Synthesis of iron(II), manganese(II), cobalt(II) and ruthenium(II) complexes containing tridentate nitrogen ligands and their application in the catalytic oxidation of alkanes

Article abstract of DOI:10.1039/b414813d

A series of Fe(II), Mn(II), Co(II) and Ru(II) complexes containing bis(imino) pyridine or bis(amino)pyridine ligands and weakly coordinating triflate (OTf^-) or non-coordinating SbF₆^- anions have been prepared. The complexes have been fully characterized including several solid-state structure analyses. Two unusual mono-chelate six-coordinate bis(imino)pyridine Fe(II) and Mn(II) complexes have been observed. The catalytic properties of the complexes for the oxidation of cyclohexane with H₂O₂ have been evaluated. Only the Fe(II) complexes have shown catalytic activity, which is mainly due to Fenton-type free radical auto-oxidation. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b414813d

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F U L L P A P E R
Synthesis of iron(II), manganese(II), cobalt(II) and ruthenium(II)
complexes containing tridentate nitrogen ligands and their
application in the catalytic oxidation of alkanes†
George J. P. Britovsek,* Jason England, Stefan K. Spitzmesser, Andrew J. P. White and
David J. Williams
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
Received 24th September 2004, Accepted 13th January 2005
First published as an Advance Article on the web 4th February 2005
A series of Fe(II), Mn(II), Co(II) and Ru(II) complexes containing bis(imino)pyridine or bis(amino)pyridine ligands
and weakly coordinating triflate (OTf−) or non-coordinating SbF6 anions have been prepared. The complexes have
−
been fully characterized including several solid-state structure analyses. Two unusual mono-chelate six-coordinate
bis(imino)pyridine Fe(II) and Mn(II) complexes have been observed. The catalytic properties of the complexes for the
oxidation of cyclohexane with H2O2 have been evaluated. Only the Fe(II) complexes have shown catalytic activity,
which is mainly due to Fenton-type free radical auto-oxidation.
1932,20 has gained strong support recently through the first
Introduction
crystallographic characterization of an Fe(IV) oxo complex.21
The functionalisation of hydrocarbons has been identified as a
In recent years, several non-heme iron-based catalysts for
key research strategy for the development of economical and
the oxidation of alkanes with H2O2 have been reported.17,22
sustainable global carbon management.1 Undoubtedly one of
the most important functionalisations is selective oxidation, in
Important examples are the tetradentate TPA (A) and BPMEN
(B) complexes shown in Fig. 1, where X represents a weakly
particular the selective oxidation of methane to methanol would
bound co-ligand, typically CH3CN or triflate (OTf−). The com-
be of great industrial interest.2 Oxidising hydrocarbons is not
bination of catalytic amounts of these complexes and H2O2 as the
difficult, but oxidising hydrocarbons selectively, i.e. halting the
oxidant have shown unusual product selectivities, for example
oxidation process at the stage of a useful product rather than
remarkably high selectivities for the formation of cyclohexanol
the thermodynamically favoured total oxidation to CO2 and
in the oxidation of cyclohexane.23–25 Oxygen labeling studies have
H2O, whilst using environmentally benign oxidants such as O2
established that, unlike in Fenton chemistry, O2 is not involved in
or H2O2, presents the major challenge in catalyst design today.3–6
these oxidations.24 In addition, stereospecific hydroxylation has
Some success has been achieved, for example the oxidation of
been observed with a prochiral substrate.26,27 These observations,
cyclohexane to a mixture of cyclohexanol and cyclohexanone7
combined with relatively large kinetic isotope effects, have led
and the oxidation of p-xylene to terephthalic acid8 are large-scale
to the suggestion that alkane hydroxylations, catalysed by A
industrial processes. Both processes operate in homogeneous
solution, using transition metal catalysts and O2 as the terminal
or B operate via a metal-based mechanism rather than the
unselective radical chain auto-oxidation mechanism. Activities
oxidant. This combination of an oxidant with a transition metal
are still rather low however, partly due to decomposition of
dates back to the discovery by Fenton, who reported in 1894 on
H2O2 (‘catalase’ activity)5 and probably also due to oxidative
the strongly oxidising nature of a mixture of H2O2 with catalytic
degradation of the catalyst, a common problem in oxidation
amounts of Fe(II) salts.9 During the last century, many studies
on the nature of the active species and the mechanism of this
reaction have been carried out,10–12 as well as attempts to modify
the iron catalyst with ligands.13,14 In Fenton chemistry, hydroxyl
radicals are believed to be the active oxidant, giving rise to a
radical chain auto-oxidation mechanism, and such free radicals
are probably also involved in other related systems.14–16
Over the years, some iron-based catalyst systems have shown
a product selectivity that is different from Fenton chemistry,
and a mechanism based on high-valent iron(IV) or iron(V) oxo
species has been invoked.17 In heme-containing oxygenases such
as cytochrome-P450 for example, the active species is believed
to be a high valent iron oxo complex, often represented as an
=
Fe(V) O complex but perhaps more accurately described as a
18,19
=
Fe(IV) O coordinated by a porphyrin radical cation.
The
intermediacy of high-valent iron–oxo species, first proposed in
† Electronic supplementary information (ESI) available: Molecular
structures of [Ru(2)(CH3CN)3](SbF6)2 (molecule B), [Fe(1)(OTf)-
(CH3CN)]SbF6, [Fe(1)(OTf)(CH3CN)](SbF6), [Fe(2)(CH3CN)3](SbF6)2,
[Ru(2)(CH3CN)3](SbF6)2 (molecules A and B), [Mn(2)(OTf)2(H2O)].
DQF-COSY 1H NMR spectrum of the paramagnetic complex
[Fe(1)Cl2]. See http://www.rsc.org/suppdata/dt/b4/b414813d/
Fig. 1 Metal complexes used in alkane oxidation.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5
9 4 5
©

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catalysis.28 These problems may be overcome by improvements
in catalyst design, which is the aim of our research.
far. Tris(pyrazolyl)borate iron complexes have been studied,41
and more recently, the use of bis(benzimidazolyl)pyridine,42
triazacyclononane (tacn)43 and bis(pyridylmethyl)amine44 have
been investigated. It appears that all of these systems operate via
a radical chain auto-oxidation mechanism.
Here we report our results on the synthesis and characteri-
sation of a series of metal(II) complexes containing tridentate
bis(imino)pyridine (C) and bis(amino)pyridine (D) ligands.
Provided sufficiently bulky substituents are used, these ligands
generally form five-coordinate complexes of the type LMX2,
whereby the X ligands are cis to each other.45,46 In addition to
iron(II), we have also prepared complexes of the neighbouring
metals manganese(II), cobalt(II) and ruthenium(II). The catalytic
properties of these complexes for the oxidation of cyclohexane,
using H2O2 as the oxidant, have been evaluated.
Compared to iron, the next door neighbours manganese and
cobalt have received much less attention as potential catalysts for
the oxidation of alkanes,29–31 despite their large-scale industrial
application in the aforementioned oxidation of cyclohexane and
p-xylene. The downstairs neighbour ruthenium has also been
investigated for the oxidation of alkanes.32 Noteworthy in this
context is the work by Meyer on the stoichiometric oxidising
properties of [(bipy)2Ru(O)2]2+ and [(bipy)2Ru(L)(O)]2+.33 Due
to ligand degradation or ligand loss in the original Meyer
complexes,34,35 substituted bipyridine ligands have been em-
ployed by others, resulting in catalytic oxidation of alkanes.
Either chloro36,37 or methyl38,39 substituents in the 2 and 9
positions have been employed. Ruthenium complexes containing
the tetradentate TPA ligand (as in A) have also been investigated
for the oxidation of alkanes.40
Results and discussion
Based on the success with the non-heme iron catalysts A and B
it appears that multidentate ligands containing nitrogen (prefer-
ably pyridine) donors and a cis orientation of two labile co-
ligands are desirable criteria in catalyst design. Tetradentate and
pentadentate ligands have received most attention,22 whereas
the application of tridentate ligands has been little explored so
Synthesis of ligands and complexes
The bis(imino)pyridine and bis(amino)pyridine ligands 1–6
(Schemes 1 and 2) were prepared according to previously pub-
lished procedures.45,47 Iron(II) bis(triflate) complex [Fe(1)(OTf)2]
Scheme 1
9 4 6
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5

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procedure did not work for the larger 2,6-bis(N-methyl-N-
phenylaminomethyl)pyridine ligand 6. The dichloro complex
[Fe(6)Cl2], prepared via a different route,46 did react with
Ag(OTf) but no pure [Fe(6)(OTf)2] could be isolated from the
product mixture.
