Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

Article abstract of DOI:10.1021/acs.inorgchem.9b03175

A series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O2 activation by copper(I). The reaction of the copper(I) complex of N,N,N′,N′-tetramethypropylenediamine with a series of para-substituted nitrosobenzene deriva

Full text of DOI:10.1021/acs.inorgchem.9b03175

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Article
Tuning Inner-Sphere Electron Transfer in a Series of Copper/
Nitrosoarene Adducts
Mohammad S. Askari, Farshid Effaty, Federica Gennarini, Maylis Orio, Nicolas Le Poul,*
and Xavier Ottenwaelder*
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ABSTRACT: A series of copper/nitrosoarene complexes was
created that mimics several steps in biomimetic O2 activation by
copper(I). The reaction of the copper(I) complex of N,N,N′,N′-
tetramethypropylenediamine with a series of para-substituted
nitrosobenzene derivatives leads to adducts in which the nitro-
soarene (ArNO) is reduced by zero, one, or two electrons, akin to
the isovalent species dioxygen, superoxide, and peroxide, respec-
tively. The geometric and electronic structures of these adducts were
characterized by means of X-ray diffraction, vibrational analysis,
ultraviolet−visible spectroscopy, NMR, electrochemistry, and
density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the
ArNO moiety: κN for ArNO, mononuclear η2-NO and dinuclear μ-η2:η1 for ArNO•−, and dinuclear μ-η2:η2 for ArNO2−. 15N
isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm−1 for κN-ArNO, 1226 cm−1 for
η2-ArNO•−, 1133 cm−1 for dinuclear μ-η2:η1-ArNO•−, and 875 cm−1 for dinuclear μ-η2:η2 for ArNO2−). The 15N NMR signal
disappears for the ArNO•− species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction
waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O2 analogues.
DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer
transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks
through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides
snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion.
usually too oxidative to be stable above −60 °C. By contrast,
Cu/ArNO adducts have been shown to have geometric and
electronic structures very similar to those in Cu/O2 adducts
but were advantageously characterized at ambient temper-
ature.17,23−25
Owing to the asymmetric structure of ArNO in comparison
with that of O2, the structural variety of metal/ArNO
complexes exceeds that of metal/O2 compounds. Some of
the main bonding modes of ArNO to metal ions are shown in
Scheme 1,13,14 with the most common one being through the
N atom (κN). The other bonding modes are more prevalent
when the ArNO moiety is reduced to the mono- or dianion.
The NO bond length in metal/ArNO complexes depends on
the ArNO bonding mode, the nature and oxidation state of the
metal, and the structure of the supporting ligands, but alone is
insufficient to characterize the degree of reduction of the
INTRODUCTION
■
The interaction of nitrosoarenes (ArNO) with metal centers
has drawn much attention because of its relevance to biological
pathways1−7 and catalytic C−N bond formation processes.8−12
Chemists now have a good understanding of the geometric
structure of transition metal/nitrosoarene complexes.13,14 In
addition, ArNO species are redox-noninnocent ligands,15−17
which portends a large landscape of electronic structures and
reactivity types upon interaction with redox-active metal ions.
Because ArNO species are isovalent with O2, the reduction
of ArNO by a transition metal is akin to the reduction of O2 to
the superoxide ion (O2•−, 1e reduction) or the peroxide ion
(O22−, 2e reduction). Therefore, metal/ArNO adducts are
often regarded as surrogates for metal/O2 adducts. In
particular, and with relevance to the present paper, the
activation of O2 by Cu(I) centers is paramount in the
biological world. This process fuels enzymes such as
dopamine-β-hydroxylase, tyrosinase, and particulate methane
monooxygenase, to name but a few.18,19 This has inspired
numerous biomimetic studies in which an electron-rich Cu(I)
species is reacted with O2.20−22 Without protection of the
protein backbone, however, the ensuing Cu/O2 complexes are
Received: October 29, 2019
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arylnitrosyl radical anion) was confirmed by magnetic
measurements and vibrational and computational studies.17,37
(iii) Dinuclear μ-η1:η1 (end-on) complexes present varying
degrees of ArNO reduction: by 0e (NO = 1.257−1.32
Å),28,29,38−41 1e (1.33−1.35 Å),42 and 2e (1.37−1.49 Å;
Scheme 2c).43,44
Scheme 1. Some Bonding Modes in Metal/Nitrosoarene
Complexes, with Typical NO Bond Lengths13
(iv) In η2-NO complexes, the NO bond length (1.323−
1.432 Å)16,23,31,45−52 is significantly longer than that in free
nitrosoarenes. 1e reduction of the ArNO moiety has been
confirmed in Cu and Ni complexes (Scheme 2d).23,52 Further
reduction of the Ni complex led to a doubly reduced PhNO2−
moiety.52 2e reduction of the ArNO moiety was also confirmed
in a square-planar Pd(II) species upon reaction of a Pd0 species
with TolNO.16
ArNO moiety, as was already shown with metal/O2 adducts.26
A few studies have scrutinized the electronic structure on
metal/ArNO complexes, particularly the oxidation state of the
ArNO moiety, by means of techniques such as X-ray
absorption spectroscopy or vibrational analysis with isotopic
labeling (Scheme 2 for group 10 and 11 complexes). Their
main conclusions are the following:
(v) Alongside several ArNO2− examples (N−O = 1.40−1.53
Å),31,53−58 dinuclear μ-η2:η1 complexes have been found in the
solid-state structures of Cu complexes with shorter NO bond
lengths (1.322−1.375 Å).23,37 Typically, the 1e-reduced
ArNO•− moiety binds η2 to a Cu(II) center and η1 to a
Cu(I) center (Scheme 2e). These species are thought to be in
equilibrium with the mononuclear form [CuII(η2-ArNO•−)] in
solution.23,37
(i) In the majority of mononuclear κN nitrosoarene
complexes, the NO bond length, 1.209−1.31 Å, shows little
or no elongation compared with that in free nitrosoar-
enes,13,14,27−29 unless back-bonding from the metal becomes
significant.16 A radical character of the κN-ArNO moiety, and
thus formally a 1.5 bond order, has been confirmed or inferred
in a few species (Scheme 2a).16,30,31
(vi) Dinuclear μ-η2:η2 complexes are quite rare, and only a
few examples with Rh,57 Zr,59 Hf,59 Ni,23 and Cu25 are
reported in the literature. With a NO bond length in the range
of single bonds (1.422−1.500 Å), these complexes possess a
doubly reduced ArNO2− moiety. In the case of the Cu complex
(Scheme 2f), this 2e reduction was made possible by using a
very electron-poor nitrosoarene bearing a p-NO2 substituent.
