Luminescent Two- and Three-Dimensional Zinc Coordination

CRYSTAL GROWTH & DESIGN

Luminescent Two- and Three-Dimensional Zinc Coordination Polymers Containing Isomers of Phenylenediacetate and a Kinked Tethering Organodiimine

2007 VOL. 7, NO. 11 2343–2351

Maxwell A. Braverman and Robert L. LaDuca* Lyman Briggs College and Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48825 ReceiVed June 29, 2007; ReVised Manuscript ReceiVed August 2, 2007

ABSTRACT: Hydrothermal synthesis has afforded a series of zinc-containing coordination polymers incorporating the kinked organodiimine 4,4′-dipyridylamine (dpa) and phenylenediacetate (phda) isomers. {[Zn(1,4-phda)(dpa)] · H2O}∞ (1) manifests a twodimensional (2D) corrugated layer morphology. While both {[Zn(1,2-phda)(dpa)] · 2H2O}∞ (2) and [Zn(1,3-phda)(dpa)]∞ (3) contain three-dimensional 4-fold interpenetrated coordination polymer networks, 2 possesses the SrAl2 structure type (sra, 42638 topology) but the acentric material 3 adopts the diamond structure type (dia, 66 topology). The varying morphologies thus reveal a significant structure-directing effect of the position of the acetate groups during self-assembly of these coordination polymers. Hydrogenbonding mechanisms imparted by the central amine group of the dpa ligand provide ancillary supramolecular structure-directing effects in all three cases. All three materials undergo blue–violet luminescence upon irradiation with ultraviolet light. Introduction Zinc-based coordination polymers containing aromatic dicarboxylate ligands have attracted significant interest because of their capabilities in hydrogen storage,1 gas absorption,2 photoluminescence,3 and nonlinear optical4 applications. The molecular structures of these materials tend to be predicated on the arrangement and binding modes of the carboxylate moieties and the varying coordination geometries possible at divalent zinc because of its lack of crystal-field stabilization. A successful approach toward the elaboration of these materials has been the inclusion of neutral tethering organodiimines, such as 4,4′-bipyridine (4,4′-bpy), that can link together zinc dicarboxylate subunits into higher dimensionalities.5–7 This class of materials also shows substantial promise toward functional properties. For example, the two-dimensional (2D) layered coordination polymer {[Zn(isophthalate)(4,4′-bpy)(H2O)] · 1.5 H2O}∞ manifested intense blue luminescence,5 the interpenetrated three-dimensional (3D) phase {[Zn(2,6-naphthalenedicarboxylate)(4,4′-dipyridylethylene)] · 5DMF · H2O}∞ underwent reversible structural reorganization upon solvent exchange and can absorb a modest amount of H2,6 and the 3D phase [Zn(terephthalate)(4,4′-bpy)0.5]∞ exhibited a striking ability to chromatographically separate linear from branched alkanes.7 In comparison to rigid tethering ligands, such as 4,4′-bpy and 4,4′-dipyridylethylene, the use of organodiimines with a kinked disposition of nitrogen donor atoms in the construction of coordination polymers is much less common. Hanton and coworkers8 and our group9 have shown that 4,4′-dipyridylamine (dpa) can act in oxoanion coordination polymer systems both as a covalent linking agent and a supramolecular structure director through its central amine subunit. Recent results in our group have illustrated that coordination polymers with novel structural motifs can be prepared from the hydrothermal selfassembly of zinc ions with aliphatic dicarboxylates and dpa.10,11 The 3D phase {[Zn(succinate)(dpa)] · H2O}∞ displayed a rarely encountered 4-fold interpenetrated SrAl2-type framework,10 * To whom correspondence should be addressed: Lyman Briggs College, E-30 Holmes Hall, Michigan State University, East Lansing, MI 48825. E-mail: [email protected].

