Phosphorus-based functional groups as hydrogen bonding templates for rotaxane formation

We report on the use of the hydrogen bond acceptor properties of some phosphorus-containing functional groups for the assembly of a series of [2]rotaxanes. Phosphinamides, and the homologous thio- and selenophosphinamides, act as hydrogen bond acceptors that, in conjunction with an appropriately positioned amide group on the thread, direct the assembly of amide-based macrocycles around the axle to form rotaxanes in up to 60% yields. Employing solely phosphorus-based functional groups as the hydrogen bond accepting groups on the thread, a bis(phosphinamide) template and a phosphine oxide-phosphinamide template afforded the corresponding rotaxanes in 18 and 15% yields, respectively. X-ray crystallography of the rotaxanes shows the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and various hydrogen bond accepting groups on the thread, including rare examples of amide-to-phosphinamide, -thiophosphinamide, and -selenophosphinamide groups. With a phosphine oxide-phosphinamide thread, the solid-state structure of the rotaxane is remarkable, featuring no direct intercomponent hydrogen bonds but rather a hydrogen bond network involving water molecules that bridge the H-bonding groups of the macrocycle and thread through bifurcated hydrogen bonds. The incorporation of phosphorus-based functional groups into rotaxanes may prove useful for the development of molecular shuttles in which the macrocycle can be used to hinder or expose binding ligating sites for metal-based catalysts.     

Theoretical investigation of the hydrogen bond interactions of methanol and dimethylamine with hydrazone and its derivatives

The interactions between hydrogen bond donors (dimethylamine (DMA) and methanol (MeOH)) and acceptors (formaldehyde dimethylhydrazone, acetaldehyde N,N-dimethylhydrazone and N-nitrosodimethylamine) were theoretically investigated by DFT. The hydrogen bonding interactions were found on several bonding sites of the acceptors. The important properties of structure, binding energy, enthalpy of formation, Gibbs free energy of formation and equilibrium constant were investigated. Compared to the monomer, the DMA complexes show a small red shift of the NH-stretching vibrational transition but a significantly intensity enhancement. On the other hand, the MeOH complexes have a large red shift but a relatively small intensity enhancement of the OH-stretching transition. Atoms-in-molecules analysis revealed that several types of hydrogen bond interaction were present in the complexes. Since natural bond orbital analysis overestimated the effect of charge transfer, the more reliable localized molecular orbital energy decomposition analysis was performed and it shows that the major contribution to the total interaction energy is the electrostatic interaction. All these parameters suggest that the hydrogen bond donor strength of MeOH is substantially greater than DMA.

     

[2]Rotaxane End-Capping Synthesis by Click Michael-Type Addition to the Vinyl Sulfonyl Group

We report the application of the click Michael-type addition reaction to vinyl sulfone or vinyl sulfonate groups in the synthesis of rotaxanes through the threading-and-capping method. This methodology has proven to be efficient and versatile as it allowed the preparation of rotaxanes using template approaches based on different noncovalent interactions (i.e., donor-acceptor π–π interactions or hydrogen bonding) in yields of generally 60–80 % and up to 91 % aided by the mild conditions required (room temperature or 0 °C and a mild base such as Et₃N or 4-(N,N-dimethylamino)pyridine (DMAP)). Furthermore, the use of vinyl sulfonate moieties, which are suitable motifs for coupling-and-decoupling (CAD) chemistry, implies another advantage because it allows the controlled chemical disassembly of the rotaxanes into their components through nucleophilic substitution of the sulfonates resulting from the capping step with a thiol under mild conditions (Cs₂CO₃ and room temperature).     

