Reactivity of an All-Ferrous Iron-Nitrogen Heterocubane under Reductive and Oxidative Conditions

Article abstract of DOI:10.1002/chem.201502530

The reactivity of the all-ferrous FeN heterocubane [Fe₄(Ntrop)₄] (1) with i)Bronsted acids, ii)σ-donors, iii)σ-donors/π-acceptors, and iv)one-electron oxidants has been investigated (trop = 5H-dibenzo[a,d]cyclo-hepten-5-yl). 1 showed self-re-assembling after reactions with i) and proved surprisingly inert in reactions with ii) and iii), with the exception of CO. Reductive and oxidative cluster degradation was observed in reactions with CO and TEMPO, respectively. These reactions yielded new cluster compounds, namely a trinuclear bis(μ₃-imido) 48 electron complex in the former case and a tetranuclear all ferric μ-oxo-μ-imido species in the latter case. Characterization techniques include NMR and in situ IR spectroscopy, single crystal X-ray analysis, M?ssbauer spectroscopy, cyclic voltammetry, magnetic susceptibility measurements, and DFT calculations.

Full text of DOI:10.1002/chem.201502530

DOI: 10.1002/chem.201502530
Full Paper
&
Heterocubanes
Reactivity of an All-Ferrous Iron–Nitrogen Heterocubane under
Reductive and Oxidative Conditions
Crispin Lichtenberg,*[a] Demyan E. Prokopchuk,[a, b] Mario Adelhardt,[c] Liliana Viciu,[a]
Karsten Meyer,[c] and Hansjçrg Grützmacher*[a]
Abstract: The reactivity of the all-ferrous FeN heterocubane
[Fe4(Ntrop)4] (1) with i) Brønsted acids, ii) s-donors, iii) s-
donors/p-acceptors, and iv) one-electron oxidants has been
tively. These reactions yielded new cluster compounds,
namely a trinuclear bis(m3-imido) 48 electron complex in the
former case and a tetranuclear all ferric m-oxo-m-imido spe-
cies in the latter case. Characterization techniques include
NMR and in situ IR spectroscopy, single crystal X-ray analysis,
Mçssbauer spectroscopy, cyclic voltammetry, magnetic sus-
ceptibility measurements, and DFT calculations.
investigated (trop
=
5H-dibenzo[a,d]cyclo-hepten-5-yl).
1 showed self-re-assembling after reactions with i) and
proved surprisingly inert in reactions with ii) and iii), with the
exception of CO. Reductive and oxidative cluster degrada-
tion was observed in reactions with CO and TEMPO, respec-
Introduction
the sulfur analogues include larger Fe-N-Fe and smaller N-Fe-N
angles as well as systematically more negative potentials for
(quasi-)reversible redox events.[1a,4,6] While the physicochemical
characteristics of iron nitrogen heterocubanes have been in-
vestigated, the understanding of their chemical reactivity is
little developed. Previous work reports that [Fe4(NPh)4(S-2,4,6-
C6H2Me3)4] reacts with toluidine in an amine/imide exchange
reaction[3a] and that [Fe4(NtBu)4Cl4] does not react with aniline,
PPh3 or styrene.[3b]
Iron sulfur heterocubanes have been studied extensively as
they are prominent structural motifs in cofactors and pro-
teins.[1] Their most important function is the mediation of elec-
tron transfer events. Furthermore, they can act as catalytically
active sites or modulate the structure of proteins via bonding
interactions with side chain functional groups.[1,2] The isolation
and characterization of synthetic analogues, Fe4Q4, which con-
tain Q=nitrogen instead for Q=sulfur, is much less ex-
plored.[3,4,5] Lee and others explored different strategies for the
synthesis of these species including salt elimination,[3c] NÀN
bond cleavage,[3a,f] transamination,[3e] and reductively[3d] or ther-
mally[3c] induced association of dinuclear iron imides. The varie-
ty of possible synthetic routes to access FeN heterocubanes
suggests that this structural motif exhibits an inherent thermo-
dynamic stability. Structural, spectroscopic, electronic, and
magnetic properties of FeN heterocubanes have been investi-
gated in some detail.[3,6] Important differences compared to
We recently described an all-ferrous iron nitrogen heterocu-
bane [Fe4(Ntrop)4] (1), which can undergo four (quasi-)revers-
ible redox events at very negative potentials (trop=5H-diben-
zo[a,d]cyclo-hepten-5-yl).[7] The self-exchange rate between
1 and its mono-reduced form [1] is relatively fast (k>3.2”
104 Hz) indicating the possibility of using 1 as an outer-sphere
electron transfer catalyst. Moreover, 1 selectively catalyzes re-
ductive and oxidative CÀC coupling reactions which implies
that substrate molecules may interact with the Fe4N4 core.[7]
We therefore investigated the reactivity of 1 towards simple re-
agents such as protons, two electron donors and oxidants.
Against many of these reagents 1 is remarkably stable but it
reacts with CO and TEMPO with reductive or oxidative cluster
degradation, respectively.
[a] Dr. C. Lichtenberg, Dr. D. E. Prokopchuk, Dr. L. Viciu,
Prof. Dr. H. Grützmacher
Department of Chemistry and Applied Biosciences, ETH Zürich
8093 Zürich (Switzerland)
E-mail: clichtenberg@ethz.ch
hgruetzmacher@ethz.ch
Results and Discussion
[b] Dr. D. E. Prokopchuk
Current address: Department of Chemistry, University of Toronto
80 Saint George St., Toronto, Ontario M5S 3H6 (Canada)
Reaction of 1 with closed shell ligands
Substrate coordination is often the initiating step in metal cen-
tered transformations. Thus, a range of common ligands with
s-donor and s-donor/p-acceptor properties were reacted with
compound 1 (Scheme 1) in order to evaluate if they form
bonding interactions with the metal center in solution. No re-
action was observed between compound 1 and an eightfold
[c] M. Adelhardt, Prof. Dr. K. Meyer
Department of Chemistry and Pharmacy, Inorganic Chemistry
Friedrich-Alexander University Erlangen-Nürnberg
91058 Erlangen (Germany)
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201502530:
Experimental and computational details.
