A kinetic study of the competition between carbon monoxide and alkenes in chloro complexes of platinum(II)

Article abstract of DOI:10.1021/om060499r

Alkenes (cyclohexene; 1-octene) react with cis-PtCl₂ (CO) ₂, leading to cis-PtCl₂(CO)(alkene) (1a, alkene = cyclohexene; 1b, alkene = 1-octene). The substitution kinetics to 1a and 1b in 1, 2-dichloroethane in the temperature range 273-308 K have the following second-order rate constants at 298 K, respectively: cyclohexene, (0.94 ± 0.07) × 10^-3 M^-1 s^-1 1-octene, (2.33 ± 0.05) × 10^-2 M^-1 s^-1. The activation parameters are ΔH* = 37 ± 4 kJ mol^-1, ΔS* = -180 ± 10 J K^-1 mol^-1 for the reaction affording la and ΔH* = 36.5 ± 0.7 kJ mol ^-1, ΔS* = -154 ± 2 J K^-1 mol ^-1 for the reaction to 1b. The large negative values of the activation entropies are consistent with an associative mechanism. The reverse reactions proceed with a similar rate constant.

Full text of DOI:10.1021/om060499r

Organometallics 2006, 25, 4913-4916
4913
A Kinetic Study of the Competition between Carbon Monoxide and
Alkenes in Chloro Complexes of Platinum(II)
Daniela Belli Dell’Amico*, Riccardo Bini, Fausto Calderazzo, Laura Carbonaro,
Luca Labella, and Anna Vitullo
Dipartimento di Chimica e Chimica Industriale, UniVersity of Pisa,
Via Risorgimento 35, I-56126 Pisa, Italy
ReceiVed June 6, 2006
Summary: Alkenes (cyclohexene; 1-octene) react with cis-PtCl2-
(CO)2, leading to cis-PtCl2(CO)(alkene) (1a, alkene ) cyclo-
hexene; 1b, alkene ) 1-octene). The substitution kinetics to 1a
and 1b in 1,2-dichloroethane in the temperature range 273-
308 K haVe the following second-order rate constants at 298
respectively.5c The CO exchange in [MCl3(CO)]- is consider-
ably slower, k2298 values being, in CDCl3 as solvent, 3.4 × 10-3
and 5.70 × 10-1 M-1 s-1 for platinum and palladium,
respectively.4a It is interesting to note that ethene exchange is
quite fast for both platinum and palladium, while the reactions
involving CO are rather slow, especially for platinum, which is
more than 100 times slower than palladium. As these reactions
proceed by an associative path, the energy of a pentacoordinated
activated complex with respect to the starting compound should
play the major role in determining the rate constant. In this
connection, it is to be noted that platinum(II) adopts pentaco-
ordination in some alkene or carbonyl complexes normally in
the presence of a bidentate nitrogen donor ligand.6,7
K, respectiVely: cyclohexene, (0.94 ( 0.07) × 10-3 M-1 s-1
;
1-octene, (2.33 ( 0.05) × 10-2 M-1 s-1. The actiVation
parameters are ∆H* ) 37 ( 4 kJ mol-1, ∆S* ) -180 ( 10
J K-1 mol-1 for the reaction affording 1a and ∆H* ) 36.5 (
0.7 kJ mol-1, ∆S* ) -154 ( 2 J K-1 mol-1 for the reaction to
1b. The large negatiVe Values of the actiVation entropies are
consistent with an associatiVe mechanism. The reVerse reactions
proceed with a similar rate constant.
