New Molecular Rectangles and Coordination Polymers Constructed

DOI: 10.1021/cg901079s

New Molecular Rectangles and Coordination Polymers Constructed from Binuclear Phenoxo-Bridged [Cu2] and [Zn2] Tectons

2010, Vol. 10 2096–2103

Gabriela Marinescu,†,‡ Geanina Marin,‡ Augustin M. Madalan,‡ Alis Vezeanu,‡ Carmen Tiseanu,§ and Marius Andruh*,‡ †

Institute of Physical Chemistry of the Romanian Academy, Coordination Chemistry Laboratory, Bucharest, Spl. Independentei nr. 202, 060021-Bucharest Romania, ‡University of Bucharest, Faculty of Chemistry, Inorganic Chemistry Laboratory, Str. Dumbrava Rosie nr. 23, 020464-Bucharest, Romania, and §National Institute for Laser, Plasma and Radiation Physics, Laboratory of Solid-State Quantum Electronics, P.O. Box MG-36, Magurele, 077125, Romania Received September 4, 2009; Revised Manuscript Received February 10, 2010

ABSTRACT: The self-assembly processes between binuclear [Zn2] and [Cu2] complex cations and exo-bidentate ligands [trans4,40 -azo-pyridine (azpy), 4,40 -bipyridine (4,40 -bipy), 1,2-bis(4-pyridyl)ethane (bpeta), 1,3-bis(4-pyridyl)propane (bpp)] generate two types of complexes: tetranuclear species with a rectangular topology and one-dimensional (1-D) coordination polymers: [{L2(μ-OH)Cu2}(μ-azpy)2{Cu2(μ-OH)L2}][{L2(μ-OH)Cu2(H2O)}(μ-azpy)2{(H2O)Cu2(μ-OH)L2}](ClO4)8 (1); [{L1Cu2(H2O)2}(μ-azpy)2{Cu2L1(H2O)2}](ClO4)4 3 (azpy) 3 2H2O (2); 1¥[{Cu2(μ-OH)L3}(μ-bpp)](ClO4)2 (3); 1¥[{L3Zn2(μ-OH)}(μ-4,40 -bipy)](ClO4)2 3 2H2O (4); 1¥[{L3Zn2(μ-OH)}(μ-bpeta)](ClO4)2 (5a); 1¥[{L3Zn2(μ-OH)}(μ-bpeta)](ClO4)2 3 THF (5b); 1¥[{L2Zn2(μ-OH)}(μ-4,40 -bipy)2](ClO4)2 3 2H2O (6) (H2Ln are compartmental Schiff-base ligands resulting from condensation reactions between 2,6-diformyl-p-cresol with, respectively, 1,3-diamino-propane, 2-aminoethyl-pyridine, and N,N-dimethyl-ethylenediamine). The zinc complexes exhibit luminescence properties.

Introduction One of the most important research areas in crystal engineering concerns the design of coordination polymers with interesting properties and, ultimately, technologically useful functions.1 The general strategy in obtaining coordination polymers consists of self-assembly processes between metal ions (assembling cations) and bridging ligands. The assembling cations are complex species that result by dissolving the metal salt in water or in nonaqueous solvents. In constructing coordination polymers, the metal ions act as nodes, being connected by exo-dentate ligands (spacers).2 Metal ions play a double role: a structural one (directing and sustaining the solid-state architecture) and a functional one (carrying magnetic, optical, or redox properties). The metal ions can also carry auxiliary ligands (usually chelating species), which block part of the coordination sites of the metal ion. The role of the blocking ligands is to control: (1) the dimensionality of the coordination polymers; (2) the topology of the metal centers. In a series of papers, we have shown that coordination polymers with interesting topologies can be obtained employing oligonuclear complexes as nodes.3 The incorporation of the oligonuclear complexes into extended frameworks occurs through: (i) formation of the nodes in a preliminary step, followed by the reaction with appropriate spacers; (ii) formation of the nodes as a result of the interaction of the metal ions with the spacer; (iii) serendipitous assembly of the metal ions into clusters which are then interconnected by spacer molecules. The formation of the dinuclear nodes in a preliminary step succeeds by employing compartmental ligands, which hold together the two metal ions. Such ligands can be either *To whom correspondence should be addressed. E-mail: marius. [email protected]. pubs.acs.org/crystal

