Investigation of novel and reinvestigation of known cyclopentadienylphosphanes: News on [1,5] sigmatropic rearrangements

Article abstract of DOI:10.1016/j.jorganchem.2010.05.001

Ph₂P(C₅HMe₄) (1) was found to undergo a prototropic rearrangement contrary to previous reports. The first detailed investigation of a cyclopentadienylphosphane to undergo such a reaction is presented. One of the two tautomers of 1 which were observed represents the kinetic product, the other one is the thermodynamic product. The two isomers exhibit pronounced differences with respect to their chemical reactivity. Thermodynamic and kinetic data of the isomerisation process were determined. A novel, electron rich cyclopentadienylphosphane, the phosphorus diamide (Me ₂N)₂P(C₅HMe₄) (2), was synthesised and was also found to undergo a prototropic rearrangement. The characteristics of the isomerisation process are fully consistent with the findings on 1. The molecular structure of the kinetic product of 2 was established by means of single crystal X-ray diffraction analysis. Me₂P(C₅H ₄tBu) (3) is presented as another novel cyclopentadienylphosphane. The mixture of isomers of 3 in thermodynamic equilibrium was investigated in detail with respect to the molecular structures of the tautomers. The results of this study suggested a reinvestigation of the parent compound Ph ₂P(C₅H₄tBu) (4) and findings differing from an earlier report in some details are presented. Ring-substituted cyclopentadienyl phosphanes are valuable building blocks for bifunctional P(III) and P(V) ligands. The compounds Ph₂P(C₅HMe₄) (1), (Me₂N)₂P(C₅HMe₄) (2), Me ₂P(C₅H₄tBu) (3), and Ph₂P(C ₅H₄tBu) (4) have been synthesized and investigated with respect to their [1,5] sigmatropic rearrangements. Experimental results differ from earlier reports. An isomer of 2, 2a, has been characterized by XRD analysis.

Full text of DOI:10.1016/j.jorganchem.2010.05.001

Journal of Organometallic Chemistry 695 (2010) 2000e2006
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Investigation of novel and reinvestigation of known cyclopentadienylphosphanes:
News on [1,5] sigmatropic rearrangements
*
Crispin Lichtenberg, Michael Elfferding, Lars Finger, Jörg Sundermeyer
Fachbereich Chemie, der Philipps-Universität Marburg, Hans-Meerwein-Str., 35032 Marburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Ph2P(C5HMe4) (1) was found to undergo a prototropic rearrangement contrary to previous reports. The
first detailed investigation of a cyclopentadienylphosphane to undergo such a reaction is presented. One
of the two tautomers of 1 which were observed represents the kinetic product, the other one is the
thermodynamic product. The two isomers exhibit pronounced differences with respect to their chemical
reactivity. Thermodynamic and kinetic data of the isomerisation process were determined. A novel,
electron rich cyclopentadienylphosphane, the phosphorus diamide (Me2N)2P(C5HMe4) (2), was
synthesised and was also found to undergo a prototropic rearrangement. The characteristics of the
isomerisation process are fully consistent with the findings on 1. The molecular structure of the kinetic
product of 2 was established by means of single crystal X-ray diffraction analysis. Me2P(C5H4tBu) (3) is
presented as another novel cyclopentadienylphosphane. The mixture of isomers of 3 in thermodynamic
equilibrium was investigated in detail with respect to the molecular structures of the tautomers. The
results of this study suggested a reinvestigation of the parent compound Ph2P(C5H4tBu) (4) and findings
differing from an earlier report in some details are presented.
Received 3 February 2010
Received in revised form
6 April 2010
Accepted 2 May 2010
Available online 10 May 2010
Keywords:
Cyclopentadienylphosphanes
Prototropic rearrangement
Phosphatropic rearrangement
Kinetics
Kinetic product
Thermodynamic product
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
phosphatropic shifts in such molecules [19,20] have been studied in
detail. Different isomers of cyclopentadienylphosphanes resulting
Cyclopentadienylphosphanes have found manifold use as
synthetic building blocks in organometallic chemistry. On the one
hand this is due to the fact that the monoanions derived from these
molecules can act as bidentate ligands with the two coordination
sites being in close proximity. This characteristic has frequently
been taken advantage of for the syntheses of dihetero metallic
[1e7] as well as tri- [8e10] and multimetallic [11,12] compounds.
On the other hand cyclopentadienylphosphanes can easily be
derivatised via oxidative addition at the phosphorus atom with
group 14, 15 or 16 electrophiles opening up synthetic routes to
many different classes of chelating ligands [13e15]. Beside the
importance of cyclopentadienylphosphanes as synthetic building
blocks, these molecules have also been subjects of academic
research, when the fluxional behaviour of cyclopentadienes
from prototropic rearrangements have been described and in some
cases the course of these reactions could be monitored [21e23].
However, a detailed investigation of the kinetic parameters of such
a reaction has not been reported yet. Also, there is no evidence in
the literature of different isomers of cyclopentadienylphosphanes
to show pronounced differences in their chemical reactivity.
During our studies involving oxidative addition reactions with
diphenyltetramethylcyclopentadienylphosphane, Ph2PCp# (1)
(Cp# ¼ C5HMe4), we found that this phosphane is a suitable
substrate for closing this gap of knowledge. In addition, our find-
ings on the isomerisation of 1 e a molecule which is known to the
literature since 1990 [24,25] and has been used in numerous
derivatisation reactions [1,2,5,7,8,26e32] e are not fully consistent
with earlier reports, which in turn are not in congruency with each
other (see section 2.1). Moreover we would like to report the
synthesis of two novel cyclopentadienylphosphanes, (Me2N)2PCp#
(2) and Me2PCptBu (3) (CptBu ¼ C5H4tBu), and the detailed NMR
spectroscopic analysis of the isomers present in thermodynamic
equilibrium. The results obtained on 3 prompted us to reinvestigate
the mixture of tautomers of the parent compound Ph2PCptBu
(4) [33] with respect to the molecular structures of the single
isomers.
was investigated [16]. After the first example of
a cyclo-
pentadienylphosphane to exhibit fluxional behaviour due to the
migration of a phosphanyl moiety had been reported [17], the
kinetic parameters [18] as well as the stereochemical aspects of
* Corresponding author. Tel.: þ49 (0)6421 2825693; fax: þ49 (0)6421 2828917.
