Oxomolybdenum(VI) complexes with glycine bisphenol [O,N,O,O'] ligand: Synthesis and catalytic studies

Article abstract of DOI:10.1080/00958972.2014.928287

The oxomolybdenum(VI) complex [MoOCl(L)] with a tetradentate glycine bisphenol ligand (H₃L) was prepared by reaction of [MoO ₂Cl₂(DMSO)₂] with a ligand precursor in hot toluene. The product was isolated in moder

Full text of DOI:10.1080/00958972.2014.928287

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Oxomolybdenum(VI) complexes with
glycine bisphenol [O,N,O,O′] ligand:
synthesis and catalytic studies
a
b
a
Tero Heikkilä , Reijo Sillanpää & Ari Lehtonen
a
Laboratory of Materials Chemistry and Chemical Analysis,
Department of Chemistry, University of Turku, Turku, Finland
b
Laboratory of Inorganic Chemistry, Department of Chemistry,
University of Jyväskylä, Jyväskylä, Finland
Accepted author version posted online: 28 May 2014.Published
online: 26 Jun 2014.
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To cite this article: Tero Heikkilä, Reijo Sillanpää & Ari Lehtonen (2014) Oxomolybdenum(VI)
complexes with glycine bisphenol [O,N,O,O′] ligand: synthesis and catalytic studies, Journal of
Coordination Chemistry, 67:11, 1863-1872, DOI: 10.1080/00958972.2014.928287
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 11, 1863–1872, http://dx.doi.org/10.1080/00958972.2014.928287
Oxomolybdenum(VI) complexes with glycine bisphenol
[O,N,O,O′] ligand: synthesis and catalytic studies
TERO HEIKKILĆ, REIJO SILLANPÄć and ARI LEHTONEN*†
†Laboratory of Materials Chemistry and Chemical Analysis, Department of Chemistry, University of
Turku, Turku, Finland
‡Laboratory of Inorganic Chemistry, Department of Chemistry, University of Jyväskylä, Jyväskylä,
Finland
(
Received 28 August 2013; accepted 11 April 2014)
tBu
O
O
Cl
O
Mo
N
tBu
OH
O
tBu
tBu
O
tBu
cis or trans
MoO2Cl2(DMSO)2
tBu
N
HO
O
tBu
O
O
tBu
OH
tBu
O
Mo
tBu
HO
O
N
tBu
O
tBu
The oxomolybdenum(VI) complex [MoOCl(L)] with a tetradentate glycine bisphenol ligand (H
was prepared by reaction of [MoO Cl (DMSO) ] with a ligand precursor in hot toluene. The product
was isolated in moderate yield as separable cis and trans isomers along with the third minor compo-
nent, [MoO (HL)]. The solid-state structure of trans-[MoOCl(L)] was determined by X-ray diffrac-
3
L)
2
2
2
2
tion. The ligand has tetradentate coordination through three oxygens and one nitrogen, which is
located trans to the terminal oxo whereas the sixth coordination site is occupied by a chloride. Both
cis and trans isomers of [MoOCl(L)] are active catalysts for epoxidation of cis-cyclooctene and sul-
foxidation of tolyl methyl sulfide. The cis isomer gave higher activity in epoxidation and sulfoxida-
tion reactions at room temperature than the trans isomer but they performed identically at 50 °C.
Keywords: Molybdenum complex; Epoxidation; Sulfoxidation
1. Introduction
Coordination chemistry of molybdenum is often studied due to its active role in several bio-
logical oxidation and oxo-transfer reactions [1]. Synthetic approaches to model the active
*
Corresponding author. Email: ari.lehtonen@utu.fi
This article was originally published without the graphical abstract. This version has been corrected. Please see
Erratum (http://dx.doi.org/10.1080/00958972.2014.941217).
