Tri(pyridylmethyl)phosphine: The elusive congener of TPA shows surprisingly different coordination behavior

Article abstract of DOI:10.1021/ic4005196

Tri(pyridylmethyl)phosphine (TPPh), the remarkably elusive congener of tri(pyridylmethyl)amine (TPA), has been prepared, as well as the relative tri(N-methyl-pyridylamino)phosphine (TPAMP). The coordination properties of these new ligands have been evaluated for chromium(III), iron(II), and ruthenium(II) complexes and compared with the related TPA complexes. In all cases, a different coordination behavior has been observed whereby TPPh and TPAMP always act as tridentate ligands. A chromium(III) complex [Cr(TPPh)Cl ₃] has been prepared, which has shown low ethylene oligomerization activity. Octahedral low spin iron(II) complexes [Fe(TPPh)₂] ²⁺ and [Fe(TPAMP)₂]²⁺ were obtained with two ligands bound to the metal center. Ruthenium(II) chloro complexes of TPA and TPPh undergo ligand exchange reactions in acetonitrile, and the ruthenium(II) complex [Ru(MeCN)₂(TPA)]²⁺ can be oxidized by m-CPBA in acetonitrile to give a transient ruthenium(IV) oxo complex [Ru(O)(MeCN)(TPA)] ²⁺. Attempts to generate high valent ruthenium(IV) oxo TPPh or TPAMP complexes could not be achieved, probably due to insufficient stabilization by these strong field ligands.

Full text of DOI:10.1021/ic4005196

Article
pubs.acs.org/IC
Tri(pyridylmethyl)phosphine: The Elusive Congener of TPA Shows
Surprisingly Different Coordination Behavior
Christopher J. Whiteoak, James D. Nobbs, Evgeny Kiryushchenkov, Sandro Pagano, Andrew J.P. White,
and George J.P. Britovsek*
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AY, United Kingdom
S
* Supporting Information
ABSTRACT: Tri(pyridylmethyl)phosphine (TPPh), the re-
markably elusive congener of tri(pyridylmethyl)amine (TPA),
has been prepared, as well as the relative tri(N-methyl-
pyridylamino)phosphine (TPAMP). The coordination proper-
ties of these new ligands have been evaluated for chromium-
(III), iron(II), and ruthenium(II) complexes and compared
with the related TPA complexes. In all cases, a different
coordination behavior has been observed whereby TPPh and
TPAMP always act as tridentate ligands. A chromium(III) complex [Cr(TPPh)Cl3] has been prepared, which has shown low
ethylene oligomerization activity. Octahedral low spin iron(II) complexes [Fe(TPPh)2]2+ and [Fe(TPAMP)2]2+ were obtained
with two ligands bound to the metal center. Ruthenium(II) chloro complexes of TPA and TPPh undergo ligand exchange
reactions in acetonitrile, and the ruthenium(II) complex [Ru(MeCN)2(TPA)]2+ can be oxidized by m-CPBA in acetonitrile to
give a transient ruthenium(IV) oxo complex [Ru(O)(MeCN)(TPA)]2+. Attempts to generate high valent ruthenium(IV) oxo
TPPh or TPAMP complexes could not be achieved, probably due to insufficient stabilization by these strong field ligands.
important feature of the TPA ligand appears to be the ability to
stabilize high valent metal complexes such as Fe(IV), Ru(IV),
and Re(V) oxo complexes, which are often invoked as
intermediates in oxidation catalysis.4,24,28 This has led to the
development of catalytic systems based on ruthenium
complexes with polypyridyl ligands including TPA that, in
combination with a chemical oxidant, are able to oxidize
hydrocarbons.29−31 Particularly interesting are recent reports
on the use of water as the oxygen source in hydrocarbon
oxygenations or indeed the oxidation of water, using ruthenium
TPA complexes in combination with the one-electron oxidant
cerium ammonium nitrate (CAN),23,32−34 or the photo-
generated oxidant [Ru(bipy)3]3+.35,36
INTRODUCTION
■
Tetradentate tripodal ligands are an important ligand class in
coordination chemistry, and tripodal tripyridyl ligands in
particular have received widespread application in catalysis
and materials chemistry.1,2 Tri(pyridylmethyl)amine (TPA)3
has acquired iconic status, and many transition metal complexes
containing TPA are known today (Figure 1). In most
Many modifications to the TPA ligand have been reported,
either to the pyridyl moieties,1 or to the linkers between the
pyridine and the central amine.37 The higher congener,
tri(pyridylmethyl)phosphine (TPPh),38 where the central
nitrogen donor is substituted for a phosphorus, has so far
remained remarkably elusive (Figure 1). The only sighting of
TPPh was reported by Chiswell in 1967.39 TPPh was
reportedly obtained by the reaction of 3 equiv of α-picolyl
lithium with 1 equiv of PCl3 in 5% yield, but aside from
elemental analysis, no characterization was provided.
