Synthesis, structures, electrochemistry and properties of dioxo-molybdenum(VI) and -tungsten(VI) complexes with novel asymmetric N 2OS, and partially symmetric N2S2, NOS 2 N-capped tripodal ligands

Article abstract of DOI:10.1016/j.ica.2004.07.008

The synthesis and full characterisation of a range of Mo(VI) and W(VI) dioxo complexes with anionic asymmetric tripodal tetradentate ligands with various combinations of N,O and S donor atoms are described. Their redox chemistry and ability to catalyse the oxidation of primary and secondary allylic alcohols and benzylic alcohols with DMSO without addition of peroxide oxidant are discussed. These oxidations are shown to proceed selectively to the corresponding aldehydes or ketones with Mo complexes with at least two sulfur donors giving the highest yields. A new class of asymmetric N-capped (dianionic/trianionic) tripodal proligands [H_x(Lⁿ)] (x = 2, n = 1-6; x = 3, n = 7, 8) which possess pendant arms with N₂OS, N₂S₂ or NOS₂ donor groups and with different chelate ring sizes {5,5,5} or {5,6,5} has been prepared. Treatment of these ligands with [WO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) in the presence of base (triethylamine or KOH) leads to the formation of cis-dioxotungsten(VI) complexes of the types [WO₂(Lⁿ)] (n = 1-6) and K[WO₂(Lⁿ)] (n = 7, 8). Reaction of these tetradentate ligands with [MoO₂(acac)₂] (acac = acetylacetonate) gives the corresponding Mo(VI) analogues [MoO ₂(Lⁿ)] (n = 1-6) and K[MoO₂(Lⁿ)] (n = 7, 8). Moreover, a new five coordinate dioxomolybdenum(VI) complex with an NS₂ tridentate ligand [MoO₂(L⁹)] has been synthesised using similar procedure. All these compounds have been spectroscopically characterised and the molecular structures of [MoO ₂(Lⁿ)] (n = 2, 6) and [WO₂(L⁶)] have been established by X-ray diffraction analysis. The electrochemistry and the catalytic activity for oxidation of allylic and benzylic alcohols of these dioxo complexes have also been investigated.

Full text of DOI:10.1016/j.ica.2004.07.008

Inorganica Chimica Acta 357 (2004) 4358–4372
www.elsevier.com/locate/ica
Synthesis, structures, electrochemistry and properties of
dioxo-molybdenum(VI) and -tungsten(VI) complexes with
novel asymmetric N2OS, and partially symmetric N2S2, NOS2
N-capped tripodal ligands
*
Yee-Lok Wong, Andrew R. Cowley, Jonathan R. Dilworth
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK
Received 3 November 2003; accepted 10 July 2004
Available online 20 August 2004
Abstract
A new class of asymmetric N-capped (dianionic/trianionic) tripodal proligands [Hx(Ln)] (x = 2, n = 1–6; x = 3, n = 7, 8) which
possess pendant arms with N2OS, N2S2 or NOS2 donor groups and with different chelate ring sizes {5,5,5} or {5,6,5} has been pre-
pared. Treatment of these ligands with [WO2Cl2(dme)] (dme = 1,2-dimethoxyethane) in the presence of base (triethylamine or KOH)
leads to the formation of cis-dioxotungsten(VI) complexes of the types [WO2(Ln)] (n = 1–6) and K[WO2(Ln)] (n = 7, 8). Reaction of
these tetradentate ligands with [MoO2(acac)2] (acac = acetylacetonate) gives the corresponding Mo(VI) analogues [MoO2(Ln)]
(n = 1–6) and K[MoO2(Ln)] (n = 7, 8). Moreover, a new five coordinate dioxomolybdenum(VI) complex with an NS2 tridentate lig-
and [MoO2(L9)] has been synthesised using similar procedure. All these compounds have been spectroscopically characterised and
the molecular structures of [MoO2(Ln)] (n = 2, 6) and [WO2(L6)] have been established by X-ray diffraction analysis. The electrochem-
istry and the catalytic activity for oxidation of allylic and benzylic alcohols of these dioxo complexes have also been investigated.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Asymmetric tripodal ligands; Dioxomolybdenum(VI) complexes; Dioxotungsten(VI) complexes; Crystal structures; Electrochemistry;
Catalytic oxidation
1. Introduction
particular interest in the context of modeling the asym-
metric active metal sites such as those found in nitrile
hydratase [11], and horse liver alcohol dehydrogenase
(HLADH) [12]. We have initially selected the well-
known dioxo Mo(VI) and W(VI) cores to explore the
coordination chemistry potential of these new ligand
systems.
Molybdenum(VI) dioxo-complexes have been extre-
mely thoroughly investigated and there are many litera-
ture reports of their syntheses and reactivity,
particularly in terms of oxo-transfer reactions [13].
Recently, high-valent oxotungsten complexes have
attracted attention owing to their roles in various cata-
lytic processes such as alcohol oxidation [14], olefin
epoxidation [15], and olefin metathesis [16]. It is believed
There has been considerable recent interest in sym-
metrical N-capped tripodal proligands of the type
N[CH2(CH2)nX]3 {n = 1: X = SH [1], SR [2], NH2 [3],
NHMe, NMe2 [4], NH(SiMe3) [5], NH(C6F5) [6], OH
[7]; n = 2: X = NH2 [8]}. However, the chemistry of
asymmetric N-capped tripodal ligands which possess
three pendant arms with different donor groups has
not been well explored [9,10]. This type of ligand is of
*
Corresponding author. Tel.: +44-1865-272639; fax: +44-1865-
272690.
E-mail address: jon.dilworth@chem.ox.ac.uk (J.R. Dilworth).
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.07.008

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
4359
that most of these organic transformations and biologi-
cal processes involve oxygen atom transfer (OAT) reac-
tions as one of the crucial steps. Recent studies of
tungstoenzymes have stimulated research into the chem-
istry of tungsten oxo-complexes with sulfur ligands [17],
and in particular for those containing dithiolene-type
ligands, which provide both structural and functional
models for the pterin-containing tungstoenzymes [18].
However, tungsten-mediated oxo-transfer reactions
have been rarely studied compared with the related
molybdenum chemistry [13] and relatively few examples
have been reported [19,20].
We herein report a new series of dioxo-tungsten(VI)
and -molybdenum(VI) complexes containing either com-
pletely asymmetric (N2OS with {5,5,5}/{5,6,5} chelate
ring-size) or partially symmetric (N2S2 or NOS2) N-
capped tripodal ligands, and also a new five coordinate
dioxomolybdenum(VI) complex with an NS2 tridentate
ligand; including their synthesis, structures, electrochem-
istry, and a brief discussion of their catalytic activities for
the oxidation of a wide range of allylic and benzylic
alcohols without additional peroxide type oxidants.
SiMe4 (d = 0). IR spectra were recorded on a Perkin–El-
mer 1710 spectrophotometer as KBr pellets. Atmos-
pheric pressure chemical ionization (APCI) mass
spectra were recorded on a Hewlett-Packard 1050 Series
Mass spectrometer. Electrospray ionization (ESI) mass
spectra were recorded on a Micromass LCT time-of-
flight (TOF) mass spectrometer. Liquid secondary ion
(LSI) mass spectra were measured on a Bruker APEX
47e Fourier transform ion cyclotron resonance (FT-
ICR) mass spectrometer with 3-nitrobenzyl alcohol as
matrix. Elemental analyses were performed by the
microanalysis laboratory of the Inorganic Chemistry
Laboratory, University of Oxford, UK. Electrochemical
measurements were carried out on a CH Instruments
Model 600A electrochemical analyzer. The cell com-
prised inlets for a platinum-sphere working electrode,
a platinum wire counter electrode and a silver wire pseu-
do-reference electrode. Typically, a 0.1 M solution of
[Bu4N][BF4] in DMF containing 1.0 mM of sample
was purged with nitrogen for 20 min and then the vol-
tammograms were recorded at scan rates ranging from
100 to 2000 mV sÀ1 at ambient temperature. Ferrocene
was added as an internal reference and all potentials
are reported with respect to the ferrocene/ferrocenium
couple at 0.45 V versus SCE [28].
2. Experimental
2.3. Synthesis of the ligands
2.1. General information
2.3.1.
2-[N-(2-mercaptoethyl)-N-(2-pyridylmethyl)]
All reactions were carried out using standard Schlenk-
line techniques under an atmosphere of dinitrogen;
workups were performed in air. Dichloromethane was
aminophenol (H2L1)
A
pale brown suspension of 2-N-(2-pyridyl-
methyl)aminophenol (1.00 g, 5.0 mmol) and ethylene
sulfide (0.30 ml, 5.0 mmol) in toluene (5 ml) was heated
in a sealed ampoule at 110 ꢁC for 18 h. After cooling to
room temperature, the pale brown mixture was loaded
onto a silica gel column using CHCl3 followed by
CHCl3/ethyl acetate (5:1) as eluant. The second band
was collected and concentrated to give a pale yellow liq-
˚
pre-dried over 4 A molecular sieves and distilled from
calcium hydride. Diethyl ether, tetrahydrofuran (THF)
and toluene were distilled from sodium-benzophenone.
Methanol (MeOH) was distilled from magnesium meth-
oxide. N,N-Dimethylformamide (DMF) was dried over
barium oxide and distilled under reduced pressure. Silica
gel (70-230 mesh) for flash column chromatography was
purchased from Fluka. [MoO2(acac)2] (acac = acetyl-
acetonate) [21], [WO2Cl2(dme)] (dme = 1,2-dimeth-
oxyethane) [22], 2-N-(2-pyridylmethyl)aminophenol
[23], 2-N-(2-pyridylmethyl)amino-4-methylphenol [20d],
2-N-(2-pyridylmethyl)amino-4-tert-butylphenol [20d],
2-[(2-mercaptoethyl)aminomethyl]pyridine [24], 3,5-di-
tert-butyl-2-hydroxybenzyl bromide [25], 2-[bis(2-
mercaptoethyl)aminomethyl]pyridine (H2L6) [26] and
N,N-bis(2-mercaptoethyl)benzylamine (H2L9) [27] were
prepared according to literature procedures with minor
modification. All reagents were used directly as received.
1
uid. Yield: 0.49 g (38%). H NMR: d 8.63–8.66 (m, 1H,
ArH), 7.66–7.72 (m, 1H, ArH), 7.19–7.27 (m, 2H, ArH),
7.12–7.16 (m, 1H, ArH), 7.02–7.08 (m, 1H, ArH), 6.95–
6.99 (m, 1H, ArH), 6.79–6.86 (m, 1H, ArH), 4.12 (s, 2H,
PyCH2), 3.24–3.34 (m, 2H, NCH2CH2), 2.38–2.48 (m,
2H, NCH2CH2). 13C{1H} NMR: d 158.6, 153.6, 148.9,
137.1, 137.0, 125.8, 122.7, 122.5, 119.1, 116.4, 116.1,
61.7, 56.1, 23.3. MS (APCI): m/z 261 (45%) [M + H]+.
2.3.2.
2-[N-(2-mercaptoethyl)-N-(2-pyridylmethyl)]
amino-4-methylphenol (H2L2)
This compound was prepared by a procedure analo-
gous to that for H2L1. 2-N-(2-Pyridylmethyl)amino-4-
methylphenol (1.07 g, 5.0 mmol) was treated with ethyl-
ene sulfide (0.30 ml, 5.0 mmol) in toluene (5 ml) to give
2.2. Instrumentation
1
1
All H and 13C{1H} NMR spectra were recorded on
a Varian Mercury VX300 spectrometer (1H, 300; 13C,
75.4 MHz). Chemical shifts were relative to internal
H2L2 as a pale yellow liquid. Yield: 0.74 g (54%). H
NMR: d 8.64 (d, J = 4.2 Hz, 1H, ArH), 7.67 (t, J = 7.7
Hz, 1H, ArH), 7.18–7.25 (m, 2H, ArH), 6.93 (s, 1H,

