New organometallic dioxotungsten(VI) complexes containing N2S tridentate ligands: The synthesis and reactivity of chloro and alkyl derivatives

Article abstract of DOI:10.1039/b108994n

A new series of N-benzyl aliphatic N₂S tridentate proligands HLSⁿ (n = 2-4) derived from 2-[(2-mercaptoethyl)-aminomethyl]pyridine (HL¹) has been prepared via the sequential reactions of trimethylsilylation, N-benzylation, and hydrolysis. A previously reported aromatic N₂S tridentate proligand, 2-[(2-mercaptophenyl)aminomethyl]pyridine (HL⁵), is also employed as an ancillary ligand. Reaction of these proligands HLⁿ (n = 1-5) with [WO₂Cl₂(DME)] (DME = 1,2-dimethoxyethane) in the presence of triethylamine leads to the formation of cis-dioxotungsten(VI) complexes [WO₂(Lⁿ)Cl] (n = 1-5). Treatment of the chloro derivatives [WO₂(Lⁿ)Cl] (n = 1, 5) with the Grignard reagents RMgX (R= CH₂SiMe₃, C₆H₄^tBu-4; X = Cl, Br) resulted in ligand substitution reaction and the formation of the first reported alkyl complexes of the type [WO₂(Lⁿ)R] (n = 1, 5; R = CH₂SiMe₃, C₆H₄^tBu-4).

Full text of DOI:10.1039/b108994n

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New organometallic dioxotungsten(VI) complexes containing N2S
tridentate ligands: the synthesis and reactivity of chloro and alkyl
derivatives
Yee-Lok Wong and Jonathan R. Dilworth*
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, UK OX1 3QR.
E-mail: jon.dilworth@chem.ox.ac.uk
Received 3rd October 2001, Accepted 11th April 2002
First published as an Advance Article on the web 1st May 2002
A new series of N-benzyl aliphatic N2S tridentate proligands HLn (n = 2–4) derived from 2-[(2-mercaptoethyl)-
aminomethyl]pyridine (HL1) has been prepared via the sequential reactions of trimethylsilylation, N-benzylation,
and hydrolysis. A previously reported aromatic N2S tridentate proligand, 2-[(2-mercaptophenyl)aminomethyl]-
pyridine (HL5), is also employed as an ancillary ligand. Reaction of these proligands HLn (n = 1–5) with
[WO2Cl2(DME)] (DME = 1,2-dimethoxyethane) in the presence of triethylamine leads to the formation of cis-
dioxotungsten() complexes [WO2(Ln)Cl] (n = 1–5). Treatment of the chloro derivatives [WO2(Ln)Cl] (n = 1, 5)
t
with the Grignard reagents RMgX (R = CH2SiMe3, C6H4 Bu-4; X = Cl, Br) resulted in ligand substitution reaction
t
and the formation of the first reported alkyl complexes of the type [WO2(Ln)R] (n = 1, 5; R = CH2SiMe3, C6H4 Bu-4).
techniques under an atmosphere of dinitrogen; workups were
Introduction
performed in air. Dichloromethane was pre-dried over 4 Å
Applications of high-valent oxo-organometallic compounds as
molecular sieves and distilled from calcium hydride. Diethyl
catalysts for various reactions such as ring-opening metathesis
ether, tetrahydrofuran (THF) and toluene were distilled from
polymerisation (ROMP), epoxidation, and oxidation have
sodium–benzophenone. Methanol (MeOH) was distilled from
magnesium methoxide. Silica gel (70–230 mesh) for flash
received considerable attention.1 Although some of these
compounds have been widely used as catalysts in industrial
column chromatography was purchased from Fluka. All other
processes, the exact nature and mechanism of the interaction
reagents and solvents were of reagent grade and used as
between the catalyst and substrate have rarely been explored.
In order to have a better understanding of their chemical
received. [WO2Cl2(DME)]10 (DME = 1,2-dimethoxyethane),
2-(2-mercaptoethyl)aminomethylpyridine11 (HL1), and 2-(2-
behaviour, a substantial number of organometallic oxo-
mercaptophenyl)aminomethylpyridine12 (HL5) were prepared
complexes have been synthesised and their chemistry investi-
according to the literature procedures with minor modification.
