Tuning Reduced States of Group 4 and Rare Earth Metals
Organometallic compounds with cyclopentadienyl ligands (Cp), called metallocenes, are an important substance of coordination chemistry. Numerous examples of complexes with cyclopentadienyl groups with various substituent patterns exist. Early on it was recognized that alkylated variations of the Cp-ligand such as the pentamethylcyclopentadienyl group (Cp*) are more electron donating and also cause a stronger ligand field influence.
The current proposal is especially concerned with investigations of the influence of silylated Cp ligands and the structurally related silole, germole, and stannole units on the orbital energies of lanthanides and group 4 metallocenes.
Over the last years it was recognized that these two substance classes in reduced oxidation states are showing most interesting electron transfer reactivity. For instance reduction of dinitrogen to N22-, of CO2 to oxalate, and of CO to diynolates were reported. Considering the facts how much energy is going into the Haber-Bosch process, every reaction that deals with nitrogen activation deserves a closer look.
Recently, Evans and co-workers have shown that it is possible to prepare stable cyclopentadienyl complexes of all lanthanide complexes in the oxidation state +2. Given the redox properties of naked lanthanide ions this should be impossible at least for Er, Ce, Tb, and Gd. However, Evans’ work suggests that the extra electron of these unlikely Ln(II) compounds is not located in an f-orbital but rather in a d-orbital. The energetic accessibility of this d-orbital is caused by the ligand field of a silylated cyclopentadienyl ligand, which decreases the energy of the 5dz2-orbital, so that it is able to compete with the 4f orbitals.
The concept of manipulating d-orbital energies of lanthanides and group 4 metallocenes by use of suitable ligands is at the core of the present research proposal. By deliberate stabilization of destabilization of these orbital energies it will be possible to control the reactivity of the associated compounds. This way we will be able to tune not only the orbital energies but also the stability and the ease or electron transfer.
The scientific questions addressed in the proposal require a number of methods and techniques to obtain the desired information. A number of distinguished collaborators will be involved to provide practical and theoretical assistance. Spectro-electrochemical studies will be done in close collaboration with Prof. Slava Jouikov at the University of Rennes. Prof. Heinz Krenn (University Graz) will help us to study the magnetism using SQUID measurements. And finally Prof. Thomas Müller at the University of Oldenburg and Dr. Tibor Szilvási at the University of Wisconsin will provide theoretical support to study group 4 metallocene and lanthanide chemistry.