Reactivity of Copper Complexes with Bis(piperidinyl)methane and Bis(quinolinyl)methane Ligands

Article abstract of DOI:10.1002/ejic.201700755

Copper complexes with bis(piperidinyl)methane (BP) and bis(quinolinyl)methane (BQ) ligands were investigated. A dinuclear carbonato-bridged copper(II) complex as well as a dinuclear copper(I) complex with BP ligands were structurally characterized. The reaction of a copper(I)–BP complex was investigated by low-temperature stopped-flow techniques, which showed that the very fast formation of a bis(μ-oxido)copper intermediate leads to an intramolecular ligand hydroxylation. However, substrates such as benzyl alcohol could also be oxidized with this complex. In contrast, no intermediates could be observed spectroscopically for complexes with the BQ ligand or derivatives of this ligand. Instead, complex reaction behavior was observed when copper(II) chloride was treated with BQ in air or under inert conditions. Several crystal structures helped to establish mechanisms for these reactions. A coordination polymer was obtained as an interesting reaction product.

Full text of DOI:10.1002/ejic.201700755

A Journal of
Accepted Article
Title: Reactivity of Copper Complexes with Bispiperidine and
Bisquinoline Ligands
Authors: Tim-Daniel Stumpf, Manfred Steinbach, Christian Würtele,
Jonathan Becker, Sabine Becker, Roland Fröhlich, Richard
Göttlich, and Siegfried Schindler
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700755
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10.1002/ejic.201700755
European Journal of Inorganic Chemistry
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Reactivity of Copper Complexes with Bispiperidine and
Bisquinoline Ligands
Tim-Daniel J. Stumpf,[a,b] Manfred Steinbach,[a,b] Christian Würtele,[a] Jonathan Becker,[a] Sabine
Becker,[a] Roland Fröhlich,[c] Richard Göttlich[b] and Siegfried Schindler*[a]
Abstract: Copper complexes with the ligands bispiperidine (BP) or
bisquinoline (BQ) were investigated. A dinuclear carbonato bridged
copper(II) complex as well as a dinuclear copper(I) complex with BP
as ligand were structurally characterized. The reaction of a copper(I)
BP complex investigated by low temperature stopped-flow techniques
showed the very fast formation of a bis-µ-oxido copper intermediate
leading to an intramolecular ligand hydroxylation. However,
substrates such as e. g. benzyl alcohol could be oxidized as well with
this complex. In contrast, no intermediates could be spectroscopically
observed when BQ or derivatives of this ligand were used. Instead
complex reaction behaviour was observed when copper(II) chloride
was reacted with BQ as a ligand in air or under inert conditions.
Several crystal structures helped to establish a mechanism for these
reactions. An interesting reaction product obtained that way was a
new coordination polymer.
Different ligands have been applied to optimize these reactions
by modifications of type and number of donor atoms, chelate ring
sizes, sterically hindrance and electronic properties.[12,16,17,19] In
that regard, different bidentate ligand systems proved to be
promising.[16] Their copper-complexes can be useful for selective
[21]
hydroxylation reactions.[20]
In the past Schönecker et al. had
demonstrated that attaching such a simple unit to a steroid system
could be applied in synthetic chemistry of complex organic
molecules.[22] More recently this approach had been optimized by
Baran and co-workers who could very efficiently make use of
these compounds for selective C-H oxidation reactions.[23] Stack
and co-workers applied very simple diamine ligands that allowed
to study binding of dioxygen in great detail.[24–26] Modification of
such ligands with guanidine groups allowed the selective
oxidation of organic substrates.[19]
Due to these successful applications, we thought it might be
useful to apply a different set of bidentate ligands based on either
a bispiperidine or a bisquinoline system (Scheme 1: a with n = 2
and b). These new ligands are close analogues of bisoxazolines
(Scheme 1, c), well known C2-symmetric N-heterocyclic ligand
systems. The latter were introduced by Masamune[27] and
optimized by Evans[28] and now have proven very successful in
asymmetric transformations.[29]
Introduction
Copper is a ubiquitous used element in nature and as trace
element indispensable for bacteria, plants, animals and
humans.[1] Especially for the redox processing of molecular
dioxygen, copper ions are used in the active sites of important
biologically active metalloproteins[2] (only a few examples are
given in the references), such as hemocyanin,[3] tyrosinase[4,5,6]
and methane monooxygenase.[7–9] To fully understand the
interactions of dioxygen with copper metalloproteins it is an
important area of research to study the reactivity of copper(I)
complexes with dioxygen under controlled conditions.[6–8,10,11–17]
The goal of utilization copper-dioxygen complexes as catalytical
and selective oxidizing agents under ambient conditions is
relevant for organic synthesis in lab scale as well as a large scale
in industry.[5,6,11–13,15-18]
Scheme 1: Structural comparison of the ligand systems of methylene-bridged
aliphatic N-heterocycles (a, only the ligand with n=2 is relevant here),
methylene-bridged bisquinoline (b) and bisoxazoline (c).
[a]
Institute for Inorganic and Analytical Chemistry
Justus-Liebig-University Gießen
Heinrich-Buff-Ring 17
We were interested in similar methylene-bridged five membered
and six membered N-heterocycles as new ligands for transition
metal catalysis and copper-dioxygen complexes, which were
35392 Gießen, Germany
E-mail: siegfried.schindler@anorg.chemie.uni-giessen.de
https://www.uni-giessen.de/fbz/fb08/Inst/iaac/schindler
Institute for Organic Chemistry
Justus-Liebig-University Gießen
Heinrich-Buff-Ring 17
35392 Gießen, Germany
Institute for Organic Chemistry
WWU Münster
Corrensstraße 40
48149 Münster, Germany
Supporting information for this article is given via a link at the end of
the document.
constructed by linking
a
CH2-unit with two aliphatic
[b]
[c]
N-heterocycles such as piperidine. In comparison to the
bisoxazoline-nitrogen the bispiperidine-nitrogen is a better donor,
due to its hybridization. Thus, different complexation behavior of
these ligands might be expected. Furthermore, by varying the
substituents attached to the nitrogen, the electronic and steric
properties of these ligands can be modified. In that regard, as an
extension of the bispiperidine system, we furthermore became
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interested in the sterically more demanding aromatic C-bridged
bisquinoline system (Scheme 1). So far, these ligands have not
been reported as ligands for copper complexes. However, a
modified ligand based on the methylene-bridged bisquinoline core
was used as ligand in a palladium(II) complex that could be
applied for ethylene polymerization.[30] Herein, we now report the
reactivity of copper complexes with ligands of type a (n = 2) and
b (Scheme 1).
pyramidal geometry while τ = 1 describes an ideal trigonal
bipyramidal coordination polyeder) one copper ion has a more
square-pyramidal geometry (τ = 0.21) while the second one has
a coordination polyeder exactly between square-pyramidal and
tripodal-bipyramidal (τ = 0.52).
