Effect of Monoelectronic Oxidation of an Unsymmetrical Phenoxido-Hydroxido Bridged Dicopper(II) Complex

Article abstract of DOI:10.1021/acs.inorgchem.8b02127

A (μ-hydroxido, μ-phenoxido)Cu^IICu^II complex 1 has been synthesized using an unsymmetrical ligand bearing an N,N-bis(2-pyridyl)methylamine (BPA) moiety coordinating one copper and a dianionic bis-amide moiety coordinating the other copper(II) ion. Electrochemical mono-oxidation of the complex in DMF occurs reversibly at 213 K at E_1/2 = 0.12 V vs Fc⁺/Fc through a metal-centered process. The resulting species (complex 1⁺) is only stable at low temperature and has been spectroscopically characterized by UV-vis-NIR cryo-spectroelectrochemical and EPR methods. DFT and TD-DFT calculations, consistent with experimental data, support the formation of a Cu^IICu^III phenoxido-hydroxido complex. Low-temperature chemical oxidation of 1 by NOSbF₆ yields a tetranuclear complex 2(SbF₆)(NO₂) which displays two binuclear Cu^IICu^II subunits. The X-ray crystal structure of 2(SbF₆)(NO₂) evidences that the nitrogen of the terminal amide group is protonated and the coordination of the amide occurs via the O atom. The bis-amide moiety appears to be a non-innocent proton acceptor along the redox process. Alternatively, protonation of complex 1 leads to the complex 2(ClO₄)₂, as evidenced by X-ray crystallography, cyclic voltammetry, and ¹H NMR.

Full text of DOI:10.1021/acs.inorgchem.8b02127

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Effect of Monoelectronic Oxidation of an Unsymmetrical Phenoxido-
Hydroxido Bridged Dicopper(II) Complex
Aurore Thibon-Pourret,*,† Federica Gennarini,‡ Rolf David,† James A. Isaac,† Isidoro Lopez,‡
†
†
‡
†
§
‡
̀
Gisele Gellon, Florian Molton, Laurianne Wojcik,† Christian Philouze, David Flot, Yves Le Mest,
∥
‡
†
́
́
̀
Marius Reglier, Nicolas Le Poul, Helene Jamet, and Catherine Belle
†
́
́
Universite Grenoble Alpes, CNRS, DCM, 38000 Grenoble, France
‡
Universite de Bretagne Occidentale, CNRS UMR 6521, Laboratoire CEMCA, 6 Avenue Le Gorgeu, CS 93837, 29238 Brest Cedex
3, France
§ESRF European Synchrotron 71, Avenue des Martyrs, 38000 Grenoble, France
∥Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille, France
S
* Supporting Information
ABSTRACT: A (μ-hydroxido, μ-phenoxido)CuIICuII complex
1 has been synthesized using an unsymmetrical ligand bearing
an N,N-bis(2-pyridyl)methylamine (BPA) moiety coordinating
one copper and a dianionic bis-amide moiety coordinating the
other copper(II) ion. Electrochemical mono-oxidation of the
complex in DMF occurs reversibly at 213 K at E1/2 = 0.12 V vs
Fc+/Fc through a metal-centered process. The resulting species
(complex 1+) is only stable at low temperature and has been
spectroscopically characterized by UV−vis−NIR cryo-spectroe-
lectrochemical and EPR methods. DFT and TD-DFT
calculations, consistent with experimental data, support the formation of a CuIICuIII phenoxido-hydroxido complex. Low-
temperature chemical oxidation of 1 by NOSbF6 yields a tetranuclear complex 2(SbF6)(NO2) which displays two binuclear
CuIICuII subunits. The X-ray crystal structure of 2(SbF6)(NO2) evidences that the nitrogen of the terminal amide group is
protonated and the coordination of the amide occurs via the O atom. The bis-amide moiety appears to be a non-innocent
proton acceptor along the redox process. Alternatively, protonation of complex 1 leads to the complex 2(ClO4)2, as evidenced
1
by X-ray crystallography, cyclic voltammetry, and H NMR.
have been reported,17−19 and investigations of their reactivity
INTRODUCTION
■
are limited despite that studies on such mixed-valent CuIICuIII
Over the past couple of years, there has been a growing interest
species are of considerable interest for industrial synthesis. The
toward the development of systems capable of stabilizing high-
search for new candidates for strong hydrocarbon functional-
valent copper species.1 Besides type III dinuclear copper
ization under mild conditions is required to produce useful
enzymes, e.g., tyrosinase yielding peroxo-bridged (μ-η2-η2)-
chemical products with low energy costs and high selectivity.20
CuIICuII species in the presence of dioxygen, which can be
converted into reactive bis(μ-oxo)CuIIICuIII intermediates,2−5
new types of species have been suggested. This is in relation to
Our objective was to characterize this type of mixed-valent
species starting from an unsymmetrical bimetallic copper(II)
advances in oxidation of alkanes by copper-loaded zeolites6 as
complex, designed with a moiety able to stabilize copper(III)
and featuring a short Cu−Cu separation. One of the most
well as in enzymatic systems such as lytic polysaccharide
monooxygenases (LPMOs)7,8 and particulate methane mono-
extensively employed compartmental ligands capable of
oxygenase (pMMO).9−11 So far for pMMO, the nature and
bringing two copper ions in close proximity is the family of
phenoxido-bridged ligands bearing two coordinating arms.
nuclearity of the intermediates involved in activation of the
Numerous CuIICuII-phenoxido bridged complexes have been
strong C−H bond of methane remain debated, moving
reported in the literature,21−24 but few studies have been
carried out on the mono-oxidation of such dicopper
complexes.25−27 Since this monoelectronic oxidation could
occur either on the ligand or on the metal ion, specific design is
necessary to control the location of the oxidation process.
recently from a dinuclear copper site9,11,12 toward a possible
mononuclear site.13,14 Nevertheless, from computational
analysis, Yoshizawa and co-workers have suggested that the
reactivity toward methane could arise from dicopper mixed-
valent species such as bis(μ-oxo)CuIICuIII or (μ-oxo)(μ-
hydroxo)CuIICuIII.12,15,16 Unlike mononuclear CuIII species
in the field of bioinorganic chemistry, descriptions of relevant
synthetic models are still largely unexplored. Few compounds
Received: July 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02127
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Instrument cryostat, and an ER4131VT Bruker temperature
controller. All spectra presented were recorded under nonsaturating
conditions. Simulation of EPR spectra was carried out with EasySpin
software.
UV−vis absorption spectra were obtained by using a Varian Cary
50 spectrophotometer operating in the 200−1000 nm range with
quartz cells. It was equipped with a Hellma low-temperature
immersion probe (1 cm path length quartz cell) for experiments at
low temperatures. The temperature was controlled with a Lauda RK8
KS cryostat.
Mono-oxidation processes yielding a phenoxyl radical in
accordance with a ligand-centered oxidation have been
reported.27 However, very recently, we have described an
interesting feature from a series of phenoxido-hydroxido-
bridged dicopper(II) species. Upon electrochemical mono-
oxidation, the unsymmetrical environment in the complex
bearing the ligand 2-[bis(2-pyridylethyl)-aminomethyl]-6-[bis-
(2-pyridylmethyl)-aminomethyl]-4-methylphenol (CH3-
BPMEP)28 (Chart 1) supported the formation of a transient
Electrospray mass spectra were recorded on an Esquire 3000 plus
Bruker Daltonics with nanospray inlet.
