2,4,6-Triphenylphosphinine and 2,4,6-triphenylposphabarrelene revisited: Synthesis, reactivity and coordination chemistry

Article abstract of DOI:10.1039/c5dt03609g

The synthesis of 2,4,6-triphenylphosphinine has been revisited and a general protocol for the preparation of such low-coordinate phosphorus compounds in good to excellent yields could be established. This allows to investigate several aspects of the chemistry of 2,4,6-triarylphosphinine, such as the reaction with in situ generated benzyne to give 2,4,6-triphenylphosphabarrelene. The corresponding 2,4,6-triphenylphosphabarrelene-selenide could be characterized crystallographically for the first time and the structural and electronic properties of this cage-compound in comparison to classical triarylphosphines could be evaluated. Moreover, [(L)W(CO)₅)] complexes of both 2,4,6-triphenylphosphinine and 2,4,6-triphenylphosphabarrelene were prepared and characterized by means of X-ray crystallography. This allowed for the first time a direct structural comparison of these related phosphorus compounds, coordinated to the same metal fragment.

Full text of DOI:10.1039/c5dt03609g

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2,4,6-Triphenylphosphinine and 2,4,6-
triphenylposphabarrelene revisited: synthesis,
Cite this: DOI: 10.1039/c5dt03609g
reactivity and coordination chemistry†
M. Rigo, J. A. W. Sklorz, N. Hatje, F. Noack, M. Weber, J. Wiecko and C. Müller*
The synthesis of 2,4,6-triphenylphosphinine has been revisited and a general protocol for the preparation
of such low-coordinate phosphorus compounds in good to excellent yields could be established. This
allows to investigate several aspects of the chemistry of 2,4,6-triarylphosphinine, such as the reaction
with in situ generated benzyne to give 2,4,6-triphenylphosphabarrelene. The corresponding 2,4,6-triphe-
nylphosphabarrelene-selenide could be characterized crystallographically for the first time and the struc-
tural and electronic properties of this cage-compound in comparison to classical triarylphosphines could
be evaluated. Moreover, [(L)W(CO)5)] complexes of both 2,4,6-triphenylphosphinine and 2,4,6-triphenyl-
phosphabarrelene were prepared and characterized by means of X-ray crystallography. This allowed for
the first time a direct structural comparison of these related phosphorus compounds, coordinated to the
same metal fragment.
Received 15th September 2015,
Accepted 3rd November 2015
DOI: 10.1039/c5dt03609g
www.rsc.org/dalton
Introduction
A milestone in the chemistry of low-coordinate phosphorus
compounds was set with the first successful preparation of the
fully unsaturated six-membered phosphorus heterocycle 1 by
Märkl in 1966 and the parent compound C5H5P by Ashe III in
1971.1,2 The stabilization of otherwise reactive PvC double
bonds by their incorporation into aromatic systems opened up
the access to formally sp2-hybridized phosphorus(III) hetero-
cycles with significantly different electronic and steric pro-
perties compared to classical ligands based on trivalent
phosphorus. Phosphinines have long been regarded as
“chemical curiosities” but state-of-the-art synthetic method-
ologies allow nowadays specific derivatizations and functiona-
lizations, including the introduction of additional donor-
functionalities.3,4 Moreover, an interesting feature is the reac-
tivity of phosphinines towards in situ generated benzyne as a
good dienophile. These [4 + 2] cycloaddition reactions afford
the corresponding phosphabarrelenes, such as 2, which have
been identified as rather bulky, mainly σ-donating phosphorus
ligands towards transition metal centers (Fig. 1).5–7
Fig. 1 2,4,6-Triphenylphosphinine (1) and 2,4,6-triphenylphosphabar-
relene (2).
considerable kinetic stability and are inert towards water,
oxygen and many acids and bases, in contrast to less substi-
tuted ones. However, despite the fact that these heterocycles
are known for almost five decades, their use in more applied
research fields is still comparatively rare, although several fas-
cinating results have recently emerged.4d,e,8–11 This also counts
for the application of the corresponding phosphabarrelenes.
Their efficient use as phosphorus(III) ligands in several homo-
geneous catalytic reaction has been recently well documented
(vide infra). In this respect, one drawback in the use of phos-
phinines is certainly their often tedious synthetic procedure,
especially the overall low yield. The first reported phosphinine
1 was prepared from 2,4,6-triphenylpyrylium tetrafluoroborate
(3) and P(CH2OH)3 in refluxing pyridine, and was obtained as
a yellow air- and moisture-stable solid in 24–30% yield.1 Alter-
natively, P(SiMe3)3 (4) in refluxing acetonitrile can be used as
phosphorus source, leading to only slightly higher yields.12
Among the known phosphinine-derivatives having different
substitution pattern, 2,4,6-triaryl-phosphinines often show a
Freie Universität Berlin, Institut für Chemie und Biochemie, Fabeckstr. 34/36,
14195 Berlin, Germany. E-mail: c.mueller@fu-berlin.de
†Electronic supplementary information (ESI) available: Experimental procedure
for the preparation of all compounds except for 1 and 4 as well as X-ray crystallo-
graphic information. CCDC 1424312–1424314 and 1424399. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c5dt03609g
Exchange of BF4 by I− in the pyrylium salt can lead to even
−
higher yields (45%) because the formation of Me3SiI as by-
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product of the synthesis is preferred over Me3SiBF4.13 Using speed. In some cases, the reaction time can vary from three
refluxing dimethoxyethane instead of acetonitrile, the yields days up to two weeks. It is consequently advisable that the
can range from 27% to 63%, depending on the pyrylium salt reaction is monitored by means of 31P NMR spectroscopy
used.14 Nevertheless, it should be mentioned here that the (unlocked), as an incomplete conversion makes the workup
time-consuming preparation of larger amounts of P(CH2OH)3 almost impossible. The colour of the reaction mixture is
and P(SiMe3)3 requires special caution, while the reported important as it turns from dark orange in the beginning to
yields can differ significantly. Despite its high toxicity the use light grey. Moreover, a clear and colourless solution can be
of PH3 gives pure 1 in 77% yield after recrystallization and only noticed at the end of the reaction when the stirrer is turned off
H2O as by-product.8,15 Performing the O+/P exchange with and the LiCl has been settled.
