1 Barry Nicolas Ingénieur de l Ecole Nationale Supérieure de Chimie de Rennes, France Titulaire d un Master de Chimie de l Université de Rennes 1, France Arene Ruthenium Assemblies for Biological Applications: From Carceplex to Host-Guest Metalla-Cages Thèse présentée à la Faculté des Sciences, Pour l obtention du grade de Docteur ès Sciences Membres du jury : Prof. G. Süss-Fink Dr. B. Therrien Prof. F. Kessler Prof. M. Pfeffer Co-directeur de thèse, Université de Neuchâtel Co-directeur de thèse, Université de Neuchâtel Rapporteur interne, Université de Neuchâtel Rapporteur externe, Université de Strasbourg Institut de Chimie de l Université de Neuchâtel Soutenue le 31 mars 2011
5 Remerciements Les travaux reportés dans la présente thèse ont été effectués au sein du Laboratoire de Chimie Organométallique et de Catalyse Moléculaire de l Université de Neuchâtel, sous la direction du Professeur Georg Süss-Fink et du Docteur Bruno Therrien. Je tiens en premier lieu à remercier le Professeur Georg Süss-Fink pour m avoir permis de réaliser ce doctorat au sein de son laboratoire. Je le remercie tout particulièrement pour les discussions qui ont accompagné la rédaction de cette thèse. J adresse mes remerciements les plus sincères au Docteur Bruno Therrien, avec qui j ai travaillé au jour le jour au cours de ces années de doctorat. Bien plus qu un encadrement, c est véritablement un apprentissage du métier de chercheur qu il m a été donné de recevoir, tant au niveau scientifique, qu au niveau rédactionnel ou humain. Je le remercie également pour les longues discussions sur mon avenir académique et les nombreux conseils donnés. Je ne saurais donc assez remercier le Docteur Bruno Therrien pour cette transmission sans réserve de ses connaissances et pour ces années que j ai vécues comme un réel «compagnonnage». Je tiens également à remercier les Professeurs Félix Kessler et Michel Pfeffer pour avoir accepté de faire partie de mon jury de thèse. Je les remercie d avoir pris connaissance de mon travail et d y avoir porté un jugement. Je remercie bien évidemment le Professeur Paul J. Dyson et le Docteur Olivier Zava pour les tests biologiques réalisés avec la grande majorité de mes produits. Je remercie particulièrement Olivier pour m avoir initié avec pédagogie à l art délicat des cultures de cellules cancéreuses et aux tests in vitro. Je remercie le Docteur Ramón Vilar et le Professeur Jérôme Lacour pour l accueil chaleureux que j ai reçu dans leurs laboratoires respectifs. Je remercie également Nurul H. Abd Karim et Martina Austeri avec qui travailler lors de ces collaborations a été un réel plaisir. Je remercie toutes les personnes qui ont participé de près à la réalisation de ce travail de doctorat et qui m ont permis d avancer rapidement dans de nombreux projets. Je pense particulièrement au Docteur Julien Furrer pour les nombreuses expériences RMN, parfois
6 complexes ou nouvelles. Je pense également à Anaïs sans laquelle toute la partie «dendrimères» de ce travail n aurait pas pu être réalisée : merci pour avoir initié l idée du projet et pour les grammes de dendrons synthétisés en un temps record. Je remercie finalement les membres du laboratoire et de l institut qui ont su faire régner une ambiance conviviale durant ces trois années. Concernant ma famille, je resterai dans le contexte de mon doctorat, car résumer ce qui a contribué à faire l essence même et la personnalité d un homme en quelques phrases serait par définition incomplet et imparfait. Je remercie donc mes parents pour m avoir donné, d une part, le goût des sciences et d autre part, le goût des lettres, deux conditions nécessaires pour la réalisation et l écriture d une thèse de doctorat. Le Palais de la Découverte, l Arboretum de Chèvreloup et les leçons de vocabulaire du lundi matin n auront finalement pas été inutiles. Paul, je te dédie la partie 4.2 de cette thèse. Les deux semaines passées dans votre foyer londonien lors des expériences décrites dans ce chapitre m ont presque fait regretter d avoir choisi les sciences plutôt que la finance. Je te remercie surtout pour avoir toujours été fier de mes réalisations, aussi petites soient-elles. Savoir que son frère est de tout cœur et en toute sincérité derrière soi est quelque chose d absolument essentiel dans la construction d un enfant, d un adolescent, mais aussi dans le développement intellectuel d un adulte. Anaïs, je te dédie le reste de ma thèse. Même en restreignant les remerciements au doctorat, une page ne suffirait pas pour être exhaustif, pas plus qu un chapitre ni même qu une nouvelle thèse. Je ne t écrirai donc qu un mot : MERCI.
