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Self-assembly of trigonal prismatic M6(μ–L)9 coordination cages
Inorganic Chemistry Communications 15 (2012) 126–129
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Self-assembly of trigonal prismatic M6(μ–L)9 coordination cages Adel M. Najar, Ceren Avci, Michael D. Ward ⁎ Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK
a r t i c l e
i n f o
Article history: Received 25 July 2011 Accepted 5 October 2011 Available online 13 October 2011 Keywords: Self-assembly Coordination cage Crystal structure Zinc Copper Cadmium
a b s t r a c t The new ligand bis-bidentate ligand L, containing two pyrazolyl-pyridine chelating units connected to a 1,8anthracene-diyl core via methylene spacers, reacts with Zn(II), Cd(II) and Cu(II) salts to form trigonal prismatic coordination cages [M6(μ–L)9] 12+ in which a metal ion occupies each vertex and a bridging ligand spans each edge; the structure is stabilised by anions which occupy the central cavity and the gaps in the centres of the triangular faces, and also by extensive inter-ligand aromatic stacking between anthracenyl and pyrazolyl-pyridine groups. © 2011 Elsevier B.V. All rights reserved.
The study by many groups of polyhedral coordination cages is a fascinating aspect of supramolecular chemistry [1]. These cage complexes generally contain an array of metal ions in a regular polyhedral shape, connected by bridging ligands that span edges (connecting two metal ions), faces (connecting three or more metal ions), or both. Correct understanding and control of the geometric principles underlying the assembly can result in the ability to assembly very large cages with a high degree of confidence, as exemplified by rational syntheses of cages varying from M4L6 tetrahedra [2] to M24L48 cages which contain seventy-two components in a single assembly [3]. Apart from the fascination with their regular structures, cages of all sizes are characterised by hollow cavities in their centres which can form the basis of host–guest chemistry and the occurrence of unusual forms of chemical reactivity in confined environments [1-3]. Our contribution to this field has consisted of the preparation and characterisation of a family of cages based on deceptively simple ligands which contain two or three pyrazolyl-pyridine chelating termini connected to a central aromatic spacer by flexible methylene ‘hinges’ [1]. Two features of the ligands have turned out to be essential: (i) the flexibility of the ligands imparted by the methylene groups, whilst it precludes rational design of cages, facilitates their formation by allowing the ligand to adopt whatever conformation is most conducive to cage formation; and (ii) the central electron-rich aromatic units become involved in inter-ligand aromatic π-stacking interactions with the electron-deficient pyrazolyl-pyridine units around the periphery of the cages, which seems to play an important role in ensuring the stability of the cages in solution. The resulting family of cages is
⁎ Corresponding author. Fax: + 44 114 2229346. E-mail address: m.d.ward@sheffield.ac.uk (M.D. Ward). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.10.007
extensive and ranges in size from M4L6 tetrahedra [4] up to M16L24 tetra-capped truncated tetrahedra [5], including both Platonic and Archimidean solids. Some noteworthy examples include a mixedligand cuboctahedral cage based on a combination of edge-bridging and face-capping ligands which self-select from a mixture [6]; and a M8L12 ‘cuneane’ which is an unusual topological isomer of a cube [7]. A tempting avenue of exploration at the moment is to incorporate fluorescent aromatic groups into bridging ligands such that the central cavity is surrounded by an array of fluorophores. This opens up the possibility of the host cage also acting as an antenna group and participating in photoinduced electron- or energy-transfer to the guest. Two series of recently-described cages have incorporated naphthyl groups in the superstructure and show fluorescence from the ligands which is modified when the cage assembles [8]; we have also prepared tetrahedral M4L6 and cubic M8L12 cages based on anthracene-containing ligands [4,8]. Here, we describe a new ligand based on a 1,8-disubstituted anthracenyl core with two pendant pyrazolyl-pyridine units, which forms unusual trigonal prismatic M6L9 cages on assembly with a range of transition metal dications. The new ligand L was prepared by reaction of 3-(2-pyridyl)pyrazole with 1,8-bis(bromomethyl)anthracene [9-11] (Scheme 1). This in turn was prepared in a multi-step procedure starting from commercial 1,8dichloro-9,10-anthraquinone using literature methods [10]. The crystal structure of L is shown in Fig. 1[12-14]; individual bond distances and angles are unremarkable. The fluorescence spectrum of L in CH2Cl2 shows the usual anthracene-based emission profile (Fig. 2). Reaction of L with Zn(BF4)2, Cd(BF4)2 or Cu(ClO4)2 in a 2:3 metal: ligand ratio, either under solvothermal conditions [15] or simply by stirring in MeOH at reflux and cooling [16], afforded solid products from which X-ray quality crystals could be grown by diffusion of diethyl ether vapour into nitromethane or MeCN solutions of the
A.M. Najar et al. / Inorganic Chemistry Communications 15 (2012) 126–129
127
Scheme 1. Synthesis of the new ligand L.
