chapter 14(2).pdf

Supramolecular Chemistry, Metallosupramolecular Chemistry and Molecular Architecture Chapter 14 Objectives: Definitions Molecular recognition Self-assembly Metallosupramolecular chemistry Encapsulated guests in metallonanostructures Molecular devices and machines 1 2 Supramolecular Che...

Supramolecular Chemistry, Metallosupramolecular Chemistry and Molecular Architecture Chapter 14 Objectives: Definitions Molecular recognition Self-assembly Metallosupramolecular chemistry Encapsulated guests in metallonanostructures Molecular devices and machines 1 2 Supramolecular Chemistry: Definitions Definitions – J. M. Lehn: the chemistry beyond the molecule, bearing organized entities of higher complexity that results from the association of two or more chemical species held together by intermolecular forces – More general: the chemistry of species made of two or more molecular components – The concept and term first introduced by Lehn in 1978 3 Lehn’s idea Just as there is a field of molecular chemistry based on the covalent bond, there is a field of supramolecular chemistry, the chemistry of molecular assemblies and of the intermolecular bond Chemistry beyond the molecule 4 DNA backbone 5 Intermolecular bonds They have different degree of strength and directionality They are – Hydrogen bonding – Electrostatic forces – Van der Waals interactions – Donor-acceptor interactions – π-π stacking interactions – Metal ion coordination 6 Bond energy comparison Intermolecular forces are generally weaker than covalent bonds Supramolecular species dynamically more flexible than molecules. Bond Energy Single 350 KJ/mol Triple (N2) 942 KJ/mol Ion-ion interactions 250 KJ/mol Hydrogen bonding 20 KJ/mol Dispersion forces 2KJ/mol 7 Examples Host-Guest chemistry: – Early example: selective binding of alkali metal cations in crown ethers and cryptands – Molecular recognition: Selective binding of a specific substrate to a receptor (Host guest chemistry) Self-assembly process: self-organization of molecular system via molecular recognition 8 Molecular recognition 1960s- Pederson, Lehn and Cram Recognition: geometrical preorganization and interaction complementarity – Preorganization: construction of host that exactly matches with guest (both sterically and electronically) – Complementarity: Host must have binding sites which attract binding sites of guests The interactions are relatively weaker than covalent bonds But multiple site interaction yields strong selective complexation without generating strong non-bonded repulsions 9 Molecular receptors (Hosts) Cation binding hosts Anion binding hosts Neutral molecule binding hosts (Inorganic, organic or biological) 10 Cation binding sites Bind via electrostatic ion-dipole interaction Crown ethers Lariat crown ethers Heterocrown Spherands Cryptands Calixarene 11 Cationic Guest 12 Cation- Binding Hosts Binding by electrostatic ion-dipole interaction Better the fit of the cation into the crown, the stronger the complex: optimal spatial fit. 13 Lariat Ethers Designed to carry and transport cation across lipophilic membranes - Higher binding constants than Crown ethers 14 Heterocrowns Incorporation of softer atoms : N, S, P Binding to Ag+ and transition metal ions Strong Complexation of toxic Cd2+, Pb2+, Hg2+ ions 15 Spherands Macrocyclic host with rigid cavity High Stability Li+ complex (Size fit) Used for Removal of Li+ 16 Cryptands Macrocyclic ligands that form a 3 dimensional cage to encapsulate (hide) the metal ion Special recognition for alkali and alkaline-earth cations Selectivity of K+ is observed for [2.2.2] 104 times stronger than its crown analog. Cation exchange decreases with increasing stability of complexes 17 Calixarenes Act as a host for cations, anions, and neutral molecules depending on the degree of functionalization π-cation interaction also observed 18 Selectivity of cation binding Several Factors – Size match between cation and host cavity – Electrostatic charge – Solvent (polarity, hydrogen bonding, coordination stability) – Degree of host preorganization – Enthalpy and entropy contribution – Cation and host free energies of solvation – Nature of counter-anion – Interaction of counter anion with solvent and the cation – Cation binding kinetics – Chelate ring size 19 Anionic Recognition Simple inorganic anions occur in range of shape and geometry – Spherical (halides) – Linear (SCN-, N3-) – Planar (NO3-, CO32-) – Tetrahedral (SO42-, PO43-) – Octahedral (PF6-) Positively charged hosts: – Polycyclic-amide ligands – Macrocyclic tetraamide –… 20  Katapinands (swallow up) Anions are relatively large, therefore receptors are large Both neutral and positively charged hosts will bind anions 21 Macrocyclic Polyamides as anion receptors Positively charged hosts bind to anions such as [PdCl4]2- and [Fe(CN)6]422 Neutral Molecules Recognition: binding Neutral Molecules Not included in