On the configuration resulting from oxidative addition of RX to Pd(PPh3)4 and the mechanism of the cis-to-trans isomerization of [PdRX(PPh3)2] complexes (R = aryl, X = halide)

Article abstract of DOI:10.1021/om9709502

The oxidative addition of RI to Pd(O) and further cis-to-trans isomerization, which are involved in the Stille reaction and other Pd-catalyzed syntheses, have been studied. C₆Cl₂F₃I (1, C₆Cl₂F₃ = 3,5-dichlorotrifluorophenyl) adds to Pd(PPh₃)₄ in THF at room temperature giving cis-[Pd(C₆Cl₂F₃)I(PPh₃)₂] (2), which could be isolated before isomerization to the more stable trans-[Pd(C₆Cl₂F₃)I(PPh₃) ₂] (3). A ¹⁹F NMR kinetic study of the isomerization of 2 in THF at 322.6 K reveals a first-order law r_iso = K_iso[2], with k_iso = f + g[2]₀ + (h + i[2]₀)/([PPh₃] + j) (f = (1.66 ± 0.03) × 10^-4 s^-1, g = (2.5 ± 0.2) × 10^-3 mol^-1 L s^-1, h = (1.3 ± 0.7) × 10^-8 mol L^-1 s^-1, i = (4 ± 2) × 10^-7 s^-1, and 7 = (1.4 ± 0.7) × 10^-5 mol L^-1). A four-pathway mechanism accounts for these results: Two are assigned to the associative replacements Of PPh₃ coordinated to 2 by an iodide ligand of I-[Pd] (I-[Pd] = 2 or 3), both THF-assisted (coefficient h) or direct (coefficient i), leading to a monoiodidebridged intermediate cis-{Pd(C₆Cl₂F₃)I(PPh ₃)(μ-I)-[Pd]}. The later rearranges via terminal-for-bridging iodide exchange to trans-{Pd(C₆Cl₂F₃)I(PPh ₃)(μ-I)]-[Pd]}, which is finally cleaved by PPh₃ yielding complex 3. The other two concurrent pathways are assigned to the isomerization via two consecutive Berry pseudorotations in the pentacoordinated species derived from 2 by coordination of THF (coefficient f) or I-[Pd] (coefficient g). The apparent activation entropy associated with k_iso is negative (ΔS^? = -21 ± 3 J K^-1 mol^-1), in agreement with the proposed bimolecular mechanisms.

Full text of DOI:10.1021/om9709502

954
Organometallics 1998, 17, 954-959
On th e Con figu r a tion Resu ltin g fr om Oxid a tive Ad d ition
of RX to P d (P P h 3)4 a n d th e Mech a n ism of th e
cis-to-tr a n s Isom er iza tion of [P d RX(P P h 3)2] Com p lexes
(R ) Ar yl, X ) Ha lid e)†
Arturo L. Casado and Pablo Espinet*
Departamento de Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
E-47005 Valladolid, Spain
Received October 31, 1997
The oxidative addition of RI to Pd(0) and further cis-to-trans isomerization, which are
involved in the Stille reaction and other Pd-catalyzed syntheses, have been studied. C6-
Cl2F3I (1, C6Cl2F3 ) 3,5-dichlorotrifluorophenyl) adds to Pd(PPh3)4 in THF at room
temperature giving cis-[Pd(C6Cl2F3)I(PPh3)2] (2), which could be isolated before isomerization
to the more stable trans-[Pd(C6Cl2F3)I(PPh3)2] (3).
A
19F NMR kinetic study of the
isomerization of 2 in THF at 322.6 K reveals a first-order law riso ) kiso[2], with kiso ) f +
g[2]0 + (h + i[2]0)/([PPh3] + j) (f ) (1.66 ( 0.03) × 10-4 s-1, g ) (2.5 ( 0.2) × 10-3 mol-1
L
s-1, h ) (1.3 ( 0.7) × 10-8 mol L-1 s-1, i ) (4 ( 2) × 10-7 s-1, and j ) (1.4 ( 0.7) × 10-5 mol
L-1). A four-pathway mechanism accounts for these results: Two are assigned to the
associative replacements of PPh3 coordinated to 2 by an iodide ligand of I-[Pd] (I-[Pd] ) 2
or 3), both THF-assisted (coefficient h) or direct (coefficient i), leading to a monoiodide-
bridged intermediate cis-{Pd(C6Cl2F3)I(PPh3)(µ-I)-[Pd]}. The later rearranges via terminal-
for-bridging iodide exchange to trans-{Pd(C6Cl2F3)I(PPh3)(µ-I)]-[Pd]}, which is finally cleaved
by PPh3 yielding complex 3. The other two concurrent pathways are assigned to the
isomerization via two consecutive Berry pseudorotations in the pentacoordinated species
derived from 2 by coordination of THF (coefficient f) or I-[Pd] (coefficient g). The apparent
activation entropy associated with kiso is negative (∆Sq ) -21 ( 3 J K-1 mol-1), in agreement
with the proposed bimolecular mechanisms.