In the case of manganese, the reaction of the 2,6-dimethyl-
phenyl derivative 2 with Mn(OTf)2 results in the clean forma-
tion of [Mn(2)(OTf)2], whereas no reaction with the bulkier
derivative 1 was observed. As in the case of iron, [Mn(1)(OTf)2]
was prepared via metathesis of the known dichloro complex
[Mn(1)Cl2] with Ag(OTf). The reaction of the less bulky o-tolyl
ligand 3 with Mn(OTf)2 leads to a mixture of mono-chelate and
bis-chelate complexes. The cobalt complexes [Co(1)(OTf)2] and
[Co(2)(OTf)2] were prepared without difficulty from the dichloro
precursors [Co(1)Cl2] and [Co(2)Cl2] and Ag(OTf). A modified
procedure was developed for the known ruthenium complex
[Ru(2)(CH3CN)Cl2],55 which gave better and more consistent
yields. Subsequent reaction with 2 equiv. AgSbF6 gave the
octahedral complex [Ru(2)(CH3CN)3][SbF6]2 in high yield.
All complexes have been analyzed by mass spectrometry,
elemental analysis, NMR spectroscopy (for Fe(II), Co(II) and
Ru(II)) and magnetic susceptibility. Crystallographic analyses
have been carried out for compounds [Fe(1)(OTf)2], [Fe(1)(OTf)-
(CH3CN)]SbF6, [Fe(2)(CH3CN)3][SbF6]2, [Mn(2)(OTf)2(H2O)]
and [Ru(2)(CH3CN)3][SbF6]2.
Scheme 2
NMR Spectroscopy
1
Analysis of the paramagnetically shifted H NMR spectra of
(OTf− = triflate = F3CSO3−) was prepared by reacting [Fe(1)Cl2]
with two equivalents of Ag(OTf), whereas [Fe(2)(OTf)2] can be
prepared directly by mixing the ligand 2 with Fe(OTf)2(CH3CN)2
in dichloromethane (see Scheme 1). [Fe(1)Cl(CH3CN)]SbF6
was prepared from [Fe(1)Cl2] with one equivalent of AgSbF6
in acetonitrile.48 Subsequent reaction with one equivalent of
Ag(OTf) results in the formation of [Fe(1)(OTf)(CH3CN)]SbF6.
The reaction of [Fe(1)Cl2] with two equivalents AgSbF6 in
acetonitrile leads to the formation of the acetonitrile com-
plex [Fe(1)(CH3CN)2][SbF6]2. In the case of the less sterically
demanding dimethyl derivative 2, the ligand can be reacted
directly with [Fe(CH3CN)6][SbF6]2 in acetonitrile resulting, after
precipitation of the complex and drying under vacuum, in the
formation of [Fe(2)(CH3CN)2][SbF6]2 (vide supra).
Attempts to prepare iron(II) triflate complexes by mixing
the less bulky ligand 3 containing ortho-tolyl substituents
with Fe(OTf)2(CH3CN)2, resulted in a mixture of [Fe(3)(OTf)2]
and the octahedral bis-chelate complex [Fe(3)2](OTf)2. The
two complexes can be easily distinguished by their distinctly
different 1H NMR chemical shift ranges; the bis-chelate complex
[Fe(3)2](OTf)2 is diamagnetic, whereas the mono-chelate com-
plex [Fe(3)(OTf)2] is paramagnetic (d ≈ −40 to 120 ppm). The
reaction of the o-tolyl derivative 3 with FeCl2 yields exclusively
the mono-chelate complex [Fe(3)Cl2].49 The reaction of this
dichloro complex with 2 equiv. of Ag(OTf) also led to a
mixture of mono- and bis-chelate complexes. It appears that
the formation of mono- versus bis-chelate complexes is anion
dependant as was noted previously for FeCl2 vs. FeBr2 complexes
of bis(imino)pyridine ligands containing “small” substituents
such as phenyl and also for terpyridine iron(II) complexes.50–53
Only in the case of sufficiently bulky 2,6-disubstituted phenyl
groups bis-chelate formation can be prevented completely.
Unsurprisingly, the reaction of the phenyl derivative 4 with
Fe(OTf)2(CH3CN)2 results in the clean formation of the dark-
purple bis-chelate complex [Fe(4)2](OTf)2. Remarkably, 2,6-
bis(dimethylaminomethyl)pyridine ligand 5 reacts readily with
Fe(OTf)2(CH3CN)2 to yield the high-spin mono-chelate complex
[Fe(5)(OTf)2] as a white crystalline solid (Scheme 2), which
is assumed to be five-coordinate, in analogy to the reported
dichloro complex.54 No bis-chelate formation is observed, which
seems to indicate that the dimethylaminomethyl substituents
provide sufficient steric bulk to prevent bis-chelation. The same
high-spin iron(II) and cobalt(II) is generally possible because
the line widths tend to be typically less than 200 Hz, unlike
high-spin Mn(II) where line widths are in the order of 40000–
200000 Hz.56 The chemical shift ranges for H NMR in the
1
case of iron(II) and cobalt(II) are roughly between −40 and
120 ppm. The line-broadening generally results in the loss of
coupling information and accurate peak assignments can be
difficult. Provided the line-broadening is not too big (<100 Hz),
coupling information can be retrieved using COSY.57 We found
that DQF-COSY is particularly useful, revealing “true” singlets,
which are eliminated from the diagonal (see Fig. S8, ESI†).58
19F NMR spectroscopy is particularly useful for determining
whether the triflate anions are coordinated to the metal centre
or not. Uncoordinated triflate anions (as in triflic acid for
example) are typically observed around −78 ppm,59 whereas
coordinated triflate ligands are shifted downfield. The 19F NMR
spectrum of [Fe(2)(OTf)2] in CD2Cl2 shows a single peak at
−11.20 ppm, which is assigned to coordinated triflate anions.60,61
In CD3CN, also a single resonance is observed, but considerably
further upfield at −70.0 ppm, which is typical for uncoordinated
triflate anions. Acetonitrile apparently displaces the weakly
coordinating triflate ligands. A similar behaviour is observed
for the bis(amino)pyridine complex [Fe(5)(OTf)2], with chemical
shifts of −14.60 ppm in CD2Cl2 and −64.41 ppm in CD3CN.
Solid-state structures
The crystal structure of the 2,6-diisopropylphenyl substituted
complex [Fe(1)(OTf)2] (Fig. 2, Table 1) shows the iron centre to
have adopted a distorted square-based pyramidal coordination
geometry, the metal lying ca. 0.44 A out of the basal plane
in the direction of the apical atom O(11); the four atoms of
the basal plane are coplanar to within ca. 0.02 A. This out-of-
plane distortion causes a folding of the aryl substituents such
that the separation between the isopropyl methine carbon atoms
“below” the basal plane is markedly reduced (ca. 4.59 A) cf. that
“above” the plane (ca. 7.97 A). The observed geometry compares
˚
˚
˚
˚
well with other bis(imino)pyridine iron(II), cobalt(II) and man-
ganese(II) dihalide complexes containing bulky substituents.45,62
Less bulky substituents often display the more symmetrical
trigonal bipyramidal geometry in the solid state.49,63,64 The Fe–
˚
O distance (1.976(6) A) of the basal bound triflate ligand is
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5
9 4 7

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◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for [Fe(1)(OTf)2]
iron centre in this complex adopts a distorted square-based
pyramidal coordination geometry, with the metal lying ca.
Fe–N(1)
Fe–N(9)
Fe–N(7)
Fe–O(11)
2.087(5)
2.241(5)
2.219(6)
2.077(6)
Fe–O(21)
C(9)–N(9)
C(7)–N(7)
1.976(6)
1.265(9)
1.282(10)
˚
0.39 A out of the basal plane (which is coplanar to within
˚
ca. 0.07 A) in the direction of the apical atom N(35). The
folding of the aryl substituents is again seen, the “upper” and
“lower” isopropyl methine carbon separations being ca. 7.64 and
˚
4.74 A, respectively. Though the Fe–N(1) distance is essentially
O(21)–Fe–O(11)
O(11)–Fe–N(1)
O(11)–Fe–N(7)
O(21)–Fe–N(9)
N(1)–Fe–N(9)
105.3(3)
99.1(2)
105.9(2)
100.9(3)
73.6(2)
O(21)–Fe–N(1)
O(21)–Fe–N(7)
N(1)–Fe–N(7)
O(11)–Fe–N(9)
N(7)–Fe–N(9)
155.5(3)
101.5(3)
73.6(2)
96.4(2)
142.7(2)
˚
˚
unchanged (2.083(4) A here cf. 2.087(5) A in [Fe(1)(OTf)2]), it is
noticeable that the two Fe–N(imino) distances are significantly
˚
shorter in this cationic complex [2.176(4) and 2.179(4) A] than
˚
=
in the neutral species [2.219(6), 2.241(5) A]; the imine C N
bond lengths themselves are not significantly altered. Being
a less sterically demanding molecule than triflate, it is no
surprise that the acetonitrile ligand has occupied the more
sterically congested apical site; the Fe–O(triflate) separation of
˚
1.988(4) A is unchanged cf. its basal counterpart in [Fe(1)(OTf)2]
˚
[1.976(6) A].
