cis-dioxo-tungsten(VI) and -molybdenum(VI) complexes with N2O2 tetradentate ligands: Synthesis, structure, electrochemistry and oxo-transfer properties

Article abstract of DOI:10.1039/a804425b

Treatment of [WO₂Cl₂(dme)] with N₂O₂ tetradentate ligands [H₂Lⁿ (n = 1-7)] in the presence of 2 equivalents of triethylamine gave the corresponding dioxotungsten(VI) complexes [WO₂

Full text of DOI:10.1039/a804425b

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cis-Dioxo-tungsten(VI) and -molybdenum(VI) complexes with N2O2
tetradentate ligands: synthesis, structure, electrochemistry and
oxo-transfer properties
Yee-Lok Wong, Yan Yan, Edith S. H. Chan, Qingchuan Yang, Thomas C. W. Mak and
Dennis K. P. Ng*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong,
P. R. China. E-Mail: dkpn@cuhk.edu.hk
Received 11th June 1998, Accepted 16th July 1998
Treatment of [WO2Cl2(dme)] with N2O2 tetradentate ligands [H2Ln (n = 1–7)] in the presence of 2 equivalents of
triethylamine gave the corresponding dioxotungsten() complexes [WO2Ln] (n = 1–7). The molybdenum counterparts
[MoO2Ln] (n = 1–4) were prepared in good yield by treating ammonium molybdate tetrahydrate with the respective
ligands and dilute hydrochloric acid. The molecular structures of four of these complexes, namely [WO2Ln] (n = 1, 5
or 6) and [MoO2L2], as determined by single crystal X-ray analysis, revealed that the metal centre exhibits distorted
octahedral co-ordination with cis-dioxo ligands. These high-valent oxo complexes are active toward oxygen atom
transfer reactions and can catalyse the oxidation of benzoin with dmso. Their reactivity and electrochemical
properties are discussed and compared.
1
High-valent molybdenum and tungsten complexes with oxo
ligation have attracted considerable attention in the recent liter-
ature.1 Apart from their intrinsic interest, the chemistry of these
complexes has been developed with a view to gaining a better
understanding concerning their role in various industrial and
biological processes. It is well documented that high-valent
molybdenum and tungsten compounds can act as catalysts for
olefin metathesis reactions2 and are also involved in various
oxidation processes.3 Such compounds with dithiolene-type
ligands are also related to a class of enzymes known as
oxotransferases, which catalyse oxygen atom transfer to and
from a substrate.4,5 While the relevant molybdenum chemistry is
well developed and numerous oxomolybdenum complexes with
a range of supporting ligands have been studied extensively,6
the chemistry of analogous tungsten complexes remains
relatively little studied although substantial recent progress
has been stimulated by the unique nature and properties of
tungsten-containing enzymes.7–14 In this paper we describe the
preparation and characterization of a series of cis-dioxo-
tungsten() compounds containing N2O2 tetradentate ligands
with different co-ordination modes, together with their electro-
chemistry and reactivity toward oxo-transfer reactions. The
analogous molybdenum complexes have also been studied and
a comparison made between these two series of compounds
including their structures and properties.
The H and 13C NMR spectra were recorded on a Bruker
DPX 300 spectrometer (1H, 300; 13C, 75.4 MHz) in CD3SOCD3
solutions unless otherwise stated. Chemical shifts were relative
to internal SiMe4 (δ 0). The IR spectra were taken on a Nicolet
Magna 550 FT-IR spectrometer as KBr pellets. Liquid
secondary-ion (LSI) and FAB mass spectra were measured on a
Bruker APEX 47e Fourier transform ion cyclotron resonance
(FT-ICR) mass spectrometer or a Hewlett-Packard 5989B mass
spectrometer, respectively, with 3-nitrobenzyl alcohol as matrix.
Elemental analyses were performed by the Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences.
Electrochemical measurements were carried out on a BAS
CV-50W voltammetric analyser. The cell comprised inlets for a
platinum-sphere working electrode, a silver-wire counter elec-
trode and a Ag–AgNO3 (0.1 mol dmϪ3 in MeCN) reference
electrode which was connected to the solution by a Luggin
capillary whose tip was placed close to the working electrode.18
Typically, a 0.1 mol dmϪ3 solution of [NBu4][ClO4] in dmf con-
taining 1.0 mmol dmϪ3 of sample was purged with nitrogen
for 20 min, then the voltammograms were recorded at scan
rates ranging from 20 to 3000 mV sϪ1 at ambient temperature.
Ferrocene was also added as an internal reference and all poten-
tials are reported with respect to the Ag–Agϩ couple in MeCN.
Preparations
2-[Bis(2-hydroxybenzyl)aminomethyl]pyridine (H2L1).15 To a
solution of 2-(aminomethyl)pyridine (2.16 g, 20 mmol) in
MeOH (50 cm3) was added salicylaldehyde (2.44 g, 20 mmol).
The yellow Schiff base product appeared immediately and the
mixture was kept stirring for 2 h at room temperature (r.t.).
Sodium tetrahydroborate (0.76 g, 20 mmol) was then added
slowly and stirring was continued for 1 h. The volatiles of the
mixture were removed under reduced pressure and the residue
was mixed with water (50 cm3). The mixture was then neutral-
ized with acetic acid before being extracted with CH2Cl2 (3 × 30
cm3). The combined extracts was dried over anhydrous Na2SO4
and evaporated to give the monocondensed product 2-(2-
hydroxybenzylaminomethyl)pyridine (H2L8), which could be
purified by column chromatography with ethyl acetate as eluent
(3.85 g, 90% yield). The crude product was dissolved again in
MeOH (50 cm3) and salicylaldehyde (2.44 g, 20 mmol) added.
Experimental
General procedures
All reactions were carried out using standard Schlenk-line
techniques under an atmosphere of nitrogen; work-ups were
performed in air. Dichloromethane was dried over 4 Å molecu-
lar sieves and distilled from calcium hydride. Tetrahydrofuran
(thf) was distilled from sodium–benzophenone. Dimethyl-
formamide (dmf) for voltammetric studies was dried over
barium oxide and distilled under reduced pressure. The electro-
lyte [NBu4][ClO4] was recrystallized from dry acetone three
times prior to use. All other reagents and solvents were of
reagent grade and used as received. The ligands H2Ln (n = 1, 2,
5 or 6)15,16 and the tungsten complex [WO2Cl2(dme)]17 were
prepared according to literature procedures.
