Catalyst stability determines the catalytic activity of non-heme iron catalysts in the oxidation of alkanes

Article abstract of DOI:10.1002/adsc.200700462

A series of iron(II) bis(triflate) complexes [Fe(L)(OTf)₂] containing linear tetradentate bis(pyridylmethyl)diamine ligands with a range of ligand backbones has been prepared. The backbone of the ligand series has been varied from a two-carbon linkage [ethylene (1), 4,5-dichlorophenylene (2) and cyclohexyl (3)] to a three-carbon [propyl (4)) and a four-carbon linkage (butyl (5)]. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by ¹H and ¹⁹F NMR spectroscopy. Due to the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-α, cis-β and trans) are observed with ligands 2-5. In these cases, the geometry observed in the solid state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable temperature magnetic moment measurements and UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligands 1 and 2, which generate complexes [Fe(L)(OTf)₂] with a cis-α coordination geometry and the corresponding complexes [Fe(L)(CH₃CN) ₂]²⁺ display spin crossover behaviour. The catalytic properties of the complexes for the oxidation of cyclohexane, using hydrogen peroxide as the oxidant, have been investigated. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst in the oxidising environment.

Full text of DOI:10.1002/adsc.200700462

FULL PAPERS
DOI: 10.1002/adsc.200700462
Catalyst Stability Determines the Catalytic Activity ofNon-Heme
Iron Catalysts in the Oxidation ofAlkanes
Jason England,a Catherine R. Davies,a Maria Banaru,a Andrew J. P. White,a
and George J. P. Britovseka,*
a
Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AY, U.K.
Phone: (+44)-(0)20-7594-5863; fax: (+44)-(0)20-7594-5804; e-mail: g.britovsek@imperial.ac.uk
Received: September 20, 2007; Revised: January 16, 2008; Published online: March 20, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A series of iron(II) bis
ACHTRE(UNG triflate) complexes ous complexes has been investigated by variable
[Fe(L)(OTf)2] containing linear tetradentate bis(pyri- temperature magnetic moment measurements and
ACHTREUNG
dylmethyl)diamine ligands with a range of ligand UV-vis spectroscopy. The strongest ligand field is ob-
backbones has been prepared. The backbone of the served with the most rigid ligands 1 and 2, which
ligand series has been varied froma two-carbon link-
age [ethylene (1), 4,5-dichlorophenylene (2) and cy- dination geometry and the corresponding complexes
clohexyl (3)] to a three-carbon [propyl (4)) and a [Fe(L)
(CH3CN)2]2+ display spin crossover behaviour.
generate complexes [Fe(L)ACHTRE(UNG OTf)2] with a cis-a coor-
AHCTREUNG
four-carbon linkage (butyl (5)]. The coordination ge- The catalytic properties of the complexes for the oxi-
ometries of these complexes have been investigated dation of cyclohexane, using hydrogen peroxide as
in the solid state by X-ray crystallography and in so- the oxidant, have been investigated. An increased
1
lution by H and 19F NMR spectroscopy. Due to the flexibility in the ligand results in a weaker ligand
labile nature of high-spin iron(II) complexes in solu- field, which increases the lability of the complexes.
tion, dynamic equilibria of complexes with different The activity and selectivity of the catalysts appear to
coordination geometries (cis-a, cis-b and trans) are be related to the strength of the ligand field and the
observed with ligands 2–5. In these cases, the geome- stability of the catalyst in the oxidising environment.
try observed in the solid state does not necessarily
represent the only or even the major geometry pres- Keywords: alkanes; biomimetic synthesis; hydrogen
ent in solution. The ligand field strength in the vari- peroxide; iron; N ligands; oxidation
Introduction
An attractive feature of non-heme iron catalysts is
that ligand modifications and catalyst tuning are rela-
The ability to selectively oxidise unfunctionalised al- tively straightforward compared, for example, with
kanes has been a long-standing goal and challenge in heme-type ligand systems. However, many of the
catalysis research.[1–4] A number of different classes of non-heme iron complexes that have been used as cat-
oxidation catalysts have been developed during the alysts in combination with H2O2 have shown catalytic
last 50 years, for example, heme-based iron complexes behaviour that resembles simple Fenton-type chemis-
containing porphyrin-type ligands,[5] polyoxometa- try,[20] which is characterised by low conversions, low
lates[6–8] and recently also organic non-metal cata- product selectivities and extensive H2O2 decomposi-
lysts.[9–11] The catalysts are typically combined with ox- tion. In contrast, much higher conversions of H2O2
idants, which can have different oxo transfer abili- into oxidised product combined with a more selective
ties,[12] for example H2O2, O2, ClOꢀ, PhIO, O3 or N2O, oxidation reactivity have been observed with non-
whereby the first two oxidants are economically and heme iron(II) catalysts containing either tripodal tet-
environmentally the most attractive oxidants.[13,14]
A
radentate ligands such as TPA[21,22] or iso-BPMEN
relatively recent addition to the armoury of oxidation (Figure 1),[23] or linear tetradentate ligands such as
catalysts has been a class of non-heme iron catalysts, BPMEN[24] or BQEN.[25] In addition, these catalysts
inspired by natureꢀs use of non-heme iron-based en- have shown stereospecific and dioxygen-independent
[15–19]
ꢀ
ꢀ
zymes for the selective oxidation of C H bonds.
hydroxylation of alkanes[16,26,27] and the C H regiose-
lectivities and kinetic isotope effects are also indica-
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Jason England et al.
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Figure 1. Examples of non-heme iron(II) oxidation catalysts.
tive of a more selective oxidant than those responsi- non-heme iron(II) catalysts? In our attempt to ad-
ble for oxidation in Fenton-type systems. Isotope la- dress this question, we have been systematically inves-
belling studies[28] and DFT calculations[29] implicate a tigating the ligand-based requirements that are
high-valent iron oxo complex as the active oxi- needed for the formation of efficient catalysts for the
dant,[30,31] which suggests that the reaction mechanism oxidation of alkanes using H2O2 as the oxidant. Here
could be analogous to the mechanism for cytochrome we report our results regarding a series of iron(II)
P-450 systems,[5] (hydrogen atomabstraction followed
complexes with ligands 1–5 incorporating pyridyl-
methyl units and a range of different ligand back-
by a rapid oxygen rebound).[32]
Common features in these non-heme iron catalysts bones (Figure 2). The length of the backbone has
are the use of tetradentate ligands with at least two been varied from2 to 4 carbon atoms resulting in var-
pyridine-type donors and two cis labile co-ligands X, ious degrees of ligand flexibility. The coordination ge-
for example, weakly coordinating triflate anions or ometries of the iron(II) bisACHTREU(NG triflate) complexes have
acetonitrile molecules in combination with non-coor- been determined in the solid state and in solution.
dinating anions. Iron(II) complexes containing linear The spectroscopic and magnetic properties of the
tetradentate ligands such as BPMEN or BQEN are iron(II) complexes have been measured in an attempt
currently the most efficient catalysts for the oxidation to correlate the coordination geometry with the cata-
of alkanes using H2O2 as the oxidant. One of the lytic activity and selectivity of these complexes for the
main questions that immediately arises is: Why are oxidation of cyclohexane. Ligands 1, 3 and 4 and their
these particular catalysts so much better than other iron(II) complexes have been used previously as cata-
lysts for the oxidation of alkanes or alkenes.[23,33–37]
Here we will report on some novel aspects of their so-
lution behaviour and their spectroscopic and magnetic
properties. In addition, their inclusion in this series
has allowed us to establish trends on the geometrical
behaviour of iron(II) complexes containing linear tet-
radentate ligands and the relationship with their cata-
lytic activity. Ligand rigidity and a strong ligand field
appear to be critically important for high catalytic ac-
tivity, which is related to catalyst stability and lifetime
under the harsh oxidising conditions.