More on this complex will be discussed below.
(ii) Most mononuclear κO complexes with non-d10
transition metals are structurally disordered,13,14,32−36 and
conclusive statements about the extent of back-donation and
ArNO reduction cannot be made. By contrast, the non-
disordered crystal structures of [(Me6tren)Cu(κO-PhNO)]X
(X = TfO−, SbF6 ; Scheme 2b) show significant NO bond
−
elongation. The radical character of the PhNO moiety (to an
a
Scheme 2. Confirmed Examples of Group 10 and 11 Complexes in Which ArNO Gets Reduced by 1e or 2e upon Reaction
a
Asterisks indicate calculated values.
B
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To summarize, 1e reduction of the ArNO moiety is usually
indicated by NO bond lengths in the range 1.29−1.37 Å and
NO stretching frequencies in the range 1000−1300 cm−1
(Scheme 2). Reduction by 2e is revealed by NO bond lengths
of 1.36−1.46 Å and NO stretching frequencies below 950
cm−1. When no reduction occurs, the NO bonds are short
[1.261(4) and 1.268(4) Å for free PhNO] and the NO
stretching frequency is high (1506 cm−1 for free PhNO),
although these values can be modified significantly when back-
bonding is present.16 Last, 4e reduction of PhNO, with
complete NO bond cleavage, is possible with very electron-rich
metal complexes such as cobalt(I) β-diketiminate species.60
Noting that these examples comprise different supporting
ligands and metal ions, we aimed at providing a systematic
study of the degree of inner-sphere ArNO reduction by using a
single Cu(I) precursor. Thus, in the present study, we report
on adducts 3R (R = NMe2, H, Cl, Br, NO2) formed upon
intermolecular reaction of para-substituted nitrosobenzenes 2R
with the Cu(I) complex of N,N,N′,N′-tetramethyl-1,3-
propanediamine (TMPD), 1 (Scheme 3), for which analogous
Cu/O2 chemistry is known.61−63
a
Scheme 3. Formation of the 3R Adducts
Figure 1. ORTEP at 50% probability of 3NMe (one of two
2
independent molecules), 3H, 3H′, 3Cl, and 3NO , with relevant N−O
2
bond lengths. Uncoordinated TfO− anions (3NMe and 3NO ) and H
2
2
atoms were omitted for clarity.
The structure of 3NMe is that of a copper(I) arylnitroso
2
complex, i.e., mere κN coordination of 1NMe onto the [TMPD-
2
CuI]+ complex 1. The N−O bond lengths in the two
independent molecules, 1.281 and 1.293 Å, are typical for
NO double bonds. The trigonal-planar ligand field of Cu is
consistent with a Cu(I) oxidation state. In the solid state, the
species dimerizes via two weak Cu···O interactions (2.242 and
2.278 Å) between two crystallographically related 3NMe
2
cations. This fact, coupled with the back-bonding of Cu into
the nitroso π* orbital, could explain the slight elongation of the
N−O bond compared with a true double bond.16,37
a
With a TfO− counterion. 3NO2 was already reported.25
Two types of crystals were obtained in the same
crystallization pot with R = H. The minor component, of
green color, is the mononuclear [TMPDCuII(η2-PhNO•−)-
(TfO)] species (3H). This complex displays an η2-NO
coordination with an elongated N−O bond of 1.327 Å,
consistent with a 1.5 bond order.23,52 Cu sits in a square-
pyramidal environment with a TfO− anion as a weak axial
ligand (Cu···O = 2.345 Å). The brown major component, 3H′,
of formula [TMPDCuII(μ-η2:η1-PhNO•−)(μ-TfO)-
CuITMPD](TfO) also displays an elongated N−O bond
1.345 Å, consistent with a 1.5 bond order. One of the Cu
centers is bonded to both N and O of the PhNO moiety (Cu−
N = 2.019 Å; Cu−O = 1.853 Å), while the other is only
bonded to the N atom of PhNO (Cu−N = 1.904 Å; Cu···O =
RESULTS AND DISCUSSION
■
Synthesis and Crystallography. The slow addition of a
[(MeCN)4Cu](TfO) (MeCN = acetonitrile) solution to a
solution of TMPD and nitrosoarene, in a 1:1:1 ratio in
tetrahydrofuran (THF) at 25 °C (2:2:1 for R = NO2), resulted
in the formation of deeply colored complexes that remained
stable under inert conditions. Crystallization of the complexes
by slow diffusion of pentane into the reaction mixtures at −30
°C afforded crystals suitable for X-ray diffraction analysis.
Several binding motifs consisting of mono- and dinuclear
complexes were obtained depending on the para substituent of
the nitrosoarene (Figure 1 and Table S1).
C
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a
2.828 Å). Species 3H′ is therefore well described as formed
from the association of the minor green component 3H with
one molecule of 1 (Scheme 4). Such architecture and
Table 1. NO Stretching Frequencies
species
ν (Δ)/cm−1
species
ν (Δ)/cm−1
2NMe
1365 (12), 1340 (19)
1388 (27)
1286 (4), 1256 (24)
1238 (20)
3NMe
1392 (14), 1315 (6)
1162 (10), 1133 (23)
1226 (6)
2
2
b
c
2H
3H′
Scheme 4. Mononuclear/Dinuclear Equilibrium in 3H
d
2Br
3Br
a
Species
2 d
2NO
3NO
875 (15)
2
a
Measured at 25 °C on species labeled with 14N and 15N on the NO
b
moiety. Full data are given in the Supporting Information. Syn
c
ArN(O)N(O)Ar dimer. Contains a small amount of mononuclear
d
species 3H. Anti ArN(O)N(O)Ar dimer.
transfer from the Cu center(s). For mixed-valent dinuclear
species 3H′, the NO stretch is lowered from 3Br by about 70−
100 cm−1, consistent with the electron density being
delocalized onto the additional Cu(I) center.
a
TfO− anions are not shown.
