Coordination polymers constructed from tetrahedral-shaped adamantane tectons

Abstract

Two rigid tetrahedral organic linkers derived from adamantane have been employed in constructing a 3-D, 4-fold interpenetrated framework featuring a PtS topology, [CuL¹(H₂O)₂](BF₄)₂·8H₂O (1) (L¹ = 1,3,5,7-tetrakis{4-(4-pyridyl)phenyl}adamantane), and a 2-fold interpenetrated grid-like coordination polymer, [Mn(hfac)₂(L²)_0.5] (2) (L² = 1,3,5,7-tetrakis(4-cyanophenyl)adamantane).

---

Tetrahedral divergent ligands play an important role in crystal engineering.¹ By developing the “node-and-spacer” approach, Robson has shown that coordination polymers featuring a diamond-like topology can be rationally obtained by combining two tectons: a metal ion with a preference for the T_d coordination geometry, and an organic molecule with tetrahedrally-disposed coordination groups.² Several other basic inorganic structures are constructed from tetrahedral tectons (mainly metal ions): lonsdaleite (lon), quartz (qtz), SrAl₂ (sra) (tetrahedral nodes – uni- or binodal nets), PtS (pts) (a combination of tetrahedral and square planar nodes), Ge₃N₄ (tetrahedral and trigonal nodes in a 3 : 4 ratio), fluorite (flu) (tetrahedral and cubic centres), etc.³ Even though a rich diversity of topologies can be envisioned, the number of coordination polymers described in the literature relying upon tetrahedral spacers is quite low.⁴ The network topologies are different from the diamondoid one when the metal ions have other stereochemical preferences.

The coordinating-prone units attached to tetrahedral skeletons (methane, silane or adamantane derivatives) can be anionic (carboxylate,⁵ sulfonate,⁶ or phosphonate^7–9 groups), or neutral (pyridine,¹⁰ triazole,¹¹ and tetrazole^8,12,13 fragments, or cyano groups^2,14). Tetra-substituted adamantane derivatives are useful tectons in generating robust porous structures showing high connectivity topologies.^6–8,12,14c In spite of the fact that rigid organic tetrahedral tectons are encoded to orient the donor atoms towards the vertices of a regular tetrahedron, thus extending the molecular assembling into three dimensions, organic ligands with a T_d symmetry forming a 2-D layer have also been described.^6a,14

Herein, we report the crystal structures of two new coordination polymers constructed from copper(II) ions and {Mn^II(hfac)₂} entities acting as nodes: [CuL¹(H₂O)₂](BF₄)₂·8H₂O (1) and [Mn(hfac)₂(L²)_0.5] (2), where L¹ and L² are tetrahedral spacers having four phenyl–pyridyl (L¹) or 4-cyanophenyl moieties (L²) attached to an adamantane backbone (Scheme 1). The two assembling species have been chosen from the following reasons: Cu^II adopts various coordination numbers and geometries, but, only rarely tetrahedral; {Mn^II(hfac)₂} has two positions available for the interaction with the spacer (either cis or trans). In the first case, Cu^II, the topology of the resulting coordination polymer cannot be easily predicted. In the second case, a 3-D coordination polymer is expected, if the donor atoms arising from the spacer occupy the trans positions at the manganese ion, and a 2-D coordination polymer, when the donor atoms from the spacer occupy the cis positions.

Scheme 1

The two adamantane derivatives, L¹ and L², have been already described in the literature.^8,15 In the case of tetrapyridyladamantane, L¹ (Scheme S1†), relevant improvements of the synthesis, compared to the method reported by Müller,¹⁵ were elaborated. The employment of PdCl₂(dppf) instead of Pd(PPh₃)₄ leads to an increase in the yield from 27 to 41%, a reduction of the reaction time from 72 to 12 h, and an easier chromatographic separation of the target compound.‡

