Synthesis and characterization of CpMo(CO)3(CH 2-pC6H4-CO2CH3) and its inclusion compounds with methylated cyclodextrins. Applications in olefin epoxidation catalysis

Article abstract of DOI:10.1016/j.jorganchem.2012.12.019

The cyclopentadienyl molybdenum tricarbonyl complex CpMo(CO) ₃(CH₂-pC₆H₄-CO₂CH ₃) (1) (Cp = η⁵-C₅H₅) and its 1:1 inclusion complexes with heptakis(2,6-di-O-

Full text of DOI:10.1016/j.jorganchem.2012.12.019

Accepted Manuscript
Synthesis and Characterization of CpMo(CO)3(CH2-pC6H4-CO2CH3) and
its Inclusion Compounds with Methylated Cyclodextrins. Applications in Olefin
Epoxidation Catalysis
Ana C. Gomes, Sofia M. Bruno, Cátia Tomé, Anabela A. Valente, Martyn Pillinger,
Marta Abrantes, Isabel S. Gonçalves
PII:
S0022-328X(12)00719-X
10.1016/j.jorganchem.2012.12.019
JOM 17826
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 September 2012
Revised Date: 13 December 2012
Accepted Date: 14 December 2012
Please cite this article as: A.C. Gomes, S.M. Bruno, C. Tomé, A.A. Valente, M. Pillinger, M. Abrantes,
I.S. Gonçalves, Synthesis and Characterization of CpMo(CO)3(CH2-pC6H4-CO2CH3) and its Inclusion
Compounds with Methylated Cyclodextrins. Applications in Olefin Epoxidation Catalysis, Journal of
Organometallic Chemistry (2013), doi: 10.1016/j.jorganchem.2012.12.019.
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ACCEPTED MANUSCRIPT

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The complex CpMo(CO)3(CH2-pC6H4-CO2CH3) and its inclusion complexes with
heptakis(2,6-di-O-methyl)-β-cyclodextrin (CD) and heptakis(2,3,6-tri-O-methyl)-β-CD were
prepared and characterized, and investigated as catalyst precursors for the epoxidation of cis-
cyclooctene at 55 ºC. Different types of oxidants (tert-butylhydroperoxide or H2O2) and
biphasic liquid-liquid solvent systems were used with the aim of facilitating catalyst
recycling.
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Revised manuscript for J. Organomet. Chem.
13 December 2012
Synthesis and Characterization of CpMo(CO)3(CH2-pC6H4-CO2CH3) and
its Inclusion Compounds with Methylated Cyclodextrins. Applications in
Olefin Epoxidation Catalysis
Ana C. Gomes a, Sofia M. Bruno a, Cátia Tomé b, Anabela A. Valente a, Martyn Pillinger a,
Marta Abrantes b, and Isabel S. Gonçalves a,*
a
Department of Chemistry, CICECO, University of Aveiro, Campus Universitário de
Santiago, 3810-193 Aveiro, Portugal
b
Centro de Química Estrutural, Complexo Interdisciplinar, Instituto Superior Técnico,
Universidade Técnica de Lisboa, 1049-001 Lisbon, Portugal
* Corresponding author. Fax: +351 234 370084
E-mail address: igoncalves@ua.pt (I.S. Gonçalves)
Dedicated to the memory of Professor F. Gordon A. Stone in recognition of his
outstanding contributions to the development of organometallic chemistry
___________________________________________________________________________
The cyclopentadienyl molybdenum tricarbonyl complex CpMo(CO)3(CH2-pC6H4-CO2CH3)
(1) (Cp = η5-C5H5) and its 1:1 inclusion complexes with heptakis(2,6-di-O-methyl)-β-
cyclodextrin (1@DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (1@TRIMEB)
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were prepared and characterized by elemental and thermogravimetric analyses, powder X-ray
diffraction, NMR, FT-IR and Raman spectroscopy. All three compounds were investigated as
catalyst precursors for olefin epoxidation using cis-cyclooctene (Cy) as a model substrate, a
reaction temperature of 55 ºC, and different types of oxidants (tert-butylhydroperoxide in
decane (TBHPdec) or in water (TBHPaq), and aqueous hydrogen peroxide). With the aim of
facilitating recycling of the homogeneous catalytic systems, different solvent mixtures were
tested: biphasic liquid-liquid solvent systems [(water, glycerol, ionic liquid)-(n-hexane, α,α,α-
trifluorotoluene (TFT), iso-butyl methyl ketone)], and in some cases no cosolvent was added.
Excellent epoxide selectivity (100%) was observed for all tested catalytic systems. An
example of an interesting catalytic system is that of 1@TRIMEB/TBHPaq/TFT/H2O since the
catalyst can be easily separated in the form of an aqueous solution, by decantation, from the
organic phase containing the reaction products and recycled.
