Article pubs.acs.org/Organometallics
A Molecular Triangle as a Precursor Toward the Assembly of a JarShaped Metallasupramolecule Shih-Ming Lin,†,‡,§ Murugesan Velayudham,†,§ Chi-Hwe Tsai,†,‡ Che-Hao Chang,† Chung-Chou Lee,† Tzuoo-Tsair Luo,† Pounraj Thanasekaran,† and Kuang-Lieh Lu*,† †
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan Department of Chemistry, National Central University, Chungli 320, Taiwan
‡
S Supporting Information *
ABSTRACT: The reaction of Re2(CO)10 and 1,1′-carbonyldiimidazole in toluene afforded the molecular triangle [Re3(μ2Im)3(CO)12] (1; Im = imidazolate). This air-stable complex 1 acted as a precursor, which could then be further transformed into the complex [{Re(CO)3}3(μ2-Im)3(μ3-L)] (2; L = 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene) upon reaction with the flexible ligand L under solvothermal conditions. Complex 2 can also be produced directly in a one-pot reaction from Re2(CO)10, 1,1′-carbonyldiimidazole, and the flexible ligand L. A single-crystal X-ray diffraction analysis showed that compound 1 has a triangular-shaped structure, which is the smallest rhenium triangle known, as of this writing. Complex 2 adopted a jarshaped structure. The photophysical properties of complexes 1 and 2 were studied.
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triplatinum compounds.4a,5 Although self-assembly reactions directed toward the preparation of metallacycles have been extensively explored in the past two decades, reports on the characterization of reaction intermediates for elucidating the pathways of the assembly reaction are very limited, and the case of identification of such an intermediate is even more rare.9 We report herein on the structure and transformation of the unique molecular triangle [Re3(μ2-Im)3(CO)12] (1), which was found to be a precursor to the formation of the jar-shaped metallacycle 2 in the presence of the flexible tripodal ligand 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene.
he controlled buildup of discrete metallacycles having nanodimensions by the self-assembly of simple components has been an active area in the rapidly expanding field of supramolecular chemistry.1−3 Among the structurally appealing metallacyclic compounds, the molecular triangle is one of the simplest forms of polygons.4,5 In sharp contrast to many examples of metallacycles reported in the past, molecular triangles are much less common, mainly due to the rarity of suitable building blocks with proper tuning angles. In fact, there are two examples of rhenium triangles that were reported independently by the Coogan6 and Massi7 groups. The complexes were produced in a three-step synthesis by the sequential exchange of two carbonyl ligands and one chloro ligand with a terpyridine and a one-step synthesis promoted by the anionic 2-pyridyltetrazolate, respectively. A neutral trinuclear Re(I)-based heteroleptic metallacycle, [( fac-Re(CO)3)3(L′)3(L)], featuring three tritopic metal acceptors, three benzimidazolates (L′), and one 1,3,5-tris(benzimidazol-1ylmethyl)-2,4,6-trimethylbenzene (L), was recently prepared in a one-pot synthesis using an orthogonal bonding approach and structurally characterized in the solid state.8 Other examples of transition-metal-based triangles include tripalladium compounds having bridged 4(3H)-pyrimidone or 4,4′-bipyridine units and chiral functionalized or cucurbituril-decorated © 2013 American Chemical Society
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RESULTS AND DISCUSSION The yellow molecular triangle [Re3(μ2-Im)3(CO)12] (1; Im = imidazolate) was obtained in good yield by reacting Re2(CO)10 and 1,1′-carbonyldiimidazole in toluene under solvothermal conditions. In the reaction, it appears that 1,1′-carbonyldiimidazole in toluene at 160 °C decomposes to give an imidazolate, which then coordinates with Re(I) to form the triangle 1 (Scheme 1). The molecular triangle 1 is stable in air and soluble in organic solvents. Interestingly, in the presence of Received: June 2, 2013 Published: December 10, 2013 40
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Article
Scheme 1. Self-Assembly of Molecular Triangle 1 and the Jar-Shaped Metallacycle 2
Figure 1. ORTEP diagram of 1 with thermal ellipsoids set at 30% probability and a space-filling diagram. Hydrogen atoms are omitted for clarity.
