Synthesis, characterization and derivatization of hydroxyl-functionalized iron(II) bis(NHC) complexes

Article abstract of DOI:10.1039/c7dt04774f

The syntheses of a novel hydroxyl-functionalized tetradentate NHC/pyridine hybrid ligand and the corresponding Ag(i) and Fe(ii) complexes are presented. Spectroscopic and X-ray diffraction techniques are used for structural investigations and cyclic voltammetry measurements reveal interesting electronic properties. Transmetalation of the trinuclear Ag(i) complex (C1) yields a mononuclear and a dinuclear iron(ii) bis(NHC) complex (C2 and C3), which can be separated by stepwise precipitation. The former is isostructural to iron(ii) bis(NHC) complex A, which is a versatile oxidation catalyst. Furthermore, suitable conditions for esterification reactions of the ligand precursor and iron(ii) bis(NHC) complex (C2) have been established, demonstrating the utility of the hydroxyl functionality for immobilization and derivatization purposes.

Full text of DOI:10.1039/c7dt04774f

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Synthesis, characterization and derivatization of
hydroxyl-functionalized iron(II) bis(NHC)
complexes†
Cite this: DOI: 10.1039/c7dt04774f
a
a
b
b
Anja C. Lindhorst,‡ Manuel Kaspar,‡ Philipp J. Altmann, Alexander Pöthig
and
a
Fritz E. Kühn
*
The syntheses of a novel hydroxyl-functionalized tetradentate NHC/pyridine hybrid ligand and the corres-
ponding Ag(I) and Fe(II) complexes are presented. Spectroscopic and X-ray diffraction techniques are used
for structural investigations and cyclic voltammetry measurements reveal interesting electronic properties.
Transmetalation of the trinuclear Ag(I) complex (C1) yields a mononuclear and a dinuclear iron(II) bis(NHC)
complex (C2 and C3), which can be separated by stepwise precipitation. The former is isostructural to
iron(II) bis(NHC) complex A, which is a versatile oxidation catalyst. Furthermore, suitable conditions for
esterification reactions of the ligand precursor and iron(II) bis(NHC) complex (C2) have been established,
demonstrating the utility of the hydroxyl functionality for immobilization and derivatization purposes.
Received 18th December 2017,
Accepted 4th January 2018
DOI: 10.1039/c7dt04774f
rsc.li/dalton
a series of hydroxyl-functionalized bis(NHC) ligands with a
variety of different wingtips. The corresponding palladium
1
. Introduction
N-Heterocyclic carbenes (NHC) have become an important complexes could be successfully immobilized via the hydroxyl
ligand class in organometallic chemistry due to their high group and applied as catalysts for Suzuki–Miyaura reac-
1
5,16
flexibility in terms of steric and electronic properties, particu- tions.
larly since the first catalytic applications of transition metal were isolated showing antiproliferative effects on cancer cell
NHC complexes were reported in the 1990s.
Moreover, coinage metal complexes of these ligands
1
–6
17
Transition lines. Recently, Hölscher and coworkers described a carboxyl-
metal NHC complexes bearing multidentate ligand scaffolds ate-functionalized bis(NHC) iridium complex, which is water-
1
8
with methylene-bridged NHC units are particularly stable due soluble and catalyzes the hydrogenation of CO
2
to formate.
to the stabilizing chelate effect and the formation of a six- In the context of catalytic applications, iron NHC complexes
7
–9
membered metallacycle. Moreover, in addition to the wing- have gained increasing interest in the past two decades and
tips and imidazolium backbone, bridging units offer an could successfully be applied as catalysts in various homo-
additional site for the modification of NHC ligands. geneous reactions including C–C couplings, reductions and
1
9,20
Specifically designed functional groups can, for example, alter oxidations.
Iron(II) complex A for example, bearing a
the solubility of transition metal NHC complexes, be used as methylene-bridged bis(NHC/pyridine) ligand, proved to be cat-
anchoring sites for immobilization or for the extension of the alytically active in the oxidation of alkenes and arenes, olefin
21–25
ligand structure by further donor moieties or even metal epoxidation as well as aldehyde olefination reactions (Fig. 1).
7
–10
centers.
In contrast to wingtip and backbone modifi-
cations, the number of reports on bridge-functionalized bis
1
1–14
(NHC) ligands is limited.
For instance, our group reported
a
Molecular Catalysis, Catalysis Research Center and Department of Chemistry,
Technical University of Munich, Lichtenbergstr. 4, D-85747 Garching bei München,
Germany. E-mail: fritz.kuehn@ch.tum.de; Fax: (+49)-89-289-13247;
Tel: (+49)-89-289-13096
b
Catalysis Research Center and Department of Chemistry, Technical University of
Munich, Lichtenbergstr. 4, D-85747 Garching bei München, Germany
†
Electronic supplementary information (ESI) available. CCDC 1811950–1811954.
For ESI and crystallographic data in CIF or other electronic format see DOI:
Fig. 1 Iron(II) bis(NHC) complex A is a potent catalyst for the oxidation
of alkenes and arenes, olefin epoxidation as well as aldehyde olefination
reactions.
10.1039/c7dt04774f
21–25
‡
These authors contributed equally to this work.
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However, especially under oxidizing conditions, catalyst degra- high reaction temperature of 130 °C. However, the enhanced
dation is a serious issue and strategies towards higher dura- elimination of HBr from 2,2-dibromoethanol under these con-
bility are to be explored. Potentially, this could be achieved by ditions results in a moderate overall yield of 39% after anion
altering the coordination environment of the iron center with exchange with NH
particular focus in the trans-labile coordination sites to
4 6
PF .
2
.2. Synthesis and characterization of hydroxyl-
prevent unselective interactions with, e.g., radicals.
functionalized iron(II) NHC complexes
Immobilization of molecular complexes or the introduction of
additional, reversibly coordinating donor moieties may there- There is a variety of different approaches, allowing access to
1
9
fore benefit the catalytic performance and can be realized via iron NHC complexes. Unlike the reported synthesis of iron(II)
1
0
26
suitable functionalization of the tetradentate ligand.
Furthermore, synthesis of a heterogenized analogue of iron [Fe{N(SiMe ) } (THF)] with the bis(imidazolium) salt P1
complex A, aminolysis of iron(II) bis(trimethylsilyl)amide
3
2 2
NHC complex A could help to elucidate the fate of the catalyti- (see Scheme 1) yielded no product. This may be attributed to
cally active species as a spacial separation of the reactive sites the presence of the hydroxyl group, which presumably reacts
should translate into a higher turnover number if a bimolecu- with one of the internal base equivalents and subsequently
lar degradation pathway is present.
may form a stable iron-oxo bond. Alternatively, transmetala-
In this work, the synthesis and characterization of tion of silver NHC complexes with iron halides has been
1
,1′-(2-hydroxyethane-1,1-diyl) bridge-functionalized silver and reported as an access route to iron NHC compounds.27–29 In
iron bis(NHC) complexes are reported. Performing esterification line of this, silver NHC complex C1 was synthesized from P1
1
reactions, the utility of the hydroxyl group as a versatile and silver(I) oxide with 84% yield (Scheme 2). H NMR spectra
target for the derivatization of the bis(imidazolium) salt itself in solution reveal all expected signals of the ligand molecules
and the corresponding mononuclear iron NHC complex is and the splitting pattern suggests a symmetric coordination
demonstrated.
mode (cf. ESI-MS, Fig. S7†). The characteristic signals of the
1
3
1
carbene carbon atoms in
C{ H} spectra are found at
1
83.67 ppm, which is consistent with previously reported Ag
3
0,31
complexes bearing NHC/pyridine hybrid ligands.
