Platinum-catalyzed hydroamination of ethylene: Study of the catalyst decomposition mechanism

Article abstract of DOI:10.1021/om301182x

A study of the addition of nucleophilic reagents that are also strong Bronsted bases (Et₃N, pyridine, quinuclidine, MeO^-) to nBu₄P⁺[PtBr₃(C₂H ₄)]^- (1) and trans-[PtBr₂(NHEt ₂)(C₂H₄)] (7) has provided key information on the deactivation of the hydroamination PtBr₂/Br^- catalyst, leading to metallic platinum. The addition of NEt₃ to 1 in CD ₂Cl₂ is reversible and temperature dependent; the quantitative formation of the zwitterionic complex trans-[Pt ^(-)Br₂(NEt₃)(CH₂CH₂N ⁽⁺⁾Et₃)] (9) is observed only at low temperature, whereas slow deposition of metallic platinum occurs at room temperature. The addition of NEt₃ to 7 in CD₂Cl₂ is also reversible and temperature dependent, yielding trans-[Pt^(-)Br₂(NHEt ₂)(CH₂CH₂N⁽⁺⁾Et₃)] (10) quantitatively at low temperature. At room temperature, this reaction led to the deposition of metallic platinum and to the formation of a complex identified as trans-[PtBr₂(NHEt₂)(CH₂CHNEt₂)] (11). The carbyl ligand in 11 is shown by an X-ray structural study to be between the π-bonded enamine and the σ-bonded iminiumalkyl configurations. The addition of MeONa to 7 results in the formation of the same products 11 and Pt⁰. On the basis of these results, a mechanism for the base-induced decomposition of Pt^II(C₂H₄) complexes that involves Wacker-type β-H elimination followed by intermolecular hydride transfer, ligand rearrangements, and final deprotonation is proposed. Addition of more nucleophilic N-based ligands (pyridine, quinuclidine) to 7 ultimately leads to C₂H₄ and Et ₂NH substitution rather than to metal reduction, even though evidence for a kinetically controlled nucleophilic addition to the coordinated ethylene is given by the quinuclidine system. From the reaction with pyridine, the complex cis-PtBr₂(py)₂ was isolated and structurally characterized.

Full text of DOI:10.1021/om301182x

Article
pubs.acs.org/Organometallics
Platinum-Catalyzed Hydroamination of Ethylene: Study of the
Catalyst Decomposition Mechanism
Aurelien Bethegnies,†,‡ Jean-Claude Daran,†,‡ and Rinaldo Poli*,†,‡,§
́
́
†CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
‡Universite
́
de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
§Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France
S
* Supporting Information
ABSTRACT: A study of the addition of nucleophilic reagents that are
also strong Brønsted bases (Et3N, pyridine, quinuclidine, MeO−) to
nBu4P+[PtBr3(C2H4)]− (1) and trans-[PtBr2(NHEt2)(C2H4)] (7) has
provided key information on the deactivation of the hydroamination
PtBr2/Br− catalyst, leading to metallic platinum. The addition of NEt3
to 1 in CD2Cl2 is reversible and temperature dependent; the quantita-
tive formation of the zwitterionic complex trans-[Pt(−)Br2(NEt3)-
(CH2CH2N(+)Et3)] (9) is observed only at low temperature, whereas
slow deposition of metallic platinum occurs at room temperature. The
addition of NEt3 to 7 in CD2Cl2 is also reversible and temperature
dependent, yielding trans-[Pt(−)Br2(NHEt2)(CH2CH2N(+)Et3)] (10)
quantitatively at low temperature. At room temperature, this reaction led to the deposition of metallic platinum and to the
formation of a complex identified as trans-[PtBr2(NHEt2)(CH2CHNEt2)] (11). The carbyl ligand in 11 is shown by an X-ray
structural study to be between the π-bonded enamine and the σ-bonded iminiumalkyl configurations. The addition of MeONa to
7 results in the formation of the same products 11 and Pt0. On the basis of these results, a mechanism for the base-induced
decomposition of PtII(C2H4) complexes that involves Wacker-type β-H elimination followed by intermolecular hydride transfer,
ligand rearrangements, and final deprotonation is proposed. Addition of more nucleophilic N-based ligands (pyridine,
quinuclidine) to 7 ultimately leads to C2H4 and Et2NH substitution rather than to metal reduction, even though evidence for a
kinetically controlled nucleophilic addition to the coordinated ethylene is given by the quinuclidine system. From the reaction
with pyridine, the complex cis-PtBr2(py)2 was isolated and structurally characterized.
such as [PtBr3(C2H4)]− (1), trans-[PtBr2(C2H4)(PhNH2)] (2),
INTRODUCTION
■
and cis-[PtBr2(PhNH2)2] (3),8 backed up by DFT calcu-
The intermolecular olefin hydroamination process opens access
to higher amines by atom-economical addition of an amine
N−H bond across the CC bond of the olefin and is currently
lations,8−10 led to the conclusion that 1 is the catalyst resting
state, while a thorough computational investigation clarified the
mechanistic pathway as summarized in Scheme 1. This involves
attracting considerable academic interest.1−3 This is a kineti-
olefin activation, followed by nucleophilic attack of the olefin by
cally difficult reaction, especially for simple alkenes that lack
aniline, proton transfer from the ammonium function of the
activation by electron-withdrawing groups such as acrylates,
zwitterionic intermediate 4 to the metal center, and final C−H
electronic delocalization such as styrene, or ring strain such as
norbornene. Brunet has recently disclosed the remarkable
activity of the simple platinum dihalides PtX2 (X = Cl, Br, I),
reductive elimination from the PtIV hydrido derivative 5 via the
σ complex 6.11
This catalytic system is haunted, however, by slow de-
when activated by [nBu4P]X (particularly for X = Br), for the
addition of anilines to ethylene4,5 and 1-hexene.5,6 Turnover
activation to metallic platinum, which is inactive for hydro-
amination although slightly active for amine transalkylation.12
numbers up to 115 for the desired N-ethylaniline product from
This process has been traced to the effect of Brønsted basicity,
because the presence of noncoordinating bases stronger than
ethylene and aniline were reported for operation at 25 bar of
ethylene pressure in 10 h at 150 °C (i.e., a TOF of 11.5 h−1 on
aniline such as Et3N and DBU totally deactivates the catalyst
average), the selectivity being relatively high with only trace
with the generation of massive amounts of Pt0. Although de-
amounts of the double-hydroamination product, PhNEt2, and ca.
