Hydrogen bond donor functionalized dioxido-molybdenum(VI) complexes as robust and highly efficient precatalysts for alkene epoxidation

Article abstract of DOI:10.1016/j.mcat.2017.09.036

The synthesis of four novel, tridentate aminophenolate ligands HL1-HL4, bearing amide functionalities is reported. Reaction of these ligands with a dioxido molybdenum(VI) precursor led, depending on the choice of solvent, to mononuclear complexes of the type [MoO₂L(OMe)] (2, 4, 6) or dinuclear complexes [{MoO₂L}₂(μ-O)] (1, 3, 5, 7), containing one facially, tridentate ONO-ligand per metal center. This synthetic discrimination between dinuclear and mononuclear complexes allows for a comparison between structures and reactivity. Complexes 1-7 were found to be highly active catalysts in the epoxidation of several internal and terminal alkenes. With tert-butyl hydroperoxide (TBHP) as oxidant, precatalyst loadings of 0.0005 mol% (5 ppm) could be realized leading to turnover numbers of up to 110000. The precatalysts also allowed for the use of hydrogen peroxide (0.1 mol% precatalyst) as oxidant as well as various alcohols as “green” solvents, such as ethanol or even tert-butanol (usually an inhibitor of epoxidation).

Full text of DOI:10.1016/j.mcat.2017.09.036

Molecular Catalysis 443 (2017) 209–219
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Molecular Catalysis
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Editor’s choice paper
Hydrogen bond donor functionalized dioxido-molybdenum(VI)
complexes as robust and highly efficient precatalysts for alkene
epoxidation
Niklas Zwettler, Jörg A. Schachner, Ferdinand Belaj, Nadia C. Mösch-Zanetti∗
Institute of Chemistry, Inorganic Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of four novel, tridentate aminophenolate ligands HL1-HL4, bearing amide functionalities
is reported. Reaction of these ligands with a dioxido molybdenum(VI) precursor led, depending on the
choice of solvent, to mononuclear complexes of the type [MoO2L(OMe)] (2, 4, 6) or dinuclear complexes
[{MoO2L}2(␮-O)] (1, 3, 5, 7), containing one facially, tridentate ONO-ligand per metal center. This syn-
thetic discrimination between dinuclear and mononuclear complexes allows for a comparison between
structures and reactivity. Complexes 1-7 were found to be highly active catalysts in the epoxidation
of several internal and terminal alkenes. With tert-butyl hydroperoxide (TBHP) as oxidant, precatalyst
loadings of 0.0005 mol% (5 ppm) could be realized leading to turnover numbers of up to 110000. The pre-
catalysts also allowed for the use of hydrogen peroxide (0.1 mol% precatalyst) as oxidant as well as various
alcohols as “green” solvents, such as ethanol or even tert-butanol (usually an inhibitor of epoxidation).
© 2017 Elsevier B.V. All rights reserved.
Received 21 March 2017
Received in revised form
29 September 2017
Accepted 30 September 2017
Keywords:
Hydrogen bonds
Molybdenum
Epoxidation
Aminoamides
Homogeneous catalysis
1. Introduction
Initially, our research interest in high valent molybdenum
compounds focused on oxygen atom transfer as well as the acti-
The relative ease of ring-opening reactions, and thus the broad
variety of possible follow-up products, makes epoxides very impor-
tant intermediates in chemical synthesis [1–4]. One of the main
synthetic route to epoxides is the oxidation of olefins. Different
approaches exist for such a transformation, including the use of
stoichiometric amounts of organic peracids as well as hetero- and
homogeneous catalytic approaches [4,5]. Homogeneous catalytic
protocols usually allow for mild conditions and often employ high
valent metal-oxido complexes as precatalysts [3,6]. In the industrial
production of propylene oxide via the Arco/Halcon process, such a
protocol is realized with molybdenum and tungsten complexes as
well as alkyl hydroperoxides as oxidants [1,7]. With the prospect to
develop environmentally more friendly processes, the use of non-
toxic, earth-abundant metals such as molybdenum is of key interest
[8]. Especially mononuclear molybdenum(VI) dioxido complexes
have proven to be highly active and selective precatalysts for the
epoxidation of internal aliphatic olefins [9–11], recently the sub-
strate scope could to some extent be expanded to terminal olefins
[12,13]. Furthermore, the use of environmentally benign solvents
as well as oxidants remains a major objective [14].
vation of molecular oxygen [15]. We then started to examine
the catalytic behavior of the involved molybdenum complexes
in the oxidation of olefins. In an endeavor to develop “greener”
and more efficient catalytic protocols, our group tested several
mono- and dinuclear dioxidomolybdenum(VI), and subsequently
also mononuclear oxidorhenium(V) complexes for their capabil-
ity of catalytic epoxidation of olefins as well as catalytic aerobic
oxidation reactions. Whereas the investigated oxidorhenium(V)
complexes were robust and in part allowed for the use of hydrogen
peroxide as oxidant, they were limited to cyclooctene as substrate
[16]. Dioxidomolybdenum systems on the other hand were able to
catalyze the oxidation of a broad scope of substrates, albeit with
varying selectivities [9,11,17,18]. Especially the use of bidentate
Schiff-base ligands with donor sites led to very selective precat-
alysts [19]. Inspired by the work of Borovik and coworkers [20],
we expanded our library of iminophenolate ligands by introducing
amide functionalities as hydrogen bond donors and investigated
their coordination chemistry with the dioxidomolybdenum(VI)
fragment, finding unusual C C and C N coupling reactions at the
metal center [21]. To inhibit such bond formation reactions but
retain the structure of the ligands, we decided to investigate the
derived aminophenolate derivatives, accessible via reduction of the
imine C N bond [18].
∗
Corresponding author.
E-mail address: nadia.moesch@uni-graz.at (N.C. Mösch-Zanetti).
http://dx.doi.org/10.1016/j.mcat.2017.09.036
2468-8231/© 2017 Elsevier B.V. All rights reserved.

210
N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
Herein we present the synthesis of four aminophenolate lig-
of the respective precatalyst was stirred in 0.5 mL of the respective
solvent and substrate was added. Mesitylene was used as inter-
nal standard. After the experiment temperature was reached the
respective oxidant was added to start the reaction and samples
for GC–MS measurements were withdrawn at given time intervals.
GC samples were quenched with MnO2, diluted with ethyl acetate
and measured. Gas chromatography mass spectroscopy (GC–MS)
measurements have been performed with an Agilent 7890 A gas
chromatograph (column type, Agilent 19091J–433), coupled to an
Agilent 5975C mass spectrometer. All yields obtained by GC have
an esd of 5%.
ands featuring amide substituents with varying NH acidity by
hydrogenation of the respective iminophenolates with Pd/C H2 or
NaBH4. The higher flexibility of those aminophenolate ligands leads
to a monoanionic tridentate facial ONO coordination mode result-
ing in mono- and dinuclear dioxidomolybdenum(VI) complexes
of the general formulas [{MoO2L}2(␮-O)] and [MoO2L(OMe)]. The
dinuclear complexes are structurally closely related to previously
reported complexes, with the additional amide functionalities
being of crucial difference [18]. A further important aspect of the
system disclosed herein is the fact that both mono- and dinu-
clear complexes are formed depending on the reaction conditions,
thus allowing for a direct comparison of their reactivity. The
complexes were found to exhibit exceptionally high activity and
functional group tolerance in the catalytic epoxidation of various
alkene substrates with TBHP. This is in stark contrast to previously
reported structural analogs[18] surpassing the activity of homoge-
neous epoxidation precatalysts known to date without additional
hydrogen bond donors [9,12,22]. In addition we disclose herein
the catalytic activity using H2O2 as oxidant as well as the use of
environmentally benign alcoholic solvent [23].
2.3. X–ray diffraction analyses
Single–crystal X–ray diffraction analyses were measured on
a BRUKER–AXS SMART APEX II diffractometer equipped with a
CCD detector. All measurements were performed using monochro-
matized Mo K␣ radiation from an Incoatec microfocus sealed
tube at 100 K (cf. Table S1). Absorption corrections were per-
formed semi–empirical from equivalents. Structures were solved
by direct methods (SHELXS–97) [25] and refined by full–matrix
least–squares techniques against F2 (SHELXL–2014/6) [25]. CCDC
1532978-1532983 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data request/cif. Full experimental details for single–crystal
X–ray diffraction analyses of all compounds are provided in the
SI.
