Box-like Heterometallic Macrocycles Derived from Bis-Terpyridine Metalloligands

Coordination-directed self-assembly of nano-cages: metal ion-change, ligand-extending, shape-control and transdermal drug delivery

The combination of different pyridyl ligands and metal ions has proven to be a very reliable strategy for controlling the coordination mode of the heterometallic coordination nano-cages. Adjusting the length of the ligands could result in the selective synthesis of several heterometallic coordination nano-cages, either [8Rh + 2M]-4L, [8Rh + 2M]-5L or [8Rh + 4M]-6L cages, derived from the very same precursors (LH3tzdc) through half-sandwich rhodium self-assembly. Moreover, a series of [8Rh + 4M]-6L cages was chosen to exemplify the preparation. The rigidity of various pyridyl donor ligands caused the vertical nano-cage to be energetically preferred and was able to change the self-assembly process through ligand flexibility to selectively give the inclined nano-cage and cross nano-cage.

Stoichiometrically Controlled Assembly of Lanthanide Molecular Complexes of the Heteroditopic Divergent Ligand 4'-(4-Pyridyl)-2,2':6',2″-terpyridine N-Oxide in Hypodentate or Bridging Coordination Modes. Structural, Magnetic, and Photoluminescence Studies

Mononuclear rare-earth tris-β-diketonato complexes RE(tta)₃dme [RE = Y (1), La (2), Dy (3), or Eu (4); Htta = 2-thenoylacetone; dme = 1,2-dimethoxyethane] react cleanly at room temperature in a 1:1 molar ratio with the heteroditopic divergent ligand 4'-(4-pyridyl)-2,2':6',2″-terpyridine N-oxide (pyterpyNO) to yield RE₂(tta)₆(pyterpyNO)_n, where n = 2 for RE = Y (5), Dy (6), or Eu (7) and n = 3 for RE = La (8). The crystal structure of 5 revealed a dinuclear compound with two pyterpyNO's bridging through the oxygen atom in a hypodentate mode leaving the terpyridine moieties uncoordinated. Using a metal:pyterpyNO molar ratio of 2 for RE = Y (9), Dy (10), or Eu (11), it was possible to isolate the molecular complexes RE₄(tta)₁₂(pyterpyNO)₂, while using a 5:3 molar ratio, the product La₅(tta)₁₂(pyterpyNO)₃ (12) can be obtained. ⁸⁹Y nuclear magnetic resonance spectroscopy revealed two different yttrium centers at room temperature for 9. An X-ray diffraction study of 10 showed a symmetrical tetranuclear structure resulting from the coordination of two Dy(tta)₃ fragments to the two hypodentate terpyridines of the dinuclear unit and presenting two different coordination sites for metals with coordination numbers of 8 and 9. Magnetic studies of 6 and 10 revealed the presence of an antiferromagnetic interaction between the two Dy(III) atoms bound by the NO bridges. These compounds displayed a slow relaxing magnetization through Orbach (6) and Raman (10) processes in the absence of an applied magnetic field; the rate increased upon application of a 1 kOe field. 7 and 11 showed a bright red emission typical of Eu³⁺. The two complexes have similar emission properties mainly determined by the employed β-diketonato ligands.

Better Together: Functional Heterobimetallic Macrocyclic and Cage-like Assemblies

Metallosupramolecular chemistry has attracted the interest of generations of researches due to the versatile properties and functionalities of oligonuclear coordination complexes. Quite a number of different discrete cages were investigated, mostly consisting of only one type of ligand and one type of metal cation. Looking for ever more complex structures, heterobimetallic complexes became more and more attractive, as they give access to new structural motifs and functions. In the last years substantial success has been made in the design and synthesis of cages consisting of more than one type of metal cations, and a rapidly growing number of functional materials has appeared in the literature. This Minireview describes recent developments in the field of discrete heterometallic macrocycles and cages focusing on functional materials that have been used as host‐systems or as magnetic, photo‐active, redox‐active, and even catalytically active materials.

Facile synthesis of multicomponent heterobimetallic metallomacrocycles through selective metal–ligand coordination

Through selective coordination, three heterobimetallic Zn^II–Pd^II/Pt^II metallomacrocycles have been constructed by either a stepwise or a one-pot self-assembly protocol.

Molecular Borromean Rings Based on Half-Sandwich Organometallic Rectangles

Over the last two decades, interlocked molecular species have received considerable attention, not only because of their intriguing structures and topological importance, but also because of their potential applications as smart materials, nanoscale devices, and molecular machines. Through judicious choice of metal centers and their adjoining ligands, a range of interesting interlocked structures have been realized by coordination-driven self-assembly. In addition, researchers have extensively developed synthetic methodologies for the construction of organized self-assemblies. One fascinating and challenging synthetic target in this field is the family of molecular Borromean rings, which consist of three chemically independent rings that are locked in such a way that no two of the three rings are linked with each other. Toward this goal, we have developed a template-free self-assembly method for synthesizing molecular Borromean rings by rationally designing metal-containing precursors and organic ligands. In this Account, we present our recent work, focusing on interlocked structures comprising half-sandwich iridium- and rhodium-based organometallic assemblies obtained by rational design. We first describe a series of template-free self-assembled organometallic molecular Borromean rings, which we constructed from preorganized binuclear half-sandwich molecular clips and suitable pyridyl ligands. These molecular Borromean rings can be sorted into four types according to their different bridging ligands, including those based on metallaligands, dihalogenated ligands, naphthazarin and π-acceptor ligands. By single-crystal X-ray crystallographic analysis, NMR experiment, and DFT calculation, we discuss their driving forces and the inter-ring interactions. Furthermore, we took advantage of the dissimilarity in their interactions to realize selective, reversible conversions between molecular Borromean rings and monomeric rectangles by the use of suitable solvents or guest molecules. Subsequently, a stepwise chemoseparation method based on molecular Borromean rings was established, with the molecular Borromean rings used in the separation being recoverable and recyclable. Due to their structural complexity and difficult synthesis, useful guidelines or rules to help design complicated interlocked molecules are highly desirable. We also highlight our efforts to develop empirical guidelines to uncover the relationship between the aspect ratio of metallarectangles and the formation or stability of molecular Borromean rings. An empirical formula has further been established to show the approximate ratio of lengths of the short arm and the long arm in molecular Borromean rings based on π-π (or p-π) stacking. We then demonstrate how to use these guidelines to design new molecular Borromean rings and further lead to other interlocked structures, for example, [2]- and [3]catenane structures. Taken together, our results may lead to a promising future for the design of fascinating and useful interlocked structures by coordination-driven self-assembly.