Chart 1. Unsymmetrical Dinucleating Ligands Used
Previously (CH3-BPMEP)25 and in This Work (H3L)
Room temperature electrochemical studies of the copper complex
were performed in a glovebox (Jacomex) (O2 < 1 ppm, H2O < 1
ppm) with a home-designed 3-electrodes cell (WE: glassy carbon
(GC) or platinum, RE: Pt wire in a Fc+/Fc solution, CE: Pt or
graphite rod). Ferrocene was added at the end of the experiments to
determine redox potential values. The potential of the cell was
controlled by an AUTOLAB PGSTAT 100 (Metrohm) potentiostat
monitored by the NOVA software. Dimethylformamide (DMF) was
purchased from Acros. The supporting salt NBu4ClO4 (99%) was
purchased from Sigma-Aldrich and stored as received in the glovebox.
Low-temperature voltammetry and thin-layer UV−vis−NIR
spectroelectrochemistry were performed with specific home-designed
cells (WE: Pt or GC, RE: Pt wire, CE: Pt wire) inside a glovebox.
Cooling down of the solution was operated with a Lauda Pro-line
cryostated system. Some low-temperature experiments were com-
pleted at a temperature close to the freezing point of DMF, but
because of the presence of high concentrations of electrolyte and
dissolved complex, the freezing point is lower compared to a pure
solution of DMF. The UV−vis and vis−NIR optic fiber probes were
purchased from Ocean Optics. Time-resolved UV−vis detection was
performed with QEPro and NIR-Quest spectrometers (Ocean
Optics).
Syntheses. Synthesis of 3-(Chloromethyl)-2-hydroxy-5-methyl-
salicylic Acid. 5-Methylsalicylic acid (5 g, 0.033 mol) was suspended
in concentrated HCl (37%) (55 mL), and a solution of formaldehyde
(37%) (5 mL) was added. The mixture was stirred at reflux during 2 h
30 min. The resulting suspension was diluted in water and filtered
through a porous frit. The white solid was 3 times rinsed with water.
After drying under vacuum overnight, 6.23 g of white solid was
isolated. Yield 95%. 1H NMR (CDCl3, 300 MHz): δ (ppm) 10.66 (s,
1H), 7.71 (d, J = 1.2 Hz, 1H), 7.45 (d, J = 1.4 Hz, 1H), 4.68 (s, 2H),
2.32 (s, 3H). 13C NMR (CDCl3, 75 MHz) δ (ppm) 173.9, 158.2,
138.7, 131.3, 128.8, 125.9, 111.2, 40.7, 20.5.
CuIICuIII species, demonstrating a metal-centered oxidation.
Subsequently, this intermediate species evolved toward a
dicopper(II,II) phenoxyl radical species.25 Following these
studies, in the present work, we explore the coordination
chemistry of a novel unsymmetrical phenoxido bridge scaffold
designed with the aim of providing two distinct binding sites to
stabilize a CuIICuIII mixed-valent species. The synthesized
ligand (H3L) contains one bis(2-pyridylmethyl)aminomethyl
arm suited for the CuII redox state and a bis-amide arm which
can be deprotonated for stabilizing the CuIII redox state, these
two moieties being connected through a phenoxido spacer
(Scheme 1). Amidates29,30 are indeed strongly donating
ligands in the family of anionic ligands (corroles,31 thiolates,32
oxamates33), hence are fully appropriate to lead to highly
stabilized Cu(III) species as requested.
EXPERIMENTAL SECTION
■
General. NMR spectra were recorded on a Bruker AM 300 (1H at
300 MHz, 13C at 75 MHz) or a Bruker Avance (1H at 400 MHz, 13
C
Synthesis of Ligand (H3L). To the mixture of bis(2-pyridyl-
methyl)amine (BPA) (4 g, 0.02 mol) and potassium carbonate (19.4
g, 0.14 mol) in CH3CN (100 mL) was added 3-(chloromethyl)-2-
hydroxy-5-methylsalicylic acid (4.2 g, 0.02 mol) in CH3CN (50 mL).
The mixture was refluxed overnight under N2. The resulting reaction
mixture was cooled at RT, and filtered through a porous frit. The
at 100 MHz). Chemical shifts are given relative to solvent residual
peak.
X-band EPR spectra were recorded at 100 K with an EMX Bruker
spectrometer equipped with a standard ER4102ST Bruker cavity. At
helium temperature, the spectrometer is equipped with an
ER4116DM Bruker cavity, a, ESR900 continuous-flow Oxford
Scheme 1. Synthetic Route for the Ligand H3L
B
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filtrate was recovered and evaporated to dryness. The crude product
was used without any further purification. H NMR (DMSO-d6, 300
corrected for absorption with SADABS and merged with Xprep.
Crystallographic structures were solved using charge flipping methods
implemented by Superflip. C, N, O, S, Cl, and Cu atoms were refined
anisotropically by the full matrix least-squares method using ShelxL-
2013 run under Olex2. H atoms were calculated on idealized positions
and constrained on their bearing atoms.
1
MHz): δ (ppm) 8.47 (ddd, J1 = 4.9 Hz, J2 = 1.6 Hz, J3 = 0.9 Hz, 2H)
7.76 (td, J1 = 7.7 Hz, J2 = 1.8 Hz, 2H), 7.63 (dt, J = 7.8 Hz, J2 = 0.8
Hz, 2H), 7.22 (ddd, J1 = 7.4 Hz, J2 = 4.9 Hz, J3 = 1.2 Hz, 2H) 7.40
(d, J = 2.5 Hz, 1H), 7.17 (d, J = 2.3 Hz, 1H), 3.72 (s, 4H), 3.58 (s,
2H), 2.17 (s, 3H). In a Schlenk flask, the previous compound was
dissolved in dry CH2Cl2 and degassed. Then 3 equiv of triethylamine
were added and the mixture was cooled to 0 °C. One equivalent of
N,N-dimethylmethaniminium hexafluoro-phosphate (HATU) was
dissolved in 3 mL of dry DMF and added to the mixture. A white
precipitate appeared. After 1 h at 0 °C, a solution of glycine N-
methylamide (1.3 equiv) in dry CH2Cl2 was added dropwise at 0 °C.
After the addition, the cooling bath was removed and the reaction
mixture was stirred overnight at 20 °C. A white solid formed was
removed by filtration and washed with CH2Cl2. The mixture was
evaporated to dryness; the residue was dissolved in EtOAc and
washed with a saturated solution of NaCl. The organic layer was dried
over Na2SO4 and evaporated. The resulting crude product was
purified by column chromatography on silica using acetone as an
eluent. The pure ligand was crystallized from a CH2Cl2−diethyl ether
Measurements for 2(ClO4)2·DMF·EtOAc were undertaken at the
ESRF ID23_1 beamline on a Maatel mini-diffractometer (MD2M)
with a DECTRIS Pilatus_6M_F detector and a Silicon 111
monochromatized radiation (λ = 0.72932 Å) at 100 K. The data
were collected with mxCuBE in phi scan mode and integrated and
corrected for Lorentz and polarization factors with XDS, then
corrected for absorption with SADABS and finally merged with
Xprep. The crystallographic structure was solved and refined as above.
