [Pb2O(C7H4(CH3)O2)2·0.25(H2O)](1), lead oxide 4bromobenzoate [Pb2O(C7H4BrO2)2·0.23(C2H5OH)] (2), lead oxide salicylate [Pb2O(C7H4O3)] (3), and lead oxide 4-fluorosalicylate [Pb3O2(C7H3FO3)·0.71(H2O)] (4). Each contains chiral substructures, though the arrangement of these moieties in the unit cells leads to different overall symmetries. For these and the previously reported lead oxide carboxylate structures, we propose a volume ratio of non-coordinating and coordinating portions of each carboxylate as a means of quantifying ligand shape. We have considered this metric along with ligand acidity to determine the role of each in influencing structural dimensionality.



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Experimental Section General Information. PbO (Aldrich), p-toluic acid (Aldrich), 4-bromobenzoic acid (Aldrich), salicylic acid (Aldrich), 4-fluorosalicylic acid (Aldrich), and absolute ethanol were used as received. Hydrothermal reactions were carried out in 23-mL Teflon cups enclosed in stainless steel autoclaves (Parr). Instrumentation. Powder X-ray diffraction (PXRD) was carried out on a Rigaku Ultima IV diffractometer with CuKα radiation. Infrared spectra were collected using a Thermo-Mattson Satellite FT-IR spectrometer with a Specac Golden Gate Single Reflection ATR accessory. Thirty-two scans were accumulated with a resolution of 8 cm-1. Thermogravimetric analysis was carried out under air (50 mL/min) using a Mettler-Toledo TGA/DSC 1 STARe System. Samples were heated in alumina crucibles from ambient temperature to either 600 oC or 800 oC at a rate of 10 oC/min. Elemental analysis was conducted by Micro Analysis, Inc., Wilmington, DE. Measurement of Second Harmonic Generation. Second harmonic generation (SHG) activities were measured for polycrystalline samples using a modified Kurtz-NLO system with a 1064 nm pulsed Nd:YAG laser.27 Light at the second-harmonic frequency, 532 nm, was collected in reflection mode and detected with a photomultiplier tube equipped with a 532 nm narrow-bandpass filter. The intensity of 532 nm light was compared with that generated by a standard sample of polycrystalline α-SiO2. Synthesis of 1. Single crystals of Pb2O(C7H4(CH3)O2)2·0.25(H2O) were prepared by grinding PbO (0.45 g, 2.0 mmol) and p-toluic acid (0.14 g, 1.1 mmol) with an agate mortar and pestle. The powder was transferred to a 23 mL Teflon cup where ethanol (5 mL) and deionized water (5 mL) were added. The mixture was sealed in an autoclave and heated to 160 oC. The oven was held at temperature for 15 h and cooled at 0.5 oC/min. Single crystals were removed for diffraction experiments. Phase-pure powder of 1 was prepared by grinding together PbO (0.45 g, 2.0 mmol) 6 ACS Paragon Plus Environment

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and p-toluic acid (0.28 g, 2.0 mmol). The powder was transferred to a 23 mL Teflon cup and 15 drops of ethanol were added. The mixture was sealed in an autoclave and heated to 195 oC. The oven was shut off after 24 hours, and the autoclave assembly was allowed to cool gradually. The white powdered product did not require further drying. Phase purity was checked by comparison of the PXRD pattern of the solid with that calculated from the single crystal structure of lead oxide p-toluate (SI, Figure S5). Yield 0.70 g (97%). Elemental analysis and thermogravimetric analysis (vide infra) suggested that the bulk powder did not include the water of crystallization found in the single crystal structure. Predicted (found) for C64H56O20Pb8: C 27.4% (27.6%), H 2.0% (1.9%). IR (ATR, cm-1): 2917, 1607, 1582, 1507, 1369, 1175, 1017, 847, 766. Synthesis of 2. Single crystals of Pb2O(C7H4BrO2)2·0.23(C2H5OH) were prepared by grinding PbO (0.22 g, 1.0 mmol) and 4-bromobenzoic acid (0.20 g, 1.0 mmol) with an agate mortar and pestle. The powder was transferred to a 23 mL Teflon cup where ethanol (2 mL) deionized water (1 mL) and an aqueous solution of 0.1 M tetramethylammonium hydroxide (7 mL) were added. The mixture was sealed in an autoclave and heated to 210 oC. The oven was held at temperature for 15 h and cooled at 0.5 oC/min. Single crystals were removed for diffraction experiments. Phase-pure powder of 2 was prepared by grinding together PbO (0.22 g, 1.0 mmol) and 4-bromobenzoic acid (0.20 g, 1.0 mmol). The powder was transferred to a 23 mL Teflon cup and 35 drops of ethanol were added. The mixture was sealed in an autoclave and heated to 230 oC. The oven was held at temperature for 15 h and then cooled at 0.5 oC/min. The solid product was white with brown discoloration around the edge and did not require further drying. The white portion of the sample was physically separated from the discolored portion before yield measurement and characterization. Phase purity was checked by comparison of the PXRD pattern of the solid with that calculated from the single crystal structure of lead oxide 4-bromobenzoate (SI, Figure S6). Yield 0.24 g (58%). Elemental analysis and thermogravimetric analysis (vide infra) suggested that the bulk powder did not include the ethanol solvent of crystallization found in the single crystal



