Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2016, 45,
15994
Received 15th June 2016,
Accepted 28th July 2016
DOI: 10.1039/c6dt02391f
www.rsc.org/dalton
Changing the chemical and physical properties of
high valent heterobimetallic bis-(µ-oxido) Cu–Ni
complexes by ligand effects†
Maria-Chrysanthi Kafentzi,
a
Maylis Orio,
a
Marius Réglier,
a
Shenglai Yao,
b
Uwe Kuhlmann,
b
Peter Hildebrandt,
b
Matthias Driess,*
b
A. Jalila Simaan*
a
and
Kallol Ray*
c
Two new heterobimetallic [LNiO
2
Cu(RPY2)]
+
(RPY2 = N-substituted bis 2-pyridyl(ethylamine) ligands with
R = indane, 3a or R = Me, 3b) complexes have been spectroscopically trapped at low temperatures. They
were prepared by reacting the mononuclear side-on LNi
II
superoxo precursor bearing a β-diketiminate
ligand (L = [HC-(CMeNC
6
H
3
(iPr)
2
)
2
]) with the Cu(I) complexes. In contrast to the oxo groups in known
high-valent [M
2
(μ-O)
2
]
n+
(M = Fe, Co, Ni, Cu) cores that display electrophilic reactivities, 3a and 3b display
rather nucleophilic oxo cores active in aldehyde deformylation reactions. However, the spectroscopic and
reactivity properties of 3a/3b are found to be distinct relative to that of the previously reported [LNiO
2
-
Cu(MeAN)]
+
complex containing a more basic (nucleophilic) N,N,N’,N’,N’-pentamethyl-dipropylenetriamine
(MeAN) ligand at the copper centre. The geometry and electronic properties of the copper ligands affect
the electron density of the oxygen atoms of the heterodinuclear {Ni(μ-O)
2
}coreand3a/3b undergo slower
nucleophilic and faster electrophilic reactions than the previously reported [LNiO
2
Cu(MeAN)]
+
intermediate.
The present study therefore demonstrates the tuning of the electrophilicity/nucleophilicity of the oxygen
atoms of the heterobimetallic [Ni(μ-O)
2
Cu]
2+
cores by controlling the electron donation from the ancillary
ligands, and underlines the significance of subtle electronic changes in the physical and chemical properties
of the biologically relevant heterobimetallic metal–dioxygen intermediates.
Introduction
The synthesis and characterisation of metal–oxygen intermedi-
ates are very important for understanding the mechanisms of
transition metal mediated dioxygen activation reactions taking
place in chemical and biological systems. In this context
homobimetallic bis(µ-oxido) complexes with [M
2
(µ-O)
2
]
n+
cores
have attracted much attention and they have been extensively
studied for systems involving Cu,
1
Mn,
2
Ni,
3
Co,
4
Fe,
5
and Pt
centres.
6
In particular, homobimetallic copper dioxygen cores
have been the subject of extensive investigations due to their
possible involvement in biological oxidation reactions as key
intermediates in phenol hydroxylation by tyrosinase,
7
as well
as in methane oxidation by particulate methane
monooxygenase.
8
In contrast to the homobimetallic systems, the properties of
mixed-metal dioxygen species have been less explored. These
types of intermediates are probably involved in various cata-
lytic processes in metalloenzymes; for example, in cytochrome
c oxidase, where a Cu
I
ion cooperates with an Fe
II
(heme) to
activate dioxygen or in Cu–Zn superoxide dismutase, in which
the superoxido ligand disproportionation is mediated through
a mixed Cu–Zn metal site. The generation and the study of
such mixed metal species are therefore of significant interest
due to the unique properties obtained by the combination of
two different metal ions. The synthesis of heterodinuclear
complexes, however, presents a special challenge because of
the possibility of disproportionation and selectivity problems
resulting in the formation of mixtures of homo- and hetero-
dinuclear complexes, as well as metal-site isomers. In order to
overcome these problems two main strategies were previously
used in the literature (Fig. 1).
9
The first one involves the synthesis of ligands providing
different coordination environments that are specific for each
of the two metal ions. An alternative route is the reaction of a
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6dt02391f
a
Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille, France.
