Interplane Distances Modulation in Lanthanide-Based Coordination

Jun 6, 2003 - compositions, both structures adopt the same general arrangement: that is, a succession, along the cb-axis, of erbium atoms planes and ...
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Interplane Distances Modulation in Lanthanide-Based Coordination Polymers A. Deluzet, W. Maudez, C. Daiguebonne, and O. Guillou* INSA Rennes, GRCM - Laboratoire de Chimie Inorganique des Lanthanides, 20, Avenue des Buttes de Coe¨ smes, CS 14315, 35043 Rennes Cedex, France

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 475-479

Received September 30, 2002

ABSTRACT: We present here two new materials, Er2(BDC)3(H2O)6 (1) and Er2(NDC)3(H2O)6 (2), where BDC stands for 1,4-benzenedicarboxylate and NDC stands for 2,6-naphthalenedicarboxylate ligand. In addition to their compositions, both structures adopt the same general arrangement: that is, a succession, along the b c-axis, of erbium atoms planes and organic spacers-ligands planes. The longer naphthalene derivative ligand increases the c parameter, but the organization inside the erbium cations planes is kept, including the nature and the geometry of the coordination spheres of the lanthanide ions, the number and the repartition of the water molecules, and the network of hydrogen bonding. The chemical nature, the flexibility, and the topology of the organic ligands allow the preservation of the organic-inorganic interface within the materials. This study clearly shows the opportunity to imagine new lanthanide-based materials with the tools of crystal engineering. Introduction In the past few years, important progress has been made in the elaboration of new inorganic coordination frameworks,1-7 where large pores are kept to allow the adsorption or the removal of guest molecules (for example, ion exchange properties,8 gas sorption,9 guest inclusions from liquids10). In particular, chemists have used the metal-ion geometry, the topology, and the chemical functions of the organic ligands to design new robust open zeolite-like frameworks.11-15 This strategy of crystal engineering16 can also be transposed successfully in lanthanide chemistry to generate new hybrid materials. The design of these materials should take advantage of the potential applications of lanthanidecontaining materials such as biological,17 optical,18-20 ionic conductive,21 catalytic,22-25 or magnetic applications.26-28 So coordination polymers containing erbium ion could exhibit strong fluorescent emission at 1.54 µm and could be used in amplifier fibers. However, two great difficulties should be overcome: the high number of O-H groups (especially in water molecules in the coordination sphere of rare earth) and the closeness of the erbium atoms. We show here that a crystal engineering strategy could be applied to modulate the intermetallic distances by use of appropriate organic ligands. It is commonly admitted that, for lanthanide ions, in contrast with transition metal ions, covalence plays a minor role in metal-ligand bonds, and the nature of the coordination sphere is controlled by a tenuous balance between Coulombic interactions and ligand steric hindrance.29 Aromatic acids have often been used to prepare rare earth salts because of the rigidity of the aromatic part which favors the formation of great and ordered single crystals and because of the high affinity of carboxylate function and lanthanide ion. Several examples of material with unifunctional ligand, such as benzoate ligand;30,31 bifunctional, such as 1,4-benzene* To whom correspondence should be addressed. Fax: +33 223 238 697. E-mail: [email protected].

dicarboxylate (noted BDC and also named terephthalate);32,33 trifunctional, such as 1,3,5-benzenetricarboxylate34,35 or 1,3,5-cyclohexanetricarboxylate;36 tetrafunctional, such as 1,2,4,5-benzenetetracarboxylate;37-39 or hexafunctional, such as benzenehexacarboxylate or mellitate ligand,40,41 have already been reported. In addition, one carboxylate group offers the opportunity of different modes of coordination with the lanthanide ions such as unidentate, bidentate, or bridging mode.42 To develop our strategy, we have chosen the 1,4benzenedicarboxylate ligand. Only one structure, Tb2(BDC)3(H2O)4, has been reported, which consists of a succession, along the a axis, of lanthanide-water molecules planes and organic ligands planes.33 The use of physical or chemical gels for slow diffusion allows one to obtain new materials with different structural arrangements compared with the one observed for the phases prepared by direct contact of reactive compounds or by hydrothermal methods. In our case, by using a tetraethyl orthosilicate gel, we obtain a new phase, hereafter noted 1, namely, the Er2(BDC)3(H2O)6. To increase the erbium-erbium distance, while keeping the general arrangement, we have used a larger ligand, namely, the 2,6-naphthalenedicarboxylate (noted NDC), to prepare an isomorphic phase 2 with a similar formulation Er2(NDC)3(H2O)6. Our choice was driven by several factors. First, the NDC ligand possesses two carboxylate groups that preserve two negative charges and the same coordination type with erbium ions. Moreover, the aromatic and rigid part of naphthalene may lead to high quality single crystals. Finally, the flexibility of the carboxylate group bounded to the aromatic part is kept. Actually, this ability of the carboxylate group to be out of the conjugated plane observed in terephthalate salt has already been noted in naphthalene-like carboxylic acid.43 Results and Discussion The slow diffusion, through gel in a U-shaped tube, of an erbium chloride solution and a sodium terephtha-