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structure. Predicted (found) for C14H8Br2O5Pb2: C 20.2% (19.6%), H 0.97% (0.60%). IR (ATR, cm1):

3058, 1581, 1513, 1367, 1168, 1135, 1066, 1009, 834, 767, 685.

Synthesis of 3. Single crystals of Pb2O(C7H4O3) were prepared by grinding PbO (0.67 g, 3.0 mmol) and salicylic acid (0.14 g, 1.0 mmol) with an agate mortar and pestle. The powder was transferred to a 23 mL Teflon cup where ethanol (8 mL) and deionized water (2 mL) were added. The mixture was sealed in an autoclave and heated to 225 oC. The temperature was held for 12 hours and the autoclave assembly was allowed to cool gradually in the oven. Single crystals were removed for diffraction experiments. In spite of many attempts, it has not been possible to isolate a phase-pure powder of 3. A sample containing 3 as its major phase was prepared by grinding together PbO (0.67 g, 3.0 mmol) and salicylic acid (0.14 g, 1.0 mmol). The powder was transferred to a 23 mL Teflon cup and 35 drops of ethanol were added. The mixture was sealed in an autoclave and heated to 180 oC. The oven was held at temperature for 15 h then turned off and allowed to cool naturally with the autoclave inside. The white powdered product did not require further drying. Phase composition was determined by comparing of the PXRD pattern of the solid with that calculated from the single crystal structure of lead oxide salicylate (SI, Figure S7). IR (ATR, cm-1): 3010, 1591, 1552, 1492, 1401, 1354, 1296, 1230, 1133, 867, 799, 756, 681, 654. Synthesis of 4. Single crystals of Pb3O2(C7H3FO3)·0.71(H2O) were prepared by grinding PbO (0.45 g, 2.0 mmol) and 4-fluorosalicylic acid (0.16 g, 1.0 mmol) with an agate mortar and pestle. The powder was transferred to a 23 mL Teflon cup where ethanol (2 mL) and deionized water (8 mL) were added. The mixture was sealed in an autoclave and heated to 180 oC. The temperature was held for 15 hours, and the autoclave assembly allowed to cool gradually in the oven. Single crystals were removed for diffraction experiments. Phase-pure powder of 4 was prepared by dissolving 0.33 g (1.0 mmol) of lead nitrate in 15 mL of deionized water. In a second vessel, 15 mL of deionized water and 10 mL 0.8 M aqueous potassium hydroxide were mixed, and 0.16 g (1.0 mmol) of 4-fluorosalicylic acid was dissolved in this solution.


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The two mixtures were combined and briefly stirred at room temperature. The product mixture was transferred to test tubes and isolated through centrifugation. The white product was washed three times with absolute ethanol and dried under vacuum in an Abderhalden drying pistol. Phase purity was checked by comparison of the PXRD pattern of the solid with that calculated from the single crystal structure of lead oxide 4-fluorosalicylate (SI, Figure S8). Yield 0.19 g (69%). Elemental analysis of the bulk powdered product was consistent with full occupancy by the water of crystallization, while occupancy of 0.71 was found in the single crystal experiment. Predicted (found) for C7H3FO5Pb3(H2O): C 10.2% (10.2%), H 0.61% (0.54%), F 2.3% (2.3%). IR (ATR, cm-1): 3569, 3018, 1604, 1557, 1490, 1421, 1348, 1298, 1254, 1150, 1127, 1089, 980, 849, 822, 779. Crystal Structure Determination. Single crystal X-ray diffraction at 100 K for 1 and at 296 K for 3 and 4 was performed on a Bruker AXS SMART APEX CCD system using MoKα laboratory radiation. Single crystal X-ray diffraction at 100 K for 2 was performed on a Bruker AXS D8 Quest CMOS diffractometer using MoKα radiation. For each dataset, initial unit cells were determined and data collected using Apex228 and data integration and unit cell refinement were performed using SAINT.29 Absorption corrections were performed analytically using face-indexing (2 and 3) or multi-scan methods (1 and 4) through either SADABS30 (1-3) or TWINABS (4).31 The structures were solved using direct methods and refined by full-matrix least-squares on Fo2 using SHELXL.32 All non-hydrogen atoms were refined anisotropically. Refinement was completed with hydrogen atoms constrained to ride on carrying atoms. Refinement data for the four compounds are summarized in Table 1. Table 1. Refinement Data for Compounds 1-4 data

