Triazolyl, Imidazolyl, and Carboxylic Acid Moieties in the Design of Molybdenum Trioxide Hybrids: Photophysical and Catalytic Behavior

Article abstract of DOI:10.1021/acs.inorgchem.6b02986

Three organic ligands bearing 1,2,4-triazolyl donor moieties, (S)-4-(1-phenylpropyl)-1,2,4-triazole (trethbz), 4-(1,2,4-triazol-4-yl)benzoic acid (trPhCO₂H), and 3-(1H-imidazol-4-yl)-2-(1,2,4-triazol-4-yl)propionic acid (trhis), were prepared to evaluate their coordination behavior in the development of molybdenum(VI) oxide organic hybrids. Four compounds, [Mo₂O₆(trethbz)₂]·H₂O (1), [Mo₄O₁₂(trPhCO₂H)₂]·0.5H₂O (2a), [Mo₄O₁₂(trPhCO₂H)₂]·H₂O (2b), and [Mo₈O₂₅(trhis)₂(trhisH)₂]·2H₂O (3), were synthesized and characterized. The monofunctional tr-ligand resulted in the formation of a zigzag chain [Mo₂O₆(trethbz)₂] built up from cis-{MoO₄N₂} octahedra united through common μ₂-O vertices. Employing the heterodonor ligand with tr/-CO₂H functions afforded either layer or ribbon structures of corner- or edge-sharing {MoO₅N} polyhedra (2a or 2b) stapled by tr-links in axial positions, whereas -CO₂H groups remained uncoordinated. The presence of the im-heterocycle as an extra function in trhis facilitated formation of zwitterionic molecules with a protonated imidazolium group (imH⁺) and a negatively charged -CO₂^- group, whereas the tr-fragment was left neutral. Under the acidic hydrothermal conditions used, the organic ligand binds to molybdenum atoms either through [N-N]-tr or through both [N-N]-tr and μ₂-CO₂^- units, which occur in protonated bidentate or zwitterionic tetradentate forms (trhisH⁺ and trhis, respectively). This leads to a new zigzag subtopological motif (3) of negatively charged polyoxomolybdate {Mo₈O₂₅}_n^2n- consisting of corner- and edge-sharing cis-{MoO₄N₂} and {MoO₆} octahedra, while the tetradentate zwitterrionic trhis species connect these chains into a 2D net. Electronic spectra of the compounds showed optical gaps consistent with semiconducting behavior. The compounds were investigated as epoxidation catalysts via the model reactions of achiral and prochiral olefins (cis-cyclooctene and trans-β-methylstyrene) with tert-butylhydroperoxide. The best-performing catalyst (1) was explored for the epoxidation of other olefins, including biomass-derived methyl oleate, methyl linoleate, and prochiral dl-limonene.

Full text of DOI:10.1021/acs.inorgchem.6b02986

Article
pubs.acs.org/IC
Triazolyl, Imidazolyl, and Carboxylic Acid Moieties in the Design of
Molybdenum Trioxide Hybrids: Photophysical and Catalytic Behavior
Andrey B. Lysenko,*,† Ganna A. Senchyk,† Konstantin V. Domasevitch,† Merten Kobalz,‡
Harald Krautscheid,‡ Jakub Cichos,§ Miroslaw Karbowiak,§ Patrícia Neves,∥ Anabela A. Valente,∥
and Isabel S. Gonca̧
lves*,∥
†Inorganic Chemistry Department, Taras Shevchenko National University of Kyiv, Volodimirska Street 64, Kyiv 01033, Ukraine
‡Institut fur Anorganische Chemie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
̈
̈
§Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Street, 50-383 Wroclaw, Poland
∥Department of Chemistry, CICECO - Aveiro Institute of Materials, University of Aveiro, Campus Universitar
3810-193 Aveiro, Portugal
́
io de Santiago,
S
* Supporting Information
ABSTRACT: Three organic ligands bearing 1,2,4-triazolyl
donor moieties, (S)-4-(1-phenylpropyl)-1,2,4-triazole
(trethbz), 4-(1,2,4-triazol-4-yl)benzoic acid (trPhCO2H), and
3-(1H-imidazol-4-yl)-2-(1,2,4-triazol-4-yl)propionic acid
(trhis), were prepared to evaluate their coordination behavior
in the development of molybdenum(VI) oxide organic
hybrids. Four compounds, [Mo2O6(trethbz)2]·H2O (1),
[Mo4O12(trPhCO2H)2]·0.5H2O (2a), [Mo4O12(trPhCO2H)2]·
H2O (2b), and [Mo8O25(trhis)2(trhisH)2]·2H2O (3), were
synthesized and characterized. The monofunctional tr-ligand
resulted in the formation of a zigzag chain [Mo2O6(trethbz)2]
built up from cis-{MoO4N2} octahedra united through
common μ2-O vertices. Employing the heterodonor ligand
with tr/−CO2H functions afforded either layer or ribbon structures of corner- or edge-sharing {MoO5N} polyhedra (2a or 2b)
stapled by tr-links in axial positions, whereas −CO2H groups remained uncoordinated. The presence of the im-heterocycle as an
extra function in trhis facilitated formation of zwitterionic molecules with a protonated imidazolium group (imH+) and a
−
negatively charged −CO2 group, whereas the tr-fragment was left neutral. Under the acidic hydrothermal conditions used, the
organic ligand binds to molybdenum atoms either through [N−N]-tr or through both [N−N]-tr and μ2-CO2− units, which occur
in protonated bidentate or zwitterionic tetradentate forms (trhisH+ and trhis, respectively). This leads to a new zigzag
2n−
subtopological motif (3) of negatively charged polyoxomolybdate {Mo8O25}n
consisting of corner- and edge-sharing cis-
{MoO4N2} and {MoO6} octahedra, while the tetradentate zwitterrionic trhis species connect these chains into a 2D net.
Electronic spectra of the compounds showed optical gaps consistent with semiconducting behavior. The compounds were
investigated as epoxidation catalysts via the model reactions of achiral and prochiral olefins (cis-cyclooctene and trans-β-
methylstyrene) with tert-butylhydroperoxide. The best-performing catalyst (1) was explored for the epoxidation of other olefins,
including biomass-derived methyl oleate, methyl linoleate, and prochiral DL-limonene.
leading to thermally stable materials with potential to operate in
the high-temperature range of up to ∼400 °C. These solids
were found to demonstrate remarkable catalytic efficiency in
epoxidation of olefins,2−4 oxidation of alcohols,5 etc. Another
interesting application arises from their promising semi-
conducting behavior that unites multiple attributes such as
tunable band gaps, reduced thermal conductivity, enhanced
dielectric constant, and negative thermal expansion.6
INTRODUCTION
■
The chemistry of polyoxomolybdate organic solids rightfully
occupies one of the most striking and intriguing domains of
materials science that, because of multiple potential applica-
tions, is of extremely high impact and has received intense
interest.1 In this regard, the employment of organic ligands with
different donor functions offers abundant opportunities for
generating new organically modified molybdenum(VI) oxide
hybrids which have unusual structures and possess desirable
properties. Among a wide choice of accessible organic donors,
N-heterocyclic derivatives such as pyridine, 1,2,4-triazole,
imidazole, and carboxylic acids provide sufficiently stiff and
strong interactions with the molybdenum trioxide matrix,
Interaction of layered β-MoO3·H2O with nonchelating 1,2,4-
triazolyl ligands can be rationalized as follows (Scheme 1).
Received: December 9, 2016
© XXXX American Chemical Society
A
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Scheme 1. Representation of Known Structure Types of Molybdenum Trioxide Modified by 1,2,4-Triazolyl-Based Linkers:
a
Layered Perovskite and One-Dimensional Motifs (Ribbon, Chain, Hydrated Chain, and Helix)
a
The coordination of organic [N−N]-tr and inorganic μ2/μ3-O-bridges displays a complementary role (for simplicity, molybdenyl bonds are not
shown).