Synthesis and exploring the excited-state PES of photochromic hydrogen bond-assembled [2]rotaxane based on 1,3-Diazabicyclo-[3.1.0]hex-3-enes

Here, the synthesis of photochromic hydrogen bond-assembled [2]rotaxanes using bis-fumarate as a thread for the first time is reported. In fact, photochromic 1,3-diazabicyclo[3.1.0]hex-3-ene moieties were used as stoppers and two-atom spacers managed good binding sites for the tetralactam macrocycles in clipping reactions. Moreover, the yields of photochromic [2]rotaxanes highly depended on the NO₂ substituent stoppers. While the thread with a para –NO₂ substituent as stopper units was shown to be an excellent template for the synthesis of photochromic [2]rotaxanes. The structures of the [2]rotaxanes are established clearly in solution by chemical shifts of the ¹H ¹³C NMR signals and UV–Vis spectra. A pronounced bathochromic shift was occurred in the excitation wavelength of photochoromic [2]rotaxanes compared with the absorption band of photochromic threads. Therefore, these organizations can be applied in light-driven molecular switches and motors. The reversible transformation of trans and cis geometric photoisomers under UV radiation was identified. In other efforts, the possibility of the process of trans to cis interconversion of the fumarate linker under UV irradiation has been examined computationally and it has appeared that it may cause the transverse of the bis-fumarate linker inside the tetralactam macrocycle to some extent.

     

Synthesis, structure, and dynamic properties of hybrid organic-inorganic rotaxanes

The synthesis and characterization of a series of hybrid organic-inorganic [2]rotaxanes is described. The ring components are heterometallic octa- ([Cr(7)MF(8)(O(2)C(t)Bu)(16)]; M = Co, Ni, Fe, Mn, Cu, Zn, and Cd) nuclear cages in which the metal centers are bridged by fluoride and pivalate ((t)BuCO(2)(-)) anions; the thread components feature dialkylammonium units that template the formation of the heterometallic rings about the axle to form the interlocked structures in up to 92% yield in conventional macrocyclization or one-pot 'stoppering-plus-macrocyclization' strategies. The presence in the reaction mixture of additives (secondary or tertiary amines or quaternary ammonium salts), and the nature of the stoppering groups (3,5-(t)Bu(2)C(6)H(3)CO(2)- or (t)BuCONH-), can have a significant effect on the rotaxane yield. The X-ray crystal structures of 11 different [2]rotaxanes, a pseudorotaxane, and a two-station molecular shuttle show two distinct types of intercomponent hydrogen bond motifs between the ammonium groups of the organic thread and the fluoride groups of the inorganic ring. The different hydrogen bonding motifs account for the very different rates of dynamics observed for the heterometallic ring on the thread (shuttling slow; rotation fast).     

The effect of mechanical interlocking on crystal packing: predictions and testing

The first statistical analyses of the X-ray crystal structures of mechanically interlocked molecular architectures, the first molecular mechanics-based solid-state calculations on such structures and atomic force microscopy (AFM) experiments are used in combination to predict and test which types of benzylic amide macrocycle-containing rotaxanes possess mobile components in the crystalline phase and thus could form the basis of solid-state devices that function through mechanical motion at the molecular level. The statistical studies and calculations show that crystals formed by rotaxanes possess similarities and unanticipated differences with respect to the crystal packing of noninterlocked molecules. Trends in the rotaxane series correlate quantities related to crystal packing, molecular size, stoichiometry, and H-bonding. In accordance with the findings of Gavezzotti et al. for conventional molecular architectures, a principal component analysis (PCA) showed that three vectors related to the size, packing parameters, and stoichiometry are sufficient to describe the crystal properties of benzylic amide macrocycle-containing rotaxanes. When hydrogen bond-related quantities are included in a second PCA, they combine with the size and the stoichiometry vectors but not with packing-related parameters, indicating that the intramolecular "saturation" of the H-bonds (between the interlocked components) takes precedence over crystal assembly (i.e., intermolecular packing) in these systems. However, cluster analyses also suggest a major role for the energy of interaction between the macrocycle and its crystal environment. The identification of such a "privileged" interaction is of fundamental importance to the development of rotaxanes with in-crystal mobility of one or more of their interlocked components, a prerequisite for the exploitation of molecular level mechanical motion in the solid state. The set of trends found, together with the calculated energies, was used to propose guidelines for which benzylic amide macrocycle-containing rotaxanes are best suited to become building blocks for systems with mobile submolecular units in the crystalline phase. An experimental test of the predictive power of such guidelines was carried out using AFM on a rotaxane and its thread, identified by the study as a promising candidate for solid-state mobility. Intuitively, the rotaxane should be less mobile in the solid state since it has multiple sets of both hydrogen bond donors and acceptors that can form strong inter- and intramolecular H-bonds. Conversely, the thread has no hydrogen bond donors and cannot form such bonds. The AFM experiments, however, confirm the statistical analysis prediction that the rotaxane is considerably more mobile in the solid than the thread.     