Published on Web 03/30/2010

end-off, side-off, or macrocyclic species, most of them being obtained by reacting 2,6-diformyl-p-cresol with various diamines (Scheme 1). The metal ions can interact through their axial positions with a large variety of ligands. Let us recall here two examples, which are related to our present work, both relying upon the macrocyclic ligand resulting from the condensation of 2,6-diformyl-p-cresol with 1,3-diamino-propane (Scheme 1 - H2L1). The reaction of the dinuclear cooper complex with the dianion of the acetylenedicarboxylic acid leads to a linear coordination polymer.4 The copper atoms are quadruply bridged: two pre-existing phenoxo oxygen atoms arising from the macrocyclic ligand, and two carboxylato groups, exhibiting the classical syn-syn bridging mode. The copper(II) ions display an elongated octahedral geometry. The self-assembly process involving the zinc complex with the same ligand and 4,40 -bipy generates a ladder-like one-dimensional (1-D) coordination polymer with {Zn2} platforms.5 For a 1:1 molar ratio between the binuclear complex and the exo-dentate ligand, two types of polynuclear complexes can be assembled: discrete (tetranuclear rectangles) and 1-D coordination polymers (Scheme 2A,B). We have shown that the reactions between dinuclear copper(II) complexes with endoff ligands (Scheme 1 - HL2, HL3) and exo-bidentate ligands (4,40 -bipyridine, bis(4-pyridyl)ethane, bis(4-pyridyl)ethylene, isonicotinato) lead to a family of molecular rectangles.6 The similar zinc complex, which undergoes an interesting photodimerization in the solid state, has been obtained by McGillivray et al., by connecting the dinuclear zinc moieties through bis(4-pyrdyl)ethylene.7 The crystals were obtained from a methanol/water mixture. When the crystallization occurs from an ethanolic solution, a one-dimensional coordination polymer is obtained, with the {Zn2} nodes connected by double bis(4-pyridyl)ethylene bridges.8 In this case, the molar ratio between the two components (dinuclear tecton and spacer) is 1:2 (Scheme 2C). We have also shown that the reaction of a r 2010 American Chemical Society

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Scheme 1

Scheme 2

heterobinuclear [CuZn] complex (in which the two metal ions are held by a dissymmetric macrocyclic bicompartmental ligands) with bis(4-pyridyl)ethylene affords a heterometallic rectangle, [Cu2Zn2].4 The aim of this work is to answer the following questions: (1) Is it possible to obtain 1-D coordination polymers starting from binuclear copper(II) complexes with end-off ligands (molar ratio = 1:1)? (2) What kind of polynuclear complexes are obtained by reacting similar binuclear complexes of zinc(II) with exo-dentate ligands? (3) Do the new polynuclear zinc complexes exhibit luminescence properties? Here we report on new polynuclear complexes of copper(II) and zinc(II), which are obtained employing binuclear complexes with Robson-type ligands as tectons. Experimental Section Syntheses of [{L2(μ-OH)Cu2}(μ-azpy)2{Cu2(μ-OH)L2}][{L2(μ-OH)Cu 2 (H 2 O)}(μ-azpy)2 {(H 2 O)Cu 2 (μ-OH)L 2 }](ClO 4 )8 (1), [{L 1 Cu 2 (H 2 O)2 }(μ-azpy)2 {Cu 2 L 1 (H 2 O)2 }](ClO 4 )4 3 (azpy) 3 2H 2 O (2), 1 ¥ [{Cu 2 (μ-OH)L 3 }(μ-bpp)](ClO 4 )2 (3), 1 ¥ [{L 3 Zn 2 (μ-OH)}(μ-4,4 0 -bipy)](ClO 4 )2 3 2H 2 O (4), 1 ¥ [{L 3 Zn2 (μ-OH)}(μ-bpeta)](ClO 4 )2 (5a), 1 ¥ [{L 3 Zn 2 (μ-OH)}(μ-bpeta)](ClO 4 )2 3 THF (5b), 2 ¥ 0 1 [{L Zn 2 (μ-OH)}(μ-4,4 -bipy)2 ](ClO 4 )2 3 2H 2 O (6). All starting materials were of reagent grade and were used without further purification. The dinuclear cooper complexes, used as precursors, have been synthesized as already described.9 In the case of zinc complexes (4, 5a, 5b, 6), the dinuclear precursors were synthesized in situ, by reacting the Schiff bases with zinc perchlorate. [{L2(μ-OH)Cu2}(μ-azpy)2{Cu2(μ-OH)L2}][{L2(μ-OH)Cu2(H2O)}(μ-azpy)2{(H2O)Cu2(μ-OH)L2}](ClO4)8 (1). The copper(II) tetranuclear complex was prepared by the reaction between trans-4,40 -azopyridine and [L2(μ-OH)Cu2](ClO4)2. 4,40 -Azopyridine was prepared by

