Synthesis and Catalysis of Redox-Active Bis(imino)acenaphthene (BIAN) Iron Complexes

Article abstract of DOI:10.1002/cctc.201700144

Reactions of various substituted bis(imino)acenaphthenes (R-BIANs) with FeCl₂(thf)_1.5 afforded the tetrahedral complexes (R-BIAN)FeCl₂ (2) from bulky α-diimines and the octahedral complexes [Fe(R-BIAN)₃][FeCl₄]₂ (3) from less bulky ligands. The driving force for the formation of complexes 3 is the high ligand-field stabilization of the low-spin Fe^II center. The two sets of complexes exhibit distinct charge-transfer band intensities and redox activities. (R-BIAN)FeCl₂ complexes showed reversible ligand-centered reductions at ?0.9 V (vs. FcH/FcH⁺; FcH: ferrocene); further reduction led to decomposition. Irreversible oxidations were observed at 0.2 and 0.4 V, associated with a reduction at ?0.4 V, as well as a ligand-centered redox event at 1.0 V. First applications of the Fe(BIAN) complexes to hydrogenations of alkenes indicated good catalytic activity under mild conditions.

Full text of DOI:10.1002/cctc.201700144

Accepted Article
Title: Synthesis and Catalysis of Redox-active Bis(imino)acenaphthene
(BIAN) Iron Complexes
Authors: Axel Jacobi von Wangelin, Dieter Schaarschmidt, Matteo
Villa, Dominique Miesel, Alexander Hildebrandt, and Fabio
Ragaini
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ChemCatChem
10.1002/cctc.201700144
FULL PAPER
Synthesis and Catalysis of Redox-active Bis(imino)acenaphthene
BIAN) Iron Complexes
(
[
a]
[b]
[b]
[c]
[a]†
Matteo Villa, Dominique Miesel, Alexander Hildebrandt, Fabio Ragaini, Dieter Schaarschmidt,*
[
a]
and Axel Jacobi von Wangelin*
th
Dedicated to Prof. Jürgen Heck on the occasion of his 66 birthday.
[
8]
Abstract: Reactions of various substituted bis(imino)acenaphthenes
polymerizations. The key characteristic of PDI ligands is their
ability to reversibly exchange electrons with the coordinated
metal. Elementary steps within the catalytic cycle involving the
transfer of electrons between the metal complex and the
substrate are facilitated by such ligand participation as
uncommon oxidation states at the metal atom can be avoided.
Although bis(imino)acenaphthenes (BIANs) have been known
for more than 50 years, they have received much less attention
as redox-active ligands in coordination chemistry and
(
R-BIAN) with FeCl
R-BIAN)FeCl (2) from bulky α-diimines and the octahedral
][FeCl (3) from less bulky ligands. The
2
(thf)1.5 afforded the tetrahedral complexes
(
2
complexes [Fe(R-BIAN)
3
4 2
]
driving force of the formation of complexes 3 is the high ligand-field
stabilization of the low-spin Fe(II). The two sets of complexes exhibit
distinct CT band intensities and redox activities. (R-BIAN)FeCl
complexes showed reversible ligand-centered reductions at –0.9 V
2
+
(vs. FcH/FcH ); further reduction led to decomposition. Irreversible
[
9],[10]
oxidations were observed at 0.2 and 0.4 V associated with a
reduction at –0.4 V as well as a ligand-centered redox event at 1.0 V.
First applications of the Fe(BIAN) complexes to hydrogenations of
alkenes documented good catalytic activity under mild conditions.
catalysis.
Introduction
The past years have witnessed an increasing interest in the
synthesis of well-defined iron complexes and catalysts. These
developments have mainly been driven by sustainability criteria
Figure 1. Generic structures of important diimine ligands: bis(imino)pyridine
PDI), 1,4-diazabutadiene (DAB), bis(imino)acenaphthene (BIAN).
[
1]
(
(
low price, low toxicity) and a lack of available complexes and
[2]
The rigidity of the acenaphthene backbone forces these
molecules to adopt an s-cis conformation which facilitates the
formation of stable metal complexes. The stereoelectronic
properties of BIANs can easily be tuned by the incorporation of
substituted primary amines during imine formation. Such ligands
have been reported to efficiently coordinate almost all main-
mechanistic insight. Besides iron nanoparticles and various
precursors forming such particles under the reaction
conditions, a broad range of well-defined iron complexes have
been applied in various catalytic transformations. Very recently,
non-innocent, redox-active ligands have complemented the
[
3]
[
4]
earlier examples of phosphine and N-heterocyclic carbene
complexes and have tremendously enriched the landscape of
[
10]
[11]
[
5]
group elements
extensively studied in olefin polymerizations
shown to be active in many other catalytic transformations.
