C-H activation at Pt(II) to form stable Pt(IV) alkyl hydrides

Full text of DOI:10.1021/ja971952g

J. Am. Chem. Soc. 1997, 119, 10235-10236
10235
C-H Activation at Pt(II) To Form Stable Pt(IV)
Alkyl Hydrides
Douglas D. Wick and Karen I. Goldberg*
Department of Chemistry, Box 351700
UniVersity of Washington, Seattle, Washington 98195-1700
ReceiVed June 13, 1997
The search for selective practical methods of alkane func-
tionalization has led to intensive investigation of the mechanistic
details of Shilov’s oxidation of alkanes by Pt salts in aqueous
solution.1-3 Strong support has been presented for a pathway
in which activation of the alkane C-H bond occurs by oxidative
addition to a Pt(II) species and generation of a Pt(IV) alkyl
hydride as an undetected intermediate.2 Consistent with this
proposal, the first well-defined example of intermolecular C-H
bond activation of alkanes by model Pt(II) centers was recently
reported.1 Although C-H bond activation was clearly estab-
lished by observation of Pt(II) alkyl/aryl exchange products,
Pt(IV) alkyl hydrides were not directly observed. This com-
plicates the distinction between this exchange occurring by a
true oxidative addition/reductive elimination sequence or by a
σ-bond metathesis pathway. Unfortunately, in the reported
system (Pt(II) complex ) [(tmeda)PtMe(NC5F5)]+), the Pt(IV)
C-H oxidative addition products would not be stable to C-H
reductive elimination.1
Figure 1. ORTEP drawing of 1b (PPN[Tp′PtMe2]‚2THF‚Et2O).
Ellipsoids are shown at the 50% probability level. Hydrogens, the PPN
cation, THF, and Et2O molecules are not shown.
Scheme 1
complexes and create an open coordination site at the metal
center.11 Similar reactivity was recently observed upon reaction
with Pt(II) methyl complexes.12 We took advantage of this
methodology and reacted B(C6F5)3 with K[Tp′PtMe2] with the
intent of abstracting the Pt(II) methyl group and generating
“Tp′PtMe” (η2-Tp), a three-coordinate Pt(II) species, which
might activate a solvent molecule RH.
Recent reports of unusually stable Pt(IV) alkyl hydrides,
TpR′PtR2H (R ) Ph, Me; TpR′ ) Tp′ (hydridotris(3,5-dimeth-
ylpyrazolyl)borate) and Tp (hydridotris(pyrazolyl)borate)),4 led
us to examine potential C-H bond activation reactions with
Pt(II) complexes that would generate analogous stable Pt(IV)
alkyl hydrides. We report here the reaction of K[Tp′PtMe2]4a
with B(C6F5)3 in arene and alkane solvents, RH (RH ) benzene,
pentane, and cyclohexane), to yield the Pt(IV) products,
Tp′PtMeRH (Scheme 1). This constitutes the first example of
the intermolecular oxidative addition of arene and alkane C-H
bonds by a Pt(II) species resulting in stable Pt(IV) compounds.