NMR Properties. In CDCl3, CD2Cl2, or acetone-d6
solutions, all 3R species display diamagnetic 1H and 13C
NMR spectra (Figures S33−S43). For 3H, 3Cl, and 3Br, this
indicates a singlet ground state, as was observed for similar η2-
ArNO complexes.23,24,37 By analogy with structurally similar
η2-superoxocopper(II) species, this ground-state singlet is
expected to be highly delocalized.64 This situation also
contrasts with the end-on topology, where end-on
superoxocopper(II) complexes have a S = 1 ground
state,65−68 as do CuII(κO-ArNO•−) complexes when Cu−
O−N−CAr is coplanar.17
association were already described in the literature.23,37
Because the mononuclear complex prevails in solution (the
solution is green, and a Job titration confirms a 1:1
stoichiometry; Figure S14), formation of the dinuclear
compound is an artifact of crystallization.
For 3Cl, only green crystals of [TMPDCuII(η2-2Cl•−)(TfO)]
were formed. The molecular structure is very similar to that of
3H, with a 1.5 N−O bond order (1.3315 Å), except that the
TfO− anion and the aromatic ring are on the same side of the
CuNO plane.
The 15N NMR spectra of the 15N-labeled 3R species are
most informative on the degree of electron transfer (Figure 2
The complex with the most electron-poor ArNO moiety,
25
3NO , was characterized in a previous communication. It is a
2
dinuclear species, [TMPDCuII(μ-η2:η2-PhNO2−)(μ-TfO)-
CuIITMPD](TfO), where 2NO is reduced by 2e (N−O =
2
1.456 Å) and both Cu centers are in the 2+ oxidation state.
Overall, the crystallographic study concludes on an increased
degree of electron transfer from 1 to 2R inasmuch as the p-R
H
substituent is made more electron-poor: 0e in 3NMe , 1e in 3 /
2
3H′ and 3Cl, and 2e in 3NO . The lability of Cu complements
2
the self-assembly process by allowing TfO− or extra Cu(I)
coordination when necessary.
IR Properties. Vibrational analysis by IR spectroscopy was
conducted on 2R precursors and 3R complexes, where the N
atom of the nitroso moiety is either 14N or 15N. Synthesis of
the 15N-labeled 2R precursors is provided in the Supporting
Information. Isotopic labeling enables one to isolate the
vibrations near the nitroso moiety from the rest of the
molecule. In parallel, density functional theory (DFT)
calculations were conducted to identify the nature of the
modes observed (especially NO versus CN stretches in the
ArNO moiety).
Comparing the IR properties of the organic precursors 2R is
Figure 2. 15N NMR data (50.7 MHz) of the 3R species (R = NMe2,
H, Br, NO2), 15N-enriched at the NO position, in CDCl3 at 25 °C.
Species 3H was made in situ by mixing equimolar amounts of TMPD,
[Cu(MeCN)4](TfO), and 2H.
tentative because they have different structures in the solid
H
state: monomeric for 2NMe , syn dimeric for 2 , and anti
2
dimeric for 2Br (Tables 1 and S2 and Figures S1−S6). Still, the
correlation between the experimental and calculated spectra is
excellent, providing confidence that the calculations can enable
us to locate the NO stretch accurately.
and Table 2). For comparison, the 15N NMR spectra of the
15N-labeled 2R species reveal a logical downfield shift of the
signal inasmuch as the R substituent becomes more electron-
Drastic changes in the NO stretching frequency are seen in
3R complexes, consistent with NO bond weakening upon
electron transfer (Tables 1 and S3 and Figures S7−S12). While
the symmetry of the complexes is different and some
complexes have multiple vibrational modes involving the NO
stretch, the NO stretching energy decreases from 1315/1392
poor. Cu(I) coordination on 2NMe to form 3NMe leads to an
upfield shift of the signal by 119 ppm, consistent with the
presence of a partial charge transfer from Cu(I) to the ArNO
2
2
moiety. On the other end of the series, the formation of 3NO
2
2 25
cm−1 for 3NMe to 1226 cm−1 for 3Br to 875 cm−1 for 3NO
.
leads to a dramatic upfield shift of the signal by 550 ppm,
consistent with the ArNO moiety being doubly reduced
(ArNO2−) and therefore quite electron-rich. Interestingly, no
2
This trend, supported by DFT calculations, is fully consistent
with reduction of the bond order upon inner-sphere electron
D
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a
Table 2. 15N NMR Data
the X-ray crystallographic data (Figure S13). The calculated
H
NO bond lengths are 1.243 Å for 3NMe , 1.287−1.288 Å for 3 ,
2
species
δ(15N)/ppm
species
δ(15N)/ppm
3Cl, and 3Br, 1.313 Å for 3H′, and 1.405 Å for 3NO . While these
2
2NMe2
787.58
885.83
878.67
913.23
3NMe2
668.58
values are all underestimated (up to 0.05 Å), they lie within the
typical error range of DFT and provide a fair trend along the
series, being thus informative on the redox state of the ArNO
moiety. The DFT-optimized structures are very close to those
observed experimentally, with root-mean-square deviations of
0.543, 0.427, 0.407, 0.419, 0.444, and 0.338 from the crystal
2H
3H
not observed
not observed
363.67
2Br
2NO2
3Br
3NO2
a
Measured in CDCl3 at 25 °C on a 500 MHz instrument; ν(15N) =
50.7 MHz.
H
Cl
Br
H
NO2
molecular structures of 3NMe , 3 , 3 , 3 , 3 ′, and 3
,
2
15N signal was observed for 3H and 3Br under the same
recording conditions or with a wider acquisition window. This
behavior is consistent with the radical character of the ArNO•−
moiety in these species. A small amount of triplet character
admixture in the ground-state singlet at room temperature
could lead to a paramagnetic shift of the 15N NMR resonance
outside the acquisition window (Fermi contact at 15N).69
Hence, the lack of a signal in a standard acquisition window
can be used as a local diagnostic of radical character on N.
Overall, the NMR data confirm, in solution, the assignments
that were made in the solid state.
respectively.