Triflate anions are generally considered weakly coordinat-
ing ligands and are easily displaced by other ligands or
1
donor solvent molecules. For example, the H NMR spectrum
of [Fe(2)(OTf)2] in CD3CN is identical to the spectrum of
[Fe(2)(CH3CN)2][SbF6]2 in the same solvent, which indicates
that [Fe(2)(OTf)2] in CH3CN solution is predominantly present
as an acetonitrile complex. In addition, the aforementioned 19
F
NMR spectrum in CD3CN shows a single peak at −70.0 ppm,
which is typical for uncoordinated triflate anions. Removal of
the solvent under vacuum can cause re-coordination of the
triflate anion, which is indeed observed in the synthesis of
[Fe(1)(OTf)(CH3CN)]SbF6.
The addition of two equivalents of AgSbF6 to a solution
of [Fe(2)Cl2] in acetonitrile results in the precipitation of
AgCl and the formation of a new complex, which after
filtration and prolonged exposure to high vacuum, analyses
according to elemental analysis as the bis(acetonitrile) complex
[Fe(2)(CH3CN)2][SbF6]2. X-Ray quality crystals of this complex,
grown from a MeCN/Et2O solution, showed three acetonitrile
molecules to have coordinated to the metal centre giving
a severely distorted octahedral geometry [cis angles in the
range 73.4(3)–119.0(3)◦, trans angles of 146.4(3), 163.1(3) and
170.0(3)◦] (Fig. 4, Table 3). The bis(imino)pyridine ligand is
folded with respect to the equatorial coordination plane, the
plane of the pyridine ring being inclined by ca. 29◦ to the
[FeN(7)N(9)N(30)] plane (which is coplanar to better than
Fig. 2 The molecular structure of complex [Fe(1)(OTf)2].
˚
noticeably shorter than the apical Fe–O distance (2.077(6) A),
indicating a stronger bond between iron and the basal bound
triflate ligand.
In the molecular structure of complex [Fe(1)(OTf)-
(CH3CN)]SbF6 (Fig. 3, Table 2) the triflate ligand is bound
in a basal position. Similar to its bis-triflate counterpart, the
˚
˚
0.01 A with N(1) lying ca. 0.44 A out of this plane). The folding
of the aryl substituents seen in the five-coordinate square-based
pyramidal iron complexes of the 2,6-diisopropylphenyl ligand
1 is not seen here, the separations of the “upper” and “lower”
Fig. 3 The molecular structure of complex [Fe(1)(OTf)(CH3CN)]SbF6.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for [Fe(1)(OTf)-
(CH3CN)](SbF6)
Fe–N(1)
Fe–N(9)
Fe–N(7)
Fe–O(11)
2.083(4)
2.176(4)
2.179(4)
1.988(4)
Fe–N(35)
C(9)–N(9)
C(7)–N(7)
2.051(5)
1.295(7)
1.280(7)
O(11)–Fe–N(35)
N(35)–Fe–N(1)
N(35)–Fe–N(9)
O(11)–Fe–N(7)
N(1)–Fe–N(7)
106.4(2)
99.0(2)
96.2(2)
104.5(2)
73.7(2)
O(11)–Fe–N(1)
O(11)–Fe–N(9)
N(1)–Fe–N(9)
N(35)–Fe–N(7)
N(9)–Fe–N(7)
154.5(2)
99.6(2)
74.2(2)
99.5(2)
146.0(2)
Fig. 4 The molecular structure of complex [Fe(2)(CH3CN)3](SbF6)2.
9 4 8
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5

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◦
Table 4 Selected bond lengths (A) and angles (◦) for the two
independent molecules (A and B) present in the crystals of
[Ru(2)(CH3CN)3](SbF6)2
˚
˚
Table 3 Selected bond lengths (A) and angles ( ) for [Fe(2)(CH3-
CN)3](SbF6)2
Fe–N(1)
Fe–N(9)
Fe–N(7)
Fe–N(27)
2.115(6)
2.218(7)
2.222(7)
2.149(8)
Fe–N(30)
C(7)–N(7)
Fe–N(33)
C(9)–N(9)
2.112(7)
1.283(11)
2.231(8)
1.267(11)
Molecule A
Molecule B
Ru–N(1)
Ru–N(9)
Ru–N(30)
C(7)–N(7)
Ru–N(7)
Ru–N(27)
Ru–N(33)
C(9)–N(9)
1.950(11)
2.115(10)
2.050(13)
1.33(2)
2.094(10)
2.021(12)
2.024(11)
1.25(2)
1.896(9)
2.090(11)
2.015(12)
1.34(2)
2.101(11)
2.018(12)
2.028(12)
1.29(2)
N(30)–Fe–N(1)
N(1)–Fe–N(27)
N(1)–Fe–N(9)
N(30)–Fe–N(7)
N(27)–Fe–N(7)
N(30)–Fe–N(33)
N(27)–Fe–N(33)
N(7)–Fe–N(33)
163.1(3)
107.1(3)
73.4(3)
119.0(3)
84.9(3)
89.1(3)
170.0(3)
89.9(3)
N(30)–Fe–N(27)
N(30)–Fe–N(9)
N(27)–Fe–N(9)
N(1)–Fe–N(7)
N(9)–Fe–N(7)
N(1)–Fe–N(33)
N(9)–Fe–N(33)
86.0(3)
94.6(3)
97.7(3)
73.8(3)
146.4(3)
79.5(3)
91.4(3)
N(1)–Ru–N(7)
N(1)–Ru–N(27)
N(1)–Ru–N(33)
N(7)–Ru–N(27)
N(7)–Ru–N(33)
N(9)–Ru–N(30)
N(27)–Ru–N(30)
N(30)–Ru–N(33)
N(1)–Ru–N(9)
N(1)–Ru–N(30)
N(7)–Ru–N(9)
N(7)–Ru–N(30)
N(9)–Ru–N(27)
N(9)–Ru–N(33)
N(27)–Ru–N(33)
79.3(4)
84.8(5)
97.3(4)
90.6(4)
88.9(4)
105.1(4)
90.3(5)
87.5(5)
78.3(4)
174.1(5)
157.3(4)
97.5(4)
91.1(4)
90.3(4)
177.6(5)
78.3(4)
85.0(4)
95.1(4)
89.9(5)
89.8(4)
96.2(4)
91.2(4)
88.8(5)
78.9(4)
173.6(4)
156.9(4)
106.8(4)
91.3(5)
89.0(4)
179.7(5)
˚
methyl carbon atoms (ca. 6.21 and 6.55 A respectively) being
different by only ca. 0.3 A. The acetonitrile ligand in equatorial
˚
position appears to be most strongly bound (Fe–N 2.112(7)
˚
A). The Fe–N distance of one of the axially bound acetonitrile
˚
ligands is significantly longer (Fe–N 2.231(8) A). This CH3CN
ligand is probably rather labile in solution and a rapid equi-
librium between five- and six-coordinated species seems likely,
as seen for triazacyclononane iron(II) complexes.60 Exposure
to vacuum removes this ligand and leaves the bis(acetonitrile)
complex [Fe(2)(CH3CN)2][SbF6]2. It should be noted that all
bis(imino)pyridine iron(II) complexes containing bulky aryl
substituents have been invariably five-coordinate with either a
trigonal bipyramidal, or a square-based pyramidal geometry.
Only in the case of [Fe(1)(hfacac)]SbF6 was a tendency towards
octahedral coordination noted, based on a weak Fe · · · F interac-
tion between the metal centre and the SbF6− counteranion.48 The
complex [Fe(2)(CH3CN)3][SbF6]2 represents the first truly six-
coordinate mono-chelate bis(imino)pyridine iron(II) complex.
With less bulky aryl substituents, for example p-methoxyphenyl,
the formation of octahedral bis-chelate complexes is observed.52
The octahedral distortion can be compared with the ruthe-
nium analogue [Ru(2)(CH3CN)3][SbF6]2. This complex, which
has crystallised with two independent molecules A and B having
˚
[RuN(7)N(9)N(30)] plane (which is coplanar to within ca. 0.02 A
˚
˚
˚
[0.02 A] with N(1) lying ca. 0.21 A [ca. 0.17 A] out of this plane).
By contrast, the folding of the aryl substituents is similar to that
seen in [Fe(2)(CH3CN)3](SbF6)2, the separations of the “upper”
˚
and “lower” methyl carbon atoms being ca. 6.62 and 6.04 A [6.55
and 6.03 A], respectively. Despite the larger ionic radius of Ru(II)
˚
vs. Fe(II), the Ru–N distances of the coordinated acetonitrile
ligands are shorter due to the low spin nature of the metal centre
and they are more or less equal, indicating a much more stable
octahedral geometry for ruthenium compared to iron.
