Bridge-splitting kinetics, equilibria and structures of trans-biscyclooctene complexes of platinum(II)

Article abstract of DOI:10.1039/b302482m

Cyclooctene (C₈H₁₄, cot) complexes of platinum(II) have been synthesised and characterised by means of multi-nuclear NMR, UV-Vis spectroscopy and X-ray crystallography. The bridge-splitting equilibrium constants of the dinuclear, chloride-bridged trans-[PtCl₂(cot)] ₂, 1, in dichloromethane solvent at 298 K with MeOH, MeCN and cot, yielding trans-[PtCl₂(cot)(MeOH)], 2, trans-[PtCl ₂(cot)(MeCN)], 3, and trans-[PtCl₂(cot)₂], 4, were determined by UV-Vis measurements as K₁₂ = 0.0169 ± 0.0015, K₁₃ = 9.7 ± 0.9 and K₁₄ = 2.05 ± 0.06 mol^-1 dm³, respectively. Substitution of one cyclooctene from 4 by MeOH gives K₄₂ = 0.110 ± 0.009 and by MeCN K₄₃ = 2.25 ± 0.03. The bis-cyclooctene complexes 1 and 4 react quantitatively with chloride to give [PtCl₃(cot)]^-, 5. The kinetics for bridge-splitting of 1 with MeOH, MeCN and cot was studied by conventional and cryo-stopped-flow spectroscopy. Second-order rate constants at 298 K are 0.128 ± 0.003, 4.93 ± 0.02 and 0.0637 ± 0. 0009 mol^-1 dm³ s^-1, respectively. The corresponding activation parameters are ΔH^≠ = 43.7 ± 1.8, 42.0 ± 0.8 and 39.6 ± 0.9 kJ mol^-1 and ΔS^≠ = - 115 ± 6, -91 ± 3 and - 135 ± 3 J K^-1 mol^-1, indicating associative activation with a relatively large -T ΔS^≠ contribution to ΔG ^≠. Crystal and molecular structures of 1, 4, and the tetraphenylphosphonium salt of 5 have been determined, indicating a moderate ground-state trans influence of cyclooctene. The molecular structure of 4 with two C=C double bonds coordinated trans to each other and perpendicular to the platinum coordination plane, features a significant Pt-C bond lengthening compared to cyclooctene complexes with chloride or nitrogen trans to cyclooctene, leading to high reactivity and thermodynamic instability. Equilibrium data confirm that the thermodynamic stability of a cyclooctene coordinated trans to another cyclooctene is much lower than trans to a ligand with less π-back-bonding capacity. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b302482m

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Bridge-splitting kinetics, equilibria and structures of
trans-biscyclooctene complexes of platinum(II)†
Stefanus Otto,*‡a,b Andreas Roodt*b and Lars I. Elding*a
a Inorganic Chemistry, Department of Chemistry, Lund University, P.O. Box 124,
SE-221 00 Lund, Sweden. E-mail: LarsI.Elding@inorg.lu.se
b Department of Chemistry & Biochemistry, Rand Afrikaans University, P.O. Box 524,
Auckland Park 2006, Johannesburg, South Africa. E-mail: aroo@rau.ac.za
Received 4th March 2003, Accepted 17th April 2003
First published as an Advance Article on the web 13th May 2003
Cyclooctene (C8H14, cot) complexes of platinum() have been synthesised and characterised by means of multi-
nuclear NMR, UV-Vis spectroscopy and X-ray crystallography. The bridge-splitting equilibrium constants of the
dinuclear, chloride-bridged trans-[PtCl2(cot)]2, 1, in dichloromethane solvent at 298 K with MeOH, MeCN and cot,
yielding trans-[PtCl2(cot)(MeOH)], 2, trans-[PtCl2(cot)(MeCN)], 3, and trans-[PtCl2(cot)2], 4, were determined by
UV-Vis measurements as K12 = 0.0169 0.0015, K13 = 9.7 0.9 and K14 = 2.05 0.06 molϪ1 dm3, respectively.