Surprisingly, only very few examples of iron complexes with
BIAN ligands have been published.
and transition metals.
BIANs have been
[
12]
[
6]
and have been
coordination chemistry and catalytic applications. One of the
most prominent classes of redox-active ligands are diimines of
which several classes of complexes and catalysts have been
reported. α-Diimine iron complexes with the simplest ligands of
this class, 1,4-diazabutadiene (DAB), have been studied since
[
13]
[
14]
Most of these reports
involve modification of the general ligand structure by a pendant
[
15]
[
7]
donor arm with the aim of mimicking the PDI behavior.
the 1970s. Bis(imino)pyridine (PDI) ligands were successfully
applied by Brookhart et al. and Gibson et al. to olefin
Recently, Fe complexes with sterically hindered dipp
2
BIAN
BIAN
(
(
dipp=Ar=2,6-diisopropylphenyl, see Figure 1) and mes
2
mes=Ar=mesityl, see Figure 1) have been successfully applied
[
16]
[
a]
M. Villa, Prof. A. Jacobi von Wangelin
as precatalysts to hydrosilylations of carbonyl compounds and
Institute of Organic Chemistry, University of Regensburg
Universitaetsstr. 31, 93040 Regensburg, Germany
E-mail: axel.jacobi@ur.de
[17]
olefins.
These studies documented only moderate activity of
the complexes and involved no full electrochemical
characterization of the complexes despite the strongly reducing
reaction conditions and the postulation of an active catalyst
species in lower oxidation states.
In an effort to enhance the knowledge of well-defined Fe
complexes of the BIAN ligand family (1), we herein report the
[
[
b]
c]
Dr. D. Miesel, Dr. A. Hildebrandt
Institute of Chemistry - Inorganic Chemistry, TU Chemnitz, Germany
Prof. F. Ragaini
Department of Chemistry, University of Milan, Italy
Dr. D. Schaarschmidt
Current address: Department of Chemistry, University of Hamburg
Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
E-mail: dieter.schaarschmidt@chemie.uni-hamburg.de
†
synthesis of several high-spin ((R-BIAN)FeCl
complexes ([Fe(R-BIAN) ][FeCl , 3, and [Fe(R-BIAN)
and document their structural, optoelectronic,
electrochemical properties.
2
, 2) and low-spin
][BF , 4)
and
3
4
]
2
3
4 2
]
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201700144
FULL PAPER
Results and Discussion
toluene gave slightly green filtrates for complexes 2 and orange-
to-red filtrates for 3. The synthesis of the [Fe(R-BIAN)
3 4 2
][FeCl ]
Synthesis. The reaction of equimolar amounts of FeCl
2
(thf)1.5
complexes 3 from FeCl (thf)1.5 is stoichiometrically unbalanced
2
with bis(imino)acenaphthenes (1a–g) in toluene at 100 °C
resulted in the formation of isolable iron complexes for all cases
studied. However, the nature of the N-aryl substituents of the
BIANs had a crucial role on the composition of resultant
Fe(BIAN) complexes. BIANs containing at least one ortho-
substituent in the N-aryl groups gave tetrahedral 1:1 complexes
regarding chloride anions and electrons. Assuming strict
exclusion of air during the preparation, the oxidation of Fe(II) to
Fe(III) must be accompanied by reduction of BIAN which is also
documented by the color of the toluene filtrates of 3. The
synthesis of derivative 3f was also performed from FeCl
2
(thf)1.5,
FeCl and 1f in a 1:2:3 ratio, which increased the yield of the Fe
3
2
; the less bulky phenyl- and 4-tolyl-BIAN derivatives gave the
octahedral 1:3 complexes (Scheme 1). Both series of
complexes could clearly be distinguished by mass spectrometry
complex from 75% to 87 % and gave a more accurate elemental
analysis.