activation by Brønsted basicity is often considered indicative of
10 turnovers for the formation of 2-methylquinoline (quinal-
Brønsted acid catalysis (typically related to the hydroamination
dine) as a byproduct. The process could also be extended to
of more reactive olefins),13 this is not the case here because
aqueous biphasic conditions, using NaBr as activator and K2PtCl4
as precatalyst,7 and tolerates the presence of air. Isolation and
equilibrium studies of simple platinum complexes containing cata-
lytically relevant species (ethylene, aniline, bromide) as ligands,
Received: December 7, 2012
Published: January 14, 2013
© 2013 American Chemical Society
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involving deprotonation followed by loss of a hydride from the
β position of the aminoalkyl ligand, was proposed as illustrated in
Scheme 2. However, the reason for the spontaneous hydride loss and
the detailed mechanism of this transformation were not clarified.
Scheme 1. Mechanism of the Aniline Addition to Ethylene
Catalyzed by PtBr2/Br−
Scheme 2. Proposed Mechanism for the Decomposition of
trans-[PtCl2(NHRR′)(CH2CH2NHRR′)]37
control experiments show the complete inactivity of strong
Brønsted acids for the aniline addition to ethylene.12 The
basicity causes the catalyst reduction to Pt0 in this case. Stability
tests in the presence of only one of the two reagents (ethylene
alone or aniline alone) did not result in the formation of any
metallic Pt,14 indicating that the catalyst decomposition occurs
only after the formation of the zwitterion 4 in the catalytic cycle.
Since the intermediate 4 was not experimentally observable
for the catalytic system in agreement with the DFT study, a
model study was carried out with a more basic amine, Et2NH,
which is too basic to yield any hydroamination because it leads
to rapid catalyst decomposition.12 The desired zwitterion could
indeed be observed after addition of Et2NH to 1, after an initial
ligand exchange step yielding trans-[PtBr2(Et2NH)(C2H4)] (7).
A second Et2NH molecule attacked the coordinated olefin in an
equilibrium process with formation of trans-[Pt(−)Br2(Et2NH)-
Another phenomenon is of relevance to our observed catalyst
deactivation. Whereas alkoxyalkyl complexes with the [PtCl-
(N−N)(CH2CH2OR)] (R = alkyl) stoichiometry, obtained by
nucleophilic addition of RO− to coordinated ethylene, are
stable,22,38−40 those containing 2,9-Me2phen were recently re-
ported by Fanizzi et al. to spontaneously decompose to give Pt0
and the corresponding vinyl ethers, CH2CHOR (R = Me, Bu),
via a proposed Wacker-type mechanism involving β-H elimination
(see Scheme 3), which was supported by mass spectrometric and
Scheme 3. Proposed Mechanism for the Decomposition of
[PtCl(2,9-Me2phen)(CH2CH2OR)]22
1
(CH2CH2N(+)HEt2)] (8), which could be characterized by H
NMR and IR spectroscopy. The NMR study gave further evi-
dence of a thermally accessible equilibrium deprotonation by the
excess Et2NH to yield a trans-[PtBr2(Et2NH)(CH2CH2NEt2)]−-
[Et2NH2]+ ion pair intermediate.15 Subsequent monitoring of the
solution at room temperature led to slow Pt0 precipitation.
The nucleophilic addition to olefins coordinated to PtII has
been intensively investigated.16−23 In particular, amines (am)
add to [PtCl2Y(C2H4)] (Y = Cl−, DMSO, amine, phosphine) to
yield [Pt(−)Cl2Y(CH2CH2am(+))] which then may evolve in
different ways depending on the nature of the amine and Y.24−36
For complexes of the type trans-[PtCl2(C2H4)(am)], the ob-
servation of the zwitterionic product, which included an X-ray
crystallographically characterized example,26 was limited to amines
of high basicity, in agreement with our observations for the
bromide analogues. Occasionally, slow decomposition of these
products with deposition of metallic platinum was reported.
One specific contribution by Balacco and Natile (henceforth
abbreviated as BN)37 reported the 1H NMR characterization of
the product that results from the decomposition of trans-
[Pt(−)Cl2(NHRR′)(CH2CH2N(+)HRR′)], which was obtained
from the addition of secondary amines NHRR′ (R, R′: cyclo-
(CH2)5; Et, Et; Me, CHMePh) to [PtCl3(C2H4)]−. The reaction
led to the formation, in addition to metallic platinum, of the
complexes trans-[PtCl2(NHRR′)(CH2CHNRR′)] according to
the stoichiometry of eq 1. A mechanism for this transformation,
NMR evidence of the [PtCl(H)(2,9-Me2phen)] intermediate.41
The reason advanced by the authors for this reduced stability is
the steric destabilization of the square-planar configuration by
the two phenanthroline methyl substituents, promoting chelate
ring opening and triggering the β-H elimination process. The
observed Pt0 product obviously results from the action of a
base on the PtII hydride intermediate. This observation
suggested us the possibility that β-H elimination may also
cause the decomposition of our catalytic system, as well as that
of the aforementioned aminoalkyl complexes (Scheme 2).
