Vanadium Aminophenolate Complexes and Their Catalytic Activity in Aerobic and H2O2-Mediated Oxidation Reactions

Article abstract of DOI:10.1002/ejic.201600068

Vanadium compounds supported by tetradentate amino-bis(phenolate) ligands, [VO(OMe)(O₂NO^BuMeMeth)] (1), [VO(OMe)(ON₂O^BuMe)] (2), [VO(OMe)(O₂NN^BuBuPy)] (3), and [VO(OMe)(O₂NO^BuBuFurf)] (4) [where (O₂NO^BuMeMeth) = MeOCH₂CH₂N(CH₂ArOH)₂, Ar = 3,5-C₆H₂-Me, tBu; (ON₂O^BuMe) = HOArCH₂NMeCH₂CH₂NMeCH₂ArOH, Ar = 3,5-C₆H₂-Me, tBu; (O₂NN^BuBuPy) = C₅H₄NCH₂N(CH₂ArOH)₂, Ar = 3,5-C₆H₂-tBu₂; (O₂NO^BuBuFurf) = C₄H₃OCH₂N(CH₂ArOH)₂, Ar = 3,5-C₆H₂-tBu₂] were synthesized and characterized by NMR spectroscopy, MALDI-TOF mass spectrometry and UV/Vis data. The catalytic activity of 1–4 as homogeneous catalysts in the aerobic oxidation of 4-methoxybenzyl alcohol and 1,2-diphenyl-2-methoxyethanol was explored. 1 and 2 showed moderately superior activity compared with 3 and 4, which might be due to increased stability of these complexes. 1–4 showed limited reactivity in H₂O₂-mediated oxidation of diphenyl ether and benzyl phenyl ether.

Full text of DOI:10.1002/ejic.201600068

DOI: 10.1002/ejic.201600068
Full Paper
Oxidation Catalysis
Vanadium Aminophenolate Complexes and Their Catalytic
Activity in Aerobic and H O -Mediated Oxidation Reactions
2
2
[a]
[a]
[b]
[a]
Ali I. Elkurtehi, Andrew G. Walsh, Louise N. Dawe, and Francesca M. Kerton*
Abstract: Vanadium compounds supported by tetradentate MALDI-TOF mass spectrometry and UV/Vis data. The catalytic
BuMeMeth
amino-bis(phenolate) ligands, [VO(OMe)(O NO
)] (1), activity of 1–4 as homogeneous catalysts in the aerobic oxid-
)] (3), and ation of 4-methoxybenzyl alcohol and 1,2-diphenyl-2-methoxy-
2
BuMe
BuBuPy
[
[
VO(OMe)(ON O
)] (2), [VO(OMe)(O NN
2
2
BuBuFurf
BuMeMeth
VO(OMe)(O NO
)] (4) [where (O NO
) = Me- ethanol was explored. 1 and 2 showed moderately superior ac-
2
2
BuMe
OCH CH N(CH ArOH) , Ar = 3,5-C H -Me, tBu; (ON O
) = tivity compared with 3 and 4, which might be due to increased
2
2
2
2
6
2
2
HOArCH NMeCH CH NMeCH ArOH, Ar = 3,5-C H -Me, tBu; stability of these complexes. 1–4 showed limited reactivity in
2
2
2
2
6 2
BuBuPy
(
(
O NN
) = C H NCH N(CH ArOH) , Ar = 3,5-C H -tBu ; H O -mediated oxidation of diphenyl ether and benzyl phenyl
2
5
4
2
2
2
6
2
2
2 2
BuBuFurf
O NO
) = C H OCH N(CH ArOH) , Ar = 3,5-C H -tBu ] ether.
2
4
3
2
2
2
6
2
2
were synthesized and characterized by NMR spectroscopy,
Introduction
benzylic, allylic, and propargylic alcohols.[8] Hanson and co-
workers have shown that the complex (HQ) VO(OiPr) (HQ = 8-
2
Nowadays, chemistry related to biomass (in particular, lignocel-
lulose) is of significant interest because of the desire for renew-
quinolinate) (I, Figure 1) efficiently catalyzed the oxidation of
benzylic, allylic, and propargylic alcohols with air in the pres-
[
1]
able chemical and fuel production. This has been brought
about for a number of reasons including political conflicts, and
the negative effects of petroleum on the environment and the
advantages of renewable resources, such as their abundance,
sustainability and often low cost. Among the three components
of lignocellulose (cellulose, hemicellulose, and lignin), a large
amount of research has been directed towards biofuel produc-
[
9]
ence of NEt3. It should be noted that using air instead of
pure oxygen is advantageous, as it reduces the safety hazard
associated with heating organic solvents under elevated O
pressure.
2
[
10]
[
2]
tion (e.g. ethanol) from cellulose. In contrast, the utilization of
lignin as a feedstock has been hampered due to lignin's struc-
tural complexity, which leads to challenges in its use as a feed-
[
3]
stock. However, the development of new homogeneous cata-
lysts to depolymerize lignin in a selective fashion may lead to
efficient conversion technologies, which could provide lignin-
[
4]
derived aromatic feedstocks for chemical production. Many of
these catalysts perform aerobic oxidation or H O -mediated C–
2
2
O bond cleavage reactions.
Recently, the field of aerobic oxidation catalysis has grown
significantly as the search for new green oxidation processes
continues. Many 3d transition metals (V, Mo, Fe, Cu) are attract-
ive in terms of their abundance and low cost as the active cen-
tres for aerobic oxidation processes.[
3d,5]
In particular, vanadium
[
6,7]
complexes are well known as an attractive class of catalysts,
mediating the aerobic oxidation of organic substrates including
[
[
a] Department of Chemistry, Memorial University of Newfoundland,
St. John's, NL, A1B 3X7, Canada
E-mail: fkerton@mun.ca
http://www.chem.mun.ca/zfac/fmk.php
b] Department of Chemistry and Biochemistry, Wilfrid Laurier University,
Waterloo, ON, Canada
Figure 1. Structures of vanadyl-derived catalysts for transformations of model
lignin compounds.
