Synthesis and catalytic properties in olefin epoxidation of octahedral dichloridodioxidomolybdenum(VI) complexes bearing N,N-dialkylamide ligands: Crystal structure of [Mo2o4(μ2-o)CI2(dmf) 4]

Article abstract of DOI:10.1002/ejic.200900544

The catalytic performance of the complexes [MoO₂Cl ₂(L)₂] [L = N,N-dimethylformamide (dmf), N,N-dimethylacetamide (dma), N,N-dimethylpropionamide (dmpa), N,N-diethylformamide (def) and N,N-diphenylformamide (dpf)] was examined in the epoxidation of cis-cyclooctene with ieri-butyl hydroperoxide (tbhp) at 55 °C and in the absence of a cosolvent. The complexes showed high turnover frequencies in the range of 561-577 molmolMo^-1h ^-1r giving the epoxide as the only product in 298 % yield after 6 h. The reaction rates decreased significantly in consecutive runs carried out by recharging the reactors with olefin and oxidant. On the basis of the IR spectroscopic characterisation of the solids recovered at the end of the catalytic reactions, the decrease in activity is attributed to the formation of dioxido(n-oxido)-molybdenum(VI) dimers. Accordingly, the treatment of [Mo0 ₂Cl₂(dmf)₂] with an excess amount of tbhp led to the isolation of [Mo₂0₄(μ₂-0)Cl ₂(dmf)₄], which was characterised by single-crystal X-ray diffraction and found to exhibit a catalytic performance very similar to that found in the second runs for the mononuclear complexes. The kinetics of the reaction of [Mo0₂Cl₂(dmf)₂] with tbhp was further examined by UV/Vis spectroscopy, allowing rate constants and activation parameters to be determined. For the dpf adduct, the effect of different solvents on cyclooctene epoxidation and the epoxidation of other olefins, namely, (R)-(+)-limonene,αpinene and norbornene, were investigated. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejic.200900544

FULL PAPER
DOI: 10.1002/ejic.200900544
Synthesis and Catalytic Properties in Olefin Epoxidation of Octahedral
Dichloridodioxidomolybdenum(VI) Complexes Bearing N,N-Dialkylamide
Ligands: Crystal Structure of [Mo2O4(µ2-O)Cl2(dmf)4]
Sandra Gago,[a] Patrícia Neves,[a] Bernardo Monteiro,[a] Márcia Pessêgo,[b]
André D. Lopes,[b] Anabela A. Valente,[a] Filipe A. Almeida Paz,[a] Martyn Pillinger,[a]
José Moreira,[b] Carlos M. Silva,[a] and Isabel S. Gonçalves*[a]
Keywords: Molybdenum / O ligands / Homogeneous catalysis / Epoxidation / Kinetics
The catalytic performance of the complexes [MoO2Cl2(L)2]
[L = N,N-dimethylformamide (dmf), N,N-dimethylacetamide
(dma), N,N-dimethylpropionamide (dmpa), N,N-diethylform-
amide (def) and N,N-diphenylformamide (dpf)] was exam-
molybdenum(VI) dimers. Accordingly, the treatment of
[MoO2Cl2(dmf)2] with an excess amount of tbhp led to the
isolation of [Mo2O4(µ2-O)Cl2(dmf)4], which was characterised
by single-crystal X-ray diffraction and found to exhibit a
ined in the epoxidation of cis-cyclooctene with tert-butyl hy- catalytic performance very similar to that found in the second
droperoxide (tbhp) at 55 °C and in the absence of a cosolvent.
The complexes showed high turnover frequencies in the
range of 561–577 molmolMo–1 h–1, giving the epoxide as the
only product in Ն98% yield after 6 h. The reaction rates de-
runs for the mononuclear complexes. The kinetics of the re-
action of [MoO2Cl2(dmf)2] with tbhp was further examined
by UV/Vis spectroscopy, allowing rate constants and acti-
vation parameters to be determined. For the dpf adduct, the
creased significantly in consecutive runs carried out by re- effect of different solvents on cyclooctene epoxidation and
charging the reactors with olefin and oxidant. On the basis
of the IR spectroscopic characterisation of the solids reco-
vered at the end of the catalytic reactions, the decrease in
activity is attributed to the formation of dioxido(µ-oxido)-
the epoxidation of other olefins, namely, (R)-(+)-limonene, α-
pinene and norbornene, were investigated.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
reactions, such as the oxidation of triphenylphosphane to
triphenylphosphane oxide with dimethyl sulfoxide (dmso),[2]
the oxidation of thiols to disulfides with dmso,[2b,2c,14] the
aerobic oxidation of activated benzyl alcohols to the corre-
sponding aldehydes,[15] the epoxidation of olefins[7,16] and
the reductive cyclisation of nitrobiphenyls and nitrostyrenes
to carbazoles and indoles.[17] The epoxidation of cyclooc-
tene with tert-butyl hydroperoxide (tbhp) in the presence of
[MoO2Br2(RCN)2] (R = CH3 or C6H5) quickly reached a
conversion of about 65% but did not proceed significantly
further.[7] This was attributed to the pronounced water sen-
sitivity of the complex. In contrast, the complex
[MoO2Cl2(dmf)2] displayed a much higher catalytic activity
(95% cyclooctene epoxide after 2 h at 55 °C; 100% after
24 h).[16b] The catalytic stability of this system was investi-
gated by recharging the reaction vessel with substrate and
oxidant after 24 h. Initially, the reaction was somewhat
slower in the second run, but after 1 h the kinetic curves for
the two runs converged to give the same conversion of
about 98% after 6 h. We have now extended this study to
cover four other N,N-disubstituted amides in order to inves-
tigate the electronic and steric effects of the substituents (H,
methyl, ethyl, phenyl) on the carbonyl group and nitrogen
atom on the catalytic performance of the resultant
Introduction
Dioxidomolybdenum(VI) complexes of the type
[MoO2X2(L)n] are of interest as models for molybdo-
enzymes,[1] as oxo-transfer agents[2] and as (pre)catalysts for
the liquid-phase epoxidation of olefins.[3] A broad range of
complexes have been studied with different anionic X (F,
Cl, Br, alkyl, OR, OSiR3, SR) and neutral L ligands.
The L ligands may involve either two monodentate ligands
(e.g., dmf,[2c,4] thf,[5] R2SO,[1d,2,6] RCN,[7] H2O,[2c,8]
OPR3[1e,2c,3e–3g,9]) or one bidentate ligand containing oxy-
gen,[3f,10] nitrogen[1d,3a–3d,11] or sulfur[12] donor atoms. With
a few exceptions, all of these complexes adopt a cis-oxido,
trans-X, cis-L configuration at the metal centre and a dis-
torted octahedral coordination geometry.[13] Adducts of
MoO2X2 (X = Cl, Br) with the solvent molecules dmf, thf,
R2SO and RCN have been investigated in several catalysed
[a] Department of Chemistry, CICECO, University of Aveiro,
3810-193 Aveiro, Portugal
E-mail: igoncalves@ua.pt
[b] Faculty of Science and Technology, Department of Chemistry,
Biochemistry and Pharmacy, University of the Algarve,
Campus de Gambelas, 8005-136 Faro, Portugal
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900544.
