Dr. Eckhard Bill (*1953)
MPI für Bioanorganische Chemie
Stiftstrasse 34 - 36 / D - 45470 Mülheim an der Ruhr
PO Box 10 13 65 / D - 45413 Mülheim an der Ruhr
Reception +49 (0)208 306 - 4
Tel.: +49 (0)208 306 - 3724
Fax: +49 (0)208 306 - 3951
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.
‘Ordinary’ Iron Compounds Can Have Very Complex Electronic Structures
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.
Figure 1. Molecular structure of the dianion A = [(pda)2Fe]2-and the dication C = [(dad)3Fe]2+ recorded at 120 K. The ellipsoids mark the 30% probability level of the distribution of atomic positions due to thermal vibrations.
Figure 2. Schematic structures of the alpha-diimine ligands ‘dad’ and ‘pda’ and metric details of the diimine backbone in neutral, monoanion and dianionform (top row). The distribution of ‘N-C-C-N’ bond lengths (numbers in red) reflects surprisingly well the pattern of (long) single bonds and (short) double bonds predicted from the corresponding Lewis formulae for the three forms.
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.
How to read the structures of A and C
Molecule A: The two ‘pda’ ligands are very similar in details, exhibiting the typical bond length pattern expected for the fully reduced state (pda2-), having a short C=C bond and two long N-C bonds in the ‘N-C=C-N’ back-bone. Thus, the two ligands provide formally four negative charges and should not contribute to the magnetism of A because of closed-shell character.
Moreover, the Fe-N distances of A are remarkably long, compared to those of related compounds, indicating the presence of iron(II) in high-spin configuration with four unpaired electrons and a local spin of SFe= 2. Thus, the structural details predict A to be in fact a dianion of the type [(pda2-)2Fe(II)]2-. The compound should have a quintet ground state with total spin SA= 2, which arises purely from iron.
Molecule C: Since the anion-cation pair must be neutral, C can only be a dication. The ‘dad’ ligands of the octahedral compound show all the same long C-C bond and two short N=C bonds, that are typical of neutral diimine (dad0). Therefore also this iron must have the oxidation number (II), and the corresponding structure for C should be [(dad0)3Fe(II)]2+. The Fe-N bonds, however, are short in this case, which is compatible only with a low-spin configuration, SFe= 0. Hence, compound C should be diamagnetic with SC= 0.
Magnetic properties corroborate the conclusions drawn from structure
Apparently the interpretation of the structural details can be examined by magnetic measurements, due to the exact predictions of the spin states of the compounds. Solid samples of (A+C) were found in fact to exhibit an effective magnetic moment of μeff≈4.9 μB in the temperature range 30 – 235 K, which fits nicely to the spin-onlyvalue for S= 2.
Thus, the actual spin of the system corresponds to the prediction SA= 2, SC= 0 (Fig. 3).
The decline of µeff below 30 K reveals zero-field splitting, as expected for the 3d6 configuration of high-spin iron(II). This results from spin-orbit coupling and depends on the strength of the ligand-field experienced by the metal ion. The temperature range affected by zero-field splitting (ca. 0 – 30 K) provides an estimate for the size of the interaction. A simulation yields an exact value, D = 4.5 cm-1, which is again typical for iron(II). The value is a measure for the bonding properties of iron and the (pda2-) ligands, and can be used for further interpretation based on quantum chemical calculations.
Basically, the magnetic susceptibility measurements corroborate the above conclusions, except that there is a minor kink and a little jump in μeff(T) at about 235 K. One certainly is attempted to ignore and assign this little glitch to an experimental error. In reality, however, this is a minor response of the magnetic data to a major rearrangement of the whole electronic structure of the A+C ion pair, as could be discovered by 57Fe-Mössbauer spectroscopy.
