A Cyclometalated NHC Iridium Complex Bearing a Cationic (η5-Cyclopentadienyl)(η6-phenyl)iron Backbone**

Article abstract of DOI:10.1002/chem.202102520

Nucleophilic substitution of [(η⁵-cyclopentadienyl)(η⁶-chlorobenzene)iron(II)] hexafluorophosphate with sodium imidazolate resulted in the formation of [(η⁵-cyclopentadienyl)(η⁶-phenyl)iron(II)]imidazole hexafluorophosphate. The corresponding dicationic imidazolium salt, which was obtained by treating this imidazole precursor with methyl iodide, underwent cyclometallation with bis[dichlorido(η⁵-1,2,3,4,5-pentamethylcyclopentadienyl]iridium(III) in the presence of triethyl amine. The resulting bimetallic iridium(III) complex is the first example of an NHC complex bearing a cationic and cyclometallated [(η⁵-cyclopentadienyl)(η⁶-phenyl)iron(II)]⁺ substituent. As its iron(II) precursors, the bimetallic iridium(III) complex was fully characterized by means of spectroscopy, elemental analysis and single crystal X-ray diffraction. In addition, it was investigated in a catalytic study, wherein it showed high activity in transfer hydrogenation compared to its neutral analogue having a simple phenyl instead of a cationic [(η⁵-cyclopentadienyl)(η⁶-phenyl)iron(II)]⁺ unit at the NHC ligand.

Full text of DOI:10.1002/chem.202102520

Accepted Article
Accepted Article
Title: A Cyclometalated NHC Iridium Complex Bearing a Cationic (η5-
Cyclopentadienyl)(η6-phenyl)iron Backbone
Authors: Christian Malchau, Tom Milbert, Tobias Eger, Daniela V.
Fries, Pascal J. Pape, Benjamin Oelkers, Yu Sun, Sabine
Becker, Marc H. Prosenc, Gereon Niedner-Schatteburg, and
Werner R. Thiel
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Chemistry - A European Journal
10.1002/chem.202102520
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A Cyclometalated NHC Iridium Complex Bearing a Cationic
5
6
(
η -Cyclopentadienyl)(η -phenyl)iron Backbone
[a]
[a]
[a]
[b]
[a]
Christian Malchau, Tom Milbert, Tobias R. Eger, Daniela V. Fries, Pascal J. Pape, Benjamin
[
a]
[a]
[a]
[a]
[b]
Oelkers, Yu Sun, Sabine Becker, Marc H. Prosenc,* Gereon Niedner-Schatteburg, Werner R.
Thiel*[a]
[
[
a]
b]
C. Malchau, T. Milbert, T. R. Eger, P. J. Pape, Dr. B. Oelkers, Dr. Y. Sun, Jun.-Prof. Dr. S. Becker, Dr. Marc H. Prosenc, Prof. Dr. W. R. Thiel
Fachbereich Chemie
Technische Universität Kaiserslautern
Erwin-Schrödinger-Straße 54, 67663 Kaiserslautern
E-mail: thiel@chemie.uni-kl.de
D. V. Fries, Prof. Dr. G. Niedner-Schatteburg
Fachbereich Chemie, Research Center OPTIMAS
Technische Universität Kaiserslautern
Erwin-Schrödinger-Straße 52, 67663 Kaiserslautern
Supporting information for this article is given via a link at the end of the document.
5
6
biology.[8] In general, they are easily accessible via a base-
assisted C-H activation step.[9] Numerous examples for the use of
Abstract: Nucleophilic substitution of [(η -cyclopentadienyl)(η -chlo-
robenzene)iron(II)] hexafluorophosphate with sodium imidazolate
5
6
resulted in the formation of [(η -cyclopentadienyl)(η -phenyl)iron(II)]-
imidazole hexafluorophosphate. The corresponding dicationic
imidazolium salt, which was obtained by treating this imidazole
precursor with methyl iodide, underwent cyclometallation with
cyclometalated iridium(III) complexes in homogenous catalysis
have been reported in the past: hydrogenation,[
10]
transfer
hydrogenation,[ dehydrogenation,[11c,12] reductive amination,
11]
[13]
hydroamination,[
14]
hydrosilylation[11c,15]
and racemization
5
[16]
bis[dichlorido(η -1,2,3,4,5-pentamethylcyclopentadienyl]iridium(III) in
reactions
are catalyzed by iridium compounds.
