Organometallic Chemistry in Directed Assembly

6.12

Organometallic Chemistry in Directed Assembly

D Stı´bal and B Therrien, University of Neuchatel, Neuchatel, Switzerland Ó 2017 Elsevier Ltd. All rights reserved.

6.12.1 6.12.2 6.12.3 6.12.4 6.12.5 6.12.6 6.12.7 6.12.8 References

6.12.1

Introduction Ferrocenyl and Sandwich Complexes Metal Carbonyl Complexes Cyanometallates Metal Carbene Complexes h5-Half-Sandwich Complexes h6-Half-Sandwich Complexes Conclusion

305 305 308 316 318 324 325 327 327

Introduction

Organometallic chemistry, like all other fields in chemistry, has reached a level of sophistication that was unthinkable 65 years ago when ferrocene was discovered.1 The complexity of organometallic compounds has progressed in parallel to the techniques available for their characterization, and nowadays applications are driving the field. While catalysis and the development of new organometallic catalysts remain a major research domain in organometallic chemistry,2 the contribution of synthetic organometallic compounds in biology3 and in supramolecular chemistry4 is growing rapidly. As emphasized in this article, organometallic building blocks have been used to prepare discrete metalla-assemblies, and to generate supramolecular systems with various applications in mind. The organometallic entities can either be found at the vertices, on the edges, on the faces, or simply being covalently linked to the metal-containing supramolecular assembly (Fig. 1). At the beginning, most metal-containing assemblies were prepared for basically aesthetical reasons, having in mind the generation of discrete objects.5 However, the concept of making devices for specific applications rapidly drove the field. These functional metal-containing assemblies have been used as sensors,6 as carriers,7 as containers,8 or as molecular flasks.9 And logically, the next step was to generate molecular machinery.10 Molecular machinery incorporating metallic entities remains in its infancy, but offers great perspectives.11 The use of organometallic complexes to prepare directed assemblies is following the same pathway. In this article, to keep the same logic, we have regrouped the different organometallic assemblies by ligands, thus providing for each family, a separate progression of the research, which often starts from purely aesthetical assemblies and finishes with applications. Therefore, we are covering the following families of organometallic complexes (Fig. 2): ferrocenyl derivatives (including sandwich complexes), carbonyl complexes, cyanometallates, carbenes, h5-half-sandwich, and h6-half-sandwich complexes. These organometallic entities are certainly the most common derivatives, and to allow a comprehensive review of the field, we have limited our discussion to what we considered to be the most prominent examples. Given space constraints, it was not possible to include every example in the literature, and no offence is intended by any omission.

6.12.2

Ferrocenyl and Sandwich Complexes

Among organometallic complexes, ferrocene occupies a special place, having significantly contributed to the establishment of organometallic chemistry as a specific branch of chemistry. Still today, ferrocene remains one of the most popular building blocks, being highly stable and redox active, whilst the cyclopentadienyl rings are easily functionalized. Ferrocene offers endless possibilities for designing discrete assemblies.12

Figure 1

Positions of organometallic entities in metalla-assemblies.

Comprehensive Supramolecular Chemistry II, Volume 6

http://dx.doi.org/10.1016/B978-0-12-409547-2.12592-2

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Figure 2

Organometallic Chemistry in Directed Assembly

Families of organometallic complexes covered in this article.

To our knowledge, the first ferrocenyl-based assemblies were obtained from ferrocenyl-pyridyl derivatives, using the pyridyl moieties to form in combination with metal ions such as Ag(I), Cu(II), and Zn(II), hetero-bimetallic metalla-rectangles. For instance, 1,10 -(bispyridin-4-yl)ferrocene was coupled with silver to generate a 3.5  15.1 Å2 rectangle (Fig. 3), [{Fe(h5-C5H4-1C5H4N)2}2Ag2]2 þ.13 Analogous hetero-bimetallic assemblies were obtained by mixing 1,10 -bis{(3-pyridylamino)carbonyl}ferrocene (3-BPFA) and metal ions.14 Interestingly, the Ni(II) and Co(II) complexes show a bridging chloride anion between the two metal ions (Fig. 4), while the Cu(II) derivative possesses an empty cavity. Encapsulation of a chloride anion was crucial for the formation of the Ni(II) and Co(II) derivatives, suggesting a template effect. Following a similar strategy, a flexible bis-pyrazolato-ferrocenyl linker was used in combination with Cp2Ni2 entities (Cp ¼ cyclopentadienyl) to form a relatively spacious heteronuclear square, [Fc{(CH2)2-3,5-Me2PzNiCp}2]2.15 In the solid state, a toluene molecule was found to sit inside the cavity (Fig. 5), thus offering possible charge transfer p-interactions between guest and host. Analogous but larger hetero-bimetallic metalla-cycles were obtained by combining Fe(C5H4COOH)2 (1,10 -ferrocenedicarboxylic acid) and the dinuclear platinum-based complex, 2,9-bis[trans-Pt(PEt3)2]phenanthrene.16 Solvent molecules were found in the cavity of this redox active metalla-rectangle. The combination of dinuclear arene ruthenium clips and tetrapyridyl metallo-ligands {Cp*Fe(h4-C4Py4)} (Cp* ¼ pentamethylcyclopentadienyl) has generated a bimetallic octacationic cube of the general formula [{(p-cymene)2Ru2(m4-oxalato)}4{Cp*Fe(h4C4Py4)}2]8 þ.17 The single crystal X-ray structure shows a separation of only 4.6 Å between the two cyclobutadiene rings of the parallel {Cp*Fe(h4-C4Py4)} metallo-ligands, despite a Ru/Ru distance of 5.5 Å (Fig. 6). Reaction between pentaphosphaferrocene [Cp*Fe(h5-P5)] and Cu(I) halides with Cp2Fe, Cp*Fe(h5-P5), or (CpCr)2(m,h5-As5) present as templates led to the formation of fullerene-like systems.18 Interestingly, with CuBr and the template (CpCr)2(m,h5As5), a 90 vertex spheroid was formed (Fig. 7). However, when CuCl was used, cleavage of the template occurs and an 80 vertex assembly was obtained, providing a protecting shield to the unstable 16 electrons species CpCr(h5-As5). Other phosphaorganometallic derivatives were used to assemble molecular squares. A hetero-bimetallic square was obtained by mixing 1,3-diphosphacyclobutadiene cobalt complexes and AuCl(PPh3).19 Using a combination of solid-state Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray structure analysis, and Density Functional Theory (DFT) calculations, the metalla-cyclic structure was confirmed. This strategy opens new possibilities for preparing organometallic assemblies with sandwich-like units.

Figure 3

Silver-linked ferrocenyl-based rectangle, [{Fe(h5-C5H4-1-C5H4N)2}2Ag2]2 þ.13

Organometallic Chemistry in Directed Assembly

Figure 4

Hetero-bimetallic assembly, [Ni2(m-Cl)(3-BPFA)4(H2O)2]3 þ.14

Figure 5

Toluene molecule in the cavity of [Fc{(CH2)2-3,5-Me2PzNiCp}2]2.15 Hydrogen atoms are omitted for clarity.

Figure 6

Octacationic metalla-cube, [{(p-cymene)2Ru2(m4-oxalato)}4{Cp*Fe(h4-C4Py4)}2]8 þ.17 Hydrogen atoms are omitted for clarity.

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Figure 7 Encapsulation of CpCr(h5-As5) (van der Waals representation) in a pentaphosphaferrocene-based cage.18 Hydrogen atoms are omitted for clarity.