The solid state structures of bis(imino)pyridine manganese(II)
complexes containing bulky aryl substituents on the imine
donors are, like iron(II), typically five-coordinate and, depending
on the size of the aryl substituents, possess either a trigonal
bipyramidal or a square-based pyramidal geometry.62,63,65 X-
Ray quality crystals of [Mn(2)(OTf)2] were grown from a
dichloromethane–pentane solution. In this case the structure
analysis (Fig. 6, Table 5) revealed a six-coordinate Mn(II)
complex containing an equatorial H2O ligand, presumably due
˚
similar conformations (rms deviation ca. 0.11 A), also has
a distorted octahedral geometry, though here the distortions
are less marked with cis angles in the range 78.3(4)–105.1(4)◦
[78.3(4)–96.2(4)◦] and trans angles of 157.3(4), 174.11(5) and
177.6(5)◦ [156.9(4), 173.6(4), 179.7(5)◦], the values in square
parentheses being for the second independent molecule (Table 4,
Figs. 5 and S1, ESI†). The folding of the bis(imino)pyridine
ligand with respect to the equatorial coordination plane is
noticeably less than seen in the iron analogue, the plane of
the pyridine ring being inclined by ca. 11◦ [ca. 10◦] to the
Fig. 5 The molecular structure of one (A) of the two independent
molecules present in the crystals of [Ru(2)(CH3CN)3](SbF6)2.
Fig. 6 The molecular structure of complex [Mn(2)(OTf)2(H2O)].
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5
9 4 9

View Article Online
◦
˚
Table 5 Selected bond lengths (A) and angles ( ) for [Mn(2)(OTf)2-
Catalytic oxidation of cyclohexane
(H2O)]
The catalytic properties of the iron(II), manganese(II), cobalt(II)
and ruthenium(II) bis(imino)pyridine and bis(amino)pyridine
complexes for the oxidation of cyclohexane, using H2O2 as
the oxidant, have been investigated (eqn. (1)). The oxidation
Mn–N(1)
Mn–N(9)
Mn–O(41)
C(7)–N(7)
2.183(3)
2.298(3)
2.266(3)
1.283(4)
Mn–N(7)
Mn–O(31)
Mn–O(50)
C(9)–N(9)
2.301(3)
2.189(2)
2.104(3)
1.286(4)
O(50)–Mn–N(1)
N(1)–Mn–O(31)
N(1)–Mn–O(41)
O(50)–Mn–N(9)
O(31)–Mn–N(9)
O(50)–Mn–N(7)
O(31)–Mn–N(7)
N(9)–Mn–N(7)
164.89(11)
108.52(9)
83.80(9)
104.96(12)
91.29(10)
113.25(12)
90.17(10)
141.78(10)
O(50)–Mn–O(31)
O(50)–Mn–O(41)
O(31)–Mn–O(41)
N(1)–Mn–N(9)
O(41)–Mn–N(9)
N(1)–Mn–N(7)
O(41)–Mn–N(7)
86.01(10)
81.67(10)
167.67(9)
71.47(10)
92.36(10)
71.90(10)
94.22(10)
(1)
reactions were carried out in acetonitrile as the solvent at room
temperature under air, similar to the reaction conditions used
by Que and co-workers.27 Hydrogen peroxide solution (70 mM,
10 equiv.) was added to an acetonitrile solution containing the
catalyst (2.1 lmol, 1 equiv.) and cyclohexane (1000 equiv.). A
large excess of substrate was used to minimize over-oxidation of
the product. The addition of dilute H2O2 was carried out slowly
using a syringe pump, in order to minimise H2O2 decomposition
via catalase-type reactivity.5 The yields are based on the amount
of oxidant (H2O2) converted into oxygenated products. The
results are summarized in Table 6. For comparison, we have
also prepared the previously reported catalysts [Fe(TPA)(OTf)2]
and [Fe(BPMEN)(OTf)2],66 which have been tested under the
same conditions.
It can be seen from Table 6 that the bis(imino)pyridine and
bis(amino)pyridine iron(II) complexes do affect the oxidation
of cyclohexane to the oxygenated products cyclohexanol (A)
and cyclohexanone (K) (runs 1–9). However, compared to
[Fe(TPA)(OTf)2] and [Fe(BPMEN)(OTf)2] (runs 10–13), the
conversion of oxidant H2O2 into oxidized product is low,
typically less than 5%, and low alcohol to ketone ratio’s (A/K =
1–3) are obtained. Perhaps more importantly, there appears to
be very little difference in product yield and product distribution
for the various ligands and anions (chloride vs. triflate) or
compared to Fe(OTf)2(CH3CN)2. The manganese(II), cobalt(II)
and ruthenium(II) complexes do not show any activity under the
conditions used.
to adventitious moisture in the solvent used for crystallization
combined with the hygroscopic nature of these triflate com-
plexes. This neutral manganese(II) complex [Mn(2)(OTf)2(H2O)]
adopts a severely distorted octahedral coordination geometry
at the metal centre with cis angles in the range 71.47(10)–
113.25(12)◦ and trans angles of 141.78(10), 164.89(11) and
167.67(9)◦. In common with the other six-coordinate structures,
the bis(imino)pyridine ligand is folded with respect to the
equatorial coordination plane, here the plane of the pyridine
ring being inclined by ca. 25◦ to the [MnN(7)N(9)O(50)] plane
˚
(which is coplanar to within ca. 0.01 A with N(1) lying ca.
˚
0.58 A out of this plane). The folding of the aryl substituents is
also non-existent, the separations of the “upper” and “lower”
methyl carbon atoms (ca. 6.45 and 6.55 A, respectively) being
different by only ca. 0.1 A. Surprisingly, both triflate ligands
˚
˚
are bound in the sterically more congested axial positions,
whereas the smaller H2O ligand occupies the equatorial posi-
tion, which is presumably a consequence of the two sterically
demanding triflate ligands wanting to be mutually trans. This
structure represents another rare example of a mono-chelate
six-coordinate bis(imino)pyridine metal(II) complex containing
bulky aryl substituents.
So far the general consensus has been that bis(imino)pyridine
ligands containing bulky aryl substituents generally form five-
coordinate complexes with first row metals. An important
outcome from these structural investigations is that a sixth
coordination site is apparently accessible, both for Fe(II) and
Mn(II). However, whatever the geometry or the nature of the
co-ligands, whether chloride, acac or as shown here, triflate
or acetonitrile co-ligands, all first row metal complexes have
shown a high-spin electronic configuration at the metal centre.
In the case of ruthenium, a larger second row metal, the
preferred coordination geometry is octahedral with a low spin
configuration.
In the case of [Fe(TPA)(OTf)2] and [Fe(BPMEN)(OTf)2], a
larger hydrogen peroxide concentration (100 equiv.) results in
a lower percentage yield of A + K (i.e. a lower percentage
conversion of H2O2) and the A/K ratio is reduced. The lower
relative yield at higher H2O2 concentration is most likely
due to an increase in non-productive processes such as the
decomposition of H2O2 and the oxidation of cyclohexanol to
cyclohexanone. The latter becomes more likely as the amount of
cyclohexanol produced increases and will result in a lower A/K
ratio. In the case of complexes [Fe(1)(OTf)2] and [Fe(2)(OTf)2],
a
Table 6 Catalytic oxidation of cyclohexane with H2O2
Run
Catalyst
H2O2 (equiv.)
Yield of A + Kb (%)
A/Kc
Pd
1
2
3
4
5
6
7
8
9
10
11
12
13
Fe(OTf)2(CH3CN)2
Fe(OTf)2(CH3CN)2
Fe(1)Cl2
10
100
10
3.8
3.4
1.4
5.2
1.4
1.9
4.9
2.0
3.9
32
1.6
2.4
1.3
1.0
1.8
2.1
1.5
3.1
1.3
12.0
5.9
9.5
2.5
Y
Y
Y
N
Y
Y
N
Y
N
N
N
N
N
Fe(1)(OTf)2
10
Fe(1)(OTf)2
100
100
10
100
10
10
100
10
Fe(2)Cl2
Fe(2)(OTf)2
Fe(2)(OTf)2
Fe(5)(OTf)2
Fe(TPA)(OTf)2
Fe(TPA)(OTf)2
Fe(BPMEN)(OTf)2
Fe(BPMEN)(OTf)2
29
65
48
100
a Conditions: see Experimental section. b Total percentage yield of cyclohexanol (A) + cyclohexanone (K), expressed in moles of product per mole of
H2O2. c Ratio of cyclohexanol (A) to cyclohexanone (K). d Qualitative analysis of cyclohexyl hydroperoxide (P), Y: observed in GC, N: not observed
in GC.
9 5 0
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5

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a ten-fold increase in the hydrogen peroxide concentration also
reduces the percentage yield, but appears to increase the A/K
ratio slightly. In these runs, cyclohexyl hydroperoxide (P) could
also be identified by GC/MS as one of the reaction products.
Cyclohexyl hydroperoxide is a relatively stable peroxide which
can be isolated in pure form,67,68 but does decompose to some
extent during GC analysis.69 This will affect the A/K ratio and,
combined with the relatively large error in the GC analysis
due to the small amounts of product produced with these
catalysts, precludes any definite explanation for the observed
change. We have therefore also refrained from quantifying the
amount of cyclohexyl hydroperoxide (P) produced and have
merely indicated a qualitative (Y/N) observation in Table 6.