gated.2 Alkoxy and phenoxy groups have most commonly been
1
All H and 13C{1H} NMR spectra were recorded on a Varian
used as ancillary ligands.3 Recently, calix[4]arenes containing
Mercury VX300 spectrometer (1H, 300; 13C, 75.4 MHz). Chem-
pre-organized sets of alkoxy donor atoms have been employed
ical shifts were relative to internal SiMe4 (δ = 0). IR spectra were
to mimic metal oxide surfaces.4 High-valent dioxotungsten
recorded on a Perkin-Elmer 1710 spectrophotometer as KBr
complexes with hydrocarbon ligands have been reported
pellets. Atmospheric pressure chemical ionization (APCI) mass
sporadically. The most typical examples contain cyclopenta-
spectra were recorded on a Hewlett-Packard 1050 Series mass
spectrometer. Liquid secondary ion (LSI) mass spectra were
measured on a Bruker APEX 47e Fourier transform ion cyclo-
tron 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.
dienyl ligands, e.g. [WO2(η5-C5R5)Cl], [{WO2(η5-C5R5)}2O], and
[WO2(η5-C5R5)RЈ] (R = H, Me; RЈ = alkyl, alkynyl).5 Apart
from these, 2,2Ј-bipyridine (bipy) and hydridotris(3,5-dimethyl-
pyrazolyl)borate [HB(Me2pz)3]Ϫ have also been used as
co-ligands in complexes such as [WO2(bipy)R2] (R = alkyl,
phenyl) and [WO2HB(Me2pz)3R] (R = alkyl, phenyl, alkenyl).6,7
However, examples of organometallic dioxotungsten complexes
containing other types of supporting ligands are extremely
Preparations
rare.8 Recently, a new series of N2O-type ligands derived from
2-(2-Mercaptoethyl)aminomethylpyridine (HL1). A colourless
2-N-(2-pyridylmethyl)aminophenol [H(L–N2O)] and its N-alkyl
mixture of 2-aminomethylpyridine (0.54 g, 5.0 mmol) and
ethylene sulfide (0.30 g, 5.0 mmol) in toluene (5 cm3) was heated
in a sealed ampoule at 110 ЊC for 38 h. After cooling to room
temperature, the pale yellow mixture was loaded onto a silica
gel column, which was eluted with ethyl acetate first followed by
ethyl acetate–ethanol (1 : 1). The third band was collected and
concentrated with a rotary evaporator to give a colourless
derivatives, which can stabilize the high-valent dioxotungsten
complexes [WO2(L–N2O)Cl] and [WO2(L–N2O)(R)] (R = Me,
t
Et, CH2SiMe3, C6H4 Bu-4) has been reported by Wong et al.9
The molecular structure of the alkyl complex [WO2(L–N2O)-
(CH2SiMe3)] has also been confirmed by X-ray diffraction
analysis. In this paper, we describe the synthesis of a new series
of dioxotungsten() complexes containing new N2S tridentate
ligands which can be functionalised via N-benzylation.
1
pungent liquid. Yield: 0.57 g (68%). H NMR (CDCl3): δ 8.56
(d, J = 4.8 Hz, 1 H, PyH), 7.65 (dd, J = 1.8, 7.8 Hz, 1 H, PyH),
7.32 (d, J = 7.5 Hz, 1 H, PyH), 7.17 (t, J = 6.2 Hz, 1 H, PyH),
3.93 (s, 2 H, PyCH2), 2.87 (t, J = 6.2 Hz, 2 H, NCH2CH2), 2.70
(t, J = 6.2 Hz, 2 H, NCH2CH2). 13C{1H} NMR (CDCl3):
δ 159.5, 149.2, 136.4, 122.1, 121.9, 54.6, 51.9, 24.9. MS (APCI):
m/z 169 (80%) [M ϩ H]ϩ.
Experimental
General procedures
All reactions were carried out using standard Schlenk-line
2366
J. Chem. Soc., Dalton Trans., 2002, 2366–2370
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b108994n

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General procedure for the preparation of HLn (n ؍ 2–4). To a
colourless mixture of HL1 and triethylamine (1 equiv.) in THF
was added chlorotrimethylsilane (1 equiv.) dropwise at 0 ЊC. A
white solid began to form during the addition. The mixture was
stirred at this temperature for 1 h, allowed to warm to ambient
temperature and stirred for a further 12 h. A second portion of
triethylamine (1 equiv.) and the appropriate substituted benzyl
bromide (1 equiv.) in THF were added. After heating under
reflux for 18 h, the resulting white solid was filtered off and
discarded. The pale yellow filtrate was concentrated using
a rotary evaporator and the residue was chromatographed on
a silica gel column using CH2Cl2 followed by ethyl acetate as
eluent. The second band was collected and concentrated to give
a colourless liquid.
