Spin-State-Dependent Redox-Catalytic Activity of a Switchable Iron(II) Complex

Article abstract of DOI:10.1002/ejic.201700454

The spin state of catalytically active 3d metal centers plays a significant role in their activity in enzymatic processes and organometallic catalysis. Here the catalytic activity of an Fe^II coordination compound that can undergo a cooperative switch between low-spin (LS) and high-spin (HS) states is reported. Catalytic measurements in the temperature range 291–318 K revealed a drastic drop of the catalytic activity on conversion of the metal centers from the LS to the HS form. For thermoswitchable complex [Fe(NH₂trz)₃]Br₂ (NH₂trz = 4-amino-1,2,4-tiazole; T_up = 305 K), the activation energy is found to be considerably lower for the LS state (158 kJ mol^–1) compared to the HS state (305 kJ mol^–1). M?ssbauer analysis revealed that this is related to higher conversion of the LS complex on oxidation. Comparisons with another polymorph of [Fe(NH₂trz)₃]Br₂ (T_up = 301 K) and [Fe(NH₂trz)₃](ClO₄)₂ (T_up = 240 K) are made. These results show the potential of spin-crossover compounds to compare the catalytic activity of different spin states of the same material when other means of differentiation are limited.

Full text of DOI:10.1002/ejic.201700454

A Journal of
Accepted Article
Title: Spin-State Dependent Redox Catalytic Activity of a Switchable
Iron(II) Complex
Authors: Il'ya A. Gural'skiy, Sergii I. Shylin, Vadim Ksenofontov, and
Wolfgang Tremel
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700454
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
FULL PAPER
Spin-State Dependent Redox Catalytic Activity of a Switchable
Iron(II) Complex
Il’ya A. Gural’skiy,*[ab] Sergii I. Shylin,[ab] Vadim Ksenofontov[a] and Wolfgang Tremel[a]
Abstract: The spin state of catalytically active 3d metal centers plays
a significant role for their activity in enzymatic processes and
organometallic catalysis. Here we report on the catalytic activity of a
Fe(II) coordination compound that can undergo a cooperative switch
between low-spin (LS) and high-spin (HS) states. Catalytic
measurements within 291 – 318 K temperature region reveal a drastic
drop of the catalytic activity upon conversion of metallic centers from
the LS to the HS form. For a thermoswitchable [Fe(NH2trz)3]Br2
complex (Tup = 305 K), an activation energy is found to be
considerably lower for the LS state (158 kJ mol-1) comparing to the
HS state (305 kJ mol-1). Mössbauer analysis reveals that this is related
to a higher conversion of a LS complex upon oxidation. The
comparisons with another polymorph of [Fe(NH2trz)3]Br2 (Tup = 301 K)
and with [Fe(NH2trz)3](ClO4)2 (Tup = 240 K) are made. These results
show the perspective of spin-crossover compounds to compare a
catalytic activity of different spin states within the same material when
other differentiations are minimized.
A typical example are alpha-diimine iron atom transfer radical
polymerization (ATRP) catalysts, where the metal spin state
correlates with the polymerization mechanism.[7] For high-spin
Fe(III) species (S
=
5/2), living atom transfer radical
polymerization predominates, whereas for catalysts in an
intermediate spin state (S = 3/2) an organometallic pathway has
led to catalytic chain transfer.[8] It has been shown that spin
transitions between HS and LS states play a key role in β-hydride
elimination reactions of high-spin alkyl complexes. This leads to a
spin-accelerated mechanism with the transition state having a
lower-spin electronic configuration than both reactants and
products.
