Base-promoted benzene C-H activation chemistry at an amido pincer complex of platinum(II)

Article abstract of DOI:10.1021/om011044z

Base-promoted benzene C-H activation chemistry at an amido pincer complex of platinum(II) was investigated. The thermally robust platinum(II) complex (BQA)Pt(OTf) underwent benzene C-H bond activation at 150 °C but required the presence of NⁱPr₂Et. The reaction products were the phenyl complex (BQA)Pt(Ph) and a stoichiometric equivalent of [HNⁱPr₂Et]]OTf].

Full text of DOI:10.1021/om011044z

Organometallics 2002, 21, 1753-1755
1753
Ba se-P r om oted Ben zen e C-H Activa tion Ch em istr y a t a n
Am id o P in cer Com p lex of P la tin u m (II)
Seth B. Harkins and J onas C. Peters*
Division of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, California Institute of Technology, Pasadena, California 91125
Received December 7, 2001
Summary: The thermally robust platinum(II) complex
(BQA)Pt(OTf) undergoes benzene C-H bond activation
at 150 °C but requires the presence of NiPr2Et. The
reaction products are the phenyl complex (BQA)Pt(Ph)
and a stoichiometric equivalent of [HNiPr2Et][OTf].
The potential for inorganic complexes to catalytically
mediate the selective functionalization of simple R-H
substrates has engendered widespread interest within
the organometallic community.1 Much recent attention
has been paid to systems with late transition elements,
largely because they offer promise for selectively coup-
ling alkane C-H activation with nonradical oxidation.
Motivating much of the interest in platinum chemistry
is the early work of Shilov and co-workers, who dem-
onstrated that platinum(II) salts can mediate the cata-
lytic conversion of methane to methanol in aqueous
solution.2 It is presumed that alkane activation in the
Shilov system (Figure 1, I) initially proceeds through
an electrophilic activation step to release aqueous
hydrochloric acid and to generate a divalent platinum
alkyl (Pt-Cl + R-H f Pt-R + HCl).3 A related step is
presumed to occur in the system reported by Catalytica
(Figure 1, II).4 Unfortunately, neither of these systems
is well-suited to an intimate study of this initial activa-
tion process.
F igu r e 1. Previously described platinum(II) systems
(I-IV) that mediate C-H activation processes, and the
system featured herein (V).
of a stable alkane, such as methane, typically drives
these activation reactions.8 The distinct feature of the
C-H activation system described herein is that triflic
acid is the formal byproduct of the intermolecular C-H
activation process.9
Addition of a bis(8-quinolinyl)amine ligand, BQAH,
to (COD)PtCl2 in the presence of triethylamine base
affords the thermally robust complex (BQA)PtCl (1).5
Suspensions of complex 1 can be superheated in benzene
at 150 °C for days without decomposition. To test
whether soluble bases might promote reactivity between
1 and benzene, we focused our attention on the rela-
tively noncoordinating tertiary amine NiPr2Et. However,
stirring a heterogeneous solution of 1 in benzene with
NiPr2Et at 150 °C resulted in no net reaction: the
starting chloride complex 1 was quantitatively recovered
from the reaction solution (Scheme 1) and none of the
anticipated salt byproduct, [HNiPr2Et][Cl], was formed.
To derive a more reactive platinum complex, we ex-
changed the chloride ligand for a labile triflate group.
This was most effectively accomplished by a two-step
procedure: addition of the lithium salt [Li][BQA]5 to
(COD)PtMeCl afforded the methyl complex (BQA)PtMe
(2) in excellent yield. Protonation of 2 with triflic acid
Our group is currently exploring whether pincer-like
amido complexes of platinum(II) can mediate intermo-
lecular C-H bond activation processes.5,6 Herein we
communicate a base-promoted, intermolecular benzene
C-H activation reaction in which triflic acid is formally
lost from a divalent Pt-OTf precursor. While several
groups have studied benzene C-H activation processes
at cationic platinum(II) (e.g., Figure 1, III and IV),7 loss
(1) For some leading references see: (a) Cho, J .-Y.; Iverson, C. N.;
Smith, M. R. J . Am. Chem. Soc. 2000, 122, 12868-12869. (b) Chen,
H. Y.; Schlecht, S.; Semple, T. C.; Hartwig, J . F. Science 2000, 287,
1995-1997. (c) J ensen, C. M. Chem. Commun. 1999, 2443-2449. (d)
Peterson, T. H.; Golden, J . T.; Bergman, R. G. J . Am. Chem. Soc. 2001,
123, 455-462. (e) Eisenstein, O.; Crabtree, R. H. New J . Chem. 2001,
25, 665-666. (f) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-
J espersen, K.; Goldman, A. S. J . Am. Chem. Soc. 2000, 122, 11017-
11018.
(2) Kushch, K. A.; Lavrushko, V. V.; Misharin, Yu. S.; Moravsky,
A. P.; Shilov, A. E. New J . Chem. 1983, 7, 729-733.
(3) (a) Stahl, S. S.; Labinger, J . A.; Bercaw, J . E. Angew. Chem.,
Int. Ed. 1998, 37, 2181-2192. (b) Sen, A. Acc. Chem. Res. 1998, 31,
550-557.
(7) (a) Holtcamp, M. W.; Labinger, J . A., Bercaw, J . E. J . Am. Chem.
Soc. 1997, 119, 848-849. (b) J ohansson, L.; Tilset, M. J . Am. Chem.
Soc. 2001, 123, 739-740. (c) Thomas, J . C.; Peters, J . C. J . Am. Chem.
Soc. 2001, 123, 5100-5101.
(8) Goldberg and co-workers showed that an octahedral platinum-
(IV) species, resulting from C-H activation, can be stable to alkane
loss by use of a potentially tripodal ligand. See: Wick, D. D.; Goldberg,
K. I. J . Am. Chem. Soc. 1997, 119, 10235-10236.
(9) Cycloplatination reactions that result from orthometalation and
generate acid have been described. See, for example: Ryabov, A. D.;
van Eldik, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 783-784.
(4) Periana, R. A.; Taube, D. J .; Gamble, S.; Taube, H.; Satoh, T.;
Fujii, H. Science 1998, 280, 560-564.
(5) Peters, J . C.; Harkins, S. B.; Brown, S. D.; Day, M. W. Inorg.
Chem. 2001, 40, 5083-5091.
(6) For a recent review of traditional “pincer” complexes of the
platinum group metals see: Albrecht, M.; van Koten, G. Angew. Chem.,
Int. Ed. 2001, 40, 3750-3781.
10.1021/om011044z CCC: $22.00 © 2002 American Chemical Society
Publication on Web 04/02/2002

