An Acentric Buckled Grid a - American Chemical Society

Jan 21, 2011 - Two-Dimensional Coordination Polymers: An Acentric Buckled Grid ... dimensional (2-D) coordination polymers, wherein the length of the ...
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DOI: 10.1021/cg100972s

Tether Length Control of Topology in Cadmium 4-Methylphthalate Two-Dimensional Coordination Polymers: An Acentric Buckled Grid and a Rare Self-Catenated Layer

2011, Vol. 11 678–683

Gregory A. Farnum, Amy L. Pochodylo, and Robert L. LaDuca* Lyman Briggs College and Department of Chemistry, Michigan State University, East Lansing, Michigan 48825, United States Received July 22, 2010; Revised Manuscript Received December 10, 2010

ABSTRACT: Hydrothermal synthesis has afforded a pair of dual-ligand divalent cadmium 4-methylphthalate (mph) twodimensional (2-D) coordination polymers, wherein the length of the dipyridyl co-ligand plays a predominant role in the overall structural topology. [Cd2(mph)2(dpe)4]n (1, dpe = 1,2-di(4-pyridyl)ethane) possesses an acentric buckled (4,4) grid topology, with a pendant, monodentate dpe ligand bound to each Cd atom. Use of a slightly longer dipyridyl tether allowed generation of the 2-D phase [Cd2(mph)2(dpp)3]n (2, dpp = 1,3-di(4-pyridyl)propane). The dpp ligands in 2 all bridge two cadmium atoms, promoting the formation of a very rare 4-connected 66 self-catenated layer topology. Luminescent and thermal properties of 1 and 2 are also discussed.

Introduction

Scheme 1. Ligands Used in This Study

The synthesis and characterization of coordination polymers remains under intensive investigation, with efforts geared toward the design of materials tailored to diverse applications such as hydrogen storage,1 molecular separations,2 ion exchange,3 catalysis,4 and luminescence.5 Dicarboxylate ligands are among the most commonly employed linkers in coordination polymers, imparting structural rigidity to the resulting crystalline lattice, and preventing inclusion of small anionic species in most cases.6 The disposition of the donor groups around the periphery of the dicarboxylate ligands, the coordination preferences at the specific metal ions, and the possibility of different carboxylate binding and bridging modes act synergistically to provide access to vast numbers of structural topologies. Neutral dipyridyl-type coligands, such as 4,40 -bipyridine (bpy), can pillar dicarboxylate coordination polymer motifs into higher dimensions, appreciably altering the overall structural topology.7,8 Previous results have indicated that dipodal nitrogen-based ligands with flexible central tethers can be successfully utilized to prepare coordination polymers with topologies that vary considerably from their bpy-based analogues.9,10 In several of these cases, the ligand flexibility permits the stabilization of self-penetrated networks, where rods of the same framework can penetrate through the shortest internodal circuits. For example, [Ni2(oba)2(1,4-bix)(H2O)2] (oba = 4,40 -oxybis(benzoate), 1,4-bix = 1,4-bis(imidazol-1-ylmethyl)benzene) adopts a rare 6-connected three-dimensional (3-D) 44611 topology built from 2D þ 2D f 3D interpenetrated structural units,11 {[Cu2(glutarate)2(dpe)] 3 5H2O}n (dpe = 1,2-di(4-pyridyl)ethane) possesses an uncommon 6-connected 48668 (rob) network.12 Rarer still are self-penetrated two-dimensional (2-D) layer topologies, with very few examples having been reported.13-15 Among examples of these very uncommon self-catenated layers are *To whom correspondence should be addressed. Mailing address: Lyman Briggs College, E-30 Holmes Hall, Michigan State University, East Lansing, MI 48825 USA. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 01/21/2011