Substitution of one cyclooctene from 4 by MeOH gives K42 = 0.110 0.009 and by MeCN K43 = 2.25 0.03. The
bis-cyclooctene complexes 1 and 4 react quantitatively with chloride to give [PtCl3(cot)]Ϫ, 5. The kinetics for bridge-
splitting of 1 with MeOH, MeCN and cot was studied by conventional and cryo-stopped-flow spectroscopy. Second-
order rate constants at 298 K are 0.128 0.003, 4.93 0.02 and 0.0637 0.0009 molϪ1 dm3 sϪ1, respectively. The
corresponding activation parameters are ∆H≠ = 43.7 1.8, 42.0 0.8 and 39.6 0.9 kJ molϪ1 and ∆S≠ = Ϫ115 6,
Ϫ91 3 and Ϫ135 3 J KϪ1 molϪ1, indicating associative activation with a relatively large -T ∆S≠ contribution to
∆G≠. Crystal and molecular structures of 1, 4, and the tetraphenylphosphonium salt of 5 have been determined,
indicating a moderate ground-state trans influence of cyclooctene. The molecular structure of 4 with two C᎐C double
᎐
bonds coordinated trans to each other and perpendicular to the platinum coordination plane, features a significant
Pt–C bond lengthening compared to cyclooctene complexes with chloride or nitrogen trans to cyclooctene, leading
to high reactivity and thermodynamic instability. Equilibrium data confirm that the thermodynamic stability of a
cyclooctene coordinated trans to another cyclooctene is much lower than trans to a ligand with less π-back-bonding
capacity.
facilitating mechanistic investigations of their reactions.
Cyclooctene is also easier to handle in quantitative experiments
than the volatile ethene. The aim of the present study was to
elucidate the structure–reactivity properties of the three cyclo-
octene complexes 1, 4, and 5 in order to gain more information
on binding modes and reaction mechanisms that would be
applicable also to ethene complexes. Equilibria and kinetics for
bridge-splitting and substitution processes have been studied
Introduction
Substitution processes trans to alkenes in platinum()
complexes are extremely fast due to their large kinetic trans
effect.1 We have previously studied such fast exchange2 and
substitution3 processes quantitatively using NMR and cryo-
stopped-flow spectroscopy. During this work we prepared
the cyclooctene analogues of Zeise’s ethene complexes, viz. the
chloride-bridged dinuclear complex trans-[PtCl2(cot)]2, 1, and
and structures determined. Scheme 1 gives an overview of the
system studied and the notation used.
the monomeric [PtCl3(cot)]Ϫ, 5. Splitting of 1 with cyclooctene
results in the mononuclear complex trans-[PtCl2(cot)2], 4,
so far the only example of a mononuclear trans-bis-alkene
platinum() complex stable at ambient conditions.4,5 It is
expected to display unusual reactivity properties due to the
mutual labilization of the two trans alkene ligands. It is
relevant as a model for the unstable trans-[PtCl2(C2H4)2], which
has been proposed to exist as a possible short-lived inter-
mediate in the reaction mechanism for ethene exchange at
[PtCl3(C2H4)]Ϫ.2,6 The bis-ethene complex was first reported by
Chatt and Wilkins 50 years ago as an unstable yellow solid that
could be precipitated from acetone at Ϫ80 ЊC.7,8 It has also been
identified as an elusive compound in THF9 and chloroform
solution.2,10
Experimental
Chemicals
Methanol (Riedel-de Haën), acetonitrile (Merck) and di-
chloromethane (Riedel-de Haën) solvents were of analytical
grade and were freshly distilled from CaH2 under a dinitro-
gen atmosphere prior to use. Cyclooctene (C8H14, cot) (Acros)
was dried over molecular sieves and PPh4Cl (Aldrich) was
dried under vacuum before use. K2PtCl4 (Johnson Matthey
Chemicals Ltd.) was used directly.
Substitution reactions trans to cyclooctene proceed more
slowly than those trans to ethene, due to the steric and elec-
tronic properties of the cyclooctene. Cyclooctene complexes are
more stable than their ethene analogues at ambient conditions,3
Preparation of complexes
trans-[PtCl2(C2H4)]2 was prepared from K2PtCl4 via [PtCl3-
(C2H4)]Ϫ according to literature procedures.8 Complex 1 was
prepared by treating trans-[PtCl2(C2H4)]2 with 2.2 molar equiv-
alents of cyclooctene in dichloromethane. Using a larger excess
of cyclooctene resulted in formation of 4. Complexes 2 and
3 were prepared in situ by reacting 1 with excess MeOH
(ca. 1000×) or MeCN (ca. 100×), respectively, as specified in the
ESI (stability constant determinations).† Crystals of 1, 4 and
PPh4[5] suitable for crystallography were grown as described
below.
† Electronic supplementary information (ESI) available: observed
pseudo-first order rate constants for bridge splitting reactions, absorb-
ance versus added ligand concentrations for equlibrium constant
determinations and complete crystallographic details in CIF format.
See http://www.rsc.org/suppdata/dt/b3/b302482m/
‡ Present address: Sasol Technology R&D, Sasol, P.O. Box 1, Sasolburg
1947, South Africa. E-mail: fanie.otto@sasol.com
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 2 5 1 9 – 2 5 2 5
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Table 1 Crystallographic data and refinement parameters for 1, 4 and PPh4[5]
1
4
PPh4[5]
Empirical formula
Formula weight
Crystal system
Space group
C16H28Cl4Pt2
752.36
Monoclinic
P21/n
C16H28Cl2Pt
486.37
C32H34Cl3PPt
751.00
Monoclinic
P21/c
Triclinic
¯
P1
a/Å
b/Å
c/Å
α/Њ
8.2310(16)
5.9600(12)
20.991(4)
90
99.68(3)
90
1015.1(3)
2
14.287
7730
2168
0.0877
1887
0.0650
0.1622
0.0734
0.1668
5.8407(12)
7.4886(15)
10.069(2)
84.01(3)
88.45(3)
73.59(3)
420.17(15)
1
8.653
4315
2547
0.0343
2542
11.227(2)
14.746(3)
18.901(4)
90
107.18(3)
90
2989.5(10)
4
5.035
β/Њ
γ/Њ
V/Å3
Z
µ/mmϪ1
Collected reflections
Independent reflections
Rint
Observed reflections [I > 2σI]
R (I > 2σI )
wR (I > 2σI )
R (all data)
wR (all data)
31554
9316
0.0647
5336
0.0354
0.0675
0.0914
0.0777
0.0428
0.1129
0.0429
0.1131
1
trans-[PtCl2(C8H14)2], 4. H NMR: 1.42 (m, 8H), 1.64 (m,
8H), 2.15 (m, 8H), 5.58 (m, 4H). 195Pt NMR: Ϫ2747. ε277 6372,
340 1182 cmϪ1 molϪ1 dm3.
ε
1
PPh4[PtCl3(C8H14)], PPh4[5]. H NMR: 1.40 (m, 3H), 1.64
2.5
(m, 3H), 2.10 (m, 3H), 2.40 (m, 3H), 4.99 (tm, 2H,
J
PtH
70 Hz), 7.60–7.70 (m, 8H), 7.76–7.84 (m, 8H), 7.86–7.94
(m, 4H). 195Pt NMR: Ϫ2681. ε340 313, ε342 232 cmϪ1 molϪ1 dm3.
NMR measurements
All NMR spectra were recorded at 295 K in CDCl3 on a Varian
Unity 300 spectrometer operating at 299.78 and 64.27 MHz for
the 1H and 195Pt nuclei, respectively. 1H spectra were calibrated
on the residual CHCl3 peak at 7.25 ppm relative to TMS.
195Pt NMR spectra were recorded with a WALTZ-16 proton
decoupling sequence. Peak resonances are reported in ppm
2Ϫ
relative to PtCl6 (1 g H2PtCl6 in 3 cm3 1.0 mol dmϪ3 HCl
containing 50% D2O, δ = 0 ppm) as external reference.
X-Ray crystallography data collection and refinement
Crystals of 1 were grown from dichloromethane and those of 4
from a dichloromethane solution of 1 containing a few drops
of cyclooctene. Crystals of PPh4[5] were obtained by treating
any of complexes 1, 2, 3 or 4 with PPh4Cl in dichloromethane.
Data were collected on a Siemens SMART CCD diffractometer
using MoKα (0.71073 Å) and ω-scans at 293(2) K. After com-
pleted collection, the first 50 frames were repeated to check for
decay, which was not observed. All reflections were merged and
integrated using SAINT11 and were corrected for Lorentz,
polarization and absorption effects using SADABS.12 The
structures were solved by the heavy atom method and refined
through full-matrix least-squares cycles using the SHELXL9713
software package with Σ(|Fo| Ϫ |Fc|)2 being minimised. All non-
H atoms were refined with anisotropic displacement param-
eters, while the H atoms were constrained to parent sites using a
riding model. The high platinum content in the crystals of 1
(52%) resulted in a large µ value, which complicated the absorp-
tion corrections and resulted in a fairly high residual electron
density. However, this was located within 1.3 Å of the metal
centres indicating no physical meaning. The DIAMOND14
Visual Crystal Structure Information System software was used
for the graphics. Crystal data and details of data collection and
refinement are given in Table 1.
Scheme 1 Reaction scheme and notation for equilibrium constants.
1
trans-[PtCl2(C8H14)]2, 1. H NMR: 1.45 (m, 8H), 1.55 (m,
4H), 1.83 (m, 4H), 2.01 (m, 4H), 2.29 (m, 4H), 5.47 (m, 4H).
195Pt NMR: Ϫ2434, Ϫ2453. ε248 17505, ε271 7370, ε353 760 cmϪ1
molϪ1 dm3.
trans-[PtCl2(C8H14)(MeOH)], 2. 195Pt NMR: Ϫ2696 (7.1
mmol dmϪ3 of 1, 4 mol dmϪ3 MeOH in CDCl3). ε286 1088, ε342
285 cmϪ1 molϪ1 dm3.
trans-[PtCl2(C8H14)(MeCN)], 3. 195Pt NMR: Ϫ2892 (6.3
mmol dmϪ3 of 1, 38 mmol dmϪ3 MeCN in CDCl3). ε248 3722,
CCDC reference numbers 205486–205488.
See http://www.rsc.org/suppdata/dt/b3/b302482m/ for crys-
tallographic data in CIF format.
ε
270 2567, ε301 1290 cmϪ1 molϪ1 dm3.
D a l t o n T r a n s . , 2 0 0 3 , 2 5 1 9 – 2 5 2 5
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Table 2 Selected bond lengths (Å) and angles (Њ) for 1, 4 and PPh4[5];
symmetry related atoms primed
Equilibrium measurements
Series of solutions of complex 1 in dichloromethane containing
the same total concentration of platinum and varying concen-
trations of methanol, acetonitrile or cyclooctene, respectively,
were prepared and their UV-Vis spectra were recorded in 1.00
cm quartz cells at 298.2 K on a Cary 300 Bio UV-Vis spectro-
photometer. Solutions were aged long enough to ensure that
equilibrium was established before measurement. Least-squares
fits of the appropriate equations to the absorbance/concen-
tration data (vide infra) were performed using the SCIEN-
TIST15 non-linear least-squares minimising programme.
Absorbance/concentration data and wavelengths are given as
ESI.†
Bond/angle
1
4
PPh4[5]
Pt–Cl(1)
Pt–Cl(1)Ј
Pt–Cl(2)
Pt–Cl(3)
Pt–C(1)
Pt–C(2)
C(1)–C(2)
2.340(3)
2.371(3)
2.265(4)
2.3068(18)
2.3068(18)
2.3632(12)
2.3022(12)
2.2913(12)
2.160(4)
2.161(4)
1.399(6)
2.132(13)
2.143(13)
1.41(2)
2.259(6)
2.287(6)
1.375(8)
Cl(1)–Pt–Cl(1)Ј
Cl(1)–Pt–Cl(2)
Cl(1)–Pt–Cl(3)
Cl(2)–Pt–Cl(3)
Cl(1)–Pt–C(1)
Cl(1)–Pt–C(2)
Cl(2)–Pt–C(1)
Cl(2)–Pt–C(2)
Cl(3)–Pt–C(1)
Cl(3)–Pt–C(2)
Cl(1)Ј–Pt–C(1)
Cl(1)Ј–Pt–C(2)
83.84(12)
173.74(12)
180
90.68(5)
86.81(5)
177.16(4)
160.96(12)
161.19(12)
88.80(12)
87.64(13)
94.00(12)
94.30(13)
Kinetic measurements
96.2(4)
96.5(4)
89.8(4)
89.4(2)
86.04(17)
86.19(18)
For rapid reactions, the bridge splitting of 1 with methanol,
acetonitrile and cyclooctene were monitored using a Hi-Tech
SF-61DX-2 diode-array stopped-flow system equipped with a
Hi-Tech Ltd. (Salisbury, UK) Cryo Flow temperature unit
described previously.3 Slower reactions were followed by use of
the Cary 300 Bio UV-Vis spectrophotometer. The Cary cell
compartment was flushed with dry nitrogen to minimize con-
densation on the cell’s windows at low temperature. Based on
the equilibrium measurements, the following wavelengths were
selected for the kinetics: 275 nm (MeOH), 300 nm (MeCN) and
280 nm (cot). Ligand concentrations were sufficiently large to
ensure pseudo first-order reaction conditions and complete
conversion of the reactants to products. In all cases, single
exponential kinetics was observed. To determine activation
parameters, reactions were studied between Ϫ10 and 25 ЊC for
MeOH, Ϫ5 and 25 ЊC for CH3CN, and between 5 and 32 ЊC for
cyclooctene. Data were collected and observed pseudo first-
order rate constants were calculated using the Hi-Tech software
package KinetAsyst2.16 All least-squares fits were performed
using the SCIENTIST15 non-linear minimising programme.
Complete experimental data with experimental errors are given
as ESI.†
157.0(4)
164.5(4)
93.96(17)
93.81(18)
C(1)–C(2)–Pt–Cl(1)
C(1)–C(2)–Pt–Cl(2)
C(1)–C(2)–Pt–Cl(3)
91.8(8)
90.7(8)
88.5(4)
90.0(3)
91.1(3)
Results and discussion
Preparation and characterization of complexes
Complexes 1, 4 and and the tetraphenylphosphonium salt of 5
were conveniently prepared and isolated as solids while com-
plexes 2 and 3 could only be prepared in situ by bridge splitting
of 1 with methanol or acetonitrile, respectively. Compound 4
seems to be the only structurally characterized mononuclear
platinum() complex so far containing two monodentate alkene
ligands coordinated trans to each other.17
In the 1H NMR spectra of complexes 1, 2, 3 and 4 all reson-
ances were broad and no coupling of the alkene protons to the
Pt nucleus could be observed, suggesting that the complexes are
in fast exchange with free ligand (cot, CH3CN or MeOH). The
2.5
J
of 70 Hz for 5 is in good agreement with the 64 Hz
PtH
recorded for the analogous ethene complex.3
Solid state structures
Molecular diagrams showing the numbering schemes and
thermal ellipsoids for 1, 4 and 5 are given in Fig. 1. Selected
geometrical parameters are summarised in Table 2. In all com-
pounds, the cyclooctene molecules are coordinated to the metal
centres through the double bonds in a ‘side-on’ fashion as pre-
dicted by the Dewar–Chatt–Duncanson model forming an
angle very closely perpendicular to the coordination plane.18
The cyclooctene molecules adopt a twisted chair conformation
in all cases. The dihedral angles between the coordination plane
and that formed by the Pt–C(1)–C(2) moieties are 89.8(3),
88.2(6) and 88.9(2)Њ for complex 1, 4, and 5, respectively,
Fig. 1 Molecular diagram of 1, 4 and 5 showing the numbering
scheme and thermal ellipsoids (30% probability level).
indicating that the perpendicular coordination mode is virtually
not influenced by different packing modes.
The molecular diagram of 1 shows its trans configuration, as
expected for an analogue of Zeise’s dimer (Fig. 1). Even though
D a l t o n T r a n s . , 2 0 0 3 , 2 5 1 9 – 2 5 2 5
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Table 3 Bond lengths (Å) in dinuclear chloride-bridged cyclopentene,
-heptene and -octene complexes, trans-[PtCl2(alkene)]2
In Table 4, structural data for free and coordinated cyclo-
octene in platinum() complexes are presented. There is a sub-
stantial and significant lengthening of the Pt–C bond distances,
from ca. 2.14 to 2.16 Å in the complexes with cyclooctene trans
to chloride or nitrogen,23 to ca. 2.27 Å in 4, where the two
cyclooctene ligands are trans to each other. Thus, as expected,
there is a significant labilisation of the cyclooctene in the
ground state of 4, since the two trans-cyclooctene ligands share
the same orbitals. This ground-state destabilisation will con-
tribute to the observed thermodynamic instability and the
extreme reactivity of the trans-alkene complexes in solution. It
is also obvious that the C(1)–C(2) bond distances in the com-
plexes are significantly longer than in free cyclooctene,24 reflect-
ing a weakening of the C(1)–C(2) double bond due to back
donation of electrons from the metal center to the antibonding
orbitals of the alkene and loss of electrons in the bonding C–C
orbital. This effect is smaller for 4 than for the complexes with
chloride or nitrogen trans to cyclooctene.
The crystal structure of [PPh4]5 is related to the previously
reported [Bu4N]5.3 Although the Pt–Cl(1) and Pt–C bond dis-
tances are significantly longer than in the tetrabutyl ammonium
salt, the C(1)–C(2) bond distance remains nearly the same
(Table 4).
In Table 5, Pt–Cl bond distances trans to different ligands L
in complexes [PtCl3(L)]Ϫ are reported.3,25–33 There is a progres-
sive increase from 2.266(2) in [PtCl3(MeCN)]Ϫ to 2.382(4) Å in
[PtCl3(PEt3)]Ϫ. The increase in bond length might indicate a
moderate increase of ground state trans influence in the series:
a
b
Alkene
Pt–Clb
Pt–Clt
Pt–C
C(1)–C(2)
Ref.
C5H8
C7H12
C8H14
2.320(5)
2.349(5)
2.328(6)
2.362(6)
2.340(3)
2.371(3)
2.264(6)
2.257(6)
2.265(4)
2.20(2)
1.40(2)
19
2.14(2)
2.10(2)
2.132(13)
2.143(13)
1.38(3)
—
1.41(2)
19
This work
a Clb = bridging. b Clt = terminal.
several halide-bridged dinuclear alkene complexes have been
prepared, only three crystal structure determinations of such
complexes seem to have been reported, viz. the cyclopentene,
cycloheptene and tetramethylallene analogues.19,20 The struc-
tural parameters of the cyclopentene, -heptene and -octene
complexes given in Table 3 show that there is a gradual increase
of the Pt–Cl(bridging) distances from cyclopentene to -octene,
corresponding to a gradual weakening of the chloride double
bridge in this order. In 1, the terminal Pt–Cl(2) bond distance
trans to bridging chloride, 2.265(4) Å, is significantly shorter
than the two bridging Pt–Cl(1) bond distances, 2.371(3) Å trans
to cyclooctene, and 2.340(3) Å trans to terminal chloride. It is
also significantly shorter than the Pt–Cl distance trans to chlor-
ide in for example PtCl42Ϫ, which is 2.317(2) Å (Table 5 below).
The situation is similar in the cyclopentene and -heptene com-
plexes, reflecting the ground state trans influence of the alkenes,
and the weakened trans influence of the bridging chlorides
compared to the terminal ones.
MeCN < CO < py < ClϪ, CH2CHSiMe3
< Me2SO < NH3 < CH2CHOEt < alkene < PEt3
The data for complex 4 given in Table 2 agree well with those
for an isostructural version of the compound reported recently,
prepared by an alternative method.4 Only two other structures
of monomeric bis-alkene complexes have been found in the
literature. They are both cis isomers, viz. cis-[PtCl2(CH2CHC6-
H5)2] and cis-[PtCl2(CH2CH2)2].21,22
But this over-all effect is small, and packing effects in the
crystal lattice might well contribute to some of the observed
changes. For instance, the Pt–Cl bond distances in PPh4[5] and
Bu4N[5] differ by 0.04 Å, from 2.363(1) to 2.324(2) Å, whereas
that in the potassium salt of Zeise’s anion is intermediate,
Table 4 Bond lengths (Å) in cyclooctene complexes of platinum()
Complex
LЈa
Pt–LT
Pt–Ca
C(1)–C(2)
Ref.
‘Free’ cot
[PtCl(Me2NCH2C6H4)(cot)]
—
N
—
2.139(9)
—
1.22(4)
1.38(2)
24
23
2.15(1)
2.16(1)
2.13(1)
2.14(1)
2.139(4)
2.147(4)
2.160(4)
2.161(4)
2.259(6)
2.287(6)
[PtCl2(cot)]2 (1)
Cl
Cl
Cl
cot
2.371(3)
2.324(2)
2.363(1)
1.41(2)
This work
3
Bu4N[PtCl3(cot)]
1.391(7)
1.399(6)
1.375(8)
PPh4[PtCl3(cot)] (PPh4[5])
trans-[Pt(cot)2Cl2] (4)
This work
This work
2.287(6)
2.259(6)
a Ligand/donor atom trans to cot.
Table 5 Geometrical parameters for complexes of the type [PtCl3(L)]Ϫ
Complex
Pt–Cl/Å
Pt–La/Å
C–C/Å
Ref.
Et4N[PtCl3(MeCN)]
Bu4N[PtCl3(CO)]
Et4N[PtCl3(py)]
Bu4N[PtCl3(CH2CHSiMe3)]
K2[PtCl4]
K[PtCl3(Me2SO)]
2.266(2)
2.289(3)
2.305(2)
2.314(2)
2.317(2)
2.318(5)
2.321(7)
2.324(2)
2.324(2)
2.340(2)
2.363(1)
2.382(4)
—
1.82(1)
—
—
—
—
1.44(1)
—
—
—
1.38(1)
1.391(7)
1.375(4)
1.399(6)
—
25
26
27
3
28
29
30
31
3
2.13(3)
2.317(2)
2.139(5)
—
2.14(2)
2.14(1)
2.131(5)
2.161(5)
2.215(4)
K[PtCl3(NH3)]
Bu4N[PtCl3(CH2CHOEt)]
Bu4N[PtCl3(C8H14)]
K[PtCl3(C2H4)]
Ph4P[PtCl3(C8H14)] (PPh4[5])
Et4N[PtCl3(PEt3)]
32
This work
33
a Average of the two Pt–C alkene bonds.
D a l t o n T r a n s . , 2 0 0 3 , 2 5 1 9 – 2 5 2 5
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2.340 Å.32 The important conclusion is that the ground state
trans influence of the alkenes is moderate, compared to for
instance the much larger trans influence of strongly σ-bonded
aryl or silyl ligands, where Pt–Cl distances up to ca. 2.47 Å have
been observed.34
Table 6 Equilibrium constants K at 298 K in dichloromethane as
defined in Scheme 1
K
Method
Value
K12/molϪ1 dm3
Eqns. (2)–(4)
Eqns. (2)–(4)
Eqns. (2)–(4)
(K13/K12)0.5
K24/K34
0.0169 0.0015
9.7 0.9
2.05 0.06
K13/molϪ1 dm3
K14/molϪ1 dm3
Equilibria
K23
K23
K24
K24
K34
K34
20
20
2
4
More or less complete studies of the kinetics of bridge-splitting
reactions of square-planar dinuclear complexes with amines,35
alkenes,36,37 iodide,38 pyridines39 and carbon monoxide40 have
been published earlier. In the present system we also observed
equilibria between the dinuclear complex 1 and the monomeric
complexes formed in the bridge-cleavage process, as shown in
Scheme 1. The bridge-splitting reaction with the nucleophiles L
= MeOH, CH3CN, cyclooctene and chloride occurs according
to eqn. (1) with complexes 2, 3, 4 and 5, respectively, as
products.