In this contribution, we present our studies on the decom-
position of [PtBr3(C2H4)]− in the presence of amines (NEt3,
py, quinuclidine) and the methoxide anion (MeO−), providing
additional insight into the decomposition pathway of the
hydroamination catalyst.
EXPERIMENTAL SECTION
2trans‐[PtCl2(NHRR′)(CH2CH2NHRR′)]
■
General Considerations. All operations were carried out in
Schlenk glassware under an argon atmosphere. CH2Cl2 was of HPLC
grade and was distilled on CaH2 under argon. CD2Cl2 for the NMR
studies was obtained from Eurisotop as flame-sealed 0.6 mL ampules.
→ trans‐[PtCl2(NHRR′)(CH2CHNRR′)] + Pt
+ C2H4 + 2NH2RR′+Cl−
(1)
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NEt3 (99+%, Acros Organics), HNEt2 (Fluka), and C5H5N (Jansen)
were distilled and kept under argon in the dark. MeONa (Fluka) was
dehydrated and stored in a Schlenk tube under argon. Quinuclidine
was purified by recrystallization according to the literature process.42
PtBr2 (99.9%, Aldrich), nBu4PBr (98%, Acros Organics), and Ph4PBr
(98+%, Avocado) were used as received. The complexes Ph4P[PtBr3-
(C2H4)], nBu4P[PtBr3(C2H4)], and trans-[PtBr2(C2H4)(HNEt2)] were
synthesized according to the literature process.8,14 Ethylene (purity
≥99.5%) was purchased from Air Liquide.
After a series of vacuum/argon cycles, 0.6 mL of CD2Cl2 was added at
−80 °C, dissolving the platinum complex. Since no reaction was ob-
served over 3 h at −80 °C, the tube was warmed to room temperature
and monitored.
X-ray Crystallography. A single crystal of each compound was
mounted under inert perfluoropolyether at the tip of a cryoloop and
cooled in the cryostream of either an Agilent Technologies Xcalibur
CCD diffractometer for [PtBr2(NC5H5)2] or a Bruker Apex2 CCD
diffractometer for 11. Data were collected using monochromatic Mo
Kα radiation (λ = 0.71073 Å). The structures were solved by direct
methods (SIR97)43 and refined by least-squares procedures on F2
using SHELXL-97.44 In the compound PtBr2(NC5H5)2 all H atoms
attached to carbon were introduced in idealized positions and treated
as riding on their parent atoms in the calculations. In compound 11,
the H atoms attached to the Pt-bonded carbon atom could not be
located and were not calculated. The other atoms were introduced in
idealized positions and treated as riding on their parent atoms in the
calculations. The drawings of the molecules was realized with the help
of ORTEP3.45,46 Crystal data and refinement parameters are shown in
Table S1 (Supporting Information). Crystallographic data (excluding
structure factors) have been deposited with the Cambridge Crystallo-
graphic Data Centre as Supplementary Publication Nos. CCDC 914166
and 914167. Copies of the data can be obtained free of charge on
application to the Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, U.K. (fax, (+44) 1223-336-033; e-mail, deposit@ccdc.cam.ac.uk).
Instrumentation. The 1H NMR investigations were carried out at
298 K on Bruker DPX300 and AV400 instruments operating re-
spectively at 300.13 and 400.10 MHz. The low-temperature NMR
studies were carried out on a Bruker AV500 instrument at 218 K,
operating at 500.33 MHz. The spectra were calibrated with the
residual solvent resonance and reported with positive chemical shifts in
ppm relative to TMS.
Reaction of [Ph4P][PtBr3(C2H4)] with NEt3. The compound
Ph4P[PtBr3(C2H4)] (10.2 mg, 0.0127 mmol) was introduced in a
5 mm NMR tube. After a series of vacuum/argon cycles, the solid was
dissolved in 0.6 mL of CD2Cl2. NEt3 (17 μL, 12.34 mg, 0.122 mmol)
was then slowly added by microsyringe. The solution was then
homogenized and used for the NMR investigation.
Reaction of trans-[PtBr2(NHEt2)(C2H4)] with NEt3. (a). NMR
Monitoring. In two parallel experiments, samples of the compound
trans-[PtBr2(NHEt2)(C2H4)] (ca. 10 mg, ca. 0.022 mmol) were
introduced in 5 mm NMR tubes. After a series of vacuum/argon
cycles, each sample was dissolved in 0.6 mL of CD2Cl2 and the tubes
were cooled to −80 °C in an acetone/liquid nitrogen bath. To each
tube was then slowly added NEt3 (6.2 μL, 4.5 mg, 0.044 mmol for
one tube; 15.4 μL, 11.2 mg, 0.110 mmol for the other tube) by
microsyringe through a rubber septum. The resulting mixture was
homogenized and analyzed by NMR.
(b). Synthesis of the Complex [(Et2NH)PtBr2(CH2CHNEt2)]. The
complex trans-[PtBr2(NHEt2)(C2H4)] (20.5 mg, 0.029 mmol) was
placed in a Schlenk tube and dissolved in dichloromethane (2 mL).
Triethylamine (5 μL, 0.036 mmol) was then added by microsyringe.
The mixture was then refluxed for 48 h, after which the solvent was
removed by evaporation under reduced pressure. The reaction residue
was purified according to the procedure described by BN:37 it was
chromatographed through a silica gel column using dichloromethane
as eluent. The first colored fraction was collected and evaporated
to dryness. It was characterized by 1H NMR spectroscopy, mass
spectrometry, and X-ray diffraction. 1H NMR (CDCl3): δ 7.71
(t, J = 9.2 Hz, 1H, −CH2CHN), 3.57 (br, 2H, CH-NCH2CH3), 3.46
(d, J = 9.2 Hz, 2H, −CH2CHN), 3.21 (m, 2H, CH3CH2N-Pt), 2.73
(m, 2H, CH3CH2N-Pt), 1.50 (t, J = 7.0 Hz, 6H, CH3CH2N-Pt), 1.30
(m, 2H, CH-NCH2CH3), 1.25 (t, J = 7.2 Hz, 3H, CH-NCH2CH3),
0.90 (t, J = 7.2 Hz, 3H, CH-NCH2CH3). MS (DCI, NH3): 529.1
(M+ + H). A single crystal for the X-ray analysis was grown by slow
evaporation of the CDCl3 solvent from the NMR tube.
