Communications
Organometallics, Vol. 25, No. 20, 2006 4735
Scheme 1. Synthesis of Pt(NNC)X Complex 3
binding of the hydrocarbon, RH, is not the rate-determining step,
then the activation barrier for the overall CH activation reaction
will depend on the energy differences between the ground state,
L-M-X, and the CH cleavage transition state (i.e., the
energetics for binding RH is not relevant to the overall activation
barrier). Under these circumstances, the key to obtaining a net
reduction in the overall barrier for CH activation in weakly
acidic solvents will be to ensure that the degree of destabilization
of the ground state brought about by ligand modifications is
greater than that for the CH cleavage transition state.
and elemental analysis. All nine expected aromatic resonances
were observed, and platinum satellites (JPt-H ) 33 Hz) were
observed for the ortho proton on the metalated phenyl ring of 3.
Control studies carried out by heating a trifluoroacetic acid
solution of 3 for 11 h at 200 °C under argon confirmed that 3
was thermally stable to acidic solutions. No insoluble Pt metal
was visible and analysis by NMR (after workup) showed that
no significant decomposition occurred as 3 was recovered with
∼91% mass balance (1,3,5-trimethoxybenzene as an internal
standard) with minor amounts of what is believed to be the ion-
pair or solvento complex.
Significantly, unlike non-transition metal ions such as Hg(II),
Pt(II) can potentially interact in the TS for CH cleavage with
both σ-acceptor and π-donor orbitals.8 In the Pt(bpym)Cl2/H2SO4
system, the CH bond was shown theoretically to be cleaved by
an electrophilic substitution (ES) pathway that involves CH
donor interactions to σ-acceptor orbitals on Pt(II).5 Increasing
electron density at the metal center in a weaker acid solvent,
HX, could be expected to destabilize the transition state for this
mode of cleavage (either minor or significant changes), assuming
that the metal center still behaves as an electrophilie. However,
if the dominant pathway for CH cleavage switches to an inser-
tion (so-called oxidative addition (OA)) pathway (by interactions
between the CH σ* antibonding orbitals and π-donor orbitals
on Pt(II)), followed by rapid proton loss, then a net decrease in
the overall barrier for CH activation could be possible by ligand
modifications that increase electron density at the Pt center.
Therefore, it should be possible to stabilize the CH cleavage by
insertion as well as destabilize water coordination by increasing
the electron density at Pt(II). Herein, we report on the synthesis
of an electron-rich, thermally stable, monoanionic, tridentate,
pincer, platinum NNC complex, Pt(NNC)TFA, and experimental
and theoretical comparison of the reactivity for CH activation
and functionalization relative to the Pt(bpym)(TFA)2 system.
It is now well established that the tridentate pincer ligand
motif can impart both thermal stability and reactivity to transi-
tion metal complexes.9 This has been demonstrated with the
thermally stable PCP-Ir systems of Kaska, Jensen, and Gold-
man10 that catalyze the thermal dehydrogenation of alkanes. The
tridentate ligand 6-phenyl-2,2′-bipyridine (NNC-H) and its
derivatives have been shown to readily form stable, cyclom-
etalated complexes with platinum, Pt(NNC)X.11 However, most
of the interest in these complexes has been focused on the
photoluminescent properties, and to our knowledge no catalysis
studies have been reported. The corresponding cyclometalated
NNC platinum(II) chloride complex, 1, Pt(NNC)Cl (NNC )
κ3-6-phenyl-4,4′-di-tert-butyl-2,2′-bipyridine), was prepared
through a modified procedure reported by Lu et al.,11c by heating
the NNC-H ligand with K2PtCl4 in glacial acetic acid. To create
a potentially active catalyst, the chloride was replaced by a more
labile leaving group using silver trifluoroacetate to obtain the
corresponding trifluoroacetate complex (3), Pt(NNC)TFA, in
good yields (Scheme 1). Complex 3 is air stable and was fully
characterized by 1H and 13C NMR, as well as mass spectroscopy
Dissolution of 3, an orange solid, in CH2Cl2 results in an
orange solution. However, consistent with the hoped-for destabili-
zation of the ground state and lability of the TFA- by use of the
more electron-rich NNC ligand, dissolution in CF3CO2H leads
to a blue solution, which we believe is due to acid-assisted disso-
ciation of the TFA- ligand and formation of [Pt(NNC)]+TFA-
as either a three-coordinate ion-pair or solvento complex. Some
evidence for this is that removal of the CF3CO2H in vacuo leads
to a blue residue that after treatment with a mixture of water and
CH2Cl2 and subsequent evaporation of the CH2Cl2 layer leads
to quantitative recovery of 3 as a yellow-orange solid. Additional
evidence that the blue solution results from dissociation of the
TFA- ligand is the observation that when orange CD2Cl2 solu-
tions of 3 are treated with the poorly coordinating, strong Lewis
acid B(Ph-F5)3, a dark blue-black solid is generated. Consistent
with the expected lability of the TFA-, reaction of the blue
solution of 3 in CF3CO2H with excess LiCl leads to immediate
formation of the yellow-orange chloride derivative Pt(NNC)Cl,
1. While a three-coordinate ion-pair or solvento complex best
fits these observations, another possibility is that the complex
becomes protonated in the presence of added CF3CO2H to gener-
ate a Pt(IV) hydride. However, 1H NMR studies in which HTFA
(3.8 equiv) was added to a solution of 3 in CD2Cl2 at -70 °C
did not show any evidence of a protonated platinum species.
Having established that 3 was thermally stable under protic
conditions and the TFA ligand was labile in CF3CO2H solvent,
the efficiency for catalyzing the H/D exchange reactions between
methane and CF3CO2D or D2SO4 was investigated and com-
pared to the reference system, Pt(bpym)(TFA)2. As was the case
for the Pt(bpym)(TFA)2 system, 3 was also found to be inactiVe
for catalyzing the H/D exchange between methane and CF3CO2D
at temperatures as high as 200 °C. Significantly, however, while
solutions of the Pt(bpym)(TFA)2 in CF3CO2D are stable in both
the presence and absence of methane, 3 decomposed at 200 °C
to colloidal platinum in the presence of methane. Since the
control studies showed that trifluoroacetic acid solutions of 3
are thermally stable at 200 °C in the absence of methane, these
results could suggest that reaction with methane occurred with
the Pt(NNC)TFA complex in CF3CO2D. Given the instability
of 3 with methane that precludes rigorous study, this potentially
interesting result was not further investigated. Interestingly,
catalytic H/D exchange between methane and sulfuric acid could
be observed with Pt(NNC)Cl, 1, without obvious decomposition,
but at a factor of ∼10 lower than the Pt(bpym) system. Thus,
heating 1 in D2SO4 with methane (500 psi) for 10 h at 180 °C
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