Shape-persistent macrocycles: structures and synthetic approaches from arylene and ethynylene building blocks

Shape-persistent arylene ethynylene macrocycles have attracted much attention in supramolecular chemistry and materials science because of their unique structures and novel properties. In this Review we describe recent examples of macrocycle synthesis by cross-coupling (Sonogashira: aryl acetylene macrocycle or Glaser: aryl diacetylene macrocycle) and dynamic covalent chemistry. The primary disadvantage of the coupling methods is the kinetically determined product distribution, since a significant portion of oligomers grow beyond the length of the cyclic targets ("overshooting"). Better results have been obtained recently by a dynamic covalent approach involving reversible metathesis reactions that afford macrocycles in one step. Mechanistic studies demonstrate that macrocycle formation is thermodynamically controlled by this route. Remaining synthetic challenges include the efficient preparation of site-specifically functionalized structures and larger, more complex two- and three-dimensional molecules.     

Shape-persistent macrocycles: building blocks for complex organic and polymeric architectures

Alkyl- and Oligostyrene substituents were attached to shape-persistent macrocycles based on a phenyl-ethynyl backbone. In good solvents for both the rigid core and the flexible corona no aggregation occurred. Whereas, addition of a solvent that selectively solubilizes the corona induced a solvophobic aggregation. For alkyl substituted rings the experimental data were described by a monomer-dimer equilibrium. In contrast, the oligostyrene substituted rings formed more expanded aggregates which were investigated by scattering and by imaging methods. The superstructures are consequently described as hollow supramolecular cylindrical brushes.     

A versatile building block for pyrazole-pyrrole hybrid macrocycles

This contribution describes the synthesis of a novel pyrazole-pyrrole building block and its use in the formation of a non-aromatic, Schiff base-type macrocycle incorporating a chromophore and H-bonding donor and acceptor functionalities inside and outside of the macrocycle, which makes it predestined for molecular recognition systems.     

Synthesis of novel steroid-peptoid hybrid macrocycles by multiple multicomponent macrocyclizations including bifunctional building blocks (MiBs)

Two new groups of cholane-peptoid hybrid macrocycles were produced by implementing novel combinations of the MiB methodology. Steroid-based hybrid macrolactams including heterocycle and aryl moieties were obtained by utilizing cholanic dicarboxylic acids and diamines in a bidirectional double Ugi-Four-Component (Ugi-4CR) based macrocyclization protocol. Alternatively, N-substituted cyclocholamides were produced from a cholanic pseudo-amino acid by an Ugi-4CR-based cyclooligomerization approach. Both types of macrocycles are steroid-peptoid hybrid macrocycles containing exocyclic peptidic chains. These novel frameworks are a result of the use of bile acids bifunctionalized with carboxylic and amino functionalities as bifunctional building blocks of the Ugi-MiB approach.     

N-monofunctionalized 1,4,7-triazacyclononane macrocycles as building blocks in inorganic crystal engineering

The syntheses of N-(3-prop-1-ene)-1,4,7-triazacyclononane molybdenum tricarbonyl (2), N-(4-but-1-ene)-1,4,7-triazacyclononane molybdenum tricarbonyl (3), N-(3-prop-1-ene)-1,4,7-triazacyclononane molybdenum trioxide (5), N-(4-but-1-ene)-1,4,7-triazacyclononane molybdenum trioxide (6), N-(hydroxyethyl)-1,4,7-triazacyclononane molybdenum trioxide (7), and N-(2-methylpyridyl)-1,4,7-triazacyclononane molybdenum trioxide (8) have been achieved. The objective of this work is to systematically vary the functionality of the pendant group in order to create different crystal packing in the solid state. This is evidenced in comparing the structures of 1,4,7-triazacyclononane molybdenum trioxide (4) and 5-8, which were determined using X-ray crystallography. The synthesis and characterization of the new ligand N-(2-methylpyridyl)-1,4,7-triazacyclononane (L5) is reported.     