4528
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4528–4537

Catalytic Olefin Epoxidation of Dichloridodioxidomolybdenum(VI) Complexes
[MoO2Cl2(RCONRЈ2)2] complexes. With the aim of ob-
taining a better understanding of the catalytically active
species formed under the reactions conditions used, the re-
action of the complexes with tbhp was examined in more
detail by (1) isolating and characterising the product
formed upon treatment of [MoO2Cl2(dmf)2] with an excess
amount of the oxidant in dmf and (2) using UV/Vis spec-
troscopy to follow the kinetics, allowing rate constants and
activation parameters to be determined.
Results and Discussion
Synthesis and Characterisation of the
Dioxidomolybdenum(VI) Complexes
Bis(N,N-dimethylformamide)dichloridodioxidomolyb-
denum(VI) (1) was synthesised as described previously by Version 5.30, November 2008 with 1 update).[21] Further-
treatment of an aqueous hydrochloric acid solution of more, the indicative R factor was quite high (Ͼ10%), and
MoO3 with an excess amount of dmf.[8a] For the other the original paper does not contain the fractional atomic
amides, the direct reaction of MoO2Cl2 with two equiva- coordinates. For these reasons, the structure of 6 was rede-
lents of the ligand in CH2Cl2 solution gave adducts 2–5 in termined by single-crystal X-ray diffraction at the low tem-
good yields as pale-yellow powders, soluble in CH2Cl2, and perature of 100(2) K by using a significantly better resolu-
insoluble in diethyl ether, hexane and pentane. In contrast tion of up to 0.70 Å. Since the paper by Atovmyan et al.,[20]
to the acetonitrile and thf adducts, both of which are air only three other structures of binuclear oxido-bridged MoVI
and moisture sensitive and can only be handled and stored complexes bearing dmf ligands have been reported, namely,
under dry inert gas atmosphere, compounds 1–5 are air [Mo2O5(hpb)2(dmf)4] (hpb = 2-o-hydroxyphenylbenzimid-
stable. The IR and Raman spectra contain a pair of bands azole) by Caparelli et al.[22] and [Mo2O3(3-t-Bussp)2(dmf)2]
in the ranges 940–950 and 905–915 cm–1, which are as- and [Mo2O3(3-EtOssp)2(dmf)2]·2dmf by Holm and co-
signed to the symmetric and asymmetric stretching vi- workers (3-t-Bussp = 3-tert-butylsalicylaldehyde; 3-EtOssp
brations of the cis-[MoO2]2+ group, respectively. The corre- = 3-ethylsalicylaldehyde).[23]
sponding frequencies for the acetonitrile and thf adducts lie
The binuclear [Mo2O5Cl2(dmf)4] molecular unit in 6 has
at higher values (ca. 960 and 920 cm–1), consistent with the crystallographic C2 symmetry around the central µ2-bridg-
stronger coordination of the amide ligands in 1–5.[18] The ing oxido group (Figure 1), which imposes a Mo(1)···
IR spectra of all five compounds afford conclusive evidence Mo(1)i distance of 3.783(1) Å and subtends a Mo(1)–(µ2-
that the amide is coordinated through the oxygen atom. O)–Mo(1)i angle of 175.02(15)° [symmetry code: (i) 1 – x,
Thus, the carbonyl stretching frequency, which occurs at y, 1.5 – z]. The crystallographically unique MoVI centre is
1640–1670 cm–1 in the free ligands, is lowered by 20– coordinated by two dmf molecules, one chlorido ligand, two
50 cm–1 to 1602–1653 cm–1 in the complexes. This result is terminal oxido groups and the central µ2-bridging oxido
explained by the decrease in the double-bond character of moiety, with the overall {MoClO5} coordination geometry
C=O and the subsequent increase in the C–N double bond resembling a highly distorted octahedron. Whereas the Mo–
character.[19] The relatively large range of shifts in ν(C=O) (O,Cl) bond lengths fall in the range 1.6983(19)–
upon complexation (which could be interpreted to mean 2.4394(6) Å, the internal cis and trans octahedral angles are
MoVI–carbonyl interactions of varying strengths) are not in the ranges 78.08(7)–103.46(9)° and 159.47(4)–167.87(8)°,
paralleled by the stretching frequencies for the cis- respectively (Figure 1 and Table 1). The wide ranges in
[MoO2]2+ group, which all fall within relatively narrow ran- these geometrical values arise from the different chemical
ges.
nature of each ligand composing the first coordination
Treatment of [MoO2Cl2(dmf)2] with a large excess of sphere. Indeed, the Mo=O bonds clearly exert the typical
tbhp (15 equiv.) in decane/dmf led to the isolation of the µ- trans effect by displacing the MoVI centre from the
oxido binuclear molybdenum(VI) complex [Mo2O4(µ2-O)- O(1)ǞCl(1) vector by about 0.35 Å, with the former bonds
Cl2(dmf)4] (6). The IR spectrum of 6 supports the presence being the shortest registered for the binuclear complex
of the cis-[MoO2]2+ group (νMo=O = 903 and 948 cm–1), the [1.6984(18) and 1.6983(19) Å], contrasting well with those
Mo–O–Mo linkage (νMo–O–Mo = 767 cm–1) and dmf ligands to the trans-oxido-coordinated dmf molecules [Mo–O bond
coordinated through oxygen (νCO = 1649 cm–1). Even lengths of 2.2581(18) and 2.2321(17) Å], also composing the
though the ambient temperature crystallographic descrip- equatorial plane of the octahedron. The two symmetry-re-
tion of this material was reported by Atovmyan et al. in lated chlorido ligands are located in the apical positions,
1970,[20] the 3D coordinates are not deposited in the most thereby minimising overall steric repulsion. The Mo=O and
recent version of the Cambridge Structural Database (CSD, Mo–Cl bond lengths (Table 1) are typical of oxidomolybde-
Eur. J. Inorg. Chem. 2009, 4528–4537
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4529

I. S. Gonçalves et al.
FULL PAPER
Figure 1. (a) Schematic representation of [Mo2O4(µ2-O)Cl2(dmf)4] (6) with all non-hydrogen atoms being represented as thermal ellipsoids
drawn at the 70% probability level, hydrogen atoms as small spheres with arbitrary radii, and showing the labelling scheme for the atoms
composing the first coordination sphere of Mo(1). (b) Top and side views of the {Mo2Cl2O9} core. For selected bond lengths and angles
see Table 1. Symmetry transformation used to generate equivalent atoms: (i) 1 – x, y, 1.5 – z.
num complexes, as revealed by systematic searches in the
CSD [dMo=O = 1.60–2.51 Å (1246 entries, median of ca.
1.71 Å); dMo–Cl = 2.17–2.74 Å (67 entries, median of ca.
2.37 Å)]. Interestingly, the Mo–O bonds with dmf are re-
markably shorter than those reported for related diox-
idomolybdenum complexes with this ligand [values found
in the range 2.31–2.70 Å].[4b,23–25] This difference is ex-
plained by the fact that the crystallographic data used here
were collected at the low temperature of 100(2) K, whereas
the data used in the cited references were all collected at
ambient temperature.