2. Experimental section
2.1. General
Unless specified otherwise, experiments were performed under
inert conditions using standard Schlenk equipment. Commer-
cially available chemicals were purchased from Sigma–Aldrich
and used as received. No further purification or drying opera-
tions have been performed. The metal precursor [MoO2(acac)2]
[24] was synthesized according to known procedures. The used
primary amines 2-amino-N-(tert-butyl)acetamide and 2-amino-
N-phenylacetamide were synthesized according to a previously
disclosed procedure [21]. Solvents were purified via a Pure–Solv
MD–4–EN solvent purification system from Innovative Technol-
ogy, Inc. Methanol was refluxed over activated magnesium for at
least 24 h and then distilled prior to use. The 1H, 13C and HSQC
NMR spectra were recorded on a Bruker Optics instrument at
300/75 MHz. Peaks are denoted as singlet (s), broad singlet (bs),
doublet (d), doublet of doublets (dd), triplet (t), pseudo–doublet
(“d”), pseudo–triplet (“t”) and multiplet (m). Used solvents and
peak assignment are mentioned at the specific data sets. HR-MS
(ESI+) measurements were performed at the University of Graz,
Department of Analytical Chemistry using a Thermo Scientific Q-
Exactive mass spectrometer in positive ion mode, the used solvent
was acetonitrile. Peaks are denoted as cationic mass peaks, and
the unit is the according ions mass/charge ratio. For dinuclear
compounds the denoted mass peak corresponds to the highest
found intensity, full calculated and found isotopic patterns are
provided within the supporting information. Gas chromatography
mass spectroscopy (GC–MS) measurements have been performed
with an Agilent 7890A gas chromatograph (column type Agilent
19091J–433), coupled to an Agilent 5975C mass spectrometer. Sam-
ples for infrared spectroscopy were measured on a Bruker Optics
ALPHA FT–IR Spectrometer. IR bands are reported with wavenum-
ber (cm−1) and intensities (s, strong; m, medium; w, weak). All
elemental analyses were measured at the Technical University of
Graz, Institute of Inorganic Chemistry using a Heraeus Vario Ele-
mentar automatic analyzer.
2.4. Ligand synthesis
All ligands are stable towards air and moisture. They can be
stored at ambient conditions for several weeks without decompo-
sition.
2.5. Synthesis of
N-(tert-butyl)-2-((2-hydroxybenzyl)amino)acetamide (HL1)
For the synthesis of HL1,
1 equiv. of 2-amino-N-(tert-
butyl)acetamide [21] (1.75 g, 13.5 mmol) as well as 0.2 g of Pd/C
(5 wt% Pd) were added to a solution of 1 equiv. of salicylic aldehyde
(1.64 g, 13.5 mmol) in 80 mL of MeOH. The resulting dark-yellowish
clouded solution was subsequently stirred at 50 ◦C in a pressure
reactor under H2 atmosphere (5 atm) for 24 h. The reaction solu-
tion was subsequently filtered and the reddish filtrate evaporated
in vacuo. The crude reddish oily solid was purified via column chro-
matography on silica gel (ethyl acetate/cyclohexane gradient from
20% to 100% ethyl acetate) to give HL1 as off-white solid (2.05 g,
65%). 1H NMR (300 MHz, (CD3)2SO, 25 ◦C, OH & amine NH obscured)
ı: 7.51 (bs, 1H, NH), 7.09-7.04 (m, 2H, ArH), 6.76-6.70 (m, 2H,
ArH), 3.66 (s, 2H, CH2), 3.02 (s, 2H, CH2), 1.25 (s, 9H, tBu) ppm;
13C NMR (75 MHz, (CD3)2SO, 25 ◦C) ı: 169.95 (C O), 156.47 (Ar-O),
129.24, 128.00, 124.95, 118.60, 115.19 (Ar), 51.39, 49.84, 49.33 (2x
CH2, q-tBu), 28.51 (tBu) ppm; HR-MS: (ESI+) m/z [M+H]+ calcd. for
13H20O2N2: 273.1598, found: 237.1594; IR (ATR, cm−1) ꢀ: 1591
˜
C
(m), 1537 (m), 1453 (m), 1391 (w), 1251 (w), 1224 (m), 754 (s), 435
(w);
2.6. Synthesis of N-(tert-butyl)-2-((3,5-di-tert-butyl-2-
hydroxybenzyl)amino)acetamide (HL2)
For the synthesis of HL2, 1.1 equiv. of 2-amino-N-(tert-
butyl)acetamide [21] (1.68 g, 12.9 mmol) as well as 0.5 g of Pd/C
(5 wt% Pd) were added to a solution of 1 equiv. of 3,5-di-tert-butyl-
2-hydroxybenzaldehyde (2.75 g, 11.7 mmol) in 100 mL of MeOH.
The resulting yellowish clouded solution was subsequently stirred
2.2. Catalytic epoxidation
A Heidolph Parallel Synthesizer 1 was used for all experiments.
In a typical epoxidation experiment an aliquot of a stock solution

N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
211
at 50 ◦C in a pressure reactor under H2 atmosphere (5 atm) for
24 h. The reaction solution was subsequently filtered and the pale
violet filtrate evaporated in vacuo. The crude pale violet pow-
der was purified via column chromatography on silica gel (ethyl
acetate/cyclohexane gradient from 20% to 100% ethyl acetate) to
give HL2 as white solid (2.69 g, 66%). 1H NMR (300 MHz, (CD3)2SO,
25 ◦C) ␦: 11.44 (bs, 1H, OH), 7.53 (bs, 1H, NH), 7.08 (d, 1H, ArH),
6.84 (d, 1H, ArH), 3.78 (s, 2H, CH2), 3.25 (bs, 1H, NH), 3.09 (s, 2H,
CH2), 1.35 (s, 9H, tBu), 1.26 (s, 9H, tBu), 1.22 (s, 9H, tBu) ppm;
13C NMR (75 MHz, (CD3)2SO, 25 ◦C) ␦: 169.24 (C O), 154.29 (Ar-
O), 139.35, 134.46, 123.21, 122.49, 121.67 (Ar), 51.87, 50.17 (CH2),
50.11, 34.42, 33.74 (q-tBu), 31.55, 29.54, 28.55 (tBu) ppm; HR-
MS: (ESI+) m/z [M + H]+ calcd. for C21H36O2N2: 349.2850, found:
˜
IR (ATR, cm−1) ꢀ: 1646 (s), 1574 (m), 1451 (s), 1297(s), 1217 (s),
1166 (m), 886 (m), 735 (s), 458 (m);
2.9. Complex syntheses
All dinuclear complexes are stable towards air and moisture
in solid state and can be stored at ambient conditions for several
weeks without decomposition. Mononuclear complexes are stable
towards air but slightly sensitive towards moisture. They can be
stored in a desiccator over P2O5 for several weeks without decom-
position.
349.2846; IR (ATR, cm−1) ꢀ: 1674 (s), 1542 (m), 1439 (m), 1360
˜
2.10. Synthesis of [{MoO2(L1)}2(ꢁ-O)] (1)
(m), 1228 (m), 578 (w);
For the synthesis of 1, 1 equiv. of [MoO2(acac)2] (111 mg,
0.34 mmol) was added to a solution of 1 equiv. of HL1 (80 mg,
0.34 mmol) in 5 mL of MeCN. The orange reaction mixture was
stirred for 2 h at 60 ◦C whereupon the formed dark yellow pre-
cipitate was filtered off and washed with a small amount of cold
MeCN and twice with 5 mL of pentane. Evaporation of all volatiles
in vacuo led to 1 as dark yellow powder (90%, 113 mg). Crystals
suitable for single–crystal X–ray diffraction analysis were obtained
via recrystallization from a concentrated MeCN solution at 5 ◦C.