Results and Discussion
Synthesis ofLgands and Complexes
The synthesis and characterisation of the ligands 1, 3
and 4 and their corresponding iron(II) bistriflate com-
plexes have been reported previously.[32,33,34,37,38]
Ligand 5 was prepared by the condensation of pyri-
AHCTREUNG
Figure 2. Backbone variations in bis(pyridylmethyl)diamine
ligands 1-5.
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Catalyst Stability Determines the Catalytic Activity of Non-Heme Iron Catalysts
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This procedure does not work for ligand 2. The prepa- yielded the corresponding lithiumdiamide, which
ration of the corresponding non-methylated secon- after subsequent reaction with methyl iodide gave the
dary amine 1,2-bis(2-pyridylmethylamino)benzene has desired ligand 2 in near quantitative yield [Eq. (2)].
previously been reported by the reaction of benzene-
The
iron(II)
1,2-diamine with pyridine-2-carbinol at elevated tem- [Fe(2)(OTf)2], [Fe(3)
peratures in the presence of catalytic amounts of pyri- [Fe(5)AHCTRE(UNG OTf)2] were prepared by mixing tetrahydrofur-
complexes
[Fe(1)
ACHTREUNG
ACHTREU(NG OTf)2],
G
A
dine N-oxide.[40] However, we were unable to repro- an (THF) solutions of equimolar quantities of ligands
duce these results and only starting materials were and FeACTHERNGU(OTf)2ACHTRE(UGN CH3CN)2 at roomtemperature. The
obtained using this method. The attempted prepara- complexes precipitate from THF and are easily puri-
tion of 1,2-bis(2-pyridylmethylamino)benzene by re- fied by recrystallisation. Interestingly, the formation
ductive amination of 1,2-diaminobenzene using pyri- of the complexes
C2-bridged
[Fe(1)AHCERT(UNG OTf)2],
dine-2-carboxaldehyde and sodiumtriacetoxyborohy- [Fe(2)(OTf)2] and [Fe(3)CAHTRE(UGN OTf)2] typically requires less
A
dride was complicated by the formation of an aminal than a day before product precipitation occurs from
side-product [Eq. (1)]. Attempts to separate the THF, whereas several days are required for [Fe(4)-
aminal product from the desired amine product were
ACHRTUNEG(OTf)2] and several weeks for complex [Fe(5)ACHTRE(UGN OTf)2],
unsuccessful, as were attempts to eliminate its forma- which indicates quite different rates of complex for-
tion by modification of the reaction conditions. Fur- mation. The solid state structures of complexes
thermore, the aminal was found to emerge unscathed [Fe(2)
ACHRTUNEG(OTf)2] and [Fe(4)HCATRE(UGN OTf)2] have been deter-
fromattempted hydrolysis and reduction, even under mined by X-ray analysis and the coordination behav-
particularly harsh conditions (e.g.. reduction using iour in solution has been investigated for all com-
1
LiAlH4). Attempts to circumvent this problem by plexes by H and 19F NMR, UV-Vis spectroscopy and
using a one-step preparation of the targeted ligand by magnetic susceptibility measurements.
reductive amination of N,N’-dimethyl-1,2-diaminoben-
zene using pyridine-2-carboxaldehyde and sodiumtri-
acetoxyborohydride as the reductant yielded only the Solid State Structures
corresponding aminal product.
It was found that the use of the less nucleophilic Solid state X-ray analysis of crystals of [Fe(2)
4,5-dichloro-1,2-diaminobenzene in place of 1,2-di- revealed cis-a coordination mode (Figure 3,
ACHTREUNGaminobenzene in the reductive amination procedure, Table 1). The angle f between the {Fe,O(31),O(41)}
ACHTREUNG(OTf)2]
a
using excess pyridine-2-carboxaldehyde and sodium plane and the {Fe,N(8),N(15)} plane is 178 (Figure 4).
triacetoxyborohydride, suppressed aminal formation. The analogous angle f in the complex [Fe(1)Cl2] is
Unlike the non-chlorinated analogue, the diamine 148,[41] which indicates a less favourable orbital over-
product was found to be a solid. This allowed straight- lap between the amine donors and the iron centre in
forward purification by recrystallisation fromethanol.
the case of complex [Fe(2)ACHTRE(UNG OTf)2] due to the rigid
Deprotonation with 2 equivalents of n-butyllithium phenylene backbone. The configurations of the amine
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Jason England et al.
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Figure 4. Definition of the angle f.
this earlier report, the tetradentate ligand in our de-
termination is highly disordered, we have reported
our structure in the Supporting Information.
Geometrical Isomerisations and Ligand Dynamics
Figure 3. The molecular structure of [Fe(2)CAHTRE(UGN OTf)2].
Linear tetradentate ligands can adopt three coordina-
tion modes in octahedral metal complexes: cis-a, cis-b
and trans. The two internal donors become chiral
nitrogen atoms are the same [(R,R) in the stereoiso- upon coordination to the metal and can therefore
mer shown in Figure 3]. The Fe bond distances are have either the same or opposite configuration. As-
typical for high-spin iron(II) complexes, with those to suming that the configuration of these internal two
the trans-pyridyl nitrogens being ca. 0.1 Š shorter donors remains unchanged, this increases the number
than those to the amino nitrogens N(8) and N(15). A of possible isomers to five pairs of enantiomers: cis-a
similar pattern is seen in the related complex (R,R or S,S), cis-b (R,R or S,S) or cis-b (R,S or S,R)
[Fe(1)(Cl)2], though it is notable that in this C2-sym- and trans (R,R or S,S) or trans (R,S or S,R). Frommo-
ꢀ
ꢀ
metric cis-a complex, the Fe N
(amino) distances [2.195(4) and 2.278(4) Š respective- (R,S or S,R) geometry is not possible due to severe
ly] are both longer than seen here in [Fe(2)
(OTf)2].[41] bite-angle strain within the complex. Whichever ge-
During this work the single crystal X-ray structure ometry a linear tetradentate ligand will adopt prefer-
of the dichloromethane solvate of [Fe(4)(OTf)2] was entially in a metal complex is not easily predicted and
ACHTRE(UGN pyridyl) and Fe N- lecular models it is immediately evident that the cis-a
ACHTREUNG
AHCTREUNG
AHCTREUNG
determined. The structure is essentially the same as depends on a number of factors, including the
that of the non-solvated complex reported recently by number of atoms between the donors, the solvent and
Que and coworkers,[37] and shows that the propyl- the temperature. In addition, if the co-ligands are
bridged ligand adopts the trans coordination geome- labile such as triflate or acetonitrile ligands, equilibria
try, whereby the amine N centres have the opposite can exist between the different geometries in solution,
configurations (R,S or S,R). Given that, in addition to which will be concentration- and temperature-depen-
Table 1. Selected bond lengths (Š) and angles (8) for the two independent complexes (I and II) present in the crystals of
[Fe(2)ACHTREUNG(OTf)2].
