Kinetico-mechanistic information about alkene hydroamination with aniline in bromide-rich ionic media: Importance of solvolysis

Article abstract of DOI:10.1021/ic2003222

The study of the [PtBr₄]^2- reactivity with hexene and aniline in highly ionic (Bu₄P)Br/CH₂Br₂ media has been studied from a Kinetico-Mechanistic perspective. The results indicate bromide ion association to the square-planar starting material to produce a stable diamagnetic compound that can be described as an ion pair of a [PtBr ₅]^3- square-pyramidal complex stabilized by several phosphonium countercations. While this species reacts rapidly with aniline, producing the known square-planar complex [PtBr₃(PhNH ₂)]^- with release of the apical bromide of the square-pyramidal intermediate, the reaction with hexene, producing the square-planar [PtBr₃(hexene)]^- complex, is much slower. The thermal and pressure activation parameters determined for these processes fully agree with the proposed reactivity. The gross features of the platinum-catalyzed hydroamination mechanism, occurring via much higher energy transition states, are not necessarily altered by these new findings, given the fact that all ligand exchange reactions occur with relatively low activation barriers. Nevertheless, the nature of the catalyst resting state needs revision as demonstrated. The importance of explicitly considering the solvent for reactions conducted in noninnocent highly organized media is also highlighted.

Full text of DOI:10.1021/ic2003222

ARTICLE
pubs.acs.org/IC
Kinetico-Mechanistic Information about Alkene Hydroamination with
Aniline in Bromide-Rich Ionic Media: Importance of Solvolysis
||
Gabriel Aullꢀon,† Kerman Gꢀomez,‡ Gabriel Gonzꢀalez,‡ Susanna Jansat,† Manuel Martínez,*,† Rinaldo Poli,§,
and Mireia Rodríguez-Zubiri§
†Departament de Química Inorgꢁanica, Universitat de Barcelona, Martí i Franquꢁes 1-11, E-08028 Barcelona, Spain
‡Institut Catalꢁa d’Investigaciꢀo Química, Avinguda dels Països Catalans 16, E-43007 Tarragona, Spain
§CNRS, LCC (Laboratoire de Chimie de Coordination), Universitꢀe de Toulouse, UPS, INP, 205 Route de Narbonne,
F-31077 Toulouse, France
Institut Universitaire de France, 103 Boulevard Saint Michel, 75005 Paris, France
S Supporting Information
b
ABSTRACT: The study of the [PtBr4]2ꢀ reactivity with hexene
and aniline in highly ionic (Bu4P)Br/CH2Br2 media has been
studied from a Kinetico-Mechanistic perspective. The results
indicate bromide ion association to the square-planar starting
material to produce a stable diamagnetic compound that can be
described as an ion pair of a [PtBr5]3ꢀ square-pyramidal complex
stabilized by several phosphonium countercations. While this
species reacts rapidly with aniline, producing the known square-
planar complex [PtBr3(PhNH2)]ꢀ with release of the apical
bromide of the square-pyramidal intermediate, the reaction with hexene, producing the square-planar [PtBr3(hexene)]ꢀ complex,
is much slower. The thermal and pressure activation parameters determined for these processes fully agree with the proposed
reactivity. The gross features of the platinum-catalyzed hydroamination mechanism, occurring via much higher energy transition
states, are not necessarily altered by these new findings, given the fact that all ligand exchange reactions occur with relatively low
activation barriers. Nevertheless, the nature of the catalyst resting state needs revision as demonstrated. The importance of explicitly
considering the solvent for reactions conducted in noninnocent highly organized media is also highlighted.
’ INTRODUCTION
composed by ordinary commercially available PtBr2 activated
in the presence of an excess of (Bu4P)Br and functions in the
absence of additional expensive ligands. It is also one of the most
practical catalytic systems because it is stable, can operate in the
presence of air and water, and provides the highest activities
among all Pt-based catalysts reported to date. Thus, the im-
provement of the catalytic efficiency will only be attained by a
comprehensive knowledge of the mechanism and the effect of the
noninnocent medium used (0.13 M (Bu4P)Br). In particular,
synthetic work using ethylene as a representative olefin and low-
temperature equilibrium studies, backed up by theoretical calcu-
lations, have assessed the relative stability in solution (mostly
using dichloromethane, but also aniline and DMF) of a variety of
simple PtII complexes containing all ligands that are present
under catalytic conditions (ethylene, aniline, bromide ions).24,26ꢀ28
Subsequent computational investigations on the high-energy
portion of the catalytic cycle, which is experimentally inaccessible
to stoichiometric studies, have confirmed the olefin-activation
mechanism as the operating cycle for the Pt-catalyzed process
and unveiled intimate details of each elementary step of the
Hydroamination, one of the available processes to form CꢀN
bonds, consists in the direct addition of an NꢀH bond across an
unsaturated CꢀC bond and represents an atom-economical
approach for the synthesis of amines from ammonia or lower
amines. Since amines are a very important class of chemicals in
today’s world market, the hydroamination reaction generates a
lot of interest for academic and industrial researchers.1,2 The
reaction is particularly challenging for alkenes, less reactive than
other unsaturated substrates, such as alkynes, dienes, and allenes,
particularly when they are not activated, and for the intermole-
cular version. Catalytic approaches have been considered for the
latter on nonactivated olefins, and moderate success has been
achieved using precatalysts based on lanthanides,3,4 Zr,5 Fe,6
Ru,7,8 Rh,9,10 Ir,10 Pd,11,12 Pt,13ꢀ19 Ag,20 and Au.21ꢀ23 Among
other metals, for Pt-catalyzed systems two basic types of catalyst-
dependent mechanisms have been indicated, one where the
metal center first activates the olefin, followed by nucleophilic
attack by the amine, the second one involving amine activation
and subsequent olefin insertion into the metalꢀN bond.24,25
Recent work carried out in one of our groups has addressed
the mechanism for the system introduced by Brunet, which is
legitimately the simplest platinum-based catalyst since it is
Received: February 16, 2011
Published: May 17, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
Figure 1. (a) Time-resolved 195Pt NMR spectra of a 0.1 M (Bu4P)2[PtBr4] and 1.0 M (Bu4P)Br solution in CDCl3 at room temperature (time between
spectra 9 h). (b) Time evolution of the signal integrals.
