Dioxidomolybdenum(VI) and -tungsten(VI) complexes with tripodal amino bisphenolate ligands as epoxidation and oxo-transfer catalysts

Article abstract of DOI:10.1016/j.poly.2017.04.036

The molybdenum(VI) and tungsten(VI) complexes [MO₂(L)] (M?=?Mo (1), W (2), H₂L?=?bis(2-hydroxy-3,5-di-tert-butybenzyl)morpholinylethylamine) were synthesized and the complexes were used to catalyze oxotransfer reactions, viz. sulfoxi

Full text of DOI:10.1016/j.poly.2017.04.036

Accepted Manuscript
Dioxidomolybdenum(VI) and -tungsten(VI) Complexes with Tripodal Amino
Bisphenolate Ligands as Epoxidation and Oxo-transfer Catalysts
Md. Kamal Hossain, Jörg A. Schachner, Matti Haukka, Ari Lehtonen, Nadia C.
Mösch-Zanetti, Ebbe Nordlander
PII:
S0277-5387(17)30327-3
DOI:
http://dx.doi.org/10.1016/j.poly.2017.04.036
POLY 12618
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
22 December 2016
27 April 2017
Please cite this article as: d.K. Hossain, J.A. Schachner, M. Haukka, A. Lehtonen, N.C. Mösch-Zanetti, E.
Nordlander, Dioxidomolybdenum(VI) and -tungsten(VI) Complexes with Tripodal Amino Bisphenolate Ligands
as Epoxidation and Oxo-transfer Catalysts, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.04.036
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Dioxidomolybdenum(VI) and -tungsten(VI) Complexes with Tripodal Amino
Bisphenolate Ligands as Epoxidation and Oxo-transfer Catalysts
Md. Kamal Hossain,a Jörg A. Schachner,b Matti Haukka,c Ari Lehtonen,d,* Nadia C.
Mösch-Zanetti,b,* Ebbe Nordlandera,*
a. Chemical Physics, Department of Chemistry, Lund University, P. O. Box 124, SE-
22100 Lund, Sweden
b. Institute of Chemistry, Department of Inorganic Chemistry, University of Graz
Schubertstraße 1, 8010 Graz, Austria
c. Department of Chemistry, P.O. Box 35, University of Jyväskylä, FI-40014 Jyväskylä,
Finland
d. Inorganic Materials Chemistry Research Group, Laboratory of Materials Chemistry
and Chemical Analysis, Department of Chemistry, University of Turku, 20014, Turku,
Finland
Corresponding author
E-mail address: Ebbe.Nordlander@chemphys.lu.se (E. Nordlander)
Abstract
The molybdenum(VI) and tungsten(VI) complexes [MO2(L)] (M= Mo (1), W (2), H2L =
bis(2-hydroxy-3,5-di-tert-butybenzyl)morpholinylethylamine)) were synthesized and the
complexes were used to catalyse oxotransfer reactions, viz. sulfoxidation, epoxidation and
benzoin oxidation. For comparison, the same reactions were catalysed using the known
complexes [MO2(L')] (M= Mo (3), W (4), H2L' = bis(2-hydroxy-3,5-di-tert-
butybenzyl)ethanolamine) and [MO2(L'')] (M= Mo (5), W (6), H2L'' = bis(2-hydroxy-3,5-di-
tert-butybenzyl)diethyleneglycolamine). The oxo atom transfer activity between DMSO and
benzoin at 120 °C was identical for all studied catalysts. Reasonable catalytic activity was
observed for sulfoxidation by the molybdenum complexes, but all tungsten complexes were
found to be inactive. Similarly, the molybdenum complex 1 exhibited relatively good
epoxidation activity, while the corresponding tungsten complex 2 catalysed only the
epoxidation of cis-cyclooctene with low activity.
Keywords: Molybdenum • Tungsten • Sulfoxidation • Epoxidation • Oxygen Atom transfer
1