___________________________________________________________________________
Keywords: Molybdenum; Cyclopentadienyl complexes; Carbonyl ligands; Methylated
cyclodextrins; Olefin epoxidation
1. Introduction
During the last decade cyclopentadienyl oxomolybdenum(VI) complexes have received
much interest as catalysts of oxygen transfer reactions [1]. Although the ability of
organometallic (η5-C5R5)MoO2X derivatives to act as catalysts for the epoxidation of olefins
by alkyl hydroperoxides was first reported by Trost and Bergman in 1991 (for the complex
with X = Cl and R = Me) [2], the study of these complexes in olefin epoxidation only gained
momentum with the discovery of a general and straightforward one-step synthesis involving
reaction of the corresponding and readily available (η5-C5R5)Mo(CO)3Cl precursors (R = H,
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CH3, CH2Ph) with tert-butylhydroperoxide (TBHP) [1a]. In fact, the parent tricarbonyls can
be used directly as catalyst precursors since they are transformed into (η5-C5R5)MoO2Cl
under the operating catalytic conditions. A number of half-sandwich molybdenum dicarbonyl
and tricarbonyl complexes have since been investigated as catalyst precursors for the
epoxidation of olefins [3], the cis-dehydroxylation of olefins [4], and the oxidation of amines
[5], alcohols [6] and sulfides [7]. Apart from dioxomolybdenum(VI) derivatives of the type
Cp'MoO2X (Cp' = η5-C5H5 (Cp), η5-C5H4R, η5-C5R5), the species formed upon oxidative
decarbonylation of these precursors include Cp'MoO(O2)X [1e], [Cp'MoO2]2(µ-O) and
[Cp'MoO(O2)]2(µ-O) [1d] from Cp'Mo(CO)3X, [CpMoO2(NHC)]+ from
[CpMo(CO)2(NHC)(NCMe)]+ (NHC = N-heterocyclic carbene) [3j], and [CpoxMoO2]+ from
[CpoxMo(CO)2(NCMe)]+ (Cpox = cyclopentadienyl functionalized with oxazoline bridging
unit) [3k].
The above-mentioned catalytic systems are homogeneous in nature. Although the active
catalysts can sometimes be recovered from the reaction mixtures and reused, the catalytic
activities are prone to decrease with successive recycling runs, and catalyst losses are
inevitable. Some of the approaches investigated to overcome these drawbacks include the
heterogenization of either the parent carbonyls or the organometallic oxides on porous
inorganic supports [8], immobilization in room temperature ionic liquids (RTILs) [3d,3e,9],
and encapsulation in cyclodextrins (CDs) [10]. We found that CD inclusion complexes
containing CpMo(CO)3X precatalysts (X = Cl, CH2CONH2) could be used in solid-liquid,
homogeneous or liquid-liquid (aqueous/organic) biphasic systems for the epoxidation of
cyclooctene with TBHP or H2O2 [10]. Of particular note was the observation that the
introduction of native β-CD or permethylated β-CD (TRIMEB) to the system with
CpMo(CO)3Cl and H2O2 was able to instill some catalytic epoxidation activity under certain
reaction conditions. Due to their protective effects on encapsulated guests, the use of CDs in
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organometallic oxidation catalysis may be particularly useful in cases where the free catalyst
is initially highly active but loses activity during the reaction due to decomposition.
Immobilization in a CD may help to stabilize the catalyst and also facilitate subsequent
recycling. Furthermore, CDs are excellent organic supports to work in aqueous solutions,
allowing more “environmentally friendly” oxidants to be used such as hydrogen peroxide.
As part of our ongoing work on the applications of molybdenum carbonyl complexes
and their cyclodextrin inclusion complexes in catalytic olefin epoxidation, we now wish to
report on the encapsulation of the tricarbonyl complex CpMo(CO)3(CH2-pC6H4-CO2CH3) in
heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and heptakis-2,3,6-tri-O-methyl-β-
cyclodextrin (TRIMEB), and an investigation of the catalytic behavior of the inclusion
compounds in the epoxidation of cyclooctene using TBHP or H2O2 as oxidant.
2. Experimental
2.1. Materials and methods
Microanalyses (CH) were obtained at the University of Aveiro (by M. Marques) and the
Analysis Laboratory of the Instituto Superior Técnico, Technical University of Lisbon. ICP
measurements for Mo were performed at the latter Analysis Laboratory. Powder X-ray
diffraction (XRD) data were collected on a Philips X’pert MPD diffractometer equipped with
an X’Celerator detector, a graphite monochromator (Cu-Kα X-radiation filtered by Ni (λ =
1.5418 Å)) and a flat-plate sample holder, in a Bragg-Brentano para-focusing optics
configuration (45 kV, 50 mA). Samples were step-scanned in 0.02º 2θ steps with a counting
time of 50 s per step. Thermogravimetric analysis (TGA) studies were performed using a
Shimadzu TGA-50 system at a heating rate of 5 ºC min–1 under air.
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Transmission FT-IR spectra were measured on a Mattson 7000 spectrometer. FT-
Raman spectra were recorded on a RFS-100 Bruker FT-Spectrometer equipped with a
Nd:YAG laser with an excitation wavelength of 1064 nm. Solution NMR spectra were
recorded on a Bruker Avance II+ 400 MHz (UltraShieldTM Magnet) spectrometer at ambient
temperature. Chemical shifts are given in ppm relative to TMS (1H, 13C{1H}, 13C,1H-HSQC)
or 1 M Na2MoO4 in D2O (95Mo). Solid-state magic-angle-spinning (MAS) NMR spectra were
recorded 125.76 MHz for 13C on a Bruker Avance 500 spectrometer. 13C CP MAS NMR
spectra were recorded with 3.5 ms 1H 90˚ pulses, 2 ms contact time, spinning rates of 7 and
11 kHz, and 4 s recycle delays.
