A closed molecular cube and an open book: Two different products from assembly of the same metal salt and bridging ligand

Article abstract of DOI:10.1039/b515296h

Reaction of the bis-bidentate bridging ligand L¹ with Co(ClO₄)₂ or Zn(BF₄)₂ affords a mixture of complexes [M₈(L¹)₁₂]X₁₆ and [M₆(L¹)

Full text of DOI:10.1039/b515296h

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A closed molecular cube and an open book: two different products from
assembly of the same metal salt and bridging ligand
Stephen P. Argent,a Harry Adams,a Lindsay P. Hardingb and Michael D. Ward*a
Received 27th October 2005, Accepted 15th November 2005
First published as an Advance Article on the web 28th November 2005
DOI: 10.1039/b515296h
Reaction of the bis-bidentate bridging ligand L1 with
Co(ClO4)2 or Zn(BF4)2 affords a mixture of complexes
[M8(L1)12]X16 and [M6(L1)9]X12 having the same metal : ligand
ratio: the former is a molecular cube with a metal ion at each
vertex and a bridging ligand spanning each edge, whereas
the latter has a metal framework like that of an ‘open book’
containing cross-linked double helical metal–ligand subunits.
that ligands of this type, with four donors, react with six-
coordinate M2+ ions to afford a range of structures based on a
2M : 3L ratio,8–11 with cages varying in size from an M4L6
tetrahedron8 to an M16L24 tetra-capped truncated tetrahedron.9
The 2M : 3L ratio is necessary to match the number of donors
provided by the ligands with the number of coordination sites
available at the metal ions, and all members of this series described
so far are based on a metal polyhedron in which there is a vertex :
edge ratio of 2 : 3, such that a metal ion occupies each vertex and
a bridging ligand spans each edge.
In the general area of metallosupramolecular chemistry, three-
dimensional coordination cages have achieved particular promi-
nence recently due to (i) their elegant and appealing polyhedral
structures, (ii) the insights they provide into how careful control
of ligand structure and metal type can be used as a powerful
synthetic tool to give elaborate structures from simple components
structures, and (iii) for the host–guest chemistry associated with
their central cavities.1,2 In many cases the cages appear to be robust
structures which form as the unique product from a particular
combination of metal and ligand. In other cases, however, two or
more different structural forms based on the same components
can arise having the same metal : ligand ratio; these may be
in equilibrium (a ‘dynamic combinatorial library’), or can be
interconverted by guest-induced interactions which favour one
form of the assembly over another.3–7 Thus, Lehn and co-workers
demonstrated how a linear trinuclear triple helicate M3L3 could be
converted to a cyclic pentanuclear helicate M5L5 in the presence
of a templating chloride ion.3 In the field of three-dimensional
cages, Raymond et al. and Albrecht et al. have demonstrated how
tetrahedral cage complexes can form in the presence of a suitable
guest which matched the tetrahedral cavity; in the absence of the
template, other simpler species were formed.4 In each of these
examples the metal : ligand ratio is necessarily the same between
the different self-assembled forms. In other cases, different sizes of
cage complexes,5 and cyclic helicates,6 and molecular grids7 are in
dynamic equilibrium with one another in solution.
L1 (above) was prepared by reaction of 3-(2-pyridyl)pyrazole
with 1,3-bis(bromomethyl)benzene according to a standard
procedure.8–12† Reaction of L1 with Co(ClO4)2 in MeNO2 afforded
a pink solution, whose ES mass spectrum showed peaks at m/z
1555.1, 1226.9, 1144.6, 1007.8, 898.2, 734.0 and 617.7. These do
not fit to a single product, but can be separated into two distinct
sequences. Peaks at m/z 1555.1, 1226.9, 1007.8 and 734.0 can be
x+
assigned to [Co8(L1)12(ClO4)16−x
]
(x = 4, 5, 6, 8), all arising from
the same octanuclear complex core but with different numbers of
counter-ions. Peaks at 1555.1, 1144.6, 898.2, 734.0 and 617.7 in
x+
contrast can be assigned to [Co6(L1)9(ClO4)12−x
]
(x = 3, 4, 5, 6,
7), arising from a hexanuclear complex but with the same Co : L1
ratio. Clearly, there are some peaks in common between the two
species: m/z 1555.1 could arise from either [Co8(L1)12(ClO4)12]4+
or [Co6(L1)9(ClO4)9]3+ (or both), and m/z 734.0 could arise
from either [Co8(L1)12(ClO4)8]8+ or [Co6(L1)9(ClO4)6]6+ (or both).
