Syntheses, Structures, and Properties of Novel Cagelike Complexes

Department of Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China, and College of Pharmacy,. Tianjin Medical UniVersity, Tianjin 300070, P. R. C...
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Syntheses, Structures, and Properties of Novel Cagelike Complexes Based on Dodecanuclear Lanthanide with a Large Cavity Xu,†,‡

Zhao,†

Jing-Yuan Bin He-Dong Dai-Zheng Liao,† and Pan-Wen Shen†

Bian,†,§

Wen

Gu,†

Shi-Ping

Yan,*,†

Peng

Cheng,†

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1044-1048

Department of Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China, and College of Pharmacy, Tianjin Medical UniVersity, Tianjin 300070, P. R. China ReceiVed July 28, 2006; ReVised Manuscript ReceiVed March 11, 2007

ABSTRACT: The syntheses, crystal structures, thermal gravimetric analyses (TGA), and magnetic properties of three novel hybrid organic-inorganic dodecanuclear lanthanide-based complexes, [Ln12Na(L)5 (HL) (µ4-NO3)6(µ3-OH)2(µ2-H2O)6(H2O)12]‚(H2O)n (Ln ) La (1), n ) 23.5; Pr (2) and Nd (3), n ) 39; L ) 2-hydroxy-5-methyl-m-phenylenedimethylenedinitrilotetraacetate anion), are reported. X-ray diffraction analyses revealed that three complexes were isostructural, exhibited C3 symmetry, and crystallized in the rhombohedral crystal system, space group R3h. In 1-3, 12 lanthanide atoms regularly arrayed and aggregated into a closed capsulelike cage by anion-bridged ligands containing polycarboxylate, phenolate, nitrate, and hydroxyl groups. Remarkably, these nanosized discrete molecular cages afford large three-compartment cavities with [Na(H2O)6]+ ions encapsulated in the center site. Introduction The current enormous interest in high-nuclearity metal oligomers stems not only from their fascinating topological structures but also from their wide magnetic, optical, electronic, and catalytic applications, which may ultimately lead to promising new materials.1-3 However, studies on high-nuclearity metal oligomers featuring cage structures hitherto have mainly focused on transition metals.1 As far as we know, cagelike complexes based on high-nuclearity lanthanide metals are relatively rare, although many lanthanide clusters4 have been reported with a variety of spectacular structures such as a cube,4d,4e ring,4f,4g,4h and wheel.4i,4j Compared with 3d or 4d metals, constructing cagelike structures containing 4f metal ions may be quite a challenge, owing to the low stereochemical preferences and the versatile coordination behavior of 4f metal ions. On the other hand, on the basis of reports on high-nuclearity cagelike complexes, smaller anions as templates, for example, Cl-,1d,1f OH-,1i O2-,1g BF4-,5 and ClO4-,5a were frequently encapsulated within the cages to improve the stabilities of the cages. Unfortunately, few metal cations trapped in the cavities of these cages have been observed to date,1h although cations such as tetraalkylammonium6 or ammonium7 as guests capsulated in cages have appeared in the literature. Lately, we have been investigating the preparation and reactions of lanthanide-based complexes that employ 2-hydroxy5-methyl-m- phenylenedimethylenedinitrilotetraacetate anion (L) as a directed ligand. Considering the robust and flexible geometry of such a bridging spacer, we wondered if this ligand could be used as an “organic claw” to chelate and bridge lanthanide ions into high-nuclearity metal oligomers. In this article, we report the synthesis and structural characterization of three unprecedented dodecanuclear Ln(III) cagelike complexes: [Ln12Na(L)5(HL)(µ4-NO3)6(µ3-OH)2(µ2-H2O)6(H2O)12]‚ (H2O)n (Ln ) La (1), n ) 23.5; Pr (2) and Nd (3), n ) 39). These discrete molecular complexes were isostructural and * To whom correspondence should be addressed. E-mail: yansp@ nankai.edu.cn. Fax: +86-22-23502458. † Nankai University. ‡ Tianjin Medical University. § Present address: Department of Chemistry, Guangxi Normal University, Guilin 541004, P. R. China.

