Combined low temperature rapid scan and1H NMR mechanistic study of the protonation and subsequent benzene elimination from a (diimine)platinum(ll) diphenyl complex relevant to arene C-H activation

Article abstract of DOI:10.1021/ic9005746

A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (dilmine)Pt^II dlphenyl complex (denoted as (N-N)PtPh₂) has been undertaken in dichloromethane solution with and without acetonitrile as a c

Full text of DOI:10.1021/ic9005746

9092 Inorg. Chem. 2009, 48, 9092–9103
DOI: 10.1021/ic9005746
Combined Low Temperature Rapid Scan and 1H NMR Mechanistic Study of the
Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II)
Diphenyl Complex Relevant to Arene C-H Activation
†
,†
,‡
ꢀ
ꢀ ‡
Jerome Parmene, Ivana Ivanovic-Burmazovic, Mats Tilset,* and Rudi van Eldik*
†Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box
1033 Blindern, N-0315 Oslo, Norway, and ‡Inorganic Chemistry, Department of Chemistry and Pharmacy,
::
University of Erlangen-Nurnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany
Received March 24, 2009
A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (diimine)PtII diphenyl
complex (denoted as (N-N)PtPh2) has been undertaken in dichloromethane solution with and without acetonitrile as
a cosolvent. Spectroscopic monitoring of the reactions by UV-vis stopped-flow and NMR techniques over the
temperature range -80 to +27 °C allowed the assessment of the effects of acid concentration, coordinating solvent
(MeCN) concentration, temperature, and pressure. Protonation of (N-N)PtPh2 with HBF4 Et2O in CH2Cl2/MeCN
3
occurs with a kinetic preference for protonation at the metal, rather than at a phenyl ligand, and rapidly produces
(N-N)PtPh2H(NCMe)+ (ΔHq = 29 ( 1 kJ mol-1, ΔSq = -47 ( 4 J K-1 mol-1). At higher temperatures,
(N-N)PtPh2H(NCMe)+ eliminates benzene to furnish (N-N)PtPh(NCMe)+. This reaction proceeds by rate-limiting
MeCN dissociation (ΔHq = 88 ( 2 kJ mol-1, ΔSq = +62 ( 6 J K-1 mol-1, ΔVq = +16 ( 2 cm3 mol-1). Protonation of
(N-N)PtPh2 in dichloromethane in the absence of MeCN cleanly produces the Pt(II) π-benzene complex
(N-N)PtPh(η2-C6H6)+ at low temperatures. Addition of MeCN to a solution of the π-benzene complex causes
an associative substitution of benzene by acetonitrile, the kinetics of which were monitored by 1H NMR (ΔHq = 39 ( 2
kJ mol-1, ΔSq = -126 ( 11 J K-1 mol-1). When the stronger triflic acid is employed in dichloromethane/acetonitrile,
a second protonation-induced reaction also occurs. Thus, (N-N)PtPh(NCMe)+ produces (N-N)Pt(NCMe)22+ and
benzene with no detectable intermediates (ΔHq = 69 ( 1 kJ mol-1, ΔSq = -43 ( 3 J K-1 mol-1). The mechanisms
for all steps are discussed in view of the accumulated data. Interestingly, the data allow a reinterpretation of a previous
report on proton exchange between the phenyl and benzene ligands in (N-N)PtPh(η2-C6H6)+. It appears that the
exchange occurs by a direct σ-bond metathesis pathway, rather than by the oxidative cleavage/reductive coupling
sequence that was proposed.
Introduction
from detailed experimental studies in aqueous media, and
on model systems in other solvents, and the activation and
functionalization of hydrocarbons in the often-termed
“Shilov system” are considered to take place by the general
three-step mechanism2,8-14 depicted in Scheme 1.
A considerable amount of mechanistic insight has been
gained from studies of the microscopic reverse of the C-H
activation step, that is, protonolysis of Pt-alkyl and Pt-aryl
complexes. We and others have investigated relevant reactions
The selective, catalytic functionalization of hydrocarbons
remains a golden target for contemporary chemists, with
tremendous technological and industrial implications.1-3 In
the late 1960s, the groups of Garnett4,5 and Shilov6,7 estab-
lished that Pt(II) salts in aqueous acidic solutions are capable
of activating and even functionalizing hydrocarbon C-H
bonds. Considerable mechanistic insight has been gained
*To whom correspondence should be addressed. E-mail: mats.tilset@
kjemi.uio.no (M.T.), rudi.vaneldik@chemie.uni-erlangen.de (R.v.E.).
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pubs.acs.org/IC
Published on Web 09/10/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9093
Scheme 1. Proposed Mechanism for Alkane Functionalization in the
Shilov System
In Scheme 2, associative ligand substitutions (first of H2O
by TFE = trifluoroethanol, then of TFE by benzene) initially
lead to a Pt(II) η2-benzene complex. Oxidative C-H cleavage
furnishes a putative Pt(IV) hydrido species which has not
been directly observed under actual C-H activating condi-
tions. Reversal of the latter process, encompassing the species
enclosed in brackets in Scheme 2, facilitates the experimen-
tally observed isotopic scrambling from C6D6 into the Pt-
CH3 group during benzene C-H(D) activation reactions.16
Importantly, the η2-arene Pt(II) species have been observed
during low-temperature protonation of (diimine)PtMePh
and (diimine)PtPh2 complexes in poorly coordinating sol-
vents such as dichloromethane.18,23 Indeed, there is substan-
tial evidence that oxidative addition of an arene at an
unsaturated metal center usually, although not without
exceptions,41,42 proceeds via η2-(C,C) pre-complexation of
the arene.9,43-52 This may be followed by an arene “slip” to
an η2-(C,H)18,45,53 or even η1-(C)54 coordination mode from
which the oxidative cleavage of the C-H bond occurs. We
recently described that protonation of (diimine)PtPh2 species
in dichloromethane at low temperatures produced observa-
ble (diimine)Pt(C6H5)(η2-C6H6)+ species. By NMR ex-
change spectroscopy (EXSY), it was demonstrated that
rapid site exchange of protons between the phenyl and π-
benzene moieties occurred. The kinetics of the exchange
processes were established by quantitative 2D EXSY mea-
surements. A rapid proton exchange that involved oxidative
cleavage to furnish a putative (diimine)PtPh2H(L)+ inter-
mediate (L = loosely coordinated ligand or vacant site)
originally postulated.23 However, a direct proton transfer
by a σ-bond metathesis (or a σ-complex assisted metathesis,
σ-CAM55) pathway has been proposed to be more likely as
suggested by recent density functional theory (DFT) calcula-
tions;56 this possibility will be further elaborated in the
Discussion section here.
at (diimine)Pt(II) and Pt(IV) complexes.8,15,16 One impor-
tant finding has been that low-temperature protonation of
(diimine)Pt(II) dialkyl and diaryl complexes leads to observa-
ble, but thermally sensitive, Pt(IV) hydridoalkyl and hydri-
doaryl complexes that eliminate the respective hydrocarbons
upon heating.17-23 Such Pt(IV) species have been proposed to
be involved in arene C-H activation at (diimine)PtMe(solv)+
species, where solv is a solvent molecule, in solution. Scheme 2
summarizes the mechanistic picture that has emerged for these
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9094 Inorganic Chemistry, Vol. 48, No. 19, 2009
Parmene et al.