Manganese(II), nickel(II), and cobalt(II) complexes were
reported and analyzed as [MCl(TPPh)]X (X = Cl− or
Figure 1. Tripodal ligands TPA, TPPh, and TPAMP.
complexes, TPA coordinates in a tetradentate fashion, although
some examples are known where TPA acts as a tridentate
ligand.4−8 Metal complexes have been prepared with either one
or with two TPA ligands.5,9,10
Iron TPA complexes, including many derivatives, have been
investigated extensively as catalysts in alkane and alkene
oxidation.11−18 We have previously reported a systematic
study on pyridine versus amine donors in TPA and related
ligands and their effect on the catalytic oxidation properties of
their respective iron(II) complexes.19 Alkane and alkene
oxidation catalysts with TPA ligands have also been used
with other metals, notably ruthenium20−26 and nickel.27 An
−
ClO4 ) according to elemental analysis. As will be shown
here, these results are highly questionable, and to the best of
Received: February 27, 2013
© XXXX American Chemical Society
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our knowledge, there are no other reports in the literature on
the synthesis of TPPh or its metal complexes. This is surprising,
considering the popularity of TPA and that related mono- and
bis(pyridylmethyl)phosphine ligands and their metal complexes
have been known for some time,40−49 as well as the ethylene-
bridged tri(pyridylethyl)phosphine.50
We present here the first reliable and high yielding synthesis
for TPPh. In order to increase the basicity of the central donor,
we also prepared the tri(pyridylamine)phosphine ligand
TPAMP (see Figure 1). This ligand also appears to be new,
although the unmethylated ligand51 as well as several mono-
and bis(pyridylamine)phosphine derivatives have been re-
ported.40,44,52−54
The coordination behavior of the TPPh and TPAMP ligands
has been investigated for several transition metals including
chromium, iron, and ruthenium and compared with their TPA
analogues. In the case of ruthenium(II) TPPh and TPAMP
complexes, the formation of high-valent ruthenium(IV) oxo
complexes has been investigated using both electrochemical
and chemical methods, and the results have been compared
with ruthenium(II) TPA complexes. Both TPPh and TPAMP
show a surprisingly different coordination behavior compared
with TPA.
(eq 2). After workup, TPPh was obtained in high purity and
76% yield, and the 1H NMR spectrum contains four resonances
for the pyridyl group and a doublet for the methylene bridge at
3.12 ppm with 2JHP = 1.2 Hz, a much smaller coupling than that
for the phosphonium salt. The 31P NMR spectrum shows one
resonance at −13.0 ppm. The melting point is 81 °C, which is
significantly lower than the value of 115−120 °C reported by
Chiswell,39 and we therefore do not believe that TPPh was
cleanly obtained by his method.
Tri(N-methyl-pyridylamino)phosphine (TPAMP) was pre-
pared by the reaction of the potassium salt of N-methylamino
pyridine with PCl3 in THF and obtained as a white powder in
54% yield (eq 3). The methyl signal is observed at 3.00 ppm
3
with a small coupling of JHP = 2 Hz, and the 31P NMR
spectrum shows a singlet at 96.8 ppm.
Synthesis of Chromium and Iron Complexes. In order
to explore the coordination properties of the new ligands TPPh
and TPAMP, several transition metal complexes have been
prepared. For TPA, mononuclear Cr(II) and Cr(III) complexes
have been reported previously, including X-ray analyses of
[CrCl2(TPA)] and [CrCl2(TPA)](BPh4), where TPA acts as a
tetradentate ligand.59 Here, the reaction of TPPh with
CrCl3·3THF in THF resulted in the formation of the complex
[CrCl3(TPPh)] (eq 4). The complex was analyzed by IR, MS,
RESULTS AND DISCUSSION
■
Synthesis of Ligands. Several attempts to repeat the
synthesis of TPPh as described by Chiswell in 1967 failed in
our hands, which was not surprising considering the reported
yield of 0−5%.39 In our first alternative approach, P(SiMe3)3
was reacted with 3 equiv of freshly prepared 2-picolylchloride in
dichloromethane. The product that was isolated was the novel
salt tetra(pyridylmethyl)phosphonium chloride in 33% yield
(eq 1.). Similar observations have been made in related
CHN analysis and single crystal X-ray diffraction, which
revealed a tridentate coordination mode as shown in Figure
2. One of the pyridyl arms does not coordinate to the metal
center, and the ligand adopts a facial coordination mode. The
coordination is very similar to that observed in the related
chromium complex with the tridentate bis(pyridylmethyl)-
phenylphosphine ligand.45,46 Noticeably, the Cr−P−CH2 angle
for the noncoordinated arm is substantially larger at 122.25(5)°
than those to the coordinated arms [102.97(5)° and
103.72(5)°]. A similar pattern, but to a lesser extent, is seen
in the structure of the chromium(II) complex [CrCl2(TPA)],
where the Cr−N−CH2 angle for the loosely bound pyridyl arm
[112.6° and 113.7° for the two independent complexes] is
larger than those of the coordinated arms [105.3° to 107.1°].59
reactions with P(SiMe3)3.55−57 The 1H NMR spectrum
contains a doublet at 4.71 ppm for the methylene protons
(2JHP = 16.0 Hz), in addition to the aromatic signals, and the
31P NMR spectrum has one resonance at δ 32.6 ppm (see
Supporting Information).
The synthesis of TPPh was achieved by the reaction of 3
equiv of 2-(trimethylsilylmethyl)pyridine58 with 1 equiv of PCl3
B
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Figure 2. Crystal structure of [CrCl3(TPPh)]. Selected bond lengths
(Å) and angles (deg): Cr−Cl(1) 2.3481(4), Cr−Cl(2) 2.3119(4),
Cr−Cl(3) 2.3004(4), Cr−P(1) 2.3333(4), Cr−N(4) 2.1486(11), Cr−
N(11) 2.1256(11), P(1)−Cr−N(4) 78.62(3), P(1)−Cr−N(11)
81.42(3), N(4)−Cr−N(11) 85.17(4).
Figure 3. Structure of the C2-symmetric cation present in the crystals
of [Fe(TPPh)2][FeBr4]. Selected bond lengths (Å) and angles (deg):
Fe(1)−P(1) 2.1547(11), Fe(1)−N(3) 2.085(3), Fe(1)−N(13)
2.021(3), P(1)−Fe(1)−N(3) 82.09(9), P(1)−Fe(1)−N(13)
85.95(10), N(3)−Fe(1)−N(13) 83.88(12).