catalytic cycle.28,29 Nevertheless, in these studies the involvement
of the highly ionic medium, found definitively relevant empiri-
cally, has not been fully addressed given the obvious limitations
in the calculation procedures.
aniline-free, [PtBr4]2ꢀ/Brꢀ mixtures at temperatures much
higher than those utilized for its reaction with aniline. The results
suggest that the catalytic cycle should include the [PtBr5]3ꢀ
resting state that undergoes a bromide by aniline substitution to
generate either an off-loop species from the catalytic cycle or a
slow subsequent reaction with hexene, thus slightly modifying
the sequence generally accepted.
Kinetico-Mechanistic studies on hydroamination processes
have been carried out in the presence of [PtL2(OTf)2] as the
catalyst (L2 = bipy, diimine, diphosphine),19 and the prepara-
tion of several possible low-energy intermediates in the pro-
posed catalytic cycle has been attained for the ligand-free
PtBr2/(Bu4P)Br catalyst.30 For the latter catalytic system, despite
the obvious stoichiometric implications of the noninnocent
medium (0.13 M (Bu4P)Br) that has been established as ideal
from a catalysis perspective,13 its importance in the kinetics and
mechanism of the cycle has not been explored. Given our interest
in the effect of the medium on the kinetics and mechanisms of
many well established reaction processes31ꢀ38 and our experience in
Kinetico-Mechanistic studies on organometallic complexes,33,38ꢀ42
we have decided to explore the importance of such a highly ionic
medium in the PtBr2/(Bu4P)Br-catalyzed hydroamination pro-
cess through a Kinetico-Mechanistic study.
In this Article we present the unexpected solution chemistry of
the [PtBr4]2ꢀ species in highly ionic (Bu4P)Br medium (from
0.016 to 2 M in CH2Br2) and its further reaction with aniline
and/or hexene. The results indicate the association of bromide
ions to the square-planar starting material to produce a stable
diamagnetic compound that can be described as an ion pair of a
[PtBr5]3ꢀ square-pyramidal complex. This species, which is
associated with the nature of the starting PtII center under
catalytic conditions (0.13ꢀ1.0 M (Bu4P)Br in aniline), reacts
readily with aniline in a series of consecutive equilibria on very
different time scales via the stepwise substitution of the bromido
ligands by PhNH2, as seen by the inverse reaction rate depen-
dence on the (Bu4P)Br concentration. Addition of hexene, at
very high concentrations, to preformed [PtBr5]3ꢀ solutions in
CH2Br2 does not produce any measurable reaction under time
and temperature conditions equivalent to those used in the
reaction with aniline. This fact, in clear contrast with that observed
for simple [PtBr4]2ꢀ plus hexene systems, suggests that the
sequence indicated in the proposed catalytic cycle for the
hydroamination of ethylene needs revision when hexene is
considered as the substrate in a bromide-rich environment.
The only reactivity with hexene was observed on equilibrated,
’ RESULTS AND DISCUSSION
Reactivity of [PtBr4]2ꢀ in (Bu4P)Br-Rich Media. Given the
fact that the PtBr2-catalyzed hydroamination of alkenes occurs
with much better results in the presence of (Bu4P)Br, a pre-
liminary study about the solution chemistry and stability of the
presumed (Bu4P)2[PtBr4] precatalyst in CH2Br2/(Bu4P)Br
medium was pursued. The choice of CH2Br2 as the accompany-
ing solvent rather than CH2Cl2 was based on its high boiling
point (97 °C), which allows the work at high temperature under
conditions rather similar to those encountered in the catalytic
processes and in the absence of undesirable halide exchange
processes. The high solubility of (Bu4P)Br in this solvent (ca. 3 M)
is also convenient for the purposes of this study.
Preliminary studies on CH2Br2 solutions of (Bu4P)2[PtBr4],
prepared as indicated in the literature,43 showed important color
changes with time that are associated with the loss of bromido
ligands and subsequent polymerization, as described in the
literature.43ꢀ45 These processes should be easily suppressed by
the addition of an excess of (Bu4P)Br. In this respect, 195Pt NMR
experiments were conducted on chloroform solutions of
(Bu4P)2[PtBr4] containing an excess of (Bu4P)Br to establish
which reactivity was taking place under these conditions. Figure 1
collects the time evolution of a 195Pt NMR spectrum of a 1 M
(Bu4P)Br/CDCl3 solution of (Bu4P)2[PtBr4] (0.10 M) at room
temperature. It is evident that at time zero only the broad signal
corresponding to [PtBr4]2ꢀ at ꢀ2500 ppm is relevant, while its in-
tensity decreases with time in favor of a final sharp single signal at
ꢀ2000 ppm, not related to any known PtII/Br or PtIV/Br
complexes.43,44 Monitoring by 1H or 31P NMR spectroscopy
of similar experiments indicated that no reaction involving the
Bu4Pþ cation occurs. Furthermore, the observed important
UVꢀvis spectral changes in CH2Br2 (see below) are also
evident both in CHCl3 and when (NEt4)Br solutions of the
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Inorganic Chemistry
ARTICLE
Figure 2. (a) Time-resolved UVꢀvis spectra of (Bu4P)2[PtBr4] (4.5 ꢂ 10ꢀ4 M) and (Bu4P)Br (0.27 M)/CH2Br2 solution at 70 °C (the inset
corresponds to the absorbance changes at 375 nm, total time 2 h). (b) Dependence of kobs on [(Bu4P)Br] at different temperatures for the reaction
observed.