A growing number of vanadium(IV) and (V) complexes of
multidentate N-O donor ligands have been reported as very
active catalysts for C–O and C–C bond cleavage reactions in
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600068.
Eur. J. Inorg. Chem. 0000, 0–0
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Full Paper
model compounds of lignin, which show a significant diversity para positions of the phenolate donor. We also report their use
[
3a,9,11]
of reactivity.
In 2010, Son and Toste reported that a series as catalysts for the aerobic oxidation of alcohols and C–O bond
of vanadium(V) complexes bearing Schiff base ligands (II–IV, cleavage reactions in simple lignin model compounds.
Figure 1), which could be used as catalysts for C–O bond cleav-
age in dimeric lignin model compounds containing a ꢀ-O-4
[
3a]
Results and Discussion
linkage.
dium catalysts to treat lignin samples isolated from Miscanthus
More recently, Toste's group used the same vana-
The amine-bis(phenol) protio ligands H [O NO]BuMeMeth,
2
2
[
11e]
giganteus at 80 °C for 24 h with air as the oxidant.
The
BuMe
BuBuPy
BuBuFurf
H [ON O]
, H [O NN]
, and H [O NO]
were pre-
2
2
2
2
2
2
results showed that the catalytic system was effective in depo-
lymerizing actual lignin at ꢀ-O-4′ linkages, also, it was select-
ively cleaved in the degradation process, just as in the case of
lignin models. Furthermore, they investigated the effect of the
[14]
pared according to previously reported procedures.
gands contain 2,4-alkyl-substituted phenols but contain differ-
All li-
ent pendant donor groups (methoxy, pyridyl and furfuryl), or
two tertiary amines situated in the backbone of the ligand. VV
8
-quinolinate vanadium complex (VII, Figure 1) which was pre-
complexes 1–4 were synthesized via reaction of the protonated
ligands in methanol with VO(acac) for 24 h, affording dark
black, purple or blue solids complex in 70–85 % yield (Scheme 1
and Scheme 2). The methoxide ligand is apparent in the NMR
and MALDI-TOF mass spectra of the crude and purified prod-
ucts. In some cases, the compounds decompose upon recrystal-
[
11c]
viously reported by Hanson et al.,
on organosolv lignins,
2
and showed that catalyst VII lowers the molecular weights of
organosolv lignins to a similar degree as catalyst V.
In a study by Hanson et al. dipicolinate vanadium(V) com-
plexes (VII a, b, Figure 1) were used as catalysts to oxidize lignin
[
11a,11b]
model compounds.
The results showed that both C–H
lization to form oxo-bridged complexes (Scheme 1). Related
and C–C were cleaved in these reactions. They also reported
in another study that vanadium catalysts VII, I[11c,3a] showed a
significant difference in selectivity for the aerobic oxidation of
phenolic and non-phenolic lignin model compounds compared
with earlier work by Toste. The vanadium catalysts allowed the
cleavage of the C–C bond between the aryl ring and the adja-
cent alcohol group, and C–O bond cleavage products were also
[15]
oxo-bridged complexes have been prepared by others.
1–4
1
were characterized using H NMR spectroscopy, MALDI-TOF
mass spectrometry, X-ray crystallography, UV/Vis spectroscopy
and elemental analysis (see Supporting Information for spectra).
The absence of acetylacetonate ligands was confirmed by FT-IR
1
spectroscopy. The H NMR spectra of 1–4 contain the expected
signals typical of diamagnetic transition metal aminophenolate
complexes. They also all contain a diagnostic signal close to δ =
[
11c,12]
afforded.
Our group, and others, have been studying the chemistry
and catalytic behaviour of aminophenolate metal complexes
over the past decade. As the ligands contain a mixed set of N-
and O-donor atoms, they have shown a great ability to coordi-
nate to a range of metal centres and their steric and electronic
properties are easily varied by changing their backbone and
5
5
.00, which was assigned to the methoxide ligand (δ = 5.30,
.05, 4.89, 5.33 for complexes 1–4, respectively).
[
13]
phenol substituents.
Recently vanadium(V) complexes of
bis(phenolate) pyridine and amino-bis(phenolate) ligands have
been reported by Hanson's group to be excellent catalysts (Fig-
ure 1, IX and X) for aerobic oxidative C–O bond cleavage reac-
Scheme 2. Synthesis of vanadium complex 2.
The MALDI-TOF mass spectra were obtained via charge trans-
tions in several different non-phenolic lignin model com- fer ionization in the presence of the neutral UV-absorbent ma-
[
11f]
[16,17]
The mass spectra generated for the VV
pounds,
with similar reactivity to their earlier dipicolinate trix, anthracene.
catalysts VII a and b.
complexes described herein showed characteristic fragment
BuMeMeth
+·
Herein, we describe the syntheses and structures of amino- ions. In particular they exhibited {[O NO]
bis(phenolate) vanadyl complexes containing tetradentate tri- {[ON O]
podal ligands [ONOL] where L is a neutral N or O-containing V=O} (4) fragments at m/z 492.16, 505.20, 609.24 and 602.25
donor group and R/R′ are alkyl groups found in the ortho and respectively. The experimental isotopic distribution pattern for
V=O} (1),
2
BuMe
+·
BuBuPy
+·
BuBuFurf
V=O} (2), {[O NN]
V=O} (3), {[O NO]
2
+·
2
2
RR′
Scheme 1. Synthesis of vanadium complexes (1, 3, and 4) and oxo-bridged complexes (5 and 6).
Eur. J. Inorg. Chem. 0000, 0–0
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each of these ions shows good agreement with the theoretical
patterns (Figures S5–S8). Additional peaks in the lower mass
region of the spectra correspond to ligand fragments. For the
oxo-bridged complexes 5 and 6, there are small peaks at m/z
1
234.6 and 1212.6, which correspond to their respective molec-
ular ions.