Metals are required to circumvent spin restrictions imposed for
reactions of triplet oxygen with singlet organic molecules.[9] Thus
Nature uses transition metals in different spin states to practice
catalytic oxidation chemistry on a large scale, examples being
Mn-catalyzed water oxidation to evolve O2[10] or the activation of
carbon hydrogen bonds involving the heme-containing enzymes
cytochrome P450[11] and chloroperoxidase.[12] Here, the electronic
structures and spin states of heme-related Fe-porphyrins are
crucial determinants of their reactivity.[13]
Introduction
Iron compounds are well known for their catalytic activity with iron
intermediates in high oxidation states.[14] Our approach to probe
the effect of spin state on the catalytic activity was to use Fe(II)
complexes that can exist in both, low-spin (LS) and high-spin (HS)
states depending upon external triggers such as temperature,
pressure or external fields. [15–18] This spin crossover (SCO) effect
resulting from an equilibrium of high- and low-spin states is the
prototype of a switchable molecular solid with applications in
molecular electronics,[19,20] actuating devices,[21,22] displays,[23]
microthermometry,[24,25] and chemical sensing[26] in solid state and
The spin states of metals in transition metal complexes and the
active sites of enzymes are in the focus of (bio) inorganic
chemistry, catalysis and materials science as determinants of
their magnetic properties and chemical reactivity.[1] The
coordination chemistry and the variable oxidation states of
transition metals provide the mechanistic machinery for a
multitude of metal-catalyzed transformations.[2] For reactions
involving paramagnetic intermediates and proceeding to form
radical intermediates it is likely that the spin states of reacting
intermediates (and spin-orbit coupling effects) require
consideration.[3] This is particularly relevant in biological oxidation
catalysis which involves high-valent manganese[4] or iron[5]
intermediates with variable spin states depending on the co-
ligands involved, but also in chemical catalysis,[6] where 3d metals
become increasingly important and have attracted theoretical and
experimental attention.[1]
coordination chemistry, biochemistry, geology, and minerology.
[27,28]
Results and Discussion
In order to observe the effect of spin state on the catalytic activity
of iron complexes in toluene suspensions we have studied the
redox catalytic activity of tris(µ2-4-amino-1,2,4-triazole)iron(II)
bromide [Fe(NH2trz)3]Br2 (NH2trz = 4-amino-1,2,4-tiazole) 1 – a
one-dimensional coordination polymer built up from iron-triazole
chains with bromide anions situated in the inter-chain
channels.[29,30] Members of the family of Fe(II)-triazole complexes
are known for their spin crossover behavior at or close to ambient
temperature.[31,32] [Fe(NH2trz)3]Br2 displays a hysteretic spin
transition around room temperature. The exact transition
temperature strongly depends on the synthetic procedure. Two
samples of [Fe(NH2trz)3]Br2 prepared from water (1a) or ethanol
(1b) were used for further investigations.
[a]
[b]
Dr. I.A. Gural’skiy, S.I. Shylin, Dr. V. Ksenofontov, Prof. W. Tremel
Institute of Inorganic and Analytical Chemistry
Johannes Gutenberg University of Mainz
Duesbergweg 10-14, Mainz 55099, Germany
E-mail: illia.guralskyi@univ.kiev.ua
Dr. I.A. Gural’skiy, S.I. Shylin
Department of Chemistry
Taras Shevchenko National University of Kyiv
Volodymyrska St. 64, Kyiv 01601, Ukraine
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
FULL PAPER
spectroscopy. TCC and CPBA do not absorb at 390 nm (Figure
S5). Without catalyst the reaction is very slow and takes hours,
whereas the catalytic reaction may occur within seconds.
The kinetic curves for this model reaction for different quantities
of 1a in Figure 2a demonstrate a pronounced catalytic efficiency
of the Fe(II)-triazole complex in the solid state, i.e. the reaction
was carried out in a heterogeneous fashion. The reaction was
zeroth order for catechol, whereas the rate depends on the
catalyst concentration. For identical starting concentrations of
catechol and peroxide dA/dt grows linearly with the catalyst
concentration (Figure 2b). For a given precursor concentration
the reaction follows a pseudo-zeroth order kinetics that can be
summarized as
dcchinone dAchinone

 keff
(1)
dt
(l)dt
where ε is the molar extinction of the chinone, l the optical
pathlength, and keff the effective rate constant.
To demonstrate the SCO effect on the catalytic activity of 1a, we
studied the temperature dependent kinetics for the reaction
between 18 and 45 °C. The corresponding kinetic curves are
displayed in Figure 3a. Conversion after 1 min in the reactions
carried out at different temperatures are shown in Figure 3b. All
reactions are pseudo-zeroth order in catechol irrespective of the
temperature.
