Communication pubs.acs.org/Organometallics
A Ruthenium−Iron Bimetallic Supramolecular Cage with D4 Symmetry from a Tetrapyridyl Iron(I) Metalloligand Ji Yeon Ryu,†,§ Ji Min Lee,†,§ Yu Jin Park,† Nguyen Van Nghia,‡ Min Hyung Lee,*,‡ and Junseong Lee*,† †
Department of Chemistry, Chonnam National University, Gwangju 500−757, Korea Department of Chemistry and EHSRC, University of Ulsan, Ulsan 680−749, Korea
‡
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S Supporting Information *
ABSTRACT: The novel iron(I) sandwich compound [Cp*Fe(η4-C4Py4)] (1) was prepared and characterized by various methods, including X-ray crystallography. Coordination-driven self-assembly of a diruthenium acceptor with the tetrapyridyl metalloligand [Cp*Fe(I)(η4-C4Py4)] led to a D4-symmetric three-dimensional M4L2 tetragonal supramolecular cage. This cage was characterized by IR and high-resolution electrospray ionization mass spectrometry. Its solid-state structure was confirmed by X-ray crystallography, showing a novel D4 cage system.
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can form enantiomers of MMM and PPP.11 Although trigonal and tetragonal prisms containing tetratopic ligands based on cobalt complexes have been reported, their solid-state structures have not been confirmed by X-ray crystallography.10,12 To gain insight into the formation and structural aspects of tetragonal M4L2 cages, we designed a new tetragonal-prismatic supramolecule using a planar tetratopic metalloligand based on an iron complex and a molecular binder acceptor based on a diruthenium complex. It should be also noted that there are no examples of tetragonal prisms containing diruthenium molecular binders. Herein, we report on the synthesis and characterization of a novel M4L2 tetragonal-prismatic cage using a tetratopic pyridyl donor ligand containing iron (1) and a diruthenium acceptor (2). To access a tetratopic pyridyl star connector, we designed the iron(I) sandwich complex [Cp*Fe(η4-C4Py4)] (1). However, 17-electron iron(I) sandwich complexes of the type CpFeI(η4C4Ar4) are rare because of their instability. Wolf and co-workers have reported the diiron(I) sandwich complexes [Cp*Fe(μ,η4:η4-L)FeCp*], where L is naphthalene or anthracene,13 but the monoiron analogue was isolated as anionic Cp*Fe(η4C4Ph4), which contains zerovalent iron.14 Fortunately, 1 could be obtained from an analogous reaction applied to the synthesis of [CpCo(η4-C4Py4)] using Cp*Fe(CO)215 and 2 equiv of 1,2diethynylpyridine (Scheme 1). The reaction provided an improved yield (23%) in comparison to that of the [CpCo(η4-C4Py4)] complex (15%).10 Complex 1 was highly soluble in chlorinated solvents, as well as in nitromethane and DMF, but
upramolecular chemistry has provided a great contribution to the design and synthesis of various two-dimensional (2D) and three-dimensional (3D) nanoscale architectures.1 Directional noncovalent dative metal−ligand bonding has been considered as an efficient method for the construction of highdimensional well-defined abiological 3D architectures.2,3 3D cage complexes, including trigonal bipyramidal, adamentanoid, cuboctahedron, and dodecahedron, have been synthesized successfully by coordination-driven self-assembly.4 These cages are of particular interest because they have inner spaces that can be used in host−guest chemistry with appropriate functional guest molecules, which are very useful in many applications, including specific organic or organometallic reactions, optical sensors, and anticancer agents.5 The simplest and most abundant example of 3D assembly is the M3L2 trigonal-prismatic cage, where M represents mono- or dinuclear metal species. Therrien et al. prepared several hexanuclear arene−ruthenium prisms,5b,6 and similar hexanuclear prismatic cages are also known as active anticancer agents.5a Stang and co-workers have recently demonstrated the formation of trigonal-prismatic supramolecules containing metalloligands composed of octahedral aluminum and gallium centers.7 In comparison with trigonal-prismatic cages, however, there have been limited examples of tetragonal M4L2 cages.8 M4L2-type tetragonal cages are more robust than other structures, rendering them suitable for host−guest chemistry. Costas and Ribas recently reported a tetragonal-prismatic cage based on a palladium porphyrin donor and showed that it can be used as a selective host for small guest species.9 Tetrakis(4pyridyl)cyclobutadienecyclopentadienyl cobalt ([CpCo(η4C4Py4)])10 is also one of the interesting “star” connectors in the construction of tetragonal cages, because its trigonal prism © 2013 American Chemical Society
Received: November 25, 2013 Published: December 3, 2013 7272
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Communication
Scheme 1. Synthesis of Tetratopic Metalloligand 1a
a
Scheme 2. Coordination-Driven Self-Assembly of M4L2 Cage from Tetratopic Metalloligand 1 and Diruthenium Acceptor 2
Legend: (i) Reflux for 48 h in xylene, 23%.
