Unexpected selectivities in C-H activations of toluene and p-xylene at cationic platinum(II) diimine complexes. New mechanistic insight into product-determining factors

Article abstract of DOI:10.1021/ja010277e

The C-H activation of toluene and p-xylene at cationic Pt^II diimine complexes (N-N)Pt(CH₃)(H₂O)⁺BF₄^- (N-N = Ar-N=CMe-CMe=N-Ar; 1(BF₄^-), N^f-N^f, Ar = 3,5-(CF₃)₂C₆H₃); 2(BF₄^-), N′N′, Ar = 2,6-(CH₃)₂C₆H₃) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Ptbenzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe)⁺ > (N-N)Pt(p-tolyl)(NCMe)⁺ > (N-N)Pt(o-tolyl)(NCMe)⁺ for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N′-N′)Pt(benzyl)(NCMe)⁺ were concurrently formed in the C-H activation of toluene at 2. The C-H activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (N-N)Pt(xylyl)(NCMe)⁺ and (N-N)Pt(p- methylbenzyl)(NCMe)⁺ were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (N^f-N^f)Pt(Tol)₂ species (3a-c; a, Tol= o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF₄ was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (N^f-N^f)Pt(mtolyl)(NCMe)⁺ (4b) and (N^f-N^f)Pt(p-tolyl)(NCMe)⁺ (4c) from all three isomeric bis(tolyl) complexes 3a-c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl)(CH₄)⁺ (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition -state structures for the solvent-induced associative elimination of methane from these intermediates.

Full text of DOI:10.1021/ja010277e

J. Am. Chem. Soc. 2001, 123, 6579-6590
6579
Unexpected Selectivities in C-H Activations of Toluene and
p-Xylene at Cationic Platinum(II) Diimine Complexes. New
Mechanistic Insight into Product-Determining Factors
Lars Johansson,1a,b Olav B. Ryan,1c Christian Rømming,1a and Mats Tilset*,1a,d
Contribution from the Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern,
N-0315 Oslo, Norway, and Department of Hydrocarbon Process Chemistry, SINTEF Applied Chemistry,
P.O. Box 124 Blindern, N-0314 Oslo, Norway
ReceiVed January 31, 2001
Abstract: The CsH activation of toluene and p-xylene at cationic PtII diimine complexes (NsN)Pt(CH3)-
-
(H2O)+BF4 (NsN ) ArsNdCMesCMedNsAr; 1(BF4-), NfsNf, Ar ) 3,5-(CF3)2C6H3); 2(BF4-), N′s
N′, Ar ) 2,6-(CH3)2C6H3) has been investigated. The reactions were performed at ambient temperature in
2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-
benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts.
In the reactions with toluene, the relative amounts of products resulting from aromatic CsH activation were
found to decrease in the order (NsN)Pt(m-tolyl)(NCMe)+ > (NsN)Pt(p-tolyl)(NCMe)+ > (NsN)Pt(o-tolyl)-
(NCMe)+ for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product
(N′sN′)Pt(benzyl)(NCMe)+ were concurrently formed in the CsH activation of toluene at 2. The CsH
activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios
of (NsN)Pt(xylyl)(NCMe)+ and (NsN)Pt(p-methylbenzyl)(NCMe)+ were found to be 90:10 and 7:93 for
reactions at 1 and 2, respectively. The elimination of toluene from (NfsNf)Pt(Tol)2 species (3a-c; a, Tol )
o-tolyl; b, Tol ) m-tolyl; c, Tol ) p-tolyl) after protonolysis with 1 equiv of HBF4 was investigated. Most
notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (NfsNf)Pt(m-
tolyl)(NCMe)+ (4b) and (NfsNf)Pt(p-tolyl)(NCMe)+ (4c) from all three isomeric bis(tolyl) complexes 3a-c.
The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This
behavior is explained by an associative mechanism for the product-determining displacement of toluene by
the solvent. For the CsH activation reactions, our findings suggest the existence of a dynamic equilibrium
between the isomeric intermediates (NsN)Pt(aryl)(CH4)+ (aryl ) tolyl/benzyl from 1; xylyl/p-methylbenzyl
from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate
transition-state structures for the solvent-induced associative elimination of methane from these intermediates.
Introduction
Scheme 1
The development of practical methods for mild and selective
functionalization of unactivated CsH bonds in hydrocarbons
remains a challenge to chemists, motivated by important
potential applications in natural gas conversion as well as
organic synthesis.2 During the past decades, important contribu-
tions toward this goal have been demonstrated in the field of
homogeneous organometallic chemistry. In particular, ap-
proaches based on the remarkable “Shilov system”,3 discovered
almost 30 years ago, hold promise for future advances.4 In the
Shilov system, simple alkanes are catalytically converted to
mixtures of alcohols and chloroalkanes in aqueous solutions of
PtII and PtIV salts. The mechanism for this reaction has been
the subject of extensive investigations, and the three primary
steps shown in Scheme 1 have been proposed: (i) CsH
activation at PtII, (ii) oxidation by PtIV, and (iii) nucleophilic
attack to yield the products and regeneration of the catalyst.5 It
has been considered of particular interest to elucidate the nature
of the activation process (i) in the cycle, since this step is
believed to dictate both the rate and the unusual chemo- and
regioselectivity in the reaction. Insight into the mechanism has
been gained through studies on stoichiometric model reactions
between cationic PtII complexes and hydrocarbons,6 as well as
the reverse reactionselimination of alkanes from PtIV hydri-
doalkyl species.7 The CsH activation mechanism at PtII has
also been addressed in theoretical investigations.8
(1) (a) University of Oslo. (b) Current address: Department of Organic
Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Sweden.
(c) SINTEF. (d) E-mail: mats.tilset@kjemi.uio.no.
(2) (a) Crabtree, R. H. Chem. ReV. 1995, 95, 987. (b) Arndtsen, B. A.;
Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28,
154. (c) Shilov, A. E.; Shulpin, G. B. Chem. ReV. 1997, 97, 2879. (d) Stahl,
S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180.
(e) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer:
Boston, 2000.
(3) Goldshlegger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A.
Zh. Fiz. Khim. 1972, 46, 1353. See also ref 2e.
(4) (a) Sen, A. Acc. Chem. Res. 1998, 31, 550. (b) Periana, R. A.; Taube,
J. D.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560.
10.1021/ja010277e CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/19/2001

