Synthesis, structure and reactivity of the compound [Cu(C7H7NH2)Cl]4 derived from CuCl and benzylamine (C7H7NH2)

Article abstract of DOI:10.1002/zaac.201400395

The facile synthesis, molecular structure, and reactivity of [Cu(C₇H₇NH₂)Cl]₄ (1) towards dioxygen and derivatives is reported. The compound could easily be prepared in good yields by mixing CuCl and benzylamine under inert conditions in dichloromethane. Surprisingly this copper(I) compound, a copper(I) tetramer with Cu^I???Cu^I interactions of 2.89 ?, was formed instead of an expected cubane clus-ter. Oxidation reactions led to formation of μ-oxido-species as intermediates, however 1 did not show any catalytic activity in the activation of CH bonds. In contrast it turned out that 1 was quite stable towards oxidation. Analogous reactions with CuBr or CuI were different and neither cluster units or polynuclear copper(I) complexes were obtained.

Full text of DOI:10.1002/zaac.201400395

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DOI: 10.1002/zaac.201400395
Synthesis, Structure and Reactivity of the Compound [Cu(C7H7NH2)Cl]4
derived from CuCl and Benzylamine (C7H7NH2)
Sabine Becker,[a] Ulrich Behrens,[b] and Siegfried Schindler*[a]
Keywords: Copper–copper ion interactions; Copper; Dioxygen activation; μ-Oxido species
Abstract. The facile synthesis, molecular structure, and reactivity of ter. Oxidation reactions led to formation of μ-oxido-species as interme-
[Cu(C7H7NH2)Cl]4 (1) towards dioxygen and derivatives is reported.
The compound could easily be prepared in good yields by mixing
CuCl and benzylamine under inert conditions in dichloromethane. Sur-
diates, however 1 did not show any catalytic activity in the activation
of CH bonds. In contrast it turned out that 1 was quite stable towards
oxidation. Analogous reactions with CuBr or CuI were different and
prisingly this copper(I) compound, a copper(I) tetramer with CuI···CuI neither cluster units or polynuclear copper(I) complexes were obtained.
interactions of 2.89 Å, was formed instead of an expected cubane clus-
Introduction
Copper(I) cubane clusters of the type [Cu4OX6L4] (X = Cl,
Br, I, see Figure 1) are well known and have been investigated
intensively in the past mainly because of their interesting op-
tical properties.[1] In most cases synthesis is facile and prepara-
tion is accomplished through self-assembly of the cubane
cores.[2] Therefore, simply mixing a copper(I) halide with an
according ligand usually leads to a cubane cluster. Especially
Davies and El-Sayed previously investigated copper(I) cubane
systems in regard to their reactivity towards dioxygen.[2a–2c,3]
In this context they also reported the formation of the μ4-oxido
Figure 1. Schematic representation of the structural motif of a cop-
per(I) cubane core (left) and a μ4-oxido copper(II) cluster (right).
copper(II) cluster [Cu4OCl6L4] (L = N,N-dimethylaminome-
thylferrocene) from the reaction of the cubane cluster
[Cu4Cl4L4] with dioxygen.[2a] This is another well known
structural motif of a copper cluster; here a copper(II) system, the copper(II) cubane cluster [Cu4(O-L)4Cl4] (L = C11NH14)
that had been already reported in the 1960s.[4]
(cluster II).[5]
In our efforts to better understand the role of CuCl as well
The role of compound 2 in this equilibrium was not clari-
as CuCl2 in organic syntheses, especially oxidation reactions, fied. Now we report the synthesis and reactivity of [CuL1Cl]4
we previously investigated complexes derived from the reac- (1). Besides the crystallization behavior (trimer or tetramer in
tion of CuCl2 with benzylamine.[5] The reaction behavior of the asymmetric units) compound 1 and 2 are chemically iden-
this system turned out to be quite complex. Thus we observed tical. Therefore our conclusions derived for 1 should be valid
the formation of compound [CuL1Cl]3 (2), which is in a for 2 as well. In order to gain more information about the
chemical equilibrium with the μ4-oxido-copper cluster reactivity of these copper(I) complexes towards dioxygen in
[Cu4OCl6L1 ] [CuL1 Cl2] (cluster I, L1 = benzylamine) and general and thus the role of compound 1/2 in the described
4 2
2
chemical equilibrium we investigated this system using UV/
Vis spectroscopy. Furthermore its ability in catalytic oxygena-
tion reactions was tested. Herein we now describe the synthe-
sis, structure, and reactivity of [Cu(C7H7NH2)Cl]4 (1).
