Mechanistic insight into the protonolysis of the Pt-C bond as a model for C-H bond activation by platinum(II) complexes

Article abstract of DOI:10.1021/om060217n

The kinetic and NMR features of the protonolysis reactions on platinum(II) alkyl complexes of the types cis-[PtMe₂L₂], [PtMe ₂(L-L)], cis-[PtMeClL₂], and [PtMeCl(L-L)] (L = PEt ₃, P(Prⁱ)₃, PCy₃, P(4-MePh) ₃, L-L = dppm, dppe, dppp, dppb) in methanol suggest a rate-determining proton attack at the Pt - C bond. In contrast, a multistep oxidative-addition - reductive-elimination mechanism characterizes the methane loss on protonation of the corresponding trans-[PtMeClL₂] species. Tools that were particularly diagnostic in suggesting different reaction pathways for the two systems were (i) the different results of kinetic deuterium isotope experiments, (ii) the detection or absence of Pt(IV) hydrido alkyl intermediate species by low-temperature ¹H NMR experiments, and (iii) the detection or absence of isotope scrambling and incorporation of deuterium into Pt - CH₃, combined with the loss of a range of CH _nD_n-4 isotopomers. For all systems, the rates of protonolysis are retarded by ligand steric congestion, accelerated by ligand electron donation, and almost unaffected by the chain length along the series of chelate complexes. A straight line correlates the rates of protonolysis of cis-dialkyl and cis-monoalkyl complexes, the difference in reactivity between the two systems being almost 5 orders of magnitude (slope of the line = 6 × 10⁴). Factors controlling the dichotomy of behavior between complexes of different geometry have been taken into consideration. Application of the principle of microscopic reversibility suggests the reason why platinum complexes with nitrogen donor ligands appear to be far more efficient than platinum phosphane complexes in activating the C-H bond.

Full text of DOI:10.1021/om060217n

Organometallics 2006, 25, 3435-3446
3435
Mechanistic Insight into the Protonolysis of the Pt-C Bond as a
Model for C-H Bond Activation by Platinum(II) Complexes
Raffaello Romeo* and Giuseppina D’Amico
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, and CIRCMSB,
UniVersita` di Messina, Salita Sperone, 31, Vill. S. Agata, 98166 Messina, Italy
ReceiVed March 7, 2006
The kinetic and NMR features of the protonolysis reactions on platinum(II) alkyl complexes of
the types cis-[PtMe2L2], [PtMe2(L-L)], cis-[PtMeClL2], and [PtMeCl(L-L)] (L ) PEt3, P(Pri)3, PCy3,
P(4-MePh)3, L-L ) dppm, dppe, dppp, dppb) in methanol suggest a rate-determining proton attack at the
Pt-C bond. In contrast, a multistep oxidative-addition-reductive-elimination mechanism characterizes
the methane loss on protonation of the corresponding trans-[PtMeClL2] species. Tools that were particularly
diagnostic in suggesting different reaction pathways for the two systems were (i) the different results of
kinetic deuterium isotope experiments, (ii) the detection or absence of Pt(IV) hydrido alkyl intermediate
species by low-temperature 1H NMR experiments, and (iii) the detection or absence of isotope scrambling
and incorporation of deuterium into Pt-CH3, combined with the loss of a range of CHnDn-4 isotopomers.
For all systems, the rates of protonolysis are retarded by ligand steric congestion, accelerated by ligand
electron donation, and almost unaffected by the chain length along the series of chelate complexes. A
straight line correlates the rates of protonolysis of cis-dialkyl and cis-monoalkyl complexes, the difference
in reactivity between the two systems being almost 5 orders of magnitude (slope of the line ) 6 × 104).
Factors controlling the dichotomy of behavior between complexes of different geometry have been taken
into consideration. Application of the principle of microscopic reversibility suggests the reason why
platinum complexes with nitrogen donor ligands appear to be far more efficient than platinum phosphane
complexes in activating the C-H bond.
Scheme 1. Alternative Reaction Pathways for Electrophilic
Attack of the Proton at Alkylplatinum(II) Complexes
Introduction
The mechanism of electrophilic cleavage of metal-carbon
bonds in transition-metal complexes is especially intriguing,
because of the problem of the selectivity of the site attack. This
complicating feature is shown particularly well in Pt(II) pro-
tonolysis chemistry. Protonation may take place by (i) a
concerted attack at the metal-carbon bond (SE2 mechanism),
as for electrophilic substitution on main-group organometallics,1
or (ii) a stepwise prior oxidative addition on the central metal
followed by reductive elimination (SE(ox) mechanism),2 as
illustrated approximately in Scheme 1. Other reaction pathways,
typical of alkyl mercury compounds3 or of metals that do not
have an accessible higher oxidation state, can in principle take
place. These include concurrent attack of the two ends of the
reagent on the polarized metal-carbon bond, forming cyclic
or open transition states, or attack at the aromatic ring in the
case of metal-aryl derivatives.4 For instance, the SE(ox)
mechanism is unlikely for Au(III) because of the unavailable
+5 oxidation state,5 and an SE2 mechanism is consistent with
the experimental findings. The reverse applies to Pt(II), where
Pt(IV) is easily accessible.
It is difficult to decide from kinetic evidence alone whether
the attack of the proton commences at one or the other center,
and it becomes impossible if the mechanism entails a rate-
determining proton transfer. Early kinetic investigations on the
protonolysis of platinum compounds focused almost exclusively
on platinum(II) halo alkyl, diaryl, and alkyl aryl complexes
containing the soft donor phosphanes, and the factors taken into
consideration, among others, to support a given reaction pathway
include (i) the form of the rate law, (ii) the halide ion de-
pendence, (iii) the magnitude and sign of the entropies and vol-
umes of activation, (iv) competition experiments, (v) the relative
energies of the Pt-C σ-bonding MO and nonbonding 5d orbi-
tals, (vi) the selectivity of alkyl vs aryl cleavage in mixed alkyl
* To whom correspondence should be addressed. Tel: +39-090-6765717.
Fax: +39-090-393756. E-mail: rromeo@unime.it.
(1) (a) Abraham, M. H. In ComprehensiVe Chemical Kinetics; Bamford,
C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1973. (b) Eaborn, C.
J. Organomet. Chem. 1975, 100, 43. (c) Kochi, J. K. Organometallic
Mechanisms and Catalysis; Academic Press: New York, 1978; pp 292-
340.
(2) Johnson, M. D. Acc. Chem. Res. 1978, 11, 57.
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nometallics 1996, 15, 1465. (b) Barbaro, P.; Cecconi, F.; Ghilardi, C. A.;
Midollini, S.; Orlandini, A.; Vacca, A. Inorg. Chem. 1994, 33, 6163. (c)
Nugent, W. A.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 5979. (d) Jensen,
F. R.; Rickborn, B. Electrophilic Substitution of Organomercurials;
McGraw-Hill: New York, 1968.
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10.1021/om060217n CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/09/2006

3436 Organometallics, Vol. 25, No. 14, 2006
Romeo and D’Amico
Chart 1. Sketch of the Halide-Assisted Transition State in
an SE2 Mechanism
at -78 °C revealed another phenomenon, incorporation of
deuterium into the Pt-CH3 site of unreacted trans-[PtMeCl-
(PEt3)2]. All these findings are against an SE2 mechanism and
strongly support a multistep SE(ox) mechanism which was
thought to involve (i) reversible chloride- or solvent-mediated
protonation of Pt(II) to produce the observed Pt(IV) hydrido
alkyl intermediate, (ii) reversible solvent or chloride dissociation,
yielding a five-coordinate platinum(IV) species, and (iii) reduc-
tive C-H bond formation to give a σ-alkane complex which
eventually loses alkane through either an associative or dis-
sociative substitution pathway.12 The nature of the solvent or
of the ancillary ligands will influence the stability of the
intermediates and transition states and, therefore, will dictate
the choice of the rate-determining step.
Unmistakable evidence for the operation of an SE(ox)
mechanism for systems containing hard ligands (nitrogen donor
atoms) or weak trans-activating groups (such as Cl- in trans-
[PtMeCl(PEt3)2]) does not necessarily imply that this multistep
mechanism accounts for protonolysis in all platinum(II) systems.
All of the previous experimental findings for electron-rich cis-
dialkyl, cis- and trans-diaryl, and mixed aryl-alkyl phosphane
complexes of platinum(II), and in particular the largely negative
values for volumes and entropies of activation,13 together with
the largely positive isotope effect mentioned before, provide
strong evidence that the primary kinetic step in the protonolysis
pathway is a one-step proton transfer to the substrate.
In view of the fundamental mechanistic relevance of the
protonation of the Pt-C bond as the microscopic reVerse of
C-H bond activation by platinum complexes,12,14 we considered
it of interest to perform a systematic examination of how
changes in the Pt(II) ligand environment affect the protonolysis
process. Thus, we studied in detail the kinetics of protonolysis
of a series of dialkyl platinum(II) complexes of the types cis-
[PtMe2L2] and [PtMe2(L-L)] and monoalkyl complexes cis-
[PtMeClL2] and [PtMeCl(L-L)], and trans-[PtMeClL2] (L )
PEt3, P(Pri)3, PCy3, P(4-MePh)3; L-L ) dppm, dppe, dppp,
dppb) according to a protocol which includes the measurement
of the dependence of the rate on proton and chloride concentra-
tion, the measurement of the primary deuterium isotope effect,
the search for the presence of Pt(IV) intermediates or for deu-
terium incorporation into the methyl positions, and, in the case
of the dialkyl complexes, the measurement of the rate depen-
dence on the temperature. We can anticipate that trans-
aryl complexes, and finally, (vii) the observed dependence of
the rates upon the structural properties of these organometallic
compounds.6 A particular emphasis has been placed on the
results of deuterium isotope experiments, as a diagnostic tool
for the extent of proton involvement in the formation of the
transition state.6c,e,g There is no general consensus on the
operation of a common mechanism. According to the SE(ox)
mechanism, the halide-dependent term in the rate law was
thought to reflect stabilization of the Pt(IV) intermediate by
halide coordination. On the other hand, a fast preequilibrium
between the uncharged substrate and X-, combined with slow
protonation and breakage of the metal-carbon σ bond, ac-
counted for a variety of the observed cases, including linear,
nonlinear, no dependence, and even retardation of the rate on
[X-] (Chart 1).
