Chelating bis-N-heterocyclic carbene complexes of iron(II) containing bipyridyl ligands as catalyst precursors for oxidation of alcohols

Article abstract of DOI:10.1039/c6dt02718k

Chelating bis-N-heterocyclic carbene (bis-NHC) complexes of iron(ii) containing pyridyl ligands have been prepared by the reaction of [FeCl₂L] [L = bipy (1), phen (2)] with [LiN(SiMe₃)₂] and a bis(imidazolium) salt. The [F

Full text of DOI:10.1039/c6dt02718k

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Chelating bis-N-heterocyclic carbene complexes of iron(II)
containing bipyridyl ligands as catalyst precursors for oxidation of
alcohols
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
a*
DOI: 10.1039/x0xx00000x
Mara F. Pinto, Bernardo de P. Cardoso, Sonia Barroso, Ana M. Martins, and Beatriz Royo
www.rsc.org/
Chelating bis-N-heterocyclic carbene (bis-NHC) complexes of iron(II) containing pyridyl ligands have been prepared by the
reaction of [FeCl L] [L = bipy (1), phen (2)] with [LiN(SiMe ] and a bis(imidazolium) salt. The [Fe(bis-NHC)L(I) ] complexes
2
3
)
2
2
were active pre-catalysts in the oxidation of 1-phenylethanol with tert-butyl hydroperoxide in neat conditions, affording
quantitative yield of acetophenone in 4.5 h. The catalyst could be reused up to six cycles giving a turnover number (TON)
of 1500. Various secondary alcohols, both aromatic and aliphatic were selectivity oxidised to the corresponding ketones in
excellent yields. Compound 1 is stable in acetonitrile solution for ca. 4 h, although after 16 h, it evolves to a mixture of
2
+
[
Fe(bis-NHC)(bipy)
2
]I
2
, (3), [Fe(bipy)
3
]
and bis-imidazolium salt. The molecular structure of 3 has been determined by X-
ray diffraction studies.
applied in catalysis, artificial photosynthesis, molecular
electronics, and metallasupramolecular chemistry, among
Introduction
4
others. Iron complexes bearing NHCs and polypyridines
The low price, high abundance, and non-toxicity of iron, along
with the great popularity of NHC ligands in catalysis have
triggered the growing interest in the development of iron-NHC
independently bound to the metal center are very rare. The
first heteroleptic tetrakis(NHC) Fe(II) complex containing a
mesoionic bis(1,2,3-triazol-5-ylidene) and a bipyridine ligand
1
chemistry. Several iron complexes supported by chelating bis-
2
NHC ligands have been described in the literature, but their
5
was recently described by Sundström. In contrast, iron-NHCs
containing polypyridine ligands as part of multidentate
6
frameworks have been intensively studied in the last years.
use as catalysts is still rather rare. To the best of our
knowledge, only three reports on catalytic applications of iron
bis-NHCs can be found in the literature. In 2011, Meyer
described the activity of four-coordinate iron(II) complexes
Recently, Herrmann and Kühn reported exciting results
describing the catalytic activity of iron(II) complexes bearing
tetradentate NHC-pyridine ligands in the epoxidation of
2
a
Fe(bis-NHC)X
reported their use in homocoupling of Grignard reagents.
Same year, Driess described the catalytic activity of the iron(0)
2
in cross-coupling catalysis, and later Hazari
2
d
olefins, and in hydroxylation of toluene and benzene with
6
d,e
H
2
O
2
.
Based on these precedents, and on our own
7
6
experience on the preparation of iron-NHC based complexes,
we now report the synthesis of a series of chelating bis-NHC
iron(II) pyridyl complexes and their application as catalysts for
the oxidation of alcohols.
Fe(
η -arene)(bis-NHC) complex in the reduction of amides to
amines with silanes.