The reaction of L¹ with copper(II) tetrafluoroborate, in the presence of monoethanolamine, affords blue-green single crystals of [CuL¹(H₂O)₂](BF₄)₂·8H₂O 1.‡ The investigation of the crystal structure§ of 1 reveals that each L¹ molecule connects four copper(II) ions (Fig. 1) in a 3-D network (Fig. 2). The copper ions display a distorted octahedral coordination geometry with four nitrogen atoms in the equatorial plane arising from four organic linker molecules (Cu1–N1 = 2.026(8) Å) and two semi-coordinated water molecules in trans positions (Cu1–O1W = 2.626(16) Å). The Cu⋯Cu distances are 18.130 Å (Cu1⋯Cu1^a, a = 1 + x, 1 + y, z), and 20.744 Å (Cu1⋯Cu1^b, b = 0.5 + x, 2.5 − y, 0.5 + z). The 3-D cationic framework can be described as a binodal (4,4)-connected network with a platinum sulfide topology, where L¹ molecules play the role of tetrahedral sulfide nodes while the equatorial CuN₄ units replace platinum square planar centres. The analysis of the packing diagram shows the interpenetration of four independent 3-D nets of 1 (Fig. 3). The structural changes of L¹, in comparison with the homolog derived from methane,^10b are reflected in an increase of the interpenetration degree. To the best of our knowledge, this is the highest interpenetration degree encountered for molecular assemblies built from a rigid organic linker with T_d symmetry bearing pyridine moieties. The void space resulting upon interpenetration is filled with BF₄⁻ anions and solvent molecules. Since the number of the water molecules could not be directly determined by the crystallographic measurements, we estimated the presence of eight water molecules per formula (ESI†) from the data obtained by elemental and thermogravimetric analyses. The diffuse reflectance electronic spectrum of 1 (Fig. S3†) shows a band at 604 nm, due to the hexacoordinated Cu^II ions.

Fig. 1 Perspective view of the coordination polymer in crystal 1 with the atom numbering scheme.

Fig. 2 Perspective view of the 3-D coordination framework of 1 and the corresponding representation of the platinum sulfide topology.

Fig. 3 Crystal packing diagram in crystal 1, showing the four-fold interpenetration.

Compound [Mn(hfac)₂(L²)_0.5] 2 crystallizes in the tetragonal centrosymmetric I4₁/a space group.§ Taking into consideration the weak coordinating ability of nitrile-containing molecules, we have employed a hexafluoroacetylacetonate metal complex, as the electron-withdrawing F atoms increase the Lewis acidity of the metal ion, thus favouring the coordination of the nitrile groups to the manganese ion, by replacing the weakly bonded aqua ligands. A neutral coordination structure is assembled by reacting [Mn(hfac)₂(H₂O)₂] with L² (Hhfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione).‡

The {Mn(hfac)₂} nodes are linked by L² molecules resulting in neutral (4,4)-connected grid-like layers. The Mn^II ions adopt a slightly distorted octahedral geometry with two chelating hfac⁻ ligands (Mn1–O1 = 2.128(4) Å, Mn1–O2 = 2.154(5) Å) and two nitrogen ligand atoms disposed in cis positions (Mn1–N1 = 2.239(7) Å) (Fig. 4). Since L² acts as a 4-connecting knot, the low dimensionality of 2 can be ascribed exclusively to the cis-arrangement of nitrogen donor atoms in {Mn(hfac)₂N₂} units, while a trans coordination of the nitrogen L² atoms to the metal centre would afford a 3-D net. In the case of honeycomb type layers^14b,c where the tetrahedral-shaped ligand acts as a 3-connecting node, the promotion into the third dimension is disrupted by the presence of solvent molecules or coordinating anions. The inherent chirality of the cis-isomer of the metal building blocks enables the formation of chains where two L² arms bridge metal centres of the same octahedral delta (Δ) conformation and the other two arms bridge metal centres of lambda (Λ) conformation. Viewed along the c axis, the structure of 2 can be regarded as sets of zig-zag chains constructed, each one, by Mn^II nodes with the same chirality, and running perpendicular to each other. Layers with meshes of square cross section are thus formed (Fig. 5). Two such layers interpenetrate (Fig. 6a). Channels within the ab plane are formed (Fig. 6b).

Fig. 4 Fragment of the crystal structure of 2 showing the interaction of the tetrahedral ligand L² with two {Mn(hfac)₂} units of opposite chirality.

Fig. 5 View of a layer in crystal 2 (see text: green – delta, violet – lambda configuration) (CF₃ groups were omitted for clarity).

Fig. 6 (a) Packing diagram of 2 showing the two-fold interpenetration of 2-D nets; (b) perspective view of channels.