However the remaining members of each series are unambiguous
and confirm formation of two distinct polynuclear assemblies.
Exactly comparable results were obtained from reaction of L1 with
Zn(BF4)2 in a 3 : 2 ratio in MeCN; ESMS of the resulting solution
x+
showed sequences of peaks arising from both [Zn8(L1)12(BF4)16−x
]
We describe here how a simple bis-bidentate bridging ligand L1,
containing two pyrazolyl-pyridine termini, reacts with Co(II) or
Zn(II) to form a mixture of two types of polynuclear complex
with the same metal : ligand ratio: an [M8(L1)12]16+ cage with a
cubic topology, and an [M6(L1)9]12+ open-framework structure.
The structures are unusual in their own right, but the appearance
of both types in the same reaction illustrates how the same
‘instructions’ provided by the ligands and metal ions can be
interpreted in two quite different ways. We have shown recently
x+
(x = 4, 5, 6, 8) and [Zn6(L1)9(BF4)12−x
]
(x = 3, 4, 5, 6, 7).
Slow diffusion of diethyl ether vapour into the product solutions
afforded a mixture of small crystals in each case; fortuitously, it
turned out that the best crystal from each batch corresponded
to a different component of the mixture (see Fig. 1 and 2).‡§
[Zn8(L1)12][BF4]16·20MeCN·Et2O (Fig. 1) is an approximate cube
with a metal ion at each vertex and a bridging ligand along each
edge, affording the necessary metal : ligand ratio of 2 : 3. The
˚
Zn · · · Zn distances are in the range 9.72 to 10.27 A, averaging
˚
9.96 A; the cube is slightly slanted, with angles at the corners
in the range 76.2–103.7◦. Zn(1)–Zn(4) all have the same optical
configuration, with their symmetry-eqivalent partners generated
by an inversion centre; the cube is accordingly achiral. Also Zn(2),
Zn(3), Zn(4) have a mer tris-chelate geometry whereas Zn(1) has a
aDepartment of Chemistry, University of Sheffield, Sheffield, UK S3 7HF.
E-mail: m.d.ward@Sheffield.ac.uk; Fax: +44 (0)114 2229346
bDepartment of Chemical and Biological Sciences, University of Hudders-
field, Huddersfield, UK HD1 3DH
542 | Dalton Trans., 2006, 542–544
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Fig. 2 Two views of the structure of the metal complex framework of
[Co6(L1)9][ClO4]12·(CH3NO2)9.5 (only one independent complex is shown).
Top: the ‘open-book’ array of metal ions, and three of the edge-bridging lig-
ands. Bottom: the whole metal–ligand assembly, with symmetry-equivalent
ligands coloured the same.
Fig. 1 Two views of the structure of the metal complex framework of
[Zn8(L1)12][BF4]16·20MeCN·Et2O. Top: the near-cubic array of metal ions,
two of the edge-bridging ligands, and the two anions in the central cavity.
Bottom: the whole metal–ligand assembly, with symmetry-equivalent
ligands coloured the same.
In contrast, [Co6(L1)9][ClO4]12·(CH3NO2)9.5 has an unusual open
framework structure based on an array of Co(II) centres (Fig. 2)
consisting of two squares that share an edge.
The array is folded about the central two Co(II) ions, like a book
that is not completely open, with a Co(II) ion at each corner and
two at either end of the central spine. The 2 : 3 metal : ligand ratio
requires nine ligands; there are two spanning each of the terminal
pairs of Co(II) ions (the opposed open edges of the book-red and
blue ligands in the figure) in a double helical arrangement, with
all remaining Co–Co vectors (from each corner of the book to
the spine, and along the spine) having one bridging ligand. The
two double helical sections are homochiral as they are related by
a C2 rotation through the centre of the complex. The Co · · · Co
fac geometry. This results in a (non-crystallographic) S6 axis along
the long diagonal of the cube, joining Zn(1) and Zn(1A). The
central cavity contains two (symmetry-related) [BF4]− anions. Ex-
tensive aromatic p-stacking between parallel, overlapping sections
of ligands around the periphery of the complex is clear. There are
a few other examples of molecular cubes in the literature, prepared
using a range of different design principles;11,13 this one is similar
to an example we described recently based on a related bridging
ligand.11 The relatively low symmetry of the cube (two independent
˚
˚
separations lie in the range 9.66–9.97 A (average 9.79 A), with the
Co–Co–Co angle at the spine (i.e. the extent of folding) being 119◦.