2
e
We recently became interested in exploring complexes
containing the “Fe-bis-NHC” fragment combined with 2,2-
bipyridine (bipy) and 1,10-phenanthroline (phen) aiming to
explore reactivity in oxidation reactions. Phenanthroline and
bipyridine are classical ligands that play important role in
Results and discussion
3
coordination chemistry. Their related transition metal
2
The iron bis-NHC complex [Fe(bis-NHC)(bipy)I ] (1) (NHC = 1,1´-
complexes often show interesting properties that can be
methylene-3,3´-di-benzylimidazole-2-ylidene) was prepared as
shown in Scheme 1, by the reaction of [FeCl (bipy)] with two
2
equivalents of [LiN(SiMe ) ] followed by addition of 1,1´-
3
2
methylene-3,3´-di-benzylimidazolium diiodide in THF. This
procedure is based on the method disclosed by Danopoulos
8
for the preparation of Fe bis-NHCs. Complex
1 precipitated
from the reaction as a deep purple crystalline solid. The
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a
Table 1 Oxidation of alcohols with TBHP using 1.
analogous phenanthroline complex [Fe(bis-NHC)(phen)I
was prepared using [FeCl (phen)] by following a similar
synthetic procedure. The identity of complexes and was
established by elemental analysis and ESI-mass spectroscopy.
The ESI mass spectrum of recorded in acetonitrile, displayed
prominent signal at m/z 666.9 assignable to the
2
] (2)
2
OH
O
cat. 1 (2 mol%)
1
2
R
R´
TBHP
R
R´
1
a
+
b
monocationic iron species [Fe(bis-NHC)(bipy)I] . A lower
intensity signal at m/z 270.0 was attributed to the dicationic
Entry Oxidant/Solvent Substrate
Time
Yield (%)
(h)
2
+
species [Fe(bis-NHC)(bipy)] . The ESI-MS spectrum of complex
displayed an intense signal at m/z 691.0 due to the cation
1
2
3
4
5
6
7
TBHP/neat
TBHP/THF
TBHP/toluene 1-phenylethanol
TBHP/MeCN
TBHP/water
TBHP/neat
TBHP/neat
TBHP/neat
1-phenylethanol
1-phenylethanol
4.5
8
>99
15
2
+
Fe(bis-NHC)(phen)I] . Consistent with their paramagnetic
8
51
[
1-phenylethanol
1-phenylethanol
cyclohexanol
cyclopentanol
1,2,3,4-tetrahydro-1-
naphthol
4.5
4.5
8
>99
>99
91
1
nature, the H NMR spectra of
1 and 2 displayed broad signals,
so structural information could not be obtained
8
96
8
95
8
9
10
11
TBHP/neat
TBHP/neat
1-phenyl-1-propanol
1-hexanol
1-phenylethanol
6
9
24
90
61
0
N
N
1) 2 LiN(SiMe
3
)
2
Cl
Cl
[
Fe(bis-NHC)L(I)
2
]
Fe
2)
H
2
2
O /neat
Bn
Bn
N
N
2I
L = bipy (1), phen (2)
a
N
Reaction conditions: substrate (0.5 mmol), catalyst 1 (2 mol%), TBHP (0.75
b 1
N
mmol), 80 °C. Yield determined by H NMR using diphenylmethane as an
internal standard.
Scheme 1 Synthesis of iron complexes 1 and 2.
Encouraged by these results, we explored the reactivity of 1
under oxidative conditions in different solvents. When the reaction
was carried out in THF or toluene, low yields were obtained,
probably due to the insolubility of the catalyst (Table 1, entries 2
and 3). In contrast, quantitative conversion of 1-phenylethanol to
acetophenone was observed when polar solvents (acetonitrile or
water) were used (Table 1, entries 4 and 5). We also performed the
stoichiometric reaction of 1 with TBHP in the presence of one
equivalent of 1-phenylethanol in acetonitrile. The reaction mixture
was heated at 80 °C for 30 min, and the resulting acetonitrile
solution was filtered and subjected to ESI-MS analysis, showing a
Both compounds
1 and 2 were insoluble in non-polar
solvents such as hexane and toluene, and also in THF, yet
soluble in polar solvents. They were air and moisture stable in
the solid state, and were stored for weeks under air without
noticeable decomposition.