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Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
ArH), 6.82–6.88 (m, 2H, ArH), 4.11 (s, 2H, PyCH2),
3.29 (t, J = 6.5 Hz, 2H, NCH2CH2), 2.42 (t, J = 6.5
Hz, 2H, NCH2CH2), 2.26 (s, 3H, CH3). 13C{1H}
NMR: d 158.7, 151.1, 148.9, 137.0, 128.3, 126.2, 123.1,
122.6, 122.3, 115.9 (1 peak overlapping), 61.8, 56.3,
23.3, 20.6. MS (APCI): m/z 275 (12%) [M + H]+.
2.3.5. 2-[N-(2-hydroxy-5-nitrobenzyl)-N-(2-mercapto-
ethyl)]aminomethyl-pyridine (H2L5)
This compound was prepared by a procedure analo-
gous to that for H2L4 by using 2-hydroxy-5-nitrobenzyl
chloride instead of 3,5-di-tert-butyl-2-hydroxybenzyl
bromide. Yield: 0.60 g (90%). 1H NMR: d 8.62 (d,
J = 7.5 Hz, 1H, ArH), 8.11 (dd, J = 2.4, 9.0 Hz, 1H,
ArH), 7.99 (d, J = 3.0 Hz, 1H, ArH), 7.74 (dt, J = 1.8,
7.7 Hz, 1H, ArH), 7.26–7.30 (m, 2H, ArH), 6.93 (d,
J = 9.6 Hz,, 1H, ArH), 3.94 (s, 2H, ArCH2), 3.82 (s,
2H, ArCH2), 2.82 (t, J = 7.1 Hz, 2H, NCH2CH2), 2.62
(t, J = 7.1 Hz, 2H, NCH2CH2). 13C{1H} NMR: d
164.2, 156.6, 148.7, 139.8, 137.3, 125.9, 125.6, 123.1,
122.9, 122.7, 117.1, 58.1, 56.7, 56.3, 22.0. MS (APCI):
m/z 320 (10%) [M + H]+.
2.3.3.
2-[N-(2-mercaptoethyl)-N-(2-pyridylmethyl)]
amino-4-tert-butylphenol (H2L3)
This compound was prepared by a procedure analo-
gous to that for H2L1. 2-N-(2-Pyridylmethyl)amino-4-
tert-butylphenol (1.28 g, 5.0 mmol) was treated with eth-
ylene sulfide (0.30 ml, 5.0 mmol) in toluene (5 ml) to give
1
H2L3 as a pale yellow liquid. Yield: 1.07 g (68%). H
NMR: d 8.65 (d, J = 4.2 Hz, 1H, ArH), 7.68 (dt,
J = 1.5, 7.7 Hz, 1H, ArH), 7.19–7.27 (m, 2H, ArH),
7.11 (d, J = 2.1 Hz, 1H, ArH), 7.06 (dd, J = 2.4, 8.3
Hz, 1H, ArH), 6.90 (d, J = 8.1 Hz, 1H, ArH), 4.14 (s,
2H, PyCH2), 3.33 (t, J = 6.5 Hz, 2H, NCH2CH2),
2.3.6. 2-[Bis(2-mercaptoethyl)amino]-3,5-dimethylphe-
nol (H3L7)
A dark brown suspension of 6-amino-2,4-dimethyl-
phenol (0.69 g, 5.0 mmol) and ethylene sulfide (0.60
ml, 10.0 mmol) in toluene (10 ml) in a sealed ampoule
was heated at 110 ꢁC for 36 h. After cooling to room
temperature, the pale brown mixture was loaded onto
a silica gel column using CH2Cl2 as eluant. The first
faint orange band was collected and concentrated to
t
2.39–2.45 (AB q, 2H, NCH2CH2), 1.29 (s, 9H, Bu).
13C{1H} NMR: d 158.8, 151.0, 148.9, 141.9, 137.1,
136.6, 122.7, 122.4, 122.3, 119.5, 115.6, 61.7, 56.1,
34.1, 31.6, 23.3. MS (APCI): m/z 317 (100%) [M + H]+.
1
give an orange pungent liquid. Yield: 0.36 g (28%). H
NMR: d 6.77 (d, J = 2.0 Hz, 2H, ArH), 3.07 (t, J = 6.6
Hz, 4H, NCH2CH2), 2.56 (t, J = 6.6 Hz, 4H,
NCH2CH2), 2.23 (s, 6H, CH3). 13C{1H} NMR: d
149.6, 134.3, 129.1, 128.4, 123.7, 120.7, 58.2, 22.9,
20.7, 15.9. MS (APCI): m/z 258 (55%) [M + H]+.
2.3.4.
2-[N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N-(2-
mercaptoethyl)]aminomethyl-pyridine (H2L4)
To a colourless mixture of 2-[(2-mercaptoethyl)ami-
nomethyl]pyridine (0.35 g, 2.1 mmol) and triethylamine
(0.29 ml, 2.1 mmol) in THF (20 ml) was added chloro-
trimethylsilane (0.23 g, 2.1 mmol) dropwise at 0 ꢁC.
Some white solid began to form during addition. The
mixture was stirred at this temperature for 1 h, then al-
lowed to warm to room temperature. After stirring for
12 h, a second portion of triethylamine (0.29 ml, 2.1
mmol) was added followed by 3,5-di-tert-butyl-2-
hydroxybenzyl bromide (0.63 g, 2.1 mmol), and the
mixture was heated under reflux for 3 h. The mixture
was cooled and the resulting white solid was filtered
and discarded. The yellow filtrate was concentrated
with a rotary evaporator and the residue was chroma-
tographed on a silica gel column using ethyl acetate/
hexane (1:2) as eluant. The second band was collected
and concentrated to give colourless viscous oil. Yield:
0.63 g (78%). 1H NMR: d 8.56 (d, J = 5.4 Hz, 1H,
PyH), 7.66 (dt, J = 1.8, 7.7 Hz, 1H, PyH), 7.34 (d,
J = 7.8 Hz, 1H, PyH), 7.23 (d, J = 2.4 Hz, 1H, ArH),
7.19 (t, J = 6.2 Hz, 1H, PyH), 6.86 (d, J = 2.4 Hz,
1H, ArH), 3.82 (s, 4H, PyCH2 and ArCH2), 2.83 (t,
J = 7.4 Hz, 2H, NCH2CH2), 2.69 (t, J = 7.4 Hz, 2H,
NCH2CH2), 1.44 (s, 9H, tBu), 1.28 (s, 9H, tBu).
13C{1H} NMR: d 157.5, 153.8, 149.1, 140.8, 136.6,
135.8, 123.9, 123.7, 123.2, 122.4, 121.2, 59.4, 59.0,
57.0, 34.9, 34.1, 31.7, 29.6, 21.7. MS (APCI): m/z 387
(90%) [M + H]+.
2.3.7. 2-[Bis(2-mercaptoethyl)amino]-4-tert-butylphenol
(H3L8)
This compound was prepared by a procedure analo-
gous to that for H3L7. 2-Amino-4-tert-butylphenol
(1.65 g, 10.0 mmol) was treated with ethylene sulfide
(1.2 ml, 20.0 mmol) to give H3L8 as an orange pungent
1
liquid. Yield: 0.86 g (30%). H NMR: d 7.11–7.14 (m,
2H, ArH), 6.88 (d, J = 9.0 Hz, 1H, ArH), 3.11 (t,
J = 6.5 Hz, 4H, NCH2CH2), 2.57 (t, J = 6.5 Hz, 4H,
t
NCH2CH2), 1.27 (s, 9H, Bu). 13C{1H} NMR: d 151.2,
143.2, 134.5, 124.1, 119.9, 114.2, 58.1, 34.3, 31.6, 22.9.
MS (APCI): m/z 286 (100%) [M + H]+.
2.4. Synthesis of the complexes
2.4.1. [MoO2(L1)]
To a yellowish-brown solution of H2L1 (0.96 g, 3.7
mmol) in MeOH (50 ml) was added [MoO2(acac)2]
(1.21 g, 3.7 mmol) and the mixture was allowed to stir
overnight. The resulting dark brown solid was collected
by filtration, washed thoroughly with ethyl acetate,
Et2O and hexane, and then dried in vacuo. Yield: 0.43
1
g (30%). H NMR (DMSO-d6): d 9.00 (d, J = 3.9 Hz,
1H, ArH), 7.95 (t, J = 7.7 Hz, 1 H, ArH), 7.75 (d,