oxidative coupling of 4-aminopyridine by hypochlorite.10 The copper(II) precursor complex (0.214 g, 0.3 mmol) was dissolved in 30 mL of methanol, while 0.110 g of 4,40 -azopyridine (0.6 mmol) was dissolved in 10 mL of methanol. The ligand was added with stirring into the flask containing the copper(II) solution. Slow evaporation of the reaction mixture gave a crystalline product after a few days. Yield ca. 70%. Elemental chemical analysis: 43.67% C, 3.66% H, 12.35% N (calcd); 43.2% C, 3.6% H, 12.1% N (found). IR data (KBr, cm-1): 3519s, 3436s, 2924w, 2862w, 1636s, 1605s, 1560s, 1481w, 1448 m, 1412s, 1339m, 1311w, 1239 m, 1191w, 1106vs, 1003w, 974w, 932w, 840m, 781m, 624s, 589w, 569w, 524w. [{L1Cu2(H2O)2}(μ-azpy)2{Cu2L1(H2O)2}](ClO4)4 3 (azpy) 3 2H2O 2. The methanolic solution (30 mL) of the homobinuclear precursor [Cu2L1]Cl2 3 6H2O (0.218 g, 0.3 mmol) was added with stirring to a methanolic solution (10 mL) containing 4,40 -azopyridine (0.110 g, 0.6 mmol). Green single crystals appeared after several days by slow evaporation of the solution. Yield ca. 60%. Elemental chemical analysis: 44.24% C, 4.19% H, 13.23% N (calcd); 44.5% C, 4.2% H, 12.9% N (found). IR data (KBr, cm-1): 3429s, 3058w, 2928w, 2865w, 1637vs, 1592s, 1565s, 1441m, 1413s, 1372w, 1325s, 1274w, 1244w, 1201w, 1095vs, 1003w, 971w, 936w, 847m, 817m, 760m, 737w, 623s, 571m. 3 ¥ 1 [{Cu2(μ-OH)L }(μ-bpp)](ClO4)2 3. The ethanolic solution (20 mL) containing 0.1 mmol of the homobinuclear precursor [Cu2L3](ClO4)2 3 H2O was added with stirring to a ethanolic solution (10 mL) of 1,3-bis(4-pyridyl)propane (0.1 mmol). Green single crystals appeared after several days by slow evaporation of the resulting solution. Yield ca. 80%. Elemental chemical analysis: 42.66% C, 5.01% H, 9.95% N (calcd); 42.6% C, 5.1% H, 9.7% N (found). IR data (KBr, cm-1): 1648s, 1628s, 1614s, 1554s, 1455m, 1423m, 1408m, 1338s, 1273w, 1254 m, 1239w, 1188w, 1096vs, 1006m, 945 m, 893w, 813 m, 765 m, 668w, 622s, 523w, 464 m, 418w. 3 ¥ 0 1 [{L Zn2(μ-OH)}(μ-4,4 -bipy)](ClO4)2 3 2H2O 4. Ethanolic solutions containing stoichiometric amounts of 2,6-diformyl-4-methylphenol (0.1 mmol, 10 mL) and N,N0 -dimethyl-ethylenediamine (0.2 mmol, 5 mL) were mixed and kept under continuous stirring for 30 min at 50 C. This solution was then reacted with the stoichiometric amounts of LiOH (0.1 mmol, 5 mL H2O) and Zn(ClO4)2 3 6H2O (0.2 mmol, 10 mL EtOH) and kept under stirring for 30 min. An ethanolic solution (10 mL) containing the exo-bidentate ligand 4,40 -bipyridine (0.1 mmol) was added to the resulting mixture containing the dinuclear precursor [L3Zn2(μ-OH)](ClO4)2. Slow evaporation at room temperature of the solution led yellow crystals after several days. Yield ca. 75%. Elemental chemical analysis: 38.50% C, 4.79% H, 9.98% N (calcd); 38.7% C, 4.7% H, 10.1% N (found). IR data (KBr, cm-1): 3584m, 2967w, 1644s, 1548s, 1458s, 1414m, 1340m, 1230m, 1071vs, 890w, 820s, 775m, 620s. ¥ ¥ 3 3 1 [{L Zn 2 (μ-OH)}(μ-bpeta)](ClO 4 )2 5a; 1 [{L Zn 2 (μ-OH)}(μ-bpeta)](ClO4 )2 3 THF 5b. Compounds 5a and 5b have been obtained following the same procedure described for 1¥[{L3Zn2(μ-OH)}(μ-4,40 -bipy)](ClO4)2 3 2H2O (4), but the solvent was EtOH for 5a and THF for 5b. Slow evaporation of the solutions at room temperature led to yellow crystals after several days. Yield ca. 80%. Elemental chemical analysis: 41.75% C, 4.83% H, 10.07% N (calcd); 41.9% C, 4.7% H, 9.8% N (found). IR data (KBr, cm-1): 3552s, 2964m, 2921m, 2873m, 2803w, 1645vs, 1620vs, 1550vs, 1457m, 1433m, 1343m, 1098vs, 943w, 890m, 835m, 777m, 622s, 548s, 449m (5a). Yield ca. 85%. Elemental chemical analysis: 43.73% C, 5.34% H, 9.27% N (calcd); 43.5% C, 5.4% H, 9.4% N (found). IR data