E-mail: [email protected]
b
Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 135,
c
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2,
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well-defined dioxygen metal subunit L
n
M-O
2
with a low-valent
metal complex fragment L′
m
M′. This last strategy has been suc-
cessfully used to generate heterobimetallic oxygen-containing
species with PdMo,
10
CuNi,
11
CuGe,
12
PtCu
13
and CuPd
13
com-
binations, starting from well-defined metal–dioxygen precur-
sors. Our contribution in the field involves the synthesis of a
novel family of NiO
2
M complexes generated by using a mono-
nuclear side-on LNi
II
superoxo precursor bearing a β-diket-
iminate (Nacnac) ligand (L = [HC-(CMeNC
6
H
3
(iPr)
2
)
2
]) system.
14
Notably, this precursor is remarkably stable and can be
handled at room temperature, which made it a suitable precur-
sor for the synthesis of heterobimetallic metal–dioxygen com-
plexes. Interestingly, in our previous studies, the electronic
structure of the [NiO
2
M]
n+
core in such complexes was found
to be dependent on the nature of the second metal ion, M;
whereas a peroxido species
15
[Ni(µ-O
2
)K]
n+
was stabilized in the
presence of potassium, replacement of the monovalent potass-
ium ion by LZn
+15
(L = Nacnac) or LFe
+16
ions initiated peroxido
O–O bond scission and subsequent H-atom abstraction to
generate the heterobimetallic bis(μ-hydroxo) Ni–Zn or alkoxo
hydroxido Ni–Fe species, respectively. Although the fleeting
[LNiO
2
ML] (M = Fe, Zn) could not be isolated, in a subsequent
study a related [LNiO
2
Cu(MeAN)]
+
species was stabilized at low
temperature by reacting the [LNi
II
O
2
] precursor with a copper
triamine [Cu(MeAN)]
+
reductant (Scheme 1A). Interestingly, the
presence of Cu in the second metal binding site was shown to
result in nucleophilic oxo groups in the high-valent bis-µ-oxido
Ni–Cu species, and it was able to react towards aldehyde sub-
strates, in contrast to the reported electrophilic properties of
the homobimetallic NiO
2
Ni, CuO
2
Cu and heterobimetallic
NiO
2
Fe counterparts.
17
In the present manuscript we now try
to systematically explore how the nature of the ancillary ligand
can modulate the formation, structure, electronic properties
and reactivity of such high-valent heterobimetallic dioxygen
species. For this purpose, we report on the synthesis, spec-
troscopy, DFT derived description, and reactivity studies of two
new bis-µ-oxido CuNi intermediates, where the triamine MeAN
ligand at Cu is replaced by relatively weaker bases/nucleophiles
in the bis 2-pyridyl(ethylamine) (RPY
2
; R = methyl or indane)
ligand systems. Notably, the RPY
2
-type ligands have been pre-
viously utilised in the formation of binuclear CuII
2-peroxido
complexes, which were shown to exist in equilibrium with the
isomeric CuIII
2(µ-oxido) species depending on the reaction con-
ditions.
1a
The side-on peroxido dicopper(II) intermediates
exhibited electrophilic character towards C–H bond oxygen-
ation of substrates.
1f,h,i
For example, on using an “indane”sub-
strate covalently attached to the ligand (IndanePY
2
, Scheme 1),
intramolecular oxygen atom transfer was shown to take place
in a highly regio- and stereo-selective manner leading to the
cis-hydroxylated diastereoisomers of the ligand.
18
The success-
ful generation of the [LNiO
2
Cu(RPY
2
)]
+
complexes now offers
the opportunity to compare and contrast the reactivity of
the present complexes with that of the previously reported
heterodinuclear [LNiO
2
Cu(MeAN)]
+
and homodinuclear
[{Cu(RPY
2
)}
2
O
2
]
2+
complexes.
