A high-valent heterobimetallic [CuIII(μ-O)2Ni III]2+ core with nucleophilic oxo groups

Article abstract of DOI:10.1002/anie.201300861

Cores and effect: Unlike homobimetallic analogues, the heterobimetallic CuNi bis(μ-oxo) diamond core has nucleophilic oxo groups. A similar heterobimetallic core may, therefore, act as a viable intermediate during the deformylation of fatty aldehydes by cyanobacterial aldehyde decarbonylase. Copyright

Full text of DOI:10.1002/anie.201300861

Angewandte
Chemie
DOI: 10.1002/anie.201300861
Bioinorganic Chemistry
A High-Valent Heterobimetallic
[CuIII(m-O)2NiIII]2+ Core with Nucleophilic Oxo Groups**
Subrata Kundu, Florian Felix Pfaff, Enrico Miceli, Ivelina Zaharieva, Christian Herwig,
Shenglai Yao, Erik R. Farquhar, Uwe Kuhlmann, Eckhard Bill, Peter Hildebrandt, Holger Dau,
Matthias Driess,* Christian Limberg,* and Kallol Ray*
The study of heterobimetallic dioxygen complexes is inter-
esting with regard to their potential in modeling the structures
and reactivities of metalloenzymes containing two different
metal ions at their active sites.[1] Moreover, such investiga-
tions may lead to the discovery of novel species exhibiting
alternative reactivity patterns with respect to their symmetric
counterparts. However, in contrast to the numerous reports of
biomimetic synthetic dinuclear complexes involving homo-
metallic dioxygen cores,[2] reports on mixed-metal [MO2M’]n+
cores are relatively rare,[3] presumably because of intrinsic
challenges associated with their preparation by the methods
usually used to obtain their symmetric analogues. A limited
number of heterobimetallic dioxygen complexes has been
made in the past, either by the reaction of heterobimetallic
M–M’ precursors with dioxygen, or by reaction of a well-
defined metal–dioxygen subunit with a reducing heterometal
complex.[3a] The latter strategy has also been used recently by
some of us[3d] to synthesize and structurally characterize the
first example of a heterodinuclear peroxo complex, {Ni(m-
O2)K}, by a one-electron reduction of a room-temperature
stable nickel superoxo complex [LNiII(O2)] (1; L = [HC-
(CMeNC6H3(iPr)2)2])[4] with elemental potassium. Further-
more, replacement of the potassium ion in the [Ni(m-O2)K]n+
strongly dependent on the nature of the second metal ion.
Unfortunately, these intermediates could not be directly
detected by spectroscopic methods and hence the predictions
of the theoretical calculations could not be verified exper-
imentally .
We now demonstrate the low-temperature trapping of the
proposed heterobimetallic bis(m-oxo) intermediate during the
one-electron reduction of 1 using a CuI triamine complex,
=
[Cu(MeAN)](BF4) [2-BF4; MeAN N,N,N’,N’,N’’-penta-
methyl dipropylenetriamine],[5] as a reductant. We report
the synthesis of the [LNiIII(m-O)2CuIII(MeAN)]+ complex 3
(Scheme 1), in nearly quantitative yield, and its character-
ization with optical, electron paramagnetic resonance (EPR),
resonance Raman (rRaman), and X-ray absorption spectros-
copies (XAS), as well as electrospray ionization mass
spectrometry (ESI-MS), reactivity studies, and density func-
tional theory (DFT) calculations. Interestingly, the oxo
groups of 3 act as nucleophiles, in sharp contrast to the
electrophilic oxo groups of the well characterized homome-
tallic [Ni2(m-O)2]2+ and [Cu2(m-O)2]2+ analogues. In fact,
complex 3 represents the only example of a high-valent[6]
bis(m-oxo)dimetal core involving nucleophilic oxo groups that
can perform deformylation of aldehydes. Until this report,
only metal-bound peroxides[7] were believed to be sufficiently
nucleophilic to attack aldehydes leading to production of
formate and oxidized coproducts. This study, therefore,
underlines the importance of subtle electronic changes in
the reactivity of the biologically relevant metal–dioxygen
intermediates.
