Low temperature kinetic study of very fast substitution reactions
at platinum(II) trans to olefins†
Stefanus Otto ‡ and Lars I. Elding *
Inorganic Chemistry, Chemical Center, Lund University, P. O. Box 124, SE-221 00 Lund,
Sweden. E-mail: LarsI.Elding@inorg.lu.se
Received 4th January 2002, Accepted 19th March 2002
First published as an Advance Article on the web 2nd May 2002
Ultra-fast substitution of chloride for bromide, iodide, azide and thiocyanate trans to ethene in Zeise’s anion,
[PtCl
3
(C
2
H
4
)]
, 1, has been investigated in methanol solvent by use of cryo temperature diode array stopped-flow
spectrophotometry. Reactions follow the usual two-term rate law for square-planar substitutions, k
obs
= k
1
k
2
[Y]
(where k
1
= k
MeOH
[MeOH]), with k
1
= 118 ± 10 s
1
and k
2
= (5.1 ± 0.2) × 10
2
, (3.51 ± 0.07) × 10
3
, (11.8 ± 0.2) × 10
3
,
and (56 ± 4) × 10
3
mol
1
dm
3
s
1
for Y = Br
, I
, N
3
and SCN
, respectively, at 223 K. Activation parameters for
MeOH, Br
, I
and N
3
are ∆H
≠
= 23 ± 2, 21 ± 2, 17 ± 1.0 and 11.9 ± 1.5 kJ mol
1
and ∆S
≠
= 124 ± 10, 96 ± 9,
98 ± 4 and 111 ± 6 J K
1
mol
1
, respectively. Recalculation of k
1
to second-order units gives the sequence
of nucleophilicity MeOH < Br
< I
< N
3
< SCN
(1 : 100 : 700 : 2500 : 12000) at 223 K. This nucleophilic
discrimination decreases with increasing temperature. Chloride for iodide substitution trans to allyl alcohol,
vinyltrimethylsilane and cyclooctene at [PtCl
3
(L)]
, (L = CH
2
CHCH
2
OH, 2; CH
2
CHSiMe
3
, 3; C
8
H
14
, 4) follow the
same rate law with k
1
= 116 ± 5, 31.0 ± 0.3 and 23.6 ± 0.1 s
1
and k
2
= (2.65 ± 0.06) × 10
3
, (0.273 ± 0.005) × 10
3
and (0.119 ± 0.002) × 10
3
mol
1
dm
3
s
1
at 223 K. Activation parameters are ∆H
≠
(k
MeOH
) = 24.4 ± 1.3, 28.4 ± 0.6
and 29.9 ± 0.8 kJ mol
1
, ∆S
≠
(k
MeOH
) = 120 ± 5, 114 ± 2 and, 108 ± 3 J K
1
mol
1
, ∆H
≠
(k
2
) = 19.9 ± 1.2,
24.6 ± 1.7 and 24 ± 3 kJ mol
1
and ∆S
≠
(k
2
) = 88 ± 5, 84 ± 7 and 93 ± 10 J K
1
mol
1
, for 2, 3 and 4 respectively.
The free energies of activation are dominated by the T ∆S
≠
terms. The crystal and molecular structures of
Bu
4
N[PtCl
3
(CH
2
CHSiMe
3
)] and Bu
4
N[PtCl
3
(C
8
H
14
)] show slight Pt–Cl bond lengthening to 2.314(2) Å and
2.3238(16) Å trans to the olefins, similar to that found trans to ethene in Zeise’s anion. All experiments support a
model for the very fast substitution reactions trans to the olefins in which ground state labilisation is much less
significant than transition state stabilisation. Extrapolation to ambient temperature together with literature data for
related reactions in methanol solvent gives a quantitative measure of the trans effect of ethene as: SR
2
< Me
2
SO <
AsEt
3
< PR
3
< P(OR)
3
< C
2
H
4
(1 : 5 : 400 : 3500 : 7000 : 3 × 10
6
). The relative trans effect of the olefins studied is
C
2
H
4
∼ CH
2
CHCH
2
OH > CH
2
CHSiMe
3
∼ C
8
H
14
, spanning a factor of between 5 and 30 depending on the
nucleophile, and reflecting minor differences in steric and electronic properties of the olefins.
Introduction
It is well known that ligand substitution processes trans to
olefins are extremely rapid due to the high π trans effect of these
ligands.
1
Quantitative studies of direct substitution kinetics
trans to olefins in unhindered systems such as Zeise’s anion,
[PtCl
3
(C
2
H
4
)]
, 1, do not seem to have been published so far.
Preliminary temperature jump experiments in our laboratory 25
years ago indicated an approximate half-life for the hydrolysis
of the trans chloride of 1 as short as 5 × 10
5
s in aqueous
solution at 25 C.
2
Based on that rough estimation and related
kinetics data, a trans effect order H
2
O : Cl
: Me
2
SO : C
2
H
4
of
1 : 330 : 2 × 10
6
: ∼10
11
was derived.
2
Tobe and co-workers,
using sterically hindered substituents resulting in much slower
reactions, estimated a qualitative trans effect sequence as
R
2
S < R
2
SO < PR
3
< P(OR)
3
< CO < C
2
H
4
.
3
We have previously studied the ethene exchange at Zeise’s
anion, [PtCl
3
(C
2
H
4
)]
, 1,
4
and have postulated a mechanism in
which the very labile chloride ligand co-ordinated trans to the
ethene is involved in the exchange process. It was therefore of
† Electronic supplementary information (ESI) available: observed
pseudo-first order rate constants for all reactions investigated, absorb-
ance versus chloride concentrations for the K
Cl
determinations. See
http://www.rsc.org/suppdata/dt/b2/b200197g/
‡ On leave from University of the Free State, P.O. Box 339, Bloem-
fontein 9300, South Africa.
interest to investigate the very fast substitution kinetics of
chloride trans to ethene directly. We report here studies of such
reactions of complex 1 as well as of the related olefinic com-
plexes [PtCl
3
(L)]
, L = CH
2
CHCH
2
OH (allyl alcohol), 2;
CH
2
CHSiMe
3
(vinyltrimethylsilane), 3 and C
8
H
14
(cyclo-
octene), 4. These studies were made feasible by use of recently
developed reliable low temperature stopped-flow diode-array
equipment. The experiments give, for the first time, a quantita-
tive evaluation of the kinetic parameters for the very fast substi-
tution processes trans to olefins co-ordinated to platinum() in
sterically unhindered systems. A quantitative measure of the
trans effect of ethene and related olefins compared to other
trans-labilising ligands can also be derived from the kinetics
data. In order to elucidate the relative importance of ground
state and transition state effects for the reactivity, the crystal
and molecular structures of the tetrabutylammonium salts of 3
and 4 have also been determined.
Experimental
Chemicals
Allyl alcohol (ACROS, 99%), vinyltrimethylsilane (Fluka,
>97%) and cyclooctene (ACROS, 95%) were dried over molecu-
lar sieves, while ethene (AGA, 99.95%) was used as received.
Methanol solvent of analytical grade (Riedel-de Haën) was
DALTON
FULL PAPER
2354 J. Chem. Soc., Dalton Trans., 2002, 2354–2360 DOI: 10.1039/b200197g
This journal is © The Royal Society of Chemistry 2002
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