Time-dependent DFT (TD-DFT) calculations were per-
formed on the 3R complexes, and the predicted spectroscopic
data provide calculated spectra that compare well with the
experimental observations (Tables S4−S8 and Figures S15−
S20). Our computations support that the UV−vis spectra of
3H, 3Cl, 3Br, and 3NO are similar and dominated by two main
2
absorption bands of different intensities, while that of 3NMe
2
displays two intense electronic transitions. For the latter, the
band at 538 nm is assigned to a metal-to-ligand charge transfer
(MLCT), and the band at 426 nm is attributed to a ligand-to-
ligand charge transfer (LLCT). For both transitions, the
acceptor states mainly involve the nitroso moiety (Figure S16).
Ultraviolet−Visible (UV−Vis) Absorption Properties.
The electronic structure of the complexes was probed by UV−
vis absorption spectroscopy (Figure 3 and Table 3).
The electronic transitions for 3NO were already analyzed.25
2
The 345 and 644 nm bands correspond to MLCT transitions
involving the μ-η2:η2-NO2− moiety, in a very similar manner to
the transitions in the (μ-η2:η2-O22−)CuII2 cores that mimic the
active sites of oxytyrosinase and oxyhemocyanin.22 For 3H, 3Cl,
and 3Br, the absorptions near 320 nm are due to a combination
of MLCT and LLCT, with the acceptor state involving the NO
moiety, while the transitions in the visible around 610 nm
display a mixed character with similar contributions from the
metal and the nitroso moiety in both donor and acceptor states
(Figures S17−S19). Our TD-DFT calculations thus adequately
reproduce the energy of the key features of the experimental
spectra, which further support the geometries and electronic
properties of the 3R complexes. Vibrational analysis also
confirmed the experimental observations (vide supra).
Figure 3. UV−vis spectra of 3R species in CH2Cl2 at 25 °C.
a
Table 3. UV−Vis Data of 3R Complexes
Consequently, 3NMe can be described as a Cu(I) complex,
2
while 3H, 3Cl, and 3Br are CuII-(ArNO•−) species. The dimer
b
species
λmax (ε)
3H′ is assigned to CuII−CuI-(ArNO•−) species, while 3NO was
2
3NMe
426 (32.8)
538 (34.7)
690 (2.3)
609 (1.9)
614 (1.1)
614 (1.2)
644 (1.5)
2
previously shown to be a Cu(II)−Cu(II) complex with a
3H
3Cl
3Br
313 (8.0), 338 (7.4)
319 (10.2), 350 (7.3)
325 (10.4), 350 (8.2)
345 (19.4)
440 (sh)
440 (sh)
440 (sh)
440 (sh)
ArNO2− moiety (2e-reduced ArNO).
Electrochemical Studies. Because this work aims at
tuning the redox properties by simple substitution, we studied
the electrochemical behavior of both precursors 2R and
complexes 3R for the whole series of R substituents (NMe2,
H, Cl, Br, and NO2). The goal was to correlate the
electrochemical properties with the reactivity (0e, 1e, or 2e
transfer) observed upon reaction with the [(TMPD)CuI]+
complex 1 and the H-atom-transfer (HAT) reactivity of the 3R
complexes (see below). The data were also compared to
existing records for analogous ArNO and O2 complexes. Cyclic
voltammetry (CV) studies were performed at a glassy-carbon
working electrode in dry CH2Cl2 with 0.1 M NBu4OTf as the
supporting electrolyte. In what follows, all potentials are
referenced to the Fc+/0 couple.
3NO
2
a
b
Measured in CH2Cl2 at 25 °C. λmax/nm (ε/mM−1 cm−1).
Complexes 3H, 3Cl, and 3Br display sensibly the same UV−
vis spectrum, with an intense band around 330 nm and a less
intense feature around 610 nm. Compound 3NO exhibits the
2
same spectral shape, but the 345 nm band is twice as intense.
The spectrum for complex 3NMe is very different from the
2
other four spectra. It shows two very intense bands at 426 and
538 nm, while the weaker feature is red-shifted to 690 nm.
These absorptions will be analyzed in the next section.
DFT Calculations. DFT calculations have been undertaken
on the 3R complexes to gain insight into the nature of the
species observed experimentally and to correlate their
electronic structures to the experimental data. The structures
of the 3R species were subjected to geometry optimization, and
their electronic properties were investigated. A good agreement
is found upon a comparison of the molecular geometries with
Substituted Nitrosoarenes, 2R. CV studies of the free
nitrosoarenes, 2R, led to the summary in Scheme 5. 2H was first
studied for a comparison with the literature.70−72 When
scanned negatively, it displays two reversible responses at E1A/2
= −1.40 V (process A) and EB1/2 = −1.86 V versus Fc+/0
(process B) (Figure S21 and Table 4). An irreversible
E
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Scheme 5. Reduction Steps of 2R Species
a
Table 4. Electrochemical Data of the 2R Nitrosoarenes
b
species
EA1/2(2R)
EB1/2(2R)
σpara
c
2NMe
−1.69 (120)
−1.40 (90)
−1.32 (110)
−1.30 (90)
−0.93 (90)
d
−0.83
0
0.227
0.232
0.78
2
2H
2Cl
2Br
−1.86 (100)
−1.79 (90)
−1.75 (90)
−1.33 (100)
e
2NO
2
a
In CH2Cl2 containing 0.1 M NBu4OTf at 25 °C (glassy-carbon
working electrode); scan rate v = 0.1 V s−1, E/V versus Fc+/0 (ΔEp/
b
c
d
mV). σpara Hammett parameters. Determined at v = 0.5 V s−1. Not
e
determined. An intermediate wave at −1.17 V was observed at faster
scan rates.
oxidation peak is also detected at 0.63 V on the backscan after
reduction at −1.90 V (Figure S22). Variation of the scan rate v
induces a significant modification of the redox behavior
(Figure S23), which is typical of two successive electron-
transfer processes coupled to a chemical reaction, namely, an
ECE mechanism (E = electrochemical and C = chemical;
Scheme 5). In agreement with previous electrochemical
studies,70,71 process A corresponds to the monoelectronic
reduction of 2H (Scheme 5), while process B corresponds to 1e
reduction of the azoxybenzene formed in situ by reaction of
the radical anion with residual water. This dimerization
hypothesis is supported by the ratio of the cathodic peak
currents, iBpc/ipAc ≈ 0.5 (assuming similar diffusion coefficients).
