Iron(ii), manganese(ii) and cobalt(ii) complexes containing tetradentate biphenyl-bridged ligands and their application in alkane oxidation catalysis

Article abstract of DOI:10.1039/b513886h

A series of manganese(ii), iron(ii) and cobalt(ii) bis(triflate) complexes containing linear tetradentate bis(imine) and bis(amine) ligands with a biphenyl bridge have been synthesized. The twist in the ligand backbone due to the biphenyl unit leads in the case of the bis(imine) ligands (1 and 2) containing sp² hybridised N donors, to a distorted cis-α coordination geometry, whereas in the case of the biphenyl- and biphenylether-bridged bis(amine) ligands (7-9 and 12), a trans coordination geometry is observed. The catalytic properties of the complexes for the oxidation of cyclohexane, using H₂O₂ as the oxidant, have been evaluated. Only the iron complexes show any catalytic activity under the conditions used, but the low conversions and selectivies observed indicate that these catalysts lead predominantly to free radical auto-oxidation. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513886h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Iron(II), manganese(II) and cobalt(II) complexes containing tetradentate
biphenyl-bridged ligands and their application in alkane oxidation catalysis†
George J. P. Britovsek,* Jason England and Andrew J. P. White
Received 30th September 2005, Accepted 22nd December 2005
First published as an Advance Article on the web 20th January 2006
DOI: 10.1039/b513886h
A series of manganese(II), iron(II) and cobalt(II) bis(triflate) complexes containing linear tetradentate
bis(imine) and bis(amine) ligands with a biphenyl bridge have been synthesized. The twist in the ligand
backbone due to the biphenyl unit leads in the case of the bis(imine) ligands (1 and 2) containing sp2
hybridised N donors, to a distorted cis-a coordination geometry, whereas in the case of the biphenyl-
and biphenylether-bridged bis(amine) ligands (7–9 and 12), a trans coordination geometry is observed.
The catalytic properties of the complexes for the oxidation of cyclohexane, using H2O2 as the oxidant,
have been evaluated. Only the iron complexes show any catalytic activity under the conditions used, but
the low conversions and selectivies observed indicate that these catalysts lead predominantly to free
radical auto-oxidation.
and activities for the formation of cyclohexanol in the oxidation
of cyclohexane.3–5 Oxygen labeling studies have established that,
Introduction
unlike in Fenton chemistry, O2 is not involved in these oxidations.4
In addition, stereospecific hydroxylation has been observed with
a prochiral substrate.6,7 These observations, combined with rel-
atively large kinetic isotope effects, have led to the suggestion
that alkane hydroxylations catalysed by A or B operate via a
metal-based mechanism rather than the unselective radical chain
auto-oxidation mechanism. The existence of high-valent iron-
oxo species, first proposed in 1932,8 has gained strong support
recently through the first crystallographic characterization of two
Fe(IV) oxo complexes.9,10 Catalytic efficiencies are still rather low,
partly due to decomposition of H2O2 (‘catalase’ activity)11 and
probably also due to oxidative degradation of the catalyst, a
common problem in oxidation catalysis.12 These problems may
be overcome by improvements in catalyst design, which is the aim
of our research.
In recent years, several non-heme iron-based catalysts for the
oxidation of alkanes with H2O2 have been reported.1,2 Important
examples are the tetradentate tpa (A) and bpmen (B) complexes
shown in Fig. 1, where X represents a weakly bound co-ligand,
typically CH3CN or triflate (OTf−). The combination of catalytic
amounts of these complexes and H2O2 as the oxidant have shown
unusual product selectivities such as remarkably high selectivities
Based on the success obtained with non-heme iron catalysts A
and B it appears that multidentate ligands containing nitrogen
(preferably pyridine) donors and a cis orientation of two labile co-
ligands are desirable criteria in catalyst design. We have previously
reported the use of metal complexes containing tridentate pyridine
bis(imine) and pyridine bis(amine) ligands and their application as
catalysts for the oxidation of cyclohexane. It was found that instead
of just two, sometimes three labile sites were present at the metal
centre and these catalyst systems resulted in Fenton-type reactivity
only.13 We therefore turned our attention to tetradentate ligands.
Initially, tripodal ligands of type A and the effect of pyridine versus
amine donors on the catalytic properties of iron(II) bis(triflate)
complexes in cyclohexane oxidation was investigated.14 Here we
report our results on the synthesis and characterisation of a series
of metal(II) complexes containing linear tetradentate bis(imine)
and bis(amine) ligands containing a biphenyl-bridged ligand
backbone (types C and D). The biphenyl bridge, in particular
the dimethyl-substituted biphenyl unit, has been used previously
to enforce cis-a geometries in transition metal complexes.15 This
geometry, which leaves two labile cis co-ligands at the metal centre,
is also seen in bpmen B complexes and is believed to be important
Fig. 1 Non-heme iron complexes containing tetradentate ligands.
Department of Chemistry, Imperial College London, Exhibition Road, South
Kensington, London, UK SW7 2AY. E-mail: g.britovsek@imperial.ac.uk;
Tel: +44-(0)20-75945863
† Electronic supplementary information (ESI) available: Fig. S1–S10. See
DOI: 10.1039/b513886h
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for high catalytic activity in the oxidation of cyclohexane with
H2O2. Compared to iron, the next-door neighbours manganese
and cobalt have received much less attention as potential catalysts
for the oxidation of alkanes,16–18 despite their large-scale industrial
application in the oxidation of p-xylene and cyclohexane.19,20 In
addition to iron(II), we have therefore also prepared and evaluated
manganese(II) and cobalt(II) complexes of the biphenyl-bridged
ligands.
catalytic properties for olefin polymerization were investigated.26
The ketimine derivative 2 was also described but could not be
obtained in pure form. We found that by using Si(OEt)4 as the
dehydrating agent, compound 2 could be obtained cleanly. Both
ligands 1 and 2 react readily with various metal salts and the
manganese(II), iron(II) and cobalt(II) bis(triflate) complexes have
been successfully prepared (see Scheme 1).
The non-methylated pyridylamine precursors 3–6 have been
generally prepared by two-step procedures via reduction of the
bis(pyridylimine).21,25,27,28 We have prepared compounds 3–6 by
a more convenient high yielding one-pot reductive amination
protocol using NaBH(OAc)3. These secondary amines were sub-
sequently methylated to give the tertiary amines 7–10 (Scheme 2).
The potentially pentadentate 12, containing a biphenylether
backbone was prepared by a similar procedure. The unsubstituted
ligand 7 and the ligands bearing two methyl groups 8 and 9 reacted
readily with Fe(OTf)2(CH3CN)2 in dichloromethane to give the
corresponding iron(II) bis(triflate) complexes. In the case of ligand
8, containing methyl groups in the pyridyl 6-position, the resulting
complex [Fe(8)OTf2] was found to be significantly more air-
sensitive than the other complexes. The reaction of ligand 10, con-
taining four methyl substituents, with Fe(OTf)2(CH3CN)2 did lead
to a product but this could not be isolated in pure form due to its
instability.
Results and discussion
Synthesis of ligands and complexes
Biphenyl bridged bis(pyridylimines) have been known for some
time,21 but their coordination chemistry has only recently been in-
vestigated, in particular binaphthyl-bridged22–24 and 6,6-dimethyl
biphenyl-bridged variations such as 1,25 which form atropisomers
and may find application as ligands in asymmetric catalysis.