6580 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Johansson et al.
Scheme 2
via exchange reactions with solvent and water (e-f) to afford
the corresponding PtII cations (NsN)Pt(R)(H2O)+. In addition,
we have observed that reversible deprotonation of the putative
(NsN)PtIV(CH3)(R)(H)+ intermediate takes place in the pres-
ence of considerable amounts of water (g).11 These reactions
are believed to be reasonable models for the activation in the
Shilov system.
A key feature in the Shilov-type systems is the unprecedented
regio- and chemoselectivity. Thus, contrary to the selectivity
in radical and superacid reactions, the reactivity order 1° CsH
> 2° > 3° is generally observed for alkanes.12 Furthermore,
from studies on the oxidation of ethane and ethanol in the
aqueous PtII/PtIV system, the relative rate of CsH bond
activation was found to decrease in the order HsCH2CH3 >
HsCH2CH2OH > HsCH(OH)CH3,5c,13 contrary to the trend
expected from the homolytic bond strengths. The CsH activa-
tion step has been proposed to govern the unusual selectivity
in the Shilov system. However, since the detailed mechanism
for this process has been unclear, definite explanations for the
observed selectivities have not been given.
A specific regioselectivity issue in CsH activation processes
is related to reactions with alkylarenes in which competition
between different aromatic and benzylic CsH bonds is possible.
Results from studies on such reactions have been reported for
some homogeneous organometallic systems. Jones and Feher
observed a statistical 2:1 ratio of Cp*(PMe3)Rh(m-Tol)(H) and
Cp*(PMe3)Rh(p-Tol)(H) as the only products from photolysis
of Cp*(PMe3)RhH2 in toluene under conditions of thermody-
namic product control.10,14 When the reaction was performed
at low temperature, small amounts of Cp*(PMe3)Rh(o-Tol)(H)
and traces of Cp*(PMe3)Rh(Bz)(H) were also detected. The
kinetic meta/para selectivity in this system was explained in
terms of arene π-precoordination and steric effects. Whitesides
and co-workers reported that thermal decomposition of trans-
(PMe3)2Pt(Np)(OTf) in toluene at 133 °C followed by treatment
with chloride yielded a statistical 2:1 ratio of trans-(PMe3)2Pt-
(m-Tol)(Cl) and trans-(PMe3)2Pt(p-Tol)(Cl).15 A kinetic prefer-
ence for aromatic CsH activation was found when (Ps
P)Pt(Np)(H) (PsP ) bis(dicyclohexylphosphino)ethane) was
heated at 69 °C in mesitylene.16 Prolonged heating resulted in
a slight excess of the benzylic activation product (PsP)Pt(3,5-
Me2Bz)(H) relative to the aromatic product (PsP)Pt(Mes)(H)
(54:46), which was proposed to reflect the thermodynamic
product distribution. The photolysis of Tp′Rh(CNsNp)(η-PhNd
CdNsNp) in toluene at 25 °C was reported by Jones and
Hessell to produce a mixture of Tp′Rh(R)(H)(CNsNp) species
with R ) m-tolyl, p-tolyl, and benzyl in a 2:4:3 ratio.17 Heating
of this mixture at 100 °C for 12 h resulted in complete
conversion to a statistical 2:1 ratio of the m-tolyl and p-tolyl
We have recently reported the development of PtII diimine
(diimine ) NsN ) ArsNdCMesCMedNsAr)-based sys-
tems that activate methane and benzene CsH bonds.9,10 The
aqua complex (NfsNf)Pt(CH3)(H2O)+BF4- (1(BF4-); Ar ) 3,5-
(CF3)2C6H3) was demonstrated to react with benzene and
methane in the hydroxylic solvent 2,2,2-trifluoroethanol (TFE)
under unusually mild conditions.9a,b The benzene CsH activa-
tion at the closely related complex (N′sN′)Pt(CH3)(H2O)+BF4
-
(2(BF4-); Ar ) 2,6-(CH3)2C6H3) was investigated in detail.9c
Kinetic studies, labeling experiments, and low-temperature NMR
characterization of possible intermediates in combination with
theoretical calculations9b have led to the proposal of a common
mechanism for these reactions (Scheme 2). A preequilibrium
between the aqua and solvento complexes (NsN)Pt(CH3)-
(H2O)+ and (NsN)Pt(CH3)(TFE)+ (a) is followed by rate-
determining coordination of the hydrocarbon RsH (b) to give
a hydrocarbon adduct (NsN)Pt(CH3)(RsH)+. These exchange
processes appear to proceed via associative mechanisms.9c,d The
hydrocarbon adduct rapidly interconverts via a transient PtIV
hydride intermediate (NsN)Pt(CH3)(R)(H)+ (c) with an η2-C,H-
methane adduct (NsN)Pt(R)(CH4)+ (d). The reaction is com-
pleted by irreversible methane loss from (NsN)Pt(R)(CH4)+
(5) (a) Kushch, L. A.; Lavrushko, V. V.; Misharin, Y. S.; Moravsky, A.
P.; Shilov, A. E. NouV. J. Chim. 1983, 7, 729. (b) Horvath, I. T.; Cook, R.
A.; Millar, J. M.; Kiss, G. Organometallics 1993, 12, 8. (c) Hutson, A. C.;
Lin, M.; Basickes, N.; Sen, A. J. Organomet. Chem. 1995, 504, 69. (d)
Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.; Bercaw, J.
E.; Horvath, I. T.; Eller, K. Organometallics 1993, 12, 895. (e) Luinstra,
G. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1993, 115, 3004.
(f) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E.
Organometallics 1994, 13, 755. (g) Luinstra, G. A.; Wang, L.; Stahl, S. S.;
Labinger, J. A.; Bercaw, J. E. J. Organomet. Chem. 1995, 504, 75.
(6) (a) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1997, 119, 848. (b) Holtcamp, M. W.; Henling, L. M.; Day, M. W.;
Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467. (c) Wick,
D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235.
(7) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1995, 117, 9371. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 1996, 118, 5961. (c) Fekl, U.; Zahl, A.; van Eldik, R.
Organometallics 1999, 18, 4156. (d) Hill, G. S.; Rendina, L. M.; Puddephatt,
R. J. Organometallics 1995, 14, 4966. (e) Jenkins, H. A.; Yap, G. P. A.;
Puddephatt, R. J. Organometallics 1997, 16, 1946.
(11) In a reaction between (NfsNf)PtII(CH3)(H2O)+ (1) and C6H6 in a
4.0 M solution of D2O in TFE-d3, the formation of (NfsNf)PtII(C6H5)-
(H2O)+ and CH4, CH3D, CH2D2, and traces of CHD3 was observed by 1H
NMR spectroscopy (the analogous experiment in pure TFE-d3 gives only
CH4). This experiment demonstrates that reversible deprotonation of the
putative (NfsNf)PtIV(CH3)(R)(H)+ intermediate is readily induced by
addition of water to the reaction medium, presumably due to the higher
basicity of water compared to TFE. Importantly, this finding strengthens
the relevance of the PtII diimine CsH activation reactions with respect to
the Shilov system. Tilset, M.; Johansson L. Unpublished results.
(12) (a) Hodges, R. J.; Webster, D. E.; Wells, P. B. J. Chem. Soc., Chem.
Commun. 1971, 462. (b) Goldshlegger, N. F.; Lavrushko, V. V.; Khrush,
A. P.; Shteinman, A. A. IzV. Akad. Nauk., Ser. Khim. 1976, 10, 2174.
(13) Sen, A.; Benvenuto, M. A.; Lin, M.; Hutson, A. C.; Basickes, N. J.
Am. Chem. Soc. 1994, 116, 998.
(8) (a) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996,
118, 4442. (b) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17,
1478. (c) Heiberg, H.; Swang, O.; Ryan, O. B.; Gropen, O. J. Phys. Chem.
A 1999, 103, 10004. (d) Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J.
Am. Chem. Soc. 1999, 121, 4633. (e) Mylvaganam, K.; Bacskay, G. B.;
Hush, N. S. J. Am. Chem. Soc. 2000, 122, 2041. (f) Bartlett, K. L.; Goldberg,
K. I.; Borden, W. T. J. Am. Chem. Soc. 2000, 122, 1456.
(9) (a) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999,
121, 1974. (b) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang
O.; Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831. (c) Johansson, L.; Tilset,
M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846. (d)
Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739.
(10) Abbreviations: Cp* ) η5-C5Me5; Tp′ ) hydridotris(3,5-dimeth-
ylpyrazol)borate; Np ) neopentyl; Tol ) tolyl; Bz ) benzyl; Xyl ) xylyl;
Mes ) mesityl; OTf- ) triflate; NfsNf ) ArNdCMeCMedNAr, Ar )
3,5-(CF3)2C6H3; N′sN′ ) ArNdCMeCMedNAr, Ar ) 2,6-(CH3)2C6H3;
TFE ) trifluoroethanol.
(14) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650.
(15) Brainard, R. L.; Nutt, R.; Lee, R.; Whitesides, G. M. Organome-
tallics 1988, 7, 2379.
(16) Hackett, M.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 1449.
(17) Jones, W. D.; Hessell, E. T. J. Am. Chem. Soc. 1993, 115, 554.

C-H ActiVation of Toluene and p-Xylene at PtII Diimines
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6581
Scheme 3
complexes. The authors remarked that this system exhibits an
unusually large kinetic preference for benzylic activation.
Bergman and Burger reported that reaction between Cp*(PMe3)-
Ir(CH3)(OTf) and toluene resulted in a 62:38 ratio of Cp*-
(PMe3)Ir(m-Tol)(OTf) and Cp*(PMe3)Ir(p-Tol)(OTf) as the only
products, i.e., with a somewhat less than statistical amount of
the meta isomer.18 The absence of o-tolyl product was explained
by steric effects. CsH activation of toluene has also been
studied in reactions with the PtIV salt H2PtCl6 in acidic media
at 50-90 °C.19 The products were trapped as the anionic species
trans-PtCl4(NH3)(Tol)-, and at short reaction times the p-tolyl
isomer was obtained as the dominant product. However,
prolonged heating gave the statistical 2:1 ratio of the m-tolyl
and p-tolyl complexes as the only observed products. Legzdins
and co-workers20 recently reported that thermal toluene CsH
activation at Cp*W(NO)(CH2CMe3)2 produced Cp*W(NO)-
(CH2CMe3)(Tol) in a 1:47:33 o:m:p ratio. The immediate
product of benzylic CsH activation, Cp*W(NO)(CH2CMe3)(CH2-
Ph), was observed as an intermediate that reacted further to give
other species that were also results of benzylic activation.
Significantly, whereas prolonged heating of the o-tolyl or m-tolyl
products separately resulted in isomerization to the 1:47:33
mixture of tolyl isomers, there was no interconversion of
products of aromatic CsH activation into those of benzylic
activation (or vice versa).
of (NfsNf)Pt(Tol)2 (3a-c) and (NfsNf)Pt(Tol)(NCMe)+ (4a-
c) complexes (Tol ) o-tolyl, a; m-tolyl, b; p-tolyl, c) was
accomplished by the reactions outlined in Scheme 3. Since these
species are new, their preparation and characterization will be
described in some detail. Starting from Pt(SMe2)2Cl2,21 the cis-
Pt(SMe2)2(Tol)2 species were synthesized by treatment with the
appropriate tolyllithium reagent by methods similar to those
described in the literature.22a Subsequent reaction with the
diimine ligand NfsNf gave the corresponding (NfsNf)Pt(Tol)2
complexes 3a-c in excellent yields. For the m- and p-tolyl
compounds 3b and 3c, protonation with 1 equiv of HBF4‚Et2O
-
in acetonitrile led to the formation of the BF4 salts of the
corresponding species (NfsNf)Pt(m-Tol)(NCMe)+ (4b) and
(NfsNf)Pt(p-Tol)(NCMe)+ (4c). In the latter reaction, small
amounts of 4b were observed as a byproduct (ca. 4% by 19F
NMR spectroscopy). Analytically and spectroscopically pure
4c(BF4-) was obtained by recrystallization. Attempted prepara-
tion of (NfsNf)Pt(o-Tol)(NCMe)+ (4a) by protonation of 3a
with HBF4‚Et2O in acetonitrile led, however, to extensive
positional isomerization and formation of a mixture of 4a, 4b,
and 4c in ca. 1:7:2 ratio. Consequently, an alternative synthetic
route was worked out for the production of 4a. Treatment of a
dichloromethane solution of cis-Pt(o-Tol)2(SMe2)2 with 1 equiv
of HCl in ether gave a product mixture that exhibited a very
1
complex product H NMR spectrum, presumably due to the
In this paper, regioselectivity issues in the PtII diimine system
are addressed through investigations of the CsH activation of
toluene (in which four different types of CsH bonds are
available) and p-xylene (in which two different CsH bonds
are available) at (NfsNf)Pt(CH3)(H2O)+ (1) and (N′sN′)Pt-
(CH3)(H2O)+ (2). The regioselectivities of these reactions
depend strongly on the identity of the substrate and the metal
complex and on the solvent composition. The results provide
important mechanistic insight that helps us to understand the
nature of the product-determining steps of these reactions.
formation of a mixture of cis- and trans-Pt(o-Tol)(Cl)(SMe2)2
and the dimeric complexes cis- and trans-Pt2(o-Tol)2(Cl)2(µ-
SMe2)2. Further reaction of this mixture with the diimine ligand
NfsNf produced (NfsNf)Pt(o-Tol)(Cl) (8). However, trans-Pt-
(o-Tol)(Cl)(SMe2)2 was found to be unreactive toward the
ligand, even at elevated temperatures. Hence, a mixture of this
species, free NfsNf ligand, and 8 was obtained after completion
of the reaction. Separation of 8 from the unreacted material was
accomplished by column chromatography on silica. Finally,
treatment of 8 with AgOTf in THF/acetonitrile yielded (Nfs
Nf)Pt(o-Tol)(NCMe)+OTf- (4a(OTf-)).
All new compounds were characterized by 1H and 19F NMR
Results
1
spectroscopy (key data are given in Table 1). The H and 19F
Synthesis and Characterization of Diimine Platinum(II)
Tolyl Complexes. To facilitate product identification in the Cs
H activation reactions with toluene and p-xylene, preparation
(20) Adams, C. S.; Legzdins, P.; Tran, E. J. Am. Chem. Soc. 2001, 123,
612.
(21) Hill, G. S.; Irwin, M. J.; Levy, C. J.; Redina, L. M.; Puddephatt, R.
J. Inorg. Synth. 1998, 32, 149.
(22) (a) Rashidi, M.; Hashemi, M.; Khorasani-Motlagh, M.; Puddephatt,
R. J. Organometallics 2000, 19, 2751. (b) Puddephatt, R. J.; Thomson, M.
A. J. Organomet. Chem. 1982, 238, 231.
(18) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462.
(19) (a) Shulpin, G. B.; Kitaygorodskiy, A. N. J. Organomet. Chem. 1981,
212, 275. (b) Shulpin, G. B.; Nizova, G. V.; Nikitaev, A. T. J. Organomet.
Chem. 1984, 276, 115.

