Coordination Polymers of Lanthanide (III): Toward Encapsulation of

Jan 11, 2011 - Synopsis. Coordination polymers [{La(Ben)3(H2O)2}·(4,4′-BPNO)0.5·HBen·DMF]n (1), [{Ce(Ben)3(4,4′-BPNO)(H2O)2}·DMF]n (2), ...
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DOI: 10.1021/cg101432k

Coordination Polymers of Lanthanide(III): Toward Encapsulation of Well-Defined Assembly of Guest Molecules

2011, Vol. 11 547–554

Rupam Sarma,† Himangshu Deka,† Athanassios K. Boudalis,‡ and Jubaraj B. Baruah*,† †

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India, and ‡Institute of Materials Science, NCSR “Demokritos”, 15310 Aghia Paraskevi Attikis, Greece Received October 27, 2010; Revised Manuscript Received December 9, 2010

ABSTRACT: A series of lanthanide(III) coordination polymers [{La(Ben)3(H2O)2} 3 (4,40 -BPNO)0.5 3 HBen 3 DMF]n (1), [{Ce(Ben)3(4,40 -BPNO)(H2O)2} 3 DMF]n (2), [Tb(Ben)3(4,40 -BPNO)0.5(H2O)]n (3), [{Ln(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n [Ln = La (4), Eu (5), Gd (6), Tb (7)] are synthesized and structurally characterized (where Ben = benzoate; HBen = benzoic acid; 4,40 BPNO = 4,40 -bipyridyl-N,N0 -dioxide; DMF = dimethylformamide). The metal centers in 1 have nine coordinated geometry, while the metal centers in all other coordination polymers are eight coordinated. Polymer 1 comprises one-dimensional chains of lanthanum(III) benzoate that encapsulate hydrogen-bonded assemblies formed by HBen 3 3 3 4,40 -BPNO 3 3 3 HBen interactions in its interstitial space. The polymer 2 and 3 are eight coordinated with square antiprism geometry around the metal node. The other four coordination polymers 4-7 have the same general composition and possess three-dimensional honeycomb like structures that encapsulates benzoic acid molecules in the interstices. The complex 5 (solid) exhibits a strong red luminescence emission characteristic of Eu(III). Magnetic susceptibility of 6 revealed weak antiferromagnetic interactions between the Gd(III) ions.

Introduction Coordination polymers of transition metal ions are frequently studied but less frequently those of post-transition metals.1 Lanthanides can expand their coordination numbers as high as 10 or 12 and are expected to lead to unusual and unprecedented structures due to such coordination flexibilities.2 Coordination complexes of lanthanide(III) with eight or nine coordination around the metal centers are well established.2,3 These complexes can adopt different geometries; for example, a seven coordinated complex can have either a capped octahedral geometry or a pentagonal bipyramidal geometry; on the other hand, an eight coordinated complex can adopt square antiprismatic, triangular dodecahedral geometry, or bicapped trigonal prismatic type of structures.4 Thus, coordination polymers arising from any of such geometries would have a size and shape that is dependent not only on the interconnecting units between the repeated motifs but also on the coordination environments. Though a few reports on coordination complexes of aromatic N-oxide with lanthanides were available during the early 1960s, structural studies were scarce.5 The complex [La(2,20 -BPNO)4](ClO4)3 by Karaghouli et al. was the first example of a structurally characterized lanthanide(III) N-oxide complex.6 Recently, numbers of examples of lanthanide-based coordination polymers employing multidentate ligands have appeared in the literature.7 The tendency of lanthanides to adopt a high coordination number makes the f-block metal ions attractive for the design of coordination polymers. As such, coordination polymers of lanthanides can adopt various new and unusual network topologies and attract interest due to their important magnetic, catalytic, as well as luminescence properties.8,9 Numbers of reports on coordination networks of lanthanides with aromatic N-oxidebased ligands have also appeared in the literature.10,11 Various two-dimensional (2D) lanthanide coordination polymers *To whom correspondence should be addressed. E-mail: [email protected]. r 2011 American Chemical Society