RESULTS AND DISCUSSION
■
Addition of Triethylamine to [PtBr3(C2H4)]−. In order
to probe whether decomposition of the zwitterion, e.g. 4 in
Scheme 1, requires deprotonation to yield an aminoalkyl ligand
as proposed by BN (Scheme 2), we wished to investigate the
reactivity of complex 1 toward nondeprotonatable bases, start-
ing with Et3N. Charging the solution of 1 with up to a 10-fold
excess of Et3N did not reveal any change in the room-temperature
1H NMR spectrum of the starting compound, and no new
resonances appeared except for those of free NEt3. This is con-
sistent with literature reports on the analogous chloride system,
according to which the formation of equilibrium amounts of the
zwitterionic product was only possible for highly basic secondary
amines such as Et2NH and piperidine. Indeed, we found in our
previous study15 that treatment of compound 7 (obtained from
1 and 1 equiv of NHEt2) with an additional 2 equiv of NHEt2 is
sufficient to promote an essentially quantitative amine addi-
tion to the coordinated ethylene. However, cooling the 1/NEt3
(10 equiv) solution to −80 °C in the NMR probe revealed the
extensive formation of a new product that can be assigned to
1
trans-[Pt(−)Br2(NEt3)(CH2CH2N(+)Et3)], 9. The H NMR spec-
Reaction of [nBu4P][PtBr3(C2H4)] with Pyridine. The complex
nBu4P[PtBr3(C2H4)] (24.6 mg, ca. 0.034 mmol) was introduced in a
5 mm NMR tube. After a series of vacuum/argon cycles, the solid was
dissolved in 0.6 mL of CD2Cl2. To the solution was added pyridine
(9.2 μL, 9.0 mg, 0.114 mmol) by microsyringe through a rubber
septum. No evidence for an immediate reaction was obtained from the
1H NMR spectrum. Standing at room temperature overnight led to the
trum is shown in Figure 1. Note that in order to keep the aliphatic
proton resonances clear from the interference of cation reson-
ances, the Ph4P+ salt of complex 1 was used in this experiment.
The identity of the compound is demonstrated by the two
characteristic triplets in a 1:1 ratio of the mutually coupled
ethanediyl protons of the −CH2CH2N(+)Et3 ligand at δ 3.18
and 2.04 (3JHH = 8.2 Hz). For comparison, those observed for
trans-[Pt(−)Br2(NHEt2)(CH2CH2N(+)HEt2)] appear at δ 2.86
and 2.09 with 3JHH = 7.0 Hz.15 The resonances of the two com-
pounds are compared in Table 1. The ammonium −N(+)Et3
group exhibits the sharper methylene and methyl proton reson-
ances with the greater downfield shift relative to those of the
excess free NEt3, whereas those of the coordinated NEt3 ligand
are shifted less downfield and are broader, the latter feature
probably being caused by an incompletely frozen exchange with
the free ligand. These trends parallel those previously reported
for the NHEt2 analogue (see Table 1). Although there is no
deposition of single crystals, which were used for the X-ray structural
analysis.
Reaction of trans-[PtBr2(NHEt2)(C2H4)] with Quinuclidine.
The complex trans-[PtBr2(NHEt2)(C2H4)] (10.2 mg, 0.0236 mmol)
and quinuclidine (4.8 mg, 0.0432 mmol) were placed separately into
5 mm NMR tubes, placed under argon and dissolved in CD2Cl2 (ca.
0.3 mL) at −80 °C. The platinum complex solution was then
transferred into the quinuclidine solution NMR tube. The resulting
solution was homogenized before being analyzed by NMR spectros-
copy at regular intervals.
Reaction of trans-[PtBr2(NHEt2)(C2H4)] with MeONa. The
complex trans-[PtBr2(NHEt2)(C2H4)] (10.2 mg, 0.0236 mmol) and
MeONa (2.6 mg, 0.048 mmol) were placed into a 5 mm NMR tube.
1
specific feature in the H NMR spectrum to distinguish the
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The solution of 1 and excess NEt3 used for this NMR ex-
periment was not indefinitely stable, giving rise to the slow
precipitation of metallic platinum upon standing for a long time
at room temperature. This observation is the first evidence that
the zwitterion decomposition does not require deprotonation
of the ammonium function. On the basis of the recent report
on the decomposition of alkoxylated olefin complexes (see
Scheme 3 in the Introduction)41 and the well-known mech-
anism of the Wacker process, it is possible to envisage occurrence
of a β-H elimination to afford a PtII hydride intermediate, which
would be deprotonated by the excess base, ultimately leading to
the formation of a vinylammonium cation (see Scheme 4).
Scheme 4. Hypothesis for the Decomposition of
[PtBr3(C2H4)]− in the Presence of NEt3
Figure 1. 1H NMR spectrum in CD2Cl2 of a solution of
[Ph4P][PtBr3(C2H4)] and NEt3 (10 equiv) at −80 °C. The off-scale
resonances belong to the methylene and methyl protons of the excess
free NEt3.
1
Table 1. H NMR Resonances Observed for Various
trans-[Pt(−)Br2(NEt2R′)(CH2CH2N(+)Et2R)] Compounds
(R, R′ = H, Et)
Vinylammonium cations are scarcely described in the litera-
ture, but a few examples are indeed known: for instance,
[CH2CHNMe2R]+ (R = Me, Et),47 with the vinyl protons
being observed by 1H NMR in D2O in the 5.5−7.0 ppm region.