6582 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Johansson et al.
1
Table 1. Key H and 19F NMR Data for New Isolated Complexes
1H NMR, δa
19F NMR, δ
aryl-CF3
(relative intensity)
Tol-Me
diimine-Me
aryl-Ho
Pt-NCMe
compound
(4J(195Pt-H))
(4J(195Pt-H))
(relative intensity)
(4J(195PtsH))
syn-(NfsNf)Pt(o-Tol)2 (syn-3a)
2.31 (6.7)
1.71
7.17 (1)
7.37 (1)
7.28
7.25
7.24
7.44 (1)
7.47 (1)
8.03 (2)
7.39 (1)
7.96 (1)
7.37 (1)
7.96 (1)
-
-62.98 (1)
-62.85 (1)
-63.35
anti-(NfsNf)Pt(o-Tol)2 (anti-3a)
(NfsNf)Pt(m-Tol)2 (3b)
2.13 (6.4)
1.89
2.04
1.71
1.81
1.79
2.23 (9.1)
2.37
-
-
-63.10
(NfsNf)Pt(p-Tol)2 (3c)
-
-63.28
(NfsNf)Pt(o-Tol)(NCMe)+ (4a)
2.37
2.08 (14.0)
-63.58 (1)
-63.24 (1)
-62.93 (2)
-63.22 (1)
-62.95 (1)
-63.45 (1)
-62.94 (1)
(NfsNf)Pt(m-Tol)(NCMe)+ (4b)
(NfsNf)Pt(p-Tol)(NCMe)+ (4c)
1.96
2.09
2.25 (9.3)
2.36
2.25 (9.8)
2.36
2.09 (13.5)
2.09 (13.8)
a Dichloromethane-d2.
Table 2. Crystal Data and Structure Refinement for syn-3b
empirical formula
formula weight
temperature
wavelength
crystal system
space group
C34H26F12N2Pt‚CH2Cl2
968.57
150(2) K
0.71073 Å
orthorhombic
Pnma
a ) 14.659(2) Å
b ) 25.664(3) Å
c ) 9.624(2) Å
3620.9(9) Å3, 4
1.657 Mg/m3
4.113 mm-1
F(000)
crystal size
1880
0.18 × 0.8 × 0.8 mm
θ-range for data
reflections collected
independent reflections
refinement method
data/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
∆F(max)
10.24 to 30.50°
45877
5386 [R(int) ) 0.056]
full-matrix least- squares on F2
5386/241
unit cell dimensions
1.219
R1 ) 0.055, wR2 ) 0.121
R1 ) 0.068, wR2 ) 0.129
2.53 e‚Å-3
volume, Z
density (calculated)
absorption coefficient
∆F(min)
-2.73e‚Å-3
(as indicated in Figure 1), thereby blocking rotation of the
diimine aryl rings. Efficient rotation of the aromatic rings may
require concerted movements of all four rings. Restricted
rotation causes the diimine aryl-H(2,6) and CF3 groups “above”
and “below” the coordination plane in syn-3a to be chemically
nonequivalent, which in turn leads to separate chemical shifts
in the NMR spectra.
Figure 1. Rotamers syn-3a and anti-3a. The steric interactions indicated
for syn-3a are proposed to account for the restricted rotation of the
diimine aryl moieties and the observation of separate signals for the
The H and 19F NMR spectra of the cationic acetonitrile
1
species (NfsNf)Pt(m-Tol)(NCMe)+ (4b) and (NfsNf)Pt(p-Tol)-
(NCMe)+ (4c) are in accordance with complexes of lower
symmetry compared to the precursors 3b and 3c. Separate
signals for the CF3 substituents on each of the two diimine aryl
CF3 and CF′ groups in the 19F NMR spectrum.
3
NMR spectra of 3b and 3c clearly reflect the molecular
symmetry of these species. Thus, the aryl CF3 groups gave rise
to one signal in the 19F NMR spectra for each complex. The
two compounds exhibit quite similar 1H NMR spectra, with the
exception of the signal pattern for the tolyl group resonances.
For the closely related species 3a, the NMR analysis revealed
a more complex situation. Thus, in the 1H NMR spectrum, two
sets of signals were seen in ca. 54:46 ratio, each corresponding
to the (NfsNf)Pt(o-Tol)2 formulation. We believe that the two
species are rotamers, differing in having the two tolyl-Me groups
oriented mutually syn or anti with respect to the coordination
plane of Pt (Figure 1). Apparently, the interconversion of the
two rotamers must be slow on the NMR time scale. Furthermore,
a significant difference between the two species was observed
by NMR spectroscopy. Whereas the minor isomer exhibited only
one signal for the diimine aryl-H(2,6) and as well as for the
CF3 groups in the 1H and 19F NMR spectra, the corresponding
groups in the major isomer gave rise to two resonances in a
1:1 ratio in each spectrum. This observation suggests that for
the major isomer, rotation around the diimine NsCAryl axes must
also be restricted. We tentatively propose the identities of the
major and minor rotamer to be syn-3a and anti-3a, respectively.
Steric interactions between the two tolyl-Me groups in syn-3a
is suggested to restrict the rotational freedom of the tolyl rings
1
rings were seen by 19F NMR spectroscopy. In the H NMR
spectra, 195Pt satellites were observed for one of the methyls
on the backbone of the diimine ligand (4J(195PtsH) ca. 9 Hz).
Based on relative trans effect arguments, these methyl groups
are presumably the ones transoid to the acetonitrile ligand.23
Once again, somewhat different NMR features were found
for the o-tolyl analogue (4a). In the 19F NMR spectrum, three
signals for the aryl CF3 groups appeared in a 2:1:1 ratio. A
similar pattern of signals was observed for the diimine aryl-
1
H(2,6) signals in the H NMR spectrum. Apparently, steric
interactions between the o-tolyl-Me group and the neighboring
diimine aryl moiety prevent free rotation of these two rings. In
analogy to the discussion above, this gives rise to chemically
nonequivalent substituents “above” and “below” the coordina-
tion plane on this diimine aryl ring. The second diimine aryl
ring is sterically less encumbered and rotates freely.
Importantly, the differences in the 19F NMR signal patterns
for the cationic complexes 4a-c were of particular value for
(23) Complete 1H and 13C NMR assignment of the closely related species
-
(NsN)Pt(CH3)(NCMe)+BF4 (NsN ) ArsNdCHsCHdNsAr; Ar )
4-MeOC6H4) demonstrated that the trends in J(195PtsH) data might be
rationalized by relative ligand trans effects. Johansson, L.; Ryan, O. B.;
Rømming, C.; Tilset, M. Organometallics 1998, 17, 3957.

C-H ActiVation of Toluene and p-Xylene at PtII Diimines
Table 3. Selected Bond Lengths (Å) and Angles (deg) for syn-3b
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6583
Pt(1)-C(11)
N(1)-C(1)
C(1)-C(1A)
2.010(6)
1.285(7)
1.501(11) C(1)-C(2)
Pt(1)-N(1)
N(1)-C(3)
2.086(4)
1.437(7)
1.485(8)
171.6(2)
N(1)-Pt(1)-C(11) 97.0(2)
N(1)-Pt(1)-N(1A) 75.7(2)
N(1)-Pt(1)-C(11A)
C(11)-Pt(1)-C(11A) 90.0(3)
Pt(1)-N(1)-C(3)
N(1)-C(1)-C(2)
C(2)-C(1)-C(1A)
Figure 3. Cationic aqua species (NfsNf)Pt(CH3)(H2O)+ (1) and (N′s
N′)Pt(CH3)(H2O)+ (2).
Pt(1)-N(1)-C(1)
C(1)-N(1)-C(3)
N(1)-C(1)-C(1A) 114.3(3)
117.4(4)
121.4(5)
121.2(4)
125.8(5)
119.9(4)
Scheme 4
Scheme 5
Figure 2. ORTEP plot of the structure of syn-3a.
the analysis of the product mixtures in the toluene activation
reaction (vide infra).
X-ray Crystallographic Structure Determination of syn-
3a. Crystals of quality suitable for X-ray diffraction analysis
were obtained by slow evaporation of a dichloromethane
solution of 3a. Details about the structure determination are
given in the Experimental Section. Table 2 lists experimental
and crystallographic data, and selected bond distances and angles
are given in Table 3. Figure 2 shows an ORTEP drawing of
the X-ray crystal structure of 3a, revealing that the complex
has crystallized in the geometry where the tolyl-Me groups are
oriented mutually syn with respect to the coordination plane of
Pt. (No attempts have been made to establish whether 3a
crystallizes exclusively in the syn form, or whether crystals of
the anti form might also be present in the isolated solid.) A
crystallographic mirror plane is located perpendicular to the
coordination plane, bisecting the chelate CsC bond and passing
through the Pt atom, and gives rise to a molecular Cs symmetry.
The diimine arene rings are twisted away from the tolyl-Me
groups at an angle relative to the chelate ring plane of 86.1°.
This geometry is probably assumed in order to minimize steric
repulsions between the diimine arene moieties and the tolyl-
Me and the chelate-Me groups. Interestingly, the orientation of
the two tolyl rings is essentially perpendicular to the chelate
ring (chelate ring/tolyl ring interplanar twist angle 90.4°). This
orientation brings the two tolyl-Me groups into close proximity
with each other. The steric repulsions caused by this interaction
are presumably alleviated through a significant distortion of the
square-planar geometry at the metal center: the PtsCTol bonds
are bent down relative to the chelate ring plane in Figure 2 by
an angle of 7.8°.
temperature. After complete reaction (ca. 3 h), acetonitrile was
added to convert all species to the corresponding acetonitrile
adducts. The resulting mixture of products was then analyzed
1
in dichloromethane-d2 by H and 19F NMR spectroscopy. As
shown in Scheme 4, a mixture of 4a, 4b, and 4c was found in
a 9:70:21 ratio. No traces of the benzylic CsH activation
product 4d were observed in the reaction. The reaction of
2(BF4-) with toluene was carried out in TFE-d3 under identical
conditions. As anticipated from the observed reaction rates with
benzene,9b,c the activation of toluene at 2 was found to be
somewhat slower than that of 1 (ca. 24 h for complete
conversion). Identification of the acetonitrile adducts in the
resulting product mixture was based solely on assignment of
the signals in the 1H NMR spectra in dichloromethane-d2 (i.e.,
the complexes 5a-d were not independently synthesized).
However, the 1H NMR spectroscopic features of the analogous
species 4a-c significantly aided the analysis and assignments
of the products. As can be seen in Scheme 4, the product mixture
consisted of 5a, 5b, 5c, and 5d in a 4:60:15:21 ratio. Complex
5d was readily identified by its Ptsmethylene signal at δ 2.68,
accompanied by characteristic 195Pt satellites (2J(195PtsH) )
102 Hz). Thus, unlike the reaction at 1, a substantial amount of
benzylic CsH activation occurs at 2. The ratio between the o-,
m- and p-tolyl species 5a, 5b, and 5c was found to be
approximately the same as in the activation at 1.
CsH Activation of p-Xylene. The complexes 1(BF4-) and
2(BF4-) were treated with p-xylene in experiments (Scheme 5)
that were conducted in the same manner as the toluene activation
reactions described above. In the reaction with 1, the products
CsH Activation of Toluene. The CsH activation of toluene
was performed at ambient temperature in TFE-d3. To probe
possible diimine ligand effects on the selectivity, both 1 and
the sterically more congested 224 were studied under otherwise
identical reaction conditions (Figure 3). The reaction between
1(BF4-) and 30 equiv of toluene (ca. 0.4 M) in TFE-d3 was
monitored by 1H and 19F NMR spectroscopy at ambient
1
were identified as 6a (93%) and 6b (7%) (key H NMR data
are given in Table 4), establishing that aromatic CsH activation
is preferred over benzylic reaction. In sharp contrast to this,
the reaction between 2 and p-xylene led to almost exclusive
formation of the benzylic product 7b (90%). A small amount
of 7a (10%) was also observed in the reaction.
(24) In TFE solution, 2 is in equilibrium with a minor solvento complex
(N′sN′)Pt(CH3)(TFE)+BF4-; Keq ) [(N′sN′)Pt(CH3)(TFE)+][H2O]/[2]-
[TFE] ≈ 1.2 ( 0.3 × 10-3 at 25 °C. See ref 9c.
Protonolysis of the (NfsNf)Pt(Tol)2 Complexes 3a-c and
(N′sN′)Pt(p-Tol)2 (9c). As mentioned above, attempted syn-

