Access to a CuII-O-CuII motif: Spectroscopic properties, solution structure, and reactivity

Article abstract of DOI:10.1021/ja406721a

We report a complex with a rare Cu^II-O-Cu^II structural motif that is stable at room temperature, which allows its in-depth characterization by a variety of spectroscopic methods. Interest in such compounds is fueled by the recent discovery that a Cu^II-O-Cu ^II species on the surface of Cu-ZSM-5 is capable of oxidizing methane to methanol, and this in turn ties into mechanistic discussions on the methane oxidation at the dicopper site within the particulate methane monooxygenase. For the synthesis of our Cu₂O complex we have developed a novel, neutral ligand system, FurNeu, exhibiting two N-(N′,N′-dimethylaminoethyl) (2-pyridylmethyl)amino binding pockets connected by a dibenzofuran spacer. The reaction of FurNeu with CuCl yielded [FurNeu](Cu₂(μ-Cl))(CuCl ₂), 1, demonstrating the geometric potential of the ligand to stabilize Cu-X-Cu moieties. A Cu^I precursor with weakly coordinating anions was chosen in the next step, namely [Cu(NCCH₃) ₄]OTf, which led to the formation of [FurNeu](Cu(NCCH ₃))₂(OTf)₂, 3. Treatment of 3 with O ₂ or PhIO led to identical green solutions, whose UV-vis spectra were markedly different from the one displayed by [FurNeu](Cu)₂(OTf) ₄, 4, prepared independently from FurNeu and Cu(OTf)₂. Further investigations including PhIO consumption experiments, NMR and UV-vis spectroscopy, HR-ESI mass spectrometry, and protonation studies led to the identification of the green product as [FurNeu](Cu₂(μ-O))(OTf) ₂, 5. DOSY NMR spectroscopy confirmed its monomeric character. Over longer periods of time 5 decomposes to give [Cu(picoloyl)₂], formed through an oxidative N-dealkylation reaction followed by further oxidation of the ligand. Due to its slow decomposition reaction, all attempts to crystallize 5 failed. However, its structure in solution could be determined by EXAFS analysis in combination with DFT calculations, which revealed a Cu-O-Cu angle that amounts to 105.17. Moreover, TDDFT calculations helped to rationalize the UV-vis absorptions of 5. The reactivity of complex 5 with 2,4-di-tert- butylphenol, DTBP, was also investigated; the initially formed biphenol product, TBBP, was found to further react in the presence of excessive O₂ to yield 2,4,7,9-tetra-tert-butyloxepino[2,3-b]benzofuran, TBOBF, via an intermediate diphenoquinone. It turned out that 5, or its precursor 3, can even be employed as a catalyst for the oxidation of DTBP to TBBP or for the oxidation of TBBP to TBOBF.

Full text of DOI:10.1021/ja406721a

Article
pubs.acs.org/JACS
Access to a CuII−O−CuII Motif: Spectroscopic Properties, Solution
Structure, and Reactivity
Peter Haack, Anne Kargel, Claudio Greco,† Jadranka Dokic, Beatrice Braun, Florian F. Pfaff, Stefan Mebs,
̈
Kallol Ray, and Christian Limberg*
Institut fur Chemie, Humboldt-Universitat zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: We report a complex with a rare CuII−O−CuII
structural motif that is stable at room temperature, which
allows its in-depth characterization by a variety of spectro-
scopic methods. Interest in such compounds is fueled by the
recent discovery that a CuII−O−CuII species on the surface of
Cu-ZSM-5 is capable of oxidizing methane to methanol, and
this in turn ties into mechanistic discussions on the methane
oxidation at the dicopper site within the particulate methane
monooxygenase. For the synthesis of our Cu2O complex we
have developed a novel, neutral ligand system, FurNeu,
exhibiting two N-(N′,N′-dimethylaminoethyl)(2-pyridylmethyl)amino binding pockets connected by a dibenzofuran spacer. The
reaction of FurNeu with CuCl yielded [FurNeu](Cu2(μ-Cl))(CuCl2), 1, demonstrating the geometric potential of the ligand to
stabilize Cu−X−Cu moieties. A CuI precursor with weakly coordinating anions was chosen in the next step, namely
[Cu(NCCH3)4]OTf, which led to the formation of [FurNeu](Cu(NCCH3))2(OTf)2, 3. Treatment of 3 with O2 or PhIO led to
identical green solutions, whose UV−vis spectra were markedly different from the one displayed by [FurNeu](Cu)2(OTf)4, 4,
prepared independently from FurNeu and Cu(OTf)2. Further investigations including PhIO consumption experiments, NMR
and UV−vis spectroscopy, HR-ESI mass spectrometry, and protonation studies led to the identification of the green product as
[FurNeu](Cu2(μ-O))(OTf)2, 5. DOSY NMR spectroscopy confirmed its monomeric character. Over longer periods of time 5
decomposes to give [Cu(picoloyl)2], formed through an oxidative N-dealkylation reaction followed by further oxidation of the
ligand. Due to its slow decomposition reaction, all attempts to crystallize 5 failed. However, its structure in solution could be
determined by EXAFS analysis in combination with DFT calculations, which revealed a Cu−O−Cu angle that amounts to
105.17°. Moreover, TDDFT calculations helped to rationalize the UV−vis absorptions of 5. The reactivity of complex 5 with 2,4-
di-tert-butylphenol, DTBP, was also investigated; the initially formed biphenol product, TBBP, was found to further react in the
presence of excessive O2 to yield 2,4,7,9-tetra-tert-butyloxepino[2,3-b]benzofuran, TBOBF, via an intermediate diphenoquinone.
It turned out that 5, or its precursor 3, can even be employed as a catalyst for the oxidation of DTBP to TBBP or for the
oxidation of TBBP to TBOBF.
Chart 1. Possible Products after Dioxygen Activation at Two
Copper(I) Centers (L = Ligand)
1. INTRODUCTION
In the last decades significant efforts have been dedicated to
systems composed of ligated CuI and O2, which were fueled by
the biological relevance and the quest for efficient oxidation
catalysts.1−9 Accordingly, several Cux/Oy species have been
characterized over the years, and detailed analysis of their
electronic structures has helped to establish a structure−
function correlation for this important class of com-
pounds.1,2,8−14 Their composition (nuclearity) and stability
have been found to be mainly dependent on the nature of the
supporting ligands employed.3,15
Dioxygen activation at two copper(I) centers mostly yields
either a (μ-1,2)peroxo dicopper(II) adduct TP, a (μ-η2:η2)-
peroxo complex P, or a bis(μ-oxo)dicopper(III) complex O
with a fully cleaved O−O bond (Chart 1). Especially the
binding motif P was found to be relevant to the biological
dioxygen fixation and activation by copper enzymes like the
oxygen carrier protein hemocyanin (Hr), the monooxygenase
tyrosinase (Tyr), and catechol oxidase.11,16 Moreover, studies
on bioinorganic model compounds mimicking these active sites
indicated that the energy barriers for the interconversion
between P and O complexes are very low,17 and, therefore,
factors like electronic and steric properties of the supporting
ligands,8,15,18,19 temperature,13,20 solvent,15,21 concentra-
tion,13,20 and identity of the counterion19,22 may have a strong
impact on the equilibrium position between P and O.3,5,6,23
Received: July 4, 2013
Published: October 17, 2013
© 2013 American Chemical Society
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Compared to the Cu2O2 systems mentioned above, only few
(μ-oxo)dicopper(II) complexes, Cu2O, have been described in
the literature,24−26 and only a small number of these
compounds are characterized unequivocally.27−32 Mostly, they
have been obtained via a cooperative 4e− reduction of one
dioxygen molecule by a total of four CuI ions (Scheme 1), and
better understanding of biological as well as heterogeneous
catalysis.46
Recently we have described a procedure that allowed for the
synthesis and isolation of novel dinuclear CuI complexes based
on a bis(diiminato) framework provided by the ligand
[Xanthdim]2− (Chart 2). One of them, [Xanthdim](Cu-
Scheme 1. Formation and Reactivity of Cu2O
Chart 2. Comparison of the Two Ligand Systems:
Xanthdim48 and FurNeu (this work); R = 2,3-
Dimethylphenyl
they were reported as undesired products in connection with
oxyHc modeling chemistry in the 1980s.33 Alternatively, they
can be formed by the reaction of 2 equiv of a CuI compound
with 1 equiv of an oxygen atom transfer reagent like
iodosobenzene, PhIO.27,29,32
(NCCH3))2, was shown to react with O2 or a soluble derivative
of iodosobenzene at −80 °C to give a green intermediate,
which was identified as a Cu2O species based on the results of
UV−vis and vibrational studies.47 However, the reported
instability of the Cu2O core precluded any structural assign-
ment on the basis of extended X-ray absorption fine structure
(EXAFS) or X-ray diffraction studies.