1H NMR (300 MHz, (CD3)2SO, 25 ◦C, major isomer) ␦: 8.40 (s, 2H,
CONH), 7.10-7.02 (m, 4H, ArH), 6.67-6.62 (m, 2H, ArH), 6.54-6.52
(m, 2H, ArH), 5.50 (d, 2H, NH), 4.18-4.14 (m, 2H, CH2), 4.00-3.87
(m, 2H, CH2), 3.60-3.56 (m, 2H, CH2), 3.14-3.08 (m, 2H, CH2), 1.00
(s, 18H, tBu) ppm; 1H NMR (300 MHz, (CD3)2SO, 25 ◦C, minor iso-
mer) ␦: 8.40 (s, 2H, CONH), 7.10-7.02 (m, 4H, ArH), 6.67-6.62 (m,
2H, ArH), 6.54-6.52 (m, 2H, ArH), 5.18 (d, 2H, NH), 4.18-4.14 (m,
2H, CH2), 4.00-3.87 (m, 2H, CH2), 3.60-3.56 (m, 2H, CH2), 3.14-
3.08 (m, 2H, CH2), 1.02 (s, 18H, tBu) ppm; 13C NMR (75 MHz,
(CD3)2SO, 25 ◦C, major isomer) ␦: 173.33 (C O), 162.79 (Ar-O),
129.93, 129.29, 123.42, 119.20, 118.31 (Ar), 51.73 (CH2), 51.67
2.7. Synthesis of 2-((3,5-di-tert-butyl-2-hydroxybenzyl)amino)-
N-phenylacetamide (HL3)
For the synthesis of HL3,
1
equiv. of 2-amino-N-
phenylacetamide [21] (540 mg, 3.6 mmol) as well as 120 mg
of Pd/C (5 wt% Pd) were added to a solution of 1 equiv. of 3,5-di-
tert-butyl-2-hydroxybenzaldehyde (842 mg, 3.6 mmol) in 50 mL of
MeOH. The resulting yellowish clouded solution was subsequently
stirred at 50 ◦C in a pressure reactor under H2 atmosphere (5 atm)
for 24 h. The reaction solution was subsequently filtered and
the brownish filtrate evaporated in vacuo. The crude beige solid
was purified via column chromatography on silica gel (ethyl
acetate/cyclohexane gradient from 20% to 100% ethyl acetate)
to give HL3 as white solid (601 mg, 45%). 1H NMR (300 MHz,
(CD3)2SO, 25 ◦C, amine NH obscured) ␦: 11.43 (bs, 1H, OH), 10.00
(bs, 1H, NH), 7.61 (“d”, 2H, ArH), 7.31 (“t”, 2H, ArH), 7.10 (d, 1H,
ArH), 7.05 (“t”, 1H, ArH), 6.89 (d, 1H, ArH), 3.88 (s, 2H, CH2), 3.41 (s,
2H, CH2), 1.37 (s, 9H, tBu), 1.23 (s, 9H, tBu) ppm; 13C NMR (75 MHz,
(CD3)2SO, 25 ◦C) ␦: 169.06 (C O), 154.28 (Ar-O), 139.47, 138.88,
134.56, 128.77 (2x), 123.33, 123.25, 122.42, 121.77, 119.05 (2x)
(Ar), 51.97, 50.40 (CH2), 34.45, 33.77 (q-tBu), 31.56, 29.55 (tBu)
ppm; HR-MS: (ESI+) m/z [M+H]+ calcd. for C23H32O2N2: 369.2537,
(q-tBu), 27.95 (tBu) ppm; IR (ATR, cm−1) ꢀ: 1624 (m), 1480 (w),
˜
1262 (m), 930 (m), 899 (s), 878 (s), 753 (s, br), 726 (s), 617 (s),
496 (m), 478 (m), 388 (w); HR-MS: (ESI+) m/z [M+H]+ calcd. for
C26H38O9N4Mo2: C, 743.0829, found: 743.0829; Anal. calcd. for
found: 369.2534; IR (ATR, cm−1) ꢀ: 1665 (m), 1518 (s), 1481 (m),
˜
1437 (s), 1234 (m), 1103 (w), 879 (w), 761 (s), 694 (s), 537 (w), 500
(m);
C26H38Mo2N4O9: C, 42.06; H, 5.16; N, 7.55. Found: C, 42.50, H, 5.08;
N, 7.37%.
2.8. Synthesis of N-(tert-butyl)-2-((3,5-dichloro-2-
hydroxybenzyl)amino)acetamide (HL4)
2.11. Synthesis of [MoO2(L1)(OMe)] (2)
For the synthesis of 2, 1 equiv. of [MoO2(acac)2] (200 mg,
0.61 mmol) was added to a solution of 1 equiv. of HL1 (142 mg,
0.61 mmol) in 5 mL of MeOH. The yellowish reaction mixture was
stirred for 4 h at 50 ◦C whereupon the formed pale yellow precip-
itate was filtered off and washed twice with a small amount of
cold MeOH. Evaporation of all volatiles in vacuo led to 2 as pale
yellow powder (86%, 246 mg). Crystals suitable for single–crystal
X–ray diffraction analysis were obtained via recrystallization from
a concentrated MeOH solution at 5 ◦C. 1H NMR (300 MHz, (CD3)2SO,
25 ◦C) ␦: 8.57 (s, 1H, CONH), 7.12-7.05 (m, 2H, ArH), 6.69-6.64
(m, 1H, ArH), 6.57-6.55 (m, 1H, ArH), 5.92 (d, 1H, NH), 4.20-4.15
(m, 1H, CH2), 4.01 (s, 3H, OMe), 3.51-3.42 (m, 2H, CH2), 3.18-
3.12 (m, 1H, CH2), 1.00 (s, 9H, tBu); 13C NMR (75 MHz, (CD3)2SO,
25 ◦C) ı: 173.26 (C O), 162.33 (Ar-O), 130.02, 129.42, 119.16,
118.50 (Ar), 64.36 (OMe), 52.15 (CH2), 51.86 (q- tBu), 51.13 (CH2),
For the synthesis of HL4,
1 equiv. of 2-amino-N-(tert-
butyl)acetamide [21] (687 mg, 5.2 mmol) as well as 1 equiv. of
3,5-dichloro-2-hydroxybenzaldehyde (1000 mg, 5.2 mmol) in 5 mL
of MeOH. The initially deep yellow solution was subsequently
stirred at 50 ◦C for 4 h, whereupon a dark yellow precipitate started
to form. The precipitate was subsequently filtered off, washed with
2 × 5 mL of pentane and dried in vacuo. The resulting orange solid
was subsequently dissolved in 10 mL of MeOH and 4 equiv. of
NaBH4 (787 mg, 20.8 mmol) were added portionwise, accompa-
nied by gas evolution and decoloration of the reaction solution.
Subsequently the clear reaction solution was stirred overnight at
room temperature. After evaporation in vacuo, the crude oily sub-
stance was purified via column chromatography on silica gel (ethyl
acetate/cyclohexane gradient from 20% to 100% ethyl acetate) to
give HL4 as pale pink solid (600 mg, 38%). 1H NMR (300 MHz,
(CD3)2SO, 25 ◦C) ␦: 7.85 (bs, 2H, NH2+), 7.55 (bs, 1H, NH), 7.34 (d,
1H, ArH), 7.10 (d, 1H, ArH), 3.81 (s, 2H, CH2), 3.10 (s, 2H, CH2),
1.26 (s, 9H, tBu) ppm; 13C NMR (75 MHz, (CD3)2SO, 25 ◦C) ␦: 169.10
(C O), 153.00 (Ar-O), 127.35, 127.07, 127.02, 121.43, 120.55 (Ar),
50.14, 50.06 (CH2), 50.04 (q-tBu), 28.49 (tBu) ppm; HR-MS: (ESI+)
m/z [M+H]+ calcd. for C13H18O2N2Cl2: 305.0818, found: 305.0815;
27.76 (tBu); IR (ATR, cm−1) ꢀ: 1615 (s), 1574 (m), 1485 (m),
˜
1450 (m), 1267 (m), 1050 (m), 927 (s), 878 (s), 769 (s), 620 (s),
550 (s), 500 (m), 475 (s); HR-MS: (ESI+) m/z [M-OMe]+ calcd.
for C14H22MoN2O5: 365.0394, found: 365.0391; Anal. calcd. for
C14H22MoN2O5: C, 42.65; H, 5.62; N, 7.10. Found: C, 42.55, H, 5.46;
N, 6.93%.

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N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
2.12. Synthesis of [{MoO2(L2)}2(ꢁ-O)] (3)
C46H62Mo2N4O9·0.5CH2Cl2: C, 53.22; H, 6.05; N, 5.34. Found: C,
53.11, H, 6.01; N, 5.51%.