©
2014 Taylor & Francis

1
864
T. Heikkilä et al.
sites of molybdenum enzymes have been either biomimetic (structural modeling) or bio-
inspired (functional modeling), and rather simple coordination compounds can be studied as
models for sophisticated biological systems [2]. In general, active structures involve molyb-
denum ions in high-oxidation states; thus, the reactivity of many active species can be mod-
eled with isolated oxomolybdenum species which are stabilized by biologically relevant
hard donors, for instance amine, carboxylate, and aryloxido ligands. Multidentate phenolic
ligands are generally used to make such model compounds. For example, the reactions of
MoO Cl derivatives with tetrapodal amine trisphenols or aminoalcohol bisphenols are
2
2
known to lead to neutral mononuclear oxochloromolybdenum(VI) complexes which can
catalyze a number of oxidations including sulfoxidation, epoxidation, and haloperoxidation
reactions [3, 4]. Similarly, sterically demanding tetradentate diamino bisphenols form active
dioxomolybdenum(VI) complexes [5–7].
Although metal complexes with different aminobisphenol ligands are generally well
known, the chemistry of such ligands with a carboxylate acid side-arm is scantly studied.
There are several reports on iron, cobalt, and vanadium complexes with glycine bisphenols
[
8–13]. These ligands carry two phenol groups, a carboxylic acid functionality and amino
nitrogen, so they mimic nicely many biomolecules for example, tyrosine. In the present
article, we describe the synthesis, structure, and catalytic applications of oxochloromolybde-
num(VI) complexes with N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminoacetic acid (H L).
3
2
2
. Experimental
.1. Materials and methods
The starting materials [MoO Cl (DMSO) ] [14], [MoO (acac) ] [15], and H L [11] were
2
2
2
2
2
3
prepared according to literature procedures. tBuOOH was purified by vacuum distillation
prior to use. Other chemicals and solvents were obtained from commercial sources and
were used as purchased. All syntheses were carried out under ambient atmosphere. IR spec-
tra were measured in compressed KBr pellets using a Nexus 870 FT-IR spectrometer. NMR
spectra were recorded at 25 °C with a Bruker Avance 500 spectrometer equipped with a
broadband observe probe (BBO-5 mm-Zgrad). The samples were dissolved in CDCl or
3
DMSO-d6 with TMS as an internal chemical shift reference (0 ppm for both nuclei).
UV–VIS spectra were measured in CHCl solutions.
3
2
.2. Syntheses
The stirred suspension of finely ground [MoO Cl (DMSO) ] (356 mg, 1.00 mM) in 60 mL
2
2
2
of toluene was treated with H L (498 mg, 0.98 mM). The reaction mixture turned dark blue
3
upon heating and was stirred under reflux for 16 h, filtered, and evaporated in vacuum. The
isomeric mixture was chromatographed over silica using CH Cl as an eluent to obtain two
2
2
separated blue fractions, which were identified as [trans-MoOCl(L)] (trans-1, the phenolate
moieties are at trans positions) and [cis-MoOCl(L)] (cis-1, the phenolate moieties at cis
positions). A yellow fraction (2) was collected as a trace quantity by adding gradually 10%
of acetonitrile to the eluent.
trans-1: IR: 1240 vs 1203 s, 1170 vs 1126 w, 1101 w, 1025 w, 958 vs 933 m, 914 vs
1
8
92 m, 863 s, 808 w, 761 m, 721 w, 694 w, 655 w, 597 m, 580 s, 559 m. H NMR (500
MHz, CDCl ): δ 7.47 (2 H, d, J = 2.0 Hz), 7.14 (2 H, d, J = 2.0 Hz), 4.69 (2 H, d, J = 13.0
3

Oxomolybdenum(VI) complexes
1865
1
3
Hz), 3.65 (2 H, d, J = 13.5 Hz), 3.42 (2 H, s), 1.56 (18 H, s), 1.35 (18 H, s) ppm. C NMR
101 MHz, CDCl ): δ 171.55, 160.68, 152.16, 140.92, 128.38, 126.21, 124.56, 62.92,
(
3
5
5.65, 35.65, 35.20, 31.43, 30.17 ppm. UV–VIS (CHCl ): λmax, nm (ε) 678 (10,703). Yield
3
1
77 mg (27%).