Scheme 2. Mechanism for Benzene C-H Activation, As Previously Proposed16,18,25
Scheme 3. Sequence of Reactions Studied in This Contributiona
a Kinetic studies have been performed for all reactions except the lower-left protonation reaction.
from this Pt(IV) species.22 The latter reaction was found, by
variable-temperature and variable-pressure kinetic measure-
ments, to proceed by initial and rate-limiting MeCN disso-
ciation. In the present contribution, we report the details of
kinetic studies of protonation reactions and benzene-produ-
cing reactions at Pt(II) and Pt(IV) species that are derived
from (N-N)PtPh2, where N-N represents the specific
diimine Ar-N=CMe-CMe=N-Ar; Ar = 2,6-Me2C6H3.
Concentration, temperature, and pressure-dependent kinetic
measurements allow us to evaluate the kinetics for most of
the reactions that are depicted in Scheme 3.
was recently obtained from two successive protonation/
methane elimination sequences from (N-N)PtMe2 and
TfOH.58 This reaction sequence has now been subjected
to a systematic kinetic investigation by time-resolved
stopped-flow techniques with UV-vis monitoring of
the reaction, with HBF4 Et2O or TfOH as acids, in
3
mixtures of dichloromethane/acetonitrile, in the tempera-
ture range -80 °C to +27 °C. To our satisfaction, the
same three-step sequence was clearly observed by NMR
and by UV-vis stopped-flow spectroscopic methods.
The first reaction step, the protonation of (N-N)-
1
PtPh2, was carefully studied by H NMR at -78 °C in
Results
MeCN/CH2Cl2 mixtures of different compositions. In all
cases, (N-N)PtPh2 reacted promptly to give mostly
(N-N)PtPh2H(NCMe)+, with trace quantities (ca.
2-10%) of (N-N)PtPh(NCMe)+ and benzene. There
was no clear trend in the relative quantities of
(N-N)PtPh(NCMe)+ as a function of [MeCN] in the
range 0.27-8.7 M. This contrasts with the findings from
analogous experiments in which the protonation of
(diimine)PtMe2 complexes were studied: a consistent
decrease in the yield of (diimine)PtMe(NCMe)+, relative
to (diimine)PtMe2H(NCMe)+, with increasing [MeCN]
present was taken as evidence that the metal, rather than
the methyl ligand, was the kinetically preferred site of
protonation.20,21 The lack of a clear-cut trend in the
present system renders this question unresolved for the
time being concerning the protonation of (N-N)PtPh2.
The presence of both products might indicate that two
reaction channels are available. In the absence of other,
more conclusive evidence, we surmise that the fact that
(N-N)PtPh2H(NCMe)+ is the predominant product
1. Protonation of (N-N)PtPh2 in the Presence of
MeCN. The protonation of (N-N)PtPh2 with HBF4
3
Et2O in mixtures of CD2Cl2 and CD3CN was previously
studied by 1H NMR spectroscopy,23 and provides
(N-N)PtPh2H(NCMe)+ as indicated in Scheme 3. While
this Pt(IV) species is stable at -78 °C, benzene is released
at about -40 °C with concomitant formation of
(N-N)PtPh(NCMe)+ with no detectable intermediates.
Furthermore, we now find that when TfOH in MeCN
(expected to be a stronger acid than HBF4 Et2O;57 pro-
3
tonated Et2O rather than HBF4 is here the actual proton
source) is employed, a second elimination of benzene is
seen at ambient temperature conditions, producing the
Pt(II) dication (N-N)Pt(MeCN)22+. The same species
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Organometallics 2007, 26, 294–301.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9095
Figure 2. kobs vs [HBF4 Et2O] for the protonation of (N-N)PtPh2 in
3
CH2Cl2 at -80 °C. [Pt] = 0.125 mM, [MeCN] = 5.74 M (30% v/v).
Figure 1. Time-resolved spectra for the protonation of (N-N)PtPh2
with HBF4 Et2O in CH2Cl2 at -80 °C. Experimental conditions: [Pt] =
3
0.125 mM, [MeCN] = 5.74 M (30% v/v), [HBF4 Et2O] = 0.0124 M. The
total duration of the experiment was 3.0 s.
3
suggests (but does not prove) that protonation at the
metal is kinetically preferred. This assumption is sup-
ported by recent DFT calculations.56 This subject will be
further addressed in the Discussion section.
The protonation event was monitored in the tempera-
ture range -80 to -50 °C by time-resolved UV-vis
stopped-flow spectroscopy. The concentrations of acid
and acetonitrile in the dichloromethane solvent were
systematically changed at -80 °C, always in a large excess
relative to (N-N)PtPh2 so as to ensure pseudo-first-order
conditions. The protonation results in a characteristic
decay in absorbance, and the changes observed in a
typical time-resolved experiment are depicted in Figure 1.
The spectral changes could in all cases be nicely fitted to a
pseudo-first-order decay. The resulting pseudo-first-or-
der rate constant was independent of [MeCN] in the range
0.95-5.7 M (5 to 30% v/v; see data in Table S1, Support-
ing Information). Lower [MeCN] could not be used
because of the poor solubility of the acid in dichloro-
methane at low temperatures. The dependence of the
Figure 3. Eyring plot for the protonation of (N-N)PtPh2 with
HBF4 Et2O in CH2Cl2. Experimental conditions: [Pt] = 0.125 mM,
[MeCN] = 5.74 M (30% v/v), [HBF4 Et2O] = 0.0124 mM.
3
3
were determined and are suggestive of an associative
mechanism. The rate constant was reported as 15200 (
400 M-1 s-1 at -78 °C. Protonation of the present
(N-N)PtPh2 system appears to be about 40 times slower
than protonation of (N-N)PtMe2 at -78 °C. This may
reflect a combined effect of the relative electron withdraw-
ing effect of Ph versus Me at the metal center and the
increased steric bulk of Ph versus Me. It was suggested that
(N-N)PtMe2 underwent rate-determining protonation to
yield a transient five-coordinate species which was rapidly
trapped by the apical coordination of MeCN. We assume
on the basis of the kinetic data and qualitative similarities
(acid and MeCN dependence) that the same holds true for
the current system, although a concerted protonation/
acetonitrile addition cannot be ruled out on the basis of
the available experimental evidence.
2. Elimination of Benzene from (N-N)PtPh2H(NC-
Me)+. Upon increasing the temperature to above about
-40 °C, (N-N)PtPh2H(NCMe)+ starts to release ben-
zene to cleanly furnish (N-N)PtPh(MeCN)+ without
detectable intermediates, as previously seen by 1H
NMR spectroscopy.23 In the following, we describe the
details of a kinetic investigation of this process in the
temperature range -10 to +27 °C. The stopped-
flow technique allows instant mixing of separately
pseudo-first-order rate constant on [HBF4 Et2O] (8.8-
3
37.2 mM) at -80 °C in the presence of 30% (v/v) MeCN
is shown in Figure 2 (data in Table S2, Supporting
Information). Clearly, a first-order dependence is seen,
and the calculated second-order rate constant by linear
regression gives k (-80 °C) = 290 ( 20 M-1 s-1 (see
Table S3, Supporting Information).