For the synthesis of 3, 1 equiv. of [MoO2(acac)2] (100 mg,
0.31 mmol) was added to a solution of 1 equiv. of HL2 (107 mg,
0.31 mmol) in 3 mL of MeCN. The orange reaction mixture was
stirred for 6 h at 60 ◦C whereupon the formed orange precipitate
was filtered off and washed thrice with 5 mL of pentane. Evapora-
tion of all volatiles in vacuo led to 3 as an orange solid (90%, 135 mg).
Crystals suitable for single–crystal X–ray diffraction analysis were
obtained via slow evaporation of a concentrated (CD3)2SO solution
at room temperature. 1H NMR (300 MHz, (CD3)2SO, 25 ◦C, major
isomer) ␦: 8.23 (s, 2H, CONH), 7.06 (d, 2H, ArH), 6.91 (d, 2H, ArH),
5.55 (d, 2H, NH), 4.11-3.93 (m, 4H, CH2), 3.51-3.47 (m, 2H, CH2),
3.20-3.10 (m, 2H, CH2), 1.28 (s, 18H, tBu), 1.23 (s, 18H, tBu), 0.95 (s,
18H, tBu) ppm; 1H NMR (300 MHz, (CD3)2SO, 25 ◦C, minor isomer)
␦: 8.29 (s, 2H, CONH), 7.07 (d, 2H, ArH), 6.89 (d, 2H, ArH), 5.09 (d,
2H, NH), 4.11-3.93 (m, 4H, CH2), 3.51-3.47 (m, 2H, CH2), 3.20-3.10
(m, 2H, CH2), 1.30 (s, 18H, tBu), 1.23 (s, 18H, tBu), 0.96 (s, 18H, tBu)
ppm; 13C NMR (75 MHz, (CD3)2SO, 25 ◦C, major isomer) ␦: 173.38
(C O), 159.46 (Ar-O), 139.43, 136.93, 124.60, 122.86, 122.28 (Ar),
52.79, 51.51 (CH2), 51.41, 34.32, 33.78 (q-tBu), 31.64, 29.78, 28.17
2.15. Synthesis of [MoO2(L3)(OMe)] (6)
For the synthesis of 6, 1 equiv. of [MoO2(acac)2] (100 mg,
0.31 mmol) was added to a solution of 1 equiv. of HL3 (89 mg,
0.31 mmol) in 5 mL of MeOH. The yellowish reaction mixture was
stirred for 6 h at 50 ◦C whereupon the formed yellow precipitate
was filtered off and washed twice with a small amount of cold
MeOH. Evaporation of all volatiles in vacuo led to 6 as a bright yel-
low solid (82%, 116 mg). Crystals suitable for single–crystal X–ray
diffraction analysis were obtained via recrystallization from a con-
centrated MeOH solution at 5 ◦C. 1H NMR (300 MHz, (CD3)2SO,
25 ◦C) ␦: 10.65 (s, 1H, CONH), 7.26-7.21 (m, 2H, ArH), 7.14-7.10 (m,
1H, ArH), 7.04-6.97 (m, 4H, ArH), 6.09 (d, 1H, NH), 4.21-4.17 (m,
1H, CH2), 4.03 (s, 3H, OMe), 3.79-3.71 (m, 1H, CH2), 3.57-3.53 (m,
1H, CH2), 3.37-3.31 (m, 1H, CH2), 1.23 (s, 9H, tBu), 1.12 (s, 9H, tBu);
13C NMR (75 MHz, (CD3)2SO, 25 ◦C) ␦: 173.17 (C O), 158.69 (Ar-O),
140.37, 137.30, 135.77, 128.41 (2x), 125.61, 124.86, 123.24, 122.43,
121.79 (2x) (Ar), 66.58 (OMe), 52.74, 51.56 (CH2), 34.30, 33.76 (q-
(tBu) ppm; IR (ATR, cm−1) ꢀ: 1621 (m), 1440 (w), 1266 (w), 910
˜
tBu), 31.41, 29.69 (tBu); IR (ATR, cm−1) ꢀ: 1627 (m), 1593 (w), 1568
˜
(m), 872 (s), 853 (s), 738 (vs, br), 554 (m), 482 (w); HR-MS: (ESI+)
m/z [M+H]+ calcd. for C42H70Mo2N4O9: 968.3343, found: 968.3335;
Anal. calcd. for C42H70Mo2N4O9: C, 52.17; H, 7.30; N, 5.79. Found:
C, 52.10, H, 6.82; N, 5.77%.
(w), 1062 (w), 923 (m), 884 (s), 845 (s), 753 (m), 598 (m), 506 (s);
HR-MS: (ESI+) m/z [M-OMe]+ calcd. for C24H34MoN2O5: 497.1332,
found: 497.1337, [M+H]+ calcd. for C24H34MoN2O5: 529.1600,
found: 529.1597; Anal. calcd. for C24H34MoN2O5: C, 54.75; H, 6.51;
N, 5.32. Found: C, 54.63, H, 6.03; N, 5.43%.
2.13. Synthesis of [MoO2(L2)(OMe)] (4)
2.16. Synthesis of [{MoO2(L4)}2(ꢁ-O)] (7)
For the synthesis of 4, 1 equiv. of [MoO2(acac)2] (100 mg,
0.31 mmol) was added to a solution of 1 equiv. of HL2 (107 mg,
0.31 mmol) in 3 mL of MeOH. The yellowish reaction mixture was
stirred for 6 h at 50 ◦C whereupon the formed yellow precipitate
was filtered off and washed twice with a small amount of cold
MeOH. Evaporation of all volatiles in vacuo led to 4 as a bright yel-
low solid (75%, 117 mg). Crystals suitable for single–crystal X–ray
diffraction analysis were obtained via crystallization from a con-
centrated MeOH solution at 5 ◦C. 1H NMR (300 MHz, (CD3)2SO,
25 ◦C) ␦: 8.48 (s, 1H, CONH), 7.08 (d, 1H, ArH), 6.93 (d, 1H, ArH),
5.92 (d, 1H, NH), 4.13-4.09 (m, 1H, CH2), 3.97 (s, 3H, OMe), 3.46-
3.38 (m, 2H, CH2), 3.21-3.16 (m, 1H, CH2), 1.29 (s, 9H, tBu), 1.23
(s, 9H, tBu), 0.94 (s, 9H, tBu); 13C NMR (75 MHz, (CD3)2SO, 25 ◦C) ␦:
173.19 (C O), 158.90 (Ar-O), 139.79, 136.92, 124.69, 122.98, 122.39
(Ar), 65.88 (OMe), 52.85 (CH2), 51.69 (q-tBu), 51.45 (CH2), 34.30,
For the synthesis of 7, 1 equiv. of [MoO2(acac)2] (107 mg,
0.33 mmol) was added to a solution of 1 equiv. of HL4 (100 mg,
0.33 mmol) in 5 mL of MeCN. The yellow reaction mixture was
stirred for 4 h at 60 ◦C whereupon the formed bright yellow pre-
cipitate was filtered off and dried in vacuo to give 7 as a pale yellow
solid (82%, 119 mg). 1H NMR (300 MHz, (CD3)2SO, 25 ◦C, major iso-
mer) ␦: 8.63 (s, 2H, CONH), 7.31 (d, 2H, ArH), 7.18 (d, 2H, ArH),
5.76 (d, 2H, NH), 4.10-4.01 (m, 2H, CH2), 3.92-3.77 (m, 2H, CH2),
3.71-3.67 (m, 2H, CH2), 3.19-3.13 (m, 2H, CH2), 1.07 (s, 18H, tBu)
ppm; 1H NMR (300 MHz, (CD3)2SO, 25 ◦C, minor isomer) ␦: 8.56
(s, 2H, CONH), 7.30 (d, 2H, ArH), 7.16 (d, 2H, ArH), 5.53 (d, 2H,
NH),), 4.10-4.01 (m, 2H, CH2), 3.92-3.77 (m, 2H, CH2), 3.63-3.59 (m,
2H, CH2), 3.19-3.13 (m, 2H, CH2), 1.05 (s, 18H, tBu) ppm; 13C NMR
(75 MHz, (CD3)2SO, 25 ◦C, major isomer) ␦: 173.62 (C O), 157.81
(Ar-O), 128.49, 128.36, 124.57, 120.89 (Ar), 52.79, 52.03 (q-tBu),
33.81 (q-tBu), 31.62, 29.78, 27.85 (tBu); IR (ATR, cm−1) ꢀ: 1622 (s),
˜
51.00 (2 x CH2), 27.81 (tBu) ppm; IR (ATR, cm−1) ꢀ: 1622 (s), 1458
1471 (w), 1260 (m), 1060 (m), 1032 (m), 906 (s), 888 (s), 846 (s),
600 (m), 567 (m), 520 (s), 484 (s); HR-MS: (ESI+) m/z [M-OMe]+
calcd. for C22H38MoN2O5: 477.1645, found: 477.1644; Anal. calcd.
for C22H38MoN2O5: C, 52.17; H, 7.56; N, 5.53. Found: C, 52.10, H,
7.44; N, 5.57%.
˜
(m), 922 (s), 899 (s), 785 (s), 750 (vs, br), 563 (w), 478 (w); HR-MS:
(ESI+) m/z [M+H]+ calcd. for C26H34Cl4Mo2N4O9: 882.9238, found:
882.9232; Anal. calcd. for C26H34Cl4Mo2N4O9: C, 35.48; H, 3.89; N,
6.36. Found: C, 35.29, H, 3.90; N, 6.15%.
2.14. Synthesis of [{MoO2(L3)}2(ꢁ-O)] (5)
3. Results and discussion
For the synthesis of 5, 1 equiv. of [MoO2(acac)2] (177 mg,
0.54 mmol) was added to a solution of 1 equiv. of HL3 (200 mg,
0.54 mmol) in 5 mL of MeCN. The orange reaction mixture was
stirred for 6 h at 60 ◦C whereupon the formed orange precipitate
was filtered off and washed thoroughly with CH2Cl2. Evapora-
tion of all volatiles in vacuo led to 5 as an orange solid (96%,
260 mg). Crystals suitable for single–crystal X–ray diffraction anal-
ysis were obtained via slow evaporation of a concentrated MeCN
3.1. Ligand synthesis
Ligands HL1–HL3 were prepared according to Scheme 1. Thus,
the respective hydroxybenzaldehydes were reacted with function-
alized amidoamines under H2 atmosphere (5 atm) in a pressure
reactor over Pd/C. The products were obtained in fair yields after
purification via column chromatography.
Ligand HL4 was prepared in a two-step procedure according to
Scheme 2. In the first step, 2-hydroxy-3,5 dichlorobenzaldehyde
was condensed with the respective amine. The formed Schiff base
was subsequently treated with an excess of sodium borohydride.
Column chromatography gave the desired product in fair yield. A
solution at room temperature. IR (ATR, cm−1) ꢀ: 1628 (w), 1569
˜
(w), 1452 (w), 1264 (w), 1072 (w), 921 (m), 887 (s), 853 (m), 778
(s), 752 (s), 553 (m), 491 (w); HR-MS: (ESI+) m/z [M+H]+ calcd.
for C46H62Mo2N4O9: 1008.2719, found: 1008.2713; Anal. calcd. for

N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
213
Scheme 1. Synthesis of the aminophenolate amide ligands HL1–HL3.
complex 3, accompanied by insoluble solids, if the synthesis is car-
ried out under inert conditions in dry MeCN. Interestingly, if the
protic solvent MeOH is employed in the synthesis, complexes 2,
4 and 6 of the general formula [MoO2L(OMe)] were obtained in
good to excellent yield both under ambient and inert conditions
(Scheme 3). The formation of this mononuclear complexes marks
an interesting contrast to previous observations, where the use of
standard grade methanol also led to dinuclear compounds [18]. We
attribute the formation of the mononuclear complexes to the sim-
ilar acidity of the protons in H2O and MeOH and the vast excess
of the latter in MeOH solution [26]. If less acidic alcoholic solvents
are employed (e.g. iPrOH, tBuOH), no formation of the respective
alcoholate complexes could be observed. The higher steric demand
of an isopropyl or tert-butyl group could also inhibit the formation
of such complexes.
Complexes 1–7 feature one ligand moiety per molybdenum cen-
ter, coordinated in a facial ONO motif via the anionic phenolate,
the amine and the carbonyl oxygen of the amide, as observed in
the solid state structures of 1–3, 5 and 6. The 1H NMR spectra of
the methanolate complexes 2, 4 and 6 feature the expected one set
of shifted ligand resonances, including a broadened doublet for the
proton at the coordinated amine. Interestingly, in the spectra of the
dinuclear ␮-oxido complexes two sets of ligand resonances with a
solvent dependent ratio (e.g. 3:2 in (CD3)2CO and 5:2 in (CD3)2SO
for complex 3, Fig. S4) were observed. Dissolving single crystals
of 3 and immediate 1H NMR measurement resulted in analogous
spectra, hinting towards a dynamic isomerization in solution. Con-
sidering free rotation around the ␮-oxido Mo-O bonds, X-ray data
suggests two homotopic half units (vide infra), which would result
in only one signal set observable by NMR spectroscopy. If the two
halves would be non-homotopic, two signal sets in a ratio of 1:1
should result. To gain further insight, we performed VT-NMR mea-
surements in (CD3)2SO (20 ◦C to 60 ◦C) as well as in CD3CN (−30 ◦C
to 60 ◦C, Fig. S2) which unfortunately did not show any meaning-
ful change in the ratio of the signal sets and thus do not allow for
clear assertion of the dynamics in solution. However, it is possible
that the observed resonances arise from a labile coordination of
the amide moieties, resulting in an equilibrium between five- and
a sixfold coordinated metal centers.
Scheme 2. Synthesis of the aminophenolate amide ligand HL4.
synthesis procedure using H2 and Pd/C did not yield an isolable
amount of HL4.
The employed functionalized primary amines were prepared
according to published procedures [21]. Ligands HL1–HL4 were
characterized via 1H, 13C NMR, FT-IR and HR-MS (ESI+) spec-
troscopy. Noteworthy is the difference in the shift of the amide
NH proton, depending on the substituent (7.53 ppm in HL2 and
10.00 ppm in HL3, tert–butyl vs. phenyl amide, (CD3)2SO). Another
interesting feature arises from a comparison of the 1H NMR spectra
of HL1-HL4 in (CD3)2SO. Whereas the OH and NH protons are not
observable in ligands HL1 and HL2, likely due to fast exchange, they
are clearly assignable for ligand HL3. For the dichloro-substituted
ligand HL4 on the other hand 1H NMR spectroscopy in (CD3)2SO
displays resonances which are assignable to an O-- NH2 form,
+
suggesting a zwitterionic behavior in solution (Fig. S1). Ligands
HL1-HL4 feature electron withdrawing and donating substituents
at the phenolate and amide moieties, respectively, allowing for an
assessment of electronic influences on the properties of the result-
ing molybdenum complexes.
3.2. Complex synthesis
The synthesis of the dinuclear complexes 1, 3, 5 and 7 of the
type [{MoO2L}2(␮-O)] was accomplished via the reaction of one
equiv. of HL1-HL4, respectively, with one equiv. of [MoO2(acac)2]
in MeCN under ambient conditions in excellent yields (Scheme 3).
The bridging oxygen atom likely originates from the deproto-
nation of advantageous water molecules, possibly via a hydroxido
intermediate, as previously disclosed for related complexes [18].
This assumption is corroborated by the formation of only traces of
Scheme 3. Synthesis of complexes 1–7.

214
N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
Scheme 4. Interconversion of mononuclear and dinuclear molybdenum complexes.
Table 1
Selected crystallographic data and structure refinement for complexes 1–3, 5 and 6.
1
2
3a
5
6a
Empirical formula
Formula weight
C26H38Mo2N4O9
742.48
C14H22MoN2O5
394.27
C42H70Mo2N4O9·3C2H6OS
1201.28
C46H62Mo2N4O9·3C2H3N
1130.03
C24H34MoN2O5·CH3OH
558.51
Crystal description
needle, yellow
needle,
block, yellow
block, yellow
block, yellow
colorless
monoclinic,
P 21/c
12.734(2)
6.8362(12)
18.769(4)
Crystal system, space
group
a [Å]
monoclinic,
C 2/c
35.8689(16)
7.3863(3)
27.2284(12)
orthorhombic, P 21 21 21
triclinic, P −1
triclinic, P −1
11.7746(6)
29.8324(15)
33.3041(17)
10.3527(7)
15.5833(12)
18.3910(13)
73.523(3)
77.913(4)
75.891(3)
2727.8(3)
2
9.5849(9)
16.7996(17)
18.4598(18)
113.094(3)
95.732(4)
98.174(3)
2666.6(5)
4
b [Å]
c [Å]
␣ [◦]
␤ [◦]
115.7437(13)
91.201(5)
␥ [◦]
Volume [Å3]
Z
6497.8(5)
8
1633.5(5)
4
11698.5(10)
8
Reflections
collected/unique
R(int), R(sigma)
51941/9491
18825/3211
63089/22972
64481/24013
152692/23463
0.0390, 0.0339
100.0% (30.0◦)
9491/404/4
8077
0.0493, 0.0518
99.9% (26.0◦)
3211/220/2
2621
0.0452, 0.0769
99.8% (26.0◦)
22972/1349/74
18476
0.0537, 0.0604
100.0% (35.0◦)
24013/685/4
19319
0.0438, 0.0266
99.9% (35.0◦)
23463/677/6
20033
Completeness (ꢂmax
)
Data/parameters/restraints
Signif. unique refl.