THF, n-pentane and diethyl ether were dried by standard procedures, distilled under
nitrogen, and kept over 3 Å molecular sieves; CH2Cl2 was used as received. Mo(CO)6 (Fluka,
tech.), methyl 4-(bromomethyl)benzoate (BrCH2PhCO2CH3, Sigma-Aldrich, 98%), Na
(Fluka, pure), dicyclopentadiene (Fluka, >95%), heptakis(2,6-di-O-methyl)-β-cyclodextrin
(DIMEB, Sigma-Aldrich), and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB,
Sigma-Aldrich) were purchased from commercial sources and used as received.
2.2. CpMo(CO)3(CH2-pC6H4-CO2CH3) (1)
NaCp was synthesized using a modified literature procedure [11]. Accordingly, freshly
cut sodium (0.390 g, 16.96 mmol) was added to dicyclopentadiene (20 mL, 149.2 mmol)
under nitrogen, and the mixture was refluxed overnight at 160 ºC. The resultant dark brown
suspension with white solid particles was filtered, and the residue washed with n-pentane (3 ×
2.5 mL) and vacuum-dried. The resultant light pink powder of NaCp was refluxed overnight
in THF (30 mL) with Mo(CO)6 (2.35 g, 8.90 mmol). The reaction mixture was then cooled to
room temperature and BrCH2-pC6H4-CO2Me (2.04 g, 8.90 mmol) in THF (30 mL) was added,
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and the mixture stirred for 2 h. A yellowish brown solution with a white precipitate was
obtained. The suspension was evaporated to dryness and the resultant residue washed with
warm water (complex 1 is stable in water). The collected yellow solid was vacuum-dried and
recrystallized from diethyl ether (2.49 g, 71%). Anal. Calcd for C17H14MoO5 (394.23): C,
51.79; H, 3.58. Found: C, 51.98; H, 3.65%. Selected FT-IR (KBr, cm–1):
CO), 1598 (vs), 1282 (s). 1H NMR (400 MHz,
ν = 2004 (vs, νCO),
1936 (vs, νCO), 1906 (vs, νCO), 1705 (vs, ν
CD2Cl2, 298 K):
δ = 7.79 (d, J = 6.2 Hz, 2H, pC6H4), 7.22 (d, J = 6.2 Hz, 2H, pC6H4), 5.26 (s,
5H, Cp), 3.84 (s, 3H, O-CH3), 2.88 (s, 2H, CH2-pC6H4) ppm. 13C {1H} (126 MHz, CD2Cl2,
298 K):
δ = 240.1 (CO trans to CH2-pC6H4-CO2CH3), 228.9 (2 CO cis to CH2-pC6H4-
CO2CH3), 167.4 (C8), 158.5 (C2), 129.5 (C4,6/3,7), 127.6 (C4,6/3,7), 125.5 (C5), 94.4 (Cp), 51.9
(C9), 4.1 (C1) ppm. 13C,1H HSQC (126/400 MHz, CD2Cl2, 298 K): δ 13C/δ 1H = 129.5/7.79
(C4,6/3,7H), 129.5/7.22 (C4,6/3,7H), 94.4/5.26 (Cp), 51.9/3.84 (C9H3), 4.1/2.88 (C1H2-pC6H4)
ppm. 95Mo (26 MHz, CD2Cl2, 298 K):
δ δ = 240.7 (CO),
= –1546 ppm. 13C CP MAS NMR:
229.0 (CO), 166.5 (C8), 157.9 (C2), 130.1 (C4,6/3,7), 126.8 (C4,6/3,7), 122.9 (C5), 94.4 (Cp), 51.9
(C9), 4.6 (C1) ppm.
2.3. 1@DIMEB
A solution of 1 (0.15 g, 0.38 mmol) in CH2Cl2 (6 mL) was added slowly to a solution of
DIMEB (0.50 g, 0.38 mmol) in CH2Cl2 (10 mL) under air. The mixture was stirred for 2 h at
40 ºC and then evaporated to dryness, yielding a yellow solid. Anal. Calcd for C56H98O35·
C17H14MoO5·2H2O (1761.6): C, 49.77; H, 6.64. Found: C, 49.67; H, 6.47. Selected FTIR
(KBr, cm–1):
MAS NMR:
ν
δ
= 2017 (vs,
= 165.8 (guest COOCH3), 130.5 (guest C4,6/3,7), 127.0 (guest C4,6/3,7), 124.2
νCO), 1932 (vs, νCO), 1716 (m, ν
CO), 1600 (m), 1282 (m). 13C CP
(guest C5), 101.1 (DIMEB C1), 93.7 (guest Cp), 82.3 (DIMEB C2,4), 69.6 (DIMEB C3,5,6),
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58.8 (DIMEB OCH3), 50.5 (guest C9).