Sch em e 1
In tr od u ction
The oxidative addition of aryl halides or pseudoha-
lides (RX) to zerovalent Pd complexes is the first step
in many catalyzed cross-coupling reactions, such as the
Stille reaction (Scheme 1) and related processes.1 The
isolated or postulated products of the oxidative addition
are commonly trans-[PdRXL2] (L ) neutral ligand).
Nevertheless, the mechanism of the oxidative addition
has been established to proceed usually via a concerted
insertion of highly reactive coordinatively unsaturated
species PdL2 or Pd(L-L) (L-L ) diphosphine) into the
RX σ bond.2 Consequently, even for monodentate
ligands, a cis-[PdRXL2] complex must be formed first,
although it further isomerizes to the trans isomer.
To the best of our knowledge, a cis isomer (R )
substituted uracil, L ) PPh3) has been isolated from an
oxidative addition in only one case.3 Its isomerization
to the trans isomer was retarded by addition of neutral
ligand L, and a dissociative mechanism, proceeding via
a three-coordinate intermediate,4,5 was postulated
(Scheme 2).
In the course of our studies on the mechanism of the
Stille couplings, using perhaloaryl iodides to facilitate
† Dedicated to Prof. P. Royo on ocassion of his 60th birthday.
(1) (a) Farina, V.; Roth, G. P. in Advances in Metal-Organic
Chemistry; Liebeskind, L. S., Ed.; J AI Press: Greenwich, CT, 1996,
Vol. 5, pp 1-53. (b) Farina, V. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, 1995, Vol. 12, Chapter 3.4. (c) Brown, J . M.; Cooley,
N. A. Chem. Rev. 1988, 88, 1131-1146.
(2) (a) Amatore, C.; Broeker, G.; J utand, A.; Khalil, F. J . Am. Chem.
Soc. 1997, 119, 5176-5185. (b) Amatore, C.; J utand, A.; Suarez, A. J .
Am. Chem. Soc. 1993, 115, 9531-9541. (c) Amatore, C.; Pflu¨ger, F.
Organometallics 1990, 9, 2276-2282.
(3) Urata, H.; Tanaka, M.; Fuchikami, T. Chem. Lett. 1987, 751-
754.
(4) Minniti, D. Inorg. Chem. 1994, 33, 2631-2634.
(5) (a) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J . K. Bull.
Chem. Soc. J pn. 1981, 54, 1857-1867. (b) Ozawa, F.; Ito, T.; Naka-
mura, Y.; Yamamoto, A. Bull. Chem. Soc. J pn. 1981, 54, 1868-1880.
(c) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J . K. J . Am. Chem.
Soc. 1976, 98, 7255-7265.
S0276-7333(97)00950-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/10/1998

Oxidative Addition of RX to Pd(PPh3)4
Organometallics, Vol. 17, No. 5, 1998 955
Sch em e 2
Sch em e 3
F igu r e 1. 19F NMR (282 MHz, -85.0 to -88.5 ppm region)
spectral sequence (10 min intervals) of the isomerization
of cis-[Pd(C6Cl2F3)I(PPh3)2] (2, 10-2 mol L-1) to trans-
[Pd(C6Cl2F3)I(PPh3)2] (3) at 315.7 K in THF.
the isolation or detection of intermediates, we have
found that the reaction of C6Cl2F3I (C6Cl2F3 ) 3,5-
dichlorotrifluorophenyl) with Pd(PPh3)4 (a very common
catalyst precursor) produces cis-[Pd(C6Cl2F3)I(PPh3)2],
which slowly isomerizes to trans-[Pd(C6Cl2F3)I(PPh3)2]
(the actual transmetalation catalyst in Scheme 1). Our
kinetic studies on the isomerization step demonstrate
the occurrence of four concurrent associative pathways.
F igu r e 2. Retardation effect by addition of PPh3 on the
isomerization rate of cis-[Pd(C6Cl2F3)I(PPh3)2] (2, 10-2 mol
L-1) in THF at 322.6 K.
NMR spectrum of 3 shows only one resonance at 22.6
ppm, and its 19F NMR spectrum exhibits a triplet at
-92.48 ppm corresponding to the Fortho atoms coupled
with two equivalent P nuclei (the coupling constants are
given in the Experimental Section).