In order to assess the reactivity of
reactions, we explored its catalytic activity in the oxidation of
alcohols. First we tested in the oxidation of 1-phenylethanol
in the presence tert-butyl hydroperoxide (TBHP) as oxidant in a
:1.5 molar ratio (substrate vs oxidant). The reaction was
1 toward oxidation
1
1
2
+
carried out in the absence of solvent at 80 °C, with a catalyst
loading of 2 mol%, which afforded a quantitative yield of
acetophenone in 4.5 h (Table 1, entry 1). The study of the
reaction profile showed conversions already at very early
reaction times (5 min, 12% yield), thus indicating that the
active species formed rapidly upon addition of TBHP.
significant signal at m/z 270 (z = 2), due to [Fe(bis-NHC)(bipy)] .
This experiment strongly suggests that the NHC ligand remains
bound to the iron center under the oxidative conditions used for
9
the catalytic reactions.
The stability of
1
in acetonitrile was explored by ESI-MS
was dissolved in
spectroscopy. In a typical experiment
1
Interestingly, the reaction mixture remained active upon 6
consecutive additions of substrate and oxidant, reaching
quantitative conversion to the ketone after 4.5 h. An
accumulated turnover number (TON) of 1500 was achieved.
Using the same conditions, a series of aromatic and aliphatic
secondary alcohols were oxidised to the corresponding
ketones in excellent yields (Table 1, entries 6-9). Moderate
production of hexanaldehyde was obtained in the oxidation of
acetonitrile, and then solution was stirred for 2 or 4 hours
before the ESI-MS spectra were taken. The mass spectra
displayed the same pattern as those recorded with freshly
prepared solutions, showing the diagnostic signal at 666.9
+
assigned to the iron cation [Fe(bis-NHC)(bipy)I] . This result is
strongly indicating that
1 is stable in acetonitrile solutions for
at least a few hours. After 16 h in MeCN, the colour of the
solution changed from the initial deep purple to purplish red.
1
The H NMR spectrum (in CD
1
-hexanol (61%, Table 1, entry 10), while mixtures of several
compounds were obtained when primary benzyl alcohols were
used. Catalyst was inactive when the reaction was performed
using H as oxidant instead of TBHP (Table 1, entry 11). The
phenanthroline iron complex displayed similar catalytic
activity compared to , indicating that substitution of
3
CN) became well-resolved and
suggested the formation of a diamagnetic product mixture.
The observed resonances were attributed to [Fe(bis-
1
2
+
2
O
2
NHC)(bipy)
of bis-imidazolium salt (in a proportion of 46.5 %, 40.8 %, and
2.4 %, respectively). The identity of complex was confirmed
by single crystal X-ray diffraction (vide infra). The complex
2 2 3
]I (3) and [Fe(bipy) ] , along with small quantities
2
1
1
3
bipyridine by phenanthroline was irrelevant to the
performance of the catalyst.
2
]
+
[Fe(bipy)
3
and the bis-imidazolium salt were identified by
comparing their spectra with the ones taken from pure
samples. As a diagnostic of the presence of the NHC ligand
2
| J. Name., 2012, 00, 1-3
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1
3
a
Table 2 Oxidation of 1-phenylethanol using Fe species
OH
bound to the metal, the C NMR spectrum of
at 189 ppm, which is a clear indication of the presence of a
carbene carbon. Similar evolution of was observed in water
and methanol. The NMR spectra of in D O and MeOD
3 shows a signal
O
1
cat. (2 mol%)
TBHP
1
2
recorded after 16 h displayed the same NMR pattern as in the
acetonitrile solution.
The molecular structure of
3
is displayed in Figure
1
. The
b
Entry
Catalysts
Fe(bis-NHC)(bipy)I
Yield (%)
cation displays a slightly distorted octahedral geometry
defined by the carbon atoms of the bis-NHC ligand (C1 and
C12) and the nitrogen atoms of the two bipy ligands (N5, N6,
N7 and N8). The equatorial plane is defined by N5, N6, N8 and
C12 while the axial positions are occupied by C1 and N7. The
angles between the mutually trans ligands range between
1
2
3
4
5
6
2
(1)
>99
33
42
30
22
0
Fe(bipy)Cl
[Fe(bipy) ]Cl
2
3
2
FeCl
FeCl
2
3
[bis-NHC]I
2
a
Reaction conditions: alcohol (0.5 mmol), catalyst (2 mol%), TBHP (0.75
mmol), MeCN (0.4 mL), 80 °C, 4.5 h. Yield determined by H NMR using
1
71.8(1)° and 175.1(2)°. The Fe1-C1 (1.976(4) Å) and Fe1-C12
bond lengths (1.965(4) Å) are within the range observed for
other dicationic bis-NHC iron complexes (1.818 Å < Fe-C
b
1
diphenylmethane as a internal standard.