Co(2) and Co(3) have the same optical configuration as each other,
and a fac tris-chelate geometry, with Co(1) having the opposite
optical configuration and a mer tris-chelate geometry. There are
numerous regions of aromatic p-stacking between near-parallel,
overlapping sections of different ligands (e.g. red/orange/green,
purple/green/purple and blue/purple/yellow triple stacks, and
red/blue and blue/purple stacks between pairs of ligand sections,
1
ligand environments) means that H spectra were uninformative
due to numerous overlapping signals in the aromatic region. 11B
NMR spectra gave only one signal for the [BF4]− anions, indicative
of either fast exchange of internal/external anions through the
cube faces, or interconversion of the closed cube with more open
structures in solution.
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using the colour scheme in Fig. 2). This type of open-framework
structure is unprecedented in our investigations with this series of
ligands.
function was applied to remove areas of diffuse electron density which
could not be modelled. Consequently only 21 of the expected 24 counter-
ions could be located, which is reflected in the molecular formula and
formula mass given above. Only the metal atoms could be refined
anisotropically. In both cases however the structure of the metal/ligand
assembly is clearly defined, although detailed analysis of metal–ligand
bond distances etc. is not appropriate. CCDC reference numbers 287971
and 287972. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b515296h.
This pair of complexes represents an unusual example of how
a single combination of a metal salt and a ligand can follow two
quite different self-assembly pathways to give a mixture of different
products which nevertheless obey the same basic stoichiometric
principle of having a 2M : 3L ratio. Whether they are in dynamic
equilibrium in solution is not clear as we have not yet been able
to isolate enough of one type of crystal completely pure to see if
it establishes an equilibrium with the other form in solution; the
two crystal types are intimately mixed following crystallisation.
This suggests in itself, however, that the closed cage and open
framework species are very similar in energy.
1 Reviews: (a) G. F. Swiegers and T. J. Malefetse, Coord. Chem. Rev.,
2002, 225, 91; (b) M. Fujita, M. Tominaga, A. Hori and B. Therrien,
Acc. Chem. Res., 2005, 38, 369; (c) D. Fiedler, D. H. Leung, R. G.
Bergman and K. N. Raymond, Acc. Chem. Res., 2005, 38, 349; (d) M.
Yoshizawa and M. Fujita, Pure Appl. Chem., 2005, 77, 1107.
2 Representative recent examples:(a) M. Schweiger, T. Yamamoto, P. J.
Stang, D. Blaser and R. Boese, J. Org. Chem., 2005, 70, 4861; (b) R. W.
Saalfrank, B. Demleitner, H. Glaser, H. Maid, D. Bathelt, F. Hampel,
W. Bauer and M. Teichert, Chem. Eur. J., 2002, 8, 2679; (c) J. K.
Clegg, L. F. Lindoy, B. Moubaraki, K. S. Murray and J. McMurtrie,
Dalton Trans., 2004, 2417; (d) A. V. Davis and K. N. Raymond, J. Am.
Chem. Soc., 2005, 127, 7912; (e) K. Nakabayashi, M. Kawano and M.
Fujita, Angew. Chem., Int. Ed., 2005, 44, 5322; (f) S. Karthikeyan and
V. Ramamurthy, Tetrahedron Lett., 2005, 46, 4495; (g) D. J. Bray, L. L.
Liao, B. Antonioli, K. Gloe, L. F. Lindoy, J. C. McMurtrie, G. Wei and
X. Y. Zhang, Dalton Trans., 2005, 2082; (h) I. M. Muller and D. Moller,
Angew. Chem., Int. Ed., 2005, 44, 2969; (i) T. Haino, M. Kobayashi, M.
Chikaraishi and Y. Fukazawa, Chem. Commun., 2005, 2321.
