and Three-Dimensional Divalent Metal Isophthalate Coordination

Nov 15, 2007 - Lyman Briggs College and Department of Chemistry, E-30 Holmes Hall, Michigan State UniVersity,. East Lansing, Michigan 48825. ReceiVed ...
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Two- and Three-Dimensional Divalent Metal Isophthalate Coordination Polymers Incorporating Flexible Bispyridylmethylpiperazine Tethers: Structure Direction through Coordination Geometry Preferences, Carboxylate Binding Mode, and Ligand Conformation

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2609–2619

David P. Martin, Maxwell A. Braverman, and Robert L. LaDuca* Lyman Briggs College and Department of Chemistry, E-30 Holmes Hall, Michigan State UniVersity, East Lansing, Michigan 48825 ReceiVed July 18, 2007; ReVised Manuscript ReceiVed September 11, 2007

ABSTRACT: Hydrothermal synthesis has afforded a family of divalent metal coordination polymers incorporating both the isophthalate dianion (ip) and the flexible organodiimine N,N′-bis(4-pyridylmethyl)piperazine (bpmp). The compounds of type {[M(ip)(bpmp)] · H2O} (M ) Co, 1; M ) Cd, 2) are isomorphous, possessing puckered two dimensional (2D) rectangular grid layers built from the linkage of [M2(ip)2] double chains through bpmp tethers. In contrast, the nickel congener {[Ni(ip)(bpmp)(H2O)] · H2O} (3) displays a corrugated, interpenetrated 2D structure. {[Zn(ip)(bpmp)] · H2O} (4) adopts a 5-fold interpenetrated three-dimensional diamondoid topology due to the tetrahedral coordination at Zn. Analysis of the variable temperature magnetic susceptibility of 1 indicated the presence of ferromagnetic coupling and zero-field splitting effects within the dinuclear subunits along with possible long-range ferromagnetic interactions transmitted through the ip ligands. The d10 derivatives 2 and 4 exhibited blue-violet luminescence upon exposure to ultraviolet radiation. The materials also exhibited high thermal stability. Introduction The synthesis and design of coordination polymer solids remain a popular research focus because of their potential applicability in diverse applications such as gas storage,1 small molecule shape-selective absorption and separation,2 ion exchange,3 catalysis,4 luminescence,5 and nonlinear optical properties.6 In these phases, dianionic dicarboxylate ligands tend to be an advantageous choice, as they can bridge cationic metal nodes while providing the necessary charge balance for the formation of a neutral framework. As a result, the incorporation of porosity-curtailing smaller anions can be avoided, enhancing the materials’ capabilities in gas storage or guest absorption. A wide range of structural morphologies is observed in these coordination polymer systems, predicated on the coordination geometry preferences at the metal atoms, the disposition of the donor groups within the dicarboxylate groups, and an array of available carboxylate-binding modes. Structural elaboration of dicarboxylate-containing coordination polymers has been achieved through the incorporation of neutral organodiimines such as the rigid-rod tether 4,4′bipyridine (4,4′-bpy) or the flexible tether 1,3-di-4-pyridylpropane (dpp), either of which can connect metal cations by means of their pyridyl nitrogen loci. Several structurally interesting solids with intriguing physicochemical properties have been prepared by employing this mixed ligand approach.7–12 For instance, the two-dimensional (2D) layered phase {[Zn(isophthalate)(4,4′-bpy)(H2O)] · 1.5H2O} manifested intense blue luminescence,8 [Co(thiophene-2,5-dicarboxylate)(4,4′-bipy)] displayed field-dependent spin-flop magnetic behavior at low temperatures,9 and the interpenetrated three-dimensional (3D) material [Zn(terephthalate)(4,4′-bpy)0.5] exhibited a striking ability to separate linear and branched alkanes when used as a * To whom correspondence should be addressed. E-mail: [email protected]