insoluble in hexane. Recrystallization from an ethyl acetate/ diethyl ether mixture afforded an air- and moisture-stable yellow crystalline solid, which is suitable for X-ray crystallography. The unusually high stability of 1 might be attributed to the presence of a strong Cp* ligand.14 Although NMR spectroscopy was not informative due to its paramagnetic nature, the characterization of 1 was accomplished by IR and mass spectrometry. In addition, cyclic voltammetry measurements indicated that 1 undergoes one reversible oxidation at −1.38 V (vs Fc/Fc+), which is in a range similar to that observed for the [Cp*Fe(μ:η4:η4-L)FeCp*] complexes above, confirming the formation of a paramagnetic Fe(I) complex (Figure S2, Supporting Information).13 Finally, the solid-state structure of 1 was unequivocally determined by X-ray crystallography (Figure 1).16 The structure of 1
counterions [M − 2OTf]2+ (m/z 2115.9) and [M − 4OTf]3+ (m/z 1360.9), where M represents the intact assemblies (Figure 2).
Figure 2. Calculated (blue) and experimental (red) ESI-MS spectra of tetragonal-prismatic cage 3. Figure 1. X-ray crystal structure of tetrapyridyl iron donor 1. Color code: orange, Fe; blue, N. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−C3 2.005(3), Fe1−C1 2.013(3), Fe1−C2 2.014(3), Fe1−C4 2.015(3), Fe1−C25 2.130(4), Fe1−C26 2.102(3), Fe1−C27 2.110(4), Fe1−C28 2.151(3), Fe1− C29 2.168(4), C1−C2 1.469(4), C2−C3 1.466(4), C3−C4 1.466(4), C1−C4 1.457(4); C1−C2−C3 89.9(2), C2−C3−C4 89.8(2), C3− C4−C1 90.4(2), C4−C1−C2 90.0(2).
Furthermore, X-ray-quality single crystals of complex 3 could be grown from a methanol solution by the diffusion of diethyl ether.17 As shown in Figure 3, the two tetratopic ligands are arranged in a face-to-face fashion and coordinated to the four diruthenium acceptors to form a tetragonal-prismatic structure. The structure of 3 can be described as a rectangular parallelepiped having square top and bottom faces (height, ca. 10 Å; a side of the square, 14 Å). The size of the inside cavity, which can be estimated using eight Ru vertices, is about 450 Å3 (ca. 4.5 Å in height and ca. 10 Å in width). Interestingly, no electron density was observable in the inside cavity of the prism and the triflate counterions were located around the edge of the tetragonal prism despite the large space and highly charged state(8+) of the inner cavity. While the distance between the two Ru centers in the dimeric acceptors is ca. 5.5 Å, two cyclobutadiene rings are forced toward the inside cavity, affording a separation of ca. 4.6 Å. This feature indicates the existence of strain in the structure and might be responsible for the absence of any other species in the cavity. Complex 3 has a point group reduced from D4h to D4 due to the deviation of pyridine rings from the eclipsed conformation, generating a “double-rosette” type helicity.6c
resembles that of the reported cyclopentadienyl cobalt complex featuring η5 and η4 coordination of the Cp* and cyclobutadiene ligands, respectively, to the Fe(I) center.10 The tetrakis(4pyridyl)cyclobutadiene ligand has 4-fold rotation symmetry (C4), and the pyridyl groups are twisted in one direction. The dihedral angles between the cyclobutadiene and pyridine rings are found to be about 12°. In order to construct a tetragonal prism linked with the pyridyl star connector, the heterometallic tetragonal-prismatic cage 3 was prepared by the [2 + 4] self-assembly of metalloligand 1 with diruthenium acceptor 2,5,6 which has often been employed in 3D supramolecular cages, such as rectangles and trigonal prisms (Scheme 2). The formation of cage 3 was characterized by IR and mass spectrometry. The ESI/MS peaks of 3 are attributable to the loss of triflate 7273
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2010-0003141 for J.L. and 2012039773 for M.H.L.) and Priority Research Center Program (2009-0093818 for M.H.L.) through the National Research Foundation of Korea (NRF).
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Figure 3. X-ray structure of the tetragonal-prismatic cage 3 (50% ellipsoids): side view (top) and top view (bottom). Color code: green, Ru; red, O; blue, N. H atoms, isopropyl and methyl groups of pcymene, and counteranions are omitted for clarity.
We have prepared and characterized the 3D tetragonalprismatic cage 3 using tetratopic pyridyl donors as a modular subunit and flexible diruthenium complexes as an acceptor by means of coordination-driven self-assembly. The crystal structure of the bimetallic M4L2 complex 3 showed the formation of a tetragonal-prismatic cage composed of the two tetratopic ligands as a face and the four diruthenium acceptors as a side of the prism. The complex constitutes a novel example of a 3D tetragonal prismatic cage having tetrapyridyl metalloligands. Studies of the host−guest chemistry of cage 3 with small aromatic systems are in progress.
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ASSOCIATED CONTENT
S Supporting Information *
Text and figures giving experimental details with a full-range mass spectrum of 3 and cyclic voltammogram of 1 and a table and CIF files giving crystallographic data for complexes 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.H.L.:
[email protected]. *E-mail for J.L.:
[email protected]. Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest. 7274
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