In addition, the value of EB1/2 is in good agreement with the
standard potential values for the azoxy species in organic
solvents.70−72
Figure 4. CV cycles at a glassy-carbon working electrode (E/V vs
Fc+/0; v = 0.1 V s−1) of 3R (1.0 mM) in dry CH2Cl2 and 0.1 M
NBu4OTf: (a) R = Br; (b) R = NO2.
a
Table 5. Electrochemical Data of 3R Complexes
b
ECpc(3R)
ED1/2(3R)
EE1/2(3R)
σpara
3NMe
3H
−1.05
−0.92
−0.82
−0.72
−1.75
−1.45
−1.22 (180)
−1.17 (145)
−0.36 (140)
0.28
0.08 (130)
0.11 (120)
−0.83
0.00
0.23
2
c
3Br
3NO
0.78
2
a
In CH2Cl2 containing 0.1 M NBu4OTf at 25 °C; scan rate v = 0.1 V
b
s
−1. E/V versus Fc+/0 (ΔEp/mV). σpara Hammett parameters.
Irreversible cathodic peak.
The processes described in Scheme 5 occur for 2R with
different para substituents (R = NMe2, H, Cl, Br, NO2) but at
different redox potentials (Figure S21 and Table 4). Under the
same experimental conditions, 2H, 2Cl, and 2Br display almost
the same redox pattern, i.e., one quasi-reversible redox system
at ca. EA1/A2 = −1.3 V and a second one at ca. EB1/2 = −1.8 V. For
c
controlled by electronic effects, which is confirmed by the
linear variation of EpCc versus σpara Hammett parameters (Figure
S29). Whatever the nature of R, the first system remains
irreversible at moderate scan rates (v < 10 V s−1;Figures 4 and
S26 and S27a). This is indicative that a fast chemical reaction
occurs upon electrochemical reduction. This EC mechanism
was confirmed, for R = NO2, by the linear behavior of EpCc
versus log v (33 mV decade−1; Figure S27b) and the constancy
of the normalized current intensity (iCpcv−1/2) with v (inset
Figure S27a), hence excluding an ECE process.
2NMe , E1/2 is shifted negatively by ca. 300 mV with respect to
2
EA1/2 (2H), and the (2NMe
)
•− radical anion is relatively unstable,
2
as seen by the irreversibility of process A for v < 0.2 V s−1
A
(Figure S21a). For 2NO , E1/2 and E1B/2 are shifted positively by
2
ca. 500 mV (Figure S21b).
Thus, a span of +760 mV is observed for EA1/2 upon NMe2/
NO2 substitution, consistent with the electron-donating/
withdrawing properties of the substituents. Fittingly, plots of
EA1/2 versus the σpara Hammett parameter follow a linear trend,
indicating that the value of the redox potential is mainly
controlled by electronic effects (Figure S29).
CV scanning until −1.8 V leads to the appearance of a
second system (process D) at ED (−1.75 V < ED < −1.21 V),
which is quasi-reversible or irreversible, depending on R
(Figures 4 and S31). Increasing the scan rate induces a
decrease of the relative peak currents iCpc and ipDc (Figure S27c,d
[Cu(TMPD)(ArNO)](OTf) Complexes 3R. CV studies of the
3R complexes (R = NMe2, H, Br, NO2) were performed under
the same experimental conditions as those for 2R ligands
(Figure 4 and Table 5). Adding 2R to a solution of 1 under CV
monitoring led to the same conclusions as those below (Figure
for 3NO ), without modification of the peak potential values.
2
Altogether, this data set confirms that the chemical species that
is reduced reversibly through a simple electron transfer at ED
originates from the first electrochemical reduction of the 3R
complex. As shown in Table 5, the potential value at ED is
highly dependent on the substituting group R, meaning that
the chemical species or complex contains the ArNO moiety.
Possibly, reduction of the complex induces breaking of the
S25). All 3R complexes display a first irreversible reduction
C
peak Epc (process C) extending from −1.05 V for 3NMe to
2
−0.72 V for 3NO (Figure 4 and Table 5). As was the case with
2
process A for the 2R ligands, the redox potential is mainly
F
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Cu−ArNO bond, liberating 2R and explaining the similarity of
the ED and EA values. Such a hypothesis is verified in all cases
Table 6. Electrochemical Data of 3R and Related Nin/ArNO
and Cun/O2 Complexes
except for 3NO (Figures S28 and S30).
2
Exhaustive electrolyses at ECpc and coulometric measurements
confirm that system C is a 1e process per mole of 3R. For
example, electrochemical reduction of 3NMe leads to its
2
disappearance, while a new wave appears at ED1/2 (Figure
S31), together with a significant color change of the solution
(purple to orange). A new system also appears in oxidation at
−0.2 , +0.45, and +0.65 V, suggesting a release of TMPD
(Figure S25a).
In a general manner, 1e reduction of the 3R complexes is
accompanied by fast chemical processes that lead to partial
decomplexation and release of the TMPD ligand and/or 2R.
The transient reduced species may thus be implicated in
several reactions: radical dimerization and simple decoordina-
tion, which themselves seem dependent on R.
