Iron(II) complexes with functionalized amine-pyrazolyl tripodal ligands in the cross-coupling of aryl Grignard with alkyl halides

Article abstract of DOI:10.1039/c1dt10258c

Structurally distinctive Fe(ii) complexes with furan, thiophene and pyridine functionalized amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. The tether substituent at the central amine plays an active role in determining the coordination mode of the ligand and the metal geometry. All complexes are catalytically active towards cross-coupling of aryl Grignard reagents with primary and secondary alkyl halides with β-hydrogen under ambient conditions. ESI-MS spectra analysis revealed the ligand-stabilised Fe(ii) and Mg(ii) species. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10258c

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PAPER
Iron(II) complexes with functionalized amine-pyrazolyl tripodal ligands in the
cross-coupling of aryl Grignard with alkyl halides†
Fei Xue,a Jin Zhao*a,b and T. S. Andy Hor*a,c
Received 16th February 2011, Accepted 24th March 2011
DOI: 10.1039/c1dt10258c
Structurally distinctive Fe(II) complexes with furan, thiophene and pyridine functionalized
amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized.
The tether substituent at the central amine plays an active role in determining the coordination mode of
the ligand and the metal geometry. All complexes are catalytically active towards cross-coupling of aryl
Grignard reagents with primary and secondary alkyl halides with b-hydrogen under ambient
conditions. ESI-MS spectra analysis revealed the ligand-stabilised Fe(II) and Mg(II) species.
some advantages over the in situ mixtures of FeCl3/TMEDA
Introduction
system.3b Low-valent iron complexes, [Li(tmeda)]2[Fe-II(C2H4)4] is
Use of iron in catalytic reactions is of topical significance in
effective towards the coupling of a large range of alkyl halides with
view of the abundance of iron and its relatively low toxicity.1
Among the many reactions of interest, the cross-coupling
of primary and secondary alkyl halides with aryl Grignard
reagents is noteworthy due to their high cross-coupling se-
lectivity with minimum b-elimination by-products, the latter
of which still plague many nickel- and palladium-catalysed
systems.1a,2–6 Some common iron compounds have been proven
to be active toward such cross coupling. For example, in the
presence of excess of an amine additive, such as TMEDA
diverse functionalities.2e,6
Encouraged by the promotional effect of amines, notably
TMEDA, in these reactions and the recent discovery that
FeIIAr2(tmeda) and FeIIArBr(tmeda) (Ar = aryl) are possible
intermediates in the FeCl3/TMEDA catalyzed coupling reactions
of ArMgX with alkylbromides,2f we are interested to apply
some easy-to-prepare, air-stable and structurally distinctive Fe(II)
complexes to aryl Grignard cross-couplings with alkyl halides
with b-hydrogen. In line with our current interest in hybrid
ligands tethered by dissociable ligands with enhanced hemilabile
functions,7–9 the ligands of interest here are amine-pyrazolyl
derivatised by a pendant function. Apart from the structural
and catalytic findings, we also report the ESI-MS species of the
catalytic mixtures which could shed some light on the active species
involved.
3a
(N,N,N¢,N¢-tetramethylethylenediamine), FeCl3 and Fe(acac)3
(acac = acetylacetonate)3b are active towards the cross-coupling
of aryl Grignard reagents with secondary bromides or chlorides.
The preparative simplicity offers another advantage of the use of
iron complexes in catalysis. For example, some amine, phosphine,
arsine and N-heterocyclic carbene complexes of iron formed
in situ from iron chlorides (mainly anhydrous FeCl3) could
give up to 90% of the cross coupling product of bromocy-
clohexane with p-tolymagnesium bromide.4 Structurally defined
complexes have also been applied, such as [FeIIICl3L] (L = 1,3,5-
trimethylhexahydro-1,3,5-triazine),4a [FeII(IPr)Br3](HIPr)3·C7H8
(IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene),5a ionic
liquid [bmim][FeIIICl4] (bmim = butylmethyl imidazolium),5b
[FeIIIClL] (L = amine-bis(phenolate)ether)5c and [FeIIICl(salen)]5d
The discrete molecular complex [(FeIIICl3)2(tmeda)3] also shows
Results and discussions
Synthesis and characterization of Fe(II) complexes 1–4
Four furan, thiophene and pyridine functionalized amine-
pyrazolyl tripodal ligands L1-L4 have been prepared from the
reactions between (3,5-dimethyl-1H-pyrazol-1-yl)methanol and
the corresponding amine based on adapted literature methods.10
They are characterized by NMR, ESI-MS spectroscopic analyses
and elemental analysis.
L1–L4 complex readily to FeCl2·4H2O in THF giving the
corresponding Fe(II) complexes 1–4 in good yields (Scheme 1).