2
. Results and discussion
Single crystals suitable for X-ray diffraction were obtained
by slow diffusion of diethyl ether into an acetonitrile solution
2
.1. Synthesis of a hydroxyl-functionalized ligand precursor
In order to introduce a suitable functional group for derivatiza- of C1. Similarly to the structure of the silver(I) complex bearing
tion and immobilization purposes to iron NHC complex A, the analogous un-functionalized NHC/pyridine hybrid
3
0
bridge-functionalized bis(imidazolium) salt P1 was synthesized ligand, three silver ions are coordinated by two ligand mole-
in a nucleophilic substitution reaction from 2,2-dibromo- cules (Fig. 2). Two of the silver atoms are coordinated almost
ethanol and 2-(imidazol-1-yl)pyridine in analogy to litera- linearly by the carbene moieties (bond angles of 175.5(5)° and
1
5–17
ture-known procedures.
Due to the low nucleophilicity of 177.6(5)°) and the third one is bound to two pyridine units
the pyridine-substituted imidazole, no reaction was observed and two acetonitrile molecules. The Ag–CNHC distances range
using 2,2-dichloroethanol as a substrate. Instead, the more between 2.089(13) and 2.105(13) Å corresponding well to pre-
3
2
reactive 2,2-dibromoethanol has to be employed as well as a viously reported silver–NHC bond distances. The three silver
Scheme 1 Synthesis of bis(imidazolium) salt P1.
Scheme 2 Synthesis of silver and iron bis(NHC) complexes bearing a pendant hydroxyl group.
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Fig. 2 ORTEP style representation of the cationic fragment of C1 with
ellipsoids shown at the 50% probability level. Hydrogen atoms, cocrys-
tallized solvent molecules, and counter ions are omitted for clarity.
Selected geometrical parameters: Ag1–Ag3 2.9153(14) Å, Ag2–Ag3
Fig. 3 ORTEP style representation of the cationic fragment of C2 with
ellipsoids shown at the 50% probability level. Hydrogen atoms, cocrys-
tallized solvent molecules, and counter ions are omitted for clarity.
Selected geometrical parameters: Fe1–C1 1.826(4) Å, Fe1–C10 1.833(4) Å,
Fe1–N7 1.913(4) Å, Fe1–N8 1.923(4) Å, Fe1–N3 2.082(3) Å, Fe1–N6
2.9024(14) Å, Ag1–C1 2.089(13) Å, Ag1–C19 2.105(13) Å, Ag3–N3
2.348(11) Å, Ag3–N12 2.385(11) Å, Ag3–N13 2.611(14) Å, Ag3–N14
2.585(14) Å, C1–Ag1–C19 177.6(5)°, C10–Ag2–C28 175.5(5)°,
N3–Ag3–N12 137.7(3)°.
2.104(4) Å, Fe1–O1 4.061(4) Å, N7–Fe1–N8 173.99(15)°.
atoms form a triangle and the rather short Ag1–Ag3 and Ag2–
Ag3 distances of 2.9153(14) Å and 2.9024(14) Å suggest argen- ial coordination plane and no iron–oxygen interaction is
3
2
tophilic interactions.
observed (distance Fe1–O1 4.061(4) Å). These data confirm
Transmetalation of C1 with FeBr (THF) at room tempera- that complex C2 is isostructural to A and that a hydroxyl-
2
2
ture affords a mixture of the expected mononuclear iron(II) functionalization of the ligand methylene bridge does not
complex C2 and a dinuclear iron(II) complex C3 in almost have any major influence on the coordination geometry.
equal amounts (Scheme 2). To balance the stoichiometry an
In contrast, complex C3, which is formed simultaneously
additional equivalent of AgPF is added during the reaction. with C2 in the transmetalation reaction, possesses a dinuclear
6
1
By increasing the reaction temperature to 50 °C, the ratio of structure as indicated by ESI-MS. H NMR spectra in solution
the two product complexes can be shifted in favor of the further reveal that the symmetry of the coordination of the
mononuclear complex C2 (C2 : C3 2 : 1). The two complexes, tetradentate ligand is broken as for each proton of the pyridine
which are both air-stable, can be separated by stepwise and imidazolylidene units a separate signal can be observed
addition of diethyl ether to the acetonitrile solution. As the (cf. ESI-MS, Fig. S13†). Interestingly, the proton signal corres-
dinuclear complex C3 is less soluble, it precipitates first and ponding to the methylene group adjacent to the hydroxyl group
can be collected by filtration. Further addition of diethyl ether is split into two independent signals indicating that the two
1
3
1
affords C2 as a red solid.
CH protons are diastereotopic in this complex. C{ H} NMR
2
The monomeric structure of C2 was proven by ESI-MS tech- spectra of C3 in solution exhibit two signals corresponding to
1
niques and H NMR spectroscopic investigations confirm the the carbenic carbon atoms at 209.92 and 200.85 ppm, respect-
equatorial coordination geometry of the tetradentate ligand as ively. The structure could be further confirmed by X-ray diffrac-
only one set of signals is present corresponding to the pyridine tion of single crystals obtained by slow diffusion of diethyl
1
3
1
and imidazolylidene units (cf. ESI-MS, Fig. S10†). The C{ H} ether into a solution of C3 in acetonitrile (Fig. 4). It shows that
signals of the carbenic carbon atoms appear at a chemical each of the NHC/pyridine units of the tetradentate ligand coor-
shift of 217.06 ppm and lie in the range of previously reported dinates to a different iron atom thus bridging the two metal
1
9,20
iron(II) NHC complexes.
X-ray diffraction measurements of centers and creating a distorted octahedral coordination geo-
single crystals obtained by slow diffusion of diethyl ether into metry, which is completed by two cis-coordinated acetonitrile
an acetonitrile solution reveal the structural similarity of C2 to ligands. The two NHC/pyridine units of each ligand molecule
2
6
the un-functionalized complex A (Fig. 3). C2 exhibits a dis- are twisted against each other with a torsion angle of 87.70°.
torted octahedral coordination geometry bearing two axially The Fe–CNHC bond distances amount to 1.9209(18) and
coordinating acetonitrile molecules and the tetradentate NHC/ 1.9333(18) Å and are thus slightly longer compared to complex
pyridine hybrid ligand covering the equatorial positions. The C2, yet still lie in the range of previously reported iron NHC
1
9,20
Fe–CNHC bond distances of 1.826(4) and 1.833(4) Å are close to complexes.
those of complex A as are the Fe–NPy distances of 2.082(3) and groups bound to the ligand methylene bridge are directed away
.104(4) Å and the Fe–NMeCN distances (1.913(4) and 1.923(4) Å). from the iron centers and no iron–oxygen interactions are
Likewise complex C2, the hydroxymethyl
2
The hydroxymethyl group is pointing outwards of the equator- observed.
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tions of both ligand precursor P1 and the functionalized
iron(II) NHC complex C2 were performed to demonstrate the
feasibility of such reactions and identify suitable reaction con-
ditions (Scheme 3).
Esterification reactions with ligand precursor P1 were per-
formed using hexanoyl, benzoyl and nicotinoyl chloride to
demonstrate the feasibility of such reactions with alkylic, aro-
matic and heteroaromatic organic acids (Scheme 4). With
acetonitrile as solvent and in the presence of pyridine as base,
the reactions proceed smoothly at room temperature within
12–21 h depending on the acid chloride used. After workup
and repeated anion exchange with NH PF P2a–P2c are
4
6
obtained in yields of 44–82%. The reaction process can be fol-
lowed by H NMR as the signal of the hydroxyl proton at
Fig. 4 ORTEP style representation of the cationic fragment of C3 with
ellipsoids shown at the 50% probability level. Hydrogen atoms, cocrys-
tallized solvent molecules, and counter ions are omitted for clarity.