7 Résumé Le développement depuis les années 1980 de la chimie supramoléculaire a permis la synthèse d objets discrets toujours plus complexes. Le rôle des métaux a été prépondérant, grâce notamment à leur mode de coordination qui permet d orienter de façon rationnelle l organisation moléculaire au sein même de l architecture supramoléculaire. Ainsi, les métalla-cages ont été au centre de l attention des chimistes, de par leur facilité d accès au niveau synthétique mais aussi de par leur large spectre d utilisation. Depuis la mise sur le marché et le succès clinique du cisplatine comme agent antitumoral, les métaux de transition sont également particulièrement étudiés dans une telle optique d application. Parmi les plus prometteurs, les complexes mononucléaires arèneruthénium ont démontré un fort potentiel antiprolifératif tout en étant moins toxiques que les complexes de platine. Le but de cette thèse de doctorat était de combiner les propriétés d assemblage des complexes arène-ruthénium en chimie supramoléculaire avec leurs propriétés antiprolifératives. Pour cela, plusieurs métalla-cages ont été construites ; leurs habilités à encapsuler de façon permanente ou réversible des molécules invitées ont été étudiées par différentes méthodes spectroscopiques, et les comportements antiprolifératifs des systèmes ainsi obtenus ont été établis in vitro envers différentes lignées de cellules cancéreuses. Mots clés Complexes organométalliques, complexes arène-ruthénium, chimie supramoléculaire, autoassemblage, métalla-cycles, complexes dinucléaires, complexes tétranucléaires, complexes hexanucléaires, complexes octanucléaires, carceplex, host-guest, activité anti-cancéreuse, G- quadruplexe,
9 Summary Since the development of supramolecular chemistry in the 1980s more and more discrete molecular objects have been synthesised. In this research area metals play a key role. Indeed, the coordination-driven self-assembly allows a directional-bonding approach that organises the different building blocks into supramolecular objects according to the coordination modes of the transition metal. Therefore, the versatility of metalla-cages and their application potential raised an increasing interest for this chemistry. On the other hand, ever since the clinical success of cisplatin as an antitumour drug, transition metals have raised considerable expectations for the treatment of cancer. So far, mononuclear arene ruthenium complexes are of central interest due to established cytotoxicity towards cancer cells and low general toxicity. The aim of this thesis was to combine the assembling properties of arene ruthenium complexes in supramolecular chemistry with their antiproliferative activity. Thus, different metalla-cages were synthesised, the permanent or reversible hosting ability of which was studied by various spectroscopic methods with different guest molecules. The antiproliferative behaviour of the resulting systems was established in vitro towards different cancer cell lines. Keywords Organometallic complexes, arene ruthenium complexes, supramolecular chemistry, selfassembly, metalla-cycles, dinuclear complexes, tetranuclear complexes, hexanuclear complexes, octanuclear complexes, carceplex, host-guest, anticancer activity, G-quadruplex.