{[Ni(oba)(dpa)]•H2O} (dpa = 4,40 -dipyridylamine), which has a 4-connected nondiamondoid 66 topology,13 and {[Zn3(OH)3(dpp)3](NO3)3•8.67H2O}n (dpp =1,3-di(4-pyridyl)propane), which has flexible dpp ligands penetrating through six-membered [Zn3(OH)3] rings.14 The ortho-dicarboxylate phthalate (pht) ligand has proven to be an efficacious choice for the generation of topologically novel coordination polymers,16-18 including the unique example of a 4-connected 7482 yyz self-penetrated 3-D net in {[Cd(pht)(dpa)(H2O)]•4H2O}n.16 Herein we report the synthesis and structural characterization of luminescent layered coordination polymer phases [Cd2(mph)2(dpe)4]n (1, mph = 4-methylphthalate) and [Cd2(mph)2(dpp)3]n (2). In this system, despite similar structural components, the length of the dipyridyl tether appears to dictate whether a “typical” but acentric (4,4) grid or a very rare self-penetrated layer topology is formed during self-assembly. Experimental Section General Considerations. Cadmium perchlorate hexahydrate, 1,2di(4-pyridyl)ethane, and 4-methylphthalic acid were purchased from Aldrich. 1,3-di(4-pyridyl)propane was purchased from TCI America. Water was deionized above 3 MΩ 3 cm in-house. IR spectra were recorded on polycrystalline samples using a PerkinElmer Spectrum One instrument. The luminescence spectra were obtained with a Hitachi F-4500 Fluorescence Spectrometer on solid crystalline samples anchored to quartz microscope slides with Rexon Corporation RX-22P ultraviolet-transparent epoxy adhesive. Powder X-ray diffraction (XRD) was performed on a Bruker D8 Davinci instrument using Cu KR radiation. Thermogravimetric r 2011 American Chemical Society

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Table 1. Crystal and Structure Refinement Data for 1 and 2 Data

1

2

C57H54Cd2N6O8 C66H60Cd2N8O8 1318.04 1175.86 orthorhombic orthorhombic Pbcn P212121 10.1155(9) 22.0525(17) 21.3388(18) 23.1373(18) 27.370(2) 10.1729(8) 5908.0(9) 5190.6(7) 4 4 1.482 1.505 0.784 0.881 0.830 0.736 -12 e h e 12, -26 e h e 26, -25 e k e 25, -27 e k e 27, -33 e l e 33 -12 e l e 12 total reflections 71561 72392 unique reflections 10891 4792 R(int) 0.0435 0.0550 parameters 757 357 0.0325 0.0571 R1 (all data) a 0.0281 0.0405 R1 (I > 2σ(I)) a 0.0688 0.1001 wR2 (all data) b b 0.0644 0.0912 wR2 (I > 2σ(I)) 0.932/-0.264 0.778/-0.472 max/min residual (e-/ A˚3) G.O.F. 1.042 1.061 P P P 2 2 2 P a b R1 = ||Fo| - |Fc||/ |Fo|. wR2 = { [w(Fo - Fc ) ]/ [wFo2]2}1/2. empirical formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) V (A˚3) Z Dcalc (g cm-3) μ (mm-1) min/max trans. hkl ranges

analysis was performed on a TA Instruments high-resolution Q500 thermal analyzer under flowing N2. Preparation of [Cd2(mph)2(dpe)4]n (1). Cd(ClO4)2•6H2O (58 mg, 0.185 mmol), dpe (68 mg, 0.370 mmol) and 4-methylphthalic acid (33 mg, 0.185 mmol) were placed into 5 mL distilled H2O in a 15 mL screw-cap vial. The vial was sealed and heated in an oil bath at 80 °C for 72 h, whereupon it was cooled slowly to 25 °C. Colorless plates of 1 (43 mg, 35% yield based on Cd) were isolated after washing with distilled water and acetone, and drying in air. Anal. Calc. for C66H60Cd2N8O8 1: C, 60.14; H, 4.59; N, 8.50% Found: C, 60.43; H, 4.21; N, 8.68%. IR (cm-1): 3053 (w), 2960 (w), 2923 (w), 1552 (s), 1497 (m), 1423 (m), 1400 (s), 1373 (m), 1222 (m), 1151 (w), 1072 (w), 1012 (m), 841 (m), 827 (w), 806 (s), 779 (m), 680 (w). Preparation of [Cd2(mph)2(dpp)3]n (2). The preparative method for 1 was followed but with the use of dpp (74 mg, 0.370 mmol) as the dipyridyl component. Colorless plates of 2 (64 mg, 59% yield based on Cd) were isolated after washing with distilled water and acetone, and drying in air. Anal. Calc. for C57H54Cd2N6O8 2: C, 58.22; H, 4.63; N, 7.15% Found: C, 57.88; H, 4.50; N, 6.84%. IR (cm-1): 3060 (w), 2860 (w), 1609 (m), 1565 (s), 1494 (w), 1426 (m), 1404 (s), 1369 (w), 1224 (m), 1040 (w), 1007 (w), 846 (m), 805 (m), 795 (m), 780 (m), 749 (w), 679 (w). X-ray Crystallography. Single-crystal XRD was performed on colorless blocks of 1 and 2 using a Bruker-AXS ApexII CCD instrument at 173 K. Reflection data were acquired using graphitemonochromated Mo-KR radiation (λ = 0.71073 A˚). The data was integrated via SAINT.19 Lorentz and polarization effect and empirical absorption corrections were applied with SADABS.20 The structures were solved using direct methods and refined on F2 using SHELXTL.21 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. Relevant crystallographic data for 1-2 is listed in Table 1.

Results and Discussion Synthesis and Spectral Characterization. Compounds 1 and 2 were prepared cleanly as crystalline products by hydrothermal reaction of cadmium perchlorate and 4-methylphthalic acid with the requisite dipyridyl ligand. Phase match verification was obtained by powder XRD (Figures S1-S2, Supporting Information). The infrared spectra of 1 and 2 were

Figure 1. Coordination environments of 1, with thermal ellipsoids at 50% probability and partial atom numbering scheme. The symmetry codes are as in Table 2. Most hydrogen atoms have been omitted. Only one of the disordered methyl positions is shown.

consistent with their structural characteristics as determined by single-crystal XRD. Medium intensity bands in the range of ∼1600 cm-1 to ∼1200 cm-1 can be ascribed to stretching modes of the pyridyl rings of nitrogen-based ligands.22 Puckering modes of the pyridyl rings are evident in the region between 820 cm-1 and 600 cm-1. Asymmetric and symmetric C-O stretching modes of the fully deprotonated mph ligands correspond to the strong, broadened features at 1552 cm-1 and 1400 cm-1 (for 1) and 1565 cm-1 and 1404 cm-1 (for 2). Structural Description of [Cd2(mph)2(dpe)4]n (1). Compound 1 crystallizes in the acentric orthorhombic space group P212121 with an asymmetric unit consisting of two divalent Cd atoms, two mph ligands (mph-A, O1-O4; mph-B, O5-O8), and four dpe ligands (dpe-A, N3/N4; dpe-B, N5/N7; dpe-C, N1/N8; dpe-D, N2/N6) (Figure 1). The coordination environment at each of the crystallographically distinct Cd atoms is a distorted {CdN3O4} pentagonal bipyramid, in which the equatorial planes are occupied by chelating carboxylate groups from two mph ligands and a nitrogen donor atom from a dpeB ligand. Pyridyl nitrogen donor atoms from dpe-A and dpe-C rest in the apical positions at Cd1; nitrogen donors from dpe-A and dpe-D take up similar coordination sites at Cd2. Thus the nitrogen donor atoms within the {CdN3O4} coordination spheres rest in a pseudomeridional arrangement. Bond lengths and angles about the Cd atoms are listed in Table 2. The bis(chelating) mph ligands connect neighboring Cd1 and Cd2 atoms into [Cd(mph)]n chain motifs that are arranged along the a crystal direction. Along the chain, the mph-A and mph-B ligands alternate, providing slightly different Cd 3 3 3 Cd contact distances (6.478 and 6.505 A˚, respectively). These subtle differences are provided by conformational variances within the crystallographically distinct mph ligands. In mphA, the carboxylate groups are twisted by 39.9 and 48.6° relative to the plane of the aromatic ring. The related torsions in mph-B are 43.5 and 47.2°. Nearest-neighbor Cd1 and Cd2 atoms in adjacent [Cd(mph)]n chain motifs are connected along the b crystal direction by tethering dpe-B ligands, which span a Cd 3 3 3 Cd distance of 8.700 A˚. Additionally, next-nearest neighbor Cd1 and Cd2 atoms in juxtaposed [Cd(mph)]n chains are bridged by dpe-A ligands with a 13.965 A˚ Cd 3 3 3 Cd distance. The gauche conformation of dpe-B (59.9° torsion angle across the ethylene tether) provides a much shorter Cd 3 3 3 Cd contact distance than that of the anti conformation dpe-A ligands, which have a corresponding torsion angle of 172.7°. The linkage of [Cd(mph)]n chain motifs through dpe-A and dpe-B

