Benzene C-H activation at a charge neutral zwitterionic platinum(II) complex

Full text of DOI:10.1021/ja0058987

5100
J. Am. Chem. Soc. 2001, 123, 5100-5101
Scheme 2
Benzene C-H Activation at a Charge Neutral
Zwitterionic Platinum(II) Complex
J. Christopher Thomas and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering
Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, California Institute of Technology
Pasadena, California 91125
ReceiVed December 18, 2000
Cationic, coordinatively unsaturated metal centers exhibit a
wide range of both stoichiometric and catalytic transformations.1
Such species are frequently generated by methide abstraction with
a strong Lewis acid,2 or alternatively by protonation with an acid
whose conjugate base is noncoordinating or weakly coordinating.3
We are targeting charge neutral, zwitterionic complexes that
display reaction chemistry reminiscent of these reactive metal
centers. Of particular interest is the development of species that
exhibit transformations at X-H bonds, where an X-H bond refers
generally to a C-H or other robust σ bond. A conceptual diagram
illustrating this strategy is shown in Scheme 1.
(ASN ) 5-azonia-spiro[4.4]nonane) and examine its ability to
promote C-H bond activation chemistry at a platinum(II) center.
Synthesis of 1 was readily achieved by low-temperature
addition of a toluene solution of diphenylchloroborane to a diethyl
ether solution of Ph2PCH2Li(TMEDA) (TMEDA ) N,N,N′,N′-
tetramethylethylene-1,2-diamine). This generates [Ph2B(CH2-
PPh2)2][Li(TMEDA)2], whose structure is shown in Figure 1.7
Cation exchange provides the ASN salt, 1, in high yield.
Scheme 1
Reaction of 1 with either (COD)Pt(Me)2 or (COD)Pt(Me)(Ph)
in THF forms the expected anionic platinum(II) products
[{Ph2B(CH2PPh2)2}Pt(Me)2][ASN] (2) and [{Ph2B(CH2PPh2)2}-
Pt(Me)(Ph)][ASN] (3) in high yield (Scheme 2). To generate the
key neutral platinum complex, {Ph2B(CH2PPh2)2}Pt(Me)(L),
several strategies were surveyed including protonolysis by acid
and methide abstraction by B(C6F5)3. All of these strategies
effected the removal of one methyl group from 2, as determined
by 31P NMR spectroscopy; however, most routes did not enable
the clean isolation of a {Ph2B(CH2PPh2)2}Pt(Me)(L) complex.
We were fortunate to find that protonation of 2 in THF with the
bulky ammonium salt [iPr2EtNH][BPh4] did enable both the clean
generation of {Ph2B(CH2PPh2)2}Pt(Me)(THF) (4) and its isolation.
The salt byproduct, [ASN][BPh4], precipitates from THF and is
readily removed. Solid 4 can be subsequently isolated by rapid
precipitation from THF with pentane, a procedure that also
Herein we describe a neutral platinum(II) alkyl complex
supported by the novel, anionic bidentate phosphine ligand
[Ph2B(CH2PPh2)2].4 This platinum system was examined for
several reasons. First, C-H activation at Pt(II) metal centers is
now well-established, particularly for systems with N-donor
ligands.2a,3,5 In contrast, there are limited examples of intermo-
lecular C-H bond activation at platinum(II) centers supported
by phosphine donor ligands: these phosphine-supported systems
require relatively high temperatures (125-150 °C).6 We envi-
sioned that zwitterionic, bis(phosphino)borate platinum complexes
would promote transformations at C-H bonds. Furthermore, it
was hoped that such complexes would be soluble in relatively
nonpolar media, contrasting their discrete salt relatives. Systems
thus designed should be amenable to mechanistic study due to
the presence of a useful spectroscopic 31P NMR handle. In
addition, designing systems that attenuate or eliminate counter-
anion effects may provide an important mechanistic simplification.
We describe below the synthesis of [Ph2B(CH2PPh2)2][ASN] (1)
i
removes the neutral amine byproduct, Pr2EtN. It is noteworthy
that the protonation of 2 directly contrasts with the reactivity of
a related compound, (dppp)PtMe2 (dppp ) 1,3-bis(diphenylphos-
phino)propane), which did not exhibit reactivity with [iPr2EtNH]-
[BPh4] at 50 °C in THF solution over 24 h.