11 Table of Contents Table of Contents Chapter 1: Introduction Arene Ruthenium Complexes Ruthenium: The Element The Arene Ligand Dimeric Arene Ruthenium Chloro Complexes Supramolecular Chemistry of Host-Guest Systems Coordination-Driven Self-Assembly and Molecular Recognition Host-Guest Systems and Carceplexes Pyrene, the Guest of Honour Metalla-Assemblies, Versatile Host Systems Arene Ruthenium Metalla-Assemblies as Host Systems Mononuclear Arene Ruthenium Complexes for Anticancer Treatment Metals in Medicine Cisplatin and other Platinum-Containing Anticancer Drugs Ruthenium versus Platinum NAMI-A and KP1019: Promising Ruthenium(III) Anticancer Agents Mononuclear Arene Ruthenium(II) Complexes as Potential Anticancer Agents Arene Ruthenium Assemblies with Anticancer Properties Drug Delivery and Enhanced Permeability and Retention Effect Arene Ruthenium Metalla-Rectangles Against Cancer Trojan Horse Concept and Arene Ruthenium Metalla-Prisms Aim of the Present Thesis Chapter 2: Arene Ruthenium Metalla-Rectangles Arene Ruthenium Metalla-Rectangles as Host Systems Overview of Arene Ruthenium Metalla-Rectangles Arene Ruthenium Metalla-Rectangles and Host-Guest Chemistry... 32
12 Table of Contents Synthesis and Characterisation of Arene Ruthenium Metalla-Clips C 5 C Synthesis of a Series of Arene Ruthenium Metalla-Rectangles (R 13 R 21 ) In- or Out-of-Cavity Interactions? Host-Guest Behaviour Antiproliferative Activity of Arene Ruthenium Metalla-Rectangles General Synthesis of a Series of Arene Ruthenium Metalla-Rectangles (R 9, R 19 R 31 ) Antiproliferative Activity Arene Ruthenium Metalla-Rectangles as Templates for [2 + 2] Cycloaddition General Intramolecular Cycloaddition of [(η 6 -p-cym) 4 Ru 4 (ox) 2 (bpe) 2 ] 4+ (R 22 ) Isolation of the Free Organic Linker Chapter 3: Arene Ruthenium Metalla-Prisms Arene Ruthenium Metalla-Prisms as Host Systems From Carceplex to Host-Guest Chemistry Synthesis of a Series of Arene Ruthenium Metalla-Prisms (P 2 P 4 ) Host-Guest Properties Arene Ruthenium Metalla-Prisms as Water-Soluble Nano-Capsules General Synthesis and Host-Guest Properties of [pyrene-x P] 6+ (P = P 2 P 4 ) Antiproliferative Study Arene Ruthenium Metalla-Prisms and Dendrimers General Encapsulation of Pyrenyl-Modified Dendrimers and Host-Guest Properties Antiproliferative Study Chapter 4: Arene Ruthenium Metalla-Cubes Antiproliferative Activity of Arene Ruthenium Metalla-Cubes... 85
13 Table of Contents General Synthesis and Characterisation of Arene Ruthenium Metalla-Cubes (Q 1 Q 8 ) Unsymmetrical Metalla-Cubes (Q 9 Q 11 ) Antiproliferative Study Arene Ruthenium Metalla-Cubes as G-Quadruplex Binders Background to G-Quadruplexes Biological Relevance of G-quadruplex DNA Synthesis of Arene Ruthenium Metalla-Cubes Q 12 and Q Interactions of some Arene Ruthenium Metalla-Cubes with DNA Open Arene Ruthenium Metalla-Cubes General Synthesis of a Series of Arene Ruthenium Metalla-Cubes Q 14 Q Antiproliferative Study Chapter 5: General Conclusion and Perspectives Arene Ruthenium Metalla-Rectangles Arene Ruthenium Metalla-Prisms Arene Ruthenium Metalla-Cubes Perspectives Chapter 6: Experimental General Remarks Solvents, Lamp and Products NMR Experiments Analytical Instruments Biological Studies DNA Interactions Syntheses and Characterisations Arene Ruthenium Metalla-Clips C 5, C 7, C 9, C 10, C 11, and C
14 Table of Contents Arene Ruthenium Metalla-Rectangles R 13 R Arene Ruthenium Metalla-Prisms P 2 P Host-Guest and Carceplex Systems [guest P][CF 3 SO 3 ] 6 (P = P 2 P 4 ) Arene Ruthenium Metalla-Cubes Q 2 Q Chapter 7: References Abbreviations List of Structures Metalla-Clips Metalla-Rectangles Metalla-Prisms Metalla-Cubes Tables of Figures, Schemes and Tables List of Publications