E-mail address: jsu@staff.uni-marburg.de (J. Sundermeyer).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.05.001

C. Lichtenberg et al. / Journal of Organometallic Chemistry 695 (2010) 2000e2006
2001
2. Results and discussion
2.1. Diphenyltetramethylcyclopentadienylphosphane, Ph2PCp# (1)
Ph2PCp# (1) has been described by two different groups
independently in 1990 [24,25]. Their conclusions concerning the
structure of this molecule differed. The results we obtained are not
fully consistent with either one of these reports. In order to clarify
the differences the results published earlier shall very briefly be
summarised.
Scheme 2. Isomerisation of Ph2PCp# (1) via a prototropic rearrangement.
2.1.1. Results on Ph2PCp# (1) obtained by other groups
Szymoniak et al. presented the first synthesis of 1 [24], which
authors usually refer to when using 1 as a synthetic building block.
They reported 1a to be the only isomer present and classified 1 to
be unstable under inert gas atmosphere at ambient temperature,
which was why they suggested a straightforward derivatisation.
Wong et al. described Ph2PCp# as a 1:1 mixture of two isomers, 1x
and 1y (Scheme 1) [25]. They did not mention the number of signals
obtained in 1H NMR spectra of 1. 13C NMR or two dimensional NMR
spectra were not reported. According to their interpretation, the sp3
hybridised carbon atom, which is part of the five-membered ring, is
not in plane with the other four carbon atoms forming this cycle,
which would let 1x and 1y be diastereomers, and the transfer of
this carbon atom from the re to the si side of the molecule (and vice
versa) is slow on the NMR time scale. In another publication two
isomers of Ph2PCp# have briefly been mentioned without any
details concerning their number, structures, molar ratio or analyt-
ical data given [8].
which can act as an electron donor with its lone pair or as an
electron acceptor with its PeC(Ph)-orbitals.
s*
The molecular structure of 1b was established on the basis of one
and two dimensional NMR spectroscopic experiments. Especially
the NOE spectrum provides evidence for the structure suggested. In
this spectrum, the resonance due to the proton, which is directly
bonded to the five-membered ring, shows a cross peak to the
resonances which were assigned to the methyl groups in 5- and in
4-position of the cyclopentadienyl fragment. A cross peak to a third
resonance arising from a methyl group could not be observed, which
rules out the molecular structures 1x and 1y which have been
suggested earlier (Scheme 1) [25]. The other cross peaks arising in
the NOE spectrum of 1 are fully consistent with the molecular
structure of 1b and are graphically summarised in Scheme 3. The full
NOE spectrum of 1b can be found in the electronic supplement.
In order to find further evidence for 1b to be an isomer of
Ph2PCp#, an equimolar mixture of 1a and 1b was dissolved in
benzene and deprotonated using nBuLi as a base in eightfold excess.
As expected both isomers were transformed into the lithium
organyl Li(PPh2(C5Me4)) (5), which was observed via 1H and 31P
NMR spectroscopy. However, the kinetics of this deprotonation
reaction, which was also monitored by 1H and 31P NMR spectro-
scopic measurements, differed significantly for the two isomers 1a
and 1b. Whereas 1a was completely deprotonated after a reaction
time of t ꢀ 15 min, a reaction time as high as t ¼ 35 h was necessary
to achieve the deprotonation of 98% of the isomer 1b. This is the
first example of two isomers of a cyclopentadienylphosphane
differing significantly in their chemical reactivity, namely their
kinetic basicity. The course of the reaction is visualised by the graph
shown in Fig. 1.
2.1.2. Isomerisation of Ph2PCp# and investigations concerning the
kinetics thereof
1 was synthesised with minor modifications to Szymoniak’s
protocol resulting in a simplification of the synthesis. The cyclo-
pentadienylphosphane 1 was obtained as a single isomer (1a) in
congruency with Szymoniak’s results [24]. Upon storage at ambient
temperature under inert gas atmosphere the formation of another
substance was observed via NMR spectroscopic measurements.
However, this substance proved not to be a product of decompo-
sition processes as suggested earlier [24], but a product of an
isomerisation process with the molecular structure 1b (Scheme 2).
This isomerisation takes place when 1 is stored as a neat product as
well as in solution of diethylether or pentane. The equilibrium of
this slow isomerisation process is reached at a molar ratio of
1a:1b ¼ 30:70 (Scheme 2), i. e. the isomer 1b is thermodynamically
When less than eight equivalents of nBuLi are used, the result
does not change within the estimated range of error (ꢁ5% for
integrals in NMR spectra). These observations lead to the conclu-
sion that under the chosen reaction conditions the deprotonation
of 1b is first order with respect to 1b and zero order with respect to
the base. The situation described in Scheme 4 gives a possible
explanation for these results: the deprotonation of 1a is signifi-
cantly faster than the isomerisation of 1, whereas the deprotona-
tion of 1b is significantly slower than the isomerisation of 1. In
more stable. This can be ascribed to the fact that the p-system of the
cyclopentadienyl ring in 1b is extended by a phosphanyl moiety,
Scheme 1. Structures of Ph2PCp# (1) according to earlier publications; Szymoniak et al.
reported 1a to be the only isomer present [24]; Wong et al. described the structures 1x
and 1y [25].