Table 1. Crystal and Structure Refinement Data for 1–3 data

1

empirical formula formula weight collection T 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

C20H19N3O5Zn 446.75 293(2) 0.710 73 monoclinic P21/n 10.392(8) 11.996(9) 14.882(11) 90.648(13) 1855(2) 4 1.600 1.363 0.9023 -13 e h e 13, -16 e k e 15, -18 e l e 18 total reflections 20 889 unique reflections 4376 R(int) 0.0530 parameters/restraints 271/4 R1 (all data)a 0.0691 R1 (I > 2σ(I)) 0.0408 wR2 (all data)b 0.0933 wR2 (I > 2σ(I)) 0.0836 max/min residual 0.390/-0.338 (e-/Å3) GOF 1.028 a

2

3

C40H38N6O10Zn2 893.50 173(2) 0.710 73 monoclinic P21/c 18.6730(5) 12.8309(3) 17.4384(4) 109.707(1) 3933.38(17) 4 1.509 1.286 0.754 -23 e h e 24, -15 e k e 17, -23 e l e 23 42 919 9789 0.0403 542/8 0.0532 0.0367 0.1057 0.0952 1.330/-0.346

C20H17N3O4Zn 428.74 293(2) 0.710 73 orthorhombic Pna21 8.972(11) 13.188(17) 15.573(19) 90 1843(4) 4 1.545 1.365 0.7866 -11 e h e 11, -16 e k e 17, -19 e l e 20 19 324 4228 0.0772 256/2 0.1040 0.0541 0.1146 0.1001 0.599/-0.610

1.025

1.042

R1 ) ∑||Fo| – |Fc||/∑|Fo|. wR2 ) {∑[w(Fo – Fc ) ]/∑[wFo2]2}1/2. b

2

2 2

while the mutually inclined 2d + 2d f 3D polycatenated material {[Zn(adipate)(dpa)] · H2O}∞ manifested both blue luminescence upon ultraviolet irradiation and a reversible guestdependent structural reorganization.11 Herein, we report the results of our synthetic explorations toward luminescent zincbased coordination polymers containing both the kinked diimine dpa and the three isomers of phenylenediacetate (phda). Experimental Section General Considerations. Zn(NO3)2 · 6H2O (Fisher) and all phenylenediacetic acids (Aldrich) were obtained commercially. The organodiimine 4,4′-dipyridylamine (dpa) was prepared by a published

10.1021/cg070599f CCC: $37.00  2007 American Chemical Society Published on Web 10/16/2007

2344 Crystal Growth & Design, Vol. 7, No. 11, 2007

Braverman and LaDuca

Figure 1. Coordination environment of 1, with thermal ellipsoids at 50% probability. Most hydrogen atoms have been removed for clarity. Table 2. Selected Bond Distance (Å) and Angle (deg) Data for 1 Zn1–O2 Zn1–O4a Zn1–N1b Zn1–N3 O1–C18 O2–C18 O3–C20 O4–C20

1.916(2) 1.926(2) 2.008(2) 2.029(2) 1.219(3) 1.255(3) 1.232(3) 1.256(3)

a

O2–Zn1–O4 O2–Zn1–N1b O4a–Zn1–N1b O2–Zn1–N3 O4a–Zn1–N3 N1b–Zn1–N3 O1–C18–O2 O3–C20–O4

107.79(10) 113.61(10) 111.26(10) 95.84(10) 125.63(9) 102.02(10) 125.0(3) 122.8(3)

a Symmetry transformations to generate equivalent atoms: x – 1, y, z. Symmetry transformations to generate equivalent atoms: x – 1/2, –y – 3 /2, z – 1/2. b

procedure.9a Water was deionized above 3 MΩ in-house. Thermogravimetric analysis was performed on a TA Instruments TGA 2050 thermogravimetric analyzer, with a heating rate of 10 °C/min, up to 900 °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. 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. Preparation of {[Zn(1,4-phda)(dpa)] · H2O}∞ (1). Zn(NO3)2 · 6H2O (110 mg, 0.37 mmol), 1,4-phenylenediacetic acid (72 mg, 0.37 mmol), dpa (127 mg, 0.74 mmol), and 10 mL of distilled H2O were placed into a 23 mL Teflon-lined Parr acid-digestion bomb. The bomb was

