Molybdenum η3-allyl dicarbonyl complexes as a new class of precursors for highly reactive epoxidation catalysts with tert-Butyl hydroperoxide

Article abstract of DOI:10.1021/om700348w

New Mo(II) diimine derivatives of [Mo(η³-allyl)X(CO) ₂(CH₃CN)₂] (allyl = C₃H₅ and C₅H₅O; X = Cl, Br) were prepared, and [Mo(η³-C₃H₅)Cl(CO)₂(BIAN)] (BIAN = 1,4-(4-chloro)phenyl-2,3-naphthalenediazabutadiene) (7) was structurally characterized by single-crystal X-ray diffraction. This complex adopted an equatorial-axial arrangement of the bidentate ligand (axial isomer), in contrast with the precursors, found as the equatorial isomer in the solid and fluxional in solution. The new complexes of the type [Mo(η³-allyl)X(CO) ₂(N-N)] (N-N is a bidentate chelating dinitrogen ligand) were tested for the catalytic epoxidation of cyclooctene using tert-butyl hydroperoxide as oxidant. All catalytic systems were 100% selective toward epoxide formation. While their turnover frequencies paralleled those of related Mo(II) carbonyl compounds or Mo(VI) compounds bearing similar N-donor ligands, they exhibited similar olefin conversions in consecutive catalytic runs. The acetonitrile precursors were generally more active than the diimine complexes, and the chloro derivatives more active than the bromo ones. Combined vibrational and NMR spectroscopy and computational studies (DFT) were used to investigate the nature of the molybdenum species formed in the catalytic system with [Mo(η³-C₃H₅)Cl(CO)₂{1,4-(2,6- dimethyl)-phenyl-2,3-dimethyldiazabutadiene}] (4) and to propose that the resulting species may be dimeric bearing oxide bridges.

Full text of DOI:10.1021/om700348w

5548
Organometallics 2007, 26, 5548-5556
Molybdenum η3-Allyl Dicarbonyl Complexes as a New Class of
Precursors for Highly Reactive Epoxidation Catalysts with tert-Butyl
Hydroperoxide
Joa˜o C. Alonso,† Patr´ıcia Neves,‡ Maria Joa˜o Pires da Silva,§ Susana Quintal,§
Pedro D. Vaz,§ Carlos Silva,‡ Anabela A. Valente,*,‡ Paula Ferreira,†
Maria Jose´ Calhorda,*,§ Vitor Fe´lix,‡ and Michael G. B. Drew|
Departamento de Engenharia Ceraˆmica e do Vidro, CICECO, UniVersidade de AVeiro,
3810-193 AVeiro, Portugal, Departamento de Qu´ımica, CICECO, UniVersidade de AVeiro,
3810-193 AVeiro, Portugal, Departamento de Qu´ımica e Bioqu´ımica, CQB, Faculdade de Cieˆncias,
UniVersidade de Lisboa, 1749-016 Lisboa, Portugal, and School of Chemistry, UniVersity of Reading,
Whiteknights, Reading RG6 6AD, U.K.
ReceiVed April 10, 2007
New Mo(II) diimine derivatives of [Mo(η3-allyl)X(CO)2(CH3CN)2] (allyl ) C3H5 and C5H5O; X )
Cl, Br) were prepared, and [Mo(η3-C3H5)Cl(CO)2(BIAN)] (BIAN ) 1,4-(4-chloro)phenyl-2,3-naphtha-
lenediazabutadiene) (7) was structurally characterized by single-crystal X-ray diffraction. This complex
adopted an equatorial-axial arrangement of the bidentate ligand (axial isomer), in contrast with the
precursors, found as the equatorial isomer in the solid and fluxional in solution. The new complexes of
the type [Mo(η3-allyl)X(CO)2(N-N)] (N-N is a bidentate chelating dinitrogen ligand) were tested for
the catalytic epoxidation of cyclooctene using tert-butyl hydroperoxide as oxidant. All catalytic systems
were 100% selective toward epoxide formation. While their turnover frequencies paralleled those of
related Mo(II) carbonyl compounds or Mo(VI) compounds bearing similar N-donor ligands, they exhibited
similar olefin conversions in consecutive catalytic runs. The acetonitrile precursors were generally more
active than the diimine complexes, and the chloro derivatives more active than the bromo ones. Combined
vibrational and NMR spectroscopy and computational studies (DFT) were used to investigate the nature
of the molybdenum species formed in the catalytic system with [Mo(η3-C3H5)Cl(CO)2{1,4-(2,6-dimethyl)-
phenyl-2,3-dimethyldiazabutadiene}] (4) and to propose that the resulting species may be dimeric bearing
oxide bridges.
completing the coordination sphere.10-17 Since the discovery
of pseudooctahedral molybdenum(II) complexes of the type
[Mo(η3-allyl)X(CO)2(CH3CN)2] (X ) halide) in 1968,18 a series
of derivatives containing the {Mo(η3-allyl)(CO)2}+ moiety has
been easily prepared by substitution of the labile nitrile ligands
and/or the anion X.2,12,14,18,19-27 In particular, compounds of the
type [Mo(η3-allyl)X(CO)2(N-N)], where N-N ) 2,2′-bipyri-
Introduction
The synthesis of materials with controlled compositions,
architectures, and functionalities is of great scientific interest
in the field of catalysis. An important group of such versatile
materials consists of organometallic complexes since their
properties can be fine-tuned by changing the metal (electron
configuration, oxidation state, coordination number, and geom-
etry) and the ligands (binding mode and electronic and steric
properties). Organometallic complexes have been used as
homogeneous catalysts for the industrial production of chemicals
for many years, and the future prospects seem quite promising.1
The availability of suitable starting materials has always been
of considerable importance to the synthetic chemist. η3-Allyl
dicarbonyl complexes of Mo and W play an important role in
both coordination chemistry2-9 and catalytic organic transforma-
tions, and their reactivity depends on the nature of the ligands
(6) Ward, Y. D.; Villanueva, L. A.; Allred, G. D.; Payne, S. C.; Semones,
M. A.; Liebeskind, L. S. Organometallics 1995, 14, 4132.
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(8) Chowdhury, S. K.; Nandi, M.; Joshi, V. S.; Sarkar, A. Organome-
tallics 1997, 16, 1806.
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Organometallics 1998, 17, 820.
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(12) Trost, B. M.; Merlic, C. A. J. Am. Chem. Soc. 1990, 112, 9590.
(13) Faller, J. W.; Linebarrier, D. Organometallics 1988, 7, 1670.
(14) Krska, S. W.; Hughes, D. L.; Reamer, R. A.; Mathre, D. J.; Sun,
Y.; Trost, B. M. J. Am. Chem. Soc. 2002, 124, 12656 and references therein.
(15) Trost, B. M.; Dogra, K.; Hachiya, I.; Emura, T.; Hughes, D. L.;
Krska, S.; Reamer, R. A.; Palucki, M.; Yasuda, N.; Reider, P. J. Angew.
Chem., Int. Ed. 2002, 41, 1929.
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1338.
* To whom correspondence should be addressed.
† Departamento de Engenharia Ceraˆmica e do Vidro, CICECO, Univer-
sidade de Aveiro.
‡ Departamento de Qu´ımica, CICECO, Universidade de Aveiro.
§ Faculdade de Cieˆncias, Universidade de Lisboa.
| School of Chemistry, University of Reading.
(1) Blaser, H.-U.; Indolese, A.; Schnyder A. Curr. Sci. 2000, 78, 1336.
