Iodine oxidative addition to isomeric platinum (II) phosphine complexes

Article abstract of DOI:10.1021/om050507o

The reaction between cis and trans isomers of (Et₃P) ₂PtAr₂ (Ar = 4-FC₆H₄) (1a and 1b, respectively) with I₂ was studied in detail. At low temperatures, the clean formation of an intermediate was observed for each of the isomers. Upon warming, both intermediates provided the single thermodynamic product trans-(Et₃P)₂PtAr₂(I)₂ (2), where each of the iodo ligands is located trans to the aryl group in the equatorial plane. The reactions followed first-order kinetics with the intermediate formed from 1b converting into 2 at a faster rate than that from 1a. The conversion was significantly faster in toluene than in acetone, with the reaction in toluene giving a very large negative activation entropy (ca. -40 cal/mol· K ^-1). On the basis of the NMR analysis and kinetic data, both intermediates are proposed to be the corresponding products of the trans oxidative addition. Subsequent iodide dissociation and rearrangement of the resulting pentacoordinate Pt(IV) complex provides complex 2.

Full text of DOI:10.1021/om050507o

5654
Organometallics 2005, 24, 5654-5659
Iodine Oxidative Addition to Isomeric Platinum(II)
Phosphine Complexes
Anette Yahav, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, Tel Aviv 69978, Israel
Received June 21, 2005
The reaction between cis and trans isomers of (Et3P)2PtAr2 (Ar ) 4-FC6H4) (1a and 1b,
respectively) with I2 was studied in detail. At low temperatures, the clean formation of an
intermediate was observed for each of the isomers. Upon warming, both intermediates
provided the single thermodynamic product trans-(Et3P)2PtAr2(I)2 (2), where each of the iodo
ligands is located trans to the aryl group in the equatorial plane. The reactions followed
first-order kinetics with the intermediate formed from 1b converting into 2 at a faster rate
than that from 1a. The conversion was significantly faster in toluene than in acetone, with
the reaction in toluene giving a very large negative activation entropy (ca. -40 cal/mol‚
K-1). On the basis of the NMR analysis and kinetic data, both intermediates are proposed
to be the corresponding products of the trans oxidative addition. Subsequent iodide
dissociation and rearrangement of the resulting pentacoordinate Pt(IV) complex provides
complex 2.
Scheme 1
Introduction
Two-electron oxidative addition to a square planar d8
late transition metal center represents the key step in
a variety of catalytic processes and has been extensively
studied for several decades.1,2 It is generally accepted
that while the C-H, H-H, and X-H bonds are cleaved
in a concerted three-center fashion to give the cis-
oxidative addition product,3,4 C-X and X-X bonds (X
) halogen) are activated heterolytically in an SN2-like
process, giving the trans isomer.5 In the latter case, the
initial formation of an η1-coordinated X2 adduct was
proposed.6 Such a complex was reported in an amine-
based pincer Pt(II) system; however, it did not lead to
the oxidative addition product (Scheme 1a).7A more
electron rich Pt-aryl complex of the same pincer ligand
underwent a smooth oxidative addition reaction, giving
the formal cis-addition product (Scheme 1b). While
extensive studies on the SN2-type oxidative addition to
Pt(II) have been performed on complexes bearing the
chelating nitrogen ligands, most notably by the groups
of van Koten, Puddephatt, and Hughes,8 very little
mechanistic information regarding the corresponding
* To whom correspondence should be addressed. E-mail: avigal@
post.tau.ac.il.
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Atwood, J. D.
Inorganic and Organometallic Reaction Mechanisms, 2nd ed.; VCH
Publishers: New York, 1997. (c) Tsuji, I. Palladium Reagents and
Catalysts; John Wiley & Sons: New York, 1997.
(7) (a) van Beek, J. A. M.; van Koten, G.; Smeets, W. J. J.; Spek, A.