While the reaction of copper(II) chloride with BPmeso did lead to a
complex in which the copper(II) cation was coordinated to both
nitrogen atoms of one BPmeso ligand this was different for the
reaction with copper(I) salts [Cu(CH3CN)4]+. According to Scheme
2 a dinuclear copper(I) BPmeso complex formed in which two
copper(I) ions are coordinated to a nitrogen donor atom of each
ligand
Results and Discussion
Synthesis, characterization and reactivity of copper
complexes with the ligand BP
Synthesis of the ligand (bis-2-piperidyl-)methane (= BP,
Scheme 1: n = 2, R1 = R2 = R3 = H rac and meso) has been
described elsewhere.[31] BP was obtained and used as a bidentate
ligand in both diastereomeric forms. However, in regard to the
reactivity of the according copper complexes described below no
difference in reaction behavior towards dioxygen was observed.
Scheme 2: Reaction of [Cu(CH3CN)4]OTf with BPmeso in THF at room
temperature under inert conditions.
When reacting BP with copper(II) chloride in a stoichiometric ratio
in methanol it was expected that [Cu(BP)Cl2] would form. From
the reaction mixture (with BPmeso) a few crystals were obtained
that could be structurally characterized. Due to the small number
of crystals obtained no further characterization could be carried
out. In contrast to our expectations it turned out that a binuclear
complex with the two copper ions bridged by a carbonate anion
had formed. The molecular structure of this complex is shown in
Figure 1. Crystallographic data are reported in Table 1 and in
supporting information.
For the stoichiometric reaction with triflate as anion and the BPmeso
ligand in THF crystals could be obtained (after many attempts)
and the molecular structure of [Cu(BPmeso)]2(OTf)2 (2) is shown in
Figure 2.
Figure 1: Molecular structure of [Cu2(BPmeso)2(CO3)Cl2] (1).
The bridging carbonate ligand in [Cu2(BPmeso)2(CO3)Cl2] (1) is not
an unusual feature and has been described previously e. g. by
Sorrell and co-workers.[32] The copper(II) complex with the ligand
BPmeso obviously picked up carbon dioxide from air during the
crystallization process. A little bit surprising is the fact that
carbonate is coordinated instead of two more chloride ions.
Chloride anions usually are good ligands in comparison with a
carbonate anion. Both copper(II) ions are coordinated by five
donor atoms. Interestingly the coordination geometry of the two
copper(II) ions is different. According to the τ–parameter
established by Addison[33] (τ = 0 represents an ideal square
Figure 2: Molecular structure of [Cu(BPmeso)]2(OTf)2 (2). Triflate anions are not
shown for clarity.
Obviously the BPmeso ligand did not act as a chelating ligand as
expected to form [Cu(BP)(CH3CN)X]OTf (with X = 1 or 2). Instead
a more unusual linear copper(I) complex system crystallized
where one piperidine ring system had flipped around of about
180° to avoid the syn-pentane interaction thus leading to the
observed molecular structure.[34] Unfortunately, for the copper
complex with the rac configured BP ligand no crystals could be
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obtained under the same conditions. The BPrac ligand should act
most likely as a normal chelating ligand because in this
configuration no syn-pentane interactions exist.
step the copper(I) complex reacts with dioxygen to form a
mononuclear copper superoxido complex. The reaction can be
described according to the following equation (L = BP):
The reaction of [Cu(BP)]2(OTf)2 (meso and rac) with dioxygen has
been investigated in THF using low temperature stopped-flow
techniques. The complex was obtained in situ prior to the reaction
by mixing the copper(I) salt solution (under argon) with the ligand
solution (under dioxygen) in the syringes of the stopped-flow unit.
When the reaction was performed an unstable reaction
intermediate was detected spectroscopically (Figure 3).
Using excess of dioxygen (pseudo first order conditions) leads to
the simplified rate law:
v = kobs x c([Cu(BP)]+) with kobs = k1 x [O2]
Concurrent reactions are much faster and thus do not allow
observation of this intermediate. Therefore, only formation of the
bis-µ-oxido complex can be observed. Most recently this has
been supported for a related complex in a detailed computational
study.[35] In contrast to previous investigations on the formation of
bis-µ-oxido complexes the reaction described here is much faster.
Therefore, no detailed kinetic investigations could be performed.
Warming the solution did lead to a color change from a deep blue
to a purple color and a purple colored solid could be obtained.
Efforts to obtain crystals from a reaction were perchlorate as an
anion (2 with perchlorate instead of triflate) was used, failed.
However, when the solid was recrystallized from hot methanol (a
further color change to a green color was observed) crystals were
obtained and the molecular structure of this complex is shown in
Figure 4. Crystallographic data are reported in Table 1 and
supporting information.
Figure 3: Time resolved UV-Vis spectra of the low temperature stopped flow
measurement of the reaction of [Cu(BP)]2OTf2 (c = 0.5 ∙ 10ꢀꢁ mol/L ) with
dioxygen (c = 3.95·10-3 mol/L) in THF at -80.0 °C (overall reaction time: 0.75 s).
The inset shows the absorbance versus time trace at 394 nm.
This intermediate complex was not stable over time and
decomposed quite fast especially at higher temperatures. As
already mentioned above no difference was observed in reactivity
if BPmeso or BPrac were used as ligands.
Due to its instability the intermediate complex could not be
isolated and structurally characterized. However, the spectral
features, 390 nm and a second absorbance band close to 300 nm
(limitation of the detector did not allow to measure UV data at
lower wavelengths) clearly indicated that this intermediate was a
bis-µ-oxido copper complex. The UV-vis data fit well with previous
investigations on the formation of bis-µ-oxido copper
complexes.[13,19,20,35,36] This is surprising in so far that it would
have been expected that a bis-µ-oxido unit would not be stable in
the presence of amine hydrogen atoms. Usually amines used
previously in these studies have been fully alkylated.[19,24–26]
Figure 4: Molecular structure of [Cu2(BPrac)2(OMe)2]ClO4)2 (3). Perchlorate
anions are not shown for clarity.
From the molecular structure of [(BPrac)Cu(OMe)2Cu(BP)](ClO4)2
(3) it is clear that now each copper(II) ion is again coordinated to
one BP ligand with the two complex units bridged by two methoxy
groups. While the copper(I) ions have been oxidized to copper(II)
ions no effects on the ligand were observed. There is nothing
unusual about this compound and more than a 100 crystal
structures for copper complexes bridged by two methoxy groups
are reported in the Cambridge Crystallographic Data Base (only
a few examples for this complex type are given in the
references).[37–39] In principle these complexes are observed if no
ligand hydroxylation takes place. Depending on the solvent
The fast formation reaction can be followed spectroscopically. An
absorbance vs. time trace is shown as an inset in Figure 3. The
data could be fitted with one exponential function leading to: kobs
= 34 s-1. The rate law derives from the fact that in a first reaction
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formation of bis-µ-hydroxido copper complexes can occur and
has been observed previously.[37,39]
The second product in Scheme 3 was not isolated. Instead some
crystals of complex 4 were obtained and were structurally
characterized (Figure 5). Obviously further oxidation took place
leading to this complex. Reactions of this type are well known and
have been described in general for different metal complexes by
Constable.[41] In more detail this has been reported previously for
a related copper complex.[42] A simplified mechanism for this
reaction is presented in Scheme 4 (instead of the binuclear
complex from Scheme 3 only half of the complex unit is used and
without the OH group of the ligand coordinated). In the end this
well-known mechanism explains why complex 4 was obtained as
a final reaction product.