core by Zn2+ or Fe+ ions initiated peroxido O O bond
À
scission and subsequent H-atom abstraction to generate the
heterobimetallic bis(m-hydroxo) Ni–Zn[3d] or alkoxo hydrox-
ido Ni–Fe[3e] complexes, respectively. Theoretical studies[3d,e]
suggested the involvement of heterobimetallic Ni(m-O)2M’
(M’ = Zn, Fe) cores in the H-atom abstraction process; their
electronic structures and reactivities were predicted to be
[*] S. Kundu, F. F. Pfaff, E. Miceli, Dr. C. Herwig, Prof. Dr. C. Limberg,
Dr. K. Ray
Dr. E. Bill
Max-Plank-Institut fꢁr chemische Energiekonversion, Mꢁlheim an
der Ruhr (Germany)
Humboldt-Universitꢀt zu Berlin, Institut fꢁr Chemie
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: christian.limberg@chemie.hu-berlin.de
kallol.ray@chemie.hu-berlin.de
[**] We gratefully acknowledge financial support of this work from the
Cluster of Excellence “Unifying Concepts in Catalysis” (EXC 314/1),
Berlin. XAS data were obtained on beamline X3B of the National
Synchrotron Light Source (Brookhaven National Laboratory) and
beamline KMC-1 of BESSY, Berlin (Germany). 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. We also thank Prof. William B. Tolman for
helpful suggestions.
Dr. I. Zaharieva, Prof. Dr. H. Dau
Freie Universitꢀt Berlin, FB Physik, Berlin (Germany)
Dr. S. Yao, Dr. U. Kuhlmann, Prof. Dr. P. Hildebrandt,
Prof. Dr. M. Driess
Technische-Universitꢀt Berlin, Institut fꢁr Chemie
Strasse des 17 Juni 135, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
Dr. E. R. Farquhar
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300861.
Case Western Reserve University Center for Synchrotron Biosci-
ences and Center for Proteomics and Bioinformatics, Brookhaven
National Laboratory (USA)
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X-ray absorption spectroscopic studies
were performed at Cu and Ni K-edges to
directly probe the metal oxidation states in 3.
Figure 2A shows a comparison of the Cu K-
edge spectrum of 3 with the spectra of its CuI
precursor and the previously reported
[(AN)CuII(NHTs)] [4, AN = 3,3’-imino-
bis(N,N-dimethyl
propylamine;
Ts =
tosyl][10] complex. A progressive blue shift
in the edge energy from 2-BF4 to 4 to 3 is in
accord with a stepwise increase in the oxida-
tion state of copper in the series. Most
importantly, a 2 eV blueshift, from 8978 to
8980 eV, and a broadening of the pre-edge
are observed from 4 to 3, which strongly
suggest[11] the presence of a CuIII ion in 3. In
addition, the Ni K-edge data (Figure 2B),
which display a subtle blueshift and reduced
intensity for the pre-edge, as well as a broader
blueshift of the overall edge in 3 relative to its
nickel(II) superoxide precursor, are consis-
tent with a nickel-based oxidation from 1 to
Scheme 1. Formation, DFT calculated structure, and products of the reactions of 3 with
different substrates.
Combination of equimolar amounts of 1 and 2-BF4 at
À908C in CH2Cl2 leads to the immediate generation of
a metastable intermediate 3 (t1/2 = 900 s at À508C) with
absorption maxima lmax (emax; mÀ1 cmÀ1) centered at 300
(20000), 370 (13500), and 895 (5000) nm (Figure 1). The
electrospray mass spectrum (Supporting Information Fig-
ure S1) of 3 exhibits the most prominent peak at a mass–
charge ratio (m/z) of 771.38, with a mass and isotope
distribution pattern corresponding to [LNi(O)2Cu(MeAN)]+.
When the reaction is carried out with 18O enriched 1, a mass
peak corresponding to [LNi(18O)2Cu(MeAN)]+ appears at
m/z 775.38, thereby revealing the presence of two oxygen
atoms in 3. Although, we are currently unable to assign the
low-energy band in the absorption spectrum of 3, the
similarity of the features at lmax = 300–400 nm to ones typical
of homometallic [Ni2(m-O)2]2+ and [Cu2(m-O)2]2+ cores[2] may
suggest an analogous formation of a bis m-oxo core in 3.
Indeed, the rRaman (Figure 1) spectrum of 3 using 413 nm
laser excitation in resonance with the 370 nm transition
supports the view of a [Ni(m-O)2Cu]2+ structure. The band at
625 cmÀ1 of 3 [n(3-16O16O)] shifts down to 595 cmÀ1 [n(3-
18O18O)] in the corresponding 18O2-substituted complex 3.
Furthermore, single oxygen-isotope labeling experiments[8]
show an additional intermediate band at 615 cmÀ1 corre-
sponding to the vibration of the partially labeled species [n(3-
16O18O)]. In analogy to the well-studied [M2(m-O)2]2+ (M = Fe,
Ni, Cu) systems, for which similar rRaman signatures and
oxygen isotope effects were reported,[9] we assign the rRaman
feature at 625 cmÀ1 to a tetra-atomic vibration (Figure S2)
mode of the [Ni(m-O)2Cu]2+ core. This assignment is also
supported by DFT calculations (see below). Furthermore, the
fact that the frequency of n(3-16O18O) is not halfway between
n(3-16O16O) and n(3-18O18O), and the lack of isotope-sensitive
bands in the 700–1200 cmÀ1 range, strongly argue against the
presence of any peroxo or superoxo species.
Figure 1. Top: Absorption spectra of 2-BF4 (solid trace), 1 (dash–dot
trace), 3 (dotted trace) in CH2Cl2 at À908C. Inset: X-band EPR
spectrum of 3 in CH2Cl2 (0.47 mm) at 10 K (frequency 9.470251 GHz;
power 0.05 mW; modulation 0.5 mT). Bottom: Calculated (DFT, see
Supporting Information for details) and experimental (CH2Cl2; À908C;
413 nm excitation) rRaman spectra of 3-16O2 (gray, solid trace),
3-18O2 (dotted trace), and 3-16O18O (dash–dot trace). Bands originating
from the solvent are marked by asterisks.
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Figure S4; Table S3) with a four-coordinate planar
geometry at both the copper and the nickel centers,
and geometrical parameters in excellent agreement
with that obtained from XAS studies (Table S2). In the
calculated structure MeAN acts as a bidentate ligand,
with one of the terminal tertiary amine nitrogen atoms
remaining out of the coordination sphere of Cu. The
ground state is calculated to be S = 1/2, consistent with
the EPR studies. The unpaired spin is distributed such
that the nickel center has 0.7 spin, whilst the copper and
oxygen centers carry an average spin density of only
0.01 and 0.06, respectively (Table S4). These results
reveal the presence of NiIII (S = 1/2; 3d7) and CuIII (S =
0; 3d8) centers in 3, thereby, supporting a [LNiIII(m-
O)2CuIII(MeAN)]+ assignment. Vibrational analysis on
the DFT-optimized structure of 3 in the S = 1/2 ground
state predicts a strong [NiIII(m-O)2CuIII]2+ core vibra-
tional mode at 640 cmÀ1 (Figures S5–S6; Table S5). This
frequency and in particular the calculated 18O/16O
isotopic shifts are in excellent agreement with the
experimental data (Figure 1). Thus, the spectroscopic
results validate the calculated molecular and electronic
structures of 3.
Figure 2. A) Normalized Cu K-edge XANES spectra for 2-BF4 (dotted line),
3 (dash–dot line), and 4 (solid line). The inset depicts an expansion of the pre-
edge region for 3 and 4. B) Normalized Ni K-edge XANES spectra for 1 (solid
line) and 3 (dash–dot line). The inset shows an expansion of the pre-edge
region. C) Fourier-transformed Cu K-edge EXAFS spectra of 2-BF4 and 3
(Experiment: dotted line; simulation: bold line). D) Fourier-transformed Ni K-
edge EXAFS spectra of 1 and 3. For the EXAFS data on a wavevector scale
before calculation of the Fourier transform see Figure S3.
The oxo groups in high-valent [M2(m-O)2]n+ (M =
Fe, Co, Ni, Cu) cores act as strong electrophiles and are
typically unreactive towards other electrophiles.[2] Con-
sistent with these observations, addition of cyclohexane
carboxaldehyde (CCA) or 2-phenylpropionaldehyde (2-
PPA) to [(AN)2Cu2(m-O)2]2+ [5,14] or [(L)2Ni2(m-O)2],[15] which
are the homometallic analogues of 3, at À908C does not
change the UV/Vis spectra. In contrast, similar reactions with
3 result in instantaneous bleaching and disappearance of the
bands at 370 and 895 nm in the UV/Vis absorption spectrum
(Figure S7). Product analysis of the reaction mixture reveals
the formation of the deformylated[16] products in 65–90%
yields (Scheme 1; Table S6). The reactivity of 3 was further
investigated using benzoyl chloride at À508C, to confirm the
nucleophilic properties of 3. A pseudo-first order decay of the
absorption feature at 895 nm and the subsequent formation of
benzoic acid (Figure S8) were observed upon adding benzoyl
chloride to a preformed solution of 3 at À508C. The rate
constant increases proportionally with the substrate concen-
tration, affording a second-order rate constant, k2, of
0.11mÀ1 sÀ1 at À508C. Additional mechanistic studies with
para-substituted benzoyl chloride (para-Y-C7H4OCl; Y=
OMe, Me, H, Cl, CN) reveal a good linear correlation of
the rate to the sP+ values of the para substituents. A positive
Hammett 1 value of 2.5 (Figure S9) is obtained, which further
supports the nucleophilic character of the [LNiIII(m-O)2CuIII-
(MeAN)]+ core in the oxidation of benzoyl chloride. To our
knowledge, 3 is the only example of a high-valent[6] bis(m-oxo)
dimetal complex containing nucleophilic oxo groups. To
investigate in how far the dangling amine group assists
reactivity or is even responsible for the observed nucleophi-
licity, experiments with N,N,N’,N’-tetramethylethylenedia-
mine (TMEDA), as a co-ligand for Cu instead of MeAN
were envisaged. Unfortunately, unlike 2-BF4, the CuI-
(TMEDA) complex has been reported to be unstable,[17] so
that it had to be generated in situ prior to the reaction with 1.
3.[12] The X-band EPR spectrum (Figure 1) of 3 is also found
to be in good agreement with the [LNiIII(m-O)2CuIII(MeAN)]+
assignment and is dominated by a typical S = 1/2 NiIII rhombic
signal (gx = 2.006, gy = 2.14, gz = 2.41) that lacks the hyperfine
splitting characteristic of CuII,[13] and corresponds, by inte-
gration, to 90% of the amount of 2-BF4 added. The absence of
hyperfine splitting indicates copper in the diamagnetic 3d8
low-spin state, which moreover reveals planar coordination.
Extended X-ray absorption fine structure (EXAFS)
analysis reveals further structural details (Figure 2C; Fig-
ure 2D and S3; Table S1). For 1 a shell of four N/O scatterers
at 1.84 ꢀ is obtained from Ni EXAFS, in good agreement
with the molecular structure determined previously by X-ray
crystallography (Table S2).[4] On the other hand, a fit to the
data for 3 requires two subshells of N/O scatterers at 1.79 ꢀ
(assigned to the two NiIII O units) and 1.88 ꢀ (assigned to the
À
two NiIII N units), consistent with the structure derived from
À
DFT calculations (Table S2). In addition, Ni EXAFS of 3
shows an additional peak at 2.81 ꢀ corresponding to the Cu
scatterer, thereby strongly supporting the presence of a het-
erodinuclear Ni···Cu center in 3. Further support for including
a Ni···Cu distance at 2.81 ꢀ comes from Cu EXAFS, which
also detects the Ni scatterer at 2.81 ꢀ in 3, but not in the
mononuclear CuI precursor. Furthermore, the best fit of the
Cu EXAFS of 3 can be obtained with two subshells, each
containing two O/N scatterers at 1.80 ꢀ and 2.00 ꢀ, thereby
supporting a four coordinate Cu center in 3. It is important to
note that a fit with two O/N scatterers at 1.88 ꢀ and three O/
N scatterers at 2.00 ꢀ gave a high Debye–Waller factor, which
together with the DFT calculations (see below) preclude the
presence of a five-coordinate Cu center in 3 (Table S1).
DFT calculations (see Supporting Information for details)
on 3 yield a minimum energy for a C1 structure (Scheme 1;
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Preliminary experiments with the resulting complex [LNiIII(m-
O)2CuIII(TMEDA)]+ (5, Scheme S1),[18] structurally similar to
3, but lacking any dangling amine group, suggest that the
dangling amine group may not be the origin of the nucleo-
philic behavior of the oxido ligands within 3: complex 5 reacts
with benzoyl chloride at À908C in acetone at a rate much
faster than 3 (Figure S12). However, the possibility that the
nucleophilic reactivity is actually initiated by the slight excess
of TMEDA present in the solution because of incomplete
complexation cannot be excluded at this point. In this context
it is also worth mentioning that [(AN)2Cu2(m-O)2]2+ may also
be expected to have dangling or at least loose amine arms and
is an electrophilic oxidant.
In conclusion, we have reported the synthesis and
spectroscopic characterization of the mixed-metal Ni–Cu bis
(m-oxo) complex 3 that is potent in the deformylation of
aldehydes. Whether or not a similar asymmetric bis(m-oxo)
core also acts as an active species in the catalytic cycle of
cyanobacterial AD is now a particularly intriguing question.
Received: January 31, 2013
Published online: && &&, &&&&
Keywords: bridging oxo ligands · copper · diamond core ·
.
nickel · nucleophilicity
Complex 3 also performs hydrogen atom abstraction from
À
À
the C H (or O H) bonds of 1,3-cyclohexadiene (CHD), 9,10-
dihydroanthracene (DHA), 2,4-di-tert-butylphenol (2,4-
DTBP), and 2,6-di-tert-butylphenol (2,6-DTBP); the
second-order rate constants derived from these studies in
CH2Cl2 solution are listed in Table S6. Surprisingly, the rates
for reactions with DHA and 2,6-DTBP are found to be
significantly lower than those determined for CHD and 2,4-
DTBP, respectively. Since DHA/CHD and 2,4-DTBP/2,6-
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À
À
DTBP have comparable C H and O H bond dissociation
energies,[19] the large rate differences observed for 3 suggest
that the b-diketiminate ligand impedes access of the bulkier
DHA or 2,6-DTBP to the [LNiIII(m-O)2CuIII(MeAN)]+ core.
Such sterically derived mitigation of reactivity has been
observed for other b-diketiminate-ligated systems.[20] No
reaction of 3 with triphenyl phosphine has been observed.
A number of enzymes[21,22] use a bimetallic active site to
bind and activate dioxygen to form reactive species involving
superoxide, peroxide, and metal oxide cores, which are then
responsible for carrying out a variety of electrophilic and
nucleophilic oxidation reactions. While nucleophilic reactions
are typically carried out by superoxides or peroxides, electro-
philic reactions are attributed to metal oxides. For example,
the [Cu2(m-O)2]2+ and [Fe2(m-O)2]4+ cores have been consid-
ered as active oxidants in the hydroxylation of phenol and
methane by tyrosinase and soluble methane monooxygenase,
respectively.[2,21] On the other hand, a homo- or heterodinu-
clear peroxo core[22] has been proposed as the active species
responsible for the deformylation of fatty aldehydes by
cyanobacterial aldehyde decarbonylase (AD); the metal
centers in the dimetal-cofactor of AD have yet to be
identified. In this work, we have now reported the synthesis
and spectroscopic characterization of a high-valent hetero-
dinuclear [NiIII(m-O)2CuIII]2+ core, which can act as nucleo-
phile to attack (among other electrophiles) aldehydes leading
to production of formate and oxidized coproducts. The
nucleophilic property of 3, which is in sharp contrast to the
electrophilicity of oxo groups in the homodinuclear analogues
containing [Cu2(m-O)2]2+ or [Ni2(m-O)2]2+ cores, may be
related to a reduced covalency of the metal–oxygen bonds
owing to the mismatch of the bonding orbitals in the
asymmetric [NiIII(m-O)2CuIII]2+ core. Further experimental
and theoretical studies are, however, warranted to understand
the origin of the nucleophilic behavior of 3. These studies are
ongoing.