Whilst this work was in progress, the synthesis of iron(II) and
cobalt(II) dichloro complexes of ligand 1 was reported and their
All metal complexes have been characterised by MS, CHN
analysis and magnetic moment. The complexes are paramagnetic,
with magnetic moments corresponding to a high-spin d5, d6 and d7
electronic configuration for Mn(II), Fe(II) and Co(II), respectively.
1H and 19F NMR spectra have been recorded and assigned for
Fe(II) and Co(II) complexes (see Experimental and ESI†). In
1
some cases H COSY NMR spectra have been recorded to aid
peak assignments (see ESI†). The metal complexes of the diimine
ligands are generally brightly coloured, whereas the diamine
iron(II) complexes are typically off-white solids, displaying rather
Scheme 1
Scheme 2
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◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for complex [Fe(1)OTf2]
Differences between [Fe(1)Cl2] and [Fe(1)OTf2] can also be seen
in the iron coordination distances. Though the Fe–N(pyridine)
bond lengths are just about statistically the same, being 2.199(2)
Fe–N(1)
Fe–O(1)
C(9)–C(9ꢀ)
2.199(2)
2.097(2)
1.501(6)
Fe–N(7)
C(7)–N(7)
2.190(2)
1.262(4)
˚
and 2.214(3) A in [Fe(1)OTf2] and [Fe(1)Cl2], respectively, those to
˚
the imine nitrogens differ by ca. 0.1 A {2.190(2) and 2.296(4)
N(1)–Fe–N(7)
N(1)–Fe–N(1ꢀ)
N(1)–Fe–O(1ꢀ)
N(7)–Fe–N(7ꢀ)
O(1)–Fe–O(1ꢀ)
74.67(9)
168.17(14)
84.62(10)
80.03(13)
103.6(2)
N(1)–Fe–O(1)
N(1)–Fe–N(7ꢀ)
N(7)–Fe–O(1)
N(7)–Fe–O(1ꢀ)
88.07(10)
114.98(10)
93.33(11)
152.77(11)
˚
A in [Fe(1)OTf2] and [Fe(1)Cl2], respectively}, presumably a
consequence of the varying trans influences exerted by triflate and
˚
chloride; the respective Fe–X distances are 2.097(2) A to O(1) in
˚
Fe(1)OTf2, and 2.397(2) A to Cl in [Fe(1)Cl2]. Additionally, the
X–Fe–Xꢀ angle between the two cis anionic co-ligands is 103.6(2)◦
in [Fe(1)OTf2] (cf. 111.6(1)◦ in [Fe(1)Cl2]).
featureless UV-VIS spectra (see ESI†). Complexes [Fe(1)OTf2] and
[Fe(9)OTf2] have been characterised by X-ray crystallography.
In contrast to [Fe(1)OTf2], the X-ray crystal structure of
[Fe(9)OTf2] (which has molecular rather than crystallographic C2
symmetry about an axis that passes through the metal centre and
bisects the C(14)–C(15) aryl–aryl linkage) shows the tetradentate
N,Nꢀ,Nꢀꢀ,Nꢀꢀꢀ diamine ligand 9 to have adopted a so-called trans ge-
ometry around the metal centre (Fig. 3), the two triflate co-ligands
being mutually trans [O(1A)–Fe–O(2A) 173.75(15)◦]. The geome-
try at the metal is again distorted octahedral (Table 2), though the
distortions are less than seen in [Fe(1)OTf2]. The bite angles [N(1)–
Fe–N(8) 78.63(16)◦, N(21)–Fe–N(28) 77.26(16)◦] compare to the
bite angle of 74.67(9)◦ for the equivalent N(pyridyl),N(imine)
chelate ring in [Fe(1)OTf2]. Similar to the conformations seen in
[Fe(1)OTf2], here in [Fe(9)OTf2] both of the 5-membered chelate
rings have envelope conformations, though distinctly more folded
than in the earlier structure. For the N(1),N(8) ring the methylene
Solid state structures
A solid state structure of [Fe(1)OTf2] has been determined
previously, but was not discussed in any detail.26 The two structures
for [Fe(1)OTf2] are identical and similar to the dichloro analogue
[Fe(1)Cl2],26 again having crystallographic C2 symmetry about an
axis that passes through the metal centre and bisects the C(9)–
C(9ꢀ) aryl–aryl linkage of the ligand (Fig. 2). The geometry at the
iron centre is significantly distorted octahedral (Table 1) and the
ligand has adopted a distorted cis-a geometry. The restricted bite
[74.67(9)◦] of the two 5-membered chelate rings [involving N(1)
and N(7)] causes the two imine nitrogens N(7) and N(7ꢀ) to be
displaced out of the equatorial {FeO2} coordination plane such
that the {Fe,O(1),O(1ꢀ)} and {Fe,N(7),N(7ꢀ)} planes are inclined
by ca. 35◦. These 5-membered chelate rings have envelope con-
˚
formations, N(1) lying ca. 0.23 A out of the {Fe,C(6),C(7),N(7)}
˚
plane which is coplanar to within ca. 0.01 A. Though larger, the
twisted 7-membered N(7),N(7ꢀ) chelate ring also has a restricted
ꢀ
˚
bite angle [N(7)–Fe–N(7 ) 80.03(13) A]; the aryl–aryl linkage is a
clear single bond [C(9)–C(9 ) 1.501(6) A] with a torsion angle of ca.
ꢀ
˚
67◦. Compared to the dichloro analogue, the pyridine nitrogens in
[Fe(1)OTf2] are bent slightly towards the triflate ligands, the N(1)–
Fe–O(1) and N(1)–Fe–O(1ꢀ) angles being 88.07(10) and 84.62(10)◦
(cf. 91.4(1) and 89.4(1)◦ in [Fe(1)Cl2]) and this is reflected in
the axial N(pyridine)–Fe–N(pyridine) angles of 168.17(14)◦ in
[Fe(1)OTf2], and 178.6(2)◦ in [Fe(1)Cl2].
Fig. 3 The molecular structure of Fe(9)OTf2.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complex [Fe(9)OTf2]
Fe–N(1)
2.151(4)
2.282(4)
2.144(4)
1.504(7)
Fe–N(8)
Fe–N(28)
Fe–O(2A)
2.295(4)
2.153(4)
2.116(4)
Fe–N(21)
Fe–O(1A)
C(14)–C(15)
N(1)–Fe–N(8)
78.63(16)
101.96(17)
88.50(16)
171.56(16)
93.54(15)
94.80(15)
89.17(16)
173.75(15)
N(1)–Fe–N(21)
N(1)–Fe–O(1A)
N(8)–Fe–N(21)
N(8)–Fe–O(1A)
N(21)–Fe–N(28)
N(21)–Fe–O(2A)
N(28)–Fe–O(2A)
169.85(16)
95.30(16)
103.66(15)
82.39(14)
77.26(16)
81.51(15)
94.89(16)
N(1)–Fe–N(28)
N(1)–Fe–O(2A)
N(8)–Fe–N(28)
N(8)–Fe–O(2A)
N(21)–Fe–O(1A)
N(28)–Fe–O(1A)
O(1A)–Fe–O(2A)
Fig. 2 The molecular structure of the C2 symmetric complex Fe(1)OTf2.
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˚
bis(pyridylmethylamine) ligands with sufficient flexibility in the
ligand backbone, the preferred coordination geometry is trans.