coordination chemistry Cavitands: host molecules with intrinsic cavity present in both solid-state and solution – eg: Calixarene, cyclodextrines, cucurbituril… 23 Cyclodextrins Host-guest Drug delivery Cyclic oligosaccharides with six-to eight D-glucopyranoside units linked by a 1,4-glycosidicbond. (truncated funnel with two different faces) Hydrophilic due to the –OH groups in the faces Hydrophobic in the cavity (glycoside oxygen and methylene hydrogen) 24 Carbonic acid as a guest Cyclophane: organic host molecules containing a bridged aromatic ring 25 Multiple recognition Able to extract a range of salts into chloroform solution e.g. LiNO3, NaNO3, KNO3… Polytopic receptors are homotopic and heterotopic 26 Self-assembly Lehn at al. 1987 Spontaneous connection of a few/many components, resulting in the formation of discrete/extended entities at either molecular or supramolecular level The components that undergo self-assembly were named tectons 27 Molecular and Supramolecular self-assembly Molecular self-assembly is static (strong bonds) Supramolecular self-assembly is dynamic and reversible (weak/labile bonds) 28 Metallosupramolecular Chemistry: Strategies and types Introduced by E.C Constable Non-covalent interaction are coordinative bonds Metal ions provide – Set of coordination geometries – A range of binding strengths (from weak to strong) – Range of formation and dissociation kinetics (from labile to inert) 29 Helicates Introduced by Lehn(1987) 30 Tetrahedral versus Octahedral coordination in double stranded helicates M= Cd, Mn, Ni, Fe, Cu 31 Triple stranded helicates 32 Cyclic helicates Reported by Lehn 33 Cyclic helicates Reported by Lehn 28 Å 11 Å 34 Grid-type Metal ion Architectures A-Cu(I) or Ag(I) B-Co(II) 35 Polygons Reported 1990 36 Polygons and Polyhedra 37 Polyhedra 38 Supramolecular Equilibrium Triangle - square 39 Supramolecular Chemistry, Metallosupramoleular Chemistry and Molecular Architecture Chapter 14 Objectives: Definitions Molecular recognition Supramolecular Dynamics: Reactivity, Catalysis and Transport Self-assembly Metallosupramolecular chemistry Supramolecular assistance in the synthesis of Molecular structures Molecular devices and machines 40 Interlocked molecules 41 Catenanes Introduced by Sauvage (mid-1980) Interlocking of two cyclic structures Also Multiple interlocking 42 Example of Catenane synthesis The Template effect of Cu(I) is used 43 Fujita Catenates Polar solvents favor hydrophobic interactions between ligands, favoring formation of the catenate 44 Rotaxanes A stoppered filamentous molecule threaded through a cyclic one Stoppers: terpyridine, porphyrine, fullerene etc. 45 Knots A single strand Alternatively passes over and under itself in a loop 46 (Supra)Molecular Devices and Machines A supramolecular device may be defined as a complex system made up of molecular components with definite individual properties The components can be connected via covalently, hydrogen-bonded, coordination interaction etc. The component should contribute something unique and identifiable : Molecular machines Common components in molecular devices are photochemically, redox, or chemically active molecules 47 Requirements Most of them use the stereoelectronic requirements of Cu(I) and Cu(II) Cu(I), CN = 4, tetrahedral Cu(II), CN = 5, spy or tbp Cu(II), CN = 6, Octahedral + J-T distortion 48 Nobel Prize in Chemistry in 2016 shared equally “for the design and synthesis of molecular machines.” 49 Molecular machines Molecular machines are single-molecules that behave much like the machines They have controllable movements and can perform a task with the input of energy Examples: Tiny elevator that goes up and down with changes in pH and a super-small motor that spins in one direction when exposed to light and heat. Many in the field speculate that molecular machines could find use in computing, novel materials, and energy storage. 50 Examples of Molecular Devices and Machines Pirouetting of ring in a catenate: 51 Examples of Molecular Devices and Machines 52 Translation in rotaxanes 53 Contracting and Stretching motion 54 Contracting and Stretching Motion 55 Animation of the world’s smallest electric car. Its motion is powered by electrically driven rotors made of nanoscale molecules Reported in Nature volume 479, pages208–211 (2011) Electrically driven directional motion of a four-wheeled molecule on a metal surface By Tibor Kudernac, Nopporn Ruangsupapichat, Manfred Parschau, Beatriz Maciá, Nathalie Katsonis, Syuzanna R. Harutyunyan, Karl-Heinz Ernst & Ben L. Feringa 56 57 58 Contracting and Stretching motion 59 Contracting and Stretching motion 60  Supramolecular Chemistry, Metallosupramolecular Chemistry and Molecular Architecture Chapter 14 Objectives: Definitions Molecular recognition Self-assembly Metallosupramolecular chemistry Encapsulated guests in metallonanostructures Molecular devices and machines 61