Mössbauer spectra reveal the full truth of A+C
A zero-field Mössbauer spectrum of A+C recorded at 80K yields a superposition of two quadrupole doublets of equal intensities, as shown in Fig. 4 bottom, corresponding to the different iron sites of the two compounds (a weak third subspectrum represents a minor contaminant from deterioration of the very sensitive sample).
A fit with Lorentzian lines yields the isomer shifts δ = 0.68and 0.38 mm/s and the quadrupole splitting ΔEQ= 5.20 and 0.53 mm/s, for the contributions of A and C, respectively. The large quadrupole splitting and high isomer shift for the dianion A (marked in blue), which are typical for iron(II) high-spin, clearly corroborate the 3d6 configuration of the four-coordinate iron and the spin state SFe= 2. The low isomer shift and small quadrupole splitting of C on the other hand, also support clearly the assignment of iron(II) low-spin with SFe= 0. This was also quantitatively established from quantum chemical calculations by usingt he spectroscopy-based DFT package ORCA by F. Neese.
Figure 4. Zero-field Mössbauer spectra recorded with a powder sample of A+C at 80K (bottom) and 260 K (top). The subspectra marked in green are a small contaminant from some decay of the sensitive sample.
Apparently, the 80 K Mössbauer spectra readily support the model suggested above for the electronic structures prevailing for A and C at low temperatures.
However, the spectra change drastically when measured at 260 K, which is just above the little step in μeff(T) at 235 K (Fig. 4, top). Both iron sites, apparently, experience a major rearrangement of their electronic structures, as seen from the changes in δ and ΔEQ, compared to those obtained at 80 K (with δ= 0.76 mm/s, ΔEQ= 1.80 mm/sfor C’; δ= 0.16 mm/s, ΔEQ= 4.19 mm/s for A’). The new state C’ of the cation shows a significantly increased isomer shift (component in red), whereas the value for the new anion A’ is clearly lower than that for C’ (component in blue). The quadrupole splittings also show reversed trends for both iron sites. From a detailed analysis, one can infer that the high-spin iron(II) of A has been oxidized and is now iron(III) in A’, having the unusual intermediate spin state, SFe= 3/2, whereas iron in the cation C’ has kept its oxidation state (II), but flipped its spin state from low-spin to high-spin, SFe= 2.
This magnetic transition observed for A+C at 235 K apparently involves an electron transfer from A to C. The apparent question is, where the transferred electron resides on C’, or whether one can prove the corresponding reduction of C’, which is obviously not metal-centered. This was possible from X-ray diffraction data measured at room temperature. One of the ligands in fact showed the metric details predicted for the (dad1-) state. It is apparently singly charged and therefore has changed to an organic radical. Since this must carry a ligand spin, Srad= 1/2, the change is reflected in the magnetic data.
The paramagnetic moment of both molecules A’ and C’ prevailing above the transition temperature results from intermediate spin iron(III) in A’, SA’= SFe(A’)= 3/2, and the contribution from high-spin iron(II) in C’ with SFe (C’) = 2,as well as that of the radical spin Srad(C’)= 1/2. Since the radical is coordinated to iron(II), both are very strongly antiferromagnetically coupled to a quartet state, SC’= 3/2,due to the vector addition of angular moment, SC’= SFe– Srad= 2 – 1/2. The sum of the resulting effective magnetic moments for both molecules, having SA’= 3/2 andSC’= 3/2 yields , which is amazingly close to the experimental result measured in the temperature range 235 – 300 K. The general features of the electronic structure and magnetic properties of the ion pair A and C below 235 K and in the new states A’+C’ above 235 K are summarized in Fig. 5.
Figure 5. Schematic presentation of the electronic structure and spin of the ion pair complexes A+C.