3
The number of transition metal complexes in which the metal is
+
2+
[14c],[19]
(
2: [M] ; 3: [M] ) and UV/Vis spectroscopy (vide infra).
coordinated by three BIAN ligands is quite limited
and
Complexes 2 and 3 were green solids forming green solutions in
acetonitrile. They are soluble in polar organic solvents, such as
tetrahydrofuran or dichloromethane, and to a lesser extent in
toluene, but are insoluble in hydrocarbons. In general, the 1:3
complexes exhibited higher solubility. The tetrahedral iron
most of these examples are best described with at least one
ligand being reduced to a radical anion. To our knowledge,
[
19d]
[14c]
[Cr((3,5-Xyl)
2
BIAN)
3
][PF
6
]
3
2 3 3 2
and [Fe(H BIAN) ][FeBr (thf)]
are the only exclusions. The latter example shows obviously
some similarities to compounds 3; in both cases equimolar
amounts of an Fe(II) salt and a BIAN were reacted. However,
the ferrate anions in 3 contain Fe(III), which might be caused by
the use of iron dichloride instead of iron dibromide, the
application of toluene instead of THF or the different
[
18]
complexes 2 were sensitive toward oxidation
and hydrolysis.
The addition of water to an acetonitrile solution of 2a (Ar = dipp)
instantly caused a color change from green to yellow consistent
with the formation of the free BIAN ligand 1a. In contrast, the
octahedral complexes
3
were less sensitive: exposure of
electrochemical behavior of 1f and 1g and H
2
BIAN.
solutions thereof to aerobic conditions did not result in visible
changes of the appearances over a couple of hours.
The presence of the paramagnetic and redox-active counterion
–
[FeCl
by NMR and cyclic voltammetry (CV). Therefore, complexes 4f-g
were prepared from [Fe(H O) ][BF in acetonitrile (Scheme 2).
4
] in complexes 3f–g resulted in difficult characterizations
2
6
4 2
]
Scheme 2. Synthesis of Fe(BIAN) complexes 4f and 4g.
Characterizations. For comparison of spectroscopic data of the
ligand-derived complexes, selected Zn complexes (5a,f,g) were
synthesized by reaction of [Zn(H
2
O)
6
][BF
4
]
2
with 1f,g in
acetonitrile and, according to a literature procedure, by reaction
[
20]
2
of ZnCl with 1a in glacial acetic acid.
Scheme 1. Ligands 1a–g and synthesis of Fe(BIAN) complexes 2a–e, 3f–g.
UV/Vis spectra of the Fe and Zn complexes and the free ligands
were recorded in acetonitrile solution at room temperature. The
free R-BIANs 1a–g exhibited intense absorptions in the UV
region and a broad absorption at around 400 nm which are
commonly assigned to π-π* transitions of the aryl substituents
and the acenaphthene backbone and to an intra-ligand charge
The conclusion whether tetrahedral (2) or octahedral (3)
complexes were formed could also be drawn from the presence
of unreacted BIAN or by-products thereof in the crude reaction
mixture. Purification of the crude mixtures by washing with
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ChemCatChem
10.1002/cctc.201700144
FULL PAPER
[
21]
transfer, respectively.
These bands are slightly shifted in the
independent molecules were found in the asymmetric units. The
Fe and Zn complexes (Figure 2, Table S1). Additionally, all Fe
complexes exhibited charge transfer (CT) absorptions in the
range of 550 and 800 nm. The intensity of these absorptions
strongly depends on the coordination geometry and serves as
an excellent probe to distinguish tetrahedral (molar absorptivity
metal ions are coordinated in a distorted tetrahedral N
2 2
Cl or
distorted octahedral N geometry. The distortion is caused by
6
the rigidity of the planar BIAN ligands which results in N–M–N′
bond angles (N, N′ of the same BIAN moiety) between 77 and
82 ° depending on the M–N distance. For the Zn and the
tetrahedral Fe complexes, M–N bond lengths between 2.11 and
2.18 Å were found. These separations are 1.99–2.00 Å in the
octahedral complexes 3f,g and 4g which clearly indicates the
low-spin state of Fe(II). The Fe–Cl separation in the counterions
–
1
–1
-1
–1
150–500 M ∙cm ) and octahedral (6200–14100 M ∙cm )
Fe(BIAN) complexes. Complex 2a exhibited two weak
absorptions (ligand field bands) in the near infrared at 1435 and
–
1
–1
1
830 nm with extinction coefficients of 8 and 16 M ∙cm ,
–
[23]
respectively, which is in good agreement with other tetrahedral
of 3f and 3g is typical for [FeCl
4
] .