Figure 2. Single 2D corrugated [Zn(1,4-phda)(dpa)] layer in 1.

sealed and heated to 120 °C for 48 h, after which it was gradually cooled to 25 °C. Colorless plates of 1 (100 mg, 60% based on Zn) were obtained after washing with distilled H2O and acetone and drying in air. Anal. Calcd for C20H19N3O5Zn (1): C, 53.77; H, 4.29; N, 9.41%. Found: C, 52.79; H, 3.96; N, 9.01%. IR (KBr, cm-1): 3532 (m), 3326 (m), 3196 (m), 3108 (m), 3070 (m), 2948 (m), 1629 (s), 1610 (s), 1568 (s), 1350 (s), 1495 (m), 1457 (m), 1213 (m), 1068 (m), 1034 (m), 908 (w), 858 (w), 828 (m), 805 (w), 733 (m), 694 (w), 664 (w), 611 (w), 538 (w). Preparation of {[Zn2(1,2-phda)2(dpa)2] · 2H2O}∞ (2). The preparation for 1 was followed with the exception of the use of 1,2phenylenediacetic acid (72 mg, 0.37 mmol). Colorless blocks of 2 (82 mg, 50% based on Zn) were obtained. Anal. Calcd for C40H38N6O10Zn2 (2): C, 53.77; H, 4.29; N, 9.41%. Found: C, 53.52; H, 3.93; N, 9.78%. IR (cm-1): 2927 (w), 1591 (s), 1513 (s), 1476 (m), 1454 (m), 1437 (m), 1420 (m), 1383 (m), 1336 (s), 1310 (m), 1292 (m), 1233 (w), 1202 (m), 1082 (m), 1058 (m), 1020 (m), 1014 (m), 903 (m), 873 (m), 826 (s), 801 (m), 731 (s), 717 (m). Preparation of [Zn(1,3-phda)(dpa)]∞ (3). The preparation for 1 was followed with the exception of the use of 1,3-phenylenediacetic acid (72 mg, 0.37 mmol). Colorless blocks of 3 (110 mg, 69% based on Zn) were obtained. Anal. Calcd for C20H17N3O4Zn (3): C, 56.03; H, 4.00; N, 9.80%. Found: C, 55.53; H, 3.86; N, 9.87%. IR (KBr, cm-1): 3265 (m), 3166 (m), 3082 (m), 3002 (m), 2933 (m), 1648 (s), 1602 (s), 1526 (s), 1453 (s), 1392 (s), 1350 (s), 1274 (m), 1213 (s),

Luminescent 2D and 3D Zinc Coordination Polymers

Crystal Growth & Design, Vol. 7, No. 11, 2007 2345

Figure 3. Stacking of adjacent corrugated layers in 1. Hydrogen bonds are represented by dashed lines. Table 3. Hydrogen-Bonding Distance (Å) and Angle (deg) Data for 1–3 d (H · · · A)

∠DHA

d (D · · · A)

O1W–H1WA · · · O3 O1W–H1WB · · · O3 N2–H2N · · · O1W

2.065(2) 1.910(2) 1.988(2)

170(4) 170(3) 169(3)

2.894(4) 2.767(3) 2.841(4)

O1W–H1WA · · · O4 O1W–H1WB · · · O5 O2W–H2WA · · · O8 O2W–H2WB · · · O2 N2–H2N · · · O1W N5–H5N · · · O2W

1.86(2) 1.92(2) 1.95(2) 1.94(2) 1.92(2) 1.96(2)