1

2

3

4

empirical formula

C64H58O21Pb8

C14.46H9.37Br2O5.23 Pb2

C7H4O4Pb2

C7H4.42FO5.71Pb3

wavelength (Å)

0.71073

0.71073

0.71073

0.71073



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temperature (K)

100(2)

100(2)

296(2)

296(2)

crystal system

tetragonal

tetragonal

orthorhombic

monoclinic

space group

I41/a

I41/a

Fdd2

P21

a (Å)

25.637(3)

25.9762(9)

12.323(2)

7.379(3)

49.951(8)

5.672(2)

5.6202(10)

13.304(5)

b (Å) c (Å)

10.7104(14)

10.7689(4)

β (degrees)

94.648(7)o

V (Å3)

7039(2)

7266.5(6)

3459.6(10)

554.9(4)

Z

4

16

16

2

ρ (g·cm–3)

2.661

3.075

4.350

4.910

µ (mm–1)

19.130

22.931

38.855

45.413

θ range for data

3.050 – 31.404°

2.218 – 33.201°

3.263 – 31.423°

1.536 – 31.333°

5378 / 6 / 223

6954 / 27 / 238

2728 / 1 / 119

3285 / 136 / 146

99.7 %

99.9 %

99.9 %

100.0 %

GOF on F2

1.020

1.070

1.072

1.065

R indices [I > 2σ(I)]a

R1 = 0.0312,

R1 = 0.0256,

R1 = 0.0279,

R1 = 0.0405,

wR2 = 0.0697 R1 = 0.0446,

wR2 = 0.0396 R1 = 0.0381,

wR2 = 0.0537 R1 = 0.0338,

wR2 = 0.0961 R1 = 0.0538,

wR2 = 0.0747

wR2 = 0.0424

wR2 = 0.0553

wR2 = 0.1019

-0.007(14)

Not refinedb

1.017 and-1.476

2.644 and -2.497

collection data / restraints / parameters completeness

R indices (all data)a Flack parameter Largest diff. peak and hole (e·Å–3) a

2.707 and-1.463

1.207 and -1.962

R1 = ΣFo – Fc/ΣFo wR2 = [Σw(Fo2-Fc2)2/Σw(Fo2)2]1/2

bBASF

parameters for inversion twin components each refined to zero with a small esd, 0.001(1) and 0.009(4), indicating a correct absolute structure. Structures 1 and 2 are isostructural to one another and each contain disordered solvent molecules. In 1, two disordered water molecules summed to one per site. In 2, a partially occupied ethanol molecule was found around a four-fold rotoinverion axis. The thermal parameters of the 10 ACS Paragon Plus Environment

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three atoms were restrained with a rigid bond and a similarity restraint (RIGU 0.001, SIMU 0.02). Subject to these conditions, the occupancy of each site refined to 0.229(6), or 0.916(6) of an ethanol molecule for the site as a whole. Final diffraction data for 4 were collected at 296 K after a dataset collected at low temperature showed incommensurate modulation. Modulation was not observed at ambient temperature, but crystals were twinned by non-merohedry. The orientation matrices of the two twin components for the crystal under investigation were identified using the program Cell Now,29 with the two components being related by a 180o rotation around the reciprocal c-axis. Some constraints and restraints were applied in order to avoid chemically unreasonable geometrical parameters resulting from pseudosymmetry in the inorganic portion of the structure. The benzene ring was constrained to resemble an ideal hexagon with C-C distances of 1.39 Å. The distances of the carboxylate carbon atom to the two ortho benzene carbon atoms were restrained to be similar. All atoms were subjected to a rigid bond restraint. Oxygen atoms O4 and O5, related by pseudo-mirror symmetry, were constrained to have identical ADPs. The final refinement indicated absence of disorder and of inversion twin domains. The BASF values refined to 0.347(5), 0.001(1) and 0.009(4). Volume Calculations. Atomic coordinates for each ligand were extracted from crystallographically determined structures for 1-4 and the previously reported lead oxide carboxylates.24-26 To calculate the ligand volumes, spheres of van der Waals radii were placed at each atomic position, and the total volumes formed by the overlapping spheres of the coordinating and non-coordinating portions were calculated. Oxygen atoms were assigned as coordinating, and all other atoms were assigned as non-coordinating. Overlap between O and C atoms were handled by evenly dividing their overlapping volume between the coordinating and non-coordinating categories. Finally, the ratio of the non-coordinating to coordinating volumes was calculated for each ligand. The same process was repeated using a set of coordinates obtained for each ligand