1. Introduction
Electronic and steric properties of ligands are routinely used to tune the reactivity of metal
species. For example, to obtain robust high-valent metal complexes that are stable under
catalytic conditions, a number of bulky anionic ligands with hard oxygen donor atoms have
been used. Various phenolate ligands, in particular, have shown great promise towards this
end [1–10]. Amino bisphenols with different side-arm donors are potentially tetradentate
ligands that can form stable complexes with all transition metals [11]. Such ligands can
occupy four coordination sites around the metal centre and thus keep the coordination number
sufficiently high to prevent the undesirable dimerization of reactive intermediates in the
catalytic reaction cycles. On the other hand, if the side-arm donor is sufficiently labile, it may
dissociate, if necessary, during the catalytic reaction. Thus, the nature of the side-arm donor
may play a crucial role for the stability and activity of the catalytically active species. A
number of amino phenolate complexes with transition metals have been studied as catalysts
for oxidation reactions, as well as model compounds for the active sites of metal enzymes.
[12–23].
We are interested in the development of new molybdenum(VI) and tungsten(VI) based
catalysts for several biologically as well as industrially important catalytic oxidation
reactions, for example such oxygen atom transfer (OAT) processes as epoxidation and
sulfoxidation [24–26]. We have previously reported the catalytic potential of
dioxidomolybdenum and dioxidotungsten complexes bearing tripodal tetradentate amino
bisphenolate ligands with either a hydroxyethylene substituent (R = H) (L') or a
hydroxyglycolene substituent (R = (CH2)2OH) (L'') (complexes 3-6 in Figure 1) [10a]. These
catalysts exhibit high activity in the epoxidation of cis-cyclooctene with either tert-butyl
hydroperoxide (tBuOOH) or hydrogen peroxide (H2O2) as ultimate oxidants [10a]. In this
study, we have used molybdenum and tungsten complexes of a morpholine-substituted amino
bisphenolate ligand (L) (complexes 1 and 2 in Figure 1). The dangling oxygen atom in the
morpholine motif may make the ligand more prone to accept H-bonds from hydroperoxide
oxidants and thus bring oxidant molecules closer to the reaction centre, and may thus render
the complexes more potent for the catalysis. Complexes 1 and 2 were studied as catalysts for
the epoxidation of olefins while all compounds 1-6 were tested as catalysts in sulfoxidation
and benzoin oxidation.
Figure 1. Molybdenum and tungsten complexes used in this study.
2

2. Results and discussion
2.1 Syntheses
The synthesis of the tripodal amino bisphenol proligand H2L has been reported earlier [27]. In
this study, it was prepared by a solvent-free Mannich condensation reaction applying a
published procedure [28]. Recrystallization from hexane gave crystals suitable for X-ray
crystallographic analyses. The reaction of H2L with [MoO2(acac)2] in acetonitrile solution at
room temperature led to the crystallization of the acetonitrile adduct [MoO2(L)]·MeCN
(1·MeCN) in a yield of 69%. Complex 1 may alternatively be prepared by the reaction of H2L
with other starting materials, such as [MoO2Cl2], in refluxing toluene followed by
recrystallization from hot acetonitrile. The molecular structure of the complex was found to
be identical to the methanol adduct published recently [27].
The tungsten complex 2 was prepared from [W(eg)3] (eg = the glycolate dianion) and H2L in
a CHCl3-MeOH mixture. The yellow reaction mixture was stirred at 70 °C for an hour to
obtain 2·MeOH as a white crystalline solid in 62% yield. The air- and moisture-stable
compounds 1 and 2 are soluble in polar chlorinated and aromatic solvents but insoluble in
non-polar solvents. Complex 2 was characterized by IR, 1H and 13C NMR spectroscopies. The
1
coordination of the ligand to the metal centre was seen in the H spectrum as a disappearance
1
of the broad signal of the phenolic OH proton. In CDCl3 solutions, the H spectrum shows
sharp signals for aromatic protons as well as for the tert-butyl groups. The benzylic protons
are seen as doublets, but the signals for all CH2 protons of the morpholine moiety are broad
and overlapping. This indicates some dynamic process in solution, e.g. the equilibrium of
different conformers of the ligand such as boat vs. chair conformation of the morpholine unit.
The 13C NMR spectrum of 2 shows 6 signals for aromatic carbons, 5 signals for CH2 carbons,
2 signals for tBu central carbons and 2 signals for tBu methyl groups, which is consistent with
the expected Cs symmetry of the complexes. This indicates that only the configurational
isomer that involves trans orientation of the phenolate moieties of the ligand, and
coordination of the basic amine moiety of the ligand trans to the oxido units is present (cf.
Figure 1).
For complex 2, stretching frequencies at 900 and 946 cm-1 were assigned as asymmetric and
symmetric stretches, respectively [10a,24,29]. The ESI-MS spectrum showed a few major
peaks; the molecular ion peaks for [2+Na]+ (m/z = 803), [2+H]+ (m/z = 781) and the found
isotopic distributions were in a good agreement with the calculated spectra. The mass
spectrum also shows a ligand molecular ion at [H2L+H]+ (m/z = 567), as well as a peak
envelope (m/z = 349) for a fragmented amino monophenolate moiety.
3