10.1021/cg020052v CCC: $25.00 © 2003 American Chemical Society Published on Web 06/06/2003

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Deluzet et al. Table 1. Atomic Coordinates for 1

Figure 1. View along the b a axis of 1 showing the succession of organic and inorganic planes. Thermal ellipsoids are at 50% level.

Figure 2. View along the (a b+B b +b c) axis of the inorganicorganic hybrid planes in 1. The channels shown are too narrow to contain host water molecules.

late solution leads to colorless and parallelepipedic crystals of 1. The general arrangement (see Figure 1) consists of a succession, along the b c axis, of erbium atoms planes and organic planes. Consequently, the shortest erbium-erbium interplane distance is 10.693 Å. (Figure 1) The material exhibits two-dimensionality by means of lanthanide-anion links. The organicinorganic hybrid planes so formed spread along the (a b + B b + b c) axis (see Figure 2). The erbium atoms present a dodecaedric coordination polyhedron made of three oxygen atoms from water molecules and five oxygen atoms from carboxylate groups. All in all, four different terephthalate ligands are connected with each lanthanide atom. As shown in Figure 1, there is one crystallographically independent organic molecule in the general position (labeled “A”) and one-half located in an inversion center (labeled “B”). The coordination modes of these two organic molecules are different: the oxygen atoms of the “A” ligand generate three bonds with three

atoms

x

y

z

Beq

Er1 O1 O2 O3 O1A O2A O3A O4A C1A C2A C3A C4A C5A C6A C7A C8A O1B O2B C1B C2B C3B C4B

0.26636(2) 0.3028(6) 0.3440(5) 0.0012(5) 0.8615(6) 1.0451(5) 0.5623(5) 0.8288(6) 0.9368(6) 0.8894(7) 0.9831(9) 0.7504(10) 0.9343(9) 0.6995(11) 0.7899(6) 0.7240(7) 0.3255(5) 0.4165(5) 0.3955(6) 0.4515(7) 0.5419(10) 0.4092(10)

0.794254(19) 1.0505(4) 0.5904(4) 0.6664(5) 1.1012(4) 0.8845(4) 0.7911(5) 0.6437(4) 0.9721(5) 0.9177(5) 0.7852(8) 0.9980(7) 0.7317(8) 0.9420(8) 0.8092(5) 0.7442(5) 0.5817(4) 0.7888(4) 0.6528(6) 0.5741(5) 0.6437(7) 0.4296(7)

0.006597(16) -0.0679(5) -0.0883(4) 0.0734(4) 0.1734(4) 0.1673(3) 0.8875(4) 0.8959(4) 0.2311(4) 0.3876(5) 0.4565(6) 0.4634(6) 0.6007(6) 0.6069(6) 0.6763(5) 0.8314(5) 0.1977(3) 0.1779(3) 0.2434(5) 0.3750(5) 0.4219(6) 0.4540(6)

0.01714(13) 0.0349(9) 0.0294(8) 0.0315(8) 0.0300(8) 0.0289(8) 0.0334(8) 0.0287(8) 0.0192(8) 0.0199(9) 0.0440(16) 0.0491(19) 0.0435(17) 0.051(2) 0.0206(9) 0.0207(9) 0.0251(7) 0.0252(7) 0.0200(9) 0.0220(9) 0.0365(14) 0.0376(14)

different erbium atoms, and the last oxygen atom remains free. On the opposite, the four oxygen atoms belonging to the “B” ligand are bonded to only two different erbium atoms. Actually, the “A” ligand exhibits a bridging carboxylate function and an unidentate carboxylate group, whereas the “B” ligand only coordinates erbium atoms in a bidentate way.35 This is a great difference between the already reported material in which all the carboxylate groups were involved in bridging links with four erbium atoms per organic molecule.33 Consequently, the free oxygen atom in this structure allows one supplementary water molecule to end the coordination sphere of the lanthanide atom. The water molecule oxygen atoms and the carboxylate groups oxygen atoms generate a complex network of hydrogen bonding that we will detail later (Table 1). The planes of organic molecules are constituted by dimers of “A” molecules separated by one “B”-type molecule. The middle planes of the two kinds of ligands make an angle of roughly 78°, that makes impossible a π-π interaction between “A” and “B” terephthalate anions. This important angle between benzene cycles is classical in packing of the aromatic molecules. This is usually attributed to an hydrogen bonding between an aromatic hydrogen atom of a first cycle and the electronic cloud of the second aromatic cycle.44 However, such a measured distance in our case appears a little bit long (nearly 3.40 Å from the phenyl group center to the closest hydrogen atom) to create strong hydrogen bonding. Within a dimer of “A”-type molecules, short C-C distances are found (3.794(2), 3.759(2), and 3.815(2) Å), but the shortest contact found between two neighboring ligands exists between two oxygen atoms. This seems to show that the interactions between aromatic rings are very modest. In contrast with the other terephthalate structure, the carboxylate functions do not largely deviate from the mean aromatic plane (Figure 2). The torsion angles of the three independent carboxylate groups (defined as the angle between the aromatic plane and the carboxylate plane) are roughly 6.37°, 9.52°, and 20.36° and very similar to the ones of the terbium compound already reported (6.99°, 11.06°, and 27.95°).33 Note however that the anion is less planar than the

Lanthanide-Based Coordination Polymers

Crystal Growth & Design, Vol. 3, No. 4, 2003 477 Table 3. Compared Erbium-Erbium Distances and Angles in the Inorganic Planes in 1 and 2a Er-Er distances (Å) and angles (°)

in 1

in 2

Er-Era Er-Erb Er-Erc Er-Erd Er-Ere Er-Erf Era-Er-Erb Erb-Er-Erc Erc-Er-Erd Erd-Er-Ere Ere-Er-Erf Erf-Er-Era

7.837(0) 5.963(3) 4.965(4) 7.837(0) 7.884(3) 5.919(4) 39.3 91.17 49.53 44.23 67.46 68.31

7.989(0) 6.036(7) 5.032(10) 7.989(0) 7.846(9) 5.954(10) 39.02 91.95 49.03 44.16 69.2 66.64

a With the following symmetry operations: (a) x-1, y, z, (b) 1-x, 1-y, 2-z, (c) 2-x, 1-y, 2-z, (d) 1+x, y, z, (e) 2-x, 2-y, 2-z, and (f) 1-x, 2-y, 2-z.

Figure 3. View along the b a axis of 2 showing the succession of organic and inorganic planes. Thermal ellipsoids are at 50% level. Table 2. Atomic Coordinates for 2 atoms

x

y

z

Beq

Er1 O1 O2 O3 O1A O2A O3A O4A C1A C2A C3A C4A C5A C6A C7A C8A C9A C10A C11A C12A O1B O2B C1B C3B C4B C5B C6B C11B

0.73447(2) 0.7073(4) 0.6589(4) 0.9832(4) 0.8978(4) 1.0684(4) 0.5409(4) 0.7936(4) 0.9251(5) 0.7756(5) 0.6984(8) 0.7595(7) 0.9024(5) 0.9847(5) 1.0051(7) 0.9370(6) 0.7829(5) 0.7067(6) 0.9605(5) 0.7012(5) 0.6657(4) 0.5618(4) 0.5613(5) 0.3604(6) 0.4063(6) 0.5295(5) 0.6024(6) 0.5872(5)

0.705579(19) 0.4560(4) 0.9217(4) 0.8357(4) 0.6163(4) 0.3957(4) 0.2872(4) 0.1446(4) 0.3294(5) 0.4247(5) 0.5458(7) 0.5709(6) 0.4731(5) 0.3570(5) 0.2097(7) 0.1807(7) 0.2720(5) 0.3897(5) 0.4959(5) 0.2322(5) 0.8925(3) 0.6927(4) 1.0317(5) 0.8590(5) 0.8108(5) 0.8737(5) 0.9834(5) 0.8167(5)

0.997814(14) 0.0517(3) 0.0771(3) 0.9413(3) 0.1295(3) 0.1437(3) 0.8922(3) 0.9251(3) 0.5017(4) 0.5502(4) 0.4787(5) 0.3641(4) 0.3141(4) 0.3821(4) 0.5738(5) 0.6876(4) 0.7366(4) 0.6690(4) 0.1879(4) 0.8608(4) 0.8424(3) 0.8654(3) 0.5135(4) 0.5690(4) 0.6727(4) 0.6989(4) 0.6217(4) 0.8081(4)

