Synthesis, structures and catalytic properties of iron(III) complexes with asymmetric N-capped tripodal NO3 ligands and a pentadentate N 2O3 ligand

Article abstract of DOI:10.1039/b804414g

A new family of N-capped tripodal NO₃ proligands N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2'-hydroxy-5'-R-phenyl)amine [H₃(Lⁿ)] [when R= Me, n = 1; R= ^tBu, n = 2; R = Cl, n = 3] with different substituents in one of the aryl rings and N,N-bis(2-hydroxy-3-tert-butylbenzyl)-N-(2'-hydroxy-5'-methylphenyl)amine [H₃(L⁴)] were synthesised. The preparation of a new pentadentate proligand N-methyl-N,N',N'-tris(2-hydroxy-3,5-di-tert-butylbenzyl) ethane-1,2-diamine [H₃(L⁵)] with an N₂O ₃ donor set is also reported. Reaction of the proligands [H ₃(Lⁿ)] (n = 1-4) with iron(iii) chloride in the presence of base (triethylamine) and 1-methylimidazole (1-Meim) as co-ligand led to the formation of iron complexes of the type [Fe(Lⁿ)(1-Meim)] (n = 1-4) (1-4) respectively, while treatment of the trilithium salt of [H ₃(L⁵)] with iron(iii) chloride afforded [Fe(L ⁵)] (5). All complexes were structurally characterised by X-ray crystallography. In complexes 1-4, the ligands form five- and six-membered chelate rings with the iron centres which have distorted trigonal bipyramidal geometry with an N₂O₃ coordination environment. Complex 5 adopts a similar distorted trigonal bipyramidal geometry also with N ₂O₃ coordination around the iron centre. The catalytic activity of these iron complexes towards epoxidation of styrene was examined.

Full text of DOI:10.1039/b804414g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structures and catalytic properties of iron(III) complexes with
asymmetric N-capped tripodal NO3 ligands and a pentadentate N2O3 ligand†
Lok H. Tong,a Yee-Lok Wong,a Sofia I. Pascua,b and Jonathan R. Dilworth*a
Received 14th March 2008, Accepted 23rd May 2008
First published as an Advance Article on the web 15th July 2008
DOI: 10.1039/b804414g
A new family of N-capped tripodal NO3 proligands N,N-bis(2-hydroxy-3,5-
t
di-tert-butylbenzyl)-N-(2ꢀ-hydroxy-5ꢀ-R-phenyl)amine [H3(Ln)] [when R= Me, n = 1; R= Bu, n = 2;
R = Cl, n = 3] with different substituents in one of the aryl rings and
N,N-bis(2-hydroxy-3-tert-butylbenzyl)-N-(2ꢀ-hydroxy-5ꢀ-methylphenyl)amine [H3(L4)] were
synthesised. The preparation of a new pentadentate proligand N-methyl-N,Nꢀ,Nꢀ-
tris(2-hydroxy-3,5-di-tert-butylbenzyl)ethane-1,2-diamine [H3(L5)] with an N2O3 donor set is also
reported. Reaction of the proligands [H3(Ln)] (n = 1–4) with iron(III) chloride in the presence of base
(triethylamine) and 1-methylimidazole (1-Meim) as co-ligand led to the formation of iron complexes of
the type [Fe(Ln)(1-Meim)] (n = 1–4) (1–4) respectively, while treatment of the trilithium salt of [H3(L5)]
with iron(III) chloride afforded [Fe(L5)] (5). All complexes were structurally characterised by X-ray
crystallography. In complexes 1–4, the ligands form five- and six-membered chelate rings with the iron
centres which have distorted trigonal bipyramidal geometry with an N2O3 coordination environment.
Complex 5 adopts a similar distorted trigonal bipyramidal geometry also with N2O3 coordination
around the iron centre. The catalytic activity of these iron complexes towards epoxidation of styrene
was examined.
Introduction
The chemistry of N-capped tripodal NO3 ligands (Fig. 1) has re-
ceived considerable attention over recent years.1–21 The symmetric
amine triphenolate type ligands (type I) bearing three identical
benzyl side arms have demonstrated rich coordination chemistry
with a wide variety of main group and transition metals.3–18 Their
complexes show a range of catalytic properties in reactions such
as aza-Diels–Alder,7 lactide polymerisation12 and sulfoxidation.14
All these have taken advantage of the tetradentate nature of
the ligands to stabilise well-defined monomeric complexes under
catalytic conditions. Substituents on the phenolate rings have
been shown to alter not only the binding mode,16,17 but also
the stability17 and catalytic activities of the metal complexes18
by modifying the electronic and steric properties of the metal
centres. Amine triphenolate-type ligands with one or two aromatic
side arms replaced by the aliphatic methoxy side-arm donors
(CH2CH2OCH3) (type II)19,20 or alkanol groups (CH2CH2OH)
(type III)21 have also been investigated for their binding modes
and catalytic behaviour over a range of metal complexes. Re-
cently, an amine triphenolate ligand with a stereogenic centre
incorporated into one of the methylene carbons (type IV) has
been synthesised with a view to potential applications in chiral
recognition and asymmetric catalysis by the resulting enantiopure
titanium complex.11 However, to our knowledge, examples of
Fig. 1 Structures of N-capped tripodal ligands.
aChemistry Research Laboratory, Department of Chemistry, University of
Oxford, 12 Mansfield Road, Oxford, UK OX1 3TA. E-mail: jon.dilworth@
metal complexes supported by amine triphenolate-type ligands
chem.ox.ac.uk; Tel: +44 (0)1865 272 639
containing two different arm lengths, have not been reported.