1754 Organometallics, Vol. 21, No. 9, 2002
Communications
3,3′-iPr2-BQA ligand. For comparison, we obtained
X-ray data for the corresponding phenyl complex (3,3′-
iPr2-BQA)Pt(Ph) (10), whose solid-state structure is also
shown (Figure 2). The Pt-N(amido) bond distance in
10 is appreciably elongated relative to its distance in 9,
due to the relative trans influence of the phenyl versus
triflate ligands, respectively.
Sch em e 1
Similar to the parent system, heating a benzene
solution of 9 at elevated temperatures in the presence
of NiPr2Et produced a stoichiometric equivalent of [HNi-
Pr2Et][OTf]. The anticipated phenyl product 10 is
formed (ca. 60-65% as determined by 1H NMR integra-
tion).11 While no platinum metal is deposited, nor free
BQAH observed, other ligand-containing platinum prod-
ucts are observed by 1H NMR; their identity has not
yet been established. A control experiment established
that the independently prepared phenyl complex 10 is
stable in benzene with NiPr2Et at elevated tempera-
tures. Thus, its decomposition under the reaction condi-
tions is not likely responsible for the decreased yield of
the phenyl product.
Because the parent system affords a cleaner reaction
with benzene, we have begun to probe its chemistry
more closely. To determine whether the benzene C-H
activation reaction is reversible under the conditions of
the experiment, we independently examined the ability
of complexes 3 and 5 to catalyze proton scrambling from
[HNiPr2Et][OTf] into C6D6. We found that neither 3 nor
5 catalyzes detectable proton scrambling into deuteri-
obenzene at 150 °C over 36 h, even when a 20-fold excess
of [HNiPr2Et][OTf] was used. Apparently, [HNiPr2Et]-
[OTf] is too weak an acid, at least in benzene solvent,
to drive the reverse reaction at an appreciable rate at
150 °C. Use of a much stronger acid, however, drives
the reverse process readily. Thus, addition of HOTf to
a benzene slurry of 5 generates the triflato complex 3
rapidly and quantitatively at 25 °C (Scheme 1).12
Likewise, addition of HCl to 5 rapidly and quantitatively
generates 1.
The reaction of 3 to generate 5 offers the first well-
defined example of a Pt-X species that accesses an
intermolecular C-H bond activation process with con-
comitant release of acid. That the conversion of 3 to 5
might proceed by the elimination of HOTf, which would
then be trapped by NiPr2Et, seems unlikely. Were this
to be the case, sterically more encumbered proton traps
should also promote the reaction. We tested 2,6-di-tert-
butylpyridine and found that it does not promote the
benzene activation reaction at 150 °C. We have also
found that the rate of formation of 5 is not increased by
addition of excess NiPr2Et (10 equiv), consistent with a
reaction whose rate is dependent on triflate dissociation,
the C-H bond activation step, or both steps.
Several of the possible scenarios which might explain
the base-promoted activation reaction described are
shown in Scheme 2. We suspect that the operative
pathway is one in which benzene associatively displaces
triflate from 3 (Scheme 2, path b) and undergoes a
subsequent oxidative C-H bond cleavage to generate a
in dichloromethane quantitatively generates the target
triflato complex (BQA)Pt(OTf) (3), which readily pre-
cipitates from solution and is isolable in 99% yield as
an analytically pure red solid. Spectroscopic NMR data
for 3 were best obtained in CD3CN, which quantitatively
produces the adduct complex [(BQA)Pt(NCCD3)][OTf]
(4).
Like its chloride counterpart, the triflato complex 3
was stable when dissolved in benzene solution over 36
h at 150 °C. However, heating 3 in benzene with 1 equiv
of NiPr2Et changed the reaction course dramatically.
Under relatively dilute conditions (0.0054 M in 3) at 100
°C, a homogeneous red solution was obtained. After
several hours, the ammonium salt [HNiPr2Et][OTf] and
a new platinum complex, (BQA)Pt(Ph) (5), could be
detected (1H NMR). Raising the reaction temperature
to 150 °C and stirring for 36 h results in the com-
plete consumption of 3, the stoichiometric production
of [HNiPr2Et][OTf], and the generation of the phenyl
product 5 in high yield (∼90% by 1H NMR spectroscopy;
Scheme 1). Air-stable 5 can be isolated in pure form by
chromatographing the solution through a silica plug
followed by removal of the volatiles (70% yield). The
identity of product 5 has been confirmed from (i) FAB
MS data obtained from the crude reaction mixture and
(ii) its independent synthesis by the addition of [Li]-
[BQA] to (COD)PtPhCl and comparison of their spectral
data.
Owing to the ease with which 3 and 5 precipitate from
a homogeneous solution during attempts to obtain
X-ray-quality crystals, we turned our attention to a more
soluble system in which the BQA ligand is substituted
by isopropyl groups at the 3-position of the quinoline
rings. The target ligand, 3,3′-iPr2-BQAH (6), proved
accessible from a multistep procedure.10 Lithiation of 6
generated [Li][3,3′-iPr2-BQA] (7), and its subsequent
metathesis with (COD)PtMeCl afforded the benzene-
soluble methyl complex (3,3′-iPr2-BQA)PtMe (8). Sto-
ichiometric addition of triflic acid to 8 quantitatively
produces the triflato complex (3,3′-iPr2-BQA)PtOTf (9),
which is modestly soluble in hot benzene and very
soluble in dichloromethane. The increased solubility of
9 did enable us to obtain X-ray-quality crystals suitable
for a diffraction study, and its solid-state structure is
shown in Figure 2. As expected, the complex adopts a
square-planar structure and the triflato ligand is
positioned trans to the amido N-donor atom of the
(10) See the Supporting Information.
(11) The yield of 10 formed in this reaction critically depends on
the purity of 9. Samples of 9 that do not give satisfactory combustion
analysis invariably give much lower yields of 10.
(12) This also occurs for the (3,3′-iPr2-BQA) system: addition of triflic
acid to a benzene solution of 10 rapidly generates 9.