L. J. Am. Chem. Soc. 1986, 108, 5010-5011. (b) Gossage, R. A.; Ryabov,
A. D.; Spek, A. L.; Stufkens, D. J.; van Beek, J. A. M.; van Eldik, R.;
van Koten, G. J. Am. Chem. Soc. 1999, 121, 2488-2497. (c) An example
of a six-coordinate Pt(II) I2 complex was reported: Makiura, R.;
Nagasawa, I.; Kimura, N.; Ishimaru, S.; Kitagawa, H.; Ikeda, R. Chem.
Commun. 2001, 1642-1643.
(8) (a) van Koten, G. Pure Appl. Chem. 1990, 62, 1155-1159. (b)
van Beek, J. A. M.; van Koten, G.; Dekker, G. P. C. M.; Wissing, E.;
Zoutberg, M. C.; Stam, C. H. J. Organomet. Chem. 1990, 394, 659-
678. (c) Puddephatt, R. J. Angew. Chem., Int. Ed. 2002, 41, 261-263.
(d) Aye, K.-T.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J. D.;
Watson, A. A. Organometallics 1989, 8, 1518-1522. (e) Hughes, R. P.;
Meyer, M. A.; Tawa, M. D.; Ward, A. J.; Williamson, A.; Rheingold, A.
L.; Zakharov, L. N. Inorg. Chem. 2004, 43, 747-756. (f) Hughes, R.
P.; Sweetser, J. T.; Tawa, M. D.; Williamson, A.; Incarvito, C. D.;
Rhatigan, B.; Rheingold, A. L.; Rossi, G. Organometallics 2001, 20,
3800-3810. (g) Hughes, R. P.; Ward, A. J.; Rheingold, A. L.; Zakharov,
L. N. Can. J. Chem. 2003, 81, 1270-1279. (h) Ruiz, J.; Lopez, J. F. J.;
Rodriguez, V.; Perez, J.; de Arellano, M. C. R.; Lopez, G. J. Chem. Soc.,
Dalton Trans. 2001, 2683-2689.
(2) (a) Maitlis, P. M.; Haynes, A.; Sunley, G. J.; Howard, M. J. J.
Chem. Soc., Dalton Trans. 1996, 2187-2196. (b) Rendina, L. M.;
Puddephatt, R. J. Chem. Rev. 1997, 97, 1735-1754.
(3) (a) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514.
(b) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471-2526 (c) Johnson,
C. E.; Eisenberg, R. J. Am. Chem. Soc. 1985, 107, 3148-3160. (d)
Johnson, C. E.; Eisenberg, R. J. Am. Chem. Soc. 1985, 107, 6531-
6540. (e) Abu-Hasanayn, F.; Goldman, A. S.; Krogh-Jespersen, K. Inorg.
Chem. 1994, 33, 5122-5130.
(4) Musaev, D. G.; Morokuma, K. Adv. Chem. Phys. 1996, 95, 61-
128.
(5) (a) van Koten, G.; Terheijden, J.; van Beek, J. A. M.; Wehman-
Ooyevaar, I. C. M.; Muller, F.; Stam, C. H. Organometallics 1990, 9,
903-912. (b) Canty, A. J. Acc. Chem. Res. 1992, 25, 83-90.
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4405-4408. (b) Alsters, P. L.; Engel, P. F.; Hogerheide, M. P.; Copijn,
M.; Spek, A. L.; van Koten, G. Organometallics 1993, 12, 1831-1844.
(c) Bickelhaupt, F. M.; Baerebds, E. J.; Ravenek, W. Inorg. Chem. 1990,
29, 350-354.
10.1021/om050507o CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/12/2005

I2 Oxidative Addition to Isomeric Pt(II) Complexes
Organometallics, Vol. 24, No. 23, 2005 5655
Scheme 2
Scheme 3
The metal-carbon bond distances of 2.089(5) and 2.101-
(6) Å are within the range for the Pt-C bonds in other
Pt(IV) complexes.5a Similarly, the Pt-I bond lengths of
2.7719(5) and 2.7961(4) (s) Å compare well with those
in Pt(IV) complexes, being trans to a carbon atom.8f
phosphine complexes is available.9 Since tertiary phos-
phines are present in a large number of catalytically
active d8 metal complexes, mechanistic studies of the
oxidative addition processes in metal-phosphine com-
plexes are an important area of research.