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Table 1. Crystallographic Data, Details of Data Collection and Structure Refinement Parameters for Compounds 1-6 compound

1

2

3

chemical formula M (g mol-1) temperature, (K) wavelength, (A˚) crystal system space group a (A˚) b (A˚) c (A˚) R () β () γ () V (A˚3) Z Dc (g cm-3) μ (mm-1) F(000) goodness-of-fit on F2 final R1, wR2 [I>2σ(I)] R1, wR2 (all data) largest diff peak and hole (e A˚-3)

C132H132Cl8Cu8N32O42 3630.58 293(2) 0.71073 triclinic P1 11.9447(12) 18.0489(32) 18.5751(26) 66.6475(13) 86.003(11) 81.7715(11) 3638.3(6) 1 1.655 1.387 1848 1.009 0.0587, 0.1099 0.1593, 0.1373 1.181, -0.588

C78H88Cl4Cu4N20O26 2117.55 293(2) 0.71073 monoclinic P21/n 12.8047(10) 20.6190(16) 16.9607(14) 90 98.402(10) 90 4429.9(6) 2 1.579 1.156 2152 0.806 0.0609, 0.1148 0.2011, 0.1538 0.558, -0.337

C30H42Cl2Cu2N6O10 844.68 293(2) 0.71073 monoclinic P21/c 10.8633(10) 15.3513(10) 21.4333(17) 90 93.062(9) 90 3569.2(5) 4 1.572 1.404 1744 0.939 0.0677, 0.1142 0.1977, 0.1451 0.717, 0.717

compound chemical formula M (g mol-1) temperature, (K) wavelength, (A˚) crystal system space group a (A˚) b (A˚) c (A˚) R () β () γ () V (A˚3) Z Dc (g cm-3) μ (mm-1) F(000) goodness-of-fit on F2 final R1, wR2 [I>2σ(I)] R1, wR2 (all data) largest diff peak and hole (e A˚-3)

4 C27H40Cl2N6O12Zn2 842.26 293(2) 0.71073 monoclinic P21/n 9.0397(3) 33.9825(14) 12.4503(4) 90 110.173(3) 90 3590.0(2) 4 1.551 1.551 1720 0.974 0.0525, 0.1327 0.0876, 0.1485 0.648, -0.455

5a C29H40Cl2N6O10Zn2 834.31 293(2) 0.71073 monoclinic P21/c 10.9507(6) 15.2354(7) 21.4721(12) 90 94.931(4) 90 3569.1(3) 4 1.553 1.555 1720 1.085 0.0803, 0.1435 0.1552, 0.1695 0.449, -0.973