Shortening the backbone, for example by using a propylene bridge,
results in a move of one of the pyridylmethyl arms out of the
coordination plane to give a cis-b geometry. Further restriction
of the flexibility, as in bpmen, leads to the least favourable cis-a
geometry. Considering the requirement for cis labile sites in non-
heme alkane oxidation catalysis, these observations may be related
to the catalytic activity of the complexes.
carbon C(7) lies ca. 0.59 A out of the {Fe,N(1),C(6),N(8)} plane
˚
(which is coplanar to within ca. 0.07 A), whilst for the N(21),N(28)
ring it is the amino nitrogen N(21) which lies ca. 0.74 A out of
˚
the {Fe,C(22),C(23),N(28)} plane (which is coplanar to within
˚
ca. 0.02 A). One of the more noticeable differences between the
coordination behaviour of ligands 1 and 9 is the bite angle of
the 7-membered N,Nꢀ chelate ring. Here in [Fe(9)OTf2] the bite
angle for this ring is 103.66(15)◦, (cf. 80.03(13)◦ in [Fe(1)OTf2]),
very clearly reflecting the release of strain on going from imino to
amino nitrogens. Concurrently, the torsion angle about the aryl–
◦
◦
˚
aryl linkage C(14)–C(15) [1.504(7) A] increases to ca. 81 (cf. 67
Solution behaviour
in [Fe(1)OTf2]). The equatorial plane formed by the iron centre
and the four donor nitrogen atoms of the tetradentate ligand 9
is noticeably twisted, the {Fe,N(1),N(8)} and {Fe,N(21),N(28)}
planes being inclined by ca. 15◦.
The most obvious difference in the Fe–N bond lengths seen in
[Fe(1)OTf2] and those in [Fe(9)OTf2] is unsurprisingly associated
with the change from imine in [Fe(1)OTf2] [Fe–N(imine) 2.190(2)
The solution behaviour of the iron(II) and cobalt(II) complexes
1
was investigated by H and 19F NMR. The bis(imine) complexes
[Fe(1)OTf2], [Fe(2)OTf2] and [Co(1)OTf2] show nine paramagnet-
ically shifted resonances in the 1H NMR (cf. Fig. S5, ESI†), which
is consistent with the C2-symmetric cis-ageometry seen in the solid
state for [Fe(1)OTf2]. The peaks have been assigned on the basis
of their integration and the proximity of the protons to the metal
centre.26 In addition, correlation spectroscopy (COSY) was used
to identify coupling interactions (Fig. S6, see ESI†).
The 1H NMR spectra of the bis(amine) complexes [Fe(7)OTf2]
and [Fe(9)OTf2] show 11 signals in CDCl3, consistent with the C2-
symmetric trans geometry seen in the solid state for [Fe(9)OTf2]
(see Fig. S7, ESI†). Complex [Fe(8)OTf2] also shows a single
C2-symmetric species in CDCl3 solution (11 signals), but the
chemical shifts are significantly different, indicating the possibility
of the less favourable cis-a geometry in this case. This may be
caused by a steric clash between the two methyl groups in the
pyridyl 6-position. These opposing effects, the biphenyl bridge
dictating a trans geometry and the clash between the methyl groups
distorting the geometry to cis-a, should lead to a tension within
the complex, which may be the reason for the decreased stability
of this complex in solution and the notably increased sensitivity
to air oxidation. In addition, complex [Fe(10)OTf2] containing the
6,6ꢀ-dimethylbiphenyl bridge, was found to be very unstable and
could not be isolated in pure form.
˚
˚
A] to amine in [Fe(9)OTf2] [Fe–N(amine) 2.282(4), 2.295(4)
A]. The Fe–N(pyridine) distances also vary between the two
˚
structures {2.199(2) A in [Fe(1)OTf2], cf. 2.151(4) and 2.153(4)
˚
A in [Fe(9)OTf2]}. The distances to the mutually trans triflate
˚
co-ligands [2.116(4) and 2.144(4) A] are comparable with the
˚
unique Fe–O bond length of 2.097(2) A seen for the cis oriented
triflate ligands in [Fe(1)OTf2]; the reason for the slight asymmetry
between the two Fe–O bonds in [Fe(9)OTf2] is not immediately
apparent, especially given the approximate C2 symmetry of the
complex. There are no intermolecular packing interactions of
note.
Conformational analysis of linear tetradentate ligands in
6-coordinate complexes
Linear tetradentate ligands are capable of arranging their donors
around a metal centre in three different ways: cis-a, cis-b and trans
(see Table 3).15,29,30 The trans geometry observed in [Fe(9)OTf2]
was initially surprising as the twisted biphenyl backbone was
expected to enforce a cis-a coordination mode. However, when
comparing coordination geometries observed in metal complexes
of a series of linear tetradentate bis(pyridylmethyl) diamine ligands
that have been structurally characterised, as shown in Table 3, a
trend becomes apparent. Metal complexes of the ethylene-bridged
bpmen ligand (I) are invariably cis-a, whereas for the slightly more
constrained cyclohexyl backbone, both cis-a and cis-b conforma-
tions have been observed. The more flexible propylene-bridged
derivative III yields, with a few exceptions, cis-b geometries.
The very rigid double ethylene-bridged derivative IV can only
give a trans geometry, which is distorted to a trigonal prismatic
geometry when the bridges are replaced by propylene units as
in V. The ethylene-bridged bis(imine) ligand VI, containing sp2
hybridized nitrogen donors, also prefers the trans geometry, unless
the backbone is twisted by a biphenyl bridge, as shown for ligand
1. There is, however, considerable strain in this conformation
and the addition of stronger field co-ligands such as iso-nitriles
has been shown to result in a cis-b coordination geometry.26
Finally, the twist-effect of the biphenyl unit is significantly reduced
when the nitrogen donors are sp3 hybridized, as in 9, and the
backbone behaves as a relatively flexible C4-linkage, resulting in a
distorted trans geometry. Thus, it appears that for singly bridged
Triflate anions are generally weakly coordinating ligands and
are often displaced by acetonitrile ligands in solution. In CD3CN,
1
[Fe(7)OTf2] and [Fe(9)OTf2] gave H NMR spectra containing
large numbers of resonances, indicative of the presence of
multiple species. This could be due to geometrical isomers, but
more likely is the presence of an equilibrium mixture between
bis(triflate) [Fe(L)OTf2], monotriflate [Fe(L)(OTf)(CD3CN)]+ and
bis(acetonitrile) [Fe(L)(CD3CN)2]2+ complexes. One of the con-
tributing species in the CD3CN spectra was found to exhibit a set
of peaks with very similar chemical shifts compared to the CDCl3
spectra, indicating the presence of the bis(triflate) complex. In
contrast, [Fe(8)OTf2] gave cleanly a spectrum containing 11 lines
in CD3CN, consistent with a single C2-symmetric complex, most
likely [Fe(8)(CD3CN)2]2+ with a cis-ageometry (Fig. S8, see ESI†).
Complex [Fe(12)OTf2] displayed 10 signals both in CDCl3 and in
CD3CN solution (see Fig. S9, see ESI†), indicative of the trans
complexes [Fe(12)OTf2] and [Fe(12)(CD3CN)2]2+, respectively.
In order to understand the complex solution behaviour of the
iron complexes containing ligands 7–9 and 12 in more detail,
we have carried out a series of 19F NMR experiments. 19F
NMR spectroscopy is particularly useful for determining whether
triflate anions are coordinated to a metal centre. In diamagnetic
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compounds the 19F chemical shift for a triflate group in CD2Cl2 at
(2.1 mmol, 1000 equiv.). A large excess of substrate was used
to minimize over-oxidation of cyclohexanol (A) to cyclohexanone
(K). The addition of dilute H2O2 was carried out slowly using
a syringe pump, in order to minimise H2O2 decomposition. The
yields are based on the amount of oxidant H2O2 converted into
oxygenated products. Two series of catalytic experiments were
carried out using 10 (70 mM) and 100 (700 mM) equiv. of H2O2. In
some cases, cyclohexyl hydroperoxide (P) could also be identified
by GC-MS as one of the reaction products. Cyclohexyl hydro-
peroxide is a relatively stable peroxide which can be isolated in
pure form,36,37 but does decompose to some extent during GC
analysis.38 We have therefore not quantified the amount of P,
but have merely indicated a qualitative (Y/N) observation in
Table 4.