Selected geometrical parameters: Fe1–C10 1.9209(18) Å, Fe1–N6
1
6.26 ppm disappears and the signals corresponding to the
CH protons adjacent to the hydroxyl group show a distinct
2
2
1
.0183(5) Å, Fe1–C1a 1.9333(18) Å, Fe1–N3a 1.9996(15) Å, Fe1–N7 downfield shift from 4.55 ppm for P1 to 5.21, 5.44, and
.9299(15) Å, Fe1–N8 1.9676(16) Å.
5.44 ppm for P2a, P2b, and P2c as the reaction proceeds. The
same accounts for signal of the bridging NCHCH proton,
which is shifted downfield from 7.18 ppm to 7.53, 7.70, and
.67 ppm for P2a, P2b, and P2c. Only minor changes of the
2
2
.3. Esterification of hydroxyl-functionalized ligand
7
precursor P1 and iron(II) NHC complex C2
1
chemical shifts of all other signals are observed by H NMR.
In accordance to the strategy presented by Danopoulos
1
0,12,13,18
In analogy to literature-known NHC ligand precursors,
3
3
the hydroxyl functionality of P1 may serve as a site for further et al. the esterified ligand precursors P2a–P2c can be con-
functionalization of the ligand scaffold or as an anchoring site verted to the corresponding complexes C2a–C2c in good yields
for immobilization approaches. Therefore, esterification reac- between 52 and 76% by aminolysis with iron(II) bis(trimethyl-
silyl)amide (Scheme 4). The product complexes have been
characterized by NMR spectroscopy, ESI-MS and elemental
analysis. The data suggest a monomeric structure in each case
with the iron center being coordinated in a distorted octa-
hedral geometry analogous to the structures of C2 and A.
1
Similar to the ligand precursors P2a–P2c in the H NMR
2
spectra of C2a–C2c the chemical shift of the CH proton
signals is moved downfield upon esterification. For the
hydroxyl-functionalized complex C2 the signal is observed at
4.19 ppm, while it is found at 4.60, 4.83, and 4.78 ppm for
C2a, C2b, and C2c, respectively. The signal of the adjacent
NCHCH2 proton is similarly shifted downfield compared to
complex C2 (7.28 ppm (C2) to 7.43 (C2a), 7.59 (C2b), and 7.58
ppm (C2c)).
Alternatively to ligand precursor P2, iron(II) complex C2 can
also be used as a substrate for esterification reactions. This is
Scheme 3 Strategies towards the synthesis of esterified iron(II) NHC
complexes. The tetradentate NHC/pyridine hybrid ligand P1 is depicted
schematically.
Scheme 4 Esterification reactions of bis(imidazolium) salt P1 with various acid chlorides and subsequent synthesis of iron(II) NHC complexes.
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3
5
especially important in the context of catalyst immobilization tion reagents for NHC complexes, proved to be suitable
as often the precise characterization of heterogenized metal reagents to realize such reactions (Scheme 5). Depending on
complexes poses difficulties and by grafting a defined metal the reactivity of the organic acid two different methods can be
complex in the last synthetic step, e.g. via an esterification applied: one possibility is reacting the organic acid with tri-
reaction, characterization can be facilitated and incomplete fluoroacetic anhydride (TFAA) to form a mixed anhydride,
1
0,34
metalation avoided.
However, acid chlorides were found to which is subsequently added to a solution of C2 leading to the
be inapplicable as no products could be isolated and degra- formation of the esterified complexes C2a and C2b (Method A).
dation of C2 was observed. On the other hand, organic acid However, in case of nicotinic acid instead of the desired
anhydrides, which have been used before as mild derivatiza- product C2c, complex C2d is formed bearing a trifluoroacetic
a
ester group. In comparison to hexanoic and benzoic acid (pK :
4
.48 (hexanoic acid), 4.20 (benzoic acid)), nicotinic acid has a
3
6,37
significantly lower pK
a
of 2.03,
which is accompanied by a
better stabilization of the corresponding carboxylate.
Apparently, as the difference in pK between trifluoroacetic
a
3
6
a
acid (pK : 0.23) and the respective organic acid decreases,
the reactivity of the mixed anhydride is affected significantly
and the selectivity of the esterification reaction declines. As an
alternative, C2 can be reacted with nicotinic anhydride in the
presence of diisopropylethylamine (DIPEA) as base yielding the
desired complex C2c in 52% yield (Method B). This method is
also applicable to benzoic anhydride and complex C2b is
obtained in 48% yield. The product yields achieved applying
method B are lower compared to method A. This observation
can be attributed to the necessity of using a base to initiate the
reaction. In this context, the reaction time is an important
parameter as, despite the high steric demand of DIPEA, slow
degradation of C2 is observed in its presence.
Single crystals suitable for X-ray diffraction measurements
of C2a and C2b have been obtained by slow diffusion of
diethyl ether into an acetonitrile solution of the respective
complexes. As Fig. 5 reveals, both complexes are isostructural
to C2 and A as the iron center is coordinated in a distorted
octahedral fashion by the tetradentate NHC/pyridine hybrid
ligand and two apical acetonitrile molecules. Again, no inter-
Scheme 5 Esterification of iron(II) complex C2 by organic acid
anhydrides.
Fig. 5 ORTEP style representation of the cationic fragments of C2a and C2b with ellipsoids shown at the 50% probability level. Hydrogen atoms,
cocrystallized solvent molecules, and counter ions are omitted for clarity. Selected geometrical parameters: C2a Fe1–C1 1.835(5) Å, Fe1–C10
1
1
.833(4) Å, Fe1–N7 1.910(4) Å, Fe1–N8 1.924(4) Å, Fe1–N3 2.091(4) Å, Fe1–N6 2.077(4) Å, N7–Fe1–N8 175.16(16)°. C2b Fe1–C1 1.830(2) Å, Fe1–C10
.829(2) Å, Fe1–N7 1.9212(18) Å, Fe1–N8 1.9252(18) Å, Fe1–N3 2.0863(18) Å, Fe1–N6 2.0798(19) Å, N7–Fe1–N8 171.26(7)°.
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action of the functional group attached to the ligand back-
bone, in this case ester groups, and the metal center were
observed. Compared to both C2 and A the distances between
the iron atoms and the coordinating atoms remain nearly
unchanged for C2a and C2b.
2
.4. Electrochemical characterization
To evaluate the influence of the methylene bridge functionali-
zation of complex A with hydroxymethyl and ester groups on Fig. 7 Cyclic voltammogram of dinuclear complex C3. The half-wave
potentials are determined to E1/2 = 793 mV and 912 mV. All potentials
are referenced to the half-wave potential of the Fc/Fc redox couple.
the electronic properties of the iron center, cyclic voltammetry
+
(CV) measurements in acetonitrile solution were performed.
Complexes C2 and C2a–C2d show a one-electron redox
process, which can be assigned to the Fe(II)/Fe(III) redox couple
+
with a hydroxymethyl group has no significant influence on
the electronic situation of the iron center. Furthermore, the
required potentials for the oxidation of the esterified com-
plexes C2a–C2d are 50–70 mV higher, which can be attributed
to the electron withdrawing properties of the ester groups. As
the electron density at the iron center is decreased, a higher
half-wave potential is observed. Within the series of C2a to
C2d, the variation of the acid substituents has little influence
26 on the half-wave potentials, which increase slightly with the
acid strength. However, compared to the effect of axial substi-
tution reactions with phosphine-, pyridine- or isocyanide-
(
Fig. 6). The half-wave potentials E1/2 vs. Fc/Fc are 427 mV for
C2, 478 mV for C2a, 480 mV for C2b, 490 mV for C2c, and
02 mV for C2d and the oxidation proved to be fully reversible
5
for all complexes for at least 20 cycles. The peak separation ΔE
of 74 mV to 82 mV lies within the typical range for a reversible
one-electron process and corresponds well to previously
described, related iron complexes.
of C2 deviates only marginally from the reported potential of
the un-functionalized complex A of 423 mV vs. Fc/Fc .