All four complexes give the characteristic molecular ion peaks of
[FeClL]+ in the positive mode of ESI-MS spectra. Diffusion of
Et2O into the complex solutions in CH2Cl2 at r.t. afforded yellow
single crystals of 1, 3 and 4 suitable for X-ray crystallographic
analysis.
aDepartment of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore, 117543, Singapore. E-mail: andyhor@nus.edu.sg,
chmzhaoj@nus.edu.sg; Fax: 656779 1691; Tel: 65 6516 2675
bGerman Institute of Science and Technology-TUM Asia Pte. Ltd., 10
Central Exchange Green, #03-01, Pixel Building, Singapore, 138649,
Singapore
cInstitute of Materials Research and Engineering, Agency for Science,
Technology and Research, 3 Research Link, Singapore, 117602, Singapore
† CCDC reference numbers 812217–812219. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt10258c
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8935–8940 | 8935
©

Change in the pendant function and adjustment in the spacer
between the pendant and the central amine nitrogen impart a
significant difference on the coordination motif of the ligand and
metal geometry. This is witnessed in the tridentate coordination
of amine-pyrazolyl ligand and trigonal bipyramidal Fe(II) in 3
(Fig. 2) and tetradentate coordination of the pyridyl-amine-
pyrazolyl ligand and octahedral Fe(II) in 4 (Fig. 3).
Fig. 2 ORTEP diagram of 3 (30% probability ellipsoids). Hydrog◦en
˚
atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe1–N1 2.1185(17), Fe1–N3 2.1209(16), Fe1–N5 2.4064(15), Fe1–Cl1
2.3086(6), Fe1–Cl2 2.3747(6). N1–Fe1–N3 113.18(6), N1–Fe1–Cl1
110.67(5), N1–Fe1–Cl2 103.68(4), N1–Fe1–N5 73.24(6), N3–Fe1–Cl1
126.02(5), N3–Fe1–Cl2 92.29(4), N3–Fe1–N5 72.43(6), N5–Fe1–Cl1
92.12(4), N5–Fe1–Cl2 160.87(4), Cl1–Fe1–Cl2 106.40(2).
Scheme 1
Molecular structure of compounds 1, 3 and 4 determined by
single-crystal X-ray diffraction
Although all three (1, 3 & 4) are Fe(II) complexes, they show
distinctly different metal geometries influenced by the tripodal
ligands with hemilabile function.
Complex 1 is (distorted) tetrahedral with bidentate chelating
coordination of L1 through the nitrogen atoms of pyrazolyl
rings (Fig. 1). The central amine is notably non-coordinating
thus giving an 8-membered metallocyclic ring. The furan arm is
understandably pendant. Similar structure has been found in the
Fe(II) bis(2-oxazolin-2,5,5-trimethyl)phenylphosphine complexes
with the ligand in a N,N coordination mode.11
Fig. 3 ORTEP diagram of4 (30% probability ellipsoids). Hydrogen atoms
◦
˚
are omitted for clarity. Selected bond lengths (A) and angles ( ):Fe1–N1
2.219(3), Fe1–N3 2.338(5), Fe1–N4 2.209(5), Fe1–Cl1 2.3512(16), Fe1–Cl2
2.4190(15). N1–Fe1–N3 74.29(8), N1–Fe1–N4 84.39(9), N1–Fe1–N1A
148.43(16), N1–Fe1–Cl1 105.16(8), N1–Fe1–Cl2 92.18(9), N3–Fe1–N4
74.6(2), N3–Fe1–Cl1 169.65(14), N3–Fe1–Cl2 92.38(14), N4–Fe1–Cl1
95.03(14), N4–Fe1–Cl2 166.99(13), Cl1–Fe1–Cl2 97.98(6).
In 3, although the pendant (thiophene) remains non-
coordinative, the slight lengthening (by a methylene) of the
spacer between the central amine nitrogen and the heterocyclic
substituent is sufficient to push the amine nitrogen from non-
˚
bonding in 1 (Fe1 ◊ ◊ ◊ N5 3.979(2) A) to within bonding distance
Fig. 1 ORTEP diagram of1 (30% probability ellipsoids). Hydrogen atoms
˚
with the metal in 3 (Fe1–N5 2.4064(15) A), thus bisecting
◦
˚
are omitted for clarity. Selected bond lengths (A) and angles ( ): Fe1–N1
2.0856(18), Fe1–N3 2.1038(19), Fe1 ◊ ◊ ◊ N5 3.979(2), Fe1–Cl1 2.2486(7),
Fe1–Cl2 2.2663(7). N1–Fe1–N3 108.75(7), N1–Fe1–Cl1 100.46(5),
N1–Fe1–Cl2 111.57(5), N3–Fe1–Cl1 116.46(6), N3–Fe1–Cl2 101.16(5),
Cl1–Fe1–Cl2 118.51(3).
