Self-assembly of Ru<sub>4</sub> and Ru<sub>8</sub> assemblies by coordination using organometallic Ru(II)<sub>2</sub> precursors: Synthesis, characterization and properties
Coordination-driven self-assembly of binuclear half-sandwich p-cymene ruthenium(II) complexes [Ru 2 (μ-η 4 1b) separately with an imidazole-based tetratopic donor L in methanol affords two tetranuclear metallamacrocycles 2a and 2b, respectively. Conversely, the similar combination of L with 2,5-dihydroxy-1,4-benzoquinonato (dhbq 1c) in 1:2 molar ratio resulted in an octanuclear macrocyclic cage 2c. All the selfassembled macrocycles 2a-2c were isolated as their triflate salts in high yields and were characterized fully by multinuclear ( 1 H, 13 C and 19 F) NMR, infrared (IR) and electrospray ionization mass spectrometry (ESIMS). In addition, the molecular structure of macrocycle 2a was established unequivocally by single-crystal X-ray diffraction analysis and adopts a tetranuclear rectangular geometry with the dimensions of 5.53 Å × 12.39 Å. Furthermore, the photo-and electrochemical properties of these newly synthesized assemblies have been studied by using UV-vis absorption and cyclic voltammetry analysis.
Introduction<!>Synthesis and characterization of the tetranuclear complexes<!>Synthesis and characterization of the octanuclear cage<!>UV-vis absorption and electrochemical studies<!>Conclusion<!>Experimental Materials and methods
<p>Self-assembly of metal-based molecular architectures through coordination has emerged as an active field of research as large numbers of intricate structural motifs can easily be derived in a single step from predesigned molecular building units. In the last two decades several interesting molecular architectures of defined shapes, sizes and functionality have been discovered with the aid of this powerful protocol [1,2]. The basic requirement for successful metal-ligand-directed self-assembly resides Scheme 1: Possible octanuclear prism and tetranuclear macrocycle resulting from the combination of a tetradendate donor with a clip-type acceptor unit.</p><p>in the judicious designing of information-encoding molecular building blocks having complementary binding sites. Large numbers of molecular polygons and polyhedra have been synthesized mostly from cis-blocked Pd(II)-and Pt(II)-based building units due to their rigid square-planar coordination geometry as well as to their interesting photophysical properties [3][4][5]. However, we and others have recently reported several shape-persistent discrete macrocycles/cages using halfsandwich octanuclear Ru(II) piano-stool complexes in combination with polypyridyl or polycarboxylate donors [6][7][8][9][10]. Ligands with an imidazole functionality are of considerable interest due to the presence of imidazole in many biological systems, and also the better donor character of imidazole compared to widely used pyridyl donors. Owing to these properties, we were interested in incorporating an imidazole functionality into discrete molecules of defined shapes, and also to investigate their influence in terms of directionality in controlling the geometry of the resulting molecular architectures. Herein, we report the selfassembly reactions of [Ru 2 (μ-η 4   13 C and 19 F) NMR, IR and ESIMS analyses. The molecular structure of tetranuclear macrocycle 2a was determined by single-crystal X-ray diffraction analysis, which reveals a tetranuclear geometry with the dimensions of 5.53 Å × 12.39 Å. In addition to their synthesis and characterization, the UV-vis absorption and cyclic voltammetry studies are reported.</p><!><p>According to the "directional-bonding approach" and "symmetry-interaction model", one can expect the formation of either a [4 + 2] assembled octanuclear tetragonal prism or a [2 + 1] assembled tetranuclear 2D structure upon reaction of a binuclear "clip"-type acceptor and a tetratopic donor (Scheme 1) [11].</p><p>Instead, the exclusive formation of two tetranuclear assemblies (2a, 2b) upon mixing of binuclear Ru(II) 2 acceptors 1a or 1b with an imidazole-based tetratopic donor L is interesting. The exclusive self-sorting of tetranuclear macrocycles (2a and 2b) rather than the expected octanuclear molecular cage is, presumably, due to the perfect matching of the distance between two Ru(II) metal centers with the distance between the adjacent imidazole donor sites in L. Moreover, from an entropic point of view such [2 + 1] self-assembly is expected to be preferred over [4 + 2] assembly as the loss of entropy in the latter case is more. As shown in Scheme 2, the binuclear acceptor 1a or 1b was treated separately with an imidazole-based tetratopic donor L in a 2:1 molar ratio in methanol at room temperature to obtain [2 + 1] self-assembled macrocycles 2a and 2b, respectively, in high yields. The addition of solid L into a methanolic solution of the acceptor (1a or 1b) lead to the immediate consumption of the solid and showed a sharp visible color change from light yellow to intense yellow. Both 2a and 2b are isolated as their triflate salts and are highly soluble in common organic solvents such as (CH 3 ) 2 CO, CH 3 CN, CH 3 OH and CH 2 Cl 2 . The formation of the complexes (2a and 2b) was initially characterized by IR, multinuclear ( 1 H, 13 C and 19 F) NMR and ESIMS spectrometric analyses.</p><p>The IR spectra (Figure S1, Supporting Information File 1) of the macrocycles showed strong bands at 1624.6 cm −1 (2a) and Scheme 2: Formation of the tetranuclear complexes (2a and 2b) upon the reaction of Ru(II) 2 based acceptors 1a or 1b and tetraimidazole ligand L in methanol at room temperature. 1602 cm −1 (2b) corresponding to the symmetrical stretching frequencies (ν CO ) of the carbonyl groups of the coordinated bisbidentate oxalato (2a) or N,N'-diphenyloxamidato (2b) ligands. These bands, due to symmetrical stretching (ν CO ) in the complexes, are slightly shifted to a higher energy region compared to those of the starting acceptors (1a, ν CO = 1623.7 cm −1 ; 1b, ν CO = 1605.0 cm −1 ) due to the ligand-tometal coordination. The appearance of a single peak in the 19 F NMR spectrum at −81.03 ppm (2a) or −79.8 ppm (2b) indicated the presence of triflate counter anions in the same chemical environment in the resultant complexes (Figure 1 and Figure S3, Supporting Information File 1).</p><p>Four peaks correspond to the donor L in the aromatic region (δ 6.95-8.10 ppm) were found in the 1 H NMR spectra of 2a and 2b. Protons corresponding to the capped p-cymene moiety appeared in the range of δ 5.14-5.81 ppm. Notably, the 1 H signals of the protons of L exhibited a significant downfield shift compared to the unbound linker due to the loss of electron density upon ligand-to-metal coordination (Figure 1 and Figure S2, Supporting Information File 1). The high solubility of the assemblies in common organic solvents and the symmetric nature of 1 H NMR spectra primarily ruled out the formation of a polymer with extended coordination. Although the initial characterization of the self-assembled species by NMR spec- troscopy intimated the ligand-to-metal coordination, it does not provide any information about the exact composition and nuclearity of the final products.</p><p>Electrospray ionization mass spectrometry (ESIMS) can be used as a soft-ionization method to elucidate the exact composition of the product [12]. The ESIMS analysis confirmed the formation of the rather unexpected [2 + 1] self-assembled tetranuclear macrocycles 2a and 2b by the appearance of multiply charged fragmented ions (Figure 2 and Figure S4, Supporting Information File 1). The multiply charged ions for 2a at m/z = 1906.9</p><p>were observed and these peaks were well resolved isotopically (Figure 2 and Figure S4, Supporting Information File 1). The appearance of the expected peaks along with the isotopic patterns was in strong support of the formation of [2 + 1] selfassembled products in both cases.</p><p>Finally, the molecular structure of the tetranuclear metallamacrocycle [(p-cymene) 4 Ru 4 (μ-η 4 -oxalato) 2 (L)] 4+ (O 3 SCF 3 − ) 4</p><p>(2a) was unambiguously determined by single-crystal X-ray diffraction analysis. Single crystals of 2a of high enough quality for X-ray diffraction were grown by slow vapor diffusion of diethyl ether into a methanolic solution of 2a at room temperature. Macrocycle 2a was crystallized in a tetragonal crystal system with I41/a space group having sixteen formula units per unit cell. A perspective view of the macrocycle 2a is depicted in Figure 3 with atom numbering, and its selected bond parameters are summarized in Table S1, Supporting Information File 1.</p><!