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
4361
J = 7.5 Hz, 1H, ArH), 7.57 (t, J = 6.5 Hz, 1 H, ArH),
7.43 (d, J = 8.1 Hz, 1H, ArH), 7.10 (t, J = 8.0 Hz, 1
H, ArH), 6.89 (t, J = 7.5 Hz, 1H, ArH), 6.56 (d,
J = 8.4 Hz, 1 H, ArH), 4.76 (d, J = 16.2 Hz, 1H,
PyCH2), 4.40–4.51 (m, 2H, PyCH2 and NCH2CH2),
3.98–4.07 (m, 1H, NCH2CH2), 3.72–3.82 (m, 1H,
NCH2CH2), 3.58–3.65 (m, 1H, NCH2CH2). 13C{1H}
NMR: d 160.5, 154.1, 149.1, 140.1, 129.6, 125.1, 124.2,
123.3, 120.4, 117.7 (one peak overlapping), 64.7, 61.9,
31.3. IR (cmÀ1): 1604m, 1587m, 1542m, 1483s, 1451w,
1420w, 1359w, 1273s, 1157w, 1109w, 1025w, 937s
m(MoO2), 901s m(MoO2), 809s, 781s, 756s, 671w, 648w,
631m, 611w, 452w. MS (APCI): m/z 389 (15%)
[M + H]+. Anal. Calc. for C14H14MoN2O3S: C, 43.5;
H, 3.7; N, 7.3. Found: C, 43.6; H, 3.8; N, 7.3%.
1272s, 1127w, 1075w, 1056w, 1022w, 923s m(MoO2),
903s m(MoO2), 837s, 681w, 647w, 631w, 608w, 541w.
MS (APCI): m/z 445 (60%) [M + H]+. Anal. Calc. for
C18H22MoN2O3S: C, 48.9; H, 5.0; N, 6.3. Found: C,
48.9; H, 5.4; N, 6.6%.
2.4.4. [MoO2(L4)]
To a colourless solution of H2L4 (0.42 g, 1.1 mmol)
in MeOH (30 ml) was added [MoO2(acac)2] (0.36 g, 1.1
mmol). After stirring at room temperature for 1.5 h,
the resulting mixture was filtered and the volatiles were
removed under reduced pressure. The yellow residue
was loaded onto a silica gel column using CH2Cl2 fol-
lowed by ethyl acetate as eluant. The intense yellow
band was collected and concentrated to give an orange
oil, which was triturated with hexane to give an orange
2.4.2. [MoO2(L2)]
1
yellow solid. Yield: 0.48 g (86%). H NMR (DMSO-
This compound was prepared by a procedure analo-
gous to that for [MoO2(L1)]. [MoO2(acac)2] (0.98 g, 3.0
mmol) was treated with H2L2 (0.82 g, 3.0 mmol) in
d6): d 9.03 (d, J = 4.5 Hz, 1H, PyH), 7.78 (dt,
J = 1.5, 7.6 Hz, 1H, PyH), 7.38 (t, J = 6.5 Hz, 1H,
PyH), 7.14 (d, J = 7.5 Hz, 1H, PyH), 7.10 (d, J = 3.0
Hz, 1H, ArH), 7.04 (d, J = 3.0 Hz, 1H, ArH), 4.86
(d, J = 12.9 Hz, 1H, ArCH2), 4.22 (s, 2H, ArCH2),
4.03–4.15 (m, 2H, ArCH2 and NCH2CH2), 3.37–3.53
(m, 2H, NCH2CH2), 3.15–3.24 (m, 1H, NCH2CH2),
1
MeOH (30 ml) to give [MoO2(L2)] (0.98 g, 82%). H
NMR (DMSO-d6): d 9.00 (d, J = 3.9 Hz, 1H, ArH),
7.95 (dt, J = 1.8, 7.5 Hz, 1H, ArH), 7.54–7.58 (m, 2H,
ArH), 7.44 (d, J = 7.8 Hz, 1H, ArH), 6.91 (dd,
J = 1.2, 8.1 Hz, 1H, ArH), 6.45 (d, J = 8.1 Hz, 1H,
ArH), 4.73 (d, J = 16.5 Hz, 1H, PyCH2), 4.48 (d,
J = 16.5 Hz, 1H, PyCH2), 4.36–4.42 (m, 1H,
NCH2CH2), 3.96–4.06 (m, 1H, NCH2CH2), 3.70–3.79
(m, 1H, NCH2CH2), 3.56–3.63 (m, 1H, NCH2CH2),
2.25 (s, 3H, CH3). 13C{1H} NMR: d 158.2, 154.1,
149.1, 140.0, 130.1, 129.5, 125.0, 124.1, 123.3, 117.2
(one peak overlapping), 64.6, 61.9, 31.1, 20.4. IR
(cmÀ1): 2922w, 1605w, 1497s, 1458w, 1430w, 1376w,
1276s, 1119w, 1075w, 1053w, 1022w, 925s m(MoO2),
903s m(MoO2), 824m, 813m, 771w, 638w, 611w, 587w,
531w. MS (APCI): m/z 403 (13%) [M + H]+. Anal. Calc.
for C15H16MoN2O3S: C, 45.0; H, 4.0; N, 7.0. Found:
C, 44.9; H, 4.0; N, 7.0%.
t
t
1.22 (s, 9H, Bu), 1.17 (s, 9H, Bu). 13C{1H} NMR: d
158.9, 155.9, 150.9, 142.6, 140.1, 137.8, 125.4, 124.2,
124.1, 122.6, 122.5, 66.0, 63.3, 60.1, 35.4, 34.7, 31.9,
30.4, 29.9. IR (cmÀ1): 2994w, 2951m, 2903w, 2863w,
1605m, 1473m, 1441m, 1424w, 1361w, 1317w, 1300w,
1260m, 1239m, 1205w, 1174w, 1130w, 1022w, 908s
m(MoO2), 887s m(MoO2), 849m, 762m, 558m, 479w.
MS (APCI): m/z 515 (56%) [M + H]+. Anal. Calc. for
C23H32MoN2O3S: C, 53.9; H, 6.3; N, 5.5. Found: C,
53.6; H, 5.9; N, 5.5%.
2.4.5. [MoO2(L5)]
This compound was prepared by a procedure analo-
gous to that for [MoO2(L1)]. [MoO2(acac)2] (0.65 g,
2.0 mmol) was treated with H2L5 (0.64 g, 2.0 mmol) in
2.4.3. [MoO2(L3)]
1
MeOH (30 ml) to give [MoO2(L5)] (0.78 g, 88%). H
This compound was prepared by a procedure analo-
gous to that for [MoO2(L1)]. [MoO2(acac)2] (0.91 g,
2.8 mmol) was treated with H2L3 (0.88 g, 2.8 mmol) in
NMR (DMSO-d6): d 8.82 (d, J = 4.2 Hz, 1H, ArH),
8.17 (d, J = 2.7 Hz, 1 H, ArH), 7.74–7.80 (m, 2H,
ArH), 7.41 (t, J = 6.5 Hz, 1H, ArH), 7.07 (d, J = 8.1
Hz, 1H, ArH), 6.36 (d, J = 9.3 Hz, 1H, ArH), 4.63 (d,
J = 13.5 Hz, 1H, ArCH2), 4.26–4.32 (m, 2 H, ArCH2),
4.02–4.12 (m, 2H, ArCH2 and NCH2CH2), 3.46–3.53
(m, 3H, NCH2CH2). 13C{1H} NMR: d 167.3, 154.8,
149.3, 140.1, 139.3, 126.4, 125.7, 124.4, 123.9, 122.5,
118.8, 64.2, 60.0, 58.3, 29.4. IR (cmÀ1): 1607m, 1579w,
1509m m(NO2), 1479m, 1439w, 1421w, 1339s m(NO2),
1289s, 1089m, 917s m(MoO2), 895s m(MoO2), 841w,
800w, 670m. HRMS (LSI): m/z calc. for C15H16Mo-
N3O5S [M + H]+ 447.9865, found 447.9867. Anal. Calc.
for C15H15MoN3O5S: C, 40.5; H, 3.4; N, 9.4. Found: C,
40.2; H, 3.9; N, 9.1%.
1
MeOH (30 ml) to give [MoO2(L3)] (1.06 g, 86%). H
NMR (DMSO-d6): d 9.01 (d, J = 3.9 Hz, 1H, ArH),
7.95 (t, J = 7.8 Hz, 1 H, ArH), 7.72 (s, 1H, ArH), 7.56
(t, J = 6.5 Hz, 1H, ArH), 7.44 (d, J = 7.8 Hz, 1H,
ArH), 7.12 (d, J = 8.7 Hz, 1H, ArH), 6.47 (d, J = 9.0
Hz, 1H, ArH), 4.74 (d, J = 17.1 Hz, 1H, PyCH2),
4.48–4.55 (m, 2H, PyCH2 and NCH2CH2), 3.96–4.05
(m, 1H, NCH2CH2), 3.67–3.79 (m, 1H, NCH2CH2),
3.56–3.63 (m, 1H, NCH2CH2), 1.26 (s, 9H, tBu).
13C{1H} NMR (DMSO-d6): d 158.1, 154.2, 149.1,
143.4, 140.0, 139.7, 126.4, 124.9, 123.4, 120.5, 116.7,
64.6, 62.0, 34.4, 31.5, 31.1. IR (cm1): 2963m, 2867w,
1604m, 1496s, 1459w, 1441w, 1421m, 1365w, 1287s,