The small amounts of oxidation products and the relatively
low A/K ratio’s, combined with the presence of cyclohexyl hy-
droperoxide strongly indicate that the oxidation of cyclohexane
mediated by the iron(II) complexes containing ligands 1, 2 and
5 are indicative of Fenton type chemistry, which involves a
radical chain auto-oxidation mechanism. The hydroxyl radicals
are generated by Haber-Weiss type reactions from the Fe(II)
complex with H2O2.10 These hydroxyl radicals can decompose
hydrogen peroxide to oxygen and water or generate cyclohexyl
radicals, which in turn react with dioxygen to give cyclohexyl
hydroperoxide radicals, the precursors to cyclohexanol, cyclo-
hexanone and cyclohexyl hydroperoxide.14,22 The Mn(II), Co(II)
and Ru(II) complexes appear to be unable to initiate this free
radical Fenton type chemistry.
2,6-bis(N,N-dimethylaminomethyl)pyridine 5,47 as well as the
complexes [Fe(1)Cl2] and [Co(1)Cl2],45 [Fe(1)Cl(CH3CN)]SbF6
48
and [Mn(1)Cl2]62 and [Co(2)Cl2]64 were prepared according to
published procedures. All other chemicals and NMR solvents
were obtained commercially and used as received.
2,6-Bis[1-(N -methyl-N -phenylamino)methyl]pyridine (6).
1.6 M butyl lithium, in hexanes, (4.72 ml, 7.55 mmol) was
added dropwise to a solution of N-methylaniline, in THF. The
solution thus formed was allowed to stir for 30 min. It
was subsequently cooled to −78 ◦C and a solution of 2,6-
bis(bromomethyl)pyridine, which had also been cooled to a
temperature of −78 ◦C, added dropwise to it. Upon completion
of the addition the mixture was allowed to warm to room
temperature and stir overnight. The reaction mixture was
quenched by addition of saturated aqueous sodium hydrogen
carbonate solution and extracted with diethyl ether. The organic
layer was dried over magnesium sulfate, filtered and reduced to
dryness to give a cream coloured solid (1.04 g, 87%). 1H NMR
(CDCl3): d 7.49 (t, H), 7.24 (t, 4H), 7.03 (d, 2H), 6.73 (m, 6H),
4.67 (s, 4H), 3.13 (s, 6H). 13C NMR (CDCl3): d 159.3, 149.2,
137.5, 129.2, 118.8, 116.6, 112.1, 58.76 (CH2), 39.07 (CH3). MS
(EI): m/z (%) 317 (15) [M+], 212 (100) [(M − NMePh)+], 106
(40) [(NMePh)+]. Anal. Calc. (found) for C21H23N3: C, 79.46
(79.56); H, 7.30 (7.43); N, 13.24 (13.10%).
{2,6-Bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine}bis(tri-
flato)iron(II) [Fe(1)(OTf)2]. Silver(I) triflate (940 mg, 3.7 mmol)
and [Fe(1)Cl2] (995 mg, 1.6 mmol) were dissolved in 80 ml
CH3CN. After stirring at room temperature for 16 h, the
solvent was removed and the residue was extracted with
dichloromethane. The solution was concentrated to 5 ml and
the product precipitated by the addition of pentane (50 ml).
The reaction mixture was filtered and the product washed
with diethyl ether. After drying in vacuo, the product was
obtained as an orange solid. (0.93 g, 68%). Crystals suitable for
X-ray diffraction were grown from a dichloromethane solution,
In conclusion, we have shown that bis(imino)pyridine Mn(II),
Fe(II), Co(II) and Ru(II) complexes containing bulky aryl
substituents and weakly or non-coordinating anions can be
prepared. Coordinated triflate ligands are easily displaced in
coordinating solvents such as acetonitrile. The acetonitrile com-
−
plexes can be isolated when non-coordinating anions (SbF6
)
are employed, resulting in the case of Fe(II), in an unusual
six-coordinate complex containing a bulky bis(imino)pyridine
ligand. The application of these Fe(II) complexes as catalysts for
the oxidation of hydrocarbons leads predominantly to Fenton
type reactivity. Based on the success in oxidation catalysis
obtained with ligands such as TPA and BPMEN we are currently
exploring the use of tetradentate ligands.
1
layered with pentane. H NMR (CD2Cl2): d 88.2 (2H, PyHm),
=
16.3 (1H, PyHp), 13.8 (4H, PhHm), 8.8 (6H, N C–Me), 0.1
(24H, iPrMe), −7.3 (4H, iPrCH), −14.1 (2H, PhHp); MS
(+FAB): m/z (%) 835 [M+], 686 [(M − OTf)+], 537 [(M −
(OTf)2)+], 482 [(M − Fe(OTf)2)+]; Anal. Calc. (found) for
C35H43F6FeN3O6S2: C, 50.30 (50.27); H, 5.19 (5.18); N, 5.03
(5.09)%. leff = 4.90 lB.
Experimental
General
Acetonitrile{2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyri-
dine}(triflato)iron(II) hexafluoroantimonate [Fe(1)(OTf)(CH3-
CN)]SbF6. Silver(I) triflate (164 mg; 0.64 mmol) and
[Fe(1)Cl(CH3CN)]SbF6 (540 mg, 0.64 mmol) were dissolved
in 40 ml CH3CN. A beige coloured precipitate (AgCl) and a
dark orange solution was formed immediately. After stirring
at room temperature for 16 h, the solution was removed by
filtration and the solvent was removed in vacuo. The residue was
washed with diethyl ether (20 ml). After drying under vacuum,
the product was obtained as an orange–red microcrystalline
solid. Crystals suitable for X-ray analysis were grown from a
concentrated dichloromethane solution, layered with pentane
(0.49 g, 80%). 1H NMR (CD2Cl2): d 104.3 (2H, PyHm),
29.3 (4H, PhHm), 12.2 (4H, iPrCH), 2.3 (30H, iPrCH3 and
All moisture-sensitive compounds were manipulated using
standard vacuum line, Schlenk or cannula techniques, or in
a conventional nitrogen-filled glove box. NMR spectra were
recordedeither ona Bruker AC-250 or aDRX-400 spectrometer;
chemical shifts for 1H 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 carried out 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.48
=
N C–Me), −16.4 (2H PhHp), −24.8 (1H, PyHp); MS (+FAB):
m/z (%) 686 [(M − SbF6 − CH3CN)+]. Anal. Calc. (found) for
C36H46F9FeN4O3SSb: C, 44.88 (45.01); H, 4.81 (5.00); N, 5.82
(5.77%). leff = 5.10 lB.
Solvents and reagents
Toluene and pentane were dried by passing through a col-
umn, 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 potas-
sium metal with a benzophenone ketyl indicator, whereas
{2,6-Bis[1-(2,6-dimethylphenylimino)ethyl]pyridine}bis(tri-
flato)iron(II) [Fe(2)(OTf)2]. A solution of 0.47 g (1.08 mmol)
of Fe(OTf)2(CH3CN)2 and 0.40 g (1.08 mmol) of 2 in 30 ml
of DCM was stirred overnight, under a nitrogen atmosphere.
The solution was filtered and the volume of the solution was
reduced to approximately 5 ml, under vacuum, and pentane
added to precipitate the product. This solid was subsequently
washed with pentane once and diethyl ether twice to give the
dichloromethane and acetonitrile were dried over CaH2.
70
The metal complex precursors FeCl2(thf)3/2
,
Mn(OTf)2 and
73
Fe(OTf)2(CH3CN)2,71,72 and [Fe(CH3CN)6][SbF6]2 have been
reported previously. The bis(imino)pyridine ligands 1–345,49 and
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5
9 5 1

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product as a dark red powder (0.72 g, 92%). 1H NMR (CD2Cl2):
d 82.5 (2H, PyHm), 16.3 (1H, PyHp), 13.0 (4H, PhHm), 6.98
(0.25 g, 0.98 mmol) in 30 ml of DCM was stirred overnight,
under a nitrogen atmosphere. The mixture obtained was filtered
through Celite and the Celite washed with further DCM. The
filtrate 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
pentane and diethyl ether and dried under vacuum to give the
product as a bright yellow powder (0.27 g, 66%). MS (+FAB):
m/z (%) 685 [(M − OTf)+], 536 [(M − (OTf)2)+], 482 [(M −
Mn(OTf)2)+]; Anal. Calc. (found) for C35H43F6MnN3O6S2: C,
50.36 (50.21); H, 5.19 (5.28); N, 5.03 (4.97%). leff = 6.26 lB.
1
=
(12H, PhMe), −0.6 (6H, N C–Me), −14.4 (2H, PhHp); H
NMR (CD3CN): d 72.1 (2H, PyHm), 21.6 (1H, PyHp), 13.6
=
(12H, PhMe), 12.2 (4H, PhHm), 0.05 (6H, N C–Me), −17.9
(2H, PhHp);19F NMR (CD2Cl2): d −11.20; 19F NMR (CD3CN):
d −70.0. MS (+FAB): m/z (%) 723 [M+], 574 [(M − OTf)+], 425
[(M − (OTf)2)+], 370 [(M − Fe(OTf)2)+]; Anal. Calc. (found)
for C27H27F6FeN3O6S2: C, 44.82 (44.89); H, 3.76 (3.93); N, 5.81
(5.81%). leff = 5.36 lB.
