Evidence for associative methane loss following protonation of (diimine)PtII(CH3)2: Three-coordinate 14-electron cations L2Pt(CH3)+ are not necessarily intermediates in C-H activation at ca

Full text of DOI:10.1021/ja002505v

J. Am. Chem. Soc. 2001, 123, 739-740
739
Scheme 1
Evidence for Associative Methane Loss Following
Protonation of (Diimine)PtII(CH3)2:
Three-Coordinate 14-Electron Cations L2Pt(CH3)+
Are Not Necessarily Intermediates in C-H
Activation at Cationic Pt Complexes
Scheme 2
Lars Johansson and Mats Tilset*
Department of Chemistry, UniVersity of Oslo
P.O. Box 1033 Blindern, N-0315 Oslo, Norway
ReceiVed July 12, 2000
The development of methods for direct, selective oxidation of
methane to value-added products remains a major challenge to
chemists.1 During the past decade, important advances toward
this goal have been demonstrated in processes2 related to the
classical Shilov system,3 in which methane is catalytically
converted to methanol by PtII/PtIV salts in aqueous media. The
nature of the C-H activation at PtII in the Shilov system has
been the subject of experimental4 and theoretical5 investigations.
Considerable mechanistic insight has been obtained from sto-
ichiometric model reactions between cationic PtII complexes and
hydrocarbons,4b-d as well as studies on the reverse reaction-
elimination of alkanes from PtIV hydridoalkyl species.4a,e-g
Relatively stable PtIV hydridomethyl complexes are available if
a suitable ligand occupies the coordination site trans to the
hydride,4a,e-g,6 and have been extensively used to study the
mechanism for reductive elimination of methane (Scheme 1).
Substantial evidence has demonstrated the need to dissociate the
ligand trans to the hydride (a) before reductive elimination
occurs.4a,e-g,5e The resulting 5-coordinate intermediate undergoes
reductive C-H coupling (b) to form a methane σ complex before
the final loss of methane and ligand reattachment (c).
The former necessitates the existence of a highly reactive,
3-coordinate, 14-electron L2Pt(CH3)+ species on the reaction
coordinate. The principle of microscopic reversibility then dictates
that the first step in methane C-H activation is also dissociative,
with methane attacking the unsaturated metal center.
Recently, we reported that methane C-H activation occurs at
the aqua complex (Nf-Nf)Pt(CH3)(H2O)+BF4- (1; Nf-Nf ) ArNd
CMe-CMedNAr, Ar ) 3,5-(CF3)2C6H3)) under unusually mild
conditions in the poorly coordinating solvent 2,2,2-trifluoroethanol
(TFE).9 The reaction between 1 and CD4 led to extensive
deuterium scrambling and formation of CHnD4-n isotopomers, as
previously seen for the related species (tmeda)Pt(CH3)(NC5F5)+-
BArf-.4b,c,10 This phenomenon was explained in terms of the
dynamic equilibrium (b) in Scheme 1. The reaction between 1
and methane was inhibited by addition of ∼0.3 M water,
indicating preequilibrium loss of water prior to the rate-limiting
step. Two alternative scenarios that account for this behavior
might be envisioned: (1) a preequilibrium dissociatiVe pathway
via the coordinatively unsaturated 14-electron intermediate (Nf-
Nf)Pt(CH3)+ followed by methane coordination to give the
σ-methane complex (Nf-Nf)Pt(σ-CH4)(CH3)+, or (2) a solvent-
assisted associative pathway via the TFE complex (Nf-Nf)Pt(CH3)-
(TFE)+.11 We report here results from studies of the microscopic
reverse reaction, the elimination of methane through protonolysis
of PtII dimethyl complexes, that strongly suggest that the latter
mechanism operates at least in some cases.
Nevertheless, there are still mechanistic details that remain
unclear. For example, it has not been clearly established whether
the last step (c) of the reaction is dissociative or associative.7,8
(1) (a) Crabtree, R. H. Chem. ReV. 1995, 95, 987. (b) Arndtsen, B. A.;
Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28,
154. (c) Shilov, A. E.; Shulpin, G. B. Chem. ReV. 1997, 97, 2879. (d) Stahl,
S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180.
(e) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer: Boston,
2000.
(2) (a) Sen, A. Acc. Chem. Res. 1998, 31, 550. (b) Periana, R. A.; Taube,
J. D.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560.
(3) Goldshlegger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A.
Zhur. Fiz. Khim. 1972, 46, 1353. See also ref 1e.
(4) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961. (b) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1997, 119, 848. (c) Holtcamp, M. W.; Henling, L. M.; Day, M. W.;
Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta. 1998, 270, 467. (d) Wick,
D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235. (e) Fekl, U.;
Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156. (f) Hill, G. S.;
Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966. (g) Jenkins,
H. A.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1997, 16, 1946.
(5) (a) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118,
4442. (b) Heiberg, H.; Swang, O.; Ryan, O. B.; Gropen, O. J. Phys. Chem. A
1999, 103, 10004. (c) Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J. Am.