0.02547(7) 0.0417(8) 0.0397(8) 0.0426(8) 0.0380(7) 0.0386(7) 0.0409(8) 0.0390(8) 0.0306(9) 0.0313(9) 0.0604(18) 0.0518(15) 0.0286(9) 0.0343(10) 0.0576(17) 0.0526(15) 0.0308(9) 0.0375(10) 0.0277(8) 0.0298(9) 0.0346(7) 0.0360(7) 0.0283(9) 0.0383(11) 0.0363(10) 0.0306(9) 0.0321(9) 0.0293(9)

neutral carboxylic acid.45-47 No crystallization water molecule has been found, and the view shown in Figure 2 explains this fact. The channels created along the b c axis are not large enough to allow the insertion of free water molecules. Slow diffusion, through water in a H-shaped tube, of solutions of erbium chloride and NDC potassium salt leads to colorless parallelepipedic crystals. The structure arrangement (see Figure 3) is exactly the same as the one observed in the previously described material (compound 1), the NDC ligand being substituted by the BDC ligand. The b a and B b axis remain unchanged (and consequently γ too), whereas the b c axis (the largest axis of the cell) increases from 10.693 to 12.429 Å (Figure 3) (Table 2). The geometry around lanthanide centers and the coordination spheres are exactly the same in both structures. Even more, despite the discrepancy of the

Table 4. Cone Angles around Lanthanide Ion in 1 and 2 label and coordination mode of ligands

cone angle in 1 (°)

cone angle in 2 (°)

B bidentate A bridging A bridging A unidentate

121.63 92.69 94.65 91.96

122.15 99.24 96.71 98.86

ligands, the distances and angles between a given Er ion and its six nearest Er ions neighbors are almost exactly the same as can be seen in Table 3. This is because four of the nearest Er ions are not connected directly by a ligand and the others are connected via a bridging carboxylate group. The number of water molecules and the coordination modes of the organic ligands are also the same. In addition, there is one independent ligand located in the general position (labeled “A”) and another located on an inversion center (labeled “B”). Even in the organization of the organic ligand-based planes, no significant difference is observed. The angle between the “A” and “B” molecules is 72° (compared with 78° for the terephthalate-based compound) and contacts between neighboring ligands are shorter and, according to the number of aromatic carbon atoms, more numerous (3.556(5), 3.585(5), 3.630(5), 3.698(5), and 3.750(5) Å for C-C contacts and 3.588(5) Å for O-O contacts). For compound 2, two hydrogen bonds per molecule could be found within the organic planes, that is, between C-H of an aromatic cycle and the electronic delocalized cloud from the other aromatic cycle. The distances between the center of the cycle of six atoms and the hydrogen atoms are 3.47 and 3.53 Å. The coordination modes of carboxylate groups are exactly the same in the two materials (one unidentate, one bidentate, and one bridging). The calculation of cone angles as defined in the literature48 shows that the steric demand of BDC and NDC is approximately the same (Table 4). Consequently, despite its larger spatial extension, the hindrance created by the ligand in the neighboring of the lanthanide ion only depends on the chemical coordination function. Whereas the aromatic part of ligands do not deviate from flatness (all carbon atoms are located less than 0.1 Å from mean plane), the carboxylate groups exhibit larger distortion. The torsion angles (the angle defined as the angle made by the mean plane of naphthalene part and by the carboxylate plane) are 18.04°, 27.12°, and 40.86°. This deformation has already been reported in nickel and cobalt salt43 and in