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excess of weak s-donors such as THF or pyridine in benzene
solution as evidenced by 1H NMR spectroscopy. The same
result was obtained, when compound 1 was reacted with
a fourfold excess of stronger s-donor/p-acceptor ligands such
as 1,5-cyclooctadiene, PPh3, P(p-C6H4F)3 or P(OMe)3 in benzene
solution at temperatures up to 808C as shown by 1H NMR
spectroscopy (and 31P, 19F NMR spectroscopy where applica-
ble).[8] Furthermore, 1 did not react with H2 in benzene solution
at temperatures up to 1208C. No hydrogenation occurred in
the presence of 10 equivalents of 1,1-diphenylethylene (up to
1208C) or benzophenone (up to 608C) under these conditions.
When compound 1 was reacted with four equivalents of the
Brønsted acids [HNEt3][BPh4] or HOCHPh2, full degradation of
the loss of two (Ntrop)2À ligands and one FeII ion. In the pres-
ence of CO, the two (Ntrop)2À ligands are degraded to the CÀC
coupling product [trop2] (3) and two equivalents of cyanate
anions (NCO)À. These are found in the FeII complex 4. The
redox events involved in this reaction sequence are depicted
in Scheme 2b. They include i) two-electron oxidation of CO to
give (NCO)À (two times), ii) one-electron reduction of two trop
fragments to give the coupling product [trop2] (3), and iii) one-
electron reduction of two FeII centers to give two (formal) FeI
centers in trinuclear 2.
1
1 was indicated by H NMR spectroscopy at early stages of the
reaction, but compound 1 could be detected again at later
stages of the reaction along with H2Ntrop. This suggests that
the intermediates formed upon protonation can undergo dis-
proportionation as in [Fe4(Ntrop)4] (1)+4 HX!intermediates!
0.5 [Fe4(Ntrop)4] (1)+2 H2Ntrop+2[Fe(solv)n][X]2. Indeed, com-
pound 1 could be re-isolated from the reaction with [HNEt3]
[BPh4] in THF. In this reaction an iron containing precipitate
was formed as a byproduct which was characterized as
[Fe(NC5H5)6][BPh4]2 after recrystallization from pyridine.[9] Alto-
gether, these results demonstrate the robustness of 1 and the
thermodynamic stability of the iron imido heterocubane struc-
tural motif as it spontaneously re-assembles in solution after
initial degradation.
Scheme 2. Reaction of [Fe4(Ntrop)4] (1) with CO. a) Experimental conditions
and yields (based on 1); py=NC5H5. b) Idealized stoichiometric redox equa-
tion; atoms that undergo formal reduction/oxidation are shown in bold, to-
gether with their formal oxidation states.
Scheme 1. Inertness of [Fe4(Ntrop)4] (1) towards potential ligands/substrates
L; COD=1,5-cyclooctadiene.
The isolated yields of 2–4 are low to moderate, which is
mainly due to difficulties in separating these compounds. How-
ever, 1H NMR spectroscopic monitoring of the reaction re-
vealed an 84% spectroscopic yield of 2 suggesting that the
transformation outlined in Scheme 2 is the main reaction path-
way.[10] This reaction is unusual because the bis(m3-NR)-capped
tri(iron) carbonyl complex is obtained on a reductive route.[11]
Furthermore, the formation of cyanate anions from CO and
a nitrogen source in the coordination sphere of iron is rare
and has so far only been reported for Fe nitride species.[12]
Recently, the reaction of an iron sulfur heterocubane,
[Fe4S4(PCy3)4][BPh4], with CO has been reported to yield
[Fe3(m3-S)2(PCy3)2(CO)4], a trinuclear 50 electron complex that is
related to 2.[13] Notably, a high CO pressure of 28 bar in combi-
nation with a reaction time of 2 d was necessary to obtain this
species in 27% yield. The formation of further (inevitable) by-
products or possible side reactions remained unknown. The
Reaction of 1 with CO
In contrast to the reluctance of 1 to react with neutral s-basic/
p-acidic compounds such as PPh3 or P(OMe)3, it readily reacts
with carbon monoxide (1 bar) in toluene at ambient tempera-
ture. The reaction of 1 with CO leads to a well-defined cluster
fragmentation yielding three different products, all of which
could be isolated and characterized (Scheme 2a). These are
i) the trinuclear species [Fe3(m3-Ntrop)2(CO)6] (2), ii) the hydro-
carbon [trop2] (3) and iii) iron(II) cyanate, which was isolated as
its pyridine adduct {[Fe(NCO)2(NC5H5)4]·2(NC5H5)} (4) after re-
crystallization from pyridine. The overall reaction between
1 and excess CO can formally be rationalized as depicted in
Scheme 2. The formation of the trinuclear Fe species 2 requires
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relatively harsh conditions necessary to enforce a reaction of
the iron sulfur heterocubane with CO in combination with the
relatively low yield suggest a higher and at the same time
more selective reactivity of the iron nitrogen heterocubane
1 towards CO.
Compounds 3 and 4 were previously independently synthe-
sized using other methods.[14,15] The analytical data we ob-
tained are in agreement with the literature data. In addition,
we determined the structures of these compounds by single
crystal X-ray analysis (Figure 1b, c).[16] The new compound 2
was isolated as intensely colored, dark brown single crystals
(monoclinic, P21/m, Z=2; Figure 1a), which are soluble in
common organic solvents such as toluene, Et2O or THF. Com-
plex 2 is a trinuclear bis(m3-imido)-iron carbonyl complex. Liter-
ature-known compounds of this type, [Fe3(m3-NR)2(CO)9-n(L)n]
(R=alkyl, aryl; L=neutral ligand; n=0–2), are exclusively 50
electron species (summation of metal/ligand valence elec-
trons).[17]
Scheme 3. Arrangements of Fe and N atoms for 50 electron (a) and 48 elec-
tron (b) bis-imido-tri-iron complexes.
