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Catalytic Pt+-Mediated Oxidation of Methane by Molecular Oxygen in the Gas Phase.

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1992. 11. 24x3 2847: b) [Cp:LaNHCH,(NH,CH,)] La-N
2.323(1) and 2302(10)A. L a + N 2.70(1)A. M. R. Gagne. C. L. Stern, T. .I.
Marks. .1. A i i i . Clieiii. So(.. 1992. 114. 275-294
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i.ieii.\ i i i C~riii/~i~f~i/i~ri7~i/
C ' h o i i i . \ i r j ~(Eds.: K . B. Llpkowiu. D B. Boyds). VCH.
Weinheim. 1990. p. 45 ff.
(151 x) R. Hacker-. E . Kaul'mann. P \'oii RapuO Schleyet-. W. Mahdi. H. Dtetrich.
. r r . 1987. 120, 1533-1538: b) K . Gregory. M . Bremer. P \ o n Rague
Schleycr, P. A . A . Kluicuer. L. Brandsma, A i i x e i l . C h i i 1989, 101. 1261 1264: Q i i g r i i C'hmii. l r i / Ed f.;i,q/. 1989, 3'.1224-1226.
K r d -
Catalytic Pt +-Mediated Oxidation of Methane
by Molecular Oxygen in the Gas Phase**
Ralf Wesendrup, Detlef Schroder, and
Helmut Schwarz *
As reported earlier,15] thermalized Pt ions dehydrogenate
methane to yield the carbene complex [PtCH,]' (Scheme 1, process I). Although the rate constant k , ( k , = 8.2 x 10-" cn13
molecule- ' s-) is somewhat smaller than the collision rate constant kADO(k,,,, = 9.8 x 1 0 - 1 0 c m ~ 3 m o l e c u l e ~ ' sA- ~D;O =
average dipole orientation) .[91 the reaction is quite efficient if
one takes into account that CH, is a small, symmetric molecule
with low polarizability and strong C - H bonds. Thermochemical considerations"'] lead to the conclusion that. since reaction I occurs at thermal energies. the reaction enthalpy (AHr) for
methane dehydrogenation is negative, and, thus, the heat of
formation ( A H , ) of [PtCH2]+ is smaller than 316 kcalmol-':
furthermore. the observation of reaction I1 (see below) sets a
lower limit for AH,[PtCH,]+ at 308 kcalmol-I; in the following, we will use an averaged value of AH,[PtCH2+] =312
4 kcalmol-'; from this one derives a Pt+-CH, bond dissociation energy of BDE(Pt+-CH,) = 115 & 4 kcalmol-'.l'Obl
Pt' + CH,
[PtCH,l+ + H,
Drciilc c i r d to Plofe\tor Hein--Geoig Wiigner
on ihcj o(ccis10i1 of /I15 6Srh hirthdci1
The activation of methane represents a fundamental challenge in chemistry, and the catalytic conversion of methane to
methanol or formaldehyde is both of scientific and economic
interest."] At metal surfaces activation of methane often commences with a homolytic C-H bond cleavage and proceeds by
reactions of the so-formed methyl radicals with an oxidant.
Alternately, the reaction of methane with a transition metal
center [MI may lead either to a metal-bonded hydrido methyl
fragment. H-M-CH,.
or a carbene species, M = C H 2 . with
subsequent 0-atom transfer to form the oxygenated products.