In contrast when the same sample was recrystallized from cold
methanol two sets of crystals were isolated that could be
structurally characterized. One set was identified as the same
complex 3, as obtained from hot methanol described above.
However, additionally the molecular structure from crystals of the
second set showed that ligand oxidation had occurred. The
molecular
structure
of
the
dinuclear
complex
[(OMeBPracO)2Cu2](ClO4)2 (4) is presented in Figure 5.
Crystallographic data are reported in Table 1 and supporting
information.
Figure 5: Molecular structure of [(OMeBPracO)2Cu2](ClO4)2 (4). Anions are not
shown for clarity.
It is obvious that a ligand hydroxylation of the ligand BP had
occurred in the 5-position. Furthermore, in the final product each
ligand contains an additional OMe group in the 6-position of the
piperidine ring. From the overall results we suggest a reaction
mechanism according to Scheme 3.
Because we did not observe a difference in reaction behavior
using either BPmeso or BPrac we think that the mononuclear copper
complex [Cu(BP]+ is present in solution (Scheme 3; most likely
with one or two acetonitrile molecules coordinated as additional
ligands) and reacts with dioxygen to form a binuclear bis-µ-oxido
copper complex, [(BP)Cu(O)2Cu(BP)]2+ (Scheme 3). No further
intermediates prior to the formation of this complex could be
detected. However, as discussed above, this is not surprising and
has been described previously for a guanidine/amine copper
system.[35] The bis-µ-oxido copper complex then undergoes
intramolecular ligand hydroxylation leading to the binuclear
product (Scheme 3) in which the two copper ions are bridged
through a hydroxide anion and the deprotonated alcohol group of
the hydroxylated ligand. Due to the chemical oxidation reaction
only one ligand molecule can be hydroxylated leading to a
maximum yield of 50 %, well known for related systems.[20,40]
However, during recrystallization from methanol a rearrangement
took place leading to the observation that the bis-methoxido
bridged binuclear copper complex 3 (Scheme 3) could be isolated
and structurally characterized.
Scheme 3: Postulated mechanism for the reaction of copper(I) BP complexes
with dioxygen and concurrent reactions.
Due to the finding that ligand hydroxylation was observed during
the reaction of the copper(I) complex with the ligand BP it seemed
to be interesting to furthermore test its potential for substrate
oxidation reactions (Scheme 5 and Table 1). As shown by Stack
and co-workers[43] a copper(I) complex with a derivative of an
ethylene diamine ligand could be used together with dioxygen to
oxidize different compounds such as e. g. 3,5-di-tert-butylcatechol
to the respective quinone (>95 % yield). Furthermore, good yields
were achieved when benzyl alcohol was oxidized to
benzaldehyde (80 %) and benzylamine to benzonitrile (70 %).
Karlin and co-workers[44] demonstrated that a copper(I) complex
with a tetradentate anisole containing polypyridylamine ligand
could be applied for the oxidation of toluene to benzaldehyde in
about 40 % yield with respect to the toluene. The oxidation of
toluene to benzaldehyde is a four-electron process. But since the
oxidizing agent, the bis-µ-oxido copper complex, only provides
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10.1002/ejic.201700755
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two electrons, the yield was about 80 % with respect to the
oxidizing agent.
used as substrate. Two oxidized compounds could be detected
and analyzed as cyclohex-2-ene-1-ol (16 %) and cyclohex-2-ene-
1-one (31 %). The observed allyl alcohol can be regarded as an
intermediate that had been further oxidized to the corresponding
unsaturated ketone. For this oxidation process again four
electrons are needed. The [Cu2(BP)2(O)2]2+ complex provides two
electrons for oxidation reactions. Therefore, two equivalents of
the oxidizing agent are needed for the reaction of cyclohexene to
the corresponding ketone. In summary the total conversion of the
oxidizing agent was 78 % for cyclohexene as the substrate. The
same electron counting is valid for the oxidation of benzylamine
to benzonitrile Here the best conversion could be observed with
84 % with respect to the [Cu2(BP)2(O)2]2+ complex.
Scheme 5: Application of the [Cu2(BP)2(O)2]2+ complex as oxidizing agent in
oxidizing experiments with various substrates.
Table 1: Results of the application of the [Cu2(BP)2(O)2]2+ complex in oxidizing
experiments with various substrates.
Scheme 4: Mechanism of the formation of the methoxylated hemiaminalether
4 (Figure 5).
substrate
product
Yield[a]
41 %
84 %
16 %
31 %
0 %
Benzylalcohol
Benzylamine
Cyclohexene
Benzaldehyde
Benzonitrile
To test for the reactivity of the copper BP complex system the
corresponding copper(I) triflate complex was prepared in situ in a
glove box. A stoichiometric amount of different substrates
according to Scheme 5 was added. The reaction mixture was then
transferred out of the glovebox, cooled to -80 °C and
subsequently O2 was bubbled through the solution for 10 minutes.
For comparison with previous work yield were referred to the
copper complex neglecting possible oxidation by excess oxygen.
The results of the oxidation experiments showed that no reaction
was observed when catechol or toluene were used as substrates.
In the case of 3,5-di-tert-butylcatechol a moderate yield of 49 %
of 3,5-di-tert-butyl-1,2-quinone determined via GC-MS was
obtained. Benzaldehyde was formed in 41 % yield when benzyl
alcohol was used as a substrate. Most likely some benzaldehyde
was further oxidized but could not be detected with GC-MS so that
the total conversion cannot be specified. The effect of double
oxidation of the substrate was observed when cyclohexene was
Cyclohex-2-ene-1-ol
Cyclohex-2-ene-1-one
o-Quinone
Catechol
3,5-Di-t-butylcatechol
Toluene
3,5-Di-t-butyl-1,2-quinone
Benzylalcohol
49 %
0 %
[a] based on GC-MS
Synthesis, characterization and reactivity of copper
complexes with the bisquinoline ligand and derivatives.
In our attempts to obtain a copper(II) complex with a bisquinoline
ligand (BQ, b with R1 = R2 = H in Scheme 1) CuCl2 x H2O was
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reacted with BQ in a 1:1 ratio in methanol. During this reaction,
the color of the solution changed to a deep red color and after
60 minutes another color change to a green color was observed.
We assume that at the beginning of this reaction the copper(II)
complex [Cu(BQ)Cl2] (5) had formed (this assumption is
supported by the molecular structure for [Cu(DMBQ)Cl2]
described below, Figure 12). However, no copper complex could
be crystallized from this solution. Instead, after decomplexation of
the copper ions with aqueous ammonia and extraction with
dichloromethane, crystals could be obtained after addition of
pentane. The molecular structure of one of these crystals is
shown in Figure 6. Crystallographic data are reported in Table 1
and supporting information.
When the reaction was repeated under inert conditions, a few red
crystals could be obtained that were structurally characterized.
The molecular structure of this complex (9) is shown in Figure 7
and revealed that a binuclear copper complex had formed.
Crystallographic data are reported in Table 1 and supporting
information.
Figure 6: Molecular structure of bisquinolineketone.
The molecular structure clearly showed that the ligand BQ was
oxidized at the bridging carbon to the according ketone (Scheme
6). At this carbon, a slight torsion could be observed leading to π
stacking.
Scheme 7: Postulated mechanism for the oxidation of the bisquinoline ligand.
