Half-sandwich Cp*Ir and Cp*Rh metalacycles have been successfully applied in traditional domains encompassing organic transformations and catalysis in recent years, especially the catalytic activation of C-H bonds. Cyclometalation has proven to be a highly attractive and versatile synthetic method for the formation of organometallic metalacycles. This review intends to describe isolated and well-defined cyclometalated iridium/rhodium complexes that contain a Cp*M-C (M = Ir, Rh) bond stabilised by the intramolecular coordination of neutral donor atoms (N, C, O or P). The formation of metalamacrocycles and cages employing cyclometalated approaches is discussed. In focusing on selected mechanistic insights garnered from iridium/rhodium-catalysed functionalisation of C-H bonds involving cyclometalated complexes, a limited number of substrates will be discussed, but a broad range of mechanistic features is highlighted.

Since the emergence of the concept of chemical topology, interlocked molecular assemblies have graduated from academic curiosities and poorly defined species to become synthetic realities. Coordination-directed synthesis provides powerful, diverse, and increasingly sophisticated protocols for accessing interlocked molecules. Originally, metal ions were employed solely as templates to gather and position building blocks in entwined or threaded arrangements. Recently, metal centers have increasingly featured within the backbones of the integral structural elements, which in turn use noncovalent interactions to self-assemble into intricate topologies. By outlining ingenious recent examples as well as seminal classic cases, this Review focuses on the role of metal−ligand paradigms in assembling molecular links. In addition, the ever-evolving approaches to efficient assembly, the structural features of the resulting architectures, and their prospects for the future are also presented.

Molecular squares obtained from two olefin-bridged bis(NHC) ligands, NHC-Ar-C═C-Ar-NHC, and two Ag(+) or Au(+) ions undergo postsynthetic modifications via a UV-irradiation-initiated [2 + 2] cycloaddition reaction to yield the corresponding cyclobutane-bridged dinuclear tetrakis(NHC) complexes. The tetrakis(NHC) ligand can be liberated from the Ag(I) complexes as the tetraimidazolium salt. For the Au(I) complexes, the substituents at N3 and N3' of the dicarbene ligands determine the outcome of the reaction in the solid state.

Picolyl-functionalized N-heterocyclic carbene complexes have been synthesized by a route involving carbene transfer from Ag(I) carbene precursors. The Ag complexes undergo facile reaction with Ni(PPh 3 ) 2 Cl 2 to yield the carbene complexes Ni(C∧N) 2 Cl 2 (C∧N ) 3-methyl-1-picolylimidazolin-2-ylidene (2a), 3-benzyl-1-picolylimidazolin-2-ylidene (2b)). Complexes 2a,b have been characterized by IR and 1 H and 13 C NMR spectra and elemental analyses. The molecular structures of complexes 2a,b have been confirmed by X-ray single-crystal analyses. The carbene complex 2a shows high catalytic activities of up to 2.6 × 10 7 g of PNB (mol of Ni) -1 h -1 for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst and exhibits moderate catalytic activity (3.3 × 10 5 g of PE (mol of Ni) -1 h -1 ) for ethylene polymerization. Catalytic activities, polymer yield, molecular weights, and molecular weight distributions of polynorbornene have been investigated under the various reaction conditions.

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 ...

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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.

Over the past decade, supramolecular compounds with organometallic Ir, Rh, Ru based half-sandwich complexes have received considerable attention as materials with a variety of potential applications. The major emphasis of this tutorial review lies on the self-assembly of such organometallic half-sandwich molecular rectangles, prisms and cages with half-sandwich corners and two different rigid bifunctional ligands using an approach of stepwise construction. Synthetic methods and the structural and functional properties of the target complexes are discussed in detail.