cis-1: IR: 1238 vs 1203 s, 1170 vs 1128 w, 1027 w, 978 w, 956 s, 931 m, 915 vs 871 m,
1
8
48 m, 808 w, 759 m, 721 m, 701 m, 653 m, 605 m, 578 s, 566 s. H NMR (500 MHz,
CDCl ): δ 7.46 (1 H, d, J = 1.9 Hz), 7.44 (1 H, d, J = 1.9 Hz), 7.24 (1 H, d, J = 1.7 Hz), 7.18
3
(1 H, d, J = 1.6 Hz), 5.01 (1 H, d, J = 13.9 Hz), 3.87 (1 H, d, J = 16.3 Hz), 3.76 (1 H, d,
J = 11.0 Hz), 3.73 (1 H, d, J = 11.0 Hz), 3.47 (1 H, d, J = 16.4 Hz), 3.40 (1 H, d,
1
3
J = 13.0 Hz), 1.55 (9 H, s), 1.49 (9 H, s), 1.33 (9 H, s), 1.31 (9 H, s) ppm. C NMR
101 MHz, CDCl ): δ 170.33, 162.74, 157.01, 151.48, 150.06, 140.86, 140.51, 129.38,
(
3
1
28.73, 125.29, 125.10, 124.76, 124.16, 62.79, 61.74, 59.56, 35.86, 35.14, 34.49 (two over-
lapping signals), 31.48, 31.32, 30.43, 29.88 ppm. UV–VIS (CHCl ): λmax, nm (ε) 693
3
(10,573). Yield 70 mg (11%).
Complex 2 was also prepared independently by the reaction of [MoO (acac) ] (165 mg,
2
2
0
.50 mM) and H L (255 mg, 0.50 mM) in 15 mL of CH Cl . The reaction mixture was stir-
3 2 2
red at room temperature for 3 h to give a dark solution. The solution was filtered through a
short pad of silica and the yellow product was collected by adding methanol to the solvent.
The brownish yellow filtrate was evaporated to obtain 190 mg (60%) of yellow solid. IR
and NMR spectra of this product were identical with those measured for the yellow product
obtained in the reaction of [MoO Cl (DMSO) ] with H L.
2
2
2
3
2
: IR: 1238 vs 1203 m, 1168 s, 1126 m, 1106 w, 1074 w, 1029 w, 977 m, 933 s, 914 vs
1
8
94 vs 846 vs 808 m, 775 w, 759 s, 698 w, 673 w, 649 w, 607 m, 559 s. H NMR
(
500 MHz, DMSO-d ): δ 7.11 (2 H, d, J = 2.5 Hz), 6.97 (2 H, d, J = 2.5 Hz), 4.44 (2 H, d,
6
J = 13.0 Hz), 3.62 (2 H, d, J = 13.0 Hz), 2.96 (2 H, s), 1.32 (18 H, s), 1.25 (18 H, s) ppm.
13
C NMR (126 MHz, DMSO-d ): δ 173.03, 160.68, 140.24, 136.88, 124.67, 123.42,
6
1
22.45, 61.71, 57.78, 49.07, 35.02, 34.29, 32.05, 30.26 ppm.
