Radical Behavior of CO2 versus its Deoxygenation Promoted by Vanadium Aryloxide Complexes: How the Geometry of Intermediate CO2-Adducts Determines the Reactivity.

Article abstract of DOI:10.1002/chem.201702943

The reactivity of carbon dioxide with vanadium(III) aryloxo complexes has been investigated. The formation of either carbon monoxide or incorporation into the ligand system with the ultimate formation of organic ester was observed depending on the overall

Full text of DOI:10.1002/chem.201702943

A Journal of
Accepted Article
Title: Radical behavior of CO2 versus its deoxygenation promoted by
vanadium aryloxide complexes: how the geometry of intermediate
CO2-adducts determines the reactivity.
Authors: Sandro Gambarotta, camilo Viasus, Nicholas Alderman, Ilia
Korobkov, and Sebastiano Licciulli
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To be cited as: Chem. Eur. J. 10.1002/chem.201702943
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Radical behavior of CO versus its deoxygenation promoted by
2
vanadium aryloxide complexes: how the geometry of
intermediate CO -adducts determines the reactivity.
2
Camilo J. Viasus, Nicholas P. Alderman, Sebastiano Licciulli, Ilia Korobkov and Sandro Gambarotta*
Abstract: The reactivity of carbon dioxide with vanadium(III)
aryloxo complexes has been investigated. The formation of
either carbon monoxide or incorporation into the ligand system
with the ultimate formation of organic ester was observed
depending on the overall electron donor ability of the ligand field.
DFT calculations were carried out to investigate the proposed
mechanism for carbon dioxide coordination and reduction.
coordination, likely to be labile, and attacking the metal center
[
27]
instead.
Ligand migratory insertion, as in the case of
organometallic derivatives, may generate organic functions
[77–79]
(
carboxylates, carbonates, carbamates, etc).
and not
[10,80–82]
necessarily through stoichiometric transformations.
When the electron transfer can be limited to only one electron,
(
•-)
the reaction initially transforms the coordinated CO
2
into a
radical anion, which, by mainly being carbon centered, eventually
[
82]
Introduction
may dimerize to form the oxalate dianion.
There are a few
examples in the literature showing that this transformation is
indeed possible while using low-valent lanthanides and transition
The high thermodynamic stability of CO
2
makes any catalytic
[
28,83–88]
transformation and activation particularly challenging, The main
metals.
also implies C-C bond formation starting from CO
desirable process which is central to the naturally occurring
This is not only an irreversible transformation but
[
1–16]
[42–
foci on this chemistry remain for its reduction,
2
, a much
[
8,17–30]
[31–41]
incorporation,
sequestration-storage
with the number of electrons transferred to the CO
atom (either one or two) being the factor determining the type of
and recycling.
4
7]
[89–91]
2
carbon
photosynthesis.
In this respect, the most spectacular
advance probably remains the closure of a step-by-step catalytic
[
48–63]
[92]
reactivity.
Therefore, being capable of controlling the extent
cycle obtained by using a Cu(I) complex.
However, the
of electron transfer is central to drive the reactivity towards
specific transformations.
utilization of complexes acting as one electron donor is a
necessary but non-sufficient condition since two electrons,
leading to a deoxygenation or disproportionation, may also be
A two-electron transfer causes three sorts of possible events.
The first is a simple deoxygenation during which an oxygen atom
is extracted by a low/mid-valent metal to afford a metal oxide and
obtained via cooperative attack of two metals on the same CO
2
[
84]
unit. Thus, one-electron reducing complexes need to provide a
sufficiently long lifetime to the intermediate CO radical-anion to
enable head to head coupling and consequent irreversible C-C
2
[
56,64–66]
CO.
Even though this reaction is potentially useful, the high
[
85]
stability of metal oxides, formed as by-products, makes their
further reduction to the original state energy-intensive and,
therefore, unlikely suitable for making the transformation
bond formation. It is conceivable to expect that for this purpose,
the type of initial coordination of CO (end-on, side-on, mono- or
2
binucleating, etc.) is of pivotal importance.
[
67,68]
catalytic.
The second possibility is a disproportionation to CO
[
56,69–73]
and carbonate dianion.
Again, the formation of a metal
Low-valent, early transition metals are normally highly
oxophilic and therefore unlikely to embark in overall one electron
carbonate suffers from the same limitations as the oxo species.