Scheme 3. Cross peaks in the NOE spectrum of 1b (indicated by arrows between the
relevant substituents).

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C. Lichtenberg et al. / Journal of Organometallic Chemistry 695 (2010) 2000e2006
Fig. 1. Plot of the relative concentration of 1b versus time in the deprotonation of an
equimolar mixture of 1a and 1b with eight equivalents of nBuLi in C6D6 at ambient
temperature.
Scheme 5. Schematic energy diagram for phosphatropic and prototropic shift in
Ph2PCp# (1).
other words, the isomerisation of 1b to 1a is the rate determining
step of the deprotonation of 1b under the chosen reaction
conditions.
margin of error given for kꢂ1 is due to the error determined in the
linear regression analysis:
The difference in chemical reactivity of the isomers 1a and 1b in
this deprotonation reaction can only arise from different kinetic
barriers, as the product 5 is the same in both cases, which rules out
thermodynamic reasons. The height of kinetic barriers of reactions
with lithiated compounds acting as nucleophilic reagents are
known to be strongly affected by precoordination effects in some
cases [34e36]. In the deprotonation of 1 with nBuLi, a precoordi-
nation of the lithium atom by the lone pair of the phosphorus atom
is a plausible initiating reaction step. In the isomer 1b, electron
k
¼ ð2:8 ꢁ 0:1Þ$10ꢂ5 sꢂ1
The equilibrium constant, K, can be calculated from the data
:
ꢂ1
listed in Scheme 2; in this case, the margin of error is due to the
estimated range of error (5% for integrals in NMR spectra). For
completeness, the Gibbs energy can also be determined from K:
K ¼ 2:3 ꢁ 0:2
density from this lone pair can also be donated into the p-system of
D
G ¼ ð ꢂ 2:1 ꢁ 0:3Þ kJ:
When the relation between the equilibrium constant and the
the cyclopentadienyl moiety. This would result in a decreased
coordination ability of 1b compared with 1a offering a possible
explanation for the differences of these two isomers with respect to
their chemical reactivity.
rate constants of the isomerisation of 1, K ¼ k1/kꢂ1, is taken into
account, the second rate constant, k1, can also be determined:
The data on the deprotonation of 1 (Fig. 1) do not only reveal the
differences of 1a and 1b in their chemical reactivity, but also allow
for the kinetic parameters of the isomerisation of 1 to be deter-
mined (details of these procedures can be found in the electronic
k1 ¼ ð6:4 ꢁ 0:8Þ$10ꢂ5 sꢂ1
:
This is the first report of kinetic data on a prototropic shift in
a cyclopentadienylphosphane. The values of the rate constants are
by approximately 17 orders of magnitude lower than the value
which has been determined for the prototropic shift in cyclo-
pentadiene [37]. The fact that a phosphatropic shift was not
observed in Ph2PCp# is not surprising as in all reported cases of
supplement). The rate constant k
can be obtained from the
ꢂ1
slope of the halflogarithmic plot of the graph shown in Fig. 1; the
Scheme 4. Kinetics of the isomerisation of Ph2PCp# (1) relative to those of the
deprotonation of 1a and 1b.
Scheme 6. Highly selective formation of the isomer 1a via two different synthetic routes;
in both cases 1a isomerises to form 1b until the thermodynamic equilibrium is reached.

C. Lichtenberg et al. / Journal of Organometallic Chemistry 695 (2010) 2000e2006
2003
Within this ligand family, the outstanding characteristic of 2 is its
electron richness which is induced by the two dimethylamino
moieties. This characteristic is of interest as electron rich phos-
phanes proved to be superior when used in some derivatisation
reactions [40,41] or as ligands in catalytic applications [42,43].
When the cyclopentadienylphosphane 2 was synthesised, the
isomer 2a was obtained exclusively.
The molecular structure of 2a was established via one and two
dimensional NMR experiments and confirmed by single crystal
X-ray analysis (Fig. 2). In solid state, the bulky Cp# substituent is
oriented away from the two dimethylamino moieties, which was
ascribed to steric interactions. The bond lengths within the cyclo-
pentadienyl unit exhibit the typical pattern of two short (C2eC3,
C4eC5), one medium (C3eC4) and two long (C1eC2, C1eC5) CeC
distances. These values provide evidence for C1 to be sp3 hybridised
and to bear the proton H1. In addition, H1 was found in the final
density map and refined with isotropic displacement parameters.
Fig. 2. ORTEP drawing of the molecular structure of 2a; the thermal ellipsoids are drawn
at the 30% probability level. All hydrogen atoms except for H1 are omitted for clarity.
Selected bond lengths [Å] and angles [ꢃ]: P1eC11.869(3), P1eN11.690(3), P1eN2 1.686
(3), C1eC2 1.522(4), C2eC3 1.340(5), C3eC4 1.477(5), C4eC5 1.338(5), C1eC5 1.520(5),
C1eH1 0.967(35), C1eP1eN1 101.1(1), C1eP1eN2 100.5(2), N1eP1eN2 109.9(1),
C1eC2eC3 109.2(3), C2eC3eC4 109.6(3), C3eC4eC5 109.3(3), C4eC5eC1 109.5(3),
C2eC1eC5 102.3(3), P1eN1eC10 126.5(2), P1eN1eC11118.2(2), C10eN1eC11113.2(3),
P1eN2eC12 127.8(2), P1eN2eC13 117.5(2), C12eN2eC13 111.8(3).