J. Chem. Soc., Dalton Trans., 1998, 3057–3064
3057

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The mixture was stirred at r.t. for 2 h, then NaBH4 (0.76 g, 20
mmol) added. The mixture was kept stirring for 1 h and the
volatiles were removed in vacuo. The residue was dissolved
in water (50 cm3) and then neutralized with acetic acid. The
resulting mixture was extracted with CH2Cl2 (3 × 30 cm3) and
the combined extracts dried over anhydrous Na2SO4 and con-
centrated, then loaded onto a silica gel column which was
eluted with CH2Cl2 followed by ethyl acetate. The second band
was collected and evaporated to give a white solid which was
recrystallized from CH2Cl2. Yield: 2.88 g (45%), mp 168–
169 ЊC. 1H NMR (CDCl3): δ 8.66 (d, J = 4.5, 1 H, py), 7.71 (dt,
J = 1.7, 7.7, 1 H, py), 7.31–7.26 (m, 1 H, py), 7.21–7.13 (m, 3 H,
aryl and py), 7.07 (dd, J = 1.4, 7.4, 2 H, aryl), 6.90 (dd, J = 0.8,
8.1, 2 H, aryl), 6.79 (dt, J = 1.0, 7.4 Hz, 2 H, aryl), 3.91 (s, 2 H,
C5H4CH2) and 3.84 (s, 4 H, C6H4CH2).
and 29.6. HRMS (LSI): m/z calc. for C28H36N3O4, [M ϩ H]ϩ,
478.2706, found 478.2683.
N,NЈ-Bis(2-hydroxy-5-nitrobenzyl)-N,NЈ-dimethylethane-1,2-
diamine (H2L7). To a solution of 2-hydroxy-5-nitrobenzyl
chloride (1.12 g, 6 mmol) in thf (50 cm3) were added N,NЈ-
dimethylethane-1,2-diamine (0.26 g, 3 mmol) and triethylamine
(0.83 cm3, 6 mmol). The mixture was refluxed overnight to give
a yellow suspension which was cooled and filtered. The filtrate
was rotary evaporated to give a yellow solid which was washed
thoroughly with diethyl ether then recrystallized from CH2Cl2–
1
hexane. Yield: 1.03 g (88%), mp 192–196 ЊC. H NMR: δ 8.11
(d, J = 3.0, 2 H, aryl), 8.00 (dd, J = 3.0, 9.0, 2 H, aryl), 6.78
(d, J = 9.0 Hz, 2 H, aryl), 4.08 (br s, 2 H, OH), 3.75 (s, 4 H,
C6H3CH2), 2.78 (s, 4 H, CH2CH2) and 2.26 (s, 6 H, CH3).
13C-{1H} NMR: δ 166.2, 137.9, 126.5, 125.5, 123.6, 116.5,
56.2, 52.8 and 40.9. HRMS (LSI): m/z calc. for C18H23N4O6,
[M ϩ H]ϩ, 391.1617, found 391.1546.
2-[(2-Hydroxybenzyl)(2-hydroxy-5-nitrobenzyl)aminomethyl]-
pyridine (H2L3). To a solution of 2-hydroxy-5-nitrobenzyl
chloride (0.56 g, 3 mmol) in thf (30 cm3) were added H2L8
(0.64 g, 3 mmol) and triethylamine (0.4 cm3, 3 mmol). The
mixture was refluxed overnight to give a yellow suspension
which was cooled and filtered. The filtrate was concentrated
and chromatographed using CHCl3 as eluent. The first band
was collected and evaporated to give the product as a yellow
General procedure for the preparation of [WO2Ln] (n ؍ 1–7).
To a mixture of [WO2Cl2(dme)] (0.38 g, 1.0 mmol) and H2Ln
(n = 1Ϫ7) (1.0 mmol) in CH2Cl2 (ca. 30 cm3) was added tri-
ethylamine (0.28 cm3, 2.0 mmol). The mixture was refluxed
overnight, then concentrated and chromatographed using
CH2Cl2 (for n = 1 or 4), CHCl3 (for n = 3) or ethyl acetate (for
n = 5 or 6) as eluent. The complexes [WO2Ln] (n = 2 or 7) were
only sparingly soluble in CH2Cl2 and were purified by filtration
followed by washing with CH2Cl2, diethyl ether and hexane.
The other tungsten complexes were further purified by recrystal-
lization from CH2Cl2–hexane.
1
solid. Yield: 0.84 g (77%), mp 141–142 ЊC. H NMR (CDCl3):
δ 8.67 (dd, J = 0.6, 5.1, 1 H, py), 8.10 (dd, J = 3.0, 9.0, 1 H, aryl),
8.05 (d, J = 3.0, 1 H, aryl), 7.77 (dt, J = 1.7, 7.7, 1 H, py), 7.36–
7.32 (m, 1 H, py), 7.22–7.16 (m, 2 H, aryl and py), 7.08 (dd,
J = 1.5, 7.5, 1 H, aryl), 6.96 (d, J = 9.0, 1 H, aryl), 6.87 (dd,
J = 1.0, 8.1, 1 H, aryl), 6.81 (dt, J = 1.0, 7.5, 1 H, aryl), 3.98 (s,
2 H, CH2), 3.91 (s, 2 H, CH2) and 3.89 (s, 2 H, CH2). 13C-{1H}
NMR (CDCl3): δ 163.9, 157.0, 155.5, 148.0, 140.0, 138.1, 130.3,
129.7, 126.5, 125.9, 123.4, 123.0, 122.0, 120.8, 119.5, 117.6,
117.0, 56.8, 56.3 and 56.0 (Found: C, 65.65; H, 5.29; N, 11.48.
Calc. for C20H19N3O4: C, 65.74; H, 5.24; N, 11.50%).
General procedure for the preparation of [MoO2Ln] (n ؍ 1–4).
A mixture of ammonium molybdate tetrahydrate [NH4]6-
[Mo7O24]ؒ4H2O (0.18 g, 0.14 mmol), H2Ln (n = 1–4) (1.0 mmol)
and dilute hydrochloric acid (0.1 , 5 cm3, 0.5 mmol) in EtOH
(40 cm3) was stirred at r.t. overnight. For [MoO2Ln] (n = 1–3)
a yellow solid was collected after filtration which was washed
with EtOH, diethyl ether and hexane, then dried in vacuo. The
orange tert-butyl analogue [MoO2L4] was obtained by chrom-
atography with CHCl3 as eluent.