2
.3. X-ray structure determination
Single crystals of trans-1 were grown from hot acetonitrile as an acetonitrile solvate. The
X-ray data were collected on a Bruker–Nonius Kappa CCD diffractometer equipped with an
APEXII detector, with Mo Kα radiation (λ = 0.71073 Å) at 123(2) K. Empirical absorption
corrections using SADABS 2008 were applied [16]. The structure was solved by direct meth-
2
ods using SIR97 [17] program and full-matrix least-squares refinements on F were per-
formed using the SHELXL-97 program [18]. Figures 1 and S1 (see online supplemental
material at http://dx.doi.org/10.1080/00958972.2014.928287) were drawn with ORTEP3 for
Windows [19] and S2 with Mercury [20]. All non-hydrogen atoms were refined first isotropi-
cally and then anisotropically. All hydrogens were placed in their calculated positions with
fixed isotropic thermal parameters and refined as a riding model. Crystallographic data for
trans-1: formula C H ClMoNO ∙C H N, M = 697.14, orthorhombic, space group Pcab,
3
2
46
5
2
3
r
3
a = 10.3960(2), b = 21.4576(4), c = 31.7701(7) Å, α = β = γ = 90°, Z = 8, V = 7087.1(2) Å ,
T = 123(2) K, ρ = 1.307 g cm , F(0 0 0) = 2928, μ(Mo–K ) = 0.486 mm , 21,272 data,
−
3
−1
c
α
7
709 unique (Rint = 0.0351), 432 parameters, final R (I > 2σ (I)) = 0.0338, wR = 0.0795,
1 2
GOF = 1.038. CCDC 956,883 contains the supplementary crystallographic data for trans-1.

1
866
T. Heikkilä et al.
Table 1. Catalytic activities of cis-1 and trans-1.
a
b
−1 c
Entry
Catalyst
% Cat
T (°C)
t
1/2 (min)
Conversion (%)
TON
TOF (h )
Epoxidation of cis-cyclooctene
1
2
3
4
5
6
7
8
9
1
cis-1
trans-1
cis-1
trans-1
cis-1
trans-1
cis-1
trans-1
cis-1
trans-1
5
5
5
25
25
40
40
40
40
50
50
50
50
23
–
4
13
7
22
4
7
4
12
74
18
84
83
85
84
81
84
80
83
14.7
3.6
46
6
73
54
132
91
62
70
154
125
16.8
16.6
34.0
33.6
16.2
16.8
32.0
33.2
5
2,5
2,5
5
5
2,5
2,5
0
Sulfoxidation of tolyl methyl sulfide
1
1
cis-1
trans-1
cis-1
trans-1
cis-1
trans-1
cis-1
trans-1
cis-1
trans-1
5
5
5
25
25
40
40
40
40
50
50
50
50
20
40
8
13
12
15
7
8
5
8
84
85
84
82
82
77
87
87
84
81
16.8
17.0
16.8
16.4
32.8
30.8
17.4
17.4
33.6
32.4
76
29
66
56
120
89
84
83
151
132
12
13
14
15
16
17
18
19
20
5
2,5
2,5
5
5
2,5
2,5
a
Time required for a 50% decrease of the initial concentration of substrate.
b
1
Yield of product measured by H NMR after 24 h.
c
−1
−1.
TOF calculated at 10 min reaction as (mol product)∙(mol catalyst) ∙(t/h)
2.4. Catalytic studies
Catalytic experiments were run in deuterated chloroform solutions while the reactions were
1
monitored by H NMR spectroscopy using a three-minute interval. Solutions of tBuOOH
(0.3 mL, 0.2 M) and substrate (0.3 mL, 0.2 M of cis-cyclooctene or methyl-p-tolylsul-
fide +0.1 M of C H Cl as an internal standard) were added in a 5 mm NMR tube. The cata-
2
4
2
lyst solution (0.030 or 0.015 mL, 0.1 M) was added and the NMR tube was subsequently
placed in the NMR probe. The reaction rates were estimated from integrated intensities of
substrate and product spectra. In the epoxidation, the alkene multiplet at 5.6 ppm was turned
to the epoxide multiplet at 2.9 ppm whereas in the sulfoxidation the sulfide methyl singlet at
2
.45 was turned to the sulfoxide methyl singlet at 2.71 ppm. The results are shown in table 1.
3
3
. Results and discussion
.1. Synthesis
There are a handful of somewhat different methods for the synthesis of H L [8, 9, 11]. In
3
our studies, we found the procedure reported by Weyhermüller et al. the most reliable, so
H L was synthesized from glycine, 2,4-di-tert-butyl phenol, and paraformaldehyde in meth-
3
anol under reflux [11].