Eqn. (6)
9.1 0.7
11
(K14/K12)0.5
Eqn. (6)
1
0.444 0.006
0.46 0.01
(K13/K14)0.5
that chloride splits the bridge quantitatively gives the following
sequence of efficiency for the bridge-splitting ligands: MeOH
< cot < MeCN < ClϪ. Similarly, if we compare pair-wise the
equilibrium constants for the substitution reactions at the
monomeric complexes, i.e. K24 and K23, K34 and K32 and K43 and
K42, we arrive at the stability order MeOH < cot < MeCN for
the competition between the ligands at the coordination site
trans to cyclooctene. Summarizing, the qualitative order of
thermodynamic stability for these cyclooctene complexes is:
MeOH < cot < MeCN < ClϪ. These results confirm the conclu-
sion from the structural study that the thermodynamic stability
of a cyclooctene coordinated trans to another cyclooctene is
much lower than trans to a ligand with less π-back-bonding
capacity.
The equilibrium constant for the bridge splitting of the
styrene analogue of Zeise’s dimer, trans-[PtCl2(CH2CHC6H4)]2,
with styrene to form trans-[PtCl2(CH2CHC6H4)2] has been
determined in chloroform as 0.0235 molϪ1 dm3.41 This value is
almost two orders of magnitude smaller than our value of K14
for splitting of 1 with cyclooctene given in Table 6. On the other
hand, cleavage of the styrene complex with ethanol in chloro-
form, resulting in trans-[PtCl2(CH2CHC6H4)(EtOH)], gave an
equilibrium constant of 0.0110 molϪ1 dm3, which is close to our
value of K12 for the splitting of 1 with methanol (Table 6). Thus,
in the case of this styrene complex, the stability order is also
alcohol < alkene, but the magnitude of the bridge splitting equi-
librium constant is obviously highly dependent on the nature of
the splitting alkene.
trans-[PtCl2(cot)]2 ϩ 2L
2 trans-[PtCl2(cot)(L)] (1)
In the case of chloride, the bridge cleavage is quantitative.
In the other three cases, the equilibrium constants K, viz. K12,
K13 and K14 as defined in Scheme 1 can be calculated by least-
squares fitting of eqns. (2)–(4) to the experimental concen-
tration/absorbance data with CPt fixed and K, ε1 (molar absorp-
tivity of 1) and εM (molar absorptivity of the monomeric
product) as adjustable parameters. A denotes the observed
absorptivity (equal to the absorbance since 1.00 cm cells were
used).
K = [M]2/([1][L]2)
(2)
(3)
A = ε1[1] ϩ εM[M]/2
[M]3K Ϫ 2[M]2(CPtK ϩ K [L]a Ϫ 1) ϩ
2
[M](4CPt[L]aK ϩ K [L]a ) Ϫ 2KCPt[L]a2 = 0 (4)
In eqns. (2)–(4), CPt and [M] refer to the total starting concen-
tration of 1 (as Pt2) and the concentration at equilibrium of
the monomeric product, respectively. The equilibrium concen-
trations of [1] = CPt – [M]/2 and [L] in eqns. (2) and (3) were
calculated for each solution by use of eqn. (4). The concen-
tration of L added is [L]a = [L] ϩ [M].
Bridge-splitting kinetics
The equilibrium constants K24 and K34 for the substitution
processes of eqn. (5) were also calculated by least-squares fit-
ting of eqn. (6) to the experimental absorbance/concentration
data with an excess of cyclooctene added, keeping [cot] and CM
(total monomeric Pt concentration) fixed and K, ε4 and εP as
adjustable parameters. Here, ε4 and εP denote the molar absorp-
tivities of species 4 and of the product, respectively, at the
specific wavelength.
For the experimental conditions used in the kinetics experi-
ments, all traces showed well-defined first-order behaviour with
the bridge-cleavage processes of eqn. (1) resulting in complete
conversion to trans-[PtCl2(cot)(L)], L = MeOH, MeCN or
cyclooctene, as shown in eqn. (1). The kinetics is monophasic,
and the plots of observed rate constants vs. the concentration
of the bridge-splitting nucleophile given in Fig. 2 are linear with
negligible intercepts, corresponding to the simple rate law of
eqn. (8).
trans-[PtCl2(cot)2] ϩ L
trans-[PtCl2(cot)(L)] ϩ cot (5)
(6)
kobs = k1n[L]
(8)
A = CM(ε4[cot] ϩ εPK [L])/([cot] ϩ K [L])]
This is compatible with a two-step process with splitting
of the first chloride bridge being rate-determining and with
negligible reverse reactions and solvent paths. Rate constants at
298 K and activation parameters calculated from the Eyring
plots in Fig. 3 are given in Table 7.