We recently found that addition of XeF2 to the cis
phosphine complex 1a gives the Pt(IV) oxidative addi-
tion product where both phosphine atoms occupy the
axial position of an octahedron and each of the fluoro
ligands sits trans to the aryl group.10 Although the
resulting product is the thermodynamic one for the
Pt(IV) complex bearing two phosphines, halides, and
aryl groups,11 it cannot be formed in a single step. While
studying the reaction in detail, we found that the
addition of XeF2 to the trans complex 1b gave the same
(thermodynamic) Pt(IV) product (Scheme 2). Attempts
to directly investigate the mechanism of this oxidative
addition reaction were encumbered by low solubility and
high reactivity of XeF2. Thus, we were interested in
investigating a model reaction where two halogen
ligands are added to a square planar d8 metal center
bearing two monodentate phosphines and aryl groups.
Herein, we present our studies of the mechanism of I2
oxidative addition to Pt(II) phosphine complexes.
Figure 1. ORTEP structure of a molecule of 2 (50%
probability). The hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Pt-C1 2.089-
(5), Pt-C7 2.101(6), Pt-I1 2.7719(5), Pt-I2 2.7961(4),
C1-Pt-C7 94.1(2), P2-Pt-P1 177.23(5), I1-Pt-I2 83.812-
(13), C1-Pt-I2 89.39(16).
Performing the reaction between 1a and I2 in toluene
at -90 °C showed the immediate formation of the new
complex 3 (Scheme 3), which rapidly disappeared upon
warming to room temperature to cleanly produce 2
(Figure 2b,c). The NMR spectra of complex 3 demon-
strate the equivalent aryl groups (a singlet in the 19F
NMR at -119.79 ppm) as well as phosphine ligands.
The latter give rise to a singlet at a very high field
(-44.22 ppm) with a low Pt-P coupling constant of
1104.8 Hz. The NMR signals in acetone were virtually
identical to those in toluene (Table 1). The 13C NMR
spectrum of 3 reveals a large phosphine splitting for the
ipso-carbon atoms of the aryl ligands (Jobs ) 145.5 Hz),
effectively excluding the trans positioning of the phos-
phine ligands. In addition, no virtual coupling, com-
monly found in the phosphine-trans-to-phosphine sys-
tems,13 was observed for the ethyl groups, indicating the
mutual cis arrangement of the phosphines. Thus, the
NMR features of 3 could be indicative of the two
intermediate complexes, namely, the trans Pt(IV) diio-
dide (shown in Scheme 3) or η1 platinum(II) addition
complex of I2 (vide infra). A similar complex was
previously reported by van Koten and co-workers in the
amine pincer system and proposed to be an intermediate
in the I2 oxidative addition reaction to a d8 metal
center.7 While the reported Pt(II) adduct was inert in
Results and Discussion
We found that addition of a toluene solution of iodine
to a solution of 1a immediately resulted in a color
change to dark yellow and the formation of complex 2
was observed by 31P NMR spectroscopy (Scheme 3).12
The 31P NMR spectrum of 2 shows a singlet at -21.99
ppm (JPtP ) 1749.4 Hz). A singlet at -116.76 ppm was
also observed in the 19F NMR spectrum. Complex 2 was
crystallized at -30 °C from pentane, and its X-ray
structure is shown in Figure 1. The platinum atom is
located in an octahedral environment with the two
phosphine ligands occupying the apical positions. Each
of the iodo ligands is located trans to the aryl group.
(9) Formation of kinetic Pt(IV) products with phosphine ligands was
observed but not studied in detail: (a) Bennett, M. A.; Bhargava, S.