By means of this analysis, the isomer 2a with H1 in
a-position
relative to the phosphanyl substituent was unambiguously estab-
lished as the product primarily formed in the synthesis of 2. The
lengths of the phosphorus nitrogen bonds (P1eN1, P1eN2) and
the phosphorus carbon bond (P1eC1) were found to lie within
the range of values typical for diaminoarylphosphanes [44e48]. The
PeC and PeN bonds in other diaminocyclopentadienylphosphanes
and the PeC bonds in iminocyclopentadienylphosphanes, which
have been characterised by means of single crystal X-ray analyses,
are slightly longer [49,50], but of comparable length when such
molecules are coordinated to a transition metal [51,52]. Remarkably,
both of the nitrogen atoms in compound 2a, N1 and N2, are centres
of a trigonal planar configuration with angle sums (CeNeC/P) of
357.1(7)ꢃ and 357.9(7)ꢃ respectively. This allows for more effective
such a rearrangement one of two criteria was met, which do not
apply to Ph2PCp#. Either at least one strongly electronegative
substituent was bonded to the phosphorus atom of the posphanyl
moiety [17,38] or five sterically demanding substituents were
bonded to the cyclopentadienyl core in addition to the phosphanyl
fragment [18,39]. In Ph2PCp# there are no highly electronegative
substituents and only four sterically demanding substituents
bonded to the cyclopentadienyl core apart from the phosphanyl
moiety. In addition, a phosphatropic rearrangement in 1 would
result in a geminally methyl and phosphanyl substituted product,
which would be sterically highly unfavourable. On the basis of
these findings a schematic energy diagram describing the fluxional
behaviour of Ph2PCp# can be formulated (Scheme 5).
nN-s
*
interactions, which provides experimental evidence for
PeC
the high potential donor ability of the phosphorus atom P1.
In the light of our results on the isomerisation of Ph2PCp# (1)
(see section 2.1) it was not surprising, that 2a is partially trans-
formed into the isomer 2b via a prototropic shift upon storage at
room temperature in a solution of toluene. The molecular structure
of 2b was established by one and two dimensional NMR spectro-
scopic experiments, a detailed analysis of which can be found in the
electronic supplement. The two isomers 2a and 2b were found to be
in thermodynamic equilibrium at a ratio of 2a:2b ¼ 40:60. From
these data can be concluded, that 2a is formed as the kinetic
As pointed out above, 1a is formed as the kinetic product in the
synthesis of Ph2PCp#, when the protocol established by Szymoniak
et al. [24] or minor modifications thereof are applied (left part,
Scheme 6).
A different synthetic approach to the cyclo-
pentadienylphosphane 1 was investigated in an attempt to syn-
thesise the isomer 1b in high selectivity prior to isomerisation. In
this respect, Li(PPh2(C5Me4)) (2) was protonated using ammonium
hexafluorophosphate as a Brønsted acid. However, 1a was again
exclusively formed (right part, Scheme 6) prior to the beginning
slow isomerisation process (middle part, Scheme 6). So the carbon
atom in 1-position of the lithium cyclopentadienide 5 is the
kinetically most basic site of this molecule.
product in the synthesis of 2. The isomer 2b, in which the p-system
of the cyclopentadienyl group is in conjugation with a phosphanyl
moiety, represents the thermodynamic product. A phosphatropic
rearrangement was not observed. These results are in good agree-
ment with our findings on Ph2PCp# (1) (see section 2.1), i.e. the
schematic energy diagram shown in Scheme 5 also holds for 2.
The lithiated species Li(P(NMe2)2(C5Me4)) (6) was prepared as
a useful building block for further derivatisation reactions or
coordination of the (P(NMe2)2(C5Me4))- anion to one or more metal
centres (Scheme 7).
2.2. Synthesis and isomerisation of (Me2N)2PCp# (2)
(Cp# ¼ C5HMe4)
Having found evidence for slow prototropic shifts in Ph2PCp# (1)
and (Me2N)2PCp# (2) e two tetramethylcyclopentadienylphos-
phanes which differ significantly in their electronic characteristics e
(Me2N)2PCp# (2), the second tetramethylcyclopentadienyl
substituted phosphane we would like to report on, was synthesised
in order to enrich the spectrum of this small class of compounds.
Scheme 7. In the synthesis of 2, the isomer 2a is exclusively formed prior to the beginning slow isomerisatin process; lithiation of 2 yields Li(P(NMe2)2(C5Me4)) (6).

2004
C. Lichtenberg et al. / Journal of Organometallic Chemistry 695 (2010) 2000e2006
reactivity towards nBuLi. 1b is much more inert towards this base
than 1a, which makes the isomerisation of 1b to 1a the rate limiting
step of this reaction.
(Me2N)2PCp# (2) is presented as a novel highly nucleophilic
cyclopentadienylphosphane. In general, the observations concern-
ing the isomerisation processes of 1 also hold for 2; i. e. the
symmetric isomer 2a is exclusively formed as the kinetic product
independent of the synthetic approach to compound 2 and 2a
undergoes a prototropic rearrangement to form the thermody-
namically more stable isomer 2b (2a:2b ¼ 40:60 in thermodynamic
equilibrium). The molecular structure of the kinetic product 2a was
established by means of single crystal X-ray analysis.
Me2PCptBu (3) (CptBu ¼ C5H4tBu) was synthesised as another
new cyclopentadienylphosphane. The mixture of isomers of 3
present in thermodynamic equilibrium was studied in detail with
respect to the molecular structures of the single tautomers. The
results of these studies suggested a reinvestigation of the mixture
of isomers of the parent compound Ph2PCptBu (4). Deviations from
an earlier report [33] with respect to the molecular structures of
two of the four the isomers of 4 were found.
Scheme 8. Tautomers which were observed in the mixture of isomers of R2PCptBu; in
parenthesis: molar percentage of each single isomer present in the mixture of isomers;
3: R ¼ Me, 4: R ¼ Ph.
this type of rearrangement is likely to be a viable reaction pathway
for the entity of this small class of compounds. The CS-symmetric
isomers, 1a and 2a, are formed exclusively prior to the beginning
isomerisation processes.
