An NMR study on the reaction of [(κ 2-P,N)-Ph 2PCH2CH2NMe2]PtMe2 with activated chlorocarbons: C-Cl and C-C activation, followed by C-N cleavage of the phosphinoalkylamine ligand and formation of an ylide complex

Article abstract of DOI:10.1007/s00706-014-1158-0

Reaction of [(κ ²-P,N)-Ph₂PCH ₂CH₂NMe₂]PtMe₂ with either ethyl chloroacetate or benzyl chloride results in complex reaction mixtures, which were investigated by a combination of NMR techniques. The reactions are initiated by oxidative addition of either the C-Cl bond or of the C-CH ₂Cl bond, respectively. The latter is followed by a rearrangement of the intermediate Pt(IV) complex with concomitant C-N bond cleavage of the phosphinoalkylamine ligand and formation of the ylide complexes Pt[CH ₂=PPh₂(vinyl)](NMe₂R)Me₂ (R = COOEt or Ph). Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-014-1158-0

Monatsh Chem
DOI 10.1007/s00706-014-1158-0
ORIGINAL PAPER
An NMR study on the reaction of [(j2-P,N)-
Ph2PCH2CH2NMe2]PtMe2 with activated chlorocarbons: C–Cl
and C–C activation, followed by C–N cleavage
of the phosphinoalkylamine ligand and formation of an ylide
complex
•
Christian Martschitsch Ulrich Schubert
Received: 5 November 2013 / Accepted: 10 January 2014
Ó Springer-Verlag Wien 2014
Abstract Reaction of [(j2-P,N)-Ph2PCH2CH2NMe2]
PtMe2 with either ethyl chloroacetate or benzyl chloride
results in complex reaction mixtures, which were investi-
gated by a combination of NMR techniques. The reactions
are initiated by oxidative addition of either the C–Cl bond
or of the C–CH2Cl bond, respectively. The latter is fol-
lowed by a rearrangement of the intermediate Pt(IV)
complex with concomitant C–N bond cleavage of the
phosphinoalkylamine ligand and formation of the ylide
complexes Pt[CH2=PPh2(vinyl)](NMe2R)Me2 (R = COO-
Et or Ph).
bond activation by Pt complexes see Refs. [7–9]). The
complexes (P\N)Pt(Me)Cl (2), (P\N)Pt(Me)3Cl (3) and
(P\N)Pt(CCl=CH2)Cl were isolated from the reaction
with CCl4 (Fig. 1). The vinyl complex is formed by
oxidative addition of CCl4, followed by MeCl elimina-
tion and rearrangement of the thus obtained complex
(P\N)Pt(Me)CCl3 to (P\N)Pt(CCl2Me)Cl, from which
HCl is eliminated. Oxidative addition of the eliminated
MeCl to 1 gives complex 3, whereas complex 2 results
from addition of HCl to 1 followed by CH4 elimination
or by ethane elimination from 3 (Fig. 1). It was found
that reactivity towards
1 decreased in the order
Keywords Platinum complexes ꢀ Ylide complexes ꢀ
Phosphinoalkylamine ligands ꢀ C–Cl activation ꢀ
C–C activation
CCl4 [ CHCl3 [ CH2Cl2 [ CH3Cl. The mechanism of
these reactions was also investigated by density func-
tional theory (DFT) calculations [10].
These reactions prompted us to investigate the
reactions of compounds with activated C–Cl bonds,
such as ethyl chloroacetate or benzyl chloride. These
chlorides were chosen because b-elimination is not
possible once C–Cl oxidative addition has occurred.
The results reported in this article show that apart
from the reactions leading to 1–3, a new reaction
occurs (Scheme 1) which involves cleavage of the
P\N ligand.