Bis{2,6-bis[1-(phenylimino)ethyl]pyridine}iron(II) bis(triflate)
[Fe(4)2](OTf)2. A mixture of 0.17 g (0.4 mmol) of Fe(OTf)2-
(CH3CN)2 and two equivalents (0.25 g, 0.8 mmol) of 4 in 25 ml
of DCM was stirred overnight, under a nitrogen atmosphere.
The solution was filtered and reduced to dryness to give a sticky
solid, which was recrystallised from a dichloromethane solution
layered with pentane. The solid thus obtained was dried under
vacuum to give a dark purple microcrystalline solid (0.18 g,
45%). 1H NMR (d6-DMSO): d 8.42 (d, 4H, PyHm), 8.22 (t, 2H,
{2,6-Bis[1-(2,6-dimethylphenylimino)ethyl]pyridine}bis(tri-
flato)manganese(II) [Mn(2)(OTf)2].
A solution of 0.38 g
(1.08 mmol) of Mn(OTf)2 and 0.40 g (1.08 mmol) of 2 in 30 ml
of DCM was stirred overnight, under a nitrogen atmosphere.
The solution obtained was filtered, reduced to approximately
5 ml, under vacuum, and pentane added to precipitate the
product. This solid was subsequently washed with pentane and
diethyl ether to give the product as a yellow–orange powder
(0.52 g, 67%). Crystals suitable for X-ray diffraction were
grown from a CH2Cl2–pentane solution. MS (+FAB): m/z (%)
723 [M+], 573 [(M − OTf)+], 424 [(M − (OTf)2)+], 370 [(M −
Mn(OTf)2)+]; Anal. Calc. (found) for C27H27F6MnN3O6S2: C,
44.88 (44.79); H, 3.77 (3.83); N, 5.82 (5.72%). leff = 5.96 lB.
PyHp), 7.17 (m, 12H, PhHp and PhHo), 6.27 (m, 8H, PhHm),
13
=
=
2.61 (s, 12H, N C–Me); C NMR (CDCl3): d 179 (N C–Me),
=
160, 144, 137, 131, 129, 128, 119, 20 (N C–Me); MS (+FAB):
m/z (%) 831 [(M − OTf)+], 682 [(M − 2OTf)+]; Anal. Calc.
(found) for C44H38F6FeN6O6S2: C, 53.88 (53.78); H, 3.91 (3.87);
N, 8.57 (8.47%).
Bis(acetonitrile){2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]-
pyridine}iron(II) bis(hexafluoroantimonate) [Fe(1)(CH3CN)2]-
[SbF6]2. A solution of 0.50 g (0.82 mmol) of [Fe(1)Cl2] and 2
equivalents (0.57 g, 1.64 mmol) of silver(I) hexafluoroantimo-
nate, AgSbF6, in 25 ml of acetonitrile was stirred overnight,
under a nitrogen atmosphere. The solution was subsequently
reduced to dryness, 10 ml of DCM added and the solid AgCl
filtered off. The DCM solution was reduced to about 5 ml
and pentane added to cause precipitation of a solid. This solid
was washed once with pentane and twice with diethyl ether to
give the product as an orange powder (0.61 g, 68%). 1H NMR
(CD2Cl2): d 92.9 (2H, PyHm), 45.4 (6H, CH3CN), 31.6 (6H,
{2,6-Bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine}bis(tri-
flato)cobalt(II) [Co(1)(OTf)2]. A mixture of [Co(1)Cl2] (0.30 g,
0.49 mmol) and silver(I) triflate (0.25 g, 0.98 mmol) in 50 ml
of CH2Cl2 was stirred overnight, under a nitrogen atmosphere.
The mixture obtained was filtered through Celite and the Celite
washed with further CH2Cl2. The filtrate 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 prior to drying under
vacuum to give [Co(1)(OTf)2] as a lime green powder (0.32 g,
1
77%). H NMR (CD2Cl2, 250 MHz, r.t., all peaks appear as
broad singlets): d 114.2 (2H, PyHm), 42.8 (1H, PyHp), 23.6 (6H,
=
=
N C–Me), 12.5 (4H, PhHm), 8.5 (1H, PyHp), 4.9 (12H, iPrMe),
N C–Me), 19.6 (4H, PhHm), −6.5 (2H, PhHp), −6.8 (12H,
4.1 (12H, iPrMe), −18.1 (2H PhHp); MS (+FAB): m/z (%) 841
iPrMe), −9.4 (12H, iPrMe); MS (+FAB): m/z (%) 689 [(M −
OTf)+], 540 [(M − (OTf)2)+], 482 [(M − Co(OTf)2)+]; Anal.
Calc. (found) for C35H43CoF6N3O6S2: C, 50.12 (50.27); H, 5.17
(5.21); N, 5.01 (4.97%). leff = 4.67 lB.
[(M − (SbF6) − (CH3))+], 792 [(M − (SbF5) − (CH3CN)2)+],
556 [(M − (Sb2F11) − (CH3CN)2)+], 537 [(M − (SbF6)2
−
(CH3CN)2)+], 482 [(M − Fe(SbF6)2(CH3CN)2)+]; Anal. Calc.
(found) for C37H49F12FeN5Sb2: C, 40.73 (40.86); H, 4.53 (4.43);
N, 6.42 (6.56%). leff = 5.13 lB.
{2,6-Bis[1-(2,6-dimethylphenylimino)ethyl]pyridine}bis(tri-
flato)cobalt(II) [Co(2)(OTf)2]. A mixture of [Co(2)Cl2] (0.23 g,
0.46 mmol) and silver(I) triflate (0.24 g, 0.92 mmol) in 40 ml
of CH2Cl2 was stirred overnight, under a nitrogen atmosphere.
The mixture obtained was filtered and the filtrate 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 pentane and diethyl ether
and dried under vacuum to give the product as a lime green
powder (0.21 g, 63%). Poor solubility in most solvents and
Bis(acetonitrile){2,6-bis[1-(2,6-dimethylphenylimino)ethyl]py-
ridine}iron(II) bis(hexafluoroantimonate) [Fe(2)(CH3CN)2]-
[SbF6]2. A mixture of 0.31 g (0.40 mmol) of [Fe(CH3CN)6]-
(SbF6)2 and 0.15 g (0.40 mmol) of 2 in 35 ml of acetonitrile was
stirred overnight, under a nitrogen atmosphere. The acetonitrile
was removed and the residue dissolved in the minimum amount
of DCM. Subsequently, the volume of the solution was reduced
to approximately 3 ml, under vacuum, and pentane added to
precipitate the product. This solid was subsequently washed
with pentane once, diethyl ether twice and dried under vacuum
to give the product as a dark orange powder. Crystals of
[Fe(2)(CH3CN)3](SbF6)2 suitable for X-ray analysis were grown
from an acetonitrile solution, layered with diethyl ether (0.30 g,
76%). 1H NMR (CD2Cl2): d 69.3 (2H, PyHm), 25.4 (1H, PyHp),
1
decomposition in d6-DMSO prevented H NMR analysis; MS
(+FAB): m/z (%) 577 [(M − OTf)+], 428 [(M − (OTf)2)+]; Anal.
Calc. (found) for C27H27CoF6N3O6S2: C, 44.63 (44.51); H, 3.75
(3.67); N, 5.78 (5.69%). leff = 4.67 lB.
(Acetonitrile)dichloro{2,6-bis[1-(2,6-dimethylphenylimino)-
ethyl]pyridine}ruthenium [Ru(2)(CH3CN)Cl2]. A mixture of 2
(0.39 g, 1.06 mmol) and [RuCl2(p-cymene)]2 (0.31 g, 0.5 mmol)
and 1 ml of acetonitrile was refluxed in 10 ml of ethanol for
24 h, under a nitrogen atmosphere. After cooling the solution
was reduced to dryness, the residue dissolved in a minimum
amount of DCM, and pentane added to precipitate solid. This
solid was then washed twice with pentane and dried under
vacuum to give the product as a burgundy-coloured solid. Yield:
0.55 g (90%). 1H NMR (d6-DMSO): d 8.37 (d, 2H), 8.16 (t, 1H),
7.16–6.88 (m, 6H), 2.54 (s, 6H), 2.12 (s, 12H), 2.06 (s, 3H). In
=
13.7 (12H, PhMe), 12.2 (4H, PhHm), 3.0 (6H, N C–Me), −18.0
(2H, PhHp), no peak corresponding to CH3CN is observed;
MS (+FAB): m/z (%) 728 [(M − (SbF6) − (CH3))+], 680 [(M −
(SbF5) − (CH3CN)2)+], 661 [(M − (SbF6) − (CH3CN)2)+],
444 [(M − (Sb2F11) − (CH3CN)2)+], 425 [(M − (SbF6)2
−
(CH3CN)2)+]; Anal. Calc. (found) for C29H33F12FeN5Sb2: C,
35.58 (35.74); H, 3.40 (3.52); N, 7.15 (7.09%). leff = 5.17 lB.