three iron atoms. In contrast, the related 50-electron species
show a more irregular arrangement, in which the N atoms are
positioned towards the non-bonded Fe atoms.[19] Due to the
chelating nature of the (Ntrop)2À ligand and the
lower coordination number of Fe1, the Fe1–N distan-
ces in 2 (1.84 Š) are shorter than the Fe2/3–N distan-
ces
(1.92 Š)
(cf.,
1.87–1.99 Š
in
[Fe3(m3-
NR)2(CO)9Àn(L)n]).[17b,e,h–k] The C=Ctrop bonds coordinat-
ed to Fe1 are significantly elongated (1.432(2) Š)
compared to the free ligand (1.340(2) Š)[7] or to
H2Ntrop in the coordination sphere of RhI (1.376(12)–
1.398(5) Š)[20] and indicate strong metal-to-ligand
back-bonding. The six terminal CO ligands bound to
Fe2 and Fe3 show FeÀCO and CꢀO bond lengths in
the range of those observed in related 50 electron
compounds.[17b,e,j] The zero field 57Fe Mçssbauer spec-
trum of 2 at 77 K consists of two subspectra with
a 2:1 intensity ratio (Figure 2). This is in contrast to
the 50 electron cluster [Fe3(m3-S)2(CO)9] that shows
one signal in the 57Fe Mçssbauer spectrum.[21,22] It
confirms that the two Fe(CO)3 groups in 2 are elec-
tronically equivalent, but distinct from the Fe(olefin)2
group. The Mçssbauer parameters of the Fe(CO)3
Figure 1. Molecular structures of [Fe3(m3-Ntrop)2(CO)6] (2) (a), [trop2] (3) (b), and {[Fe(N-
CO)2(NC5H5)4]·2(NC5H5)} (4). Displacement ellipsoids are shown at the 50% probability
level. Hydrogen atoms, annelated C6H4 groups of 2 and solvent molecules in the lattice
are omitted for clarity. Selected bond lengths [Š] and angles [8] for 2: Fe1ÀN1,
1.8398(13); Fe2ÀN1, 1.9234(13); Fe3ÀN1, 1.9218(13); Fe1ÀFe2, 2.6020(4); Fe2ÀFe3,
2.5131(5); Fe1ÀFe3, 2.5932(5); Fe1ÀC4, 2.0849(16); Fe1ÀC5, 2.0801(16); Fe1À(C4=C5),
1.9554(16); Fe2ÀC16, 1.809(3); Fe2ÀC17, 1.8131(17); Fe3ÀC18, 1.813(3); Fe3ÀC19,
1.8171(17); C16ÀO1, 1.147(3); C17ÀO2, 1.132(2); C18ÀO3, 1.138(3); C19ÀO4, 1.132(2);
C4ÀC5, 1.432(2); Fe-Fe-Fe, 57.859(13)-61.246(14).
Accordingly, they show two metalÀmetal bonds. Interactions
of the N atoms with all three Fe atoms results in a V-shaped ar-
rangement of the metal centers with two short and one long
Fe–Fe distances, the latter of which is not associated with MÀ
M bonding (Scheme 3a). In contrast, compound 2 is a 48 elec-
tron complex and can be expected to adopt a triangular ar-
rangement with three FeÀFe bonds (Scheme 3b).
groups d=0.00(1) mmsÀ1 and jDEQ j =0.79(1) mmsÀ1 are simi-
lar to those reported for [Fe3(m3-S)2(CO)9] (d=À0.040(2) mmsÀ1
;
jDEQ j =0.543(2) mmsÀ1).[21] The Fe(olefin)2 group shows
a
higher isomer shift and quadrupole splitting (d=
0.35(1) mmsÀ1; jDEQ j =2.47(1) mmsÀ1).
Compound 2 is diamagnetic as determined by the Evans’
method. NMR spectroscopic analyses indicate C2v symmetry in
solution. The CHolefin groups show resonances at d = 6.90 and
Indeed, the three Fe–Fe distances in compound 2 (2.51–
2.60 Š) fall into the range of metal-metal bonding with the
Fe2ÀFe3 bond being by 0.09 Š shorter than the Fe1ÀFe2/3
bonds.[18] These three FeÀFe bonds are, however, slightly
longer compared to the two FeÀFe bonds in the 50-electron
complexes (2.42–2.50 Š), but more than 0.4 Š shorter than the
distance between the non-bonded Fe(2)–Fe(3) centers in these
compounds.[17b,e,h–k] As a result, the largest Fe-Fe-Fe angle in 2
(618) is about 158 smaller than that in related 50 electron com-
plexes. Two symmetry-equivalent, m3-bound nitrogen atoms,
N1 and N1’, are located in the axial positions of the trigonal bi-
pyramidal structure above and below the plane defined by the
1
89.42 ppm in the H and 13C NMR spectra. Compared to the
free ligand, this corresponds to an upfield shift by about
0.2 ppm and 40 ppm, respectively,[7] indicating coordination of
the olefin groups and efficient d(Fe)!p*(C=C) back donation
in solution in agreement with the structural data.[23] The car-
bonyl groups of 2 give rise to one resonance in the 13C NMR
spectrum suggesting rapid exchange on the NMR time scale
between the two magnetically inequivalent positions in solu-
tion at 298 K.
Detailed electrochemical studies of [Fe3(m3-NPh)2(CO)9] and
related compounds showed that the singly reduced radical
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Scheme 4. Calculated energies of 48 electron complex 2, 50 electron adduct
2-CO, and transition state TS-2. Relative free energies (G8) and enthalpies
(H8, in parentheses) are presented in kcalmolÀ1
.
relative to 2. Thus, a slow interconversion between 2 and 2-CO
(and possibly further CO adducts of 2) is possible. We therefore
suggest that the isolation of 2 rather than the more stable 2-
CO is caused by the lower solubility of 2 which is removed
from the equilibrium reaction by selective crystallization. Both,
dissociative and associative mechanisms have been discussed
for CO ligand exchange reactions in 50 electron species
[Fe3(m3-NR)2(CO)9Àn(L)n].[17d] Taking the calculated 50-electron
species 2-CO as a starting point, our results indicate the viabili-
ty of dissociative pathways in such reactions.
Figure 2. Mçssbauer spectrum of [Fe3(Ntrop)2(CO)6] (2) at 77 K. Solid lines
represent fitted data (solid fill: major component; striped pattern fill: minor
component; no fill: superimposed). Component 1 (38%): d=0.35(1) mmsÀ1
jDEQ j =2.47(1) mmsÀ1; GFWHM =0.27(1) mmsÀ1; component 2 (62%):
;
d=0.00(1) mmsÀ1; jDEQ j =0.79(1) mmsÀ1; GFWHM =0.32(1) mmsÀ1
.