Monitoring the decomposition of the CD2Cl2 solution of 1 +
NEt3 by 1H NMR over 7 days, however, did not reveal the gen-
eration of any resonance in this region.
Addition of Triethylamine to trans-[PtBr2(NHEt2)-
(C2H4)]. From our DFT studies of the catalytic cycle of the
PtBr2/Br− catalyzed aniline addition to ethylene we have learned
that nucleophilic addition to the coordinated ethylene is both
kinetically promoted and thermodynamically more favored
when the trans Br− ligand is replaced by aniline.11 Therefore,
we envisaged promoting the nucleophilic addition of NEt3 by
using 7 as substrate.
R, R′
(a)
(b)
(c)
(d)
(e)
b
(f)
(g)
(h)
7.9
a
H, H (8)
2.09 2.86 3.37 1.42 n.o. ca. 3.08 1.40
3.21
ca. 2.79
2.92
c
Et, Et (9) 2.04 3.18 2.99 1.24
ca. 1.18
1.38
c
Et, H (10) 1.71 3.19 2.99 1.21
2.92
4.23
2.66
b
a
c
In CD2Cl2 at room temperature (from ref 15). Not observed. In
CD2Cl2 at −80 °C.
trans and cis stereochemistries, a trans configuration can be
assigned to the product, on the basis of the reasonable hypo-
thesis that the stronger trans effect of the ethylene ligand
directs the initial ligand exchange process. As suggested by our
previous DFT study,11 replacement of the Br− ligand trans to
ethylene with an amine is expected to favor the ethylene
nucleophilic attack both kinetically and thermodynamically.
Thus, in contrast to previous reports, nucleophilic addition of
Et3N to a PtII-coordinated olefin may indeed be observed, at
least for complex 1, although only at low temperatures. Warm-
ing of the solution back to room temperature and subsequent
1H NMR measurement indicated total reversal of the reaction
to regenerate complex 1 quantitatively, with no trace of the
zwitterionic product 9 left in solution. This can be rationalized
by the fact that the addition reaction (eq 2) is severely exoentropic
In the presence of 2 equiv of NEt3 in CD2Cl2, no changes in
1
the H NMR spectrum of 7 were observed and no new re-
sonance appeared, with the exception of those of the added
Et3N. This behavior parallels that reported above for the addi-
tion of NEt3 to 1 but differs from that observed for the addition
of Et2NH to 7, where formation of the zwitterionic product
was nearly quantitative at room temperature under the same
conditions. However, as for the Et3N addition to 1 described
above, extensive formation of the expected zwitterionic prod-
uct, trans-[Pt(−)Br2(NHEt2)(CH2CH2N(+)Et3)] (10), was ob-
served at low temperature (see Figure 2). Once again, the high
negative reaction entropy accounts for the observed behavior.
Upon warming the solution to room temperature, the initial
complex 7 again forms quantitatively, confirming the reversi-
bility of the nucleophilic addition process.
The Pt-CH2CH2-N moiety is characterized in this case by
mutually coupled (3JHH = 8.8 Hz) triplet resonances at δ 3.19
and 1.71. In spite of the absence of observable platinum coupl-
ings, probably because of the strong chemical shift anisotropy
relaxation mechanism and the high field of the used instrument,
this feature is a clear signature for zwitterion formation (cf. the
resonances of this compound with those of the similar com-
pounds in Table 1). The coordinated diethylamine ligand is
characterized by two signals for the diastereotopic methylene
[PtBr (C2H4)]− + 2NEt3
3
1
→ trans‐[Pt(−)Br (NEt3)(CH2CH2N(+)Et3)] + Br−
2
9
(2)
and hence highly temperature dependent. Being obviously
exothermic, the process becomes favored at low temperatures,
where the impact of the −TΔS term is reduced.
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overnight under argon and protected from light, the NMR tube
gave rise to the growth of yellow crystals and no observable
formation of metallic platinum. The crystals were analyzed by
X-ray diffraction, revealing the formation of the complex cis-
[PtBr2(py)2] (see Figure 3). The greater coordinating power of
Figure 2. 1H NMR spectrum in CD2Cl2 of a solution of trans-
[PtBr2(Et2NH)(C2H4)] and NEt3 (2 equiv) at −80 °C. The re-
sonances marked with asterisks belong to the methylene and methyl
protons of the excess free NEt3.
protons at δ 2.66 and 2.92 and one signal for the Me protons at
δ 1.38, shifted upfield from those in the precursor compound
7,15 whereas the triethylammonium group shows the methylene
and methyl resonances shifted downfield from the free amine,
at δ 2.99 and 1.21, respectively. A 1H COSY NMR experiment
confirmed the coupling pattern in the compound.
Figure 3. ORTEP view of the complex cis-[PtBr2(py)2]. Selected
parameters (distances in Å, angles in deg): Pt1−Br1, 2.4253(8); Pt1−
Br2, 2.4082(8); Pt1−N1, 2.023(6); Pt1−N2, 2.007(6); Br1−Pt1−Br2,
91.66(3); Br1−Pt1−N1, 178.61(17); Br1−Pt1−N2, 91.82(18); Br2−
Pt1−N1, 88.89(16); Br2−Pt1−N2, 176.53(18); N1−Pt1−N2,
87.6(2).
When the solution containing compound 7 and NEt3 (2 equiv)
was left standing at room temperature for an extended period, a
black precipitate started to deposit after ca. 24 h. Monitoring this
1
transformation by H NMR showed no residual starting complex
after 1 week. Hence, even though this zwitterionic product has
once again a nondeprotonatable ammonium group, its decom-
position to metallic platinum can nevertheless take place. The
pyridine is clearly responsible for the ligand exchange process,
overruling the nucleophilic addition to coordinated ethylene
and leading to loss of ethylene from the coordination sphere.