2.2 Crystal and molecular structures of H2L and [MO2(L)] (M=Mo (1), W (2))
The proligand H2L was crystallised from hexane and characterized by single-crystal X-ray
crystallography as shown in Figure 2. Single crystals of 1 and 2 suitable for X-ray diffraction
were isolated from the reaction mixtures. Complex 1 crystallizes with one molecule of MeCN
in the asymmetric unit, while 2 crystallizes with a molecule of MeOH. The molecular
structures of 1 and 2 are shown in Figure 2, together with a capped-sticks representation of the
superimposed structures of the two complexes. Relevant crystallographic data are summarized
in Table 1, and selected bond lengths and angles are collated in Table 2. Both complexes are
hexacoordinate with a nearly Cs-symmetric distorted octahedral geometry, which is consistent
with the observed NMR spectra (vide supra and supporting material). The amino bisphenolate
ligand is coordinated to the cis-MO2 (M = Mo, W) moiety through two anionic phenolate
oxygens and two neutral nitrogen donor atoms of the central amine and the morpholine
moiety, with the Ophenolate-donors in cis positions and the N-donors in trans position to the
oxido ligands. The Ooxido-M-N angles deviate from 180°, presumably because of ligand
constraints; for the central amine nitrogen of the ligand, the O4-M-N1 angle is 157.5(8)° for
Mo and 156.3(2)° for W, and for the morpholine nitrogen the O3-M-N2 angle is 167.6(8)° for
Mo, 169.6(2)° for W. The positioning of the nitrogens trans to the oxido -donor ligands
results in long M-N bond distances, i.e. M-N1 distance is 2.3629(19) for 1 and 2.373(5) Å for
2 whereas the M-N2 distance is 2.445(2) for 1 and 2.455(5) Å for 2. Complex 2·MeOH is
essentially isostructural with the recently reported complex 1·MeOH [27] (see Table 2). All
O=M=O angles and M=O distances resemble those found in other six-coordinate
dioxidomolybdenum(VI) and -tungsten(VI) complexes [27,29–36]. The M-Ophenolate angles are
125.82(17) and 120.28(16)° for 1 and 122.7(4) and 127.8(4)° for 2, whereas the
corresponding M-Ophenolate distances are 1.9393(19) and 1.9673(18) Å for 1 and 1.947(4) and
1.921(5) Å for 2, respectively. The larger angles are associated with shorter M-Ophenolate bond
distances and vice versa. The value of the M-O-C angle can in theory reflect the degree of π
donation from oxygen to the metal, but it has been found to be a flexible parameter and no
clear correlation between the metal-phenolate angles and distances for d-block transition
metal complexes has been established [37,38].
4

(a)
(b)
(c)
(d)
Figure 2. The solid-state structures of proligand H2L (a), complexes 1 (b) and 2 (c) as well as
thesuperimpoed molecular structures of 1 (red) and 2 (green) (d). For clarity reasons, the C-H
hydrogen atoms and the co-crystallized solvent molecules have been omitted. Thermal
ellipsoids are drawn at 50% probability level.
5

