Aerobic oxidation reactions catalyzed by vanadium complexes of bis(phenolate) ligands

Article abstract of DOI:10.1021/ic3007525

Vanadium(V) complexes of the tridentate bis(phenolate)pyridine ligand H₂BPP (H₂BPP = 2,6-(HOC₆H₂-2,4- ^tBu₂)₂NC₅H₃) and the bis(phenolate)amine ligand H₂BPA (H₂BPA = N,N-bis(2-hydroxy-4,5-dimethylbenzyl)propylamine) have been synthesized and characterized. The ability of the complexes to mediate the oxidative C-C bond cleavage of pinacol was tested. Reaction of the complex (BPP)V ^V(O)(OⁱPr) (4) with pinacol afforded the monomeric vanadium(IV) product (BPP)V^IV(O)(HOⁱPr) (6) and acetone. Vanadium(IV) complex 6 was oxidized rapidly by air at room temperature in the presence of NEt₃, yielding the vanadium(V) cis-dioxo complex [(BPP)V^V(O)₂]HNEt₃. Complex (BPA)V ^V(O)(OⁱPr) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium(IV) dimer [(BPA)V^IV(O)(HO ⁱPr)]₂. Complexes 4 and 5 were evaluated as catalysts for the aerobic oxidation of 4-methoxybenzyl alcohol and arylglycerol β-aryl ether lignin model compounds. Although both 4 and 5 catalyzed the aerobic oxidation of 4-methoxybenzyl alcohol, complex 4 was found to be a more active and robust catalyst for oxidation of the lignin model compounds. The catalytic activities and selectivities of the bis(phenolate) complexes are compared to previously reported catalysts.

Full text of DOI:10.1021/ic3007525

Article
pubs.acs.org/IC
Aerobic Oxidation Reactions Catalyzed by Vanadium Complexes of
Bis(Phenolate) Ligands
Guoqi Zhang, Brian L. Scott, Ruilian Wu, L. A. “Pete” Silks, and Susan K. Hanson*
Chemistry, Bioscience, and Materials Physics Applications Divisions, Los Alamos National Laboratory, Los Alamos, New Mexico
87544, United States
S
* Supporting Information
ABSTRACT: Vanadium(V) complexes of the tridentate
bis(phenolate)pyridine ligand H2BPP (H2BPP = 2,6-
(HOC6H2-2,4-tBu2)2NC5H3) and the bis(phenolate)amine
ligand H2BPA (H2BPA = N,N-bis(2-hydroxy-4,5-
dimethylbenzyl)propylamine) have been synthesized and
characterized. The ability of the complexes to mediate the
oxidative C−C bond cleavage of pinacol was tested. Reaction
of the complex (BPP)VV(O)(OiPr) (4) with pinacol afforded
the monomeric vanadium(IV) product (BPP)VIV(O)(HOiPr) (6) and acetone. Vanadium(IV) complex 6 was oxidized rapidly
by air at room temperature in the presence of NEt3, yielding the vanadium(V) cis-dioxo complex [(BPP)VV(O)2]HNEt3.
Complex (BPA)VV(O)(OiPr) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium(IV) dimer
[(BPA)VIV(O)(HOiPr)]2. Complexes 4 and 5 were evaluated as catalysts for the aerobic oxidation of 4-methoxybenzyl alcohol
and arylglycerol β-aryl ether lignin model compounds. Although both 4 and 5 catalyzed the aerobic oxidation of 4-methoxybenzyl
alcohol, complex 4 was found to be a more active and robust catalyst for oxidation of the lignin model compounds. The catalytic
activities and selectivities of the bis(phenolate) complexes are compared to previously reported catalysts.
INTRODUCTION
■
Recent years have seen tremendous growth in the field of
aerobic oxidation catalysis.1−3 In 2007, the American Chemical
Society Green Chemistry Institute Pharmaceutical Roundtable
identified the search for new green oxidation processes as a key
future synthetic research and development challenge.4 As
catalysts for aerobic oxidation, earth-abundant metals offer
practical advantages in terms of abundance and cost.5 In
particular, vanadium complexes have emerged as an attractive
class of catalysts,6 mediating the aerobic oxidative kinetic
resolution of α-hydroxy esters,7 acids,8 amides,9 and ketones,10
and the aerobic oxidation of benzylic, allylic, and propargylic
alcohols.11−13 Vanadium exhibits a remarkable diversity of
reactivity, also catalyzing oxidative processes that are proposed
to involve radical intermediates, such as the oxidative
dimerization of phenols14,15 and the C−C bond cleavage of
α-hydroxyketones.3
Beyond synthetic applications, vanadium complexes have
been investigated as catalysts for C−O and C−C bond cleavage
reactions in lignin model compounds.16,17 Lignin, an irregular
polymer composed of methoxy-substituted phenolic subunits, is
an integral constituent of nonfood biomass and potential source
of valuable aromatic compounds.18 The development of new
methods to break lignin down in a selective fashion could
provide access to renewable building blocks for chemicals or
fuels.19−21 Recently, Son and Toste reported a vanadium(V)
complex 1 (Figure 1) that cleaves the C−O bond in a
nonphenolic lignin model compound.16 We have demonstrated
that the vanadium complexes (dipic)VV(O)(OiPr) (2, dipic =
Figure 1. Vanadium catalysts for the aerobic oxidation of lignin model
compounds.
dipicolinate) and (HQ)VV(O)(OiPr) (3, HQ = 8-quinolinate)
(Figure 1) catalyze aerobic oxidative C−C bond cleavage
reactions in several different lignin model compounds.17
Although these reports illustrate the promise of vanadium to
catalyze aerobic oxidation processes and break C−C and C−O
bonds in lignin, the impact of varying the ligand backbone on
the activity and selectivity of vanadium catalysts is not well
understood.
