Formation of a sandwich-type supercomplex through second-sphere coordination of functionalized macrocyclic polyamines

Abstract

A novel sandwich-type supercomplex, namely {[Er(TETA)]₂Er(H₂O)₈}NO₃·10H₂O (1), has been self-assembled through the conjoined hydrogen bonds by using a tetraaza-functionalized macrocyclic polyamine ligand, showing complete incorporation of the octahydrate lanthanide cation between the second coordination spheres.

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The rational design and construction of the second coordination sphere, referring to any non-covalent interactions with the ligands directly bound to the primary coordination sphere,¹ represent an active area of current research in supramolecular chemistry.² As well as the effective synthetic strategy for supramolecular assemblies,³ second-sphere coordination is also of major importance in biological systems.⁴ Crown ethers and macropolycyclic ethers have been widely employed as host receptors through hydrogen bonding or π–π interaction.⁵ Numerous studies of cyclodextrins and calixarenes coordinated to metal complexes using a hydrophobic cavity as second-sphere ligands have also been reported.⁶ More recently, Wisner et al. have prepared interlocked molecular compounds, such as rotaxanes and catenanes via second-sphere coordination, demonstrating its potential as an effective synthetic paradigm.⁷ However, macrocyclic polyamines or their functionalized derivatives, receiving increasing interest in the last two decades for their widespread applications in magnetic resonance imaging (MRI), luminescent probes and in radioimmunotherapy,⁸ have rarely been employed for such second-sphere coordination. On the other hand, the primary coordination sphere commonly consists of transitional metal complexes containing hydrogen-bonding donor groups (e.g. NH₃, H₂O); however, the application of aqua or amino lanthanide complexes as a primary coordination sphere has been studied to a much smaller degree.⁹ Herein, for the first time to the best of our knowledge, we present the complete incorporation of an octahydrate lanthanide cation within the second coordination sphere between two functionalized macrocyclic polyamines, forming a novel sandwich-type supercomplex through second-sphere coordination (see Fig. 1).

Fig. 1 Schematic representation of the formation of a sandwich-type structure through second-sphere coordination for ML₈ species.

A tetraaza-functionalized macrocyclic polyamine ligand, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (H₄TETA) was prepared according to a previously reported procedure.¹⁰ Two equivalents of Er(NO₃)₃·5H₂O were dissolved in deionized water to which one equiv of H₄TETA was added, then the mixture was refluxed at pH = 6 for 12 h. Slow evaporation of the solution yielded single crystals of the Er(III) supercomplex {[Er(TETA)]₂Er(H₂O)₈}NO₃·10H₂O (1), suitable for X-ray analysis after ∼6 weeks.‡

Single-crystal X-ray diffraction analysis§ reveals that compound 1 crystallizes in a primitive orthorhombic lattice with a chiral space groupP2(1)2(1)2(1). The asymmetric unit is composed of three crystallographically independent trivalent erbium centers, two complete deprotonated H₄TETA, one dissociative nitrate anion and eighteen water molecules, among which eight coordinate to the erbium center and the others exist as crystallized molecules. The local environments of the lanthanide ions, all of which are eight-coordinated, are illustrated in Fig. 2.

Fig. 2 ORTEP representation of the local environment of the eight-coordinate erbium motifs in complex 1, with the thermal ellipsoids at the 30% probability level. The H atoms are omitted for clarity.

The complete encapsulation of Er1 and Er2 by two TETA^4–macrocycles, each of which forming a tight anionic hemisphere {Er(TETA)}, generates the whole second coordination sphere. In both independent motifs, the erbium ion is coordinated to four nitrogen atoms and four oxygen atoms from the four monodentate carboxyl groups of the TETA^4– ligand (see Fig. 2a). The Er–O distances range from 2.275(3) to 2.305(4) Å for motif Er1 and from 2.281(4) to 2.292(3) Å for motif Er2. The Er–N distances range from 2.547(5) to 2.599(4) Å for Er1 and from 2.547(4) to 2.633(5) Å for Er2. These values are in agreement with those reported for other Er(III) macrocyclic polyamine complexes.¹¹ The resulting coordination polyhedron adopts a highly distorted dodecahedron geometry which can be deduced by calculating the least-squares equation of the mean planes of the nitrogen atoms and of the carboxylate oxygen atoms. Two trans-located nitrogen atoms (N1 and N3) are displaced by 0.281 Å from the mean nitrogen atoms plane, while the other pair lie on the opposite direction with the same offset. Similarly, the four oxygen atoms split into two groups with O1 and O5 above, O3 and O7 below the mean oxygen atoms plane. The remaining uncoordinated carboxylate oxygen atoms are just employed as hydrogen-bonding receptors in the second-sphere coordination.

For the {Er(H₂O)₈} cationic moiety, serving as the primary coordination sphere, the coordination geometry around the Er(III) centre is a slightly distorted square-antiprism. The erbium ion is bonded to eight water molecules, with Er(III)–O distances varying from 2.317(4) to 2.364(4) Å. As shown in Fig. 2b, four water oxygen atoms (O3W, O4W, O7W, and O8W) and the remainder form square planes that are nearly parallel to each other. The calculation of torsion angles between pairs of adjacent faces, ranging from 47.2 to 48.9°, indicates a slightly distorted square-antiprismatic geometry.