4. Experimental
2.3. Synthesis of Me2PCptBu (3) and reinvestigation of Ph2PCptBu (4)
(CptBu ¼ C5H4tBu)
4.1. General considerations
When there are less than five sterically demanding substituents
All experimental procedures were carried out in an atmosphere
of purified argon or nitrogen using standard Schlenk techniques or
a glovebox. The solvents used were dried and degassed according to
standard protocols. Chemicals were purchased from Alfa Aesar or
Aldrich and used without further purification. PClMe2 [53], LiCp#
(Cp# ¼ C5HMe4) [54], PCl(NMe2)2 [55] and LiCptBu [56] were syn-
thesised according to literature procedures. NMR measurements
were performed using a Bruker AVANCE 300, DRX 400 or DRX 500
spectrometer at 25 ꢃC. Elemental analyses were carried out at the
Analytical Laboratory of the Department of Chemistry/Philipps-
Universität Marburg. EI mass spectra were recorded using a Fin-
nigan MAT CH7 instrument (E ¼ 70 eV). ATR FT IR spectra were
obtained using a Bruker Alpha-P spectrometer.
bonded
to
the
five-membered
ring
of
a
cyclo-
pentadienylphosphane, a mixture of isomers is usually obtained in
the syntheses of these compounds. This also proved to be the case
when the novel cyclopentadienylphosphane Me2PCptBu (3) was
synthesised in analogy to the protocol which was established for
the parent compound Ph2PCptBu (4) [33]. All isomers of 3 exhibit
the typical substitution pattern, in which the two bulky substitu-
ents are separated by a C1-bridge, which is attributed to steric
repulsion. With this restriction in mind five isomers of 3 could
possibly be formed, four of which were actually observed (Scheme
8). The molecular structures of these compounds were established
by means of one and two dimensional NMR spectroscopic methods.
The detailed analysis of the NMR data is presented in the electronic
supplement. The molecular structures established for 3 differ in
some points from those which were reported for the parent
compound 4 [33]. In the course of our studies we also synthesised 4
for further derivatisation reactions. The analysis of the tautomeric
mixture of compound 4 let us suggest the molecular structures
shown in Scheme 8 for the four isomers observed (details can be
found in the electronic supplement). These findings are in full
congruency with our results on Me2PCptBu (3). However, they
deviate in some points from the conclusions drawn in an earlier
publication [33], in which the same number of isomers and similar
molar percentages of the single isomers were reported, but
different molecular structures were proposed for the isomers 4b
and 4c.
4.1.1. Synthesis of Ph2PCp# (1) [24]
LiCp# (1.95 g, 15.2 mmol) was suspended in pentane (80 mL)
and chlorodiphenyl phosphane (3.14 g, 14.2 mmol) was added at
ambient temperature. The liquid phase slowly turned yellow. After
7 h the suspension was filtered. The solid was washed with pentane
(3 ꢄ15 mL) and the solvent was removed from the filtrate under
reduced pressure yielding a yellow oil, which was dried in vacuo for
2 h. The product 1 consisting exclusively of the isomer 1a was
obtained as a yellow oil in quantitative yield (4.35 g, 14.2 mmol).
A mixture of the isomers 1a and 1b (1a:1b ¼ 30:70) was obtained
upon storage of 1 under inert gas atmosphere at ambient temper-
ature as a neat substance or in a solution of diethylether or pentane
for at least 19 d.
1H NMR (300.1 MHz, C6D6): 1a:
d
¼ 1.53 (s, 6H, 3,4-CpMe2), 1.79
3. Conclusion
(s, 6H, 2,5-CpMe2), 3.67 (d, 1H, 2JHP ¼ 1.1 Hz, HCp), 7.04e7.07 (m, 6H,
m-, p- Ph), 7.44e7.48 (m, 4H, o-Ph) ppm. 1b:
d
¼ 0.84 (d, 3H,
In the synthesis of the cyclopentadienylphosphane Ph2PCp# (1)
(Cp# ¼ C5HMe4) the symmetric isomer 1a is e at first e exclusively
formed, independent of the synthetic route chosen. In contrast to
earlier reports [24,25] 1a was found to undergo a prototropic
rearrangement yielding 1b as a thermodynamically more stable
isomer. A mixture of these two isomers in a ratio of 1a:1b ¼ 30:70
was found to be present in thermodynamic equilibrium. The rate
constants of this slow isomerisation process, k1 and kꢂ1, were
determined and are in the order of 5/10ꢂ5 sꢂ1. Unexpectedly, the
isomers 1a and 1b differ significantly with respect to their chemical
3JHH ¼ 6.9 Hz, 5-CpMe), 1.63 (s, 3H, 4-CpMe), 1.65 (s, 3H, 3-CpMe),
4
3
2.00 (d, 3H, JHP ¼ 1.6 Hz, 2-CpMe), 2.84 (q, 1H, JHH ¼ 6.9 Hz, HCp),
7.07e7.12 (m, 6H, m-, p- Ph), 7.51e7.55 (m, 4H, o-Ph) ppm.
