C-H bond activation by air-stable [(diimine)MII(μ 2-OH)]22+ dimers (M = Pd, Pt)

Article abstract of DOI:10.1021/ja076740q

Air- and water-tolerant C-H activation is observed in reactions of [(diimine)Pt(μ₂-OH)]₂²⁺ dimers with allylic and benzylic C-H groups. The reactions proceed in good yields under mild conditions. Mechanistic studies indicate that the active species is the monomeric [(diimine)Pt(OH₂)]²⁺ dication. The related palladium species, [(diimine)Pd(μ₂-OH)]₂²⁺, exhibit similar stoichiometric activations and also effect catalytic oxidation of cyclohexene to benzene with molecular oxygen as the terminal oxidant. Copyright

Full text of DOI:10.1021/ja076740q

Published on Web 02/01/2008
2+
C-H Bond Activation by Air-Stable [(Diimine)MII(µ2-OH)]2 Dimers (M ) Pd, Pt)
Travis J. Williams,† Andrew J. M. Caffyn,‡ Nilay Hazari, Paul F. Oblad,
Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received September 6, 2007; E-mail: bercaw@caltech.edu; jal@caltech.edu
Scheme 1
Selective C-H bond functionalization is a potentially valuable
approach for synthesis in areas ranging from fuels and commodity
chemicals to pharmaceuticals.1 C-H activation studies in our
laboratory have focused on models of the Shilov system,2 particu-
larly methylplatinum cations [(diimine)PtII(Me)(solv)]+ (2, diimine
) ArNdC(Me)-C(Me)dNAr; solv ) 2,2,2-trifluoroethanol (TFE),
H2O).3 These cations are able to activate a variety of R-H bonds
to form [(diimine)PtII(R)(solv)]+ complexes (3), as shown in
Scheme 1, because the H can be removed as methane.4 Under some
conditions double protonolysis of 1a generates dicationic platinum-
(II) complex 4a, which can activate certain C-H bonds: those that
are part of a benzylic or allylic system, or can otherwise form a
chelate product (illustrated for the allylic case in Scheme 1).5
These systems are limited because productive catalytic func-
tionalization, as in the original Shilov system, requires activation
by a metal complex that does not include a sacrificial organic group.
An alternative, non-organometallic route to complexes of the general
form [(diimine)M(solv)2]2+ involves metathesis of [(diimine)MCl2]
complexes with silver salts (in the presence of ∼1% water), which
leads to the formation of dimeric bis(hydroxo)-bridged complexes
for both palladium6 and platinum.7 No C-H activation reactions
have been reported for these species, and in some oxidative
functionalization schemes such dimers appear to be the thermody-
namic “final resting places”.8 As water will usually be a byproduct
of oxidative functionalization, aquo- or hydroxo-ligated complexes
must not be dead ends.
Scheme 2
Scheme 3
We here report that the air- and water-tolerant dimeric hydroxy-
2+
bridged dimers [(diimine)M(OH)]2 (M ) Pt, Pd), prepared and
used without the intermediacy of any M-Me bond, are not
necessarily resting states. In fact, they can effect not only stoichio-
metric activation of a variety of C-H bonds, but also catalytic
conversion of cyclohexene to benzene, using dioxygen as the
terminal oxidant, when the metal is palladium.
Metathesis reactions of [(diimine)MCl2] complexes 6 (M ) Pt)
and 7 (M ) Pd) proceed as shown in Scheme 2. For platinum with
the diimine ligands Ar ) 3,5-t-Bu2C6H3 (6a) or 2,4,6-Me3C6H2 (6b)
mixtures of dimeric hydroxo-bridged (i) and monomeric bis(aquo)
(ii) dications are formed initially, but the dimeric forms can be
isolated cleanly.9 In the case of palladium, the dimers 9ai6 and 9bi
could be cleanly isolated in low yield, while the monomeric bis-
(aquo)dication 9aii was synthesized following a literature proce-
dure.6 8ai and 8aii equilibrate slowly in the presence of added
aqueous HBF4 (Scheme 3), as demonstrated by the dependence of
speciation on [H+] and temperature (K1 ) 1.33(14) × 103 M-1 at
80 °C; ∆H° ) 17(1) kcal/mol and ∆S° ) 63(4) eu from 20 to
80 °C).10
cyclohexenylplatinum(II) complex 10a;9 the reaction is very slow
in the absence of added acid. The proposed mechanism for this
transformation, involving conversion of 8ai to 8aii, coordination
of cyclohexene at a comparable rate, and fast conversion of the
cyclohexene adduct to 10a with the loss of H3O+, is shown in
Scheme 3. The kinetic data for a series of reactions with varying
[HBF4] and [cyclohexene] fit the predicted behavior for this system
of consecutive (pseudo) first-order reactions, with the first step being
first-order in [HBF4] and the second in [cyclohexene], as expected.10
Surprisingly, the second step is also first-order in [HBF4]; this may
suggest that general-acid catalysis is needed to facilitate displace-
ment of coordinated water by cyclohexene.11
Platinum dimer 8ai reacts with cyclohexene in the presence of
acid (delivered as BF3-TFE-d3, HBF4 (aq), or 8aii) to yield
† Present address: Loker Hydrocarbon Research Institute and Department of
Chemistry, University of Southern California, 837 Bloom Walk, Los Angeles,
California 90089-1661.
8a (generally used as a mixture of 8ai and 8aii) activates a variety
of C-H bonds, subject to the apparent requirement that a
multidentate ligand can be thus produced (Scheme 4). Substrates
‡ On scholarly leave from Department of Chemistry, University of the West
Indies, St. Augustine, Trinidad and Tobago.
9
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J. AM. CHEM. SOC. 2008, 130, 2418-2419
10.1021/ja076740q CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4
reaction mixture. Moreover, the consumption of a stoichiometric
amount of dioxygen (measured by Toepler pump) corresponding
to that needed for catalytic oxidation of cyclohexene to benzene
was observed.10
Disproportionation of cyclohexene effected by simple palladium
salts has been previously reported, and proposed to involve
heterogeneous catalysis by precipitated metallic palladium(0);12
there are few examples of homogeneously catalyzed oxidative
dehydrogenation.13 To confirm that the oxidation described in this
work was homogeneous, the filtrate obtained from the reaction
mixture (at various stages of the reaction) was found to be
catalytically active, whereas the precipitated solid was not; fur-
thermore the reaction was successfully performed in the presence
of a drop of elemental mercury.10
b
a Isolated yield (each >95% by NMR). NMR yield.
Work toward increasing mechanistic understanding and expand-
ing the scope of these stoichiometric and catalytic C-H activation
processes involving bis(aquo) complexes is ongoing.
Scheme 5
Acknowledgment. This work was supported by the BP MC2
program, the NIH (NRSA fellowship GM075691 to T.J.W.), and
Caltech (Switzer fellowship to P.F.O.). We thank V. Rostovtsev,
L. Henling, and M. Day for assistance with X-ray crystallography.
Supporting Information Available: Kinetics analysis, van’t Hoff
data, experimental details, and characterization data of all new
compounds (including X-ray information for 8ai and 12).14 This material
is available free of charge via the Internet at http://pubs.acs.org.
References
bearing coordinative directing groups undergo cyclometallation,
giving the allylbenzene and aryl oxime adducts 11 and 12 in high
yield.9 Allylic and benzylic C-H bonds are activated to give η3
platinum species: 10 and 14 are formed quantitatively by NMR,
and 13 is formed in 40% yield (NMR). In contrast, neither benzene
nor cyclohexane reacts detectably with 8a in trifluoroethanol-d3
solution after 20 h at 90 °C. The reaction of 8a with indene to give
14 proceeds smoothly in a variety of solvents, including dichlo-
romethane-d2, trifluoroethanol-d3, or benzene-d6. Although the
presence of water retards the rate of reaction, only minimal
precautions are needed to exclude air or water: 14 can be formed
quantitatively from a dichloromethane-d2 solution prepared in air
on the bench top.