2.4. 1@TRIMEB
A solution of 1 (0.14 g, 0.35 mmol) in CH2Cl2 (6 mL) was added slowly to a solution of
TRIMEB (0.50 g, 0.35 mmol) in CH2Cl2 (10 mL) under air. The mixture was stirred for 2 h at
40 ºC and then evaporated to dryness, yielding a yellow solid. Anal. Calcd for C63H112O35·
C17H14MoO5·3H2O (1877.82): C, 51.17; H, 7.08. Found: C, 51.23; H, 6.80. Selected FTIR
(KBr, cm–1):
MAS NMR:
ν
δ
= 2017 (vs,
= 239.3 (CO), 228.3 (CO), 165.8 (guest COOCH3), 130.5 (guest C4,6/3,7), 128.5
νCO), 1930 (vs, νCO), 1718 (m, ν
CO), 1600 (m), 1282 (m). 13C CP
(guest C4,6/3,7), 124.2 (guest C5), 99.25 (TRIMEB C1), 93.7 (guest Cp), 81.6 (TRIMEB C2,4),
70.4 (TRIMEB C3,5,6), 58.8 (TRIMEB OCH3), 49.8 (guest C9).
2.5. Cyclooctene epoxidation catalysis
The catalytic reactions were carried out under air (autogeneous pressure) and stirred
magnetically (1000 rpm) in a closed borosilicate micro reactor (10 mL capacity) equipped
with a valve for sampling, and immersed in an oil bath thermostated at 55 ºC. Typically, the
micro reactor was loaded with 18 µmol of molybdenum complex, 1.8 mmol of cis-cycloctene
(Cy) and 2.75 mmol of oxidant, without cosolvent, or using a biphasic liquid-liquid solvent
mixture. The biphasic liquid-liquid solvent mixtures consisted of (A) a polar solvent (H2O,
glycerol (Gly, Aldrich, ≥99%) or the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride
(Aldrich, 98%)) and (B) an immiscible cosolvent (n-hexane (Hex, Sigma-Aldrich, PA), α,α,α-
trifluorotoluene (TFT, Aldrich, >99%) or iso-butyl methyl ketone (IBMK, Merck, PA)). The
total volume of solvent mixture (A+B) was 2 mL with an A:B ratio of 1:3 (v/v), except for the
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IL:hexane mixture for which the total volume was 1.65 mL and the A:B ratio was 1:10 (v/v).
The oxidants used were tert-butylhydroperoxide dissolved in water (TBHPaq, Sigma-
Aldrich, 70 wt% aq.) or in decane (TBHPdec, Sigma-Aldrich, 5-6 M in decane, <4% water),
or aqueous hydrogen peroxide (H2O2, Sigma-Aldrich, 30% w/v aq.). Prior to use, TBHPdec
was pre-dried using activated molecular sieves (4 Å), while the IL was pre-dried under
vacuum for 2 h, at 100 ºC.
The olefin, oxidant and cosolvents were pre-heated in separate vessels (10 min at the
desired reaction temperature) and then the corresponding desired amounts were transferred to
the micro reactor (with pre-heated walls) containing the catalyst. Zero time (initial instant of
the reaction of Cy) was taken as the instant the oxidant was added to the reactor. After each
24 h-batch run at 55 ºC, the reaction mixtures were cooled to ambient temperature,
centrifuged, and their features (color and type of physical states) noted. The course of the
reactions was monitored using a Varian 3800 GC equipped with a BR-5 (Bruker) capillary
column (30 m × 0.25 mm; 0.25 µm) and a flame ionization detector, using H2 as the carrier
gas and undecane as internal standard.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the complex CpMo(CO)3(CH2-pC6H4-CO2CH3) (1) (Chart 1) was
adapted from literature procedures for analogous compounds [12]. Complex 1 presents three
strong terminal carbonyl absorptions at 2004, 1936 and 1906 cm–1 in the FT-IR spectrum
(KBr disc), in agreement with that reported for other complexes of the type Cp'Mo(CO)3X
[1a,8c,10a]. A strong band at 1705 cm–1 is assigned to the carbonyl absorption of the ester
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group. The successful synthesis of 1 was confirmed by elemental analysis, 1H and 13C NMR
spectroscopy. Of particular note is the appearance of two resonances for the carbonyl ligands
at 228.9 and 240.1 ppm, which are attributed to the CO groups that are cis (×2) and trans (×1)
to the CH2-pC6H4-CO2CH3 group, respectively. Inclusion of molybdenum complex 1 in
DIMEB and TRIMEB was performed by mixing solutions of the host and guest in
dichloromethane, followed by vacuum-drying of the resultant solution to isolate 1@DIMEB
and 2@TRIMEB as yellow solids. This technique works particularly well for the methylated
cyclodextrins due to their high solubility in some organic solvents.
((Chart 1 here))
1@DIMEB and 2@TRIMEB were characterized in the solid state by elemental
analysis, powder XRD, TGA, FT-IR, FT-Raman, and 13C CP MAS NMR spectroscopy.