6584 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Johansson et al.
1
Table 4. Key H NMR Data for Products Formed in the CsH Activation of p-Xylene at 1 and 2
1H NMR, δa
PtCH2
diimine
NdC(Me)
Pt-NCMe
Compound
(2J(195Pt-H))
Xyl/Bz Me
(4J(195Pt-H))
(NfsNf)Pt(Xyl)(NCMe)+ (6a)
-
1.96 (Mem)
2.34 (Meo)
na
1.97 (Mem)
2.46 (Meo)
2.12
2.21
2.35
na
2.11
2.20
2.02
2.07
2.08 (13.8)
(NfsNf)Pt(p-MeBz)(NCMe)+ (6b)
(N′-N′)Pt(Xyl)(NCMe)+ (7a)
2.78 (101.9)
-
1.81 (na)
1.86 (13.7)
(N′-N′)Pt(p-MeBz)(NCMe)+ (7b)
2.64 (101.4)
1.68 (13.9)
a Dichloromethane-d2. na, not applicable.
Table 5. Product Mixtures Obtained after Protonation of Complexes 3a-c with 1 Equiv of HBF4‚Et2O
a (A) TFE; addition of MeCN after completed reaction. (B) 1.7 M MeCN in TFE. (C) MeCN.
Scheme 6
thesis of 4a by protonation of the corresponding bis(o-tolyl)
complex 3a in acetonitrile gave a mixture of 4a, 4b, and 4c.
Since this behavior was in striking contrast to that of the
analogous complexes 3b and 3c, we wanted to better understand
the factors that caused this outcome. Each of the three bis(tolyl)
species 3a-c were protonated with 1 equiv of HBF4‚Et2O in
neat acetonitrile, 1.7 M acetonitrile in TFE, and neat TFE. For
the reactions that were performed in neat TFE, acetonitrile was
added after completed reactions to trap the products as the
corresponding acetonitrile adducts. As can be seen from the
results (Table 5), the extent of isomerization was found to be
highly dependent on the solvent system. Most notably, a 77:23
mixture of 4b and 4c was obtained from all three bis(tolyl)
complexes 3a-c in the reactions that were done in neat TFE.
The presence of 1.7 M acetonitrile clearly inhibited the
isomerization, and in neat acetonitrile the reactions of 3b and
3c proceeded to almost exclusively yield the corresponding
cationic acetonitrile adducts 4b and 4c. The absence of the
o-tolyl product 4a in all reactions (except the 7% produced in
the protonation of 3a in acetonitrile) manifests that its formation
is highly unfavorable when compared to the isomeric products.
product mixture consisted of 5a, 5b, 5c, and 5d in an 8:65:15:
12 ratio. This product distribution is similar to that obtained in
the toluene activation at 2. Importantly, the formation of
significant amounts of the benzylic isomer 5d in this reaction
implies that toluene is capable of rapidly rearranging between
“aromatic” and “benzylic” CsH bond coordination modes prior
to elimination.25
(25) In a TFE-d3 solution of (N′sN′)Pt(C6H5)(NCMe)+ 9c in the presence
of 0.4 M toluene, no formation of the corresponding (N′sN′)Pt(Tol)-
(NCMe)+ complexes could be observed by 1H NMR spectroscopy after 24
h at ambient temperature. This control experiment suggests that the observed
isomerization in the protonation experiment is not due to re-coordination
and activation of toluene at the produced (N′sN′)Pt(Tol)(NCMe)+ com-
plexes.
An experiment was also performed where (N′sN′)Pt(p-Tol)2
(9c; prepared from cis-Pt(SMe2)2(p-Tol)2 and the diimine ligand
N′sN′) was protonated with 1 equiv of HBF4‚Et2O in a 0.05
M solution of acetonitrile in TFE. As shown in Scheme 6, the

C-H ActiVation of Toluene and p-Xylene at PtII Diimines
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6585
Scheme 7
be explained by steric and electronic effects for the associative
elimination of methane, as will be discussed in the following
section.
CsH Activation of Toluene. The most striking aspect of
the CsH activation reactions with toluene is the observed meta
preference which, to our knowledge, has never been previously
reported for this type of reaction. Even after statistical correction
for two meta positions vs one para position, meta activation is
preferred over para activation by a factor of ca. 2 in the reactions
of 1 and 2. The product ratio corresponds to a relatively small
energy difference in the activation barriers that lead to meta
and para products. However, the greater than statistical m/p ratio
is consistently seen in our experiments and apparently has not
been previously observed in other systems, and in our view
deserves special attention.
It is reasonable to assume that the product ratios in these
reactions predominantly reflect a kinetically controlled selectiV-
ity. In agreement with this, a control experiment showed that
the activation of toluene at the produced (NfsNf)Pt(Tol)(H2O)+
species is considerably slower than the activation at 1 (see
Experimental Section). In addition, the CsH activation reactions
were quenched by acetonitrile addition once the starting Pt
complexes were consumed, as judged by the NMR spectra. This
treatment further minimizes the extent to which repeated
activation events might take place.
Some additional features of the toluene activation are inferred
from the mechanism of the analogous benzene activation
reactions at 1 and 2.9b,c Thus, we expect that (1) the intercon-
version between the hydrocarbon adducts in Scheme 7 is fast
compared to the elimination of methane and toluene from these
intermediates, and (2) the η2-C,C- or η2-C,H-coordinated27
toluene adduct undergoes facile migration from one CsC/Cs
H bond to another. The extensive positional isomerization
observed in the protonation reactions of the bis(tolyl) species
3a-c in pure TFE clearly supports this scenario. This gives
rise to a situation where all the three isomeric (NsN)Pt(Tol)-
(σ-CH4)+ intermediates (A-C) are in rapid equilibrium with
each other (Figure 4, indicated by the dashed lines). According
to the Curtin-Hammett principle, the product distribution will
then be determined solely by the differences in the corresponding
transition-state energies for the rate-limiting steps that lead to
the products. By applying the suggested solvent-assisted as-
sociative mechanism for the elimination of methane from the
(NsN)Pt(Tol)(σ-CH4)+ intermediates, we propose that the
trigonal bipyramidal structures Aq-Cq depicted in Figure 4 are
reasonable models for the respective transition states. The
relative positions of these structures in the energy diagram reflect
qualitatively the observed product distributions. In the following,
we will elaborate on how our results might be explained by
this model.
The relative energies of the ground-state structures A-C are
not known (and, applying the Curtin-Hammett principle, not
relevant for the product distribution), but we assume that the
ortho isomer A is somewhat destabilized relative to the meta
(B) and para (C) isomers due to steric effects. In the transition
states, steric interactions between the o-tolyl-Me group and
coordinated methane or TFE in Aq should lead to an even more
pronounced destabilization when compared to isomeric struc-
tures Bq and Cq. This effect provides a straightforward rationale
for the nearly complete absence of the o-tolyl products 4a and
5a. However, steric arguments do not seem to be applicable in
Discussion
We expect that the reactions of 1 and 2 with toluene and
p-xylene proceed analogously to the general pathways estab-
lished for benzene and methane in previous investigations
(Scheme 2).9b,c Thus, rate-limiting exchange of the aromatic
substrate for coordinated water at the starting complex gives
the corresponding toluene or p-xylene adduct intermediate
(Scheme 7).26 Subsequent reversible cleavage of an arene Cs
H bond produces a PtIV hydride which undergoes methyl-H
coupling to yield a methane adduct. Irreversible elimination of
methane and coordination of water complete the reaction.
Substantial experimental evidence obtained from the mecha-
nistic studies on the benzene and methane activation systems
suggests that a TFE-assisted associative mechanism operates
for the water/hydrocarbon exchange at platinum (i.e., the first
and the last steps in Scheme 7). For the benzene CsH activation
at 2, this was corroborated by (1) a rate law first-order in [C6H6]
as well as [2] and inverse first-order in [H2O], and (2) rather
negative activation entropy (-16 cal mol-1 K-1) for the
reaction.9c The methane activation at 1 was also found to be
inhibited by addition of water, in accordance with reversible
predissociation of water prior to methane coordination. The
microscopic reverse of the methane activationselimination of
methane through protonolysis of (NfsNf)Pt(CH3)2 and (N′s
N′)Pt(CH3)2 with DOTf in TFE-d3swas also examined.9d In
these experiments, the extent of methyl/methane H/D scrambling
(effected by an equilibrium analogous to the one within the
brackets in Scheme 7) was found to be dependent on the
concentration of small amounts of added acetonitrile. Thus, at
higher [MeCN], less H/D scrambling occurred. This finding
strongly suggests that the mechanism for acetonitrile/methane
exchange is also associative, given that the final methane loss
is irreversible under the reaction conditions.
The results from the CsH activation and protonation reactions
in the present study provide further evidence for associative
pathways for the hydrocarbon/solvent exchange in these systems.
Unequivocal evidence for meta preference is observed in the
toluene CsH activation as well as the protonation reactions of
the bis(tolyl) complexes 3a-c and 9c in TFE in the absence of
acetonitrile. This regioselectivity is in sharp contrast to the ortho/
para preference that is normally obtained in electrophilic
aromatic substitution reactions with toluene. This result implies
that resonance stabilization of positive charge by the tolyl group
is not of major importance in the transition state for the product-
determining step. In the protonation reactions, the inhibition of
the isomerization process by the presence of acetonitrile is highly
indicative of an associative hydrocarbon/solvent exchange, since
the hydrocarbon loss is effectively irreversible on the experi-
mental time scale. A dissociative mechanism is not readily
reconciled with either the observed product distributions in the
toluene activation or the acetonitrile inhibition of the isomer-
ization processes in the protonation reactions. The unusual
selectivities in the CsH activation of toluene and p-xylene might
(26) In agreement with rate-limiting hydrocarbon coordination, the
activation of trifluoromethylbenzene at 1 was found to be on the order of
10 times slower than the activation of toluene. The identity of the products
obtained in this reaction was, however, not determined.
(27) Evidence for a π-coordinated PtII benzene complex (N′sN′)Pt(CH3)-
(η2-C,C-C6H6)+ was recently reported. However, the cleavage of a CsH
bond from this species is believed to take place via an η2-C,H-coordinated
benzene intermediate. See ref 9c.