Table 1. Kinetic, Equilibrium, and Thermal and Pressure
Activation Parameters for the Reaction Observed between
(Bu4P)2[PtBr4] and (Bu4P)Br in CH2Br2 Solution
same concentrations are used, indicating that the bromide ion
is responsible for the observed changes. When the experi-
ments were conducted using acetonitrile instead of dibromo-
methane, no significant changes were observed by UVꢀvis
spectroscopy. It seems clear that a low-polarity solvent is
needed for the reaction with high bromide concentrations to
occur; alternatively, the reason could also be associated with
the coordinating characteristics of CH3CN, which should
allow its competition with Brꢀ. A similar behavior in both
UVꢀvis spectra and 195Pt NMR (Δδ ≈ 1000 ppm downfield)
has been observed in the formation of crystallographically
characterized pentacoordinated adducts of organometallic
pincer complexes with SO2.46,47
328k
ΔH
ΔS
328ΔV
K
OS
q
þ
q
þ
q
þ
328
/sꢀþ1
/kJ molꢀ1
/J Kꢀ1 molꢀ1
/cm3 molꢀ1
/Mꢀ1
6.4 ꢂ 10ꢀ4
76 ( 7
ꢀ22 ( 22
ꢀ20 ( 1
3.0
q
ꢀ
q
328k
ΔH
ΔS
ꢀ
/sꢀꢀ1
/kJ molꢀ1
/J Kꢀ1 molꢀ1
1.0 ꢂ 10ꢀ4
117 ( 10
30 ( 30
equilibrium constant, defined as k /k , they show a small increase
þ
at low temperatures (from 3 (79 °ꢀC) to 15 (40 °C)), which
indicates a higher presence of the [PtBr5]3ꢀ at the lower tem-
peratures in the study.
UVꢀvis monitoring of the reaction progress, using pseudo-
first-order conditions, produced very well behaved time-resolved
spectra (Figure 2a) that could be perfectly fitted to by a single
exponential from which kobs values could be derived. These were
found independent of the platinum concentration but showed a
limiting dependence on [(Bu4P)Br] and a nonzero intercept.
The fact that the spectral changes increase with increasing value of
[(Bu4P)Br] suggests the establishment of an equilibrium (k /k =
The presence of a definite value of Keq is surprising given the
full disappearance of the 195Pt NMR signal of the starting
[PtBr4]2ꢀ material (Figure 1) in CDCl3. This seems to point
to the need for a very low polarity medium to stabilize the new
species, which again is in line with the lack of reactivity observed
in CH3CN indicated above (empirical solvent polarity scale
being 0.26 for CHCl3, 0.27 for CH2Br2, and 0.46 for CH3CN).50
Finally, considering that the outer-sphere complex {[PtBr4]2-;
Brꢀ} occurs between two negatively charged species, the rather
high value determined for the outer-sphere complex formation
equilibrium constant, KOS, is a clear indication of the complicated
nature of the “strictly” nonbonding interactions.
þ
ꢀ
Keq) with a preformed outer-sphere species (KOS), i.e., kobs
=
k þ {k KOS[(Bu4P)Br]/(1 þ KOS[(Bu4P)Br])}.48,49 Figure 2b
ꢀ
þ
collects the observed trend at different temperatures. Derivation
of k , k , and KOS at different temperatures produced the
þ
ꢀ
thermal activation parameters collected in Table 1 by the use
of Eyring plots. The dependence of ln kobs on pressure at 1.0 M
(Bu4P)Br (where kobs ≈ k ) was taken as a measure of the
þ
activation volume for the forward process, and the value is also
collected in Table 1.
This type of pentacoordinate species has already been fully
established in solution, apart from the SO2 adduct of the above-
mentioned PtII-pincer complex,46,47 by EXAFS measurements
and theoretical calculations on aqueous acidic solutions of
All the above indications are consistent with the reaction being
the addition of a bromide ligand to the system, as depicted in eq 1.
[Pt(OH2)4]2þ 51,52 The existence of [Pd(OH2)5]2þ and hy-
.
k
KOS
þ
2ꢀ
2ꢀ
3ꢀ
½PtBr4ꢁ þ Brꢀ a f½PtBr4ꢁ ; Brꢀg a ½PtBr5ꢁ
ð1Þ
drated forms of [PtCl4]2ꢀ and [PdCl4]2ꢀ (showing a geometry
that can be better described as an octahedral diamagnetic complex
with a very strong JahnꢀTeller effect) has also been indicated.53,54
Even the formation of square-pyramidal [Ni(CN)5]3ꢀ in solu-
tion has been reported, as well as other NiII five-coordinated
complexes.55,56 An analysis of the Cambridge database indicates
a definite preferential formation of square-pyramidal pentacoor-
dinated PtII complexes for nontripoidal ligands. Given the filled
dz2 orbital on the PtII center, their apical bond lengths are
k
ꢀ
The negative activation entropy and particularly the negative
activation volume are strong indicators of the associative character
of the forward process. The relatively small value determined for
q
ΔS agrees with the already important ordering of the ionic
þ
atmosphere prior to bromide ion addition. Similarly, the reverse
reaction shows a set of values for ΔH q and ΔS q that are in line
ꢀ
ꢀ
with a dissociatively activated process. As for the values of the
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Inorganic Chemistry
ARTICLE
Figure 3. (a) Time-resolved UVꢀvis spectra of the evolution of a CH2Br2 solution of putative [PtBr5]3ꢀ (2 ꢂ 10ꢀ4 M), (Bu4P)Br (1.0 M), and
C6H5NH2 (0.6 M) at 20 °C and 1 atm (the inset corresponds to the absorbance changes at 390 nm, total time 20 s). (b) Dependence on [aniline] of the
value of kobs1 at differenct temperatures for the reaction observed. (c) ln koff1 for the same process at different pressures at 20 °C.