A series of polynuclear titanium compounds were obtained by selective C-C coupling reactions.20 In the case of pyrazine, direct formation of a metalla-square of 7.26 Å2 was observed (Fig. 8A). However, with pyrimidine, C-C coupling spontaneously occurs, and an octanuclear metalla-square of 11.25 Å2 was obtained (Fig. 8B). Treatment of a di-uranium-N,N0 -ferrocenyl m–h6–h6-toluene complex with quinoxaline in toluene at 85 C afforded the tetranuclear uranium-ferrocenyl square, in which the ferrocenyl units, Fec(NSitBuMe2)2, surrounded the tetra-uranium square.21 The internal U4 square is 7.13 Å2, while the Fe4 external, square is 12.10 Å2.

6.12.3

Metal Carbonyl Complexes

An early example of a square-shaped metalla-cycle with carbonyl ligands appeared in 1983.22 Five different metalla-cycles containing M(CO)4 corners (M ¼ Cr, W, or Mo) were synthesized by a two-step process between (nbd)Cr(CO)4, (nbd)W(CO)4, or cis-(tmpa)Mo(CO)4 [nbd ¼ norbornadiene, tmpa ¼ tris(2-pyridylmethyl)amine] and [P(OCH2)3P0 ]. Heterometallic squares were obtained by reacting cis-(CO)4M[P(OCH2)3P0 ]2 and cis-(CO)4M0 [tmpa]2 (M ¼ Cr, W; M0 ¼ Mo) (Fig. 9). The squares were characterized by 31P and 1H NMR, infrared (IR) spectroscopy and also by Fast Atom Bombardment Mass Spectrometry (FAB-MS).

Figure 8

Tetranuclear and octanuclear titanocene squares with pyrazine (A) and pyrimidine (B) linkers, respectively.20

Organometallic Chemistry in Directed Assembly

Figure 9

309

Molecular squares (M ¼ Cr, W, Mo).22

A molecular triangle with rhenium carbonyl corners was prepared in 90% yield by heating Re(CO)5Br and 1,4-bis(40 -pyridylethynyl)-2,5-dihexyloxybenzene at reflux in benzene. The assembly was characterized by IR and NMR spectroscopies, FAB-MS, and elemental analysis.23 Similarly, a reaction using Re(CO)5Cl as the metal precursor, 1,4-dipyridylbutadiyne as the bridging ligand, and a 1:1 THF/toluene (THF ¼ tetrahydrofurane) solvent mixture afforded a molecular square in 92% isolated yield.23 No trace of the triangular complex was observed in the product, a phenomenon ascribed to poor solubility of the square and its subsequent precipitation from solution upon formation. The electronic absorption spectra of both complexes were investigated. These complexes were found to be luminescent in solution at room temperature; the emission of the triangle supposedly originating from p-p* excited states, while that of the square was assigned to arise from metal-to-ligand charge transfer. Molecular squares represent a large group of carbonyl-containing assemblies. One of the reasons for this might be the relative ease of their preparationdthe strong trans effect of the CO ligand activating two (and only two) adjacent carbonyls and allowing their substitution by a bridging ligand, thus leading to a self-assembly of square structures. Applications of these metalla-cycles include solution-phase catalysis, chemical sensing, and host-guest chemistry by the group of Hupp.24 A molecular square containing rhenium carbonyl metal centers was synthesized in 1995, by the extension of the previously known strategy of cis-bridging ligation of transition metals. The cationic supramolecular square composed of two Pd [Ph2P(CH2)3PPh2] corners, two fac-Re(CO)3Cl corners, and four bipyridyl bridging ligands (Fig. 10) was isolated as chloride salt.25 The incorporation of luminescent rhenium carbonyl centers led to the induction of photoluminescent characteristics for the square and opened up possible applications in molecular sensing and electronic excited-state reactivity. Emission from the square was shown to be enhanced by addition of tetraethylammonium perchlorate, a feature ascribed to encapsulation of ClO4 - in the cationic square cavity. The authors speculated that this enhancement stemmed from the attenuation of Pd(II) quenching kinetics by the guest. These results demonstrated the possibility of host-guest interactions of the assembly with small ionic species and suggested a possible application of charged supramolecular squares in anion sensing.25

Figure 10

Molecular square incorporating Pd[Ph2P(CH2)3PPh2] and fac-Re(CO)3Cl corners, and bipyridine linkers.25

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Organometallic Chemistry in Directed Assembly

Figure 11

Neutral Re(CO)3Br molecular squares built from pyrazine (A) and 4,40 -bipyridine (B).26

The same group reported the use of this synthetic strategy for the preparation of neutral metalla-cycles containing solely octahedral rhenium carbonyl centers. The squares were synthesized by a one-pot self-assembly strategy from a Re(CO)5Br precursor and N-donor ligands such as pyrazine (Fig. 11A) and 4,40 -bipyridine (Fig. 11B).26 A combination of FAB-MS, NMR spectroscopy, and single crystal X-ray structure analysis showed that higher order assemblies were not formed in addition to the molecular squares. These squares (Fig. 11) were later studied for their molecular sieving properties. Electrochemical experiments showed thin films of the squares to be exceptionally porous and selective toward permeants of varying sizes. Control experiments with the vertex Re(CO)3Cl(bipy)2 resulted in the efficient exclusion of all permeant molecules evaluated.27,28 The data suggests that the membrane-like permeation through pores or tunnels formed in thin films is the main mode of transport of permeant species. These results show the potential of tailored supramolecular squares for molecular separation and recognition applications. This was later confirmed by Hupp and coworkers who demonstrated that the squares are capable of selectively interacting with volatile organic compounds such as cyclohexene, toluene, benzene, fluorobenzene, and 4-fluorotoluene through both van der Waals and weak charge-transfer interactions.29 Trends in guest preference were also observed depending on variation in their character (e.g., aromatic and good electron donors), as well as size selectivity toward a range of cyclic ethers. The same synthetic strategy was used to prepare rhenium-containing molecular squares from porphyrins. The squares were formed by the self-assembly of 2,8,12,18-tetrabutyl-3,7,13,17-tetramethyl-5,15-bis(4-pyridyl)-porphyrin and Re(CO)5Cl. Hostguest properties of the assemblies were investigated, showing a strong interaction with pyridine and the formation of a 1:1 hostguest complex with 5,10,15,20-tetrapyridylporphyrin.30 Self-assembly methodology was used to construct a series of molecular squares of general formula [fac-Br(CO)3Re(m-(pyterpy)2M)]4(PF6)8, (pyterpy ¼ 40 -(400 -pyridyl)-2,20 :60 ,200 -terpyridine; M ¼ Fe, Ru, Os).31 Reaction of Re(CO)5Br and a bridging metal complex in a CH3CN/THF mixture at reflux afforded these new complexes in quantitative yield (Scheme 1).