Results and discussion
Synthesis and UV-Vis spectroscopy
The complexes 1a and 1b were prepared by mixing one molar
eq. of Cu
I
(CH
3
CN)
4
(OTf) with one molar eq. of ligand in di-
chloromethane. Equimolar amounts of 1a {(IndanePY
2
)
Cu
I
(OTf)} or 1b {(CH
3
PY
2
)Cu
I
(OTf)} and 2were then mixed at
−90 °C in dry dichloromethane leading to the immediate for-
mation of the brownish intermediates 3a {[IndanePY
2
]Cu(O
2
)
Ni[C-(CMeNC
6
H
3
(iPr)
2
)
2
](OTf)} and 3b {[CH
3
PY
2
]Cu(O
2
)Ni[C-
(CMeNC
6
H
3
(iPr)
2
)
2
](OTf)}, respectively (Scheme 1B). The gene-
ration of these species was monitored by UV-Vis spectroscopy
following the appearance of a characteristic absorption band
centred at 885 nm for 3a and 870 nm for 3b (Fig. 2). The inter-
mediates are stable at −90 °C, but they are thermally sensitive,
as indicated by bleaching of their UV-Vis absorption features
Fig. 1 Two main strategies for the development of heterobimetallic
metal–dioxygen complexes.
Scheme 1 A: Generation of the [LNi
III
O
2
Cu
III
(MeAN)]
+
complex which is
already reported.
17
B: Generation of [LNi
III
O
2
Cu
III
(RPY
2
)]
+
species 3a (3b)
upon mixing a solution of the LNi
II
superoxo complex 2with the (RPY
2
)
Cu
I
complexes 1a (or 1b).
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upon warming to higher temperatures (see the ESI†) with t
1/2
=
2000 s at −60 °C.
EPR spectroscopy
The EPR spectra were recorded in frozen CH
2
Cl
2
solutions
(77 K) for 3a and 3b and are very similar for both complexes.
The EPR spectrum obtained in the case of 3a is presented in
Fig. 2. The spectra are composed of typical S= 1/2 rhombic
signals with g
1
= 2.39, g
2
= 2.13, and g
3
= 2.005 for 3a and g
1
=
2.39, g
2
= 2.13, and g
3
= 2.006 for 3b. These EPR signals are
very similar to the one reported previously for the [LNiO
2
Cu-
(MeAN)]
+
complex.
17
Notably, no interaction of the electron
spin with the nuclear spin of copper is observed for both 3a/
3b, consistent with their Ni
III
(µ-O)
2
Cu
III
assignment, in agree-
ment with the data previously reported in the literature for the
Cu–Ni analog.
17
Resonance Raman spectroscopy
The resonance Raman (rRaman) spectra of 3a and 3b were
recorded using 413 nm laser excitation. rRaman bands are
detected at 605 cm
−1
and 612 cm
−1
for 3a and 3b, respectively
(Fig. 3, ESI†), which are sensitive to
18
O substitution (per-
formed by using Ni
18
O
2
as a starting reagent) and downshift
(Δ
18
O) by 31 cm
−1
and 29 cm
−1
, respectively. Notably, these
bands are shifted to lower energies relative to the tetra-atomic
vibrational mode of the [LNi(µ-O
2
)Cu(MeAN)]
+
complex
observed at 625 cm
−1
.
17
Furthermore, as previously observed,
the absence of any isotope sensitive bands in the
700–1200 cm
−1
region excludes the possibility of the presence
of peroxo or superoxo units in 3a or 3b. Interestingly, a second
18
O sensitive band is also detected at 556 cm
−1
(Δ
18
O=
−21 cm
−1
) and 579 cm
−1
(Δ
18
O=−19 cm
−1
) for 3a and 3b
respectively, which was not observed in the previously reported
spectrum of the [LNiO
2
Cu(MeAN)]
+
complex.
DFT calculations
DFT calculations were performed on 3a and 3b in an effort to
understand their structural and electronic properties as well as
the origin of the additional
18
O sensitive band in the rRaman
spectra. DFT-optimised structures revealed a four-coordinate
distorted square-planar geometry around the nickel centre.