The molybdenum(VI) oxo chloro complex 1 was synthesized applying a practice
developed by us earlier for preparation of corresponding complexes with tetrapodal amine

Oxomolybdenum(VI) complexes
1867
trisphenols or aminoalcohol bisphenols (scheme 1) [4, 21]. Solid [MoO Cl (DMSO) ] was
2
2
2
mixed with a stoichiometric amount of a ligand precursor in hot toluene and the resulting
mixture was kept at reflux temperature to obtain an intense blue solution. Under such condi-
1
tions, the reaction leads to a mixture of several compounds as seen by TLC. H NMR spec-
trum of the evaporated mixture shows signals attributable to cis and trans isomers of 1 in
ca. 1 : 1 ratio. The reaction was repeated several times and the isomeric mixtures were frac-
tioned by silica column chromatography. In a typical experiment, trans-1 was obtained in
about 30% isolated yield whereas the yield for cis-1 was just around 10% due to the partial
decomposition upon separation and purification steps. Both compounds are readily soluble
in common organic solvents. Isolated solid isomers are stable under ambient atmosphere
but they slowly lose their color in solution. Generally, these carboxylate complexes, cis-1
and trans-1, are less stable than corresponding aminoalkoxide bisphenolates [4, 21].
In our attempts to prepare aforesaid isomers, minor amounts of some yellow 2 were also
isolated as a crystalline solid. The air-stable 2 is soluble in MeOH, MeCN, and DMSO
although, it did not form single crystals suitable for XRD analyses. The molecular identity
of 2 was studied by IR and NMR spectroscopy, which indicate that the compound is a
dioxo complex of amine bisphenolate with a neutral carboxylic acid side-arm (see
Section 3.2. Structure description). Complex 2 was also formed when [MoO (acac) ] was
2
2
used as a starting material in reaction with H L in a dichloromethane solution at room
3
temperature yielding an identical yellow product in a good yield.
3
.2. Structure description
1
The H NMR spectra of cis-1 and trans-1 show anticipated resonances for deprotonated
ligands, which readily differentiate between the C1 (cis-1) and C (trans-1) isomers.
s
Especially, benzylic methylene protons in C symmetric cis-1 are seen as four separated
1
one-proton doublets at 5.01, 3.87, 3.47, and 3.40 ppm whereas corresponding protons in
trans-1 are seen as two two-proton doublets at 4.69 and 3.65 ppm, respectively; these struc-
tures resemble those found for corresponding MoOCl complexes with aminoethanol bisphe-
−
1
nols [21]. The IR spectra of isolated solids show a strong IR absorption at ca. 915 cm
which can be assigned as Mo = O stretch but they lack the typical doublets for the MoO2
moiety, which indicates that the reaction includes also the removal of an oxo group.
1
13
The H and C NMR spectra of the yellow 2 are consistent with a C symmetric octahe-
s
dral structure with a mirror plane passing through the two oxo groups and a side-arm oxy-
gen donor. Upon complexation, the methylene protons adjacent to the phenolate group
1
become diastereotopic. In the H NMR spectrum of 2, there are two doublets at 4.44 and
3
.62, both with a geminal coupling constant J = 13.0 Hz.
There are few reports on structurally well-characterized dioxomolybdenum complexes
with tripodal tetradentate aminobisphenolate ligands [6, 7, 22–24]. These compounds present
−
1
typically a couple of medium to strong IR absorptions at 850–950 cm . The strong absorp-
−
1
tions at 886–905 and 908–944 cm
are assigned as asymmetric and symmetric
M = O stretches, respectively, while these two absorptions show generally a difference of ca.
−
1
−1
2
0–35 cm . Complex 2 shows four strong IR-absorptions in the range of 846–933 cm .
−
1
The strongest absorption maxima are at 894 and 914 cm , which is characteristic for
octahedral complexes of MoO2.
Crystallization of trans-1 from acetonitrile leads to formation of a solvate trans-1∙MeCN.
