An octanuclear molybdenum(VI) complex containing coordinatively bound 4,4′-di-tert-butyl-2,2′-bipyridine, [Mo8O 22(OH)4(di-t Bu-bipy)4]: Synthesis, structure, and catalytic epoxidation of bio-derived olefins

Article abstract of DOI:10.1021/ic202640a

The reaction of [MoO₂Cl₂(di-tBu-bipy)] (1) (di-tBu-bipy = 4,4′-di-tert-butyl-2,2′-bipyridine) with water at 100-120 °C in a Teflon-lined stainless steel autoclave, in an open reflux system, or in a microwave synthesis system gave the octanuclear complex [Mo ₈O₂₂(OH)₄(di-tBu-bipy)₄] (2) as a microcrystalline powder in good yields. Single crystals of 2 suitable for X-ray diffraction were obtained by the reaction of MoO₃ and di-tBu-bipy in water at 160 °C for 3 days. The molecular structure of 2 comprises a purely inorganic core, Mo₄O₈(μ₃-OH) ₂(μ₂-O)₂, attached to two peripheral oxo-bridged binuclear units, Mo₂O₄(μ₂-O) ₂(OH)(di-tBu-bipy)₂. The inorganic core is composed of a unique assembly of four {MoO₅} distorted square pyramids connected to each other via edge-sharing. Overall, the octanuclear complex adopts a highly distorted form strongly resembling an "S"-shaped molecular unit. Complex 2 was applied in the catalytic epoxidation of the biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using tert-butylhydroperoxide (TBHP) as an oxygen donor, under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high selectivities at high conversions (89% 1,2-epoxy-p-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable "non-productive" decomposition of TBHP.

Full text of DOI:10.1021/ic202640a

Article
pubs.acs.org/IC
An Octanuclear Molybdenum(VI) Complex Containing Coordinatively
Bound 4,4′-di-tert-Butyl-2,2′-Bipyridine, [Mo O (OH) (di-tBu-bipy) ]:
8
22
4
4
Synthesis, Structure, and Catalytic Epoxidation of Bio-Derived Olefins
†
†
†
‡
‡
†
Tatiana R. Amarante, Patrícia Neves, Cat
́
ia Tome,
́
Marta Abrantes, Anabela A. Valente,
†
,†
Filipe A. Almeida Paz, Martyn Pillinger, and Isabel S. Gonca
̧
lves*
†
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
‡
Centro de Química Estrutural, Complexo Interdisciplinar, Instituto Superior Tec
Pais, 1, 1049-001 Lisboa, Portugal
́
nico, Universidade Tec
́
nica de Lisboa, Av. Rovisco
*
S Supporting Information
ABSTRACT: The reaction of [MoO Cl (di-tBu-bipy)] (1)
2
2
(
1
di-tBu-bipy = 4,4′-di-tert-butyl-2,2′-bipyridine) with water at
00−120 °C in a Teflon-lined stainless steel autoclave, in an
open reflux system, or in a microwave synthesis system gave
the octanuclear complex [Mo O (OH) (di-tBu-bipy) ] (2) as
8
22
4
4
a microcrystalline powder in good yields. Single crystals of 2
suitable for X-ray diffraction were obtained by the reaction of
MoO and di-tBu-bipy in water at 160 °C for 3 days. The
3
molecular structure of 2 comprises a purely inorganic core,
Mo O (μ -OH) (μ -O) , attached to two peripheral oxo-
4
8
3
2
2
2
bridged binuclear units, Mo O (μ -O) (OH)(di-tBu-bipy) . The inorganic core is composed of a unique assembly of four
2
4
2
2
2
{
MoO } distorted square pyramids connected to each other via edge-sharing. Overall, the octanuclear complex adopts a highly
5
distorted form strongly resembling an “S”-shaped molecular unit. Complex 2 was applied in the catalytic epoxidation of the
biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using tert-butylhydroperoxide (TBHP) as an oxygen donor,
under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high
selectivities at high conversions (89% 1,2-epoxy-p-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-
epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable
“
non-productive” decomposition of TBHP.
INTRODUCTION
Oxomolybdenum hybrid materials have been of interest for
several years due to their potentially interesting catalytic,
Molybdenum oxide/organoamine hybrids are generally
prepared by the hydrothermal treatment at 160−200 °C of
aqueous solutions containing the organonitrogen compound
and the molybdenum source, which is usually Na MoO ,
■
1
,2
magnetic, electronic, and optical properties. One class of
molybdenum oxide composites that has attracted particular
interest is that in which the organic component is an
organonitrogen compound. Zubieta and co-workers identified
three subclasses for these materials based on the role of the
organic molecule: (1) a charge-compensating, space-filling, and
structure-directing cation, (2) a ligand bonded to a heterometal
center which is incorporated into the metal oxide backbone
either as peripheral moieties or as complex bridging units, and
2
4
MoO , or (NH ) Mo O . This method frequently affords
3
4 6
7
24
crystals suitable for X-ray diffraction. On the other hand, yields
can be low, and mixtures of phases are sometimes obtained that
require mechanical separation. We have been exploring
alternative approaches that use monomeric molybdenum
complexes as precursors. Thus, the oxidative decarbonylation
of the tetracarbonyl complexes [Mo(CO) (L)] (L = 2,2′-bipy,
4
pzpy) by reaction with tert-butylhydroperoxide at room
temperature gives microcrystalline [MoO (2,2′-bipy)] and
(
3) a ligand bonded directly to a molybdenum site of the oxide
3
3
,4
1
[
Mo O (pzpy) ] in excellent yields. In these compounds,
substructure. Ligands examined for the latter include 2,2′-
4 12 4
2
a−c
2d−f
{
MoO N } octahedra share corners to form 1D chains in the
bipyridine (2,2′-bipy),
4,4′-bipyridine,
4,4′-bipyridylami-
4 2
2
g
2h
former and tetranuclear square-type species in the latter. For L
4,4′-di-tert-butyl-2,2′-bipyridine (di-tBu-bipy), the octanuclear
complex [Mo O (di-tBu-bipy) ] containing a central cubane-
ne,
(
1,10-phenanthroline,
pzpy), pyrazine,2 2,4,6-tripyridyltriazine, 1,2,3-triazole,
2-[3(5)-pyrazolyl]pyridine
2
i
2j
2k
2l
=
m
and 1,2,4-triazoles.
Owing to the pronounced structure-
8
24
4
3
type Mo (μ -O) core is obtained. The controlled hydrolysis
directing influence of organoamines, the structural chemistry of
these materials encompasses one-dimensional (1D) chains,
two-dimensional (2D) sheets, and three-dimensional (3D)
networks, as well as discrete clusters.
4
3
4
Received: December 7, 2011
Published: February 28, 2012
©
2012 American Chemical Society
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Inorganic Chemistry
Article
and condensation of monomeric complexes of the type
[
MoO Cl L] is another potentially interesting route to
2 2
temperature on an X’Pert MPD Philips diffractometer, equipped with
an X’Celerator detector and a flat-plate sample holder in a Bragg−
Brentano para-focusing optics configuration (40 kV, 50 mA). Intensity
data were collected by the step-counting method (step 0.04°), in
continuous mode, in the ca. 3.5 ≤ 2θ ≤ 70° range. The microwave-
assisted syntheses were carried out in a Discover S-Class (CEM
Corporation, USA) microwave oven using a glass vessel with a capacity
of 35 mL. A dynamic method was used in which the power was
automatically controlled on the basis of the temperature feedback
measured using a vertical focused IR sensor.
molybdenum oxide/organic hybrids. The reaction of
[
{
MoO Cl (2,2′-bipy)] with water at 100−120 °C gives
2
2
[MoO (2,2′-bipy)][MoO (H O)]} with a crystal structure
3 3 2 n
containing 1D inorganic and organic−inorganic polymers
5
linked by O−H···O hydrogen bonds.