2-[N-Benzyl-N-(2-mercaptoethyl)]aminomethylpyridine
(HL2). According to the general procedure, HL1 (0.57 g, 3.4
mmol) was stirred with triethylamine (0.47 cm3, 3.4 mmol) and
chlorotrimethylsilane (0.37 g, 3.4 mmol) in THF (40 cm3) for 12
h. Then a second portion of triethylamine (0.47 cm3, 3.4 mmol)
and benzyl bromide (0.58 g, 3.4 mmol) were added, and the
mixture was heated under reflux for 18 h to give HL2 (0.48 g,
55%). 1H NMR (CDCl3): δ 8.51 (d, J = 4.2 Hz, 1 H, PyH), 7.66
(dt, J = 1.8, 7.7 Hz, 1 H, PyH), 7.55 (d, J = 8.1 Hz, 1 H, PyH),
7.22–7.40 (m, 5 H, ArH), 7.15 (t, J = 5.9 Hz, 1 H, PyH), 3.78 (s,
2 H, PyCH2), 3.67 (s, 2 H, ArCH2), 2.73–2.77 (m, 2 H,
NCH2CH2), 2.61–2.68 (m, 2 H, NCH2CH2). 13C{1H} NMR
(CDCl3): δ 159.6, 148.8, 138.8, 136.3, 128.8, 128.2, 127.0,
122.9, 121.9, 59.9, 58.5, 56.7, 22.4. MS (APCI): m/z 259 (15%)
[M ϩ H]ϩ.
[WO2(L1)Cl]. According to the general procedure, [WO2Cl2-
(DME)] (3.32 g, 8.8 mmol) in CH2Cl2 (20 cm3) was treated with
HL1 (1.48 g, 8.8 mmol) and triethylamine (1.2 cm3, 8.8 mmol) in
MeOH (30 cm3) to give [WO2(L1)Cl] (1.14 g, 31%) as a yellow
solid. IR (cmϪ1): 3135m, 2925w, 1607m, 1484w, 1440m, 1340w,
1290m, 1227w, 1157w, 1106w, 1076w, 1025w, 943s ν(WO2),
893s ν(WO2), 819m, 773s, 731m, 645w, 425m. Anal. calc. for
C8H11ClN2O2SW: C, 23.0; H, 2.7; N, 6.7%. Found: C, 22.6; H,
2.9; N, 6.5%.
[WO2(L5)Cl]. According to the general procedure, [WO2Cl2-
(DME)] (1.70 g, 4.5 mmol) in CH2Cl2 (20 cm3) was treated with
HL5 (0.97 g, 4.5 mmol) and triethylamine (0.62 cm3, 4.5 mmol)
in MeOH (30 cm3) to give [WO2(L5)Cl] (0.94 g, 45%) as a grey
solid. IR (cmϪ1): 3055w, 1708w, 1608m, 1585m, 1475m, 1439m,
1297m, 1249w, 1158w, 1083m, 1046m, 997w, 945s ν(WO2),
905s ν(WO2), 814m, 755s, 729m, 681w, 651w. Satisfactory
analytical data for this compound could not be obtained.
General procedure for the preparation of [WO2(Ln)Cl]
(n ؍ 2–4). A solution of [WO2Cl2(DME)] in THF was added
dropwise to a solution of HLn (n = 2–4, 1 equiv.) in THF
with vigorous stirring. The pale yellow mixture turned dark
purple immediately. This mixture was stirred at room temper-
ature for 15 min, then triethylamine (1 equiv.) was added, and
the mixture was kept stirring overnight. The resulting mixture
was loaded onto a silica gel column, which was then eluted with
ethyl acetate. The yellow band was collected and concentrated
to give a pale yellow solid.
[WO2(L2)Cl]. According to the general procedure, [WO2Cl2-
(DME)] (0.60 g, 1.6 mmol) was treated with HL2 (0.41 g,
1.6 mmol) and triethylamine (0.22 cm3, 1.6 mmol) in THF
(50 cm3) to give [WO2(L2)Cl] (0.39 g, 48%). 1H NMR (DMSO-
d6): δ 9.23 (d, J = 4.5 Hz, 1 H, PyH), 8.13 (dt, J = 1.8, 7.8 Hz,
1 H, PyH), 7.60–7.69 (m, 4 H, PyH and ArH), 7.46–7.48 (m,
3 H, PyH and ArH), 4.89 (d, J = 14.1 Hz, 1 H, ArCH2), 4.77
(d, J = 15.3 Hz, 1 H, ArCH2), 4.68 (d, J = 14.1 Hz, 1 H, ArCH2),
4.04 (d, J = 15.3 Hz, 1 H, ArCH2), 3.85–3.89 (m, 1 H,
NCH2CH2), 3.56–3.62 (m, 1 H, NCH2CH2), 3.18–3.22 (m, 1 H,
NCH2CH2), 2.26–2.33 (m, 1 H, NCH2CH2). 13C{1H} NMR:
δ 155.5, 150.4, 141.1, 132.5, 132.2, 129.1, 128.8, 125.6, 125.0,
62.3, 61.7, 61.4, 30.2. IR (cmϪ1): 3032w, 2930w, 1608m, 1485w,
1440m, 1421m, 1366w, 1308w, 1294m, 1204w, 1160w, 1047m,
1028m, 951s ν(WO2), 905s ν(WO2), 847w, 783m, 764m, 751m,
723w, 697m, 604w, 546w. HRMS (LSI): m/z calc. for
C15H18ClN2O2SW [M ϩ H]ϩ 509.0287, found 509.0262. Anal.
