New dioxo-molybdenum(vi) and -tungsten(vi) complexes with N-capped tripodal N2O2 tetradentate ligands: Synthesis, structures and catalytic activities towards olefin epoxidation

Article abstract of DOI:10.1039/b926864b

A series of N₂O₂ tripodal tetradentate ligands derived from di-/tetra-tert-butyl substituted 2-[bis(2-hydroxybenzyl) aminomethyl]X (X = pyridine and benzimidazole) (H₂Lⁿ (n = 1-4)) and 8-[bis(3,5-di-tert-butyl-2-hydroxybenzyl)]aminoquinoline (H ₂L⁵) were synthesised through a 4-step reaction scheme involving sequential formylation, reduction, bromination and alkylation. Treatment of H₂Lⁿ (n = 1-5) with [WO₂Cl ₂(dme)] (dme = 1,2-dimethoxyethane) in the presence of triethylamine gave the corresponding cis-dioxotungsten(vi) complexes [WO₂(L ⁿ)] (n = 1-5). The corresponding molybdenum analogues [MoO ₂(Lⁿ)] (n = 1-5) were also prepared from the reaction of [MoO₂(acac)₂] (acac = acetylacetonate) with H ₂Lⁿ (n = 1-3) or [MoO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) with H₂Lⁿ (n = 4 and 5). All these compounds were fully characterised by a wide range of spectroscopic methods. The molecular structures of [MoO₂(Lⁿ)] (n = 2, 4) and [WO₂(L²)] were also confirmed by single-crystal X-ray diffraction analysis. The catalytic activities of [MO₂(L ⁿ)] (M = Mo, W; n = 1-4) towards epoxidation of styrene were also examined.

Full text of DOI:10.1039/b926864b

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PAPER
www.rsc.org/dalton | Dalton Transactions
New dioxo–molybdenum(VI) and –tungsten(VI) complexes with N-capped
tripodal N2O2 tetradentate ligands: Synthesis, structures and catalytic
activities towards olefin epoxidation†
Yee-Lok Wong,a,b Lok H. Tong,b,c Jonathan R. Dilworth,*b Dennis K. P. Ng*a and Hung Kay Lee*a
Received 22nd December 2009, Accepted 15th March 2010
First published as an Advance Article on the web 12th April 2010
DOI: 10.1039/b926864b
A series of N2O2 tripodal tetradentate ligands derived from di-/tetra-tert-butyl substituted
2-[bis(2-hydroxybenzyl)aminomethyl]X (X = pyridine and benzimidazole) (H2Ln (n = 1–4)) and
8-[bis(3,5-di-tert-butyl-2-hydroxybenzyl)]aminoquinoline (H2L5) were synthesised through a 4-step
reaction scheme involving sequential formylation, reduction, bromination and alkylation. Treatment of
H2Ln (n = 1–5) with [WO2Cl2(dme)] (dme = 1,2-dimethoxyethane) in the presence of triethylamine gave
the corresponding cis-dioxotungsten(VI) complexes [WO2(Ln)] (n = 1–5). The corresponding
molybdenum analogues [MoO2(Ln)] (n = 1–5) were also prepared from the reaction of [MoO2(acac)2]
(acac = acetylacetonate) with H2Ln (n = 1–3) or [MoO2Cl2(dme)] (dme = 1,2-dimethoxyethane) with
H2Ln (n = 4 and 5). All these compounds were fully characterised by a wide range of spectroscopic
methods. The molecular structures of [MoO2(Ln)] (n = 2, 4) and [WO2(L2)] were also confirmed by
single-crystal X-ray diffraction analysis. The catalytic activities of [MO2(Ln)] (M = Mo, W; n = 1–4)
towards epoxidation of styrene were also examined.
Introduction
Metal complexes supported by N-capped tripodal tetradentate
ligands have attracted considerable attention over past decades.
These are highly versatile ligands and their steric and elec-
tronic requirements can readily be tuned by incorporation of
appropriate functionalised pendant arms onto the bridgehead
nitrogen. Using this approach, a variety of tripodal ligands with
different combinations of neutral and anionic N-, O-, P- or
S-donor sets have been developed.1 The ligands containing an
amine-bis(phenol) core with an appended nitrogen-donor pendant
arm (Fig. 1) can provide a N2O2 coordination environment for
metal complexes for both bioinorganic and catalytic studies. For
example, Cu(II) complexes supported by tripodal N2O2-donor
ligands were reported as functional mimics for galactose oxidase.2
In addition, a few Fe(III) complexes containing N-capped tripodal
tetradentate ligands with two phenolate and one pyridyl pendant
arms were reported as synthetic analogues for catechol 1,2-
dioxygenases.3 The use of tetradentate amine-bis(phenol) type
tripodal ligands for the preparation of metal-based catalysts has
Fig. 1 Structure of N-capped tripodal N2O2 type tetradentate ligands
containing an amine-bis(phenol) core appended with a nitrogen-donor
arm. (R and R1 are substituents on the aryl rings).
also been documented. A range of main group and transition metal
complexes supported by this type of ligands have been studied for
the ring-opening polymerisation of lactides4 and lactones,5 as well
as polymerisation6 and co-polymerisation of alkenes.7
High-valent dioxo–molybdenum(VI) and –tungsten(VI) com-
plexes belong to a fascinating class of compounds due to their bi-
ological relevance to molybdenum and tungsten oxotransferases,8
as well as their catalytic properties towards oxygen-atom-transfer
(OAT) reactions,9 olefin metathesis10 and oxidation of alcohols.11
Moreover, their epoxidation activities have been a focus of
attention in the development of efficient catalysts for production
of useful epoxides in industry.12 A number of epoxidation-
active dioxo–molybdenum(VI)13,14 and –tungsten(VI)15,16 complexes
supported by bidentate and tridentate ligands have been reported.
By contrast, the use of tripodal tetradentate ligands in this area
is rare,17,18 an exception being the use of Salen-type Schiff base
ligands.19
We have a long-standing interest in the structures and cat-
alytic properties of dioxo–molybdenum(VI) and –tungsten(VI)
complexes ligated by various [N,O] donor ligands.16,18,20 Previ-
ously, we have reported the preparation and OAT activities of
dioxo–molybdenum(VI) and –tungsten(VI) complexes supported
by unsubstituted 2-[bis(2-hydroxybenzyl)aminomethyl]pyridine17
aDepartment of Chemistry, The Chinese University of Hong Kong, Shatin,
New Territories, Hong Kong SAR, People’s Republic of China. E-mail:
hklee@cuhk.edu.hk, dkpn@cuhk.edu.hk; Tel: +852 2609 6331, +852 2609
6375
bChemistry Research Laboratory, Department of Chemistry, University
of Oxford, 12 Mansfield Road, Oxford, UK OX1 3TA. E-mail: jon.
dilworth@chem.ox.ac.uk; Tel: +44 (0) 1865 272 689
cSection of Chemistry and Biochemistry, University of Geneva, 30 Quai
Ernest-Ansermet, 1211, Geneva, Switzerland
† Electronic supplementary information (ESI) available: Synthetic pro-
cedures for mono- and di-tert-butyl substituted 2-hydroxybenzaldehydes
and 2-hydroxybenzyl bromides, and molecular structure of [WO2(L2)]
with atom labelling. CCDC reference numbers 759487–759489. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b926864b
4602 | Dalton Trans., 2010, 39, 4602–4611
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Scheme 1 Synthesis of the N2O2 tetradentate ligands H2Ln (n = 1–5).
and the unsymmetrical nitro-substituted derivatives.18 Herein we
report on a series of dioxo–molybdenum(VI) and –tungsten(VI)
complexes supported by a new family of N-capped tripodal
tetradentate ligands carrying a N2O2 donor set (L1–L5, Scheme 1).