><p>The construction of an octanuclear macrocyclic cage 2c was accomplished following a similar procedure as adopted for the synthesis of tetranuclear rectangular complexes 2a and 2b. Drop-wise mixing of a methanolic solution of binuclear acceptor 1c into a suspension of tetradendate linker L in methanol in 2:1 molar ratio afforded the exclusive formation of the [4 + 2] self-assembled macrocyclic cage 2c in good yield after 24 h of stirring at room temperature (Scheme 3). The obvious reason for the formation of an octanuclear macrocyclic cage 2c in contrast to the tetranuclear rectangles 2a and 2b is attributed to the increased length of the acceptor unit 1c compared to 1a or 1b. The immediate consumption of the suspended donor L in the clear solution and the observable color change from purple to deep red indicated the progress of the self-assembly reaction. The initial characterization of the isolated macrocycle by multinuclear ( 1 H, 13 C and 19 F) NMR suggested the formation of a single and symmetric macrocyclic complex.</p><p>The infrared spectra (Figure S1, Supporting Information File 1) of the macrocycle (2c) showed a strong band at 1521.2 cm −1 corresponding to the symmetric stretching frequency (ν CO ) of the carbonyl groups of the coordinated bis-bidentate quinonato ligand, and this symmetric stretching frequency (ν CO ) in the isolated macrocycle is slightly shifted to the higher energy region compared to that of the starting acceptor (1c, ν CO =  1515.8 cm −1 ) due to the ligand-to-metal coordination. The 1 H NMR spectra of the macrocycle 2c established the expected peaks in the aromatic region corresponding to the tetratopic donor L and the proton resonance of capped p-cymene ligand. Moreover, the peak of L in the macrocyclic complex exhibits a significant downfield shift due to the ligand-metal coordination (Figure 4). The assignment of the proton signals in Unfortunately, all efforts to obtain diffraction-quality single crystals of the macrocycle 2c have so far been unsuccessful. However, the analysis of multinuclear NMR ( 1 H, 13 C and 19 F) in concurrence with the ESIMS study is fully consistent with the formation of a [4 + 2] self-assembled octanuclear metallacage. In view toward gaining further insight into the structural characteristics of the newly designed macrocycle, the energyminimized structure of the macrocycle 2c was obtained by means of molecular mechanics universal-force-field simulation (MMUFF) [13]. A perspective view of the energy-minimized structure of the macrocycle 2c is depicted in Figure 5. The optimized structures of the macrocycle 2c suggested the formation  of an octanuclear macrocycle having an internal diameter of 1.54 nm. Notably, the quinonato-bridged ruthenium clips are tilted out of the plane of the imidazole donor in 2c in order to minimize the steric influence between the acceptor clips. Therefore, macrocycle 2c adopts a staggered conformation as was observed in similar types of 3D cages [14].</p><!><p>One of the strongest driving factors to design multifarious functional supramolecular complexes is attributed to their interesting functional properties. In general, the utilization of functional properties would be realized more from macrocycles containing transition metals, due to their high sensitivity towards various external stimuli, compared to purely organic macrocycles. In this regard, substantial efforts have been devoted to the design of novel, nanoscopic and functional metallamacrocycles and also to the study of their functional properties [15]. The photo-and electrochemical properties of our newly synthesized macrocycles (2a and 2b) are studied by UV-vis absorption and cyclic voltammetry investigations and the obtained results are summarized in Table 1.</p><p>The UV-vis absorption spectra were recorded from a CH 3 CN solution (1.0 × 10 −5 M) of the macrocycles at room temperature. The electronic absorption spectra (Figure 6) of the macrocycles 2a-2c exhibit intense bands at λ max (ε) 309 nm (2.3 × 10 4 M −1 cm −1 ), 383 nm (6.8 × 10 3 M −1 cm −1 ) for 2a; λ max (ε) = 328 nm (5.8 × 10 4 M −1 cm −1 ) for 2b and λ max (ε) = 293 nm (6.3 × 10 4 M −1 cm −1 ), 499 nm (3.2 × 10 4 M −1 cm −1 ) for 2c. The peaks in the ranges of 293-309 nm and 328-499 nm can be ascribed to both intra-and intermolecular π-π* and metal-toligand charge-transfer transitions associated with the capped p-cymene-ruthenium moiety, respectively. The electrochemical behavior of the macrocycles (2a-2c) was examined by using a Pt-disk electrode in dichloromethane