Iron complexes have been shown to display catalytic activities
for a range of organic transformations.22 However, their role in
bDepartment of Chemistry, University of Bath, Bath, UK BA2 7AY
† CCDC reference numbers 669890–669893 and 670101. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b804414g
4784 | Dalton Trans., 2008, 4784–4791
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the epoxidation of olefins has received relatively less attention
compared with other transition metal complexes.23 Much of
the previous effort has been directed to mechanistic studies
of porphyrin-based catalysts24 and only a handful of examples
of non-heme iron-based epoxidation catalysts are known.25–28
Recently, Beller and co-workers reported a series of iron complexes
supported by N-donor bi- and tridentate ligands which have
demonstrated excellent activity for epoxidation of olefins with
hydrogen peroxide as oxidant.29
We report herein a new series of iron(III) complexes supported
by the asymmetric N-capped tripodal NO3 ligands with {5,6,6}
chelate ring-sizes and substituents with different steric and elec-
tronic properties on the aryl rings (type V). We discuss their
synthesis, structures and catalytic properties for the epoxidation
of styrene in the presence of tert-butylhydroperoxide as oxidant.
The structure and catalytic activity of a related iron(III) complex
supported by a new pentadentate ligand offering an N2O3 coordi-
nation were also examined.
(96%). 1H NMR: d 7.74 (s, 1H, ArOH), 7.26 (dd, J = 1.2, 7.8 Hz,
1H, ArH), 6.90 (d, J = 6.9 Hz, 1H, ArH), 6.80 (t, J = 7.5 Hz, 1H,
ArH), 4.86 (s, 2H, ArCH2), 1.44 (s, 9H, tBu).
3-tert-Butyl-2-hydroxybenzylbromide. To a solution of 3-tert-
butyl-2-hydroxybenzyl alcohol (4.22 g, 23.4 mmol) in CHCl3
(50 cm3) was added PBr3 (1.10 cm3, 11.7 mmol). White fumes
appeared immediately during addition and stirring was continued
for 1 h at room temperature. Then cold water (30 cm3) was
added with vigorous stirring for 2 min. The organic layer was
separated and the aqueous residue was extracted with CHCl3 (2 ×
50 cm3). The combined extracts were dried over anhydrous MgSO4,
concentrated and dried in vacuo to give a pale-yellow liquid, which
was used without further purification. Yield: 5.57 g (98%).
General procedure for the preparation of (H3Ln) (n = 1–4).
To a solution of 2-hydroxy-5-R-aniline (R = Me, tBu, Cl)
in THF (20 cm3) was added a solution of 3,5-di-tert-butyl-
2-hydroxybenzylbromide or 3-tert-butyl-2-hydroxybenzylbromide
(2 equiv.) in THF (20 cm3) followed by triethylamine (2 equiv.)
at room temperature. After heating under reflux overnight, the
mixture was cooled, and the resulting white solid was filtered
and discarded. The brown filtrate was concentrated with a rotary
evaporator and the residue was chromatographed on a silica gel
column. The second band was collected and concentrated to give
a pale-yellow solid.
Experimental
General procedures
All reactions were carried out using standard Schlenk line
techniques under an atmosphere of dinitrogen; workups were
performed in air. Tetrahydrofuran (THF), diethyl ether (Et2O)
and toluene were distilled from sodium benzophenone. Methanol
(MeOH) and triethylamine (Et3N) were distilled from mag-
nesium methoxide and sodium respectively. Anhydrous tert-
butylhydroperoxide30 (TBHP) was obtained according to the
literature procedures. 3,5-di-tert-butyl-2-hydroxybenzylbromide31
was prepared according to the literature procedures with mi-
nor modification. Silica gel (70–230 mesh) for flash-column
chromatography was purchased from Fluka. All other reagents
N,N-Bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2ꢀ-hydroxy-5ꢀ-
methylphenyl)amine (H3L1). According to the general procedure,
2-hydroxy-5-methylaniline (0.98 g, 8.0 mmol) was treated with
3,5-di-tert-butyl-2-hydroxybenzylbromide (4.78 g, 16.0 mmol) and
triethylamine (2.23 cm3, 16.0 mmol) to give H3L1. The crude
product was purified by flash-column chromato◦graphy using
CH2Cl2 as eluant. Yield: 3.67 g (82%). Mp: 169–171 C. 1H NMR:
d 7.16 (d, J = 2.4 Hz, 2H, ArH), 7.01 (s, 1H, ArH), 6.96 (m,
2H, ArH), 6.73 (m, 2H, ArH), 4.11 (s, 4H, ArCH2), 2.21 (s, 3H,
1
were purchased from Aldrich and used as received. All H and
t
t
1
ArCH3), 1.39 (s, 18H, Bu), 1.27 (s, 18H, Bu). 13C{ H} NMR: d
152.0, 148.5, 141.4, 135.4, 135.0, 129.7, 125.9, 125.8, 123.3, 122.7,
121.6, 115.6, 56.3, 34.7, 34.2, 31.7, 29.8, 20.9. MS (APCI): m/z
560 (20%) [M + H]+. Anal. calc. for C37H53NO3: C, 79.4; H, 9.5;
N, 2.5%. Found: C, 79.1; H, 9.3; N, 2.1%.