Table 2. Kinetic, Equilibrium, and Thermal Activation Parameters for the Reaction between Putative [PtBr5]3ꢀ and C6H5NH2 in
1.0 M (Bu4P)Br/CH2Br2 Solution
293
step
293k
K
/Mꢀ1
ΔHq/kJ molꢀ1
ΔSq/J Kꢀ1 molꢀ1
ΔVq/cm3 molꢀ1
eq
on first
0.16 Mꢀ1 sꢀ1
0.31 sꢀ1
4.9 ꢂ 10ꢀ3 Mꢀ1sꢀ1
4.3 ꢂ 10ꢀ4 sꢀ1
5.4 ꢂ 10ꢀ4 Mꢀ1sꢀ1
3.9 ꢂ 10ꢀ5 sꢀ1
0.5
0.5
10
71 ( 13
91 ( 5
ꢀ20 ( 44
53 ( 15
∼0
19 ( 2
off first
on second
off second
on third
off third
94 ( 4
30 ( 14
10
15
15
52 ( 10
94 ( 16
105 ( 19
ꢀ134 ( 35
13 ( 52
29 ( 65
definitively longer than their basal counterparts; from all these
available data, the apical PtꢀBr bond length can be reasonably
estimated around the 2.90 Å value. This rather long distance can
also be related to the relatively small absolute value of the
activation entropy for the association process. In this respect,
the reverse reaction should be dissociatively activated as expected
cycles.14,24 Solutions of (Bu4P)2[PtBr4] (ca. 4 ꢂ 10ꢀ4 M) and
(Bu4P)Br (1.0 M) in CH2Br2 were heated at 50 °C until
equilibration (2ꢀ3 h, k /k = 7; see above). Aliquots of these
þ
ꢀ
solutions were mixed with different aniline solutions of different
concentrations and at the same ionic strength in CH2Br2; the
final platinum concentration was usually in the (1.0ꢀ2.5) ꢂ
10ꢀ4 M range. As expected, time-resolved UVꢀvis monitoring of
the reaction showed a series of processes, the relative importance
of which was highly dependent on the temperature, time, and
aniline concentration. Nevertheless, very long reaction times or
high temperatures were avoided to overcome the problems
caused by side reactions of the aniline. In all cases a blank
experiment in the absence of added platinum was conducted to
establish the reliability limit of the experiments.
for the microreversibility principle. Effectively, as indicated before,
q
the value for ΔS is positive, although small, correspond-
ꢀ
ing to the dissociation of an already elongated PtꢀBr bond.57
These facts are a good example of the need for considering the
effects of noninnocent Lewis base media in reactions occurring at
low solute concentrations (i.e., very large solvent:solute ratios) that
allow the existence of otherwise undefined solvento species.33,58
Given the nonfeasibility of the isolation, workup, and further
characterization of the species formed in the above-mentioned
reaction conditions, DFT calculations were tried for the putative
[PtBr5]3ꢀ anion and its ion-paired species. Although in other
systems these have led to reliable results,59 for our system no
optimization of the desired species could be achieved.60
The first monitored process is very fast and can be observed
within the stopped-flow equipment limits, despite the high
viscosity and density of the solutions used. The process is very
clean, and a good retention of isosbestic points is observed
(Figure 3a). Furthermore, this step is an equilibrium reaction, as
detected by the important intercepts of the plots of the pseudo-
first-order observed rate constants (kobs1) versus [aniline]
(Figure 3b); Figure 3c also collects the trend observed for the
ln k1off versus P plots for the intercepts measured as a function of
pressure. Table 2 collects the series of relevant kinetic and
thermal and pressure activation parameters for the reaction.
Following this very fast reaction, a second process on a very
different time scale is also observed with similar UVꢀvis spectral
changes, yielding a set of values for kobs2, also varying linearly
with the concentration of aniline (Table S1, Supporting In-
formation) and showing a definite intercept. Before the side
reactions of aniline take place, with a dramatic exponential
increase of the absorbance of the spectrum, a third process is
Reactivity of [PtBr4]2ꢀ with Aniline in (Bu4P)Br Medium.
Given the reactivity observed for [PtBr4]2ꢀ in concentrated
CH2Br2 solutions of (Bu4P)Br, the reactivity of these equili-
brated reaction mixtures with PhNH2 was further pursued. This
is especially interesting after our recent report30 about all the
species present on equilibration of [PtBr4]2ꢀ/PhNH2/C2H4
systems in CH2Cl2 solution at room temperature. From the data
it is clear that a rather complex series of equilibria exist, even in
the absence of important relative concentrations of (Bu4P)Br.
Nevertheless, in the reported cases the nature of the starting
material was neatly set to (Bu4P)2[PtBr4], which is not the case
in the present report that deals with a set of reaction conditions
much closer to those needed for the improved catalytic
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ARTICLE
Figure 4. (a) Values of kobs1 versus [(Bu4P)Br] for the reaction of a solution of 2 ꢂ 10ꢀ4 M of putative [PtBr5]3ꢀ and 0.11 M [C6H5NH2] (20 °C).
