C-H activation at cationic platinum(II) centers

Full text of DOI:10.1021/ja9620595

848
J. Am. Chem. Soc. 1997, 119, 848-849
Scheme 1
C-H Activation at Cationic Platinum(II) Centers
Matthew W. Holtcamp, Jay A. Labinger,* and
John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical
Synthesis, California Institute of Technology
Pasadena, California 91125
ReceiVed June 19, 1996
The oxidation of alkanes to alcohols by aqueous solutions
containing [PtCl4]2- and [PtCl6]2- 1 has been the subject of
recent mechanistic studies.2 These support a multistep mech-
anism, in which the key first step, the actual C-H bond
activation, involves reaction of alkane with a PtII complex.
Most of our evidence concerning the nature of this step comes
indirectly, from studies on the microscopic reverse reactions
protonolysis of PtII alkyls such as (tmeda)Pt(CH3)2 (tmeda )
N,N,N′,N′-tetramethylethylenediamine)swhich implicate both
a σ complex, PtII(RH), and an oxidative adduct, PtIV(R)(H), as
intermediates.3 To date, however, neither the intermolecular
reaction of an alkane and a PtII complex to give a stable organo-
platinum species nor the oxidation of alkanes by PtII complexes
containing amine or phosphine ligands has been reported.4 Our
findings suggested that a sufficiently electrophilic [(tmeda)-
Pt(CH3)(solvent)]+ complex should be able to activate alkane
C-H bonds. We report here our preliminary results that
demonstrate such reactivity and strengthen the relevance of our
model studies to the alkane oxidation mechanism.
Addition of 1 equiv of [H(OEt2)n][BArf] {BArf ) B(3,5-C6H3-
(CF3)2)4}5 to an ether solution of (tmeda)Pt(CH3)2 at -70 °C
gives [(tmeda)Pt(CH3)(OEt2)][BArf] (1).6 This complex is stable
in solution at low temperature and may be isolated as a solid;7
but at room temperature in solution it slowly reacts, ultimately
giving methane (92% collected by Toepler pump) and the
carbene hydride complex 3.8 This transformation presumably
proceeds via C-H activation to give 2 (Scheme 1).9 Only
coordinated ether undergoes C-H activation, as 1 dissolved in
Et2O-d10 gives only CH4, whereas 1-d10 in (C2H5)2O gives CH3D
(as well as some CH4, CH2D2, and CHD3; see below).10 Similar
chemistry is observed in THF.11
Demonstration of intermolecular C-H activation requires a
solvent that is not subject to activation itself, does not coordinate
so strongly that alkane activation is blocked, and dissolves ionic
complexes.12 Pentafluoropyridine proves to be suitable: reac-
tion of (tmeda)Pt(CH3)2 with [H(NC5F5)n][BArf]13 in NC5F5 at
0 °C gives [(tmeda)Pt(CH3)(NC5F5)][BArf] (5).14 At 85 °C in
the presence of benzene 5 is converted to the phenyl analog
(81% of methane collected by Toepler pump);15 while under
30 atm 13CH4 the methyl exchange shown in eq 1 is demon-
strated by the slow growth of the Pt-13CH3 resonance in the
13C NMR spectrum at -21 ppm (1JPtC ) 725 Hz). The latter
reaction is accompanied by slow deposition of metallic plati-
num, which is connected to the C-H activation process: no
Pt0 is observed upon heating 5 to 85 °C in the absence of
hydrocarbon. Reaction with neopentane gives rapid decomposi-
tion of 5 to Pt0, while reaction with benzene gives only traces
of metallic Pt.16
Upon heating 5 at 85 °C with cyclohexane-d12, toluene-d8,
or benzene-d6 in pentafluoropyridine, the methane isotopomers
(9) At early stages of the reaction 1H NMR reveals several species which
appear to include 4 and a vinyl ether complex in addition to 3. Over the
course of a week all gradually convert to 3. Although 2 is shown as five-
coordinate, we believe such coordinatively unsaturated species exist only
as transient intermediates (see ref 3b); most probably in its stable form 2 is
six-coordinate, with solvent ether (or, conceivably, O from the ethoxyethyl
group) completing the octahedron. The same applies to other five-coordinate
structures shown (or implied) here.
(1) Kushch, K. A.; Lavrushko, V. V.; Misharin, Yu. S.; Moravsky, A.
P.; Shilov, A. E. New J. Chem. 1983, 7, 729.