In solid state, it forms monomeric molecules in which the glycine bisphenolate has

1
868
T. Heikkilä et al.
Figure 1. The molecular structure of trans-1∙MeCN. The hydrogens are omitted for clarity. Selected bond
lengths (Å) and angles (°) are: Mo–O1, 1.6766(15); Mo–O2, 1.8724(14); Mo–O3, 1.8811(14); Mo–O4, 2.0149
(
1
14); Mo1-N8, 2.4266(17); Mo1-Cl1, 2.3513(5); O1-Mo1-N8, 168.80(7); O2-Mo1-O3 162.11(6); O4-Mo1-Cl1,
62.15(4).
coordinated as a tetradentate trianionic ligand through three oxygen donors and one nitro-
gen donor (figure 1). The nitrogen donor is located trans to the terminal oxo, which is typi-
cal for oxomolybdenum aminophenolates [3–7, 21–24]. The Mo–N bond is relatively long
(
ca. 2.43 Å) due to the strong structural trans effect of a multiple bonded oxo ligand [25].
The chloride is trans to the carboxylate oxygen. A mean plane defined by the chelate ring
atoms Mo1, O4, C17, C16, and N8 as well as terminal ligands O1 and Cl1 divides the mol-
ecule of trans-1 into two similar, although not crystallographically identical halves.
The carboxylate Mo–O4 bond length of 2.015 is ca. 0.11 Å longer than Mo–O alkoxide
(1.897–1.903 Å) [4, 21] or aryloxide (1.873–1.903 Å) [3, 25] bonds in structurally related
aminoalcoholate bisphenolates or amine trisphenolates. Otherwise, the bonding parameters
between the metal center and the trianionic aminocarboxylate bisphenolate ligand are in
accord with previous studies on oxomolybdenum(VI) complexes with tripodal aminoalcohol
bisphenolates and amine trisphenolates [3, 4, 21, 26]. In general, trans-1 has a structure
very similar to [MoOCl((tBuPhO) N)] ((tBuPhO) N = 6,6′,6″-(nitrilotris(methylene))tris
3
3
(2-(tert-butyl)phenolate), which is an efficient epoxidation and sulfoxidation catalyst [3].
Trans-1 crystallizes as distinct molecules without any strong hydrogen bonds (only weak H
bonds with CH∙∙∙O distances from 2.359 to 2.697 Å are found, see figure S1) or other
non-covalent interactions between complex units, which also explains the high solubility of
the complex. Bulky tBu substituents in the phenolate groups obviously prevent any strong
stacking interactions; therefore, the shortest distances between the aromatic rings from
adjacent molecules are 5.208 Å which indicates that there is no π-stacking as seen in
figure S2. The solvate acetonitrile molecules are located in the cavities of the structure with
C35H⋯O5 distance of 2.359 Ε (figure S1) and with the N⋯H distances of 2.881 and

Oxomolybdenum(VI) complexes
1869
Scheme 1. Reaction of glycine bisphenol H3L with MoO2Cl2(DMSO)2].
3.100 Ε to the nearest benzylic hydrogens of the organic ligand. Otherwise the structure is
stabilized by van der Waals forces to have a 3-D close packing of molecules in the lattice.
3
.3. Catalytic activity
Dioxomolybdenum complexes with different organic ligands are frequently used as catalysts
for oxygen transfer reactions, e.g. oxidation reactions of alkenes and sulfides with H O or
2
2
tBuOOH [7, 27–34]. However, the use of corresponding mono-oxo chloro complexes is not
extensively studied [3, 4]. In our experiments, the oxo chloro complexes cis-1 and trans-1
were tested as catalysts for epoxidation of cis-cyclooctene and sulfoxidation of methyl-p-tol-
ylsulfide with one equivalent of tert-butyl hydroperoxide as an oxidant (scheme 2). The
Scheme 2. Studied oxidation reactions.