(b) Values of kobs versus [(Bu4P)Br] for the reaction of a solution of 2 ꢂ 10ꢀ4 M of putative [PtBr5]3ꢀ and 0.50 M hexene (45 °C).
neatly observed during the monitoring of UVꢀvis spectral
changes. The spectral changes and [aniline] dependence of these
kobs3 values are equivalent to those observed for the first two
processes, and the relevant parameters are also collected in
the fact that the plot in Figure 4a does not reach an asymptotic
k
ꢀ2 limit also indicates that the second approximation cannot be
applied for the [Brꢀ] used in the study. The alternative approach,
involving an initial bromide decoordination followed by a simple
aniline by bromide substitution, can be disregarded in view of the
very fast processes (within stopped-flow mixing time) observed
for the [PtBr4]2ꢀ plus PhNH2 reaction. This fact should produce
an as effective substitution reaction rate as that measured for the
back reaction indicated in the previous section, which is orders of
magnitude slower than that observed.
Table 2 together with those for kobs2
.
As seen in Table 2, all the processes measured show a definite
equilibrium nature that must be very little displaced at the high
bromide concentrations existing at the catalytic conditions of the
target hydroamination process. All of them are, consequently,
related to sequential bromide by aniline substitution. Some
experiments were also carried out to establish the importance
As a consequence, the values of the apparent thermal activa-
tion parameters derived from the measurements have to be taken
very carefully and cannot be considered as a reliable tool for the
assignation of the intimate mechanism operating. The apparent
values for the on process indicate either an overall concerted
mechanism for the bromido by aniline substitution or a dissociation
of an already elongated PtꢀBr bond (as indicated in the previous
section). Clearly, the square-pyramid structure of the putative
[PtBr5]3ꢀ species formed on dissolution of the [PtBr4]2ꢀ in con-
centrated (Bu4P)Br solutions is responsible for this behavior as
found for other NiII complexes.56 For the reverse reaction, off, an
overall dissociative exit of the aniline ligand seems to be operat-
ing, which agrees with the second possibility indicated. The
complex nature of the rate constants determined (eq 4) makes
any further discussion useless. That is, according to eqs 2 and 3,
k1 seems to be dominant for the on process while kꢀ1 for the off
reactions, in agreement with the microreversibility principle.
Interestingly, the values of ΔVq determined for this faster process
(kon first, koff first) are very much in line with those measured for
ΔSq, which normally indicates the nonactuation of determinant
hydrogen bonding in the transition state of the process.61,64,65
Evidently, the sequential second and third processes observed
must have a different character if they are derived from square-
planar [PtBr3(PhNH2)]ꢀ and [PtBr2(PhNH2)2] complexes.
Given the high complexity involved, the nature of the second
and third processes observed was not further explored. Never-
theless, the formation of cis-[PtBr2(PhNH2)2], as the more readily
formed bisaniline compound28,30 in a second step from
[PtBr3(PhNH2)]ꢀ or [PtBr4(PhNH2)]2ꢀ, can be easily under-
stood according to the set of reactions in eqs 2 and 3. Its
transformation to the more stable trans-isomer, as the process
that originates the third observed step, seems a reasonable
speculation. The [PhNH2] dependence of kobs3 agrees with this
assumption, taking into account the intermolecular character
of the bromide concentration for the first relevant process, kobs1
.
As seen in Figure 4a, an important increase of the reaction rate
occurs on decreasing the amount of bromide ions in solution,
which is indicative of the need of a bromido dissociation for the
formation of the final complex with the amine ligand in the PtII
coordination sphere. Given the high complexity of the sequential
reactivity observed, no further studies were conducted at other
[(Bu4P)Br]:[C6H5NH2] ratios. The same type of trend has also
been observed in other systems,61ꢀ63 where an equilibrium
leading to a lower coordination number species is responsible
for the final substitution reaction observed. Summarizing, the
process agrees with the set of reactions indicated in eqs 2 and 3,
which produce the rate law indicated in eq 4,
k1
2ꢀ
½PtBr5ꢁ3ꢀ þ PhNH2 a ½PtBr4ðPhNH2Þꢁ þ Brꢀ ð2Þ
k
ꢀ1
k2
2ꢀ
½PtBr4ðPhNH2Þꢁ a ½PtBr3ðPhNH2Þꢁꢀ þ Brꢀ
ð3Þ
k
ꢀ2
k1k2½PhNH2ꢁ
k
ꢀ1k ½Brꢀꢁ
ꢀ2
kobs1
¼
þ
k
ð4Þ
k
ꢀ1
½Brꢀꢁ þ k2
½Brꢀꢁ þ k2
ꢀ1
once the steady-state approximation has been applied to the
[PtBr4(PhNH2)]2ꢀ intermediate species.48 Equation 4 becomes
kobs1 = kon first[PhNH2] þ kofffirst, with kon first ={k1k2/(kꢀ1[Brꢀ] þ
k2)} and koff first = {kꢀ1kꢀ2[Brꢀ]/(kꢀ1[Brꢀ] þ k2)}. While only
for k2 . kꢀ1[Brꢀ] does the actual rate constant measured have
any specific significance with respect to kon first (=k1), the
k2 , kꢀ1[Brꢀ] approximation is needed for koff first to reduce
to a significant rate constant, kꢀ2. Nevertheless, the decrease of
kobs1 with [Brꢀ] observed in Figure 4a needs the kꢀ1[Brꢀ] term
to be relevant, not allowing for the first approximation. Further,
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Figure 5. (a) Time-resolved UVꢀvis spectra of the evolution of a CH2Br2 solution of putative [PtBr5]3ꢀ (2.5 ꢂ 10ꢀ4 M), (Bu4P)Br (0.13 M), and
hexene (0.08 M) at 55 °C and 900 atm (the inset corresponds to the absorbance changes at 375 nm, total time 15000 s). (b) Dependence on [hexene] of
k
obs at different temperatures.
