Oxidative addition of methyl iodide to a new type of binuclear platinum(II) complex: A kinetic study

Article abstract of DOI:10.1021/ic0511064

A new organodiplatinum(II) complex cis,cis-[Me₂Pt(μ-NN)(μ- dppm)PtMe₂] (1), in which NN = phthalazine and dppm = bis(diphenylphosphino)methane, is synthesized by the reaction of cis,cis-[Me₂Pt(μ-SMe₂)(μ-dppm)PtMe₂] with 1 equiv of NN. Complex 1 has a 5d_π(Pt) → π*(imine) metal-to-ligand charge-transfer band in the visible region, which was used to easily follow the kinetics of its reaction with MeI. Meanwhile, the complex contains a robust bridging dppm ligand that holds the binuclear integrity during the reaction. A double MeI oxidative addition was observed, as shown by spectrophotometry and confirmed by a low-temperature ³¹P NMR study. The classical S_N2 mechanism was suggested for both steps, and the involved intermediates were suggested. Consistent with the proposed mechanism, the rates of the reactions at different temperatures were slower in benzene than in acetone and large negative ΔS^? values were found in each step. However, some abnormalities were observed in the related rate constants and ΔS^? values, which were demonstrated to be due to the associative involvement of the polar acetone molecules in the reactions. The rates are almost 6 times slower in the second step as compared to the first step because of the electronic effects transmitted through the ligands and the steric effects.

Full text of DOI:10.1021/ic0511064

Inorg. Chem. 2005, 44, 8594−8601
Oxidative Addition of Methyl Iodide to a New Type of Binuclear
Platinum(II) Complex: a Kinetic Study
Sirous Jamali,† S. Masoud Nabavizadeh,‡ and Mehdi Rashidi*,‡
Chemistry Department, Persian Gulf UniVersity, Bushehr 75168, Iran, and Chemistry Department,
College of Sciences, Shiraz UniVersity, Shiraz 71454, Iran
Received July 3, 2005
A new organodiplatinum(II) complex cis,cis-[Me2Pt(
µ
-NN)(
µ
-dppm)PtMe2] (1), in which NN phthalazine and dppm
)
)
bis(diphenylphosphino)methane, is synthesized by the reaction of cis,cis-[Me2Pt(
µ
-SMe2)(µ-dppm)PtMe2] with 1
equiv of NN. Complex 1 has a 5dπ(Pt) f π*(imine) metal-to-ligand charge-transfer band in the visible region,
which was used to easily follow the kinetics of its reaction with MeI. Meanwhile, the complex contains a robust
bridging dppm ligand that holds the binuclear integrity during the reaction. A double MeI oxidative addition was
observed, as shown by spectrophotometry and confirmed by a low-temperature 31P NMR study. The classical SN2
mechanism was suggested for both steps, and the involved intermediates were suggested. Consistent with the
proposed mechanism, the rates of the reactions at different temperatures were slower in benzene than in acetone
and large negative
related rate constants and
∆
Sq values were found in each step. However, some abnormalities were observed in the
Sq values, which were demonstrated to be due to the associative involvement of the
∆
polar acetone molecules in the reactions. The rates are almost 6 times slower in the second step as compared to
the first step because of the electronic effects transmitted through the ligands and the steric effects.
Introduction
and contain characteristic sharp metal-to-ligand charge-
transfer (MLCT) bands in the visible region, and therefore
kinetics of their reactions can usually be investigated easily
using spectrophotometry.5 The reactions involving the ad-
dition of alkyl halides are shown to proceed normally by
the classical SN2 mechanism, although in some relatively
rare cases, the reactions proceed by a radical mechanism.6
Another alternative mechanism involves a concerted three-
center transition state that has been proposed in the oxidative
addition of C-H7 or O-O4c bonds to diimineorganoplati-
num(II) complexes. Despite these, the related reactions with
binuclear complexes are scarce.8 The addition to the binuclear
Oxidative addition is a fundamental reaction and often
serves as the critical activation step in many homogeneous
catalytic processes.1 Recently, oxidative addition reactions
have been used in preparing supramolecular and polymeric
materials.2 Oxidative addition of alkyl halides or other
reagents to mononuclear transition-metal complexes has been
extensively studied. In particular, oxidative addition of a wide
variety of reagents to monomeric organoplatinum(II) com-
plexes, especially those containing diimine ligands, has been
extensively studied.3,4 These diimine complexes are colored
* To whom correspondence should be addressed. E-mail: rashidi@
chem.susc.ac.ir.
† Persian Gulf University.
(3) Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735.
(4) (a) Rashidi, M.; Esmaeilbeig, A. R.; Shahabadi, N.; Tangestaninejad,
S.; Puddephatt, R. J. J. Organomet. Chem. 1998, 568, 53. (b) Rashidi,
M.; Momeni, B. Z. J. Organomet. Chem. 1999, 574, 286. (c) Rashidi,
M.; Nabavizadeh, S. M.; Hakimelahi, R.; Jamali, S. J. Chem. Soc.,
Dalton Trans. 2001, 3430. (d) Rashidi, M.; Shahabadi, N.; Nabaviza-
deh, S. M. Dalton Trans. 2004, 619. (e) Rashidi, M.; Nabavizadeh,
S. M.; Akbari, A.; Habibzadeh, S. Organometallics 2005, 24, 2528.
(f) De Felice, V.; Giovannitti, B.; De Renzi, A.; Tesauro, D.; Panunzi,
A. J. Organomet. Chem. 2000, 593-594, 445.
‡ Shiraz University.