15 Chapter 1: Introduction Chapter 1: Introduction 1.1 Arene Ruthenium Complexes Ruthenium: The Element 44 Ruthenium is one of the six elements, Ru, Os; Rh, Ir; Pd, and Pt commonly called platinum metals which together with the lighter homologues Fe, Co, Ni form Group VIII of the periodic table. Karl Karlovitch Klaus discovered this transition metal in 1844 in ores from the Urals and named the new body, in honor of [his] motherland ruthenia, the latinised name for Russia. 1 Ruthenium is a rare element with a terrestrial abundance in the earth s crustal rocks estimated at around ppm, and was the last element of the platinum metals to be discovered. Ruthenium is found in the metallic state along with the other metals of Group VIII, in particular in the nickel-copper sulphide ores found in South Africa, Canada (Grand Sudbury) and in the river sands of the Urals. These ores are purified by treatment with aqua regia, in order to remove Pt, Pd and Au, and by treatment with hot nitric acid to remove Ag as its soluble nitrate. Fusing with NaHSO 4, and then with Na 2 O 2, to eliminate Rh and Ir, finally leads to the extraction of ruthenium as a lustrous and silvery metal that crystallises in a hexagonal close-packed (hcp) structure. 2 Ruthenium is the element 44 of Mendeleev s periodic table, its molecular weight is g mol -1, its density is g cm -3 at 20 C, and seven isotopes are naturally occurring. The electronic configuration of ruthenium is Kr [4d 7 ] [5s 1 ]. The organometallic chemistry of ruthenium mainly revolves around the oxidation states (0), (II) and (IV), the latter being more stable in the presence of strongly donating ligands, such as cyclopentadienyl (Cp) or pentamethylcyclopentadienyl (Cp*). 3 Other formal oxidation states are known such as (I) in [Ru(CO) 2 Cp] 2 or (III) in [RuCl 2 Cp*] n. 4 In mononuclear complexes the ligands usually contribute 10 electrons to give the metal an 18- electron configuration, although sterically demanding ligands may stabilize electronically unsaturated derivatives, as in the triisopropylphosphine complex [RuCl(PPr i 3)Cp] (16 electrons). 5 Other complexes of ruthenium have a square-planar stereochemistry with a 16- electron configuration, due to the high level of energy of the orbital p z that cannot be filled in this square-planar geometry. This type of complexes can react to give an 18-electron intermediate before eliminating a ligand, thus coming back to the stable 16-electron 1
16 Chapter 1: Introduction configuration. This reactivity provides to these complexes remarkable catalytic properties. 6 The wide range of oxidation states, the different geometries associated along with an important variety of coordination numbers as well as the possible tuning of the complex by ligands are the basis of the rich chemistry of ruthenium and more generally of the transition metals. However, these are also limitations and drawbacks in this chemistry and for this reason chemists need to control the reactivity of the metal centre. In particular, the control of the metal oxidation state is crucial The Arene Ligand Arenes or aromatic hydrocarbons are defined by the International Union of Pure and Applied Chemistry (IUPAC) as hydrocarbons with a conjugated cyclic molecular structure that is much more stable than the hypothetical localized structure. 7 Arenes are nowadays some of the most popular ligands in ruthenium(ii) chemistry. This popularity is particularly due to its strong π-acceptor behaviour that enhances the stability of the transition metal s d orbitals. This stabilisation of the orbitals of the ruthenium leads to a great stability against oxidation, in particular from Ru(II) to Ru(III). Another consequence is that the metal centre shows a strong preference for π-donor ligands. The stabilisation of the transition metal s d orbitals can be explained by the theory of the molecular orbitals. 8 An arene is a L 3 ligand and consequently gives 3 electron pairs to the metal in a coaxial way from p orbitals to vacant d orbitals of the metal, but also receives lateral bonding in return from filled d orbitals of the metal to π* antibonding p orbitals of the ligand. This bonding mode is called back π-bonding (see Figure 1). Figure 1. Bonding modes between transition metal and π-acceptor ligand 2
17 Chapter 1: Introduction This high stabilisation of the transition metal s d orbitals by arene ligand via back π- bonding is described by molecular orbital diagrams. In the case of an octahedral or pseudooctahedral complex ML 6, the simplified molecular orbital diagram established without back π-bonding (see Figure 2, A) shows that 3 d orbitals of the metal are nonbonding, molecular orbitals called t 2g. However, if the ligand L is a π-acceptor, like in the case of the arene ligand, these nonbonding d orbitals will be involved in the back π-bonding with the π* antibonding orbitals of L (see Figure 2, B). Therefore, these d orbitals will become bonding orbitals, and the 18-electron complex will be stabilised. 9 Figure 2. Molecular orbital diagram of an octahedral complex ML 6 In addition to this electronic protection of the oxidation state of the ruthenium centre, the arene ligands present other advantages. They are widely available and readily derivatised 3