For the further development of this dicopper chemistry we
have now designed a novel ligand system, FurNeu, which
differs from the Xanthdim ligand48 in the following features: (i)
denticity and structure of the two N-donor binding pockets, (ii)
overall charge of the ligand, and (iii) nature of the ligand
backbone (Chart 2).
We show that FurNeu provides access to a rare room
temperature stable Cu2O core, which enables for the first time
an extensive characterization by NMR, DOSY NMR, EPR,
UV−vis spectroscopy, HR-ESI mass spectrometry, extensive
theoretical considerations as well as by EXAFS spectroscopy.
The reactivity of the Cu2O core in presence of externally added
substrates has also been studied in considerable detail. In
particular, we demonstrate an unprecedented reactivity for a
Cu2O core toward sterically hindered phenols under aerobic
conditions.
The rarity of the Cu2O cores can possibly be explained by
the electron density a single oxido ligand experiences between
two electron-rich CuII centers, which makes it reactive, for
instance, toward water: the presence of H2O in the reaction
system was found to lead to the formation of the corresponding
bis(μ-hydroxo)dicopper(II) compounds.27−29,32 This basic/
nucleophilic character of the oxo group has also been held
responsible for the ability of the Cu2O complexes to react with
CO2 forming the corresponding (μ-carbonato)dicopper(II)
complexes.32,34 Surprisingly, some of the representatives also
exhibit a rather effective oxygen atom transfer capability onto
phosphines,27,29,30,35 a reactivity that is usually reserved for
electrophilic metal oxygen units (Scheme 1).4,5,36
While Cu2O complexes have lived a shadowy existence
during the last few decades, recently they moved into the spot
light: An oxygen-activated Cu-ZSM-5 (Figure 1A) has been
2. RESULTS AND DISCUSSION
2.1. Ligand Synthesis. FurNeu. Compared to the
Xanthdim system, the completely different framework of the
two potential metal coordinating sites within the FurNeu
ligand suggested an extension of the distance between these
binding pockets. From Pacman porphyrin chemistry it is known
that the space between two cofacially located porphyrin units is
increased, if a xanthene derived spacer is replaced by a
dibenzofuran moiety.49 With this background and inspired by
related contributions of Hagadorn et al., FurNeu was
synthesized by a direct tethering of N-(N′,N′-dimethylamino-
ethyl)(2-pyridylmethyl)amine to 4,6-diiododibenzofuran, FurI2,
mediated by CuI [Scheme 2, see Supporting Information (SI,
experimental section) for details].50 FurNeu was isolated in
21% yield, and its identity was proved with the aid of NMR and
IR spectroscopy, HR-ESI mass spectrometry, and a CHN
analysis.
Figure 1. (A) Simplified model of the methane oxidizing active site in
Cu-ZSM-5.38 (B) Structure of the active site in pMMO from
Methylococcus capsulatus (Bath), PDB code: 1YEW.41
shown to selectively oxidize methane to methanol at [CuII−O−
CuII]2+ cores.37−39 Furthermore, contemporaneous crystallo-
graphic analyses (Figure 1B) combined with activity studies on
particulate methane monooxygenase (pMMO) and engineered
protein variants have indicated that the catalytic function is also
based on a dicopper site,40−44 and (among others) a Cu2O
motif is discussed as a very likely candidate for the methane
oxidizing species in the enzyme active site.24−26,45 Accordingly,
the synthesis and investigation of Cu2O complexes advanced to
one of the major goals in the current Cu/O2 chemistry:24−26,45
Any progress in Cu2O model studies could set the basis for a
2.2. Synthesis and Characterization of FurNeu Copper
Chloride Complexes. [FurNeu](Cu2(μ-Cl))(CuCl2), 1. The
addition of 2 equiv of CuCl to a solution of FurNeu in
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a
Scheme 2. Synthesis of FurNeu
Scheme 3. Syntheses of 1 and 2 and Oxidation of 1 Yielding
2
a
a
CuI (cat.), (CH2OH)2, K3PO4, iPrOH.
tetrahydrofuran (THF) and appropriate workup led to a yellow
product, which after crystallization could be investigated by
single-crystal X-ray diffraction analysis. The molecular structure
of [FurNeu](Cu2(μ-Cl))(CuCl2), 1, is shown in Figure 2.
a
In this case the exact stoichiometry remains unconsidered.
[FurNeu](CuCl2)2, 2. Upon exposure to air or dioxygen the
color of a yellow solution of 1 in acetonitrile turned green
within seconds. NMR spectroscopic investigations pointed to
the formation of a paramagnetic product containing CuII ions.
Slow evaporation of the solvent from the reaction mixture at
room temperature led to the formation of single crystals, and
[FurNeu](CuCl2)2, 2, could be clearly identified as an
oxidation product of 1 through an X-ray diffraction analysis
(Scheme 3, for further details see SI). Alternatively, complex 2
could be synthesized in about 78% yield by the reaction of
FurNeu with 2 equiv of CuCl2 in acetonitrile (Scheme 3).
Complete characterization of 2 was performed by a variety of
spectroscopic methods as well as an elemental analysis (for
further details see SI).
2.3. Synthesis and Characterization of FurNeu Copper
Triflate Complexes. The potential binding/activation of
molecules by metal complexes can be hampered by the
presence of halide anions. These anions can be bound to the
metal centers very tightly and thus block the access of
substrates due to the lack of any free coordination site.
Consequently, for activation studies with FurNeu copper
complexes the synthesis of a corresponding dinuclear precursor
containing weakly coordinating anions was pursued.
[FurNeu](Cu(NCCH3))2(OTf)2, 3. To a THF solution of
FurNeu 2 equiv of [Cu(NCCH3)4]OTf were added to obtain
[FurNeu](Cu(NCCH3))2(OTf)2, 3, as a beige solid in 61%
yield. Although the molecular structure of 3 could not be
determined by X-ray crystallography, it was characterized in
detail by NMR, UV−vis (Figure 3, yellow line) and IR
spectroscopy, mass spectrometry, and an elemental analysis
(Scheme 4).
Complex 3 is readily soluble in acetonitrile and shows a
distinct sensitivity against air and dioxygen. A 1H NMR
spectrum of an acetonitrile-d3 solution of 3 showed the
complete set of signals expected for the FurNeu ligand.