From the molecular structure, it is evident that this is not a Cu(II)
complex anymore but instead a binuclear Cu(I) complex, bridged
by two chloride ions, had formed. The geometry around the two
copper(I) ions can be best described as a distorted tetrahedron.
Furthermore, a hydroxyl function on the bridged carbon of each
ligand can be observed (Figure 7a) In this redox reaction, Cu(II)
was apparently reduced to Cu(I) and the bridging carbon atom
was hydroxylated. However, the crystal structure analysis
revealed that only 23% of the bridged carbon are substituted in
this way. 37% are substituted with a chlorine atom (Figure 7b) and
40% do not show substitution at all. The hydroxy group is believed
to originate from the water of the CuCl2 x H2O salt used (see
proposed mechanism in Scheme 9.
Scheme 6: Oxidation of BQ to the according bisquinoline ketone during
complexation with CuCl2 in methanol or dichloromethane and the presence of
dioxygen from air.
This type of oxidation is a well-known reaction, quite typical for
copper and iron complexes.[41] It was reported previously for
example by Sprecher and Zuberbühler (who worked with bis(1-
methyl-benzimidazol-2-yl)methane)[45] or by Forlani et al.[46]. A
proposed mechanism for this reaction leading to a copper(II)
complex (7) that could not be characterized either (and therefore
was demetalated to lead to the isolated product
bisquinolineketone) is presented in Scheme 7. In a first step the
enamine formed from 5 can be deprotonated leading to two
resonance structures that finally end up with a copper(I) ligand
radical complex (6). Reactions of this type again are well known
and have been described in general previously.[41] Oxidation with
After the hydroxylation reaction in the glove box the solution was
transferred into air leading to a further reaction a second oxidation
step takes place with the aid of oxygen from the air to form the
bisquinoline ketone. When the previously obtained, red crystals
were exposed to dioxygen, green crystalline leaflets were isolated
and examined by X-ray crystallography, the molecular structure
of this complex (10) is shown in Figure 8. Crystallographic data
are reported in Table 1 and supporting information.
dioxygen from air leads to copper(II) complex
7
and
decomplexation finally to the crystallized bisquinoline ketone
shown in Figure 6.
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10.1002/ejic.201700755
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ion to such an extent that it subsequently reacted with a second
BQ ligand at the carbon bridge leading to an oxidative C-C
coupling with the formation of a double bond. In addition to the
oxidative coupling of the two ligands, the reduction of Cu(II) to
Cu(I) was observed again. The oxidative coupled di-BQ ligand
coordinates to one Cu(I) ion with each side. The copper ions are
bridged by two chloride ions on the one side and the oxidative
coupled di-BQ ligand on the other side thus forming a coordination
polymer. Crystals could be obtained of this compound (11) and its
molecular structure is presented in Figure 9. Crystallographic data
are reported in Table 1 and supporting information.
a
Figure 9: Molecular structure of the linear chain of the coordination polymer
[Cu2(di-BQ)Cl2]x (11) with the oxidative coupled bisquinoline as ligand (solvent
molecules omitted).
b
Figure 7: Molecular structure of the binuclear complexes formed under inert
conditions a) [Cu2(BQOH)2Cl2] (9) and b) [Cu2(BQCl)2Cl2].
One of the units with the bridging chloride anions is shown in
scheme 8.
Scheme 8: Detail of the molecular structure of the [Cu2(di-BQ)Cl2]x complex (11)
with focus on the oxidative coupled bisquinoline ligand.
Figure 8: Molecular structure of the mononuclear copper(II) dichlorido methoxy
bis chino-2-yl methanol complex (10) (solvent molecules omitted).
From the overall findings a mechanism for this reaction can be
proposed according to scheme 9. When no dioxygen is present
one copper(II) unit still present in solution can further oxidise
radical copper(I) complex 6 to complex 8 with a formal positive
charge at the bridging carbon atom (alternatively, oxidation of the
carbon radical by copper(II) chloride to the alkyl chloride is a
possible side reaction leading to [Cu2(BQCl)2Cl2] shown in Figure
7b). Reaction with water then causes formation of the dimeric
complex 9 that had been crystallized (Figure 7a). If this complex
then is reacting with air in the presence of methanol the same
oxidation steps occur as described above, finally leading to the
The molecular structure shows that the dinuclear Cu(I) complex
(9) had been oxidized further to a mononuclear Cu(II) complex by
atmospheric dioxygen. A further oxidation to the hemiacetal took
place at the bridged carbon atom by introduction of a methoxy
group. The methoxy group coordinates additionally to the Cu(II)
ion whereby the Cu(II) is coordinated fivefold in a nearly quadratic
pyramidal geometry.
In contrast if the reaction was carried out under inert gas
conditions with anhydrous CuCl2, no hydroxylation reaction was
observed. However, still the bridging carbon atom was chemically
activated during complexation of the ligand BQ with a copper(II)
hemiacetal complex
9 that again could be characterized
structurally (Figure 8). If neither water and dioxygen are present
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it is easy to imagine how complex 8 can react further leading to
the observed copper(I) polymer 11 (Figure 9).
bisquinoline ketone as oxidized product could be detected by GC-
MS.
Due to the fact that the bridging carbon atom in the BQ ligand
obviously is very sensitive to oxidation it did not seem promising
to use this ligand further in an effort to observe intermediate
copper "dioxygen adduct" complexes. Therefore, in a first attempt
to reduce the reactivity the carbon atom in BQ the fully methylated
ligand DMBQ was synthesized (Scheme 10). This ligand could be
obtained in a good yield and crystallographic data are reported in
Table 1 and the molecular structure is reported in supporting
information. Furthermore, it was possible to obtain a few crystals
of the corresponding copper(II) complex [Cu(DMBQ)Cl2]·¼H2O
(12) and its molecular structure is presented in Figure 10.
Crystallographic data are reported in Table 1 and supporting
information. As described above the characterization of this
complex supports formation of analogous copper(II) complex 5
prior to further reaction. Due to the missing hydrogen atom at the
bridgehead carbon (so no deprotonation according to Scheme 7
can occur) this compound is stable towards oxidation reaction and
therefore additionally supports the proposed mechanism
presented in Scheme 7.
Scheme 9: Proposed mechanism for oxidation reactions.
While we have been fortunate with "crystal picking" to establish a
reasonable mechanism for the complex reaction system we tried
to get more insight into these reactions by studying the reaction
of the copper(I) BQ complex with dioxygen using low temperature
stopped-flow techniques. For that purpose and to compare the
reaction with the copper BP complex described above we reacted
dioxygen with the copper(I) complex prepared in situ by mixing
stoichiometric amounts of BQ with [Cu(CH3CN)4]OTf in
anhydrous THF solution under inert conditions. Time resolved
UV-Vis spectra for this reaction are shown in supporting
information. In contrast to our findings described above for the
copper complex with BP as ligand we did not observe a bis-µ-
oxido copper complex or any other reaction intermediate. The
absorbance changes are a consequence of the oxidation of
copper(I) to copper(II) and the formation of the bisquinoline
ketone. After warming of the solution to room temperature the
Scheme 10: Synthetic modifications on the bisquinoline ligand (BQ).