18 Chapter 1: Introduction by attaching different substituents on the benzene ring through electrophilic substitution. 10 Moreover, the structural occupation of three coordinating sites forces the arene ruthenium complex to adopt a pseudo-octahedral geometry. Finally, from a chemical reactivity point of view, the arene ligands are quite inert towards substitution reactions. It means that arene ruthenium units are relatively stable against a large variety of chemical reagents and reaction conditions Dimeric Arene Ruthenium Chloro Complexes The first arene ruthenium complex was prepared by Winkhaus and Singer in 1967 by reduction in ethanol of ruthenium tri-chloride hydrate (RuCl 3 x H 2 O) in the presence of 1,3- cyclohexadiene (C 6 H 8 ). This neutral diamagnetic complex was obtained as a brown precipitate, for which a polymeric structure with the formula [RuCl 2 (C 6 H 6 )] n was proposed. 12 In 1972 Baird showed this polymeric formula to be erroneous and suggested a dimeric structure 13 by analogy with the complexes [(η 5 -C 5 H 5 )M(μ 2 -Cl)Cl] 2 (M = Ir, Rh) published by Maitlis in Eventually, in 1974, Bennett confirmed this dimeric structure [(η 6 - C 6 H 6 )Ru(μ 2 -Cl)Cl] 2 in which each ruthenium atom is at the oxidation state II, and where the hapticity (η) of the arene is equal to 6. The two ruthenium centres are linked by two chloro bridges. 15 The recognition of the dimeric nature of these arene ruthenium complexes had marked the beginning of 40 years of arene-ruthenium-based chemistry. Thus, many different arene ruthenium dimers were synthesised and the method developed by Winkhaus and Singer is still 4
19 Chapter 1: Introduction employed nowadays with electronically poor dienes. For instance, the dehydrogenation of (-)- (α)-phellandrene by ruthenium tri-chloride hydrate in ethanol leads to the formation of the well-used para-cymene (p-cym) dimer [(η 6 -p-cym)ru(μ 2 -Cl)Cl] However, this method cannot be employed with the electronically rich arenes, such as hexamethylbenzene (hmb). In this case, the arene exchange method at elevated temperature can lead to the desired dimer. In 1982, following this method, Bennett succeeded in synthesising the hmb ruthenium dimer by exchange of the para-cymene ligand with the hexamethylbenzene arene at around 180 C. 16 The enthusiasm for arene ruthenium dimers did not wane during the 1990s and found a revival with the emergence of functionalised arenes. These functionalisations are of interest because they are expected to combine the properties of the arene ruthenium unit with the properties of the grafted functions. 17 5
20 Chapter 1: Introduction Figure 3. Examples of functionalised arenes in arene ruthenium dimers A convenient method to introduce these functions is the preliminary functionalisation of an arene, followed by a Birch-type reduction 18 that converts the aromatic ring into a diene, which is then used as reagent for Winkhaus and Singer s method. For example, the dinuclear complexes [(η 6 -C 6 H 5 (CH 2 ) 3 OCO-p-C 6 H 4 -OC 8 H 17 )Ru(μ 2 -Cl)Cl] 2 (A) and [(η 6 -p- C 6 H 4 (CH 2 COOCH 2 CH 3 ) 2 )Ru(μ 2 -Cl)Cl] 2 (B) were obtained in 2006 (see Figure 3). 19 6
21 Chapter 1: Introduction 1.2 Supramolecular Chemistry of Host-Guest Systems Coordination-Driven Self-Assembly and Molecular Recognition Supramolecular chemistry has been defined by Jean-Marie Lehn as the chemistry beyond the molecule. 20 This means that supramolecular chemistry is not focused on the synthesis of a single molecule constructed gradually via the formation of direct covalent linkages but deals with the assembly of a molecular object constituted by several single molecules held together by a variety of weak interactions, such as hydrogen bonds, hydrophobic and π-π interactions, and electrostatic effects. 21 These weak interactions are used by Nature to construct large and ordered molecular arrays with precise functions at the subcellular and cellular levels. 