C-H Bond Activation by Unsymmetrical 2-(N-Arylimino)pyrrolide Pt Complexes: Geometric Effects on Reactivity

Article abstract of DOI:10.1021/ja036511d

Reactions of chloroplatinum methyl complexes with N-(arylimino)pyrrolide anions afford cis and trans neutral platinum methyl complexes. Isomers with methyl trans to the pyrrolide nitrogen activate benzene C-H bonds at 85 °C more than 80 times faster than

Full text of DOI:10.1021/ja036511d

Published on Web 09/24/2003
C-H Bond Activation by Unsymmetrical 2-(N-Arylimino)pyrrolide Pt
Complexes: Geometric Effects on Reactivity
Carl N. Iverson,† Charles A. G. Carter,† R. Tom Baker,*,† John D. Scollard,‡ Jay A. Labinger,*,‡ and
John E. Bercaw*,‡
Los Alamos Catalysis InitiatiVe, Chemistry DiVision, MS J514, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545, and Arnold and Mabel Beckman Laboratory for Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received June 4, 2003; E-mail: bakertom@lanl.gov
Scheme 1
N-Ligated chelate complexes of Pt(II) provide useful models for
the C-H bond activation step in selective oxidation of alkanes by
aqueous solutions of chloroplatinate salts (the Shilov system)1 and
are effective catalysts for methane oxidation by sulfuric acid.2
Cationic complexes of the form [(N-N)PtMeL]+, where (N-N)
is a neutral bidentate ligand, have been extensively studied.3 Our
mechanistic picture of the Shilov system4 suggests that C-H
activation followed by oxidation to Pt(IV) could provide a pathway
to catalytic alkane oxidation by dioxygen. However, in contrast to
neutral Pt dimethyl complexes (N-N)PtMe2,5 recent experimental
evidence suggests cationic methyl complexes can be resistant to
oxidation by dioxygen to Pt(IV).6 Alkane activation by Pt(II) and
a monoanionic bidentate N-N ligand would lead to a neutral
complex (N-N)PtRL that might be more readily oxidized; several
such complexes have been shown to be capable of C-H bond
activation.7 We report here on neutral Pt(II) complexes of unsym-
metrical 2-(N-arylimino)pyrrolide ligands, which are easily syn-
Scheme 2
thesized,8 and also present two types of nitrogen ligation with widely
different steric and electronic attributes, leading to dramatic
geometric effects on C-H bond activation chemistry.
Treatment of (N-arylimino)pyrroles 1a,b with n-BuLi followed
by trans-PtMeCl(SMe2)2 affords (N-N′)PtMe(SMe2) as a mixture
of cis and trans isomers9 of 2. Structures are assigned on the basis
of NMR10,11 coupled with a crystal structure of cis-2c (Vide infra).
On heating in C6D6 at 85 °C, both isomers react to give the cis
isomer of the corresponding phenyl complex 3 along with CH3D,
but at quite different rates: approximate half-lives are 1 and 4 h
for trans-2a and trans-2b and 3 and 14 d for cis-2a and cis-2b,
respectively (Scheme 1). We suggest that rate-determining associa-
tive displacement of SMe2 by benzene accounts for these differ-
ences.12 The greater electronegativity of the ligand in 2a would
tend to favor associative displacement. Furthermore, the trigonal
bipyramidal intermediate for such displacement in trans-2 would
have the better π-acceptor imine ligand equatorial and the better
σ-donor pyrrolide axial, the strongly preferred conformation13
(Scheme 2). Slower displacement in the cis isomer must pass
through a higher-energy intermediate and is accompanied by
isomerization to the thermodynamically preferred cis isomer of 3.