Table 1. Summary of crystallographic data for H2L, 1 and 2
H2L
C36H58N2O3
1
2
empirical
formula
fw
C38H59MoN3O5
C37H60N2O6W
566.84
170(2)
0.71073
Triclinic
P1
10.6957(2)
11.4601(2)
14.7028(3)
90.3150(10)
99.7710(10)
101.8420(10)
1736.72(6)
2
733.82
120(2)
1.54184
Monoclinic
C2/c
812.72
170(2)
temp (K)
0.71073
Monoclinic
C2/c
(Å)
cryst syst
space group
a (Å)
b (Å)
c (Å)
27.0047(6)
11.5353(2)
26.9948(6)
90
115.291(3)
90
7603.1(3)
8
1.282
26.8098(3)
11.3140(2)
26.9685(4)
90
α (deg)
 (deg)
γ (deg)
V (Å3)
Z
113.8780(10)
90
7480.1(2)
8
1.443
3.133

calc (Mg/m3)
1.084
0.068
K)
3.166
(Mo
(mm-1)
No. reflns.
33698
59934
7991
1.053
0.0801
0.0399
50391
9228
1.096
0.0840
0.0588
0.1155
Unique reflns. 10108
GOOF (F2)
Rint
1.093
0.0418
0.0673
0.1500
R1a (I  2)
wR2b (I  2)
0.1050
a R1 = ||Fo| – |Fc||/|Fo|. b wR2 = [[w(Fo – Fc2)2]/ [w(Fo ) ]]1/2.
2
2 2
Table 2. Selected bond lengths [Å] and angles [°] for 1, 2 and the reported 1.MeOH [27].
2
1·MeOH
1
M1-O1
M1-O2
M1-O3
M1-O4
M1-N1
M1-N2
O(1)-M(1)-O(2)
O(3)-M(1)-O(4)
N(1)-M(1)-N(2)
O(1)-M(1)-O(3)
O(1)-M(1)-O(4)
1.9393(19)
1.9673(18)
1.7062(19)
1.7157(18)
2.3629(19)
2.445(2)
156.25(7)
108.31(9)
73.51(6)
1.947(4)
1.921(5)
1.724(5)
1.728(5)
2.373(5)
2.455(5)
155.90(19)
106.7(2)
72.68(17)
95.3(2)
1.938(4)
1.960(4)
1.699(4)
1.699(4)
2.381(4)
2.466(5)
155.73(16)
107.6(2)
72.98(14)
95.2(2)
95.12(9)
98.49(8)
98.6(2)
98.76(19)
6

2.3 Catalysis studies
2.3.1 Catalytic epoxidation
Figure 3. Schematic depiction of catalytic epoxidation of substrates S1-5, using complexes 1
and 2 as catalysts.
Catalysts 1 and 2 were tested for the epoxidation of five different olefin substrates S1-5 using
three equiv. of tert-butyl hydroperoxide (tBuOOH, 5.5 M in decane) as oxidant and with 1
mol% of catalyst in CHCl3 solution at 50 °C. In addition, epoxidation of cis-cyclooctene S1
was also tested under more green conditions, using hydrogen peroxide (H2O2, 30%) as
oxidant in CHCl3 and dimethyl carbonate (DMC) as an eco-friendly solvent. Using H2O2
yields water as the only by-product instead of tBuOH from tBuOOH, and dimethyl carbonate
has proven to be an eco-friendly, non-hazardous substitute for chlorinated organic solvents
[39]. Conversions were determined by GC mass spectrometry. The five investigated olefin
substrates cis-cyclooctene S1, styrene S2, 1-octene S3, racemic limonene S4 and racemic α-
terpineol S5 are depicted in Figure 3 and results are summarized in Table 3. Whereas complex
1 showed catalytic activity with all substrates, the W analogue 2 remained inactive under the
same reaction conditions. The lower catalytic epoxidation activity of tungsten complexes
relative to their molybdenum analogues has been observed before [9,40–43]. In all cases,
blank experiments without catalysts were run.
Complex 1 converts S1 quantitatively to its epoxide with excellent selectivity within 24 h,
whereas yields were generally lower for the more challenging substrates S2-5. With S2, low
selectivity (<30%) to styrene oxide was observed, due to over-oxidation to
phenylacetaldehyde and benzaldehyde. In case of S3, a yield of 1-octene oxide of 28% with
high selectivity (>95%) after 24 h was obtained. For limonene S4, low selectivity due to
epoxidation of the exocyclic double bond was observed. Finally, for substrate S5, complex 1
showed a similar performance than for S4, but with a low yield (24%) of epoxide. Since
alcohols are known to compete for the vacant coordination site with the substrate, the low
conversion for S5 is most likely caused by the same effect.
7

