Supramolecular structures based on dimetal units: simultaneous utilization of equatorial and axial connections

Abstract

As determined by X-ray crystallography, tetranuclear metal–metal bound molecular loops [Rh₂(DAniF)₂]₂- (O₂CCH₂CO₂)₂ (DAniF = N,N′-di-p-anisylformamidinate) react with 2-N donor linkers to give a tubular structure {[Rh₂(DAniF)₂]₂(O₂CCH ₂CO₂)₂(NC₅H₄CHCHC ₅H₄N)₂}_n 1, and a sheet-like structure with interstitial dichloromethane molecules for [Rh₂(DAniF)₂]₂(O₂CCH₂ - CO₂)₂(NCC₆H₄CN) ₂}_n 2; when the assembly unit was changed from [Rh₂(DAniF)₂(O₂CCH₂CO₂ )]₂ to the square [Rh₂(DAniF)₂(O₂CCO₂)]₄ and the linker NCC₆F₄C₆F₄CN was used, the compound {[Rh₂(DAniF)₂]₄(O₂C CO₂)₄(NCC₆F₄C₆ F₄CN)₄}_n 3, was formed, in which there are infinite tubes of square cross section having entrained CH₂Cl₂ molecules.

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Oligomers and polymers in which a key organizing element is a coordinated metal atom, for example, a square-coordinated Pd^II or Pt^II atom or a tetrahedral zinc atom, are currently the subjects of vigorous study in many laboratories.¹ This work is driven by the search for novel materials, distinguished by special magnetic properties,² micro- and meso-porous structures,³ or catalytic properties.⁴

Several years ago we began to explore the possibilities of employing strongly bonded dimetal units, such as Mo₂⁴⁺ and Rh₂⁴⁺, that can be suitably complexed to control their reactivity and then linked to form linear,^5a,b square,^5c,d triangular^5c and polyhedral^5e structures. The linking process entails the use of di- and tri-carboxylic acids which attach themselves to adjacent pairs of equatorial sites in the M₂⁴⁺ entities, as shown schematically in A. The arrays that can be built up in this way have as great a range of sizes as those previously made² but differ from most of those in forming initially as neutral species rather than as highly charges ones. However, charge may readily be introduced—in a controlled, stepwise fashion^5b,d—by oxidation of the Mo₂⁴⁺ units.

Here we report that by using axial linking, as shown schematically in B, it is possible to take simple oligomers and connect them to form one- and two-dimensional polymers. These, again, are initially neutral but can be oxidized. This is the first time such architectures have been created by using dimetal building blocks. We also show how the nature of the polymeric structure can be controlled by choosing axial linkers of the right length, a principle that will be of importance in all future work.

In the examples reported here we have utilized dirhodium, Rh₂⁴⁺, building blocks shown as II and III in Fig. 1. The square, III, has been reported before^5c but the loop, II, is a new unit, not reported before, though it is somewhat similar to loops that have been made with Mo₂⁴⁺ units.⁶ Dirhodium units were used here because they have a strong affinity for axial ligands, unlike Mo₂⁴⁺ units. As axial linkers we have employed IV, V, and VI, (Fig. 1).

Fig. 1 Synthesis of compounds 1·3CH₂Cl₂·0.5Et₂O, 2·4CH₂Cl₂ and 3·12.36CH₂Cl₂. Views of the extended structures are given in the middle section as follows: (1a) structure of 1; (2a) structure of 2; (2b) intercalating architecture in 2; (3a) structure of 3 and (3b) a space filling drawing of 3 showing CH₂Cl₂ molecules inside the square tube. A schematic view of the corresponding structure is shown in the lower section. In the top section, there are axial CH₃CN molecules (not shown for clarity) at each Rh atom in I and III. For II, there are two CH₃CN molecules distributed on the four Rh atoms. The p-anisyl or DAniF groups have also been omitted for clarity. Color labels: Rh, red; N, blue; O, green; C, gray; F, yellow; Cl, orange; H, turquoise.

With IV, {[Rh₂(DAniF)₂]₂(O₂CCH ₂CO₂)₂(NC₅H₄- CHCHC₅H₄N)₂}_n1 is obtained.‡ It has a tubular structure shown in Fig. 1(a). In Fig. 1(b) there is a schematic representation of this structure§ showing how the rings (II) are related alternately by centers of inversion and two-fold axes.

While the formation of such tubes might be considered the ‘obvious’ outcome of linking units of type II by axial bridges, it can only result when the linkers are long enough to keep the bulky p-anisyl groups from clashing with each other. With a shorter linker, V, major clashes would occur and therefore a different structure arises in {[Rh₂(DAniF)₂]₂(O₂CCH ₂- CO₂)₂(NCC₆H₄CN) ₂}_n2. This sheet-like structure is shown as 2(a), where dichloromethane molecules are omitted, as 2(b) where the sheets are viewed edge-on and the CH₂Cl₂ molecules are included, and in schematic form as 2(c). The CH₂Cl₂ molecules were well ordered and refined without difficulty. The sheet belongs to the two-dimensional space group Cmm, the highest symmetry possible in a rectangular sheet structure.

When the assembly unit was changed from II to III and the linker VI was used, the compound {[Rh₂(DAniF)₂]₄(O₂C CO₂)₄(NCC₆F₄C₆ F₄CN)₄}_n, 3 was formed, in which there are infinite tubes of square cross section. A portion of the entire structure is shown as 3(a) while 3(b) shows a portion of one of the units, including one of the entrained CH₂Cl₂ molecules; 3(c) shows a schematic representation of this structure.