Finally, reversible coordination to electron-rich elements, MOF’s
transfers to CO
reductants, the enhanced stability of the M-O bond is the drive for
subjecting CO to a two-electron cooperative attack by two
metals. However, this might be moderated or perhaps
2
. Even in the case of purely one electron
[
23,74–76]
and metal-free systems is also a possibility.
phenomenon is conceptually very appealing, it might preclude
any realistic possibility for further incorporation of CO into
substrates. Any given reagent expected to attack the coordinated
CO (either electro- or nucleophile) may simply revert the
Although this
2
[
86–88]
2
prevented by tuning the redox potential of the transition metal, via
the judicious choice of the ligand field, and by increasing the steric
bulk around the metal center.
2
Mr. C. J. Viasus, Dr. N. P. Alderman, Dr. S. Licciulli, Dr. I. Korobkov and
Prof. Dr. Sandro Gambarotta.
2
To verify this point, we have studied the reactivity of CO with
selected vanadium aryloxide complexes, obtaining both two- and
one-electron transfer depending on ligand selection, thus
resulting in completely different reactivity patterns. The choice of
vanadium aryloxides was advised by the lower tendency of the V-
O bond to perform migratory insertion reactions in turn providing
better substrates for studying the extent of electron transfer from
Department of Chemistry and Biomolecular Science
University of Ottawa
10 Marie Curie, Ottawa, ON, Canada
E-mail: sgambaro@uottawa.ca
Supporting information for this article is given via a link at the end of the
document.
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the metal to CO
2
. In addition, the aryloxydes ligands are readily
residual THF in the metal coordination sphere has been evaluated
available with an ample choice of molecular frameworks, thus
enabling the fine tuning of steric environment. Herein we report
our findings.
2
by computing CO adducts with and without THF. Convergence
could be obtained only in the case of a THF-free adduct. In this
event, the calculation revealed that the optimal geometry adopted
by the coordinated CO
2
is a bent end-on mode. It should be
reminded that this is a unique bonding mode, observed in the sole
[
96]
Results and Discussion
case of one trivalent uranium compound. The deviation from
the linearity appears to be caused by the transfer of electron
density, roughly corresponding to one electron, from the metal to
the CO2 carbon atom (Chart 1). The effective valence electron
configuration (Natural Electron Configuration) in terms of the
Natural Population Analysis (NPA) indicates that in the orbitals of
V there is the equivalent of 0.97 electrons and which roughly
2
The d trivalent state of vanadium was selected as a starting point
for this work. It was argued that lower oxidation states would have
too strong reducing power. It is worth reminding that simple
homoleptic V(III) complexes already are sufficiently reducing to
[
93,94]
attack N
2
.
Thus, homoleptic vanadium complexes both
O) V(THF) ] (1) and anionic {[(2,4,6-
[Li(THF) ]} (2) and {[(2,4,6-
O)(µ-η6-2,4,6-Me O)Li]} (3)
neutral [(2,4,6-Me
3
C
6
H
2
3
2
1
corresponds to the d configuration of V(IV). The electron
Me
Me
3
3
C
C
6
6
H
H
2
2
O)
O)
2
2
V](µ-2,4,6-Me
V](µ-2,4,6-Me
3
3
C
C
6
6
H
H
2
2
O)
2
2
2
transferred by vanadium to CO is mostly located on its carbon
3
C
6
H
2
2
atom displaying 0.67 electrons in an orbital of mainly p-character.
have been prepared (Scheme 1) for the purpose of reacting with
[
95]
Therefore, this intermediate is formally composed by tetravalent
CO
and 3 are instead unknown and therefore we have authenticated
their structures by X-ray diffraction methods (Figure 1 and ESI).
2
. Complex 1 was previously reported by Thiele. Complexes
vanadium and a CO
2
radical anion. Furthermore, the two
C atom) appear
2
electrons (one on vanadium and one on the CO
2
to be decoupled since the frequency analysis presents the triplet
state as the lower energy transition state.
OH
OLi
THF
4 2
LiV(MesO) (THF)
2
4.4 nBuLi
3 3
VCl (THF)
4
4
[
4 2
LiV(MesO) ]
Toluene
3
Scheme 1.
Exposure of toluene solutions of the above complexes to
anhydrous CO at atmospheric pressure afforded a simple
deoxygenation with formation of CO and the corresponding
vanadyl derivative (2,4,6-Me O) V(O) (4). The formation of
2
3
C
6
H
2
3
Figure 1. Thermal ellipsoids plot of 2 and 3 with ellipsoids drawn at the 50%
probability level. Hydrogen atoms were omitted for simplicity.