1
13C{ H} NMR spectra were recorded on a Varian Mercury VX300
spectrometer (1H, 300 MHz and 13C, 75.4 MHz) using CDCl3 as
solvent. Chemical shifts were relative to internal SiMe4 (d = 0).
Atmospheric-pressure chemical-ionization (APCI) mass spectra
were recorded on a Hewlett-Packard 1050 Series Mass Spectrom-
eter. Elemental analyses were performed by the microanalysis
laboratory of the Inorganic Chemistry Laboratory, University of
Oxford, UK. Gas-chromatography (GC) analyses were performed
on an Unicam Pro-GC equipped with a flame ionization detector
(FID). The column had internal diameter 4 mm and was packed
with 3% Carbowax 20M on Supercoport 100/120 mesh support.
N,N-Bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2ꢀ -hydroxy-5ꢀ-
tert-butylphenyl)amine (H3L2). According to the general pro-
cedure, 2-hydroxy-5-tert-butylaniline (1.32 g, 8.0 mmol) was
treated with 3,5-di-tert-butyl-2-hydroxybenzylbromide (4.78 g,
16.0 mmol) and triethylamine (2.23 cm3, 16.0 mmol) to give H3L2.
The crude product was purified by flash-column chromatography
using CHCl3–hexane (1 : 2) as eluant. Yield: 1.97 g (41%). Mp:
◦
1
Preparations
182–184 C. H NMR: d 6.76–7.29 (m, 7H, ArH), 4.17 (s, 4H,
t
t
t
ArCH2), 1.41 (s, 18H, Bu), 1.29 (s, 18H, Bu), 1.27 (s, 9H, Bu).
3-tert-Butyl-2-hydroxybenzyl alcohol. To a solution of 3-tert-
butyl-2-hydroxybenzaldehyde (4.34 g, 24.4 mmol) in MeOH
(50 cm3) was added NaBH4 (1.84 g, 48.8 mmol) slowly. During
addition, the mixture turned from pale yellow to colourless and
stirring was continued for 1 h at room temperature. The volatiles
were then removed using a rotary evaporator and the residue was
mixed with water (50 cm3). The pH of this mixture was neutralised
with glacial acetic acid before being extracted with CH2Cl2 (3 ×
150 cm3). The combined extracts were dried over anhydrous
MgSO4, concentrated to give a pale-yellow liquid. Yield: 4.22 g
13C{ H} NMR: d 152.4, 148.7, 143.5, 141.7, 135.7, 134.9, 125.8,
1
123.8, 122.4, 122.0, 119.4, 115.5, 56.7, 35.0, 34.7, 34.5, 32.0, 31.9,
30.2. MS (APCI): m/z 602 (100%) [M + H]+. Anal. calc. for
C40H59NO3: C, 79.8; H, 9.9; N, 2.3%. Found: C, 79.9; H, 9.5;
N, 2.5%.
N,N-Bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2ꢀ -hydroxyl-5ꢀ-
chlorophenyl)amine (H3L3). According to the general procedure,
2-hydroxy-5-chloroaniline (1.15 g, 8.0 mmol) was treated with
3,5-di-tert-butyl-2-hydroxybenzylbromide (4.78 g, 16.0 mmol) and
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triethylamine (2.23 cm3, 16.0 mmol) to give H3L3. The crude
product was purified by flash-column chromato◦graphy using
CH2Cl2 as eluant. Yield: 2.64 g (57%). Mp: 153–155 C. 1H NMR:
d 6.75–7.31 (m, 7H, ArH), 4.08 (s, 4H, ArCH2), 1.39 (s, 18H, tBu),
1.28 (s, 18H, Bu). 13C{ H} NMR: d 153.6, 148.1, 144.4, 142.8,
135.9, 134.6, 127.8, 124.1, 122.2, 122.1, 119.7, 114.1, 56.7, 34.3,
34.0, 32.3, 30.8. MS (APCI): m/z 580 (100%) [M + H]+. Anal.
calc. for C36H50ClNO3: C, 74.5; H, 8.7; N, 2.4; Cl, 6.1%. Found:
C, 74.8; H, 8.8; N, 2.4; Cl, 5.6%.
70%). Anal. calc. for C41H56FeN3O3: C, 70.9; H, 8.1; N, 6.1%.
Found: C, 71.0; H, 8.3; N, 6.3%.