the presence of triethyl amine. The resulting bimetallic iridium(III)
complex is the first example of a NHC complex bearing a cationic and
The application of transition metal complexes as catalysts for the
transfer hydrogenation is known since 1967.[17] At least since the
trailblazing work of Noyori et al. from 1995 and the following years,
catalytic transfer hydrogenation has developed into a highly
regarded field of chemical research.[18] In comparison to
conventional hydrogenation, hazardous hydrogen and expensive
pressure vessels can be avoided.[11a] The challenge is then to
design suitable catalysts bringing the reaction to equilibrium as
fast as possible.[19] Ruthenium is by far the most commonly used
transition metal to mediate hydrogen transfer, though iridium
catalysts are in general the most active ones.[19c] Pioneering work
on the field of iridium catalyzed transfer hydrogenation was
reported by Mestroni et al. in the late 1970s.[20] Since then,
numerous examples have been reported. While most transfer
hydrogenation catalysts are electronically neutral, there are also
some cationic representatives. One of the most famous might
be Crabtree's catalyst invented in the 1970s.[22] The usage of
iridium-NHC complexes in transfer hydrogenation in general is
even more widespread.[23]
In 2017 Choudhury et al. prepared a broad variety of five- and six-
membered cyclometalated iridium imidazolylidene complexes
and have introduced them as catalysts for transfer
hydrogenation.[24] Their study was focused in particular on the
stereoelectronic properties of such catalysts by comparison of
their activities in a model transfer hydrogenation reaction. The
cyclometalated complex derived from 3-methyl-1-phenyl-1H-
imidazol-3-ium iodide showed moderate activity yielding less than
5
6
+
cyclometallated [(η -cyclopentadienyl)(η -phenyl)iron(II)] substituent.
As its iron(II) precursors, the bimetallic iridium(III) complex was fully
characterized by means of spectroscopy, elemental analysis and
single crystal x-ray diffraction. In addition, it was investigated in a
catalytic study, wherein it showed high activity in transfer
hydrogenation compared to its neutral analogue having a simple
5
6
+
phenyl instead of a cationic [(η -cyclopentadienyl)(η -phenyl)iron(II)]
unit at the NHC ligand.
Introduction
N-Heterocyclic carbenes, commonly abbreviated as NHCs, are
an important class of ligands for organometallic complexes, which
became established in the 1960s until late 1970s.[1] They are
characterized as strong σ-donors and are therefore resembling
phosphines in terms of their ligand properties.[2] Depending on
their structure they are often more resistant to air, moisture and
heat though.[3]
The development of new NHC-based catalysts accelerated in
particular in the late 1990s.[4] The certainly most prominent
examples are the second-generation Grubbs catalysts for olefin
metathesis, which were obtained from the phosphine-substituted
species of the first generation.[5] However, the structural motif of
an NHC can also be employed to generate cyclometalated
transition metal complexes.[6] The resulting compounds are
defined by the presence of at least one carbon-metal bond in
addition to the NHC-metal bond.[7] Cyclometalated iridium(III)
compounds in particular are well known for specific applications
in photochemistry and as antibacterial and anticancer agents in
[21]
40% of racemic 1-phenylethanol after 3 h at a reaction
temperature of 100 °C and with the use of 20 mol-% KOH as the
base and isopropanol as the hydrogen source. The complex is
therefore considered as a catalytically less active structural motif
compared to various six-membered iridacycles providing more
efficient hydride transfer. To the best of our knowledge, this is the
1
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FULL PAPER
only catalytic application that has been reported with this
compound to date.
6
5
Our group recently investigated cationic [(η -arene)(η -
+
cyclopentadienyl)iron(II)] complexes as the part of a ligand’s
backbone. Such 18 VE compounds were first described in the late
1
950s.[25] In addition to some cationic phosphines that are
structurally related to commonly used triphenylphosphine, our
interest turned on the possibility of generating bimetallic N-
6
heterocyclic carbene complexes based on the cationic [(η -
5
+
[26]
arene)(η -cyclopentadienyl)iron(II)] motif.
Results and Discussion
Synthesis of the dinuclear iridium(III)/iron(II) complex
In 1992, Roberts had reported some imidazole and triazole deriv-
6
5
+
atives with [(η -arene)(η -cyclopentadienyl)iron(II)] substituents.
These compounds were considered to be suitable precursors for
our structural aims and thus mark the starting point of our
research efforts.[27] In a first step, sodium imidazolide, prepared
Figure 1. Molecular structure of complex 2 in the solid state. Characteristic bond
lengths (Å) and angles (deg), hydrogen atoms are omitted for clarity: Fe1−Cp
1.6627(2), Fe1-Ar 1.5305(2), N2-C6 1.409(2), C6-N2-C12 126.25(16), Cp-Fe-
5
[
28]
Ar 178.81(2), wherein Cp denotes the centre of the η -coordinated
according to a procedure published by Collmann et al.,
was
6
cyclopentadienyl ring. Ar denotes the centre of the η -coordinated arene ring.
6
5
reacted with (η -chlorobenzene)(η -cyclopentadienyl)iron(II)
hexafluorophosphate (1) in a nucleophilic aromatic substitution
reaction yielding 73% of the cationic imidazole 2 (Scheme 1).
Cyclopentadienyl and arene rings form a sandwich complex in
combination with the iron center, in which the angle between
these planes of the five and six-membered rings was calculated
as 1.6°. Thus, these two rings are almost parallel. The imidazole
ring is inclined with respect to the phenyl ring by about 28°.