CO was monitored and quantified by GC-TCD analysis of the gas
mixture.
The reaction appears to be the result of a much expected two-
2
electron attack of CO by V(III) with consequent extraction of the
oxo-atom, release of CO and oxidation of the metal center to the
pentavalent state (Scheme 2).
O
V
CO2
+ CO
1
or 2 or 3
MesO
MesO
OMes
Chart 1. a) Carbon dioxide interaction with vanadium(III) tris-(2,4,6-
trimethylphenoxide). V1-O5 = 1.720Å, C1-C28 = 2.729Å, V1-O1 = 1.888 Å, C1-
o
o
o
4
O1 = 1.308 Å, V1-O5-C28 = 128.9 , O5-C28-O4 = 115.8 and O1-V1-O5 = 93.8 .
b) HOMO-Spin density contours.
Scheme 2.
The end-on orientation of the coordinated CO
2
molecule in
combination with its bending, predicted by the calculations, is
somewhat unique. It appears as the transition from the linearity of
Assuming that the reaction proceeds via an intermediate V-CO
2
adduct, DFT calculations have been performed to elucidate the
possible structure of this elusive intermediate. Calculations were
performed at B3LYP level using TZVP basis set for all atoms with
Gaussian 09 package. Berny algorithm was performed to find a
transition state using the QST3 method. The possible presence of
the observed end-on labile coordination on
a
sterically
[
96]
encumbered trivalent uranium atom
side-on coordination on zerovalent Ni complexes.
present case, the predicted angle in the O=C=O unit is close to a
to the well-established
[97]
In the
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2
o
sp configuration [V1-O5-C28 = 128.9 ]. The incipient formation
of the vanadyl function (V1-O5 = 1.720Å) is accompanied by the
expected elongation of the O5—C28 bond (1.729Å). The residual
C-O multiple bond instead undergoes further shortening [C(28)-
O(4) 1.152Å] implying an increase of the CO bond multiplicity
vanadium(III) are being paired because of the CO
2
coordination,
or the two electrons are transferred to CO
diamagnetic specie, ts1.
2
thus generating a
An Intrinsic Reaction Coordinate diagram was calculated as
a function of the relative Gibbs energy, thus verifying that
reagents and products can indeed be connected via the
prior to the final release of CO. The peculiar bonding mode of CO
2
indicated by DFT calculations implies, from the formal point of
intermediate active CO
The progress of the reaction nicely shows the change of all the
geometrical parameters in synchrony with the increase of CO
2
-adduct sited at the highest energy point.
2
view, that substantial spin density is transferred from the d
vanadium with consequent formal oxidation of the metal center.
Accordingly, Mulliken analysis (Table 1) shows that the carbon
atom gathers a significant spin density (0.260989), even larger
than that of the two oxygen atoms (0.027629 and 0.113987
respectively).
2
bending until the final deoxygenation. The calculated relative
Gibbs free energy for the overall reaction is 7.08 kcal/mol (Chart
4
).
2
Table 1. Mulliken charges and spin density for V-CO adduct.ꢀ
Atom
Mulliken charges
.331914
0.481092
.153548
Atomic-atomic spin density
1
1.010518
0.116675
0.260989
0.027629
0.113987
V1
O1
C28
O4
-
0
-0.159974
-0.527752
O5
Alternative bonding modes of CO
2
to the metal center (structures
A-E, Chart 2) have been also analyzed by DFT. Calculations
evaluated the lowest energy and eventually the most probable
transition/ground state for the pathway of carbon dioxide
reduction by vanadium(III) homoleptic compound (Chart 3).
Chart 4. Energy profile diagram and Intrinsic Reaction Coordinate diagram.
Chart 2. Binding modes calculated as possible intermediates during the
reduction of carbon dioxide.
2 3
The deoxygenation of CO to form a pentavalent (RO) V=O
function and CO was somewhat expected for a relatively oxophilic
element such as vanadium. We reason that by increasing the
steric hindrance around the metal center and creating a pocket,
2
It was found that the linearity of CO in the bonding mode C is
almost unaffected by the multiplicity (gs4 and gs5, Chart 3). As
expected, higher multiplicity only results in lower energy. Once
the bonding mode A is input, calculations show the structure as
where the bending of the coordinated CO
2
could be moderated
[
96]
similarly to the case of uranium , potentially an adduct could be
energetically unstable, shifting towards the D or E bonding modes. trapped and isolated. In an attempt to simulate such an
environment, we have probed the reactivity of a sterically
encumbered trivalent vanadium tris(2,6-bis-phenylaryloxide
complex) (Scheme 3).