The temperature dependence of the rate constant for
protonation was measured by monitoring the reaction of
(N-N)PtPh2 with a 12.4 mM HBF4 Et2O solution in
3
dichloromethane-30% MeCN (v/v; 5.74 M) in the tem-
perature range -80 °C to -50 °C (data in Table S3,
Supporting Information). The Eyring plot in Figure 3
shows an excellent linear fit, and the resulting kinetic
parameters are k(-80 °C) = 290 ( 20 M-1 s-1, ΔHq =
28.8 ( 0.8 kJ mol-1, and ΔSq = -47 ( 4 J K-1 mol-1
.
The protonation has a rather small ΔHq and substan-
tially negative ΔSq under the studied conditions. This is in
accordance with the associative nature of the reaction.
These data may be compared with our previously pub-
lished data22 on the protonation of (N-N)PtMe2,
namely, ΔHq = 15 kJ mol-1 and ΔSq = -85 J K-1 mol-1
prepared solutions of (N-N)PtPh2 and HBF4 Et2O in
3

9096 Inorganic Chemistry, Vol. 48, No. 19, 2009
Parmene et al.
Figure 5. Eyring plot for the kinetic data for the elimination of
benzene from (N-N)PtPh2H(NCMe)+ when HBF4 Et2O was the
acid used to initially protonate (N-N)PtPh2 (see text). Experimental
conditions: [Pt] = 0.125 mM, [MeCN] = 5.7 M (30% v/v), [HBF4] =
0.0124 M.
3
Figure 4. Time-resolved spectra for the elimination of benzene from
(N-N)PtPh2H(NCMe)+ in CH2Cl2 at 0 °C. Experimental conditions:
[Pt] = 0.125 mM, [MeCN] = 5.74 M (30% v/v), [HBF4 Et2O] = 12.4
mM. The total duration of the experiment was 35 s.
3
dichloromethane/acetonitrile mixtures to immediately
furnish (N-N)PtPh2H(NCMe)+. The ensuing, much
slower, benzene elimination can be conveniently moni-
tored by UV-vis spectroscopy at varying [HBF4],
[MeCN], temperatures, and pressures. A typical time-
resolved UV-vis spectrum of the reaction is depicted in
Figure 4 and is characterized by an increase in absorbance
in the selected wavelength region with time. The spectral
changes are nicely described by first-order kinetics. The
kinetic measurements were done by monitoring the spec-
tral changes over the whole spectral range. The excellent
first-order kinetic fits that were obtained suggest the
smooth transformation of (N-N)PtPh2H(NCMe)+ to
(N-N)PtPh(NCMe)+ without any observable inter-
mediates.
The rate of the reaction was studied at different acet-
onitrile concentrations. In the concentration range 0.95 to
5.7 M, only a slight effect of [MeCN] on the rate of
benzene elimination was seen (ca. 20% rate increase in
the concentration range; data in Table S4, Supporting
Information). The kinetics of the benzene elimination
Figure 6. Pressure-dependent rate constants for the elimination of
benzene from (N-N)PtPh2H(NCMe)+ at 20 °C. Experimental condi-
tions: [Pt] = 0.125 mM, [MeCN] = 5.7 M (30% v/v), [HBF4 Et2O] =
0.0062 M.
3
that has been previously thoroughly described,59 using a
0.125 mM solution of (N-N)PtPh2 and 0.0062 M
were also evaluated at varying [HBF4 Et2O] in dichloro-
3
HBF4 Et2O in dichloromethane with 5.7 M MeCN
methane containing acetonitrile (5.7 M, 30% v/v) at
27 °C (Table S5, Supporting Information). No significant
effect of the acid concentration on the reaction rate could
be discerned.
3
(30% v/v). The measured rate constants are given in
Table S7, Supporting Information. The good linear fit
that is evident from Figure 6 provides an activation
volume ΔVq = +16 ( 2 cm3 mol-1
.
Variable-temperature kinetic measurements of the ben-
zene elimination were done by recording the time-re-
solved UV-vis spectra during the elimination of
benzene when the initial protonation was done with
In summary, the kinetics for the benzene elimination
from (N-N)PtPh2H(NCMe)+ exhibit a rather high
ΔHq, a large positive ΔSq, and a large positive ΔVq. The
collective data strongly suggest a dissociative character of
the rate-limiting step, believed to be MeCN dissociation,
for the benzene elimination. The data are qualitatively
similar to those determined22 for the elimination of
methane from (N-N)PtMe2H(NCMe)+, for which
12.4 mM HBF4 Et2O in dichloromethane containing
3
5.74 M MeCN (30% v/v) in the temperature range -10
to +20 °C. The kinetic data are summarized in Table S6,
Supporting Information. Figure 5 shows an Eyring plot
of the kinetic data, from which the following kinetic
parameters were extracted: k (20 °C) = 2.3 s-1, ΔHq =
ΔHq = 75 ( 1 kJ mol-1, ΔSq = +38 ( 5 J K-1 mol-1
,
and ΔVq = +18 ( 1 cm3 mol-1, and for which a
mechanism that is dissociative with respect to MeCN
was proposed.
88 ( 2 kJ mol-1, and ΔSq = +62 ( 6 J K-1 mol-1
.
Finally, pressure-dependent kinetic measurements
were performed for benzene elimination at pressures
between 10 and 125 MPa at 20 °C. The volume of
activation ΔVq was calculated for the reaction from the
slope of plots of ln k versus pressure (Figure 6) in the way
(59) van Eldik, R.; Hubbard, C. D. In Chemistry at Extreme Conditions;
Manaa, M. R., Ed.; Elsevier: Amsterdam, 2005, pp 109-164.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9097
Figure 8. kobs vs [TfOH] for the second protonation/elimination se-
quence in CH2Cl2. Experimental conditions: [Pt] 0.125 mM, [MeCN] =
5.74 M (30% v/v), and T = 25 °C.
Figure 7. Time-resolved UV-vis spectra for the protonation/benzene eli-
mination sequence from (N-N)PtPh2 in dichloromethane/acetonitrile at 25
°C. Experimental conditions: [Pt] = 0.125 mM, [MeCN] = 5.74 M (30% v/v),
[TfOH] = 0.013 M. The total duration of the experiment was 30 min.
3. Second Protonation and Benzene Elimination. As
mentioned in the Introduction, (N-N)PtPh2 reacts with
TfOH in acetonitrile at ambient temperature to immedi-
ately furnish (N-N)PtPh(NCMe)+ and then, more
2+
slowly, the (N-N)Pt(NCMe)2
dication. Analogous
behavior has been reported for (diimine)PtMe2 species,
where treatment with TfOH can ultimately lead to the
dicationic Pt complexes.58
The kinetics for this protonation/benzene elimination
reaction was pursued with the stopped-flow UV-vis
spectroscopic method in dichloromethane/acetonitrile
mixtures. At 25 °C, the time-resolved UV-vis spectra
(Figure 7) show characteristic absorption bands decaying
in intensity at 320 and 430 nm, and bands of increasing
intensity at 280 and 350 nm. Three well-defined isosbestic
points are seen at 300, 346, and 360 nm. The first spectrum
is that of (N-N)PtPh(NCMe)+ and the last matches that
of (N-N)Pt(NCMe)22+, and the isosbestic points estab-
lish that no detectable intermediates build up during
the reaction. The reaction rate was extracted from the
changes in absorbance at 410 nm, and the data were nicely
described by a single exponential decay suggestive of
pseudo-first-order kinetic behavior in [Pt].