Table 1. Selected bond lengths [Å] and angles [°] for the MoVI coor-
dination environment present in complex [Mo2O4(µ2-O)Cl2(dmf)4]
(6).[a]
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
1.8934(3)
1.6984(18)
1.6983(19)
82.69(5)
82.87(8)
159.47(4)
99.30(7)
167.87(8)
90.26(8)
94.61(6)
Mo(1)–O(4)
Mo(1)–O(5)
Mo(1)–Cl(1)
2.2581(18)
2.2321(17)
2.4394(6)
103.46(9)
87.77(8)
164.70(8)
90.04(6)
80.64(5)
78.08(7)
82.04(5)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–O(5)
O(1)–Mo(1)–Cl(1)
O(2)–Mo(1)–O(1)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–O(5)
O(2)–Mo(1)–Cl(1)
O(3)–Mo(1)–O(1)
O(3)–Mo(1)–O(2)
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–O(5)
O(3)–Mo(1)–Cl(1)
O(4)–Mo(1)–Cl(1)
O(5)–Mo(1)–O(4)
O(5)–Mo(1)–Cl(1)
101.22(9)
Mo(1)i –O(1)–Mo(1) 175.02(15)
[a] Symmetry transformation used to generate equivalent atoms: (i)
1 – x, y, 1.5 – z.
Figure 2. Perspective views of the crystal packing along the (a)
[100] and (b) [001] crystallographic directions in the structure of
[Mo2O4(µ2-O)Cl2(dmf)4] (6). MoVI metal centres are represented as
Individual [Mo2O4(µ2-O)Cl2(dmf)4] complexes close
pack in the solid state with the self-assembly being driven distorted {MoClO5} octahedra.
mainly by the need to effectively fill the space while medi-
ated by a few rather weak C–H···(O,Cl) interactions (not
shown). Undulated layers placed in the ac plane of the unit
Catalytic Tests
cell and composed of binuclear complexes pack in a parallel
Complexes 1–5 are active catalysts for the epoxidation of
fashion along the direction of the b axis (Figure 2) to give cis-cyclooctene at 55 °C, using tbhp (5.5  in decane) as the
the crystal structure of 6.
oxidant without a cosolvent, and give 1,2-epoxycyclooctane
4530
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4528–4537

Catalytic Olefin Epoxidation of Dichloridodioxidomolybdenum(VI) Complexes
as the only product. The noncatalytic contribution is negli- the active species into less-active ones, such as oxidising
gible. The kinetic profiles are nearly coincident (Figure 3) species with lower electrophilicity at the peroxido function-
and the observed initial TOF values lie in the narrow range ality, cannot be ruled out.
of 561–577 molmolMo–1 h–1 (Table 2). These are the first in-
dications that the active species and reaction mechanism
may be similar for all five complexes. Initially, cyclooctene
conversion increased abruptly reaching at least 92% at
Table 2. cis-Cyclooctene epoxidation with tbhp (in decane) at 55 °C
in the presence of the Mo complexes.
Molybdenum
TOF[a]
Conversion[b] [%]
Run 1 Run 2
10 min and 98–99% after 6 h. The sudden slowdown of the complex
[molmolMo–1 h–1]
reaction with time is typical of dioxidomolybdenum(VI)
complexes of the type [MoO2X2(L)n] [where (L)n is one bi-
dentate or two monodentate Lewis base N,O ligands] used
as catalysts in the same reaction, under similar condi-
tions.[3a,26,27] One explanation for this behaviour is that the
accumulation of tert-butyl alcohol, formed as a byproduct
of the decomposition of tbhp, during the batch-wise reac-
tion is detrimental because it is able to coordinate to the
1
565
561
577
574
575
102
573
571
98/99
99/100
99/100
98/99
99/100
94/100
99/100
99/100
90/99
90/99
91/100
83/97
91/100
89/99
89/100
91/100
2
3
4
5
6
[MoO2Cl2(thf)2]
[MoO2Cl2(CH3CN)2]
[a] Turnover frequency calculated at ca. 10 min. [b] Cyclooctene
metal centre, blocking free coordination sites for the acti- conversion calculated at 6/24 h for runs 1 and 2. The epoxide selec-
tivity was always 100%.
vation of the olefin. On the other hand, transformations of
The influence of the solvent on cyclooctene epoxidation
in the presence of diphenylformamide adduct 5 was investi-
gated at 55 °C (Table 3, Figure 4). Depending on whether
the solvent was ethanol, acetonitrile or 1,2-dichloroethane
(dce), the catalytic activity changed considerably without
affecting epoxide selectivity, which was always 100%. Al-
though the complex is more soluble in CH3CN or ethanol
(transparent solutions were obtained) than in dce (the mix-
ture was opaque, as observed without cosolvent), the reac-
tion was faster in the latter case, leading to more than 99%
conversion after 10 min, which is somewhat higher than
that observed when no cosolvent was used. The negative
effect of CH3CN and especially of ethanol on initial cata-
lytic activity may be due to the coordinating abilities of
these solvent molecules, consistent with the above dis-
cussion concerning the negative effect of the byproduct tert-
butyl alcohol on the overall reaction rate. It is also possible
that metal–ligand dissociation (discussed below) is en-
hanced in the presence of these coordinating solvents, origi-
nating less-active species.
Table 3. Olefin epoxidation at 55 °C with the use of complex 5,
tbhp (in decane) and different cosolvents.
Olefin
Cosolvent
TOF[a]
Conv.[b]
[%]
Select.[b]
[%]
[molmolMo–1h–1]
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
(R)-(+)-Limonene
α-(+)-Pinene
Norbornene
none
ethanol
CH3CN
dce
dce
dce
575
91
516
605
205
52
99/100
86/99
89/91
100/100
92/98
21/28
100/100
100/100
100/100
100/100
90/73[c]
Ͻ1[d]
dce
321
96/100
92/81[e]
Figure 3. (A) Cyclooctene concentration as a function of reaction
time in two consecutive 24 h runs at 55 °C by using tbhp (5.5  in
decane) without a cosolvent, in the presence of 1 (ϫ), 2 (᭺), 3
[a] Turnover frequency calculated at ca. 10 min. [b] Olefin conver-
sion and epoxide selectivity calculated at 6/24 h. [c] Sum of selec-
tivities to 1,2-epoxy-p-menth-8-ene and 1,2–8,9-diepoxy-p-men-
(᭝), 4 (+), 5 (-), 6 (ᮀ), MoO2Cl2 (Ɇ), [MoO2Cl2(thf)2] (᭛) and thane. At 24 h, the epoxide/diepoxide mol ratio was 0.7. [d] Selec-
[MoO2Cl2(CH3CN)2] (–). The inset shows in detail the behaviour
during the first 6 h of the first run for all complexes except
[Mo2O4(µ2-O)Cl2(dmf)4] (6). (B) Cyclooctene conversion as a func-
tion of time (in the first run) for all complexes except 6.
tivity to campholenic aldehyde at 6/24 h was 61/51%. Epoxy cam-
pholenic aldehyde and small amounts of other unidentified prod-
ucts were formed. [e] Dihydroxybicyclo[2,2,1]heptane and small
amounts of other unidentified products were formed.
Eur. J. Inorg. Chem. 2009, 4528–4537
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4531

I. S. Gonçalves et al.
FULL PAPER
deactivation through, for example, hydrolysis. Nevertheless,
after 24 h, cyclooctene conversion reached 93% and epox-
ide selectivity was 100%.