The kinetics of the isomerization of 2 in THF and in
chlorobenzene were studied by 19F NMR spectroscopy
(Figure 1) under a strictly oxygen-free N2 atmosphere
(see cautionary note in the Experimental Section). The
reactions followed first-order rate laws ln([2]/[2]0) ) kisot.
The values of kiso measured under different experimen-
tal conditions are given in Table 1.6
The results in Table 1 show that at a given temper-
ature kiso varies with [PPh3] and with the initial
concentration of complex 2 ([2]0). These experimental
data have been analyzed by means of suitable repre-
sentations in order to establish the rate law for the
process.
Resu lts a n d Discu ssion
The oxidative addition of C6Cl2F3I (1) to Pd(PPh3)4
yielded, after 5 h in toluene at 60 °C, a 1:2 mixture of
cis-[Pd(C6Cl2F3)I(PPh3)2] (2) and trans-[Pd(C6Cl2F3)I-
(PPh3)2] (3). When the reaction was carried out in THF
at room temperature for 1 h, only complex 2 was formed
in quantitative yield. Upon heating, the trans isomer
3 was then formed from 2 (Scheme 3, a). A sample of
3, prepared in a different way (Scheme 3, b), could not
be isomerized to 2, proving that 3 is the thermodynami-
cally more stable isomer. These results suggest that
under catalytic conditions (usually 50 °C, 20:1 ratio
organic iodide:Pd) the first product formed is 2, which
then isomerizes to 3.
The cis or trans configurations of 2 and 3 were
established by multinuclear NMR spectroscopy: The
31P{1H} NMR spectrum of 2 in CDCl3 shows two
resonances at 29.6 and 16.7 ppm, whereas the 19F NMR
spectrum shows a doublet of doublets at -90.63 ppm
corresponding to the Fortho atoms coupled with two
inequivalent P atoms. On the other hand, the 31P{1H}
The rate dependence on the concentration of added
PPh3 is shown in Figure 2. The isomerization of 2 in
THF is at first sharply retarded by small amounts of
free PPh3, but at [PPh3] > 10-2 mol L-1 the isomeriza-
tion rate constant reaches a nearly constant kiso value.
(6) (a) Wilkins, R. G. Kinetics and Mechanism of Reactions of
Transitions Metal Complexes, 2nd ed.; VCH: Weinheim, 1991. (b)
Espenson, J . H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;
McGraw-Hill: Singapore, 1995.

956 Organometallics, Vol. 17, No. 5, 1998
Casado and Espinet
Ta ble 1. F ir st-Or d er Ra te Con sta n ts kiso for th e
Isom er iza tion of cis-[P d (C6Cl2F 3)I(P P h 3)2] (2) to
tr a n s-[P d (C6Cl2F 3)I(P P h 3)2] (3)
[2]0/10-3
[PPh3]/10-3
mol L-1
solvent
T/K
mol L-1
kiso/10-5 s-1
THF 296.2 ( 0.2 10.0 ( 0.2
0
0
0
0
0
0
6.5 ( 0.2
11.4 ( 0.4
20.0 ( 0.7
39.2 ( 1.5
72 ( 5
THF 300.9
THF 304.9
THF 310.4
THF 315.7
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 322.6
THF 326.1
PhCl 322.6
PhCl 322.6
PhCl 322.6
PhCl 322.6
PhCl 322.6
PhCl 322.6
10.0
10.0
10.0
10.0
4.72 ( 0.04
4.72
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
15.0 ( 0.2
15.0
20.0 ( 0.2
20.0
25.1 ( 0.3
25.1
10.0
4.01 ( 0.03
10.0
10.0
15.0
20.0
25.1
117 ( 4
36.3 ( 0.5
0
17.7 ( 0.17
149 ( 9
0.320 ( 0.002 21.9 ( 0.2
1.024 ( 0.006 25.8 ( 0.2
1.190 ( 0.006 18.0 ( 0.2
2.112 ( 0.006 18.4 ( 0.2
2.602 ( 0.006 17.5 ( 0.2
3.448 ( 0.006 16.7 ( 0.3
6.272 ( 0.006 17.2 ( 0.2
16.48 ( 0.06
36.3
70.6 ( 0.6
0
36.3
0
36.3
0
36.3
0
0
0
36.3
0
0
0
F igu r e 3. Dependence of the isomerization rate of cis-[Pd-
(C6Cl2F3)I(PPh3)2] (2) with the initial concentration [2]0 at
322.6 K: (a) in THF; (b) in THF with excess (3.63 × 10-2
mol L-1) of PPh3; (c) in PhCl.