6586 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Johansson et al.
the above argumentation. Thus, solvent exchange reactions at
(LsL)Pt(CH3)(DMSO)+ complexes (LsL ) chelating diamines
and diimines) were generally found to take place by associative
mechanisms.28c Furthermore, the fastest reactions were observed
at the most electron-deficient metal centers. The authors
rationalized the fast exchange rates for the electron-poor
complexes by (1) high affinity of the metal center for the
entering nucleophile and (2) good ability to stabilize a transition
state with a higher coordination number. Clear-cut dissociative
exchange processes have, on the other hand, been recognized
only for electron-rich PtII complexes, in particular with strong
σ-activators trans to the leaving group, e.g., cis-PtR2L2 (R )
Me, Ph; L ) thioethers or DMSO)28a and (bph)Pt(SR2)2 (bph
) 2,2′-biphenyl; R ) Me, Et).28d The electron-rich nature of
these complexes has been suggested to promote the changeover
from associative to dissociative pathways mainly by (1) thwart-
ing the possibility for nucleophilic attack at the metal centers
and (2) enhancing the stability of the corresponding transient
three-coordinate, 14-electron PtII intermediates.28b
The apparent lack of meta/para selectivity (after statistical
correction) in other CsH activation reactions between toluene
and organometallic complexes, as discussed in the Introduction,
may well indicate that the product-determining steps in these
cases are associated with completely different types of processes,
e.g., CsH bond breaking/formation or dissociative alkane loss.
Another explanation could be that the present diimine PtII system
is unusually sensitive to electronic effects.
Figure 4. Qualitative reaction coordinate diagram for the reaction path
from the square planar (NsN)Pt(Tol)(CH4)+ intermediates A-C to
the corresponding trigonal bipyramidal transition states Aq-Cq accord-
ing to a TFE-assisted associative mechanism for elimination of methane.
order to account for the required energy difference between Bq
and Cq. Thus, we have to consider possible electronic effects.
In the species in Figure 4, pπ(tolyl)-dπ(Pt) interactions should
be present. From simple resonance structure considerations, it
may be argued that the m-tolyl ligand less efficiently delocalizes
the positive charge at Pt than does the p-tolyl ligand. This leaves
a greater positive charge at Pt in B than in C, rendering the Pt
center of B more susceptible to nucleophilic attack. The
resonance effect is indicated by a somewhat higher ground-
state energy for B versus C in the diagram. However, for the
corresponding transition-state structures Bq and Cq, the experi-
mentally observed product distributions dictate that this ordering
must be reversed. Two major factors, seemingly working in
opposing directions, might be envisioned to govern the relative
energies of Bq and Cq. Resonance effects will favor structure
Cq for the same reason as for the ground-state species C, but to
a lesser extent because the effective positive charge at Pt is
diluted as a consequence of the ligand association. On the other
hand, the greater positive charge at Pt in Bq might be predicted
to lead to enhanced stabilization due to stronger binding of the
electron-donating TFE ligand when compared to Cq in providing
a stronger electrostatic component to the bonding of the
incoming nucleophile. If this effect is more important, the
observed meta preference can, indeed, be rationalized with this
model. However, it must be emphasized that the exact explana-
tion for how the electronic effects contribute to the stabilization
of Bq versus Cq may be considerably more complicated. We
hope to address this point in future contributions.
CsH Activation of p-Xylene. Whereas the tolyl complexes
4a-c were observed as the exclusive products in the reaction
between 1 and toluene, a considerable amount of the benzyl
product 5d was obtained in the reaction of 2. This difference
in selectivity between 1 and 2 became even more prominent in
the reactions with p-xylene. Thus, the ratios of Pt(xylyl) (6a/
7a) versus Pt(p-methylbenzyl) (6b/7b) products were 90:10 and
7:93 in the reactions with 1 and 2, respectively. This remarkable
diversity in selectivity is most likely a result of differences in
the steric influence from the diimine ligands in 1 and 2.
If we assume that the mechanistic model that was utilized in
the discussion about the toluene reaction still applies (i.e. rapidly
equilibrating isomeric PtII p-xylene adduct intermediates), the
product-determining factors might once again be found in the
trigonal bipyramidal transition-state structures (Figure 5). For
the reaction with 1, the product distribution implies that that
the energy for transition state Dq is lower than that for Eq. The
reasons for this may be that (1) MsC(sp2) bonds in general
are stronger than MsC(sp3) bonds and (2) the positive charge
at Pt is stabilized by resonance. Steric interactions, which should
favor structure Eq, appear to be of lesser importance.
In contrast, the reaction of complex 2 led to almost exclusive
CsH activation of the p-xylene methyl groups. This finding is
most readily rationalized by the steric properties of the diimine
ligand in 2. Steric repulsions between the diimine aryl 2,6-Me
groups and the o-tolyl-Me substituent lead to a significant
destabilization of the transition-state structure Fq (Figure 5).
Consequently, product formation via the less crowded transition-
state structure Gq becomes the dominant pathway. By analogy,
the same effects explain the observation of considerable amounts
of benzylic product in the reaction between 2 and toluene.
The above argumentation is based on the assumption that
the equilibration of all involved intermediates is fast compared
Importantly, the meta selectivity appears to be in direct
conflict with an alternative dissociative mechanism for the
elimination of methane. The putative unsaturated three-
coordinate intermediates (NsN)Pt(Tol)+ on the reaction coor-
dinate for this pathway should clearly be stabilized by electron-
donating ligands. Since the donating capacities for the tolyl
ligands decrease in the order p-tolyl ≈ o-tolyl > m-tolyl, an
ortho/para preference would be expected if this mechanism were
operative.
(28) (a) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem.
Soc. 1989, 111, 8161. (b) Romeo, R.; Grassi, A.; Scolaro, L. M. Inorg.
Chem 1992, 31, 4383. (c) Romeo, R.; Scolaro, L. M.; Nastasi, N.; Arena,
G. Inorg. Chem. 1996, 35, 5087. (d) Plutino, M. R.; Scolaro, L. M.; Romeo,
R.; Grassi, A. Inorg. Chem. 2000, 39, 2712.
Romeo and co-workers have investigated in detail the
influence of electronic and steric properties on reaction rates
and mechanisms for simple ligand exchange processes in square
planar PtII complexes.28 Their observed trends seem to support

C-H ActiVation of Toluene and p-Xylene at PtII Diimines
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6587
Figure 5. Proposed transition-state structures for the displacement of methane by TFE during the CsH activation reactions of p-xylene at 1 (Dq,
Eq) and 2 (Fq, Gq).
Scheme 8
isomeric (NfsNf)Pt(Tol)(toluene)+ species increase in the order
(NfsNf)Pt(m-Tol)(toluene)+ < (NfsNf)Pt(p-Tol)(toluene)+
,
(NfsNf)Pt(o-Tol)(toluene)+. This is entirely analogous to the
trend for elimination of methane from the putative (NfsNf)Pt-
(Tol)(CH4)+ intermediates in the toluene activation reaction at
1. Again, the results appear incompatible with a dissociative
mechanism since the most facile toluene elimination takes place
from the meta intermediate, which is presumably least capable
of stabilizing the positive charge. The total absence of o-tolyl
product in the protonation reactions implies that the five-
coordinate transition-state structure [(NfsNf)Pt(o-Tol)(toluene)-
(TFE)+]q is even more unfavorable relative to the m- and p-tolyl
analogues than was the case for [(NfsNf)Pt(o-Tol)(CH4)-
(TFE)+]q (cf. Figure 4). Since toluene is considerably larger
than methane, this difference may be attributed to enhanced
steric effects in the corresponding transition states for elimination
of toluene when compared to methane.
In the presence of 1.7 M acetonitrile, considerably less
isomerization is seen in the reactions. This indicates that
acetonitrile, which is a considerably better incoming nucleophile
and coordinating ligand than TFE, displaces toluene at platinum
before complete equilibriation of the intermediates has been
established. This result strongly indicates that the toluene/
acetonitrile exchange also occurs in an associative fashion.
In neat acetonitrile, protonation of 3b gives no positional
isomerization, and only traces of isomerization occur in the
reaction of 3c. Contrary to this, the protonation of 3a led to
extensive isomerizationsonly 7% of the product mixture was
identified as the corresponding o-tolyl product 4a. From these
observations it is evident that (1) the extent of isomerization is
dependent on the acetonitrile concentration and (2) the elimina-
tion of toluene from the sterically most congested metal center,
(NfsNf)Pt(o-Tol)(toluene)+, is highly unfaVorable when com-
pared to the isomeric meta and para intermediates. Once again,
these results appear to be in conflict with a dissociative
mechanism for the toluene elimination and provide further
support for the proposed associative pathway for this process.
to methane losssi.e., that coordinated toluene and p-xylene
molecules are capable of rapidly rearranging between the
“aromatic” and “benzylic” CsH bond coordination modes. This
scenario is, indeed, supported by the observation of significant
amounts of the benzyl product (N′sN′)Pt(Bz)(NCMe)+ (5d)
arising from the protonation of the bis(p-tolyl) complex 9c in
the presence of 0.05 M MeCN in TFE. If, alternatively, the
aromatic/benzylic coordination rearrangement is slow compared
to elimination of methane in the CsH activation reactions, then
initial hydrocarbon coordination must control the discrimination
between aromatic versus benzylic activation. However, since
the transition-state structures for the coordination of toluene and
p-xylene at the starting platinum species 1 and 2 will be
analogous to those depicted in Figure 5 (with CH4 and xylyl/
p-methylbenzyl ligands being replaced by p-xylene and methyl,
respectively), an extrapolation of the steric arguments above
will lead to similar explanations for the observed selectivities.
Protonation Reactions. Protonolysis of the three bis(tolyl)
complexes 3a-c in neat TFE led to extensive positional
isomerization (Table 5). In analogy with the H/D scrambling
observed in the activation reactions with deuterated methane
and benzene,9b,c this is straightforwardly explained by a rapid
equilibrium between (NfsNf)PtIV(Tol)2(H)+ and (NfsNf)PtII-
(Tol)(toluene)+ intermediates (Scheme 8). In addition, fast
rearrangement of toluene between different η2-C,H- and η2-
C,C-coordination modes in (NfsNf)PtII(Tol)(toluene)+ must take
place. The elimination of toluene may be confidently assumed
to be irreversible under these conditions since (1) the reaction
between toluene and the (NfsNf)Pt(Tol)(H2O)+ products is slow
(vide supra), (2) the reaction times were short (ca. 5 min) and
the products were irreversibly trapped by acetonitrile addition
prior to analysis, and (3) toluene is present only in low
concentrations throughout the reactions.
Concluding Remarks
The cationic aqua complexes 1 and 2 undergo facile CsH
activation of toluene and p-xylene in TFE at ambient temper-
ature. Analysis of the product mixtures obtained in these
reactions and mechanistic insight gained from the study on the
elimination of toluene through protonolysis of the bis(tolyl)
complexes 3a-c and 9c have allowed us to address several
important questions regarding the factors that govern regio-
selectivity in these systems.
The selectivity in the CsH activation appears to be dictated
by the following key mechanistic features: (1) a rapid dynamic
equilibrium between the isomeric (NsN)Pt(Ar)(CH4)+ inter-
mediates (Ar ) tolyl/benzyl or xylyl/p-methylbenzyl) precedes
(2) a relatively slow exchange of TFE or H2O for methane from
these species by (3) an associatiVe mechanism. Items (1) and
The products 4b and 4c were obtained in a 77:23 ratio in all
experiments performed in neat TFE, regardless of which bis-
(tolyl) complex 3a-c was protonated. This product distribution
must therefore reflect the true kinetic selectivity of toluene
elimination when the isomeric intermediates within the brackets
in Scheme 8 are at equilibrium. This is again a situation where
the Curtin-Hammett principle applies, and consequently, the
transition-state energies for elimination of toluene from the