The Cr−P bond in the TPPh complex [2.3333(4) Å] is longer
than the corresponding Cr−N bond in the TPA complex
[2.152(5) and 2.164(5) Å]. The chromium(III) complex
[CrCl2(TPA)](BPh4) has all three pyridyl arms coordinated
to the metal center, with associated Cr−N−CH2 angles of ca.
106.7°, 107.2°, and 109.7°. This pattern of two similar and one
larger M−N−CH2 angles is seen in many complexes with a
tetradentate TPA ligand (see Table S1).
NMR analysis did not provide any meaningful character-
ization due to severe paramagnetic line broadening. Our
interest in chromium-based ethylene oligomerization cata-
lysts60,61 prompted us to assess the activity of [CrCl3(TPPh)]
for the oligomerization of ethylene, using 500 equiv MAO as
the cocatalyst. A low activity of only 10 g/mmolhbar was
observed, similar to the activities reported for chromium
complexes of TPA and bis(pyridylmethyl)phenylphos-
phine.49,59
The reaction of TPPh with FeBr2 in acetonitrile at room
temperature results in a brick-red product (eq 5). The 1H NMR
spectrum shows resonances in the diamagnetic region,
indicative of a low spin iron(II) complex. However, the signals
are broad which suggests the presence of a paramagnetic
species or fluxional behavior. The 31P NMR spectrum displays a
single resonance at 96.4 ppm, which is a significant change from
−13.0 ppm for TPPh. Crystals suitable for single crystal X-ray
crystallography were obtained by slow diffusion of diethylether
into a concentrated solution of the complex in acetonitrile. The
product was not a mono-ligand complex, but a combination of
a bis-ligand dicationic iron(II) complex together with a
dianionic iron(II) tetrabromide complex [Fe(TPPh)2][FeBr4]
(see Figure 3). The TPPh ligand coordinates again as a
tridentate ligand, and the cationic complex has crystallographic
C2 symmetry. As was seen for [CrCl3(TPPh)], the M−P−CH2
angle for the noncoordinated arm is substantially larger
[133.12(17)°] than those to the coordinated arms
[102.66(13)° and 103.84(14)°]. The six-coordinate [Fe-
(TPPh)2]2+ cation is related to the [Fe(TPA)2]2+ complex,
previously reported by Hagen,5 which shows a similar
coordination, but less pronounced. The Fe−N−CH2 angles
for the noncoordinated pyridyl arms are ca. 111.6° and 115.3°,
while those for the coordinated arms range between 103.7° and
107.9°. The bonds to the central atom of the ligand (P for
TPPh, and N for TPA) are very different for [Fe(TPPh)2]-
[FeBr4] [2.1547(11) Å] compared to [Fe(TPA)2](OTf)2
[2.317(5) and 2.330(5) Å]. The different ligand field strength
of P versus N donors results in a low spin complex in the case
of [Fe(TPPh)2]2+ and a high spin complex for [Fe(TPA)2]2+.
Metal ligand bond lengths are typically 10% shorter for low
spin iron(II) complexes, compared to high spin analogues.
Consequently, the Fe−Npyr bonds are also noticeably shorter in
the low spin cation [Fe(TPPh)2]2+ [2.021(3) and 2.085(3) Å]
compared to the high spin cation [Fe(TPA)2]2+ [2.134(5)−
2.215(5) Å].5 In both cases, the central donors N versus P are
cis to each other. The presence of the paramagnetic anion
[FeBr4]2‑ is likely to be the reason for the signal broadening in
1
the H NMR spectrum of [Fe(TPPh)2][FeBr4].
The reaction of TPAMP with Fe(OTf)2 in THF resulted in a
similar bis-ligand complex [Fe(TPAMP)2](OTf)2. Despite the
1:1 ratio between iron and ligand used in the synthesis, the
formation of the bis(ligand) complex appears to be
1
thermodynamically preferred (eq 6). The H NMR spectrum
shows 12 aromatic signals and two N-methyl signals for two
coordinated and one uncoordinated pyridine units. Single
crystals were obtained from acetonitrile/diethyl ether. The low
spin cations in the structures of [Fe(TPAMP)2](OTf)2 (Figure
4) and its analogue [Fe(TPAMP)2][FeCl4]2 (see below and the
Supporting Information) are similar to that observed for
[Fe(TPPh)2][FeBr4]. Each TPAMP ligand binds in a tridentate
fashion with one of the pyridyl arms not bound to the metal,
with the Fe−P−N angle for this noncoordinated arm being in
each case substantially larger [in the range 133.45(6)−
136.36(6)° across all the independent TPAMP ligands in
both structures] than those for the coordinated arms [which
range between 102.17(11)° and 104.65(5)°]. The Fe−P bonds
to the central phosphorus atom in each ligand [2.0972(9)−
2.1154(4) Å] are shorter than that seen in the TPPh complex
[2.1547(11) Å], although the Fe−N(py) bonds are compara-
C
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Figure 5. 1H NMR spectra of the CH2 region of the reaction of
[RuCl2(TPA)] in CD3CN over time at 298 K.
to an amine donor. A third species C with two AB doublets at δ
4.89 and 4.97 ppm (JAB = 15.1 Hz) and a singlet at δ 4.44 ppm
is assigned as [Ru(MeCN)2(TPA)]Cl2. During the first 24 h,
the color changes from red to yellow, and the mixture gradually
evolves into essentially C only.