3 B. Hasenknopf, J.-M. Lehn, B. O. Kneisel, G. Baum and D. Fenske,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1838; B. Hasenknopf, J.-M.
Lehn, N. Boumidiene, A. Dupont-Gervais, A. Van Dorsselaer, B.
Kneisel and D. Fenske, J. Am. Chem. Soc., 1997, 119, 10956.
4 M. Scherer, D. L. Caulder, D. W. Johnson and K. N. Raymond, Angew.
Chem., Int. Ed., 1999, 38, 1588; M. Albrecht, I. Janser, J. Runsink, G.
Raabe, P. Weis and R. Frohlich, Angew. Chem., Int. Ed., 2004, 43, 6662.
5 S. J. Park, D. M. Shin, S. Sakamoto, K. Yamaguchi, Y. K. Chung, M. S.
Lah and J. I. Hong, Chem. Eur. J., 2004, 11, 235.
Acknowledgements
We thank the University of Sheffield for financial support.
Notes and references
† A mixture of 1,3-bis(bromomethyl)benzene (1.20 g, 4.55 mmol), 3-(2-
pyridyl)pyrazole (1.35 g, 9.32 mmol), aqueous NaOH (10 M, 20 cm3),
toluene (50 cm3) and Bu4NOH (40% aqueous solution, 3 drops) was
stirred vigorously at room temperature for 40 min. The mixture was
diluted with H2O (100 cm3) and the organic layer separated, dried over
MgSO4 and concentrated before purification using an alumina column
(5% THF/dichloromethane) to give L1 as a yellow oil (Yield: 1.48 g, 83%).
1H-NMR (270 MHz, CDCl3): d 8.61 (2H, ddd, J 4.8, 1.8, 0.9; pyridyl H6),
7.90 (2H, d, J 7.9; pyridyl H3), 7.68 (2H, td, J 7.6, 1.8; pyridyl H4), 7.39
(2H, d, J 2.4; pyrazolyl H5), 7.21–6.98 (6H, m; pyridyl H5 and 4 × phenyl),
6.89 (2H, d, J 2.4; pyrazolyl H4), 5.35 (4H, s, CH2). EIMS m/z 392 (M+).
Found: C, 73.0; H, 5.1; N, 20.9%. Required for C24H20N6: C, 73.4; H, 5.1;
N, 21.4%.
‡ Elemental analyses were performed on samples of the crystalline
materials that were dried in vacuo to remove lattice solvent. The
results indicated some uptake of atmospheric water after drying.
For [Zn8(L1)12][BF4]16·5H2O: found C, 51.5; H, 3.8; N, 14.5. Re-
quired for C288H240B16F64N72Zn8·5H2O: C, 51.4; H, 3.8; N, 15.0%. For
[Co6(L1)9][ClO4]12·3H2O: found C, 50.2; H, 3.5; N, 15.0. Required for
6 G. Baum, E. C. Constable, D. Fenske, C. E. Housecroft and T. Kulke,
Chem. Commun., 1999, 195; F. Tuna, J. Hamblin, A. Jackson, G.
Clarkson, N. W. Alcock and M. J. Hannon, Dalton Trans., 2003, 2141;
C. S. Campos-Fernandez, B. L. Schottel, H. T. Chifotides, J. K. Bera, J.
Basca, J. M. Koomen, D. H. Russell and K. R. Dunbar, J. Am. Chem.
Soc., 2005, 127, 12909.
7 J. R. Nitschke and J.-M. Lehn, Proc. Natl. Acad. Sci. USA, 2003, 100,
11970.
8 J. S. Fleming, K.L. V. Mann, C.-A. Carraz, E. Psillakis, J. C. Jeffery,
J. A. McCleverty and M. D. Ward, Angew. Chem., Int. Ed., 1998, 37,
1279; R. L. Paul, Z. R. Bell, J. C. Jeffery, J. A. McCleverty and M. D.
Ward, Proc. Natl. Acad. Sci. USA, 2002, 99, 4883; R. L. Paul, Z. R.
Bell, J. C. Jeffery, L. P. Harding, J. A. McCleverty and M. D. Ward,
Polyhedron, 2003, 22, 781; R. L. Paul, S. P. Argent, J. C. Jeffery, L. P.