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10.1021/om700348w CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/09/2007

Molybdenum η3-Allyl Dicarbonyl Complexes
Organometallics, Vol. 26, No. 23, 2007 5549
Chart 1
dine or 1,10-phenanthroline, were found to possess some air
and water tolerance.28,23 These compounds have been success-
fully used as catalyst precursors for imine aziridination,29
oxidation of triphenylphosphine with molecular oxygen,30 or
as catalysts for allylic alkylations.10-12 Recent studies have
revealed that complexes [Mo(η3-C3H5)X(CO)2(CH3CN)2] act as
single component initiators for ring-opening metathesis polym-
erization of norbornene and polymerization of terminal acety-
lenes31 and catalyze the oligomerization of Ph(CtC)nPh (n )
1, 2).20
Olefin epoxidation with molybdenum complexes is a par-
ticularly relevant catalytic reaction since homogeneous molyb-
denum catalysts are the basis of important industrial processes,
such as ARCO-Halcon, for epoxidation with alkyl hydroper-
oxides as oxidant.32,33 Recently, it has been found that halo-
carbonyl Mo(II) complexes of the type MoCpCl(CO)3 (Cp)η5-
cyclopentadienyl) react readily with tert-butyl hydroperoxide
(TBHP) in decane to give the corresponding dioxomolybdenum-
(VI) complexes CpMoO2Cl,34 which in turn are active and
selective catalysts for olefins epoxidation using alkyl hydrop-
eroxides as oxidants.35-37 Under certain reaction conditions,
turnover frequencies of up to 21 000 mol mol-1 h-1 were
Mo
found that even surpass that of the well-known MeReO3/H2O2
epoxidation system.34 The carbonyl precursors were applied
directly as catalyst precursors without isolation of the dioxo
complexes (less stable than the former) prior to use, and by
adjustment of the reaction conditions such as temperature, the
oxidative decarbonylation of the catalyst precursor was not rate-
determining in the epoxidation mechanism.37
of the type [Mo(η3-allyl)X(CO)2(N-N)] (X ) Br, Cl) by
reaction of the parent species [Mo(η3-C3H5)X(CO)2(CH3CN)2]
with several bidentate chelating 1,4-diazabutadiene ligands and
to examine, for the first time, how these Mo(II) η3-allyl
dicarbonyl complexes and their precursors perform as catalyst
precursors for the epoxidation of olefins using TBHP. The
different stereochemical and electronic characteristics of the X
and N-N ligands may impart distinct reactivities. For com-
parison, congener organometallic complexes were prepared by
modifying the allyl group from η3-C3H5 to η3-C5H5O.
1,4-Diaza-1,3-butadienes are favorable supporting ligands for
complexes of the family [MoO2X2L] (L ) Lewis base),
enhancing catalyst stability and activity for olefin epoxidation
with TBHP.38-41 In this work, we set out to prepare complexes
(21) Curtis, M. D.; Fotinos, N. A. J. Organomet. Chem. 1984, 272, 43.
(22) tom Dieck, H.; Friedel, H. J. Organomet. Chem. 1968, 14, 375.
(23) Baker, P. K.; Jenkins, A. E. J. Organomet. Chem. 1997, 545-546,
125.
(24) Borgmann, C.; Limberg, C.; Kaifer, E.; Pritzkow, H.; Zsolnai, L.
J. Organomet. Chem. 1999, 580, 214.
(25) Brisdon, B.J.; Day A. J. Organomet. Chem. 1981, 221, 279.
(26) Hunger, M.; Limberg, C.; Kircher, P. Organometallics 2000, 19,
1044.
(27) Chowdhury, S. K.; Samanta, U.; Puranik, V. G.; Sarkar, A.
Organometallics 1997, 16, 2618.
(28) Brisdon, B. J.; Cartwright, M.; Hodson, A. G. J. Organomet. Chem.
1984, 277, 85.
(29) Morales, D.; Pe´rez, J.; Riera, L.; Riera, V.; Corzo-Sua´rez, R.; Garc´ıa-
Granda, S.; Miguel, D. Organometallics 2002, 21, 1540.
(30) Joshi, V. S.; Nandi, M.; Zhang, H.; Haggerty, B. S.; Sarkar, A.
Inorg. Chem. 1993, 32, 1301.
(31) Yamaguchi, Y.; Fujita, A.; Suzuki, N.; Ito, T. J. Mol. Catal., A:
Chem. 2005, 240, 226.
Results and Discussion
Synthesis and Characterization. Derivatives of the MoII-
(η3-allyl)X(CO)2 fragment with bidentate nitrogen ligands,
namely, 1,4-(2,6-dimethyl)phenyl-2,3-dimethyldiazabutadiene,
1,4-(2,6-diisopropyl)phenyldiazabutadiene (DAB), or 1,4-(4-
chloro)phenyl-2,3-(1,9-naphthalene)diazabutadiene (BIAN), were
synthesized from the well-known precursors [Mo(η3-
C3H5)Cl(CO)2(CH3CN)2] (1), [Mo(η3-C3H5)Br(CO)2(CH3CN)2]
(2),22 or [Mo(η3-C5H5O)Br(CO)2(CH3CN)2] (3).42 All the
complexes are depicted in Chart 1. The complexes 4-8 were
obtained by displacement of the acetonitrile ligands of com-
plexes 1-3 and subsequent coordination of the bidentate ligand
to the molybdenum center.
Two stable isomers, axial and equatorial, are known for this
family of complexes, as shown in Chart 2. The preferences
between them are not clear, and often they interconvert in
solution.43 In many structurally characterized species, it has been
observed that bulky ligands tend to favor the formation of the
axial isomer.
(32) Bre´geault, J.-M. Dalton Trans. 2003, 3289.
(33) Jørgensen, K. A. Chem. ReV. 1989, 89, 431.
(34) Abrantes, M.; Santos, A. M.; Mink, J.; Ku¨hn, F. E.; Roma˜o, C. C.
Organometallics 2003, 22, 2112.
(35) Trost, M. K.; Bergman, R. G. Organometallics 1991, 10, 1172.
(36) Martins, A. M.; Roma˜o, C. C.; Abrantes, M.; Azevedo, M. C.; Cui,
J.; Dias, A. R.; Duarte, M. T.; Lemos, M. A.; Lourenc¸o, T.; Poli, R.
Organometallics 2005, 24, 2582.
(37) Valente, A. A.; Seixas, J. D.; Gonc¸alves, I. S.; Abrantes, M.;
Pillinger, M.; Roma˜o, C. C. Catal. Lett. 2005, 101, 127.
(38) Valente, A. A.; Moreira, J.; Lopes, A. D.; Pillinger, M.; Nunes, C.
D.; Roma˜o, C. C.; Ku¨hn, F. E.; Gonc¸alves, I. S. New J. Chem. 2004, 28,
308.
(39) Nunes, C. D.; Pillinger, M.; Valente, A. A.; Lopes, A. D.; Gonc¸alves,
I. S. Inorg. Chem. Commun. 2003, 6, 1228 and references therein.
(40) Ku¨hn, F. E.; Lopes, A. D.; Santos, A. M.; Herdtweck, E.; Haider,
J. J.; Roma˜o, C. C.; Santos, A. G. J. Mol. Catal., A: Chem. 2000, 151,
147.
(41) Ku¨hn, F. E.; Santos, A. M.; Lopes, A. D.; Gonc¸alves, I. S.;
Herdtweck, E.; Roma˜o, C. C. J. Mol. Catal., A: Chem. 2000, 164, 25.