exhibited large three-compartment cavities with [Na(H2O)6]+ ions trapped in the center of the closed cages. Experimental Section Materials and Physical Techniques. During the syntheses of 1-3, all reagents and solvents employed were commercially available and used directly without further purification. Analyses for C, H, and N were carried out on a Perkin-Elmer analyzer at the Institute of ElementoOrganic Chemistry, Nankai University. Variable-temperature magnetic susceptibilities were measured on a Quantum Design MPMS-7 SQUID magnetometer. Diamagnetic corrections were made with Pascal’s constants for all the constituent atoms. X-ray powder diffraction measurements were performed on a D/Max-2500 X-ray diffractometer using Cu KR radiation. Thermal analyses (under oxygenated atmosphere, heating rate of 5 °C/min) were carried out in a Labsys NETZSCH TG 209 Setaram apparatus. Experimental Procedures. Trisodium salt Na3H2L was synthesized by the literature method.8 [La12Na(L)5(HL)(µ4-NO3)6(µ3-OH)2(µ2-H2O)6(H2O)12]·(H2O)23.5 (1). Na3H2L (0.5 mmol, 0.232 g) was added as a solid to an ethanol-water (1:1) solution (20 mL) of La(NO3)3‚6H2O (1.0 mmol, 0.433 g) under stirring. The resulting suspension was refluxed for ca. 1 h. An aqueous solution of NaOH (1 M) was added dropwise to the above mixture until all the precipitate dissolved and sequentially the pH value was adjusted to 8-9. After two weeks of evaporation at room temperature, perfect block-shaped crystals suitable for X-ray analysis were formed, which were isolated in yields of 67% (based on La). Element analysis (%) calcd for 1: C 23.51, H 3.64, N 4.84; found: C 22.94, H 4.01, N 4.77. Atomic emission spectrometry (AES) analysis found Na 0.43% for 1. [Pr12Na(L)5(HL)(µ4-NO3)6(µ3-OH)2(µ2-H2O)6(H2O)12]·(H2O)39 (2) and [Nd12Na(L)5(HL)(µ4-NO3)6(µ3-OH)2(µ2-H2O)6(H2O)12]·(H2O)39 (3). The synthetic procedures were similar to that of 1. The corresponding amounts were Pr(NO3)3‚6H2O (1.0 mmol, 0.435 g) for 2 and Nd(NO3)3‚6H2O (1.0 mmol, 0.438 g) for 3. The pH value was adjusted to 7-8 for 2 and 3. Yield: 54% (based on Pr in 2) and 63% (based on Nd in 3). Element analysis (%) calcd for 2: C 22.24, H 3.98, N 4.57; found: C 22.37, H 4.12, N 4.77. Element analysis (%) calcd for 3: C 22.08, H 3.95, N 4.54; found: C 22.34, H 4.21, N 4.76. AES analysis found Na: 0.41% for 2 and 0.40% for 3. Crystal Structure Determination. Diffraction intensities for the three complexes were collected at 293 K on a computer controlled Bruker SMART 1000 CCD diffractometer equipped with graphitemonochromated Mo-KR radiation with radiation wavelength of 0.71073 Å by using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved by direct methods and refined with the full-matrix least-square technique

10.1021/cg0605032 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007

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Crystal Growth & Design, Vol. 7, No. 6, 2007 1045

Table 1. Crystal Data and Structure Refinement for 1-3

empirical formula formula weight crystal system space group a b c γ V (Å3) Z F [Mg/m3] abs coeff (mm-1) F(000) crystal size (mm3) θ limiting indices reflections collected/unique data/restraints/parameters GOF R1 [I > 2σ(I)]a wR2 (all data) largest diff peak and hole (e Å-3) a

1

2

3

C102H188La12N18NaO115.5 5207.28 rhombohedral R3h 18.549(7) Å 18.549(7) Å 50.77(3) Å 120° 15127(11) 3 1.715 2.584 7639 0.30 × 0.25 × 0.25 2.05 to 25.02° -21 e h e 22 -22 e k e 17 -53 e l e 60 20105/5745 [R(int) ) 0.3693] 5745/54/420 0.962 0.0695 0.2182 1.044 and -1.124

C102H219Pr12N18NaO131 5507.86 rhombohedral R3h 25.329(4) Å 25.329(4) Å 45.031(6) Å 120° 25020(6) 3 1.097 1.785 8172 0.24 × 0.22 × 0.18 mm 1.61 to 25° -30 e h e 17 -24 e k e 29 -53 e l e 52 28926/9727 [R(int) ) 0.1370] 9727/0/419 1.097 0.0653 0.2192 1.637 and -1.163

C102H219Nd12N18NaO131 5547.82 rhombohedral R3h 25.305(8) Å 25.305(8) Å 45.08(3) Å 120° 24996(19) 3 1.105 1.902 8208 0.20 × 0.15 × 0.10 mm 1.61 to 25.03° -30 e h e 29 -28 e k e 29 -34 e l e 53 30803/9661 [R(int) ) 0.1656] 9661/0/414 1.069 0.0617 0.1938 1.815 and -1.133