Distances between the iron(II) site and the centroids of
cyclopentadienyl and arene ring are 1.6627(2) and 1.5305(2) Å,
respectively, which reflects the size of the two rings. The newly
formed C6-N2 bond in compound 2 has a length of 1.409(2) Å
which is slightly shorter than the typical value for a carbon
nitrogen single bond.[29] Furthermore, the solid state structure of
Scheme 1. Synthesis of the imidazole complex 2.
1H NMR spectroscopy unambiguously proves the successful
synthesis of compound 2. The resonances for the imidazole
protons of 2 appear at 8.15, 7.67 and 7.22 ppm, while the signals
the corresponding derivative carrying
a 4-methylimidazole
6
substituent instead of an imidazole substituent could also be
obtained. It is included in the Supporting Information (Figure S49).
To generate a suitable NHC precursor, complex 2 was converted
into a dicationic imidazolium salt. Iodomethane turned out to be a
suitable alkylating reagent.[30] The reaction of compound 2 with an
excess of iodomethane in acetonitrile at room temperature for 16
h provided the 18 VE iron(II) complex 3 in good yields of 71%
(Scheme 2).
of the η -coordinated phenyl ring are observed between 6.71 and
6
.28 ppm, thus shifted to higher field compared to a free phenyl
group. They exhibit the expected resonances with a doublet at
.71 ppm for the hydrogen atoms in the ortho-position and two
triplets for the hydrogen atoms in the meta- and para-position at
.43 and 6.28 ppm, respectively. The protons of the
6
6
cyclopentadienyl ring are assigned to a singlet at 5.04 ppm. The
presence of the hexafluorophosphate anion is proved by 31P NMR
spectroscopy, which reveals the typical septet for PF
-
6
at -144.3
ppm. In addition to the NMR spectra, elemental analysis and ESI
mass spectrometry further support the structural identification of
2. For the cation of complex 2 for example a peak at m/z = 265.02
(
calcd. 265.04) with a matching isotope pattern was observed.
Single crystals of 2 that were suitable for an X-ray structure
analysis were obtained by storing a saturated solution of 2 in
acetonitrile at -20 °C for several days. In Figure 1 the molecular
structure is depicted and typical bond parameters are
summarized.
Scheme 2. Generation of the dicationic NHC precursor 3.
In both, the 1H and the 13C NMR spectrum of 3 the additional
signal of the methyl group at 4.17 respectively 35.0 ppm is
noticeable at first sight. Furthermore, all proton resonances are
shifted to lower field compared to compound 1 due to the
additional positive charge, which is particularly distinct for the
protons of the imidazolium moiety (10.12, 8.30 and 7.73 ppm).
2
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The signal exposed at 10.12 ppm can be assigned to the proton
between the two nitrogen atoms of the imidazolium group. Its
chemical shift indicates a rather high acidity implying a smooth
generation of an N-heterocyclic carbene site in the following step.
By ESI-MS measurements multiple cationic complexes can be
identified. Molecular masses of m/z = 406.90 and 424.95
correspond to the dicationic ligand scaffold in combination with
either an iodide (calcd. 406.97) or a hexafluorophosphate (calcd
found up to date in single crystals, which might be explained by a
poorer solubility of the homoanionic salts.
With the imidazolium salt in hands, cyclometalation with half an
5
5
5 5 2 2 5 5
equiv. of the iridium(III) precursor [(η -C Me )IrCl ] (η -C Me =
1,2,4,5-pentamethylcyclopentadienyl) was carried out (Scheme
3). According to the presumed high acidity of imidaziolium salt 3,
triethylamine was chosen as the base, which has proven to be
well suitable for the synthesis of NHC complexes in combination
with rhodium and iridium.[31] The polar solvent acetonitrile ensured
425.03) anion resulting in monocationic species. Interestingly, the
cleavage of hydrogen iodide is observed under ESI conditions too, sufficient solubility of the dication.
leading to the cationic free carbene with m/z = 279.00 (calcd
279.06). The presumed structure of 3 was finally clarified by X-ray
structure analysis. Single crystals were obtained from a saturated
solution of 3 in acetonitrile by slow vaporization of the solvent at
room temperature. Figure 2 exhibits the molecular structure of 3*
(
the asterisk denotes the presence of two iodide anions in the
crystal) and summarizes typical bond parameters of the dicationic
species which is accompanied by two iodide counter anions.
Scheme 3. Synthesis of the cyclometalated iridium complex 4.
The synthesis according to Scheme 3 provided 4 in 85% yield as
an orange-colored powder in analytically pure form. First
evidence for a successful complexation of iridium(III) and the
cyclometallation of the iron(ii) coordinated phenyl ring can be
1
derived from its H NMR spectrum. One indicator is the absence
of the two-fold nitrogen-neighbored imidazolium proton. Both the
imidazole protons as well as the phenyl protons show a significant
shift to higher field compared to the precursor 3 due to the
reduced positive charge of the product. The absence of one of the
phenyl protons is also conspicuous as
a result of the
cyclometalation reaction. The signals of the four remaining
protons occur as a combination of two doublets and two triplets
(
ABCD spin system) in the range from 6.55 to 5.86 ppm whereas
Figure 2. Molecular structure of 3* with two iodide anions in the solid state.
