Disassembly and Reassembly of Polyoxometalates: The Formation of

Jan 8, 2010 - School of Chemistry, University of Melbourne, Victoria, 3010, Australia. Cryst. Growth Des. , 2010, 10 (2), pp 488–491. DOI: 10.1021/c...
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DOI: 10.1021/cg901400k

Disassembly and Reassembly of Polyoxometalates: The Formation of Chains from an Adaptable Precursor

2010, Vol. 10 488–491

Chris Ritchie and Colette Boskovic* School of Chemistry, University of Melbourne, Victoria, 3010, Australia Received November 8, 2009; Revised Manuscript Received December 7, 2009

ABSTRACT: Two one-dimensional chain complexes with building blocks comprised of polyoxotungstate-ligated lanthanoid centers have been synthesized from the adaptable precursor [As2W19O67(H2O)]14- through combined disassembly and reassembly processes. Careful control of the reaction and crystallization conditions affords compounds formulated as H3K7Na6[Nd3As4W41O141OH(H2O)10] 3 60H2O (1) and KNa[Dy4As2W22O76(H2O)19(C2H5NO2)2] 3 20.5H2O (2). Polyoxometalates (POMs) of the early transition metals (V, Mo, W, Nb, and Ta) are a large and expanding subset of polynuclear metal complexes.1 They display an extraordinarily diverse array of structural topologies, with their formation and decomposition driven by series of complex condensation and hydrolysis processes.2 It is however the complexity of these processes that enables the isolation of some very interesting structural architectures, which have potential applications in areas such as materials chemistry, photochemistry, catalysis, molecular magnetism, and medicine.3 Some of the most structurally complex POMs reported to date incorporate lanthanoid metal centers.4 This is due to several key properties of the lanthanoid metals that facilitate binding of lanthanoid centers to the vacant sites of lacunary POMs. These include their highly oxophilic nature and ability to display high coordination numbers and diverse coordination geometries. However, relatively few lanthanoid-containing POMs have been crystallographically characterized compared to those containing 3d metals. Similarly, there are even fewer examples of one-, two-, and three-dimensional network complexes that contain POM-ligated lanthanoid centers.5 The difficulty in growing single crystals of these materials is predominantly due their insolubility and thus rapid precipitation upon formation. This is in contrast to the slowness of the aggregation of the highly anionic POM units that is required to form these polymeric structures. The reported preparation of the polyanions [As6W65O217(H2O)7]26- and [As4W42O143(H2O)4]22- from [As2W19O67(H2O)]14- suggests the potential utility of this dilacunary polyoxotungstate as a valuable precursor for higher nuclearity POMs or POM-based network complexes.6 Although its nucleophilic properties toward 3d or 4f metals have been little explored, the presence of As(III) heteroatoms within the {B-R-AsIIIW9O33} capping units implies considerable promise for [As2W19O67(H2O)]14- as a POM ligand precursor. These As(III) atoms introduce a stereodirecting lone pair which affects the further condensation of the polyanion, critically preventing the formation of plenary structures including Keggin and Dawson complexes. Conversely, the lone pair induces the formation of more open architectures through the inclusion of bridging electrophiles such as lanthanoid metal centers.7-9 In addition, the presence of the bridging {WO(H2O)} linker in the precursor provides a fracture point where the molecule can be disassembled (Figure S1). This yields the {B-R-AsIIIW9O33} species and releases an additional tungstate fragment that can further aggregate with free tungstate units, or react with other ligands. *To whom correspondence should be addressed. E-mail: c.boskovic@ unimelb.edu.au. pubs.acs.org/crystal