the 8-membered ring into two 5-membered rings. The latter
Fe1–N5 bond is however significantly longer, and presumably
weaker, than the coordination of the pyrazolyl ring Fe1–N1
˚
and Fe1–N3 of 2.1185(17) and 2.1209(16) A, respectively. This
8936 | Dalton Trans., 2011, 40, 8935–8940
This journal is The Royal Society of Chemistry 2011
©

Table 1 Catalytic activity of Fe(II) compounds on the coupling of
structure is similar to some compounds formed between FeCl2
and a-substituted tris(2-pyridylmethyl)amine (TPA) ligands,12 for
example, FeCl2(R2TPA) (R = Br, Ph) in which the tripod is
tridentate with a substituted pyridyl arm being pendant. The
sharp increase of iron-amine contact from 1 to 3 exemplifies the
hemilability nature of this type of hybrid ligand which adds the
skeletal flexibility to the resultant metallomacrocyclic ring. Such
coordination and conformational freedom allows the metal to
adjust its geometry to adapt to the environmental change, which
is an important attribute in a catalytic metal moiety.
PhMgBr with bromocyclohexanea
Catalyst (5 mol% Fe)
Yieldb of B (%)
Conversionb of A (%)
FeCl2
33
54
FeCl2·4H2O
88/41c
100
100
100
100
100
100
FeCl2·4H2O/L4
96
96
85
1
2
3
4
95
99/85d/62e
a Conditions: bromocyclohexane (1.0 mmol), PhMgBr (2.0 mmol), catalyst
(5 mol%), THF, r.t., 40 min; ArMgBr was added dropwise for 30 min.
b Yield and conversion are determined by 1H NMR (mesitylene as internal
standard). c bromocycloheptane was used as substrate. d 1 mol% Fe.
e ArMgBr was added in one portion.
The ligand-initiated geometric change is substantiated in 4 when
a more basic pyridyl substituent is introduced to the amine. As
expected, all the functional groups (pyrazolyl, amine & pyridyl)
are coordinated, thus giving a tetradentate ligand binding to
the metal with three orthogonal 5-membered metallomacrocyclic
rings. Significantly, the pyridyl coordination has further shortened
˚
the iron-amine bonding contact to Fe1–N3 2.338(5) A (from Fe1–
methylene spacer, gives the lowest yield of B (85%). The pendant
effect is marginal, similar to those found in some Fe(III) amine-
bis(phenolate) complexes.5d FeCl2 is active but not effective whilst
FeCl2·4H2O has a similar effect to 2. However, the yield of B
increases (from 88 to 96%) when L4 is added stoichiometrically
to FeCl2·4H2O, which suggests clearly the promoting effect of the
ligand. It also suggests that the present system does not require the
presence of ligand in excess to achieve high catalytic activities. The
ligand effect is also witnessed when 4 gives better yield (76%) (Table
2, entry 2) than FeCl2·4H2O (41% yield) when bromocycloheptane
is used as the substrate.
˚
N5 2.4064(15) A in 3). This formation provides crystallographic
evidence that a functional tether at the center of a tripodal ligand
could influence not only the coordination mode of the ligand
but also the strength of its central anchor. The Fe–Cl lengths
understandably increase as the coordination number increases
from 1 (Fe–Clave . = 2.258 A) to 3 (Fe–Cl ave . = 2.342 A) to 4 (Fe–
Clave . = 2.385 A). Pyridyl coordination in 4 weakens the trans Fe1–
Cl2 to 2.4190(15) A, which is the longest in this series, suggesting
that it may be possible to use the tether effect to labilise the Fe–Cl
bond to achieve metal activation in catalysis.
˚
˚
˚
˚
When the catalyst load is reduced to 1 mol%, otherwise under
similar conditions, 4 maintains its high conversion, but slightly
lower selectivity towards the cross coupling product B (Table 1).
Similar to some other iron catalysts,2f,3a a slow addition of the
Grignard substrate would ensure higher selectivity of the cross
coupling product.
Cross-coupling of aryl Grignard reagents with alkyl halides
Iron-catalysed cross-coupling reactions of aryl Grignard reagents
with primary or secondary alkyl halides with b-hydrogen are
mostly conducted on Fe(III) complexes2–5 Recently, Sun et al.5a
reported a highly active anionic iron(II) NHC complex towards
cross-coupling of p-tolylmagnesium bromide with bromocyclo-
hexane. Little is known on the use of heterocycle-based hybrid
ligands in Fe(II) complexes in this type of catalysis. We are
interested in applying the structurally characterized complexes
1–4 and examine if the tether-imposed structural features could
have any influence on the catalytic activities.
Initial screening was carried out on the r.t. reaction of bromocy-
clohexane (A) with PhMgBr as a representation of aryl Grignard–
secondary alkyl coupling catalysed by 1–4 (Scheme 2), with FeCl2
and FeCl2·4H2O included for comparison (Table 1).
These results suggest that the octahedral complex 4 performs
the best and hence it was examined further for the coupling
between other primary or secondary alkyl halides and different
aromatic Grignard substrates (viz. PhMgBr, p-TolMgBr &
p-FC6H4MgBr (Tol = tolyl; FC6H4 = fluorophenyl)) in THF under
similar conditions (Table 2). The catalyst load was kept at 5 mol%
Fe for direct comparison. In general, bromocyclopentane and
bromocycloheptane give higher cross-coupling yields than the
open chain secondary alkyl bromides (comparing entries 1–2 with
entries 3–4; entries13–15 with entry 16). (Bromomethyl)benzene
is moderately active with PhMgBr, similar to those catalysed by
Fe(III) amine-bis(phenolate),5d but could achieve higher yield at
lower temperature (-10 ◦C) (entry 7). Electron-rich p-TolMgBr
shows similar activities (entries 13–17) as PhMgBr. Although
chlorocyclohexane generally couples poorly with Grignard
reagents,3b,4a,5c its current reactions with PhMgBr and p-TolMgBr
give quantitative conversions with satisfactory yields (82% and
83%, respectively) at 45 ◦C (entries 6 and 17), which are similar to
the FeIIICl(salen)-catalyzed reactions.4 Primary alkyl halides are
also reactive with these Grignard reagents at elevated temperature
(45 ◦C) (entries 9–12, 17–21 and 25–26), with the chain length
showing little effect on the catalytic activity. Catalyst 4 is also
tolerant of p-FC6H4MgBr with bromocyclohexane, giving rise
to near-quantitative yield of the desired coupling product (entry
22). In contrast, Nakamura3a and Fu¨rstner6c observed moderate
activities in their systems when the electron-poor Grignard
Scheme 2
In the absence of catalyst, only trace amount of homo-coupled
product, biphenyl, was detected (GC-MS). The ligand L1–L4
without metal show no activity. All complexes (1–4) at 5 mol%
at r.t. show high yields of cyclohexylbenzen (B). These activities
are comparable or better than those reported molecular systems.5
The differences in this series are not significant but among
them, 4 is most effective, with quantitative conversion and yield
whereas 2, with a thiophene pendant connected through a short
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8935–8940 | 8937
©

Table 2 Cross-coupling of ArMgBr with alkyl halides using 4 as catalysta
Table 2 (Contd)
Entry ArMgBr
1
Alkyl halide
Product
Yield%b
88/78c
Entry ArMgBr
24
Alkyl halide
Product
Yield%b
60
bromocyclopentane
bromocycloheptane
84c
85c
2
bromocycloheptane
76
25
26
1-bromooctane
1-bromodecane
3
4
3-bromopentane
2-bromopropane
62
65
a Conditions: alkyl halide (1.0 mmol), ArMgBr (2.0 mmol), catalyst
(5 mol%), THF, r.t., 40 min. b Yield of the cross coupling products and
the conversion of alkyl halide are determined by 1H NMR (mesitylene as
internal standard). The conversion of alkyl halide for all reactions is 100%
after 40 min except in Entry 6 which is 65% for the r.t. reaction. c 45 ◦C.
5
2-iodopropane
69
35/82c
46/70d
79
-10 ◦C.
d
6
7
8
chlorocyclohexane
(bromomethyl)benzene
1-iodobutane
reagent is coupled with bromocyclohexane. The reactions
of p-FC6H4MgBr with different alkyl bromides also lead to the
desired products in high yield (entries 23–26).
Compound 4 can be reused several times; the conversion of
bromocyclohexane maintains 100% both for the 2nd and 3rd
runs but the yields of B (see Scheme 2) lower to 75% and 66%,
respectively. The loss of yield may be traced to the presence of
excess Grignard reagent in the reaction system after the 1st and
the 2nd runs. The +ve mode ESI-MS spectrum of the catalytic
reaction mixtures using 3 as the catalyst upon 10 min of reaction
gives a major peak of [MgBrL3]+ (448 m/z (75%)) and a minor
peak of [FeBrL3]+ (478 m/z (17%)). For the mixture with 4 as
catalyst, [MgBrL4]+ (427 m/z (63%)) and [FeBrL4]+ (459 m/z
(100%)) are also observed. Their parent species could be the aryl-
halo intermediates [MgArBrL] and [FeArBrL] as proposed by
Nagashima et al.2f The benefit of the hybrid ligands is exemplified
in their complexation and hence stabilization of not only the Fe(II)
catalysts but also the Grignard reagents, which could explain the
good catalytic performance of this system.