The acenaphthene
[
22]
Fe(II)N
2
Cl
2
complexes (Figure S6).
backbone is essentially planar; the N-aryl substituents are
orthogonal to this plane. The bond lengths of the N=C–C=N
moiety are indicative of a C–C single bond and two C=N double
2
2
1
1
5
1a
bonds, which is in full agreement with the formulation of a
0
.8
[24],[25]
2
5
a
a
neutral 1,2-diimine ligand.
It is noteworthy that the
octahedral Fe complexes 3 and 4 contain slightly shortened C–C
bonds whereas the C=N bonds are slightly longer. This deviation
does not necessarily indicate a partial reduction of the BIAN
ligands but could be a consequence of an increased metal-to-
ligand backbonding. Comparison of Zn complex 5a with the
0
0.6
0
0
0
.4
.2
.0
5
0
5
0
[
16]
[26]
corresponding Fe(II) complex
that they form an isomorphous family. The complexes 2b,c and
g exhibit in the solid-state parallel displaced π∙∙∙π interactions
and Co(II) complex
shows
4
00 450 500 550 600 650 700 750 800
3
between the N-aryl substituents, the acenaphthene backbone
and co-crystallized toluene molecules. Graphical representations
along with geometrical data can be found in the ESI.
2
00
300
400
500
600
700
800
Wavelength / nm
4
3
2
1
0
0
0
0
0
1f
4f
5f
Figure 3. ORTEP diagram (50% probability level) of the molecular structure of
2b with the atom-numbering scheme. All H atoms and one molecule n-
pentane are omitted for clarity. Symmetry code: A: –x, y, –z+0.5. Selected
bond lengths (Å), angles (°): C7–C7A 1.521(5), C7–N1 1.274(3), C8–N1
3
00
400
500
600
700
800
1.444(3), Fe1–Cl1 2.2217(7), Fe1–N1 2.1368(19); C7–N1–C8 119.09(19),
Wavelength / nm
C7–N1–Fe1 112.92(16), C8–N1–Fe1 112.92(16), N1–Fe1–Cl1 113.65(6), N1–
Fe1–Cl1A 113.86(5), N1–Fe1–N1A 78.64(10), Cl1–Fe1–Cl1A 117.24(5).
Figure 2. Top: UV/Vis spectra of 1a (black), 2a (Fe, blue), and 5a (Zn, red) in
–
4
–3
acetonitrile at 10 M (inset: 2a and 5a at 10 M). Bottom: UV/Vis spectra of 1f
(
–5
–4
black), 4f (Fe, blue), and 5f (Zn, red) in acetonitrile at 10 –10 M.
Selected Fe and Zn complexes were analyzed by single crystal
X-ray diffraction. The molecular structures and important bond
lengths (Å) and bond angles (°) are given in Figures 3–4 (2b,c)
and S10,11,13–15 (3f,g 4g, 5a,f). The crystal and structure
refinement data are summarized in Tables S3,4. The
asymmetric units of 2b and 5f contain only half of the molecule
as the Fe compound and the Zn cation lie at a site of
2
crystallographic C symmetry. Contrary, for 3g and 5a two
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and will therefore be discussed on the example of 2a (Table 1).
Fe complex 2a showed a reversible reduction at –0.91 V which
is also present in the Zn complex 5a at slightly more negative
potential (Figure 5). This corresponds to the reduction of the
neutral BIAN to the radical anion state. Upon further decrease of
potential, additional cathodic waves were observed (e.g. BIAN
reduction to dianion, reduction of Fe(II)). However, the following
redox events gave complex CVs which were indicative of rapid
decomposition of the analyte. An increased potential range to
+1.5 V afforded irreversible oxidations at 0.20, 0.42 and 0.99 V
and an irreversible reduction at –0.36 V. The reduction event
also occurred when the potential was raised to +0.7 V (Figure 5,
bottom, dashed line). A comparison with the respective Zn
compound showed that solely the oxidation at 0.99 V is ligand-
centered (Figure 5, bottom).