(42) Liebeskind, L. S.; Bombrun, A. J. Am. Chem. Soc. 1991, 113, 8736.
(43) Ascenso, J. R.; de Azevedo, C. G.; Calhorda, M. J.; Carrondo, M.
A. A. F. de C. T.; Costa, P.; Dias, A. R.; Drew, M. G. B.; Galva˜o, A. M.;
Roma˜o, C. C.; Fe´lix, V. J. Organomet. Chem. 2001, 632, 197.
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K. J. Chem. Soc., Dalton Trans. 1997, 3599.

5550 Organometallics, Vol. 26, No. 23, 2007
Alonso et al.
Chart 2
state is the axial one. The equatorial coordination plane is
determined by the two carbonyls, bromine, and one nitrogen
donor from 1,4-(4-chloro)phenyl-2,3-naphthalenediazabutadiene,
while the axial positions are occupied by the remaining nitrogen
atom of this ligand and the centroid of η3-C3H5 fragment.
This geometric arrangement is comparable to that found in
related complexes.43,45,46 The reported distances of the atom
donors to the molybdenum center compare well with those
quoted in Table 2 for 7.
The two phenyl rings intersect the 2,3-naphthalenediazab-
utadiene fragment at interplanar angles of 88.8(2) and 79.0-
(1)°, indicating that they are almost perpendicular to this
extended aromatic system.
Furthermore, in the unit cell, the naphthalenediazabutadiene
aromatic rings from two molecules of 7 are at a distance of ca.
3.8 Å. This suggests that they are involved in π-π stacking
interactions, as shown in Figure 2.
Catalytic Epoxidation of Olefins. All complexes were tested
as catalyst precursors for olefin epoxidation using cis-cy-
clooctene as a model substrate and tert-butyl hydroperoxide
(TBHP in decane) as oxygen donor, without cosolvent, at
55 °C. Practically no olefin reacted without a catalyst, whereas
the addition of a Mo complex led to a catalytically efficient
process, yielding 88-100% 1,2-epoxycyclooctane as the only
product within 24 h (Table 3).
Table 1. FTIR Wavenumbers of νC≡O and νC≡N Modes for
Complexes 1-8
complex
ν
C≡O (cm-1
)
ν )
C≡N (cm-1
1
2
3
4
5
6
7
8
1949, 1851
1941, 1851
2320, 2287
2316, 2284
2312a
1969, 1887a
1938, 1847
1956, 1865
1928, 1869
1946, 1869
1906, 1982
a Taken from ref 44.
Table 2. Selected Distances (Å) and Angles (deg) for the
Molybdenum Coordination Sphere of Complex 7
Mo-C(100)
Mo-N(11)
Mo-Br
1.957(6)
2.277(4)
2.6989(7)
Mo-C(200)
Mo-N(24)
1.983(6)
2.229(4)
The highest reaction rate was observed for 1; nearly 90%
cyclooctene conversion was achieved within ca. 10 min reaction,
corresponding to a turnover frequency (TOF) of 472 mol
C(100)-Mo-C(200)
C(100)-Mo-N(11)
C(100)-Mo-N(24)
N(11)-Mo-Br
C(100)-Mo-Br
Cent-Mo-O(100)a
Cent-Mo-N(11)
Cent-Mo-Br
79.2(2)
104.6(2)
86.0(2)
83.6(1)
162.7(2)
93.7
N(24)-Mo-N(11)
72.7(1)
C(200)-Mo-N(11) 165.5(2)
C(200)-Mo-N(24)
N(24)-Mo-Br
C(200)-Mo-Br
Cent-Mo-O(200)
Cent-Mo-N(24)
93.8(2)
82.0(1)
89.3(2)
96.6
molMo h-1 (Table 3). This is comparable to that reported in
-1
the literature for several halocarbonylmolybdenum(II) complexes
of the type [MoCpCl(CO)3] and dioxomolybdenum(VI) com-
plexes of type [MoCpClO2] (Cp ) η5-cyclopentadienyl)34,37,47,48
and better than that reported for [Mo(O)2(Cl)2(CH3CN)2] used
as catalyst precursors in the same reaction. In the case of the
complexes 4-8, the TOFs are comparable to some of the most
active complexes of the type [MoCl2O2(DAB)] reported previ-
ously (using similar reaction conditions).38,39,40 The highest TOF
97.9
100.3
170.2
a Cent denotes the centroid defined by the three allylic carbon atoms.
Elemental analysis and FTIR and H and 13C NMR spec-
troscopy support the proposed structures. The FTIR spectra of
1
1-8 show the νC≡O stretching modes of the cis-CtO groups
-1
was observed for 6 (426 mol molMo h-1).
in the expected range (1780-2000 cm-1 10
) and the νC≡N of the
The ligand dependency of Mo-catalyzed epoxidation of
olefins has been extensively investigated for complexes of the
type [MoCpCl(CO)3], [MoCpClO2], and [MoX2O2L] (L )
bidentate neutral N-ligand),50,51 but to the best of our knowledge
until now nothing has been reported for (η3-allyl)dicarbonyl-
molybdenum(II) complexes. The initial rate of cyclooctene
epoxidation in the presence of 1-8 is different depending on
the nature of the X, N-donor ligands, and π-allyl ligands of the
original compounds. Higher initial reaction rates are observed
for the chloro complexes in comparison to their bromo
congeners (compare couples 1/2 and 6/7; Table 3). This is
precursors 1-3 (Table 1). A new band assigned to the νCdN
mode of the nitrogen ligands and slightly deviated from those
of the free ligands appears in the range 1600-1660 cm-1. The
vibrations assigned to the allyl group (∼3000 cm-1 νC-H
stretching, 1410-1420 cm-1 δC-H in plane bending, 1000-
1050 cm-1 νC-H stretching, 950-900 cm-1 δC-H out of plane
bending) are always present, while the absence of the νC≡N
stretching modes at ca. 2300 cm-1 in 4-8 reflects the loss of
the nitrile ligands from the precursors.
1
The H NMR spectroscopic data for all complexes show
signals assigned to the corresponding bidentate nitrogen and
allyl ligands. For example, the spectrum of [Mo(η3-C3H5)Cl-
(CO)2{1,4-(2,6-dimethyl)phenyl-2,3-dimethyldiazabutadi-
ene}] (4) displays a multiplet in the range 7.50 < δ < 7.24
ppm, assigned to the protons of the aromatic rings, and signals
at δ ≈ 3.36, 2.46, and 1.17 ppm, which are related to the meso,
syn, and anti protons of the allyl group, respectively. The signals
of the methyl protons are observed at 2.23 (CH3Ph) and 2.13
ppm (CH3-DAB).
(45) Costa, P. M. F. J.; Mora, M.; Calhorda, M. J.; Fe´lix, V.; Ferreira,
P.; Drew, M. G. B.; Wadepohl, H. J. Organomet. Chem. 2003, 687, 57.
(46) Martinho, P. N.; Quintal, S.; Costa, P.; Losi, S.; Fe´lix, V.; Gimeno,
M. C.; Laguna, A.; Drew, M. G. B.; Zanello, P.; Calhorda, M. J. Eur. J.
Inorg. Chem. 2006, 4096.
(47) Braga, S. S.; Gago, S.; Seixas, J. D.; Valente, A. A.; Pillinger, M.;
Santos, T. M.; Gonc¸alves, I. S.; Roma˜o, C. C. Inorg. Chim. Acta 2006,
359, 4757.
