Crown ethers: sensors for ions and molecular scaffolds for materials and biological models.

Ammonia–borane, H3NBH3, is an intriguing molecule for chemical hydrogen storage applications. With both protic N–H and hydridic B–H bonds, three H atoms per main group element, and a low molecular weight, H3NBH3 has the potential to meet the stringent gravimetric and volumetric hydrogen storage capacity targets needed for transportation applications. Furthermore, devising an energy-efficient chemical process to regenerate H3NBH3 from dehydrogenated BNHx material is an important step towards realization of a sustainable transportation fuel. In this perspective we discuss current progress in catalysis research to control the rate and extent of hydrogen release and preliminary efforts at regeneration of H3NBH3.

Ammonia-borane: the hydrogen source par excellence?

Chemistry welcomes a new bond: The mechanical bond has endowed molecules with component parts whose movements can be controlled and monitored. In his Nobel Lecture, J. F. Stoddart describes how being able to template the formation of mechanically interlocked molecules has led to the design and synthesis of shuttles, switches, and machines at the nanoscale.

Mechanically Interlocked Molecules (MIMs)-Molecular Shuttles, Switches, and Machines (Nobel Lecture).

We have collated various supramolecular designs that utilize organic donor–acceptor CT complexation to generate noncovalently co-assembled structures including fibrillar gels, micelles, vesicles, nanotubes, foldamers, conformationally restricted macromolecules, and liquid crystalline phases. Possibly inspired by nature, chemists have extensively used hydrogen bonding as a tool for supramolecular assemblies of a diverse range of abiotic building blocks. As a structural motif, CT complexes can be compared to hydrogen-bonded complexes in its directional nature and complementarities. Additional advantages of CT interactions include wider solvent tolerance and easy spectroscopic probing. Nevertheless the major limitation is their low association constant. This article shows different strategies have evolved over the years to overcome this drawback by reinforcing the CT interactions with auxiliary noncovalent forces without hampering the alternate stacking mode. Emerging reports on promising CT complexes in organic electronics are intimately related to various supramolecular designs that one can postulate based on donor–acceptor CT interactions.

Supramolecular assemblies by charge-transfer interactions between donor and acceptor chromophores.

Zwar steht der Entwicklung von nanometergrosen Strukturen prinzipiell nichts im Wege, doch setzt sich immer mehr die Auffassung durch, das sich Strukturminiaturisierungen unter die gegenwartig durch lithographische Techniken erreichbare 1-μ-Grenze als nicht mehr praktikabel erweisen werden. Es wurde daher deutlich, das nur durch ein grundlegendes Verstandnis der Selbstorganisation von funktionellen makroskopischen biologischen Strukturen mit Abmessungen im Nanometerbereich und sogar darunter (Verkleinerungsansatz) und durch die Erweiterung unseres Wissens uber die chemische Synthese von mikroskopischen Strukturen (Vergroserungsansatz) die Brucke zwischen Anspruch und Wirklichkeit bei Nanosystemen geschlagen werden kann. Die Konstruktion von Nanostrukturen und -systemen aus kleinen Molekulbausteinen ist das „engineering-up” zum Aufbau von molekularen Funktionseinheiten. Bedeutende Fortschritte konnen auf dem Gebiet der Nanowissenschaften erzielt werden, wenn die Konzepte, die in der Biologie gefunden wurden, auf die Chemie ubertragen werden. Im Zentrum dieser Aufgabe steht die Entwicklung von einfachen chemischen Systemen, die sich selbst durch gegenseitige Erkennung zu groseren Molekulaggregaten organisieren konnen. Die genaue Programmierung derartiger Erkennungsprozesse und somit auch der korrekte Aufbau der Uberstrukturen setzen ein fundamentales Verstandnis und die Nutzung inter- sowie intramolekularer nichtkovalenter bindender Wechselwirkungen voraus. Die supramolekulare Chemie – eine Chemie, die in jeder Hinsicht uber die Chemie der Molekule hinausgeht – hat begonnen, den grosen Graben zwischen molekularen und makromolekularen Strukturen zu schliesen. Durch Nutzung von so unterschiedlichen Wechselwirkungen wie aromatischen π-Stapel- und Metall-Ligand-Koordinationswechselwirkungen als Informationsquellen der Aufbauprozesse haben Chemiker in den letzten zehn Jahren biologische Konzepte wie die Selbstorganisation zur Konstruktion von Nanostrukturen und Uberstrukturen mit einer Vielzahl von Formen und Funktionen herangezogen. Wir wollen hier einen Eindruck davon vermitteln, wie die Selbstorganisation in naturlichen Systemen funktioniert und wie diese Prinzipien nutzbringend auf nichtnaturliche Systeme angewendet werden konnen.

Selbstorganisation in natürlichen und in nichtnatürlichen Systemen

Second Sphere Coordination of Cationic Platinum Complexes by Crown Ethers— The X‐Ray Crystal Structure of [Pt(bpy)(NH3)2. Dibenzo[30]crown‐10]2+[PF6] 2−xH2O (x≈0.6)