Reactions of heterocubane cluster 1 with small molecules
H2, P(OMe)3, and CO were investigated using closed-shell den-
sity functional theory (DFT) calculations (Figure 4).[26] The most
sterically accessible coordination trajectories are at the four
faces of the cubane orthogonal to the molecule’s C2 rotational
axis. The small molecules can easily approach one of four iden-
tical “iron binding pockets” at these faces. When H2 or P(OMe)3
are introduced into this vacant coordination site, the optimized
ground state structures 1-H2 and 1-P(OMe)3 reveal a modestly
increased free energy (DG8=7.1 and 7.0 kcalmolÀ1, respective-
ly) and a modestly decreased reaction enthalpy (DH8=À2.4
and À8.9 kcalmolÀ1, respectively). This suggests that reversible
coordination of these small molecules is possible. However,
the experimental observations suggest that the equilibria for
these reactions lie sufficiently far towards the reactant side to
make the observation of the adducts impossible. Dissociation
of one of the olefin ligands from an Fe center in 1 is unlikely
to be an initial event before coordination, because complex 1’
with one dissociated alkene ligand is much higher in energy
(DG8=35.5 kcalmolÀ1; Figure 4).
Figure 3. Cyclic voltammogram of [Fe3(Ntrop)2(CO)6] in 0.1m solution of
[nBu4N][PF6] in THF versus Fc/Fc+ at a scan rate of 20 mVsÀ1 at 238C; work-
ing electrode: glassy carbon; counter electrode: Pt; reference electrode: Ag;
E
1/2 =À1.29 V versus Fc/Fc+.
CÀ
species [Fe3(m3-NPh)2(CO)9] can be generated, but is metasta-
ble with respect to ligand dissociation, which in turn induces
decomposition in the absence of additional p-acidic ligands.[17i]
Cyclic voltammetry of 2 in THF/0.1m [nBu4N][PF6] revealed a re-
versible redox event at À1.29 V vs. Fc/Fc+ (Figure 3), which is
about 0.2 V more negative than the redox event observed for
[Fe3(m3-NPh)2(CO)9]. No indications for decomposition of the
electrochemically generated radical anion [Fe3(m3-Ntrop)2-
CÀ
(CO)6] were obtained, which we ascribe to the electronic and
steric stabilization by the chelating trop groups and the lower
CÀ
total electron count (49 e) compared to [Fe3(m3-NPh)2(CO)9]
(51 e).
The interaction of 1 with small and more p-accepting CO li-
gands was investigated. Terminal CO coordination to iron (1-
COt) is an exergonic process with a relative free energy of
À16.7 kcalmolÀ1. 1-COt shows the expected elongation of the
FeÀN and FeÀalkene bonds surrounding the iron-carbonyl
moiety (see Supporting Information). From this point, three dif-
ferent types of reactions may be considered: i) CO insertion
into an FeÀN bond; ii) alkene dissociation; iii) coordination of
another CO molecule. Carbonyl insertion into a FeÀN bond
leading to the carbamoyl compound 1-COc was found to be
endothermic (DG8=6.4 kcalmolÀ1) and is associated with a siz-
able activation free energy of 22.0 kcalmolÀ1 (TS1t) relative to
1. With respect to ii), the ground state energy of species 1’-COt
with a dissociated alkene is only slightly higher in energy than
that of 1-COt (DDG8=5.2 kcalmolÀ1), especially when com-
DFT calculations and mechanistic considerations
As 50 electron species [Fe3(m3-NR)2(CO)9-n(L)n] are prevalent
among trinuclear bis(m3-imido)-iron carbonyl complexes, the
addition of CO to compound 2 to give such a 50 electron clus-
ter was investigated by DFT calculations (Scheme 4). The result-
ing product, 2-CO, is thermodynamically favorable by DH8=
À20.0 kcalmolÀ1 (DG8=À9.7 kcalmolÀ1).[24] As expected, CO as-
sociation is accompanied by the cleavage of one FeÀFe bond
(Supporting Information). Performing
a relaxed potential
energy surface scan followed by a transition state geometry
optimization allowed us to locate a bent[25] CO association
transition state TS-2, with a modest barrier of 13.6 kcalmolÀ1
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rapidly (full conversion of 1 after
ꢁ10 min), whereas compound 2
is only slowly formed from this
reaction mixture (84% after 5 d).
Reacting 1 with only 4 equiv CO
gas (18 h, 298 K) results in a dis-
tribution of unreacted 1 (39%),
complex 2 (21%), CÀC coupling
product 3 (38%), and a precipi-
tate (presumably [Fe(NCO)2]).
This experiment indicates that
the calculated end-on carbonyl
compounds might well be
formed rapidly as intermediates
towards further cluster degrada-
tion. Indeed, when the reaction
was monitored by IR spectrosco-
py in toluene solution, at least
four new IR bands in the CO
region were observed after
10 min reaction time. The inten-
sity of these bands decreases
with increasing reaction time
giving evidence for the inter-
mediate character of the corre-
sponding CO species (Support-
ing Information). The frequencies
of three of the IR bands with n˜ =
1956, 1980, 1992 cmÀ1 corre-
spond well to those calculated
for the carbonyl adducts 1-COt–
1-COt,t,t,t (scale factor:[28] 0.972,
Figure 4. Energy profile for cluster degradation relative to 1. Relative free energies (DG8) and enthalpies (DH8, in
parentheses) are presented in kcalmolÀ1. Annelated benzo groups of the trop substituents are omitted for clarity.