Halotriazolium Axle Functionalised [2]Rotaxanes for Anion Recognition: Investigating the Effects of Halogen‐Bond Donor and Preorganisation

The anion‐templated synthesis of three novel halogen‐bonding 5‐halo‐1,2,3‐triazolium axle containing [2]rotaxanes is described, and the effects of altering the nature of the halogen‐bond donor atom together with the degree of inter‐component preorganisation on the anion‐recognition properties of the interlocked host investigated. The ability of the bromotriazolium motif to direct the halide‐anion‐templated assembly of interpenetrated [2]pseudorotaxanes was studied initially; bromide was found to be the most effective template. As a consequence, bromide anion templation was used to synthesise the first bromotriazolium axle containing [2]rotaxane, the anion‐binding properties of which, determined by ¹H NMR spectroscopic titration experiments, revealed enhanced bromide and iodide recognition relative to a hydrogen‐bonding protic triazolium rotaxane analogue. Two halogen‐bonding [2]rotaxanes with bromo‐ and iodotriazolium motifs integrated into shortened axles designed to increase inter‐component preorganisation were also synthesised. Anion ¹H NMR spectroscopic titration experiments demonstrated that these rotaxanes were able to bind halide anions even more strongly, with the iodotriazolium axle integrated rotaxane capable of recognising halides in aqueous solvent media. Importantly, these observations suggest that a halogen‐bonding interlocked host binding domain, in combination with increased inter‐component preorganisation, are requisite design features for a potent anion receptor.     

Selective product amplification of thymine photodimer by recognition-directed supramolecular assistance

Two symmetric ditopic supramolecular templates (1 and 2) each presenting two hydrogen bonding recognition subunits were synthesized. Each such subunit comprises the same donor and acceptor pattern, capable of binding a substrate molecule with complementary hydrogen bonding groups to form a supramolecular complex. Substrate molecules, such as thymine or uracil derivatives, yield 2 : 1 complexes with the acceptors involving two hydrogen bonds to each subunit with ideal orientation for subsequent [2 + 2] dimerization upon photoirradiation. Selective syn photoproduct formation and concomitant suppression of the trans isomer are favored by orientation of the two guest nucleobases within the template cleft. Complementary donor and acceptor hydrogen bonding induced positioning of the two substrates and steric hindrance within the template clefts are responsible for the selective product formation.     

Iodide Discrimination by Tetra-Iodotriazole Halogen Bonding Interlocked Hosts

Whilst the exploitation of interlocked host frameworks for anion recognition is widely established, examples incorporating halogen bond donor groups are still relatively rare. Through the integration of a novel tetra(iodotriazole)-pyridinium motif into macrocycle and axle components, a family of halogen bonding catenane and rotaxanes are constructed for anion recognition studies in a competitive aqueous-organic solvent mixture. Importantly, the degree of anion selectivity displayed is dictated by the topological nature and charged state of the respective interlocked host cavity. All the interlocked hosts exhibit iodide anion selectivity over other halides and sulfate, with the level of discrimination being the greatest with the mono-cationic rotaxane. Arising from greater electrostatic interactions working in tandem with halogen bonding and hydrogen bonding, the di-cationic rotaxane displays stronger anion association at the expense of a relatively lower degree of iodide selectivity.     