The catalytic properties of the present dioxo complexes towards
epoxidation of styrene are also presented.
were performed in air. Dichloromethane was pre-dried over
˚
4 A molecular sieves and distilled from calcium hydride.
Tetrahydrofuran (THF) and toluene were distilled from sodium-
benzophenone. tert-Butyl hydroperoxide (TBHP) in decane
˚
4 A molecular
(5–6 M) was pre-dried by storing over
sieves. [MoO2Cl2(dme)] (dme = 1,2-dimethoxyethane),21 [WO2Cl2-
(dme)],22 2-(aminomethyl)benzimidazole dihydrochloride23 and
2-(3,5-di-tert-butyl-2-hydroxybenzyl)aminomethylpyridine18 were
prepared as described previously with minor modification. Mono-
and di-tert-butyl substituted 2-hydroxybenzaldehydes and 2-
hydroxybenzyl bromides were prepared according to published
procedures (see also Supplementary Information†).24 All other
reagents and solvents were of reagent grade and used as received.
The ligand precursors H2L1–H2L5 were readily synthesised by
a four-step procedure, which provided a good alternative to the
direct condensation of aldehydes and amines, the latter being a
common method for the preparation of tetradentate tripods.1-3
The new ligands L1–L5 share a similar amine-bis(phenol) core
but are distinguished by a different N-heterocyclic pendant side
arm (namely a pyridyl, benzimidazolyl or quinolinyl pendant
group). Compared to other N2O2 tripods reported previously,17,18
the symmetrical Ln (n = 1–5) ligands employed in this work contain
two or four tert-butyl substituents, which greatly enhance the
solubility of the corresponding dioxo metal complexes in common
organic solvents. This modification in ligand design is crucial
in OAT reactivity as reactions for highly selective epoxidation
catalysts are usually carried out in non-polar organic solvents
under mild reaction conditions. Moreover, it was also expected that
the incorporation of different N-donor pendant groups on L1–L5
may modify the epoxidation reactivity of the corresponding dioxo
complexes.
1
1H and 13C{ H} NMR spectra were recorded on Bruker DPX 300
or Varian Mercury VX300 spectrometer (1H, 300; 13C, 75.4 MHz).
Chemical shifts are reported relative to internal SiMe4 (d = 0).
IR spectra were recorded on Nicolet Magna 550 or Perkin-
Elmer 1710 FT-IR spectrophotometer as KBR pellets. Electron
impact (EI) mass spectra were obtained on a Hewlett-Packard
5989B mass spectrometer at 70 eV. Atmospheric pressure chemical
ionisation (APCI) mass 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 cyclotron resonance (FT-ICR) mass spectrometer
with 3-nitrobenzyl alcohol as matrix. GC-MS analyses were
carried out on a Hewlett Packard GC(HP 6890)–MS(HP 5973)
analyser equipped with a HP-5MS (5% phenylmethylsiloxane)
column. Elemental analyses were performed by MEDAC Ltd.,
Brunel University, UK or the microanalysis laboratory of the
Inorganic Chemistry Laboratory, University of Oxford, UK.
Experimental
General procedures
All the reactions were carried out using standard Schlenk-
line techniques under an atmosphere of dinitrogen; workups
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Preparations
7.8 Hz, 1 H, PyH), 6.92 (d, J = 7.2 Hz, 2 H, ArH), 6.72 (t, J =
7.5 Hz, 2 H, ArH), 3.77 (s, 6 H, ArCH2 and PyCH2), 1.40 (s, 18 H,
2-(3-tert-Butyl-2-hydroxybenzyl)aminomethylpyridine. This
compound was synthesised from 3-tert-butyl-2-hydroxybenzal-
dehyde (1.07 g, 6.0 mmol), 2-aminomethylpyridine (0.65 g,
6.0 mmol), and NaBH4 (0.23 g, 6.0 mmol) in MeOH (50 cm3) using
a procedure similar to that for the preparation of 2-(3,5-di-tert-
butyl-2-hydroxybenzyl)-aminomethylpyridine.18 The compound
was isolated as a pale yellow liquid. Yield: 1.59 g (98%). 1H NMR
(CDCl3): d 8.59 (dd, J = 1.8, 5.4 Hz, 1 H, ArH), 7.70 (dt, J =
1.8, 7.7 Hz, 1 H, ArH), 7.20–7.27 (m, 3 H, ArH), 6.86 (d, J =
7.5 Hz, 1 H, ArH), 6.74 (t, J = 7.7 Hz, 1 H, ArH), 4.00 (s, 2 H,
1
tBu). 13C{ H} NMR (CDCl3): d 156.2, 156.0, 148.0, 137.3, 137.2,
128.5, 126.4, 123.3, 122.4, 122.1, 118.3, 56.4, 55.2, 34.8, 29.5. MS
(EI): m/z 432 (100%) [M]+.
2 - [Bis(5 - tert - butyl - 2 - hydroxybenzyl)aminomethyl] - pyridine
(H2L3). This compound was synthesised from 2-(5-tert-butyl-2-
hydroxybenzyl)aminomethylpyridine (1.03 g, 3.8 mmol), 5-tert-
butyl-2-hydroxybenzyl bromide (0.92 g, 3.8 mmol), and triethy-
lamine (0.53 cm3, 3.8 mmol) in THF (60 cm3) using a similar
procedure as described for the preparation of H2L1. The product
was isolated as a sticky white solid. Yield: 1.28 g (78%). 1H NMR
(CDCl3): d 8.65 (dd, J = 0.9, 5.1 Hz, 1 H, PyH), 7.69 (dt, J = 1.5,
7.7 Hz, 1 H, PyH), 7.26 (dt, J = 0.9, 6.5 Hz, 1 H, PyH), 7.20 (dd,
J = 2.4, 8.4 Hz, 2 H, ArH), 7.15 (d, J = 7.8 Hz, 1 H, PyH), 7.06 (d,
J = 2.4 Hz, 2 H, ArH), 6.83 (d, J = 8.7 Hz, 2 H, ArH), 3.91 (s, 2 H,
ArCH2), 3.95 (s, 2 H, ArCH2), 1.44 (s, 9 H, tBu). 13C{ H} NMR
1
(CDCl3): d 157.2, 156.9, 148.8, 137.2, 137.0, 127.0, 126.1, 123.2,
122.7, 122.3, 118.4, 52.3, 51.7, 34.6, 29.5. HRMS (LSI): m/z calc.
for C17H23N2O [M + H]+ 271.1810, found 271.1799.
2-(5-tert-Butyl-2-hydroxybenzyl)aminomethylpyridine. This
compound was synthesised from 5-tert-butyl-2-hydroxybenzal-
dehyde (0.93 g, 5.2 mmol), 2-aminomethylpyridine (0.56 g,
5.2 mmol), and NaBH4 (0.20 g, 5.2 mmol) in MeOH (50 cm3)
using a procedure similar to that for the preparation of 2-(3,5-
di-tert-butyl-2-hydroxybenzyl)-aminomethylpyridine. The com-
pound was isolated as a pale yellow liquid. Yield: 1.38 g (98%).
1H NMR (CDCl3): d 8.59 (d, J = 4.5 Hz, 1 H, ArH), 7.67 (dt,
J = 1.8, 7.7 Hz, 1 H, ArH), 7.18–7.24 (m, 3 H, ArH), 6.97 (d, J =
2.4 Hz, 1 H, ArH), 6.79 (d, J = 8.4 Hz, 1 H, ArH), 4.00 (s, 2 H,
1
PyCH2), 3.83 (s, 4 H, ArCH2), 1.28 (s, 18 H, tBu). 13C{ H} NMR
(CDCl3): d 155.9, 154.8, 148.1, 141.7, 137.5, 127.0, 126.2, 123.6,
122.6, 120.7, 116.4, 56.6, 55.7, 33.9, 31.6. HRMS (LSI): m/z calc.
for C28H37N2O2, [M + H]+ 433.2855, found 433.2840.