derived from 3,5-bis(3-pyridyl-N-oxide)-4-amino-1,2,4-triazole have been reported recently.10b The polymer {[La(μ-L)2L(H2O)2](CF3SO3)3}n (L = bis(4-pyridyl)ethane-N,N0 -dioxide) exhibits a three-dimensional (3D) cationic coordination framework {[La(μ-L)2L(H2O)2]3þ that encapsulates uncoordinated (CF3SO3)- counteranions.10c A 3D coordination polymer prepared from predesigned [V6O13{(OCH2)3C(NHCH2C6H4-4-CO2)}2]4-, 4,40 -bipyridine N,N0 -dioxide, and Tb(III) ions is a catalyst for oxidation reactions.10d Inspired by these versatilities in the chemistry of aromatic N-oxide containing Ln(III) complexes, we have studied the synthesis of coordination polymers of Ln(III) benzoate (Ln = La, Ce, Eu, Gd, Tb) with 4,40 -bipyridyl-N,N0 -dioxide from one-pot multicomponent reactions to understand the role of the central metal ions and stoichiometry on their structures. A total of seven coordination polymers are synthesized and each of them are characterized by conventional spectroscopy along with single crystal and powder X-ray diffraction techniques. Experimental Section Synthesis of [{La(Ben)3(H2O)2} 3 (4,40 -BPNO)0.5 3 HBen 3 DMF]n (1). To a solution of benzoic acid (1.5 mmol, 0.183 g) in methanol (15 mL) lanthanum(III) acetate hydrate (0.5 mmol, 0.158 g) was added and stirred for 10 min. To this reaction mixture 4,40 -bipyridyl-N,N0 -dioxide hydrate (0.25 mmol, 0.047 g) was added with constant stirring at room temperature. The resulting precipitate was dissolved with addition of water (5 mL). Three milliliters of DMF was added and the mixture was stirred for another 30 min. The solution was kept standing at room temperature. Block-shaped colorless crystals appeared after 7 days and were dried in air. Yield of the pure crystalline complex was found to be >70% (based on La). IR (KBr, cm-1): 3431 (bs), 1660 (s), 1625 (m), 1592 (m), 1542 (s), 1522 (m), 1477 (w), 1408 (s), 1276 (m), 1219 (m), 1184 (m), 843 (m), 722 (s), 712 (s). Elemental Anal. Calc. for C36H36LaN2O12: C, 52.28; H, 4.38. Found: C, 52.41; H, 4.47%. Synthesis of [{Ce(Ben)3(4,40 -BPNO)(H2O)2} 3 DMF]n (2). To a solution of sodium benzoate (1.5 mmol, 0.216 g) in methanol (15 mL) cerium(III) nitrate hexahydrate (0.5 mmol, 0.217 g) was added and stirred for 10 min. The resulting precipitate was dissolved with Published on Web 01/11/2011