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10.1002/ejic.201700755
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The copper(I) complex of the ligand DMBQ was subsequently
prepared in situ by reaction of DMBQ with [Cu(CH3CN)4]OTf in
anhydrous THF under inert conditions as well. The reaction of this
complex solution with dioxygen was investigated in THF using low
temperature stopped-flow techniques. No intermediates could be
spectroscopically detected and only a slow oxidation reaction to a
copper(II) complex was observed. Therefore, no further
investigations were performed with this complex system.
hydroxylated at the bridging carbon atom had formed. The
reaction obviously follows the same pathway as described above
for the copper complexes with BQ as ligand in Scheme 9 and is
shown for this ligand in Scheme 11.
With the introduction of a benzyl group at the bridged carbon of
the BQ ligand the bidentate benzyl-BQ ligand (BnBQ, Scheme 10)
was synthesized in an excellent yield. Unfortunately, for this
system again no intermediate complex could be spectroscopically
detected when the copper(I) complex of this ligand was reacted
in situ with dioxygen. However, more interesting reactions were
observed when CuCl2 x H2O was reacted with the BnBQ ligand in
a dichloromethane/methanol mixture at room temperature. This
reaction did lead to a green colored solution and green colored
crystals could be isolated for structural characterization. The
molecular structure of this complex (13) is shown in Figure 11.
Crystallographic data are reported in Table 1 and supporting
information.
Figure 10: Molecular structure of the [Cu(DMBQ)Cl2] (12) complex.
Scheme 11: Proposed mechanism of the oxidation of the BnBQ ligand.
Interestingly, when CuCl2 x H2O was added without methanol to
a solution of BnBQ in dichloromethane a second set of green
crystals could be isolated after a slightly longer reaction and
cooling time. The molecular structure of this crystallized
[Cu2(BnBQ-O)2Cl2] compound (13b) is shown in Figure 12.
Crystallographic data are reported in Table 1 and supporting
information. Again, a dinuclear complex had formed. The newly
introduced deprotonated alkoxy groups bridge both copper(II)
ions, which are thus coordinated together with one chloride ion in
a trigonal-bipyramidal geometry.
Figure 11: Molecular structure of the [Cu2(BnBQ-O)(CH3OH)Cl3] complex (13a)
as reaction product of BnBQ in dichloromethane with CuCl2 x H2O dissolved in
CH3OH (co-crystallized solvent omitted).
Clearly substitution of the bridging carbon atom with only one
substituent does not reduce the oxidation sensitivity of this
The molecular structure shows that a dinuclear Cu(II) complex
[Cu2(BnBQ-O)(CH3OH)Cl3] (13a) with
a
BnBQ ligand
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10.1002/ejic.201700755
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position. Therefore, it is not surprising that no copper-dioxygen
adduct species could be detected under controlled conditions
even at low temperatures. For clarification a simplified picture of
the hydroxylation reaction of the ligand BnBQ is shown in Scheme
11.
Figure 12: Molecular structure of the [Cu2(BnBQ-O)2Cl2] complex (13b) as
reaction product of BnBQ with CuCl2 x H2O in dichloromethane.
Efforts to change the reactivity of the BQ ligands by introducing a
third donor group thus leading to the tripodal ligands PyBQ and
PicBQ (Scheme 10) were not really successful. While it was
possible to prepare both ligands no crystals of either copper(I) or
copper(II) complexes for structural characterization could be
obtained. Furthermore, low temperature stopped flow
investigations of the copper(I) complexes with these ligands
prepared in situ showed no reactive intermediates that could be
spectroscopically detected.
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Table 2. Crystallographic data from the X-ray diffraction studies
1
2
3
4
BQ-ketone
9
C38H27.28Cl2.72N4O0.53
Cu2
Empirical Formula
C23H44Cl2Cu2N4O3
C12H22F3N2CuSO3
C12H25N2O5ClCu
C12H23N2O6ClCu
C19H12N2O
Mr
622.60
394.91
376.33
390.31
284.31
771.94
Temperature [K]
Radiation used (λ [Å])
223(2)
190(2)
190(2)
190(2)
223(2)
193(2)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Crystal size [mm]
0.30·0.20·0.10
triclinic
0.30·0.10·0.07
triclinic
ꢂ1
0.15·0.12·0.08
monoclinic
P21/c
0.75·0.40·0.20
monoclinic
P21/n
0.35·0.25·0.15
orthorhombic
Pbca
0.40·0.24·0.08
monoclinic
P21/n
Crystal system
Space group
a [Å]
ꢂ1
10.6637(3)
11.4106(4)
12.5781(4)
65.038(1)
81.040(2)
85.034(1)
1370.22(8)
2
9.0400(18)
9.960(2)
10.040(2)
88.66(3)
76.82(3)
67.78(3)
812.8(3)
2
11.153(2)
10.221(2)
14.974(3)
90
9.4490(19)
10.167(2)
16.178(3)
90
21.5383(4)
8.3878(2)
31.0331(7)
90
9.815(2)
9.952(2)
16.772(3)
90
b [Å]
c [Å]
α [°]
β [°]
109.21(3)
90
92.08(3)
90
90
104.09(3)
90
γ [°]
90
V [Å3]
Z
1611.9(6)
4
1553.2(5)
4
5606.4(2)
16
1589.0(6)
2
F(000)
652
408
788
812
2368
784
ρcalcd. [g·cm-3
µ [mm-1
]
1.509
1.614
1.511
1.669
1.347
1.613
]
1.778
1.515
1.542
1.608
0.675
1.606
Total reflections
Unique reflections
R(int)
14420
14603
12100
15831
35425
13683
4802
3573
2952
3402
4979
3833
0.035
0.03111
2.505 to 27.104
99.8
0.0832
2.459 to 25.350
99.8
0.0346
2.366 to 27.102
99.4
0.041
0.0842
2.961 to 28.173
99.6
Scan range θ [°]
Completeness to θmax [%]
Index ranges
1.80 to 25.00
99.5
4.10 to 67.88
97.9
0 ≤ h ≤ 12
-11 ≤ h ≤ 11
-12 ≤ k ≤ 12
-12 ≤ l ≤ 12
-13 ≤ h ≤ 12
-11 ≤ k ≤ 12
-17 ≤ l ≤ 18
-12 ≤ h ≤ 8
-13 ≤ k ≤ 12
-19 ≤ l ≤ 20
0 ≤ h ≤ 25
0 ≤ k ≤ 9
-12 ≤ h ≤ 11
-13 ≤ k ≤ 13
-22 ≤ l ≤ 22
-13 ≤ k ≤ 13
-14 ≤ l ≤ 14
-37 ≤ l ≤ 0
Data/restraints/parameters
R1,[a][b] wR2 [I > 4σ(I)][c]
R1,[a][d] wR2 (all data)[c][d]
Goodness-of-fit on F2 [e]
4802/0/323
0.0276, 0.0664
0.0333, 0.0696
1.035
3573/2/205
0.0293, 0.0699
0.0369, 0.0726
1.040
2952/2/197
0.0576, 0.1118
0.1046, 0.1274
1.050
3402/294/252
0.0299, 0.0764
0.0355, 0.0795
1.064
4979/0/397
0.0405, 0.1005
0.0479, 0.1060
1.007
3833/2/224
0.0480, 0.1059
0.0889, 0.1215
0.911
Max./min. el. dens. [e.Å-3
]
0.312, -0.362
0.289, -0.558
0.467, -0.346
0.319, -0.481
0.136, -0.173
0.620, -0.621
[b]
[a] ꢃ1 =
ꢄ
ꢅ
−
ꢄ
ꢆ
/
ꢄ . Denotes the value of the residual considering only the reflections with I > 4σ(I). [c] ꢇꢃ2 =
ꢇ ꢄꢈ − ꢄꢈ
/
ꢄꢈ
ꢉ/ꢈ;ꢇ = 1/ ꢊꢈ ꢄꢈ ꢋꢂ ꢈ + ꢌꢂ , ꢂ =
+
ꢅ
ꢈ
ꢈ
ꢅ
ꢅ
ꢆ
ꢅ
ꢍꢋꢎ ꢄꢈ ꢏꢐ 0 + 2 ꢄꢈ /3. [d] Denotes the value of the residual considering all the reflections. [e] ꢑ =
ꢇ ꢄꢈ − ꢄꢈ
/
ꢒ − ꢓ
, n: number of data, p: parameters used.