2-[Bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminomethyl]-benzi-
midazole (H2L4). To a pink suspension of 2-(aminomethyl)-
benzimidazole dihydrochloride (0.66 g, 3.0 mmol) in THF
(100 cm3) was added
a solution of 3,5-di-tert-butyl-2-
hydroxybenzyl bromide (1.79 g, 6.0 mmol) in THF (50 cm3)
under N2. The yellowish brown mixture was stirred for 5 min
before triethylamine (1.7 cm3, 12.0 mmol) was added. The mixture
was then heated under reflux overnight to give an orange-
red suspension, which was cooled and filtered. The filtrate was
concentrated and chromatographed using CH2Cl2 as eluent. The
second band was collected and evaporated to give the product
ArCH2), 3.95 (s, 2 H, ArCH2), 1.27 (s, 9 H, tBu). 13C{ H} NMR
1
(CDCl3): d 157.1, 155.3, 149.1, 141.6, 136.7, 125.6, 125.5, 122.6,
122.4, 121.2, 115.7, 52.6, 51.6, 33.8, 31.4. HRMS (LSI): m/z calc.
for C17H23N2O [M + H]+ 271.1810, found 271.1791.
2-[Bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminomethyl]-pyridine
(H2L1). A solution of 3,5-di-tert-butyl-2-hydroxybenzyl bromide
(0.90 g, 3.0 mmol) in THF (25 cm3) was added to a solution of 2-
(3,5-di-tert-butyl-2-hydroxybenzyl)aminomethylpyridine (0.97 g,
3.0 mmol) in THF (25 cm3) under N2. The pale yellow mixture was
stirred at room temperature for 5 min, followed by the addition of
triethylamine (0.42 cm3, 3.0 mmol). The mixture was then heated
under reflux for 8 h to give a pale brown suspension, which
was cooled and filtered. The filtrate was rotary evaporated and
chromatographed using CHCl3 as eluent. The second band was
collected and evaporated to give ◦the product as a white solid.
◦
1
as a white solid. Yield: 0.73 g (42%). Mp 130–132 C. H NMR
(CDCl3): d 10.37 (bs, 2 H, ArOH), 8.99 (bs, 1 H, NH), 7.87 (d, J =
6.6 Hz, 1 H, ArH), 7.31–7.39 (m, 3 H, ArH), 7.28 (d, J = 2.4 Hz,
2 H, ArH), 6.98 (d, J = 2.4 Hz, 2 H, ArH), 3.89 (s, 2 H, ArCH2),
1
3.84 (s, 4 H, ArCH2), 1.45 (s, 18 H, tBu), 1.32 (s, 18 H, tBu). 13C{ H}
NMR (CDCl3): d 153.6, 149.4, 141.3, 137.1, 125.0, 124.0, 123.6,
122.9, 122.6, 121.5, 119.5, 110.7 (1 peak overlapping), 56.1, 46.9,
35.1, 34.2, 31.7, 29.8. HRMS (LSI): m/z calc. for C38H54N3O2,
[M + H]+ 584.4216, found 584.4251.
1
Yield: 1.36 g (83%). Mp 175–178 C. H NMR (CDCl3): d 8.73
8 - [Bis(3,5 - di - tert - butyl - 2 - hydroxybenzyl)]amino - quinoline
(H2L5). To a suspension of 8-aminoquinoline (0.29 g, 2.0 mmol)
in THF (50 cm3) was added a solution of 3,5-di-tert-butyl-2-
hydroxybenzyl bromide (1.20 g, 4.0 mmol) in THF (30 cm3)
under N2. The yellowish brown mixture was stirred for 5 min
before triethylamine (0.56 cm3, 4.0 mmol) was added. The mixture
was then heated under reflux overnight to give a yellowish-
brown suspension, which was cooled and filtered. The filtrate was
concentrated and chromatographed using CH2Cl2 as eluent. The
second band was collected and evaporated to give the product as
a pale yellow solid. Yield: 0.77 g (66%). Mp 200–202 ◦C. 1H NMR
(CDCl3): d 10.65 (s, 2 H, ArOH), 8.97 (dd, J = 1.5, 4.4 Hz, 1 H,
ArH), 8.18 (dd, J = 1.5, 8.3 Hz, 1 H, ArH), 7.40–7.50 (m, 4 H,
ArH), 7.19 (d, J = 2.4 Hz, 2 H, ArH), 7.05 (d, J = 2.4 Hz, 2 H,
(d, J = 4.5 Hz, 1 H, PyH), 7.68 (dt, J = 1.7, 7.7 Hz, 1 H, PyH),
7.22–7.28 (m, 3 H, ArH and PyH), 7.11 (d, J = 8.1 Hz, 1 H, PyH),
6.94 (d, J = 2.4 Hz, 2 H, ArH), 3.85 (s, 2 H, PyCH2), 3.81 (s, 4
1
H, ArCH2), 1.40 (s, 18 H, tBu), 1.29 (s, 18 H, tBu). 13C{ H} NMR
(CDCl3): d 156.2, 153.8, 148.1, 140.4, 137.3, 136.3, 125.1, 123.7,
123.4, 122.4, 121.2, 56.8, 55.3, 35.1, 34.1, 31.7, 29.6. HRMS (LSI):
m/z calc. for C36H53N2O2, [M + H]+ 545.4107, found 545.4103.
2 - [Bis(3 - tert - butyl - 2 - hydroxybenzyl)aminomethyl] - pyridine
(H2L2). This compound was synthesised from 2-(3-tert-butyl-2-
hydroxybenzyl)aminomethylpyridine (1.22 g, 4.5 mmol), 3-tert-
butyl-2-hydroxybenzyl bromide (1.09 g, 4.5 mmol), and triethy-
lamine (0.63 cm3, 4.5 mmol) in THF (60 cm3) using a similar
procedure as described for the preparation of H2L1. The product
was obtained as a white solid. Yield: 1.56 g (80%). Mp 45–48 ◦C.
1H NMR (CDCl3): d 8.63 (d, J = 4.8 Hz, 1 H, PyH), 7.57 (t, J =
7.7 Hz, 1 H, PyH), 7.15–7.21 (m, 3 H, PyH and ArH), 6.99 (d, J =
t
ArH), 4.43 (bs, 4 H, ArCH2), 1.38 (s, 18 H, Bu), 1.33 (s, 18 H,
1
tBu). 13C{ H} NMR (CDCl3): d 153.6, 147.7, 144.9, 142.2, 140.0,
137.8, 136.0, 129.5, 126.8, 125.3, 123.4, 123.1, 121.2, 120.9, 120.6,
4604 | Dalton Trans., 2010, 39, 4602–4611
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57.5, 35.1, 34.1, 31.8, 29.6. MS (APCI): m/z 581 (5%) [M + H]+.
Anal. calc. for C39H52N2O2: C, 80.6; H, 9.0; N, 4.8%. Found: C,
80.8; H, 8.9; N, 4.8%.
m/z calc. for C28H35MoN2O4 [M + H]+ 561.1651, found 561.1688.
Anal. calc. for C28H34MoN2O4: C, 60.2; H, 6.1; N, 5.0%. Found:
C, 59.8; H, 6.2; N, 4.8%.
General procedure for the preparation of [MoO2(Ln)] (n = 1–
3). To a colourless solution of H2Ln (n = 1–3) in MeOH was
added [MoO2(acac)2] (1 equiv.) and stirring was continued for
6 h. The resulting orange-yellow solid was collected by filtration,
redissolved in a minimum amount of CHCl3 and loaded onto
a silica gel column then eluted using CHCl3. The first intense
orange band was collected. Removal of the solvent gave the desired
product as an orange-yellow solid.