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Table 2. Selected Bond Distance (A˚) and Angle (°) Data for 1a Cd1-N1 Cd1-O6 Cd1-O1 Cd1-N4#1 Cd1-N5#2 Cd1-O2 Cd1-O5 Cd2-N2 Cd2-O3#3 Cd2-N3 Cd2-O7 Cd2-N7 Cd2-O8 Cd2-O4#3 N1-Cd1-O6 N1-Cd1-O1 O6-Cd1-O1 N1-Cd1-N4#1 O6-Cd1-N4#1 O1-Cd1-N4#1 N1-Cd1-N5#2 O6-Cd1-O5#2 O1-Cd1-N5#2 N4#1-Cd1-N5#2 N1-Cd1-O2 O6-Cd1-O2 O1-Cd1-O2 N4#1-Cd1-O2

2.324(3) 2.329(2) 2.339(2) 2.342(3) 2.430(3) 2.492(2) 2.543(2) 2.337(3) 2.337(2) 2.354(3) 2.365(2) 2.442(3) 2.450(2) 2.506(2) 94.98(9) 92.86(9) 88.43(7) 168.34(10) 92.24(9) 96.48(9) 83.8(1) 138.23(9) 133.34(9) 84.72(10) 92.82(9) 142.25(8) 54.28(7) 87.02(9)

N5#2-Cd1-O2 N1-Cd1-O5 O6-Cd1-O5#2 O1-Cd1-O5 N4#1-Cd1-O5 N5#2-Cd1-O5 O2-Cd1-O5 N2-Cd2-O3#3 N2-Cd2-N3 O3#3-Cd2-N3 N2-Cd2-O7 O3#3-Cd2-O7 N3-Cd2-O7 N2-Cd2-N7 O3#3-Cd2-N7 N3-Cd2-N7 O7-Cd2-N7 N2-Cd2-O8 O3#3-Cd2-O8 N3-Cd2-O8 O7-Cd2-O8 N7-Cd2-O8 N2-Cd2-O4#3 O3#3-Cd2-O4#3 N3-Cd2-O4#3 O7-Cd2-O4#3 N7-Cd2-O4#3 O8-Cd2-O4#3

79.33(9) 86.01(9) 53.98(8) 142.01(8) 90.88(9) 84.36(9) 163.67(8) 99.02(9) 164.27(10) 90.84(9) 90.94(9) 87.87(7) 101.71(9) 81.33(10) 138.14(9) 83.15(10) 133.95(9) 93.25(10) 140.57(8) 86.71(10) 54.41(8) 80.61(9) 83.78(9) 54.10(7) 92.27(9) 139.83(7) 84.66(9) 165.26(8)

a Symmetry transformation to generate equivalent atoms: #1: -x, y 1/2, -z þ 3/2; #2: -x þ 1, y - 1/2, -z þ 3/2; #3: x - 1, y, z.