After the isolation of some platinum(IV) aryl hydrido
complexes,7 in 1995 three different groups reported that
protonation of [PtMe2(N-N)] complexes (N-N ) 2,9-dimethyl-
1,10-phenanthroline;8 N-N ) 2,2′-bipyridine, 4,4′-di-tert-butyl-
2,2′-bipyridine, 1,10-phenanthroline9) or [Pt(CH2Ph)Cl(N-N)]
complexes (N-N ) N,N,N′,N′-tetramethylethylenediamine10)
with HX leads to the formation of platinum(IV) alkyl hydrido
species that were isolated as solids8 or detected as transient
intermediates using low-temperature 1H NMR spectroscopy9,10
before their reductive elimination, at higher temperatures, to
yield the corresponding monoorgano or dihalide compounds.
Since then, the path to detection and characterization of stable
platinum(IV) methyl hydrido complexes has been open, espe-
cially with the use of ligands that do not easily dissociate.11
Formation of platinum(IV) alkyl hydrido species was not limited
to diamine- or diimine-containing Pt(II) species. Addition of
HCl to a CD2Cl2 solution of trans-[PtMeCl(PEt3)2] at -78 °C
generates [PtMe(H)Cl2(PEt3)2].12 Addition of DOTf in CD3OD
(6) (a) Belluco, U.; Giustiniani, M.; Graziani, M. J. Am. Chem. Soc. 1967,
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D. S.; Bancroft, G. M.; Bozek, J. D.; Puddephatt, R. J.; Tse, J. S. Inorg.
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Pt-C Bond Protonolysis and C-H Bond ActiVation
Organometallics, Vol. 25, No. 14, 2006 3437
[PtMeClL2] complexes exhibit totally different mechanistic
features compared to dialkyl and monoalkyl complexes of cis
geometry. We are inclined to think that this mechanistic
difference is difficult to fit into a unified mechanism (SE2 or
SE(ox) mechanism). Rather, the different kinetic behaviors offer
valuable information on the role of trans-activating groups and
of the ensuing polarization of the platinum-carbon bond in
governing the selectivity of the proton attack, the form of the
reaction profiles, and the stability of intermediates and transition
states. Mechanistic implications of the mechanism of C-H bond
activation by platinum(II) complexes will be discussed.
preparation of the pure monomer 5b.23 In compounds 1b-8b
the methylplatinum resonance appears as four peaks of equal
intensity due to coupling with two nonequivalent 31P atoms,
with 195Pt satellites. The two nonequivalent phosphane ligands
can be distinguished on the basis of the different magnitudes
1
observed for the JPtP coupling constants of the two 31P
resonances (low value for PA trans to carbon, high value for PB
trans to chloride).
The remaining trans-[PtMeClL2] complexes (1c-4c) were
prepared by two reaction routes: (i) spontaneous isomerization
in methanol of the corresponding cis-monoalkyl species for L
) PEt3, P(4-MeC6H4)3 and (ii) reaction of a slight excess of
phosphane with trans-[PtMeCl(Me2SO)2] for L ) PPri3, PCy3.
Both procedures proved to be fast and easy, but method (ii) is
to be preferred when using bulky phosphanes. The NMR
characterizations of these complexes are in good agreement with
their structures. Selected NMR data for all complexes in the
text are reported in Tables 1 and 2. A complete list and
assignment of NMR data are given in Table S1 in the Supporting
Information. Figure S1 in the Supporting Information displays
Results
Synthesis and Characterization of Complexes. The reaction
illustrated in eq 1 is fast and clean, and the desired products
can be obtained in high yield and purity, as reported previously
for an extended series of phosphanes of relatively high basicity
and low cone angle.15
cis-[PtMe2(Me2SO)2] + 2L f cis-[PtMe2L2] + 2Me2SO
1
typical H and 31P NMR features that are useful to distinguish
(1)
between compounds of classes a-c, respectively.
Protonolysis of Dialkyl Complexes. (a) NMR Measure-
ments. The cleavage of the first Pt-C bond in complexes
1a-8a takes place according to the reaction
Complications arise, as for [PtMe2(cod)],16,17 only for the
reactions with the most sterically demanding or poorly σ-donat-
ing phosphanes, because of the easy re-entry of the leaving
sulfoxide. In such a case, a valid alternative seems to be the
use as precursors of [Pt2Me4(µ-SMe2)2] or of [PtMe2(nbd)].18
In the 1H NMR spectra of 1a-8a the phosphorus-coupled
methyl resonance appears as an apparent triplet (3JPH ≈ 7 Hz)
due to virtual coupling. The 31P{1H} NMR spectra show a single
resonance. The cis stereochemistry for 1a-4a was clearly
+
cis-[PtMe2L2] [H ]8 cis-[PtMeCIL2] + CH4
(2)
[Cl-]
The reaction is too fast to be followed by NMR, where an
immediate and sharp change in the spectrum is observed on
addition of the acid. The final cis product can be isolated by
evaporation of the solvent in a synthetic workup or can be
recognized by NMR, at low temperature when dealing with
compounds with sterically demanding phosphanes (2b and 3b).
In this particular case the cis configuration that results from
Pt-C bond breaking is hardly retained at room temperature
because of a subsequent geometrical isomerization. Attempts
to detect by low-temperature NMR the presence of possible
platinum(IV) alkyl hydrido intermediates of the type [PtMe2-
(H)Cl2L2] were unsuccessful. The addition of acid to 1a-8a,
according to the procedure described in the Experimental
Section, leads exclusively to the quantitative formation of cis-
[PtMeClL2] and methane. The same result was obtained when
Me3SiCl in wet CD2Cl2 was used to give 1 equiv of HCl.
Treatment of 1a-8a in CD3OD/CD2Cl2 (8/1) with DOTf
(1/10) and LiCl at -70 °C results in the immediate formation
of CH3D and of the complexes 1b-8b. Similar results were
obtained by Holtcamp, Labinger, and Bercaw in the protonolysis
and deuteriolysis of [PtMe2(depe)].24
(b) Spectrophotometric Studies. Typical spectral changes
are associated with the protonolysis of 1a-8a. The reaction
takes place in a single step according to eq 2. The systematic
kinetics of these reactions were studied at different proton and
chloride concentrations and required the use of stopped-flow
techniques. The reaction rate is independent of the concentration
of Cl-, within a wide range of concentrations. At the concentra-
tions of acid used, the reactions went to completion and excellent
fits were obtained from the regression analysis of the absorbance
1
indicated by the low values observed for JPtP (∼1850 Hz),
which are diagnostic for a phosphane ligand trans to an alkyl
or aryl group.19 The 31P chemical shifts for 5a-8a follow the
pattern characteristic of chelated bis(tertiary phosphane) alkanes
of ring size 4-7.20 The abnormally low value of the coupling
1
constant JPtP (1422 Hz) for 5a is correlated to the bite-angle
strain for the four-membered dppm ring.
Careful stoichiometric protonolysis of cis-[PtMe2L2] in non-
polar solvents usually gave clean and isolable monoalkyl
products. This was not the case for complexes with the sterically
demanding phosphanes PPri3 and PCy3, because of the spon-
taneous easy conversion of cis monoalkyl products to the
corresponding trans isomers.21 The characterization of 2b and
3b has been performed “in situ”, using low-temperature 1H and
31P{1H} NMR, soon after the addition of acid to 2a and 3a.
The reaction of trans-[PtMeCl(Me2SO)2] with stoichiometric
amounts of dppe, dppp, and dppb gave the required chelated
compounds (6b-8b). The same procedure applied to dppm
mainly gave the dimeric species [PtMeCl(µ-dppm)]2 in high
yield, as reported previously for the corresponding reaction of
dppm with [PtMeCl(cod)]22 in benzene. Careful protonolyis of
[PtMe2(dppm)], using methanolysis of acetyl chloride as the
source of the proton, proved to be a convenient route to the
(15) Romeo, R.; Alibrandi, G. Inorg. Chem. 1997, 36, 4822-4830.
(16) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock,
A.; Giering, W. P. Organometallics 1999, 18, 474-479.
(17) Butikofer, J. L.; Hoerter, J. M.; Peters, R. G.; Roddick, D. M.
Organometallics 2004, 23, 400-408.
(18) Appleton, T. G.; Hall, J. R.; Williams, M. A. J. Organomet. Chem.
1986, 303, 139.
(19) (a) Allen, F. H.; Pidcock, A. J. Chem. Soc. A 1968, 2700. (b)
Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973, 10,
335.
(22) Appleton, G. T.; Bennett, M. A.; Tomkins, I. B. J. Chem. Soc.,
Dalton Trans. 1976, 439.
(23) Cooper, S. J.; Brown, M. P.; Puddephatt, R. J. Inorg. Chem. 1981,
20, 1374.
(20) Garrou, P. E. Chem. ReV. 1981, 81, 229.
(21) Romeo, R. Comments Inorg. Chem. 2002, 23(1), 79.
(24) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta
1997, 265, 117.