crude materials. In all cases the crystals scattered weakly due to a combination of disorder of anions / solvent molecules, and rapid solvent loss which compromised the crystal quality. Consequently refinements are relatively poor although the gross structures and connectivities of the complex cations are clear. We use as an illustration the structure of [Zn6L9](BF4)8(SiF6)2•6MeCN for which the R1 value is 13.3% [17]. The structure is that of a hexanuclear trigonal prismatic cage, with a bridging ligand at each vertex and a bridging ligand spanning every edge (Figs. 3, 4). Zn–N distances are unremarkable. The 2:3 metal:ligand ratio arises from the fact that each metal ion is a tris-chelate which requires six donor atoms, and each ligand can provide only four donors, so coordinative saturation requires 1.5 ligands per metal cation. As in other polyhedral cages of this family this stoichiometric condition can be met by cages which have a 2:3 ratio of vertices (metal ions) to bridging ligands (edges) and this limitation defines the range of polyhedral structures that can form [1]. Thus an octahedron—a sterically lower energy array of six metal ions than a trigonal prism – is not possible here as it would require twelve bridging ligands acting as edges, with each metal ion required to be 8coordinate. The arrangement of ligands is such that the two triangular faces of the prism have circular helical M3L3 structures with threefold
symmetry due to the C3 axis running through the centres of the triangular faces. These two cyclic helical subunits are enantiomeric, although they are not crystallographically related by an inversion centre such that the structure as a whole is still chiral. The two M3L3 faces are then linked by three vertical ‘pillar’ ligands, each of which connects a Zn(1) and a Zn(2) centre and which are all therefore crystallographically equivalent. We have noted before how many of the polyhedral cages in this family can be considered to be based on highly conserved M3L3 cyclic helicates that form triangular faces and are connected in a range of different ways [5]. Connecting two such faces in an approximately eclipsed manner with three bridging ligands is the simplest way in which this can be achieved. The separation between the Zn(1) centres in one face is 10.50 Å; and the separation between Zn(2) ions in the other face is 10.14 Å. The Zn(1)•••Zn(2) separation along the edges of the trigonal prism between the triangular faces is 10.48 Å. The structure is characterised by regions of aromatic π-stacking between ligands around the periphery (Fig. 5). These stacks are invariably based on alternating sequences of relatively electrondeficient aromatic units (pyrazolyl-pyridine ligands coordinated to M 2+ cations) and electron-rich units (anthracenyl fragments) [1]. Within each triangular helical face, for example, every anthracenyl group is sandwiched between two coordinated pyrazolyl-pyridine
Fig. 1. Molecular structure of L taken from crystallographic data (thermal ellipsoids shown at the 40% probability level).
Fig. 2. Fluorescence spectrum of L in CH2Cl2 (room temperature).
Fig. 3. Two partial views of the complex cation of [Zn6L9](BF4)8(SiF6)2•6MeCN. (a) The trigonal prismatic core of Zn(II) ions with three of the bridging ligands shown; the triangular faces which form M3L3 cyclic helicates are coloured in yellow, and the central tetrafluoroborate anion is also shown. (b) An alternative view looking down the crystallographic C3 axis showing the three ‘pillar’ ligands which connect the two triangular faces. The offset between the two triangular units is also clear.
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A.M. Najar et al. / Inorganic Chemistry Communications 15 (2012) 126–129
Fig. 4. A view of one of the triangular M3(μ–L3) faces of [Zn6L9](BF4)8(SiF6)2•6MeCN; the three ligands around the cyclic helical face are shown completely; only the coordinated pyrazolyl-pyridine units of the ‘pillar’ ligands are shown. The π-stacking in which each anthracenyl unit is sandwiched between two coordinated pyrazolyl–pyridine units is clear, and the hexafluorosilicate anion in the centre of the triangular face is also shown.
fragments; and for the three Zn(1) ions around one face, the anthracenyl units of the ‘pillar’ ligands also participate giving a fourcomponent stack. Around the other face of the trigonal prism, composed of Zn(2) ions, only three-component stacks are present within the cyclic helical array. This gives a total of 15 pairwise donor/acceptor stacking interactions between ligands within each cage unit in the crystal structure. The additional enthalpic stabilisation provided by donor/acceptor stacks such as this is likely to play a significant role in stabilising these large assemblies whose formation is clearly unfavourable based on entropy considerations. In addition there are anions closely associated with the cage: a [BF4] − ion in the centre of the cavity lying on a threefold axis, and [SiF6] 2− anions (arising from reaction of fluoride ions in the tetrafluoroborate anions with the glass vials, a quite common phenomenon [5,18]) lying the in the centres of the triangular helical faces, again on threefold axis. Thus the 12+ charge on the cage is partly offset by the 5-charge of anions that are closely associated with the cage. These anions are involved in CH•••F H-bonding interactions with the cage superstructure, such as H(21A)•••F(43) (2.28 Å) involving one the hexafluorosilicate anions and H(315)•••F(13) (2.40 Å) involving the central tetrafluoroborate anion. Trigonal prismatic cage structures are relatively rare and we have only observed one previous example [5]. This is in part because a trigonal prismatic arrangement of bulky groups is inherently less stable than an octahedral arrangement because the two triangular faces are eclipsed rather than staggered, increasing steric repulsions between
Fig. 5. A view of the same M3 face [based on Zn(1)] as in Fig. 4, showing the fourcomponent π-stack sequence which is replicated three times around the face. The anthracenyl units of the ‘pillar’ ligands are aligned to participate in this stacking (right-hand component) and therefore do not participate in the stacking around the Zn(2) faced which consequently only contains three-component stacks.