Table 3. Summary of epoxidation results conversion of substrate (selectivity to epoxide)
[%][a]) for complex 1.
S4[b]
1
S1
S2
S3
S5
TBHP, CHCl3
>95 (>95)
65 (27)
28 (>95)
>95 (32)
81 (30)
Conditions: 1 mol% cat. loading, 3 equiv. oxidant, 50 °C; 2 showed zero to low (< 10%) conversions, results are
not shown. [a] conversion of substrate (selectivity to epoxide) after 24 h; [b] sum of cis and trans epoxide is given.
Due to the low activities observed for S2-5, only S1 was further tested with H2O2 as oxidant.
In order to also test a non-chlorinated solvent, the eco-friendly solvent dimethyl carbonate
(DMC) was used instead of CHCl3; the results are summarized in Table 4.
Table 4. Epoxidation results (Yield (selectivity) [%][a]) with hydrogen peroxide as oxidant.
S1, H2O2
CHCl3
DMC
1
2
84 (89)
95 (>95)
8 (81)
60 (>95)
Conditions: 1 mol% cat. loading, 3 equiv. H2O2, 50 °C [a] yield of epoxide (selectivity to) after 24 h.
Similar to previous observations with the published complexes 3-6 (cf. Figure 1), tungsten
complex 2 is more active in DMC compared to CHCl3 when using H2O2 as oxidant [10a].
Under otherwise similar conditions, complex 2 is 7.5 times more active in the eco-friendly
solvent DMC compared to chlorinated solvent CHCl3. Also the selectivity towards cyclo-
octene oxide is enhanced in DMC (Table 4). Furthermore, complex 1 showed an increased
activity in DMC, reaching essentially full conversion of substrate after already 7 h (Figure 4).
Figure 4. Time-conversion plot of complex 1 for epoxidation of S1 with two different
oxidants and solvents (DMC = dimethyl carbonate).
8

The overall lower activity of W complex 2 with H2O2 as oxidant compared to Mo complex 1
is somewhat unexpected. The exact opposite activity was overserved with previously
published examples, including complexes 3-6 [10a,40,43–45]. Examples of Mo complexes
like 1 giving high conversions with S1 and H2O2 are scarce in literature [10a,43,46].
2.3.2 Catalytic sulfoxidation
Complexes 1-6 were tested as catalysts for sulfoxidation of methyl-p-tolyl sulfide with two
equivalents of tert-butyl hydroperoxide or aqueous H2O2 as oxidants (Scheme 1). The
reactions were run in CDCl3 or CD3CN or CD3CD2OD solutions at 25 °C while the reaction
course was monitored by 1H NMR. In the control experiments, no reactions occurred without
any catalyst. All W complexes proved inactive under applied conditions. Conversely, with the
Mo complexes the catalytic reactions commenced instantly without any noticeable induction
times. The sulfoxidation of methyl-p-tolylsulfide in CDCl3 using tBuOOH as oxidant was
accomplished by all molybdenum complexes with high selectivity and without any sign of the
formation of sulfone. Complex 3 exhibited the fastest reaction with a half-life of
approximately 5 minutes compared to the other two complexes 1 and 5, which showed half-
lives of approximately 15 minutes under similar conditions.
Scheme 1
Under the experimental conditions used, the sulfoxidation reactions gave good yields (> 90%)
while the calculated turnover frequencies (TOFs) for complexes 1, 3 and 5 were 245 h-1, 485
h-1 and 157 h-1, respectively (Table 5). Importantly, the sulfoxidation was found to be
selective, without any sign of the formation of sulfone. The (possible) enantioselectivities of
the sulfoxidation reactions were not evaluated.
The molybdenum complexes were also tested as catalysts for sulfoxidation of the same
substrate in MeCN-d3 using hydrogen peroxide as oxidant. All complexes showed almost
equal activities (Table 6), and these activities were clearly lower than in the analogous
reactions employing tBuOOH (Table 5). After completion of the reaction, only small amounts
of sulfone as a by-product were observed.
The sulfoxidations were run in ethanol-d6 in order to compare different oxidants under
identical conditions. All catalysts showed very low epoxidation activities when tBuOOH was
used as an oxidant, but the activities were high with H2O2 as an oxygen source (see Tables 5
and 6). After completion of the reactions, negligible amounts of sulfone as a by-product were
observed when H2O2 was used.
9