R1 ) ∑||Fo| - |Fc||/|Fo|, wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2. Table 2. Selected Bond Lengths [Å] and Bond Angles [°] for 2a Pr(1)-O(8) Pr(1)-O(1) Pr(1)-O(6) Pr(1)-N(3)#1 Pr(2)-O(4) Pr(2)-O(2) Pr(2)-O(3)#2 N(3)-Pr(1)#5

2.445(10) 2.498(9) 2.551(11) 2.885(14) 2.456(10) 2.508(10) 2.684(10) 2.885(14)

O(8)-Pr(1)-O(6) O(10)-Pr(1)-O(6) O(8)-Pr(1)-O(13) O(1)-Pr(1)-O(13) O(6)-Pr(1)-O(13) O(10)-Pr(1)-N(2) O(14)-Pr(1)-N(2) O(8)-Pr(1)-Pr(2) N(2)-Pr(1)-Pr(2) O(16)#4-Na(1)-O(16) O(13)-Pr(1)-Pr(2)

Pr(1)-O(12)#1 Pr(1)-O(10)#1 Pr(1)-O(13) Pr(1)-Pr(2) Pr(2)-O(11) Pr(2)-O(15) Pr(2)-N(1) O(10)-Pr(1)#5 125.7(4) 139.8(3) 83.5(4) 138.9(3) 133.1(3) 136.8(3) 72.7(4) 118.8(3) 100.5(3) 180.0(1) 124.3(3)

2.457(9) 2.498(9) 2.586(5) 4.0846(12) 2.463(9) 2.566(10) 2.746(12) 2.498(9)

Pr(1)-O(10) Pr(1)-O(14) Pr(1)-N(2) Pr(2)-O(1) Pr(2)-O(12)#1 Pr(2)-O(16) Na(1)-O(16)#2 Na(1)-O(16)#4

O(8)-Pr(1)-O(10) O(8)-Pr(1)-O(1) O(10)-Pr(1)-O(1) O(10)-Pr(1)-O(14) O(12)#1-Pr(1)-O(6) O(14)-Pr(1)-O(6) O(10)-Pr(1)-O(13) O(14)-Pr(1)-O(13) O(8)-Pr(1)-N(2) O(1)-Pr(1)-N(2) O(6)-Pr(1)-N(2)

2.493(9) 2.549(10) 2.748(13) 2.448(9) 2.477(9) 2.578(9) 2.387(9) 2.387(9) 86.3(3) 87.6(3) 73.2(3) 135.1(3) 67.8(3) 79.7(4) 66.2(3) 69.9(4) 64.2(4) 74.6(3) 61.8(4)

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

using the SHELXS-97 and SHELXL-97 programs.9 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond lengths and angles for 1-3 are listed in Tables 1 and 2 and Tables SF-7 and SF-8 (Supporting Information). CCDC reference numbers are 188517 for 1, 205529 for 2, and 205530 for 3.

Results and Discussion Syntheses of 1-3. The three complexes were obtained by an analogous route: Na3H2L ligand and corresponding lanthanide nitrate were mixed under stirring and refluxing conditions in ethanol-water (1:1), NaOH solution was added to make the resulting suspension clear, and then the pH value was adjusted to 8-9 for 1 and 7-8 for 2 and 3. Block-shaped crystals suitable for X-ray analysis were obtained by slow solvent evaporation after two weeks.

Crystal Structures of 1-3. Single-crystal X-ray diffraction analyses revealed that three isostructural complexes crystallized in the rhombohedral crystal system, space group R3h. The molecular motifs of three complexes displayed extreme similarity with slight differences in the numbers of lattice water molecules among them, which is 23.5 in 1 and 39 in both 2 and 3. Each complex consists of two crystallographically independent Ln atoms (Ln1 and Ln2), and Ln1‚‚‚Ln2 separations are 4.192, 4.085, and 4.072 Å, respectively, in 1-3. The dinuclear Ln motif packs into a distorted trigonal prism by a corresponding symmetrical operation; consequently, the resulting hexanuclear Ln(III)-based trigonal prism may further aggregate into a large Ln12 cagelike structure by employing a Na1 atom as a symmetry center. As a result, a [Na(H2O)6]+ ion was perfectly encapsulated in the closed nanosized cage. Here complex 2 was taken as an example to depict the cagelike structure in detail. As shown in Figure 1, two crystallographically independent Pr atoms (Pr1, Pr2) exist, each coordinated with eight oxygen atoms and one nitrogen atom to

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Figure 1. ORTEP diagram of a crystallographically unique dinuclear Pr unit in 2. H atoms and uncoordinated water molecules were omitted for clarity.