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from geometries optimized using Gaussian 09 through the WebMO interface.33,34 Calculations were run using the Hartree-Fock method with the 6-31G(d) basis set. Calculations of Electron Localization Function. Calculations of the electron localization function (ELF) for 2-4 were performed with the Vienna Ab initio Simulation Package (VASP),35-38 which implements the Kohn–Sham formulation of density functional theory (DFT) using a plane wave basis set and the projector augmented wave formalism39,40. The disordered ethanol molecules in 2 were not taken into account in the calculation, and the occupancy of the water of crystallization in 4 was set to 1 in the input file. The generalized gradient approximation was employed using the revised exchange and correlation functional of Perdew, Burke, and Ernzerhof for solids (GGA– PBEsol).41 Electrons were included in the valence as follows: H, 1s1; C, 2s22p2; O, 2s22p4; F, 2s22p5; Br, 4s24p5; Pb, 6s25d106p2. The plane wave basis set cutoff energy (600 eV) and k-point mesh density (2: 1×1×3; 3: 3×1×7; 4: 5×5×3; all Monkhorst–Pack sampling)42 were chosen based on convergence of the Kohn–Sham energy. The crystal structures and ELFs were visualized using the VESTA suite of programs.43 Results and Discussion Synthesis. For each structure, single crystals were grown using solvothermal reactions between PbO and a carboxylic acid. These reactions were based upon our previous syntheses of lead oxide carboxylates, which were in turn modeled upon the corrosion reactions that occur when a carboxylic acid dissolves a PbO layer on a lead-rich object.25 Modified synthetic conditions were required to obtain phase-pure powders. A phase-pure sample of compound 4 was prepared through room-temperature precipitation from aqueous solution. For 1-2, pure samples were prepared through a slurry method using a minimal amount of solvent along with the reactant solids. At the reaction temperature for 1, above the melting point of p-toluic acid (180 oC),44 the ligand is thought to function as both a reactant and a flux. The melting point of 4-bromobenzoic acid is considerably higher at 255 oC.44 Above this temperature, the reactants for 2 decomposed to a powder that was identified through PXRD as Pb9O4Br10.45 At 230oC, slight discoloration around


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the edge of the reaction vessel occurred. The white portion of the sample was physically removed for further characterization. The PXRD pattern of this separated product showed an excellent match to the pattern calculated from the single crystal structure (SI, Figure S6). Elemental analysis resulted in larger-than-usual deviations from expected values, consistent with some remaining contamination by degradation products. At reaction temperatures below 230 oC, unreacted PbO remained, and an unidentified contaminant phase accompanied the desired product. For compounds 1, 2, and 4, bulk powders showed different extents of solvation than the single crystals used for structure determination. Comparison of the PXRD patterns of the powders with those calculated from the single crystal structures indicated that major structural changes did not accompany the differences in solvation. Despite many attempts involving modified solvothermal reactions, slurry reactions, roomtemperature precipitation, and crystal picking, a phase-pure sample of 3 has not been obtained. The product of a water slurry reaction of PbO and salicylic acid in a 2:1 stoichiometric ratio – matching that in the desired final product – was identified by PXRD as a mixture of 3 and lead salicylate.46 In order to promote formation of the lead oxide carboxylate, the reactant ratio was increased to 3:1. This change enriched the product mixture in 3, but a significant fraction of lead salicylate still remained. Unreacted PbO was visible as a red solid as part of the product mixture, indicating that further increasing the ratio of the oxide to the acid would not be a good strategy for eliminating formation of the simple salicylate. In the absence of a phase-pure powder of 3, observation of a powder pattern that can be accounted for as a combination of those of the new structure and a known compound corroborates the result of our single crystal diffraction experiment (SI, Figure S7). Thermal Analysis. The new compounds showed high thermal stability for hybrid materials, all undergoing major mass loss only around 400 oC. The mass remaining after heating of 1 to 800 oC was 62.7%, compared with a predicted 63.3% for decomposition to PbO. No mass loss steps for 1