Having previously studied catalysts 2 and 3 with dipicolinate
and 8-quinolinate ligands, we were interested in how a more
electron-donating ligand set might influence the catalysis.
Specifically, it was envisioned that a tridentate scaffold with
phenolate donor groups could provide a more electron-rich
environment and an additional open site at the metal center
that might facilitate the reaction of VIII or VIV with oxygen. As
Received: April 16, 2012
Published: June 18, 2012
© 2012 American Chemical Society
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Figure 2. Ligands H2BPA, H2BPP, and H2BPB.
Figure 3. (a) X-ray structure of vanadium(V) complex 4 (thermal ellipsoids at 50% probability, hydrogen atoms omitted for clarity). Selected bond
lengths (Å): V1−O1 = 1.589(2), V1−O4 = 1.772(2), V1−O2 = 1.823(2), V1−O3 = 1.830(2), V1−N1 = 2.277(3). (b) X-ray structure of
vanadium(V) complex 5 (one of two crystallographically independent molecules, thermal ellipsoids at 50% probability, hydrogen atoms omitted for
clarity). Selected bond lengths (Å): V1−O1 = 1.589(3), V1−O4 = 1.762(3), V1−O3 = 1.814(3), V1−O2 = 1.848(3), V1−N1 = 2.272(4).
possible ligand candidates, we identified the bis(phenolate)-
pyridine ligand H2BPP (H2BPP = 2,6-(HOC6H2-
2,4-tBu2)2NC5H3), the bis(phenolate)benzene ligand H2BPB
(H2BPB = 2,6-(HOC6H2-2,4-tBu2)2C6H4), and the bis-
(phenolate)amine ligand H2BPA (H2BPA = N,N-bis(2-
hydroxy-4,5-dimethylbenzyl)propylamine), (Figure 2).22,23
None of the bis(phenolate) ligands have previously been
explored in the context of vanadium-mediated aerobic oxidation
catalysis.
This work describes the synthesis of vanadium complexes
bearing bis(phenolate) ligands H2BPP and H2BPA. The
reactivity of the complexes has been explored, and their ability
to catalyze aerobic oxidation reactions has been evaluated for
several substrates. While both complexes (BPP)VV(O)(OiPr)
(4) and (BPA)VV(O)(OiPr) (5) catalyzed aerobic oxidation
reactions, complex 4 was found to be a more stable and active
catalyst.
Complexes 4 and 5 were characterized by NMR and IR
spectroscopy, X-ray crystallography, and elemental analysis.
Distinctive features in the 1H NMR spectra of complexes 4 and
5 include the signals for the vanadium-isopropoxide ligand,
which are shifted significantly downfield from free isopropanol.
The V-isopropoxide methine signal (V-OCH) appears as a
septet at 5.61 ppm for complex 4 and 5.54 ppm for complex 5,
as compared to 6.35 ppm (CD3CN) for (dipic)VV(O)(OiPr)
(2) and 6.25 ppm for (HQ)2VV(O)(OiPr) (3). Previously, for a
series of substituted dipicolinate ligands, it was found that the
chemical shift of the isopropoxide methine V-OCH signal
correlated with the electronic nature of the ligand, shifting
downfield as the electron-withdrawing character of the ligand
increased. This suggests that the bis(phenolate) ligand
frameworks are more electron donating than the dipicolinate
or bis(8-quinolinate) ligand scaffolds.
The X-ray structures of 4 and 5 are shown in Figure 3. For
complex 5, single crystal X-ray diffraction analysis revealed two
crystallographically independent molecules in the unit cell. The
two molecules display very similar bond lengths (see
Supporting Information for details of the other crystallo-
graphically independent molecule). Both complexes 4 and 5
have trigonal bipyramidal geometries where the VV centers are
raised out of the ONO plane. Selected bond lengths for 4 and 5
are listed in Figure 3; complexes 4 and 5 have nearly identical
vanadium-oxo bond distances, and the V−OiPr distances are
also quite similar (1.772(2) vs 1.762(3) Å).
RESULTS AND DISCUSSION
■
Synthesis of Vanadium Complexes. The bis(phenolate)-
pyridine ligand (H2BPP) and bis(phenolate)benzene ligand
(H2BPB) were prepared via Negishi cross-coupling reactions,
as previously described by Agapie and Bercaw.23 Using a
previously published procedure,22 H2BPA was prepared by a
Mannich condensation of 3,4-dimethylphenol, propylamine,
and formaldehyde in CH3OH. The bis(phenolate)pyridine
ligand H2BPP reacted with VV(O)(OiPr)3 at room temperature
in isopropanol solution, affording dark red-brown solid complex
(BPP)VV(O)(OiPr) (4). A similar reaction between H2BPA
and VV(O)(OiPr)3 in isopropanol yielded complex (BPA)-
VV(O)(OiPr) (5). Surprisingly, no reaction was observed
between H2BPB and VV(O)(OiPr)3 in isopropanol-d8 solvent;
1H and 51V NMR spectra of the reaction mixture showed only
signals corresponding to the starting material, even after heating
at 50 °C for 2 h.
