A New Domain of Reactivity for High-Valent Dinuclear [M(μ-O)2M′] Complexes in Oxidation Reactions

Article abstract of DOI:10.1002/anie.201607611

The strikingly different reactivity of a series of homo- and heterodinuclear [(M^III)(μ-O)₂(M^III)′]²⁺(M=Ni; M′=Fe, Co, Ni and M=M′=Co) complexes with β-diketiminate ligands in electrophilic and nucleophilic oxidation reactions is reported, and can be correlated to the spectroscopic features of the [(M^III)(μ-O)₂(M^III)′]²⁺core. In particular, the unprecedented nucleophilic reactivity of the symmetric [Ni^III(μ-O)₂Ni^III]²⁺complex and the decay of the asymmetric [Ni^III(μ-O)₂Co^III]²⁺core through aromatic hydroxylation reactions represent a new domain for high-valent bis(μ-oxido)dimetal reactivity.

Full text of DOI:10.1002/anie.201607611

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International Edition: DOI: 10.1002/anie.201607611
German Edition: DOI: 10.1002/ange.201607611
Dioxygen Activation
A New Domain of Reactivity for High-Valent Dinuclear [M(m-O)2M’]
Complexes in Oxidation Reactions
Xenia Engelmann+, Shenglai Yao+, Erik R. Farquhar, Tibor Szilvꢀsi, Uwe Kuhlmann,
Peter Hildebrandt, Matthias Driess,* and Kallol Ray*
Abstract: The strikingly different reactivity of a series of
homo- and heterodinuclear [(MIII)(m-O)2(MIII)’]2+ (M = Ni;
M’ = Fe, Co, Ni and M = M’ = Co) complexes with b-diket-
iminate ligands in electrophilic and nucleophilic oxidation
reactions is reported, and can be correlated to the spectroscopic
features of the [(MIII)(m-O)2(MIII)’]2+ core. In particular, the
unprecedented nucleophilic reactivity of the symmetric [NiIII-
(m-O)2NiIII]2+ complex and the decay of the asymmetric
[NiIII(m-O)2CoIII]2+ core through aromatic hydroxylation reac-
tions represent a new domain for high-valent bis(m-oxido)di-
metal reactivity.
T
he synthesis and investigation of heterobimetallic bis(m-
oxido) complexes with [M(m-O)2M’]n+ cores are of particular
interest for modeling the structures and reactivities of active
sites in chemically related metalloenzymes containing two
different metal ions.[1] Moreover, such studies can lead to the
discovery of novel species[2] exhibiting reactivity patterns
different from their homodinuclear counterparts. The illus-
trative example (Scheme 1 inset) is the recently reported
[L1Ni(m-O)2Cu(MeAN)]+ (1; L1 = [HC(CMeNC6H3(iPr)2)2];
MeAN = N,N,N’,N’,N’’-pentamethyl dipropylenetriamine)[3]
complex, which, in sharp contrast to the presence of electro-
philic oxido groups in symmetric [M2(m-O)2]n+ (M = Ni, Co,
Fe and Mn) analogues,[4] possesses nucleophilic oxido ligands
and deformylates aldehydes.
Scheme 1. Generation and self-decay of the heterodinuclear [NiIII(m-
O)2CoIII]2+ complex 2 and its homodinuclear [MIII(m-O)2MIII]2+ ana-
logues 3 and 4 (M=Ni, Co). Inset: The structure of the previously
reported[3] asymmetric bis(m-oxido)NiIIICuIII compound with nucleo-
philic oxido ligands.