The two iron bis(triflate) complexes containing the ligands
bpmen and tpa are used as benchmarks against which we
compare our other catalysts. Using 10 equiv. of H2O2 the most
active catalyst [Fe(bpmen)OTf2] converts 65% of the H2O2 added
into oxygenated products, with a large A/K ratio. The complex
[Fe(tpa)OTf2] gives a conversion of 32%, but with a better A/K
ratio. These results are consistent with those reported previously
by Que and co-workers for complexes [Fe(bpmen)(CH3CN)2]-
(ClO4)2 and [Fe(tpa)(CH3CN)2](ClO4)2, respectively.7 Similar to
previous observations with manganese and cobalt complexes
containing bis(imino)pyridine ligands,13 the complexes [Mn(1)-
OTf2] and [Co(1)OTf2] showed no catalytic activity under these
conditions.
room temperature can vary between −78.7 ppm for a covalently
bound triflate such as Me3SiOTf31 and −80.5 ppm for ionic triflate
in [PPN]OTf (PPN = Ph3P N PPh3 ).32 Diamagnetic transition
metal triflate complexes also generally show 19F resonances
between −77 and −79 ppm.33 In paramagnetic iron(II) complexes
much larger differences in chemicals shifts are observed, ranging
from ca. +60 ppm (bridging triflate), to ca. −10 ppm (terminal)
and ca. −80 ppm (free triflate).14,34,35 In CDCl3 or CD2Cl2, the
19F NMR spectra of complexes [Fe(7)OTf2] and [Fe(9)OTf2] show
a single peak at −11.1 and −12.1, respectively, indicating that
for these complexes the triflate anions are coordinated. The same
applies to complex [Fe(12)OTf2] with a chemical shift of −0.3 ppm,
which also indicates that the ether oxygen donor is probably
not coordinated to the iron centre. Complex [Fe(8)OTf2] shows
a peak at −20.5 ppm, slightly upfield, which indicates increased
lability of the triflate ligands in CD2Cl2. In CD3CN, the complexes
+
=
=
[Fe(8)OTf2] and [Fe(12)OTf2] show relatively sharp peaks (m1/2
<
100 Hz) at −77 ppm, indicative of uncoordinated triflate anions.
Complexes [Fe(7)OTf2] and [Fe(9)OTf2] on the other hand, give
significantly broadened peaks (m1/2 > 1000 Hz) between −70 and
−72 ppm. This suggests partial coordination of the triflate anions,
1
which is consistent with the multiple species observed in the H
NMR spectra of these complexes.
Catalytic oxidation of alkanes
When using 70 mM solutions of hydrogen peroxide, the
complexes [Fe(1)OTf2], [Fe(7)OTf2] and [Fe(9)OTf2] exhibit higher
yields of product (A + K) (10, 9.5 and 14%, respectively) than
Fe(OTf)2(CH3CN)2 (3.8%). Although no cyclohexyl hydroperox-
ide P is observed in the product mixtures, it should be noted
that the A/K ratios are small (1.3, 2.1 and 1.9, respectively).
This implies that Fenton type chemistry is dominant, although
partial involvement of iron-based oxidants is possible. In line with
previous results,13,14 at increased hydrogen peroxide concentration
(700 mM) using complex [Fe(1)OTf2] as the catalyst, the A/K
ratio increased and the production of P was observed. The yield
of (A + K), however, was seen to decrease dramatically from 10%
to 2.6%. This apparent deactivation of the catalyst is assumed to
be the result of catalyst decomposition, most likely via oxidation
or hydrolysis of the imine functionality.
The catalytic properties of the iron(II), manganese(II) and
cobalt(II) bis(triflate) complexes containing ligands 1, 7–9 and 12
for the oxidation of cyclohexane with H2O2 have been evaluated
(eqn (3)).
(3)
The oxidation reactions were carried out in acetonitrile as
the solvent at room temperature under air. Hydrogen peroxide
solution (70 mM, 10 equiv.) was added to an acetonitrile solution
containing the catalyst (2.1 lmol, 1 equiv.) and cyclohexane
Table 4 Catalytic oxidation of cyclohexane with hydrogen peroxidea
Run
Catalyst
Equiv. of H2O2
Yield of A + K (%)b
A/Kc
Pd
1
2
3
4
5
6
7
8
9
Fe(OTf)2(CH3CN)2
Fe(OTf)2(CH3CN)2
[Fe(tpa)(OTf)2]
[Fe(tpa)(OTf)2]
[Fe(bpmen)(OTf)2]
[Fe(bpmen)(OTf)2]
[Fe(1)(OTf)2]
10
100
10
100
10
100
10
100
10
3.8
3.4
32
29
65
48
10
2.6
14
0
1.6
2.4
12.0
5.9
9.5
2.5
1.3
2.0
1.9
0
Y
Y
N
N
N
N
N
Y
N
N
N
Y
[Fe(1)(OTf)2]
[Fe(7)(OTf)2]
10
11
12
[Fe(8)(OTf)2]
10
10
10
[Fe(9)(OTf)2]
9.5
3.7
2.1
2.8
[Fe(12)(OTf)2]
a Conditions: b Total percentage yield of cyclohexanol (A) + cyclohexanone (K), expressed as moles of product per mole of H2O2. c Ratio of moles of
cyclohexanol (A) to moles of cyclohexanone (K). d Indication of detection of cyclohexyl hydroperoxide (P): Y = Yes, N = No.
1404 | Dalton Trans., 2006, 1399–1408
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Considering the moderate activity of the complexes [Fe(7)OTf2]
and [Fe(9)OTf2], it was rather surprising that [Fe(8)OTf2] displayed
no catalytic activity under the conditions used here. However,
as mentioned previously, exposure of an acetonitrile solution
of [Fe(8)OTf2] to air caused the colourless solution to turn
yellow–brownwithinminutes. This colour change probablyreflects
oxidation by dioxygen and it is the product of oxidation that is
found to be unable to catalyse the oxidation. The robustness of
the product of oxidation of [Fe(8)OTf2] indicates either formation
of multimetallic oxo-bridged species, or metal-mediated ligand
oxidation. The high sensitivity of [Fe(8)OTf2] to oxidation by
dioxygen is likely to be related to the destabilising effect of the
methyl groups substituted at the 6-position of the pyridyl rings.
The complex [Fe(12)OTf2] was found to be a very poor
catalyst for the oxidation of cyclohexane with an (A + K)
yield and A/K ratio of 3.7% and 2.8, respectively. Additionally,
cyclohexyl hydroperoxide (P) was observed in the GC trace of the
product mixture. These observations are all indicative of catalysis
proceeding predominantly via Fenton-type chemistry.