(KBr, cm-1): 3555s, 2923m, 2874m, 2802w, 1646vs, 1621vs, 1550vs, 1463m, 1432m, 1345m, 1089vs, 941w, 891m, 833m, 777m, 623s, 549s (5b). 2 ¥ 0 1 [{L Zn2(μ-OH)}(μ-4,4 -bipy)2](ClO4)2 3 2H2O 6. Compound 6 has been obtained following the same general procedure described for 1¥[{L3Zn2(μ-OH)}(μ-4,40 -bipy)](ClO4)2 3 2H2O 4, using L2 instead of L3 in MeOH. Slow evaporation of the solution at room temperature led after several days yellow crystals. Yield ca. 80%. Elemental chemical analysis: 48.42% C, 4.16% H, 10.51% N (calcd); 48.6% C, 4.0% H, 10.4% N (found). IR (KBr, cm-1): 3616m, 3539m, 3059w, 2912w, 2873w, 1638v.s, 1601v.s, 1572s, 1543m, 1487m, 1440m, 1349m, 1309w, 1262m, 1222m, 1102v.s, 975w, 875m, 816m, 777s, 690w, 623m, 584s, 485m. Physical Measurements. The IR spectra were recorded on KBr pellets on a BIO-RAD FTS-135 spectrophotometer or a Bruker FT-IR Tensor 37 spectrophotometer in the 4000-400 cm-1 range. Absorption spectra were made with a UV4 UNICAM spectrophotometer (MgO as a standard). The photoluminescence measurements were carried out at room-temperature by using a JASCO FP 6500, and a Fluoromax 4 (Horiba) spectrofluorometer. For the measurements of the relative quantum yield of emission, the diffuse reflectance spectra of the complexes and the standard phosphor were recorded with the diffuse reflectance accessory of the UV4 UNICAM spectrophotometer (with MgO as a reference). The solidstate quantum yields were determined against sodium salicylate as the standard (NaSal ΦST = 60% at λex = 350 nm) as described by Bril11a by using the following equation: ΦX ¼

1 - RST IX ΦST 1 - RX IST

ð1Þ

5b C33H48Cl2N6O11Zn2 906.41 293(2) 0.71073 triclinic P1 11.1869(12) 11.3689(11) 18.3793(19) 83.302(8) 72.225(8) 66.953(8) 2048.3(4) 2 1.470 1.363 940 0.976 0.0749, 0.1383 0.1912, 0.1791 0.519, -0.521

6 C43H44Cl2N8O12Zn2 1066.47 293(2) 0.71073 orthorhombic Fdd2 11.7602(8) 21.1860(15) 35.794(3) 90 90 90 8918.0(11) 8 1.583 1.269 4352 0.938 0.0635, 0.1021 0.1420, 0.1237 0.506, -0.805

where Φ is the quantum yield, R is the diffuse reflectance, and I is the integrated emission spectrum of sample (X) and standard (ST). The samples together with the standard were finely ground in order to reduce self-absorbance of the emitted light and to avoid refractive index corrections to the quantum yield. The powders were deposited with a thickness of ∼2 mm onto the solid-state holder of the YvonJobin mounted at 30 deg relative to the excitation. Slit widths were 1 nm in both excitation and emission and the data (as photons/s) were collected in fluorescence mode using an integration time of 0.1 ms. Emission spectra were corrected against the spectral response of the detectors. The errors in the quantum yield values associated with this technique lie within 10%.11b X-ray Structure Determination. X-ray diffraction measurements were performed on a Nonius Kappa CCD diffractometer for the compounds 1 and 3, on a STOE IPDS I diffractometer for compound 2 and STOE IPDS II diffractometer for compounds 4, 5, and 6, operating with Mo-KR (λ=0.71073 A˚) X-ray tube with graphite monochromator. The structures were solved by direct methods and refined by full-matrix least-squares techniques based on F2. The non-H atoms were refined with anisotropic displacement parameters. Calculations were performed using SHELX-97 crystallographic software package. A summary of the crystallographic data and the structure refinement for crystal 1-6 are given in Table 1. CCDC reference numbers: 746575-746581.

Results and Discussion New Molecular Rectangles. As expected, the self-assembly process involving the dinuclear copper(II) complex

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Figure 1. (a, b) Crystal structures of the two tetranuclear cations in 1.