K.; Ke, M.; Willis, A. C. J. Chem. Soc., Dalton Trans. 2000, 3537-
3545. (b) Crespo, M.; Puddephatt, R. J. Organometallics 1987, 6, 2548-
2550.
(10) Yahav, A.; Goldberg, I.; Vigalok, A. Inorg. Chem. 2005, 44,
1547-1553.
(11) (a) Appleton, T. G.; Clark, H. C.; Manzer, L. E. J. Organomet.
Chem. 1974, 65, 275-287. (b) Edelbach, B. L.; Lachicotte, R. J.; Jones,
W. D. J. Am. Chem. Soc. 1998, 120, 2843-2853.
(13) Fackler, J. P., Jr.; Fetchin, J. A.; Mayhew, J.; Seidel, W. C.;
Swift, T. J.; Weeks, M. J. Am. Chem. Soc. 1969, 91, 1941-1947.
(12) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 4020-4033.

5656 Organometallics, Vol. 24, No. 23, 2005
Yahav et al.
Figure 3. Eyring plot of conversion of 3 to 2 in acetone
([) and toluene (4).
significant rate acceleration in a nonpolar solvent versus
acetone was reported for the isomerization of an Ir(III)
dihydride phosphine complex.3c The efficient solvation
by acetone might stabilize 3 and deactivate it toward
further transformations. The formation of a six-coordi-
nate metal complex might also account for this effect.7c
It is plausible that acetone stabilizes the unobserved
cationic intermediate, slowing the cis-trans rearrange-
ment reaction (vide infra).
The observation of cis complex 3 alone, however, does
not explain the mutual trans orientation of the phos-
phine ligands in the final product. The formation of 2
must be preceded by ligand dissociation from 3 or
another, unobserved intermediate. Addition of free Et3P
to a solution of 3 in toluene at -90 °C resulted in the
immediate precipitation of the Et3P(I)n species and
regeneration of complex 1a, ruling out the possibility
of phosphine ligand dissociation under the reaction
conditions. Attempts to follow the reaction by UV-vis
spectroscopy did not provide new mechanistic informa-
tion. A new shoulder at 444 nm was observed in the
UV-vis spectrum upon addition of I2 to 1a at low
temperatures, this shoulder rapidly disappearing upon
conversion to 2. The latter shows signals at 355 and 327
nm. Similar inconclusive results were observed for the
previously reported Pt-I2 complexes.7b Little evidence
in support of the intermediate structures could be
obtained from the 195Pt NMR measurements. While the
Pt(II) complex 1a shows a triplet at -4551.29 ppm in
its 195Pt NMR spectrum, the Pt(IV) complex 2 exhibits
a triplet at -3555.03 ppm. Complex 3, on the other
hand, gives a signal at -4333.70 ppm, making it diffi-
cult to unequivocally assign the structure of 3. The
correlation between the complex structure and position
of its 195Pt NMR signal should be treated with caution,
as it can vary dramatically with the ligand environ-
ment.15
As complex 2 appears to be the thermodynamic
product for (Et3P)2Pt(Ar)2I2, we were also interested to
investigate its formation in the reaction of the trans
isomer of 1a with I2. Addition of I2 to a solution of the
trans complex 1b at -90 °C showed no formation of 3
(Scheme 4). Instead, the formation of the new complex
4 was observed. The same product was formed even at
-110 °C in Et2O. Complex 4 rapidly converted to 2 even
at -30 °C. Complex 4 shows the 31P NMR signal in the
area expected for the Pt(IV) diiodo complex (-28.33
ppm) with a JPtP value of 1667.3 Hz, close to that in 2.
The 195Pt NMR spectrum of 4 showed a triplet at
-4524.80 ppm, compared with -4422.63 for the starting
Figure 2. 31P NMR spectra of complexes 1a (a) and 3 (b)
and mixture of 3 and 2 at ca. 50% conversion (c).