The precursor complex K[Tp′PtMe2] (1a)5 was prepared by
Reaction of B(C6F5)3 with K[Tp′PtMe2] in C6H6 does indeed
lead to C-H bond activation of the solvent producing Tp′PtMe-
1
(Ph)H (2) and K[MeB(C6F5)3].13 2 was characterized by H
NMR spectroscopy in C6D6, which showed a diagnostic PtIV-H
signal at δ -19.35 (1JPt-H ) 1368 Hz).14 A significant amount
of Tp′PtMe2H (3)4a,15 (ca. 20%, δ -20.39 (1JPt-H ) 1358 Hz))
is also formed during these reactions. We attribute the formation
6
reaction of [PtMe2(µ-SMe2)]2 with KTp′ in THF at ambient
temperature.4a A metathesis reaction of 1a with [PPN]Cl7
generated [PPN]Tp′PtMe2 (1b)5 and single crystals of 1b,
suitable for X-ray diffraction, were grown from a THF solution
layered with Et2O at -33 °C. An ORTEP of 1b is shown in
Figure 1.8 The geometry of the platinate is a distorted square
plane, and the Tp′ ligand is coordinated to the Pt(II) center in
a bidentate fashion with the nitrogen of the noncoordinated
pyrazolyl ring directed away from the Pt center.9,10
(9) The six-membered PtN4B ring is in a boat conformation with the Pt
and B being respectively, 0.890 and 0.498 Å above the plane defined by
N1-N2-N3-N4. The dihedral angle defined by the planes N1-Pt-C1
and N3-Pt-C2 is 3°, and that defined by the planes Pt-N3-N1 and C2-
Pt-C1 is 2.8°. Selected bond distances (Å): Pt-C1, Pt-C2, Pt-B,
2.030(8), 2.032(8), 3.366(5), respectively. Selected bond angles (°): N1-
Pt-N3, N1-Pt-C1, N3-Pt-C1, N1-Pt-C2, N3-Pt-C2, C1-Pt-C2,
86.0(2), 94.2(3), 178.0(3), 178.0(3), 94.1(3), 85.7(4), respectively.
(10) (a) Rush, P. E.; Oliver, J. D. J. Chem. Soc., Chem. Commun. 1974,
996. (b) Oliver, J. D.; Rice, N. C. Inorg. Chem. 1976, 15, 2741.
(11) See for example: (a) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J.
Organometallics 1997, 16, 842 and references therein. (b) Gillis, D. J.;
Quyoum, R.; Tuderet, M. J.; Wang, Q.; Jeremic, D.; Roszak, Q. W.; Baird,
M. C. Organometallics 1996, 15, 3600.
The highly electrophilic reagent B(C6F5)3 has been used to
abstract a methyl anion from early transition-metal methyl
(1) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1997, 119, 848.
(2) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961 and references therein.
(3) See: (a) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.;
Bercaw, J. E. J. Organomet. Chem. 1995, 504, 75 and references therein.
(b) Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.; Bercaw,
J. E.; Horva´th, I. T.; Eller, K. Organometallics 1993, 12, 895. (c) Hutson,
A. C.; Lin, M.; Basickes, N.; Sen, A. J. Organomet. Chem. 1995, 504, 69
and references therein.
(12) (a) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R.
J. Organometallics 1997, 16, 525. (b) Hill, G. S.; Rendina, L. M.;
Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1996, 1809.
(13) Comparison of the 11B-NMR spectrum of the reaction mixture
-
containing 2 in C6D6 (br s at δ -15) with that of a sample of MeB(C6F5)3
,
made from the reaction of MeLi and B(C6F5)3 in C6D6 (br s at δ -15),
confirmed that abstraction of methyl anion from 1a had occurred.
1
(14) (a) Tp′PtMe(Ph)H (2): 1H NMR (C6D6): δ -19.35 (s, JPt-H
)
(4) (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.
1368 Hz, 1 H, PtH), 1.62 (s, 3 H, Tp′CH3), 1.84 (s, 2JPt-H ) 69.4 Hz, 3 H,
PtCH3), 1.87, 2.10, 2.11, 2.18, 2.23 (all s, each 3 H, Tp′CH3), 5.48 (s, 4JPt-H
4
) 6.2 Hz, 1 H, Tp′CH), 5.53 (s, JPt-H ) 6.4 Hz, 1 H, Tp′CH), 5.59 (s, 1
(5) 1H NMR data (for 1a and 1b) and 13C{1H} NMR data (for 1b) are
provided in the Supporting Information.