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Fink, R. J. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, CA, 1987. (b) Crabtree, R. H. Organo-
metallic Chemistry of the Transition Metals, 3rd ed.; John Wiley &
Sons: New York, 2001. (c) Atwood, J. D. Inorganic and Organo-
metallic Reaction Mechanism, 2nd ed.; Wiley-VCH: New York, 1997.
(2) (a) Mana, J.; Kuehl, C. J.; Whiteford, J. A.; Stang, P. J.; Organome-
tallics 1997, 16, 1897. (b) Fraser, C. S. A.; Eigler, D. J.; Jennings, M.
C.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 2002, 1224. (c)
Fraser, C. S. A.; Jennings, M. C.; Puddephatt, R. J. J. Chem. Soc.,
Chem. Commun. 2001, 1310. (d) Achar, S.; Puddephatt, R. J. J. Chem.
Soc., Chem. Commun. 1994, 1895.
(5) (a) Jawad, J. K.; Puddephatt, R. J. J. Organomet. Chem. 1976, 117,
297. (b) Jawad, J. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans.
1977, 1466.
(6) Hill, R. H.; Puddephatt, R. J. J. Am. Chem. Soc. 1985, 107, 1218.
8594 Inorganic Chemistry, Vol. 44, No. 23, 2005
10.1021/ic0511064 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/06/2005

OxidatiWe Addition of MeI to a Diplatinum(II) Complex
Scheme 1
transition-metal complexes is particularly interesting because
cooperative steric or electronic effects between the two
adjacent metal centers can give rise to reaction pathways or
products not possible in the mononuclear analogues.9 In some
cases, cooperative effects between two metal centers are
thought to enhance the reactivity of binuclear systems,8i,j but
in other cases, increased steric hindrance in binuclear
complexes leads to a decrease in reactivity.8d One main
problem faced in such investigations is that binuclear
integrity is usually broken up during the reactions, and
therefore the reactions are complicated by the parallel
reactions involving the monomeric complexes formed by the
binuclear fragmentation. One way to tackle the problem is
to lock the metallic centers by suitable bidentate phosphine
ligands such as bis(diphenylphosphino)methane, dppm, or
its amine analogue bis(diphenylphosphino)amine, dppa.
These ligands are known to easily bridge between a wide
range of transition-metal centers in many binuclear or
polynuclear complexes and at the same time strongly hold
the molecular integrity during different reactions.10 The
corresponding binuclear complexes are held together by
usually two such bridging ligands and as such are normally
experiencing a great steric bulk for any oxidative addition
to occur.8d However, recently a new series of unusual
organodiplatinum(II) complexes with a general formula
cis,cis-[R2Pt(µ-SMe2)(µ-PP)PtR′2], in which PP is either
dppm or dppa and R and R′ are methyl or simple aryl ligands,
have been reported in which the labile bridging SMe2 ligand
can easily be replaced by other phosphine ligands.11 In the
present study, in an attempt to study the oxidative addition
of MeI to a suitable binuclear system, we have used the
methyl analogue, cis,cis-[Me2Pt(µ-SMe2)(µ-dppm)PtMe2],
and replaced the bridging SMe2 ligand by phthalazine,
represented by NN, to produce cis,cis-[Me2Pt(µ-NN)(µ-
dppm)PtMe2]. This new diplatinum(II) complex showed two
main advantages in the study of its reaction with MeI. First,
it has a distinct color because of the presence of the imine
ligand and so its reactions could easily be followed by
spectrophotometry in the visible region. Second, the complex
contains a robust bridging ligand, which strongly prevents
fragmentation of the dimer during the reaction.
(7) (a) Anderson, C. M.; Puddephatt, R. J.; Ferguson, G.; Lough, A. J. J.
Chem. Soc., Chem. Commun. 1989, 1297. (b) Anderson, C. M.; Crespo,
M.; Jennings, M. C.; Lough, A. J.; Ferguson, G.; Puddephatt, R. J.
Organometallics 1991, 10, 2672.
Results and Discussion
Synthesis and Characterization of Complex cis,cis-
[Me2Pt(µ-NN)(µ-dppm)PtMe2] (1). When a solution of the
binuclear complex cis,cis-[Me2Pt(µ-SMe2)(µ-dppm)PtMe2]
was treated with 1 equiv of phthalazine, NN, the color of
the reacting mixture turned red and the product cis,cis-[Me2-
Pt(µ-NN)(µ-dppm)PtMe2] (1) was formed by displacement
of the bridging SMe2 ligand by NN (reaction 1 in Scheme
1). The product was separated in good yield in an analytically
pure form as a stable pale red solid. Complex 1 is stable in
an acetone solution for several hours and was fully charac-
terized by multinuclear (1H, 31P, and 195Pt) NMR spectros-
copy. It is to be noted that phthalazine and its derivatives
have been used before as ligand-forming mono- and binuclear
complexes.12
(8) (a) Halpern, J. Inorg. Chim. Acta 1982, 62, 31. (b) Finke, R. G.;
Gaughan, G.; Pierpont, C.; Noordik, J. H. Organometallics 1983, 2,
1481. (c) Shyu, S.; Wojcicki, A. Organometallics 1985, 4, 1457. (d)
Ling, S. S. M.; Jobe, I. R.; Muir, L. M.; Muir, K. W.; Puddephatt, R.
J. Organometallics 1985, 4, 1198. (e) Puddephatt, R. J.; Scott, J. D.
Organometallics 1985, 4, 1221. (f) Ling, S. S. M.; Payne, N. C.;
Puddephatt, R. J. Organometallics 1985, 4, 1546. (g) Poilblanc, R.
Inorg. Chim. Acta 1982, 62, 75. (h) Schmidbaur, H.; Franke, R. Inorg.