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Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
2.4.6. [MoO2(L6)]
NCH2CH2), 3.06–3.11 (m, 2H, NCH2CH2), 2.94 (dt,
J = 3.9, 12.3 Hz, 2H, NCH2CH2), 1.23 (s, 9H, Bu).
To a colourless solution of H2L6 (0.66 g, 2.9 mmol) in
MeOH (60 ml) was added [MoO2(acac)2] (0.95 g, 2.9
mmol). The mixture turned from yellow to orange and
then gave a dark brown suspension. After stirring at
room temperature for 3 h, the resulting orange-brown
solid was collected by filtration, washed thoroughly with
MeOH, Et2O and hexane, and dried in vacuo. Yield:
t
13C{1H} NMR (acetone-d6): d 163.0, 138.7, 135.7,
125.0, 118.3, 117.9, 66.0, 34.6, 32.1, 31.8. IR (cmÀ1):
2958m, 2861w, 1702w, 1603w, 1498s, 1460w, 1362w,
1307s, 1275m, 1128w, 1077w, 913s m(MoO2), 878s
m(MoO2), 835s, 758m, 671w, 603w, 442w. MS (ESIÀ):
m/z 412 (100%) [M]À. Anal. Calc. for C14H20KMo-
NO3S2: C, 37.4; H, 4.5; N, 3.1. Found: C, 37.4; H, 5.0;
N, 3.0%.
1
0.94 g (92%). H NMR (DMSO-d6): d 8.97 (d, J = 5.1
Hz, 1H, PyH), 8.00 (dt, J = 1.8, 7.7 Hz, 1H, PyH),
7.48–7.53 (m, 2H, PyH), 4.42 (s, 2H, PyCH2), 3.84–
3.94 (m, 2H, NCH2CH2), 3.46–3.52 (m, 2H,
NCH2CH2), 3.33–3.40 (m, 2H, NCH2CH2), 3.07–3.17
(m, 2H, NCH2CH2). 13C{1H} NMR: d 155.4, 149.2,
139.4, 124.3, 121.9, 66.2, 60.0, 32.3. IR (cmÀ1): 1603m,
1439m, 1297m, 1212w, 1052w, 1019w, 915s m(MoO2),
889s m(MoO2), 810m, 761w. MS (APCI): m/z 357
(50%) [M + H]+. Anal. Calc. for C10H14MoN2O2S2: C,
33.9; H, 4.0; N, 7.9. Found: C, 34.0; H, 3.7; N, 7.9%.
2.4.9. [MoO2(L9)]
To a warmed solution of [MoO2(acac)2] (0.98 g, 3.0
mmol) in MeOH (60 ml) at 60 ꢁC was added a colourless
solution of H2L9 (0.68 g, 3.0 mmol) in MeOH. The mix-
ture turned from yellow to orange and then to a dark
brown suspension. After heating for 30 min, the result-
ing orange-brown solid was collected by filtration,
washed thoroughly with MeOH, diethyl ether and hexa-
1
nes, then dried in vacuo. Yield: 0.72 g (68%). H NMR:
2.4.7. K[MoO2(L7)]
7.43–7.45 (m, 3H, ArH), 7.26–7.32 (m, 2H, ArH), 4.65
(s, 2H, ArCH2), 3.85–3.93 (m, 2H, NCH2CH2), 3.44–
3.57 (m, 4H, NCH2CH2), 2.74–2.82 (m, 2H,
NCH2CH2). 13C{1H} NMR: d 131.9, 130.7, 129.2,
128.9, 58.2, 58.0, 33.3. MS (ESI+): m/z 356 (22%) [M
+ H]+. IR (cmÀ1): 934s m(MoO2), 903s m(MoO2). Anal.
Calc. for C11H15MoNO2S2: C, 37.4; H, 4.3; N, 4.0.
Found: C, 37.9; H, 4.6; N, 3.8%.
A colourless mixture of H3L7 (0.26 g, 1.0 mmol)
and KOH (0.056 g, 1.0 mmol) in MeOH (100 ml)
was added to [MoO2(acac)2] (0.33 g, 1.0 mmol). The
mixture turned deep red immediately. After stirring
at room temperature for 1 h, the solution was filtered
and the volatiles were removed on a rotary evapora-
tor. The deep red residue was redissolved in acetone
(50 ml) and stirred for 15 min. Then the insoluble
white solid was filtered and discarded. The filtrate
was concentrated to ca. 5 ml and triturated with hex-
ane. The red solid formed was isolated by filtration
and then redissolved in ethyl acetate and precipitated
with hexane to give a reddish brown solid which was
collected by filtration and dried in vacuo. Yield: 0.19
2.4.10. [WO2(L1)]
To a pale yellow mixture of H2L1 (0.39 g, 1.5 mmol)
and triethylamine (0.42 ml, 3.0 mmol) in MeOH (30 ml)
was added a colourless solution of [WO2Cl2(dme)] (0.57
g, 1.5 mmol) in CH2Cl2 (20 ml). The mixture turned or-
ange-brown during addition. After stirring for 15 min,
some white solid began to form and this mixture was al-
lowed to stir overnight. The white solid formed was col-
lected by filtration, washed with ethanol, ethyl acetate,
Et2O and hexane, and dried in vacuo. Yield: 0.38 g
(53%). 1H NMR (DMSO-d6): d 9.07 (d, J = 5.4 Hz,
1H, ArH), 8.02 (t, J = 7.5 Hz, 1H, ArH), 7.78 (d,
J = 8.4 Hz, 1H, ArH), 7.61 (t, J = 6.2 Hz, 1H, ArH),
7.51 (d, J = 8.1 Hz, 1H, ArH), 7.15 (t, J = 7.1 Hz, 1H,
ArH), 6.94 (t, J = 7.7 Hz, 1H, ArH), 6.61 (d, J = 8.1
Hz, 1H, ArH), 4.72–4.87 (AB q, 2H, PyCH2), 4.55–
4.60 (m, 1H, NCH2CH2), 3.84–3.96 (m, 2H,
NCH2CH2), 3.58–3.64 (m, 1H, NCH2CH2). 13C{1H}
NMR (DMSO-d6): d 159.6, 155.0, 149.4, 140.5, 139.9,
129.9, 125.1, 124.3, 123.6, 121.0, 118.8, 65.2, 61.4,
29.6. IR (cmÀ1): 1607m, 1589m, 1484s, 1451m, 1420m,
1306w, 1273s, 1253m, 1233w, 1159w, 1108w, 1058w,
1028w, 948s m(WO2), 907s m(WO2), 859w, 825w, 781m,
757m, 649w, 637m. MS (APCI): m/z 475 (10%)
[M + H]+. Anal. Calc. for C14H14N2O3SW: C, 35.5; H,
3.0; N, 5.9. Found: C, 35.6; H, 3.1; N, 6.0%.
1
g (46%). H NMR (acetone-d6): d 6.75 (s, 1H, ArH),
6.63 (s, 1H, ArH), 4.24 (dt, J = 5.1, 11.4 Hz, 2H,
NCH2CH2), 3.55–3.59 (m, 2H, NCH2CH2), 3.02–3.08
(m, 2H, NCH2CH2), 2.94 (dt, J = 3.9, 12.3 Hz, 2H,
NCH2CH2), 2.13 (s, 6H, CH3). 13C{1H} NMR: d
162.2, 135.3, 129.9, 126.2, 123.6, 119.4, 65.9, 31.8,
20.7, 17.5. IR (cmÀ1): 2926w, 2860w, 1482s, 1309s,
1262m, 902s m(MoO2), 873m m(MoO2), 819w, 790w,
757w, 641w, 593w. MS (ESIÀ): m/z 384 (85%) [M]À.
Anal. Calc. for C12H16KMoNO3S2: C, 34.2; H, 3.8;
N, 3.3. Found: C, 34.6; H, 4.0; N, 3.4%.
2.4.8. K[MoO2(L8)]
This compound was prepared by a procedure analo-
gous to that for [MoO2(L7)]. [MoO2(acac)2] (0.46 g,
1.4 mmol) was treated with H3L8 (0.40 g, 1.4 mmol)
and KOH (0.079 g, 1.4 mmol) in MeOH (100 ml) to give
1
K[MoO2(L8)] (0.35 g, 56%). H NMR (acetone-d6): d
7.14 (d, J = 2.1 Hz, 1H, ArH), 6.96 (dd, J = 2.4, 8.1
Hz, 1H, ArH), 6.53 (d. J = 8.4 Hz, 1H, ArH), 4.25 (dt,
J = 5.1, 11.8 Hz, 2H, NCH2CH2), 3.65–3.70 (m, 2H,

