Tectonic Molecules Show Suprising Strength

Tectonic Molecules Show Suprising Strength

by Dr. John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Jul- Aug 98	Rank Last Period May- Jun 98
1	A. Thess, et al., "Crystalline ropes of metallic carbon nanotubes," Science, 273(5274):483-7, 26 July 1996. [Rice U., Houston, TX; U. Pennsylvania, Philadelphia; Inst. Charles Sadron, Strasbourg, France; Michigan St. U., E. Lansing] *UY983	37	2
2	A.P. Scott, L. Radom, "Harmonic vibrational frequencies: an evaluation of Hartree-Fock, Moller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors," J. Phys. Chem., 100(41):16502-13, 10 October 1996. [Australian. Natl. U., Canberra] *VL579	19	1
3	G.N. Murshudov, A.A. Vagin, E.J. Dodson, "Refinement of macromolecular structures by the maximum-likelihood method," Acta Cryst., D53:240-55, May 1997. [U. York, England; Free U. Brussels, Belgium] *XB256	15	†
4	S.J. Tans, et al., "Individual single-wall carbon nanotubes as quantum wires," Nature, 386(6624):474-7, 3 April 1997. [Delft U. Technol., Netherlands; Rice U., Houston, TX] *WR256	14	3
5	P. Brunet, M. Simard, J.D. Wuest, "Molecular tectonics, porous hydrogen-bonded networks with unprecedented structural integrity," J. Amer. Chem. Soc., 119(11):2737-8, 15 March 1997. [U. Montreal, Canada] *WN855	9	†
6	J.W.G. Wildoer, et al., "Electronic structure of atomically resolved carbon nanotubes," Nature, 391(6662):59-62, 1 January 1998. [Delft U. Technol., Netherlands; Rice U., Houston, TX] *YP888	9	†
7	R.N. Warrener, et al., "A new building BLOCK technique based on cycloaddition chemistry for the regiospecific linking of alicyclic sub-units as a route to large, custom-functionalised structures," Chem. Comm., (11):1023-4, 7 June 1997. [Central Queensland U., Rockhampton, Australia; Deakin U., Geelong, Australia] *XD466	9	†
8	M.W. Wong, "Vibrational frequency using density functional theory," Chem. Phys. Lett., 256(4,5):391-9, 5 July 1996. [U. Queensland, Australia] *UX017	8	†
9	C. Granel, et al., "Controlled radical polymerization of methacrylic monomers in the presence of a Bis(ortho-chelated) arylnickel (II) complex and different activated aklyl halides," Macromolecules, 29(27):8576-82, 30 December 1996. [U. Liege, Belgium] *WB741	7	†
10	A. Harriman, R. Ziessel, "Making photoactive molecular-scale wires," Chem. Comm., (15):1707-16, 7 August 1996. [[Ecole. Chim. Polymeres Mat. Strasbourg, France] *VA968	7	†

SOURCE: ISI's Hot Papers Database. the full legend.

Slowly but surely chemists are moving to uncover the secret of how simple molecules were able to organize themselves in a way that must once have led to the first "living" unit. This requires a molecule to attract the components from which it is made and then to manipulate these to make a replica of itself. Such molecules will of necessity be large, and must preserve structural integrity yet permit reactivity. In the current Hot Ten are two papers, #5 and #7, which report the assembly of structures with some of the necessary features.

Paper #5 comes from the group of James Wuest, based at the University of Montreal, Canada, and is concerned with the behavior of so-called "tectons," which are molecules that have specific attractive forces enabling them to assemble into large aggregates with structural integrity. The paper concentrates on a relatively simple tecton molecule that has four protruding 2,4-diaminotriazine rings. It is the ability of these to form hydrogen bonds that allows the molecule to create a vast three-dimensional network, with cavities big enough (ca.12Ĺ in diameter) to trap other molecules.

One structure, determined by X-ray crystallography, has formic acid and dioxane solvent molecules in the cavities, to the extent that they fill 42% of the crystal’s volume. When this material was suspended in water, the trapped molecules escaped, to be replaced by water molecules, which being smaller could pack better into the vacated holes, yielding crystals with 21 waters per tecton unit. The molecular array was robust enough for this to happen without the structure having to break down and reassemble to accommodate the change.

In this respect Wuest’s new material bears comparison with zeolites and other inorganic materials whose structures can withstand the removal and replacement of trapped molecules. What makes Wuest’s tectonic array so remarkable is that it is only held together by the weakest of bonding forces, hydrogen bonds, rather than by strong covalent bonds as in the zeolites. However, it is the sheer number of these bonds that gives the construct its overall stability, and in this respect it bears comparison with the sturdy nature of other hydrogen-bonded macro-molecular arrays such as DNA.

In more recent work, yet to be published, Wuest has been investigating tectons that associate to form porous anionic networks that can undergo ionic exchange. In a typical network of this new type 74% of the volume of the crystal is occupied by disordered and mobile guests, including the cation and the solvent.

"These new materials raise interesting questions about how small a fraction of the volume of an ordered tectonic solid can be occupied by the tectons themselves,"says Wuest, adding that "because of the obvious analogies that exist between porous tectonic solids and zeolites, molecular tectonics is becoming an extremely attractive strategy for making new types of catalysts and chromatographic sorbents."

Meanwhile paper #7 reports on another kind of structure-building chemistry. This comes as a result of joint work from two Australian teams, that of Ronald Warrener, working at Central Queensland University, Australia, and that of Richard Russell, at Deakin University, Victoria. The paper reveals a way of making large "custom-functionalized" structures using two types of building-block molecule which clip together. The method is a development of the discovery that cyclobutene epoxides, activated by the presence of nearby ester groups, will couple with norbonadienes to form a larger molecule, which can then be used to continue the process.

The building blocks can incorporate reactive components such as crown ethers, ligands, and redox centers, offering a variety of possibilities when it comes to interacting with the molecule’s environment. A particular attraction of the methodology is that it permits the incorporation of chemical groupings that would normally be incompatible with the reaction conditions used. More recently the Australians have been using the ubiquitous porphyrins, so abundant in nature, as building blocks, and in work soon to be published in Chemical Communications, Warrener will announce a molecule which incorporates a "universal joint" composed of two rigid U-shaped bis-porphyrins held together by a tetrapyridyl porphyrin.

Dr. John Emsley is Science Writer in Residence
at the Department of Chemistry, University of Cambridge, U.K.