PH4I as PH3-source in butanol at T = 110–120 °C, 1 is obtained
in 61% isolated yield.16 PH4I is, however, not commercially P(SiMe3)3 (4), doubling the amount and obtaining reproduci-
available anymore. ble results, which are reported in Table 1. It turned out that by
We optimized and scaled up the synthetic procedure for
P(SiMe3)3 as the most often used synthon for the using lithium granulate handled under air a reaction time of
preparation of 2,4,6-triarylphosphinines has been initially syn- 1 week was observed (Table 1, entries 1 and 2). This time
thesized by reaction of NaPH2 (from large amounts of PH3 and period can be reduced to 30 h by stirring the lithium vigor-
Na in liquid ammonia) and Me3SiCl in diglyme.17 Becker and ously with a large stirring bar (5 cm, Ø = 2 cm, 34.65 g) under
Hölderich reported on an improved synthesis starting from P4, an argon atmosphere prior to the addition of the solvent and
Na/K alloy and Me3SiCl.18 A by far less dangerous procedure reactants. In this way, the passivated lithium surface can be
was reported by Niecke and Westermann. The authors started partially removed, leading to an activation of the metal. This
from N-(dichlorophosphino)piperidine, lithium and Me3SiCl.19 procedure takes approximately 4 h at a stirring speed of 1000
During the preparation of arene complexes of Sm, Eu, Tm, rpm and should not be extended, as an aggregation of the Li
Yb, Arnold, Sergeev and Cloke reported on the use of granulate was observed. This effect increases the reaction time
Li[P(SiMe3)2] (5) for the preparation of 2,4,6-tri-tert-butylphos- again most likely due to the presence of a smaller metal
phinine in 75% yield.20 Inspired by these results, we started to surface (entries 3 and 4 vs. 5 and 6). Furthermore, we found
investigate the general use of M[(P(SiMe3)2] (M = Li, Na, K) in that both the morphology as well as the amount of lithium has
more detail, especially for the preparation of also 2,4,6-triaryl- also an effect on the reaction times. The best results were
phosphinines. In line with our recent results on the obtained using lithium foil, as the reaction was complete
preparation of novel coordination compounds based on phos- within one day (entry 9). By doubling the amount of lithium to
phinines and phosphabarrelenes, we also started to explore 14 equivalents, the reaction was complete within 16 h. It
the coordination chemistry of 1 and 2 towards W(0) in order to turned out that the use of sliced lithium rods is not rec-
get more information on the structural and electronic pro- ommended (entries 11/12). It should also be mentioned that
perties of these structurally related heterocycles. The results the solvent must be removed in vacuo immediately after the
will be presented in this paper.
reaction is complete, as the generation of unidentified species
was otherwise observed by 31P NMR spectroscopy. The crude
product can be stored at T = −20 °C for several weeks under an
argon atmosphere and can be finally worked up by suspending
the grey mixture with 300 mL of dry pentane. The solid is
Results and discussion
Optimization of P(SiMe3)3 synthesis
A simple and convenient method for the preparation of tris(tri-
methylsilyl)phosphine is the reaction of the N-(dichlorophos-
phino)piperidine with lithium and chlorotrimethylsilane
in boiling tetrahydrofurane, according to Niecke and
Westermann (Scheme 1).19
Table 1 Optimization of P(SiMe3)3 synthesis
Stirring speed
Entry
Li source
Li eq.
[rpm]
Time
Yield
From experience, however, this reaction is not as straight-
forward as reported. It strongly depends on the scale, the type
and source of lithium, the order of reagents and the stirring
1
2
3
4
5
6
7
8
Granulara
Granulara
Granularb
Granularb
Granularc
Granularc
Granularc,d
Granularc,d
Foilc
7
7
7
7
7
7
7
7
7
750
750
1000
750
1000
1000
1000
1000
1000
1000
1000
1000
7 d
7 d
30 h
34 h
2 d
2 d
2 d
69%
70%
70%
74%
72%
70%
71%
70%
68%
72%
69%
69%
2 d
9
20 h
16 h
7 d
10
11
12
Foilc
14
7
7
Rodc,e
Rodc, f
12 d
a Li handled under air. b Li handled under air, activation by stirring.
c Exclusively Ar atmosphere (no activation). d Li, 99%, trace metals
basis. e Horizontally and vertically sliced. f Vertically sliced.
Scheme 1 Synthesis of P(SiMe3)3 (4).
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removed by filtration under argon atmosphere and the solvent
is subsequently removed in vacuo. P(SiMe3) in obtained in an
average yield of 70% after vacuum distillation (∼55 °C,
0.5 mbar).