Figure 4. ORTEP diagram (50% probability level) of the molecular structure of
2c with the atom-numbering scheme. All H atoms and one molecule toluene
are omitted for clarity. Selected bond lengths (Å), angles (°): C11–C12
1
1
.510(2), C11–N1 1.284(2), C12–N2 1.281(2), C13–N1 1.4350(19), C23–N2
.4378(19), Fe1–Cl1 2.2283(5), Fe1–Cl2 2.2182(5), Fe1–N1 2.1422(13), Fe1–
N2 2.1251(12); C11–N1–C13 119.69(13), C11–N1–Fe1 111.83(10), C13–N1–
Fe1 127.60(9), C12–N2–C23 120.82(13), C12–N2–Fe1 112.78(10), C23–N2–
Fe1 125.92(10), N1–Fe1–N2 78.41(5), N1–Fe1–Cl1 104.18(4), N1–Fe1–Cl2
1
22.51(4), N2–Fe1–Cl1 112.62(4), N2–Fe1–Cl2 113.81(4), Cl1–Fe1–Cl2
Table 1. Cyclic voltammetry data of 2a,c,d and 5a.
118.635(18).
o
E ′ (ΔE ) / mV
1
p
E
pa1 / mV
E
pa2 / mV
E
pa3 / mV
E
pc / mV
2
a
–910 (87)
195
185
160
–
420
410
380
–
990
1015
n.d.
–360
–385
–355
–
The presence of the octahedral Fe complexes in a low-spin state
explains the different reactivity of FeCl and ZnCl toward BIANs
2c
–920 (78)
–925 (76)
–995 (88)
2
2
bearing only H atoms in the 2- and 6-positions of the N-aryl
substituents. The thermodynamic driving force of the formation
2d
of Fe(BIAN)
3
complexes is most likely due to a significant gain in
with
5a
1010
ligand field stabilization. Consequently, the reaction of ZnCl
2
[
21c]
1
g gave the tetrahedral 1:1 complex.
n.d.: not determined. Epa: anodic peak potential. Epc: cathodic peak potential.
1
13
1
H and C{ H} NMR spectra were recorded of the octahedral Fe
o
1
The difference between E ′ of 2a,c,d and 5a of 70–85 mV
and Zn complexes 4f,g and 5f,g. Unlike in the free BIAN ligands
documented that the BIAN ligand in the Fe complexes is slightly
less electron-rich than in the Zn complex. A more pronounced
metal-to-ligand back bonding in the Zn complex or a stronger π-
donation of the BIAN ligands toward Fe would in principle be in
agreement with the observation. Though, the inspection of the
C–C and C=N bond lengths of the N=C–C=N moiety, which are
affected by both interactions, showed no significant differences
1
f,g, two sets of resonances were observed for the ortho/ortho′
and meta/meta′ positions of the aryl substituents, which are also
significantly broader than the remaining resonances of the
acenapthene backbone. This clearly indicates that free rotation
of the N-aryl substituents is hindered in Fe and Zn complexes.
The longer M–N distances of 5f,g indicate a smaller rotation
barrier and thus give broader H resonances of the ortho and
meta protons in comparison with the respective Fe complexes.
The C{ H} NMR spectra confirmed the presence of (up to) 13
different carbon atoms in the aromatic region. The magnetic
1
(
vide supra). It is instructive to note that Krüger and co-workers
recorded a CV of the octahedral Fe(II) complex of 1f with N,N′-
dimethyl-2,11-diaza[3.3](2,6)pyridinophane that showed
1
3
1
reversible BIAN reduction to the radical anion (–1.01 V) and
dianion (–1.52 V) and reversible Fe(II)→Fe(III) oxidation
moments of 2a–e in thf-d
(Evans NMR method), which indicates the presence of high-
spin Fe(II) centers.
8
solutions were between 4.9 and 5.3
µ
B
+
[14d]
(
+0.73 V vs. FcH/FcH ).
The striking difference to the
behavior of the tetrahedral complexes may result from the
degree of anion coordination to the metal. The reduction of the
BIAN ligands facilitates the cleavage of coordinated chloride
anions forming highly reactive tri- or di-coordinated metal
species in solution. Due to the relatively low concentration of the
analyte, a stabilization of these species by dimerization is rather
The redox chemistry of bis(imino)acenaphthenes has
extensively been studied in the past. The cyclic voltammograms
(
CVs) are strongly dependent on the electronic nature of the N-
aryl substituents; however, the common features are two
separate reductions to the monoanionic and dianionic forms and
[21b],[25]
–
oxidation of the terminal N-aryl substituents.