A solution of the product in dichloromethane yielded red
crystals, which were analyzed by X-ray crystallography as
[RuCl2(TPA)] (A). The initial red product is therefore
complex A, which slowly undergoes substitution of the chloride
ligands for MeCN ligands, to give the yellow complexes B and
C, as shown in eq 7. Despite several attempts, it was not
Figure 4. Structure of the cation present in the crystals of
[Fe(TPAMP)2](OTf)2. Selected bond lengths (Å) and angles (deg):
Fe−P(1) 2.1050(4), Fe−P(2) 2.1154(4), Fe−N(1) 2.0088(14), Fe−
N(11) 2.0865(13), Fe−N(31) 2.0963(13), Fe−N(41) 2.0077(14),
P(1)−Fe−N(1) 84.64(4), P(1)−Fe−N(11) 78.99(4), N(1)−Fe−
N(11) 83.01(5), P(2)−Fe−N(31) 78.18(4), P(2)−Fe−N(41)
84.76(4), N(31)−Fe−N(41) 84.58(5).
ble, ranging between 1.990(3) and 2.0963(13) Å in the
TPAMP complex, cf. 2.021(3) and 2.085(3) in [Fe(TPP)2]-
[FeBr4].
The reaction of TPAMP with FeCl3 in DCM resulted in a
red solid, which according to MS and elemental analysis
corresponds to the complex [FeCl3(TPAMP)], probably
similar to the chromium(III) complex mentioned above.
Attempts to crystallize this complex resulted in single crystals,
which upon analysis gave [Fe(TPAMP)2][FeCl4]2 (see
Supporting Information). This compound contains iron(II) in
the cation and iron(III) in the anion. Reduction of one of the
iron(III) centers to iron(II) presumably took place during the
crystallization process, but the origin of this reduction is unclear
at this stage.
possible to isolate complex B or C in pure form. Noteworthy,
only one isomer has been reported from the reaction of
[RuCl2(PhCN)4] with TPA in methanol, which was assigned as
[RuCl(PhCN)(TPA)]Cl, analogous to isomer B.25
Synthesis of Ruthenium(II) Complexes. The reaction of
TPA and [RuCl2(p-cymene)]2 in acetonitrile was expected to
give either [RuCl2(TPA)], in analogy to the reaction of TPA
with FeCl2,18 or [RuCl(MeCN)(TPA)]Cl, analogous to the
reaction between [RuCl2(PhCN)4] and TPA, which gave
The molecular structure of the mononuclear dichloro
ruthenium(II) complex [RuCl2(TPA)] (A) is shown in Figure
6, together with the relevant bond lengths and angles. The
structure is similar to the Ru(III) analogue [RuCl2(TPA)]-
(ClO4) and the cationic Ru(II) complexes [RuCl(S-DMSO)-
(TPA)](PF6) and [RuCl(TPA)]2(ClO4)2, although the Ru−N
bonds in [RuCl2(TPA)] are slightly shorter.20,26,62 The Ru−
N−CH2 angles for the tetradentate TPA ligand in these three
mononuclear structures show the same pattern as seen for the
first row metal TPA complexes, with two similar angles ranging
between ca. 105.2° and 106.4(4)° and one larger angle of
110.0(4)°, 110.05(12)°, and ca. 111.4°. For the dimeric
complex [RuCl(TPA)]2(ClO4)2, the same pattern is present
but to a lesser extent, with the three Ru−N−CH2 angles being
ca. 106.2°, 106.5°, and 107.8° (Table 1).
[RuCl(PhCN)(TPA)]Cl.25 However, the H NMR spectra in
1
CD3CN and in CD2Cl2 indicate the presence of multiple
species (see Figure S1). A solution of the product in CD3CN
was monitored by 1H NMR spectroscopy over time (17 days),
and the region of the CH2 protons is shown in Figure 5. Three
species, A, B, and C, can be distinguished. Each species shows a
double doublet (AB pattern) for the two pyridylmethyl arms
trans to each other and a singlet for the other pyridylmethyl
group, a common motif that is also observed in other
ruthenium(II) TPA complexes.7,25 Species A, which is least
abundant and shows two doublets at 5.09 and 5.25 ppm (JAB
=
15.2 Hz) together with a singlet at 4.73 ppm, is assigned to
[RuCl2(TPA)]. Product B with two doublets at 4.77 and 5.37
ppm (JAB = 14.6 Hz) and a singlet at δ 4.61 ppm is assigned to
[RuCl(MeCN)(TPA)]Cl. Only one isomer of this complex is
observed, most likely the isomer with MeCN trans to a pyridine
due to the stronger trans labilizing effect of pyridine compared
Despite the mixture of three species A, B, and C, exchange of
the Cl− anions for noncoordinating SbF6 counterions in
−
acetonitrile results in a single complex [Ru(MeCN)2(TPA)]-
(SbF6)2, whose 1H NMR spectrum is very similar to the
spectrum of complex C, which was assigned as [Ru-
(MeCN)2(TPA)]Cl2 (See Figure S2). The ortho protons of
D
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neutral analogue [RuCl2(TPA)], with the TPA ligand
coordinated in a tetradentate fashion with the same pattern
for the Ru−N−CH2 angles, though the Ru−N bonds in the
cationic complex are slightly larger than in the neutral species.
The reaction of TPPh with [RuCl2(p-cymene)]2 in
acetonitrile results in the formation of [RuCl-
1
(MeCN)2(TPPh)]Cl (see Scheme 1). The H NMR spectrum
for [RuCl(MeCN)2(TPPh)]Cl at 283 K in CD3CN shows two
different environments for the pyridine protons in a 2:1 ratio,
whereby the minor set of signals is broadened (see Figure S3).
The CH2 protons appear as two sets of signals in a ratio of 2:1,
but surprisingly, in this case the major signals are broadened.