In conclusion, we have prepared and characterized transition
metal complexes of biphenyl-bridged bis(pyridylmethylimine) and
bis(pyridylmethylamine) ligands. The twist of the biphenyl bridge
was shown in the case of the iron(II) bis(triflate) complexes
containing the bis(imine) ligand 1 to give a distorted cis-a coordi-
nation geometry, whereas the amine ligand 9, rather unexpectedly,
gave a trans geometry. A similar geometry is assumed for the
unsubstituted derivative 7. The introduction of methyl groups in
the 6 position of the pyridyl groups, as in [Fe(8)OTf2], is believed to
distort this trans geometry to give a cis-a geometry. This complex
is inherently less stable than the previous complexes, leading to
an increased sensitivity to air oxidation. Complex [Fe(10)OTf2],
containing both methyl groups on the pyridine rings and on
the biphenyl backbone, was too unstable to be isolated in pure
form. The biphenylether ligand 12 forms an iron(II) bis(triflate)
complex where, based on 19F NMR studies, both triflate ligands
are coordinated in CH2Cl2 solution. This indicates that the ether
oxygen is not coordinated and the coordination geometry is most
likely trans. The catalytic properties of the iron(II) complexes for
the oxidation of cyclohexane with H2O2 in acetonitrile have been
evaluated and it can be concluded that all iron(II) bis(triflate)
complexes oxidize cyclohexane predominantly via a radical chain
auto-oxidation mechanism, which results typically in low con-
versions (<10% of the oxidant), low selectivity (A/K ≈ 1) and
extensive decomposition of H2O2. In the case of the bis(imine)
ligands this is believed to be due to the sensitivity of the imine
moieties to oxidation or hydrolysis, whereas the biphenyl bridged
bis(amine) ligands give rise to complexes with the unfavourable
trans geometry. These results highlight the importance of a cis
disposition of the labile co-ligands in non-heme alkane oxidation
catalysis, i.e. the use of tetradentate ligands that enforce a rigid
cis-a coordination geometry around the iron centre.
either on a Bruker AC-250 or a DRX-400 spectrometer; chemical
1
shifts for H and 13C NMR are referenced to the residual protio
impurity and to the 13C-NMR signal of the deuterated solvent.
Mass spectra were recorded on either a VG Autospec or a VG
Platform II spectrometer. Elemental analyses were performed by
the Science Technical Support Unit at the London Metropolitan
University. GC analysis was carried out on an Agilent 6890A
gas chromatograph with a HP-5 column (30 m × 0.32 mm,
film thickness 0.25 lm). Toluene was used as the standard for
quantitative analysis. Magnetic moments were determined by the
Evans’ NMR method in CD2Cl2, unless otherwise stated.39
Solvents and reagents
Toluene and pentane were dried by passing through a column,
filled with commercially available Q-5 reagent (13 wt% CuO
on alumina) and activated alumina (pellets, 3 mm). Diethyl
ether and tetrahydrofuran were dried over potassium metal
with a benzophenone ketyl indicator, whereas dichloromethane
and acetonitrile were dried over CaH2. The metal complex
precursors Mn(OTf)2 and Fe(OTf)2(CH3CN)2,40,41 and N,Nꢀ-(6,6ꢀ-
dimethylbiphenyl-2,2ꢀ-diyl)diamine42 and N,Nꢀ-(6,6ꢀ-dimethyl-
biphenyl-2,2ꢀ-diyl)bis(2-pyridylmethyl)diimine (1)25 as well as the
cobalt complex [Co(1)Cl2]26 have been reported previously. All
other chemicals and NMR solvents were obtained commercially
and used as received.
Synthesis of ligands
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)bis(2-pyridylethyl)diimine
(2). 0.56 g of 2,2ꢀ-diamino-6,6ꢀ-dimethylbiphenyl (2.64 mmol),
2.2 equiv. of 2-acetylpyridine (0.65 ml, 5.80 mmol), 2.2 equiv. of
tetraethoxy silane (1.29 ml, 5.80 mmol) and 1 drop of sulfuric
acid were heated, in a flask equipped for distillation, at 160 ◦C for
16 h. Upon cooling, the residue was dissolved in diethyl ether and
washed with saturated sodium hydrogen carbonate solution. The
organic layer was separated, washed with water, dried (magnesium
sulfate) and reduced to dryness. The residue was recrystallised
from ethanol to give 2 as a brick red crystalline solid (0.49 g,
1
44.3%). H-NMR (CDCl3): d 8.54 (d, 2H, 6-PyH), 7.60 (m, 4H,
3-PyH and 4-PyH), 7.24 (m, 2H, 5-PyH), 7.17 (t, 2H, PhHp), 7.02
=
(d, 2H, PhHm), 6.55 (d, 2H, PhHm), 2.27 (s, 6H, MeC N), 2.13
(s, 6H, PhMe); MS (EI): m/z (%) 418 (100), 403 (34), 340 (45)
[M–Py], 285 (48), 202 (47); Anal. Calcd. (found) for C28H26N4: C,
80.35 (80.47); H, 6.26 (6.23); N, 13.39 (13.28).
N,Nꢀ-(biphenyl-2,2ꢀ-diyl)-N,Nꢀ-bis(2-pyridylmethyl)diamine (3).
Two equiv. of 2-pyridine carboxaldehyde (0.52 ml, 5.42 mmol) was
added to a mixture of 1 equiv. of 2,2ꢀ-diamino-biphenyl (0.50 g,
2.71 mmol) and 2.8 equiv. of sodium trisacetoxyborohydride
(1.61 g, 7.60 mmol) in DCM and stirred overnight, under a nitro-
gen atmosphere. Saturated aqueous sodium hydrogen carbonate
solution was added to the resultant mixture, stirred for 15 min
and extracted with ethyl acetate. The organic layer was dried
(magnesium sulfate) and reduced to dryness. The oil obtained
was dissolved in diethyl ether, filtered to remove any insoluble
material and reduced to dryness to give 3 as a pale yellow oil
(0.70 g, 70%). Compounds 4, 5 and 6 were also prepared using
this procedure in 97%, 96% and 92% yield, respectively. Analytical
data was identical to that previously reported.25,28
Experimental
General
All moisture-sensitive compounds were manipulated using stan-
dard vacuum line, Schlenk or cannula techniques, or in a con-
ventional nitrogen-filled glove box. NMR spectra were recorded
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N,Nꢀ -(biphenyl-2,2ꢀ -diyl)-N,Nꢀ -bis(2-pyridylmethyl)-N,Nꢀ -
dimethyldiamine (7). 2.29 ml of aqueous formaldehyde (37%)
was added to a solution of 0.68 g (1.86 mmol) of 3 and 7 ml of
acetic acid in acetonitrile (50 ml), and allowed to stir for 30 min.
Subsequently, 0.47 g (12.4 mmol) of sodium borohydride was
added and the resultant mixture was stirred overnight. All solvents
were removed and the residue was made strongly basic with 3 M
aqueous sodium hydroxide. This aqueous mixture was extracted
several times with DCM. The organic layers were combined, dried
(magnesium sulfate) and reduced to dryness to give 7 as a pale
2H, 3-PhH), 3.95 (dd, 4H, PhNCH2), 2.45 (s, 6H, PyMe), 2.41 (s,
6H, NMe), 2.01 (s, 6H, PhMe). 13C-NMR (CDCl3): d159.5, 156.7,
152.3, 137.5, 136.4, 135.6, 127.6, 125.3, 123.8, 120.9, 118.7, 63.4
(NCH2), 40.7 (NMe), 24.3 (PyMe), 20.3 (PhMe). MS (EI): m/z
(%) 450 (4) [M+], 359 (32) [(M − Pic)+], 344 (25) [(M − PicCH2)+],
222 (18) [(M − (PicCH3)(PicCH2)(CH3))+], 107 (100) [(PicCH3)+].