gas chromatographic stationary phase.10 The longer, more flexible tether dpp has been recently utilized to prepare coordination polymers with novel structural morphologies.11–13 For instance, the luminescent dpp-bearing coordination polymer {[Zn(1,3,5-benzenetricarboxylate)(Hdpp)] · H2O} crystallized in a noncentrosymmetric space group and represented the first reported case of a noninterpenetrated (10,3)-d topology (utp) network.11 To probe the effect of lengthening the organodiimine tether in the hopes of increasing either porosity or interpenetration levels with coordination polymer frameworks, we investigated the synthesis of divalent metal isophthalate-extended solids incorporating the conformationally flexible N,N′-bis(4-pyridylmethyl)piperazine (bpmp) ligand. In addition to being able to adopt a number of energetically accessible conformations, bpmp can also provide an ancillary structure direction capability through hydrogen-bonding acceptance at the nitrogen lone pairs within its central piperazine moiety. While some silver- and mercury-based coordination polymers incorporating bpmp have been observed,14,15 there have been no prior reports of this ligand’s inclusion in a dicarboxylate coordination polymer to the best of our knowledge. In this contribution, we report the synergistic structure-directing effects of metal coordination geometry, isophthalate-binding mode, and bpmp conformation in the self-assembly of three new 2D (1–3) or 3D (4) coordination polymers. We also report herein the thermal properties of these materials, a variable temperature magnetic susceptibility study of the cobalt derivative, and the photoluminescent behavior of the cadmium and zinc derivatives. Experimental Section General Considerations. Hydrated metal nitrates and isophthalic acid were obtained commercially. The organodiimine bpmp was prepared via a published procedure.14 Water was deionized above 3 MΩ in-house. Thermogravimetric analysis was performed on a TA

10.1021/cg700664u CCC: $37.00  2007 American Chemical Society Published on Web 11/15/2007

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Table 1. Crystal and Structure Refinement Data for 1–4 compound empirical formula formula weight collection T (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) min/max T hkl ranges total reflections unique reflections R (int) parameters/restraints R1 (all data) R1 [I > 2σ(I)] wR2 (all data) wR2 [I > 2σ(I)] max/min residual (e-/Å3) G.O.F.





C24CoH24N4O4 491.40 293(2) 0.71073 monoclinic P21/c 9.617(2) 10.046(2) 23.158(5) 98.590(4) 2212.4(9) 4 1.475 0.815 0.911 -12 e h e 12 -13 e k e 13 -30 e l e 29 23688 5036 0.0207 298/0 0.0311 0.0270 0.0738 0.0715 0.289/-0.272 1.037

C24H24CdN4O4 544.87 293(2) 0.71073 monoclinic P21/c 9.663(2) 10.332(3) 23.208(6) 100.398(4) 2279.0(10) 4 1.588 0.997 0.901 -12 e h e 12 -13 e k e 13 -30 e l e 30 24638 5305 0.0528 298/0 0.0633 0.0395 0.0839 0.0764 0.485/-0.406 1.017

C24H28N4NiO6 527.21 293(2) 0.71073 monoclinic P21/c 9.3093(18) 25.642(5) 10.235(2) 98.818(3) 2414.3(8) 4 1.450 0.851 0.905 -12 e h e 12 -32 e k e 32 -13 e l e 13 26947 5545 0.0452 331/6 0.0623 0.0389 0.0927 0.0838 0.318/-0.284 1.011

C24H26N4O5Zn 515.86 173(2) 0.71073 monoclinic C2/c 15.982(5) 15.038(4) 20.859(6) 109.946(4) 4713(2) 8 1.454 1.085 0.873 -20 e h e 20 -19 e k e 20 -26 e l e 27 27734 5692 0.0246 313/3 0.0331 0.0267 0.0657 0.0631 0.324/-0.332 1.051

Instruments TGA 2050 Thermogravimetric Analyzer with a heating rate of 10 °C/min up to 850 °C. Elemental analysis was carried out using a Perkin Elmer 2400 Series II CHNS/O Analyzer. IR spectra were recorded on powdered samples on a Perkin Elmer Spectrum One instrument. Preparation of [Co(bpmp)(ip)] (1). Co(NO3)2 · 6H2O (54 mg, 0.185 mmol), bpmp (100 mg, 0.37 mmol), and isophthalic acid (31 mg, 0.185 mmol) were added to 10 mL of distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb. The bomb was sealed and heated to 120 °C for 48 h, at which point the bomb was gradually cooled to 23 °C. Pink crystals of 1 (40 mg, 44% yield based on Co) were obtained after vacuum filtration and washing with distilled water and acetone, followed by air drying. Crystals of 1 were stable indefinitely in air. Anal. calcd for C24H24N4O4Co: C, 58.66; H, 4.92; N, 11.40%. Found: C, 58.27; H, 4.61; N, 11.64%. IR (cm-1): 2937 w, 2821 w, 2773 w, 1612 m, 1565 m, 1545 w, 1482 w, 1434 m, 1421 m, 1389 s, 1346 m, 1320 m, 1295 m, 1270 m, 1231 m, 1157 m, 1132 m, 1109 m, 956 w, 930 w, 861 w, 830 w, 800 m, 735 s, 715 s. Preparation of [Cd(bpmp)(ip)] (2). A similar procedure to that for 1 was used, with Cd(NO3)2 · 6H2O (57 mg, 0.185 mmol) as the metal