Table 1. Selected NMR Data for Complexes 1-4 in
Different Solvents
complex
δ 31P{1H}
JPtP
1798.9d
δ 195Pt
-4551.29d
-4546.70c
-4422.63a
δ 19F{1H}
-120.91d
1a
3.18d
3.34c
1b
9.36a
2790.4a
2786.3b
2781.3c
1104.8a
1114.3c
1667.3a
1658.8c
1749.4a
1764.7b
1759.8c
1763.3d
-121.79a
9.44b
10.68c
-120.79c
-119.79a
-119.16c
-118.52a
3
4
2
-44.22a
-43.96c
-28.33a
-28.45c
-21.99a
-22.24b
-20.59c
-21.23d
-4333.70a
-4524.8a
-3555.03a
-116.76a
-116.18b
-116.36d
a In toluene. b In benzene. c In acetone. d In CDCl3.
the actual oxidative addition step, probably as a result
of the rigid square planar structure of the tridentate
pincer ligand and low electron density on the metal, the
more electron rich aryl complex gave the cis oxidative
addition product.7,11 In our case, the observed interme-
diate underwent smooth conversion to the oxidative
addition product upon warming to room temperature.
Kinetic analysis of the conversion of 3 to 2 at different
temperatures in toluene, obtained using 31P NMR
spectrometry, gave the activation parameter values of
9.97 kcal/mol and -40.5 cal/mol‚K-1 for ∆Hq and ∆Sq,
respectively (Table 2). The highly negative value of the
activation entropy was previously observed in the C-X
bond activation as an indication of the highly ordered
SN2-type process.8d The conversion followed first-order
kinetics and no intermediates were observed. Interest-
ingly, when the same reaction was performed in acetone,
the conversion of 3 to 2 occurred at a much slower rate.
∆Hq of 23.25 kcal/mol and ∆Sq of 4.0 cal/mol‚K-1 were
obtained in this case (Figure 3). The solvent effect on
multistep reactions, such as an oxidative addition-
rearrangement sequence, is not always straightforward
to rationalize. For example, oxidative addition to an
Ir(I) chelate was assisted by polar solvents,14 while a
(14) Imhoff, P.; Gulpen, J. H.; Vrieze, K.; Smeets, W. J. J.; Spek, A.
L.; Elsevier, C. J. Inorg. Chim. Acta 1995, 235, 77-88.
(15) (a) Pregosin, P. S. Coord. Chem. Rev. 1982, 44, 247-291. (b)
Hope, E. G.; Levason, W.; Powell, N. A. Inorg. Chim. Acta 1986, 115,
187-192.

I2 Oxidative Addition to Isomeric Pt(II) Complexes
Organometallics, Vol. 24, No. 23, 2005 5657
Table 2. Observed Kinetic Parameters for Conversion of Complexes 3 and 4
complex
solvent
k263, (s-1
)
∆Gq (kcal/mol)
∆Sq (cal/mol‚K-1
)
∆Hq (kcal/mol)
3
toluene
acetone
acetone
(4 ( 1) × 10-5
20.63 ( 0.4
22.19 ( 0.05
19.79 ( 0.1
-40.5 ( 2.3
+4.0 ( 1.2
-4.8 ( 0.4
9.97 ( 1.0
23.25 ( 0.5
18.52 ( 0.2
(2 ( 0.02) × 10-6
(2 ( 0.1) × 10-4
4
Scheme 4
ture, rapid isomerization to 1b was observed, akin to
that reported for the I--promoted cis-to-trans isomer-
ization of Pt(II) complexes.16 The complex behavior of
the I- anion in organometallic chemistry is well docu-
mented.17
Although the proposed structures of 3 and 4 cannot
be unambiguously confirmed without X-ray analysis, the
31P NMR features suggest that 3 corresponds to the
Pt(IV) product of the trans addition of I2 to 1. A large
chemical shift difference from 1 and significantly lower
JPtP are indicative of the Pt(IV) complex.9,18 Due to
nearly identical 31P NMR signals of 3 in toluene and
acetone, the cationic Pt(IV) structure is highly unlikely.9b
Similar to the reaction with 1a, the addition of I2 to
(dmpe)PtAr2 gave the trans addition product, which
showed a greater than 45 ppm upfield shift in the 31P
NMR spectrum (-25.85 ppm, versus 21.46 ppm for
(dmpe)PtAr2) and a low Pt-P coupling constant of
1195.8 Hz versus 1709.3 in (dmpe)PtAr2. The trans
addition structure assigned to 3 also explains the
dramatic difference in the reaction rates in toluene and
acetone. Since no phosphine dissociation occurs, the
formation of 2 must be preceded by the dissociation of
the iodide ligand. If this step is faster than the rear-
rangement of the resulting pentacoordinate intermedi-
ate, the latter would be stabilized by a coordinating
solvent, such as acetone. In toluene, the iodide dissocia-
tion is unfavorable and most probably produces a highly
unstable tight ion pair. The reorganization of this
intermediate, via the pseudorotation or turnstile routes,19
accounts for the large negative activation entropy. In
the trans complex 4, the phosphine ligands are already
located in the “right” position and only a minor rotation
is required. Together with the lower thermodynamic
stability of 4, this results in a significantly faster
conversion to 2.