them. The few examples that are known are often based on rigid triangular ligands which provide the top and bottom faces of the assembly and thereby impose threefold symmetry (cf. examples from Stang [19], Kaim [20], and Mukherjee [21]). The closest related example to ours – with a metal ion at each of the six vertices and a bridging ligand along each of the nine edges – appears to be [{Mo(CO)3}6(μCN)9] 9− described by Rauchfuss and co-workers [22]. The complexes with Cd(II) and Cu(II) are basically identical. In both cases however the quality of the refinement is poorer (R1 > 0.2) so no detailed analysis of the structural parameters is presented here. We just note that the gross structures are the same in all important respects as that of the Zn(II) complex discussed above, with the same arrangement of metal ions and ligands, the same aromatic stacking between ligand components, and the same presence of anions in the cavity centre and the centres of the triangular faces (cf. Figs. 3–5). Unusually for cages of this series [1] the complexes do not appear to be stable in solution. Electrospray mass spectrometry on MeCN solutions do not show peaks characteristic of intact cages but only show peaks at low m/z value characteristic of mononuclear fragments; 1H NMR spectra of the Zn(II) and Cd(II) complexes show a poorlyresolved overlapping set of a large number of peaks in the aromatic region indicative of fragmentation of the cage in solution. Thus, the luminescence spectra of dissolved complexes are qualitatively the same as the luminescence spectrum of the free ligand and solutionbased host–guest chemistry is not possible. However, the possibly to encapsulate different anionic guests in the solid state is possible and will be the subject of future studies. We thank the University of Garyounis in Libya for a PhD studentship to Mr. Adel Najar, and Mr. Harry Adams for assistance with the crystallography. Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10. 1016/j.inoche.2011.10.007. References [1] (a) D. Fiedler, D.H. Leung, R.G. Bergman, K.N. Raymond, Accounts of Chemical Research 38 (2005) 349; (b) M. Fujita, M. Tominaga, A. Hori, B. Therrien, Accounts of Chemical Research 38 (2005) 369; (c) S.R. Seidel, P.J. Stang, Accounts of Chemical Research 35 (2002) 972; (d) M.D. Ward, Chemical Communications (2009) 4487; (e) J.J. Perry, J.A. Perman, M.J. Zaworotko, Chemical Society Reviews 38 (2009) 1400; (f) S. Alvarez, Dalton Transactions (2006) 2209; (g) B. Breiner, J.K. Clegg, J.R. Nitschke, Chemical Science 2 (2011) 51. [2] (a) R.W. Saalfrank, R. Burak, A. Breit, D. Stalke, R. Herbst-Irmer, J. Daub, M. Porsch, E. Bill, M. Muther, A.X. Trautwein, Angewandte Chemie (International Ed. in English) 33 (1994) 1621; (b) J.K. Clegg, L.F. Lindoy, B. Moubaraki, K.S. Murray, J.C. McMurtrie, Dalton Transactions (2004) 2417; (c) A.V. Davis, D. Fiedler, M. Ziegler, A. Terpin, K.N. Raymond, Journal of the American Chemical Society 129 (2007) 15354; (d) P. Mal, B. Breiner, K. Rissanen, J.R. Nitschke, Science 324 (2009) 1697; (e) C.J. Brown, R.G. Bergman, K.N. Raymond, Journal of the American Chemical Society 131 (2009) 17530. [3] Q.-F. Sun, J. Iwasa, D. Ogawa, Y. Ishida, S. Sato, T. Ozeki, Y. Sei, K. Yamaguchi, M. Fujita, Science 328 (2010) 1144. [4] (a) J.S. Fleming, K.L.V. Mann, C.-A. Carraz, E. Psillakis, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Angewandte Chemie International Edition 37 (1998) 1279; (b) R.L. Paul, Z.R. Bell, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Proceedings of the National Academy of Sciences of the United States of America 99 (2002) 4883; (c) I.S. Tidmarsh, B.F. Taylor, M.J. Hardie, L. Russo, W. Clegg, M.D. Ward, New Journal of Chemistry 33 (2009) 366. [5] (a) S.P. Argent, H. Adams, T. Riis-Johannessen, J.C. Jeffery, L.P. Harding, M.D. Ward, Journal of the American Chemical Society 128 (2006) 72; (b) A. Stephenson, S.P. Argent, T. Riis-Johannessen, I.S. Tidmarsh, M.D. Ward, Journal of the American Chemical Society 133 (2011) 858. [6] N.K. Al-Rasbi, I. Tidmarsh, S.P. Argent, H. Adams, L.P. Harding, M.D. Ward, Journal of the American Chemical Society 130 (2008) 11641.
A.M. Najar et al. / Inorganic Chemistry Communications 15 (2012) 126–129 [7] A. Stephenson, M.D. Ward, Dalton Transactions 40 (2011) 7824. [8] (a) I.S. Tidmarsh, T.B. Faust, H. Adams, L.P. Harding, L. Russo, W. Clegg, M.D. Ward, Journal of the American Chemical Society 130 (2008) 15167; (b) N.K. Al-Rasbi, C. Sabatini, F. Barigelletti, M.D. Ward, Dalton Transactions (2006) 4769. [9] A mixture of 1,8-bis(bromomethyl)anthracene [10] (0.5 g, 1.37 mmol), 3-(2-pyridyl)-pyrazole [11] (0.4 g, 2.70 mmol), aqueous NaOH (5.5 M, 5 cm3), and tetrahydrofuran (60 cm3) was stirred at reflux for 21 h. After cooling, the organic phase was separated, dried over MgSO4, filtered, and evaporated to dryness. The crude product was dissolved in dichloromethane; addition of diethylether precipitated a white powder which was filtered and dried to yield pure L (0.40 g, 56 %). Anal. calcd for C32H24N6•0.5H2O: C , 76.6; H, 5.0; N, 16.8 %. Found: C , 76.6; H, 4.8; N, 16.8 %. ESMS: m/z 493 (M + H)+, 247 (M + 2H)2+. 1H NMR (400 MHz, CDCl3): δ 8.93 (1H, s, anthryl H9 or H10); 8.62 (2H, d, pyridyl H6); 8.48 (1H, s, anthryl H10 or H9); 8.03 (2H, d, pyridyl H3); 7.99 (2H, d, anthryl H2/H7 or H4/H5); 7.67 (2H, td, pyridyl H4); 7.43 (2H, dd, anthryl H3/6); 7.40 (2H, d, pyrazolyl H5); 7.27 (2H, d, anthryl H4/H5 or H2/H7); 7.17 (2H, ddd, pyridyl H5); 6.89 (2H, d, pyrazolyl H4); 5.93 (4H, s, CH2 [10] (a) J. Re, X.