Two conformations around the copper ion were computed for
each system (conformers A and B). In all cases, the copper site
is in a five-coordinate square pyramidal geometry but with
either the amine group (conformer A) or one pyridine group
(conformer B) weakly bonded to the metal ion in the axial
position. Energetic analysis conducted on both 3a and 3b
showed that the minimum free energy is obtained for the con-
formers (A) which make these structures the most favoured
one (see Table S3; ESI†). The computed structures for 3a and
3b (conformers A and B) are presented in Fig. 4, S11 and S12.†
Both spin density plots and Singly Occupied Molecular
Orbitals (SOMO) reveal that 3a and 3b bear one unpaired elec-
tron located at the nickel centre (see Fig. S13 and S14; ESI†).
These results confirm the presence of one low-spin Ni
III
centre
(S= 1/2, 3d
7
) and one low-spin Cu
III
(S=0,3d
8
) centre in both
3a and 3b. This assignment is further confirmed by the com-
puted EPR parameters, which are in fair agreement with the
experimental data (see Table S4; ESI†). Frequency calculations
were conducted on the DFT-optimised structures of the two
species and Raman spectra were calculated. Vibration modes
involving the oxygen ligands were predicted at 596 cm
−1
and
621 cm
−1
for 3a and 3b (Fig. S15 and S16; Table S5†), respec-
tively, in agreement with the assignment previously proposed
for the MeAN-based Cu–Ni analog.
17
A second mode was calcu-
lated to be at 553 cm
−1
and 528 cm
−1
for 3a and 3b, respec-
tively, originating from the coupling of the Ni(µ-O
2
)Cu core
and the Ni-coordinated β-diketiminate ligand coordinates (see
Fig. S17 and S18; ESI†). Both sets of the calculated results are
in good agreement with the observed rRaman bands for each
complex (Table S5; ESI†). The five-coordination geometry at
the Cu ions obtained in the case of RPY
2
-type ligands is
Fig. 2 Absorption spectra of the Cu
I
complex (solid trace) 1a,Ni
II
superoxo complex 2(dotted trace) and the 0.15 mM [Ni
III
(µ-O
2
)Cu
III
]
+
species 3a (dashed trace) in dichloromethane at −90 °C. Inset: X-band
EPR spectrum of 3a in dichloromethane at 77 K.
Fig. 3 Raman shifts obtained using a 413 excitation laser for 3a involv-
ing
16
O (top) and
18
O (bottom) atoms. Bands originated from the solvent
are marked by asterisks.
Paper Dalton Transactions
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affecting the coordination sphere of the Ni ion in 3a and 3b
and the geometry around the Ni ion appears to be significantly
distorted from planarity (dihedral angle of 11.2 and 16.2°,
respectively) while it is perfectly planar in the MeAN-based
complex (dihedral angle of 2.3°). This explains the appearance
of a new vibration in the rR spectra of 3a/3b. The spectroscopic
data thus support the DFT-calculated structure and validate
the formation of both 3a and 3b.
Reactivity studies
The reactivity of the complexes 3a and 3b has been investi-
gated in a number of nucleophilic and electrophilic reactions.
The nucleophilicity of 3a and 3b has been studied in reactions
with benzaldehyde, and cyclohexane carboxaldehyde (CCA).
The ability of 3a (3b) to undergo electrophilic Hydrogen Atom
Transfer (HAT) reactions was also evaluated in reactions with
9,10-dihydroanthracene (DHA) and 1,3-cyclohexadiene (CHD)
for C–H bonds and with 2,4-di-tert-butylphenol (2,4-DTBP) and
2,6-di-tert-butylphenol (2,6-DTBP) for O–H bonds. In a typical
reaction preformed 3a and 3b at −60 °C were treated with an
excess of the above-mentioned substrates under a pseudo first-
order condition.
The disappearance of the characteristic band at 885 nm in
the UV-Vis absorption spectrum was then monitored over time
(Fig. 5 for reaction of 3a with CCA). Rate constants in CH
2
Cl
2
at −60 °C or at −90 °C were obtained from the pseudo-first
order fit of the time trace of the decay of the 885 nm band
(Fig. 5). Second-order rate constants were then determined
from the dependence of the first-order rate constants on the
substrate concentration (Fig. 6). Second-order rate constants
derived from those studies are listed in Table 1 and compared
with that of the results obtained for the previously reported
[LNiO
2
Cu(MeAN)]
+
intermediate. Notably, complex 3b reacts
with CCA and benzaldehyde at rates significantly slower than
[LNiO
2
Cu(MeAN)]
+
.