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were identified under 300 oC, suggesting that the bulk powder did not contain water of crystallization. Compound 4 showed a small mass loss under 100 oC that was attributed to loss of water of crystallization. The mass remaining for 4 at 600 oC was 82.2% compared with 81.1% predicted for decomposition to PbO. Thermal behavior of the mixed-phase product rich in 3 is shown in the SI (Figure S12). For 2, no mass loss steps were identified under 300 oC, suggesting that the bulk powder did not contain ethanol solvent of crystallization. The decomposition product at 600 oC was identified by PXRD as Pb3O2Br2, crystallizing in the mendipite structure type.47 The mass remaining at 600 oC was 57.8%, markedly less than the 64.6% calculated for decomposition to Pb3O2Br2. The volatility of the PbO-PbBr2 system is, however, well-documented.48-50 The lower-than-expected remaining mass is consistent with loss of PbBr2 at elevated temperature. When heating was continued beyond 600 oC, the mass remained approximately constant up to 700 oC, above which it decreased further. This observation is consistent with the reported melting point of 709 oC51 for Pb3O2Br2 and the enhanced volatility of the PbO-PbBr2 melt above that temperature. The material remaining after cooling from 800 oC appeared to be a solidified melt and was fused to the alumina cup, preventing analysis by PXRD. Asymmetric Pb2+ Coordination Environments. Lead atoms in 1-4 are coordinated by oxide anions as well as oxygen atoms of the carboxylate ligands (Figure 1). Contacts between Pb and oxide anions are in the range of 2.06 – 2.41 Å, compared with 2.21 – 2.30 Å in the two polymorphs of PbO.52 The carboxylate oxygens coordinate Pb2+ at longer distances of 2.37 – 3.07 Å. In the analysis of the structures, maximum Pb-O contacts to be considered as bonds were determined by looking for gaps in histograms of contact distances and by performing bond valence sum calculations for each lead center (SI, Table S1).53,54 In 1 and 2, which are isostructural to one another, both lead centers have coordination numbers of 6 in irregular geometries. In 3, the two lead centers are five-coordinate. In 4, two lead centers have coordination numbers of 6, while the


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third is four-coordinate with a distorted disphenoidal geometry. As in previously reported lead oxide carboxylate structures, the coordination environments are hemidirected, with ligands asymmetrically situated around the metal centers.24,25 The gaps in the coordination spheres are attributed to stereochemically active 6s2 lone pairs on the metal centers. This qualitative interpretation of the crystal structures is supported by calculations of the electron localization functions for 2-4 (SI, Figure S9). In these structures, a lobe of elevated electron density was located on each Pb2+ atom, directed away from the coordinating oxygen atoms.55

Figure 1. Asymmetric Pb2+ coordination environments in a) 1 and 2, b) 3, and c) 4. Part a uses the numbering scheme of 2. One-Dimensional Chiral Substructures. The structures of 1-4 all contain chiral substructures, though their arrangement with respect to symmetry elements in the unit cells lead to differences in the overall symmetries of the compounds. The essential building element of the inorganic portion 15 ACS Paragon Plus Environment


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of each compound is an oxygen-centered Pb4O tetrahedron. In these compounds, as well as in previously reported lead oxide carboxylates, the tetrahedra share edges to form extended inorganic chains.24,25 In 1 and 2, which are isostructural to one another, distorted Pb4O tetrahedra share edges to form one-dimensional, helical Pb2O2+ chains. The organic ligands coordinate the chains through both carboxylate oxygen atoms, forming a brushlike arrangement around the inorganic cores. The helices are situated on 4-fold screw axes in the I41/a cell. Right- and left-handed helices are related through inversion, resulting in a structure that is centrosymmetric overall (Figure 2). There are nominal similarities between 1 and 2 and two polymorphs of lead oxide benzoate, which also contain helical Pb2O2+ chains with brushlike benzoate coordination.24 However, right- and lefthanded helices are not related through inversion in the unit cells of the previously reported structures, which occur in the Fdd2 and P21 space groups. Although 1 and 2 contain hemidirected Pb2+ coordination sites, we believe that these local asymmetries are not the cause of the chirality of the one-dimensional chains. While ELF calculations have not been reported for other members of the lead carboxylate family, qualitative analysis of coordination geometries and bond valence sum calculations have shown asymmetric local environments even when no longer-range asymmetry results.24,25 For example, in lead oxide formate, Pb2O2+ chains occur with a single hemidirected Pb2+ site, yet the chains are linear and not chiral. Thus, the stoichiometry Pb2O2+ does not always lead to long-range asymmetry. We propose that in 1 and 2, as in the polymorphs of lead oxide benzoate, the distortion of the chains instead arises from the influence of the ligands. The interdigitation of the aromatic rings may stabilize particular ligand orientations that, in turn, favor the distortion of the Pb4O tetrahedra. The approach between bromine atoms on ligands coordinating adjacent chains in 2 is consistent with weak halogen-halogen interactions. The C-Br-Br angles between adjacent rings are 83.5o and 138.7o, which fall into the type II, or bent, classification for halogen-halogen interactions.56 The difference between these angles, 55.2o, is in the range of 55-60o most commonly observed for type II


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Br-Br contacts.57 In 2, the single crystallographically unique Br-Br distance is 3.804 Å, compared with the van der Waals sum of 3.700 Å. Desiraju and coworkers have found that electrostatically mediated interactions can persist even at distances greater than the van der Waals separation.57 However, while halogen-halogen distances of up to 0.10 Å greater than the van der Waals contact have been considered in surveys of halogen-halogen bonding, the long contact in 2 suggests that only a very weak interaction occurs. Moreover, the isostructural relationship between 1, where no halogen bonding is possible, and 2 suggests that halogen-halogen interactions don’t have a strong influence in stabilization of 2.



