β-hydrogen kinetic effect

Article abstract of DOI:10.1021/ja0702162

A combined kinetic and DFT study of the uncatalyzed isomerization of cationic solvent complexes of the type cis-[Pt(R′)(S)(PR₃) ₂]⁺ (R′ = linear and branched alkyls or aryls and S = solvents) to their trans isomers has sho

Full text of DOI:10.1021/ja0702162

Published on Web 04/05/2007
â-Hydrogen Kinetic Effect
Raffaello Romeo,*,† Giuseppina D’Amico,† Emilia Sicilia,‡ Nino Russo,‡ and
Silvia Rizzato§
Contribution from the Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,
UniVersita` di Messina, Salita Sperone, 31-Vill. S. Agata-98166 Messina, Italy, Dipartimento di
Chimica, UniVersita` della Calabria, I-87030 ArcaVacata di Rende (Cs) Italy, and Dipartimento
di Chimica Strutturale (DCSS) and Facolta` di Farmacia, UniVersita` di Milano, Via Venezian 21,
Milano, Italy
Received January 11, 2007; E-mail: rromeo@unime.it
Abstract: A combined kinetic and DFT study of the uncatalyzed isomerization of cationic solvent complexes
of the type cis-[Pt(R′)(S)(PR3)2]+ (R′ ) linear and branched alkyls or aryls and S ) solvents) to their trans
isomers has shown that the reaction goes through the rate-determining dissociative loss of the weakly
bonded molecule of the solvent and the interconversion of two geometrically distinct T-shaped 14-electron
three-coordinate intermediates. The Pt-S dissociation energy is strongly dependent on the coordinating
properties of S and independent of the nature of R′. The energy barrier for the fluxional motion of [Pt(R′)-
(PR3)2]+ is comparatively much lower (≈8-21 kJ mol-1). The presence of â-hydrogens on the alkyl chain
(R′ ) Et, Prn, and Bun) produces a great acceleration of the reaction rate. This accelerating effect has
been defined as the â-hydrogen kinetic effect, and it is a consequence of the stabilization of the transition
state and of the cis-like three-coordinate [Pt(R′)(PR3)2]+ intermediate through an incipient agostic interaction.
The DFT optimization of [Pt(R′)(PMe3)2]+ (R′ ) Et, Prn, and Bun) reproduces a classical dihapto Pt····η2-HC
agostic mode between the unsaturated metal and a dangling C-H bond. The value of the agostic stabilization
energy (in the range of ≈21-33 kJ mol-1) was estimated by both kinetic and computational data and
resulted in being independent of the length of the hydrocarbon chain of the organic moiety. A better
understanding of such interactions in elusive reaction intermediates is of primary importance in the control
of reaction pathways, especially for alkane activation by metal complexes.
Introduction
platinum(II) chemistry, T-shaped three-coordinate 14-electron
intermediates were assumed as key intermediates in reactions
Four-coordinated square-planar 16-electron transition metal
compounds show a great steric and electronic propensity to add
a fifth ligand and to form five-coordinate 18-electron species
either as discrete compounds1 or as reaction intermediates.2 Yet,
there is a great interest in the possibility of activating alternative
low-energy dissociative pathways.3 Many organometallic reac-
tions are thought to take place through intermediates with open
coordination sites, and dissociation, usually of a neutral ligand,
is often proposed as the initial step in many reactions involving
square-planar d8 organometallic complexes.4 This dissociation
enables subsequent processes or initiates catalytic cycles. In
of â-hydrogen elimination,5 thermal decomposition of dialkyls,6
trans-7 and cis-8 monoalkyl compounds, insertion of olefins into
the M-H bond,9 electrophilic attack at the Pt-C bond,10
(4) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. C. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987. (b) Crabtree, R. H. The Organometallic
Chemistry of the Transition Metals; Wiley-Interscience: New York, 1994.
(c) Yamamoto, A. Organotransition Metal Chemistry; Wiley: New York,
1986. (d) James, B. R. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford,
1982; Ch. 51. (e) Parshall, G. W. Homogeneous Catalysis; Wiley-
Interscience: New York, 1980.
(5) For general reviews of â-elimination reactions, see: (a) Cross, R. J. The
Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.; John
Wiley: New York, 1985; Vol. 2, Ch. 8. (b) Whitesides, G. M. Pure Appl.
Chem. 1981, 53, 287-292. (c) Yamamoto, A.; Yamamoto, T.; Komiya,
S.; Ozawa, F. Pure Appl. Chem. 1984, 56, 1621-1634.
† Universita` di Messina.
‡ Universita` della Calabria.
§ Universita` di Milano.
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1981, 103, 3396-3403. (b) Griffiths, D. C.; Young, G. B. Organometallics
1989, 8, 875-886.
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67-114. (b) Maresca, L.; Natile, G. Comments Inorg. Chem. 1993, 14,
349-366.
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1557. (b) Brainard, R. L.; Miller, T. M.; Whitesides, G. M. Organometallics
1986, 5, 1481-1490.
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Wesley Longman: Essex, UK, 1999. (b) Wilkins, R. G. Kinetics and
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Inorganic Reactions; Wiley: New York, 1968. (f) Langford, C. H.; Gray,
H. B. Ligand Substitution Processes; W. A. Benjamin: New York, 1965.
(3) For a discussion on dissociative pathways in platinum(II) chemistry, see
Romeo, R. Comments Inorg. Chem. 1990, 11, 21-57.
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9
5744
J. AM. CHEM. SOC. 2007, 129, 5744-5755
10.1021/ja0702162 CCC: $37.00 © 2007 American Chemical Society

â
-Hydrogen Kinetic Effect
A R T I C L E S
geometrical isomerization,11 exchange reactions,12 and fluxional
motions of coordinated ligands.13 Another promising feature of
unsaturated platinum(II) chemistry is ligand cycloplatination14,15
or successful C-H activation under mild conditions by com-
plexes of the form [Pt(N-N)(CH3)(solv)]+, where N-N is a
bidentate nitrogen-centered ligand and solv is a weakly coor-
dinating solvent.16 Likewise, dissociative pathways were pro-
posed for a number of cases in palladium(II) chemistry,
isomerization, ligand rotation, â-elimination, and palladium-
catalyzed cross-coupling reactions.17
Chart 1. View of Some Cationic T-Shaped Platinum(II) Molecules
Stabilized by Agostic Interactions
complexes trans-[Pt(CH3)L2]+ supported by a δ-agostic interac-
tion from the bulky phosphine PR2-(2,6-Me2C6H3), [R ) Ph or
Cy] (complex C in Chart 1).20
Despite the kinetic perception of the intermediacy of such
elusive coordinatively unsaturated intermediates in a number
of fundamental organometallic processes, and their recognized
importance in homogeneous catalysis and bond activation, direct
proof of 14-electron platinum(II) complexes has been difficult
to find. The reason is that unsaturation on the metal can be
easily relieved by coordination of the solvent or of anions on
forming 16-electron species, and dimerization is also a reason-
able alternative. Very few papers have been published so far
that describe the structural characterization of such compounds.
Spencer et al. have described the cationic [Pt(R)(P-P)]+
complexes (P-P ) chelating ligand and R ) ethyl, norbornyl)
that show â-agostic C-H interactions (complex A in Chart 1),18
Ingleson et al. have reported on the complex [Pt(CH3)(iPr3P)2]+
that is stabilized by â-agostic C-H interactions (complex B in
Chart 1),19 and Baratta et al. have prepared Pt(II) cationic
Related complexes of isoelectronic Ni(II),21 Rh(I),22,23 and
Pd(II)24-27 have also been described. Judging from the bonding
and structural features of the species mentioned previously, the
challenge to isolate stable 14-electron T-shaped compounds
relies on the use of non-innocent, preferably bulky, ligands that
guarantee some form of protection of the fourth coordination
site and a certain extent of electron transfer through C-H‚‚‚‚
M agostic interactions to a vacant orbital of the metal. Agostic
interactions appear in a number of highly diverse transition and
lanthanide metal compounds and have been scrutinized inten-
sively by X-ray crystallography, NMR, or computational
techniques.28 The C-H‚‚‚‚M interaction seems to be particularly
favored by steric constraints brought about by bulky ligands.29
Much less understood is the role of agostic interactions on the
reactivity, although it appears to be crucial in the reaction of
â-hydride elimination5 as well as in the control of the tacticity
of polymers.30
When an incipient â-agostic interaction by a dangling C-H
bond stabilizes a T-shaped transient three-coordinate transition
state or intermediate, this results in a sharp increase of the
reaction rate. We discovered this phenomenon in kinetic studies
of dissociative processes in platinum(II) chemistry, as ligand
exchange and substitution performed on the novel complex cis-
[Pt(Hbph)2(dmso)2] (Hbph- ) η1-biphenyl monoanion).15,31 The
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A R T I C L E S
Romeo et al.