Mol I
Mol II
Mol I
Mol II
ꢀ
ꢀ
Fe N(1)
Fe N(15)
2.154(2)
2.236(2)
2.069(2)
93.63(8)
170.21(8)
94.33(8)
94.36(8)
89.37(7)
96.62(8)
95.49(9)
95.19(9)
2.154(2)
2.233(2)
2.0958(19)
94.69(8)
171.12(8)
93.60(8)
94.77(8)
90.07(8)
92.58(8)
94.55(8)
98.38(8)
Fe N(8)
Fe N(22)
2.2340(19)
2.138(2)
2.1037(18)
78.83(7)
90.76(9)
80.49(7)
168.95(9)
78.20(8)
165.65(8)
92.60(8)
2.227(2)
2.154(2)
2.0798(19)
78.68(8)
91.06(8)
80.68(7)
167.13(7)
78.21(8)
166.12(8)
92.38(8)
ꢀ
ꢀ
ꢀ
ꢀ
Fe O(31)
Fe O(41)
ꢀ
ꢀ
ꢀ ꢀ
N(1) Fe N(15)
N(1) Fe N(8)
ꢀ
ꢀ
ꢀ ꢀ
N(1) Fe N(22)
N(1)-Fe-O(41)
N(8) Fe N(22)
N(8) Fe O(41)
N(15) Fe O(31)
N(1) Fe O(31)
N(8) Fe N(15)
N(8) Fe O(31)
N(15) Fe N(22)
N(15) Fe O(41)
N(22) Fe O(41)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
ꢀ ꢀ
N(22) Fe O(31)
O(31) Fe O(41)
ꢀ
ꢀ
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Catalyst Stability Determines the Catalytic Activity of Non-Heme Iron Catalysts
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Scheme 1.
dent. The coordination geometries observed in the twist or an envelope (sometimes called half chair) con-
solid state are therefore not necessarily the same as formation (Scheme 1).
those in solution, as we have previously established
In the case of complexes featuring BPMEN (1) or
for a series of iron(II) complexes containing bis- related ligands with three 5-membered chelate rings,
A
the cis-a (R,R or S,S) geometry with a twist confor-
Bosnich and co-workers have analysed the relative mation of the central chelate ring is the only geome-
(III) complexes containing try seen in the solid state.[24,41,43–46] Loss of one of the
stability of a series of CoACHTREUNG
the linear tetradentate tetraamine ligands, 2,2,2-tet, weakly bound X ligands (for example OTfꢀ or
3,2,3-tet and 2,3,2-tet, whereby the numbers denote CH3CN) would result in a five-coordinate intermedi-
the number of bridging atoms between the four ate (Scheme 1). A clockwise twist of 1208 around one
amine donors.[42] The preference for any of the three of the N Fe bonds and re-coordination of the X
ꢀ
possibly topologies, cis-a, cis-b or trans, depends on ligand would convert the cis-a (R,R) isomer into the
the cumulative bite-angle strain of the three chelate cis-b (R,R) isomer with an envelope conformation of
rings formed by the linear tetradentate ligand, which the central five-membered chelate ring. Another twist
ꢀ
in turn is related to the absolute configuration of the around the other N Fe bond would give the trans
chirality at the internal amine donors. Unlike the ki- (R,R) topology. These latter two ligand rearrange-
netically inert low-spin CoCAHTRE(UNG III) complexes, for which ments are highly unfavourable in the case of iron(II)
different conformational isomers can often be isolat- complexes containing the BPMEN ligand due to the
ed, the labile high-spin iron(II) complexes here show bite-angle strain.
rapid equilibria between different isomers and the
isomer that crystallises preferentially may not be the
dominant isomer in solution.
19F NMR Spectroscopy
If we assume that, once an iron(II) complex has
been formed with a tetradentate bis(pyridylmethyl)di- The 19F NMR spectrumof [Fe( 1)
ACHTRE(UNG OTf)2] in CD2Cl2
amine ligand, the absolute configuration of the inter- solution consists of a single peak at ꢀ28 ppmat
nal amine donors is fixed as being either R or S and 298 K, which shifts to ꢀ14 ppmat 188 K due to Curie
only the triflate or acetonitrile co-ligands can dissoci- behaviour (Supporting Information, Fig. S10). This
ate and re-coordinate, a series of equilibria between signal is assigned to a C2-symmetric complex with a
six- and five-coordinated complexes can be proposed. cis-a (R,R or S,S) coordination mode, which is the ge-
In the case of a C2-bridged ligand, the configuration ometry found in the solid state structures of similar
at the amine N donor centres can in theory be either complexes such as [Fe(1)Cl2][41] and which is generally
the same (R,R or S,S) or different (R,S or S,R) and the most stable geometry for tetradentate ligands fea-
the central five-membered chelate can adopt either a turing three 5-membered chelate rings.[42]
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Jason England et al.
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of the resolved signals at lower temperatures, with
those observed for complexes [Fe(3)
ACHTRE(UNG OTf)2] and
[Fe(4)(OTf)2], which consist predominantly as the cis-
AHCTREUNG
b and trans isomer, respectively (vide infra). The
amount of cis-a isomer is about 40% and the amounts
of cis-b and trans isomers are approximately 20% and
40%, respectively and this ratio does not appear to
change significantly over the temperature range 298–
198 K. As shown in Scheme 2, the cis-b (R,S) enantio-
mer can interconvert via the trans (R,S) isomer into
its mirror image. The trans (R,S) isomer is identical to
the trans (S,R) isomer and can thus be regarded as
the meso form. This situation is akin to the stereo-
chemistry of cis-1,2-dimethylcyclohexane, which in
the planar representation appears as a meso form, but
in the chair representation is actually a racemate
whereby the enantiomers rapidly interconvert.
Both topological isomers, the cis-a (R,R or S,S) and
the cis-b (R,S or S,R) geometry, of complex
[Fe(3)ACHTRE(UNG OTf)2] have been separately prepared by Que
and co-workers using two different synthetic proce-
dures.[33,35] The cis-b (R,S or S,R) isomer was obtained
by mixing equimolar amounts of the ligand BPMCN
Figure 5. VT 19F NMR spectra of complex [Fe(2)
ACHTREU(NG OTf)2] in
CD2Cl2 between 198–298 K.