Characteristic bond lengths (Å) and angles (deg), hydrogen atoms are omitted
for clarity: Fe1−Cp 1.6652(9), Fe1-Ar 1.5359(9), C6-N2 1.432(8), C15-N1
the resonance of the cyclopentadienyl ring is shifted to 4.89 ppm.
In addition to the singlet of the nitrogen-bound methyl group at
3
.88 ppm, an intense singlet at 1.77 ppm is assigned to the
1
.473(9), C12-N2-C6 124.4(5), C13-N2-C6 126.8(6). Cp denotes the centre of
1,2,3,4,5-pentamethylcyclopentadienyl ligand. The 13C NMR
spectrum exhibits the familiar signals with the resonance of the
carbene carbon atom being located at 163.8 ppm. Additionally,
31P NMR data as well as elemental analysis confirm the presence
of the hexafluorophosphate counter anion, obviously due to the
poorer solubility of triethylammonium chloride compared to
triethylammonium hexafluorophosphate. ESI-MS measurements
reveal a peak at m/z = 732.93, which is in accordance with the
simulation for 4 (733.03). Suitable single crystals for an X-ray
structure analysis were obtained by recrystallization of 4 by
keeping a solution of the compound in 1,2-difluorobenzene
overlaid with toluene for several days at -30 °C. Figure 3 depicts
the molecular structure of the bimetallic complex in the solid state
and summarizes typical bond parameters.
5
6
the η -coordinated cyclopentadienyl ring. Ar denotes the centre of the η -
coordinated arene ring.
The molecular structure of compound 3* closely resembles that of
compound 2. The distance between the iron center and the
centroids of cyclopentadienyl ring and arene ring are 1.6652(9)
and 1.5359(9) Å, respectively, which are within the expected
range. In addition, the newly established nitrogen-carbon bond
between N1 and C15 is evident with 1.473(9) Å, which
corresponds to a typical carbon-nitrogen bond. A slight elongation
of the C6-N2 bond (1.432(8) Å) in comparison to the molecular
structure of 2 with 1.409(2) Å can be noted. The reason might be
the electron-withdrawing effect of both the cyclopentadienyl-
iron(II) and the imidazolium cations on each side of the bond.
Considering the C12-N2-C6 angle, only a very small change from
126.25(16)° to 124.4(5)° can be observed.
Additional structural data for the dication of 3 in combination with
two hexafluorophosphate counter anions (3**) can be found in the
Supporting Information (Figure S50). Although it would have been
consistent with the statistical distribution of counterions, the
permutation of a single iodide and a hexafluorophosphate was not
3
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performance. Surprisingly, rather good conversions were
observed with a catalyst loading of just 1 mol-% (Table 1).
Different bases were examined. Among them potassium tert-
butoxide was found to be the most active, followed by potassium
hydroxide and potassium phosphate. 10 mol-% of base in the
reaction mixture gave the best conversions, lower ratios (5 and
7.5 mol-%) led to an increase of reaction time. Based on our
experience with transfer hydrogenation reactions, the
temperature was set to 82 °C.[32] The activity of the catalyst
decreases rapidly with temperature: At 70 °C, only 4% of
conversion could be detected after 10 min. In the first five minutes
the catalyst shows little activity, either due to a warm-up phase of
the reaction mixture or an activation period of the catalyst.
In comparison to the studies of Choudhury et al. employing a
related neutral cyclometalated iridium NHC complex without the
+
[24]
[
CpFe] complex fragment, the transfer hydrogenation reaction
Figure 3. Molecular structure of 4 in the solid state. Characteristic bond lengths
of acetophenone induced by complex 4 is faster by at least two
orders of magnitude. While Choudhury et al. observed less than
40% of racemic 1-phenylethanol after 3 h using 20 mol-% of KOH
and a reaction temperature of 100 °C, the reaction is completed
within minutes in our case. This is even more remarkable since
we were able to decrease the reaction temperature to 80 °C and
the concentration of the comparable base potassium tert-butoxide
to 10 mol-%. In this case, it should also be explicitly pointed out
that we obtained similar results with the base KOH instead of
potassium tert-butoxide as well. Thus, the difference in activity is
obviously not mainly due to the change of base. In adddition to
the model substrate acetophenone, we were able to achieve
excellent yields in short reaction times for a series of related
substrates (Table 1).
(
Å) and angles (deg), hydrogen atoms are omitted for clarity: Fe1−Cp 1.6633(5),
Fe1−Ar 1.5578(4), N2-C6 1.409(3), Ir1-C11 2.043(2), Ir1-C12 1.999(2), Ir1-I1
.6882(5), Ir1-Cp* 1.8619(5), C12-Ir1-C11 77.79(8), C12-N2-C6 115.36(2), N2-
2
C6-C11 112.14(2), C12-Ir1-I1 86.55(6), C11-Ir1-I1 97.19(6), C11-C6-N2-C12
5
6
.40(5). Cp denotes the centre of the η -coordinated cyclopentadienyl ring. Ar
6
denotes the centre of the η -coordinated arene ring. Cp* denotes the centre of
the η -coordinated 1,2,3,4,5-pentamethylcyclopentadienyl ring.