<
NHC
1
0
2
.007 Å). The Fe-Nbipy distances are also in agreement with
1
1
Finally, we have performed spin trap experiments, which
t
showed that radicals such as BuOO or BuO could be involved
in the oxidation reaction catalysed by . When the catalytic
commonly reported values.
t
ꢀ
ꢀ
1
oxidation of 1-phenylethanol with TBHP in acetonitrile was
performed in the presence of an oxygen-radical trap, such as
Ph NH, the yield of acetophenone was reduced to 5%,
2
indicating that the oxidation reaction proceeds via a radical
mechanism involving oxygen-centered radicals. In contrast, the
presence of CBrCl , a carbon-radical trap did not affect the
3
catalytic reaction.
Conclusions
In summary, we prepared two iron(II) bis-NHC complexes
bearing bipypidyl and phenanthroline ligands in good yields.
These complexes were used in the oxidation of alcohols with
TBHP. The reaction can be performed in the absence of
solvent, and the active species can be reused over 6 cycles
without measurable loss of activity. The catalytic reaction can
also be performed in polar solvents such as acetonitrile and
water, affording quantitative yield of the corresponding
Figure 1 ORTEP-3 diagram of the cation of 3, using 30% probability level
ellipsoids. Hydrogen atoms and counter-anions (two iodides) are omitted for
clarity. Selected distances (Å): Fe1-C1 1.976(4), Fe1-C12 1.965(4), Fe1-N8
1
.967(3), Fe1-N5 1.983(3), Fe1-N7 1.987(3), Fe1-N6 2.007(3). Selected
ketones. Compound
1 is stable in acetonitrile solution for at
angles (º): N8-Fe1-N5 171.8(1), C1-Fe1-N7 175.1(2), C12-Fe1-N6 173.1(2).
least 4 h, although after 16 h evolves to a mixture of [Fe(bis-
2
+
NHC)(bipy) ]I , (
2
2
3
), [Fe(bipy)3] and bis-imidazolium salt. The
In light of these results, we decided to explore the catalytic molecular structure of
has been determined by X-ray
activity of acetonitrile solutions of after being incubated for diffraction studies.
6 h. Then, TBHP and 1-phenylethanol were added and heated
Our work constitutes the first example of an iron(II)-NHC
3
1
1
at 80 °C for 4.5 h. Under these conditions, 40% yield of complex catalysing the oxidation of alcohols with TBHP. We
acetophenone was obtained. We also evaluated the catalytic believe that our research may inspire future research in the
activity of a series of iron salts for comparative purposes. As field of metal-abundant-based catalysts.
shown in Table 2, FeCl , FeCl , FeCl (bipy), and [Fe(bipy) ]Cl
2
3
2
3
2
catalysed the reaction to a much lower extend (<45 %) than
1.
These results strongly support that complex 1 is a pre-catalyst
in the catalytic oxidation reaction, thus indicating a beneficial Experimental details
role of the NHC in the coordination sphere of the metal.
General methods
All manipulations were carried out under nitrogen atmosphere
using standard Schlenk techniques, and solvents were purified
from appropriate drying agents. Deuterated solvents were
1
2
2
degassed and stored over molecular sieves. FeCl (bipy),
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Journal Name
1
3
FeCl
diiodide
described in the literature. All other reagents were purchased solid. Data for
2
(phen), and 1,1´-methylene-3,3´-di-benzylimidazolium methylene-3,3´-di-benzylimidazolium diiodide (377 mg, 0.65
1
4
were synthesised according to the method mmol) afforded
2
(275 mg, 0.34 mmol, 52%) as deep purple
: Data for : Anal. Calc for C33 FeI
2
2
H
28
N
6
2
from commercial suppliers and used without further (818.27): C, 48.44; H, 3.45; N, 10.27. Found: C, 48.30; H, 3.77;
1
purification. H and C NMR spectra were recorded on a N, 9.87. MS (ESI-TOF) in acetonitrile: m/z [M] calcd for
13
+
+
Bruker Avance III 500 MHz. Assigment of resonances was made [C33
from HMQC and HMBC experiments. Electrospray mass Characterisation of [Fe(bis-NHC)(bipy)
spectra (ESI-MS) were recorded on a Micromass Quatro LC complex was obtained by slow diffusion of Et
instrument; nitrogen was employed as drying and nebulising solution of
28 6
H N FeI] , 691, found: 691.0.