The binding mode B with a multiplicity of 3 also initially leads to C
and then eventually to D while attempting to obtain a transition
state. With a multiplicity of 1, B remains instead unchanged but
with high energy (sg 8 and 9, Chart 3). The linear bonding mode
C (gs4 and 5, Chart 3) is the starting point for reaching the
transition state D as the final coordination mode (ts1 and 2, Chart
3
) prior to product formation. Energetically speaking, the binding
mode E (gs6 and 7 with multiplicity of 1 and 3 respectively) is more
stable than D. However, not a single imaginary frequency was
found implying that no transition state could be reached while
using E as a starting point. In this case, the complex with a
multiplicity of 3 (ts2) was found higher in energy than 1. This
Scheme 3
2
implies that either the electrons in the d configuration on
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The complex (2,6-Ph
2
C
6
H
3
O)
3
V(THF) (5) was prepared and
In addition, the reaction seems to proceed through initial
formation of intermediates given that the original green color of
the reaction solution slowly turned blue within the next two hours
and eventually red after twelve. When the reaction was halted at
the initial stage by degassing the reactor after the color turned
analyzed. The connectivity and composition were confirmed by
an X-Ray crystal structure (Figure 2) (see ESI). Even in this
case, the reaction of (2,6-Ph
afforded deoxygenation with the formation of the corresponding
pentavalent (2,6-Ph O) V(O) (6) and CO albeit through a
susbstantially slower reaction. (Figure 3)
2 6 3 3
C H O) V(THF) (5) with CO2
2
C
6
H
3
3
decisively blue, a different species was isolated as a
paramagnetic, blue crystal.
Chart 3.
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The dimeric nature of this paramagnetic complex, along with
the composition and connectivity, were yielded by an X-ray crystal
Figure 2. Thermal ellipsoids plot of
5 with ellipsoids drawn at the 50%
probability level. Hydrogen atoms were omitted for simplicity.
structure (Figure 4).
Figure 4. Thermal ellipsoids plot of 9 with ellipsoids drawn at the 50%
probability level. Hydrogen atoms were omitted for simplicity.
The crystal structure confirmed the analytical data and revealed a
monomeric
Ph O)
tetravalent
complex
formulated
as
(2,6-
2
C
6
H
3
2
V(O)(THF) (7) (Figure 3).
2
Since even in this case its formation implies loss of one ligand
and one-electron, the reaction mother liquor was analyzed. The
accurate mass was calculated on the product+Na+ (305.1492
m/z) peak using lock Mass Accuracy on a micromass TOF-MS-
ESI spectrometer. The analysis indicated the presence in solution
of an organic molecule whose molecular mass corresponds to the
ester 10 depicted in Scheme 4. The structure was confirmed as
1
by H-NMR in CDCl
ESI).
3
and fragmentation pattern of MS-EI (see
Figure 3. Thermal ellipsoids plot of
6 and 7 with ellipsoids drawn at the
5
0% probability level. Hydrogen atoms were omitted for simplicity.
Scheme 4
The formation of 7 does not have a straightforward
explanation since it implies a one-electron transfer and the loss of
one ligand anion. It is thus unlikely to be the intermediate for the
formation of 6. In any event, it hints at the occurrence of a
reactivity alternative to simple deoxygenation. Unfortunately,
attempts to isolate other species or to identify other products
Although the formation of the ester via coupling of the ligand
aryl residues and CO looks surprising, it may have in fact a
2
simple rationalization (Scheme 5), but others are also possible.
failed. We have therefore examined
aryloxide complex containing terminally bonded electro-
withdrawing chlorine atom in the hope of moderating the electron
transfer to CO , and to arrest the vanadium oxidation to the
tetravalent state. In turn, this implies that only one electron may
indeed be provided by the metal for CO reduction.
a trivalent vanadium
a
2
2
Scheme 5. ꢀProposed rationalization for the formation of the ester.