Harding, J. M. Lynam and M. D. Ward, Dalton Trans., 2004, 3453;
S. P. Argent, T. Riis-Johannessen, J. C. Jeffery, L. P. Harding and M. D.
Ward, Chem. Commun., 2005, 4647.
9 S. P. Argent, H. Adams, T. Riis-Johannessen, J. C. Jeffery, L. P. Harding
and M. D. Ward, J. Am. Chem. Soc., DOI: 10.1021/ja056993o.
10 Z. R. Bell, J. C. Jeffery, J. A. McCleverty and M. D. Ward, Angew.
Chem., Int. Ed., 2002, 41, 2515.
C
216H180Cl12Co6N54O48·3H2O: C, 50.5; H, 3.7; N, 14.7%.
§ Crystallography. A crystal of [Zn8(L1)12][BF4]16·20MeCN·Et2O (0.33 ×
0.20 × 0.15 mm) was mounted on a Bruker-AXS SMART-1000 diffrac-
tometer at 150 K. Formula: C331H302B16F64N92O2Zn8; formula weight
˚
7512.6; triclinic, P-1; a = 19.440(3), b = 21.◦855(4), c = 23.828(4) A;
3
˚
a = 106.286(3), b = 105.450(3), c = 107.424(3) ; V = 8566(3) A ; Z = 1;
q = 1.456 g cm−3; l(Mo-Ka) = 0.655 mm−1; k = 0.71073 A. 30121 unique
˚
data were collected; refinement of 1538 parameters with 34 restraints
converged at R1 = 0.0896 [selected data with I > 2r(I)], wR2 = 0.2651
(all data). The asymmetric unit contains one half of the complex molecule
adjacent to an inversion centre. A crystal of [Co6(L1)9][ClO4]12·(CH3NO2)9.5
(0.50 × 0.25 × 0.18 mm) was mounted on a Bruker-AXS SMART-
1000 diffractometer at 100 K. Formula: C451H417Cl21Co12N127O122; formula
weight 11019.7; monoclinic, P2/n; a = 29.108(5), b = 26.238(4), c =
11 Z. R. Bell, L. P. Harding and M. D. Ward, Chem. Commun., 2003, 2432.
12 Z. R. Bell, J. A. McCleverty and M. D. Ward, Aust. J. Chem., 2003, 56,
665.
◦
3
−3
˚
˚
37.671(6) A; b = 91.369(4) ; V = 28762(8) A ; Z = 2; q = 1.272 g cm
;
l(Mo-Ka) = 0.516 mm−1; k = 0.71073 A. 65214 unique data were collected;
refinement of 1052 parameters with 1549 restraints converged at R1 =
0.1519 [selected data with I > 2r(I)], wR2 = 0.2682 (all data). The
asymmetric unit contains two independent half-molecules astride C2 axes.
Both sets of crystals diffracted very poorly due to a combination of
immediate solvent loss on removal from the mother liquor and extensive
disorder of counter-ions and lattice solvent molecules. Extensive use of
restraints was necessary to keep the geometries of anions and lattice solvent
molecules reasonable. For [Co6(L1)9][ClO4]12·(CH3NO2)9.5 a ‘SQUEEZE’
˚
13 S. Roche, C. Haslam, H. Adams, S. L. Heath and J. A. Thomas, Chem.
Commun., 1998, 1681; K. K. Klausmeyer, T. B. Rauchfuss and S. R.
Wilson, Angew. Chem., Int. Ed., 1998, 37, 1694; B. F. Abrahams, S. J.
Egan and R. Robson, J. Am. Chem. Soc., 1999, 121, 3535; M. C. Hong,
Y. J. Zhao, W. P. Su, R. Cao, M. Fujita, Z. Y. Zhou and A. S. C.
Chan, J. Am. Chem. Soc., 2000, 122, 4819; E. J. Schelter, A. V. Prosvirin
and K. R. Dunbar, J. Am. Chem. Soc., 2004, 126, 15004; Y. Liu, V.
Kravtsov, R. D. Walsh, P. Poddar, H. Srikanth and M. Eddaoudi,
Chem. Commun., 2004, 2806 and refs. therein.
544 | Dalton Trans., 2006, 542–544
This journal is
The Royal Society of Chemistry 2006
©