2
]I
2
(
3
)
.
Crystals of
O into a MeCN
3
. H NMR (298 K, 400MHz, CD CN): δ = 8.26 (d, 2H,
3
2
1
1
3
3
gas. Elemental analyses were performed in our ITQB
laboratory services.
J
HH = 8.1 Hz, Hbipy), 8.08 (d, 2H, JHH = 8.0 Hz, Hbipy), 7.97 (t, 2H,
HH = 7.8 Hz, Hbipy), 7.48-7.43 (m, 4H, HAr), 7.43-7.38 (m, 4H,
3
J
H
Ar), 7.20-7.15 (m, 2H, HAr), 7.13-7.07 (m, 4H, HAr), 7.07-7.02
3
(m, 4H, HAr), 6.71 (t, 2H, JHH = 6.5 Hz, Ph), 6.31 (s ap., 2H,
X-ray crystallography
Crystals suitable for single-crystal X-ray analysis were obtained
2
Himid), 6.29 (s ap., 2H, Himid), 6.27 (s, 2H, NCH N), 4.50 (d, 2H,
2
2
Ph), 4.22 (d, 2H, JHH= 16.9 Hz, NCH
for complex
of
. The data were collected using graphite monochromated 13
Mo-K
2
3 by slow diffusion of Et O in acetonitrile solutions
J
HH = 16.9 Hz, NCH
2
2
Ph).
1
C NMR (298 K, 400MHz, CD
3
CN): δ = 189.8 (Fe-CNHC), 161.0,
α
radiation (λ = 0.71073 Å) on a Bruker AXS-KAPPA APEX
1
58.8, 158.3, 155.3 (Cipso), 152.0, 138.3, 137.6 (CH, Ar), 137.5
II diffractometer equipped with an Oxford Cryosystem open-
flow nitrogen cryostat. Cell parameters were retrieved using
Bruker SMART software and refined using Bruker SAINT on all
observed reflections. Absorption corrections were applied
(
Cipso), 129.2 128.1, 127.1, 126.8, 126.4 (CH, Ar), 125.9 (Cimid),
1
2 2
24.6, 124.0 (CH, Ar), 63.2 (NCH N), 53.0 (CH Ph).
15
Typical procedure for the oxidation of alcohols
using SADABS. The structures were solved and refined using
direct methods with program SIR2004 using WINGX-Version
1
6
An open air flask was charged with catalyst, and alcohol (0.5
1
7
18
2
014.1 SHELXL system of programs. All non-hydrogen mmol). Then, TBHP (5.0-6.0 M in decane, 0.75 mmol) was
atoms were refined anisotropically and the hydrogen atoms added. The progress of the reactions were monitored by taking
1
were inserted in idealised positions and allowed to refine aliquots of the reaction mixtures and subjecting them to H
riding on the parent carbon atom. The molecular diagrams
3
NMR in chloroform-d , indicating the presence of the
1
9
were drawn with ORTEP-3 for Windows included in the
software package. For crystallographic experimental data and
structure refinement parameters see Table S2. CCDC 1455061
contains the supplementary crystallographic data for this
paper. The data can be obtained free of charge from The
corresponding ketones and unreacted alcohols. The yield was
1
determined by H NMR using diphenylmethane as internal
standard. The corresponding ketones were extracted in
dichloromethane, and identified by comparison of their NMR
2
0
spectral data to the literature.
Cambridge Crystallographic
www.ccdc.cam.ac.uk/structures.