The coordination chemistry of the proton

It is well known that an acidic hydrogen atom can form hydrogen bonds to a hydrogen bond acceptor, a Lewis base. It is considerably less known that the proton can coordinate two or more atoms conveniently in bonding modes that cannot be described as hydrogen bonding. Agostic interactions, bridging hydrides, 3-centre-2-electron bonds in boranes, bifurcated hydrogen atoms, they are all elements of the coordination chemistry of the proton and, of course, the hydrogen bond comes in more than one facette as well.     

Red- and blue-shifted hydrogen bonds: the Bent rule from quantum theory of atoms in molecules perspective

MP2/6-311++G(3pd,3df) calculations were performed on complexes of acetylene and fluoroform acting as the proton donating systems and different Lewis bases being the proton acceptors since these complexes are linked through C-H···Y hydrogen bonds. Quantum Theory of Atoms in Molecules (QTAIM) is applied to explain the nature of these interactions. The characteristics of bond critical points are presented for these complexes. The inter-relations between energetic and geometrical parameters as well as the parameters derived from the Natural Bond Orbital (NBO) theory are analyzed here. Red- and blue-shifted hydrogen bonds are detected for the complexes investigated and the differences between those interactions are analyzed from the QTAIM perspective. It is shown that such differences are in agreement with the Bent rule. The position of the bond critical point of the proton donating bond is connected with the nature of hydrogen bonding, that is, if it is blue- or red-shifted.     

Synthesis of [1]rotaxane via covalent bond formation and its unique fluorescent response by energy transfer in the presence of lithium ion

Although there have been a lot of reports on the synthesis and properties of [n]rotaxanes (mainly n = 2), only a few reports on the synthesis of [1]rotaxane has been published by V?gtle's group and others (see ref 5). Generally speaking, [1]rotaxane might be expected to exhibit properties different from other rotaxanes, because the rotor and the axle in the [1]rotaxane is bound covalently and closely. We report on a novel method to make [1]rotaxanes via covalent bond formation from a macrocyclic compound. That is, we first prepared a bicyclic compound from macrocycle and then proceeded to [1]rotaxane by aminolysis. This is the first synthetic example of preparation of [1]rotaxane via covalent bond formation, not utilizing weak interactions such as hydrogen bonding, charge transfer, via metal complexation, etc. This method might provide a powerful and new tool for construction of [1]rotaxane as a new supramolecular system. In addition, we investigated energy transfer from rotor to axle using [1]rotaxane that we prepared. Energy transfer occurred perfectly from the naphthalene ring of the rotor to the anthracene ring of the axle. We found also that only lithium ion among alkali ions can drastically enhance the fluorescence intensity. This finding could be applicable to ion-sensing systems, switching devices, and so on.     

Organic chlorine as a hydrogen-bridge acceptor: evidence for the existence of intramolecular O--H...Cl--C interactions in some gem-alkynols