The catalyst stability was investigated for 1–5 by carrying
out two consecutive runs of cyclooctene epoxidation at
55 °C by using tbhp (in decane) without cosolvent. After
the first run of 24 h, the reactor was recharged with olefin
and oxidant in equivalent amounts to those used in the first
run. As found for the first runs, the kinetic curves for the
second runs were practically coincident for all five systems
(Figure 3), and epoxide selectivity was always 100%. Al-
though the initial reaction rate decreased from the first to
the second run, the epoxide yield after 24 h was practically
the same for both runs (Ͼ97%). These results are far supe-
rior to the reported behaviour of [MoO2Br2(CH3CN)2],
which was completely deactivated after approximately 2 h,
giving a maximum conversion of 65%.[7] For comparison,
two consecutive runs of cyclooctene epoxidation were car-
ried out by using MoO2Cl2, [MoO2Cl2(thf)2] and
[MoO2Cl2(CH3CN)2] as (pre)catalysts, under similar condi-
tions to those used for 1–5. Upon exposure to air, the ini-
tially pale-yellow MoO2Cl2 rapidly turned blue, which is in-
dicative of MoV species. However, the yellow colour was
Figure 4. Kinetic profiles of cyclooctene epoxidation in the pres-
ence of 5 by using tbhp (5.5  in decane) without a cosolvent at
55 °C (ϫ) or 30 °C (-) or by using 1,2-dichloroethane (ᮀ), ethanol
(᭺) or acetonitrile (᭝) as cosolvents at 55 °C, or using aqueous
tbhp (70 wt.-%) without a cosolvent at 55 °C (+).
The epoxidation of other olefins, namely, (R)-(+)-limon- restored in the presence of tbhp. The catalytic performance
ene, α-pinene and norbornene, was investigated for 5 with of the three systems was very similar to that for 1–5, giving
the use of dce as cosolvent, at 55 °C (Table 3). The reaction a TOF in the range of 571–576 molmolMo–1 h–1 for the first
of limonene gave 1,2-epoxy-p-menth-8-ene and 1,2–8,9-diep- run, and practically coincident kinetic profiles for both runs
oxy-p-menthane as the main products and the respective di- (Figure 3, Table 2). This implies that the coordinated sol-
ols as byproducts; after 24 h the total epoxide yield was 72% vent molecules in the starting complexes of the type
against 26% of diols (small amounts of other unidentified [MoO2Cl2(L)2] do not have a significant influence on the
products were also formed). The reaction of α-pinene was outcome of the catalytic olefin epoxidation. Possibly, the
much slower and gave mainly campholenic aldehyde (until steric and electronic (supported by the FTIR data discussed
28% conversion), an important intermediate used in the fra- above) effects on the activity of the complexes are similar
grance industry for the synthesis of santalol, which is the (activation parameters obtained by UV/Vis are similar, as
principle component of sandalwood oil.[28,29] The amount of discussed below) for the distinct monodentate, labile ligands
α-pinene oxide detected was residual (Ͻ1% yield), possibly (which may explain the similar initial TOFs for the fresh
because it undergoes Lewis acid catalysed isomerisation to catalysts) and/or the latter become at least partially dissoci-
give campholenic aldehyde (13% yield after 6 h).[28,29] Nor- ated under the reaction conditions used, originating active
bornene was reactive with tbhp in the absence of a catalyst, species that are somewhat alike in the second run.
giving epoxynorbornane as the only product. Although the
As described above, treatment of [MoO2Cl2(dmf)2] (1)
initial reaction rate was relatively slow, 63% conversion was with a large excess of tbhp (15 equiv.) in decane/dmf led
achieved after 24 h. In the presence of 5, the TOF was much to the isolation of the µ-oxido binuclear molybdenum(VI)
higher (321 molmolMo–1h–1) than without catalyst, and complex [Mo2O4(µ2-O)Cl2(dmf)4] (6). This complex was
selectivity to epoxynorbornane was 96% at 62% conversion, tested in two consecutive runs of catalytic cis-cyclooctene
decreasing to 81% at 100% conversion.
epoxidation at 55 °C, under similar conditions to those used
When the temperature of the cyclooctene reaction in the for 1–5. The epoxide was the only observed product. The
presence of 5 was lowered from 55 to 30 °C, the initial TOF profiles for 6 in runs 1 and 2 were very similar (Figure 3).
decreased from 575 to 496 molmolMo–1 h–1, without affect- Whereas in the first run the reaction was slower for 6 (TOF
ing epoxide selectivity. However, after 1 h, approximately
= = 565–
102 molmolMo–1 h–1) than for 1–5 (TOF
the same conversion was reached (94–97%, Figure 4). The 577 molmolMo–1 h–1), in the second run the kinetic profiles
use of 70 wt.-% aqueous tbhp instead of the 5.5  decane were essentially coincident for 1–6. This suggests that the
solution gave a liquid–liquid biphasic system, and the reac- formation of µ-oxido binuclear molybdenum(VI) species
tion rate at 55 °C decreased significantly (TOF decreased from starting complexes 1–5 is likely to play an important
from 575 to 88 molmolMo–1 h–1, Figure 4). The following role with respect to the main species responsible for the
factors may contribute to the lower catalytic activity: (1) catalytic reactions in the second runs.
Mass transfer limitations and the distribution of tbhp in the
The reaction systems formed using complexes 1–6 and
liquid–liquid biphasic system. (2) The coordinating ability MoO2Cl2 are not homogeneous when no solvent is added.
of the water molecules (inhibitory effect). (3) Partial catalyst After catalytic runs of 24 h for cyclooctene epoxidation at
4532
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4528–4537

Catalytic Olefin Epoxidation of Dichloridodioxidomolybdenum(VI) Complexes
55 °C, the solids were separated by centrifugation, washed a large excess over the catalyst precursor, the reactions follow
thoroughly with hexane, dried at room temperature (giving reversible pseudo-first-order kinetics. A typical set of spectra
ca. 10 wt.-% of the amounts of complexes initially charged are given in the Supporting Information for the reaction of
to the reactor) and characterised by FTIR spectroscopy. [MoO2Cl2(dmf)2] (1) (4ϫ10–4 moldm–3) with tbhp
The spectra in the region 500–1000 cm–1 were very similar (0.03 moldm–3) at 25 °C. Over 4 h an absorbance at 300 nm
for all recovered materials, showing six medium-strong non- decreased with time and an absorbance in the range 323–
ligand absorptions at 555–570, 620–630, 685–700, 778–798, 400 nm increased (due to the formation of a new species)
882–902 and 950–960 cm–1, which resemble the six nonli- generating an isosbestic point at 323 nm. The absorbance–
gand bands exhibited by complex 6 at 567, 630, 681, 767, time curves fit to a first-order exponential equation, At =
903 and 948 cm–1. In contrast, the amide ligand absorptions (A0 – Aϱ)exp(–kobst) + Aϱ, where At, A0 and Aϱ are the
in the region 1000–1700 cm–1 were either absent or reduced absorbance values at time t, at t = 0 and at equilibrium,
to very weak intensity (compared with complexes 1–6) for respectively, and kobs is the observed rate constant. Figure 5
the recovered solids. This was backed up by elemental shows a typical fit to absorbance (λ = 300 nm) versus time
analysis of the solid recovered after catalysis using dmf ad- data for the reaction of [MoO2Cl2(dmpa)2] (3) with tbhp at
duct 1, which revealed a significant reduction in the amount 25 °C; all kobs values are given in the Supporting Infor-
of C (from 8.0 to 2.7 wt.-%), N (from 20.5 to 8.9 wt.-%) mation. The fit of kobs values against oxygen donor concen-
and H (from 4.3 to 2.5 wt.-%). For complex 5, GC–MS tration shows a clear first-order dependence on [tbhp], as
analysis of the reaction solution showed the presence of free exemplified in Figure 6 for [MoO2Cl2(dma)2] (2) at 25 °C,
N,N-diphenylformamide.
