Fig. 1: tma-molecules bond in a flat adsorption geometry at a copper surface are resolved as equilateral triangle in STM. The sequence of STM images reveals how the thermal motion of molecules at the surface proceeds. Following rotational motions and displacements, a single Cu atom is captured whereupon a cloverleaf-shaped Cu(tma)4 coordination compound evolves (second image for t = 80 s; see also [1]). Image: Max Planck Institute for Solid State Research

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.

Fig. 2: Synthesis of Fe(tma)4 complexes at a copper substrate. Observable are unidentate bonding and the correlated orientation of the four tma molecules surrounding the central Fe atom. As a consequence two mirror-symmetric compounds can be formed, designated with R and S, i.e., the system is chiral in two dimensions. Image: Max Planck Institute for Solid State Research

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.

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).