source. Colorless crystals of 2 (50 mg, 50% yield based on Cd) were obtained. Anal. calcd for C24H24CdN4O4: C, 52.90; H, 4.44; N, 10.28%. Found: C, 52.77; H, 4.36; N, 10.38%. IR (cm-1): 2839 w, 1674 w, 1608 m, 1563 w, 1502 w, 1424 m, 1334 w, 1301 m, 1233 m, 1160 m, 1067 s, 1006 m, 934 w, 835 m, 795 s, 779 s, 650 s. Preparation of {[Ni(bpmp)(ip)(H2O)] · H2O} (3). A similar procedure to that for 1 was used, with Ni(NO3)2 · 6H2O (54 mg, 0.185 mmol) as the metal source. Green crystals of 3 (52 mg, 53% yield based on Ni) were obtained. Anal. calcd for C24H24N4O4Ni: C, 54.65; H, 5.35; N, 10.62%. Found: C, 54.24; H, 5.06; N, 10.68%. IR (cm-1): 3448 w, 3129 w, 2952 w, 2816 w, 1666 w, 1600 m, 1563 m, 1535 m, 1446 m, 1429 m, 1397 m, 1380 s, 1341 m, 1288 m, 1220 w, 1229 m, 1007 m, 943 m, 881 m, 803 m, 735 s, 716 s. Preparation of {[Zn(bpmp)(ip)] · H2O} (4). A similar procedure to that for 1 was used, with Zn(NO3)2 · 6H2O (55 mg, 0.185 mmol) as the metal source. Colorless crystals of 4 (56 mg, 59% yield based on Zn) were obtained. Anal. calcd for C24H26N4O5Zn: C, 55.88; H, 5.08; N, 10.86%. Found: C, 55.71; H, 5.02; N, 11.17%. IR (cm-1): 3508 w

Figure 1. Coordination environment of 1 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Hydrogen atoms have been omitted for clarity.

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Figure 2. Coordination environment of 2 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Hydrogen atoms have been omitted for clarity. Table 2. Selected Bond Distances (Å) and Angles (deg) for 1 and 2a 1 Co1-O2 Co1-O1#1 Co1-O4#2 Co1-N1 Co1-N4#3 Co1-O3#2 O2-Co1-O1#1 O2-Co1-O4#2 O1#1-Co1-O4#2 O2-Co1-N1 O1#1-Co1-N1 O4#2-Co1-N1 O2-Co1-N4#3 O1#1-Co1-N4#3 O4#2-Co1-N4#3 N1-Co1-N4#3 O2-Co1-O3#2 O1#1-Co1-O3#2 O4#2-Co1-O3#2 N1-Co1-O3#2 N4#3-Co1-O3#2

2 2.0317(11) 2.0498(11) 2.1109(11) 2.1741(13) 2.1883(12) 2.2581(12) 105.59(4) 155.01(4) 99.07(4) 89.21(4) 89.55(4) 86.93(4) 96.91(5) 87.04(5) 88.27(4) 173.61(5) 94.98(4) 158.84(4) 60.12(4) 85.77(5) 95.50(5)


Cd1-O4 Cd1-O3#5 Cd1-O2 Cd1-N1 Cd1-N4#6 Cd1-O1 O4#4-Cd1-O3#5 O4#4-Cd1-O2 O3#5-Cd1-O2 O4#4-Cd1-N1 O3#5-Cd1-N1 O2-Cd1-N1 O4#4-Cd1-N4#6 O3#5-Cd1-N4#6 O2-Cd1-N4#6 N1-Cd1-N4#6 O4#4-Cd1-O1 O3#5-Cd1-O1 O2-Cd1-O1 N1-Cd1-O1 N4#6-Cd1-O1

2.194(2) 2.292(2) 2.316(2) 2.328(3) 2.352(3) 2.413(2) 109.08(9) 150.62(8) 99.76(8) 100.40(9) 85.48(9) 87.11(9) 88.61(9) 87.75(9) 86.96(9) 170.12(9) 94.97(8) 154.57(8) 55.70(8) 98.52(9) 84.61(9)