Other studies of these compounds and the syntheses of other assemblies of different topologies containing different dimetal units and axial ligands are in progress.

Acknowledgements

We are grateful to the National Science Foundation for support of this work, and to Johnson Matthey for a generous loan of RhCl₃.

Notes and references

1. See, for example: S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853 and references therein;; M. Fujita, Chem. Soc. Rev., 1998, 27, 417 and references therein;; P. Espinet, K. Soulantica, J. P. H. Charmant and A. G. Orpen, Chem. Commun., 2000, 915; J. A. R. Navarro and B. Lippert, Coord. Chem. Rev., 1999, 185–186, 653; D. Whang and K. Kim, J. Am. Chem. Soc., 1997, 119, 451; S. Mann, G. Huttner, L. Zsolnai and K. Heinze, Angew. Chem., Int. Ed. Engl., 1996, 35, 2808; M. Scherer, D. L. Caulder, D. W. Johnson and K. N. Raymond, Angew. Chem., Int. Ed., 1999, 38, 1588; S.-W. Lai, M. C.-W. Chan, S.-M. Peng and C.-M. Che, Angew. Chem., Int. Ed., 1999, 38, 669; C. J. Jones, Chem. Soc. Rev., 1998, 27, 289; K. K. Klausmeyer, S. R. Wilson and T. B. Rauchfuss, J. Am. Chem. Soc., 1999, 121, 2705; H. Li, M. Eddaoudi, M. O’Keeffe and O. M. Yaghi, Nature, 1999, 402, 276.
2. O. Kahn and C. J. Martinez, Science, 1998, 279, 44; Ø. Hatlevik, W. E. Buschmann, J. Zhang, J. L. Manson and J. S. Miller, Adv. Mater., 1999, 11, 914.
3. T. J. Barton, L. M. Bull, W. G. Klemperer, D. A. Lay, B. McEnaney, M. Misono, P. A. Monson, G. Pez, G. W. Sherer, J. C. Vartuli and O. M. Yaghi, Chem. Mater., 1999, 11, 2633.
4. M. Fujita, Y. J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 1994, 116, 1151.
5. F. A. Cotton, C. Lin and C. A. Murillo, J. Chem. Soc., Dalton Trans., 1998, 3151; F. A. Cotton, J. P. Donahue, C. Lin and C. A. Murillo, Inorg. Chem., in press;; F. A. Cotton, L. M. Daniels, C. Lin and C. A. Murillo, J. Am. Chem. Soc., 1999, 121, 4538; F. A. Cotton, C. Lin and C. A. Murillo, Inorg. Chem., in press;; F. A. Cotton, L. M. Daniels, C. Lin and C. A. Murillo, Chem. Commun., 1999, 841.
6. F. A. Cotton, C. Lin and C. A. Murillo, Inorg. Chem., in press;; E. Whelan, M. Deveraux, M. McCann and V. McKee, Chem. Commun., 1997, 427.

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Footnotes

† Most of the results reported here were presented in a poster at Contemporary Inorganic Chemistry, II, March 12–15, 2000, College Station, TX, USA.

‡ The general experimental conditions were described in ref. 1. Elemental analyses were satisfactory for all compounds. The following procedure describes the preparation of 1·3CH₂Cl₂·0.5Et₂O. A CH₂Cl₂ solution (10 mL) of II (82 mg, 0.05 mmol) was carefully layered with a CH₂Cl₂–Et₂O solution (1:1, 20 mL) of trans-1,2-bis(4-pyridyl)ethylene (18 mg, 0.10 mmol). Red crystals formed after several days. A similar method was used for 2·4CH₂Cl₂ and 3·12.36CH₂Cl₂. The yields are essentially quantitative.

§ Crystal data: for 1·3CH₂Cl₂·0.5Et₂O: C₉₅H₉₅Cl₆N₁₂O_16.5Rh ₄, M = 2293.17, monoclinic, space group C2/c, a = 56.521(4), b = 19.016(2), c = 19.564(2) Å, β = 103.053(2)°, V = 20484(3) ų, Z = 8, μ(Mo-Kα) = 0.857 mm⁻¹, T = 213(2) K. The structure, refined on F², converged for 13458 unique reflections and 613 parameters to give R1 = 0.082 and wR2 = 0.182 and a goodness-of-fit = 1.031.For 2·4CH₂Cl₂: C₈₆H₈₀Cl₈N₁₂O₁₆Rh ₄, M = 2232.86, triclinic, space group P1̄, a = 12.708(2), b = 14.378(2), c = 14.997(3) Å, α = 65.810(3), β = 72.693(3), γ = 74.766(3)°, V= 2355.0(7) ų, Z = 1, μ(Mo-Kα) = 0.984 mm⁻¹, T = 213(2) K. The structure, refined on F², converged for 6163 unique reflections and 578 parameters to give R1 = 0.051 and wR2 = 0.119 and a goodness-of-fit = 1.010.For 3·12.36CH₂Cl₂: C_196.36H_144.72Cl_24.72F₃₂N ₂₄O₃₂Rh₈, M = 5660.01, triclinic, space group P1̄, a = 18.235(6), b = 18.430(6), c = 18.665(6) Å, α = 81.839(7)°, β = 81.775(6), γ = 80.742(7)°, V= 6082(3) ų, Z = 1, μ(Mo-Kα) = 0.891 mm⁻¹, T = 223(2) K. The structure, refined on F², converged for 15333 unique reflections and 814 parameters to give R1 = 0.094 and wR2 = 0.211 and a goodness-of-fit = 1.014. CCDC 182/1852. See http://www.rsc.org/suppdata/cc/b0/b007347o/ for crystallographic files in .cif format.

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