On the oxidation side, a quasi-reversible system (process E)
is detected at EE1/2 (Table 5), with varying peak potential and
intensity values as R is varied. EE1/2 is in the same range as that
reported by Warren et al. (Epa = +0.48 V in MeCN) for a
similar side-on arylnitrosylcopper(II) complex with a diketi-
minate ligand.23
The reduction data obtained for the 3R complexes can be
compared with the few redox processes reported for Nin/
ArNO and Cun/O2 analogues (Table 6). The side-on
arylnitrosyl 3H species (entry 1) gets reduced at a potential
similar to that of Warren’s side-on arylnitrosylnickel(II)
complex.52 When ArNO binds in a 1,2-fashion (end-on)
between two Ni(II) centers, the potential for ArNO2−/•−
conversion is decreased by ca. 650 mV (entry 3).44 Comparing
ArNO with O2 complexes would be interesting, but so far there
is no reported redox data for monocopper superoxo species
that would be similar in structure to 3H. The exception is the
recent work by Reinaud et al., which showed by spectroelec-
trochemistry that an in situ generated calix[6]amino-tren end-
on superoxo complex could not be reduced above −0.90 V
versus Fc at −60 °C (113 K) in acetone.73 On the other hand,
a few dicopper peroxo and superoxo species have been well
characterized by electrochemistry with the help of low-
temperature approaches (entries 4−7). Here, the irreversible
1e reduction of 3R is comparable to the monoelectronic and
reversible electron-exchange reactions detected for the end-on
superoxo/peroxo pyrazolate- and xylO-based complexes
(entries 4 and 5).74,75 Interestingly, the reduction potential
a
The reaction is written in a way it was carried out either as an
oxidation or as a reduction. Abbreviations: n/c, compound not
characterized; P, cis-end-on peroxo; CS, cis-end-on peroxo; P, side-
C
S
b
c
on peroxo; O, bis(μ-oxo). Potential versus Fc+/0
. Irreversible
d
e
f
cathodic peak. Reversible reduction. Reversible oxidation. Con-
verted from the value versus saturated calomel electrode (SCE) using
E
1/2(Fc+/0) = 560 mV versus SCE in these conditions.
of 3NO (entry 8) is close to that of Kodera’s side-on
2
Scheme 6. Thermodynamic Analysis of HAT Reactivity
peroxodicopper(II) species (entry 7), although the latter is a
2e process.76 Overall, using such comparisons to make
educated assignments of the electrochemical processes remains
tentative given the large variety of ligands, charge, nuclearity,
and bonding topology of the reported complexes. While data
for μ-hydroxodicopper complexes that are reminiscent of Cun/
O2 species is readily available,77−82 comparisons with the 3R
complexes would be even more tentative.
HAT Reactivity. We evaluated the reactivity of 3Cl, 3Br, and
3NO for HAT reactivity (Scheme 6). In previous work, Warren
2
et al. reported a NiII-(η2-ArNO•−) that converts into the
related NiII-(η2-ArN(H)O) complex (monoprotonated hy-
droxylamine) upon reaction with 9,10-dihydroanthracene
(DHA).52 Conversely, Meyer et al. reported a dinuclear
NiII -(μ-η1:η1-ArN(H)O) species that released its H atom to a
mol−1].44 Following these examples, we reacted 3Cl, 3Br, and
2
phenoxyl radical to form the related NiII -(μ-η1:η1-ArNO•−)
3NO with DHA (bond dissociation free energy BDFE = 76.0
2
2
species [bond dissociation energy BDE(N−H) ≈ 62−65 kcal
mol−1)83 under UV−vis monitoring. A significant decrease of
G
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91
(2NO
)
were prepared via literature procedures. Isotopically labeled
the 3R spectrum was observed upon the addition of 40 mol
equiv of DHA in THF at 45 °C, which was corrected for self-
decomposition of the 3R complexes at this temperature. By
analogy with the above examples, we presume that the reaction
yielded complexes of ArN(H)O, labeled 3R-H hereafter
(Scheme 6), but their instability prevented further analysis of
the reaction and its mechanism.
2
15N-4-nitrosonitrobenzene and [(TMPDCu)2(μ-TfO)(μ-η2:η2-p-
NO2-C6H4NO)](TfO) were prepared following the procedure
reported earlier.25 4-Bromoaniline-15N was prepared from 15N-
acetamide as reported in the Supporting Information. 15N derivatives
of 3H and 3Br were prepared similarly to the 14N samples (see the
Supporting Information).90
Characterization Methods. NMR spectra were recorded on a
Varian Innova-500 MHz instrument. All spectra were recorded in
CDCl3 unless otherwise noted. 1H and 13C NMR spectra were
referenced to internal tetramethylsilane. For 3R species, the signal for
the TfO− anion is not reported; it is observed at 119.5 ppm in
concentrated samples. 15N NMR spectra were referenced to external
formamide in dimethyl sulfoxide. IR spectra were recorded on a
Nicolet iS5 (Thermo Scientific) attenuated-total-reflectance instru-
ment. UV−vis spectra were recorded on an Agilent 8453
spectrophotometer or a B&W Tek i-Trometer. Elemental analysis
The electron-withdrawing NO2 group induces a faster
oxidation of DHA, consistent with 3NO being a stronger
2
oxidant (higher E1C/2) than 3Br and 3Cl. The initial rates of
reaction depend on the R substituent: 0.029, 0.021, and 0.051
0.005 mM min−1 for 3Cl, 3Br, and 3NO , respectively. Using
2
eq 1 (Scheme 6)83 with a temperature correction, the value of
EC1/2 for 3R taken as ECpc, and using C = 66 kcal mol−1 in THF,84
the pKA value of the N−H bond in 3R-H is evaluated around
18 and 19.5 for the NO2 and Br adducts, respectively, in order
́
́
was performed by the Laboratoire d’Analyze Elementaire de
to perform HAT from DHA. Similarly, 2NO reacts, but slowly,
2
́
́
l’Universite de Montreal. The presence of F atoms in the samples
interfered with the normal integration peak for H atoms. The value
for H is not necessarily trustworthy.
with 1,2-diphenylhydrazine (BDFE = 67.1 kcal mol−1) at 25
°C. This brings the pKA value to around 13, but this reaction is
complicated by the byproduct azobenzene, which can interact
X-ray Crystallography. Crystallographic analysis was performed
on a Bruker APEX-DUO diffractometer. The frames were integrated
with the Bruker SAINT software package using a narrow-frame
algorithm. Data were corrected for absorption effects using the
multiscan method (SADABS or TWINABS). The structures were
solved by direct methods and refined using the APEX3 software
package. All non-H atoms were refined with anisotropic thermal
parameters. H atoms were generated in idealized positions, riding on
the carrier atoms with isotropic thermal parameters.
Electrochemistry. Room temperature electrochemical studies of
the nitrosoarene ligands and their copper complexes were performed
in a glovebox (Jacomex; O2 < 1 ppm and H2O < 1 ppm) with a home-
designed three-electrode cell (WE, glassy carbon or platinum; RE,
platinum wire in a Fc+/Fc solution; CE, platinum or graphite rod).