[I > 2␴(I)]
Goodness-of-fit on F2
Final R indices [I
> 2␴(I)]
1.040
1.144
1.114
1.037
1.047
R1 = 0.0264,
wR2 = 0.0580
R1 = 0.0353,
wR2 = 0.0603
0.746 and
−0.516
R1 = 0.0402,
wR2 = 0.0728
R1 = 0.0574,
wR2 = 0.0824
0.845 and
−0.828
R1 = 0.0546, wR2 = 0.1420
R1 = 0.0344,wR2 = 0.0833
R1 = 0.0239, wR2 = 0.0577
R indices (all data)
R1 = 0.0678, wR2 = 0.1480
1.276 and −1.098
1532980
R1 = 0.0477,wR2 = 0.0893
0.890 and −1.415
1532981
R1 = 0.0332, wR2 = 0.0631
0.836 and −0.667
1532982
Largest difference peak
and hole [e/Å3]
CCDC - Number
1532978
1532979
a
Parameters corresponding to the second observed modification are provided within the SI.
We were interested if the different complexes formed in MeCN
and MeOH are able to interconvert. For this reason, the follow-
ing observation is of importance. If the dinuclear complexes are
dissolved in MeOH, the color of the solution brightened within
minutes and removal of the solvent after 24 h of standing at room
temperature yielded the respective methanolate complex in quan-
titative yield. A similar effect was observed upon dissolving the
methanolate complexes in benchtop MeCN (Scheme 4).
To obtain insight into the behavior of the complexes under cat-
alytic conditions (vide infra), a mixture of complex 3 and ten equiv.
of TBHP was dissolved in benchtop CD3CN (the solubility in CDCl3
was not sufficient to obtain meaningful spectra) and stirred for 24 h
at 50 ◦C. A 1H NMR spectrum of the reaction solution recorded after
24 h showed no observable difference to the spectrum of the ini-
tial complex (Fig. S3), revealing a surprisingly high stability under
these conditions. The NMR measurement repeated after leaving the
sample for 30 days under ambient conditions and also after subse-
quent addition of ∼500 equiv. of H2O still showed no substantial
decomposition, again in contrast to other published precatalysts
[9,12,17].
observed, and for 6 two independent molecules (labelled A and B,
only molecule A discussed here, for B please refer to the SI) with dis-
tinct structural differences were found in the solid state. Molecular
views of 1, 3 and 5 are given in Fig. 1, and of 2 and 6 in Fig. 2. Selected
crystallographic data for complexes 1-3 and 5–6 are provided in
Table 1, selected bond lengths and angles are given in Table 2, full
crystallographic details such as intra- and intermolecular hydro-
gen bonding, structure refinement and experimental details are
provided within the SI.
In the ␮-oxido bridged dinuclear complexes 1, 3 and 5 each
molybdenum center is coordinated in a distorted octahedral
fashion by two oxido ligands, one facially ONO-coordinating tri-
dentate ligand as well as the bridging oxygen atom. General
features of these complexes are the rather long distances between
the molybdenum atoms and the bonding amino and carbonyl
groups, respectively (Table 2). Despite the similarity of these com-
plexes, clear differences can be observed with respect to the
Mo1–O1–Mo2 angle, i.e. the linearity of the ␮-oxido bridge, as well
as the dihedral angles between the oxido groups on Mo1 and Mo2
(e.g. O2–Mo1–Mo2–O4). Whereas the Mo1–O1–Mo2 angles in 1
and 3 exhibit smaller deviations from 180◦ (166.94(7)◦ in 1 and
175.2(4)/170.1(3)◦ in 3), the Mo1–O1–Mo2 angle observed for com-
plex 5 is 146.96(5)◦. Both, linear (Mo-O-Mo angle close to 180◦)
and bent structures (Mo-O-Mo angle <180◦) have been observed in
previously reported dinuclear dioxidomolybdenum(VI) complexes
[18]. Also, whereas the dihedral O-Mo-Mo-O angles in complexes
3.3. Molecular structures
Molecular structures of dinuclear complexes 1, 3 and 5, as well as
mononuclear complexes 2 and 6 were determined by single–crystal
X–ray diffraction analysis. For complex 3, racemic twinning was

N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
215
Fig. 1. Molecular views (50% probability level) of 1 (top), 3 (middle) and 5 (bottom); H atoms (except nitrogen bound protons) as well as solvent molecules are omitted for
clarity reasons. Open bonds depict unusual long Mo–L contacts. The tert–butyl substituents on the phenyl rings in 3 and 5 are drawn with small spherical atoms and line
bonds for clarity reasons.
Fig. 2. Molecular views (50% probability level) of 2 (left), and 6 (right); H atoms (except nitrogen bound protons) as well as solvent molecules are omitted for clarity reasons.
Open bonds depict unusual long Mo–L contacts.
1 and 5 are comparable (300◦ and 302◦), the complex half units in 3
are rotated around the ␮-oxido bridge (243◦), further corroborating
the geometric flexibility of the complexes (Table 2).
In the mononuclear complexes 2 and 6, the molybdenum cen-
ter is coordinated in a distorted octahedral fashion that closely
resembles the coordination environment of the related dinuclear

216
N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
Table 2
Selected bond lengths [Å] and angles [◦] for complexes 1–3, 5 and 6.
1
2
3a
5
6a
Mo1 O1
Mo1 O2
Mo1 O3
Mo1 O21
Mo1 O14
Mo1 N12
Mo2 O1
Mo2 O4
Mo2 O5
Mo2 O6
Mo2 O41
Mo2 O34
1.8994(12)
1.7149(12)
1.7202(11)
1.9555(12)
2.2231(11)
2.3952(13)
1.8795(12)
1.7131(12)
1.7180(12)
–
1.9797(13)
2.2222(11)
2.3915(13)
166.94(7)
105.20(6)
105.19(6)
300.94(6)
194.55(7)
1.923(2)
1.710(3)
1.728(3)
2.002(2)
2.227(3)
2.361(3)
–
–
–
–
–
–
–
–
1.885(5)
1.707(6)
1.715(5)
1.979(5)
2.307(5)
2.355(5)
1.919(5)
1.723(6)
1.721(5)
–
1.987(5)
2.245(5)
2.390(4)
175.2(4)
105.6(3)
105.7(3)
243.03(27)
135.46(27)
1.9049(9)
1.7245(9)
1.7122(10)
1.9791(9)
2.2613(9)
2.3623(11)
1.9141(9)
1.7178(10)
1.7129(10)
–
1.9807(9)
2.2929(9)
2.3650(11)
146.96(5)
105.97(5)
106.79(5)
302.10(5)
196.51(5)
1.9237(8)
1.7172(8)
1.7221(8)
1.9520(7)
2.2257(7)
2.3617(9)
–
1.7137(8)
1.7245(8)
1.9333(8)
1.9570(7)
2.2833(7)
2.3378(8)
–
Mo2 N32
Mo1 O1 Mo2
O2 Mo1 O3
O4 Mo2 O5
O2 Mo1 Mo2 O4
O2 Mo1 Mo2 O5
107.11(13)
107.11(4)
107.89(4)
–
–
–
–
–
a
Only data for one molecule shown, bond lengths and angles for other molecule shown in SI.
Table 3
Overview of conversion of substrate (selectivity to epoxide) with TBHP as oxidant.
1
2
3
4
5
6
7
S1
S2
S3
S4
S5
59 (92)
no conv.
unsel.
61 (34)
60 (10)
41 (89)
no conv.