6588 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Johansson et al.
(200 MHz, dichloromethane-d2): major isomer (64%), δ 2.06 (s,
(2) imply that the presumed rate-determining step, coordination
of the arene, does not affect the selectivity in the reactions.
Another consequence is that differences in CsH bond strengths
within the hydrocarbon substrates are irrelevant for the CsH
activation selectivity.29 Most importantly, (3) implies that the
product-determining factors are found in the pentacoordinate
transition-state structures [(NsN)Pt(Ar)(CH4)(TFE)+]q for the
elimination of methane. Steric as well as electronic effects
influence the relative stabilities of these structures.
The observed selectivity order for the aromatic positions in
the toluene activation, meta > para > ortho, appears incompat-
ible with the alternative dissociative mechanism for the product-
determining step. According to our mechanistic model, the
disfavored formation of o-tolyl products is explained by steric
destabilization of the corresponding transition-state structure
[(NsN)Pt(o-Tol)(CH4)(TFE)+]q. The meta over para preference
is, on the other hand, caused largely by electronic effects. Thus,
the poorer capacity of the m-tolyl versus the p-tolyl ligand to
disperse the positive charge at Pt is proposed to lead to enhanced
bonding of the incoming TFE ligand.
4
3J(195PtsH) ) 24.6 Hz, 12 H, SMe2), 2.67 (s, J(195PtsH) ) 7.3 Hz,
6 H, TolMe), 6.66-6.93 (m, 6 H, TolHm and TolHp), 6.97 (“dd”, 3J(Hs
H) ) 7.0 Hz, 3J(195PtsH) ) 73.2 Hz, 2 H, TolHo); minor isomer (36%),
δ 2.08 (s, 3J(195PtsH) ) 24.2 Hz, 12 H, SMe2), 2.57 (s, 4J(195PtsH) )
6.9 Hz, 6 H, TolMe), 6.66-6.93 (overlap with major isomer, 6 H, TolHm
and TolHp), 6.97 (overlap with major isomer, 2 H, TolHo).
General Procedure for the Preparation of Complexes (NfsNf)-
Pt(Tol)2 (3a-c) and (N′sN′)Pt(p-Tol)2 (9c). One equivalent of the
diimine ligand NfsNf or N′sN′ was added to a solution of the
appropriate Pt(Tol)2(SMe2)2 complex in toluene. The solution was stirred
at 45 °C overnight. The solvent was removed, and the residue was
recrystallized from a dichloromethane/pentane mixture. The product
was obtained after drying in vacuo.
syn-(NfsNf)Pt(o-Tol)2 (syn-3a) and anti-(NfsNf)Pt(o-Tol)2 (anti-
3a). This compound was prepared from cis-Pt(o-Tol)2(SMe2)2 (482 mg,
0.961 mmol) and the diimine ligand NfsNf (489 mg, 0.962 mmol) in
toluene (50 mL). Dark purple microcrystals formed (720 mg, 85%).
1H NMR (200 MHz, dichloromethane-d2): syn-3a (54%), δ 1.71 (s, 6
H, NC(Me)C(Me)N), 2.31 (s, 4J(195PtsH) ) 6.7 Hz, 6 H, TolMe), 6.34-
6.66 (m, 6 H, TolHm and TolHp), 6.90 (“d”, 3J(HsH) ) 7.5 Hz,
3J(195PtsH) ) 76.8 Hz, 2 H, TolHo), 7.17 (s, 2 H, ArHo), 7.37 (s, 2 H,
ArHo′), 7.58 (s, 2 H, ArHp); anti-3a (46%), δ 1.71 (s, 6 H, NC(Me)C-
Differences in the steric influences from the diimine ligands
in 1 and 2 have dramatic consequences for the selectivity in
the reactions with p-xylene. By utilizing the same mechanistic
model as for the toluene activation reaction, the selectivities
are again explained by the relative energies for the corresponding
transition-state structures for the associative methane elimination
processes to the products.
4
(Me)N), 2.13 (s, J(195PtsH) ) 6.4 Hz, 6 H, TolMe)), 6.34-6.66 (m,
6 H, TolHm and TolHp), 7.04 (“d”, 3J(HsH) ) 8.0 Hz, 3J(195PtsH) )
70.5 Hz, 2 H, TolHo), 7.28 (s, 4 H, ArHo), 7.56 (s, 2 H, ArHp). 19F
NMR (188 MHz, dichloromethane-d2): syn-3a, δ -62.98 (s, 6 F,
ArCF3) -62.85 (s, 6 F ArCF3′); anti-3a, δ -63.35 (s, 12 F, ArCF3).
Anal. Calcd for C34H26F12N2Pt: C, 46.11; H, 2.96; N, 3.16. Found: C,
44.26; H, 3.12; N, 2.98.
(NfsNf)Pt(m-Tol)2 (3b). This compound was prepared from cis-
Pt(m-Tol)2(SMe2)2 (121 mg, 0.241 mmol) and the diimine ligand Nfs
Nf (123 mg, 0.242 mmol) in toluene (15 mL). Dark purple microcrystals
In more general terms, the results illuminate an important
aspect of product formation by reductive elimination from Xs
MsY species. Product distributions in such reactions are
commonly discussed in terms of the driving force as measured
by XsY bond strengths in the alternative products. The present
examples show that this factor may be of subordinate importance
in cases where associative displacement of the coordinated Xs
Y molecule is rate-determining. It remains to be seen whether
effects similar to those described in this paper contribute to
determine the selectivity in alkane activation and functional-
ization reactions in Shilov-type systems.
1
formed (179 mg, 84%). H NMR (300 MHz, dichloromethane-d2): δ
1.81 (s, 6 H, NC(Me)C(Me)N), 1.89 (s, 6 H, TolMe), 6.30 (“d”, 3J(Hs
H) ) 7.3 Hz, 2 H, TolHp), 6.49 (“dd”, 3J(HsH) ) 7.3 Hz, 2 H, TolHm),
3
3
6.53 (s, J(195PtsH) ) 72.0 Hz, 2 H, TolHo), 6.55 (“d”, J(HsH) )
3
7.3 Hz, J(195PtsH) ) 72.9 Hz, 2 H, TolHo′), 7.25 (s, 4 H, ArHo),
7.60 (s, 2 H, ArHp).19F NMR (188 MHz, dichloromethane-d2):
δ
-63.10 (s, ArCF3). Anal. Calcd for C34H26F12N2Pt: C, 46.11; H, 2.96;
N, 3.16. Found: C, 46.47; H, 3.14; N, 3.51.
(NfsNf)Pt(p-Tol)2 (3c). This compound was prepared from cis-Pt-
(p-Tol)2(SMe2)2 (448 mg, 0.893 mmol) and the diimine ligand NfsNf
(455 mg, 0.895 mmol) in toluene (60 mL). Dark purple microcrystals
Experimental Section
1
formed (791 mg, 82%). H NMR (200 MHz, dichloromethane-d2): δ
General Considerations. Trifluoroethanol (TFE) and TFE-d3 were
distilled from CaSO4 and a small amount of NaHCO3 and stored over
3-Å molecular sieves. All other solvents were purified according to
standard procedures. 1H, 13C{1H}, and 19F NMR spectra were recorded
on Bruker Advance DXP 200 and 300 instruments. 1H and 13C chemical
shifts are reported in ppm relative to tetramethylsilane, with the residual
solvent proton resonance as internal standards. 19F chemical shifts are
reported in ppm relative to CFCl3. For TFE-d3, the solvent fluorine
resonance was used as internal standard (δ -77.72). Elemental analyses
were carried out at Ilse Beetz Mikroanalytisches Laboratorium, Kronach,
Germany. The preparation of the diimine ligands NfsNf 9a and N′s
1.79 (s, 6 H, NC(Me)C(Me)N), 2.04 (s, 6 H, TolMe), 6.42 (“d”, 3J(Hs
H) ) 8.0 Hz, 4 H, TolHm), 6.53 (“d”, 3J(HsH) ) 7.3 Hz, 3J(195PtsH)
) 73.7 Hz, 4 H, TolHo), 7.24 (s, 4 H, ArHo), 7.61 (s, 2 H, ArHp).19F
NMR (188 MHz, dichloromethane-d2): δ -63.28 (s, ArCF3). Anal.
Calcd for C34H26F12N2Pt: C, 46.11; H, 2.96; N, 3.16. Found: C, 46.65;
H, 3.15; N, 3.56.
(N′sN′)Pt(p-Tol)2 (9c). This compound was prepared from cis-Pt-
(p-Tol)2(SMe2)2 (193 mg, 0.385 mmol) and the diimine ligand N′sN′
(113 mg, 0.386 mmol) in toluene (20 mL). Dark purple powder formed
(229 mg, 89%). 1H NMR (300 MHz, dichloromethane-d2): δ 1.60 (s,
6 H, NCMeCMeN), 1.99 (s, 6 H, TolMe), 2.19 (s, 12 H, ArMe), 6.33
(“d”, 3J(HsH) ) 7.9 Hz, 4 H, TolHm), 6.68 (“d”, 3J(HsH) ) 7.9 Hz,
3J(195PtsH) ) 70.1 Hz, 4 H, TolHo), 6.85-6.94 (m, 6 H, ArH). Anal.
Calcd for C34H38N2Pt: C, 60.97; H, 5.72; N, 4.18. Found: C, 61.11;
H, 5.59; N, 4.21.
N′ 30 as well as the aqua complexes 1(BF4 9a and 2(BF4 9c is described
) )
-
-
elsewhere. PtCl2(SMe2),21 cis-Pt(m-Tol)2(SMe2)2,22a and cis-Pt(p-Tol)2-
(SMe2)222b were prepared by methods similar to those described in the
literature.
cis-Pt(o-Tol)2(SMe2)2. The complex was prepared by adaptation of
(NfsNf)Pt(o-Tol)(Cl) (8). A 1.0 M solution of HCl in diethyl ether
(0.65 mL, 0.65 mmol) was added with a syringe to a solution of cis-
Pt(o-Tol)2(SMe2)2 (308 mg, 0.613 mmol) in dichloromethane (40 mL).
The mixture was stirred for 1 h. The solvent was removed, and the
22a
the reported synthesis for cis-Pt(m-Tol)2(SMe2)2 (39%, obtained as
a mixture of two isomers, tentatively proposed to be the syn-(cis-Pt-
(o-Tol)2(SMe2)2) and anti-(cis-Pt(o-Tol)2(SMe2)2) complexes). 1H NMR
1
solid was washed with cold diethyl ether portions. According to H
(29) The fact that the selectivity cannot be predicted from the substrate
CsH bond strengths appears to be a general feature of the majority of
nonradical hydrocarbon CsH activation reactions with metal complexes.
However, since various mechanisms seem to operate in different CsH
activation reactions, there is probably not a general explanation for this
behavior. See refs 2 and 20 and references therein.
NMR spectroscopy, this material consisted of a complex mixture of
products, presumably cis-Pt(Cl)(o-Tol)(SMe2)2, trans-Pt(Cl)(o-Tol)-
(SMe2)2, cis-Pt2(Cl)2(o-Tol)2(µ-SMe2)2, and trans-Pt2(Cl)2(o-Tol)2(µ-
SMe2)2. This material (271 mg) and the diimine ligand NfsNf (312
mg, 0.614 mmol) were dissolved in toluene (40 mL), and the mixture
(30) tom Dieck, H.; Svoboda, M.; Grieser, T. Z. Naturforsch. 1981, 36B,
823.
1
was stirred at 70 °C for 14 h. The solvent was removed, and the H