9
1-bromobutane
1-bromooctane
1-bromodecane
1-bromododecane
63c
10
11
12
57/72c
76c
61c
13
14
15
bromocyclohexane
bromocyclopentane
bromocycloheptane
99
79
77
16
17
18
3-bromopentane
chlorocyclohexane
1-bromobutane
72
Experimental
83c
61c
All preparations and manipulations were performed using stan-
dard Schlenk techniques under a nitrogen atmosphere. Solvents
were dried by standard procedures. FeCl2·4H2O (99.99%, trace
metal based) was purchased from Sigma–Aldrich. Elemental
analyses for C, H, and N were performed on a Perkin–Elmer PE
2400 CHNS elemental analyzer. The ligands are chemically pure
but isolated as oils which are susceptible to solvent occlusion,
thus leading to imperfection in some analytical data. 1H and
13C NMR analyses were measured in CDCl3 with an AMX500
500 MHz FT NMR spectrometers. IR spectra were recorded on
a Shimadzu IR-470 spectrometer using KBr pellets as IR matrix.
GC-MS analyses were recorded on Agilent 6890N/5973N system.
ESI-MS was performed on a Finnigan LCQ quadrapole ion trap
mass spectrometer. Sample was introduced into the ESI-source
using a syringe pump. The following ESI-MS parameters were
kept constant for ◦all measurement: Spray voltage = 4.5 kV;
Capillary T = 200 C; Flow rate = 5 mL min-1; Tube lens offset =
35 V and capillary voltage = 35 V. The spectra were obtained as
an average of at least 20 scans.
19
20
21
1-bromooctane
1-bromodecane
1-bromododecane
66c
73c
67c
22
23
bromocyclohexane
bromocyclopentane
98
85
8938 | Dalton Trans., 2011, 40, 8935–8940
This journal is
The Royal Society of Chemistry 2011
©

Synthesis of ligand L1–L4
Na+] = 347.2 (70%). For C18H24N6: C, 66.43; H, 7.74; N, 25.82.
Found C, 66.22; H, 7.29; N, 24.85.
Ligands L1–L4 were prepared by a common procedure as follows:
The mixture of (3,5-dimethyl-1H-pyrazol-1-yl)methanol (1.26 g,
10 mmol) and 5 mmol of the corresponding amine (furfurylamine,
0.435 g; 2-thiophenemethylamine, 0.566 g; 2-thiopheneethylamine,
0.636 g; 2-picolylamine, 0.504 g) in MeCN (20 mL) was stirred in
a closed vessel at r.t. for ~5d. The solvent MeCN was removed
under reduced pressure. The resulted oil was re-dissolved in
Et2O (20 mL). The solution was washed by water (3 ¥ 15
ml). The ether layer was dried by anhydrous Na2SO4. After
filtration, the solvent was removed on a rotatory evaporator. The
product was obtained as pale yellow oil. For bis[(3,5-dimethyl-
1H-pyrazolyl)methyl][(2-furanyl)methyl]amine (L1): yield: 0.207 g
Synthesis of compounds 1–4
Complexes 1–4 were synthesized by a common procedure as
follows: To a THF solution (2 mL) of FeCl2·4H2O (199 mg, 1.0
mmol) was added THF solution (8 mL) of L1 (313 mg, 1.0 mmol),
L2 (329 mg, 1.0 mmol), L3 (343 mg, 1.0 mmol) or L4 (324 mg,
1.0 mmol). The reaction mixture was stirred at r.t. for 6 h, then
the solvent removed under reduced pressure. The resulting solid
residue was washed with Et2O (3 ¥ 20 mL) to afford complexes
1–4 as yellow solid. For 1, Yield: 312 mg (71%). IR (KBr)/cm:
u(C N(Pz)) 1552(s). ESI-MS (m/z (%)): [FeL1Cl]+ = 404.0 (100).
Anal. Calcd. for C17H23N5OFeCl2: C, 46.39; H, 5.27; N, 15.91.
Found: C, 46.11; H, 5.18; N, 15.79. For 2, Yield: 337 mg (74%). IR
(KBr)/cm: u(C N(Pz)) 1551(s). ESI-MS (m/z (%)): [FeL2Cl]+ =
420.0 (100). Anal. Calcd. for C17H23N5SFeCl2: C, 44.76; H, 5.08;
N, 15.35. Found: C, 44.12; H, 4.74; N, 14.65. For 3, Yield: 371
mg (79%). IR (KBr)/cm: u(C N(Pz)) 1553(s). ESI-MS (m/z
(%)): [FeL3Cl]+ = 434.0 (100). Anal. Calcd. for C18H25N5SFeCl2:
C, 45.97; H, 5.36; N, 14.89. Found: C, 45.84; H, 5.37; N, 14.45.
For 4, Yield: 383 mg (85%). IR (KBr)/cm: u(C N(Pz)) 1554(s).
ESI-MS (m/z (%)): [FeL4Cl]+ = 415.1(100). Anal. Calcd. For
C18H24N6FeCl2: C, 47.92; H, 5.36; N, 18.63. Found: C, 47.86; H,
5.20; N, 18.15.