-L. Zhao, Q.-C. Wang, C.-F. Ku, D.-H. Qu, C.-P. Chang, H. Tian, Dyes and Pigments 64 (2005) 179; (b) M. Rogers, B.A. Averill, The Journal of Organic Chemistry 51 (1986) 3308. [11] (a) A.J. Amoroso, A.M.W. Cargill Thompson, J.C. Jeffery, P.L. Jones, J.A. McCleverty, M.D. Ward, Journal of the Chemical Society, Chemical Communications (1994) 2751; (b) H. Brunner, T. Scheck, Chemische Berichte 125 (1992) 701; (c) Y. Lin, S.A. Lang, Journal of Heterocyclic Chemistry 14 (1977) 345. [12] Crystal data for L (C32H24N6): monoclinic, space group P21/c, Mr = 492.57, a = 5.4905(2), b = 14.2024(4), c = 31.5698(9) Å, β = 94.904(2)˚, V = 2452.75 (13) Å3, Z = 4, Dc = 1.334 g cm-3, μ(Mo-Kα) = 0.082 mm-1. A crystal of size 0.50 x 0.20 x 0.19 mm3 was mounted on a Brujer APEX-2 diffractometer under a stream of cold N2 and intensity data were collected at 100 K. 22417 reflections were collected with 2θmax = 55˚ which after merging afforded 5575 independent data with Rint = 0.0415. Refinement of 343 parameters converged at R1 [selected data with I > 2σ(I)] = 4.94%; wR2 (all data) = 16.6%. Software used for structure solution and refinement was SHELXS-97 and SHELX-97 [13]; the absorption correction was applied with SADABS [14]. [13] G.M. Sheldrick, Acta Crystallographica Section A: Foundations 64 (2008) 112. [14] G.M. Sheldrick, SADABS: A program for absorption correction with the Siemens SMART system, University of Göttingen, Germany, 1996. [15] Synthesis of [Zn6L9][BF4]12: A Teflon-lined autoclave was charged with Zn(BF4)2• xH2O (0.016 g, 0.07 mmol), L (0.05 g, 0.1 mmol) and methanol (9 cm3). Heating to 100 °C for 12 h followed by slow cooling to room temperature yielded the product as a white powder in 68% yield. X-ray quality crystals were grown by slow diffusion of diethylether into a solution of the complex in CH3NO2. Anal. calcd. for [Zn6L9][BF4]12: C, 58.9; H, 3.7; N, 12.9%. Found: C, 59.2; H, 3.8; N, 12.6%. The Cd(II) analogue was made in an exactly similar way. [16] Synthesis of [Cu6L9][ClO4]12: A mixture of Cu(ClO4)2•6H2O (0.041 g, 0.11 mmol) and L (0.081 g, 0.16 mmol) in MeOH (10 cm3) was heated to reflux with stirring
[17]
[18]
[19]
[20]
[21] [22]
129
for 2 h and then allowed to cool. The solid product was collected by filtration (70% yield) and X-ray quality crystals were grown by slow diffusion of diethylether into a solution of the complex in CH3NO2. Anal. calcd. for [Cu6L9] [ClO4]12•10H2O: C, 55.9; H, 3.8; N, 12.2%. Found: C, 55.6; H, 3.7; N, 12.0%. Crystal data for [Zn6L9](BF4)8(SiF6)2•6MeCN (C300H234B8F44N60Si2Zn6): cubic, space group I23, Mr = 6050.35, a = 40.9005(2), V = 68420.4(6) Å3, Z = 8, Dc = 1.175 g cm-3, μ(Mo-Kα) = 0.501 mm-1. A crystal of size 0.42 x 0.32 x 0.15 mm3 was mounted on a Bruker APEX-2 diffractometer under a stream of cold N2 and intensity data were collected at 123 K. 403173 reflections were collected with 2θmax = 55˚ which after merging afforded 26416 independent data with Rint = 0.065. Refinement of 1059 parameters with 1846 restraints converged at R1 [selected data with I > 2σ(I)] = 13.3%; wR2 (all data) = 37.6%. Software used for structure solution and refinement was SHELXS-97 and SHELX-97 [13]; the absorption correction was applied with SADABS [14]. The complex cation lies on a threefold axis such that one third of it lies in the asymmetric unit: there are 24 asymmetric units in the unit cell, giving eight complete formula units. The central tetrafluoroborate anion, and the two hexafluorosilicate anions in the centres of the triangular faces, all lie on the same threefold axis; another tetrafluoroborate anion is in a general position. Thus for the complete cage (charge 12+) anions totalling a charge of 8– (two hexafluorosilicate and two tetrafluoroborate) were located; the remaining anions, totalling 1.333 tetrafluoroborates per asymmetric unit, could not be located and are assumed to be in regions of diffuse residual electron density that could not be satisfactorily modelled. Geometric restraints were applied to the tetrafluoroborate and hexafluorosilicate anions to keep their geometries reasonable. In order to keep the refinement stable only the atoms associated with the complex cage were refined with anisotropic displacement parameters; extensive use of restraints on these displacement parameters was required to keep the refinement stable and to ensure that adjacent atoms did not have unreasonably different displacement parameters. Atoms associated with anions were left isotropic. (a) J.S. Fleming, K.L.V. Mann, S.M. Couchman, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Journal of the Chemical Society Dalton Transactions (1998) 2047; (b) P.J. van Koningsbruggen, J.G. Haasnoot, R.A.G. de Graaff, J. Reedijk, Journal of the Chemical Society Dalton Transactions (1993) 483. (a) C.J. Kuehl, Y.K. Kryschenko, U. Radhakrishnan, S.R. Seidel, S.D. Huang, P.J. Stang, Proceedings of the National Academy of Sciences 99 (2002) 4932; (b) C.J. Kuehl, T. Yamamoto, S.R. Seidel, P.J. Stang, Organic Letters 4 (2002) 913; (c) Y.-R. Zheng, H.-B. Yang, K. Ghosh, L. Zhao, P.J. Stang, Chemistry A European Journal 15 (2009) 7203. (a) C.-Y. Su, Y.-P. Cai, C.-L. Chen, F. Lissner, B.S. Kang, W. Kaim, Angewandte Chemie International Edition 41 (2002) 3371; (b) C.-Y. Su, Y.-P. Cai, C.-L. Chen, M.D. Smith, W. Kaim, H.-C. zur Loye, Journal of the American Chemical Society 125 (2003) 8595. S. Ghosh, P.S. Mukherjee, Organometallics 27 (2008) 316. S.M. Contakes, T.B. Rauchfuss, Angewandte Chemie International Edition 39 (2000) 1984.