Several VV complexes of bis(phenolate)-24 and tris-
(phenolate)-amine25,26 ligands have previously been reported.
Ali and co-workers prepared vanadium alkoxide complexes of
bis(phenolate)amine ligands similar to H2BPA;27,28 these
complexes were evaluated for the catalytic oxidation of toluene
and xylene using H2O2. In these proposed radical reactions,
high yields of benzoic acid products were obtained at elevated
temperatures.27 Several multidentate phenolate-amine type
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Scheme 1. Oxidation of Pinacol by Vanadium(V) Complex 4
Scheme 2. Reaction of Complex 5 with Pinacol
ligands have also been investigated for the assembly of mixed
valent VV−VIV species.29,30
Stoichiometric Oxidation of Pinacol. To initially assess
the potential of the vanadium complexes to mediate oxidative
C−C bond cleavage reactions, the reactions of 4 and 5 were
tested with pinacol. Complex 4 reacted readily with pinacol (5
equiv) at room temperature, affording dark green crystals of the
vanadium(IV) complex 6 in 95% yield (Scheme 1). When the
reaction was conducted in CD2Cl2 and monitored by 1H NMR
spectroscopy, the C−C bond cleavage product acetone was
detected (1 equivalent based on V), consistent with the overall
stoichiometry shown in Scheme 1. Complex 6 was charac-
terized by IR and UV−vis spectroscopy, elemental analysis, and
X-ray crystallography. The X-ray structure of complex 6 (see
Supporting Information, Figure S2) indicates that it is a
mononuclear VIV complex. In the X-ray structure of 6, the
coordinated solvent molecule is disordered (see Supporting
Information for details). Most likely, the bulky tert-butyl
substituents on the phenolate donors contribute to the
monomeric nature of complex 6, preventing dimerization via
bridging phenolate groups.
Figure 4. X-ray structure of vanadium(IV) dimer 7 (thermal ellipsoids
at 50% probability, hydrogen atoms and co-crystallized acetonitrile
omitted for clarity). Selected bond lengths (Å): V1−O1 = 1.601(2),
V1−O2 = 1.928(2), V1−O3 = 2.131(2), V1−O3′ = 2.004(2), V1−O4
= 2.152(2), V1−N1 = 2.164(2).
(2.131(2) Å) is significantly longer than the distance between
the vanadium center and the phenolate oxygen trans to the
isopropanol ligand (1.928(2) Å).
When excess pinacol was added to a suspension of 5 in
CH3CN, the vanadium complex reacted over the course of 20 h
at room temperature, affording light purple crystals of
vanadium(IV) dimer 7 in nearly quantitative yield (97%).
When the reaction was conducted in CD3CN and monitored
by H NMR spectroscopy, acetone was detected as a reaction
product, suggesting the overall stoichiometry shown in Scheme
2.
As depicted in Figure 4, in the solid state complex 7 is a
centrosymmetric VIV dimer, in which each ligand binds to a VIV
center in a chelating mode and one of the phenolate groups
bridges to another VIV center by a μ-oxo bridging mode. In
complex 7 the phenolate oxygen atoms and amine nitrogen
atom are in a facial arrangement, with one of the phenolate
donor groups positioned trans to the vanadium-oxo ligand.
Isopropanol molecules are also present in the structure of the
VIV product, saturating the coordination sphere of each VIV
center.31 Selected bond lengths for 7 are given in Figure 4; the
vanadium oxo bond distance of 1.601(4) Å is longer than those
of VV complexes 4 and 5. The distance between the vanadium
center and the phenolate oxygen located trans to the oxo ligand
A vanadium(IV) dimer of a tetradentate bis(phenolate)-
amine-glycine ligand with bridging phenolate moieties has
previously been described by Ceccato and co-workers.32 Unlike
complex 7, in this previously reported example the phenolate
oxygen atoms and amine nitrogen atom are in a meridonal
arrangement. A nonoxo VIV complex bearing two bis-
(phenolate)amine ligands has also previously been reported.33
Having demonstrated that both complexes 4 and 5 mediated
the stoichiometric C−C bond cleavage of pinacol, we then
tested the reaction of the vanadium(IV) complex 6 with air,
which would be required to complete a catalytic cycle. When a
solution of paramagnetic complex 6 in CD2Cl2 was exposed to
air at room temperature in the presence of NEt3 (2 equiv), an
instantaneous color change from green to brown was observed.