There are many noticeable things concerning this structure: the
crystal is a twin with roughly a 75%−25% ratio. Surprisingly enough,
there is a pseudo inversion center located at 0.354 0.500 0.682, giving
rise to strong correlations. We believe that the combination of the
twin together with this pseudo inversion center could explain why we
observe high electronic density residues around the copper ions.
The crystal data of 1·DMF, 2(SbF6)(NO2)·solvate, and 2(ClO4)2·
DMF·EtOAc and details of the data collections are given in Table S1.
Supplementary data are available from the CCDC, quoting the
deposition numbers CCDC 1578061, 1578062, and 1578063 for 1·
DMF, 2(SbF6)(NO2)·solvate, and 2(ClO4)2·DMF·EtOAc, respec-
tively.
Computational Section. All calculations were performed using
the Gaussian 09 package.34 To compute oxidation potential value, a
calibration (choice of the functional, basis set) was already done on
similar systems.25 Geometry optimizations were carried out using the
hybrid exchange-correlation PBE0 functional accounting for CH3CN
solvation effects using the IEFPCM continuum solvent model. A
mixed basis set composed of the Def2-TZVPD for copper atoms, the
6-311+G(d,p) for N and O atoms, and 6-311G(d,p) for C and H
atoms was used. Then energetic calculations were carried out from
additional single point calculations on the previously optimized
geometries using the M11-L functional and always the IEFPCM
continuum solvent model and the same basis set as stated above.
TD-DFT calculations were performed using the functional
wB97XD associated with the IEFPCM solvent model and the same
basis set as described above. Benchmarks were already done on similar
systems.18 Solutions for the 50 lowest states were computed at this
level. For the calculations, different spin states are possible. The
ground state of dicopper (II,II) complexes can be either a triplet or a
singlet state. For the mono-oxidized species, according to the
oxidation site, only a doublet state (oxidation on a copper atom) or
a doublet state and quartet state (oxidation on the ligand) were
possible. Thus, we have first considered calculations on a doublet state
to determine the oxidation site. Mulliken populations and spin
densities have allowed us quickly to perform this task. To compute
the oxidation potential value when oxidation occurs on the ligand,
since two spin states are possible, we consider the high spin state, i.e.,
the quartet state. Similarly, for the dicopper (II,II) complexes, energy
of the triplet state is used even if it is not necessarily the ground state.
It is justified since the energy difference between spin states is very
small compared to that between two redox states.
1
solution and isolated as a light beige powder. Overall yield: 34%. H
NMR (CDCl3, 300 MHz): δ (ppm) 9.16 (t, J = 5.5 Hz, 1H), 8.59 (d,
J = 4.4 Hz, 2H) 7.87 (d, J = 1.5 Hz, 1H), 7.66 (td, J = 7.3 Hz, J = 1.5
Hz, 2H), 7.30 (d, J = 7.8 Hz, 2H) 7.21 (dd, J = 7.6 Hz, J = 5.3 Hz,
2H), 7.03 (d, J = 1.8 Hz, 1H), 6.71 (s, 1H), 4.17 (d, J = 6.0 Hz, 2H),
3.92 (s, 4H), 3.84 (s, 2H), 2.82 (d, J = 4.8 Hz, 3H), 2.27 (s, 3H). 13C
NMR (CDCl3, 75 MHz) δ(ppm) 170.9, 167.5, 157.7, 154.7, 149.1,
137.1, 135.0, 131.2, 128.5, 123.8, 123.4, 122.6, 118.8, 59.0, 56.8, 44.7,
26.3, 20.4. UV−vis (DMF): λ (ε, M−1 cm−1) = 313 (4700) nm;
HRMS − ESI/QTOF: m/z [L + H]+ calcd 434.2192; found
434.2188; elemental analysis calc. (%) for C24H27N5O3·0.25H2O: C
65.81, H 6.33, N 15.99, found (%): C 65.81, H 6.48, N 16.19.
Synthesis of [Cu2(L)(μ-OH)](1). The ligand H3L (0.150 g, 0.35
mmol) was dissolved in methanol (6 mL), and 4 equiv of
tetramethylammonium hydroxide (25% wt methanolic solution, 1.4
mmol) was added. An immediate color change to red was observed. A
solution of Cu(ClO4)2·6H2O (0.263 g, 0.71 mmol) in MeOH (5 mL)
was added to the previous solution of ligand, which resulted in an
immediate precipitate. The appearing precipitate was filtered off and
the solution was evaporated to dryness. Then, by adding 4 mL of
THF, the remaining NMe4ClO4 was completely extracted. The brown
product was filtered, washed with acetone, and dried under vacuum.
Single crystals were obtained by slow diffusion of acetone into a
concentrated solution of complex in water. Yield: 0.113 g, 57%. A
recrystallization was done by slow diffusion of ethyl acetate into a
concentrated solution of previous crystals in DMF. UV−vis (DMF): λ
(ε, M−1 cm−1) = 313 (sh, 5900), 460 (780), 723 (140), 798 (150)
nm; elemental analysis calcd (%) for C24H25Cu2N5O4·3.5H2O: C
45.21, H 5.06, N 10.98; found (%): C 45.39, H 5.02, N 10.96.
Synthesis of [(Cu2(LH)(μ-OH))2](ClO4)2·DMF·EtOAc) (2(ClO4)2·
DMF·EtOAc). Complex 1 (7.9 mg, 0.012 mmol) was dissolved in 3
mL of DMF, and 1 equiv of HClO4 (0.1 M) was added. The brown
solution turned immediately to green after the addition of perchloric
acid. Single blue crystals were obtained by slow diffusion of ethyl
acetate into the concentrated solution prepared in DMF. Yield: 3.8
mg, 48%. UV−vis after dissolution of the crystals (DMF): λ (ε, M−1
cm−1) = 315 (11600), 410 (sh, 850), 700 (100), 820 (170) nm; ESI-
MS: m/z [(Cu2(LH)(μ-OH))2 + ClO4]+ calcd 1249.06; found
1248.92.
RESULTS AND DISCUSSION
■
Ligand and Complex Synthesis. The preparation of the
dinucleating ligand is obtained in a three-step reaction starting
from 5-methylsalicylic acid (Scheme 1). The reaction of the 5-
methylsalicylic acid with paraformaldehyde in hydrochloric
acid gave the corresponding chloromethylated product
quantitatively. The N-alkylation of N,N-bis(2-pyridyl)-
methylamine (BPA) and the subsequent peptide coupling
reaction with the glycine N-methylamide afforded the ligand
H3L as a crystalline material with a global yield of 34%
(Figures S1−S3). The dinuclear copper(II) complex 1 was
Safety Note. Caution! Although no problems were encountered
during the preparation of the perchlorate salts, suitable care should be
taken when handling such potentially hazardous compounds.