[Fe(L2)(1-Meim)] (2). According to the general procedure,
FeCl3 (0.16 g, 1.0 mmol) was treated with H3L2 (0.60 g, 1.0 mmol),
triethylamine (0.42 cm3, 3.0 mmol) and 1-methylimidazole
(0.48 cm3, 6.0 mmol) to give [Fe(L2)(1-Meim)] (0.38 g, 51%). Anal.
calc. for C44H62FeN3O3: C, 71.7; H, 8.5; N, 5.7%. Found: C, 71.4;
H, 8.6; N, 5.9%.
t
1
[Fe(L3)(1-Meim)] (3). According to the general procedure,
FeCl3 (0.16 g, 1.0 mmol) was treated with H3L3 (0.58 g, 1.0 mmol),
triethylamine (0.42 cm3, 3.0 mmol) and 1-methylimidazole
(0.48 cm3, 6.0 mmol) to give [Fe(L3)(1-Meim)] (0.43 g, 60%). Anal.
calc. for C40H53ClFeN3O3·0.36(CH3OH): C, 66.7; H, 7.6; N, 5.8;
Cl, 4.9%. Found: C, 66.9; H, 7.6; N, 5.8; Cl, 4.9%. The result of
elemental analysis suggests the presence of 0.36 equiv. of methanol
molecule incorporated per unit cell in the crystal structure.
N,N-Bis(2-hydroxy-3-tert-butylbenzyl)-N-(2ꢀ-hydroxy-5ꢀ-methyl-
phenyl)amine (H3L4). According to the general procedure,
2-hydroxy-5-methylaniline (0.50 g, 4.1 mmol) was treated with
3-tert-butyl-2-hydroxybenzylbromide (1.99 g, 8.2 mmol) and
triethylamine (1.14 cm3, 8.2 mmol) to give H3L4. The crude
product was purified by flash-column chromatography using
CH2Cl2 as eluant. Yield: 1.43 g (78%). Mp: 164–166 ◦C. 1H
NMR: d 7.16 (dd, J = 1.8, 7.7 Hz, 2H, ArH), 7.03 (d, J = 1.8 Hz,
1H, ArH), 6.96 (dd, J = 1.5, 7.2 Hz, 2H, ArH), 6.77–6.69 (m, 4H,
ArH), 4.12 (s, 4H, ArCH2), 2.23 (s, 3H, ArCH3), 1.38 (s, 18H,
[Fe(L4)(1-Meim)] (4). According to the general procedure,
FeCl3 (0.19 g, 1.2 mmol) was treated with H3L4 (0.54 g, 1.2 mmol),
triethylamine (0.50 cm3, 3.6 mmol) and 1-methylimidazole
(0.57 cm3, 7.2 mmol) to give [Fe(L4)(1-Meim)] (0.44 g, 63%). Anal.
calc. for C33H40FeN3O3: C, 68.0; H, 6.9; N, 7.2%. Found: C, 68.2;
H, 7.1; N, 7.0%.
1
tBu). 13C{ H} NMR: d 154.6, 148.3, 136.4, 135.0, 130.1, 128.9,
126.5, 126.2, 122.7, 122.3, 119.2, 115.7, 56.1, 34.5, 29.7, 20.9. MS
(APCI): m/z 448 (25%) [M + H]+. Anal. calc. for C29H37NO3: C,
77.8; H, 8.3; N, 3.1%. Found: C, 77.5; H, 8.1; N, 3.2%.
[Fe(L5)] (5). To a mixture of FeCl3 (0.16 g, 1.0 mmol) and
H3L5 (0.73 g, 1.0 mmol) in MeOH (20 cm3) was added with MeOLi
(0.11 g, 3.0 mmol). The mixture was heated under reflux overnight.
The volatiles were removed using a rotary evaporator and the
residue was chromatographed on a silica gel column using CH2Cl2
as eluant. The second band was collected and concentrated to give
a brown solid. Yield: (0.35 g, 45%). Anal. calc. for C48H73FeN2O3:
C, 73.7; H, 9.4; N, 3.6%. Found: C, 73.7; H, 9.1; N, 3.7%.
N -Methyl-N ,Nꢀ,Nꢀ -tris(2-hydroxy-3,5-di-tert-butylbenzyl)-
ethane-1,2-diamine (H3L5). To
a
solution of N-methyl-
ethylenediamine (0.54 ml, 6.2 mmol) in THF (50 cm3) was added
a solution of 3,5-di-tert-butyl-2-hydroxybenzylbromide (5.56 g,
18.6 mmol) in THF (50 cm3) followed by triethylamine (2.59 cm3,
18.6 mmol). After stirring at room temperature for three days,
the resulting white solid was filtered and discarded. The brown
filtrate was concentrated with a rotary evaporator and the residue
was chromatographed on a silica gel column using CHCl3–hexane
(1 : 1) as eluant. The second band was collected and concentrated
to give a pale yellowish–brown solid. Yield: 1.81 g (40%). Mp: 66–
68 ◦C. 1H NMR: d 6.89–7.31 (m, 6H, ArH), 3.76 (m, 4H, ArCH2),
3.71 (m, 2H, ArCH2), 2.86 (m, 2H, NCH2CH2N), 2.76 (m, 2H,
NCH2CH2N), 2.24–2.25 (s, 3H, NCH3), 1.53 (s, 9H, tBu), 1.51 (s,
9H, tBu), 1.48 (s, 9H, tBu), 1.47 (s, 9H, tBu), 1.39 (s, 9H, tBu), 1.38
General procedure for catalytic activity study of the com-
plexes towards oxidation of styrene under anaerobic condition.
A mixture of metal complex (1, 2, 3 and 5) (5 mmol%), tert-
butylhydroperoxide (TBHP) (2.5 mmol) and styrene (1 mmol) in
toluene (10 cm3) was heated under reflux for 24 h (Scheme 3).