[Cu2L2(μ-OH)](ClO4)2 (with an end-off compartmental ligand) and trans-4,40 -azo-pyridine (azpy) leads to tetranuclear cationic species with the copper ions describing a rectangle. The short edges are formed by the pre-existing μ-OH and μ-phenoxo groups, and the long ones are constructed from the exo-bidentate azpy ligands (HL2 results from the reaction of 2,6-diformyl-p-cresol with 2-aminoethyl-pyridine Scheme 1). Actually, the crystal contains two different tetranuclear complexes and has the overall composition [{L2(μ-OH)Cu 2 }(μ-azpy)2 {Cu 2 (μ-OH)L 2 }][{L 2 (μ-OH)Cu 2 (H 2 O)}(μazpy)2{(H2O)Cu2(μ-OH)L2}](ClO4)8 (1) (Figure 1). In the first type (Figure 1a), all the copper ions are pentacoordinated, with a square pyramidal geometry, the basal plane being formed by two nitrogen atoms arising from the Schiffbase ligand, and two oxygen atoms (one from the phenolato endogenous bridge, and the other one from the hydroxo bridge); the apical positions are occupied by the nitrogen atoms of the spacers [Cu(1)-N(9) = 2.348(4) A˚ and Cu(2)N(10) = 2.374(4) A˚]. The coordination geometries of the copper ions are different within the second type (Figure 1b): two of them are square-pyramidal, with the apical positions occupied by the nitrogen atoms arising from the bridging azpy molecules. For these Cu(II) ions, the Cu-N(apical) distances are Cu(4)-N(13) = 2.284(4) A˚. The two other copper ions display an elongated octahedral geometry, the sixth position being occupied by an aqua ligand [Cu(3)-N(14) = 2.492(6) A˚; Cu(3)-O(1w) = 2.563(5) A˚]. The dimensions of the two rectangles are 3.08 13.70, and, respectively, 3.04 13.77 A˚. The complexes reported by us and others suggest that the formation of the tetranuclear rectangles (Scheme 2A) is favored by end-off compartmental ligands, while the reaction between binuclear complexes with macrocyclic compartmental ligands and exo-dentate ligands affords 1-D coordination polymers, with the binuclear nodes connected by two spacer molecules (Scheme 2B).4-7 Surprisingly, by reacting [(H2O)2Cu2L1](ClO4)2 3 2H2O (H2L1 results from the reaction of 2,6-diformyl-p-cresol with 1,3-diamino-propane Scheme 1 - H2L1) with azpy in a molar ratio 1:2, we obtained again a molecular rectangle, [{L1Cu2(H2O)2}(μ-azpy)2{Cu2L1(H2O)2}](ClO4)4 3 (azpy) 3 2H2O (2) (Figure 2). The copper(II) ions display an elongated octahedral stereochemistry, the basal plane being formed by two nitrogen atoms from the Schiff-base ligand, and two oxygen atoms from the phenolato endogenous bridges. The apical positions are occupied by the nitrogen atoms arising from the bridging azpy molecules and aqua ligands [Cu(1)-O(1w) = 2.815(13); Cu(2)-O(2w) = 2.808(15); Cu(1)-N(6) = 2.388(6) and Cu(2)-N(5) = 2.385(6) A˚]. The intramolecular Cu 3 3 3 Cu distances are 3.113(2) and 13.778(2) A˚. Interestingly, the crystal contains one uncoordinated azpy molecule per tetranuclear complex. In spite of the initial molar ratio, (binuclear complex)/(spacer)

Figure 2. Perspective view of the molecular rectangle 2.

= 1:2, and of the preference of copper(II), in such complexes, for hexacoordination, the ladder-like 1-D coordination polymer is not formed. On the other hand, we notice that the metal centers are hexacoordinated, like in the railroad type coordination polymers constructed from dinuclear nodes generated by macrocyclic Robson ligands. The presence of an aqua ligand coordinated to each copper ion prevents the propagation of the structural motif into a 1-D coordination polymer. Taking into account the presence of the four coordinated aqua ligands, which are oriented outward from the rectangle, the packing of the tetranuclear complexes in the crystal can be easily predicted: the tetranuclear entities are connected through hydrogen bonds involving the aqua ligands as well the crystallization water molecules [O(1w) 3 3 3 O(2w) = 2.771(18), O(1w) 3 3 3 O(2w0 ) = 2.447(20), O(1w) 3 3 3 O(3w) = 2.909(28) A˚], resulting in supramolecular chains (Figure 3). Selected bond distances for compounds 1 and 2 are collected in Table 2. A Coordination Polymer Constructed from Binuclear Copper(II) Nodes and a Flexible Spacer. By reacting the binuclear complex [Cu2(μ-OH)L3](ClO4)2 with a flexible exodentate ligand [1,3-bis(4-pyridyl)propane - bpp], we succeeded to obtain a 1-D coordination polymer, 1¥[{Cu2(μ-OH)L3}(μ-bpp)](ClO4)2 3 (HL3 results from the condensation of 2,6-diformyl-p-cresol with N,N-dimethyl-ethylenediamine Scheme 1). All the bpp spacer molecules, which are trans coordinated with respect to the (Cu2O2) plane, adopt the same conformation, anti-gauche (Figure 4). The copper ions are pentacoordinated with a square pyramidal geometry. The pyridyl nitrogen atoms are coordinated into the apical positions of the two copper ions within the node [Cu(1)-N(5) = 2.336(5), Cu(2)-N(6) = 2.268(5) A˚]. The intra- and internode Cu 3 3 3 Cu distances are 3.000(1) A˚ and 13.825(2) A˚, respectively. Other bond distances are gathered in Table 2. The formation of a 1-D coordination polymer instead of a rectangle is more probably due to the high flexibility of the spacer.

