Transcript Folie 1

Trinuclear Mixed-Valent Oxo-Centered Iron Complexes:
Fully Localised, Partially and Fully Delocalised Valencies
R.W. Saalfrank, A. Scheurer, Universität Erlangen-Nürnberg, Germany
V. Schünemann, Technische Universität Kaiserslautern, Germany
A.X. Trautwein, Universität zu Lübeck, Germany
Motivation
Our interest in polynuclear supramolecular {Fe3O} complexes stems from the
importance of oxo-centered polyiron aggregates as model compounds for iron-oxo
proteins, oxidation catalysts, and corrosion inhibitors. Such oxo-centered complexes,
related to basic iron acetate [FeIII3O(O2C-R)6(H2O)3]+, are easily accessible (Fig. 1, left):
Deprotonation of pentadentate ligand H2L1 with triethylamine and treatment of the
resulting dianion L2- with a solution of iron(III) chloride, yielded after workup the triplehelical, μ3-oxo-centered, trinuclear and mixed-valent complex [FeIIFeIII 2O(L1)3] (2) as
dark-green crystals (Fig. 1, right: stereoview) [1].
Stereoview of 2: FeII dark blue, FeIII gold, C white, N blue, O red
Fig. 1
The lack of counterion in the solid state implies intramolecular charge compensation and
therefore mixed-valence character for [FeIIFeIII 2O(L1)3] (2). This is confirmed by a Mössbauer
spectrum of a powder sample recorded at 4.2K. The spectrum exhibits two quadrupolar doublets
with a peak area ratio of about 1:2 (Fig. 2). The doublet with quadrupole splitting DEQ=2.64 mms-1
and an isomer shift d=0.95 mms-1 suggests a high-spin iron(II) species and the doublet with
DEQ=1.83 mms-1 and d=0.53 mms-1 a high-spin iron(III) species. From this view 2 appears as a
localised mixed-valent trinuclear complex with ground state of the type Fe2+-Fe3+-Fe3+ as
observed also in other cases [2]. A more rigorous characterisation of the valencies in 2, however,
requires a more detailed investigation (vide infra), including the variation of temperature.
Fig. 2
Fully localised vs. fully delocalised valency
The complex [Fe3O(OAc)6(3-Et-py)3].CH3CCl3 (3), with 3-Et-py representing 3-ethylpyridine
(Fig. 3, left), exhibits dI=0.54 mms-1, DEQI=1.04 mms-1 for the ferric and dII=1.26 mms-1,
DEQII=1.54 mms-1 for the ferrous site at T=43 K [3]. At room temperature only one doublet
with d =0.65 mms-1 and DEQ=0.93 mms-1 is seen (Fig. 3, right); this corresponds to full
delocalisation of the “excess” electron over the three iron sites. If the solvate molecule
CH3CCl3 is replaced by toluene or benzene, however, the complex remains valence trapped
up to room temperature.
Stereoview of 3, Fe gold, C white, N blue, O red, S yellow
Fig. 3 (Mössbauer spectra taken from [3])
Partially valence delocalised states
The Mössbauer spectra of [FeIIFeIII 2O(L1)3]
(2) in the temperature range between 4.2
and 332 K are shown in Fig. 4 [4]. A
consistent analysis of all spectra requires
four doublets. They are pairwise related by
the area ratio I:II and III:IV of 1:2. From dI
and dII 2 appears as mixed-valent iron
complex which exhibits a partially
delocalised ground state: Fe2.2+-Fe2.9+Fe2.9+ (vide infra). With increasing
temperature the subspectra III and IV are
gaining weight. They represent Fe2.5+Fe2.75+-Fe2.75+, a structurally slightly
different molecular state of [FeIIFeIII 2O(L1)3]
(2) in the solid. The two complexes Fe2.2+Fe2.9+-Fe2.9+ and Fe2.5+-Fe2.75+-Fe2.75+ do
not significantly change their respective
delocalisation pattern of the “excess”
electron, however, their relative amounts
do change with increasing temperature: at
4.2 K the relative amounts are 90 % and 10
%, and at 332 K they are 45 % and 55 %.