1
(94%). H-NMR (500 MHz, CDCl3): d (ppm) 2.03 (s, 6H, Pz-
CH3), 2.19 (s, 6H, Pz-CH3), 3.76 (s, 2H, furan-CH2), 4.88 (s,
4H, Pz-CH2), 5.79 (s, 2H, Pz-H), 6.22–6.23 (d, 1H, furan-H),
6.30–6.31 (m, 1H, furan-H), 7.33–7.34 (m, 1H, furan-H). 13C-
NMR (125 MHz, CDCl3): d (ppm) 10.4 (s, CH3-Pz), 13.4 (s,
CH3-Pz), 45.7 (s, N-CH2-furan), 64.2 (s, N-CH2-Pz), 105.8 (s,
CH(Pz)), 109.0 (s, CH(furan)), 110.3 (s, CH(furan)), 140.0 (s,
C(Pz)), 142.0 (s, CH(furan)), 147.5 (s, C(Pz)), 151.4 (s, C(furan)).
ESI-MS (m/z (%)): [L1 + H+] = 313.9 (20%), [L1 + Na+] = 336.1
(70%). Anal. Calcd. for C17H23N5O: C, 65.15; H, 7.40; N, 22.35.
Found C, 65.09; H, 6.71; N, 21.53. For bis[(3,5-dimethyl-1H-
pyrazolyl)methyl][(2-thiophenyl)methyl]amine (L2): 0.213 g (93%).
1H-NMR (500 MHz, CDCl3): d (ppm) 2.10 (s, 6H, Pz-CH3), 2.19
(s, 6H, Pz-CH3), 3.88 (s, 2H, thiophene-CH2), 4.95 (s, 4H, Pz-
CH2), 5.80 (s, 2H, Pz-H), 6.89–6.91 (m, 2H, thiophene-H), 7.17–
7.19 (m, 1H, thiophene-H). 13C-NMR (125 MHz, CDCl3): d (ppm)
10.7 (s, CH3-Pz), 13.4 (s, CH3-Pz), 46.9 (s, N-CH2-thiophene),
64.6 (s, N-CH2-Pz), 105.7 (s, CH(Pz)), 124.9 (s, CH(thiophene)),
126.1 (s, CH(thiophene)), 126.5 (s, CH(thiophene)), 140.0 (s,
C(Pz)), 141.9 (s, C(thiophene)), 147.7 (s, C(Pz)). ESI-MS (m/z
(%)): [L2 + H+] = 329.8 (30%), [L2 + Na+] = 352.1 (30%). Anal.
Calcd. for C17H23N5S: C, 61.97; H, 7.04; N, 21.26. Found C, 61.37;
H, 6.89; N, 19.60. For bis[(3,5-dimethyl-1H-pyrazolyl)methyl][(2-
thiophenyl)-ethyl] amine (L3): 0.225 g (95%). 1H-NMR (500 MHz,
CDCl3): d (ppm) 2.15 (s, 6H, Pz-CH3), 2.20 (s, 6H,Pz-CH3), 2.71,
2.72 and 2.74 (t, 2H, thiophene-CH2), 2.94, 2.95 and 2.97 (t, 2H,
N–CH2), 4.93 (s, 4H, Pz-CH2), 5.80 (s, 2H, Pz-H), 6.61 - 6.63 (m,
1H, thiophene-H), 6.85–6.86 (m, 1H, thiophene-H), 7.06–7.07
(m, 1H, thiophene-H). 13C-NMR (125 MHz, CDCl3): d (ppm)
10.8 (s, CH3-Pz), 13.4 (s, CH3-Pz), 28.1 (s, CH2-CH2-thiophene),
51.5 (s, CH2-CH2–N), 65.7(s, N-CH2-Pz), 105.87 (s, CH(Pz)),
123.2 (s, CH(thiophene)), 124.7 (s, CH(thiophene)), 126.7 (s,
CH(thiophene)), 139.7 (s, C(Pz)), 142.4 (s, C(thiophene)), 147.7 (s,
C(Pz)). ESI-MS (m/z (%)): [L3 + H+] = 343.8 (100%), [L3 + Na+] =
366.1(30%). Anal. Calcd. for C18H25N5S: C, 62.94; H, 7.34; N,
20.39. Found C, 63.20; H, 7.10; N, 19.77. For bis[(3,5-dimethyl-
X-ray crystallography
Diffraction measurements were conducted at 100(2)–293(2) K
on a Bruker AXS APEX CCD diffractometer by using Mo-Ka
˚
radiation (l = 0.71073 A). The data were corrected for Lorentz
and polarization effects with the SMART suite of programs and
for absorption effects with SADABS.13 Structure solutions and
refinements were performed by using the programs SHELXS-9714a
and SHELXL-9714b The structures were solved by direct methods
to locate the heavy atoms, followed by difference maps for the
light non-hydrogen atoms. Anisotropic thermal parameters were
refined for the rest of the non-hydrogen atoms. Hydrogen atoms
were placed geometrically and refined isotropically.