1H and 51V NMR spectra of the resulting solution were
consistent with the formation of a new VV species [(BPP)-
VV(O)2]HNEt3 (8). Complex 8 could also be synthesized
independently from the reaction of 4 with water and NEt3. The
X-ray structure of 8 is shown in Figure 5. The vanadium oxo
distance (V1−O1 = 1.621(2) Å) is longer than that of complex
4. The distance between the O2 oxo ligand and the nitrogen on
1
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Table 1. Aerobic Oxidation of 4-Methoxybenzylalcohol
a
Using Bis(phenolate)pyridine Complex 4
additive
(10 mol %)
T
(°C)
time
(h)
% yield
(NMR)
entry
solvent
1
none
1,2-dichloroethane
1,2-dichloroethane
THF
60
60
60
60
80
80
80
80
70
80
80
80
80
80
80
24
24
24
24
65
24
48
24
24
65
65
65
65
65
65
65
0
2
NEt3
trace
trace
trace
99
45
81
0
3
NEt3
4
NEt3
toluene
5
NEt3
toluene
6
NEt3
toluene
7
NEt3
toluene
8
none
toluene
9
NEt3
toluene
2
10
11
12
13
14
15
N(iPr)2Et
piperidine
DBU
toluene
91
99
89
14
1
toluene
Figure 5. X-ray structure of vanadium(V) cis-dioxo complex 8
(thermal ellipsoids at 50% probability, hydrogen atoms and co-
crystallized CH3OH omitted for clarity). Selected bond lengths (Å):
V1−O1 = 1.621(2), V1−O2 = 1.644(2), V1−O3 = 1.919(2), V1−O4
= 1.920(2), V1−N1 = 2.172(2).
toluene
DMAP
K2CO3
CH3COONa
NEt3
toluene
toluene
toluene
9
b
16
toluene
80
80
a
b
Conditions: 2 mol % of 4, 10 mol % additive. 2 mol % of 5 was used
the HNEt3 counterion is about 2.657 Å, consistent with a
hydrogen bonding interaction.
as a catalyst. Yields determined by integration against an internal
standard (hexamethylbenzene).
The rapid reactivity of the vanadium(IV) complex 6 with air
stands in contrast to that previously observed for the complex
(dipic)VIV(O)(pyr)2, which reacted with air only upon heating
for several days at 100 °C.17c This indicates that the ligand
framework has a significant influence on the rate of the reaction
of VIV with air, with the more electron-donating BPP ligand
framework promoting the reaction relative to dipicolinate.
carbonate, and sodium acetate proved to be less effective for
promoting the reaction. Bis(phenolate)amine VV complex 5
was also found to catalyze the aerobic oxidation of 4-
methoxybenzyl alcohol under the optimized reaction con-
ditions, but afforded 4-methoxybenzaldehyde in somewhat
lower yield (80%) (Table 1, entry 16).
To further investigate the interaction of VV catalyst 4 with
the substrate, the reaction of 4 with 4-methoxybenzyl alcohol
was monitored by NMR spectroscopy. When 1 equiv of 4-
methoxybenzyl alcohol was added to a toluene-d8 solution of 4,
new signals corresponding to the vanadium(V) species
CATALYTIC OXIDATIONS
■
We recently reported that the 8-quinolinate vanadium complex
3 catalyzed the aerobic oxidation of activated alcohols, and
evaluated several different vanadium complexes as catalysts for
the aerobic oxidation of 4-methoxybenzyl alcohol using a base
additive (NEt3).13 To compare with the previously reported
results and optimize reaction conditions, VV complexes 4 and 5
were tested for the catalytic aerobic oxidation of 4-
methoxybenzyl alcohol. Initially, the aerobic oxidation of 4-
methoxybenzyl alcohol was attempted using bis(phenolate)-
pyridine complex 4 (2 mol %) as a catalyst at 60 °C in 1,2-
dichloroethane. However, no significant oxidation of the
substrate was observed under these conditions (Table 1,
entry 1). Addition of a base (NEt3) and use of a different
solvent (THF or toluene) did not prompt the oxidation of 4-
methoxybenzyl alcohol. However, increasing the reaction
temperature to 80 °C afforded 4-methoxybenzaldehyde in
∼99% NMR yield after 65 h under air in toluene solvent (Table
1, entry 5).
1
(BPP)VV(O)(OCH2C6H4-p-OCH3) (9) appeared in the H
and 51V NMR spectra within 5 min at room temperature.
Addition of an excess (3 equiv) of 4-methoxybenzyl alcohol to
complex 4 allowed for the isolation of complex 9 (Scheme 3),
Scheme 3. Reaction of Complex 4 with 4-Methoxybenzyl
Alcohol
Compared to previously reported 8-quinolinate catalyst 3,
complex 4 was a slower catalyst for the aerobic oxidation of 4-
methoxybenzyl alcohol, requiring both a longer reaction time
and a higher temperature (Table 1, entry 6).13 However, as was
the case for catalyst 3, a base promoter was found to be crucial
for the catalytic reaction (Table 1, entry 8). We tested several
different bases as promoters for the oxidation of 4-
methoxybenzyl alcohol using 4 in toluene at 80 °C (Table 1,
entries 10−15). In addition to triethylamine, diisopropylethyl-
amine, piperidine, and DBU (DBU = 1,8-diazabicyclo[5.4.0]-
undec-7-ene) were effective promoters of the vanadium
complex, affording 4-methoxybenzaldehyde in high yields
after 65 h. Dimethylaminopyridine (DMAP), potassium
which was characterized by NMR and IR spectroscopy,
elemental analysis, and X-ray crystallography. Complex 9
shows a singlet at 6.39 ppm in the 1H NMR spectrum
(CD2Cl2) for the V-OCH2 protons, consistent with an
environment of Cs symmetry. For the previously reported
complex (HQ)2VV(O)(OCH2C6H4-p-OCH3), the V-OCH2
protons appear at downfield chemical shifts of 6.72 and 6.57
ppm, suggesting that the bis(8-quinolinate) ligand framework is
somewhat more electron-withdrawing.13 The X-ray structure of
9 is shown in Figure 6. The vanadium-oxo bond and vanadium
benzyloxy bond distances in 9 are 1.577(2) and 1.783(2) Å,
respectively, which differ only slightly from those in 4.