In contrast, in the presence of DHA, CHD, 2,4-DTBP, or 2,6-
DTBP, complex 3b is found to be a better oxidant than
[LNiO
2
Cu(MeAN)]
+
. These studies point to the presence of less
nucleophilic oxygen atoms in 3b relative to [LNiO
2
Cu(MeAN)]
+
.
Notably, replacement of the methyl group of the MePy
2
ligand
in 3b with a bulkier indane group in 3a did not lead to any
significant change in the rate constants for the reactions with
Fig. 4 Optimised molecular structures for 3a; conformers A (top) and B
(bottom).
Fig. 5 Spectral changes of 3a in the presence of 50 mM CCA at −60 °C.
Inset: Time trace of the decay at 885 nm band upon addition of the sub-
strate (black trace) and the first-order fit (inset: red trace).
Fig. 6 Pseudo first-order rate constants as a function of substrate con-
centration; the reaction of 3a with cyclohexane carboxaldehyde (left)
and 2,4 di-tert-butylphenol (right) in CH
2
Cl
2
.
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DHA and benzaldehyde. However, for reactions with bulkier
substrates like 2,6-DTBP, 2,4-DTBP, or CCA, a significantly
reduced rate constant is observed for 3a relative to 3b. This
may be attributed to the sterically induced hindered access of
the substrates in the presence of the covalently attached
indane group of the IndanePY
2
ligand in 3a. Analysis of the
resultant solution after reactions of 3a or 3b with various sub-
strates showed the formation of the oxidised products in
45–70% yields (Table S1; ESI†). Interestingly, for reactions with
3a no evidence of the hydroxylation of the indane moiety was
observed, which is in contrast to the previous report of the
intramolecular indane hydroxylation reaction mediated by the
homodinuclear copper–oxygen intermediate.
18
Additional DFT calculations were performed in order to
further characterise the reactivity differences. Natural popu-
lation analysis was performed for 3a/3b and the results were
compared with those obtained for [LNi
III
O
2
Cu
III
(MeAN)]
+
(see
the ESI†). The natural negative charges found at the O atoms
are smaller (by −7% to −8%) in 3a and 3b with respect to
those of the corresponding atoms in [LNi
III
O
2
Cu
III
(MeAN)]
+
.
These data indicate that the oxido cores in 3a and 3b are less
electron-rich compared to that of [LNi
III
O
2
Cu
III
(MeAN)]
+
.It
thus correlates with the decreased nucleophilicity presently
observed.
Conclusions
Examples of heterobimetallic dioxygen intermediates are extre-
mely rare in the literature and hence, very little is known about
their reactivity. This is unfortunate, as metal dioxygen cores
involving non-equivalent metal-sites have been proposed as
the active species in a number of biological and chemical oxi-
dation processes, and a detailed knowledge about their reactiv-
ity can provide vital mechanistic insights. In our continuous
effort to uncover structure–reactivity relationships of un-
precedented dinuclear metal–dioxygen intermediates we now
investigate the role of ancillary ligands in controlling the spec-
troscopic and reactivity properties of a heterobimetallic
[Ni(μ-O)
2
Cu]
+
core. The first (and only example before this
study of such a core) has been spectroscopically identified in
our groups and previously shown to possess a nucleophilic oxo
group that led to the deformylation of aldehydes. We now
show that the nucleophilicity of the oxo groups can be tuned
by varying the electron donation of the ancillary ligand at the
copper centre. Thus replacement of the MeAN ligand in the
previously reported [LNi(µ-O
2
)Cu(MeAN)]
+
complex by less
basic/nucleophilic RPY
2
ligands in 3a/3b led to slower nucleo-
philic and faster electrophilic reactions in the latter. Based on
the DFT calculations, the difference in reactivity has been
attributed to the presence of a five coordinate copper centre
and a significant distortion of the nickel centre from planarity
in 3a/3b. These result in the development of lower negative
charges at the oxygen atoms of 3a/3b relative to the [LNi(µ-O
2
)-
Cu(MeAN)]
+
complex, where a four coordinate square planar
structure has been reported to be prevalent at both the Cu and
Ni centres. Furthermore, for reactions with 3a no hydroxylation
of the indane moiety occurred, in contrast to the previous
report of the intramolecular indane hydroxylation reaction
mediated by the homodinuclear {(indane-py
2
)CuO
2
Cu(indane-
py
2
)} complex. This again emphasizes the initiation of novel
reactivity properties in heterodinuclear systems relative to their
homobimetallic counterparts. The present results therefore
underline the significance of subtle geometric and electronic
changes in the physical and chemical properties of this
important class of biologically relevant metal–dioxygen
intermediates.
