The metal is the kinetic site of protonation of (Diimine)Pt dimethyl complexes

Article abstract of DOI:10.1021/ja027649j

Protonolysis of (diimine)PtMe₂ (1) complexes in CD₂Cl₂ containing CD₃CN at -78 °C yields (diimine)PtMe₂(H)(NCCD₃)⁺ (4), (diimine)PtMe(NCCD₃)⁺ (5), and methane. The relative yields of 5 and methane decrease with increasing concentrations of CD₃CN. This is consistent with protonation of 1 occurring directly at the metal, rather than at a methyl group. The principle of microscopic reversibility then implies that the deprotonation in Shilov-type C-H activation occurs from a Pt(IV) hydridomethyl intermediate, rather than from a Pt σ-methane complex. Copyright

Full text of DOI:10.1021/ja027649j

Published on Web 09/24/2002
The Metal Is the Kinetic Site of Protonation of (Diimine)Pt Dimethyl
Complexes
Bror Johan Wik, Martin Lersch, and Mats Tilset*
Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
Received July 11, 2002
Scheme 1
The development of methods for direct, selective oxidation of
methane to value-added products continues to pose a major
challenge to chemists.1 Thirty years ago, Shilov and co-workers
reported the catalytic conversion of methane to methanol and
chloromethane by aqueous Pt salts.1e,2 Since then, the classical
Shilov system as well as model systems have been the subject of
detailed investigations. The accumulated evidence suggests that
three major steps, each of which gives rise to intriguing mechanistic
possibilities, constitute the catalytic system (Scheme 1).1d
Experimental3 and theoretical3d,4 studies of relevant Pt systems
Scheme 2
have focused mostly on the initial C-H activation step, which is
considered to yield a Pt(IV) hydridoalkyl species by oxidative
addition of R-H. H/D scrambling between labeled hydride and
methyl ligands during alkane elimination imply the intermediacy
of (as yet unobserved) Pt(II) σ-alkane complexes, as well as their
facile interconversion with Pt(IV) hydridoalkyl species.3c-d,5 The
mechanism requires the loss of a proton as X-H, but it has not
been firmly established whether the deprotonation occurs from the
Pt(IV) hydridoalkyl or the Pt(II) σ-alkane complex. Experimentally,
these reactions have been investigated through the microscopic
reverse of the C-H activation, that is, protonation of Pt(II) alkyl
precursors. Hydridoalkyl Pt(IV) species have been observed during
low-temperature protonations,3a,d,5a-c,6 and it has been asserted that
the metal must be the preferred site of protonation. However, the
observation of such hydrides only identifies the thermodynamic site
of protonation. It is still possible that these hydrides are preceded
Hammett principle applies,9 and the product ratio will only depend
by unobserved kinetic products that rapidly rearrange to the
observed hydrides. We now present evidence that at least under
the applied conditions (vide infra), it is the metal center of Pt(II)
dialkyl complexes that is the kinetically preferred site of protonation.
The strategy that has been applied to probe this issue is delineated
in Scheme 2. Initial protonation of the Pt(II) dimethyl complex 1
produces a five-coordinate7 Pt(IV) hydridomethyl complex 2 by
metal protonation or a Pt(II) σ-methane complex 3 by protonation
at a methyl ligand. It is well established5 that species such as 2
and 3 undergo facile interconversion. Intermediate 2 is irreversibly
(vide infra) trapped by MeCN to produce the observed 4. The σ
complex 3 is also irreversibly trapped by MeCN by associative
displacement8 of the methane ligand to give Pt(II) complex 5. Both
trapping processes are intermolecular and should exhibit first-order
dependencies on [MeCN], whereas the intramolecular interconver-
sion between 2 and 3 is [MeCN] independent. This results in a
diagnostic dependence of the 5:4 product ratio on [MeCN],
on the relative magnitudes of k2 and k3. On the other hand, if k1
and k-1 are very slow compared to k2[MeCN] and k3[MeCN], the
5:4 ratio will reflect the relative occurrence of Pt versus methyl
protonation but will still be independent of [MeCN]. Finally, if the
interconversion between 2 and 3 and trapping by MeCN occur at
comparable rates, the 5:4 ratio will be [MeCN]-dependent: If initial
protonation occurs at Pt to give 2, an increase of [MeCN] will trap
2 more efficiently, inhibiting the interconversion of 2 to 3.
Increasing [MeCN] therefore decreases the 5:4 ratio. In contrast,
initial protonation at a methyl group to give 3 will cause the 5:4
ratio to increase with increasing [MeCN].
A series of experiments has been conducted in which (diimine)-
PtMe2 complexes 1a-c were protonated with ca. 4 equiv10 of
HBF4‚Et2O in CD2Cl2 at -78 °C in the presence of variable
amounts of MeCN-d3.11-13 The products 4 and 5 were readily
observed by 1H NMR at this temperature, and the 5:4 product ratios