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Scheme 5. Aerobic Oxidation of Lignin Model Compound
11-13C2 Using Vanadium Catalyst 4 (10 mol %)
14-13C2 has not been observed previously, and is tentatively
assigned as the alcohol 1-(3,5-dimethoxyphenyl)-3-hydroxy-
propan-1-one (Scheme 4), on the basis of NMR spectroscopy.
The 13C{1H} NMR spectrum of 14-13C2 shows two signals
arising from the labeled carbons in 14 at 58.2 and 40.8 ppm
(1JC−C = 38 Hz). In the proton-coupled carbon NMR spectrum,
the signal at 58.2 appears as a multiplet, displaying coupling to
the two protons on the labeled carbon (1JC−H = 105 Hz), the
adjacent labeled carbon (1JC−C = 38 Hz), and the two protons
on the adjacent carbon (2JC−H = 4 Hz) (See Supporting
Information for details). A similar pattern is observed for the
labeled carbon at 40.8 ppm, which shows one-bond coupling to
two protons (1JC−H = 124 Hz), coupling to the adjacent labeled
carbon (1JC−C = 38 Hz), and coupling to the two protons on
the adjacent labeled carbon (2JC−H = 2 Hz).
For the oxidation of nonphenolic lignin model compound
10-13C2, the selectivity of catalyst 4 bears similarity to
previously reported dipicolinate catalyst 2 (Figure 1),17b giving
predominantly a mixture of C−O and C−H bond cleavage
products. This stands in contrast to catalyst 1, which was
previously reported by Son and Toste to give a high selectivity
(94%) for C−O bond cleavage in a closely related nonphenolic
lignin model.16 This is also distinct from 8-quinolinate catalyst
3, which gave only C−H bond cleavage products (ketone
12-13C2) upon aerobic oxidation of 10-13C2 (Table 2, entry
1).34 Previously, Son and Toste have suggested that the
selectivity for C−O bond cleavage vs C−H bond cleavage
products is influenced by the ligand bite angle.16 Although the
origin of selectivity remains under investigation, the C−O and
Figure 6. X-ray structure of complex 9 (thermal ellipsoids at 50%
probability, hydrogen atoms omitted for clarity). Selected bond
lengths (Å): V1−O1 = 1.583(2), V1−O2 = 1.785(2), V1−O4 =
1.833(2), V1−O3 = 1.841(2), V1−N1 = 2.236(2).
Lignin Model Compounds. Having found that both
complexes 4 and 5 catalyzed the aerobic oxidation of 4-
methoxybenzylalcohol, we evaluated 4 and 5 as catalysts for the
aerobic oxidation of the previously reported arylglycerol β-aryl
ether lignin model compounds 10-13C2 and 11-13C2 (Schemes
4 and 5).17b,34 When nonphenolic lignin model 10-13C2 was
heated in toluene-d8 solution with bis(phenolate)pyridine
complex 4 (10 mol %), approximately 84% conversion occurred
after 48 h at 100 °C. 1H and 13C NMR analysis of the resulting
reaction mixture revealed the formation of a mixture of
products, including ketone 12-13C2, alkene 13-13C2, 14-13C2, 2-
methoxyphenol (15), 3,5-dimethoxybenzaldehyde (16), formic
acid-13C1 (17-13C1), and 3,5-dimethoxybenzoic acid (18)
(yields given in Scheme 4). Products 12-13C2 and 13-13C2
have been reported previously,17b and the identity of 3,5-
dimethoxybenzaldehyde and 3,5-dimethoxybenzoic acid was
confirmed by comparison with commercial samples. Product
Scheme 4. Aerobic Oxidation of Lignin Model Compound 10-13C2 Using Vanadium Catalyst 4 (10 mol %)
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Table 2. Comparison of Products Obtained from Vanadium Catalysts 3 and 4
percent yield
entry
substrate
catalyst
% conv.
12
13
14
16
18
19
20
21
a
1
2
3
4
10-13C2
10-13C2
11-13C2
11-13C2
3
65
55
27
0
0
0
4
0
6
b
4
84
26
14
a
3
100
20
27
19
30
trace
34
b
4
trace
a
b
Conditions: 10 mol % catalyst, pyridine-d5, 80 °C, 48 h. Conditions: 10 mol % catalyst, toluene-d8, 100 °C, 48 h.
C−H bond cleavage reactions likely proceed through different
mechanisms, as suggested by the varied selectivities observed
for vanadium catalysts 1−4.
CONCLUSIONS
■
New vanadium(V) complexes of bis(phenolate) pyridine and
bis(phenolate)amine ligands have been synthesized and
characterized. Both complexes 4 and 5 react with pinacol at
room temperature, affording acetone and vanadium(IV)
products 6 and 7, respectively. Complexes 4 and 5 were
evaluated as catalysts for the aerobic oxidation of 4-
methoxybenzylalcohol. Complex 4 (2 mol %) was an active
catalyst, affording 4-methoxybenzaldehyde in nearly quantita-
tive yield, while a lower conversion was observed with complex
5. The vanadium complexes were also tested for the aerobic
oxidation of lignin model compounds 10-13C2 and 11-13C2.
