Ligand and Metal Based Multielectron Redox Chemistry of Cobalt Supported by Tetradentate Schiff Bases

Article abstract of DOI:10.1021/jacs.7b03604

We have investigated the influence of bound cations on the reduction of cobalt complexes of redox active ligands and explored the reactivity of reduced species with CO₂. The one electron reduction of [Co^II(^Rsalophen)] with

Full text of DOI:10.1021/jacs.7b03604

Article
pubs.acs.org/JACS
Ligand and Metal Based Multielectron Redox Chemistry of Cobalt
Supported by Tetradentate Schiff Bases
Julie Andrez,† Valentin Guidal,‡ Rosario Scopelliti,† Jacques Pecaut,‡ Serge Gambarelli,‡
́
and Marinella Mazzanti*,†
†Institut des Sciences et Ingen
́ ́ ́
ierie Chimiques Ecole Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland
‡Univ. Grenoble Alpes, CEA, CNRS, INAC, SYMMES, F-38000 Grenoble, France
S
* Supporting Information
ABSTRACT: We have investigated the influence of bound
cations on the reduction of cobalt complexes of redox active
ligands and explored the reactivity of reduced species with
CO2. The one electron reduction of [CoII(Rsalophen)] with
alkali metals (M = Li, Na, K) leads to either ligand-centered or
metal-centered reduction depending on the alkali ion. It
affords either the [CoI(Rsalophen)K] complexes or the
[CoII (bis-salophen)M2] (M = Li, Na) dimers that are present
2
in solution in equilibrium with the respective [CoI(salophen)-
M] complexes. The two electron reduction of
[CoII(OMesalophen)] results in both ligand centered and
metal centered reduction affording the Co(I)−Co(II)−Co(I)
[Co3(tris-OMesalophen)Na6(THF)6], 6 complex supported by
a bridging deca-anionic tris-OMesalophen10− ligand where three
OMesalophen units are connected by two C−C bonds. Removal of the Na ion from 6 leads to a redistribution of the electrons
affording the complex [(Co(OMesalophen))2Na][Na(cryptand)]3, 7. The EPR spectrum of 7 suggests the presence of a Co(I)
bound to a radical anionic ligand. Dissolution of 7 in pyridine leads to the isolation of [CoI2(bis-OMesalophen)Na2Py4][Na-
(cryptand)]2, 8. Complex 6 reacts with ambient CO2 leading to multiple CO2 reduction products. The product of CO2 addition
to the OMesalophen ligand, [Co(OMesalophen-CO2)Na]2[Na(cryptand)]2, 9, was isolated but CO32− formation in 53% yield was
also detected. Thus, the electrons stored in the reversible C−C bonds may be used for the transformation of carbon dioxide.
active ligands in complexes of d-block37−41 and f-block6,42,43
INTRODUCTION
■
metals leading to intramolecular or intermolecular C−C bond
Multielectron redox processes play a key role in chemical and
formation. Moreover, reactivity studies suggest that the
enzymatic transformations catalyzed by transition metals. An
electrons stored in the C−C bond may become available for
increasingly used strategy, both in d-block and f-block
various chemical transformations including CO2 fixation.37
chemistry, for the design of complexes capable of multielectron
transfer, involves the use of redox-active ligands.1−6 These
Schiff base complexes of cobalt have been reported to act as
catalyst in the electrochemical reduction of CO2,44 and in
ligands can be used to store and release electrons during
hydrogen production,45 but the role of ligand-based redox
substrate transformation, without a change in the metal
oxidation state, and as such to enable novel reactivity.7−10
processes in these transformations has not been elucidated.
The chemical monoelectronic reduction of the
Notably, the excellent properties of bis(imino)pyridine as
[CoII(Rsalophen)] (R= H and OMe) was shown not to affect
supporting ligands in catalytic and stoichiometric reactions
mediated by first row transition metals have been interpreted in
the metal center but to afford the reduction of the imine group
and C−C formation.37 The resulting complex was reported to
bind reversibly carbon dioxide.37 Although the structure of the
carbon dioxide adduct was not characterized, it was proposed to
be very similar to that of the complex [CoI(Rsalen)K(CO2)].46
The latter complex forms reversibly from the reaction of CO2
with the bimetallic Co(I) complex [CoI(Rsalen)K] obtained
from the reduction of the Schiff base complex [CoII(Rsalen)] (R
= H, Et and nPr) with K.46 This reaction was the first example
terms of their redox-active character.8,11−14 The redox-active
behavior of ligands such as polypyridines or unsaturated
azamacrocycles also plays a key role during electrocatalytic CO2
reduction by metal complexes of these ligands impacting on
reactivity and product selectivity.15−19 Other ligands with well
identified redox-active behavior include amido-phenolates,20
dipyrromethane,21 diazadienes,22−25 azopyridine,26 bipyri-
dine,27,28 phenanthroline,29 terpyridine.30
Redox events at the ligand may be accompanied by the
formation of a new C−C bond29,31−34 that in some cases is
reversible.29,31,32,35,36 Tetradentate Schiff bases can act as redox-
Received: April 10, 2017
© XXXX American Chemical Society
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of cooperative binding of carbon dioxide by a heterobimetallic
complex. Multimetallic cooperative binding is thought to play a
key role in electrocatalytic reduction of carbon dioxide and in
its biological transformation,47−49 but examples of carbon
dioxide reduction at molecular heterobimetallic complexes
remain very rare up to date.50−54
Recent studies from Bart and co-workers have shown that
subtle variation of the donor properties of coligands structure in
uranium complexes of bis(imino)pyridine leads to drastic
changes in the distribution of the charge in these complexes
and in their reactivity.55 In contrast the possible effect of the
presence and nature of bound counterions on the electronic
structure of complexes of redox active ligands has not been
investigated. The heterobimetallic [CoI(salophen)M] (M = Li,
Na, K) complex is an ideal candidate to investigate how bound
cations affect the redox chemistry in metal complexes of redox
active ligands. Here we show that the presence of bound
cations and the nature of the cation result in dramatic changes
in the structure of the reduced species.
Figure 1. ORTEP of the [Co(salophen)K(THF)]n 1 (right) polymer
and of the [Co(salophen)K(THF)] unit (left). Ellipsoids at 50%.
Carbon atoms in gray, hydrogen atoms omitted for clarity.