An X-ray diffraction study on single crystals of 4 confirmed
its structural assignment (Figure 2).8 To date, this represents the
third crystallographically characterized example of a platinum-
THF adduct and is the only charge neutral species thus character-
ized for divalent platinum.9 Importantly, the coordinated THF
molecule in 4 is weakly bound: it is readily substituted by a
variety of neutral ligands (CO, pyridine, H2O, acetone, Et3N) and
(1) (a) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1976, 98, 2134-
2143, 2143-2147, and 4450-4455. (b) Crabtree, R. Acc. Chem. Res. 1979,
13, 331-338. (c) Veghini, D.; Henling, L. M.; Burkhardt, T. J.; Bercaw, J.
E. J. Am. Chem. Soc. 1999, 121, 564-573. (d) Korolev, A. V.; Guzei, I. A.;
Jordan, R. F. J. Am. Chem. Soc. 1999, 121, 11605-11606. (e) Tellers, D.
M.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122, 954-955.
(2) (a) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235-
10236. (b) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998,
120, 1772-1784.
(7) [Ph2B(CH2PPh2)2][Li(TMEDA)2] (C50H65BLiN4P2), MW ) 801.75,
colorless prism, collection temperature ) 98 K, monoclinic, space group )
P21/c, a ) 11.7922(6) Å, b ) 11.7081(6) Å, c ) 33.1336(18) Å, R ) 90°, â
) 94.0620(10)°, γ ) 90°, V ) 4563.1(4) Å3, Z ) 4, R1 ) 0.061 [I > 2σ(I)],
GOF ) 1.952.
(8) 4‚2(THF) (C43H45BOP2Pt‚2(C4H8O)), MW ) 845.67 × 2(72.10),
colorless block, collection temperature ) 98 K, triclinic, space group ) P1h,
a ) 12.210(4) Å, b ) 12.803(4) Å, c ) 16.205(5) Å, R ) 109.614(5)°, â )
104.361(5)°, γ ) 96.489(5)°, V ) 2257.6(12) Å3, Z ) 2, R1 ) 0.043 [I >
2σ(I)], GOF ) 1.404.
(9) (a) Butts, M. D.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc. 1996,
118, 11831-11843. (b) Schlect, S.; Magull, J.; Fenske, D.; Dehnicke, K.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1994-1995.
(3) (a) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999,
121, 1974. (b) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang,
O.; Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831-10845.
(4) A lithium adduct of an anionic, bis(phosphino)aluminate species has
been previously reported. See: Karsch, H. H.; Appelt, A.; Mu¨ller, G.
Organometallics 1985, 4, 231-238.
(5) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1997, 119, 848-849.
(6) (a) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides, G. M.
Organometallics 1988, 7, 2379-2386. (b) Edelbach, B. L.; Lachicotte, R. J.;
Jones, W. D. J. Am. Chem. Soc. 1998, 120, 2843-2853. (c) Peters, R. G.;
White, S.; Roddick, D. M. Organometallics 1998, 17, 4493-4499.
10.1021/ja0058987 CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/05/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 21, 2001 5101
Analogous to its reactivity with benzene, 4 reacts with toluene
preferentially at the C-H bonds of the aryl ring. There is no
evidence for competitive benzylic C-H activation. Spectroscopic
evidence suggests that the predominant isomer formed is the
p-tolyl platinum complex and we tentatively assign the product
ratio as 3:1:1 for the para:meta:ortho isomers, respectively.