Data
Centre
via
Acknowledgements
General procedure for the synthesis of complexes 1 and 2
Two equivalents of [LiN(SiMe
of [FeCl
L] (L = bipy, phen) (1 equiv.) in toluene at 0 °C, and the (UID/Multi/04551/2013,
3 2
)
] were added to a suspension We are grateful to FCT-Fundação para a Ciência e a Tecnologia
UID/QUI/00100/2013, and
PTDC/QEQ-QIN/0565/2012). B.R., M.F.P. and S.B thank FCT for
consolidation contract IF/00346/2013, grant
PD/BD/105994/2014, and SFRH/BPD/73941/2010,
2
mixture was stirred overnight at room temperature. The
toluene solution was evaporated and all the volatiles were
removed under vacuum to yield a green residue, which was
redissolved in THF (10 mL). Solid 1,1´-methylene-3,3´-di-
benzylimidazolium diiodide (1 equiv.) was added at once to the
THF solution, and the reaction mixture was stirred for 16 h at
room temperature. A deep purple crystalline solid insoluble in
respectively. We thank Analytical Services at ITQB for
elemental analyses, and the UniMS-Mass Spectrometry Unit,
ITQB/IBET. The authors thank the National NMR and X-ray
Facilities supported by the FCT (RECI/BBB-BQB/0230/2012 and
RECI/QEQ/QIN/0189/2012). We are grateful to COST Action
THF was isolated by filtration, washed several times with THF CM1205 for networking, and Prof. Martin Albrecht for helpful
and Et
Preparation and characterisation of [Fe(bis-NHC)(bipy)I
Following the general procedure, [LiN(SiMe ] (150 mg, 0.90
mmol), [FeCl (bipy)] (127 mg, 0.45 mmol) and 1,1´-methylene-
,3´-di-benzylimidazolium diiodide (475 mg, 0.81 mmol)
2
O to yield the pure product.
discussions.
2
] (1).
3 2
)
Notes and references
2
3
afforded
Data for
1
(520 mg, 0.65 mmol, 80%) as deep purple solid.
1
(a) K. Riener, S. Haslinger, A. Raba, M. P. Högerl, M. Cokoja,
W. A. Herrmann, F. E. Kühn, Chem. Rev., 2014, 114, 5215; (b)
D. Bézier, J.-B. Sortais, C. Darcel, Adv. Synth. Catal., 2013,
1: Anal. Calc for C31 N FeI (794.25): C, 46.88; H,
6
H
28
2
3
.55; N, 10.58. Found: C, 47.10; H, 3.86; N, 10.27. MS (ESI-TOF)
+
FeI] , 667, found:
3
2
55, 19; (c) M. J. Ingleson, R. A. Layfield, Chem. Commun.,
012, 48, 3579.
+
in acetonitrile: m/z [M] calcd for [C31
H
28
N
6
2
Fe] , 270 found: 270; [C42
+
+
FeI] .
666.9; [C31
H
28
N
6
H
40
N
8
2
(a) S. Meyer, C. M. Orben, S. Demeshko, S. Dechert, F.
Meyer, Organometallics, 2011, 30, 6692; (b) S. Zlatogorsky,
C. A. Muryn, F. Tuna, D. J. Evans, M. J. Ingleson,
Preparation and characterisation of [Fe(bis-NHC)(phen)I
Following the general procedure, [LiN(SiMe ] (119 mg, 0.71
mmol), [FeCl (phen)] (109 mg, 0.36 mmol), and 1,1´-
2
] (2).
3 2
)
Organometallics, 2011, 30, 4974; (c) S. Zlatogorsky, M. J.
2
4
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Journal Name
ARTICLE
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R. H. Crabtree, J. Organomet. Chem., 2014, 751, 174.
0 A CCDC search for hexacoordinated Fe(II) cations with similar
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1
1
1 A CCDC search for Fe(II) cations with bipy ligands returned 64
hits with Fe(II)-Nbipy bond distances ranging between 1.919
and 2.230 Å. See for example: Y. Wang, C. Lin, X. Ma, Z. Xue,
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2 F. F. Charron Jr., W. M. Reiff, Inorg. Chem,. 1986, 25, 2786.
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