with 0.1 M n-Bu 4 NPF 6 as the supporting electrolyte and at 100 mV s −1 scan rate. The obtained redox responses of the macrocycles were found to be entirely different. Macrocycle 2c shows three well-anchored quasi-reversible reductions at −0.39, −1.09 and −1.54 V and their corresponding anodic peak current at −0.16, −0.81 and −1.38 V (Figure 6). On the other hand, complex 2b showed a single quasi-reversible redox response of the cathodic peak current at −1.53 V and the anodic peak counterpart potential approximately at −1.24 V (Figure S7, Supporting Information File 1). Surprisingly, macrocycle 2a showed no anodic or cathodic peak current intensity within the range of +0.2 to −0.2 V, which implies the electrochemical inertness of 2a (Figure S7, Supporting Information File 1). The observed anodic and cathodic peak currents of 2c are almost equal, indicating the high chemical stability on the time scale of the voltammetry experiments. Notably, the cathodic peak current intensity of 2b is much higher than its anodic peak current, and this can be attributed to the deposition of macrocycles on the electrode surface. Based on the electrochemical activity of other reported ruthenium complexes, the observed cathodic peaks are roughly assigned to one-electron reductions of the bridged quinonato or diphenyloxamidato ligands, and the oxidation is attributed to the Ru(II)/Ru(III) redox couples [16,17].</p><!><p>In conclusion, we synthesized two new tetranuclear macrocycles 2a, 2b and an octanuclear molecular prism 2c, by coordination-driven self-assembly of an imidazole-based tetratopic donor linker L in combination with Ru(II)-based binuclear "clip" acceptors. All three of the self-assembled macrocycles were fully characterized by multinuclear ( 1 H, 13 C and 19 F) NMR, IR and ESIMS spectroscopic studies. In addition, the formation of a tetranuclear structure of 2a was unambiguously established by analysis of single-crystal X-ray diffraction data. Though the formation of discrete structures from pyridyl donors has been well established, the quantitative formation of 2a, 2b and 2c from an imidazole-type donor is new and suggests the feasibility of using imidazole derivatives as potential building units. The lengths of 1a and 1b fit well with the distance between two imidazole units in L to form the entropically favored [2 + 1] self-assembled structures 2a and 2b, respectively. As the distance between the two Ru(II) in 1c is greater compared to the distance between the neighboring imidazole units in L, a similar reaction with 1c allowed the formation of the 3D cage 2c. The use imidazole-based donor linkers in combination with organometallic half-sandwich ruthenium(II) precursors may generate a wide variety of complex molecular architectures with interesting functional properties.</p><!><p>The acceptor clips</p><p>were synthesized under a dry nitrogen atmosphere by means of a standard Schlenk technique following the reported procedures [18][19][20]. The solvents were dried and distilled according to the standard literature procedures. 1,2,4,5-tetrabromobenzene and α-phellan-drene were purchased from commercial sources and used without further purification. 1,2,4,5-Tetrakis(imidazol-1yl)benzene (L) was synthesized by following the reported procedure [21]. NMR spectra were recorded on a Bruker 400 MHz spectrometer. The chemical shifts (δ) in the 1 H NMR spectra are reported in ppm relative to tetramethylsilane (Me 4 Si) as the internal standard (0.0 ppm) or the proton resonance resulting from incomplete deuteration of the NMR solvents: CD X-ray-quality crystals were mounted on a glass fiber with traces of viscous oil. Intensity data were collected by using graphitemonochromatized Mo Kα radiation (0.7107 Å) at 150 K. The structures were solved by direct methods using SHELX-97 incorporated in WinGX [23][24][25][26][27]. Empirical absorption correc-tions were applied with SADABS [25]. All nonhydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were assigned isotropic displacement coefficients, U(H) = 1.2U(C) or 1.5U (C-methyl), and their coordinates were allowed to ride on their respective carbons.</p><p>CCDC-845980 (for complex 2a) contains the supplementary crystallographic data reported in this paper. The data can be obtained free of charge from the Cambridge Crystallographic Data Center via http://www.ccdc.cam.ac.uk/data_request/cif.</p>