Electronic absorption spectra of complexes 1–4 in CH Cl
2
2
show multiple bands in the UV and visible regions (Table 1 and
Figures S9–S13). The highest energy bands (<300 nm) result
from ligand π→π* transitions. Other intense bands also present
in the UV region (300–350 nm), were assigned to charge-trans-
fer from the out-of-plane pπ orbital (HOMO) of the phenolate
oxygen to the empty d
x2–y2
/dz orbitals of the V centre. The
2
lowest energy bands (visible region) arise from charge-transfer
transitions from the in-plane π orbital of the phenolate to the
empty d orbitals on V.
Table 1. UV/Vis spectroscopic data of the vanadium complexes in CH
2
Cl
2
.
π→π*
λ/nm (ε/
Out-of-plane pπ→d
In-plane pπ→d
λ/nm (ε/M
–1
cm–1
–1
–1
–1
cm–1)
M
)
λ/nm (ε/
M
cm
)
1
2
3
4
248(10900)
243 (99000)
240 (98570)
276 (115000)
252 (92000)
275 (105000)
278(115000)
684 (34000)
601 (56000)
594 (10250)
603 (47500)
240 (105000)
Figure 4. X-ray structure of 5 (thermal ellipsoids at 50 % probability, H atoms
Single crystals suitable for X-ray analysis were obtained after
several weeks from a saturated methanol solution cooled to
20 °C (1, 2) or a mixture of methanol, pentane and acetonitrile
stored at 6 °C (3, 4). ORTEP drawings are shown in Figures 2, 3,
, and 5 with selected bond lengths and angles given in Table 2.
omitted for clarity).
–
4
Figure 2. X-ray structure of 1 (thermal ellipsoids at 50 % probability, H atoms
omitted for clarity).
Figure 5. X-ray structure of 6 (thermal ellipsoids at 50 % probability, H atoms
omitted for clarity).
ronments. For 1 O(1)–V(1)–O(2), O(3)–V(1)–O(4) and N(1)–V(1)–
O(5) angles are 159.78(7)°, 172.13(8)°, and 156.59(7)° respec-
tively; for 2, O(1)–V(1)–O(2), O(4)–V(1)–N(1), and O(3)–V(1)–N(2),
angles are 164.06(15)°, 161.33(17)° and 167.28(16)° respectively.
The phenolate oxygen atoms are trans orientated for 1 and 2
with V(1)–O(1) distances, 1.899(20) and 1.858(3) Å and V(1)–
Figure 3. X-ray structure of 2 (thermal ellipsoids at 50 % probability, H atoms
omitted for clarity).
The solid-state structures of 1 and 2 (VNO ) and (VN O ) O(2), distances, 1.886(18) and 1.878(3) Å respectively. The N1
5
2 4
V
confirm that the V centres reside in distorted octahedral envi- atom in 1 is trans to the methoxide oxygen O(5) and cis to the
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Table 2. Selected bond lengths [Å] and angles [°] for 1, 2, 5 and 6.
complexes.[18,19] Unfortunately, we were not able to isolate suf-
ficient quantities of 5 and 6 to study in detail their behaviour
in subsequent catalytic reactions.
1
2
5
6
V(1)–N(1)
V(1)–N(2)
V(1)–O(1)
V(1)–O(2)
V(1)–O(3)
V(1)–O(4)
V(1)–O(5)
2.244(2)
2.272(4)
2.355(4)
1.858(3)
1.878(3)
1.579(4)
1.766(3)
2.377(3)
2.243(3)
1.865(3)
1.896(3)
1.620(3)
1.8071(7)
2.344(2)
Inspired by the recent results obtained by the group of Han-
son,[
11f]
complexes 1–4 were screened for their activity in the
1.8994(20)
1.8860(18)
2.2567(16)
1.5851(16)
1.7923(20)
159.78(7)
83.68(7)
83.95(6)
95.16(9)
94.91(7)
172.13(8)
83.35(7)
77.86(8)
73.65(7)
98.49(9)
97.79(7)
96.54(9)
83.20(7)
104.67(9)
156.59(7)
1.8576(18)
1.8525(18)
2.1655(19)
1.7737(4)
1.6052(17)
164.71(8)
84.75(8)
83.75(8)
93.66(6)
94.26(6)
162.40(5)
83.89(7)
83.08(7)
72.56(7)
89.83(5)
94.78(9)
95.59(9)
91.71(8)
105.90(7)
164.27(8)
aerobic oxidation of 4-methoxybenzyl alcohol at several differ-
ent temperatures 80–140 °C using toluene as the solvent in the
absence of a base additive. We postulated, that in the case of 3,
the pendant pyridyl might act as an internal base. No significant
oxidation of the substrate was observed under the following
O(1)–V(1)–O(2)
O(1)–V(1)–O(3)
O(2)–V(1)–O(3)
O(4)–V(1)–O(1)
O(4)–V(1)–O(2)
O(3)–V(1)–O(4)
O(1)–V(1)–N(1)
O(2)–V(1)–N(1)
O(3)–V(1)–N(1)
O(4)–V(1)–N(1)
O(1)–V(1)–O(5)
O(2)–V(1)–O(5)
O(3)–V(1)–O(5)
O(4)–V(1)–O(5)
N(1)–V(1)–O(5)
O(1)–V(1)–N(2)
O(2)–V(1)–N(2)
O(3)–V(1)–N(2)
O(4)–V(1)–N(2)
N(2)–V(1)–N(1)
164.06(15)
97.32(16)
94.00(15)
93.88(13)
93.62(14)
106.49(18)
80.61(12)
87.85(12)
91.94(17)
161.33(17)
165.16(12)
94.86(14)
95.22(13)
94.79(9)
conditions: 0.5 mmol of alcohol, 2 mol-% of V (1–4), in [D ]tolu-
8
ene 100 °C (Table 3, entry 1). However, addition of a base (NEt3)
facilitated the oxidation of 4-methoxybenzyl alcohol with con-
versions of up to 90 % (Table 3, entry 2). It should be noted that
at shorter reaction times there was little difference in activities
between the four complexes studied. They all achieved approxi-
mately 30 % conversion in 12 h. This suggests that complexes
92.71(9)
106.69(10)
83.25(12)
84.35(11)
166.04(13)
87.27(8)
3
and 4, which give lower conversions after 72 h compared
with 1 and 2, are less stable catalysts. Based on the structures
described above, we propose that they decompose to form cat-
alytically inactive oxo-bridged compounds such as 5 and 6.