5
The distances between both the cyclopentadienyl ring and the
phenyl ring, and the iron(II) center resemble those of the previous
structures, indicating only minor differences in this part of the
molecule. However, the C12-N2-C6 angle is decreased from
124.4(5) to 115.36(2) °C due to the chelating coordination of the
iridium(III) site. It is remarkable that the bond distance between
Ir1 and the carbene-type carbon atom C12 is significantly shorter
(
1.999(2) Å) than the bond distance between Ir1 and the
carbanion-type carbon atom C11 (2.043(2) Å). The angles
between the arene and the imidazole ring distort slightly from
planarity as the torsion angle C12-C6-N2-C11 is 6.4° and angles
of 97.19° for C11-Ir1-I1 and 85.66° for C12-Ir1-I1 differ due to
nonbonding repulsions in the ligand environment.
Catalytic transfer hydrogenation
To gain information on the cooperative influence of the [CpFe]+
moiety in 4 on the catalytic activity of the iridium(III) site, we
examined its performance in the catalytic transfer hydrogenation
of ketones and compared the results with those reported by
Choudhury et al. for analogue iridium(III) complexes having no
+
[24]
[
CpFe] function.
hydrogenation of acetophenone to phenyl ethanol was chosen
Scheme 4).
As
a
model reaction the transfer
(
Scheme 4. Optimized reaction conditions for the transfer hydrogenation with 4.
Since transfer hydrogenation includes the transfer of a metal-
bound hydrido ligand to the carbonyl carbon atom, it might be
expected that the chelating and positively charged iridium-bound
NHC ligand of 4 could have a detrimental effect on the catalytic
4
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Table 1. Substrate scope of the iridium catalyzed transfer
hydrogenation.[a]
Entry
1
Substrate
Yield
Time
97 %[b]
10 min
95 %[b]
97 %[c]
15 min
15 min
2
3
4
5
6
98 %[c]
30 min
30 min
30 min
93 %[b]
Scheme 5. Possible mechanisms for the transfer hydrogenation with compound
4
(the C,C chelating ligand is abbreviated by its both carbon donor sites): top,
-hydrogen transfer reaction with preservation of the C,C coordination; middle,
-hydrogen transfer reaction after protolytic cleavage of the Ir-phenyl bond;
>69 %[b]

bottom: Meerwein-Pondorf-Verley-type mechanism.
7
8
>99 %[b]
>99 %[b]
60 min
90 min
On one side, a -hydrogen transfer from an iridium-coordinated
isopropanolato ligand can take place. The ensuing acetone ligand
is then substituted in equilibrium by the aromatic ketone, to which
the hydrido ligand is transferred. Alternatively, the -hydrogen
atom is directly transferred from the iridium-coordinated
isopropanolate to the carbonyl carbon atom of an iridium-
coordinated ketone (inner or outer sphere).[33]
[
a] Reaction conditions: 1 mmol of substrate, 5 mL of isopropanol, 1 mol-% of
catalyst 4, 10 mol-% of KO Bu, 100 µL of tetradecane (internal standard), 80 °C,
b] yields determined by GC, [c] yields determined by NMR.
t
Therefore, in the DFT-calculated sequences, the isopropanolato
complex A has to be generated for both cases in the first step,
[
starting from the monocationic 18 VE complex 4 by substitution of
Table 1 reveals that beside acetophenone, the sterically more
demanding substrates 2-acetonaphthone and benzophenone can
be converted at similar rates. Various para-substituted
acetophenone derivatives (entries 5 to 7) also provide rapid
conversions to the corresponding alcohol in equilibrium
regardless of the nature of the substituent. Moreover, even
electron-rich aliphatic ketones such as 2-hexanone and
cyclohexanone can be hydrogenated efficiently. In contrast, the
transfer hydrogenation of benzaldehyderequired 24 h for a
conversion of just 48% and the reduction of nitrobenzene to
aniline (or intermediates) was not observed within one day.
the iodido ligand by isopropanolate.[34] The structure of complex
A is depicted in Figure 4.
Computational study
The generally high activity of the cationic catalyst 4 in transfer
hydrogenation reactions raises the question for the mechanism.
According to the transfer hydrogenation process depicted in
Scheme 4 dihydrogen is transferred from the hydrogen donor
isopropanol to the aromatic ketone. While the oxygen-bound
hydrogen atom can be transferred via a simple proton exchange
between an acid and a base, there are at least three possible
mechanisms for the carbon-bound hydrogen atom to be
transferred, which are summarized in Scheme 5.
Figure 4. Sequence A-B-C.
5
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Geometry optimization of the isopropanolato complex A revealed
the geometric parameters listed in Table S2. An Ir-H distance of
331 pm and an Ir-O-C angle of 126° indicate that the -hydrogen
atom of the isopropanolate is not bonded to the iridium(III) center.