3438 Organometallics, Vol. 25, No. 14, 2006
Romeo and D’Amico
1
Table 1. Selected H and 31P{1H} NMR Spectroscopic Data for Compounds Discussed in the Texta
1H NMR
31P{1H} NMR
compd
T, K
solvent
δ(PtCH3)
2JPtH
3JPH
δ
1JPtP
cis-[Pt(Me)2(PEt3)2] (1a)
cis-[Pt(Me)2(PPri3)2] (2a)
cis-[Pt(Me)2(PCy3)2] (3a)
cis-[Pt(Me)2(P(4-MePh)3)2] (4a)
[Pt(Me)2(dppm)] (5a)
[Pt(Me)2(dppe)] (6a)
[Pt(Me)2(dppp)] (7a)
[Pt(Me)2(dppb)] (8a)
cis-[Pt(Me)(Cl)(PEt3)2] (1b)
298
298
298
298
298
298
298
298
298
CD2Cl2
CD2Cl2
CD2Cl2
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CD2Cl2
0.27
0.31
0.28
0.37
0.94
0.69
0.33
0.28
0.50
66.2
66.9
66.9
69.2
74.2
70.4
68.6
68.3
54.4
6.6
6.3
5.5
7.1
7.3
7.5
5.7
7.7
7.4
11.3
33.7
23.5
26.1
-38.8
46.5
3.5
18.9
17.5
11.2
32.8
27.6
23.2
18.7
25.6
20.3
-36.3
40.8
44.0
43.2
3.4
1858
1860
1824
1904
1422
1782
1766
1847
1737
4225
1741
4250
1753
4328
1775
4450
1248
3876
1725
4264
1654
4178
1711
4385
cis-[Pt(Me)(Cl)(PPri3)2] (2b)
cis-[Pt(Me)(Cl)(PCy3)2] (3b)
cis-[Pt(Me)(Cl)(P(4-MePh)3)2] (4b)
[Pt(Me)(Cl)(dppm)] (5b)
[Pt(Me)(Cl)(dppe)] (6b)
225
225
298
298
298
298
298
CD3OD/CD2Cl2 (8/1 v/v)
0.58
0.67
0.62
0.72
0.61
0.42
0.39
53.2
56.2
52.2
62.0
55.0
53.4
54.3
7.0
7.6
7.3
8.9
7.7
7.1
6.8
CD3OD/CD2Cl2 (8/1 v/v)
CDCl3
CD2Cl2
CDCl3
CDCl3
CDCl3
[Pt(Me)(Cl)(dppp)] (7b)
5.7
19.5
18.7
[Pt(Me)(Cl)(dppb)] (8b)
2
3
1
a Resonances (δ) are in ppm and coupling constants JPtH, JPH and JPtP in hertz.
1
Table 2. Selected H NMR Spectroscopic Data for the Trans Monoalkyl Compounds 1c-4c and Related Pt(IV)
Hydride Speciesa
1H NMR
δ(Pt-CH3)b
δ(Pt-H)c
31P{1H} NMR δd
trans-[Pt(Me)Cl(PEt3)2] (1c)
[Pt(Me)Cl2(H)(PEt3)2] (1d)
trans-[Pt(Me)Cl(P(Pri)3)2] (2c)
[Pt(Me)Cl2(H)(P(Pri)3)2] (2d)
2d′
trans-[Pt(Me)Cl(PCy3)2] (3c)
[Pt(Me)Cl2(H)(PCy3)2] (3d)
3d′
0.25 (84.0)
0.96 (62.5)
0.30 (83.4)
1.14 (57.1)
1.14 (57.1)
0.20 (81.4)
0.81
16.2 (2817)
9.17 (1848)
32.0 (2852)
27.4 (2363)
29.0 (2785)
21.0 (2821)
-18.8 (1228)
-17.4 (1362)
-18.7 (1676)
-18.8 (1300)
-17.6 (1354)
0.81
-0.26 (79.6)
trans-[Pt(Me)Cl(P(4-MePh)3)2] (4c)
27.7 (3122)
b 2
c 1
a In CD2Cl2 at 220 K; resonances (δ) are in ppm.
PtP
J
values are reported in parentheses in hertz.
J
values are reported in parentheses in hertz.
PtH
PtH
d 1
J
values are reported in parentheses in hertz.
Table 3. Second-Order Rate Constants, kH and kD, and
Deuterium Kinetic Isotope Effects for the Protonolysis of the
Platinum-Methyl Bond in the Complexes cis-[PtMe2L2]
(1a-8a) and cis-[PtMeClL2] (1b-8b)a
compd
L
kH,b M-1 s-1
kD,c M-1 s-1
kH/kD
1a
2a
3a
4a
5a
6a
7a
8a
1b
2b
3b
4b
5b
6b
7b
8b
PEt3
63400 ( 2150
3030 ( 30
24500 ( 300
1740 ( 10
2.59
1.74
2.60
2.66
3.51
4.74
4.30
3.52
4.73
1.36
1.70
2.90
1.72
2.99
1.82
2.53
P(Pri)3
PCy3
8050 ( 50
3090 ( 420
P(4-MePh)3
1/2 dppm
1/2 dppe
1/2 dppp
1/2 dppb
PEt3
6960 ( 200
2610 ( 80
30650 ( 1130
30400 ( 1220
25170 ( 340
14900 ( 70
8730 ( 300
6420 ( 20
5860 ( 470
4230 ( 380
1.068 ( 0.003
0.0208 ( 0.0008
0.124 ( 0.001
0.119 ( 0.004
0.129 ( 0.003
0.172 ( 0.006
0.195 ( 0.007
0.222 ( 0.013
0.226 ( 0.005
0.0153 ( 0.0005
0.0727 ( 0.0009
0.0410 ( 0.0008
0.0751 ( 0.0006
0.0575 ( 0.0020
0.107 ( 0.005
0.0876 ( 0.0026
P(Pri)3
PCy3
P(4-MePh)3
1/2 dppm
1/2 dppe
1/2 dppp
1/2 dppb
Figure 1. Dependence of the pseudo-first-order rate constants
(kobs/s-1) on proton concentration for reactions of cis-[PtMe2L2]
complexes (1a-4a, eq 2) in methanol at 298.2 K. For the sake of
clarity the diagrams for compounds 5a-8a have been omitted.
a At 298 K. b In CH3OH. c In CH3OD.
vs time data. The dependence of the pseudo-first-order rate
constants (Table S2, in the Supporting Information) is described
by a family of straight lines with zero intercept (Figure 1) and
obeys eq 3.
The values of kH, from linear regression analysis of the
dependence of kobs on [H+], are given in Table 3, together with
their standard deviations. The values of kH at different temper-
atures are set forth in Table S3 (Supporting Information), and
kobs ) kH[H+]
(3)

Pt-C Bond Protonolysis and C-H Bond ActiVation
Organometallics, Vol. 25, No. 14, 2006 3439
Table 4. Rate Constants and Activation Parameters for the
Protonolysis of the Pt-Me Bond in cis-[PtMe2L2]
Complexesa
Chart 2. Structures of Stereoisomers Compatible with the
31P{1H} NMR Experimental Data for Pt(IV) Bis(phosphane)
Dichloride Hydrido Alkyl Reaction Intermediates
b
compd
L
10-4kH
∆Hqc
∆Sqd
1a
2a
3a
4a
5a
6a
7a
8a
PEt3
6.34 ( 0.2
0.30 ( 0.01
0.80 ( 0.01
0.69 ( 0.02
3.06 ( 0.1
3.04 ( 0.1
2.52 ( 0.03
1.49 ( 0.01
26.3 ( 1.0
16.0 ( 0.5
36.1 ( 0.7
48.1 ( 2.4
35.7 ( 0.4
42.8 ( 0.7
35.7 ( 2.7
44.2 ( 5.3
-67 ( 4
-125 ( 2
-49 ( 3
-12 ( 8
-43 ( 1
-18 ( 2
-44 ( 9
-22 ( 18
P(Pri)3
PCy3
P(4-MePh)3
1/2 dppm
1/2 dppe
1/2 dppp
1/2 dppb
S4, in the Supporting Information). The values of kH and
kD derived from eq 3 and eq 4, respectively, are given in
Table 3 together with the values of the calculated kinetic isotope
effect.
a In methanol at 298 K. b In units of M-1 s-1
d In units of J K-1 mol-1
.
c In units of kJ mol-1
.
.
Protonolysis of Monoalkyl trans-[PtMeClL2] Complexes.
(a) NMR Measurements. Addition of an excess of HOTf
(1/10, as a CD2Cl2 solution) to a solution of 1c and Ph4AsCl
(1/10) in CD2Cl2 at -53 °C results in partial conversion to the
oxidative addition product 1d. The same treatment applied to
2c and 3c yields a pair of Pt(IV) compounds (2d, 2d′ and 3d,
3d′) for each starting material. Selected 1H and 31P resonances
for all compounds are reported in Table 2, along with those of
the starting Pt(II) complexes. When the temperature is raised,
reductive elimination of methane takes place, accompanied by
the formation of trans-[PtCl2L2]. Protonation of 1c-3c with
DOTf in CD3OD results in deuterium incorporation into the
coordinated methyl site prior to CH3D, CH2D2, and CHD3 loss,
which implies the presence of unstable PtIV(D)Me intermediates
not detected by 1H and 31P NMR. These results were not
unexpected, in light of similar previous findings on 1c and on
trans-[PtMeBr(PEt3)2], trans-[PtEtCl(PEt3)2], and trans-[PtBzCl-
(PEt3)2] by Bercaw et al.,12,24 which have been explained by (i)
oxidative addition of HCl to the planar substrate and (ii) isotopic
exchange on PtIVMe(D) through the intermediacy of a four-
coordinate σ-alkane complex.
the associated activation parameters are given in Table 4. The
same pattern of behavior is observed when the kinetic runs are
carried out with DOTf and LiCl in CH3OD, the only difference
being a marked decrease of reactivity with respect to the
protonolysis. The values of kD derived from eq 4 are given in
Table 3 together with the values of the calculated kinetic primary
isotope effect.
kobs ) kD[D+]
(4)
Protonolysis of Monoalkyl cis-[PtMeClL2] and [PtMeCl-
(L-L)] Complexes. (a) NMR Measurements. Although all of
the complexes 1b-8b can be obtained easily “in situ” from
the corresponding dialkyls, the NMR measurements were carried
out using pure synthesized compounds as starting materials,
except for 2b and 3b, which were prepared by acid attack from
2a and 3a, respectively. The cleavage of the Pt-C bond takes
place according to the reaction
+
cis-[PtMeClL2] [H ]8 cis-[PtCl2L2] + CH4
(5)
[Cl-]
(b) Stereochemistry of HCl Addition. For [PtMeCl2(H)-
(PEt3)2] (1d) the characteristic Pt-H resonance appears far
upfield (δ -18.8 ppm) with the corresponding platinum satellites
(1JPtH ) 1228 Hz). The Pt-Me resonance moves downfield with
respect to that of 1c (δ 0.96 ppm), showing a lower value for
the 195Pt satellite signal (2JPtH ) 63 Hz). These data correspond
closely to those reported by Bercaw et al. for [PtMeCl2(H)-
(PEt3)2].12 The theory predicts six possible stereoisomers for
an octahedral species of the type Ma2b2cd. Only three of them
are compatible with a single 31P resonance for two magnetically
equivalent phosphorus atoms (Chart 2).