18.2 ( 0.2
18.5 ( 0.2
18.7 ( 0.2
153 ( 6
20.7 ( 0.2
175 ( 7
21.7 ( 0.2
186 ( 6
22.5 ( 0.2
208 ( 0.2
11.8 ( 0.2
12.4 ( 0.2
3.05 ( 0.07
12.6 ( 0.2
13.0 ( 0.2
13.5 ( 0.2
Ta ble 2. Coefficien ts d a n d e for Eq 2 a t 322.6 K
conditions
d/10-4 s-1
e/10-3 mol-1 L s-1
THF
10.8 ( 0.7
1.66 ( 0.03
1.09 ( 0.03
32 ( 4
2.5 ( 0.2
1.1 ( 0.2
a
THF with excess of PPh3
Chlorobenzene
a
(3.63 ( 0.05) × 10-2 mol L-1
.
way (slope e). The term which is independent of [2]0 is
clearly more important for THF than for chlorobenzene
by about 1 order of magnitude. THF and chlorobenzene
have a similar polarity, but the former coordinates more
effectively to Pd(II) than the later.8,9 Hence, this
observation suggests that the first term corresponds to
a solvent-assisted pathway.
Similar behavior was observed with chlorobenzene as
the solvent. The nonlinear least-squares fitting of the
data in Figure 2 gives eq 1,7 where a ) (1.81 ( 0.10) ×
According to eq 2, each term in eq 1 must consist of
two contributions, one constant and one proportional to
[2]0. After suitable numerical analysis, the four-term
rate law given in eq 3 is obtained, where f ) (1.66 (
0.03) × 10-4 s-1, g ) (2.5 ( 0.2) × 10-3 mol-1 L s-1, h
b
kiso ) a +
(1)
[PPh3] + c
) (1.3 ( 0.7) × 10-8 mol L-1 s-1, i ) (4 ( 2) × 10-7 s-1
,
and j ) (1.4 ( 0.7) × 10-5 mol L-1. The coefficients
h + i[2]0
10-4 s-1, b ) (1.7 ( 0.9) × 10-8 mol L-1 s-1, and c )
(1.3 ( 0.7) × 10-5 mol L-1. This equation reveals two
contributions to the isomerization rate. The first term,
a, is independent of [PPh3]. The second term is in-
versely dependent on [PPh3] and corresponds to a
pathway starting with PPh3 dissociation from 2. The
later is responsible for the dramatic retarding effect of
the added phosphine, which led to the suggestion of a
dissociative mechanism in the literature.3
The dependence of the isomerization rate constant on
the initial concentration of the starting complex 2 ([2]0)
is shown in Figure 3. Both in the absence (Figure 3a)
and in the presence of an excess of PPh3 (Figure 3b),
kiso varies following the two terms in eq 2, whose
coefficients in each case are given in Table 2.
kiso ) f + g[2]0 +
(3)
[PPh3] + j
given hold for THF at 322.6 K.
For each kinetic experiment, [PPh3] and [2]0 in eq 3
remain constant, rendering kiso also constant and giving
first-order dependence on [2].10 A simple dissociative
pathway (Scheme 2) would also give first-order kinetics,
but in such a case the rate constant kiso should be
independent of the initial concentration of reagent,
[2]0.6,11 Consequently, it must be discarded. The com-
plexity of eq 3 reveals a more complicated situation
involving at least four parallel pathways. The isomer-
ization mechanism outlined in Scheme 4 accounts for
this complexity.
kiso ) d + e[2]0
(2)
(8) Catala´n, J .; Lo´pez, V.; Pe´rez, P.; Mart´ın-Villamil, R.; Rodriguez,
J .-G. Liebigs Ann. Chem. 1995, 241-252.
(9) Complexes with THF coordinated to Pd(II) have been reported,
see: Uso´n, R.; Fornie´s, J . Adv. Organomet. Chem. 1988, 28, 219-297.
(10) In other words, although the isomerization is a second-order
reaction (bimolecular), it shows (pseudo)first-order kinetics ln([2]/[2]0)
) kisot (see ref 6b, pp 16).
Equation 2 reveals two parallel contributions to the
isomerization rate, one independent of the total con-
centration of the complexes (intercept d) and one
proportional to [2]0, suggesting an autocatalytic path-
(7) The study of the retarding effect of PPh3 is extremely difficult
because of the very sharp effect observed. For this reason, some
coefficients in eq 3 have an appreciable imprecision. This difficulty has
been noticed before in other kinetic work involving PPh3; see, for
example, ref 1a.