(48) Abrantes, M.; Gago, S.; Valente, A. A.; Pillinger, M.; Gonc¸alves,
I. S.; Santos, T. M.; Rocha, J.; Roma˜o, C. C. Eur. J. Inorg. Chem. 2004,
4914.
(49) Ku¨hn, F. E.; Herdtweck, E.; Haider, J. J.; Herrmann, W. A.;
Gonc¸alves, I. S.; Lopes, A. D.; Roma˜o, C. C. J. Organomet. Chem. 1999,
583, 3.
The crystal structure of [Mo(η3-C3H5)Br(CO)2{1,4-(4-chloro)-
phenyl-2,3-naphthalenediazabutadiene}] (7) was determined by
single-crystal X-ray diffraction. Selected bond distances and
angles in the molybdenum coordination sphere, listed in Table
2, show that the metal center is surrounded by the ligands in a
pseudooctahedral environment. The perspective view of 7
presented in Figure 1 confirms that the isomer found in the solid
(50) Freund, C.; Abrantes, M.; Ku¨hn, F. E. J. Organomet. Chem. 2006,
691, 3718.
(51) Ku¨hn, F. E.; Santos, A. M.; Gonc¸alves, I. S.; Roma˜o, C. C.; Lopes,
A. D. Appl. Organomet. Chem. 2001, 15, 43.

Molybdenum η3-Allyl Dicarbonyl Complexes
Organometallics, Vol. 26, No. 23, 2007 5551
Figure 1. ORTEP diagram showing the molecular structure of [Mo(η3-C3H5)Br(CO)2{1,4-(4-chloro)phenyl-2,3-naphthalenediazabutadiene}]
(7) with thermal ellipsoids drawn at 50% probability level.
Table 3. Olefin Epoxidation Using TBHP as Oxygen Donor
in the Presence of Complexes 1-8
TOFa
convb
(%)
convc (%)
run 1/run 2
complex
substrate
(mol molMo-1 h-1
)
1
2
3
4
5
6
7
8
8
8
8
8
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
cyclooctene
(R)-(+)-limonene
cyclododecene
trans-2-octene
1-octene
472
222
310
297
305
426
307
99/100
99/100
97/100
88/95
93/81
88/91
82/63
68/88
99/100
99/100
97/100
100/100
82/94
93/84
91/78
88/83
284/265d
287d
100d
42d
67/85
22/40
2d
a Turnover frequency calculated at 10 min, at 55 °C. b Cyclooctene
conversion after 6/24 h reaction in the first run, at 55 °C. c Cyclooctene
conversion in the first/second runs, at 45 °C. d 1,2-Dichloroethane was used
as cosolvent, at 55 °C.
Figure 2. Crystal packing diagram illustrating the π-π stacking
in the unit cell of naphthalenediazabutadiene fragments of two
molecules of 7.
typical of the [MoX2O2L]-type complexes with the same L and
has been attributed to the enhanced Lewis acidity at the metal
center in the case of X ) Cl,40,52-54 though this interpretation
has not been confirmed.54 In the case of the complexes with X
= Cl, the epoxidation reaction is slower for 4-6 (bearing
bidentate N-ligands) than for the corresponding precursor 1: this
effect is less pronounced for X = Br (Figure 3). A comparative
study for couples 2/3 and 7/8 shows that the initial reaction
rate is somewhat affected by the type of π-allyl ligand (η3-
Figure 3. Cyclooctene epoxidation using TBHP as oxygen donor,
without cosolvent, at 55 °C, in the presence of (A) [Mo(η3-allyl)-
Cl(CO)2(N-N)] (1 (0); 4 (4); 5 (+); 6 (×)) or (B) [Mo(η3-allyl)-
Br(CO)2(N-N)] (2 (O); 3 (]); 7 (-); 8 (/)).
C3H5 or η3-C5H5O-) present in the original compounds. The
-
(52) Petrovski, Z.; Pillinger, M.; Valente, A. A.; Gonc¸alves, I. S.; Hazell,
A.; Roma˜o, C. C. J. Mol. Catal., A: Chem. 2005, 227, 67.
(53) Al-Ajlouni, A.; Valente, A. A.; Nunes, C. D.; Pillinger, M.; Santos,
A. M.; Zhao, J.; Roma˜o, C. C.; Gonc¸alves, I. S.; Ku¨hn, F. E. Eur. J. Inorg.
Chem. 2005, 1716.
(54) Ku¨hn, F. E.; Groarke, M.; Bencze, E.; Herdtweck, E.; Prazeres, A.;
Santos, A. M.; Calhorda, M. J.; Roma˜o, C. C.; Gonc¸alves, I. S.; Lopes, A.
D.; Pillinger, M. Chem.sEur. J. 2002, 8, 2370.
different catalytic activities are probably a consequence of the
complex interplay of steric and electronic factors, which govern
among others the type of isomers formed (Chart 2). The more
labile solvent adducts 1-3 may exist as a mixture of isomers
that interconvert, while the complexes bearing bulkier and
bidentate N-N ligands may be less fluxional in solution,

5552 Organometallics, Vol. 26, No. 23, 2007
Alonso et al.
2 h reaction (Figure 4A), without affecting product selectivity.
The addition of a cosolvent led to a drop in the initial reaction
rate, which may be partly due to the decrease in catalyst
concentration as a result of the dilution. Nevertheless, between
15 and 120 min reaction conversion follows the order DCE >
none > toluene > hexane. Whereas the reaction system with
DCE is homogeneous, for hexane a heterogeneous solid-liquid
system is obtained, which may explain, at least in part, the
observed differences in catalytic activity. Eventually, at 6 h,
nearly quantitative epoxide yield is obtained for all solvents.
The catalytic performance of 8 was further investigated for
the epoxidation of other olefins, namely, cyclododecene, trans-
2-octene, 1-octene, and (R)-(+)-limonene, using DCE as solvent,
at 55 °C. For all substrates, with the exception of limonene,
the corresponding epoxide is the only product. The initial rate
of olefin conversion decreases in the order cyclooctene =
limonene > cyclododecene > trans-2-octene >1-octene (Table
3). The kinetic profiles are similar, suggesting that a similar
reaction mechanism may be involved for all olefins (Figure 4B).
In the case of limonene, 95% epoxide products are obtained at
24 h and byproducts include ca. 4% diols. Regioselectivity is
high in favor of the epoxidation of the internal cyclic double
bond, yielding limonene oxide and 1,2:8,9-diepoxy-p-menthane
in a molar ratio of 7. These results are consistent with the
mechanistic aspects mentioned above, since the higher electronic
density of an internal olefinic double bond (compared with the
terminal CdC bond) should favor nucleophilic attack on an
electrophilic oxidizing specie. The slower reaction of cy-
clododecene in comparison to that of cyclooctene may be due
to the larger molecular dimensions of the former substrate.
Reaction Mechanism. The clear understanding of the mech-
anism of olefin epoxidation using the previous compounds
requires the elucidation of the nature of the active specie(s)
involved. Joshi30 proposed that the oxidation of [Mo(π-allyl)-
Cl(CO)2L] (L ) bis(3,5-dimethylpyrazolyl)methane) with mo-
lecular oxygen resulted in the loss of an allyl radical and
formation of a dimeric Mo(V) oxo complex featuring the neutral
bidentate chelate and the halide ligand. This dimeric complex
efficiently catalyzed the oxidation of triphenylphosphine with
O2, possibly via a peroxo intermediate. Attempts at character-
izing the metal complex after catalysis were made for 4. After
cooling of the sample to room temperature, a small amount (ca.