Moreover, a singlet at 1.96 ppm was observeda shift identical
to free acetonitrile, suggesting a dynamic exchange of the
coordinated CH3CN ligands with the corresponding trideu-
terated CD3CN solvent molecules.53
Figure 2. Molecular structure of [FurNeu](Cu2(μ-Cl))(CuCl2), 1. All
hydrogen atoms have been omitted for clarity. Selected bond lengths
[Å] and angles [°]: Cu1···Cu2, 3.119(1); N3···N6, 5.431(2); Pycenter
−
Pycenter, 4.08089(10); Cu1−Cl1, 2.2430(4); Cu1−N1, 1.9949(14);
Cu1−N2, 2.1813(14); Cu1−N3, 2.3803(14); Cu2−Cl1, 2.2122(14);
Cu2−N4, 1.9955(14); Cu2−N5, 2.1892(13); Cu2−N6, 2.3510(13);
Cu3−Cl2, 2.1097(5); Cu3−Cl3, 2.1086(5); Cu1−Cl2−Cu2,
88.883(16); Cl1−Cu1−N1, 129.19(4); Cl1−Cu2−N4, 139.88(4);
Cl2−Cu3−Cl3, 172.16(2).
As expected the two N3-binding sites of FurNeu coordinate
one copper(I) ion each. Both binding pockets are arranged in a
vis-a-vis mode, and the two copper atoms (Cu1, Cu2) are
̀
bridged by one chloride ligand (Cl1). Considering the plane
spanned by the ligand backbone the pyridylmethyl and
dimethylaminoethylene units, respectively, exhibit a cis-
configuration. This arrangement might be favored due to a
π−π stacking interaction between the two pyridyl units.
However, as judged by the distance of these aromatic units
(4.08089(10) Å) the strength of this interaction is rather
weak.51 The atoms Cu1 and Cu2 are located at a distance of
3.119(1) Å from each other, and the Cu1−Cl1−Cu2 angle
amounts to 88.883(16) °. All of the Cu−N bond lengths are
similar to those in comparable systems.52 The single charge of
the complex cation [FurNeu](Cu2(μ-Cl))+ in complex 1 is
compensated by an almost linear dichlorocuprate anion (Cl2−
−
Cu3−Cl3, 172.16(2)°), CuCl2 , which must have been formed
in the course of the reaction, and hence the FurNeu/CuCl ratio
amounts to 1:3 within [FurNeu](Cu2(μ-Cl))(CuCl2), 1.
Consequently, employment of 3 equiv of CuCl gave 1 in
59% yield after workup (Scheme 3). 1 has been fully
characterized with the aid of NMR (see Figure S1) and IR
spectroscopy, HR-ESI mass spectrometry, and an elemental
analysis. The structure of 1 clearly shows that with respect to
geometric arguments the FurNeu system allows for an
intermolecular activation of small substrates between the
coordinated metal ions.
In the solid state, in contrast, the acetonitrile ligands are
found to be comparatively tightly bound to the copper ions;
even after drying 3 for several days in vacuum no decrease of
the intensity of the corresponding singlet signal at 1.96 ppm
was noted upon redissolution in acetonitrile-d3. Compared to
the 1H NMR spectrum of 1, some signals corresponding to the
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Figure 3. UV−vis spectra of a solution of 3 dissolved in acetonitrile (2
mM) before (yellow line) and 20 min after the reaction with O2 (blue
line); employing PhIO as the oxidant an analogous spectrum with a
somewhat higher absorbance is obtained (see Figure S5). The dotted
gray line represents the UV−vis absorption behavior of a solution of 4
in acetonitrile (2 mM). The reported extinction coefficient of the
oxygenation product of 3 is based on the PhIO reaction. The inset
shows the spectral changes associated with the oxidation of 3 in the
ultraviolet region, the investigation of which required a reduction of
the concentration to 0.1 mM.
Figure 4. Comparison of the aromatic regions in the 1H NMR spectra
of solutions of (a) 5 and (b) 3, respectively, in acetonitrile-d3. The
signals detected and the positions of the corresponding H-atoms
within the molecule are denoted with the help of the capital letters.
Moreover, spectrum (a) contains the signal set of the byproduct
iodobenzene (PhI).
Scheme 4. Syntheses of 3 and 4
reactivity of 3 toward dioxygen, it was dissolved in acetonitrile,
and after treatment with excessive O2 at −40 °C, the initially
formed pale yellow solution slowly changed its color to green.
Annealing of the reaction mixture to room temperature
accelerated this process, and repetition of the experiment
without any cooling immediately gave a green solution. In both
cases UV−vis measurements showed that during the reaction of
3 with dioxygen, three new absorption features evolved (Figure
3, blue line): two broad absorptions between 400 and 500 nm
and between 800 and 1100 nm, and the appearance of a distinct
peak at 644 nm. At room temperature the formation of the 644
nm band was completed after 20 min, but at −40 °C, the
reaction was not finished even after 90 min.
In particular the absorption band at 644 nm is reminiscent of
the UV−vis spectrum reported previously for the temperature-
sensitive xanthene-based Cu2O species.47 On the other hand
the absorption features inherent to the product of the
oxygenation of 3 is clearly distinct from the absorption features
of the simple one-electron oxidation product 4. For
comparison, the UV−vis spectrum of an acetonitrile solution
of 4 (Figure 3, dotted gray line) exhibits an absorption feature
at 612 nm (ε = 260 M−1 cm−1) and does not show the broad
features between 400 and 500 nm and between 800 and 1100
nm that are present in the product of the oxygenation of 3.
Accordingly, the formation of 4 in course of the reaction of 3
with dioxygen can be clearly ruled out. The reaction of 3 with
iodosobenzene, PhIO, resulted in absorption features identical
to what was obtained for the 3/O2 system.
Titration experiments confirmed that only 1 equiv of PhIO is
necessary for the quantitative oxygenation of 3, and ε = 140
M−1 cm−1 was determined for the absorption band at 644 nm.
The addition of more than 1 equiv of PhIO did not lead to any
further changes in the absorption spectrum, while the reaction
between 3 and 0.5 equiv of iodosobenzene generated the
oxidized product in only 50% yield (Figure S9). Accordingly,
we tentatively assign the oxygenation product of 3 to a room
ligand within 3 (for example in the aromatic region, see Figure
S1) did not show the expected coupling pattern and appeared
broadened (compare Figure 4b); either the fast exchange of the
acetonitrile coligands in solution or a lack of fixation of the
metal coordination sites by a bridging ligand may cause this line
broadening.
[FurNeu](Cu)2(OTf)4, 4. It was conceivable that the reaction
of 3 with dioxygen either leads to copper−oxygen products
associated with dioxygen activation or to a dicopper(II) triflate
species arising from simple one electron oxidation, as was
observed in case of 1 (giving 2). In order to be able to
distinguish between these possibilities the analogous copper(II)
triflate compound was synthesized as a reference compound
directly via the reaction of FurNeu with 2 equiv of Cu(OTf)2 in
acetonitrile as the solvent. Complexation of the copper(II) ions
was indicated by an immediate color change of the reaction
mixture from light beige to deep blue. Workup gave blue 4 in
50% yield, and it was fully characterized (Scheme 4, for further
details see SI) by a variety of spectroscopic methods and an
elemental analysis.
2.4. Access to a CuII−O−CuII Motif (Cu2O) and Its
Characterization. UV−vis Spectroscopy. To investigate the
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temperature stable Cu2O complex, [FurNeu](Cu2(μ-O))-
(OTf)2, 5 (Scheme 5), which in the following is further
supported by subsequent ESI-MS, NMR, EXAFS, and DFT
studies (see below).
DOSY NMR measurements were also performed, which
indicated the existence of 5 predominantly in its monomeric
form (see Figures S20−S22), thus excluding the possibility of
any oligomerization processes associated with the formation of
5, as observed in the comparable system supported by the
Xanthdim ligand.47
Mass Spectrometry. While IR (Figures S11 and S12) and
Raman (λexc = 442, 514, 647, and 1064 nm) experiments,
unfortunately, did not lead to the observation of isotope
sensitive signals, HR-ESI mass spectrometry strongly supported
the formation of the FurNeu-based Cu2O complex 5. The
reaction of 3 with either O2 or PhIO led to the development of
a new signal (the intensity of which varied, depending on the
mass spectrometer used) in the mass spectrum, which can be
assigned to the doubly charged complex cation [FurNeu]-
(Cu2(μ-16O))2+ (experiment with PhI16O: 332.0864, calcd
332.0919, Figure 5a). As expected, upon usage of the
Scheme 5. Access to 5 Starting From 3 through the Reaction
with O2 and PhIO, Respectively
Notably, while 5 is generated in near-quantitative yield in the
reaction of 3 with PhIO, the oxidation of 3 with dioxygen
yielded 5 in only 85% yield (Figure S10). The lower efficiency
of the dioxygen reaction can be attributed to the more
complicated steps associated with the dioxygen activation and
O−O bond cleavage processes involving the 4 electrons
delivered by a total of 4 CuI ionsthat are necessary for the
generation of 5 from 3. The iodosobenzene reaction, in
contrast, involves a simple oxygen-atom transfer to the
dinuclear copper(I) precursor, which may explain the near-
quantitative conversion of 3 to 5.
EPR Spectroscopy. EPR experiments are also consistent with
the formation of 5 as a result of oxygenation of 3. Similar to the
previously reported Cu2O complexes [(TpMeCuII)2(μ-O)]30,31
(TpMe = tris(3,5-dimethylpyrazolyl)borate) and [MePy2-
(CuII)]2(μ-O)(X)2, (X = ClO4 , [B[3,5-(CF3)2C6H3]4]−,
−
MePy2 = N-methyl-N,N-bis[2-(2-pyridyl)ethyl]amine)27,29 the
product of the reaction between 3 and 1 equiv of PhIO was
found to be EPR silent, suggesting strong antiferromagnetic
coupling of the two copper(II) centers mediated by the
bridging oxo ligand. This finding again excludes the formation
of 4, whose EPR spectrum showed the typical signals associated
with a copper(II) center, thereby revealing that the copper
centers in a [[FurNeu](Cu)2]4+ entity are not interacting with
each other in the absence of any bridging ligand.
Figure 5. Characteristic peaks in the HR-ESI mass spectra of solutions
of 5 (black lines), and [FurNeu](Cu2(μ-18O))(OTf)2, 518O (red
lines), respectively, in acetonitrile. Signals of the corresponding
dications [[FurNeu](Cu2(μ-16/18O))]2+ and [[FurNeu]-
(Cu2(μ-16/18O)) + CH3COOH]2+ were also observed after the
reaction of 3 with both 16O2 (18O2) and PhI16O (PhI18O). The gray
lines represent the calculated peak patterns. For clarity reasons the
intensities of the signals shown were equalized. The full spectra are
given in Figures S13 and S14.
NMR Spectroscopy. The 1H NMR spectrum of 5 also
corroborates our Cu2O assignment. In Figure 4 we show the
1H NMR spectra of 5 and 3, which are found to be nearly
identical to each other. Only small differences in the chemical
shifts are observed, and altogether the signals of 5 are
somewhat sharper, comparable to those of the chloro bridged
dicopper(I) complex 1 (see Figure S1). As outlined above we
have rationalized the decreased line widths in comparison with
3 in case of 1 with the rigidity provided by the chloro (Cl−)
bridge and consistently it can be attributed to the oxo (O2−)
bridge for the case of 5. In general, these findings are
comparable to those of a previous study by Karlin et al., who
corresponding 18O containing O atom sources (18O2 or
PhI18O) the characteristic peak shifted by one mass unit
(experiment with PhI18O: 333.0833, calcd 333.0944, Figure
5b), and in all cases the experimentally observed isotopic
patterns matched the calculated ones excellently. An additional
isotope sensitive signal was observed in the ESI-MS spectrum at
m/z = 362.0980, which also shifted by one unit when an 18O
source was employed in the reaction. The intensity of this peak
was found to be much higher as compared to the previously
mentioned peak at m/z = 332.0864 (see Figures S13 and S14).
With the aid of tandem mass spectrometry experiments the
origin of this signal could be assigned to an adduct between the
complex cation [FurNeu](Cu2(μ-O))2+ and acetic acid (Figure
5c,d). The adduct formation presumably involves the
protonation of the oxo ligand in 5, followed by the binding
of the acetate ligand to one of the CuII centers, which would be
consistent with the expected basicity of the oxido ligand in 5
(see below in the reactivity part: protons). The origin of the
acetic acid in our reaction mixture is not clear at this point and
possibly arises from a contamination of the acetonitrile that we
employed as the solvent.
1
also reported that the H NMR spectra of [MePy2(CuII)]2(μ-
O)(X)2,27,29 containing a Cu2O core, and those of its
mononuclear CuI precursors are very similar to each other.
Moreover, the absence of paramagnetic line broadening and
shifts in the 1H NMR spectrum of 5 is a further indication of its
diamagnetic ground state. In addition to the signals associated
with 5, the 1H NMR spectrum of the reaction mixture of 3 and
PhIO exhibits a set of signals associated with iodobenzene
(PhI); relative integration of the corresponding signals
established a 1:1 stoichiometry for 5 and PhI, thereby
suggesting a complete conversion of the PhIO employed.
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To further confirm the adduct formation between a
carboxylic acid and 5 under the conditions of the ESI-MS
measurements, we repeated our studies in presence of different
externally added carboxylic acids. The results of these studies
are summarized in Figure 6. After exposing an acetonitrile
Figure 7. Overlay of the k3 weighted EXAFS of 5 and the best fit (no.
8 in Table S2) in R and k space, respectively. Main figure: Black line
shows the Fourier transformed EXAFS data, and the red line
represents the best fit (no. 8 in Table S2) in R space. Inset: Black
line shows the weighted raw data, and the red line the back-Fourier
transformed fit (range R = 2−13 Å) in k space.
Table 1. Summary of the Structural and Spectroscopic
Properties of 5 and Oxygen-Activated Cu-ZSM-538
Determined Theoretically and Experimentally
5
Cu-ZSM-5/O2
Figure 6. Cutouts of the HR-ESI mass spectra of solutions of 5 in
acetonitrile in the presence of (a) formic acid, (b) acetic acid, and (c)
propionic acid.
Cu···Cu/Å
Cu−O/Å
2.844
3.29
2.91 (EXAFS)
≈2.9 (EXAFS, not
significant)
1.791
1.75/1.76
solution of 3 to an atmosphere of dioxygen in presence of
externally added formic acid at room temperature a new peak at
m/z = 355.0875 indicated the formation of the corresponding
adduct [[FurNeu](Cu2(μ-16O)) + HCOOH]2+ (calcd
355.0946, Figure 6a). Its intensity was slightly higher than
the acetic acid pendant, [[FurNeu](Cu2(μ-16O)) +
CH3COOH]2+, which presumably originates from the trace
amount of the acetic acid impurity in the solvent. In a further
experiment the presence of additional acetic acid only led to the
detection of the known signal at m/z = 362.0970 ([[FurNeu]-
(Cu2(μ-16O)) + CH3COOH]2+, calcd 362.1024, Figure 6b).
The usage of propionic acid afforded the formation of the
dication [[FurNeu](Cu2(μ-16O)) + C2H5COOH]2+, as evi-
denced by a new signal at m/z = 369.1024 (calcd 369.1102,
Figure 6c)).