Table 5. Catalytic sulfoxidation of methyl-p-tolylsulfide by tert-BuOOH at 25 °C in CDCl3
and EtOH-d6.
Catalyst
Solvent
CDCl3
Yield (%)a TOF (h-1) b Solvent Yield (%)c TOF (h-1) b
92
97
90
245
485
157
EtOH-d6 84
12
0
1
3
5
47
54
9
a
Reaction conditions: 1 mol-% of catalyst, 2.0 equivalents of tert-BuOOH. Yield of the
product measured by 1H NMR after an hour. bTOF calculated after 10 min of reaction as (mol
product) (mol catalyst)−1 (t/h)−1. cYield of the product measured by 1H NMR after 24 h.
Table 6. Catalytic sulfoxidation of methyl-p-tolylsulfide by H2O2 at 25 °C in MeCN-d3 and
ethanol-d6.
Catalyst
Solvent
CDCl3
Yield (%)a TOF (h-1) b Solvent
Yield (%)c TOF (h-1) b
87
99
94
32
49
43
EtOH-d6 99
78
1
3
5
98
98
120
114
Reaction conditions: 1 mol-% of catalysts, 2.0 equivalents of 35% H2O2. aYield of the product
1
b
measured by H NMR after three hours. TOF calculated after 60 min of reaction as (mol
product) (mol catalyst)−1 (t/h)−1. cYield of the product measured by 1H NMR after two hours.
dTOF calculated after 10 min of reaction as (mol product) (mol catalyst)−1 (t/h)−1.
2.3.3 Catalytic oxidation of benzoin
The oxo transfer reactivities of complexes 1-6 (10 mol-% catalyst loadings) were investigated
using benzoin as the substrate and deuterated dimethylsulfoxide (DMSO-d6) as an oxygen
source at 120 °C. Such reactivity of complex 1 has been investigated earlier although under
different conditions, i.e. heating benzoin in the presence of an excess of DMSO in acetonitrile
1
13
at 80 °C [27]. The reaction mixtures were characterized by H and C NMR spectroscopy.
After 24 hours, benzil was formed in all experiments in nearly quantitative (> 95%) yields,
while no reactions were seen when the complexes were absent. During the reactions, all
studied catalysts showed almost the same performance, although molybdenum catalysts are
usually reported to show higher oxotransfer activities than their tungsten counterparts [31].
13
Formation of the co-product dimethyl sulfide (DMS) was detected qualitatively using C
1
NMR spectroscopy. In the H NMR spectra some unidentified signals were observed after
prolonged heating. During the course of the reaction, the MoO2(VI) and WO2(VI) complexes
are proposed to react with C6H5CH(OH)C(O)C6H5 and are subsequently converted to
MoO(IV) and WO(IV) complexes while C6H5C(O)C(O)C6H5 is formed as another product
(Scheme 2) [47].
10