Table 1. Average Bond Distances in Reduced Co(salophen)
Complexes
Co−Nimino
Co−Namido
Co−O
C−Nimino
C−Namido
1
2
3
5
6
7
1.829(8)
1.833(6)
1.852(5)
1.892(4)
1.830(6)
1.84(1)
/
1.90(1)
1.884(8)
1.87(3)
1.916(5)
1.90(3)
1.86(3)
1.303(3)
1.327(4)
1.306(8)
1.298(6)
1.359(8)
1.34(2)
/
/
/
1.824(5)
1.875(4)
1.857(4)
/
1.464(7)
1.449(5)
1.457(1)
/
RESULTS AND DISCUSSION
■
One Electron Reduction of Co(Rsalophen) Complexes
by Alkali Metals. The reduction of the Co(II) complexes
[Co(salophen)] and [Co(OMesalophen)] with one equivalent of
potassium metal in THF affords the polymeric complex
[Co(salophen)K(THF)]n, 1 and the monomeric complex
[Co(OMesalophen)K(THF)], 2 (Scheme 1).
found in the [Co(salen) (Na(THF)]n polymer (Co−O =
1.899(3) Å and Co−N = 1.82(1) Å). A lengthening of the Co−
O distances and a shortening of the Co−N distances compared
to those found in [Co(salophen)] (Co−O = 1.843(4) Å and
Co−N = 1.873(4) Å) is observed.58 The lengthening of the
Co−O distances is due to the K+ binding, while the shortening
of the Co−N bond is consistent with the presence of cobalt in a
lower oxidation state.57
Scheme 1. Reduction of [Co(Rsalophen)] (R = H, OMe)
with Alkali Metals
Crystals of [Co(OMesalophen)K] were obtained by slow
diffusion of DIPE into a THF solution of complex 2. The solid
structure of 2 (Figure 2), shows the presence of a
Figure 2. ORTEP of [Co(OMesalophen)K(THF)]2 2. Ellipsoids at
50%. Carbon atoms in gray. Hydrogen atoms omitted for clarity.
The solid state structure of 1 determined by X-ray diffraction
(Figure 1) shows the presence of a heterobimetallic CoK
complex where the phenolate oxygen atoms bind both the Co
and the K cations. Each K ion binds two THF molecules, two
oxygen atoms from one Co(salophen) moiety and two oxygen
atoms from another Co(salophen) moiety resulting in a
polymeric structure. The Co ion is tetracoordinated by the
two oxygen and two nitrogen atoms of the salophen ligand with
a square planar geometry. The structure of complex 1 is similar
to that previously reported for the polymeric complex
[Co(salen)Na(THF)]n, prepared by reduction of [Co(salen)]
with sodium metal.56,57 The mean Co−O (1.90(1) Å and Co−
N distances (1.829(8) Å) (selected bond distances for all
complexes are summarized in Table 1) are comparable to those
tetracoordinate cobalt ion in the O2N2 pocket of the ligand
with a square planar geometry. The average Co−O and Co−N
distances are 1.884(8) Å and 1.833(6) Å respectively and are in
the range of those found in complex 1. The two distances C7−
N1 and C14−N2 (1.329(2) Å and 1.324(2) Å) are in
agreement with the presence of CN double bonds. The
potassium counterion binds the four oxygen atoms of the ligand
slightly below the oxygen atoms plane (0.65 Å). A THF
molecule completes its coordination sphere. The methoxy
substituents of the OMesalophen ligand provide a binding pocket
for the potassium cation thus preventing polymerization.
The structure of complexes 1 and 2 differs from the solid
state structures previously reported for the compound obtained
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from the reduction of [Co(salophen)] and [Co(OMesalophen)]
with sodium metal.37 Notably, the reduction of [Co(salophen)]
and [Co(OMesalophen)] by sodium does not lead to the
reduction of the Co(II) center to Co(I), but to the reduction of
the imino function of the ligand yielding the dimeric complexes
one in the diamagnetic region and one in the paramagnetic
region, is clearly identified. The first set of seven signals in the
1
6.0−10.6 ppm region is similar to that found in the H NMR
spectrum of 1 (see Supporting Information) and accordingly
suggests the presence of a square planar Co(I) d8 complex of
analogous [Co(salophen)Na(THF)] formula. A second set of
14 signals is also found in the paramagnetic region between 50
and −130 ppm. These signals are in agreement with the
presence in solution of the dimeric Co(II) complex [Co2(bis-
salophen)Na2(THF)6].
of cobalt(II) [Co2 (bis-salophen)Na2 (THF)6 ],
3
[Co2(bis-OMesalophen)Na2(THF)4], 4 (Scheme 1). These
complexes contain the reduced hexa-anionic bis-salophen
ligands arising from the formation of a C−C bond between
two reduced imino carbons of two different salophen units.
The difference in the molecular structure of the compounds
obtained from the reduction of [Co(salophen)] and [Co-
Proton NMR studies of this system (Figure 4) show that the
ratio of Co(II) versus Co(I) species in solution changes with
(
OMesalophen)] with K and Na, indicates that the nature of the
alkali ion has an important effect on the outcome of the
reduction. While in the presence of potassium the reduction
occurs on the metal, in the presence of sodium it occurs on the
ligand.
In order to gain more information on these compounds we
have investigated and compared the solution structure of
complexes 1 and 2 with that of the previously reported
[Co2(bis-salophen)Na2(THF)6] complex. The [Co2(bis-
salophen)Na2(THF)6], 3 and [Co2(bis-OMesalophen)-
Na2(THF)4], 4 complexes were prepared using a modified
literature procedure.37 Notably, in the published procedure the
dimeric complexes 3 and 4 were prepared by reacting
[Co(salophen)] and [Co(OMesalophen)] with an excess sodium
(1.5 to 2.5 equiv) for 4 h. Considering that excess sodium can
lead to further reduction (see following sections) we prepared
the complexes 3 and 4 by reacting the Co(II) precursors with
one equivalent of sodium for 24 h.
The proton NMR spectra of 1 and 2 in deuterated THF (up
to the solubility limit) show the presence of only one set of
seven signals in the diamagnetic region between 6.0 and 10.4
ppm in agreement with the presence of C2v symmetric solution
species containing a diamagnetic square planar Co(I) d8
complex. The X band (9.6473 GHz) EPR spectrum of a 10
mM solution of 1 in THF/DIPE 4:1 at 10.1 K does not show
any signal in agreement with the presence of a square planar
Co(I) d8 complex.
Figure 4. Zoom of the 1H NMR spectra (298 K, 400 MHz) (between
32 and −30 ppm) in THF-d8 of complex 3 at variable concentrations.
1
The H NMR spectrum in THF at 298 K of the complex 3
the concentration in agreement with the presence of an
equilibrium between the monomeric Co(I) complex and the
dimeric Co(II) complex. Notably, for starting concentrations of
the [Co(salophen)] complex equal to or lower than 10 mM
only the set of signals assigned to the monomeric Co(I)
complex is observed. Due to the low solubility of [Co2(bis-
salophen)Na2(THF)6] it was not possible to attain concen-
trations where only this complex is present in solution. The
measurement of the diffusion coefficients by Pulsed-Field
Gradient Stimulated Echo (PFGSTE) NMR in THF-d8 at 298
prepared “in situ” by reduction of a 0.353 M solution of
[Co(salophen)] with one equivalent of sodium metal shows a
significantly more complex pattern compared to the 1H
spectrum of 1 (Figure 3). The presence of two sets of signals,
K for the two species (DCo(I) = 5.92.10−10 m2/s and DCo(II)
=
5.17.10−10 m2/s) is in agreement with the presence of a
monomeric and a dimeric complex in solution. Finally the
presence of the previously reported complex [Co2(bis-
salophen)Na2] in the concentrated mixture was confirmed by
X-ray diffraction of the crystallized product.
