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Mössbauer & MCD

Molecular Paramagnetism and Bioinorganic Spectroscopy

Paramagnetic transition metal ions in molecules continue to interest chemists and physicists. They occur in metalloproteins, and the functional sites of industrial catalysts used for amazing chemical reactions, such as the activation of inert small molecules. The wide gamut of optical and magnetic properties of molecular transition metal centers and clusters stimulates extended research in material sciences and development of molecular devises. The decisive factor in controlling all of the fascinating properties of transition metal centers is the electronic structure, more than merely the spatial arrangement and the molecular structure. Even geometrically very similar systems may display drastically different properties, and undergo very different reactions.

Therefore, in order to make rational use of transition metal ions in future chemistry, one needs to understand the origin of the observed features and functions in terms of the electronic structure. This can be attained with notable success by a combination of spectroscopy and quantum chemistry.

A major technique for the investigation of iron compounds is 57Fe-Mössbauer spectroscopy. Measurements of the electric hyperfine parameters isomer shift δ and quadrupole splitting ΔEQ with or without using applied magnetic fields provide most valuable insight in the oxidation state and the coordination of the specific metal ion. Paramagnetic properties, which in Mössbauer spectroscopy can be probed with elaborate applied-field measurements, are particularly interesting, because the magnetism originates directly from the spin of valence electrons. Therefore magnetic techniques in general are particularly powerful tools of inorganic spectroscopy.

In our lab we combine 57Fe-Mössbauer spectroscopy, multi-frequency electron-paramagnetic-resonance spectroscopy (EPR), static magnetic susceptibility measurements, and magnetic circular-dichroism spectroscopy (MCD) for the study of bioinorganic compounds and metalloproteins. Low temperature measurements and magnetic fields (flux up to 7 and 10 Tesla) are regularly applied for the investigation of complex systems.

The spectra obtained from the different techniques can be consistently parameterized within a common theoretical frame, which is provided by spin-Hamiltonian simulations. This phenomenological concept, which is an application of ligand-field theory, yields descriptions of the electronic structure of paramagnetic ions in terms of zero-field splitting (D and E/D values), Zeeman interaction (g values), and spin coupling (J values). Moreover, the Spin Hamiltonian parameters D, E/D, g, J can be derived theoretically from sophisticated quantum chemical electronic structure calculations, and thus provide an eminently useful and convenient link between spectroscopy and quantum chemistry, and their common interface. Chemical information is obtained from the combination of both techniques: about the oxidation states of metals and ligands, the nature of valence orbitals, coordination symmetry and strength, and putative interaction between metal sites. In the following some recent research projects shall be presented.

Emek Öner

Emek Öner

Research Topics

Ordinary Iron Compounds Can Have Very Complex Electronic Structures

Ordinary Iron Compounds Can Have Very Complex Electronic Structures

(A project together with Marat Khusniyarov, Thomas Weyhermüller and Karl Wieghardt, published in Angew. Chem. Int. Ed. 47, 1228 (2008). Dr. Khusniyarov is presently a "Habilitant" at the Department of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-Nürnberg).

For many enzymes and technical processes the functionality of the catalytic site is owed to the accessibility of variable oxidation states of transition metal ions, i.e. their ability to accept or donate electrons from or to the reaction partners when needed. Iron, in this respect, is a very versatile element because it exhibits rich redox chemistry with a large range of oxidation states easily available for storing oxidation equivalents. Most abundant in the environment, and best investigated in chemistry, are iron compounds with the oxidation numbers (II) and (III) of the metal. Low-valent iron(I) and iron(0) are known to occur in key enzymes such as hydrogenases, whereas high-valent iron(IV) and iron(V) centers play a decisive role in reaction intermediates of oxygen-activating enzymes. Hexavalent iron occurs also, but it was known only in ferrates; recently in this institute for the first time an iron(VI) molecule was reported. In material sciences, iron with high valence states holds some promises for the development of new batteries for electric energy storage.

Oxidative reactions in biochemistry are often catalyzed by metalloenzymes, for which not only the transition metal ion takes part in redox reactions, but also the organic environment or even the ligands to the metal. The archetypical examples are the heme proteins horseradish peroxidase and the cytochrome P-450 family of enzymes from liver. Their iron porphyrin complexes are oxidized during the catalytic cycle up to the formal oxidation number (V). However, it is long known that this highly reactive intermediate in fact is an iron(IV) and a porphyrin pi-cation radical complex, as shown for horse radish peroxidase 1969 by Mössbauer spectroscopy. Since then a large number of metalloenzymes has been found that contain redox-active ligands taking part in the catalytic process by donating or accepting electrons under formation of organic radicals. Among them are so different examples as ribonucleotide reductase, photosystem II of green plants, galactose oxidase, prostaglandin H synthase and amine oxidase.

Fascinated by the variety of such systems, a major part of our research activity was dedicated to transition metal complexes with small synthetic ligands, which were known or could be suspected to be redox-active. Recently, M. Khusniyarov synthesized two unusual iron compounds with the α-diimine ligands 'dad' (= 2,6-iPr2-C6H3-N=C(Me)-C(Me)=N-2,6-iPr2-C6H3) and 'pda' (=tetrakis-(3-methylphenyl)-N,N,N′,N′-2,5-phenylene -diamine), which turned out to be paramount examples of complexes with simple basic structure but rich and complex electronic properties. The two compounds crystallized together as cation-anion pairs with molecular structures as shown in Fig. 1. The cation (C) is the six-coordinate complex [(dad)3Fe]n+, whereas the anion (A) is the four-coordinate pda complex [(pda)2Fe]n-. Other ions are not present in the crystal structure. Compound C is quasi octahedral, whereas the structure of A is intermediate between tetrahedral and planar.

Apparently the charges of the two ionic molecules cannot be directly inferred from the structures of A and C, because both iron may be ferric or ferrous, and each of the five ligands may take one of three oxidation states shown.

Therefore, even if the molecular charges were known, the electronic structure of the compounds could not be inferred, because the ligands and the metal ions exhibit that variety of possible oxidation states. The electronic structures can be deciphered only from a careful evaluation of magnetic susceptibility and Mössbauer data, probing the electronic structure of iron, in conjunction with inspection of the metric details of the ligand structure. In this latter program, the values of certain atomic distances may reveal the oxidation state of the ligand, because they reflect the characteristic distribution of single and double bonds expected from the Lewis structure for the different oxidation states, as sketched in Fig. 2.

 

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