The unprecedented nucleophilic reactivity of 1 may be
attributed to the asymmetric nature of the bis(m-oxido)dime-
tal core. Alternatively, the nature of the ancillary ligand may
[*] M. Sc. X. Engelmann,[+] Prof. Dr. K. Ray
Department of Chemistry, Humboldt-Universitꢀt zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: kallol.ray@chemie.hu-berlin.de
also play a vital role; notably, 1 possesses a dangling amine
group, which may induce the change in reactivity, leading to
the observed nucleophilicity. Understanding the origin of the
nucleophilicity in 1, therefore, warrants a systematic compar-
ison of the reactivity of a series of symmetric and asymmetric
dinuclear bis(m-oxido) complexes involving similar ancillary
ligands, which preferably lack any dangling basic/nucleophilic
group that could affect the reactivity.
In our continuous effort to uncover structure–reactivity
relationships of dinuclear metal–dioxygen intermediates, we
now demonstrate spectroscopic trapping of a series of homo-
and heterodinuclear [MIII(m-O)2(MIII)’]2+ (M = Ni; M’ = Co,
Ni and M = M’ = Co) complexes of b-diketiminate ligands. A
comparison of the reactivity of these complexes in various
oxidation reactions reveals a distinct reactivity pattern
dependent on the spectroscopic properties of the bis(m-
oxido)dimetal core. Furthermore, the fully symmetric [L1NiIII-
Dr. S. Yao,[+] Dr. U. Kuhlmann, Prof. Dr. P. Hildebrandt,
Prof. Dr. M. Driess
Department of Chemistry, Technische Universitꢀt Berlin
Straße des 17. Juni 135, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
Dr. E. R. Farquhar
Case Center for Synchrotron Biosciences, NSLS-II
Brookhaven National Laboratory
Upton, NY 11973 (USA)
Dr. T. Szilvꢁsi
Department of Inorganic and Analytical Chemistry
Budapest University of Technology and Economics
Szent Gellꢂrt tꢂr 4, 1111 Budapest (Hungary)
[+] These authors contributed equally to this work.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607611.
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(m-O)2NiIIIL1]2+ (3) complex demonstrates a nucleophilic
property, which is in contrast to the observed electrophilic
behavior of all homodinuclear bis(m-oxido) cores reported to
date.[4] This, together with the self-decay of the heterodinu-
(75% yield based on spin-quantification) with gx = 1.98, gy =
2.12, gz = 2.76, and observable 59Co hyperfine splitting at gx
and gz. The electronic structure of 2 is thus best represented as
[L1NiIII(m-O)2CoIIIL3], where antiferromagnetic coupling
between NiIII (3d7; S = 1/2) and CoIII (3d6; S = 1) ions leads
to an S = 1/2 ground state.
clear [L1Ni(m-O)2CoL3] species
2
(L3 = [HC(CHNC6H3-
(iPr)2)2]) by an unprecedented hydroxylation-mediated 1,3-
migration of an aryl nitrogen atom, represents a novel domain
for high-valent bis(m-oxido)dimetal reactivity.
X-ray absorption spectroscopy (XAS) studies were per-
formed at the Ni and Co K-edges to directly probe the metal
oxidation states in 2. Notably, the Ni K-edge spectrum of 2 is
virtually indistinguishable to that reported previously for
1 (where the oxidation state of Ni has been unambiguously
determined to be + 3; Figure S3A),[3] although the pre-edge is
slightly more intense and sharper in 2 (Figure S3A inset). The
higher pre-edge intensity (Table S1) of 2 (6.6 units) relative to
1 (5.4 units) at the Ni K-edge may originate from the larger
deviation from the ideal square (sq) planar geometry[7] at the
Ni center of 2 (dihedral angle q = 1728; Table S2) relative to
1 (q = 178.18; Table S2) in the optimized structures derived
from density functional theory (DFT) calculations. In addi-
tion, a comparison of the Co K-edge spectrum of 2 with that
of [L3Co(C7H8)] shows a blue-shift in the edge energy
(Figure S3B) together with an upshift (from 7709 eV in
[L3Co(C7H8)] to 7709.8 eV in 2; Table S1) and intensity
increase in the pre-edge peak of 2 (16.9 units) vs. [L3Co-
(C7H8)] (6.2 units). This strongly suggests a change in Co
oxidation from + 1 to + 3 during the conversion of [L3Co-
(C7H8)] to 2. Thus XAS data also are consistent with
a [L1NiIII(m-O)2CoIIIL3] assignment for 2.