Crystal Structure Determination and Refinement. Measure-
ments for 1·DMF and 2(SbF6)(NO2)·solvate were made on a Bruker-
Nonius Kappa diffractometer with an APEXII detector and a
multilayer mirror monochromatized Mo(Kα) radiation (λ =
0.71073 Å) from an Incoatec microsource at 200 K. The data were
collected with phi and omega scans. Data were integrated and
corrected for Lorentz and polarization factors with Eval 14, then
C
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prepared in 57% yield by the successive addition of 4 equiv of
tetramethylammonium hydroxide to a solution of the ligand
H3L in methanol, followed by the addition of 2 equiv of
Cu(ClO4)2·6H2O.
Description of the Crystal Structure. Crystals suitable
for X-ray diffraction of the complex were obtained by slow
diffusion of ethyl acetate into a solution of the complex
solubilized in dimethylformamide (DMF) at RT. The crystal
obtained of [Cu2(L)(μ-OH)]·DMF (1·DMF) contains one
associated solvent molecule. The structure of complex 1·DMF
reveals that the ligand is deprotonated three times and that the
copper centers are doubly bridged by the phenoxido and by a
hydroxido group leading to a neutral complex (Figure 1). The
Table 1. Selected Bond Lengths and Angles for Complex 1
bond lengths (Å) and angles (deg)
Cu1−O1
Cu1−N2
Cu1−O2
Cu1−N3
Cu2−O1
Cu2−O2
1.960(2)
1.914(3)
1.925(2)
1.928(3)
2.118(2)
1.903(2)
Cu2−N4
Cu2−N5
C7−N2
C7−O3
C11−N3
C11−O4
2.036(3)
2.043(3)
1.335(4)
1.266(4)
1.328(4)
1.251(4)
Cu2−N1
2.024(3)
O1−Cu1−N2
O1−Cu1−O2
O1−Cu1−N3
N2−Cu1−O2
N2−Cu1−N3
O2−Cu1−N3
O1−Cu2−O2
O1−Cu2−N1
O1−Cu2−N4
91.40(11)
81.54(10)
167.29(11)
167.66(12)
85.82(12)
103.23(11)
78.06(9)
O1−Cu2−N5
O2−Cu2−N1
O2−Cu2−N4
O2−Cu2−N5
N1−Cu2−N4
N1−Cu2−N5
N4−Cu2−N5
Cu1−O1−Cu2
Cu1−O2−Cu2
103.68(10)
171.05(11)
102.77(12)
97.85(12)
82.95(12)
82.52(12)
134.46(11)
94.92(9)
93.15(10)
120.03(10)
103.55(12)
modes υCO are located at 1675 and 1638 cm−1 for the free
ligand and at 1608 and 1564 cm−1 for the complex.
Solution Studies of Complex 1. UV−vis Spectroscopy.
The electronic spectrum of this complex was recorded in DMF
and is shown in Figure S6. A shoulder observed at λ = 313 nm
(ε = 5900 M−1 cm−1) can be attributed to the π → π*
transition of the ligand. In the visible region, a broad band at λ
= 460 nm (ε = 780 M−1 cm−1) that dominates can be assigned
to a pair of different overlapping LMCT transitions between
the phenoxido and copper ions and between the hydroxido and
copper ions, according to previous studies.38,40 These
transitions are responsible for the brown color of the solution.
At lower energy, d−d transitions are also present at 723 and
800 nm (ε = 140 M−1 cm−1 and ε = 150 M−1 cm−1).41−43
1H NMR Spectroscopy. The spectrum of complex 1 was
recorded in DMF-d7 and exhibits paramagnetically shifted
resonances within the range −60 to 90 ppm. The chemical
shifts are attributed by reference to the spectrum of the
symmetrical complex [Cu2(BPMP)(OH)](ClO4)2 previously
Figure 1. Crystal structure of complex 1 [Cu2(L)(μ-OH)] crystallized
in DMF/EtOAc with selected numbering scheme. All hydrogen atoms
have been omitted for clarity except the hydroxido hydrogen.
unsymmetrical ligand confers different coordination environ-
ments for the copper centers. Cu1 is tetracoordinated by
oxygen atoms from the phenoxido (O1) and hydroxido (O2)
bridges, and by the two deprotonated amide groups (N2, N3),
forming a distorted square planar geometry around Cu1 with
the angle between the N2−Cu1−O1 and N3−Cu1−O2 planes
of 14.5°. On the other hand, the Addison parameter35 (τ) of
the pentacoordinated Cu2 is 0.59, indicating that the geometry
is a distorted trigonal bipyramid with N1 and O2 occupying
the axial positions. The intermetallic distance is 3.007 Å, and
the bond distances observed in complex 1 are in agreement
with those found in copper(II) complexes coordinated by
similar N,O-donor groups.36−38 Interestingly, within the bis-
amide core, the CO bonds decrease (the C7−O3 and the
C11−O4 bonds of 1.266 and 1.251 Å, respectively) while the
C−N ones lengthen (the C7−N2 and the C11−N3 of 1.335
and 1.328 Å, respectively), indicating a double bond character
for the carbonyl groups. Selected bond distances and angles are
given in Table 1. Considering the angles, a dihedral angle of
44.4° exists between the Cu1−O1−Cu2 and the plane
containing the phenyl ring of the spacer, indicating that the
ligand is relatively twisted.
Infrared Spectra. The FT-IR spectra in the solid state of
the ligand H3L and the complex 1 show some differences
(Figures S4 and S5). An intense band at 3276 cm−1 is observed
for the free ligand H3L which indicates the presence of an N-H
stretching band of the amide groups.39 This band disappears in
the spectrum of the complex, supporting the deprotonation of
these amide groups. Furthermore, shifts are observed for the
stretching vibrations υCO from the protonated ligand to the
complex. These bands corresponding to the two vibrational
reported44,45 and by the two-dimensional COSY H NMR
1
performed in the range of −5 to 30 ppm (Figures S7 and S8).
Electrochemistry. Cyclic voltammetry (CV) of complex 1
was recorded in DMF containing tetra-n-butyl ammonium
perchlorate (NBu4ClO4) as the supporting electrolyte (0.1 M).
At room temperature (T = 293 K), three oxidation peaks were
detected at Epa = 0.19, 0.64, and 0.84 V vs Fc+/Fc (Figure 2A).
The first system is irreversible at low scan rates, but becomes
quasi-reversible when the scan rate (v) is increased (v > 0.5 V/
s) (Figure S10). This behavior is typical of an EC mechanism
(E = electrochemical, C = chemical), for which a rather slow
chemical process follows the electrochemical oxidation.
Lowering the temperature to 213 K induced a modification
of the redox behavior as shown by cyclic voltammetry (Figure
2A,B). Indeed, the first system became fully reversible at E1/2
(1) = 0.12 V vs Fc+/Fc (ΔEp = 75 mV at v = 0.1 V/s). Increase
of the scan rate (0.02 V/s < v < 1 V/s) resulted in an increase
of peak current and peak separation, without loss of chemical
reversibility (Figure S11A). At more positive potential values, a
single irreversible system was detected at Epa = 1.01 V (Figure
2A). When scanning toward negative potential values, CV
displayed a quasi-reversible system at E1/2 (2) = −1.27 V (ΔEp
= 91 mV at v = 0.1 V/s) (Figure 2B). When comparing the
reduction and the oxidation processes (Figure 2B), both
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Figure 2. CVs of complex 1 in DMF/NBu4ClO4 0.1 M (C WE, E/V vs Fc+/Fc): (A) at two different temperatures (T = 293 and 213 K) on the
−1.8 to 1.2 V potential range (v = 0.1 V/s); (B) CV at 0.1 V/s at T = 213 K for the first oxidation and reduction processes.