Experimental
All chemicals used in this study except for the ligands and the
complexes were commercial products of the highest available
purity. Commercial starting materials were used without
further purification, except for 2-vinylpyridine which was dis-
tilled prior to use. Preparation and handling of air sensitive
materials were performed in a N
2
glove box OMNI-Lab 2 (VAC)
with O
2
and moisture concentrations less than 2 ppm.
1
HNMR
spectra were recorded on a Bruker Avance III 300 MHz NMR
spectrometer. UV-Vis spectra were recorded by using an Agilent
8453 diode array spectrophotometer from 300–1100 nm with
samples in a 1.0 path length quartz cuvette, connected with a
cryostat from Unisoku Scientific Instruments, Japan.
Synthetic procedures
General procedure of synthesis of ligands
CH
3
PY2. The ligand has been synthesized in a slightly
different way from the already reported procedure.
1g
To absol-
ute MeOH (80 mL) were added 2-vinylpyridine (11 g,
0.210 mol), 2-(2-methylaminoethyl) pyridine (4.2 g, 0.030 mol),
and glacial acetic acid (1.8 g, 0.060 mol). After refluxing for 5
days under argon, MeOH was evaporated and 15% NaOH was
Table 1 Second order rate constants determined for different substrates by kinetic studies by UV-Vis spectroscopy
Substrates
Benzaldehyde
(−60 °C)
CCA
(−60 °C)
DHA
(−60 °C)
CHD
(−60 °C)
2,4 DTBP
(−90 °C)
2,6 DTBP
(−60 °C)
3a k
2
/M
−1
s
−1
0.021 0.088 0.015 0.053 4.60 0.0065
3b k
2
/M
−1
s
−1
0.0178 0.557 0.015 0.014 Too fast 0.0985
LNi
III
[(µ-oxo)Cu
III
(MeAN)]
+
k
2
/M
−1
s
−1
0.11 (−50 °C) Too fast (−90 °C) Too slow (−70 °C) 0.0005 (−70 °C) 2.44 (−90 °C) 0.0001 (−80 °C)
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added. The product was extracted with CH
2
Cl
2
(×3) and the
combined organic layers were dried over Na
2
SO
4
. Evaporation
of CH
2
Cl
2
under reduced pressure left the crude product.
Flash chromatography (neutral alumina, ethylacetate/MeOH,
98 : 2) afforded the ligand as a yellow product (yield: 40%).
IndanePY
2
.The ligand has been synthesized according to the
already reported procedure.
18a
General procedure of the synthesis of complexes
The complexes 1a and 1b have been synthesized in situ by the
dropwise addition of a solution of 1 eq. Cu
I
(CH
3
CN)
4
OTf in
anhydrous dichloromethane in a solution of 1 eq. dissolved
ligand in the same solvent.
General procedure for the synthesis of [LNi
III
(O
2
)Cu
III
(RPY2)]
The greenish solution of 2in anhydrous CH
2
Cl
2
(2.5 mL;
0.15 mM) was cooled to −90 °C. Then 1 equivalent of 1a (1b)
in anhydrous CH
2
Cl
2
(0.1 mL) was added to yield the deep
brown intermediate 3a (3b). The generation of the deep brown
species 3a (3b) was monitored by the growth of the 885 nm
band in the UV/vis spectrum.
18
O labelled samples for rRaman
experiments were prepared using the same procedure but start-
ing from
18
O labelled Ni precursor 2.
EPR spectroscopy
EPR measurements were performed on a Bruker ESP 300
X-band EPR spectrometer equipped with a fused quartz Dewar
for measurements at liquid nitrogen temperature.