15 wt % of the initial amount of 4) of solid compound (lighter
in color than the original complex and hereafter denoted as 4*)
could be separated from the reaction solution by centrifugation.
Compound 4* was thoroughly washed with n-hexane and dried
at room temperature prior to characterization.
Figure 4. Olefins epoxidation using TBHP as oxygen donor in
the presence of 8, at 55 °C: (A) solvent effect on cyclooctene
epoxidation without a cosolvent (+) or with 1,2-dichloroethane (O),
toluene (4), or n-hexane (-), at 55 °C; (B) epoxidation of
cyclooctene (O), (R)-(+)-limonene (/), cyclododecene (-), trans-
2-octene (4), or 1-octene (+), using 1,2-dichloroethane as cosolvent.
preferring the axial isomer, as attested for 7 by 1H NMR
spectroscopy. The nature of the ligands may affect the electronic
density of the metal center, accounting for different reactivity
of the complexes with, for example, TBHP (a necessary step
to originate active intermediate species, since no reaction occurs
without TBHP). Differences in solubility are unlikely to be
relevant, since the solubility of these compounds in the reaction
medium is apparently similar.
The kinetic profiles of 1-8 display an initial abrupt increase
in cyclooctene conversion (no induction periods were observed)
after which the reaction slows down considerably with the
course of time (Figure 3). This behavior has typically been found
for cyclooctene epoxidation with complexes of the type MoCp-
Cl(CO)3 and MoCpClO2,34,36,37,48 as well as for several dioxo-
molybdenum(VI) complexes belonging to the [MoX2O2L]
family, under similar reaction conditions.51,53,54 Kinetic and DFT
studies of molybdenum epoxidation reactions catalyzed by
[MoX2O2L] complexes have led to the proposal that the TBHP
first reacts with the metal species and afterward the olefin attacks
the resulting intermediate Mo-alkyl(hydro)peroxo complex in
a second step to give the corresponding epoxide and tert-butanol.
The latter byproduct may compete with t-BuOOH for coordina-
tion to the metal center, gradually retarding the overall reac-
tion.53,54,55
The catalyst reusability was examined at 45 °C, by initiating
a second reaction cycle after 24 h, without catalyst separation
and recharging equimolar amounts of cyclooctene and TBHP.
In general, the catalytic performance was quite efficient in the
second run (Table 3). The catalytic results obtained for 1 and 2
are outstanding compared to those found in the literature for
solvent adducts of the type [MoX2O2(CH3CN)2], used as
catalysts in the same reaction.49 The latter undergo catalyst
deactivation during a first catalytic run, accounting for a
maximum of 65% cyclooctene conversion, at 55 °C. Compounds
1 and 2 gave 100% conversion, which decreased by a factor of
only 0.13 and 0.03, respectively, from the first to the second
run. Compounds 2 and 8 also gave approximately the same
conversion in the first and second runs, and in the cases of 1,
4, 6, and 7 conversions at 24 h decreased by a factor of 0.10-
0.24.
The FTIR and Raman spectra of 4* were substantially
different from that of the original compound. In contrast to that
observed for 4, the IR spectrum of 4* did not show the νC≡O
bands in the region 1780-2000 cm-1 but contained two new
strong νModO bands at 920 and 956 cm-1, characteristic of the
asymmetric and symmetric stretching vibrations of the cis-
[MoO2]2+ fragment.53,54,56 In the Raman spectrum of 4* the
ModO stretching modes are observed in the range 880-980
cm-1, whereas the bands in the range 430-510 cm-1, assigned
to Mo-C bonds, practically disappeared.54 On the basis of these
results, it seems possible that, under the reaction conditions, 4
undergoes complete oxidative decarbonylation in the presence
of TBHP, to give a dioxomolybdenum complex. The IR
spectrum of 4* contained a broad band of strong intensity
centered at 714 cm-1, not present in the spectrum of the original
Further tests to assess the influence of the solvent on the
epoxidation of cyclooctene were carried out for 8, using
n-hexane, toluene, or 1,2-dichloroethane (DCE), at 55 °C. The
type of solvent influences the reaction rate, mainly for the first
(55) Veiros, L. F.; Prazeres, Aˆ .; Costa, P. J.; Roma˜o, C. C.; Ku¨hn, F.
E.; Calhorda, M. J. Dalton Trans. 2006, 1383.
(56) Nunes, C. D.; Valente, A. A.; Pillinger, M.; Rocha, J.; Gonc¸alves,
I. S. Chem.s Eur. J. 2003, 9, 4380.

Molybdenum η3-Allyl Dicarbonyl Complexes
Organometallics, Vol. 26, No. 23, 2007 5553
compound, which may be assigned to Mo-O-Mo stretching
vibrations.56,57 Materials containing binuclear species with two
Mo(VI) centers, each with two ModO groups and each linked
by one or two oxo-bridges, have been identified as active species
for olefin epoxidation and oxidation of alcohols with TBHP,
under reaction conditions similar to those applied in this work.56
The IR-absorption band at ca. 1646 cm-1 assigned to the νCdN
mode in the DAB group of 4 shifted to 1620 cm-1 after catalysis,
possibly due to changes in the oxidation state and first
coordination sphere of Mo. Weak bands in the range 2850-
3060 cm-1 assigned to C-H bonds of the DAB ligand were
present for both 4 and 4*. The Raman spectra of 4 and 4*
differed in the 190-240 cm-1 range, possibly owing to changes
in the Mo-N stretching vibrations, as would be expected from
the discussion above.54 After catalysis, the intensities of the
Raman-absorption bands centered at ca. 250 and 360 cm-1
assigned to Mo-Cl vibrations decreased drastically. The band
at 1213 cm-1, tentatively assigned to the π-allyl group of 4, is
no longer present after catalysis. The 1H NMR spectrum of 4*
in CD2Cl2 displayed signals in the range 7-8.0 ppm, assigned
to the aromatic rings, as well as two methyl resonances at 1.96
and 1.60 ppm, but showed no signals of allyl groups. These
results indicate that the Mo(II) complex reacts with TBHP to
give the active oxidizing species for olefin epoxidation, which
may be dimeric dioxomolybdenum(VI) complexes of the type
[{MoXO2(N-N)}2(µ-O)] and/or [{MoO2(N-N)}2(µ-O)2]. The
4* sample contains less than 1% Cl (ascertained by ionic
chromatography), which is much lower than that expected for
[{MoClO2(N-N)}2(µ-O)] (ca. 7.7% Cl). Hence, if species of
the type [{MoClO2(N-N)}2(µ-O)] are formed during catalysis,
they are not predominant. Elemental analysis of 4* gave 59.5%
C, 3.2% N, and 5.4% H. The expected values for [{MoO2(N-
N)}2(µ-O)2] are 55.1% C, 6.4% N, and 5.5% H and for
[{MoXO2(N-N)}2(µ-O)] are 51.8% C, 6.0% N, and 5.2% H.
The relatively high carbon content of 4* may be due to reaction
products and/or solvent present in the recovered solid (the
presence of decane and tert-butanol signals was detected in the
1H NMR). The ESI mass spectra of 4* showed several ions,
and above m/z 750, the spectrum was quite noisy, making it
difficult to interpret. Three parts of the spectrum (m/z ) 177,
487, 517) showed isotopic patterns corresponding to Mo species
that were not identified. Degradation of the complex may occur
to a certain extent under the conditions of the ESI source
(350 °C).