calc. for C15H17ClN2O2SW: C, 35.4; H, 3.4; N, 5.5%. Found: C,
36.0; H, 3.9; N, 5.3%.
2-[N-(4-tert-Butylbenzyl)-N-(2-mercaptoethyl)]aminometh-
ylpyridine (HL3). According to the general procedure, HL1
(1.23 g, 7.3 mmol) was stirred with triethylamine (1.0 cm3, 7.3
mmol) and chlorotrimethylsilane (0.92 g, 7.3 mmol) in THF
(60 cm3) for 12 h. Then a second portion of triethylamine (1.0
cm3, 7.3 mmol) and 4-tert-butylbenzyl bromide (1.66 g, 7.3
mmol) were added, and the mixture was heated under reflux
for 18 h to give HL3 (0.69 g, 30%). 1H NMR (CDCl3): δ 8.51 (d,
J = 4.2 Hz, 1 H, PyH), 7.67 (dt, J = 1.8, 7.7 Hz, 1 H, PyH), 7.57
(d, J = 7.5 Hz, 1 H, PyH), 7.29–7.36 (m, 4 H, ArH), 7.15 (t,
J = 6.2 Hz, 1 H, PyH), 3.77 (s, 2 H, PyCH2), 3.64 (s, 2 H,
t
ArCH2), 2.63–2.78 (m, 4 H, NCH2CH2), 1.31 (s, 9 H, Bu).
13C{1H} NMR (CDCl3): δ 159.7, 149.9, 148.7, 136.4, 135.5,
128.4, 125.1, 122.9, 121.9, 59.8, 58.0, 56.6, 34.4, 31.3, 22.4. MS
(APCI): m/z 315 (20%) [M ϩ H]ϩ.
2-[N-(3,5-Di-tert-butylbenzyl)-N-(2-mercaptoethyl)]amino-
methylpyridine (HL4). According to the general procedure,
HL1 (0.82 g, 4.9 mmol) was stirred with triethylamine (0.68 cm3,
4.9 mmol) and chlorotrimethylsilane (0.54 g, 4.9 mmol) in THF
(60 cm3) for 12 h. Then a second portion of triethylamine (0.68
cm3, 4.9 mmol) and 3,5-di-tert-butylbenzyl bromide (1.39 g, 4.9
mmol) were added, and the mixture was heated under reflux
for 18 h to give HL4 (0.58 g, 32%). 1H NMR (CDCl3): δ 8.50 (d,
J = 4.5 Hz, 1 H, PyH), 7.65 (dt, J = 1.8, 7.7 Hz, 1 H, PyH), 7.58
(d, J = 7.5 Hz, 1 H, PyH), 7.22–7.29 (m, 3 H, ArH), 7.14 (t, J =
5.3 Hz, 1 H, PyH), 3.77 (s, 2 H, PyCH2), 3.67 (s, 2 H, ArCH2),
[WO2(L3)Cl]. According to the general procedure,
[WO2Cl2(DME)] (0.83 g, 2.2 mmol) was treated with HL3 (0.69
g, 2.2 mmol) and triethylamine (0.30 cm3, 2.2 mmol) in THF
(60 cm3) to give [WO2(L3)Cl] (0.25 g, 20%). 1H NMR (CDCl3):
δ 9.46 (d, J = 5.1 Hz, 1 H, PyH), 7.95 (dt, J = 1.8, 7.8 Hz, 1 H,
PyH), 7.53 (t, J = 6.5 Hz, 1 H, PyH), 7.48 (d, J = 8.7 Hz, 2 H,
ArH), 7.33–7.36 (m, 3 H, PyH and ArH), 4.93 (s, 2 H, ArCH2),
4.89 (d, J = 15.0 Hz, 1 H, PyCH2), 4.07–4.14 (m, 1 H,
NCH2CH2), 3.86 (d, J = 15.0 Hz, 1 H, PyCH2), 3.62–3.70 (m,
1 H, NCH2CH2), 3.30–3.35 (m, 1 H, NCH2CH2), 2.31–2.41 (m,
t
2.67–2.81 (m, 4 H, NCH2CH2), 1.32 (s, 18 H, Bu). 13C{1H}
NMR (CDCl3): δ 159.8, 150.5, 148.7, 137.8, 136.4, 136.2, 123.0,
122.9, 121.9, 120.9, 59.9, 59.1, 56.8, 34.7, 31.5, 22.6. MS
(APCI): m/z 371 (25%) [M ϩ H]ϩ.