Selective formation of discrete versus polymeric copper organophosphates: DNA cleavage and cytotoxic activity

Copper phosphate metalloligands [Cu(X-dipp)(Pyterpy)]₂ [X = H (1), Br (2)], exemplifying expanded 4,4'-bipyridine type molecules, have been synthesized by reacting 4'-(4-pyridyl)-2,2':6',2''-terpyridine (Pyterpy) and para substituted 2,6-diisopropylphenyl phosphate (X-dippH₂) with copper acetate. The pendant N,N-ends of dimeric copper phosphates 1 and 2 have been forced to engage in further coordination by limiting the concentration of Pyterpy in the reaction mixture to yield rare Pyterpy bridged corner-shared polymeric copper phosphates [Cu₂(X-dippH)(X-dipp)(Pyterpy)(H₂O)]_n [X = Cl (3), Br (4), I (5)]. The formation of 1-5 is supported by spectroscopic and analytical data. The solid state structures of these compounds have further been confirmed by single-crystal X-ray diffraction studies. Soluble dimeric complexes 1 and 2 have been assessed for their in vitro anti-tumour properties against human breast and colorectal cancer cell lines. The DNA cleavage, protein cleaving and cytotoxicity assays revealed that these compounds are effective in cleaving DNA, while the activity of 1 as an anti-tumor agent is better than 2.

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.

Light-initiated reversible conversion of macrocyclic endoperoxides derived from half-sandwich rhodium-based metallarectangles

A series of metallarectangles were synthesized by an anthracene-based ligand and three different half-sandwich rhodium precursors. The photochemical reactions show that these metallarectangles can be reversibly converted to the macrocyclic endoperoxides.

Terpyridine-based metallosupramolecular constructs: tailored monomers to precise 2D-motifs and 3D-metallocages

Comprehensive summary of the recent developments in the growing field of terpyridine-based, discrete metallosupramolecular architectures.

A nonanuclear triangular macrocycle and a linear heptanuclear heterometallic complex based on a 2-substituted imidazole-4,5-dicarboxylate ligand

A Cp*Rh-based nonanuclear triangular macrocycle complex [(Cp*Rh)9L3(NO3)4.5(MeOH)](OTf)4.5 (1), a Cp*Ir-based trinuclear complex [(Cp*Ir)3L(MeCN)4](OTf)3 (2) and a linear heptanuclear heterometallic complex [(Cp*Ir)6ZnL2(MeCN)8(MeOH)2](OTf)8 (3) (Cp* = η(5)-pentamethylcyclopentadienyl) have been synthesized from a 2-(4-(pyridin-4-yl)phenyl)-1H-imidazole-4,5-dicarboxylic acid proligand. These complexes were further characterized by X-ray crystallography, (1)H NMR, DOSY NMR, IR spectroscopy, and elemental analyses.

[NO]- and [NN]-coordination mode rhodium complexes based on a flexible ligand: synthesis, reactivity and catalytic activity

Two rhodium(i) complexes were synthesized by the reaction of an [NON]-flexible ligand with Rh(COD)₂BF₄ under different reaction conditions.

Crystal structures of the water and acetone monosolvates of bis-[4'-(pyridin-4-yl)-2,2':6',2''-terpyridine]-manganese(II) bis-(hexa-fluorido-phosphate)

The crystal structures of bis-[4'-(pyridin-4-yl)-2,2':6',2''-terpyridine]-man-gan-ese(II) bis-(hexa-fluorido-phosphate) monohydrate, [Mn(C20H14N4)2](PF6)2·H2O, (1), and bis-[4'-(pyridin-4-yl)-2,2':6',2''-terpyridine]-manganese(II) bis(hexa-fluorido-phosphate) acetone monosolvate, (2), [Mn(C20H14N4)2](PF6)2·CH3COCH3, are described. At 150 K, (1) and (2) have monoclinic (P21/c) and ortho-rhom-bic (C2221) symmetries, respectively. Both structures exhibit octahedrally coordinated Mn(II) atoms and disorder. They display weak inter-actions, such as C-H⋯F, C-H⋯N, C-H⋯π, F⋯π and π-π. The twofold rotation axis in the molecule of (2) is coincident with a twofold rotation axis of the crystal.

Hydrothermal syntheses, structural characterizations, and magnetic properties of five MOFs assembled from C2-symmetric ligand of 1,3-di(2′,4′-dicarboxylphenyl)benzene with various coordination modes

Five new complexes with appealing structural features from 0D paddle wheel {Cu₂(COO)₄} SBUs to 3D frameworks were reported to better understand the synthon selectivity in multifunctional crystal structures.

Structural diversities and related properties of four coordination polymers synthesized from original ligand of 3,3′,5,5′-azobenzenetetracarboxylic acid

Four CPs, derived from the original 3,3′,5,5′-azobenzenetetracarboxylic acid ligand were obtained under solvothermal conditions.

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.