In agreement, trans-2a undergoes rapid exchange (τ1/2 ≈ 5 min)
with S(CD3)2 at room temperature, whereas cis-2a is only partly
exchanged after 2 d, during which time trans-2a is completely
isomerized to cis-2a.14
X-ray crystallography.11 For 1a,b, an intermediate may be observed
by NMR at earlier stages; spectral parameters11,15 suggest the agostic
structure shown in Scheme 3. Remarkably, when the reaction is
carried out in C6D6, the remaining Pt-methyl group is about 90%
deuterated, whereas the liberated methane is completely undeuter-
ated.
Bulkier ligands 1d,e with ortho-disubstituted aryl groups behave
completely differently: reaction in benzene leads rapidly and
directly to the C-H activation products trans-3d,e,11 along with 2
equiv of liberated methane (Scheme 3). Significant deuteration of
the methane is observed in C6D6;16 for 3d, the methyl groups of
the isopropyl substituents are partially deuterated as well.17 In
contrast, reaction of 1d with 4 in acetonitrile leads smoothly to the
MeCN-methyl complex cis-5d; the assigned geometry has been
confirmed by a crystal structure.11 5d does not react with benzene
even at reflux.
In principle, methyl complexes 2 could be prepared directly from
1 and [PtMe2(µ-SMe2)]2 (4) by protonolysis of one methyl group
by the pyrrole proton. Indeed, 1a-c react with 4 in benzene at
ambient temperature over several days to form cis-2a-c with
methane evolution. The structure of cis-2c has been confirmed by
It appears that the C-H activation of benzene, whether leading
all the way to phenyl products as in 3d,e or just to H/D exchange
† Los Alamos.
‡ Caltech.
9
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J. AM. CHEM. SOC. 2003, 125, 12674-12675
10.1021/ja036511d CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
G. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1993, 115, 3004.
(c) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E.
Organometallics 1994, 13, 755. (d) Luinstra, G. A.; Wang, L.; Stahl, S.
S.; Labinger, J. A.; Bercaw, J. E. J. Organomet. Chem. 1995, 504, 75. (e)
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1995, 117,
9371-9372. (f) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1996, 118, 5961-5976.
as in 2a,b, must precede chelation, because preformed complexes
2 react with benzene only under considerably more stringent
conditions. The fact that only CH4 is produced in the reactions of
1a,b with 4 further suggests that protonolysis by the pyrrole proton
precedes any benzene activation, implying that the pyrrolide-Pt
bond is formed while the imine N is not coordinated and hence
that the intermediate observed in these reactions must be formed
reversibly. The fact that only trans-3d,e are observed is presumably
a consequence of strong steric interactions between the ortho-aryl
substituents and a cis-SMe2 ligand, disfavoring the cis geometry
which otherwise appears preferred. Indeed, reaction of 1d with
trans-PtMeCl(SMe2)2 affords only trans-2d. A more complete
mechanistic interpretation of the observations of Scheme 3 is not
possible at this time; yet it is clear that C-H activation chemistry
in these Pt(II) systems is highly dependent upon ligand electronic
and steric properties, as well as subtle geometric factors. Exploita-
tion of this behavior for the design of alkane functionalization
catalysts is the subject of ongoing investigation in our labs.
(5) (a) Rostovtsev, V. V.; Labinger, J. A.; Bercaw, J. E.; Lasseter, T. L.;
Goldberg, K. I. Organometallics 1998, 17, 4530-4531. (b) Rostovtsev,
V. V.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002,
41, 3608-3619.
(6) Scollard, J. D.; Day, M.; Labinger, J. A.; Bercaw, J. E. HelV. Chim. Acta
2001, 84, 3247-3268.
(7) Several reports relating C-H bond activation chemistry of neutral
platinum-methyl complexes utilizing monoanionic bi- or tridentate ligands
have recently appeared: (a) Wick, D. D.; Goldberg, K. I. J. Am. Chem.
Soc. 1997, 119, 10235. (b) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J.
Am. Chem. Soc. 2001, 123, 6579-6590. (c) Fekl, U.; Goldberg, K. I. J.
Am. Chem. Soc. 2002, 124, 6804-6805. (d) Thomas, J. C.; Peters, J. C.
J. Am. Chem. Soc. 2001, 123, 5100-5101. (e) Reinartz, S.; White, P. S.;
Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 12724-
12725. (f) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L.
Organometallics 2001, 20, 1709-1712. (g) Lo, H. C.; Haskel, A.; Kapon,
M.; Keinan, E. J. Am. Chem. Soc. 2002, 124, 3226-3228. (h) Iron, M.
A.; Lo, H. C.; Martin, J. M. L.; Keinan, E. J. Am. Chem. Soc. 2002, 124,
7041-7054.
(8) Tanaka, T.; Yamauchi, O. Chem. Pharm. Bull. 1961, 9, 588.
Acknowledgment. Parts of this work were supported by Akzo-
Nobel; a joint DOE-OIT grant; the Los Alamos LDRD program;
and BP. We thank Ulrich Fekl and Dave Thorn for helpful
discussions. J.D.S. thanks Larry Henling and the X-ray Crystal-
lography Facility of the Beckman Institute at Caltech for assistance
with crystal structure determinations.
(9) The cis:trans ratio of the products is 1:1 for 2a and 1:4 for 2b. We
arbitrarily label the geometries cis and trans according to the relationship
of the anionic (methyl or phenyl) group and the pyrrolide arm of the
chelating ligand.
(10) Coupling constants in 195Pt satellites appear to be reliably diagnostic, with
3
2JPtH for Pt-Me (79-80 Hz vs 73 Hz) and JPtH for SMe2 (51 Hz vs 58
Hz), for cis (Me cis to pyrrolide) versus trans isomers, respectively.
(11) Synthetic details, full NMR spectroscopic characterization of all new
compounds, and X-ray crystallographic results are presented in the
Supporting Information.
Supporting Information Available: Preparative details and NMR
data for all new compounds; X-ray crystallographic results for cis-2c
and cis-5d (PDF and CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(12) Hydrocarbon substitution of labile ligands in C-H bond activation studies
has been observed previously. See, for example, refs 3b, 3e, and 7a.
(13) See, for example: Tobe, M. L.; Burgess, J. Inorganic Reaction Mecha-
nisms; Addison-Wesley Longman: New York, 1999; pp 101-103
References
(14) Thermolysis of the isolable, more stable isomer, cis-2, in C6D6 results
only in the activation of benzene solvent, and we do not observe trans-2
in the reaction solution. However, given the difference in qualitative rates,
it is possible that cis-2 isomerizes to trans-2 and then undergoes C-H
bond activation.
(15) Interaction between the pyrrole N-H and the platinum metal center is
suggested by a substantial downfield shift of the signal for the former, to
δ 13.9 and JPt-H ) 24 Hz. This is the lowest value observed for this
coupling, however, and another possibility may be an H-bonding interac-
tion with the sulfur.
(1) (a) Gol’dshleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman, A. A.
Zh. Fiz. Khim. (Engl. Transl.) 1972, 46, 785. (b) Shilov, A. E. ActiVation
of Saturated Hydrocarbons by Transition Metal Complexes; Reidel:
Boston, 1984.
(2) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H.
Science 1998, 280, 560-564.
(3) (a) Holtcamp, M. H.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1997, 119, 848-849. (b) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am.
Chem. Soc. 1999, 121, 1974-1975. (c) Johansson, L.; Tilset, M.; Labinger,
J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846-10855. (d)
Johansson, L.; Ryan, O. B.; Romming, C.; Tilset, M. J. Am. Chem. Soc.
2001, 123, 6579-6590. (e) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E.
J. Am. Chem. Soc. 2002, 124, 1378-1399.
(16) Added SMe2 decreases the amount of methane deuteration, increases the
amount of trans-2 formed, and slows the rate of the reaction.
(17) Similar exchange has been observed in a related system: Fekl, U.;
Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804-6805.
(4) (a) Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.; Bercaw,
J. E.; Horvath, I. T.; Eller, K. Organometallics 1993, 12, 895. (b) Luinstra,
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