H, Tp′CH), 6.80 (bt, 4JPt-H ) 13.8 Hz, 1 H, Hm), 7.01 (d, JH-H ) 7.6 Hz,
3JPt-H ) 42.8 Hz, 1 H, Ho), 7.03 (tt, JH-H ) 2.3 Hz, 7.3 Hz, 1 H, Hp), 7.19
(bt, 4JPt-H ) 13.3 Hz, 1 H, Hm′), 7.82 (d, JH-H ) 7.3 Hz, 3JPt-H ) 64.7 Hz,
1 H, Ho′).
(6) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643.
(7) PPN ) bis(triphenylphosphoranylidene)ammonium.
(8) 1b‚2(THF)‚Et2O (C53H58BN7P2Pt‚C8H16O2‚C4H10O), MW ) 1279.28,
pale yellow rhombohedron, triclinic, space group ) P1h, a ) 13.666(2) Å,
b ) 14.538(3) Å, c ) 17.631(3) Å, R ) 70.85(2)°, â ) 88.63(2)°, γ )
65.92(2)°, V ) 2996(1) Å3, Z ) 2, R1 ) 0.0436 [I > 2σ(I)], GOF ) 1.053.
(15) Tp′PtMe2H (3): 1H NMR (C6D6): δ -20.39 (s, 1JPt-H ) 1358 Hz,
1 H, PtH), 1.66 (s, 2JPt-H ) 67.8 Hz, 3 H, PtCH3), 2.09 (s, 6 H, Tp′CH3),
2.15 (s, 3 H, Tp′CH3), 2.19 (s, 6 H, Tp′CH3), 2.33 (s, 3 H, Tp′CH3), 5.51
4
(s, JPt-H ) 6.2 Hz, 2 H, Tp′CH), 5.57 (s, 1 H, Tp′CH).
S0002-7863(97)01952-5 CCC: $14.00 © 1997 American Chemical Society

10236 J. Am. Chem. Soc., Vol. 119, No. 42, 1997
Communications to the Editor
of this material to the presence of adventitious water. As
recently reported by Puddephatt and co-workers, H2O complexes
to B(C6F5)3 to make the very strong acid H[HOB(C6F5)3] that
can protonate Pt(II) centers.12 Protonation of 1a by acid was
previously reported to generate 3.4a To prevent large amounts
of 3 from being produced, it was necessary to take scrupulous
care to avoid any source of H2O.16
Support for this analysis of the reaction pathway is obtained
by considering the microscopic reverse of this process, reductive
elimination of C-H bonds from Pt(IV). We and others have
documented the consistent involvement of five-coordinate
intermediates (B) in C-C and C-H reductive elimination from
octahedral Pt(IV) and Pd(IV) centers (A; eq 1).2,20,21 The
When the reaction of B(C6F5)3 with 1a was carried out in
C6D6, the hydride signal for the protonation product, Tp′PtMe2H
(3), appeared alone in the upfield region. However, the PtCH3
signal assignable to Tp′PtMe(D)(C6D5) (2-d6) still appeared at
1.84 (2JPt-H ) 69.4 Hz) along with a set of Tp′ signals identical
with those of 2. This experiment confirmed that the hydride
of 2 resulted from solvent activation but that of 3 did not.
Reaction of B(C6F5)3 with 1a in C6H6 followed by removal
of the solvent under vacuum left an oily residue containing
Tp′PtMe(Ph)H (2), Tp′PtMe2H (3), and K[MeB(C6F5)3]. Initial
attempts to purify this mixture have been unsuccessful. Dis-
solution of this product mixture in C6D6 and heating of the
solution for 1 day at 63 °C (or maintaining the solution at
ambient temperature for 1 month) results in no changes in the
number of products or product ratios. The high thermal stability
of complex 2 is similar to that previously reported for 3 and
TpPtMe2H and is likely due to the inhibition of formation of a
five-coordinate species by the chelate effect.4
principle of microscopic reversibility then requires that a ligand
must be lost from the square-planar Pt(II) complex (D) prior to
oxidative addition (reverse of eq 1). Complexes such as C (four-
coordinate C-H/C-C σ-complexes) and/or a three-coordinate
Pt(II) species (C′) are likely species along the reaction pathway.