In conclusion, two coordination polymers with different dimensionalities have been synthesized from rigid tetrahedral-shaped tectons with adamantane scaffolds. We have emphasized the role played by the coordination preference of the metal centre, along with the organic linker's symmetry, in dictating the network topology. Compound 1 features a PtS-type topology of network, with four interpenetrating nets. The low dimensionality of compound 2 arises from the employment of a neutral building block displaying a cis-conformation that limits the propagation into the third dimension. Studies to check the ability of the ligands to afford high connectivity networks with other metal ions are in progress.

Acknowledgements

This work was financially supported by UEFISCDI (Grant PNII-ID-PCCE-2011-2-0050) and by a grant of the Romanian National Authority for Scientific Research and Innovation, POCPOLIG, project nr. 67/08.09.

Notes and references

1. (a) R. Robson, J. Chem. Soc., Dalton Trans., 2000, 3735; (b) R. Robson, Dalton Trans., 2008, 5113; (c) G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311.
2. B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1990, 112, 1546.
3. S. R. Batten, S. M. Neville and D. R. Turner, Coordination polymers: Design, analysis and applications, Royal Society of Chemistry, Cambridge, 2008.
4. See, for example T. Muller and S. Bräse, RSC Adv., 2014, 4, 6886 and references therein.
5. (a) B. Chen, M. Eddaoudi, T. M. Reineke, J. W. Kampf, M. O'Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2000, 122, 11559; (b) J. Kim, B. Chen, T. M. Reineke, H. Li, M. Eddaoudi, D. B. Moler, M. O'Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2001, 123, 8239; (c) M. Zhang, Y.-P. Chen, M. Bosch, T. Gentle, K. Wang, D. Feng, Z. U. Wang and H.-C. Zhou, Angew. Chem., Int. Ed., 2014, 53, 815; (d) H. Chun, D. Kim, D. N. Dybtsev and K. Kim, Angew. Chem., Int. Ed., 2004, 43, 971; (e) L. Wen, P. Cheng and W. Lin, Chem. Commun., 2012, 48, 2846; (f) L. Wen, P. Cheng and W. Lin, Chem. Sci., 2012, 3, 2288; (g) C. Tan, S. Yang, N. R. Champness, X. Lin, A. J. Blake, W. Lewis and M. Schröder, Chem. Commun., 2011, 47, 4487; (h) Y. E. Cheon and M. P. Suh, Chem. Commun., 2009, 2296; (i) L. Ma, A. Jin, Z. Xie and W. Lin, Angew. Chem., Int. Ed., 2009, 48, 9905; (j) M. Zhang, Y.-P. Chen and H.-C. Zhou, CrystEngComm, 2013, 15, 9544; (k) R. P. Davies, R. Less, P. D. Lickiss, K. Robertson and A. J. P. White, Cryst. Growth Des., 2010, 10, 4571; (l) J. M. Gotthardt, K. F. White, B. F. Abrahams, C. Ritchie and C. Boskovic, Cryst. Growth Des., 2012, 12, 4425; (m) D. Liu, Z. Xie, L. Ma and W. Lin, Inorg. Chem., 2010, 49, 9107; (n) G. A. Senchyk, A. B. Lysenko, H. Krautscheid, E. B. Rusanov, A. N. Chernega, K. W. Krämer, S.-X. Liu, S. Decurtins and K. V. Domasevitch, Inorg. Chem., 2013, 52, 863.
6. (a) D. J. Hoffart, A. P. Côté and G. K. H. Shimizu, Inorg. Chem., 2003, 42, 8603; (b) D. J. Hoffart, S. A. Dalrymple and G. K. H. Shimizu, Inorg. Chem., 2005, 44, 8868.
7. (a) J. M. Taylor, A. H. Mahmoudkhani and G. K. H. Shimizu, Angew. Chem., Int. Ed., 2007, 46, 795; (b) M. V. Vasylyev, E. J. Wachtel, R. Popovitz-Biro and R. Neumann, Chem. – Eur. J., 2006, 12, 3507; (c) M. Vasylyev and R. Neumann, Chem. Mater., 2006, 18, 2781.
8. I. Boldog, K. V. Domasevitch, I. A. Baburin, H. Ott, B. Gil-Hernández, J. Sanchiz and C. Janiak, CrystEngComm, 2013, 15, 1235.
9. A. Bulut, Y. Zorlu, E. Kirpi, A. Çetinkaya, M. Wörle, J. Beckmann and G. Yücesan, Cryst. Growth Des., 2015, 15, 5665.
10. (a) K. Matsumoto, M. Kannami, D. Inokuchi, H. Kurata, T. Kawase and M. Oda, Org. Lett., 2007, 9, 2903; (b) C. B. Caputo, V. N. Vukotic, N. M. Sirizzotti and S. J. Loeb, Chem. Commun., 2011, 47, 8545; (c) F. L. Geyer, F. Rominger, M. Vogtland and U. H. F. Bunz, Cryst. Growth Des., 2015, 15, 3539; (d) S. Shankar, R. Balgley, M. Lahav, S. R. Cohen, R. Popovitz-Biro and M. E. van der Boom, J. Am. Chem. Soc., 2015, 137, 226; (e) X.-Y. Chen, H.-Y. Shi, R.-B. Huang, L.-S. Zheng and J. Tao, Chem. Commun., 2013, 49, 10977; (f) X.-Y. Chen, R.-B. Huang, L.-S. Zheng and J. Tao, Inorg. Chem., 2014, 53, 5246.
11. (a) G. A. Senchyk, A. B. Lysenko, I. Boldog, E. B. Rusanov, A. N. Chernega, H. Krautscheid and K. V. Domasevitch, Dalton Trans., 2012, 41, 8675; (b) G. A. Senchyk, A. B. Lysenko, E. B. Rusanov, A. N. Chernega, J. Jezierska, K. V. Domasevitch and A. Ozarowski, Eur. J. Inorg. Chem., 2012, 5802; (c) A. B. Lysenko, G. A. Senchyk, J. Lincke, D. Lässig, A. A. Fokin, E. D. Butova, P. R. Schreiner, H. Krautscheid and K. V. Domasevitch, Dalton Trans., 2010, 39, 4223.
12. (a) I. Boldog, K. V. Domasevitch, J. Sanchiz, P. Mayerd and C. Janiak, Dalton Trans., 2014, 43, 12590; (b) I. Boldog, K. Domasevitch, J. K. Maclaren, C. Heering, G. Makhloufi and C. Janiak, CrystEngComm, 2014, 16, 148.
13. M. Dincă, A. Dailly and J. R. Long, Chem. – Eur. J., 2008, 14, 10280.
14. (a) D. Venkataraman, S. Lee, J. S. Moore, P. Zhang, K. A. Hirsch, G. B. Gardner, A. C. Covey and C. L. Prentice, Chem. Mater., 1996, 8, 2030; (b) F.-Q. Liu and T. D. Tilley, Inorg. Chem., 1997, 36, 5090; (c) K. M. Patil, M. E. Dickinson, T. Tremlett, S. C. Moratti and L. R. Hanton, Cryst. Growth Des., 2016, 16, 1038.
15. C. I. Schilling, O. Plietzsch, M. Nieger, T. Muller and S. Bräse, Eur. J. Org. Chem., 2011, 1743.

---

Footnotes

† Electronic supplementary information (ESI) available: Details of synthetic procedures, crystallographic data, figures of asymmetric units, spectral data, TGA curve and experimental and calculated powder diffraction patterns. CCDC 1508551 and 1508552. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ce02146h

‡ Synthesis of L¹: to a 100 mL three-necked round-bottom flask equipped with a reflux condenser were added 4-pyridylboronic acid (0.258 g, 2.11 mmol), Pd(dppf)Cl₂ (0.043 g, 0.05 mmol), and Na₂CO₃ (0.452 g, 4.27 mmol) in dioxane/water (2.5 : 1 v/v, 35 mL) and the solution was flushed with argon for 20 min. After, 1,3,5,7-tetrakis(4-iodophenyl)adamantane (0.25 g, 0.26 mmol) was added under an argon atmosphere, and the reaction mixture was stirred at 80 °C for 12 h. The solvent was evaporated, and the crude product was dissolved in chloroform and washed with water (2 × 50 mL) and a satd. NH₄Cl solution (2 × 50 mL). The separated organic layer was dried over MgSO₄, filtered and concentrated under vacuum. After purification by column chromatography (silica gel, pentane/DCM/MeOH: 1.5 : 1 : 0.2, R_f = 0.45), the product was isolated as a white solid (0.082 g, 41% yield). Tetrakis(4-cyanophenyl)adamantane (L²) was obtained (Scheme S2†) in good yield (80%) starting from 1,3,5,7-tetrakis(4-iodophenyl)adamantane applying the procedure described in the literature.⁸

Synthesis of 1: a methanol solution (4 mL) of Cu(BF₄)₂·6H₂O (0.0024 g, 0.007 mmol) was reacted with 0.0018 g (0.029 mmol) of monoethanolamine in 4 mL of methanol and the resulting mixture was left to slowly diffuse through an ethanol layer (5 mL) into a 5 mL CHCl₃ solution of L¹ (0.0019 g, 0.0025 mmol). Blue green single crystals suitable for X-ray measurements appeared in two–three weeks. Yield (based on L¹): 0.0013 g, 46%. Selected IR data (KBr): 3315 (m), 3068 (w), 3033 (w), 2929 (w), 2897 (w), 2849 (w), 1664 (s), 1614 (vs), 1542 (m), 1493 (s), 1431(m), 1401 (w), 1356 (w), 1295 (w), 1236 (w), 1123 (s), 1073 (s), 1028 (s), 1007 (s), 1025 (m), 825 (w), 792 (s), 739 (w), 667 (w), 637 (w).

Synthesis of 2: [Mn(hfac)₂(H₂O)₂]·H₂O (0.0038 g, 0.0072 mmol) was solubilized in 4 mL of boiling n-heptane. Upon cooling to room temperature, 4 mL of CH₂Cl₂ was added and the solution was carefully layered on top of a 2 mL CH₂Cl₂ solution of L² (0.002 g, 0.0037 mmol) in a test tube. Yellow single crystals were formed in a few days. Yield (based on L²): 0.0015 g, 28%. Selected IR data (KBr): 3401 (m), 2939 (w), 2858 (w), 2262 (m), 2224 (m), 1647 (s), 1606 (w), 1523 (w), 1493 (s), 1405 (w), 1369 (w), 1252 (s), 1196 (s), 1149 (vs), 1096 (w), 836 (w), 794 (w), 764 (w), 665 (w).

§ Crystal data for [CuL¹(H₂O)₂](BF₄)₂·8H₂O (1): C₅₄H₆₄CuB₂F₈N₄O₁₀, M = 1166.26, T = 293(2) K, tetragonal, space group I422, a = 12.8199(9), c = 32.617(4) Å, V = 5360.5(1) ų, D_c = 1.310 g cm⁻³, μ = 1.218 mm⁻¹, Z = 4, Flack parameter = 0.42(7), reflections collected = 6049, independent reflection = 1932 [R_int = 0.0566]. Before SQUEEZE: R indices [I > 2σ(I)]: R₁ = 0.1033, wR₂ = 0.2671, R indices (all data): R₁ = 0.1507, wR₂ = 0.3199, GoF = 1.086 with Δρ_min/Δρ_max = −0.244/1.258 e Å⁻³. After SQUEEZE: R indices [I > 2σ(I)]: R₁ = 0.0750, wR₂ = 0.1876, R indices (all data): R₁ = 0.1138, wR₂ = 0.2237, GoF = 0.938 with Δρ_min/Δρ_max = −0.216/0.939 e Å⁻³.

Crystal data for [Mn(hfac)₂(L²)_0.5] (2): C₂₉H₁₆MnF₁₂N₂O₄, M = 739.38, T = 293(2) K, tetragonal, space group I4₁/a, a = 11.645(5), c = 52.700(5) Å, V = 7146(6) ų, D_c = 1.374 g cm⁻³, μ = 0.465 mm⁻¹, Z = 8, reflections collected = 32 305, independent reflection = 3122 [R_int = 0.2541]. R indices [I > 2σ(I)]: R₁ = 0.0604, wR₂ = 0.1342, R indices (all data): R₁ = 0.1674. wR₂ = 0.1807, GoF = 0.885 with Δρ_min/Δρ_max = −0.249/0.534 e Å⁻³.

---