the flexible ligand 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6trimethylbenzene (L), triangle 1 was found to be a reaction precursor, which after being formed couples with the flexible ligand L to form the jar-shaped metallacycle [{Re(CO)3}3(μ2Im)3(μ3-L)] (2; L = 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6trimethylbenzene) under solvothermal conditions (Scheme 1). The preparation of 1 confirms that the reaction of 1,1′carbonyldiimidazole takes place with the rhenium precursor at an early stage of the reaction, which was transformed to imidazolate and stabilized by the structure 1. The latter molecular triangle then reacts with L to produce the jar-shaped product 2. Alternatively, the metallacycle 2 could be directly assembled in high yield from Re2(CO)10, 1,1′-carbonyldiimidazole, and L in a one-step process under similar reaction conditions (Scheme 1). Compounds 1 and 2 were structurally characterized by various spectroscopic methods, and their structures were further confirmed by single-crystal X-ray diffraction analyses. The 1H NMR data for 1 showed that, upon coordination of the Im ligand with the Re metal center, the Hb,c and Ha hydrogen atoms lose more electron density as a result of being adjacent to donor N atoms and their signals are shifted to 7.09 and 6.41 ppm, respectively. The Ha signal for Im in 1 is affected more than that of Hb,c, indicating that Ha is more shielded through the ring current once compound 1 is formed. FAB mass spectral data for 1 yielded a peak at m/z 1096.9 [M+], implying
that compound 1 has a triangular structure. This conclusion was further unambiguously confirmed by an single-crystal X-ray diffraction analysis (vide infra). In compound 2, the chemical shifts for the H4−7 proton signals were shifted downfield, whereas H2, H8, and methyl proton signals were shifted upfield in comparison to those of the free ligand L. A remarkable change was observed for the H2 proton signal for L in 2, which was shifted upfield by 1.99 ppm due to the ring current effect, indicating that the Bim rings in L retained the face-to-face arrangement with a head-to-head syn conformation.10 The chemical resonances for the Ha and Hb,c protons of the Im ligand in 2 were observed at δ 8.80 and 5.94 ppm, respectively. A large downfield shift (Δδ = 2.39 ppm) was observed for the Ha proton signal in 2, due to the loss in electron density of the imidazolyl nitrogen in the L ligand upon coordination. On the other hand, the Hb,c proton signal in 2 was shifted upfield, implying that the proton is located in close proximity to the π cloud of the aromatic ring of L. These results show that in solution compounds 1 and 2 retain the same structure as those found in the solid state (vide infra). The FAB mass spectrum of 2 showed a molecular ion peak at m/z 1522.5 [M+]. The ORTEP diagram of 1 (Figure 1) showed that it adopts a structure consistent with that of an isosceles triangular structure with alternating Re(CO)4 vertices and imidazole edges. It is likely that the imidazolate ligands in 1 are produced via the disintegration of 1,1′-carbonyldiimidazole. All of the rhenium 41
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Figure 2. ORTEP diagram of 2: side view of the jar (left); top view (right). Thermal ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.