Table 1. Selected 1H and 31P{1H} NMR Data for cis- and trans-Monoalkyl/aryl Solvent Species Obtained upon Fast Protonolysis of
cis-[Pt(R)(R′)(PEt3)2] (1-9) and Subsequent Isomerizationa
cis-[Pt(R)(CD3CN)(PEt3)2]+
trans-[Pt(R)(CD3CN)(PEt3)2] +
R
δ
(1H)
δ
(
31PA)b
δ
(
31PB)c
δ
(1H)
δ (
31P)
1
2
CH3
C2H5
0.27 (3H)
1.62 (2H)
0.70 (3H)
19.1 (1775)
18.3 (1621)
11.8 (4170)
13.2 (4345)
0.16 (3H)
1.76 (2H)
0.84 (3H)
22.8 (2687)
22.0 (2871)
3
4
C2D5
n-C3H7
18.6 (1609)
12.9 (1657)
13.0 (4352)
18.3 (4312)
22.5 (2848)
21.9 (2857)
0.95-0.72 (4H)
0.45 (3H)
1.83-1.57 (4H)
0.47 (3H)
5
6
n-C4H9
1.85-0.85 (6H)
18.3 (1648)
18.3 (1890)
13.0 (4331)
10.3 (4226)
1.77-1.63 (6H)
21.7 (2861)
18.9 (2721)
0.53 (3H)
0.53 (3H)
CH2Si(CH3)3
0.25 (2H)
0.16 (2H)
-0.15 (9H)
-0.21 (9H)
7
8
9
C6H5
2-MeC6H4
2,4,6-C6H2
15.0 (1690)
15.2 (1653)
15.6 (1644)
7.4 (4200)
5.7 (4228)
2.8 (4249)
20.1 (2680)
19.1 (2694)
17.8 (2713)
2.25 (3H)
2.24 (6H)
1.95 (3H)
2.20 (3H)
2.20 (3H)
1.94 (6H)
a Recorded in MeCN-d3 as the solvent at 263 K for 2a-4a and at 298 K for 1a, 5a-8a, and for all the trans derivatives (1b-9b). Chemical shifts (δ)
are in ppm from TMS and H3PO4; coupling constants 1JPtP in Hz are given in parentheses. b PA, phosphorus atom trans to the alkyl or to the aryl group.c PB,
phosphorus atom trans to the coordinated MeCN-d3.
effect was found to be by far more significant in the uncatalyzed
cis to trans isomerization of solvent cis-[Pt(R)(S)(PEt3)2]+ (R
) linear or branched alkyls and S ) MeOH) complexes.32
This paper focuses on a phenomenon that so far has had only
sporadic evidence and reports a combined kinetic and DFT study
of the spontaneous isomerization of cationic cis-[Pt(R)(S)-
(PR3)2]+ complexes (kinetic data in acetonitrile for PR3 ) PEt3;
S ) CH3CN; R ) Me, Et, Et-d5, Prn, Bun, CH2SiMe3, Ph,
2-MeC6H4, or 2,4,6-Me3C6H2; computational data for PR3 )
PMe3; R ) Me or Et; S ) CH3CN, MeOH, DMSO, or benzene;
R ) Bun; S ) CH3CN or MeOH). Besides confirming a rare
dissociative mode of activation, inferred from the kinetic
analysis, the computational data allow for a measure of the
energies and a better understanding of the bonding and structural
features of the species intercepted along the potential energy
surface (PES) of the reaction. In particular, the T-shaped 14-
electron species, formed upon solvent dissociation from cis-
[Pt(R)(S)(PMe3)2]+, appear to be stabilized by interactions with
the electron pair of a dangling C-H bond giving origin to what
has been called the â-hydrogen kinetic effect. The structural
and bonding characteristics of cis-[Pt(R)(PMe3)2]+ have been
analyzed in detail. The agostic stabilization energy was found
to affect also the fluxional motion of the T-shaped cis 14-
electron species toward its trans form. All this information is
crucial in exploring the role of these fascinating elusive species
in fundamental processes such as C-H bond activation.16
complexes were synthesized by literature methods and charac-
terized by 1H and 31P NMR. The dialkyls species 1-5 showed
only one phosphorus resonance in the 31P NMR spectra, and
the alkyl-aryls 6-9 showed two resonances, with low values
1
of JPtP coupling constants, typical of phosphorus atoms trans
to carbon in platinum(II) complexes. A complete characteriza-
tion and assignment of the resonances is reported in Supporting
Information Table SI1. Addition of the stoichiometric amount
of an ethereal solution of HBF4 to a solution of the platinum
complex in acetonitrile results in a selective cleavage of the
Pt-C(alkyl) σ-bond, and in the formation of the cationic solvent
complex cis-[Pt(R)(CH3CN)(PEt3)2]+ (1a-9a) and alkane lib-
eration. Care must be taken to avoid a local excess of acid that
leads to the protonolysis of the remaining R group.
1
The cis products can be recognized by H and 31P NMR, at
room temperature for 1a and 6a-9a complexes, but a lower
temperature (263 K) is required when R is an alkyl group
containing â-hydrogens (compounds 2a-5a). Satisfactory in-
dications of the stereochemistry of 1a-9a came from their 31
P
NMR spectra that showed two 31P resonances for two non-
equivalent phosphane ligands, characterized by completely
different values of a coupling constant with 195Pt, as a
consequence of the diverse trans influence of the alkyl/aryl or
CH3CN ligands (1JPtP ≈ 1700 Hz for PA, in a trans position to
the carbon atom, and 1JPtP ≈ 4200 Hz for PB trans to CH3CN).
There was no evidence for the build-up of any intermediate
species upon electrophilic attack by the proton, even at low
temperature. The subsequent isomerization reactions were
monitored at appropriate times until the cis to trans conversion
Results
Synthesis, Characterization, and Structure of the Com-
plexes. The dialkyl and mixed alkyl-aryl cis-[Pt(R)(R′)(PEt3)2]
1
was complete. A collection of selected H and 31P NMR data
(31) Dissociative substitution is rather common for complexes of the type cis-
[Pt(C,C)(S,S)] (C is a strong σ-donor carbon group; S ) thioether or
sulfoxide), no matter whether the carbon atom comes from an alkyl, an
aryl, or a metalated aryl ligand. See (a) Plutino, M. R.; Monsu` Scolaro, L.;
Romeo, R.; Grassi, A. Inorg. Chem. 2000, 39, 2712-2720. (b) Frey, U.;
Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem. Soc. 1989, 111, 8161-
8165. (c) Alibrandi, G.; Minniti, D.; Monsu` Scolaro, L.; Romeo, R. Inorg.
Chem. 1989, 28, 1939-1943. (d) Alibrandi, G.; Bruno, G.; Lanza, S.;
Minniti, D.; Romeo, R.; Tobe, M. L. Inorg. Chem. 1987, 26, 185-190. (e)
Minniti, D.; Alibrandi, G.; Tobe, M. L.; Romeo, R. Inorg. Chem. 1987,
26, 3956-3958. (f) Lanza, S.; Minniti, D.; Moore, P.; Sachinidis, J.; Romeo,
R.; Tobe, M. L. Inorg. Chem. 1984, 23, 4428-4833. (g) Lanza, S.; Minniti,
D.; Romeo, R.; Moore, P.; Sachinidis, J.; Tobe, M. L. J. Chem. Soc., Chem.
Commun. 1984, 542-543. (h) Minniti, D. J. Chem. Soc., Dalton Trans.
1993, 1343-1345.
for the monoalkyl and monoaryl cis and trans solvent isomers
is reported in Table 1. A complete characterization and
assignment of the resonances is given in the Experimental
Procedures. Typical examples of 31P NMR spectra of cationic
cis- and trans-[Pt(Et)(CD3CN)(PEt3)2]+ solvent complexes are
given in Supporting Information Figure SI2.
X-ray quality crystals of 5 were grown by diffusion of
n-hexane into a dichloromethane solution of the complex. Details
of the structure determination are given in the Experimental
Procedures. Table 2 lists experimental and crystallographic data.
Selected bond distances and angles are given in Table 3. An
(32) (a) Romeo, R.; Plutino, M. R.; Elding, L. I. Inorg. Chem. 1997, 36, 5909-
5916.
9
5746 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

â
-Hydrogen Kinetic Effect
A R T I C L E S
Table 2. Experimental Data for the X-ray Diffraction Study of 5
formula
mol wt
T (K)
C20 H48P2Pt
545.61
120 (2)
diffractometer
cryst syst
space group (no.)
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Brucker SMART CCD
triclinic
P1h (2)
8.9295(9)
10.267(1)
14.388(1)
99.620(2)
107.679(2)
98.996(2)
1208.5(2)
2
Z
F
calcd (g cm-3
)
1.499
5.94
µ (mm-1
)
radiation
Mo KR (λ ) 0.71073 Å)
1.53 < θ < 26.03
11022
4751
4588
θ range (deg)
no. of data collected
no. of independent data
no. of obsd reflns (no)
Figure 2. Electronic spectrum of 5 in CH3CN at 298 K (a) and typical
spectral changes (b f c) associated with the cis to trans isomerization of
5a generated by protonolysis in situ. Cycle time for isomerization was 20
s. The inset in the figure gives the time dependence of the absorbance at
230 nm.
2
2
[|Fo| > 2.0σ(|F| )]
no. of param. refined (nv)
Rint
208
0.0196
0.0148
0.0369
1.075
Ra (obsd refln)
R2 b (obsd refln)
w
GOFc
tions are in the upper range of the values found for other
phosphane-dialkyl complexes of platinum(II).33,34
2
2
2 2
a R ) Σ(|Fo (1/k)Fc|)/Σ|Fo|. b R2 ) [Σw(Fo - (1/k)Fc )2/Σw|Fo | ].
-
w
Kinetics. The isomerization of cis-[Pt(R)(CH3CN)(PEt3)2]+
complexes can be monitored by NMR techniques through the
decrease of the two associated 31P signals and the parallel and
matching increase in the signal of the corresponding trans
complex, which appears as a singlet with platinum satellites
(1JPtP ≈ 2800 Hz). However, the systematic kinetics of isomer-
ization were followed spectrophotometrically by repetitive
scanning of the spectrum in the UV region. The conversion was
100% complete, and the spectral changes showed well-defined
isosbestic points (Figure 2).
2
2
c GOF ) [Σw(Fo - (1/k)Fc )2/(no - nv)]1/2
.