1b. The 13C NMR spectrum showed a virtual triplet for
the CH2 group of the Et3P ligands, indicating their
mutual trans arrangement. Also, no large JPC coupling
was observed for the aromatic ipso-carbon, showing the
cis positioning of the aryl group versus the phosphine
ligands. The combined NMR features suggested the all-
trans structure for complex 4. As in the case of 3, no
significant changes in the NMR spectra of 4 were
observed upon performing the measurements in acetone
instead of toluene. Kinetic investigation of the isomer-
ization of 4 to 2 in acetone using 31P NMR spectroscopy
(Figure 4b,c) resulted in the activation enthalpy and
The combined data provide some quantitative infor-
mation about the proposed mechanism for the I2 oxida-
tive addition to the d8 square planar Pt(II) phosphine
complexes (Scheme 5). While not detected in this work,
the formation of the pentacoordinate η1 iodine adduct
might take place in the SN2-type mechanism. The trans
oxidative addition step produces the intermediate com-
plex where the two iodo ligands are located at the axial
positions of an octahedron, with the equatorial ligands
retaining the configuration of the parent square planar
complex. If such a product is not the thermodynamically
Figure 4. 31P NMR spectra of complexes 1b (a) and 4 (b)
and mixture of 4 and 2 at ca. 50% conversion (c).
entropy of 18.52 kcal/mol and -4.8 cal/mol‚K-1, respec-
tively (Table 2). The activation barrier for this rear-
rangement reaction is ca. 2.5 kcal/mol lower than that
observed in 3. Thus, it appears that the oxidative
addition of I2 to the geometric isomers 1a and 1b does
not proceed via a common intermediate.
Finally, we tried to investigate the effect of the added
I- on the reaction rates. Unfortunately, the results were
inconclusive. Addition of 1-10 equiv of Bu4N+I- to a
solution of 3 in acetone significantly accelerated the
reaction rate. However, other unstable products were
also observed along with the starting material. Similar
results were also obtained upon addition of Bu4N+I- to
a solution of 4 in acetone at -50 °C. Large amounts of
the starting Pt(II) complex 1b were observed almost
immediately after the addition along with 2 and other
products. This reactivity is attributed to aggregation
between I- and I2. When a preformed 1:1 mixture of
Bu4N+I- and I2 was reacted with 1a at room tempera-
(16) Mueller, W. D.; Brunea, H. A. Chem. Ber. 1985, 118, 4347-
4355.
(17) (a) Maitlis, P. M.; Haynes, A.; James, B. R.; Catellani, M.;
Chiusoli, G. P. Dalton Trans. 2004, 3409-3419. (b) Svensson P. H.;
Kloo, L. Chem. Rev. 2003, 103, 1649-1684.