31P NMR (121.5 MHz, C6D6): 1a:
d
¼ 0.6 ppm. 1b: ¼ ꢂ24.8 ppm.
d
13C NMR (75.5 MHz, C6D6): 1a:
d
¼ 11.2 (s, 3,4-CpMe2), 14.2
3
(d, JCP ¼ 6.7 Hz, 2,5-CpMe2), 57.1 (d, 1JCP ¼ 26.3 Hz, 1-Cp), 128.0 (d,
2JCP ¼ 19.5 Hz, o-Ph), 128.6 (s, p-Ph), 133.5 (d, JCP ¼ 19.5 Hz, m-Ph),
3
2
1
133.8 (d, JCP ¼ 3.5 Hz, 2,5-Cp), 136.8 (d, JCP ¼ 19.9 Hz, ipso-Ph),
2
137.3 (d, JCP ¼ 3.4 Hz, 3,4-Cp) ppm. 1b:
d
¼ 11.1 (s, 3-CpMe), 12.2
3
3
(s, 4-CpMe), 14.2 (d, JCP ¼ 15.6 Hz, 2-CpMe), 15.3 (d, JCP ¼ 2.4 Hz,

C. Lichtenberg et al. / Journal of Organometallic Chemistry 695 (2010) 2000e2006
2005
5-CpMe), 53.5 (d, 2JCP ¼ 3.7 Hz, 5-Cp), 128.6 (d, 3JCP ¼ 5.5 Hz, m-Ph),
1054 (w), 953 (s), 800 (w), 764 (w), 661 (s), 632 (m), 504 (s), 452
(m) cmꢂ1
2
4
133.1 (d, JCP ¼ 6.3 Hz, o-Ph), 133.4 (d, JCP ¼ 5.8 Hz, p-Ph), 134.2 (d,
.
3
2JCP ¼ 12.7 Hz, 2-Cp), 135.0 (d, JCP ¼ 6.1 Hz, 3-Cp), 138.3 (d,
1
1JCP ¼ 11.8 Hz, ipso-Ph1/2), 139.1 (d, JCP ¼ 10.6 Hz, ipso-Ph2/1), 147.1
4.1.4. Synthesis of Li((Me2N)2P(C5Me4) (6)
(d, 3JCP ¼ 2.5 Hz, 4-Cp), 155.0 (d, 1JCP ¼ 15.2 Hz, 1-Cp) ppm.
4.1.2. De- [24] and Reprotonation of Ph2PCp# (1a:1b ¼ 50:50)
(Me2N)2PCp# (100 mg, 0.43 mmol) was dissolved in hexane and
cooled to 0 ꢃC. A solution of nBuLi in hexane (1.6
M, 300 mL,
0.48 mmol) was added. After stirring at ambient temperature for
16 h and at 55 ꢃC for another 24 h a precipitate had formed, which
was separated by filtration and washed with pentane (2 ꢄ 5 mL).
After drying in vacuo 6 was obtained as a yellow solid in 67% yield
(72 mg, 0.29 mmol).
A mixture of 1a and 1b (1a:1b ¼ 50:50, 10 mg, 33
mmol) was
dissolved in 600
m
L of perdeuterated benzene. A solution of nBuLi in
L, 75 mol) was added at ambient temperature.
hexane (2.5 , 30
M
m
m
98% of the 1 were found to be converted to the product 2 after
a reaction time of 36 h (judged by the 1H and 31P NMR spectra)
during which the yellow colour of the reaction mixture intensified.
1H-NMR (300.1 MHz, d8-THF):
d
¼ 1.93 (d, 6H, CpMe2), 2.01
3
(s, 6H, CpMe2), 2.57 (d, JHP ¼ 8.5 Hz, 12H, NMe2) ppm.
Ammonium hexafluorophosphate (46 mg, 282
mmol) was added to
13C-NMR (100.7 MHz, d8-THF):
d
¼ 11.4 (s, CpMe2), 12.5
3
2
the reaction mixture. After ultrasonic treatment for 1 h and an
overall reaction time of 6 h the intensity of the yellow colour of the
reaction mixture had decreased and a complete conversion of 2 to
the starting material 1 was observed with the isomer 1a being
formed exclusively (judged by the 1H and 31P NMR spectra; for
NMR spectroscopic data of 1a see section 4.1.1). After 6 more hours
of reaction time the beginning isomerisation of 1a to 1b was
observed by 1H and 31P NMR spectroscopic analysis (for detailed
NMR spectroscopic data of 1b see section 4.1.2).
(d, JCP ¼ 4.7 Hz, CpMe2), 41.4 (d, JCP ¼ 16.4 Hz, NMe2), 103.4
1
2
(d, JCP ¼ 10.4 Hz, 1-Cp), 111.9 (d, JCP ¼ 6.8 Hz, 2,5-Cp), 112.9
(d, 3JCP ¼ 14.4 Hz, 3,4-Cp) ppm.
31P-NMR (121.5 MHz, d8-THF):
d
¼ 109.3 ppm.
4.1.5. Synthesis of Me2PCptBu (3)
A suspension of LiCptBu (500 mg, 3.90 mmol) in toluene (12 mL)
was cooled to ꢂ78 ꢃC. A solution of PClMe2 in toluene (3.94
M,
1.04 mL, 4.10 mmol) was added. After warming the reaction
mixture up to room temperature over a period of 16 h the solid was
separated by filtration and washed with hexane (2 ꢄ 6 mL). The
solvents were removed from the filtrate under reduced pressure at
0 ꢃC. 3 was obtained as a slightly yellowish oil of low viscosity in
2: 1H NMR (300.1 MHz, C6D6/Hex (20:1)):
d
¼ 2.00 (bs, 6H,
CpMe2), 2.10 (bs, 6H, CpMe2), 7.04e7.12 (m, 6H, Ph), 7.43e7.48 (m,
4H, Ph) ppm.
31P-NMR (121.5 MHz, C6D6/Hex (20:1)):
d
¼ ꢂ27.8 ppm.
92% yield (650 mg, 3.57 mmol).