Scheme 1

Synthesis of the heterometallic squares [fac-Br(CO)3Re(m-(pyterpy)2M)]4 8 þ from Re(CO)5Br.31

Organometallic Chemistry in Directed Assembly

Figure 12

311

Molecular structure of the triangle [{Ru2(CO)4(PMe3)2}3(O2CC6H4CO2)3].34

All of these complexes display strong visible absorptions that can be ascribed to metal-to-ligand (pyterpy) charge transfer. This occurs in the region of 400–600 nm and the osmium derivative in Scheme 1 was found to be highly luminescent at room temperature in deoxygenated solution. In contrast, the Ru and Fe derivatives show no detectable luminescence under analogous conditions. The positively charged squares are also potential host materials for anions. A preliminary binding study showed an increase in luminescence in the presence of the BF 4 anion. The detailed binding mechanism of the anions was not reported, but the authors suggested an electrostatic stability effect from the BF4  ion. To our knowledge, this study is the first example of the incorporation of Fe, Os, and Ru complexes as bridging units of carbonyl-containing molecular squares.31 In 1990, the synthesis of a molecular triangle with diruthenium corners bridged by dicarboxylic ligands was reported.32 The treatment of Ru3(CO)12 with tartaric acid (R,R, S,S, and R,S) leads to the formation of the triangles {Ru6(CO)12[OOC(CHOH)2COO]3L6} (L ¼ PPh3, MeCN). Single-crystal X-ray structure analysis of the acetonitrile derivative shows the Ru2(CO)4 units to be linked via three tartrato bridges. The angle defined by the two adjacent Ru2(CO)4 corners was c.80 degrees. The all-R,R, all-S,S, and all-R,S diastereomers were isolated. Similar triangles such as [{Ru2(CO)4(PPh3)2}(m-4,40 -O2CC6H4C6H4CO2)]3 and [{Ru2(CO)4(L)2}3(O2CC6H4CO2)3] (L ¼ PMe3, 3,5-Me2NC5H3) were also reported.33,34 The structure of [{Ru2(CO)4 (PMe3)2}3(O2CC6H4CO2)3] is shown in Fig. 12. To our knowledge, the first porphyrin-containing molecular square with an octahedral rhenium carbonyl center was reported in 2000.35 The metal precursors trans-[RuCl2(Me2SO)3(CO)] and trans-[RuCl2(Me2SO)2(CO)2] (Me2SO bound through O) undergo replacement of two Me2SO ligands with 5,10-bis(40 -pyridyl)-15,20-diphenylporphyrin (40 -cisDPyP), forming a 90-degree corner complex. The addition of (CF3SO3)2Pd(dppp) [dppp ¼ 1,3-bis(diphenylphosphino)propane] yields heterometallic squares (Fig. 13). Another porphyrin-derived molecular square was later synthesized and demonstrated the use of a metal-directed self-assembly approach in catalysis.36 Inspired by natural enzymes, the authors assembled a molecular square from fac-Re(CO)3Cl corners and 5,15-di(4-pyridyl)-10,20-diphenylporphyrin (dpyp) linkers. This structure was then used to encapsulate a manganese-based catalyst (Mndpyp) and the resulting host-guest system was tested in catalytic epoxidation reactions. This enzyme-like system was found to possess significantly higher stability and substrate selectivity than the free catalyst. Styrene epoxidation reaction revealed a 10-fold increase in the turnover number (TON) following complex formation, although the rate of reaction decreased, presumably due to the steric hindrance of the catalytic active site. In addition, the catalyst lifetime under reactive conditions was extended from approximately 10 min (free catalyst) to more than 3 h for the encapsulated system. When the Mndpyp catalyst was replaced by the more strongly coordinating complex meso-tetra(4-pyridyl)porphinatomanganese(III) chloride, the TON increased approximately twofold. The encapsulated catalyst also showed higher selectivitydfor example, the competitive epoxidation reactions with both cis-stilbene and cis-3,30 ,5,50 -tetra-tert-butylstilbene showed that the sterically more encumbered substrate is 3.5 times less reactive.36 This study showed that self-assembled structures can impart stability and substrate selectivity to a conventional catalyst. With appropriate cavity functionalization, this approach has the potential to be used to construct highly selective artificial enzyme-like systems. Chiral molecular squares of the general formula [Cl(CO)3Re(L*)]4 were reported in 2002 (L* ¼ enantiopure 4,40 -bis(pyridyl)0 1,1 -binaphthyls).37 These complexes showed interesting enantioselective luminescence quenching by chiral aminoalcohols. The squares, synthesized by a self-assembly approach from an equimolar mixture of Re(CO)5Cl and the chiral ligands (L*), were characterized by FAB-MS and 1H and 13C{1H} NMR spectroscopy. The (R) and (S) square enantiomers showed different rates of luminescence quenching by the two isomers of 2-amino-1-propanol. Chirality-based luminescence-quenching selectivity was

312

Organometallic Chemistry in Directed Assembly

Figure 13

Heterometallic porphyrin-containing squares.35

demonstrated with the (S)-enantiomer in the presence of 2-amino-1-propanol. Overall, the study showed the clear potential for application of such chiral self-assembled structures in enantioselective catalysis. Rectangular-shaped assemblies were reported in parallel by two groups in 1998.38,39 In the first case, the rectangles were synthesized by one-step self-assembly of Re(CO)5(OTf) (OTf ¼ CF3SO3) and 4,40 -bipyridine (bipy) using water, methanol, ethanol, or ethane-1,2-diol as solvents. The resulting metalla-cycles were composed of fac-Re(CO)3 corners with m-alkoxy/m-hydroxy and 4,40 -bipyridine bridges on alternating sides of the assembly.38 The self-assembled rectangles showed luminescence in the solid state, but not in solution. Single crystal X-ray structure analysis of the 4,40 -bipyridine derivative shows a Re/Re separation of 3.38 Å. The second example of CO-containing rectangles reported a two-step approachdfirst, the stable thiolato or selenato dimers [(CO)4Re(m-QR)]2 (Q ¼ S, Se; R ¼ CH2CH2CH3 or C6H5) were synthesized, and these were then reacted with pyrazine (Fig. 14A) or 4,40 -bipyridine (Fig. 14B) in refluxing CHCl3 to afford the corresponding rectangles.39 Similar structures were observed with identical dimensions. The 4,40 -bipyridyl ligands were near coplanar, as was the case in the previous study. In both reports, the size of the cavity was deemed too small for successful inclusion of a small guest. Nevertheless, both studies demonstrate a possibility to construct rectangular-shaped assemblies, which, if the cavity size is carefully tailored, can serve as hosts for various planar aromatic compounds. Building upon this strategy, the synthesis of rectangles with difunctional chelating bridges formed by 2,20 -bipyrimidine was achieved. By reacting these dirhenium units with bipy and 1,2-bispyridylethylene, new rectangles were formed. The cavity