* Prof. Dr. S. Schindler
Fax: +49-641-99-34149
E-Mail: siegfried.schindler@anorg.chemie.uni-giessen.de
[a] Institut für Anorganische und Analytische Chemie
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 58
35392 Gießen, Germany
Results and Discussion
[b] Institut für Anorganische und Angewandte Chemie
Universität Hamburg
When mixing CuCl and benzylamine the formation of a
copper(I) cubane cluster was expected due to previous reports
in the literature on related systems. However, mixing CuCl
with benzylamine under inert conditions led to the formation
Martin-Luther-King-Platz 6
20146 Hamburg, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201400395 or from the au-
thor.
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of an unexpected copper(I)-tetramer (1). Stoichiometric varia-
tions led to the same result. The molecular structure of the
colorless compound obtained is shown in Figure 2.
Figure 3. UV/Vis spectra of the reaction of compound 1 towards O2
(solvent: dcm, T = 25 °C, c = 2ϫ10–5 mol·L–1, t = 20 min). λmax
=
259 nm, 746 nm, 829 nm, shoulder at 357 nm.
Figure 2. Molecular structure of 1. Ellipsoids are drawn at the 50%
probability level.
The spectra show the development of absorption bands at
259, 746, and 824 nm. Additionally a shoulder at ca. 357 nm
The asymmetric unit of the elemental cell shows half of was formed. In contrast the absorption band at 570 nm de-
the tetramer with copper–copper distances of 2.89 Å. CuI···CuI creased. The formed species was stable for days at room tem-
interactions are well known in literature.[6] So far reasons for perature before it decomposed. It is difficult to relate the ab-
these interactions are not quite clear, especially if there is no sorption maxima to certain Cu/O2 species, because only little
ligand environment, which enforces the copper(I) ions in such is known about dioxygen activation with tetranuclear copper
an arrangement. It is known that due to the small bandgap compounds. The formation of these “oxygen adduct” com-
between the 3d and 4s band electrons can transfer from the plexes follow a more complex mechanism, since the 4e– pro-
occupied 3d band into the empty 4s band und thus covalent vided by the tetranuclear clusters do not correlate with the 2e–
Cu–Cu bonds are possible. However, the reason for this ex- reduction of O2 to peroxide.[7] Reim et al. reported a tetranu-
traordinary small bandgap is not understood yet. As already clear μ4-peroxido complex and provided UV/Vis data of this
described above, the formation of 1 was unexpected. Only the compound.[8] However these data do not correlate with the
crystal structure revealed the linear character of the formed data presented herein. In addition there are no reports of di-
tetramer instead of the expected cubane structure. Standard oxygen activation with compounds that exhibit CuI···CuI inter-
spectroscopy (e.g. IR spectroscopy, cryoscopy, elemental actions. However we do not have clear evidence that there is
analysis) would not have allowed here to distinguish between only the tetramer 1 in solution. We already observed formation
l and the cluster. In our case most likely only crystal structures of trimer 2 and the formation of other compounds is possible.