(CH3CN)2 in THF,[34] whereas to
(3) and FeACTHER(UNG OTf)2·ACHTREUGN
The 19F NMR spectrumof the phenylene derivative
[Fe(2)(OTf)2] in CD2Cl2 at roomtemperature shows a
obtain the cis-a (R,R or S,S) isomer, the dichloro
complex [Fe(3)Cl2] was first prepared by mixing the
ligand with FeCl2 in CH3CN, followed by halide ab-
ACHTREUNG
sharp signal at ꢀ18 ppmand a broad resonance
around ꢀ5 ppm. Upon cooling of this solution, the straction with AgOTf.[33] This interesting observation
sharp signal shifts downfield, whereas the broad suggests that the nature of the counterions can con-
signal splits into 4 new resonances (Figure 5). The trol the binding of the ligand to the metal centre, to
sharp resonance at ꢀ18 ppmis assigned to the cis-a give either a cis-a topology where both amine nitro-
(R,R or S,S) geometrical isomer, based on its similari- gens have the same configuration (R,R or S,S) or a
ty with the previous complex [Fe(1)CATHRE(UGN OTf)2] and be- cis-b topology, where the configurations are opposite
cause this is the geometry seen in the solid state struc- (R,S or S,R). It is thus clear that these two isomers
ture (Figure 3). The broad signal is assigned to an in- cannot interconvert. In this study, the complex [Fe(3)-
terconverting mixture of the cis-b (R,S or S,R) and
ACHTREUNG
the trans (R,S or S,R) geometries, as shown in was prepared fromracemic
Scheme 2. This assignment is based on the similarity hexane, with FeACTHER(UNG OTf)2·ACHTRENUG
Scheme 2.
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Catalyst Stability Determines the Catalytic Activity of Non-Heme Iron Catalysts
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isomer (ca. 3%). A small amount of cis-a (R,R or S,S)
isomer can be observed above 238 K, which converts
to the cis-b (R,R or S,S) isomer at very low tempera-
tures (Scheme 1). The major cis-b (R,S or S,R) geo-
metrical isomer is therefore not the only isomer in so-
lution and, at least in CD2Cl2, is in equilibriumwith
the trans (R,S or S,R) isomer (Scheme 2). It should be
noted that because in the case of cis-b complexes, we
are dealing with interconverting enantiomers with re-
spect to the chirality at the N centres, the additional
chirality in the case of complex [Fe(3)CATHRE(UGN OTf)2] created
by the racemic trans-cyclohexyldiamine unit at the
two C centres therefore leads to an interconverting
mixture of diastereomers, which may explain why
only one set of signals is observed for the cis-b and
the trans isomer.
As shown in Scheme 3, the central chelate in the
propyl-bridged complex [Fe(4)ACHTRE(UNG OTf)2] forms a six-
membered ring, which can adopt either a twist or a
chair conformation (or the less stable boat conforma-
tion, dashed line). The complexity and number of pos-
sible isomers has increased as cis-b (R,R or S,S) and
trans (R,R or S,S) isomers are now more easily acces-
Figure 6. VT 19F NMR spectra of complex [Fe(3)
ACHTREU(NG OTf)2] in
CD2Cl2 between 198–298 K.
complex should therefore have the cis-b geometry. sible. The order of relative stabilities of topological
Indeed, the 1H NMR spectrumin CD CN looks exact- isomers of low-spin Co
(III) complexes containing the
related 2,3,2-tet ligand is known to be as follows:
AHCTREUNG
3
ly like the previously reported spectrumby Que and
co-workers,[33] showing 24 (12 pairs) of proton reso- trans (R,S or S,R) > trans (R,R or S,S) > cis-a (R,R
nances due to the lack of symmetry of the cis-b geom- or S,S) @ cis-b (R,R or S,S) and cis-b (R,S or S,R).[42]
etry (see Supporting Information, Fig. S13). However, This order may be similar in the case of high-spin
the 19F NMR spectrumof this complex in CD 2Cl2 so- Fe(II) complexes. Indeed, the X-ray analysis of com-
lution at roomtemperature shows one very broad
plex [Fe(4)
ACHTREU(NG OTf)2], obtained froma CH Cl2/pentane
2
signal centred around ꢀ21 ppm(Figure 6). Cooling
solution, shows the trans (R,S) geometry (see Sup-
this solution to 198 K reveals 6 peaks, which corre- porting Information, Fig. S7). In addition, the cis-a
spond to three isomers: the cis-b (R,S or S,R) isomer (R,R or S,S) geometry has been previously observed
(ca. 56%) which is in equilibriumwith the trans (R,S in the solid state structure of the dichloro complex
or S,R) isomer (ca. 41%) and the cis-b (R,R or S,S) [Fe(4)Cl2].[41] In CD2Cl2 solution, the 19F NMR spec-
Scheme 3.
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Jason England et al.
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trumof [Fe( 4)
roomtemperature, whereas at lower temperatures
A
trans (R,R or S,S), trans (R,S or S,R) and cis-b (R,S or
five peaks emerge, assumed to correspond to the cis-a S,R) isomers.
isomer (ca. 2%), a cis-b (R,S or S,R) isomer (ca.
16%) and the major Cs-symmetric trans (R,S or S,R)
isomer (ca. 82%) (see Supporting Information, Magnetic Susceptibilities
Fig. S11). These spectra are identical to the 19F NMR
spectra observed by Que and co-workers.[37]
Magnetic susceptibility measurements have been used
The butyl bridge in complex [Fe(5)CATHRE(UGN OTf)2] gener- in this study to measure the relative ligand field
ates a central seven-membered chelate ring, which strength of the different ligands 1–5 in octahedral
has four possible conformers of which the twist-chair iron(II) complexes. The magnetic moments of the
[47]
formis generally the most stable.
The 19F NMR iron(II) bis
ACHTREUNG
spectrumin CD 2Cl2 solution at roomtemperature
shows three broad peaks at ꢀ13, ꢀ22 and ꢀ42 ppm. using the Evansꢀ NMR method, are listed in Table 2
At 218 K, five relatively sharp signals are observed, together with previously reported data for [Fe(1)-
which have been tentatively assigned to the trans (R,S
or S,R), cis-b (R,S or S,R) and trans (R,R or S,S) iso- erted by triflate anions, CD2Cl2 solutions of all the
mers (see Supporting Information, Fig. S12). The bis(triflate) complexes display magnetic moments
(OTf)2].[23] As expected fromthe weak ligand field ex-
ACHTREUNG
AHCTREUNG
trans (R,R or S,S) isomer, which is C2-symmetric gives consistent with high-spin (HS) iron(II) (S=2) centres.
rise to only one 19F NMR signal. A related biphenyl- In CD3CN solution, the triflate anions are displaced
bridged complex, which also has a seven-membered by stronger field acetonitrile ligands to yield dication-
(CH3CN)n]2+. In the
central chelate ring, gives exclusively the trans (R,R ic complexes of the form [Fe(L)ACHTREUNG
or S,S) geometry.[48] The 19F NMR spectrumshows a
single peak at ꢀ11 ppmat 298 K in this case, which
shifts to +27 ppmat 198 K.