Figure 14

Pyrazine (A) and 4,40 -bipyridine (B) rhenium-based rectangles.39

Organometallic Chemistry in Directed Assembly

Figure 15

313

Gondola-shaped dinuclear Re(CO)3-based complex.43

dimensions of these complexes were larger, thus showing host-guest interactions with 2,6-naphthalenedisulfonic acid disodium salt (a planar aromatic species with a length of  10 Å). The association constant was 2.3  103 M 1, showing that the cavity can bind aromatic guests.40 Similarly, neutral rhenium and manganese rectangles [(CO)3M(N,N0 -bzim)(bipy)]2 (bzim ¼ benzimidazole, M ¼ Mn, Re) were prepared and their host behavior toward small aromatic guests was studied by quartz crystal microgravimetry.41 The rectangles show high-affinity constants for toluene and 4-fluorotoluene, whereas the affinity constants for benzene, fluorobenzene, and hexafluorobenzene are lower, suggesting an influence of electron donor/acceptor interactions. The anticancer activity of a self-assembled supramolecular rectangle [(CO)3Re(m-OC5H11)2Re(CO)3]2{m-(4,40 -bpy)}2 was evaluated on cancerous cell lines.42 The rectangle was characterized spectroscopically and by single-crystal X-ray structure analysis and was tested against melanoma, fibrosarcoma, breast, and prostate cancer cells. It was found to be cytotoxic against all the cell lines tested, most notably against fibrosarcoma cells. A gondola-shaped assembly was prepared from Re2(CO)10, 2,20 -bisbenzimidazole, and a flexible ligand a,a0 -bis(benzimidazol1-yl)-o-xylene.43 The resulting assembly was analyzed by IR spectroscopy, FAB-MS and 1H NMR, and single crystal X-ray structure analysis and features a Re/Re distance of 5.7 Å and a distorted octahedral geometry around the rhenium centers (Fig. 15). A similar strategy was employed by the same group in 2009 to prepare metalla-cyclic molecular rotors in one step and in high yields.44 The dirhenium unit, composed of fac-Re(CO)3 corners and 6,11-dihydroxy-5,12-naphthacenedione or dihydroxyanthraquinone, represented the stator; the rotor was formed from the a,a0 -bis(benzimidazol-1-yl)-p-xylene or a,a0 -bis[(1-imidazolyl)methyl]-p-xylene. The resulting air/moisture-stable products were soluble in polar organic solvents, allowing for detailed 1H NMR, Nuclear Overhauser Effect Spectroscopy (NOESY) NMR, and variable temperature 1H NMR experiments to be undertaken. These proved that the para-phenylene unit rotates in solution, thus confirming that the complexes can be depicted as molecular rotors. Gondola-shaped metalla-cycles were also investigated for their anticancer properties.45 Ester functionalized Re(I)-based oxamidato-bridged neutral dinuclear metalla-cycles of general formula [(CO)3Re(m-L)(m-L0 )Re(CO)3] were self-assembled by means of oxidative addition of oxamide ligands (H2L ¼ N,N0 -diphenyloxamide, and N,N0 -dibenzyloxamide) to Re2(CO)10 with flexible, ester-containing ditopic pyridyl ligands (L0 ¼ o-phenylene-diisonicotinate, ethane-diyl-di-4-pyridine carboxylate, or 1,4butane-diyl-di-4-pyridine carboxylate) in the reaction mixture. These assemblies were tested for activity against lung (A549), cervical (HeLa), and colon (HCT-15) cancer cell lines, and were found to have IC50 values in the mM range whilst being inactive against normal peripheral blood mononuclear cell (PBMC) cells. Subsequent live cell experiments, coupled with fluorescence imaging, revealed the best candidate to induce apoptosis in lung and colon cancer cells. A weak-link approach, previously developed by Farrel et al. in 1988, can be employed for the synthesis of 3D structures.46 Indeed, cage-like cylindrical supramolecular structures composed of two metalla-cycles connected either by 4,40 -biphenyldicarbonitrile or by 4,40 -biphenyldiisocyanide ligands were prepared (Fig. 16). Single crystal X-ray structure analysis revealed that each Rh(I) center is of square planar geometry and possesses two trans-phosphine ligands within the coordination sphere. The remaining coordination sites are occupied by trans-isocyanide and acetonitrile ligands, all of which give rise to the cylindrical structure in Fig. 16. The dimensions of the assembly can be defined by the Rh(I)/Rh(I) distances that are found to be  12 and  16 Å. Porphyrin-containing macrocycles with ruthenium carbonyl corners were synthesized by reacting 5,15-bis(40 -pyridyl)-2,8,12,18tetra-n-propyl-3,7,13,17-tetramethylporphyrin or 5,15-bis(40 -pyridyl)-10,20-diphenylporphyrin with a pre-assembled diruthenium porphyrin square.47 When the ligand is 5,15-bis(40 -pyridyl)-2,8,12,18-tetra-n-propyl-3,7,13,17-tetramethylporphyrin, single crystal X-ray structure analysis revealed that the assemblies stack into polymers, whereas with 5,15-bis(40 -pyridyl)-10,20diphenylporphyrin the resulting structure was found to be a discrete supramolecular assembly. As shown in Fig. 17, the latter comprises metallo-porphyrin squares connected by linkers that are found to be separated by  11 Å.

314

Organometallic Chemistry in Directed Assembly

Figure 16

Cylindrical structure composed of two rhenium-based metalla-cycles connected by two 4,40 -biphenyldicarbonitrile.46

Figure 17

Porphyrin-containing macrocycles with ruthenium carbonyl corners.47

A report in 2001 detailed the self-assembly of molecular organometallic trigonal prisms that comprise a total of 11 components.48 Notably this involved the use of two distinct bridging ligands and could be achieved by either a two-step or one-pot method. The two-step method involved the synthesis and isolation of 2,20 -bipyrimidine-bridged Re(I) pillars, followed by ligand exchange and insertion of tridentate panels (1,3,5-tris(20 -ethynyl-400 -pyridyl)benzene or 2,4,6-tris(40 -pyridyl)-1,3,5triazine). The one-pot strategy involved refluxing triflate anions, Re(CO)5, and the pyridyl linkers in a 6 þ 3 þ 2 ratio in tetrahydrofuran (THF) for 1 week. The 2,20 -bipyrimidine bridged dimer is assumed to be thermodynamically more stable due to the stabilization through the chelation of 2,20 -bipyrimidine and to form first, allowing this facile one-step synthesis of the 3D

Organometallic Chemistry in Directed Assembly

Figure 18

315

Structure of the metalla-prism {(CO)3Re(m-OEt)2Re(CO)3}3(m-tpt)2.49

assemblies. A study of host-guest chemistry revealed that these prisms displayed a preference for aromatic guests, with additional shape-selectivity being observed. A similar one-pot synthesis using 2,4,6-tri-4-pyridyl-1,3,5-triazine (tpt) and Re2(CO)10 in the presence of 1-butanol, 1-octanol, or 1-dodecanol resulted in the assembly of trigonal prisms with the general formula [{(CO)3Re(m-OR)2Re(CO)3}3(m-tpt)2], an example of which is shown in Fig. 18. The octanol and dodecanol derivatives exhibited large solvatochromism, showing that 49 the lab max value can be tuned by changing the polarity of the solvent. Gondola-shaped assemblies that showed selective recognition of metal ions and planar aromatic molecules were reported in 2006.50 These luminescent metalla-cycles were composed of 2,5-bis(5-tert-butyl-2-benzoxazolyl)-thiophene (Ls) and 1,4dihydroxy-9,10-anthraquinone (H2-dhaq) or 1,2,4-trishydroxy-9,10-anthraquinone (H2-thaq) and four rhenium carbonyl metal centers (Fig. 19). These systems contained crown-ether-like recognition sites with the uncoordinated sulfur atoms of the thiophene ligands, allowing selective binding of Hg(II) ions (K ¼ 1.3  103 M 1). In addition to this, anthracene was found to have a higher affinity (K ¼ 3.8  103 M 1) than pyrene, naphthalene, or benzene, with the value of K for the latter being undetectable. The contribution of p systems of the bridging dhaq and thaq ligands, as well as a shape complementarity of anthracene with the anthraquinone moiety, could be at the origin of the reported selectivity. A metallo-bridged cavitand and its binding properties toward organic cavitands and calix[4]arenes were reported in 2007.51 The overall structure was stabilized by the fac-Ru(CO)3 corners, preventing the collapse of the cavity (Fig. 20). The assembly was characterized by single-crystal X-ray structure analysis, electron spray ionization mass spectrometry (ESI-MS), IR, and NMR spectroscopy. Interactions with calixarenes or organic cavitands as guests were investigated by 1H NMR and luminescence studies, showing that the nature of the substituents on the lower rim of the calixarene was playing an important role in recognition by the host. A rhenium polypyridine-based supramolecular vessel was used to demonstrate a “Sleeping Trojan Horse” approach, a new way of targeting cancer cells.52 The trimeric, rhenium-based molecular vessel with a prismatic-shaped cavity was found to efficiently take up Agþ and Cu2 þ ions, which formed a “stopper” at one side of the cavity of the vessel (Fig. 21). The “empty” form was impermeable to cellular membranes, whereas the ion-containing species were able to enter cells by passive membrane transport. Confocal microscopy confirmed the uptake of the silver derivative into MCF-7 cancer cells and a co-localization experiment confirmed its accumulation in the nucleoli.