can provide clear evidence for a cubane core. So in our opinion Hence it is not possible to exclude that 1 does not dissociate
previously proposed cubane clusters, without a molecular in solution leading to mononuclear complexes [CuL1Cl]. Spec-
structure reported, should therefore be considered with care. troscopic features have to be compared to those of mononu-
An attempt to distinguish between both structural units could clear and polynuclear Cu/O2 species. However, the UV/Vis
be fluorescence analysis. Copper(I) cubane cores often show features reported herein closely resemble UV/Vis data reported
fluorescence. In contrast to these observations 1 does not show by Churchill et al. previously.[2c] They observed maxima at
any emission properties. At the moment we do not know if 235, 255, 770, and 850 nm for the reaction of the cubane clus-
that is a unique feature of compound 1 or if this is a general ter [Cu4Cl4(denc)4] (denc = N,N-diethylnicotineamide) with di-
phenomenon of these structure types. To confirm this assump- oxygen.[2c] Assigning these findings with the spectroscopic
tion more investigations on derivatives of 1 have to be carried features of 1 the formation of a μ-oxido intermediate (“Cu2O”)
out in the future.
is suggested. These species are well known in literature[9] and
In addition we investigated the reactivity of 1 towards di- μ-oxido intermediates are currently discussed to play an impor-
oxygen and derivatives. For this purpose a large number of tant role in the enzymatic hydroxylation of methane (copper
experiments were performed. Bubbling dioxygen through a based methane monooxygenase).[10] Thus μ-oxido species
solution of compound 1 in dichloromethane led to a color could gain more importance in the search for suitable model
change from colorless to green. However, even at room tem- systems for enzymatic CH activation.[11] Despite of the pro-
perature this reaction was very slow. Figure 3 shows the time posed catalytic activity of μ-oxido species, the system investi-
resolved UV/Vis spectra of this reaction over a time range of gated by Churchill et al. did not show any catalytic activity.[2c]
20 min.
Davies et al. reported the reaction of the cubane cluster
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[Cu4Cl4(pyridine)4] with dioxygen. However, this system
showed similar UV/Vis features in the range of 700–900 nm
as the denc system and as 1 (Figure 4). The formation of two
complexes was proposed: a μ4-oxido species with one oxygen
coordinated in the center of the cubane core and a μ-oxido
species, a cubane, where each of the two oxygen ions bridges
two copper ions. The appearance of two maxima was assigned
to these two species. It was furthermore reported that the μ4-
oxido species was active in oxygenation reactions (see be-
low).[2b,12]
Figure 5. Molecular structure of [CuL1 Cl2] (3). Ellipsoids are drawn
2
at the 50% probability level.
comparison is shown in Figure 6. Compound 3 exhibits an in-
tense absorption maximum at 285 nm and a shoulder at
350 nm. The absorption maxima at 285 nm is assigned to
πǞπ* of the coordinated benzylamine group. The shoulder at
350 nm is identified as a LMCT band. Weaker dd or ligand
field transitions are observed at 660 nm. The comparison of
the UV/Vis spectra of the Cu/O2 species and cluster I shows
strong similarities. For cluster I absorption maxima at 286,
754, and 834 nm and a shoulder at 360 nm are observed. These
similarities suggest that 1 reacted with dioxygen to form clus-
ter I as an intermediate. This finding goes along with the equi-
librium between cluster I and 1/2. However, analysis of the
UV/Vis data is complicated due to the structural similarities
between the “oxygen intermediate” complexes proposed by
Davies et al. and cluster I (both contain a μ4-oxido motif).
Thus it is currently not possible to explain the UV/Vis features
completely. However, we suggest the simultaneous formation
of a μ-oxido species and a μ4-oxido species (cluster I). This is
consistent with two absorption maxima in the range of 700–
900 nm as previously proposed by Davies et al. and Endres et
al.[2b,12]
Figure 4. Infrared and near-infrared part of the UV/Vis spectra of the
reaction of compound 1 towards O2 (solvent: dcm, T = 25 °C, c =
1ϫ10–4 mol·L–1). λmax = 746 and 829 nm.