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addition of DMF (3 mL) and water (5 mL). Then, a solution of 4,40 bipyridyl-N,N0 -dioxide hydrate (0.5 mmol, 0.094 g) in methanol (5 mL) was added to the above mixture with continuous stirring for about 30 min. The resultant solution was filtered and left to stand at room temperature. Yellow needle-shaped crystals suitable for X-ray analysis was produced by slow evaporation of the solvent for 5 days. Yield of the pure crystalline complex was found to be >70% (based on Ce). IR (KBr, cm-1): 3430 (bs), 1661 (s), 1594 (s), 1550 (s), 1473 (m), 1402 (s), 1259 (w), 1230 (s), 1181 (m), 1100 (w), 1068 (w), 1025 (w), 838 (s), 718 (s), 670 (m). Elemental Anal. Calc. for C34H34CeN3O11: C, 50.99; H, 4.27. Found: C, 50.45; H, 4.21%. Synthesis of [Tb(Ben)3(4,40 -BPNO)0.5(H2O)]n (3). Complex 3 was synthesized in a similar procedure as complex 1 except for the use of terbium(III) acetate hydrate as the metal source. Pure block crystals suitable for X-ray analysis were collected after about 6 days and dried in air. Yield of the pure crystalline complex was found to be >70% (based on Tb). IR (KBr, cm-1): 3337 (bw), 1604 (s), 1560 (s), 1522 (s), 1492 (w), 1482 (m), 1427 (s), 1408 (s), 1245 (s), 1190 (w), 1069 (w), 1023 (w), 847 (s), 720 (s), 691 (m), 683 (m). Elemental Anal. Calc. for C26H21NO8Tb: C, 49.22, H, 3.33. Found: C, 49.54; H, 3.62%. Synthesis of [{La(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n (4). The same synthetic procedure as that for complex 1 was used except that here the metal/4,40 -BPNO ratio was 1:1. Diffraction quality crystals were collected after about 4 days and dried in air. Yield of the pure crystalline complex was found to be >65% (based on La). IR (KBr, cm-1): 3437 (bs), 1590 (m), 1544 (s), 1476 (s), 1408 (s), 1219 (s), 1185 (m), 1071 (w), 835 (s), 724 (s), 552 (m). Elemental Anal. Calc. for C38H33LaN2O12: C, 53.78, H, 3.92. Found: C, 53.81, H, 3.95%. Synthesis of [{Eu(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n (5). Complex 5 was synthesized in a similar procedure as for complex 4 but using europium(III) actetate hydrate as the metal source. Diffraction quality crystals were collected after 7 days and dried in air. Yield of the pure crystalline complex was found to be >70% (based on Eu). IR (KBr, cm-1): 3368 (bm), 1591 (s), 1548 (s), 1475 (s), 1412 (s), 1321 (w), 1241 (s), 1218 (s), 1185 (m), 1071 (w), 1025 (w), 841 (s), 723 (s) 552 (w). Syntesis of [{Gd(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n (6). Complex 6 was synthesized with the same procedure as that for complex 4 except that here gadolinium(III) actetate hydrate was used as the metal source. Diffraction quality crystals were collected after 5 days and dried in air. Yield of the pure crystalline complex was found to be >70% (based on Gd). IR (KBr, cm-1): 3233 (bs), 1700 (bm), 1591 (s), 1551 (s), 1476 (m), 1414 (s), 1217 (s), 1184 (m), 1072 (w), 839 (m), 835 (m), 724 (s). Synthesis of [{Tb(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n (7). The same synthetic procedure as that for complex 3 was used except that here the metal/4,40 -BPNO ratio was 1:1. Pure block crystals suitable for X-ray analysis were collected after 6 days and dried in air. Yield of the pure crystalline complex was found to be >70% (based on Tb). IR (KBr, cm-1): 3239 (bs), 1699 (bm), 1591 (s), 1552 (s), 1476 (m), 1414 (s), 1216 (s), 1184 (m), 1072 (w), 835 (m), 724 (s), 658 (w). X-ray Crystallography. The X-ray crystallographic data were collected at 296 K with Mo KR radiation (λ = 0.71073 A˚) using a Bruker Nonius SMART CCD diffractometer equipped with a graphite monochromator. The SMART software was used for data collection and also for indexing the reflections and determining the unit cell parameters; the collected data were integrated using SAINT software. The structures were solved by direct methods and refined by full-matrix least-squares calculations using SHELXTL software.12 All the non-H atoms were refined in the anisotropic approximation against F2 of all reflections. The H-atoms, except those attached to O, were placed at their calculated positions and refined in the isotropic approximation; those attached to heteroatoms (O) were located in the difference Fourier maps and refined with isotropic displacement coefficients. The crystallographic parameters of the complexes are given in Table 1. Magnetic Measurements. Variable-temperature dc magnetic susceptibility studies were carried out on a polycrystalline sample of 6 (2-300 K) using a Quantum Design MPMS SQUID susceptometer under a magnetic field of 0.1 T. AC studies were carried out on a

Sarma et al. Quantum Design PPMS system on a polycrystalline sample 7. Diamagnetic corrections for the complexes were estimated from Pascal’s constants. The magnetic susceptibility of 6 has been computed by exact calculation of the energy levels associated with the spin Hamiltonian through diagonalization of the full matrix with a general program for axial symmetry.13 Least-squares fits were accomplished with an adapted version of the function-minimization program MINUIT.14 The error-factor R is defined as R = Σ(((χexp2 - χcalc)2)/(Nχexp )), where N is the number of experimental points.