Figure 2. A single [Cd2(mph)2(dpe)4]n layer in 1: (a) face view; (b) side view. The mph, gauche-conformation dpe, and anti-conformation dpe ligands are shown in red, blue, and green, respectively. Monodentate dpe ligands are shown in orange.

provides a [Cd2(mph)2(dpe)2]n acentric buckled (4,4) grid coordination polymer layer (Figure 2). The Cd 3 3 3 Cd 3 3 3 Cd angles around the grid apertures measure 46.1, 66.1, 102.3, and 113.9°. Monodentate gauche conformation dpe-C (50.8° torsion) and dpe-D (59.1° torsion) ligands, which are bound to Cd1 and Cd2, respectively, curl above and below the layer motifs, interdigitating between the gauche dpe-B tethers.

Figure 3. Network perspective of the buckled (4,4) layer in 1. The mph ligands, gauche-conformation dpe, and anti-conformation dpe ligands are shown as red, blue, and green rods, respectively. Table 3. Selected Bond Distance (A˚) and Angle (°) Data for 2a Cd1-O2 Cd1-N2 Cd1-O4#1 Cd1-N1 Cd1-N3#2 Cd1-O3#1 Cd1-O1 O2-Cd1-N2 O2-Cd1-O4#1 N2-Cd1-O4#1 O2-Cd1-N1 N2-Cd1-N1 O4#1-Cd1-N1 O2-Cd1-N3#2

2.329(3) 2.352(3) 2.368(3) 2.374(4) 2.443(4) 2.493(3) 2.512(3) 92.87(10) 90.40(9) 100.20(11) 93.51(11) 168.59(12) 89.21(11) 140.05(12)

N2-Cd1-N3#2 O4#1-Cd1-N3#2 N1-Cd1-N3#2 O2-Cd1-O3#1 N2-Cd1-O3#1 O4#1-Cd1-O3#1 N1-Cd1-O3#1 N3#2-Cd1-O3#1 O2-Cd1-O1 N2-Cd1-O1 O4#1-Cd1-O1 N1-Cd1-O1 N3#2-Cd1-O1 O3#1-Cd1-O1

92.42(13) 127.40(12) 76.65(14) 141.80(9) 83.06(11) 53.54(9) 97.64(11) 78.16(11) 54.21(9) 93.45(12) 142.77(9) 82.66(12) 85.95(12) 163.54(9)

a Symmetry transformation to generate equivalent atoms: #1: -x þ 1/ 2, -y þ 1/2, z - 1/2; #2: -x, y, -z - 1/2.

[Cd2(mph)2(dpe)4]n layer motifs (Figure 3) are stacked along the c crystal direction in an ABAB pattern, related by crystallographic screw axes (Figure S1). Structural Description of {[Cd2(mph)2(dpp)3]n (2). In contrast to 1, compound 2 crystallizes in a centrosymmetric space group (Pbcn). Its asymmetric unit consists of a single cadmium atom, one mph ligand, and one-and-one-half dpp ligands (dpp-A, N1, N3; dpp-B, N2) (Figure 4). The coordination environment at cadmium is a {CdN3O4} pentagonal bipyramid, with the axial positions taken up by pyridyl nitrogen atoms from dpp-A and dpp-B ligands. The equatorial plane is occupied by chelating carboxylate groups from two mph ligands, and a pyridyl nitrogen donor atom is from another dpp-A ligand. As in 1, the nitrogen donors at cadmium rest in a pseudomeridional arrangement. Bond lengths and angles about the Cd atoms in 2 are listed in Table 3. The mph ligands bridge adjacent cadmium ions with a bis(chelating) binding mode, as in 1, to construct [Cd(mph)]n chain motifs that are oriented along the c crystal direction. The Cd 3 3 3 Cd contact distances of 6.395 A˚ are uniform along the chain motifs, shorter than those in 1 by ∼0.1 A˚, and without the distance alternations seen along similar motifs in the dpe derivative. The carboxylate groups of the mph ligands are twisted relative to their respective aromatic ring planes by 69.8 and 73.4°. Parallel [Cd(mph)]n chains are aggregated into [Cd2(mph)2(dpp)3]n 2-D coordination polymer layers oriented parallel to the ac crystal planes (Figure 5a,b) by pairs of gauche-anti conformation dpp-A ligands (four-C atom torsion angles = 63.7, 170.0°) and by single anti-anti conformation dpp-B ligands (torsion angles = 170.0, 170.0°). The Cd 3 3 3 Cd distances across the gauche-anti dpp-A pairs and anti-anti dpp-B ligands are 11.301 and 14.101 A˚, respectively, enforced by the different conformations of the trimethylene tethers. A closer examination of the layer motif reveals that each Cd atom within a single [Cd(mph)]n chain connects to two Cd atoms in one