ꢈ
ꢉ/ꢈ
ꢅ
ꢆ
ꢅ
ꢆ
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10.1002/ejic.201700755
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FULL PAPER
Table 3. Crystallographic data from the X-ray diffraction studies, continued
10
11
DMBQ
C21H18N2
12
13a
13b
Empirical Formula
Mr
C22H24Cl2N2O4Cu
514.87
C40.8H26.8O0.7Cl2N4Cu2
782.24
C84H74Cl8N8Cu4O
1749.27
C55H48Cl8N4O4Cu4
1366.73
C26H19ClN2CuO
474.42
298.37
Temperature [K]
Radiation used (λ [Å])
223(2)
193(2)
190(2)
190(2)
190(2)
190(2)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Mo-Kα (0.71073)
Crystal size [mm]
0.25·0.25·0.06
monoclinic
P21/n
0.20·0.10·0.05
triclinic
ꢂ1
0.56·0.08·0.07
monoclinic
P21
0.36·0.33·0.20
orthorhombic
Pnma
0.46·0.30·0.26
triclinic
0.30·0.10·0.08
trigonal
ꢃ3
Crystal system
Space group
a [Å]
ꢂ1
14.9211(3)
9.5702(2)
16.1093(3)
90
10.981(2)
12.106(2)
14.495(3)
81.29(3)
86.48(3)
63.53(3)
1705.0(7)
2
9.4845(19)
6.2235(12)
13.661(3)
90
10.344(2)
17.330(4)
10.517(2)
90
8.8410(18)
9.0220(18)
18.470(4)
78.62(3)
85.06(3)
83.39(3)
1431.6(5)
1
32.209(11)
32.209(11)
13.231(3)
90
b [Å]
c [Å]
α [°]
β [°]
98.1740(10)
90
96.72(3)
90
90
90
γ [°]
90
120
V [Å3]
Z
2277.00(8)
4
800.8(3)
2
1885.3(7)
1
11887(9)
18
F(000)
1060
794
316
894
690
4374
ρcalcd. [g·cm-3
µ [mm-1
]
1.502
1.524
1.237
1.541
1.585
1.193
]
1.225
1.443
0.073
1.450
1.887
0.945
Total reflections
Unique reflections
R(int)
13855
15617
8153
14153
24291
18826
4004
7640
2839
2830
7084
2966
0.0606
1.74 to 25.00
99.9
0.0933
2.521 to 28.154
93.6
0.0367
2.773 to 25.682
99.4
0.0317
2.265 to 30.015
99.8
0.0320
0.1653
2.470 to 21.334
99.6
Scan range θ [°]
Completeness to θmax [%]
Index ranges
2.254 to 28.327
99.7
-12 ≤ h ≤ 17
-11 ≤ k ≤ 10
-17 ≤ l ≤ 19
-14 ≤ h ≤ 14
-15 ≤ k ≤ 15
-19 ≤ l ≤ 19
-11 ≤ h ≤ 11
-7 ≤ k ≤ 7
-14 ≤ h ≤ 13
-24 ≤ k ≤ 19
-13 ≤ l ≤ 14
-11 ≤ h ≤ 11
-12 ≤ k ≤ 11
-24 ≤ l ≤ 24
-32 ≤ h ≤ 31
-32 ≤ k ≤ 32
-13 ≤ l ≤ 13
-16 ≤ l ≤ 16
Data/restraints/parameters
R1,[a][b] wR2 [I > 4σ(I)][c]
R1,[a][d] wR2 (all data)[c][d]
Goodness-of-fit on F2 [e]
4004/0/286
0.0407, 0.0986
0.0524, 0.1050
1.045
7640/804/461
0.0565, 0.1161
0.1434, 0.1436
0.809
2839/1/210
0.0419, 0.0893
0.0583, 0.0957
1.053
2830/2/133
0.0282, 0.0723
0.0348, 0.0759
1.052
7084/0/359
0.0414, 0.1251
0.0480, 0.1311
1.065
2966/0/280
0.0663, 0.1208
0.0922, 0.1282
1.130
Max./min. el. dens. [e.Å-3
]
0.727, -0.710
0.918, -0.998
0.130, -0.170
0.410, -0.514
1.942, -0.839
0.412, -0.388
[b]
[a] ꢃ1 =
ꢄ
ꢅ
−
ꢄ
ꢆ
/
ꢄ . Denotes the value of the residual considering only the reflections with I > 4σ(I). [c] ꢇꢃ2 =
ꢇ ꢄꢈ − ꢄꢈ
ꢉ/ꢈ
/
ꢄꢈ
ꢉ/ꢈ;ꢇ = 1/ ꢊꢈ ꢄꢈ ꢋꢂ ꢈ + ꢌꢂ , ꢂ =
+
ꢅ
ꢈ
ꢈ
ꢅ
ꢅ
ꢆ
ꢅ
ꢍꢋꢎ ꢄꢈ ꢏꢐ 0 + 2 ꢄꢈ /3. [d] Denotes the value of the residual considering all the reflections. [e] ꢑ =
ꢇ ꢄꢈ − ꢄꢈ
/
ꢒ − ꢓ
, n: number of data, p: parameters used.
ꢈ
ꢅ
ꢆ
ꢅ
ꢆ
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10.1002/ejic.201700755
European Journal of Inorganic Chemistry
FULL PAPER
Oswego, USA). Details on the setup and kinetic measurements have been
described previously.[14,49]
Conclusions
X-ray crystallography: Details on X-ray crystallography are reported in
Supporting Information. CCDC-1557630 (1), CCDC-1557629 (2), CCDC-
1557630 (3), CCDC-1557640 (4), CCDC-1557632 (BQ-Ketone), CCDC-
1557634 (9), CCDC-1557631 (10), CCDC-1557635 (11), CCDC-1557628
(DMBQ), CCDC-1557636 (12), CCDC-1557637 (13a) and CCDC-
1557638 (13b) 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/data_request/cif.