From the values of K24 and K34 a value of K23 for the substi-
tution process of eqn. (7) can also be derived.
trans-[PtCl2(cot)(MeOH)] ϩ MeCN
trans-[PtCl2(cot)(MeCN)] ϩ MeOH (7)
As illustrated in Fig. 4, the nucleophilic discrimination of
1 at 298 K is cot < MeOH < MeCN, or k14 < k12 < k13,
in relative numbers 1 : 2 : 80. In agreement with previous
findings, acetonitrile is a good nucleophile,42 whereas alkenes
are known to be poor.43 In the present case, the difference in
nucleophilicity is mainly due to the different entropy contri-
butions to the free energy of activation. The ϪT ∆S≠ term
varies in the order cot < MeOH < MeCN as 40 : 34 : 27 kJ
molϪ1 (Table 7).
All equilibrium constants are given in Table 6. The agreement
between K23, K24, and K34 calculated by the different methods is
good. The magnitude of the equilibrium constant is highly
dependent on the nature of the nucleophile and the solvent.
Complexes 1 and 4 react quantitatively with a stoichiometric
amount of chloride in the dichloromethane solvent.
The comparison of the bridge splitting equilibrium constants
given in Table 6 shows that K12 < K14 < K13. Including the fact
D a l t o n T r a n s . , 2 0 0 3 , 2 5 1 9 – 2 5 2 5
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Table 7 Second-order rate constants at 298 K and activation parameters for bridge splitting of 1 with neutral ligands L in dichloromethane solvent
L
k1n
k1n/molϪ1dm3sϪ1
∆H≠/kJ molϪ1
∆S≠/J KϪ1 molϪ1
%T ∆S≠ a
MeOH
MeCN
cot
k12
k13
k14
0.128 0.003
4.93 0.02
0.0637 0.0009
43.7 1.8
42.0 0.8
39.6 0.9
Ϫ115
Ϫ91
Ϫ135
6
3
3
44
39
50
a Contribution to ∆G≠.
Fig. 4 Bridge splitting of 1 with MeOH (Y2), MeCN (Y1) and
cyclooctene at 25 ЊC in dichloromethane.
The three bridge-splitting reactions proceed with the charac-
teristic negative values of ∆S≠ usually observed for associatively
activated square-planar substitution reactions. As observed
previously for substitution reactions trans to alkenes, the
ϪT ∆S≠ terms contribute relatively much (between 40 and 50%)
to the overall free energy of activation.2,3
Conclusions
The chloride-bridged dinuclear complex trans-[PtCl2(C8H14)]2
splits into monomeric complexes on addition of chloride,
acetonitrile, cyclooctene and methanol. With chloride, bridge
cleavage is quantitative, whereas equilibria between monomeric
and dimeric complexes are established with the other three
ligands. Bridge-splitting is fast with chloride and acetonitrile,
much slower with methanol and cyclooctene. The bridge-
splitting reactions are monophasic, but in principle two-step
processes, with the splitting of the first bridge rate-determining.
This step is characterised by largely negative entropies of activ-
ation, and a relatively large contribution from the entropy term
to the overall free energy of activation. Splitting with cyclo-
octene results in the isolation of trans-[PtCl2(C8H14)2]. The
molecular structure of this complex with the two C᎐C double
᎐
Fig.
2
Bridge splitting of 1 with MeOH (a), MeCN (b) and
bonds coordinated trans to each other and perpendicular to the
platinum coordination plane, features a significant Pt–C bond
lengthening compared to cyclooctene complexes with chloride
or nitrogen trans to cyclooctene. This weakening of the plat-
inum–cyclooctene bond contributes to the high reactivity and
relative thermodynamic instability of this compound, which
can serve as a model for other monomeric trans-bis-alkene
complexes that have been postulated as short-lived intermedi-
ates in chemical reactions. The cyclooctene complexes react
slower than their ethene analogues.
cyclooctene (c) in dichloromethane at various temperatures.
Acknowledgements
Financial support from the Swedish Science Research Council
(VR), the South African NRF and the Rand Afrikaans
University, and the Swedish International Development
Cooperation Agency (SIDA), including a post-doctoral fellow-
ship at Lund University for S. O. is gratefully acknowledged.
The Crafoord Foundation is thanked for funding of the diode
array and cryo-stopped-flow units.
Fig. 3 Eyring plots for bridge splitting of 1 with MeOH, MeCN and
cyclooctene in dichloromethane.
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23 S. Otto, P. V. Samuleev, V. A. Polyakov, A. D. Ryabov and
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