8
In earlier papers we have reported that cis-PtCl2(CO)2
Introduction
undergoes CO substitution by cyclohexene or 1-octene (see eq
1) through equilibrium reactions favoring the dicarbonyl.9 Here
we report the kinetic studies carried out on the same systems
operating in 1,2-dichloroethane in the temperature range 273-
308 K.
Ligand substitution kinetics of square-planar complexes
containing d8 metal centers have been widely studied,1 especially
for platinum(II), which, being characterized by moderate reac-
tion rates, can be monitored by conventional methods.2 How-
ever, to the best of our knowledge, kinetic studies concerning
competitive CO/alkene substitution reactions are not available
in the literature, although they are certainly relevant to processes
involving both carbon monoxide and olefins, such as hydro-
formylation and alkene/CO copolymerization.3 On the other
hand, the kinetics of exchange of carbon monoxide have been
reported for Pt(II), Pd(II), Rh(I), and Ir(I),4 and a certain number
of data concerning olefin exchange,5a-d or the displacement of
an olefin by another olefin,5f,g are available in the literature.
Olefin exchange in [MCl3(C2H4)]- (M ) Pd, Pt) shows that
the second-order rate constant at 298 K (k2298) in THF as solvent
is 9 × 102 and 1.3 × 103 M-1 s-1 for platinum and palladium,
cis-PtCl2(CO)2 + alkene h cis-PtCl2(CO)(alkene) + CO
(1)
Earlier data have established spectroscopically that only the cis
geometrical isomer is obtained through reaction 1.9 This has
been confirmed crystallographically in the case of alkene )
cycloheptene.9b
Experimental Section
General Procedures. K2PtCl4 was purchased from Aldrich
Chemical Co. or from Chimet S.p.A., Badia al Pino, Arezzo, Italy.
Carbon monoxide (research grade) was purchased from Rivoira and
purified by passing it through concentrated sulfuric acid and KOH.
1,2-Dichloroethane was freshly distilled from P4O10 under a
dinitrogen atmosphere. Toluene and heptane were distilled from
sodium. Benzene-d6 (99.9%) was purchased from Aldrich and
distilled after treatment with activated molecular sieves.
(1) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition
Metal Complexes, 2nd ed.; VCH: Weinheim, 1991.
(2) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions. A
Study of Metal Complexes in Solution, 2nd ed.; Wiley: New York, 1967.
(3) (a) Pino, P.; Piacenti, F.; Bianchi, M. Reactions of Carbon Monoxide
and Hydrogen with Olefinic Substrates: The Hydroformylation (Oxo)
Reaction. In Organic Syntheses Via Metal Carbonyls; Wender, I., Pino, P.,
Eds.; J. Wiley: New York 1977; Vol. 2. (b) van Leeuwen, P. N. W. M.;
Claver C. Rhodium Catalysed Hydroformylation; Kluwer: Dordrecht, 2000.
(c) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663. (d) Rix, F.
C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118, 4746. (e)
Consiglio, G. In Late Transition Metal Polymerization Catalysis; Wiley-
VCH: Weinheim, 2003; pp 279-305.
(4) (a) Olsson, A.; Roodt, A.; Bojczuk, M.; Heaton, B. T.; Elding, L. I.
J. Chem. Soc., Dalton Trans. 1994, 3723. (b) Churlaud, R.; Frey, U.; Metz,
F.; Merbach, A. E. Inorg. Chem. 2000, 39, 304.
(5) (a) Cramer, R. Inorg. Chem. 1965, 4, 445. (b) Natile, G.; Maresca,
L.; Cattalini, L. J. Chem. Soc., Dalton Trans. 1977, 651. (c) Olsson, A.;
Kofod, P. Inorg. Chem. 1992, 31, 183. (d) Plutino, M. R.; Otto, S.; Roodt,
A.; Elding, L. I. Inorg. Chem. 1999, 38, 1233. (e) Otto, S.; Samuleev, P.
V.; Polyakov, V. A.; Ryabov, A. D.; Elding, L. I. Dalton Trans. 2004,
3662. (f) Hupp, S. S.; Dahlgren, G. Inorg. Chem. 1976, 15, 2349. (g) Miya,
S.; Kashiwabara, K.; Saito, K. Inorg. Chem. 1980, 19, 98.
IR spectra were obtained with a Perkin-Elmer 1725X Fourier
transform spectrophotometer with an IR Data Manager station.