(10) A referee suggested an alternate possibility, that coordinated ether
undergoes not oxidative addition to give 2 but rather electrophilic activation
to liberate H+ which subsequently protonolyzes a Pt-CH3 bond. This is
ruled out by the following crossover experiment: reaction of a mixture of
[(tmeda)Pt(CH3)(O(C2D5)2)][BArf] and [(tmeda)Pt(13CH3)(OEt2)][BArf] in
C5F5N gives no methane containing both 13C and D.
(2) (a) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw,
J. E. J. Organomet. Chem. 1995, 504, 75. (b) Luinstra, G. A.; Wang, L.;
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Organometallics 1994, 13, 755.
(c) Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.; Bercaw,
J. E.; Horvath, I. T.; Eller, K. Organometallics 1993, 12, 895. (d) Labinger,
J. A.; Herring, A. M.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 5628.
(e) Luinstra, G. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1993,
115, 3004. (f) Kao, L.-C.; Hutson, A. C.; Sen, A. J. Am. Chem. Soc. 1991,
113, 700.
(11) [(tmeda)Pt(CH3)(THF)][BArf]:
Elem. Anal. Calcd for
C43H39N2F24BOPt: C, 40.94; H, 3.12; N, 2.22. Found: C, 41.19; H, 3.41;
N, 2.19. 1H NMR (THF-d8). δ 0.586 (s, 3H, JPt-H ) 83 Hz); 1.43 (t, 6H);
2.76 (s, 6H); 2.82 (s, 6H); 2.6-3.0 (m, 4H); 3.98 (q, 4H); 7.58 (s, 4H);
7.78 (s, 8H). This compound also slowly converts to a carbene hydride
analogous to 3.
(3) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1995, 117, 9371. (b) Stahl, S. S.; Labinger, J. A.; Bercaw. J. E. J. Am.
Chem. Soc. 1996, 118, 5961.
(12) The reaction of (tmeda)Pt(CH3)2 with [H(Et2O)2][BArf] in chlori-
nated solvents such as dichloromethane and chlorobenzene results in C-Cl
(4) A 1H NMR signal attributed to [Pt(CH3)Cl5]2- was detected in a
cooled reaction mixture following oxidation of methane;1 however, no
corresponding signal was observed under reaction conditions using a high-
pressure NMR cell: Horva´th, I. T.; Cook, R. A.; Millar, J. M.; Kiss, G.
Organometallics 1993, 12, 8. The electrophilic activation of arenes by trans-
(Me3P)2Pt(neopentyl)(triflate) has been observed: Brainard, R. L.; Nutt,
W. R.; Lee, T. R.; Whitesides, G. M. Organometallics 1988, 7, 2379.
(5) Brookhart, M.; Grant, B.; Volpe, J. Organometallics 1992, 11, 3920.
(6) Related complexes with a different chelating ligand (a substituted
2,2′-bipyridine) have recently been reported: Hill, G. S.; Rendina, L. M.;
Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1996, 1809.
activation; the dimer [(tmeda)PtCl]2[BArf]2 can be isolated. The initial H
1
NMR spectrum reveals a complex mixture of products, but after several
hours a crystalline product is isolated in 60% yield and identified as
[(tmeda)PtCl]2[BArf]2 by X-ray crystallography, elemental analysis, and 1H
NMR. Elem. Anal. Calcd for C38H28N2F24BClPt: C, 37.72; H, 2.33; N,
2.32. Found: C, 37.5; H, 2.52; N, 2.37. 1H NMR (CD2Cl2): δ 2.5-3.2 (m,
16H); 7.6 (s 4H); 7.7 (s 8H).
(13) Prepared in situ by dissolving [H(Et2O)2][BArf] in pentafluoropy-
ridine and removing solvent in Vacuo (repeated several times).
(14) [(tmeda)Pt(CH3)(NC5F5)][BArf] (5). Elem. Anal. Calcd for
C44H31N3F29BPt: C, 38.9; H, 2.3; N, 3.09. Found: C, 38.5; H, 2.7; N,
3.35. 1H NMR (THF-d8). δ 0.30 (s, 3H, JPt-H ) 75 Hz); 2.65 (s, 6H); 2.96
(s, 6H); 2.6-3.2 (m, 4H); 7.58 (s, 4H); 7.78 (s, 8H).
(15) [(tmeda)Pt(C6H5)(NC5F5)][BArf]‚NC5F5. Elem. Anal. Calcd for
C49H33N3F29BPt‚NC5F5: C, 40.8; H, 2.09; N, 3.52. Found: C, 40.43; H,
2.39; N, 3.43. 1H NMR (NC5F5). δ 2.68 (s, 6H); 2.89 (s, 6H); 2.6-3.2 (m,
4H); 6.26 (t, 1H); 6.56 (t, 2H); 7.24 (sh, 1H, peak and Pt satellites obscured
by BArf hydrogens); 7.26 (s, 4H); 7.67 (s, 8H).