Ferrocene was added at the end of the experiments to determine the
redox potential values. The potential of the cell was controlled by an
AUTOLAB PGSTAT 100 (Metrohm) potentiostat monitored by the
NOVA 1.11 software. Dichloromethane (Acros) was distilled over
CaH2 under an inert atmosphere and stored in a glovebox. The
supporting salt NBu4PF6 was synthesized from NBu4OH (Acros) and
HPF6 (Aldrich). It was then purified, dried under vacuum for 48 h at
100 °C, and then kept under argon in the glovebox. NBu4OTf
(Aldrich, 99%) was stored as received in the glovebox. Electrolytic
solutions were prepared in the glovebox and dried for a few days over
molecular sieves (3 Å) to remove traces of water before use.
Computational Details. All theoretical calculations were
performed with the ORCA program package.92 Full geometry
optimizations were carried out for all complexes using the generalized
gradient approximation functional BP8693−95 in combination with the
TZV/P96 basis set for all atoms and by taking advantage of the
resolution of the identity (RI) approximation in the Split-RI-J
variant97 with the appropriate Coulomb fitting sets.98 Increased
integration grids (Grid4 in the ORCA convention) and tight self-
consistent-field convergence criteria were used. IR spectra were
obtained from numerical frequency calculations performed on DFT-
optimized structures. Isotope shift effects (14N/15N) were taken into
account using the orca_vib utility program, and vibrational normal
modes were visualized with Chemcraft99 software. Solvent effects were
accounted for according to the experimental conditions. For that
purpose, we used the CH2Cl2 (e = 9.08) solvent within the framework
of the conductor-like screening (COSMO) dielectric continuum
approach.100 Single-point optical properties were predicted from
additional single-point calculations using the same functional/basis set
as that employed previously. Electronic transition energies and dipole
moments for all models were calculated using TD-DFT101−103 within
the Tamm−Dancoff approximation.104,105 To increase the computa-
tional efficiency, the RI approximation106 was used to calculate the
Coulomb term. At least 40 excited states were calculated in each case,
and difference transition density plots were generated for each
with Cu(I) and dissociate 2NO . Further studies with different
2
substrates are necessary to decipher how nitrosarene
complexes perform this reaction, i.e., in a concerted or
sequential manner.85,86
CONCLUSIONS
■
In summary, placing a synthetic handle at the para position of
nitrosoarenes enables control over the degree of electron
transfer from Cu(I) complexes, from 0e with electron-donating
substituents to 1e with electron-neutral substituents and 2e
with electron-poor substituents. As the Cu/ArNO adducts are
undergoing self-assembly, the geometric preferences of the Cu
center will prevail.37 Thus, Cu(I) will be found in trigonal
geometries, with κN-ArNO coordination, whereas a square-
pyramidal Cu(II) will force η2-ArNO•−/2− coordination. One
of the novel features of this work is the use of 15N NMR as a
direct, local probe for the redox level of the ArNO moiety.
Thus, the absence of a 15N NMR signal coincides with the
radical state. A side effect of the self-assembly is, however, the
relative instability of the adducts upon external electron-
transfer events. Notwithstanding, this series of complexes
provides structural snapshots of the isovalent Cu/O2
chemistry, without the complication of thermal sensitivity of
Cu/O2 species. It also enables redox studies to be performed,
although much remains to be done before a proper ArNO/O2
redox benchmark can be established. This series also highlights
the variety of intermediates that could occur during Cu-
catalyzed ArNO transformations and suggests that, perhaps,
bond-forming events from ArNO precursors may proceed via
radical states.
EXPERIMENTAL SECTION
■
Materials. Chemicals were obtained from Sigma-Aldrich and Alfa
Aesar, except acetanilide-15N, which was purchased from Cambridge
Isotope Laboratories. Air-sensitive samples were handled under an
inert atmosphere (N2) in a dry nitrogen glovebox (O2 < 0.1 ppm;
H2O < 0.1 ppm) or using standard Schlenk techniques. Solvents were
dried by standard procedures, degassed, and stored over 4 Å
molecular sieves in the glovebox. N,N,N′,N′-Tetramethyl-1,3-
propanediamine (TMPD) was distilled over CaH2 under nitrogen
and stored in the glovebox. The copper salt [(MeCN)4Cu](TfO) was
prepared by adapting the Kubas procedure using TfOH.87 4-
88,89
Dimethylaminonitrosobenzene (2NMe ),
4-chloronitrosobenzene
2
(2Cl),90 4-bromosonitrobenzene (2Br),90 and 4-nitrosonitrobenzene
H
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Article
transition. For each transition, difference density plots were generated
using the ORCA plot utility program and visualized with the
Chemcraft program. The same procedure was also employed to
generate and visualize spin-density plots as well as molecular orbitals.
Synthetic Procedures. General Procedure for the Synthesis of
3R Complexes (R = NMe2, H, Cl, Br).25 To a stirring solution of
TMPD (0.28 mmol, 1.1 equiv) and the corresponding nitrosobenzene
2R (0.27 mmol, 1.05 equiv) in 5 mL of THF was added dropwise at
25 °C a solution of [(MeCN)4Cu](TfO) (0.26 mmol, 1 equiv) in 2
mL of THF. The solution was stirred for 15 min and then cooled to
−30 °C. Dropwise addition of the solution to 15 mL of swirling
pentane previously cooled to −30 °C resulted in the precipitation of a
solid. The solid was isolated, washed with diethyl ether and pentane,
and dried in vacuo (yields typically 70−85%). Crystals suitable for X-
ray structure determination were grown through the slow layered
diffusion of pentane into a concentrated solution of the complex in
THF at −30 °C.