Resonance Raman
rRaman spectra were recorded in CH
2
Cl
2
at −92 °C (Bruker
cryostat) using 413 nm excitation with a Horiba Jobin-Yvon
LabRAM HR800 confocal Raman spectrometer. The concen-
trations of samples were from 3 mM to 8 mM.
DFT calculations
All theoretical calculations were performed with the ORCA
program package.
19
Full geometry optimizations were carried
out for all complexes using the GGA functional BP86
20–22
in
combination with the TZV/P
23
basis set for all atoms and by
taking advantage of the resolution of the identity (RI) approxi-
mation in the Split-RI-J variant
24
with the appropriate
Coulomb fitting sets.
25
Increased integration grids (Grid4 in
ORCA convention) and tight SCF convergence criteria were
used. Solvent effects were accounted within the framework of
the conductor like screening model (COSMO) which is a
dielectric continuum approach.
26
Vibrational frequency calcu-
lations were performed to ensure that each geometry optimi-
zation converged to a real minimum. IR and rRaman spectra
were obtained from numerical frequency calculations per-
formed on the optimised structures using the B3LYP
27,28
func-
tional together with the TZV/P
23
basis set. The relative energies
were obtained from single-point calculations using the same
combination of functional and basis sets. They were computed
from the gas-phase optimised structures as a sum of electronic
energy, thermal corrections to free energy, and free energy of
solvation. g-Tensors
29–32
were evaluated from additional single
point calculations using the B3LYP functional. Scalar relativistic
effects were included with ZORA paired using the SARC
def2-TZVP(-f) basis sets
33,34
and the decontracted def2-TZVP/J
Coulomb fitting basis sets for all atoms. Increased integration
grids (Grid4 and GridX4 in ORCA convention) and tight SCF
convergence criteria were used. Picture change effects were
applied and the integration grids were increased to an
integration accuracy of 11 (ORCA convention) for the metal
centre. Natural charges were obtained from additional single-
point calculations using the Natural Population Analysis (NPA)
and the B3LYP functional together with the TZV/P basis set.
35
Molecular orbitals, spin density plots and vibrational normal
modes were visualized with Chemcraft software.
36
Reactivity studies
Exogenous substrate oxidation. In a typical experiment,
0.15 mM to 0.2 mM of 3a (3b) was generated at −90 °C, as dis-
cussed previously. The reactivity studies of 3a (3b) were per-
formed at −60 °C under an inert atmosphere, by injecting the
CH
2
Cl
2
solution of the substrate (varying from 1 mM to
60 mM). Spectral changes at the 885 nm band were monitored
by using a UV-Vis spectrum every 1 second. The pseudo-first
order oxidation rate was then extracted and compared with the
self decay of 3a (3b) in the absence of the substrate. If the rate
of oxidation in the presence of the substrate was higher than
that in the absence of the substrate, it was concluded that 3a
(3b) reacted with that substrate. The pseudo-first order fitting
of the decay curves yielded the rate constants (k
obs
), which
follow linear dependence with the concentration of the sub-
strate. The slope of the rate constant (k
obs
)versus substrate
concentration fitting plot provided the second order rate con-
stants (k
2
).
Intramolecular ligand oxidation. The brownish solution of
3a was warmed up at room temperature. The solution was
treated with 35% ammonia solution and after extraction with
dichloromethane (×2) the organic phases were washed with
brine (×2); the organic phases were collected, dried over
MgSO
4
and the solvent was removed under vacuum. The
resulting solid was analysed by
1
H NMR spectroscopy and
mass spectrometry which showed the absence of the hydroxyl-
ated product.
Acknowledgements
This work has been carried out in the framework of COST
action CM1305 (ECOSTBio) including two STSMs
(STSM-CM1305-28897 and STSM-CM1305-32182). Financial
support by the French Research Agency (COMEBAC/ANR-13-
BSO7-0018) as well as from the DFG (Cluster of Excellence
“Unifying Concepts in Catalysis”; EXC 314-2) is gratefully
acknowledged. K. R. also thanks the Heisenberg-Program of
DFG for financial support.
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