Decomposition of vanadium catalyst 5 resulting from oxidation
of the bis(phenol)amine ligand was observed during the
attempted aerobic oxidation of both lignin model compounds.
However, complex 4 did catalyze the aerobic oxidation of lignin
model compounds 10-13C2 and 11-13C2. In general, complex 4
is a somewhat slower catalyst for the aerobic oxidation reactions
than previously reported 8-quinolinate catalyst 3. The NMR
features of complexes 4 and 9 are consistent with the
bis(phenolate)pyridine ligand scaffold being more electron
rich than the bis(8-quinolinate) ligand set. Overall, in
considering a complete catalytic cycle, the evidence suggests
that while the more electron rich bis(phenolate) ligand set may
accelerate the reaction of vanadium(IV) with air, the opposite
trend is expected for the alcohol oxidation step, which would be
promoted by more electron deficient ligands. Designing more
active vanadium catalysts for aerobic oxidation reactions will
require balancing the electronic requirements of both steps.
Bis(phenolate)amine catalyst 5 was also tested as a catalyst
for the aerobic oxidation of 10-13C2. However, when 10-13C2
was heated in toluene-d8 (100 °C, 48 h) with 5 (10 mol %), a
low conversion was observed and signals corresponding to 4,5-
dimethylsalicylaldehyde were detected in the 1H NMR
spectrum, suggesting the decomposition of the vanadium
catalyst. The identity of the 4,5-dimethylsalicylaldehyde was
confirmed by GC-MS analysis, which showed a peak arising
from this product with the expected mass (m/z = 150). The
4,5-dimethylsalicylaldehyde likely results from oxidation of the
benzylic position of the BPA ligand and subsequent C−N bond
cleavage. Previously, Kol and co-workers have reported
reactivity at the benzylic position of a closely related
bis(phenolate)amine ligand, which underwent β-hydrogen
abstraction upon thermolysis of a tantalum tribenzyl complex.35
Decomposition of vanadium catalyst 5 was also observed
during the attempted catalytic oxidation of phenolic lignin
model 11-13C2.
Complex 4 catalyzed the oxidation of the phenolic lignin
model compound 11-13C2. When 4 (10 mol %) was heated
with phenolic lignin model 11-13C2 in toluene-d8 solvent under
air (100 °C, 48 h), only partial conversion of 11-13C2 was
observed (ca. 20%), and the reaction afforded ketone 19-13C2
(19%) (Scheme 5). Surprisingly, although catalyst 4 generates
C−O bond cleavage products upon oxidation of the non-
phenolic lignin model compound 10-13C2, no C−O bond
cleavage products were observed in the oxidation of phenolic
lignin model compound 11-13C2 using catalyst 4. Previously,
alkyl-phenyl bond cleavage products (20 and 21-13C2, Table 2)
were observed as the major products when the oxidation of 11
was carried out with 8-quinolinate catalyst 3;34 using catalyst 4
only trace alkyl-phenyl bond cleavage products were detected
EXPERIMENTAL SECTION
■
General Considerations. Unless specified otherwise, all reactions
were carried out under a dry argon or nitrogen atmosphere using
standard glovebox and Schlenk techniques. Deuterated solvents were
purchased from Cambridge Isotope Laboratories. CD2Cl2 and CD3CN
were dried over CaH2. Anhydrous grade solvents were obtained from
Acros and used as received. 1H, 13C, and 51V NMR spectra were
obtained at room temperature on a Bruker AV400 MHz spectrometer,
1
by H and 13C NMR spectroscopy (Table 2, entries 3 and 4).
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Inorganic Chemistry
Article
with chemical shifts (δ) referenced to the residual solvent signal (1H
and 13C) or referenced externally to VOCl3 (0 ppm). IR spectra were
obtained on a Perkin-Elmer Spectrum One instrument. GC-MS
analysis was obtained using a Hewlett-Packard 6890 GC system
equipped with a Hewlett-Packard 5973 mass selective detector.
Elemental analyses were performed by Atlantic Microlab in Norcross,
GA. Ligands H2BPA,22 H2BPP,23 and H2BPB23 were synthesized
according to previously published procedures.
7.72 (d, J = 8.0 Hz, 1H, Py), 7.56 (d, J = 2.4 Hz, 2H, aryl), 7.46 (d, J =
2.4 Hz, 2H, aryl), 2.68 (q, J = 7.2 Hz, 6H, HNEt3), 1.49 (s, 18H,
C(CH3)3), 1.38 (s, 18H, C(CH3)3), 1.07 (t, J = 7.2 Hz, 9H, HNEt3).
51V NMR (105 MHz, CD2Cl2) δ −552 (s). IR (thin film): νVO = 934
cm−1, 905 cm−1.
Preparation of (BPP)VV(O)(OCH2C6H4-p-OCH3) (9). In a glass
vial, complex 4 (30.6 mg, 0.050 mmol) was dissolved in CH2Cl2 (3
mL) and 4-methoxybenzyl alcohol (20.7 mg, 0.150 mmol) was added.
n-Hexane (1 mL) was then added, and the resulting reaction mixture
was slowly evaporated at room temperature over 5 days, during which
time dark crystals formed. The supernatant was decanted, and the
crystals washed with n-hexane (2 × 1 mL) and dried under vacuum.