Table 3. Kinetic, Equilibrium, and Thermal Activation Parameters for the Reaction between Putative [PtBr5]3ꢀ and Hexene in
0.1 M (Bu4P)Br/CH2Br2 Solution
average
step
328k
K /Mꢀ1
ΔHq/kJ molꢀ1
ΔSq/J Kꢀ1 molꢀ1
ΔVq/cm3 molꢀ1
eq
on
2.4 ꢂ 10ꢀ4 Mꢀ1sꢀ1
1.6 ꢂ 10ꢀ5 sꢀ1
13
70 ( 2
65 ( 5
ꢀ105 ( 6
ꢀ145 ( 15
ꢀ11 ( 2
ꢀ28 ( 3
off
normally actuating for this type of isomerization process (Figures
S1 and S2, Supporting Information).66
scale. Solutions of (Bu4P)2[PtBr4] in the (2ꢀ5) ꢂ 10ꢀ4 M range
in (Bu4P)Br/CH2Br2 (0.02ꢀ2.0 M (Bu4P)Br concentration
margin) treated with varying amounts (0.1ꢀ0.9 M) of hexene
only showed a significant reaction when very long periods or high
temperatures were used. As a consequence, the (Bu4P)Br con-
centration was set to 0.08 M to facilitate the monitoring. Figure 5
collects some relevant information about these experiments.
From the time-resolved recorded spectra, it is clear that under
these conditions a neat process occurs. The observed rate
constants show an important degree of equilibrium, as indicated
by the intercepts in the plot of kobs versus [hexene] (Figure 5b).
The temperature and pressure dependences of the slopes and
intercepts derived from these plots produce the thermal and
pressure activation parameters collected in Table 3 using the
standard Eyring and ln kon and ln koff versus P plots. The effect of
the (Bu4P)Br concentration on these reaction rates was also
studied in view of the results found for the (Bu4P)2[PtBr4]/
(Bu4P)Br/aniline system. Figure 4b shows the observed changes
for the kobs value at 55 °C when CH2Br2 solutions of putative
[PtBr5]3ꢀ (2.5 ꢂ 10ꢀ4 M) were reacted with hexene (0.5 M) in
the 0.05ꢀ2.0 M (Bu4P)Br concentration range.
Reactivity of [PtBr4]2ꢀ with Hexene in (Bu4P)Br Medium.
The reactivity observed for (Bu4P)2[PtBr4] with PhNH2 in a
(Bu4P)Br/CH2Br2 medium, indicated in the previous section, as
well as the reported partial ethylene dissociation from
[PtBr3(C2H4)]ꢀ in a (Bu4P)Br/CH2Cl2 medium,30 prompted
us to parallel the above-described study with a (Bu4P)2[PtBr4]/
(Bu4P)Br/alkene system in CH2Br2. The chosen alkene was
1-hexene (hexene from here on) given its relatively high boiling
point that enables its easy and safe manipulation, as well as the
reliability of the desired concentration levels during prolonged
periods. This study is relevant given the preferential stability
observed for [PtBr3(C2H4)]ꢀ, with respect to [PtBr3(PhNH2)]ꢀ,
when cis- or trans-[PtBr2(C2H4)(PhNH2)] complexes are dis-
solved in (Bu4P)Br/CH2Cl2 solutions. The improved catalyzed
hydroamination reaction of 1-hexene with aniline in a (Bu4P)Br-
rich environment has also been reported within the 0.10ꢀ0.23
range for [(Bu4P)Br]/[hexene],14 which is included in the
0.11ꢀ0.53 margin used in this study (Table S1, Supporting
Information).
In this process the values obtained for kobs are close to
those observed for the [PtBr4]2ꢀ/Brꢀ equilibration process
indicated before, and the alternative mechanism shown in
eqs 5ꢀ7,
To establish whether CH2Br2 solutions of the starting
[PrBr4]2ꢀ complex react with hexene, preliminary UVꢀvis
monitoring of solutions of (Bu4P)2[PtBr4] in dibromomethane
with varying amounts of hexene was conducted in the absence of
added (Bu4P)Br to suppress the formation of the putative
[PtBr5]3ꢀ. Under these conditions reactivity is effectively ob-
served (Figure S3, Supporting Information), leading to the
probable formation of [PtBr3(hexene)]ꢀ, by analogy with com-
plex [PtBr3(C2H4)]ꢀ that has been previously prepared under
similar conditions.30 Nevertheless, when the experiments were
conducted on the same (Bu4P)[PtBr4]/(Bu4P)Br equilibrated
mixtures used for the aniline reactivity studies indicated in the
previous section, the process occurred on a much longer time
k
ꢀ
2ꢀ
½PtBr5ꢁ3ꢀ a ½PtBr4ꢁ þ Brꢀ
ð5Þ
ð6Þ
ð7Þ
k
þ
k3
2ꢀ
½PtBr4ꢁ þ hexene a ½PtBr3ðhexeneÞꢁꢀ þ Brꢀ
k
ꢀ3
2
k k3½hexeneꢁ
ꢀ
k k ½Brꢀꢁ
þ ꢀ3
kobs
¼
þ
k ½Brꢀꢁ þ k3½hexeneꢁ k ½Brꢀꢁ þ k3½hexeneꢁ
þ
þ
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Inorganic Chemistry
ARTICLE
involving the preliminary dissociation of a bromido ligand from
coordination provides lower stabilization relative to ethylene.29
Given the fact that steric hindrance of the alkene cannot be held
responsible for the difference in such an open environment, the
origin should be electronic. Hexene is a better σ-donor but a
poorer π-acceptor than ethylene, thus a worse ligand if back-
bonding is dominant for the stabilization of the Ptꢀalkene bond
in the complex. Nevertheless, the reaction media for the two
studies are by no means equivalent. The important excess of
bromide ions in the reaction medium enables the actuation of the
equilibrium reactions indicated by eqs 2 and 3 (k1, kꢀ1, k2, kꢀ2) as
the putative [PtBr5]3ꢀ, could be operating. If the k [Brꢀ] .