Published on Web 01/08/2010

The compounds H3K7Na6[Nd3As4W41O141OH(H2O)10] 3 60H2O (1) and KNa[Dy4As2W22O76(H2O)19(C2H5NO2)2] 3 20.5H2O (2) were both synthesized using benchtop reaction conditions in acidic (pH < 2) aqueous media in the presence of 1 M NaCl.10 A feature of both syntheses is the heating of the reaction mixture to 90 °C. The heating and acidic conditions appear to both be essential for the conversion of the [As2W19O67(H2O)]14- precursor into the components necessary for the formation and crystallization of 1 and 2.11 A large excess of sodium cations is also required to obtain 1 and 2. This may be due to the ability of the potassium cations from the precursor compound to stabilize [As2W19O67(H2O)]14- by binding into the vacancy, whereas [As2W19O67(H2O)]14- is apparently destabilized in the presence of an excess of smaller sodium cations. It is noteworthy that the crystallization of both compounds is very slow, occurring over a number of weeks. A systematic survey of the reaction of [As2W19O67(H2O)]14- with salts of various lanthanoid metals under similar conditions afforded crystalline samples only for compounds 1 and 2. This selectivity may be based on the ability of the POM framework to only accommodate lanthanoid ions with particular ionic radii. Compound 2 was obtained following the addition of glycine to the reaction solution, with a Dy/glycine ratio of 1:1. The investigation of the possibility of incorporating glycine into these systems was inspired by the focus in recent years on the derivatization of POMs with organic ligands.12 Such species are of interest due to the ability of the organic component to induce supramolecular interactions,13 chirality,14 and aid the assembly of more complex architectures.15 The polyanion in compound 1 is constructed from four main polyoxotungstate subunits with three {B-R-AsIIIW9O33} and one {B-β-AsIIIW8O29(OH)} fragments constituting the more recognizable POM units (Figure 1). Other mono- and dinuclear tungstate components, along with the three Nd(III) centers, complete the [Nd3As4W41O141(OH)(H2O)10]16- assembly. Within the tetrameric structure is an {AsIII2W19O68} subunit that is formed by the loss of the water ligand on the {WO(H2O)} bridging unit in the [As2W19O67(H2O)]14- precursor. This rearrangement highlights a site of structural versatility within an otherwise rigid cluster, with the formation of new species depending on subtle condensation processes at this site. Unusually, the repeating tetrameric polyoxotungstate species found in 1 is composed of different polyanion building blocks, a circumstance that occasionally results in asymmetric POM assemblies.16 In this instance, the asymmetric arrangement of the repeating POM unit results in C1 point symmetry, rendering each unit chiral. The material is however achiral and crystallizes in the centrosymmetric space group P1. A number of novel features are evident in 1, including the first observation of the {B-β-AsIIIW8O29(OH)} polyoxotungstate subunit, which has a structure similar to the r 2010 American Chemical Society

Communication

Figure 1. Graphical representation of the repeating unit in 1. {B-RAsW9O33} = yellow, {B-β-AsW8O29(OH)} = brown, W = green spheres, Nd = violet spheres, As = pale blue spheres, O = red spheres.

Figure 2. Graphical representation of the chain structures in 1 (top), 2 (bottom). Color scheme as in Figure 1. WO6 = green polyhedra.

other previously reported POM fragments {AsVW8O31},17 {SiIVW8O30(OH)},18 and {GeIVW8O30(OH)}.19 However, it is noteworthy that there have been no reports to date of any lanthanoid-containing complexes with any of these structurally related polyanion subunits. The {B-β-AsIIIW8O29(OH)} anion is likely formed through initial isomerization of {B-R-AsIIIW9O33} to the known {B-β-AsIIIW9O33} followed by cleavage of a single tungstate unit. The protonated site is characterized as a terminal hydroxo ligand on the basis of bond valence sum calculations,20 with a W-O bond length of 1.88(1) A˚. At the core of the chain structure in 1 are the Nd metal centers which act as both templates for the polyanion assembly and linkers to form the chains (Figure 2). Within 1, there are three crystallographically unique Nd atoms, each displaying square antiprismatic coordination geometry. Nd1 is located at the center of each tetrameric polyanion, with six of its eight bonds to oxo ligands of the additional tungstate units not belonging to the {AsW9} or {AsW8} subunits. The Nd-O bond lengths fall in the range 2.36(2)-2.63(2) A˚. The remaining two coordination sites are filled by a terminal oxo ligand of the {AsW8} unit, and

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Figure 3. Graphical representation of the repeating unit in 2. Color scheme as in Figure 1. Na=cream spheres, C=black spheres, N= dark blue spheres.

