Dancing molecules on the make

December 16, 2002

Towards the end of the 19th century Alfred Werner formulated the basis of coordination chemistry. He provided long searched for explanations on the formation of chemical compounds consisting of a central transition metal atom surrounded by a set of molecular ligands. Coordination compounds are of great scientific interest: they play an important role in many biological processes and are employed in the synthesis of novel supramolecular architectures and materials. A research team at the Max Planck. Institute for Solid State Research in Stuttgart have succeeded in directly observing and controlling the formation of surface-supported metal-organic complexes at a molecular scale.

The development of the scanning tunneling microscope (STM) in the early 1980s brought a radical change of the way we regard the atomic and molecular world. This technique allows in particular the in situ observation of molecules and chemical processes at the atomic scale, provided that the investigated components are adsorbed at a surface. Moreover it is possible to monitor rotational and translational movements of single atoms and molecules. In recent studies it was even possible to perform detailed analyses of supramolecular systems, where functional molecular building blocks self-assemble into complex architectures. The driving force for the self-assembly are so-called non-covalent interactions, such as hydrogen bonding or metal-ligand interactions.



In order to gain direct insight into the formation of coordination compounds at a surface, the scientists deposited a simple molecular building block - 1,3,5-tricarboxylic acid (tma) - on a copper substrate. At room temperature there is a gas of highly mobile Cu atoms at copper surfaces, which can interact with the reactive ligands of the molecule, i.e., the deprotonated carboxylic acid groups. In sequences of STM images the movements of single molecules could be monitored and it could be revealed how rotating tma molecules act as dynamic atom traps for individual Cu atoms (the corresponding STM movies can be found at [1]). Thus single events of association and dissociation of cloverleaf-shaped Cu(tma)4 coordination compounds were directly observed. Furthermore it appears that the lifetime of such complexes is crucially dependent upon the local chemical environment.

In a further step the scientists succeeded in the deliberate synthesis of a related cloverleaf-shaped complex consisting of iron atoms and tma molecules. Again the reaction took place with the constituents adsorbed at a copper substrate. A stronger interaction between the central Fe atom and the carboxylic acid ligands is, however, encountered in this system. As a consequence there is an increased thermal stability and a different orientation of the complex. A detailed analysis of the bonding geometry revealed in particular that the complexes exist in two mirror-symmetrical configurations at the surface, in analogy to the mirror symmetry of left and right hands in three-dimensional space. This phenomenon is called "chirality" (from gr. ceir : hand). Chiral molecules play an important role in biology and pharmacology. In the present case, the Fe(tma)4-complexes are chiral in two dimensions. This represents the first observation of a chiral coordination compound at a surface.

These experiments are the first steps in the exploration of the nature and bonding mechanisms in coordination compounds at surfaces, a research field where our current knowledge is far from complete. It is expected that a systematic understanding of the underlying chemistry and physics will be of significant value for the deliberate synthesis of surface-supported functional supramolecular architectures and nanostructures.
-end-
Original publications:
"Real-time single-molecule imaging of the formation and dynamics of coordination compounds", N. Lin, A.
Dmitriev, J. Weckesser, J.V. Barth and K. Kern, Angewandte Chemie Int. Ed. vol, pp (2002).
"Direct observation of chiral metal-organic complexes assembled on a Cu(100) surface"
P. Messina, A. Dimitriev, N. Lin, H. Spillmann, M. Abel, J.V. Barth and K. Kern, Journal of the American
Chemical Society vol, pp (2002).

Related Link:
[1] http://www.mpi-stuttgart.mpg.de/kern/Res_act/supmat_2.html
[2] http://www.mpi-stuttgart.mpg.de/kern/Res_act/supmat_2_1.html Animation (3.2 MB): Molecular rotors working as dynamical atom trap: Sequence of STM images revealing the molecular mobility in association and dissociation of a Cu(TMA)4 compound.
[3] http://www.mpi-stuttgart.mpg.de/kern/Res_act/supmat_2_2.html Animation (2.3 MB):Reactivity of single chemical reactions is strongly influenced by the local chemical enviroment.

Max-Planck-Gesellschaft

Related Physics Articles from Brightsurf:

Helium, a little atom for big physics
Helium is the simplest multi-body atom. Its energy levels can be calculated with extremely high precision only relying on a few fundamental physical constants and the quantum electrodynamics (QED) theory.

Hyperbolic metamaterials exhibit 2T physics
According to Igor Smolyaninov of the University of Maryland, ''One of the more unusual applications of metamaterials was a theoretical proposal to construct a physical system that would exhibit two-time physics behavior on small scales.''

Challenges and opportunities for women in physics
Women in the United States hold fewer than 25% of bachelor's degrees, 20% of doctoral degrees and 19% of faculty positions in physics.

Indeterminist physics for an open world
Classical physics is characterized by the equations describing the world.

Leptons help in tracking new physics
Electrons with 'colleagues' -- other leptons - are one of many products of collisions observed in the LHCb experiment at the Large Hadron Collider.

Has physics ever been deterministic?
Researchers from the Austrian Academy of Sciences, the University of Vienna and the University of Geneva, have proposed a new interpretation of classical physics without real numbers.

Twisted physics
A new study in the journal Nature shows that superconductivity in bilayer graphene can be turned on or off with a small voltage change, increasing its usefulness for electronic devices.

Physics vs. asthma
A research team from the MIPT Center for Molecular Mechanisms of Aging and Age-Related Diseases has collaborated with colleagues from the U.S., Canada, France, and Germany to determine the spatial structure of the CysLT1 receptor.

2D topological physics from shaking a 1D wire
Published in Physical Review X, this new study propose a realistic scheme to observe a 'cold-atomic quantum Hall effect.'

Helping physics teachers who don't know physics
A shortage of high school physics teachers has led to teachers with little-to-no training taking over physics classrooms, reports show.

Read More: Physics News and Physics Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.