C-H ActiVation of Toluene and p-Xylene at PtII Diimines
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6589
NMR spectrum of the solid material revealed a mixture of 8, trans-
Pt(Cl)(o-Tol)(SMe2)2, and NfsNf ligand in approximately a 1:1:1 ratio.
The mixture was separated by flash chromatography (silica/dichlo-
romethane), and 8 was collected as the first dark red fraction. The
solvent was removed, and the product was washed with pentane and
dried in vacuo, giving a dark red powder (235 mg, 46%).1H NMR (200
Reaction between 2 and Toluene. The 1H NMR spectrum showed
a product mixture consisting of (N′sN′)Pt(o-Tol)(NCMe)+ (5a, 4%),
(N′sN′)Pt(m-Tol)(NCMe)+ (5b, 60%), (N′sN′)Pt(p-Tol)(NCMe)+ (5c,
1
15%), and (N′sN′)Pt(CH2C6H5)(NCMe)+ (5d, 21%). H NMR (300
MHz, dichloromethane-d2): 5a (characteristic signals), δ 1.86 (s,
4J(195PtsH) ) 13 Hz, 3 H, PtNCMe), 2.01 (s, 3 H, ArMe), 2.11 (s, 3
H, NCMeC′MeN), 2.22 (s, 3 H, NCMeC′MeN), 2.32 (s, 3 H, ArMe),
2.37 (s, 3 H, ArMe) 2.45 (s, 4J(195PtsH) ) 6 Hz, 3 H, TolMe), 2.47 (s,
3 H, ArMe) [The observation of four separate ArMe signals for this
species indicates that rotation of both the diimine aryl groups around
the NsCAryl axes is restricted.]; 5b, δ 1.88 (s, 4J(195PtsH) ) 13 Hz, 3
4
MHz, dichloromethane-d2): δ 1.43 (s, J(195PtsH) ) 10.5 Hz, 3 H,
4
NCMeC′MeN), 2.02 (s, 3 H, NCMeC′MeN), 2.26 (s, J(195PtsH) )
7.0 Hz, 3 H, TolMe), 6.42-6.56 (“m”, 3 H, TolHm, TolHm′, TolHp),
3
6.82 (“m”, J(195PtsH) ) 35 Hz, 1 H, TolHo), 7.24 (s, 2 H, ArHo),
7.56 (s, 1 H, ArHp), 7.72 (s, 2 H, Ar′Ho), 7.95 (s, 1 H, Ar′Hp). 19F
NMR (188 MHz, dichloromethane-d2): δ -63.55 (s, 3 F, ArCF3),
-63.21 (s, 3 F, ArCF3′), -63.02 (s, 6 F, Ar′CF3).
4
H, PtNCMe), 1.96 (s, 3 H, TolMe), 2.11 (s, 3 H, J(195PtsH) ) 9.4
Hz, NCMeC′MeN), 2.16 (s, 6 H, ArMe), 2.21 (s, 3 H, NCMeC′MeN),
2.40 (s, 6 H, Ar′Me), 6.4-6.6 (m, 4 H, TolH), 6.9-7.0 (m, 3 H, ArH)
7.2-7.3 (m, 3 H, Ar′H); 5c (characteristic signals), δ 1.87 (s, 3 H,
PtNCMe), 2.08 (s, 3 H, TolMe), 2.10 (s, 3 H, NCMeC′MeN), 2.16 (s,
6 H, ArMe), 2.20 (s, 3 H, NCMeC′MeN), 2.40 (s, 6 H, Ar′Me); 5d
(characteristic signals), δ 1.67 (s, 4J(195PtsH) ) 13.9 Hz, 3 H, PtNCMe),
2.04 (s, 3 H, NCMeC′MeN), 2.08 (s, 3 H, NCMeC′MeN), 2.22 (s, 6
H, ArMe), 2.28 (s, 6 H, Ar′Me), 2.68 (s, 2J(195PtsH) ) 102.0 Hz, 3 H,
PtCH2), 6.75 (m, 2 H, PtCH2PhHo).
(NfsNf)Pt(o-Tol)(NCMe)+OTf- (4a(OTf-)). Solid AgOTf (20 mg,
0.077 mmol) was added to a solution of 8 (62 mg, 0.075 mmol) in
THF (10 mL) and acetonitrile (3 mL). After being stirred for 20 min,
the mixture was filtered, and the solvent was removed. The oily residue
was dissolved in a small amount of diethyl ether, and pentane was
added to the solution. After the solution was cooled to -20 °C, the
1
product precipitated as orange needles (54 mg, 74%). H NMR (200
4
MHz, dichloromethane-d2): δ 2.08 (s, 3 H, J(195PtsH) ) 14.0 Hz,
PtNCMe), 2.23 (s, 4J(195PtsH) ) 9.1 Hz, 3 H, NCMeC′MeN), 2.37 (s,
3 H, NCMeC′MeN), 2.37 (s, 3 H, TolMe), 6.5-6.6 (“m”, 3 H, TolHm,
1
Reaction between 1 and p-Xylene. The H and 19F NMR spectra
showed a product mixture consisting of (NfsNf)Pt(Xyl)(NCMe)+ (6a,
93%) and (NfsNf)Pt(p-MeBz)(NCMe)+ (6b, 7%). 1H NMR (300 MHz,
3
TolHm′, TolHp), 6.82 (“m”, J(195PtsH) ) 37 Hz, 1 H, TolHo), 7.44
4
dichloromethane-d2): 6a, δ 1.96 (s, 3 H, XylMem), 2.08 (s, J(195Pts
(s, 1 H, ArHo), 7.47 (s, 1 H, ArHo′), 7.55 (s, 1 H, ArHp), 7.99 (s, 1 H,
Ar′Hp), 8.03 (s, 2 H, Ar′Ho). 19F NMR (188 MHz, dichloromethane-
d2): δ -78.70 (s, 3 F, CF3SO3-), -63.58 (s, 3 F, ArCF3), -63.24 (s,
3 F, ArCF3′), -62.93 (s, 6 F, Ar′CF3). Anal. Calcd for C30H22BF15N3O3-
PtS: C, 36.59; H, 2.25; N, 4.27. Found: C, 37.00; H, 2.78; N, 4.38.
H) ) 13.8 Hz, 3 H, PtNCMe), 2.21 (s, 3 H, NCMeC′MeN), 2.34 (s, 3
3
H, XylMeo), 2.35 (s, 3 H, NCMeC′MeN), 6.38 and 6.43 (“d”, J(Hs
3
H) ) 7.7 Hz, 1 H each, XylHm and XylHp), 6.59 (s, J(195PtsH) )
32.7 Hz, 1 H, XylHo), 7.38 (s, 1 H, ArHo), 7.41 (s, 1 H, ArHo′), 7.56
(s, 1 H, ArHp), 7.96 (s, 2 H, Ar′Ho), 8.00 (s, 1 H, Ar′Hp); 6b
(characteristic signals), δ 1.81 (s, 3 H, PtNCMe), 2.78 (s, 2J(195PtsH)
) 101.9 Hz, 3 H, PtCH2), 6.76 (d, 3J(HasHb) ) 7.7 Hz, 2 H,
p-MeBzHa), 6.80 (d, 3J(HasHb) ) 7.7 Hz, 2 H, p-MeBzHb). 19F NMR
(NfsNf)Pt(m-Tol)(NCMe)+BF4 (4b(BF4-)). A 54% solution of
-
HBF4 in ether (14 µL, 0.10 mmol) was added with a syringe to a
solution of 3b (87 mg, 0.098 mmol) in acetonitrile (15 mL). The mixture
was stirred for 20 min. The solvent was removed, and the solid was
washed with diethyl ether portions and dissolved in a minimum amount
of acetone. The product was obtained as an orange powder (80 mg,
-
(188 MHz, dichloromethane-d2): 6a, δ -150.19 (s, 4 F, BF4
counterion), -63.50 (s, 3 F, ArCF3), -62.95 (s, 3 F, ArCF3′), -62.92
(s, 6 F, Ar′CF3); 6b, δ -150.19 (s, 4 F, BF4- counterion), -63.14 (s,
6 F, ArCF3), -63.00 (s, 6 F, Ar′CF3).
1
88%) by the addition of diethyl ether. H NMR (200 MHz, dichlo-
4
romethane-d2): δ 1.96 (s, 3 H, TolMe), 2.09 (s, 3 H, J(195PtsH) )
13.5 Hz, PtNCMe), 2.25 (s, 4J(195PtsH) ) 9.3 Hz, 3 H, NCMeC′MeN),
2.36 (s, 3 H, NCMeC′MeN), 6.4-6.6 (m, 4 H, TolH), 7.39 (s, 2 H,
ArHo), 7.60 (s, 1 H, ArHp), 7.96 (s, 2 H, Ar′Ho), 7.99 (s, 1 H, Ar′Hp).
19F NMR (188 MHz, dichloromethane-d2): δ -150.39 (s, 4 F, BF4-),
-63.22 (s, 6 F, ArCF3), -62.95 (s, 6 F, Ar′CF3). Anal. Calcd for C29H22-
BF16N3Pt: C, 37.76; H, 2.40; N, 4.56. Found: C, 38.14; H, 2.51; N,
4.34.
1
Reaction between 2 and p-Xylene. The H and 19F NMR spectra
showed a product mixture consisting of (N′sN′)Pt(Xyl)(NCMe)+ (7a,
10%) and (N′sN′)Pt(p-MeBz)(NCMe)+ (7b, 90%). 1H NMR (300
MHz, dichloromethane-d2): 7a (characteristic signals), δ 1.86 (s,
4J(195PtsH) ) 13.7 Hz, 3 H, PtNCMe), 1.97 (s, 3 H, XylMem), 2.11
(s, 3 H, NCMeC′MeN), 2.20 (s, 3 H, NCMeC′MeN), 2.46 (s, 3 H,
XylMeo), 6.27 (s, 3J(195PtsH) ) 33.4 Hz, 1 H, XylHm), 6.37 and 6.53
(d, 3J(HasHb) ) 7.6 Hz, 1 H each, XylHm and XylHp), 6.73 (d, 3J(Has
Hb) ) 7.6 Hz, 1 H, ArHm), 6.94 (“t”, 3J(HasHb) ) 7.6 Hz, 1 H, ArHp),
7.08 (d, 3J(HasHb) ) 7.6 Hz, 1 H, ArHm′) [The three latter signals are
assigned to the diimine aryl ring neighboring the xylyl ligand; the
observation of two separate ArHm signals suggests that rotation around
the NsCAr axis is restricted, cf. complex 5a.]; 7b, δ 1.68 (s, 4J(195Pts
(NfsNf)Pt(p-Tol)(NCMe)+BF4 (4c(BF4-)). A 54% solution of
-
HBF4 in ether (20 µL, 0.15 mmol) was added with a syringe to a
solution of 3c (125 mg, 0.141 mmol) in acetonitrile (15 mL). The
mixture was stirred for 20 min. The solvent was removed, and the solid
was washed with diethyl ether portions and dissolved in a minimum
amount of acetone. The product was obtained as an orange powder
4
H) ) 13.9 Hz, 3 H, PtNCMe), 2.02 (s, J(195PtsH) ) 9.2 Hz, 3 H,
1
(130 mg, 85%) by the addition of diethyl ether. H NMR (200 MHz,
4
dichloromethane-d2): δ 2.09 (s, 3 H, TolMe), 2.09 (s, 3 H, J(195Pts
NCMeC′MeN), 2.07 (s, 3 H, NCMeC′MeN), 2.12 (s, 3 H, p-MeBz),
2.21 (s, 6 H, ArMe), 2.27 (s, 6 H, Ar′Me), 2.63 (s, 2J(195PtsH) ) 101.4
Hz, 3 H, PtCH2), 6.62 (d, 3J(HasHb) ) 7.8 Hz, 2 H, p-MeBzHa), 6.80
(d, 3J(HasHb) ) 7.7 Hz, 2 H, p-MeBzHb), 7.14-7.24 (m, 3 H, ArH),
7.29 (m, 3 H, Ar′H).
H) ) 13.8 Hz, PtNCMe), 2.25 (s, 4J(195PtsH) ) 9.3 Hz, 3 H,
NCMeC′MeN), 2.36 (s, 3 H, NCMeC′MeN), 6.50 (“m”, 2 H, TolHm),
3
6.57 (“m”, J(195PtsH) ) 35.2 Hz, 2 H, TolHo), 7.37 (s, 2 H, ArHo),
7.61 (s, 1 H, ArHp), 7.96 (s, 2 H, Ar′Ho), 8.00 (s, 1 H, Ar′Hp). 19F
NMR (188 MHz, dichloromethane-d2): δ -150.40 (s, 4 F, BF4-),
-63.45 (s, 6 F, ArCF3), -62.94 (s, 6 F, Ar′CF3). Anal. Calcd for C29H22-
BF16N3Pt: C, 37.76; H, 2.40; N, 4.56. Found: C, 38.15; H, 2.55; N,
4.35.
General Procedure for the Protonation of (NfsNf)Pt(Tol)2
Complexes (3a-c) and (N′sN′)Pt(p-Tol)2 (9c). Three round-bottomed
flasks were loaded with the appropriate (NfsNf)Pt(Tol)2 species (10
mg, 0.011 mmol). Three mixtures of a 48% aqueous solution of HBF4
(1.6 µL, 0.012 mmol) in TFE (5.0 mL), TFE (5.0 mL)/acetonitrile (500
µL), or acetonitrile (5.0 mL), respectively, were then added to the flasks.
The reaction mixtures were stirred until all the starting material had
reacted to give orange product solutions (ca. 5 min). Acetonitrile (500
µL) was added to the reaction that was performed in neat TFE. The
solvent was removed in vacuo, and the resulting materials were
dissolved in dichloromethane-d2 and analyzed by 1H and 19F NMR
spectroscopy. The protonation of 9c in 0.05 M acetonitrile in TFE was
performed in a similar manner.
General Procedure for the Reactions between 1/2 and Toluene/
p-Xylene. The aromatic substrate (ca. 0.24 mmol, 0.4 M) was added
to a solution of 1(BF4-) or 2(BF4-) (ca. 8 × 10-3 mmol) in TFE-d3
1
(0.6 mL) in an NMR tube. The reactions were followed by H NMR
spectroscopy. After complete conversion to the products, acetonitrile
was added to the solution. The solvent was removed, and the residue
was dissolved in dichloromethane-d2. The product mixtures were
analyzed by NMR spectroscopy.
1
Reaction between 1 and Toluene. The H and 19F NMR spectra
showed a product mixture consisting of 4a (9%), 4b (70%), and 4c
(21%).
Examination of the Rates for Toluene Activation at 1 versus
Mixture of Isomeric (NfsNf)Pt(Tol)(H2O)+ Complexes. Toluene (16