174(3) 165(3) 167(3) 176(3) 166(3) 167(3)

2.722(3) 2.763(3) 2.777(3) 2.793(3) 2.774(3) 2.801(3)

N2–H2N · · · O4

1.94(2)

164(5)

2.815(7)

D–H · · · A 1

2

3

1152 (w), 1068 (m), 1026 (s), 908 (w), 824 (m), 717 (m), 653 (m), 611 (m), 534 (m). X-ray Crystallography. A colorless plate of 1 (with dimensions 0.35 × 0.20 × 0.05 mm), a colorless block of 2 (0.62 × 0.39 × 0.11 mm), and a colorless block of 3 (0.35 × 0.35 × 0.15 mm) were subjected to single-crystal X-ray diffraction using either a Bruker-AXS SMART 1k CCD instrument (1 and 3) or a Bruker-AXS Apex II CCD instrument (2). Reflection data were acquired using graphite-monochomated Mo KR radiation (λ ) 0.710 73 Å). The data was integrated via SAINT.12 The Lorentz and polarization effects and empirical absorption corrections were applied with SADABS.13 The structures were solved using direct methods and refined on F2 using SHELXTL.14 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 the central nitrogens of the dpa moieties, and any water molecules were found via Fourier difference maps, then restrained at fixed positions, and refined isotropically. Extended thermal ellipsoids within one of the phenylenediacetate aromatic rings within the structure of 2 are indicative of a slight amount of torsional disorder. Initial attempts to model this behavior were not successful; further efforts in this direction

symmetry transformation for A -x + 2, –y + 1, -z + 1

-x - 1, -y + 2, -z –x - 1, y - 1/2, -z + 1/2 x, -y + 3/2, z - 1/2 x, -y + 3/2, z + 1/2 -x + 2, -y, z + 1/2

were abandoned because the atoms in question were not especially germane to the coordination polymer connectivity. Relevant crystallographic data for 1–3 are listed in Table 1. Crystallographic data (excluding structure factors) for 1–3 have been deposited with the Cambridge Crystallographic Data Centre under 651035–651037, 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: 441223336033. E-mail: [email protected]).

Results and Discussion Synthesis and Spectral Characterization. The coordination polymers 1–3 were obtained cleanly as uniform-phase crystalline products [as judged by elemental analysis and powder X-ray diffraction analysis (XRD)] under hydrothermal conditions through the combination of zinc nitrate, 4,4′-dipyridylamine, and the appropriate isomer of phenylenedicarboxylic acid. The infrared spectra of 1–3 were consistent with their formulations. Sharp and medium-intensity bands in the range of ∼1600–1200



Figure 4. Expanded asymmetric unit of 2, with symmetry-related atoms to complete the coordination environments at the zinc atoms and thermal ellipsoids shown at 50% probability. Most hydrogen atoms have been removed for clarity. Water molecules of crystallization are not shown. Table 4. Selected Bond Distance (Å) and Angle (deg) Data for 2 a

Zn1–O5 Zn1–O1 Zn1–N1 Zn1–N3b Zn2–O3 Zn2–O7 Zn2–N4 Zn2–N6c O1–C8 O2–C8 O3–C10 O4–C10 O5–C18 O6–C18 O7–C20 O8–C20

1.970(2) 1.928(2) 2.051(2) 2.010(2) 1.956(2) 1.981(2) 2.026(2) 2.050(2) 1.284(3) 1.224(3) 1.284(3) 1.231(3) 1.291(3) 1.214(3) 1.275(3) 1.227(3)

a

O1–Zn1–O5 O1–Zn1–N3b O5a–Zn1–N3b O1–Zn1–N1 O5a–Zn1–N1 N3b–Zn1–N1 O7–Zn2–O3 O3–Zn2–N4 O7–Zn2–N4 O3–Zn2–N6c O7–Zn2–N6c N4–Zn2–N6c O2–C8–O1 O4–C10–O3 O6–C18–O5 O8–C20–O7