4.1.3. Synthesis of (Me2N)2PCp# (2)
1H-NMR (300.1 MHz, C6D6): 3a:
d
¼ 1.06 (d, JHP ¼ 2.9 Hz, 6H,
2
LiCp# (520 mg, 4.06 mmol) was suspended in diethylether/
hexane (1:1, 30 mL). At 0 ꢃC PCl(NMe2)2 (553 mg, 3.58 mmol) was
added dropwise and the resulting reaction mixture was warmed to
room temperature over a period of 16 h. The volume of the liquid
phase was reduced to half in vacuo, after which the solid was
separated by filtration and washed with pentane (2 ꢄ 8 mL). All
volatiles were removed from the filtrate under reduced pressure
yielding a white solid. After drying in vacuo 2 was obtained in 70%
yield (601 mg, 2.50 mmol) as a colourless solid. At first 2a was
exclusively formed. After keeping 2 at 100 ꢃC for 24 h in d8-toluene,
a mixture of 2a and 2b was obtained with a ratio of 2a:2b ¼ 40:60,
which did not change upon storage at room temperature or further
PMe2),1.17 (s, 9H, C(CH3)3), 2.88 (m, 2H, CH2), 6.03 (m,1H, HCp), 6.80
2
(m, 1H, HCp) ppm. 3b:
9H, C(CH3)3), 2.95 (m, 2H, CH2), 6.11 (m, 1H, HCp), 6.55 (m, 1H, HCp
d
¼ 1.08 (d, JHP ¼ 2.9 Hz, 6H, PMe2), 1.10 (s,
)
2
ppm. 3c:
d
¼ 1.13 (d, JHP ¼ 2.8 Hz, 6H, PMe2), 1.16 (s, 9H, C(CH3)3),
2.84 (m, 2H, CH2), 6.23 (m, 1H, HCp), 6.32 (m, 1H, HCp) ppm. 3d:
2
2
d
¼ 0.86 (d, JHP ¼ 4.5 Hz, 3H, PMe2), 0.88 (d, JHP ¼ 4.5 Hz, 3H,
PMe2), 1.10 (s, 9H, C(CH3)3), 3.20 (m, 1H, HCp), 6.04 (m, 1H, HCp), 6.38
(m, 1H, HCp), 6.60 (m, 1H, HCp) ppm.
13C-NMR (75.5 MHz, C6D6): 3a:
d
¼ 14.6 (d, 1JCP ¼ 12.7 Hz, PMe2),
30.0 (s, C(CH3)3), 32.5 (s, C(CH3)3), 42.0 (d, 3JCP ¼ 8.7 Hz, CH2), 125.7
2
2
(d, JCP ¼ 3.1 Hz, CHCp), 137.2 (d, JCP ¼ 20.0 Hz, CHCp), 150.6 (d,
1JCP ¼ 16.1 Hz, ipso-CCp), 157.5 (d, JCP ¼ 5.7 Hz, CC(CH3)3) ppm. 3b:
3
heating at 100 ꢃC.
d
¼ 14.7 (d, 1JCP ¼ 12.8 Hz, PMe2), 31.0 (s, C(CH3)3), 33.6 (s, C(CH3)3),
4
2
1H-NMR (300.1 MHz, d8-Tol): 2a:
d
¼ 1.78 (d, JHP ¼ 2.4 Hz, 6H,
41.3 (d, JCP ¼ 9.9 Hz, CH2), 125.0 (d, JCP ¼ 6.0 Hz, CHCp), 136.9 (d,
3
1
2,5-CpMe2), 1.92 (s, 6H, 3,4-CpMe2), 2.59 (d, JHP ¼ 8.6 Hz, 12H,
JCP ¼ 18.9 Hz, CHCp), 146.6 (d, JCP ¼ 15.3 Hz, ipso-CCp), 162.4 (d,
4
1
NMe2), 3.38 (bs, 1H, 1-HCp) ppm. 2b:
d
¼ 1.24 (d, JHP ¼ 5.4 Hz, 3H,
3JCP ¼ 3.1 Hz, CC(CH3)3) ppm. 3c:
d
¼ 13.5 (d, JCP ¼ 12.8 Hz, PMe2),
5-CpMe), 1.72 (s, 3H, 3-CpMe), 1.82 (s, 3H, 4-CpMe), 1.93 (s, 3H, 2-
30.2 (s, C(CH3)3), 33.5 (s, C(CH3)3), 41.6 (d, 2JCP ¼ 5.5 Hz, CH2), 125.1
CpMe), 2.66 (d, 3JHP ¼ 9.1 Hz, 12H, NMe2), 2.86 (m, 1H, 5-HCp) ppm.
(d, JCP ¼ 15.0 Hz, CHCp), 132.9 (d, JCP ¼ 14.4 Hz, CHCp), 148.4 (d,
13C-NMR (100.7 MHz, d8-Tol): 2a:
d
¼ 11.6 (s, 2,5-CpMe2), 12.6
1JCP ¼ 14.4 Hz, ipso-CCp), 160.3 (d, JCP ¼ 4.5 Hz, CC(CH3)3) ppm. 3d:
3
(s, 3,4-CpMe2), 41.7 (d, JCP ¼ 16.0 Hz, NMe2), 57.1 (d, 1JCP ¼ 21.1 Hz,
d
¼ 12.2 (d, 1JCP ¼ 18.6 Hz, PMe2), 12.4 (d, 1JCP ¼ 18.7 Hz, PMe2), 31.1
2
2
3
ipso-CCp), 133.7 (d, JCP ¼ 9.2 Hz, 2,5-Cp), 137.9 (d, JCP ¼ 5.4 Hz,
(s, C(CH3)3), 32.5 (s, C(CH3)3), 54.2 (d, 1JCP ¼ 19.9 Hz, ipso-CCp), 124.3
(d, JCP ¼ 5.2 Hz, CHCp), 132.6 (d, JCP ¼ 3.6 Hz, CHCp), 135.0 (d,
JCP ¼ 4.8 Hz, CHCp) ppm. (The resonance due to CC(CH3)3 of the
isomer 3d is overlapped by the resonance arising from CC(CH3)3 of
the isomer 3a (157.5 ppm), but could be identified by two dimen-
sional NMR experiments.)