[
95]
The reaction of (2,4,6-Me
was performed under the previous reaction conditions. After 48
hours exposure to CO at atmospheric pressure, blue crystals of
new compound formulated as the tetravalent [(2,4,6-
O)V(O)Cl] (µ-2,4,6-Me O) (9) were obtained at
3 6 2 2 2 2
C H O) V(Cl)(THF) (8) with CO
Under any mechanistic scenario however, the formation of
the tetravalent 9 clearly indicates that only one electron has been
2
a
transferred by vanadium to CO
reaction requires for CO undergoing a two-electron reduction as
in the case of the simple de-oxygenation. The striking difference
2
. On the other hand, the overall
Me
3
C
6
H
2
2
3
C
6
H
2
2
2
room temperature.
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of the product (ester instead of CO) possibly indicates that the two
electrons have been delivered at two different stages and, very
possibly, by the cooperative interaction of two metal centers. In
any event, it is conceivable that the preliminary interaction of the
Secondly, the linearity of carbon dioxide remained almost
unaffected by the coordination as a result of a minimal electron
transfer from the metal center (Chart 6). This means that the
chlorine atom is interfering in the coordination and preventing the
two-electron transfer to carbon dioxide by limiting the charge
2
V(III) center of chlorine containing 8 with CO is more limited in
term of charge transfer. To probe the likelihood of this assumption, transfer to only one electron. The NPA indicated that the electron
as well as obtaining information on the initial interaction in the
mechanistic pathway for ester formation, DFT calculations were
performed.
density is mostly on the vanadium atom (see ESI). Accordingly,
the Mulliken charges shows the carbon of the CO molecule
remains more positive than in the case of the trisaryloxo
vanadium(III)-CO adduct. The longer distance for V—O4 (Chart
6) compared to V1—O5 (Chart 1) and the minor perturbation of
the C=O bond are all in agreement with a minimum extent of CO
reduction. Surprisingly, the spin density distribution on the HOMO
of 8-CO clearly indicates that one of the two aryloxide oxygen
atoms of 8 has gained a considerable amount of charge as a
result of the CO coordination (Chart 7). Whether this may or may
not lead to a radical attack by the ligand, instead of the metal, to
the coordinated or free CO to generate a ROC(O) radical species
2
The geometrical parameters as obtained from the crystal
structures have been recalculated, obtaining a good match with
the experimental values (Chart 5). This lends credibility to the
appropriateness of the selection of the functional and basis set.
2
2
Berny algorithm was used to find
a
possible initial
2
interaction/transition state. The result indeed confirmed the
formation of an adduct but with some striking differences from the
case of the homoleptic(III) adduct.
2
2
is impossible to establish at this stage.
Chartꢀ5.ꢀComparative table of experimental and computed bond distances and
angles.
2
Chart 7. HOMO-spin density of compound 8 and 8-CO adducts with and
without THF. Selected bond distances and angles for compound8-CO
2
adduct
o
V1-O4 = 2.190Å, V1-O5 = 1.827Å, C28-O5 = 1.348Å, V1-O5-C28 = 157.9 ,
Firstly, in the homoleptic case the preliminary dissociation
o
o
O4-C27-O6 = 178.3 and O5-V1-O4 = 100.08 .
2
of THF is energetically downhill and the CO adduct does not
contain THF. With the partly chlorinated compound instead, the
presence of THF does not seem to influence the stability of the
structure, as well as geometry of the activated complex. The THF
2
Regrettably, CO -adducts could not be isolated and
chemically characterized in spite of reiterated attempts. To probe
at least that the metal reduction potential is indeed substantially
modified by replacing one aryloxide ligand with chlorine. Cyclic
voltammetry measurements have been carried out on THF
solutions of both 1 and 8. In both cases only irreversible vanadium
oxidations could be detected. However, for compound 1, the two
stages of oxidation, due to formation of tetra and pentavalent
states are clearly visible at +0.23V and +0.42V. In compound 8
there is only a single broad oxidation at substantially higher
potential +0.77V (Chart 8). The observed difference in oxidation
potentials is in line with the computed lower ability of 8 to transfer
and THF-free CO
2
-adducts show in this case almost identical
(see ESI).
arrangements of the coordinated CO
2
electrons to CO
outcome.
2
and which explains the different reaction
Chartꢀ6.ꢀa) Computed carbon dioxide interaction with 8. V1-O4 = 2.190Å, V1-
o
o
O5 = 1.827Å, C28-O5 = 1.348Å, V1-O5-C28 = 157.9 , O4-C27-O6 = 178.3
o
and O5-V1-O4 = 100.08 . b) HOMO-Spin density for adduct compound 8-
2
CO . (See also ESI).
