Investigating a Redox Active Samarium Complex in Catalytic Reactions

Article abstract of DOI:10.1002/ejic.202100391

Herein the synthesis of a new ethynyl ferrocenyl amidinate ligand [Fc?C≡C-{C(Ndipp)₂H}] (Fc=ferrocenyl; dipp=diisopropylphenyl) and the subsequent formation of the corresponding samarium amido complex [Sm{Fc?C≡C-[C(Ndipp)₂]}₂

Full text of DOI:10.1002/ejic.202100391

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European Journal of Inorganic Chemistry
Accepted Article
Title: Investigating a redox active samarium complex in catalytic
reactions
Authors: Sebastian Kaufmann and Peter Werner Roesky
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100391
Link to VoR: https://doi.org/10.1002/ejic.202100391
01/2020

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Investigating a redox active samarium complex in catalytic
reactions
Sebastian Kaufmann[a] and Peter W. Roesky*[a]
[a]
Dr. S. Kaufmann and Prof. Dr. P. W. Roesky
Institute of Inorganic Chemistry
Karlsruhe Institute of Technology (KIT)
Engesserstrasse 15
76131 Karlsruhe, Germany
Email: Roesky@kit.edu
https://www.aoc.kit.edu/AK Roesky.php
Supporting information for this article is given via a link at the end of the document. NMR and IR spectra, kinetic measurements, XRD data
and ORTEP plots; CCDC 2082238-2082239:
Abstract: Herein the synthesis of a new ethynyl ferrocenyl
amidinate ligand [Fc-C≡C-{C(Ndipp)2H}] (Fc = ferrocenyl;
dipp = diisopropylphenyl) and the subsequent formation of
the corresponding samarium amido complex [Sm{Fc-C≡C-
metalation of the ferrocene in the backbone was realized
by using suitable group 8 metal precursors (scheme 1,
B and C).
[C(Ndipp)2]}2{N(SiMe3)2}]
is
reported.
[Fc-C≡C-
{C(Ndipp)2H}] and [Sm{Fc-C≡C-[C(Ndipp)2]}2{N(SiMe3)2}]
were fully characterized including the study of the complex’
redox properties by cyclovoltammetry. Furthermore, we
investigated the catalytic properties of the samarium
complex in the intramolecular hydroamination reaction and
intermolecular dehydrocoupling of pinacol borane with
various amines.
Scheme 1: Some examples of known ferrocenyl amidinate
complexes and their different binding modes in coordination
chemistry.[16, 18]
Introduction
Amidinates are very popular ligands because they have
the inherent advantage of being easily accessible and
show a high flexibility and tunability in terms of their
steric demand as well as their electronic properties.[1-4]
These advantages lead to the employment of these
ligands for almost any stable metal going from main
group[5-6] over transition metals[1, 7] to f-elements.[8-12] In
lanthanide chemistry, they now compete with the
formerly omnipresent cyclopentadienyl ligands in nearly
every aspect.[9, 13] Nevertheless, they still have a large
potential for further improvements.[1, 5-8, 10, 14]
However, in all these compounds the ferrocenyl
substituent is bound directly to the central carbon atom
of the amidinate backbone. Herein, we describe the
synthesis of a new ethynyl tethered ferrocene dipp
amidinate ligand, in which an ethynyl linker is localized
between the ferrocene moiety and the dipp amidinate.
Due to the remote binding of the ferrocenyl unit there is
less steric displacement in the backbone of the ligand,
resulting in a release of the steric strain of the ligand.
Herein, we showcase the coordination of the new
ferrocenyl ethynyl functionalized amidinate ligand to
samarium. The resulting complex was subsequently
used as a catalyst in intramolecular hydroamination and
dehydrocoupling reactions.[21-27] Furthermore, the redox
activity of the ferrocene backbone of the complex was
investigated by cyclovoltammetry (CV) as well as in
aforementioned catalytic reactions.
Despite the broad scope of substituents, there are only
few known examples of amidinates functionalized by a
ferrocenyl entity in the backbone (scheme 1, A-C).[15-18]
The corresponding complexes are mainly transitional
19]
metal derivatives,[15-16,
though there is one report
including rare earth elements.[20] In most of these
complexes cyclohexyl substituents are bound to the
nitrogen atoms of the amidinate function (scheme 1, A).
Recently, we introduced the more sterically demanding
2,6-diisopropylphenyl (dipp) and mesityl groups as a
substituents on the nitrogen atoms.[18] Owing to the high
steric demand, we could alter the coordination mode of
the amidinate ligand. Besides the typical k2N, an ortho-