A 4e -
[
28]
unlikely
and consequently, rapid decomposition of the
reduction of 1a was achieved by Fedushkin et al. by reaction
with sodium metal in diethyl ether. The solid-state structures
showed that the first two electrons reduce the acenaphthene
diimine to the acenaphthylene diamine, and the following two
reduced metal complexes is to be expected. This may explain
why for singly reduced 2a,c,d and 5a an irreversible electro-
chemical behavior is observed upon further decrease of the
applied potential. However, it should not be concealed that free
bis(imino)acenaphthenes very often do not show reversible
[
27]
reductions occurred at the naphthalene moiety.
CVs were
recorded for the tetrahedral complexes 2a,c and 5a and the
octahedral complexes 4f,g and 5f,g in acetonitrile using [N(n-
[
21b],[25]
redox events in CVs.
Bu)
4 6
][PF ] as electrolyte (Tables 1, S2 and Figures 5, S16–25).
The electrochemical features of compounds 2 were very similar
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FULL PAPER
(
BIAN)FeCl
2
complex to a low-valent (BIAN)Fe species which
possibly contains the BIAN ligand in the radical anion or dianion
[
16],[17],[32]
state (Scheme 3).
Attempts to disclose the chemical
identity of this fraction have not yet been successful. Application
of the 2a/BuLi catalyst solution to the hydrogenation of various
alkenes resulted in excellent yields of the corresponding alkane
ia
5
µA
ic
2
a
2
products (Scheme 4). Similar reactions with catalytic FeCl (thf)1.5
[
33]
(
and no ligand) gave much lower yields. It is important to note
reversible
ligand-centered
e-reduction
that clean hydrogenations of tri- and tetra-substituted alkenes
were achieved at elevated H
2
pressure and temperature.
1
-
1600 -1400 -1200 -1000
-800
-600
-400
inactive
+
Potential in mV vs FcH/FcH
ia
ic
5
µA
5a
ia
active
hydrogenation
catalyst
1
0 µA
ic
inactive
2a
Scheme 3. Reductive activation of pre-catalyst 2a.
-1500 -1000
-500
0
500
1000
1500
+
Potential in mV vs FcH/FcH
–1
–3
Figure 5. Cyclic voltammograms (scan rate 200 mV∙s ) of 2a (10 M (solid
–3
line: –1.3 to 1.4 V, dashed line: –1.3 to 0.7 V)) and 5a (0.5∙10 M).
Catalytic studies. We have then applied the tetrahedral
[
29],[30]
complexes of type 2 to the hydrogenation of alkenes.
Good
catalytic activities were only observed when the complexes were
activated by addition of a strong reductant. Treatment of 2a with
3
equiv. n-butyllithium (n-BuLi) afforded the active hydrogenation
[31]
catalyst.
The observation of octane (70%), octenes (18%),
and hexadecane (12%) formation from the reaction of 2a with 3
equiv. n-octyllithium in toluene correspond to a reduction of the
metal complex by two or more electrons. However, the reduction
of 2a with n-BuLi on a preparative scale (150 mg 2a) afforded a
mixture of species. The major component (~70 %) could be
Scheme 4. Catalytic hydrogenations of alkenes (yield of reactions with 3 mol%
a
2
FeCl (thf)1.5 in parentheses); traces of isomerized product.
Conclusion
separated by extraction of the dried residue with hexane and
6
A
(
series of Fe complexes of the general composition
R-BIAN)FeCl (2) and [Fe(R-BIAN) ][FeCl (3) was prepared
by reaction of FeCl (thf)1.5 with various bis(imino)acenaphthenes
BIANs). The presence of N-aryl substituents in 2- or 6-position
was identified as (dipp
been prepared by Findlater and co-workers. This formal two-
electron reduction species was inactive in the hydrogenation of
2
BIAN)Fe(η -C
7
H
8
), which had previously
[16]
2
3
4 2
]
2
(
2
α-methylstyrene (1.9 bar H , 20 °C, 3 h). In contrast, the minor
governs the formation of tetrahedral or octahedral complexes.
The low-spin configuration of Fe(II) in the octahedral complexes
component of the reduction of 2a with n-BuLi which was
obtained from extraction with toluene and washing with hexane
showed identical catalytic activity to the in-situ generated
3
suggests that a gain in ligand-field stabilization is the
thermodynamic driving force of this pathway. UV/Vis
spectroscopy allowed the unambiguous distinction between
catalyst mixture ((dipp
recent literature, we postulate a three-electron reduction of the
2 2
BIAN)FeCl , n-BuLi). In accordance with
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ChemCatChem
10.1002/cctc.201700144
FULL PAPER
complexes of type 2 and 3 as the latter exhibited much stronger
CT absorptions in the visible region. This interpretation is in full
accordance with the results obtained from mass spectrometry
and X-ray diffraction. The electrochemical properties of
We acknowledge excellent technical support from F. Riedlberger
(CV) and R. Hoheisel (UV). M.V. was a Kekulé doctoral fellow of
the Fonds der Chemischen Industrie (FCI). D.S. is currently an
independent research fellow of the Deutsche Forschungs-
gemeinschaft (DFG).