EPR studies were also carried out for 4/1 THF:DIPE
solutions of the [Co(salophen)] complex and of its reduction
products at different concentrations (Figure 5). The EPR
spectrum of a 4/1 THF:DIPE solution of [Co(salophen] shows
a signal typical for low-spin square planar Co(II) complexes.59
In spite of the broadening, a characteristic eight-line pattern
arising from the interaction of the electronic spin with the
Figure 3. 1H RMN (298 K, 400 MHz) spectrum in THF-d8 of a 0.353
M solution of [Co(salophen)] reacted with one equivalent of Na.
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for this complex shows the presence of a very weak C−C bond
with ΔG close to zero. Comparable values have only been
found for a C−C bonded phenoxyl radical dimer (bond
dissociation energy of 6.1(5) kcal mol−1)62 and in the
[Cp*2Yb(phen)] complex (bond dissociation energy of 8.1
(2) kcal mol−1).29
In contrast to what observed for 3, the proton NMR of the
previously reported [Co2(bis-OMesalophen)Na2(THF)4], 4
(prepared in situ by reduction of the [Co(OMesalophen)] with
one equivalent of sodium metal) shows the presence of only
one set of 7 signals in the diamagnetic region in agreement with
the presence in solution of the [Co(OMesalophen)Na] complex.
In this case dimerization is not detectable by proton NMR in
solution up to the solubility limit (10 mM) of the complex but
occurs in the solid state.
In order to further investigate the effect of the alkali ions we
have also performed the reduction of the [Co(salophen)]
complex with Li metal (Scheme 1). Proton NMR studies of the
reaction mixture obtained from the reduction of the [Co-
(salophen)] complex with 1 equiv of Li metal show the
presence of two sets of 7 and 14 signals that where assigned to
a monomeric Co(I) [Co(salophen)Li(THF)x] complex and to
a Co(II) dimeric complex [Co2(bis-salophen)Li2(THF)x]. This
suggests that the reduced complex behaves similarly in the
presence of Na+ and Li+ cations. X-ray quality crystals of the
dimeric complex [Co2(bis-salophen)Li2(Py)4], 5 were obtained
by recrystallization of the reaction mixture in pyridine. The
structure of 5 (Figure 6) shows the presence of a reduced hexa-
Figure 5. X-Band X-Band (9.64 GHz, 15 dB, T = 10 K) EPR spectra
of 4:1 THF:DIPE solutions of (a) [Co(salophen)], 10 mM, (b)
[Co(salophen)Na], 10 mM, (c) [Co(salophen)Na]/[Co2(bis-
salophen)Na2], 30 mM. (d) X-Band EPR spectra of isolated powders
of [Co2(bis-salophen)Na2(THF)6] suspended in hexane.
cobalt nucleus (ICo = 7/2) is unambiguously observed in the
high field component. In order to measure the EPR of the
complex 3 at different concentrations we prepared the complex
in situ because its solubility is very low once it is crystallized.
The EPR spectrum of the reaction mixture obtained after
reduction of [Co(salophen)] with one equivalent sodium metal
has been measured for 10 mM and 30 mM concentrations both
at room temperature (no signal observed) and at 10 K after
rapidly freezing the solutions prepared at room temperature.
No EPR signal is observed at 10 K for the frozen 10 mM
solution in agreement with the only presence of the
diamagnetic square-planar d8 Co(I) complex [Co(salophen)-
Na] (Figure 5b). In contrast, the EPR spectrum of the frozen
30 mM solution shows (at 10 K) the presence of a complex
anisotropic signal in agreement with the presence of several
paramagnetic Co(II) species in solution (Figure 5c). The EPR
spectrum obtained for powders of the complex [Co2(bis-
salophen)Na2(THF)6] suspended in n-hexane is simpler and
exhibit a pattern characteristic of an axial g tensor (g// = 2.926,
g⊥ = 2.012) with no observable hyperfine coupling (Figure 5d).
Upon close observation, this pattern can also be found in the
previous spectrum (Figure 5c).
The results presented here are in agreement with the
presence in solution of an equilibrium between a Co(I)-
salophen and Co(II)-bis-salophen species (Scheme 1). The
proposed radical intermediate containing a Co(II) bound to a
reduced salophen ligand could not be clearly identified by EPR
solution studies. We note that the equilibrium between the
Co(II)/Co(I) species is reached immediately after the
concentration adjustment. Moreover, the ratio Co(II)/Co(I)
increases when the temperature is decreased. A variable
temperature NMR study allows the determination of the
thermodynamic parameters for the interconversion Co(II)/
Co(I). The Van’t Hoff analysis for this equilibrium leads to
values of ΔH0 = −5.4(1) kcal/mol and of ΔS0 = −14(1) cal/
mol/K. The value of ΔG0 (−1.2(1) kcal/mol) at 298 K is close
to 0 kcal/mol indicating that the C−C bond formation is
thermodynamically favored at room temperature but the bond
dissociation energy is close to zero. Several examples of
reversible C−C coupling with low dissociation energies (10−20
kcal mol−1) have been reported in metal complexes29,60 and in
sterically hindered organic compounds such as the Gomberg’s
dimer.61 However, the bonding dissociation energy measured
Figure 6. ORTEP of the structure of [Co2(Py)2(bis-salophen)-
Li2(Py)4], 5. Ellipsoids at 50%. Carbon atoms in gray, C−C bond
between reduced imino groups in orange, hydrogen atoms omitted for
clarity.
anionic bis-salophen ligand resulting from the formation of a
C−C bond (1.581(8) Å) between the two carbon atoms of the
reduced imino groups from two different Co(salophen) units
related by the symmetry center of the P21/n space group. Each
cobalt center is five coordinated by two oxygens and two
nitrogens of the bis-salophen ligand and one pyridine nitrogen
with a square pyramidal geometry. Each lithium cation is
tetracoordinated by two oxygens of the bis-salophen ligand and
two pyridine nitrogens with a tetrahedral geometry. Signifi-
cantly different C−N distances are found for the imino
(1.298(6) Å) and amido (1.449(5) Å) nitrogens in agreement
with the presence of double and single bond, respectively.
A variable temperature NMR study was than performed to
determine the thermodynamic parameters for the interconver-
sion Co(II)/Co(I) in the presence of Li+ (Figure 7). The Van’t
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1
Figure 7. Variable temperature H NMR spectra (400 MHz) from
−40 to 60 °C of a 0.206 M solution in THF-d8 of [Co(salophen)]
reacted with one equivalent of Li.
Figure 8. ORTEP of the structure of [Co3(tris-OMesalophen)-
Na6(THF)6]·hex, 6·hex. Ellipsoids at 50%. Carbon atoms in gray,
C−C bond between reduced imino groups in orange. Solvent
molecules and hydrogen atoms omitted for clarity.
