Activation of C-H bonds of arenes: Selectivity and reactivity in bis(pyridyl) platinum(II) complexes

Article abstract of DOI:10.1021/ja055436z

The reaction of [PtMe₂(NN)] and B(C₆F₅)₃/H₂O in CF₃CH₂OH with arenes Ar-H gives [PtAr{HOB(C₆F₅)₃}(LL)] if the bis(pyridyl) ligand NN forms a six-membered, but not five-membered, chelate ring; methyl-substituted arenes give selectivity for metalation of meta > para ? ortho, but methoxy-substituted arenes give ortho ? meta, para. Copyright

Full text of DOI:10.1021/ja055436z

Published on Web 09/24/2005
Activation of C-H Bonds of Arenes: Selectivity and Reactivity in Bis(pyridyl)
Platinum(II) Complexes
Fenbao Zhang, Christopher W. Kirby, Douglas W. Hairsine, Michael C. Jennings, and
Richard J. Puddephatt*
Department of Chemistry, The UniVersity of Western Ontario, London, Canada N6A 5B7
Received August 9, 2005; E-mail: pudd@uwo.ca
There has been rapid progress in understanding the factors that
influence selectivity and reactivity in C-H bond activation in arenes
or alkanes by organoplatinum(II) compounds.1-4 The reactions with
arenes are often reversible and proceed through “arene platinum-
(II)” and “aryl(hydrido)platinum(IV)” complex intermediates. In
arene activation reactions, the rate-determining step can be arene
coordination or C-H oxidative addition, and the C-H oxidative
addition/reductive coupling steps can be reversible and either faster
or slower than methane displacement. Since the design of catalysts
for reactions involving C-H activation requires knowledge of the
factors affecting reactivity and selectivity in C-H activation
reactions, we are prompted to report an arene C-H bond activation
reaction in which the chelate ring size of the bidentate ligand, NN,
and its effect on the ligand geometry (Chart 1), is a critical factor.
In addition, it is shown that methyl and methoxy substituents on
the arene give opposite selectivity.
Figure 1. The structures of the methyl and phenylplatinum complexes
[PtRX(DPK)]: (a) R ) Me, 2a; and (b) R ) Ph, 4a.
Chart 1. Planar and Bowed Bipyridine Groups in 1d and 1a-1c,
Respectively (note the orientation of the o-hydrogen atoms in the
plane in 1d and out of the plane in 1a-1c)
Reactions of [PtMe2(NN)], 1a, NN ) di-2-pyridyl ketone (DPK);
1b, NN ) di-2-pyridylamine (DPA); 1c, NN ) di-2-pyridylmethane
(DPM); 1d, NN ) 4,4′-di-tert-butyl-2,2′-bipyridyl (bu2bpy) (Chart
1), with B(C6F5)3/H2O in CF3CH2OH occurred rapidly at room
temperature with loss of methane to give the corresponding
complexes [PtMe{HOB(C6F5)3}(NN)], 2a-2d.5 The anion [HOB-
(C6F5)3]- acts as a good leaving group; replacement by CO gives
the complexes [PtMe(CO)(NN)][HOB(C6F5)3], 3a-3d, whose
carbonyl stretching frequencies are 2119, 2105, 2109, and 2109
cm-1 when NN ) DPK, DPA, DPM, and bu2bpy, respectively,
indicating that DPK is the weakest donor, while the other ligands
have similar electronic effects. The structures of 2a (Figure 1) and
3b were confirmed crystallographically.5
Scheme 1. X ) HOB(C6F5)3
Reactions of 2a-2c (or, more conveniently, a mixture of 1a-
1c and B(C6F5)3/H2O) with benzene or with several methyl- or
methoxy-substituted arenes Ar-H in CF3CH2OH gave the corre-
sponding complexes [PtArX(NN)], with X ) HOB(C6F5)3 and
NN ) DPK, DPA, or DPM. However, complex 2d, NN ) bu2-
bpy, failed to react with arenes under these conditions. Because it
is established that the ligands DPA, DPM, and bu2bpy have similar
electronic effects, the enhanced reactivity of 2a-2c compared to
that of 2d must arise from the presence of bowed six-membered
Pt(NN) chelate rings in 2a-2c compared to the more planar five-
membered chelate ring in 2d (see Chart 1, which shows the different
orientations of the ortho-hydrogen atoms of the chelate ligands).6
The complexes 2a-2c, with NN ) (2-C5H4N)2X and X ) CO,
NH, and CH2 respectively, show similar reactivity to arenes,
although the carbonyl group is more electron-withdrawing than
either the NH or CH2 group, further suggesting that structural rather
than electronic effects are responsible for the difference in reactivity
of 2d compared to that of 2a-2c.