pared to the large energy cost computed for the alkene disso-
ciation in 1 (DG8=35.5 kcalmolÀ1). Finally with respect to iii),
end-on coordination of another molecule of CO is an exergon-
ic process (DDG8=À15.2 kcalmolÀ1) leading to compound 1-
COt,t at DG8=À31.9 kcalmolÀ1. A CO insertion into a FeÀN
bond of 1-COt,t to give the carbamoyl complex 1-COt,c is with
DDG=4.0 kcalmolÀ1 only weakly endothermic when com-
pared to the comparable insertion reaction in the mono-car-
bonyl adduct 1-COt. After several attempts, we could not suc-
cessfully locate a transition state going from 1-COt,t to 1-COt,c;
however, we predict that the TS free energy barrier will be sig-
nificantly lower (<38.7 kcalmolÀ1) than for TS1t. Incorporating
a third and fourth end-on bound CO ligand to the Fe4N4
cubane is calculated to be thermodynamically favorable for
each addition leading to 1-COt,t,t at DG8=À43.2 kcalmolÀ1 and
1-COt,t,t,t at DG8=À46.7 kcalmolÀ1 (not shown in Figure 4, see
Supporting Information). These computational studies on
a closed-shell (i.e., simplified) model system[26] suggest that
two or more CO molecules coordinate to the Fe4N4 core as ini-
tiating reaction steps. Only after the binding of these p-acidic
CO molecules, the cluster rearrangement (alkene dissociation
and CO insertion) are facilitated which finally lead to fragmen-
tation.[27]
Supporting Information). The presence of an additional band
(n˜ =2015 cmÀ1) indicates that additional intermediates are
long-lived enough to be observed by IR spectroscopy. IR Bands
were also observed in the region expected for carbamoyl spe-
cies, but could not unequivocally be distinguished from bands
arising from the solvent (toluene). Furthermore, weak CO
bands identical with those of compound 2 were also detected.
As expected, their intensity increases with increasing reaction
time. After a reaction time of 1 d, compound 2 is clearly the
dominating carbonyl species in the reaction mixture. Based on
our computational, NMR and IR spectroscopic studies, we sug-
gest that carbonyl species such as 1-COt–1-COt,t,t,t are formed
as intermediates at early stages of the reaction en route to
compound 2. The formation of carbamoyl species as inter-
mediates is well possible, but remains speculative at this point.
Reaction of 1 with TEMPO-radical
Previous electrochemical studies showed that [Fe4(Ntrop)4] (1)
can undergo up to four chemically reversible one-electron re-
ductions at remarkably negative redox potentials down to E0 =
À3.59 V vs. Fc/Fc+.[7] In contrast, compound 1 is not suscepti-
ble to electrochemically reversible oxidations. In order to
obtain hints at possible degradation pathways under oxidative
conditions, 1 was reacted with four equivalents of (2,2,6,6-tet-
Monitoring the reaction between 1 and excess CO by
1H NMR spectroscopy showed that the initial reactions proceed
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Scheme 5. Reaction of [Fe4(Ntrop)4] (1) with (2,2,6,6-tetramethylpiperidin-1-
yl)-oxidanyl (TEMPO) to give 5.
Figure 5. Molecular structure of [Fe4(m2-5H-dibenzo[a,d][7]annulen-5-im-
ido)4(O)2(TEMPO)4] (5). Displacement ellipsoids are shown at the 50% proba-
bility level; carbon atoms are shown as wireframe. Hydrogen atoms and an-
nelated C6H4 groups are omitted for clarity. Selected bond lengths [Š], bond
angles [8] and interatomic distances [Š]: Fe1ÀN1, 2.046(5); Fe1ÀN2, 2.055(5);
Fe1ÀO1, 1.860(5); Fe1ÀO3, 1.780(4); Fe2ÀN1, 2.052(5); Fe2ÀN2, 2.012(5);
Fe2ÀO2, 1.810(6); Fe2ÀO3’, 1.786(4); C1ÀN1, 1.289(7); C16ÀN2, 1.260(7); N1-
Fe1-N2, 86.3(2); N1-Fe1-O1, 133.7(2); Fe1-N1-Fe2, 90.12(19); Fe1-O3-Fe2’,
154.3(2); Fe1···Fe2, 2.9008(12); Fe1···Fe2’, 3.4766(12).
ramethylpiperidin-1-yl)-oxidanyl (TEMPO), which can act as
a one electron oxidant towards molecular iron compounds.[29]
Remarkably, the tetranuclear cluster 5 was isolated from this
reaction as dark brown single crystalline plates, however in
low yield (Scheme 5).
In the course of the reaction, the four (Ntrop)2À ligands have
been oxidized to the corresponding mono-anionic ketiminato-
ligands, that is the benzylic H atom in the 5-position of the
5H-dibenzo[a,d]cyclo-hepten-5-yl unit has been abstracted
under formation of a C=N double bond. Furthermore, TEMPO
served as an oxygen transfer agent under NÀO bond cleavage
and two Fe-O-Fe bridges are observed in 5.[30] In addition, each
Fe center is coordinated by one (TEMPO)À ligand. Accordingly,
the yield of 5 could be increased to 26% by reacting 2 with
a tenfold excess of TEMPO and TEMPOH could be detected as
a by-product of this reaction (see Supporting Information for
idealized redox equation). Single crystals of 5 were used for X-
ray diffraction experiments to determine its structure (mono-
clinic, C2/c, Z=4, Figure 5). Compound 5 shows Ci symmetry
in the solid state. The four iron atoms form a twisted rectangu-
lar array without any direct metal-metal interactions. Each iron
atom is linked to one neighboring Fe center by an oxygen
atom (Fe1-O3-Fe2’, 154.3(2)8) and to another neighboring Fe
center by two m2-ketiminato groups (Fe1-N1-Fe2, 90.12(19)8).