Figure 5. DFT-optimized geometries of two possible Mo(VI)
dimers [{MoO2Cl(N-N)}2(µ-O)] (4ma*) and [{MoO2(N-N)}2-
(µ-O)2] (4mb*).
The structures of the two most likely optimized models of 4*
are shown in Figure 5.
The Mo-O-Mo angle was 156.6 and 102.6°/98.6° for 4ma*
and 4mb*, respectively, with Mo-O distances of 1.886 and
1.925/1.980 Å, in the same order. The terminal ModO bonds
were slightly shorter for 4ma* (1.676/1.706 Å) than for 4mb*
(1.690/1.717 Å). These values are in the range usually found
for other Mo(VI) oxide complexes. The calculated νMo-O-Mo
stretching frequency was 798 and 661 cm-1 for 4ma* and
4mb*, respectively, compared to 714 cm-1 for 4*. The ModO
stretching frequencies appeared in the range 936-1012 cm-1
for 4ma* and 911-990 cm-1 for 4mb*, the former being closer
to the experimental values. Both models give for the CdN
stretching vibrations (1595-1706 cm-1) values similar to that
observed experimentally for 4* (1620 cm-1). These results,
together with the lack of clear evidence for Mo-Cl vibrations,
suggest that the reaction of 4 with TBHP originates a complex
of the type [(MoO2(N-N))2(µ-O)2], though a mixture of the
two dimeric species cannot be ruled out. If species of the type
[(MoO2(N-N))2(µ-O)2] were also formed from 6-8, one could
expect similar epoxidation rates for these three complexes, at
least after an initial period of time. In fact, for 6-8, cyclooctene
conversions at 1 h differ only in 10%, tending to similar values
(Figure 3A,B). Similarly, in the case of complexes 1-3,
conversions differ in 8% after 1.5 h reaction. When the solid is
removed from the reaction mixture (using a Whatman 0.2 µm
(PVDF w/GMF) membrane at the reaction temperature) after
30 min and the solution is left to react further 5.5 h, cyclooctene
conversion is the same as that achieved when no solid is
removed, suggesting that the reaction is homogeneous in nature.
The recovered solid 4* is active in a second reaction run, giving
75% conversion at 24 h compared to 95% when no solid is
removed during the reaction.
A proposal of the catalytic cycle for olefin epoxidation is
sketched in Scheme 1, based on our previous studies of similar
reactions.54,55 We assume that the precursor complex is con-
verted by TBHP in the active species, the Mo(VI) dimer 4*.
Another molecule of TBHP adds to the ModO bond to form
the heptacoodinate environment of Mo(VI), with two new
ligands, OH and OOR.
Notice that a similar reaction may happen in the second side
of the molecule (not shown for clarity). The olefin then adds to
the Mo-O bond, forming a large cycle with hydrogen bond
assistance from the OH ligand. This hydrogen bond positions
the reagents for the loss of alcohol in the final step, which takes
place with formation of the epoxide and regeneration of the
catalyst.
DFT/B3PW9158 calculations (Gaussian03 program; see Com-
putational Details) were performed to try to identify the species
4*. Several models based on available oxide complexes of Mo-
(VI), including monomers and dimers, were built. 1,4-Phenyl-
diazabutadiene was taken as model for the DAB ligand in 4
(methyl groups were replaced by hydrogen atoms). The
monomers did not converge easily, suggesting that they are not
stable species in the absence of another coordinating ligand.
Two model dimers, [{MoO2Cl(N-N)}2(µ-O)] (4ma*) and
[{MoO2(N-N)}2(µ-O)2] (4mb*), were studied. For [{MoO2-
Cl(N-N)}2(µ-O)] (4ma*), the molybdenum is coordinated to
three oxygen atoms, which can exhibit a facial or meridional
arrangement. Structures available in the CSD59 contain the fac-
Mo(O)3 group, and it was found that during the optimization
procedure the meridional isomers converted into the facial ones.
(57) Xiao, Z.; Enemark, J. H.; Wedd, A. G.; Young, C. G. Inorg. Chem.
1994, 33, 3438.
Conclusions
(58) Parr, R. G.; Yang, W. In Density Functional Theory of Atoms and
Molecules; University Press: Oxford, U.K., New York, 1989.
(59) Allen, F. H. Acta Crystallogr. 2002, B58, 380.
Several new complexes of the Mo(η3-allyl)X(CO)2 fragment
with derivatives of diazabutadiene were synthesized and char-

5554 Organometallics, Vol. 26, No. 23, 2007
Alonso et al.
Scheme 1
acterized. The X-ray structure of [Mo(η3-C3H5)Br(CO)2{1,4-
(4-chloro)phenyl-2,3-naphthalenediazabutadiene}] (7) showed
the formation of the axial isomer, and it was found by NMR
spectroscopy that only one isomer exists in solution. The
complexes were tested in the epoxidation of cyclooctene with
TBHP, giving excellent epoxide selectivity and turnover fre-
quencies, comparable to those reported previously for some well-
known molybdenum(VI) catalysts but displaying as well an
extremely efficient second run, not observed in any of the other
Mo(VI) systems. To identify the active species, combined
experimental (vibrational and NMR spectroscopy, analysis) and
computational (DFT) studies were carried out for one of the
diimine complexes. The probable active species is a dimer of
the type [{MoO2(N-N)}2(µ-O)2], which requires more TBHP
to perform epoxidation and complete the catalytic cycle, as
discussed in previous works for Mo(VI) catalysts. This formula-
tion is also supported by the fact that similar olefin conversions
are achieved after a relatively short period of time for complexes
with the same N-donor ligands, independently of X and allyl
ligands. More experimental work is being carried out to clarify
the details of the mechanism. Probably, the dimeric framework
of the active species is more stable under reaction conditions,
contributing to the high catalytic activity after the first run. On
the other hand, preliminary experiments carried out for 8 (which
has no symmetry) showed no induction ability for enantiose-
lective epoxidations with TBHP. This disappointing result is
somewhat in agreement with the symmetric structure proposed
for 4*.
was carried out on a DX100 equipment using a AG4ASC column
and an aqueous solution composed of 1.7 mM Na2CO3 plus 1.8
mM NaHCO3 as eluent.
Complexes [Mo(η3-C3H5)Cl(CO)2(CH3CN)2] (1) and [Mo(η3-
C3H5)Br(CO)2(CH3CN)2] (2) were prepared by following the
synthetic procedure reported in literature by tom Dieck et al.22 [Mo-
(η3-C5H5O)Br(CO)2(CH3CN)2] (3) was prepared as described by
Liebeskind et al.,42 and the ligands 1,4-(2,6-dimethyl)phenyl-2,3-
dimethyldiazobutadiene, 1,4-(2,6-diisopropyl)phenyldiazabutadi-
ene,60 and 1,4-(4-chloro)phenyl-2,3-naphthalenediazabutadiene were
from literature procedures.61
Preparation of [Mo(η3-C3H5)Cl(CO)2{1,4-(2,6-dimethyl)phe-
nyl-2,3-dimethyldiazabutadiene}] (4). 2,6-DimethylPh-DAB (2,6-
MeC6H3)NdC(CH3)C(CH3)CdN(2,6-MeC6H3)) (0.5 mmol, 0.146
g) was added to a yellow solution of [Mo(η3-C3H5)Cl(CO)2(CH3-
CN)2] (0.5 mmol, 0.155 g) in CH2Cl2 (20 mL), under N2 and
stirring, to give a dark blue solution. This solution was stirred for
2 h, and the volume was reduced under vacuum. After addition of
n-hexane, the mixture was left in the fridge during 2 days and
gave dark blue precipitate. It was washed with 3 × 5 mL of diethyl
ether and dried under vacuum.