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Self-assembly of trigonal prismatic M6(μ–L)9 coordination cages Adel M. Najar, Ceren Avci, Michael D. Ward ⁎ Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK
a r t i c l e
i n f o
Article history: Received 25 July 2011 Accepted 5 October 2011 Available online 13 October 2011 Keywords: Self-assembly Coordination cage Crystal structure Zinc Copper Cadmium
a b s t r a c t The new ligand bis-bidentate ligand L, containing two pyrazolyl-pyridine chelating units connected to a 1,8anthracene-diyl core via methylene spacers, reacts with Zn(II), Cd(II) and Cu(II) salts to form trigonal prismatic coordination cages [M6(μ–L)9] 12+ in which a metal ion occupies each vertex and a bridging ligand spans each edge; the structure is stabilised by anions which occupy the central cavity and the gaps in the centres of the triangular faces, and also by extensive inter-ligand aromatic stacking between anthracenyl and pyrazolyl-pyridine groups. © 2011 Elsevier B.V. All rights reserved.
The study by many groups of polyhedral coordination cages is a fascinating aspect of supramolecular chemistry [1]. These cage complexes generally contain an array of metal ions in a regular polyhedral shape, connected by bridging ligands that span edges (connecting two metal ions), faces (connecting three or more metal ions), or both. Correct understanding and control of the geometric principles underlying the assembly can result in the ability to assembly very large cages with a high degree of confidence, as exemplified by rational syntheses of cages varying from M4L6 tetrahedra [2] to M24L48 cages which contain seventy-two components in a single assembly [3]. Apart from the fascination with their regular structures, cages of all sizes are characterised by hollow cavities in their centres which can form the basis of host–guest chemistry and the occurrence of unusual forms of chemical reactivity in confined environments [1-3]. Our contribution to this field has consisted of the preparation and characterisation of a family of cages based on deceptively simple ligands which contain two or three pyrazolyl-pyridine chelating termini connected to a central aromatic spacer by flexible methylene ‘hinges’ [1]. Two features of the ligands have turned out to be essential: (i) the flexibility of the ligands imparted by the methylene groups, whilst it precludes rational design of cages, facilitates their formation by allowing the ligand to adopt whatever conformation is most conducive to cage formation; and (ii) the central electron-rich aromatic units become involved in inter-ligand aromatic π-stacking interactions with the electron-deficient pyrazolyl-pyridine units around the periphery of the cages, which seems to play an important role in ensuring the stability of the cages in solution. The resulting family of cages is
⁎ Corresponding author. Fax: + 44 114 2229346. E-mail address: m.d.ward@sheffield.ac.uk (M.D. Ward). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.10.007
extensive and ranges in size from M4L6 tetrahedra [4] up to M16L24 tetra-capped truncated tetrahedra [5], including both Platonic and Archimidean solids. Some noteworthy examples include a mixedligand cuboctahedral cage based on a combination of edge-bridging and face-capping ligands which self-select from a mixture [6]; and a M8L12 ‘cuneane’ which is an unusual topological isomer of a cube [7]. A tempting avenue of exploration at the moment is to incorporate fluorescent aromatic groups into bridging ligands such that the central cavity is surrounded by an array of fluorophores. This opens up the possibility of the host cage also acting as an antenna group and participating in photoinduced electron- or energy-transfer to the guest. Two series of recently-described cages have incorporated naphthyl groups in the superstructure and show fluorescence from the ligands which is modified when the cage assembles [8]; we have also prepared tetrahedral M4L6 and cubic M8L12 cages based on anthracene-containing ligands [4,8]. Here, we describe a new ligand based on a 1,8-disubstituted anthracenyl core with two pendant pyrazolyl-pyridine units, which forms unusual trigonal prismatic M6L9 cages on assembly with a range of transition metal dications. The new ligand L was prepared by reaction of 3-(2-pyridyl)pyrazole with 1,8-bis(bromomethyl)anthracene [9-11] (Scheme 1). This in turn was prepared in a multi-step procedure starting from commercial 1,8dichloro-9,10-anthraquinone using literature methods [10]. The crystal structure of L is shown in Fig. 1[12-14]; individual bond distances and angles are unremarkable. The fluorescence spectrum of L in CH2Cl2 shows the usual anthracene-based emission profile (Fig. 2). Reaction of L with Zn(BF4)2, Cd(BF4)2 or Cu(ClO4)2 in a 2:3 metal: ligand ratio, either under solvothermal conditions [15] or simply by stirring in MeOH at reflux and cooling [16], afforded solid products from which X-ray quality crystals could be grown by diffusion of diethyl ether vapour into nitromethane or MeCN solutions of the
A.M. Najar et al. / Inorganic Chemistry Communications 15 (2012) 126–129
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Scheme 1. Synthesis of the new ligand L.