The reaction mixture was filtered through a short-bed of celite.
The catalytic product (styrene oxide) was identified by comparing
the retention time with the authentic sample and the percentage
conversion of the product was determined by gas chromatography.
t
1
(s, 9H, Bu). 13C{ H} NMR: d 153.6, 152.6, 152.1, 141.8, 141.4,
140.5, 135.8, 135.7, 125.0, 124.5, 123.8, 123.6, 123.4, 123.0, 121.4,
121.0 (two overlapping signals), 71.4, 62.5, 57.3, 53.5, 50.8, 41.3,
35.0, 34.9 (one overlapping signal), 34.3, 34.2 (one overlapping
signal), 31.7 (one overlapping signal), 30.1, 29.8, 29.7, 29.6. MS
(APCI): m/z 729 (100%) [M + H]+. Anal. calc. for C48H76N2O3: C,
79.1; H, 10.5; N, 3.8%. Found: C, 78.8; H, 10.3; N, 3.3%.
X-Ray crystallographic analysis†
Crystal data and data processing parameters for the five iron(III)
structures are given in Tables 1 and 2. X-Ray quality crystals
of 1, 3, 4 and 5† were grown by slow evaporation of MeOH
solution of the metal complexes at room temperature. X-Ray
quality crystals of 2† were obtained by slow diffusion of Et2O into
the MeOH solution of the metal complex at room temperature.
Crystals were mounted on a glass fibre using perfluoropolyether
oil and cooled rapidly to 150 K in a stream of cold nitrogen
using an Oxford Cryosystems CRYOSTREAM unit, except 2
which was cooled to 250K. All diffraction data were collected
on an Enraf-Nonius KappaCCD diffractometer with graphite-
General procedure for the preparation of [Fe(Ln)(1-Meim)] (n =
1–4) (1–4). To a mixture of H3Ln (n = 1–4) and FeCl3 (1 equiv.)
in MeOH (20 cm3) was added with triethylamine (3 equiv.) and
1-methylimidazole (1-Meim) (6 equiv.). After heating under reflux
overnight, the mixture was cooled and left at room temperature
for slow evaporation. The brown crystalline solids grown in the
dark solution were collected, washed with cold Et2O and hexane,
and dried in vacuo.
[Fe(L1)(1-Meim)] (1). According to the general procedure,
FeCl3 (0.16 g, 1.0 mmol) was treated with H3L1 (0.56 g, 1.0 mmol),
triethylamine (0.42 cm3, 3.0 mmol) and 1-methylimidazole (1-
Meim) (0.48 cm3, 6.0 mmol) to give [Fe(L1)(1-Meim)] (0.49 g,
monochromated Mo Ka (k = 0.71073 A). The h range for all data
˚
collection was 5.0 ≤ h ≤ 27.5◦. Intensity data were processed using
the DENZO-SMN package.32 All these structures were solved
using the direct-methods program SIR92,33 which located all
4786 | Dalton Trans., 2008, 4784–4791
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Table 1 Crystallographic data† for complexes 1, 2 and 3
Compound
1
2
3
Empirical formula
Formula weight
Temperature/K
Colour
C41H56FeN3O3
694.76
150
C44H62FeN3O3·1.5[(C2H5)2O]
C40H53ClFeN3O3·0.36(CH3OH)
848.03
726.63
250
150
Brown prism
0.08 × 0.20 × 0.20
Monoclinic
P21/n
10.1469(1)
32.0173(3)
12.6860(1)
90
103.5726(4)
90
4006.28(6)
4
Brown plate
0.04 × 0.20 × 0.24
Monoclinic
P21/c
14.3625(2)
13.6632(2)
26.7778(4)
90
100.8828(5)
90
5160.31(13)
4
Dark-brown block
0.18 × 0.18 × 0.22
Triclinic
¯
P1
11.2778(2)
13.6398(3)
14.0391(3)
94.0494(11)
94.7940(11)
112.5924(11)
1974.47(7)
2
Crystal size/mm3
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
D
calcd/Mg m−3
1.152
0.415
1.091
0.335
1.222
0.490
l/mm−1
F(000)
1492
0.95, 0.97
33 694
1840
0.94, 0.99
39 433
774
0.92, 0.92
27 291
Transmission coefficients (min, max)
Number of data measured
Number of unique data (Rint
)
9281 (0.039)
5947
470
0.0437
0.0505
1.0592
+0.36 and −0.36
12 027 (0.060)
6508
505
0.0656
0.0657
1.1413
+0.66 and −0.43
8924 (0.041)
6725
442
0.0461
0.0655
1.0417
+1.16 and −1.05
Number of observed data (I > 3r(I))
Number of parameters, p
R
wR
S (GOF)
−3
˚
Largest difference peak and hole/e A
Table 2 Crystallographic data† for complexes 4 and 5
Compound
4
5
Empirical formula
Formula weight
Temperature/K
Colour
C33H40FeN3O3
582.55
150
C48H73FeN2O3
781.97
150
Black block
0.10 × 0.20 × 0.28
Monoclinic
P21/c
13.3141(2)
10.7638(1)
22.3380(3)
90
102.1568(6)
90
3129.48(7)
4
Brown fragment
0.08 × 0.20 × 0.20
Monoclinic
P21/n
15.4928(3)
19.4211(4)
16.9802(4)
90
115.7166(7)
90
4603.07(17)
4
Crystal size/mm3
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
D
calcd/Mg m−3
1.236
0.518
1.128
0.367
l/mm−1
F(000)
1236
0.90, 0.95
41 442
1700
0.93, 0.97
40 111
Transmission coefficients (min, max)
Number of data measured
Number of unique data (Rint
)
7470 (0.057)
4882
361
0.0380
0.0490
1.0218
+0.35 and −0.41
10 776 (0.053)
7387
487
0.0390
0.0453
1.0376
+0.46 and −0.43
Number of observed data (I > 3r(I))
Number of parameters, p
R
wR
S (GOF)
−3
˚
Largest difference peak and hole/e A
non-hydrogen atoms. Subsequent full-matrix least-squares refine-