Symmetry transformations to generate equivalent atoms: #1, -x + 1, -y, -z + 2; #2, x, y + 1, z; #3, x - 1, -y + 1/2, z + 1/2; #4, x, y - 1, z; #5, -x + 1, -y + 2, -z; and #6, x - 1, -y + 3/2, z + 1/2. a

br, 2931 w, 2830 w, 1619 s, 1568 m, 1429 m, 1362 s, 1320 m, 1297 m, 1273 w, 1230 w, 1144 m, 1129 m, 1070 w, 1036 w, 1003 m, 951 w, 923 w, 860 w, 829 m, 809 m, 740 s, 726 s. X-ray Crystallography. A pink block of 1 (with dimensions 0.75 mm × 0.30 mm × 0.20 mm), a colorless block of 2 (0.40 mm × 0.30 mm × 0.25 mm), a green block of 3 (0.40 mm × 0.20 mm × 0.15 mm), and a colorless block of 4 (0.44 mm × 0.32 mm × 0.24 mm) were subjected to single-crystal X-ray diffraction using a BrukerAXS SMART 1k CCD instrument. Reflection data were acquired using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). The data were integrated via SAINT.16 Lorentz and polarization effect and empirical absorption corrections were applied with SADABS.17 The structures were solved using direct methods (1, 2, and 4) or Patterson synthesis (3) and refined on F2 using SHELTXL.18 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. The hydrogen atoms bound to any water molecules were found via Fourier difference maps and then restrained at fixed

Table 3. Selected Bond Distances (Å) and Angles (deg) for 3a Ni1-O3#1 Ni1-O5 Ni1-N1 O3-Ni1-O5#1 O3#1-Ni1-N1 O5-Ni1-N1 O3#1-Ni1-O1 O5-Ni1-O1 N1-Ni1-O1 O3#1-Ni1-N4#2 O5-Ni1-N4#2

2.0159(14) 2.0543(17) 2.1093(18) 94.65(7) 88.82(7) 86.42(7) 165.84(6) 99.44(7) 90.64(7) 88.38(7) 95.63(8)

Ni1-O1 Ni1-N4#2 Ni1-O2 N1-Ni1-N4#2 O1-Ni1-N4#2 O3#1-Ni1-O2 O5-Ni1-O2 N1-Ni1-O2 O1-Ni1-O2 N4-Ni1-O2#2

2.1185(15) 2.1362(19) 2.1382(16) 176.65(7) 91.63(7) 103.81(6) 160.50(6) 87.71(7) 62.03(6) 91.19(7)

a Symmetry transformations to generate equivalent atoms: #1, x, y, z - 1; and #2, -x +1, y - 1/2, -z + 1/2.

positions and refined isotropically. Relevant crystallographic data for 1–4 are listed in Table 1.

Results and Discussion Synthesis and Spectral Characterization. The coordination polymers 1–4 were obtained cleanly under hydrothermal conditions through the combination of the appropriate metal nitrate, isophthalic acid, and bpmp in a 1:1:2 mol ratio. The infrared spectra of 1–4 were consistent with their single crystal structures. Sharp and medium intensity bands in the range of ∼1600 to ∼1200 cm-1 were attributed to stretching modes of the pyridyl rings of the bpmp moieties. Features corresponding to pyridyl and piperazinyl ring puckering exist in the region between ∼930 and ∼600 cm-1. Asymmetric and symmetric C-O stretching modes of the fully deprotonated, ligated ip linkers were indicated by broadened bands at ∼1600 and ∼1400 cm-1. The lack of bands in the region of ∼1710 cm-1 is diagnostic of complete deprotonation of all isophthalate carboxylate groups in 1-4. Broad bands in the area of ∼3400 to ∼3200 cm-1 represent O-H stretching modes within water molecules of crystallization (3 and 4) and/or ligated water molecules (3 and 4). The broadness of the latter features is attributed to hydrogen-bonding pathways (vide infra). Structural Description of [M(bpmp)(ip)]n M ) Co, Cd (1, 2). The asymmetric unit of compound 1 contains one cobalt atom, one isophthalate ligand, and one bpmp ligand. The

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Figure 3. View of a single [Co(ip)]n chain within 1.

Figure 4. Single [Co(ip)(bpmp)]n layer within 1.

