Synthesis of Iron(0) Complexes Bearing Protic NHC Ligands: Synthesis and Catalytic Activity

Article abstract of DOI:10.1021/acs.organomet.9b00260

Preparation of iron(0) pNHC complexes by oxidative addition of N-phosphine-tethered azoles to [Fe₃(CO)₁₂] is described. Phosphine-tethered imidazoles 1 and 2 react with [Fe₃(CO)₁₂] to give mononuclear iron(0) co

Full text of DOI:10.1021/acs.organomet.9b00260

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Iron(0) Complexes Bearing Protic NHC Ligands:
Synthesis and Catalytic Activity
†
†
‡
†
Christoph Muhlen, Jenny Linde, Lena Rakers, Tristan T. Y. Tan, Florian Kampert,†
̈
Frank Glorius,*,‡ and F. Ekkehardt Hahn*,†
†Institut fu
Munster, Germany
̈
̈ ̈
̈
r Anorganische und Analytische Chemie, Westfalische Wilhelms-Universitat Munster, Corrensstraße 28-30, 48149
̈
‡
̈
̈
̈
̈
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstraße 40, 48149 Munster, Germany
S
* Supporting Information
ABSTRACT: Preparation of iron(0) pNHC complexes by oxidative
addition of N-phosphine-tethered azoles to [Fe3(CO)12] is described.
Phosphine-tethered imidazoles 1 and 2 react with [Fe3(CO)12] to give
mononuclear iron(0) complexes of type [Fe(CO)3(CpNHC^PR2)] ([5]: R =
Ph; [6]: R = Cy) bearing a CpNHC^phosphine chelate ligand. In addition, the
phosphine-tethered imidazolium salt 4-Br was C2-deprotonated and reacted
with [Fe(bda)(CO)3] (bda = benzylidene acetone) to give the iron(0)
chelate complex [Fe(η2-4)(CO)3] (7). Complexes [5], [6], and [7] act as
moderate to good catalysts in the hydrogenation of acetophenone with
elemental hydrogen. The N−H group of the protic NHC ligands in [5] and
[6] is insignificant to the catalytic outcome.
solation of the first stable N-heterocyclic carbene (NHC)
of the C2−H bond. The hydrido complex generated this way is
often not stable and reductively eliminates a proton which
protonates the ring-nitrogen atom of the azolato ring, leading
to complexes with protic NH,NR-NHC (pNHC) ligands. Such
pNHC^donor chelate complexes have been described for Ru,12
Rh,13 and Ir.14 The N−H unit of the NH,NR-NHC donor can
be deprotonated12−14 and has been shown to function as a
molecular recognition unit.15
Iron NHC complexes have received attention due to the low
cost of iron and its nontoxic and environmentally benign
properties. Apart from iron(II) and iron(III) NHC complex-
es,9f only a few examples for iron(0) NHC complexes have
been reported. The known Fe0 complexes were obtained by
treatment of [Fe(CO)5] with a free NHC16 in the presence of
an FeI catalyst or by reaction of imidazolium halides with
[Fe3(CO)12]17 proceeding via a currently unknown mecha-
nism. All known Fe0 NHC complexes bear classical NR,NR-
NHC ligands.
We became interested in the synthesis of the currently
unknown iron(0) complexes bearing bidentate pNHC^donor
chelate ligands. The stability of such complexes might benefit
from the chelating coordination while they feature an N−H
recognition unit. Herein, we describe the preparation of two
new iron(0) complexes bearing imidazole−phosphine-derived
pNHC^PR2 chelate ligands. In addition, the iron(0) chelate
complex bearing a phosphine-tethered classical NR,NR-NHC
I
by Arduengo et al.1 in 1991 initiated intense research on
NHCs. Over the last two decades, NHCs and their metal
complexes have gained attention, and various types of NHCs
are known today.2 Due to their useful electronic and steric
properties,3 NHCs have found numerous applications as
ligands in catalytically active complexes,4 as organocatalysts5
in different fields of coordination chemistry,6 or as building
blocks for metalosupramolecular complexes.7
NHC complexes of transition metals, including iron
complexes, are generally synthesized either by the Ag2O
method,8 by the in situ deprotonation of azolium cations
followed by coordination of the NHC ligand to a metal
center2a,9a−c or by cleavage of entetramines and coordination
of the free NHC ligand to the metal center.9d−f These methods
lead to classical NHC complexes bearing NR,NR-substituted
NHC ligands.