Catalytic reactions
General method for cross-coupling catalysis runs: After stan-
dard cycles of evacuation and back-fill with pure N2, catalyst
(0.05 mmol, 5.0 mol%) was introduced into a 25 mL Schlenk
tube equipped with a magnetic stir bar. To the catalyst were added
THF (2 mL), alkyl halide (1.0 mmol) and the solution was stirred
at r.t.. Aryl Grignard (2.0 mmol) was added dropwise within 30
min and the resulting mixture was stirred for another 10 min. The
reaction was quenched with HCl (aq., 2 M, 5 mL). The organic
phase was extracted with Et2O (3 ¥ 5 mL) and dried over MgSO4.
Mesitylene as internal standard (0.5 mmol, 0.069 mL) was added
and then the solvent was carefully removed on a rotary evaporator.
1
1H-pyrazolyl)methyl][(2-pyridinyl) methyl]amine (L4): H-NMR
(500 MHz, CDCl3): d (ppm) 2.01(s, 6H, Pz-CH3), 2.18 (s, 6H,
Pz-CH3), 3.87 (s, 2H, Py-CH2, 4.98 (s, 4H, Pz-CH2), 5.76 (s, 2H,
Pz-H), 7.11–7.16 (m, 2H, Py-H), 7.56–7.59 (m, 1H, Py-H), 8.47–
8.48 (d, 1H, Py-H). 13C-NMR (125 MHz, CDCl3): d (ppm) 10.6
(s, CH3-Pz), 13.4 (s, CH3-Pz), 54.6 (s, N-CH2-Py), 64.9 (s, N-CH2-
Pz), 105.7 (s, CH(Pz)), 122.0 (s, CH(Py)), 123.1 (s, CH(Py)), 136.4
(s, CH(Py)), 139.9 (s, C(Pz)), 147.7 (s, C(Pz)), 148.8 (s, CH(Py)),
158.7 (s, C(Py)). ESI-MS (m/z (%)): [L4 + H+] = 324.8 (100), [L4 +
Conclusions
We have demonstrated the catalytic efficacy of Fe(II) amine-
pyrazolyl complexes towards aryl Grignard coupling with alkyl
halides. The coordination mode of the hybrid ligands in these
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8935–8940 | 8939
©

complexes depends on the nature of the tether function on the
central amine moiety whereas the metal geometry is in turn
governed by the coordination state of the ligand. The use of
such flexible multidentate hybrid ligands can stabilise both metals
(Fe(II) and Mg(II)) through complexation and solubilisation as
well as catalyse Grignard cross-couplings under ambient condi-
tions. They could in principle also be able to give intermetallic
(Fe–Mg) species, whose formation could support the aryl transfer
across the metals. Current direction in our laboratory is directed
at the design of intermetallic complexes supported by suitable
tripodal or polydentate ligands and their use as single-site catalysis
for cross-couplings.
4 (a) R. B. Bedford, D. W. Bruce, R. M. Frost and M. Hird, Chem.
Commun., 2005, 4161; (b) R. B. Bedford, M. Betham, D. W. Bruce, A.
A. Danopoulos, R. M. Frost and M. Hird, J. Org. Chem., 2006, 71,
1104.
5 (a) H. Gao, C. Yan, X. Tao, Y. Xia, H. Sun, Q. Shen and Y. Zhang,
Organometallics, 2010, 29, 4189; (b) K. Bica and P. Gaertner, Org. Lett.,
2006, 8, 733; (c) R. R. Chowdhury, A. K. Crane, C. Fowler, P. Kwong
and C. M. Kozak, Chem. Commun., 2008, 94; (d) R. B. Bedford, D. W.
Bruce, R. M. Frost, J. W. Goodby and M. Hird, Chem. Commun., 2004,
2822.
6 (a) A. Fu¨rstner and R. Martin, Chem. Lett., 2005, 34, 624; (b) A.
Fu¨rstner, R. Martin, H. Krause, G. Seidel, R. Goddard and C. W.
Lehmann, J. Am. Chem. Soc., 2008, 130, 8773; (c) R. Martin and A.
Fu¨rstner, Angew. Chem. Int. Ed., 2004, 43, 3955.
7 (a) F. Li, J. J. Hu, L. L. Koh and T. S. A. Hor, Dalton Trans., 2010, 39,
5231; (b) S. Bai, S. Leelasubcharoen, X. Chen, J. L. Zuo, L. L. Koh and
T. S. A. Hor, Cryst. Growth Des., 2010, 10, 1715; (c) S. Bai, J. Y. Kwang,
L. L. Koh, D. J. Young and T. S. A. Hor, Dalton Trans., 2010, 39, 2631;
(d) S. K. Yen, L. L. Koh, H. V. Huynh and T. S. A. Hor, Eur. J. Inorg.