N,Nꢀ-(biphenylether-2,2ꢀ-diyl)-N,Nꢀ-bis(2-pyridylmethyl)diamine
(11). 11 was prepared using the same procedure as for 3
from 0.84
g (4.19 mmol) bis(2-aminophenyl)ether, 80 ml
(8.39 mmol) 2-formylpyridine and 2.49 g (11.7 mmol) sodium
triacetoxyborohydride to give a pale yellow oil (1.60 g, 88%).
1H-NMR (CDCl3): d 8,54 (d, 2H, 6-PyH), 7.60 (t, 2H, 4-PyH),
7.30 (d, 2H, 3-PyH), 7.15 (t, 2H, 5-PyH), 6.96 (t, 2H, 5-PhH),
6.83 (d, 2H, 3-PhH), 6.64 (t, 4H, 4-PhH and 6-PhH), 5.29
(broad s, 2H, NH), 4.54 (s, 2H, NCH2). 13C-NMR (CDCl3): d
158.9, 149.3, 143.7, 139.4, 136.7, 124.3, 122.1, 121.3, 117.6, 117.1,
111.6, 49.2 (NCH2). MS (EI): m/z (%) 382 (22) [M+], 291 (50)
[(M − PyCH)+], 211 (36) [(M − (PyH)(PyCH2))+], 183 (100)
[(PhNHCH2Py)+], 93 (50) [(PyCH3)+].
1
yellow oil (0.58 g, 79.2%). H-NMR (CDCl3): d 8.40 (d, 2H, 6-
PyH), 7.41 (t, 2H, 4-PyH), 7.33 (m, 4H, 5-PyH and 6-PhH),
7.17 (d, 2H, 3-PyH), 7.07 (t, 4H, 4-PhH and 5-PhH), 6.76 (d,
2H, 3-PhH), 4.11 (dd, 4H, PhNCH2), 2.54 (s, 6H, NMe). 13C-
NMR (CDCl3): d 159.6, 151.1, 148.5, 136.2, 135.5, 132.0, 128.0,
122.4, 122.0, 121.6, 120.1, 62.8 (NCH2), 40.3 (NMe). MS (EI):
m/z (%) 394 (19) [M+], 317 (36) [(M − C5H3N)+], 302 (28)
[(M–PyCH2)+], 225 (62) [(M − (Py)(PyCH))+], 209 (46) [(M −
(PyCH2)(PyCH3))+], 194 (65) [(M − (PyCH3)(PyCH2)(CH3))+],
180 (39) [(M − (PyCH2NH)(PyCH3))+], 93 (100) [(PyCH3)+].
N,Nꢀ-(biphenylether-2,2ꢀ-diyl)-N,Nꢀ-bis(2-pyridylmethyl)-N,Nꢀ-
dimethyldiamine (12). 12 was prepared using the same procedure
as for 7 from 1.50 g (3.92 mmol) of 11, 10.0 ml of acetic acid,
70.0 ml of acetonitrile, 3.46 ml of aqueous formaldehyde (37%)
and 0.70 g (18.5 mmol) of sodium borohydride to give a pale
yellow oil (1.15 g, 72%). 1H-NMR (CDCl3): d 8.46 (d, 2H,
6-PyH), 7.44 (t, 2H, 4-PyH), 7.18 (d, 2H, 3-PyH), 7.04 (m, 6H,
4-, 5-, and 6-PhH), 6.89 (m, 2H, 5-PyH), 6.78 (d, 2H, 3-PhH),
4.39 (s, 4H, NCH2), 2.70 (s, 6H, NMe). 13C-NMR (CDCl3): d
159.7 (ipso), 149.3, 148.7, 143.6, 136.4, 129.4, 123.7, 122.0, 121.7,
119.4, 119.3, 61.5 (NCH2), 39.9 (NMe). MS (EI): m/z (%) 410
(10) [M+], 333 (27) [(M–C5H3N)+], 318 (10) [(M − PyCH2)+], 254
(100) [(M − (Py)2)+], 210 (29) [(M − (PyCH2)(PyCH3)(CH3))+],
N,Nꢀ-(biphenyl-2,2ꢀ-diyl)-N,Nꢀ-bis[(6-methyl-2-pyridylmethyl)]-
N,Nꢀ-dimethyldiamine (8). 8 was prepared using the same proce-
dure as for 7 from 1.20 g (3.04 mmol) of 4, 16.5 ml of acetic acid,
110 ml of acetonitrile, 5.36 ml of aqueous formaldehyde (37%)
and 1.09 g (28.8 mmol) of sodium borohydride to give 8 as a pale
1
yellow oil (0.84 g, 65.4%). H-NMR (CDCl3): d 7.32 (m, 4H, 4-
PyH and 4-PhH), 7.24 (d, 2H, 6-PhH), 7.14 (d, 2H, 3-PyH), 7.02
(t, 2H, 5-PhH), 6.89 (d, 2H, 5-PyH), 6.63 (d, 2H, 3-PhH), 4.10
(dd, 4H, NCH2), 2.53 (s, 6H, NMe), 2.45 (s, 6H, PyMe). 13C-NMR
(CDCl3): d 159.1, 157.1, 151.1, 136.4, 135.2, 132.0, 127.9, 122.1,
121.0, 120.0, 118.7, 62.6 (NCH2), 40.4 (NMe), 24.3 (PyMe). MS
(EI): m/z (%) 422 (16) [M+], 316 (20) [(M − PicCH2)+], 223 (24)
[(M − (PicH)(PicCH2))+], 209 (18) [(M − (PicCH2)(PicCH3))+],
194 (34) [(M − (PicCH3)(PicCH2)(CH3))+], 107 (100) [(PicCH3)+].
197 (38) [(PhNMeCH2Py)+], 134 (50) [(OPhN(CH2)2 ], 120 (65)
+
[(PhNMe2)+], 93 (72) [(PyCH3)+].
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)-N,Nꢀ-bis(2-pyridylmethyl)-
N,Nꢀ-dimethyldiamine (9). 9 was prepared using the same
procedure as for 7 from 0.47 g (1.19 mmol) of 5, 3.0 ml of acetic
acid, 20 ml of acetonitrile, 0.98 ml of aqueous formaldehyde
(37%) and 0.20 g (5.29 mmol) of sodium borohydride to give 9
Synthesis of complexes
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)bis(2-pyridylmethyl)diimine
manganese(II) bis(triflate).
[Mn(1)(OTf)2]. A solution of 0.30 g (0.77 mmol) of the ligand
1 and 0.27 g (0.77 mmol) of Mn(OTf)2 in 25 ml of DCM was
stirred overnight, under a nitrogen atmosphere. The solution was
then filtered, reduced to approximately 5 ml, under vacuum,
and pentane added to precipitate the product. This solid was
subsequently washed with diethyl ether and dried under vacuum
to give [Mn(1)(OTf)2] as a yellow powder (0.31 g, 55%). 19F-NMR
(CD3CN): d −53 (m1/2 ≈ 4000 Hz). MS (+FAB): m/z (%) 743
[M+], 594 [(M − OTf)+], 445 [(M − (OTf)2)+]; Anal. Calcd. (found)
for C28H22F6MnN4O6S2: C, 45.23 (45.19); H, 2.98 (2.91); N, 7.54
(7.55). leff (CD2Cl2) = 5.99 lB.
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)bis(2-pyridylmethyl)diimine
iron(II) bis(triflate).
