Self-Assembled Hexanuclear Organometallic Cages: Synthesis, Characterization, and Host-Guest Properties

Selective Formation of Small and Large Coordination Cages and Their Catalytic Differences

Self-assembly of CuX2 (X- = BF4-, ClO4-, and CF3SO3-) with a new tridentate 5,5',5″-(((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris(oxy))triisoquinoline (L) gives rise to single-crystal pairs consisting of small and large cages, [X@Cu2X2L4]X and [Cu6X12L8], respectively, via selection of solvents. In particular, the large cage is transformed into a small cage in acetonitrile above 50 °C. A significant difference in heterogeneous catechol oxidation catalysis between the small and large cages is observed. Such notable catechol-oxidation-catalytic effects can be explained by maintenance of the Cu···Cu distance at the catalytic center. This research is a direct systematic example of both cage-size control via solvent selection and the significance of the Cu···Cu distance in catechol oxidation catalysis with copper (Cu).

Preparation of cage-shaped hexakis(spiroborate)s

In previous research studies, various types of prismatic cage-shaped molecular containers have been prepared and evaluated in terms of their guest inclusion properties. Basically, most of these molecular cages have a cationic or electron-deficient nature, and exhibit strong affinity mainly toward electron-rich aromatic guests. On the other hand, there is no report concerning anionic prismatic cages that are expected to recognize cationic polyaromatic guests with various structures and functions. In this manuscript, we present the preparation of hexakis(spiroborate)-type molecular cages, which was achieved by the reaction of phenylene- or biphenylenebis(dihydroxynaphthalene), hexahydroxytriphenylene, and boric acid in N,N-dimethylformamide. Their triangular prismatic hollow structures were confirmed by X-ray crystallographic analysis, and it was found that both phenylene- and biphenylene-bridged spiroborate cages have internal cavities of the corresponding size. It was also revealed that tetra(n-butylammonium) cations located inside the cavity and between the two adjacent spiroborate cages resulted in the formation of a one-dimensional columnar array. The molecular recognition behavior of the spiroborate cages was evaluated using tris(pyridinium)triazines as tricationic aromatic guests. ¹H NMR measurement implied that a discrete 1 : 1 host-guest complex was formed when 1 equiv. of guest was added to the cage, whereas infinite one-dimensional aromatic stacks were constructed by the addition of 2 equiv. of guest.

On/off fluorescence emission induced by encapsulation, exchange and reversible encapsulation of a BODIPY-guest in self-assembled organometallic cages

A reversible fluorescence turn off/on switch induced by the encapsulation and release of a guest molecule within an organometallic cage was presented.

Gold-Clip-Assisted Self-Assembly and Proton-Coupled Expansion-Contraction of a Cofacial FeIII -Porphyrin Cage

A molecular cage {Au₈ (μ-PAnP)₄ [Fe(H₂ O)₂ (TPyP)]₂ (OTf)₂ }(OTf)₈ (1) composed of two cofacial Fe^III -porphyrin can be self-assembled from the gold clip [Au₂ (PAnP)Cl₂ ] and Fe³⁺ (H₂ O)₂ (TPyP)⁺ (PAnP=9,10-bis(diphenylphosphino)anthracene, TPyP=meso-tetra(4-pyridyl)porphyrinato). The height of the cage is 8.579(3) Å. The addition of a base to a solution of the cage leads to a contracted and twisted cage {[Au₈ (μ-PAnP)₄ [Fe₂ (μ-O)(TPyP)₂ ]}(OTf)₈ (2), which has a height of ≈4.4 Å and porphyrin-porphyrin torsional angle of ≈20°. The contracted cage can be synthesized independently from the gold clip and Fe₂ (μ-O)(TPyP)₂ . The spectroscopy and crystal structure of an unclipped analog of the contracted cage, {[AuPPh₃ )₈ [Fe₂ (μ-O)(TPyP)₂ ]}(OTf)₈ (3), supports the DFT-calculated structure of 2. NMR and UV/Vis titrations show that the expansion-untwisting and contraction-twisting of the cage is reversible.