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Figure 4. Coordination environment of 2, with thermal ellipsoids at 50% probability and partial atom numbering scheme. The symmetry codes are as in Table 3. Hydrogen atoms have been omitted.

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Figure 6. Network perspective of the rare 66 self-catenated layered topology in 2. The mph ligands, pairs of gauche-anti-conformation dpp, and anti-anti-conformation dpp ligands are represented as red, blue, and green rods, respectively. Edge crossing in the real network is avoided by the projection of the gauche-conformation dpp ligands above and below the layer plane.

Figure 5. A single [Cd2(mph)2(dpp)3]n layer in 2: (a) face view; (b) side view. The mph, gauche-anti-conformation dpp, and anti-anti-conformation dpp ligands are shown in red, blue, and green, respectively.

Figure 7. Close-up view of the self-catenation in the layers in 2. Cadmium atoms within the self-catenating circuits are marked by yellow and black spheres.

other [Cd(mph)]n chain, with one interchain connection provided by a pair of gauche-anti conformation dpp-A ligands (blue in Figure 5), and the other provided by a single anti-anti conformation dpp-B ligand (green in Figure 5). Each Cd atom also connects via two mph ligands (red in Figure 5) to Cd atoms within the same [Cd(mph)]n chain. The topology of the [Cd2(mph)2(dpp)3]n 2-D layer can be investigated by considering each Cd atom as a 4-connected node, treating the dpp-A pairs as a single connection. However, because each Cd atom within a single [Cd(mph)]n chain connects only to one other [Cd(mph)]n chain, the underlying topology cannot be that of a common (4,4) 2-D grid. Analysis afli symbol for with TOPOS software23 indicates that the Schl€ the 4-connected layer in 2 is 66, indicating that all smallest internodal circuits comprise six Cd atom nodes in a (6,4) grid (Figure 6). Edge-crossing in the actual network of 2 is avoided by alternation of curling of the pairs of gauche-anti dpp-A ligands above or below the plane of the layer motif (Figure 5b). There are only two prior examples of a 2-D layered net with this 66 topology in the literature, in {[Ni(oba)(dpa)]•H2O}n13 and in [Zn(cpdpa)(dpp)]n (cpdpa = 4-(4-carboxyphenylamino)3,5-dinitrobenzoate).24 The extended Vertex Symbol of

62.62.63.66.64.64 for 2 is similar to that of {[Ni(oba)(dpa)]•H2O}n, despite the absence of helical subunits in 2. However this differs from the Vertex symbol of [Zn(cpdpa)(dpp)]n, which is a regular 62.62.62.62.62.62. The topology of 2 is another rare example of a selfcatenated layer, in which six-membered nodal circuits weave through each other, and yet connect covalently through other tethering ligands, in this case via mph ligands (Figure 7). It is very plausible that the longer span of the anti-anti conformation dpp ligands permits generation of this rare self-catenated layer in 2. The shorter dpe ligands in 1 do not have the requisite girth to span cadmium atoms across the grid apertures, and thus can remain pendant monodentate donors. Adjacent [Cd2(mph)2(dpp)3]n layers stack in an ABAB pattern along the b crystal direction (Figure S2). Small pockets in the interlayer regions, bracketed by the anti-anti conformation dpp-B and mph ligands, occupy 1.7% of the unit cell volume according to PLATON.25 Luminescence Spectra. Irradiation of crystalline samples of complexes 1-2 with ultraviolet light (λex = 330 nm for 1, 345 nm for 2) in the solid state resulted in moderately intense