(1) For recent reviews on C-H activation and functionalization, see: (a)
Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc.
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The analogous palladium(II) complexes 9a and 9b also exhibit
C-H activation reactivity: cyclometalates 15a and 15b and indenyl
complexes 16a and 16b are obtained in good yield (Scheme 5).9
With palladium catalytic reactions can also be observed: solutions
containing a mixture of 9ai and 9aii, clean 9aii, or 9bi all convert
cyclohexene to benzene and cyclohexane, with up to forty turnovers
(9aii, 6:1 dichloroethane-d4/trifluoroethanol-d3, 1 atm O2, 60 °C, 3
days).10 The benzene/cyclohexane ratio is influenced by reaction
conditions, particularly oxygen pressure, mixing, and the size and
shape of the reaction vessel. Under certain conditions (2 atm O2 or
thorough mixing), cyclohexane formation can be stopped com-
pletely.10 When the reaction is performed under argon, the ratio of
benzene to cyclohexane is 1:2, which is consistent with dispropor-
tionation being the sole process in the absence of oxygen.
We suggest that in both the presence and absence of oxygen,
the reaction involves the C-H activation of cyclohexene by the
palladium complex, followed by dehydrogenation; the resulting
Pd-H species can be recycled either by hydrogenation of additional
cyclohexene (disproportionation) or oxidation by dioxygen. Forma-
tion of water is indicated by a broad peak in the 1H NMR spectrum
at δ 1.75 ppm in C2D4Cl2, which corresponds to averaged signals
of free and coordinated water (which are in rapid exchange). This
signal is greatly diminished and shifted when D2O is added to the
(5) Driver, T. G.; Williams, T. J.; Labinger, J. A.; Bercaw, J. E. Organome-
tallics 2007, 26, 294-301.
(6) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D. M.; Labinger, J. A.; Bercaw, J.
E. Organometallics 2003, 22, 3884-3890.
(7) Kannan, S.; James, A. J.; Sharp, P. R. Polyhedron 2000, 19, 155-163.
(8) See for example: Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A.
N. Organometallics 2007, 26, 2402-2413.
(9) Preparative details and characterization data for all new complexes are
provided in the Supporting Information.
(10) Experimental details and more extensive discussion are provided in the
Supporting Information.
(11) Previous studies imply a rather high kinetic barrier to simple displacement
of water by hydrocarbon ligands in related systems.4
(12) (a) Wolfe, S.; Campbell, P. G. C. J. Am. Chem. Soc. 1971, 93, 1497-
1499. (b) Trost, B. M.; Metzner, P. J. J. Am. Chem. Soc. 1980, 102, 3572-
3577. We have confirmed that the latter case is heterogeneous; see
Supporting Information.
(13) PdCl2 in DMF/H2O catalyzes the reaction under conditions similar to ours,
but a quinone co-oxidant is required: Sheldon, R. A.; Sobczak, J. M. J.
Mol. Catal. 1991, 68, 1-6. An earlier report on a Pd cluster catalyst,
Pd5(PPh3)2, works with O2 alone: Berenblum, A. S.; Knizhnik, A. G.;
Mund, S. L.; Moiseev, I. I. IzV. Akad. Naukk SSSR Ser. Khim. 1980, 2700-
2704. Also, Pd{OOC(CF3)}2 aerobically catalyzes the dehydrogenation
of cyclohexanone to phenol and cyclohexenone: Muzart, J.; Pete, J. P. J.
Mol. Catal. 1982, 15, 373-376.
(14) Crystallographic data have been deposited at the CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K., and copies can be obtained on request, free
of charge, by quoting the publication citation and the deposition numbers
655348 (8ai) and 665648 (12).
JA076740Q
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