Taking into account the structural data presented above, this
indicates that the functionalization of the ligand backbone
3
8–42
The half-wave potential
+
4
0,43
based ligands on the electronic properties of A,
the
observed changes in the half-wave potentials upon ester-
functionalization of the ligand methylene bridge are small.
The CV data of diiron complex C3 reveals two reversible
waves at E1/2 = 793 mV and 912 mV (Fig. 7). The separation of
the two oxidation events amounts to 119 mV, indicating weak
4
4–46
electronic coupling between the two metal centers.
However, as charge delocalization through the ligand is un-
likely due to the saturated nature of the bridge between the
two NHC/pyridine units, the peak splitting in the CV data
4
7
must result from a through-space electrostatic interaction.
3
. Conclusions
The synthesis and characterization of a new 1,1′-(2-hydroxy-
ethane-1,1-diyl) bridge-functionalized bis(imidazolium) salt
has been presented providing access to tunable silver and iron
bis(NHC) complexes. In addition to the targeted mononuclear
analogue of iron(II) NHC complex A, a dinuclear iron complex
was synthesized displaying interesting structural features and
a weak electronic coupling of the two iron centers. Performing
esterification reactions and determining suitable reaction con-
ditions, the utility of the hydroxyl group as a versatile platform
for the derivatization of the ligand precursor on the one hand
and the corresponding mononuclear iron NHC complex on the
other hand has been demonstrated. Thus, four iron(II) bis
(
NHC) complexes bearing aliphatic, aromatic or heteroaro-
matic ester groups could be isolated and fully characterized.
These reactions serve as model reactions for the design of
extended ligand scaffolds and lay the foundations for the
immobilization of molecular iron NHC complexes analogous
Fig. 6 Cyclic voltammograms of complexes C2 and C2a–C2d. Half-
wave potentials are determined to E1/2 = 427 mV (C2), 478 mV (C2a),
4
80 mV (C2b), 490 mV (C2c), and 502 mV (C2d). All potentials are refer-
+
enced to the half-wave potential of the Fc/Fc redox couple.
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complex A, which is an active catalyst in a wide range of reac- on the top of a microsampler, transferred to the diffractometer
tions. Experiments following these strategies are currently and frozen under a stream of cold nitrogen. A matrix scan was
ongoing. Moreover, the presented iron NHC complexes are to used to determine the initial lattice parameters. Reflections
be used as catalysts in oxidation reactions in order to evaluate were corrected for Lorenz and polarization effects, scan speed,
5
3
the influence of the ligand functionalization on their catalytic and background using SAINT.
Absorption corrections,
behavior in comparison to complex A. including odd and even ordered spherical harmonics were per-
5
3
formed using SADABS. Space group assignments were based
upon systematic absences, E statistics, and successful refine-
ment of the structures. Structures were solved by direct
methods with the aid of successive difference Fourier maps,
and were refined against all data using the APEX III software
in conjunction with SHELXL-2014 and SHELXLE. H atoms
bound to oxygen atoms were allowed to refine freely with an
O–H distance of 0.84 Å. Other H atoms were placed in calcu-
lated positions and refined using a riding model, with methyl-
ene and aromatic C–H distances of 0.99 and 0.95 Å, respect-
4
. Experimental section
4
.1. General remarks
5
4
55
All chemicals were purchased from commercial suppliers and
used without further purification. Anhydrous solvents were
obtained from an MBraun solvent purification system and
degassed by freeze–pump–thaw techniques. All syntheses were
performed under standard Schlenk conditions unless stated
4
8
otherwise. 2-(Imidazol-1-yl)pyridine
and iron(II) bis(tri-
ively, and Uiso(H) = 1.2U (C). Full-matrix least-squares refine-
eq
4
9
2
2 2
methylsilyl)amide were synthesized according to previously
published procedures. Liquid NMR spectra were recorded on
Bruker Avance DPX 400 and Bruker CRX 400 spectrometers.
Chemical shifts are provided in parts per million (ppm) and
spectra were referenced using the residual solvent shifts as
ments were carried out by minimizing Δw(F
o
− F
c
) with
5
6
SHELXL-97
weighting scheme. Neutral atom scattering
factors for all atoms and anomalous dispersion corrections for
the non-hydrogen atoms were taken from International Tables
5
7
for Crystallography. The image of the crystal structure was
1
1
13
5
8
internal standards (CDCl
δ 39.52, CD CN, H δ 1.94, C δ 118.26). Elemental analyses
3
3
, H δ 7.26, DMSO-d
6
, H δ 2.50,
C
generated by PLATON. CCDC 1811950–1811954† contains
1
13
the supplementary data for the structures.
were performed by the microanalytical laboratory of TUM and
a Thermo Scientific LCQ/Fleet spectrometer was employed to
obtain ESI-MS data.
4
.3. Syntheses
4.3.1. 2,2-Dibromoethanol. In
a
three-necked flask
A Metrohm Autolab PGSTAT302N potentiostat was used
together with the Nova 1.11 software for cyclic voltammetry
measurements. A graphite electrode was chosen as counter
electrode, together with platinum as working electrode and Ag/
equipped with a dropping funnel and a reflux condenser
LiAlH (4.35 g, 114.8 mmol, 2.0 eq.) is suspended in diethyl
4
ether (80 mL). At 0 °C a solution of 2,2-dibromoacetic acid
(
12.5 g, 57.4 mmol, 1.0 eq.) in diethyl ether (25 mL) is added
3
AgNO (0.01 M in acetonitrile with 0.1 M tetrabutylammonium
dropwise. After completion of the addition the reaction
mixture is stirred at room temperature for 2.5 h. Subsequently
hexafluorophosphate) as reference electrode. For all measure-
ments, 2.0–3.0 mg of the respective compound were dissolved
in 1.0 mL of a 0.1 M solution of tetrabutylammonium hexa-
fluorophosphate in acetonitrile under inert conditions. The
4 2
excess LiAlH is quenched by slowly adding H O (30 mL)
under ice cooling followed by addition of aqueous H SO
2
4
(
100 mL, 10% w/w). After separation of the organic phase, the
−1
potential was scanned with 100 mV s and the obtained
aqueous phase is extracted with diethyl ether (3 × 50 mL). The
combined organic phases are washed with saturated NaHCO3
+
values were referenced against the Fc/Fc redox couple as
5
0
internal standard (0.48 V versus SCE).
solution (50 mL) and H
solvent is removed in vacuo yielding 2,2-dibromoethanol
5.18 g, 25.4 mmol, 44% yield) as a colorless oil.
2 4
O (50 mL), dried over MgSO and the
4
.2. Single-crystal X-ray diffraction
The X-ray intensity data of C1–C3 were collected on an X-ray
single crystal diffractometer equipped with a CCD detector, a 1H, CH), 4.04 (dd, J = 7.1, 5.9 Hz, 2H, CH
(
1
H NMR (400.13 MHz, 295.1 K, CDCl
3
): δ 5.66 (t, J = 5.9 Hz,
), 2.43 (t, J = 7.4 Hz,
2
1
3
1
Mo fine-focus tube with MoK radiation (λ = 0.71073 Å) and a 1H, OH). C{ H} NMR (100.62 MHz, 295.5 K, CDCl ): δ 70.0
α
3
graphite monochromator by using the APEX II software (CH
package. The X-ray intensity data of C2a were collected on an
2
), 47.1 (CH).