(6) Albano, V. G.; Natile, G.; Panunzi, A. Coord. Chem. ReV. 1994, 133,
67.
(7) De Felice, V.; De Renzi, A.; Ferrara, M. L.; Panunzi, A. J.
Organomet. Chem. 1996, 513, 97.
(8) Bagnoli, F.; Belli Dell’Amico, D.; Calderazzo, F.; Englert, U.;
Marchetti, F.; Herberich, G. E.; Pasqualetti, N.; Ramello, S. J. Chem. Soc.,
Dalton Trans. 1996, 4317.
(9) (a) Alper, H.; Huang, Y.; Belli Dell’Amico, D.; Calderazzo, F.;
Pasqualetti, N.; Veracini, C. A. Organometallics 1991, 10, 1665. (b) Belli
Dell’Amico, D.; Calderazzo, F.; Pasqualetti, N.; Maichle-Mo¨ssmer, C.;
Stra¨hle, J.; Wurst, K Organometallics 1994, 13, 1582. (c) Belli Dell’Amico,
D.; Bini, R.; Calderazzo, F.; Carbonaro, L.; Labella, L.; Vitullo, A.
Organometallics 2005, 24, 4427.
10.1021/om060499r CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/25/2006

4914 Organometallics, Vol. 25, No. 20, 2006
Notes
Figure 2. UV-vis spectra of the reaction of cis-PtCl2(CO)2 (∼1
× 10-4 M) with 1-octene (∼5 × 10-3 M) in 1,2-dichloroethane at
25.0 ( 0.2 °C, under a dinitrogen atmosphere. Following the
arrows: spectrum of cis-PtCl2(CO)2 and spectra recorded 75, 150,
240, 330, 420, and 600 s after mixing the reagents.
were also conducted by mixing, under a CO atmosphere, solutions
of cis-PtCl2(CO)(1-octene) and saturated solutions of CO containing
an excess of olefin, i.e., under pseudo-first-order conditions for both
the forward and reverse reactions. Good first-order kinetic curves
were normally obtained for both the forward and the reverse
reactions, which were quantified by nonlinear least-squares fitting
of the absorbance versus time plots to the equation A ) A∞ + (A0
- A∞) exp(-kobst), where A, A∞, and A0 are absorbances at time t∞
and 0, respectively. Satisfactory first-order dependence was nor-
mally observed for at least 6 half-lives. The analysis of data was
accomplished with the nonlinear least-squares fitting program
OriginLab 5.0. The reported errors correspond to one standard
deviation.
Figure 1. (a) UV-vis spectra of the reaction of cis-PtCl2(CO)2
(∼1 × 10-4 M) with cyclohexene (∼0.7 M), in 1,2-dichloroethane
at 25.0 ( 0.2 °C, under a dinitrogen atmosphere. Following the
arrows: cis-PtCl2(CO)2 and spectra recorded 45, 180, 1080, and
3900 s after mixing the reagents. (b) Spectra for the reaction of
cis-PtCl2(CO)(cyclohexene) (∼1 × 10-4 M) with CO (5.5 × 10-3
M in 1,2-dichloroethane at 25.0 °C under a CO atmosphere.
Following the arrows: cis-PtCl2(CO)(cyclohexene) and spectra
recorded 60, 180, 270, 480, and 1140 s after mixing the reagents.
Results
Spectrophotometric and kinetic measurements were carried out by
using 1.00 or 0.10 cm quartz Suprasil cells, on a Perkin-Elmer
Lambda9 spectrophotometer equipped with a cell compartment
maintained at constant temperature ((0.2 °C).
All reactions and manipulations were carried out under a
dinitrogen atmosphere using Schlenk tube techniques. cis-PtCl2-
(CO)2,8 cis-PtCl2(CO)(cyclohexene), 1a,9a,b and cis-PtCl2(CO)(1-
octene), 1b,9c were prepared as previously described. Because of
the high sensitivity of the platinum(II) carbonyls, extreme care was
devoted to avoiding moisture traces, especially when dilute solutions
were used.
Kinetics Measurements. All glassware was thoroughly dried
prior to use. The spectrophotometric measurements were carried
out in quartz cells, 1 cm thick, of about 5 mL capacity, supplied
with a stopcock for their use under a controlled atmosphere. A
freshly prepared 1,2-dichloroethane solution of cis-PtCl2(CO)2 of
known concentration (between 2 × 10-4 and 5 × 10-4 M) was
mixed with a solution of the olefin (between 2 × 10-2 and 5 ×
10-1 M) in the quartz cell under a dinitrogen atmosphere. The
reaction was monitored by UV-vis spectroscopy at the wavelength
of the largest change of absorbance, i.e., between 265 and 275 nm.
For the reverse reaction, solutions of complexes 1a,b (ca. (2-5)
× 10-4 M) were similarly prepared in 1,2-dichloroethane under a
dinitrogen atmosphere. An aliquot of such a solution (ca. 0.2 mL,
corresponding to an amount ranging from 0.4 × 10-4 to 1 × 10-4
mmol of 1a or 1b) was introduced into the quartz cell. The required
amount (about 2.5 mL) of 1,2-dichloroethane saturated with CO at
atmospheric pressure (1 mL contains about 5 × 10-3 mmol of CO10a
under our conditions, taking into account the vapor pressure of the
solvent10b) was added under CO. Identical kinetic curves were
obtained either when the cell was closed with its Teflon stopcock
or when it was connected to a 500 mL CO reservoir. Kinetic runs
Pure cis-PtCl2(CO)(alkene), 1a,b, were obtained by reacting
cis-PtCl2(CO)2 with excess olefin and by pumping off the
produced CO. The olefin complex cis-PtCl2(CO)(alkene), alkene
) cyclohexene, 1-octene, dissolved in 1,2-dichloroethane
saturated with CO at room temperature, was completely
converted into cis-PtCl2(CO)2 in a few minutes. The IR spectra
of the resulting solution, recorded ca. 1 h after mixing the
reagents, confirmed the formation of cis-PtCl2(CO)2, as moni-
tored by the νCO bands at 2178 and 2136 cm-1
.
For the kinetic studies reactions 2 and 3 were monitored by
UV-vis spectroscopy (Figures 1 and 2) in 1,2-dichloroethane
as solvent, and the forward reactions were initiated under
dinitrogen (T ) 25.0 °C) by mixing the appropriate volumes of
cis-PtCl2(CO)2 +
kalk
cis-PtCl (CO)(cyclohexene)
cyclohexene y z
+ CO (2)
2
kCO
1a
kalk
cis-PtCl (CO)(1-octene)
cis-PtCl2(CO)2 + 1-octene y z
+
2
kCO
1b
CO (3)
cis-PtCl2(CO)2 and olefin solutions, under pseudo-first-order
conditions (high excess of olefin), directly in the quartz cell.
After addition of the olefin, the intensity of the maximum at
255 nm (ꢀ ) 4831 M-1 cm-1), due to cis-PtCl2(CO)2, started
to decrease and a new band at 260 (ꢀ ) 4839 M-1 cm-1) or
262 nm (ꢀ ) 4225 M-1 cm-1) was observed, due to the
formation of 1a or 1b, respectively. For both reactions the
presence of isosbestic points (at about 240 and 260 nm) suggests
that two absorbing species dominate in solution. In both cases,
the reverse sequence of the spectra recorded in the carbonylation
(10) (a) Lu¨hring, P.; Schumpe, A. J. Chem.-Eng. Data 1989, 34, 250.
(b) Handbook of Chemistry and Physics, 82nd ed.; Lide, D. R., Ed.; CRC
Press: Boca Raton, FL, 2001-2002; pp 6-72.

Notes
Organometallics, Vol. 25, No. 20, 2006 4915
Table 1. Temperature Dependence of kI and kII for the
Substitution Reaction of CO by Alkene in cis-PtCl2(CO)2
Affording the Complexes 1a,b (eqs 2 and 3) in
1,2-Dichloroethane
T/°C
102 × kII/M-1 s-1
102 × kI /s-1
reaction 2
reaction 3
15.0
25.0
35.0
45.0
15.0
25.0
35.0
45.0
0.053 ( 0.006
0.094 ( 0.007
0.18 ( 0.02
0.24 ( 0.02
1.38 ( 0.03
2.33 ( 0.05
4.0 ( 0.1
0.001 ( 0.003
0.015 ( 0.003
0.018 ( 0.009
0.10 ( 0.01
0.01 ( 0.01
0.01 ( 0.01
0.06 ( 0.01
0.1 ( 0.02
Figure 3. Observed rate constant kalk of reaction 2 vs cyclohexene
concentration in 1,2-dichloroethane at different temperatures.
6.3 ( 0.3
Figure 4. Observed rate constant kalk of reaction 3 vs 1-octene
concentration in 1,2-dichloroethane at different temperatures.
Figure 5. Eyring plots for the forward reactions 2 and 3 carried
out in 1,2-dichloroethane at p ) 1 atm. Solid circles for reaction 2
and triangles for reaction 3 at the following temperatures ((0.2):
15.0; 25.0; 35.0; 45.0 °C.
of the olefin complexes (shown in Figure 1b for reaction 2)
confirms the reversible nature of the processes.
The processes were studied at different olefin concentrations.
The kinetic traces, recorded at the suitable wavelength under
pseudo first-order conditions, after exclusion of the first few
data points, can be described by single exponentials with rate
∆H* ) 36.5 ( 0.7 kJ mol-1, ∆S* ) -154 ( 2 J K-1 mol-1
for reaction 3. The large negative values of the activation
entropies strongly support an associative mechanism. Thus, the
activated complex must be characterized by well-formed Pt-
alkene and Pt-CO bonds.
The reaction may proceed through one step with synchronous
bond formation and bond breaking or, alternatively, with a two-
step mechanism and a five-coordinate intermediate (Scheme 1).
In the latter case, by applying the steady-state approximation
and neglecting the intercepts as discussed above, the expressions
5 and 6 for the pseudo-first-order kinetic constants of the
forward (kalk) and the reverse (kCO) reactions, respectively, can
be derived.
constants kalk
.
Figures 3 and 4 report plots of kalk versus the olefin
concentration at different temperatures for the direct reactions
2 and 3, respectively. Linear dependences were obtained up to
the highest olefin concentrations.
These findings are in agreement with the accepted scheme
for ligand substitution in square-planar complexes, as for the
rate law of eq 4.
kalk ) kI + kII[alkene]
(4)
Although we have no spectroscopic or kinetic evidence for
the formation of a reaction intermediate, we cannot exclude its
presence in a low concentration, as required by the steady-state
approximation. For this reason we propose the following scheme
(Scheme 1) and the corresponding eqs 5 and 6.
The second-order rate constant kII refers to the olefin-dependent
pathway, while kI may include contributions related to (a) the
reverse reaction, (b) the solvent-dependent substitution pathway,
and (c) concurrent decomposition reactions. In our case the
intercept values are usually small, the contribution of these paths
appearing not to be relevant, and the error is fairly large relative
to their magnitude. The relatively high kI values at the highest
temperatures are probably due to a decomposition reaction.
As reported in Table 1 and Figures 3 and 4, the temperature
dependence of the forward reaction was studied between 15
and 45 °C. The overall activation parameters were calculated
from Eyring plots (Figure 5) and correspond to ∆H* ) 37 ( 4
kJ mol-1, ∆S* ) -180 ( 10 J K-1 mol-1 for reaction 2 and
k1k2
kalk
)
[olefin]
[CO]
(5)
(6)
k-1 + k2
k-1k-2
kCO
)
k-1 + k2
The reverse reactions were studied under pseudo-first-order
conditions, the concentration of dissolved carbon monoxide
Scheme 1

4916 Organometallics, Vol. 25, No. 20, 2006
Notes
The intercept, (3.6 ( 0.1) × 10-3 M-1 s-1, satisfactorily
matches the value of the rate constant for the reaction carried
out in the absence of olefin (see above). The slope of (2.06 (
0.05) × 10-2 M-1 s-1 is in good agreement with the result
obtained for reaction 3 followed under a dinitrogen atmosphere.
It can be noted that the ratios {kalk/[alkene]}/{kCO/[CO]} give
the equilibrium constants of eq 2 and 3 [K2 ) (1.5 ( 0.2) ×
10-3 and K3 ) (3.6 ( 0.3) × 10-2 at 25 °C], in good agreement
with the data previously reported (calculated from ∆G° at T )
25 °C: K2 ) 2.5 × 10-3, K3 ) 4.5 × 10-2).9c
Figure 6. Plot of kobsd vs 1-octene concentration, for the reactions
carried out under CO (p ) 1 atm; [CO] ) 5.5 × 10-3 M) between
(a) cis-PtCl2(CO)2 (∼1 × 10-4 M) and 1-octene, solid circles, b;
(b) cis-PtCl2(CO)(1-octene) (∼1 × 10-4 M) and CO in the presence
of 1-octene, open squares, 0; and in the absence of 1-octene
(rectangle). Solvent is 1,2-dichloroethane, T ) 25.0 ( 0.2 °C.
As discussed in the Introduction, the second-order rate
constant of the L exchange for the substrates [PtCl3L]- is
reported to be about 10-3 M-1 s-1 for L ) CO4a and about 103
M-1 s-1 for L ) C2H4,5c the large difference being related to
the differences in both activation parameters, ∆H* and ∆S*,
acting in the same direction. Our results show that the CO
substitution by a simple olefin in cis-PtCl2(CO)2 proceeds with
a second-order rate constant (10-2-10-3 M-1 s-1) that depends
on the nature of the olefin, as expected for an associative
process, the terminal olefin 1-octene substituting CO about 10
times faster than the cyclic C6H10. This difference has to be
ascribed to the different activation entropy of the two processes,
the terminal olefin showing a less negative value. It is to be
noted that the two reverse reactions proceed, probably by a
fortuitous combination of factors, essentially with the same rate
constant.
being 5.5 × 10-3 M,10 about 40-fold greater than that of the
metal complex. The pseudo-first-order rate constant for the
disappearance of the olefin complex was determined from a
one-exponential plot of the absorbance versus time up to 80-
90% conversion of the reagent with respect to the equilibrium
concentration. Data fitting for at least three independent kinetic
runs gave a mean value of kCO ) (3.5 ( 0.2) × 10-3 s-1 for
reaction 2 and kCO ) (3.6 ( 0.2) × 10-3 s-1 for reaction 3.
Kinetic runs for the reverse reaction 3 were also carried out
in the presence of an excess olefin over the metal complex,
under CO at atmospheric pressure at 25.0 °C. The kinetics were
largely controlled by one exponential. A very small drift of the
absorbance at longer reaction times, likely depending on some
side reactions, was observed in the reaction profile only by
increasing the concentration of the olefin. By fitting the data of
the first part of the reaction to one exponential, the kobsd were
determined. Under these conditions, the rate constant is a
combination of the forward and reverse reaction constants, kobs
) kalk + kCO. It is linearly dependent on the concentration of
olefin, as shown in Figure 6.
Acknowledgment. The authors wish to thank the Ministero
dell’ Istruzione, dell’ Universita` e della Ricerca (MIUR),
Programmi di Ricerca Scientifica di Rilevante Interesse Nazio-
nale, Cofinanziamento 2004-2005, and the COST ACTION
D17/003 of the European Community for promoting this
scientific activity. Thanks are due to CRCC (Consorzio Inter-
universitario Reattivita` Chimica e Catalisi) for a postgraduate
fellowship to R.B.
OM060499R