Figure 1. Temperature-dependent magnetic properties of [Fe(NH2trz)3]Br2 (1a
and 1b) and [Fe(NH2trz)3](ClO4)2 (2) in toluene. 1a and 1b show spin-crossover
behavior between 260 and 320 K, whereas 2 does not change its spin state. A
photograph of the capillary with sample is inserted. Magnetization is given in
arbitrary units. It was not turned in molar units since precise values cannot be
obtained for a very elongated capillary with a liquid diamagnetic toluene inside.
Scheme 1. Model redox reaction to demonstrate the redox catalytic activity of
[Fe(NH2trz)3]Br2.
The spin behavior of the complexes was monitored (in closed
capillaries under liquid toluene, Figure S1) by SQUID
magnetometry (Figure 1). 1a and 1b displayed a cooperative spin
transition around room temperature (1a: Tup = 305 K, Tdown = 296
K; 1b: Tup = 301 K, Tdown = 283 K). These temperatures are similar
to those firstly reported for this complex by L.G. Lavrenova et al.
in 1990 (Tup = 312 K, Tdown = 302 K for the hydrated form; Tup
=
302 K, Tdown = 284 K for the dehydrated form).[29] PXRD patterns
of two isomorphic forms obtained by us are given in Figure S2.
For comparison we used the analogous complex
[Fe(NH2trz)3](ClO4)2 (2) containing a perchlorate counter anion
which has no spin transition in the given temperature range
(LS→HS transition occurs at ~240 K, Figure S3). 3,4,5,6-
tetrachlorochatechol (TCC) was used as substrate because its
oxidation product (3,4,5,6-tetrachloro-1,2-chinone, Figure S4)
could easily be detected spectrophotometrically (Scheme 1). The
oxidizing agent was 3-chloroperoxobenzoic acid (CPBA) in
toluene. Nonpolar toluene was specifically used to avoid any
dissolution (solvolysis) of the iron complexes. Tetrachloroquinone
shows a specific adsorption band at 390 nm (Figure 2a) that was
used to monitor the progress of the reaction by UV-Vis
Figure 2. Catalytic effect of [Fe(NH2trz)3]Br2 on the kinetics of the TCC oxidation
at 39 °C. (a) Kinetic curves for different concentrations of 1a. The insert shows
the evolution of the UV-Vis spectrum upon catechol conversion. (b) Conversion
rate as a function of the catalyst concentration.
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
FULL PAPER
Figure 3. Effect of a spin state on the catalytic activity in the reaction of TCC oxidation (cTCC = 0.309 mM, CCPBA = 4.48 mM, Ccomplex = 2.14 mM). (a) Kinetic curves
of TCC oxidation catalyzed by 1a at different temperatures within 18-45 °C region. (b) Yield of tetrachlorochinone after 1 min of reactions conducted at different
temperatures. (c) Dependence of the reaction rate constant on temperature. (d) Arrhenius plot demonstrates the presence of two kinetic regimes for different spin
states of the catalyst.
Final concentration of tetrachloroquinone (concentration on plato)
is the same for all reactions made at different temperatures (~
0.32–0.36 mmol L-1). This compound can be considered as a
principal oxidation product (intermediate) which is stable in the
timeframes of these reactions, that is why its concentration was
monitored here. It should be noted that this quinone may undergo
further condensation and dechlorination transformations. These
following transformations of tetrachloroquinone are responsible
for the gradual decrease of its concentration after reaching the
After the change of spin state was complete, the temperature
dependence between 39°C and 45 °C showed Arrhenius behavior
again. In essence, Figure 3 clearly differentiates two distinct
temperature regions associated with the FeII(HS) and FeII(LS)
states of 1a. Using Equation 2 the activation energies for the spin
LS
HS
states could be extracted as Ea = 158(11) kJ mol-1 and Ea
=
305 kJ mol-1. This abrupt increase corroborates with the
observation that the LS state was more active than the HS state.
Its origin is assumed to be related to the electronic structures of
two forms and may be due to differences in the electronic
multiplicity, redox potential, lattice energies, etc.
plato, with
a pronounceable slope observed at elevated
temperatures (see 45°C curve in Figure 3a).
The rate constants keff vs. temperature (extracted from the
gradients) are shown in Figure 3c, and the temperature-
dependence of rate constants according to
To confirm the effect of SCO on this redox reaction, we carried
out analogous experiments with 1b ([Fe(NH2trz)3]Br2 (precipitated
from ethanol rather than from water) that displayed a lower
transition temperature than 1a (Figure 1). Rate constants as a
function of temperature are shown in Figure 4a. The most
important conclusion is that the effect of the spin transition on this
reaction was observed as well. The reaction rate followed an
Arrhenius behavior between 23°C and 29 °C and showed a
pronounced drop after the SCO.