However, most of these assignments came from spectro-
scopic studies and only a small number of the proposed cluster
structures actually could be experimentally confirmed by crys-
tal structures (e.g. only the crystal structure of [Cu4I4(denc)4]
is reported, not of [Cu4Cl4(denc)4]). As discussed above the
formation of a linear tetramer such as 1 instead of a cubane
cluster is possible as well.
This could explain the observed similarities in the UV/Vis
spectra. Attempts to obtain crystals of the intermediate species
formed during the oxidation were not successful and actually
resulted in recrystallization of compound 1. This emphasized
the extraordinary stability of compound 1 towards dioxygen.
However, after partial decomposition of the “Cu/O2” species
crystals could be obtained from the green solution. The mole-
cular structure of this final product, the simple copper(II) com-
plex [CuL1 Cl2] (3) is shown in Figure 5.
2
The molecular structure of 3 had been reported pre-
viously.[5,13] The UV/Vis spectrum of pure 1 was compared
with those of the “oxygen adduct”, compound 3, and cluster I.
As described above the μ4-oxido cluster “cluster I” is in a
chemical equilibrium with compound 2/1. In addition Davies
et al. also observed the formation of a μ4-oxido copper(II) clus-
ter [Cu4OCl6L4] (L = N,N-dimethylaminomethylferrocene) af-
ter the reaction of the cubane cluster [Cu4Cl4L4] with di-
oxygen.[2a] Thus the comparison of the UV/Vis data of the
Figure 6. UV/Vis spectra of compound 1, 3, the “oxygen adduct” and
cluster I (solvent: dcm, T = 25 °C, c(1) = 2ϫ10–5 mol·L–1, c(3, cluster
“oxygen adduct” compound with cluster I is essential. The I) = 1ϫ10–4 mol·L–1).
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As mentioned above the oxygen intermediate described by investigated the reactivity of CuBr and CuI towards benzyl-
Davies et al. and Endres et al. was active as a catalyst for amine as well. Interestingly neither CuBr nor CuI reacted with
polymerization of phenols.[2b,12] Since compound 1 showed benzylamine under inert conditions. So the syntheses were re-
similar UV/Vis data for the reaction with dioxygen and μ-ox- peated under atmospheric conditions. Due to the large stability
ido species are proposed to be catalytically active, it was obvi- of both copper salts, syntheses of cubane clusters are some-
ous to test its ability to catalyze oxidation reactions for reac- times carried out under atmospheric conditions as well.[1b,2d]
tions that had been investigated previously with cluster I. As However, formation of a cubane cluster or a tetramer was nei-
already described above 2/1 was isolated for the first time from ther observed in dichloromethane nor methanol.
a redox equilibrium of cluster I with other copper(I) and cop-
The reaction of CuBr with benzylamine led to dark green
per(II) species. This equilibrium caused the oxidation and hy- solutions and colorless crystals were obtained from the meth-
droxylation of solvent molecules and additives. We already as- anol solution. However no copper complex was isolated but
sumed that a copper(I) compound is the active species in these the carbamate of benzylamine (HL1)(C7H7NHCOO) was ob-
oxidation reactions.[5] In order to find out if 1 was the active tained, that has been described previously (its molecular struc-
species we now tested the ability of 1 to oxygenate aliphatic ture is reported in the Supporting Inofrmation).[15] Most likely
and/or aromatic CH bonds instead of repeating the reactions, benzylamine reacted with carbon dioxide present in air to give
which were performed by Davies et al. Thus compound 1 was this carbamate. In contrast when reacting CuI with benzyl-
dissolved either in dichloromethane or acetonitrile, cyclohex- amine in dichloromethane the solution turned purple, but no
ane was added, and dioxygen was bubbled through the solu- product could be isolated. When reacting CuI and benzylamine
tion. However, no oxygenation product was observed. So H2O2 in methanol the solution turned blue and green crystals
was added instead of O2 to the solution. But again no oxygena- were obtained from this solution that turned out to be
tion occurred. In another attempt H2O2 was substituted with [CuL1 (OMe)]2[Cu3,5I6](HL1)0.5·nH2O·nMeOH (4). The mo-
2
the urea adduct (H2N)2CO·H2O2 in order to avoid the presence lecular structure of the cation of 4 is shown in Figure 7.
of water as much as possible. But again no oxygenation reac-
tion could be observed. Furthermore, the substrate was
changed from aliphatic cyclohexane to aromatic benzene. Still,
compound 1 proved to be unreactive as an oxygenation cata-
lyst.