Mechanisms proposed to date indicate that metal-alkoxide
80.61(12)
82.63(14)
167.28(16)
86.00(15)
75.73(14)
87.44(12)
81.22(12)
92.80(13)
160.09(10)
73.33(12)
[
20]
bonds are crucial in these reactions,
and these bonds do
not exist in oxo complexes 5 and 6. Increasing the reaction
temperature from 80 to 100 °C led to moderate improvements
in conversion (100 %, 93 % for 1 and 2, respectively, after 60 h
under air in toluene, Table 3, entry 3). Data for the reactions at
oxo atom O(4). In 2, the methoxide bond (V–O4) is slightly
shorter than the corresponding bond in 1 (V–O5). This might
be due to steric considerations when the orientation of the
tert-butyl groups are taken into account. The longer V(1)–O(3)
distance in 1 [2.2567(16) Å] implies weak binding of the ether
group in the sidearm to the vanadium, and also reflects the
trans effect of the oxo group, as has been observed in other
1
40 °C is also reported (Table 3, entry 4) and similar conversions
can be achieved after 36 h. A control reaction with 4-methoxy-
benzyl alcohol and base only (no vanadium) in [D ]toluene un-
der air showed no reaction when heated at 140 °C for 3 d
8
(Scheme 3). Reactions were also performed in [D ]acetonitrile,
3
as it is a widely used solvent in oxidation chemistry. Conversion
levels in this, even at longer reaction times, were significantly
lower than in toluene (Table 3, entry 5). 1 and 2 are slightly
[
18]
complexes.
The V–Ophenolate distances are within the values
that have been observed for these types of complexes previ-
[
18,19]
ously.
Table 3. Table caption. Aerobic oxidation of 4-methoxybenzyl alcohol using
vanadium complexes (1–4).
Upon recrystallization in air, 3 and 4 formed oxo-bridged
[a]
complexes. The solid-state structures of the resulting com-
Entry
Additive
T
Time [h]
Conversion [%][b]
V
pounds 5 and 6 confirm that the V centres reside in distorted
[
°C]
octahedral environments (Figures 4 and 5). Although only small
amounts of 5 and 6 were obtained, mass spectrometric and
elemental analytical data confirmed their formulation. In 5,
O(1)–V(1)–O(2), O(3)–V(1)–N(1) and N(2)–V(1)–O(4) angles are
1
2
3
4
5
6
7
none
80
80
100 60
140 36
120 106
72
72
trace (1–4)
NEt
NEt
3
3
(1) 90 (2) 85 (3) 70 (4) 75 (X) 93
(1) 100 (2) 93 (3) 77 (4) 85
(1) 100 (2) 100 (3) 84 (4) 95
(1) 52
NEt3
[c]
1
65.16(12)°, 166.04(13)°, and 160.09(10)° respectively; in 6, O(1)–
V(1)–O(2), O(4)–V(1)–O(3), and O(5)–V(1)–N(1), angles are
64.71(8)°, 162.40(5)° and 164.27(8)° respectively. For 5 atoms
NEt
NEt
NEt
3
3
3
[11f]
80
60
65
24
(IX) 99 (X) 80
(VIII) 99[
11c]
1
[
a] Conditions: 2 mol-% of 1, 2, 3 or 4, 10 mol-% additive, in [D
8
]toluene.
O(1), O(2), N(2), and O(4) lie in the plane and cis to the terminal
oxo O(3) group and the nitrogen atom N(1), and for 6 atoms
O(1), O(2), O(3), and O(4) lie in the plane and are cis to the
terminal oxo O(5) group and the nitrogen atom N(1). The two
vanadium centres are linked by a linear V–O–V bridge; both
complexes 5 and 6 have nearly identical vanadium-oxo bond
lengths, which are longer than in a similar complex.[ The ter-
minal V=O bond length for both complexes are also consistent,
but moderately shorter than the values that were reported for
Vanadium complex identified in parentheses. [b] Conversion determined by
1
3
H NMR spectroscopy. [c] In [D ]acetonitrile.
19]
[
19]
similar complexes.
The V–Ophenolate distances are in agree-
Scheme 3. Catalytic aerobic oxidation of p-methoxybenzyl alcohol using V
ment with those of 1, 2 and with those observed in related
complexes.
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less reactive than related compounds (Figure 1, VIII, IX and X) the same reaction conditions in [D ]toluene solvent, a third
8
previously studied by Hanson and co-workers (up to 99 %, 60– product distribution was observed (Table 4, entry 3). The major
[
11c,11f]
80 °C, 24–48 h), respectively.
These differences in activi- products were methyl benzoate (57 %) and benzaldehyde
ties are likely associated with the donor ability of the pendant (51 %), and a small amount of benzoic acid (2 %) was also de-
group.
We propose that our homogeneous vanadium(V) catalysts
tected. Neither benzoin methyl ether nor methanol were ob-
served. For comparison, Hanson and co-workers under similar
reaction conditions in [D ]DMSO or [D ]pyr as the solvent ob-
perform these oxidations via a mechanism similar to that pro-
posed by Hanson and co-workers, i.e. a two-electron base-as-
6
5
served a different product distribution at conversion levels of
95 %; benzaldehyde 73 % (DMSO), 9 % (pyr), methyl benzoate
[
21,22]
sisted redox dehydrogenation mechanism.
The alkoxide li-
6
9 % (DMSO), 6 % (pyr), benzoic acid 5 % (DMSO), 85 % (pyr),
gand on the vanadium complex exchanges readily with free
benzyl alcohol in the first step. The base then deprotonates the
benzyl alkoxide at the methylene, which leads to the formal
dehydrogenation of the benzyl alcohol and two-electron reduc-
and methyl benzoate 5 % (DMSO), 84 % (pyr).
tion of the metal centre (V5 to V ).