6590 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Johansson et al.
µL, 0.15 mmol) was added to an NMR tube loaded with 1 (4.1 mg,
5.0 µmol) dissolved in TFE-d3 (600 µL). After 65 min at 25 °C, the 1H
NMR spectrum showed that ca. 95% of 1 had reacted to (NfsNf)Pt-
(Tol)(H2O)+ products and CH4. The reaction was allowed to proceed
for an additional hour, and the solvent was removed in vacuo. The
resulting mixture of (NfsNf)Pt(Tol)(H2O)+ species was dissolved in
TFE-d3 (600 µL), and the solution was transferred to an NMR tube.
Toluene-d8 (16 µL, 0.15 mmol) was added to the tube, and the following
determination were carried out with the SAINT and XPREP programs.31
Absorption corrections were applied by the use of the SADABS
program.32
The structures were determined and refined using the SHELXTL
program package.33 The non-hydrogen atoms were refined with
anisotropic thermal parameters; hydrogen positions were calculated from
geometrical criteria with isotropic thermal parameters. Final figures of
merit are included in Table 2. The structure was refined in the space
group Pnma (the use of Pna21 did not alter the geometrical features).
1
reaction was followed by H NMR spectroscopy. After 65 min at 25
°C, the PtsTolH signals for the (NfsNf)Pt(Tol)(H2O)+ complexes had
decreased by ca. 16% relative to the original intensity due to deuterium
incorporation from toluene-d8. Albeit an analysis of the rate of toluene
activation at the mixture of (NfsNf)Pt(Tol)(H2O)+ complexes is
complicated by H/D scrambling processes and possible kinetic isotope
effects, this experiment indicates that the reaction is considerably slower
than the activation of toluene at 1.
Acknowledgment. We gratefully acknowledge generous
support from the Norwegian Research Council, NFR (stipend
to L.J.).
Supporting Information Available: X-ray crystallographic
file, listings of crystal data, X-ray experimental details, atomic
coordinates, thermal parameters, and bond distances and angles
for syn-3b (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
X-ray Crystallographic Structure Determination of 3a. Crystals
were obtained by slow evaporation of a dichloromethane solution of
3a. X-ray data were collected on a Siemens SMART CCD diffracto-
meter31 using graphite-monochromated Mo KR radiation. Data collec-
tion method: ω-scan, range 0.6°, crystal-to detector-distance 5 cm;
further information is given in Table 2. Data reduction and cell
JA010277E
(32) Sheldrick, G. M. Private communication, 1996.
(33) Sheldrick, G. M. SHELXTL, Version 5; Siemens Analytical X-ray
Instruments Inc., Madison, WI, 1998.
(31) SMART and SAINT Area-detector Control and Integration Software;
Siemens Analytical X-ray Instruments Inc., Madison, WI, 1996.