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naphthalenedicarboxylic acid.49 It has been calculated that this distortion brings destabilization of only 9 kcal mol-1, this loss of energy being compensated by more effective lanthanide ion coordination. Conclusion These results show the opportunity of using crystal engineering tools to design new coordination polymers with expected physical properties. For example, such an approach can be developed to increase the size of pores for sorption/desorption properties. For optical properties, the separation of erbium centers is a prerequisite to prepare efficient fluorescent materials for amplification fibers. Magnetic properties should be examined when lanthanide magnetic centers are close together. In all cases, the forces at the organicinorganic interface are strong enough to accommodate a slight modulation of the organic part. Despite the weak chemical role of lanthanide ions, building elementary brick with rigid coordination sphere is possible if chemical functions and flexibility properties of the organic ligands are kept identical. The tunability of the intermetallic distances demonstrated here, together with the well-known chemical likeness of the lanthanide ions, could offer us the opportunity of designing new classes of materials exhibiting tunable physical properties. Experimental Section Synthesis of 1. Disodium terephthalate salt is prepared by addition of 2 equiv of sodium hydroxide to a suspension of terephthalic acid in deionized water until complete dissolution of terephthalic acid. Then, the solution was evaporated to dryness. The residue is put in suspension in ethanol, stirred, and refluxed during 1 h. After filtration and drying of the sample in a desiccator, a white powder of disodium 1,4benzenedicarboxylate was obtained. Erbium chloride was obtained by reaction of hydrochloric acid with erbium oxide. An amount of erbium oxide was put in suspension in water at 50-60 °C with magnetic stirring, and hydrochloric acid was added until complete dissolution of powder. Then, the solution was evaporated to dryness, and the residue was dissolved in ethanol. After precipitation by addition of ether and filtration, the pink powder was collected, dried, and stored in a desiccator. 1 is obtained by slow diffusion of deionized aqueous solutions of erbium chloride and TBC sodium salt, through an tetraethyl orthosilicate gel bridge in a U-shaped tube. After a month, colorless single crystals were obtained. C24H24O18Er2 (934) Anal. Calcd. C 30.84, H 2.57; found C 30.68, H 2.49. Synthesis of 2. Dipotassium 2,6-naphthalenedicarboxylate was purchased from Aldrich Chemical Incorporation and used without further purification. Erbium chloride is synthesized as described above. Slow diffusion through water, in a H-shaped tube, of deionized aqueous solutions of erbium (III) chloride and NDC potassium salt leads to colorless single crystals of 2 after two months. C36H30O18Er2 (1084) Anal. Calcd. C 39.85, H 2.77; found C 39.58, H 2.65. X-ray Crystallographic Study. The determination of the unit cell and the data collection for colorless crystals of compounds 1 and 2 were performed on a Nonius Kappa CCD, and the data were collected using graphite-monochromated Mo-KR radiation (λ ) 0.71073 Å) at 293 K. The structures were solved by direct methods and refined by full-matrix leastsquares methods, which were performed using the SHELXS97 and SHELXL-97 software packages.50 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of organic

Deluzet et al. ligands have been calculated by the SHELXL-97 package and hydrogen atoms of water molecules have not been refined. Crystal data for 1: C6H3Er0.5O4.5, Mr ) 230.71, a ) 7.8373(1)Å, b ) 9.5854(2)Å, c ) 10.6931(2)Å, R ) 68.7770(8)°, β ) 70.8710(8)°, γ ) 75.3330(12)°, Z ) 4, d ) 2.192 g.cm-3, triclinic, P1 h. Data collection for 1: crystal size 0.55 × 0.42 × 0.18 mm3, θ range: 2.12° to 27.51°, 9063 measured reflections, 3067 of symmetry-independent reflections (I > 2σ(I)), µ ) 6.046 mm-1, analytical absorption correction by WinGX program (Tmin ) 0.1322, Tmax ) 0.4255),51 200 parameters, 0 constraint F(000) ) 436. R1 ) 0.0329, wR2 ) 0.0329 Goodness of fit 1.120 for 3067 reflections with I > 2σ(I), (R1 ) 0.0342, wR2 ) 0.0865 for all the reflections). CCDC-188931 contains the supplementary crystallographic data for 1. Crystal data for 2: C9H4.5Er0.5O4.5, Mr ) 268.26, a ) 7.9891(3)Å, b ) 9.5539(4)Å, c ) 12.4286(6)Å, R ) 75.9870(15)°, β ) 74.8250(15)°, γ ) 75.889(3)°, Z ) 4, d ) 2.044 g cm-3, triclinic, P1 h. Data collection for 2: crystal size 0.19 × 0.11 × 0.045 mm3, θ range: 1.73° to 30.09°, 12789 measured reflections, 4273 of symmetry-independent reflections (I > 2σ(I)), µ ) 4.865 mm-1, analytical absorption correction by WinGX program (Tmin ) 0.5849, Tmax ) 0.8285),51 253 parameters, 0 constraint, F(000) ) 514. R1 ) 0.0377, wR2 ) 0.0803. Goodness of fit 1.069 for 4273 reflections with I > 2σ(I), (R1 ) 0.0522, wR2 ) 0.0872 for all the reflections). CCDC-188991 contains the supplementary crystallographic data for 2. These data can be obtained free of charge at www. ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 IEZ, UK; fax: (internat.) +44-1223/336-033; E-mail: [email protected]].

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