Figure 5. View down b of the layer stacking pattern in 1.

coordination environment (Figure 1) about Co is best described as a slightly distorted [CoN2O4] octahedron, wherein the terminal nitrogen atoms from two crystallographically identical bpmp ligands are arranged in a trans disposition. The four oxygen donors belong to one chelating and two monodentate carboxylate groups of three different ip ligands. The asymmetric unit of 2 is nearly identical (Figure 2), with longer bond distances in accordance with known periodic trends.19 Bond distances and angles are consistent with distorted octahedral geometry about

the metal atoms in both 1 and 2; these are listed in Tables 2 and 3, respectively. Further expansion of the structure reveals a 2D layer consisting of [Co(ip)]n chains running parallel to the b crystal axis (Figure 3) connected by bpmp tethering ligands along the j crystal direction (Figure 4). The [Co(ip)]n chains are [2 0 1] comprised of {Co2} dinuclear kernels formed by bridging carboxylate groups from two different ip moieties, which in turn are linked to each other through the chelating carboxylate ip

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Figure 6. Coordination environment of 3 with thermal ellipsoids at 50% probability and partial atom numbering scheme. Hydrogen atoms have been omitted for clarity.

Figure 7. View down a of a single rhomboid layer in 3. Hydrogen bonds are displayed as dotted lines. Table 4. Hydrogen Bonding Contact Distances (Å) and Angles (deg) in 3 and 4a (D-H · · · A)


r(D · · · A)


3 O1W-H1WA · · · N3#1 O1W-H1WB · · · O4 O5-H5A · · · O1W#2 O5-H5B · · · O4#3

2.047(19) 2.05(3) 1.864(17) 1.91(2)

2.915(3) 2.866(3) 2.687(3) 2.704(2)

175(4) 158(5) 171(3) 155(3)

4 O1W-H1WA · · · N2#4 O1W-H1WB · · · O3

2.360(17) 2.089(16)

3.213(2) 2.938(2)

170(2) 169(2)

Symmetry transformations to generate equivalent atoms: #1, x + 2, -y + 2, -z + 1; #2, x, -y + 3/2, z - 1/2; #3, x, y, z - 1; and #4, -x + 1, -y, -z + 1. a

ligand termini. Thus, the ip dianions in 1 and 2 act as exotridentate bridging ligands. The Co-Co distance within a single {Co2} dinuclear kernel is 4.558 Å, while the closest

distance from one Co atom to its nearest neighbor in the next kernel along the [Co(ip)]n chain is 6.910 Å. The corresponding distances are 4.629 and 7.136 Å for its isostructural Cd congener 2. The length of the metal-metal contacts through the bpmp ligands within the layer motifs is determined largely by the angle (defined as κ) between the two terminal nitrogen atoms through the centroid of the piperazine ring. The bpmp linkers are slightly bent, with a κ angle of 171° in both 1 and 2. As expected, the central piperazine ring adopts a standard “chair” conformation. The metal-metal contact distances through the bpmp tethering ligands are 16.175 and 16.448 Å in 1 and 2, respectively. The structures of 1 and 2 are very comparable to that of [Ni(ip)(4,4′bpy)],20 where one-dimensional (1D) [Ni(ip)]n chains are linked into a 2D grid through shorter 4,4′-bpy rigid rod tethers. However, these structures contrast markedly with the 3D primitive cubic (R-Po morphology, pcu) strucutral morphology exhibited by [Cd4(ip)4(dpp)2],21 which contains a flexible, yet

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Figure 8. Interpenetrating layers of 3 as viewed down c. Independent layers are shown in different colors, and unligated water molecules are colored orange. Hydrogen bonding interactions are shown as dashed lines.

Figure 9. View down the c crystal axis of stacked doubly interpenetrated layers in 3. Unligated water molecules are shown in orange. The hydrogenbonding patterns are represented by dashed lines.

shorter, tethering ligand than in 1 and 2. Individual [M (ip)(bpmp)] layers then stack in an ABA pattern to give the overall 3D structure of 1 and 2 (Figure 5), primarily through crystal-packing forces. The closest metal-metal through-space contact distances between the layers are 9.617 and 9.663 Å for 1 and 2, respectively. Structural Description of {[Ni(bpmp)(ip)(H2O)] · H2O} (3). Single crystal X-ray diffraction revealed that 3 possesses an asymmetric unit consisting of a nickel atom, one bpmp moiety, one ip ligand, and one ligated water molecule (Figure 6). There is also one unligated water molecule of crystallization, resulting in a formulation for 3 of {[Ni(ip)(bpmp)(H2O)] · H2O}. Operation of the crystallographic symmetry results in a distorted [NiN2O4] coordination octahedron with nitrogen donors from two different bpmp ligands arranged in a trans fashion. The four equatorial oxygen donors belong in turn to the ligated water molecule, a single chelating ip carboxylate group, and a monodentate carboxylate group from a second, but symmetry related, ip ligand. Bond distances and angles about nickel are consistent with a slightly distorted octahedral coordination geometry (Table 3). The junction of neighboring nickel atoms through the bpmp and bisbridging chelating/monodentate ip ligands creates a corrugated rhomboid grid motif with (4,4) topology, running parallel to the bc crystal plane (Figure 7). In contrast to 1 and 2, the ip dianion only bridges two metal atoms. In the b