Figure 3. (a) Schematic diagram of an O-monocapped triangular prism of 2 including a NO3--coordinated mode; the NO3- anions on the other two side faces were omitted for clarity. (b) Schematic diagram of the dodecanuclear Pr-based cage consisting of two identical triangular prisms bridged by six carboxy groups.

Figure 2. DIAMOND view of a dodecanuclear Pr cagelike cluster. H atoms and water molecules were omitted for clarity. Color scheme: Pr, green; Na, yellow; N, blue; O, red; C, white.

complete the nine-coordinated geometry configuration. The average bond length of Pr1-O is 2.509 Å and that of Pr1-N2 is 2.748 Å, while those of Pr2-O and Pr2-N1 are 2.522 and 2.746 Å, respectively. Compared with previous work,10 the distances of Pr-O and Pr-N fall in the normal range. The dinuclear Pr1‚‚‚Pr2 motif, bridged by one L5- ligand and two NO3- anions, aggregates into a distorted Pr6-based triangular prism and then into a Pr12-based three-compartment capsulelike cage (Figure 2) through C3 and centrosymmetric operations, respectively. More interestingly, each L5- ligand, which generally acted as dinucleating roles as reported,8,11 here however adopt a [9.11121111] mode (as described using Harris notation, see Supporting Information) and not only locked Pr1 and Pr2 atoms through its phenolate, two amine nitrogen atoms, and four oxygens of carboxylate groups to produce a crystallographically basic dinuclear unit (Figure 1), which acts as an exo-ligand of the Pr1‚‚‚Pr2 side-edge of the trigonal prism, but also interacted with the carboxylate O3 atom linked Pr2C ion from the opposite trigonal prism generated via a centrosymmetric operation at the Na1 point. For one trigonal prism as

shown in Figure 3a, three side edges Pr1‚‚‚Pr2, Pr1A‚‚‚Pr2A, and Pr1B‚‚‚Pr2B display equal lengths at 4.085 Å, and three Pr1 series (Pr1, Pr1A, and Pr1B) and three Pr2 series (Pr2, Pr2A, and Pr2B) ions constitute two triangular planes of the trigonal prism giving equal edge lengths of 4.231 and 6.433 Å, respectively. On top of the smaller triangular plane of the trigonal prism, a O-monocapped prism emerged with an O13Pr1 (Pr1A, Pr1B) distance of 2.586(5) Å; outside the bigger side, Na1 atoms linked with Pr2 (Pr2A, Pr2B) by the water molecule O16 (O16A, O16B) at distances of O16-Pr2 2.578 Å and O16-Na1 2.387 Å and a Na1-O16-Pr2 angle of 132.0(4)°. From the view of the side face of the trigonal prism, an NO3- anion connects four Pr ions, and all the atoms on it (for example: Pr1, Pr2, Pr1B, Pr2B, N3, O10, O11, and O12 in Figure 3a) deviate from their mean plane by ca. (0.2934 Å.qj It should be noted that the NO3- anion plays a key role in efficiently chelating four Pr ions to generate a fastened trapezoid to prevent small molecules entering the semiclosed trigonal prism cavity from the side faces; here the NO3- anion takes a unique [4.221] coordination mode, which, to the best of our knowledge, has never been reported before. Most interestingly, the two opposite prisms with an offset of 60°, linked by six carboxylate groups of L5- ligands with responsibility for the middle cavity diameter of 8 Å (see Figure 4), offer a perfectly closed three-compartment nanocage (∼22 × 16 × 16 Å3, van der Waals radii of atoms have been included) (Figure 3b). The Na+ with octahedral geometry linked six Pr3+ ions (Pr2, Pr2A, Pr2B, Pr2C, Pr2D, and Pr2E) from two opposite prisms by water molecules to further stabilize the capsule-like cage and large bulk [Na(H2O)6]+ ion dwelling in the inner cavity. Unfortunately, all our efforts to trap Li+ or K+ into the cavity of the cage and/or employ other lanthanide ions to crystallize

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Crystal Growth & Design, Vol. 7, No. 6, 2007 1047

Figure 4. Schematic diagram of a cagelike structure with a large cavity. The large purple sphere indicates the size of the central cavity. Color scheme: Pr, green; N, blue; O, red; C, gray. Figure 6. A space-filling diagram of the nanosized holes in 2 (Pr, green; Na, yellow; N, blue; O, red; C, light gray).