t
1 H, NCH2CH2), 1.37 (s, 9 H, Bu). 13C{1H} NMR (CDCl3):
δ 154.8, 152.3, 151.3, 140.1, 131.7, 128.7, 125.8, 125.1, 123.8,
62.9, 61.6, 61.1, 34.7, 31.2, 30.8. IR (cmϪ1): 2960m, 2869m,
1655w, 1609m, 1476m, 1445m, 1365m, 1297m, 1270w, 1218w,
1158w, 1109w, 1077w, 1051w, 1027m, 953s ν(WO2), 910s
ν(WO2), 833m, 815w, 777m, 763w, 726w, 689w, 587w, 557w.
HRMS (LSI): m/z calc. for C19H26ClN2O2SW [M ϩ H]ϩ
565.0913, found 565.0889. Anal. calc. for C19H25ClN2O2SW: C,
40.4; H, 4.5; N, 5.0%. Found: C, 41.0; H, 4.7; N, 4.7%.
General procedure for the preparation of [WO2(Ln)Cl] (n ؍ 1,
5). Triethylamine (1 equiv.) was added to a colourless solution
of HLn (n = 1, 5) in MeOH. After stirring at room temperature
for 15 min, the mixture was then transferred to a solution
of [WO2Cl2(DME)] (1 equiv.) in CH2Cl2 via a cannula. This
colourless mixture turned yellow immediately and stirring
was continued overnight. Due to the poor solubility of [WO2-
(Ln)Cl] (n = 1, 5), the resulting solid was collected by filtration,
washed with MeOH, ethyl acetate and hexanes, and dried
in vacuo.
[WO2(L4)Cl]. According to the general procedure,
[WO2Cl2(DME)] (0.60 g, 1.6 mmol) was treated with HL4 (0.59
g, 1.6 mmol) and triethylamine (0.22 cm3, 1.6 mmol) in THF
(60 cm3) to give [WO2(L4)Cl] (0.46 g, 46%). 1H NMR (CDCl3):
J. Chem. Soc., Dalton Trans., 2002, 2366–2370
2367

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δ 9.48 (d, J = 5.4 Hz, 1 H, PyH), 7.94 (dt, J = 1.2, 7.8 Hz, 1 H,
PyH), 7.50–7.56 (m, 2 H, PyH and ArH), 7.32 (d, J = 8.1 Hz,
1 H, PyH), 7.19 (d, J = 1.8 Hz, 2 H, ArH), 4.95 (s, 2 H, ArCH2),
4.90 (d, J = 15.3 Hz, 1 H, PyCH2), 4.04–4.08 (m, 1 H,
NCH2CH2), 3.83 (d, J = 15.3 Hz, 1 H, PyCH2), 3.64–3.71 (m,
1 H, NCH2CH2), 3.27–3.32 (m, 1 H, NCH2CH2), 2.32–2.40 (m,
2 H, ArH), 7.15 (d, J = 7.8 Hz, 2 H, ArH), 7.04–7.10 (m, 3 H,
ArH and NH), 4.41 (d, J = 16.2 Hz, 1 H, PyCH2), 3.87–3.93 (m,
t
1 H, PyCH2), 1.18 (s, 9 H, Bu). IR (cmϪ1): 3386m, 3110w,
2966w, 1610m, 1491s, 1284s, 1058w, 1025w, 949s ν(WO2), 900s
ν(WO2), 867m, 820w, 780m, 752m, 635m, 562w. HRMS
(LSI): m/z calc. for C22H25N2O2SW [M ϩ H]ϩ 565.1146, found
565.0991. Anal. calc. for C22H24N2O2SW: C, 46.8; H, 4.3; N,
5.0%. Found: C, 46.7; H, 3.8; N, 4.6%.
t
1 H, NCH2CH2), 1.37 (s, 18, Bu). 13C{1H} NMR (CDCl3):
δ 154.7, 151.4, 151.3, 140.1, 130.8, 126.2, 125.2, 123.7, 123.0,
62.8, 61.1 (one peak overlapping), 34.8, 31.4, 30.7. IR (cmϪ1):
2962s, 2867m, 1609m, 1477m, 1459m, 1445m, 1363m, 1298m,
1249m, 1201m, 1158w, 1081w, 1053w, 1027w, 953s ν(WO2), 911s
ν(WO2), 877w, 841w, 776m, 763m, 728m, 702w, 647w, 553w.