2-(3,5-Di-tert-butyl-2-hydroxybenzylaminomethyl)pyridine
(H2L9). 3,5-Di-tert-butyl-2-hydroxybenzaldehyde (1.17 g, 5
mmol) was added to a stirred solution of 2-(aminomethyl)-
pyridine (0.54 g, 5 mmol) in MeOH (30 cm3). The yellow Schiff
base product appeared immediately and stirring was continued
for 1 h at r.t. Sodium tetrahydroborate (0.19 g, 5 mmol) was
then added slowly and the mixture stirred for 4 h. The volatiles
were removed in vacuo and the residue was dissolved in water
(30 cm3). The mixture was then neutralized with acetic acid and
extracted with CH2Cl2 (3 × 30 cm3). The combined extracts
were dried over anhydrous Na2SO4, concentrated and chrom-
atographed using ethyl acetate as eluent. The first band was
collected and evaporated to give a pale yellow solid. Yield: 1.62
X-Ray crystallographic analysis of [WO2Ln] (n ؍ 1, 5 or 6) and
[MoO2L2]
Crystal data and data processing parameters are given in Table
2. Data collection was performed at 293 K on a MSC/Rigaku
RAXIS IIc imaging-plate system using Mo-Kα radiation
(λ = 0.71073 Å) from a Rigaku RU-200 rotating-anode gener-
ator operating at 50 kV and 90 mA {for [WO2L1], 3Њ oscillation
frames in the range of 0–159Њ, exposure 8 min per frame; for
[WO2L5], 5Њ oscillation frames in the range of 0–150Њ, exposure
5 min per frame; for [WO2L6] and [MoO2L2], 3Њ oscillation
1
g (99%), mp 71–74 ЊC. H NMR (CDCl3): δ 8.56 (d, J = 4.5,
1 H, py), 7.64 (dt, J = 1.7, 7.7, 1 H, py), 7.24–7.17 (m, 3 H, aryl
and py), 6.84 (d, J = 2.4 Hz, 1 H, aryl), 3.98 (s, 2 H, CH2), 3.93
(s, 2 H, CH2), 1.44 (s, 9 H, But) and 1.28 (s, 9 H, But). 13C-{1H}
NMR (CDCl3): δ 157.9, 154.6, 149.4, 140.4, 136.6, 135.8,
123.4, 123.0, 122.6, 122.3, 121.7, 53.3, 52.7, 34.9, 34.1, 31.6 and
29.6.
frames in the range of 0–150Њ, exposure 8 min per frame}.19
A
self-consistent semiempirical absorption correction based on
Fourier coefficient fitting of symmetry-equivalent reflections
was applied by using the ABSCOR program.20
The structures were solved by direct methods, which yielded
the positions of all non-hydrogen atoms, which were refined
anisotropically. Hydrogen atoms in [WO2L1] were also located
by subsequent Fourier difference syntheses, while those in the
remaining three structures were placed in their idealized posi-
tions (C᎐H 0.96 Å) with fixed isotropic thermal parameters and
allowed to ride on their parent carbon atoms. All the H atoms
were held stationary and included in structure factor calcu-
lations in the final stage of full-matrix least squares refinement.
All computations were performed on an IBM-compatible
personal computer with SHELXTL PC21 (for structure solu-
tion) and SHELXL 9322 (for refinement) program packages.
CCDC reference number 186/1095.
2-[(3,5-Di-tert-butyl-2-hydroxybenzyl)(2-hydroxy-5-nitro-
benzyl)aminomethyl]pyridine (H2L4). According to the pro-
cedure described for H2L3, this ligand was prepared from 2-
hydroxy-5-nitrobenzyl chloride (0.75 g, 4 mmol) and H2L9 (1.30
g, 4 mmol) as a yellow solid. Yield: 1.57 g (82%), mp 87–90 ЊC.
1H NMR (CDCl3): δ 8.67 (dd, J = 0.6, 5.1, 1 H, py), 8.12–8.07
(m, 2 H, aryl), 7.77 (t, J = 7.8, 1 H, py), 7.37–7.35 (m, 1 H, py),
7.24–7.20 (m, 2 H, aryl and py), 6.95 (d, J = 8.7, 1 H, aryl),
6.89 (d, J = 2.4 Hz, 1 H, aryl), 3.94 (s, 2 H, CH2), 3.90 (s, 2 H,
CH2), 3.86 (s, 2 H, CH2), 1.38 (s, 9 H, But) and 1.27 (s, 9 H,
But). 13C-{1H} NMR (CDCl3): δ 163.9, 155.5, 153.5, 147.8,
140.9, 140.0, 138.1, 136.3, 127.2, 125.8, 124.6, 123.8, 123.7,
123.0, 122.2, 120.3, 117.8, 58.1, 55.7, 55.3, 35.0, 34.1, 31.6
See http://www.rsc.org/suppdata/dt/1998/3057/ for crystallo-
graphic files in .cif format.
3058
J. Chem. Soc., Dalton Trans., 1998, 3057–3064

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12,23
The tungsten() compounds Na2WO4ؒ2H2O,11,14 WO2Cl2
Results and discussion
and [WO2(acac)2]7a,23b,24 are commonly used to prepare other
dioxotungsten() complexes. These starting materials, however,
have some limitations. The first reagent can only be used in an
acidic medium in which the species [WO2]2ϩ is generated. Reac-
tions employing this reagent appear to be susceptible to the pH
of the reaction mixtures.25 The complex WO2Cl2 possesses a
polymeric structure which renders it insoluble in most organic
solvents.1b Although its derivative [WO2(acac)2] has better solu-
bility, its preparation is not always reproducible.7a The dme
adduct [WO2Cl2(dme)] was first reported by Andersson and co-
workers in 1991.17 In contrast to WO2Cl2 and its derivatives
[WO2Cl2L2] [L = dmf, dmso, MeCN, Ph3PO, etc.],26 this com-
pound is highly soluble in organic solvents such as CH2Cl2,
MeCN and dmf. The dme and Cl ligands are also labile and can
be displaced readily.27 It thus serves as a good alternative as
the starting material for other tungsten() compounds. We
employed this reagent to complex with the tetradentate ligands
H2Ln (n = 1–7). Treatment of [WO2Cl2(dme)] with 1 equivalent
of the ligand and 2 of triethylamine in CH2Cl2 produced the
corresponding [WO2Ln] in moderate to good yields (Table 1).
All of these tungsten() complexes are air-stable and white
except for [WO2L4] which is pale yellow.