Figure 19

Structure of the gondola-shape assembly {(CO)3Re(m-dhaq)Re(CO)3}2(m-Ls)2.50

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Organometallic Chemistry in Directed Assembly

Figure 20

Cavitand-like assembly obtained from Re(CO)3Br units.51

Figure 21

Molecular structure of a silver trapped inside the cavity of a Re(CO)3-based triangle.52

Helicates and mesocates were synthesized with fac-Re(CO)3 corners and semi-rigid 1,4-bis(2-(2-hydroxyphenyl)benzimidazol1-ylmethyl)benzene (H2-pbc) or 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (H2-mbc) ligands.53 A one-step process for the synthesis of metallo-cavitands from Re2(CO)10, rigid N,N-donors benzimidazole, benzotriazole, or imidazole and flexible hexatopic N-donor ligand 1,2,3,4,5,6-hexakis(benzimidazol-1-ylmethyl)-benzene (Fig. 22) was also reported.54

6.12.4

Cyanometallates

Cyanometallates are mainly found in metal-organic frameworks,55 and discrete assemblies incorporating cyanometallate units remain scarce and are often obtained serendipitously. Nevertheless, examples of directed assemblies built from cyanometallates are known. It began in the 1990s with the synthesis of a mixed manganese-palladium cluster bridged by cyano linkers (Fig. 23).56

Organometallic Chemistry in Directed Assembly

Figure 22

Helicate (A) and mesocate (B) with fac-Re(CO)3 corners.53,54

Figure 23

A mixed manganese-palladium cluster linked by cyano ligands.56

317

The main development in the field came from the group of Rauchfuss, which over the years generated all kinds of boxes and defect boxes.57 These organometallic cages are cationic, anionic, or neutral, and therefore able to interact with ions. For instance, anionic boxes with Mo(CO)3 vertices can adapt their geometry according to the size of the cation. In the presence of Liþ and Naþ a tetrahedral structure was found (Fig. 24A), while in the presence of Kþ and Csþ a trigonal prismatic assembly was observed (Fig. 24B). The labile nature of the cyano bridges allows such geometrical flexibility. The synthesis of mixed metal cobalt-rhodium or cobalt-ruthenium boxes was achieved by combining [CpCo(CN)3] and [Cp*M(NCMe)3]þ (M ¼ Rh, Ru) in solution.58 These neutral [(CpCo)4(m-CN)12(Cp*M)4] cages accommodate ions in their  50 Å3 cavity (Fig. 25). Small ions show rapid exchange in solution, while larger ions such as NH4 þ are trapped inside and become unreactive.59 Defect boxes can be obtained by stoichiometric control, using, for example, 4 equiv. of [CpCo(CN)3] with 3 equiv. of [Cp*M(NCMe)3]þ, thus affording a cationic seven vertex box of the general formula [(CpCo)4(m-CN)12(Cp*M)3]2 þ.60 Defect boxes can also be obtained by steric control.61

318

Organometallic Chemistry in Directed Assembly

Figure 24

Monocations trapped in a tetrahedral assembly (A) and a prismatic cage (B).57

Figure 25

Metalla-cage [(CpCo)4(m-CN)12(Cp*Ru)4].58

6.12.5

Metal Carbene Complexes

To our knowledge, the first carbene-based molecular triangle was reported in 1970,62 and was obtained in near quantitative yield by addition of chloro(triphenylarsine)gold(I) to a solution of 2-pyridyllithium at d40 C, followed by warming to d5 C. Five other derivatives were synthesized and these were found to be stable to light. Later, the solvoluminescence of an analogous gold-carbene trinuclear complex, [Au3(CH3N]COCH3)3] (Fig. 26), was reported.63 When crystals of the complex, previously irradiated by UV light at 366 nm, came into contact with a solvent (chloroform, dichloromethane, toluene, methanol, hexane, or water), they showed naked-eye-visible luminescence (522 nm) that presumably stemmed from the stacking of the triangles into supramolecular columns by Au/Au interactions. Luminescent squares were also reported.64 They were composed of cis-ligated Pt(dtbpy) (dtbpy ¼ 4,40 -di-tert-butyl bipyridine) corners and 4,40 -bis(alkynyl) derivatized 1,10 -bi-2-naphthol (BINOLs) as linear bridging units. The squares were found to be in equilibrium with triangles, composed of three Pt(II) corners and three linear units. The luminescence studies of the chiral squares showed very strong phosphorescence at  565 nm in addition to weak fluorescence at  440 nm. After the incorporation of the thin films of the squares into model light-emitting diode (LED) devices, the supramolecular assemblies showed superior efficiency to that of simple platinum acetylide complexes, demonstrating the potential of this approach for the construction of light-emitting devices. Another example of organometallic molecular triangle was formed by a self-assembly of methylaquacobaltoxime or ethylaquacobaltoxime and 3-aminophenylboronic acid. This occurs through condensation of the boronic acid with the oximic oxygens, with

Organometallic Chemistry in Directed Assembly

Figure 26

319

Molecular structure of [Au3(CH3N]COCH3)3].63 Hydrogen atoms are omitted for clarity.

pyridine occupying the remaining coordination sites of the octahedral cobalt center.65 The reaction was found to be reversible and strongly pH dependentdthe two triangles precipitate from water at pH 7, but dissolve in both acidic (pH < 4) and basic (pH > 9) solutions. Tin-containing triangles can be prepared via the reaction of dimethyltin(IV) or di-n-butyltin(IV) oxide and isophthalic acid.66 The reaction results in a mixture of cyclooligomeric dinuclear, trinuclear, and tetranuclear species in solution. For the n-butyl derivative, only the trinuclear species was isolated in the solid state and was analyzed by single crystal X-ray crystallography; the structure of one of these 24-membered macrocycle is shown in Fig. 27. The macrocycle presents a near-planar structure akin to 18-crown-6, with six oxygens pointing toward the center. Six Sn-n-butyl groups are arranged nearly perpendicular to the plane of the macrocycle due to the distorted octahedral geometry of each Sn center. The cavity dimensions were large enough (Sn/Sn distances > 9.00 Å) for this to accommodate linear alkyl groups from a symmetry equivalent neighbor, forming a non-covalently bound bis-triangle as a result. An enantiomerically pure metalla-cycle comprising three Pd(II) centers and chiral C-metallated b-ketosulfoxide (R)-3-p-tolylsulfinyl-2-propanone linkers has been prepared.67 Single crystal X-ray structure analysis (Fig. 28) revealed that the coordination sphere of each Pd center is occupied by a sulfur of a sulfinyl moiety, a carbonyl oxygen, a methine carbon, and finally a p-tolyl ortho-carbon. The metalla-cycle is thus a nine-membered ring, and is found to have a diameter of  4 Å.68 Chiral platinum-containing triangles were also synthesized,69 and they were used as asymmetric homogeneous catalysts. The triangles were composed of cis-ligated platinum(II) corners and BINOL linkers. All four triangles were characterized by 1H,

Figure 27

Molecular structure of a triangular dibutyltin assembly.66 Hydrogen atoms are omitted for clarity.

320

Organometallic Chemistry in Directed Assembly

Figure 28

Chiral palladium-based metalla-cycle.68 Hydrogen atoms are omitted for clarity.