octahedral geometries with an N3C3 donor environment. Each pair of rhenium atoms is bridged by an imidazolate ligand. Moreover, the tridentate L ligand adopts an all-syn conformation,10 with three benzimidazole arms on the same side, and coordinates to three rhenium centers. The trimethylbenzene moiety of L in 2 forms the bottom of the jar with a diameter of 5.83 Å, and the imidazolate ligand covers the sides of the jar. The three rhenium centers bridged by imidazolate form the neck of the jar. The height of the jar, from the centroid of the trimethylbenzene unit of L at the bottom to the centroid of the top, is 5.40 Å. The average distance between the centroid of imidazolate and the diagonally opposite methylene carbon of the benzimidazole moiety in L is 6.18 Å. Compound 2 has crystallographic mirror symmetry and, therefore, has only two different Re···Re distances of 6.34 and 6.38 Å. Thermogravimetric analysis data showed that compounds 1 and 2, in the solid state, are stable up to a temperature of 400 °C, and a representative TGA curve of 2 is shown in Figure S2 in the Supporting Information. The absorption and emission spectra of the free ligands Im and L and compounds 1 and 2 were recorded under ambient conditions, and the data are shown in Table 1. Complex 1 showed a strong band at 2σ(I)) and wR2 = 0.2349 (all data)). These calculations indicate the presence of 373 e/unit cell. This may be attributed to one toluene (50 e) and one pyridine molecule (42 e) per formula unit. Experimental details for Xray data collection and the refinements are summarized in Table S1 (Supporting Information). CCDC-826935 (1), CCDC-826936 (2), and CCDC-826937 (2squeezed·C7H8·C5H5N) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www. ccdc.cam.ac.uk/data_request/cif.
Supporting Information). The emission spectrum of 1, on excitation at 318 nm, displayed a vibronically structured emission centered at λmaxem 368 nm with additional peaks (Figure 3) and showed a biexponential decay pattern with
Figure 3. Emission spectra of 1 and 2 in CH2Cl2 and DMF, respectively.
lifetime values of 1.20 ns (73.1%) and 4.01 ns (26.9%) at 368 nm. An inspection of the absorption spectrum of 2 (Figure S5 in the Supporting Information) indicates the presence of an intense absorption band in the UV region due to not only the π−π* transition associated with the ligands but also the overlapping MLCT transitions directed toward the Im and L ligands, which account for the significantly increased absorption coefficient.13 Upon excitation, compound 2 exhibited emission bands at 314 and 450 nm. The emission decay at 314 nm consisted mainly of 1.18 ns with a 100% intensity fraction (Figure S7 in the Supporting Information), whereas that at 450 nm lacked the nanosecond decay components, indicating the presence of two different excited states in 2. Hence, the observation of a small Stokes shift with short τ values in 1 and 2 indicates that emission is predominantly of 1π−π* character, although a small contribution from the MLCT state may be possible.14
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CONCLUSIONS The structural characterization and transformation of the unique molecular triangle 1 is described. The air-stable triangular compound 1 was found to be a reaction precursor, which could be further transformed to form the jar-shaped complex 2 in the presence of the flexible tripodal ligand L. To the best of our knowledge, 1 is the smallest rhenium triangle reported to date. Although many self-assembly reactions resulting in the formation of metallacycles are known in the literature, the identification of a reaction precursor in a selfassembly reaction, as observed in this study, is extremely rare.
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EXPERIMENTAL SECTION
Reagents were used as received without further purification. The solvents used in this study were of spectroscopic grade. The ligand 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (L) was prepared by a reported procedure.10 IR spectra were recorded on a Perkin-Elmer 882 FT-IR spectrophotometer. 1H NMR spectra were recorded on a Bruker AV 400 NMR spectrometer. Elemental analyses were performed using a Perkin-Elmer 2400 CHN elemental analyzer. FAB-MS data were obtained using a JMS-700 double-focusing mass
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ASSOCIATED CONTENT
S Supporting Information *
Tables, figures, and CIF files giving experimental details for Xray data collection and the refinements, X-ray crystallographic 43
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data of 1 and 2, IR spectra of 1 and 2, the TGA curve of 2, and absorption, emission, and excitation spectra of ligands and metallacycles. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for K.-L.L.:
[email protected]. Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the Academia Sinica and the National Science Council of Taiwan for financial support. We also express our gratitude to Mr. Ting-Shen Kuo, National Taiwan Normal University, for assistance with the X-ray structure analysis.
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