Figure 7. PXRD patterns for 2 (a) taken at room temperature, (b) after heating to 150 °C for 12 h. Figure 5. The structure of 2 viewed along the 3-fold axis, which passes through O13, Na1, and O13A, exhibiting inside and outside homocentric circles based on six Pr atoms, respectively.

the corresponding complexes were unsuccessful to date; however, expanding the investigation for this system is underway. The angle of O13-Na1-O13A is 180°, and its bond axis is collinear with the principal C3 axis, while the spectacular topological motifs including two co-center circles each constructed by six Pr ions were observed in Figure 5. Another interesting feature of this structure is the intermolecular hydrogen bond that exists among dodecanuclear Pr(III) clusters and uncoordinated water molecules, which also affords approximately 18 × 16 Å2 sized holes suitable to trap guest molecules (Figure 6). Thermal Gravimetric Analysis (TGA) and PXRD. TGA was performed on crystalline samples of the compounds in the range of 20-600 °C (see Supporting Information). The weight loss of 12.03% for 1, 13.78% for 2, and 13.05% for 3 between 20 and 150 °C corresponds to the loss of all uncoordinated water molecules (calcd 11.36% for 1, 12.75% for 2, and 12.65% for 3). Taking 2 as an example, we explored whether the framework would break down upon removal of the guest water molecules (uncoordinated water molecules). Water molecules were re-

moved by heating 2 at 150 °C for 12 h. The powder X-ray diffraction patterns of hydrated and dehydrated samples for 2 are considerably different, which suggests that the framework loses its integrity after the guest water molecules have been removed (Figure 7). Magnetic Properties of 2 and 3. Pr3+ and Nd3+ possess a rather large unquenched orbital angular momentum associated with the internal nature of the valence f orbitals and simultaneously have orbitally degenerate ground states, which are easily split by spin-orbit coupling and crystal-field effects. The magnetic properties of Pr3+ and Nd3+ ions are strongly influenced by this. The magnetic susceptibilities for complexes 2 and 3 were measured in the temperature range from 5 to 300 K (Figure 8) under an applied field of 1 T. The χMT values are equal to 19.01 and 19.62 cm3 K mol-1 at room temperature, respectively, which are slightly lower than the theoretical values of 19.2 and 19.68 cm3 K mol-1, those expected for 12 isolated Pr ions in the 3H4 ground state (g ) 4/5) and 12 isolated Nd ions in the 4I9/2 (g ) 8/11),12 respectively. Although the χMT values for 2 and 3 smoothly decrease on cooling, according to discussion above, the anti-ferromagnetic coupling between adjacent Pr or Nd ions could not be deduced due to the existence of strong spin-orbit coupling of the lanthanide atoms.12 The decrease in χMT possibly originates in the thermal depopulation of the highest Stark

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Figure 8. Plots of χMT vs T for 2 (4) and 3 (9).

components derived from the splitting of the free ion ground state 3H4 for Pr ion and 4I9/2 for Nd ion by the crystal field.13 Conclusion Three isostructural dodecanuclear lanthanide-based nanocages with C3 symmetry were successfully isolated and structurally characterized. Such a high-symmetry and high-nuclearity lanthanide nanocage motif, to our knowledge, is seldom observed in other metallosupramolecular species, and it has a number of novel features: (i) Notwithstanding that the structural chemistry of HXTA4- polycarboxylates has been studied to some extent with transition metals,8,11 examples of structurally characterized LnIII-HXTA complexes have not been reported hitherto. (ii) The Ln12 cage was perfectly fabricated by the L5- ligand, µ3-OH and µ4-NO3- bridges, showing a fascinating unique centrosymmetric cagelike structure with a large cavity with a diameter of ∼8 Å; a larger bulk cation [Na(H2O)6]+ is strongly trapped in the center site. (iii) Examples of bridging µ4-NO3- connecting four lanthanum atoms are scarce in the literature. (iv) More importantly, this contribution demonstrates an available route to these otherwise hard-to-produce lanthanide-based substances at high pH values. These results provide an opening into a promising new field of cagelike lanthanide-based nanomaterials. Further studies are currently in progress. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20331020, 20501012) and Tianjin Natural Science Foundation, China (No. 06YFJMJC12700). J. Y. Xu thanks Prof. Dr. Siegfried Schindler and Dr. Simon P. Foxon, Institut fu¨r Anorganische and Analytische Chemie der Justus-Liebig-Universita¨t Giessen of Germany, for their helpful advice.

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Supporting Information Available: CIFs, TGA plots of 1-3, the coordination modes of L5- ligand, structural drawing of 2, and selected bond lengths and bond angles for 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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