The protonolysis rates appear to be many orders of magnitude
slower than those of the corresponding dialkyls. As a conse-
quence, in contrast to reaction 2, the process can be monitored
by 1H or 31P NMR, modulating conveniently the reaction
temperature and the concentration of added acid. This circum-
stance facilitated the search for the presence of possible
platinum(IV) alkyl hydrido intermediates, performed by fol-
lowing the general procedure described in the Experimental
Section. The 1H and 31P NMR spectra monitored soon after the
treatment of 1b-8b with HOTf and Ph4AsCl, in CD2Cl2 at -78
°C correspond closely to that of the starting complexes. When
the temperature is increased to -33 °C, even after 1 h there is
no evidence for reactions or for the buildup of any intermediate
species. At room temperature slow formation of cis-[PtCl2L2]
and methane release can be monitored, except for 2b and 3b,
which exhibit a concurrent geometrical isomerization leading
to 2c and 3c. The same phenomena were observed in the reaction
with DOTf in CD3OD, with the only difference being the re-
lease of CH3D instead of CH4. When the reaction mixture was
left at low temperature for a long time, no deuterium incorpora-
tion into the Pt-CH3 site of unreacted cis-[PtMeClL2] was
observed.
1
A large value of the JPtH coupling constant is considered
diagnostic for a hydride trans to a weakly trans-activating group
(Cl) rather than to a strong σ-donor methyl ligand.7 In addition,
1
the magnitudes of the coupling constants JPtP are consistent
with mutual trans positions of the two phosphanes.25 Thus,
structure A would be preferred over structures B and C for 1d.
This stereochemical arrangement should be the result of trans
addition of HCl to 1c. By analogy, structure A should be
associated with the product formed on protonation of 2c and
3c. The observation of two oxidation products, each character-
1
ized by a single 31P resonance and large values of JPtP, could
be tentatively explained by the concurrent formation of species
having the solvent or OTf - in the position trans to the hydride
together with the chloride [PtMeCl2(H)(PPri3)2] and [PtMeCl2-
(H)(PCy3)2] species.
(b) Spectrophotometric Studies. The spectral changes
associated with eq 5 can be monitored through conventional
spectrophotometry and exhibit well-defined isosbestic points.
The systematic kinetics of these reactions were studied at vari-
ous concentrations of H+, D+, and Cl-. Once again, the reac-
tion rates were independent of the concentration of Cl-.
The dependence of the pseudo-first-order rate constants on
[H+] was consistent with eq 3 and that of [D+] with eq 4 (Table
(25) (a) Anderson, D. W. W.; Ebsworth, E. A. V.; Rankin, D. W. H.
J. Chem. Soc., Dalton Trans. 1973, 854. (b) Blacklaws, I. M.; Brown, L.
C.; Ebsworth, E. A. V.; Reed, F. J. S. J. Chem. Soc., Dalton Trans. 1978,
877.

3440 Organometallics, Vol. 25, No. 14, 2006
Romeo and D’Amico
All of the rate data appear to fit the rate law
kobs ) kH[H+] + kCl[H+][Cl-]
which includes a chloride-dependent term. The values of kobs
(7)
,
[H+], and [Cl-] were fitted to this expression by using a
multiple-linear nonweighted regression. The best values of the
constants kH and kCl are given in Table 5, together with their
standard deviations. The kinetic runs with DOTf in CH3OD were
carried out over a range of [D+] and [Cl-] concentrations, and
the values of kD and k′Cl derived from eq 8 are given in Table
5 together with the values of the calculated kinetic primary
isotope effect.
kobs ) kD[D+] + k′Cl[D+][Cl-]
(8)
Figure 2. 3D plot showing the dependence of the observed pseudo-
first-order rate constants (kobs/s-1) for reactions of trans-[PtMeCl-
(PEt3)2] as a function of the proton and chloride ion concentrations
in methanol at 298 K.
Discussion
The synthesis of the complexes 1a-8a, 1b-8b, and 1c-4c
has been performed by adopting slight modifications to well-
established procedures. The various steps (Scheme 2) involve
the use of cis-[PtMe2(Me2SO)2] as a precursor in the synthesis
of 1a-8a and of trans-[PtMeCl(Me2SO)2] as a precursor in the
synthesis of 1c-4c and a series of protonolysis reactions.
Problems arise only when using sterically demanding phos-
phanes such as PPri3 and PCy3. With such ligands, the
substitution reaction at cis-[PtMe2(Me2SO)2] often does not go
to completion, giving a mixture of mono- and bis-substituted
cis products. In addition, protonolysis of both cis-[PtMe2(PPri3)2]
(2a) and cis-[PtMe2(PCy3)2] (3a) with HCl at room temperature
gives directly the trans monochloride derivatives 2c and 3c, as
a result of a well-known spontaneous geometrical isomerization
of cis monoalkyl halide compounds13,15 that is accelerated by
the encumbrance of the phosphane ligand.21 In any case, the
product formed upon electrophilic attack by the proton maintains
the stereochemistry of the starting complex. A preliminary
search for the buildup of Pt(IV) intermediates species upon
protonation of the substrate in CD2Cl2 and/or H/D exchange
and deuterium incorporation in the reagents and products in
CD3OD has been carried out systematically for all compounds
by NMR at low temperature. In methanol, at room temperature,
1H and 31P NMR and UV/vis spectral changes of these reactions
always showed the presence of only two species, the starting
complex and the final product.
Kinetic Features of trans-[Pt(R)XL2] Complexes. Several
studies have provided a detailed understanding of the mech-
anism of protonation of trans-[PtMeCl(PEt3)2] (1c). A two-step
SE(ox) mechanism was originally proposed by Belluco et al,6a
based essentially on the form of the rate law equation, which
includes a chloride-dependent term and the chemical evidence
for easy access to the Pt(IV) oxidation state. Interestingly, new
findings by Bercaw12,24 were consistent with the metal being
the kinetically preferred site of protonation and ruled out the
alternative mechanism of protonation at the methyl group.
Indeed, several factors such as (i) observation of [PtMeCl2(H)-
Table 5. Second-Order Rate Constants, kH and kD, and
Deuterium Kinetic Isotope Effects for the Protonolysis of the
Platinum-Methyl Bond in the Complexes trans-[PtMeClL2]
(1c-4c)a
c
103kH,b
103kCl,b
103kD,c
103k′Cl
,
compd
L
M-1 s-1
M-2 s-1
M-1 s-1
M-2 s-1 kH/kD
1c
2c
3c
4c
PEt3
2.08 ( 0.3 326 ( 6
0.29 ( 0.01 0.52 ( 0.04 0.74 ( 0.04 2.62 ( 0.6 0.39
3.46 ( 0.5 413 ( 6 0.60
P(Pri)3
PCy3
0.81 ( 0.02 9.4 ( 0.4
2.14 ( 0.04 26.7 ( 1
2.77 ( 0.2 110 ( 3
0.38
0.69
P(4-MePh)3 1.92 ( 0.1 116 ( 2
a At 298 K. b In CH3OH. c In CH3OD.
(c) Spectrophotometric Studies. Protonolysis of trans-
[PtMeClL2] complexes has been the subject of several kinetic
studies in the past, particularly as far as compound 1c is con-
cerned.6a,i,12 As in previous investigations, we utilized UV-vis-
ible spectroscopy to monitor these reactions, which take place
smoothly and quantitatively according to eq 6.
+
trans-[PtMeClL2] [H ]8 trans-[PtCl2L2] + CH4 (6)
[Cl-]
Several clean isosbestic points characterize the spectral
changes. Obviously, the Pt(IV) intermediates responsible for
deuterium exchange into the methyl sites in CD3OD (see above)
cannot be seen in the electronic spectrum. The systematic
kinetics of these reactions were studied at different proton and
chloride concentrations, and the calculated values of the pseudo-
first-order rate constants kobs are available in the Supporting
Information (Table S5). The dependence of the rates on [H+],
at constant chloride concentration, is represented by a straight
line passing through the origin. At constant proton concentration
the rates were linearly dependent on [Cl-] (with a nonzero
intercept). A global 3-D representation of the [H+] and [Cl-]
dependence of the kinetic data for the protonolysis of 1c is given
in Figure 2.
Scheme 2. Synthesis of Platinum(II) Complexes

Pt-C Bond Protonolysis and C-H Bond ActiVation
Organometallics, Vol. 25, No. 14, 2006 3441
Scheme 3. Currently Accepted
Oxidative-Addition-Reductive-Elimination Mechanism
this multistep pathway is characterized by inVerse KIE values
(kH/kD < 1), as expected for a sequence of proton-transfer
equilibria followed by a rate-determining reaction of the
protonated substrate. Similar KIE’s have been observed in many
cases of reductive elimination from alkyl hydride and alkyl
deuteride complexes26 and were interpreted in terms of the
inverse equilibrium isotope effect Ke,H/Ke,D < 1, which controls
C-H(D) bond formation in the intermediate σ-alkane complex
(the passage from I(H) to I(σ) in the coordinate reaction diagram),
combined with comparable values for the rates of hydrocarbon
dissociation from M(R-H) and M(R-D). Interestingly, the
contributions of the chloride-dependent pathways, indicated by
the values of the third-order rate constants kCl and k′Cl, are
comparable but not identical for protonolysis and deuteriolysis
on the same substrate. This contribution is significant compared
to that of kH or kD in the unhindered complexes 1c and 4c but
is likely to vanish when the steric hindrance is increased at the
central metal, as shown by the rate data for 2c and 3c. This
variability of the importance of the chloride-dependent term with
the steric hindrance of the substrates is a further indication that
the form of the rate law can hardly be assumed as a clear-cut
diagnostic tool in assessing the reaction mechanism. The bulk
at the metal site plays a much less marked role in controlling
the protonation rates kH or kD along the series of examined
complexes 1c-4c. The number of data points is insufficient to
perform a statistically satisfactory quantitative analysis of ligand
effects (QALE)27 on the reactivity, but it is clear that protonation
at the metal is favored by a transfer of electronic density from
the ligand to the metal and is impeded by steric congestion.
An important question pertaining to these protonation reac-
tions is the extent to which a change in coordinative environment
can promote a change of mechanism and of site attack. A survey
of literature data for closely related compounds can shed some
light on this matter. A change in the organic moiety along the
series of complexes trans-[Pt(R)Cl(PEt3)2] (R ) Et, n-Pr, n-Bu,
benzyl)6i does not apparently produce any change with respect
to the kinetic features observed for 1c-4c: (i) same rate law
and (ii) sequence of reactivity for kH, Et > n-Pr > n-Bu > Me
> benzyl, which parallels the rate of deuterium incorporation
into the R-position of the alkyl group in CD3OD/DOTf, Et >
Me > benzyl.24 The mechanistic picture changes markedly on
increasing the trans-activating effect of X in trans-[PtMe(X)-
(PEt3)2] complexes. When X ) Br-, a bivariate rate law6a and
multiple H/D exchange24 are still observed. With X ) I-, the
halide-dependent term vanishes6a and only monodeuterated
methane is observed on deuteriolysis.24 Introducing in the system
a strong σ-donor carbon ligand R (R ) o-, m-, and p-substituted
aryl ring)6e the proton attack occurs selectively at the methyl
group, with the simplified rate law kobs ) kH[H+] and an increase
of reactivity of about 7 orders of magnitude with respect to
that of the chloride analogue (kH(Ph)/kH(Cl) ) 20 300/0.002 )
1.01 × 107). In addition, deuteriolysis leads to the formation of
only CH3D and, most importantly, the KIE is strongly positiVe
(kH/kD ≈ 7). All of these findings are consistent with a rate-
determining proton transfer to the substrate, whatever the site
of the proton attack is, and will be discussed in connection with
other results obtained in this work. A recent report by Puddephatt
et al.28 on the protonolysis of phenyl-platinum(II) bonds in
[PtPh(NCN)] (NCN ) 2,6-C6H3(CH2NMe2)2), a complex with
(PEt3)2] at -78 °C in CD2Cl2, (ii) the kinetics of its reductive
elimination, and (iii) multiple D incorporation into unreacted
Pt-CH3 with concurrent loss of CHnDn-4 isotopomers in
methanol all concur with a verification and refinement of the
original reaction scheme that currently include (Scheme 3) the
intermediacy of a square-pyramidal five-coordinate Pt(IV)
hydride (I(H) in Scheme 3) and the formation of an elusive
σ-alkane complex (I(σ) in Scheme 3).
The results of this work confirm and extend to complexes
2c and 3c some fundamental features exhibited by protonolysis
reactions of 1c such as (i) the form of the rate law, (ii) the
buildup of Pt(IV) hydride intermediates on protonation of the
substrates in dichloromethane, and (iii) the evidence for H/D
scrambling in methanol prior to methane loss and subsequent
observation of the CHnD4-n isotopomers. Thus, we can confi-
dently assume that the SE(ox) mechanism in Scheme 3 applies
to complexes 1c-4c and to closely related species. Figure 3
shows a qualitative energy profile based on Scheme 3 that
accounts for the kinetic features exhibited by the protonolysis
of trans-[PtMeClL2] complexes in methanol. The rate-determin-
ing step is loss of alkane from the σ-complex.
Examination of the rate data in Table 5 adds a number of
significant observations to the mechanistic picture. The mea-
surement of the solvent kinetic isotope effect (KIE) reveals that
Figure 3. Qualitative reaction coordinate diagram (based on
Scheme 3) for the protonolysis reactions of trans-[Pt(R)Cl(PR3)2]
(26) Jones, W. D. Acc. Chem. Res. 2003, 36, 140-146.
(27) http://www.bu.edu/qale. This Web site collects sets of data, protocol
for the analysis, program package, parameters of ligands, leading references,
etc. Each set of data, taken from the literature, is subjected to a QALE
analysis with commentary on how each analysis is done and how successful
it is. All of the material is updated with recent data sets and analyses.
complexes in methanol. S represents the starting complex, I(HCl)
,
I
(H), and I(σ) the transient intermediates [Pt(R)Cl2(H)(PR3)2],
[Pt(R)Cl(H)(PR3)2]+, and [PtCl(PR3)2(σ-RH)], respectively, and P
the reaction product.

3442 Organometallics, Vol. 25, No. 14, 2006
Romeo and D’Amico
a phenyl trans to a carbon donor, seems to be in contrast with
what is expected for complexes having mutually trans Pt-C-
bonded groups, because of the unambiguous disposition showed
by the complex to an oxidative-reductive behavior. However,
it is possible that the π-bonding system of the in-plane aryl
ring of the cyclometalated NCN ligand in [PtPh(NCN)] is able
to relieve the excess electron donation by the trans phenyl ligand,
promoting kinetic features similar to those of [PtMe2(N-N)]
complexes.
Kinetic Features of cis-[Pt(R)XL2] Complexes. In the
systems analyzed in this study (R ) Me, X ) Cl; L ) PEt3,
P(Pri)3, PCy3, P(4-MePh)3, 1b-4b; L-L ) dppm, dppe, dppp,
dppb, 5b-8b) the trans-activating groups are phosphanes of
widely different steric and electronic characteristics with chelate
rings of increasing size. The rate law is kobs ) kH[H+], and the
halide ion has no influence on the rate. No Pt(IV) intermediates
are detected, nor is deuterium incorporation into the coordinated
methyl observed. No methane isotopomers different from CH3D
are observed, as reported before for cis-[PtMeCl((MeO)3P)2].24
Interestingly, the KIE’s are positiVe in all cases, with a mean
value of 2.5 ( 1. None of the diagnostic tools that supported
the SE(ox) mechanism for the corresponding trans analogues
are readily available. Rather, these results are strongly in favor
of a rate-determining proton transfer to the substrate, whatever
the site of the proton attack is.
The nature of the P-donor ligands L significantly affects the
rates (see data in Table 3). The efficiency of the spectator
P-donor ligands in accelerating the proton attack increases in
the order P(Pri)3 < P(4-MeC6H4)3 ≈ PCy3 < PEt3, the difference
in reactivity between the first and the last members of the series
being about 1 order of magnitude. The sequence of reactivity
reflects the extent of steric congestion or electron donation by
the P-donor ligands. The rate of methane loss on protonation is
at least 100 times higher for the complexes cis-[PtMeCl(PR3)2]
(1b-4b) than for the corresponding trans complexes (1c-4c).
The ring size does not appear to have a significant effect on
the rates of protonation and deuteriolysis (cf. 5b-8b). More
electrophilic Pt centers, such as [PtMeX(dfpe)] (dfepe ) (C2F5)2-
PCH2CH2P(C2F5)2; X ) O2CCF3, OSO2H, OSO2CF3, OSO2F)
complexes, are much more resistant to protonolysis, requiring
prolonged reaction times and heating in the neat solvents.29 The
absence of kinetic dependence on chloride concentration, the
absence of Pt(IV) intermediates by variable-temperature NMR
in dichloromethane, and positive KIE values were observed in
this study. In contrast, trans-[PtMeX(PMe(C2F5)2)2] complexes
show a significant tendency toward deuterium incorporation into
the Pt-methyl group.17 Thus, apart from a remarkable difference
of reactivity, the (fluoroalkyl)phosphane systems show a clear
analogy of kinetic behavior with the corresponding alkylphos-
phanes, with a propensity of trans isomers (Cl trans to CH3)
toward an SE(ox) mechanism and of the cis isomers (phosphane
trans to CH3) toward an SE2 mechanism.
Figure 4. Correlation between the second-order rate constants
kH (M-1 s-1) for protonolysis of the dialkyl complexes cis-
[PtMe2L2] (1a-8a) and related monoalkyl compounds cis-
[PtMeClL2] (1b-8b) in methanol at 298 K. Numbers refer to
compounds as listed in Table 3.
group. Interestingly, treatment of [PtMe2(depe)] with DOTf in
CD3OD at -70 °C was reported to result in the immediate
formation of CH3D and a monomethyl solvento species.24
Deuteriolysis of 1a-8a leads to the formation of only CH3D,
and the KIE values are always positiVe, with a mean value of
3.2 ( 1. The reactions are characterized by low values of
activation enthalpy and highly negative values of activation
entropy. At this stage it is worth recalling that negative volumes
of activation are associated with the protonolysis of the parent
complexes cis-[PtR2(PEt3)2] (R ) linear or branched alkyl
groups) and cis-[Pt(R)(R′)(PEt3)2] (R′ ) Me). As for the
compounds 5b-8b, chelation and ring size do not affect
significantly the reactivity of compounds 5a-8a.
The series of trans-activating effects by the P-donor in
determining the rate of proton attack is the same as that for
compounds 1b-8b. Moreover, a straight line correlates the two
series of second-order rate constants values kH (Figure 4).
Linear regression analysis of this plot gives the equation
kH(PtMe ) ) (1780 ( 1180) + (57900 ( 2400)[kH(PtMeCl)]. Thus,
2
the rate of methane loss on protonation of 1a-8a is about 5
orders of magnitude higher than that for the corresponding cis-
monoalkyl chloride compounds. Deviation from linearity in the
plot, observed for some data points pertaining to chelate
complexes, can be explained by differences in torsion angles
between dialkyls and monoalkyl compounds having the same
ring size (especially in the case of 5a and 5b, with dppe as
chelating ligand).
A marked difference in rates of protonolysis was already
measured between the complexes [PtMe2(dfepe)] and [PtMeCl-
(dfepe)] and was interpreted by a possible diversity of reaction
mechanism dictated by the wide difference in electron densities
at the metals of the two complexes.29
Apart from the extreme similarity of the kinetic features, the
good linear correlation in Figure 4 rules out this possibility for
dialkyl complexes 1a-8a and monoalkyl complexes 1b-8b and
strongly indicates a common mechanism. This is confirmed by
the very similar KIE values measured for the attack at the first
and at the second methyl group. All the kinetic evidence in the
two cases militates against a multistep mechanism and is in favor
of a direct proton transfer in the transition state.
Primary Attack at Metal or Ligand? In this work we have
evaluated the kinetic features for electrophilic attack by the
proton on three systems, (i) cis-[PtMe2L2], (ii) cis-[PtMeClL2],
and (iii) trans-[PtMeClL2], through a variety of kinetic and NMR
measurements and have examined their correspondence with
literature data for closely related compounds. In an attempt to
Kinetic Features of cis-[Pt(R)(R′)L2] Complexes. The
systems analyzed in this study were those in which R )
R′ ) Me and L ) PEt3, P(Pri)3, PCy3, P(4-MePh)3 (1a-4a)
and L-L ) dppm, dppe, dppp, dppb (5a-8a). The rate law is
kobs ) kH[H+], and no kinetic influence of the chloride ion
was observed. The search for Pt(IV) intermediates by low-
temperature NMR in dichloromethane was unsuccessful, as
was the search for deuterium incorporation into the Pt-methyl
(28) Ong, C. M.; Jennings, M. C.; Puddephatt, R. J. Can. J. Chem. 2003,
81, 1196.
(29) Bennett, B. L.; Hoerter, J. M.; Houlis, J. F.; Roddick, D. M.
Organometallics 2000, 19, 615.

Pt-C Bond Protonolysis and C-H Bond ActiVation
Organometallics, Vol. 25, No. 14, 2006 3443
Figure 5. Qualitative coordinate reaction diagram for a rate-
determining initial protonation in an SE(ox) mechanism. The
symbols S, I(HCl), I(H), and I(σ) and P have the same meaning as in
Figure 3.
Figure 6. Qualitative coordinate reaction diagram for a rate-
determining proton attack at the Pt-C σ-bond of S with release of
alkane in a three-center transition state (SE2 mechanism).
rationalize the diverse kinetic results within a unified picture,
indirect arguments can never be completely conclusive. For
example, we can hardly rely on the form of the kinetic rate law
or on the analysis of steric or inductive effects of substituents
on the ligands. Thus, electron release should increase and steric
congestion should decrease the rate in either a SE(ox) or SE2
reaction pathway. Much more reliable diagnostic tools are the
detection of transient intermediates and/or the magnitude and
sign of kinetic parameters (such as entropies of activation,
volumes of activation, and isotope effects), which are indicative
of the extent of proton involvement in the transition state. From
these criteria two different categories of reactions can be
distinguished: (i) those involving a rate-determining proton
transfer to the substrate (either to the Pt-C σ-bond or to the
metal) and (ii) multistep reactions, where the important target
of detecting a hydride intermediate with an intact Pt-C bond
can be achieved. Protonolyses on cis-[PtMe2L2] and cis-
[PtMeClL2] belong to the first category, as presumed by the
lack of direct or inferred evidence of Pt(IV) intermediates, by
the largely negative values of entropies and volumes of
activation, and by the positiVe values of KIE. In contrast, the
kinetic features of the third system (trans derivatives), which is
simply a different geometrical shape of the second, are perfectly
in keeping with a multistep SE(ox) mechanism, as suggested
by the intermediacy of Pt(IV) species and by inVerse values of
KIEs.
Scheme 4. Three-Center-Two-Electron Transition State in
the Formation of the σ-Alkane Complex
density at the Pt-alkyl σ-bond of the four-coordinate starting
complex, combined with a significant LnPtδ+-Cδ- bond
polarization. As a consequence, either the carbon atom or the
Pt-alkyl σ-bond become the favored sites of protonation. If
the synchronous attack takes place at the Pt-C σ-bond, the
energy profile has the form illustrated in Figure 6, where the
point of highest energy is a three-center transition state in which
there is extensive involvement of the proton, considerable
stretching of the Pt-C bond, and no assistance by halide ions.
This could still be the most likely pathway if the Pt-C σ bond
is the highest occupied molecular orbital.6g
In conclusion, the similarity of the energy profiles in Figures
5 and 6, both describing the coordinate reaction for a rate-
determining proton transfer to the substrate, suggests that, under
these circumstances, any discussion of the site of proton attack
risks becoming semantic in nature. This is still more under-
standable if one considers that a three-center-two-electron
species could well be found along the path toward the formation
of the σ-alkane complex from the platinum(IV) hydrido alkyl
intermediate in an SE(ox) process (Scheme 4).
We can conclude that (i) isotopic scrambling and detection
of platinum(IV) alkyl hydrido intermediates are the most
definitive evidence for the oxidative-addition mechanism in
complexes of the type [PtMe2(N-N)], [PtMeCl(N-N)], and trans-
[PtMeX(PR3)2] (X ) Cl, Br) and (ii) when the trans-labilizing
effect of donor atoms is increased along the series I- < PR3 <
C(alkyl), the initial protonation becomes the rate-determining
step and it is difficult or impossible to identify the site of attack
and decide whether a SE(ox) mechanism is still operating or an
SE2 mechanism has taken over.
Relevance to C-H Bond Activation. The role of platinum-
(IV) hydrido intermediates is well-established in the protonolysis
of [PtMe2(N-N)] complexes11 and in the microscopic reverse
activation of alkanes or arenes by the related cationic compounds
[PtMe(N-N)(S)]+ (S ) weakly coordinating solvent).14 A result
of this work is that full oxidation of the metal is not a
prerequisite for all protonolysis reactions and there is no reason
to assume, “by analogy”, SE(ox) as a general mechanism for
electrophilic breakage of the Pt-C bond. In contrast to the
expectation of a facile oxidation of platinum(II) electron-rich
complexes, it was shown that the possibility of “one-step”
We are inclined to think that the geometry of the compounds
hardly plays a significant role in determining such kinetic
differences. Rather, the nature of the group in the position trans
to the Pt-C bond is crucial. If we agree to the intermediacy of
platinum(IV) hydrido species as a general rule for protonolysis
of the metal-carbon bond for electron-rich alkyl complexes of
platinum(II), the explanation that could be offered, according
to the SE(ox) mechanism, is that the nature of the trans ligand
markedly affects the relative energies and stabilization of the
intermediates I(HCl), I(H), and I(σ) in the energy profile of Figure
3. When the trans effect is increased along the series Cl-
<
Br- < I- < PR3 < C(alkyl), the qualitative form of the reaction
coordinate diagram changes from that in Figure 3 to that in
Figure 5.
As a result, the mechanism must entail a rate-determining
oxidative-addition reaction, with undetectable small concentra-
tions of oxidized intermediate. This qualitative coordinate
reaction diagram is consistent also with a very fast extrusion
of the alkane from the σ-alkane complex I(σ) upon nucleophilic
attack by the nucleophile (halide ion or solvent), the lack of
multiple H/D exchange, and the formation of only CH3D upon
deuteriolysis.
A reasonable alternative is that strong electron donation from
the trans-activating group promotes an increase of electron

3444 Organometallics, Vol. 25, No. 14, 2006
Romeo and D’Amico
Figure 7. Comparison of qualitative coordinate reaction diagrams for electrophilic attack by the proton at a platinum-alkyl substrate
occurring with an SE(ox) mechanism (A) or with an SE2 mechanism (B).
electrophilic attack at the Pt-C bond increases with increasing
electron donation brought about by the trans ligand. The
probability of intercepting Pt(IV) hydrido alkyl intermediates
along the energy profile decreases. The reason for this seems
to be a preferential electron enrichment and increase of energy
at the Pt-C σ-bond combined with a significant LnMδ+-Cδ-
bond polarization. Thus, the Pt-C bond becomes the preferential
site of attack in a one-step transfer of the proton characterized
by a three-center transition state. In addition, a strong donor
makes the complex more susceptible to protonation. Apart from
the kinetic data discussed above, a number of examples in the
literature prove this assumption. Interestingly, a comparison of
the kinetic behavior between trans-[Pt(Ph)(Me)(PEt3)2] and
trans-[PtCl(Me)(PEt3)2] is paradigmatic for a concurrent change
of site attack (Pt-C vs Pt) and of increase of reactivity (7 orders
of magnitude on going from Cl to Ph).
In light of the kinetic results of this work, there is sufficient
reason to consider a phosphane as a strongly trans-activating
ligand promoting an SE2 mechanism. Thus, it is of interest to
evaluate the effect of the change from N- to P-donor ligands,
because a choice of ligands has a great effect on the efficiency
of C-H activation at Pt(II) centers. The situation is summarized
in simplified form in Figure 7, which illustrates in terms of
qualitative reaction coordinate diagrams the chemical charac-
teristics of two systems containing nitrogen (A) or phosphorus
donor atoms (B) as trans ligands, respectively. The scheme is
also consistent with the general view that N-based ligands are
able to stabilize the Pt(IV) oxidation state better than P-based
ligands.30 Looking at the two plots in Figure 7 from the right
to the left side can help in drawing some conclusions on the
relative efficiency of platinum(II) phosphane or diimine com-
plexes in activating C-H bonds.31
(dissociative or associative). The facile access of the alkane (or
arene) to the coordination sphere of the metal in B is balanced
by its much easier removal, the rate-determining step shifts to
the actual C-H bond activation, and the overall energetics will
be less favorable in B than in A. A similar analysis was
performed by Puddephatt on the qualitative energy profiles of
two SE(ox) processes to predict the efficiency in activating the
arene C-H bond by [PtX(NCN)], a cyclometalated complex
characterized by a strong trans activating carbon atom, as
compared to the parent [PtPhX(NN)] complex.28
The good efficiency of Pt(II) bis(N-donor) alkyl complexes
in activating the C-H bond is reflected in the rapidly growing
number of examples that can be found in the literature for these
reactions.14 Relatively fewer cases refer to Pt(II) phosphane alkyl
complexes.32 However, several examples of intramolecular C-H
bond activation of phosphane ligands have been reported for
cycloplatination reactions.33 Cases in which cyclometalation is
initiated by ligand dissociation followed by an agostic interaction
of the resulting three-coordinate T-shaped 14-electron interme-
diate with the σ(C-H) orbital of a methyl group have been
recently reported.34 The exact mechanism for these C-H bond-
cleavage reactions is still unknown and deserves further study
because of its strong connection with the design of a model for
the still elusive platinum(II) σ-alkane complexes.
Experimental Section
General Procedures and Chemicals. All syntheses were
performed on a double-manifold Schlenk vacuum line under a dry
(32) (a) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides, G. M.
Organometallics 1988, 7, 2379. (b) Konze, W. V.; Scott, B. L.; Kubas, G.
J. J. Am. Chem. Soc. 2002, 124, 12550. (c) Peters, R. G.; White, S.; Roddick,
D. M. Organometallics 1998, 17, 4493.
The two strong donor phosphane ligands in B will markedly
affect the rate of ligand substitution. Under a strong trans-
activating effect, the alkane will enter the coordination sphere
of the metal more easily in B than in A. Likewise, the activation
energy for alkane loss from the σ-alkane complex will be much
lower in B than in A, whatever the details of this reaction step
(33) (a) Fornies, J.; Martin, A.; Navarro, R.; Sicilia, V.; Villarroya, P.
Organometallics 1996, 15, 1826. (b) Chappell, S. D.; Cole-Hamilton, D. J.
J. Chem. Soc., Dalton Trans. 1983, 1051. (c) Cheney, A. J.; Shaw, B. L. J.
Chem. Soc., Dalton Trans. 1972, 754. (d) Thorn, D. L. Organometallics
1998, 17, 348. (e) Alyea, E. C.; Ferguson, G.; Malito, J.; Ruhl, B. L.
Organometallics 1989, 8, 1188. (f) Alyea, E. C.; Malito, J. J. Organomet.
Chem. 1988, 340, 119. (g) Scheffknecht, C.; Rhomberg, A.; Mu¨eller, E.
P.; Peringer, P. J. Organomet. Chem. 1993, 463, 245. (h) Clark, H. C.;
Goel, A. B.; Goel, R. G.; Goel, S. Inorg. Chem. 1980, 19, 3220.
(34) (a) Plutino, M. R.; Monsu` Scolaro, L.; Albinati, A.; Romeo, R. J.
Am. Chem. Soc. 2004, 126, 6470. (b) Plutino, M. R.; Romeo, R.; Romeo,
A. HelV. Chim. Acta 2005, 88, 507. (c) Baratta, W.; Stoccoro, S.; Doppiu,
A.; Herdtweck, E.; Zucca, A.; Rigo, P. Angew. Chem., Int. Ed. 2003, 42,
105. (d) Ingleson, M. J.; Mahon, M. F.; Weller, A. S. Chem. Commun.
2004, 2398.
(30) (a) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17, 1478.
(b) Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735. (c) van
Asselt, R.; Rijnberg, E.; Elsevier, C. J. Organometallics 1994, 13, 706.
(31) The position of the two profiles with respect to one another is
completely arbitrary, and therefore, no quantitative comparison should be
made between the energies of the various species (P vs N). Only the relative
barrier heights at each (separate) profile are significant.

Pt-C Bond Protonolysis and C-H Bond ActiVation
Organometallics, Vol. 25, No. 14, 2006 3445
and oxygen-free dinitrogen atmosphere using freshly distilled, dried,
and degassed solvents. Solvents employed in the synthetic proce-
dures (Analytical Reagent Grade, Lab-Scan, Ltd.) were distilled
under dinitrogen from sodium-benzophenone ketyl (tetrahydro-
furan, diethyl ether, toluene) or barium oxide (dichloromethane).35
Spectrophotometric grade methanol, phosphane ligands, HCF3SO3,
DCF3SO3, Chloroform-d (99.8+%, CIL, Inc.), toluene-d8 (99+%),
CH3OD, and CD3OD were purchased from Aldrich Chemical Co.
and used as received. Elemental analyses were performed by the
Microanalytical Laboratory, Department of Chemistry, University
College Dublin, Belfield, Dublin 4, Ireland.
Instrumentation and Measurements. NMR analyses were
performed on a Bruker AMX-R 300 spectrometer equipped with a
broad-band probe operating at 300.13 and 121.49 MHz for 1H and
31P nuclei, respectively. 1H chemical shifts are reported in ppm (δ)
units with respect to Me4Si and referenced to the residual protiated
impurities of the deuterated solvent. Coupling constants are given
in hertz. 31P chemical shifts, in parts per million, are given relative
to external phosphoric acid. The temperature within the probe was
checked using the methanol method.36 Slow reactions were carried
out in a silica cell in the thermostated cell compartment of a rapid-
scanning Hewlett-Packard Model 8452A spectrophotometer or a
JASCO V-560 UV/vis spectrophotometer, with a temperature
accuracy of 0.02 °C. Fast reactions required the use of an Applied
Photophysics Bio Sequential SX-17 MX stopped-flow spectropho-
tometer.
in diethyl ether. After evaporation of the solvent the residues were
crystallized as white compounds from a petroleum ether mixture.
Complications arose with the bulky phosphanes L ) P(Pri)3, P(Cy)3
because of the easy geometrical conversion of the cis-monoalkyl
chloride species to their trans isomers. Therefore, a solution of a
weighed amount of cis-[PtMe2L2] in a 8/1 CD3OD/CD2Cl2 mix-
ture was frozen in the NMR tube and then added with a solution
of DCF3SO3 and LiCl in CD3OD (in a 1/10 ratio with respect to
the complex). The temperature was slowly increased to 225 K, and
the 1H and 31P NMR spectra of the ensuing cis-monoalkyl chloride
species were recorded.
1
cis-[PtMe(Cl)(P(Pri)3)2] (2b). H NMR (CD3OD/CD2Cl2 (8/1
v/v, T ) 225 K): δ 2.52 (m, 6H, PCHMe2), 1.29 (m, 36H,
PCHMe2), 0.58 (m, 2JPtH ) 53.2 Hz, 3JPH ) 7.0 Hz, 3H, Pt-CH3).
1
31P{1H} NMR (CD3OD/CD2Cl2, T ) 225 K): δ 32.8 (d, JPtP
)
A
1
1741 Hz, PA trans to CH3), 27.6 (d, JPtP ) 4250 Hz, PB trans to
A
Cl).
cis-[PtMe(Cl)(PCy3)2] (3b). 1H NMR (CD3OD/CD2Cl2 (8/1 v/v,
T ) 225 K): δ 2.72-1.54 (m, 66H, Cy), 0.67 (m, 3JPH ) 7.6 Hz,
2JPtH ) 56.2 Hz, 3H, Pt-CH3). 31P{1H} NMR (CD3OD/CD2Cl2, T
) 225 K): δ 23.2 (d, 1JPtP ) 1753 Hz, PA trans to CH3), 18.7 (d,
A
1JPtP ) 4328 Hz, PB trans to Cl).
A
Compounds with L-L ) dppe, dppp, dppb were obtained by
reacting a weighed amount of trans-[PtMeCl(Me2SO)2] with a
stoichiometric amount of the chelating phosphane in distilled and
degassed CH2Cl2. After evaporation of most of the solvent, the
complexes separated out as solids on adding light petroleum and
cooling.
Preparation of Complexes. cis-[PtMe2(Me2SO)2],37 trans-
[PtMeCl(Me2SO)2],37 [PtMe2(cod)],38 [PtMeCl(cod)],38 and [Pt2Me4-
(µ-SMe2)2],39 used as precursors for the synthesis of the alkylphos-
phane complexes, were prepared by literature methods and
characterized by elemental analysis and 1H and 31P{1H} NMR
spectra. All dialkyl and monoalkyl compounds in the text were
synthesized using a number of variations to the literature methods
in order to improve the yield and purity of old and new compounds.
Dialkyl Substrates. The complexes cis-[PtMe2L2] and [PtMe2-
(L-L)] (L ) PEt3,40 P(4-MeC6H4)3;38 L-L ) dppm,22,41 dppe,22,41
dppp,22,41 dppb22,41) were synthesized by following a well-
established general procedure: a weighed amount of cis-[PtMe2-
(Me2SO)2] was reacted in degassed dichloromethane with a
stoichiometric amount of the phosphane, and the reaction mixture
was set aside for a few hours. After evaporation of most of the
solvent, the complex separated out as an oil or a solid on adding
light petroleum (bp 60-80 °C) and cooling. The residue was crystal-
lized from a suitable solvent. The identity and purity of the com-
pounds were established by elemental analysis and by their NMR
spectra. In the case of the most sterically demanding phosphanes
P(Pri)3 and P(Cy)3 the solvent used was toluene and, for L ) P(Pri)3,
the starting material was the dimeric complex [Pt2Me4(µ-SMe2)2].