[MoO2(L4)]. To a colourless solution of [MoO2Cl2(dme)]
(0.58 g, 2.0 mmol) in CH2Cl2 (20 cm3) was added a solution of H2L4
(1.17 g, 2.0 mmol) in CH2Cl2 (30 cm3), followed by triethylamine
(0.55 cm3, 4.0 mmol). After heating under reflux overnight, the
dark brown mixture was loaded onto a silica gel column using
first CH2Cl2, followed by ethyl acetate as eluents. The second
orange band was collected and evaporated to give the product
as an orange-yellow solid. Yield: 1.08 g (76%). 1H NMR (acetone-
d6): d 8.31 (d, J = 6.9 Hz, 1 H, ArH), 7.11–7.33 (m, 7 H, ArH),
5.02 (d, J = 12.9 Hz, 2 H, ArCH2), 4.22 (s, 2 H, ArCH2), 3.98
(d, J = 12.9 Hz, 2 H, ArCH2), 1.23 (s, 18 H, tBu), 1.12 (s, 18 H,
[MoO2(L1)]. According to the general procedure, [MoO2-
(acac)2] (0.33 g, 1.0 mmol) was treated with H2L1 (0.54 g, 1.0 mmol)
1
in MeOH (30 cm3) to give [MoO2(L1)] (0.56 g, 83%). H NMR
1
tBu). 13C{ H} NMR (acetone-d6): d 160.2, 153.3, 142.2, 140.0,
(CDCl3): d 9.12 (d, J = 4.9 Hz, 1 H, PyH), 7.51 (dt, J = 1.2,
7.7 Hz, 1 H, PyH), 7.10–7.14 (m, 3 H, ArH and PyH), 6.94 (d, J =
2.2 Hz, 2 H, ArH), 6.75 (d, J = 7.8 Hz, 1 H, PyH), 4.99 (d, J =
13.1 Hz, 2 H, ArCH2), 3.98 (s, 2 H, PyCH2), 3.77 (d, J = 13.1 Hz, 2
138.3, 134.5, 125.8, 124.6, 124.2, 123.4, 123.3, 119.7, 112.2, 63.5,
51.2, 35.4, 34.7, 31.9, (one peak overlapped with solvent peak).
IR (cm-1): 2960 m, 2904w, 2870w, 1476 s, 1458 s, 1390w, 1362 m,
1310w, 1263 s, 1241 s, 1205 m, 1170 m, 1132w, 1053w, 932 m, 915 s
n(MoO2), 886 s n(MoO2), 849 s, 809w, 746 m, 559m. MS (LSI):
m/z 712 [M + H]+. Anal. calc. for C38H51MoN3O4: C, 64.3; H, 7.2;
N, 5.9%. Found: C, 64.7; H, 7.9; N, 6.1%.
1
H, ArCH2), 1.28 (s, 18 H, tBu), 1.24 (s, 18 H, tBu). 13C{ H} NMR
(CDCl3): d 159.3, 155.3, 151.3, 142.1, 139.0, 137.5, 123.9, 123.7,
122.6, 121.5, 121.1, 63.7, 59.2, 34.9, 34.1, 31.6, 29.8. IR (cm-1):
2952 s, 2902 m, 2867 m, 1606w, 1472 s, 1442 s, 1363w, 1258 s,
1169 m, 905 s n(MoO2), 845 s, 757 m, 558 m, 479w. HRMS (LSI):
m/z calc. for C36H51MoN2O4 [M + H]+ 673.2903, found 673.2896.
Anal. calc. for C36H50MoN2O4: C, 64.5; H, 7.5; N, 4.2%. Found:
C, 64.6; H, 7.6; N, 4.1%.
[MoO2(L5)]. To a colourless solution of [MoO2Cl2(dme)]
(0.29 g, 1.0 mmol) in CH2Cl2 (20 cm3) was added a solution of H2L5
(0.58 g, 1.0 mmol) in CH2Cl2 (30 cm3), followed by triethylamine
(0.28 cm3, 2.0 mmol). After heating under reflux overnight, the
dark brown mixture was loaded onto a silica gel column using
first CH2Cl2, followed by ethyl acetate as eluents. The second
orange band was collected and evaporated to give the product
as an orange-yellow solid. Yield: 0.55 g (78%). 1H NMR (CDCl3):
d 9.41 (dd, J = 1.5, 4.7 Hz, 1 H, ArH), 8.06 (dd, J = 1.5, 8.3 Hz, 1
H, ArH), 7.80 (dd, J = 1.8, 7.2 Hz, 1 H, ArH), 7.35–7.52 (m, 3 H,
ArH), 6.85 (d, J = 2.7 Hz, 2 H, ArH), 6.56 (d, J = 2.1 Hz, 2 H,
ArH), 5.69 (d, J = 12.6 Hz, 2 H, ArCH2), 4.25 (d, J = 12.6 Hz,
[MoO2(L2)]. According to the general procedure, [MoO2-
(acac)2] (0.26 g, 0.8 mmol) was treated with H2L2 (0.35 g, 0.8 mmol)
1
in MeOH (30 cm3) to give [MoO2(L2)] (0.42 g, 93%). H NMR
(CDCl3): d 9.20 (d, J = 4.8 Hz, 1 H, PyH), 7.61 (dt, J = 1.8,
7.7 Hz, 1 H, PyH), 7.15–7.23 (m, 3 H, ArH and PyH), 7.02 (dd,
J = 1.2, 7.5 Hz, 2 H, ArH), 6.88 (d, J = 7.8 Hz, 1 H, PyH), 6.75
(t, J = 7.5 Hz, 2 H, ArH), 5.07 (d, J = 13.2 Hz, 2 H, ArCH2),
4.05 (s, 2 H, PyCH2), 3.85 (d, J = 13.2 Hz, 2 H, ArCH2), 1.33 (s,
t
t
1
2 H, ArCH2), 1.14 (s, 18 H, Bu), 1.05 (s, 18 H, Bu). 13C{ H}
NMR (CDCl3): d 157.2, 153.0, 142.8, 142.4, 141.5, 139.0, 137.6,
128.1, 126.7, 126.5, 124.6, 124.1, 123.3, 121.6, 121.3, 64.6, 34.7,
34.0, 31.4, 29.8. IR (cm-1): 2954 s, 2867w, 1511 m, 1478 s, 1442
m, 1415w, 1391w, 1362w, 1321w, 1261 s, 1240 s, 1204w, 1170 m,
1130w, 1094w, 1021w, 903 s n(MoO2), 853 s, 790w, 756 m, 715w,
556 m, 492w, 471w. MS (APCI): m/z 709 (70%) [M + H]+. Anal.
calc. for C39H50MoN2O4: C, 66.3; H, 7.1; N, 4.0%. Found: C, 66.5;
H, 7.5; N, 3.7%.
1
18 H, tBu). 13C{ H} NMR (CDCl3): d 161.3, 155.1, 151.3, 139.4,
138.7, 127.5, 126.9, 122.7, 122.4, 121.2, 119.8, 63.4, 58.7, 34.7,
29.7. IR (cm-1): 2957 m, 2910w, 2867w, 1606 m, 1586w, 1429 s,
1390 m, 1357w, 1266 s, 1244 s, 1188 m, 1146w, 1093w, 1056w,
1022w, 976w, 922 s n(MoO2), 903 s n(MoO2), 875 s, 807w, 787w,
753 s, 686 m, 646w, 576w, 502w, 454w. HRMS (LSI): m/z calc. for
C28H35MoN2O4 [M + H]+ 561.1651, found 561.1651. Anal. calc.
for C28H34MoN2O4: C, 60.2; H, 6.1; N, 5.0%. Found: C, 59.8; H,
6.2; N, 4.8%.
General procedure for the preparation of [WO2(Ln)] (n = 1–3).
To a colourless solution of [WO2Cl2(dme)] in CH2Cl2 was added
a solution of H2Ln (n = 1–3, 1 equiv) in CH2Cl2 followed by
triethylamine (2 equiv). The resulting mixture was heated under
relux overnight. All the volatiles were then removed in vacuo. The
dark brown crude product was purified by column chromatogra-
phy using CHCl3 as eluent. The first UV-active chromophore was
collected and evaporated to give the desired products as a white
solid.