Scheme 2. Under Typical Synthetic Conditions Used Either Pyridinecarboxylic Acids or Alpha-Amino Acids Serve as Neutral
−
Zwitterionic Anchors Supporting the Inorganic Oxide Matrix through Negatively Charged −CO2 Donor Groups
Similar to pyridine, pyrazine, and 4,4′-bipy building blocks,7−10
the employment of 1H-1,2,3-triazole and 1H-1,2,4-triazole
molecules affords rare organic−inorganic perovskite-like
MoO3(1H-1,2,3-tr)0.5 and MoO3(1H-1,2,4-tr)0.5.8b,10 However,
various nonchelating 4-substituted 1,2,4-tr derivatives clearly
tend to support one-dimensional molybdenum(VI) oxide
topological motifs (e.g., chains, hydrated chains, ribbons, and
helixes). Recently, we showed a rational strategy for preparing
the preprogrammed 2D and 3D metal oxide organic frame-
works (MOOFs) by the linkage of 1D Mo(VI) oxide motifs
through suitable bistriazolyl ligand modules.11 On the other
hand, the presence of an extra donor group such as amine,
pyrazole, or 1H-tetrazole in the structure of tr-based ligands did
not lead to high-dimensional coordination networks, because
the secondary function typically behaves as a noncoordinating
H-donor/H-acceptor unit.11b
Unlike this case, the coordination of some heteroaryl-
carboxylic acids (nicotinic, isonicotinic acids)12a,b and even
tripeptide glycyl−glycyl−glycine12c revealed interesting differ-
ences. Under typical reaction conditions the molecules behave
in neutral zwitterionic forms as short syn,syn-μ2-η1:η1 −COO−
links toward oxomolybdates, whereas pyridyl and amino groups
act as proton-acceptor sites (Scheme 2). The resultant
polymeric array [Mo2O6(RCO2)]n reminds one of
molybdenum(VI) oxide 1,2,4-triazolyl-based organic solids
having a ribbon-like scaffold [Mo2O6(tr)], shown in Scheme
1. Both bridges, syn,syn-μ2-η1:η1 carboxylate and μ2-[N−N]-tr,
can be regarded as competing and complementary donor units,
B
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Burke−Ernzerhof (PBE) exchange−correlation functional (xc) were
used in calculations.
and thus, heterofunctional tr/CO2H ligands are of extraordinary
interest. In order to comprehend how the ligand choice can
further modify the structure and properties of molybdenum
trioxide, we have launched the current project starting from the
single-tr molecule through heterobifunctional tr/−CO2H and
then to heterotrifunctional tr/−CO2H/plus extra heteryl units.
In this paper we report on 1,2,4-triazolyl derivatives
(enantiopure (S)-4-(1-phenylpropyl)-1,2,4-triazole, trethbz; 4-
(1,2,4-triazol-4-yl)benzoic acid, trPhCO2H; and racemic 3-(1H-
imidazol-4-yl)-2-(1,2,4-triazol-4-yl)propionic acid, trhis) con-
taining different kinds and numbers of donor substituents for
fabricating extended Mo(VI) oxide organic solids (Scheme 3).
Crystal Structure Determination. The diffraction data were
collected with graphite-monochromated Mo Kα radiation (λ =
0.710 73 Å) using a Stoe image plate diffraction system (face-indexed
numerical absorption correction using X-RED and X-SHAPE14). The
structures were solved by direct methods and refined by full-matrix
least-squares on F2 using the SHELX-97 package.15 In the structure of
2a, very high thermal motion parameters for the solvate water
molecules suggested 0.5 partial occupancies of their positions. These
partial contributions are further equally disordered over closely
situated positions across a 2-fold axis, and, therefore, no hydrogen
atoms were added to these molecules. In all other cases, CH-hydrogen
atoms were constrained, while NH and OH hydrogen atoms were
located and then included with fixed bond distances (0.87 and 0.85 Å,
respectively) and Uiso values at the level of 1.5Ueq of the carrier N (O)
atoms. In the structure of 2b, a set of similarity restraints was applied
to the thermal motion of the noncoordinated carboxylic group in order
to improve the refinement stability. Graphical visualization of the
structures was made using the program Diamond 2.1e.16
Scheme 3. 1,2,4-Triazolyl-Based Ligands Containing
Different Numbers of Functional Groups (Enantiopure (S)-
trethbz with a Single tr-Group, trPhCO2H with a tr/−CO2H
Couple, and Racemic trhis with Three Different Donors, tr/
im/−CO2H) Utilized in This Work
Crystallographic data and experimental details for structural
analyses are summarized in Table S1. The crystallographic material
can also be obtained from the CCDC, through deposition numbers
CCDC 1519096−1519099 for 1, 2a, 2b, and 3, respectively.
Catalytic Performance Tests. Catalytic experiments were carried
out under autogenous pressure in borosilicate reactors (5 mL)
equipped with a magnetic stirring bar and a Teflon valve for sampling.
cis-Cyclooctene (Cy, 95%, Alfa Aesar), methyl oleate (Ole, 99%,
Aldrich), methyl linoleate (LinOle, 95%, Alfa Aesar), 1-octene (1C8,
98%, Aldrich), trans-2-octene (2C8, 97%, Aldrich), cyclododecene
(Cy12; mixture of cis and trans isomers, 96%, Aldrich), DL-limonene
(DL-Lim, ≥ 95%, Merck), and trans-β-methylstyrene (99%, Aldrich)
were the substrates tested, and tert-butylhydroperoxide (5.0−6.0 M in
decane, Aldrich) and H2O2 (30% aqueous, Aldrich) were chosen as
oxidants. The reactors were charged with the molybdenum catalyst
(using an amount equivalent to 18 μmol of molybdenum), 1.8 mmol
of substrate, and 2 mL of cosolvent (α,α,α-trifluorotoluene (TFT,
≥99%, Aldrich) or acetonitrile (CH3CN, ≥99%, Aldrich)). Sub-
sequently, the reactors were immersed in an oil bath thermostated at
the desired reaction temperature (55 or 70 °C), under stirring for 10
min, followed by the addition of 2.75 mmol of oxidant. The initial
instant of the catalytic reaction was taken to be the moment that the
oxidant solution was added to the reactor. The reactions were carried
out under air; molecular oxygen did not influence the catalytic
reaction, ascertained by the fact that no reaction of Cy occurred in the
presence of the molybdenum catalysts without TBHP. The evolutions
of the catalytic reactions were monitored via sampling throughout 24
h. The samples were analyzed using a Varian 3900 GC equipped with
an FID detector and a capillary column (J&W Scientific DB-5, 30 m ×
0.25 mm × 0.25 μm), and undecane (>99%) or methyl decanoate
(99%) was used as internal standard for the reactions of Cy/1C8/
2C8/Cy12/DL-Lim and Ole/LinOle, respectively. The reaction
products were identified by GC-MS (Trace GC 2000 Series Thermo
Quest CE Instruments GC; Thermo Scientific DSQ II), using He as
the carrier gas.
The ligands have been investigated in the hydrothermal
reaction with an oxomolybdate matrix, although 4-(1,2,4-
triazol-4-yl)benzonitrile was preferably used as a precursor to
trPhCO2H. These afforded diverse 1D and 2D assemblies,
including homo- and heterochiral types. The photophysical
properties of the resultant solids have been investigated in
detail. Subsequently, the molybdenum(VI) oxide complexes
have been examined as epoxidation catalysts via the model
reactions of achiral and prochiral olefins (cis-cyclooctene (Cy)
and trans-β-methylstyrene (TBMS)) with tert-butylhydroper-
oxide (TBHP), at 55−70 °C. The best-performing catalyst (1)
was further explored for the epoxidation of several olefins with
TBHP, including biomass-derived methyl oleate, methyl
linoleate, and prochiral DL-limonene. The influence of the
type of oxidant (TBHP or H2O2) was studied.
EXPERIMENTAL SECTION
■
Measurements. Elemental analysis (C, H, and N) was carried out
with a Vario EL-Heraeus microanalyzer. IR spectra (400−4000 cm−1)
were measured with a PerkinElmer FTIR spectrometer (KBr pellets)
and with a Mattson 7000 spectrometer, using a globar source, a DTGS
detector, and KBr cells, with 2 cm−1 resolution and triangular
apodization. 1H NMR spectra recorded on a Bruker 400 MHz
spectrometer are illustrated in Figures S10 and S11. The room
temperature (rt) powder X-ray diffraction (PXRD) patterns were
measured using a Stoe STADIP (Cu Kα1, using a Mythen detector).
The temperature-dependent X-ray measurements were recorded on a
Stoe STADIP with a high-temperature attachment and an image plate
detector system. Simultaneous thermogravimetric/differential thermal
analysis/mass spectrometry (TG/DTA-MS) studies were carried out
on a Netzsch F1 Jupiter device connected to an Aeolos mass
spectrometer. Samples were heated at a rate of 10 K min−1.
Catalyst Stability Studies. The reaction mixtures using TBHP as
oxidant were biphasic solid−liquid after 24 h. After a 24 h batch run,
the reaction mixture was cooled to ambient temperature. The solid
phase was separated from the liquid phase by centrifugation (3500
rpm); the solid was washed with diethyl ether and pentane and
vacuum-dried (ca. 0.1 bar) at 55 °C for 1 h, giving the solid denoted as
i-TBHP-T (i corresponds to compound 1, 2a, or 3*, and T = 55 or 70
(°C)). In a similar fashion, solids (trPhCO2H-TBHP-55 and trhis-
TBHP-55) were isolated from a 24 h batch run using the free ligands
trPhCO2H and trhis of compounds 2a and 3*, respectively, for Cy/
TBHP reaction, at 55 °C.