Synthesis and spectroscopic characterization
The ligands H2Ln (n = 1, 5 or 6) were prepared previously by
reductive amination starting from salicylaldehyde and the cor-
responding amines.15 By using the stronger reducing agent
NaBH4 under neutral conditions instead of NaBH3(CN) at pH
5, we found that the reaction time could be significantly short-
ened and the yield of ligand H2L1 was greatly increased from
3.8 to 45%. This simplified procedure is described in detail in
the Experimental section. The ligand H2L2 was synthesized by
treating 2-(aminomethyl)pyridine with 2-hydroxy-5-nitrobenzyl
chloride in the presence of triethylamine.16 These two method-
ologies can be integrated to prepare unsymmetrical ligands
such as H2Ln (n = 3 or 4). Thus reductive amination using 2-
(aminomethyl)pyridine and 1 equivalent of salicylaldehyde or
3,5-di-tert-butyl-2-hydroxybenzaldehyde gave the respective
secondary amines H2Ln (n = 8 or 9), which underwent nucleo-
philic substitution with 2-hydroxy-5-nitrobenzyl chloride to
give H2Ln (n = 3 or 4) in good yield. The ligand H2L7 was pre-
pared similarly from N,NЈ-dimethylethane-1,2-diamine and 2
equivalents of 2-hydroxy-5-nitrobenzyl chloride.
The molybdenum counterparts [MoO2Ln] (n = 1–4) were also
prepared for comparison. The synthesis involved the treatment
of ammonium molybdate tetrahydrate with the ligands H2Ln
(n = 1–4) in the presence of HCl in EtOH.28 Filtration followed
by washing with organic solvents led to the isolation of pure
[MoO2Ln] (n = 1–3). The unsubstituted [MoO2L1] was prepared
previously by using [MoO2(acac)2] as the starting material.15
The tert-butyl analogue [MoO2L4] was purified readily by
chromatography. All these molybdenum complexes are also
stable to air.
Ligands:
R′′
OH HO
OH HO
R
R′
N
N
N
N
H2L1 R = R′ = R′′ = H
H2L5
Purification of the dinitro complexes [WO2Ln] (n = 2 or 7)
was found to be rather difficult because of the poor solubility in
organic solvents. In contrast, the ligand H2L4 and its molyb-
denum and tungsten complexes [MO2L4] (M = Mo or W) were
highly soluble and could be purified by chromatography.
H2L2 R = R′ = NO2, R′′ = H
H2L3 R = NO2, R′ = R′′ = H
H2L4 R = NO2, R′ = R′′ = But
R
1
Although the H NMR spectra of all these compounds sug-
HO
gested that the samples were essentially pure, the analytical data
were still not entirely satisfactory even after repeating the
chromatographic procedure and recrystallization. These com-
pounds, however, were unambiguously characterized with
accurate mass measurements together with other spectroscopic
techniques. Table 1 summarizes the analytical and spectroscopic
data for the dioxo complexes [WO2Ln] (n = 1–7) and [MoO2Ln]
(n = 2–4).
OH
HO
HN
R
R
R
N
N
N
H2L6 R = H
H2L7 R = NO2
H2L8 R = H
H2L9 R = But
The IR spectra of all these dioxo compounds showed two
Complexes:
strong bands within 907–914 and 934–949 cmϪ1 attributable to
the asymmetric and symmetric M᎐O stretches, respectively in
᎐
O
O
O
O
O
O
a cis-dioxo moiety.9b,10d The only exceptions were those for
[MoO2L2] and [MoO2L4], which exhibited only one strong and
broad band at 918 or 910 cmϪ1, respectively. There was no
difference in these stretching frequencies between the molyb-
denum and tungsten complexes of H2L3. The spectra of com-
pounds containing nitro group(s) also displayed strong bands at
1333–1343 and 1510–1518 cmϪ1 which arise from the C᎐NO2
R
M
W
O
N
O
N
R′′
N
N
R′
1
stretching vibrations.29 The H and 13C NMR data for these
M = Mo, W
[MO2Ln] (n = 1–4)
[WO2L5]
complexes were in accord with the proposed octahedral struc-
ture shown. For complexes with a symmetrical ligand, they
possess either an internal mirror plane, [WO2Ln] (n = 1, 2 or 5)
and [MoO2L2], or a C2 rotation axis, [WO2Ln] (n = 6 or 7). Upon
complexation the two methylene protons next to the benzene
ring are no longer equivalent as expected. For example, two
doublets at δ 4.83 and 4.33, both with a geminal coupling con-
stant of 14.4 Hz, appeared in the 1H NMR spectrum of
[WO2L2] recorded in CD3SOCD3. For complexes with an
unsymmetrical ligand such as [MO2Ln] (M = Mo or W, n = 3 or