ment was carried out using the CRYSTALS program suite.34 In 1,
one of the tert-butyl groups was disordered over two orientations
and the occupancies of these were refined, subject to constraint
of the total occupancy to unity. Geometric similarity restraints
were applied to the two positions. One of the tert-butyl groups
in 2 was refined isotropically. The methyl carbon atoms of a
tert-butyl group in 2 were disordered over two orientations. One
of these was disordered over two orientations not related by
crystallographic symmetry whereas the second was disordered
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over two orientations related by a crystallographic centre of
inversion. Coordinates, isotropic thermal parameters and, where
appropriate, site occupancies of disordered non-hydrogen atoms
were refined, with chemically equivalent atoms constrained to have
common thermal parameters. Geometric restraints were applied
to bond lengths and angles within the disordered fragments. In 3,
a solvated methanol molecule is disordered over a crystallographic
centre of inversion. The thermal parameters of the solvent carbon
and oxygen atoms refined to unreasonably large values, which
was taken to indicate that the solvent site was only partially
occupied. Coordinates and anisotropic thermal parameters of
all non-hydrogen atoms were refined. Hydrogen atoms were
positioned geometrically after each cycle of refinement. 4-Term
{for complex 1} and 3-Term {for complexes 2–5} Chebychev
polynomial weighting schemes were applied.
di-tert-butyl-2-hydroxybenzylbromide with differently substituted
t
2-hydroxy-5-R-aniline [R = Me, n = 1; R = Bu, n = 2; R =
Cl, n = 3] in the presence of base (triethylamine) afforded the
proligands in 41–82% yield. Reaction of the same bromide with
commercially available N-methylethylenediamine under similar
conditions gave the new N2O3 pentadentate proligand H3L5 in
40% yield (Scheme 2). The proligand H3L4 was synthesised by a
similar procedure using 3-tert-butyl-2-hydroxybenzylbromide and
2-hydroxy-5-methylaniline as reagents in 78% yield (Scheme 1).
Synthesis of iron(III) complexes
Reaction of the tripodal proligands H3Ln (n = 1–4) with iron(III)
chloride in the presence of three equivalents of triethylamine
and six equivalents of 1-methylimidazole (1-Meim) in MeOH
afforded the corresponding complexes 1–4 in 51–70% (Scheme 1).
Treatment of the trilithium salt of H3L5 with iron(III) chloride
in MeOH afforded the five coordinate complex 5 in 45% yield
(Scheme 2). All these iron complexes possessed good solubility
in common organic solvents such as methanol, chloroform and
toluene, except 4 which was sparingly soluble in toluene. Crystals
of all iron complexes suitable for X-ray crystallography were
obtained by slow evaporation of a methanolic solution of the
complexes, except for 2, for which crystals were grown by slow
Results and discussion
Synthesis of ligands
The synthesis of a series of previously unknown N-capped
tripodal NO3-type proligands H3Ln (n = 1–4) is shown in
Scheme 1. The sole differences in H3Ln (n = 1–3) are the
substituents on one of the aryl rings. The coupling between 3,5-
Scheme 1 Synthesis of tripodal ligands H3Ln (n = 1–4) and iron(III) complexes 1–4.
Scheme 2 Synthesis of pentadentate ligand H3L5 and iron(III) complex 5.
4788 | Dalton Trans., 2008, 4784–4791
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diffusion of diethyl ether into the methanolic solution of the
complex. Elemental analysis data of these complexes were in
good agreement with the structural formula determined from X-
ray crystallography,† except 2 where the solvated diethyl ether
molecules were not observed.
Structural studies
The molecular structures of 1–4 are shown in Fig. 2–5 respectively.
Selected bond distances and angles for these complexes are sum-
marised in Tables 3 and 4. The crystallographic data are in Tables 1
and 2.† Complex 1 crystallises in the monoclinic space group
P21/n. Complexes 2 and 4 crystallise in the monoclinic space group
¯
P21/c, while 3 crystallises in the triclinic space group P1. For 1,
the iron centre is surrounded by three equatorial phenolate oxygen
atoms (O(1)–O(3)) and two axial nitrogen atoms (bridgehead N(1)
and imidazole N(2)) in a distorted trigonal bipyramid geometry,
with the N(1)–Fe–N(2) angle of 166.55(7)◦. The iron atom is out
Fig. 3 ORTEP representation (40% probability ellipsoids) of the molecu-
lar structure and atom labeling scheme for [Fe(L2)(1-Meim)] 2. Hydrogen
atoms are omitted for clarity.