[MoO2(L3)]. According to the general procedure, [MoO2-
(acac)2] (0.46 g, 1.4 mmol) was treated with H2L3 (0.61 g, 1.4 mmol)
1
in MeOH (30 cm3) to give [MoO2(L3)] (0.61 g, 78%). H NMR
(CDCl3): d 9.19 (d, J = 5.1 Hz, 1 H, PyH), 7.53 (dt, J = 1.2,
7.7 Hz, 1 H, PyH), 7.22 (t, J = 6.4 Hz, 1 H, PyH), 7.04–7.08 (m, 4
H, ArH), 6.74 (d, J = 7.7 Hz, 1 H, PyH), 6.56 (d, J = 8.2 Hz, 2 H,
ArH), 4.87 (d, J = 13.2 Hz, 2 H, ArCH2), 3.95 (s, 2 H, PyCH2),
t
1
3.78 (d, J = 13.2 Hz, 2 H, ArCH2), 1.24 (s, 18 H, Bu). 13C{ H}
NMR (CDCl3): d 160.9, 155.1, 150.9, 143.4, 139.2, 126.3, 125.7,
123.5, 122.2, 121.4, 117.5, 63.0, 59.5, 34.0, 31.5. IR (cm-1): 2958
m, 2901w, 2866w, 1607 m, 1495 s, 1460 m, 1443 m, 1385w, 1363w,
1270 s, 1204w, 1188w, 1130 m, 1098w, 1056w, 1024w, 968w, 916 s
n(MoO2), 900 s n(MoO2), 872 m, 850 m, 839 s, 827 m, 793 m, 765w,
728w, 701w, 679w, 647w, 597w, 552 m, 520w, 457w. HRMS (LSI):
[WO2(L1)]. According to the general procedure, [WO2Cl2-
(dme)] (0.38 g, 1.0 mmol) was treated with H2L1 (0.54 g, 1.0 mmol)
and triethylamine (0.28 cm3, 2.0 mmol) in CH2Cl2 (40 cm3) to give
[WO2(L1)] (0.67 g, 88%). 1H NMR (CDCl3): d 9.15 (d, J = 5.4 Hz,
1 H, PyH), 7.53 (dt, J = 1.5, 7.7 Hz, 1 H, PyH), 7.10–7.15 (m, 3 H,
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ArH and PyH), 6.92 (d, J = 2.3 Hz, 2 H, ArH), 6.74 (d, J =
7.8 Hz, 1 H, PyH), 5.16 (d, J = 13.1 Hz, 2 H, ArCH2), 4.09 (s, 2
H, PyCH2), 3.78 (d, J = 13.1 Hz, 2 H, ArCH2), 1.28 (s, 18 H, tBu),
137.7, 133.4, 125.2, 124.0, 123.3, 122.6, 118.3, 112.0 (one peak
overlapping), 62.0, 50.4, 34.5, 34.0, 31.6, 29.8. IR (cm-1): 2957 s,
1477 s, 1457 s, 1362 m, 1266 s, 1241 s, 1204 m, 1170 m, 1132w,
1055w, 938 m n(WO2), 915 m n(WO2), 884 s, 864 s, 749 m, 559 m,
473w. MS (LSI): m/z 798 [M + H]+. Anal. calc. for C38H51N3O4W:
C, 57.2; H, 6.4; N, 5.3%. Found: C, 57.3; H, 6.4; N, 5.3%.
1
1.23 (s, 18 H, tBu). 13C{ H} NMR (CDCl3): d 157.2, 156.0, 152.0,
142.6, 139.4, 138.5, 124.1, 123.8, 122.8, 121.4, 120.9, 64.0, 59.4,
34.8, 34.1, 31.6, 29.8. IR (cm-1): 2957 s, 2903 m, 2866 m, 1607 m,
1475 s, 1445 s, 1359w, 1308w, 1254 s, 1205w, 1168 m, 1130w, 941 s
n(WO2), 908 s n(WO2), 855 s, 761 m, 558 m, 480w. HRMS (LSI):
m/z calc. for C36H51N2O4W [M + H]+ 759.3358, found 759.3308.
Anal. calc. for C36H50N2O4W: C, 57.0; H, 6.6; N, 3.7%. Found: C,
57.0; H, 6.7; N, 3.7%.
[WO2(L5)]. To a colourless solution of [WO2Cl2(dme)] (0.38 g,
1.0 mmol) in CH2Cl2 (20 cm3) was added a solution of H2L5
(0.58 g, 1.0 mmol) in CH2Cl2 (30 cm3) followed by triethylamine
(0.28 cm3, 2.0 mmol). After heating under reflux overnight, the
dark brown mixture was loaded onto a silica gel column using first
CH2Cl2, followed by ethyl acetate as eluents. The second UV-active
chromophore was collected and evaporated to give the product as
a white solid. Yield: 0.60 g (75%). 1H NMR (CDCl3): d 9.50 (dd,
J = 1.8, 4.7 Hz, 1 H, ArH), 8.12 (dd, J = 1.5, 8.4 Hz, 1 H, ArH),
7.81 (dd, J = 1.8, 7.4 Hz, 1 H, ArH), 7.41–7.55 (m, 3 H, ArH), 6.90
(d, J = 2.7 Hz, 2 H, ArH), 6.56 (d, J = 2.1 Hz, 2 H, ArH), 5.80 (d,
J = 12.6 Hz, 2 H, ArCH2), 4.28 (d, J = 12.6 Hz, 2 H, ArCH2), 1.15
[WO2(L2)]. According to the general procedure, [WO2Cl2-
(dme)] (0.57 g, 1.5 mmol) was treated with H2L2 (0.65 g, 1.5 mmol)
and triethylamine (0.42 cm3, 3.0 mmol) in CH2Cl2 (50 cm3) to give
[WO2(L2)] (0.80 g, 83%). 1H NMR (CDCl3): d 9.19 (d, J = 4.8 Hz,
1 H, PyH), 7.58 (dt, J = 1.7, 7.7 Hz, 1 H, PyH), 7.13–7.18 (m, 3
H, ArH and PyH), 6.96 (dd, J = 1.4, 7.4 Hz, 2 H, ArH), 6.83 (d,
J = 7.8 Hz, 1 H, PyH), 6.70 (t, J = 7.6 Hz, 2 H, ArH), 5.20 (d, J =
13.2 Hz, 2 H, ArCH2), 4.11 (s, 2 H, PyCH2), 3.82 (d, J = 13.2 Hz,
1
(s, 18 H, tBu), 1.05 (s, 18 H, tBu). 13C{ H} NMR (CDCl3): d 155.3,
1
2 H, ArCH2), 1.26 (s, 18 H, tBu). 13C{ H} NMR (CDCl3): d 159.2,
153.7, 143.0, 142.9, 141.9, 139.5, 138.3, 128.4, 127.2, 126.9, 124.5,
124.4, 123.6, 121.6, 121.5, 64.6, 34.6, 34.0, 31.4, 29.8. IR (cm-1):
2954 s, 2867w, 1511 m, 1478 s, 1442 m, 1415w, 1391w, 1362w,
1321w, 1261 s, 1240 s, 1204w, 1170 m, 1130w, 1094w, 1021w, 903 s
n(WO2), 853 s 809w, 790w, 756 m, 715w, 556 m, 492w, 471w. MS
(APCI): m/z 795 (50%) [M + H]+. Anal. calc. for C39H50N2O4W:
C, 58.9; H, 6.3; N, 3.5%. Found: C, 58.2; H, 6.3; N, 3.5%.
155.9, 152.0, 139.9, 139.7, 127.4, 127.3, 122.8, 122.3, 121.1, 120.2,
63.7, 59.0, 34.7, 29.7. IR (cm-1): 2957 m, 2911w, 2867w, 1608 m,
1586 m, 1431 s, 1391w, 1358w, 1323w, 1264 s, 1244 s, 1188 s, 1146w,
1091 m, 1058w, 1026w, 977w, 936 s n(WO2), 900 s n(WO2), 880 s,
821w, 809w, 784w, 749 s, 688 m, 664w, 648w, 579w, 565w, 508w,
456w, 443w, 429w. MS (LSI): m/z 647 [M + H]+. Anal. calc. for
C28H34N2O4W: C, 52.0; H, 5.3; N, 4.3%. Found: C, 52.2; H, 5.3;
N, 4.3%.