Chem., 2009, 4288; (e) F. Li and T. S. A. Hor, Chem. Eur. J., 2009, 15,
10585; (f) S. Bai, L. L. Koh and T. S. A. Hor, Inorg. Chem., 2009, 48,
1207; (g) F. Li, S. Bai and T. S. A. Hor, Organometallics, 2008, 27, 672.
8 (a) S. Teo, Z. Weng and T. S. A. Hor, Organometallics, 2008, 27, 4188;
(b) Z. Weng, S. Teo and T. S. A. Hor, Dalton Trans., 2007, 3493; (c) Z.
Weng, S. Teo and T. S. A. Hor, Acc. Chem. Res., 2007, 40, 676; (d) Z.
Weng, S. Teo and T. S. A. Hor, Organometallics, 2006, 25, 4878; (e) S.
Teo, Z. Weng and T. S. A. Hor, Organometallics, 2006, 25, 1199; (f) Z.
Weng, S. Teo and T. S. A. Hor, Chem. Commun., 2006, 12, 1319; (g) Z.
Weng, S. Teo, L. L. Koh and T. S. A. Hor, Organometallics, 2004, 23,
4342.
Acknowledgements
We acknowledge the National University of Singapore and the
Ministry of Education (WBS No: R-143-000-361-112) for financial
support.
Notes and references
1 See recent reviews (a) W. M. Czaplik, M. Mayer, J. Cvengrosˇ and A. J.
von Wangelin, ChemSusChem., 2009, 2, 396; (b) C. Bolm, J. Legros, J. Le
Paih and L. Zani, Chem. Rev., 2004, 104, 6217; (c) S. Enthaler, K. Junge
and M. Beller, Angew. Chem. Int. Ed., 2008, 47, 3317; (d) Iron Catalysis
in Organic Chemistry (Ed.: B. Plietker), Wiley-VCH, Weinheim, 2008.
2 (a) Metal-Catalyzed Cross-Coupling Reactions (ed.: A. De Meijere, F.
Diederich), 2nd ed., Wiley-VCH, Weinheim, 2004; (b) Transition Metals
for Organic Synthesis (ed.: M. Beller, C. Bolm), 2nd ed., Wiley-VCH,
Weinheim, 2004; (c) D. J. Ca´rdenas, Angew. Chem. Int. Ed., 2003, 42,
384; (d) A. C. Frisch and M. Beller, Angew. Chem. Int. Ed., 2005, 44, 674;
(e) B. D. Sherry and A. Fu¨rstner, Acc. Chem. Res., 2008, 41, 1500; (f) D.
Noda, Y. Sunada, T. Hatakeyama, M. Nakamura and H. Nagashima,
J. Am. Chem. Soc., 2009, 131, 6078 and the references therein; (g) O.
Vechorkin, V. Proust and X. Hu, J. Am. Chem. Soc., 2009, 131, 9756;
(h) L. Ackermann, A. R. Kapdi and C. Schulzke, Org. Lett., 2010, 12,
2298.
9 (a) J. Zhang, P. Braunstein and T. S. A. Hor, Organometallics, 2008, 27,
4277; (b) J. Zhang, A. Li and T. S. A. Hor, Organometallics, 2009, 28,
2935; (c) J. Zhang, A. Li and T. S. A. Hor, Dalton Trans., 2009, 9327.
10 S.-C. Sheu, M.-J. Tien, M.-C. Cheng, T.-l. Ho, S.-M. Peng and Y.-C.
Lin, J. Chem. Soc. Dalton. Trans., 1995, 3503.
11 A. Kermagoret and P. Braunstein, Dalton Trans., 2008, 585.
12 (a) D. Mandon, A. Machkour, S. Goetz and R. Welter, Inorg. Chem.,
2002, 41, 5364; (b) A. Machkour, D. Mandon, M. Lachkar and R.
Welter, Inorg. Chem., 2004, 43, 1545.
13 SADABS: Area-Detection Absorption Correction: Bruker AXS Inc.,
Madison, WI, 1995.
14 (a) G. M. Sheldrick, SHELXS-97 Program for crystal structure
solution, University of Go¨ttingen, Go¨ttingen, Germany, 1997; (b) G.
M. Sheldrick, SHELXL-97 Program for crystal structure refinement,
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
3 (a) M. Nakamura, K. Matsuo, S. Ito and E. Nakamura, J. Am. Chem.
Soc., 2004, 126, 3686; (b) G. Cahiez, V. Habiak, C. Duplais and A.
Moyeux, Angew. Chem., Int. Ed., 2007, 46, 4364.
8940 | Dalton Trans., 2011, 40, 8935–8940
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The Royal Society of Chemistry 2011
©