(NN)], 5a, NN ) DPK; 5b, NN ) DPA (Scheme 1). NMR analysis
showed that the tolyl complexes exist as a mixture of isomers with
meta:para:ortho ) 70:27:3 for 5a and 75:22:3 for 5b, consistent
with results for related diimine complexes.2-4 Reactions of 2a or
2c with o-xylene and of 2a or 2b with m-xylene gave only the
isomers shown in Scheme 1,5 with no products of ortho C-H or
benzylic C-H bond activation detected. Mesitylene, which has only
ortho and benzylic C-H bonds, failed to react with complex 2a
under similar conditions. The structures of 6a, 6c, and 7a were
confirmed crystallographically.5
Anisole reacted with 2a to give [Pt(C6H4OMe)X(DPK)], 8a, as
a mixture of o-, m-, and p-isomers in a ratio 90:8:2, from which
the pure ortho isomer was crystallized, and m-dimethoxybenzene
reacted to give [Pt{2,4-C6H3(OMe)2}X(DPK)], 9a, respectively
Benzene reacted with complexes 2a-2c to give the correspond-
ing phenyl complexes [PtPhX(NN)], 4a-4c,5 while toluene reacted
with 2a or 2b to give the corresponding complex [Pt(C6H4Me)X-
9
14196
J. AM. CHEM. SOC. 2005, 127, 14196-14197
10.1021/ja055436z CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. X ) HOB(C6F5)3 (the inset shows the structure of
10a)
References
(1) (a) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259. (b) Labinger,
J. A.; Bercaw, J. E. Nature 2002, 417, 507.
(2) (a) Heyduk, A. F.; Driver, T. G.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 15034. (b) Zhong, H. A.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 2002, 124, 1378.
(3) Norris, C. E.; Reinartz, S.; White, P. S.; Templeton, J. L. Organometallics
2002, 21, 5649.
(4) (a) Johansson, L.; Ryan, O. B.; Romming, C.; Tilset, M. J. Am. Chem.
Soc. 2001, 123, 6579. (b) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105,
2471.
(5) Experimental details, including synthetic procedures, spectroscopic data,
and X-ray data for complexes 2a, 3b, 4a, 6a-10a, and 6c, are given in
the Supporting Information. The NMR data for the complexes support
the structures and show that the [B(OH)(C6F5)3]- anion remains coordi-
nated in CD2Cl2 solution. In each case, the OH proton appeared as a septet
in the 1H NMR at room temperature through coupling to all six ortho-
fluorine atoms, and the 19F NMR contained only three resonances (o, m,
p). However, at -80 °C, all fluorine atoms were inequivalent due to
restricted rotation about the B-O and B-C bonds. For example, complex
2a gave at 20 °C δ(OH) ) 3.30, sept., J(HF) ) 3 Hz, but at -80 °C,
δ(OH) ) 3.36, d, J(HF) ) ca. 20 Hz, indicating H-bonding of the OH
proton to only one ortho-fluorine atom at low temperature. At -80 °C,
δ(o-F) ) -127.0, -132 (3F), -139.6, -142.2; δ(p-F) ) -157.5, -160.0,
-161.3; δ(m-F) ) -164.1, -164.4, -164.5, -164.6; -165.0 (2F).