case of L=3, 4 and 5, the meff values are consistent
with high-spin iron(II) complexes at room tempera-
ture. In contrast, the meff value measured for [Fe(1)-
In summary, the size and conformation of the cen-
ACHRTUNEG(OTf)2] and [Fe(2)AHCTRE(UGN OTf)2] at 298 K in CD3CN were
tral chelate ring formed by linear tetradentate bis(pyr- found to be lower than expected for an HS iron(II)
idylmethyl)diamine ligands, affects the flexibility of centre, which is suggestive of the existence of spin
these ligands which can lead to more than one coordi- crossover (SC) behaviour in these complexes.[49,50]
nation geometry in solution. Due to the labile nature
The magnetic behaviour of these iron(II) com-
of high-spin iron(II) complexes several equilibria can plexes in CD3CN solution was further investigated by
exist between different topological isomers. The ethyl- measurement of their magnetic moments over the
ene-bridged BPMEN complex [Fe(1)
clusively the cis-a (R,R or S,S) geometry. The phenyl- method (Figure 7). The magnetic moment of complex
ene-bridged complex [Fe(2)(OTf)2] consists of ca. [Fe(5)(OTf)2] (and also of complex [Fe(4)(OTf)2],
ACHTRE(UNG OTf)2] gives ex- temperature range 233–343 K using the Evansꢀ NMR
A
N
ACHTREUNG
40% cis-a (R,R or S,S) isomer and 60% cis-b (R,S or which is not shown for clarity) remains fully HS upon
S,R) isomer which is in equilibrium with the trans cooling to 233 K, whereas the magnetic moment of
(R,S or S,R) isomer. The cyclohexyl-bridged complex the phenylene derivative [Fe(2)
[Fe(3)(OTf)2] prepared here is predominantly the cis- meff =4.0 BM and of the cyclohexyl derivative
b (R,S or S,R) isomer, which is in equilibrium with [Fe(3)(OTf)2] to meff =2.5 BM. Complex [Fe(1)(OTf)2]
the trans (R,S or S,R) isomer. In the case of the undergoes a full spin transition within the tempera-
propyl-bridged complex [Fe(4)(OTf)2], the trans (R,S ture range, which shows a similar magnetic behaviour
or S,R) isomer is the major isomer, whereas the butyl-
ACHTRE(UNG OTf)2] decreases to
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
Table 2. Selected physical data for complexes [Fe(1)CAHTER(UGN OTf)2]–[Fe(5)ACHTRENUG
[c]
[c]
Complex
19F in CD3CN[a]
lmax[b] [nm]
emax
G
meff CD2Cl2 (BM)
meff CD3CN (BM)
d [ppm]
n1/2 [Hz]
[Fe(1)
[Fe(2)
[Fe(3)
[Fe(4)
[Fe(5)
C
ꢀ77
ꢀ68
ꢀ67
ꢀ69
ꢀ68
850
2150
340
2100
2200
375
366
376
341
333
3800
2200
1800
770
5.27
5.34
nd
nd
nd
4.26
4.80
5.01
5.51
5.40
560
[a]
At 298 K.
[b]
[c]
c=0.5 mM in CH3CN.
Evansꢀ NMR method at 298 K.
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Catalyst Stability Determines the Catalytic Activity of Non-Heme Iron Catalysts
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Figure 7. The magnetic moments of selected iron(II) complexes in CD3CN solution as a function of temperature.
as the previously reported complex [Fe
(CH3CN)2]
(ClO4)2.[51]
C
One of the most noticeable facets of the UV-vis
spectra in Figure 8 is that the extinction coefficient of
G
ACHTREUNG
A
=
ACHTREUNG
Such mixtures of isomers are likely to exist also in nature of this complex, which equates to an occupa-
CD3CN solutions and the magnetic moments deter- tion of the low-spin configuration and results in an
mined in solution are therefore an average value of enhancement of the MLCT band.[56] As might be ex-
the various magnetic moments of the individual iso- pected, a minor enhancement is also observed for
mers. Notwithstanding, these magnetic moment meas- [Fe(2)
ACHRTUNEG(OTf)2] and [Fe(3)CAHTRE(UGN OTf)2], which at roomtem-
urements show clearly that the pyridylmethyl ligands perature show partial occupation of the low-spin con-
that formexclusively 5-membered chelate rings upon
figuration (see Figure 7).
coordination (BPMEN 1, BPMPhN 2 and BPMCN 3)
It should be noted again that, except for complex
exert a stronger ligand field than those that incorpo- [Fe(1)ACHTRE(UNG OTf)2], all complexes exist as a mixture of geo-
rate 6- or 7-membered chelate rings (BPMPN 4 and metrical isomers in solution and the UV-vis spectra of
BPMBN 5). The BPMEN ligand in complex these complexes consist therefore of several superim-
[Fe(1)ACHTREUNG(OTf)2], which gives rise to the cis-a isomer posed spectra, which makes correct peak assignments
only, exerts the strongest ligand field. Ligands 2 and 3 extremely difficult.
result in complexes that are topological mixtures of
isomers in solution, but with an average ligand field
strength that is stronger than in the case of ligands 4 Catalytic Oxidation ofCyclohexane
and 5.
The catalytic properties of the iron(II) bisACHTER(UNG triflate)
complexes containing ligands 1–5 for the oxidation of
cyclohexane with H2O2 have been evaluated [Eq.(3)].
The oxidation reactions were carried out in acetoni-
UV-Vis Spectroscopy
The UV-vis spectra of the iron(II) complexes of bis- trile as the solvent at roomtemperature (see Experi-
ACHTREUNG(pyridylmethyl)diamine ligands are dominated by mental Section). The H2O2 is added slowly during
high intensity bands in the UV region and broad tran- 25 min via a syringe pump to the reaction mixture,
sitions in the near-visible (Figure 8, Table 2). By anal- containing the solvent, cyclohexane (1000 equiv.) and
ogy to spectra of similar iron(II) complexes of pyri- the catalyst (1 equiv.). The addition of H2O2 is carried
dine-alkylamino ligands, these two sets of bands can out slowly to avoid decomposition to water and di-
be assigned to ligand-centred p–p* transitions and oxygen, although we have established that the rate of
MLCT bands between the iron(II) t2g orbitals and the addition only marginally affects the conversion (see
pyridine p*-orbitals, respectively.[52–55]
Supporting Information, Fig. S14). All the individual
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Jason England et al.
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Figure 8. UV-vis spectra of iron bistriflate complexes of ligands 1–5 in CH3CN (c=0.5 mM).
Table 3. Results for the catalytic oxidation of cyclohexane.[a]
Run Complex
Equiv. H2O2 A+K[b] (%) A/K[c]
1[d]
2[d]
3
[Fe(1)
[Fe(1)(OTf)2] 100
[Fe(2)(OTf)2] 10
[Fe(2)(OTf)2] 100
[Fe(3)(OTf)2] 10
(OTf)2] 10
[Fe(3)(OTf)2] 100
[Fe(4)(OTf)2] 10
[Fe(4)(OTf)2] 100
[Fe(5)(OTf)2] 10
[Fe(5)(OTf)2] 100
C
65
48
62
23
18
19
6
42
6
0
9.5
2.5
2.4
2.2
1.2
0.9
2.3
1.4
1.4
0
4
5
catalytic runs were performed at least twice. Two
series of catalytic experiments were carried out initial-
ly, using 10 and 100 equiv. of H2O2. The amounts of
cyclohexanol (A) and cyclohexanone (K) produced
using the different catalysts are collected in Table 3.
5a[e] [Fe(3)
6
7
8
9
The iron bisACHTREUNG(triflate) complex [Fe(1)CAHTRE(UGN OTf)2], contain-
10
0.9
1.1
ing the ligand BPMEN, is used as a benchmark
against which the other catalysts are compared. We
have previously reported that this catalyst, when
using 10 equiv. of H2O2, converts 65% of the H2O2
added into oxygenated products, with a large ratio of
cyclohexanol to cyclohexanone (A/K ratio) of 9.[23]
These results are consistent with those reported previ-
ously by Que and co-workers for the complex [Fe-
[a]
[b]
Catalytic conditions: see Experimental Section.