Previously we had observed that cluster I was converted to
a different copper(II) cubane cluster (cluster II) when stored in
denaturated ethanol (here 2-butanone is added to the ethanol
to avoid drinking it).[5] Scheme 1 shows the proposed mecha-
nism for the formation of the new ligand.
Scheme 1. Proposed reaction for the formation of 3-benzylimino-bu-
tan-2-olate.[5]
Figure 7. Molecular structure of [L1 (OMe)CuII]22+, the cation of 4.
Ellipsoids are drawn at the 50% probability level.
2
The ligand benzylamine reacts with 2-butanone present in
solution to form an imine. Intramolecular hydroxylation of this
Schiff base leads to the ligand 3-benzylimino-butane-2-olate
2–
For clarity the anion [CuI3.5I6]HL1
is not shown. The
0.5
and finally to cluster II. Therefore, it seemed to be interesting structure of the cation shows the copper(II) complex
to test if compound 1 actually was responsible for this conver- [L1 (OMe)CuII]22+. Here the copper(II) ions have a square-
2
sion. However, adding 1 to a solution of 2-butanone and bub- planar arrangement. They coordinate two benzylamine groups
bling dioxygen through this solution did not cause formation per ion and are bridged via methoxy groups derived from the
of cluster II and thus 1 seems not to be involved in this reac- solvent methanol.
tion. In conclusion and quite surprising 1 is extraordinary
Methanolate bridged copper compounds are well known.
stable concerning reactivity towards dioxygen or derivatives. Only some examples for such compounds with monodentate
Generally solutions of 1 were stable at room temperature and N-donor ligands are listed in the references.[14] The question
under atmospheric conditions for hours before they turned of the origin of the copper(II) ions is not easy to answer. It is
green. Crystals of 1 were even more stable and quite often not very likely that the copper(I) ions disproportionated be-
the pure copper(I) complex was obtained again after treating a cause the precipitation of elemental copper was not observed.
solution of it with dioxygen as discussed above. So far we do Probably a part of the dissolved copper(I) ions were oxidized
not understand this extraordinary stability towards oxidation.
by atmospheric oxygen to give copper(II) ions. In addition an
Having observed previously that also copper(II) cluster acid-base-reaction must have occurred between methanol and
[5]
complexes were obtained using CuBr2 instead of CuCl2 we benzylamine allowing the formation of the methoxy bridged
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0.89 mmol) was added to the stirred solution. The solution was al-
lowed to stand until crystallization occurred.
complex and the benzylammonium ions. The larger part of CuI
probably did not react with either benzylamine or atmospheric
dioxygen and did not form long polymeric chains. The voids
of these chains were filled with solvent molecules, methanol
and water.
In summary for both CuBr as well as CuI syntheses of a
cubane cluster or an analogue to 1 were not successful.
Attempted Oxygenation of Cyclohexane:
With O2: Compound 1 (10 mg, 13 μmol) was dissolved in acetonitrile
(ca. 3 mL) and cyclohexane (840 mg, 10 mmol) was added. Afterwards
dry O2 was bubbled through the solution for 20 min. The solution was
analyzed by GC-MS. The reaction was also performed in dichloro-
methane.
Conclusions
With H2O2: Compound 1 (21 mg, 25 μmol) was dissolved in aceto-
nitrile (ca. 10 mL) and cyclohexane (840 mg, 10 mmol) was added.
Afterwards H2O2 (aqueous 30%, 2.0 mL, 20 mmol) was added to the
stirred solution. The solution was stirred for ca. 6 h and then analyzed
by GC-MS. The reaction was also performed in dichloromethane.