General procedure for epoxidation experiments. In a typical
epoxidation experiment, an ampoule equipped with a Young’s tap
was charged with a mixture of the catalyst (0.01 mmol), tert–
butyl hydroperoxide (0.20 cm3, 1.0 mmol, 5–6 M in decane),
[WO2(L3)]. According to the general procedure, [WO2Cl2-
(dme)] (0.68 g, 1.8 mmol) was treated with H2L3 (0.78 g, 1.8 mmol)
and triethylamine (0.50 cm3, 3.6 mmol) in CH2Cl2 (50 cm3) to give
[WO2(L3)] (1.01 g, 87%). 1H NMR (CDCl3): d 9.17 (d, J = 5.3 Hz,
1 H, PyH), 7.52 (dt, J = 1.5, 7.7 Hz, 1 H, PyH), 7.20 (t, J =
6.4 Hz, 1 H, PyH), 7.05–7.09 (m, 4 H, ArH), 6.71 (d, J = 7.9 Hz,
1 H, PyH), 6.55 (d, J = 8.2 Hz, 2 H, ArH), 5.08 (d, J = 13.4 Hz,
2 H, ArCH2), 4.05 (s, 2 H, PyCH2), 3.84 (d, J = 13.4 Hz, 2 H,
˚
benzophenone (0.0091 g, 0.050 mmol), and 4 A molecular sieves
(ca. 0.05 g) in toluene (3.0 cm3). The mixture was gently warmed
to 35 ◦C under a nitrogen atmosphere for 30 min, followed by
the addition of styrene (0.042 g, 0.40 mmol) and the reaction
mixture was heated at 65 ◦C under nitrogen for 26 h. The resulting
mixture was filtered through a column containing a short bed of
silica gel using i–PrOH/hexane (1 : 5) as eluent to remove all metal
compounds. The amount of styrene oxide in the product mixture
was quantified by GC–MS analysis, using benzophenone as an
internal standard.
t
1
ArCH2), 1.23 (s, 18 H, Bu). 13C{ H} NMR (CDCl3): d 158.7,
155.8, 151.2, 143.8, 139.5, 126.5, 125.6, 123.6, 121.9, 121.3, 118.4,
63.4, 59.5, 34.0, 31.5. IR (cm-1): 2959 m, 2903w, 2868w, 1609 m,
1498 s, 1459w, 1440w, 1388w, 1364w, 1270 s, 1205w, 1130 m, 1096w,
1057w, 1026w, 936 s n(WO2), 901 s n(WO2), 855 m, 843 m, 828 m,
795 m, 768w, 728w, 703w, 681w, 648w, 597w, 552 m, 521w, 460w.
HRMS (LSI): m/z calc. for C28H35N2O4W [M + H]+ 647.2106,
found 647.2060. Anal. calc. for C28H34N2O4W: C, 52.0; H, 5.3; N,
4.3%. Found: C, 52.0; H, 5.2; N, 4.3%.
X-Ray crystallography†
Crystallographic data for complexes [MoO2(L2)], [WO2(L2)] and
[MoO2(L4)]·H2O are provided in Table 1. Single crystals of
[MoO2(L2)] and [WO2(L2)] for X-ray diffraction were grown from
ethyl acetate solutions and suitable crystals of [MoO2(L4)]·H2O
were obtained by slow evaporation of a methanol–CH2Cl2 solution
in air. X-Ray data were collected on a Rigaku RAXIS-IIC
diffractometer at 293 K using graphite-monochromatised Mo-Ka
[WO2(L4)]. To a colourless solution of [WO2Cl2(dme)] (0.57 g,
1.5 mmol) in CH2Cl2 (20 cm3) was added a solution of H2L4
(0.88 g, 1.5 mmol) in CH2Cl2 (30 cm3), followed by triethylamine
(0.42 cm3, 3.0 mmol). After heating under reflux overnight, the
dark brown mixture was loaded onto a silica gel column using
CH2Cl2, followed by ethyl acetate as eluents. The second UV-active
chromophore was collected and evaporated to give the product
˚
radiation (l = 0.71073 A) by taking oscillation photos. Com-
putations were performed using the SHELX-97 PC programme
package on a PC computer. The structures were solved by direct
phase determination and refined by full-matrix least squares with
anisotropic thermal parameters for the non-hydrogen atoms.25
Hydrogen atoms were introduced in their idealised positions and
included in structure factors calculations with assigned isotropic
temperature factors.
1
as a white solid. Yield: 0.67 g (56%). H NMR (acetone-d6): d
8.39 (d, J = 9.3 Hz, 1 H, ArH), 7.13–7.32 (m, 7 H, ArH), 5.20
(d, J = 12.3 Hz, 2 H, ArCH2), 4.33 (s, 2 H, ArCH2), 4.01 (d,
t
J = 12.3 Hz, 2 H, ArCH2), 1.22 (s, 18 H, Bu), 1.10 (s, 18 H,
1
tBu). 13C{ H} NMR (DMSO-d6): d 156.7, 154.5, 141.6, 138.4,
4606 | Dalton Trans., 2010, 39, 4602–4611
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Table 1 Selected crystallographic data for [MoO2(L2)], [WO2(L2)] and [MoO2(L4)]·H2O
[MoO2(L2)]
[WO2(L2)]
[MoO2(L4)]·H2O
Molecular formula
Molecular weight
Colour
C28H34MoN2O4
558.51
yellow block
0.30 ¥ 0.20 ¥ 0.20
Monoclinic
P21/n
8.5617(17)
14.180(3)
25.984(5)
90
90.00(3)
90
3154.5(11)
4
1.176
C28H34N2O4W
646.42
colourless prism
0.20 ¥ 0.20 ¥ 0.20
Orthorhombic
Pbca
20.257(4)
12.703(3)
20.780(4)
90
C38H51MoN3O5
725.76
orange prism
0.40 ¥ 0.10 ¥ 0.10
Monoclinic
P21/c
16.327(3)
10.457(2)
25.688(5)
90
114.18(3)
90
4000.9(14)
4
1.205
0.369
9861
Crystal size/mm
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
3
˚
V/A
5347.2(19)
8
1.606
4.356
12458
Z
Density/g cm-3
m/mm-1
0.445
6273
Reflections collected
Unique data measured
Observed data with I ≥ 2s(I)
No. of parameters
Final R indices [I ≥ 2s(I)]a
4078 (Rint = 0.0639)
4113 (Rint = 0.1555)
5739 (Rint = 0.1102)
3307
3544
4734
317
317
435
R1 = 0.1067
wR2 = 0.2800
R1 = 0.1269
wR2 = 0.2938
R1 = 0.1008
wR2 = 0.2307
R1 = 0.1159
wR2 = 0.2514
R1 = 0.0758
wR2 = 0.2054
R1 = 0.0966
wR2 = 0.2217
R indices (all data)a
ꢀ
ꢀ
ꢀ
ꢀ
a R1 =
ꢀFo| - |Fcꢀ/ |Fo|; wR2 = { [w(Fo - Fc2)2]/ [w(Fo2)2]}
.
2
1/2
lamine afforded H2L1. The same strategy was adopted for
the synthesis of H2L2 and H2L3 by reacting 2-(n-tert-butyl-2-
hydroxybenzyl)aminomethylpyridine (n = 3 or 5 respectively) with
n-tert-butyl-2-hydroxybenzyl bromide (n = 3 or 5 respectively).