Reaction of equimolar amounts of the isolable NiII–
superoxo complex [L1NiO2][5] with the CoI precursor [L3Co-
(C7H8)][6] at ꢀ788C in toluene leads to immediate formation
of the intermediate 2 (Figure 1A and Scheme 1) with
absorption maxima lmax (e; mꢀ1 cmꢀ1) centered at 576 (1651)
and 834 nm (2700). Notably, the near-IR absorption feature of
2 at 834 nm is blue-shifted relative to 1 (Table 1).[3] Moreover,
the thermal stability of 2 (t1/2 = 245 s in toluene at ꢀ508C) is
significantly lower than that of 1 (t1/2 = 900 s in CH2Cl2 at
ꢀ508C).[3] The electrospray mass spectrum (ESI-MS; Fig-
ure S1 in the Supporting Information) of 2 exhibits the most
prominent peak at m/z = 955.4904, with a mass and isotope
distribution pattern corresponding to [L1Ni(m-O)2CoL3]. The
rRaman spectrum of 2 using 413 nm laser excitation supports
the [Ni(m-O)2Co]2+ assignment of 2 (Figure 1C). The band at
n = 634 cmꢀ1 of 2-16O16O shifts down to 600 cmꢀ1 [n(2-18O18O)]
in the corresponding 18O2-substituted complex 2. The X-band
EPR spectrum (Figure S2) shows a rhombic S = 1/2 signal
Extended X-ray absorption fine structure (EXAFS)
analyses revealed further structural details. For 2 a shell of
four N/O scatterers at 1.86 ꢀ is obtained from Ni EXAFS
(Table S3–1, Figure S4A), which is in excellent agreement
ꢀ
with the DFT calculated average Ni N/O distance of 1.88 ꢀ
for 2. In addition, a peak at 2.78 ꢀ corresponding to the Co
scatterer is observed, thereby strongly supporting the pres-
ence of a heterodinuclear Ni···Co center in 2. Further support
for a Ni···Co separation of ca. 2.78 ꢀ comes from Co EXAFS
(Table S3–2, Figure S4B), which detects a Ni scatterer at
2.72 ꢀ. In addition the best fit of the Co EXAFS can be
obtained with four N/O scatterers at 1.82 ꢀ. Notably, the DFT
ꢀ
ꢀ
calculated average Co N/O and Co Ni distances of 1.87 and
2.77 ꢀ, respectively, are again in reasonable agreement with
the experiment.
The [L1NiIII(m-O)2NiIIIL1]2+ (3) and [L3CoIII(m-O)2-
CoIIIL3]2+ (4) complexes, which are homometallic (symmetric)
analogues of 2, have also been synthesized by reacting
[L1NiIIO2] with [(L1Ni)2(C7H8)] and [L3Co(C7H8)] with dioxy-
gen, respectively, in toluene at ꢀ788C (Scheme 1). Table 1
lists rRaman, absorption spectra and t1/2 data for 1–4. Notably,
complex 4 is unique in the series with a significantly blue-
shifted absorption band (Figure 1a), higher energy metal–oxo
(n(M-O)) stretching vibration frequency and higher stability
relative to 1–3. Thermal stability of 4 at room temperature
enabled its isolation as dark blue crystals in 80% yields; the
Figure 1. A) Absorption spectra of 2–4 at ꢀ788C in toluene. B–
D) rRaman spectra (413 nm excitation) of 2–4 in toluene at ꢀ908C:
B) 3-16O2 in [D8]toluene (solid trace) and 3-18O2 in toluene (dash-dotted
trace). C) 2-16O2 in [D8]toluene (solid trace) and 2-18O2 in toluene
(dash-dotted trace). D) 4-16O2 in [D8]toluene (solid trace) and 4-18O2 in
[D8]toluene (dashed trace). See also Figure S22.