For complex 5, the solid (denoted 5-S) and liquid (de-
noted 5-L) phases were separated after a first run by centri-
fugation and filtration using a 0.2 µm PVDF w/GMF
Whatman membrane. After washing with n-hexane and
drying at room temperature, colourless 5-S was tested in a
second run by adding oxidant and olefin in amounts pro-
portional to those used in the first run. The reaction of
cyclooctene in the presence of 5-S gave 55%/95% conver-
sion at 30 min/6 h, indicating that the solid is active albeit
the reaction rate is lower than that observed for 5 (95%/
99% conversion at 30 min/6 h). The activity of the homo-
geneous phase was investigated by recharging oxidant and
olefin (in the same amounts as those used in the first run
for 5) to the 5-L solution and monitoring the reaction for
a further 6 h, during which time no solid precipitated. The
Figure 5. Absorbance (λ = 300 nm) against time data for the reac-
tion of [MoO2Cl2(dmpa)2] (3) with tbhp (the solid line shows the
fit to a first order decay equation). Reaction conditions: concentra-
recycled 5-L gave 51%/85% conversion after 1 h/6 h, which
is comparable with that observed for the second run of 5
without separating the solid (58%/91% after 1 h/6 h, Fig-
ure 3A). Hence, the catalytic reaction is essentially homo-
geneous, albeit a minor amount of active solid is present in
the first run.
tion of complex
3
=
4.0ϫ10–4 moldm–3, [tbhp]
=
3.0ϫ10–2 moldm–3, solvent = CH3CN with less than 1% of dec-
ane, T = 25 °C.
Taken together with the catalytic performances described
above, it seems that (1) the nature of the active species
formed under the reaction conditions used is similar for all
the investigated complexes, (2) (in)soluble active species
possessing the [(MoO2)2(µ-O)]2+ core are formed during the
first run, (3) the catalytic reaction is mainly due to active
species in homogeneous phase and (4) the solid fraction
contains active species formed upon metal–ligand dissoci-
ation.
UV/Vis Kinetic Studies of the Reaction of 1–5 with tbhp
The reaction of [MoO2Cl2(L)2] (1–5) with tbhp at 20–
50 °C was followed by UV/Vis spectroscopy (in acetonitrile).
Control experiments carried out in the absence of tbhp
showed no spectral changes with time. Under the conditions
Figure 6. Reversible pseudo-first-order observed rate constants,
kobs, at different [tbhp] values. Reaction conditions: concentration
of [MoO2Cl2(dma)2] (2) = 3.0ϫ10–4 moldm–3, solvent = CH3CN
described in the Experimental Section, with tbhp present in with less than 1% of decane, T = 25 °C.
Eur. J. Inorg. Chem. 2009, 4528–4537
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4533

I. S. Gonçalves et al.
FULL PAPER
and the slope gives the apparent second-order rate constant epoxidation reaction. The kinetic studies suggest that the
(k) for the reaction of the Mo complex with tbhp. The k reaction of the precursor with tbhp is a primary rate-de-
values increased significantly with temperature (Table 4).
termining elementary step.
Table 4. Measured second-order rate constants, k, at different tem-
peratures for the reaction of complexes 1–5 with tbhp.[a]
Table 5. Values of activation parameters obtained for the different
molybdenum complexes.
T
[°C]
Complex Complex Complex Complex Complex
Complex
∆H϶ [kJmol–1]
∆S϶ [Jmol–1 K–1]
1
2
3
4
5
[MoO2Cl2(dmf)2] (1)
[MoO2Cl2(dma)2] (2)
[MoO2Cl2(dmpa)2] (3)
[MoO2Cl2(def)2] (4)
[MoO2Cl2(dpf)2] (5)
68Ϯ2
67Ϯ4
73Ϯ6
66Ϯ2
67Ϯ3
–38Ϯ7
–42Ϯ14
–23Ϯ18
–45Ϯ7
20
25
35
40
50
5.0Ϯ0.2 4.87Ϯ0.07 3.1Ϯ0.1
4.2Ϯ0.2
7.3Ϯ0.3
18.7Ϯ0.4
28Ϯ1
5.0Ϯ0.2
7.7Ϯ0.3
21Ϯ1
7.1Ϯ0.4
19.7Ϯ0.8
32Ϯ2
8.7Ϯ0.2
25Ϯ1
34Ϯ2
5.6Ϯ0.4
18Ϯ1
28Ϯ2
36Ϯ1
–40Ϯ11
70Ϯ3
67Ϯ4
51Ϯ1
58Ϯ3
65Ϯ2
[a] Values are 102k [mol–1 dm3 s–1].
Concluding Remarks
Following the Eyring equation (for an ideal system),
plots of ln(kh/kBT) versus 1/T, where h is Planck’s constant
and kB is the Boltzmann constant, gave straight lines with
slope –∆H϶/R and intercept ∆S϶/R (Figure 7). The acti-
vation parameters are collected in Table 5. The k values at
a given temperature and the ∆H϶ values are similar for all
five complexes, which parallels the similar initial activity
observed for all five complexes in the epoxidation of cyclo-
octene with tbhp. Furthermore, the values of the activation
parameters are of the same order of magnitude as those
determined previously (using the same method) for com-
plexes of the type [MoO2Cl2L], where L is a bidentate Lewis
base ligand.[26] The negative ∆S϶ values were interpreted to
support an associative mechanism involving the formation
of one adduct between the oxygen donor, tbhp and the
metal centre. It is also possible that solvation of the reagents
and intermediate species leads to increased ordering in the
surrounding solvent. For the catalytic systems studied in
the present work, we assume that the active intermediate is
mainly responsible for the very high initial olefin epoxid-
ation rates in the first runs. One hypothesis for the catalytic
behaviour at longer reaction times and in the second runs
is that the intermediate rapidly reacts with tbhp and/or re-
sidual water to give an oxido-bridged bimetallic species,
which may further react with tbhp to give a second type of
active intermediate that is increasingly responsible for the
The solvent adducts [MoO2Cl2(L)2] containing N,N-di-
alkylamide ligands exhibit very high initial activities in the
epoxidation of cis-cyclooctene with tbhp under mild condi-
tions (30–55 °C) and in the absence of cosolvents. These
complexes are easy to prepare and are relatively stable to
air and moisture. The turnover frequencies at 55 °C lie at
the top end of the range (10–600 molmolMo–1 h–1) found for
the same reaction under similar conditions with the use of
[MoO2Cl2(LЈ)n] complexes containing two monodentate li-
gands or one bidentate ligand, with most falling below
250 molmolMo–1 h–1. It must be noted that the catalytic
epoxidation reactions with the [MoO2Cl2(L)2] complexes
were very clean, giving the epoxide quantitatively within a
few hours. Changing the nature of the substituents (H,
methyl, ethyl, phenyl) on the carbonyl group and nitrogen
atom of the N,N-dialkylamide does not lead to measurably
different catalytic behaviour, and indeed very similar results
are obtained by using the complexes with L = CH3CN and
thf, or even the precursor MoO2Cl2. The very high initial
activities are offset by the decreased reaction rates in the
second runs. Whereas the accumulation of tert-butyl
alcohol coupled with the dilution of the reaction mixture
in the second run could have some influence, the evidence
suggests that the major cause is the formation of binuclear
species with a structure similar to that determined for
[Mo2O4(µ2-O)Cl2(dmf)4], which was isolated from the reac-
tion of [MoO2Cl2(dmf)2] with an excess amount of tbhp.