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Figure 2. a) Helical Pb2O2+ chain in 1 and 2. b) Unit cell for 1. c) Unit cell for 2. d) Right-handed (red) and left-handed (purple) helices in 1 and 2. Disordered solvent molecules are omitted for clarity in a) and b) but occupy the channels between the inorganic helices. Lead atoms are shown in gray, oxygen atoms in red, carbon atoms in black, hydrogen atoms in blue, and bromine atoms in brown. 18 ACS Paragon Plus Environment

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Compounds 3 and 4 also contain chiral substructures, though the nature of their chirality is different from that of the chains in 1 and 2. In 3, Pb4O tetrahedra share edges to form linear Pb2O2+ chains (Figure 3). Inversion symmetry is precluded by propeller-like chelation of the chains by the salicylate ligands. The inorganic units are linked into layers by carboxylate bridges. More extensively edge-shared Pb2O32+ chains occur on 21 screw axes in 4. The single-crystal diffraction experiment showed that the inorganic lead oxide fraction alone has pseudo-inversion symmetry, emulating the centrosymmetric space group P21/m. The organic ligands break the pseudosymmetry, chelating the inorganic cores along the 21 axes and linking the chains into layers. The Pb2+ coordination environments are locally asymmetric in 3 and 4. However, the extended inorganic components of these structures, considered alone, have higher pseudo-symmetries than the overall structural symmetries, suggesting that the geometry around Pb2+ is not the source of the non-centrosymmetries of the crystal structures. As noted above, it is the ligands that introduce chirality through their propeller-like coordination of the inorganic chains. The chiral components of 3 and 4 relate in different ways to determine the overall symmetries of the structures. In the Fdd2 space group of 3, chiral chains are related through glide planes but not inversion so that the overall structure is achiral but polar and non-centrosymmetric. In the P21 space group of 4, only one type of helix occurs, leading to a structure that is polar, chiral, and noncentrosymmetric. Measurement of second-harmonic generation by 4 showed an intensity of 30 times the standard α-silica, corroborating the crystallographic finding of non-centrosymmetry. Second-harmonic generation was not measured for 3 since a phase-pure powder has not yet been prepared.



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Figure 3. Linear Pb2O2+ chain (a) and unit cell (c) in 3. Pb3O22+ double chain (b) and unit cell (d) in 4. Lead atoms are shown in gray, oxygen atoms in red, carbon atoms in black, hydrogen atoms in blue, and fluorine atoms in green.


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Ligand Parameters and Structural Dimensionality. Examination of 1-4 and the previously reported members of the lead oxide carboxylate family shows that while all members have onedimensional inorganic substructures, the overall dimensionalities of the structures vary from oneto three-dimensional. Table 2 classifies the compounds by their dimensionalities, using the suggestion of Cheetham and coworkers that extended inorganic hybrids can be classified as ImOn, with m denoting the inorganic dimensionality and n the dimensionality of the metal-organic-metal linkages.1 The group of carboxylic acids considered, with pKas that vary from 1.9 to 4.7, have a variety of structures, denticities, and ring substituents. A correlation was not observed between overall dimensionality and acid strength. The strongest acids – maleic, salicylic, and 4fluorosalicylic – formed both 1- and 2-D structures. Acetic acid, the weakest in the group, led to a 2D structure, while several acids with pKas surrounding 4.0 formed both 1- and 3-D structures.

Table 2. Structural classification and ligand acidities for lead oxide carboxylate family. ligand

structural classificationa

total dimensionality

pKa of conjugate acid

formate

I1O2

3

3.75b

maleate

I1O2

3

1.92b

acetate

I1O1

2

4.756b

4-fluorosalicylate

I1O1

2

2.85c

salicylate

I1O1

2

2.98d

benzoate

I1O0

1

4.204b

4-bromobenzoate

I1O0

1

3.96b

p-toluate

I1O0

1

4.37b

a) Classifications are in the form ImOn (m = inorganic dimensionality, n = dimensionality of metalorganic-metal linkages) b) Reference 44 c) Reference 58 d) Reference 59