Spectrophotometric investigations of equimolar mixtures of diquat bis(hexafluorophosphate)(2) and a range of dibenzo-3n-crown-n ethers [n=6–12; (10)–(16)] in acetonitrile reveal the existence of charge-transfer absorption bands at ca.λmax. 400 nm. These absorptions are attributable to intermolecular π–π charge transfer between the electron-rich catechol units of the dibenzo-crown ethers and the electron-deficient bipyridinium ring system of the diquat dication. The qualitative conclusion from these experiments, that the most stable 1 : 1 complex is formed between dibenzo-30-crown-10 (14) and diquat bis(hexafluorophosphate)(2). led to the isolation from dichloromethane methanol–n-heptane of red crystals of [diquat.dibenzo-30-crown-10][PF6]2 suitable for X-ray crystallography. Although the crystal structure analysis revealed that there are two independent sets of 1 : 1 complexes (I and II) in the unit cell, the gross structural features of the two complexes are very similar. In addition to the parallel alignment of their three aromatic rings to accommodate the stabilising intermolecular π-π charge-transfer interaction, there is probably some further host–guest stabilisation to be gained on account of favourable electrostatic interactions between the phenolic oxygen atoms in the host and the nitrogen atoms in the pyridinium rings of the guest. Moreover, there is some evidence for weak C–H ⋯ O hydrogen bonding involving principally H-6 and H-6′ on the bipyridinium ring system of the guest and certain –CH2OCH2– oxygen atoms in the host. As evidenced by 1H n.m.r. spectroscopy in CD3COCD3, these non-covalent bonding interactions are probably responsible for the formation of stable and ordered 1 : 1 complexes with similar gross structural features in solution, at least in the cases where dibenzo-30- crown-10 (14). dibenzo-33-crown-11 (15), and dibenzo-36-crown-12 (16) are the hosts. Further evidence for the 1 : 1 stoicheiometry of these solution complexes, as well as for the complex involving dibenzo-27-crown-9 (13), has come from equilibrium constant measurements for the association between the dibenzo-3n-crown-n(n=9–12) hosts (13)–(16) and diquat bis(hexafluorophosphate)(2) in acetone. A quantitative treatment of the charge-transfer absorption bands at 400 nm, which affords Kavalues of 410, 17 500, 10 800, and 2 000 M–1 for n=9, 10, 11, and 12, respectively, provides convincing quantitative evidence for (a) 1 : 1 stoicheiometry and (b) the relative stabilities of the 1 : 1 complexes in solution. In the case of dibenzo-24-crown-8 (12), a complex of 2:l (guest:host) stoicheiometry is believed to be formed in acetone with a Kavalue of 385 000 M–2, as shown by a successful quantitative treatment of the charge-transfer absorption data by an independent method.

The complexation of the diquat dication by dibenzo-3n-crown-n ethers

An X-ray crystallographic investigation of the complex between the diquat dication and dibenzo-30-crown-10, [Diquat,DB30C10][PF6]2, reveals gross structural features for host–guest complexes (I) and (II) in which weak [C–H ⋯ O]hydrogen bonding and charge transfer are the major components of host–guest binding; these same components are also responsible for the formation of stable and orderde 1 : 1 solution complexes between dibenzo-3n-crown-n(n 9–12) and diquat bis(hexafluorophosphate)(2).

Complex formation between dibenzo-3n-crown-n ethers and the diquat dication

Koordination kationischer Platinkomplexe in der zweiten Sphäre durch Kronenether: Struktur von [Pt(bpy)(NH3)2·Dibenzo‐[30]krone‐10]2+[PF6−]2·xH2O (x ≈︂ 0.6)

In addition to hydrogen bonding and Coulombic forces, charge transfer interactions stabilise the 1:1 adducts formed between [Pt(bipy)(NH3)2] 2+ and a series of dibenzo-3n-crown-n (n = 6–12) ethers as evidenced by 1H NMR and UV-visible spectra in solution and by X-ray crystal structures (for n = 8 and 10) in the solid state. Mutual diamagnetic (ring current) shielding by the aromatic systems of host and guest results in dramatic upfield shifts of certain aromatic proton resonances on adduct formation, especially when n = 10 or 11. A broad charge-transfer absorption band at 350 nm attains its maximum intensity at n = 11 though stability constant measurements indicate optimum binding when n = 10. X-ray diffraction studies reveal that, when n = 8, only one of the crown ether benzo-rings interacts with the bipyridyl ligand in a charge-transfer sense. However, when n = 10, the macrocycle is sufficiently large and flexible to allow both benzo-rings to enter into stabilising interactions with the bipyridyl ligand such that the platinum complex is encapsulated by the host in a U-shaped cavity.

Second‐sphere Coordination of [Pt(bipy)(NH3)2]2+ by Dibenzo‐crown Ethers. Solution Spectroscopic Studies and the Crystal and Molecular Structures of [Pt(bipy)(NH3)2.dibenzo‐30‐crown‐10] [PF6]2.0.6 H2O and [Pt(bipy)(NH3)2.dibenzo‐24‐crown‐8] [PF6]2

John B. Wolstenholme

Papers

Second Sphere Coordination of Cationic Platinum Complexes by Crown Ethers— The X‐Ray Crystal Structure of [Pt(bpy)(NH3)2. Dibenzo[30]crown‐10]2+[PF6] 2−xH2O (x≈0.6)

The complexation of the diquat dication by dibenzo-3n-crown-n ethers

Complex formation between dibenzo-3n-crown-n ethers and the diquat dication

Koordination kationischer Platinkomplexe in der zweiten Sphäre durch Kronenether: Struktur von [Pt(bpy)(NH3)2·Dibenzo‐[30]krone‐10]2+[PF6−]2·xH2O (x ≈︂ 0.6)

Second‐sphere Coordination of [Pt(bipy)(NH3)2]2+ by Dibenzo‐crown Ethers. Solution Spectroscopic Studies and the Crystal and Molecular Structures of [Pt(bipy)(NH3)2.dibenzo‐30‐crown‐10] [PF6]2.0.6 H2O and [Pt(bipy)(NH3)2.dibenzo‐24‐crown‐8] [PF6]2