direction, bpmp linkers connect nickel atoms to form an undulating chain with a Ni-Ni contact distance of 13.421 Å. The shorter M-M contact distance through bpmp linkers in 3, relative to 1 and 2, can be attributed to its more kinked disposition, with a κ angle of 119°. Adjacent nickel atoms are joined in the c crystal direction by the exobidentate ip ligands, separated by a through-ligand distance of 10.235 Å. The Ni-Ni-Ni angles within the ruffled grid measure 106.9 and 73.1°, reflective of a relatively pinched rhomboid morphology. Covalent connectivity within the grid motif is supplemented by a single intralayer hydrogen-bonding pattern. The oxygen atom of the monodentate carboxylate group (O4) accepts hydrogen bonding from the water molecule (O5, via H5B) ligated to the same nickel atom to which it is bound. Pertinent data regarding the hydrogen-bonding supramolecular contacts are given in Table 4. The rhomboid voids within a single grid motif (14.32 Å × 19.10 Å as measured by through-space Ni-Ni distances) allow interpenetration of an identical [Ni(ip)(bpmp)(H2O)]n sinusoidal layer to form a double layer motif (Figure 8). The remaining lone pair on the unligated carboxylate oxygen atom (O4) accepts a hydrogen bond from the unligated water molecule (via H1WB). In turn, O1W accepts a hydrogen bond from the ligated water molecule (via H5A) in the opposing grid. The aromatic rings of the ip ligands also participate in interlayer π-π stacking [centroid-to-centroid distance ) 3.964(2) Å].22 The closest

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Figure 10. Expanded asymmetric unit of 4 with thermal ellipsoids at 50% probability and partial atom numbering. Hydrogen atoms have been omitted for clarity. Table 5. Selected Bond Distances (Å) and Angles (deg) for 4a Zn1-O4 Zn1-O2#1 O4-Zn1-O2#1 O4-Zn1-N4 O2#1-Zn1-N4

1.9492(11) 1.9986(12) 129.46(5) 104.22(5) 115.12(5)

Zn1-N4 Zn1-N1 O4-Zn1-N1 O2#1-Zn1-N1 N4-Zn1-N1

2.0367(13) 2.0874(13) 108.50(5) 95.66(5) 99.14(5)

a Symmetry transformations to generate equivalent atoms: #1, -x + 1/2, y + 1/2, -z + 1/2.

intergrid Ni-Ni distance is 6.715 Å. The “wavelength” of the undulations with the corrugated layer patterns is 25.64 Å, defining the b lattice parameter. These 2D layers stack in the a crystal direction through further supramolecular interaction via the unligated water molecules of crystallization, which donate hydrogen bonds (through H1WA) to the lone pairs of nitrogen atoms (N3) within the piperazine subunits of the bpmp ligands in neighboring sheets (Figure 9). The closest interlayer Ni-Ni distance is 9.309 Å, demarcating the a lattice parameter. The incipient interlamellar void space occupied by the water molecules of crystallization represents only 1.9% of the unit cell volume of 3, as determined by PLATON.22 Structural Description of {[Zn(bpmp)(ip)] · H2O} (4). The structure of 4, which crystallizes in the centrosymmetric monoclinic space group C2/c, contains an asymmetric unit consisting of one zinc atom, two halves of two crystallographically distinct bpmp units (bpmp-A, marked by N1-N2; bpmpB, denoted by N3-N4) and a single ip ligand (Figure 10). There is also one unligated water molecule of crystallization per asymmetric unit, resulting in a formula for 4 of {[Zn(ip)(bpmp)] · H2O}. The zinc atom is bound by two oxygen