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
́
Nicolas Le Poul − Laboratoire de Chimie, Electrochimie
́
Moleculaires et Chimie Analytique, UMR, CNRS 6521,
́
Universite de Bretagne Occidentale, Brest 29238, France;
orcid.org/0000-0002-5915-3760; Email: nicolas.lepoul@
univ-brest.fr
Xavier Ottenwaelder − Department of Chemistry and
Biochemistry, Concordia University, Montreal, Quebec H4B
1R6, Canada; orcid.org/0000-0003-4775-0303;
Email: dr.x@concordia.ca
[(TMPDCu)(TfO)(κN-p-NMe2-C6H4NO)](TfO) (3NMe ). Dark purple
2
1
solid. H NMR (500 MHz, CDCl3): δppm 1.76 (m, 2H), 2.51 (s,
Authors
12H), 2.84 (m, 4H), 3.22 (s, 6H), 6.81 (br, 2H), 9.09 (very br, 2H).
13C NMR (125 MHz, CDCl3): δppm 22.89, 40.80, 48.69, 61.75, 112.2,
122.05, 156.26, 158.23. 15N NMR (50.7 MHz, CDCl3): δppm 668.58
(NO moiety). Anal. Calcd for C16H28CuF3N4O4S: C, 38.98; H, 5.72;
N, 11.36; S, 6.50. Found: C, 37.86; H, 5.82; N, 11.13; S, 6.61.
[(TMPDCu)2(μ-TfO)(μ-η2:η1-PhNO)](TfO) (3H′). Brown solid. 1H
NMR (500 MHz, CDCl3): δppm 1.72 (br, 4H), 2.55 (br, 24H), 2.69
(br, 8H), 7.49 (t, 2H), 7.67 (t, 1H), 8.09 (d, 2H). 13C NMR (125
MHz, CDCl3): δppm 22.36, 48.59, 60.59, 120.97 (d upon 15N labeling,
J(13C−15N) = 3 Hz), 130.94 (d upon 15N labeling, J(13C−15N) = 2
Hz), 131.78, 160.93. Anal. Calcd for C22H41Cu2F6N5O7S2: C, 33.33;
H, 5.21; N, 8.83; S, 8.09. Found: C, 31.28; H, 5.35; N, 8.37; S, 8.19
(precision is lacking because this compound contains a minor
quantity of 3H in the solid state).
Mohammad S. Askari − Department of Chemistry and
Biochemistry, Concordia University, Montreal, Quebec H4B
1R6, Canada; orcid.org/0000-0002-1746-5141
Farshid Effaty − Department of Chemistry and Biochemistry,
Concordia University, Montreal, Quebec H4B 1R6, Canada;
orcid.org/0000-0002-2389-4903
Federica Gennarini − Department of Chemistry and
Biochemistry, Concordia University, Montreal, Quebec H4B
́
1R6, Canada; Laboratoire de Chimie, Electrochimie
́
Moleculaires et Chimie Analytique, UMR, CNRS 6521,
́
Universite de Bretagne Occidentale, Brest 29238, France;
orcid.org/0000-0001-5679-512X
[(TMPDCu)(TfO)(η2-PhNO)](TfO) (3H). Prepared in situ (green
́
Maylis Orio − Aix Marseille Universite, CNRS, Centrale
1
solution). H NMR (500 MHz, CDCl3): δppm 1.69 (br, 2H), 2.47
Marseille, iSm2, Marseille 13007, France; orcid.org/0000-
(12H), 2.63 (4H), 7.43 (t, 2H), 7.63 (t, 1H), 7.97 (d, 2H). 13C NMR
(125 MHz, CDCl3): δppm 20.37, 46.59, 59.24, 119.34 (d upon 15N
labeling, J(13C−15N) = 5 Hz), 127.97 (d upon 15N labeling,
J(13C−15N) = 2 Hz), 130.78, 160.67 (d upon 15N labeling,
J(13C−15N) = 6 Hz). 15N NMR (50.7 MHz, CDCl3): not observed.
[(TMPDCu)(TfO)(η2-p-ClC6H4NO)](TfO) (3Cl). Green solid. 1H
NMR (500 MHz, acetone-d6): δppm 2.42 (br, 2H), 3.06 (s, 12H),
3.45 (br, 4H), 7.77 (d, 2H), 7.97 (d, 2H). 13C NMR (125 MHz,
acetone-d6): δppm 20.17, 43.19, 55.50, 122.16, 126.10, 130.01, 166.47.
Anal. Calcd for C14H22ClCuF3N3O4S: C, 34.71; H, 4.58; N, 8.67; S,
6.62. Found: C, 34.27; H, 4.49; N, 8.16; S, 6.38.
0002-9317-8005
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03175
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
[(TMPDCu)(TfO)(η2-p-Br-C6H4NO)](TfO) (3Br). Green solid. 1H
NMR (500 MHz, CDCl3): δppm 1.74 (br, 2H), 2.65 (br, 12H),
2.78 (br, 4H), 7.54 (d, 2H), 7.91 (d, 1H). 13C NMR (125 MHz,
CDCl3): δppm 22.28, 48.52, 60.29, 122.81 (d upon 15N labeling,
J(13C−15N) = 5 Hz), 126.01, 133.55 (d upon 15N labeling,
J(13C−15N) = 2.5 Hz), 160.59 (d upon 15N labeling, J(13C−15N) =
5 Hz). 15N NMR (50.7 MHz, CDCl3): not observed. Anal. Calcd for
C14H22BrCuF3N3O4S: C, 31.80; H, 4.19; N, 7.95; S, 6.06. Found: C,
31.29; H, 4.49; N, 7.81; S, 6.37.
Financial support was provided by the Natural Sciences and
Engineering Council of Canada (Discovery Grant for X.O. and
graduate scholarship to M.S.A.) and the Centre de Chimie
Verte et Catalyze (Quebec). The authors are also thankful for
French financial support through Grant ANR-13-BSO7-0018
and thank the University of Bretagne Occidentale for a
mobility grant (F.G.).
Notes
H
H
Cl
X-ray data for 3NMe , 3 , 3 ′, and 3 are available as CCDC
2
The authors declare no competing financial interest.
1959040−1959043, respectively. Note that 3NO is CCDC 1029423.
2
ACKNOWLEDGMENTS
■
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03175.
■
We thank the Capobianco lab and Biofins platform
(Concordia) for help with IR spectral measurement and
analysis and Alexey Denisov (Concordia) for help with 15N
NMR.
sı
Experimental supplements, including a crystallography
table, a Job plot, 15N labeling, IR data, electrochemistry
supplements, DFT data, and NMR spectra (PDF)
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I
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