Ea
1
(2)
ln(keff )  ln( A) 
R T
is given in Figure 3d, with A as pre-exponential factor, R the
universal gas constant, and Ea as activation energy.
The reaction rate shows Arrhenius behavior and gradually
increases with temperature between 18 and 36 °C. A drastic drop
of the catalytic activity at 36 °C is associated with the SCO.
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
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Figure 4. Temperature-dependent catalytic activity of Fe(II)-triazolic complexes (cTCC = 0.309 mM, CCPBA = 4.48 mM, C1b = 2.1 mM, C2 = 2 mM). (a) Rate constants
of TCC oxidation catalyzed by 1b at different temperatures within 23-42 °C. (b) Rate constants of TCC oxidation catalyzed by 2 at different temperatures within 26-
47 °C. (c) Arrhenius plot for the oxidation catalyzed by 1b demonstrates an effect of spin state on the catalytic activity. (d) Arrhenius plot for the oxidation catalyzed
by a HS complex 2 demonstrates a classical dependence of the reaction rate on temperature.
The simultaneous presence of the FeII(HS) and FeII(LS) states in
the spin equilibrium range around 32°C leads to a superposition
of the reactivities of both states in Figure 3 (for 1a) and Figure 4
(for 1b). After the transition was complete for 1b at 36 °C the
reaction rate increased again in an Arrhenius-type manner. This
sudden change in the reaction rate agrees with the transition
temperature of 1b (a systematic shift related to the thermalization
in kinetic experiments should be considered when comparing with
magnetic measurements). The slight difference in the
temperature of the “activity drop” for 1a and 1b is assumed to be
related to the different transition temperatures.
because iron in high valence states is assumed to play a crucial
role as intermediate. Mössbauer spectroscopy is the most
convenient way to monitor the relative amounts of iron in different
oxidation and spin states in solid samples.[33] Mössbauer spectra
of 1a and 2 have hyperfine parameters very well corroborating
with those obtained by Lavrenova et al.[29] We recorded
Mössbauer spectra of 1a and 2 that were treated with an excess
of CPBA to achieve a partial (but considerable) oxidation of 1a
and 2 (named 1a-ox and 2-ox). Moreover, a reduction by TCC
was used to reduce iron to its initial form (1a-ox-red and 2-ox-
red) and thus reproduce a whole catalytic cycle. Mössbauer
spectra of precursors and oxidized samples are shown in Figure
5. Each oxidized sample contains iron in Fe(II) and Fe(III)
oxidation states (oxidized form can be different within a catalytic
cycle, e.g. Fe(IV) frequently plays a role of an intermediate). Their
principal hyperfine parameters are summarized in Table 1.
Oxidation does not affect the spin state of Fe(II) centers. In its
order, the oxidation state of newly formed Fe(III) centers is hardly
definable just from hyperfine parameters.[34] What is also
important, TCC can majorly reduce these Fe(III) fractions back to
initial Fe(II) forms (samples 1a-ox-red and 2-ox-red) as shown in
Figure 5. Complex 1a used for the kinetic measurements does
not show any considerable content of the oxidized form after one
catalytic cycle (Figure S8).
Even more informative is the comparison with a sample that
displays no spin transition. We have synthetized 2 that contains
iron-triazole chains as 1a and 1b, but the chains are separated by
perchlorate rather than bromide anions. As a result, 2 adopts the
HS state in toluene suspension above room temperature (Figure
1).
LS
HS
The activation energies of Ea = 325(41) kJ mol-1 and Ea
=
504(46) kJ mol-1 derived for the LS and HS states 1b (Figure 4c)
are slightly higher than those for 1a; comparison of HS and LS
activation energies indicates that the LS state is more reactive
than the HS state. Complex 2 showed Arrhenius behavior (Figure
HS
4d) without deviations from linearity. The activation energy Ea
=
74(2) kJ mol-1 was small and lower than that observed for the LS
and HS states of 1a and 1b.