The proligands H2Ln (n = 4 and 5) were prepared in a sim-
ilar way by reacting 3,5-di-tert-butyl-2-hydroxybenzyl bromide
(2 equiv.) with 2-(aminomethyl)benzimidazole dihydrochloride
or 8-aminoquinoline (1 equiv.), respectively. With the exception
of H2L1 and H2L2, all the other tetradentate ligands are new
compounds and were fully characterised.29,30
It is worth-noting that the reduction of aldehydes and the
bromination of alcohols were all performed in high yields
(> 95%), and respectable yields were obtained in the N-alkylation
step (42–83%). We believe that this four-step synthetic procedure
is of general utility for the preparation of a range of new
N2O2 tripodal tetradentate ligands, using appropriate phenols and
amines as starting materials.
Results and discussions
Synthesis of ligands
The preparation of the proligands H2Ln (n = 1–5) is shown in
Scheme 1. H2L1–H2L3 contain an identical methylene pyridyl
pendant arm but possess different numbers of tert-butyl groups
on the 2-hydroxybenzyl unit. H2L4 and H2L5 are structurally
related to H2L1, with an identical tetra-tert-butyl-substituted
amine-bis(phenol) core. HL1 possesses a pendant pyridyl group,
whereas H2L4 and H2L5 bear benzimidazole and quinoline groups,
respectively.
The synthesis of H2L1 was first reported by Wong and co-
workers in 1999 via repetitive reductive amination of 3,5-di-tert-
butyl-2-hydroxybenzaldehyde and 2-aminomethylpyridine in 90%
yield.26 Unfortunately, attempts to prepare the related proligands
H2Ln (n = 2–5) using a similar procedure were unsuccessful.
Later, an alternative synthetic route for H2L1 was reported by
Shimazaki and co-workers.27 A modification of the synthetic
procedures for H2Ln (n = 1–5) was developed in this work.
The key step of our new procedure involved N-alkylation of
an appropriate amine with mono- or di-tert-butyl substituted
2-hydroxybenzyl bromides (Scheme 1). The latter compounds
were readily obtained in three steps starting with the respective
substituted phenols.24 Formylation of the phenols was carried
out using tin(IV) chloride, paraformaldehyde and tributylamine
as reagents.28 The resulting aldehydes were then reduced using
NaBH4 to afford the corresponding benzyl alcohols. Subsequent
reaction of the benzyl alcohols with PBr3 in CHCl3 afforded the
desired benzyl bromides. The bromides were freshly prepared
and used without further purification. Treatment of 2-(3,5-
di-tert-butyl-2-hydroxybenzyl)aminomethylpyridine with 3,5-di-
tert-butyl-2-hydroxybenzyl bromide in the presence of triethy-
Synthesis of dioxo–molybdenum(IV) and –tungsten(IV) complexes
The synthesis of the dioxo complexes is summarised in Scheme 2.
The tungsten complexes [WO2(Ln)] (n = 1–5) were prepared in
fair to good yields (56–88%) by the reaction of [WO2Cl2(dme)]
(dme = 1,2-dimethoxyethane) with the appropriate proligands
H2Ln (n = 1–5) in the presence of triethylamine. The advantages
of using [WO2Cl2(dme)] as a reagent are its good solubility in
common organic solvents as well as its versatile ligand substitution
properties.16,18,20 The molybdenum complexes [MoO2(L4)] and
[MoO2(L5)] were prepared in a similar fashion from [Mo2Cl2(dme)]
in 76% and 78% yields, respectively. On the other hand, the
[MoO2(Ln)] (n = 1–3) counterparts were synthesised by the re-
actions of [MoO2(acac)2] (acac = acetylacetonate) with the corre-
sponding H2Ln (n = 1–3) in methanol. All dioxo–molybdenum(IV)
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Scheme 2 Synthesis of the dioxo complexes [MO2(Ln)] (M = Mo, W; n = 1–5).
and –tungsten(IV) complexes were isolated as orange-yellow and
Structural studies
white solids, respectively. All of the complexes are stable to air and
moisture. Purifications of these complexes were readily achieved
by column chromatography.
The synthesis of [MoO2(L1)] and [WO2(L1)] was first reported
by Wong and co-workers in 1999,26 and has recently been reported
by Lehtonen and co-workers, who employed a different synthetic
route.30 All the other dioxo complexes synthesised in this work are
new compounds.
The solid-state structures of complexes [MoO2(L2)], [WO2(L2)]
and [MoO2(L4)]·H2O were established by single-crystal X-ray
diffraction analysis. The molecular structures of the molybdenum
complexes are shown in Fig. 2 and 3, whereas the structure of
the tungsten analogue [WO2(L2)] is illustrated in Figure S1 in
Supplementary Information.† Selected bond lengths and angles
of the three complexes are provided in Table 2.
The metal complexes were characterised with NMR (1H and
13C) and infra-red spectroscopies. All of the tungsten complexes
showed two characteristic cis-WO2 vibrational bands at 900–915
and 936–941 cm-1 in their IR spectra,18,31 except for [WO2(L5)]
of which the spectrum showed only one band at 903 cm-1.
The IR spectra of the molybdenum analogues displayed similar
=
asymmetric and symmetric Mo O stretches at 886–903 and 915–
922 cm-1, respectively.18,31 Exceptions were found in the spectra of
[MoO2(L1)] and [MoO2(L5)], which showed only one band at 905
and 903 cm-1, respectively. The 1H and 13C NMR spectra of these
dioxo complexes were consistent with an octahedral structure
with a mirror plane passing through the two oxo groups and
heterocyclic donor as shown in Scheme 2.18 Upon complexation,
the two methylene protons adjacent to the aromatic ring become
diastereotopic. For example, two doublets at d 5.20 and 4.01, both
with a geminal coupling constant of 12.3 Hz, were observed in the
1H NMR spectrum of [WO2(L4)].
Fig. 2 Molecular structure and atom labelling scheme for [MoO2(L2)].
Hydrogen atoms are omitted for clarity.
All of the three complexes exhibit a distorted octahedral geome-
try, which consists of a cis-dioxo–metal(VI) core coordinated by the
The metal complexes were also characterised by mass spectrom-
etry using either APCI or LSI technique to detect the [M + 1]+
ions. The isotopic distribution of the [M + 1]+ peak was in
good agreement with the corresponding calculated spectrum.
Satisfactory microanalysis data were also obtained for all of the
complexes.