4.3.2. Ligand precursor P1. In an ACE pressure tube 2,2-
X-ray single crystal diffractometer equipped with a CMOS dibromoethanol (2.45 g, 12.0 mmol, 1.0 eq.) and 2-(imidazol-1-
detector (Bruker Photon-100), an IMS microsource with MoK yl)pyridine (4.35 g, 30.0 mmol, 2.5 eq.) are dissolved in toluene
radiation (λ = 0.71073 Å) and a Helios mirror optic by using (12 mL) in air. The tube is sealed tightly and heated to 130 °C
5
1
α
5
2
the APEX III software package. The X-ray intensity data of for 5 days. After cooling to room temperature the brown pre-
C2b were collected on an X-ray single crystal diffractometer cipitate is washed subsequently with acetonitrile, ethanol and
equipped with a CMOS detector (Bruker Photon-100), a rotat- dichloromethane (3 × 10 mL each). Recrystallization from
ing anode (Bruker TXS) with MoK radiation (λ = 0.71073 Å) methanol yields an off-white powder. The intermediate
α
and a Helios mirror optic by using the APEX III software product is dissolved in water (5 mL) and added dropwise to a
5
2
package. The measurements were performed on single crys- solution of ammonium hexafluorophosphate (1.96 g,
tals coated with perfluorinated ether. The crystals were fixed 12.0 mmol, 1.0 eq.) in water (5 mL). The resulting white pre-
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cipitate is washed carefully with water and dried in vacuo yield- for C22
H
22
F
12FeN
8
OP
2
: C 34.76, H 2.92, N 14.74. Found:
ing imidazolium salt P1 as a white solid (2.92 g, 4.68 mmol, C 34.39, H 2.85, N 14.27.
3
9%).
4.3.5. Complex C3. C1 (102.0 mg, 67.8 µmol, 1.0 eq.),
): δ 10.35 (t, J = FeBr (THF) (48.8 mg, 135.6 µmol, 2.0 eq.) and silver(I) hexa-
1
H NMR (400.13 MHz, 295.1 K, DMSO-d
6
2
2
1
4
4
5
.7 Hz, 2H, HIm), 8.75–8.66 (m, 4H, HIm + H ), 8.35–8.24 (m, fluorophosphate (18.9 mg, 74.6 µmol, 1.1 eq.) are dissolved in
H, HIm + HPy), 8.08 (d, J = 8.2 Hz, 2H, HPy), 7.71 (dd, J = 7.5, acetonitrile (3 mL). The resulting solution is stirred under
Py
.8 Hz, 2H, HPy), 7.18 (t, J = 5.4 Hz, 1H, NCHCH
.7 Hz, 1H, OH), 4.55 (t, J = 5.5 Hz, 2H, NCHCH2). C{ H} is cautiously added to the orange solution until an orange
): δ 149.89 (CPy), 146.47 solid precipitates. The solution is decanted and the precipitate
Py), 141.34 (CPy), 136.82 (NCHNIm), 126.29 (CPy), 123.03 (CIm), is washed with diethyl ether and dried in vacuo. The remaining
20.24 (CIm), 114.94 (C ), 71.65 (NCHCH ), 60.23 (NCHCH ). solution still contains complex C2. C3 is obtained as an
2
), 6.26 (t, J = light exclusion at 50 °C for 12 h. After filtration, diethyl ether
1
3
1
NMR (100.62 MHz, 295.5 K, DMSO-d
6
(
C
1
Py
2
2
+
MS-ESI (m/z): [P1–PF
6
]
calcd: 479.12, found: 478.92; orange solid in 41% yield (42.6 mg, 28.0 µmol).
2
+
1
[P1–2PF
6
]
calcd: 167.08, found: 167.16. Anal. calcd for
3
H NMR (400.13 MHz, 295.1 K, CD CN): δ 9.30 (d, J =
C H F N OP : C 34.63, H 2.91, N 13.46. Found: C 34.15, 5.6 Hz, 2H, H ), 8.56 (d, J = 2.4 Hz, 2H, HIm), 8.53 (m, 2H,
1
8
18 12
6
2
Py
H 2.96, N 13.36.
H
Py), 8.34 (d, J = 8.3 Hz, 2H, HPy), 8.11 (m, 4H, HIm′ + HIm),
4
.3.3. Complex C1. In air P1 (250.0 mg, 0.4 mmol, 1.0 eq.) 7.91–7.83 (m, 4H, HPy + HPy′), 7.63 (d, J = 8.2 Hz, 2H, HPy′),
is dissolved in acetonitrile (5 mL) and Ag O (278.1 mg, 6.98 (ddd, J = 7.3, 5.9, 1.2 Hz, 2H, HPy′), 6.48 (d, J = 5.9 Hz,
2
1
.2 mmol, 3.0 eq.) is added. The suspension is stirred at 2H, HPy′), 6.38 (d, J = 2.4 Hz, 2H, HIm′), 5.59 (dd, J = 6.9, 2.3
room temperature under light exclusion for 12 h. After fil- Hz, 2H, NCHCH ), 4.64 (ddd, J = 13.0, 7.0, 3.3 Hz, 2H,
tration, diethyl ether is added slowly to the brown solution NCHCH ), 4.56 (ddd, J = 13.0, 6.1, 2.4 Hz, 2H, NCHCH ), 3.56
2
2
2
1
3
1
until it becomes colorless. It is decanted from the brown oil (dd, J = 6.0, 3.2 Hz, 2H, OH). C{ H} NMR (100.62 MHz,
and the product is precipitated by further addition of diethyl 295.5 K, CD CN): δ 209.92 (CIm), 200.85 (CIm′), 155.63 (CPy),
ether. The precipitate is isolated and washed with diethyl 155.32 (CPy′), 154.13 (C ), 151.70 (C ), 142.84 (C ), 140.98
3
Py
Py′
Py
ether yielding C1 as a white solid (266.1 mg, 0.2 mmol, (CPy′), 126.48 (CIm), 125.90 (C Im′), 125.31 (CPy), 124.03 (CPy′),
8
4%).
121.61 (CIm), 119.08 (C Im′), 114.26 (CPy), 112.76 (CPy′), 75.16
1
H NMR (400.13 MHz, 295.1 K, CD CN): δ 7.91 (d, J = (NCHCH ),62.95 (NCHCH2). MS-ESI (m/z): [C3–4CH CN–
.1 Hz, 4H, HIm), 7.88 (d, J = 4.7 Hz, 4H, HPy), 7.86 (d, J = 2PF
3
2
3
2
+
2
2
8
5
5
6
]
calcd: 533.04, found: 533.12. Anal. calcd for
.0 Hz, 4H, HIm), 7.82 (dt, J = 7.9, 1.8 Hz, 4H, HPy), 7.65 (d, J =
.1 Hz, 4H, H ), 7.32 (brs, 2H, NCHCH ), 7.26 (dd, J = 6.9, C 34.46, H 3.13, N 14.24.
.0 Hz, 4H, HPy), 4.63 (t, J = 5.3 Hz, 4H, NCHCH
.2 Hz, 2H, OH). C{ H} NMR (100.62 MHz, 295.5 K, CD
C
44
H
44
F
24Fe
2
N
16
O
2
P
4
C 34.76, H 2.92, N 14.74. Found:
Py
2
2
), 4.01 (t, J =
4.3.6. Ligand precursor P2a. P1 (500.0 mg, 0.8 mmol,
CN): 1.0 eq.) is dissolved in dried acetonitrile (3 mL) and hexanoyl
1
3
1
3
δ 183.67 (CIm), 150.78 (C ), 149.61 (C ), 141.12 (C ), 125.41 chloride (224 µL, 1.6 mmol, 2.0 eq.) and pyridine (129 µL,
Py
Py
Py
(
6
C
Py), 122.33 (CIm), 121.60 (CIm), 116.45 (CPy), 78.13 (NCHCH
2.00 (NCHCH ). Anal. calcd for C40 38Ag
C 31.92, H 2.54, N 13.03. Found: C 31.05, H 2.39, N 12.09.