The saturated 18 VE situation of the iridium(III) center and the
nonbonding repulsion of the isopropyl CH groups with other the
3
ligands around appear to be the cause. To calculate the reaction
pathway of a -hydrogen transfer to the metal center we
elongated the C-H
path. The transition state B was located E = +147 kJ/mol above
complex A. In transition state B, a short distance Ir-C of 2.17 Å

bond and optimized the geometries along the
#
1
and an angle of Ir-C-C of 119° are indicative of a  -bonded
pentamethylcyclopentadienyl ligand. By the shift in bonding from
5
to  , the formally 18 valence electron iridium complex
1
generates a vacant coordination site at the metal center in the
transition state B. IRC calculations revealed the reaction path
5
1
depicted in Figure 4 including a ring slippage from  to  of the
cyclopentadienyl ligand. Geometry optimization of the
intermediate C revealed a short Ir-H distance of 1.60 Å and Ir-C11
and Ir-C12 distances of 2.04 respectively 2.01 Å as well as similar
Ir-C distances for the pentamethylcyclopentadienyl ring. These
parameters are indicative of a nearly tetrahedral 18 VE iridium
complex in the final state. In addition, the resulting ketone is not
bound to the iridium center. We found that the ketone is bound via
a weak O-H hydrogen bond to the methylimidazolylidene ring. The
energy of complex C was calculated to be -105 kJ/mol below
complex A.
The catalytic transfer hydrogenation (Table 1, Entry 1) was
performed at 80°C and is completed after 10 minutes employing
acetophenone, indicating a much lower activation barrier. Thus,
the -hydrogen transfer to the saturated iridium center is not very
likely. In addition, the strong stabilization of intermediate C will
result in a very high activation barrier for the reverse reaction, a
migratory insertion of a ketone into the Ir-H bond, here ca. 260
kJ/mol.
Figure 5. Optimized structures along the beta-hydrogen transfer paths D-E-F.
For this path, a proton has been transferred to the phenyl ring.
To reveal the activation barriers for the -hydrogen transfer from
the isopropanolato ligand to the iridium center, we calculated the
reaction paths which are presented in Figure 5. A transition state
E was located 89 kJ/mol above D which is about 54 kJ/mol lower
in relative energy than transition state B for the saturated 18 VE
complex (Figure 4). Thus, the H-transfer to the iridium center is
orders of magnitude slower in the 18VE system. To complete the
catalytic cycle the acetone in complex F will be replaced by the
substrate ketone following back the reaction path from F to D The
alcoholato ligand is then replaced by isopropanol revealing an
isopropanolato ligand and the finally hydrogenated alcohol.
For a related path starting from a 16VE iridium complex with a
protonated imidazolidine ligand we calculated an activation
barrier of 109 kJ/mol (transition state E’, see the Supporting
Information Figure S51), which is about 20 kJ/mol higher than the
barrier shown in Figure 4 (protonated phenyl ring). Thus, this path
was excluded.
From our calculations we conclude, that an additional
coordination site at an unsaturated iridium center is deemed
necessary. Under the reaction conditions with an excess of
isopropanol,
CpFe(phenylimidazolylidene)
a
proton transfer might occur to the
ligand generating free
a
coordination site accompanied with a protonated ligand as
depicted in Figure 5 (structure D). To decide, whether such a
reversible protic ligand cleavage is possible in isopropanol,
t
compound 4 was stirred with 10 mol-% of KO Bu in isopropanol-
An alternative path includes the direct outer-sphere hydride
transfer to a ketone as suggested in a Meerwein-Ponndorf-Verley
(MPV) type of reaction or to an outer-sphere assembled ketone in
the reaction mixture. Due to our experimental results that the
hydrogenation of substrate 7 is slower by a factor of about six an
outer-sphere hydride transfer seems unlikely.[24]
8
d for 1 h at 80 °C. The reversibility of the cyclometallation reaction
2
was detected by means of D NMR spectroscopy, which provided
one resonance at 5.69 ppm, that is assigned to the deuteration of
the ortho-position of the iron(II) coordinated phenyl ring. From this
observation it not only can be concluded, that the cyclometallation
is reversible in isopropanol (in the presence of a base). It can also
be concluded, that there must be a second, minor isomer of
compound 4, wherein the (CpFe) and the (Cp*Ir) units are not
oriented in anti- but in syn-position to each other. Calculation of
complex A-syn revealed nearly the same energy than for complex
A-anti which is in accord with the experimental observation.
6
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Chemistry - A European Journal
10.1002/chem.202102520
FULL PAPER
For the calculation of a MPV reaction path of the isenthalpic
hydrogen transfer from isopropanol to acetone we started from
the dicationic complex G which was generated by addition of an
acetone ligand to complex D depicted in Figure 6. A short bond
Ir-O1 of 1.94 Å and a very long Ir-O2 distance of 4.41 Å indicate,
that in the initial complex G the ketone is only weakly hydrogen
bonded to the complex as depicted in Figure 6. The energy for
complex G was calculated 31 kJ/mol below complex D. Following
the hydrogen transfer path from the alcoholato to the ketone
ligand a further a transition state H was reached at C-H
distances of 1.31 Å and 1.34 Å respectively as depicted in Figure
A suitable route for the preparation of a dicationic imidazolium
5
6
compound carrying
a (η -cyclopentadienyl)(η -phenyl)iron(II)
substituent was established. Both the mono cationic imidazole
precursor and the dicationic imidazolium salt were fully
characterized and crystal structures could be obtained.