Figure 3. (A) RDEVs of complex 1 in DMF/NBu4ClO4 0.1 M (C WE, E/V vs Fc+/Fc) at T = 233 K (ω = 600 rpm, v = 0.05 V/s) before (black)
and after (red) low-temperature electrolysis (Eapp = 0.25 V vs Fc+/Fc). (B) X-Band EPR spectrum of complex 1 after 1 h electrolysis, T = 10 K.
Black: experimental, Blue: Simulated. See text for simulated EPR data.
display the same peak heights by CV (Figure 2B) and Rotating
Disk Electrode Voltammetry (RDEV) (Figure S11B). This
indicates that oxidation and reduction of complex 1 involve the
same number of electrons (n). The determination of n was
carried out by a combination of different electrochemical
methods (CV, RDEV (Figure S12) and CA (chronoamper-
ometry, Figure S13)) at room temperature, as previously done
for an analogous species.18 This calculation was performed in
the frame of an EC mechanism, as detected by CV at 293 K
(see SI for details). The three techniques converge toward n =
1, which sounds reasonable considering the unsymmetrical
topology of the ligand and assuming that oxidation/reduction
processes occur on the metal ions. Thus, electrochemical
studies suggest that complex 1 is monoelectronically oxidized
at E1/2 (1) and monoelectronically reduced at E1/2 (2).
electrolyzed solution displayed a weak axial signal at g = 2 with
a four-line pattern at 10 K (Figure 3B) while no EPR signal
was observed in frozen solution of DMF (X band at 6 and 100
K) for complex 1.38,46 Simulation of the experimental result
provided the following parameters: g∥ = 2.246, g⊥ = 2.0575, A∥
= 193 × 10−4 cm−1, A⊥(14N) = 17 × 10−4 cm−1, which are
typical of a mononuclear Cu(II) center. These results support
electrochemical oxidation of one Cu(II) ion into an EPR silent
Cu(III) ion, leading to a localized mixed-valent CuIICuIII
complex 1+ similar to the examples previously reported.17−19
This transient species decomposed rapidly. CV of the solution
was also recorded after electrolysis at low temperature. A new
anodic wave was detected at Epa (3) = 0.58 V vs Fc+/Fc in
addition to the first anodic oxidation at E1/2 (1). All these data
are consistent with the conclusion that the electrochemically
generated mono-oxidized complex 1+ evolves toward a new
species at 233 K, and can be oxidized at Epa (3).
Cryo-spectroelectrochemical Studies of Complex 1.
To further investigate the electrochemical oxidation of
complex 1, we employed a cryo-spectroelectrochemical setup
which allows the generation and characterization of transient
species in a thin layer (optical path = 200 μm) at the electrode
surface. This method offers the advantage of characterizing the
generated species quickly (a few seconds) which is of interest
Electrochemical Generation and Characterization of
the One-Electron Oxidized Species 1+. Attempts to
prepare the one-electron oxidized species by exhaustive
electrolysis of a solution of complex 1 at low temperature (T
= 233 K) were carried out. After 1 h, low-temperature RDEV
of the solution confirmed that concentration of 1 has
decreased, and the presence of a small cathodic wave at
E
1/2(1) indicates that the one-electron species 1+ was formed
in solution (Figure 3A). X-band EPR characterization of the
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Figure 4. (A) 3D UV−vis spectroscopic monitoring of the electrochemical oxidation of complex 1 in DMF/NBu4ClO4 at T = 213 K under thin-
layer conditions (200 μm). The red curve represents the current intensity (i) response (vs time) upon variation of potential. (B) 2D differential (vs
initial spectrum) spectroscopic response of complex 1 upon electrochemical oxidation. The red curves on the right panels represent the variation of
the current intensity (i) and potential (E) with time.
Figure 5. Spin density (purple) plots of 1 (A) (triplet state), 1b+ (B) (doublet state), 1a+ (C) (quartet state).
when the generated species is not very stable, even at low
temperatures, as found for complex 1. Figure 4A displays the
UV−vis monitoring of the complex 1 upon electrochemical
oxidation by CV (red curve) at T = 213 K. Clearly, when
electrochemical oxidation occurs, an increase was observed of
the band at around 400 nm. This band decreased when the
potential was swept back to its initial value. An important point
here is that the redox process is fully reversible as attested by
the CV shape, meaning that the species detected by UV−vis
spectroscopy is effectively the one-electron oxidized complex
1+. In order to better analyze the variation of the spectroscopic
data with applied potential, we extracted the initial UV−vis
spectrum (t = 0 s) from the set of data. The resulting UV−vis
spectra are displayed in a 2D manner in Figure 4B, together
with the applied potential and resulting current (red curves). It
shows the appearance of a band at λmax = 386 nm at
approximately 17 s, which corresponds to the foot of the
reduction wave (i.e., before the electrochemically generated 1+
gets reduced). This band can be confidently ascribed to a UV
feature of the one-electron oxidized species 1+.
Theoretical Calculations. In an effort to characterize the
species formed during the electrochemical oxidation of 1,
theoretical calculations were performed. For the mono-
oxidized species, oxidation can occur on the ligand or on
one copper(II) atom. In the first case, the ligand-centered
radical species 1a+, the spin state of the system can be a
doublet or quartet state, while, in the other case, the CuIICuIII
mixed-valent species 1b+, it is a doublet state. Table S3 gives
the primary computed geometric parameters and Figure 5 the
spin-density plots (the difference between the α and the β
densities) for 1, 1a+, and 1b+. Comparisons of the spin
densities clearly indicate that oxidation occurs for 1a+ on the
phenoxido and for 1b+ on one copper atom. Values of the
oxidation potential of complex 1 associated with, respectively,
1a+ and 1b+ are computed at the M11-L/IEFPCM level using
the protocol already published.25 Theory predicted a standard
potential for 1b+/1 equal to 0.10 V vs Fc+/Fc (and 0.54 V for
1a+/1). Compared to the experimental data (E1/2 (1) = 0.12 V
vs Fc+/Fc), 1b+ appears as the best description for the mono-
oxidation species. Then we performed for 1b+ time-dependent
(TD-DFT) calculations and compared them with the UV−vis
spectra of 1+. Different benchmarks for TD-DFT calculations
were already done on similar systems,18 and good results were
obtained at the wB97XD/IEFPCM level of theory. The main
absorption bands are given in Table 2. A transition at λ = 385
Table 2. TD-DFT Data for the Main Absorption Bands
(Oscillator > 0.01 and Weak Spin Contamination) of 1b+
(Doublet State) at wB97XD/IEFPCM Level
excited
state
energy
(nm)
oscillator strengths spin contamination
system
(f)
⟨S2⟩
16
17
18
24
386
385
376
347
0.026
0.034
0.019
0.026
0.85
0.83
0.82
0.85
1b+
F
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nm with a correct value of oscillator strength (0.0340) and a
weak spin contamination (S2 = 0.831 against 0.75 for a doublet
state) was found for 1b+. This is in agreement with the band at
λ = 386 nm observed by cryo-spectroelectrochemical studies.