2
), 1.6 mmol, 2.0 eq.) are subsequently added. After stirring the
2
H
18
F
3
F
18
N
14
O
2
P
3
:
reaction mixture at room temperature for 21 h the white pre-
cipitate is filtered off and the resulting solution is dried
4
.3.4. Complex C2. C1 (102.0 mg, 67.8 µmol, 1.0 eq.), in vacuo. The resulting oily residue is redissolved in acetone
FeBr (THF) (48.8 mg, 135.6 µmol, 2.0 eq.) and silver(I) hexa- (1 mL) and slowly added to a solution of ammonium hexa-
2
2
fluorophosphate (18.9 mg, 74.6 µmol, 1.1 eq.) are dissolved in fluorophosphate (260.8 mg, 1.6 mmol, 2.0 eq.) in water (3 mL)
acetonitrile (3 mL). The resulting solution is stirred under yielding a brown solution, which is subjected to reduced
light exclusion at 50 °C for 12 h. After filtration, diethyl ether pressure until the acetone evaporates and a brown residue pre-
is cautiously added to the orange solution until an orange cipitates from the aqueous phase. After decantation and
solid precipitates (complex C3). The solution is decanted and washing with water (3 mL) the oily residue is dissolved in
addition of further diethyl ether yields another orange precipi- acetone and precipitates by slow addition of diethyl ether. The
tate, which is filtered off, washed with diethyl ether and dried off-white precipitate is isolated by filtration, washed with
in vacuo. C2 is obtained as an orange solid in 40% yield diethyl ether (5 mL) and dried in vacuo yielding P2a (471.5 mg,
(
41.0 mg, 54.5 µmol).
0.65 mmol) in 82% yield.
1
1
H NMR (400.13 MHz, 295.1 K, CD CN): δ 9.57 (d, J =
H NMR (400.13 MHz, 295.1 K, DMSO-d ): δ 10.43 (t, J =
3
6
4
.9 Hz, 2H, HPy), 8.35 (m, 2H, HPy), 8.32 (d, J = 2.3 Hz, 2H, 1.6 Hz, 2H, HIm), 8.76–8.68 (m, 4H, HIm + HPy), 8.35 (t, J =
H
Im), 8.06 (d, J = 8.3 Hz, 2H, HPy), 7.90 (d, J = 2.3 Hz, 2H, 2.0 Hz, 2H, HIm), 8.33–8.25 (m, 2H, HPy), 8.08 (dt, J = 8.3,
HIm), 7.76 (ddd, J = 7.6, 5.4, 1.1 Hz, 2H, H ), 7.28 (t, J = 0.9 Hz, 2H, H ), 7.72 (ddd, J = 7.6, 4.8, 0.9 Hz, 2H, H ), 7.53
Py
Py
Py
3
3
2
1
.4 Hz, 1H, NCHCH
2
), 4.19 (dd, J = 6.6, 3.4 Hz, 2H, NCHCH
2
), (t, J = 5.8 Hz, 1H, NCHCH
2
2
), 5.21 (d, J = 5.8 Hz, 2H, NCHCH ),
1
3
1
.39 (t, J = 6.7 Hz, 1H, OH). C{ H} NMR (100.62 MHz, 2.38 (t, J = 7.4 Hz, 2H, HHex), 1.48 (p, J = 7.3 Hz, 2H, HHex),
1
3
1
95.5 K, CD CN): δ 217.06 (CIm), 155.29 (C ), 153.74 (C ), 1.24–1.12 (m, 4H, HHex), 0.78 (t, J = 6.9 Hz, 3H, HHex). C{ H}
3
Py
Py
42.29 (CPy), 125.99 (CIm), 124.35 (CPy), 120.19 (CIm), 113.05 NMR (100.62 MHz, 295.5 K, DMSO-d
Py), 76.94 (NCHCH ), 66.39 (NCHCH ). MS-ESI (m/z): (CPy), 145.95 (CPy), 140.95 (CPy), 136.78 (CIm), 125.96 (CPy),
C2–2CH CN–2PF ] calcd: 194.04, found: 194.23. Anal. calcd 122.33 (CIm), 120.06 (CIm), 114.50 (C ), 68.68 (NCHCH ), 60.78
6
): δ 172.04 (CvO), 149.49
(
[
C
2
2
2
+
3
6
Py
2
Dalton Trans.
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Paper
(
(
2
NCHCH
C5,Hex), 13.75 (C6,Hex). MS-ESI (m/z): [P2a–2PF6]
16.11, found: 216.16. Anal. calcd for
2
), 33.03 (C2,Hex), 30.49 (C3,Hex), 23.84 (C4,Hex), 21.76 295.5 K, DMSO-d
6
): δ 163.87 (CvO), 154.18 (CNic), 150.54
2+
calcd: (CNic), 149.49 (C ), 146.06 (C ), 140.93 (C ), 137.43 (CNic),
Py
Py
Py
C
24
H
28
N
6
O
2
P
2
F
12 137.02 (CIm), 125.91 (CPy), 124.85 (CNic), 124.04 (CNic), 122.46
(CIm), 120.05 (CIm), 114.53 (CPy), 68.97 (NCHCH ), 62.15
.3.7. Ligand precursor P2b. P1 (1000.0 mg, 1.6 mmol, (NCHCH ). MS-ESI (m/z): [P2c–2PF ] calcd: 219.59, found:
C 39.90, H 3.91, N 11.63. Found: C 39.51, H 3.92, N 11.45.
2
2
+
4
2
6
+
1
(
8
.0 eq.) is dissolved in dried acetonitrile (15 mL) and pyridine 219.93. [P2c–PF
387 µL, 4.8 mmol, 3.0 eq.) and benzoyl chloride (992 µL, 12FeN
.0 mmol, 5.0 eq.) are subsequently added under ice cooling. H 3.00, N 13.17.
4.3.9. Complexes C2a–C2c. A solution of the respective
2 h the solvent is removed in vacuo. The resulting solid is dis- ligand precursor P2a–P2c (0.07 mmol, 1.0 eq.) in acetonitrile
solved in methanol (20 mL) and slowly added to a solution of (1.5 mL) is added to a frozen solution of [Fe{N(SiMe ) } (THF)]
6
] calcd: 584.14, found: 584.04. Anal. calcd for
C
24
H
21
F
7 2 2
O P C 39.52, H 2.90, N 13.44. Found: C 38.95,
After stirring the reaction mixture at room temperature for
1
3
2 2
ammonium hexafluorophosphate (239.4 mg, 3.2 mmol, (0.09 mmol, 1.3 eq.) in acetonitrile (1 mL) and after warming
.0 eq.) in water (5 mL). The white precipitate is isolated by fil- to room temperature it is stirred for 14 h. Subsequently the
2
tration and washed with water (5 mL) and diethyl ether (5 mL) solvent is removed in vacuo and the residual solid is redis-
and dried in vacuo yielding P2b (571.3 mg, 0.78 mmol) in 49% solved in acetonitrile. By addition of diethyl ether the solution
yield.
is precipitated stepwise. The red product complexes C2a–C2c
1
H NMR (400.13 MHz, 295.1 K, DMSO-d ): δ 10.53 (s, 2H, are washed with diethyl ether and dried in vacuo.