a terminal water ligand with bond lengths 2.42(2) and 2.63(2) A˚, respectively. The other Nd centers are involved in connecting the polyanions into chains, with four W-O-Nd linkages existing between neighboring clusters. The shared oxygen atoms are bound to the Nd centers through bonds of 2.40(2) and 2.43(2) A˚ and to the W centers through bonds of 1.77(2) and 1.76(2) A˚. As is evident in 1, the polyanion in compound 2 is a polymeric 1D chain of lanthanoid-containing POM repeat units (Figure 3). However, 2 is an uncommon example of a POM-based material that incorporates an amino acid ligand.21 It displays Cs point symmetry. The structure of the [Dy4As2W22O76(H2O)19(C2H5NO2)2]2- polyanion in 2 differs from the Nd-containing chain in 1, with the POM component assembled from two {B-RAsIIIW9O33} clusters, that are then connected via two unprecedented {W2O5(gly)} linking units and templated by a single Naþ cation. The glycine molecules bridge the W centers in a bidentate fashion with W-O bond lengths of 2.25(1) A˚. Because of the presence of the glycine molecules, the two {B-R-AsIIIW9O33} units are crystallographically unique with hydrogen bonding donation from the protonated amine to only one {B-R-AsIIIW9O33} unit. One hydrogen bond is observed between a W-bound terminal oxo ligand, with a N to O distance of 2.99(2) A˚. Furthermore, two additional hydrogen bonds are located between the protonated amine and a neighboring polyanion unit with N-O distances of 3.07(2) and 2.98(2) A˚. Hydrogen atoms were not located in the difference map due to the presence of the multiple heavy atoms. Overall, the sodium templated polytungstate arrangement provides an open structure that accommodates four Dy(III) centers. Three of the four Dy atoms are crystallographically unique with Dy1 and Dy3 displaying square antiprismatic coordination geometries, while Dy2 has dodecahedral coordination geometry. Both Dy1 and Dy3 coordinate to the polyanion surface through four terminal oxo ligands belonging to the polyanion; meanwhile, the two equivalent Dy2 centers form only two DyO-W linkages. The Dy-O(W) bond lengths are in the range 2.25(1)-2.47(1) A˚ and average 2.36 A˚. The remaining coordination sites on each of the Dy centers are occupied by terminal water ligands with an average Dy-(OH2) bond length of 2.38 A˚, except for one additional Dy-O-W connection which is responsible for the chain structure of 2. Each repeating POM unit in 2 is connected into chains through two crystallographically equivalent Dy-O-W linkages which occur through coordination of Dy3 to a terminal oxo ligand in the {W3} cap of the {B-RAsIIIW9O33} unit. The Dy-O and W-O bond lengths are 2.41(2) and 1.76(2) A˚, respectively. As is common for POMbased materials, there are extensive intermolecular interactions