Results and Discussion Seven new coordination polymers (1-7) of lanthanide(III) ions with 4,40 -bipyridyl-N,N0 -dioxide as ancillary ligand and benzoate as anionic ligand has been prepared and characterized. The structures represent the coordination polymerization of hybrid benzoate/4,40 -BPNO-Ln(III) arrays of different dimensionality. Three of them are of one-dimensional (1D) architecture and the other four are 3D wherein the metal nodes are bridged by at least one or both of the organic tectones. All these coordination polymers are synthesized under solution state reaction conditions and it is observed that in general when the metal to ligand (4,40 -BPNO) ratio is 1:0.5 1D polymers are formed while a 1:1 ratio results in 3D coordination polymers. A schematic presentation for the synthesis of the coordination polymers of Tb(III) is depicted in Scheme 1. The infrared spectra of the synthesized polymers (1-7) are very similar to each other, νas (COO) stretching appearing around 1550 cm-1 and νs (COO) stretching around 1410 cm-1. The N-O stretching appears at 1276 cm-1 for complex 1 in which the N-oxo group is uncoordinated. In all the other complexes, the N-O stretching appears in the range 12151240 cm-1 signifying electron donation from the aromatic N-oxide ligand. In addition, the band at ca. 3400 cm-1 indicates the existence of coordinated water in the structures. Reaction of La(III) acetate, benzoic acid, and 4,40 -BPNO (1:3:0.5 equiv) in methanol/DMF reaction mixture led to the formation of the 1D coordination polymer 1 with the composition [{La(Ben)3(H2O)2}.(4,40 -BPNO)0.5 3 HBen 3 DMF]n. It crystallizes in the triclinic space group P1 and the asymmetric unit contains the metal center coordinated by three benzoate ligands and two aquo ligands along with one each of uncoordinated 4,40 -BPNO, benzoic acid, and DMF molecules. Thus, the coordination polymer 1 can also be described as a multicomponent molecular complex where a 1D coordination polymer cocrystallizes with three other organic molecules. The coordination polymer 1 comprises 1D linear chains where the nearby La(III) centers are bridged by benzoate ligands with a M-M separation of 4.20 A˚. As shown in Figure 1b, each La(III) ion is nine-coordinated in a triply capped trigonal prism formed by seven carboxylate oxygen atoms from six benzoate ligands and two oxygen atoms from two coordinated water molecules. The La-O distances (Table S2, Supporting Information) range from 2.470(1) to 2.861(1) A˚ and the longer La-O distances are associated with the benzoate oxygen atom O6 that acts as a μ2-O bridge linking two La(III) centers. The benzoate ligands connect the metal centers through two different binding modes: the η2:μ2 mode and η2:μ3 mode. A closer look at the coordination polymer 1 shows that it contains dinuclear La(III) cores that act as repeating units connected by an O4-C8-O8 benzoate bridge through the η2:μ2 binding mode. This bridging extends the polymer along the a crystallographic axis. The 4,40 -BPNO, benzoic acid, and DMF molecules remain uncoordinated but are encapsulated between the 1D chains with the aid of weak interactions. The

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Table 1. Crystallographic Parameters of the Complexes 1-7 compound no.

1

C36H36LaN2O12 827.58 triclinic P1 9.5174(3) 12.5384(3) 16.3714(4) 80.0080(10) 81.4190(10) 69.0050(10) 1788.22(8) 2 1.537 1.260 838 23453 6583 25.50 -11 e h e11 -15 e k e13 -19 e l e18 completeness to 2θ (%) 98.5 data/restraints/parameters 6583/0/478 1.084 GOF (F2) R indices [I > 2σ(I)] 0.0206 R indices (all data) 0.0227 0.0534 wR2 = {Σ[wFo2 - Fo2)2] /Σ[w(Fo2)2]}1/2 formulas formula wt crystal system space group a/A˚ b/A˚ c/A˚ R/ β/ γ/ V/A˚3 Z density/Mg m-3 abs coeff /mm-1 F(000) total no. of reflections reflections, I > 2σ(I) max θ/ ranges (h, k, l)

2

3

4

5

6

7

C34H34CeN3O11 800.76 monoclinic P21/c 12.8612(12) 10.2716(9) 26.254(2) 90.00 112.500(4) 90.00 3448.4(5) 4 1.542 1.384 1620 31305 5886 25.00 -14 e h e15 -11 e k e11 -31 e l e33 96.8 5886/0/444 1.076 0.0329 0.0433 0.0916