[Fe(1)OTf2]. This complex was prepared by a similar proce-
dure as used for complex [Mn(1)OTf2] using a solution of 0.40 g
(1.02 mmol) of 1 and 0.45 g (1.02 mmol) of Fe(OTf)2(CH3CN)2
1
as a pale yellow oil (0.43 g, 85.5%). H-NMR (CDCl3): d 8.37
(d, 2H, 6-PyH), 7.36 (t, 2H, 4-PyH), 7.23 (d, 2H, 3-PyH), 7.17
(d, 2H, 3-PhH), 7.00 (m, 4H, 5-PyH and 4-PhH), 6.39 (d, 2H,
5-PhH), 3.96 (dd, 4H, PhNCH2), 2.41 (s, 6H, NMe), 2.03 (s, 6H,
PhMe). 13C-NMR (CDCl3): d 160.0 (ipso), 152.2, 148.2, 147.5,
136.3, 135.5, 127.5, 125.4, 121.8, 121.5, 118.7, 63.4 (NCH2),
40.5 (NMe), 20.2 (PhMe). MS (EI): m/z (%) 422 (45) [M+], 407
(11) [(M − Me)+], 330 (100) [(M − PyCH2)+], 237 (44) [(M−
(PyCH3)(PyCH2))+], 223 (32) [(M − (PyCH2NMe)(Py))+], 208
(23) [(M − (PyCH2NMe)(PyMe))+], 93 (76) [(PyCH2NH)+].
N,Nꢀ -(6,6ꢀ -dimethylbiphenyl-2,2ꢀ -diyl)-N,Nꢀ -bis[(6-methyl-2-
pyridylmethyl)]-N,Nꢀ-dimethyldiamine (10). 10 was prepared
using the same procedure as for 7 from 0.88 g (2.08 mmol)
of 6, 6.65 ml of acetic acid, 45 ml of acetonitrile, 2.17 ml of
aqueous formaldehyde (37%) and 0.44 g (11.6 mmol) of sodium
borohydride to give 10 as a pale yellow oil (0.61 g, 65.2%).
1H-NMR (CDCl3): d 7.24 (m, 4H, 4-PyH and 4-PhH), 7.12 (d,
2H, 3-PyH), 6.97 (d, 2H, 5-PyH), 6.87 (d, 2H, 5-PhH), 6.27 (d,
in 25 ml of DCM to give [Fe(1)OTf2] as a burgundy powder
1
=
(0.66 g, 86%). H-NMR (CD2Cl2): d 180.2 (2H, N CH), 142.0
(2H, PyHa), 52.5 (2H, PyHb), 44.3 (2H, PyHbꢀ), 13.6 (2H, PyHc),
1406 | Dalton Trans., 2006, 1399–1408
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9.9 (2H, 4-PhH), −2.4 (6H, PhMe), −7.1 (4H, 3-PhH and 5-PhH);
MS (+FAB): m/z 744 [M+], 595 [(M − OTf)+], 446 [(M − (OTf)2)+];
Anal. Calcd. (found) for C28H22F6FeN4O6S2: C, 45.17 (45.22); H,
2.98 (2.89); N, 7.53 (7.60). leff (CD2Cl2) = 4.95 lB.
PyHbꢀ), 8.9 (2H, PyHc ), 4.5 (2H, 4-PhH), 3.5 (2H, 3-PhH),
−2.5 (2H, 6-PhH), −10.7 (2H, 5-PhH). 1H-NMR (CD3CN):
very complex spectrum arising from multiple species in solution.
19F-NMR (CDCl3): d −11.1. 19F-NMR (CD3CN): d −71.8. MS
(+FAB): m/z (%) 748 [M+], 599 [(M − OTf)+], 395 [(M −
Fe(OTf)2)+]. Anal. Calcd. (found) for C28H26F6FeN4O6S2: C, 44.93
(44.54); H, 3.50 (3.36); N, 7.49 (7.33). leff (CD2Cl2) = 5.23 lB.
Crystal data for [Fe(1)OTf2]. C28H22F6FeN4O6S2, M = 744.47,
monoclinic, C2/c (no. 15), a = 16.7167(13), b = 11.4815(8), c =
◦
3
˚
˚
16.6129(10) A, b = 95.470(5) , V = 3174.0(4) A , Z = 4 (C2
symmetry), Dc = 1.558 g cm−3, l(Mo-Ka) = 0.690 mm−1, T =
293 K, orange/red prisms; 2787 independent measured reflections,
F2 refinement, R1 = 0.043, wR2 = 0.099, 2119 independent ob-
N,Nꢀ-(biphenyl-2,2ꢀ-diyl)-N,Nꢀ-bis[(6-methyl-2-pyridylmethyl)]-
N,Nꢀ-dimethyl-diamine iron(II) bis(triflate).
[Fe(8)(OTf)2]. Similar procedure as for [Fe(7)(OTf)2] using
0.74 g (1.75 mmol) of 8 and 0.69 g (1.59 mmol) of Fe(OTf)2-
(CH3CN)2, to give [Fe(8)(OTf)2] as an off-white crystalline solid
served absorption-corrected reflections [|Fo| > 4r(|Fo|), 2hmax
=
50◦], 242 parameters. CCDC 270626. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b513886h.
1
(0.67 g, 55%). H-NMR (CDCl3): d 112.6 (6H, NMe), 72.2 (2H,
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)bis(2-pyridylethyl)diimine
iron(II) bis(triflate).
[Fe(2)OTf2]. This complex was prepared by a similar proce-
dure as used for complex [Mn(1)OTf2] using 2 (0.20 g, 0.48 mmol)
and Fe(OTf)2(CH3CN)2 (0.21 g, 0.48 mmol) in DCM to give
[Fe(2)OTf2] as a dark red solid (0.24 g, 64%). 1H-NMR (CD2Cl2):
d 130.1 (2H, PyHa), 65.1 (2H, PyHb), 49.3 (2H, PyHbꢀ), 15.0 (6H,
PyCH2), 55.1 (2H, PyHb), 53.7 (2H, PyHbꢀ), 8.3 (2H, PyHc ), 5.8
(2H, 4-PhH), −1.0 (2H, 6-PhH), −9.6 (2H, 5-PhH), −16.4 (4H,
1
3-PhH and PyCH2), −29.7 (6H, PyMe). H-NMR (CD3CN): d
139.3 (2H, PyCH2), 133.8 (6H, NMe), 71.3 (2H, PyHb), 64.9
(2H, PyHbꢀ), 9.9 (2H, PyHc ), 1.5 (2H, 4-PhH), −16.6 (2H,
6-PhH), −18.5 (2H, 5-PhH), −24.2 (2H, PyCH2), −32.7 (2H,
3-PhH), −62.4 (6H, PyMe). 19F-NMR (CDCl3): d −20.5. 19F-
NMR (CD3CN): d −76.7. MS (+FAB): m/z (%) 573 [(M −
(C2F6O2S)+], 423 [(M − Fe(OTf)2)+]. Anal. Calcd. (found) for
C30H30F6FeN4O6S2: C, 46.40 (46.58); H, 3.89 (3.82); N, 7.22 (7.16).
leff (CD2Cl2) = 5.10 lB.
=
N C–Me), 11.2 (2H, 4-PhH), 11.1 (2H, PyHc ), −3.9 (6H, PhMe),
−11.7 (2H, 5-PhH), −18.5 (4H, 3-PhH); 19F-NMR (CD2Cl2): d
−25 (m1/2 ≈ 530 Hz). 19F-NMR (CD3CN): d −78.3 (m1/2 = 80 Hz).
MS (+FAB): m/z 772 [M+], 623 [(M − OTf)+], 474 [(M − (OTf)2)+];
Anal. Calcd. (found) for C26H22Cl2FeN4: C, 46.64 (46.80); H, 3.39
(3.25); N, 7.25 (7.13). leff (CD2Cl2) = 5.08 lB.
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)-N,Nꢀ-bis(2-pyridylmethyl)-
N,Nꢀ-dimethyl-diamine iron(II) bis(triflate).
N,Nꢀ-(6,6ꢀ-dimethylbiphenyl-2,2ꢀ-diyl)bis(2-pyridylmethyl)diimine
cobalt(II) bis(triflate).
[Fe(9)(OTf)2]. Similar procedure as for [Fe(7)(OTf)2], using
0.43 g (1.02 mmol) of 9 and 0.42 g (0.97 mmol) of Fe(OTf)2-
(CH3CN)2, to give [Fe(9)(OTf)2] as an off-white crystalline solid
(0.49 g, 65.4%). Crystals suitable for X-ray diffraction were grown
by slow diffusion from a DCM solution layered with pentane. 1H-
NMR (CDCl3): d 96.2 (2H, PyHa), 92.9 (6H, NMe), 69.8 (4H,
PyHb and PyCH2N), 53.6 (2H, PyCH2N), 47.6 (2H, PyHbꢀ), 8.5
(2H, PyHc ), 5.0 (4H, 4-PhH and 3-PhH), −9.8 (2H, 5-PhH),
[Co(1)(OTf)2]. A mixture of 0.19 g (0.37 mmol) of [Co(1)Cl2]
and 0.19 g of silver(I) triflate (0.73 mmol) in 80 ml of DCM
was stirred overnight, under a nitrogen atmosphere. The solution
was then filtered through Celite and the Celite washed with
further DCM. The filtrate was reduced to dryness and a Soxhlet
extraction, using DCM, performed on the solid obtained. The
DCM solution obtained was reduced in volume to approximately
5 ml, under vacuum, and pentane added to precipitate the product.
This mixture was subsequently filtered and the solid washed with
diethyl ether and dried under vacuum to give [Co(1)(OTf)2] as
1
−10.3 (6H, PhMe). H-NMR (CD3CN): very complex spectrum
arising from multiple species in solution. 19F-NMR (CDCl3):
d −12.1. 19F-NMR (CD3CN): d −70.1. MS (+FAB): m/z (%)
776 [M+], 627 [(M − OTf)+], 423 [(M − Fe(OTf)2)+]. Anal. Calcd.
(found) for C30H30F6FeN4O6S2: C, 46.40 (46.72); H, 3.89 (4.05); N,
7.22 (7.06). leff (CD2Cl2) = 5.16 lB.
1
an orange–yellow powder (0.14 g, 58.7%). H-NMR (CD2Cl2):
=
d 144.0 (2H, N CH), 111.2 (2H, PyHa), 80.1 (2H, PyHb), 60.5
(2H, PyHbꢀ), 23.1 (2H, PyHc), 2.6 (2H, 4-PhH), −3.7 (6H, PhMe),
−14.6 (2H, 5-PhH), −52.3 (2H, 3-PhH); 19F-NMR (CD2Cl2): d
−39 (m1/2 ≈ 2000 Hz). 19F-NMR (CD3CN): d −73.1 (m1/2 = 46
Hz). MS (+FAB): m/z 747 [M+], 598 [(M − OTf)+], 449 [(M −
(OTf)2)+]; Anal. Calcd. (found) for C28H22CoF6N4O6S2: C, 44.99
(45.17); H, 2.97 (2.80); N, 7.49 (7.35). leff (CD2Cl2) = 3.86 lB.
Crystal data for [Fe(9)OTf2]. C30H30F6FeN4O6S2, M = 776.55,
¯
˚
triclinic, P1 (no. 2), a = 10.2990(17), b = 11.717(4), c =
◦
15.587(2) A, a = 94.10(3), b = 102.471(18), c = 115.10(2) ,
3
−3
˚
V = 1634.6(7) A , Z = 2, Dc = 1.578 g cm , l(Mo-Ka) =
0.673 mm−1, T = 203 K, colourless blocks; 5645 independent
measured reflections, F2 refinement, R1 = 0.062, wR2 = 0.172, 4137
independent observed absorption-corrected reflections [|Fo| >
4r(|Fo|), 2hmax = 50◦], 503 parameters. CCDC 270627. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b513886h.
N,Nꢀ -(biphenyl-2,2ꢀ -diyl)-N,Nꢀ -bis(2-pyridylmethyl)-N,Nꢀ -
dimethyldiamine iron(II) bis(triflate).
[Fe(7)(OTf)2]. A mixture of 0.55 g (1.39 mmol) of 7 and
0.55 g (1.27 mmol) of Fe(OTf)2(CH3CN)2 was stirred in DCM
(50 ml) overnight. The mixture obtained was reduced in volume
and pentane added to precipitate a solid. The solid was washed
with diethyl ether and dried under vacuum. It was, subsequently,
recrystallised by slow diffusion from a DCM solution layered
with pentane to give [Fe(7)(OTf)2] as an off-white crystalline solid
N,Nꢀ-(biphenylether-2,2ꢀ-diyl)-N,Nꢀ-bis(2-pyridylmethyl)-N,Nꢀ-
dimethyldiamine iron(II) bis(triflate).
[Fe(12)(OTf)2]. Similar procedure as for [Fe(7)(OTf)2], using
0.22 g (0.52 mmol) of 12 and 0.20 g (0.46 mmol) of Fe(OTf)2-
(CH3CN)2, to give [Fe(12)(OTf)2] as a tan-coloured crystalline
1
1
(0.53 g, 55%). H-NMR (CDCl3): d 90.3 (8H, PyHa and NMe),
solid (0.23 g, 65%). H-NMR (CDCl3): d 75.4 (2H, PyHa), 67.5
69.4 (4H, PyCH2N and PyHb), 55.6 (2H, PyCH2N), 47.4 (2H,
(2H, PyCH2), 49.1 (6H, NMe), 48.0 (2H, PyHb), 42.1 (2H, PyHbꢀ),
Dalton Trans., 2006, 1399–1408 | 1407
This journal is
The Royal Society of Chemistry 2006
©

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21.3 (2H, PyHc ), 10.7 (2H, 5-PhH), 5.3 (2H, 3-PhH), 2.2 (2H,
4-PhH), −2.4 (2H, 6-PhH). 1H-NMR (CD3CN): d 70.1 (2H,
PyHa), 62.0 (2H, PyCH2), 43.9 (6H, NMe), 42.7 (2H, PyHb),
36.7 (2H, PyHbꢀ), 16.0 (2H, PyHc ), 5.1 (2H, 5-PhH), 0.2 (2H, 3-
PhH), −2.8 (2H, 4-PhH), −7.4 (2H, 6-PhH). 19F-NMR (CDCl3):
d −0.3. 19F-NMR (CD3CN): d −76.8. MS (+FAB): m/z (%) 615
[(M − OTf)+], 411 [(M − Fe(OTf)2)+]. Anal. Calcd. (found) for
C28H26F6FeN4O7S2: C, 43.99 (44.12); H, 3.43 (3.63); N, 7.33 (7.11).
leff (CD2Cl2) = 5.04 lB.
31 V. A. Jones, S. Sriprang, M. Thornton-Pett and T. P. Kee, J. Organomet.
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Acknowledgements
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We are grateful to BP Chemicals Ltd. for a CASE award to J. E. Mr
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1408 | Dalton Trans., 2006, 1399–1408
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The Royal Society of Chemistry 2006
©