Unlike the previously reported inclusion compounds CpMo(CO)3CH2CONH2@TRIMEB
[10a] and CpMo(CO)3Cl@TRIMEB [10b,13], which were prepared in the same way,
1@DIMEB and 2@TRIMEB form amorphous phases, showing only two or three very broad
reflections in their powder XRD patterns (Fig. 1). Although no structural information can be
extracted from these data, the absence of reflections due either to non-included 1 or the pure
methylated CD shows that no separation of the two components occurred, which is an early
indication of the stability of the respective inclusion compounds.
((Figure 1 here))
Fig. 2 compares the TGA curves of 1@DIMEB and 1@TRIMEB with those of DIMEB,
TRIMEB, and nonincluded 1. DIMEB decomposes abruptly in the temperature range 230-370
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°C with a mass loss of 76%, followed by a smoother loss of 24% up to 730 °C. TRIMEB
starts to melt and decompose at 170 ºC, and at 630 ºC 100% mass loss is reached. Complex 1
decomposes in two main steps up to 550 ºC (70% mass loss). The first step occurs between
100 and 230 ºC (30%) and is attributed to disruption of the Mo–CO bonds and subsequent
liberation of CO, as well as partial decomposition of the remaining organic ligands. A further
mass loss of 33% between 400 and 550 ºC comprises the decomposition/combustion of all
residual organic matter. The TGA profiles for 1@DIMEB and 1@TRIMEB are quite similar.
Decomposition takes place between 140 and 300 ºC, followed by a more gradual loss up to
500 ºC, leaving a residual mass of 8.7% for 1@DIMEB and 7.8% for 1@TRIMEB, which are
close to the calculated values assuming the formation of MoO3 (8.2% and 7.7%, respectively).
Both 1@DIMEB and 1@TRIMEB reach a mass loss of 19% at a temperature that is about 70
ºC lower than that required to reach the same loss with the corresponding pure methylated
CD. On the other hand, the onset of decomposition for the organometallic guest seems to be
delayed by 40-50 ºC compared with that for the nonincluded complex. The early degradation
of the organic macrocycle in the inclusion compounds may be due to a combination of
factors, such as the lower crystallinity of the samples, the presence of different crystal packing
arrangements, and the promoting effect of the decomposition of the organometallic guest on
the decomposition of the host molecules.
((Figure 2 here))
The FT-IR spectra of 1@DIMEB and 1@TRIMEB showed several
absorption bands assigned to the guest species in addition to the characteristic bands of the
hosts. In the CO (carbonyl ligand) stretching region (1900–2050 cm–1), the spectra of both
compounds contain a strong, sharp peak at 2017 cm–1, and a strong, broader peak at 1931 cm–
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1 (Fig. 3). The former absorption band is red-shifted by 13 cm–1 compared with the
corresponding band for 1, while the latter probably comprises overlapping absorptions
equivalent to the bands found at 1906 and 1936 cm–1 for 1. Another notable difference
between the spectra for the inclusion compounds and that for nonincluded 1 concerns the shift
of the carbonyl absorption of the ester group from 1705 cm–1 for 1 to 1717 cm–1 for
1@DIMEB and 1@TRIMEB.
((Figure 3 here))
In the 13C CP MAS NMR spectra of 1@DIMEB and 1@TRIMEB (Fig. S1 in the
supporting information), the signals for the cyclodextrin carbon atoms appear mainly as single
broad peaks, and additional signals due to the organometallic guest are found at 50 (CH3),
93.7 (Cp), 124-131 (phenyl-C), 165.8 (COOCH3), 228.3 and 239.3 ppm (CO, only detectable
for 1@TRIMEB), which are essentially unshifted compared with the corresponding signals
for the free complex 1.
3.2. Catalytic studies
Complex 1 and the corresponding inclusion compounds were evaluated as pre-catalysts
for the epoxidation of olefins by using cis-cyclooctene (Cy) as a model substrate and a
reaction temperature of 55 ºC. Different types of oxidants (TBHP in water or decane, and
H2O2) and biphasic liquid-liquid solvent systems [(water, glycerol, ionic liquid)-(n-hexane,
α,α,α-trifluorotoluene, iso-butyl methyl ketone)] were used; in some cases no cosolvent was
added (nsolv). For all reaction systems tested, 1,2-cyclooctene oxide (CyO) was the only
product of the reaction of Cy (CyO selectivity was always 100%).
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The catalytic performances were initially investigated using the conditions
TBHPdec/nsolv. The catalytic results for 1 and 1@TRIMEB are fairly comparable (100%
conversion at 3 h reaction), and superior to those for 1@DIMEB (74% conversion at 3 h), Fig.
4. The similar catalytic results for the free and TRIMEB-included complex parallels that
reported in the literature for CpMo(CO)3CH2CONH2 [10a] and CpMo(CO)3Cl [10b], and the
corresponding TRIMEB inclusion compounds used as pre-catalysts in the same reaction
under similar conditions; when compared with the literature data, the catalytic activities for 1
and 1@TRIMEB are higher. For 1, 1@DIMEB and 1@TRIMEB a pale yellow solid and a
single pale yellow liquid phase were obtained after a 24-h batch run (Table 1). The type of
CD, acting as a second sphere ligand for the molybdenum center, may influence the steric and
electronic properties of the active species [10a]. The DIMEB host possesses hydroxyl groups
which may participate in guest-host hydrogen bonding interactions, thereby influencing the
formation of transition states.