þ
k3[hexene] approximation applies, the rate law in eq 7 fits the
trend of the data in Figures 4b and 5b with kon = k k [hexene]/
ꢀ 3
k [Brꢀ] and koff = kꢀ3[Brꢀ]. Nevertheless, at 36 °C the values of
þ
k
þ3 and kꢀ3 were determined as 2.0 ꢂ 10ꢀ3 Mꢀ1 sꢀ1 and 2.1 ꢂ
10ꢀ3 ꢀ1, respectively (Figure S3b, Supporting Information),
s
which should produce a value for koff ≈ (1ꢀ2) ꢂ 10ꢀ4 sꢀ1 under
the conditions of the study at 35 °C. The fact that the experi-
mental value determined is 2 orders of magnitude smaller (3.1 ꢂ
10ꢀ6
s
ꢀ1, Figure 5b) indicates that the alternative mechanism
well as the equivalent for hexene (k1 , kꢀ1 , k2 , kꢀ2 ) and thus
0
0
0
0
does not effectively fit the data obtained. That is, the mechanism and
rate law shown in eqs 2ꢀ4 with hexene as the entering ligand
generates a much more complex system.
Summarizing, the present study is fully relevant to shed light
on the mechanism of olefin hydroamination in a highly ionic
medium in terms of the identification of the dormant species of
the catalytic cycle. Although it has been established as
[PtBr3(C2H4)]ꢀ (for the hydroamination of ethylene in fully
organic solvents),28,30 the results indicated that the kinetically
preferred species is an aniline complex without coordinated
olefin, such as [PtBr3(PhNH2)]ꢀ or possibly [PtBr2(PhNH2)2],
when dealing with the hydroamination of 1-hexene in (Bu4P)Br/
CH2Br2 medium. Nevertheless, the considerations about the in-
timate hydroamination mechanism, occurring via much higher
energy transition states, according to the recent computational
study,28 do not have to be altered so far given the fact that all ligand
exchange reactions occur with relatively low activation barriers.
0
0
0
0
instead of aniline (k1 , kꢀ1 , k2 , and kꢀ2 ) applies. In this respect, it is
important to note that at high [Brꢀ] values there seems to be a
limiting value for kobs at 55 °C of ca. 1 ꢂ 10ꢀ4 sꢀ1 (Figure 4b), the
same within error as k for the [PtBr4]2ꢀ/Brꢀ system. Even though
ꢀ
eq 7 can explain this fact via a compensation of the k [Brꢀ] and
þ
kþkꢀ3[Brꢀ]2 terms, we are tempted to believe that at high Brꢀ
concentrations the coordination of hexene to the system is totally
suppressed and that only the re-equilibration reaction to the new
lower [Brꢀ] (from 1.0 to 0.08 M; see before) is being observed.
Similarly to the aniline substitution process, the complex
0
0
0
0
nature (k1 , kꢀ1 , k2 , and kꢀ2 ) of the values obtained for kon
and koff does not allow for a reliable discussion about the intimate
reaction mechanism, given the fact that the activation parameters
are only apparent values. These values, collected in Table 3, are
fairly negative for both ΔSq and ΔVq, which can be associated
with an associative character of the overall substitution process.
This is accompanied by relatively small values for the activation
enthalpies. The values are in clear contrast with the correspond-
ing data in Table 2 for the reaction with aniline where a concerted
or dissociative character can be interpreted from the activation
parameters. It is evident that the mechanism operating for the
alkene coordination to the platinum center is not the same as that
’ EXPERIMENTAL SECTION
Instruments. 195Pt NMR spectra were recorded on a Bruker 500
spectrometer in CDCl3 at room temperature at the Institut Catalꢁa
d’Investigaciꢀo Química; chemical shifts are reported in parts per million,
1
referred to H2[PtCl6]. 31P and H NMR spectra were recorded on a
Varian Unity 300 (31P, 121.42 MHz) or on a Mercury 400 (1H, 400
MHz) instrument and referenced to SiMe4 (1H) or H3PO4 (31P) at the
Unitat de RMN d’Alt Camp (Universitat de Barcelona) .
occurring for the aniline. While for the aniline entry the k1/k
ꢀ1
The UVꢀvis spectra and the kinetic monitoring of the reactions were
recorded in a Hewlett-Packard 8453A, a Cary 50, or a J&M TIDAS
instrument, depending on the experiment. The pressurizing system
used for the experiments run at variable pressure has already been
described.67ꢀ69
reaction path fits the activation parameters determined, in this
case the values for ΔHq, ΔSq, and ΔVq are all indicative of a
process with a very important associative character much more in
0
0
line with what would be expected for the k2 /k reaction path.