3,4-Cp) ppm. 2b:
d
¼ 10.9 (s, 3-CpMe), 11.9 (s, 4-CpMe), 13.4
3
3
(d, JCP ¼ 1.4 Hz, 2-CpMe), 17.2 (d, JCP ¼ 9.7 Hz, 5-CpMe), 41.7
2
2
(d, JCP ¼ 15.8 Hz, NMe2), 42.4 (d, JCP ¼ 16.2 Hz, NMe2), 53.4
2
3
(d, JCP ¼ 9.4 Hz, 5-Cp), 135.4 (d, JCP ¼ 2.8 Hz, 3-Cp), 140.8
1
2
(d, JCP ¼ 3.0 Hz, ipso-Cp), 144.5 (d, JCP ¼ 5.8 Hz, 4-Cp), 146.5
(d, 3JCP ¼ 17.6 Hz, 2-Cp) ppm.
31P-NMR (121.5 MHz, C6D6): 3a:
(33%) ppm. 3c:
d
¼ -57.2 (46%) ppm. 3b: ¼ ꢂ57.5
d
31P-NMR (121.5 MHz, d8-Toluol): 2a:
¼ 100.0 ppm.
d
¼ 100.6 ppm. 2b:
d
¼ ꢂ59.7 (13%) ppm. 3d: ¼ ꢂ41.9 (8%) ppm.
d
d
EI/MS (70 eV): m/z (%) ¼ 182 (45) [Mþ], 167 (100), 126 (16), 111
(18), 57 (23).HR-EI/MS (70 eV): Calc. for C11H19P: 182.1224. Found:
182.1224.
EI/MS (70 eV): m/z (%) ¼ 240 (8) [Mþ], 196 (22.0), 153 (7), 119
(100), 105 (4), 76 (31).HR-EI/MS (70 eV): Calc. for C13H25N2P:
240.1755. Found: 240.1790.
Anal. Calc. for C11H19P (182.24): C, 72.50; H, 10.51. Found:
C, 68.19; H, 9.85.
Anal. Calc. for C13H25N2P (240.32): C, 64.97; H, 10.49; N, 11.66.
Found: C, 65.00; H, 10.53; N, 11.29.
IR: 2961 (w), 2912 (w), 2885 (m), 2828 (m), 2781 (w), 1439 (m),
1378 (w), 1257 (m), 1215 (m), 1186 (s), 1135 (w), 1098 (w), 1068 (w),
IR: 3054 (w), 2959 (s), 2900 (m), 1591 (w), 1426 (w), 1362 (m),
1260 (m), 1127 (m), 1064 (m), 1018 (m), 938 (m), 903 (s), 881 (s),
809 (s), 752 (w), 709 (m), 595 (m), 462 (w) cmꢂ1
.

2006
C. Lichtenberg et al. / Journal of Organometallic Chemistry 695 (2010) 2000e2006
[13] K.A. Rufanov, B. Ziemer, M. Hummert, S. Schutte, Eur. J. Inorg. Chem. (2004)
4.2. X-ray crystallographic studies
4759e4763.
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Dalton Trans. (2008) 909e915.
4.2.1. Crystallography
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45 (2006) 4562e4570.
[16] For a comprehensive review about the fluxional behaviour of cyclopentadienyl
compounds of main group elements including cyclopentadienylphosphanes
see: P. Jutzi Chem. Rev. 86 (1986) 983e996.
Crystal data were collected with a Stoe-IPDSIII area-detector
diffractometer using graphite-monochromatised Mo-Ka-radiation
(l
¼ 71.073 pm) at 100 K. Data reduction was carried out by using
the IPDS software X-Area (Stoe) [57]. The data were empirically
corrected for absorption and other effects by using multiscans [58].
The structures were solved by direct methods (Sir-97) [59] and
refined by full-matrix least-squares techniques against Fo2 (SHELXL-
97) [60]. The programs PLATON [61] and PLUTON [62] were used to
check the results of the X-ray analysis. Diamond was used for the
structure representation [63]. CCDC-760488 (2a) contains the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Polyhedron 9 (1990) 2901e2909.
Storage of an oversaturated solution of 2a in hexane at -30 ꢃC
yielded colourless single crystals of this substance.
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153e160.
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4.2.2. Crystal data for 2a
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K.A. Lyssenko, A.V. Tsvetkov, I.P. Beletskaya, J. Organomet. Chem. 690
(2005) 1710e1717.
C13H25N2P, Mr ¼ 240.32, triclinic, P1, a ¼ 5.7450(4) Å, b ¼ 9.3979
(7) Å, c ¼ 13.0274(10) Å,
a
¼ 89.456(6)ꢃ,
b
¼ 89.310(6)ꢃ,
g
¼ 88.029
(6)ꢃ, V ¼ 702.87(9) Å3, Z ¼ 2,
r
(calc.) ¼ 1.136 g cmꢂ3
,
m
¼ 0.175 mmꢂ1
,
T ¼ 100 K. A total of 4923 reflections were collected, 2443 of which
were independent. The refinement on all data converged at
R1 ¼5.09%, wR2 ¼18.05% and the goodness of fit was 1.147. Hydrogen
atoms were included in idealised positions and refined with
isotropic displacement parameters except for the hydrogen atom H1
bonded to the Cp(ipso)-carbon atom, which was found in the final
density map and refined with isotropic displacement parameters.
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Angew. Chem. Int. Ed. 44 (2005) 2700e2704.
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B. Gautheron, J. Organomet. Chem. 598 (2000) 365e373.
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6625e6637.
Acknowledgements
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The authors would like to thank Benjamin Oelkers for helpful
discussions concerning kinetic experiments.
[40] Y.G. Gololobov, L.F. Kasukhin, Tetrahedron 48 (1992) 1353e1406.
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Appendix. Supplementary data
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Trans. (2003) 104e113.
CCDC-760488 (2a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.05.001.
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