Synthesis of chiral building blocks for cephalostatin analogues

The present paper describes the synthesis of 5‐azido‐6‐ketones (14) and 6‐hydroxy‐5‐ketone (20) from Hajos Wiechert ketone as chiral building blocks for cephalostatin analogues. The synthesis of symmetric cephalostatin analogue from 6‐hydroxy‐5‐ketone has also been reported. The characterization of the each synthesized compounds was carried out by IR, ¹H‐NMR, ¹³C‐NMR and High resolution Mass Spectrometry.     

Microporous metal-organic frameworks formed in a stepwise manner from luminescent building blocks

A series of trivalent lanthanides (Sm, Eu, Gd, Tb, Dy) have been complexed to the dianionic ligand, 4,4'-disulfo-2,2'-bipyridine-N,N'-dioxide, L, in a 3:1 ratio to form trianionic complex building blocks. These units were then cross-linked into a network solid by addition of BaCl2 to form mixed-metal networks of formula {Ba2(H2O)4[LnL3(H2O)2](H2O)nCl}infinity, Ln = Sm3+ (1), Eu3+ (2), Gd3+ (3), Tb3+ (4), Dy3+ (5). The networks were isostructural and contained open channels which readily absorbed and desorbed water accompanied by a spongelike shrinkage and expansion of the host. CO2 sorption measurements confirmed microporosity giving a DR surface area of 718 m2/gm and an average pore size of 6.4 A. Ligand L sensitized all the lanthanide ions with the exception of Gd3+. Studying the series of Ln complexes allowed the determination of the triplet state energy of L which is itself a new ligand for sensitization purposes. The luminescent properties of the lanthanide building blocks were retained in the porous network solid. From the luminescence data, it was possible to attribute the spongelike properties of the network to the Ba2+ coordination sphere rather than the Ln3+ center. Networks were characterized by X-ray crystallography, PXRD, DSC/TGA, water vapor and gas sorption, and luminescence spectroscopy.     

Giant macrocycles composed of thiophene, acetylene, and ethylene building blocks

Fully conjugated giant macrocyclic oligothiophenes with 60pi, 90pi,120pi, 150pi, and 180pi frames (1, 2, 3, 4 and 5) have been designed, and their butyl-substituted derivatives (1a, 2a, 3a, 4a, and 5a) have been synthesized using modified Sonogashira and McMurry coupling reactions as key steps. The 60-180pi systems 1-5 are circular with 1.8-6 nm inner cavities and 3.3-7.5 nm outside molecular diameters. Compound 1a containing ten 3,4-dibutyl-2,5-thienylene, eight ethynylene, and two vinylene units has been converted into macrocyclic oligo(3,4-dibutyl-2,5-thienylene-ethynylene) 6a using bromination/dehydrobromination procedure. Giant macrocycles 1a-6a exhibit a red shift of their absorption spectra and a fairly strong fluorescence with a large Stokes shift as compared to a linear conjugated counterpart having five thiophene rings. Compounds 1a-6a exhibit multistep reversible redox behaviors with fairly low first oxidation potentials, reflecting their cyclic conjugation. Furthermore, chemical oxidation of 1a-6a with FeCl3 shows drastic changes of spectroscopic properties due to intramolecular and intermolecular pi-pi interactions. Doping of 1a-3a with iodine forms semiconductor due to its pi-donor properties and pi-pi stacking ability. X-ray analysis of 1a confirmed a round, planar structure with nanoscale inner cavity, and revealed host ability for alkanes and unique packing structure. Interestingly, 2a and 3a self-aggregate in the solid state to form "molecular wires," which are about 200 nm thick and more than 1 mm long. The internal structures of fibrous aggregates have been investigated by optical microscope, scanning electron microscopy, atomic force microscopy, and X-ray diffraction analyses.     