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present in 2, which likely aid the stabilization of the chains and the crystallization process. Multiple intrachain hydrogen bonds are present between the terminal water ligands of the Dy centers and the terminal oxo ligands of the polytungstate. Furthermore, several interchain hydrogen bonds exist, such as those between the protonated amine of the bound glycine and oxo ligands belonging to the polytungstate portions of neighboring chains. In conclusion, two novel chain complexes have been synthesized from the precursor [As2W19O67(H2O)]14- and crystallographically and chemically analyzed. The polyanion precursor has been demonstrated to be highly adaptable and a useful source of {B-R-AsIIIW9O33} and the unprecedented {B-β-AsIIIW8O29(OH)} POM subunits. These recombine in the presence of lanthanoid ions (and glycine), under carefully controlled conditions, to form compounds 1 and 2. A systematic investigation is presently underway into the potential of [As2W19O67(H2O)]14for the formation of other novel POM compounds. This includes monitoring the self-assembly processes in solution using techniques such as mass spectrometry and NMR spectroscopy. The results of this study will be reported in due course. Acknowledgment. We thank the Australian Research Council for funding. Supporting Information Available: Crystallographic information in CIF format and a schematic representation of the disassembly of [As2W19O67(H2O)]14- into the structural subunits found in 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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98.5%), HNO3, Dy(NO3)3 3 H2O (Aldrich 99.9%), Nd(NO3)3 3 6H2O (Aldrich, 99.9%). The precursor K14[As2W19O67(H2O)] was prepared as described previously.6a Compound 1: solid K14[As2W19O67(H2O)] (1.00 g, 0.189 mmol) was dissolved in an aqueous solution (20 mL) containing 1 M NaCl, followed by the addition of solid Nd(NO3)3 3 5H2O (0.166 g, 0.450 mmol). The resulting precipitate dissolved on stirring. The solution was then acidified to pH 2 using 12 M HCl, and then to pH 1.7 using 2 M HCl, before being heated to 90 °C for 10 min, followed by cooling to room temperature. Lavender-colored needle-shaped crystals of the desired product crystallized over the course of 8-12 weeks. Yield: 0.135 g (13% based on W). Anal. calcd for 1, H3K7Na6[Nd3As4W41O141OH(H2O)10] 3 60H2O, H144As4K7Na6Nd3O212W41: As, 2.45; K, 2.24; Na, 1.13; W, 61.69. Found: As, 1.90; K, 2.30; Na, 1.09; W, 59.30. Selected IR (KBr, cm-1): 3423 (br), 1617(m), 959 (s), 855(s), 698 (s), 631(s), 478(s). Compound 2: A solution of Dy(NO3)3 3 H2O (0.622 g, 0.090 mmol) in water was added to an aqueous solution (15 mL) containing 1 M NaCl, followed by the addition of glycine (0.9 mL, 1 M). The pH of the solution was then lowered to 1.6 using 3 M HCl solution. Solid K14[As2W19O67(H2O)] (0.396 g, 0.0750 mmol) was then added and the pH of the resulting solution was increased to 1.95. This afforded a precipitate, which was then collected and dissolved in 3 M HCl solution, giving rise to a solution with a pH of 1.55. This solution was heated to 90 °C for 10 min then cooled to room temperature. Colorless rodshaped crystals of 2 then formed in 6 weeks. Yield: 0.075 g (17% based on W). Anal. Calcd for 2, KNa[Dy4As2W22O76(H2O)19(C2H5NO2)2] 3 20.5H2O, C8H178As4Dy8K2N4Na2O239W44: C, 0.69; H, 1.28; N, 0.40; As, 2.15; K, 0.56; W 57.91. Found: C, 1.04; H, 1.24; N, 0.47; As, 1.90; K, 0.40 W, 53.40. Selected IR (KBr, cm-1): 3426 (br), 2921 (wk), 2360(wk), 1619(m), 1476(wk), 1414(wk), 1336(wk), 956 (s), 872(s), 799 (s), 699(s), 492(s).The low yields for both compounds 1 and 2 is due to the collection of pure crystalline samples prior to the deposition of poorly defined solids as the solvent evaporated to small volume. Single crystal X-ray data for compound 1: Mr =12218.49, triclinic, P1, a = 20.454(3), b = 21.893(4), c = 23.565(4), R = 87.502, β = 81.844(3), γ = 70.296(3), V = 9834(3), Z = 2, R1 = 0.0974, wR2 = 0.2141, GOF=1.110. The crystallographic data for 1 was collected on a Bruker Apex I Diffractometer at 130 K using graphite monochromatic Mo-KR radiation (0.71073 A˚). Some of the K and Na cations were not located due to disorder within the solvent filled voids between independent chains in 1. A closest fit formula was achieved through the combined interpretation of the X-ray and elemental data. Single crystal X-ray data for compound 2: Mr = 13968.21, orthorhombic, Pnma, a=22.1515(2), b=19.8626(2), c= 24.6483(2), V=10844.92, Z=2, R1=0.0403, wR2=0.1051, GOF= 1.037. The crystallographic data for 2 was collected at 130 K on a Gemini Oxford Diffractometer using graphite-monochromated Cu-KR radiation (1.5418 A˚). Analytical numerical absorption corrections were carried out using a multifaceted crystal model and the ABSPACK routine within the CrysAlis software package.22 The structures were solved by direct methods and refined by full-matrix least-squares method on F2 using the SHELXS-9723 and SHELXL-9724 crystallographic packages via WinGX.25 (a) Peng, Z. Angew. Chem., Int. Ed. 2004, 43, 930. (b) Hussain, F.; Ritchie, C.; Gable, R. W.; Moubaraki, B.; Murray, K. S.; Boskovic, C. Polyhedron 2009, 28, 2070. (a) Zhu, Y.; Xiao, Z.; Ge, N.; Wang, N.; Wei, Y.; Wang, Y. Cryst. Growth Des. 2006, 6, 1620. (b) Streb, C.; McGlone, T.; Br€ucher, O.; Long, D.-L.; Cronin, L. Chem.;Eur. J. 2008, 14, 8861. (c) Fang, X.; K€ogerler, P.; Isaacs, L.; Uchida, S.; Mizuno, N. J. Am. Chem. Soc. 2009, 131, 432. (d) Li, Y.; Hao, N.; Wang, E.-B.; Yuan, M.; Hu, C.; Hu, N.; Jia, H. Inorg. Chem. 2003, 42, 2729. (a) Hasenknopf, B.; Micoine, K.; Lac^ ote, E.; Thorimbert, S.; Malacria, M.; Thouvenot, R. Eur. J. Inorg. Chem. 2008, 2008, 5001. (b) Qin, C.; Wang, X.-L.; Yuan, L.; Wang, E.-B. Cryst. Growth Des. 2008, 8, 2093. (c) Fang, X.; Anderson, T. M.; Hill, C. L. Angew. Chem., Int. Ed. 2005, 44, 3540. (a) Mialane, P.; Dolbecq, A.; Secheresse, F. Chem. Commun. 2006, 3477. (b) Li, J.; Huth, I.; Chamoreau, L.; Hasenknopf, B.; Lacote, E.; Thorimbert, S.; Malacria, M. Angew. Chem., Int. Ed. 2009, 48, 2035. (c) Song, Y.-F.; Abbas, H.; Ritchie, C.; McMillian, N.; Long, D.-L.; Gadegaard, N.; Cronin, L. J. Mater. Chem. 2007, 17, 1903. (a) Mitchell, S. G.; Ritchie, C.; Long, D-L; Cronin, L. Dalton Trans. 2008, 1415. (b) Bi, L.; Hou, G.; Wu, L.; Kortz, U. CrystEngComm. 2009, 11, 1532. Wang, J.; Wang, W.; Niu, J. Inorg. Chem. Commun. 2007, 10, 1054.