The key structural parameters are given in the legend of Figure 3.
A similar structure has previously been described for the aniline
adduct cis-[PtBr2(PhNH2)2], obtained by double ligand exchange
from [nBu4P]2[PtBr4] and aniline.8 It is worth noting that the
study of pyridine addition to trans-[PtCl2(py)(C2H4)] has only
resulted in reversible addition to the coordinated ethylene and
pyridine exchange, with no reported exchange of the olefin
ligand.49
1
evolution of the H NMR spectrum is shown in the Supporting
Information (Figure S1). The notable feature of the final spectrum
is the absence of any resonance attributable to a vinylammonium
product, as could be expected on the basis of the decomposition
pathway shown in Scheme 4. On the other hand, the spectrum
clearly shows two mutually coupled resonances (confirmed by a
1H COSY NMR spectrum): a triplet at δ 7.70 and a doublet at δ
3.37 in a 1:2 ratio with 3JHH = 9 Hz. Other visible resonances are
attributable to free ethylene (δ 5.40) and to protonated tri-
ethylamine (single set of resonances shifted downfield from those
of free triethylamine, as can be expected from a rapid degenerate
exchange between Et3N and Et3NH+). These signals have the
same pattern and are observed at positions very similar to those of
the enamine complexes trans-[PtCl2(NHRR′)(CH2CHNRR′)] re-
ported by BN as the decomposition products of [PtCl3(C2H4)]− +
2NHRR′ as mentioned in the Introduction.37 Obviously, the
zwitterionic intermediate 10 could not lead to this type of
product by the proposed pathway (Scheme 2), because the
ammonium function cannot be deprotonated. Our next ex-
periments were aimed at probing the generality of this unusual
transformation and at confirming the identity of the observed
decomposition product, before attempting to rationalize the
decomposition mechanism.
Addition of Quinuclidine to trans-[PtBr2(NHEt2)(C2H4)].
Quinuclidine is electronically similar to triethylamine but has
a much greater nucleophilic power, given its much smaller
steric demand. The addition of 2 equiv of quinuclidine to
1
compound 7 at −80 °C in CD2Cl2 and immediate H NMR
monitoring shows evidence not only of quinuclidine nucleo-
philic addition to the coordinated ethylene with formation of a
zwitterionic product (mutually coupled triplets for the Pt-
CH2CH2-N moiety at δ 3.16 and 1.78, J = 8.8 Hz) but also of
incipient ligand exchange with the release of free Et2NH (see
the Supporting Information, Figure S2). A yellow precipitate
appeared rapidly at low temperature, and its production was
increased by subsequent warming to room temperature, while
there was no evidence for the formation of metallic platinum. It
is possible that this solid, which was not isolated and further in-
vestigated, corresponds to a bis(quinuclidine) complex similar
to that obtained with pyridine and presented in the above
section. Since it seems that strongly nucleophilic amines divert
the reactivity toward exchange of the ethylene ligand and prevent
the reductive decomposition process, we have abandoned this
strategy.
Addition of Pyridine to [PtBr3(C2H4)]−. Searching for
other nondeprotonatable amines, we considered the use of
pyridine, with the additional advantage over triethylamine of a
greater nucleophilic power. Addition of 4 equiv of pyridine to
the CD2Cl2 solution of 1 (as the nBu4P+ salt) did not produce
1
any immediate change in the room-temperature H NMR
spectrum. A low-temperature NMR analysis was not carried
out in this case, but related low-temperature NMR studies
have previously provided evidence of the reversible addition
of pyridine to trans-[PtCl2(py)(C2H4)].48,49 Upon standing
Addition of Sodium Methoxide to trans-[PtBr2(NHEt2)-
(C2H4)]. Given the recently reported evidence that the product
of MeO− addition to certain PtII−ethylene complexes results
in the elimination of methyl vinyl ether and reduction to Pt0,41
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Article
we wished to test the reactivity of compound 7 with this
nucleophile. NaOMe is insoluble in dichloromethane; thus, we
could not observe the formation of the methoxyethyl product at
low temperatures. Nevertheless, treatment of the CD2Cl2 solution
of 7 at room temperature with 5 equiv of NaOMe led to
complete degradation of the starting material over 48 h and to
the generation of methanol (CH3 resonance at δ 3.45). The
decomposition was accompanied by the massive precipitation
correct relative intensities. All of these resonances are very
close to those previously reported for the analogous dichloride
derivative.37
The formulation of the product 11 was further confirmed by
an X-ray single crystal diffraction study. A view of the molecule
is shown in Figure 4. The coordination geometry is as expected
1
of black Pt0. The final H NMR spectrum of the solution is
available in the Supporting Information (Figure S3). In this
spectrum, the most striking feature is the presence of two
mutually coupled resonances (confirmed by a 1H COSY NMR
experiment) at δ 7.71 (triplet) and 3.38 (doublet) with JHH = 9 Hz.
These resonances are identical in chemical shift and coupling con-
stant with those obtained for the product of the decomposition of
compound 7 in the presence of Et3N.
This evidence suggests that the addition of either NEt3 or
MeO− to compound 7 leads to the same product, which there-
fore contains neither NEt3 nor MeO−. The two mutually
coupled triplet and doublet resonances are reminiscent of the
Pt-CH2CHNRR′ derivatives reported by BN.37 We therefore
speculated that the common product is trans-[PtBr2(Et2NH)-
(CH2CHNEt2)] (11), containing two diethylamine functions,
one bonded as a ligand to platinum and the second covalently
bonded as an iminium function to a carbon atom. The basic re-
agents (NEt3 and MeO−) start the reaction by the experi-
mentally observed (for Et3N) nucleophilic addition to the
olefin carbon but ultimately participate to the reaction stoichio-
metry just as Brønsted bases by capturing the two protons. The
resulting proposed stoichiometry is a revised version of that
advanced by BN,37 as shown in eq 3.