There is no change to the spectrum when the temperature is
decreased, but at higher temperatures, the broad signals
sharpen. We propose that this fluxionality is due to a ligand
exchange process between [RuCl(MeCN)2(TPPh)]Cl and the
dicationic complex [Ru(MeCN)3(TPPh)]Cl2, as indicated in
Scheme 1.
The reaction of TPAMP with [RuCl2(p-cymene)]2 in
acetonitrile gives a complex that shows no fluxional behavior
as seen for [RuCl(MeCN)2(TPPh)]Cl. The 1H NMR spectrum
is very similar to that of [Ru(MeCN)3(TPAMP)](SbF6)2 (see
below), which suggests that this complex exists mainly as
[Ru(MeCN)3(TPAMP)](Cl)2 in acetonitrile solution, due to a
similar equilibrium as shown for the TPPh complex in Scheme
1. The P-donor in the TPAMP ligand appears to have a much
stronger trans effect compared to TPPh, resulting in a highly
labile chloro ligand, and consequently the equilibrium is shifted
toward the dicationic complex.
Figure 6. Crystal structure of [RuCl2(TPA)]. Selected bond lengths
(Å) and angles (deg): Ru−Cl(1) 2.4558(5), Ru−Cl(2) 2.4473(5),
Ru−N(1) 2.0529(17), Ru−N(3) 2.0534(18), Ru−N(13) 2.0279(18),
Ru−N(23) 2.0471(17), N(1)−Ru−N(3) 83.26(7), N(1)−Ru−N(13)
83.20(7), N(1)−Ru−N(23) 81.34(7), N(3)−Ru−N(13) 85.36(7),
N(3)−Ru−N(23) 164.57(7), N(13)−Ru−N(23) 93.78(7).
all three pyridine moieties appear in a 2:1 ratio and are shifted
downfield compared to the free ligand. Two signals are
observed for coordinated acetonitrile ligands, which implies
that TPA is coordinated as a tetradentate ligand. Single crystals
were obtained of complex [Ru(MeCN)2(TPA)](SbF6)2, and
the molecular structure is shown in Figure 7. The structure of
[Ru(MeCN)2(TPA)](SbF6)2 is very similar to that of the
Subsequent reactions with AgSbF6 in acetonitrile result in the
dicationic complexes [Ru(MeCN)3(TPPh)](SbF6)2 and [Ru-
(MeCN)3(TPAMP)](SbF6)2. Also in these complexes the
TPPh and TPAMP ligands remain coordinated as tridentate
1
rather than tetradentate ligands. The H NMR spectrum of
[Ru(MeCN)3(TPPh)](SbF6)2 at 298 K in CD3CN shows two
different environments for the pyridine and the CH2 protons,
both in a ratio of 2:1. The CH2 protons from the
uncoordinated pyridine appear as a doublet at δ 4.06 ppm
2
with JHP = 12.6 Hz. The diastereotopic CH2 protons of the
coordinated pyridines appear as two double doublets at δ 4.14
2
2
2
and δ 3.88 ppm, with JHH = 18.4 Hz, JHP = 11, and JHP =15
Hz, respectively. The acetonitrile ligands give two singlet
resonances: one integrates to six protons and the other to three
protons, and the latter is coordinated trans to phosphorus. This
acetonitrile ligand exchanges with the solvent CD3CN during
1
the course of several hours at room temperature. VT H and
31P NMR measurements have shown no evidence for other
dynamic behavior of this complex over the temperature range
from 243 to 343 K.
In the 1H NMR spectrum of [Ru(MeCN)3(TPAMP)]-
(SbF6)2, the coordinated pyridines give four resonances, and
the uncoordinated pyridine gives a separate set of four
resonances. There are two doublets for the aminomethyl
3
groups at δ 3.50 and 3.15 ppm, with JHP = 5.9 Hz for the
coordinated pyridines and 3JHP = 9.5 Hz for the uncoordinated
pyridine. The exchange of the CH3CN ligand trans to P for
CD3CN occurs within 10 min, significantly faster than for
[Ru(MeCN)3(TPPh)](SbF6)2, which required several hours
for complete exchange. This is most likely due to the stronger
trans effect of the phosphorus donor in TPAMP compared to
TPPh. The 31P{1H} NMR spectrum displays a single resonance
at δ 165.1 ppm (Figure S13).
Figure 7. Structure of the cation present in the crystals of
[Ru(MeCN)2(TPA)](SbF6)2. Selected bond lengths (Å) and angles
(deg): Ru−N(1) 2.0628(18), Ru−N(3) 2.0681(17), Ru−N(13)
2.0427(18), Ru−N(23) 2.0693(17), Ru−N(30) 2.0448(18), Ru−
N(40) 2.0376(19), N(1)−Ru−N(3) 82.65(7), N(1)−Ru−N(13)
82.71(7), N(1)−Ru−N(23) 81.47(7), N(3)−Ru−N(13) 85.08(7),
N(3)−Ru−N(23) 164.09(7), N(13)−Ru−N(23) 93.89(7).
E
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Scheme 1
Single crystals of [Ru(MeCN)3(TPPh)](SbF6)2, suitable for
single crystal X-ray crystallography, were obtained by layering
an acetonitrile solution of the complex with diethyl ether. The
structure obtained and selected bond lengths and angles are
shown in Figure 8. Complex [Ru(MeCN)3(TPPh)](SbF6)2
128.3(3)°] than those for the coordinated arms [in the range
101.7(4)−105.1(3)°].