Scheme 2
A simplified catalytic cycle for the biomimetic oxidation of benzoin with dimethyl sulfoxide
(DMSO) to give dimethyl sulfide (DMS) and benzil is shown in Scheme 3. The colour of the
reaction mixtures remained yellow throughout the reaction, suggesting that μ-oxido-M(V)
(M=Mo, W) complexes, which are expected to be of purple colour with relatively high
extinction coefficients were not formed.[48] The reduced Mo(IV) complexes are not stable,
and are presumably rapidly oxidized to the original compounds to complete the catalytic cycle
[49,50].
Scheme 3 (adapted from reference 31)
3. Summary and conclusions
The new tungsten complex [WO2(L)] (2) based on the tripodal tetradentate amino bisphenol
proligand H2L has been synthesized and characterized. Complex 2 and its molybdenum
analogue [MoO2(L)] (1) were examined as catalysts for epoxidation of various alkenes using
tert-BuOOH or H2O2 as terminal oxidants. The molybdenum complex 1 exhibited higher
epoxidation activities than its tungsten analogue, but the latter complex showed markedly
improved catalytic activity when the solvent medium was changed from CHCl3 to dimethyl
carbonate. These results demonstrate that the use of the morpholine side-arm in complexes 1
and 2 yields active, stable epoxidation catalysts. Since the ligand also allows for epoxidations
with aqueous H2O2 in the eco-friendly solvent dimethyl carbonate, a pronounced stability
towards H2O, the side product from H2O2, is demonstrated. Especially Mo complex 1 shows
catalytic activity in both the biphasic system CHCl3/H2O2 as well as the single-phasic
DMC/H2O2. This might also point to a certain phase transfer ability of complex 1 between the
organic phase of the substrate and the aqueous phase of the oxidant.
The six complexes used in the present study are active catalysts for sulfoxidation and
oxidation of benzoin. Among them, the three molybdenum complexes proved to be highly
selective and effective sulfoxidation catalysts while the analogous tungsten complexes did not
show any reactivity under the same conditions. All complexes showed almost similar catalytic
activities for oxidation of benzoin to benzil, leading to nearly quantitative (> 95%) yields. The
overall lower catalytic activities exhibited by the tungsten complexes may be related to the
11

higher bond dissociation energies for tungsten-oxido bonds relative to analogous
molybdenum-oxido bonds.
4. Experimental
4.1 General and instrumentation
Unless otherwise specified, all experiments were run under atmospheric conditions. [W(eg)3]
was prepared by a literature method [51]. The ligand precursors H2L' and H2L'' were also
prepared by published methods [28,52,53], while H2L was synthesized by a solvent-free
version of a published method [28] (see Supplementary Material). All other chemicals were
1
13
from commercial sources and were used as received. The H and C NMR spectra were
recorded on a Bruker 300 MHz instrument or a Varian Inova 500 MHz spectrometer using
deuterated chloroform as solvent, and referenced to the residual signal of the solvent. The IR
spectra were recorded on a Bruker Vertex Optics 70 spectrometer. Infrared bands are reported
with wave number (cm-1) and intensities (br = broad, vs = very strong, s = strong, m =
medium, w = weak). UV–vis measurements were performed on a Varian 300 Bio UV-vis
spectrophotometer. Mass spectrometry was performed with a Waters ZQ 4000 spectrometer
using CsI as a calibrant. Gas chromatography-mass spectroscopy measurements (GC-MS)
were performed with Agilent 7890 A (column type Agilent 19091J-433), coupled to an
Agilent 5975 C mass spectrometer. A Heidolph Parallel Synthesizer 1 was used for all
epoxidation experiments.
[WO2(L)]·MeOH (2·MeOH). The ligand precursor H2L (0.31 g, 0.549 mmol) was dissolved
in 5 mL CHCl3 and added to a suspension of [W(eg)3] (0.20 g, 0.549 mmol) in 5 mL of
methanol. The orange mixture was stirred and heated at 70 °C. After 1 h of stirring, the
reaction mixture was allowed to concentrate by slow evaporation at room temperature until
white crystals were obtained. Compound 2 was isolated in 62% yield (0.28 g). 1H NMR (500
MHz, CDCl3) δ 7.35 (d, J = 2.4 Hz, 2H), 6.98 (d, J = 2.4 Hz, 2H), 4.34 (s, 2H), 4.27 (d, J =
14.2 Hz, 2H), 4.08 (d, J = 14.2 Hz, 2H), 4.01 (t, J = 5.8 Hz, 2H), 3.86 – 3.78 (m, 2H), 3.51 (s,
3H, CH3OH), 3.19 (t, J = 5.8 Hz, 2H), 3.06 (t, J = 5.8 Hz, 2H), 2.92 (d, J = 14.2 Hz, 2H), 1.48
13
(s, 18H), 1.30 (s, 18H), 1.03 (s, CH3OH). C NMR (126 MHz, CDCl3) δ 159.00, 143.19,
137.48, 124.01, 123.16, 122.50 (Ar-C), 70.73, 62.14, 62.08, 56.18, 53.84 (CH2), 50.85(CH3,
CH3OH), 34.94, 34.23 [C(CH3)3], 31.61, 30.44 (CH3). Selected FT-IR (cm-1) 946s (W=O),
900s (W=O). UV-vis in CH3CN: max, nm (, M–1cm–1): 381 (3530). ESI-MS: m/z = 803
[2+Na]+, 781 [2+H]+, 567 [H2L+H]+.
4.2 Epoxidation of olefins
In a typical experiment, 2-3 mg of catalyst (1 mol-%) were dissolved in CHCl3 (0.5 mL) and
mixed with a specific substrate (1 equiv.) and 10 µL of mesitylene (internal standard) at 50
°C. To start the reaction, 3 equiv. of oxidant, tert-butylhydroperoxide (5.5 M in decane) and
H2O2 (50% in water) were added. Aliquots for GC–MS (20 μL) were withdrawn at given time
intervals, quenched with MnO2 and diluted with HPLC-grade ethyl acetate. The reaction
products were analyzed by GC–MS (Agilent Technologies 7890 GC System), and the epoxide
12