Table 1: Spectroscopic and reactivity properties of 1–4.
[d]
[d]
t1/2 [s]
n(M-O)
[cmꢀ1
lmax [nm] k2(PhCOCl)
k2(CHD)
]
[ꢃ10ꢀ4 mꢀ1 sꢀ1
]
[ꢃ10ꢀ4 mꢀ1 sꢀ1
]
1[a] 900[b,c]
625
634
895
834
853
582
550
240
305
52.2
5
338
97
X-ray crystal structure (Scheme 1, Figure S5, Table S4) con-
3
2
3
4
245[b,d]
ꢀ
ꢀ
sists of two L Co O fragments (Co O distance of 1.786(1) ꢀ)
related by inversion and separated by Co···Co = 2.6715(5) ꢀ.
The short metal–metal and metal–oxo distances found in 4
compare favorably to those in related “diamond core”
4200[b,d] 632
stable[d] 647
60.9
[a] Ref. [3]. [b] ꢀ508C. [c] CH2Cl2. [d] Toluene.
2
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structures.[4] A solution magnetic moment (meff, C6D6, 298 K)
of 2.55 mB for 4 suggests population of a higher spin state at
elevated temperatures. The Ni sites in 3 are, however,
antiferromagnetically coupled at 298 K stabilizing the singlet
(S = 0) state. DFT calculations provide some insight into the
differences in the magnetic properties of 4 and 3. The parallel
spin coupling in 4 can be ascribed to the slightly folded Co2O2
structure, in which two cobalt planes make an angle (f) of
1788 (calculated f = 171.78; Table S2) and initiates ferromag-
netic interaction due to loss of orbital overlap. In contrast,
a nearly planar Ni2O2 structure (f = 179.88; Table S2) is
predicted by DFT for 3, which may explain the strong
antiferromagnetism leading to an S = 0 ground state. Similar
diminution of antiferromagnetic couplings due to a folded
Co2O2 core have been observed earlier.[4g,h] Moreover, the
Co–Co separation of 2.6715 ꢀ (DFT: 2.66 ꢀ) in 4 is signifi-
cantly shorter than the DFT calculated Ni–Ni separation of
2.77 ꢀ (Table S9) for 3. This reduction favors the direct
interaction, increasing also the ferromagnetic contribution.
Table 1 (see also Figure S6 and Table S5) also compares
the kinetic data for the four complexes with respect to their
rates of nucleophilic and electrophilic reactions. In one set of
experiments, the nucleophilic oxidation of benzoyl chloride
(PhCOCl) by 2–4 was investigated in toluene solutions,
resulting in about 80% yield of benzoic acid (Figure 2 and
Figure 3. Linear correlation of the logarithms of k2(PhCOCl) vs. lmax (filled
circles and dashed line) or n(M-O) (filled squares and solid line) of the
bis(m-oxido)dimetal units in 1–4.
For comparison, the hydrogen atom abstraction (HAT)
rates of 1–4 were also investigated with 1,4-cyclohexadiene
(CHD) and substituted phenols. They follow the reactivity
order 2 > 3 > 4 > 1 (Figure S11, Table S5). Interestingly, the
linear correlation observed for the logk2 values for PhCOCl
oxidation versus lmax or n(M-O) values does not extend to HAT
rates of 1–4 (Figure S12B, Table S5). While the respective rate
values associated with 1–3 for the oxidation of CHD follow
a linear correlation similar to that observed for PhCOCl, 4
deviates significantly from this pattern and exhibits HATrates
that are few orders of magnitude below what are predicted by
these lines. Clearly, the nucleophilic and electrophilic oxida-
tion reactions are not governed by a common set of factors.
