Molecular Box Research Articles

A Molecular Box with an Edge Length of 1.5 nm: Four Ga−Ga Bonds Bridged by Dicarboxylato and Hydroxo Ligands

The four Ga−Ga bonds of the title compound 2 are bridged by two 4,4‘-biphenyldicarboxylato and four hydroxo ligands to yield a molecular box with an edge length of 1.5 nm. 2 was obtained by treatment of the digallium compound R2Ga−GaR2 (1; R = CH(SiMe3)2) with 4,4‘-biphenyldicarboxylic acid in the presence of water.

Electronic Energy Transfer in a Multiporphyrin‐Based Molecular Box

The molecular box 1 comprises of two zinc-porphyrin metallacycles connected by two free-base 4'-trans-dipyridylporphyrins, axially coordinated to the zinc centers. The photophysics of 1 were studied in chloroform by emission and ultrafast absorption spectroscopy. In the molecular box, fast singlet energy transfer (main component, tau=32 ps) is observed to occur from the zinc-porphyrin metallacycles to the free-base chromophores. From wavelength-dependent spectrofluorimetric data, the efficiency of the energy-transfer (ET) process is estimated as 0.5. The lower-than-unity value is tentatively attributed to the possibility of a competing electron-transfer quenching pathway. Molecular box 1 can be considered to be a simple, self-assembling, six-chromophore antenna system. It has an inner cavity, 11.4 Angstrom wide, that could be used, in principle, to host a variety of guest molecules and obtain higher-order assemblies.

Pyrazine as a Building Block for Molecular Architectures with PtII

A series of pyrazine (pz) complexes containing cis-(NH(3))(2)Pt(II), (tmeda)Pt(II) (tmeda = N,N,N',N'-tetramethylethylenediamine), and trans-(NH(3))(2)Pt(II) entities have been prepared and characterized by X-ray crystallography and/or 1H NMR spectroscopy. In these compounds, the pz ligands act as monodentate (1-3) or bidentate bridging ligands (4-7). Three variants of the latter case are described: a dinuclear complex [Pt(II)]2 (4b), a cyclic tetranuclear [Pt(II)](4) complex (5), and a trinuclear mixed-metal complex [Pt2Ag] (7). Mono- and bidentate binding modes are readily differentiated by 1H NMR spectroscopy, and the assignment of pz protons in the case of monodentate coordination is aided by the observation of (195)Pt satellites. Formation of the open molecular box cis-[{(NH3)2Pt(pz)}4](NO3)8.3.67H2O (5) from cis-(NH3)2Pt(II) and pz follows expectations of the "molecular library approach" for the generation of a cyclic tetramer.

Metal-directed 1-D molecular-box based coordination polymers with mono- and di-nuclear nodes – Construction of 3-D supramolecular networks via hydrogen bonding and S⋯S interactions

Self-assemblies of 4-pyridylthioacetic acid (Hpyta) with CdCl 2 or HgCl 2 yield the polymeric metal-organic coordination frameworks [Cd(pyta) 2(H 2O)] n ( 1) and [Hg(pyta)Cl] n ( 2). X-ray single-crystal diffraction results reveal that both complexes display similar 1-D neutral coordination arrays containing approximate rectangular molecular box subunits, which are, however, linked via mononuclear Cd II and dinuclear Hg 2O 2 nodes. For 1, the adjacent 1-D motifs are interconnected to form a 2-D layer via O–H⋯O hydrogen bonds and further engender a 3-D network by weak S⋯S interactions; whereas complex 2 possesses a novel 3-D supramolecular architecture with the aid of abundant C–H⋯Cl and S⋯S contacts. Interestingly, due to the intrinsic difference of the metal ions, the carboxylate group of the pyta ligand displays symmetric bidentate chelate mode for 1 and monoatomic bridging fashion for 2; the latter case is unprecedented in coordination chemistry of pyta. These results clearly suggest that it is an effective synthetic strategy by employing coordinative forces and concomitant noncovalent contacts to construct the fine-tuning supramolecular networks.

Synthesis, Characterization, and Electrochemical Properties of Dinuclear Complexes Assembled from Asymmetric CoIII Bis(dioximates) and Boronic Acids