Attempt of an interpretation
In summary, structural and spectroscopic data consistently reveal a very unusual and interesting transition of the electronic structure of the A+C ion pair at 235 K, which involves an inter-molecular electron transfer as well as changes of the local spin states of iron. The question arises what may be driving the processes. Spin transitions are well known for iron and have been studied in depth, also because of the potential applications in molecular devices and switches. In this case here, however, the dianion A switches from high-spin at low temperature to low-spin at high temperature, which has never been observed before. Usually the transition is opposite, as shown here by the cation C. It is generally accepted that the increase of enthalpy related to the formation of the high-spin state (concerning the number of possible spin vibronic states) is the major driving force for ‘regular’ spin transitions as shown by C. At first glance, the behavior of A seems to violate this rule.
On the other hand, electron transfer between molecules is the basis of redox chemistry, and particularly in biochemistry transfer over remarkable distances is also known. The direction of such electron hopping is always determined by the potential of the associated sites and always down-hill. The transfer observed here, however, is temperature-dependent and reversible. The observation of a hysteresis of the magnetic data at the transition indicates a ‘collective’ phenomenon. For such transitions, a ‘signal’ must exist (like a change in volume, etc.) in order to synchronize the process within large domains of particles in the solid. Since something like this is not known at all for usual redox transfer processes, but as many spin transitions are collective, we may speculate that the reversible transition A+C ↔ A’+C’ is triggered by the ‘regular’ spin transition observed for C ↔ C’, for which iron(II) switches between SFe= 0 at low temperature and SFe= 2 above 235 K. An electron transfer may follow this, swapping a charge from iron(II) in A to the ligand of C, caused by the structural changes of the primary spin transition. The ‘inverse’ spin flip of iron(III) in A from SFe= 1/2 to SFe= 3/2 then would be the consequence of the charge transfer.
In summary, we have presented an example of a pair of two charged complexes that crystallize together and exhibit amazing and challenging electronic structures, which could be deciphered from high-resolution structural data, combined with magnetic susceptibitlity and Mössbauer measurements. For the first time a complex transition in the solid could be detected and described which involves coupled electron transfer and two spin transitions.
|Extract from the latest Research Report|
Marlin, D. S., Bill, E., Weyhermüller, T., Bothe, E. and Wieghardt, K.: Magnetic interactions in dinuclear Mn(III)Mn(IV) complexes covalently tethered to organic radicals: Spectroscopic models for the S2YZ* state of photosystem II. J. Am. Chem. Soc. 127, 6095-6108 (2005)
Ghosh, P., Bill, E., Weyhermüller, T. and Wieghardt, K.: Molecular and electronic structures of iron complexes containing N,S-coordinated, open-shell o- iminothionebenzosemiquinonate(1-) p-radicals. J. Am. Chem. Soc. 125, 3967-3979 (2003)
Hans, M., Bill, E., Cirpus, I., Pierik, A. J., Hetzel, M., Alber, D. and Buckel, W.: Adenosine triphosphate-induced electron transfer in 2-hydroxyglutaryl-CoA dehydratase from Acidaminococcus fermentans. Biochemistry 41, 5873-5882 (2002)
Burdinski, D., Bill, E., Birkelbach, F., Wieghardt, K. and Chaudhuri, P.: Long-range exchange interactions and integer-spin St=2 EPR spectra of a Cr(III)Zn(II)Cr(III) species with multiplet mixing. Inorg. Chem. 40, 1160-1166 (2001)
Grapperhaus, C. A., Mienert, B., Bill, E., Weyhermüller, T. and Wieghardt, K.: Mononuclear (nitrido)iron(V) and (oxo)iron(IV) complexes via photolysis of (cyclam-acetato)Fe(III)(N3)(+) and ozonolysis of (cyclam-acetato)Fe(III)(O3SCF3)(+) in water/acetone mixtures. Inorg. Chem. 39, 5306-5317 (2000)
Meyer, K., Bill, E., Mienert, B., Weyhermüller, T. and Wieghardt, K.: Photolysis of cis- and trans-[Fe(III)(cyclam)(N3)2](+) complexes: Spectroscopic characterization of a nitridoiron(V) species.