˚
of the plane (ca. 0.11 A) of the three oxygen donors towards the
imidazole nitrogen. The bond distances and angles are comparable
to those in a related structure [Fe{N(CH2-o-C6H4O)3}(1-Meim)]
reported by Koch et al.3 Not surprisingly, the five-membered
chelate ring displays different geometric features with respect to
those in the six-membered systems: (i) the angle O(1)–Fe–N(1)
[80.11(6)◦] is considerably smaller than that for O(2)–Fe–N(1) and
O(3)–Fe–N(1) [90.51(6)◦ and 90.11(6)◦ respectively], and (ii) the
˚
Fe–O(1) bond distance [1.9226(17) A] is longer than Fe–O(2) and
˚
Fe–O(3) bonds [1.8823(15) and 1.8602(15) A respectively]. It is
also observed that the imidazole nitrogen bends towards the five-
membered chelate ring, indicated by the smaller angle of N(2)–
Fe–O(1) compared with N(2)–Fe–O(2) and N(2)–Fe–O(3). Such
a conformation is likely to minimise the steric hindrance between
the axial ligand and the tert-butyl groups in the ortho positions on
the other two aryl rings.
Fig. 4 ORTEP representation (40% probability ellipsoids) of the molecu-
lar structure and atom labeling scheme for [Fe(L3)(1-Meim)] 3. Hydrogen
atoms are omitted for clarity.
Fig. 2 ORTEP representation (40% probability ellipsoids) of the molecu-
lar structure and atom labeling scheme for [Fe(L1)(1-Meim)] 1. Hydrogen
atoms are omitted for clarity.
Fig. 5 ORTEP representation (40% probability ellipsoids) of the molecu-
lar structure and atom labeling scheme for [Fe(L4)(1-Meim)] 4. Hydrogen
atoms are omitted for clarity.
The structural features of 2–4 show strong resemblances to
those in 1, with similar bond distances and angles as in the parent
compound. The steric and electronic demands of the substituents
t
(Me, Bu and Cl) on the aryl ring appear to have no significant
incorporate approximately 1.5 Et2O per molecule of the complex
effect on the overall structure in these complexes. The crystals of 2
while that in 3 contains 0.36 MeOH of solvation.
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◦
˚
Table 3 Selected bond distances (A) and angles ( ) for complexes 1, 2 and 3
[Fe(L1)(1-Meim)] 1
[Fe(L2)(1-Meim)] 2
[Fe(L3)(1-Meim)] 3
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(2)
1.9226(17)
1.8823(15)
1.8602(15)
2.2160(16)
2.0938(18)
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(2)
1.918(2)
1.866(2)
1.864(2)
2.212(3)
2.099(3)
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(2)
1.9362(16)
1.8848(14)
1.8740(14)
2.2137(17)
2.1000(18)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
O(2)–Fe(1)–O(3)
O(1)–Fe(1)–N(1)
O(2)–Fe(1)–N(1)
O(3)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(2)
O(3)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
129.00(8)
118.61(8)
111.35(7)
80.11(6)
90.51(6)
90.11(6)
86.92(7)
94.92(7)
99.31(7)
166.55(7)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
O(2)–Fe(1)–O(3)
O(1)–Fe(1)–N(1)
O(2)–Fe(1)–N(1)
O(3)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(2)
O(3)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
124.26(11)
121.00(11)
113.74(11)
80.18(9)
91.4(1)
89.1(1)
87.3(1)
96.58(11)
96.52(11)
167.5(1)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
O(2)–Fe(1)–O(3)
O(1)–Fe(1)–N(1)
O(2)–Fe(1)–N(1)
O(3)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(2)
O(3)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
125.69(7)
119.20(7)
113.74(7)
79.66(6)
90.12(6)
89.17(6)
88.30(7)
94.50(7)
99.45(7)
167.58(7)
◦
˚
crystallographic data in Table 2. Complex 5 crystallises in the
monoclinic space group of P21/n. The iron resides in a distorted
trigonal bipyramidal geometry with the unmethylated nitrogen
N(1) and one of the phenolate oxygens O(3) occupying the two
axial positions, giving an O(3)–Fe(1)–N(1) angle of 159.71(5)◦.