Anal. Calcd for 4·1.5CH2Cl2: C, 49.10; H, 4.97; N, 4.32.
Found: C, 48.86; H, 4.60; N, 4.53.
1H NMR (400 MHz, CDCl3, room temperature, ppm): δ 7.24
(m, H3′-H5′, 6H), 3.36 (m, Hmeso, 1H), 2.46 (br, Hsyn + CH3Ph,
5H), 2.23 (br, CH3Ph, 9H), 2.13 (s, CH3-DAB, 6H), 1.17 (d, Hanti
,
2H).
13C{1H} NMR (100.51 MHz, CDCl3, room temperature, ppm):
δ 172.38 (C2/C3), 129.75 (C3′ or C5′), 129.48 (C1′), 128.78 (C3′
or C5′), 127.21 (C2′/C6′), 126.99 (C4′), 73.71 (Cmeso), 63.08
(Canti/syn), 20.43 (CH3Ph), 18.52 (CH3-DAB).
Selected IR (KBr pellets, cm-1): 1938, 1847 (νC≡O), 1630
(νCdN), 1417 (δp(C-H)allyl), 1210 (νs(C-C-C)allyl), 983 (δop(C-H)allyl).
Preparation of [Mo(η3-C3H5)Cl(CO)2{1,4-(2,6-diisopropyl)-
phenyldiazabutadiene}] (5). 2,6-DiisopropylPh-DAB [2,6-i-
PrC6H3)NdC(CH3)C(CH3)CdN(2,6-iPrC6H3)] (0.5 mmol, 0.188 g)
was added to a yellow solution of [Mo(η3-C3H5)Cl(CO)2(CH3CN)2]
in CH2Cl2 (20 mL), under N2 and stirring, to give a dark green
solution. This solution was stirred for 2 h, and the volume was
reduced by vacuum. After addition of n-hexane, the mixture was
left in the fridge during several days and gave a dark green
precipitate. It was washed three times with 5 mL of n-hexane and
dried under vacuum.
Experimental Section
General Considerations. All air-sensitive reactions and ma-
nipulations were performed using standard Schlenk techniques under
an oxygen-free and water-free nitrogen atmosphere. Commercially
available reagents and all solvents were purchased from standard
chemical suppliers. Solvents were dried by standard procedures
(THF, n-hexane, and Et2O over Na/benzophenone ketyl; CH2Cl2
and CH3CN over CaH2), distilled under nitrogen, and kept over 4
Å molecular sieves (3 Å for CH3CN).
Infrared spectra were recorded on a Unican Mattson Mod 7000
FTIR spectrophotometer as KBr pellets. 1H and 13C solution NMR
spectra were obtained with a Bruker Avance-400 spectrometer.
Chemical shifts are quoted in parts per million (ppm) from TMS.
Microanalyses for C, H, and N were performed at the Departments
of Chemistry, Universities of Aveiro and Vigo. The ESI (in negative
mode) mass spectral data were obtained at the Mass Spectrometry
Center, University of Aveiro, using a Finnigan LXP (Thermo
Electron Corp.) equipment operated at an acceleration voltage of 5
kV and ESI source temperature of 350 °C. Ionic chromatography
(60) Schadt, M. J.; Gresalfi, N. J.; Lees, A. J. Inorg. Chem. 1985, 24,
2942.
(61) van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L.; Benedix,
R. Recl. TraV. Chim. Pays-Bas 1994, 113, 88.

Molybdenum η3-Allyl Dicarbonyl Complexes
Organometallics, Vol. 26, No. 23, 2007 5555
Anal. Calcd for 5·1.5C6H14: C, 65.42; H, 8.51; N, 3.81. Found:
(5.5 M in decane), and different substrates were studied, namely
cyclooctene, (R)-(+)-limonene, 1-octene, trans-2-octene, and cy-
clododecene. The reactions were carried out without additional
solvent (other than the decane present in the TBHP solution) or
using n-hexane, 1,2-dichloroethane, or toluene as cosolvent (2 mL
solvent for 1.8 mmol substrate). The course of the reaction was
monitored using a gas chromatograph (Varian 3800) equipped with
a capillary column (DB-5, 30 m × 0.25 mm) and a flame ionization
detector. The products were identified by gas chromatography-
mass spectrometry (HP 5890 Series II GC; HP 5970 Series Mass
Selective Detector) using He as carrier gas.
1
C, 65.85; H, 7.61; N, 3.30. H NMR (400 MHz, CDCl3, room
temperature, ppm): δ 8.02 (s, H-DAB, 2H), 7.30-7.20 (m, H3-
H5, Ph, 6H), 3.67 (m, Hmeso + CHipr, 5H), 2.86 (br, Hsyn, 2H),
i
i
1.44 (d, Hanti, 2H), 1.30 (d, CH3 pr, 6H), 1.23 (d, CH3 pr, 6H), 1.01
i
i
(d, CH3 pr, 6H), 0.93 (d, CH3 pr, 6H). 13C{1H} NMR (100.51 MHz,
CDCl3, room temperature, ppm): δ 159.88 (C2/C3), 141.10 (C2′,
C6′), 139.48 (C1′), 128.34, 124.77 (C3′, C5′), 123.77 (C4′), 78.29
(Cmeso), 65.26 (Canti/syn), 26.47, 22.63 (CH3 ipr,), 28.16 (CHipr).
Selected IR (KBr pellets, cm-1): 1938, 1847 (νC≡O).
Preparation of [Mo(η3-C3H5)Cl(CO)2{1,4-(4-chloro)phenyl-
2,3-naphthalenediazabutadiene}] (6). To a yellow solution (CH2-
Cl2, 10 mL) of [Mo(η3-C3H5)Cl(CO)2(CH3CN)2] (1.0 mmol, 0.310
g) was added N,N’-bis(p-chlorophenylimino)acenaphthene (1 mmol,
0.396 g) under stirring and N2. A dark green precipitate was formed.
The stirring was continued overnight. The green precipitate was
filtered off, washed with 3 × 10 mL of CH2Cl2, and dried under
vacuum.
Crystallography. Crystals of [Mo(η3-C3H5)Br(CO)2{1,4-(4-
chloro)phenyl-2,3-naphthalenediazabutadiene}] (7) with suitable
quality for single-crystal X-ray determination were grown up from
CH2Cl2/hexane solution. Crystal data: C29H19BrCl2MoO2, Mr )
674.21; triclinic, space group P1h, Z ) 2; a ) 9.705(12), b ) 10.499-
(17), c ) 13.612(17) Å; V ) 1335.2 Å3; R ) 77.65(10), â ) 87.16-
(10), γ ) 80.25 (10)°; F(calcd) ) 1.677 Mg m-3, µ(Mo KR) )
2.218 mm-1
.