Introduction
We have previously pointed out that the reactivity of Pt(II)
complexes towards organosilanes ([1] and references cited
therein) or chlorocarbons [2, 3] is greatly enhanced when
heterofunctional chelating ligands R2N-R0-PPh2 [4–6] are
employed. It was shown, inter alia, that the complex
(P\N)PtMe2 (1, P\N = (j2-P,N)-Ph2PCH2CH2NMe2)
reacts with CHxCl4-x by initial oxidative addition of the
C–Cl bond (for selected examples of (non-activated) C–Cl
Scheme 1
C. Martschitsch ꢀ U. Schubert (&)
Institute of Materials Chemistry, Vienna University of
Technology, Getreidemarkt 9, 1060 Wien, Austria
e-mail: ulrich.schubert@tuwien.ac.at
123

C. Martschitsch, U. Schubert
1H NMR investigations
The main metal-containing product was complex 2. In the
1:1 reaction with ethyl chloroacetate, ethyl acetate was
identified as an organic product after complete con-
sumption of complex 1 (hydrodehalogenation). Neither
the methylation product ethyl propionate nor dehaloge-
native coupling product diethyl succinate was observed
by either NMR or GC/MS investigations. The proportion
of ethyl acetate (molar ratio of 1:8, as calculated from the
Fig. 1 Complexes 1–3
Results and discussion
The reactions of 1 with ethyl chloroacetate or benzyl
chloride at 70 °C in a 1:1 molar ratio were relatively slow.
With benzyl chloride, only about 10 % of 1 was consumed
after 10 days at 70 °C, whereas the reaction with ethyl
chloroacetate was finished after 3 days at the same tem-
perature. Reaction was faster when an excess of the
chloride was employed; this did not influence the product
distribution. For example, only 10 % of 1 was consumed
after 10 days in the reaction with one molar equivalent of
benzyl chloride, whereas reaction with a 15-fold excess of
benzyl chloride was finished after 1 day (disappearance of
the NMR signals of 1).
1
integrated signals in the H NMR spectrum), however,
was too low relative to complex 2. Closer examination of
1
the H NMR spectra revealed two additional features:
1. A singlet at 0.16 ppm was assigned to methane. The
only reaction that could yield methane is the reaction
of a methyl complex (P\N)Pt(Me)Z (any group Z)
with HCl and concomitant formation of (P\N)Pt(Cl)Z.
The presence of HCl contaminations in the starting
compounds can be ruled out, because of the high
proportion of the protonated products and the fact that
they are only slowly formed. A control experiment
carried out between complex 1 and HCl showed that 1
is converted to 2 within seconds even at room
temperature. The possible mechanism leading to HCl
formation will be elucidated later in the article.
When monitoring the reaction by NMR spectroscopy it
was observed that a variety of intermediates were formed.
Because of the slow reaction, it was possible to identify
these intermediates by a combination of various NMR
techniques. To this end the reactions were carried out in
C6D6, and NMR spectra of the reaction mixture were
recorded at regular intervals. No attempts were made to
isolate the products. Apart from the organic by-products,
the outcome of the reactions with ethyl chloroacetate or
benzyl chloride was very similar, indicating that the same
sequence of reactions takes place. In the following, the
NMR experiments with ethyl chloroacetate will be dis-
cussed in detail, followed by a brief section on the benzyl
chloride experiments. Identification of the intermediates, as
discussed in the following, allowed the likely mechanism
of the overall reaction to be deduced as will be outlined in
the ‘‘Conclusions’’ section.
2. Two groups of signals were observed in the olefinic
region of the spectra (Fig. 2). Group 1 and 3 represent
two protons (according to the integration values)
which couple with each other. The signal groups 3
and 4 in Fig. 2 consisted of two doublets of doublets
indicative of a vinylic CH group. The phosphorus-
1
decoupled H NMR spectrum showed that the addi-
tional splitting of the signals is due to the phosphorus
atom of the P\N ligand. In a P/H-HMBC experiment,
the vinylic protons correlate with a signal at 21.6 ppm
in the 31P NMR spectrum as well as with two signals in
the aromatic region of the 1H NMR spectrum. The 31
P
signal also correlates with two Pt–Me groups.
