Hemilabile ligands versus rigid spacers: Synthesis, coordination chemistry, and catalytic applications

Already in 1912 the famous expert in chemistry Emil Fischer said with respect to catalysis: "Catalysts help to make miraculous conversions with hydrogen, oxygen, nitrogen, or carbon monoxide possible at temperatures several hundred degrees lower than those conditions in which these gases reacted earlier. This chapter of catalysis is nearly unlimited" Especially, in the present situation of a diminishing supply of mineral oil and / or the necessity to reduce the release of carbon dioxide responsible for the green house effect, the idea to use less energy for the production of important molecular building blocks is of growing significance. In this connection a new branch of catalysis has developed using light and hence solar energy for catalytic reactions and thus being ecologically harmless. Nowadays with the help of catalysts essential building blocks for pharmaceuticals, agrochemicals, light- sensitive, current-conducting, and other synthetic materials, and fragrances are produced. Our working group is concerned with the chemistry of a special kind of catalysts, all containing the element phosphorus. Compounds containing phosphorus have been proven to be very useful to increase the efficiency and stability of catalysts. In this connection so-called phosphines form "spacers", which remain stable during catalysis and prevent unwelcome side reactions. We succeeded in preparing sophisticated "spacers" by simple and hence cheap methods, where otherwise tedious procedures have been necessary. With the help of metals as building blocks, which exert the so- called "template effect" the foreseen reaction is controlled and afterwards the metals can be separated again and recycled. Interestingly, only a balanced combination of rigidity and flexibility of these "spacers" containing phosphines leads to optimal results for their catalytic applications. Thus, we have been able to obtain several catalysts, which are comparable to the industrial catalysts with respect to their productivity and therefore the world company Shell financed a protecting patent. These compounds catalyse the copolymerization of carbon monoxide and ethene producing polyketones which are important innovative materials. However, unpublished results show, that both the quality of these catalysts can be enhanced and their application to other important fields of catalysis is also possible. The goal of this project is to optimize the efficiency of these catalysts containing phosphines and therefore to reduce the energy consumption during catalysis. Since many of theses catalysts contain expensive metals like platinum or palladium, it is also planned to replace these metals by the cheaper nickel.

Several new ligands either recently designed in the wider context of our international cooperations or derived from natural products have been under close investigations. With respect to their catalytic applications the bis(bidentate) behavior of some of these ligands leading to rigid backbones and very robust complexes has already proved to show dramatic and beneficial effects. However, since an excess of ligands produces a favourable entropy-effect, they can be turned into bidentate ligands. This opens the way for heterobimetallic compounds and tandem catalysts. Furthermore, the unprecedented steric control of excited-state lifetimes due to the difference between bidentate and bis(bidentate) behavior has already led to an enormous enlargement of luminescent lifetimes at ambient temperature. A multidentate, but hemilabile ligand has been shown to lead to self-assembly. This Weak-Link Approach to supramolecular coordination complexes opens a new synthetic route to functional materials capable of binding reversible selected guest molecules. This nanoporous material is important in the wider context of nanotechnology. Most of these ligands have been prepared in our laboratory for the first time. They have been designed to fulfil the criteria of two main topics: 1) Homo- and heterobinuclear complexes containing Ru(II), Os(II), and various additional metals attract considerable attention as model catalytic systems for biomimetic photochemical conversion of solar energy. Our challenge is to study bidentate versus bis(bidentate) behavior, saturated versus conjugated backbones, and steric effects. 2) Only recently, it has been recognized that homogeneous catalysis is part of the most efficient technological principles, which are able to match economical with ecological advantages. Our challenge is to find structurally precisely defined catalysts for oligomerization and polymerization reactions, oxidative carbonylations, and hydrogenations.