{2,6-Bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine}bis(tri-
flato)manganese(II) [Mn(1)(OTf)2].
A mixture of 0.30 g
(0.49 mmol) of [Mn(1)Cl2] and 2 equivalents of silver(I) triflate
9 5 2
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5

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other solvents, such as CD2Cl2 or CDCl3, mixtures of products
are observed. Other analytical data as previously reported.55
Tris(acetonitrile){2,6-bis[1-(2,6-dimethylphenylimino)ethyl]-
pyridine}ruthenium(II)
bis(hexafluoroantimonate)
[Ru(2)-
(CH3CN)3][SbF6]2. A mixture of [Ru(2)Cl2(CH3CN)] (0.10 g,
0.17 mmol) and silver(I) hexafluoroantimonate (0.12 g,
0.34 mmol) was stirred in 20 ml of acetonitrile overnight,
under a nitrogen atmosphere. Subsequently, the acetonitrile
was removed, under vacuum, to yield a solid that was dissolved
in DCM and filtered. The volume of the DCM solution was
reduced to approximately 3 ml, under vacuum, and pentane
added to precipitate the product. This solid was washed with
pentane and diethyl ether to give the product as a brown powder
(0.11 g, 61%). Crystals suitable for X-ray diffraction were grown
from a CH2Cl2–pentane solution. 1H NMR (d6-DMSO): d 8.59
(d, 2H, PyHm), 8.39 (t, 1H, PyHp), 7.20 (6H, PhH), 6.7 (12H,
=
PhMe), 2.58 (s, 6H, N C–Me), 2.36 (6H, axial CH3CN), 2.21
(3H, equatorial CH3CN), 2.07 (12H, PhMe); MS (+FAB): m/z
(%) 829 [(M − (SbF6))+], 788 [(M − (SbF6) − (CH3CN))+],
747 [(M − (SbF6) − (CH3CN)2)+], 706 [(M − (SbF6) −
(CH3CN)3)+], 512 [(M − (SbF6)2 − (CH3CN)2)+], 471 [(M −
(SbF6)2 − (CH3CN)3)+], 369 [(M − Ru(SbF6)2(CH3CN)3)+].
{2,6-Bis[1-(N,N-dimethylamino)methyl]pyridine}bis(triflato)-
iron(II) [Fe(5)(OTf)2]. The mixture obtained by adding a
solution of 5 (0.23 g, 1.19 mmol) in THF (25 ml) to a solution
of Fe(OTf)2(CH3CN)2 (0.47 g, 1.00 mmol) in THF (25 ml),
was stirred overnight at room temperature. Subsequently,
the solution was reduced in volume and pentane added to
precipitate a solid. The solid was washed with diethyl ether twice
and dried under vacuum. The off-white powder obtained was
recrystallised by slow diffusion from a CH2Cl2 solution layered
with pentane to give a white crystalline solid (0.21 g, 36%).
1H NMR (CD2Cl2): d 132.2 (12H, NMe), 73.8 (2H, PyHm),
56.5 (4H, CH2), −12.3 (1H, PyHp); 1H NMR (CD3CN): d 75.6
(12H, NMe), 62.6 (2H, PyHm), 49.5 (4H, CH2), −8.90 (1H,
PyHp); 19F NMR (CD2Cl2): d −14.60; 19F NMR (CD3CN): d
−64.41. MS (+FAB): m/z (%) 398 [(M − OTf)+], 194 [(M −
(Fe(OTf)2)+]; Anal. Calc. (found) for C13H19F6FeN3O6S2: C,
28.53 (28.54); H, 3.50 (3.40); N, 7.68 (7.77%). leff = 4.84 lB.
{2,6-Bis[1-(N-methyl-N-phenylamino)methyl]pyridine}dichloro-
iron(II) [Fe(6)Cl2]. A mixture of 0.80 g (2.52 mmol) of 6
and 0.59 g (2.52 mmol) of FeCl2(thf)3/2 in toluene (75 ml) was
allowed to heat at reflux for 48 h. It was subsequently hot filtered
and the solution filtrate was reduced in volume and cooled. The
resultant mixture was filtered and the solid obtained was dried
under vacuum to give [Fe(6)Cl2] as a lime green crystalline solid
(0.84 g, 75%). 1H NMR (CDCl3): d 74.2 (6H, PhMe), 57.9 (2H,
PyHm), 44.2 (4H, CH2), 10.0 (4H, PhHm), −2.5 (2H, PhHp),
−7.5 (4H, PhHo), −18.9 (1H, PyHp); MS (+FAB): m/z (%)
443 [M+], 408 [(M − Cl)+], 318 [(M − FeCl2)+]; Anal. Calc.
(found) for C21H23Cl2FeN3: C, 56.78 (56.96); H, 5.22 (5.37); N,
9.46 (9.25%). leff = 5.06 lB.
Procedure for the oxidation experiments
In a 20 ml sample vial, 2.1 lmol catalyst and 2.1 mmol
(0.23 ml) cyclohexane were dissolved in 2.70 ml acetonitrile
solvent. To this stirred solution, 0.30 ml of a 70 mM (or
700 mM) H2O2 solution (prepared from commercial 30%
aqueous H2O2 and acetonitrile) was added at room temperature
via syringe pump during 25 min. Final concentrations: catalyst:
0.7 mM. H2O2: 7 mM and cyclohexane: 700 mM or a molar
ratio of 1/10/1000. In the case of a 700 mM H2O2 solution, the
ratio was 1/100/1000. After the addition of the H2O2 solution
was complete, the reaction mixture was stirred for another
15 min and then immediately filtered through silica to remove
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5
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the catalyst. The reaction products were analyzed by GC. All
values are the average of at least 2 runs.
26 C. Kim, K. Chen, J. Kim and L. Que, Jr., J. Am. Chem. Soc., 1997,
119, 5964.
27 K. Chen and L. Que, Jr., J. Am. Chem. Soc., 2001, 123, 6327.
28 T. J. Collins, Acc. Chem. Res., 1994, 27, 279.
29 G. V. Nizova, C. Bolm, S. Ceccarelli, C. Pavan and G. B. Shul’pin,
Adv. Synth. Catal., 2002, 344, 899.
Crystallographic details
Table 7 provides a summary of the crystallographic data for
compounds [Fe(1)(OTf)2], [Fe(1)(OTf)(CH3CN)]SbF6, [Fe(2)-
(CH3CN)3](SbF6)2, [Ru(2)(CH3CN)3](SbF6)2 and [Mn(2)(OTf)2-
(H2O)]. Data were collected on Bruker P4 diffractometers, and
the structures were refined based on F2 using the SHELXTL
program system.74 The absolute structures of [Fe(1)(OTf)2]
and [Fe(2)(CH3CN)3](SbF6)2 were determined by a combina-
tion of R-factor tests and use of the Flack parameter [for
30 C. A. Tolman, J. D. Druliner, P. J. Krusic, M. J. Nappa, W. C. Seidel,
I. D. Williams and S. D. Ittel, J. Mol. Catal., 1988, 48, 129.
31 L. Saussine, E. Brazi, A. Robine, H. Mimoun, J. Fischer and R.
Weiss, J. Am. Chem. Soc., 1985, 107, 3534.
32 T. Naota, H. Takaya and S.-I. Murahashi, Chem. Rev., 1998, 98,
2599.
33 T. J. Meyer, J. Electrochem. Soc., 1984, 131, 221C.
34 L. Roecker, W. Kutner, J. A. Gilbert, M. Simmons, R. W. Murray
and T. J. Meyer, Inorg. Chem., 1985, 24, 3784.
35 J. C. Dobson and T. J. Meyer, Inorg. Chem., 1988, 27, 3283.
36 T.-C. Lau, C.-M. Che, W.-O. Lee and C.-K. Poon, J. Chem. Soc.,
Chem. Commun., 1988, 1406.
[Fe(1)(OTf)2], R1 = 0.0492, R1 = 0.0506, x+ = −0.09(8); for
+
−
[Fe(2)(CH3CN)3](SbF6)2, R1 = 0.0394, R1 = 0.0411, x+
=
+
−
+0.06(6), x− = +0.94(6)].
37 C.-M. Che, K.-W. Cheng, M. C. W. Chan, T.-C. Lau and C.-K. Mak,
J. Org. Chem., 2000, 65, 7996.
38 A. S. Goldstein and R. S. Drago, J. Chem. Soc., Chem. Commun.,
1991, 21.
CCDC reference numbers 246556–246560. See http://www.
rsc.org/suppdata/dt/b4/b414813d/ for crystallographic data
in CIF or other electronic format.
39 A. S. Goldstein, R. H. Beer and R. S. Drago, J. Am. Chem. Soc.,
1994, 116, 2424.
Acknowledgements
40 T. Kojima, H. Matsuo and Y. Matsuda, Inorg. Chim. Acta, 2000,
300–302, 661.