+
3+
Studying model compounds of lignin instead of lignin itself
is a good way to get a better understanding of the reactivity
of corresponding lignin substructures because of the difficulty
associated with the characterization of structural changes of
lignin. In this study, 1,2-diphenyl-2-methoxyethanol, diphenyl
ether, and benzyl phenyl ether were chosen to represent the
lignin model compounds, air and hydrogen peroxide were us-
ing as oxidizing agents under the catalytic influence of the va-
nadium complexes.
After the promising results concerning the aerobic oxidation
of 4-methoxybenzyl alcohol, the vanadium complexes were
evaluated as catalysts for the aerobic oxidation of the lignin
model compound 1,2-diphenyl-2-methoxyethanol (Scheme 4).
The substrate was heated with 5 mol-% of vanadium complex
Scheme 4. Reaction of vanadium complexes with 1,2-diphenyl-2-methoxyeth-
anol and potential products from aerobic oxidation.
(
1–4) at 100 °C for 54 h. The product distribution for the cata-
lytic oxidation differed in [D ]DMSO, [D ]pyr and [D ]toluene
Table 4. Aerobic oxidation of 1,2-diphenyl-2-methoxyethanol using 4 in a
6
5
8
range of solvents.[
a]
solvents but similar conversions were achieved (Table 4, data
provided for 4, although 1–3 also gave similar yields of each
product). Product identities were confirmed via GC–MS analy-
ses. Control reactions with 1,2-diphenyl-2-methoxyethanol only
Yield of products [%][b]
Entry
Conv.
[%]
Solvent
1
2
3
4
5
(
no vanadium) in [D ]DMSO, [D ]pyr and [D ]toluene solvents
1
2
3
90
99
99
[D
6
]DMSO
18
46
36
90
0.2
6
5
8
under air showed no reaction when heated at 140 °C for 3 d.
Table 5 provides a direct comparison of compounds 1–4 for the
conversion of 1,2-diphenyl-2-methoxyethanol over time. These
data suggest that there is very little ligand effect in these cata-
lytic reactions within this family of complexes. The nature of
the amine used in preparing ligand (leading to the presence or
absence of a pendant arm, or the type of pendant donor) does
not significantly influence the conversion levels achieved, which
were in all cases 90–100 %. As the substrate can yield multiple
molecules upon oxidation via bond cleavage reactions, it
should be noted that the sum of the product yields can be
[D
5
]Pyr
15
–
2
–
67
57
1
–
2
[D
8
]toluene
51
[
a] 0.5 mmol of lignin model, 5 mol-% of 4, 54 h. All runs were carried out
at 100 °C, using [D
sions and yields determined by integration against an internal standard (p-
xylene). [b] Up to 5 products were identified as shown in Scheme 4.
5
6 8
]pyridine, [D ]DMSO, or [D ]toluene as solvents, conver-
Table 5. Comparison of conversion levels for aerobic oxidation of 1,2-di-
phenyl-2-methoxyethanol using 1–4.[
a]
Entry
Time
1
2
3
4
in excess of 100 %. In [D ]DMSO, 4 gave benzaldehyde (90 %),
6
1
2
3
12 h
24 h
36 h
62
90
94
75
86
100
72
84
75
78
100
methanol (46 %), methyl benzoate (36 %) and benzoin methyl
ether (18 %) as the major products and a small amount of ben-
zoic acid (Table 4, entry 1). A different product distribution was
observed when the catalytic reaction was run under the same
100
[
a] 5 mol-% 1–4, 0.5 mmol of lignin model, 140 °C, [D
8
]toluene. Conversion
determined by integration against an internal standard (p-xylene).
reaction conditions in [D ]pyr solvent. After 54 h, the starting
5
material had been completely consumed, and the organic prod-
Diphenyl ether and benzyl phenyl ether were then studied
ucts consisted of, methyl benzoate (67 %), benzoin methyl ether as simple substrates for catalytic oxidation reactions using H O2
2
(
15 %), and methanol (2 %). Benzaldehyde was detected as a as the oxidant to determine if benzylic ether cleavage was se-
minor product (1 %) and no benzoic acid was detected (Table 4, lective compared with aromatic, Ph-O-Ph, cleavage. A solution
–1
entry 2). Whereas, when the catalytic reaction was run under of H O was added over a period of 1 h at 1 mL h using a
2
2
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5
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
syringe pump to a solution of the substrate and catalyst, 1–4 however, the conversions were quite low. Future work will focus
(
2 mol-%) at 80 °C. However, no significant oxidation of the on designing more active and stable catalysts for aerobic oxid-
substrate was observed under these conditions. Addition of a ation reactions as well as examining the reactivity of these spe-
base (NEt ) and use of a different solvent (acetone or toluene) cies in other catalytic processes.
3
did not prompt the oxidation of either compound (Scheme 5).
However, when acetonitrile was used as a solvent diphenyl
ether, and benzyl phenyl ether, gave 10 % and 9 % yield of
Experimental Section
phenol, and 15 % yield of benzaldehyde for benzyl phenyl
ether, after stirring the reaction mixture at 80 °C for 72 h
General Methods and Materials: Starting materials for all synthe-
ses were purchased from Strem, Aldrich or Alfa Aesar and were
(
Table 5). This shows that no selectivity in terms of C-O-C cleav- used as received. Ligands were prepared via literature proce-
[
11c]
age was observed and reactivity towards such bonds is poor dures.
CDCl , [D ]toluene, [D ]DMSO were purchased from Cam-
3 8 6
bridge Isotope Laboratories and [D ]pyridine from Aldrich. HPLC
especially when compared with reactions of 1,2-diphenyl-2-
methoxyethanol using 1–4 in air. Control reactions showed no
reaction when base, solution of H O and diphenyl ether or
benzyl phenyl ether (no vanadium) were heated and stirred in
acetonitrile under air at 80 °C for 3 d (Table 6).