4), up to four doublets could be resolved for these CH2 protons.
All these complexes were also characterized with mass spec-
O
M
O
O
R
O
N
N
R
M = Mo, W
[MO2Ln] (n = 6, 7)
J. Chem. Soc., Dalton Trans., 1998, 3057–3064
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Table 1 Analytical and spectroscopic data for [WO2Ln] (n = 1–7) and [MoO2Ln] (n = 2–4)
Analysisa (%)
Compound
Yield (%)
48
C
H
N
Spectroscopic datab
1H NMR: δ 8.95 (d, J = 5.4, 1 H, py), 7.79 (dt, J = 1.3, 7.7, 1 H, py), 7.40 (t, J = 6.5, 1 H, py), 7.19 (d,
[WO2L1]
44.29
(44.97)
3.21
5.07
(3.40) (5.24) J = 7.5, 2 H, aryl), 7.09–7.04 (m, 3 H, aryl and py), 6.77 (t, J = 7.5, 2 H, aryl), 6.52 (d, J = 8.1, 2 H,
aryl), 4.75 (d, J = 13.5, 2 H, C6H4CH2), 4.09–4.05 (m, 4 H, C6H4CH2 and C5H4CH2)
13C-{1H} NMR: δ 160.7, 156.2, 150.4, 140.9, 129.7, 129.4, 124.3, 123.2, 123.1, 120.8, 118.3, 61.6, 58.6
IR: ν(WO2) 937s, 907s
MS (FAB): a cluster peaking at m/z 535 (100%), [M ϩ H]ϩ
[WO2L2]
[WO2L3]
83
57
1H NMR: δ 9.00 (d, J = 5.1, 1 H, py), 8.28 (d, J = 3.0, 2 H, aryl), 8.00 (dd, J = 3.0, 9.0, 2 H, aryl), 7.88
(dt, J = 1.5, 7.8, 1 H, py), 7.49 (t, J = 6.0, 1 H, py), 7.19 (d, J = 7.8, 1 H, py), 6.78 (d, J = 9.0, 2 H,
aryl), 4.83 (d, J = 14.4, 2 H, C6H3CH2), 4.33 (d, J = 14.4, 2 H, C6H3CH2), 4.24 (s, 2 H, C5H4CH2)
13C-{1H} NMR: δ 166.2, 156.2, 150.7, 141.8, 140.7, 126.2, 125.5, 125.0, 124.1, 123.6, 119.5, 60.7, 58.7
IR: ν(WO2) 949s, 914s; ν(NO2) 1510s, 1338s
HRMS (LSI):a m/z 625.0544 (625.0535)
41.11
(41.47)
2.84
7.05
1H NMR: δ 8.97 (d, J = 5.1, 1 H, py), 8.21 (d, J = 3.0, 1 H, aryl), 7.96 (dd, J = 3.0, 9.0, 1 H, aryl), 7.84
(2.96) (7.25) (dt, J = 1.5, 7.8, 1 H, py), 7.45 (t, J = 6.3, 1 H, py), 7.25 (dd, J = 1.2, 7.5, 1 H, py), 7.15–7.08 (m, 2 H,
aryl), 6.82 (dt, J = 0.9, 7.5, 1 H, aryl), 6.71 (d, J = 9.0, 1 H, aryl), 6.58 (d, J = 7.5, 1 H, aryl), 4.84 (d,
J = 13.8, 1 H, aryl CH2), 4.75 (d, J = 13.8, 1 H, aryl CH2), 4.27 (d, J = 14.1, 1 H, aryl CH2), 4.23–4.09
(m, 3 H, aryl CH2 and C5H4CH2)
13C-{1H} NMR: δ 166.8, 160.4, 156.2, 150.5, 141.3, 140.3, 129.9, 129.6, 126.1, 125.5, 124.7, 124.3,
123.4, 122.8, 121.5, 119.4, 118.4, 61.7, 60.7, 58.6
IR: ν(WO2) 944s, 908s; ν(NO2) 1512s, 1339s
MS (FAB): a cluster peaking at m/z 590 (69%), [M ϩ H]ϩ
[WO2L4]
67
1H NMR (CDCl3): δ 9.23 (d, J = 4.5, 1 H, py), 8.07 (d, J = 2.7, 1 H, aryl), 7.97 (dd, J = 2.7, 9.0, 1 H,
aryl), 7.64 (dt, J = 1.4, 7.7, 1 H, py), 7.30 (t, J = 6.1, 1 H, py), 7.22 (d, J = 2.2, 1 H, aryl), 6.99
(d, J = 2.2, 1 H, aryl), 6.90 (d, J = 7.7, 1 H, py), 6.71 (d, J = 9.0, 1 H, aryl), 5.19–5.09 (m, 2 H, aryl),
4.27 (d, J = 16.5, 1 H, aryl CH2), 4.05–3.91 (m, 3 H, aryl CH2 and C5H4CH2), 1.28 (s, 9 H, But), 1.25
(s, 9 H, But)
13C-{1H} NMR (CDCl3): δ 166.8, 157.0, 155.5, 151.8, 143.9, 140.5, 140.3, 138.7, 125.9, 125.4, 124.6,
124.1, 124.0, 123.0, 121.9, 121.1, 119.8, 63.9, 62.4, 59.3, 34.9, 34.3, 31.5, 29.9
IR: ν(WO2) 946s, 911s; ν(NO2) 1517s, 1338s
MS (FAB): a cluster peaking at m/z 691 (100%), [M ϩ H]ϩ
HRMS (LSI):a m/z 692.1769 (692.1936)
[WO2L5]
[WO2L6]
[WO2L7]
80
65
43
41.61
(42.04)
4.25
5.34
1H NMR: δ 7.30 (t, J = 8.1, 2 H, aryl), 7.23 (d, J = 7.5, 2 H, aryl), 6.90 (t, J = 7.5, 2 H, aryl), 6.79 (d,
(4.31) (5.45) J = 8.1, 2 H, aryl), 4.46 (d, J = 14.4, 2 H, C6H4CH2), 4.08 (d, J = 14.4, 2 H, C6H4CH2), 2.76–2.74 (m,
2 H, CH2CH2), 2.57 (s, 6 H, CH3), 2.52–2.50 (m, 2 H, CH2CH2)
13C-{1H} NMR: δ 159.4, 129.9, 129.8, 123.8, 121.2, 118.2, 61.6, 57.1, 52.0, 49.7
IR: ν(WO2) 938s, 912s
MS (FAB): a cluster peaking at m/z 515 (66%), [M ϩ H]ϩ
41.82
(42.04)
4.31
5.34
1H NMR: δ 7.28 (t, J = 8.1, 2 H, aryl), 7.15 (d, J = 7.5, 2 H, aryl), 6.90 (t, J = 7.5, 2 H, aryl), 6.79 (d,
(4.31) (5.45) J = 8.1, 2 H, aryl), 4.73 (d, J = 14.4, 2 H, C6H4CH2), 3.83 (d, J = 14.4, 2 H, C6H4CH2), 2.82 (d, J = 9.6,
2 H, CH2CH2), 2.63 (s, 6 H, CH3), 2.43 (d, J = 9.6, 2 H, CH2CH2)
13C-{1H} NMR: δ 157.5, 130.0, 129.5, 122.9, 121.2, 119.0, 63.5, 51.9, 47.6
IR: ν(WO2) 942m, 907s
MS (FAB): a cluster peaking at m/z 515 (100%), [M ϩ H]ϩ
1H NMR: δ 8.21–8.18 (m, 4 H, aryl), 7.03 (d, J = 7.8, 2 H, aryl), 4.74 (d, J = 14.7, 2 H, C6H3CH2),