HRMS (LSI): m/z calc. for C23H34ClN2O2SW [M ϩ H]ϩ
621.1539, found 621.1602. Anal. calc. for C23H33ClN2O2SW: C,
44.5; H, 5.4; N, 4.5%. Found: C, 44.7; H, 5.4; N, 4.4%.
Results and discussion
A new series of aliphatic N2S tridentate proligands HLn (n =
2–4), derived from the previous reported 2-[(2-mercaptoethyl)-
aminomethyl]pyridine (HL1), were synthesised by a reaction
involving four steps, two of which were performed in situ
(Scheme 1). The first step was based on the literature procedure
General procedure for the preparation of [WO2(Ln)(R)] (n ؍ 1,
t
5; R ؍ CH2SiMe3, C6H4 Bu-4). To an ice-cooled suspension
of [WO2(Ln)Cl] (n = 1, 5) in THF was added a solution of
Grignard reagent (3 equiv.) in diethyl ether over a period of
30 min. The mixture turned to dark brown during addition. The
mixture was allowed to warm to room temperature and stirred
overnight. The volatiles were then removed under reduced
pressure, and water was added. The mixture was extracted with
CH2Cl2 (3 × 80 cm3) with vigorous shaking until a yellowish
brown solution was achieved. The combined organic extracts
were dried over anhydrous magnesium sulfate, then con-
centrated to ca. 2 cm3 to afford a pale brown solid, which was
collected by filtration and washed thoroughly with ethyl
acetate–hexanes (1 : 1), and dried in vacuo.
[WO2(L1)(CH2SiMe3)]. According to the general pro-
cedure, [WO2(L1)Cl] (0.42 g, 1.0 mmol) in THF (30 cm3) was
treated with Me3SiCH2MgCl (3.0 mmol) in diethyl ether
(20 cm3) to give [WO2(L1)(CH2SiMe3)] (85 mg, 18%). 1H NMR
(DMSO-d6): δ 8.97 (d, J = 5.1 Hz, 1 H, PyH), 7.99 (t, J = 7.7 Hz,
1 H, PyH), 7.47–7.54 (m, 3 H, PyH and NH), 4.41–4.50 (m,
1 H, PyCH2), 4.11 (d, J = 17.7 Hz, 1 H, PyCH2), 3.48–3.52 (m,
1 H, NCH2CH2), (two aliphatic proton signals obscured by
the residual solvent peaks), 1.91–2.00 (m, 1 H, NCH2CH2), 1.20
(d, J = 12.9 Hz, 1 H, WCH2), Ϫ0.01 (s, 9 H, CH3), Ϫ0.36 (d,
J = 12.9 Hz, 1 H, WCH2). IR (cmϪ1): 2946w, 1607m, 1438m,
1291w, 1241m, 1158w, 1055w, 1025w, 945s ν(WO2), 904s
ν(WO2), 850s, 832s, 761m, 713w, 682w, 499w. HRMS (LSI): m/z
calc. for C12H23N2O2SSiW [M ϩ H]ϩ 471.0759, found 471.0720.
Anal. calc. for C12H22N2O2SSiW: C, 30.7; H, 4.7; N, 6.0%.
Found: C, 31.1; H, 3.9; N, 6.6%.
Scheme 1
with minor modifications.11 Reaction of 2-aminomethylpyridine
with 1 equivalent of ethylene sulfide gave 2-[(2-mercaptoethyl)-
aminomethyl]pyridine (HL1) in 68% yield. This compound was
purified by flash column chromatography rather than vacuum
distillation in order to prevent the possibility of regenerating
the starting materials by decomposition during prolonged
heating. Treatment of HL1 with chlorotrimethylsilane in the
presence of triethylamine gave the silyl thioether adduct which
was then N-alkylated with a variety of benzyl bromides to
afford the silyl-protected N2S proligands. Hydrolysis of these
potential ligands during workup and purifying procedures led
to the formation of a new series of N2S tridentate proligands
HLn (n = 2–4) in 30–55% yield. Although silyl reagents have
been conventionally 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.13 By employing this synthetic scheme with the silyl
reagent, we were able to introduce a wide range of substituents
on the nitrogen donor atom having different steric and elec-
tronic requirements with the goal of studying their effects on
complex formation and properties of the resulting complexes.
The versatility of this synthetic route using silyl reagents has
been further explored through the preparation of a new class of
asymmetric N2OS tripodal ligands.14 Apart from the aliphatic
analogues, the previously reported aromatic N2S tridentate
proligand 2-[(2-mercaptophenyl)aminomethyl]pyridine12 (HL5)
has also been investigated as a comparison to the aliphatic
analogues. All these tridentate proligands HLn (n = 1–5) were
found to be rather unstable, and the colourless oily liquids
darkened gradually upon exposure to air over 1–2 days. The
proligands were therefore freshly prepared and stored under
dinitrogen.