Furthermore, a monocationic, cyclometalated iridium(III)/iron(II)
complex was prepared from the imidazolium salt. This bimetallic
iridium(III)/iron(II) compound provides unexpectedly high activity
in transfer hydrogenation reactions. Especially in comparison with
the neutral cyclometalated iridium(III) complex derived from 3-
methyl-1-phenyl-1H-imidazol-3-iumiodide the increase in catalytic
activity is substantial. The cationic cyclopentadienyl iron
complexation appears to be the cause. The new bimetallic
catalyst allows a reduced amount of base as well as moderate
temperatures. DFT-calculations on the reaction path revealed,
that a reaction sequence starting from aH-transfer to the
iridium center, replacement of the resulting ketone and reinsertion
into the Ir-H bond is energetically favored over an outer-sphere
hydride transfer along a Meerwein-Pondorf-Verley type of path
starting from a 16 valence electron complex. However, further
investigations of the reaction mechanism are mandatory. In
particular, we are interested in whether the increased catalytic
activity is due to the positive charge of the molecule or due to a
direct influence of the iron center. Moreover, the application of the
catalyst in tandem reactions seems feasible.
6. Distances Ir-O1 of 2.16 Å and Ir-O2 of 2.15 Å indicate a six
membered ring consisting of two carbonyl groups one hydrogen
and one iridium center in a twist-like conformation (cf. Figure 6).
The energy for transition state H is calculated +133 kJ/mol above
complex G. Transition state H is calculated to be 13 kJ/mol less
favored than transition state E starting from complex G upon
dissociation of acetone. Thus, an outer-sphere hydride transfer
from the alcoholate to an acetone molecule is calculated to be
much slower than by a sequence of -hydride transfer to the
iridium center, exchange of the ketone and reinsertion of the
ketone into the Ir-H bond.
Experimental Section
6
5
(
η -Chlorobenzene)(η -cyclopentadienyl)iron(II) hexafluorophosphate (1)
was prepared following a procedure published in the literature.[
27b]
All
reactions were carried out under an argon atmosphere using standard
Schlenk techniques. The solvents were either freshly distilled or dried and
degassed before use according to standard techniques. Commercially
available chemicals were purchased from ABCR, Alfa Aesar, Sigma
Aldrich, Strem or TCI. 1H, 13C and 31P NMR spectra were recorded on
BRUKER Spectrospin Avance 400 and 600 spectrometers. The chemical
shifts are referenced to internal solvent resonances. The multiplicities are
reported as s = singlet, d = doublet, t = triplet, q = quartet, sept = septet
and m = multiplet. ESI-mass spectrometric measurements were performed
on an AmaZon ETD by introducing solutions of the compound in
acetonitrile. Elemental analyses were carried out with a Vario MICRO
Cube elemental analyzer at the Analytical Laboratory of the Technische
Universität Kaiserslautern. For GC analyses, a Clarus 580 GC equipped
with an FID-detector was used.
Figure 6. Optimized structures along the reaction path G-H of the MPV-type
reaction sequence.
5
6
(
η -Cyclopentadienyl)(η -1-phenyl-1H-imidazol)iron(II)
hexafluoro-
6
5
phosphate (2): 4.96 g (13.1 mmol) of (η -chlorobenzene)(η -cyclopenta-
dienyl)iron(II) hexafluorophosphate (1) and 2.03 g (22.5 mmol) of freshly
prepared sodium imidazolide were stirred in 50 mL of acetonitrile at room
temperature for 2 h. The reaction mixture was filtered over neutral alumina
and the solvent was removed under vacuum. The product precipitated as
a yellow powder after the addition of 25 mL of diethyl ether. Yield: 3.92 g
73%). Anal. calcd. for C14
0.50, H 2.91, N, 6.82%. H NMR (400 MHz, CD
s, 1H), 7.22 (s, 1H), 6.71 (d, J = 6.7 Hz, 2H), 6.43 (t, J = 6.5 Hz, 2H), 6.28
(t, J = 6.2 Hz, 1H), 5.04 (s, 5H). 13C NMR (101 MHz, (CD ) CO):  137.4,
We conclude from our DFT-calculations, that
a transfer
hydrogenation proceeding via a 18 VE species with a carbanion
and NHC coordinating (chelating) ligand is much slower than the
same reaction occurring with a complex, where the Ir-C(phenyl)
(
4
(
H
13
F
6
FeN
2
P: C 41.01, H 3.20, N 6.83; found: C
CN):  8.15 (s, 1H), 7.67
bond is initially cleaved by proton transfer. The presence of the
1
3
+
[
CpFe] fragment resulting in a weakened Ir-C(phenyl) bond
appears to be the cause for this reaction path. We also observe a
decrease in the reactivity of catalyst 4 by switching from
acetophenone to 4-Cl-acetophenone as the substrate in contrast
to the observations of Choudhury et al..