5
grade solvents were used as purchased. All syntheses were per-
formed under ambient laboratory atmosphere unless otherwise
specified. NMR spectra were recorded on Bruker Avance-500 or Av-
ance III-300 spectrometers and referenced internally to TMS. MALDI-
TOF mass spectra were recorded in reflectron mode on an Applied
Biosystems Voyager DE-PRO equipped delayed ion extraction and
high performance nitrogen laser (337 nm). Samples were prepared
2
2
–
1
at a concentration of 0.02 mg L in toluene. Anthracene was used
–1
as the matrix, which was mixed at a concentration of 0.02 mg L .
UV/Vis spectra were recorded on an Ocean Optics USB4000+ spec-
trophotometer. GC–MS data were obtained on an Agilent Technolo-
gies 7890 GC with 5975 MSD. Elemental analyses were carried out
by Canadian Microanalytical Service Ltd., Delta, BC, Canada. X-ray
data were collected for single crystals of 1, 2, 5, and 6 (details
provided below).
Scheme 5. Reaction of vanadium complexes in H
aromatic ethers.
2 2
O -mediated oxidation of
BuMeMeth
Synthesis of Vanadium Complexes: [O NO]
V(O)(OMe) (1):
] (0.5970 g,
2
BuMeMeth
Table 6. Catalytic oxidation of diphenyl ether and benzyl phenyl ether using
H
2
[O
2
NO]
(1.00 g, 2.25 mmol) and [VO(acac)
2
[
a]
1
–4 with H
2
O
2
.
2.25 mmol) were placed in a flash, methanol (50 mL) was added
and the mixture was heated to reflux overnight to yield a dark blue
solution. The solvent was removed under vacuum and a dark blue
Entry
Substrate
Addi-
tive
Conv.
[%]
Yield [%]
Phenol
solid (0.88 g) was obtained, 73 % yield. Compounds 2–4 were pre-
Benzalde-
hyde
BuMe
pared in a similar fashion. [ON O]
V(O)(OMe) (2) was isolated as
2
BuBuPy
a dark blue solid 0.95 g, 80 % yield. [O NN]
isolated as a black solid 0.92 g, 78 % yield. [O NO]
V(O)(OMe) (3) was
2
1
2
diphenyl ether
benzyl phenyl
ether
none
none
< 1
< 1
–
–
–
–
BuBuFurf
V(O)(OMe)
2
(4) was isolated as a dark blue solid with yield 0.95 g, 80 % yield.
Using a Johnson–Matthey magnetic susceptibility balance, all com-
pounds were confirmed to be diamagnetic and did not give rise to
any signals in EPR experiments. Upon recrystallization in air, crystals
of 1 and 2 suitable for X-ray structure determination were obtained.
Upon storing in a refrigerator, solutions of 3 and 4 afforded oxo-
bridged compounds 5 and 6.
3
4
diphenyl ether
benzyl phenyl
ether
Et
Et
3
N
3
N
10
15
10
9
–
15
[
2 2
a] 0.5 mmol of ether, 2 mol-% 4, 10 mol-% additive if used, 1.5 mmol H O
in MeCN, 72 h, conversions determined by GC using dodecane as an internal
standard. Note: similar reactivity was observed for compounds 1–3.
1
: C H NO V (526.61): calcd. C 66.26, H 8.34, N 2.76; found C 65.88,
28 45 5
–
1 1
H 8.12, N 2.99. IR: ν˜ = 951 cm . H NMR (300 MHz, CDCl ): δ = 7.11
3
(d, J = 1.8 Hz, 2 H, ArH), 6.82 (d, J = 1.6 Hz, 2 H, ArH), 5.30 (s, 3 H,
Conclusions
V-OCH ), 4.57 (d, J = 13.7 Hz, 2 H, OCH -C), 3.75 (d, J = 13.8 Hz, 2
3
2
H, NCH -CH ), 3.38 (s, 3 H, C-OCH ), 3.23 (s, 2 H, ArC-CH -N), 2.82 (s,
2
2
3
2
Four new vanadium(V) complexes of amino-bis(phenolate) li-
gands have been synthesized and four structures determined
by single-crystal X-ray diffraction. Crystallography showed that
some of the alkoxide derivatives decompose to yield oxo-
bridged dimers upon storage in air. Their parent complexes
were also less active in oxidation reactions of 4-methoxybenzyl
alcohol, which leads us to propose that the bridged complexes
2
H, ArC-CH -N), 2.35–2.29 (m, 6 H, ArCH ), 1.47 [s, 18 H, ArC(CH ) ]
2 3 3 3
ppm.
2
: C H N O V (536.63): calcd. C 66.52, H 8.37, N 5.54; found C
29 45 2 4
–
1 1
6
6.75, H 8.20, N 5.68. IR: ν˜
= 951 cm . H NMR (300 MHz, CDCl ):
V=O 3
δ = 7.12–7.03 (m, 2 H, ArH), 6.73 (d, J = 1.6 Hz, 1 H, Ar), 6.66 (d, J =
.8 Hz, 1 H, ArH), 5.05 (s, 3 H, OCH ), 4.67 (d, J = 14.6 Hz, 1 H, N-
1
3
CH -C), 4.43 (d, J = 13.6 Hz, 1 H, N-CH -C), 3.75–3.62 (m, 1 H, N-
2
2
might be catalytically inactive species formed during oxidation CH -C), 3.63–3.34 (m, 1 H, N-CH -C), 3.11 (d, J = 13.6 Hz, 2 H, ArC-
2
2
processes. All complexes were also tested for catalytic aerobic CH -N), 3.08–2.97 (m, 2 H, ArC-CH -N), 2.57 (s, 3 H, NCH ), 2.53 (s, 3
2
2
3
oxidative C–C bond cleavage of 1,2-diphenyl-2-methoxyethan- H, NCH
), 2.28 (s, 3 H, ArCH
ArC(CH ) ], 1.45 [s,9 H, ArC(CH ) ] ppm.