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Because the ligand acidities did not correspond to structural dimensionality, our focus turned to the role of ligand shape. Ligands were considered as being partitioned into coordinating portions composed of the oxygen donor atoms and non-coordinating portions comprising the remaining atoms. A ratio of the volumes of the non-coordinating to coordinating portions of each ligand is proposed as a means of quantifying its shape (Table 3). There is a strong inverse relationship between the ligand volume ratio and the overall dimensionality of structures in this family, with a particular distinction between the ratios of ligands forming 1-D structures and those forming hybrid structures of higher dimensionalities (Figure 4). Table 3. Ratios of non-coordinating to coordinating volumes for carboxylate ligands. ligand

total dimensionality

ratio based on crystal structures

ratio based on ab initio geometry optimization

ratio based on numbers of atoms

formate

3

0.72

0.75

1.0

maleate

3

1.1

1.2

1.5

acetate

2

1.5

1.6

2.5

salicylate

2

2.6

2.7

3.7

4-fluorosalicylate

2

2.7

2.7

3.7

benzoate

1

4.1

4.3

6.0

p-toluate

1

4.9

5.1

7.5

4-bromobenzoate

1

5.1

5.3

6.0


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Figure 4. Ratios of non-coordinating to coordinating volumes for carboxylate ligands and designation of overall structural dimensionalities of resulting extended hybrid structures. Values shown are based on crystallographically determined atomic coordinates from the current work and from references 24-26.

To test the sensitivity of the calculated volume ratios to details of our method, ratios were also calculated using atomic coordinates obtained through ab initio geometry optimizations for the carboxylate ligands. These ratios were consistently slightly higher than those based on the crystallographic coordinates, but the correspondence to structural dimensionality was still observed. Simple ratios were also calculated using the numbers of non-coordinating atoms and coordinating oxygen atoms in each ligand. Here, each ratio was larger than that obtained through more nuanced volume calculations. This is to be expected as the method of atom counting placed the same weight on small hydrogen atoms as on other atoms in the molecules and led to an inflated non-coordinating volume. Nevertheless, even this crude approach produced ratios that inversely correlated with the dimensionalities of the extended inorganic hybrids in this family.



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Notions of volume fraction have previously been used in analysis of the crystallographic topologies of organic solids60,61 and a series of Ag+ coordination networks.62 In these studies, amphiphilic organic molecules and ligands were partitioned into hydrophilic and hydrophobic regions, with metal ions considered as either hydrophilic or intermediate components of the structures. The hydrophilic volume fraction was shown to correlate with the crystal topology, following the same pattern known for amphiphilic diblock copolymers.63 That is, structures could be described alternatively as columnar, layered, bicontinuous, or inverse columnar as the hydrophilic volume fraction increased. The topologies found in lead oxide carboxylates also parallel those formed by diblock copolymers. Here, a useful perspective is to identify continuous regions bound through inorganic bonding and bridging through carboxylate coordination. In the remaining space, only non-covalent interactions occur. From this viewpoint, structures 1 and 2, as well as both previously reported polymorphs of lead oxide benzoate, consist of inorganic columns separated by regions in which only non-covalent interactions are possible (Figure 5). Structures 3 and 4, as well as lead oxide acetate, contain layers bound by inorganic and coordination bonding. Between the layers, only van der Waals interactions occur. Lead oxide formate and lead oxide maleate show three-dimensional inorganic and coordination networks containing channels lined by C-H groups. The three categories of structures can be described as columnar, lamellar, and inverse columnar, respectively. The evolution of structures through these topological types follows the increase in the coordinating fraction of the organic ligand, corresponding to a decrease in the non-coordinating:coordinating volume ratio. An important distinction between the current analysis and previous studies is that our volume calculations did not partition the entire occupied volume of each structure. Instead, we have shown that volume ratios found by partitioning the organic component alone correlate with dimensionality and topology of the overall structures of lead oxide carboxylates.


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Figure 5. Topologies formed by a) lead oxide benzoate, b) 1, c) 2, d) lead oxide acetate, e) 3, f) 4, g) lead oxide formate, h) lead oxide maleate. One unit cell is shown for each compound except for 3, for which half the cell volume is shown. Regions linked through inorganic bonding and carboxylate coordination bridging are red. The remaining components are shown in blue.

Conclusions. Four newly synthesized lead oxide carboxylates add to the structural diversity of this family, showing that the ligand influences the overall dimensionality and symmetry of the structure. The chiral elements found within these structures include helical Pb2O2+ chains and linear Pb2O2+ and Pb3O22+ chains with propeller-like chelation by organic ligands. The arrangement of these components around other symmetry elements leads to structures that are centrosymmetric (1 and 2), achiral but non-centrosymmetric (3) and both chiral and noncentrosymmetric (4). For compound 3, the crystallographic finding of non-centrosymmetry has been corroborated by measurement of SHG activity. We have found that the acidity of the carboxylic acid used in each synthesis does not correlate with the resulting dimensionality. There is, however, a strong inverse correlation between the ratio 25 ACS Paragon Plus Environment