Complexes bearing protic NHC ligands (NH,NR-NHC
ligands, pNHCs) were initially obtained by the template-
controlled cyclization of β-functionalized isocyanides.10 This
methodology is significantly influenced by the template
metal10a,d and therefore not generally applicable. More
recently, it was found that neutral 2-halogenoazoles react
with low-valent metals in an oxidative addition to give C2-
metalated intermediates, which upon subsequent protonation
yield complexes with pNHC ligands.11 For the challenging
oxidative addition of the C2−H bond of neutral azoles, a
donor function tethered to one of the ring nitrogen atoms is
required. Such azoles are assumed to coordinate first with the
N1-tethered donor function followed by the oxidative addition
Received: April 23, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00260
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Organometallics
Communication
ligand is described, and the catalytic activity of all three
complexes in the hydrogenation of acetophenone is discussed.
Neutral proligands N-[2-(diphenylphosphino)ethyl]-
imidazole 1 and N-[2-(dicyclohexylphosphino)ethyl]imidazole
2 (Scheme 1) were prepared from 1-vinylimidazole18 following
in the reaction. Complex [6a] was characterized by an X-ray
diffraction study (see the Supporting Information for a
molecular plot of [6a]).
Formation of the diamagnetic chelate complexes [5] and [6]
1
was confirmed by H, 13C{1H}, and 31P{1H} NMR spectros-
copy, mass spectrometry, IR spectroscopy, and X-ray
diffraction studies (see the Supporting Information). Small
amounts of paramagnetic impurities were previously removed
by filtration of the reaction solution. The 31P{1H} NMR
spectra indicate coordination of the phosphorus donor.
Whereas the proligands exhibit resonances at δ −21.9 ppm
(1) and δ −9.4 ppm (2), the equivalent resonances for
complexes [5] and [6] were recorded at δ 51.3 and δ 60.7
ppm, respectively. In addition, characteristic resonances at δ
185.8 (d, 2JCP = 30.3 Hz) and δ 189.0 (d, 2JCP = 31.5 Hz) were
recorded in the 13C{1H} NMR spectra for the carbene carbon
Scheme 1. Synthesis of Iron(0) pNHC^PR2 Chelate
Complexes [5] and [6]
2
published procedures (see the Supporting Information for
experimental details).13,14 Alternatively, N3-alkylation of 1-
vinylimidazole followed by reaction with diphenyl phosphine/
KOtBu yielded the phosphine-tethered imidazolium salt 4-Br
(Scheme 2; see the Supporting Information for experimental
details).
atoms of complexes [5] and [6]. The observation of JCP
coupling for both complexes confirms that the NHC and the
phosphine donor are coordinated to the same metal center.
The IR spectra of complexes [5] and [6] show the
characteristic carbonyl stretching absorptions at ν = 1968,
1895, and 1878 cm−1 for [5] and at slightly lower
wavenumbers at ν = 1963, 1882, and 1858 cm−1 for [6].
This observation is in good agreement with the higher basicity
of the dicyclohexyl phosphine donor, resulting in an increased
π-backbonding for [6].
Scheme 2. Synthesis of Iron(0) NHC Complex [7]
The conclusions drawn from the spectroscopic investiga-
tions have been confirmed by X-ray diffraction studies with
crystals of [5] and [6]·0.25CH2Cl2 (see the Supporting
Information). Molecular plots of [5] and [6] are depicted in
Figure 1. The iron centers in [5] and [6] are surrounded in a
trigonal-bipyramidal fashion. The pNHC^PR2 chelate ligand
occupies an equatorial (−PR2) and an axial (−NHC)
The phosphine-substituted imidazoles 1 and 2 react with
1/3 equiv of [Fe3(CO)12] in refluxing THF over 2 h under
cleavage of the trinuclear iron cluster and formation of the
mononuclear iron(0) complexes [5] and [6] bearing bidentate
pNHC^phosphine chelate ligands. Standard workup proce-
dures yielded [5] and [6] as reddish brown, air-sensitive solids
in reproducibly low yields of 28 and 31%, respectively (Scheme
1).
Various reaction conditions such as changing the solvent, the
reaction temperature, the reaction time, or the addition of
Me3NO were tested to improve the yield of [5] and [6], but
no improvement was observed. Based on previous studies,12−14
we assume that the chelate complexes formed by an initial
coordination of the phosphine donors of 1 and 2 to the iron
atoms of [Fe3(CO)12]. In the absence of a base for the
imidazole C2-deprotonation, the next step is presumably the
loss of a CO ligand followed by the oxidative addition of the
C2−H bond to the iron(0) center. Reductive elimination of a
proton from the FeII center with concurrent protonation of the
azolato-ring nitrogen atom leads to the complexes [5] and [6].