The methylated nitrogen N(2) along with the other two phenolate
atoms O(1) and O(2) define the equatorial plane. The iron atom
Table 4 Selected bond distances (A) and angles ( ) for complexes 4 and
5
[Fe(L4)(1-Meim)] 4
[Fe(L5)] 5
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(2)
1.9202(13)
1.8612(13)
1.8686(13)
2.2005(15)
2.0957(16)
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1)–N(2)
1.8712(12)
1.8728(12)
1.8895(12)
2.2407(14)
2.2185(14)
˚
is 0.158 A out of the equatorial plane towards the axial oxygen
O(3). The two nitrogen atoms form a five-membered chelate ring
with the iron(III) centre whereas all three phenolate arms form
six-membered chelate rings. The Fe–N and Fe–O bond distances
are within the range of those reported for other iron(III) complexes
containing related ligands.35 In general, the structural details of 5
show resemblance to a recently reported five-coordinate iron(III)
complex supported by a similar pentadentate ligand.36
O(1)–Fe(1)—O(2)
O(1)–Fe(1)—O(3)
O(2)–Fe(1)—O(3)
O(1)–Fe(1)—N(1)
O(2)–Fe(1)—N(1)
O(3)–Fe(1)—N(1)
O(1)–Fe(1)—N(2)
O(2)–Fe(1)–N(2)
O(3)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
130.67(6)
117.15(6)
111.37(6)
80.68(6)
90.77(6)
90.47(6)
87.90(6)
92.53(6)
99.69(6)
167.32(6)
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–O(3)
O(2)–Fe(1)–O(3)
O(1)–Fe(1)–N(1)
O(2)–Fe(1)–N(1)
O(3)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(2)–Fe(1)–N(2)
O(3)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
114.36(6)
91.90(5)
106.81(5)
87.33(5)
91.89(5)
159.71(5)
137.45(5)
106.17(5)
88.32(5)
78.74(5)
Catalysis studies
The catalytic properties of 1, 2, 3 and 5 for the epoxidation of
styrene were briefly examined (Scheme 3). In a typical experiment,
5 mmol% of the complex, 1 mmol of styrene and 2.5 mmol
TBHP were dissolved in toluene and heated under reflux for 24 h
under nitrogen.30,37 The percentage conversion of the substrate
was determined by gas chromatography. All these iron complexes
were found to be catalytically active for the oxidation of styrene
to styrene oxide with a similar percentage conversion (45–55%).
Despite their modest conversions compared with the recently
reported iron catalysts where quantitative conversions of aromatic
alkenes to corresponding epoxides were observed,29 the results
here suggest a new type of catalytically active iron complex
for epoxidation. Formation of additional products was also
observed, which may be attributable to the instability of the
styrene in the reaction medium. No transformation from styrene
to styrene oxide was observed in the absence of the iron complexes
under similar conditions. The electronic and steric properties
Fig. 6 ORTEP representation (40% probability ellipsoids) of the molec-
ular structure and atom labeling scheme for [Fe(L5)] 5. Hydrogen atoms
are omitted for clarity.
The molecular structure of 5 is shown in Fig. 6. Selected bond
distances and angles for the complex are given in Table 4 and
Scheme 3
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of the substituent in the ligands only showed a little effect on
the percentage conversion in the epoxidation. Owing to limited
solubility in toluene, the catalytic activity of 4 was not examined.
The fact that the five-coordinate complex 5 bearing no co-
ligand (1-methylimidazole) showed similar catalytic activity as 1–
3 suggests that the catalytic process may involve a six- or even
higher-coordinate metastable intermediate. Other examples of
amine triphenolate metal complexes with catalytic activity towards
styrene epoxidation have been reported in the literature but their
conversion rates are comparatively slow (2–3 turnovers per day).17
16 S. Groysman, S. Segal, I. Goldberg, M. Kol and Z. Goldschmidt, Inorg.
Chem. Commun., 2004, 7, 938.
17 S. Groysman, I. Goldberg, Z. Goldschmidt and M. Kol, Inorg. Chem.,
2005, 44, 5073.
18 S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt and M. Kol, Inorg.
Chem., 2006, 45, 4783 .
19 (a) E. Y. Tshuva, I. Goldberg, M. Kol and Z. Goldschmidt,
Organometallics, 2001, 20, 3017; (b) E. Y. Tshuva, S. Groysman, I.
Goldberg, M. Kol and Z. Goldschmidt, Organometallics, 2002, 21,
662.
20 (a) S. Groysman, E. Y. Tshuva, I. Goldberg, M. Kol, Z. Goldschmidt
and M. Shuster, Organometallics, 2004, 23, 5291; (b) S. Groysman, E.
Sergeeva, I. Goldberg and M. Kol, Inorg. Chem., 2005, 44, 8188; (c) S.
Groysman, E. Sergeeva, I. Goldberg and M. Kol, Eur. J. Inorg. Chem.,
2006, 2739.
21 Y. Kim and J. G. Verkade, Organometallics, 2002, 21, 2395.
22 C. Bolm, J. Legros, J. Le, Paih and L. Zani, Chem. Rev., 2004, 104,
6217.
Conclusions
In summary, a series of iron(III) complexes with asymmetric
N-capped tripodal NO3 ligands and a pentadentate N2O3 lig-
and were synthesised and structurally characterised by X-ray
crystallography.† These iron complexes showed catalytic activity
towards epoxidation of styrene and despite their modest conver-
sions, they represent new types of iron-based epoxidation catalysts.
The substituents with different steric and electronic properties on
the five-membered chelate ring were shown to have little impact
on the overall molecular structure and the catalytic activity of the
complexes.
23 (a) K. A. Joergensen, Chem. Rev., 1989, 89, 431; (b) B. S. Lane and K.
Burgess, Chem. Rev., 2003, 103, 2457.
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Acknowledgements
We thank the Croucher Foundation (Dr Yee-Lok Wong) and Royal
Society (Dr Sofia I. Pascu) for financial support. We are grateful
to Dr Andrew Cowley for crystallographic assistance.
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©