110.84(8) 121.99(8) 115.41(8) 102.08(7) 99.91(7) 102.44(8) 97.31(8) 110.09(8) 122.35(8) 111.85(8) 109.13(8) 105.98(8) 124.4(2) 123.5(2) 123.1(2) 123.2(2)

a Symmetry transformation to generate equivalent atoms: x + 1, -y + 3/2, z + 1/2. b Symmetry transformation to generate equivalent atoms: x, -y + 1/2, z + 1/2. c Symmetry transformation to generate equivalent atoms: x, -y + 5/2, z - 1/2.

cm-1 were ascribed to stretching modes of the pyridyl rings of the dpa moieties and the aromatic rings of respective phda ligands.15 Features corresponding to pyridyl and benzene ring puckering exist in the region between ∼820 and ∼600 cm-1. Asymmetric and symmetric C–O stretching modes of the fully deprotonated, ligated phda linkers were substantiated by strong, broadened bands at ∼1600 and ∼1400 cm-1. The lack of bands in the region of ∼1710 cm-1 is indicative of complete deprotonation of all carboxylate groups in 1-3. Broad bands in the area of ∼3400–3200 cm-1 in all cases represent N–H stretching modes within the dpa ligands and O–H stretching

modes within water molecules of crystallization found in 1 and 2. The broadness of the latter features is attributed to widespread hydrogen-bonding pathways observed therein. Structural Description of {[Zn(1,4-phda)(dpa)] · H2O}∞ (1). Compound 1 possesses an asymmetric unit consisting of one zinc atom, one ligated dpa molecule, one 1,4-phda moiety, and one water molecule of crystallization. The coordination environment around Zn is a slightly distorted [ZnO2N2] tetrahedron, where the two nitrogen donor atoms belong to two different, symmetry-related dpa ligands and the two oxygen donor atoms are part of two separate 1,4-phda ligands (Figure 1). Selected bond length and angle information for 1 is given in Table 2. Extension of the structure through bisbridging/bis(monodentate) 1,4-phda ligands reveals neutral one-dimensional (1D) undulating [Zn(1,4-phda)]n chains traveling along the a crystal direction with a through-ligand Zn–Zn contact distance of 10.39 Å. Each acetate side chain is twisted relative to the aromatic ring by varying extents, where arm-A (denoted by C17–C18) exhibits a slightly synclinal torsion angle of 81.6° (through C18– C17–C11–C12) and arm-B (denoted by C19–C20) possesses a periplanar torsion angle of 20.29° (through C20–C19–C14–C15). The Zn–Zn–Zn angle through the 1,4-phda ligands is exactly 180°, likely accommodated by the geometrical disposition of the acetate side chains. The virtually linear [Zn(1,4-phda)]n chains are conjoined into 2D corrugated (4,4) rhomboid gridlike layers running parallel to the ab crystal plane through linking dpa moieties with a through-ligand Zn–Zn distance of 11.65 Å (Figure 2). The dimensions of the intralayer incipient



Figure 5. Single 42638 topology sra network in 2.