Furthermore, direct oxidation of methane can be achieved by
reactive 0 x 0 species. for example. high-valent transition metal
oxides, which are recovered in the catalytic process. In a more
classical fashion. methane functionalization may also proceed
by ionic mechanisms in which either a proton or a hydride is
abstracted or protonation take
The high catalytic activity of platinum has been known since
the last century, and platinum-based catalysts are widely used in
reduction as well oxidation processes.[31Although heterogeneous catalysis is one of the most actively pursued research
areas. this approach often fails to provide an understanding of
the chemistry at a molecular level. In contrast, more direct
mechanistic information may be obtained in gas-phase experiments in which the reactions of "naked" metal ions with neutral
substrates can be probed under relatively well-defined condition~.'~]
Recently. Irikura and Beauchamp[lb. 'I discovered that
"naked" third-row transition metal cations are capable of dehydrogenating methane in a stoichiometric Fashion in the gas phase
to yield the inethylene complexes [M=CH,] + . I 6 ] Here, we report
the design and realization of a catalytic cycle['] for the gas-phase
oxidation of methane under the conditions of Fourier transform
ion cyclotron resonance (FT-ICR) mass spectrometry.'*]
[*] Prof. Dr. H. Schwarr. Dr. D. Schroder, R. Wcsendrup
lnstitut fur Organische Chemie der Technischen Universitil
Strasse dca 17. Junr 135. D-10623 Berlin ( F R G )
Telcfax: Int. codc + (30)311-21102
Thii Kork was supported by the Deutsche Forschungsgemeinschaft a n d the
Fonds der Chemischen Industrie. We arc _eratel'ul to Spectrocpin AG.
Fillanden. for the loan of ii glow-discharge ion source. and Degussa AG.
Hanau. for thc Senerour supply of platinum tarzeta.
PtO+ + CH,O
+ "CH,O,"
[PtCH,l+ + H,O
+ CH,
Pt+ + CH,OH
Schemc 1. Formation and reactions of [PtCH,] ' a n d [PtO]', as well as the formation of the methane oxidation products CH,OH. CHZO. a n d "CH,O,".
The intramolecular kinetic isotope effects (KIEs) associated
with the Pt+-mediated dehydrogenation of CH,D, can be deduced from the relative abundance of [PtCD,]', [PtCHD]'.
and [PtCH,]', which are formed in a 2.5:6.5: 1.0 ratio. Because
formation of [PtCHD]+ from CHzD, is favored by a factor of
4 over the other two isotopomers. the statistically weighted ratio
is 2.5 : 1.6:1 .O for H,, HD. and D2 losses, respectively. According to the mechanism depicted in Scheme 2, these data can be
H- P+t - C H D ,
Ptt+ CH,D,
H / P t =CHD
+ D,
Scheme 7 . Scheme for the kinetics of the rcactions of Pt' and CH,D,
analyzed in terms of intramolecular kinetic isotope effects associated with the rates of the insertion step ( ' k ) , the %-hydrogen
transfer ( 2 k ) . and the reductive elimination of dihydrogen
(3k),[111as described in Equations (a)-(c).
+ CH,
Scheme 3. Formation of [PtCH,]' from [PtO]' and CH,
L ~ i ~ ~ k=i l(ll ). 5 ( ' k i l ~ ' k+
D ' k , , i ' k , ) ~ ~ k , , ~ ~ ~ k=1.6
( ' X , , ~ ' k , ) ( L k , l i ' k , ) ( ~ k H ~=
~ 32.5
k,,D/ku>= 0.5('kIi/'k, + 2kH:2ko)x JkHi,/3kD,= 1.6
Although an exact solution of the set of equations (a)-(c) is
not possible, from (a) and (c) we can conclude that 3 k H ,=
( 3 k H x2 JkDZ)1;2.