13

C{1H}, and 31P{1H} NMR spectroscopy, mass spectrometry, elemental analysis, and IR, UV–vis, and circular dichroism spectroscopies. The catalytic properties of the triangles were investigated, showing conversion superior to 95% with e.e. over 85%, demonstrating the potential of similar chiral assemblies in asymmetric catalysis. In 2003, the group of Stang described the formation of three molecular triangles of varying sizes, composed of organometallic 2,9-bis[trans-Pt(PEt3)2(NO3)] phenanthrene corner units and linear bridging ligands.70 The triangles self-assembled in several hours and were isolated in near quantitative yields, with no traces of higher order macrocycles being detected. The three triangles were characterized by multinuclear NMR, ESI-MS, elemental analysis, and, in the case of [{Pt(PEt3)2}6(m-phenanthrene)3(m-bipy)3]6 þ, by single-crystal X-ray structure analysis (Fig. 29). Molecular hexagons, synthesized by a similar strategy to that described previously for the construction of triangles, were formed by 4,7-phenanthroline and organo-palladium dinuclear complexes.71 As shown in Fig. 30, the hexagons featured a planar Pd6

Figure 29

Molecular structure of [{Pt(PEt3)2}6(m-phenantherene)3(m-bipy)3]6 þ.70 Hydrogen atoms are omitted for clarity.

Organometallic Chemistry in Directed Assembly

Figure 30

321

Molecular structure of a Pd6 macrocycle.71

macrocycle, with the planes of the two different fragments being nearly perpendicular. The cavity of the hexagons was approximately 1.2 nm in diameter. Exploiting their ligand-directed approach to other shapes, the synthesis of supramolecular hexagons from rigid linear linkers and 120 degree corners was extended.72 Carbene-based squares with platinum and palladium corners were published in 1994.73 These squares comprised two (dppp) Pt(-C6H4-p-CN)2 and two (dppp)M(CN)2 corners (dppp ¼ diphenylphosphinopropane, M ¼ Pt, Pd), and were synthesized by selfassembly and characterized by 31P, lH, and l3C{lH} NMR, as well as IR spectroscopy. A similar strategy was later used to prepare chiral organometallic square assemblies with two [2,20 -bis(diphenylphosphino)-1,10 -binaphthyl] and two [2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)-butane] ligands at the corners. The chiral squares were able to complex two Agþ ions by p-interaction with the triple-bonds; five of the resulting silver complexes were isolated. The squares also interacted with the neutral aromatic guests tetramethylpyrazine or phenazine; seven of the resulting inclusion complexes were isolated in essentially quantitative yields. Square formation, silver ion complexation, and guest inclusion was observed by NMR spectroscopy, circular dichroism, and UV spectroscopy. Silver ion complexation was also used to encapsulate small organic molecules in homometallic squares of the general formula [{(cis-(dppp)M(4-ethynylpyridyl)2)(cis-(L)M)}2Ag2]6 þ (M ¼ Pt(II) or Pd(II) and L ¼ dppp or two equivalents of PEt3). The silver ions are bound via the “p-tweezer effect” and afford a series of host-guest interactions with aromatic guests such as pyridine, pyrazine, phenazine, and 4,40 -dipyridyl ketone, a number of which were characterized crystallographically.74 Supramolecular squares that employed organometallic units as linear linkers were reported in 1996.75 The 4,40 -bis[transPt(PEt3)2]biphenyl units were reacted with 90 degree corners containing iodonium ions or Pt(II) centers to form two metallacycles. The assemblies were characterized by multinuclear NMR as well as by Matrix-assisted laser desorption/ionization (MALDI) and electron spray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Similar squares with various sizes were later synthesized by adding longer aromatic spacers (1,4-phenyl, 4,40 -biphenyl and 4,400 -terphenyl) to the linear Pt2 metal centers. The squares were able to complex four Agþ ions via the p-tweezer effect of the two adjacent triple-bonds in the corners. N-heterocyclic carbenes (NHCs) can also be used to construct supramolecular square-shaped assemblies. Hahn and coworkers recently reported the synthesis of platinum, nickel, and rhodium-containing squares based on NH, O-, or NH, N-NHC ligands. The structures of two squares are presented in Fig. 31.76,77 The ligand-directed approach to molecular rectangles based upon a double-oxidative addition strategy was reported in 2000.78 This strategy used 1,8-bis[trans-Pt(PEt3)2(NO3)]anthracene as a rigid unit with two parallel coordination sites facing in the same direction, a so-called molecular “clip.” This clip was then combined with an equimolar amount of 4,40 -bipyridyl or 4,400 -dipyridyl-10 ,40 -diethynylbenzene, yielding two molecular rectangles.45 With the same strategy, the group later synthesized similar rectangles and reported a detailed study of their structure and physical properties. An interesting effect of the counterions on the  stability of the assemblies was also describeddthe replacement of nitrate anions by weakly nucleophilic ions, PF 6 , BF4 , or  79 ClO4 , led to kinetic stabilization of the rectangles that became significantly more stable in solution. These metalla-rectangles were also studied for their electron-transfer capacity and were shown to undergo reversible multielectron reduction and oxidation.80

322

Organometallic Chemistry in Directed Assembly

Figure 31

Molecular squares incorporating NiCp (A) or RhCp* (B) corners.76,77 Hydrogen atoms are omitted for clarity.

Using the same rigid clip, a supramolecular optical sensor for Ni(II), Cd(II), and Cr(III) ions has been synthesized.81 The resulting assembly showed an optical response to metal ions; the binding constants were 2.01  0.05  107 for Ni(II), 3.39  0.5  104 for Cd(II), and 7.53  0.4  103 for Cr(III). 1,8-Bis[trans-Pt(PEt3)2(NO3)]anthracene can also be used as a molecular clip for the construction of rectangles with dicarboxylatebased oxygen donor ligands.82 The synthesis of such rectangles with terephthalate or maleate bridging ligands was reported. The methodology was shown to be highly versatile, allowing the synthesis of rectangles with various linkers such as squarate, croconate and oxalate,83 1,10 -ferrocenedicarboxylates,16 p-carborane dicarboxylates,84 crown ethers,85 or optically pure D- or L-tartrate,86 endowing the resulting assemblies with advantageous physicochemical properties. Similarly, if the clip of the metalla-rectangle was composed by a rigid organic molecule such as 1,3-bis(3-pyridyl)isothalamide, 1,3-bis(3-pyridyl)ethynylbenzene, or 1,8-bis(4pyridyl)ethynylanthracene and the linear linker being the organometallic unit 4,40 -bis[trans-Pt(PEt3)2-(O3SCF3)(ethynyl)]biphenyl, rectangles with promising sensitivity to nitroaromatics87 or to acyclic dicarboxylic acids88 were obtained. A recent report describes the two-step synthesis of an unprecedented rectangular structure, composed of four gold carbene moieties embedded in a macrocylic stereoidal cavity containing four estrone nuclei (Fig. 32), which could have potential applications as a chiral catalyst and as a tumor growth inhibitor.89 The combination of rigid-shaped organometallic linkers with organic N-donor ligands was shown to be a highly versatile strategy for the construction of assemblies with various functionalities. Indeed, pyridine-derived cavitands and platinumcontaining organometallic clips can self-assemble into hexagonal [2 þ 2] or [3 þ 3] structures, depending on the choice of the organo-platinum clip.90 When a 120-degree organo-platinum clip was derived with crown ethers and combined with

Figure 32

Molecular structure of four gold carbene moieties embedded in a stereoidal macrocycle.89