Figure 4. ORTEP view of the complex trans-[PtBr2(Et2NH)-
(CH2CHNEt2)] (11). Selected parameters (distances in Å, angles in
deg): Pt1−Br1, 2.4287(6); Pt1−Br2, 2.4173(6); Pt1−N2, 2.133(4);
Pt1−C1, 2.097(5); C1−C2, 1.414(8); C2−N1, 1.313(7); Br1−Pt1−
Br2, 177.434(18); Br1−Pt1−N2, 88.34(12); Br1−Pt1−C1, 90.63(17);
Br2−Pt1−N2, 89.94(12); Br2−Pt1−C1, 90.96(17); N2−Pt1−C1,
176.16(18); Pt1−C1−C2, 96.3(4).
for a d8 PtII complex with four ligands occupying approximately
the four corners of a square plane. The two Br ligands are
located on opposite corners, the other two hosting the Et2NH
and CH2CHNEt2 ligands. Unfortunately, the crystal quality did
not allow the H atoms of the Pt-bonded CH2 group to be located,
but the single H atom of the C atom bonded to the −N(+)Et2
moiety was indeed observed at the expected position in the dif-
ference Fourier map. The Pt−Br and Pt−N distances are similar
to those reported for the structure of the precursor complex 7
(2.4191(5) and 2.4201(6) Å for the former, 2.089(4) Å for the
latter). The most interesting feature of this structure is the con-
figuration adopted by the −CH2CHNEt2 ligand. As mentioned in
the Introduction (Scheme 2), this ligand may adopt in principle an
enamine configuration (CH2CH-NEt2) or an iminiumalkyl
configuration ((−)CH2−CHN(+)Et2). The Pt atom is too far
from atom C2 (2.655 Å) for the expected π coordination of the
enamine limiting structure, whereas the Pt1−C1−C2 angle is
too narrow (96.3°) for the expected σ coordination of the
iminiumalkyl ligand.
2trans‐[PtBr (NHEt2)(C2H4)] + 2Y
2
7
→ trans‐[PtBr (NHEt2)(CH2CHNEt2)] + Pt0 + C2H4
2
11
+ 2YH+ + 2Br−
(3)
Y = Et3N, MeO−, NHEt2
Preparation and Characterization of trans-
[PtBr2(NHEt2)(CH2CHNEt2)]. In order to confirm the validity
of the above assumptions, namely the stoichiometry of eq 3 and
the identity of the PtII product 11 as containing the CH2CHNEt2
ligand, we have repeated the reaction between compound 7 and
only 1 equiv of Et3N per Pt on a larger scale. After 1 week at
room temperature, the mixture contained a black Pt0 precipitate,
while the starting material had completely disappeared. Sub-
sequent purification of the soluble product by chromatography
on silica gel using dichloromethane as eluent produced a spectro-
scopically pure material, where all resonances (see Figure S4 in
the Supporting Information) could be clearly identified and
assigned, also with the help of a COSY spectrum (Figure S5 in the
Supporting Information), to the corresponding protons of the pro-
posed formulation of 11 as trans-[PtBr2(NHEt2)(CH2CHNEt2)].
Notably, we observe again the mutually coupled (JHH = 9.2 Hz)
triplet and doublet resonances at δ 7.71 and 3.46 corresponding
to one and two protons, the methylene and methyl resonances
of the Pt-coordinated Et2NH ligand (the diastereotopic
methylene protons are diagnostically present as two multiplets,
whereas the Me protons are equivalent), and those of the two
inequivalent Et groups of the enamine NEt2 moiety, with the
The observed configuration may be compared with those
reported for cis-[PtCl2(PPh3)(iPrCHCHNH(o-C6H4Cl))],50
which is best described as having an enamine ligand, and for
trans-[PtCl2(2,4,6-NC5H2Me3)(CH2CHN(OH)tBu)],51 to
which the iminiumalkyl configuration was assigned (see Table 2).
Whereas the pattern of Pt−C distances and Pt−C1−C2 angle for
11 appear closer to that of the iminiumalkyl complex, the trends of
the C1−C2 and C2−N distances in the three complexes clearly
point to compound 11 having a configuration intermediate
between the two extremes. The interaction between atoms Pt1
and C2 in 11 must be very weak to account for the equivalence
1
of the methylene protons in the H NMR spectrum.
Discussion of the Zwitterion Decomposition Mecha-
nism. The results presented in the previous sections de-
monstrate that compound 7 leads to the deposition of Pt0 and
formation of the enamine complex 11 on treatment with a
strong base. This occurs not only when the base is Et2NH
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Table 2. Comparison of Relevant Structural Parameters (Distances in Å and Angles in deg) for Enamine Complexes of
Square-Planar PtII
compd
Pt−C1
Pt−C2
C1−C2
C2−N
Pt−C1−C2
a
cis-[PtCl2(PPh3)(iPrCHCHNH(o-C6H4Cl))]
2.21
2.097
2.09
2.31
2.655
2.70
1.38
1.414
1.53
1.37
1.313
1.22
76.2
96.3
95
trans-[PtBr2(Et2NH)(CH2CHNEt2)] (11)
b
trans-[PtCl2(2,4,6-NC5H2Me3)(CH2CHN(OH)tBu)]
a
b
From ref 50. From ref 51.