(11) The rate equation derived by applying the steady-state as-
sumption to the dissociative Scheme 2 can be developed as in ref 4,
giving an expression for the first-order rate constant riso ) kiso[2], where
kiso ) (k1′k2′)/(k-1′[PPh3] + k2′). The first-order rate constant kiso is
independent of the initial concentration of the reagent [2]0.

Oxidative Addition of RX to Pd(PPh3)4
Organometallics, Vol. 17, No. 5, 1998 957
Sch em e 4a
Sch em e 5a
a
I-[Pd] stands for either 2 or 3; R1 ) C6Cl2F3; L ) PPh3;
p-r ) pseudorotation.
be THF-assisted (step k3, intermediate A is considered)
or direct (step k4); this two-term rate law is well-known
in many substitution processes involving d8 square-
planar complexes.13 The intermediate B rearranges
further (step k6) by terminal-for-bridging iodide ex-
change to give C, which is finally cleaved by PPh3 to
yield complex 3. The retarding effect of added free PPh3
is due to the capture of intermediate B.
The two PPh3-insensitive pathways, via k1 and k2, are
less important in THF. For [2]0 ) 10-2 mol L-1 and
[PPh3] ) 0 at 322.6 K, reaction via k1 accounts for 12%
of the observed isomerization rate whereas reaction via
k2 accounts for 2%. Since both pathways are unaffected
by the addition of free PPh3, the mechanism should not
involve the release of the neutral ligand. Plausible
solvent-assisted and direct pathways are given in
Scheme 5. The coordination of THF or I-[Pd] to
complex 2 gives a pentacoordinated species, which can
evolve through two consecutive Berry pseudorotations.
Note that the release of PPh3 from such pentacoordi-
nated intermediates leads, respectively, to the interme-
diates A and B considered in Scheme 4; therefore, the
a
I-[Pd] stands for either 2 or 3; R1 ) C6Cl2F3; L ) PPh3.
There are two PPh3-insensitive (k1 and k2) and two
PPh3-sensitive (k3 and k4) pathways. The corresponding
theoretical rate expression (eq 4) is obtained by applying
the steady-state approximation (see Appendix), under
the simplification that complexes 2 and 3 catalyze the
isomerization of 2 (autocatalytical pathways) with simi-
lar efficiency.12
k3k6 + k4k6[2]0
riso ) kiso[2] ) k1 + k2[2]0 +
[2] (4)
{
}
k-4[PPh3] + k6
The theoretical rate expression (eq 4) is consistent
with our kinetic results. Comparing with the experi-
mental rate law (eq 3), the following values for the
elemental constants are obtained: k1 ) (1.66 ( 0.03) ×
10-4 s-1, k2 ) (2.5 ( 0.2) × 10-3 mol-1 L s-1, k3 ) (9 (
5) × 10-4 s-1, k4 ) (2.9 ( 1.4) × 10-2 mol-1 L s-1, k6/k-4
) (1.4 ( 0.7) × 10-5 mol L-1
.
The PPh3-sensitive pathways (Scheme 4, via k3 and
k4) are the most important in THF. For [2]0 ) 10-2 mol
L-1 and [PPh3] ) 0 at 322.6 K, reaction via k3 accounts
for 67% of the observed isomerization rate and reaction
via k4 for 21%. These pathways correspond to an
associative substitution of PPh3 from complex cis-
[PdR1IL2] (2) by an iodide ligand coordinated to either
2 or 3 (both represented by I-[Pd] in Scheme 4) to give
a mono-bridged dimer B. The ligand substitution can
(12) If the isomerization was catalyzed only by 2, a second-order
rate law would be expected, riso ) k[2]2, giving
a second-order
dependence in time, 1/[2] ) 1/[2]0 + kt. On the other hand, if the
isomerization was catalyzed only by 3, an autocatalytic law should hold,
riso ) k([2]0 - [2])[2], giving an S-shaped dependence in time, ln([2]/
([2]0 - [2])) ) kt. Neither of these simple models explains the first-
order kinetics, whereas eq 4 does.
(13) This two-terms rate law is common for substitution processes
in planar complexes, see, for example: Belluco, U.; Cattalini, L.; Basolo,
F.; Pearson, R. G.; Turco, A. J . Am. Chem. Soc. 1965, 87, 241-246.

