Organometallic, Physical Organic, and Analytical Chemistry
Motivation and Overview
Our research aims at the detailed understanding of chemical processes at the molecular level. We believe that the thus gained mechanistic insight is not only of interest in its own right, but in the long term will provide the best way for reagent and catalyst optimization. Here, our main focus lies on organometallics due to their chemical diversity and complexity as well as their great practical value. For identifying elusive reactive intermediates and probing their reactivity, we employ a variety of different methods, among which electrospray-ionization (ESI) tandem mass spectrometry is of particular importance.
Accordingly, our research activities cluster around three different main topics:
We use ESI-mass spectrometry, NMR spectroscopy, and various other techniques to identify and characterize reactive intermediates. First, we probe the species formed by synthetically important organometallics reagents, such as Grignard reagents, organozinc reagents, and organocuprates in solution. In particular, we seek to find out how simple additives, such as LiCl, affect the association and aggregation behavior of the organometallic species. Second, we aim at identifying the catalytically active intermediates of cross-coupling reactions, trifluoromethylations, and coordinative anionic polymerizations. Here, we foucs on complexes of the late transition metals Fe, Co, Ni, Pd, Cu, and Ag. Finally, we are also interested in stabilized carbanions, which are intermediates in a wide range of nucleophilic additions and anionic polymerizations. Besides investigating the speciation of these different systems, we also attempt to determine their reactivity by kinetic measurements.
For a true comprehension of organometallic reactivity, the constituent elementary reactions must be thoroughly understood. We aim at contributing to this understanding chiefly by probing the microscopic reactivity of organometallic ions isolated in the gas phase. Here, mass selection achieves ultimate control over the homogeneity of the reactant ions while the electrostatic repulsion of the ions of like charge prevents any bimolecular exchange processes, which typically operate in solution. We then probe the uni- and/or bimolecular reactivity of the ions by subjecting them to collisions with an inert gas or with substrate molecules, respectively. Apart from establishing the outcome of the individual elementary reactions, we seek to determine the effect of different organyl groups, ligands, or solvent molecules bound to the metal center as well as different coordination numbers and different oxidation states of the latter. Elementary reactions of interest include oxidative additions/halogen-transfer reactions, transmetalations, reductive eliminations, β-hydrogen eliminations, and protodemetalations. For obtaining additional insight into the relevant potential energy surfaces, we also make use of quantum chemical calculations.
Given our active use of tandem mass-spectrometric methods, we have a keen interest in better understanding and improving these methods. First, we focus on the ionization process. Thus, we develop routines permitting the ESI-mass spectrometric analysis of highly oxygen- and moisture-sensitive species in intact form. Furthermore, we try to identify the factors controlling the ESI respone of different classes of analytes. We also seek to determine the internal energy imparted to the analyte ions during the ionization process and their consecutive transfer through the mass spectrometer. To this end, we employ so-called thermometer ions, which dissociate once their internal energy exceeds a known threshold. Second, we focus on gas-phase reactivity studies. We have customized a quadrupole-ion trap to allow for the controlled infusion of volatile substrates and the analysis of ion-molecule reactions. We are now in the process of implementing a thermostat to examine the temperature dependence of the reactions. In addition, we seek to simulate the experimental results by statistical-rate theory calculations based on quantum chemical predictions. In this way, the latter can be directly compared to the former and, thus, assessed in their accuracy.