Natural transition orbital analysis describes this band as a
ligand-to-metal charge transfer essentially between the
phenoxido ligand and the CuIII ion (Figure 6). All of these
Figure 7. Molecular structure of the dication 2 obtained after
chemical oxidation of 1. Hydrogen atoms (except for the hydroxido
bridge and the amide arm), solvent molecules, and counteranions
were removed for clarity.
Figure 6. Natural transition orbitals of the excited state of 1b+
associated with the absorption band detected by cryo-spectroelec-
trochemical studies.
The copper centers are doubly bridged by the phenoxido and
by a hydroxido group, leading to a distance of 2.9014 and
2.8936 Å between the Cu1A···Cu2A and Cu1B···Cu2B copper
centers, respectively. The polyhedron of coordination around
Cu1A and Cu1B is a distorted square pyramid with the copper
coordinated by the N atom from the amide groups (N2A,
N2B) and also the O atom from the other amide groups (O4A,
O4B), by the oxygen of the phenoxido (O1A, O1B) and
hydroxido (O2A, O2B) groups. However, a close examination
of the coordination around Cu2A and Cu2B indicates that
these two copper centers have different geometries. A distorted
square pyramid for Cu2A and a distorted octahedron for Cu2B
explains why we have an opened cubane as a resulting structure
for this complex. The distance between Cu2A and O2B is quite
long at 2.823 Å, which does not allow a coordination between
these two atoms. Selected bond distances and angles are given
in Table 3. On the bis-amide side, although the terminal amide
has been reprotonated and coordinates the metal center by the
oxygen atom, the bond distances C11A−O4A and C11B−O4B
are comparable to the bond distances C7A−O3A and C7B−
O3B and are compatible with a double bond character for the
carbonyl group.
Interestingly, a switch in the coordination of the amide
occurs after chemical oxidation of dinuclear complex 1 at low
temperature, and the complex obtained is a tetranuclear one
constituted of two monoprotonated dinuclear units in the solid
state. Because the final product obtained shows a protonation
of the N atom of the terminal amide and consequently a
change in the coordination of this amide, we investigated the
direct effect of protonation on a solution of complex 1.
Solution Studies of the Protonated Species Obtained
from Complex 1. Protonation of complex 1 by perchloric
acid was followed by 1H NMR and UV−vis spectroscopies and
electrochemistry.
results support the formation of a mixed-valent CuIICuIII
species for the mono-oxidized species in agreement with the
mono-CuII signature observed in EPR. Furthermore, a close
inspection in the NIR wavelength range shows no appearance
of any new band. The lack of NIR band and the observed EPR
feature are in accordance with a class I mixed-valent species in
the Robin−Day classification.47
Chemical Oxidation of Complex 1. In order to better
understand the oxidative processes, chemical oxidation was
performed by using NOSbF6 as oxidizing agent in a DMF
solution of 1 at T = 243 K. After addition of 1 equiv of the
oxidant, the brown solution turned immediately to green. No
UV−vis absorptions associated with a transient species could
be detected (Figure S14). From the final solution, crystals
suitable for X-ray diffraction were obtained by slow diffusion of
ethyl acetate into a solution of the oxidized complex 1 in DMF.
Description of the Crystal Structure of Complex 2
Obtained after Chemical Oxidation of the Complex 1.
Single crystals of complex 2 were obtained as [(Cu2(LH)(μ-
OH))2](SbF6)(NO2)·solvate (2(SbF6)(NO2)·solvate) where
solvate = 1.54(C4H8O2)·0.16(H2O). The nitrite anion present
in the crystal structure of complex 2(SbF6)(NO2)·solvate
comes from the nitrosonium added in as an oxidant and acts as
a counteranion. The structure consists of a tetranuclear
complex which can be described as two dinuclear units
connected together at an average distance of 2.6 Å to form an
opened cubane (Figure 7). Moreover, the equatorial pyridine
rings of each dimer are facing each other separated by
interplanar distances consistent with the existence of a π−π
interaction which contributes to the stability of the tetranuclear
complex. Unlike the initial complex, the ligand is deprotonated
twice and the terminal amide function remains protonated.
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paramagnetic species possessed a hydroxido bridge charac-
terized by the signals at −47 ppm and −42 ppm.
Table 3. Selected Bond Lengths and Angles for the Product
(Complex 2) Recovered after Oxidation of Complex 1
Electrochemistry. The redox behavior of complex 1 in the
presence of acid was evaluated by cyclic voltammetry (Figure
9A). As for the NMR studies, aliquots of perchloric acid in
acetonitrile (1 M) were added stepwise (from 0.25 to 1 equiv)
to a solution of complex 1 (C = 0.65 mM) in DMF containing
tetra-n-butyl ammonium perchlorate (TBAP) as the support-
ing electrolyte at T = 233 K. The addition of 1 equiv of acid
produces a monoprotonated species with an anodic peak at Epa
(2) = 0.58 V at v = 0.1 V/s. The shift toward the anodic region
of potential indicates that the new species formed is much
harder to oxidize, and the irreversibility of this wave suggests
that a chemical reaction occurs after the electron transfer. An
interesting comparison could be made between the oxidation
potential of this monoprotonated species generated by direct
protonation and the new anodic wave detected at Epa (2) =
0.58 V vs Fc+/Fc during low-temperature electrolysis. These
species display the same Epa value, hence corroborating that
complex 1+ evolves toward a monoprotonated species.