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2
,6-diphenylphenol 98% were purchased from Aldrich. Carbon dioxide
(
2 5
grade 4.0) was passed through a P O column prior to use. Complex 1
[
95]
and 8 were prepared according to published procedures. The formation
of CO was monitored by gas chromatography using an Agilent
Technologies 7820A GC-TCD system with a Molsieve column. All other
chemicals used were purchased from Sigma-Aldrich and used as received.
Magnetic susceptibilities were measured using
a Johnson Matthey
magnetic susceptibility balance at room temperature. NMR experiments
were performed with either Bruker 300 or 400 MHz instruments using
3 6 6
CDCl or C D as solvents. Mass spectrometric analysis for compound 10
was carried out using a Micromass QToF I ESI Mass Spectrometer, with
Lock Mass HRes Exact Mass Calculation and Kratos Concept 1S, HRes
EI Mass Spectrometer and for compounds 4, 5 and 6, in a Bruker
UltrafleXtreme MALDI-TOF/TOF mass spectrometer interfaced to an
MBraun glovebox for anaerobic analysis using pyrene as matrix. Sample
preparation: a 100 mM solution of pyrene in C H was prepared and mixed
6
6
Chart 8.ꢀCyclic voltammetry diagrams for 1 and 8
with 3.1 mM analyte solutions to give a ratio of ca. 500:1 (molar) pyrene:
analyte, spotted ca. 1-2 uL on the MALDI target, allowed solvent to
evaporate via the dried droplet method before inserting. TOF spectra
collected in positive reflection mode, with the accelerating voltage (IS1)
fixed at 20.16 kV. To minimize fragmentation, laser energy maintained at
minimum intensity required to observe a signal within 1000 shots; samples
Conclusions
In conclusion, vanadium(III) aryloxide compounds present strong
reducing power towards carbon dioxide, performing partial
deoxygenation and ultimately producing vanadyl-oxo species.
The occurrence of linear end-on coordination, instead of the
commonly observed side-on, is the starting point of the reactivity.
[98,99]
were analyzed promptly to prevent matrix evaporation.
2
Electron-transfer from the metal bends the coordinated CO and
Crystallography and magnetic moment.
triggers the reaction. During this stage, the extent of charge
transfer, as determined by the selection of the ligands, affects the
Single x-ray crystal data were obtained using a Bruker diffractometer
equipped with a Smart CCD area detector and a Bruker Kappa APEXII
CCD diffractometer. The magnetic moments were measured at room
temperature from samples sealed in calibrated tubes prepared inside a
drybox.
bending of the coordinated CO
2
. A larger amount of electron
, leading to CO.
transfer severely bends the coordinated CO
2
Simple coordination with less charge-transfer enhances instead
the radical nature of the adduct, in the end triggering ligand
Computational method
2
fragmentation and attack to CO .
Preliminary ground state geometries were optimized using semi-empirical
Questions remains about why one-electron transfer from
vanadium afforded the ester instead of an oxalate. It is tempting
to speculate at this stage that this could be again ascribed to the
[100]
PM7 method implemented in MOPAC2016.
DFT calculations have
[101–104]
[105,106]
been performed at the B3LYP
level by using TZVP
basis set
for all atoms with Gaussian 09® suite package. Berny algorithm was
particular bonding mode adopted by CO
2
with these complexes.
[
107,108]
performed to find a transition state using QST3 method.
Vibrational
The formation of oxalates with one-electron reductants has been
[
83–86]
frequencies analysis was performed to characterize the nature of the
transition state, one imaginary frequency. The transition state was probed
related
2
also to an inward bending of coordinated CO ,
instead of outward observed in this work, in an overall chelating
bonding mode with two oxygen atoms both pointing towards the
same metal center. Identifying the factors determining the type of
[
109,110]
by performing an intrinsic reaction coordinate (IRC) procedure
to
confirm that the two minima are connected. Solvation effects were also
[
111,112]
bending (inward versus outward) of linearly coordinated CO
be obviously the next challenge.
2
will
considered using PCM solvation model.
Natural population analysis
was performed to obtain the electronic distribution in the transition state.
[
113]
[114]
The graphical user interface Gabedit
input/output and structure handling.
Electrochemistry
and Avogadro
were used for
Experimental Section
All cyclic voltammetry experiments have been carried out in a three neck
round bottom flask. All experiments were done in a N2 filled glovebox.