61 (8)
60 (11)
no conv.
>95 (>95)a
33 (70)
69 (22)
65 (69)
>95 (86)
68 (>95)
no conv.
unsel.
51 (42)
81 (31)
75 (91)
no conv.
unsel.
87 (43)
85 (22)
79 (>95)
no conv.
83 (34)
>95 (49)
no conv.
>95 (>95)b
32 (92)
27 (30)
66 (69)
56 (58)
General conditions: 2 equiv. TBHP (5.5 M in decane), 50 ◦C, CHCl3, 0.001 mol% precatalyst loading; conversion of substrate (%) after 24 h, as determined by GC–MS (selectivity
in % for epoxide in brackets); no conv. = substrate was not converted; unsel.= substrate was converted, but epoxide was not formed or consumed over reaction time.
a
After 8 h reaction time.
After 4 h reaction time.
b
Table 4
compounds 1 and 5. Instead of the ␮-oxido bridge, the sixth coor-
Overview of conversion of substrate (selectivity to epoxide) with H2O2 as oxidant.
dination site in complexes 2 and 6 is occupied by a monoanionic
methoxide ligand. Most bond lengths are similar to the respec-
tive dinuclear complexes with the exception of the Mo1–O1 bond
to the methoxido ligand which is longer than to the ␮-oxido
bridging atom. In general, the observed molybdenum oxido bond
lengths are within expected ranges for all reported complexes
(Table 2) [27]. For all structurally characterized complexes, inter-
and intra-molecular hydrogen bonding is observed (Tables S4, S7,
S10, S13, S16 and S19). For dinuclear complexes 1, 3 and 5 intra-
molecular hydrogen bonding is especially pronounced in 5, and
less pronounced in 1 and 3. In these complexes, hydrogen bonds
are spanning the two halves of the molecule across the ␮-oxido
bridging atom, between the amine N-H atom and a terminal oxido
ligand (Fig. S12). In addition, all three complexes 1, 3 and 5 show
inter-molecular hydrogen bonding to neighboring complexes in the
solid state. For mononuclear complexes 2 and 6, hydrogen bonds
between the amide N-H atom and the methoxide oxygen atom are
observed, validating the ability of ligands HL1-4 to provide hydro-
gen bonding.
1
3
4
5
S1
S2
S3
S4
S5
32 (>95)
no conv.
no conv.
no conv.
no conv.
74 (87)
30 (77)
no conv.
64 (27)
unsel.
89 (95)
no conv.
no conv.
39 (31)
no conv.
87 (93)
28 (93)
no conv.
65 (10)
unsel.
General conditions: 2 equiv. H2O2 (30 wt% in water), 50 ◦C, CHCl3, 0.1 mol% pre-
catalyst loading; conversion of substrate (%) after 24 h, as determined by GC–MS
(selectivity in % for epoxide in brackets); no conv. = substrate was not converted;
unsel.= substrate was converted, but no epoxide was formed or consumed over
reaction time; 2, 6 and 7 were unproductive/unselective, results not shown.
With TBHP as oxidant, all complexes reached average to high
substrate conversions, even at very low precatalyst loadings of
0.001 mol%, except for substrate S2 (Table 3). For 1-octene S2, only
complex 3 and 7 gave low conversions under these conditions. For
aqueous oxidant H2O2, only complexes 1, 3, 4 and 5 gave noticeable
conversions at the higher precatalyst loading of 0.1 mol% (Table 4).
A comparison of turnover numbers (TON) for all seven complexes
with both oxidants is given in Table 5.
3.4. Olefin epoxidation
In accordance to literature, also precatalysts 1-7 show higher
epoxidation activity with organic oxidant TBHP compared to
aqueous oxidant H2O2. Whilst conversion of cyclooctene S1 and
selectivity for epoxide with TBHP was high in all cases, the more
challenging substrates S2-5 showed a different picture. Only two
(3 and 7) out of the seven complexes showed activity for 1-octene
S2, albeit with low conversions. For styrene S3 epoxidations suf-
fered from un-selectivity (0–34%) due to over-oxidation of the
initially formed epoxide to phenylacetaldehyde and benzaldehyde.
Epoxidation of cyclic substrate limonene S4 was selective for the
endocyclic double bond, but selectivity overall (11–69%) suffered
Complexes 1-7 were screened in the catalytic epoxidation of
various internal and terminal olefins using two equiv. of tert-
butylhydroperoxide (TBHP, 5.5 M in decane) or hydrogen peroxide
(H2O2, 30% in water) as oxygen source (Fig. 3). All catalyst loadings
as well as TONs are calculated per complex molecule (vide infra).
Blank reactions without added catalysts were performed for all sub-
strates and showed conversions below 10% (uncatalyzed reactions).
In an initial round, the five substrates S1-S5 were tested under
previously optimized reaction conditions (50 ◦C, 2 equiv. oxidant,
CHCl3).

N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
217
Fig. 3. Epoxidations of olefinic substrates S1-S5 by complexes 1-7.
Table 5
Overview of TONs for epoxidations using TBHP/H2O2.
1
2
3
4
5
6
7
S1
S2
S3
S4
S5
75,500/270
0/0
0/0
46,400/0
29,500/0
36,700/0
0/0
4500/0
6800/0
0/0
95,300/650
23,100/230
15,000/0
44,700/270
83,500/0
66,400/830
0/0
0/0
21,500/120
25,000/0
68,400/790
0/200
0/0
37,600/60
18,600/0
77,600/0
0/0
28,400/0
48,000/0
0/0
110,000/0
32,400/0
27,200/0
66,300/0
56,200/0
TON is calculated per complex molecule.
from subsequent over-oxidation of the formed epoxide to the cyclo-
hexanone carvone. An almost similar catalytic profile was observed
for hydroxy containing substrate ␣-terpineol S5, with the exception
of precatalyst 3. For precatalysts 1, 4, 5 and 7, selectivity for epoxide
with S5 was low (10–58%) due to over-oxidation, whereas mononu-
clear complexes 2 and 6 showed no conversion with S5 at all. Only
precatalyst 3 both displayed high conversion (>95%) and selectiv-
ity (86%) (Table 3). In summary, dinuclear complexes 1, 3, 5 and
7 were in all cases more or at least similarly active (per molecule
complex) than mononuclear precatalysts 2, 4 and 6. This could, in
part, be caused by a dinucleation in presence of adventitious water,
resulting in a lowering of the precatalyst loading (per molecule).
Some of the trends observed in epoxidation with TBHP are also
valid for epoxidations with H2O2, although the overall catalytic
activity of precatalysts 1-7 was greatly reduced (Tables 4 and 5).
Again complex 3, in this case together with complex 5, showed
the best performances with H2O2, both displaying catalytic activ-
ity with four (S1-2 and S4-5) out of the five substrates. Mononuclear
complexes 2 and 6 showed no conversion with any substrate, mak-
ing these two complexes the least active precatalysts out of the
seven complexes. In contrast to TBHP epoxidations, also complex
7 remained unproductive in epoxidations of S1-4 and unselective
for S5. This lack of catalytic activity with the aqueous oxidant H2O2
might be attributed to the enhanced water sensitivity of 7. Inter-
estingly all seven complexes failed to give any conversion with
aromatic substrate styrene S3 when H2O2 was the oxidant (Table 4).
In order to rule out a catalase-like activity of precatalysts 1-7, test
reactions under catalytic conditions without added substrate were
conducted. The concentration of H2O2 was determined after 24 h
via iodometric titration, showing that complexes 1-7 do not decom-
pose H2O2 under catalytic conditions.
Fig. 4. Epoxidations of olefinic substrates S6-S9 by complexes 3 and 7.
comparison of the catalytic activities of complexes 3 and 5, it is evi-
dent that the tert-butyl substituted dinuclear compound 3 exhibits
a better performance over a broader scope of substrates. Whereas
this can partly be attributed to the low solubility of complex 5, it is
also possible that the more acidic phenyl-amide moiety is prone to
side-reactions such as deprotonation.
For complex 7 the precatalyst loading for S1 could be even
reduced to 0.0005 mol% (5 ppm), giving 54% yield of cyclooctene
oxide after 24 h. At 0.001 mol% loading, full conversion of S1 was
obtained within 24 h, indicating a 5 ppm loading of 7 to be the
limit of activity. Complex 3 on the other hand showed a remarkable
activity with S5, hinting towards a high hydroxy functional group
tolerance. Also complex 3 showed activity for three out of the five
substrates with H2O2 as oxidant (Table 4). Taking into account the
observed catalytic activities as well as the complexes stability and
solubility, precatalysts 3 exhibits the best overall performance.