3
2
31
132.6, 119.0, 108.0, 88.6, 88.2, 81.1, 79.2. P NMR (162 MHz, CD
 -144.3 (sept, J = 711.3 Hz). ESI-MS (CD CN): m/z 265.02 [C14
3
CN):
+
3
H
2
13FeN ] .
5
6
(
η -Cyclopentadienyl)(η -1-phenyl-4-methyl-1H-imidazol)iron(II)
hexafluorophosphate (2a): This complex was synthesized as described
above for compound 2 using 0.14 g (1.32 mmol) of freshly prepared
sodium 4-methylimidazolide and 0.50 g (1.32 mmol) of 1. Yield: 390 mg
Conclusion
15 6 2
(70%) of a yellow solid. Anal. calcd. for C15H F FeN P C 42.48, H, 3.56
7
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Chemistry - A European Journal
10.1002/chem.202102520
FULL PAPER
1
N, 6.61; found: C 42.11, H 3.45, N 6.49%. H NMR (600 MHz, CD
8
6
3
CN): 
.01 (s, 1H), 7.38 (s, 1H), 6.65 (d, J = 6.4 Hz, 2H), 6.40 (t, J = 6.4 Hz, 2H),
.25 (t, J = 6.1 Hz, 1H), 5.01 (s, 5H), 2.23 (s, 3H). 13C NMR (151 MHz,
CN):  142.1, 136.8, 118.4, 115.3, 108.0, 88.3, 87.9, 80.5, 79.0, 13.9.
All hydrogen atoms were placed in calculated positions and refined by
using a riding model. CCDC 2092872-2092876 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk.
CD
3
3
1
P NMR (162 MHz, CD
3
CN):  -144.6 (sept, J = 706.2 Hz). ESI-MS
+
(
3
CD CN): m/z 279.00 [C15
H
15FeN
2
] .
5
6
(
η -Cyclopentadienyl)(η -1-phenyl-3-methylimidazolium)iron(II)
Acknowledgements
hexafluorophosphate iodide (3): 6.00 g (14.6 mmol) of compound 2
were dissolved in 25 mL of acetonitrile. 22.8 g (159 mmol, 10 mL) of
iodomethane were added. After stirring overnight at room temperature, a
saffron-yellow solid had precipitated. It was washed with diethyl ether
The authors wish to thank the DFG-funded transregional
collaborative research center SFB/TRR 88 “Cooperative effects
in homo- and heterometallic complexes (3MET)” for financial
support. C. M. would also like to sincerely thank Nelly Nunheim
and Moritz Knöringer for fruitful discussions and Tobias Grimm for
providing GC blank samples of some substrates for catalysis.
Furthermore, thanks are due to Dr. Jonathan Becker of Justus-
Liebig-Universität Gießen for measuring single crystal structures.
(
3x10 mL) and isolated with a yield of 71% (5.70 g, 10.3 mmol). Anal. calcd.
for C15 FeIN P: C 32.64, H 2.92, N 5.07; found: C 32.91, H 2.97, N
.16%. H NMR (600 MHz, CD NO ):  10.12 (s, 1H), 8.30 (s, 1H), 7.73 (s,
H), 7.29 (d, J = 6.5 Hz, 2H), 6.75 (t, J = 6.4 Hz, 2H), 6.62 (d, J = 6.2 Hz,
H), 5.41 (s, 5H), 4.17 (s, 3H). 13C NMR (151 MHz, CD
NO ):  138.5,
27.0, 123.0, 105.7, 90.2, 89.2, 83.8, 80.9, 38.2. 3 P NMR (162 MHz,
H
16
F
6
2
1
5
1
1
1
3
2
3
2
1
CD
3
NO
2
):  -142.4 (sept, J = 711.3 Hz). ESI-MS (CD
3
CN): m/z 424.95
+
+
+
[
C
15
H16FeN
2 6 2
PF ] , 406.90 [C15H16FeN I] , 279.00 [C15H15FeN
2
] .
Keywords: cyclometalation • iridium • iron • N-heterocyclic
carbene • transfer hydrogenation
5
2
2,7
6
[
(η -Cyclopentadienyl)κ C -(η -1-phenyl-3-methylimidazol-2-yliden)-
5
iron(II)](iodido(η -1,2,3,4,5-pentamethylcyclopentadienyl)iridium(III))
hexafluorophosphate (4): 410 mg (0.52 mmol) of bis(dichlorido(η -
,2,3,4,5-pentamethylcyclopentadienyl)iridium(III)) and 569 mg (1.03
5
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This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202102520
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Iron Power: A cyclometalated Fe(II) and Ir(III) complex based on the cationic (η -cyclopentadienyl)(η -phenyl)iron(II)]imidazole skeleton
is reported. Compared to its neutral analogue without the [CpFe]+ fragment, it is surprisingly active in the transfer hydrogenation of
ketones. Mechanistic investigation on the reaction paths performed by DFT calculations provided a better understanding of the reaction
mechanism, possible intermediates and the active species.
1
0
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