3 4 2
complexes 2 and [Fe(BIAN) ][BF ] (4) were determined by
cyclic voltammetry. All compounds showed reversible BIAN-
–
centered 1e -reductions in the potential range of –0.7 to –1.2 V
vs FcH/FcH , whereby the BIAN ligand in 2 is reduced to a
radical anion. A further decrease of the potential leads to
Keywords: iron • imines • redox-active ligands •
+
electrochemistry • hydrogenations
irreversible reductions. For (BIAN)FeCl
2
,
a ligand-related
[
1]
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a reduction at –0.4 V.
3
Applications (Ed.: B. Plietker), Wiley-VCH, Weinheim, 2008; c) W. M.
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(
i) The facile synthetic access to iron BIAN complexes, (ii) their
stereoelectronic modulation by variation of the N-aryl
substituents, stoichiometry, and counterion, and (iii) the non-
innocent character of the BIAN ligands are a prime motivation to
apply such complexes as catalysts to redox reactions. Initial
catalytic studies in hydrogenations of olefins documented the
[
[
2]
3]
See for example: R. B. Bedford, Acc. Chem. Res. 2015, 48, 1485–1493.
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2 2
excellent activity of (dipp BIAN)FeCl (2a). Our group is currently
investigating further avenues toward new iron-catalyzed
reduction, hydrofunctionalization and dehydrogenation protocols
with these complexes.
4493–4505.
[
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Experimental Section
General procedure for the synthesis (Ar
2 2
BIAN)FeCl (2): A Schlenk
flask was charged with FeCl (thf)1.5 (1.0 mmol) and the BIAN ligand (1.1
2
mmol, 1.1 equiv.) in toluene (20 mL). The mixture was heated to 100 °C
and stirred for 16 h during which a precipitate formed. After cooling to r.t.,
the suspension was concentrated in vacuo and filtered. The solid residue
was washed with toluene (3×5 mL) and dried.
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[Fe(Ph BIAN)
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][FeCl
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(3f):
A
Schlenk flask was charged with
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2
3
(0.4 mmol, 2 equiv.) and Ph BIAN (0.6
2
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and stirred for 16 h during which a green precipitate formed. After cooling
to r.t., the suspension was concentrated in vacuo and filtered. The solid
residue was washed with toluene (3×5 mL) and dried to afford the 3f as a
green complex in 87% yield (251 mg).
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General procedure for the synthesis [Fe(Ar
Schlenk flask was charged with [Fe(H O) ][BF
3.3 mmol, 3.3 equiv.) in acetonitrile (40 mL). After stirring for 20 h at r.t.,
the green solution was filtered. The filtrate was evaporated to dryness
and the solid residue was washed with thf/toluene (1:1, 4×10 mL) and
toluene (3×5 mL) and then dried.
General procedure for catalytic hydrogenations of alkenes: A 4 mL
vial was charged with a freshly prepared solution of 2a in toluene (2.50
mL, 0.003 M) and a solution of n-BuLi in toluene (45 μL, 0.5 M) under
argon. The alkene (0.25 mmol) was added and the vial transferred to a
2
BIAN)
] (1.0 mmol) and BIAN
3
][BF
4
]
2
(4): A
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4 2
(
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3
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10.1002/cctc.201700144
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Entry for the Table of Contents
FULL PAPER
A set of redox-active tetrahedral and
M. Villa, D. Miesel, A. Hildebrandt, F.
Ragaini, D. Schaarschmidt,* and A.
Jacobi von Wangelin*
octahedral
bis(imino)acenaphthene
iron(II) complexes were prepared and
fully characterized. The complexes
–
showed reversible BIAN-centered 1e -
Page No. – Page No.
reductions and could be applied to
catalytic hydrogenations of bulky
alkenes upon reductive activation.
Synthesis and Catalysis of Redox-
active Bis(imino)acenaphthene
(
BIAN) Iron Complexes
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