C26H21NO8Tb 634.36 monoclinic P21/n 12.9343(7) 8.2555(4) 24.1694(13) 90.00 102.5180(10) 90.00 2519.4(2) 4 1.672 2.856 1252 12841 4648 25.50 -15 e h e15 -3 e k e10 -28 e l e29 99.4 4648/0/329 0.964 0.0366 0.0435 0.0864

C38H33LaN2O12 848.57 orthorhombic Pnaa 10.139 17.804 20.010 90.00 90.00 90.00 3612.0 4 1.560 1.250 1712 10490 3334 25.49 -12 e h e12 -20 e k e18 -24 e l e 17 99.0 3334/2/241 1.004 0.0802 0.1502 0.1766

C38H33EuN2O12 861.62 orthorhombic Pccn 17.7412(8) 19.8116(7) 10.0546(3) 90.00 90.00 90.00 3534.0(2) 4 1.619 1.843 1736 29134 3473 26.00 -18 e h e12 -9 e k e6 -25 e l e 26 99.7 3473/0/248 1.024 0.0266 0.0493 0.0513

C38H33GdN2O12 866.91 orthorhombic Pnaa 10.040 17.788 19.786 90.00 90.00 90.00 3533.6 4 1.630 1.946 1740 30827 3458 26.00 -12 e h e12 -21 e k e21 -24 e l e 23 99.3 3458/1/241 1.051 0.0399 0.0749 0.0910

C38H33N2O12Tb 868.58 orthorhombic Pccn 17.7423(5) 19.7457(5) 10.0389(3) 90.00 90.00 90.00 3516.97(17) 4 1.640 2.080 1744 28661 3244 25.50 -21 e h e21 -23 e k e23 -7 e l e 11 99.0 3244/2/248 0.945 0.0194 0.0290 0.0439

Scheme 1. Synthesis of N-Oxide Coordination Polymers of Tb(III)

4,40 -BPNO molecules play a vital role in the extension of the dimensionality of 1. The aquo ligands (bearing the oxygen O3) are involved in O3-H3a 3 3 3 O9 interaction with the 4,40 -BPNO molecules. Thus, both the N-oxo groups of the 4,40 -BPNO molecule are connected to the aquo groups of two nearby 1D chains of La(III) benzoate thereby acting as a hydrogenbonded spacer. This weak interaction imparts the molecule a 2D hydrogen bonded architecture extended parallel to the ab plane (Figure 1c). Apart from this, another hydrogen bond interaction exists (O10-H10a 3 3 3 O9) between the 4,40 -BPNO molecules and the benzoic acid molecules that holds the benzoic

acid molecules in the crystal lattice. The DMF molecules are held in the lattice through the O3-H3b 3 3 3 O12 and O5-H5a 3 3 3 O12 interactions with the coordinated aquo groups. The detailed hydrogen-bond parameters are listed in Table S3 (please refer to Supporting Information). Although there are reports on nine coordinated lanthanide complexes,10 complex 1 is important in terms of a new avenue generated for encapsulating assemblies of guest molecules constructed through hydrogen bonding between benzoic acid and 4,40 -BPNO. The hydrogen-bond interaction between the benzoic acid and 4,40 BPNO produces HBen 3 3 3 4,40 -BPNO 3 3 3 HBen assemblies in

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Figure 1. (a) Asymmetric unit of 1, (b) one-dimensional La(III) benzoate chain showing the coordination around the metal center, (c) weak interactions between the components of 1 leading to 2D hydrogen bonded assembly, (d) encapsulation of HBen 3 3 3 4,40 -BPNO 3 3 3 HBen units and DMF in between the 1D chains of 1 through weak interactions.

a steplike arrangement so as to have a 22.38 A˚ length of each unit and these units are placed in the interstices of the polymer (Figure 1d). Identification of such assemblies is important as they may lead to isolation of intermediate species that would lead to chemical reactions in interstitial sites.10d Similar reactions with Ce(III) acetate led to the formation of a 1D coordination polymer 2, [{Ce(Ben)3(4,40 -BPNO)(H2O)2} 3 DMF]n, with monodentate coordination by 4,40 BPNO ligands. The Ce1 center in 2 coordinates to five carboxylate oxygen atoms of bridging benzoate groups, one oxygen atom of N-oxide, and two water molecules. The coordination geometry around the Ce1 center can be described as a distorted square antiprism with O-Ce1-O bond angles ranging from 69.1(1) to 149.7(1). The Ce(III) centers in 2 are doubly bridged by the carboxylate groups of benzoate ligands to form an infinite 1D chain (Figure 2a) with the closest Ce-Ce distance of 5.51 A˚. The benzoate ligands connect the metal centers through the η2:μ2 binding mode, while there are a set of benzoate ligands that coordinate the metal center with monodentate binding mode. The bridging extends the polymer along the crystallographic b axis. The N-oxide ligand coordinates the metal center with a monodentate binding mode