This results in two four-membered Fe2N2 rings annelated to an
eight-membered Fe4O2N2 ring. To our knowledge, this structur-
al motif with two FeIII centers bridged by two m2-ketiminato li-
gands is unprecedented. The Fe-N distances (2.05–2.06 Š) and
Fe-N-Fe angles (90.1–91.08) are within the range observed for
(Fe2(m2-ketiminato)2) groups with Fe in the oxidation state of
+2 (Fe–N, 2.01–2.08 Š; Fe–Fe, 72.8–96.78).[31] The FeÀO3(’) bond
lengths of 1.78–1.79 Š compare well to those found in other
compounds with two FeIII centers linked by an O2À bridge
(1.78–1.80 Š).[32,33] The average Fe1/2–O1/2 and N3/4–O1/2 dis-
tances of 1.835(6) and 1.377(7) Š, respectively, suggest the for-
mation of anionic (TEMPO)À ligands.[34–37]
tion state of 3+ for Fe.[38,39] Magnetic susceptibility measure-
ments of 5 were performed in the temperature range of 2–
300 K at 1 T (Figure 6). At 300 K, the observed value of c·T=
1.36 emuKmolÀ1 is clearly smaller than that expected for four
non-interacting Fe3+ centers in a high spin electron configura-
tion (for g=2: c·Tcalcd =17.51 emuKmolÀ1). When lowering the
temperature, the c·T value decreases indicating antiferromag-
netic interactions between the paramagnetic iron centers. At
temperatures of about 50 K or lower, the sample shows dia-
magnetic behavior. A simulation[40] of the data with four high
spin Fe3+ centers revealed two pairs of antiferromagnetic inter-
actions with JFeOFe =À107.8 cmÀ1 (Fe1···Fe2’ and Fe1’···Fe2 in
Figure 5) and JFeNFe =À36.3 cmÀ1 (Fe1···Fe2 and Fe1’···Fe2’ in
Figure 5).[41] The stronger antiferromagnetic coupling was as-
signed to the two Fe-O-Fe units as this value is typical for two
magnetically coupled oxo-bridged FeIII centers.[32a,39] The less
negative J value was thus assigned to the two Fe(m2-ketimina-
to)2Fe units. There are no comparable molecules known, but
similar binding motifs were observed with two FeII ions or one
FeII and one FeIII ion. In these known compounds, the magnetic
interactions between the two metal centers range from weakly
The zero field 57Fe Mçssbauer spectrum of 5 at 77 K showed
one quadrupole doublet confirming the equivalence of the
four Fe atoms per molecule. The Mçssbauer parameters d=
0.38(1) mmsÀ1 and jDEQ j =1.58(1) mmsÀ1 did not allow an un-
ambiguous assignment of the spin state, but support an oxida-
Figure 6. Temperature dependence of c·T for compound 5 in the solid state.
Fit of experimental data with four high spin Fe3+ centers gave
J1 =J2 =À107.8 cmÀ1, J3 =J4 =À36.3 cmÀ1, g1 =g2 =g3 =g4 =2.07.
Chem. Eur. J. 2015, 21, 15797 – 15805
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1
shifts are reported relative to SiMe4 using the residual H and 13C
ferromagnetic (J=2.24 cmÀ1) to strongly antiferromagnetic (J=
À235 cmÀ1).[31a,d] The poor solubility of 5 in common organic
solvents such as benzene, THF, acetonitrile or DMSO hampered
a solution NMR spectroscopic characterization.[42]
chemical shifts of the solvent as a secondary standard. Infrared
spectra were collected on a Perkin-Elmer-Spectrum 2000 FT-IR-
Raman spectrometer. UV/Vis spectra were recorded on UV/vis/NIR
lambda-19-spectrometer in a cell with a 0.5 cm path length. Ele-
mental analyses were performed at the Mikrolabor of ETH Zürich.
Single crystals suitable for X-ray diffraction were coated with poly-
isobutylene oil in a dry-box, transferred to a nylon loop and then
transferred to the goniometer of an Oxford XCalibur, a Bruker
SMART APEX or a Bruker APEX II diffractometer equipped with
a molybdenum X-ray tube (l=0.71073 Š). The structures were
solved using direct methods (SHELXS) completed by Fourier syn-
thesis and refined by full-matrix least-squares procedures. 57Fe
Mçssbauer spectra were recorded on a WissEl Mçssbauer spec-
trometer (MRG-500) at 77 K in constant acceleration mode. 57Co/Rh
was used as the radiation source. WinNormos for Igor Pro software
has been used for the quantitative evaluation of the spectral pa-
rameters (least-squares fitting to Lorentzian peaks). The minimum
experimental line widths were 0.20 mmsÀ1. The temperature of the
samples was controlled by an MBBC-HE0106 MÖSSBAUER He/N2
cryostat within an accuracy of Æ0.3 K. Isomer shifts were deter-
mined relative to a-iron at 298 K. Magnetic susceptibility measure-
ments were carried out with a superconducting quantum interfer-
ence device (SQUID) magnetometer (Quantum Design, MPMS-5S)
in the temperature range of 2–300 K in a field of 1 T. Both field-
cooled (FC) and zero-field-cooled (ZFC) data were obtained. The
data were corrected for diamagnetic contributions using Pascal’s
constants.
Conclusion
In summary, the reactivity of the all-ferrous heterocubane
[Fe4(Ntrop)4] (1) with Brønsted acids, H2, s-donors, p-acceptors,
and a stable O-radical was investigated with first experiments
(trop=5H-dibenzo[a,d]cyclo-hepten-5-yl). Compound 1 under-
goes initial degradation in the presence of four equivalents of
Brønsted acids, but re-assembles with concomitant formation
of H2Ntrop. No reactivity of 1 was observed towards H2 or
common s-donor ligands such as THF or pyridine. Also, bulky
s-donor–p-acceptor ligands such as PPh3, P(OMe)3, or 1,5-
cyclo-octadiene fail to react with 1. These results underline the
remarkable stability and robustness of the Fe4N4 heterocubane
structural motif in compound 1. With CO as a two electron s-
donor/p-acceptor that has a low steric profile, 1 undergoes an
unusual reductive cluster degradation to give trinuclear
[Fe3(m3-Ntrop)2(CO)6] (2) along with the by-products [trop2] (3)
and [Fe(NCO)2], which was isolated as its pyridine adduct. This
reaction includes both metal- and ligand-centered redox
events. Combined DFT computational, NMR- and IR spectro-
scopic studies suggest that the iron centers in 1 are unsaturat-
ed and each easily accepts one CO ligand prior to cluster rear-
rangements which likely precede the fragmentation reactions.
Compound 2 is a rare example of a trinuclear bis-(m3-imido)
complex with a 48 valence electron count. It clearly differs
from the more common 50 electron species with respect to
(metal–metal-) bonding and, importantly, its electrochemical
properties (specifically, the enhanced stability of the radical
CCDC 1407333 (2), 1407334 (3), 1407335 (4), and 1407336 (5) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre.