Cyclic voltammetry was performed using a VersaSTAT 3 (Princeton
Applied Research) potentiostat. A conventional three electrode system
was employed. A glassy carbon electrode (diameter = 0.4 cm) was used
as the working electrode, a Pt wire as the auxiliary electrode, and an Ag
Chemical and physical measurements
All manipulations were performed under a nitrogen atmosphere with
rigorous exclusion of oxygen and water by using standard Schlenk and
glovebox techniques. n-Heptane, ether, toluene and THF were dried with
an activated Al
2
O
3
column and deoxygenated prior to use by several
(THF) , Mesithylphenol 97%,
vacuum/nitrogen purges. Pyrene 98%, VCl
3
3
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Chemistry - A European Journal
10.1002/chem.201702943
FULL PAPER
wire was used as a pseudo-reference electrode. Ferrocene was added as
an internal reference. Tetrabutylammoniumhexafluorophosphate ((n-
mmmol. 60 %). µeff=2.64 BM. HRES-MALDI-TOF-TOF [M-THF]•+ m/z
3 54
calculated for VO C H39 786.234, found 786.283.
Bu)
with dried methanol and dried in vacuum at 90 °C for 24 h before use. The
electrolyte solution, 0.1 (n-Bu) NPF in THFwas used in each
experiment. The concentration of compounds 1 and 8 were 1.1 mM and
.4 mM (10 mL THF) in each experiment.
4 6
NPF , TBAHFP), the supporting electrolyte, was crystallized two times
6 5 2 6 3 2 2
Preparation of VO(2,6-C H ) C H O) (THF) (6).
Similar procedure as for compound 5 was followed with the reaction being
stopped after 2h, when the color of the solution was blue. No formation of
carbon monoxide was observed at this stage. The solution was degassed
M
4
6
1
o
to remove carbon dioxide. The blue solution was cooled to -30 C and after
filtration blue crystals were obtained. (0.068 g, 0.100 mmol. 19 %).
µeff=1.46 BM.
Synthesis of vanadium(III) aryloxides
3 6 2 4
Preparation of [Li(THF) V(2,4,6-Me C H O) ] (2).
2
MesOH (mesythylphenol) (0.375 g, 2.75 mmol) was dissolved in 15 mL of
6 5 2 6 3 3
Preparation of VO(2,6-C H ) C H O) (7).
o
n-heptane, and the solution cooled to -35 C. n-BuLi (2.5 M, 1.2 mL) was
Complex 5 (0.250 g, 0.291 mmol) was dissolved in 15 mL of a 1:1 mixture
of toluene:THF. The mixture was stirred and placed under carbon dioxide
atmosphere. The color of the solution changed from green to dark red
passing through an intermediate blue color. The formation of CO as
byproduct was monitored by GC-TCD. The solvents were evaporated in
vacuo and a dark solid was obtained. 5 mL of pure toluene was used to
added drop wise. The mixture was stirred for approximately 12 hours and
the solvent evaporated in vacuo. The resulting solid was re-dissolved in
3 3
THF (20 mL) and VCl (THF) (0.257 g, 0.70 mmol) added and stirred for
2
4 hours. The initial color of the solution turned purple and then green. The
resulting solution was centrifuged to remove the lithium salt produced and
o
allowed to stand undisturbed at -30 C for 4 days. Green crystals of 2 were
o
dissolve the solid and after one week at -35 C, red crystals were collected
obtained (0.104 g, 0.22mmol., 31 %). µeff = 2.83 BM. E.A. found (Calcd)
C 70.91 (71.14), H 8.22 (8.14).
(
0.088g, 0.102 mmol, 35 %). HRES-MALDI-TOF-TOF [M-H]•+ m/z
4
54 39
H
51
o
calculated for VO C
MHz, C ) δ: -521.1 ppm. VOCl
00 MHz, C ) δ: 7.58 (d, J=7.1Hz, 4H), 7.49 (t, J=7.4Hz, 4H), 7.40 (t,
801.221, found 801.118. V NMR (22 C., 300
1
o
6
D
6
3
was used as reference. H NMR (22 C.,
3 6 2 4 2
Preparation of [LiV(2,4,6-Me C H O) ] (3).