Such a correlation is typical for a rate-determining reductive elimination of molecular hydrogen from a symmetrical transition structure." 21 Accordingly, 'k,/'k, as well as
'k,,;'k, are close to unity, and the observed intramolecular kinetic isotope effect associated with the rate-determining reductive elimination step ( 3 k )is about 1.6 per H atom. It is noteworthy that in contrast to other methane activation processes,['] for
P t + the initial C-H bond insertion does not contribute to the
rate-determining step. In view of the differences usually expected for intra- and intermolecular kinetic isotope effects,['31 our
findings are in good agreement with the minor KIE associated
with the Pt+-mediated activation of CD, (k,jk, = 1.3).[']
A catalytic process for the gas-phase oxidation of methane
must regenerate the consumed P t c for further cycles. The
[PtCH,]' intermediate must therefore be oxidized by an appropriate reagent. Interestingly, and in contrast to other exa m p l e ~ . [nitrous
' ~ ~ oxide does not serve as an oxidant donating
a single 0 atom, and N,O is completely unreactive towards
P t C H i . although the formation of Pt'. formaldehyde, and nitrogen is exothermic by 24 kcalmol-l. However, [PtCH,]' reacts with molecular oxygen to form Pt' and the monoxide PtO+
as the ionic products (Scheme 1 , reaction 11); thermalized Pt'
does not react with 0,, but with N,O yields the monoxide
cation PtO'. Although reaction IT is exothermic for both products (see below), it is relatively slow relative to the collision rate
( k H = = 0 . 2 ~ l O - ' ~ cmolecule-'s-';
kADo= 5 . 6 ~ 1 0 - ' ~ c m
inolecule-ls- '). which indicates that a barrier is associated
with the activation of dioxygen by [PtCH,]+. We cannot deduce
the structures of the neutral products in our experimental ICR
setup; however, thermodynamic considerations imply that
formaldehyde is formed in reaction I I a (AHR= - 4 44 kcal
mol-I). In reaction IT b the situation is not as straightforward,
since several possibilities such as formic acid or its thermolysis
products carbon monoxide/water or carbon dioxide/dihydrogen are thermochemically accessible (AHR= - 69 k 4 kcal
mol-' (HC0,H). -63 k 4 kcalmol-' (CO/H,O), and -73 i
4 kcal mol (CO,/H,O)) .[I
The PtO cation also contributes to the catalytic cycle, since
it is recovered as P t + after reaction with methane (Scheme 1,
reaction 111). Not unexpected for a transition metal oxide cation
(e.g. FeO+).[''I
reaction I11 is collision-controlled ( k , =
10.5 + 1 0 - " cm3 molecule-'s-';
k,,, = 9.8 x 1 0 - ' o c m 3
molecule ~I s-') and gives rise to [PtCH,]' and neutral water
(AHR= - 61 i 4 kcalmol-I), or P t + and methanol (AHR =
- 30 kcalmol-I). When CH,D, is employed as reaction partner. reaction 111 leads to [PtCD,]', [PtCHD]', and [PtCH2]+,
which are formed in a ratio of 2.1 : 5.3: 1 .O. An analysis similar
to that performed for the reactions in Scheme 2 indicates that
in reaction IIIa the reductive elimination of water from a
[(HO)(H)PtCH,]+ intermediate contributes most to the ratedetermining step; however, contributions of the other steps to
the measured intramolecular isotope effect cannot be ignored
entirely (Scheme 3).
Combining reactions 1-111 of Scheme 1 leads to a catalytic
cycle under FT-ICR conditions in which Pt' ions mediate the
oxidation of methane by molecular oxygen. This is the first
successful demonstration of catalytic methane activation by a
naked metal ion under oxidative conditions. Since the conversion of [PtCH,]' (reaction 11) is the slowest step in the cycle and
[PtCH,]+ is also formed in reaction 111, a large excess of oxygen
is necessary for a satisfactory catalytic rate (typically, a ratio of
CH4:0, = 1 :20 is sufficient).
The deviation of the Pt' concentration from the pseudo-first
order exponential decrease (Fig. 1 ) clearly demonstrates the op-
' 80
Fig. I . Temporal evolution of the species involved in the Pt ' -cataly/cd inethanc
~oxidation (CH,: 0, = 1 :20). The dashed curve shows thc decrease of thc Pt + signal
In the presence of methane only. All curbes are based on a numerial solution of the
appropriate differential equations on use of the k , values given in the tcxt. For thc
sake of clarity the experimentally measured data are omitted: these data agree with
the computed ion abundancies within experimental crror (i5 % ) . Ion losses at long
storage time ( > X O s) are taken into account.
eration of a catalytic process in the present system; this supposition is further substantiated by the steady state concentrations
for the reactive species Pt'. [PtCH,]', and [PtO]' at longer
reaction times (>SO s). The slow reaction of [PtCH,]' with
methane"] ( k R= 0.2 x 1 0 - l o versus k , = 8.2 x 10- l o cm3
molecule- I s- for the activation of methane by naked Pt' )
that generates Pt[C,,H,]' (Scheme 4) and, after subsequent oxidation, [PtCO]', Pt[C,O,H]+, and [PtH]', which neither react
with oxygen nor with methane, limits the turnover of the overall
process by the irreversible formation of unreactive products.