Based on the observed HAT rates 2 can be considered as the
most electrophilic of all the complexes 1–4. Replacement of
the Co atom in [L1Ni(m-O)2CoL3] with Fe further increases
the electrophilicity of the bis(m-oxido)dimetal core to an
extent that the [L1Ni(m-O)2FeL3] (5) complex becomes
extremely unstable against self-decay by an intramolecular
ligand hydroxylation pathway (Scheme 1). Thus, monitoring
of the reaction of [L1NiIIO2] with [L3Fe(C7H8)][8] in toluene by
means of UV/Vis spectroscopy (Figure S13) at ꢀ908C did not
lead to the observation of any intermediate species. The
molecular structure of the resultant product 6 (> 90% yields;
Figure S14, Table S7) reveals the presence of b-diketiminato-
ligated NiII and FeII centers[9] in sq planar and tetrahedral (Td)
coordination environments, respectively. The two metals are
bridged by m-hydroxido (Figure S15) and m-alkoxido units
Figure 2. Reactivity studies of complexes 2–4. Yields are given relative
to the metal complexes.
Figure S7). Comparison of the reaction rates with that
reported previously for 1[3] reveals a reactivity order of 1 >
3 > 2 > 4. This trend is also found to be valid for reactions with
2-phenylpropionaldehyde (2-PPA), to form acetophenone
and formate (Figure 2, Figure S8, Table S5). Furthermore,
when the logarithms of the second order rate constants
(logk2) were plotted versus lmax or n(M-O), an excellent linear
correlation was observed (Figure 3), demonstrating that lmax
or n(M-O) reflect the relative nucleophilicities of the bis(m-
oxido)dimetal units in 1–4. The reactivity of 2 and 3 was
further investigated using a series of para substituted
benzaldehydes (Figure S9). Positive 1 values of 1.33 and
2.26 in the Hammett plots were obtained for 2 and 3,
respectively (Figure S10), which, when compared to the
1 value of 2.83 reported previously for 1,[3] again supports
the nucleophilicity order of 1 > 3 > 2. These results demon-
strate that the bis(m-oxido)dimetal cores of 2 and 3 are active
nucleophilic oxidants, similar to that reported previously
for 1.
ꢀ
derived from C H activation of an isopropyl (iPr) group
belonging to the b-diketiminato-ligand bound at the nickel
site.
In contrast, decay of a solution of 2 led to the hydroxyl-
ation of one of the arene residues in the major (> 75% yield
relative to 2) decay product 7 (Scheme 1, Figure S16,
Table S7). The X-ray crystal structure analysis of 7 shows
a heterodinuclear complex, with b-diketiminato-ligated NiII
and CoII centers in sq planar and Td coordination environ-
ments, respectively. The oxidation state assignments are based
on XANES studies (Figure S17); the Ni K-edge of 7 is near-
identical to that of the [L1NiIIO2] precursor (consistent with sq
planar NiII) and the Co K-edge of 7 reveals an edge energy
intermediate between the CoI precursor, [L3Co(C7H8)], and 2
(consistent with CoII). Notably, the metal atoms are bridged
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by m-hydroxido and m-phenoxido units (Figure S18) derived
from hydroxylation induced 1,3-migration of the aryl nitrogen
atom of the b-diketiminato ligand bound at the Ni site. The
conversion of 2 to 7 is therefore reminiscent of the NIH shift
reaction suggested for enzymatic aromatic hydroxylations.[10]
This involves 1,2-migration of an aryl-X (X = H, CH3 or Cl)
atom during rearomatization of an oxygenated intermediate
(that prevents its isolation even at low temperatures),
although identical QO values have been calculated in both
cases (Table S10). For comparison calculations were also
performed on the hypothetical [L1Ni(m-O)2CuL3] complex
(8)[13] that unlike 1 contains a b-diketiminate ligand attached
to the Cu center. A trend of increasing QO and decreasing
covalency of the [Ni(m-O)2M] (M = Fe, Co, Ni, Cu) cores with
increasing atomic weight of M is now obvious, as also
ꢀ
(Figure S19A). The cleavage of the aromatic C N bond
ꢀ
during the conversion of 2 to 7 similarly suggests a decay
evidenced by the decreasing M O Mayer bond order and
mechanism (Scheme 2; also supported by DFT, Figure S19B)
increasing Ni–M distances (Tables S9 and S10).