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
4363
1
2.4.11. [WO2(L2)]
pale yellow solid. Yield: 0.59 g (43%). H NMR: d 9.20
(d, J = 4.8 Hz, 1H, PyH), 7.66 (dt, J = 1.8, 7.8 Hz, 1H,
PyH), 7.26 (t, J = 6.5 Hz, 1H, PyH), 7.09 (d, J = 2.4
Hz, 1H, ArH), 6.89–6.94 (m, 2H, PyH and ArH), 5.05
(d, J = 12.9 Hz, 1H, ArCH2), 4.28 (d, J = 16.2 Hz, 1H,
ArCH2), 4.10–4.19 (m, 2H, ArCH2 and NCH2CH2),
3.81 (d, J = 12.9 Hz, 1H, ArCH2), 3.60–3.67 (m, 1H,
NCH2CH2), 3.50–3.57 (m, 1H, NCH2CH2), 3.20–3.29
This compound was prepared by a procedure analo-
gous to that for [WO2(L1)]. [WO2Cl2(dme)] (0.87 g, 2.3
mmol) was treated with H2L2 (0.63 g, 2.3 mmol) and tri-
ethylamine (0.64 ml, 4.6 mmol) in MeOH (20 ml)/
CH2Cl2 (20 ml) to give [WO2(L2)] (0.68 g, 61%). 1H
NMR (DMSO-d6): d 9.06 (d, J = 5.1 Hz, 1H, ArH),
8.01 (t, J = 7.7 Hz, 1H, ArH), 7.59–7.62 (m, 2H,
ArH), 7.51 (d, J = 8.1 Hz, 1H, ArH), 6.96 (d, J = 8.1
Hz, 1H, ArH), 6.50 (d, J = 8.1 Hz, 1H, ArH), 4.72–
4.85 (AB q, 2H, PyCH2), 4.52–4.56 (m, 1H, NCH2CH2),
3.79–3.97 (m, 2H, NCH2CH2), 3.56–3.62 (m, 1H,
NCH2CH2), 2.26 (s, 3H, CH3). 13C{1H} NMR
(DMSO-d6): d 157.2, 155.0, 149.4, 140.4, 139.5, 130.4,
130.1, 125.1, 124.2, 123.6, 118.2, 65.0, 61.4, 29.5, 20.4.
IR (cmÀ1): 2924w, 1607m, 1499s, 1458w, 1431m,
1377w, 1276s, 1153w, 1117w, 1054w, 1025w, 949s
m(WO2), 909s m(WO2), 827m, 813m, 772w, 641w, 533w.
MS (APCI): m/z 489 (20%) [M + H]+. Anal. Calc. for
C15H16N2O3SW: C, 36.9; H, 3.3; N, 5.7. Found: C,
36.6; H, 3.4; N, 5.9%.
t
(m, 1H, NCH2CH2), 1.22 (s, 9H, Bu), 1.18 (s, 9H,
tBu). 13C{1H} NMR: d 156.1, 154.9, 151.1, 142.9,
139.3, 138.4, 124.4, 124.0, 123.5, 121.1, 120.8, 66.0,
63.3, 60.4, 34.8, 34.2, 31.6, 30.0, 28.2. IR (cmÀ1):
2956s, 2868m, 1608m, 1476s, 1443s, 1361w, 1304w,
1261s, 1240s, 1208w, 1171w, 943s m(WO2), 900s
m(WO2), 849s, 759m, 559m. MS (APCI): m/z 601 (95%)
[M + H]+. Anal. Calc. for C23H32N2O3SW: C, 46.0; H,
5.4; N, 4.7. Found: C, 46.7; H, 6.2; N, 4.4%.
2.4.14. [WO2(L5)]
This compound was prepared by a procedure analo-
gous to that for [WO2(L1)]. [WO2Cl2(dme)] (0.75 g, 2.0
mmol) was treated with H2L5 (0.64 g, 2.0 mmol) and tri-
ethylamine (0.55 ml, 4.0 mmol) in MeOH (20 ml)/
CH2Cl2 (20 ml) to give [WO2(L5)] (0.83 g, 78%). 1H
NMR (DMSO-d6): 8.88 (d, J = 4.5 Hz, 1H, ArH), 8.19
(d, J = 3.0 Hz, 1H, ArH), 7.81–7.86 (m, 2H, ArH),
7.46 (t, J = 6.5 Hz, 1H, ArH), 7.15 (d, J = 7.5 Hz, 1H,
ArH), 6.44 (d, J = 8.7 Hz, 1H, ArH), 4.66 (d, J = 13.5
Hz, 1H, ArCH2), 4.24–4.42 (m, 3H, ArCH2), 3.95–4.05
(m, 1H, NCH2CH2), 3.64–3.68 (m, 1H, NCH2CH2),
3.54–3.56 (m, 2H, NCH2CH2). 13C{1H} NMR
(DMSO-d6): d 165.7, 155.7, 149.6, 140.6, 139.9, 126.3,
125.8, 124.6, 124.1, 122.6, 119.1, 64.5, 59.9, 58.8, 27.6.
IR (cmÀ1): 1609m, 1581w, 1513m m(NO2), 1481m,
1440w, 1422w, 1341s m(NO2), 1285s, 1088m, 942s
m(WO2), 900s m(WO2), 842w, 801w, 673w. HRMS
(LSI): m/z calc. for C15H16N3O5SW [M + H]+
2.4.12. [WO2(L3)]
This compound was prepared by a procedure analo-
gous to that for [WO2(L1)]. [WO2Cl2(dme)] (0.75 g, 2.0
mmol) was treated with H2L3 (0.63 g, 2.0 mmol) and tri-
ethylamine (0.55 ml, 4.0 mmol) in MeOH (30 ml)/
CH2Cl2 (20ml) to give [WO2(L3)] (0.84 g, 79%). 1H
NMR (DMSO-d6): 9.07 (d, J = 4.8 Hz, 1H, ArH), 8.02
(t, J = 7.7 Hz, 1H, ArH), 7.74 (d, J = 2.4 Hz, 1H,
ArH), 7.60 (t, J = 6.5 Hz, 1H, ArH), 7.52 (d, J = 7.8
Hz, 1H, ArH), 7.17 (dd, J = 1.8, 8.7 Hz, 1H, ArH),
6.52 (d, J = 8.4 Hz, 1H, ArH), 4.73–4.86 (AB q, 2H,
PyCH2), 4.64–4.69 (m, 1H, NCH2CH2), 3.79–3.93 (m,
2H, NCH2CH2), 3.56–3.62 (m, 1H, NCH2CH2), 1.26
(s, 9H, tBu). 13C{1H} NMR (DMSO-d6): d 157.1,
155.2, 149.4, 144.0, 140.5, 139.4, 126.7, 125.0, 123.6,
120.6, 117.8, 65.1, 61.5, 34.4, 31.5, 29.5. IR (cm1):
2959m, 2904w, 2869w, 1607m, 1498s, 1425m, 1363w,
1278s, 1233w, 1127w, 1056w, 1024w, 949s m(WO2),
909s m(WO2), 837s, 820m, 788w, 769w, 682w, 649w,
633w. MS (APCI): m/z 531 (35%) [M + H]+. Anal. Calc.
for C18H22N2O3SW: C, 40.8; H, 4.2; N, 5.3. Found: C,
40.7; H, 4.6; N, 5.3%.
534.0320,
found
534.0303.
Anal.
Calc.
for
C15H15N3O5SW: C, 33.8; H, 2.8; N, 7.9. Found: C,
33.5; H, 3.5; N, 7.6%.
2.4.15. [WO2(L6)]
A colourless solution of H2L6 (0.57 g, 2.5 mmol) in
MeOH (20 ml) was added triethylamine (0.69 ml, 5.0
mmol) with vigorous stirring. After stirring at room
temp for 15 min, a colourless solution of [WO2Cl2(dme)]
(0.94 g, 2.5 mmol) in CH2Cl2 (30 ml) was added drop-
wise via a cannula to this solution. The mixture turned
cloudy immediately and some pale yellow solid began
to form. This mixture was allowed to stir for 5 h. Then
the pale yellow solid formed was isolated by filtration,
washed thoroughly with MeOH, ethyl acetate, Et2O
and hexane, and dried in vacuo. Yield: 0.62 g (56%).
1H NMR (DMSO-d6): d 9.00 (dd, J = 1.2, 5.9 Hz, 1H,
PyH), 8.06 (dt, J = 1.8, 7.7 Hz, 1H, PyH), 7.52–7.56
(m, 2H, PyH), 4.58 (s, 2H, PyCH2), 3.74–3.82 (m, 2H,
2.4.13. [WO2(L4)]
To a colourless mixture of H2L4 (0.89 g, 2.3 mmol)
and triethylamine (0.64 ml, 4.6 mmol) in CH2Cl2 (20
ml) was added a solution of [WO2Cl2(dme)] (0.87 g,
2.3 mmol) in CH2Cl2 (30 ml). The mixture turned dark
brown immediately. After stirring overnight, the vola-
tiles were removed under reduced pressure and the resi-
due was transferred to a silica gel column using CH2Cl2
first followed by ethyl acetate as eluant. The second
band was collected and concentrated to give a pale yel-
low oil which was then triturated with hexane to give a