Controllable assembly of rectangular macrocycles bearing different numbers of unsaturated sites based on half-sandwich iridium fragments

A series of hexanuclear rectangular macrocycles were designed and synthesized by utilizing multifunctional pyrazine-derived (pyrazine-2,3-diamine (H4L1)) and quinoxaline-derived ligands (2,3-dihydroxy-quinoxaline (H2L2)) featuring two monodentate sites and one pair of chelating sites. X-ray crystallography in combination with 1H NMR spectroscopy elucidated that both half-sandwich iridium diimine and dihydroxy moieties are located on either side of the rectangular macrocycles, making them centrosymmetric. Thereby, the prepared diimine-functionalised complexes were found to have unsaturated metal sites on account of their strongly bound Ir-N-C-C-N arrangement. However, all the iridium atoms in the rectangular macrocycles containing dihydroxy groups were found to adopt an 18-electron coordination configuration, indicating that the O,O'-bonded iridium centers had bound additional ligands, such as Cl-, MeOH, MeCN, etc. Notably, a rare rectangular macrocycle containing a single coordinatively unsaturated metal site was achieved when the ligands H2L12- and L22- were introduced simultaneously.

Half-sandwich rhodium and iridium metallamacrocycles constructed via C-H activation

Half-sandwich rhodium and iridium complexes with carboxylic acid ligands were combined with pyrazine, 4,4'-bipyridine (bpy) or trans-1,2-bis(4-pyridyl)-ethylene (bpe) to give a series of tetranuclear macrocycles. The metallamacrocycles [(Cp*Rh)4()2(pyrazine)2][OTf]2 (), [(Cp*Rh)4()2(bpy)2][OTf]2 (), [(Cp*Rh)4()2(bpe)2][OTf]4 () and [(Cp*Ir)4()2 (pyrazine)2] () ( = 3-(2-pyridyl)acrylic acid, = 1,4-di(4-carboxyphenyl)benzene) were characterized by elemental analysis, NMR, IR and single-crystal X-ray analyses. Due to the different structures of the carboxylate ligands, the complexes , and were synthesized through double-site C-H activation, and complexes were obtained by one-site C-H activation.

A stepwise assembly of a molecular box from 16-electron half-sandwich precursors [Cp*M(pdt)] (M = Rh, Ir)

The coordinatively unsaturated 16-electron half-sandwich precursors [Cp*M(pdt)] (M = Rh and Ir, pdt = pyrazine-2,3-dithiol) have been synthesized. Their bridging and unsaturated attributes are further used in stepwise assembly reactions with binuclear blocks to give a closed molecular box.

Half-sandwich iridium- and rhodium-based organometallic architectures: rational design, synthesis, characterization, and applications

CONSPECTUS: Over the last two decades, researchers have focused on the design and synthesis of supramolecular coordination complexes, which contain discrete functional structures with particular shapes and sizes, and are similar to classic metal-organic frameworks. Chemists can regulate many of these systems by judiciously choosing the metal centers and their adjoining ligands. These resulting complexes have unusual properties and therefore many applications, including molecular recognition, supramolecular catalysis, and some applications as nanomaterials. In addition, researchers have extensively developed synthetic methodologies for the construction of discrete self-assemblies. One of the most important challenges for scientists in this area is to be able to synthesize target structures that can be controlled in both length and width. For this reason, it is important that we understand the factors leading to special shapes and sizes of such architectures, especially how starting building blocks and functional ligands affect the final conformations and cavity sizes of the resulting assemblies. Towards this goal, we have developed a wide range of different organometallic architectures by rationally designing metal-containing precursors and organic ligands. In this Account, we present our recent work, focusing on half-sandwich iridium- and rhodium-based organometallic assemblies that we obtained through rational design. We discuss their synthesis, structures, and applications for the encapsulation of guests and enzyme-mimicking catalysis. We first describe a series of self-assembled organometallic metallarectangles and metallacages, which we constructed from preorganized dinuclear half-sandwich molecular clips and suitable pyridyl ligands. We extended this strategy to tune the size of the obtained rectangles, creating large cavities by introduction of larger molecular clips. The cavity was found to exhibit selective and reversible CH2Cl2 adsorption properties while retaining single crystallinity. By using suitable molecular clips, we found we could use a number of metallacycles as organometallic templates to direct photochemical [2 + 2] cycloaddition reactions, even in the solid state. Due to their chemical stability and potential applications in catalytic reactions, researchers are giving significant attention to complexes with cyclometalated backbones. We also highlight our efforts to develop efficient approaches to utilize cyclometalated building blocks for the formation of organometallic assemblies. By incorporation of imine ligands or benzoic acids, bipyridine linking subunits, and half-sandwich iridium or rhodium fragments, we built up a series of cationic and neutral metallacycles through cyclometalation-driven self-assembly. In addition, we have developed an efficient route to carborane-based metallacycles, involving the exploitation of metal-induced B-H activation. The method can provide prism-like metallacages, which are efficient hosts for the recognition of planar aromatic guests. This effort provides an incentive to generate new building blocks for the construction of organometallic assemblies. Taken together, our results may lead to a promising future for the design of complicated enzyme-mimetic-catalyzed systems.