The TPPh and TPAMP ligands in the complexes presented
here coordinate in all cases in a tridentate fashion with one of
the pyridyl arms not coordinated, which is in stark contrast to
related TPA complexes, where the TPA ligand typically
coordinates in a tetradentate mode. The hypodentate
coordination behavior63 of TPPh and TPAMP is believed to
be a consequence of the larger M−P distances, which are
typically 7−8% larger than the M−N distances in related TPA
complexes (see Table S1). In TPA complexes, all three
pyridylmethyl arms can normally reach the metal and result in a
tetradentate coordination mode, although this typically results
in two smaller and one large M−N−CH2 angle. In some cases,
the third arm cannot effectively bind to the metal center, and
the TPA ligand coordinates as a tridentate ligand.4−8
Cyclic Voltammetry and UV−Vis Spectroscopy. Cyclic
voltammetry measurements have been carried out in MeCN
under N2 at room temperature with 0.1 M [NBu4][PF6] as the
electrolyte. The dicationic complex [Ru(MeCN)2(TPA)]-
(SbF6)2 displays pseudoreversible redox behavior with E1/2
=
1.31 V (vs Ag/AgCl) for the Ru(II)/Ru(III) redox couple
(Figure 9). This value is higher compared to related
monocationic ruthenium(II) TPA complexes such as [RuCl-
(DMSO)(TPA)]PF6 (E1/2 = 0.98 V, vs Ag/AgCl) or
[RuCl(PhCN)(TPA)]Cl (E1/2 = 0.77 V, vs Ag/AgCl), both
of which contain anionic chloro ligands which lower the redox
potential.25 Further oxidation to Ru(IV) is not observed for
[Ru(MeCN)2(TPA)](SbF6)2, similar to observations with the
other ruthenium(II) TPA complexes. It is noteworthy that the
Ru(III)/Ru(IV) redox couple has been observed for
ruthenium(III) TPA complexes such as [RuCl2(TPA)]+ at
E1/2 = 1.73 V (vs Ag/AgCl), which contains two chloro
ligands.20,62
Figure 8. Structure of one (A) of the two independent cations present
in the crystals of [Ru(MeCN)3(TPPh)](SbF6)2. Selected bond lengths
(Å) and angles (deg) (values in square brackets refer to cation B):
Ru(1A)−P(1A) 2.210(2) [2.208(2)], Ru(1A)−N(3A) 2.098(6)
[2.095(6)], Ru(1A)−N(13A) 2.085(7) [2.095(7)], P(1A)−Ru(1A)−
N(3A) 83.53(17) [83.98(18)], P(1A)−Ru(1A)−N(13A) 80.8(2)
[81.18(19)], N(3A)−Ru(1A)−N(13A) 86.8(3) [88.1(2)].
The Ru(II)/Ru(III) redox potential is observed at E1/2
=
1.47 V for [Ru(MeCN)3(TPAMP)](SbF6)2 and at a slightly
lower potential for [Ru(MeCN)3(TPPh)](SbF6)2 (Figure 9).
Changing the nitrogen amine donor in TPA for a strong field P-
donor (and one pyridine ligand for an acetonitrile ligand)
increases the Ru(II)/Ru(III) redox potential considerably.
Interestingly, a further Ru(III)−Ru(IV) redox couple is
crystallized with two independent cations (and four independ-
ent anions) in the asymmetric unit; the two cations are very
similar, with the rms fit of all the non-hydrogen atoms of the
two complexes being ca. 0.05 Å. The TPPh ligand binds in a
tridentate fashion; the M−P−CH2 angle involving the
noncoordinated arm is substantially larger [128.1(3)° and
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Figure 9. Cyclic voltammograms of [Ru(MeCN)2(TPA)](SbF6)2
(- - -), [Ru(MeCN)3 (TPPh)](SbF6 )2 (), and [Ru-
(MeCN)3(TPAMP)](SbF6)2 (···): 1 mM in acetonitrile, 0.1 M
[NBu4][PF6] vs Ag/AgCl.
Figure 11. UV−vis spectra of [Ru(MeCN)2(TPA)](SbF6)2 (···),
[Ru(MeCN)3(TPPh)](SbF6)2 (), and [Ru(MeCN)3(TPAMP)]-
(SbF6)2 (- - -); c = 0.5 mM in CH3CN at 298 K.
spectrum of [Ru(MeCN)2(TPA)](SbF6)2 shows two MLCT
bands at λmax = 371 nm (ε = 2200 M−1 cm−1) and a distinct
shoulder at 316 nm (ε = 1400 M−1 cm−1), probably due to the
two different pyridine environments. The complexes [Ru-
(MeCN)3(TPPh)](SbF6)2 and [Ru(MeCN)3(TPAMP)]-
(SbF6)2, where only two of the three pyridine donors are
coordinated, show only one MLCT band at λmax = 318 nm (ε =
1900 M−1 cm−1) and 299 nm (ε = 1900 M−1 cm−1),
respectively.
observed, which is pseudoreversible for both the TPPh and the
TPAMP complex with E1/2 values of 1.63 and 1.69 V,
respectively. These results suggest that high valent ruthenium-
(IV) oxo TPPh and TPAMP complexes might become
accessible upon oxidation of the ruthenium(II) complexes
with a suitable oxidant. This has been investigated in the next
section.
The substitution of acetonitrile ligands for anionic weak field
chloro ligands generally lowers the redox potential. This is seen
here for [RuCl(MeCN)2(TPPh)]Cl, where the Ru(II)/Ru(III)
redox process occurs at ca. 1.0 V, compared to ca. 1.4 V in the
case of [Ru(MeCN)3(TPPh)](SbF6)2 (both irreversible, see
Ruthenium Oxo Complexes and Oxidation Studies.