voids are 18.71 × 11.73 Å, as measured by through-space Zn–Zn distances. As determined by a through-space Zn–Zn contact distance, the wavelength of the “sawtooth” corrugation pattern is 14.882 Å, which defines the c lattice parameter. The Zn–Zn–Zn angle through conjoining dpa entities is 101.5°, which is influenced by the kinked nitrogen donor disposition, the inter-ring torsion observed between pyridyl subunits of each dpa subunit (17.84°), and the tetrahedral metal coordination geometry. Parallel corrugated networks stack in an interdigitating fashion (interlayer Zn–Zn distance of 6.14 Å) along the b crystal plane and are pinned by hydrogen bonding through water molecules of crystallization (Figure 3). Each water molecule of crystallization accepts one hydrogen bond from the central amine of a dpa moiety while engaging in both weak inter- and intralayer hydrogen bonding with unligated carboxylate oxygens of arm-B. The water molecules of crystallization occupy very small regions, with no apparent void space registered by a calculation with PLATON.16 Hydrogen-bonding details for 1 are given in Table 3. Structural Description of {[Zn2(1,2-phda)2(dpa)2] · 2H2O}∞ (2). The asymmetric unit of 2 contains two crystallographically unique Zn atoms, two ligated dpa moieties, two 1,2-phda dianions, and two water molecules of crystallization. The coordination environment around each Zn is best portrayed as a slightly distorted [ZnO2N2] tetrahedron, with each Zn atom ligated by two nitrogen atoms from two symmetry-related dpa subunits and a pair of carboxylate oxygen atoms originating from crystallographically distinct 1,2-phda ligands (Figure 4). Selected bond length and angle information for 2 is given in Table 4.

Extension of this network through the bisbridging/bis(monodentate) 1,2-phda linkers (phda-1, C1–C10/O1–O4; phda-2, C11–C20/O5–O8) shows the formation of neutral 1D [Zn(1,2phda)]n chains running along the [01–1] crystal direction. These are constructed by the junction of Zn1–Zn2 through both phda-1 and phda-2. The acetate arms in the phda-1 ligands differ in torsion angle with respect to the aromatic ring, where arm-A (denoted by C7–C8) exhibits a synclinal conformation (86.1° torsion angle through C8–C7–C6–C5) and arm-B (denoted by C9–C10) rests in a gauche orientation (67.2° torsion angle through C10–C9–C6–C5). In phda-2, arm-C (denoted by C17–C18) adopts an anticlinal conformation (90.5° through C18–C17–C11–C16) and arm-D (denoted by C19–C20) is found in a gauche conformation (53.7° through C20–C19–C16–C15). The Zn–Zn contact distance mediated by the phda-1 ligand is 9.97 Å; because of the differences in the torsion angles of the acetate arms, the Zn–Zn distance through phda-2 is lengthened to 10.34 Å. Exobidentate dpa ligands (dpa-1, denoted by N1–N3; dpa-2, denoted by N4–N6) connect [Zn(1,2-phda)]n chains to form a 3D network (Figure 5), wherein Zn1 is bound to two other Zn1 atoms through dpa-1 and to two Zn2 atoms through dpa-2. The Zn–Zn distances through dpa-1 and dpa-2 are 11.42 and 11.18 Å, respectively. Differences observed in Zn–Zn distances through the dpa ligands can be attributed to the slight adjustments in the torsion angles between the pyridyl rings (36.4° in dpa-1 and 34.6° in dpa-2). An investigation of the topology of this 3D coordination polymer best treats Zn1 and Zn2 as identical connecting nodes, because only relatively minor torsional differences within the ligands cause the need for two



Figure 6. Framework view of the SrAl2-type 4-fold interpenetrated network found in 2. Each individual network is shown in a different color.

Figure 7. Coordination environment of 3, with thermal ellipsoids shown at 50% probability. Most hydrogen atoms have been removed for clarity.

crystallographically distinct Zn atoms. Examination of this uninodal 3D framework with TOPOS17 reveals a four-connected SrAl2 type (sra, 42638 topology) with 4-fold interpenetration (Figure 6). The interpenetration belongs to class IIIa (both spacegroup symmetry and translational relation of the individual nets) with Zt ) 2 and Zn ) 2, where Zt represents the number of interpenetrated nets related by translation and Zn denotes the number of interpenetrated nets related by crystallographic symmetry.18 Intermolecular forces encourage network interpenetration because water molecules of crystallization enable hydrogen bonding between independent frameworks. The central N–H unit of both dpa-1 and dpa-2 donate hydrogen bonds to different