crude materials. In all cases the crystals scattered weakly due to a combination of disorder of anions / solvent molecules, and rapid solvent loss which compromised the crystal quality. Consequently refinements are relatively poor although the gross structures and connectivities of the complex cations are clear. We use as an illustration the structure of [Zn6L9](BF4)8(SiF6)2•6MeCN for which the R1 value is 13.3% [17]. The structure is that of a hexanuclear trigonal prismatic cage, with a bridging ligand at each vertex and a bridging ligand spanning every edge (Figs. 3, 4). Zn–N distances are unremarkable. The 2:3 metal:ligand ratio arises from the fact that each metal ion is a tris-chelate which requires six donor atoms, and each ligand can provide only four donors, so coordinative saturation requires 1.5 ligands per metal cation. As in other polyhedral cages of this family this stoichiometric condition can be met by cages which have a 2:3 ratio of vertices (metal ions) to bridging ligands (edges) and this limitation defines the range of polyhedral structures that can form [1]. Thus an octahedron—a sterically lower energy array of six metal ions than a trigonal prism – is not possible here as it would require twelve bridging ligands acting as edges, with each metal ion required to be 8coordinate. The arrangement of ligands is such that the two triangular faces of the prism have circular helical M3L3 structures with threefold
symmetry due to the C3 axis running through the centres of the triangular faces. These two cyclic helical subunits are enantiomeric, although they are not crystallographically related by an inversion centre such that the structure as a whole is still chiral. The two M3L3 faces are then linked by three vertical ‘pillar’ ligands, each of which connects a Zn(1) and a Zn(2) centre and which are all therefore crystallographically equivalent. We have noted before how many of the polyhedral cages in this family can be considered to be based on highly conserved M3L3 cyclic helicates that form triangular faces and are connected in a range of different ways [5]. Connecting two such faces in an approximately eclipsed manner with three bridging ligands is the simplest way in which this can be achieved. The separation between the Zn(1) centres in one face is 10.50 Å; and the separation between Zn(2) ions in the other face is 10.14 Å. The Zn(1)•••Zn(2) separation along the edges of the trigonal prism between the triangular faces is 10.48 Å. The structure is characterised by regions of aromatic π-stacking between ligands around the periphery (Fig. 5). These stacks are invariably based on alternating sequences of relatively electrondeficient aromatic units (pyrazolyl-pyridine ligands coordinated to M 2+ cations) and electron-rich units (anthracenyl fragments) [1]. Within each triangular helical face, for example, every anthracenyl group is sandwiched between two coordinated pyrazolyl-pyridine
Fig. 1. Molecular structure of L taken from crystallographic data (thermal ellipsoids shown at the 40% probability level).
Fig. 2. Fluorescence spectrum of L in CH2Cl2 (room temperature).
Fig. 3. Two partial views of the complex cation of [Zn6L9](BF4)8(SiF6)2•6MeCN. (a) The trigonal prismatic core of Zn(II) ions with three of the bridging ligands shown; the triangular faces which form M3L3 cyclic helicates are coloured in yellow, and the central tetrafluoroborate anion is also shown. (b) An alternative view looking down the crystallographic C3 axis showing the three ‘pillar’ ligands which connect the two triangular faces. The offset between the two triangular units is also clear.
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Fig. 4. A view of one of the triangular M3(μ–L3) faces of [Zn6L9](BF4)8(SiF6)2•6MeCN; the three ligands around the cyclic helical face are shown completely; only the coordinated pyrazolyl-pyridine units of the ‘pillar’ ligands are shown. The π-stacking in which each anthracenyl unit is sandwiched between two coordinated pyrazolyl–pyridine units is clear, and the hexafluorosilicate anion in the centre of the triangular face is also shown.
fragments; and for the three Zn(1) ions around one face, the anthracenyl units of the ‘pillar’ ligands also participate giving a fourcomponent stack. Around the other face of the trigonal prism, composed of Zn(2) ions, only three-component stacks are present within the cyclic helical array. This gives a total of 15 pairwise donor/acceptor stacking interactions between ligands within each cage unit in the crystal structure. The additional enthalpic stabilisation provided by donor/acceptor stacks such as this is likely to play a significant role in stabilising these large assemblies whose formation is clearly unfavourable based on entropy considerations. In addition there are anions closely associated with the cage: a [BF4] − ion in the centre of the cavity lying on a threefold axis, and [SiF6] 2− anions (arising from reaction of fluoride ions in the tetrafluoroborate anions with the glass vials, a quite common phenomenon [5,18]) lying the in the centres of the triangular helical faces, again on threefold axis. Thus the 12+ charge on the cage is partly offset by the 5-charge of anions that are closely associated with the cage. These anions are involved in CH•••F H-bonding interactions with the cage superstructure, such as H(21A)•••F(43) (2.28 Å) involving one the hexafluorosilicate anions and H(315)•••F(13) (2.40 Å) involving the central tetrafluoroborate anion. Trigonal prismatic cage structures are relatively rare and we have only observed one previous example [5]. This is in part because a trigonal prismatic arrangement of bulky groups is inherently less stable than an octahedral arrangement because the two triangular faces are eclipsed rather than staggered, increasing steric repulsions between
Fig. 5. A view of the same M3 face [based on Zn(1)] as in Fig. 4, showing the fourcomponent π-stack sequence which is replicated three times around the face. The anthracenyl units of the ‘pillar’ ligands are aligned to participate in this stacking (right-hand component) and therefore do not participate in the stacking around the Zn(2) faced which consequently only contains three-component stacks.