AbstractBis(methylphenylglyoximate)cobalt(III) complexes exist both as cis and trans isomers due to the asymmetry of the equatorial ligand, and, when the axial ligands are different, the trans isomer is chiral. The reaction of racemic trans‐[CH3Co(mpgH)2py] (1) with either 3‐ or 4‐pyridylboronic acid affords dimeric units arranged on a crystallographic symmetry center such that the pyridyl nitrogen of one moiety coordinates to the Co atom of the symmetry‐related unit. In principle, three structurally different dimeric species (two homodimers and one heterodimer) can be obtained. Time‐resolved 1H NMR spectra of a 1:1 mixture of racemic 1 and either 3‐ or 4‐pyridylboronic acid in CDCl3/CD3OD show that the reaction does not converge toward a unique species in solution. Nevertheless, X‐ray structures show that the heterochiral dimers are the only products that crystallize from the reaction mixture. The nature of the dioximate side groups does not affect the geometry of the dimeric arrangements assembled by 4‐pyridylboronic acid (“molecular box”). On the contrary, the geometry of the species assembled by 3‐pyridylboronic acid varies from the “molecular parallelogram” obtained from the bis(dimethylglyoximates) to the highly squeezed “molecular box” obtained from bis(methylphenylglyoximates). Cyclic voltammetry studies show that the metal centers in the dimeric species do not interact with each other and undergo a simultaneous redox process. However, depending on the geometry of the systems, the redox process involves a single four‐electron reduction for 3 and 5 or two consecutive two‐electron reduction steps for 4 and 6. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Selective Self-Organization of Guest Molecules in Self-Assembled Molecular Boxes

This article describes the synthesis and binding properties of highly selective noncovalent molecular receptors 1(3).(DEB)6 and 3(3).(DEB)6 for different hydroxyl functionalized anthraquinones 2. These receptors are formed by the self-assembly of three calix[4]arene dimelamine derivative molecules (1 or 3) and six diethylbarbiturate (DEB) molecules to give 1(3).(DEB)6 or 3(3).(DEB)6. Encapsulation of 2 occurs in a highly organized manner; that is, a noncovalent hydrogen-bonded trimer of 2 is formed within the hydrogen-bonded receptors 1(3).(DEB)6 and 3(3).(DEB)6. Both receptors 1(3).(DEB)6 and 3(3).(DEB)6 change conformation from staggered to eclipsed upon complexation to afford a better fit for the 2(3) trimer. The receptor selectivity toward different anthraquinone derivatives 2 has been studied using 1H NMR spectroscopy, X-ray crystallography, UV spectroscopy, and isothermal microcalorimetry (ITC). The pi-pi stacking between the electron-deficient center ring of the anthraquinone derivatives 2a-c and 2e-g and the relatively electron-poor melamine units of the receptor is the driving force for the encapsulation of the guest molecules. The selectivity of the hydrogen-bonded host for the anthraquinone derivatives is the result of steric interactions between the guest molecules and the calix[4]arene aromatic rings of the host.

ZnII–Salphen Complexes as Versatile Building Blocks for the Construction of Supramolecular Box Assemblies

Zn(II)-salphen complexes are readily accessible and interesting supramolecular building blocks with a large structural diversity. Higher-order supramolecular assemblies, such as molecular boxes based on a bis-Zn(II)-salphen building block and various ditopic bipyridine ligands, have been constructed by means of supramolecular, coordinative Zn(II)-N(pyr) interactions. The use of bipyridine ligands of differing sizes enables the construction of structures with predefined box diameters. The features of the 2:2 box assemblies were investigated in detail by (variable temperature) NMR spectroscopy, UV-visible spectroscopy, NMR titrations, and X-ray crystallographic studies. The spectroscopic studies reveal a high association constant for the Zn(II)-salphen-pyridyl motif, which lies in the range 10(5)-10(6) M(-1). The strong interaction between the Zn(II) center and pyridine donors was supported by PM3 calculations that showed a relatively high Lewis acid character of the metal center in the salphen complex. Titration curves monitored by UV-visible show a cooperative effect between the two bipyridine ligands upon complexation to the bis-Zn(II) template, suggesting the formation of 2:2 complexes. The crystal structures of two supramolecular boxes have been determined. In both examples such a 2:2 assembly is present in the solid state, and the box size is different because they consist of different building blocks. Interestingly, the box assemblies line up in the solid state to form porous channels that are potentially useful in a number of applications.

Molecular Boxes on a Molecular Printboard: Encapsulation of Anionic Dyes in Immobilized Dendrimers

Fifth-generation poly(propylene imine) dendrimers, modified with 64 apolar adamantyl groups, have been immobilized on cyclodextrin host monolayers ("molecular printboards") on glass by supramolecular microcontact printing. The immobilized dendrimers retain their guest-binding properties and function as "molecular boxes" that can be filled with fluorescent dye molecules from solution. Alternatively, part of the immobilized dendrimers were filled with dye molecules by cross-microcontact printing while the remaining, empty dendrimers were filled with a different dye from solution, resulting in alternating patterns of dye molecules. In addition, we demonstrate that encapsulation of dyes in immobilized dendrimers is reversible: immobilized molecular boxes can be filled with a dye, emptied, and subsequently refilled with a different dye.