The synthesis of the aforementioned molybdenum oxide/
organodiamine compounds in high yields under mild
conditions has paved the way to catalytic applications. In
preliminary investigations these materials have proved to be
active, selective, and stable catalysts for the epoxidation of cis-
[
Mo O (OH) (di-tBu-bipy) ] (2). Method A. A mixture of 1
8 22 4 4
3
−5
(0.25 g, 0.53 mmol) and water (25 mL) was stirred and heated
to 120 °C inside a microwave oven and maintained at this
temperature for 4.5 h. The resultant white solid was separated
from the pink aqueous mother liquor (pH ≅ 2) by filtration;
washed with water, acetone/diethyl ether (1:1), and diethyl
ether (10 mL each); and finally vacuum-dried. Yield: 0.092 g,
61% (based on Mo). Anal. Calcd for C H Mo N O
26
cyclooctene, used as a model substrate.
Future work will
need to address the epoxidation of more demanding olefins
and/or other types of oxidation reactions.
As part of our ongoing investigations into the use of
molybdenum complexes as precursors to oxomolybdenum
hybrids, we now report on results with the complex
7
2
100
8
8
[
MoO Cl (di-tBu-bipy)] (1). Several studies in the literature
(2261.12): C, 38.24; H, 4.45; N, 4.95. Found: C, 37.80; H,
4.31; N, 4.94. Selected FT-IR (cm ): ν = 3262 (m), 2967 (s),
936 (sh), 2906 (m), 2870 (m), [1615 (vs), 1548 (m), 1410
2
2
−1
indicate that di-tBu-bipy is a good choice as a ligand for
oxometal complexes to be used as catalysts for oxidation
reactions. For example, in addition to the octanuclear complex
mentioned above, the complexes [MoO (NCS) (di-tBu-
2
6
(vs) (bipy CC and CN str)], [935 (vs), 923 (s), 914 (vs),
mode), [829 (vs), 777 (vs), 751 (s), 719 (s), 640 (vs,br)
3
899 (vs), 882 (vs), 865 (vs) ν(MoO)], 850 (vs) (ligand
2
2
6
a−d
6d
bipy)],
[Mo O (NCS) (di-tBu-bipy) ], and [V O (μ-
−1
2
5
2
2
2
2
ν(Mo−O−Mo) and ν(OMo )]. Selected FT-Raman (cm ): ν
6g
3
= 3072 (m), 2972 (s), 2929 (m), 2905 (m), 1608 (s), 1541
MeO) (μ-MO ) (di-tBu-bipy) ] (M = Mo, W) are effective
as oxygen atom transfer agents and/or (pre)catalysts for
alcohol oxidation and olefin epoxidation. Kodama et al.
2
4 2
2
(
863 (s), 827 (m), 802 (m), 781 (m), 718 (s). H NMR (400
vs), 1415 (s), 1318 (vs), 957 (s), 934 (vs), 922 (vs), 900 (s),
1
concluded that the affinity of the heterotetranuclear V W2
MHz, CD CN, 25 °C, ppm): δ = 8.56 (d, bipy), 8.44 (s, bipy),
2
3
7.43 (d, bipy), 1.37 (s, C(CH
13
complex to organic substrates was improved by the selection
3
)
1.6 ((CH ) C, (CH ) C′), 34.9, 36.0 ((CH ) C, (CH ) C′),
17.3−120.0, 126.3, 150.1, 151.1, 151.9, 153.4, 162.7, 163.9,
67.3 (C(2,2′)-C(6,6′)).
3
). C CP MAS NMR: δ = 31.0,
3 3
6g
3
1
1
of di-tBu-bipy as the organic ligand.
3
3
3
3
3
3
Hydrolysis of 1 using various heating methods gives
Mo O (OH) (di-tBu-bipy) ] (2) with a structure that
[
8
22
4
4
Method B. A Schlenk tube was charged with 1 (0.97 g, 2.08 mmol)
comprises a purely inorganic core, Mo O (μ -OH) (μ -O) ,
4
8
3
2
2
2
and water (60 mL), and the mixture was refluxed for 18 h under air
using an external oil bath as a heating source. The resultant white solid
was separated from the pink aqueous mother liquor (pH ≅ 2) by
filtration; washed with water, n-hexane, and diethyl ether (10 mL
each); vacuum-dried; and identified as 2 (0.42 g, 72%; based on Mo)
by elemental analysis, FT-IR spectroscopy, and PXRD.
attached to two peripheral oxo-bridged binuclear units,
Mo O (μ -O) (OH)(di-tBu-bipy) . Octanuclear 2 can also be
2
4
2
2
2
obtained by the hydrothermal treatment of an aqueous solution
containing MoO and di-tBu-bipy. Complex 2 was applied in
3
the catalytic epoxidation of R-(+)-limonene and methyl oleate,
which were chosen as substrates due to their relevance to the
use of plant biomass as a renewable feedstock for the chemical
Method C. A Teflon-lined stainless steel autoclave was charged with
(0.16 g, 0.34 mmol) and water (10 mL) and heated in an oven at
1
7
100 °C for 19 h. The resultant white solid was separated from the pink
aqueous mother liquor (pH ≅ 2) by filtration; washed with water, n-
hexane, and diethyl ether (10 mL each); vacuum-dried; and identified
as 2 (0.065 g, 68%; based on Mo) by elemental analysis, FT-IR
spectroscopy, and PXRD.
industry.
EXPERIMENTAL SECTION
Materials and Methods. The monomer [MoO Cl (di-tBu-bipy)]
1) was synthesized according to the published procedure. MoO3
■
(
2
2
8
Method D. A Teflon-lined stainless steel autoclave was charged with
(
99.5%, AnalaR), MoO Cl2 (Sigma-Aldrich), 4,4′-di-tert-butyl-2,2′-
MoO (0.34 g, 2.4 mmol), di-tBu-bipy (0.64 g, 2.4 mmol), and water
2
3
bipyridine (Sigma-Aldrich), diethyl ether (Sigma-Aldrich, puris, p.a.),
acetone (Fluka, puris, p.a.), toluene (Riedel-de-Haen, puris, p.a.), and
(25 mL) and heated in an oven at 160 °C for 3 days with rotation at
25 rpm. A light pink solid and small single crystals suspended in a
colorless solution (pH ≅ 5) were obtained. Crystals were directly
harvested from the reaction vessel and immediately immersed in
highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC
140/13) for X-ray diffraction analysis. The remaining solid material
was isolated by filtration; washed with water, n-hexane, and diethyl
ether (10 mL each); vacuum-dried; and identified as 2 (0.47 g, 70%;
based on Mo) by elemental analysis, FT-IR spectroscopy, and PXRD.