Synthesis of steroid-biaryl ether hybrid macrocycles with high skeletal and side chain variability by multiple multicomponent macrocyclization including bifunctional building blocks

Utilizing the multiple multicomponent macrocyclization including bifunctional building blocks (MiB) strategy, a library of nonracemic, nonrepetitive peptoid-containing steroid-biaryl ether hybrid macrocycles was built. Up to 16 new bonds, including those of the macrocyclization, can be formed in one pot simultaneously while introducing varied elements of diversity. Functional diversity is generated primarily by choosing Ugi-reactive functional building blocks, bearing the respective recognition or catalytic motifs. These appear attached to the peptoid backbone of the macrocyclic cavity, similar to side chains of amino acids found in enzyme active sites. Likewise, skeletal diversity is based on the variation of defined bifunctional building blocks which allow the parallel formation of macrocyclic cavities that are highly diverse in shape and size and thus perspectively in function. This straightforward approach is suitable to generate multifunctional macrocycles for applications in catalysis, supramolecular, or biological chemistry.     

Cyclic semipeptoids: peptoid-organic hybrid macrocycles

[structure: see text] Cyclic semipeptoids 1 and 2 represent constrained, secondary structure mimics where the R(1) and R(2) side chains correspond to those of amino acids. Solid-phase syntheses and conformational analyses of these compounds are described.     

Tetra- and octalactam macrocycles and catenanes with exocyclic metal coordination sites: versatile building blocks for supramolecular chemistry

The synthesis of a new tetralactam macrocycle and the simultaneous formation of catenanes and larger octalactam macrocycles is reported. These species bear 2,2'-biquinoline moieties suitably positioned to bind a metal center at the outer periphery of the macrocycles. (1)H NMR chemical shifts permit the unambiguous distinction of transoid and cisoid conformations of the biquinoline moiety, thereby allowing an unequivocal identification of the catenane and octalactam structures, despite the fact that both have the same elemental composition and bear identical structural subunits. With the aid of an anion template effect, rotaxanes can be prepared from the smaller tetralactam macrocycle. These reveal significantly altered requirements in terms of the stopper size as compared to previously reported tetralactam wheels. Several copper(I)-mediated dimers and a (bpy)(2)Ru(II) complex (bpy=2,2'-bipyridine) have been synthesized from the tetralactam macrocycle and the rotaxanes. The anion binding abilities of the tetralactam macrocycle and its (bpy)(2)Ru(II) complex in DMSO have been compared by (1)H NMR titration experiments, which revealed significantly enhanced binding by the metal complex. Mass spectrometry has been used to study the potential formation of larger assemblies of copper(I) and the catenane built-up from two tetralactam macrocycles. Indeed, a 2:2 complex was identified. In contrast, the octalactam macrocycle of the same elemental composition yields only 1:1 complexes, with the Cu(I) ion connecting its two biquinoline moieties in the center of a figure-eight-shaped molecule. Molecular modeling studies support the structural assignments made.     

Novel photochromic macrocycles composed of thiophene and ethylene building blocks: synthesis, structure, and photochromic property

Two novel photochromic macrocycles composed of thiophene and ethylene building blocks have been synthesized, and their crystal structure and photochromic properties are described. Macrocycle 2 shows good photochromic properties.     

Interpenetrating metal-organic frameworks formed by self-assembly of tetrahedral and octahedral building blocks