To understand this difference in the catalytic activity, we have
analyzed the tendency of the two spin states to be oxidized,
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
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Table 1. Mössbauer hyperfine parameters of 1a, 2, 1a-ox and 2-ox. A higher conversion to FeIII is displayed by a HS complex comparing to a LS complex (cTCC
= 300 mM, CCPBA = 130 mM, Ccomplex = 20 mM).
Reduced form (FeII)
Spin state
Oxidized form (FeIII)
Content (%)
δ
ΔEQ
(mm s-1)
Content (%)
δ
ΔEQ
(mm s-1)
(mm s-1)
(mm s-1)
[Fe(NH2trz)3]Br2 (1a)
[Fe(NH2trz)3](ClO4)2 (2)
1a-ox
100
LS
HS
LS
HS
LS
HS
0.436(3)
1.041(2)
0.37(1)
1.04(1)
0.40(1)
1.044(1)
0.207(7)
2.781(5)
0.2(fixed)
2.78(2)
100
25(3)
71.0 (8)
82(3)
85(1)
75(3)
29.0(8)
18(5)
15(2)
0.37(1)
0.22(1)
0.31(2)
0.19(4)
0.78(2)
0.73(2)
0.79(3)
0.77(6)
2-ox
1a-ox-red
0.21(2)
2-ox-red
2.818(3)
When using model peroxidase substrates instead of TCC, e.g.
3,3’,5,5’-tetramethylbenzidine or 2,2’-azino-bis(3-
ethylbenzothiazoline-6-sulphonic acid), triazolic complexes do not
display any catalytic activity that excludes a peroxidase-like
mechanism. We believe that the studied catalytic cycle has two
principal steps: (i) oxidation of the iron complex by peroxoacid and
(i) reduction of the complex by TCC. As revealed by Mössbauer
measurements, both of these steps can be reproduced separately.
The higher catalytic activity of the LS species can reasonably be
referred to their higher inclination towards oxidation under
otherwise identical conditions.
Both complexes (1a and 2) undergo a partial oxidation, while the
Fe(III) content is different in each case. When the 1a in its LS state
(at room temperature) is oxidized, the ferric form is present in
amounts of 75(3) %. However, when 2 reacts in its HS state (at
room temperature), the Fe(III) content is just 29(1) %. This
considerable difference reveals that for very similar complexes,
the spin state plays a prominent role in the oxidation process. This
observation is in line with the results of the catalytic oxidation of
TCC.
Conclusions
We have found that in the SCO compound [Fe(NH2trz)3]Br2 the
spin state of the Fe(II) centers plays a crucial role in determining
their redox catalytic activity. The LS species are characterized by
higher reaction rates and smaller activation energies compared to
the HS analogues. We demonstrated that this difference is driven
by a higher tendency of LS iron(II) to be oxidized. Such spin-
dependent activity will be analyzed for other switchable SCO
complexes and other types of chemical reactions in order to
derive a broader picture of the effect of spin state on the catalytic
metal centers, in particular as photochemical and electrochemical
activities may be very sensitive to the spin state of the catalyst.
Application of chiral switchable complexes[35,36] may lead to a
switchable stereoselectivity in catalysis.
Figure 5. Mössbauer spectra of 1a and 2, their oxidized (1a-ox and 2-ox) and
newly reduced (1a-ox-red and 2-ox-red) forms. Fe(II) species are shown in
gray and Fe(III) are shown in black. The content of the Fe(III) species reflects a
higher conversion of LS Fe(II) (vs. HS Fe(II)) upon oxidation.
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
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Experimental Section
Keywords: iron • spin crossover • spin state • catalysis •
Mössbauer spectroscopy
Synthesis. [Fe(NH2trz)3]Br2 (1a). NH2trz (250 mg, 2.98 mmol) in water (1
mL) and FeBr2 (210 mg, 0.97 mmol)) in water (1 mL) were mixed under
[1]
[2]
M. Swart, M. Costas, Eds. , Spin States in Biochemistry and Inorganic
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2314.
stirring. The precipitate was formed within
1 h and separated by
centrifugation (13000 rpm, 4 min), washed with water and dried in air. Yield
is 344 mg (74 %). Anal. Calcd for C6H12N12Br2Fe: C, 15.40; N, 35.92; H,
2.59. Found: C, 15.75; N, 36.12; H, 2.31.[Fe(NH2trz)3]Br2 (1b). The sample
was obtained as described for 1a, by replacing water with ethanol in all
steps. Yield is 382 mg (82 %). Anal. Calcd for C6H12N12Br2Fe: C, 15.40; N,
35.92; H, 2.59. Found: C, 15.51; N, 35.96; H, 2.48.