958 Organometallics, Vol. 17, No. 5, 1998
Casado and Espinet
Sch em e 6a
∆Sqiso ) -21 ( 3 J K-1 mol-1. Under these conditions,
the rate law given in eq 4 simplifies to eq 5.
kiso ) k1 + k3 + (k2 + k4)[2]0
(5)
The composition of the associative steps (bimolecular
ligand substitutions) considered in Scheme 4 should give
negative values of the apparent ∆Sq for the overall
process.6a On the contrary, the dissociative mechanism
given in Scheme 2 should give very positive values of
∆Sq.15,16 Thus, the negative apparent value of ∆Sq
obtained is, at least, not in contradiction with the
mechanism proposed.
a
R1 ) C6Cl2F3; L ) PPh3.
two PPh3-insensitive and the two PPh3-sensitive mech-
anisms are connected.14
Con clu sion s
In summary, our results prove that the apparently
simple oxidative-addition step framed in Scheme 1 is
in fact rather complex: A cis isomer is formed first in
the addition of RI to Pd(0), which then undergoes
isomerization to the more stable trans isomer via at
least four concurrent bimolecular pathways, two auto-
catalytic and two solvent-assisted, as shown in Scheme
4.
In chlorobenzene, a much less coordinating solvent,
the isomerization rate is much lower and the contribu-
tion independent of [2]0 is also very small. In this
solvent, the contribution of a dissociative mechanism
via a three-coordinate intermediate (as shown in Scheme
2), rather than a solvent-assisted pathway, could also
be responsible for the constant contribution to kiso
(intercept in Figure 3c). In fact, with our data we
cannot completely discount the occurrence of dissocia-
tive paths as an additional general contribution to the
topomerization mechanism, even in THF. However, the
lifetime of a three-coordinate intermediate in THF
should be very short, as the coordinating solvent present
in a large excess would quickly transform it into the
THF-complex A, and consequently, its contribution
should be unimportant. Moreover, it is known that the
topomerizations of three-coordinate species are not fast
in the case of palladium.5a
The associative mechanisms proposed here are fur-
ther supported by some other experimental facts. When
the isomerization of 2 was carried out in THF saturated
with NaI (scarcely soluble), complex 3 was formed very
quickly, revealing a remarkable catalytic effect of the
iodide ion. The rate of this disfavors a mechanism based
on iodide dissociation from complex 2, similar to that
previously proposed for haloorganoplatinum(II) com-
plexes,15 and supports the double-associative ligand
substitution outlined in Scheme 6 and, indirectly, the
autocatalyzed mechanism considered in Scheme 4. The
iodide ligand, either free or complexed as I-[Pd], is
playing a similar role in both schemes, but the lower
nucleophilicity of I-[Pd] compared to that of the free
iodide should produce a lower isomerization rate, as
observed.
Exp er im en ta l Section
The reactions were carried out under N2. Solvents were
purified using standard methods. Pd(PPh3)4 and [Pd2(C6-
Cl2F3)2(µ-Cl)2(tht)2] were prepared as reported in the litera-
ture.17,18 Infrared spectra (in cm-1) were recorded on a Perkin-
Elmer FT-IR 1720 X spectrometer. Combustion analyses were
performed on a Perkin-Elmer 2400 CHN microanalyzer. 1H,
13C{1H}, 19F, and 31P{1H} NMR spectra were taken on a Bruker
ARX-300 spectrometer equipped with a VT-110 variable-
temperature probe; chemical shifts are reported in ppm from
Me4Si (1H, 13C), CCl3F (19F), or H3PO4 (31P) in CDCl3 at room
temperature.
C6Cl2F 3I (1). To a solution of [Li(C6Cl2F3)]18 (21.3 mmol)
in diethyl ether (60 mL) at -78 °C was added I2 (5.65 g, 22.2
mmol), and the mixture was stirred for 2 h while allowing the
temperature to increase slowly. The resulting colorless solu-
tion was treated with aqueous NaHCO3, washed with water
(2 × 30 mL), and dried over MgSO4. After evaporation, the
solid residue was flash chromatographed (silica/n-hexane) and
further purified by sublimation in a vacuum (75 °C, 3 mm),
yielding white needles of 1 (6.08 g, 87%): mp 42-43 °C. IR
(KBr, cm-1): 1591 (m), 1435 (vs), 1069 (vs), 786 (vs), 709 (vs).
19F NMR (CDCl3/THF): δ -91.90/-89.47 (d, J FF ) 2.5 Hz,
4
o-CF), -110.03/-108.53 (t, 4J FF ) 2.5 Hz, p-CF). 13C{1H} NMR
1
3
3
(CDCl3): δ 156.9 (ddd, J FC ) 247.4 Hz, J FC ) 7.1 Hz, J FC
)
4.3 Hz, o-CF), 155.6 (dt, 1J FC ) 252.5 Hz, 3J FC ) 4.6 Hz, p-CF),
2
2
4
106.9 (ddd, J FC ) 27.2 Hz, J FC ) 21.9 Hz, J FC ) 5.6 Hz,
CCl), 66.0 (dt, 2J FC ) 31.0 Hz, 4J FC ) 4.8 Hz, CI). Anal. Calcd
for C6Cl2F3I: C, 22.05. Found: C, 22.03. MS: m/z 326 (71)
[M+], 199 (79) [M+ - I], 164 (72), 149 (68), 127 (100), 79 (94).