Hoff analysis leads to values of ΔH0 = −4.1(1) kcal/mol and of
ΔS0 = −11(1) cal/mol/K. The value of ΔG0 (at 298 K) is of
−0.7(1) kcal/mol indicating once again that the C−C bond
formation is thermodynamically favored at room temperature
but the bond dissociation energy is close to zero.
These results show a very similar behavior for the
heterobimetallic Co-M complexes of the salophen ligand for
M= Na or Li. In both cases Co(II) complexes containing a
reduced ligand are isolated in the solid state, but in solution the
species arising from metal-centered reduction and ligand-
centered reduction are in equilibrium trough reversible C−C
bond formation. In contrast for M = K only the Co(I) complex
is formed both in THF solution and in the solid state.
moieties trough formation of two C−C bonds between the
reduced immino carbon atoms of three different ligands. Six
sodium cations counterbalance the charge of the complex. In
complex 6, two sodium cations are positioned into the O4
pockets formed by the phenolate and methoxy groups of the
tris-OMesalophen10− ligand. The four remaining Na+ are bound
to the phenolate groups and to the amido groups of the ligand.
The planarity of the OMesalophen4− fragment binding the
Co(II) cation is distorted in a zigzag fashion compared to the
[Co(OMesalophen)] complex due to the presence of the two
C−C bonds (1.586(9) Å) connecting this tetra-anionic
fragment to the two OMesalophen3− fragments in the
tris-OMesalophen10− ligand. Each cobalt center is tetracoordi-
nated by two oxygen and two nitrogen atoms of the
tris-OMesalophen10− ligand with a distorted square planar
geometry. Significantly different average C−N bond distance
are found for the imino (1.358(8) Å) and amido (1.458(8) Å)
functions.
Two Electron Reduction of the [Co(OMesalophen)]
Complex by Alkali Metals. Proton NMR studies show that
the reduction of the Co(II) complexes [Co(salophen)] and
[Co(OMesalophen)] with two equivalents of potassium metal
leads to the formation of further reduced products. Here we
have further explored the products of the two-electron
reduction of [Co(OMesalophen)]. The addition of 2 equiv of
Na to a suspension of [CoII(OMesalophen)] complex leads to a
green solution (Scheme 2). Crystals of the trimeric complex
Scheme 2. Two Electron Reduction of [Co(OMesalophen)]
(Left) and the Ligand Tris-OMesalophen10− (Right)
The proton NMR spectra of the crude reaction mixture
obtained after addition of 2 equiv of Na to a suspension of
Co(II) [Co(OMesalophen)] and of the crystals of 6 both show
the presence of two sets of 14 signals between 13 and 2 ppm
with integral ratio 2:1. This is in agreement with the presence
of two symmetry related OMesalophen moieties and a third
independent OMesalophen unit as observed in the solid state
structure. The same proton NMR spectrum is also obtained for
the reaction mixture formed by the reduction of
[Co2(bis-OMesalophen)Na2(THF)4] with one equivalent of
sodium per cobalt ion. Variable temperature NMR studies
from −40 to 60 °C of a 3.3 mM solution of complex 6 in THF-
d8 do not reveal any change in the ratio of the two sets of 14
1
[Co3(tris-OMesalophen)Na6(THF)6]·hex, 6·hex, were obtained
from the slow diffusion of hexane into a THF solution of 6
(Figure 8). The crystal structure of 6 shows that reduction has
occurred both on the metal and on the imino groups of the
ligand affording a Co(I)−Co(II)−Co(I) complex supported by
the deca-anionic tris-OMesalophen10− ligand. This ligand is
formed from the coupling of three reduced OMesalophen
signals, but shifts and overlapping of the broad signals. H
NMR spectra at variable concentrations from 5 mM to
saturation (21.6 mM of Co monomer) of complex 6 in
THF-d8, do not show any equilibrium in solution. These results
suggest that the trinuclear structure is maintained in solution
without any equilibrium occurring between possible isomeric
forms.
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The molecular structure of complex 6 shows that the two
electron reduction of the [CoII(OMesalophen)] complex does
not lead to the reduction of Co(II) to Co(0) but results in the
formation of a trinuclear Co(I)−Co(II)−Co(I) compound as a
result of the reduction of both the ligand and the metal.
Notably the six electrons reduction of three Co(II) complexes
yields two Co(I), while the remaining four electrons are stored
in two C−C bonds formed from the reduction of two imino
groups.
(Scheme 4). The solid state structure of complex 8 (see
Supporting Information) shows the presence of the hexa-
Scheme 4. Formation of the C−C Bond by Solvent Change
In order to assess the role of the cation in the formation of
complex 6 and to further investigate the stability of the C−C
bond we added cryptand to a solution of 6 in THF.
The addition of 6 equiv of cryptand to complex 6 leads to the
formation of the dinuclear complex, [(Co(OMesalophen))2Na]-
[Na(cryptand)]3, complex 7 (Scheme 3). Dark needles suitable
anionic bis-OMesalophen ligand restored via the formation of a
C−C bond (1.598(13) Å) between the two carbon atoms of
imino groups from two different Co(OMesalophen) units. Each
cobalt center is tetracoordinated by two oxygens and two
nitrogens of the bis-OMesalophen ligand. Two sodium cations
are hexacoordinated by four oxygens of the bis-OMesalophen
ligand and two pyridine nitrogens. The two additional sodium
cations are encapsulated by cryptand moieties. The C−N
distances found for the imino (1.350(9) Å) and amido
(1.462(9) Å) nitrogens are significantly different confirming
the presence of double and simple bond, respectively.
Scheme 3. Cleavage of C−C Bond upon Cryptand Addition
These results show that, while it was not possible to identify
equilibria between different solution species for 6 in THF,
changes in the nature of the cation and in the nature of solvent
lead to redistribution of the electrons in the overall structure.
This indicates that the two-electrons introduced on the
[CoII(OMesalophen)] complex upon reduction by alkali metal
in THF can easily redistribute between metal and ligand.
In order to further analyze the structure of these species, EPR
spectra were measured for complexes 6 and 7.
for X-ray diffraction of 7 were obtained by slow diffusion of a
solution of 2.2.2-cryptand in THF into a THF solution of 6.
The structure of complex 7 shows the presence of a trianionic
cobalt dimer and of three (Na(cryptand))+ cations (Figure 9).
The EPR of a solution of complex 6 in THF/DIPE shows a
characteristic signal of a Co(II) species with a rhombic g tensor
and a well-defined hyperfine tensor with the I = 7/2 59Co
nuclear spin (Figure 10b). Similarly, the EPR of powdered
crystals of complex 6 in hex/DIPE shows a characteristic signal
of a Co(II) species with a rhombic g tensor and a well-defined
hyperfine tensor with the I = 7/2 59Co nuclear spin (Figure
10c). Upon closer analysis, it is possible to observe in the EPR
spectrum of crystals of 6 the presence of at least one very minor
Figure 9. ORTEP of the structure of the [(Co(OMesalophen))2Na]3−
anion in 7. Ellipsoids at 50%. Hydrogen atoms omitted for clarity.