(18) There is very limited and inconclusive data on changes in JPtP
upon coordination of the fifth ligand to square planar Pt(II) phosphine
complexes: (a) Bruggeller, P. Inorg. Chem. 1987, 26, 4125-4127. (b)
MacDougall, J. J.; Nelson, J. H.; Mathey, F. Inorg. Chem. 1982, 21,
2145-2153. (c) Holt, M. S.; Nelson, J. H.; Alcock, N. W. Inorg. Chem.
1986, 25, 2288-2295. (d) Salem, G.; Wild, S. B. Inorg. Chem. 1992,
31, 581-586.
(19) (a) Springer, C. S., Jr. J. Am. Chem. Soc. 1973, 95, 1459-1467.
(b) Ugi, I.; Marquarding, D.; Klusacek, H.; Gillespie, P.; Ramirez, F.
Acc. Chem. Res. 1971, 4, 288-296.

5658 Organometallics, Vol. 24, No. 23, 2005
Yahav et al.
Scheme 5
minutes, the 31P{1H} NMR spectrum showed the disappear-
ance of the starting material and formation of 2. Evaporation
of the solvent gave 2 in a quantitative yield. A precise amount
of I2 must be used to avoid the formation of side products and
a tedious cleaning process.
stable one, it undergoes isomerization to give the most
stable isomer. The isomerization most likely proceeds
via the pseudorotation pathway preceded by the dis-
sociation of I- from the metal center. Coordinating
solvents, such as acetone, stabilize the cationic inter-
mediate and slow the rearrangement process.
Low-Temperature Experiments. Freshly prepared tolu-
ene or acetone solutions (0.032 mmol) of 1a (or 1b) were cooled
to -90 °C in an NMR tube using an ethanol/liquid N2 bath. A
cold I2 solution (1 equiv) was then added at once, the mixture
was rapidly shaken to ensure complete mixing, and the tube
was placed into the NMR spectrometer precooled to the desired
temperature.
Experimental Part
General Procedures. All operations with air- and moisture-
sensitive compounds were performed in a nitrogen-filled
Innovative Technology glovebox. All solvents were degassed
and stored under high-purity nitrogen over activated 4 Å
molecular sieves. All deuterated solvents were stored under
high-purity nitrogen on 3 Å molecular sieves. Commercially
available reagents were used as received. The NMR spectra
were recorded on Bruker AC 200 MHz and Bruker AMX 400
2 (CDCl3, 23 °C): 31P{1H} -21.23 (s, JPtP ) 1763.3 Hz); 19
F
{1H} -116.36 (s, Ar-F); 1H 0.99 (m, CH3, 18H), 2.24 (m, CH2,
12H), 6.70 (br t, J ) 8.5 Hz, Ar-H, 4H), 7.12-7.72 (br m,
Ar-H, 4H); 13C{1H} 10.15 (s, CH3), 19.20 (t, JPC ) 17.6 Hz,
CH2), 113.94 (br m, JPC) 18.2 Hz, Ar), 128.68 (br m, Ar-Pt),
141.10 (br s, Ar), 161.34 (d, C-F, JF-C ) 245.4 Hz); 195Pt NMR
(C6D6) -3555.03 (t). Anal. for C24H38F2I2P2Pt: found (calc): C
32.11 (32.93), H 4.35 (4.38). FAB-MS M-I: found (calc) 748
(748).
1
MHz spectrometers. H and 13C NMR signals are reported in
ppm downfield from TMS. 1H signals are referenced to the
residual proton of a deuterated solvent (7.26 ppm for CDCl3,
7.15 ppm for C6D6, 2.11 ppm for toluene-d8, 2.09 ppm for
acetone-d6). For 13C NMR spectra, the following signals were
used as a reference: 77.36 ppm for CDCl3, 128.62 ppm for
C6D6, 21.10 ppm for toluene-d8, and 30.60 for acetone-d6. 31P
chemical shifts are reported in ppm downfield from H3PO3 and
referenced to an external 85% phosphoric acid sample. 19F
chemical shifts are reported in ppm downfield from CClF3.