Organometallic Chemistry in Directed Assembly

323

di-2-pyridyl ketone, a series of rhomboidal and hexagonal poly[2]pseudorotaxanes were obtained. The structures were characterized by 1D and 2D multinuclear NMR, ESI-MS, and molecular force-field simulations. The crown-ether moieties were shown to bind dibenzylammonium ions as guests.91 Similar hexagons decorated with dendrimers were later synthesized and used as stimuli-responsive host-guest systems for fluorescent dye release. The existence of vesicle-like structures composed of hexagons was demonstrated, a phenomenon attributable to intermolecular interactions (p-p, CH-p, and H-bonding) acting in concert between peripheral dendrons.92 It was also shown that it is possible to induce a vesicle to micelle transition through the addition of bromide ions. This process is proposed to proceed due to disruption of the hexagonal core through this chemical stimulus. As a result, when fluorescent dyes such as boron-dipyrromethene (BODIPY) or sulforhodamine B were encapsulated in the hexagonal cavity of the assembly, the addition of bromide anions to the vesicles resulted in the controlled release of the encapsulated molecules. Supramolecular hexagons held together by hydrogen bonding were recently synthesized.93 The 4,40 -biphenyldiplatinum(II) and nicotinic acid formed a molecular [3 þ 6] hexagon, where the linear platinum complex was bound to two molecules of nicotinic acid and the non-covalent interactions between the COOH groups of the acid facilitated the formation of the hydrogen-bonded metallacycle. Other polygons synthesized by a ligand-directed approach include saccharide-functionalized rhomboids and hexagons,94 hexagons decorated with ferrocenes95, or fluorescent naphthalimide-containing hexagons.96 Emissive rhomboidal metalla-cycles were also synthesized and tested in vivo on a breast cancer mouse xenograft model. The metalla-cycles were non-toxic and significantly inhibited the tumor growth, showing 64% reduction of the tumor volume after 30 days.97 The shape of supramolecular assemblies can also be varied post-synthetically.98 This was elegantly demonstrated with a molecular [6 þ 6] hexagon composed of 4,40 -bis(trans-Pt(PPh3)2(OTf))benzophenone and 4,40 -bipyridine. Reaction of this hexagon with Co2(CO)6 resulted in its reassembly into a [3 þ 3] hexagon. Similarly, the combination of triangles and squares formed a mixture of products upon reaction with Co2(CO)6, where the majority ( 80%) was a rhomboid system. This entropy-driven process demonstrated the possibility to alter the shape of assembled polygons and to control their supramolecular architecture by including a reactive center in the bridging ligands. Another method of post-synthetic modification of supramolecular assemblies was reported in 2012.99 In this case, hexagons were assembled from a building unit, 2,3-bis(2-methyl-5-(pyridin-4-yl)thiophen-3-yl)benzo[b]thiophene-1,1-dioxide (bto), that possesses the desired directing angle of 120 degrees. As the aforementioned dipyridyl ligand is photochromic, irradiation of the hexagons with UV light (365 nm) causes photocyclization from its “opened” to “closed” form, modifying the shape and size of the assemblies. The cyclization process was found to be completely reversible by irradiation with visible light (> 510 nm). The transformation was quantitative and no degradation of the supramolecular hexagons was observed after 10 cycles. A similar strategy was used for the synthesis of a chiral N-heterocyclic carbene silver(I) cylinderda positively charged chiral macrocyclic triimidazoline salt assembled into a cylindrical structure with two NHC ligands being bridged by three silver ions (Fig. 33).100 The catalytic activity of the assembly was tested for the cyanosilylation of imines with Me3SiCN; the reaction gave nearly quantitative yields with sterically unhindered substrates, although with low e.e. An analogous strategy was used to prepare prism- and cube-shaped assemblies of gold, silver, and later copper NHCs.101,102

Figure 33

Chiral N-heterocyclic carbene silver(I) cylinder.100

324

Organometallic Chemistry in Directed Assembly

Gold(I)–diphosphine helical cages of the general formula [Au8(m4-CCOPh)2(PPh2-X-PPh2)4](PF6)2 can be constructed from rigid phosphine linkers PPh2-X-PPh2 (X ¼ 1,4-benzene, 4,40 -biphenyl, 4,400 -p-terphenyl, 3,6-pyridazine, or 1,3-diethynyl benzene) and tetragold {Au4(m4-CCOPh)} centers with hypercoordinate m4-bridging carbon atoms. The 1,3-diethynyl benzene derivative was able to encapsulate CH2Cl2 and CS2 molecules inside its cavity.103 A bioorganometallic guanosine-based Au(I) isonitrile complex was shown to self-assemble into a quartet or a 3D octamer in the presence or absence of potassium ions. The system exhibited a switchable luminescence resulting from the Au(I)–Au(I) interactions of the sandwich-like octamer.104 A recent report showed another possible application of supramolecular gold-carbene assemblies.105 Tetrahedral cages were synthesized by the self-assembly of gold NHC units with zinc or cadmium salts and their use in gold nanoparticle synthesis was demonstrated. Addition of Au(2,4,6-trimethoxybenzonitrile)2SbF6 to solutions of the aforementioned assemblies was found to trigger cage disassembly. This stimulus caused concomitant release of the Au-NHC subcomponent into solution, the result of which was growth of gold nanoparticles. In addition, system design was also found to affect the rate of Au-NHC release, offering additional routes toward controlling gold nanoparticle morphology. In 1999, the self-assembly of two different cuboctahedra composed of tridentate subunits and organo-platinum linkers was reported.106 The compounds were synthesized in a single-step with quantitative yields. Supramolecular nanoscale dodecahedra composed of 50 components were also reported in the same year.107 Using nitrogen donor ligands and organo-platinum complexes, two different cuboctahedra were synthesized in quantitative yield via single-step process. The combination of tritopic building blocks with palladium or platinum corner units resulted in the self-assembly of a series of truncated tetrahedra with large cavities. When the platinum complex was used as the tritopic unit and a dipyridyl-derivatized porphyrin as the corner unit, a porphyrin-containing truncated tetrahedron was formed.108 Chiral nanoscale adamantanoids can also be constructed by the ligand-directed approach, using a chiral derivative of the nitrogen donor ligand in combination with bent platinum organometallic units.109 The previously mentioned rigid linker 1,8bis[trans-Pt(PEt3)2(NO3)]anthracene was also employed in the construction of three-dimensional assemblies. Supramolecular cages or prisms can be prepared from the combination of the linker with ligands. The planar structure leads to the formation of prisms,110 whereas bent ligands in combination with 1,8-bis[trans-Pt(PEt3)2(NO3)]anthracene produced propeller-type cages.111 A similar method was later used to construct supramolecular prisms from related subunits.112 It was also demonstrated that with careful design of the supramolecular 3D structures, it is possible to use certain nanoscale assemblies in applications such as nitro-aromatic sensing 113; other structures could be decorated by ferrocene or crown ether moieties and might therefore find applications as multielectron catalysts or sensors.114

6.12.6

h5-Half-Sandwich Complexes

Early on, the potential of using half-sandwich complexes to form directed assemblies for sensing was recognized. Fish and coworkers prepared a series of cyclic trimeric organometallic complexes in the 1990s.4 The single crystal X-ray structure of the rhodium pentamethylcyclopentadienyl (RhCp*) 9-methyladenyl derivative shows a bowl-shaped cavity with three adenine planes for the sides, and three half-sandwich units forming the base of the bowl (Fig. 34). Introduction of functional groups on the nitrogen, N9 of the adenenyl ligands, modifies the property and depth of the cavity, thus allowing synthetic flexibility for sensing. Replacement of the 9-methyladenyl by 20 -deoxyadenosine connectors has generated a deeper hydrophobic cavity able to recognize the aromatic ring of L-tryptophan through p-p interactions.115

Figure 34

Molecular structure of the bowl shape trimeric complex [{Cp*Rh(9-methyladenyl)}3]3 þ.4 Hydrogen atoms are omitted for clarity.