Two [Co(OMesalophen)]2− moieties in 7 are bridged by a
sodium cation in a quasi-perpendicular fashion (85.4° between
the planes defined by the N2O2 sites of the ligands). The Co
atoms are tetracoordinated in a square planar geometry with an
average Co−N distance of 1.84(1) Å and an average Co−O
distance of 1.86(3) Å.
The short C−N bond lengths (1.34(2) Å in average) are
consistent with the presence of two imino functions on each
ligand.
Complex 7 is completely insoluble in THF but soluble in
pyridine. However, the H NMR does not show any signal
suggesting the presence of fluxional species or free-radical
species in solution. Slow diffusion of hexane into a pyridine
solution of 7 leads to the isolation of the Co(I) bis-OMesalophen
dimer [Co2(bis-OMesalophen)Na2Py4][Na(cryptand)]2, 8
Figure 10. X-Band (9.64 GHz) EPR spectra of (a) powdered
[(Co(OMesalophen))2Na][Na(cryptand)]3 7, as a suspension in a
mixture 4:1 hex/DIPE at 10K and 32 dB, (b) a solution of
[Co3(tris-OMesalophen)Na6(THF)6] 6, in a mixture 4:1 THF/DIPE
at 20K and 23 dB, (c) powdered [Co3(tris-OMesalophen)Na6(THF)6]
6, as a suspension in a mixture 4:1 hex/DIPE at 20K and 21 dB.
1
F
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species. These Co(II) impurities are present in sizable quantity
in the raw reaction mixture and are gradually removed by
successive recrystallization as demonstrated by EPR spectra
(see Supporting Information). The EPR of solutions of 6 shows
the presence of the same species, the only difference arising
from the broader lines that blur out the smallest hyperfine
interactions (Figure 10b). The origin of this broadening could
be the presence of sizable g and A strain due to slight
geometrical changes around the Co(II) ion.
The EPR spectrum at 10 K of the complex 7 isolated by
addition of cryptand to a solution of complex 6 in THF does
not show any trace of an EPR active Co(II) species. Rather, it
consists of a narrow (11 G peak to peak) and nearly isotropic
line (g = 1.968) that points toward the presence of a
paramagnetic species with a strong radical character. This
spectrum could be explained by the presence of a low spin
Co(I) system with a ligand centered radical. The presence of a
Co(0) species can be ruled out by the absence of any sizable
hyperfine coupling with the 59Co I = 7/2 nuclear spin.63,64 Two
minor signals can be noted on this spectrum. From batch to
batch, the relative intensity of these species changes. They
could be due to impurities or to a system with a different
electron localization.
Reactivity. Proton NMR studies show that a THF solution
of complex 6 reacts readily with 3 equiv of AgOTF affording
the Co(I) [Co(OMesalophen)Na] complex as the only NMR
active product (see Supporting Information). This suggests that
the electrons stored in the C−C bonds are available for the
reduction of substrates.
Figure 11. ORTEP of the structure of the dianion [Co(OMesalophen-
2−
CO2)Na]2 in 9 (right) and of the [Co(OMesalophen-CO2)Na]−
fragment (left). Ellipsoids at 50%. Lattice solvent molecules and
hydrogen atoms omitted for clarity.
presence of a CO2−C−N single bond. The two C−Ccarboxylate
bond lengths found in the dimer are significantly different
(1.590(7) Å and 1.556(8) Å) but both comparable to values
found for C−C bonds formed from reductive coupling of the
salophen imino groups in complexes 4 and 6. The O−C−O
angle values of the carboxylate moieties are 126.4(5)° and
128.9(6)°. The two carboxylate functions feature a slightly
longer C−Ocarboxylate bond distance for the oxygen atoms
coordinated to the sodium cation (1.257(6) Å in average)
compared to the unbound oxygen atoms (1.230(4) Å in
average). However, these C−O distances remain short in
agreement with the presence of a delocalized double bond.
Unfortunately, because of the low solubility of 9 and of the
presence of multiple reaction products, complex 9 could not be
isolated in reasonable amounts for further characterization.
The dissolution in deuterated water of the residue obtained
after removal of volatiles from the reaction mixture obtained
from the reaction of 6 with excess 13CO2 leads to partial
dissolution of the solid. The 13C NMR spectrum of this water
Since the electrons stored in the complexes formed from the
reduction of [CoII(OMesalophen)] are easily available at the
metal center, we have investigated the possibility of using them
for the reductions of carbon dioxide.
The reaction of complex 3 with CO2 resulted only in the
reversible coordination of CO2 as previously reported for 3 and
4 by Floriani and co-workers.37 The exposure of complex 3 to
CO2 (1 atm) in THF-d8 lead to the formation of a completely
1
insoluble product as previously described. HNMR studies of
the reaction of 3 with CO2 show that the diamagnetic Co(I)
complex first reacts with CO2 followed by complete
disappearance of all signals (Co(I) and Co(II)) and formation
of an insoluble product. Suspension of this product in THF
leads to CO2 release and restoration of the proton NMR signals
of complexes 3. In contrast complex 6 reacts irreversibly with
excess 13CO2 (1 atm) in THF-d8 to yield a brown suspension.
2−
solution shows a signal at 167 ppm, assigned to the CO3
dianion.65 The addition of 13C labeled sodium acetate as
internal standard allows the determination of the yield in
carbonate product that amounts to 53% of CO3 (per Co
2−
atom). When only 1eq of 13CO2 per Co atom is added onto a
THF-d8 solution of 6, a comparable yield in carbonate was
measured (58%). Carbon monoxide should also form in the
disproportionation reaction of CO2 to carbonate, but it could
not be observed by 13C NMR spectroscopy probably because of
the CO coordination to the cobalt center.66 These results show
that the electrons stored into the C−C bond become available
1
The H NMR spectrum in THF-d8 of the supernatant after 1
night is NMR silent. Single crystals suitable for X-ray diffraction
of the complex [Co(OMesalophen-CO2)Na]2[Na(cryptand)]2,
9, were obtained by slow diffusion of DIPE into a THF solution
of 6 reacted with CO2 after addition of cryptand. The structure
of complex 9 shows the presence of the product of ligand-based
CO2 reduction The structure of complex 9.(THF)2 (Figure 11)
is composed of the dimeric dianion [Co(OMesalophen-CO2)-
Na]22− and of two [Na(cryptand)]+ cations. Each Co(II) atoms
is tetracoordinated in a square planar geometry by the tetra-
‑
for the reduction of CO2 to CO2 which subsequently
disproportionates to yield carbonate and CO or may add
onto the ligand framework (Scheme 5). The formation of
complex 9 may result either from direct two-electron reduction
of CO2 by complex 6 that makes available the two electrons
stored in the C−C bonds or by the one electron reduction
followed by electrophilic addition to a radical localized on the
C−N moiety.
4−
anionic OMesalophen-CO2 ligand resulting from the addition
of CO2 to one imino carbon of the OMesalophen ligand. Each
Na+ cation bind a carboxylate and a phenolate from an adjacent
[Co(OMesalophen-CO2)Na]− anion yielding a [Co-
2−
OMesalophen-CO2)Na]2 dianion. A significant lengthening
CONCLUSION
(
■
of the C−N bond distances is observed for the CO2−C−N
bonds (1.456(7) and 1.474(8) Å) compared to the imino C−N
bonds (1.302(8) and 1.320(8) Å) in agreement with the
To summarize, the one electron reduction of [Co(Rsalophen)]
with alkali metals leads to the formation of heterometallic Co-
M complexes that have undergone either ligand-centered or
G
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Article
Elemental Analyses. Analyses were performed under argon by
Analytische Laboratorien GMBH at Lindlar, (Germany) or by the
elemental analyses department of the EPFL using a Thermo Scientific
Flash 2000 Organic Elemental Analyzer.
Scheme 5. Reaction of Complex 6 with CO2, and the Ligand
OMeSalophen-CO2
4−
Starting Materials. Unless otherwise noted, reagents were
purchased from commercial suppliers and used without further
purification. 2,2,2-Cryptand was recrystallized from THF prior to
use. Commercial anhydrous CoCl2 was purified by extraction in THF
to yield CoCl2(THF)2. [Co{N(SiMe3)2}2(THF)] was prepared as
previously reported.67 The solvents were purchased from Aldrich,
Eurisotop or Cortecnet (deuterated solvents) in their anhydrous form,
conditioned under argon and vacuum distilled from K/benzophenone
(pyridine, DIPE, DME and THF) or sodium dispersion (hexane). All
reagents were dried under high-vacuum for 7 days prior to use.
Schiff-base ligands were prepared in air by the condensation of 1,2-
phenylenediamine with the corresponding salicylaldehyde derivatives
(1:2 stoichiometric ratio) in ethanol under reflux similarly to earlier
procedures.68
The potassium salts of the Schiff base ligands were prepared as
previously described42,69 by addition of KH or NaH to a THF solution
R
of the corresponding Schiff base. The resulting M2 salophen (yellow
to orange), salts were obtained in 70−95% yield.
K2salophen. 1H NMR (200 MHz, THF-d8, 298 K) δ = 8.4 (s, 2H),
7.2 (s, 2H), 7.1 (s, 2H), 7.0 (s, 2H), 6.8 (s, 2H), 6.4 (s, 2H), 6.1 (s,
2H).
metal-centered reduction depending on the alkali ion. The two
electron reduction of [Co(Rsalophen)] results in both ligand
centered and metal centered reduction affording a Co(I)−
Co(II)−Co(I) complex supported by a bridging deca-anionic
tris-OMesalophen10− ligand where three OMesalophen units are
connected by two C−C bonds. In this case the structure of the
trimer is maintained in solution. However, the electrons stored
in the two C−C bonds may easily become localized on the
metal center upon removal of the bound Na cation from the
neutral complex [Co3(tris-OMesalophen)Na6(THF)6]. Notably,
removal of the cations leads to the cleavage of the C−C bonds
and isolation of a new species, [(Co(OMesalophen))2Na][Na-
(cryptand)]3 where the formal oxidation state of the cobalt is
(0). The EPR spectrum of this species suggests the presence of
Co(I) bound to a radical anion ligand. Thus, these results
demonstrated the reversible storage of electrons on redox active
ligands bound to cobalt ions and the key role of bound alkali
cations in such processes. Moreover, The Co(I)−Co(II)−
Co(I) complex reacts with CO2 in ambient conditions by
transferring the electrons stored in the C−C bonds. The
reaction results in the formation of carbonate, but also on the
addition of the reduced CO2 onto the ligand framework. These
results show that electrons stored in reversible C−C bond may
be used for the transformation of carbon dioxide. We anticipate
that careful tuning of the ligand and counterion might allow to
prevent the addition of the carbon dioxide on the ligand and
studies in this direction will be performed in the future.
1
Na2salophen. H NMR (200 MHz, THF-d8, 298 K) δ = 8.6 (s,
2H), 7.4 (d, 2H), 6.9−7.3 (m, 8H), 6.5 (t, 2H).
K2OMesalophen. H NMR (200 MHz, DMSO-d6, 298 K) δ = 8.5
1
(s, 2H), 7.0−6.9 (m, 4H), 6.7 (t, 2H), 6.4 (d, 2H), 5.7 (t, 2H), 3.6 (s,
6H).
Na2OMesalophen. 1H NMR (200 MHz, pyridine-d5, 298 K) δ = 8.6
(s, 2H), 7.3 (s, 4H), 7.1 (d, 2H), 6.6 (d, 2H), 6.4 (t, 2H), 3.3 (s, 6H).
Complex Syntheses. [Co(salophen)], and [Co(OMesalophen)]
were prepared from CoCl2.THF according to the published
procedure,37 or from addition of protonated H2salophen/
H2OMesalophen ligand to [Co{N(SiMe3)2}2(THF)].
[Co(salophen)K], 1. [CoCl2(THF)] (551.0 mg, 2.7 mmol, 1 equiv)
is added on a suspension of K2salophen (1.071 g, 2.7 mmol, 1 equiv)
in THF (18 mL) and the red suspension was stirred for 5 h at room
temperature. Then potassium chunks (106.7 mg, 2.7 mmol, 1 equiv)
were added. The mixture was stirred for 24 h affording a brown
suspension. A brown precipitate was collected by filtration and dried
under vacuum (945.1 mg, 80%). 1H NMR (400 MHz, THF-d8, 298 K)
δ = 10.4 (s, 2H), 7.9 (d, 2H), 7.4 (t, 2H), 7.2 (m, 2H), 6.5 (m, 2H),
6.1 (s, 2H), 5.9 (s, 2H). Anal. Calcd for [Co(salophen)]K.(KCl)0.33
C20H14N2O2CoK1.33Cl0.33: C, 54.97; H, 3.23; N, 6.41, Cl, 2.68. Found:
C, 55.14; H, 3.55; N, 6.61; Cl, 4.33. ES-MS m/z = 373.2 [M − K+]−.
Single crystals of [Co(salophen)K(THF)] suitable for X-ray diffraction
were obtained by slow diffusion of diisopropylether into a THF
solution of complex.
[Co(OMesalophen)K], 2. Potassium chunks (6.5 mg, 0.2 mmol, 1
equiv), was added onto a brown suspension of [Co(OMesalophen)
(THF)0.5].(KCl)0.2 (80.4 mg, 0.2 mmol, 1 equiv) in THF (4 mL). The
suspension was stirred 6 days at room temperature affording a greenish
suspension. The precipitate was collected by filtration and dried under
vacuum (79.0 mg). 1H NMR (200 MHz, THF-d8, 298 K) δ = 10.3 (s,
2H), 7.5 (d, 2H), 7.2 (s, 2H), 7.0 (s, 2H), 6.6 (s, 2H), 6.0 (s, 2H), 3.6
(s, 6H). Anal. Calcd for [Co(OMesalophen)K].(THF)0.5.(KCl)0.02
C24H22N2O4.5CoK1.02Cl0.02 C, 56.28; H, 4.33; N, 5.47. Found: C,
55.94; H, 3.96; N, 5.11. Single crystals of [Co(OMesalophen)K(THF)]
suitable for X-ray diffraction were obtained by slow diffusion of
diisopropylether into a THF solution of complex.