((Figure 4 and Table 1 here))
Using TBHPaq instead of TBHPdec would be preferable since decane is more
expensive than water, and the final reaction mixture using TBHPdec is more demanding with
respect to product separation/purification (b.p. of decane is 174 ºC). The reaction of Cy in the
presence of 1 or 1@TRIMEB was slower using TBHPaq (64-73% conversion at 3 h reaction)
instead of TBHPdec (Fig. 4). These results somewhat parallel those reported in the literature
for the free and TRIMEB-included complex CpMo(CO)3Cl used as pre-catalysts in the same
reaction under similar conditions [10b]. The reaction mixtures using 1, 1@DIMEB and
1@TRIMEB, and TBHPaq, are triphasic liquid-liquid-solid (L-L-S), and mass transfer
limitations may at least partly explain the lower overall reaction rate in comparison to the
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TBHPdec-based systems (Table 1). Nevertheless, in the case of 1@DIMEB the catalytic
results using TBHPaq or TBHPdec are similar (96% conversion at 24 h). Based on the
different features of the reaction mixture for 1@DIMEB in relation to those for 1 and
1@TRIMEB, the DIMEB host seems to enhance the solubility of the catalyst in the aqueous
phase in comparison to TRIMEB (somewhat more evident and in parallel with these
observations are those when comparing the three compounds using the biphasic H2O/Hex
solvent system as discussed below, Table 1). Notably, the presence of water (in TBHPaq)
does not seem to have a detrimental effect on the catalytic reaction. According to the
literature, water molecules can participate in the formation (and stabilization) of transition
states through hydrogen bonding [1h].
The performances of 1, 1@DIMEB and 1@TRIMEB with TBHPaq were further
investigated using the biphasic solvent mixtures H2O/Hex (Hex = hexane) and Gly/Hex (Gly
= glycerol); under these conditions, no solid phases were obtained, i.e., the metal species were
always completely dissolved. Preliminary solubility tests for the pre-catalysts showed that
1@DIMEB was the most soluble in water, and the opposite was the case with Gly. Although
these differences may not necessarily be maintained for the corresponding active species
under the reaction conditions, it motivated us to explore the possibility of using a polar
medium containing the catalyst combined with an immiscible organic cosolvent for the in situ
extraction of the reaction products, and facilitated catalyst recycling. For each compound, the
catalytic activity was considerably higher for the solvent system H2O/Hex than for Gly/Hex
(conversions at 24 h were 80-87% for H2O/Hex and 7-18% for Gly/Hex, Table 1). The same
applies for the IL/Hex solvent systems (less than 10% conversion at 24 h, 55 ºC). The
detrimental effects on the catalytic reaction associated with the use of Gly and IL may be
partly due to major mass transfer limitations (Gly and IL possess relatively high viscosities in
comparison to water).
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In the case of 1@TRIMEB/TBHPaq, the catalytic performance was further explored
using different aqueous-organic solvent systems (H2O/(Hex, IBMK, TFT); IBMK = iso-butyl
methyl ketone, TFT = α,α,α-trifluorotoluene). The reaction rate (based on conversions at 6
h/24 h, Table 1) followed the order H2O/IBMK << TFT/H2O H2O/Hex. Of the three organic
solvents tested, IBMK possesses the highest solubility in water (ca. 19 g L–1 compared to less
than 1 g L–1 for the other two organic solvents, at 20 ºC) and thus one would expect superior
catalytic results for this solvent (enhanced water-organic solvent miscibility), which was not
the case. It has been generally accepted that for these types of molybdenum complexes the
olefin epoxidation reaction involves oxygen atom transfer from active oxidizing species
(formed via reaction of the metal complex with the hydroperoxide oxidant) to the olefin [1e-
1h,3d]. Accordingly, the negative effect of IBMK may be partly due to the fact that it can act
as a Lewis base and coordinate to the metal center, inhibiting the formation of transition states
in the catalytic epoxidation. Based on the BF3 affinities (kJ mol–1) of Lewis bases measured in
dichloromethane (25 ºC, 1 atm), the Lewis basicity of IBMK (BF3 affinity of ca. 75.1 kJ mol–
1) is intermediate between those of N,N-dimethylformamide (DMF, ca. 110 kJ mol–1) and
tetrahydrofuran (THF, ca. 90.4 kJ mol–1), and that of acetonitrile (ca. 60.4 kJ mol–1); THF,
DMF and CH3CN are sufficiently strong to form solvent adducts of oxomolybdenum(VI)
complexes [14].