Total percentage yield of cyclohexanol (A)+cyclohexa-
none (K), expressed as moles of product per mole of
H2O2.
[c]
Ratio of moles of cyclohexanol (A) to moles of cyclohex-
anone (K).
[d]
[e]
Values fromref. [23]
Values fromref. [33]
A
(ClO4)2.[26]
ACHTREUNG(CH3CN)2]ACHTREUGN
leads to more oxidised product but, as can be seen in clohexanone (K), but the relationship is far from
Table 3, this generally results in a lower percentage linear. If we consider that a second equivalent of oxi-
conversion of H2O2. The activity profile for the addi- dant is required to oxidise cyclohexanol to cyclohexa-
tion of up to 430 equivalents of H2O2 has been exam- none, the total conversion of H2O2 to generate the
ined in more detail for complex [Fe(1)CAHTRE(UNG OTf)2] and the oxygenated products is given by the sumof the equiv-
results are shown in Figure 9, where the number of alents of cyclohexanol and twice the equivalents of
equivalents of product is plotted versus the number of cyclohexanone produced, which is the A+2K curve
equivalents of H2O2 added. A dashed diagonal line is in Figure 9. Beyond approximately 100 equiv. of
included to represent the total number of equivalents H2O2, the A+2K curve increases only marginally and
of oxidised product that can be obtained fromany
falls continually further behind the theoretical maxi-
amount of H2O2 added. It appears that the addition mum. In order to identify the differences in activity
of more H2O2 leads to more cyclohexanol (A) and cy- for the different catalysts, the activity profiles for all
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Catalyst Stability Determines the Catalytic Activity of Non-Heme Iron Catalysts
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Figure 9. Product distribution as a function of the equiva-
lents of hydrogen peroxide added. Conditions: catalyst: 2.1
mmol [Fe(1)OTf2], 2.1 mmol cyclohexane (1000 equiv.),
roomtemperature in acetonitrile. The dashed line represents
the total number of equivalents of oxidised product that can
be obtained.
catalysts have been examined in more detail up to the
addition of 50 equivalents of H2O2. The reaction pro-
files for the catalysts [Fe(1)ACHTER(UGN OTf)2] and [Fe(2)CAHTRE(UGN OTf)2]
are given in Figure 10 (the other profiles can be
found in the Supporting Information, Fig. S15).
Upon examining the reaction profile for
[Fe(1)ACHTREUNG(OTf)2] (Figure 10), it can be seen that the in-
crease in production of cyclohexanol (A) observed
upon the addition of increasing amounts of H2O2 de-
viates negatively fromlinearity, whereas the produc-
tion of cyclohexanone (K) shows the opposite devia-
tion (i.e., a decreasing A/K ratio). This is consistent
with secondary oxidation of cyclohexanol to cyclohex-
anone, which increases in significance as the alcohol
Figure 10. Product composition at various amounts of H2O2
in the oxidation of cyclohexane catalyzed by a)
[Fe(1)ACTHERNGU(OTf)2] and b) [Fe(2)CAHTRE(UGN OTf)2]. The dashed line repre-
sents the total number of equivalents of oxidised product
that can be obtained.
product builds up (cf. the decrease in A/K ratio from H2O2 added) a comparable catalytic activity, but
9.5 to 2.5 observed upon moving from 10 to 100 equiv. levels off at higher conversions. Complex
of H2O2 in runs 1 and 2). Presumably, with further ad- [Fe(3)
ACHTREUNG
ditions of oxidant, cyclohexanone would become the complex [Fe(4)AHCTREUNG
major product (A/K <1). This over-oxidation process H2O2 added) a good performance, but this rapidly
can be viewed as non-productive because it does not levels off. It appears that for these catalysts the cata-
represent further conversion of cyclohexane to oxy- lytic activity levels off at higher conversions due to
genated product, which is reflected in the decrease in catalyst deactivation and that this catalyst deactiva-
the percentage A+K yield observed upon moving tion is related to the stability of the complexes under
from10 to 100 equiv. H O2 (runs 1 and 2) and the de- the oxidising conditions, which in turn is related to
2
viation of the A+K curve away fromlinearity in
Figure 10.
the coordination mode of the ligands. Complex
[Fe(1)(OTf)2] contains the BPMEN ligand with a cis-
ACHTREUNG
In order to determine the efficiency of the various a coordination geometry, which provides the strongest
catalysts, the A+2K curves for the most active cata- ligand field, as shown by magnetic susceptibility and
lysts containing the ligands 1–4 have been determined UV-vis measurements. This complex is the most
and are collected in Figure 11. It can be seen that for stable and most active catalyst for cyclohexane oxida-
complex [Fe(1)ACHTREUNG(OTf)2] the A+2K curve is closest to tion and only after the addition of approximately
the theoretical maximum (diagonal), which represents 100 equiv. of H2O2 does the catalytic activity start to
the highest conversion of H2O2 into oxidised products. level off, as shown in Figure 9. Complex [Fe(2)-
Complex [Fe(2)
A
ACHTRE(UNG OTf)2], which only partially (ca. 40%) exists as the
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Jason England et al.
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Figure 11. Conversion of H2O2 into oxygenated products (A+2K curves) for different catalysts. The dashed line represents
the total number of equivalents of oxidised product that can be obtained.
cis-a isomer, is considerably less active. The other co- has been prepared and characterised. The coordina-
ordination geometries, cis-b and trans, result in tion geometries of these complexes have been deter-
weaker ligand fields and less active catalysts that de- mined both in the solid state and in solution by
activate more readily, as shown by the performance of 19F NMR and these studies have shown that only the
complexes
[Fe(3)
A
and
[Fe(4)ACHTREUNG
(Figure 11).
ACHTREUNG
Catalyst deactivation is a common but often over- ometry which, according to magnetic susceptibility
[57]
looked problemin oxidation catalysis.
In heme- measurements has the strongest ligand field. All the
based oxidation catalysts, the mode of deactivation other ligand backbones in the complexes of ligands 2–
has been shown to involve oxidation of the porphyrin 5 result in various amounts of other coordination geo-
ligand.[58–61] The exact deactivation pathway of the metries (cis-b or trans), which generally show a lower
non-heme catalysts studied here is not known at this ligand field strength. The catalytic activity of the com-
stage. However, based on the results presented here plexes as catalysts for the oxidation of cyclohexane
and recent observations of the oxidation of (pyridyl- using H2O2 as the oxidant has been investigated. The
methyl)amine-type ligands,[62,63] we believe that oxida- changes in product distribution during the addition of
tive degradation of the ligand is the main pathway for the oxidant to the substrate have been determined for
catalyst deactivation. The reason why iron catalysts all catalysts and complex [Fe(1)CAHTRE(UGN OTf)2] was found to
containing ligands 2–5 deactivate more rapidly is that be the most productive catalyst amongst the series
these complexes exist mostly as kinetically labile studied here. The better performance of this catalysts
high-spin cis-b or trans complexes at room tempera- compared to the other catalysts is believed to be due
ture in acetonitrile, whereas the cis-a complex to the higher stability and the lifetime of this particu-
[Fe(1)ACHTREUNG(OTf)2] exists at least partly as a kinetically lar catalysts under the harsh oxidising conditions. Oxi-
inert low-spin complex. We are currently investigating dation of the ligand is believed to be the main source
the mechanism of this deactivation pathway with the of catalyst degradation.
aimto improve the stability and efficiency of these
oxidation catalysts.