X-ray Absorption Spectroscopy. X-ray absorption spectro-
scopic studies were performed at Cu K edge in order to directly
probe the metal oxidation state in 5. The X-ray absorption near
edge spectral (XANES) feature of complex 5 was found to be
typical of a copper(II) center (edge =8987.7 eV; Figure S23,
SI).54 In particular, the absence of a low energy peak maximum
due to the 1s to 4p transition in the region between 8983 and
8986 eV excludes the possibility of the presence of any Cu(I)
center in 5.
1.79 (Cu−O/N,
EXAFS)
Cu−O−Cu/°
νs/cm−1
105.17
565
139
456
456 (exptl)
852
νas/cm−1
619
236
870 (exptl)
253
Cu−O−Cu bend/
cm−1
237 (exptl)
λ (CuII d−d)/nm
648
644 (exptl)
λ (O → CuII LMCT/ <450
752 (exptl)
441 (exptl)
nm
<450 (exptl)
this points toward more covalent Cu−N bonds in 5. Cu EXAFS
of 5 also showed a peak at 2.91 Å corresponding to the Cu
scatterer, thereby strongly supporting the presence of a
homodinuclear Cu···Cu center in 5. Fits 11−15 show that
the inclusion of a Cu scatterer is an absolute necessity for
obtaining a reasonable simulation (fit 8) of the experimental
data; no good simulation could be obtained by considering
outer-shell contributions from C and N only. Although the
additional outer-shell features could be satisfactorily accounted
for by considering single scattering paths involving 8 carbon
and 3 N/O scatters, the fit could be significantly improved by
introducing multiple-scattering pathways (fits 7 and 8 in Table
S2).
EXAFS analysis revealed further structural details. Figure 7
shows the Fourier transforms (r space) of the Cu K-edge
EXAFS data and the best-fit.
The first coordination sphere can be fit well by two subshells
of N/O atoms at absorber-scatterer distances (r) of 1.79 and
1.95 Å. Splitting of the first coordination shell into two
subshells was necessary in order to significantly improve the
quality of the fit (fits 8−10 in Table S2). The shorter-distance
shell is attributed to the oxygen scatterer of the CuII−O−CuII
moiety, which is also supported by DFT calculation (vide infra,
Tables 1 and S3). The shell at 1.95 Å with 3 scatterers
represents the donor nitrogen atoms of the polydentate ligand
FurNeu. Notably, this distance in 5 is significantly shorter than
the average CuII−N distances of 2.18 and 2.07 Å, as obtained
by X-ray crystallography for complexes 1 and 2, respectively;
Theoretical Considerations. DFT analysis of a rare
molecular entity like the dicopper μ-oxido species reported
here cannot rely on a specific validation of the level of theory to
be adopted, due to the obvious lack of benchmark. However,
the vast literature available on the assessment of density
functionals can be used to define reliable and stringent
approaches for the calculation of molecular geometries, relative
stabilities, and spectroscopic properties using DFT.
In the case of geometry optimization of 5, we decided to
adopt the B3LYP functional.55 In fact, such hybrid functional
was shown to be capable of excellent performances in the
reproduction of experimentally derived metal−ligand bond
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lengths56 (see in particular ref 56a, a benchmark study in which
metal oxides had a prominent role). The latter are clearly key
geometric parameters for a comparison with experimental
EXAFS data. Energy minimization of 5 was thus carried out at
the B3LYP/def2-TZVP level,57 and broken-symmetry58 was
applied to represent the antiferromagnetic coupling between
the two Cu centers that results in the experimentally observed S
= 0 ground state (see also SI for further details). Based on the
crystal structure of 1 we set out with a model featuring a cis
arrangement of the pyridylmethyl and dimethylaminoethylene
units of the ligand backbone. The minimum geometry obtained
is shown in Figure 8. In this model the Cu···Cu interatomic
plexes.61 All B3LYP/def2-TZVP and BP86/def2-TZVP energy
differences are reported in Table 2 (see SI for further
Table 2. Relative Energies Computed for the cis and trans
Isomers of 5 (singlet and triplet states) at Either the BP86/
a
def2-TZVP or B3LYP/def2-TZVP Levels
model
B3LYP/def2-TZVP
BP86/def-TZVP
trans (singlet)
trans (triplet)
cis (singlet)
cis (triplet)
0.0
+5.1
+4.3
+1.8
0.0
+1.0
+6.9
+5.2
a
See also the Theory section in SI for details. All values are given in
kcal/mol.
methodological details). Surprisingly, such values evidence
discrepancies between the selected levels of theory which are
unusually large in the case of cis−trans isomerization
(ΔΔEBP86‑B3LYP up to 10.2 kcal/mol). As far as singlet−triplet
splitting is concerned, results are much more consistent
(ΔΔEBP86‑B3LYP up to 1.8 kcal/mol), but for both BP86 and
B3LYP the values of the energy splitting are smaller than 2
kcal/mol, which is within the uncertainty of DFT. Such a
picture prevented us to safely draw conclusions as far as the
general features of the PES of 5 are concerned. Clearly, the
above-reported experimental results already provide conclusive
demonstration of the S = 0 state of 5, while the cis−trans
isomerization issue still requires further insights, which are
provided below by means of time-dependent DFT (TDDFT)
analysis.
TDDFT calculation on UV−vis absorption features of the
modelboth with cis and trans arrangementwas carried out
using the CAM-B3LYP62 functional and a smaller basis set
(def2-SVP) or a larger one (def2-TZVP) as explained in
Methods (SI). The choice of the CAM-B3LYP functional stems
from the improvement it allows in the treatment of charge-
transfer states, as compared to conventional functionals.62,63
Let us consider the case of the cis ligand arrangement first. As
expected in case of correct structural assignment, the larger
basis allowed for the calculation of a spectrum closer to the
experimental spectra recorded for 5 (see below and Figure
S28). Figure 9 illustrates the outcome of CAM-B3LYP/def2-
TZVP calculations: it becomes obvious that TDDFT results
reproduce the dominant absorption features derived from
experiments. In particular, the band centered at 648 nm in
Figure 9 closely matches the experimentally observed 644 nm
band (Figure 3 (blue line) and Figure S5 (green line)).
Moreover, two predicted signals at 837 and 843 nm are also in
reasonable agreement with the experimental spectrum, which
reveals a broad absorption in the 800−1100 nm region. Finally,
TDDFT bands around and below 450 nm nicely fit the
experimental data in the same spectral region. As far as relative
intensities of the theoretical bands are concerned, the overall
picture appears satisfactory notwithstanding the well-known
limitations of TDDFT in computing oscillator strengths.64 It is
interesting to note that calculations on the UV−vis absorption
features of the corresponding trans isomer of 5 led to a
distinctly different spectrum (see Figure S31) thus supporting a
cis configuration of the ligand in 5, as had been found also in
case of 1 (vide supra).
Figure 8. Structure of the cis isomer of the complex cation
[[FurNeu](Cu2(μ-O))]2+ of 5 as optimized at the B3LYP/def2-
TZVP level. All hydrogen atoms have been omitted for clarity.
Selected bond lengths [Å] and angles [°]: Cu2···Cu3, 2.844; Cu2−O1,
1.791; Cu3−O1, 1.791; Cu2−N4, 2.407; Cu2−N5, 2.022; Cu2−N6,
2.163; Cu3−N7, 2.415; Cu3−N8, 2.024; Cu3−N9, 2.157; Cu2−O1−
Cu3, 105.17.
distance is 2.844 Å, while the Cu−O bonds are 1.791 Å long
and the Cu−N bonds lengths vary between 2.415 and 2.022 Å.
These results are in reasonable agreement with the EXAFS data
reported above (see also Table 1). The calculated Cu2−O1−
Cu3 angle in 5 amounts to 105.17°. Vibrational frequency
calculations evidenced three bands that involve the [CuII−O−
CuII]2+ motif: the Cu−O−Cu bending mode is calculated at
236 cm−1, while the symmetric and antisymmetric metal−oxo
stretches are predicted at 565 (νs) and 619 cm−1 (νas),
respectively, in the theoretical spectrum. Taking all calculated
geometric and vibrational information together significant
differences between the Cu2O core in complex 5 and the
[CuII−O−CuII]2+ entity in the oxygen-activated Cu-ZSM-5
species (see Table 1) become evident: For the latter the Cu−
O−Cu angle was calculated to be much larger (139°38 in Cu-
ZSM-5 vs 105.17° in 5), and the angle in turn has a significant
influence on the respective positions of νs and νas.
Next, we decided to explore the potential energy surfaces
(PES) of model 5, in terms of calculation of singlet−triplet
energy splitting as well as cis−trans isomerization. Due to
known limitations of B3LYP in computing spin-state splitting
(an effect of the Hartree−Fock exchange contribution to the
total B3LYP energy), we decided to perform energy
minimizations also with an additional functional to check
coherence of computed energies.59 In particular, we chose the
BP86 functional,60 as it completely lacks Hartree−Fock
exchange and was extensively and successfully used in our
laboratories for studies on first-row transition metal com-
An analysis of the nature of transitions was also performed,
based on the individuation of dominant molecular orbital
(MO) contributions to the excitations (Table S6) and on the
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Scheme 6. Proposed Mechanism for the Decomposition
Route of 5 Giving [Cu(picoloyl)2]
Figure 9. Calculated absorption spectra for 5 using TDDFT at the
CAM-B3LYP/def2-TZVP level (compare experimental spectrum,
Figures 3 (blue line) and S5 (green line), respectively. The molecular
orbitals involved in leading excitations for the transition at 648 nm are
also shown (see also Table S6 for a detailed analysis of the other main
transitions). The thicker line represents the Gaussian-broadened
spectrum with s = 900 cm−1.
characterization of [Cu(picoloyl)2] was possible only by
means of IR spectroscopy.66 On the basis of the determined
low yield (7%) of [Cu(picoloyl)2] and the unsuccessful
identification of the other byproducts (e.g., hydroxylated
species) of the reaction we can only speculate on the relevant
decomposition mechanism of 5. The conversion of 5 to
[Cu(picoloyl)2] involves the dioxygenation of a methyl pyridyl
residue of FurNeu proceeding via cleavage of the bond to the
amino N atom to which it had been connected originally.
Reactions of this type have already been reported for a small
number of transition-metal complexes exhibiting 2-pyridyl-
methyl units under aerobic conditions: For example the
evaluation of fractional contribution of basis set functions in the
various MOs (Table S7). It turned out that the band at 648 nm
originates predominantly from a CuII d−d transition. The band
around 450 nm, in contrast, mainly derives from a ligand-to-
metal charge transfer (LMCT) with prominent involvement of
the oxo ligand; the one at around 850 nm originates from
several transitions of similar relevance, which also correspond
to LMCTs. The good agreement between the computed and
experimental absorption spectra also provides a strong support
for the Cu2O assignment of 5.
collapse of the monoanionic mebpena ligand (mebpena−
=
N-methyl-N,N′-bis(2-pyridylmethyl)ethylendiamine-N′-ace-
tate) was reported to proceed via C−N bond scission in the
presence of cobalt(II) perchlorate, dioxygen, and water.67 In
this context a radical mechanism was assumed, initialized by an
H-atom abstraction from the CH2 group of the pyridylmethyl
unit. Also O complexes (Chart 1) have been shown to undergo
N-dealkylation reactions, wherein a tertiary amine is cleaved
into a secondary one and an aldehyde species.68 In the further
course of such reactions the aldehyde gets converted into the
corresponding carboxylic acid and carboxylate, respectively, by
a transient oxidant, which was, however, not experimentally
detected. A similar reaction sequence is also conceivable for the
decomposition of the dicopper(II) complex 5 (Scheme 6),
although specific experiments with added substrates (like 1-
benzyl-1,4-dihydronicotinamide and 9,10-dihydroanthracene)
did not lead (over several hours at room temperature) to
evidence for a pronounced H-atom abstraction reactivity of 5.
However, as mentioned above, formation of [Cu(picoloyl)2]
takes days and is highly reproducible so that a decomposition of
5 initiated by a slow H-atom abstraction step is feasible,
nevertheless. In any case, this behavior is unique to 5 since
other FurNeu-based copper(II) complexes 2 and 3 did not
form [Cu(picoloyl)2] under the same conditions.
Reactivity of the Cu2O Complex 5 Toward Exogenous
Substrates: Protons. Many of the previously reported Cu2O
compounds possessed highly basic oxo groups,27−29,32 which
made them extremely susceptible to protons that often led to
formation of the corresponding [CuII−(OH)−CuII] cores, even
in the presence of weak proton donors like water (pKa =14).
The higher thermal stability of 5, however, already demon-
strates a reduced basicity of its oxo group, which is also
reflected in its inability to react with water. Thus no significant
changes in the absorption spectrum were observed upon
It is important to note that the position of the CuII d−d
transition absorption band at 644 nm in 5 differs substantially
from the value of 752 nm, reported for the Cu-ZSM-5/O2
system (see also Table 1),26 while the temperature-sensitive
Cu2O core supported by the Xanthdim ligand showed
absorption features comparable to those of 5.47 The deviation
of the Cu-ZSM-5/O2 system can be possibly rationalized by the
different environment the [CuII−O−CuII]2+ entity is embedded
in: In Cu-ZSM-5 it is linked to the oxido platform of the zeolite
surface, while the FurNeu ligand system in 5 provides an N-
based donor sphere. As pyridine and alkyl amine units are
comparatively strong donors, FurNeu leads to a much stronger
ligand field than the zeolite surface and thus to higher energy
d−d excited states for 5 as compared to the Cu−O−Cu units in
the Cu-ZSM-5/O2 system. The nature of the coordination
sphere, however, does not have a decisive influence on the
positions of the band resulting from the corresponding O→
CuII LMCT transition (5: <450 nm, Cu-ZSM-5/O2: 441 nm).
2.5. Reactivity of the Cu2O Complex 5. Decomposition
of 5. Compared to the Xanthdim-based Cu2O,47 which was
only stable at −80 °C, 5 is far less sensitive to temperature:
Although temperatures higher than 30 °C led to the
discoloration of the reaction mixture within 2 h, there was no
evidence for the decomposition of the deep green acetonitrile
solutions of 5 at room temperature. However, from solutions of
5 in CH3CN, when stored for more than 3 days at room
temperature, reproducibly purple crystals precipitated, the
molecular structure of which, as determined by single-crystal
X-ray diffraction analysis, revealed the formation of [Cu-
(picoloyl)2] (see Scheme 6, and Figure S24).65 Due to its low
solubility in common organic solvents and H2O, further
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addition of 1−20 equiv of water to a CH3CN solution of 5 at
25 °C. In contrast, addition of 1.2 equiv of a stronger proton
donor like acetic acid (pKa = 4.5), that had been found to react
already in the ESI-MS studies described above, led to the
disappearance of the characteristic absorption features
associated with the Cu2O core; the broad bands in the region
between 350 and 500 nm (assigned as an O→CuII LMCT
transitions) and between 800 and 1100 nm (assigned to LMCT
transitions) vanished, and the 644 nm band got broadened and
shifted to 690 nm (ε = 100 M−1 cm−1; Figures S17−19).