High valent ruthenium oxo complexes are often invoked as
intermediates in ruthenium-based oxidation catalysis.64 Several
stable ruthenium(IV) oxo complexes have been isolated and
characterized, for example [Ru(O)(bipy)2(py)]2+ (bipy = 2,2′-
bipyridine),65−72 [Ru(O)(terpy)(bipy)]2+ (terpy = 2,2′,6′,2″-
terpyridine),73−75 and [Ru(O)(terpy)(pic)]+ (pic = picoli-
nate).76 Ruthenium(IV)-oxo TPA complexes have been
prepared by the oxidation of ruthenium(II) complexes using
a suitable oxidizing agent such as cerium(IV) ammonium
nitrate (CAN), as was previously shown for the oxidation of
[Ru(H2O)2(TPA)](PF6)2 to give the mononuclear ruthenium-
(IV) complex [Ru(O)(H2O)(TPA)](PF6)2.23 Catalytic oxida-
tion reactions of various hydrocarbons have been described
using ruthenium TPA complexes in combination with oxidants
such as meta-chloroperoxybenzoic acid (m-CPBA).20,25,26
Attempts to generate high valent ruthenium oxo complexes
were made by reacting [Ru(MeCN)2(TPA)](SbF6)2 in
acetonitrile with an excess of m-CPBA (10 equiv). The UV−
vis spectrum shows a gradual change during the course of 30
min, as shown in Figure 12. The analogous reaction of
[Ru(MeCN)3(TPPh)](SbF6)2 with 10 equiv of m-CPBA in
acetonitrile results in a slow color change to give a rather
featureless UV−vis spectrum with no clearly discernible
absorptions (Figure S16). Similarly, the addition of m-CPBA
to a solution of [Ru(MeCN)3(TPAMP)](SbF6)2 in acetonitrile
showed a very slow change over several hours to give again a
featureless spectrum (Figure S17).
Figure 10). The Ru(III)/Ru(IV) redox process occurs at E1/2
=
Figure 10. Cyclic voltammograms of [RuCl(MeCN)2(TPP)]Cl (- - -)
and [Ru(MeCN)3(TPPh)](SbF6)2 (): 1 mM in CH3CN, 0.1 M
[NBu4][PF6], vs Ag/AgCl.
1.32 V, compared to E1/2 = 1.63 V for [Ru(MeCN)3(TPPh)]-
(SbF6)2. A similar trend is seen when comparing [RuCl-
(MeCN)2(TPAMP)]Cl and [Ru(MeCN)3(TPAMP)](SbF6)2,
where the results for the Ru(II)/Ru(III) redox potentials are
E1/2 = 0.95 and 1.47 V, respectively, and the Ru(III)/Ru(IV)
potentials are observed at ca. E1/2 = 1.3 V (irreversible) and
1.69 V, respectively (see Figure S14).
The UV−vis spectra of [Ru(MeCN)2(TPA)](SbF6)2, [Ru-
(MeCN)3(TPPh)](SbF6)2, and [Ru(MeCN)3(TPAMP)]-
(SbF6)2 in acetonitrile at 298 K are shown in Figure 11. The
The formation of a shoulder in the UV−vis spectrum around
500 nm upon oxidation of [Ru(MeCN)2(TPA)](SbF6)2 with
m-CPBA is similar to the observations reported for the reaction
of [Ru(H2O)2(TPA)](PF6)2 with CAN, which resulted in the
formation of the ruthenium(IV) oxo complex [Ru(O)(H2O)-
(TPA)](PF6)2 (see Figure S15).23 Analysis of the reaction
mixture by mass spectrometry (LSIMS) also showed the
G
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Inorganic Chemistry
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formation of a ruthenium oxo species (see Figure S18), and we
therefore conclude that a ruthenium(IV) oxo complex
[Ru(O)(MeCN)(TPA)](SbF6)2 is probably generated as a
transient complex when TPA is the supporting ligand. In the
case of TPPh and TPAMP, the results are less clear. It is
possible that upon reaction with m-CPBA a ruthenium(IV) oxo
complex is formed, but immediate reaction with the ruthenium-
(II) starting complex could result in oxo-bridged dinuclear
ruthenium(III) complexes such as [{Ru(MeCN)2(TPPh)}2(μ-
O)]4+ or [{Ru(MeCN)(TPPh)}2(μ-O)(μ-meta-chloroben-
zoate)]3+. Complexes of this type, for example [{RuCl-
(TPA)}2(μ-O)]2+ and [{Ru(TPA)}2(μ-O)(μ-acetate)]3+, have
been previously isolated and crystallographically characterized
by Sasaki and co-workers.77
Although cyclic voltammetry studies have shown that the
ruthenium(II) complexes containing TPPh and TPAMP can be
electrochemically oxidized to ruthenium(IV), the strong field
TPPh and TPAMP ligands are probably unable to sufficiently
Figure 12. UV−vis spectrum of [Ru(TPA)(MeCN)2](SbF6)2 (5 mM)
after addition of 10 equiv of m-CPBA in acetonitrile at 298 K.
Readings taken every 3 min.