Fig. 2 Vinylic region of the
1
500 MHz H NMR spectrum of
the reaction solution between 1
and ethyl chloroacetate
123

An NMR study on the reaction of [(j2-P,N)-Ph2PCH2CH2NMe2]PtMe2 with activated chlorocarbons
Fig. 3 31P NMR signals of octahedral Pt complexes formed by reaction of 1 with ethyl chloroacetate
31P NMR investigations
To gain deeper insight in the reactions taking place another
experiment with two equivalents of ethyl chloroacetate was
carried out. The mixture was only heated to 70 °C for 1 h to
investigate the reaction at an early stage, where the 31P NMR
signals of the different Pt complex intermediates can be
Fig. 4 Complexes 4 and 5
observed. These signals can be divided into three groups,
which will be discussed separately in the following: the
couples with proton signals in the 1H NMR spectrum at 0.53
region of 0–12 ppm typical of octahedral complexes, the
region of 26–40 ppm typical of square planar complexes and
the peak at 21.2 ppm. The last of these is the only peak which
is also observed at the end of the reaction.
and 2.04 ppm and another signal at 2.98 ppm. The resonance
at 0.53 ppm apparently belongs to a CH3 group out of the P–
Pt–N plane; the other signal at 2.04 ppm has a Pt–P coupling
constant of 3JPtH = 70 Hz and is therefore in the trans position
to the nitrogen of the P,N ligand. The peak at 2.98 ppm was
assigned to a CH2 group. The ligand in the trans position to the
phosphorus can definitely not be chlorine, because this would
shift the 31Pt signal upfield relative to that of complex 3. Also
JPtP of 1,593 Hz is remarkably high compared to JPtP of
1,208 Hz in 4, where the chlorine is located trans to the
phosphorus. A possible explanation for these unusual obser-
vations could be that the H2ClC–COOEt bond was activated,
yieldingcomplex5. This interpretation wasalso supported by a
C/H-HMBC measurement, where no coupling was observed
between the protons of the CH2Cl ligand and the ethyl CH2
carbon of the COOEt group (this coupling is observed in ethyl
chloroacetate). Oxidative addition of a CCO–C bond to a Rh
centre was, for example, described by Suggs and Jun [13].
Octahedral complexes
The 31P NMR spectrum showed three signals with plati-
num satellites in the region of octahedral Pt complexes
(0–12 ppm, complexes A, B and C, Fig. 3).
Complex A is 3, formed by oxidative addition of CH3Cl to
1, as previously reported [2]. P/H-HMBC measurement
showed that the 31P NMR signals of complex B couple with
protons at 0.41 and 1.68 ppm. According to an APT NMR
measurement, the two ligands of complex B are CH3 groups.
The chemical shift indicated that the signal at 0.41 ppm is a
CH3 group out of the P–Pt–N plane. The peak at 1.68 ppm has
a coupling constant 3JPtH = 73 Hz, typical for a CH3 ligand in
the trans position to the nitrogen of theP,N ligand(as in 3). The
corresponding 31P NMR signal is shifted upfield relative to
complex 3; this is typical for replacing the ligand trans to
phosphorus by a group with lower trans influence [11]. Since
there is obviously no organic ligand in this position, this ligand
could be chlorine, which has a very weak trans influence [12].
Therefore, complex B appears to be (P\N)PtMe2Cl2 (4),
formed by reaction of 3 with HCl (Fig. 4).
Square-planar complexes
Three compounds were observed in the range of planar
Pt(II) complexes in the 31P NMR spectrum (Fig. 6), viz. the
known complexes 1 and 2 and a new complex 6.