This analysis is consistent with our results and also with the
recent report by Labinger and Bercaw of intermolecular alkane
C-H activation by [(tmeda)PtMe(NC5F5)]B(Arf)4 and White-
sides’s report of intermolecular arene C-H activation by trans-
(PMe3)2Pt(CH2CMe3)(SO3CF3).1,22 Both of these Pt(II) com-
plexes are square-planar species with one particularly weakly
bound ligand. Bercaw and Labinger have also reported strong
evidence for a Pt(II) C-H σ-complex, (tmeda)PtMe(CH4)+ (an
intermediate analogous to C), on the reaction path of C-H
reductive elimination of methane from the five-coordinate
Pt(IV) species, (tmeda)PtMe2H+.2
In summary, oxidative addition to Pt(II) of the C-H bonds
of alkanes to form Pt(IV) alkyl hydride complexes, a reaction
which has been proposed as the C-H activation step in Shilov
alkane oxidation stystems, has now been documented in a model
system. The success of demonstrating this reactivity has relied
on two factors: (1) the generation of a reactive three-coordinate
Pt(II) species and (2) rapid trapping of the five-coordinate
Pt(IV) species. We will use these principles as we continue
our studies of the mechanism and selectivity of oxidative
addition/reductive elimination reactions at Pt(II)/Pt(IV).
The reaction of 1a with B(C6F5)3 was also carried out in
alkane solvents, and C-H bond activation of pentane and
cyclohexane was observed. Due to the limited solubility of 1a
in alkanes, 1a was stirred as a suspension with the boron reagent
in pentane or cyclohexane at 50 °C for 2 days. Removal of the
solvent and dissolution of the residue in C6D6 allowed observa-
tion of the Pt(IV) alkyl hydrides, Tp′PtMeRH (R ) n-C5H11
1
(4) and Cy (5)), by H NMR.17 The PtIV-H signals appear at
δ -20.73 (1JPt-H ) 1393 Hz) and δ -21.37 (1JPt-H ) 1442
Hz) for 4 and 5, respectively. The production of Tp′PtMe2H
(3) is a significant competing reaction (ca. 43%) in these alkane
activation reactions, and once again, its formation is attributed
to adventitious water.
The reaction of the Pt(II) complex with pentane is highly
selective. Only two hydride signals, assignable to 4 and 3, were
observed in the 1H NMR (C6D6) after reaction of 1a with
B(C6F5)3 in pentane at 50 °C. The assignment of 4 as the
n-pentyl derivative (the product resulting from C-H activation
at the primary carbon) is based on the observation of the four
methylene carbon resonances of the n-pentyl ligand in a
13C{1H}-DEPT NMR spectrum of 4.18 The signal for CR
appears at δ 1.3 (1JPt-C ) 637 Hz).
A plausible pathway for these C-H bond activation reactions
is abstraction of an anionic methyl group from [Tp′PtMe2]- by
B(C6F5)3 generating the very reactive three-coordinate “Tp′Pt-
Me” species (η2-Tp′), which then undergoes oxidative addition
of a C-H bond of the solvent. The five-coordinate Pt(IV)
species, η2-Tp′PtMeRH, is then rapidly trapped by the third
pyrazolyl ring to form the thermally stable Tp′PtMeR(H).19
Acknowledgment is made for support from the National Science
Foundation, the Union Carbide Innovation Recognition Program, the
DuPont Educational AID Program, and the University of Washington
(UW). K.I.G. is an Alfred P. Sloan Research Fellow. We are grateful
for a loan of K2PtCl4 from Johnson Matthey/Aesar/Alfa and gifts of
B(C6
F ) from Prof. D. McConville (University of British Columbia)
5
3
and Dr. R. Fisher (Exxon Chemical). Prof. R. Hughes (Dartmouth
College) is acknowledged for assistance in glassware silylation
techniques. The X-ray structural determination of 1b was performed
by Dr. D. Barnhart (UW).