1
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Results and Discussion
While the monoamidinate [Sm(DippLPh){N(SiMe3)2}2] (D)
was obtained by using the bulky N,N-bis(2,6-
Dipp
diisopropylphenyl)benzamidinate
ligand
(
L )
Ph
Synthesis
(scheme 3), the bisamidinate samarium compound
[Sm{iPrLPhen}{N(SiMe3)2}(THF)] (E) (scheme 3) was
formed by employing the rigid ligand system 4,5-
Since our attempts to coordinate ferrocenyl dipp and
mesityl amidinates shown in scheme 1[18] to the
lanthanides failed, we were interested in designing a
ligand, which has a lower steric strain upon coordination
of the lanthanides. Therefore, we synthesized a new
ethynyl ferrocene amidinate ligand, in which the
ferrocene is bridged to the amidinate via an ethynyl
function.
bis(diisopropylamidinate)phenanthrene
(iPrLPhen).[23-24]
Both complexes are functionalized by N(SiMe3)- groups,
which act as leaving groups of the precatalyst in the
hydroamination reaction.
The
synthesis
of
this
ethynyl
ferrocene
bis(diisopropylphenyl)amidine [Fc-C≡C-{C(Ndipp)2H}]
was achieved through an in situ lithiation of ethynyl
ferrocene,[28-29] which was followed by subsequent
reaction
with
bis(diisopropylphenyl)carbodiimide
(scheme 2).[1, 15, 30] The resulting lithium compound was
protonated and recrystallized from hot toluene.
Figure 1: Molecular structure of [Fc-C≡C-{C(Ndipp)2H}] in solid
state. Hydrogen atoms and the distorted toluene molecule are
omitted for clarity (except H1 and H1’) and the second molecule is
slightly transparent to show the symmetry. Selected bond lengths [Å]
and -angles [°]: N1-C1 1.338(5), N1- C1' 1.345(5), N2-C1 1.294(5),
N2'-C1' 1.310(4), C1-C2 1.456(6), C1'-C2' 1.442(5), C2-C3 1.189(5),
C2'-C3' 1.198(5), C3-C4 1.442(6), C3'-C4' 1.428(6), N2-C1-N1
119.6(3), N2'-C1'-N1' 118.7(3), C1-N1-C14 126.3(3), C1’-N2'-C14'
121.7(3), C1-N2-C26 122.7(3), C1' N1'-C26'-125.0(3).
Scheme 2: Synthesis of [Fc-C≡C-{C(Ndipp)2H}] .
[Fc-C≡C-{C(Ndipp)2H}] crystallizes in the monomeric
space-group Pn (figure 1) with two molecules in the
asymmetric unit. The low symmetry of the unit cell is
most likely caused by a cocrystalized distorted toluene
molecule, which prevents higher symmetry operations.
The two molecules form a dimer, which is bridged by
hydrogen bonds between the respective imine nitrogen
of one amidine and the amine proton of the other
amidine function. As expected, the amidine units show a
longer bond between the central carbon and the amine
(N1-C1 1.338(5) and N1-C1' 1.345(5) Å) and a shorter
bond to the imine nitrogen atoms (N2-C1 1.294(5) and
N2'-C1' 1.310(4) Å). The dipp-groups are turned towards
the ferrocene moiety to avoid a steric clash upon
dimerization forming an angle between the amidine
carbon (C1), the amidine nitrogen and the dipp carbon
atom of about 124° at all nitrogen atoms. The ferrocene
moieties in the backbone are arranged orthogonally to
the amidinate functions.
The 1H NMR spectrum of [Fc-C≡C-{C(Ndipp)2H}]
exhibits the expected resonances of all functional
groups, though DMSO-d6 is needed as NMR solvent to
resolve the signals. Due to the dynamic proton
exchange between the nitrogen atoms of the amidine
function broad and uninterpretable spectra were
obtained in less polar solvents. However, in DMSO-d6
the resonance of the proton of the amidine function was
detected at 8.52 ppm. The 13C{1H} NMR spectrum also
shows the expected set of signals.
Scheme 3: The mono and bisamidinate samarium amido complexes
[Sm(DippLPh){N(SiMe3)2}2] (D) and [Sm{iPrLPhen}{N(SiMe3)2}(thf)] (E).
[23-24]
For comparison in synthesis and catalysis, we desired
to synthesize a related samarium amido complex by
using our new ligand [Fc-C≡C-{C(Ndipp)2H}]. Following
the synthetic route established for D and E, we directly
reacted two equivalents of the amidine [Fc-C≡C-
{C(Ndipp)2H}] with [Sm{N(SiMe3)2}3] to obtain [Sm{Fc-
C≡C-[C(Ndipp)2]}2{N(SiMe3)2}] (1) in 33% yield after
recrystallization from hot toluene (scheme 4). In contrast
to D a bisamidinate compound is formed. Obviously, the
formal substitution of the phenyl group in the amidinate
backbone by ethynyl ferrocene reduces the steric bulk
of the ligand and thus enables the formation of the
bisamidinate samarium complex 1.
Recently, we reported on samarium amidinate
complexes as catalysts for the hydroamination reaction.
2
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complex and even in the 1H, 1H-COSY and 1H,13C-
