Supramolecular assemblies adducts

Many of the reactions observed in molecular s)mthetic cages are reactions that enzymes are not known to catalyze examples are Diels-Alder cycloaddition and 1,3-dipolar cycloaddition. This behavior constitutes a reason to explore further the range of applications of these supramolecular assemblies. The fact that cycloaddition reactions are t) ically catalyzed is probably a result of the similarity of the transition state and the reactants and products. This characteristic of the cycloadditions makes product inhibition inevitable, unless additional reactions are operative that alter the shape and functionality of the adduct that is formed and thus reduce its association constant with the cage. 

Before discussing specific D—[H]—A systems, it is appropriate to identify common experimental properties and modalities that are pertinent to their assembly and electron transfer reactivity. Because of the dynamic nature of noncovalent interactions, the supramolecular assemblies of this section have, for the most part, been characterized by spectroscopic techniques. Association constants for adduct for-... 

This entry to supramolecular chemistry allows one, at the same time, to develop a host-guest chemistry because the space inside the cucurbituril barrel is sufficient to confine small "guesf molecules. This is illustrated with the crystallization of the supramolecular adduct [W3S4(H20)8Cl](pyH ccuc) Cl4a 5.5 H2O with a pyridinium cation inside the cucurbituril cavity [48]. The introduction of guest molecules may vary the net charge on the assembly and consequently the whole packing in the solid state. 

A complete study concerning a new class of supramolecular structures (supraminols) has been recently published. These supramolecular structures are formed by an enantiodifferentiating self-assembly between (If , 2f )-diaminocyclohexane and various trans-1,2-cyclohexane diols. An example is shown in eq 27. When racemic diols were used, a homochiral cristalline adduct is formed with an efficient kinetic resolution. 

Stabilization and crystallization of [Cr f/x-OI 0/0112)J(3j y>+ cations have been achieved by exploiting the supramolecular interactions with large anionic assemblies initially with mesitylene-2-sulfonate or its adduct with 18-crown-6,837-839 and subsequently with the adducts of calixarene sulfonates with N- or O-macrocycles.840,841 This approach, illustrated in Scheme 27, allowed the crystal structures of di-, tri- and tetrameric Cr111 complexes, (165)-(167),837,839-841 as well as that of a heteropolynuclear complex, [(H20)4Rh(/i-0H)2Cr(0H2)4]4+,838 to be determined. 

As unambiguously evidenced by NMR spectroscopy, titration of 9Zn in CDCI3 solution with one equivalent of a linear ditopic N-ligand leads rapidly to the quantitative assembling of sandwich-like 2 2 supramolecular adducts of formula [(9Zn)2(Ar-L)2j (L = 4,4 -bipy, 13 L = 4 -fransDPyP, 14 L = 4 -tra sDPyP-npm, 15), formed by two parallel metallacycles connected by two bridging ligands which are axially bound to the zinc-porphyrins (Scheme 3) [68,72,73]. 

A unique assembly of a crystalline hybrid supramolecular salt was achieved by reaction of 2,4-bis (3-pyridylmethyl)oxy -(l,3)-p-bu -calix[4]-crown-6 (33), hexadecafluoro-l,8-diidooctane and Csl (2(X) 4951). In its solid state, the cesium ion of the supercation was encapsulated inside the cavity created by the crown ether loop, the picolyl and inverted phenyl moieties, whereas the two iodide ions of the superanion, formed a discrete five component aggregate held together by hydrogen and halogen bonds. Similar adduct existed in solution. A combination of this calix-crown 33 and octafluoro-l,4-diiodobutane acted as an effective binary host for the selective extraction of Csl from aqueous to fluonous phase. 

Supramolecular self-assembly via bridging cyclopentadienyl ligands occurs, for example, in the metallocenes [Zn(C5Hs)2] , 86 [84], and [Cd(C5Hs)2] , 87. In the crystalline state these compounds form coordination polymers and dissolve only in strongly coordinating solvents, thereby forming monomeric adducts. From their... 

The development of new generations of probes has been crucial for the advancement of molecular imaging. Supramolecular chemistry has emerged as a powerful tool in this area. Host-guest chemistry and molecular self-assembly have been exploited in MRI, optical, and nuclear imaging to form supramolecular adducts with superior sensitivity and specificity than many standard small molecule probes. 

Simply mixing solutions of the tetracationic diphosphine (175) with a tetraanionic calix[4]arene leads to the formation of supramolecular heterocapsules that can bind a transition metal ion within the cavity of the assembly, providing a new class of potential supramolecular catalysts. Sulfonated aryl-phosphines and -diphosphines form supramolecular adducts with cyclodextrins that are also of interest as new ligand systems,and the mechanism of such inclusion processes has attracted a theoretical study. ... 

Scheme 6 Operation mechanism of a supramolecular solar cell with a ZnP-Cgo donor-acceptor dyad assembled by axial metal-ligand coordination of the Cao-imidazole adduct to a ZnP polymer film. Reprinted with permission from Subbaiyan NK, Obraztsov I, Wijesinghe CA, Tran K, Kutner W, D Souza F (2009) Supramolecular donor-acceptor hybrid of electropolymaized zinc porphyrin with axially coordinated fulleiene ftnmation, characterization, and photoelectrochem-ical properties. J Phys Chem C 113(20) 8982-8989. Copyright (2009) American Chemical Society...

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