The performances of 1, 1@DIMEB and 1@TRIMEB as pre-catalysts in the reaction of
Cy were further investigated using aqueous H2O2 as oxidant instead of TBHPaq, and the
solvent system H2O/Hex, at 55 ºC (Table 1). For 1 and 1@TRIMEB, a conversion of ca. 80%
was reached at 24 h, which is far superior to that measured previously for free and CD-
included CpMo(CO)3Cl tested as (pre-)catalysts in the same reaction under similar conditions
(5% and 16% Cy conversion, respectively) [10b]. For these two catalytic systems the organic
phase was pale yellow in color while the aqueous one was colorless, suggesting that the active
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species are in the former phase where they are mixed with the reaction products (making
catalyst separation and reuse somewhat troublesome). The catalytic results for
1@DIMEB/H2O2 are considerably poorer (13% conversion at 24 h), which may be due to
differences in the partition ratio of the catalyst between the two liquid phases and/or in the
type of active species (no yellow colored phase was obtained for 1@DIMEB in contrast to
that observed for 1 and 1@TRIMEB). All of the tested catalytic systems which were
completely colorless were sluggish in contrast to those possessing at least one colored phase,
suggesting that the yellow color is associated with the presence of active species (Table 1).
Due to the high solubility of the metal species in the L-L biphasic reaction media, for
these systems we did not succeed in isolating metal species for characterization. According to
the literature, complexes of the type Cp'Mo(CO)3R (R = alkyl) can be oxidized into the
corresponding dioxo Cp'MoO2R and oxoperoxo Cp'MoO(O2)R derivatives, using TBHP
[1b,1e,1f,3a,3d] or H2O2 [15] as oxidants. For example, CpMoO2Me can be oxidized with
H2O2 into the oxoperoxo derivative CpMoO(O2)Me (yellow-colored solids) [15]. The
Cp'MoO2R complexes are intermediates in the formation of Cp'MoO(O2)R, and the two types
may play a catalytic role [1e,1f,3a,16]. Attempts to identify the types of species were made
for the system 1@TRIMEB/TBHPdec/nsolv. The ATR FT-IR spectrum of the recovered solid
(not shown) did not exhibit the bands characteristic of the carbonyl ligands, which is
consistent with oxidative decarbonylation of the pre-catalyst during the catalytic reaction. The
unequivocal identification of bands assignable to metal-oxo or η2-peroxo groups was not
possible due to overlap with bands attributed to the host.
As discussed above, the separation (by simple decantation) and reuse of the aqueous
solution of the catalyst which is immiscible with the organic solvent containing the reaction
products seems quite attractive. Based on the results of the catalytic tests (Table 1), the most
interesting catalytic systems in terms of activity and recyclability seem to be
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1@TRIMEB/TBHPaq/TFT/H2O and 1@DIMEB/TBHPaq/H2O/Hex (in contrast to, for
example, (1, 1@TRIMEB)/TBHPaq/H2O/Hex in which the catalyst and products are
apparently in the same liquid (organic) phase). The catalyst recyclability in the aqueous-
organic biphasic reaction system was investigated for 1@TRIMEB/TBHPaq/TFT/H2O. After
a 24 h batch run the resultant aqueous and organic phases obtained were separated by
decantation and transferred to separate reactors. ICP analysis of the TFT phase indicated
negligible amount of molybdenum (< 0.05 wt% of the initial amount of molybdenum). The
reagents Cy and TBHPaq were added to each reactor in similar amounts to those used in the
first batch, and the reaction solutions were stirred for 24 h at 55 ºC. Whereas the colorless
organic phase had a negligible catalytic contribution, the pale yellow aqueous phase led to
63% conversion. These results are consistent with the above discussion relating the yellow
color of the phases to the catalyst, and suggest that the catalyst can be easily separated in the
form of an aqueous solution, by decantation, from the organic phase containing the reaction
products and recycled. A third run of the water-catalyst system gave similar results to those of
the second run (conversion was 0.91 of that in the first run and epoxide selectivity was always
100%). A recycling test was carried out for the system 1@DIMEB/TBHPaq/H2O/Hex in a
similar fashion to that for 1@TRIMEB/TBHPaq/TFT/H2O: the recycled organic phase
accounted for 33% conversion (100% epoxide selectivity). Hence, it seems that TFT allows a
better partitioning of the metal species in the aqueous phase relative to the organic one, when
compared with Hex as cosolvent.
4. Conclusions
For the three compounds prepared in this work, the catalytic epoxidation activity was
investigated using cis-cyclooctene as a model substrate and different types of hydroperoxide
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oxidants at 55 ºC. The type of CD host can have a determinant effect on the catalytic reaction,
possibly due to an interplay of different factors such as catalyst solubility and stability,
electronic and steric effects on the formation of transition states. For example, the influence of
the type of oxidant on the catalytic reaction is different depending on the type of CD host:
while 1@TRIMEB was effective with aqueous TBHP or H2O2 under similar reaction
conditions, 1@DIMEB was only effective with TBHP. On the other hand, the inclusion
compounds may present advantages in relation to the corresponding free complex in terms of
homogeneous catalyst recycling. Optimized compromises between catalytic activity and
facilitated catalyst reuse may be achieved by using different types of solvent mixtures,
depending on the type of CD host. An interesting catalytic system is, for example, that of
1@TRIMEB/TBHPaq/TFT/H2O where the catalyst is in the aqueous phase immiscible with
the organic extracting solvent (TFT) containing the reaction products: the catalyst can be
separated in the form of an aqueous solution, by decantation, from the organic phase
containing the reaction products and recycled.