DFT calculations were performed using Gaussian 09[44] at the
M06L[45]/TZVP[46]/TZVPfit level of theory. Either normal (opt) or tight
(opt=tight) convergence criteria were used for all optimizations,
and a pruned (99590) integration grid was used throughout
(grid=ultrafine). Optimizations were performed in a toluene sol-
vent continuum using the integral equation formalism polarizable
continuum model (IEF-PCM)[47] with radii and non-electrostatic
terms from the SMD solvation model (scrf=smd).[48] Full vibrational
and thermochemical analyses (1 atm, 298 K) were performed on
optimized structures to obtain solvent-corrected free energies (G8)
and enthalpies (H8). Optimized ground states were found to have
zero imaginary frequencies while transition states had one imagi-
nary frequency. Open-shell structures were not considered in this
study. Calculations were performed by the facilities of the Shared
Hierarchical Academic Research Computing Network (SHARC-
NET)[49] and Compute/Calcul Canada. Three-dimensional visualiza-
tions of calculated structures were generated by ChemCraft.[50]
CÀ
anion (2) ). In agreement with the reluctance of 1 to undergo
chemically reversible one-electron oxidations, reaction of
1 with one-electron oxidant TEMPO led to an oxidative cluster
degradation. This reaction includes ligand and metal oxidation
(along with NÀO bond cleavage of TEMPO) to give a tetranu-
clear FeIII cluster with a diamagnetic ground state, which was
isolated and fully characterized.
Experimental Section
General considerations: All air- and moisture-sensitive manipula-
tions were carried out using standard vacuum line Schlenk tech-
niques or in an MBraun inert atmosphere dry-box containing an at-
mosphere of purified argon. Toluene, hexanes, and THF were de-
gassed and purified using an Innovative Technologies PureSolv
system. C6D6 and MTBE were distilled before use from sodium ben-
zophenone ketyl; 1,1-diphenylethylene, 1,5-cyclooctadiene, and
pyridine were distilled before use from CaH2. Benzophenone was
sublimed prior to use. P(OMe)3 was distilled from Na prior to use.
EtOH was dried by addition of Na, distilled and degassed prior to
use. TEMPO, P(4-C6H5F)3, PMe3 (1m in THF) were obtained from
Sigma Aldrich or ABCR and used as received. [Fe4(Ntrop)4] and
[HNEt3][BPh4] were synthesized according to the literature.[7,43] NMR
spectra were recorded on Bruker instruments operating at 200,
If not otherwise noted, 2 was isolated with one molecule of tolu-
ene in the lattice, which can be removed in vacuo.
Reaction of 1 with H2, Brønsted acids, s-donors, and s-donors/
p-acceptors: Typical reactions were carried out on the 10 mmol
scale with respect to 1. The requested amount of reagent was
1
added to a solution of 1 in C6D6 (0.5 mL) and analyzed by H NMR
spectroscopy (and 31P and 19F NMR spectroscopy where applicable).
Two freeze/pump/thaw cycles (À788C) were performed for reac-
tions with H2.
The reaction of 1 with four equiv of [HNEt3][BPh4] was also carried
out on the 40 mmol scale (with respect to 1) in THF. The reaction
mixture was filtered. The filtrate was layered with hexanes at
À308C to give single crystalline 1 after several days (30% yield
based on maximum theoretical yield of 20 mmol). The solid residue
1
1
250, 300, or 500 MHz with respect to H. H and 13C NMR chemical
Chem. Eur. J. 2015, 21, 15797 – 15805
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
was dissolved in pyridine and layered with MTBE at ambient tem-
perature to give single crystalline [Fe(NC5H5)6][BPh4]2 after 1 d.
(w), 994 (w), 948 (w), 928 (m), 884 (w), 825 (m), 797 (s), 780 (s), 767
(s), 717 (m), 702 (m) cmÀ1; zero-field 57Fe Mçssbauer (d, jDEQ j
(mmsÀ1)): (77 K): 0.38(1), 1.58(1) (GFWHM =0.33(1) mmsÀ1); UV/VIS
(THF): lmax =215, 227, 283 nm; elemental analysis (%) calcd for
C96H112N8O6Fe4 ”(C7H8) (1789.52 gmolÀ1): C 69.13, H 6.76, N 6.26;
found: C 69.11, H 6.76, N 6.04.
Reaction of [Fe4(Ntrop)4] (1) with CO: A solution of [Fe4(Ntrop)4]
(1) (65 mg, 62 mmol) in toluene (2 mL) was degassed in two freeze/
pump/thaw cycles and exposed to an atmosphere of carbon mon-
oxide (1 bar) at À1968C. The reaction mixture was warmed to
room temperature by removing the liquid nitrogen bath and
stirred for 2 h. The reaction mixture was filtered and layered with
hexanes (8 mL). After 1 d a colorless precipitate was filtered off and
the filtrate (filtrate 1) stored at À308C. The residue was extracted
with pyridine (2 mL), the resulting suspension was filtered, and the
filtrate (filtrate 2) stored at À308C. After 1 d 4 had precipitated
from filtrate 2 as a pale yellow solid, which was isolated by decant-
ation and dried in stream of argon. Yield: 7 mg, 11 mmol, 18%
(based on [Fe4(Ntrop)4]).
When the reaction was carried out on NMR scale (10 mg 1, 15 mg
TEMPO) in C6D6 (0.6 mL), TEMPOH could be detected in the liquid
phase of the reaction mixture after 45 min.[51]
Upon addition of dry, degassed ethanol (20 mL) to a suspension of
1
5 (3 mg) in [D6]DMSO (0.5 mL), a suspension was obtained. H NMR
spectroscopic analysis of the liquid part revealed resonances in
agreement with those expected for 5H-dibenzo[a,d][7]annulen-5-
imine.[52] 1H NMR (300 MHz, [D6]DMSO): d=7.01 (s, 2H, 10,11-H),
7.36–7.52 (m, 7H, 1,2,3,6,7,8,9-H), 7.74–7.76 (m, 1H, 4-H), ppm
10.46 (s, NH). Resonances for toluene and EtOH were also detect-
ed.