Table 2 Optimization of 2,4,6-triphenylphosphinine synthesis
Ratio
Time
[h]
Isolated
yield [%]
Entry
3/LiP(SiMe3)2
Solvent
T (°C)
1
2
3
4
5
6
7
8
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1.4
1 : 1.6
1 : 2
16
3
1
6
16
6
6
6
6
6
THF
THF
THF
THF
r.t
100
28
31
33
56
55
—
44
—
48
52
54
76
Optimization of 2,4,6-triarylphosphinine synthesis
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
80
Reflux
Reflux
Reflux
As mentioned above, the reaction of 2,4,6-triarylpyrylium tetra-
fluoroborates with P(SiMe3)3 gives the corresponding 2,4,6-
triarylphosphinines in generally rather poor yields, although
some improvements have been reported in literature. Me3SiO-
SiMe3 as well as Me3SiBF4 are the by-products of this conver-
sion. It is believed, that a nucleophilic attack of P(SiMe3)3 to
the Cα atom of the pyrylium salt is initiating the reaction, with
subsequent elimination of Me3SiBF4.12 The neutral species
undergoes ring-opening, rearrangement and ring-closing
under elimination of Me3SiOSiMe3 and formation of the
aromatic phosphorus heterocycle. Nevertheless, mechanistic
investigations are lacking, although one derivative of a pro-
posed intermediate has been isolated in one case.12
THF
MeCN
DME
Toluene
No solvent
THF
THF
THF
9
10
11
12
6
6
Conditions: pyrylium salt: 2.0 mmol, solvent: 25 mL.
Table 3 Optimization of 2,4,6-triphenylphosphinine synthesis
In order to increase the yield of the reaction, we envisaged
to start from M[P(SiMe3)2] (M = Li, Na, K) as a phosphorus
source, since the formation of MBF4 as a salt, which would
precipitate from the reaction mixture, should entropically
favour the formation of the 2,4,6-triarylphosphinine core
(Scheme 2). Moreover, [P(SiMe3)2]− is more nucleophilic than
P(SiMe3)3. Therefore, we started to synthesize Li[P(SiMe3)2] (5)
by reaction of the above prepared P(SiMe3)3 with nBuLi in
THF, according to the literature.21 We first examined the reac-
tion of 2,4,6-triphenylpyrylium tetrafluoroborate (3) with
Li[P(SiMe3)2] (5) under different reaction conditions and the
results are listed in Table 2.
Ratio py-salt/
M[P(SiMe3)]2
Isolated yield
[%]
Entry
M[P(SiMe3)2]
4
12
13
14
15
16
1 : 1
1 : 2
1 : 1
1 : 2
1 : 1
1 : 2
Li[P(SiMe3)2]
Li[P(SiMe3)2]
Na[P(SiMe3)2]
Na[P(SiMe3)2]
K[P(SiMe3)2]
K[P(SiMe3)2]
56
76
45
47
46
51
Conditions: pyrylium salt: 2.0 mmol, solvent: THF (25 mL, reflux),
reaction time: 6 h.
It turned out, that the optimal reaction time is 6 h, while
the solvent of choice is THF under refluxing conditions (entry
4). Interestingly, the use of 2 equivalents of Li[P(SiMe3)2] has a
beneficial effect on the yield of the reaction, as 76% of isolated
2,4,6-triphenylphosphinine could be obtained (entry 12).
Next to the use of Li[P(SiMe3)2] (5) we also used
Na[P(SiMe3)2] (6) as well as K[P(SiMe3)2] (7) as phosphorus
source for the preparation of 2,4,6-triphenylphosphinine.21
The optimized conditions with respect to solvent, temperature
and reaction time (Table 1) were used and the results are listed
in Table 3.
A comparison between 5, 6 and 7 shows that reasonable
yields are obtained with all three bis-trimethylsilylphosphides,
although there is no significant difference between using one
or two equivalents of 6 and 7, in contrast to 5. However, it can
be concluded that the optimal reaction conditions for the
preparation of 1 are 2 equivalents of Li[P(SiMe3)2] (5) in reflux-
ing THF and a reaction time of 6 h, leading to a highest iso-
lated yield of 76%.
Next, we were wondering whether we can transfer our obser-
vations to the preparation of other 2,4,6-triarylphosphinines
for which the isolated yields are normally rather low. We there-
fore chose for the synthesis of 2-(2-pyridyl)-4,6-diphenyl-phos-
phinine (8) and 2,4-diphenyl-5-methyl-6-(2,3-dimethylphenyl)-
phosphinine (9).22,23 Starting from the corresponding pyrylium
salts and P(SiMe3)3, phosphinine 8 is typically obtained in an
average yield of 25%, while the atropisomeric phosphinine 9 is
typically isolated in only 10–15% yield (Fig. 2).
Interestingly it turned indeed out, that phosphinines 8 and
9 can be prepared in significantly higher yields under the
same optimized reaction conditions as described in Table 1,
Scheme 2 Improved synthesis of 1.
Fig. 2 Functionalized 2,4,6-triarylphosphinines 8 and 9.
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entry 12. Both compounds could be obtained in 37% isolated
yield, using 2 equivalents of Li[P(SiMe3)2] (5) in refluxing THF
and a reaction time of 6 h.
2,4,6-Triphenylphosphabarrelene: synthesis and reaction with
selenium
Scheme 4 Synthesis of phosphabarrelene-selenide 10.