Experimental Section
Materials and Methods: Microanalyses for CHN were performed
at the University of Aveiro. IR spectra were obtained as KBr pellets
by using an FTIR Mattson-7000 infrared spectrophotometer. Ra-
man spectra were recorded with a Bruker RFS100/S FT instrument
(Nd:YAG laser, 1064 nm excitation, InGaAs detector). 1H NMR
spectra were acquired at room temperature by using a Bruker CXP
300 spectrometer and referenced to TMS. All preparations and ma-
nipulations were carried out by using standard Schlenk techniques
under an atmosphere of N2. Anhydrous dichloromethane and
MoO2Cl2 were obtained from Aldrich and used as received. Hex-
ane and diethyl ether were dried by standard procedures (Na/
benzophenone ketyl), distilled, and kept under an atmosphere of
N2 over 4 Å molecular sieves. The complex [MoO2Cl2(dmf)2] (1)
was synthesised as described previously by treatment of an aqueous
Figure 7. Fit of the second-order rate constants, k, to the Eyring
equation (T = 20–50 °C) for complex [MoO2Cl2(def)2] (4; def =
N,N-diethylformamide).
4534
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4528–4537

Catalytic Olefin Epoxidation of Dichloridodioxidomolybdenum(VI) Complexes
hydrochloric acid solution of MoO3 with an excess amount of
dmf[8a] and gave satisfactory elemental analyses, FTIR and NMR
spectroscopic data. The tbhp (5.5  in decane, ca. 5 wt.-% water)
was used as received.
SAINT+,[33] and data were corrected for absorption by the
multiscan semiempirical method implemented in SADABS.[34] The
structure was solved by using the Patterson synthesis algorithm
implemented in SHELXS-97,[35] which allowed the immediate loca-
tion of the crystallographically independent MoVI centre and the
coordinated chlorido ligand. All remaining non-hydrogen atoms
were directly located from difference Fourier maps calculated from
successive full-matrix least-squares refinement cycles on F2 by using
SHELXL-97.[36] Non-hydrogen atoms were successfully refined by
using anisotropic displacement parameters. Hydrogen atoms were
located at their idealised positions by using appropriate HFIX in-
structions in SHELXL (43 and 137 for the formamide and methyl
groups of dmf, respectively) and included in subsequent refinement
cycles in riding-motion approximation with isotropic thermal dis-
placement parameters (Uiso) fixed at 1.2 or 1.5, respectively, times
Ueq of the carbon atom to which they are attached. The last differ-
ence Fourier map synthesis showed the highest peak (0.871 eÅ–3)
and deepest hole (–1.031 eÅ–3) located at 0.83 Å and 0.78 Å from
Mo(1). Selected bond lengths and angles for the MoVI coordination
environment are collected in Table 1. CCDC-654609 contains the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
General Method for the Preparation of Complexes 2–5: A suspen-
sion of MoO2Cl2 (1.0 g, 5.0 mmol) in CH2Cl2 (30 mL) was treated
with the ligand (2 equiv.). The solution was stirred for 2 h, filtered
and evaporated to dryness in vacuo, and the resultant light-yellow
solid was washed with diethyl ether and dried in vacuo.
[MoO2Cl2(dma)2] (2; dma = N,N-dimethylacetamide): Yield: 1.80 g,
96%. C8H18Cl2MoN2O4 (373.09): calcd. C 25.75, H 4.85, N 7.5;
1
found C 25.85, H 5.2, N 7.45. H NMR (300 MHz, CDCl3): δ =
2.42 [s, 6 H, CH C(O)], 3.14 [s, 12 H, N(CH ) ] ppm. IR (KBr): ν
˜
3
3 2
= 1608 (vs, νCO), 1509 (m, νCN of OC–N), 950 (s), 911 (vs, νMo=O
)
cm–1. Raman: ν = 1505 (w), 947 (s), 909 (w) cm–1.
˜
[MoO2Cl2(dmpa)2] (3; dmpa = N,N-dimethylpropionamide): Yield:
1.73 g, 86%. C10H22Cl2MoN2O4 (401.14): calcd. C 29.95, H 5.55,
N 7.0; found C 30.15, H 5.95, N 6.95. 1H NMR (300 MHz,
CDCl3):
δ = 1.29 [t, 6 H, CH3CH2C(O)], 2.74 [q, 4 H,
CH CH C(O)], 3.17 [s, 12 H, N(CH ) ] ppm. IR (KBr): ν = 1602
˜
3
2
3 2
(vs, νCO), 1510 (m, νCN of OC–N), 950 (s), 911 (s, νMo=O) cm–1.
Raman: ν = 1514 (w), 946 (vs), 906 (w) cm–1.
˜
[MoO2Cl2(def)] (4; def = N,N-diethylformamide): Yield: 1.85 g,
92%. C10H22Cl2MoN2O4 (401.14): calcd. C 29.95, H 5.55, N 7.0;
found C 30.2, H 5.9, N 6.95. 1H NMR (300 MHz, CDCl3): δ =
1.22–1.33 [m, 12 H, N(CH2CH3)2], 3.42–3.54 [m, 8 H, N(CH2-
Table 6. Crystal and structure refinement data for [Mo2O4(µ2-O)
Cl2(dmf)4] (6).
Formula
C12H28Cl2Mo2N4O9
635.16
monoclinic
C2/c
Formula weight
Crystal system
Space group
a [Å]
CH ) ], 8.35 [s, 2 H, HC(O)] ppm. IR (KBr): ν = 1637 (vs, νCO),
˜
3 2
945 (s), 908 (s, νMo=O) cm–1. Raman: ν = 943 (vs), 915 (w) cm–1.
˜
12.6523(7)
[MoO2Cl2(dpf)2] (5; dpf = N,N-diphenylformamide): Yield: 2.7 g,
b [Å]
c [Å]
13.3656(7)
13.1889(7)
91%. C26H22Cl2MoN2O4 (593.31): calcd. C 52.65, H 3.75, N 4.7;
1
found C 52.45, H 3.55, N 5.1. H NMR (300 MHz, CDCl3): δ =
β [°]
90.952(4)
2230.0(2)
4
1.892
1.414
0.18ϫ0.16ϫ0.08
colourless blocks
3.78 to 30.49
–17 Յ h Յ 17
–19 Յ k Յ 17
–18 Յ l Յ 18
20032
7.12–7.42 (m, 20 H, phenyl-H), 8.73 [s, 2 H, HC(O)] ppm. IR
Volume [Å3]
Z
(KBr): ν = 1630 (vs), 1618 (vs, νCO), 950 (s), 910 (s, νMo=O) cm–1.
˜
Raman: ν = 951 (s), 912 (w) cm–1.