To investigate the relationship between topological types and molecular building blocks (MBBs), we have designed and synthesized a series of three-dimensional (3D) interpenetrating metal-organic frameworks based on different polygons or polyhedra under hydrothermal conditions, namely [Cd(bpib)_0.5(L¹)] (1), [Cd(bpib)_0.5(L²)]·H₂O (2), [Cd(bpib)_0.5(L³)] (3) and [Cd(bib)_0.5(L¹)] (4), where bpib=1,4-bis(2-(pyridin-2-yl)-1H-imidazol-1-yl)butane, bib=1,4-bis(1H-imidazol-1-yl)butane, H₂L¹=4-(4-carboxybenzyloxy)benzoic acid, H₂L²=4,4′-(ethane-1,2-diylbis(oxy))dibenzoic acid and H₂L³=4,4′-(1,4-phenylenebis(methylene))bis(oxy)dibenzoic acid, respectively. Their structures have been determined by single crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and thermogravimetric (TG) analyses. Compounds 1–3 display α-Po topological nets with different degrees of interpenetration based on the similar octahedral [Cd₂(–COO)₄] building blocks. Compound 4 is a six-fold interpenetrating diamondoid net based on tetrahedral MBBs. By careful inspection of these structures, we find that various carboxylic ligands and N-donor ligands with different coordination modes and conformations, and metal centers with different geometries are important for the formation of the different MBBs. It is believed that different topological types lie on different MBBs with various polygons or polyhedra. Such as four- and six-connected topologies are formed by tetrahedral and octahedral building blocks. In addition, with the increase of carboxylic ligands’ length, the degrees of interpenetration have been changed in the α-Po topological nets. And the luminescent properties of these compounds have been investigated in detail.     

Supramolecular architectures from phthalocyanine building blocks

Different procedures for preparing columnar stacks and layered structures from phthalocyanine molecules are presented and discussed, including organization of the molecules by one of the following means: (i) with the help of liquid crystallinity, (ii) by using crown ether rings attached to the phthalocyanine core, (iii) by polymerization, and (iv) by Langmuir-Blodgett film formation.     

Versatile building blocks from disaccharides: glycosylated 5-hydroxymethylfurfurals

A practical protocol for the elaboration of O-glycosyl-HMF’s from glycosyl-(1→6)-glucoses is reported, the two steps involving aluminate-promoted isomerization to the respective 6-O-glycosyl-fructoses and subsequent selective dehydration of the fructose portion. Accordingly, melibiose, gentiobiose, and primeverose are converted into the corresponding 2-uloses and, then, into α-GalMF 11, β-GMF 12, and β-XylMF 13. Pt/C-catalyzed oxidation with oxygen in NaOH at 25 °C efficiently generated the respective furoic acids from α-GalMF and α-GMF, whilst Pt/O₂ in water at 50 °C also oxidizes the primary OH to give the dicarboxylic acids 15 and 17–key building blocks for the generation of novel types of polyesters and polyamides.     

Octahedral niobium chloride clusters as building blocks of templated prussian blue framework analogues

The preparation, structure, and magnetic properties of the first three-dimensional framework containing octahedral niobium cyanochloride clusters as building units are reported. Reactions of aqueous solutions of (Me(4)N)(2)K(2)[Nb(6)Cl(12)(CN)(6)] (2) with aqueous solutions of MnCl(2) result in the precipitation of the compound (Me(4)N)(2)[MnNb(6)Cl(12)(CN)(6)] (3). The structure of 3 was determined from single-crystal X-ray diffraction study (crystal data: cubic, Fm3macrom(No. 225), a = 15.513(4) A, V = 3733.2(12) A(3), Z = 4). Its 3D framework is based on edge-bridged [Nb(6)Cl(12)](2+) clusters and Mn(2+) ions bridged by cyanide ligands to form a cfc lattice [MnNb(6)Cl(12)(CN)(6)](2)(-) in which all tetrahedral sites are occupied by the cations (Me(4)N)(+) which act as charge compensating template. The structure of 3 can be considered as an expansion of the Prussian blue framework in which [Fe(CN)(6)](4)(-) is replaced by the cluster [Nb(6)Cl(12)(CN)(6)](4)(-). Magnetic susceptibility measurements indicate that Mn(2+) is present in a high spin d(5) configuration. No magnetic ordering is observed.     

Ligands derived from tetrathiafulvalene: building blocks for multifunctional materials

The last decade has witnessed many advances in the coordination chemistry of tetrathiafulvalene (TTF). Various ligands, in which a metal-binding functionality is attached to the TTF unit, have been synthesized and used for the preparation of metal complexes. This Perspective summarizes the main types of TTF-containing ligands and their metal complexes and outlines the potential for the use of these building blocks in the design and assembly of multifunctional molecular materials.