4.15 (d, J = 14.7, 2 H, C6H3CH2), 2.84 (d, J = 9.6, 2 H, CH2CH2), 2.67 (s, 6 H, CH3), 2.54 (d, J = 9.6,
2 H, CH2CH2)
13C-{1H} NMR: δ 163.1, 141.0, 126.4, 125.7, 124.1, 120.2, 62.5, 52.3, 47.8
IR: ν(WO2) 934s, 910s; ν(NO2) 1518s, 1343s
MS (FAB): a cluster peaking at m/z 605 (33%), [M ϩ H]ϩ
HRMS (LSI):a m/z 605.0846 (605.0848)
[MoO2L2]
[MoO2L3]
90
87
44.64
(44.79)
2.97
10.54 1H NMR: δ 9.01 (d, J = 5.1, 1 H, py), 8.25 (d, J = 3.0, 2 H, aryl), 7.95 (dd, J = 3.0, 9.0, 2 H, aryl), 7.86
(3.01) (10.45) (t, J = 7.8, 1 H, py), 7.48 (t, J = 6.4, 1 H, py), 7.19 (d, J = 7.8, 1 H, py), 6.74 (d, J = 9.0, 2 H, aryl),
4.75 (d, J = 14.4, 2 H, C6H3CH2), 4.26 (d, J = 14.4, 2 H, C6H3CH2), 4.11 (s, 2 H, C5H4CH2)
13C-{1H} NMR: δ 168.1, 155.2, 150.4, 141.4, 140.4, 126.3, 125.4, 124.9, 124.1, 123.7, 118.8, 60.6, 58.5
IR: ν(MoO2) 918s (br); ν(NO2) 1511s, 1339s
HRMS (LSI):a m/z 539.0064 (539.0106)
48.50
(48.89)
3.29
8.35
1H NMR: δ 8.99 (d, J = 4.8, 1 H, py), 8.20 (d, J = 2.7, 1 H, aryl), 7.92 (dd, J = 2.7, 9.0, 1 H, aryl), 7.83
(3.49) (8.55) (dt, J = 1.5, 7.5, 1 H, py), 7.45 (t, J = 6.3, 1 H, py), 7.25 (dd, J = 1.5, 7.5, 1 H, py), 7.15 (d, J = 7.8,
1 H, aryl), 7.08 (dt, J = 1.5, 7.8, 1 H, aryl), 6.83 (dt, J = 0.9, 7.2, 1 H, aryl), 6.67 (d, J = 9.0, 1 H,
aryl), 6.58 (d, J = 7.8, 1 H, aryl), 4.76 (d, J = 14.1, 1 H, aryl CH2), 4.62 (d, J = 14.1, 1 H, aryl CH2),
4.17 (d, J = 14.1, 1 H, aryl CH2), 4.08 (d, J = 14.1, 1 H, aryl CH2), 4.04 (s, 2 H, C5H4CH2)
13C-{1H} NMR: δ 168.8, 162.4, 155.3, 150.3, 141.0, 139.8, 130.0, 129.3, 126.2, 125.5, 124.6, 124.3,
123.6, 122.8, 121.4, 118.8, 117.4, 61.4, 60.5, 58.5
IR: ν(MoO2) 944s, 908s; ν(NO2) 1512s, 1339s
MS (FAB): a cluster peaking at m/z 494 (17%), [M ϩ H]ϩ
[MoO2L4]
84
1H NMR (CDCl3): δ 9.20 (d, J = 4.9, 1 H, py), 8.06 (d, J = 2.7, 1 H, aryl), 7.94 (dd, J = 2.7, 9.0, 1 H,
aryl), 7.63 (dt, J = 1.2, 7.8, 1 H, py), 7.30 (t, J = 6.4, 1 H, py), 7.19 (d, J = 2.3, 1 H, aryl), 6.99
(d, J = 2.3, 1 H, aryl), 6.89 (d, J = 7.8, 1 H, py), 6.68 (d, J = 9.0, 1 H, aryl), 5.01–4.91 (m, 2 H, aryl
CH2), 4.13 (d, J = 16.2, 1 H, aryl CH2), 3.92–3.82 (m, 3 H, aryl CH2 and C5H4CH2), 1.28 (s, 9 H, But),
1.26 (s, 9 H, But)
13C-{1H} NMR (CDCl3): δ 168.9, 159.2, 154.7, 151.3, 143.7, 139.9, 137.5, 125.7, 125.5, 124.2, 123.8,
122.9, 122.1, 121.3, 119.2, 63.5, 62.1, 59.1, 35.0, 34.3, 31.5, 29.9
IR: ν(MoO2) 910s (br); ν(NO2) 1510s, 1333s
HRMS (LSI):a m/z 606.1475 (606.1507)
a Calculated values given in parentheses. b The NMR spectra were obtained at r.t. in CD3SOCD3 unless otherwise stated; data given as chemical shift
(δ), multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet), coupling constant (Hz), relative intensity and assignment. The IR spectra (cmϪ1
)
were recorded as KBr pellets. For high resolution mass spectra the values of m/z quoted are based on the most abundant isotope of each element in
[M ϩ H]ϩ. The bands show the predicted isotopic patterns.
3060
J. Chem. Soc., Dalton Trans., 1998, 3057–3064

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Table 2 Crystallographic data for [WO2Ln] (n = 1, 5 or 6) and [MoO2L2]
[WO2L1]
[WO2L5]
[WO2L6]
[MoO2L2]
Formula
Formula weight
C20H18N2O4W
534.21
C18H22N2O4W
514.23
C18H22N2O4W
514.23
C20H16MoN4O8
536.31
Crystal system
Space group
a/Å
b/Å
Monoclinic
P21/n (no. 14)
11.7600(11)
14.4350(11)
11.8790(11)
116.120(4)
1810.6(3)
4
Orthorhombic
P212121 (no. 19)
11.7880(10)
18.285(2)
Monoclinic
C2 (no. 5)
19.0150(14)
7.4238(5)
14.8940(11)
119.400(4)
1831.7(2)
4
Monoclinic
P21 (no. 4)
9.4745(4)
11.4210(6)
9.5782(4)
96.172(3)
1030.43(8)
2
c/Å
8.2640(7)
β/Њ
U/Å3
1781.3(3)
4
6.510
3321
Z
µ/mmϪ1
6.409
2963
6.331
2778
0.695
3343
Unique data measured
Observed data [I > 2σ(I)], n
2804
317
0.0386
0.0968
3261
227
0.0570
0.1421
2671
229
0.0539
0.1453
3267
299
0.0551
0.1424
No. parameters, p
Ra
RЈb
¹
a Σ∆/Σ|Fo|. b [Σw∆2/Σw|Fo|2]² where ∆ = ||Fo| Ϫ |Fc||; weighting scheme, w = [σ2(Fo) ϩ 0.009|Fo|2]Ϫ1
.
Fig. 3 Molecular structure of [WO2L6]. Details as in Fig. 1.
Fig. 1 Molecular structure of [WO2L1] showing the atom labelling
scheme. Hydrogen atoms are omitted for clarity and thermal ellipsoids
are drawn at the 35% probability level.