1
870
T. Heikkilä et al.
reactions were run at 25, 40, and 50 °C in CDCl solutions while the reaction course was
3
1
monitored by H NMR (results are seen in table 1). The control experiments were carried
out without any catalyst where no reactions occurred. Conversely, the catalytic reactions
started immediately and no induction time was observed in these experiments. At room
temperature, trans-1 shows low activity for the epoxidation reaction, but it performs better
in higher temperatures whereas cis-1 has a good activity in all experiments. The conversions
of alkene were practically quantitative in all experiments, although the selectivity for epox-
ide was only ca. 75–85% while cis-cyclooctene oxide was the only detectable product. At
−
1
−1
5
0 °C, epoxidation activities of cis-1 (TOF = 154 h ) and trans-1 (TOF = 125 h ) are
comparable with that of several dioxomolybdenum(VI) complexes under related conditions,
e.g. [MoO (OSiPh )(2,2’-bipyridine)] [32], [MoO Cl(HC(bim) )]Cl (HC(bim) = tris(benz-
2
3
2
3
3
R
R
imidazolyl)methan) [34], and [MoO (acac)( N,O)] ( N,OH = imino-alcohol derivative of
2
α-pinene) [30]. However, activities were clearly lower than those reported for structurally
−
1
closely similar amine trisphenolate complex [MoOCl((tBuPhO) N)] (TOF = 7500 h ) [3],
3
possibly due to the lower stability of the studied carboxylate complexes. Especially, cis-1
seems not to survive applied turn-over conditions, so the actual reacting species may be
some decomposition product instead of the distinct complex.
Oxidation of organic sulfides to sulfoxides with tert-butyl hydroperoxide as oxidant can
be catalyzed by various molybdenum complexes, e.g. [MoOCl((tBuPhO) N)] [3] or
3
MoO Cl (MeTolSO) (MeTolSO = (R)-(+)-methyl-p-tolylsulfoxide) [35], whereas the sec-
2
2
2
ond oxidation step may yield corresponding sulfones as by-products. In our experiments,
the sulfoxidation of methyl-p-tolylsulfide was run readily with catalysts cis-1 and trans-1
without any formation of sulfone. Again, cis-1 shows higher activity at 25 °C although the
reaction rates are practically equal at higher temperatures. On the other hand, sulfoxidation
reactions are known to be self-catalytic, especially in high temperatures, which may affect
the reaction rates [36]. In general, these carboxylate complexes show higher activities for
oxidation reactions than corresponding alkoxide complexes.
4
. Conclusion
[
MoO Cl (DMSO) ] reacts with a tetradentate O N-type ligand precursor N,N-bis(3,5-
2
2
2
3
di-tert-butyl-2-hydroxybenzyl)aminoacetic acid (H L) to form the oxomolybdenum(VI) com-
3
plex [MoOCl(L)] as two separable isomers (cis and trans) along with minor quantities of a
dioxo product [MoO (HL)]. Isomers of [MoOCl(L)] form monomeric molecules in which
2
the ligand coordinates as a tetradentate trianionic ligand through two phenolate oxygen
donors, one carboxylate and one nitrogen donor. In the cis isomer, the phenolato moieties are
at cis positions, while the chloride is cis to the carboxylate oxygen. In the trans isomer, the
phenolato moieties are at trans positions. The overall structure of these isomers is very
similar to active oxidation catalyst [MoOCl((tBuPhO) N)], although the catalytic activities
3
are lower. Both isomers of [MoOCl(L)] catalyze selectively the epoxidation of cis-cyclooc-
tene and sulfoxidation of methyl-p-tolylsulfide with tert-butyl hydroperoxide. The cis isomer
is more active of the two and compares with a number of dioxomolybdenum(VI) complexes
used as catalysts for the epoxidation of cyclooctene.

Oxomolybdenum(VI) complexes
1871
Supplementary material
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: (+44) 1223-336-033; or E-mail: deposit@ccdc.cam.ac.uk.
Funding
This work was supported by COST Action [grant number CM1003] “Biological oxidation reactions – mechanisms
and design of new catalysts”.
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