Figure 9. Eyring plot for the kinetic data for the second protonation/
benzene elimination sequence from (N-N)PtPh2. Experimental con-
ditions: [Pt] = 0.125 mM, [MeCN] = 5.74 M (30% v/v), [TfOH] =
0.13 M in dichloromethane.
time-resolved UV-vis spectra for the reaction with
0.13 M TfOH in dichloromethane containing 5.74 M
MeCN (30% v/v) in the temperature range 7-29 °C.
The resulting kinetic data are provided in Table S9,
Supporting Information. The Eyring plot in Figure 9
shows an excellent linear fit and gives the kinetic para-
meters: k (25 °C) = 0.033 M-1 s-1, ΔHq = 68.5 ( 1.0 kJ
mol-1, and ΔSq = -43 ( 3 J K-1 mol-1
.
The kinetics were further evaluated by UV-vis mon-
itoring of the reaction at 25 °C to study the influence of
acid concentration on this step by varying [TfOH] in the
range 0.013-0.13 M. In each case, the reaction exhibited
pseudo-first-order kinetic behavior, and the observed rate
increased linearly with [TfOH] (Figure 8; data in Table
S8, Supporting Information). Thus, the reaction is first-
order with respect to the acid, analogous to the first
protonation of (N-N)PtPh2 at low temperatures. From
the slope of the plot in Figure 8, the second-order rate
constant for the protonation was determined as 0.38 (
4. Substitution of Benzene on (N-N)PtPh(η2-C6H6)+
by Acetonitrile in Dichloromethane. We have recently
reported that protonation of a series of (diimine)PtPh2
complexes (diimine=Ar-N=CMe-CMe=N-Ar with
differently substituted N-aryl groups) in dichloro-
methane at -78 °C results in the formation of (diimine)-
PtPh(η2-C6H6)+ complexes that were characterized by
1H NMR spectroscopy.23 When Ar was a 2,6-dimethyl
substituted phenyl group, these π-benzene complexes
were relatively stable at temperatures up to about
-40 °C. On the other hand, when Ar was unsubstituted
at the 2,6-positions, the π-benzene complexes rapidly
reacted by liberating benzene even at -78 °C. This might
be a result of considerably faster associative substitution
reactions at the π-benzene complexes that are 2,6-unsub-
stituted: For steric reasons, the N-aryl groups are or-
iented more or less perpendicular to the coordination
plane at Pt.23 Thence, substituents in the 2,6-positions will
sterically block access to Pt from above and below the
0.03 M-1 s-1 at 25 °C. Interestingly, when HBF4 Et2O
3
was used as the acid (0.13 M), the pseudo-first-order rate
constant was estimated to be as low as 3 ꢀ 10-5 s-1
corresponding to 2.3 ꢀ 10-4 M-1 s-1. The significantly
slower reaction is in accord with HBF4 Et2O being a
3
considerably weaker acid than TfOH.57
Variable-temperature kinetic measurements of the pro-
tonation/benzene elimination were done by recording the

9098 Inorganic Chemistry, Vol. 48, No. 19, 2009
Parmene et al.
coordination plane for any incoming nucleophile. The
substitution kinetics at the reasonably stable, N-aryl 2,6-
dimethyl substituted π-benzene complex (N-N)PtPh(η2-
C6H6)+ are described in the following.
A solution of the π-benzene complex (N-N)PtPh(η2-
C6H6)+ in dichloromethane-d2 was prepared in situ in an
NMR-tube by addition of HBF4 Et2O to a solution of
3
(N-N)PtPh2 at -78 °C. Controlled quantities of acet-
onitrile-d3 in excess (to ensure pseudo-first-order con-
ditions) were added at -78 °C, and the NMR tube was
inserted into the precooled NMR probe so that the
progress of the ensuing reaction could be monitored by
1H NMR. At the temperatures investigated, smooth
release of benzene and the formation of (N-N)PtPh-
(NCCD3)+ was observed. The rate of the reaction was
determined by integration of the Pt-Ph signals in the
NMR spectra using the residual proton signals of the
solvent as an internal standard. The disappearance of the
starting complex exhibited first-order kinetics for at least
3-4 half-lives of the reaction. The pseudo-first-order rate
constant varied linearly with [acetonitrile-d3], as illu-
strated in Figure 10. The near-zero intercept, (-6 ( 7) ꢀ
10-5 s-1, suggests that there is no MeCN-unassisted me-
chanism operating in parallel with the associative one
implied by the data. The kinetic data are summarized in
Table S10, Supporting Information. The average second-
order rate constant was determined from these data as
k = (5.70 ( 0.05) ꢀ 10-4 M-1 s-1 at -55 °C.
Figure 10. kobs vs [CD3CN] for the substitution of benzene at
(N-N)PtPh(C6H6)+ in dichloromethane-d2. Experimental conditions:
[Pt] = 0.023 mM, [MeCN] = 5.7 M (30% v/v), T = -55 °C. HBF4 Et2O
3
was used for in situ protonation of (N-N)PtPh2 before addition of
CD3CN.
Importantly, there was no evidence for formation of the
Pt(IV) hydrido complex (N-N)PtPh2H(NCCD3)+ in
these experiments. This compound, if it were formed,
would have been stable under the reaction conditions on
the experimental time scale. In a separate experiment,
EXSY spectra of (N-N)PtPh(η2-C6H6)+ were recorded
in dichloromethane-d2 in the absence of acetonitrile-d3
and in the presence60 of 54 mM acetonitrile-d3 at -48 C.
In both cases, EXSY correlation peaks between phenyl
and π-benzene signals were seen, as described previously,
and with intensities that were independent of the presence
(or not) of acetonitrile. These findings provide compelling
evidence that the exchange processes seen by EXSY
cannot proceed via the intermediacy of (N-N)Pt-
Ph2H+, which, if formed, should have been efficiently
trapped by acetonitrile to furnish the stable, readily
observable (N-N)PtPh2H(NCCD3)+. (The acetonitrile
concentration for this EXSY experiment, representing
an about 2-fold excess relative to Pt, is considerably
lower than that used for the substitution kinetics.
Higher [MeCN] led to substitution of benzene, furnish-
ing (N-N)PtPh(NCMe) exclusively, before significant
EXSY peaks could be detected).
Figure 11. Eyring plot for the kinetic data for the substitution of
benzene with acetonitrile at (N-N)PtPh(C6H6)+ when HBF4 Et2O
3
was the acid used for the in situ protonation of (N-N)PtPh2 before
addition of CD3CN to the solution in dichloromethane-d2. Experimental
conditions: [Pt] = 0.023 M, [MeCN] = 2.7 M, [HBF4] = 0.10 M.
acetonitrile. This is in agreement with recent DFT calcu-
lations which suggest a solvent-induced associative ben-
zene elimination.56
Discussion
The accumulated kinetic and mechanistic data that have
been presented in the Results section are consistent with the
overall reaction mechanism that is shown in Scheme 4 for the
protonation of (N-N)PtPh2 (A) and the following benzene-
producing reactions. The details of this mechanism will now
be the subject of discussion. It will be useful in this context to
consult the data in Table 1, which lists rate constants for each
step (where available). The data have been determined from
the activation parameters described in the Results section
or from data available elsewhere, at the temperatures -78,
-40, 0, and +30 °C.
Low-Temperature Protonation of (N-N)PtPh2. The
protonation of complex A in dichloromethane/acetoni-
trile, observed as a fast reaction even at -80 °C, is
postulated in Scheme 4 to occur as a two-step process
that initially generates the unobserved five-coordinate
intermediate (N-N)PtPh2H+ (B) followed by rapid cap-
ture of MeCN at the remaining vacant site. A concerted
protonation and MeCN coordination cannot be ruled
The temperature dependence of the substitution of
benzene by acetonitrile was evaluated in the temperature
range -55 to -70 °C. Figure 11 shows a linear Eyring plot
of the kinetic data which are given in Table S11, Support-
ing Information. The derived activation parameters are
ΔHq = 39 ( 2 kJ mol-1 and ΔSq = -126 ( 11 J K-1
mol-1. The substantially negative entropy of activation
and the first-order kinetic behavior of the rate on [MeCN]
strongly imply an associative benzene substitution by
(60) We thank a reviewer for suggesting this experiment and for useful
comments.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9099
Scheme 4. Proposed Mechanistic Scheme for the Benzene Producing Reactions under Study
Table 1. Summary of Approximate, Available First-Order or Pseudo-First-Order
Scheme 5. Two Possible Sites of Protonation at (N-N)PtPh2
Rate Constants (s-1) for the Reactions in Scheme 4 at Selected Temperaturesa
temperature
k1[H+ b
-78 °C
-40 °C
75
0 °C
770
+30 °C
3000
7.5
<430
0.85
]
3.5
2.0 ꢀ 10-8
<5.6 ꢀ 10-5
0.00011
0.00016
<0.078
0.0063
1.3 ꢀ 10-6
0.078
0.15
<18
0.14
0.00027
18
c
k-2
k-3
d
k4[MeCN]e
k5[H+ f
]
1.0 ꢀ 10-9
0.0060
430
k6
5.6 ꢀ 10-5
g
a Data are derived from activation parameters determined in this
study unless otherwise noted. b Second-order rate constant derived from
the low-temperature protonation kinetics multiplied by [HBF4] taken as
0.0124 M. c First-order rate constant derived from the kinetics of
benzene elimination from (N-N)PtPh2H(NCMe)+, assuming rate-
determining MeCN dissociation. d An upper limit to the value for k-3
is given by k6, see text. e Second-order rate constant derived from the
benzene substitution kinetics multiplied by [MeCN] taken as 2.7 M.
f Second-order rate constant derived from the high-temperature proton-
ation kinetics multiplied by [TfOH] taken as 0.13 M. g First-order rate
constant derived from kinetic data for H/H site-exchange between the
phenyl and benzene ligands in (N-N)PtPh(η2-C6H6)+, see ref 23.
(RC denotes “reductive coupling” and OC “oxidative
cleavage” as suggested elsewhere61). As argued in the case
of protonation at (diimine)PtMe2,20,21 the kinetically
preferred site can be assessed provided that a [MeCN]-
dependent product distribution is observed. Kinetic pro-
tonation at Pt should under such circumstances lead to a
C/E ratio that increases with increasing [MeCN], whereas
a kinetic preference for protonation at Ph should lead to a
C/E ratio that decreases with increasing [MeCN]. The
experimental results established that over the [MeCN]
range 0.27-8.18 M, more than 90% of C was always
produced at -78 °C; traces of E were always seen but
there was no systematic trend in the C/E ratio with
changes in [MeCN]. A C/E ratio that appears to be
independent of [MeCN] can be explained by two possible
scenarios based in Scheme 5: (i) Competing protonation
at Pt and Ph with a strong preference for the former (kPt
> kPh), combined with slow rates of interconversion kRC
and kOC. The C/E ratio simply reflects the relative kPt/kPh
rates. (ii) Protonation to give B and/or D followed by fast
interconversion rates and much slower product-forming
out, as stated in the Results section. The protonation is
slower than that at the analogous (N-N)PtMe2 complex
by a factor of about 40 at -78 °C under comparable
conditions. The difference may be attributed to a combi-
nation of the increased steric bulk and decreased donor
power of the phenyl ligands versus the methyl ligands
at Pt.
One important question regarding the protonation
event is whether the kinetically preferred site of proton-
ation is the metal (rate constant kPt) or a phenyl ligand
(kPh), see Scheme 5. A similar scheme was the basis for our
analysis of the kinetically preferred site of protonation at
(diimine)PtMe2 complexes.20,21 It was concluded that Pt
protonation was preferred in that case. In the present
context, protonation at Pt produces the five-coordinate
intermediate B, which will furnish the observed product C
when trapped by MeCN in the dichloromethane/acetoni-
trile medium. On the other hand, protonation at a phenyl
group produces the π-benzene complex D which is the
observed product in dichloromethane when the proton-
ation is conducted in the absence of MeCN. Despite these
observations, protonation at a phenyl group might be
kinetically preferred even in the presence of MeCN,
provided that the rate of interconversion between B and
D, kRC and kOC, is fast relative to the rates of each of the
product forming steps, ktrap[MeCN] and ksubst[MeCN],
both of which are first-order with respect to [MeCN]
steps. The C/E ratio then depends on the relative ktrap
/
ksubst rates and the equilibrium constant for the B/D
interconversion (Curtin-Hammett conditions62). The
latter explanation can be readily ruled out in our case:
The kinetic data in Table 1 clearly show that at -78 °C, an
upper limit to the rate of interconversion, determined
(61) Jones, W. D. Acc. Chem. Res. 2003, 36, 140–146.
(62) Seeman, J. I. Chem. Rev. 1983, 83, 83–134.

9100 Inorganic Chemistry, Vol. 48, No. 19, 2009
Parmene et al.
experimentally from the rates of proton exchange be-
tween phenyl and η2-benzene ligands in (diimine)-
PtPh(η2-C6H6)þ complexes (see next paragraph), is
orders of magnitude slower than the rate of production
of C in the protonation reaction. Therefore, C cannot be
produced via protonation at Ph to give D followed by
oxidative cleavage to furnish B. Consequently, we con-
clude that the kinetically preferred site of protonation
in this system has to be the metal, furnishing the
Pt(IV) hydride, rather than a phenyl ligand, to furnish
the Pt(II) η2-benzene complex. (An ipso protonation
leading to an unobserved η1-benzene structure, possibly
resembling a Wheland-type intermediate, can however
not be ruled out). Similar arguments were used to arrive at
the same conclusion in a recent DFT study.56 The trace
quantities of E observed at -78 °C might suggest that
protonation at Ph to furnish D is slightly competitive
with the predominant protonation at Pt. On the other
hand, the data in Table 1 suggest that substitution of η2-
C6H6 by MeCN in D should be exceedingly slow com-
pared to the overall rate of protonation at -78 °C, and
therefore E should not have time to form by this pathway
at all on the experimental time scale. We must consider
that the traces of E, which were observed by 1H NMR but
not under UV-vis kinetics conditions (under which
traces would be difficult to detect), might be caused by
experimental artifacts that are currently beyond our
control.
In order that a clear trend can be observed in the
product distribution when the trapping agent concentra-
tion is changed, a relatively delicate balance must exist
between the relative rates of the two trapping reactions
and the rates of interconversion between the intermedi-
ates that will be trapped. Apparently, the requirements
concerning relative rates were met in our previous study
on the protonation of (N-N)PtMe2 species,20 but not in
the present case. An offset of one of the rates by an order
of magnitude or so might be sufficient to prevent the
observation of the sought-after trend. We note that in the
present case, the steric bulk of the phenyl ligands may
retard the trapping by MeCN, but would also similarly
affect the rate of associative hydrocarbon substitution.
Another significant difference is that benzene is associa-
tively displaced in the present case versus methane in the
former,20 and that the barriers to reductive coupling/
oxidative cleavage tend to be lower for sp2 C-H bonds.
Altogether, it is not obvious to us exactly which of the
effects on the relative rates causes the difference in
experimental outcome.
Scheme 6. Oxidative Cleavage (top) and σ-Bond Metathesis (bottom)
Pathways for the Proton Exchange between Phenyl and Benzene
Ligands in (N-N)PtPh(η2-C6H6)þ
underlying exchange mechanism that was originally pro-
posed is the oxidative cleavage depicted in Scheme 6, top
path. The five-coordinate species resulting from the oxi-
dative cleavage of a C-H bond in the benzene ligand is
crucial for the proton site exchange, as well as for the
diimine symmetrization. Since B (whether a true inter-
mediate or a transition state in this process) is expected to
be a relatively high energy species compared to D, the rate
constant kOC can be taken as the rate constant for this
exchange process. Recent DFT calculations56 (computed
ΔGq values at -40 °C) located B about 73 kJ mol-1 higher
in energy than D and they are connected by a 86 kJ mol-1
barrier when going from D to B, that is, the barrier will be
merely 13 kJ mol-1 in the reverse direction. (For these
calculations, the polarized continuum model (PCM) was
used to include ether as the solvent). Interestingly, the
DFT results suggested that the C6H5/η2-C6H6 exchange
process is more likely to occur by a “direct σ-bond
metathesis” in which the proton maintains a weak inter-
action with Pt as it passed along from one phenyl group to
the other in the equatorial plane of the complex. We have
now established that the EXSY correlation peaks are seen
in spectra of (N-N)PtPh(η2-C6H6)þ even in the presence
of acetonitrile. This provides strong evidence for the non-
involvement of the putative intermediate (N-N)Pt-
Ph2Hþ in the exchange process, since it should have
immediately formed (N-N)PtPh2H(NCCD3)þ by trapp-
ing with acetonitrile under the reaction conditions.
The DFT-predicted direct σ-bond metathesis pathway
now is an even more attractive alternative to the oxidative
cleavage/reductive coupling pathway for exchange. Such
a σ-bond metathesis process appears to nicely explain
H/D exchanges between two phenyl groups, via a reason-
ably looking calculated transition-state structure.56 (An
analogous exchange mechanism is not necessarily in-
volved in other reported, related exchange processes; it
appears to us less likely to be involved in exchanges
between two methyl groups,17,20 or between phenyl and
methyl groups16,18). The computed transition-state free
energy for this process, which closely resembles the
transition state in what Perutz and Sabo-Etienne55 have
termed a “σ-complex assisted metathesis” (σ-CAM), was
62 kJ mol-1, that is, about 24 kJ mol-1 below that for the
oxidative cleavage pathway. If these computational re-
sults give a true description of the mechanism, then one
Reductive Coupling and Oxidative Cleavage. The ki-
netics of the proposed reversible interconversion between
B and D in Scheme 5 has been described elsewhere,23 but
some comments need to be made here. It was seen by 2D
EXSY 1H NMR spectroscopic experiments on
(N-N)PtPh(η2-C6H6)þ D in dichloromethane-d2 that
the protons of the C6H5 and η2-C6H6 ligands underwent
mutual site exchange. This was accompanied by exchange
between the two non-equivalent halves of the diimine
ligand. The kinetics of these exchange processes were
evaluated by quantitative EXSY spectroscopy, and it
was found that the diimine-based exchange occurred at
the same rate as the C6H5/η2-C6H6 exchange, with ΔGq
about 61 kJ mol-1 in dichloromethane-d2 at -40 °C. The

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9101
obvious implication will be that the numerical value that
is used for kOC is an upper limit to the real value: The
EXSY kinetics represent the energetics for reaching the
transition-state for the direct exchange process; the
transition state for the oxidative cleavage must necessa-
rily be higher in energy and thence the corresponding
rate must be even slower;which strengthens the argu-
ments that led to the conclusion in the previous para-
graph: Pt is the kinetically preferred site of protonation.
We note that kinetic isotope effects obtained by reactions
with HX versus DX acids might shed further light on the
nature of the initial protonation event, as demonstrated in
a recent contribution by the Bercaw63 and Romeo
groups,64 and we hope to report on this in a future
contribution.
there are a few literature reports on isolable, five-coordi-
nate Pt(IV) alkyl complexes.68-74
Substitution of Benzene by Acetonitrile at
(N-N)PtPh(η2-C6H6)þ. We have reported23 that
(diimine)PtPh(η2-C6H6)þ complexes are readily available
by protonation of (diimine)PtPh2 complexes in dichlor-
omethane at about -78 °C. It was qualitatively seen that
the stability of (diimine)PtPh(η2-C6H6)þ complexes with
respect to benzene loss depended highly on the substitu-
ents at the N-aryl groups of the diimine ligand. Com-
plexes that are 2,6-dimethyl substituted on the N-aryl
groups slowly start to lose benzene at about -40 °C under
these conditions, whereas 2,6-unsubstituted analogs start
to lose benzene at a substantial rate even at -78 °C. These
qualitative stability differences are readily understood if it
is assumed that benzene loss occurs by an associative
mechanism. Since the N-aryl groups are oriented more or
less perpendicularly with respect to the coordination
plane of Pt,23 the N-aryl methyl substituents will serve
to sterically protect the Pt center from nucleophilic attack
by any incoming ligand that approaches from above or
below the coordination plane. Experimental evidence
suggests that substitution of methane by acetonitrile in
(diimine)PtMe(σ-CH4)þ complexes,19 and of toluene
by acetonitrile in related (diimine)Pt(o/m/p-tolyl)(η2-
toluene)þ species, are associative in nature. However,
the kinetics of these processes have not been investigated
in any detail.
The experimental results that have been presented
herein leaves no doubt that substitution of benzene by
acetonitrile in (N-N)PtPh(η2-C6H6)þ is an associative
process. The first-order dependence of kobs on [MeCN],
with the near-zero intercept of the kobs versus [MeCN]
plot in Figure 10, as well as the substantially negative ΔSq,
strongly support this notion. Thus, it appears that hydro-
carbon loss from cationic diimine-supported complexes
will generally proceed by an associative substitution
mechanism in a good donor solvent like acetonitrile. It
is noteworthy that addition of MeCN to (N-N)PtPh(η2-
C6H6)þ produces only (N-N)PtPh(NCMe)þ and not
(N-N)PtPh2H(NCMe)þ, which means that with refer-
ence to Scheme 5, it must be required that the rate
constant for benzene substitution, k4[MeCN], is expected
to be substantially greater than that for oxidative clea-
vage, k-3. This is based on the assumption that the
trapping reaction k2 is rapid, which is suggested by the
fact that (N-N)PtPh2H(NCMe)þ is rapidly and cleanly
produced from (N-N)PtPh2 protonation in the presence
of MeCN. The data in Table 1 show that the rate of site
exchange in (N-N)PtPh(η2-C6H6)þ far outruns the rate
of substitution by MeCN under typical experimental
conditions; further evidence for this is provided by the
observation of EXSY correlation peaks even in the pre-
sence of acetonitrile. This fact provides compelling ex-
perimental evidence that site exchange cannot occur by
Elimination of Benzene from (N-N)PtPh2H(NCMe)þ.
The six-coordinate Pt(IV) hydride is quite stable on the
time scale of all our experiments at -78 °C. When heated,
the compound smoothly undergoes benzene elimination
to furnish (N-N)PtPh(NCMe)þ. The kinetic parameters,
notably the rather large ΔHq and the substantially posi-
tive values for ΔSq and ΔVq, suggest that the rate-limiting
step of this reaction must involve MeCN dissociation.
The data in Table 1 show that at temperatures well below
0 °C, MeCN dissociation (k-2) is considerably slower
than benzene substitution (k4[MeCN]), and within the
framework of Scheme 4 the elimination of benzene from
(N-N)PtPh2H(NCMe)þ to furnish (N-N)PtPh(NC-
Me)þ is expected to proceed without the build-up of
observable intermediates. At temperatures above 0 °C,
benzene substitution is competitive with or even faster
than MeCN dissociation. However, on the assumption
that MeCN dissociation is reversible, the accumulation of
the putative π-benzene intermediate (N-N)PtPh(η2-
C6H6)þ would be negligible also under these conditions.
Thus, the observations are readily reconciled with the
notion that benzene elimination is dissociative and takes
place without build-up of detectable intermediates. The
mechanism is entirely analogous to that observed for the
elimination of methane from (N-N)PtMe2H(NC-
Me)þ.22 It should be mentioned here that reductive
eliminations of X-Y from octahedral L4PtIV(X)(Y) com-
pounds are usually found to be dissociative whenever a
readily dissociable ligand is present; whether the overall
process occurs in a concerted or stepwise fashion may
strongly depend on the nature of the eliminating groups
X and Y, as well as of the ancillary ligands L.11,65-67 The
five-coordinate species (N-N)PtPh2Hþ, a crucial inter-
mediate that is postulated to be in common for the
protonation as well as for the benzene reductive elimina-
tion processes, has not been directly observed, not even
under the rapid-scan conditions utilized here. However,
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9102 Inorganic Chemistry, Vol. 48, No. 19, 2009
Parmene et al.
the oxidative cleavage/reductive coupling pathway. The
combined data are most consistent with site exchange
occurring by a separate pathway, that is, the concerted
σ-bond metathesis pathway indicated in Scheme 6 and
suggested by DFT calculations.56 It should be mentioned
here that directly observable equilibria between Pt(II)
π-benzeneand Pt(IV) hydridophenylcomplexes havebeen
previously described.75 For example, a dynamic equilibri-
um exists at low temperatures between (κ2-HTp0)PtH(η2-
measurements on a well-defined sequence of reactions that
is initiated by protonation of (N-N)PtPh2. Detailed insight
into the kinetics and mechanisms of a cascade of reactions
that ultimately leads to the release of 2 equiv of benzene has
been obtained within the temperature range of -80 to þ
27 °C. The accumulated experimental data are in agreement
with the mechanistic picture that is depicted in Scheme 4. At
low temperatures (ca. -80 °C), protonation of (N-N)PtPh2
with HBF4 Et2O in dichloromethane/acetonitrile occurs at
3
-
C6H6)þ and (κ2-HTp0)PtH2Ph(NCMe)þ (HTp0 = N-
the metal to furnish an unobserved five-coordinate Pt(IV)
complex (N-N)PtPh2Hþ that is immediately trapped to
furnish (N-N)PtPh2H(NCMe)þ. This six-coordinate Pt(IV)
hydride is stable under the low-temperature reaction condi-
tions. At higher temperatures, (N-N)PtPh2H(NCMe)þ is
kinetically less stable: The energetically costly MeCN dis-
sociation occurs and under these conditions, the following
reactions, namely, Ph-H reductive coupling to furnish
(N-N)PtPh(η2-C6H6)þ and the subsequent associative sub-
stitution of acetonitrile for benzene, are quite rapid. Conse-
quently, (N-N)PtPh2H(NCMe)þ produces (N-N)PtPh-
(NCMe)þ without observable intermediates. The next pro-
tonation occurs at a reasonable rate only when the stronger
acid TfOH is utilized and leads to benzene loss without
observable intermediates. The (N-N)PtPh(C6H6)þ complex
can be independently prepared by protonation with
protonated hydridotris(3,5-dimethylpyrazolyl)borate) in
a dichloromethane-d2 solution containing acetonitrile.
These complexes are formally cationic at the pendant,
uncomplexed, protonated pyrazole ring and, importantly,
are neutral at Pt. The latter will tend to destabilize MeCN
binding at Pt(IV) when compared to the situation in the
cationic-at-Pt diimine systems that we describe here and
therefore, a putative MeCN dissociation step should be
much more facile in the HTp0 than in the diimine systems.
Protonation and Benzene Elimination at (N-N)PtPh-
(NCMe)þ. The second protonation of (N-N)PtPh2 oc-
curs at temperatures near ambient after the Pt(II) cation
(N-N)PtPh(NCMe)þ has been generated. This proton-
ation is exceedingly slow when HBF4 Et2O is employed
3
as the acid, but occurs much more rapidly when TfOH is
used. This is in accord with the general notion that TfOH
HBF4 Et2O in dichloromethane. Substitution of benzene
3
is a stronger acid than HBF4 Et2O. The sluggishness of
by acetonitrile at this complex is clearly associative in nature
and furnishes exclusively (N-N)PtPh(NCMe)þ with no
hints for (N-N)PtPh2H(NCMe)þ. This observation pro-
vides indirect evidence that the previously reported exchange
of protons between the phenyl and benzene ligands in
(N-N)PtPh(C6H6)þ probably occurs via a σ-bond metath-
esis pathway, also suggested by DFT calculations, rather
than by an oxidative cleavage/reductive coupling sequence.
The diimine-Pt system offers rich opportunities to investi-
gate the effects of substituents and other parameters on these
and other reactions that are of relevance to C-H bond
activation reactions that are of great practical and academic
interest. Further studies are in progress and will be reported
in due time.
3
the reaction arises from the combined effect of a high ΔHq
(resulting from the necessity to protonate an already
positively charged metal complex) and a strongly negative
ΔSq. No intermediates have been observed for this reac-
tion, perhaps not too surprising, considering the rela-
tively high temperatures that are required for the
protonation. These conditions are expected to lead to
the instant elimination of benzene from a formally doubly
charged Pt(II) π-benzene complex or a doubly charged
Pt(IV) phenyl hydride complex. The kinetic data do not
allow us to draw firm conclusions regarding the site of
protonation, the involvement (or not) of Pt(IV) hydride
species, the involvement (or not) of π-benzene intermedi-
ates, or the associative (or dissociative) nature of the
benzene elimination or substitution. One difference is
particularly striking when the first and second proton-
ation are compared, in addition to the simple fact that the
second protonation is much slower than the first: In the
first protonation/benzene-forming sequence, the ben-
zene-producing reactions are rate limiting reactions fol-
lowing a rapid protonation. In the second protonation/
benzene-producing sequence, the benzene-producing
reactions are rapid reactions following a rate-limiting
protonation. This difference might be a consequence of
the different temperature regimes of the two reaction
sequences: The second protonation occurs at a tempera-
ture sufficiently high to dramatically accelerate the high-
activation energy (as expressed through ΔGq) benzene-
producing process relative to the lower-activation energy
protonation event.