J. Am. Chem. Soc. 121, 4859-4876 (1999)
Glaser, T., Beissel, T., Bill, E., Weyhermüller, T., Schünemann, V., Meyer-Klaucke, W., Trautwein, A. X. and Wieghardt, K.: Electronic structure of linear thiophenolate-bridged heterotrinuclear complexes LFeMFeL(n+) (M = Cr, Co, Fe; n = 1-3): Localized vs delocalized models.
J. Am. Chem. Soc. 121, 2193-2208 (1999)
Müller, J., Weyhermüller, T., Bill, E., Hildebrandt, P., Ould-Moussa, L., Glaser, T. and Wieghardt, K.: Why does the active form of galactose oxidase possess a diamagnetic ground state? Angew. Chem.-Int. Edit. 37, 616-619 (1998)
Müh, U., Buckel, W. and Bill, E.: Mössbauer study of 4-hydroxybutyryl-CoA dehydratase - Probing the role of an iron-sulfur cluster in an overall non-redox reaction. Eur. J. Biochem. 248, 380-384 (1997)
Bossek, U., Hummel, H., Weyhermüller, T., Bill, E. and Wieghardt, K.: The first
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|See PDF of the latest research report|
57Fe Mössbauer spectroscopy:
Three spectrometers with i.) protection gas sample cabinet for ambient temperature to 500K; ii.) liquid nitrogen flow cryostat with double window set for γ-rays and optical spectroscopy (Oxford Instruments OPTISTAT DN); iii.) liquid helium flow cryostat (Oxford Instruments VARIOX) 1.5K - 300K; iv.) cryomagnet system (Oxford Instr., SPECTROMAG) 8T, sample temperature 1.5K - 300K
Magnetic susceptibility measurements:
SQUID susceptometer (Quantum Design) 7T, 2K - 310K with acquisition software MultiVu.
EPR spectroscopy: CW-X-band Instrument (Bruker, ELEXSYS E-500) with helium flow cryostat (Oxford Instr., ESR910) 2.3K - 300K, dual-mode cavity, dispersion-mode microwave bridge, ENDOR unit; cw-instrument (Bruker, ESP300) for Sand Q-band measurements with helium flow cryostat (Oxford Instr.), S-band loop-gap resonators in the range 2-4 GHz, wide-bore resonators for Mössbauer samples.
MCD Spectroscopy: spectro-polarimeter for measurements in the range 200 - 1100 nm (JASCO J-715), cryomagnet system (Oxford Instr., SPECTROMAG) 11T, 1.5 - 300 K, with locally developed acquisition software for fully automatic operation of temperature controller, field power supply and CD spectrometer.
Software julX: this is an in-house developed simulation program for static molar magnetic susceptibilities, written in Fortran for Linux, Mac OS-X and Windows. The program can be used for:
1. simulation of magnetic susceptibility data from SQUID measurements.
2. scanning of a 2D error grid (64x64 steps) for error contour plots with two independent parameters, or a 3D view of the corresponding error surface.
3. calculation of energy level schemes for level plots as function of the applied field in any direction, or plots of the Boltzmann populations as function of temperature. The simulations are based on the usual spin-Hamilton operator for up to four coupled spins with multiplicities up to Si = 5/2. The parameters are the usual spin-Hamiltonian parameters, which are the average electronic g values, the axial zerofield splitting and rhombicity parameters D and E/D, and the exchange coupling constants Jij., where i and j number the paramagnetic sites.
Diagonalization of the respective Hamiltonian operator is performed with the routine ZHEEV from the LAPACK Library and the magnetic moments are obtained from first order numerical derivative dE/dB of the eigen values. Powder summations are done by using a 16-point Lebedev grid. Intermolecular interactions are considered by using a Weiss temperature, ΘW, as perturbation of the temperature scale, kT
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Herr Göbels, Andreas
Herr Mienert, Bernd
Herr Reikowski, Frank
Frau Stapper, Marion
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