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all manipu-
lations were carried out at ambient temperature under an inert argon
atmosphere using Schlenk techniques and an MBraun glovebox
equipped with a purifier unit. The water and oxygen level were always
kept at less than 1 ppm. Glassware was dried overnight at 130 °C
before use.
1H and 13C NMR. Experiments were carried out using NMR tubes
adapted with J. Young valves. NMR spectra were recorded on Bruker
200 and 400 MHz and on Varian Mercury 400 MHz spectrometers, at
various temperatures. NMR chemical shifts are reported in ppm with
solvent as internal reference. Diffusion coefficient measurements were
performed by NMR using a Pulsed-Field Gradient STimulated Echo
(PFGSTE) sequence (see Supporting Information), using bipolar
Gradients, at 298 K and no spinning was applied to the NMR tube.
[Co2(bis-salophen)Na2(THF)6], 3. Title compound was prepared
using a procedure slightly modified with respect to the previously
published one37 (in the published procedure an excess sodium is
added and the reaction is stopped before all sodium is consumed).
[CoCl2(THF)] (415.4 mg, 2.0 mmol, 1 equiv) is added onto a
solution of Na2salophen (741.3 mg, 2.0 mmol, 1 equiv) in THF (18
mL) and the red suspension is stirred for 5 h at room temperature.
Then sodium chunks (46.0 mg, 2.0 mmol, 1 equiv) were added. The
H
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Journal of the American Chemical Society
Article
mixture was stirred for 24 h affording a green suspension. The mixture
was filtered to remove NaCl and the THF volume of the filtrate was
reduced to 3 mL. Slow layering with diisopropylether gave a brown
precipitate which was collected by filtration and dried under vacuum
(720 mg, 87%). 1H NMR depends on the concentration. Diluted
(0.010 M) 1H NMR (400 MHz, THF-d8, 298 K) δ = 10.6 (s, 2H), 7.9
(d, 2H, 3J = 7.2 Hz), 7.4 (t, 2H, 3J = 7.3 Hz), 7.2 (dd, 2H, 3J = 3.3 Hz,
3J = 5.9 Hz), 6.6 (dd, 2H, 3J = 3.3 Hz, 3J = 5.9 Hz), 6.1 (t, 2H, 3J = 7.3
Reaction of [Co(salophen)K(THF)] with Carbon Dioxide.
[Co(salophen)K].(KCl)0.33 (100.0 mg, 0.2 mmol) was dissolved in
THF (8 mL) in a J. Young flask under argon. The solution was
degassed and an excess of 13CO2 (1 atm) was introduced in the
solution. A brown precipitate slowly appeared after 1 h of stirring. The
brown precipitate was collected by filtration and dried under vacuum
to recover 82 mg of the [Co(salophen)K(CO2) (THF)n] complex.
1
The H NMR spectrum taken (400 MHz, THF-d8, 298 K) recorded
3
Hz), 6.0 (d, 2H, J = 7.2 Hz). Concentrated (0.336 M): apparition of
for the brown solid dissolved in THF-d8 showed shows only the
presence of signals assigned to [Co(salophen)K].13C NMR (400
MHz, THF-d8, 298 K) δ = 125.8 (free 13CO2).
1
paramagnetic signals H NMR (400 MHz, THF-d8, 298 K) δ = 48.6
(s, 2H), 29.8 (d, 2H), 18.1 (s, 2H), 13.9 (s, 2H), 4.9 (s, 2H), 1.8 (s,
2H), 0.9 (s, 2H), −3.3 (s, 2H), −5.6 (s, 2H), −8.8 (s, 2H), −16.1 (s,
2H), −28.8 (s, 2H), −59.1(s, 2H), −124.8 (s, 2H). ES-MS m/z =
373.2 [Co(salophen)]−, 769.5 [Co2(bis-salophen)Na]−. Anal. Calcd
for [Co2(bis-salophen)Na2].(NaCl)0.60 C40H28N4O4Co2Na2.60Cl0.60 C,
58.05; H, 3.41; N, 6.77. Found: C, 58.11; H, 3.82; N, 6.29.
X-ray studies on single crystals isolated by slow diffusion of hexane
into a THF solution confirmed the presence of the previously reported
[Co2(bis-salophen)Na2(THF)6].37
Reaction of [Co3(tris-OMesalophen)Na6(THF)6], 6 with Carbon
Dioxide. [Co3(tris-OMesalophen)Na6(THF)2.5].(NaCl)0.3 (6.3 mg,
0.004 mmol) was dissolved in THF-d8 (0.5 mL) in a J. Young
NMR tube under argon. The solution was degassed and an excess of
13CO2 (approximately 74 equiv) was introduced in the tube. The
mixture turned purple instantaneously and then slowly became brown
1
and an important brown precipitate formed. The H NMR spectrum
[Co2(bis-salophen)Li2(Py)4], 5. Lithium chunks (2.7 mg, 0.4 mmol,
1 equiv) were added on a solution of [Co(salophen)] (145.8 mg, 0.4
mmol, 1 equiv) in THF (6 mL). The mixture was stirred for 24 h
affording a green solution. The solution was taken to dryness and the
obtained residue was crystallized by slow diffusion of hexane into a
after 1 night does not show any signal. THF was then removed under
vacuum and the residue was dried 5 min under dynamic vacuum.
THF-d8 (0.5 mL) was added, the H NMR spectrum remained silent
1
and the 13C NMR spectrum did not show the signal of free 13CO2.
THF was removed again and the residue was dried for 30 min under
dynamic vacuum. The residue was suspended in D2O where only a
part of the solid dissolves. 13C NMR spectrum (400 MHz, D2O, 298
K): 167.5 ppm (CO32−) shows the presence of carbonate in 53% yield
(using 13C labeled sodium acetate as internal standard). A similar yield
in carbonate was measured from the reaction of 6 with 3 equiv of CO2.
When the reaction’s scale was increased to 43.5 mg of complex 6,
and an excess of 2.2.2-cryptand was added after the reaction of 6 with
excess CO2, single crystals of [Co(OMesalophen-CO2)Na]2[Na-
(cryptand)]2, 9, were obtained by slow diffusion of DIPE into the
THF reaction medium.
1
pyridine solution of the complex (103.9 mg, 70%). H NMR depends
on the concentration. Diluted (0.02 M) 1H NMR (400 MHz, THF-d8,
298 K) δ = 10.4 (s, 2H), 7.9 (d, 2H), 7.3 (m, 4H), 6.7 (s, 2H), 6.1 (m,
1
4H). Concentrated (0.34 M) apparition of paramagnetic signals H
NMR (400 MHz, THF-d8, 298 K) δ = 50.8 (s, 2H), 32.8 (s, 2H), 17.9
(s, 2H), 12.6 (s, 2H), 7.5 (s, 2H), 7.0 (s, 2H), 2.4 (s, 2H), −3.5 (s,
2H), −5.6 (s, 2H), −13.0 (s, 2H), −14.0 (s, 2H), −30.0 (s, 2H),
−56.9 (s, 2H), −131.5(s, 2H). Anal. Calcd for [Co(salophen)Li].