(i.e., identical with the ligand that is already present in 7), as
also shown by BN for the analogous dichloride system,37 but
also when using Et3N or MeO−, according to eq 3. Obviously,
the mechanism proposed by BN cannot explain the formation
of compound 11, at least when using nondeprotonatable
nucleophilic reagents.
amine ligand, which then immediately attacks the newly
formed CH2CHY ligand. The course of this reaction, as
opposed to that observed for the [PtCl(2,9-Me2phen)(C2H4)]+
complex,41 may be related to the greater lability of Et2NH
relative to the chelating 2,9-Me2phen ligand.
Complex III still features a hydride ligand. However, the
latter is bonded to a highly charged platinum atom (2− formal
charge) and is thus presumably resistant to deprotonation. The
−N(+)HEt2 ammonium group, on the other hand, is acidic and
can be deprotonated by Y, leading to IV. The hydride ligand in
IV is still under the influence of a very electron rich PtII center
and is therefore not deprotonatable. It should, on the contrary,
be strongly hydridic. Therefore, we propose that a ligand
exchange involving the hydride ligand in the dianion IV and the
Et2NH ligand in neutral 7 takes place at this point, yielding the
two monoanionic complexes V and VI. Complex V may then
expel Y, leading to the observed product 11 (the expulsion of Y
may also occur on intermediate IV, before the ligand exchange
with 7), whereas complex VI, featuring the hydride ligand in a
now less electron rich environment, may be deprotonated,
leading to Pt0, free C2H4, and two Br− ions.
On the basis of first principles and previous literature evi-
dence, we propose the following alternative mechanism (Scheme 5).
Scheme 5. Proposed Mechanism for the Decomposition of
trans-[PtBr2(Et2NH)(C2H4)]− in the Presence of Brønsted
Bases
There are several key features leading to this transformation.
The first one is that Y is a poorer ligand than Et2NH and thus
cannot replace the latter and bind the metal. This is not the
case, for instance, for pyridine and quinuclidine, which are even
capable of replacing the olefin and lead to cis-bis-amine adducts.
Second, Y is also a less powerful nucleophile than Et2NH but is
nevertheless capable of attacking the olefin, as shown by the
low-temperature NMR experiments (at least when Y = NEt3).
However, it preferentially leaves the carbyl ligand in intermediate
V to yield the enamine complex 11 rather than protonation-
assisted loss of Et2NH to generate a putative trans-[PtBr2-
(Et2NH)(CH2CHY)] product. On the other hand, it is
a strong Brønsted base and can thus serve well in this capacity
to capture the protons that are generated along the transformation.
One still obscure point is the fate of the zwitterion 9, since
no protons are available either on the ammonium function or
on the coordinated amine, but the formation of the predicted
vinylammonium product could not be observed. It is possible
that the expelled vinylammonium cation is subject to further
transformations under these reaction conditions (excess tertiary
amine) and proceeds to a final product that we have not yet
been able to identify. However, even the zwitterion 9 ultimately
decomposes to generate metallic platinum, consistent with the
formation of a platinum hydride intermediate by β-H elimination.
Equilibrium addition of the Y nucleophile to the coordinated
ethylene in 7 affords the zwitterion I, as experimentally ob-
served for Y = NHEt2 in a previous contribution15 and NEt3 in
the present paper. The reaction then proceeds from this inter-
mediate by β-H elimination, to yield the hydride olefin complex
II. Olefin loss and deprotonation at this stage (as shown for the
[PtCl(H)(2,9-Me2phen)] intermediate in Scheme 3)41 would
lead to the immediate generation of Pt0 and CH2CHY
(vinylammonium if Y = NHEt2, NEt3, vinyl ether if Y = MeO−),
which are not observed for our system. Rather, we propose that
the enhanced electrophilicity of the coordinated olefin, which is
activated not only by coordination to platinum but also by the
electron-withdrawing power of Y, triggers an intramolecular
nucleophilic attack by the amine ligand, which should pre-
ferentially occur on the Y-substituted carbon atom for elec-
tronic reasons. Et2NH does not have available lone pairs when
coordinated, but the electron richness and the presence of
trans-labilizing ligands in the complex may favor its departure
from the coordination sphere and the N lone pair becomes
then available for attacking the unsaturated ligand. It is in fact
possible that the β-H elimination step occurs on a 14-electron
[Pt(−)Br2(CH2CH2Y)] species after decoordination of the
CONCLUSION
■
In pursuit of understanding the mechanism of decomposition
of the PtBr2/Br− catalyst for ethylene hydroamination by aniline,
679
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Organometallics
Article
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bases (Et3N, pyridine, quinuclidine, MeONa) to coordinated
ethylene in the model compound trans-[PtBr2(NHEt2)(C2H4)]
(7), as well as in the catalytic resting state [PtBr3(C2H4)]− (1).
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zwitterionic intermediate as the key step.
On the basis of this new understanding, it is clear that
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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, a table, and CIF files giving crystallographic data and
additional characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
(23) Barone, C. R.; Benedetti, M.; Vecchio, V. M.; Fanizzi, F. P.;
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*LCC (Laboratoire de Chimie de Coordination), 205 route de
Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France. Tel:
+33-561333173. Fax: +33-561553003. E-mail: rinaldo.poli@
lcc-toulouse.fr.
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Author Contributions
(26) Benedetti, E.; De Renzi, A.; Paiaro, G.; Panunzi, A.; Pedone, C.
Gazz. Chim. Ital. 1972, 102, 744−754.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
(30) De Renzi, A.; Di Blasio, B.; Morelli, G.; Vitagliano, A. Inorg.
Chim. Acta 1982, 63, 233−241.
We are grateful to the Centre National de la Recherche
Scientifique (CNRS), the Institut Universitaire de France
(IUF), and the Agence Nationale de la Recherche (ANR, Grant
“HYDROAM” No. NT09_442499) for financial support.
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ABBREVIATIONS
■
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BN, Balacco and Natile (ref 37); DMSO, dimethyl sulfoxide;
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