which is a rare one to be observed for 4,40 -BPNO as it generally prefers a bidentate binding mode. This binding mode makes the N-oxo functionality susceptible toward hydrogen bond formation. Thus, the 4,40 -BPNO ligands are involved in both intrachain and interchain hydrogen bond interactions with the coordinated aquo groups (Table S3, Supporting Information). The interchain hydrogen bond interaction viz. O4-H4b 3 3 3 O10 interaction of the 4,40 -BPNO ligand with the coordinated water molecule (bearing the oxygen O4) leads to formation of a 2D hydrogen bonded network of 2 (Figure 2b). Apart from the hydrogen-bond interactions, the aromatic rings of the N-oxide ligands are also involved in a very weak π-π interaction (intercentroid distance 3.8 A˚, average distance between the aromatic planes 3.497 A˚, angle between the line between centroids and the normal to the plane of the metal ions ∼139). Moreover, the coordination polymer also contains hydrogen-bonded DMF molecules that are held in the crystal lattice through O6-H6a 3 3 3 O11 interaction with the coordinated aquo groups. Tb(III) acetate generated a zigzag 1D coordination polymer [Tb(Ben)3(4,40 -BPNO)0.5(H2O)]n (3) with 4,40 -BPNO ligands coordinating through the conventional trans η2:μ2

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Figure 2. (a) One-dimensional chain of 2 showing coordination around the metal centers and (b) short-range interactions in 2 leading to a twodimensional hydrogen-bonded network.

bridging mode (Figure 3). The coordination polymer crystallizes in the monoclinic space group P21/n. Each of the Tb(III) centers in 3 is coordinated by six benzoate oxygen atoms, one N-oxo oxygen atom, and one aquo ligand. In this structure, the terbium(III) benzoate forms a dinuclear core having two benzoate η2:μ2 bridges and four chelating benzoate coordinations between two metal centers. The metal center adopts an eight coordinated geometry. The dinuclear core acts as repeating units and is connected to each other through the Noxide spacer ligand with a distance of separation of 13.04 A˚. This generates the zigzag 1D coordination polymer extended along the ab diagonal of the unit cell. The coordination polymer is shown in Figure 3. The solvent diffusion method yielded a similar 1D zigzag chain coordination polymer of 4,40 -bipyridyl-N,N0 -dioxide (4,40 -BPNO) with terbium(III) nitrate with the composition [Tb(4,40 -BPNO)(CH3OH)(NO3)3]n.10a The Tb-O bond lengths in 3 span the range 2.30-2.52 A˚ similar to those reported in the literature.10 The coordination polymer 3 is unique from the other two, that is , 1 and 2, in that here the repeating dinuclear cores are interconnected by the bridging 4,40 -BPNO ligands, whereas in the other two cases the repeating units are connected by bridging benzoate ligands. Reaction of La(III) acetate, benzoic acid, and 4,40 -BPNO in 1:3:1 equiv in a methanol/DMF reaction mixture yielded the