Monoalkyl Substrates. (a) cis-[PtMeClL2] and [PtMeCl-
[PtMe(Cl)(dppb)] (8b). 1H NMR (CDCl3, T ) 298 K): δ 7.6-
7.5 (m, 8H, Ho,o′), 7.4-7.3 (m, 12H, Hm,m′ + Hp), 2.5 (m, 2H, PHâ),
2
2.3 (m, 2H, PHR), 1.7 (m, 2H, PHR′), 0.39 (m, JPtH ) 54.3 Hz,
3JPH ) 6.8 Hz, 3H, Pt-CH3). 31P{1H} NMR (CDCl3, T ) 298 K):
1
1
δ 19.5 (d, JPtP ) 1711 Hz, PA trans to CH3), 18.7 (d, JPtP
)
A
A
4385 Hz, PB trans to Cl). Anal. Calcd for PtC29H31ClP2: C, 51.83;
H, 4.65. Found: C, 50.61; H, 4.69.
The reaction of trans-[PtMeCl(Me2SO)2] with dppm gives
quantitatively the dimer [PtMeCl(µ-dppm)]2. Preparation of the pure
monomer was achieved by applying the Puddephatt method to 5a.23
(b) trans-[PtMeClL2] Complexes. The compounds with L )
PEt3, P(4-MeC6H4)3 were obtained easily by spontaneous isomer-
ization of the corresponding cis isomers in methanol, following a
well-established synthetic procedure.43
trans-[PtMe(Cl)(P(4-MeC6H4)3)2] (4c). 1H NMR (CDCl3, T )
298 K): δ 7.60-7.13 (m, 24H, PC6H4CH3), 2.33 (s, 18H,
2
3
PC6H4CH3), -0.14 (m, JPtH ) 79.6 Hz, JPH ) 6.5 Hz, 3H,
Pt-CH3). 31P{1H} NMR (CDCl3, T ) 298 K): δ 27.7 (1JPtP
)
3122 Hz). Anal. Calcd for PtC43H45ClP2: C, 60.45; H, 5.31.
Found: C, 59.66; H, 5.26.
The complexes trans-[PtMeCl(PCy3)2]44 and trans-[PtMeCl-
(PPri3)2],45 were obtained as follows: a Schlenk tube equipped with
a magnetic bar was charged with trans-[PtMeCl(Me2SO)2] (60 mg,
0.15 mmol), phosphane (0.40 mmol), and distilled CH2Cl2 (10 mL).
The vessel was connected to a Schlenk line, where the mixture
was stirred for 4 h. The solvent was removed in vacuo, and the
residue was then taken up in a minimum amount of CH2Cl2.
Addition of diethyl ether and cooling afforded colorless crystals
of 2c and 3c.
42
(L-L)] Complexes. Compounds with L ) PEt3,40 P(4-MeC6H4)3
were prepared using the general procedure described by Chatt and
Shaw.40 A weighed amount of the corresponding dialkyl complex
in dry ether was treated with a stoichiometric quantity of dry HCl
(35) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. In Organic Solvents,
4th ed.; Weissberger, A., Ed.; Wiley: New York, 1986.
(36) (a) Van Geet, A. L. Anal. Chem. 1968, 40, 2227. (b) Van Geet, A.
L. Anal. Chem. 1970, 42, 679.
(37) Eaborn, C.; Kundu, K.; Pidcock, A. J. Chem. Soc., Dalton Trans.
1981, 933.
(38) (a) Clark, H. C.; Manzer L. E. J. Organomet. Chem. 1973, 59, 411-
428. (b) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643. (c)
Puddephatt, R. J.; Rashidi, M.; Fakhroeian, Z. J. Organomet. Chem. 1990,
406, 261. (d) Yang, D. S.; Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt,
R. J.; Tse, J. S. Inorg. Chem. 1990, 29, 2487.
(39) Hill, G. S.; Irwin, M. J.; Levy, C. J.; Rendina, L. M.; Puddephatt,
R. J. Inorg. Synth. 1998, 32, 149.
Kinetics. The rates of protonolysis of the dialkyl complexes
1a-8a were followed by stopped-flow spectrophotometry under
pseudo-first-order conditions. Since some of these complexes appear
to decompose slowly in methanol at room temperature, even in
the absence of acid, fresh solutions of the complexes were used
for all kinetic runs. Initial concentrations of starting complex were
(40) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 705.
(41) Hietkamp, S.; Stufkens, D. J.; Vrieze, K. J. Org. Chem. 1979, 44,
107-113.
(43) Romeo, R.; Minniti, D.; Trozzi, M. Inorg. Chem. 1976, 15, 1134.
(44) Strukul, G.; Michelin, R. A. J. Am. Chem. Soc. 1985, 107, 7563.
(45) Arnold, D. P.; Bennett, M. A. J. Organomet. Chem. 1980, 199, 119-
135.
(42) Cobley, C. J.; Pringle, P. G. Inorg. Chim. Acta 1997, 265, 107.

3446 Organometallics, Vol. 25, No. 14, 2006
Romeo and D’Amico
in the range 0.05-0.1 mM, and pseudo-first-order conditions were
achieved by having the proton concentration in at least 10-fold
excess. Rate constants were evaluated using the Applied Photo-
physics software package46 and are reported as average values from
five to seven independent runs. Second-order rate constants for the
protonolysis (kH) at 298.2 K were obtained by least-squares
regression analysis of the linear plots of the pseudo-first-order rate
constants vs the concentration of acid. At other temperatures, the
values of kH were obtained from the ratio of the measured pseudo-
first-order rate constants kobs to [H+]. Enthalpies and entropies of
activation for protonolysis were derived from a linear least-squares
analysis of ln(kH/T) vs T-1 data.
CH3OH and in CH3OD. Thus, kinetic runs were carried out with
DOTf and LiCl in CH3OD by following exactly the same procedure
described above for stopped-flow and conventional spectrophoto-
metric measurements.
Detection of Platinum(IV) Alkyl Hydrido Species. An ap-
propriate amount of the complex (5-6 mg, 0.01 mmol) was
combined with Ph4AsCl (42 mg, 0.1 mmol). The reagents were
dissolved in CD2Cl2 (0.5 mL) and then cooled to -78 °C before
adding via syringe HOTf (0.1 mmol) in CD2Cl2. After the con-
tents were mixed while the tube was kept as cool as possible, the
tube was placed in the precooled NMR probe. Relevant 1H and 31
P
NMR data for some identified [PtMeCl2(H)L2] species are given
in Table 2.
The slowest protonolysis reactions of the complexes cis-
[PtMeClL2] (1b-4b), [PtMeCl(L-L)] (5b-8b), and trans-
[PtMeClL2] (1c-4c) were followed by conventional spectropho-
tometry by repetitive scanning of the spectrum at suitable times in
the range 320-220 nm or at a fixed wavelength, where the
absorbance change was largest. The reaction was started by mixing
known volumes of the complex and of HCF3SO3, in the presence
of known amounts of lithium chloride, in a 1 cm quartz cell placed
in the thermostated cell compartment of the spectrophotometer. The
ionic strength was adjusted using lithium perchlorate to I(LiClO4)
) 0.2 M. Alternatively, the monoalkyl chloride complexes 1b-8b
were generated in situ in a silica cell by adding with a syringe a
prethermostated solution of [PtMe2L2] to a methanolic thermostated
solution of HA and LiCl, taking advantage of the very fast cleavage
of the first Pt-Me bond. Likewise, the trans complexes 1c-4c were
generated in situ from protonolysis of 1a-4a with HA and addition
of lithium chloride only when the ensuing cis-trans isomerization
of the cationic solvento complex cis-[PtMeL2(MeOH)]+ was
complete.6k,13 Rate constants were evaluated with the SCIENTIST47
software package by fitting the absorbance/time data to the expo-
nential function At ) A00 + (A0 - A00) exp(-kobst). Rate constants
are reported as average values from five to seven kinetic runs.
Kinetic Isotope Effect. The kinetic isotope effect was determined
systematically by comparing the rates of reactions carried out in
H/D Exchange. A similar procedure was applied to reveal H/D
exchange in methanol. Thus, the complex was dissolved in a
CD3OD/CD2Cl2 mixture (8/1) and cooled to -48 °C. After addition
of a solution of DOTf the mixture was placed in the NMR probe.
Deuterium incorporation into the methyl positions was checked at
this temperature for all compounds studied. The phenomenon was
monitored only for compounds 1c-3c, after addition of a small
excess of LiCl and warming to -20 °C, by the loss in the integration
intensity of the Pt-Me resonance and, at higher temperatures, by
the observation of the release of the full range of methane
isotopomers.
Acknowledgment. We are grateful to the Ministero
dell’Universita` e della Ricerca Scientifica e Tecnologica
(MIUR), PRIN 2004, and to the University of Messina, PRA
2003, for funding this work.
Supporting Information Available: A figure showing typical
1H and 31P NMR features useful in distinguishing between
compounds of classes a-c, text giving a full listing and assignment
of 1H and 31P NMR data for compounds of classes a-c, and tables
giving the concentration and temperature dependence of primary
kinetic data. This material is available free of charge via the Internet
at http://pubs.acs.org.
(46) Applied Photophysics Bio Sequential SX-17 MV, sequential
stopped-flow ASVD spectrofluorimeter. Software manual: Applied Pho-
tophysics, Kingstone Road, Leatherhead KT22, U.K.
(47) SCIENTIST; Micro Math Scientific Software, Salt Lake City, UT.
OM060217N