), 2.27 (s, 3 H, ArCH ), 1.55 [s, 9 H,
3
3
3
ol, which afforded benzaldehyde and methyl benzoate as the
3 3
3 3
major products. Catalytic, oxidative C–O bond cleavage of di- 3: C H N O V (640.78): calcd. C 70.91, H 8.27, N 4.59; found C
3
7
53
2 4
–
1 1
phenyl ether, and benzyl phenyl ether was also demonstrated, 71.15, H 8.03, N 4.83. IR: νV=O = 951 cm . H NMR (300 MHz, CDCl ):
˜
3
Eur. J. Inorg. Chem. 0000, 0–0
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6
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
3
δ = 9.04 (d, J = 4.7 Hz, 1 H, ArH), 7.35 (td, J = 7.7, 1.6 Hz, 1 H, ArH),
9.3036(7) Å, ꢀ = 92.780(6)°, V = 2760.1(4) Å , Z = 4, T = 123.15 K,
–
1
3
7
2
4
.08 (d, J = 2.4 Hz, 2 H, ArH), 7.02–6.96 (m, 1 H, ArH), 6.91 (d, J =
.4 Hz, 2 H, ArH), 6.49 (d, J = 7.8 Hz, 1 H, ArH), 4.89 (s, 3 H, OCH3),
.52 (d, J = 12.4 Hz, 2 H, N-CH -C), 3.75 (d, J = 14.9 Hz, 2 H, N-CH -
μ(Mo-K ) = 0.396 mm , D
= 1.291 g/cm , 25008 reflections
α
calcd.
measured (3.304° ≤ 2Θ ≤ 59.586°), 6832 unique [and 3831 with
I > 2σ(I); Rint = 0.3546, Rsigma = 0.2006] which were used in all calcu-
lations. The final R was 0.1163 [I > 2σ(I)] and wR was 0.3179 (all
2
2
C), 3.36 (d, J = 12.5 Hz, 2 H, N-CH -C), 2.28–2.17 (m, 2 H, N-CH -C),
2
2
1
2
1
.38 [s, 18 H, ArC(CH ) ], 1.24 [s, 18 H, ArC(CH ) ] ppm.
data).
3
3
3 3
4
: C H NO V (629.75): calcd. C 68.66, H 8.32, N 2.22; found C 69.15,
36 52 5
Crystal Data for 5: C H N O V (M = 1235.43 g/mol): monoclinic,
72 100 4 7 2
–
1 1
H 8.03, N 2.19. IR: ν˜
= 951 cm . H NMR (300 MHz, CDCl ): δ =
3
V=O
space group P2 /c (no. 14), a = 14.6760(11) Å, b = 17.4805(9) Å, c =
1
7
3
.42–7.25 (m, 2 H, ArH), 6.96 (dd, J = 20.3, 2.1 Hz, 2 H, ArH), 5.33 (s,
H, OCH ), 4.97 [d, J = 15.0 Hz, 1 H, CH(CH ) ], 4.50–4.21 (m,1 H,
3
1
5.9994(11) Å, ꢀ = 113.184(9)°, V = 3773.1(5) Å , Z = 2, T = 123.15 K,
μ(Mo-K ) = 0.297 mm , D
–1
3
3
2 2
= 1.087 g/cm , 18149 reflections
α
calcd.
ArH), 3.84 (d, J = 15.1 Hz, 1 H, ArH), 3.56 (ddd, J = 30.2, 20.5, 8.4 Hz,4
H, CH -N-CH ), 3.02–2.18 (m, 2 H, N-CH -C), 1.59–1.41 [m, 18 H,
ArC(CH ) ], 1.35–1.26 [m, 18 H, ArC(CH ) ] ppm.
measured (5.16° ≤ 2Θ ≤ 50.054°), 6630 unique [and 4573 with
I > 2σ(I); Rint = 0.0688, Rsigma = 0.1240] which were used in all calcu-
lations. The final R was 0.0895 [I > 2σ(I)] and wR was 0.2301 (all
2
2
2
3
3
3 3
1
2
5
: C H N O V (1235.49): calcd. C 70.00, H 8.16, N 4.53; found C
data).
7
2 100 4 7 2
6
9.97, H 8.17, N 4.66.
Crystal Data for 6: C H N O V (M = 1385.66 g/mol): monoclinic,
7
8 118 6 9 2
6
: C H N O V (1213.44): calcd. C 69.29, H 8.14, N 2.31; found C
space group I2/a (no. 15), a = 14.1027(3) Å, b = 17.1339(5) Å, c =
7
0 98 2 9 2
3
6
9.46, H 8.21, N 2.52.
32.0775(7) Å, ꢀ = 90.371(2)°, V = 7750.9(3) Å , Z = 4, T = 123.15 K,
–
1
3
μ(Mo-K ) = 0.298 mm , D = 1.187 g/cm , 34379 reflections
Experimental for Single-Crystal X-ray Determinations: Crystals
of 1, 2, 5 and 6 were mounted on a low temperature diffraction
loops. All measurements were made on a Rigaku Saturn70 CCD
α
calcd.
measured (5.08° ≤ 2Θ ≤ 53°), 8029 unique [and 6581 with I > 2σ(I);
Rint = 0.0596, Rsigma = 0.0459] which were used in all calculations.