Chem. Soc. 1999, 121, 4633. (d) Mylvaganam, K.; Bacskay, G. B.; Hush, N.
S. J. Am. Chem. Soc. 2000, 122, 2041. (e) Bartlett, K. L.; Goldberg, K. I.;
Borden, W. T. J. Am. Chem. Soc. 2000, 122, 1456.
(6) (a) O’Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc.
1996, 118, 5684. (b) Hill, G. S.; Vittal, J. J.; Puddephatt, R. J. Organometallics
1997, 16, 1209. (c) Canty, A. J.; Fritsche, S. D.; Jin, H.; Patel, J.; Skelton, B.
W.; White, A. H. Organometallics 1997, 16, 2175. (d) Prokopchuk, E. M.;
Jenkins, H. A.; Puddephatt, R. J. Organometallics 1999, 18, 2861. (e) Haskel,
A.; Keinan, E. Organometallics 1999, 18, 4677.
Protonation of (Nf-Nf)Pt(CH3)2 (2a) or (N′-N′)Pt(CH3)2 (2b;
N′-N′ ) ArNdCMe-CMedNAr, Ar ) 2,6-(CH3)2C6H3) with 1
equiv of HOTf 10 in TFE causes elimination of methane, presum-
ably via (N-N)PtIV(H)(CH3)2 and (N-N)PtII(CH3)(σ-CH4)+ in-
+
termediates.12 Coordination of a suitable ligand (L ) TFE, H2O,
MeCN) produces the observed cationic (N-N)PtII(CH3)(L)+
products 3. If the protonolysis is performed with DOTf, two
extremes might be envisioned for the outcome of the reaction
(Scheme 2). If the scrambling process is slow relative to the loss
(9) (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.
(10) Abbrevations: tmeda ) tetramethylethylenediamine; BArf- ) (3,5-
(CF3)2C6H3)4B-; OTf- ) triflate.
(11) A direct preequilibrium displacement of water by methane in the C-H
activation experiments performed in TFE can be ruled out: The methane
binding step must be rate determining in order to account for the fact that
H/D exchange within a proposed (Nf-Nf)Pt(CH3)(σ-CD4)+ intermediate is much
faster than methane binding to the substrate (see ref 9).
(7) The protonolysis of trans-(PEt3)2Pt(H)(Cl) by HCl in methanol showed
a first-order dependence on [Cl-], interpreted in terms of associative
displacement of methane by Cl-. See ref 4a.
(8) Associative mechanisms have been reported for solvent exchange
reactions at (L-L)Pt(CH3)(DMSO)+ species (L-L ) various chelating
diimines and diamines). Romeo, R.; Scolaro, L. M; Nastasi, N.; Arena, G.
Inorg Chem. 1996, 35, 5087.
(12) Two (Nf-Nf)Pt(CH3)2(H)(L)+ species can be observed after protonation
in CD2Cl2 (no MeCN) at low temperature (see ref 9b). Low-temperature
protonation of poorly soluble 2 in TFE-d3 (no MeCN) leads to production of
3 and methane with no detectable intermediates.
10.1021/ja002505v CCC: $20.00 © 2001 American Chemical Society
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740 J. Am. Chem. Soc., Vol. 123, No. 4, 2001
Communications to the Editor
Scheme 3
suggesting that H/D scrambling is effected only by the equilibrium
process depicted in Scheme 1. The decrease in the CH4:CH3D
ratio at increasing [MeCN] is most readily explained by an
associative mechanism for the exchange of acetonitrile for
methane at (N-N)PtII(CH3)(σ-CH4)+.17 By the principle of micro-
scopic reversibility, this implies that substitution of methane for
MeCN at (N-N)Pt(CH3)(NCMe)+ should occur associatiVely and
that MeCN predissociation to proVide a 3-coordinate intermediate
is not required.
Figure 1. Selected parts of the 1H NMR spectra (300 MHz) of products
obtained after treatment of 2b with 1 equiv DOTF in: (a) TFE-d3. CD3-
CN was added after complete reaction. (b) 0.5 M CD3CN/TFE-d3. (c)
2.0 M CD3CN/TFE-d3. (d) 6.0 M CD3CN/TFE-d3. Legend: (R) PtCH3
of 3b; (â) PtCH2D of 3b-d; (ø) 195PtCMe of 3b/3b-d (2J(195Pt-H) ) 75
Hz); (δ) CH4; (ꢀ) CH3D (2J(D-H) ) 1.9 Hz).