=
corresponding tetradentate N2O2 ligands. The M O distances fall
within the ranges typical for oxomolybdenum17 and oxotungsten32
complexes. The structures of complexes [MoO2(L2)] and [WO2(L2)]
are essentially identical. The pyridyl nitrogen atoms N(1) and the
amino nitrogen N(2) are located trans to the oxo atoms O(3) and
O(4), respectively. The M–N(1) and M–N(2) bond lengths in these
4608 | Dalton Trans., 2010, 39, 4602–4611
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◦
2
2
4
˚
Table 2 Selected bond distances (A) and angles ( ) for [MoO2(L )], [WO2(L )] and [MoO2(L )]·H2O
[MoO2(L2)]
[WO2(L2)]
[MoO2(L4)]·H2O
Mo(1)–N(1)
Mo(1)–N(2)
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
N(1)–Mo(1)–N(2)
N(1)–Mo(1)–O(1)
N(1)–Mo(1)–O(2)
N(1)–Mo(1)–O(3)
N(1)–Mo(1)–O(4)
N(2)–Mo(1)–O(1)
N(2)–Mo(1)–O(2)
N(2)–Mo(1)–O(3)
N(2)–Mo(1)–O(4)
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(4)
O(3)–Mo(1)–O(4)
2.34(1)
2.38(1)
1.947(8)
1.981(9)
1.690(8)
1.692(7)
70.4(3)
81.0(4)
83.7(4)
166.2(4)
85.3(4)
74.5(3)
82.6(3)
95.9(4)
155.5(4)
155.6(3)
95.3(4)
100.2(4)
95.1(4)
97.2(4)
108.5(4)
W(1)–N(1)
W(1)–N(2)
W(1)–O(1)
W(1)–O(2)
W(1)–O(3)
W(1)–O(4)
N(1)–W(1)–N(2)
N(1)–W(1)–O(1)
N(1)–W(1)–O(2)
N(1)–W(1)–O(3)
N(1)–W(1)–O(4)
N(2)–W(1)–O(1)
N(2)–W(1)–O(2)
N(2)–W(1)–O(3)
N(2)–W(1)–O(4)
O(1)–W(1)–O(2)
O(1)–W(1)–O(3)
O(1)–W(1)–O(4)
O(2)–W(1)–O(3)
O(2)–W(1)–O(4)
O(3)–W(1)–O(4)
2.336(7)
2.359(8)
1.933(7)
1.941(6)
1.723(6)
1.723(8)
71.7(3)
82.2(3)
78.8(3)
163.9(3)
89.3(3)
78.6(2)
81.8(2)
92.4(3)
160.9(3)
156.1(2)
97.0(3)
97.2(3)
97.3(3)
96.9(3)
106.7(3)
Mo(1)–N(1)
Mo(1)–N(3)
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
N(1)–Mo(1)–N(3)
N(1)–Mo(1)–O(1)
N(1)–Mo(1)–O(2)
N(1)–Mo(1)–O(3)
N(1)–Mo(1)–O(4)
N(3)–Mo(1)–O(1)
N(3)–Mo(1)–O(2)
N(3)–Mo(1)–O(3)
N(3)–Mo(1)–O(4)
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(4)
O(3)–Mo(1)–O(4)
2.292(5)
2.418(5)
1.917(4)
1.919(4)
1.716(4)
1.715(4)
71.2(1)
82.6(1)
81.0(1)
157.9(2)
94.7(2)
81.0(1)
81.9(1)
86.7(2)
165.9(2)
159.4(2)
96.1(2)
96.8(1)
94.5(2)
96.9(1)
107.4(2)
Table 3 Epoxidation of styrene using [MO2(Ln)] (M = Mo, W; n = 1–4)
as catalystsa
Entry
Catalyst
Yield (%)b
1
2
3
4
5
6
7
8
[MoO2(L1)]
[MoO2(L2)]
[MoO2(L3)]
[MoO2(L4)]
[WO2(L1)]
[WO2(L2)]
[WO2(L3)]
[WO2(L4)]
56
37
48
42
19
35
28
19
Fig. 3 Molecular structure and atom labelling scheme for [MoO2(L4)]·
H2O. Hydrogen atoms and the water solvate molecules are omitted for
clarity.
a Reactions were performed in toluene (3 cm3) with 0.4 mmol of styrene,
2.5 mol% catalyst, and 2.5 equiv. of tert-butyl hydroperoxide under N2 at
65 ◦C for 26 h. b Reaction yields were determined by GC-MS.
˚
two complexes are relatively long (> 2.30 A). This is attributed
to the strong trans effect of the oxo ligands.18 The two phenoxyl
In a typical experiment, styrene (0.40 mmol) was mixed with
oxygen atoms [O(1) and O(2)] of the L2 ligand occupy coordination
2.5 mol% of an appropriate catalyst and 2.5 equiv of tert-butyl
˚
sites trans to each other. The Mo–O(phenoxyl) bond distances in
hydroperoxide (TBHP) in toluene containing 4 A molecular
2
˚
[MoO2(L )] (1.947(8) and 1.981(9) A) are similar to those reported
sieves with benzophenone as an internal standard. The reaction
mixture was heated at 65 ◦C under nitrogen for 26 h (Scheme 3).
The reaction yield of styrene oxide was determined by GC–MS
analysis. As shown in Table 3, all of the dioxo complexes are active
towards oxidation of styrene to styrene oxide, but with moderate
reaction yields which range from 19–56%.36,37 In our hands, higher
yields (37–56%) of styrene oxide were obtained with the dioxo–
molybdenum(VI) complexes, as compared to those of 19–35% for
the dioxo–tungsten(VI) counterparts. Molybdenum(VI) complexes
are generally more active in other catalytic reactions involving
oxo-transfer (such as OAT). This has been ascribed to the less
negative reduction potential of molybdenum(VI) as well as the
for other Mo(VI) alkoxide complexes.33 On the other hand, the W–
O(phenoxyl) bond lengths (1.933(7) and 1.941(6) A) in [WO2(L )]
2
˚
are slightly longer than the corresponding distances reported for
other W(VI) alkoxides.34
Replacement of the pyridine pendant arm in the L2 ligand
by a benzimidazolyl group does not introduce any significant
differences in the bonding parameters of the solvated complex
[MoO2(L4)]·H2O35 as compared to those of [MoO2(L2)]. The
Mo–N(benzimidazolyl) (Mo–N(1)) and Mo–N(amino) (Mo–
N(3)) bond lengths in [MoO2(L4)]·H2O are comparable to the
corresponding distances in the [MoO2(L2)] complex. The Mo–
O(phenoxyl) bond distances in [MoO2(L4)]·H2O are slightly
shorter than those in the [MoO2(L2)] counterpart.
Catalytic studies
The catalytic activities of the dioxo complexes [MO2(Ln)] (M = Mo
and W, n = 1–4) towards epoxidation of styrene were examined.
Scheme 3
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Dalton Trans., 2010, 39, 4602–4611 | 4609
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weaker molybdenum–oxo bond strength as compared to those of
their tungsten(VI) counterparts.16,18,20
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Similar conversion yields for epoxidation of styrene to styrene
oxide (19–52%) were observed with dioxo–molybdenum(VI) and
–tungsten(VI) complexes supported by tridentate N2O-donor
ligands.16 Earlier, we have also reported on the OAT reactiv-
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to benzoin, though their conversion yields were low.38 Herrmann
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A straightforward relationship between the catalytic activities
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and electronic) of the supporting ligands was not discernible. It is
noteworthy that other factors such as solubility of the complexes
may also contribute to a difference in their catalytic behaviour.
The mechanisms of metal-catalysed epoxidation of olefins by
alkyl hydroperoxides have attracted considerable attention.39,40
Finney and co-workers have examined the epoxidation of olefins
mediated by dioxo–Mo(VI) complexes supported by tridentate
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epoxidation catalysts suggests a direct oxygen-atom-transfer from
the coordinated alkyl hydroperoxide to the olefin substrates.
Results of their work provide some insights to the mechanism
of the epoxidation reaction in the present catalytic system. Con-
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hydroperoxide intermediate species, which undergoes subsequent
oxygen-atom-transfer to the styrene substrate. However, other
mechanistic pathways cannot be ruled out at this stage.
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Conclusions
A new series of tripodal tetradentate N2O2-type ligands have
been prepared by an efficient four-step synthetic procedure. The
corresponding dioxo–Mo(VI) and –W(VI) complexes have been
synthesised and their catalytic activities towards epoxidation of
styrene have been examined. It is believed that the four-step
synthetic procedure described in this work would provide an
efficient route for the preparation of a variety of N2O2-type
tetradentate ligands for biological modelling and catalytic studies.
13 For dioxo–molybdenum(VI) complexes supported by bidentate ligands,
see: F. E. Ku¨hn, A. M. Santos, A. D. Lopes, I. S. Gonc¸alves, E.
Herdtweck and C. C. Roma˜o, J. Mol. Catal. A: Chem., 2000, 164, 25;
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Acknowledgements
We thank the Croucher Foundation for offering a Fellowship to
Dr Yee-Lok Wong. This work was also supported by a Direct
Grant (A/C 2060300) of The Chinese University of Hong Kong
and a grant of the Research Grants Council of the Hong Kong
Special Administrative Region, China (Earmarked Grant: CUHK
464/95P). We are grateful to Professors Thomas C. W. Mak, Ze-
Ying Zhang and Chi-Keung Lam for crystallographic assistance.
F. E. Kuhn, J. Organomet. Chem., 2001, 626, 1; M. Gomez, S. Jansat,
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