We are grateful to EPSRC and BP Chemicals Ltd. for financial
support. Dr Glenn Sunley and Dr Marco Boesveld are thanked
for helpful discussions. We thank Prof. Vernon Gibson for
allowing the use of unpublished results by S. K. S. and Mr.
Richard Sheppard for NMR measurements. We also thank
Johnson Matthey for a loan of RuCl3.
41 N. Kitajima, J. Chem. Soc. Chem. Commun., 1988, 485.
42 X. Wang, S. Wang, L. Li, E. B. Sundberg and G. P. Gacho, Inorg.
Chem., 2003, 42, 7799.
43 M. Klopstra, R. Hage, R. M. Kellogg and B. L. Feringa, Tetrahedron
Lett., 2003, 44, 4581.
44 G. B. Shul’pin, G. V. Nizova, Y. N. Kozlov, L. Gonzalez Cuervo and
G. Su¨ss-Fink, Adv. Synth. Catal., 2004, 346, 317.
45 G. J. P. Britovsek, M. Bruce, V. C. Gibson, B. S. Kimberley, P. J.
Maddox, S. Mastroianni, S. J. McTavish, C. Redshaw, G. A. Solan,
S. Stro¨mberg, A. J. P. White and D. J. Williams, J. Am. Chem. Soc.,
1999, 121, 8728.
46 G. J. P. Britovsek, V. C. Gibson, S. Mastroianni, D. C. H. Oakes, C.
Redshaw, G. A. Solan, A. J. P. White and D. J. Williams, Eur. J. Inorg.
Chem., 2001, 431.
47 A. N. Vedernikov, P. Wu, J. C. Huffman and K. G. Caulton, Inorg.
Chim. Acta, 2002, 330, 103.
48 G. J. P. Britovsek, V. C. Gibson, S. K. Spitzmesser, K. P. Tellmann,
A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 2002,
1159.
49 G. J. P. Britovsek, S. Mastroianni, G. A. Solan, S. P. D. Baugh, C.
Redshaw, V. C. Gibson, A. J. P. White, D. J. Williams and M. R. J.
Elsegood, Chem. Eur. J., 2000, 6, 2221.
50 F. Lions and K. V. Martin, J. Am. Chem. Soc., 1957, 79, 2733.
51 M. S. Brookhart and B. L. Small, (DuPont/UNC), Int. Pat., WO
98/30612, 1998 (Chem. Abstr., 1998, 129, 149375r).
52 B. de Bruin, E. Bill, E. Bothe, T. Weyhermu¨ller and K. Wieghardt,
Inorg. Chem., 2000, 39, 2936.
References
1 H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, E. J. Beckman,
A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus, D. A. Dixon, K. Domen,
D. L. DuBois, J. Eckert, E. Fujita, D. H. Gibson, W. A. Goddard,
D. W. Goodman, J. Keller, G. J. Kubas, H. H. Kung, J. E. Lyons, L. E.
Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R. A. Periana,
L. Que, Jr., J. Rostrup-Nielson, W. M. H. Sachtler, L. D. Schmidt, A.
Sen, G. A. Somorjai, P. C. Stair, B. R. Stults and W. Tumas, Chem.
Rev., 2001, 101, 953.
2 J.-P. Lange, Ind. Eng. Chem. Res., 1997, 36, 4282.
3 J. A. Labinger, J. Mol. Catal., 2004, 220, 27.
4 R. A. Sheldon and J. K. Kochi, Metal-Catalysed Oxidations of
Organic Compounds, Academic Press, New York, 1981.
5 B. Meunier, Biomimetic Oxidations Catalysed by Transition Metal
Complexes, Imperial College Press, London, 2000.
6 A. E. Shilov and G. B. Shul’pin, Activation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes, Kluwer,
Dordrecht, 2000.
7 K. U. Ingold, Aldrichim. Acta, 1989, 22, 69.
53 W. M. Reiff, N. E. Erickson and W. A. Baker, Inorg. Chem., 1969, 8,
2019.
8 W. Partenheimer, Catal. Today, 1995, 23, 69.
9 H. J. H. Fenton, J. Chem. Soc., 1894, 65, 899.
10 F. Haber and J. Weiss, Proc. R. Soc. London, Ser. A, 1934, 147, 332.
11 C. Walling, Acc. Chem. Res., 1975, 8, 125.
54 M. W. Day, B. D. Ward and R. H. Grubbs (CCDC 133691, 2001).
55 B. C¸ etinkaya, E. C¸ etinkaya, M. Brookhart and P. S. White, J. Mol.
Catal., 1999, 142, 101.
12 F. Buda, B. Ensing, M. C. M. Gribnau and E. J. Baerends, Chem.
Eur. J., 2001, 7, 2775.
56 R. S. Drago, Physical Methods for Chemists, Saunders College,
London, 1992.
13 H.-C. Tung, C. Kang and D. T. Sawyer, J. Am. Chem. Soc., 1992,
114, 3445.
57 W. Peters, M. Fuchs, H. Sicius and W. Kuchen, Angew. Chem., Int.
Ed., 1985, 24, 231.
58 S. Braun, H.-O. Kalinowski and S. Berger, 150 and More Basic NMR
Experiments, Wiley-VCH, Weinheim, 1998.
59 R. E. Noftle and G. H. Cady, Inorg. Chem., 1965, 4, 1010.
60 D. W. Blakesley, S. C. Payne and K. S. Hagen, Inorg. Chem., 2000,
39, 1979.
14 F. Gozzo, J. Mol. Catal., 2001, 171, 1.
15 C. Limberg, Angew. Chem., Int. Ed., 2003, 42, 5932.
16 M. J. Perkins, Chem. Soc. Rev., 1996, 229.
17 M. Costas, M. P. Mehn, M. P. Jensen and L. Que, Jr., Chem. Rev.,
2004, 104, 939.
18 M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson, Chem. Rev.,
1996, 96, 2841.
19 B. Meunier, Chem. Rev., 1992, 92, 1411.
61 H. Bo¨rzel, P. Comba, K. S. Hagen, Y. D. Lampeka, A. Lienke, G.
Linti, M. Merz, H. Pritzkow and L. V. Tsymbal, Inorg. Chim. Acta,
2002, 337, 407.
20 W. C. Bray and M. H. Gorin, J. Am. Chem. Soc., 1932, 54, 2124.
21 J.-U. Rohde, J.-H. In, M. H. Lim, W. W. Brennessel, M. R. Bukowski,
A. Stubna, E. Mu¨nck, W. Nam and L. Que, Jr., Science, 2003, 299,
1037.
62 D. Reardon, G. Aharonian, S. Gambarotta and G. P. A. Yap,
Organometallics, 2002, 21, 786.
63 D. A. Edwards, M. F. Mahon, W. R. Martin, K. C. Molloy, P. E.
Fanwick and R. A. Walton, J. Chem. Soc., Dalton Trans., 1990,
3161.
64 T. M. Kooistra, K. F. W. Hekking, Q. Knijnenburg, B. de Bruin,
P. H. M. Budzelaar, R. de Gelder, J. M. M. Smits and A. W. Gal,
Eur. J. Inorg. Chem., 2003, 648.
22 M. Costas, K. Chen and L. Que, Jr., Coord. Chem. Rev., 2000, 200–
202, 517.
23 J. Kim, R. G. Harrison, C. Kim and L. Que, Jr., J. Am. Chem. Soc.,
1996, 118, 4373.
24 K. Chen and L. Que, Jr., Chem. Commun., 1999, 1375.
25 G. Roelfes, M. Lubben, R. Hage, L. Que, Jr. and B. L. Feringa, Chem.
Eur. J., 2000, 6, 2152.
65 V. C. Gibson, S. J. McTavish, C. Redshaw, G. A. Solan, A. J. P. White
and D. J. Williams, J. Chem. Soc., Dalton Trans., 2003, 221.
66 M. Fujita and L. Que, Jr., Adv. Synth. Catal., 2004, 346, 190.
9 5 4
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5

View Article Online
67 C. Walling and S. A. Buckler, J. Am. Chem. Soc., 1955, 77, 6032.
68 F. A. Chavez, J. M. Rowland, M. M. Olmstead and P. K. Mascharak,
J. Am. Chem. Soc., 1998, 120, 9015.
69 G. B. Shul’pin, CPS 2001, orgchem/0106001, 1.
70 F. Calderazzo, U. Englert, G. Pampaloni and E. Vanni, C. R. Acad.
Sci. Paris Se´r. IIc, 1999, 2, 311.
71 P. S. Bryan and J. C. Dabrowiak, Inorg. Chem., 1975, 14, 296.
72 K. S. Hagen, Inorg. Chem., 2000, 39, 5867.
73 W. W. Brennessel, N. R. Brooks, M. P. Mehn, L. Que, Jr.
and V. G. Young, Jr., Acta Crystallogr., Sect. E., 2001, 57,
m545.
74 B. A. SHELXTL PC version 5.1, Madison, WI, 1997.
D a l t o n T r a n s . , 2 0 0 5 , 9 4 5 – 9 5 5
9 5 5