N
P
CH2COOEt
Me
Complex C is visible in the two-dimensional NMR spec-
trum (Fig. 5) at dPt = -1,410 ppm. P/H-HMBC
measurement showed that the 31P NMR signal of complex C
Pt
123

C. Martschitsch, U. Schubert
Fig. 5 Pt/H correlation NMR
1
of the region showing the H
signals of octahedral complexes
Fig. 6 31P NMR signals in the
shift range of the planar
complexes in the reaction
mixture of 1 with ethyl
chloroacetate
Fig. 7 Pt/H correlation
measurement of the planar
complexes formed in the
reaction of 1 with ethyl
chloroacetate
A Pt/H correlation spectrum was measured (Fig. 7) to
determine the composition of 6. Two new ligand signals
appeared at 1.31 and 2.51 ppm. According to APT
NMR, the first is a methyl group, the second a CH2
group (resulting in a larger JPtP of 2,742 Hz compared
to 2,070 Hz in 1). In a 2D C/H spectrum the CH2 group
shows correlations with COO, CH2 and CH3 groups.
Therefore, this signal appears to belong to an ethoxy-
carbonylmethyl ligand, and the composition of 6 is
(P\N)Pt(Me)(CH2COOEt), formed by methyl chloride
elimination from the octahedral intermediate (P\N)
Pt(Me)2(CH2COOEt)Cl.
Complex 6 apparently does not undergo oxidative
addition of another ethyl chloroacetate, because diethyl
123

An NMR study on the reaction of [(j2-P,N)-Ph2PCH2CH2NMe2]PtMe2 with activated chlorocarbons
Fig. 8 a Isotope pattern of the
unknown signal and
b calculated pattern of
C20H26NO2PPt ? H?
succinate (formed by reductive elimination) was not
observed in the reaction mixture. Reaction with HCl,
yielding complex 2 and ethyl acetate, is the main reaction
pathway for 6 instead.
chemical shifts reported in the literature for ylide complexes
[14, 15]. Mass spectrometry with chemical ionisation pro-
vided the final proof for the formation of an ylide complex.
Mass spectrometry investigations
The resonance at 21.2 ppm
The experiments discussed in this article show that the HCl-
producing reaction yields a complex containing a coordinated
amino group as well as a vinyl group bonded to a phosphorus
atom. The 31P NMR signal at 21.2 ppm corresponds to
This method was chosen to generate complex ions with low
fragmentation, because some fragments of the other complexes
formed in the reaction are identical. The measurements were
performed on the reaction mixtures of complex 1 with ethyl
123

C. Martschitsch, U. Schubert
chloroacetate in a molar ratio of 1:2. The mixture was heated
until 1, 2 and the possible ylide complex were observed in the
31P NMR spectrum. The crude mass spectrum showed four
signal groups: complexes 1 and 2, complex 1 less one CH3
group, and a group of unknown signals, possibly belonging to
the ylide complex. A background file, containing the spectrum
of the complexes 1 and 2, was subtracted from the spectrum to
reveal the isotope pattern of the unknown signal (Fig. 8).
For the complex from the reaction with ethyl chloro-
acetate, the elemental composition C20H26NO2PPt was thus
determined. Considering all facts from the NMR spectra
(especially the splitting of the vinylic protons in Fig. 2), the
signal appears to belong to a doubly demethylated frag-
ment of the ylide complex Pt[CH2=PPh2(vinyl)]
(NMe2COOEt)Me2 (7, R = COOEt).
Scheme 2
C–Cl bonds and associated consecutive reactions,
which were already observed for non-activated C–Cl
bonds, new reactions were observed which can be
explained by alternative oxidative addition of the
C–CH2Cl bond.
RMe2N
Me
Pt
CH2
P
On the basis of these results, the following sequence
of reactions is postulated. The HCl-producing reaction
is due to initial C–C bond activation, i.e. oxidative
addition of the R–C bond of R–CH2Cl (R = Ph or
COOEt) to the Pt atom. The resultant CH2X ligand of
the (observed) complex 5 is then attacked by the
phosphorus atom of the chelating ligand resulting in a
cyclic phosphonium intermediate, followed by migration
of the group R from the metal to the amino group and
abstraction of a proton (by the cleaved chloride) from
the CH2 group next to the nitrogen atom (Scheme 2) to
result in the observed ylide complex 7. To the best of
our knowledge this sequence of reactions, resulting in
the cleavage of the P\N ligand, has not been observed
before. Reaction of the phosphane with the CH2X ligand
might be related to the reaction of Pt(PPh3)4 with
CH2ClI which results in the formation of the cationic
ylide complex cis-Pt(CH2PPh3)Cl(PPh3)2 [15].