donors belonging to monodentate carboxylate termini of two different isophthalate ligands, and two pyridyl nitrogen donors from two crystallographically distinct bpmp ligands, forming a [ZnN2O2] coordination sphere. Bond lengths and angles around the metal center are consistent with distorted tetrahedral geometry and are given in Table 5. The largest L-M-L angle of 129.5° exists between the two carboxylate oxygen donor atoms. Extension of the structure reveals a diamondoid 66 lattice consisting of zinc centers connected by an undulating chain of bisbridging bismonodentate ip ligands extending in the b crystal direction and linked into a 3D lattice by both types of bpmp linkers (Figure 11). The Zn-Zn contact distance through the ip ligand is 10.077 Å, while the Zn-Zn distances through bpmp-A and bpmp-B are 15.442 and 16.202 Å, respectively. The κ angle is ∼180° through bpmp-A, which causes the longer Zn-Zn span. The shorter Zn-Zn contact distance through bpmp-B is promoted by the narrower corresponding κ of 138°. The longest Zn-Zn through-space distance within a single diamondoid lattice aperture is 30.18 Å. The large amount of empty space within a single diamondoid lattice allows for the mutual interpenetration of five identical frameworks within the crystal structure (Figure 12), which were analyzed by TOPOS software.23 The interpenetration within the structure of 4 belongs to Class Ia (translation only) with Full Interpenetration Vectors24 of (1/2,1/2,0) and (1/2,-1/2,0) for Zn-Zn distances of 10.97 Å and (1,0,0) for a Zn-Zn distance of 15.98 Å. Each framework is hydrogen bonded to two others through the water molecules of crystallization, which occupy an incipient void space of 8.9% of the unit cell volume as determined by PLATON.22 The unligated oxygen of one

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Figure 11. View down the b crystal axis of a single 66 diamondoid lattice within the structure of 4. Water molecules of crystallization are shown in orange.

monodentate carboxylate terminus (O3) accepts a hydrogen bond from the water of crystallization (through H1WB), which then hydrogen bonds (via H1WA) to the piperazine nitrogen atom (N2) of the bpmp-A linker (see Table 4 for metrical parameters). The remaining unligated monodentate carboxylate oxygen atom (O1), as well as the piperazinyl nitrogen atoms of bpmp-B, do not participate in hydrogen bonding. The closest internetwork through-space Zn-Zn distance is 5.77 Å. Thermogravimetric Analysis of 1–4. The decomposition patterns of 1–4 were ascertained via thermogravimetric analysis (TGA). The mass of compound 1 remained constant until ∼385 °C, at which point decomposition commenced. The 21% mass remnant at 800 °C roughly correlates to a final product of CoCO3 (24.2% expected). The decomposition of 2 was similar but showed slightly lower thermal stability. Here, mass decrease began at ∼370 °C, possibly showing the significance of the longer bond lengths in 2. A rapid mass loss was complete by 425 °C, followed by a gradual decrease in mass to a remnant of 29.3% of the original mass. This is loosely consistent with the calculated mass percent of 31.6% based on the deposition of CdCO3. Compound 3 showed an initial 3.4% weight loss at 120 °C corresponding to the loss of its unligated water molecules (3.4% predicted) followed at ∼230 °C by an additional 3.5% weight loss caused by the elimination of the aquo ligands. Gradual decomposition then occurred, which was complete by ∼750 °C. The 15.0% mass remnant signifies a final solid of NiO (15.7% calculated). The decomposition of 4 began at 100 °C by expelling its water molecules of crystallization (2.2% mass loss observed, 3.5% calculated). The mass then remained stable until ∼330 °C, with its combustion of the organic components complete by 600 °C. The final mass remnant of 17.5% correlates well

with deposition of ZnO (15.8% expected). TGA traces for 1–4 are given in Figures S1-S4 of the Supporting Information. Variable Temperature Magnetic Behavior of 1. To probe spin communication between the d7 Co2+ ions within and between the dimeric subunits in 1, a variable temperature magnetic susceptibility experiment was performed. A plot of χm-1 vs T (for T > 50 K, Figure S5 of the Supporting Information) shows that 1 follows the Curie–Weiss law with C ) 2.54 cm3 K/mol and Θ ) –1.1 K, indicative of net antiferromagnetic interactions. The value of C is higher than that predicted for a spin-only S ) 3/2 ion, consistent with spin–orbit coupling and anisotropy common for Co2+. A more detailed view of the magnetic behavior of 1 is possible through examination and modeling of the χmT vs T plot (Figure 13). The χmT product, which is 2.45 cm3 K/mol at 300 K, rises slowly to a maximum value of 2.54 cm3 K/mol at ∼115 K, marking the likelihood of ferromagnetic coupling within the {Co2} kernels. Subsequently, the value of χmT decreases to 2.20 cm3 K/mol at 18 K, possibly indicating either antiferromagnetic coupling between the dimeric subunits or zerofield splitting within the Co2+ electronic manifold, resulting in an effective S ) 1/2 ground state. The χmT behavior above 50 K was modeled successfully with eq 1, derived for an isotropically interacting dimer with two S ) 3/2 ions, which takes both intradimer coupling (J) and an interdimer coupling/zero-field splitting term (Θ) into account.26 χmT )


where x ) J/kT.