3
6 6
D
Method A. A similar procedure was used as for complex 2, but using
toluene instead of THF. From the reaction 3 was isolated as a blue
crystalline material (0.481 g, 0.80 mmol, 96 %). µeff = 2.65 BM. E.A. found
1
3
J=7.2 Hz, 2H), 7.30 (d, J=7.6Hz,2H), 7.08 (t,J=7.6 Hz, 1H). C NMR
o
(
22 C., 300 MHz, CDCl3) δ: 149.45, 137.72, 130.10, 129.50, 128.98,
1
28.89 and 122.79 ppm.
(
Calcd) C 72.46(72.23), H 7.38(7.41).
3 6 2 2 3 6 2 2
Preparation of [(2,4,6-Me C H O)V(O)Cl] (µ-2,4,6-Me C H O) (9).
Method B. A procedure identical to the formation of 1 was followed.
However, the reaction mixture was not centrifuged, but dried under
vacuum instead. Hot heptane was used to extract the complex from the
solid residue and the extracts filtered to get a clear blue solution. After slow
cooling, blue crystals of 3 were obtained (0.144 g, 0.24 mmol, 35%).
Complex 8 (0.451 g, 0.900 mmol) was dissolved in 25 mL of toluene and
exposed to carbon dioxide for 48h. The solution was concentrated to ½ of
the volume by gentle vacuum and then filtered. Layering crystallization
technique using n-heptane (3 mL) was used and as a result blue crystals
were obtained. The crystals were collected by filtration. (0.30 g, 0.05 mmol.
1
1.1%). µeff= 2.39 BM. E.A. found (Calcd) C 55.01(55.51), H 6.66(6.75).
3 6 2 3
Preparation of VO(2,4,6-Me C H O) (4).
Exposure of a toluene solution of complexes 1, 2 or 3 to carbon dioxide
changed the color of the solutions to dark-red. In all three cases, the
presence of CO as byproduct was determined by GC-TCD. After the
reaction was completed (no more CO released) the solvents were
removed with vacuum. The resulting dark solid was re-dissolved in 5 mL
of pure toluene and centrifuged to remove small amount of insoluble
Mesityl ester (C19
The filtrate from the isolation of complex 9 was quenched with 2 mL of HCl
M. 5 mL of ethyl acetate was used to extract the mesithylphenol and the
22 2
H O ) purification (10).
1
mesityl ester. The mixture was isolated by using amorphous silica gel 60
mesh with 8:2 hexanes:ethyl acetate as mobile phase. After drying out of
1
the first fraction, it was analyzed by H NMR, MS-ESI and MS-EI (ESI).
o
residue. After one week at -35 C, red crystals were obtained (0.223 g,
0
.472 mmol. 94 %). HRES-MALDI-TOF-TOF [M]•+ m/z calculated for
5
1
o
VO
4
C
18
H
33 472.182, found 472.203. V NMR (22 C., 300 MHz, C
6
D
6
) δ: -
Acknowledgements
1
o
4
85.5 ppm. VOCl
3
was used as reference. H NMR (22 C., 300 MHz, C D )
6 6
1
3
o
To John L. Holmes Mass Spectrometry Facility of University of
Ottawa, in special to Dr. Sharon Curtis who performed the mass
spectrometry analysis. To Dr. Gyaneshwar Rao for the
electrochemistry measurements. To Prof. Tom Woo and the Woo
research group at University of Ottawa to allow us to use the
Wooki cluster to perform the DFT calculations. This work was
financially supported by NSERC.
δ: 6.68 (s, 6H), 2.43 (s, 18H), 2.10 (s, 9H). C-NMR (22 C., 300 MHz,
C6D6) δ: 151.62, 129.52, 129.10, 122.93, 20.61 and 15.81 ppm.
Preparation of V((2,6-C
,6-diphenylphenol (0.50 g, 2.03 mmol) was dissolved in 15 mL of THF
and reacted with KH (0.089 g, 2.22 mmol). The mixture was stirred for a
further 12 h before addition of VCl (THF) (0.251 g, 0.67 mmol). The
6 5 2 6 3 3
H ) C H O) (THF) (5).
2
3
3
resulting solution was stirred for 24h and centrifuged to remove insoluble
material. Dark green crystals of 5 were obtained from THF (0.235 g, 0.40
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Chemistry - A European Journal
10.1002/chem.201702943
FULL PAPER
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Radical behavior of CO
by vanadium aryloxide complexes: how the geometry of
intermediate CO -adducts determines the reactivity.
2
versus its deoxygenation promoted
2
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