Fig. 4
I
II
III
IV
Fig. 4: The spectrum taken at 4.2 K is dominated by two doublets, I and II, with intensity ratio 1:2
and with the isomer shifts and quadrupole splittings of dI=0.99 mms-1, DEQI=2.65 mms-1 and dII=0.52
mms-1, DEQII=1.85 mms-1. It has been found in an accompanying study that the isomer shift of the
ferric sites of [FeIIFeIII 2O(L1)3] (2) (d=0.52 mms-1) is enhanced compared to the value of the ferric
sites of the corresponding hetero-nuclear complexes [MIIFeIII 2O(L1)3] with MII=NiII, CoII and CuII, i.e.
d=0.46 mms-1. In turn, the isomer shift of the ferrous site (d=0.99 mms-1) is significantly lower than
the corresponding d-values expected for 5N/1O ligation; e.g. isomer shifts of various octahedrally
coordinated iron(II)-pyridine complexes [FeII(pyridine)4X2] with X=Cl, Br, I, NCO are ~1.1 mms-1 at
4.2 K. Both the enhancement of the isomer shift of the ferric sites as well as the reduction of the
isomer shift of the ferrous site can be explained on the basis of partial electron (valence)
delocalisation in the ground state of [FeIIFeIII 2O(L1)3] (2) according to the relation
d,Fe(2+x)+ = xd,Fe2+ + (1-x)d,Fe3+
d,Fe(3-y)+ = yd,Fe2+ + (1-y)d,Fe3+
and x = 2y. From this it has been concluded that 2 exhibits a partially delocalised ground state at
4.2 K, i.e. Fe2.2+-Fe2.9+-Fe2.9+. A corresponding estimate with subspectra III and IV (dIII=0.80 mms-1
and dIV=0.60 mms-1 at 20K) yields Fe2.5+-Fe2.75+-Fe2.75+ [4].
The main contribution to the observed temperature dependence of isomer shifts in Fig. 4 is due to
the second-order Doppler shift (SOD). For comparison, SOD accounts for Dd ~ 0.13 mms-1 in the
temperature interval between 4.2 and 300 K for [FeII(pyridine)4X2] complexes [5]. If the temperature
dependence of isomer shifts in 2 were mainly due to the temperature-induced changes of electron
delocalisation, then the isomer shifts dI and dII as well as dIII and dIV would gradually merge pairwise
with increasing temperature according to the above relation. Such behavior was not observed in the
present case, therefore it was concluded that the clusters Fe2.2+-Fe2.9+-Fe2.9+ and Fe2.5+-Fe2.75+Fe2.75+ do not change their respective delocalisation pattern of valencies.
Solvent vs. packing effect upon
valence delocalisation
Electron (valence) delocalisation is very
much affected by solvent and packing.
Fig. 4 has shown the temperaturedependent Mössbauer spectra of
[FeIIFeIII 2O(L1)3] (2) in the solid state
with partially delocalised valencies over
the whole temperature range. Fig. 5 displays the Mössbauer spectra of 2 at
4.2K (a) and at 160 K (b) in frozen THF
[6]. Only three doublets are required to
fit the spectra at 160 K. Two of them (I
and II) are consistent with partially
localised valencies, i.e. Fe2.3+-Fe2.85+Fe2.85+, while the third (III; d=0.62 mms-1)
is consistent with full valence
delocalisation, i.e. Fe2.67+-Fe2.67+-Fe2.67+.
This apparently delocalised component
accounts for as much as 70% of the
molecules at 160 K.
Fig. 5
Electron exchange in asymmetrically
substituted hetero-nuclear {MIIFeIII2O}
complexes
The oxo-centered iron(II,III) complex
[FeIIFeIII2O(L2)3(OAc)3] (5) was
synthesised starting from HL2 (4) under
aerobic conditions with iron(II) acetate. In
addition, 5 can be converted to complexes
6-8 by co-ligand exchange or nickel(II)
acetate (Fig. 6) [7,8]. The asymmetrically
substituted µ3-oxo centered complexes
[MIIFeIII2O(L2)3(O2C-R)3] (MII=FeII, NiII;
R=Me, Ph) (5-8) possess a metal-toligand-to-co-ligand ratio (M:L:co-L) of
1:1:1, and the absence of a counterion
requires intramolecular charge
compensation, i.e. mixed-valence
character for 5-8.