water molecules (O1W · · · H2N and O2W · · · H5N). These water molecules in turn donate hydrogen bonds to unligated and ligated carboxylate oxygen atoms within phda-1 and phda-2. Details regarding these noncovalent interactions in 2 are given in Table 3. According to a calculation with PLATON, the incipient void spaces occupied by the unligated water molecules represent 6.3% of the unit-cell volume. Structural Description of [Zn(1,3-phda)(dpa)]∞ (3). With an asymmetric unit consisting of one zinc atom, one ligated dpa molecule, and one 1,3-phda moiety, 3 crystallized in the acentric orthorhombic space group Pna21. The Flack parameter19 of 0.03(2) indicates a high degree of enantiomeric excess. The

Luminescent 2D and 3D Zinc Coordination Polymers Table 5. Selected Bond Distance (Å) and Angle (deg) Data for 3 Zn1–O2 Zn1–O3a Zn1–N3 Zn1–N1b O1–C18 O2–C18 O3–C20 O4–C20

1.949(6) 1.986(5) 2.044(5) 2.075(5) 1.161(11) 1.177(8) 1.253(7) 1.246(6)

O2–Zn1–O3a O2–Zn1–N3 O3a–Zn1–N3 O2–Zn1–N1b O3a–Zn1–N1b N3–Zn1–N1b O1–C18–O2 O4–C20–O3

103.7(2) 130.5(2) 111.54(18) 103.4(3) 95.5(2) 106.52(18) 115.2(7) 124.3(6)

a Symmetry transformations to generate equivalent atoms: -x + 3/2, y - 1/2, z + 1/2. b Symmetry transformations to generate equivalent atoms: -x + 5/2, y + 1/2, z + 1/2.

[ZnO2N2] coordination environment is best described as slightly distorted tetrahedral, as is the case for 1 and 2. The two oxygen donors and two nitrogen donors belong to different symmetrically related 1,3-phda and dpa linkers, respectively, as seen in Figure 7. Selected bond length and angle information for 3 is given in Table 5. Extension of the structure through bisbridging/bis(monodentate) 1,3-phda moieties exposes neutral 1D [Zn(1,3-phda)]n ribbons coursing orthogonally to the a crystal direction, with a Zn–Zn through-ligand distance of 10.83 Å. Unique torsion angles are observed in both acetate side chains of the linking 1,3-phda moiety with respect to its aromatic ring. Arm-A (denoted by C17–C18) bears a syn conformation (30.7° torsion through C18–C17–C11–C16), whereas arm-B (denoted by C19–C20) is found in a synclinal orientation (81.4° torsion through C20–C19–C13–C12). Tethering dpa ligands, with an inter-ring torsion angle of 35.4°, form jagged [Zn(dpa)]n chains

Figure 8. Single diamondoid network in 3.


with a through-ligand Zn–Zn distance of 11.52 Å. The intersection of these with the [Zn(1,3-phda)]n ribbons results in a 3D four-connected [Zn(1,3-phda)(dpa)]∞ coordination polymer framework (Figure 8). Topological analysis of this framework with TOPOS software reveals a 4-fold interpenetrated diamond-type (dia, 66 topology) (Figure 9), permitted by the 17.97 × 15.95 Å apertures within a single framework (as measured by through-space Zn–Zn distances). Each metal center node is linked to four others, forming six six-membered circuits at each metal atom. The shortest Zn–Zn distance between the individual interpenetrated networks is 6.45 Å. As is the case for 2, the interpenetration in 3 is of class IIIa with Zt ) 2 and Zn ) 2.17 Hydrogen bonding plays a key role in the stabilization of this interpenetrated structure, via a N–H · · · O interaction mediated by the dpa central amine subunit (Table 3). Noncentrosymmetric diamondoid structural motifs are not uncommon, with even the prototypical nonlinear optical (NLO) material potassium hydrogen phthalate adopting this pattern.20 Lin has prepared several zinc and cadmium pyridinecarboxylate interpenetrated diamondoid coordination polymers that exhibit intriguing NLO properties.21 In the pyridinecarboxylate system of Lin, odd-numbered levels of interpenetration in diamondoid coordination polymer lattices tend to result in acentricity, while even-numbered interpenetration levels promote the presence of crystallographic inversion centers.22 The acentric nature of the 4-fold interpenetrated diamondoid networks in 3 stands in contrast to this previous trend. It is possible that the presence



Figure 9. Four-fold interpenetration of the diamondoid coordination polymer networks in 3. Each individual network is shown in a different color.