The final R was 0.0747 [I > 2σ(I)] and wR was 0.2223 (all data).
diffractometer using graphite monochromated Mo-K radiation. For
1
2
α
all four structures, all non-hydrogen atoms were located in differ-
ence map positions and were refined anisotropically, while all
hydrogen atoms were introduced in calculated positions and re-
CCDC 1419296 (for 1), 1419297 (for 2), 1419298 (for 5) and 1419299
for 6) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
(
fined on a riding model. In the structure of 1, no H-atoms were
–
found on O5, which suggests that this is a methoxide ion (H CO ),
3
General Procedure for the Catalytic Aerobic Oxidation of 4-
Methoxybenzyl Alcohol: In a 25 mL round-bottom flask, or in a
20 mL microwave vial, 4-methoxybenzyl alcohol (69 mg, 0.50 mmol)
was combined with a vanadium complex (1–4) (0.01 mmol, 2 mol-
therefore, for charge balance, vanadium must be in the +5 oxidation
state. Bond valence sum calculations were attempted with VaList in
order to confirm this assignment,[ however, V cannot be unam-
23]
V
III
IV
biguously assigned using this method. Calculations for V and V
%
), NEt3 (7 μL, 0.05 mmol, 10 mol-%), and p-xylene (5 μL,
0.041 mmol) as an internal standard. The mixture was dissolved in
D ]toluene (1 mL) under air, and the flask equipped with a stir bar
gave results that were high (ca. 4.8), which does suggest that the
V
species is most consistent with V and this is supported by other
[
8
experimental evidence (e.g. NMR spectroscopy). Crystals of 2 were
irregular and appeared cracked. Multiple domains with the same
unit cell were identified, but refinement using all components did
not yield satisfactory results. A single domain was used for the re-
ported results. Connectivity is similar to 1 around the vanadium
centre. Crystals of 5 were small and diffracted weakly. A solvent
accessible void is present in the unit cell, however, application of a
and an air condenser. If the reaction was performed in a microwave
vial, it was sealed. The reaction mixture was heated with stirring in
an oil bath at 80–140 °C for 36–60 h under air. The reaction mixture
was cooled to room temperature, and the yield of oxidized product
1
4
-methoxybenzaldehyde determined by integration of the H NMR
spectra against the internal standard.
solvent mask did not improve the refinement statistics, and there- General Procedure for the Catalytic Aerobic Oxidation of 1,2-
fore this was left untreated. Finally, 6 exhibited three areas of disor-
der; one of the lattice solvent acetonitrile molecules was refined as
Diphenyl-2-methoxyethanol: In a 25 mL round-bottom flask, or
in 20 mL microwave vial, 1,2-diphenyl-2-methoxyethanol (29.8 mg,
two parts [(C38,N3):(C38A,N3A) 0.136(11):0.864(11)]. Distance and 0.131 mmol) was combined with a vanadium complex (1–4)
angle restraints were used to model this solvent molecule. H-atoms (0.0066 mmol, 5 mol-%), and p-xylene (5 μL, 0.041 mmol) as an
associated with this solvent were omitted form the model, but in- internal standard. The mixture was dissolved in [D ]toluene (1 mL)
8
cluded in the formula for the calculation of intensive properties.
Distance and angle restraints were applied to this molecule. Next,
one of the ligand carbon atoms, and corresponding H-atoms, in the
pendant THF was refined as two parts [C16:C16A, 0.553(9):0.447(9)].
Finally, one of the ligand tert-butyl groups, and corresponding H-
under air, and the flask equipped with a stir bar and an air con-
denser. If the reaction was performed in a microwave vial, it was
sealed. The reaction mixture was heated with stirring in an oil bath
at 100–140 °C for 12–36 h under air. The reaction mixture was
cooled to room temperature, and the yields of oxidized products
1
atoms, was also refined as two parts [(C25,C26,C27):(C25A, were determined by integration of the H NMR spectra against the
C26A,C27A), 0.423(11):0.577(11)]. Similar disorder has been seen in
other metal complexes of this ligand.
internal standard. Product identities were further confirmed via GC–
MS analyses.
Crystal Data for 1: C H NO V (M = 523.59 g/mol), monoclinic,
2
8
42
5
General Procedure for H O -Mediated Oxidation of Diphenyl
Ether and Benzyl Phenyl Ether: Acetonitrile (9 mL) was added
2
2
space group C2/c (no. 15), a = 32.094(11) Å, b = 11.055(3) Å, c =
3
1
9.787(7) Å, ꢀ = 127.184(3)°, V = 5593(3) Å , Z = 8, T = 163(2) K,
to a vanadium complex (1–4) (0.005 mmol, 1 mol-%), NEt (7 μL,
3
–
1
3
μ(Mo-K ) = 0.391 mm , D
= 1.244 g/cm , 22592 reflections
α
calcd.
0.05 mmol, 10 mol-%), in a 50 mL of round-bottomed flask. The
ether substrate (0.5 mmol) was added to the solution at room tem-
perature in air. Aqueous H O (30 %, 170 μL, 1.5 mmol or 340 μL,
measured (5.24° ≤ 2Θ ≤ 53°), 5706 unique [and 5242 with I > 2σ(I);
Rint = 0.0610] which were used in all calculations. The final R was
1
2
2
0
.0569 [ > 2σ(I)] and wR was 0.1623 (all data).
2
3.0 mmol) was dissolved in acetonitrile (870 μL or 1660 μL, respec-
tively) and was added to the reaction mixture over a period of 1 or
2 h by a syringe pump. The flask was then equipped with a stir bar
Crystal Data for 2: C H N O V (M = 536.61 g/mol): monoclinic,
2
9 45 2 4
space group P2 /c (no. 14), a = 12.3406(8) Å, b = 24.0686(19) Å, c =
1
Eur. J. Inorg. Chem. 0000, 0–0
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7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
and an air condenser. The flask was heated with stirring in an oil
bath at 80 °C for 48 h. The reaction was cooled to room temperature
and the solvent was removed under vacuum. The residue of the
reaction mixture was dissolved in diethyl ether and dodecane (100
or 50 μL as internal standard) was added, the mixture was analyzed
by GC–MS and quantified using a calibration curve.
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Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Oxidation Catalysis
The effects of ligand variation on ho-
mogeneous catalytic oxidative trans-
formations of alcohols and ethers was
investigated.
A. I. Elkurtehi, A. G. Walsh,
L. N. Dawe, F. M. Kerton* ................ 1–9
Vanadium Aminophenolate Com-
plexes and Their Catalytic Activity in
Aerobic and H O -Mediated Oxid-
2
2
ation Reactions
DOI: 10.1002/ejic.201600068
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim