Structural characterization of an intermediate in arene C-H bond activation and measurement of the barrier to C-H oxidative addition: A platinum(II) η2-benzene adduct

Full text of DOI:10.1021/ja011765p

12724
J. Am. Chem. Soc. 2001, 123, 12724-12725
methylpyrazolyl)borate;7 R ) alkyl, aryl, or silyl8] initiates a
cascade of reactions AfBfCfD as shown in eq 1. Protonation
Structural Characterization of an Intermediate in
Arene C-H Bond Activation and Measurement of
the Barrier to C-H Oxidative Addition: A
Platinum(II) η2-Benzene Adduct
Stefan Reinartz, Peter S. White, Maurice Brookhart,* and
Joseph L. Templeton*
Department of Chemistry, UniVersity of North Carolina
Chapel Hill, North Carolina 27599-3290
ReceiVed July 20, 2001
Transition metal arene complexes display multifaceted coor-
dination chemistry,1 and they are key intermediates in aromatic
C-H bond activation.2,3 Most recently, Johansson, Tilset, Lab-
inger, and Bercaw have detected by NMR spectroscopy a
platinum(II) benzene complex, [(diimine)Pt(η2-C6H6)(CH3)]+ (1),
a precursor to arene C-H oxidative addition.4 Given the
importance of η2-arene intermediates in aromatic C-H bond
activation at late transition metals in general, and at platinum in
particular,4,5 we now report acid-assisted reductive elimination
of benzene from a Pt(IV) phenyl dihydride complex to form a
Pt(II) η2-benzene hydride complex which has been characterized
by NMR spectroscopy and by single-crystal X-ray diffraction.6
Isolation of a ground-state η2-benzene adduct with an adjacent
hydride ligand has allowed us to quantitatively assess the barrier
to oxidative addition of a benzene C-H bond to Pt(II) by
monitoring exchange of the hydride and arene signals in variable
temperature NMR experiments.
of Tp′Pt(SiEt3)(H)2 leads to an isolable five-coordinate cationic
silyl dihydride Pt(IV) complex, [κ2-(HTp′)Pt(H)2(SiEt3)][BAr′]
4
(intermediate B in eq 1) [BAr′ ) tetrakis(3,5-triflurometh-
4
ylphenyl)borate], which has been structurally characterized.9
Protonation at a pyrazole nitrogen atom promotes reductive
elimination of methane from Tp′PtMe(H)2, and ultimately leads
to isolable cationic Pt(II) hydride complexes of the type [κ2-
(HTp′)Pt(H)(L)][BAr′] (D) after addition of a trapping ligand, L
(intermediates B and4C are not observed).8g For R ) aryl, we
show here that it is possible to isolate the putative Pt(II) (RH)
adduct (C) prior to ligand exchange.
Protonation of Pt(IV) complexes of the type TpPt(R)(H)2 (A)
[Tp ) hydridotris(pyrazolyl)borate; Tp′ ) hydridotris(3,5-di-
(1) (a) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T. A.
Chem. ReV. 1982, 82, 499. (b) Wadepohl, H. Angew. Chem., Int. Ed. Engl.
1992, 31, 247. (c) Harman, W. D. Chem. ReV. 1997, 97, 1953.
Protonation of Tp′Pt(C6H5)(H)2 (2)10 with [H(OEt2)2][BAr′]11
in CD2Cl2 at -78 °C yields a single chiral compound (eq 24).12
(2) (a) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions
of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer:
Dordrecht, 2000. (b) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97,
2879. (c) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91. (d) Jones,
W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650. (e) Jones, W. D.;
Feher, F. J. J. Am. Chem. Soc. 1986, 108, 4814. (f) Jones, W. D.; Dong, L.
J. Am. Chem. Soc. 1989, 111, 8722. (g) Sweet, J. R.; Graham, W. A. G. J.
Am. Chem. Soc. 1983, 105, 305. (h) Werner, R.; Werner, H. Angew. Chem.,
Int. Ed. Engl. 1981, 20, 793. (i) Hackett, M.; Ibers, J. A.; Whitesides, G. M.
J. Am. Chem. Soc. 1988, 110, 1436.
(3) Arene C,C-η2-coordination is not necessarily involved in the bond
activation process, see, e.g.: (a) Peterson, T. H.; Golden, J. T.; Bergman, R.
G. J. Am. Chem. Soc. 2001, 123, 455. (b) Vigalok, A.; Uzan, O.; Shimon, L.
J. W.; Ben-David, Y.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 1998,
120, 12539.
(4) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2000, 122, 10846.
(5) (a) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem.
Soc. 2001, 123, 6579. (b) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc.
2001, 123, 5100. (c) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123,
739. (d) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang, O.;
Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831. (e) Johansson, L.; Ryan, O.
B.; Tilset, M. J. Am. Chem. Soc. 1999, 121, 1974. (f) Stahl, S. S.; Labinger,
J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180. (g) Wick, D. D.;
Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235. (h) Holtcamp, M. W.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1997, 119, 848. (i) Stahl, S.
S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961.
(6) For other examples of crystallographically characterized η2-arene
complexes, see: (a) Meiere, S. H.; Brooks, B. C.; Gunnoe, T. B.; Sabat, M.;
Harman, W. D. Organometallics 2001, 20, 1038 (a rare benzene adduct). (b)
Casas, J. M.; Fornie´s, J.; Mart´ın, A.; Menjo´n, B.; Toma´s, M. J. Chem. Soc.,
Dalton Trans. 1995, 2949. (c) Tagge, C. D.; Bergman, R. G. J. Am. Chem.
Soc. 1996, 118, 6908. (d) Batsanov, A. S.; Crabtree, S. P.; Howard, J. A. K.,
Lehmann, C. W.; Kilner, M. J. Organomet. Chem. 1998, 550, 59. (e) van der
Heijden, H.; Orpen, A. G.; Pasman, P. J. Chem. Soc., Chem. Commun. 1985,
1576. (f) Belt, S. T.; Duckett, S. B.; Helliwell, M.; Perutz, R. N. J. Chem.
Soc., Chem. Commun. 1989, 928. (g) Jones, W. D.; Partridge, M. G.; Perutz,
R. N. J. Chem. Soc., Chem. Commun. 1991, 264. (h) Belt, S. T.; Helliwell,
M.; Jones, W. D.; Partridge, M. G.; Perutz, R. N. J. Am. Chem. Soc. 1993,
115, 1429. (i) Higgitt, C. L.; Klahn, A. H.; Moore, M. H.; Oelckers, B.;
Partridge, M. G.; Perutz, R. N. J. Chem. Soc., Dalton Trans. 1997, 1269.
No free benzene is observed. A resonance at 11.67 ppm in the
(7) Trofimenko, S. Scorpionates-The Coordination Chemistry of Polypyra-
zolylborate Ligands; Imperial College Press: London, 1999.
(8) (a) O’Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc.
1996, 118, 5684. (b) Canty, A. J.; Dedieu, A.; Jin, H.; Milet, A.; Richmond,
M. K. Organometallics 1996, 15, 2845. (c) Wick, D. D.; Goldberg, K. I. J.
Am. Chem. Soc. 1999, 121, 11900. (d) Haskel, A.; Keinan, E. Organometallics
1999, 18, 4677. (e) Reinartz, S.; White, P. S., Brookhart, M.; Templeton, J.
L. Organometallics 2000, 19, 3748. (f) Reinartz, S.; White, P. S., Brookhart,
M.; Templeton, J. L. Organometallics 2000, 19, 3854. (g) Reinartz, S.; Baik,
M. H.; White, P. S., Brookhart, M.; Templeton, J. L. Inorg. Chem. 2001, 40,
4726.
10.1021/ja011765p CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/17/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 50, 2001 12725
1H NMR spectrum is assigned to the protonated pyrazole ring.8f,9,13
The lone platinum-bound hydride resonates unusually far upfield
1
at -30.13 ppm (1H, JPt-H ) 935 Hz), and a singlet in the
aromatic region at 6.88 ppm, which integrates for six protons,
indicates that a cationic benzene adduct, [κ2-(HTp′)Pt(H)(η2-
C6H6)][BAr′] (3), has formed. Only one 13C resonance at 115.3
4
ppm is observed for the six-ring carbon atoms at -78 °C.
Complex 3 slowly undergoes loss of benzene and forms a
dicationic hydride-bridged platinum dimer8g in CH2Cl2 solution
at 273 K.
If benzene is added to an NMR sample of complex 3 at 193
K, distinct resonances are observed for free (7.33 ppm) and
platinum-coordinated benzene (6.95 ppm), indicating that benzene
exchange does not occur on the NMR time scale. As the probe
temperature is raised to 252 K, both the platinum-bound benzene
and hydride resonances broaden significantly, suggesting that
hydrogen exchange is occurring between these positions. The
signal for free benzene remains sharp. Using the slow exchange
approximation at 252 K, a first-order rate constant of k ) 47 s-1
was determined via line broadening of the hydride resonance,
corresponding to a barrier (∆Gq) for hydrogen exchange of 12.7
kcal/mol. As expected, line broadening of the bound benzene
resonance is approximately one-sixth that of the hydride reso-
nance. Hydrogen exchange between the bound benzene and
hydride positions is also evident from a spin saturation transfer
experiment: irradiating the coordinated benzene resonance at 6.95
ppm at 243 K leads to almost complete disappearance of the
hydride resonance. We postulate that the five-coordinate Pt(IV)
aryl dihydride intermediate 4 is accessible via oxidative addition
from the Pt(II) benzene adduct 3, but the Pt(II) benzene structure
is the ground state (eq 2). This result complements protonation
studies involving Pt(IV) silyl complexes, where the five-coordinate
Pt(IV) structure is the ground state.9
Figure 1. ORTEP diagram of [κ2-((Hpz*)BHpz*2)Pt(H)(C,C-η2-C6H6)]-
[BAr′] (3); ellipsoids are drawn at the 50% probability level, and the
4
BAr′ counterion is omitted for clarity.