Molecular Architectures

V O L U M E 3 8 N U M B E R APRIL 2005 Registered in U.S. Patent and Trademark Office; Copyright 2005 by the American Chemical Society GUEST EDITORIAL Molecular Architectures The realization that simple molecular building blocks contain the information needed to assemble an astounding array of new chemical architectures has captured the imagination of chemists across the discipline. From porous metal-organic frameworks to molecular boxes, from engineered crystals to quantum dots, “shake and bake” has given way to design principles that arm the imaginative architect with blueprints for constructing novel materials across organic and inorganic chemistry. As the cover photo suggests, engineering, art, and architecture are coming together in a way that makes us aware of the endless frontier that is modern synthetic chemistry. The Molecular Architectures special issue celebrates this accomplishment with 16 Accounts of how we arrived at the present state of the art. This is in no small part due to our ability to identify and manipulate the weaker forces of chemistry. Serendipitous self-assembly still plays a role, but chemists have become increasingly clever at using labile coordinate bonds and supramolecular chemistry to create grand order and complexity from random piles of small molecular components. Among these building components, one finds many simple and familiar molecules: porphyrins, fullerenes, multifunctional heterocycles, metal complexes (particularly cyano complexes), silse- quioxanes, oxyanions, catechols, chalcogenide clusters, multifunctional carboxylates, and multi-hydrogen-bonding molecules. New building materials are undoubtedly still lying around waiting to be exploited. There are important underlying principles at work when small molecules assemble into large structures. In more than one of the present Accounts, authors reveal the guiding hand that physical principles and natural law plays in directing the develop- ment of macroscopic structure. The molecular architects and engineers of the future will increasingly find that they need to understand these principles to design new materials. This has set the stage for the next era, making beautiful structures into functional constructs. I confess to a joy at simply stopping to marvel at the beauty of molecular architecture. Anyone who walks up to the new home of the Los Angeles Philharmonic illustrated on the front cover would do the same. However, eventually, we must go 10.1021/ar050039e CCC: $30.25 Published on Web 04/19/2005  2005 American Chemical Society VOL. 38, NO. 4, 2005 / ACCOUNTS OF CHEMICAL RESEARCH

Ring opening reactions of tetrathiacyclopropenonophanes and the subsequent formation of a molecular box stabilised by hydrogen bonds

Abstract Sulfur atoms next to a cyclopropenone moiety change their reactivity completely. Tetrathiacyclopropenonophanes 6 and 10 were treated with protic solvents such as methanol and water yielding ring opened diesters 9a and 9b, and the diacid 11, respectively. This observation is in contrast to alkyl and aryl substituted cyclopropenones which are stable in the presence of protic solvents. The molecular structures of the diester and the diacid could be elucidated by means of X-ray diffraction analyses. In the latter case a molecular box was formed in such a way that the two acid moieties dimerise via hydrogen bonding. The resulting hollow core is able to host one molecule of chloroform.

Electrospray ionization mass spectrometric study of the metal cation complexation capabilities of a novel, doubly cage-annulated molecular box

An unusual molecular box, 4, obtained as a mixture of diastereoisomeric, doubly cage-annulated host molecules, i.e., 4a and 4b, has been prepared. The metal cation complexation properties of the mixture of 4a and 4b thereby obtained have been assessed, with comparison to another bis crown 2, via application of electrospray ionization mass spectrometric (ESI-MS) methods. Both host systems 2 and 4 bind selectively to Li + and Na + , thereby demonstrating preference over the other larger alkali metals. Addition of Ag + to the solutions containing alkali metals does not result in substantial Ag + complexation. Analysis of a solution containing only transition metal, not alkali metal, cations indicates that 4 forms complexes with Zn + and Ag + along with substantial complexation of Na + from residual sodium present in the solvents.

‘Twisted’ isophthalamide analogues

Steric interactions in 1,3-dicarboxamidoanthraquinones have been employed to 'twist' isophthalamide-like anion binding sites; the crystal structure of the fluoride complex of a bis-3,5-dichlorophenylamide derivative shows the receptor acting as a 'hydrogen-bonding corner' in a '2 + 2' fluoride containing molecular box.

Strong enhancement of second-harmonic generation (SHG) response through multi-chiral centers and metal-coordination

The hydrothermal reaction of CdCl2 with L (L =trans-2,3-dihydro-2-(4'-pyridyl)-3-(3"-cyanophenyl)benzo[e]indole) in the presence of NaN3 and water offers a novel route to [Cd(L-N3)2(H2O)2]n, a 1D infinite molecular box with approximate dimensions 10.37 x 6.64 è; 1 is shown to display a very strong SHG response that is 80 times that observed for urea.