The strength of Pt-L bonding at (N-N)Pt(CH3)(L)+ decreases
in the order L ) MeCN > H2O > TFE > CH4.9b,13b It is of
particular interest to determine whether an associative mechanism
also applies for methane coordination at (N-N)Pt(CH3)(L)+ for
L ) the better leaving groups H2O and TFE (i.e. in the system
where we have demonstrated true methane C-H activation).9,12b
The reaction of 2a with DOTf was therefore carried out in the
presence of D2O in TFE-d3. The CH4:CH3D ratio obtained at
[D2O] ) 0.5 M is essentially identical to that in neat TFE-d3,
that is, complete scrambling has occurred. This is consistent with
both mechanisms that may show inhibition by water for the C-H
activation (i.e. preequilibrium dissociative or solvent-assisted
associative mechanisms). Water is expected to be a considerably
poorer entering ligand than acetonitrile, less apt at competing with
the intramolecular scrambling process in an intermolecular
displacement of methane.
At higher [D2O] a completely different outcome ensued. At
[D2O] ) 6.0 M, the full range of methane isotopomers was
observed by 1H NMR (Scheme 3). The multiple D incorporation
must result from reversible protonations at 2a.18 Presumably, this
process is facilitated by the higher basicity of water relative to
TFE and acetonitrile.19
In conclusion, we have shown that methane loss from proto-
nated (N-N)Pt(CH3)2 complexes may proceed associatively, with
the very important implication that methane associatively enters
the coordination sphere of (N-N)Pt(CH3)(L)+ complexes in C-H
activation reactions.
Table 1. Methane Isotopomer Mixtures (%) Formed in the
Reaction between 2a/b and DOTf
2a
2b
solvent
CH4
CH3D
CH4
CH3D
TFE-d3
41
35
31
28
40
59
65
69
72
60
37
33
28
15
63
67
72
85
0.5 M CD3CN/TFE-d3
2.0 M CD3CN/TFE-d3
6.0 M CD3CN/TFE-d3
0.5 M D2O/TFE-d3
of methane from the σ complex, 3 and CH3D are expected as the
only products.13 On the other hand, if the scrambling occurs
rapidly compared to the methane loss, a mixture of 3/CH3D (1:
1) and 3-d/CH4 (1:1) would be formed; complete scrambling
would result in a 43:57 mixture14 of CH4 and CH3D. If scrambling
and methane loss occur at comparable rates, product distributions
between these extremes should arise. Moreover, the product
distribution should be dependent on [L] for an associative
displacement of methane by L, whereas a dissociative process
should give a product distribution independent of [L].15
Solutions of 2a and 2b in TFE-d3 with various amounts of
acetonitrile-d3 present were treated with 1 equiv of DOTf. The
relative amounts of methane isotopomers CH4 and CH3D produced
were determined by integration of the 1H NMR spectra (see Figure
1 for 2b). The results are summarized in Table 1. In the absence
of CD3CN, essentially complete scrambling occurred for 2a and
somewhat less complete for 2b. The data show that the CH4:
CH3D ratio decreases with increasing [CD3CN] for both com-
plexes.16 A concomitant increase in the 3:3-d ratio is also seen,
Acknowledgment. We thank the Norwegian Research Council, NFR,
for generous support.
JA002505V
(16) Bulk solvent effects should be insignificant since the dielectric
constants for TFE and acetonitrile are of similar order (ꢀTFE ) 26.7, Evans,
D. F.; McElroy, M. I. J. Solution Chem. 1975, 5, 405; ꢀMeCN ) 37.5, Harwood,
L. M.; Moody, C. J. Experimental Organic Chemistry; Blackwell Science:
Great Britain, 1995; pp 740).
(17) The rate of the scrambling process, i.e., step (b) and methane “rotation”
at Pt (Scheme 1) is assumed not to be affected by [MeCN].
(18) Interestingly, this behavior is consistent with Shilov’s original observa-
tion of multiple methane H/D exchange by PtII in aqueous solution, and clearly
strengthens the relevance of our model system in this respect. Goldshlegger,
N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A. Zhur. Fiz. Khim. 1969,
43, 2174. See also ref 1e.
(13) (a) No H/D exchange occurs between the intermediates (N-N)PtII(CH3)-
(σ-CH4)+/(N-N)PtIV(H)(CH3)2+ and TFE-d3 under these conditions, i.e. depro-
tonation of the intermediates is slow compared to methane loss. See refs
9b,13b. (b) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2000, 122, 10846.
(14) D will be located in a σ-CH3D ligand 57% (4/7) of the time, producing
CH3D, and in a CH2D ligand that remains bonded to the metal 43% of the
time. H/D kinetic isotope effects are ignored in this analysis.
(15) Competitive capture of a 3-coordinate intermediate by L and CH4 could
also give an [L]-dependent product ratio. However, this can be ruled out under
the actual reaction conditions, since methane loss is irreversible on the
experimental time scale (minutes at ambient temperature). Methane coordina-
tion is the rate-limiting step in the C-H activation process and takes place
on a time scale of days at 45 °C even at elevated methane pressure (ref 9b).
(19) Analogous multiple solvent D incorporation into methane was also
observed in the protonation of (tmeda)Pt(CH3)2 in methanol-d4, a solvent more
basic than TFE-d3. See ref 4a.