4364
Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
NCH2CH2), 3.61–3.67 (m, 2H, NCH2CH2), 3.51–3.57
(m, 2H, NCH2CH2), 3.12–3.22 (m, 2H, NCH2CH2).
13C{1H} NMR (DMSO-d6): d 156.5, 149.4, 139.8,
124.5, 122.1, 66.1, 60.5, 31.2. IR (cm1): 2955w, 2862w,
1605m, 1482w, 1457w, 1439w, 1422w, 1298m, 1213w,
1083w, 1052w, 945s m(WO2), 897s m(WO2), 839w,
795w, 763m. MS (APCI): m/z 443 (58%) [M + H]+. Anal.
Calc. for C10H14N2O2S2W: C, 27.2; H, 3.2; N, 6.3.
Found: C, 27.3; H, 3.4; N, 6.2%.
2.5. General procedure for screening the catalytic activity
of the complexes towards alcohol oxidation under anaer-
obic condition
A mixture of metal complex {[MO2(Ln)] (M = Mo,
W; n = 3À6, 8, 9)} (5 mol%) and benzoin (0.2 mmol)
in DMSO-d6 (0.5 ml) was placed in a NMR tube
equipped with YoungÕs Teflon tap. The mixture was de-
gassed and heated at 100 ꢁC. The catalytic product (ben-
1
zil) was identified by comparing the H NMR spectrum
2.4.16. K[WO2(L7)]
with that of the authentic sample. The percentage con-
version of the product was determined by 1H NMR
spectroscopy.
A colourless mixture of H3L7 (0.23 g, 0.9 mmol) and
KOH (0.15 g, 2.6 mmol) in MeOH (60 ml) was added to
[WO2Cl2(dme)] (0.34 g, 0.9 mmol). The mixture turned
dark purple immediately. After stirring at room temper-
ature for 12 h, the solution was filtered before being con-
centrated using rotary evaporator. The dark purple
residue was redissolved in acetone (50 ml) and stirred
for 15 min. The insoluble white solid was filtered and
discarded. The filtrate was concentrated to ca. 5 ml
and triturated with hexane to give a reddish solid which
was isolated by filtration, washed thoroughly with ethyl
acetate and acetone/hexane (1:3) to give a pale yellow
solid. Yield: 0.16 g (36%). 1H NMR (acetone-d6): d
6.76 (s, 1H, ArH), 6.68 (s, 1H, ArH), 4.21 (dt, J = 4.8,
11.7 Hz, 2H, NCH2CH2), 3.68 (dd, J = 4.2, 10.1 Hz,
2H, NCH2CH2), 3.23 (dd, J = 4.8, 11.7 Hz, 2H,
NCH2CH2), 2.94 (dt, J = 4.5, 12.5 Hz, 2H, NCH2CH2),
2.15 (s, 3H, CH3), 2.12 (s, 3H, CH3). 13C{1H} NMR
(acetone-d6): d 162.1, 135.4, 130.4, 127.4, 124.8, 119.8,
65.7, 31.1, 20.6, 17.2. IR (cmÀ1): 2925w, 2858w, 1485s,
1459m, 1310s, 1261m, 1211w, 1075w, 981w, 927s
m(WO2), 876s m(WO2), 819m, 791w, 757w, 677w, 645w,
595w, 499w, 453m. MS (ESIÀ): m/z 470 (100%) [M]À.
Anal. Calc. for C12H16KNO3S2W: C, 28.3; H, 3.2; N,
2.8. Found: C, 28.6; H, 3.5; N, 3.0%.
2.6. General procedure for the alcohol oxidation catalysed
by [MoO2(L9)] under aerobic condition
A mixture of substrate (0.2 mmol) and [MoO2(L9)] (5
mol%) in DMSO-d6 (0.5 ml) was placed in a 10 ml
round bottom flask equipped with a water condenser.
The mixture was heated at 100 ꢁC in air. The catalytic
product was identified by comparing the 1H NMR spec-
trum with that of the authentic sample. The percentage
conversion of the products was determined by 1H NMR
spectroscopy.
2.7. X-ray crystal structure determination of
[MoO2(L2)], [MoO2(L6)] and [WO2(L6)]
Single crystals of [MoO2(L2)] and [MoO2(L6)] suita-
ble for X-ray diffraction were grown from DMF by slow
evaporation. A suitable crystal of [WO2(L6)] was ob-
tained by slow cooling of a warm solution in DMSO-
d6 in an NMR tube. Crystal data and data processing
parameters are given in Table 3. Crystals were mounted
on a glass fiber using perfluoropolyether oil and cooled
rapidly to 150K in a stream of cold nitrogen using an
Oxford Cryosystems CRYOSTREAM unit. For
[MoO2(L2)] and [WO2(L6)], diffraction data were col-
lected on an Enraf-Nonius KappaCCD diffractometer
with graphite-monochromated Mo Ka (k = 0.71073
2.4.17. K[WO2(L8)]
This compound was prepared according to the proce-
dure analogous to that for K[WO2(L7)]. [WO2Cl2(dme)]
(0.41 g, 1.1 mmol) was treated with H3L8 (0.31 g, 1.1
mmol) and KOH (0.19 g, 3.4 mmol) in MeOH (60 ml)
˚
A). Intensity data were processed using the DENZO-
1
to give K[WO2(L8)] (0.22 g, 38%). H NMR (acetone-
SMN package [29]. For [MoO2(L6)], diffraction data
were collected on an Enraf-Nonius DIP2000 image-plate
diffractometer with graphite-monochromated Mo Ka
d6): d 7.15 (d, J = 2.4 Hz, 1H, ArH), 7.01 (dd, J = 2.4,
8.1 Hz, 1H, ArH), 6.51 (d. J = 8.7 Hz, 1H, ArH), 4.21
(dt, J = 4.8, 12.1 Hz, 2H, NCH2CH2), 3.79 (dd,
J = 4.8, 10.2 Hz, 2H, NCH2CH2), 3.26 (dd, J = 4.8,
11.4 Hz, 2H, NCH2CH2), 2.93 (dt, J = 4.8, 12.5 Hz,
˚
(k = 0.71069 A). Ninety frames of data were collected
(2ꢁ oscillation per frame, 900 s exposure). Intensity data
were processed using the programs DENZO and SCALE-
PACK [29]. All these structures were solved using the
direct-methods program SIR92 [30], which located all
non-hydrogen atoms. Subsequent full-matrix least-
squares refinement was carried out using the CRYSTALS
program suite [31]. Coordinates and anisotropic thermal
parameters of all non-hydrogen atoms were refined.
Hydrogen atoms were positioned geometrically after each
cycle ofrefinement. 4-Term {for[MoO2(L2)]}, 3-term {for
t
2H, NCH2CH2), 1.24 (s, 9H, Bu). 13C{1H} NMR (ace-
tone-d6): d 163.2, 139.4, 135.9, 125.5, 119.0, 118.7, 65.7,
34.6, 32.1, 31.1. IR (cmÀ1): 2960m, 2864w, 1499s,
1459w, 1363w, 1307s, 1277w, 1128w, 1076w, 927s
m(WO2), 877s m(WO2), 835m, 759m, 671w, 607w, 451w.
MS (ESIÀ): m/z 498 (100%) [M]À. Anal. Calc. for
C14H20KNO3S2W: C, 31.3; H, 3.8; N, 2.6. Found: C,
31.5; H, 4.0; N, 2.8%.

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
4365
[MoO2(L6)]} and 5-term {for [WO2(L6)]} Chebychev pol-
ynomial weighting schemes were applied.
during prolonged heating. Treatment of this N2S precur-
sor with chlorotrimethylsilane in the presence of trieth-
ylamine gave the silyl thioether adduct which was then
N-alkylated with a variety of substituted 2-hydrox-
ybenzyl bromides to afford the silyl-protected N2OS
proligands. Hydrolysis of these potential ligands during
workup and purifying procedures led to the formation
of new N2OS tetradentate proligands H2Ln (n = 4, 5)
in 78–90% yield. Although silyl reagents have been con-
ventionally used as protecting groups for alcohols and
amines, they are seldom used for thiol protection,
mainly due to the instability of the silyl thioether type
compounds in protic media [33]. By employing this syn-
thetic scheme with the silyl reagent, we were able to ex-
tend the range of asymmetric N2OS ligands H2Ln
(n = 1–3) from those {5,5,5} chelate ring systems to
those with {5,6,5}. The versatility of this synthetic route
using silyl reagent has also recently been demonstrated
by the preparation of a new class of N2S tridentate lig-
ands [34].
3. Results and discussion
3.1. Synthesis and spectroscopic characterisation
The asymmetric N2OS N-capped tripodal proligands
H2Ln (n = 1–3) were prepared by a reaction involving
two steps (Scheme 1). Reductive amination of 2-pyridin-
ecarboxaldehyde and the respective 2-aminophenol gave
the
corresponding
substituted
2-N-(2-pyridyl-
methyl)aminophenol [23]. Further reaction of these
N2O precursors with 1 equivalent of ethylene sulfide in
a sealed ampoule gave the new asymmetric N2OS tripo-
dal proligands with {5,5,5} chelate ring systems in 38–
68% yield. Interestingly, the reaction yield increased
with the size of the alkyl chain (methyl or tert-butyl)
on the phenolic ring, probably due to the improved sol-
ubility of N2O precursors, which enhanced the reaction
with ethylene sulfide. We have also tried to extend the
above synthetic pathway to prepare proligands with
{5,6,5} chelate ring systems. However, attempts to syn-
thesise H2Ln (n = 4, 5) by the reaction of 2-N-(3,5-di-
tert-butyl-2-hydroxybenzyl)aminomethylpyridine [20a]
or 2-N-(2-hydroxy-5-nitrobenzyl)aminomethylpyridine
[32] with 1 equivalent of ethylene sulfide using identical
conditions as for those of H2Ln (n = 1–3) was not suc-
cessful. After workup procedure, only the N2O precur-
sors were regenerated. The white insoluble material
formed in the reaction mixture suggested that self-po-
lymerization of ethylene sulfide occurred rather than
N-alkylation.
A modified strategy for the synthesis of H2Ln (n = 4,
5) is shown in Scheme 2. These proligands were synthe-
sised by a reaction involving four steps, three of which
were performed in situ. The first step was based on a lit-
erature procedure with minor modification [24]. Reac-
tion of 2-aminomethylpyridine with 1 equivalent of
ethylene sulfide gave 2-[(2-mercaptoethyl)amino-
methyl]pyridine in 68% yield. This compound was puri-
fied by flash column chromatography rather than
vacuum distillation in order to prevent the possibility
of regenerating the starting materials by decomposition
Due to the flexibility and length of the pendant arms,
the proligands H2Ln (n = 1–5) are capable of providing a
N2OS coordination environment for a metal without
any steric hindrance, and may generate a chiral mono-
nuclear complex upon coordination. Apart from the
strongly asymmetric systems, partially symmetric tripo-
dal tetradentate proligands HxLn [x = 2, n = 6 (N2S2);
x = 3, n = 7, 8 (NOS2)] and a NS2 tridentate proligand
H2L9 have also been prepared (Schemes 3–5). The
N2S2 proligand was first reported by reaction of 2-ami-
nomethylpyridine with 2 equivalents of ethylene sulfide
to give H2L6 in 72% yield. This proligand was again
purified by flash column chromatography rather than
vacuum distillation due to the reason stated above.
The trivalent NOS2 proligands H2Ln (n = 7, 8) and diva-
lent NS2 proligand H2L9 were prepared using similar
procedure as for H2L6. All these sulfur-containing pro-
ligands HxLn (x = 2, n = 1–6, 9; x = 3, n = 7, 8) are very
sensitive to air and easily oxidise to give the disulfide.
Therefore they must be stored under a dinitrogen atmos-
phere at low temperature (À30 ꢁC) prior to use.
In the preparation of dioxotungsten(VI) complexes,
[WO2Cl2(dme)] (dme = 1,2-dimethoxyethane) was em-
ployed as starting material [22]. This dme adduct is ex-
tremely soluble in common organic solvents such as
S
OH
N
SH
1.
N
CHO
OH
H
N
toluene
110oC
MeOH, reflux
N
N
R
R
NH2
2. NaBH4
R
HO
H2L1 R = H
H2L2 R = Me
H2L3 R = tBu
Scheme 1.