[3]
[4]
T. Risse, D. Hollmann, A. Brückner, in Catalysis 2015, 27, 1–32.
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[5]
[Fe(NH2trz)3](ClO4)2 (2). The sample was prepared as 1a starting from
NH2trz (250 mg, 2.98 mmol) and Fe(ClO4)2·6H2O (360 mg, 0.99 mmol).
Yield is 330 mg (65 %). Elemental analysis was not performed for safety
reasons.
[6]
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Catalysis. Catalytic measurements were performed in 3.5 mL quartz
cuvettes with 1 cm optical pathway. The temperature in the cuvette holder
was controlled with a Haake C50P thermostat. The concentration of the
product was monitored by measuring the absorption of quinone with a UV-
Vis Carry spectrometer at 390 ± 2.5 nm with a 5 sec interval.
[10] V. Krewald, M. Retegan, F. Neese, W. Lubitz, D. A. Pantazis, N. Cox,
Inorg. Chem. 2016, 55, 488–501.
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5859.
A catalyst (3 mg) was added to the TCC (0.23 mg) in toluene (2.9 mL). The
cuvette was thermalized for 4 min prior to each measurement. CPBA in
toluene (0.1 mL, 0.13 mol L-1) was added after and a UV-Vis monitoring
started directly. No stirring during the reaction was applied.
[14] A. Ghosh, D. Mitchell, A. Chanda, A. D. Ryabov, D. L. Popescu, E. C.
Upham, G. J. Collins, T. J. Collins, V. Pennsyl, J. Am. Chem. Soc. 2008,
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[15] S. Brooker, Chem. Soc. Rev. 2015, 44, 2880–2892.
[16] M. A. Halcrow, Spin-Crossover Materials, John Wiley & Sons Ltd, Oxford,
UK, 2013.
Oxidized samples. To produce 1a-ox and 2-ox, a Fe(II) complex (0.1
mmol) was mixed with 3-chloroperoxobenzoic acid in toluene (3 ml, 0.13
mol L-1) and allowed to stay for 5 min at 20 °C. To produce 1a-ox-red and
2-ox-red, oxidized samples (0.1 mmol) were treated with TCC solution (3
ml, 0.3 mol L-1) and allowed to stay for 5 min at 20 °C. The powders were
separated from solutions by centrifugation (13000 rpm, 1 min), washed
with toluene and dried in air.
[17] P. Gütlich, Eur. J. Inorg. Chem. 2013, 2013, 581–591.
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Carrey, G. Molnár, M. Respaud, A. Bousseksou, Adv. Mater. 2013, 25,
1745–1749.
Magnetic susceptibility measurements. Temperature-dependent
magnetic susceptibility measurements were carried out with a Quantum-
Design MPMS-XL-5 SQUID magnetometer with a heating and cooling rate
of 1 K min−1, and a magnetic field of 0.5 T. A powder sample mixed with
liquid toluene was sealed using a hydrogen torch in a glass capillary (~ 4.5
mm long, inner diameter is 1.0 mm, outer diameter is 1.4 mm). Capillary
was fixed between two gelatin capsules. Molar magnetization was not
calculated because of the considerable diamagnetic impact of toluene and
of the capillary, which could not be accurately subtracted.
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
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10.1002/ejic.201700454
European Journal of Inorganic Chemistry
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Entry for the Table of Contents
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Spin-crossover Fe(II) complex
[Fe(NH2trz)3]Br2 shows a redox
catalytic activity that changes upon
conversion of metallic centers from
the low-spin to the high-spin form.
The activation energy is
considerably smaller for the low-
spin state. This difference is
related to a higher tendency of the
diamagnetic form towards
Il’ya A. Gural’skiy,* Sergii I.
Shylin, Vadim Ksenofontov and
Wolfgang Tremel
Page No. – Page No.
Spin-State Dependent Redox
Catalytic Activity of a
Switchable Iron(II) Complex
oxidation.
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