The aromatic phosphinine heterocycle can serve as the basis
for the development of very interesting phosphorus-cages, the
so-called phosphabarrelenes. In 1971 Märkl reported on the
Diels–Alder-type [4 + 2] cycloaddition of benzyne with 1 to give
the corresponding phosphabarrelene 2.5 These molecules have
mainly σ-donating character with some π-acceptor properties
as due to low lying σ* orbitals of the P–C bonds.6,7 Breit and
co-workers showed that very active hydroformylation catalysts
could be generated in combination with [Rh(CO)2(acac)].6
Remarkably, internal alkenes were converted essentially free of
alkene-isomerization towards internal aldehydes. Le Floch and
Mézailles proved their application in the Pd-catalyzed Suzuki–
Miyaura cross-coupling reaction and the Pd-catalyzed allylation
of amines.24,25 Also very efficient Pt-complexes based on phos-
phabarrelenes were prepared for the hydrosilylation of alkynes
under mild reaction conditions.26 Breit and co-worker reported
on Rh-catalyzed hydrogenations and our group could show
that phosphabarrelenes are excellent phosphorus ligands for
the Rh-catalyzed tandem-hydroformylation-cyclization reaction
of allyl-functionalized imidazole derivatives.27,28
1
75.6 ppm to δ (ppm) = 6.6 with J31P–77Se = 834 Hz is observed
for product 10. Interestingly, the 2,4,6-triphenylphosphabarre-
lene 10 shows by far the largest coupling constant among the
differently substituted phosphabarrelene-selenides reported in
literature.29 This shows that the s-character of the phosphorus
lone-pair in 10 is significantly higher compared to other phos-
phabarrelenes as well as classical phosphine-selenides
(Ph3PvSe: J31P–77Se = 732 Hz).30 It should be noted here that
1
although the preparation of phosphinine–sulfides is possible,
phosphinine–selenides are still unknown, making a direct
comparison of the structurally related heterocycles unfortu-
nately impossible.
The two structurally characterized phosphabarrelene-
selenides, reported in literature, contain a –SiMe3 group in
2- and 6-position of the heterocycle.29 Since the molecular struc-
ture of 10 in the crystal is unknown, we attempted a crystalliza-
tion and structural characterization of this 2,4,6-triphenyl-
derivative. Crystals of 10, suitable for single crystal X-ray diffr-
action were indeed obtained from a concentrated solution of
this compound in toluene and its molecular structure is
depicted in Fig. 3. The P–Se as well as the P–C bond lengths in
10 are very similar to the ones of reported phosphabarrelene-
selenides. In contrast, the sum of the CPC angle in 10 is
approximately 6° smaller compared to the reported values of
the –SiMe3-substituted phosphabarrelene-selenides. This
As 2,4,6-triphenylphosphinine can be prepared in large
amounts using this improved synthetic procedure, we further
converted 1 into the corresponding 2,4,6-triphenylphosphabar-
relene 2, according to the literature procedure (Scheme 3).6 In
contrast to 2,4,6-triarylphosphinines, phosphabarrelenes are
prone to oxidation reactions and can easily be converted into
the corresponding phosphabarrelene-oxides, sulfides and
1
selenides.27,29 Generally the J31P–77Se coupling constant in
1
finding could explain the larger J31P–77Se coupling constant
P–Se compounds allows for the evaluation of the s-character of
the phosphorus lone pair.30 It is consequently an important
tool for the design of tailor-made phosphorus ligands and
their applications in homogeneous catalysis.
It appeared thus surprising to us that only one literature
reference deals with the preparation and structural characteri-
zation of phosphabarrelene-selenides.29 We therefore con-
verted 2 with an excess of grey selenium in refluxing toluene to
10 within 24 h and the product could be obtained as a white
solid in 95% isolated yield, according to Scheme 4.
While the starting material 2 shows a signal at δ =
−69.0 ppm in the 31P{1H} NMR spectrum, a downfield shift of
Fig. 3 Molecular structure of 10 in the crystal. Displacement ellipsoids
are shown at the 50% probability level. Only one independent molecule
is shown.
Scheme 3 Synthesis of phosphabarrelene 2.
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Table 4 Structural parameters of 2 and 10
Bond length [Å]
2
10
P(1)–Se(1)
P(1)–C(1)
P(1)–C(5)
P(1)–C(24)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(3)–C(25)
C(24)–C(25)
Bond angles (°)
C(1)–P(1)–C(5)
C(1)–P(1)–C(24)
C(5)–P(1)–C(24)
∑(CPC)
—
2.0924(7)
1.838(3)
1.839(2)
1.812(2)
1.330(3)
1.531(3)
1.538(3)
1.333(4)
1.563(3)
1.390(4)
1.8454(16)
1.8601(16)
1.8345(18)
1.331(2)
1.527(2)
1.533(2)
1.333(2)
—
—
95.57(7)
94.95(7)
92.23(7)
282.7
97.9(1)
97.4(1)
98.4(1)
293.7
Scheme 5 Reaction of 1 and 2 with [W(CO)5(THF)].