22 The most famous and telling example of supramolecular assembly created by Nature is the double helical structure of deoxyribonucleic acid (DNA), elucidated by Watson and Crick in 1953, in which two separate strands of nucleotides are connected through hydrogen bonds. 23 If the weak interactions have a predominant position in natural supramolecular assemblies, a considerable number of systems were however synthesised by chemists via metal-donor bonds strictly covalent bonds in order to assemble organic building blocks into molecular objects. Thus, supramolecular chemistry can be seen as an interface of organic chemistry with the synthesis of the different building blocks and inorganic chemistry with the coordination of these organic building blocks to a transition metal. Furthermore, this meeting point also gathers physical chemistry with the experimental and theoretical studies of interactions and biochemistry with the biological processes that start with substrate binding and recognition together. 24 The concept of supramolecular chemistry described by Lehn as the designed chemistry of the intermolecular bond 25 is based on a process by which a supramolecular species forms spontaneously from its components. 26 This process, called self-assembly is a simple convergent path giving rise to the assembled target in a straightforward manner, and this, for the majority of the synthetic systems. 27 However, it is important to notice that selfassembly is not confined to supramolecular systems, but on the contrary is ubiquitous throughout life chemistry. The growth of crystals, 28 the formation of liquid crystals, 29 the spontaneous generation of synthetic lipid bilayers, 30 and the alignment of molecules on existing surfaces 31 are some examples of self-assembly in chemical systems. 7
22 Chapter 1: Introduction A very interesting subset of self-assembly, is the coordination-driven self-assembly, 32 which allows a directional-bonding approach that organises the different building blocks following the coordination modes of the transition metal. 33 Indeed, whereas many of the weaker noncovalent interactions are nondirectional for example, hydrogen bonds, van der Waals, and hydrophobic interactions covalent metal-ligand bonds, also called dative bonds, are highly directional and relatively strong. Consequently, coordination-driven self-assembly leads to the straightforward construction of large and rigid supramolecular objects with welldefined shapes, sizes, and geometries. 34 Over the last two decades, the groups of Fujita, 34 Stang, 35 Raymond, 36 Lehn, 25 or Mirkin 37 have pioneered the use of the directional-bonding coordination-driven approach to self-assembly. The idea of this approach is to combine rigid electron-poor metal centres and complementary, rigid electron-rich organic donors to provide a wide variety of discrete two dimensional (2D) and three dimensional (3D) polygonal and polyhedral coordination assemblies. It is important to notice that metal-ligand coordination bonds are kinetically labile, also being strong as compared to most other noncovalent interactions. This lability is crucial since one important feature of supramolecular chemistry is that molecular assemblies are normally reversible. Therefore, the final product is in thermodynamic equilibrium with its components. In particular, this notion of reversibility is fundamental in the case of the systems assembled by molecular recognition. Like selfassembly, molecular recognition is a process that is widely found throughout both natural and synthetic systems. Molecular recognition is essential in a range of biological areas for instance substrates-enzymes or proteins-dna interactions and is also of central importance in a number of chemical fields sensors, separation science or catalysis for example. In particular, molecular recognition is closely linked with host-guest chemistry Host-Guest Systems and Carceplexes Host-guest chemistry is a subset of supramolecular chemistry based on molecular recognition. 39 For many biological systems, as for substrate-enzyme systems, host-guest chemistry is the link between the nature of these biological systems and their functions. In other words, the structural adaptations of both enzyme and substrate in order to accommodate their shapes and to allow interactions and catalytic processes are the visible expressions of the 8