HMQC NMR spectra not every resonance can be
assigned to its corresponding functional group. The
-
resonance of the N(SiMe3)2 ligand is observed at higher
field as expect for
a diamagnetic compound (–
Scheme 4: Synthesis of compound 1.
4.18 ppm). The signals corresponding to the iso-propyl
groups split and can be seen highfield (–2.51 ppm (4 H))
or downfield shifted (10.32 ppm (2 H) and 12.28 ppm
(2 H)), when compared to the precursor (1.27 ppm). In
contrast, the signals of the ferrocene unit do not show
significant shifts in comparison to [Fc-C≡C-
{C(Ndipp)2H}]. They are detected in the same range as
observed in the free ligand (from 4.10 to 4.90 ppm in
compound 1; from 3.93 to 4.27 ppm in [Fc-C≡C-
{C(Ndipp)2H}]). This behavior is a result of the larger
distance between the ferrocene units and the
paramagnetic center.
̅
Compound 1 crystalizes in the triclinic space-group 푃1
with one molecule in the asymmetric unit (figure 2). The
samarium atom is κ2N coordinated by two {Fc-C≡C-
-
[C(Ndipp)2]}- ligands and one additional N(SiMe3)2
,
resulting in a distorted square planar pyramid. The bond
distances between the samarium atom and the nitrogen
atoms of the amidinate units range from 2.428(6) to
2.459(7) Å. Within each amidinate ligand-samarium
interaction there is a longer (Sm-N2 2.459(7) Å and Sm-
N3 2.459(6) Å) and a shorter bond (Sm-N1 2.446(6) Å
and Sm-N4 2.428(6) Å). This bond interactions are in the
range of literature known examples (2.28 – 2.52 Å).[23-24]
The most striking difference in the IR spectra of
compounds [Fc-C≡C-{C(Ndipp)2H}] and
1 is the
-
stretching vibration of the ethinyl group at 2220 cm–1.
While the resonance is broad and split in [Fc-C≡C-
{C(Ndipp)2H}], it is significantly sharper in compound 1.
The measurement of Raman spectra failed as even the
lowest laser energy setting melted and destroyed
compound 1.
The samarium nitrogen bond to the N(SiMe3)2 ligand
(Sm-N5 2.249(5) Å) is also in the range of literature
known bonds.[23-24, 31-33] Within the amidinate units the
delocalisation of charge is confirmed by almost similar
bond lengths within the NCN units (N1-C1 1.326(9), N2-
C1 1.344(8), N3-C38 1.329(9) and N4-C38
1.343(10) Å). The above mentioned slightly lower steric
demand of the ethynyl group in {Fc-C≡C-[C(Ndipp)2]}
can be verified
by the larger bit angle of {Fc-C≡C-[C(Ndipp)2]} (N1-C1-
Cyclovoltammetry
The electrochemical properties of compound 1 were
investigated by cyclovoltammetric measurements. The
cyclovoltammogram in figure 3 shows a quasireversible
redox wave at E01/2 = 119 mV (ΔE = 83 mV, ipc/ipa = 0.87).
Despite two ferrocenyl moieties present in compound 1
only one redox event can be observed in the
Dipp
compared to the phenyl group in
L
Ph
N2 116.9(6) and N3-C38-N4 117.0(7) °) compared to
Dipp
L
Ph
(114.2(4) °).[24]
cyclovoltammogram. This indicates
a
chemical
independency of each ferrocene group. In comparison
to our previously published ferrocenyl amidinate
complexes, e.g., B (figure 1), we see a comparable
anodically shifted potential (1: 119 mV vs.
[Fc{C(Ndipp)2Rh(CO)2}: 223 mV or
[Fc{C(Ndipp)2Rh(cod)}: 138 mV).[18]
Next, we tried to replicate these electrochemical
observations chemically by employing different oxidizing
agents. In almost all cases this led to an insoluble
powder, which could not be further characterized. Only
the usage of [Ag(OC(CF3)3)4] yielded a soluble species,
which could not be recrystallized or further analysed and
decomposed upon reduction with different reducing
agents.
Figure 2: Molecular structure of compound 1 in solid state .
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and -angles [°]: Sm-N1 2.446(6), Sm-N2 2.459(7), Sm-N3 2.459(6),
Sm-N4 2.428(6), Sm-N5 2.245(6), N1-C1 1.326(9), N2-C1 1.344(8),
C1-C2 1.450(9), C2-C3 1.188(10), C3-C4 1.418(10), N3-C38
1.329(9), N4-C38 1.343(10), C38-C39 1.441(10), C39-C40
1.190(10), C40-C41 1.437(11), N1-C1-N2 116.9(6), N1-Sm-N2
55.3(2), N2-C1-C2 120.8(7), N1-C1-C2 122.3(6), C1-C2-C3
178.1(9), C2-C3-C4 173.8(10), N3-C38-N4 117.0(7), N4-Sm-N3
55.6(2), N3-C38-C39 122.0(7), N4-C38-C39 121.0(7), C38-C39-
C40 176.7(9), C39-C40-C41 176.2(9), N1-Sm-N5 104.7(2), N2-Sm-
N5 121.1(2), N3-Sm-N5 121.2(2), N4-Sm-N5 104.2(2), Si1-N5-Si2
124.8(4).
On account of the paramagnetic nature of the metal
centre, the NMR spectra of compound 1 are quite
3
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