The investigations herein showed that the copper complex with
bispiperidine (BP) is capable not only of intramolecular ligand
hydroxylation but also of oxidation of substrates e. g.
benzylalcohol to benzaldehyde. These results are promising,
however, further work on that ligand system is necessary because
for practical use it still needs to be optimized. In contrast copper
complexes with the sterically more demanding ligand BQ do not
seem to be useful for substrate oxidation reactions. The reaction
behavior of the copper complexes with this ligand had turned out
quite complicated and without the crystal structures obtained it
would have been difficult to gain a good understanding of the
reaction mechanisms. These reactions show again how easily
copper chloride can affect organic molecules (in a quite colorful
way)[47] and how a large number of different reactions can be thus
triggered. Most interestingly in that regard was a coordination
polymer that formed, however here again only a few crystals were
obtained and it could not be prepared yet on a real synthetic scale.
Methanonbis(2-quinoline): Bisquinoline (0.4 g, 1.5 mmol) was dissolved
in 50 mL of methanol under ambient conditions. While stirring CuCl2 x H2O
(0.4 g, 3 mmol) was added to the orange solution. After addition of the
copper salt the solution obtained a red color and after 1 – 2 h of stirring it
turned to a green color. This solution was diluted with 50 mL of water and
shake out with aqueous ammonia solution. The solution was extracted with
dichloromethane (50 mL), dried with Na2SO4 and evaporated. The product
(0.34 g, 1.2 mmol, 81 %) was obtained as yellow solid after flash
chromatography (pentane/TBME, 1:1 (v/v)). 1H-NMR (400 MHz, CDCl3): δ
(ppm) = 8.36 (d, 3J = 8.4 Hz, 2H), 8.24 (d, 3J = 8.5 Hz, 2H), 8.19 (d, 3J =
8.5 Hz, 2H), 7.92 (d, 3J = 8.0 Hz, 2H), 7.77 (t, 3J = 8.4 Hz, 2H), 7.65 (t, 3J
= 8.1 Hz, 2H). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 193.8, 154.5, 147.6,
136.7, 130.9, 130.2, 129.2, 128.6, 127.8, 121.6. HRMS (EI): calcd. for
C19H12N2O: 284.0950; found: 284.0954. Anal. calcd. for C19H12N2O: C,
80.27; H, 4.25; N, 9.85; found: C, 79.29; H, 4.29; N, 9.54.
Experimental Section
General: All reagents and solvents were purchased from commercial
sources. The solvents were purified by distillation and dried by standard
procedures or used from the solvent purification system SPS-800 by
MBraun (Garching, Germany). Synthesis of the bispiperidine (BP) and
bisquinoline (BQ) ligands will be published elsewhere.[31] Preparations
under anaerobic conditions were carried out in Schlenk tubes using dry
nitrogen or in a glovebox by MBraun under an argon atmosphere. 1H and
13C NMR spectra were recorded on a Bruker Avance II 200 spectrometer
(1H at 200 MHz; 13C at 50 MHz), Bruker Avance II 400 spectrometer (1H at
400 MHz; 13C at 100 MHz) and Bruker Avance III HD 400 spectrometer
(1H at 400 MHz; 13C at 100 MHz) in CDCl3 using TMS as internal standard.
The 1H- and 13C- NMR spectra were calibrated against the residual proton
and carbon signals of chloroform at δ=7.26 ppm and δ=77.16 ppm. Mass
spectra were measured with a MAT 95 sector field mass spectrometer
(Thermo Finnigan MAT GmbH, Bremen GER) with electron impact
ionization (EI) and with a MicroTOF mass spectrometer (Bruker Daltonics,
Bremen GER) with electro spray ionization (ESI). Calibration was done
with NaHCOO clusters before measurement. Reactions carried out in a
microwave reactor an apparatus from CEM GmbH (Kamp-Lintford, GER)
was used in an open system with reflux condenser. Elemental analysis
was weighted with a UMX-2 (Mettler-Toledo, Giessen GER) and analyzed
with a Carlo Erba 1106 CHN.
Propane-2,2’-bis(-2-quinoline) (DMBQ): Under inert gas conditions
bisquinoline (0.3 g, 1.1 mmol) was dissolved in 10 mL anhydrous THF and
cooled to -78 °C were n-BuLi (0.7 mL, 1.2 mmol, 1.6 M in hexane) was
dropped to the solution and stirred for 30 min at this temperature. The
reaction mixture was allowed to warm to -10 °C for a few minutes and
cooled back to -78 °C. Subsequent iodomethane (189 mg, 1.3 mmol) was
added to the mixture and the solution was allowed to stirrer overnight while
warming up to room temperature. The solvent was evaporated and the
residue was re-dissolved in anhydrous THF (10 mL) under inert gas
conditions and the procedure was repeated to substitute the second
methyl group. After this the reaction mixture was diluted with water (50 mL),
extracted with dichloromethane (2 x 50 mL), dried the combined organic
phases with Na2SO4 and evaporated to dryness. The product (0.221 g,
0.74 mmol, 67 %) was obtained as yellow-orange solid after flash
chromatography (pentane/TBME, 1:1 (v/v)). 1H-NMR (400 MHz, CDCl3): δ
(ppm) = 8.16 (d, 3J = 8.5 Hz, 2H), 7.93 (d, 3J = 8.6 Hz, 2H), 7.75 – 7.69 (m,
4H), 7.51 (dt, 3J = 7.5 Hz, 4J = 1.1 Hz, 2H), 7.16 (d, 3J = 8.6 Hz, 2H), 2.02
(s, 6H). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 167.3, 147.4, 135.9, 129.7,
129.2, 127.4, 126.8, 126.1, 121.0, 50.0, 28.1. HRMS (ESI+): calcd. for
C21H19N2+ [M + H]+: 299.1543; found: 299.1550. Anal. calcd. for C21H18N2:
C, 84.53; H, 6.08; N, 9.39; found: C, 83.19; H, 6.26; N, 8.89.
Benzylbisquinoline (BnBQ): Bisquinoline (0.4 g, 1.48 mmol) was
dissolved in 10 mL anhydrous THF under inert gas conditions and cooled
to -78 °C. To this reaction mixture n-BuLi (1.0 mL, 1.6 mmol, 1.6 M in
hexane) was added and the solution was allowed to stirrer for 30 minutes
at this temperature. The reaction mixture was allowed to warm to -10 °C
for a few minutes and cooled back to -78 °C. Subsequent benzylbromide
(0.305 g, 1.78 mmol) was added at -78 °C and the mixture was allowed to
stirrer overnight while warming up to room temperature. The solution was
diluted with water (50 mL) and extracted with dichloromethane (50 mL).