4366
Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
R
S
OH
X
Me3SiCl
N
N
toluene
110oC
R'
NH2
H
H
N
N
N
THF, Et3N
reflux
THF, Et3N
HS
Me3SiS
X = Cl, Br
R
R
SH
N
SSiMe3
OH
OH
hydrolysis
N
N
N
R'
R'
H2L4 R = R' = tBu
H2L5 R = H, R' = NO2
Scheme 2.
S
SH
SH
(Scheme 6). All these dioxotungsten(VI) compounds
are stable to air and moisture, and white in colour ex-
cept for [WO2(L6)] and K[WO2(Ln)] (n = 7, 8) which
are pale yellow.
N
NH2
N
N
toluene
100oC
H2L6
The molybdenum analogues were also prepared for
comparison. Treatment of [MoO2(acac)2] (acac = acetyl-
acetonate) with the proligands HxLn (x = 2, n = 1–6, 9;
x = 3, n = 7, 8) in MeOH (KOH was added in the case
of potentially trianionic ligands to provide potassium
as source of counter ion) gave the dioxo complexes
(Scheme 6). All these dioxomolybdenum(VI) com-
pounds are stable to air and moisture, and orange in col-
our except for [MoO2(Ln)] (n = 1–3) which are yellow.
Due to the limited solubility of most of these dioxo–
tungsten(VI) and –molybdenum(VI) complexes, the
crude products were collected by filtration, washed with
organic solvents followed by recrystallisation. This pro-
cedure was used for the complexes [MO2(Ln)] (M = Mo,
W; n = 1–3, 5, 6, 9) and K[MO2(Ln)] (M = Mo, W;
n = 7, 8); whereas [MO2(L4)] (M = Mo, W) containing
two tert-butyl groups on the ligand possess sufficient sol-
ubility to be purified by column chromatography.
All these complexes have appropriate microanalyses.
The infra-red spectra show two strong bands in the re-
gions 873–909 and 902949 cmÀ1 which are attributable
to the asymmetric and symmetric M = O (M = Mo,
W) stretches respectively in a cis-dioxo moiety [38,39].
The spectra of compounds containing nitro groups also
displayed strong bands at 1339–1341 and 1509–1513
cmÀ1 which arise from the stretching vibrations of the
Scheme 3.
R
R
OH
S
SH
SH
OH
NH2
N
toluene
110oC
R'
R'
H3L7 R = R' = Me
H3L8 R = H, R' = tBu
Scheme 4.
S
SH
N
NH2
toluene
100oC
SH
H2L9
Scheme 5.
THF, CH2Cl2 and acetonitrile which is in contrast to the
conventionally used reagents such as [WO2(acac)2]
(acac = acetylacetonate) [35] and [WO2Cl2] [35c,36].
Moreover, this dme reagent is highly reactive towards
1
C–NO2 unit [40]. The H and 13C{1H} NMR data for
a
wide range of ligand substitution reactions
these complexes are consistent with the proposed octa-
hedral structures. For complexes with asymmetric lig-
ands [MO2(Ln)] (M = Mo, W; n = 1–3), the appearance
of two doublets at d 4.40–4.72 and 4.73–4.87 for the
two methylene protons adjacent to the pyridine ring in
the 1H NMR spectra shows that two methylene protons
[20,23,34,37]. Treatment of the proligands HxLn (x = 2,
n = 1–6; x = 3, n = 7, 8) with [WO2Cl2(dme)] in the pres-
ence of base (triethylamine or KOH) gave the corre-
sponding dioxo compounds [WO2(Ln)] (n = 1–6) and
K[WO2(Ln)] (n = 7, 8) in moderate to good yield

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
4367
O
R
O
M
O
S
R'
O
O
O
M
N
N
R
N
S
N
H2(Ln) (n = 4, 5)
H2(Ln) (n = 1-3)
M = Mo, W
R = R' = tBu; R = H, R' = NO2
M = Mo, W
R = H, Me, tBu
O
O
O
Cl
O
O
O
O
O
Me
W
Mo
Cl
or
O
O
Me
base, CH2Cl2 / MeOH
MeOH
H2(L6)
H2(L9)
O
M
KOH
O
S
H3(Ln) (n = 7, 8)
N
S
S
O
O
N
N
Mo
-
O
S
S
O
S
M
M = Mo, W
O
R
K+
N
R'
M = Mo, W
R = R' = Me; R = H, R' = tBu
Scheme 6.
become diastereotopic after complexation. All dioxo
complexes have been characterised by the detection of
the [M + 1]+ ions by APCI, ESI or LSI mass spectrom-
etry and the isotopic distribution of the [M + 1]+ peaks
is also in good agreement with the calculated spectra.
seen that for the complexes [MO2(Ln)] (M = Mo, W;
n = 1–3) the substituents on the phenolic ring of the lig-
ands have marginal effects on the reduction potential
Table 1
Electrochemical data for [MO2(Ln)] (M = Mo, W; n = 1–6, 9)a
Compound
Reduction potential (V) vs. SCE
Process
[MoO2(L1)]
[MoO2(L2)]
[MoO2(L3)]
[MoO2(L4)]
[MoO2(L5)]
[MoO2(L6)]
[MoO2(L9)]
À1.41
À1.42
À1.44
À1.45
À1.65
À1.31
À1.02
Irreversible
Irreversible
Irreversible
Irreversible
Irreversible
Irreversible
Irreversible
3.2. Electrochemical studies
The electrochemical properties of the complexes
[MO2(Ln)] (M = Mo, W; n = 1–6, 9) were studied by cyc-
lic voltammetry. Ferrocene was added as an internal ref-
erence in all measurements. The electrochemical data are
summarized in Table 1. All these complexes exhibit an
irreversible one-electron reduction process attributable
to the conversion of MVI to MV. It is worth noting that
the metal-based reduction potentials for the tungsten
complexes are more negative by 0.29–0.50 V than those
of the corresponding molybdenum counterparts. This is
fully consistent with studies of other high-valent molyb-
denum and tungsten oxo-complexes [18a,41]. It can be
[WO2(L1)]
[WO2(L2)]
[WO2(L3)]
[WO2(L4)]
[WO2(L5)]
[WO2(L6)]
a
À1.86
À1.89
À1.90
À1.92
À1.94
À1.81
Irreversible
Irreversible
Irreversible
Irreversible
Irreversible
Irreversible
Recorded with [Bu4N][BF4] as electrolyte in DMF (0.1 M) at
ambient temperature. Scan rate 100 mV sÀ1
.

4368
Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
and are consistent with the electron donating character
of the methyl and tert-butyl groups. They follow the or-
der [MoO2(L1)] > [MoO2(L2)] > [MoO2(L3)] as a result
of the different electron donating abilities (tBu > -
Me > H). A similar trend can also be observed from
the tungsten analogues.
It is interesting to note that incorporation of sulfur
into the tetradentate ligand system as in L6 lowers the
reduction potential of the tungsten complex (À1.81 V
versus SCE) relative to the analogous oxygen-containing
ligands (À2.16 to À2.45 V versus SCE) [20a], although
some caution is attached to discussion of irreversible
processes such as those observed here. The reduction
potential of [WO2(L6)] having N2S2 coordination is low-
er than with N2OS coordination by 0.05–0.13 V. How-
ever, [WO2(L6)] is still more negative than those of the
synthetic models for tungstoenzymes such as [Et4N]2[-
WO2(dtc)2] (bdt = 1,2-benzenedithiolate) (Epc = À1.34 V
versus SCE) [18c] and [Et4N]2[WO2(mnt)2] (mnt = 1,2-
dicyanoethylenedithiolate) (Epc = À1.50 V versus Ag/
Ag+ in MeCN) [18c].
Fig. 1. Molecular structure and atom labeling scheme for [MoO2(L2)].
Hydrogen atoms are omitted for clarity.
C4
S2
The complex [MoO2(L9)] exhibits an irreversible
one-electron reduction process attributable to the MoVI
to MoV process at Epc = À1.02 V versus SCE in
DMF. This data are fully consistent with that of
[MoO2{HN(CH2CH2S)2}] with Epc = À0.98 V versus
C3
O1
N1
C5
C7
Mo1
C6
C1
O2
N2
C8
O
5 mol % [MO2(Ln)]
H
OH
R'
C2
C10
C9
R
R
R'
DMSO-d6 , 100oC
S1
(M = Mo, W; n = 3-6, 8, 9)
R = aryl, alkenyl
Fig. 2. Molecular structure and atom labeling scheme for [MoO2(L6)].
Hydrogen atoms are omitted for clarity.
R' = H, aryl, alkenyl
Scheme 7.
Table 2
Percentage conversion of benzoin into benzil catalysed by [MoO2(Ln)]a
Entry
Complex
Time (h)
Conversion (%)
1
2
3
4
5
6
[MoO2(L3)]
[MoO2(L4)]
[MoO2(L5)]
[MoO2(L6)]
K[MoO2(L8)]
[MoO2(L9)]
12
12
12
6
75
81
86
98
79
99
12
4
7
8
[WO2(L3)]
[WO2(L4)]
[WO2(L5)]
[WO2(L6)]
K[WO2(L8)]
98
98
98
48
98
14
19
53
87
44
9
10
11
a
5 mol% metal complex was treated with 0.2 mmol benzoin in
deoxygenated DMSO-d6 at 100 ꢁC.
Fig. 3. Molecular structure and atom labeling scheme for [WO2(L6)].