In conclusion, this study presents the first example
showing that a fully symmetric bis(m-oxido)dimetal complex
3 can act as a nucleophilic oxidant in addition to demonstrat-
ing the typical HAT reactivities. 3 reveals a characteristic
absorption band in the near infrared (IR) region, which acts
as a spectroscopic marker for nucleophilic bis(m-oxido)dime-
tal cores. Furthermore, the nature of the second metal ion (M)
is shown to systematically modulate the electronic structure
and reactivity of a series of b-diketiminate [Ni(m-O)2M]2+
complexes with M = Fe, Co Ni, and Cu. The combined
experimental and computational evidences herein indicate
that the decreasing nucleophlicity of the oxygen atoms in the
order [NiIII(m-O)2CuIII]2+ > [NiIII(m-O)2NiIII]2+ > [NiIII(m-
O)2CoIII]2+ > [NiIII(m-O)2FeIII]2+ correlates to the increasing
Scheme 2. Self-decay of 2 by an electrophilic aromatic hydroxylation
mechanism to form 7.
ꢀ
covalency of the M O bonds, owing to an increase in the
extent of O!M p-backdonation (Figure S21). For M = Fe
(d5) with half-filled p*(xz, yz) levels O!M p-backdonation is
maximum with the most electrophilic oxygen atoms. The
additional electrons in the Co(d6), Ni(d7) and Cu(d8) com-
plexes subsequently fill up the p*(xz, yz) levels and reduce
the O!M p-backdonation to an extent that there is
essentially no multiple bond character present in the [NiIII-
(m-O)2CuIII]2+ complex. Additionally, with increasing oxido
nucleophilicity, the energy of the oxygen based highest
involving intramolecular electrophilic attack of the [NiIII(m-
O)2CoIII]2+ core on an aromatic ring of the Ni-bound b-
diketiminato ligand leading to a cationic intermediate; the
ꢀ
rearomatization process now involves cleavage of a C N
bond to form a Ni-nitrene species,[11] which then effects H-
atom abstraction from an isopropyl iPr group to yield 7.
Thermal decay of a solution of 3 led to the isolation of crystals
of the previously reported[5] bis(hydroxido)dinickel(II) spe-
cies in 15% yield (Figure S20). The formation of such doubly
protonated reduced species has been previously observed in
reactions of basic bis(m-oxido)dimetal cores[12] with protons
and is consistent with the presence of nucleophilic oxido
ligands in 3. In contrast to the metastable properties of 2, 3
and 5, complex 4 is considerably more stable at 258C, and no
significant decay was observed at this temperature over
a period of 24 hours.
DFT calculations of 1–5 provided further insight into the
different reactivity patterns of the homo and heterodinuclear
bis(m-oxido) cores. The geometry and the ground spin state of
the complexes 1–4 are nicely reproduced in the calculation
(Tables S8, S9). A quintet (S = 2) ground-state is predicted for
5 akin to 4, which is consistent with the predicted short Ni–Fe
distance of 2.69 ꢀ and a folded NiO2Fe core (f = 166.78;
Table S2). Furthermore, the calculated charge densities (QO)
on the bridging O-atoms of 1–4 when plotted against the logk2
for the PhCOCl oxidation reactions also fall on a line very
similar to the linear correlation obtained from logk2 vs. lmax
and n(M-O) plots (Figure S12A). This correlation is however
not valid for electrophilic reactions. For example, the room
temperature stability of 4 contrasts with the facile decay of 5
by an electrophilic intramolecular ligand hydroxylation step
Figure 4. Nature of the HOMOs and LUMOs and their absolute
energies as obtained from DFT calculations on a series of [L1Ni(m-
O)2ML3] (M=Fe, Co, Ni and Cu) complexes. DE represents the
HOMO–LUMO energy gap.
4
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