Chim. Acta 1975, 13, 85. (i) Fackler, J. P., Jr.; Murray, H. H.; Basil,
J. D. Organometallics 1984, 3, 821. (j) Azam, K. A.; Brown, M. P.;
Hill, R. H.; Puddephatt, R. J.; Yavari, R. J. Organometallics 1984, 3,
697. (k) Scott, J. D.; Puddephatt, R. J. Organometallics 1986, 5, 2522.
(9) (a) Gavrilova, A. L.; Qin, C. J.; Sommer, R. D.; Rheingold, A. L.;
Bosnich, B. J. Am. Chem. Soc. 2002, 24, 1714. (b) Konard, M.; Wuth,
S.; Meyer, F.; Kaifer, E. Eur. J. Inorg. Chem. 2001, 2233. (c) Fackler,
J. P.; Murray, H. H. Organometallics 1984, 3, 821. (d) Lewis, N. S.;
Mann, K. R.; Gordon, J. G.; Gray, H. B. J. Am. Chem. Soc. 1976, 98,
7461. (e) Schenck, T. G.; Milne, C. R. C.; Sawyer, J. F.; Bosnich, B.
Inorg. Chem. 1985, 24, 2338. (f) Casado, M. A.; Perez-Torrenet, J.
J.; Ciriano, M. A.; Dobrinovitch, I. T.; Lahoz, F. J.; Oro, L. A. Inorg.
Chem. 2003, 42, 3956. (g) Tejel, C.; Ciriano, M. A.; Lopez, J. A.;
Lahoz, F. J.; Oro, L. A. Organometallics 2000, 19, 4977.
(10) (a) Puddephatt, R. J. Chem. Soc. ReV. 1983, 99. (b) Chaudret, B.;
Delavaux, B.; Poilblanc, R. Coord. Chem. ReV. 1988, 86, 191. (c)
Witt, M.; Roesky, H. W. Chem. ReV. 1994, 94, 1163. (d) Balakrishna,
M. S.; Reddy, S. V.; Krishnamutthy, S. S.; Nixon, J. F.; Burckett St.
Laurent, J. C. T. R. Coord. Chem. ReV. 1994, 129, 1. (e) Bhattacharyya,
P.; Woollins, D. J. Polyhedron 1995, 14, 3367.
(11) (a) Rashidi, M.; Hashemi, M.; Khorasani-Motlagh, M.; Puddephatt,
R. J. Organometallics 2000, 19, 275. (b) Rashidi, M.; Jamali, S.;
Hashemi, M. J. Organomet. Chem. 2001, 633, 105. (c) Jamali, S.;
Rashidi, M.; Jennings, M. J.; Puddephatt, R. J. Dalton Trans. 2003,
2313. (d) Rashidi, M.; Kamali, K.; Jennings, M. J.; Puddephatt, R. J.
J. Organomet. Chem. 2005, 690, 1600.
The presence of a single bridging dppm ligand in complex
1 was demonstrated simply by its 195Pt NMR spectrum, which
showed a doublet due to the coupling J(PtP) ) 1912 Hz,
although the long-range coupling J(PtP) was not resolved.
1
3
(12) (a) Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 9172.
(b) Kalyda, G. V.; Komeda, S.; Ikeda, K.; Sato, T.; Chikuma, M.;
Reedijk, J. Eur. J. Inorg. Chem. 2003, 4347. (c) Toth, A.; Floriani,
C.; Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1987, 26, 236. (d)
Hausmann, J.; Klingele, M. H.; Lozan, V.; Steinfeld, G.; Siebert, D.;
Journaux, Y.; Girerd, J. J.; Kersting, B. Chem. Eur. J. 2004, 10, 1716.
(e) Chen, L.; Thompson, L. K.; Tendon, S. S.; Bridson, J. N. Inorg.
Chem. 1993, 32, 4063. (f) Dixon, K. R.; Eadie, D. T.; Stobart, S. R.
Inorg. Chem. 1982, 21, 4318.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8595

Jamali et al.
Consistent with this, a singlet at δ ) 9.1 was observed in
the 31P NMR spectrum of the complex, which has short-
range platinum satellites with 1J(PtP) ) 1910 Hz as well as
long-range platinum satellites with 3J(PtP) ) 59 Hz.11a The
1H NMR spectrum of complex 1 was also very useful in
structure determination. Thus, two equal-intensity doublets
2
3
at δ ) 0.54 [with J(PtH) ) 70.0 Hz and J(PH) )7.5 Hz]
2
3
and δ ) 0.67 [with J(PtH) ) 88.6 Hz and J(PH) ) 7.7
Hz] were assigned to the Me ligands trans to phosphorus
and the Me ligands trans to nitrogen, respectively. Note that
2
the latter signal has a significantly higher J(PtH) value
because nitrogen has a considerably lower trans influence
than phosphorus; the 3J(PH) values are, however, similar for
both types of Me ligands.13 A singlet at δ ) 9.60, which is
3
coupled to platinum with J(PtH) ) 10.9 Hz, was easily
assigned to two CH groups of phthalazine adjacent to the
coordinating N atoms. The presence of only one such signal
confirms that the phthalazine ligand has symmetrically
bridged between the two metal centers. The overall relative
integration of protons also confirmed the presence of one of
each of the phthalazine and dppm ligands in the molecule
(see the Experimental Section). The UV-vis spectrum of
complex 1 contained a series of very strong absorptions
below λ ) 300 nm, which are due to the phthalazine
intraligand transitions;14 there was also a strong absorption
appearing as a shoulder at λ ) 375 nm (in both acetone and
benzene). This absorption is ascribed to the 5dπ(Pt) f π*-
(imine) MLCT band, which is believed to be responsible
for the reddish color of the complex and is used to study the
kinetics of its reaction with MeI (see below).