The octahydrate erbium cations are further encapsulated by two {Er(TETA)} anionic hemispheres through nine strong hydrogen bonds, forming a sandwich-type structure through second-sphere coordination. As illustrated in Fig. 3, the carboxylate oxygens from two TETA^4–macrocycle form nine O–H⋯O hydrogen bonds with the water molecules, with the O⋯O distances varying from 2.661 to 2.988 Å and the angles varying from 144.7 to 172.6°. In addition, electrostatic charge–charge interactions between the octahydrate metal cation and the anionic hemisphere stabilize the structure further. The {Er(H₂O)₈} cation of complex 1 is a rare example of aqua lanthanide complexes serving as primary coordination sphere.^5b

Fig. 3 View of the formation of the sandwich-type structure in complex 1, showing the hydrogen-bonding interactions. Lattice water molecules and dissociative nitrate anions are omitted for clarity.

The dissociative nitrate anions don't take part in the coordination but rather intersperse between the second sphere complexes in an efficient charge-compensating manner (see Fig. 4). Viewing along the b-axis, the second sphere motifs are arranged alternately in a column with the ‘naked’ nitrate centers. Comparing the orientation of these two moieties to the {Er(TETA)} hemisphere, it can obviously be concluded that the strong hydrogen bonding is the key factor to the formation of the structure. As illustrated in Fig. 5, the ten unique water molecules found in the crystallographic asymmetric unit are inserted between the layers through hydrogen-bonding interactions, which are integral to the structural stability of the crystals.

Fig. 4 View of compound 1 along the b-axis illustrating the interspersion of the nitrate anions between the second sphere motifs. Lattice water molecules and H atoms are omitted for clarity.

Fig. 5 Packing diagram of 1 showing the lattice water molecules inserted between the layers.

The thermogravimetric analysis (TGA) of compound 1 (see Fig. 6) reveals that a continuous weight loss of 18.1% occurs from 50 to 250 °C, which can be attributed to the loss of all the water molecules (calc. 18.6%). The release of crystallization guest water molecules and the subsequent slow release of coordinated water molecules demonstrate that strong hydrogen bonding is the key factor in the formation of the structure. The final mass loss, between 350 to 740 °C, corresponds to the decomposition of the supercomplex. TGA data show the enhanced stability of the primary hydration sphere through the secondary coordination sphere, which is a very significant value given that only weak interactions are involved.

Fig. 6 TGA diagram for compound 1.

To summarize, we have succeeded in constructing a novel sandwich-type supercomplex through second-sphere coordination by using a tetra-functionalized macrocyclic polyamine ligand. This work clearly illustrates that the concept of second-sphere coordination can be employed successfully as a synthetic strategy for building hydrogen-bonding materials. Furthermore, we have demonstrated a complete incorporation of an octahydrate lanthanide cation between the second coordination spheres comprised of two functionalized macrocyclic polyamines. Further studies, including the construction of analogues of supercomplex 1 using the same concept, are under current investigation in our laboratory.

Acknowledgements

This work was financially supported from 973 Program (2006CB932900/03), NSFC (90206040, 20325106, 20521101, 20333070), NSF of Fujian Province (2005HZ01-1, E0520003), Fujian Key Laboratory of Nanomaterials (2006L2005), “The Distinguished Oversea Scholar Project” and “One Hundred Talent Project” from CAS. We thank Dr Shoutian Zheng for TGA measurement and Qin Zhang for processing the figures.

Notes and references

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Footnotes

† CCDC reference number 651770. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b710633e

‡ Preparation of compound 1: Er(NO₃)₃·5H₂O (0.072 g, 0.2 mmol) was dissolved in deionized water to which H₄TETA (0.043 g, 0.10 mmol) had been added, then the mixture was refluxed at pH = 6 for 12 h. Slow evaporation of the solution at room temperature for ∼6 weeks yielded pink block crystals of compound 1. Yield: 45% (based on Er). Calc. for 1 {[Er(TETA)]₂Er(H₂O)₈}NO₃·10H₂O (1744.97): C 24.78, H 5.31, N 7.22. Found: C 24.26, H 5.12, N 7.38. IR (KBr, cm^–1): 3379 (br), 2975 (m), 2932 (m), 2876(m), 1599 (vs), 1435(m), 1401 (s), 1384 (vs), 1337 (s), 1301 (m), 1075 (m), 852(m), 721 (s), 605 (m).

§ Crystal data for {[Er(TETA)]₂Er(H₂O)₈}NO₃·10H₂O (1): M_r = 1744.97, orthorhombic, space groupP2(1)2(1)2(1), a = 17.9347(1), b = 18.1877(1), c = 18.3414(2) Å, U = 5982.80(8) ų, Z= 4, D_c = 1.937 g cm^–3, 32934 reflections measured, 11328 unique which are used in all calculations. The final R1 = 0.0274 and wR2 = 0.0522. Data collection was performed on a SIEMENS SMART CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at room temperature. All absorption corrections were performed using the SADABS program.¹² The structure was solved by direct methods and refined on F² by full-matrix least-squares using the SHELXTL–97 program package.¹³ All non-hydrogen atoms were treated anisotropically. The positions of the hydrogen atoms attached to carbon atoms were generated geometrically (C–H bond fixed at 0.99 Å). The Idealized positions of the H atoms of water molecules were located from Fourier difference maps and refined isotropically (O–H bond fixed at 0.85 Å).

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