The acceptor capabilities of "organic" halogen, CX (X=F, Cl, Br, I), with respect to hydrogen bonding are controversial, and unactivated organic chlorine is generally deemed to be a poor acceptor. Hydrogen bridges of the type O--H...Cl--C are uncommon and occur mainly in an intramolecular situation when the donor group is sterically hindered, so that the formation of intermolecular interactions is difficult. In this paper, intramolecular O--H...Cl--C interactions in a series of chloro-substituted gem-alkynols are studied. We describe various features of this interaction using crystallographic, spectroscopic and computational methods. The O--H...Cl--C interaction occurs in five of the six compounds under consideration here (CDDA, 14DDDA, 15DDDA, 18DDDA, 15MKA). Solution (1)H NMR spectroscopy shows that the interaction is intramolecular and that it is a true hydrogen bond. DFT calculations give a stabilisation energy around 4.0 kcal mol(-1). In the crystal structures of the compounds studied, the intramolecular O--H...Cl--C interactions fit into the overall scheme of cooperative interactions. These structures may be derived from that of the unsubstituted compound DDA by means of synthon exchange and the O--H...Cl--C interaction fares surprisingly well in the presence of competing stronger acceptors. The crystal structures show an unusual degree of modularity for compounds that generally form interactions that are weak and variable. It is noteworthy that the so-called "weak" acceptor, organic chlorine, is able to sustain a good intramolecular hydrogen bridge that is of an attractive and stabilizing nature and which is of potential importance in crystal engineering and supramolecular chemistry.     

Self-assembly of novel [3]- and [2]rotaxanes with two different ring components: donor-acceptor and hydrogen bonding interactions and molecular-shuttling behavior

Three of the first kind of hetero[3]rotaxanes, which comprise one linear component and one neutral and one tetracationic ring component, have been assembled by using the intermolecular hydrogen bonding and donor-acceptor interactions. Three neutral [2]rotaxanes and three tetracationic [2]rotaxanes have also been synthesized as intermediate products or for the sake of property comparison. The linear molecules are incorporated with two glycine subunits, for templating the formation of the neutral tetraamide cyclophane, and one or two hydroquinone subunits, for inducing the formation of the tetracationic cyclophane. Variable-temperature (1)H NMR investigation reveals that the shuttling behavior of the tetracationic ring component along the linear component is substantially influenced by the existence of the neutral ring component. The spatial repelling interaction of the neutral ring on the electron-deficient tetracationic ring simultaneously weakens the latter's "positioning" tendency at both electron-rich hydroquinone sites of the linear component. As a result, the activation energy associated with the shuttling process of the tetracationic ring between the two hydroquinone sites is remarkably reduced in comparison to that of the shuttling process of the corresponding neutral ring-free [2]rotaxanes. For the first time, the rotation of the dipyridinium subunit around the axis formed by the two methylene groups connecting them within the tetracationic cyclophane has been investigated by variable-temperature (1)H NMR spectroscopy and the associated kinetic data have also been successfully obtained. Furthermore, the UV-vis and fluorescent properties of the new [2]- and [3]rotaxanes have been studied. The results demonstrate that [3]rotaxanes with different ring components possess unique kinetic features that are not available in [3]rotaxanes with identical ring components.     

A Spectroscopic Overview of Intramolecular Hydrogen Bonds of NH…O,S,N Type

Intramolecular NH…O,S,N interactions in non-tautomeric systems are reviewed in a broad range of compounds covering a variety of NH donors and hydrogen bond acceptors. ¹H chemical shifts of NH donors are good tools to study intramolecular hydrogen bonding. However in some cases they have to be corrected for ring current effects. Deuterium isotope effects on ¹³C and ¹⁵N chemical shifts and primary isotope effects are usually used to judge the strength of hydrogen bonds. Primary isotope effects are investigated in a new range of magnitudes. Isotope ratios of NH stretching frequencies, νNH/ND, are revisited. Hydrogen bond energies are reviewed and two-bond deuterium isotope effects on ¹³C chemical shifts are investigated as a possible means of estimating hydrogen bond energies.     