Experimental Section
Conclusions
General Remarks
A series of iron(II) bis
bis(pyridylmethyl)diamine ligands 1–5, containing dif-
ACHTRE(UGN triflate) complexes featuring
All moisture and oxygen sensitive compounds were pre-
pared using standard high vacuumline, Schlenk, or cannula
ferent backbones with various degrees of flexibility, techniques. A standard nitrogen-filled glove-box was used
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Catalyst Stability Determines the Catalytic Activity of Non-Heme Iron Catalysts
FULL PAPERS
for any subsequent manipulation and storage of these com-
MS (+ EI): m/z (%)=358 (24) [M+], 278 (8) [(MꢀPyH2)+],
1
pounds. Standard H, 19F and 13C NMR spectra were record-
266 (20) [(MꢀPyCH2)+], 187 (32) [(Mꢀ
(PyCH2))+],
ACHRTUNEG(PyH)ACHTREUGN
93 (99) [(PyCH3)+], 80 (100) [(PyH2)+]; anal. calcd. (found)
for C18H16Cl2N4: C 60.18 (59.93), H 4.49 (4.29), N 15.60
(15.79).
ed using a Bruker AC-250 MHz spectrometer. VT NMR
spectra were recorded using a Bruker AM-500 MHz or a
1
DRX-400 MHz spectrometer. The H and 13C NMR chemi-
cal shifts were referenced to the residual protio impurity
and 13C NMR signal of the deuterated solvent, respectively.
The 19F NMR chemical shifts were referenced to CFCl3.
Mass spectra were recorded using either a VG Autospec or
a VG PlatformII spectrometer. Elemental analyses were
performed by the Science Technical Support Unit at The
London Metropolitan University. GC analysis was per-
formed using an Agilent 6890 A gas chromatograph on
either a HP-5 (30 m”0.32 mm, film thickness 0.25 mm ) or an
Innowax (30 m”0.25 mm, film thickness 0.25 mm) column.
Toluene was used as the standard for quantitative analysis
and product identities were confirmed using GC-MS. UV-vis
spectra were recorded at 298 K in acetonitrile solution using
a Perkin–Elmer Lambda 20 spectrometer.
4,5-Dichloro-1,2-bisACHTRE[UNG methyl(2-pyridylmethyl)amino]ben-
zene (2): n-Butyllithium(4.90 mL of 1.6M solution in hex-
anes, 7.79 mmol) was added dropwise to a solution of 4,5-di-
chloro-1,2-bis(2-pyridylmethylamino)benzene
(1.40 g,
3.90 mmol) in THF (75 mL), precooled to ꢀ788C. The mix-
˚
ture was stirred at ꢀ78 C for 1 hour followed by a further
hour at roomtemperature to give a dark red-brown solu-
tion. This was cooled to ꢀ788C and methyl iodide (2.43 mL,
39.0 mmol) was added, after which the cooling bath was re-
moved and the reaction mixture stirred for a further 12 h.
Subsequent to this period, saturated aqueous sodiumhydro-
gen carbonate solution and diethyl ether was added. Separa-
tion of the organic layer was followed by extraction of the
aqueous layer with DCM (2”100 mL). The organic fractions
were combined, dried (MgSO4) and the solvent removed
using the rotary evaporator. The resultant residue was ex-
tracted with diethyl ether and the extract reduced to dryness
to give a yellow viscous oil that solidifies to a waxy solid
upon prolonged standing; yield: 1.16 g (76.9%). 1H NMR
(CDCl3): d=8.53 (d, 2H, J=4.8 Hz, 6-PyH), 7.52 (dt, 2H,
J=7.6 Hz, J=1.7 Hz 4-PyH), 7.13 (dd, 2H, J=6.9 Hz, J=
5.0 Hz, 5-PyH), 7.03 (d, 2H, J=7.7 Hz , 3-PyH), 6.95 (s, 2H,
PhH), 4.56 (s, 4H, NCH2), 2.78 (s, 6H, NMe); 13C NMR
(CDCl3): d=157.7 (ipso), 148.6, 143.4, 135.8, 124.5, 122.1,
121.5, 120.8, 58.6 (NCH2), 39.3 (NMe); MS (+EI): m/z
(%)=386 (29) [M+], 294 (55) [(MꢀPyCH2)+], 215 (85)
Solvents and Reagents
Diethyl ether and tetrahydrofuran were dried by prolonged
reflux, under a nitrogen atmosphere, over sodium metal
with a benzephenone ketyl indicator and distilled freshly
prior to use. Dichloromethane and acetonitrile were treated
in a similar manner, but using calcium hydride as the drying
agent. Toluene and pentane were dried by passing through a
column, packed with commercially available Q-5 reagent
(13% CuO on alumina) and activated alumina (pellets,
3 mm), in a stream of nitrogen. The synthesis and character-
isation of the following compounds has been reported previ-
ously:
[Fe(3)
[Fe
A
G
(OTf)2],[23]
[Fe(1)ACHTREUNG
[(Mꢀ
183 (45), 93 (100) [(PyCH3)+].
N,N’-Dimethyl-N,N’-bis(2 pyridylmethyl)butylenediam-
A
G
(PyCH2))+],
ACHRTUNEG(PyCH3)ACHTREUGN
C
(OTf)2].[37] The ligands and
ligand precursors N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)-
cyclohexyldiamine (3),[35] N,N’-dimethyl-N,N’-bis(2-pyridyl-
(4)[38]
and
N,N’-bis(2-pyridyl-
AHCTREUNG
methyl)propyldiamine
ACHTREUNG
methyl)butylenediamine[39] were prepared according to pub-
lished procedures. The ligand N,N’-dimethyl-N,N’-bis(2-pyri-
dylmethyl)cyclohexyldiamine (3) was prepared fromracemic
trans-1,2-diaminocyclohexane.
Synthesis ofLigands and Complexes
4,5-Dichloro-1,2-bis(2-pyridylmethylamino)benzene: Pyri-
dine-2-carboxaldehyde (2.15 mL, 22.6 mmol) was added to a
mixture
of
4,5-dichloro-1,2-diaminobenzene
(1.00 g,
5.65 mmol) and sodium triAHCTR(UENG acetoxy)borohydride (6.23 g,
29.4 mmol) in dichloromethane (DCM) (150 mL), and the
reaction was left to stir for 48 h. Subsequent to this period,
50 mL of 3M aqueous sodium hydroxide solution were
added and the stirring continued for a further 30 min. The
organic layer was separated and the aqueous layer extracted
with DCM (2”100 mL). All the organic fractions were com-
bined, dried (MgSO4) and all volatiles removed. Trituration
of the residue in ethanol, followed by air drying gave a
yellow feathery solid; yield: 1.52 g (74.9%). 1H NMR
(DMSO-d6): d=8.54 (d, 2H, J=4.7 Hz, 6-PyH), 7.75 (dt,
2H, J=7.7 Hz, J=1.6 Hz, 4-PyH), 7.37 (d, 2H, J=7.7 Hz, 3-
PyH), 7.26 (dd, 2H, J=7.0 Hz, J=5.2 Hz, 5-PyH), 6.42 (s,
2H, PhH), 5.93 (t, 2H, J=5.6 Hz, NH), 4.41 (d, 4H, J=
5.5 Hz, NCH2); 13C NMR (DMSO-d6): d=158.7 (ipso),
148.9, 136.7, 136.1, 122.2, 121.4, 118.1, 110.3, 48.2 (NCH2);
to give
a brown oil; yield: 0.75 g (2.52 mmol, 29%).