Self-assembly of novel discrete binuclear molecular box structure from a novel bis- N, O-bidentate Schiff-base ligand and Zn II, Co II salts

In our efforts to design discrete polynuclear metal complexes with tailored structures, a novel bis- N, O-bidentate Schiff-base ligand, 2,5-bis( N-salicylidene-3-aminophenyl)-1,3,4-oxadiazole ( L), have been synthesized. The reaction of L with Zn(OAc) 2 · 2H 2O or Co(OAc) 2 · 4H 2O in a CH 3OH/CHCl 3 medium yielded two new complexes, [Zn 2 L 2](CHCl 3) 1/2 ( 1) and [Co 2 L 2](CHCl 3) 1/2 ( 2), which exhibit novel discrete dinuclear structure in which the metal centers take pseudo-tetrahedral coordination mode. In each of the two complexes, the L ligands adopt two different conformations ( Cisoid- I ( close) and Cisoid- II ( open)), respectively, to incorporate with the metal centers into box structure. π–π stacking interactions were found to play important roles in the self-assembly processes.

Structural and mechanistic studies on ion insertion into the molecular box {[CpCo(CN) 3] 4[Cp*Ru] 4}

The molecular box [CpCo(CN) 3] 4[Cp*Ru] 4 (Co 4Ru 4) reacts readily with a variety of monocations to form M⊂Co 4Ru 4 + (M=K +, Cs +, Rb +). Ion competition experiments, monitored by ESI-MS, show that the molecular box binds the smaller K + more rapidly than Cs +, but that thermodynamically Co 4Ru 4 prefers the larger ion. The rates of ion-insertion for K + and Cs + into Co 4Ru 4 were found to qualitatively follow second order kinetics with K +, 300 M −1 s −1 and Cs +, 36 M −1 s −1. The ratio k K/ k Cs qualitatively matched the ESI-MS results from ion competition experiments. The rates of ion-insertion into Co 4Ru 4 were found to depend on the counter anions. In particular, RbBF 4 reacted with Co 4Ru 4 more slowly than did RbOTf. The slower rates allowed us to establish second order kinetics. 1H NMR studies reveal that the Cp signal for Co 4Ru 4 is very sensitive to the presence of entering ions, e.g., Rb +, whereas the corresponding Cp signal for Rb⊂Co 4Ru 4 + was insensitive to the presence of Rb +. The molecular structures of [Co 4Ru 4] · 6MeCN, [K⊂Co 4Ru 4]BF 4 · 7MeCN, [Cs⊂Co 4Ru 4]BF 4 · 6MeCN and [Tl⊂Co 4Ru 4]BF 4 · 6MeCN, determined by X-ray diffraction, showed that although the compounds crystallized in the same space group I23, a correlation exists between the Ru–N/Co–C bond distances and the size of the interstitial ion.

Metal cation complexation of a novel, doubly cage-annulated molecular box by electrospray ionization mass spectrometry

Metal cation complexation of a novel, doubly cage-annulated molecular box by electrospray ionization mass spectrometry

Symmetry induced supramolecular assembly of a resorcinarene trimeric molecular box.

A novel trimeric resorcinarene molecular box is induced during hydrothermal synthesis by use of the 3-fold symmetric tripyridyl triazine linker molecule.

Self-assembly and crystal structure of a novel hetero-octametallic molecular box (Chelate complexes, Part 23)

Reaction of hexadentate ligand m-xylene-bis-(iminodiethanol) H 4L 2 ( 4) with lithium hydride and either copper(II) acetate or copper(II) chloride resulted in the formation of two completely different products. Starting from copper(II) acetate the green bis-dimetallic cyclophane [Cu 4(H 2L 2) 2(OAc) 4] · 2CH 2Cl 2 ( 5) was formed. However, when 4 was allowed to react with copper(II) chloride, the novel hetero-octametallic molecular box {[Li 4(Cu 4O)(L 2) 2](H 2O) 2(EtOH) 2}Cl 2 ( 6) was isolated. Deep violet 6 can be described as two metallacrown ether sub-units linked together by a central μ 4-bridging O 2−-ion. The structures of all new compounds were determined unequivocally by single crystal X-ray analyses.

Membership rules for a molecular box: the admission process and protection provided to guest molecules.

Membership rules for a molecular box: the admission process and protection provided to guest molecules.

Membership Rules for a Molecular Box: The Admission Process and Protection Provided to Guest Molecules

Kationen im Kasten: Das Bindungsverhalten einer Reihe von Kationen im Hohlraum des würfelförmigen Wirt-Komplexes [{CpCo(CN)3}4(Cp*Ru)4] (Cp=C5H5, Cp*=C5Me5) wurde untersucht (siehe Bild). Die Komplexierung von Cs+-Ionen ist hierbei besonders begünstigt. Eingeschlossene Ammonium-Ionen sind gegen chemische Reaktionen wie Deprotonierung oder H-D-Austausch abgeschirmt. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2003/z19562_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.