Fig. 6
The X-ray structure of [FeIIFeIII2O(L2)3(OBz)3] (6) reveals: The iron centers Fe1/Fe2 are
linked by two ligands (L2)-, Fe2/Fe3 by one (L2)- ligand and a benzoate ion, and Fe1/Fe3 by
two benzoate bridges. As a consequence, all three iron ions in the mixed-valent complex 6
are differently octahedrally coordinated (Fig. 7). The effect of the (OAc)- co-ligands on the
{FeIIFeIII2O} core in 5 is different from the effect of the (OBz)- co-ligands on the iron core in 6,
leading to the situation that in 6 the difference in coordination among iron sites is larger than
in 5. This situation is reflected in the Mössbauer spectra (Fig. 8).
Stereoview of 6 with numbering of the iron ions: FeII blue, FeIII gold, C white, N blue (small), O red, S yellow
Fig. 7
The Mössbauer spectra of [FeIIFeIII2O(L2)3(O2C-R)3]
(5,6) (R=Me, Ph) are shown in Fig. 8 [7,8]. The
spectra of 5 and 6 at 300 K are almost identical; both
exhibit two quadrupolar doublets with an area ratio of
1:2 for the FeII and the FeIII ions. However, at 4.2 K
the Mössbauer spectra of 5 and 6 differ considerably.
Unlike for 5 the spectrum of 6 shows at 4.2 K a rather
broad signature of the FeII doublet as compared with
the FeIII doublet, i.e. the experimental data (inset in
Fig. 8d) are best fitted by three doublets (DEQ=2.77,
2.66, 2.66 mms-1, d=1.19, 1.12, 0.96 mms-1, G=0.25,
0.25, 0.25mms-1), which represent FeII with a relative
area ratio of 11,11,11 %, respectively (Table 1) [7].
The presence of partial electron (valence)
delocalisation at 4.2 K in the present case is
excluded, since the hetero-nuclear all-ferric
complexes [NiIIFeIII2O(L2)3(O2C-R)3] (7,8) (R=Me, Ph)
exhibit comparable isomer shifts at 77 K, i.e. 0.50
mms-1 and 0.49 mms-1, as the ferric sites in 5 and 6
at 4.2 K (Table 1).
Fig. 8
As a result of the different coordination of the iron sites (Fe1)3N/3O, (Fe2)4N/2O, (Fe3)2N/4O in
[FeIIFeIII2O(L2)3(OBz)3] (6), there exist evidently three configurations with unlike energies
(Fe1)II(Fe2)III(Fe3)III, (Fe1)III(Fe2)II(Fe3)III, (Fe1)III(Fe2)III(Fe3)II for 5 and 6. Even if the difference
among these energies is small and can be overcome at elevated temperature by thermal
activation, at low temperature one expects that only the configuration with the lowest energy is
populated, corresponding to only one FeII doublet (instead of three doublets) in the Mössbauer
spectrum at 4.2 K. However, assuming a slow electron-tunneling exchange mechanism in 6, one
might arrive at a situation, at which all three configurations remain nearly equally populated, even
at very low temperature. Thermal fluctuations at 300 K then equalise the ligand-field strength
around each iron site in 5 and in 6, such that it is not possible to resolve the different (Fe1)II,
(Fe2)II, (Fe3)II components in the Mössbauer spectra.
Summary
Mixed-valent oxo-centered trinuclear metal complexes provide model systems for systematic
studies of the phenomenon of electron (valence) delocalisation. Such investigations have
shown that the rate of electron transfer is dramatically affected by the environment of the
complex, especially by solvent and/or packing, and by the nature of the ligands directly
coordinated to the metal sites.
References
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Stadler and J. Daub, Angew. Chemie. Int. Ed. Engl. 35, 2206 (1996)
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Biology, K. Prassides (Ed.), Kluwer, Dordrecht, 1991, p. 67
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