Fig. 4 Molecular structure of [MoO2L2]. Details as in Fig. 1.
co-workers15 in 1989. Table 3 gives selected bond distances and
angles for these complexes. The W=O bond distances (1.719–
1.742 Å) are in the range typical of oxotungsten units30 and are
only marginally longer (by 0.02–0.04 Å) than the Mo᎐O bonds
᎐
in the respective molybdenum analogues.15 The W᎐O single
bond distances (1.899–1.952 Å) are comparable with those in
other tungsten() alkoxides.31 The metal–nitrogen bonds are
relatively long (>2.30 Å) because of the strong trans effect of
the oxo ligands. Similar to the molybdenum analogues,15 the
terminal W᎐N (pyridine) bond in [WO2L1] (2.305 Å) is substan-
tially shorter than the terminal W᎐N (amino) bond in [WO2L5]
(2.455 Å), while the W᎐N (tripodal) bond lengths are relatively
close in these complexes. The N᎐W᎐N angle is also smaller in
[WO2L1] than in [WO2L5] (71.60 vs. 74.32Њ). The longer W᎐N
(amino) bond may be attributed to the weaker bonding (sp3 vs.
sp2 N) and the steric effects due to the two methyl groups at
nitrogen. Substitution of tungsten for molybdenum has only
a small influence on the overall structure. There is no signifi-
cant difference in metal–ligand bond distances and angles
for the isomeric complexes [WO2L5] and [WO2L6]. The intro-
duction of nitro groups in [MoO2L2] does not induce substan-
tial geometrical changes in the structure of the complex as
compared with [MoO2L1].15
Fig. 2 Molecular structure of [WO2L5]. Details as in Fig. 1.
trometry. The FAB or LSI spectra exhibited the corresponding
[M ϩ H]ϩ envelope of which the isotopic distribution was in
good agreement with the calculated spectra.
Structural studies
The structures of four of these complexes, namely [WO2Ln]
(n = 1, 5 or 6) and [MoO2L2], were established by single crystal
X-ray analysis. As shown in Figs. 1–4, all of these compounds
exhibit a distorted octahedral geometry with cis-dioxo ligands
which are trans to the two nitrogen atoms of the ligand Ln. For
[WO2L6] an imposed C2 axis bisects the O(1)᎐W(1)᎐O(1A)
angle and two crystallographically independent molecules are
present in the unit cell. These tungsten complexes are isostruc-
tural with their molybdenum counterparts [MoO2Ln] (n = 1, 5
or 6) of which the structures were determined by Spence and
J. Chem. Soc., Dalton Trans., 1998, 3057–3064
3061

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Table 3 Selected bond distances (Å) and angles (Њ)a
[WO2L1]
[WO2L5]
[WO2L6]
[MoO2L2]
b
M᎐Ot
M᎐Op
M᎐N
1.719(2) [O1]
1.734(2) [O4]
1.941(2) [O2]
1.943(2) [O3]
2.378(2) [N1]
2.305(2) [N2]
1.731(4) [O2]
1.721(3) [O1]
1.926(2) [O3]
1.933(3) [O4]
2.355(3) [N1]
2.455(4) [N2]
1.723(2) [O1]
1.742(2)c
1.7046(11) [O1]
1.7042(13) [O2]
1.9779(13) [O3]
1.9658(11) [O4]
2.384(2) [N1]
b
1.952(2) [O2]
1.899(2)c
2.403(3) [N1]
2.366(2)c
2.3157(12) [N2]
Ot᎐M᎐Ot
Op᎐M᎐Op
N᎐M᎐N
Ot᎐M᎐Op
107.00(8)
155.51(7)
71.60(6)
105.8(2)
107.3(2)
107.70(7)
154.39(5)
71.58(5)
108.53(14)c
158.3(2)
158.08(13)
74.32(12)
152.0(2)c
75.00(13)
74.81(11)c
98.31(9) [O1, O2]
97.33(9) [O1, O3]
97.67(8) [O4, O2]
95.63(7) [O4, O3]
88.54(8) [O1, N1]
160.09(8) [O1, N2]
164.18(6) [O4, N1]
92.91(7) [O4, N2]
96.93(13) [O2, O3]
97.98(13) [O2, O4]
95.20(14) [O1, O3]
96.09(13) [O1, O4]
89.56(14) [O2, N1]
163.9(2) [O2, N2]
164.6(2) [O1, N1]
90.3(2) [O1, N2]
95.75(10) [O1, O2]
97.09(10) [O1, O2A]
95.75(1)c
98.02(6) [O1, O3]
95.99(5) [O1, O4]
99.04(6) [O2, O3]
96.96(6) [O2, O4]
89.41(6) [O1, N1]
160.98(6) [O1, N2]
162.46(6) [O2, N1]
91.30(6) [O2, N2]
100.48(11)c
88.86(10) [O1, N1]
163.80(10) [O1, N1A]
88.46(9)c
Ot᎐M᎐N
162.69(9)c
a The numbers in parentheses are the estimated standard deviations in the last digit. The atom labels for individual ligand atoms (see Figs. 1–4)
appear in square brackets. b Ot = Terminal oxo group, Op = phenolic oxygen atom. c Data for another crystallographically independent unit.
¹
4
²
Fig. 6 Plots of ipc vs. v for [MoO2L ]: (᭿) ferrocene, (᭹) complex 1,
(᭡) 2.
the values of ∆Ep of these reduction processes with that of
ferrocene which ranges from 64 to 69 mV, these couples are
described as quasi-reversible processes. It can be further cor-
Fig. 5 Cyclic voltammograms of [MoO2L1] (——) and [MoO2L3]
(–––) in dmf containing 0.1 mol dmϪ3 [NBu4][ClO4] (v = 100 mV sϪ1).
roborated by the plots of peak current (ip) vs. square root of the
¹
scan rate (v ).32 A typical plot for [MO2L4] is shown in Fig. 6 in
²
Electrochemical studies
¹
¹
¹
Ϫ
²
²
²
which a transition clearly occurs at v ≈ 20 mV s (i.e. v ≈ 400
mV sϪ1) for the first and second cathodic peaks characterized as
a quasi-reversible process. The plot for ferrocene remains linear
throughout the range of scan rates being examined.
The electrochemical properties of [MO2Ln] (M = Mo or W;
n = 1–4) were studied by cyclic voltammetry. As shown in Fig. 5,
the voltammogram of [MoO2L1] shows a reduction couple at
ϩ
₁
E = Ϫ1.61 V vs. Ag–Ag in MeCN together with a weak and
The electrochemical behaviour of the tungsten analogues is
remarkably different. The voltammogram of [WO2L1] [Fig.
7(a)] exhibits an irreversible reduction peak at Ϫ2.16 V together
with two oxidation peaks at Ϫ1.78 and Ϫ0.85 V. It appears that
the latter peaks are due to a species generated from electro-
chemical reduction of [WO2L1]. The other tungsten complexes
behave similarly except that an extra quasi-reversible couple
appears at ca. Ϫ1.6 V which may be due to the nitro-containing
ligand as in the case of the molybdenum analogues. This is
exemplified by the voltammogram of [WO2L4] [Fig. 7(b)]
although the two oxidation peaks at Ϫ1.66 and Ϫ1.02 V are
not readily observed in this case. It is worth noting that the
metal-based reduction potentials for the tungsten complexes
are more negative by 0.34–0.53 V than those of the correspond-
ing molybdenum counterparts and the reductions are less
reversible. This is fully consistent with studies of other high
valent molybdenum and tungsten complexes.9a,10d,33
₂
broad couple centred at Ϫ1.09 V. Ferrocene was also added in
all measurements as an internal reference and its oxidation
ϩ
₁
potential (E = 0.07 V vs. Ag–Ag in MeCN) remained virtually
₂
unchanged. The complexes [MoO2Ln] (n = 2–4), which contain
one or two nitro groups in the ligands, underwent two reduction
processes. The voltammogram of [MoO2L3] is also given in Fig.