Me
Ph2
Reaction of benzyl chloride
The analogous products were observed during the (slower)
reaction of 1 with benzyl chloride. The only differences
were (i) that the analogue of intermediate 5 was not
observed spectroscopically, i.e. its reaction to give 7 is
apparently faster, and (ii) that both dibenzyl and toluene
1
were identified as organic products in the H NMR spec-
trum of the reaction solution. Dibenzyl is the product of
dehalogenative coupling, and toluene that of hydrode-
halogenation. Thus contrary to complex 6, the
corresponding complex (P\N)Pt(Me)CH2Ph can also
undergo oxidative addition of a second benzyl chloride
molecule, followed by elimination of dibenzyl and for-
mation of 2, in addition to the reaction with HCl, yielding
complex 2 and toluene. Since there is no doubt about the
formation of 7, it is safe to assume the same mechanisms as
in the case of ethyl chloroacetate (with different reaction
rates). The mass spectrum of the reaction with benzyl
chloride was only slightly different from that of the reac-
tion with ethyl chloroacetate. It led to the elemental
composition C23H26NPPt and assignment to the demethy-
lated fragment of the ylide complex Pt[CH2=PPh2
(vinyl)](NMe2Ph)Me2 (7, R = Ph).
During this reaction HCl is developed, which can either
react with the starting complex 1 to give 2 directly, the
trimethyl complex 3 to produce methane and 4 (both
compounds were identified), or with complex 6 through
formation of RCH3 (toluene or ethyl acetate).
The starting complex 1 also reacts with RCH2Cl by
activation of the C–Cl bond, as previously reported [2, 3].
The formed octahedral intermediate 8 eliminates CH3Cl
which subsequently reacts with complex 1 to form 3
(Scheme 3). As judged by the observed organic products, 2
can be formed from 6 either by addition of HCl and
elimination of RCH3 (R = Ph or COOEt) or, in the reac-
tion of benzyl chloride, also by oxidative addition of a
second PhCH2Cl molecule, followed by dibenzyl
elimination.
Conclusions
Reaction of (P\N)PtMe2 with either benzyl chloride
or ethyl chloroacetate resulted in complex reaction
mixtures. In addition to the oxidative addition of the
123

An NMR study on the reaction of [(j2-P,N)-Ph2PCH2CH2NMe2]PtMe2 with activated chlorocarbons
Scheme 3
3
Experimental
CH3-Pt, 4), 1.89 (d, with satellites, JPH = 7.0 Hz,
2JPtH = 72 Hz, CH3-Pt, 3), 2.04 (d, with satellites,
2
All compounds were handled under Ar atmosphere using
standard Schlenk techniques. All solvents were dried by
standard methods. The NMR solvents were stored over
3JPH = 5.5 Hz, JPtH = 71 Hz, CH3-Pt, 8), 2.51 (s, with
satellites, 3JPtP = 11.9 Hz, CH3-N, 2), 2.74 (s,
PhCH2CH2Ph), 2.98 (m, CH2-Pt, 8), 5.74, 6.27 and 6.34
(see Fig. 2, 3JPH = 23.8 Hz, P–CH=CH2, 7), 7.04 and 7.53
˚
molecular sieve (4 A). The NMR tubes were treated with
dichlorodimethylsilane to prevent the SiOH groups from
reacting with the educts and the complexes.
