Anisotropic Functionalization of Clusters

Inorganic-organic hybrid materials had major impact on materials science in the past 15-20 years. In this class of materials, molecular or nanoscale inorganic and organic building blocks are combined with each other giving rise to new materials properties. Various types of such materials have been prepared by various preparative routes. A relatively new sub-class of inorganic-organic hybrid materials are based on molecular inorganic clusters as pre- formed inorganic "nanobuilding blocks". The proposed work aims at stable and well characterized clusters with functional ligands in a defined spatial arrangement. Several preparative approaches will be explored. These clusters will later be used as "designer nanobuilding blocks" for hybrid materials. The clusters must have Lego brick characteristics, such as a specific shape and a defined number and geometrical arrangement of reactive sites. The major challenges of the proposed project thus will be (i) to prepare clusters with Lego brick characteristics which are stable in solution against re- organization or degradation, (ii) to prepare clusters with both functional and non-functional ligands, where the functional ligands are arranged in a pre-determined geometry, (iii) to prevent re-organization or degradation of the cluster core during introducing, modifying or exchanging ligands, (iv) to control dynamic site exchange of the ligands at the cluster surface and exchange processes with the environment in order to prevent loss of directionality and functionality. A new concept to prepare such cluster is "anisotropic functionalization" of clusters, which will take advantage of structurally or chemically non-equivalent coordination sites at the cluster surface for site- selective functionalization.

Molecular clusters are compounds containing more than two metal atoms in a three dimensional arrangement. They have typical diameters of about 1 nanometre, and their surface is saturated by organic groups (ligands). Clusters with a core of metal and oxygen atoms (oxo clusters) and carboxylate or phosphonate ligands were exemplarily investigated in this project. In order to use such clusters as nanoscale building blocks for materials syntheses, fundamental questions concerning stability or dynamics of the clusters must be answered and subsequently controlled, such as exchange reactions with the surrounding medium, stability of the cluster core upon reactions in the ligand shell, mobility of the ligands or controlled arrangement of different ligands on the surface of the clusters. Mixed-metal clusters and/or clusters containing both functional and non-functional ligands are especially interesting both for fundamental investigations and later applications. Two examples were chosen to demonstrate that the cluster core is retained when the carboxylate ligands are exchanged. To this end, new investigation methods were developed. In the course of this work a new synthesis method for mixed-ligand clusters was found. A plethora of new oxo clusters, mainly of titanium, was synthesized and structurally characterized. It was shown, among others, that clusters with phosphonate ligands are less dynamic than those with carboxylate ligands. Their structures can be traced back to a few basic motifs. The especially interesting class of mixed-metal clusters was considerably extended by simple, but efficient modifications of the synthesis method. Several clusters of this kind fulfil the requirements on nanosized building blocks for future materials syntheses. The great number of structurally characterized clusters allowed highlighting interesting construction principles for such complex chemical compounds.