cis-[P d (C6Cl2F 3)I(P P h 3)2] (2). To a suspension of Pd-
(PPh3)4 (268 mg, 0.230 mmol) in THF (11 mL) under N2 was
added 1 (280 mg, 0.860 mmol), and the mixture was stirred
at room temperature for 1 h. The solvent was evaporated, and
the solid residue was washed with diethyl ether, giving yellow
2 (yield 211 mg, 95%). IR (KBr, cm-1): 1481 (m), 1436 (vs),
1401 (vs), 1195 (s), 1145 (s), 774 (s), 752 (s), 743 (s), 693 (vs),
533 (vs), 521 (vs), 509 (vs), 494 (s). 1H NMR (CDCl3): δ 7.5-
On the other hand, apparent activation parameters
can be derived from an Eyring plot of ln(kiso/T) vs 1/T.
Since kiso does not correspond to an elemental step but
to a composite rate constant, it is strictly improper to
apply transition-state theory. For this reason, we use
the expression apparent ∆Hq and ∆Sq values. For
iso
iso
[PPh3] ) 0, these are ∆Hqiso ) 90.0 ( 1.1 kJ mol-1 and
(14) These pathways have been proposed for some L-catalyzed cis-
trans isomerizations, see: Cross, R. J . Adv. Inorg. Chem. 1989, 34,
219-292. The fact that an excess of free PPh3 does not very noticeably
catalyze the isomerization following a pentacoordinate-based mecha-
nism may be due to a higher energy barrier for the pseudorotations.
(15) The dissociation of halide has been proposed for the isomer-
ization of cis-[PtRXL2] complexes in alcoholic solvents, see: Alibrandi,
G.; Scolaro, L. M.; Romeo, R. Inorg. Chem. 1991, 30, 4007-4013 and
references therein.
(16) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J . Am. Chem.
Soc. 1989, 111, 8161-8165.
(17) Coulson, D. R. Inorg. Synth. 1972, 13, 121-123.
(18) Espinet, P.; Martinez-Ilarduya, J . M.; Pe´rez-Briso, C.; Casado,
A. L.; Alonso, M. A. J . Organomet. Chem., in press.

Oxidative Addition of RX to Pd(PPh3)4
Organometallics, Vol. 17, No. 5, 1998 959
7.3 (m, 3H, CH), 7.2-7.1 (m, 2H, CH). 19F NMR
traces of oxygen are present, easy phosphine oxidation occurs
and the defect of phosphine accelerates the isomerization very
noticeably, giving ln([2]) vs t plots with marked curvature.
OPPh3 was detected after an experimental run in the presence
of some air using 31P{1H} NMR: δ 28.4 ppm in THF at room
temperature. This phosphine oxidation interference was more
evident in THF than in chlorobenzene.
4
(CDCl3/THF/PhCl): δ -90.63/-85.87/-85.90 (dd, J FP ) 12.0
4
6
Hz, J FP′ ) 6.0 Hz, o-CF), -120.83/-118.32/-116.63 (dd, J FP
) 1.1 Hz, J FP′ ) 2.5 Hz, p-CF). 31P{1H} NMR (CDCl3/THF/
6
2
4
PhCl): δ 29.6/33.3/33.4 (ddt, J PP ) 26.9 Hz, J FP ) 5.9 Hz,
6J FP ) 2.5 Hz), 16.7/21.0/21.0 (ddt, 2J PP ) 26.9 Hz, 4J FP ) 11.9
6
Hz, J FP ) 1.2 Hz). Anal. Calcd for C42H30Cl2F3IP2Pd: C,
52.67; H, 3.16. Found: C, 52.94; H, 3.45.
tr a n s-[P d (C6Cl2F 3)Cl(P P h 3)2] (4). To a stirred suspension
of [Pd2(µ-Cl)2(C6Cl2F3)2(tht)2] (250 mg, 0.291 mmol) in acetone
(10 mL) was added PPh3 (305 mg, 1.16 mmol). After a fast
solubilization, white 4 immediately precipitated. The mixture
was stirred for 1 h, and then the acetone evaporated. The solid
residue was recrystallized from CH2Cl2/n-hexane at -28 °C,
and the resulting colorless crystals were washed with ethanol
and air-dried (yield 484 mg, 96%). IR (KBr, cm-1): 1587 (m),
1481 (s), 1435 (vs), 1402 (vs), 1096 (vs), 1048 (s), 776 (s), 743
(s), 693 (vs), 522 (vs), 512 (vs), 496 (s), 428 (s), 314 (s). 1H
NMR (CDCl3): δ 7.7-7.6 (m, 2H, CH), 7.45-7.30 (m, 3H, CH).
