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MAGNETIC PROPERTIES OF [Fe4S4(SR)4]3- CLUSTERS
G. Papaefthymiou, R. Frankel, S. Foner, E. Laskowski, R. Holm
To cite this version:
G. Papaefthymiou, R. Frankel, S. Foner, E. Laskowski, R. Holm. MAGNETIC PROPERTIES OF [Fe4S4(SR)4]3- CLUSTERS. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-493-C1-494.
�10.1051/jphyscol:19801195�. �jpa-00219685�
JOURNAL DE PHYSIQUE CoNoque C1, suppl&ment au n 1 , Tome 41, janvier 1980, page Cl-493
EVIGNETIC PROPERTIES OF
ke4s4
(SR)J 3- CLUSTERS.++I
+
G.C. Papaefthymiou, R.B. Frankel, S. Foner, E.J. Laskowski and R.H. Holm
Francis B i t t e r National Magnet ~ a b o r a t o r ~ f MIT, Cambridge, MA 02139.
+
Dept. of Chemistry, Stanford University, Stanford, CA 94305.Synthetic analogues of the active sites of bac- terial ferredoxins [l] under suitable conditions undergo the oxidation-reduction process
which serves a s a representation of the protein site electronic and structural changes in a Fd /Fd electron transfer couple in the absence of%xtr&"sd,c constraints such a s might be imposed by protein structural features [2,3]. bfrksbauer, epr, magnetiza- tion and x-ray crystallographic measurements on analogues in both oxidation states have been used to characterize the properties of the Fe4S4 core [l -51.
In the present paper we compare the results previ- ously obtained in the two oxidation states and present additional experimental data on the trianion analogue in order to gain insight into the relative strengths of exchange interactions between the iron ions and the sensitivity of the Fe4S4 core structure to terminal li- gand variations and crystal packing effects.
In the dianion with formal electronic configu- ration 2FeZf
+
2 ~ e 3 4 , various experimental measure- ments [l] show that the electrons a r e highly delocal- ized resulting in four equivalent iron ions of inter- mediate valence ~ e ~ " ~ + , The iron spins a r e strongly exchange coupled producing a singlet ground state.Furthermore, the electronic properties of the FeqSq core a r e relatively insensitive to terminal ligand sub- stitution and crystal packing effects.
In the trianion with formal electronic configu- ration 3Fe2+f 1Fe3', observed isomer shifts and quadrupole splittings indicate a high degree of electron delocalization [Z, 3,5]. However, in contrast to the dianion, the trianion i s sensitive to changes in the nature of both the terminal ligand and quaternary ammonium counterion when studied in crystalline form, Mb'ssbauer and magnetization studies show that the trianions with different R groups assume one of two possible structures (I, 11) resulting in different ground state configurations [2,3]. Furthermore, the iron ions a r e exchange coupled resulting in a ground state of S = 1/2 for structure (I) (R = phenyl, 2-tolyl) and
*Supported by National Science Foundation.
*
*Present address, Bell Laboratories, Murray Hill, New Jersey 07974.approaching S = 3/2 for structure (11) (R = benzyl,
m
-tolyl, p-tolyl, p-methoxybenzyl and p-isopropyl- phenyl) [2,5] a s indicated by magnetization measure- ments obtained at T = 1.45 and T = 4.2K. Repre-sentative data a r e shown in Fig. 1.
Fig. 1. Magnetization vs
.
HIT for (E t4N)3Fe S4(SR)4J crystals (a) R = benzyl, T = 4.2K, (b)dfi.
=benzyl, T = 1.4K, (c) R = phenyl, T = 4.2K, (d) R = phenyl, T = 1.4K.
The moment for the R = phenyl cluster approaches 1 p ~ , the expected value for spin 1/2, whereas the moment for R = benzyl approaches 3 @, the value 'for an S = 3/2 ground state. In both cases, the moment i s still increasing at the highest applied field, re:
flecting low lying excited states. Moreover, the mo- ment in both cases is not a simple function of H/T but increases with increasing temperature (see Fig. 1)
reflecting higher spin multiplicity of the excited states.
A measure of the strength of the exchange coupling in the two oxidation states can be observed in the spectrum of low energy electronic levels. Cal- culation of the magnetic susceptibility vs. tempera- ture behavior, X (T), was made using an exchange interaction model of the form
4
in the limit of the applied field H approaching zero.
Here g = 1.97, the electronic spin g-factor observed in epr experiments, is the Bohr magneton, Si i s
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801195
Cl-494 JOURNAL DE PHYSIQUE
the iron spin and J couples iron ions i,j with ature T, T0 is the lowest temperature of observation spins Sj, Sj corresponding to the formal values and m is the ratio of the degeneracy of the first ex- 5/2 and 2. For [Fe4S4(SR)4]2- the simulated X(T) c i t e d s t at e to that of the ground s t a t e . Measurements behavior with all Jjj's equal gives a best fit to the o f a glassed acetonitrile solution of (Et4N)3[Fe4S4(SPh)4]
data for J = -232 c m " ! [2,7]. This places the first structure (I) were made with the spectrometer
excited (S = 1) state at -2J or Aj ~ 460 cm_ 1above the described in [2]„ Data were collected from 18 to 83K ground state. For [Fe4S4(SR>4]3" the ground state is at 3 mw power since in this region no power satura- at least a doublet. Calculations of the susceptibility tion effects a r e present [2]. Integration of the spectra vs. temperature behavior for structure (I), c o r r e s - wag d o n e ^ h a n d # A n l o t o f in [ a Cr0) T0 ~I<T)T)/(I<T)T)]
ponding to an S = 1/2 ground state using the above y s < 1 / T ^y e s & 22 cm"1 and m ~ 4 . 5 (Fig. 3 ) . exchange interaction model in which all Jjj's a r e
equal, does not yield satisfactory fits to the data. These results show that the overall exchange Simulations using inequivalent Jjj's are better, but coupling in the trianions is more than one order of give similar fits with non-unique sets of Jjj's. magnitude lower than in the dianions with approxi-
To restrict the parameter space in which pos- mately 60 states lying within 30 c m "1 of the ground sible combinations of values for the exchange inter- state. With this constraint, efforts a r e being made to actions could range, the temperature dependence of find a unique set of Jjj's that will provide theoretical the epr signal has been studied. In Fig. 2 the energy simulations of the magnetization and x(T) data using level diagram for the states of a spin Eq. (2).