By an iterative modeling of the kinetic data with the rate constants for the reactions depicted in Schemes 1 and 4. at a
mbar, p ( 0 2 )= 8 x
certain p r e s ~ u r e [ ' ~(p(CH,)
mbar) a turnover number of about 6 is computed. Further-
lPtCH21+ + CH,
- HL
[ P t ( C 2 ,H4)]-
Scheme 4 Formation and oxiddtion of [Pt(C,,H,)]+
more, this analysis reveals that in the Pt+-mediated oxidation,
methane is converted into methanol (10 YO),formaldehyde
(25 YO),and "CH,O," (65 YO).The turnover of the system may
be enhanced by increasing the oxygen excess to favor reactions IT and 111; however. this will result in a lowering of the
overall rate of conversion. Similar to the effect observed in the
reaction of Ir' with methane in the presence of oxygen,[51reaction I1 accelerates slightly during the course of the catalysis; this
may indicate the formation of excited [PtCH,]' ions o r of the
presence of a structural isomer in reaction 111a. This point will
be addressed in forthcoming. more detailed kinetic studies.
In conclusion, the reaction sequence reported here describes
for the first time a relatively efficient catalytic system for the
gas-phase oxidation of methane by a bimolecular ion -molecule
process. However, even in the gas phase the well-known problem of over-oxidation cannot yet be circumvented: the formation of "CH,O," is observed as main reaction channel.
E-xperimental Procedure
Pt ' ions were generated in the external ion source of a Spectrospin CMS 47X FTICR mass spectrometer [I71 by either laser desorption;laser ionization by focusing
a beam of a Nd.YAG laser ( i = 1064 nm) onto a platinum target [1X] o r glow-discharge ionization o f a platinum wire In an argon plasma [19]. The cations were
extracted from the source and transferred to the analyzer cell by a system ofelectric
potentials and lenses. The isolation of the lY5Pt+isotope and all subsequent isolation steps were performed by using FERETS [20],a computer-controlled ion-ejection protocol that combines single frequency ejection pulses with frequency sweeps
to optimize ion isolation. After isolation the ions of interest were thermalized by
collisions with either pulsed-in argon gas o r the reaction gases. and the thermalized
ions were subsequently reisolated. The degree of thermalimtion was assumed to be
complete if no further change in reactivity occurs upon increasing the amount of
pulsed-in ai-gon gas [7i]. Methane and oxygen were admitted to the FTICR cell
through leak valves (typical preswres about 4 x lo-' mbar and 8 x
respectively). The pseudo-first order rate constants were derived from the decay of
the precursor ion signals and converted to absolute rate constants by calibrating the
ionization gauge measurements with rates of well-known ion-molecule processes
[14b, 21. 221; the error in the absolute rates is k 2 5 % [14b]. All functions of the
instrument were controlled by a Bruker Aspect 3000 minicomputer. Methane
(Linde AG. 99.999%). [DJmethane (Cambridge Isotope Laboratories, >98
a t o m % D), and oxygen (Linde ACT. 99.995%) were used w'ithout further purification.
Received: December 15. 1993 [Z 6562 I€]
Gerinan version: Angew. Cl7rm. 1994. 105, 1232
[l] a) H. Schwarz. Anpew. Chern. 1991, 103, 837; Angeir. Chen?. In?. Ed. Enyl.
1991. 30, 820; b) K. K . Irikura, J L. Beauchamp. J. PIijs. Chern. 1991, 95,
8344; c) J. M. Fox. