Subsequent projects

Prof. Dr. Karsten Meyer 

Lehrstuhl für Anorganische Chemie

Prof. Dr. Paula Diaconescu

University of Southern California, Los Angeles
Department of Chemistry and Biochemistry

Probing Metal-Metal Weak Orbital Interactions

We originally proposed the study of f-orbital interactions in uranium(IV) complexes and are still working on this idea. Cerium is the only lanthanide that has an accessible +4 oxidation state. In order to understand how important is the ability of uranium f orbitals to engage in covalent bonding, a project was started to investigate the chemistry of cerium complexes supported by ferrocene-based chelating ligands. Yttrium complexes were also studied to differentiate between redox-active and redox-inactive metal centers. All these metal complexes were applied to redox-switchable catalysis. The collaboration between the Erlangen and UCLA groups was vital in understanding the electronic structure of the metal complexes used as pre-catalysts for the polymerization reactions. We plan to continue to study weak metal-metal interactions and their importance in catalysis. Such interactions are important not only in influencing reactivity behavior but also in understanding fundamental concepts such as the Lewis basicity of transition metals.

Primary project: Probing f-Orbital Interactions and Magnetic Exchange Phenomena in Uranium Chemistry

Final Report

The continued BaCaTeC grant allowed the two groups, Meyer and Diaconescu, to pursue the study of metal complexes supported by ferrocene ligands. Such compounds have proved fundamental in isolating interesting bonding motifs, uncovering new reaction mechanisms, and redox-switching between two catalyst states. In all these cases, the special ligand architecture is important because a weak interaction may be established between iron and the metal of interest.

Metal-metal bonding has been the subject of intense research and discussion, and interactions between metal centers in bimetallic complexes can give rise to interesting electronic and magnetic properties and novel reactivity. Additionally, weak metal-metal bonding is important in developing and understanding the concept of transition metal Lewis basicity, which, in turn, is of fundamental interest in understanding the reactivity of corresponding complexes. Prior to our work, only X-ray crystallography, Mössbauer, X-ray absorption spectroscopy, and computational methods were employed to determine the nature of weak metal-metal interactions. Our groups added the use of resonance Raman spectroscopy to this list (publication 1). A unique compound, [fc(NH2)2]Ru(PPh3)2 (fc = 1,1’- ferrocenylene), was identified whose solid-state molecular structure revealed a short Fe-Ru distance. Because a metal-to-metal charge transfer band was observed in the electronic absorption spectrum of [fc(NH)2]Ru(PPh3)2, the Fe-Ru interaction was characterized by resonance Raman spectroscopy for the first time and also by 1H NMR, UV-Vis, NIR, Mössbauer spectroscopy, and X-ray crystallography. DFT calculations including natural bond order analysis, Bader’s atom in molecules method, and TDDFT provided further support that the iron-ruthenium bond is a weak donor-acceptor interaction with iron acting as the Lewis base.

Furthermore, we pursued a systematic study of metal-metal interactions (publication 2) in mixed valent uranium complexes. The basic premise of the study consisted of four parts. We sought to (1) understand the interaction between iron and uranium centers, (2) determine whether the iron centers in ferrocene and ferrocenium diamide ligands communicated, (3) investigate how well uranium mediated the electron transfer between iron centers, and (4) examine whether iron-uranium interaction and/or iron-iron communication can be increased by tuning the electronic and steric nature of the amino substituents. Collective interpretation of electrochemical (voltammetry and chronoamperometry), spectroscopic (1H NMR, UV-Vis/NIR, IR, Mössbauer, and EPR), structural (single crystal X-ray diffraction) and magnetic (variable field and temperature magnetization and magnetic susceptibility) data unveiled several important findings: (1) a mild oxidant can oxidize one or two iron centers to generate mixed-valent (Fe2+–U4+–Fe3+) and doubly oxidized (Fe3+–U4+–Fe3+) species, respectively, (2) bulky amido substituents increase electronic communication between iron centers by imposing greater tilt and torsion angles on the ferrocene and ferrocenium moieties, (3) deviation from linearity of the angle formed by all three metal centers (i.e. ∡Fe-U-Fe) lowers the extent of electronic communication in mixed-valence species, which suggests that uranium is directly involved in the electron transfer between ferrocene and ferrocenium ligands.

Last but not least, it is important to mention that the results obtained within this exceedingly fruitful BaCaTeC-funded collaboration resulted in a successful application for Friedrich Wilhelm Bessel Research Award (granted to PLD) from the Alexander-von-Humboldt foundation.

Publications supported by the BaCaTeC grant:

1. Characterization of an Iron-Ruthenium Interaction in a Ferrocene Diamide Complex. Green, Aaron G.; Kiesz, Matthew D.; Oria, Jeremy V.; Elliott, Andrew G.; Buechler, Andrew K.; Hohenberger, Johannes; Meyer, Karsten; Zink, Jeffrey I.; Diaconescu, Paula L. Inorg. Chem. 2013, 52(9), 5603–5610.

2. Electronic Communication in Bis(1,1’-Diamidoferrocene) Uranium Complexes. Duhović, Selma; Huang, Wenliang; Storms La Pierre, Henry; Hohenberger, Johannes; Meyer, Karsten; Diaconescu, Paula L. manuscript in preparation.


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