.
Angewandte
Communications
electron configuration, which was reported for the few other
HYSCORE experiments reveal the interaction of several
protons with the electron spin (Figure 1e). The estimation of
the different proton hyperfine anisotropies based on the
through-space point-dipole interaction using the Fe···H dis-
tances (derived from the single-crystal X-ray analysis of 2a)
yielded values very close to those observed in the spectra.
This indicates localization of the unpaired electron at one
[23,25,27]
iron imido heterocubanes.
This supports the presence
of FeÀFe bonds in compound 1. The quadrupole splitting of
3
+
2+
3+
1
(
is larger than that of Fe or Fe /Fe imido heterocubanes
À1
j DE j= 0.55–1.32 mms ), which is attributed to the chelat-
Q
2À
ing character of the (Ntrop) ligand. Reduced 2a and 2b
show each one quadrupole doublet indicating that the addi-
tional electron (compared to 1) is delocalized in the cluster at
FeNtrop subunit in a distorted Fe N4 cube rather than
4
7
7 K on the time scale of the Mçssbauer spectroscopic
delocalization over the entire cluster. Remarkably, HYS-
CORE experiments at 10 K on 2b and N-labeled 2b
15
experiment. This is in agreement with the results obtained
from single-crystal X-ray analyses at 100 K (see above).
Identical isomer shifts (0.33(1) mms ) and very similar
(Figure 1 f) reveal strong coupling with one nitrogen nucleus
À1
1
(N1, with A ꢀ 18 MHz) and interactions in the weak coupling
À1
2,3
quadrupole splittings (j DE j= 1.35(1), 1.31(1) mms ) were
regime with two nitrogen atoms (N2 and N3, with A
Q
observed for 2a and 2b, revealing that the counterion
ꢀ 4 MHz), which is again consistent with localization of the
unpaired electron. This phenomenon was not resolved by
other analytical techniques (VT NMR, Mçssbauer spectros-
copy, X-ray diffraction), which is due to their lower time
resolution and the higher temperatures at which these
measurements were performed (77–300 K). Additionally,
HYSCORE spectra revealed weak couplings with carbon
nuclei (Supporting Information). Thus the analysis of the
hyperfine interactions of the unpaired electron with the
surrounding magnetic nuclei reveals that the unpaired
electron in 2b is predominantly located at one Fe center
+
+
(
[(Co(C Me ) ] vs. [Na(thf) ] ) does not have a substantial
5 5 2 4
effect on the Mçssbauer parameters. The quadrupole split-
tings are moderately decreased compared to 1. The small but
significant increase in the isomer shift suggests that the effect
of longer FeÀN bonds slightly overcompensates the effect of
shorter FeÀC bonds in 2a/b compared to 1.
Compounds 1, 2a, and 2b were analyzed by SQUID
magnetization measurements in an applied field of 1 T
(
Supporting Information). At 300 K they show effective
magnetic moments of meff = 2.83 mB (1), and 2.27–2.30 mB
2a,b), respectively. With decreasing temperature, meff steadily
decreases to reach values of 0.77 mB (1) and 1.64–1.54 mB
2a,b). These findings indicate intramolecular antiferromag-
netic interactions as previously described for iron–sulfur
(
(spin density 1 ꢀ 80%) with some delocalization to the
Olefin
coordinated C=C
unit (1 ꢀ 18%) and to the nitrogen
(
atom that is part of the same chelating ligand (1 ꢀ 0.8%).
Only minor contributions to the total spin density are
detected at the other two nitrogen nuclei that directly interact
with the Fe radical center (1 ꢀ 0.2% each; Supporting
[41]
clusters with heterocubane structures.
Overall, the mag-
netic susceptibility data indicate S = 0 and S = 1/2 ground
states for 1 and 2a/b, respectively, with higher electronic
states being increasingly populated with increasing temper-
ature. In all cases, the experimental data were best repro-
duced in simulations using antiferromagnetic spin-coupling
models in which one Fe center shows parameters different
from the other three Fe centers. Local spin states of 4 S = 1
Information). This is in contrast to the electronic structure
+
of [M(Ntrop )(bipy)] (M = Rh, Ir), where the spin is
2
predominantly located at the nitrogen center, leading to
[44,45]
a description as stable cationic aminyl radical complexes.
Cyclic voltammetry of 1 in THF at 238C revealed four
chemically reversible redox events, which are assigned to the
[46]
(
for 1) and 3 S = 1 plus 1 S = 1/2 (for 2a,b) were assumed,
electron-transfer series denoted in Figure 2.
The redox
+
and best-fit simulations give antiferromagnetic coupling
processes take place between À1.72 and À3.59 V vs. Fc/Fc ,
which is the most negative potential range reported for iron
heterocubanes. Note that the shapes of the curves for the
redox events E and E are affected by the onset of reductive
À1
À1
constants J = À174 cm , J’ = À94 cm (for 1) and J = À244/
À1
À1
À195 cm , J’ = À244/À244 cm (for 2a/b) (Supporting Infor-
mation). It should be noted that the existence of a diamagnetic
ground state has been under debate for all-ferrous [Fe S ]
3
4
[47]
solvent decomposition. The chemical reversibility in the
cyclic voltammogram of 1 shows that the cluster can store up
to four electrons and remains intact in solution over five
distinct oxidation states, which is comparable to some iron
4
4
[42,43]
clusters in biological systems.
magnetic moments at 298 K of 1 (C D , 2.7(1) m ) and 2b
The solution effective
6
6
B
(
[D ]THF, 2.4(2) m ) determined by Evansꢀ method are close
8 B
[
8]
[19]
to those determined in the solid state, giving evidence for the
solid-state structures being essentially maintained in solution.
There are no experimental indications that 1 or 2 dissociate
heterocubanes with oxygen or sulfur as heteroatoms. The
large difference of up to 870 mV between the potentials of
consecutive redox events E ÀE suggests strong electronic
1
4
[24,26]
[48]
into dinuclear subunits.
coupling between the metal centers, especially for E /E . In
1 2
Compound 1 is EPR-silent in a 2-MeTHF glass at 10 K,
which is in agreement with an S = 0 ground state (Figure 1c,
green line). Compound 2b shows an axial X-band EPR signal
without any resolved hyperfine coupling in a 2-MeTHF glass
agreement with the electron-transfer series depicted in
Figure 2 (bottom), compound 1 did not react with [Co-
0
(C H ) ] (E ’ ꢀ À1.3 V), and stronger reducing agents such as
5
5 2
0
[Co(C Me ) ] (E ’ ꢀ À1.9 V) were necessary to obtain the
5
5 2
at 10 K (g = 2.02; g = 1.96), which is consistent with S
singly reduced species 2. Oxidation of 2a with [Co(C H ) ]I
5 5 2
k
?
4
symmetry in solution (Figure 1c). An electron spin echo
was detected from the reduced compound 2b, and its S = 1/2
ground state at T= 10 K was confirmed by comparison with
cleanly regenerates compound 1.
1
At room temperature, H NMR spectra of 1 in [D ]THF
show eleven paramagnetically shifted resonances of equal
intensity (one pair is overlapping) in the range of about 6–
8
II
a Cu standard in two-dimensional EPR spectroscopic
nutation experiments (Figure 1d). Davies ENDOR and
60 ppm. This is in agreement with an apparent S symmetry
4
1
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 13012 –13017
Lichtenberg, Crispin