1H NMR (CDCl3): d=8.46 (dd, 2H, J=1 Hz, J=4.9 Hz,
PyHa), 7.56 (dt, 2H, J=1.8 Hz, J=7.7 Hz, PyHb), 7.35 (d,
2H, J=7.8 Hz, PyHb), 7.07 (ddd, 2H, J=1 Hz, J=5.0 Hz,
J=7.7 Hz, PyHg), 3.57 (s, 4H, NCH2Py), 2.36 (t, 4H, J=
6.8 Hz, NCH2), 2.17 (s, 6H, CH3), 1.49 (q, 4H, J=3.4 Hz,
CH2); 13C NMR (CDCl3): d=160, 149, 137, 123, 122, 63.9,
57.6, 42.4, 25.1; MS (EI): m/z (%)=298 (30) [M+], 206 (40)
[MꢀACHTRE(UNG CH2Py)].
Adv. Synth. Catal. 2008, 350, 883 – 897
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
895

Jason England et al.
FULL PAPERS
a small egg-shaped stirrer bar, and the mixture stirred until
the substrate had fully dissolved. For the addition of 10
equivalents of H2O2 (relative to the amount of catalyst),
0.3 mL of 70 mM solution of hydrogen peroxide in acetoni-
trile was added dropwise over the course of 25 min, using a
syringe pump. Upon completion of addition, the solution
was stirred for a further 15 min and subsequently filtered
through a pad of silica to remove the catalyst. The silica was
then washed with 3.0 mL of acetonitrile and the washings
combined with the filtered reaction mixture. The final con-
centration of the components in the reaction mixture was:
cyclohexane=700 mM, H2O2 =7 mM and catalyst=0.7 mM.
This gave a substrate: oxidant: catalyst molar ratio of
1000:10:1. Other ratios between H2O2 and catalyst were ob-
tained by adding different amounts of H2O2.
The acetonitrile solutions of hydrogen peroxide were pre-
pared from commercially available 35% aqueous hydrogen
peroxide and reagent grade acetonitrile. The resultant aceto-
nitrile solution was used without drying. The silica pads
used for catalyst removal were prepared by inserting a glass
wool plug into a Pasteur pipette, onto which an approxi-
mately 25 mm deep layer of silica was added.
Synthesis ofMetal Complexes
The iron(II) bis
[Fe(3)(OTf)2] and [Fe(4)
[Fe(OTf)2A(CH3CN)2] and the appropriate ligand in THF at
A
ACHTREU(NG OTf)2],
A
ACHTREUNG
A
CHTREUNG
room temperature. The first two complexes precipitated
after one day, whereas several days were required for the
third complex. In all cases, the mixture was filtered and the
residue was washed with diethyl ether and dried in vacuo.
The spectroscopic details were identical to those previously
reported for [Fe(1)
[Fe(4)
(OTf)2].[37]
[Fe(2)(OTf)2]:
2.81 mmol) in THF (50 mL) was added to a stirring solution
of [Fe(OTf)2A(CH3CN)2] (1.12 g, 2.56 mmol) in THF (50 mL).
(OTf)2],[23] [Fe(3)(OTf)2][35] and
ACHRTEUNG ACHTREUGN
ACHTREUNG
A
A
solution of the ligand (1.09 g,
A
CHTREUNG
A precipitate began to formwithin minutes. After stirring
overnight, the volume of THF was reduced to approximate-
ly 10 mL and the reaction mixture filtered. The solid ob-
tained was washed with multiple small quantities of THF to
remove green impurities, followed by a single washing with
diethyl ether (75 mL) and dried under vacuum to give the
1
product as a white powder; yield: 1.46 g (77.2%). H NMR
(CD3CN): d=123.2 (2H, PyHa), 72.3 (6H, NMe), 55.5 (2H,
PyHb), 54.3 (2H, PyHb’), 32.3 (2H, CH2A), 26.4 (2H, CH2B),
18.8 (2H, PyHg), 7.0 (2H, PhH); 19F NMR (CD2Cl2): d=
ꢀ19.9; 19F NMR (CD3CN): d=ꢀ68.1; MS (+ FAB): m/z=
1332 ([2M<M->OTf]+), 740 ([M]+), 591 ([MꢀOTf]+);
Supporting Information
The supplementary crystallographic data for this paper can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/re-
trieving.html (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+
44) 1223–336–033; or deposit@ ccdc.cam.ac.uk). The Sup-
porting Information also contains VT-19F NMR spectra and
cyclohexane oxidation results.
anal. calcd. (found) for C22H20Cl2F6FeN4O6S2:
C 35.65
(35.87), H 2.72 (2.75), N 7.56 (7.41); meff (CD2Cl2)=5.34 mB;
meff (CD3CN)=4.80 mB.
Crystal data for [Fe(2)
M=741.29, monoclinic, P21/c (no. 14), a=15.1625(11), b=
23.9862(17), c=15.8454(18) Š, b=93.649(8)8, V=
5751.1(9) Š3, Z=8 (2 independent molecules), 1calcd.
1.712 gcmꢀ3, (Mo-Ka)=0.940 mmꢀ1, T=173 K, yellow
ACHTERU(NG OTf)2]: C22H20Cl2F6FeN4O6S2,
=
m
ACHTREUNG
Acknowledgements
blocks, Oxford Diffraction Xcalibur 3 diffractometer; 17726
independent measured reflections, F2 refinement, R1 =0.048,
wR2 =0.120, 12074 independent observed absorption-cor-
We are grateful to EPSRC and BP Chemicals Ltd. for a
CASE award to J.E. We also thank the British Council and
IAESTE UK for organising the training placement for M.B.
Mr. Richard Sheppard and Mr. Peter Haycock are thanked
for their assistance in NMR measurements.
rected reflections [jFo j> 4s(jFoj), 2qmax =648], 921 parame-
H
ters; CCDC 618854.
[Fe(5)
ACHTREUNG(OTf)2]: The ligand 5 (0.25 g, 0.839 mmol) and [Fe-
A
CHTREUNG
in THF for 3 weeks. The off-white precipitate formed was
filtered, washed with dry diethyl ether and dried under
vacuumto give the product as an off-white solid; yield:
References
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0.19 g (35%); H NMR (CD3CN): d=120 (2H, PyHa), 102.3
(4H, CH2), 83.1 (4H, CH2), 69.1 (2H, PyHb), 66.3 (2H,
PyHb’), 50.1 (2H, CH2APy), 46.7 (6H, NMe), ꢀ5.1 (2H,
CH2BPy), ꢀ26.4 (2H, PyHg); 19F NMR (CD2Cl2, 298 K): d=
ꢀ12, ꢀ22, ꢀ42; 19F NMR (CD3CN, 298 K): d=ꢀ67.8; MS
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