Anal. Calcd for 6: C, 55.31; H, 3.04; N, 4.45. Found: C, 54.55;
1
H, 3.06; N, 4.28. H NMR (400 MHz, dmf-d7, room temperature,
X-ray data were collected at room temperature on a MAR
Research Image plate system using graphite-monochromatized Mo
KR radiation (λ ) 0.710 73 Å) at Reading University. The crystals
were positioned at 50 mm from the detector, and the frames were
measured using an appropriated counting. Data analysis was carried
out with the XDS.62 An empirical absorption correction was carried
out using the DIFABS program.63 A total of 6691 reflections
collected were merged in 1h symmetry leading to 4539 unique data
with a Rint of 0.0308.
ppm): δ 8.30 (d, H5/H6, 2H), 7.85 (d, H3′/H5′, 4H), 7.73 (t, H4/
H7, 2H), 7.58 (d, H2′/H6′, 4H), (6.96 (br, H3/H8, 2H), 4.95 (m,
meso, 1H), 2.98 (Hsyn, overlapped with the solvent signal), 1.31
H
(d, Hanti
,
2H). 13C NMR (HMQC, dmf-d7, ppm): δ 131.6 (C5/C6), 130.5
(C3′/C5′), 128.7 (C4/C7), 124.8 (C3/C8), 122.2 (C2′/C6′), 77.3
(Cmeso), 56.2 (Canti/syn). Selected IR (KBr pellets, cm-1): 1928, 1869
(νC≡O).
Preparation of [Mo(η3-C3H5)Br(CO)2{1,4-(4-chloro)phenyl-
2,3-naphthalenediazabutadiene}] (7). N,N’-Bis(p-chlorophenylim-
ino)acenaphthene (2.6 mmol, 1.027 g) was added to a yellow
solution of [Mo(η3-C3H5)Br(CO)2(CH3CN)2] (2,0 mmol, 0.710 g)
in ethanol (30 mL) under N2 with stirring. The dark green solution
formed was stirred for 2 days, and then the volume was reduced
under vacuum. The mixture was left in the fridge during several
days. The dark blue precipitate was filtered out, washed with 2 ×
20 mL of n-hexane, and dried under vacuum. It was recrystallized
by diffusion of n-hexane into a CH2Cl2 solution of the compound.
Diffusion of n-hexane into a CH2Cl2 solution of the compound gave
good crystals for single X-ray diffraction studies.
The structure was solved by direct methods and subsequent
difference Fourier syntheses and refined by full matrix least-squares
on F2 using the SHELX-97 system programs.64 Anisotropic thermal
parameters were used for all non-hydrogen atoms. The hydrogen
atoms bonded to carbon atoms were included in the refinement in
calculated positions giving thermal parameters equivalent 1.2 times
those of the atom to which were attached. The residual electronic
density ranging from -0.49 to 0.67 e Å-3 was within the expected
values. The final refinement of 335 parameters converged to final
R and Rw indices R1 ) 0.0514 and wR2 ) 0.0970 for 3740
reflections with I > 2σ(I) and R1 ) 0.0697 and wR2 ) 0.1031 for
all unique hkl data. Molecular diagrams were drawn with PLA-
TON.65
Computational Details. DFT58 calculations were performed
using the Gaussian03 program (rev C.02)66 with the B3PW91 hybrid
functional. This functional includes a mixture of Hartree-Fock
exchange with DFT exchange-correlation given by Becke’s three-
parameter hybrid functional67 with Perdew and Wang’s 1991
Anal. Calcd for 7: C, 51.66; H, 2.84; N, 4.15. Found: C, 50.94;
1
H, 3.08; N, 4.04. H NMR (CDCl3, room temperature, ppm): δ
7.96 (d, H5/H6, 2H), 7.61 (d, H3′/H5′, 4H), 7.59 (d, H2′/H6′, 4H),
7.4′), 7.46 (t, H4/H7, 2H), 6.63 (br, H3/H8, 2H), 3.72 (m, Hmeso),
2.01 (d, Hsyn, 2H), 1.18 (d, Hanti, 2H). 13C NMR (dmf-d7, ppm): δ
132.8 (C5/C6), 130.5 (C3′/C5′), 128.7 (C4/C7), 124.9 (C3/C8),
122.3 (C2′/C6′), 77.5 (Cmeso), 55.6 (Canti/syn). Selected IR (KBr
pellets, cm-1): 1946, 1869 (νC≡O).
(62) Kabsch, W. J. J. Appl. Crystallogr. 1988, 21, 916.
Preparation of [Mo(η3-C5H5O)Br(CO)2{1,4-(4-chloro)phenyl-
2,3-naphthalenediazabutadiene}] (8). N,N’-Bis(p-chlorophenylim-
ino)acenaphthene (3 mmol, 1.203 g) was added to a yellow solution
of Mo(η3-C5H5O)Br(CO)2(CH3CN)2] (3.0 mmol, 1.184 g) in CH2-
Cl2 (20 mL) under N2 with stirring. The dark green precipitate
formed was filtered out, washed with 2 × 20 mL of n-hexane, and
dried under vacuum.
(63) Walker, N.; Stuart, D. DIFABS. Acta Crystallogr. 1983, A39, 158.
(64) Sheldrick, G. M. SHELX-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(65) Spek, A. L. PLATON, a Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 1999.
(66) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C; Pople, J. A. Gaussian
03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Anal. Calcd for 8·CH2Cl2: C, 48.09; H, 2.65; N, 3.51. Found:
1
C, 48.17; H, 2.45; N, 3.56. H NMR (dmso-d6, ppm): 8.31 (dd,
H5/H6, 2H), 7.80 (dd, H3′/H5′, 4H), 7.71 (t, H4/H7, 2H), 7.68-
7.39 (m, H2′/H6′, 4H), (6.83 (d, H3/H8, 2H), 6.20 (t, H3-Cy, 1H),
3.64 (m, H2-Cy, 1H), 3.47 (t, H4-Cy, 1H), 2.88, 1.98 (d, H5-Cy,
2H). Selected IR (KBr pellets, cm-1): 1906, 1982 (νC≡O).
Catalytic Reactions Using Compounds 1-8. The liquid-phase
catalytic epoxidations were carried out at atmospheric pressure in
a reaction vessel equipped with a magnetic stirrer and immersed
in a thermostated oil bath. Typically, 1% molar ratio of complex/
substrate and a substrate/oxidant molar ratio of 0.63 were used.
tert-Butyl hydroperoxide (TBHP) was used as the oxygen donor
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5556 Organometallics, Vol. 26, No. 23, 2007
Alonso et al.
gradient-corrected correlation functional.68 The basis set for Mo
consisted of the standard SDD basis set69 augmented with a f
polarization function in all calculations,70 while the 6-311G** basis
set71 was used on all remaining atoms. Frequency calculations were
performed at the same level of theory to confirm the nature
(minima) of the stationary points determined.
The two models 4ma* and 4mb* were based on available
structures of Mo(VI) oxide complexes taken from the CSD,59
namely GODMIV, HUXMAD, ECOYAW, and FILYIL (see the
structures in Figure 1S, Supporting Information) and the DAB
ligand from the X-ray structure of complex 7 described above, and
their structures were optimized under C2 symmetry.
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Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G.,
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Acknowledgment. We are grateful to the FCT and FEDER
for financial support (Grants POCTI/QUI/44654/2002, POCI/
QUI/58925/2004). P.N. is grateful to CICECO-University of
Aveiro for a research grant. S.Q. and P.D.V. thank the FCT for
fellowships (SFRH/BPD/11463/2002 and SFRH/BPD/27344/
2006, respectively). M.G.B.D thanks the EPSRC (U.K.) and
the University of Reading for funds for the MarResearch Image
Plate and Oxford Diffraction X-Calibur X-ray systems. We
thank Cristina M. F. R. Barros for helpful discussions on the
ESI mass spectral data and Carlos Valente of CUF-Qu´ımicos
Industriais for the ionic chromatography analysis.
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A.; Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
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Supporting Information Available: Figures giving available
structures of Mo(VI) oxides from the CSD. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM700348W