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Figure 3. Packing diagram for crystal 2, showing the formation of the supramolecular chains.

Table 2. Selected Geometric Parameters, Bond Length (A˚), of the Coordination Surroundings of the Copper(II) Ions in the Compounds 1-3 1 Cu1-N1 = 2.032(3) Cu1-N2 = 1.962(4) Cu1-O1 = 1.997(3) Cu1-O2 = 1.908(3) Cu1-N9 = 2.348(4) Cu2-N3 = 1.950(4) Cu2-N4 = 2.016(4) Cu2-N10 = 2.374(4) Cu2-O1 = 2.000(3) Cu2-O2 = 1.920(3)

Cu3-N5 = 2.024(4) Cu3-N6 = 1.982(4) Cu3-N14 = 2.492(6) Cu3-O3 = 2.000(3) Cu3-O4 = 1.914(3) Cu3-O1w = 2.563(5) Cu4-N7 = 1.963(4) Cu4-N8 = 2.021(4) Cu4-N13 = 2.284(4) Cu4-O3 = 2.011(3) Cu4-O4 = 1.915(3)

2

3

Cu1-N1 = 1.961(6) Cu1-N3 = 1.976(6) Cu1-N6 = 2.388(6) Cu1-O1 = 1.985(4) Cu1-O2 = 1.977(5) Cu1-O1w = 2.815(13) Cu2-N2 = 1.972(6) Cu2 -N4 = 1.959(6) Cu2-N5 = 2.385(6) Cu2-O1 = 1.976(5) Cu2-O2 = 1.987(4) Cu2-O2w = 2.808(15)

Cu1-N1 = 2.032(3) Cu1-N2 = 1.962(4) Cu1-O1 = 1.997(3) Cu1-O2 = 1.908(3) Cu1-N9 = 2.348(4) Cu2-N3 = 1.950(4) Cu2-N4 = 2.016(4) Cu2-N10 = 2.374(4) Cu2-O1 = 2.000(3) Cu2-O2 = 1.920(3)

Figure 4. View of the coordination polymer in 3.

Coordination Polymers Constructed from [Zn2] Nodes. We tested the ability of the binuclear zinc complexes to generate coordination polymers by employing the compartmental ligands HL2, HL3, and, as spacers, 4,40 -bipyridyl and 1,2bis(4-pyridyl)ethane (bpeta). The reaction between zinc perchlorate and HL3 in the presence of LiOH, followed by the addition of 4,40 -bipy affords a 1-D coordination polymer with the formula 1¥[{L3Zn2(μ-OH)}(μ-4,40 -bipy)](ClO4)2 3 2H2O (4). The binuclear nodes are connected by only one spacer molecule, the zinc ions being pentacoordinated with a square-pyramidal stereochemistry (Figure 5). The two zinc ions are crystallographically nonequivalent. The basal plane of each zinc ion is formed by the bridging oxygen atoms and by two nitrogen atoms from the Schiff-base. The apical

position is occupied by a pyridyl nitrogen atom. The intraand internode Zn 3 3 3 Zn distances are, respectively, 3.1172(6) and 11.1642(7) A˚. The similar reaction with bpeta as a spacer was carried out in two different solvents: ethanol (EtOH) and tetrahydrofurane (THF). In both cases, single crystals have been obtained, and the crystal structures have been solved. Their compositions correspond to the following formulas: 1¥[{L3Zn2(μ-OH)}(μ-bpeta)](ClO4)2 (5a); 1¥[{L3Zn2(μ-OH)}(μbpeta)](ClO4)2 3 THF (5b). The first compound crystallizes in the monoclinic system and the second one in the triclinic system (Table 1). In both cases, 1-D coordination polymers are formed (Figure 6). The cationic chains in 5a and 5b are similar but not identical. In both cases, the [Zn2] nodes are

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Figure 5. View of the coordination polymer in 4.