NMR study of 1-aryl-1,2-dicarba-closo-dodecaboranes: intramolecular C-H?O hydrogen bonding in solution

Dicarba-closo-dodecaborane(12) (carborane) has recently received much attention as a building block for supramolecular assemblies and bioactive compounds. Among carborane isomers, 1,2-dicarba-closo-dodecaborane(12) (o-carborane) has unique chemical properties, including the ability of the o-carborane C-H hydrogens to form hydrogen bonds. To evaluate intramolecular hydrogen bond formation between the o-carborane C-H hydrogen and various hydrogen bond acceptors in solution, we have designed and synthesized 1-aryl-o-carboranes 2. Intramolecular hydrogen bonding ability was evaluated by means of ¹H NMR measurement of the o-carborane C-H hydrogen signal of 2. The 1-(2-methoxyphenyl)-o-carborane derivative 2m appeared to form an intramolecular hydrogen bond between o-carborane C-H hydrogen and the oxygen atom acting as a hydrogen bonding acceptor. In this study, we present evidence for hydrogen bond formation in solution between the o-carborane C-H and hydrogen bond acceptors positioned with appropriate geometry.     

A novel detection technique of hydrazine hydrate: modality change of hydrogen bonding-induced rapid and ultrasensitive colorimetric assay

Ultrasensitive visual detection of hydrazine hydrate using a Au nanoparticles-based colorimetric sensing system (ANCSS) is reported for the first time, which is based on the hydrogen bonding recognition and the modality change of hydrogen bonding from "linear" (simple hydrogen bond interactions) to "nonlinear" (a complicated hydrogen bond network) between as-modified Au nanoparticles (Au NPs).     

Switching "on" and "off" the expression of chirality in peptide rotaxanes

The hydrogen-bond-directed synthesis, X-ray crystal structures, and optical properties of the first chiral peptide rotaxanes are reported. Collectively these systems provide the first examples of single molecular species where the expression of chirality in the form of a circular dichroism (CD) response can selectively be switched "on" or "off", and its magnitude altered, through controlling the interactions between mechanically interlocked submolecular components. The switching is achievable both thermally and through changes in the nature of the environment. Peptido[2]rotaxanes consisting of an intrinsically achiral benzylic amide macrocycle locked onto various chiral dipeptide (Gly-L-Ala, Gly-L-Leu, Gly-L-Met, Gly-L-Phe, and Gly-L-Pro) threads exhibit strong (10-20k deg cm(2) dmol(-1)) negative induced CD (theta;) values in nonpolar solvents (e.g. CHCl(3)), where the intramolecular hydrogen bonding between thread and macrocycle is maximized. In polar solvents (e.g., MeOH), where the intercomponent hydrogen bonding is weakened, or switched off completely, the elliptical polarization falls close to zero in some cases and can even be switched to large positive values in others. Importantly, the mechanism of generating the switchable CD response in the chiral peptide rotaxanes is also determined: a combination of semiempirical calculations and geometrical modeling using the continuous chirality measure (CCM) shows that the chirality is transmitted from the amino acid asymmetric center on the thread via the macrocycle to the C-terminal stopper of the rotaxane. This understanding could have important implications for other areas where chiral transmission from one chemical entity to another underpins a physical or chemical response, such as the seeding of supertwisted nematic liquid crystalline phases or asymmetric synthesis.     

Frequency analysis of amide-linked rotaxane mimetics

A vibrational analysis of 2-fold hydrogen bonds between an isophthalic amide donor and different acceptors is presented. These systems can be considered as mimetics for the hydrogen-binding situation of numerous supramolecular compounds such as rotaxanes, catenanes, knotanes, and anion receptors. We calculated pronounced red-shifts up to 65 cm(-1) for the stretching modes of the acceptor carbonyl as well as for the donor NH2 groups, whereas we observe a blue shift for the NH2 bending modes and an additional weak hydrogen bond between the acceptor and the middle C-H group of the donor. The red and blue shifts observed for different modes in various complexes have been correlated with the binding energy of the complexes, independently. In comparison with comparable single hydrogen bonds, we find for the 2-fold hydrogen bonds smaller red shifts for the N-H stretch modes of the donor but larger red shifts for the C=O stretch mode of the acceptor. Furthermore, our results indicate that the pronounced blue shift of the C-H stretch mode is basically caused by the fact that the acceptor is fixed directly above this group due to the 2-fold hydrogen bond.