Yield: 0.030 g (86%). 1H NMR (400 MHz, CD2Cl2) δ 8.03 (t, J = 8.0
Hz, 1H, Py), 7.79 (d, J = 8.0 Hz, 2H, Py), 7.57−7.53 (m, 4H, aryl),
7.34 (d, J = 8.4 Hz, 2H, aryl), 6.90 (d, J = 8.4 Hz, 2H, aryl), 6.39 (s,
2H, V-OCH2), 3.82 (s, 3H, -OCH3), 1.48 (s, 18H, C(CH3)3), 1.40 (s,
18H, C(CH3)3). 13C NMR (101 MHz, CD2Cl2) δ 163.1, 160.3, 159.7,
159.3, 144.6, 139.8, 136.9, 134.0, 133.2, 129.6, 129.1, 127.2, 125.3,
124.4, 123.9, 114.3, 114.1, 105.7, 66.2, 65.3, 55.8, 35.7, 35.1, 31.9, 30.4,
15.7. 51V NMR (105 MHz, CD2Cl2) δ −481.3 (s). IR (thin film):
νVO = 972 cm−1. Anal. Calcd.for C41H52NO5V: C, 71.39; H, 7.60; N,
2.03. Found: C, 71.11; H, 7.53; N, 1.94.
General Procedure for the Catalytic Aerobic Oxidation of 4-
Methoxybenzyl Alcohol. In a 25 mL round-bottom flask, 4-
methoxybenzyl alcohol (69 mg, 0.50 mmol) was combined with
vanadium complex 4 or 5 (0.01 mmol, 2 mol %), NEt3 (7 μL, 0.05
mmol, 10 mol %), and a hexamethylbenzene internal standard (5.5
mg, 0.034 mmol). The mixture was dissolved in toluene (1 mL) under
air, and the flask equipped with a stir bar and an air condenser. The
flask was heated with stirring in an oilbath at 80 °C for 65 h under air.
The reaction mixture was cooled to room temperature, the solvent
removed under vacuum, and the yield of oxidized product 4-
methoxybenzaldehyde determined by integration of the 1H NMR
spectra against the internal standard.
Preparation of (BPP)VV(O)(OiPr) (4). The ligand H2BPP (243
mg, 0.500 mmol) was suspended in isopropanol (5.0 mL) and
V(O)(OiPr)3 (122 mg, 0.500 mmol) was added. The mixture was
stirred overnight at room temperature, and then the brown solid was
collected by filtration, washed with isopropanol (2 × 2 mL), and dried
under vacuum. Yield: 0.260 g (85%). Brown blocks suitable for X-ray
diffraction were grown by slow evaporation of a CH2Cl2-iPrOH
1
solution of the complex. H NMR (400 MHz, CD2Cl2) δ 8.03 (t, J =
7.9 Hz, 1H, Py), 7.80 (d, J = 7.9 Hz, 2H, Py), 7.56 (m, 4H, aryl), 5.61
(m, 1H, V-OCH(CH3)2), 1.51 (s, 18H, C(CH3)3), 1.45 (d, J = 6.2 Hz,
6H, V-OCH(CH3)2), 1.39 (s, 18H, C(CH3)3). 13C NMR (101 MHz,
CD2Cl2) δ 160.7, 155.6, 143.6, 139.7, 136.4, 127.2, 124.9, 123.9, 123.6,
105.6, 35.7, 35.0, 31.9, 30.4, 25.3. 51V NMR (105 MHz, CD2Cl2) δ
−512.7 (s). IR (thin film): νVO = 970 cm−1. Anal. Calcd. for
C36H50NO4V: C, 70.68; H, 8.24; N, 2.29. Found: C, 70.82; H, 8.23; N,
2.35.
Preparation of (BPA)VV(O)(OiPr) (5). In a glass vial, H2BPA (305
mg, 0.932 mmol) and V(O)(OiPr)3 (228 mg, 0.934 mmol) were
suspended in isopropanol (5 mL). The mixture was stirred at room
temperature for 20 h, during which time a brown solid formed. The
brown solid was collected on a frit, washed with isopropanol (2 × 1
1
mL), and dried under vacuum. Yield: 289 mg (68%). H NMR (400
MHz, CD3CN) δ 6.92 (s, 2H, aryl), 6.48 (s, 2H, aryl), 5.54 (h, 1H, J =
6.0 Hz, V-OCH(CH3)2), 4.76 (d, 2H, J = 15.2 Hz, CH2), 3.93 (d, 2H,
J = 15.2 Hz, CH2), 2.46 (br s, 2H, N−CH2CH2CH3), 2.20 (s, 6H, aryl-
CH3), 2.19 (s, 6H, aryl-CH3), 1.41 (d, 6H, J = 6.4 Hz, V-
OCH(CH3)2), 1.35 (br s, 2H, N−CH2CH2CH3), 0.45 (br t, 3H, J =
6.0 Hz, N−CH2CH2CH3). 51V NMR (105 MHz, CD3CN) δ −518.0
(s). IR (thin film): νVO = 961 cm−1. Anal. Calcd. for C24H34NO4V:
C, 63.85; H, 7.59; N, 3.10. Found: C, 63.88; H, 7.53; N, 3.27.