Notably, the absorption spectrum of the resultant solution
formed upon protonation of 5 is distinct from that of 4. This
helps us to exclude an alternative CuII−(OH)−CuII assignment
of 5, whose protonation in such case would have resulted in the
formation 4 with concomitant loss of a water molecule. The
reaction of acetic acid with 5 also led to the disappearance of all
the signals of the originally diamagnetic Cu2O compound in
chemically and magnetically different tert-butyl groups within
one compound featuring a chemical environment resembling
the ones in DTBP and TBBP, respectively. Surprisingly, after
exposing this reaction mixture to an atmosphere of dioxygen
the color of the solution immediately changed to purple, and an
accompanying UV−vis experiment led to the observation of a
new broad absorption feature at 573 nm (ε = 260 M−1 cm−1,
see Scheme S4, bottom). In the course of 1 h at room
temperature the color of the solution changed to green and 1H
NMR spectra recorded subsequently contained only the set of
signals belonging to the species that had been minor within the
original spectrum: Beside the four singlet signals, which
exhibited now the highest intensity, two additional singlet
signals at δ = 5.61 and 6.50 ppm and two doublet signals at δ =
7.20 and 7.40 ppm could be detected (see Figures S26B and
S27B). The absence of signals belonging to the FurNeu ligand
was attributed to the paramagnetism of the resulting FurNeu-
based copper(II) species formed in the dioxygen atmosphere,
and based on the lack of signals belonging to TBBP or the
starting material DTBP, the product was anticipated to derive
from these compounds. Only an X-ray diffraction analysis
conducted for crystals that could be grown revealed the identity
of this product as 2,4,7,9-tetra-tert-butyloxepino[2,3-b]-
benzofuran, TBOBF, which to our knowledge so far has not
been structurally characterized. Its molecular structure is shown
in Scheme 7 and Figure S25.
1
the H NMR spectrum, which, together with the UV−vis data,
may suggest a proton-induced modification with respect to the
oxo bridge resulting in a paramagnetic compound (Figures S15
and S16).
Reactivity of the Cu2O Complex 5 Toward Exogenous
Substrates: PPh3. After the addition of 5 equiv of PPh3 a
solution of 5 in acetonitrile changed its color from green to
light brown within 3 h under anaerobic conditions. Monitoring
the reaction with the help of UV−vis spectroscopy indicated a
decrease of the characteristic absorption bands associated with
5, and the investigation of the reaction mixture by 31P NMR
spectroscopy proved the formation of (O)PPh3 in 8% yield
(with respect to the amount of 3 employed for the generation
of 5). The low efficiency of this oxygen atom transfer may be
rationalized either by a steric clash between 5 and the substrate
or by a potentially high nucleophilicity of the bridging oxo
ligand.4,5 Reactivity toward carbon dioxide was not observed,
and comparable studies employing carbon disulfide as a
substrate did not lead to the detection of any dithiocarbonate
species.
Scheme 7. Formation of TBOBF Starting From DTBP in the
Presence of a Suitable Oxidant ([O])
Reactivity of the Cu2O complex, 5, Toward Exogenous
Substrates: 2,4-Di-tert-butylphenol. As phenols are often
employed as test systems to explore the behavior of novel
copper complexes, the reactivity of 5 toward the sterically
hindered phenol derivative 2,4-di-tert-butylphenol, DTBP, was
investigated. A green solution of 5 in acetonitrile or acetonitrile-
d3 was found to decolorize within 1 h at room temperature in
presence of 2 equiv of DTBP. Again the characteristic UV−vis
spectroscopic features of 5 vanished, and after 3 h the product
of the oxidative coupling of DTBP, 3,3′,5,5′-tetra-tert-butyl 2,2′-
Muller and co-workers had already reported the observation
̈
of a temporary color change of reaction mixtures to purple,
though, while investigating dehydrogenation studies setting out
from sterically hindered phenol and biphenol derivatives.
Formation of a quinone-derived end product was suspected,
but a variety of valence isomers had to be discussed.69 Several
of those could be excluded with the aid of IR and NMR
spectroscopic investigations, but, as in our case, even the
knowledge of the exact sum formula did not allow for the
determination of the correct molecular structure. Almost two
decades later, in 1978, the group of Meier was able to identify
the unknown oxidation product as TBOBF combining
experimental and theoretical studies,70 but even without this
knowledge a formation mechanism starting from monophenol
derivatives was discussed:69,71 With DTBP as a starting material
an oxidative coupling reaction mediated by suitable oxidants
under anaerobic conditions first leads to TBBP. In the presence
of excessive oxidant or after reactivation of the oxidant
employed by dioxygen TBBP gets oxidized again forming the
1
biphenol, TBBP, was identified by means of H and 13C NMR
spectroscopy with a yield of 56% (based on DTBP). In
addition, equimolar amounts of water and an additional set of
signals belonging to a FurNeu containing species were detected
1
by H NMR spectroscopy (see Scheme S4, top and Figure
S26A)). The latter showed an almost identical signal pattern
and similar signal shifts compared to the ones caused by 5 and
3, respectively. Based on the absence of a copper(II) d−d
transition in the recorded UV−vis spectrum of the reaction
solution, the identity of this unknown species was assigned to a
copper(I) species, {[FurNeu](CuI)2}, derived from 3.
1
Furthermore, the H NMR spectra measured for the reaction
solution with acetonitrile-d3 as the solvent exhibited four low
intensity singlet signals of equal integrals in the aliphatic region
(Figures S26A and S27A). The positions of these signals (δ =
1.20, 1.25, 1.35, 1.44 ppm) suggested the presence of four
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corresponding quinonic form 3,3′,5,5′-tetra-tert-butyl 2,2′-
diphenoquinone, TBDQ, which causes the characteristic purple
color. Subsequently, spontaneous isomerization of TBDQ
results in the formation of pale yellow TBOBF (Scheme 7).
Even though numerous examples of the copper-mediated
oxidative coupling giving TBBP are existing,4,5,12,14,19,31,36,72 the
formation of TBOBF starting from DTBP is only rarely
reported.73−75
conversion to TBOBF in the presence of 3 and an atmosphere
of excessive dioxygen.
With the aim of evaluating the relevance of the bridging oxo
ligand in 5 for its reactivity toward phenols 4, a copper(II)
complex lacking an oxido ligand was reacted with 2 equiv of
DTBP for comparison: After the addition of the phenol
derivative the initially blue solution of 4 in acetonitrile or
acetonitrile-d3 got slowly decolorized, and the UV−vis
spectrum of the reaction solution recorded a decrease of the
characteristic absorption band of 4 at 612 nm. After 3 h the
coupling product TBBP was detected in 50% yield by 1H NMR
Catalytic Studies. Having found that 5 (which is produced
upon treatment of 3 with O2) oxidizes DTBP to TBBP with the
concomitant formation of a FurNeu copper(I) complex, which
apparently activates O2 for the conversion of TBBP to TBOBF,
the question occurred whether the copper(I) complex 3 can be
employed directly to promote the DTBP→TBOBF conversion
in the presence of O2, perhaps even as a catalyst. Indeed, while
no reaction between 3 and DTBP was observed under inert
conditions, the exposure of a solution of this mixture in
acetonitrile as the solvent to dioxygen again led to an
immediate color change from pale yellow to purple. After 15
min the complete conversion of 4 equiv DTBP to TBOBF and
4 equiv H2O was ensured with the aid of 1H NMR
spectroscopy. This finding suggests that 3 acts as a precatalyst
which in contact with dioxygen forms the active catalyst 5
(although we cannot rule out other active species like
superoxide complexes). In a dioxygen atmosphere 5 is able to
mediate the catalytic oxidative coupling of DTBP as well as the
further catalytic oxidation of the resulting TBBP giving TBOBF
(Scheme 8).