ꢀ2
Reactivity of [PtBr4]2ꢀ in (Bu4P)Br/Aniline/Hexene Med-
ium. To determine the reactivity of the platinum species
simultaneously with both relevant ligands, a set of experiments
was also conducted on preformed [PtBr5]3ꢀ, with hexene and
aniline at varying concentrations, temperatures, and times. At
short reaction times a fast reaction occurs which follows the same
kinetic pattern as that observed solely with PhNH2. When
the processes were monitored by UVꢀvis for longer times, some
further changes were observed that were fully equivalent to
those obtained in blank experiments without hexene added to
the reaction medium, thus indicating that a mere aniline side
decomposition process was being observed. These results in-
dicate that hexene has a lower coordinating ability than aniline for
PtII under this specific coordination environment (bromide-rich
solution in a polar solvent), whereas it was previously concluded
that ethylene binds to [PtBr4]2ꢀ more favorably than aniline in a
nonionic medium.30 The simplest explanation consists in con-
sidering the equilibrium indicated in eq 8
Compounds. (Bu4P)2[PtBr4] was obtained via chlorido by bromi-
do exchange on K2[PtCl4] in aqueous solution, followed by treatment
and extraction with a saturated solution of (Bu4P)Br in CH2Cl2 as
indicated in the literature.24 Repetitive washing with water of the organic
layer and evaporation to dryness produced the desired compound. The
use of commercially available K2[PtBr4] was avoided after an important
number of experiments carried out indicated that the purchased
compound did not agree with the specifications indicated. 195Pt NMR
resonance of a commercial sample indicated the presence of only minor
quantities of the tetrabromido species, while mixed bromido/chlorido
compounds were dominant. (Bu4P)Br, C4H9CHdCH2, and CH2Br2
were reagent grade chemicals and have been used without further
treatment. PhNH2 was vacuum distilled and kept in the dark under a
nitrogen atmosphere before use.
Kinetic Measurements. The progress of the different processes
studied in dibromomethane solution was followed by UVꢀvis spectros-
copy in conventional or stopped-flow (depending on t1/2) instruments
at the wavelength range where the solvent does not absorb. The general
kinetic technique for this type of processes has been described
elsewhere.40,42,70 For the kinetic runs at elevated pressure, a previously
described pressurizing system connected to a J&M TIDAS instrument
was used.69,71 Observed rate constants were derived from absorbance
½PtBr3ðalkeneÞꢁꢀ þPhNH2 a ½PtBr3ðPhNH2Þꢁꢀ þalkene ð8Þ
very displaced to the left for ethylene and to the right for hexene.
Indeed, a recent computational study confirms that 1-hexene
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ARTICLE
versus time traces at the wavelength where a maximum increase and/or
decrease of absorbance was observed by the use of the SPECFIT
software.72 No dependence of the observed rate constants on the
selected wavelength was detected; for some reactions a good retention
of isosbestic points was observed.73 The Supporting Information
collects the obtained kinetic constant values for all the reactions studied
as a function of aniline, hexene, and tetrabutylphosphonium bromide
concentrations, pressure, and temperature. The errors in all cases were
within the 10ꢀ15% margin. All postrun fittings were carried out with the
commercial software available and using nonweighted least-squares
methods. Typical errors were 10ꢀ15% for slopes and intercepts and
15ꢀ18% for asymptotic limits.
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’ ASSOCIATED CONTENT
S
Supporting Information. Values of kobs determined as a
b
function of the compounds, concentration, temperature, and
pressure and some relevant figures. This material is available free
of charge via the Internet at http://pubs.acs.org.
(32) Bell, C. A.; Bernhardt, P. V.; Gahan, L. R.; Martínez, M.;
Monteiro, M. J.; Rodríguez, C.; Sharrad, C. A. Chem.—Eur. J. 2010,
16, 3166–3175.
(33) Aullꢀon, G.; Chat, R.; Favier, I.; Font-Bardía, M.; Gꢀomez, M.;
Granell, J.; Martínez, M.; Solans, X. Dalton Trans. 2009, 8292–8300.
(34) Basallote, M. G.; Bozogliꢀan, F.; Fernandez-Trujillo, M. J.;
Martínez, M. New J. Chem. 2007, 32, 264–272.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: manel.martinez@qi.ub.es.
’ ACKNOWLEDGMENT
(35) Pꢀerez-Tejada, P.; Lꢀopez-Pꢀerez, G.; Prado-Gotor, R.; Sꢀanchez,
F.; Gonzꢀalez-Arjona, D.; Lꢀopez-Lꢀopez, M.; Bozogliꢀan, F.; Gonzꢀalez, G.;
Martínez, M. Inorg. Chim. Acta 2006, 359, 149–158.
(36) Aullꢀon, G.; Bernhardt, P. V.; Bozogliꢀan, F.; Font-Bardía, M.;
Macpherson, B. P.; Martínez, M.; Rodríguez, C.; Solans, X. Inorg. Chem.
2006, 45, 8551–8562.
(37) Bernhardt, P. V.; Bozogliꢀan, F.; Macpherson, B. P.; Martínez,
M.; Merbach, A. E.; Gonzꢀalez, G.; Sienra, B. Inorg. Chem. 2004,
43, 7187–7195.
(38) Crespo, M.; Font-Bardía, M.; Granell, J.; Martínez, M.; Solans,
X. Dalton Trans. 2003, 3763–3769.
(39) Gꢀomez, M.; Granell, J.; Martínez, M. Inorg. Chem. Commun.
2002, 5, 67–70.
Financial support from the Ministerio de Educaciꢀon y Ciencia
and the Generalitat de Catalunya, Grants CTQ2009-14443-C02-
02, CTQ2008-06670-C02-01 and 2009SGR-1459, is acknowl-
edged. We also thank the “Laboratoire European Associꢀe”
CTPMM (Chimie Trans-Pyrꢀenꢀeenne: de la Molꢀecule aux
Matꢀeriaux), cofunded by the French CNRS and the Generalitat
de Catalunya, for a travel grant to M.R.-Z.
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