Communication (18) Bassil, B. S.; Kortz, U.; Tigan, A. S.; Clemente-Juan, J. M.; Keita, B; Oliveira, P.; Nadjo, L. Inorg. Chem. 2005, 44, 9360. (19) Nsouli, N. H.; Ismail, A. H.; Helgadottir, I. S.; Dickman, M. H.; Clemente-Juan, J. M.; Kortz, U. Inorg. Chem. 2009, 5884. (20) Hormillosa, C.; Healy, S.; Tamon, S.; and Brown, I. D. Bond Valence Calculator version 2.00; Institute for Materials Research, McMaster University: Hamilton, Ontario, 1993. (21) (a) Chen, W.; Li, Y.; Wang, Y.; Wang, E-B; Su, Z. Dalton Trans. 2007, 38, 4293. (b) Kortz, U; Savelieff, M. G.; Abou Ghali,

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F. Y.; Khalil, L. M.; Maalouf, S. A. Angew. Chem., Int. Ed. 2002, 41, 4070. (a) CrysAlis RED, Version 1.171.32.15; Oxford Diffraction Ltd.: Oxfordshire, U.K., 2008. (b) Clark, R. C. Reid; J. S. Acta Crystallogr, Sect. A, 1995, 51, 887. Sheldrick, G. M. Acta Crystallogr., Sect. A 1998, 46, 467. Sheldrick, G. M. SHELXL-97. Program for Crystal Structure Analysis; University of G€ottingen: Germany, 1997. Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.