The absorption spectra of polycrystalline samples of compounds 1,
2a, and 3 were recorded in the 800−200 nm range at 298 K on a Cary-
5000 UV−vis−NIR spectrophotometer. In order to obtain the
spectrum, a well-ground mixture of the sample with some paraffin
oil was placed between two quartz plates, approximately 1 cm in
diameter, and pressed to get a transparent layer.
The theoretical calculations were carried out using the CASTEP
module13 of a commercial software package (Materials Studio 8.0).
Generalized gradient approximations (GGA) with the Perdew−
For the catalytic reaction system 1/Cy/H2O2/CH3CN/55 °C, a
yellow-colored solution was obtained after a 24 h batch run. The
solution was filtered through a 0.2 μm PTFE membrane, and after
addition of pentane and diethyl ether it was stored at 4 °C overnight.
C
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The precipitate obtained was separated by centrifugation (3500 rpm),
washed with acetone, and vacuum-dried (ca. 0.1 bar) at 55 °C for 1 h,
giving the solid 1-H2O2-55.
heated to 140 °C, held for 24 h, and cooled to rt for 48 h. The
resultant colorless crystals of the desired product were filtered off,
washed with water and CH3OH, and air-dried (yield: 26.4 mg, 78%).
Anal. Calcd for C22H28Mo2N6O7: C, 38.84; H, 4.15; N, 12.35. Found:
C, 38.80; H, 4.16; N, 12.33.
The catalytic contribution of the dissolved metal species of the
system 1/TBHP (which led to the best catalytic results) was
investigated by carrying out a contact test (CT). Hence, a mixture
of 1, oxidant, and cosolvent (without substrate) was stirred at 55 °C
for 24 h. Afterward, the mixture was cooled to ambient temperature
and centrifuged (3500 rpm) to separate the solid and liquid phases.
The liquid phase was passed through a filter equipped with a 0.2 μm
PTFE membrane, giving the solution 1-CT-L. This solution was
transferred to a 5 mL reactor, preheated for 10 min under stirring, and
then used for the homogeneous catalytic reaction of Cy/TBHP.
Specifically, Cy (which was separately preheated at 55 °C) was added
to 1-CT-L in an amount corresponding to an initial substrate
concentration of 0.7 M, and this solution was stirred for 24 h at 55 °C.
On the other hand, the undissolved metal species of the CT (1-CT-S)
were isolated as described above for the normal catalytic runs and
subsequently tested for Cy epoxidation using the same initial mass of
solid as that used for the original compound 1 under typical conditions
(the initial molar ratio olefin:oxidant was the same for all catalytic
tests). After a 24 h batch run using 1-CT-S, the solid was recovered
((1-CT-S)-run 1) and characterized.
[Mo4O12(trPhCO2H)2]·0.5H2O (2a)/[Mo4O12(trPhCO2H)2]·H2O (2b).
A mixture of MoO3 (30.0 mg, 0.208 mmol), trPhCN (17.0 mg, 0.100
mmol), water (5 mL), and a 7% aqueous solution of HF (100 μL) was
sealed in a 20 mL Teflon-lined autoclave. The presence of diluted HF
was aimed at enhancing the hydrolysis of the nitrile precursor as well
as at mineralizing molybdenum trioxide. The autoclave was then
heated to 140 °C, held for 48 h, and cooled to rt for 24 h. The
resultant mixture consisted of two crystalline phases: the prevalent
green prisms of complex 2a and traces of colorless needles of
compound 2b in ca. 60:1 weight ratio. The mixture was filtered off,
washed with water, and air-dried. Samples 2a/2b were mechanically
separated from one another under a microscope that yielded
compound 2a in 58% yield (28.0 mg) and compound 2b in 1%
yield (0.5 mg).
For complex 2a: Anal. Calcd for C18H15Mo4N6O16.5: C, 22.45; H,
1.57; N, 8.73. Found: C, 22.40; H, 1.62; N, 8.68.
For complex 2b: Anal. Calcd for C18H16Mo4N6O17: C, 22.24; H,
1.66; N, 8.65. Found: C, 22.19; H, 1.70; N, 8.64.
Several dozen identical experiments reproducibly afforded 2a and
2b in varying yields of 55−65% and 0.5−2.5%, respectively. The
dominant complex 2a was selected to evaluate the catalytic potential in
olefin epoxidation reaction. Replacing trPhCN with poorly soluble
trPhCO2H noticeably shifts the hydrothermal reaction toward
formation of a mixture of white solids containing preferably initial
trPhCO2H itself, compound 2b, and traces of complex 2a. This
emphasizes that the employment of triazolylnitriles can be an essential
synthetic approach in preparing not only triazolylcarboxylic acids but
also their molybdenum(VI) oxide hybrids.
[Mo8O25(trhis)2(trhisH)2]·2H2O (3). Trhis (21.0 mg, 0.101 mmol),
MoO3 (28.8 mg, 0.200 mol), and 3 mL of water were sealed in a 20
mL Teflon-lined autoclave. The autoclave was heated to 140 °C, held
for 24 h, and gradually cooled to rt for 48 h. The resultant white solid
contained colorless crystals of the desired complex, though as a minor
component, which were mechanically collected under a microscope,
washed with water, and dried at rt (yield: ∼1 mg, ∼2%). Anal. Calcd
for C32H42Mo8N20O35: C, 18.89; H, 2.08; N, 13.77. Found: C, 18.74;
H, 2.15; N, 13.60. A major phase of the reaction mixture was identified
by elemental analysis and PXRD as complex 3* assuming the
composition MoO3:trhis:H2O = 2:1:1 (or [Mo2O6(trhis)]·H2O). Anal.
Calcd for C8H11Mo2N5O9: C, 18.73; H, 2.16; N, 13.65. Found: C,
18.65; H, 2.22; N, 13.50. The major sample 3* was further used to
investigate the catalytic activity.
The isolated metal species were characterized by attenuated total
reflectance (ATR) FT-IR spectroscopy and PXRD. The PXRD data
were collected at ambient temperature on a Philips Analytical
Empyrean (θ/2θ) diffractometer equipped with a PIXcel1D detector,
with automatic data acquisition (X’Pert Data Collector software v4.2)
using monochromatized Cu Kα radiation (λ = 1.5406 Å). Intensity
data were collected by the step-counting method (step 0.02°), in
continuous mode, in the ca. 5° ≤ 2θ ≤ 70° range. Attenuated total
reflectance FT-IR spectra were measured on a Mattson 7000 FT-IR
spectrometer, equipped with a Specac Golden Gate Mk II ATR
accessory having a diamond top-plate and KRS-5 focusing lenses.
Caution! Hydrofluoric acid (HF) is toxic and corrosive, even if a
diluted aqueous solution is used, and must be handled in a well-ventilated
fume hood with extreme caution using appropriate protective gear.
Synthesis. All chemicals were of reagent grade and used as
received without further purification. 4-(1,2,4-Triazol-4-yl)benzonitrile
(trPhCN)17 and 4-(1,2,4-triazol-4-yl)-benzoic acid (trPhCO2H)18 were
synthesized according to the procedure described by Bartlett and
Humphrey.19
(S)-4-(1-Phenylpropyl)-1,2,4-triazole, trethbz. A solution of
(S)-(−)-α-ethylbenzylamine (or (S)-(−)-1-phenylpropylamine) (2.70
g, 20.0 mmol), N,N-dimethylformamide azine (3.36 g, 23.6 mmol),
and TsOH (0.38 g, 2.21 mmol) in toluene (5 mL) was heated at reflux
for 38 h. Afterward, toluene was removed in vacuo to give a brownish
oil, which solidified on cooling. The solid was washed with CH3CN/
Et2NH (5 mL:0.5 mL) and diethyl ether and dried yielding white
crystals (1.12 g, 30%). Anal. Calcd for C11H13N3: C, 70.56; H, 7.00; N,
22.44. Found: C, 70.50; H, 7.10; N, 22.40. 1H NMR (400 MHz,
CDCl3): 8.19 (2H, tr), 7.14−7.40 (5H, m, C6H5), 5.08 (dd, 1H, J = 8.0
and 7.6 Hz, CH), 2.17−2.35 (2H, m, CH2), 0.94 (dd, 3H, J = 7.3 and
7.0 Hz, CH3).
RESULTS AND DISCUSSION
■
Synthesis and Crystal Structures. We recently developed
a series of molybdenum(VI) oxide solids decorated with various
triazolyl-based ligands. Under slightly acidic hydrothermal
conditions (pH ∼4−6) usually applied for the synthesis, the
1,2,4-triazolyl functional group exists in an uncharged neutral
form since the pKB of 1H-1,2,4-triazole is around 2.39.20 The
simultaneous presence of basic and acidic functions in 4-(1,2,4-
triazol-4-yl)benzoic acid (trPhCO2H) requires two pK values.