[WO2(L5)(CH2SiMe3)]. According to the general pro-
cedure, [WO2(L5)Cl] (0.47 g, 1.0 mmol) in THF (30 cm3) was
treated with Me3SiCH2MgCl (3.0 mmol) in diethyl ether (20
cm3) to give [WO2(L5)(CH2SiMe3] (0.14 g, 28%). 1H NMR
(DMSO-d6): δ 9.09 (d, J = 5.4 Hz, 1 H, ArH), 8.28 (d, J = 6.6
Hz, 1 H, ArH), 7.99 (t, J = 7.5 Hz, 1 H, ArH), 7.58 (t, J = 6.5
Hz, 1 H, ArH), 7.49 (d, J = 7.8 Hz, 1 H, ArH), 7.30 (t, J = 4.2
Hz, 1 H, ArH), 7.00–7.03 (m, 3 H, ArH and NH), 4.84–4.92 (m,
1 H, PyCH2), 4.40 (d, J = 17.7 Hz, 1 H, PyCH2), 1.37 (d, J =
12.3 Hz, 1 H, WCH2), 1.13 (d, J = 12.3 Hz, 1 H, WCH2), 0.04 (s,
9 H, CH3). IR (cmϪ1): 3433w, 3067w, 2950w, 2894w, 1608m,
1481m, 1443m, 1363w, 1297w, 1281w, 1244m, 1185w, 1157w,
1132w, 1105w, 1025w, 995w, 948s ν(WO2), 897s ν(WO2), 852s,
832m, 757m, 728w, 713w, 683w, 649w. HRMS (LSI): m/z calc.
for C16H23N2O2SSiW [M ϩ H]ϩ 518.0681, found 518.0461.
Anal. calc. for C16H22N2O2SSiW: C, 37.1; H, 4.3; N, 5.4%.
Found: C, 36.8; H, 5.1; N, 5.2%.
t
[WO2(L5)(C6H4 Bu-4)]. According to the general pro-
cedure, [WO2(L5)Cl] (0.47 g, 1.0 mmol) in THF (30 cm3) was
treated with 4-tBuC6H4MgBr (3.0 mmol) in diethyl ether
t
(20 cm3) to give [WO2(L5)(C6H4 Bu-4)] (0.12 g, 22%). 1H NMR
(DMSO-d6): δ 9.35 (d, J = 5.1 Hz, 1 H, ArH), 8.18 (d, J = 6.6
Hz, 1 H, ArH), 8.00 (t, J = 5.9 Hz, 1 H, ArH), 7.71 (t, J = 5.9
Hz, 1 H, ArH), 7.60 (d, J = 7.8 Hz, 2 H, ArH), 7.37–7.44 (m,
In the preparation of the dioxo complexes, [WO2Cl2(DME)]10
was employed as the starting material. The versatility of this
2368
J. Chem. Soc., Dalton Trans., 2002, 2366–2370

View Article Online
Scheme 2
DME adduct as a precursor to a number of cis-dioxo-
suggesting that the compounds underwent decomposition. The
tungsten() complexes has been recently reported by Wong and
co-workers.9,15 Treatment of the proligands HLn (n = 1–5) with
[WO2Cl2(DME)] in the presence of triethylamine gave the
corresponding dioxo compounds [WO2(Ln)Cl] (n = 1–5)
(Scheme 2) in moderate yield. Due to the poor solubility
of [WO2(Ln)Cl] (n = 1, 5) in common organic solvents, these
compounds were isolated by filtration and could only be
purified by washing thoroughly with various solvents, thus no
solution characterisation data could be obtained. By contrast,
the dioxo complexes [WO2(Ln)Cl] (n = 2–4) containing various
benzyl groups possess sufficient solubility to be purified by
column chromatography and have been characterized with
stability of the organometallic compounds is significantly
enhanced by increasing the steric bulk of the alkyl or aryl
group. An absence of β-hydrogens may also improve
stability.