3D coordination polymer 4 having composition [{La(Ben)3(4,40 BPNO)(H2O)2} 3 HBen]n. The other three coordination polymers with Eu(III), 5; Gd(III), 6, and Tb(III) 7 have the same general composition as in 4 and possess similar 3D structures. All these coordination polymers have metal nodes with eight coordinated distorted square antiprismatic geometry (Figure 4a). As such structural details for only complex 4 is outlined here as a representative example. All the La-O bonds in complex 4 lie in the range 2.40-2.55 A˚. The key feature of this structural type is the formation of a 3D honeycomb -like structure that encapsulates benzoic acid molecules in the interstices (Figure 4c). The crystals of the coordination polymer 4 crystallize in the orthorhombic space group Pnaa. In the structure, each of the La(III) ions is coordinated by four benzoate oxygen atoms, two N-oxo oxygen atoms, and two aquo ligands. The benzoate ligands bridge the nearby metal centers through the η2:μ2 binding mode extending the polymer along the crystallographic a axis. Such a bridging leads to 1D chains with the nearest La-La distance of 5.06 A˚ (Figure 4a). These 1D chains are then interconnected through the 4,40 -BPNO ligands that spread the dimensionality of the complex to the three dimensions. The 4,40 -BPNO ligands binds the La(III) centers through the trans η2:μ2 bridging mode. This elongation of the structure around the dimension creates 1D channels of dimension 14.1  14.1 A˚ that hold benzoic acid molecules in dimeric pairs.

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Figure 3. Zig-zag one-dimensional chain of the coordination polymer 3.

Figure 4. (a) One-dimensional chain in 4 showing the coordination environment, (b) square antiprismatic geometry around La(III) in 4, and (c) 3D structure of 4 encapsulating benzoic acid molecules (hydrogen atoms are omitted for clarity).

Recently, such a 3D coordination polymer derived from 4,40 BPNO ligands is reported which is shown to be used for

separation purposes for C6-C8 aromatics.15 The 3D structure of the coordination polymer 4 is shown in Figure 4c.

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Figure 5. Solid state fluorescence spectrum of the complex 5.

Figure 6. χMT vs T experimental data and calculated curve for 6, according to the model described in the Experimental Section.

Thermogravimetric Analysis and Fluorescent Properties. Thermal stability of the complexes 1-3 and 6 is studied. Thermogram of the complexes 4, 5, and 7 are expected to tally with that of 6 and are not recorded. Thermogravimetric analysis of the coordination polymer 1 shows a weight loss of 4,40 -bipyridyl-N,N0 -dioxide, DMF, and benzoic acid molecules in different steps. In the first step, weight loss occurs in the range 70-180 C, which corresponds to 23.2% of the total weight. This loss of weight accounts for the loss of the uncoordinated DMF and benzoic acid molecules (theoretical weight loss 23.4%). The second loss occurs in the range 225-360 C corresponding to weight loss of 15.1% of the residue from the first step (calc. 14.8%) due to loss of half a 4,40 bipyridyl-N,N0 -dioxide molecule per formula. In the third step, continuous degradation takes place due to the loss of the coordinated molecules. For the coordination polymer 2, the first step within the range 45-95 C corresponds to a weight loss of 9.5% (calc. 9.1%) due to loss of the DMF molecule, and the second step, 95-360 C, is due to the loss of two water molecules and a 4,40 -bipyridyl-N,N0 -dioxide molecule (experimental 31.9% of the residue from the first step; calc. 30.8%). Coordination polymer 3 also loses weight in two steps: the first step in the range 102-350 C corresponds to weight loss of 16.8% (calc. 17.7%) due to loss of the a water and half a 4,40 -bipyridyl-N,N0 -dioxide molecules per formula. Continuous degradation then takes place for the removal of the benzoic acid molecules. For the coordination polymer 6, weight loss due to the uncoordinated benzoic acid molecule