UV−vis Spectroscopy. Electronic absorption data were
recorded during the titration of complex 1 with HClO4 as
shown in Figure 9B. Addition of acid results into the
progressive shift of the LMCT transition at 460 nm to a
shoulder around 410 nm. A ligand-centered transition at λ =
315 nm is also present as already observed for complex 1. An
isosbestic point is observed at 393 nm, indicating that
equilibrium between two species exists. We can suggest that
complex 1 can be converted to a second dicopper(II) complex,
as the d−d transition seemed to be only slightly affected. Even
after the addition of 15 equiv of Et3N, the original spectrum is
not restored, confirming the relative stability of the
monoprotonated complex.48
bond lengths (Å) and angles (deg)
Cu1A−O4A
Cu1A−O1A
Cu1A−O2A
Cu1A−O1B
Cu1A−N2A
Cu1B−O1A
Cu1B−O4B
Cu1B−O1B
Cu1B−O2B
Cu1B−N2B
C11A−O4A
C11A−N3A
C7A−O3A
C7A−N2A
1.975(6)
1.940(6)
1.920(6)
2.496(6)
1.891(8)
2.553(6)
1.980(7)
1.947(6)
1.916(6)
1.885(7)
1.243(11)
1.325(11)
1.238(12)
1.338(13)
Cu2A−O1A
Cu2A−O2A
Cu2A−N1A
Cu2A−N4A
Cu2A−N5A
Cu2B−O2A
Cu2B−O1B
Cu2B−O2B
Cu2B−N1B
Cu2B−N5B
Cu2B−N4B
C11B−O4B
C11B−N3B
C7B−O3B
2.019(6)
1.909(6)
2.023(8)
2.006(8)
2.223(9)
2.779(7)
2.016(6)
1.915(7)
2.027(8)
2.242(8)
2.000(8)
1.257(12)
1.314(13)
1.268(12)
1.327(12)
100.3(3)
84.2(3)
83.1(3)
92.7(3)
81.2(3)
94.0(3)
101.0(3)
101.7(3)
82.3(3)
93.7(3)
98.1(3)
C7B−N2B
O4A−Cu1A−O2A
O4A−Cu1A−N2A
O1A−Cu1A−O2A
O1A−Cu1A−N2A
O1A−Cu2A−O2A
O1A−Cu2A−N1A
O1A−Cu2A−N5A
O2A−Cu2A−N4A
N1A−Cu2A−N4A
Cu1A−O1A−Cu2A
Cu1A−O2A−Cu2A
100.6(3)
84.5(3)
82.7(3)
92.7(3)
80.9(3)
93.2(3)
90.8(3)
102.1(3)
82.6(3)
94.2(3)
98.5(3)
O4B−Cu1B−O2B
O4B−Cu1B−N2B
O1B−Cu1B−O2B
O1B−Cu1B−N2B
O1B−Cu2B−O2B
O1B−Cu2B−N1B
O2B−Cu2B−N5B
O2B−Cu2B−N4B
N1B−Cu2B−N4B
Cu1B−O1B−Cu2B
Cu1B−O2B−Cu2B
1H NMR Spectroscopy. Perchloric acid (1 M in acetonitrile)
was added stepwise (from 0.25 to 1 equiv) to a solution of
complex 1 in DMF-d7. After each addition, the H NMR
spectra were recorded at room temperature to follow the
1
Considering all the data collected in solution, we can suggest
that the protonation site is the nitrogen of the terminal amide
1
1
of the ligand. The detection of a H NMR signal at δ = −42
changes of the resonances of the protons (Figure 8). The H
ppm confirms that the monoprotonated species also displays a
hydroxido bridge between two copper centers. Complemen-
tary, the EPR silent spectrum (X band; 100 K) demonstrates
that both copper ions are at the +II redox state, with an
antiferromagnetic coupling. Moreover, the relatively high
anodic potential (Epa (2) = 0.58 V) in comparison to that
NMR spectrum of 1 gradually changes upon the addition of
acid; the intensities of some peaks decreased while new peaks
appeared at the same time. Half an equivalent of acid conduces
to the presence of two species in the same proportion, and the
addition of 1 equiv of acid converts completely the first species
into a new one. Analysis of the spectra indicates that these two
Figure 8. Variation of the chemical shift of the 1H signals upon subsequent addition of HClO4 to complex 1 in DMF-d7. The chemical shift of the
proton of the hydroxido bridge of the complex is reported in the inset.
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Figure 9. (A) Cyclic voltammograms of complex 1 after addition of 0 equiv (black), 0.25 equiv (orange), 0.5 equiv (red), 0.75 equiv (purple), and
1 equiv (blue) of a solution of HClO4. Conditions: 0.65 mM solution of complex 1 in DMF, 0.1 M TBAP, T = 233 K. (B) UV−vis spectra of
complex 1 before (black) and after addition of 0.25 equiv (orange), 0.5 equiv (red), 0.75 equiv (purple), and 1 equiv (blue) of a solution of HClO4.
obtained for complex 1 (Epa (1) = 0.16 V) indicates that the
generated complex is better stabilized as a consequence of a
change of coordination of the terminal amide. The protonation
of the nitrogen of the amide decreases the donating power of
the ligand and leads to the stabilization of the resulting
CuIICuII complex.
features are quite similar in both structures (selected bond
lengths and angles are given in Table S4).
Solubilization in DMSO of the blue crystals of 2 allows the
detection by ESI-MS of two peaks (Figure S16), one at m/z =
574.03 corresponding to the molecular cation [Cu2(LH)(μ-
OH)]+ with the correct isotopic distribution and a second one
at m/z = 1248.92 corresponding to the molecular ion peak
[(Cu2(LH)(μ-OH))2 + ClO4]+ which indicates the presence
of dinuclear and tetranuclear species, respectively. A possible
partial dissociation of the tetranuclear unit can be envisioned
in solution.51
Theoretical Calculations. Theoretical calculations can
help to identify the molecular structure of the monoprotonated
species obtained from complex 1 in solution. A first hypothesis
is to consider the dinuclear complex corresponding to one half-
unit of the tetranuclear complex obtained in solid state. For
this dinuclear system, theory predicts a standard potential
equal to 0.84 V vs Fc+/Fc with a spin density localized on the
phenoxido ligand. This computed value corroborates perfectly
with those found for similar complexes25 but is significantly
different from the experimental data obtained for the final
product after chemical oxidation of 1 (Epa (2) = 0.58 V vs Fc+/
Fc). This suggests that the complex in solution is not the half-
unit of the tetranuclear complex. Other structures have thus to
be considered for the monoprotonated form in solution. We
have, for example, considered a dinuclear complex derived
from the tetranuclear species but with one DMF molecule
coordinated in an axial position to the Cu1A or Cu1B atom.
According to our calculations, this complex is monoelectroni-
cally oxidized at 0.67 V vs Fc+/Fc, a value close to that found
experimentally. Here, the oxidation occurs on the phenoxido
ligand, leading to a bis-Cu(II) phenoxyl species as previously
found for the BPMP derivatives.
Isolation and Characterization of the Protonated
Species Obtained from Complex 1. Interestingly, addition
of 1 equiv of HClO4 to a solution of complex 1 in DMF,
followed by diffusion of ethyl acetate, resulted in obtaining
blue crystals that were suitable for X-ray diffraction (Figure
S15; Table S4). Complex 2 crystallizes as [(Cu2(LH)(μ-
OH))2](ClO4)2·DMF·EtOAc) (2(ClO4)2·DMF·EtOAc), with
one molecule of DMF and one molecule of ethyl acetate
associated. The perchlorate ions are not coordinated to the
copper centers as it was already evidenced by the vibrations at
1083 and 621 cm−1 in the IR spectrum (Figure S17).49,50 This
structure clearly shows the protonation of the terminal amide
and the switch in the coordination previously observed in the
structure of complex 2(SbF6)(NO2)·solvate recovered after
chemical oxidation of 1 (see above). The main geometric
In summary, the neutral dinuclear complex 1 was
synthesized and the characterization of its mono-oxidized
species 1+ was performed. EPR analysis of the one-electron
oxidized transient species formed is consistent with the
formation of a mixed-valent CuIICuIII complex. The lack of
NIR band and the observed EPR feature allow the assignment
of this species to a class I mixed-valent species in the Robin−
Day classification, meaning a localized mixed-valence complex
possessing distinguishable Cu(II) and Cu(III) sites, even
though bridged.47 The DFT calculations predicted a spin
density for 1+ localized on one copper which corroborates with
our outcome. Moreover, a band at λmax = 386 nm detected by
cryo-spectroelectrochemistry was ascribed to a UV feature of
the one-electron oxidized species 1+. Natural transition orbitals
of the excited state of 1+ provide a reasonable assignment of
this band as a ligand-to-metal charge transfer essentially
between the phenoxido ligand and the CuIII ion. In addition,
attempts to characterize to the mono-oxidized species 1+ by
chemical oxidation at low temperature were unsuccessful, but
interestingly crystals of the final product 2 obtained after
chemical oxidation were isolated. Complex 2 is a tetranuclear
complex in which a switch in the coordination of the amide has
occurred. Because complex 2 has shown a protonation of the N
atom of the terminal amide and consequently a change in the
coordination of this amide, the direct protonation of complex 1
was studied. Characterization in solution suggests that the
protonation site is indeed the N atom of the terminal amide.