Table 3. Bond Lengths (Å) and Angles (deg) for 5
Pt-C(5)
2.108 (2)
2.114 (2)
2.3002 (6)
2.3065 (6)
172.52 (6)
99.55 (2)
C(5)-Pt-C(1)
C(5)-Pt-P(1)
C(1)-Pt-P(1)
C(5)-Pt-P(2)
Pt-C(1)-C(2)
Pt-C(5)-C(6)
83.24(9)
167.75(6)
86.70(6)
91.02(6)
111.82 (6)
112.42 (6)
Pt-C(1)
Pt-P(1)
Pt-P(2)
C(1)-Pt-P(2)
P(1)-Pt-P(2)
The isomerization follows a first-order rate law. For R ) Et
(2a), Et-d5 (3a), Prn (4a), or Bun (5a), the temperature
dependence was measured with the traditional CTK method.
Specific rate constants, ki (s-1), measured at different temper-
atures are given as Supporting Information in Table SI2 and
were analyzed by least-squares regression of linear Eyring plots.
Values of ki (s-1), ∆Hq (kJ mol-1), ∆Sq (J K-1 mol-1), and
∆Gq (kJ mol-1) at 298 K, for the same reactions, are collected
in Table 4. The much slower conversions of the complexes in
which R ) CH2SiMe3 (6a), Ph (7a), 2-MeC6H4 (8a), or 2,4,6-
Me3C6H2 (9a) were measured with the VTK method,35-37 and
the kinetic and activation data calculated as described in the
Experimental Procedures are listed in Table 4. In Figure 3 is
plotted a typical example of a sigmoidal temperature-absor-
bance profile used to calculate the activation parameters for a
slow reaction (isomerization of cis-[Pt(CH2SiMe3)(CH3CN)-
(PEt3)2]+, 6a) and to construct a temperature-rate (ki vs T)
profile reported as Supporting Information Figure SI1B.
Figure 1. ORTEP view of a molecule of compound 5 showing 50%
probability ellipsoids.
ORTEP view of the molecule is given in Figure 1. The
immediate coordination sphere around the Pt center consists of
the two alkyl groups, cis to each other, and the P atoms of the
phosphane moieties. The square-planar coordination is tetrahe-
drically distorted with maximum deviation from the mean square
plane (defined by atoms C(1), C(5), P(1), P(2), and Pt) of 0.118-
(1) Å. The butyl groups lie on opposite sides of the coordination
plane with a slightly different orientation, as can be judged from
the values of the C(1)-Pt-C(5)-C(6) and C(5)-Pt-C(1)-
C(2) dihedral angles: 99.7(2) and 83.3(2)°, respectively. No
significant difference is found in the Pt-C bond lengths (2.108-
(2) and 2.114(2) Å, respectively) or in the Pt-P bond lengths
(2.3002 (6) and 2.3065 (6) Å, respectively). The Pt-P separa-
(33) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock, A.; Giering,
W. P. Organometallics 1999, 18, 474-479.
(34) International Tables for X-ray Crystallography, 2nd ed.; Wilson, A. J. C.,
Prince, E., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands,
1999; Vol. C, Ch. 9.5.
(35) Romeo, R.; Alibrandi, G. Inorg. Chem. 1997, 36, 4822-4830.
(36) (a) Alibrandi, G. J. Chem. Soc., Chem. Commun. 1994, 2709-2710. (b)
Alibrandi, G. Inorg. Chim. Acta 1994, 221, 31-34. (c) Alibrandi, G.; Micali,
N.; Trusso, S.; Villari, A. J. Pharm. Sci. 1996, 85, 1105-1108. (d)
Alibrandi, G.; Coppolino, S.; Micali, N.; Villari, A. J. Pharm. Sci. 2001,
90, 270-274.
(37) Zhang, S.; Brown, T. L. Inorg. Chim. Acta. 1995, 240, 427.
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5747

A R T I C L E S
Romeo et al.
Table 4. Effect of Varying the Nature of R on Rates and
Activation Parameters of Spontaneous Isomerization of
cis-[Pt(R)(CH3CN)(PEt3)2]+ in Acetonitrile Solution
b,c
d,e
f
n
R
106 kia
∆
Hq
∆
Sq
∆
Gq
1a
2a
3a
4a
5a
6a
7a
8a
9a
CH3
C2H5
C2D5
n-C3H7
n-C4H9
CH2Si(CH3)3
C6H5
2-MeC6H4
2,4,6-Me3C6H2
2.42
4900
2730
8310
14000
40.2
15.8
8.09
12.7
147.7c
100.5b
99.8b
139e
106.3
86.1
87.5
84.9
83.7
48.2d
41.2d
81.9d
40.9d
63.5e
92.4e
91.0e
47.4e
109.3b
95.9b
117.1c
127.9c
129.2c
115.1c
98.2
100.4
102.1
101.0
a First-order rate constants (s-1) for isomerization at 298.2 K. b Enthalpies
of activation (kJ mol-1) from constant-temperature kinetics. c Enthalpies
of activation (kJ mol-1) from variable-temperature kinetics. d Entropies of
activation (J K-1 mol-1) from constant-temperature kinetics. e Entropies of
activation (J K-1 mol-1) from variable-temperature kinetics. f Free energy
of activation (kJ mol-1) at 298.2 K.
Figure 4. Calculated DFT PES for the isomerization of the solvent cationic
complex [Pt(Me)(MeCN)(PMe3)2]+.
Figure 5. Calculated DFT PES for the isomerization of the solvent cationic
complex [Pt(Et)(MeCN)(PMe3)2]+.
Table 5. Effect of Varying Nature of R and Solvent on Relative
Energiesa of Optimized Structures of Two T-Shaped Reaction
Intermediates, TS for Their Conversion, and Product during
Isomerization of cis-[Pt(R)(S)(PEt3)2]+b Complexes
Figure 3. Change in optical density at 215 nm during the geometrical
isomerization of cis-[Pt(CH2SiMe3)(CH3CN)(PEt3)2]+, 6a, carried out in
acetonitrile at the variable-temperature T (K) ) 296 + 0.00156t (s).38
n
R
S
cis T-shaped
TS
trans T-shaped
trans product
1
Me
Et
CH3CN
CH3CN
107.0
72.9
66.9
73.8
32.5
25.9
70.5
35.1
6.2
116.6
90.8
85.6
82.7
50.5
48.9
79.4
53.1
15.1
-9.1
67.8
53.4
52.6
33.8
13.0
11.7
56.8
15.6
-7.6
-46.5
-22.9
-24.1
-24.5
-15.1
-26.4
-27.0
-10.5
-12.4
-29.6
-42.3
Computational Data. The analyzed processes, in the frame-
work of DFT, were the geometrical conversion of the methyl
and ethyl solvent complexes cis-[Pt(R)(S)(PMe3)2]+ (R ) Me
or Et and S ) acetonitrile, methanol, dimethyl-sulfoxide, or
benzene) and of cis-[Pt(Bun)(S)(PMe3)2]+ (S ) acetonitrile or
methanol). The aim was to obtain insight on the way in which
the different nature of the alkyl group or of the labile coordinated
solvent could affect the various steps along the reaction
coordinate. Energy profiles for cis-[Pt(Me)(CH3CN)(PMe3)2]+
(Figure 4) and cis-[Pt(Et)(CH3CN)(PMe3)2]+ (Figure 5) show
the key steps through which the cis to trans conversion proceeds.
The relevant computational data listed in Table 5 refer to
the energetics of the isomerization of various species of cis-
[Pt(R)(S)(PMe3)2]+ in which the nature of the organic fragment
(R ) Me, Et, or Bun) and of the leaving group (CH3CN, MeOH,
Me2SO, or benzene) has been systematically changed. Relative
energies, calculated with respect to the reactant state, of the
two T-shaped intermediates, of the transition state (TS) for their
conversion, and of the product state are in kilojoules per mol.
2
3
4
5
6
7
8
9
Bun CH3CN
Me
Et
MeOH
MeOH
Bun MeOH
Me
Et
Me
Me2SO
Me2SO
C6H6
10 Et
C6H6
-27.0
a Calculated with respect to the reactant (kJ mol-1). b S ) solvent.
(PR3 ) tertiary phosphine; R′) alkyl or aryl group; and S )
solvent), formed easily in situ by protonolysis of precursor
dialkyls or mixed alkyl-aryl platinum complexes, are short-lived
species. The reason for their instability in solution is a facile
conversion into long-standing trans isomers. The factors that
usually combine in accelerating the rate of conversion are (i)
electron release by the phosphine ligands,35 (ii) steric repulsions
and distortion of the original four-coordinate square-planar
configuration,35,11a and (iii) interaction of the â-hydrogen atoms
(38) In a VTK experiment, the temperature increases linearly according to the
Discussion
general equation T ) T0 + γt. In the kinetic run of Figure 3, T0 ) 296 K,
and the temperature gradient is γ ) 0.00156 K s-1
.
Kinetic Studies. We knew from previous studies39 that bis-
phosphane solvent complexes of the type cis-[Pt(R′)(S)(PR3)2]+
(39) (a) Alibrandi, G.; Minniti, D.; Monsu` Scolaro, L.; Romeo, R. Inorg. Chem.
1988, 27, 318-324.
9
5748 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

â
-Hydrogen Kinetic Effect
A R T I C L E S
Scheme 1 Mechanism for Isomerization of Solvent Platinum(II)
Cationic Complexes
judged from the comparison of the rate data in Table 4 for the
compounds with R ) Me, (trimethylsilyl)methyl, Ph, o-tolyl,
or mesityl. The trend of the steric effects is significantly different
from that observed for bimolecular nucleophilic substitutions40
or for bimolecular electrophilic attack by H+,32 where the steric
bulk of the cis group produces a decrease of 4-5 orders of
magnitude in the rate.