The reactivity of the chloro functionality in these complexes
towards other anions has also been examined. Treatment of the
compounds [WO2(Ln)Cl] (n = 1, 2, 5) with NaXR (X = O, S, Se;
R = Me, Et, Ph) in the presence of 18-crown-6 (cat.) in toluene
under reflux conditions caused complete decomplexation. This
is in contrast with the behaviour of [WO2HB(Me2pz)3Cl], which
can be converted to various –OR, –SR, and –SeR derivatives by
displacement of the chloro ligand.16
1
a wide range spectroscopic methods. The H NMR spectra
The IR spectra of these dioxo compounds show two char-
acteristic vibrational bands in the regions 893–911 and 943–953
cmϪ1 which were assignable to the asymmetric and symmetric
of these chloro compounds [WO2(Ln)Cl] (n = 2–4) show
two doublets at δ 3.81–4.92 which are assignable to the two
diastereotopic methylene protons adjacent to the pyridine
ring. Similarly to the dioxotungsten() analogues with N2O
ancillary ligands,9 these high-valent dioxo compounds are
stable to air, however they are slightly sensitive to moisture.
Some unidentified peaks appeared in the 1H NMR spectra after
prolonged dissolution of these compounds in undried solvent,
showing that these dioxo compounds decomposed in the
presence of moisture.
W᎐O stretches respectively.17 The liquid secondary ion (LSI)
᎐
mass spectra of all these compounds exhibited the correspond-
ing protonated molecular ion [M ϩ H]ϩ which was in good
agreement with a predicted accurate mass and isotopic distri-
bution pattern. Despite strenuous efforts we were unable to
obtain crystals of any of the dioxo complexes suitable for X-ray
structure determination.
In conclusion, a new series of chloro dioxotungsten() com-
plexes of general type [WO2(Ln)Cl] (n = 1–5) have been prepared
from the reaction of [WO2Cl2(DME)] with the N2S tridentate
ligands (HLn) (n = 1–5) in the presence of triethylamine.
Reaction of [WO2(Ln)Cl] (n = 1, 5) with an excess of the Grig-
Treatment of [WO2(Ln)Cl] (n = 1, 5) with an excess of a
t
Grignard reagent RMgX (R = CH2SiMe3, C6H4 Bu-4; X = Cl,
Br) resulted in a chloride ligand substitution reaction and the
t
formation of [WO2(Ln)(R)] (n = 1, 5; R = CH2SiMe3, C6H4 Bu-4)
t
in 18–28% yield (Scheme 2). Numerous attempts were made to
improve the yield by modifying the reaction conditions without
success. These high-valent alkyl and aryl tungsten complexes
are stable to oxygen; but they are sensitive to moisture to differ-
ent extents. Those containing a CH2SiMe3 group are more
nard reagent RMgX (R = CH2SiMe3, C6H4 Bu-4; X = Cl, Br)
gave the alkyl or aryl compounds [WO2(Ln)(R)]. To our
knowledge, these compounds are the first example of organo-
metallic dioxotungsten() complexes with N2S ancillary
ligands. The chloro derivatives show similar stability to air and
moisture to the tungsten N2O analogues, while the alkyl or aryl
t
stable than the one with C6H4 Bu-4. These complexes are com-
compounds [WO2(Ln)(R)] (n = 1, 5; R = CH2SiMe3, C6H4 Bu-4)
t
pletely insoluble in hydrocarbons and ether; and showed only
limited solubility in chlorinated solvents, but are soluble
in dipolar aprotic solvents such as N,N-dimethylformamide
(DMF) or dimethylsulfoxide (DMSO). Similarly to [WO2-
(L–N2O)(CH2SiMe3)] [L–N2O = 2-N-(2-pyridylmethyl)amino-
are relatively less stable than their counterparts with
N2O ligands. Although all these high-valent alkyl or aryl
dioxotungsten complexes [WO2(Ln)(R)] are stable to oxygen;
they are sensitive to moisture to different extents
1
t
phenolato],9 the H NMR spectrum of [WO2(L5)(CH2SiMe3)]
[CH2SiMe3 > C6H4 Bu-4 (less stable)]. The degree of sensitivity
in DMSO-d6 showed two upfield doublets at δ 1.37 and 1.13
with a geminal coupling constant of 12.3 Hz, which could be
attributed to the two diastereotopic methylene protons next to
tungsten. Moreover, no satellite resonances due to 183W and 29Si
nuclei were observed probably due to relatively poor solubility.
The complexes decomposed over the period required to acquire
13C NMR spectra. Attempts to prepare other alkyl or aryl
dioxo complexes by treating [WO2(Ln)Cl] (n = 1, 2, 5) with
RMgCl (R = Me, Et, i-Pr, CH2Ph, mesityl) were not successful.
Free proligands were regenerated after the workup procedure,
may reflect the difference in steric bulk of the organometallic
ligands.
Acknowledgements
We thank Prof. Hung Kay Lee of the Chinese University of
Hong Kong for recording the HRMS (LSI) spectra and the
Croucher Foundation for financial support (Dr. Yee-Lok
Wong).
J. Chem. Soc., Dalton Trans., 2002, 2366–2370
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View Article Online
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