Figure 10. Emission spectra of 1–3.

of the kinked tethering dpa ligand and the flexibility of the pendant arms of the 1,3-phda dianion promote the lack of inversion centers within 3. Thermogravimetric Analysis (TGA). The decomposition of 1-3 was investigated via TGA. Combustion of organics commenced at ∼325 °C for the 2D coordination polymer 1.

The mass remnant of 28% was consistent with the deposition of ZnCO3 (28% expected). For 2, a slight decrease in mass at ∼140 °C (4% predicted and 2% observed) marks the expulsion of water molecules of crystallization. Elimination of the remaining components in 2 began at ∼290 °C, with a mass remnant of 24% at ∼850 °C corresponding roughly to the deposition of ZnCO3.


Compound 3 underwent thermal decomposition at ∼360 °C, with a mass remnant of ∼24%, somewhat comparable with the production of ZnCO3 (29% expected), perhaps admixed with some ZnO. TGA traces for 1-3 are given in Figures S1–S3 in the Supporting Information, respectively. Luminescent Properties. Irradiation of complexes 1-3 with ultraviolet light (λ ) 300 nm) in the solid state resulted in blue–violet visible light emission in all cases. Similar emission maxima (1, λmax ∼ 350 nm; 2, λmax ∼ 356 nm; 3, λmax ∼ 350 nm) were observed between samples (Figure 10), with a more extensive tail in the blue region in the case of 2. These emissions can likely be attributed to the π–π* or π–n electronic transitions within ligand-centered aromatic systems of the phda or dpa moieties, by comparison with other luminescent d10 metal coordination polymers.23 Shifts in maxima and the spectral profile likely originate from subtle differences in bond length and ligand conformation or other local supramolecular effects than can also affect vibrational nonemissive radiative pathways.


(2)

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Conclusions A family of three luminescent zinc coordination polymers based on phenylenediacetate and dpa ligands has been prepared via hydrothermal self-assembly. In all of these materials, the zinc atoms are tetrahedrally coordinated and the phenylenediacetate ligands adopt a bisbridging/bis(monodentate)-binding mode. Therefore, the differences among the three structural morphologies rests largely on the disposition of the two acetate “arms” within the phenylenediacetate components and the torsion of their carboxylate termini. Because the pendant carboxylates within the 1,4-phda ligands are 180° apart, virtually straight [Zn(1,4-phda)]n chains are formed; in turn, these are linked by kinked dpa tethers to form the corrugated 2D layer motifs of 1. While both 2 and 3 manifest 4-fold interpenetrated four-connected coordination polymer networks, the shrinkage of the distance between the carboxylate termini in 1,2-phda as compared to 1,3-phda promotes a change between SrAl2 and acentric diamondoid structures. Continued synthetic efforts with organodiimines and dicarboxylate ligands with both rigid centers and flexible pendant “arms” should result in a wide scope of coordination polymer materials. Acknowledgment. The authors gratefully acknowledge Michigan State University for financial support of this work. We thank Dr. Rui Huang for performing the elemental analysis and Dr. Kathryn Severin for use of the fluorimeter. M.A.B. thanks the MSU Quality Fund Undergraduate Research Program for financial support. Supporting Information Available: TGA traces for 1–3. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG070599F

Luminescent Two- and Three-Dimensional Zinc Coordination

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