(Py)1.65 C28.25H22.25N3.65O2CoLi C, 66.46; H, 4.39; N, 10.01. Found:
C, 66.07; H, 4.44; N, 10.40. Single crystals of [Co2(Py)2(bis-
salophen)Li2(Py)4] suitable for X-ray diffraction were obtained by
slow diffusion of hexane into a pyridine solution of [Co(salophen)Li]/
[Co2(bis-salophen)Li2] mixture.
X-ray Crystallography. Experimental details for X-ray data
collections of all complexes are given in Table S1. Figure Graphics
are created using MERCURY 3.9 Supplied with Cambridge Structural
Database; CCDC: Cambridge, U.K., 2004−2016. Diffraction data of 1,
2, 5 were measured using an Oxford-Diffraction XCalibur S at low
temperature [150(2) K] The data sets were reduced by CrysAlis
(CrysAlisPro, Rigaku Oxford Diffraction) and then corrected for
absorption.70 The diffraction data of 6 and 8 were measured at low
temperature [100(2) K] using Mo Kα radiation on a Bruker APEX II
CCD diffractometer equipped with a kappa geometry goniometer. The
data sets were reduced by EvalCCD71 and then corrected for
absorption.72 The data collection of compounds 7 and 9 were
performed at low temperature [140(2) K] using Cu Kα (7) or Mo Kα
(9) radiation on a Rigaku SuperNova dual system in combination with
an Atlas CCD detector. The data reduction was carried out by Crysalis
PRO.70 The solutions and refinements were performed by SHELXT
and SHELXL.73 The crystal structures were refined using full-matrix
least-squares based on F2 with all non hydrogen atoms anisotropically
defined. Hydrogen atoms have been located in calculated positions by
means of the “riding” model. Additional electron density found in the
difference Fourier map (due to highly disordered solvent) of 6 and 7
was treated by the SQUEEZE algorithm of PLATON.74
EPR Measurements. EPR experiments were carried out using 4
mm EPR tubes adapted with J. Young valves. Solid samples were
carefully ground before the measurements. EPR of the solutions at
different concentrations were recorded after rapidly freezing the
solutions after equilibrium was reached at 298 K. EPR spectra were
recorded on a Bruker EMX continuous wave spectrometer operating
at X band frequency equipped with an Oxford instrument ESR 900
Helium flow cryostat. For each experiment, magnetic field was
recorded with a Bruker Gaussmeter ER035 M and microwave
frequency was recorded with an Hewlett-Packard Microwave
Frequency Counter 5350B.
[Co3(tris-OMesalophen)Na6(THF)6], 6. [CoCl2(THF)] (633.1 mg,
2.9 mmol, 1 equiv) is added onto a suspension of Na2OMesalophen (1.2
g, 2.9 mmol, 1 equiv) in THF (30 mL) and the resulting brown
suspension was stirred for 5 h at 323 K. Then sodium chunks (137.0
mg, 5.8 mmol, 2 equiv) were added. The resulting mixture was stirred
for 5 days at room temperature to yield a dark green suspension that
was filtered to obtain a green solid after washing with THF. A slow
diffusion of hexane into a THF solution of the complex affords
[Co3(tris-OMesalophen)Na6(THF)6] crystals suitable for X-ray dif-
1
fraction (68% yield). H NMR (400 MHz, THF-d8, 298 K) δ = 12.8
(s, 2H), 12.5 (br s, 1H), 9.5 (br s, 1H), 9.2 (br s, 1H), 8.6 (d, 2H), 8.5
(br s, 1H), 8.3 (d, 2H), 7.9 (d, 2H), 7.8 (d, 2H), 7.4 (br s, 1H), 7.2 (br
s, 1H), 7.0 (s, 2H), 6.7−6.6 (m, 4H), 6.5 (br s, 1H), 6.4 (d, 2H), 6.2
(d, 2H), 6.1 (br s, 1H), 5.9 (t, 2H), 5.3 (t, 2H), 4.9 (br s, 1H), 4.7 (br
s, 2H), 4.5 (br s, 1H), 3.4 (s, 6H, CH3), 3.1 (s, 6H, CH3), 2.4 (br s,
3H, CH3), 2.2 (br s, 3H, CH3 ). Anal. Calcd for
[ C o 3 ( t r i s - O M e s a l o p h e n ) N a 6 ( T H F ) 2 . 5 ] . ( N a C l ) 0 . 3
C76H74N6O14.5Co3Na6.3Cl0.3 C, 55.81; H, 4.56; N, 5.14. Found: C,
55.55; H, 4.78; N, 4.76.
[(Co(OMesalophen))2Na][Na(cryptand)]3, 7. A THF (20 mL)
solution of cryptand (95.1 mg, 0.2 mmol, 6 equiv) was diffused into
a green THF (50 mL) solution of [Co3(tris-OMesalophen)-
Na6(THF)2.5].(NaCl)0.3 (60.5 mg, 0.04 mmol, 1 equiv) leading to
the formation of brown needles (78.6 mg, 68% yield) and suitable for
X-ray diffraction. Complex 6 is insoluble in THF which prevent its
1
NMR characterization in this solvent and the H NMR spectrum in
deuterated pyridine is silent. Anal. Calcd for [(Co(OMesalophen))2Na]-
[Na(cryptand)]3 C98H144N10O26Co2Na4 C, 56.37; H, 6.95; N, 6.71.
Found: C, 56.44; H, 6.98; N, 6.46.
Crystals of [Co2(bis-OMesalophen)Na2Py4]-[Na(cryptand)]2, 8 were
isolated by slow diffusion of hexane into a pyridine solution of 7.
Complex 8 could not be isolated analytically pure in significant
amounts.
I
DOI: 10.1021/jacs.7b03604
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Journal of the American Chemical Society
Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b03604.
■
S
1H NMR and EPR spectra (PDF)
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8410.
X-ray crystallographic data for complex 2 (CIF)
X-ray crystallographic data for complex 1 (CIF)
X-ray crystallographic data for complex 5 (CIF)
X-ray crystallographic data for complex 7 (CIF)
X-ray crystallographic data for complex 6 (CIF)
X-ray crystallographic data for complex 9 (CIF)
X-ray crystallographic data for complex 8 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*marinella.mazzanti@epfl.ch
ORCID
Marinella Mazzanti: 0000-0002-3427-008X
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
(28) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen,
R. A. Organometallics 2002, 21, 460.
■
We thank F. Fadaei Tirani for her contribution to the X-ray
single crystal structure analyses and E. Solari for the
determination of the elemental analyses. We thank D. Toniolo
for performing some NMR measurements. We acknowledge
support from the Swiss National Science Foundation
(200021_157158) and from the Ecole Polytechnique Federale
́ ́
de Lausanne (EPFL).
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