(m, Carom, 2) ppm; 31P NMR: d = 1.7 (s with satellites,
1JPtP = 1,208 Hz, 4), 8.8 (s with satellites, JPtP
=
1
1
NMR experiments were performed on Bruker Avance
250 and 300 MHz spectrometers and on a Bruker Avance
III 500 MHz spectrometer at room temperature. 13C, 31P
and 195Pt NMR spectra were proton decoupled. All pulse
programs were obtained from the Bruker software library.
Mass spectra were measured on a Thermo Scientific ITQ
1100 with chemical ionisation. Signals of unreacted educts
are not listed.
1,183 Hz, 3), 11.3 (s with satellites, JPtP = 1,587 Hz, 8),
21.6 ppm; 13C NMR: d = -9.96 (CH3-Pt, 4), -8.61
(CH3-Pt, 3), -7.83 (CH3-Pt, 8), 17.29 (CH3-Pt, 3), 27.61
(CH3-Pt, 6), 29.87 (CH2-Pt, 8) ppm; 195Pt NMR: d =
-4,173 (6), -2,977 (3), -3,030 (8) ppm.
Reaction with benzyl chloride
Benzyl chloride (2.7 mg, 0.021 mmol) was added at room
temperature to 10 mg of 1 (0.021 mmol) in 0.6 cm3 of
C6D6. NMR spectra were recorded at regular intervals. The
following NMR data are from the reaction mixture while
the reaction was still on-going. Therefore, not all com-
pounds discussed in the ‘‘Results and discussion’’ section
are represented. 1H NMR: d = 0.16 (s, CH4), 0.80 (s,
Reaction with ethyl chloroacetate
Ethyl chloroacetate (5.1 mg, 0.042 mmol) was added at
room temperature to 10 mg of 1 (0.021 mmol) in 0.6 cm3
of C6D6. NMR spectra were recorded at regular intervals.
The following NMR data are from the reaction mixture
while the reaction was still on-going. Therefore, not all
compounds discussed in the ‘‘Results and discussion’’
3
CH3CH3), 1.31 (d, with satellites, JPH = 3.3 Hz,
2JPtH = 74 Hz, CH3-Pt, 2), 2.11 (s, Ph-CH3), 2.51 (s, with
satellites, CH3-N, 2), 2.74 (s, PhCH2CH2Ph), 5.74, 6.27
and 6.34 (see Fig. 2, 3JPH = 23.8 Hz, P–CH=CH2, 7), 7.04
(m, Carom, 2), 7.53 (m, Carom, 2) ppm; 31P NMR: d = 20.1
1
section are represented. H NMR: d = 0.16 (s, CH4), 0.41
(d, with satellites, JPH = 7 Hz, JPtH = 50 Hz, CH3-Pt,
3
2
3
4), 0.50 (d, with satellites, JPH = 7.8 Hz, JPtH = 71 Hz,
2
1
3
CH3-Pt, 3), 0.53 (d, with satellites, JPH = 6.7 Hz,
(s, 7), 27.1 (s with satellites, JPtP = 4,675 Hz, 2),
2JPtH = 69 Hz, CH3-Pt, 8), 0.80 (s, CH3CH3), 1.31 (d, with
21.6 ppm; GC/MS: Tstart = 80 °C, Tend = 280 °C,
rate = 10 °C/min. Time: 2.13 min (m/z = 106, 105, 93,
92, 91, C6H5CH2CH3), 12.33 min (m/z = 183, 182, 92, 91,
C6H5CH2CH2C6H5).
3
satellites, JPH = 3.3 Hz, JPtH = 74 Hz, CH3-Pt, 2), 1.47
2
3
(d, with satellites, JPH = 8 Hz, JPtH = 62 Hz, CH3-Pt,
2
3
3), 1.68 (d, with satellites, JPH = 6.8 Hz, JPtH = 73 Hz,
2
123

C. Martschitsch, U. Schubert
8. von Zelewsky A, Suckling AP, Stoeckli-Evans H (1993) Inorg
Chem 32:4585
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