1
spectroscopy. Additionally, the H NMR spectrum contained a
set of signals, which could be assigned to a FurNeu-based
compound as well as one further broad singlet signal at 12.04
1
ppm. By comparison with the H NMR spectra obtained after
the reactions of FurNeu and 3, respectively, with 2 equiv of a
proton-donating compound (for example 2,6-lutidium salts)
the origin of the new signal at 12.04 ppm could be ascribed to a
doubly protonated 3, [3(H)2]2+. Due to the position of the
additional signal it was concluded that the protons are trapped
by the N-donor units previously bound to the copper(I) ions.
After exposure of the reaction mixture to an atmosphere of
dioxygen all signals associated with [3(H)2]2+ vanished possibly
due to the formation of a paramagnetic FurNeu-based
copper(II) species. Moreover, neither additional oxidative
coupling of DTBP nor further oxidation of TBBP giving
TBOBF was observed. This leads to the conclusion that 3
under acidic conditions does not mediate the oxidation of
phenol-based substrates with O2. To further confirm the
inhibiting influence of acid, 3 was dissolved together with
proton donors (2,6-lutidinum salts) in acetonitrile prior to the
addition of DTBP, and after subsequent exposure to dioxygen
indeed no coupling product was detected. All the experiments
made are summarized in Scheme S5 and Table S4.
Scheme 8. Summary of the Catalytic Cycle Relevant to the
Oxidative Coupling of DTBP Giving TBBP and to the
Oxidation of TBBP Giving TBOBF Mediated by 5
Thus, both copper(II) complexes, 4 and 5, have the ability to
mediate the oxidative coupling of DTBP. In the case of 5, H2O
is formed as a byproduct, so that the further oxygenation of the
resulting 3 closes the catalytic cycle. Upon usage of 4 as the
oxidant the protons formed in the course of the coupling
reaction get bound by N-donors of the resulting copper(I)
complex 3 (instead of getting trapped by the basic oxido
ligand), and hence, it gets deactivated with respect to further
oxidation chemistry.
In order to evaluate the influence of water, which is a
byproduct of the phenol oxidation reaction, on the efficiency of
the catalytic system an excess of degassed water (23 equiv with
respect to the amount of 3) was added to an acetonitrile
1
3. CONCLUSION
solution of 3 and 4 equiv of DTBP. A H NMR spectrum
recorded again indicated inertness of the starting materials, and
after exposure to O2 once more full conversion to TBOBF via
TBDQ was evidenced by H NMR spectroscopy (after 3 h),
CuII−O−CuII cores have recently been proposed as the
potential active species responsible for the challenging
oxidation of methane to methanol realized at the surface of a
Cu-grafted zeolithe24,26,38,39 as well as in the active center of the
copper-enzyme pMMO.40−44 Hence, a fundamental under-
standing of the electronic structure and behavior of such cores
could aid the development of more general systems for C−H
bond functionalization. However, reports of CuII−O−CuII
cores are extremely rare in the literature, possibly due to the
high electron density that should be inherent to an oxo ligand
forming the sole bridge between two copper centers. Moreover,
such complexes are often thermally unstable. The lack of
sufficient kinetic and thermodynamic stability of the CuII−O−
CuII cores has previously precluded their in-depth spectro-
scopic characterization, thereby, making a definite structural
assignment difficult.
1
that is, no detrimental effect of water became evident. Hence, 3
was now reacted with 20 equiv of DTBP in a dioxygen
atmosphere. After 3 h the formation of 66% TBBP and 15%
TBOBF (based on DTBP) was detected. No further conversion
of either DTBP or TBBP was noted pointing to a deactivation
of the catalyst within the initial three hours. The difference
between the two TONs (TBBP: 16.2, TBOBF: 3.0, with
respect to 3) is consistent with the fact that the formation of
TBOBF starting from DTBP proceeds via TBBP as an
intermediate product.73,74 Further proof for that came from
two orienting experiments: (i) In the presence of substoichio-
metric amounts of O2 3 mediated the oxidative coupling of
DTBP basically giving TBBP as had been observed for the
anaerobic reaction of preformed 5 (see above); and (ii)
employing TBBP instead of DTBP led to the complete
Consequently, this report represents a significant progress in
the field. A novel ligand system FurNeu has been developed to
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Journal of the American Chemical Society
Article
pre-orientate two CuI ions in close proximity for the activation
of O2. This has been realized within the precursor complex 3
containing weakly coordinating triflate anions and acetonitrile
coligands that are readily replaced by O2. Treatment of 3 with
O2 or PhIO leads to green solutions of the oxygenation product
that could be identified as a Cu2O complex, 5, by means of
UV−vis spectroscopy, PhIO consumption experiments, NMR
as well as DOSY NMR spectroscopy, HR-ESI mass spectros-
copy, and protonation studies. EXAFS analysis in combination
with DFT has revealed the solution structure of 5, and DFT
studies could also rationalize the UV−vis spectrum. The CuII−
O−CuII core in 5 is surprisingly stable at room temperature and
decays only slowly over an extended period of time to a
[Cu(picoloyl)2] species, possibly by an N-dealkylation
mechanism. The thermal stability of 5 can be attributed to a
reduced basicity of its oxo goup, which is also reflected in its
inability to react with water. In contrast to the CuII−O−CuII
entities in Cu-ZSM-5 5 does not show a pronounced H-atom
abstraction reactivity. This may be explained by the marked
difference in the structures, as revealed in our investigation:
While the CuII−O−CuII unit in 5 is in a relaxed state, the
situation within ZSM-5 may be regarded as strained favoring
formation of a SOMO that acts as the abstracting site.38
Investigations concerning the reactivity of 5 showed that it
catalyzes the oxidation of 2,4-di-tert-butylphenol to first give the
corresponding biphenol (TBBP), which then is oxidized further
via a diphenoquinone (TBDQ) intermediate to yield TBOBF,
which has rarely been observed as an oxidation product in
copper mediated oxidations. As 5 can be generated in situ from
the precursor compound 3 via reaction with O2, 3 shows the
same catalytic activity.
the crystal structure analysis of compound 1. XAS data were
obtained on beamline X3B of the National Synchrotron Light
Source (Brookhaven National Laboratory, Upton, NY, USA).
Beamline X3B is operated by the Case Western Reserve
University Center for Synchrotron Biosciences, supported by
NIH Grant P30-EB-009998. NSLS is supported by the United
States Department of Energy, Office of Science, Office of Basic
Energy Sciences, under contract DE−AC02−98CH10886.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section, molecular structures of 2·1.5(CH3CN),
2·2(CH3CN), (4(NCCH3)2)2, [Cu(picoloyl)2], and TBOBF,
detailed discussions of 2 and 4, X-ray crystallographic data, HR-
ESI mass spectra of 5 and 518O, additional UV−vis and NMR
data, DOSY NMR spectra, XAS details and data of 5, summary
of the reactivity studies of 3−5, additional TDDFT results. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Christian.limberg@chemie.hu-berlin.de
■
Present Address
†Milano-Bicocca University, Department of Earth and Environ-
mental Sciences, Piazza della Scienza 1, 20126 Milano, Italy.
Notes
The authors declare no competing financial interest.
Hatwell, K. R.; Kaderli, S.; Zuberbuhler, A. D.; Rheingold, A. L.;
̈
ACKNOWLEDGMENTS
This work is dedicated to Professor Dr. Dr. h.c. mult. Wolfgang
A. Herrmann on the occasion of his 65th birthday.
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