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
4369
Table 3
Selected bond distances (A) and angles (ꢁ) for complexes [MoO2(L )], [MoO2(L )] and [WO2(L )]
2
6
6
˚
[MoO2(L2)]
[MoO2(L6)]
[WO2(L6)]
Mo(1)–S(1)
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–N(1)
Mo(1)–N(2)
2.3854(4)
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–S(1)
Mo(1)–S(2)
Mo(1)–N(1)
Mo(1)–N(2)
1.7131(13)
1.7065(14)
2.4052(5)
2.4344(5)
2.3493(15)
2.3797(15)
W(1)–O(1)
W(1)–O(2)
W(1)–S(1)
W(1)–S(2)
W(1)–N(1)
W(1)–N(2)
1.732(4)
1.715(5)
1.7069(12)
1.7067(13)
2.0034(12)
2.3493(13)
2.3606(14)
2.4149(17)
2.4135(15)
2.328(5)
2.354(5)
S(1)–Mo(1)–O(1)
S(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(2)
S(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(3)
O(2)–Mo(1)–O(3)
S(1)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–N(1)
S(1)–Mo(1)–N(2)
O(1)–Mo(1)–N(2)
O(2)–Mo(1)–N(2)
O(3)–Mo(1)–N(2)
N(1)–Mo(1)–N(2)
98.68(4)
96.48(5)
108.24(6)
150.69(3)
103.86(5)
94.02(6)
75.52(3)
156.13(5)
95.49(5)
76.29(5)
84.89(3)
85.89(5)
165.34(5)
78.37(5)
70.65(5)
S(1)–Mo(1)–S(2)
S(1)–Mo(1)–O(1)
S(2)–Mo(1)–O(1)
S(1)–Mo(1)–O(2)
S(2)–Mo(1)–O(2)
O(1)–Mo(1)–O(2)
S(1)–Mo(1)–N(1)
S(2)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–N(1)
S(1)–Mo(1)–N(2)
S(2)–Mo(1)–N(2)
O(1)–Mo(1)–N(2)
O(2)–Mo(1)–N(2)
N(1)–Mo(1)–N(2)
154.333(17)
97.29(5)
95.38(5)
97.72(5)
99.49(5)
107.86(7)
76.79(4)
79.92(4)
95.16(5)
156.89(6)
83.96(4)
78.57(4)
166.14(6)
85.56(6)
71.62(5)
S(1)–W(1)–S(2)
S(1)–W(1)–O(1)
S(2)–W(1)–O(1)
S(1)–W(1)–O(2)
S(2)–W(1)–O(2)
O(1)–W(1)–O(2)
S(1)–W(1)–N(1)
S(2)–W(1)–N(1)
O(1)–W(1)–N(1)
O(2)–W(1)–N(1)
S(1)–W(1)–N(2)
S(2)–W(1)–N(2)
O(1)–W(1)–N(2)
O(2)–W(1)–N(2)
N(1)–W(1)–N(2)
155.04(6)
97.5(3)
95.2(3)
96.82(17)
99.94(17)
106.7(2)
76.88(13)
80.67(13)
95.24(18)
80.67(13)
83.81(12)
79.04(12)
166.8(2)
86.1(2)
72.21(16)
Table 4
Crystallographic data for [MoO2(L2)], [MoO2(L6)] and [WO2(L6)]
[MoO2(L2)]
[MoO2(L6)]
[WO2(L6)]
Empirical formula
Formula weight
Diffractometer
Temperature (K)
Colour
Crystal size (mm3)
Crystal system
Space group
C
400.30
15H16MoN2O3S
C10H14MoN2O2S2
354.29
C10H14N2O2S2W
442.20
Enraf-Nonius KappaCCD
150
Enraf-Nonius DIP2000
150
Enraf-Nonius KappaCCD
150
yellow plate
0.1·0.3·0.3
triclinic
orange block
0.3·0.3·0.4
orthorhombic
Pna21 (No. 33)
12.963(1)
11.416(1)
8.667(1)
90
yellow block
0.06·0.14·0.20
orthorhombic
Pna21 (No. 33)
8.6513(3)
11.4311(3)
12.9956(5)
90
ꢀ
P1 (No. 2)
˚
a (A)
6.6900(1)
7.3783(2)
16.7711(5)
78.347(1)
78.495(1)
71.845(1)
762.1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
90
90
90
3
˚
V (A )
1282.6
1285.2
Z
2
1.74
4
1.83
4
2.29
Dcalc (Mg/m3)
l (mmÀ1
F(000)
)
1.01
400.56
1.34
705.51
0.71069
9.30
835.59
˚
k (Mo Ka) (A)
0.71073
0.74, 0.90
0 6 h 6 27.5
10645
0.71073
0.27, 0.57
0 6 h 6 27.5
8378
Transmission coefficients (min, max)
h Range for data collection (ꢁ)
Number of data measured
0.56, 0.67
0 6 h 6 26.37
8227
Number of unique data (Rint
)
3248 (0.017)
3139
199
2491 (0.021)
2485
155
2904 (0.042)
2399
155
Number of observed data (I > 3r(I))
Number of parameters, p
R
0.0247
0.0254
0.0186
0.0184
0.0239
0.0244
Rw
S(GOF)
1.0502
+0.45 and À0.61
0.9952
+0.51 and À1.16
1.0430
+1.50 and À1.63
À3
˚
Largest difference peak and hole (e A
)
SCE in DMSO [42]. It is worth noting that this five-co-
ordinate dioxomolybdenum complex has remarkably
lower reduction potential by ca 0.3 V compared to
in DMF) which adopts a six-coordinate geometry,
and is significantly lower than those dioxomolybde-
num(VI) complexes containing N2OS tetradentate
ligands (Epc = À1.41 to À1.65 V versus SCE).
that of [MoO2(L6)] (Epc = À1.31
V versus SCE

4370
Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
3.3. Oxidation catalysis studies
cessful on a larger scale. A 10 mmol reaction of benzoin,
2-hydroxybenzyl alcohol and trans-cinnamyl alcohol
provided benzil, salicylaldehyde and trans-cinnamalde-
hyde in 95%, 83%, 94% isolated yields, respectively.
As indicated above, the vast majority of the homoge-
neous metal-based alcohol oxidation systems studied to
date have involved the use of peroxide or hydroperox-
ides as oxidants. There have also been a number of stud-
ies where coupled systems involving electron transfer
agents in tandem with a metal complex for oxidation
of alcohols using oxygen as the oxidant [43]. The use
of [RuO4]À in the presence of copper salts [44] and
OsO4 with dimethylsulfoxide (DMSO) as co-oxidant
[45] as homogeneous oxidants for alcohols have also
been reported. There is also one example of the use of
[MoO2(acac)2] in the presence of DMSO, but ligand ef-
fects were not studied nor was the activity of compara-
ble tungsten complexes [46]. We here report briefly the
activity of the dioxo complexes [MO2(Ln)] (M = Mo,
W; n = 3–6, 8, 9) for the catalytic oxidation of primary
or secondary allylic and benzylic alcohols with DMSO
as the oxo-donor (see Scheme 7).
3.4. X-ray structural studies
The molecular structures of [MoO2(L2)], [MoO2(L6)]
and [WO2(L6)] were established by single-crystal X-ray
diffraction analysis and the perspective views of these
structures are shown in Figs. 1–3, respectively. Selected
bond distances and angles for these complexes are sum-
marized in Table 3 and crystallographic data in Table 4.
2
ꢀ
[MoO2(L )] crystallizes in the triclinic space group P1.
Since ligand L2 possesses three different pendant arms,
the complex [MoO2(L2)] is prochiral. One of the two
enantiomeric isomers in the unit cell is depicted in Fig.
1. [MoO2(L6)] and [WO2(L6)] are isomorphous and crys-
tallize in the orthorhombic space group Pna21. The
overall geometry of these three compounds about the
metal centre is pseudo-octahedral with the two oxo-
groups in the usual cis configuration and trans to the
two nitrogen donor atoms of the ligand. The five mem-
ber chelate ring systems of these three complexes cause a
bending of the NOS/NSS donor set away from the oxo-
groups which have an O–Mo–O angle of 107.9–108.2ꢁ
and O–W–O angle of 106.7ꢁ. The Mo‚O bond dis-
In a typical experiment, 5 mol% of the complex and
0.2 mmol of benzoin were dissolved in deoxygenated
DMSO-d6 (0.5 ml). The mixture was sealed in a NMR
tube and heated at 100 ꢁC. The progress of the reaction
1
was monitored by H NMR spectroscopy. All of the
complexes were initially screened for their ability to oxi-
dise benzoin and the complex [MoO2(L9)] was found to
be the most active. The results were summarized in Ta-
ble 2. The molybdenum analogues gave much higher
conversions in shorter reaction times than tungsten, con-
sistent with the lower metal-based reduction potentials
of the molybdenum complexes relative to those of tung-
sten. The donor groups were one of the major factors
that affected the performance of the catalyst and com-
plexes having ligands with more than one sulfur donor
showed significantly better performance. There were
also minor effects due to variations of substituents on
the ligand aryl groups.
˚
tances (1.694–1.713 A) and W‚O bond distances
˚
(1.715–1.732 A) are within the range typical for oxo-mo-
lybdenum [20a,38] and oxo-tungsten species [47,48]. The
˚
Mo–S single bond distances (2.347–2.434 A) and W–S
˚
single bond distances (2.414–2.415 A) are comparable
to those in other Mo(VI) and W(VI) thiolates [38].
4. Conclusions
A series of new asymmetric potentially dianionic or
trianionic tetradentate proligands have been synthesised
using both modification of known methods or new syn-
thetic routes. Their coordination chemistry has been ini-
tially explored by reaction with [MoO2(acac)2] or
[WO2Cl2(dme)] to give a series of dioxo-complexes of
the type [MO2(L)]nÀ (n = 0, 1). Variation of chelate ring
size and donor atom was shown to have relatively little
impact on the standard bond distances and bond angles
for the dioxo-complexes. The complexes have been
shown to be efficient catalysts for the oxidation of pri-
mary and secondary allylic and benzylic alcohols in
good but unexceptional yields.
We then investigated the activity of the complex
[MoO2(L9)] in DMSO for the oxidation of a range of
1
primary and secondary allylic and benzylic alcohols.
Yields in the range 68–99% were obtained. Substrates
containing phenolic OH or aromatic NO2 substituents
could also be oxidised in good yields.
Apart from the small scale reactions (0.2 mmol), this
catalytic oxidation system has been proved to be suc-
1
Substrates used in the catalytic activity studies of [MoO2(L9)]
(product, % yield, reaction time): benzoin (benzil, 99%, 3 h),
benzhydrol (benzophenone, 80%, 18 h), trans-cinnamyl alcohol
(trans-cinnamaldehyde, 99%, 18 h), 2-hydroxybenzyl alcohol (salicyl-
aldehyde, 89%, 24 h), 4-hydroxybenzyl alcohol (4-hydroxybenzalde-
hyde, 94%, 24 h), 4-methylbenzyl alcohol (p-tolualdehyde, 98%, 24 h),
4-tert-butylbenzyl alcohol (4-tert-butylbenzaldehyde, 78%, 24 h), 4-
methoxybenzyl alcohol (4-methoxybenzaldehyde, 68%, 24 h), 4-chlo-
robenzyl alcohol (4-chlorobenzaldehyde, 95%, 24 h) and 4-nitrobenzyl
alcohol (4-nitrobenzaldehyde, 99%, 20 h).
Acknowledgements
We thank Professor Hung Kay Lee of the Chinese
University of Hong Kong for recording the HRMS

Y.-L. Wong et al. / Inorganica Chimica Acta 357 (2004) 4358–4372
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