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hand, the longer and more flexible dpp ligands in 2 can adopt an anti-anti conformation with the appropriate length to bridge cadmium atoms promote formation of a rare self-catenated layer topology. Both 1 and 2 show similar ligand-based luminescent behavior. The self-catenated polymer topology of 2 appears to result in an increase in thermal robustness. Acknowledgment. We acknowledge the donors of the American Chemical Society Petroleum Research Fund for funding this work. A.L.P. thanks the MSU Honors College for her Professorial Assistantship. We thank Mr. Curtis Wang for experimental assistance. We thank Dr. Michael Rich of the Michigan State University Composite Materials and Structures Center for use of the thermogravimetric analyzer.

Figure 8. Luminescence spectra of 1 and 2.

violet visible light emission in both cases. For 1, a broad emission band centered on 380 nm was observed, while that for 2 was centered around 430 nm (Figure 8). Similar to other d10 metal coordination polymers with aromatic ligands that do not exhibit a large emission redshift,5 the origin of the visible light emission behavior of 1-2 is attributed to π-π* or π-n electronic transitions within the molecular orbitals of the phenyl rings of the mpht ligands. The redshift of the emission of the complex relative to that of the pure ligand can be ascribed to changes in energy levels due to coordination to cadmium. Thermogravimetric Analysis. According to thermogravimetric analysis under flowing N2, the mass of compound 1 remained stable until ∼150 °C. Between this temperature and ∼260 °C, a 40.0% mass loss occurred in two steps, plausibly indicating the loss of two molar equivalents of monodentate dpe ligands and one molar equivalent of bridging dpe ligands (42.0% mass loss predicted). The mass remnant at 475 °C of 25.2% corresponds with a deposition of CdCO3 (26.1% calc’d). For 2, virtually no mass loss was observed until ∼225 °C, whereupon pyrolysis of the organic ligands occurred with a rapid 45.9% mass loss, followed by a slower mass loss between ∼250 °C and ∼315 °C. The total of these two mass losses represents 49.2% of the initial mass, corresponding well with an ejection of three molar equivalents of dpp ligands (50.4% calc’d). The 22.6% mass remnant at 475 °C matches roughly with a final product of CdO (21.8% calc’d) along with some carbonaceous material. The small quantity of residual solid precluded phase determination by powder XRD. Thermograms for 1 and 2 are shown in Figures S3 and S4, respectively. Conclusions Cadmium methylphthalate coordination polymers with flexible dipyridyl tethers have been synthesized and structurally characterized. Despite virtually identical cadmium methylphthalate chain subunits in both dpe derivative 1 and dpp derivative 2, the length of the polymethylene tether within the dipyridyl ligands appears to play a predominant role in structure direction during self-assembly. The curled conformation dpe ligands in 1 are too short to span the longer Cd 3 3 3 Cd distances within the layer motif, and thus adopt monodentate pendant binding modes. On the other

Supporting Information Available: Powder X-ray spectra for phase match verification and thermogravimetric data. Crystallographic data (excluding structure factors) for 1 and 2 have been deposited with the Cambridge Crystallographic Data Centre with Nos. 770687 and 770688, respectively. Copies of the data can be obtained free of charge via the Internet at http://www.ccdc.cam.ac. uk/conts/retrieving.html or by post at CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: 44-1223336033, E-mail: [email protected]). This material is also available free of charge via the Internet at http://pubs.acs.org.

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