Dcalcd. [gcm–3]
µ(Mo-Kα) [mm–1]
Crystal size [mm]
Crystal type
θ range
˜
[Mo2O4(µ2-O)Cl2(dmf)4] (6; dmf = N,N-dimethylformamide): A
solution of [MoO2Cl2(dmf)2] (0.3 g, 0.87 mmol) in dmf (6 mL) was
treated with tbhp (5.5  in decane, 13 mmol), and the solution was
stirred in an ice bath for 2 h. A mixture of hexane and diethyl ether
was then added to initiate precipitation of a solid, and the mixture
was left to age at –30 °C for 3 d. After complete precipitation, the
solution was filtered off and the solid dried in vacuo. Yield: 0.19 g,
69%. C12H28Cl2Mo2N4O9 (635.16): calcd. C 22.7, H 4.45, N 8.8;
Index ranges
Reflections collected
Independent reflections
Completeness to θ = 30.49°
Final R indices [IϾ2σ(I)][a,b]
3364 (Rint = 0.0564)
99.2%
1
found C 22.65, H 4.5, N 8.75. H NMR (300 MHz, CD3COCD3):
R1 = 0.0331
wR2 = 0.0709
R1 = 0.0497
wR2 = 0.0774
m = 0.0362
n = 1.6573
0.871 and –1.031 eÅ–3
δ = 2.84 (s, 6 H, N-CH3), 2.93 (s, 12 H, N-CH3), 3.01 (s, 6 H, N-
CH ), 8.03 [s, 4 H, HC(O)] ppm. IR (KBr): ν = 1649 (vs, νCO), 948
˜
3
Final R indices (all data)[a,b]
Weighting scheme[c]
(s), 903 (vs, νMo=O), 767 (vs, νMo–O–Mo) cm–1. Raman: ν = 939 (vs),
˜
902 (m, νMo=O) cm–1.
Single-Crystal X-ray Diffraction: Details of the crystallographic
data collection and structure refinement are summarised in Table 6.
Single crystals of 6 suitable for X-ray analysis were obtained by
slow evaporation of a solution of the complex in dmf. A single
crystal was manually harvested from the crystallisation vial and
mounted on a Hampton Research CryoLoop using Fomblin Y per-
fluoropolyether vacuum oil (Aldrich LVAC 25/6)[30] with the help
of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses.
Data were collected at 100(2) K with a Bruker X8 Kappa APEX
II CCD area-detector diffractometer (Mo-Kα graphite-monochro-
mated radiation, λ = 0.7107 Å) controlled by the APEX2 software
package,[31] and equipped with an Oxford Cryosystems Series 700
cryostream monitored remotely by using the software interface
Cryopad.[32] Images were processed by using the software package
Largest diff. peak and hole
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = √Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2].
2
2 2
[c] w = 1/[σ2(Fo2) + (mP)2 + nP], where P = (Fo + 2Fc )/3.
2
2
Catalysis: The liquid-phase catalytic epoxidations were carried out
at atmospheric pressure in a reaction vessel equipped with a mag-
netic stirrer and immersed in a thermostatted oil bath. Typically,
the microvessel was loaded with the complex (18 µmol), olefin
(1.8 mmol) and oxidant (2.75 mmol, 5.5  tbhp in decane or 70%
aq. tbhp) without additional solvent (other than the decane present
in the tbhp solution) or with ethanol, 1,2-dichloroethane (dce) or
acetonitrile (2 mL). Different substrates were studied, namely, cis-
cyclooctene, (R)-(+)-limonene, α-(+)-pinene and norbornene in the
Eur. J. Inorg. Chem. 2009, 4528–4537
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4535

I. S. Gonçalves et al.
FULL PAPER
[5]
[6]
C. D. Nunes, A. A. Valente, M. Pillinger, J. Rocha, I. S. Gonç-
alves, Chem. Eur. J. 2003, 9, 4380–4390.
N. Manwani, M. C. Gupta, R. Ratnani, J. E. Drake, M. B.
Hursthouse, M. E. Light, Inorg. Chim. Acta 2004, 357, 939–
945.
F. E. Kühn, E. Herdtweck, J. J. Haider, W. A. Herrmann, I. S.
Gonçalves, A. D. Lopes, C. C. Romão, J. Organomet. Chem.
1999, 583, 3–10.
a) F. J. Arnaiz, R. Aguado, J. Sanz-Aparicio, M. Martinez-Ri-
poll, Polyhedron 1994, 13, 2745–2749; b) F. J. Arnáiz, R. Agu-
ado, M. R. Pedrosa, J. Mahía, M. A. Maestro, Polyhedron
2001, 20, 2781–2785; c) W. Levason, R. Ratnani, G. Reid, M.
Webster, Inorg. Chim. Acta 2006, 359, 4627–4630.
a) M. B. Hursthouse, W. Levason, R. Ratnani, G. Reid, Polyhe-
dron 2004, 23, 1915–1921; b) M. F. Davis, W. Levason, R. Rat-
nani, G. Reid, T. Rose, M. Webster, Eur. J. Inorg. Chem. 2007,
306–313.
presence of dce (2 mL). The course of the reaction was monitored
by using a Varian 3800 GC equipped with a capillary column (DB-
5, 30 mϫ0.25 mm) and a flame ionisation detector. The products
were identified by GC–MS (HP 5890 Series II GC and HP 5970
Series Mass Selective Detector) using He as the carrier gas.
Undecane was used as the internal standard added after the reac-
tion.
[7]
[8]
Kinetic Studies of Catalyst Formation: All kinetic measurements
were carried by out using a large excess of tbhp (pseudo-first-order
conditions). Typically, an appropriate small quantity of concen-
trated tbhp (5.5  in decane) was added to a thermostatted UV
quartz cell containing an appropriate amount of a solution of the
metal complex in CH3CN in order to obtain a total volume of
3 mL with final concentration of molybdenum compound equal to
3 to 4ϫ10–4 . The product formation was monitored against time
by following absorbance changes at a specific wavelength in the
range 300–500 nm by using a Varian Cary Bio50 UV/Vis spectro-
photometer equipped with a thermostatted multiple cell holder.
[9]
[10]
[11]
K. Dreisch, C. Andersson, M. Håkansson, S. Jagner, J. Chem.
Soc., Dalton Trans. 1993, 1045–1049.
a) A. M. Al-Ajlouni, A. Günyar, M.-D. Zhou, P. N. W. Baxter,
F. E. Kühn, Eur. J. Inorg. Chem. 2009, 1019–1026; b) F. E.
Kühn, A. D. Lopes, A. M. Santos, E. Herdtweck, J. J. Haider,
C. C. Romão, A. G. Santos, J. Mol. Catal. A 2000, 151, 147–
160; c) F. E. Kühn, A. M. Santos, A. D. Lopes, I. S. Gonçalves,
E. Herdtweck, C. C. Romão, J. Mol. Catal. A 2000, 164, 25–
38.
a) M. D. Brown, M. B. Hursthouse, W. Levason, R. Ratnani,
G. Reid, Dalton Trans. 2004, 2487–2491; b) M. F. Davis, W.
Levason, M. E. Light, R. Ratnani, G. Reid, K. Saraswat, M.
Webster, Eur. J. Inorg. Chem. 2007, 1903–1910.
G. Barea, A. Lledos, F. Maseras, Y. Jean, Inorg. Chem. 1998,
37, 3321–3325.
R. Sanz, R. Aguado, M. R. Pedrosa, F. J. Arnáiz, Synthesis
2002, 856–858.
K. Jeyakumar, D. K. Chand, Appl. Organomet. Chem. 2006,
20, 840–844.
a) P. Chaumette, H. Mimoun, L. Saussine, J. Fischer, A.
Mitschler, J. Organomet. Chem. 1983, 250, 291–310; b) B. Mon-
teiro, S. S. Balula, S. Gago, C. Grosso, S. Figueiredo, A. D.
Lopes, A. A. Valente, M. Pillinger, J. P. Lourenço, I. S. Gonç-
alves, J. Mol. Catal. A 2009, 297, 110–117.