Preparation of (BPP)VIV(O)(HOiPr) (6). In a glass vial, complex 4
(30.6 mg, 0.050 mmol) was dissolved in CH2Cl2 (3 mL) and pinacol
(17.7 mg, 0.150 mmol) was added. n-Hexane (1 mL) was then added
into the solution, and the resulting reaction mixture was slowly
evaporated at room temperature over a period of 3 days, during which
time dark-green crystals formed. The supernatant was decanted, and
the crystals washed with n-hexane (2 × 1 mL) and dried under
Aerobic Oxidation of 10-13C2 Using Vanadium Complex 4. In
an NMR tube, lignin model 10-13C2 (0.032 g, 0.095 mmol) was
dissolved in toluene-d8 (1 mL) containing dimethylsulfone (ca. 2 mg)
as an internal standard. An initial spectrum was recorded, and then the
solution was transferred to a 50 mL round-bottom flask containing 4
(5.8 mg, 0.0095 mmol, 10 mol %) under air. The reaction mixture was
heated under air with stirring at 100 °C for 48 h, and then cooled to
room temperature. The solution was transferred to an NMR tube, and
additional spectra were recorded. Product yields were determined by
integration against the internal standard.
Aerobic Oxidation of 11-13C2 Using Vanadium Complex 4. In
an NMR tube, phenolic lignin model 11-13C2 (0.020 mg, 0.057 mmol)
was dissolved in toluene-d8 (1 mL) containing dimethylsulfone (0.025
M) as an internal standard. An initial spectrum was recorded, and then
the solution was transferred to a 50 mL round-bottom flask containing
4 (3.5 mg, 0.0057 mmol, 10 mol %) under air. The reaction mixture
was heated under air with stirring at 100 °C for 48 h, and then cooled
to room temperature. The solution was transferred to an NMR tube
and additional spectra were recorded. Yields were determined by
integration against the internal standard.
1
vacuum. Yield: 0.029 g (95%). H NMR (400 MHz, CD2Cl2) δ 7.21
(br), 4.26 (br), 1.96 (br s), 1.57 (br), 1.43 (br s), 1.28 (br s). IR (thin
film): νVO = 953 cm−1. Anal. Calcd. for C36H49NO4V·iPrOH: C,
69.83; H, 8.56; N, 2.09. Found: C, 69.14; H, 8.12; N, 2.18.
Preparation of [(BPA)VIV(O)(HOiPr)]2 (7). In a glass vial, complex
5 (0.1534 g, 0.3401 mmol) and pinacol (0.113 g, 0.958 mmol) were
suspended in CH3CN (2 mL). The mixture was allowed to stand at
room temperature overnight (20 h), during which time all of the dark
red solid dissolved and pale purple crystals formed. The reaction
mixture was cooled to −20 °C for a further 48 h. At this time, the
supernatant was decanted, and the crystals washed with diethyl ether
1
ASSOCIATED CONTENT
(3 × 2 mL) and dried under vacuum. Yield: 0.148 g (96%). H NMR
■
(400 MHz, DMSO-d6) δ 4.34 (br s), 2.33 (br), 1.90 (br), 1.62 (br),
−0.14 (br). IR (thin film): νVO = 946 cm−1. Anal. Calcd for
C42H54N2O6V2: C, 63.71; H, 7.80; N, 3.10. Found: C, 63.93; H, 7.93;
N, 3.15.
S
* Supporting Information
CIF files, X-ray crystallographic data, and NMR spectra from
the catalytic oxidation reactions. This material is available free
of charge via the Internet at http://pubs.acs.org.
Preparation of [(BPP)VV(O)2]HNEt3 (8). Complex 4 (12.2 mg,
0.02 mmol) was dissolved in CH2Cl2/MeOH (4.0 mL, 3:1, v/v) in a
small vial, and then water (0.1 mL) was added. To the resulting pale-
brown solution was added triethylamine (10.1 mg, 0.1 mmol), and the
reaction mixture was slowly evaporated at room temperature over a
period of 3 days, brown blocks suitable X-ray diffraction analysis
formed, which were collected by decanting the supernatant and
washing with Et2O, and then dried under vacuum. Yield: 11.5 mg
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: skhanson@lanl.gov.
Notes
1
(86%). H NMR (400 MHz, CD2Cl2) δ 7.97 (t, J = 8.0 Hz, 1H, Py),
The authors declare no competing financial interest.
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Inorganic Chemistry
Article
(27) Maity, D.; Marek, J.; Sheldrick, W. S.; Mayer-Figge, H.; Ali, M. J.
Mol. Catal. A 2007, 270, 153−159.
ACKNOWLEDGMENTS
■
This work was supported by Los Alamos National Laboratory
LDRD (20100160ER) and by a Director’s Post-Doctoral
Fellowship for G. Zhang. We also thank J. C. Fettinger (UC
Davis) for the X-ray structure determination of 6.
(28) Maity, D.; Mijanuddin, M.; Drew, M. G. B.; Marek, J.; Mondal,
P. C.; Pahari, B.; Ali, M. Polyhedron 2007, 26, 4494−4502.
(29) Mondal, A.; Sarkar, S.; Chopra, D.; Row, T. N. G.; Pramanik, K.;
Rajak, K. K. Inorg. Chem. 2005, 44, 703−708.
(30) Mondal, A.; Basak, S.; Sarkar, S.; Chopra, D.; Das, R.; Rajak, K.
K. Eur. J. Inorg. Chem. 2006, 1824−1829.
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