23 Chapter 1: Introduction host-guest phenomenon. In that precise system, the enzyme can be seen as a host and the substrate as a guest of the host-guest system. The place where the guest is accommodated by the host is called binding site. Furthermore, in a given supramolecular system the degree of electronic and steric complementary between host and guest dictates the magnitude of the molecular recognition occurring. In particular, the stoichiometry of a host-guest system is defined by the number of guest molecules that one molecule of host is able to accommodate. It should be noted that host-guest chemistry is not only a natural and biological phenomenon, but is also widely used by chemists. Indeed, since D. J. Cram baptised this field of supramolecular chemistry in 1974, 40 many efforts have been invested on the design of numerous host-guest systems. In particular, a large array of synthetic hosts, capable of binding guest ions or molecules, has been developed. The field began with ionophores such as crown ethers 41 and cryptands, 25 (see Figure 4). crown ether cryptand cyclodextrin calixarene Figure 4. Some common host molecules In the 1970s Cram began to develop cavitands, 42 and in the 1980s the first calixarenes were synthesised. 43 Cavitands have enforced cavities, and therefore they cannot collapse upon themselves. Such preorganisation optimises the van der Waals contacts between host and guest. Later, Cram synthesised a spheroid compound constituted by two hemispherical cavitands link together and able to entrap guests in such manner that the loss of the guest molecule is only possible via rupture of a covalent bond. Then, in 1983 Cram proposed the name of carceplex to define these host molecules that permanently incarcerate their guests. 44 Since this beginning of synthetic host-guest chemistry, host-guest systems and the limit case 9
24 Chapter 1: Introduction of the carceplex molecules have been widely studied, with many different types of host and guest molecules Pyrene, the Guest of Honour Among all the different guest molecules that have been encapsulated via host-guest chemistry since the 1970 s, pyrene is one of the best studied. The success of pyrene as a guest molecule can be partially explained by the chemical nature of its structure. Pyrene is a highly symmetrical molecule (point group D 2h ), and is the smallest peri-fused polycyclic aromatic hydrocarbon (PAHs) compound. 45 This planar structure confers to pyrene a very interesting sterical environment, which gives it the opportunity to fit into small host s cavity. This suggests that the design of host s small pockets can be ideal for pyrene. pyrene Moreover, despite having 16 π electrons and consequently not following the Hückel s 4n + 2 π electrons rule, which states that cyclic planar molecules in which each atom has a p orbital are aromatic if they contain 4n + 2 π electrons, pyrene is an aromatic molecule, based on Clar sextet theory, 46 Platt s ring perimeter, 47 and the Randić-Herndon conjugated circuit models 48 commonly employed for qualitative characterisation of the aromaticity of polycyclic aromatic compounds. 49 This aromaticity is crucial for the use of pyrene as guest molecule. Indeed, as described in section 1.2.1, supramolecular chemistry in general, and host-guest chemistry in particular are mainly based on weak interactions such as hydrophobic or π - π stacking interactions. The presence of a rich π electronic environment is consequently susceptible to give strong intermolecular interactions, via π - π stacking, between an aromatic constituent of the host molecule and pyrene. This parallel π - π stacking can also be reinforced by T-shape interactions
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Generated by Foxit PDF reator Foxit Software Pd- AND f-blk RGANETALLIS PAn organometallic compound must contain P a carbon-metal bond. PBook, p. 459, gives list of common ligands P An over-view of organometallics
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Acids and : A Brief Review Acids: taste sour and cause dyes to change color. : taste bitter and feel soapy. Arrhenius: acids increase [H ] bases increase [OH ] in solution. Arrhenius: acid base salt water.
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