Figure 6. Crystal structures of the coordination polymers 5a (a) and 5b (b). Table 3. Selected Geometric Parameters, Bond Length (A˚), of the Coordination Surroundings of the Zinc(II) Ions in the Compounds 4-6 4

5a

5b

6

Zn1-N1 = 2.076(3) Zn1-N3 = 2.077(3) Zn1-N4 = 2.172(4) Zn1-O1 = 2.082(2) Zn1-O2 = 1.945(3) Zn2-N2 = 2.066(3) Zn2-N5 = 2.166(4) Zn2-N6 = 2.075(3) Zn2-O1 = 2.090(3) Zn2-O2 = 1.939(3)



Zn1-N1 = 2.084(4) Zn1-N2 = 2.199(4) Zn1-N3 = 2.312(4) Zn1-N4 = 2.366(4) Zn1-O1 = 2.116(3) Zn1-O2 = 1.981(3)

connected by only one spacer molecule (as in 4), the zinc ions being pentacoordinated. The main difference between the cationic chains in 5a and 5b arises from the torsion angles around the CH2-CH2 bond of the bpeta molecule: 163.5 in 5a, and 180 in 5b. The intra- and internode Zn 3 3 3 Zn distances are 3.0981(6) and 13.1697(9) A˚ [Zn(2) 3 3 3 Zn(10 )] for 5a, respectively, 3.1313(8) and 13.269(2) A˚ [Zn(2) 3 3 3 Zn(20 )] for 5b. Selected bond distances are presented in Table 3. We notice also a difference between the chains in 4, 5a, and 5b: the {L3Zn2(μ-OH)} moieties in 4 are oriented in the same direction, while in 5a and 5b they are alternatively oriented up and down. The reaction between zinc perchlorate and HL2 in the presence of LiOH, followed by the addition of 4,40 -bipy

affords an 1-D ladder-like coordination polymer, with the binuclear nodes connected by double bridges, 1¥[{L2Zn2(μ-OH)}(μ-4,40 -bipy)2](ClO4)2 3 2H2O (6) (Figure 7). In contrast to 1¥[{L3Zn2(μ-OH)}(μ-4,40 -bipy)](ClO4)2 3 2H2O (4), each Zn ion of 6 lies in an octahedral, rather than a squarepyramidal, coordination environment such that two N-atoms of two 4-pyridyl groups adopt a transoid arrangement. The remaining coordination sites of each Zn ion are occupied by a single O- and two N-atoms of pentadentate Schiff-base ligand and a single O-atom of the hydroxo bridge (Figure 7). The pentadenate Schiff-base ligand are oriented parallel along the polymer backbone. The intra-, [Zn(1) 3 3 3 Zn(10 ); -x, -y, z], and internode, [Zn(1) 3 3 3 Zn(100 ); 1 þ x, y, z], distances are 3.1680(6) and 11.7602(14) A˚ , respectively.

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Figure 7. View of the ladder-like polymer in 6.

Figure 8. Luminescence spectra of compounds 4 (a), 5a (b), 5b (c), and 6 (d).

Selected bond distances for compounds 4-6 are gathered in Table 3. Luminescence Properties. Zinc complexes with Schiff bases or aromatic heterocyclic ligands are known to exhibit interesting luminescence properties.12 The coordination of the ligands to the metal ion increases the rigidity of the molecular edifice and reduces the loss of energy by radiationless thermal vibrations. The emission spectra of compounds 4, 5a, 5b, and 6 are depicted in Figure 8. The four complexes exhibit a strong luminescence as follows: 4 λem = 490 nm; 5a λem = 463 nm; 5b λem = 453 nm; and 6 λem = 461 nm, 510 nm. For the four complexes λex=350 nm. The band located in the

region 350-400 nm in the absorption spectra of complexes is most probably due to a π-π* transition of the organic ligand. The luminescence is assignable to intraligand 1(π*-π) fluorescence. By using Bril technique11a (see Experimental Procedures), the following values for the solid-state quantum yields were obtained: Φ = 4: 0.025; 5a: 0.05; 5b: 0.11; 6: 0.02. Conclusions Herein we have illustrated with new examples that dinuclear complexes with compartmental ligands can be efficiently

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employed as building-blocks in metallosupramolecular chemistry and crystal engineering. Two types of complexes can be in principle obtained: tetranuclear species with the metal ions describing a rectangle and 1-D coordination polymers, with the binuclear nodes connected by one or two spacers. The stereochemical preference of the metal centers is not necessarily a condition for assembling rectangles or chains. On the other hand, long, flexible ligands [e.g., bis(4-pyridyl)propane] seem to favor the formation of 1-D coordination polymers. The zinc systems are particularly interesting because of their luminescence properties in the solid-state.

(4) (5) (6)

Acknowledgment. Financial support from the PNII - IDEI Program (Project 506/2009) is gratefully acknowledged. We thank Prof. Andrei Medvedovici for recording the mass spectra.

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New Molecular Rectangles and Coordination Polymers Constructed

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