195Pt chemical shifts are reported in ppm downfield from
external K2PtCl6. UV-vis measurements were recorded on a
Scinco s-1300. Elemental analysis was performed in the
laboratory for microanalysis at the Hebrew University of
Jerusalem. The platinum complexes cis- and trans-(PEt3)2Pt-
(p-FC6H4)2 (1a,b) were prepared by ligand exchange with the
corresponding (1,5-cyclooctadiene)PtAr2 and (PEt3)2PtI2, re-
spectively, as previously reported.12,20
3 (acetone-d6, -50 °C): 31P{1H} -43.96 (s, JPtP ) 1114.3 Hz)
(at the same temperature in toluene the signal appears
at -44.22 with JPt-P ) 1104.8 Hz); 19F{1H} -119.16 (s,
Ar-F); 1H NMR 1.09 (m, CH3, 18H), 2.30 (m, CH2, 12H), 6.66
(t, J ) 8.3 Hz, Ar-H, 4H), 7.92 (m, Ar-H, 4H); 13C{1H} 10.98
(s, CH3), 20.37 (m, CH2), 114.14 (m, Ar), 132.52 (dd, Ar-Pt,
JPcisC ) 5.8 Hz, JPtransC ) 145.5 Hz), 147.28 (br s, Ar), 161.67
(d, C-F, JF-C ) 240.2 Hz); 195Pt NMR (toluene, -50 °C)
-4333.70 (t).
4 (toluene-d8, -50 °C): 31P{1H} -28.33 (s, JPtP ) 1667.3 Hz)
(at the same temperature in acetone the signal appears at
-28.45 with JPtP ) 1658.8 Hz); 19F{1H} -118.52 (s, Ar-F); 1H
0.54 (m, CH3, 18H), 1.94 (br s, CH2, 12H), 6.89 (br t, J ) 8.5
Hz, Ar-H, 4H), 8.79 (br m, Ar-H, 4H); 13C{1H} 10.24 (br s,
CH3), 19.93 (br t, JPC ) 17.5 Hz, CH2), 115.14 (m, JPC ) 17.0
Hz, Ar), 148.94 (br s, JPtC ) 160.2 Hz, Ar-Pt), 162.08 (d, C-F,
JF-C ) 242.5 Hz), 130.49 (overlapped); 195Pt NMR (acetone,
-50 °C) -4524.80 (t).
Kinetic Experiments. In a typical experiment, a freshly
prepared solution of 1a or 1b, in an appropriate solvent, was
cooled to -90 °C in an NMR tube using an ethanol/liquid N2
bath. A cold I2 solution (1 equiv) was then added at once, the
Synthesis of 2. In a typical procedure, a freshly prepared
solution of I2 (1 mL of 0.018 M) in an appropriate solvent was
added dropwise (while stirring) to a solution of 1a or 1b (0.018
mmol) in the same solvent (1 mL) at 23 °C. After several
(20) (a) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59,
411. (b) Eaborn, C.; Odell, K. J.; Pidcock, A. J. Chem. Soc., Dalton
Trans. 1978, 357.

I2 Oxidative Addition to Isomeric Pt(II) Complexes
Organometallics, Vol. 24, No. 23, 2005 5659
mixture was rapidly shaken to ensure complete mixing, and
the tube was placed into the NMR spectrometer precooled to
-10 °C. The reaction progress was followed by 31P NMR
spectrometry. In each spectrum 120 scans were taken (2 min),
with the time intervals between the spectra being 20 min. At
higher temperatures, shorter time intervals and a smaller
number of scans were used. The maximal monitored conver-
sion was 45%. For the kinetic analysis, each measurement was
repeated at least twice.
Acknowledgment. This work was supported by the
Germany-Israel Foundation (GIF) Young Investigator
Award and Yigal Alon fellowship to A.V.
Supporting Information Available: Crystallographic
data for 2 (CIF), 195Pt NMR spectra for complexes 1-4, and
1H, 31P, and 19F spectra for complex 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM050507O