Acknowledgements
We are grateful to the Fundação para a Ciência e a Tecnologia (FCT, Portugal), the
Programa Operacional Ciência e Inovação (POCI) 2010, Orçamento do Estado (OE),
European Union, Fundo Europeu de Desenvolvimento Regional (FEDER), QREN,
COMPETE, the Associate Laboratory CICECO (Pest-C/CTM/LA0011/2011), and CQE
(PEst-OE/QUI/UI0100/2011) for continued support and funding. The FCT and the European
Social Fund are acknowledged for a post-doctoral grant to S.M.B. (SFRH/BPD/46473/2008).
The authors also thank the Portuguese NMR Network (IST-UTL Center) for providing access
to the NMR facilities.
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Appendix A. Supplementary material
Supplementary material (for 1, 1@DIMEB and 1@TRIMEB: full listing of FT-IR and FT
Raman bands, and 13C CP MAS NMR spectra) associated with this article can be found, in the
online version, at doi:10.1016/j.jorganchem.2013.XX.XXX.
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Table 1. Cyclooctene conversions at 55 ºC with different oxidants and solvents in the
presence of complex 1 and the CD inclusion compounds, 1@DIMEB and 1@TRIMEB.a
Sample
Oxidant
Solvents
CyO yield Features of the reaction mixturesc
(%)b
Lower layer Upper layer Solid
TBHPdec nsolv
TBHPaq nsolv
100d
single L phase, p-yellow
p-yellow
1
80 / 95
p-yellow
cless
p-yellow
yellow
cless
p-yellow
H2O/Hex 53 / 80
-
-
-
-
Gly/Hex
IL/Hex
3 / 10
5 / 9
cless
p-yellow
cless
cless
H2O2
H2O/Hex 8 / 78
yellow
1@DIMEB TBHPdec nsolv
TBHPaq nsolv
89 / 96
79 / 97
single L phase, p-yellow
p-yellow
p-yellow
p-yellow
cless
cless
cless
cless
cless
cless
p-yellow
H2O/Hex 69 /87
-
-
-
-
Gly/Hex
IL/Hex
4 / 18
8 / 9
p-yellow
cless
H2O2
H2O/Hex 2 / 13
1@TRIMEB TBHPdec nsolv
TBHPaq nsolv
100d
single L phase, p-yellow
p-yellow
80 / 88
p-yellow
cless
p-yellow
yellow
cless
p-yellow
H2O/Hex 49 /83
-
-
-
-
-
-
Gly/Hex
IL/Hex
2 / 7
4 / 8
cless
p-yellow
cless
cless
H2O/IBMK - / 20
TFT/H2O 46 / 77
H2O/Hex 2 / 81
cless
cless
p-yellow
yellow
H2O2
cless
a Reaction conditions: 1:100:150 molar ratios of Mo:Cy:Oxidant, 55 ºC, 1000 rpm.
b Yield of CyO at 6 h/24 h reaction (CyO selectivity was always 100%).
c Color of the liquid (L) and solid phases after centrifugation of the reaction mixtures for a 24
h-batch; p-yellow = pale yellow; cless = colorless; for the biphasic liquid-liquid systems, the
lower/upper layers match the order indicated in the solvents column.
d At ca. 3 h reaction.
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Figure captions
Chart 1. Guest (1) and hosts (DIMEB, TRIMEB) used in the present work.
Fig. 1. Powder XRD patterns of (a) DIMEB, (b) TRIMEB, (c) CpMo(CO)3(CH2-pC6H4-
CO2CH3) (1), (d) 1@DIMEB and (e) 1@TRIMEB.
Fig. 2. TGA curves of (a) DIMEB (– · · – · ·), TRIMEB (– – – –),
CpMo(CO)3(CH2PhCO2CH3) (1) (··········), 1@DIMEB (– · – · –) and 1@TRIMEB (——).
Fig. 3. FT-IR spectra in the range 1000-2500 cm–1 for (a) CpMo(CO)3(CH2-pC6H4-CO2CH3)
(1), (b) 1@DIMEB and (c) 1@TRIMEB.
Fig. 4. Dependence of CyO yield (CyO selectivity was always 100%) on the reaction time of
cis-cyclooctene in the presence of complex 1 (+,□) and the related CD inclusion compounds,
1@DIMEB (○,●) and 1@TRIMEB (ꢀ,▲), using TBHPaq (+,○,ꢀ) or TBHPdec (□,●,▲), at
55 ºC without additional cosolvent.
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1. Synthesis and characterization of CpMo(CO)3(CH2-pC6H4-CO2CH3) (1).
2. Synthesis of 1:1 inclusion compounds between 1 and methylated cyclodextrins.
3. Complex 1 and its TRIMEB adduct are effective precatalysts for cyclooctene epoxidation.
4. Aqueous tert-butylhydroperoxide and hydrogen peroxide can be used as oxidants.
5. Biphasic liquid-liquid systems were studied, aiming to facilitate catalyst recycling.
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