were obtained by integration of selected 1H NMR signals.14 Figure
1 summarizes the results, expressed as the yield of 5 relative to
the combined yields of 4 and 5, as a function of [MeCN] in the
0.03-5.5 M range. It can be immediately seen that for all three
series, the relative yield of 5 decreases with increasing [MeCN].
This result is consistent with initial protonation occurring exclu-
siVely or mostly at Pt.15
depending on the relative magnitudes of the rate constants k1, k-1
,
k2, and k3. If k1 and k-1 are rapid compared to k2[MeCN] and k3-
[MeCN], the 5:4 ratio will be independent of [MeCN] and also
independent of the identity of the protonation site. The Curtin-
* To whom correspondence should be addressed. E-mail: mats.tilset@
kjemi.uio.no.
9
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J. AM. CHEM. SOC. 2002, 124, 12116-12117
10.1021/ja027649j CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
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(10) Excess acid is used to ensure complete consumption of 1. The protonation
occurs irreversibly under these conditions: Protonation of 1a-d6 with
HBF4‚Et2O leads to the formation of CD3H but not CD2H2, CDH3, or
CH4 as judged by 1H NMR.
(11) A solution of 1a-c (ca. 3 mg, 5-8 µmol) in CD2Cl2 (400 µL) in an
NMR tube was layered with CD2Cl2 (100 µL). (Careful layering was used
to prevent premature mixing of 1 and acid). The contents were cooled to
-78 °C and layered with a solution of HBF4‚Et2O (3 µL) in a mixture of
MeCN-d3 (x µL) and CD2Cl2 (200 - x µL.; x ) 0-200 µL). The tube
was capped, kept at -78 °C, and shaken to mix the reactants immediately
before transfer to a precooled NMR probe. A pale, homogeneous yellow
solution was immediately obtained. Particular care was taken to minimize
any heating of the sample. The product distributions at -40 °C are not
very different from those at -78 °C, suggesting that unintentional heating
did not perturb the results.
Figure 1. Relative yield of 5 () [5]/([5]+[4]) resulting from protonation
of 1a-c with HBF4‚Et2O. Filled symbols represent experiments conducted
at -78 °C, whereas open symbols derive from protonations at -40 °C.
The N-aryl groups of the diimine are oriented more or less
perpendicularly with respect to the coordination plane in 1.3f,i Bulky
2,6-substituents exert a significant steric hindrance of the empty
apical coordination site in 2. The three complexes 1a-c qualitatively
show the same response to increasing [MeCN], but significant
quantitative differences may be understood in terms of these steric
effects. Complex 1c, unsubstituted in the 2,6-positions, is effectively
trapped after protonation at very low [MeCN]sthe relative yield
of 4c through efficient trapping of 2c is greater than 95% even at
0.1 M MeCN. For 2a, which is 2,6-dimethyl-substituted, trapping
is less efficient, and ca. 5 M MeCN is required to achieve a 95%
trapping yield of 4a. Finally, for 1b, with the much bulkier isopropyl
substituents, trapping as 4b is only 90% efficient even at this
concentration.16
In conclusion, we have shown that protonation of a series of
(diimine)PtMe2 complexes occurs preferentially at Pt as opposed
to at a methyl ligand. The principle of microscopic reversibility
then dictates that the deprotonation step in the general Shilov
mechanism must occur from the Pt(IV) hydridomethyl, rather than
the Pt(II) σ-methane, species. We anticipate that the competitive
trapping technique described herein will be useful for further
mechanistic studies and provide further insight into how the site
of protonation/deprotonation may depend on metal complex
structure, solvent, identity of acid, and other experimental param-
eters.
(12) The observed 5:4 product yields and ratios were unchanged after 2 h at
-78 °C, indicating that 2 is irreversibly trapped by MeCN (otherwise,
more 5 should be formed over time). Heating resulted in elimination of
methane from 4, yielding more 5.
(13) Compounds 1 and 5 have been previously described (see refs 3h,i). 1H
NMR data for 4: (a) δ -22.38 (2JPt-H ) 1563 Hz); (b) δ -22.66 (2JPt-H
) 1571 Hz); (c) δ -22.90 (2JPt-H ) 1547 Hz). Additional NMR data for
4 are given in the Supporting Information.
(14) Due to extensive signal overlap, relative yields were consistently
determined from the integral of the CH4 signal at δ 0.20 and the combined
Pt-CH3 signals of 4 and 5. In cases where relative yields of 4 and 5
could be determined from nonoverlapping diimine ligand signals, the
results were identical within experimental uncertainties.
Acknowledgment. We thank the Norwegian Research Council,
NFR, for generous support (stipends to B.J.W. and M.L.).
Supporting Information Available: NMR data for 4 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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