observed for 10 (vide supra), which in turn indicates a higher
s-character of the phosphorus lone-pair of phosphabarrelene 2
in comparison to the –SiMe3–substituted derivatives. The crys-
tallographic characterization of 10 further allows for the com-
parison with the molecular structure of the starting material 2
in the crystal, which has been reported by Breit and co-workers
(Table 4).6
Fig. 4 Time-dependent 31P{1H} NMR spectra for the reaction of 1 with
Upon reaction of the phosphorus atom with selenium,
a shortening of the three P–C bonds can be noticed, while the
C–C bond distances within the cage remain essentially very
similar. Moreover, a significant widening of the CPC angles
upon oxidation with selenium can be observed, as the sum of
the CPC angles changes from 282.7° in 2 to 293.7° in 10. Inter-
estingly, the ∑(CPC) angle in 2,4,6-triphenylphosphabarrelene
2 of 282.7° is indeed significantly smaller than the ones
observed for the –SiMe3-substituted derivatives reported in lit-
erature (287.7° and 290.1°; PPh3: 307.2°).29,31 Apparently, the
aryl-substituted cage compounds, such as 2, are the least
σ-donating ligands among the hitherto known series of phos-
phabarrelenes as already anticipated from the 31P{1H} spectro-
scopic data (vide supra).
[W(CO)5(THF)].
We first looked into the synthesis of [(1)W(CO)5] (11) and
stirred a suspension of [W(CO)6] in THF for 2 h under UV light
in order to generate [W(CO)5(THF)], according to a modified
procedure described by Deberitz and Nöth.32 By adding a solu-
tion of 1 in THF to this intermediate we were able to prepare
11 almost quantitatively (Scheme 5a).
Compound 11 shows a resonance in the 31P{1H} NMR at
δ (ppm) = 159.1 with 1J31P–183W = 273 Hz due to coupling to the
183W NMR active nucleus (15% natural abundancy). This value
is very similar to the one observed for [W(CO)5(PPh3)]
(1J31P–183W = 245 Hz). The reaction of 1 with [W(CO)5(THF)] can
also be directly monitored by means of 31P{1H} NMR
spectroscopy in THF (Fig. 4). Within 30 min phosphinine 1
has been completely consumed under quantitative formation
of 11, as can be nicely detected by the tungsten satellites.
Similarly, reaction of 2,4,6-triphenylphosphabarrelene 2
with [W(CO)5(THF)] affords the corresponding novel tungsten
complex 12 in 85% isolated yield as a white solid, according to
Scheme 5b. In the 31P{1H} NMR spectrum of 12 a resonance at
δ (ppm) = −8.5 with 1J31P–183W = 266 Hz can be detected. These
values are also very similar compared to the ones reported
Coordination chemistry of 2,4,6-triphenylphosphinine (1) and
2,4,6-triphenylphosphabarrelene (2)
We were further interested in the coordination chemistry of 1
and 2 towards the same metal fragment. This would allow for
a direct structural comparison of a phosphinine- and a phos-
phabarrelene-based coordination compound independently
from any substituent influence. Surprisingly, although several
structurally characterized metal complexes of phosphinines
and phosphabarrelenes have been reported in literature, no
such evaluation of the structurally related phosphorus hetero-
cycles has been made, to the best of our knowledge. Moreover,
as carbonyl-complexes provide the possibility to obtain valu-
able information on the electronic properties of the corres-
ponding ligands, we aimed for the preparation of metal
carbonyl complexes of 1 and 2 and were successful with the
[W(CO)5]-fragment for both ligands.
for
a
tungsten complex based on
a
substituted
triarylphosphabarrelene.7
We were able to grow crystals of both 11 and 12, suitable for
X-ray diffraction, by slow evaporation from a pentane solution
and the molecular structures of 11 and 12 are depicted in
Fig. 5 and 6, respectively. Although the preparation of com-
pound 11 has been reported in 1973 by Deberitz and Nöth, its
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Table 5 Structural parameters of 1, 2, 11 and 12
Bond length [Å]
1 36
11
2
12
P(1)–W(1)
P(1)–C(1)
P(1)–C(5)
P(1)–C(24)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(3)–C(25)
C(24)–C(25)
W(1)–C(25)
W(1)–C(28)
W(1)–C(31)
W(1)–C(32)
C(28)–O(5)
C(25)–O(2)
C(31)–O(2)
C(32)–O(3)
Angles (°)
—
1.757(8)
1.774(7)
—
1.367(10)
1.423(10)
1.455(11)
1.461(11)
—
—
—
—
—
—
—
—
—
—
2.5053(7)
1.737(2)
1.733(3)
—
1.390(4)
1.403(4)
1.390(3)
1.394(4)
—
—
2.5276(4)
1.8454(16)
1.8601(16)
1.8345(18)
1.331(2)
1.527(2)
1.533(2)
1.333(2)
—
—
—
—
—
—
—
—
—
—
1.8528(13)
1.8445(13)
1.8333(13)
1.3361(19)
1.5325(18)
1.5307(18)
1.3370(19)
1.5506(19)
1.406(2)
—
—
2.049(3)
1.999(3)
—
—
2.0610(16)
1.9952(15)
—
—
1.139(2)
1.156(2)
—
1.149(3)
1.140(4)
—
—
Fig. 5 Molecular structure of 11 in the crystal. Displacement ellipsoids
are shown at the 50% probability level.