6
1
H
H
Im), 8.73 (s, 2H, HIm), 8.71 (d, J = 4.7 Hz, 2H, HPy), 8.42 (s, 2H,
Im), 8.26 (t, J = 7.7 Hz, 2H, HPy), 8.08 (d, J = 8.2 Hz, 2H, HPy), CD
4.3.9.1. Complex C2a. H NMR (400.13 MHz, 295.1 K,
CN): δ 9.58 (dd, J = 5.3, 1.8 Hz, 2H, HPy), 8.41–8.33 (m, 2H,
H ), 8.31 (d, J = 2.2 Hz, 3H, HIm), 8.07 (dt, J = 8.3, 1.0 Hz, 2H,
3
8
.01 (d, J = 7.8 Hz, 2H, HPh), 7.70 (m, 4H, HPy + HPh
+
Py
NCHCH
NCHCH
2
), 7.54 (t, J = 7.7 Hz, 2H, HPh), 5.44 (d, J = 5.3 Hz, 2H,
H
Py), 7.93 (d, J = 2.5 Hz, 2H, HIm), 7.78 (t, J = 6.4 Hz, 2H, HPy),
1
3
1
2
). C{ H} NMR (100.62 MHz, 295.5 K, DMSO-d
6
): 7.43 (t, J = 5.5 Hz, 1H, NCHCH ), 4.60 (d, J = 5.5 Hz, 2H,
2
δ 164.73 (CvO), 149.48 (C ), 146.03 (C ), 140.92 (C ), 136.96 NCHCH ), 2.29 (t, J = 7.4 Hz, 7H, HAlk), 1.52 (tt, J = 7.3, 7.3 Hz,
Py
Py
Py
2
(CIm), 134.10 (CPh), 129.69 (CPh), 128.90 (CPh), 128.48 (CPh), 2H, HAlk), 1.34–1.19 (m, 4H, HAlk), 0.86 (t, J = 7.0 Hz, 3H, HAlk).
1
3
1
1
(
25.91 (CPy), 122.39 (CIm), 120.08 (CIm), 114.53 (CPy), 68.98
3
C{ H} NMR (100.62 MHz, 295.5 K, CD CN): δ 217.50 (CIm),
2
+
NCHCH ), 61.81 (NCHCH ). MS-ESI (m/z): [P2b–2PF ] calcd: 173.41 (CvO), 155.18 (C ), 153.78 (C ), 142.46 (C ), 126.58
2
2
6
Py
Py
Py
2
19.09, found: 219.17. Anal. calcd for
C
25
H
22
N
6
O
2
P
2
F
12 (CIm), 124.57 (CPy), 120.33 (CIm), 113.21 (CPy), 74.09 (NCHCH
67.98 (NCHCH ), 34.22 (C2,Hex), 31.78 (C3,Hex), 25.02 (C4,Hex),
.3.8. Ligand precursor P2c. P1 (500.0 mg, 0.8 mmol, 23.00 (C5,Hex),
.0 eq.) is dissolved in dried acetonitrile (5 mL) and nicotinoyl [C2a–2CH CN–2PF
chloride (226.5 mg, 1.6 mmol, 2.0 eq.) and pyridine (129 µL, for C28 12FeN
2
),
C 41.22, H 3.04, N 11.54. Found: C 41.79, H 3.12, N 11.77.
2
4
14.19
(C6,Hex).
calcd: 243.07, found: 243.53. Anal. calcd
: C 39.18, H 3.76, N 13.05. Found: C
MS-ESI
(m/z):
2
+
1
3
6
]
O
H
32
F
8
2
P
2
1
.6 mmol, 2.0 eq.) are subsequently added. After stirring the 39.07, H 3.78, N 12.85. Yield: 62%.
1
reaction mixture at room temperature for 17 h the white pre-
cipitate is filtered off and the filtrate is dried in vacuo. The oily CD
4.3.9.2. Complex C2b. H NMR (400.13 MHz, 295.1 K,
CN): δ 9.56 (dd, J = 5.3, 0.8 Hz, 2H, HPy), 8.35 (ddd, J = 8.3,
3
residue is redissolved in acetone (1 mL) and slowly added to a 7.5, 1.6 Hz, 2H, H ), 8.30 (d, J = 2.3 Hz, 2H, HIm), 8.06 (dt, J =
Py
solution of ammonium hexafluorophosphate (260.8 mg, 8.3, 1.0 Hz, 2H, HPy), 8.00 (d, J = 2.3 Hz, 2H, HIm), 7.99–7.94
1
.6 mmol, 2.0 eq.) in water (2 mL) yielding a brown solution, (m, 2H, HPh), 7.76 (ddd, J = 7.5, 5.4, 1.1 Hz, 2H, HPy), 7.68–7.62
which is subjected to reduced pressure until the acetone evap- (m, 1H, HPh), 7.59 (t, J = 5.5 Hz, 1H, NCHCH ), 7.53–7.47 (m,
2
1
3
1
orates and a brown residue precipitates from the aqueous 2H, HPh), 4.83 (d, J = 5.5 Hz, 2H, NCHCH
phase. After decantation, the product is subsequently extracted (100.62 MHz, 295.5 K, CD CN): δ 217.66 (CIm), 166.19 (CvO),
from the residue with water by dissolving it in a mixture of 155.19 (C ), 153.75 (C ), 142.44 (C ), 134.90 (CPh), 130.59
2
). C{ H} NMR
3
Py
Py
Py
acetone and water (1 : 1), evaporating the acetone and decant- (CPh), 129.89 (CPh), 126.67 (CIm), 124.53 (CPy), 120.37 (CIm),
ing the aqueous phase. The combined aqueous phases are 115.97 (CPh), 113.22 (CPy), 74.14 (NCHCH ), 68.89 (NCHCH ).
2
2
2
+
dried in vacuo and the raw product is dissolved in acetone and MS-ESI (m/z): [C2b–2CH CN–2 PF ] calcd: 246.05, found:
3
6
8 2 2
precipitated by slow addition of dichloromethane. The light 246.23. Anal. calcd for C29H26FeN O P F12: C 40.30, H 3.03,
orange precipitate is isolated, washed with dichloromethane N 12.96. Found: C 39.85, H 3.06, N 12.35. Yield: 52%.
1
(
5 mL) and dried in vacuo yielding P2c (257.6 mg, 0.35 mmol)
4.3.9.3. Complex C2c. H NMR (400.13 MHz, 295.1 K,
CD CN): δ 9.56 (d, J = 5.4 Hz, 2H, HPy), 9.11 (s, 1H, HNic), 8.82
): δ 10.50 (t, J = (d, J = 5.2 Hz, 1H, HNic), 8.48 (d, J = 8.0 Hz, 1H, HNic), 8.39–8.32
.6 Hz, 2H, HIm), 9.19 (d, J = 2.2 Hz, 1H, HNic), 8.85 (dd, J = (m, 2H, H ), 8.30 (d, J = 2.3 Hz, 2H, HIm), 8.07 (d, J = 8.2 Hz,
in 44% yield.