Taking into account the corresponding values of ∼2.39 for the
pKB(HtrH+/trH) of 1H-1,2,4-triazole and ∼4.19 for the
3-(1H-Imidazol-4-yl)-2-(1,2,4-triazol-4-yl)propionic acid,
trhis. A mixture of racemic histidine monohydrochloride monohydrate
((l,d)-His·HCl·H2O) (3.14 g, 15.0 mmol), N,N-dimethylformamide
azine (4.26 g, 30.0 mmol), and TsOH (0.28 g, 1.47 mmol) in toluene
(15 mL) was heated at reflux. Water, HCl, and toluene were removed
from the mixture as an azeotrope using a Dean−Stark trap, and the
refluxing was continued for 50 h.The reaction mixture was cooled to rt.
The precipitate formed was isolated by decanting the solution, washed
with toluene, CH2Cl2/CH3CN, and CH3CN, and dried, yielding a
white solid (2.16 g, 69%). Anal. Calcd for C8H9N5O2: C, 46.38; H,
−
pKA(C6H5CO2H/C6H5CO2 ) of benzoic acid, it can be readily
concluded that under the experimental conditions employed
both ionizable groups should be in their neutral forms; that is,
protonation of the tr-ring is only possible at low pH, less than 2.
This explains why the 1,2,4-triazolyl unit typically participates
in the coordination to Mo(VI) centers utilizing a [N−N]
coordination mode (complexes 1−3), while the extra donor
groups, for instance, −COOH or 1H-tetrazole, remain
1
4.38; N, 33.80. Found: C, 46.22; H, 4.50; N, 33.67. H NMR (400
MHz, D2O): 8.46 (s, 1H, im), 8.38 (s, 2H, tr), 7.00 (s, 1H, im), 5.06−
5.18 (dd, 1H, J = 9.2 and 4.9 Hz, CH), 3.32−3.56 (m, 2H, CH2).
[Mo2O6(trethbz)2]·H2O (1). trethbz (18.7 mg, 0.100 mmol),
(NH4)6Mo7O24·4H2O (35.3 mg, 28.6 μmol), and 2.5 mL of water
were sealed in a 20 mL Teflon-lined autoclave. The autoclave was
D
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uncoordinated (see compounds 2a and 2b).11,21 Another
enantiopure (S)-trethbz was employed, the self-assembly
process led to a homochiral coordination polymer as confirmed
by the Flack parameter, which refined to the value of 0.00(7).
Interactions between tr-ligands and the oxomolybdate matrix
might generally result in multiple products of five different
structural types (Scheme 1).8b,11 They, however, have the
dominant composition ratio of tr-ligands:MoVI as 1:2, for
instance [Mo2O6(tr2ad)]·6H2O and [Mo2O6(tr2ad)]·H2O11a,b
or [Mo2O6(Hpz-tr)(H2O)2] and [Mo2O6(Hpz-tr)]·H2O,11b,22
where tr2ad = 1,3-bis(1,2,4-triazol-4-yl)adamantane and Hpz-tr
= 4-(3,5-dimethylpyrazol-4-yl)-1,2,4-triazole. Under hydrother-
mal conditions trPhCO2H or its nitrile precursor (trPhCN)
reacts with MoO3, affording a mixture of the two complexes,
[Mo4O12(trPhCO2H)2]·0.5H2O (2a)/[Mo4O12(trPhCO2H)2]·
H2O (2b), which exhibit a very similar composition but totally
different structures.
situation was observed for some heteroaryl carboxylic acids
−
(e.g., for 4-pyridinecarboxylic acid: pKA(pyCO2H/pyCO2 )
∼1.77 and pKB(HpyCO2H+/pyCO2H) ∼4.84) and amino acids
(e.g., for glycine with pKA ∼2.34 and pKB ∼9.60), in which the
pKB is significantly higher than the pKA. This allows formation
of zwitterionic molecules with coordinating carboxylate
−
(−CO2 ) and noncoordinating heteroaryl or amino functions
+
(pyH+ or −NH3 , as depicted in Scheme 2). Following this line
of thought, the trhis molecule might be of special interest. A
combination of three different functional groups, as two basic
heterocycles (1,2,4-triazole&imidazole) and one acidic function
(−CO2H), would result in at least three dissociation constants.
First, a simple histidine molecule (His), a starting precursor for
the synthesis of trhis, could be a representative example for this
consideration. This molecule possesses three pK values:
−
pK1(CO2H/CO2 ) ∼1.82, pK2(HimH+/Him) ∼6.04, and
The crystal structure of 2a belongs to the perovskite type, a
rare example for triazolyl-based molybdenum(VI) oxide solids,
which was reported just for the simplest 1H-1,2,3-tr and 1H-
1,2,4-tr ligands.8b,10 The 2D sheets are built up from distorted
MoO5N octahedra, each of which is interconnected by four
common vertices of μ2-O atoms (Figures 2 and 3 and Table 1).
+
pK3(α-NH3 /-NH2) ∼9.17. The transamination of His by
introduction of an electron-withdrawing tr− instead of a strong
electron-donating group (−NH2) should lead to a decrease of
the corresponding pK and appearance of a low pK(HtrH+/trH)
+
of 2.39 versus a pK3(α-NH3 /−NH2) of ∼9.17. Apparently, at
low pH of the reaction medium (pH ∼4) the carboxylic
fragment of trhis should be deprotonated, having a negative
charge (−1), the imidazole ring should appear as a positively
charged (+1) imidazolium cation, whereas the triazolyl termini
would rather prefer to be neutral. So, formation of the
zwitterionic species is highly expected. This interesting case is
achieved in compound [Mo8O25(trhis)2(trhisH)2]·2H2O (3).
The crystal structure of [Mo2O6(trethbz)2]·H2O (1) is based
on the distorted octahedra of molybdenum atoms in a typical
cis-{N2O4} arrangement. The polyhedra are joined via common
vertexes of μ2-O atoms forming parallel zigzag chains (Figure
1). Each two Mo atoms are alternatively bridged by either a
double [N−N]−tr and μ2-O links or exceptionally through μ2-
O atoms, thus separating Mo atom pairs within the chain at
distances of 3.628 and 3.812 Å, respectively. The crystallizating
water molecules display a key role in the formation of the 2D
hydrogen-bonded network, as shown in Figure 1. Since
Figure 2. Perovskite-type solid [Mo4O12(trC6H4CO2H)2]·0.5H2O
(2a) was formed as a major product of the hydrothermal reaction
between MoO3 and 4-(1,2,4-triazol-4-yl)benzonitrile in dilute aqueous
HF.
In axial positions the octahedra are clamped by [N−N]-tr links
with a strict alternation above and below the layer, whereas the
carboxylic acid fragments are left uncoordinated. The ligand
length and anchoring binding mode predetermine the
molybdenum trioxide interlayer spacing of b = 13.6620(6) Å.
Each two ligand molecules bound on one side of the layer are
tightly stacked in a pair via π−π interactions, as suggested by
centroid−centroid distances of 3.622 and 3.686 Å between
closely disposed tr-rings. A closer inspection of the packing
reveals that eight molybdenyl oxygen atoms within the MoO3
layer form a cyclic and shape-complementary cavity around the
tr-pair (Figure 4), leaving the −C6H4CO2H fragments to be
located outside the host cavity. This perfect fit of triazoles and
oxide matrix is achieved through four unprecedented non-
covalent contacts between C atoms of tr and O atoms of
molybdenyl groups (∼2.83−2.88 Å). Likely, these interactions
can be responsible for the deep green color of the complex. The
small interlayer voids are filled by uncoordinated water
Figure 1. Formation of zigzag chains in [Mo2O6(trethbz)2]·H2O (1)
running along the c-axis. The uncoordinated water molecules cross-
link the neighboring chains through H-bonding interactions forming a
H-bonding layer (symmetry codes: (i) x, −y, −z; (ii) −x, y, −z+1/2.
(iii) −x+1, y, −z+1/2) (some Mo atoms shown as green polyhedra).
E
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Figure 3. (a) Space filling model of the 2D perovskite-like layer of compound 2a consisting of distorted MoO5N octahedra joined by vertices of μ2-O
and stapled axially through [N−N]-tr fragments; (b) set of intralayer π−π interactions and C(tr)---O contacts; (c) the −CO2H groups are involved
in H-bonding interactions between the adjacent layers (water molecules are not shown).
Table 1. Principal Structural Features of [Mo2O6(trethbz)2]·H2O (1), [Mo4O12(trPhCO2H)2]·0.5H2O (2a),
[Mo4O12(trPhCO2H)2]·H2O (2b), and [Mo8O25(trhis)2(trhisH)2]·2H2O (3)
coord.
polyhedra
oxygen atom
bridges
polyhedral
connectivity
Mo(VI) oxide-subtopology and overall
topology
compound
ligand binding mode
[Mo2O6(trethbz)2]·H2O (1)
[Mo4O12(trPhCO2H)2]·
0.5H2O(2a)
cis-{N2O4}
μ2-(N,N)tr
μ2-(N,N)tr
μ2-O
μ2-O
vertex shared
1D, ribbon
{NO5}
vertex shared
2D, perovskite-like layer
[Mo4O12(trPhCO2H)2]·H2O
(2b)
{NO5}
μ2-(N,N)tr
μ3-O
vertex shared
1D
[Mo8O25(trhis)2(trhisH)2]·
2H2O (3)
cis-{N2O4}
{O6}
μ2-(N,N)tr& μ4-(N,N)tr,
(O,O)
μ2-O, μ3-O
vertex and edge
shared
1D → 2D
molecules which engage in H-bonding interactions with
molybdenyl oxygen atoms.
The asymmetric unit of compound 3 consists of four
independent molybdenum atoms, 12 and a half oxygen atoms
of the oxide matrix, two organic ligands, and a water molecule
as shown in Figures 6 and S7. Except for atom O4, which
occupies a crystallographic inversion center with the site
occupancy factor of 0.5, all other atoms occupy general
positions. The molybdenum atoms having distorted octahedral
{O6} and cis-{N2O4} environments are linked in a wavelike
chain combining common vertices and edges from oxygen
atoms (Figure 7a). The motif contains a unique sequence of
alternating fragments [MoO2-(μ2-O)(tr)2-MoO2-(μ2-O)]2 and
[MoO2-(μ3-O)(μ2-O)(μ2-CO2)-MoO2]2, in which the coordi-
The crystal structure of the related complex
[Mo4O12(trPhCO2H)2]·H2O (2b) forming under identical
conditions belongs to the more common ribbon type (Figure
5, Scheme 1). The linear sequences of distorted MoO5N
octahedra are linked by two O−O edges and supported
alternatively by bindings of tr-fragments above and below. The
ribbons and water molecules are further self-assembled via
hydrogen bonds that lead to 2D layers. A similar structural
motif was recently described for related Mo(VI) oxide solids
decorated with heterofunctional triazolyl derivatives.11b At first
glance, the carboxylic acid group seems to be inert toward the
Mo(VI) oxide matrix.21 However, the crystal structure of
[Mo8O25(trhis)2(trhisH)2]·2H2O (3) disproves this trend.
nating groups [N−N]-tr and syn,syn-μ2-η1:η1−CO2 appear
−
simultaneously and synergistically with small inorganic bridges.
Taking into account the fact that carboxylate coordination to
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Figure 4. (a) Eight MoO3 fragments are arranged in a cyclic bowl-
shaped cavity that incorporates two π-stacked triazolyl groups from
two organic ligands. (b) Phenylene spacers and terminal carboxylic
acid groups stay away from the cavity, while the accommodation of
two tr-rings is supported through direct coordination to Mo and four
unprecedentedly short noncovalent C(tr)···OMo contacts (∼2.83−
2.88 Å).
Figure 6. In the crystal structure [Mo8O25(trhis)2(trhisH)2]·2H2O (3)
the coordination environments of the Mo atoms are completed by
−
bridging tr−, −CO2 , and μ2-O/μ3-O (symmetry code: (i) x, y+1, z;
(iii) −x, −y, −z+1; (v) x, y−1, z).
Mo oxide typically affords linear ribbons (Scheme 2), whereas
triazolyl coordination preferably leads to chainlike coordination
polymers (Scheme 1), the obtained {[MoO2-(μ2-O)(tr)2-
MoO2-(μ2-O)]2-[MoO2-(μ3-O)(μ2-O)(μ2-CO2)-MoO2]2}n
subtopology can be rationally explained as follows. Under the
above-mentioned reaction conditions the organic ligand trhis
behaves in two forms: neutral zwitterionic tr-CH(CH2-
Thermal Stability. We recently demonstrated that
molybdenum oxide solids decorated by triazolyl-based ligands
exhibit appreciably high thermal stability, usually reaching 300−
350 °C and sometimes even more. The results correlate well
with the nature of extra functional moieties introduced at the
ligand platform, in addition to the 1,2,4-tr heterocycle, as well
as the resultant network topology of the coordination polymer;
that is, the low dimension of nets dictates the relatively low
thermal stability often occurring with temperature-induced
crystal structure transformation.11 To the best of our
knowledge, the highest thermal stability was documented for
the related tungsten(VI) oxide complex with 4,4′-bipy (530
°C).9b
The thermal stability of the compounds was examined by
measurements of temperature-dependent PXRD. For the 1D
coordination polymer [Mo2O6(trethbz)2]·H2O (1), the initial
diffraction pattern shows significant changes in positions of
peaks in the temperature ranges of 120−130 °C and 250−260
°C (Figure S12b). The first thermal transition can be attributed
to dehydration, while the second one clearly indicates complete
decomposition of the coordination polymer. As evidenced by
thermogravimetry/mass spectrometry (TG-MS) studies (Fig-
ure S13), the dehydration process proceeds gradually in a wide
Him+H)-CO2 (or trhis), and positively charged protonated
−
species tr-CH(CH2-im2H+)-CO2H (or trhisH+). In 3, the
composition ratio between them is the same, thus compensat-
ing the negatively charged polyoxomolybdate matrix
{Mo8O25}n2n−. Since a racemic mixture of trhis was employed,
crystals of 3 contain both enantiomers, resulting in the
centrosymmetric space group P21/c. The neutral zwitterion
trhis acts as a tetracoordinating unit exploiting both triazolyl
and carboxylate bridging functions. This leads to tight
interconnections between nearby chains forming a 2D
coordination network (Figure 7). For the protonated trhisH+
species, only a triazolyl group serves as a [N−N]-bidentate
bridge, whereas the carboxylic acid functional group remains
uncoordinated. Thus, compound 3 can be considered as the
first example of a zwitterionic complex of Mo(VI) oxide with a
three-functional zwitterionic ligand. The polyoxomolybdate
matrix bears the negative charge, whereas the positive charge is
concentrated at the imidazolium ring of the ligand.
Figure 5. (a) Ribbon-type motif in complex [Mo4O12(trPhCO2H)2]·H2O (2b) supported by [N−N]-triazole coordination. 2b was isolated as a
minor phase in the hydrothermal process. (b) The carboxylic acid functions (−CO2H) are left uncoordinated and are involved in a wide set of H-
bonding interactions between adjacent ribbons as well as water molecules.
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Figure 7. Crystal structure of [Mo8O25(trhis)2(trhisH)2]·2H2O (3): (a) the wavelike chain consists of a sequence of two alternating domains,
[MoO2-(μ2-O)(tr)2-MoO2-(μ2-O)]2 and [MoO2-(μ3-O)(μ2-O)(μ2-CO2)-MoO2]2, which are supported by different bridging types either through
μ2-O/[N−N]-tr or μ2-O/μ3-O/μ2-[OC−O]; (b) the domains from neighboring chains are interlinked by a tetradentate zwitterionic trhis unit
forming the layered structure; (c) the adjacent layers are involved in hydrogen bonding.
Figure 8. UV−vis absorbance spectra (a) and Tauc plots (b) for thin films of samples 1, 2a, and 3.
temperature range of 60−130 °C with a weight loss of 2.71%
corresponding to one water molecule (m/z = 18, 2.64% calcd).
The TGA curve above 250−260 °C shows high weight loss
(over 40 wt %) accompanied by intense evolution of H2O and
CO2 (m/z = 18 and 44), suggesting ligand decomposition.
Unlike this case, the thermal stability of the 1D coordination
polymer [Mo4O12(trPhCO2H)2]·0.5H2O (2b) is somewhat
higher, which can be explained by the existence of a highly
effective H-bonding network cross-linking through the
polymeric structure. The complex begins to lose its crystallinity
beyond 320 °C. On the basis of its 2D perovskite-type
structure, the thermal stability of the related complex
[Mo4O12(trPhCO2H)2]·0.5H2O (2a) is even higher, reaching
up to 350 °C.
Absorption Spectra. Absorption spectra recorded for thin
films of samples 1−3 are shown in Figure 8a. For 2a a strong
absorption band starts at about 526 nm (19 000 cm−1) and
extends toward the UV region with two maxima, at ca. 25 800
cm−1 (388 nm) and 35 300 cm−1 (283 nm). Due to the
absorption in the visible region, the complex exhibits a deep
green color. The spectra of the colorless compounds 1 and 3
are dominated by strong absorption in the UV region. For
complex 3 the maximum of the absorption band is observed at
ca. 32 800 cm−1 (305 nm), whereas for 1 it is shifted toward the
H
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UV region, with the maximum at ca. 35 000 cm−1 (286 nm)
and a shoulder at ca. 39 900 cm−1 (250 nm). Unlike the related
oxomolybdate with 3,5-di(4H-1,2,4-triazol-4-yl)benzoic acid
recently reported,21 none of the three samples investigated
here exhibit any luminescence when excited at absorption
bands using a Xe lamp.
Our DFT calculations show that samples 2a, 3, and 1 are
direct band gap semiconductors with band gaps increasing from
1.67 and 2.47 to 2.56 eV, respectively (Figures S24, S26, and
S22). On the basis of the UV/vis spectra the second-power
(ahν)2 was used for the Tauc plots (Figure 8b), yielding band
gap values of 2.61, 3.44, and 3.58 eV for 2a, 3, and 1,
respectively. DFT calculations usually underestimate band gap
values,23,24 in our case by ca. 1 eV. However, the experimentally
observed trend is well reflected in the calculations. Similar
energy gaps are calculated for samples 1 and 3, and they are
larger by about 1 eV than that for sample 2a.
Table 2. Epoxidation of cis-Cyclooctene in the Presence of
Mo Compounds Studied in This Work
d
sample
T (°C); cosolvent; oxidant CyO yield at 1 h/6 h/24 h (%)
a
2a
55; TFT; TBHP
55; TFT; TBHP
55; TFT; TBHP
55; TFT; TBHP
55; TFT; TBHP
55; TFT; TBHP
5/44/82
0/12/47
28/77/97
−/16/83
−/−/<1
35/61/89
36/66/92
68/98/100
−/−/77
a
a
3*
1
MoO3
none
1-CT-L
1-CT-S
1
b
c
55; TFT; TBHPb
a
70; TFT; TBHP
a
1
55; CH3CN; H2O2
a
Initial molar ratios Mo:Cy:oxidant = 1:100:153 ([Cy]0 = 0.7 M; 2 mL
b
of cosolvent). The initial concentration of Cy was equal to that used
for a typical catalytic batch run (with 1). Using a mass amount of
c
recovered solid similar to that used for a typical catalytic batch run
d
with fresh catalyst. Cyclooctene oxide (CyO) yield (selectivity was
Partial densities of states calculated for molybdenum and
organic ligands are shown in Figures S21, S23, and S25. In each
case the bands below the Fermi level mainly consist of p-states
of ligands. Although some Mo d-states are also present, they are
centered at about −4 eV and do not contribute significantly to
the valence band. Contribution of Mo d-states is much more
pronounced in the conduction band. Moreover, these states are
lying on the edge of the conduction band, leading to the
assumption that the optical absorption is associated with energy
transfer from organic ligands to molybdenum d-levels.
Catalytic Performance and Stability Tests. The
epoxidation activity and stability of compounds 1, 2a, and 3*
were investigated via the model reaction of cis-cyclooctene with
tert-butylhydroperoxide (TBHP) as oxidant, at 55 °C (Figure 9,
100%).
For 1, 2a, and 3*, the solids recovered from a 24 h catalytic
batch run at 55 °C (i-TBHP-55, i = 1, 2a, and 3*; details given
in the Experimental Section) were characterized by ATR FT-IR
spectroscopy and PXRD. For each compound, the IR spectra
and PXRD patterns of the fresh and used solids were
comparable (Figures 10−12), suggesting that their chemical
and structural features were essentially preserved during the
catalytic reaction. The main differences in characterization data
were slight changes in the relative intensities of some weak
peaks in the case of 3*, which may be due to partial
recrystallization under the catalytic reaction conditions (which
are different from the synthesis conditions).
The molybdenum centers in compounds 1, 2a, and 3* are
responsible for the epoxidation activity, since under metal-free
conditions, in the presence of the corresponding free organic
ligands trethbz, trPhCO2H, or trhis (used in molar amounts
equivalent to those added together with the corresponding
molybdenum compounds), the Cy reaction at 55 °C was very
sluggish; the best result was 9% conversion at 24 h, for trethbz.
Hence, the active species contain molybdenum. Attempts to
isolate the ligands after a 24 h batch run were successful in the
cases of trPhCO2H and trhis, trPhCO2H-TBHP-55, and trhis-
TBHP-55, respectively. The ATR FT-IR spectra and PXRD
patterns of the original and used solids are similar (Figures 10
and 11). These results are consistent with the characterization
studies of the hybrid polymers, which indicate fairly good
stability under the catalytic conditions.
The activity trends for compounds 1, 2a, and 3* may be due
to an interplay of Lewis acidity, catalyst solubility, and/or
structural dimensionality of the hybrid compounds. It is
generally accepted that molybdenum-catalyzed epoxidation of
olefins with alkylhydroperoxides involves the reaction between
the (Lewis acid) metal centers and the (basic) oxidant
molecules, giving active species that possess linear or η2-peroxo
ligands ({Mo-O-O} or {Mo-(O)2}) involved in the O atom
transfer reaction to the olefin, finally giving the epoxide product
(and an alcohol as coproduct of the consumption of the
oxidant).25,26 Accordingly, the Lewis acidity of the metal
centers can determine the epoxidation activity and is influenced
by the type of coordinated organic ligands. The three
compounds possess different coordination environments,
which may partly account for the difference in reaction kinetics.
On the other hand, differences in catalyst solubility and initial
Figure 9. Epoxidation of cis-cyclooctene with TBHP, using the
■
○
systems 1/TFT/55 °C ( ), 2a/TFT/55 °C (×), 3*/TFT/55 °C ( ),
●
▲
1/TFT/70 °C ( ), 1-CT-L/TFT/55 °C ( ), and 1-CT-S/TFT/55
□
°C ( ). Selectivity to 1,2-epoxycyclooctane (CyO) was always 100%.
The dashed lines are visual guides.
Table 2). The three compounds led to 1,2-epoxycyclooctane
(CyO) as the only product (100% selectivity); without catalyst,
Cy conversion was negligible. The reaction rate (based on
conversion at 24 h) followed the order 1 (97%) > (2a) (82%)
> 3* (47%). The results for 1 and 2a are comparable or
superior to those for commercially available MoO3 (Scharlau,
>99% purity), tested as catalyst under identical Cy reaction
conditions (83% conversion at 24 h, Table 2).
I
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Figure 10. FT-IR ATR spectra (A) and PXRD patterns (B) of ligands trPhCO2H (a) and trPhCO2H-TBHP-55 (b) and compounds 2a (c) and 2a-
TBHP-55 (d).
Figure 11. FT-IR ATR spectra (A) and PXRD patterns (B) of ligands trhis (a) and trhis-TBHP-55 (b) and compounds 3* (c) and 3*-TBHP-55 (d).
dissolution rates may affect the overall reaction rate.
Consistently, the kinetic profiles showed an initial induction
period (10−60 min) that decreased in the order 1 > 2a > 3*;
longer initial induction periods may be partly due to slower
initial dissolution of the catalyst. The structure of the three
compounds is very different, and steric hindrance effects may
be less important for (most active catalyst) 1, in comparison to
2a and 3*. The monofunctional tr ligand in 1 led to a 1D zigzag
chain (ribbon) structure. On the other hand, 2a, bearing the
heterodonor ligand tr/CO2H, possesses a 2D structure, and 3*,
bearing the heterodonor ligand im/tr/CO2H, possesses a 2D
structure where tr and/or CO2H are coordinated to Mo
centers; these structural features may lead to the lower
accessibility/inaccessibility of some metal centers for coordina-
tion to the hydroperoxide oxidant and olefin epoxidation.
Compound 1 is a homogeneous catalyst, which was
confirmed by a contact test approach (details in the
Experimental Section). Specifically, the liquid and solid phases
(1-CT-L and 1-CT-S, respectively) obtained from contacting 1
with TBHP/TFT/55 °C for 24 h (without substrate) were
tested separately for Cy reaction and led to relatively high
conversions (89% and 92% conversion at 24 h, respectively),
which reasonably parallels that for 1 used in a typical batch run
(97% conversion) (Figure 9); some differences in reaction rates
are likely due to differences in the amount of catalyst. The
PXRD patterns and ATR FT-IR spectra of the solids 1-CT-L
and 1-CT-S and of the solid (1-CT-S run 1) recovered from
the catalytic test using 1-CT-S were similar to the
corresponding data for 1 (Figure 12). These results suggest
that the epoxidation reaction is essentially promoted by the
solubilized metal species, and the active metal species are stable
when dissolved or not in the oxidizing medium.
The catalytic conversion could be improved by increasing the
reaction temperature from 55 to 70 °C, without affecting
selectivity and catalyst stability; CyO yield at 6 h increased from
77% at 55 °C to 98% at 70 °C (Figure 9, Table 2). The FT-IR
ATR spectra and PXRD patterns of the recovered solid 1-
TBHP-70 were similar to the corresponding data for the
original compound 1, suggesting fairly good stability (Figure
12).
The catalytic performance of 1 was superior to that reported
for other molybdenum oxide-based hybrid polymers tested as
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Figure 12. FT-IR ATR spectra (A) and PXRD patterns (B) of ligand trethbz (a) and compounds 1 (b), 1-TBHP-55 (c), 1-CT-S-run 1 (d), 1-CT-S
(e), 1-TBHP-70 (f), and 1/H2O2 (g).
Table 3. Epoxidation of Different Olefins with TBHP, in the Presence of 1, Using TFT as Co-solvent
T
conv.
a
select.
b
c
d
olefin
Cy
(°C)
(%)
product
(%)
ee (%)
de (%)
70
70
70
70
70
98/100 1,2-epoxycyclooctane
97/100 1,2-epoxycyclododecane
100/100
100/100
100/100
100/100
95/92
Cy12
1C8
2C8
Ole
41/63
88/99
1,2-epoxyoctane
2,3-epoxyoctane
92/100 methyl 9,10-epoxyoctadecanoate
methyl 9,10-dihydroxyoctadecanoate
3/6
LinOle
70
63/78
9,10-epoxy-12Z-octadecenoate + methyl 12,13-epoxy-9Z-
octadecenoate
88/81
methyl 9,10;12,13-diepoxyocta-decanoate
12/19
Prochiral olefins
TBMS
55
43/66
78/90
(1R,2R)-(+)-phenylpropylene oxide
(1S,2S)-(−)-phenylpropylene oxide
1,2-epoxy-p-menth-8-ene
100/100 0/0
DL-Lim
55
85/82
9/13
13/12 ((+)-cis); 8/8
((+)-trans)
2/3 ((−)-cis); 8/8
((+)-cis)
1,2;8,9-diepoxy-p-menthane
a
b
c
Initial molar ratios Mo:Cy:oxidant = 1:100:153, 2 mL of TFT. Conversion at 6/24 h. Selectivity at 6 h/24 h. Enantiomeric excess at 6 h/24 h.
Diastereomeric excess at 6 h/24 h.
d
catalysts in the same model reaction, under somewhat
comparable conditions (Table S10); namely, [Mo2O6(trpzH)-
(H2O)2]n (trpzH = 4-(3,5-dimethyl-1H-pyrazol-4-yl)-1,2,4-
triazole),11b [MoO3(bipy)][MoO3(H2O)]}n (bipy = 2,2′-
bipyridine),3b [Mo2O6(HpypzA)]n (HpypzA = [3-(pyridi-
nium-2-yl)-1H-pyrazol-1-yl]acetate),3f [MoO3(1H-1,2,4-
tr)0.5],3g and [Mo3O9(pypz)]n (pypz = 2-[3(5)-pyrazolyl]-
and steric (ligand lability, accessibility of reactant molecules to
27
the metal centers) effects. Compounds [MoO3(bipy)]n and
28
[Mo2O6(pent-pp)]n both consist of 1D chains of corner-
sharing distorted {MoO4N2} octahedra (relatively “flexible”)
and led to higher Cy conversions than 1. Clear comparisons
with other molybdenum-based polymers (e.g., possessing
salycilidene- or pyridoxal-based ligands) are not straightforward
due to considerably different reaction conditions used between
different works.
The versatility of 1 as a catalyst was further explored for the
epoxidation of various substrates using TBHP as oxidant at 70
°C (Table 3). The reaction of the linear olefins trans-2-octene
(2C8) and 1-octene (1C8) gave the corresponding epoxide
27
pyridine).3c Compounds [MoO3(bipy)]n and [Mo2O6(pent-
pp)]n (pent-pp = (2-(1-pentyl-3-pyrazolyl)pyridine)28 led to
higher Cy conversions than 1. Differences in catalytic activity
may be associated with different structural and chemical
properties of the hybrid materials, which may lead to
differences in catalyst solubility and electronic (Lewis acidity)
K
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Inorganic Chemistry
Article
products with 100% selectivity. The reaction of 2C8 (88%/99%
conversion at 6 h/24 h) was faster than that of 1C8 (41%/63%
conversion at 6 h/24 h). This may be explained in light of the
mechanistic considerations discussed above. The internal
double bond of 2C8 is electron richer than the terminal
double bond of 1C8, and thus the nucleophilic attack of the
olefin on the active oxidizing species and the O atom transfer to
the olefin is more favorable for 2C8. Somewhat consistent with
this hypothesis, the cyclic substrate Cy is more reactive than the
linear octenes.29
The epoxidation cyclododecene (Cy12) in the presence of 1
led to 97% conversion at 6 h, which is comparable to that for
the less bulky cyclic olefin Cy. It seems that electronic effects
are more considerable than steric ones for these epoxidation
systems. Compound 1 was further explored for the epoxidation
of the biomass-derived olefins methyl oleate and methyl
linoleate, which are fatty acid methyl esters (FAMEs) produced
from vegetable oils by transesterification.30 Epoxidized FAMEs
have widespread use as, for example, PVC stabilizers and
plasticizers and as intermediates in the production of
polyurethane polyols.31
The epoxidation of Ole gave methyl 9,10-epoxyoctadeca-
noate in 87%/92% yield at 92%/100% conversion (6 h/24 h);
methyl 9,10-dihydroxyoctadecanoate was formed in 6% yield at
24 h. The reaction of LinOle gave epoxides in a total selectivity
of 100%: specifically, at 63%/78% conversion and 6 h/24 h
reaction, the monoepoxy isomers methyl 9,10-epoxy-12Z-
octadecenoate and methyl 12,13-epoxy-9Z-octadecenoate
were formed in equimolar amounts and 56%/63% total yield,
and methyl 9,10;12,13-diepoxyoctadecanoate was formed in
8%/15% yield. LinOle was less reactive than Ole, which may be
partly associated with the fact that the former possesses two
double bonds.
The reaction mixture consisted of a yellow solution, whereas
the as-synthesized compound 1 is a white powder, suggesting
that different metal species were formed from 1 during the
catalytic reaction with H2O2. The FT-IR ATR spectrum of
metal species (1-H2O2-55) isolated from a 24 h batch run at 55
°C is different from that of 1 (Figure 12). The bands at 940,
920, and 906 cm−1 associated with the MoO vibrations of 1
disappeared, and instead the solid 1-H2O2-55 exhibited two
bands at 931 and 892 cm−1, as well as a shoulder at 862 cm−1
and a weak band at 678 cm−1 assignable to ν(O−O) stretching
vibrations and symmetric Mo(O2) vibrational modes, respec-
tively; a broad band centered at 540 cm−1 assignable to Mo−
O−Mo vibrations remained present.33,34 On the other hand, 1-
H2O2-55 exhibited slight differences of the ligand coordination
modes in the range 1220−1150 cm−1, and a shoulder at 520
cm−1 disappeared. These spectral differences indicate that
considerable changes in the coordination sphere for 1 occurred,
leading to species of the type 1-H2O2-55, which seem to be
polymeric, bearing oxo and peroxomolybdenum groups.
Accordingly, the PXRD pattern of 1-H2O2-55 is very different
from that of 1 (Figure 11); the former solid is amorphous and
structurally very different. The lower catalyst stability and
formation of different types of metal species for H2O2 as
oxidant in comparison to TBHP may partly explain the lower
catalytic activity in the former case.
Catalytic Performance of 1 in the Reaction of trans-β-
Methylstyrene. The catalytic reaction of the prochiral olefin
trans-β-methylstyrene (TBMS), in the presence of the
homochiral coordination polymer 1, at 55 °C, gave 100%
total selectivity to the corresponding epoxide isomers (1R,2R)-
(+)-phenylpropylene oxide and (1S,2S)-(−)-phenylpropylene
oxide and 66% conversion at 24 h. No significant enantiomeric
excess was obtained. To the best of our knowledge, there are no
reports on the use of molybdenum(VI) oxide hybrid polymers
possessing chiral centers in the organic component, tested as
catalysts for TBMS epoxidation. A mixed Mo−Sn hybrid
polymer, namely, [((Me)2(menthyl)Sn)2MoO4(H2O)3.5]n, with
[MoO4]2-tetrahedra coordinated to [R3Sn]+ cationic spacers,
gave similar results to 1: 68% TBMS conversion and negligible
ee after 24 h (molar ratio catalyst:olefin:oxidant = 1:100:150),
using dichloromethane as cosolvent at 55 °C.35
Compound 1 was further explored for the epoxidation of DL-
limonene (DL-Lim; consisting of a racemic mixture of S-
(−)-limonene and R-(+)-limonene), using TBHP as oxidant at
55 °C, under similar reaction conditions to those used for
TBMS as substrate (Table 3). The reaction of DL-Lim gave
mainly isomers of 1,2-epoxy-p-menth-8-ene (LimOx) in a total
yield of 66%/74% at 78%/90% conversion and 6 h/24 h;
1,2;8,9-diepoxy-p-menthane (LimDiOx) was formed with 7%/
12% yield. The molar ratio LimOx/(LimOx+LimDiOx) was 0.9
(6 and 24 h) and 8,9-epoxy-p-menth-1-ene was not formed,
suggesting high regioselectivity toward the epoxidation of the
endocyclic double bond in relation to the exocyclic one. The
epoxide products, LimOx and LimDiOx, have commercial
importance for the production of paints and resins, for example,
and can be used as intermediates in the synthesis of fragrances,
agrochemicals, biodegradable polymers, and pharmaceuticals,
among others.36,37
Only a few other hybrid organic/inorganic molybdenum
compounds have been studied as catalysts for Ole epoxidation
with TBHP. On the basis of the OleOx yields at 6 h/24 h, the
performance of 1 is comparable or slightly superior to that
reported for the molybdenum oxide/bipyridine dicarboxylate
hybrid material (DMA)[MoO3(Hbpdc)]·nH2O (DMA =
dimethylammonium; Hbpdc = deprotonated 2,2′-bipyridine-
5,5′-dicarboxylic acid),3d which led to 91%/98% OleOx yield
(93%/100% conversion) at 75 °C (molar ratio Mo:Ole:TBHP
= 1:103:160, TFT as cosolvent). At the lower reaction
temperature of 55 °C, the hybrid material [Mo2O6(2-(1-
pentyl-3-pyrazolyl)pyridine)] led to 78% OleOx yield (82%
conversion) at 6 h (molar ratio Mo:Ole:TBHP = 1:113:172).28
The use of H2O2 instead of TBHP gave CyO as the only
product, and the kinetics was slower: 77% and 97% conversion
at 24 h, for H2O2 and TBHP, respectively, at 55 °C (Table 2).
The catalytic results for the system 1/H2O2 are similar to those
reported in the literature for the polymer {[MoO3(bipy)]-
32
3g
[MoO3(H2O)]}n and inferior to [MoO3(tr)0.5]n and
[Mo2O6(trpzH)(H2O)2]n,11b tested as catalysts under similar
Cy reaction conditions (Table S11). Additional comparisons
were difficult due to the different reaction conditions used for
other systems presented in Table S11.
The slower kinetics when using H2O2 as oxidant instead of
TBHP may be partly due to differences in catalyst stability. On
the other hand, water (which is added together with H2O2) is a
coordinating solvent that may compete with the oxidant
molecules in the coordination to the metal center (based on the
above mechanistic considerations).
For compound 1, the product LimOx consisted of four
isomers, namely, the diastereoisomers (1S,4S)-(−)-cis-limo-
nene-1,2-epoxide and (1R,4S)-(−)-trans-limonene-1,2-epoxide
formed from (S)-(−)-limonene with 3% diastereomeric excess
(de) of the (−)-cis-isomer at 24 h, and the diastereoisomers
L
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(1R,4R)-(+)-cis-limonene-1,2-epoxide and (1S,4R)-(+)-trans-
limonene-1,2-epoxide formed from (R)-(+)-limonene with 8%
de of the (+)-cis- isomer at 24 h. The catalytic results for 1
compare favorably to literature data for the polymeric material
[((Me)2(menthyl)Sn)2MoO4(H2O)3.5] tested for the reaction
of DL-Lim, which led to 67% total monoepoxide yield at 90%
conversion, 24 h (Mo:substrate:TBHP = 1:100:150; 55 °C;
dichloromethane); when the pure isomer (R)-(+)-limonene or
(S)-(−)-limonene was used as substrate, the corresponding cis-
isomers were formed with 5−7% de, which is somewhat
comparable to the de values for 1.35 For compound 1, the
isomers (1R,4R)-(+)-cis-limonene-1,2-epoxide and (1S,4R)-
(+)-trans-limonene-1,2-epoxide were formed in an enantio-
meric excess (ee) of 12% (relative to the (−)-cis- isomer) and
8% (relative to the (−)-trans-isomer), respectively, at 24 h. A
literature survey on molybdenum(VI) complexes bearing chiral
N/O-donor ligands indicates that the induction ability of these
types of catalysts for the epoxidation of prochiral olefins is
generally low or negligible, which has been attributed to various
factors such as the lability of the chiral ligands,38,39 considerable
distance between the chiral center(s) of the organic ligand and
the catalytic site,40,35 and/or poorly selective radical reac-
tions.41−44
(1) was compared to other molybdenum−organic ligand based
hybrid polymer catalysts reported in the literature. Differences
in catalytic activity may be associated with different structural
and chemical properties of the hybrid materials, which may lead
to differences in catalyst solubility and electronic (Lewis
acidity) and steric (ligand lability, accessibility of reactant
molecules to the metal centers) effects.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02986.
Experimental details for X-ray structural analyses, spectral
characterization data, thermal PXRD patterns, and DTA/
TG-MS data (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ab_lysenko@univ.kiev.ua (A. B. Lysenko).
*E-mail: igoncalves@ua.pt (I. S. Gonca
■
̧ lves).
ORCID
CONCLUSION
Andrey B. Lysenko: 0000-0002-0342-5122
■
We employed organic ligands bearing 1,2,4-triazolyl (enantio-
pure (S)-trethbz), tr, and carboxylic acid (trPhCO2H) and tr, −
CO2H, and imidazolyl (racemic trhis) donor moieties for the
development of Mo(VI) oxide hybrid solids. The [N−N]
coordination behavior of the tr-group is highly predictable and
can be regarded as a clear tendency of the unit to anchor to
MoO3 through two-donor sites. The coordination ability of
−CO2H strongly depends on the nature of extra donors, which
might specifically regulate the corresponding pKA values of the
ligands. In the acidic media, the −CO2H group preferably
serves as a H-bonding unit. The presence of basic imidazolyl
fragments facilitates the formation of neutral zwitterions tr-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. The Portuguese (PT) group acknowl-
edge the FCT and the European Union for a postdoctoral grant
to P.N. (SFRH/BPD/110530/2015) cofunded by MCTES and
the European Social Fund through the program POPH of
QREN. This work was developed in the scope of the project
CICECO-Aveiro Institute of Materials, POCI-01-0145-
FEDER-007679 [FCT (Fundaçao para a Ciencia e a
̂
̃
CH(CH2-Him+H)-CO2 with coordinating pairs of tr− and
−CO2 , while the imidazolium species is left uncoordinated.
−
Tecnologia) ref UID/CTM/50011/2013], financed by national
funds through the FCT/MEC and when applicable cofinanced
by FEDER (Fundo Europeu de Desenvolvimento Regional)
under the PT2020 Partnership Agreement. DFT calculations
have been carried out using resources provided by the Wroclaw
Centre for Networking and Supercomputing (http://wcss.pl),
grant no. 415, using Materials Studio 8.0 software provided by
PLATON U3, Service Platform for e-Science. J.C. acknowl-
edges financial support from the Foundation for Polish Science.
−
This leads to diverse structural motifs of Mo(VI) oxide solids
varying from zigzag chain 1 and ribbon 2b through rare
perovskite-type 2a to the new two-domain chain 3. The optical
band gaps of the complexes are in good agreement with the
semiconducting behavior of molybdenum(VI) oxide hybrids.
Compounds 1−3 were investigated as epoxidation catalysts
via the model reaction of cis-cyclooctene with tert-butylhy-
droperoxide (TBHP). The compounds are stable, based on
catalytic and characterization studies, and lead to 1,2-
epoxycyclooctane with 100% selectivity. The catalytic activity
follows the order 1 > 2a > 3*. Compound 1 effectively
promoted the epoxidation of other olefins, namely, 1-octene,
trans-2-octene, cyclododecene, and the bioderived olefins
methyl oleate and methyl linoleate. For the epoxidation of
the prochiral substrate trans-β-methylstyrene with TBHP, the
total selectivity to the epoxide isomers (1R,2R)-(+)-phenyl-
propylene oxide and (1S,2S)-(−)-phenylpropylene oxide was
100% at 66% conversion, but enantiomeric excess was
negligible. For the biobased prochiral olefin DL-limonene, four
monoepoxide isomers were formed; the isomers (1R,4R)-
(+)-cis-limonene-1,2-epoxide and (1S,4R)-(+)-trans-limonene-
1,2-epoxide were formed in an enantiomeric excess of 12%
(relative to the (−)-cis-isomer) and 8% (relative to the
(−)-trans-isomer), respectively. The best performing catalyst
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