The organic layer was dried with Na2SO4 and evaporated. The product
(0.4 g, 1.11 mmol, 75 %) was obtained as waxy slight red solid after flash
chromatography (pentane/TBME, 2:1 (v/v)). 1H-NMR (200 MHz, CDCl3): δ
(ppm) = 8.04 (d, 3J = 8.3 Hz, 2H), 7.86 (d, 3J = 8.5 Hz, 2H), 7.62 – 7.54 (m,
4H), 7.45 – 7.31 (m, 4H), 7.15 – 7.12 (m, 2H), 7.05 – 6.94 (m, 3H), 4.92 (t,
Stopped-Flow Measurements: Solutions of copper(I) complexes for
stopped-flow measurements were prepared in glass syringes under argon
in a glove box. Saturated solutions of dioxygen were prepared by bubbling
dry dioxygen (grade 5.5, Messer Griesheim, Germany) through the solvent
in a syringe for about 10 min. Dioxygen concentration in a saturated thf
solution has been reported to be 7.9 x 10-3 mol/L at 25 °C.[48] Stopped-flow
analysis was performed at
a temperature of -80 °C ± 1 °C. All
measurements were repeated about 5 times at each concentration and
temperature and gave consistent results. Data were obtained using a Hi-
Tech SF-61SX2 low-temperature stopped-flow unit equipped with a diode
array spectrophotometer (Hi-Tech, Salisbury, UK; now TgK Scientific,
Bradford-on-Avon, UK). Data were analyzed either using the integrated
software Kinetic Studio (TgK Scientific) or Igor Pro (Wavemetrics, Lake
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10.1002/ejic.201700755
European Journal of Inorganic Chemistry
FULL PAPER
3J = 7.6 Hz, 1H), 3.75 (d, 3J = 7.6 Hz, 2H). 13C-NMR (100 MHz, CDCl3): δ
(ppm) = 162.1, 147.9, 140.4, 136.3, 129.44, 129.40, 129.35, 128.2, 127.6,
127.2, 126.1, 125.9, 121.9, 59.2, 40.2.
10 min. Subsequent 1 – 2 mL of the complex solution was transferred to
the glow tube. The formation of red crystals could be achieved by diffuse
of the Et2O into the glow tube. The crystals could be structurally
characterized.
Pyridylbisquinoline (PyBQ): A 10 mL microwave vessel was charged
with bisquinoline (250 mg, 0.925 mmol) and 2-bromopyridine (500 mg,
3.16 mmol). After that one drop of ethereal hydrochloric acid (1 M in Et2O)
were added whereby some solid was formed. The vessel was heated for
10 min at 215 °C in a microwave reactor whereby the mixture turns into
dark red. The waxy mixture was dissolved in dichloromethane, washed
with 2 mol/L aqueous sodium hydroxide solution whereby the organic layer
turns deep orange. The mixture was extracted with dichloromethane (2 x
50 mL), the organic phase was dried with Na2SO4 and evaporated. The
product (116 mg, 0.48 mmol, 52 %) was obtained as orange-red solid after
flash chromatography (pentane/TBME, 1:10 (v/v)). 1H-NMR (400 MHz,
CDCl3): δ (ppm) = 8.60 (d, 3J = 4.0 Hz, 1H), 8.10 – 8.04 (m, 4H), 7.77 (d,
3J = 8.4 Hz, 2H), 7.67 – 7.60 (m, 6H), 7.50 – 7.43 (m, 4H), 7.16 – 7.13 (m,
1H), 6.39 (s, 1H). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 161.2, 161.1,
149.6, 148.0, 136.6, 136.2, 129.6, 129.3, 127.5, 127.1, 126.3, 124.5, 122.7,
121.9, 65.6. HRMS (ESI+): calcd. for C24H18N3+ [M + H]+: 348.1495; found:
348.1502.
copper(II) dichlorido methoxy bis chino-2-yl methanol complex (10):
BQ (100 mg, 0.37 mmol) was diluted in 3 – 5 mL of methanol resulted in
an orange solution. To this solution was added a solution of CuCl2 · 2H2O
(62 mg, 0.37 mmol) in 2 – 3 mL of methanol whereby the color of the
solution changed to red. After one hour the color of the solution changed
to brown and finally to green. Slow evaporation of the green solution led to
formation of a few green crystals that could be structurally characterized.
General procedure for the oxidation experiments: Under inert gas
conditions in a glovebox BP (50 mg, 0.3 mmol) and [Cu(CH3CN)4]OTf
(103.2 mg, 0.3 mmol) was dissolved in anhydrous THF and staggered with
the same equivalent of the substrate. The reaction mixture was cooled to
-80 °C and dioxygen was bubbled through the solution for 10 min. The
reaction mixture was allowed to warm to room temperature and diluted
with aqueous ammonia solution or water. The organic phase was extracted
with dichloromethane, evaporated to dryness and the yield was
determined by GC-MS.
Picolylbisquinoline (PicBQ): Under inert gas conditions, α-
picolylbromide hydrobromide (1.0 g, 5.0 mmol) was suspended with NaH
(0.18 g, 7.5 mmol) in 5 mL anhydrous THF and stirred over night at room
temperature. Bisquinoline (0.36 g, 1.33 mmol) was dissolved in 10 mL
anhydrous THF under inert gas conditions and cooled to -78 °C. To this
reaction mixture n-BuLi (0.92 mL, 1.46 mmol, 1.6 M in hexane) was added
and the solution was allowed to stirrer for 30 minutes at this temperature.
The reaction mixture was allowed to warm to -10 °C for a few minutes and
cooled back to -78 °C. Subsequent the prepared mixture of
α-picolylbromide was added without the precipitate at -78 °C and the
mixture was allowed to stirrer overnight while warming up to room
temperature. The solution was diluted with water (50 mL) and extracted
with dichloromethane (100 mL). The organic layer was dried with Na2SO4
and evaporated. The product (0.31 g, 0.85 mmol, 64 %) was obtained as
waxy slight red solid after flash chromatography (pentane/TBME, 1:5 (v/v)).
1H-NMR (400 MHz, CDCl3): δ (ppm) = 8.46 (d, 3J = 4.0 Hz, 1H), 8.10 (d,
3J = 8.4 Hz, 2H), 7.93 (d, 3J = 8.5 Hz, 2H), 7.67 – 7.61 (m, 4H), 7.51 (d, 3J
= 8.5 Hz, 2H), 7.42 (dt, 3J = 7.5 Hz, 4J = 1.1 Hz, 2H), 7.32 (dt, 3J = 7.65 Hz,
4J = 1.8 Hz, 1H), 7.12 (d, 3J = 7.8 Hz, 1H), 6.93 – 6.90 (m, 1H), 5.35 (t, 3J
= 7.7 Hz, 1H), 4.04 (d, 3J = 7.7 Hz, 2H). 13C-NMR (100 MHz, CDCl3): δ
(ppm) = 162.0, 160.2, 149.2, 147.9, 136.2, 135.9, 129.4, 129.2, 127.5,
127.0, 126.0, 123.8, 121.8, 121.0, 56.8, 42.0. HRMS (ESI+): calcd. for
C25H20N3+ [M + H]+: 362.1652; found: 362.1652.
Keywords: Copper • Dioxygen • Oxidation • Bispiperidine •
Bisquinoline
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Copper complexes with bispiperidine
(BP) as a ligand can form a bis-µ-
oxido intermediate that is capable not
only of intramolecular ligand
hydroxylation but also of oxidation of
substrates e. g. benzylalcohol to
benzaldehyde. In contrast copper
complexes with the sterically more
demanding ligand bisquinoline (BQ)
were not useful for substrate oxidation
reactions.
Selective Oxidations
Tim-Daniel J. Stumpf, Manfred
Steinbach, Christian Würtele, Jonathan
Becker, Sabine Becker, Roland Fröhlich,
Richard Göttlich, Siegfried Schindler*
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Reactivity of Copper Complexes with
Bispiperidine (BP) and Bisquinoline
Ligands
*one or two words that highlight the emphasis of the paper or the field of the study
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Author(s), Corresponding Author(s)*
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*one or two words that highlight the emphasis of the paper or the field of the study
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