Ng2β2 ex + 5e3x + 14e6x k(T - Θ) 1 + 3ex + 5e3x + 7e6x




2D and 3D Divalent Metal Isophthalate Coordination Polymers

Crystal Growth & Design, Vol. 7, No. 12, 2007 2617

Figure 12. View down a of the full 5-fold interpenetrated framework in 4. Individual networks are displayed in different colors, and ligands are represented by straight lines.

Figure 13. Plot of χmT vs T for 1.

The best fit to the data occurred with J ) 120(1) cm-1, g ) 1.889(1), and Θ ) -6.02(6) K with R ) 7.64 × 10-3, where R ) {Σ[(χmT)obs - (χmT)calcd]2/Σ[(χmT)obs]2}. The positive J value signifies that the coupling within the {Co2} kernels is ferromagnetic, in contrast to the more simplistic Curie–Weiss

perspective. Given the distorted octahedral coordination geometry about the Co2+ ions in 1, the negative Θ factor can be largely ascribed to zero-field splitting,27 reducing the net value of χmT. At low temperatures (T < 15 K), a sharp increase in χmT is observed, indicating the presence of either ferromagnetic

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Martin et al.

Figure 14. Solid state emission spectra of 2 (red) and 4 (blue).

interdimer interactions or a ferromagnetic impurity. Because only extremely weak magnetic coupling through the shorter organodiimine 4,4′-bpy has been reported,25 it is likely that any magnetic communication through the long bpmp tethers is negligible in this case. Luminescent Properties of 2 and 4. In the solid state at room temperature, both 2 and 4 exhibited blue-violet luminescence upon ultraviolet excitation (λ ) 300 nm). The emission maximum for 2 (λmax ) 440 nm) is redshifted relative to that of 4 (λmax ) 405 nm). Isophthalic acid and 4,4′-bpy are luminescent compounds, showing weak emissions with λmax ) 408 nm (λex ) 348 nm) and λmax ) 424 nm (λex ) 350 nm), respectively.8,28 Thus, the luminescent behavior of 2 and 4 is best ascribed to ligand-centered π f π* or π f n orbital transitions within the aromatic rings of the ip and bpmp ligands, by comparison with other luminescent d10 coordination polymers.5,8,28,29 Differences in intensity and emission maxima between 2 and 4 likely arise from their respective differences in metal–ligand bond lengths and specific supramolecular environments. Conclusions Hydrothermal synthetic explorations toward dicarboxylate/ bpmp coordination polymers have resulted in the preparation of four new extended solids incorporating both isophthalate and bpmp ligands. The cobalt and cadmium congeners exhibited isostructural gridlike motifs based on dinuclear {M2} subunits linked through exotridentate ip and exobidentate bpmp ligands. In the cobalt case, ferromagnetic interactions within and between the {Co2} kernels appear to be operative. The nickel congener manifested stark structural change to a doubly interpenetrated 2D network, imparted by its octahedral coordination preference. Further reduction in metal coordination number to four, with a concomitant adjustment to a bisbridging bismonodentate carboxylate binding mode, promoted the formation of 5-fold interpenetrated diamondoid lattices in the zinc derivative. All

four materials displayed high thermal stability, illustrating the vast potential for robust dicarboxylate/bpmp coordination polymer frameworks. Further work in this direction continues in our laboratory. Acknowledgment. We gratefully acknowledge Michigan State University for financial support of this work. We thank Dr. Rui Huang for performing the elemental analysis and Dr. Reza Loloee and Matthew R. Montney for assistance with the SQUID magnetometer. Supporting Information Available: Crystallographic data (excluding structure factors) for 1–4, deposited with the Cambridge Crystallographic Data Centre with Nos. 654363, 654364, 634365, and 654366, respectively, and TGA traces for 1–4, two magnetic plots for 1, and CIF files for 1–4. This material is available free of charge via the Internet at

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