Ack n ow led gm en t. Financial support by the Direc-
cio´n General de Investigacio´n Cient´ıfica y Te´cnica
(Project No. PB96-0363) and the J unta de Castilla y
Leo´n (Project No. VA40/96) and a fellowship to A.L.C.
from the Ministerio de Educacio´n y Ciencia are very
gratefully acknowledged.
Ap p en d ix: Der iva tion of Kin etic Eq 4. The
steady-state concentrations of intermediates A and B,
and the isomerization rate, are given in eqs 6-10.
4
19F NMR (CDCl3): δ -91.50 (t, J FP ) 7.2 Hz, o-CF), -121.19
k3[2] + k-5[B]
6
(t, J FP ) 2.6 Hz, p-CF). 31P{1H} NMR (CDCl3): δ 24.7 (td,
[A] )
(6)
(7)
k-3[PPh3] + k5[2]0
4J FP ) 7.2 Hz, 6J FP ) 2.6 Hz). Anal. Calcd for C42H30Cl3F3P2-
Pd: C, 58.23; H, 3.49. Found: C, 58.01; H, 3.57.
k4[2][2]0 + k5[A][2]0
tr a n s-[P d (C6Cl2F 3)I(P P h 3)2] (3). To a colorless solution
of 4 (400 mg, 0.462 mmol) in acetone/CH2Cl2 (5/5 mL) was
added an excess of NaI (200 mg, 1.33 mmol). The resulting
yellow mixture was stirred at room temperature for 1 h and
evaporated to dryness. The residue was extracted in CH2Cl2
and evaporated to dryness, yielding yellow 3, which was
washed with diethyl ether and vacuum-dried (yield 376 mg,
85%). IR (KBr, cm-1): 1589 (w), 1482 (m), 1435 (vs), 1404 (vs),
1097 (s), 1048 (s), 773 (s), 742 (s), 692 (vs), 522 (vs), 512 (vs),
495 (s). 1H NMR (CDCl3): δ 7.7-7.6 (m, 2H, CH), 7.4-7.3
(m, 3H, CH). 19F NMR (CDCl3/THF/PhCl): δ -92.48/-88.17/
[B] )
k-4[PPh3] + k-5 + k6
k3k5 + k4(k5[2]0 + k-3[PPh3])
[B] )
k-3k-5[PPh3] + (k-4[PPh3] + k6)(k-3[PPh3] + k5[2]0)
[2][2]0 (8)
riso ) {k1 + k2[2]0}[2] + k6[B]
(9)
4
-87.91 (t, J FP ) 6.6 Hz, o-CF), -120.80/-117.65/-116.34 (t,
6J FP ) 2.5 Hz, p-CF). 31P{1H}NMR (CDCl3/THF): δ 22.6/26.9/
riso ) {k1 + k2[2]0}[2] +
k3k5k6 + k4k6(k5[2]0 + k-3[PPh3])
k-3k-5[PPh3] + (k-4[PPh3] + k6)(k-3[PPh3] + k5[2]0)
4
6
26.8 (td, J FP ) 7.2 Hz, J FP ) 2.5 Hz). Anal. Calcd for
C42H30Cl2F3IP2Pd: C, 52.67; H, 3.16. Found: C, 52.99; H, 3.59.
Kin etic exp er im en ts. NMR tubes (5 mm) were charged
with appropriate amounts of 2, which was dissolved under N2
at room temperature (293 K) in THF to a fixed volume of 600
( 5 µL (obtaining the concentrations shown in Table 1). An
acetone-d6 capillary was used for deuterium lock. The tube
was placed in a thermostated probe ((0.2 K, the temperature
was measured by ethylene glycol or methanol standard
methods). Concentration vs time data were then acquired
from 19F NMR signal areas of 2 and 3 and fitted to the equation
ln([2]) vs t to obtain first-order constants kiso from least-squares
slopes (standard errors are also given). A strictly oxygen-free
N2 atmosphere is required in the kinetic experiments. When
[2]
[2]0 (10)
If we consider that k-3[PPh3] is low, eq 10 can be
simplified to eq 4.
k3k6 + k4k6[2]0
kiso ) kiso[2] ) k1 + k2[2]0 +
[2] (4)
{
}
k-4[PPh3] + k6
OM9709502