Ying-Feng Han

Formation of direct metal–metal bonds from 16-electron “pseudo-aromatic” half-sandwich complexes Cp″M[E2C2(B10H10)]

Continuous study on preparation of multimetallic clusters is stimulated by their rich coordination chemistry and promising applications in a variety of interesting fields. Although numerous efforts have been devoted to this field, the rational design of homo- and hetero-multimetallic compounds with direct metal-metal bonding supported by 1,2-dicarba-closo-dodecarborane-1,2-dichalcogenolates will still be an important step forward. This tutorial review focuses on the synthetic approach via redox reactions between the pseudo-aromatic half-sandwich oraganometallic carborane precursors and low-valent transition metal reagents. The tailoring of reaction conditions and the structural information from the resulting products are discussed extensively.

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.

Stepwise Construction of Discrete Heterometallic Coordination Cages Based on Self-Sorting Strategy.

A chelation-directed self-sorting synthesis of a series of cationic heterometallic coordination cages (HCCs) with tunable sizes is described. Two complexation modes were found in the cage-forming process. Metal-anchoring host-guest behavior and size-selective in-cage catalytic activities were found for the HCCs.

Stepwise formation of “organometallic boxes” with half-sandwich Ir, Rh and Ru fragments

Octanuclear complexes with half-sandwich Ir, Rh and Ru fragments and tetra(4-pyridyl)porphyrin (L1) and oxalate (L2) spacer ligands [(Cp*M)4L1]2[L2]4 (M = Ir (6a) M = Rh (6b)), [((cymene)Ru)4(L1)]2[L2]4 were prepared and characterized.

Reversible Photochemical Modifications in Dicarbene‐Derived Metallacycles with Coumarin Pendants

Molecular rectangles were obtained from two bis(NHC) ligands, each featuring two terminal coumarin groups and two Ag(+), Au(+), or Cu(+) ions. Upon UV irradiation (λ=365 nm), the dinuclear complexes undergo photochemical modification through a [2+2] cycloaddition reaction of two adjacent coumarin moieties to give a macrocyclic tetra(NHC) ligand. The photodimerization of the coumarin pendants proceeds stereoselectively to give the syn-head-head isomers in all cases. Subsequent irradiation at λ=254 nm initiates a photocleavage reaction with reconstitution of the initial dinuclear complexes with coumarin pendants.

H2-Initiated Reversible Switching between Two-Dimensional Metallacycles and Three-Dimensional Cylinders

Although reversible covalent activation of molecular hydrogen (H2) by transition-metal complexes is a common reaction, H2-mediated sophisticated reversible arrangements of organometallic frameworks have not yet been described. Herein, we report unusual organometallic transformations in solution that can be accomplished by uptake or release of H2. An efficient route for synthesizing air- and moisture-stable 16-electron M2L2-type metallacycles under very mild conditions has been developed. The new organometallic metallacycles favor the binding of small ligands such as MeCN, Cl(-), CO, and pyridine. The reaction of a coordinatively unsaturated 16-electron M2L2-type macrocyclic complex featuring thione ligands with 1 atm of H2 leads to the isolation of a 18-electron M2L3-type cylinder, along with hydride species. Remarkably, the obtained mixture underwent loss of H2 in a facile manner upon heating to re-form the starting M2L2-type complex. A possible mechanism is proposed for the reversible transformations.

Facile Separation of Regioisomeric Compounds by a Heteronuclear Organometallic Capsule

Owing to the often-similar physical and chemical properties of structural isomers of organic molecules, large efforts have been made to develop efficient strategies to isolate specific isomers. However, facile separation of regioisomeric compounds remains difficult. Here we demonstrate a universal organometallic capsule in which two silver centers are rigidly separated from each other by two tetranuclear [Rh4] pyramidal frustums, which selectively encapsulate a specific isomer from mixtures. Not only is the present heterometallic capsule suitable as a host for the encapsulation of a series of aromatic compounds, but also the receptor shows widely differing specificity for the various isomers. Direct experimental evidence is provided for the selective encapsulation of a series of para (p)-disubstituted benzene derivatives, such as p-xylene, p-dichlorobenzene, p-dibromobenzene, and p-diiodobenzene. The size and shape matching, as well as the Ag-π interactions, are the main forces governing the extent of molecular recognition. The encapsulated guest p-xylene can be released by using the solid-liquid solvent washing strategy, and the other guest molecules are easily liberated by using light stimulus.

Efficient formation of organoiridium macrocycles via C–H activation directed self-assembly

Versatile and efficient procedures for the construction and modification of organometallic macrocycles with half-sandwich Ir corners via C-H activation and self-assembly have been developed.

Synthesis and structural characterization of binuclear half-sandwich iridium, rhodium and ruthenium complexes containing 4,4'-dipyridyldisulfide (4DPDS) ligands.

Reactions of half-sandwich iridium and rhodium complexes [Cp*MCl2]2 (1a: M = Ir, 1b: M = Rh) and the half-sandwich ruthenium complex [(p-cymene)RuCl2]2 (1c) with 4DPDS gave the corresponding binuclear complexes [{Cp*MCl2]2}(m-4DPDS) (2a: M = Ir, 2b: M = Rh) and [{(p-cymene)RuCl2]2}(m-4DPDS) (2c), which can be converted into binuclear metallamacrocycles. The metallamacrocycles [Cp*M(m-4DPDS)Cl]2(2+) (3a: M = Ir, 3b: M = Rh) and [(p-cymene)Ru(m-4DPDS)Cl]2(2+) (3c) are composed of two half-sandwich units and two 4DPDS ligands with the same chirality. The macrocycle 3c is chiral and changes its shape slightly depending on the guests accommodated above and below the cavities.

Facile Synthesis of Size‐Tunable Functional Polyimidazolium Macrocycles through a Photochemical Closing Strategy

A synthetic strategy for the preparation of a series of polyimidazolium macrocycles from the corresponding dicarbene-derived metallacycles is described. Photodimerization of terminal cinnamic esters (UV-irradiation, λ=365 nm) produces the closed metallacycles with perfect stereoselectivity and high yields. Subsequent removal of the template from the photodimerization product results in polyimidazolium macrocycles. The size and shape of the receptor can be tuned easily by changing the length and breadth of the internal bridging groups of the ligands. Preliminary investigation shows the potential of the macrocycle as iodide sensor.