Single Crystal X-Ray Diffraction. A single-crystal of
[Mo O (OH) (di-tBu-bipy) ] (2) was mounted on a very thin
̈
dichloromethane (Fluka, puris, p.a.) were obtained from commercial
sources and used as received. Microanalyses (CHN) were provided by
the Mass Spectrometry Laboratory, New University of Lisbon. FT-IR
spectra were recorded as KBr pellets using a Unicam-Mattson 7000
spectrophotometer equipped with a DTGS CsI detector. Attenuated
total reflectance (ATR) spectra were measured on the same
instrument equipped with a Specac Golden Gate Mk II ATR accessory
having a diamond top-plate and KRS-5 focusing lenses. FT-Raman
spectra were recorded on a RFS-100 Bruker FT-Spectrometer
equipped with a Nd:YAG laser with an excitation wavelength of
8
22
4
4
9
Hampton Research CryoLoop with the help of a Stemi 2000
stereomicroscope equipped with Carl Zeiss lenses. Data were collected
at 150(2) K on a Bruker X8 Kappa APEX II CCD area-detector
diffractometer (Mo Kα graphite-monochromated radiation, λ =
13
1
064 nm and a laser power set to 94 mW. C cross-polarization (CP)
magic-angle-spinning (MAS) NMR spectra were obtained using a
Bruker Avance 400 spectrometer (9.4 T) at 100.62 MHz, with 3.6 μs
1
10
H 90° pulses, 2 ms contact time, spinning rates of 7 and 11 kHz, and
0.71073 Å) controlled by the APEX2 software package and equipped
5
(
s recycle delays. Chemical shifts are quoted in parts per million
ppm) with respect to TMS. 1H NMR spectra in solution were
recorded on a Bruker Avance II+ 400 MHz (UltraShieldTM Magnet)
with an Oxford Cryosystems Series 700 cryostream monitored
remotely using Cryopad. Because of the very small dimensions of
the available crystals (maximum dimension of ca. 0.01 mmthe real
11
3
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Inorganic Chemistry
Article
crystal dimensions could be smaller), in conjunction with the low
crystal symmetry and relatively large unit cell, diffraction at high angles
was very poor. The collected data set was ultimately composed of
frames integrated over a period of 300 s and with 1° per frame. Images
Hydrogen atoms bound to carbon were placed at their idealized
positions using appropriate HFIX instructions in SHELXL: 43 for the
aromatic and 137 for the terminal −CH methyl groups belonging to
3
the di-tBu-bipy ligands. All of these atoms were included in subsequent
refinement cycles in riding-motion approximation with isotropic
thermal displacement parameters (Uiso) fixed at 1.2 or 1.5 × Ueq of the
1
2
were processed using SAINT+, and data were corrected for
absorption by the multiscan semiempirical method implemented in
13
SADABS. Due to poor diffraction, the mean I/σ values were only
over unity for resolutions lower than 1.05 Å, hence the very high Rint
value for the employed data set (Table 1). The overall quality of the
data set may only be improved by using either a synchrotron or a
rotating anode source.
respective parent carbon atom (for CH (aromatic) and CH
respectively). The overall quality of the collected data set did not
allow the hydrogen atoms associated with the terminal (O13) and μ
3
,
-
3
bridging (O1) oxygen atoms to be sensibly located from difference
Fourier maps, and so these were also placed in calculated positions
using HFIX 147 and 13 instructions, respectively, in SHELXL and
were refined with Uiso = 1.5 × Ueq of the parent oxygen atom.
The last difference Fourier map synthesis showed the highest peak
1.500 eÅ ) and deepest hole (−1.676 eÅ ) located at 1.20 Å and
.96 Å from Mo1, respectively. Information concerning crystallo-
Table 1. Crystal and Structure Refinement Data for
[
Mo O (OH) (di-tBu-bipy) ] (2)
−3
−3
8
22
4
4
(
0
formula
fw
C H Mo N O
26
7
2
100
8
8
graphic data collection and structure refinement details is summarized
in Table 1. CCDC-856220 contains the supplementary crystallo-
graphic data for this paper (including structure factors). These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Catalysis. The catalytic epoxidation reactions were carried out in 5
mL borosilicate reactors immersed in a thermostatted oil bath at 55
°C, under magnetic stirring (800 rpm) and autogenous pressure. The
reagents were separately preheated to 55 °C prior to initializing the
catalytic reaction by adding the oxidant to the reactor containing the
olefin/catalyst. The solvents were distilled, and solvents and reagents
were predried using activated 3 or 4 Å molecular sieves. Typically, the
initial molar ratio of molybdenum/olefin/oxidant was 1:100:152 (18
μmol Mo). When 5.5 M tert-butylhydroperoxide in decane was the
oxidant solution used, the reactions were performed either without
additional solvent (TBHPdec) or with 1 mL of 1,2-dichloroethane
2261.12
triclinic
cryst syst
space group
a (Å)
P1
̅
12.90(2)
13.111(18)
13.46(2)
65.98(4)
85.61(5)
84.51(5)
2068(5)
1
b (Å)
c (Å)
α [deg]
β [deg]
γ [deg]
V [Å3]
Z
Dcalcd [g cm−3]
1.815
μ(Mo Kα) [mm−1]
cryst size [mm]
cryst type
1.252
0.01 × 0.01 × 0.01
colorless block
(
DCE), acetonitrile, ethyl acetate (EtOAc), ethanol (EtOH), or
θ range [deg]
index ranges
3.52 to 25.35
benzotrifluoride (BTF) as a cosolvent (TBHPdec/cosolvent).
Mixtures denoted as TBHP/DCE and TBHP/EtOH were prepared
by mixing 70% aqueous TBHP and a solvent and subsequently
eliminating the excess water using magnesium sulfate. TBHP
concentrations of these TBHP/cosolvent mixtures were determined
by iodometric titrations. After each 24 h batch run, the solid phase was
separated from the liquid phase by centrifugation, thoroughly washed
with hexane, dried at room temperature overnight, and characterized
by ATR FT-IR spectroscopy. The evolutions of the catalytic reactions
were monitored by using a Varian 3900 GC equipped with a flame
ionization detector and a DB-5 capillary column (30 m × 0.25 mm ×
0.25 μm); undecane and methyl decanoate were used as internal
standards for the reactions of DL-limonene and methyl oleate,
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.
−15 ≤ h ≤ 15, −15 ≤ k ≤ 15, −16 ≤ l ≤ 15
21721
reflns collected
independent reflns
7211 [Rint = 0.3014]
completeness to θ = 25.35° 95.2%
,
ab
final R indices [I > 2σ(I)]
R1 = 0.0995, wR2 = 0.2051
R1 = 0.3317, wR2 = 0.3114
m = 0.1181, n = 0.0
,
ab
final R indices (all data)
c
weighting scheme
−3
largest diff. peak and hole
1.500 and −1.676 eÅ
a
b
2
2 2
R1 = ∑∥F | − |F ∥/∑|F |. wR2 = (∑[w(F − F ) ]/
o
c
o
o
c
2
2
1/2
c
2
2
o
2
2
∑
2
[w(F ) ]) . w = 1/[σ (F ) + (mP) + nP] where P = (F
+
o
o
2
F )/3.
c
The crystal structure was solved using the Patterson synthesis
1
4
algorithm implemented in SHELXS-97, which allowed the
immediate location of the four crystallographically independent
molybdenum centers. All remaining non-hydrogen atoms were located
from difference Fourier maps calculated from successive full-matrix
RESULTS AND DISCUSSION
■
Synthesis of [Mo O (OH) (di-tBu-bipy) ] (2). The
8
22
4
4
2
14a,15
least-squares refinement cycles on F using SHELXL-97.
Non-
reaction of [MoO Cl (di-tBu-bipy)] (1) with water was carried
2
2
hydrogen atoms composing the [Mo O (OH) (di-tBu-bipy) ]
8
22
4
4
out either in a sealed glass vessel with microwave-assisted
heating (120 °C, 4.5 h, method A), in an open reflux system in
the air (18 h, oil bath heating, method B), or in a sealed Teflon-
lined stainless steel digestion bomb (autogenous pressure, 100
molecular unit were successfully refined using anisotropic displace-
ment parameters.
The oxidation states of the molybdenum centers were unequivocally
confirmed by using bond valence calculations following the theoretical
°
C, 19 h, method C). In each case, a white solid suspended in
16
17
models of Brese and O’Keeffe and of Brown and Altermatt and
based on the measured Mo−O and Mo−N interactions. Calculations
an acidic solution (pH ≅ 2) was obtained, which was recovered
by filtration, washed with water and organic solvents, and
vacuum-dried. Practically identical elemental analysis, FT-IR,
and PXRD data were obtained for all three solid products,
indicating that the heating method had no significant influence
on the outcome of the reaction (Figures S1 and S2 in the
Supporting Information). The same product could be
18
were performed using PLATON. The sums of the bond valences at
each metal atomic position are as follows (considering a +6 oxidation
state for each metal center): Mo1 +5.92, Mo2 +5.80, Mo3 +6.19, and
Mo4 +6.16. These results clearly suggest that the oxidation states of
the four crystallographically independent metal centers are +6. This
structural evidence supports the existence of four hydroxyl groups (as
ultimately modeled into the crystal structure) and is well in line with
synthesized from MoO , di-tBu-bipy, and H O in the molar
3
2
the results previously described by us for the related octanuclear
ratio 1:1:580 at 160 °C for 3 days in a Teflon-lined digestion
bomb rotating at 25 rpm. In addition to microcrystalline
3
[
Mo O (di-tBu-bipy) ] windmill-type complex.
8
24
4
3
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Inorganic Chemistry
Article
material, this reaction gave very small single crystals (maximum
limitations concerning the atomic parameters of the crystal
model (please see the Experimental Section for full details).
Nevertheless, analysis of the available data set produced a
structural model of 2 fully consistent with the other
characterization data, including powder X-ray diffraction of
the microcrystalline powders obtained by methods A−D
(Figure S2).
dimension of ca. 0.01 mm), which were subjected to X-ray
19
diffraction analyses (described below). When the latter
reaction was carried out without rotation, the main product
was contaminated with unreacted MoO . Performing the
3
reaction of MoO with di-tBu-bipy under conditions equivalent
3
to those used in methods A and B gave either a mixture of
unreacted MoO + compound 2 (method A) or a mixture of
Compound 2 crystallizes as discrete centrosymmetric
octanuclear units, which were ultimately formulated as
[Mo O (OH) (di-tBu-bipy) ]. The octanuclear complex can
3
compound 2 and the dinuclear complex [{MoO (μ-O)(di-tBu-
2
19
bipy)} ] (method B).
2
8
22
4
4
On the basis of the characterization data and the crystal
structure solution described below, the product from methods
A−D is formulated as [Mo O (OH) (di-tBu-bipy) ] (2)
be divided into two distinct sections (Figure 1): the inner core,
8
22
4
4
(
Scheme 1). Yields were 61% for method A and 70 ± 2% for
Scheme 1. Synthesis Routes to the Octanuclear Complex
a
[
Mo O (OH) (di-tBu-bipy) ] (2)
8 22 4 4
Figure 1. Schematic representation of the centrosymmetric
[
Mo O (OH) (di-tBu-bipy) ] molecular unit viewed in perspective
8 22 4 4
a
along two distinct crystallographic directions. Atoms belonging to the
asymmetric unit are represented as thermal ellipsoids drawn at the
The purely inorganic core Mo O (μ -OH) (μ -O) is highlighted in
4
8
3
2
2
2
red.
3
0% probability level. Hydrogen atoms are represented as small
spheres with arbitrary radii.
methods B−D. The microcrystalline powders were phase-pure
and the PXRD patterns were in agreement with a simulated
pattern calculated from the crystal structure data for 2 (Figure
S2). The formula for 2 implies the presence of free ligand in the
acidic mother liquors obtained for all methods A−C.
Accordingly, in previous work, crystals of 4,4′-di-tert-butyl-
composed of four Mo centers, Mo O (μ -OH) (μ -O) , is
4
8
3
2
2
2
purely inorganic and is attached to two peripheral symmetry-
related oxo-bridged dinuclear units containing N,N′-chelated di-
tBu-bipy moieties in the coordination spheres of each metal
center. Because of the considerable steric repulsion associated
with these bulky organic ligands, the octanuclear complex
adopts a highly distorted form strongly resembling an “S”-
shaped molecular unit. The related octanuclear [Mo O (di-
2
,2′-dipyridinium dichloride were isolated as a byproduct of the
20
microwave-assisted reaction of 1 with water.
In common with the molecular precursor 1 and the
8
24
21
oxodiperoxo complex [MoO(O ) (di-tBu-bipy)], the FT-IR
2
2
tBu-bipy) ] windmill-type complex can also be divided into an
4
spectrum of 2 exhibits a strong pyridyl ring stretching vibration
inorganic core and a surrounding hybrid sphere containing the
same N,N′-chelated organic ligand. However, in this latter
−1
3
at 1615 cm , which is indicative of the bidentate coordination
VI
mode of the di-tBu-bipy ligands to Mo centers. In addition, a
complex, the Mo (μ -O) inorganic core is significantly more
4
3
4
complex pattern of bands is displayed in the 600−950 cm−
range attributable to Mo−O modes. Compound 2 is slightly
soluble in acetonitrile, dichloromethane, and chloroform and
insoluble in water, ethanol, n-hexane, and diethyl ether.
Crystal Structure Description of [Mo O (OH) (di-tBu-
1
symmetrical, adopting the typical metal distribution of a
cubane-type complex. Consequently, the peripheral Mo
octahedra holding the coordinated organic ligands are
uniformly distributed around the core, leading to a highly
symmetrical complex.
8
22
4
bipy) ] (2). Because of the very small dimensions of the
The centrosymmetric [Mo O (OH) (di-tBu-bipy) ] com-
4
8
22
4
4
crystals of 2 isolated from method D, X-ray diffraction at high
angles was poor, meaning that reasonably good I/σ values
could not be obtained for all reflections, with this being directly
translated into a high Rint value (Table 1) and, consequently,
plex is composed of four crystallographically independent Mo
centers: Mo1 and Mo2 compose the aforementioned
Mo O (μ -OH) (μ -O) inorganic core, while Mo3 and Mo4
4
8
3
2
2
2
are peripheral to this core, each being coordinated to one N,N′-
3
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dx.doi.org/10.1021/ic202640a | Inorg. Chem. 2012, 51, 3666−3676

Inorganic Chemistry
Article
chelated di-tBu-bipy ligand (Figure 2). The Mo1 and Mo2
Table 2. Bond Distances (in Å) for the Four
centers exhibit significantly distorted square pyramidal
Crystallographically Independent Mo Coordination
Environments Present in [Mo O (OH) (di-tBu-bipy) ]
8
22
4
4
a
(
2)
Mo1−O1
2.283(14)
1.984(17)
1.860(16)
1.689(14)
1.688(14)
1.836(17)
1.716(13)
1.688(16)
2.174(15)
2.231(19)
2.300(17)
Mo2−O1
Mo2−O2
Mo2−O5
Mo2−O6
Mo2−O7
Mo4−O10
Mo4−O11
Mo4−O12
Mo4−O13
Mo4−N3
Mo4−N4
1.935(15)
2.073(15)
1.718(16)
1.720(14)
1.942(17)
1.764(16)
1.754(15)
1.683(15)
2.273(15)
2.264(16)
2.291(19)
i
Mo1−O1
Mo1−O2
Mo1−O3
Mo1−O4
Mo3−O7
Mo3−O8
Mo3−O9
Mo3−O10
Mo3−N1
Mo3−N2
a
Symmetry transformation used to generate equivalent atoms:
i) 1 − x, 2 − y, −z.
(
Table 3. Bond Angles (in Degrees) for the Four
Crystallographically Independent Mo Coordination
Environments Present in [Mo O (OH) (di-tBu-bipy) ]
8
22
4
4
a
(
2)
i
O1 −Mo1−O1
O2−Mo1−O1
70.9(6)
72.9(6)
139.7(6)
104.2(7)
101.6(6)
99.8(7)
153.7(6)
102.6(7)
104.5(7)
102.1(7)
85.8(6)
161.2(7)
93.4(6)
108.2(7)
90.4(6)
87.2(7)
153.3(6)
100.1(7)
104.1(7)
161.6(6)
85.9(7)
86.8(7)
83.4(6)
75.4(6)
69.0(6)
O1−Mo2−O2
O1−Mo2−O7
O5−Mo2−O1
O5−Mo2−O2
O5−Mo2−O6
O5−Mo2−O7
O6−Mo2−O1
O6−Mo2−O2
O6−Mo2−O7
O7−Mo2−O2
O10−Mo4−O13
O10−Mo4−N3
O10−Mo4−N4
O11−Mo4−O10
O11−Mo4−O13
O11−Mo4−N3
O11−Mo4−N4
O12−Mo4−O10
O12−Mo4−O11
O12−Mo4−O13
O12−Mo4−N3
O12−Mo4−N4
O13−Mo4−N4
N3−Mo4−O13
N3−Mo4−N4
76.5(6)
148.2(6)
106.6(7)
104.2(7)
107.5(8)
101.6(8)
91.6(7)
148.1(6)
93.7(7)
82.7(6)
90.6(6)
87.8(6)
157.2(7)
105.9(7)
83.8(6)
151.8(7)
89.9(7)
103.4(7)
107.0(7)
158.7(6)
93.0(6)
87.0(7)
74.5(6)
71.3(5)
71.2(6)
i
O2−Mo1−O1
O3−Mo1−O2
O3−Mo1−O1
i
O3−Mo1−O1
O4−Mo1−O1
O4−Mo1−O2
O4−Mo1−O3
i
O4−Mo1−O1
O7−Mo3−O10
O7−Mo3−N1
O7−Mo3−N2
O8−Mo3−O7
O8−Mo3−O10
O8−Mo3−N1
O8−Mo3−N2
O9−Mo3−O7
O9−Mo3−O8
O9−Mo3−O10
O9−Mo3−N1
O9−Mo3−N2
O10−Mo3−N1
O10−Mo3−N2
N1−Mo3−N2
Figure 2. (a) Detailed view of the octanuclear core of the
centrosymmetric [Mo O (OH) (di-tBu-bipy) ] molecular unit.
8
22
4
4
Atoms belonging to the asymmetric unit are represented as thermal
ellipsoids drawn at the 30% probability level. Hydrogen atoms are
represented as small spheres with arbitrary radii. The labeling scheme
is provided for all atoms composing the first coordination spheres of
the four crystallographically independent Mo centers. (b) Polyhedral
representation of the same octanuclear core. (c) Intermetallic distances
of oxygen-bridged close Mo centers. For selected bond lengths and
angles on the represented coordination polyhedra, see Tables 2 and 3,
respectively. Symmetry transformation used to generate equivalent
atoms: (i) 1 − x, 2 − y, −z.
coordination environments with the overall Mo−O bond
lengths ranging from 1.688(14) to 2.283(14) Å (Table 2 and
Figure 2a), and the cis O−Mo−Obasal and O−Mo−Oapical
polyhedral angles lying in the 70.9(6)−102.6(7)° and
a
Symmetry transformation used to generate equivalent atoms:
i) 1 − x, 2 − y, −z.
(
Mo···Mo distances ranging from 3.200(1) to 3.659(1) Å
Figures 2b and c). A search in the Cambridge Structural
9
9.8(7)−107.5(8)° ranges, respectively (Table 3). The
(
{
MoO } coordination environment of Mo1 is significantly
22
5
Database (version 5.32, November 2011 with four updates)
revealed the existence of only a handful of structures containing
edge-shared {MoO } polyhedra, in particular, various assem-
blies of three {MoO }’s fused to an octahedron,
{
more distorted than that of Mo2 due to its close proximity to a
symmetry-related (by inversion) Mo1 [Mo1···Mo1 distance of
5
2
n,23
3.480(1) Å, Figure 2c]. This creates a significant electrostatic
or two
5
2
g,24
repulsion between the two spatially close Mo cores,
concomitantly leading to a highly distorted coordination
environment.
MoO }’s fused to two octahedra;
however, cores based on
5
four edge-fused {MoO } units such as that found in 2 have yet
5
to be described. Another interesting feature concerns the fact
that the MoO apical bonds of the Mo1 and Mo2 polyhedra
are cis to each other with respect to the mean plane of the two
bases of the pyramids (Figure 2b). Even though the Mo1
The Mo O (μ -OH) (μ -O) inorganic core is composed of
4
8
3
2
2
2
a truly unique assembly of four {MoO } distorted square
5
pyramids connected to each other via edge-sharing, with the
3
670
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Inorganic Chemistry
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Figure 3. (a) Schematic representation of the way adjacent octanuclear [Mo O (OH) (di-tBu-bipy) ] complexes close pack along the [010]
8
22
4
4
direction of the unit cell, with the process being mediated by various intra- and intermolecular weak C−H···O interactions and bifurcated O−H···O
hydrogen bonds. (b) Detailed view of the supramolecular contacts present between the octanuclear core of adjacent centrosymmetric
[
Mo O (OH) (di-tBu-bipy) ] molecular units. For geometrical details on the represented contacts see Table 4. Symmetry codes associated with
8 22 4 4
symmetry-equivalent atoms have been omitted for clarity.
coordination environment is more distorted than that of Mo2,
the two Mo centers are raised from the average coordination
basal planes by approximately the same distance (0.42 Å for
Mo1 and 0.49 Å for Mo2).
the complexes. In addition, the terminal O13 hydroxyl groups
bound to Mo4 (see previous paragraph) are engaged in
relatively strong O−H···O hydrogen bonding interactions with
the O8 oxo groups of an adjacent inorganic core (Figure 3b).
The strengths of these intercore connections are further
cooperatively increased by the existence of a number of
intermolecular C−H···O interactions: for example, the C1 atom
of a coordinated di-tBu-bipy ligand interacts with the O6 oxido
group via a strong [dC···O = 3.17(2) Å] and relatively directional
[∠(CHO) = 146°] contact. A number of weak intramolecular
C−H···O contacts also exist in the crystal structure, with the
most relevant being tabulated in Table 4. The overall
combination of all of these supramolecular contacts leads to
the formation of robust one-dimensional chains running
parallel to the b-axis of the unit cell. Individual chains close
pack in the ac plane mediated by purely geometrical reasons
because the contacts between spatially close di-tBu-bipy organic
molecules are, essentially, of the van der Waals type (Figure 4).
The two remaining Mo centers, Mo3 and Mo4, exhibit highly
distorted octahedral {MoN O } coordination environments,
2
4
with each being coordinated to one N,N′-chelated di-tBu-bipy
ligand and four oxygen-containing groups. While for Mo3 two
of these oxygens belong to μ -bridging oxido groups [establish-
2
ing connections to the inorganic core (O7) and to the
neighboring Mo4 metal center (O10), Figure 2a], for Mo4 one
of these groups is a terminal charge-balancing hydroxyl moiety
(
O13, Figure 2a). The octahedral coordination environments of
Mo3 and Mo4 are highly distorted due to the presence of two
MoO groups in their coordination spheres, which exert a
marked trans effect on the coordination polyhedra by displacing
the metal cations from their geometrical centers: for the two
coordination polyhedra, while the Mo−(N,O) bond lengths
range from 1.683(15) to 2.300(17) Å, the cis and trans (N,O)−
Mo−(N,O) internal octahedral angles are found in the
Table 4. Hydrogen Bonding and Close Contact (Both Inter-
and Intramolecular) Geometrical Details (Distances in Å
6
9.0(6)−108.2(7)° and 151.8(7)−161.6(6)° ranges, respec-
tively (Tables 2 and 3). Because the connections between
and Angles in Degrees) for [Mo
8
O22(OH)
4
(di-tBu-bipy)
4
]
a
(
2)
adjacent {MoN O } octahedra, and between the Mo O (μ -
2
4
4
8
3
OH) (μ -O) inorganic core and Mo3, are based on corner-
2
2
2
D−H···A
d(D···A)
∠(DHA)
sharing (Figure 2b), the Mo···Mo separations are concom-
itantly longer: 3.724(1) and 3.773(1) Å for Mo2···Mo3 and
Mo3···Mo4, respectively (Figure 2c).
i
O13−H13···O2
O13−H13···O6
2.73(2)
2.93(2)
3.28(2)
3.17(2)
3.05(2)
3.43(2)
3.49(2)
121
124
169
146
127
165
153
C10−H10···O2 (intramolecular)
Despite the highly asymmetric shape of the octanuclear
i
C1−H1···O6 (intermolecular)
[
Mo O (OH) (di-tBu-bipy) ] complex, the close packing of
ii
8
22
4
4
C7−H7···O9 (intermolecular)
individual units is relatively efficient, being moderated by a
number of supramolecular contacts. As shown in Figure 3a,
individual complexes pack close in a parallel fashion along the
ii
C16−H16C···O9 (intermolecular)
iii
C32−H32B···O4 (intermolecular)
a
Symmetry transformations used to generate equivalent atoms:
i) 1 − x, 1 − y, −z; (ii) 2 − x, 1 − y, −z; (iii) 1 − x, 1 − y, 1 − z.
[
010] direction of the unit cell in such a way that the inorganic
(
cores are clearly separated from the external hybrid portion of
3
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Figure 4. Crystal packing of the [Mo O (OH) (di-tBu-bipy) ] compound viewed in perspective along the [010] direction of the unit cell. Weak C−
8
22
4
4
H···O interactions and O−H···O hydrogen bonds, which mediate the crystal packing along the b-axis, have been omitted for clarity.
Catalytic Epoxidation of Bioderived Olefins. The
Reaction of DL-Limonene. Complex 2 was applied in the
catalytic epoxidation of DL-limonene (Lim), one of the most
common naturally occurring monoterpenes (present in citrus
oil), to give 1,2-epoxy-p-menth-8-ene (LimOx, Scheme 2),
conversion and 24 h reaction (no LimDiol was detected). For
the two solvents, the reaction mixtures were biphasic liquid−
solid. Differences in the solvent polarity may influence the
solubility of the metal species, and consequently the reaction
rate (based on the dipole moment, BTF (2.86 D) is slightly
more polar than that for DCE (1.83 D)). For the system
TBHP/DCE (in which the oxidant solution was prepared by
mixing 70% aqueous TBHP and DCE, followed by the removal
Scheme 2. Epoxidation Reactions Studied in This Work
of excess water using MgSO ), LimDiOx was formed in 8%
4
yield at 24 h (Figure 5B). These results indicate very high
regioselectivity toward the epoxidation of the endocyclic double
bond relative to the exocyclic one. The selectivity to LimOx
tends to decrease for conversions of Lim greater than ca. 80%
(
Figure 5B): for all tested reaction systems excluding
TBHPdec/BTF, this trend is accompanied by the formation
of 1-methyl-4-(1-methylethenyl)-1,2-cyclohexanediol (denoted
LimDiol), reaching a maximum yield at 24 h of 11% for the
TBHPdec/no cosolvent system. For TBHPdec/BTF, the
decrease in selectivity to LimOx was accompanied by the
formation of LimDiOx (no LimDiol was detected). The
decrease in LimOx selectivity and concomitant increase in
selectivities to LimDiol and LimDiOx (TBHP/DCE system,
Figure 5B) suggest that LimOx is a common intermediate
product in the formation of LimDiol and LimDiOx. The
formation of LimDiol may take place via the hydrolysis of
LimOx with residual water present in the reaction media.
Accordingly, the presence of water influences the product
distribution by favoring the LimOx to LimDiol conversion,
which may take place in parallel with the LimOx to LimDiOx
conversion. The comparable LimOx yields at 24 h for the three
oxidant/solvent systems suggest that it may be feasible to use
TBHPaq (coupled with a drying procedure) instead of
TBHPdec and to substitute decane (bp ca. 174 °C) for
solvents with lower boiling points, thereby avoiding energy-
intensive separation/purification processes.
which is an interesting cycloaliphatic epoxide for the synthesis
of pharmaceuticals, fragrances, and food additives; the
manufacture of copolymers by reaction with CO ; and
2
7a,25
applications in metal coatings and varnishes.
The systems TBHPdec/no cosolvent, TBHPdec/DCE, and
TBHP/DCE (please see the Experimental Section for details)
gave comparable conversions of Lim after 6 h/24 h of reaction,
and LimOx was always the main product formed in similar
amounts at 24 h (84−89% selectivity at 94−96% conversion,
reached at 24 h, Figure 5); without a catalyst, LimOx was
formed in less than 4% yield after 24 h of reaction. A
comparison of the TBHPdec/no cosolvent and TBHPdec/
DCE systems indicates no significant dilution effects on the
observed catalytic activity. For the TBHPdec/BTF system, the
initial reaction rate of Lim was lower than that observed for
TBHPdec/DCE, although similarly high conversions were
reached after 6 h; LimOx and 1,2:8,9-diepoxy-p-menthane
The catalytic performance of 2 was compared with those for
MoO (freshly prepared by using MoO Cl as the source of
3
2
2
molybdenum and following the conditions used in method D,
omitting the organic ligand) and a commercial ammonium
heptamolybdate ((NH ) Mo O ·4H O, 99% p.a., Merck),
4
6
7
24
2
(
LimDiOx) were formed in 76% and 22% yield at 98%
under similar reaction conditions using the system
3
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Inorganic Chemistry
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in a few cases: 88% LimOx yield at 24 h/55 °C for
4
[
Mo(CO) (ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}], 82%
4
5
35
LimOx yield at 35 min for [(η -C H )Mo(CO) Me], and
9
7
3
9
5−100% LimOx yield at 24 h/55 °C for [MoO X L ], where
2
2 2
36
X = Cl or OSiPh and L = 2-(1-butyl-3-pyrazolyl)pyridine).
3
2
With the aim of replacing DCE or BTF with a more
environmentally attractive, bioderived solvent, the epoxidation
of Lim was carried out using the system TBHP/EtOH, complex
2
, and a reaction temperature of 55 °C. The reaction of Lim
was considerably slower for TBHP/EtOH than for TBHP/
DCE (Figure 5A), and selectivity to LimOx decreased from
4
7% at 27% conversion to 6% at 63% conversion (reached at 24
h reaction), with the concomitant formation of several
byproducts including LimDiol (39% selectivity at 63%
conversion, Figure 5B); without a catalyst, less than 3% Lim
conversion was reached at 24 h under similar reaction
conditions. A possible explanation for the slower reaction of
Lim when EtOH is used as a cosolvent instead of (non-
coordinating) DCE is that the EtOH molecules may compete
37
with the reagents in the coordination to the metal center. It is
worth mentioning that for the two solvents the reaction
mixtures were similarly biphasic liquid−solid. The enhanced
formation of LimDiol for the system TBHP/EtOH may be
partly due to the less efficient removal of water from this
mixture prior to the catalytic reaction. Besides LimDiol, ethoxy
alcohol products were formed possibly via the ethanolysis of
the epoxide products. The formation of ethoxy alcohol
products has been reported for the reaction of LimOx with
38
ethanol in the presence of BF ·O(C H ) or EtONa.
3
2
5 2
The tested reaction systems were always biphasic solid−
liquid, and the ATR FT-IR spectra of the recovered solids were
similar to that of the fresh catalyst (Figure S3). The solid
recovered from the catalytic reaction using the system
TBHPdec/DCE was reused in a second 24 h batch run,
under similar reaction conditions, giving comparable con-
version (74%/95%) and LimOx selectivity (89%/88%) at 6 h/
Figure 5. (A) Kinetic profiles for the reaction of DL-limonene (Lim)
at 55 °C using the systems TBHPdec/no cosolvent (black +),
TBHPdec/BTF (orange □), TBHPdec/DCE (blue ), TBHP/DCE
2
4 h to those observed for run 1 (92%/89% LimOx selectivity
at 79%/96% conversion). The “productive consumption” of
TBHP was ascertained by iodometric titrations, which indicated
no measurable decomposition of TBHP (to give tert-butanol
and molecular oxygen) in the presence of 2 (without olefin), at
(
red ×), and TBHP/EtOH (green *). (B) Dependence of selectivity
to LimOx (solid-filled symbols), LimDiol (white-filled symbols), and
LimDiOx (+,×) on Lim conversion using the systems TBHPdec/no
cosolvent (■,□), TBHPdec/BTF (orange ■,+), TBHPdec/DCE
5
5 °C.
(
▲
blue ⧫,◊), TBHP/DCE (red ●,○,×), and TBHP/EtOH (green
,△).
The homogeneous/heterogeneous nature of the catalytic
reaction performed using the system TBHP/DCE was checked
by filtering the reaction mixture through a 0.2 μm PTFE
membrane after 1 h and then leaving the filtrate to react at 55
TBHPdec/DCE. After 6 h of reaction, conversion of Lim was
negligible for both MoO and the heptamolybdate. When
3
°
C for a further 5 h. The increment in conversion between 1
compared against previously investigated oxomolybdenum
complexes or their carbonyl precursors tested as catalysts in
the same reaction, using TBHP as an oxidant at 55 °C, the
catalytic performance of complex 2 seems quite outstanding in
terms of the LimOx yield reached (at 24 h, unless otherwise
and 6 h was 32%, which is comparable to that observed in the
presence of the solid (36%). Hence, the catalytic reaction takes
place in the homogeneous phase. Compound 2 did not dissolve
completely even when the amount charged to the reactor was
decreased from 1 mol % (Mo/Lim) to 0.2 mol %.
Unsurprisingly, therefore the kinetic behavior of the homoge-
neous phase catalytic reaction using 0.2 mol % was similar
(94%/91% LimOx selectivity at 74%/91% conversion for 6 h/
24 h) to that measured for 1 mol % (94%/84% LimOx
selectivity at 79%/96% conversion).
stated): [MoO X L ] (for X = Cl, LimOx yields of 39% for L =
2
2 2
26
27
dimethylformamide, 74% for L = {OP(CH CH )(Ph) },
2
3
2
28
6
{
5% for L = bidentate salen ligand, 15−20% for L =
2
2
29
PhCHNCH CH NCHPh}; for X = CH , LimOx yield of
2 2 3
4
7% at 4 h for L = N,N-p-tolyl-2,3-dimethyl-1,4-diazabuta-
2
3
0
31
diene ); [MoO L] where L is a tetradentate oxazoline ligand
The Reaction of Methyl Oleate. Fatty acid methyl ester
epoxides are bioderived oleochemicals with a significant impact
in the chemical industry due to their wide applications profile,
e.g., as plasticizers and stabilizers in the production of polyvinyl
chloride (PVC), as plastic additives, and as intermediates in the
production of biodegradable lubricants and polyols for
2
28
(59% LimOx yield) or a tetradentate salen ligand (50%
3
LimOx yield); [CpMo(CO) L] where n = 2 and L = η -C H
n
3
5
32
(70% of LimOx plus LimDiOx) or n = 3 and L = Cl (21%
33
LimOx yield at 6 h); [Mo O (μ -O)Cl (pyrazole) ] (59%
LimOx yield at 6 h). High LimOx yields have been reported
2
4
2
2
4
34
3
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Inorganic Chemistry
Article
7a,b,39
polyurethane production.
The industrial scale epoxidation
of unsaturated plant oils is carried out by the Prileshajev
reaction using either preformed or in situ generated short chain
peroxyacids (e.g., peracetic acid) which react with the
unsaturated fatty acid CC double bond to give the
7a
corresponding epoxide. Some drawbacks of this process
include the wide range of byproducts formed (which result in
more demanding separation/purification steps and enhanced
formation of waste products), equipment corrosion hazards
(
presence of strong acid in an oxidative environment), and the
need for neutralization processes downstream. A potentially
interesting alternative that may render the epoxidation of
vegetable oils more selective is the use of catalysts based on
7
c,d,39b
high-valent transition metals such as molybdenum(VI).
The catalytic potential of 2 for the production of biobased
epoxidized vegetable oils was investigated using methyl oleate
(
denoted Ole) as a model substrate, which is a monounsatu-
rated fatty acid ester present in, for example, methyl oleate
soybean oil. The oxygen donor was TBHPdec, and the
cosolvent was DCE, BTF, EtOAc, or CH CN, at 55 °C. The
3
main reaction product was always methyl 9,10-epoxystearate
(
(
OleOx) formed in at least 91% selectivity at high conversions
up to 94%, Figure 6); without a catalyst, no reaction occurred
under similar reaction conditions. The highest initial catalytic
activity was observed for DCE as a cosolvent, and BTF led to
the highest OleOx yield (92%) at high conversion (99% OleOx
selectivity at 94% conversion, reached at 24 h reaction). When
EtOAc or CH CN was used as a cosolvent, the OleOx
3
selectivity was 95−97% at ca. 60% conversion (24 h reaction).
As found with Lim as the substrate, these reaction systems were
always biphasic solid−liquid, and the ATR FT-IR spectra of the
solids recovered after a 24 h batch run were similar to that of 2
(Figure S3). The slower reaction for the cosolvents EtOAc and
CH CN may be due to their ability to compete with the
3
reagents in the coordination to the metal center, in parallel with
that discussed above for EtOH as cosolvent in the reaction of
Lim.
Figure 6. (A) Kinetic profiles for the reaction of methyl oleate (Ole),
and (B) dependence of selectivity to OleOx on Ole conversion, using
the systems TBHPdec/DCE (blue △), TBHPdec/BTF (orange +),
To the best of our knowledge, the epoxidation of pure
methyl oleate using molybdenum-based catalytic systems has
not been reported. Wang and co-workers reported that the
epoxidation of a mixture of methyl oleate and methyl lineolate
with H O /NaHCO in the presence of the complex
TBHPdec/EtOAc (red ○), and TBHPdec/CH CN (green ×).
3
pathways, which may partly explain the fairly high catalyst
stability observed.
2
2
3
[
MoO(O ) (8-quinilinol) ] gave 92% total selectivity to
2 2 2
epoxides at 72% conversion, using CH CN as a cosolvent, at
3
7
d
CONCLUSIONS
2
h/30 °C. Sobczak and Zio
́
łkowski investigated the
■
epoxidation of oleic acid with organic hydroperoxides in the
The present work has reinforced the notion that the hydrolysis
and condensation of complexes of the type [MoO Cl L] is a
p r e s e n c e o f t h e c o m p l e x e s [ M o ( C O ) ] ,
6
2
2
H [Mo O (oxalate) (H O) ]·4H O·(CH ) CO,
potentially interesting route to molybdenum oxide/organic
assemblies with structures and properties directed to a large
extent by the type of organic ligand and its substituents. Thus,
for reactions with water at 100−120 °C, the hybrid
2
2
4
2
2
2
2
3
2
[
MoO (acac) ], and [MoO (SAP)(EtOH)] (SAP = dianionic
2 2 2
tridentate salycylideneiminophenolate chelating ligand); the
latter complex exhibited the best catalytic performance, giving
ca. 87% epoxide selectivity at 67% TBHP conversion, at 260
molybdenum oxide/bipyridine material {[MoO (2,2′-bipy)]-
3
7
e
min/80 °C. More recently, it was reported that the use of
[MoO (H O)]} is obtained with L = 2,2′-bipyridine, while the
3
2
n
[
MoO (acac) ] as a catalyst in the epoxidation of soybean oil
octanuclear species [Mo O (OH) (di-tBu-bipy) ] (2) is
2
2
8 22 4 4
with TBHP led to 77% epoxide selectivity at 70% conversion (2
h/110 °C).
obtained with L = 4,4′-di-tert-butyl-2,2′-bipyridine. The
Mo O (μ -OH) (μ -O) inorganic core in 2 is composed of a
7
c
4
8
3
2
2
2
The structure of compound 2 consists of a type of clustered
molybdenum oxide/organic assembly, and therefore steric
effects may partly explain the higher reactivity of Lim in
comparison to the bulkier substrate Ole, under similar reaction
conditions (TBHPdec/DCE). On the other hand, these
structural features of 2 may hinder catalyst autodegradation
truly unique assembly of four {MoO } distorted square
5
pyramids connected to each other via edge-sharing. In a
manner analogous to that commonly used in sol−gel chemistry,
hydrolysis of the Mo−Cl bonds in the molecular precursors
likely gives reactive Mo−OH species that condense through
oxolation and/or olation reactions to give metal-oxo clusters,
3
674
dx.doi.org/10.1021/ic202640a | Inorg. Chem. 2012, 51, 3666−3676

Inorganic Chemistry
Article
oligomers, and polymers assembled via M−O−M and/or M−
OH−M bridges. Ongoing work in our laboratories has
indicated that a wide range of structurally diverse compounds
are accessible by using this approach. These molybdenum
oxide/organic assemblies are promising catalysts for the
epoxidation of nonfunctionalized olefins, such as the conversion
of bioderived olefins into added value epoxides studied in the
present work.
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̧
2
(
̧
(
ASSOCIATED CONTENT
Supporting Information
■
S
̧
*
(
Crystallographic information file (CIF) for compound 2, FT-IR
and powder XRD patterns of 2 synthesized by methods A−D,
and ATR FT-IR spectra of the solids recovered at the end of
the catalytic runs. This material is available free of charge via
the Internet at http://pubs.acs.org.
M. J. Chem. Soc. Chem. Commun. 1995, 655. (b) Arzoumanian, H.;
Agrifoglio, G.; Krentzien, H. New J. Chem. 1996, 20, 699.
(
c) Arzoumanian, H.; Maurino, L.; Agrifoglio, G. J. Mol. Catal. A:
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Catal. Lett. 2009, 129, 350. (e) Ferreira, P.; Xue, W.-M.; Bencze, E.;
Herdtweck, E.; Kuhn, F. E. Inorg. Chem. 2001, 40, 5834. (f) Kuhn, F.
E.; Xue, W.-M.; Al-Ajlouni, A.; Santos, A. M.; Zang, S.; Romao, C. C.;
̧
́
AUTHOR INFORMATION
Corresponding Author
E-mail: igoncalves@ua.pt.
■
̈
̈
̃
Eickerling, G.; Herdtweck, E. Inorg. Chem. 2002, 41, 4468. (g) Kodama,
S.; Nomoto, A.; Yano, S.; Ueshima, M.; Ogawa, A. Inorg. Chem. 2011,
*
Notes
5
0, 9942. (h) Kodama, S.; Hashidate, S.; Nomoto, A.; Yano, S.;
Ueshima, M.; Ogawa, A. Chem. Lett. 2011, 40, 495.
7) (a) Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 2411.
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
(b) Meier, M. A. R.; Metzger, J. O.; Schubert, U. S. Chem. Soc. Rev.
2007, 36, 1788. (c) Farias, M.; Martinelli, M.; Bottega, D. P. Appl.
Catal. A: Gen. 2010, 384, 213. (d) Cai, S.-F.; Wang, L.-S.; Fan, C.-L.
We are grateful to the Fundaca
̧
o
̃
para a Cien
cia e Inovaca
mento do Estado (OE), and Fundo Europeu de
Desenvolvimento Regional (FEDER) for general funding
̂
cia e a Tecnologia
(
FCT), the Programa Operacional Cien
̂
̧ o
̃
(POCI)
Molecules 2009, 14, 2935. (e) Sobczak, J. M.; Zioł
Catal. A: Gen. 2003, 248, 261.
8) Monteiro, B.; Gago, S.; Neves, P.; Valente, A. A.; Gonca
Pereira, C. C. L.; Silva, C. M.; Pillinger, M. Catal. Lett. 2009, 129, 350.
́
kowski, J. J. Appl.
2
010, Orca̧
(
̧ lves, I. S.;
(
(
project PTDC/QUI/65427/2006) and for financial support
FCT) towards the purchase of the single-crystal diffractom-
(9) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615.
eter. We acknowledge the FCT for a Ph.D. grant to T.R.A.
(10) APEX2, Data Collection Software, version 2.1-RC13; Bruker
(
(
SFRH/BD/64224/2009) and a postdoctoral grant to P.N.
SFRH/BPD/73540/2010).
AXS; Delft, The Netherlands, 2006.
11) Cryopad, Remote monitoring and control, version 1.451; Oxford
(
Cryosystems: Oxford, UK, 2006.
(12) SAINT+, Data Integration Engine, version 7.23a; Bruker AXS:
Madison, WI, 2005.
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