Figure 1. 31P NMR spectra of a mixture of complex 1 and excess MeI in
acetone at -40 °C: (a) spectrum of pure complex 1 before adding MeI;
(b) spectrum immediately after the addition of MeI; (c-f) successive spectra
during 15-min intervals; (g) spectrum after 4 h. As can be seen, upon the
addition of MeI, complex 1 is rather quickly converted to intermediate 2,
while the latter is converted to intermediate 3 considerably slower;
intermediate 3 is very rapidly converted to the final Pt(IV)-Pt(IV) dimer
4. The assignments are shown.
Reaction of Complex 1 with MeI. MeI was oxidatively
added to both platinum(II) centers of complex 1, which was
then followed by displacement of the NN ligand by iodide
ligands to give the known Pt(IV)-Pt(IV) binuclear complex
4 as shown in Scheme 1 (reaction 2). Complex 4 had been
prepared previously either by reaction of cis,cis-[Me2Pt(µ-
SMe2)(µ-dppm)PtMe2] with MeI11b or by a reaction involving
the mixture cis,cis-[Me4Pt2(µ-SMe2)2]/MeI/dppm.15 Reaction
2 in Scheme 1 was slow enough at -40 °C to be monitored
by 31P NMR spectroscopy, as shown in Figure 1. The results
are described in Scheme 2. Immediately after the addition
of MeI, a mixture of complex 1 and an intermediate
formulated as 2 in Scheme 2 was detected. This intermediate
showed a singlet signal with parameters similar to those of
diplatinum(II) complex 1 [δ ) 10.6 and 1J(PtP) ) 2029 Hz]
and another singlet signal further upfield at δ ) -4.2 with
1J(PtP) ) 1243 Hz [close to the value of 1060 Hz obtained
for the Pt(IV)-Pt(IV) binuclear complex 4; note that, in
complex 2, the Pt(IV) center is adjacent to a Pt(II) center,
and thus a higher 1J(PtP) value is expected]. Intermediate 2
is, therefore, depicted as a rare case of a mixed-valence Pt-
(II)-Pt(IV) binuclear complex8f having structure 2; the
signals are rather broad because of the unresolved P-P
coupling of the two chemically inequivalent 31P nuclei.11a
The formation of any complex with structure 2a as an
alternative for intermediate 2 in Scheme 2 is ruled out
because, in contrast to the expectation based on the kinetic
date discussed in the next section, structure 2a is not expected
to contain any 5dπ(Pt) f π*(imine) MLCT band. Also, on
the basis of the 31P NMR spectra in Figure 1, no fragmenta-
tion to monomeric forms occurred at this stage; the param-
eters expected for the presumed Pt(IV) monomer [PtMe3I-
(dppm)]16 are far more different from the data obtained for
the Pt(IV) signal at this stage in Figure 1.
The intermediate 2 was then reacted with MeI, albeit at a
rate rather slower than that for the first step, and a singlet
with platinum satellites at δ ) -23.5 [with 1J(PtP) ) 1035
Hz] gradually formed. The latter signal is attributed to a
cationic symmetrical Pt(IV)-Pt(IV) intermediate binuclear
complex with structure 3 in Scheme 2. Note that the two
alternative structural possibilities for 3, i.e., [Me3Pt(µ-NN)-
(µ-dppm)PtMe3]2+[I-]2 or [Me3IPt(µ-NN)(µ-dppm)PtIMe3],
each of which is expected to give a similar 31P signal, are
ruled out because the former contains coordinatively unsatur-
2
3
(13) Similar trends of J(PtH) and J(PH) values for the Me ligands trans
to phosphorus and the Me ligands trans to nitrogen have been observed
in the binuclear methylplatinum(II) complex [MePt(µ-PN)(µ-NP)-
PtMe2](CF3CO2), where PN ) 2-Ph2PC5H4N. See: Rashidi, M.;
Jennings, M. C.; Puddephatt, R. J. Organometallics 2003, 22, 2612.
(14) Amstutz, E. D. J. Org. Chem. 1952, 17, 1508.
(15) Rashidi, M.; Fakhroeian, Z.; Puddephatt, R. J. J. Organomet. Chem.
1991, 406, 261.
(16) Tangestaninejad, S.; Rashidi, M.; Puddephatt, R. J. J. Organomet.
Chem. 1991, 412, 445.
8596 Inorganic Chemistry, Vol. 44, No. 23, 2005

OxidatiWe Addition of MeI to a Diplatinum(II) Complex
Scheme 2
Figure 2. Changes in the UV-vis spectrum during the reaction of complex
1 (3 × 10-4 M) and MeI (0.106 M) in benzene at T ) 25 °C: (a) initial
spectrum (before adding MeI); (b) spectrum at t ) 0. Successive spectra
were recorded at intervals of 30 s.
ated platinum centers and the latter is too crowded. The
displacement of NN with iodide ligands in the last step gave
the final Pt(IV)-Pt(IV) binuclear complex 4. Complex 4 was
easily characterized by its known NMR data.11b
Figure 3. Absorbance-time curves for the reaction of complex 1 with
MeI (0.053-0.53 M; [MeI] increases reading downward) in benzene at 25
°C. The solid lines were obtained by fitting the (A, t) data into eq 2.