Supporting Information Available: Experimental procedures,
NMR data for 1(a,b), and crystal structure data for 1b (16 pages). See
any current masthead page for ordering and Internet access instructions.
(16) The boron reagent was recrystallized from pentane prior to use. The
glassware was pretreated with bis(trimethylsilyl)acetamide and flame dried
under vacuum immediately prior to use. See: Hughes, R. P.; Rose, P. R.;
Rheingold, A. L. Organometallics 1993, 12, 3109.
JA971952G
(19) The alternative that the Pt species which activates the C-H bond
of the solvent might be η3-Tp′PtMe should also be considered. Such a four-
coordinate d8 species would be analogous to the Cp*ML fragments of Ir
and Rh well known to undergo oxidative addition of C-H bonds. See:
Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Pederson, T. H. Acc.
Chem. Res. 1995, 28, 154.
(17) Tp′PtMeRH (R ) C5H11 (4) and Cy (5)): 1H NMR (C6D6): δ
1
-20.73 (s, JPt-H ) 1393 Hz, 1 H, PtH), 0.91 (t, JH-H ) 7.3 Hz, 3 H,
Pt(CH2)4CH3), 1.37 (m, 2 H, Pt(CH2)3CH2CH3), 1.48 (m, 2 H, Pt(CH2)2CH2-
2
CH2CH3), 1.63 (s, JPt-H ) 69.0 Hz, 3 H, PtCH3), 1.66 (m, 2 H,
PtCH2CH2(CH2)2CH3), 2.08, 2.09, 2.15, 2.19, 2.25, 2.40 (all s, each 3 H,
(20) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem. Soc.
1995, 117, 6889.
2
Tp′CH3), 2.72 (m, JPt-H ∼ 68 Hz, 2 H, PtCH2(CH2)3CH3), 5.50, 5.57,
1
5.61 (all s, each 1 H, Tp′CH). 5: 1H NMR (C6D6) δ -21.37 (s, JPt-H
)
(21) Representative examples: (a) Brown, M. P.; Puddephatt, R. J.;
Upton, C. E. E. J. Chem. Soc., Dalton Trans. 1974, 2457. (b) Roy, S.;
Puddephatt, R. J.; Scott, J. D. J. Chem. Soc., Dalton Trans. 1989, 2121. (c)
Canty, A. J. Acc. Chem. Res. 1992, 25, 83 and references therein. (d) Hill,
G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics, 1995,14, 4966
(e) de Graaf, W.; Boersma, J.; Smeets, W. J. J.; Speck, A. L.; van Koten,
G. Organometallics, 1989, 8, 2907.
1442 Hz, 1 H, PtH), 1.40 (m, 2H, PtCH(CH2)5), 1.54 (m, 4 H, PtCH(CH2)5),
1.67 (s, 2JPt-H ) 69.5 Hz, 3 H, PtCH3), 1.80 (m, 4 H, PtCH(CH2)5), 2.06,
2.10, 2.16, 2.17, 2.38, 2.44 (all s, each 3 H, Tp′CH3), 3.14 (tt, JH-H ) 12.0
2
Hz, 7.2 Hz, JPt-H ) 88.0 Hz, 1 H, PtCH(CH2)5), 5.47 (s, 1 H, Tp′CH),
4
5.59 (s, JPt-H ) 6.4 Hz, 1 H, Tp′CH), 5.63 (s, 1 H, Tp′CH).
(18) Selected 13C{1H}-DEPT NMR data for 4, Pt-pentyl CH2 signals:
1.3 (s, 1JPt-C ) 637 Hz, CR), 23.0 (s, Cδ), 33.7 (s, Cγ), 36.1 (s, 2JPt-C ) 87
Hz, Câ).
(22) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides, G. M.
Organometallics 1988, 7, 2379.