5 for exemplification. The first couple is tentatively assigned to a
mainly ligand-based reduction while the second reduction is
attributed to the MoVI–MoV couple. The electrochemical data
are summarized in Table 4. It can be seen that the first reduction
potentials of [MoO2Ln] (n = 2–4) are consistent with the elec-
tron withdrawing property of the nitro group and the electron
donating character of the tert-butyl group. This is also observed
for the second reduction by considering the half-wave poten-
₁
tials (E = Ϫ1.835, Ϫ1.846 and Ϫ1.853 V for n = 2, 3 and 4
₂
respectively) but the effects are less significant. By comparing
3062
J. Chem. Soc., Dalton Trans., 1998, 3057–3064

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Table 4 Electrochemical data for [MO2Ln] (M = Mo or W; n = 1–4)a
First reduction
Second reduction
Epc (2) Epa (2)
Ϫ1644
b
b
Compound
Epc (1)
Ϫ1135
Ϫ1277
Ϫ1451
Ϫ1490
—
Epa (1)
Ϫ1036
Ϫ1202
Ϫ1382
Ϫ1412
—
∆Ep
|ipa/ipc
|
∆Ep
|ipa/ipc|
[MoO2L1]
[MoO2L2]
[MoO2L3]
[MoO2L4]
[WO2L1]
[WO2L2]
[WO2L3]
[WO2L4]
99
75
69
78
—
147
62
72
0.43
0.86
0.92
0.87
—
0.52
0.53
0.46
Ϫ1578
Ϫ1748
Ϫ1793
Ϫ1796
—
86
164
105
114
—
—
—
0.98
1.12
1.14
1.09
—
—
—
Ϫ1922
Ϫ1898
Ϫ1910
Ϫ2163
Ϫ2450
Ϫ2241
Ϫ2330
Ϫ1622
Ϫ1587
Ϫ1592
Ϫ1475
Ϫ1515
Ϫ1520
—
—
—
—
—
a Recorded with [NBu4][ClO4] as electrolyte in dmf (0.1 mol dmϪ3) at ambient temperature. Scan rate = 100 mV sϪ1. Potentials are expressed in mV vs.
Ag–Agϩ in MeCN. b ∆Ep = |Epc Ϫ Epa|.
Table 5 Percentage conversion of benzoin into benzila
Entry
Catalyst
Conversion (%)
1
2
3
4
5
6
[WO2L1]
[WO2L3]
[WO2L2]
[WO2L5]
[WO2L6]
[MoO2L2]
4
10
20
24
12
65b
a Metal complex was treated with 25 equivalents of benzoin in
deoxygenated CD3SOCD3 at 100 ЊC for 96 h. b After 7 h.
of the catalyst was deactivated and consumed. Some unidenti-
1
fied signals also appeared in the H NMR spectra after pro-
longed heating which precluded the extraction of kinetic
parameters for these catalytic systems. A qualitative com-
parison of the rate of conversion of benzoin into benzil could
however be made. Table 5 shows the percentage conversion of
benzoin after heating the mixture at 100 ЊC for 96 h. It is worth
noting that the introduction of the electron withdrawing nitro
group facilitates the oxo-transfer process (entries 1–3) and the
co-ordination environment of the metal complexes also has
substantial effects on the rates of oxo transfers as shown by the
data for the structural isomers [WO2L5] and [WO2L6] (entries 4
and 5). The reactions of these tungsten complexes with benzoin
are much slower than the corresponding reactions involving
the anionic complexes [WVIO2(bdt)2]2Ϫ (bdt = benzene-1,2-
dithiolate)9a,f and [WVIO2{O2CC(S)Ph2}]2Ϫ,11 but faster than the
reactions involving [WO2X(L)] [L = hydrotris(3,5-dimethyl-
pyrazol-1-yl)borate; X = OPh or S2PPh2-S], which are inert
toward oxygen atom transfer reactions.10d For comparison, the
oxo-transfer properties of [MoO2L2] were also investigated.
This molybdenum catalyst was much more efficient in the
oxidation of benzoin with dmso as compared with its tungsten
counterpart (entries 6 and 3) which may be rationalized by the
Fig. 7 Cyclic voltammograms of (a) [WO2L1] and (b) [WO2L4] in dmf
(0.1 mol dmϪ3 [NBu4][ClO4]; v varies from 20 to 1000 mV sϪ1).
difference in reduction potentials (Ϫ1.92 vs. Ϫ2.45 V) and M᎐O
᎐
Oxo-transfer properties
bond dissociation energies (ca. 410 vs. >580 kJ molϪ1).7c,10d,30b
In conclusion, we have synthesized and characterized a series
of dioxo-tungsten() and -molybdenum() complexes with
tetradentate ligands having an N2O2 donor set. These com-
pounds can act as catalysts in oxo-transfer reactions although
the reactivities appear to be low, in particular for the tungsten
complexes which have highly negative reduction potentials and
As the atom transfer chemistry of tungsten is poorly devel-
oped,7,8,9a,f,10d,11,34 the efficacy of these dioxotungsten() com-
plexes to mediate oxo-transfer reactions was examined using
benzoin as the reductant and dmso as the oxo donor. Typically,
0.01 mmol of the complexes [WO2Ln] (n = 1–3, 5 or 6) and 25
equivalents of benzoin were dissolved in deoxygenated
CD3SOCD3 (0.5 cm3). The mixtures were sealed in NMR tubes
and heated at 100 ЊC. The progress of the reactions was moni-
strong W᎐O bonds.
᎐
1
tored by H NMR spectrometry which allowed the quanti-
Acknowledgements
This work was supported by The Hong Kong Research Grants
Council (Earmarked Grant: CUHK 464/95P).
fication of benzoin and its oxidized product, benzil as well
as the metal complexes. All of these compounds were able to
catalyse the oxidation of benzoin by CD3SOCD3 which pro-
ceeded slowly over several days. In a control experiment no
reaction between benzoin and CD3SOCD3 was detected
when the complexes were absent. The concentration ratios of
([benzoin] ϩ [benzil]) to [WO2Ln] were found to increase
gradually during the course of the reactions indicating that part
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