â-Hydrogen Kinetic Effect. The rate of isomerization of cis-
[Pt(R)(CH3CN)(PEt3)2]+ solvent complexes, when R ) Et, Prn,
or Bun, results in being many orders of magnitude higher than
that of the remaining complexes. Inductive or steric effects
cannot account for this large rate acceleration, and a specific
interaction of â-hydrogens with the metal in the TS must be
held responsible for the effect. A tentative calculation of the
energy involved in the â-hydrogen kinetic effect, based on the
values of the free activation energies listed in Table 4 and on
the reasonable assumption that the ground state energies of 2a,
4a, and 5a are comparable or not greatly different from that of
1a, gives a mean value of 21.3 ( 1.3 kJ mol-1 for the
stabilization energy of an agostic TS relative to that of a non-
agostic TS (as the methyl compound 1a). Interestingly, experi-
mental and computational studies have revealed interaction
energies for agostic bonds in transition metal complexes in the
of the alkyl chain with the coordinatively unsaturated inter-
mediate.32,11a The solvent plays a crucial role. In weakly
coordinating solvents (benzene, dichloromethane, chloroform,
etc.), the conversion is instantaneous. In protic solvents (metha-
nol, ethanol, etc.), the process is relatively fast, and the
characterization of the solvent cationic cis-[Pt(R′)(S)(PR3)2]+
complexes by NMR requires low-temperature measurements.
In acetonitrile, a good coordinating solvent, the process can be
conveniently followed by conventional or rapid-scanning spec-
trophotometry, and the characterization of cis solvent complexes
is relatively easy as described previously. For all compounds,
the conversion is complete, the spectral changes in the UV
region show well-defined isosbestic points, the trans isomer is
the only species at the end of the reaction, and the process
follows a first-order rate law. The whole set of kinetic data is
collected in Table 4, including the activation parameters that
were calculated from the temperature dependence of the reaction
using both the traditional isothermal (CTK) method and a non-
isothermal (VTK) method. The advantages offered by the VTK
method were discussed elsewhere35,36 and are straightforward:
(i) with a single kinetic run, it is possible to obtain a k versus
T profile instead of a single rate constant, saving time and
chemicals, (ii) the whole set of data comes from a single
experiment carried out in homogeneous conditions, (iii) it
permits fast and easy collection and processing of massive
amounts of data, and (iv) the statistical error associated with
the calculated values of ∆Hq and ∆Sq is very low. The large
values of enthalpy of activation and particularly the positive
entropies of activation associated with isomerization give an
indication that the reaction proceeds through the well-established
dissociative mechanism illustrated in Scheme 1, which involves
dissociative loss of the solvent molecule and interconversion
of two geometrically distinct three-coordinate T-shaped 14-
electron intermediates.
range of 40-50 kJ mol-1 41
The activation mode would be
.
expected to exhibit a primary isotope effect when â-hydrogen
atoms are replaced with deuterium atoms since bonds to the
isotopic atoms are formed during that step.42 There will be a
preference for hydrogen over deuterium in the agostic interaction
that would lead to a positive value of KIE (the kinetic isotope
effect). Thus, the value of KIE (kH/kD ) 1.8), obtained from
the comparison of the rate data of 2a and 3a at 298.2 K, gives
a further confirmation of an extensive involvement of hydrogen
atoms in the formation of the transition state.
DFT Calculations: Mechanism, Structures, and Bonding.
The PES’s for isomerization obtained from DFT calculations
for a variety of systems evolve along the same key steps
indicated in Scheme 1, that is (i) ligand dissociation, (ii)
conversion of a cis-like to a trans-like three-coordinate inter-
mediate, and (iii) final uptake of the ligand by the latter. The
relative energies, calculated with respect to the reactant state,
of the two reaction intermediates, of the transition state for their
rearrangement, and of the product state are collected in Table
5. The corresponding PES’s, along with the structures of the
stationary points, for the isomerization of cis-[Pt(Me)(CH3CN)-
(PMe3)2]+ and cis-[Pt(Et)(CH3CN)(PMe3)2]+ are illustrated in
Figures 4 and 5, respectively. It appears that the most energy
demanding step is ligand dissociation from the starting four-
coordinate square-planar species, which can be assumed to be
the rate-determining step. The most significant changes in the
geometries of the original complexes that are observed along
this reaction path are the crucial lengthening of the Pt-S bond
A rate law of the form
ki ) kD/{1 + (k-D/kT)[S]}
(1)
can be derived, in which the term (k-D/kT)[S] measures the
retardation due to the capture of the first intermediate by the
bulk solvent. The mechanism is consistent with the great
influence of the nature of the leaving group on the reaction rate.
On going from cis-[Pt(R)(MeOH)(PEt3)2]+ (rate data in ref 32)
to cis-[Pt(R)(CH3CN)(PEt3)2]+ (rate data in Table 4), the rate
of isomerization decreases by 3 orders of magnitude with a
corresponding average increase of the free energy of activation
of ≈15-20 kJ mol-1. Electronic and steric features of cis groups
having no â-hydrogens play a negligible role, if any, as can be
(40) Faraone, G.; Ricevuto, V.; Romeo, R.; Trozzi, M. J. Chem. Soc., Dalton
Trans. 1974, 1377-1380.
(41) (a) Gonzales, A. A.; Zhang, K.; Nolan, S. P.; Lopez de la Vega, R. L.;
Mukerjee, S. L.; Hoff, C. D.; Kubas, G. J. Organometallics 1988, 7, 2429-
2435. (b) Crabtree, R. H. Chem. ReV. 1985, 85, 245-269. (c) Zhang, K.;
Gonzalez, A. A.; Mukerjee, S. L.; Chou, S.-J.; Hoff, C. D.; Kubat-Martin,
K. A.; Barnhart, D.; Kubas, G. J. J. Am. Chem. Soc. 1991, 113, 9170-
9176. (d) Lohrenz, J. C. W.; Woo, T. K.; Ziegler, T. J. Am. Chem. Soc.
1995, 117, 12793-12800. (e) Thomas, J. L. C.; Hall, M. B. Organometallics
1997, 16, 2318-2324. (f) Kawamura-Kuribayashi, H.; Koga, N.; Moro-
kuma, K. J. Am. Chem. Soc. 1992, 114, 2359-2366.
(42) Melander, L.; Saunders, W. H. Reaction Rates of Isotopic Molecules; Robert
E. Krieger: Malabar, FL, 1987; pp 170-201.
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5749

A R T I C L E S
Romeo et al.
[Pt(Et)(CH3CN)(PMe3)2]+ or 111.8° in the precursor dialkyl cis-
[Pt(Bun)2(PEt3)2], and of a similar contraction of the C(1)-
C(2)-H(3) angle to 111.4, while the other bond angles are
slightly wider than the tetrahedral value. All these data fit well
with the distinction made by Mealli et al.46 between classical
and nonclassical metal‚‚‚H3C-C interactions as a result of a
close examination of the structural properties of a large amount
of agostic compounds.
Chart 2. DFT Optimized Structures of T-shaped Three-Coordinate
Intermediates for Geometrical Isomerization of
cis-[Pt(Et)(MeCN)(PMe3)2]+ and cis-[Pt(Bun)(MeCN)(PMe3)2]+
Therefore, the structural parameters of the reaction intermedi-
ates depicted in Chart 2 can be interpreted as arising from an
agostic interaction viewed in a traditional way, which should
involve electron donation from the σ C-H bond to the metal
center and possible back-donation from suitable d orbitals of
the metal to the σ* C-H bond. Calculations carried out on cis-
[Pt(Me)(PMe3)2]+ accurately predict the T-shaped geometry and
give reasonable values of the bond distances. According to the
distortion process from trigonal planarity toward a T-shaped
geometry, the molecular orbital calculations carried out by us
reveal that the most external metal degenerate xy and x2 - y2
orbitals are split. As a result, the xy orbital is stabilized and
together with the s orbital contributes to the formation of a linear
s-d hybrid giving rise to a three-centersfour-electron interac-
tion with the alkyl and one of the phosphane ligands. The set
of bonds in the coordination plane is completed with a bond
with the remaining phosphorus atom that is characterized by a
strong electrostatic component, while all the remaining d metal
orbitals are doubly occupied. The situation is similar to that
described by Landis et al.47 in computing the shape of PdH3
-
and of other transition metal hydrides and alkyls by use of the
(HV-VB) MM method. The drawing of the LUMOs of the
non-agostic and agostic three-coordinate T-shaped species (Chart
3) highlights the disposability of a vacant σ* Pt-P bond to
accept electron donation from a weak Lewis base as the alkane
C-H bond. This kind of interaction is prevented in cis-[Pt-
(Me)(PMe3)2]+ but becomes feasible for alkyl groups containing
â-hydrogens.
To obtain deeper insight into the nature of the interaction
between the C-H bond and the valence orbitals of the metal
atom, an NBO analysis48 was carried out that identifies and
estimates the magnitude of the interaction between each donor
and each acceptor NBO. The main contribution is due to the
electron density donation from the C-H σ-bond to the anti-
bonding Pt-P σ* NBO obtained by the out-of-phase overlap
of the metal and phosphorus sd and sp3 hybrids, respectively.
It is also possible for the metal to back-donate into the empty
orbitals of the ligand, and the same analysis calculates that the
extent of this effect is greatest for the interaction between the
and a progressive narrowing of the M-C(1)-C(2) angle for
the ethyl complex. The geometrically attractive trigonal-planar
structure for the resulting three-coordinate [Pt(R)(PMe3)2]+
species turns out to be less stable than a T-shaped configuration.
In spite of the reduced symmetry, the surface maintains the same
basic features shown by the more symmetric d8 ML3 class of
compounds where a Jahn-Teller instability favors distortions
to T- and Y-shaped configurations of lower energies.43-45
As can be seen from the structures displayed in Chart 2, the
DFT optimization of the T-shaped intermediates cis-[Pt(Et)-
(PMe3)2]+ and cis-[Pt(Bun)(PMe3)2]+ reveals a classical dihapto
Pt····η2-HC agostic mode between the unsaturated metal and the
dangling C-H bond of the same molecular fragment.28 Focusing
on cis-[Pt(Et)(PMe3)2]+, the C(2)-H(3) bond lies in the Pt, C(1),
C(2) plane (the dihedral angle Pt-C(1)-C(2)-H(3) ) 0.916°),
and the agostic bond is revealed by the considerable elongation
of the bond length (1.170 Å) relative to the geminal non-agostic
C(2)-H(4)(5) bond lengths (1.092 Å) that point away from the
metal. The Pt-H(3) distance is 2.001 Å, and the close proximity
of Pt and C-H is the result of the reduction of the Pt-C(1)-
C(2) angle to 84.7°, as compared to 112.6° in the starting cis-
2
2
Pt dx -y lone pair and the C-H σ* NBO.
The energy stabilization associated with the agostic interac-
tion, at a level of T-shaped reaction intermediates, has been
estimated for cis-[Pt(Et)(PMe3)2]+ by eliminating this interaction
through a rotation of the ethyl group (60°) around the C1-C2
bond. The resulting structure with the ethyl group in the
internally staggered conformation was therefore optimized, and
the transition state for the rotation around the C(1)-C(2) bond
(43) For previous theoretical discussions of T geometries for low spin binary
three-coordinate d8 complexes, see: (a) Burdett, J. K. J. Chem. Soc.,
Faraday Trans. 2 1974, 70, 1599-1613. (b) Burdett, J. K. Inorg. Chem.
1975, 14, 375-382. (c) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14,
1058-1076.
(44) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J. K. J. Am. Chem.
Soc. 1976, 98, 7255-7265.
(45) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem. Soc.
Jpn. 1981, 54, 1857-1867.
(46) Baratta, W. M; Mealli, C.; Herdtweck, E.; Ienco, A.; Mason, S. A.; Rigo,
P. J. Am. Chem. Soc. 2004, 126, 5549-5562.
(47) Landis, C. R.; Cleveland, T.; Firman, T. K. J. Am. Chem. Soc. 1998, 120,
2641-2649.
(48) (a) Carpenter, J. E.; Weinhold, F. J. Mol Struct. 1988, 169, 41. (b) Carpenter,
J. E.; Weinhold, F. J. The Structure of Small Molecules and Ions; Plenum:
New York, 1988.
9
5750 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

â
-Hydrogen Kinetic Effect
A R T I C L E S
Chart 3. Drawings of (a) DFT Optimized Structure of
cis-[Pt(Me)(PMe3)2]+, (b) Its Computed LUMO, and (c) Computed
LUMO of Corresponding Agostic cis-[Pt(Et)(PMe3)2]+
Figure 6. Correlation between the energies of agostic and non-agostic
T-shaped intermediates.
stabilizing to allow such species to be isolated and characterized.
Almost all the isolated three-coordinate complexes of d8
transition metals mentioned in the Introduction contain an
agostic interaction at the open site of a T-shaped geometry, but
it must be borne in mind that the presence of bulky ligands
plays a major role in helping to achieve the 16-electron
configuration and in increasing the stability of the agostic bond.29
However, the recent isolation and structural characterization of
some aryl palladium amido complexes by Hartwigh et al.25 has
provided clear-cut evidence for the existence of true T-shaped
three-coordinate 14-electron species with no donor in the open
coordination site.
Leaving Group Effect. Previous detailed kinetic investiga-
tions on the spontaneous isomerization of cis-[Pt(R)X(PR3)2]
(R ) alkyl or aryl groups and X ) halide ions) complexes have
indicated that the rate is extremely sensitive to structural changes
influencing bond dissociation, viz. the nature of the halide ion,
electrophilic solvation of the leaving halide ion through
hydrogen bonding, and the amount of electronic density or strain
at the metal atom, brought about by substituents on the aromatic
ring, etc.50 The present study has confirmed that Pt-S bond
dissociation is the rate-determining step of the whole multistep
process and that the lability of the coordinated solvents, judging
from the energy involved in the formation of the cis-like
T-shaped 14-electron three-coordinate intermediate (see data in
Table 5), is CH3CN < MeOH ≈ Me2SO , benzene. This result
is not surprising since it reflects perfectly the relative donor
ability of the solvents.
A further consideration comes from the data listed in Table
5. On plotting the dissociation energies involved in the formation
of the agostic intermediates cis-[Pt(Et)(PMe3)2]+ and cis-[Pt-
(Bun)(PMe3)2]+ against those for the formation of the non-
agostic cis-[Pt(Me)(PMe3)2]+, for various leaving groups (sol-
vents), one obtains a linear plot with a value of the slope very
near to the unit (Figure 6, slope ) 0.960 ( 0.07, constant )
-35.69 ( 5.9 kJ mol-1, r2 ) 0.977). This linear relationship
strongly suggests that the energy involved in the formation of
the â-hydrogen interactions is hardly dependent on the nature
was located at 12.6 kJ mol-1 above the corresponding minimum.
However, as noted recently for the case of La(CH2-SiH3)3,49
the rotation removes the agostic interaction but changes the
conformation of the ligand from eclipsed in the agostic geometry
to staggered in the transition state. The energy stabilization with
respect to the structure with no â-agostic interaction, estimated
including the energy for the conformational change (11.9 kJ
mol-1 for CH3-CH3), was calculated as 24.5 kJ mol-1, a value
very near to that derived from the kinetic data for the TS. Values
ranging from 30.9 to 59.4 kJ mol-1, as a function of the used
level of theory, were reported by Meier et. al for [Pt(Et)(PH3)2]+,
a product resulting from C2H4 insertion into the three-coordinate
hydride [Pt(H)(PH3)2]+.9b The rotation of the agostic bond away
from the metal induces a widening of the Pt-C1-C2 angle from
84.7 to 97.6°, which testifies a residual degree of angular
distortion, possibly due to the onset of electrostatic interactions.
The energetic weakness of the agostic bond in the ethyl
intermediate, despite the strong electrophilicity of the unsaturated
metal, is not surprising since the C-H σ-bond is both strong
and nonpolar, the HOMO σ is low-lying and therefore is ill-
suited for electron donation, and the LUMO σ* is high in energy
and unsuitable for accepting electron density. Hydrogen agostic
interactions of this type, while lowering the energy of the
formally three-coordinate 14-electron species, are insufficiently
(50) (a) Faraone, G.; Ricevuto, V.; Romeo, R.; Trozzi, M. J. Chem. Soc. A
1971, 1877-1881. (b) Romeo, R.; Minniti, D.; Trozzi, M. Inorg. Chem.
1976, 15, 1134-1138. (c) Romeo, R.; Minniti, D.; Lanza, S. Inorg. Chem.
1979, 18, 2362-2368. (d) Romeo, R. Inorg. Chem. 1978, 17, 2040-2041.
(e) Romeo, R.; Minniti, D.; Lanza, S. Inorg. Chem. 1980, 19, 3663-3668.
(f) Blandamer, M. J.; Burgess, J.; Romeo, R. Inorg. Chim. Acta 1982, 65,
179-180. (g) Blandamer, M. J.; Burgess, J.; Minniti, D.; Romeo, R. Inorg.
Chim. Acta 1985, 96, 129-135. (h) Alibrandi, G.; Monsu` Scolaro, L.;
Romeo, R. Inorg. Chem. 1991, 30, 4007-4013.
(49) Perrin, L.; Maron, L.; Eisenstein, O.; Lappert, M. F. New J. Chem. 2003,
27, 121-127.
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5751

A R T I C L E S
Romeo et al.
of the group that is leaving the coordination sphere of the metal.
Even the length of the alkyl chain plays a little, if any, role on
the stability of the T-shaped species. These observations could
help to elucidate the kinetic pathway for the {C-H } + M f
{C-H‚‚‚M} process. It would seem that the progress of the
agostic interaction goes along with the departure of the leaving
group (S) from the metal without affecting significantly the
energy involved in the Pt-S bond breaking. The activation mode
seems to feature that of a dissociative interchange (Id, in the
Langford-Gray terminology)2f nucleophilic substitution reac-
tion, where the degree of assistance within a preformed
aggregate (ion pair or reaction intermediate) is small and the
reaction is primarily dissociative. The value of the constant of
the plot, 35.69 ( 5.9 kJ mol-1, represents the average difference
between the energy of cis-[Pt(Me)(PMe3)2]+ and that of the
longer alkyl chain agostic T-shaped intermediates.
process, a rarity in platinum(II) chemistry, and the energy of
the process is strongly dependent on the donor ability of S. The
bonding features of the T-shaped 14-electron cis-[Pt(R)L2]+
species formed upon dissociation of a molecule of solvent were
analyzed, with particular regard to the characteristics of the
â-hydrogen interaction developed between the unsaturated metal
ion and a dangling C-H bond of an alkyl group. The formation
of hydrogen interactions in elusive reaction intermediates is a
phenomenon strictly related to the coordination of a C-H bond
to a transition metal, a problem of fundamental interest for
catalytic reactions and for alkane functionalization via C-H
bond activation. The â-agostic interaction in the T-shaped 14-
electron cis-[Pt(R)L2]+ species developed in terms of a classical
dihapto M···η2-HC bonding, with a LUMO liable to electron
donation by the hydrocarbon moiety and back-bonding from d
orbitals to the C-H σ*-bonding orbital, with all Pt, C(1), C(2),
and H atoms lying in a plane, a long C-H bond, a short M‚‚‚H
bond, and a pronounced narrowing of the Pt-C1-C2 angle.
The stabilization of the intermediate and of the TS results in a
sharp increase of the rate of isomerization. This phenomenon
has been defined as the â-hydrogen kinetic effect to be
distinguished from that usually investigated by spectroscopic
techniques in stable transition metal and lanthanide complexes.
The strength of the â-agostic interaction in the TS and
intermediate has been estimated by both kinetic and DFT
methods ranging from 20 to 33.5 kJ mol-1, and it was found to
not depend on the length of the hydrocarbon chain. The analysis
of the computational data on the optimized structures of
T-shaped 14-electron species cis-[Pt(R)(PMe3)2]+ (R ) Me, Et,
or Bun), obtained from dissociation of different molecules of
solvents (CH3CN, MeOH, Me2SO, or benzene) suggests that
the formation of â-hydrogen interactions is entirely unaffected
by the nature of the leaving group, and therefore, such forces
play little if any role in the process of Pt-S bond breaking.
The energy barrier associated to the fluxional rearrangement
between T-shaped coordinatively unsaturated unsymmetrical [Pt-
(R)L2]+ intermediates is very small (average ) 9.1 ( 0.4 kJ
mol-1) when the cis R group is CH3 but increases when the R
group contains â-hydrogens (Et or Bun) (average ) 18.9 ( 2
kJ mol-1). An easy prediction is that as long as the strength of
the agostic interactions increases, isomerization is prevented,
and the possibility of isolation of reaction intermediates aug-
ments.
Fluxional Behavior of T-Shaped [Pt(R)(PMe3)2]+ Species.
The T-shaped configurations of the 14-electron species are
energy minima along the PES for the geometrical isomerization
of square-planar 16-electron species, and their interconversion
occurs via a Y-shaped configuration. The basic features of the
PES for such a conversion have been already obtained by
Hoffmann et al. by extended Hu¨ckel calculations for the
trialkylgold(III) complex (CH3)3Au44 and for less symmetrical
PdL2R and PdLR2 systems.45 The structure and bonding features
of cis-[Pt(R)(PMe3)2]+ have been discussed in detail previously.
As for its geometrical isomer, it makes use of a sd hybrid
orbital51 in the formation of coordination bonds with ligands
but does not exhibit agostic interactions. The energy involved
in the rearrangement of the cis-like to its trans-like T-shaped
configuration is very small (average ) 9.1 ( 0.4 kJ mol-1
)
when the cis R group is CH3 (data points 1, 4, 7, and 9 in Table
5), in agreement with the low-energy barrier indicated for the
fluxionality of other coordinatively unsaturated unsymmetrical
species such as [HPt(PH3)2]+,9c [(Et)2(PEt3)Pt]6a, or [(Me)(PH3)2-
Pd].45 The energy barrier associated to the polytopal rearrange-
ment when the R group contains â-hydrogens (Et or Bun) is
doubled (average ) 18.9 ( 2 kJ mol-1). This would suggest
that the conversion of an agostic cis-like three-coordinate to its
trans form requires a supplementary consumption of energy and
that this additional request of energy could result in an inhibition
of the whole process if the constraint (electronic interactions
or steric hindrance) in the three-coordinate species becomes
compelling. In light of the experimental remarkable stability of
species containing strong â-agostic C-H interactions, such as
the cationic [Pt(Et)(P-P)]+,18 [Pt(CH3)(iPr3P)2]+,19 or [Pd(Et)-
(P-P)]+52 complexes and the substantial energy barrier calcu-
lated for geometrical conversion of [Pd(PH3)(CH3)2],45 it is easy
to conclude that much more remains to be understood in the
polytopal rearrangement of 14-electron T-shaped species.
Experimental Procedures
General Procedures and Chemicals. All syntheses were performed
on a double-manifold Schlenk vacuum line under a dry and oxygen-
free dinitrogen atmosphere using freshly distilled, dried, and degassed
solvents. Solvents employed in the synthetic procedures (Analytical
Reagent Grade, Lab-Scan Ltd.) were distilled under nitrogen from
sodium-benzophenone ketyl (tetrahydrofuran, diethyl ether, and toluene)
or barium oxide (dichloromethane).53 Phosphane ligands (from Strem)
were used without any purification. Acetonitrile spectrophotometric
grade for kinetic studies and CD3CN for NMR measurements were
used as received from Aldrich Chemical Co.
Conclusion
The present article analyzes kinetic and computational aspects
of the spontaneous cis-trans isomerization of cationic solvent
alkyl/aryl platinum(II) complexes of the type cis-[Pt(R)(S)L2]+.
Ligand (S) dissociation is the rate-determining step of the overall
Instrumentation and Measurements. NMR analyses were per-
formed on a Bruker AMX-R 300 spectrometer equipped with a
1
broadband probe operating at 300.13 and 121.49 MHz for H and 31P
1
2
2
(51) The distortion toward the T-shaped trans isomer stabilizes the dx -y orbital,
the other component of the half-filled e′.
nuclei, respectively. H chemical shifts are reported in ppm (δ) with
(52) Clegg, W.; Eastham, G. R.; Elsegood, M. R. J.; Heaton, B. T.; Iggo, J. A.;
Tooze, R. P.; Whyman, R.; Zacchini, S. Organometallics 2002, 21, 1832-
1840.
(53) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. In Organic SolVents, 4th ed.;
Weissberger, A., Ed.; Wiley & Sons: New York, 1986.
9
5752 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

â
-Hydrogen Kinetic Effect
A R T I C L E S
1
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.54 Slow reactions were carried out in a silica cell
in the thermostated cell compartment of a JASCO V-560 or of a rapid-
scanning Hewlett-Packard Model 8452 A spectrophotometer, with a
temperature accuracy of 0.02 °C.
cis-[Pt(CH2-Si(CH3)3)(CH3CN-d3)(PEt3)2]+ (6a). H NMR (CD3-
CN, T ) 298 K): δ 1.67 (m, 3JHH ) 7.7 Hz, 12H, PA-CH2 + PB-CH2),
3
2
0.89 (m, JHH ) 7.7 Hz, 18H, PA-CH3 + PB-CH3), 0.25 (m, JPtH
)
50.6 Hz, JHH ) 8.8 Hz, 2H, Pt-CH2), -0.15 (s, 9H, Si-(CH3)3). 31P-
{1H} NMR (CD3CN, T ) 298 K): δ 18.33 (d, 1JPtPA ) 1890 Hz, 2JPAPB
) 17.8 Hz, PA trans to 2-MeC6H4), 10.28 (d, 1JPtPB ) 4226 Hz, 2JPAPB
) 17.8 Hz, PB trans to MeCN).
3
cis-[Pt(C6H5)(CH3CN-d3)(PEt3)2]+ (7a). H NMR (CD3CN, T )
1
3
3
298 K): δ 7.07 (m, JHH ) 6.6 Hz, JPtH ) 43.0 Hz, 2H, H2,6), 6.84
(dd, JHH ) 7.7 Hz, 2H, H3,5), 6.72 (dd, JHH ) 7.7 Hz, 1H, H4), 1.64
(m, JHH ) 7.7 Hz, 6H, PA-CH2), 1.40 (m, JHH ) 7.7 Hz, 6H, PB-
Synthesis of Complexes. The dialkyl substrates cis-[Pt(R)2(PEt3)2]
(R ) methyl (1),55 ethyl (2),56 ethyl-d5 (3),6a n-propyl (4),6a n-butyl
(5),31c or silyl (6)57) and the mixed alkyl-aryl complexes cis-[Pt(Me)-
(R)(PEt3)2] (R ) phenyl (7),39 o-tolyl (8),32 or mesityl (9),39) are well-
known compounds that were prepared by literature methods. The
identity and purity of these compounds were established by elemental
3
3
3
3
3
3
CH2), 0.96 (m, JHH ) 7.7 Hz, 9H, PA-CH3), 0.79 (m, JHH ) 7.7 Hz,
9H, PB-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ 15.0 (d, 1JPtPA
)
2
1
1692 Hz, JPAPB ) 17.8 Hz, PA trans to C6H5), 7.4 (d, JPtPB ) 4198
2
1
analysis and by H and 31P {1H} NMR spectra. Elemental analyses
Hz, JPAPB ) 17.8 Hz, PB trans to MeCN).
cis-[Pt(2-MeC6H4)(CH3CN-d3)(PEt3)2]+ (8a). H NMR (CD3CN,
1
were consistent with the theoretical formulas. A complete assignment
of the 1H and 31P NMR resonances of these compounds is reported in
Supporting Information Table SI1.
T ) 298 K): δ 7.1-6.9 (m, 1H, H6), 6.78 (m, 1H, H3), 6.71 (m, 2H,
H
4,5), 2.25 (s, 3H, 2-CH3), 1.72 (m, 3JHH ) 8.8 Hz, 6H, PA-CH2), 1.44
3
3
(m, JHH ) 7.7 Hz, 6H, PB-CH2), 0.98 (m, JHH ) 7.7 Hz, 9H, PB-
Monoalkyl/aryl Solvent Complexes. (a) cis-[Pt(R)(CH3CN-d3)-
(PEt3)2]+ (1a-9a). For R ) Et, Et-d5, Prn, and Bun (2a-5a), the
protonolysis was carried out at low temperature. A solution of an
appropriate amount (5-6 mg, 0.01 mmol) of cis-[Pt(R)2(PEt3)2] in CD3-
CN was frozen in the NMR tube before adding with a syringe HCF3-
SO3 in CD3CN (in a 1:1 ratio, with respect to the complex). The
CH3), 0.83 (m, JHH ) 8.8 Hz, 9H, PA-CH3). 31P{1H} NMR (CD3CN,
3
T ) 298 K): δ 15.2 (d, 1JPtPA ) 1653 Hz, JPAPB ) 17.8 Hz, PA trans
2
to 2-MeC6H4), 5.7 (d, 1JPtPB ) 4228 Hz, 2JPAPB ) 17.8 Hz, PB trans to
MeCN).
cis-[Pt(2,4,6-Me3C6H2)(CH3CN-d3)(PEt3)2]+ (9a). 1H NMR (CD3-
CN, T ) 298 K): δ 6.48 (m, 2H, H3,5), 2.24 (s, 6H, 2,2′-CH3), 1.95 (s,
1
temperature was slowly increased to 263 K, and the H and 31P NMR
3
3
3H, p-CH3), 1.73 (m, JHH ) 7.7 Hz, 6H, PA-CH2), 1.49 (m, JHH
)
spectra of the ensuing cis-monoalkylsolvent species were recorded. For
all the other compounds, (1a and 6a-9a), the acid attack was carried
out very carefully at 298 K, avoiding a local excess of acid concentra-
tion.
cis-[Pt(Me)(CH3CN-d3)(PEt3)2]+ (1a). 1H NMR (CD3CN, T ) 298
K): δ 1.71 (m, 3JHH ) 7.7 Hz, 6H, PA-CH2-CH3), 1.66 (m, 3JHH ) 7.7
Hz, 6H, PB-CH2-CH3), 0.90 (m, 3JHH ) 7.7 Hz, 9H, PA-CH2-CH3), 0.85
3
7.7 Hz, 6H, PB-CH2), 0.98 (m, JHH ) 7.7 Hz, 9H, PB-CH3), 0.78 (m,
3JHH ) 7.7 Hz, 9H, PA-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ
15.6 (d, 1JPtPA ) 1644 Hz, 2JPAPB ) 17.8 Hz, PA trans to 2,4,6-Me3C6H2),
1
2
2.8 (d, JPtPB ) 4249 Hz, JPAPB ) 17.8 Hz, PB trans to MeCN).
Monoalkyl/aryl Solvent Complexes. (b) trans-[Pt(R)(CH3CN-d3)-
(PEt3)2]+ (1b-9b). The spontaneous conversion of the cis solvent
isomers (1a-9a) into their corresponding trans derivatives was obtained
easily by setting aside at room temperature the solution resulting from
the electrophilic attack on the corresponding dialkyl/aryl precursors.
The process was complete in a few hours for R ) Et, Et-d5, Prn, and
Bun (2a-5a), while many days were necessary for the remaining
compounds (1a and 6a-9a) containing alkyl/aryl groups with no
â-hydrogens.
trans-[Pt(Me)(CH3CN-d3)(PEt3)2]+ (1b). H NMR (CD3CN, T )
298 K): δ 1.66 (m, JHH ) 7.7 Hz, 12H, P-CH2-CH3), 0.91 (m, JHH
) 7.7 Hz, 18H, P-CH2-CH3), 0.16 (m, JPtH ) 81.4 Hz, JPH ) 14.3
Hz, 3H, Pt-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ 22.8 (1JPtP
) 2687 Hz).
3
2
3
(m, JHH ) 7.7 Hz, 9H, PB-CH2-CH3), 0.27 (m, JPtH ) 51.7 Hz, JPH
) 6.6 Hz, 3H, Pt-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ 19.1
1
2
(d, JPtPA ) 1775 Hz, JPAPB ) 16.5 Hz, PA trans to CH3), 11.8 (d,
1JPtPB ) 4170 Hz, JPAPB ) 16.5 Hz, PB trans to MeCN).
2
cis-[Pt(Et)(CH3CN-d3)(PEt3)2]+ (2a). H NMR (CD3CN, T ) 263
1
K): δ 1.62 (m, 3JHH ) 7.7 Hz, 2H, Pt-CH2-CH3), 1.38 (m, 3JHH ) 7.8
3
Hz, 12H, PA-CH2 + PB-CH2), 0.70 (m, JHH ) 7.7 Hz, 3H, Pt-CH2-
1
CH3), 0.61 (m, 3JHH ) 7.7 Hz, 18H, PA-CH3 + PB-CH3). 31P{1H} NMR
3
3
(CD3CN, T ) 263 K): δ 18.3 (d, 1JPtPA ) 1621 Hz, 2JPAPB ) 15.3 Hz,
2
3
1
2
PA trans to Et), 13.2 (d, JPtPB ) 4345 Hz, JPAPB ) 15.3 Hz, PB trans
to MeCN).
cis-[Pt(Et-d5)(CH3CN-d3)(PEt3)2]+ (3a). 31P{1H} NMR (CD3CN,
+
1
trans-[Pt(Et)(CH3CN-d3)(PEt3)2] (2b). H NMR (CD3CN, T )
2
T ) 263 K): δ 18.6 (d, 1JPtPA ) 1609 Hz, JPAPB ) 17.8 Hz, PA trans
298 K): δ 1.76 (m, 3JHH ) 7.7 Hz, 2H, Pt-CH2-CH3), 1.54 (m, 3JHH
)
1
2
to Et-d5), 13.0 (d, JPtPB ) 4352 Hz, JPAPB ) 17.8 Hz, PB trans to
MeCN).
7.7 Hz, 12H, P-CH2), 0.84 (m, 3JHH ) 7.7 Hz, 3H, Pt-CH2-CH3), 0.78
(m, JHH ) 7.7 Hz, 18H, P-CH3). 31P{1H} NMR (CD3CN, T ) 298
3
cis-[Pt(Prn)(CH3CN-d3)(PEt3)2]+ (4a). 1H NMR (CD3CN, T ) 263
K): δ 22.0 (1JPtP ) 2871 Hz).
3
K): δ 1.41 (m, JHH ) 7.7 Hz, 12H, PA-CH2 + PB-CH2), 0.95-0.72
trans-[Pt(Et-d5)(CH3CN-d3)(PEt3)2]+ (3b). 31P{1H} NMR (CD3CN,
T ) 298 K): δ 22.5 (1JPtP ) 2848 Hz).
(m, 4H, Pt-(CH2)2-CH3), 0.62 (m, 3JHH ) 7.7 Hz, 18H, PA-CH3 + PB-
CH3), 0.45 (m, 3JHH ) 7.7 Hz, 3H, Pt-(CH2)2-CH3). 31P{1H} NMR (CD3-
trans-[Pt(Prn)(CH3CN-d3)(PEt3)2]+ (4b). H NMR (CD3CN, T )
1
1
2
CN, T ) 263 K): δ 18.3 (d, JPtPB ) 4312 Hz, JPAPB ) 15.3 Hz, PB
trans to MeCN), 12.9 (d, 1JPtPA ) 1657 Hz, 2JPAPB ) 15.3 Hz, PA trans
to Prn).
3
298 K): δ 1.83-1.57 (m, JHH ) 7.7 Hz, 4H, Pt-(CH2)2-CH3), 1.46
(m, 3JHH ) 7.7 Hz, 12H, P-CH2), 0.71 (m, 18H, P-CH3), 0.47 (m, 3H,
Pt-(CH2)2-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ 21.9 (1JPtP
)
cis-[Pt(Bun)(CH3CN-d3)(PEt3)2]+ (5a). 1H NMR (CD3CN, T ) 263
2857 Hz).
3
K): δ 1.48 (m, JHH ) 7.7 Hz, 12H, PA-CH2 + PB-CH2), 1.08-0.85
trans-[Pt(Bun)(CH3CN-d3)(PEt3)2]+ (5b). H NMR (CD3CN, T )
1
(m, 6H, Pt-(CH2)3-CH3), 0.69 (m, 3JHH ) 7.7 Hz, 18H, PA-CH3 + PB-
CH3), 0.53 (m, 3H, Pt-(CH2)3-CH3). 31P{1H} NMR (CD3CN, T ) 263
3
298 K): δ 1.77-1.63 (m, 6H, Pt-(CH2)3-CH3), 1.53 (m, JHH ) 7.2
Hz, 12H, P-CH2), 0.77 (m, 3JHH ) 7.2 Hz, 18H, P-CH3), 0.53 (m, 3H,
1
2
K): δ 18.3 (d, JPtPA ) 1648 Hz, JPAPB ) 15.3 Hz, PA trans to Bun),
Pt-(CH2)3-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ 21.7 (1JPtP
)
1
2
13.0 (d, JPtPB ) 4331 Hz, JPAPB ) 15.3 Hz, PB trans to MeCN).
2861 Hz).
trans-[Pt(CH2Si(CH3)3)(CH3CN-d3)(PEt3)2]+ (6b). 1H NMR (CD3-
(54) (a) Van Geet, A. L. Anal. Chem. 1968, 40, 2227-2229. (b) Van Geet, A.
L. Anal. Chem. 1970, 42, 679-680.
(55) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 705-716.
(56) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 4020-4033.
(57) Foley, P.; Di Cosimo, R.; Whitesides, G. M. J. Am. Chem. Soc. 1980, 102,
6713-6725.
3
CN, T ) 298 K): δ 1.72 (m, JHH ) 7.7 Hz, 12H, P-CH2), 0.95 (m,
3JHH ) 7.7 Hz, 18H, P-CH3), 0.16 (m, JPtH ) 85.8 Hz, JPH ) 19.8
Hz, 2H, Pt-CH2), -0.21 (s, 9H, Si-(CH3)3). 31P{1H} NMR (CD3CN, T
) 298 K): δ 18.9 (1JPtP ) 2721 Hz).
2
3
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5753

A R T I C L E S
Romeo et al.
trans-[Pt(C6H5)(CH3CN-d3)(PEt3)2]+ (7b). 1H NMR (CD3CN, T )
(b) VTK Method. Kinetic runs were carried out using a JASCO
V-560 spectrophotometer, connected to a Pentium II 800 MHz
computer. The reaction cell was equipped with a stirrer to ensure
chemical and thermal homogeneity, and the cell compartment was
connected to a HAAKE C 25 thermostating bath that allows a controlled
change of the temperature with time with an accuracy of (0.05 °C.
The temperature was checked by a platinum resistor inserted into the
spectrophotometric cell and connected to the computer (readout
resolution 0.01 °C). The reactions were started as described previously
for the CTK method. The reactions were followed at a fixed wavelength,
and the absorbance-temperature-time data were automatically ac-
quired by using a Visual Basic program. The starting temperature (T0)
was made sufficiently low to slow down the initial rate of reaction and
to make negligible the dead-time before reaching the linear part of the
temperature program. The acquisition speed was adjusted to have a
good density of points and a good fit to the mathematical model. The
processing of the stored data was performed by use of the MicroMath
SCIENTIST program62 with a Powell modified Marquadt algorithm63
for the fitting and the Eulero method for solving the differential equation
3
3
298 K): δ 6.97 (m, JHH ) 6.6 Hz, JPtH ) 60.5 Hz, 2H, H2,6), 6.73
3
3
(dd, JHH ) 7.7 Hz, 2H, H3,5), 6.65 (dd, JHH ) 7.7 Hz, 1H, H4), 1.35
(m, 3JHH ) 7.7 Hz, 12H, P-CH2), 0.84 (m, 3JHH ) 7.7 Hz, 18H, P-CH3).
31P{1H} NMR (CD3CN, T ) 298 K): δ 20.1 (1JPtP ) 2679 Hz).
trans-[Pt(2-MeC6H4)(CH3CN-d3)(PEt3)2]+ (8b). 1H NMR (CD3CN,
3
T ) 298 K): δ 6.98 (m, JHH ) 8.8 Hz, 1H, H3), 6.77-6.56 (m, 3H,
3
H
4,5,6), 2.20 (s, 3H, 2-CH3), 1.40 (m, JHH ) 8.8 Hz, 12H, P-CH2),
3
0.86 (m, JHH ) 8.8 Hz, 18H, P-CH3). 31P{1H} NMR (CD3CN, T )
298 K): δ 19.1 (1JPtP ) 2694 Hz).
trans-[Pt(2,4,6-Me3C6H2)(CH3CN-d3)(PEt3)2]+ (9b). 1H NMR (CD3-
CN, T ) 298 K): δ 6.43 (s, 2H, H3,5), 2.20 (s, 3H, P-CH3), 1.94 (s,
6H, 2,2′-CH3), 1.40 (m, 3JHH ) 7.7 Hz, 12H, P-CH2), 0.85 (m, 3JHH
)
7.7 Hz, 18H, P-CH3). 31P{1H} NMR (CD3CN, T ) 298 K): δ 17.8
(1JPtP ) 2713 Hz).
Crystallography. Crystals of cis-[Pt(Bun)2(PEt3)2], 5, suitable for
X-ray diffraction, were obtained by slow diffusion of hexane into a
concentrated toluene solution and are air-stable. The unit cell constants,
crystal symmetry determination, and data collection were carried out
at 120(2) K, on a Bruker SMART CCD diffractometer. The space group
was later confirmed by successful refinement. Selected crystallographic
and other relevant data are listed in Tables 2 and 3 and in the Supporting
Information. Data were corrected for Lorentz and polarization factors
with the data reduction software SAINT58 and empirically for absorption
using the SADABS program.59 The structure was solved by Patterson
and Fourier methods and refined by full matrix least-squares analysis60
(the function minimized being [∑w(Fo2 - (1/k)Fc2)2] using anisotropic
displacement parameters for all atoms except the hydrogen atoms; their
contribution (in calculated position: C-H ) 0.95 (Å); B (H) )
aB(Cbonded) (Å2), with a ) 1.2 for CH2 and a ) 1.5 for the CH3 groups)
was included in the refinement using a riding model. No extinction
correction was deemed necessary. The scattering factors used, corrected
for the real and imaginary parts of the anomalous dispersion, were taken
from the literature.34 All calculations were carried out by using the
WINGX,61a SHELX-97,60 and ORTEP61 programs.
d(Dt - D∞)
-
)
dt
∆Sq
∆Hq
R(T0 + Rt)
k(T0 + Rt)
exp
exp -
(Dt - D∞) (3)
[
]
[
]
h
R
where R is the temperature gradient, and D∞, ∆Hq, and ∆Sq are the
parameters to be optimized. The ki versus T profile of Figure SI1B in
the Supporting Information has been obtained simply by dividing the
time derivative of the absorbance data in Figure 3 to the normalized
value of the absorbance (Dt - D∞).
Computational Details. Geometry optimizations as well as fre-
quency calculations for all the model complexes considered here were
performed at the density functional level of theory, employing the
Becke’s three-parameter hybrid functional64 combined with the Lee-
Yang-Parr (LYP)65 correlation functional, denoted as B3LYP within
the Gaussian 03/DFT package.66 The LANL2DZ effective core
potential67 was used for the metal center. In LANL2DZ, the valence
shell is explicitly represented using double-ú and includes both n )
5(s,p,d) and n ) 6(s) orbitals. The standard 6-31+G** basis set68 was
employed for the rest of the atoms.
No symmetry restrictions were imposed during the geometry
optimizations, whereas for each optimized stationary point, vibrational
analysis was performed to determine its character (minimum or saddle
point) and to evaluate the zero-point vibrational energy (ZPVE)
corrections, which are included in all relative energies. For all the
reported transition states, it was carefully checked that the vibrational
mode associated with the imaginary frequency corresponds to the correct
movement of involved atoms. All the minima connected by a given
transition state were confirmed by intrinsic reaction coordinate (IRC)69
driving calculations (in mass-weighted coordinates) as implemented
in the Gaussian 03 program.
Kinetic Measurements. The kinetic of isomerization of the cis-
[Pt(R)(CH3CN)(PEt3)2]+ complexes (1a-9a) was followed spectro-
photometrically by both traditional method, at constant temperature
(CTK) for R ) Et, Et-d5, Prn, and Bun, and variable-temperature kinetics
(VTK)35,36,37 for R ) Me, silyl, phenyl, o-tolyl, and mesityl.
(a) CTK Method. The reactions were started by adding with a
syringe a prethermostated solution of complexes 2-5 in acetonitrile
to a thermostated acetonitrile solution of H+BF4 in a silica cell, in
-
the thermostated cell compartment of a spectrophotometer. A 1:1 acid/
complex ratio was sufficient to produce a very fast cleavage of the
first Pt-C bond. The kinetic of isomerization of 2a-5a was followed
by repetitive scanning of the spectrum at suitable times in the range of
350-200 nm. All the reactions obeyed a first-order rate law until well
over 90% of the reaction and rate constant ki (s-1) was obtained from
a nonlinear least-squares fit of the experimental data to Dt ) D∞ +
(D0 - D∞) exp(-kit) with D0, D∞, and ki as the parameters to be
optimized (D0 ) absorbance after mixing of reagents and D∞ )
absorbance at completion of reaction). Activation parameters were
derived from a linear least-squares analysis of ln(ki/T) versus T-1
according to the Eyring equation
Acknowledgment. We are grateful to the Ministero dell’
Universita` e della Ricerca Scientifica e Tecnologica (MIUR),
(62) MicroMath Scientific Software; Micromath: Salt Lake City, UT.
(63) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical
Recipes; Cambridge University Press: Cambridge, 1986.
(64) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(65) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys.
Chem. 1994, 98, 11623-11627.
(66) Frisch, M. J. et al. Gaussian 03, revision A.1; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(67) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
(68) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650-654. (b) Blaudeau, J. P.; McGrath, M. P.; Curtiss, L. A.;
Radom, L. J. Chem. Phys. 1997, 107, 5016-5021.
(69) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154-2161.
(b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523-5527.
k ) (kT/h) exp(-∆Hq/RT) exp(∆Sq/R)
and are listed in Table 4.
(2)
(58) Bruker AXS, SAINT, Integration Software; Bruker Analytical X-ray
Systems: Madison, WI, 1995.
(59) Sheldrick, G. M. SADABS, Program for Absorption Correction; University
of Go¨ttingen: Go¨ttingen, Germany, 1996.
(60) Sheldrick, G. M. SHELX-97, Structure Solution and Refinement Package;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(61) (a) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838. (b) Farrugia,
L. J. J. Appl. Crystallogr. 1997, 30, 565.
9
5754 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

â-Hydrogen Kinetic Effect
A R T I C L E S
1
PRIN 2004, and to the University of Messina, PRA 2003, for
funding this work.
assignment of H and 31P NMR data for compounds 1-9, the
temperature dependence of primary kinetic data for 1a-9a, and
Cartesian coordinates of the B3LYP optimized T-shaped cis-
and trans-[Pt(R)(PMe3)2]+ (R ) Me, Et, Bun) complexes.
Extended tables of crystallographic data, coordinates, ADPs,
bond distances, bond angles, and torsion angles. Complete list
of authors for ref 66. This material is available free of charge
via the Internet at http://pubs.acs.org.
Supporting Information Available: Figure of spectral changes
associated with isomerization of 6a at constant temperature and
ki vs T profile for the same reaction obtained from the analysis
of the VTK kinetic run in Figure 3. Figure of an example of
31P NMR spectra of cis- and trans-[Pt(Et)(PEt3)2(CD3CN)]+
compounds in acetonitrile-d3. Tables of a full listing and
JA0702162
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