This assumption is substantiated by the isolation of crystals of
this monoprotonated species in two dinuclear CuIICuII units
associated in a tetranuclear entity similar to the one obtained
by chemical oxidation of complex 1. The different redox and
proton transfer proposed to explain the presented studies are
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Scheme 2. Proposed Pathways for the Monoelectronic Oxidation and Protonation of Complex 1
depicted in Scheme 2. Interestingly, the transient CuIICuIII
phenoxido-hydroxido complex 1+ evolves slowly at low
temperature toward the tetranuclear complex 2, through the
probable formation of a dinuclear bis-Cu(II) protonated
species (alternatively detected by mass spectrometry). Strictly,
this reaction involves the gain of either one hydrogen atom
(HAT mechanism), or one proton and one electron (PCET
mechanism), by the mixed-valent species.
the ligand associated with its close proximity of the oxidized
metal center induces the rapid reactivity of this mixed-valent
species. Nevertheless, this study shows that the geometry and a
fine-tuning of the electronic properties of the ligand are
important for controlling the locus of the oxidation. We are
currently working on the ligand rigidity to avoid any switch in
the coordination around the metal center and subsequent
tetranuclear formation.
ASSOCIATED CONTENT
CONCLUSIONS
■
■
S
* Supporting Information
A novel unsymmetrical ligand based on a phenoxido spacer has
enabled obtaining a dissymmetric dicopper complex in which
the two copper centers possess different coordination environ-
ments. On one side, the ligand offers a square planar and a
strongly anionic environment which has allowed an oxidation
centered on the metal supported by cryo-spectroelectrochem-
istry, EPR spectroscopy, and DFT calculations. This strategy
has allowed the oxidation process to be directed on the metal
center rather than on the ligand site which underlines that,
even with a redox non-innocent spacer as the phenoxido, it is
possible to obtain metal ion oxidation at lower potential value
than that of the ligand. The low redox potential of this system
makes it a good candidate for reactivity toward C−H
activation. However, the mixed-valent species formed con-
duces to the formation of a new tetranuclear complex with a
protonated ligand which coordinates the metal center by the O
atom of the terminal amide. Although the ligand seems to be
perfect in theory to stabilize high-valent copper, the basicity of
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02127.
NMR spectra and IR spectrum of H3L; IR spectrum,
UV/vis spectrum, NMR spectra, mass spectrum, and
electrochemical measurements of complex 1; crystallo-
graphic data for 1·DMF, 2(SbF6)(NO2)·solvate, and
2(ClO4)2·DMF·EtOAc; determination of the number of
electrons for the oxidation of 1; main computed
geometric parameters for 1, 1a+, and 1b+; crystal
structure of compound 2(ClO4)2·DMF·EtOAc; selected
bond lengths and angles for compound 2(ClO4)2·DMF·
EtOAc; mass spectrum of complex 2(ClO4)2·DMF·
EtOAc; IR spectrum of the complex 2(ClO4)2·DMF·
EtOAc in solid phase (PDF)
J
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polysaccharide monooxygenase. Curr. Opin. Chem. Biol. 2016, 31,
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Accession Codes
CCDC 1578061−1578063 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
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.
(9) Sirajuddin, S.; Rosenzweig, A. C. Enzymatic Oxidation of
Methane. Biochemistry 2015, 54, 2283−2294.
(10) Smith, S. M.; Rawat, S.; Telser, J.; Hoffman, B. M.; Stemmler,
T. L.; Rosenzweig, A. C. Crystal structure and characterization of
particulate methane monooxygenase from Methylocystis species strain
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(11) Ross, M. O.; Rosenzweig, A. C. A tale of two methane
monooxygenase. JBIC, J. Biol. Inorg. Chem. 2017, 22, 307−319.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: aurore.thibon@univ-grenoble-alpes.fr.
ORCID
(13) Cao, L.; Caldararu, O.; Rosenzweig, A. C.; Ryde, U. Quantum
Refinement Does Not Support Dinuclear Copper Sites in Crystal
Structures of Particulate Methane Monooxygenase. Angew. Chem., Int.
Ed. 2018, 57 (1), 162−166.
Aurore Thibon-Pourret: 0000-0002-2292-391X
Rolf David: 0000-0001-5338-6267
Yves Le Mest: 0000-0001-6426-5639
Nicolas Le Poul: 0000-0002-5915-3760
(14) Ro, S. Y.; Ross, M. O.; Deng, Y. W.; Batelu, S.; Lawton, T. J.;
Hurley, J. D.; Stemmler, T. L.; Hoffman, B. M.; Rosenzweig, A. C.
From micelles to bicelles: Effect of the membrane on particulate
methane monooxygenase activity. J. Biol. Chem. 2018, 293 (27),
10457−10465.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
́
(15) Shiota, Y.; Juhasz, G.; Yoshizawa, K. Role of Tyrosine Residue
Notes
in Methane Activation at the Dicopper Site of Particulate Methane
Monooxygenase: A Density Functional Theory Study. Inorg. Chem.
2013, 52 (14), 7907−7917.
The authors declare no competing financial interest.
(16) Yoshizawa, K.; Shiota, Y. Conversion of Methane to Methanol
at the Mononuclear and Dinuclear Copper Sites of Particulate
Methane Monooxygenase (pMMO): A DFT and QM/MM Study. J.
Am. Chem. Soc. 2006, 128, 9873−9881.
ACKNOWLEDGMENTS
■
The authors thank the French research agency for financial
support ANR-13-BSO7-0018. This work has been carried out
in the framework of COST action CM1305 (ECOSTBIO) and
Labex ARCANE (ANR-11-LABX-0003-01). The authors wish
to acknowledge the support from the ICMG Chemistry
Nanobio platform for computer facilities, NMR and ESI-MS
analysis, and X-ray characterizations. The authors acknowledge
the ESRF for the allocation of beam time.
(17) Halvagar, M. R.; Solntsev, P. V.; Lim, H.; Hedman, B.;
Hodgson, K. O.; Solomon, E. I.; Cramer, C. J.; Tolman, W. B.
Hydroxo-Bridged Dicopper(II,III) and -(III,III) Complexes: Models
for Putative Intermediates in Oxidation Catalysis. J. Am. Chem. Soc.
2014, 136 (20), 7269−7272.
(18) Isaac, J. A.; Gennarini, F.; Lopez, I.; Thibon-Pourret, A.; David,
R.; Gellon, G.; Gennaro, B.; Philouze, C.; Meyer, F.; Demeshko, S.;
́
Le Mest, Y.; Reglier, M.; Jamet, H.; Le Poul, N.; Belle, C. Room-
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Inorg. Chem. XXXX, XXX, XXX−XXX