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[18] Complex 5 (t1/2 = 1.5 h at À908C) was generated by adding 1 to
a precooled (À908C) solution of equimolar amounts of [Cu-
(MeCN)4]OTf and TMEDA in acetone. The absorption spec-
trum (Figure S11) of 5 was found to be very similar to that of 3;
this further confirms that the Cu center in 3 is also four
coordinate as in 5. Reactions with benzoyl chloride were
performed with 1 mm solutions of 5 in acetone at À908C.
[19] Y.-R. Luo, Comprehensive handbook of chemical bond energies,
Taylor & Francis, New York, 2007.
[20] N. W. Aboelella, E. A. Lewis, A. M. Reynolds, W. W. Brennesel,
C. J. Cramer, W. B. Tolman, J. Am. Chem. Soc. 2002, 124, 10660.
[21] a) E. I. Solomon, P. Chen, M. Metz, S.-K. Lee, A. E. Palmer,
Angew. Chem. 2001, 113, 4702; Angew. Chem. Int. Ed. 2001, 40,
4570; b) M. H. Sazinsky, S. J. Lippard, Acc. Chem. Res. 2006, 39,
558.
[13] The wiggles in the EPR spectrum, faintly reminiscent of
hyperfine lines, arise from uneven freezing of the sample
resulting in microcrystals of defined orientation. This is con-
cluded on the basis of the observed variations upon repeated
freezing or rotating the EPR tube (Figure S10). Also labeling 3
with 63Cu did not systematically affect the pattern.
[14] The corresponding [(MeAN)2Cu2(m-O)2]2+ complex does not
exist (Ref. [5]). Rather a peroxo bridged complex [(MeAN)2Cu2-
(m-O2)]2+ is formed, which is also found to be unreactive towards
aldehydes.
[15] [(L)2Ni2(m-O)2] is formed by the reaction of 1 with its NiI
precursor (S. Pfirrmann, S. Yao, B. Ziemer, R. Stçsser, M.
Driess, C. Limberg, Organometallics 2009, 28, 6855) at À908C in
CH2Cl2. Formation of the bis(m-oxo) core was confirmed on the
basis of the appearance of the absorption band at 400 nm, which
is common to all [Ni2(m-O)2] cores, and also by ESI-MS methods.
[22] N. Li, H. Nørgaard, D. M. Warui, S. J. Booker, C. Krebs, J. M.
Bollinger, Jr., J. Am. Chem. Soc. 2011, 133, 6158.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Bioinorganic Chemistry
S. Kundu, F. F. Pfaff, E. Miceli,
I. Zaharieva, C. Herwig, S. Yao,
E. R. Farquhar, U. Kuhlmann, E. Bill,
P. Hildebrandt, H. Dau, M. Driess,*
C. Limberg,* K. Ray*
&&&& — &&&&
A High-Valent Heterobimetallic
[CuIII(m-O)2NiIII]2+ Core with Nucleophilic
Oxo Groups
Cores and effect: Unlike homobimetallic
analogues, the heterobimetallic CuNi
bis(m-oxo) diamond core has nucleophilic
oxo groups. A similar heterobimetallic
core may, therefore, act as a viable inter-
mediate during the deformylation of fatty
aldehydes by cyanobacterial aldehyde
decarbonylase.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!