R. Sanz, J. Escribano, M. R. Pedrosa, R. Aguado, F. J. Arnáiz,
Adv. Synth. Catal. 2007, 349, 713–718.
R. J. Butcher, H. P. Gunz, R. G. A. R. Maclagan, H. K. J. Pow-
ell, C. J. Wilkins, J. Chem. Soc., Dalton Trans. 1975, 1223–1227.
W. E. Bull, S. K. Madan, J. E. Willis, Inorg. Chem. 1963, 2,
303–306.
L. O. Atovmyan, Y. A. Sokolova, V. V. Tkachev, Dokl. Akad.
Nauk SSSR 1970, 195, 1355–1356.
a) F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58,
380–388; b) F. H. Allen, W. D. S. Motherwell, Acta Crys-
tallogr., Sect. B: Struct. Sci. 2002, 58, 407–422.
M. V. Caparelli, B. Piggott, S. D. Thorpe, R. N. Sheppard, In-
org. Chim. Acta 1985, 98, L53–L55.
Supporting Information (see also the footnote on the first page of
this article): UV/Vis spectra for the reaction of 1 with tbhp at
25 °C; kobs values for all complexes and reaction temperatures.
Acknowledgments
[12]
We are grateful to the Fundação para a Ciência e a Tecnologia
(FCT), the Programa Operacional “Ciência e Inovação 2010”
(POCI 2010), the Orçamento do Estado (OE) and the Fundo
Europeu de Desenvolvimento Regional (FEDER) (Project POCI/
QUI/56109/2004) for funding. B.M., P.N. and S.G. thank the FCT
for grants. We are also grateful to the FCT for financial support
towards the purchase of the single-crystal diffractometer.
[13]
[14]
[15]
[16]
[1] a) J. M. Tunney, J. McMaster, C. D. Garner, Comprehensive Co-
ordination Chemistry II (Eds.: J. A. McCleverty, T. J. Meyer),
Elsevier, Amsterdam, 2004, vol. 8, pp. 459–477; b) J. H. Ene-
mark, J. J. A. Cooney, J.-J. Wang, R. H. Holm, Chem. Rev.
2004, 104, 1175–1200; c) C. J. Whiteoak, G. J. P. Britovsek,
V. C. Gibson, A. J. P. White, Dalton Trans. 2009, 2337–2344; d)
A. L. Bingham, J. E. Drake, M. B. Hursthouse, M. E. Light,
R. Kumar, R. Ratnani, Polyhedron 2006, 25, 3238–3244; e) R. J.
Butcher, B. R. Penfold, E. Sinn, J. Chem. Soc., Dalton Trans.
1979, 668–675.
[2] a) F. J. Arnáiz, R. Aguado, J. M. Martinez de llarduya, Polyhe-
dron 1994, 13, 3257–3259; b) F. J. Arnáiz, R. Aguado, M. R.
Pedrosa, A. D. Cian, Inorg. Chim. Acta 2003, 347, 33–40; c)
F. J. Arnáiz, R. Aguado, M. R. Pedrosa, J. Mahía, M. A. Mae-
stro, Polyhedron 2002, 21, 1635–1642.
[3] a) F. E. Kühn, M. Groarke, É. Bencze, E. Herdtweck, A. Praz-
eres, A. M. Santos, M. J. Calhorda, C. C. Romão, I. S. Gonç-
alves, A. D. Lopes, M. Pillinger, Chem. Eur. J. 2002, 8, 2370–
2383; b) F. E. Kühn, A. M. Santos, M. Abrantes, Chem. Rev.
2006, 106, 2455–2475; c) S. M. Bruno, B. Monteiro, M. S. Bal-
ula, C. Lourenço, A. A. Valente, M. Pillinger, P. Ribeiro-Claro,
I. S. Gonçalves, Molecules 2006, 11, 298–308; d) S. M. Bruno,
C. C. L. Pereira, M. S. Balula, M. Nolasco, A. A. Valente, A.
Hazell, M. Pillinger, P. Ribeiro-Claro, I. S. Gonçalves, J. Mol.
Catal. A 2007, 261, 79–87; e) G. Wang, G. Chen, R. L. Luck,
Z. Wang, Z. Mu, D. G. Evans, X. Duan, Inorg. Chim. Acta
2004, 357, 3223–3229; f) A. Jimtaisong, R. L. Luck, Inorg.
Chem. 2006, 45, 10391–10402; g) L. Feng, E. Urnezius, R. L.
Luck, J. Organomet. Chem. 2008, 693, 1564–1571.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
J. A. Craig, E. W. Harlan, B. S. Snyder, M. A. Whitener, R. H.
Holm, Inorg. Chem. 1989, 28, 2082–2091.
F. J. Arnáiz, R. Aguado, M. R. Pedrosa, A. De Cian, J. Fischer,
Polyhedron 2000, 19, 2141–2147.
C. P. Rao, A. Sreedhara, P. V. Rao, M. B. Verghese, K. Ris-
sanen, E. Kolehmainen, N. K. Lokanath, M. A. Sridhar, J. S.
Prasad, J. Chem. Soc., Dalton Trans. 1998, 2383–2393.
A. Al-Ajlouni, A. A. Valente, C. D. Nunes, M. Pillinger, A. M.
Santos, J. Zhao, C. C. Romão, I. S. Gonçalves, F. E. Kühn, Eur.
J. Inorg. Chem. 2005, 1716–1723.
A. A. Valente, J. Moreira, A. D. Lopes, M. Pillinger, C. D.
Nunes, C. C. Romão, F. E. Kühn, I. S. Gonçalves, New J.
Chem. 2004, 28, 308–313.
[26]
[27]
[28]
[4] a) M. L. Larson, F. W. Moore, Inorg. Chem. 1966, 5, 801–805;
b) L. R. Florian, E. R. Corey, Inorg. Chem. 1968, 7, 722–725;
c) J. E. Drake, M. B. Hursthouse, M. E. Light, R. Kumar, R.
Ratnani, J. Chem. Crystallogr. 2007, 37, 421–427.
J. B. Lewis, G. W. Hedrick, J. Org. Chem. 1965, 30, 4271–4275.
4536
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 4528–4537

Catalytic Olefin Epoxidation of Dichloridodioxidomolybdenum(VI) Complexes
[29] P. J. Kunkeler, J. C. van der Waal, J. Bremmer, B. J. Zuurdeeg,
R. S. Downing, H. van Bekkum, Catal. Lett. 1998, 53, 135–
138.
[30] T. Kottke, D. Stalke, J. Appl. Crystallogr. 1993, 26, 615–619.
[31] APEX2, Data Collection Software, version 2.1-RC13, Bruker
AXS, Delft, The Netherlands, 2006.
[32] Cryopad, Remote Monitoring and Control, version 1.451, Ox-
ford Cryosystems, Oxford, UK, 2006.
[33] SAINT+, Data Integration Engine, version 7.23a, Bruker AXS,
Madison, WI, USA, 2005.
[34] G. M. Sheldrick, SADABS, Bruker/Siemens Area Detector Ab-
sorption Correction Program, version 2.01, Bruker AXS, Madi-
son, WI, USA, 1998.
[35] G. M. Sheldrick, SHELXS-97, Program for Crystal Structure
Solution, University of Göttingen, Göttingen, Germany, 1997.
[36] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Göttingen, Göttingen, Germany,
1997.
Received: June 16, 2009
Published Online: September 10, 2009
Eur. J. Inorg. Chem. 2009, 4528–4537
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4537