them. The few examples that are known are often based on rigid triangular ligands which provide the top and bottom faces of the assembly and thereby impose threefold symmetry (cf. examples from Stang [19], Kaim [20], and Mukherjee [21]). The closest related example to ours – with a metal ion at each of the six vertices and a bridging ligand along each of the nine edges – appears to be [{Mo(CO)3}6(μCN)9] 9− described by Rauchfuss and co-workers [22]. The complexes with Cd(II) and Cu(II) are basically identical. In both cases however the quality of the refinement is poorer (R1 > 0.2) so no detailed analysis of the structural parameters is presented here. We just note that the gross structures are the same in all important respects as that of the Zn(II) complex discussed above, with the same arrangement of metal ions and ligands, the same aromatic stacking between ligand components, and the same presence of anions in the cavity centre and the centres of the triangular faces (cf. Figs. 3–5). Unusually for cages of this series [1] the complexes do not appear to be stable in solution. Electrospray mass spectrometry on MeCN solutions do not show peaks characteristic of intact cages but only show peaks at low m/z value characteristic of mononuclear fragments; 1H NMR spectra of the Zn(II) and Cd(II) complexes show a poorlyresolved overlapping set of a large number of peaks in the aromatic region indicative of fragmentation of the cage in solution. Thus, the luminescence spectra of dissolved complexes are qualitatively the same as the luminescence spectrum of the free ligand and solutionbased host–guest chemistry is not possible. However, the possibly to encapsulate different anionic guests in the solid state is possible and will be the subject of future studies. We thank the University of Garyounis in Libya for a PhD studentship to Mr. Adel Najar, and Mr. Harry Adams for assistance with the crystallography. Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10. 1016/j.inoche.2011.10.007. References [1] (a) D. Fiedler, D.H. Leung, R.G. Bergman, K.N. Raymond, Accounts of Chemical Research 38 (2005) 349; (b) M. Fujita, M. Tominaga, A. Hori, B. Therrien, Accounts of Chemical Research 38 (2005) 369; (c) S.R. Seidel, P.J. Stang, Accounts of Chemical Research 35 (2002) 972; (d) M.D. Ward, Chemical Communications (2009) 4487; (e) J.J. Perry, J.A. Perman, M.J. Zaworotko, Chemical Society Reviews 38 (2009) 1400; (f) S. Alvarez, Dalton Transactions (2006) 2209; (g) B. Breiner, J.K. Clegg, J.R. Nitschke, Chemical Science 2 (2011) 51. [2] (a) R.W. Saalfrank, R. Burak, A. Breit, D. Stalke, R. Herbst-Irmer, J. Daub, M. Porsch, E. Bill, M. Muther, A.X. Trautwein, Angewandte Chemie (International Ed. in English) 33 (1994) 1621; (b) J.K. Clegg, L.F. Lindoy, B. Moubaraki, K.S. Murray, J.C. McMurtrie, Dalton Transactions (2004) 2417; (c) A.V. Davis, D. Fiedler, M. Ziegler, A. Terpin, K.N. Raymond, Journal of the American Chemical Society 129 (2007) 15354; (d) P. Mal, B. Breiner, K. Rissanen, J.R. Nitschke, Science 324 (2009) 1697; (e) C.J. Brown, R.G. Bergman, K.N. Raymond, Journal of the American Chemical Society 131 (2009) 17530. [3] Q.-F. Sun, J. Iwasa, D. Ogawa, Y. Ishida, S. Sato, T. Ozeki, Y. Sei, K. Yamaguchi, M. Fujita, Science 328 (2010) 1144. [4] (a) J.S. Fleming, K.L.V. Mann, C.-A. Carraz, E. Psillakis, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Angewandte Chemie International Edition 37 (1998) 1279; (b) R.L. Paul, Z.R. Bell, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Proceedings of the National Academy of Sciences of the United States of America 99 (2002) 4883; (c) I.S. Tidmarsh, B.F. Taylor, M.J. Hardie, L. Russo, W. Clegg, M.D. Ward, New Journal of Chemistry 33 (2009) 366. [5] (a) S.P. Argent, H. Adams, T. Riis-Johannessen, J.C. Jeffery, L.P. Harding, M.D. Ward, Journal of the American Chemical Society 128 (2006) 72; (b) A. Stephenson, S.P. Argent, T. Riis-Johannessen, I.S. Tidmarsh, M.D. Ward, Journal of the American Chemical Society 133 (2011) 858. [6] N.K. Al-Rasbi, I. Tidmarsh, S.P. Argent, H. Adams, L.P. Harding, M.D. Ward, Journal of the American Chemical Society 130 (2008) 11641.
A.M. Najar et al. / Inorganic Chemistry Communications 15 (2012) 126–129 [7] A. Stephenson, M.D. Ward, Dalton Transactions 40 (2011) 7824. [8] (a) I.S. Tidmarsh, T.B. Faust, H. Adams, L.P. Harding, L. Russo, W. Clegg, M.D. Ward, Journal of the American Chemical Society 130 (2008) 15167; (b) N.K. Al-Rasbi, C. Sabatini, F. Barigelletti, M.D. Ward, Dalton Transactions (2006) 4769. [9] A mixture of 1,8-bis(bromomethyl)anthracene [10] (0.5 g, 1.37 mmol), 3-(2-pyridyl)-pyrazole [11] (0.4 g, 2.70 mmol), aqueous NaOH (5.5 M, 5 cm3), and tetrahydrofuran (60 cm3) was stirred at reflux for 21 h. After cooling, the organic phase was separated, dried over MgSO4, filtered, and evaporated to dryness. The crude product was dissolved in dichloromethane; addition of diethylether precipitated a white powder which was filtered and dried to yield pure L (0.40 g, 56 %). Anal. calcd for C32H24N6•0.5H2O: C , 76.6; H, 5.0; N, 16.8 %. Found: C , 76.6; H, 4.8; N, 16.8 %. ESMS: m/z 493 (M + H)+, 247 (M + 2H)2+. 1H NMR (400 MHz, CDCl3): δ 8.93 (1H, s, anthryl H9 or H10); 8.62 (2H, d, pyridyl H6); 8.48 (1H, s, anthryl H10 or H9); 8.03 (2H, d, pyridyl H3); 7.99 (2H, d, anthryl H2/H7 or H4/H5); 7.67 (2H, td, pyridyl H4); 7.43 (2H, dd, anthryl H3/6); 7.40 (2H, d, pyrazolyl H5); 7.27 (2H, d, anthryl H4/H5 or H2/H7); 7.17 (2H, ddd, pyridyl H5); 6.89 (2H, d, pyrazolyl H4); 5.93 (4H, s, CH2 [10] (a) J. Re, X.