After 3 d, black crystals of 2 had precipitated from filtrate 1 along
with small amounts of colorless single crystalline [trop2] (3). The
crystalline material was isolated by decantation, separated manual-
ly and dried in
a
stream of argon each. Yield for
Acknowledgements
[Fe3(Ntrop)2(CO)6]·(toluene) (2·(toluene)): 20 mg, 24 mmol, 39%
(based on [Fe(Ntrop)4]). Yield for [trop2] (3): 2 mg, 5 mmol, 8%
(based on [Fe(Ntrop)4]).
C.L. is grateful for a Feodor Lynen fellowship generously
1
hosted by Prof. FranÅois Diederich.
[Fe3(Ntrop)2(CO)6] (2): m.p. >2208C; H NMR (300 MHz, C6D6): d=
3
6.18 (s, 2H, 5-H), 6.83 (t, 4H, JHH =7.3 Hz, 3,7-H), 6.90 (s, 4H, 10,11-
3
3
H), 6.90 (t, JHH =7.5 Hz, 4H, 2,8-H), 7.07 (d, 4H, JHH =7.5 Hz, 1,9-H),
7.33 ppm (d, 4H, 3JHH =7.3 Hz, 4,6-H); 13C NMR (100 MHz, C6D6):
d=87.50 (s, 5-C), 89.42 (s, 10,11-C), 124.97 (s, 1,9-C), 125.95 (s, 4,6-
C), 126.92 (s, 2,8-C), 128.58 (s, 3,7-C), 138.27 (s, 4a,6a-C), 144.96 (s,
9a,11a-C), 205.86 ppm (s, CO); resonances for toluene and trace
amounts of [trop2] were also detected; ATR IR: n˜ =3050 (w), 3015
(w), 2923 (w), 2885 (w), 2062 (m), 2007 (s), 1978 (s), 1809 (m), 1597
(m), 1577 (w), 1484 (m), 1461 (m), 1393 (m), 1381 (m), 1300 (m),
1256 (m), 1204 (m), 1160 (m), 1123 (m), 1041 (w), 968 (w), 943 (m),
902 (m), 883 (m), 863 (m), 802 (m), 763 (s), 732 (s), 708 (s) cmÀ1; IR
(toluene): n˜ =2003 (m), 2035 (m), 2066 (m) cmÀ1; UV/VIS (THF):
lmax =225, 254, 357, 437 nm; elemental analysis (%) calcd for
C36H22N2O6Fe3 ”(C7H8) (838.26 gmolÀ1): C 61.61, H 3.61, N 3.34;
found: C 61.37, H 3.39, N 3.30; zero-field 57Fe Mçssbauer (d, jDEQ j
Keywords: cluster compounds
oxidation · reduction
· heterocubanes · iron ·
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(mmsÀ1)): (77 K) component 1 (38%): 0.35(1), 2.47(1) (GFWHM
0.27(1) mmsÀ1); component 2 (62%): 0.00(1), 0.79(1) (GFWHM
0.32(1) mmsÀ1). meff =0mB (Evans’ method).
=
=
[trop2] (3):[14] 1H NMR (300 MHz, C6D6): d=4.47 (s, 2H, 5-H), 6.67
3
3
(d, 4H, JHH =7.4 Hz, (1,9/4,6)-H), 6.87 (t, 4H, JHH =7.4 Hz, (2,8/3,7)-
H), 7.05 (t, 4H, 3JHH =7.5 Hz, (3,7/2,8)-H), 7.30 ppm (d, 4H, JHH
=
3
7.5 Hz, (4,6/1,9)-H).
{[Fe(NCO)2(NC5H5)4]·2(NC5H5)} (4):[15] ATR IR: n˜ =3031 (w), 3956 (w),
2912 (w), 2192 (w), 1677 (m), 1594 (m), 1546 (w), 1481 (m), 1439
(m), 1383 (w), 1327 (w), 1212 (m), 1146 (w), 1066 (m), 1034 (m),
1005 (w), 991 (w), 806 (m), 756 (s), 745 (m), 701 (s) cmÀ1; ESI-MS
[8] Compound 1 did not react with 4 equiv of PMe3 within 1 h as evi-
denced by 1H and 31P NMR spectroscopy. After prolonged reaction
times of >1 d, small amounts of a dark precipitate had formed, the
composition of which remains unclear.
(MeOH,
negative
mode):
m/z:
519.12
[Fe(N-
CO)3(NC5H5)3(MeOH)2(H2O)2]À.
[Fe4(m2-5H-dibenzo[a,d][7]annulen-5-imido)4(TEMPO)4(m2-O)2] (5):
TEMPO (78 mg, 499 mmol) was added to a solution of [Fe4(Ntrop)4]
(52 mg, 50 mmol) in toluene (2 mL). The reaction mixture was fil-
tered after 2 h and left undisturbed for 5 d, after which dark brown
crystals had formed. The supernatant was decanted. The crystals
were washed with toluene (3”2 mL) and dried in vacuo. Even after
prolonged drying in vacuo, the bulk material contains one equiv of
toluene per formula unit. Yield: 23 mg, 13 mmol, 26%; m.p.
>2208C; ATR IR: n˜ =3062 (w), 3015 (w), 2923 (w), 1600 (m), 1584
(m), 1556 (w), 1447 (w), 1434 (w), 1408 (w), 1370 (m), 1356 (m),
1347 (m), 1292 (m), 1237 (m), 1177 (m), 1130 (w), 1097 (w), 1054
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macher, Organometallics 2015, 34, 3079–3089.
[10] Independent experiments showed, that the solubility of [trop2] in ben-
zene or toluene is low. [Fe(NCO)2] (not as its pyridine adduct) would
1
not be detectable by H NMR spectroscopy.
[11] Previous syntheses of [Fe3(m3-NR)2(CO)9] were achieved by reaction of
[Fe3(CO)12] with azides,[17a] nitro-,[17c] nitroso-,[17f] and azocompounds[17g]
or by thermolysis of [Fe3(CO)9(m3-h2-R2N2)].[17j]
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15804
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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´
[30] For examples of thermally induced NÀO bond cleavage of (TEMPO)À in
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Received: June 29, 2015
Published online on September 16, 2015
Chem. Eur. J. 2015, 21, 15797 – 15805
www.chemeurj.org
15805
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