We have begun to examine the potential of 4 to activate alkyl
C-H bonds and have not, as yet, conclusively observed such a
reaction. Incubation of a THF solution of 4 under an atmosphere
of 13CH4 at 75 °C for 10 h afforded no detectable incorporation
(13C NMR) of an isotopically enriched methyl group. This
contrasts results reported for complexes with amine and imine
donor ligands, where 13CH4 has been demonstrated to reversibly
react with the compounds [(TMEDA)Pt(CH3)(NC5F5)]+ in pen-
tafluoropyridine and [(ArNdC-CdNAr)Pt(CH3)(H2O)]+ in dichlo-
romethane.3,5 Additionally, no reaction occurred upon dissolution
of 4 in a 1:1 THF:cyclohexane mixture at 75 °C over a period of
12 h. A possible reaction between 4 and the C-H bonds of
methane or cyclohexane is likely inhibited by the presence of a
large excess of tetrahydrofuran. Complex 4, while soluble in
aromatic hydrocarbons, reacts rapidly with them relative to other,
nonaromatic solvent molecules; however, 4 is not appreciably
soluble in simpler hydrocarbons such as pentane and cyclohexane,
and hence its reactivity in the absence of solubilizing THF
equivalents has yet to be determined. Efforts to prepare more
lipophilic derivatives of 4 with new bis(phosphino)borate ligands
are currently underway.
Figure 1. 50% displacement ellipsoid representation of [Ph2B(CH2-
PPh2)2]. Hydrogen atoms and the Li(TMEDA)2 countercation are omitted
for clarity.
Figure 2. 50% displacement ellipsoid representation of {Ph2B(CH2-
PPh2)2}Pt(Me)(THF)‚2THF (4). Hydrogen atoms and THF molecules are
omitted for clarity. Selected interatomic distances (Å) and angles (deg):
Pt-P1, 2.313(2); Pt-P2, 2.193(2); Pt-O, 2.170(4); Pt-C39, 2.087(6);
P1-Pt-P2, 91.93(6); P1-Pt-O, 91.2(1); O-Pt-C39, 87.4(2); C39-
Pt-P2, 89.9(2); P1-Pt-C39, 174.9(2); P2-Pt-O, 174.4(1).
In conclusion, a monoanionic bidentate bis(phosphino)borate
ligand has been developed,11 and its influence on platinum(II)
C-H bond activation processes has been explored. Notably, the
neutral, zwitterionic platinum(II) complex 4 has been cleanly
generated and reacts preferentially with aryl C-H bonds at 50
°C. We anticipate that the reactive complex 4 and related Group
10 complexes will exhibit a rich reaction chemistry.
is very unstable under reduced pressure (placing 4 under vacuum
leads to a single new product whose identity has yet to be
determined). It should also be noted that 4 slowly degrades in
THF solution at ambient temperature.
Acknowledgment. We thank the California Institute of Technology
for its generous support of this research through a start-up grant. J.C.P.
thanks the Dreyfus Foundation for a Dreyfus New Faculty Award. J.C.T.
thanks the National Science Foundation and the California Institute of
Technology for financial support. The authors thank Prof. John Bercaw
for helpful discussions and Dr. Michael Day and Lawrence Henling for
their assistance with X-ray crystallography.
With target complex 4 in hand, its ability to engage the aryl
C-H bonds of benzene was examined. When a solid sample of
4 is dissolved and gently warmed in benzene, a solvent in which
it is appreciably soluble, formation of one major product is
observed. At 50 °C, the reaction is complete after 4 h. The major
product (≈80%) formed is {Ph2B(CH2PPh2)2}Pt(Ph)(THF) (5)
based upon spectroscopic data. To confirm its assignment, 5 was
independently generated by methide abstraction from 3 with
B(Ar)3 in THF. It is noted that addition of several molar
equivalents of THF to a benzene solution of 4 slows the rate of
benzene activation.10
Supporting Information Available: Detailed experimental proce-
dures for the preparation and characterization of compounds 1-5 and
tables of crystal, data collection, and refinement parameters, atomic
coordinates, bond distances, bond angles, and anisotropic displacement
parameters for [Ph2B(CH2PPh2)2][Li(TMEDA)2] and 4 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA0058987
(10) The conversion of 4 to 5 occurs in greater yield in the presence of
several molar equivalents of THF, albeit more slowly. Notably, H2O has been
observed to inhibit C-H activation in related systems by Tilset and co-workers
(see ref 3b).
(11) For references pertaining to bidentate borate ligands, see: (a) Ge, P.;
Riordan, C. G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem. 1996, 35, 5408-
5409. (b) Trofimenko, S. Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, UK, 1999.