Experimental Section
General Considerations. The solvents for UV-vis stopped-
flow kinetics measurements, p.a. grade and ultra dry acetonitrile
and dichloromethane, were supplied by Acros and were used
without further purification. Deuterated solvents were used as
received without further purification (CD2Cl2, CD3CN,
Et2O-d10). 1H NMR spectra were measured on a Bruker
DRX500 spectrometer. 1H NMR chemical shifts (δ) are re-
ported in ppm relative to TMS using the residual proton
resonances of the solvent (δ 5.32 in CD2Cl2, 1.93 in CD3CN).
The temperature calibration for the low-temperature experi-
ments was done using a thermocouple situated inside a thin glass
tube that was inserted into an NMR tube with methanol.
The complex (N-N)PtPh2 was prepared as described in the
literature23 from Ph2Pt(SMe2)276 and the diimine.77 (Ph2Pt(SMe2)2
has been reported78 to exist as mixtures of dimers and trimers).
Pt(II)/Pt(IV) Ratio vs [MeCN]. Competitive trapping experi-
ments were conducted by adaptation of already published
Summary and Concluding Remarks
(76) Hill, G. S.; Irwin, M. J.; Levy, C. J.; Rendina, L. M.; Puddephatt, R.
J. Inorg. Synth. 1998, 32, 149–153.
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In this contribution, we have described the combined
results from NMR and stopped-flow UV-vis spectroscopic
(75) Norris, C. M.; Templeton, J. L. Organometallics 2004, 23, 3101–3104.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9103
procedures.20,21 A solution of (N-N)PtPh2 (ca. 3 mg, 5 μmol) in
CD2Cl2 (400 μL) in an NMR tube was cooled to -78 °C and
of acetonitrile and dichloromethane was prepared and used to
dissolve the desired amount of platinum complex. With the same
solvent mixture, the acid was diluted to the desired concentra-
tion. These two solutions were mixed in the UV-vis cuvette,
providing a 0.125 mM solution of (N-N)PtPh2. Using this
procedure, different experiments were conducted in which the
acid concentration was varied at constant acetonitrile concen-
tration and vice versa.
UV-vis spectra were recorded on Shimadzu UV-2102 and
Hewlett-Packard 8542A spectrophotometers. Low-temperature
kinetic data were obtained by recording time-resolved UV-vis
spectra using a modified Bio-Logic stopped-flow module
layered with a mixture of HBF4 Et2O (3 μL) in MeCN-d3 (x μL)
3
and CD2Cl2 (300 - x μL.; x = 10-300 μL). The tube was
capped and shaken to mix the reactants immediately before
transfer to a pre-cooled NMR probe. A pale yellow solution
was immediately obtained. The relative amounts of the
Pt(II):Pt(IV) complexes were determined by integration of
suitable non-overlapping 1H NMR signals from the complexes.
Thus, the Ar-Me signal at δ 2.13 from one-half of the diimine
ligand was used for (N-N)PtPh(NCCD3)þ, whereas the signal
at δ 2.41 arising from the methyl groups at the backbone of the
diimine ligand were used for (N-N)PtPh2H(NCCD3)þ. Char-
acterization of the complexes has been already published.23 The
relative distribution of the two products was unaltered when the
tube was kept for 3 h at the same temperature.
μSFM-20 combined with
a cryo-stopped-flow accessory
(Huber CC90 cryostat) equipped with a J & M TIDAS high-
speed diode array spectrometer with a combined deuterium and
tungsten lamp (200-1015 nm bandwidth). Isolast “O” rings
were used for all sealing purposes. Data were analyzed using the
integrated Bio-Kine software, version 4.44, and the software
package Specfit/32 global analysis program. Measurements
under high pressure were carried out using a homemade high-
pressure stopped-flow instrument,79,80 for which Isolast “O”
rings were also used for all syringe seals.
Four to nine kinetic runs were recorded under all conditions,
and the reported rate constants represent the mean value. The
UV-vis spectrophotometers and stopped-flow instrument were
thermostatted at the desired temperature ( 0.1 °C. Values of
ΔHq and ΔSq were calculated from the slopes and intercepts
of plots of ln(k/T) versus 1/T, respectively, and values of ΔVq
were calculated from the slope of plots of ln(k) versus pressure in
the usual way.59
NMR Monitoring and Kinetics of Benzene Substitution by
MeCN at (N-N)PtPh(η2-C6H6)þ. The NMR investigation of
the benzene substitution kinetics was performed by slight mod-
ifications of previously reported procedures.20,22,23 In an NMR
tube, (N-N)PtPh2 (10 mg, 15 μmol) was dissolved in dichlor-
omethane-d2 (300 μL), and the tube was cooled to -78 °C on a
dry ice/acetone bath. A solution of HBF4 Et2O (10 μL, 78 μmol)
3
ina mixtureofEt2O-d10 (70μL) anddichloromethane-d2 (180 μL)
was then added, the tube was shaken in the bath, and a 1H NMR
was recorded at -78 °C to ensure thatall the starting material was
consumed and that (N-N)PtPh(η2-C6H6)þ had formed (diagno-
stic signal at δ 6.85, s, 6 H, η2-C6H6). The tube was removed from
the NMR probe and returned to the dry ice/acetone bath. A
solutionofacetonitrile-d3 (x μL) indichloromethane(200 - x μL)
was carefully layered on top of the contents in the NMR tube (x
was in the range 40-100 μL, corresponding to acetonitrile con-
centrations in the range 1.0-2.7 M). The tube was then shaken to
ensure complete mixing prior to transfer to the NMR probe and
monitoring of the progress of the reaction at predetermined tem-
peratures. A smooth conversion of (N-N)PtPh(η2-C6H6)þ to
(N-N)PtPh(NCCD3)þ (δ 2.37, s, 6 H, Ar-Me) and benzene (δ
7.3, s, 6 H) was seen. The kinetic data were obtained by integra-
tion of the peak intensity for the phenyl ligand protons (δ 6.0-
6.2) of the η2-benzene complex versus the residual proton signal
of the solvent dichloromethane-d2 as an internal standard. A plot
of ln(relative intensity) versus time was linear for 3-4 half-lives.
EXSY NMR Spectra of (N-N)PtPh(η2-C6H6)þ in the
Absence and Presence of Acetonitrile-d3. The sample preparation
was identical to that of the benzene substitution kinetics experi-
ments described above. The EXSY spectra were recorded at -48 °C
without acetonitrile-d3 or with 54 mM acetonitrile-d3. The same
experimental parameters were used as described in our previous
contribution.23 The EXSY correlation signals were of indistinguish-
able intensities in the two experiments.
Acknowledgment. We gratefully acknowledge generous fi-
nancial support from the Norwegian Research Council (stipend
to J.P.) and the Deutsche Forschungsgemeinschaft through SFB
583 “Redox-active metal complexes” (to I.I.B. and R.v.E.).
Supporting Information Available: One file containing kinetic
data for the protonation of (N-N)PtPh2 as a function of
[MeCN], [HBF4], and temperature; kinetic data for the elimina-
tion of benzene from (N-N)PtPh2H(NCMe)þ as a function of
[MeCN], [HBF4], temperature, and pressure; kinetic data for the
protonation/benzene elimination of (N-N)PtPh(NCMe)þ as a
function of [TfOH] and temperature, and kinetic data for thþe
substitution of benzene by acetonitrile at (N-N)PtPh(η2-C6H6
as a function of [MeCN] and temperature, all in tabular form.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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UV-vis Stopped-Flow Measurements. In the stopped-flow
kinetic studies the samples were prepared as follows. A mixture