C(1)–P(1)–C(5)
C(1)–P(1)–C(24)
C(5)–P(1)–C(24)
∑(CPC)
97.5(4)
—
—
103.6(1)
—
—
95.57(7)
94.95(7)
92.23(7)
282.7
95.94(6)
95.90(6)
95.46(6)
287.30
97.5(4)
103.6
substituents in 2- and 4-position are rotated considerably out
of the plane of the 6-membered ring (torsion angles P(1)–C(1)–
C(12)–C(13) = 56.13° and P(1)–C(5)–C(6)–C(7) = 69.3°). In this
way, the P-ligating ability of such 2,4,6-triaryl-substituted phos-
phinines is apparently not influenced as dramatically as
observed for SiMe3-substituted ones, which show a preference
for η6-coordination through the aromatic ring, rather than for
η1-coordination through the phosphorus lone pair.35
The molecular structure of the phosphabarrelene-based
W(0) complex 12 in the crystal also shows the expected mono-
nuclear nature of the compound (Fig. 6). Since both ligands 1
and 2 are coordinated to the same metal fragment, a direct
structural comparison of 11 and 12 is now possible, even with
the free ligands 1 and 2. Table 5 contains all relevant structural
parameters of 1, 2, 11 and 12. It can be noticed, that the P–W
bond length in 12 lies exactly in between the ones in 11 and
[(PPh3)W(CO)5]. Both the P–C bond lengths as well as the C–C
bond lengths within the phosphorus cage are very similar to
the bond lengths found for the free ligand. However, the pyra-
midalization of the phosphorus atom is more pronounced in
12 than in the corresponding phosphabarrelene-selenide 10
(∑(CPC) = 293.7°) and in [(PPh3)W(CO)5] (∑(CPC) = 308.6°), as
the sum of the CPC angles in 12 has the smallest value of
287.30°, which is close to the one of the free ligand (282.7°).
The IR-spectroscopic investigation of 11 and 12, in compari-
son with [(PPh3)W(CO)5], reveals the expected trend of the net-
donor capabilities of the corresponding ligands (Table 6). The
order of increasing basicity of the donor-atom is 1 < 2 < PPh3,
which is in line with earlier observations that phosphinines
are rather good π-accepting ligands.
Fig. 6 Molecular structure of 12 in the crystal. Displacement ellipsoids
are shown at the 50% probability level.
structural characterization (and consequently also of 12) has
so far remained elusive.32
The molecular structure of 11 in the crystal shows the
expected mononuclear nature of the compound and is very
similar to the one reported for the corresponding Cr(0)
complex [(1)Cr(CO)5].33 Fig. 5 nicely shows the distortion of the
aromatic 6-membered heterocycle due to the longer P–C bonds
in comparison to a C–C or N–C bond in benzene or pyridine.
The phosphorus–tungsten bond is 2.5053(7) Å, which is
shorter than in [(PPh3)W(CO)5] (2.545(1) Å).34 This observation
indicates the difference between a formally sp2 and a sp3-hybri-
dized phosphorus atom, as well as the stronger π-accepting
character of 1 in contrast to PPh3. Upon complexation, both
the P(1)–C(1) and P(1)–C(5) bonds as well as the C–C bonds
within the phosphorus heterocycle shorten significantly com-
pared to the free ligand 1 (Table 5). Interestingly, the phenyl-
During their investigation on the synthesis of [(1)M(CO)5]
complexes (M = Cr, Mo, W), Deberitz and Nöth reported on the
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Table 6 IR-wavenumbers ν(CO) of 11, 12, 13 and [(PPh3)W(CO)5]
˜
˜ν(CO) (cm)−1
[(1)W(CO)5] (11)
[(2)W(CO)5] (12)
[(PPh3)W(CO)5]
[(1)2W(CO)4] (13)
2073 (w); 1992 (w); 1932 (sh); 1909 (s)
2071 (w); 1989 (w); 1930 (sh); 1907 (s)
2071 (m); 1985 (w); 1929 (sh); 1909 (s)
2024 (s); 1915 (s); 1884 (s)
Scheme 6 Synthesis of bis(phosphinine)–W(0) complex 13.
Fig. 7 Molecular structure of 13 in the crystal. Displacement ellipsoids
are shown at the 50% probability level. Solvent molecules (CH3CN) are
omitted for clarity. Selected bond lengths [Å] and angles (°): P(1)–W(1):
2.494(1); P(2)–W(1): 2.500(1); P(1)–C(1): 1.744(6); P(1)–C(5): 1.744(6);
P(2)–C(24): 1.732(6); P(2)–C(28): 1.746(5). P(1)–W(1)–P(2): 82.88(5);
C(1)–P(1)–C(5): 103.6(3); C(24)–P(1)–C(28): 104.0(3).
formation of [(1)2W(CO)4] (13) during the photochemical reac-
tion of 1 and [W(CO)6] in THF.32 Indeed we also observed
traces of 13 during our NMR spectroscopic investigations on
the formation of 11 from 1 and [W(CO)5(THF)], as depicted in
Fig. 4.
THF solution of 1-(dichlorophosphanyl)piperidine (28.5 mL,
200 mmol, 1 eq.) was added to the dropping funnel. The flask
was then equipped with a reflux condenser and the solution
was heated up to reflux. At this point the 1-(dichlorophospha-
nyl)piperidine solution was added dropwise over 30 minutes
and the reaction mixture was then refluxed for 4 h. Afterwards
it was cooled down to room temperature and stirred until full
conversion (reaction followed by 31P NMR, reaction mixture
becomes colorless from orange), then the volatiles were evap-
orated. 300 mL of dry pentane were added and the suspension
was filtered. After removal of the volatiles in vacuum, the
product is distilled (∼55 °C, 0.5 mbar) in an average yield of
70% (35 g).