Organometallic Chemistry in Directed Assembly

Figure 35

325

Molecular structure of a rhodium-based helicate.118

Analogous trimeric structures have also showed great selectivity for alkali metal ions. The trinuclear complexes incorporating 2,3-dihydroxypyridine and MCp* units (M ¼ Rh, Ir) possess a high affinity for Liþ and Naþ, while the same complexes are unable to bind Kþ ions.116 Such trimeric structures with hydroxypyridine linkers possess helical chirality. With a cation present in their cavity they are positively charged, and can therefore form diastereoisomers with chiral anions. Using this basic concept, the racemic trinuclear complex [Cp*Rh(5-chloro-2,3-dioxopyridine)3] was purified by fractional crystallization after (a) encapsulation of Liþ in the trimeric cavity and (b) addition of D-trisphat.117 In recent years, the flexible nature of bis-dihydroxypyridine linkers in combination with h5-half-sandwich units has allowed the formation of helicates: two trimeric units being linked by different spacers.118 In the longest structure, the metal-metal distance reached 21.2 Å between the Rh atoms (Fig. 35). Another approach consists of using rigid or semi-rigid dinuclear clips of h5-half-sandwich complexes with multidentate ligands.119 Rational design has allowed for the formation of rectangles, prisms, and boxes of different sizes and connectivity. These systems are versatile, and offer endless possibilities for preparing organometallic assemblies.120

6.12.7

h6-Half-Sandwich Complexes

Like h5-half-sandwich complexes, the first application of assemblies built from h6-half-sandwich units was in sensing. Replacing h5-half-sandwich complexes with arene ruthenium units, trinuclear assemblies were obtained.121 The functionalized arene ruthenium complex [(h6-C6H5CO2Et)Ru(3-oxo-2-pyridonate)]3 shows a steric discrimination between Naþ and Liþ (Fig. 36). These examples show just how easily metalla-assemblies can be adjustable synthetically, as well as the importance of additional functionalization to increase the sensing ability of organometallic cages. Similar trinuclear arene ruthenium metalla-cycles with pendant aldehyde groups in combination with di- and triamine derivatives have generated large assemblies upon condensation reactions.122 Organometallic assemblies with 3, 6, and even 12 ruthenium centers were synthesized and characterized. The size of the cavities ranged from 290 to 740 Å3. An adaptable organometallic cage has been recently designed using dicarboxylic spacers (dmnc-H2 ¼ 3,6dimethoxynaphthalene-2,7-dicarboxylic acid) connected to six p-cymene ruthenium corners.123 The cavity size of the prism

Figure 36

Sensing of Liþ in the cavity of [(h6-C6H5CO2Et)Ru(3-oxo-2-pyridonate)]3.121

326

Organometallic Chemistry in Directed Assembly

Figure 37

Molecular structure of [(coronene)2 3 {(p-PriC6H4Me)Ru}6(dmnc)6(tpt)2]6 þ.123

increases from nearly 0 Å3 in the absence of guests to more than 500 Å3 in the presence of coronene (Fig. 37); two molecules of coronene are stacked inside the cavity. Following a strategy developed by Süss-Fink,124 a series of cationic tetranuclear arene ruthenium metalla-cycles able to sense simple anions and multicarboxylate anions (oxalate, citrate, tartrate) were developed.125 For example, the tetracationic tetranuclear complex, [{(p-PriC6H4Me)Ru}4(dotq)2(dpo)2]4 þ (dotq ¼ 6,11-dihydroxynaphthacene-5,12-dionato; dpo ¼ di(pyridin-4-yl)oxalamide) (Fig. 38), can interact strongly with oxalate, while analogous tetranuclear metalla-bowls such as [{(p-PriC6H4Me) Ru}4(dotq)2(bpcp)2]4 þ (bpcp-H2 ¼ 2,6-bis{N-(pyridin-4-yl)carbamoyl}pyridine) also interact with citrate and tartrate by hydrogen bonding between the inner amido groups of the metalla-cage and the oxygen atoms of the anionic guests.126 Other than sensing, arene ruthenium metalla-rectangles incorporating N,N-bridging ligands and p-cymene ruthenium clips are potential anticancer agents.7 The antiproliferative activity of rectangles was investigated on several cancer cell lines, showing excellent anticancer activity. They can also interact with DNA and proteins. Extending this strategy to metalla-prisms and metalla-cubes, hexanuclear and octanuclear metalla-cages incorporating half-sandwich complexes, [{(p-PriC6H4Me)Ru}6(dobq)3(tpt)2]6 þ (dobq ¼ 2,5-dioxido-1,4-benzoquinonato; tpt ¼ 2,4,6-tris (donq ¼ 5,8-dioxido-1,4-naphthoquinonato; (4-pyridyl)-1,3,5-triazine) and [{(p-PriC6H4Me)Ru}8(donq)4(tpvb)2]8 þ tpvb ¼ 1,2,4,5-tetrakis{2-(4-pyridyl)vinyl}benzene), were shown to act as drug delivery vectors for hydrophobic molecules.127 Interesting effects were observed when the properties of these host-guest systems were studied. First, the degree of porphin release was found to be greater in the case of the larger octanuclear cage. In addition, inclusion of the photosensitizer within the inner-space of the cages gave rise to hypochromism. The result of this phenomenon is the absence of phototoxicity through encapsulation. These host-guest systems, therefore, possess safety upon photosensitization of the skin, and ultimately can generate better efficiency during photodynamic treatment.

Figure 38 Molecular structure of the tetranuclear metalla-cycle [{(p-PriC6H4Me)Ru}4(dotq)2(dpo)2]4 þ able to interact with oxalate anions.125 Hydrogen atoms are omitted for clarity.

Organometallic Chemistry in Directed Assembly

Figure 39

327

Molecular structure of [Pt(acac)2 3 {(p-PriC6H4Me)Ru}6(dobq)3(tpt)2]6 þ.128

Protecting and shielding molecules offers great potential for biomedical application, and the development of drug delivery systems remains an active field of research. New drug delivery systems may overcome drug resistance mechanisms by better targeting diseases, and if well designed, they can potentially regulate drug release, thus increasing the pharmacokinetics of the drugs. With that in mind, arene ruthenium metalla-cages were developed for the drug delivery of palladium and platinum complexes.128 The hexacationic hexanuclear arene ruthenium metalla-cage [{(p-PriC6H4Me)Ru}6(dobq)3(tpt)2]6 þ was used to transport to cancer cells two highly hydrophobic complexes, Pd(acac)2 and Pt(acac)2 (Fig. 39). The metalla-cage was found to be moderately cytotoxic with an IC50 of 23 mM on human ovarian A2780 cancer cell line, while the host-guest system [Pd(acac)2 3 {(p-PriC6H4Me) Ru}6(dobq)3(tpt)2][CF3SO3]6 was 20 times more active with an IC50 of only 1 mM. The strategy to use the hydrophobic cavity of water-soluble organometallic cages to hide and transport lipophilic guest molecules has already been extended to other metal centers, and several other guest compounds have been encapsulated and delivered to cells, showing the adaptability of these systems.129 Association between arene ruthenium metalla-cycles and pyrenyl-functionalized dendrimers has generated supramolecular liquid-crystalline hybrid materials.130 Upon encapsulation in the hydrophobic cavity of the metalla-rectangle, the liquid crystalline phase shifts from smectic to cubic, showing that liquid crystalline phases can be maintained and modified by host-guest chemistry. This finding has opened up new perspectives for designing new materials.

6.12.8

Conclusion

Organometallic assemblies have already found applications in biology, in host-guest chemistry, and in materials science. The use of organometallic building blocks will certainly become even more popular in directed assembly, a rapidly growing field with great perspectives.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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