produced from each reaction mixture was quantified versus mesitylene as the internal
standard.
4.3 Sulfoxidation reactions
The experiments were run at room temperature in deuterated chloroform or acetonitrile or
ethanol solutions using 1:2 molar ratios of substrate/tBuOOH or H2O2 (0.19M: 0.38 M) and
10μL of 1,2-dichloroethane added as an internal standard in a 5 mm NMR tube. The reactions
1
were monitored by H NMR spectroscopy at five-minute intervals. The reaction rates were
estimated upon the integrated intensities of substrate and product spectra. In the sulfoxidation
test, the sulfide methyl singlet at 2.45 was turned to the sulfoxide methyl singlet at 2.71 ppm.
4.4 Catalytic benzoin oxidation
In a typical test reaction, 0.10 mmol (21 mg) of benzoin and 0.01 mmol of catalyst were
dissolved in 1.0 mL of deuterated dimethyl sulfoxide in a liquid scintillation vial sealed with a
screw cap and maintained at 120 ˚C for 24 h. The reaction mixture was subsequently allowed
to cool to room temperature. An aliquot of the reaction solution was placed in an NMR tube
1
and the substrate conversion was assessed by H NMR spectroscopy. The conversion of
benzoin to benzil was detected whereas the yield was over 95% in all experiments.
4.5 X-ray structure determination
The crystals of 1, 2, and H2L were immersed in cryo-oil, mounted in a MiTeGen loop, and
measured at a temperature of 120 or 170 K. The X-ray diffraction data were collected on a
Rigaku Oxford Diffraction Supernova diffractometer or on a Bruker KappaApex II using Cu
K radiation ( = 1.54184 Å) or Mo K ( = 0.71073 Å) radiation. The CrysAlisPro [54] or
Denzo/Scalepack [55] program packages were used for cell refinements and data reductions.
A multi-scan or numerical absorption correction was applied to all data (SADABS, [56]
CrysAlisPro [54]). The structures were solved by charge flipping method using the Superflip
[57] program. Structural refinements were carried out using SHELXL [58] program with the
Olex2 [59] and SHELXLE [60] graphical user interfaces. Structure 2 was refined as a twinned
crystal (twin matrix 0 0 1 0 -1 0 1 0 0). The BASF value was refined to 0.14. Hydrogen atoms
were positioned geometrically and constrained to ride on their parent atoms, with C-H = 0.95-
0.99 Å, O-H = 0.84 Å, and Uiso = 1.2-1.5 Ueq(parent atom).
Acknowledgements
This work has been carried out within the framework of COST Action CM1003 Biological
oxidation reactions - mechanisms and design of new catalysts. M.K.H thanks the European
Commission for an Erasmus Mundus predoctoral fellowship.
Appendix A. Supplementary data
CCDC 1523698-1523700 contains the supplementary crystallographic data for H2L, 1 and 2.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
13

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16

Graphical Abstract
MO2 (M=Mo,W) of three tripodal amino bisphenolate ligands have been investigated as
catalysts in oxotransfer reactions (epoxidation, sulfoxidation, benzoin oxidation). The
molybdenum complexes were found to be more active than their tungsten congeners, and are
good sulfoxidation catalysts. Complexes exhibited higher activity in dimethyl carbonate
(DMC) solvent than in CHCl3.
17