4
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complex 3
Pt1-C11
2.241(11)
2.214(11)
2.037(6)
35.5(6)
167.3(5)
99.3(5)
Pt1-N31
2.171(8)
1.360(24)
1.35(3)
94.2(4)
87.1(3)
71.2(7)
119.7(14)
Pt1-C12
C11-C12
Pt1-N21
C12-C13
C11-Pt1-C12
C11-Pt1-N21
C11-Pt1-N31
C12-Pt1-N21
C12-Pt1-N31
N21-Pt1-N31
Pt1-C11-C12
C11-C12-C13
155.6(4)
was subjected to X-ray structural analysis; an ORTEP diagram
is shown in Figure 1. The hydride ligand on platinum was not
located in the difference Fourier map, and thus it is placed in a
calculated position. The benzene ligand indeed coordinates in η2-
fashion to the square-planar Pt(II) center. The orientation of the
plane of the aromatic ring versus the metal plane is almost
perpendicular (tilt angle: 82.5°),6a,15 and the Pt-C distances (2.24
and 2.21 Å) lie in the expected range for this coordination mode.6
The orientation of the benzene ring is probably responsible for
the unusual upfield shift of the hydride resonance in the 1H NMR
spectrum of complex 3: the hydride ligand is placed in the
shielded area directly above the aromatic ring.16
In the Cp*Rh(PMe3) system [Cp* ) pentamethylcyclopenta-
dienyl], Jones has established that oxidative addition of benzene
occurs via an η2-benzene adduct Cp*(PMe3)Rh(η2-C6H6) (5), but
the ground state in this case is the Rh(III) phenyl hydride
Cp*(PMe3)Rh(H)(C6H5) (6).2c-e,14 The barrier for conversion of
5 to 6 is unknown.
A potentially attractive alternative route to benzene adduct 3,
protonation of Tp′Pt(CH3)(H)2 and subsequent addition of excess
benzene, does not yield the desired product. Note that the Pt(II)
benzene adduct 1 was also generated by protonation of a Pt(II)
phenyl complex, and not by addition of benzene to a Pt(II) solvent
species.4 Addition of 5 equiv of deuteriobenzene (C6D6) to a
solution of 3 at low temperature does not lead to observable
exchange of free and coordinated benzene even after 2 days at
243 K.
Isolation and structural characterization of a Pt(II) η2-benzene
complex provide detailed information about an arrested intermedi-
ate in aromatic C-H bond activation at platinum. The barrier
for the arene C-H oxidative addition reaction which converts a
Pt(II) η2-benzene adduct to a Pt(IV) phenyl hydride complex has
been determined and found to be 12.7 kcal/mol.
Colorless crystals of the benzene adduct [κ2-(HTp′)Pt(H)(η2-
C6H6)][BAr′] (3) were obtained in 90% yield by slow diffusion
4
Acknowledgment. We gratefully acknowledge the National Science
Foundation (Grant CHE-9727500) and the National Institutes of Health
(Grant GM 28938) for support of this research.
of pentane into a methylene chloride solution. A single crystal
(9) Reinartz, S.; White, P. S., Brookhart, M.; Templeton, J. L. J. Am. Chem.
Soc. 2001, 123, 6425.
(10) Reinartz, S.; White, P. S., Brookhart, M.; Templeton, J. L. Organo-
metallics 2001, 20, 1709.
Supporting Information Available: Complete synthetic and spec-
troscopic data (PDF) and crystallographic data, in CIF format, for complex
3. This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920.
(12) 1H NMR data for complex 3 (CD2Cl2, 193 K, δ): 11.67 (s, 1H,
pz*NH), 6.88 (s, 6H, PtC6H6), 6.26, 5.93, 5.91 (s, 1H each, HTp′CH), 2.32,
2.31, 2.27, 1.86, 1.59 (s, 6H, 3H, 3H, 3H, 3H, HTp′CH3), -30.13 (s, 1H,
1JPt-H ) 935 Hz, Pt-H).
JA011765P
(13) (a) Ball, R. G.; Ghosh, C. K.; Hoyano, J. K.; McMaster, A. D.; Graham,
W. A. G. J. Chem. Soc., Chem. Commun. 1989, 341. (b) Rheingold, A. L.;
Haggerty, B. S.; Trofimenko, S. Angew. Chem., Int. Ed. Engl. 1994, 33, 1983.
(c) Wiley: J. S.; Oldham, W. J., Jr.; Heinekey, D. M. Organometallics 2000,
19, 1670.
(15) Pt(II) ethylene complexes also show perpendicular orientation of the
ethylene ligand with respect to the platinum square plane, see, e.g.: (a) ref
8f, (b) Baar, C. R.; Jenkins, H. A.; Yap, G. P. A., Puddephatt, R. J.
Organometallics 1998, 17, 4330. (c) Jarvis, J. A. J.; Kilbourn, B. T.; Owston,
P. G. Acta Crystallogr. 1971, B27, 366.
(14) For certain substituted arenes, the η2-arene complexes are the more
stable isomers. See refs 2d, f.
(16) Bercaw et al. propose a similar benzene orientation to account for the
1
unusual upfield H chemical shift of the methyl group in complex 1.4