The spin-only Hamiltonian in Eq. (2) operating Sj = 5/2, S2 = 2 , S„ = 2, S. = 2 (3) on the system of spin states in Eq. (3) alone does not
lead to a 3/2 ground state as is observed in s t r u c - .... u ,i T t , . . ture (II). However, crystal field interactions in the system resulting when all JH S a r e equal is given. , .. ' J /TTvro1 ,„ . , Th er level " A A r-h A f lower symmetry structure (II)[2] may result in a sub- to s'Jn slates axTtnS!^!' m e n kT ^ I T o n ^ t h e S t a n t l a l Z e r° f L e l d SPI i t t U l g ^ m l X ^ f SP * c h a r"
ground state will be populated and the observed epr a< fr i n t 0 ** Sr o u n d s t a t e y i e l d mS *e °b s e r v e d r e"
signal results from transitions within the S = 1/2 s t s' __ „, . „. _ „ . The observation that the Fe4S4 core structure in the trianion is sensitive to crystal packing effects Energy Spin * suggest an important role of the crystalline electric (|8* - 8 0 J 17/2 * fields and may have implications relating to possible
3 _ \ _ constraints imposed by the protein moeity such as to
^—• *l stabilize different oxidation states of the core. This
f 4 8 ) - 6 3 J 15/2 £ \ may explain the fact that seemingly identical Fe4S4
i P \ active sites occurring in ferredoxins and in high po- (84) H? w 2 — *\ — tential iron-sulfur proteins [1] can exist in different
-48 J 13/2 j? "\ oxidation states with very different oxidation reduction ,,„«, o> \ potentials.
^ -35J | | / 2 " * \
\ References
<l 50 ' o „ , ' _ \ _
- 2 4 J 9/2 \ j-y Hoim^ R.H # and ft,ers, J.A. in Iron-Sulfur Pro- U f i l _ |5j 7 / 2 \ teins, W. Lovenberg, Ed. Academic Press, New (108) _ \ York, NY (1977).
J60) _ 3j %,\ I j 1 1—£—1~1 [2] Laskowski, E . J . , Frankel, R.B., Gillum, W.O., TieT ° l / 2 i/Txioo Papaefthymiou, G . C . , Renaud, J . , Ibers, J.A.
and Holm, R. H., J. Am. Chem. Soc. 100, 5322 (1978)
Fig- 2 Fig. 3 [3] Laskowski, E . J . , Reynolds, J . G . , Frankel, R . B . ,
„ . „ _ i i J- * . Foner, S=, Papaefthymiou, G . C . , and Holm, R„H.
F i g . 2 . Energy level diagram of an exchange , * / - * ! o m, • coupled spin system of S, = 5/2, s f = 2, J- Am- Ctiem' S o c" M i ™ P"ws.
S3 = 2 and S4 = 2. The numbers hi paren- M F r a n k ei > R . B . , AverllU B°A: • * ?d "o l m' R'H"
thesis give tha degeneracy of each spm _ '• d e ^ * P L ^ a 9 7 4£ n T
state. t5] L a n e> R*w«> Wedd, A . G . , Gillum, W . O . , L a s - Fig. 3 . Hot of logtdoT,, - IT)/(IT)] v s . 1/T. The k o w s k i' E'^ > H o l m> R'H >' F r a n k e l. R'B- ***
8 solid line is a least-squares fit to the data Papaefthymiou, G. C., J. Am. Chem. S o c , 99.,
points. 2 3 5 0 t1 9 7 7) -
[6] Papaefthymiou, G. C., Frankel, R. B., Laskowski, manifolds. Assuming that population of the excited E . J . , Holm, R. H . , Bull. Am. Phys. S o c , 24, states follows a Boltzmann distribution and states with No. 3, 345 (1979).
S > 3/2 a r e well separated in energy from the S = 1/2 [7] Better simulations a r e obtained by allowing for and S = 3/2 states, the intensity of the epr signal as slightly different J values for exchange coupling a function of temperature follows the relationship [8] between different pairs of ions (S. Frota-Pessoa,
, unpublished results).
I(T)T = I(To)To[l+ mexp(-A1/kT)]" . (4) [ 8 ] Biu m > H. , Salerno, J . C . , Prince, R. C . , Leigh, J r . , J . S . , and Ohnishi, T . , Biophys. J. 20, 23 (1977).
Here I(T) is the intensity of the epr signal at temper-