This reaction sequence has been observed multiple times for
the formation of pNHC complexes from donor-functionalized
azoles.12−14
Analyzing the reaction mixtures containing complexes [5]
and [6], we noted by NMR spectroscopy the presence of a
large amount (up to 60%) of a side product. Upon workup of
the reaction mixture leading to [6], the side product was
isolated by crystallization and identified as the [Fe(CO)4(PR2-
imidazole)] complex [6a]. We observed that complex [6a]
once formed did not react further to give [6], thus leading to
the conclusion that it is a side product and not an intermediate
Figure 1. Molecular structures of complex [5] and [6] in [6]·
0.25CH2Cl2 (50% probability ellipsoids; hydrogen atoms except for
H3 have been omitted for clarity, and only one of the two essentially
identical molecules of [6] in the asymmetric unit is depicted).
B
DOI: 10.1021/acs.organomet.9b00260
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Communication
coordination site. The axial C2−Fe−C8 angles ([5]:
176.09(5)°, [6]: 173.83(8)°) are almost linear with the larger
deviation from linearity observed for complex [6] with the
bulkier −PCy2 donor. The Fe−CNHC bond length in [5] (Fe−
C2 1.9901(11) Å) is slightly longer than that in [6] (Fe−C2
1.978(2) Å).
Table 1. Selected Results of the Catalytic Hydrogenation of
Acetophenone with Iron(0) Complexes
a
To evaluate the influence of the N-substituent (H or alkyl)
at the NHC ligand on the catalytic properties of iron(0)
NHC^phosphine complexes, we have prepared proligand 4-Br
and complex [7] (Scheme 2; see the Supporting Information).
For the synthesis of complex [7], a preparative method
different to than that used for the synthesis of [5] and [6] was
employed. Imidazolium salt 4-Br was first deprotonated with
KHMDS and then reacted with [Fe(bda)(CO)3] (bda =
benzylideneacetone19). Complex [7] bearing a classical
phosphine-tethered NHC ligand was isolated as a yellow, air-
sensitive solid in 63% yield. Formation of complex [7] was
confirmed by NMR spectroscopy and mass spectrometry (see
the Supporting Information). As seen for complexes [5] and
[6], metalation of the proligand 4-Br leads to a significant
downfield shift of the phosphorus resonance (δ −22.05 ppm in
4-Br, δ 54.6 ppm in [7]) in the 31P{1H} NMR spectrum. The
13C{1H} NMR spectrum features the characteristic resonance
for the CNHC carbon atom at δ 185.4 ppm (d, 2JCP = 27.4 Hz),
at a chemical shift almost identical to that of [5].
The N−H moiety of complexes [5] and [6] potentially plays
an important role in catalytic transformations either via
substrate recognition through N−H···substrate hydrogen
bonding15 or as part of a Noyori-type bifunctional catalyst.20
Therefore, we evaluated the catalytic activity of complexes [5]
and [6] in the hydrogenation of acetophenone and compared
the results to those obtained with complex [7] (Table 1; also
see the Supporting Information).
The catalytic activity of [5] and [6] in the hydrogenation of
acetophenone depends on the polarity of the solvent (entries
1−6). Only low yields were observed in n-hexane (entries 1
and 2), whereas the use of solvents with higher polarity such as
THF (entries 3 and 4) and EtOH (entries 5 and 6) resulted in
moderate to good yields, respectively. The complex with the
less basic PPh2 donor [5] always gave yields lower than those
of [6] with the PCy2 donor. Further optimization of the
reaction conditions enabled a catalyst loading as low as 2.5 mol
% (entries 7 and 8) with consistent yields, whereas a further
decrease led to reduced yields (entries 9 and 10). The reaction
time could be reduced to 6 h (entry 12). Both the catalyst and
the irradiation with UV light proved to be essential for the
catalytic process (entries 13−15). When the reaction was
performed under argon instead of H2, the yield decreased from
97 to 25% (entry 16), indicating that there might be an
alternative catalysis pathway that proceeds via transfer
hydrogenation instead of direct hydrogenation.