Communications
Organometallics, Vol. 21, No. 9, 2002 1755
F igu r e 2. Displacement ellipsoid representations (35%) of complexes 9 and 10. Selected bond distances (Å) and angles
(deg) are as follows. Complex 9: Pt-N1, 1.994(2); Pt-N2, 1.946(2); Pt-N3, 1.990(2); Pt-O1, 2.097(2); N2-Pt-O1, 179.13-
(9); N1-Pt-N3, 166.94(10). Complex 10: Pt-N1, 2.007(3); Pt-N2, 2.020(3); Pt-N3, 2.009(3); Pt-C25, 2.023(4); N2-Pt-
C25, 178.07(11); N1-Pt-N3, 163.45(12).
Sch em e 2
uct. The strategy employed places a labile triflate group
in a position trans to the amido N-donor of the pincer-
like amido ligand. This ligand effectively stabilizes the
platinum(II) system at the high temperatures (150 °C)
necessary to drive the forward reaction and likely
labilizes the triflate ligand to allow binding of the
benzene substrate. We are now pursuing the possibility
that base-promoted C-H activation chemistry may be
mediated by other metal systems and applied to other
R-H substrates.
Ack n ow led gm en t. We thank BP, Caltech, and the
Dreyfus Foundation for financial support of this work
and Larry Henling for crystallographic assistance.
Members of the Bercaw group are thanked for insightful
discussions.
platinum(IV) hydride (path c).13 The amine base then
displaces the platinum(II) phenyl product by deproto-
nation of the platinum(IV) hydride. Non-redox pro-
cesses, such as those that involve heterolytic activation
of the Pt-OTf bond (path a) or a simple base deproto-
nation of benzene activated by precoordination to cat-
ionic platinum(II) (path d), also need to be considered.14
Further studies are necessary to shed light on the mech-
anism by which the amine base promotes the observed
activation chemistry.
Su p p or tin g In for m a tion Ava ila ble: Text giving detailed
experimental procedures and tables giving crystallographic
information. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM011044Z
(13) Not shown in Scheme 2 is the case where amine base preco-
ordinates platinum, perhaps forming [(BQA)Pt(NiPr2Et)][OTf], prior
to the reaction with benzene.
(14) We recently suggested a related process in which deprotonation
of an sp2-hybridized C-H group of cyclooctadiene occurs on coordina-
tion to cationic platinum(II).5
To conclude, a bulky amine base promotes an inter-
molecular benzene C-H activation process at a Pt-X
center by formally accepting the reaction’s acid byprod-