Caution: It is important to remove the solvent once the
reaction is complete, to prevent side- and back-reactions. After-
wards, the crude product can be safely stored for several weeks
at −20 under argon atmosphere. P(TMS)3 is pyrophoric and
can be destroyed under inert atmosphere by reaction with
n-hexanol, isopropanol, ethanol and only at last, water. The
same procedure should be adopted with the filter cake.
The resonance of 13 can be detected at δ (ppm) = 169.9
(1J31P–183W = 263 Hz) in the 31P{1H} NMR spectrum. By using
[(CH3CN)2W(CO)4] and 1 as starting material in the ratio of
1 : 2, the authors observed the quantitative formation of 13.
Based on the IR-spectroscopic data, the presence of the cis-,
rather than the trans-isomer was suggested, although a struc-
tural characterization of this coordination compound
remained elusive. In order to clarify this point, we started to
synthesize [(1)2W(CO)4] according to the literature procedure
(Scheme 6).
We were able to obtain crystals of this compound, suitable
for X-ray diffraction from slowly cooling down a saturated solu-
tion of 13 in hot acetonitrile. The molecular structure of 13 in
the crystal along with selected bond lengths and angles is
depicted in Fig. 7 and confirms indeed the presence of the cis–
bis(phosphinine) complex. It is interesting to note that the for-
mation of the cis-isomer is apparently preferred over the trans-
isomer, despite the presence of two sterically rather demand-
ing 2,4,6-triphenylphosphinine ligands. As expected, the IR-
spectroscopic data of 13 show stretching frequencies at lower
wavenumbers compared to the mono-substituted [(1)W(CO)5]
complex 11 (Table 6).
Synthesis of 2,4,6-triphenylphosphinine (1)
2,4,6-Triphenylpyrylium tetrafluoroborate (790 mg, 2.0 mmol)
and Li[P(TMS)2] × 0.5 THF (870 mg, 2 eq.) were put together in
a 50 mL Schlenk flask under an argon atmosphere. 25 mL of
THF were added. Upon addition a dark reaction mixture was
obtained which was heated to reflux for 6 h. Subsequently, all
Experimental part
Synthesis of P(TMS)3 (4)
In a 2-necked 1 L Schlenk-flask, equipped with stirring bar volatiles were removed in vacuo to obtain a yellow-brown oil.
and dropping funnel, 9.72 g of Li (1.4 mol, 7 eq.) were sus- The crude product was eluted through a neutral alumina plug
pended in 300 mL of freshly distilled THF. TMSCl (101.8 mL, (8 cm) with toluene to afford the pure product as a yellow solid
800 mmol, 4 eq.) was then added to the flask and a 120 mL (490 mg, 76%).
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Dalton Transactions
M. Habicht, M. Bruce, G. Pfeifer and J. Wiecko, Chem. Lett.,
2014, 43, 1390.
Conclusions
We have revisited several important aspects of the synthesis,
reactivity and coordination chemistry of 2,4,6-triphenylphos-
phinine and 2,4,6-triphenylphosphabarrelene. First we opti-
mized the synthesis of P(SiMe3)3, which has been the most
frequently used phosphorus source for the preparation of
2,4,6-triarylphosphinines starting from the corresponding pyr-
ylium salts. We could further show that Li[P(SiMe3)2], prepared
from P(SiMe3)3 and nBuLi in THF, is a highly recommendable
alternative phosphorus source, as generally very high yields in
2,4,6-triarylphosphinines can be obtained. Structurally related
to these aromatic phosphorus heterocycles are 2,4,6-
triarylphosphabarrelenes. We could prepare and for the first
5 G. Märkl, F. Lieb and C. Martin, Tetrahedron Lett., 1971, 17,
1249.
6 E. Fuchs, M. Keller and B. Breit, Chem. – Eur. J., 2006, 12,
6930.
7 C. Wallis, P. G. Edwards, M. Hanton, P. D. Newman,
A. Stasch, C. Jones and R. P. Tooze, Dalton Trans., 2009,
2170.
8 B. Breit, R. Winde, T. Mackewitz, R. Paciello and K. Harms,
Chem. – Eur. J., 2001, 7, 3106.
9 L. E. E. Broeckx, A. Bucci, C. Zuccaccia, M. Lutz,
A. Macchioni and C. Müller, Organometallics, 2015, 34,
2943.
time structurally characterize 2,4,6-triphenylphosphabarre- 10 P. Roesch, J. Nitsch, M. Lutz, J. Wiecko, A. Steffen and
lene-selenide, which allows for an evaluation of the structural C. Müller, Inorg. Chem., 2014, 53, 9855.
and electronic properties of this compound in comparison to 11 J. Moussa, T. Cheminel, G. R. Freeman, L.-M. Chamoreau,
PPh3PSe. Moreover, we could prepare and structurally charac-
terize [(L)W(CO)5] complexes of both the phosphinine and
J. A. G. Williams and H. Amouri, Dalton Trans., 2014, 43,
8162.
phosphabarrelene, for which the molecular structures in the 12 G. Märkl, F. Lieb and A. Merz, Angew. Chem., Int. Ed. Engl.,
crystal remained so far elusive. This allowed for the first time 1967, 79, 475.
the direct structural comparison of these structurally related 13 E. F. DiMauro and M. C. Kozlowski, J. Chem. Soc., Perkin
phosphorus-compounds, coordinated to the same metal frag- Trans. 1, 2002, 3, 439.
ment. By means of X-ray crystallography and IR-spectroscopy, 14 C. Wallis, D. McGuinness, P. D. Newman, R. P. Tooze and
valuable structural and electronic information about these P. G. Edwards, Dalton Trans., 2009, 2178.
compound could thus be obtained. Finally we were able to 15 (a) R. Paciello, E. Zeller, B. Breit and M. Röper, DE
characterize crystallographically a bis(phosphinine)–W(CO)4
complex, which shows a cis-arrangement of the two phos-
phinine ligands.