For these reasons, 3 and 7 were selected for further epoxidation
experiments with cyclohexene substrates S6-S9 (Fig. 4).
Similar to cyclic olefin S1, precatalysts 3 and 7 showed high
activity and selectivity for substrates cyclohexene S6 and 1-methyl-
1-cylohexene S7 (Table 6). Whereas especially complex 3 was very
tolerant towards the hydroxy group in S5, in epoxidation of 2-
cyclohexen-1-ol S8 both 3 and 7 were unselective, oxidizing S8 to
2-cyclohexen-1-one (S9) as the main side product. For S9 on the
other hand, no activity was observed. To further test the tolerance
for OH groups, complex 3 and 7 were used for the epoxidation of S1
Comparing the TONs (Table 5) allowed the conclusion that
dinuclear complexes 3 and 7 exhibited the best overall catalytic
activity, being the only two precatalysts showing activity for all
five substrates S1-5 with TBHP as oxidant. Furthermore 3 and
7, bearing tert-butyl and chloride substituents at the phenolate
moieties, respectively, gave the highest TONs for S1, suggesting a
limited influence of the substituents electronic properties on the
catalytic activity. However, the increased sensitivity of complex 7
can be attributed to the electron withdrawing nature of the chlo-
ride substituents, resulting in a weaker phenolate-Mo bond. From a

218
N. Zwettler et al. / Molecular Catalysis 443 (2017) 209–219
Table 6
a different catalytically active species for mononuclear and dinu-
clear complexes. Furthermore, the high activity in tert-butanol is
especially surprising, as tert-butanol usually impedes epoxidation
activity with many other published precatalysts [18,28]. In compar-
ison with structurally related, previously reported ␮-oxido bridged
complexes (bearing hydrogen bond acceptor functionalities such as
OMe and NMe2) [18], the high activity and functional group tol-
erance of the complexes herein clearly suggest a beneficial role of
the additional hydrogen bond donors on the ligand amide func-
tionalities, which seems to be able to stabilize the oxidant and/or
substrate molecules in the second coordination sphere. Another
benefit of the presented system might be the increased lability of
the donor arm, allowing for the formation of a vacant coordination
site. It remains unclear however, whether both molybdenum cen-
ters in dinuclear complexes 1, 3, 5 and 7 are active in epoxidation
catalysis or not.
Overview of conversion (selectivity to epoxide) of substrates S6–S9.
3
7
S6
S7
S8
S9
>95 (>95)
>95 (>95)a
83 (54)
>95 (>95)
>95 (>95)a
92 (55)
no conv.
no conv.
General conditions: 2 equiv. TBHP (5.5 M in decane), 50 ◦C, CHCl3, 0.1 mol% pre-
catalyst loading; conversion of substrate (%) after 24 h, as determined by GC–MS
(selectivity in% for epoxide in brackets); no conv. = substrate was not converted.
a
Conversion after 2 h.
Table 7
Overview of conversion of substrate S1 (selectivity to epoxide) in alcoholic solvents.
entry
precatalyst
loading [mol%]
oxidant
solvent
conversiona [%]
1
2
3
4
5
6
3
3
3
3
3
3
1
1
0.1
0.1
0.1
0.1
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
EtOH
EtOH
MeOH
EtOH
iPrOH
tBuOH
49 (>95)
75 (90)
63 (>95)
59 (93)
75 (90)
89 (95)
4. Conclusions
The synthesis of four novel aminophenolate ligands HL1–HL4
with tert-butyl and phenyl amide functionalities as hydrogen
bond donors is reported. Upon reaction with the molybdenum(VI)
precursor [MoO2(acac)2], they form, dependent on the used sol-
vent, mono- and dinuclear complexes 1-7 of the general formulas
[{MoO2L}2(␮-O)] and [MoO2L(OMe)], respectively, coordinated by
one facially, tridentate ONO-ligand moiety per metal center.
Especially dinuclear complexes 1, 3, 5 and 7 were found to
be highly efficient and stable precatalysts in the epoxidation of
various olefins, leading to turnover numbers up to 110000 with
precatalysts loadings of 5–10 ppm, although a lack of chemoselec-
tivity due to over-oxidation was observed for some substrates. The
complexes furthermore showed a remarkable tolerance towards
hydroxy functionalities as well as moisture, allowing for the use
of aqueous H2O2 as oxidant as well as alcoholic solvents such
as EtOH and thus environmentally benign, “green”, reaction con-
ditions in catalysis. Furthermore also catalytic experiments in
tBuOH (a by-product in epoxidations with TBHP, usually postu-
lated to inhibit the reaction [18,28]) led to high epoxide yields
with TBHP (0.1 mol% precatalyst). The high activity and tolerance
of the complexes is remarkable in comparison with previously
reported, structurally similar compounds with acceptor function-
alities ( OMe and NMe2), which exhibited negligible activity in
alcoholic solvents and/or with H2O2 as oxidant. Furthermore the
system described herein allowed for the reduction of the precata-
lyst loading for all tested substrates by at least a factor of 10 [18].
These findings underline the significant benefit of the introduction
of hydrogen bond donor functionalities in epoxidation catalysis and
therefore clearly warrant a further exploration and implementa-
tion in systems for efficient and “green” homogeneous epoxidation
catalysis.
General conditions: 2 equiv. oxidant, 50 ◦C.
Conversion of substrate (%) after 24 h, as determined by GC–MS (selectivity in%
for epoxide in brackets).
a
in ethanol as solvent with TBHP as well as H2O2 as oxidant. Whereas
7 showed no activity in ethanol, 3 gave conversion to epoxide with
both oxidants (Table 7). Encouraged by this result we tested 3 in
the four different alcohols methanol, ethanol, isopropanol and tert-
butanol.
Precatalyst 3 is active for the epoxidation of substrate S1 in all
four tested alcoholic solvents (Table 7). With the aqueous oxidant
H2O2, precatalyst activity was lower than compared to TBHP under
the same reaction conditions (entry 1 and 2). In contrast to the
standard solvent CHCl3, a single phase is formed in the reaction
mixture, indicating that the lower activity with H2O2 cannot be
solely attributed to a miscibility problem between precatalyst, oxi-
dant and substrate. The activity of 3 in MeOH and EtOH is essentially
the same (within experimental error), with conversions of 63 and
59% respectively, for S1 (Table 7, entry 3 and 4). We could demon-
strate that complex 4 is formed upon dissolution of 3 in methanol
(Scheme 4). Based on the similar pKa value of EtOH (29.8) [26] and
MeOH (29.0) [26] the formation of an analogous ethoxido complex
is possible, hence explaining the similar catalytic activity. How-
ever, an induction period was only observed employing methanol
as solvent (Fig. S5). The lack of conversion during the first hour of
catalysis indicates the formation of the methoxido species 4 prior
to formation of the catalytic active species. Epoxidations in iso-
propanol and tert-butanol show higher catalytic activity (75 and
89% conversion of S1, entry 5 and 6), despite the fact that tert-
butanol is known to be an inhibitory by-product in epoxidations
with TBHP as oxidant [18,28]. Mononuclear methoxido complexes
2, 4 and 6 have proven to be less active in epoxidation compared
to their dinuclear congeners 1, 3, 5 and 7, suggesting that the din-
uclear complexes are stable in isopropanol or tert-butanol and are
not cleaved to the corresponding mononuclear alkoxido complexes
(vide supra).
Acknowledgements
Financial support by the Austrian Science Fund (FWF, grant
number P26264) and NAWI Graz is gratefully acknowledged.
Appendix A. Supplementary data
Significant differences in catalytic activity of dinuclear com-
pared to mononuclear precatalysts suggest a difference in the active
catalyst species formed. We have no spectroscopic evidence for a
cleavage of the dinuclear compounds in CHCl3, the standard solvent
used in epoxidations. Thus, in presence of 10 equiv. TBHP dinuclear
complex 3 remains unchanged as shown by 1H NMR spectroscopy,
indicating high stability of the complex under oxidative conditions
(vide supra, Fig. S3). Based on this experimental evidence retention
of the dinuclear structure during catalysis is suggested and thus also
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.09.
036.
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