(16) On heating a solution of 5 (0.05 mmol) and neopentane (1.5 mmol)
in C5F5N (0.5 mL), visible formation of Pt metal began almost immediately;
after several hours at 85 °C a sizable Pt mirror had formed. A similar
reaction with benzene gave nearly complete conversion of 5 after 7 h at
115 °C with only a small amount of metallic Pt visible.
(7) [(tmeda)Pt(CH3)(OEt2)][BArf] (1): Elem. Anal. Calcd for
C43H41N2F24BOPt: C, 40.87; H, 3.27; N, 2.22. Found: C, 40.98; H, 3.41;
N, 2.21%. 1H NMR (THF-d8). δ 0.648 (s with Pt satellites, JPt-H ) 77
Hz); 1.43 (t, 6H); 2.76 (s, 6H); 2.82 (s, 6H); 2.6-3.0 (m, 4H); 3.98 (q,
4H); 7.58 (s, 4H); 7.78 (s, 8H).
(8) [(tmeda)Pt(dC(CH3)(OCH2CH3)(H)][BArf] (3): Elem. Anal. Calcd
for C42H37N2F24BOPt: C, 40.43; H, 2.99; N, 2.24. Found: C, 41.19; H,
3.41; N, 2.19. 1H NMR (Et2O-d10). δ -17.4 (s, 1H, JPt-H ) 1640 Hz);
1.52 (t, 3H); 2.48 (m); 2.58 (s, 3H); 2.84 (s, 6H); 2.90 (s, 6H); 2.6-3.0
(m); 5.23 (q, 2H); 7.58 (s, 4H); 7.78 (s, 8H). 13C NMR (Et2O-d10). δ 276
(JPt-C ) 1250 Hz); 82.5 (JPt-C ) 140 Hz); 62.8; 62.7; 52.9; 52.2; 44.9;
43.4 (JPt-C ) 160 Hz) (plus anion peaks in the aromatic region).
S0002-7863(96)02059-8 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 4, 1997 849
Scheme 2
an alternative to oxidative addition as in Scheme 2, perhaps
partly because of the unattractiveness of the IrV intermediate
required for the latter.19 We cannot exclude σ-bond metathesis
for reactions of 1 and 5, although the multiple H/D exchange
requires at a minimum that there be an intermediate, such as a
σ complex (Scheme 2), with significant lifetime. Delineation
of the various modes of C-H activation and their applicability
to catalytic alkane functionalization remains the subject of
further study.
CH4, CH3D, CH2D2, and CHD3 are observed by 1H NMR after
several days.17 Similar (multiple) H/D exchange observed in
protonolysis of (tmeda)Pt(CH3)2(H)Cl3 has been interpreted in
terms of an equilibrium between PtIV dialkyl hydride and PtII
σ-complex species (Scheme 2). The possibility that multiple
exchange occurs via reversible loss and readdition of methane
is ruled out by heating 5 with C6D6 under 13CH4; no methane
containing both 13C and D is observed.
It should be noted that very similar activation of alkane
and ether C-H bonds is observed for the IrIII complexes
[Cp*Ir(PMe3)(CH3)(ClCH2Cl)][BArf] and Cp*Ir(PMe3)(CH3)-
(OTf).18 A σ-bond metathesis mechanism has been offered as
Acknowledgment. We thank the Army Research Office for support
and Shannon S. Stahl for many helpful discussions.
JA9620595
(17) In a typical experiment, using C6D6, the product distribution of CH4:
1
CH3D:CH2D2 was approximately 4:7:9 (by H NMR), with a few percent
(18) (a) Luecke, H. F.; Arndtsen, B. A.; Burger, P.; Bergman, R. G. J.
Am. Chem. Soc. 1996, 118, 2517. (b) Arndtsen, B. A.; Bergman, R. G.
Science 1995, 270, 1970.
(19) A recent theoretical study supports oxidative addition over σ-bond
metathesis for the Ir case: Strout, D. L.; Zaric, S.; Niu, S.; Hall, M. B. J.
Am. Chem. Soc. 1996, 118, 6068.
of CHD3. The CH4 probably arises from competing reaction and/or exchange
with protonic impurities on glass surfaces and elsewhere, as prewashing
the NMR tube with D2O followed by thorough drying reduces the relative
yield of CH4 by half. Only a few percent of CH4 is obtained in
decomposition of [(tmeda)Pt(CH3)(O(C2D5)2)][BArf].