19743197A1, 1999 (to BASF); (b) T. Mackewitz and
M. Röper, EP 1036796A1, 2000 (to BASF).
16 G. Märkl, F. Lieb and A. Merz, Angew. Chem., Int. Ed. Engl.,
1967, 6, 944.
17 A. B. Bruker, L. D. Balashova and L. Z. Soborovskii, Dokl.
Akad. Nauk SSSR, 1960, 135, 843.
Acknowledgements
18 G. Becker and W. Hölderich, Chem. Ber., 1975, 108, 2484.
19 E. Niecke and H. Westermann, Synthesis, 1988, 330.
20 P. L. Arnold, M. A. Petrukhina, V. E. Bochenkov,
T. I. Shabatina, V. V. Zagorskii, G. B. Sergeev, F. Geoffrey
and N. Cloke, J. Organomet. Chem., 2003, 688, 49.
21 (a) G. Fritz and W. Hölderich, Z. Anorg. Allg. Chem., 1976,
422, 104; (b) F. Uhlig, S. Gremler, M. Dargatz, M. Scheer
and E. Herrmann, Z. Anorg. Allg. Chem., 1991, 606, 105.
22 C. Müller, D. Wasserberg, J. J. M. Weemers, E. A. Pidko,
S. Hoffmann, M. Lutz, A. L. Spek, S. C. J. Meskers,
R. A. Janssen, R. A. van Santen and D. Vogt, Chem. – Eur. J.,
2007, 13, 4548.
Financial support was provided by the European Initial Train-
ing Network SusPhos (317404), the Free University of Berlin
and the Deutsche Forschungsgemeinschaft (DFG).
Notes and references
1 G. Märkl, Angew. Chem., Int. Ed. Engl., 1966, 78, 907.
2 A. J. Ashe III, J. Am. Chem. Soc., 1971, 93, 3293.
3 F. Mathey and P. Le Floch, Sci. Synth., 2005, 5, 1097.
4 (a) P. Le Floch, in Phosphorus-Carbon Heterocyclic Chemistry: 23 (a) C. Müller, E. A. Pidko, M. Lutz, A. L. Spek, R. A. van
The Rise of a New Domain, ed. F. Mathey, Pergamon, Palai-
seau, 2001, pp. 485–533; (b) G. Märkl, in Multiple Bonds
and Low Coordination in Phosphorus Chemistry, ed.
M. Regitz and O. J. Scherer, Thieme Verlag, Stuttgart, 1990,
Santen and D. Vogt, Dalton Trans., 2007, 5372;
(b) C. Müller, E. A. Pidko, A. J. P. M. Staring, M. Lutz,
A. L. Spek, R. A. van Santen and D. Vogt, Chem. – Eur. J.,
2008, 14, 4899.
p. 220; (c) L. Weber, Angew. Chem., Int. Ed., 2002, 41, 563; 24 O. Piechaczyk, M. Doux, L. Ricard and P. Le Floch, Organo-
(d) C. Müller and D. Vogt, in Catalysis and Material Science metallics, 2005, 24, 1204.
Applications, ed. M. Peruzzini and L. Gonsalvi, Springer, 25 M. Blug, C. Guibert, X.-F. Le Goff, N. Mézailles and P. Le
vol. 36, ch. 6, 2011; (e) C. Müller, in Phosphorus Ligand Floch, Chem. Commun., 2009, 2, 201.
Effects in Homogeneous Catalysis: Design and Synthesis, ed. 26 M. Blug, X.-F. Le Goff, N. Mézailles and P. Le Floch, Organo-
P. C. J. Kamer and P. W. N. M. van Leeuwen, Wiley-VCH, metallics, 2009, 28, 2360.
2012; (f) C. Müller, J. A. W. Sklorz, I. de Krom, A. Loibl, 27 B. Breit and E. Fuchs, Synthesis, 2006, 2121.
Dalton Trans.
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28 P. S. Bäuerlein, I. Arenas Gonzalez, J. J. M. Weemers, 32 J. Deberitz and H. Nöth, J. Organomet. Chem., 1973, 49,
M. Lutz, A. L. Spek, D. Vogt and C. Müller, Chem.
Commun., 2009, 4944.
453.
33 H. Vahrenkamp and H. Nöth, Chem. Ber., 1973, 106, 2227.
29 M. Blug, Ph.D. thesis, École Polytechnique, Palaiseau, 34 M. J. Aroney, I. E. Buys, M. S. Davies and T. W. Hambley,
France, 2009. J. Chem. Soc., Dalton Trans., 1994, 2827.
30 D. W. Allen and B. F. Taylor, J. Chem. Soc., Dalton Trans., 35 N. Mézailles, L. Ricard, F. Mathey and P. Le Floch, Organo-
1982, 51. metallics, 2001, 20, 3304.
31 B. J. Dunne and A. G. Orpen, Acta Crystallogr., Sect. C: 36 K. Eggers, Ph.D. thesis, University Erlangen-Nürnberg,
Cryst. Struct. Commun., 1991, 47, 345.
Germany, 2000.
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