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and the coordinated water molecules takes place in the range 95-185 C corresponding to a weight loss of 18.8% (calc. 18.1%). PXRD analysis of the bulk samples of the coordination polymers 1-4 are carried out and found to match well with the simulated patterns (please refer to the Supporting Information). This depicts the pure phase of the synthesized compounds. The solid-state fluorescence spectra of complexes 5 and 7 at room temperature are recorded. On excitation at 315 nm, complex 5 exhibits a very strong red luminescence at 618 nm, which arises from 5D0 f 7F2 transition, a typical characteristic of Eu(III).16 This is the main emission which is induced by electric dipole moment and is hypersensitive to the environment of the Eu(III). In addition, the 5D0 f 7F1 transition is also observed (Figure 5). The presence of the 4,40 -bipyridyl-N,N0 -dioxide ligand exerts some more luminescent behavior to this molecule. On irradiation at 315 nm, the molecule emits at two different wavelengths 373 nm and at 460 nm and is characteristic of the ligand. In the case of the complex 3 and 7 also, these two bands are observed. However, characteristic signals due to Tb(III) are not observed and may be due to overlap by the ligand bands. Apart from this the lanthanide based complexes are known to show absorption in the NIR region of the electromagnetic spectrum.17 The solid state NIR-UV spectra of the complex 5 was recorded that showed absorptions at 1190, 1385, and 1710 nm for complex 5 (please refer to the Supporting Information). Similar absorptions by europium(III) complexes are reported in the literature.18 Magnetic Studies. Magnetic couplings between paramagnetic Ln(III) ions are hard to determine due to orbital contributions (L 6¼ 0), which render the spin-Hamiltonian inapplicable, and due to the low strength of those interactions, due to the effective shielding of the 4f electrons by the outer-shell electrons. Only in orbitally nondegenerate (L = 0) Gd (III) complexes, magnetic interactions can be easily determined by magnetic susceptibility studies, since in all other cases the effect of magnetic exchange on the thermal variation of the magnetic susceptibility is masked by the effect of the thermal depopulation of the excited Stark levels upon cooling. For this reason, complex 6 [{Gd(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n was chosen for such studies, to obtain an estimate of the Ln(III)-Ln(III) magnetic interactions. The χMT value of 6 at 300 K is 7.88 cm3 mol-1 K, typical of a S = 7/2 spin (g = 2). This remains relatively constant upon cooling, exhibiting a sharp decrease below 20 K. This was associated with the combined effects of Zeeman splitting inside the magnetic field and possible antiferromagnetic interactions. Initial attempts to account for this drop only through consideration of Zeeman splitting were unsuccessful. Consequently, magnetic exchange couplings were also included into our model, employing a mean-field correction. The corresponding Hamiltonian was Hˆ = gμBHSˆ - 2zJÆSˆ zæSz, where z is the number of nearest neighbors and J is the magnetic exchange coupling constant. Fits to this model yielded best-fit parameters zJ = -0.022 cm-1 and g = 2.01 with R = 1.3  10-5. Considering that from the crystal structure we derive z = 2, the exchange coupling is estimated to be -0.011 cm-1. The very weak couplings between Gd (III) ions are typical of the strength of 4f-4f interactions. AC susceptibility studies were carried out on the Tb(III) complex 7 to determine possible magnetic relaxation behavior.

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However, no out-of-phase signals were detected over a large frequency range, and thus such effects were ruled out.19

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Conclusion One-pot reactions of anionic benzoate ligands and 4,40 bipyridyl-N,N0 -dioxide with various lanthanide(III) form coordination polymers whose compositions are decided by the stoichiometry of the reactants. A series of lanthanide 1D coordination polymers 1-3 are formed when the metal to 4,40 -BPNO ratio used is 1:0.5 in the reaction mixture. The structures as well as the guest inclusion by these polymers prepared by the deficiency of 4,40 -BPNO are random rather than depending on the metal ion under consideration. However, one such polymer is important in encapsulating selective 1D supramolecular units in the interstices. The coordination polymer 1 holds hydrogen bonded units of HBen 3 3 3 4,40 BPNO 3 3 3 HBen with a length of 22.38 A˚. Isostructural [{Ln(Ben)3(4,40 -BPNO)(H2O)2} 3 HBen]n where Ln = La, Eu, Gd, Tb coordination polymers prepared from similar reaction with metal to 4,40 -BPNO ratio 1:1 have eight coordinated distorted square antiprismatic geometry around the metal centers and are able to form 1D channel structures having neutral benzoic acid molecules encapsulated. Complex 5 exhibits a strong red luminescence emission in the solid state, characteristic of Eu(III) complexes. Magnetic susceptibility of 6 revealed weak antiferromagnetic interactions between the Gd(III) ions.

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Acknowledgment. The authors thank Department of Science and Technology, New-Delhi, India, for financial support, and R.S. thanks Council of Scientific and Industrial Research, New Delhi, India, for Senior Research Fellowship. Supporting Information Available: The hydrogen-bond parameters in tabular form, selected bond lengths, and angles in tabular form, NIR-UV spectra of 5, PXRD plots for 1-4, and crystallographic information files (CIFs). The CCDC numbers of the CIFs of the reported complexes are 797550-797556. This material is available free of charge via the Internet at http://pubs.acs.org.

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