Reacting CuCl with benzylamine under inert conditions led
to the linear compound [CuL1Cl]4 (1) instead of an expected
cubane cluster. 1 showed unsupported CuI···CuI interactions.
The nature of these interactions is not completely clarified yet.
In contrast formation of either cluster compounds or polynu-
clear copper(I) complexes was not observed when CuBr or CuI
was used instead of CuCl. Spectroscopically it could be shown
that μ-oxido complexes and a μ4-oxido species formed when
1 was oxidized with dioxygen. Despite the fact that μ-oxido
species are proposed to play an important role in catalytic hy-
droxylation reactions of CH bonds, 1 turned out to have an
extraordinary stability towards dioxygen/peroxides and proved
to be inreactive in oxidation reactions. Probably the CuI···CuI
interactions might somehow be the reason for the stability
towards oxidation. From these findings we can conclude that
polynuclear copper(I) complexes such as 1 or 2 do not seem to
be directly involved in catalytic oxidation reactions observed
previously using cluster I as a catalyst. Furthermore, it turned
out that formation of cubane cluster units derived from cop-
per(I) halogenides is not always as facile as expected and alter-
native reaction products are possible. Distinguishing these dif-
ferent reaction products only spectroscopically is complicated
due to their spectral similarities. Therefore, copper cubane
cluster complexes reported in the literature without a crystal
structure should be considered with care.
With (NH2)2CO·H2O2: Compound 1 (21 mg, 25 μmol) was dissolved
in dichloromethane (ca. 10 mL) and cyclohexane (840 mg, 10 mmol)
was added. Afterwards (NH2)2CO·H2O2 (1.9 g, 20 mmol) was added
to the stirred solution. The solution was stirred for ca. 6 h and then
analyzed by GC-MS.
Attempted Hydroxylation of Benzene: Compound
1 (10 mg,
13 μmol) was dissolved in absolute benzene (ca. 3 mL) and dry O2
was bubbled through the solution for 20 min. The solution was sepa-
rated into three parts. The first was analyzed by GC-MS. A small part
of H2O was added to the second part and a small part of HCl to the
third part. All samples were analyzed by GC-MS.
Attempted Hydroxylation of 2-Butanone: The reaction was per-
formed under inert conditions in a glove box. Compound 1 (8 mg,
10 μmol) was dissolved in absolute dichloromethane (ca. 20 mL) and
2-butanone (ca. 3 mL) was added. The solution was heated to ca. 30 °C
for ca. 15 min and then dry O2 was bubbled into the solution for
20 min. The solution was analyzed by GC-MS. The reaction was also
performed in absolute methanol.
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number CCDC-982979 (for 1) (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Experimental Section
All chemicals used were of p.a. quality and were purchased from
Acros, Aldrich, Fluka, or Merck, if not mentioned otherwise. Dry sol-
vents for air sensitive reactions were redistilled in an argon atmo-
sphere. Analytical data (elemental analyses, IR spectra, and crystallo-
graphic data[16]) are summarized in the Supporting Information.
Supporting Information (see footnote on the first page of this article):
Spectroscopic data, photographs, and crystallographic data.
Acknowledgements
Preparation of 1: The synthesis was performed under inert conditions
in a glove box. CuCl (165 mg, 1.67 mmol) was dissolved in absolute
dichloromethane (ca. 10 mL) and benzylamine (358 mg, 3.33 mmol)
was added dropwise to the solution whilst stirring. The mixture was
stirred until CuCl was dissolved completely and crystallization oc-
curred. The resulting colorless crystals were filtered off and dried to
give compound 1 (260 mg, 0.31 mmol, 79%).
S. Becker and S. Schindler are thankful to the Stiftung Stipendienfonds
der Chemischen Industrie e.V. for providing a Ph.D. scholarship to
S. Becker.
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Received: August 17, 2014
Published Online: November 3, 2014
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