(Scheme 2).5 The prominence of ortho-metalation is obvious, but
there is no associated acceleration of reaction. Thus, reaction of
2a with an equimolar mixture of m-xylene and m-dimethoxybenzene
gave 7a and 9a in a 3:2 ratio, and the reaction was slower than
with m-xylene alone. Reaction of 3-methylanisole with 2a gave
[PtArX(DPK)], 10a, with Ar ) 2-MeO-4-Me-C6H3 (Scheme 2),5
showing that, in the reagent that contains both methyl and methoxy
substituents, the methoxy group controls the site of metalation.7
The reaction of 1a and B(C6F5)3/H2O in CF3CH2OH with
o-xylene or anisole gave a kinetic isotope effect kH/kD ) 2.8 or
3.6, respectively,8 and the similar reactions with o-xylene-d10 or
anisole-d8 gave CH4 and CH3D, with only traces of CH2D2,
indicating that the reductive coupling step to give methane is largely
irreversible and that the C-H oxidative addition step is rate-
determining.2-4 In agreement, the arylplatinum product from
reaction of 1 with o-xylene-d10 in CF3CH2OH was very largely
10-d9, with <2% H incorporation at the aromatic sites and no H
incorporation at the methyl sites.8
In conclusion, it is shown that the chelate ring size of supporting
bipyridyl ligands is an important factor in C-H activation of arenes
and should be considered in the design of more active catalysts.9
In addition, methyl and methoxy substituents on the arene are shown
to give very different regioselectivities. It is interesting that the
selectivity in toluene activation is similar in this case, in which
C-H oxidative addition is product-determining, and with the ligand
ArNdC(Me)C(Me)dNAr, in which the methane displacement step
is most important.1-4 The methoxy group is shown for the first
time to give o-platination but without an associated rate accelera-
tion.10
(6) Hill, G. S.; Manojlovic-Muir, Lj.; Muir, K. W.; Puddephatt, R. J.
Organometallics 1997, 16, 525. The carbonyl group in DPK is known to
form hemiketals with alcohols, and this might affect reactivity, but we
did not detect any reaction of either DPK or complex 4a with trifluoro-
ethanol, as indicated by NMR.
(7) If the methyl group controlled the site of metalation, the major product
would be that with Ar ) 3-MeO-5-Me-C6H3. We suggest that the methoxy
group is a better ligand for platinum than the π-arene and directs metalation
to the least hindered ortho site, though a transient arene complex is
probably required for metalation.1-4 In the isolated 2-methoxyarylplatinum
products, the Pt‚‚‚O distance is >3 Å, and there are no bond distortions
that might suggest a bonding interaction between platinum and the
methoxy substituent. The hydroxy proton was not directly located in 8a-
10a, but it lies close to the 2-methoxyaryl oxygen atom and to one or
two ortho-fluorine atoms. For example, in 10a (Scheme 2), the short
contacts are O‚‚‚O ) 2.73, O‚‚‚F ) 2.65, 2.76 Å, all in the range for
which a hydrogen bond is possible.
(8) The mechanistic implications of the presence or absence of multiple H-D
exchange between aryl and methyl groups or between hydrocarbonyl
groups and solvent have been discussed in refs 2-4. The isotope effects
were determined by reaction of 1a with equimolar amounts of o-xylene/
o-xylene-d10 or anisole/anisole-d8, with analysis of the H/D content of
the isolated product by 1H NMR. In the reaction of 1 with anisole-d8 in
CF3CH2OH, a greater degree of H-D exchange was observed in the 5-
(ca. 8% H) and 3- (ca. 6% H) positions of the deuterated 2-methoxy-
phenylplatinum complex 8a. This is attributed largely to H/D exchange
through conventional electrophilic substitution para or ortho to the
activating methoxy group, with a small preference for para. The
arylplatinum complexes were inert to C-H activation; for example, 4a
failed to react with o-xylene or 3-methoxyanisole.
(9) The bowing of the six-membered chelate ring in the DPM complexes is
not very easily inverted. Thus, the platinum(II) complexes give two well-
separated resonances for the CH2 protons [e.g., 1c, δ(CH2) ) 4.00, 4.74].
This bowing should reduce steric hindrance between o-substituents of the
pyridyl and incipient aryl group in the transition state compared to planar
bipyridine.
Acknowledgment. We thank the NSERC (Canada) for financial
support.
(10) Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S. J. Am. Chem.
Soc. 2004, 126, 13192. This article reviews some CH activation reactions
of arenes, in which a substituent may give selectivity for ortho product
as a result of either kinetic or thermodynamic control, as well as providing
an elegant example of the second.
Supporting Information Available: Experimental procedures,
diagrams of X-ray structures, and spectroscopic data (PDF), X-ray data
for complexes 2a, 3b, 4a, 6a-10a, and 6c (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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