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of non-coordinating to coordinating volumes in the ligands and the resulting overall dimensionalities. This volume ratio is a simple metric that is robust to details of the method of determining ligand coordinates and is able to encompass a family of ligands with differing denticities. The utility of volume considerations emphasizes the importance of space filling in the formation of extended inorganic hybrids. The current study suggests that ligands with low noncoordinating-to-coordinating ratios should be selected in order to promote formation of higherdimensionality structures. Such structures will be the target of future work. Supporting Information. X-ray crystallographic data in CIF format, figures including thermal ellipsoids and labeling schemes, overlays of calculated and experimental PXRD patterns, calculated electron localization functions, thermogravimetric analysis traces, and results of bond valence sum calculations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]. Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant Number DMR-1151498. Additional support came from Oberlin College. We thank Professor P. Shiv Halasyamani and Thao Tran (University of Houston) for performing the SHG measurements and for useful discussions. We acknowledge Professor Ram Seshadri (University of California, Santa Barbara) for assistance with ELF calculations. We thank Nigel Kidder-Wolff for synthetic work and Dr. David Oertel for assistance with ligand volume calculations. Powder X-ray diffraction data and 26 ACS Paragon Plus Environment

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some single crystal X-ray diffraction data were collected on instruments obtained with support from the National Science Foundation (Grant Numbers DMR-1337296 and DMR-0922588). DHF thanks the National Science Foundation Graduate Research Fellowship Program for support under Grant DGE 1144085. We acknowledge support from the Center for Scientific Computing from the CNSI, MRL: an NSF MRSEC (DMR-1121053) and NSF CNS-0960316. REFERENCES (1)

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(63) Bates, F. S.; Schulz, M. F.; Khandpur, A. K.; Förster, S.; Rosedale, J. H.; Almdal, K.; Mortensen, K. Faraday Discuss. 1994, 98, 7–18. For Table of Contents Use Only

Ligand-Induced Variations in Symmetry and Structural Dimensionality of Lead Oxide Carboxylates Elaine E. Liu,† Calvin Gang,† Matthias Zeller,‡ Douglas H. Fabini,¥ Catherine M. Oertel†,* †Department of Chemistry and Biochemistry, Oberlin College, 119 Woodland St., Oberlin, OH 44074 ‡Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907 ¥ Materials Research Laboratory and Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106

Four new lead oxide carboxylates have been prepared solvothermally. For each of the ligands in the new structures as well as in four previously reported compounds, a ratio of non-coordinating to coordinating volumes has been calculated. This ligand volume ratio correlates inversely with the overall dimensionality of the compounds. The analysis encompasses 1- to 3-D structures and ligands of differing denticities.


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Figure 1. Asymmetric Pb2+ coordination environments in a) 1 and 2, b) 3, and c) 4. Part a uses the numbering scheme of 2. Lead atoms in 1-4 are coordina 84x109mm (300 x 300 DPI)

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Figure 2. a) Helical Pb2O2+ chain in 1 and 2. b) Unit cell for 1. c) Unit cell for 2. d) Right-handed (red) and left-handed (purple) helices in 1 and 2. Disordered solvent molecules are omitted for clarity in a) and b) but occupy the channels between the inorganic helices. Lead atoms are shown in gray, oxygen atoms in red, carbon atoms in black, hydrogen atoms in blue, and bromine atoms in brown. In 1 and 2, which are isostruc 84x177mm (300 x 300 DPI)


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Figure 3. Linear Pb2O2+ chain (a) and unit cell (c) in 3. Pb3O22+ double chain (b) and unit cell (d) in 4. Lead atoms are shown in gray, oxygen atoms in red, carbon atoms in black, hydrogen atoms in blue, and fluorine atoms in green. In 3, Pb4O tetrahedra share ed 84x177mm (300 x 300 DPI)



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Figure 4. Ratios of non-coordinating to coordinating volumes for carboxylate ligands and designation of overall structural dimensionalities of resulting extended hybrid structures. Values shown are based on crystallographically determined atomic coordinates from the current work and from references 24-26. between the ratios of ligands 84x96mm (300 x 300 DPI)


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Figure 5. Topologies formed by a) lead oxide benzoate, b) 1, c) 2, d) lead oxide acetate, e) 3, f) 4, g) lead oxide formate, h) lead oxide maleate. One unit cell is shown for each compound except for 3, for which half the cell volume is shown. Regions linked through inorganic bonding and carboxylate coordination bridging are red. The remaining components are shown in blue. by regions in which only non-c 130x82mm (300 x 300 DPI)



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Four new lead oxide carboxylates have been prepared solvothermally. For each of the ligands in the new structures as well as in four previously reported compounds, a ratio of non-coordinating to coordinating volumes has been calculated. This ligand volume ratio correlates inversely with the overall dimensionality of the compounds. The analysis encompasses 1- to 3-D structures and ligands of differing denticities. For Table of Contents Use Only 88x45mm (300 x 300 DPI)


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Ligand-Induced Variations in Symmetry and ... - ACS Publications

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