3
1
H NMR (400.13 MHz, 295.1 K, DMSO-d
6
1
4
5
8
Py
.9, 1.7 Hz, 1H, HNic), 8.73 (t, J = 2.0 Hz, 2H, HIm), 8.71 (dd, J = 2H, HPy), 8.02 (d, J = 2.3 Hz, 2H, HIm), 7.75 (dd, J = 7.7, 5.2 Hz,
.1, 1.7 Hz, 2H, HPy), 8.40 (t, J = 1.9 Hz, 2H, HIm), 8.36 (dt, J = 2H, HPy), 7.73–7.67 (m, 1H, HNic), 7.58 (t, J = 5.8 Hz, 1H,
.0, 2.0 Hz, 1H, HNic), 8.28 (td, J = 7.9, 1.9 Hz, 2H, H ), 8.08 (d, NCHCH ), 4.78 (d, J = 5.9 Hz, 2H, NCHCH ). C{ H} NMR
1
3
1
Py
2
2
J = 8.3 Hz, 2H, HPy), 7.71 (dd, J = 7.5, 4.9 Hz, 2H, HPy), 7.67 (t, (100.62 MHz, 295.5 K, CD
J = 5.0 Hz, 1H, NCHCH
3
CN): δ 217.72 (CIm), 165.23(CvO),
), 7.59 (dd, J = 8.0, 4.9 Hz, 1H, HNic), 155.20 (CNic), 155.17 (CPy), 153.77 (CPy), 151.51 (CNic), 142.45
2
1
3
1
5
.44 (d, J = 5.0 Hz, 2H, NCHCH2). C{ H} NMR (100.62 MHz, (C ), 138.15 (CNic), 126.66 (CIm), 125.97 (CNic), 124.86 (CNic),
Py
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24.56 (CPy), 120.43 (CIm), 113.23 (CPy), 73.96 (NCHCH
Dalton Transactions
1
2
), 69.06
4 L. Cavallo and C. S. J. Cazin, in N-Heterocyclic Carbenes in
Transition Metal Catalysis and Organocatalysis, ed. C. S. J.
Cazin, Springer, Dordrecht, The Netherlands, 2011,
pp. 1–22.
5 P. de Frémont, N. Marion and S. P. Nolan, Coord. Chem.
Rev., 2009, 253, 862–892.
2
+
(
NCHCH ). MS-ESI (m/z): [C2c–CH CN–2PF ] calcd: 267.06,
2
3
6
found: 267.43. Anal. calcd for C24
H
21
F
7 2 2 6
12FeN O P + HPF :
C 33.25, H 2.59, N 12.47. Found: C 32.98, H 2.90, N 11.92.
Yield: 76%.
4
.3.10. Esterification of complex C2 by organic anhydrides.
Method A. The respective organic acid (52.6 µmol, 2.0 eq.) is
dissolved in trifluoroacetic anhydride (7.4 µL, 52.6 µmol,
6 F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008,
47, 3122–3172.
2
.0 eq.) and the mixture is stirred for 5 min before adding it to
7 M. Poyatos, J. A. Mata and E. Peris, Chem. Rev., 2009, 109,
3677–3707.
8 J. A. Mata, M. Poyatos and E. Peris, Coord. Chem. Rev.,
2007, 251, 841–859.
a solution of C2 (20.0 mg, 26.3 µmol, 1.0 eq.) in 0.5 mL aceto-
nitrile. In case of trifluoroacetic acid, the anhydride is added
directly to the complex solution. After a reaction time of
1
–20 h diethyl ether is slowly added to precipitate the product
9 R. Corberán, E. Mas-Marzá and E. Peris, Eur. J. Inorg.
Chem., 2009, 2009, 1700–1716.
complexes, which are isolated by filtration.
Method B. The respective organic anhydride (28.9 µmol, 10 R. Zhong, A. C. Lindhorst, F. J. Groche and F. E. Kühn,
.1 eq.) and C2 (20.0 mg, 26.3 µmol, 1.0 eq.) are dissolved in Chem. Rev., 2017, 117, 1970–2058.
.0 mL acetonitrile and subsequently diisopropylethylamine 11 E. Mas-Marzá, M. Poyatos, M. Sanaú and E. Peris,
1
1
(4.9 µL, 28.9 µmol, 1.1 eq.) is added. After a reaction time of
Organometallics, 2004, 23, 323–325.
3
h diethyl ether is slowly added to precipitate the product 12 E. Mas-Marzá, M. Sanaú and E. Peris, J. Organomet. Chem.,
complexes, which are isolated by filtration.
2005, 690, 5576–5580.
1
4
.3.10.1. Complex C2d. H NMR (400.13 MHz, 295.1 K, 13 M. Viciano, E. Mas-Marzá, M. Poyatos, M. Sanaú,
CD
3
CN): δ 9.58 (ddd, J = 5.4, 1.6, 0.7 Hz, 2H, HPy), 8.37 (ddd,
R. H. Crabtree and E. Peris, Angew. Chem., Int. Ed., 2005,
44, 444–447.
J = 8.2, 7.5, 1.6 Hz, 2H, H ), 8.32 (d, J = 2.3 Hz, 2H, HIm),
Py
8
.07 (dt, J = 8.4, 1.0 Hz, 2H, HPy), 7.93 (d, J = 2.3 Hz, 2H, HIm), 14 O. Kühl, Germany Pat, WO2012034880A1, 2012.
7
.79 (ddd, J = 7.5, 5.4, 1.2 Hz, 2H, HPy), 7.58 (t, J = 5.6 Hz, 1H, 15 R. Zhong, A. Pöthig, S. Haslinger, B. Hofmann,
1
3
1
NCHCH ), 4.83 (d, J = 5.5 Hz, 2H, NCHCH ).
C{ H}
G. Raudaschl-Sieber, E. Herdtweck, W. A. Herrmann and
2
2
NMR (100.62 MHz, 295.5 K, CD
3
CN): δ 217.97 (CIm), 156.95 (q,
F. E. Kühn, ChemPlusChem, 2014, 79, 1294–1303.
J = 43.2 Hz, CvO), 155.05 (CPy), 153.74 (CPy), 142.48 (CPy), 16 R. Zhong, A. Pöthig, D. C. Mayer, C. Jandl, P. J. Altmann,
1
26.51 (CIm), 124.62 (C ), 120.51 (CIm), 115.28 (q, J = 284.6 Hz,
W. A. Herrmann and F. E. Kühn, Organometallics, 2015, 34,
2573–2579.
Py
CF
m/z): [C2d–2CH
Anal. calcd for C H F FeN O P : C 33.67, H 2.47, N 13.09.
3 2 2
), 113.23 (CPy), 73.03 (NCHCH ), 71.09 (NCHCH ). MS-ESI
2
+
(
3
CN–2PF
6
]
calcd: 242.03, found: 242.63. 17 J. Rieb, B. Dominelli, D. Mayer, C. Jandl, J. Drechsel,
W. Heydenreuter, S. A. Sieber and F. E. Kühn, Dalton
Trans., 2017, 46, 2722–2735.
2
4
21 15
8 2 2
Found: C 33.66, H 2.55, N 12.81. Yield: 98%.
1
8 R. Puerta-Oteo, M. Hölscher, M. V. Jiménez, W. Leitner,
V. Passarelli and J. J. Pérez-Torrente, Organometallics, 2017,
DOI: 10.1021/acs.organomet.7b00509.
Conflicts of interest
19 K. Riener, S. Haslinger, A. Raba, M. P. Högerl, M. Cokoja,
W. A. Herrmann and F. E. Kühn, Chem. Rev., 2014, 114,
There are no conflicts to declare.
5215–5272.
2
2
2
0 V. Charra, P. de Frémont and P. Braunstein, Coord. Chem.
Rev., 2017, 341, 53–176.
1 S. Haslinger, A. Raba, M. Cokoja, A. Pöthig and F. E. Kühn,
J. Catal., 2015, 331, 147–153.
2 Ö. Karaca, M. R. Anneser, J. W. Kück, A. C. Lindhorst,
M. Cokoja and F. E. Kühn, J. Catal., 2016, 344,
Acknowledgements
The authors appreciate financial support by the Fonds der
Chemischen Industrie (FCI). Peter Gänsheimer is acknowl-
edged for his valuable experimental contributions.
213–220.
2
3 J. W. Kück, A. Raba, I. I. E. Markovits, M. Cokoja and
F. E. Kühn, ChemCatChem, 2014, 6, 1882–1886.
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