Conductivity measurement was carried out to investigate
the possible formation of the ionic species during the reaction
of complex 1 with MeI. Thus, the molar conductivity of a
10-3 solution of the neutral complex 1 in acetone at -25 °C
sharply increased upon the addition of excess MeI and then
significantly decreased after 2 h. This observation supports
the formation of the ionic intermediate for 3 [Me3Pt(µ-NN)-
(µ-dppm)(µ-I)PtMe3]+I-, as was also shown above by 31P
NMR spectra.
were evaluated by nonlinear least-squares fitting of the
absorbance-time profiles to the biphasic first-order equation
(eq 2).
At ) (A0 - A∞)[exp(-kobst)] + A∞
(1)
At ) R[exp(-kobs(1)t)] + â[exp(-kobs(2)t)] + A∞ (2)
Kinetic Study of the Reaction of Complex 1 with MeI
in Benzene. As mentioned before, the reddish complex 1
contains a MLCT band in the visible region that could be
used to monitor its reaction with MeI and study the kinetics
of the reaction by using UV-vis spectroscopy. Thus, an
excess of MeI was used at 25 °C, and the disappearance of
the MLCT band at λ ) 375 nm in a benzene solution was
used to monitor the reaction. The change in the spectrum
during a typical run is shown in Figure 2.
The time-dependence curves of the spectra of the reaction
in benzene at this condition are shown in Figure 3. The data
failed to fit properly in eq 1, which corresponds to a
monophasic kinetic behavior. However, the data were
successfully fitted in eq 2 with two exponentials, showing a
clear biphasic kinetic behavior. Thus, the pseudo-first-order
rate constants kobs(1) and kobs(2) for the two steps of the reaction
Plots of the first-order rate constants, kobs(1) (for step 1)
and kobs(2) (for step 2), versus [MeI] in both cases were linear
with no intercepts (Figure 4), showing a first-order depen-
dence of the rate on the concentration of MeI in each
step. The slope in each case gave the second-order rate
constant (k2 for the first step and k′ for the second step),
2
and the results are collected in Table 1. The findings seem
to be consistent with the two-step sequence indicated
above based on the 31P NMR data (see Figure 1) for the
reaction of complex 1 with MeI. Therefore, each step of
the reaction (see Scheme 2) obeys a simple second-order
rate law (eq 3 for step 1 and eq 4 for step 2), first order
in both the corresponding dimer and MeI; note that k2 >
k′ and the reproducibility of the data was remarkable
2
((5%). The same method was used at other temperatures,
and activation parameters were obtained from the Eyring
Inorganic Chemistry, Vol. 44, No. 23, 2005 8597

Jamali et al.
Table 1. Rate Constants and Activation Parametersa for the Reaction of Complex 1 with MeI in Acetone or Benzene
Reaction in Acetone
rate constants at different temperatures
5 °C
10 °C
15 °C
20 °C
25 °C
∆Hqb/(kJ mol-1
25.1 ( 2.0
)
∆Sqb/(J K-1 mol-1
-165 ( 7
)
102k2/(L mol-1 s-1
102k1/s-1
)
26.4 ( 2.5
1.9 ( 0.4
5.6 ( 0.2
37.0 ( 2.2
1.5 ( 0.4
7.3 ( 0.2
41.1 ( 2.3
2.8 ( 0.4
9.2 ( 0.3
65.6 ( 4.6
1.7 ( 0.6
12.9 ( 0.4
85.6 ( 2.4
1.1 ( 0.2
15.8 ( 0.4
102k′2/(L mol-1 s-1
)
22.1 ( 2.1
-188 ( 7
Reaction in Benzene
rate constants at different temperatures
5 °C
15 °C
25 °C
35 °C
∆Hqb/(kJ mol-1
)
∆Sqb/(J K-1 mol-1
)
102k2/(L mol-1 s-1
102k′2/(L mol-1 s-1
)
)
13.9 ( 0.2
1.4 ( 0.1
22.4 ( 1.1
3.6 ( 0.1
39.0 ( 0.5
5.6 ( 0.1
62.2 ( 3.6
11.1 ( 0.5
33.5 ( 1.1
45.3 ( 4.4
-140 ( 4
-116 ( 15
a Values given based on 95% confidence limits from least-squares regression analysis. b Obtained from the Eyring equation.
that, as is indicated in Scheme 2, MeI first performs an
electrophilic attack on one of the electron-rich platinum(II)
centers of diplatinum(II) complex 1 with an SN2-type
mechanism with the rate constant k2 and the mixed-valence
Pt(II)-Pt(IV) binuclear intermediate 2 (Scheme 2) is formed.
The assignment of the alternative structure 2a to this
intermediate is ruled out because 2a cannot have any 5dπ-
(Pt) f π*(imine) MLCT band in order to contribute to
the absorption changes in the next step. Intermediate 2 was
then reacted with MeI in step 2 again by an SN2-type
mechanism, with the rate constant k′, which is nearly 6-10
2
times smaller than the corresponding value for k2 (see
Table 1).
Figure 4. Plots of first-order rate constants [(a) first step, kobs/s-1, and
(b) second step, ko′bs/s-1] for the reaction of complex 1 with MeI at 25 °C
versus the concentration of MeI in benzene (the inset shows plots in acetone;
note the intercept for the first step in part a).
In a two-step oxidative addition reaction of this type in
which none of the components of the reaction contains any
metal-metal bond and yet the metallic centers are not too
far from each other, it is suggested that the first addition
occurs faster than the second.8k,9e In the present study, we
strongly believe that this has actually been the case. First,
as expected based on the qualitative 31P NMR investigations,
vide supra, step 2 appears to be slower than step 1. Second,
in step 1, MeI attacks the platinum(II) center of a Pt(II)-
Pt(II) complex, 1, while in step 2, MeI reacts with the
platinum(II) center of a Pt(II)-Pt(IV) species (complex 2).
As such, the reaction should be slower in the second step
because, in contrast to complex 1, in species 2 the adjacent
Pt(IV) moiety not only creates a more severe steric hindrance
for the attacked Pt(II) center but also reduces its nucleophi-
licity through the bridging ligating groups. Despite these,
however, to confirm that the higher rate constant, k2, actually
belongs to the first step in the above sequence and the second
Figure 5. Eyring plots for the reaction of complex 1 with MeI in
benzene: (a) first step; (b) second step.
equation (see eq 5 and Figure 5). The data are collected in
Table 1.
step is proceeded with the slower rate constant, k′, we
2
considered Espenson’s approach, in which the biphasic
equation (eq 6) was used in evaluating the two rate
constants17 and it was predicted that the method might be of
general applicability.
-d[1]/dt ) k2[1][MeI]
(3)
(4)
-d[2]/dt ) k′[2][MeI]
2
∆Sq ∆Hq
k2
kB
At - A∞ ) af[exp(-kft) + as[exp(-kst)]
(6)
ln
) ln
+
-
(5)
( )
( )
T
h
R
RT
In this equation, kf and ks are the rate constants for the
fast and slow steps, respectively, but do not identify a
Large negative values of ∆Sq, which are in favor of a
classical SN2-type mechanism, were observed for both steps.
On the basis of these NMR and kinetic data, we suggest
(17) Shimura, M.; Espenson, J. H. Inorg. Chem. 1984, 23, 4069.
8598 Inorganic Chemistry, Vol. 44, No. 23, 2005

OxidatiWe Addition of MeI to a Diplatinum(II) Complex
Table 2. Rate Constants (k2 and k′) and Calculated ꢀi Values at 375
one mentioned in the last section for the reaction in benzene,
was also observed for the reaction in acetone. Thus, the
second-order rate constant for the first step, k2, is again nearly
6 times larger than the second-order rate constant for the
2
nm for Cases 1 and 2a
10-3ꢀi/(M-1 cm-1
)
103ks/
103kf/
102[MeI]/M
(L mol-1 s-1
)
(L mol-1 s-1
)
case 1
case 2
second step, k′. However, two abnormalities are seen when
2.6
5.3
7.9
10.6
13.2
15.9
18.5
21.2
26.5
53.0
2.24
3.49
5.22
6.42
7.62
10.48
10.87
12.51
15.60
28.35
10.79
19.95
30.76
40.59
49.63
69.19
74.15
86.97
108.14
200.97
3.22
3.33
3.36
3.39
3.41
3.42
3.44
3.45
3.45
3.46
-13.49
-16.75
-17.42
-18.97
-19.64
-19.99
-20.78
-21.25
-21.17
-21.75
2
the results are compared with the results usually reported
for the reaction of MeI with electron-rich transition-metal
complexes operated by a classical SN2-type mechanism:
(i) The reactions in both steps at different temperatures
are nearly 2 times faster in acetone than in benzene. This,
of course, strongly supports the operation of the SN2-type
mechanism in the reactions because in the latter mechanism,
in which after each MeI attack a polar transition state is
expected, the rate should be faster in a more polar acetone
solvent than in benzene with a significantly lower dielectric
constant. Usually, however, a rather higher rate increase is
expected. For example, [Pt(p-MeC6H4)2(NN)], in which NN
) 4,4′-di-tert-butyl-2,2′-bipyridine, reacts with MeI at 25
°C faster in acetone than in benzene by a factor of 7.5,4e
and the factor for the reaction of MeI with [PtPh2(2,2′-
bipyridine)] is 6.2 at 40 °C.5a Also, the rate of the reaction
of Vaska’s complex [IrCl(CO)(PPh3)2] with MeI at 25 °C is
6 times faster in acetone than in benzene.18
(ii) It is believed that in SN2-type reactions, where a highly
polar transition state is formed, less negative ∆Sq values are
obtained in more polar solvents, and this is attributed to
decreasing order in the transition state relative to the ground
state.19 In the three reactions of MeI just mentioned in (i),
∆Sq values in benzene and acetone, respectively, are -161
and -123 J mol-1 K-1 for the first reaction4e and -156 and
-114 J mol-1 K-1 for the second reaction;5a for the third
reaction, ∆Sq values of -213 and -49 J mol-1 K-1 in
benzene and DMF, respectively, were found. As is shown
in Table 1, a reversed trend is actually observed for the
system studied in the present work. Thus, ∆Sq in each step
has a larger negative value in acetone (-165 J mol-1 K-1 in
step 1 and -183 J mol-1 K-1 in step 2) than in benzene
(-140 J mol-1 K-1 in step 1 and -116 J mol-1 K-1 in step
2).
a The values of the extinction coefficient (ꢀi) at 375 nm were determined
by the method of nonlinear least squares (the program KaleidaGraph 3.6).
particular reaction step. Thus, we now consider the two cases
pertaining to a sequence of reactions:
Case 1: k2 ) kf > k′ ) ks
2
Case 2: k2 ) ks < k′ ) kf
2
If we consider ꢀd, ꢀi, and ꢀp as the molar absorptivities of
starting dimer 1, intermediate 2, and the final product
(complex 4), respectively, then ꢀi can be calculated from the
following equations, depending upon which case is ap-
plicable:
Case 1: ꢀi ) (ꢀd - af)(1 - ks/kf) + ꢀpks/kf
Case 2: ꢀi ) (ꢀd - as)(1 - kf/ks) + ꢀpkf/ks
The calculated ꢀi values from the experimental data for
different presumed cases are listed in Table 2. As an example,
in a benzene solvent at 25 °C and λ ) 375 nm, the end
results are as follows:
Before the MeI addition:
[complex 1]0 ) 3.0 × 10-4 M and Abs ) 1.192 w ꢀd )
Abs/[complex 1]0 ) 3973.3
To explain these abnormalities, we suggest that, in each
step of the reaction, the more polar acetone solvent would
unusually lower the entropy of the ground state as compared
to the excited state. In fact, we have observed a two-term
rate law for the first step of the reaction in acetone (see the
inset of Figure 4):
End of reaction:
[complex 4]0 ) 3.0 × 10-4 M and Abs ) 0.103 w ꢀp )
Abs/[complex 4]0 ) 344
The data for case 1 are preferable for two reasons. First,
in case 1, the value of ꢀi for the 10 experiments at 375 nm
is almost constant; ꢀi ) 3.33 × 103 M-1 cm-1 ((5% standard
deviation), whereas large scatter and negative values are
obtained for ꢀi when case 2 was assumed. Second, the value
of ꢀi at 375 nm in case 1 lies close to ꢀd (3.97 × 103 M-1
cm-1), in which both of the related complexes have MLCT
bands, and far from ꢀp, as one would expect for the final
product 4 with no MLCT band in the visible region.
Kinetic Study of the Reaction of Complex 1 with MeI
in Acetone. A similar UV-vis spectroscopic method was
used at λ ) 375 nm to study the kinetic behavior of the red-
dish complex 1 with MeI in an acetone solvent. The results
are collected in Table 1. A two-step sequence, similar to the
-d[1]/dt ) kobs[1], where kobs ) k1 + k2[MeI]
Thus, in addition to second-order term k2 for step 1, which
is suggested for the simple associative SN2-type mechanism,
a much smaller term, k1, is also observed for associative
substitutions by solvent in this step. All of the rate constants
are shown in Table 1. The associative substitution by acetone,
S, in step 1 is suggested to involve a transition state with
(18) Stieger, H.; Kelm, H. J. Phys. Chem. 1973, 77, 290.
(19) (a) Wiberg, K. B. Physical Organic Chemistry; John Wiley & Sons:
New York, 1964; pp 381-387. (b) Levy, C. J.; Puddephatt, R. J. J.
Am. Chem. Soc. 1997, 119, 10127.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8599

Jamali et al.
structure [Me2(η1-NN)Pt(µ-dppm)Pt(S)Me2], in which one
S molecule displaces one of the N donor atoms and the
binuclear integrity is held by only one bridging dppm ligand.
This structure is based on a recent report that the labile SMe2
ligand in cis,cis-[Me2Pt(µ-SMe2)(µ-dppm)PtMe2] can be
replaced with monodentate P donor ligands such as PPh3,
L, to form the stable complexes cis,cis-[Me2LPt(µ-dppm)-
PtLMe2], in which the two platinum(II) moieties are bridged
by only one dppm with no metal-metal bonding; each
platinum center also bears a monodentate ligand L and two
methyl groups.11e
Preparation of cis,cis-[Me2Pt(µ-NN)(µ-dppm)PtMe2] (1). To
a solution of cis,cis-[Me2Pt(µ-SMe2)(µ-dppm)PtMe2] (100 mg, 0.11
mmol) in acetone (15 mL) was added phthalazine, NN (14.6 mg,
0.11 mmol), in acetone (5 mL) at room temperature. The mixture
was stirred for 1 h, the solvent was then removed under reduced
pressure, and the residue was triturated with ether (3 mL) to give
a pale red solid, which was separated and dried under vacuum.
Yield: 70 mg, 65%. Mp: 170 °C (dec). Anal. Calcd for C37H40N2P2-
1
Pt2: C, 46.0; H, 4.1; N, 2.9. Found: C, 45.9; H, 4.1; N, 2.6. H
NMR data in acetone-d6: δ 0.54 [d, 2J(PtH) ) 70.0 Hz, 3J(PH) )
7.5 Hz, 6H, 2 Me ligands trans to phosphorus], 0.67 [d, 2J(PtH) )
3
88.6 Hz, J(PH) ) 7.7 Hz, 6H, 2 Me ligands trans to nitrogen],
2.61 [m, 1H of dppm], 3.90 [m, 1H of dppm]; phthalazine protons,
8.00 [m, 2H], 8.15 [m, 2H], 9.60 [s, 3J(PtH) ) 10.9 Hz, 2H, 2 CH
groups of phthalazine adjacent to N atoms]; dppm phenyl groups,
7.00, 7.10, 7.30, 7.90 [20 H]. 31P NMR in CDCl3: δ 9.1 [s, 1J(PtP)
) 1910 Hz, 3J(PtP) ) 59 Hz, dppm]. 195Pt NMR in CDCl3: δ 262
We believe that this kind of associative solvent involve-
ment by acetone molecules could most probably be respon-
sible for the above-mentioned abnormalities observed in the
kinetics of the reaction of complex 1 with MeI. Thus, in the
first step of the MeI reaction in acetone, the association of
starting complex 1 with solvent molecules will lower the
entropy of the ground state relative to the cationic exited
state (not shown in the scheme) more than is usually observed
in the absence of such an extra solvent involvement. This
makes the ∆Sq value in the acetone solvent more negative
than usual, to the extend that it abnormally becomes more
negative than the ∆Sq value in benzene. This entropy effect
should also be responsible for an abnormally lower rate
increase (nearly 2 times as compared to the usual 6-10
times) observed in the present study in going from the
acetone solvent to the nonpolar benzene solvent. The
intermediate 2, which is now the reactant for the reaction
with MeI in the second step, would similarly act to make
the ∆Sq value for step 2 more negative than usual. The
magnitude of the effect on ∆Sq values is more pronounced
in the second step; the ∆Sq value difference on going from
acetone to benzene is -25 J K-1 mol-1 for step 1, while it
is -72 J K-1 mol-1 for step 2. We believe that this is
because, in the second step, the associative substitution by
acetone is only taking place with reactant complex 2 and
not with the final kinetic product complex 3 (or more
precisely with the preceding polar transition state not shown
in Scheme 2) in which both platinum centers are Pt(IV); note
that, in the first step, both reactant complex 1 and the product
complex 2 (or, more precisely, with the preceding polar
transition state not shown in the Scheme 1) are involved in
the associative substitution by acetone and so a more modest
entropy effect is expected for step 1.
1
[d, J(PtP) ) 1912 Hz].
Reaction of 1 with MeI. An excess of MeI (40 µL) was added
to a solution of complex 1 (20 mg in 5 mL of acetone) at room
temperature. The mixture was allowed to stand at this condition
for 6 h, then the solvent was removed under reduced pressure, the
residue was washed twice with ether (2 × 3 mL), and the product
1
was dried under vacuum and identified by its H and 31P NMR
spectra as the known diplatinum(IV) complex [{PtMe3(µ-I)}2(µ-
dppm)] (4)11b,15 in a high yield.
This reaction was also monitored by 31P NMR spectroscopy at
-40 °C in an NMR tube as follows: a small sample (10 mg) of 1
was dissolved in acetone-d6 (0.3 mL) in a sealed NMR tube, and
while the tube was cooled by liquid N2, an excess of MeI (20 µL)
was added. The NMR spectra were recorded several times at -40
°C over about 4 h until the mixture was gradually converted to
complex 4 in solution (see Figure 1).
Kinetic Study. In a typical experiment, a solution of complex 1
in benzene (3 mL, 3 × 10-4M) in a cuvette was thermostated at
25 °C and a known excess of MeI was added using a microsyringe.
After rapid stirring, the absorbance at λ ) 375 nm was monitored
with time.
Conclusions
A new binuclear complex, cis,cis-[Me2Pt(µ-NN)(µ-dppm)-
PtMe2], 1, in which NN ) phthalazine and dppm ) bis-
(diphenylphosphino)methane, was synthesized, and its two-
step reaction with MeI was investigated in detail by
spectrophotometry in the visible region. The results are
consistent with the qualitative results obtained from the study
of the reaction by 31P NMR spectroscopy at -40 °C. Both
steps proceed by the classical SN2-type mechanism, as shown
in Scheme 2, with large negative ∆Sq values in both acetone
and benzene, and the rates were faster in acetone than in
benzene. An associative substitution by an acetone solvent
was suggested as being responsible for some abnormalities
observed in the trends of the quantitative kinetic measure-
ments. In the first step, MeI attacks the electron-rich platinum
centers of the Pt(II)-Pt(II) starting complex 1 to form the
intermediate 2, which is suggested to have a Pt(II)-Pt(IV)
skeleton. The rate of the reaction of the platinum(II) center
in complex 2 with MeI is therefore considerably slower (by
a factor of 6) than that of the platinum(II) center in the
starting complex 1, for a combination of steric hindrances
and the electronic effects transmitted through the ligands.
Experimental Section
The NMR spectra were recorded on a Bruker Avance DRX 500-
MHz spectrometer. The operating frequencies and references,
respectively, are shown in parentheses as follows: 1H (500 MHz,
TMS), 31P (202 MHz, 85% H3PO4), and 195Pt (107 MHz, aqueous
Na2PtCl4). All of the chemical shifts and coupling constants are in
ppm and Hz, respectively. Kinetic studies were carried out by using
a Perkin-Elmer Lambda 25 spectrophotometer with temperature
control using an EYELA NCB-3100 constant-temperature bath. The
conductivity study was carried out using a Metrohm 660 conduc-
tometer. The microanalysis was performed using a Termofinnigan
Eager 300 CHN-O elemental analyzer. The dimeric precursor
cis,cis-[Me2Pt(µ-SMe2)(µ-dppm)PtMe2] was prepared by the lit-
erature method.11b
8600 Inorganic Chemistry, Vol. 44, No. 23, 2005

OxidatiWe Addition of MeI to a Diplatinum(II) Complex
MeI in the first step reacts with complex 1 (in which each
metallic center is bonded to one P donor and one N donor)
in acetone at 25 °C with a rate constant k2 ) 0.86 ( 0.02 L
mol-1 s-1, some 50 times slower than that with the
mononuclear complex [PtPh2(2,2′-bipyridine)] (with the
metal center bonded to two N donors) at the same condition
for which k2 ) 47 L mol-1 s-1.8k This significant rate
decrease is to be expected for the reaction of complex 1 in
which each of the metallic centers is connected to a P atom
with a large π-back-bonding that considerably reduces the
electron density on the platinum centers as compared with
that in the monomeric complexes with two strong N donor
atoms. However, it would be of great interest to have more
experimental data on the oxidative addition to the binuclear
complexes of this type and also the same oxidative additions
to some monomeric complexes with P and N donors
(designed to have suitably similar overall structures to the
corresponding dimers) to actually substantiate any possible
“neighboring group effect” of the adjacent metal center in
the binuclear complexes.
Acknowledgment. We thank the Persian Gulf University
and the Shiraz University Research Councils for financial
support and Mr. Bijanzadeh for assistance with the multi-
nuclear NMR experiments. S.M.N. also thanks Dr.
Nobuyoshi Koshino for helpful discussions.
IC0511064
Inorganic Chemistry, Vol. 44, No. 23, 2005 8601