To understand the role of the N−H moiety for the catalytic
outcome, the complex bearing the N,N′-dialkylated NHC [7]
was also tested as a catalyst for the hydrogenation of
acetophenone. No difference in yield was observed for
complexes 7 and 5 under identical conditions (compare
entry 17 to entry 5). This indicates that the N−H moiety is not
involved in the hydrogen transfer process. We propose the
following catalytic cycle (Scheme 3). First, the iron(0)
complexes have to be activated by UV light, removing one
CO ligand. Hydrogen is then oxidatively added to the iron(0)
center, resulting in an iron(II) dihydride complex.21 The next
step is the hydride transfer from the catalyst to the substrate,
b
entry
catalyst
mol %
solvent
yield (%)
1
2
3
4
5
6
7
8
9
10
[5]
[6]
[5]
[6]
[5]
[6]
[6]
[6]
[6]
[6]
[6]
[6]
none
[6]
[6]
[6]
[7]
10
10
10
10
10
10
5
2.5
1
0.5
10
10
n-hexane
n-hexane
THF
0
10
12
61
46
97
97
97
52
33
36
97
0
THF
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
c
11
12
d
13
14
15
16
e
10
10
10
10
0
0
25
49
f
g
17
a
General conditions: acetophenone (0.2 mmol), iron catalyst, KOtBu
(11 mol %), solvent (1 mL), H2 (1 bar), 25 °C, UV light, 24 h.
Yields were determined by GC-FID analysis using mesitylene (17
μL, 0.2 mmol, 1 equiv) as internal standard. Reaction time 3 h.
Reaction time 6 h. No UV light. No UV light and addition of
Me3NO·2H2O (1.4 mg, 0.013 mmol, 0.11 equiv). Argon atmosphere
b
c
d
e
f
g
instead of H2.
Scheme 3. Proposed Catalytic Cycle for the Hydrogenation
of Acetophenone with Catalysts [5]−[7]
which explains the observed solvent dependency as this is
facilitated in more polar solvents.22 The second hydride is then
reductively eliminated, protonating the substrate to 1-phenyl-
ethanol and generating back the active four-coordinate iron(0)
catalyst.
We have described the reaction of phosphine-functionalized
imidazoles with [Fe3(CO)12)] to give, most likely by an
oxidative addition/reductive elimination process, iron(0)
complexes [5] and [6] bearing a pNHC^phosphine chelate
ligand. The complexes exhibit a good catalytic activity in the
direct hydrogenation of acetophenone with molecular hydro-
gen. However, the N−H moiety of the pNHC ligands has no
significance for the catalytic outcome, as was shown in control
C
DOI: 10.1021/acs.organomet.9b00260
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00260.
■
S
Description of experimental procedures and NMR
spectra for all compounds (PDF)
Accession Codes
CCDC 1901742−1901744 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
̈
4572−4573. (c) Conrady, F. M.; Frohlich, R.; Schulte to Brinke, C.;
Pape, T.; Hahn, F. E. Stepwise Formation of a Molecular Square with
Bridging NH,O-Substituted Dicarbene Building Blocks. J. Am. Chem.
Soc. 2011, 133, 11496−11499. (d) Schmidtendorf, M.; Pape, T.;
Hahn, F. E. Stepwise Preparation of a Molecular Square from NR,NR-
and NH,O-Substituted Dicarbene Building Blocks. Angew. Chem., Int.
Ed. 2012, 51, 2195−2198. (e) Sinha, N.; Roelfes, F.; Hepp, A.;
Mejuto, C.; Peris, E.; Hahn, F. E. Synthesis of Nanometer-Sized
Cylinder-Like Structures from a 1,3,5-Triphenylbenzene-Bridged Tris-
NHC Ligand and AgI, AuI, and CuI. Organometallics 2014, 33, 6898−
6904. (f) Sinha, N.; Hahn, F. E. Metallosupramolecular Architectures
Obtained from Poly-N-heterocyclic Carbene Ligands. Acc. Chem. Res.
2017, 50, 2167−2184. (g) Gan, M.-M.; Liu, J.-Q.; Zhang, L.; Wang,
Y.-Y.; Hahn, F. E.; Han, Y.-F. Preparation and Post-Assembly
Modification of Metallosupramolecular Assemblies from Poly(N-
Heterocyclic Carbene) Ligands. Chem. Rev. 2018, 118, 9587−9641.
(8) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.;
Hwang, W. S.; Lin, I. J. B. Coinage Metal−N-Heterocyclic Carbene
Complexes. Chem. Rev. 2009, 109, 3561−3598.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: glorius@uni-muenster.de.
*E-mail: fehahn@uni-muenster.de.
ORCID
Tristan T. Y. Tan: 0000-0001-5391-7232
Frank Glorius: 0000-0002-0648-956X
F. Ekkehardt Hahn: 0000-0002-2807-7232
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft (SFB 858 and IRTG 2027)
and also thank Tristan Wegner for experimental support during
preparation of the revision.
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DOI: 10.1021/acs.organomet.9b00260
Organometallics XXXX, XXX, XXX−XXX