-L. Zhao, Q.-C. Wang, C.-F. Ku, D.-H. Qu, C.-P. Chang, H. Tian, Dyes and Pigments 64 (2005) 179; (b) M. Rogers, B.A. Averill, The Journal of Organic Chemistry 51 (1986) 3308. [11] (a) A.J. Amoroso, A.M.W. Cargill Thompson, J.C. Jeffery, P.L. Jones, J.A. McCleverty, M.D. Ward, Journal of the Chemical Society, Chemical Communications (1994) 2751; (b) H. Brunner, T. Scheck, Chemische Berichte 125 (1992) 701; (c) Y. Lin, S.A. Lang, Journal of Heterocyclic Chemistry 14 (1977) 345. [12] Crystal data for L (C32H24N6): monoclinic, space group P21/c, Mr = 492.57, a = 5.4905(2), b = 14.2024(4), c = 31.5698(9) Å, β = 94.904(2)˚, V = 2452.75 (13) Å3, Z = 4, Dc = 1.334 g cm-3, μ(Mo-Kα) = 0.082 mm-1. A crystal of size 0.50 x 0.20 x 0.19 mm3 was mounted on a Brujer APEX-2 diffractometer under a stream of cold N2 and intensity data were collected at 100 K. 22417 reflections were collected with 2θmax = 55˚ which after merging afforded 5575 independent data with Rint = 0.0415. Refinement of 343 parameters converged at R1 [selected data with I > 2σ(I)] = 4.94%; wR2 (all data) = 16.6%. Software used for structure solution and refinement was SHELXS-97 and SHELX-97 [13]; the absorption correction was applied with SADABS [14]. [13] G.M. Sheldrick, Acta Crystallographica Section A: Foundations 64 (2008) 112. [14] G.M. Sheldrick, SADABS: A program for absorption correction with the Siemens SMART system, University of Göttingen, Germany, 1996. [15] Synthesis of [Zn6L9][BF4]12: A Teflon-lined autoclave was charged with Zn(BF4)2• xH2O (0.016 g, 0.07 mmol), L (0.05 g, 0.1 mmol) and methanol (9 cm3). Heating to 100 °C for 12 h followed by slow cooling to room temperature yielded the product as a white powder in 68% yield. X-ray quality crystals were grown by slow diffusion of diethylether into a solution of the complex in CH3NO2. Anal. calcd. for [Zn6L9][BF4]12: C, 58.9; H, 3.7; N, 12.9%. Found: C, 59.2; H, 3.8; N, 12.6%. The Cd(II) analogue was made in an exactly similar way. [16] Synthesis of [Cu6L9][ClO4]12: A mixture of Cu(ClO4)2•6H2O (0.041 g, 0.11 mmol) and L (0.081 g, 0.16 mmol) in MeOH (10 cm3) was heated to reflux with stirring
[17]
[18]
[19]
[20]
[21] [22]
129
for 2 h and then allowed to cool. The solid product was collected by filtration (70% yield) and X-ray quality crystals were grown by slow diffusion of diethylether into a solution of the complex in CH3NO2. Anal. calcd. for [Cu6L9] [ClO4]12•10H2O: C, 55.9; H, 3.8; N, 12.2%. Found: C, 55.6; H, 3.7; N, 12.0%. Crystal data for [Zn6L9](BF4)8(SiF6)2•6MeCN (C300H234B8F44N60Si2Zn6): cubic, space group I23, Mr = 6050.35, a = 40.9005(2), V = 68420.4(6) Å3, Z = 8, Dc = 1.175 g cm-3, μ(Mo-Kα) = 0.501 mm-1. A crystal of size 0.42 x 0.32 x 0.15 mm3 was mounted on a Bruker APEX-2 diffractometer under a stream of cold N2 and intensity data were collected at 123 K. 403173 reflections were collected with 2θmax = 55˚ which after merging afforded 26416 independent data with Rint = 0.065. Refinement of 1059 parameters with 1846 restraints converged at R1 [selected data with I > 2σ(I)] = 13.3%; wR2 (all data) = 37.6%. Software used for structure solution and refinement was SHELXS-97 and SHELX-97 [13]; the absorption correction was applied with SADABS [14]. The complex cation lies on a threefold axis such that one third of it lies in the asymmetric unit: there are 24 asymmetric units in the unit cell, giving eight complete formula units. The central tetrafluoroborate anion, and the two hexafluorosilicate anions in the centres of the triangular faces, all lie on the same threefold axis; another tetrafluoroborate anion is in a general position. Thus for the complete cage (charge 12+) anions totalling a charge of 8– (two hexafluorosilicate and two tetrafluoroborate) were located; the remaining anions, totalling 1.333 tetrafluoroborates per asymmetric unit, could not be located and are assumed to be in regions of diffuse residual electron density that could not be satisfactorily modelled. Geometric restraints were applied to the tetrafluoroborate and hexafluorosilicate anions to keep their geometries reasonable. In order to keep the refinement stable only the atoms associated with the complex cage were refined with anisotropic displacement parameters; extensive use of restraints on these displacement parameters was required to keep the refinement stable and to ensure that adjacent atoms did not have unreasonably different displacement parameters. Atoms associated with anions were left isotropic. (a) J.S. Fleming, K.L.V. Mann, S.M. Couchman, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Journal of the Chemical Society Dalton Transactions (1998) 2047; (b) P.J. van Koningsbruggen, J.G. Haasnoot, R.A.G. de Graaff, J. Reedijk, Journal of the Chemical Society Dalton Transactions (1993) 483. (a) C.J. Kuehl, Y.K. Kryschenko, U. Radhakrishnan, S.R. Seidel, S.D. Huang, P.J. Stang, Proceedings of the National Academy of Sciences 99 (2002) 4932; (b) C.J. Kuehl, T. Yamamoto, S.R. Seidel, P.J. Stang, Organic Letters 4 (2002) 913; (c) Y.-R. Zheng, H.-B. Yang, K. Ghosh, L. Zhao, P.J. Stang, Chemistry A European Journal 15 (2009) 7203. (a) C.-Y. Su, Y.-P. Cai, C.-L. Chen, F. Lissner, B.S. Kang, W. Kaim, Angewandte Chemie International Edition 41 (2002) 3371; (b) C.-Y. Su, Y.-P. Cai, C.-L. Chen, M.D. Smith, W. Kaim, H.-C. zur Loye, Journal of the American Chemical Society 125 (2003) 8595. S. Ghosh, P.S. Mukherjee, Organometallics 27 (2008) 316. S.M. Contakes, T.B. Rauchfuss, Angewandte Chemie International Edition 39 (2000) 1984.