Table 1. Crystallographic Data for Compounds [CrCl3(TPPh)], [Fe(TPPh)2][FeBr4], [Fe(TPAMP)2](OTf)2,
[Fe(TPAMP)2][FeCl4]2, [RuCl2(TPA)], [Ru(MeCN)2(TPA)](SbF6)2, and [Ru(MeCN)3(TPPh)](SbF6)2
[CrCl3(TPPh)]
C18H18Cl3CrN3P
[Fe(TPPh)2][FeBr4]
[Fe(TPAMP)2](OTf)2
[Fe(TPAMP)2][FeCl4]2
[C36H42FeN12P2](FeCl4)2
chemical formula
solvent
fw
[C36H36FeN6P2](FeBr4)
CH3CN
[C36H42FeN12P2](CF3SO3)2
CH2Cl2
1143.67
−100
465.67
1087.04
1155.91
T (°C)
space group
a (Å)
−100
−100
−100
P21/c (No. 14)
12.072 64(15)
9.898 70(11)
17.5667(2)
Ibca (No. 73)
19.8229(2)
22.6568(3)
18.5048(2)
P1 (No. 2)
P21/n (No. 14)
21.5022(2)
32.0677(2)
29.0228(2)
̅
13.1348(3)
13.5183(3)
14.2438(3)
92.8540(17)
97.9214(18)
104.5567(18)
2415.17(10)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
104.6450(13)
104.3814(9)
19 384.9(3)
2031.08(4)
4
8310.94(16)
8
a
16
ρcalcd (g cm−3
)
1.523
1.738
1.573
1.584
λ (Å)
0.710 73
1.044
1.541 84
11.129
0.0384
0.1017
0.710 73
0.658
0.710 73
μ (mm−1
)
1.435
c
R1 (obsd)
0.0283
0.0765
0.0596
0.0626
0.1811
d
wR2 (all)
0.1763
[RuCl2(TPA)]
[Ru(MeCN)2(TPA)](SbF6)2
[Ru(MeCN)3(TPPh)](SbF6)2
chemical formula
solvent
fw
C18H18Cl2N4Ru
3CH2Cl2
[C22H24N6Ru](SbF6)2
CH3CN
[C24H27N6PRu](SbF6)2
0.5C4H10O·0.5CH3CN
1060.64
717.11
986.10
T (°C)
space group
a (Å)
−100
−100
P1 (No. 2)
̅
−100
P21/n (No. 14)
P21/c (No. 14)
8.543 32(10)
24.0817(3)
13.862 64(17)
8.710 65(14)
9.043 39(16)
20.2295(3)
93.6493(14)
93.6436(14)
92.6472(14)
1585.09(4)
2
10.7301(3)
b (Å)
37.0357(11)
c (Å)
20.8357(10)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
96.5352(11)
101.078(4)
2833.54(6)
4
8125.8(5)
b
8
ρcalcd (g cm−3
)
1.681
2.066
1.734
λ (Å)
0.710 73
1.327
0.710 73
2.262
1.541 84
14.579
0.0737
0.2349
μ (mm−1
)
c
R1 (obsd)
0.0359
0.0767
0.0284
d
wR2 (all)
0.0575
a
b
There are four crystallographically independent complexes in the asymmetric unit. There are two crystallographically independent complexes in
c
d
the asymmetric unit. R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. wR2 = {Σ[w(Fo − Fc2)2]/Σ[w(Fo )2]}1/2; w−1 = σ2(Fo ) + (aP)2 + bP.
2
2
2
H
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Diffraction Xcalibur PX Ultra (compounds [Fe(TPPh)2][FeBr4] and
[Ru(MeCN)3(TPPh)](SbF6)2) and Xcalibur 3 (compounds
[CrCl3(TPPh)], [Fe(TPAMP)2](OTf)2, [Fe(TPAMP)2][FeCl4]2,
[RuCl2(TPA)] and [Ru(MeCN)2(TPA)](SbF6)2) diffractometers,
and the structures were refined on the basis of F2 using the SHELXTL
and SHELX-97 program systems.80 CCDC 926471 to 926477 contain
the relevant data.
stabilize the high valent oxo complexes. Rapid reduction by the
ruthenium(II) starting complex to give an oxo-bridged
dinuclear ruthenium(III) complex is a conceivable decom-
position pathway. In fact, there are parallels for this behavior in
iron(II) complexes. We and others have reported very similar
observations with low spin iron(II) complexes containing
strong field ligands. Iron(IV) oxo complexes, generated from
iron(II) complexes upon oxidation with PhIO, were rapidly
reduced by the iron(II) starting complex to oxo-bridged
dinuclear iron(III) complexes.78,79 Strong field ligands with π-
acceptor properties are inherently unsuited for the stabilization
of high valent metal oxo complexes and ligands with strong σ-
and π-donor properties should be used.
Oxidation reactions of various hydrocarbons have been
described using ruthenium-TPA catalysts in combination with
oxidants such as m-CPBA.20,25,26 We have carried out oxidation
reactions of both cyclohexene and cyclohexane using
iodosylbenzene or hydrogen peroxide as the oxidants and
[Ru(MeCN)2(TPA)](SbF6)2 or [Ru(MeCN)3(TPPh)](SbF6)2
as the catalyst. Under our reaction conditions, no oxidation of
cyclohexane with hydrogen peroxide was observed when using
any of the ruthenium(II) complexes. The oxidation of
cyclohexene by iodosylbenzene gave trace amounts of cyclo-
hexene oxide and a bis-chlorinated cyclohexane product, when
the reaction was carried out in chloroform as the solvent.
Formation of chlorinated species indicates that a radical
intermediate is generated under these reaction conditions.
Once the solvent was changed to a nonchlorinated solvent,
such as acetonitrile, only very low yields of cyclohexene oxide
were observed.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic files in CIF format for all complexes and
experimental details regarding synthesis and characterization of
the ligands and complexes, as well as details on the
electrochemical, spectroscopic, and catalytic procedures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: g.britovsek@imperial.ac.uk. Phone: +44-(0)20-
75945863. Fax. +44-(0)-20-75945804.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to EPSRC for financial support. E.K. thanks the
British Council for his placement under the IEASTE scheme.
We thank Johnson Matthey for the generous loan of ruthenium
trichloride, and we thank Prof. G. Cloke, University of Sussex,
for the kind donation of tris(trimethylsilyl)phosphine.
CONCLUSIONS
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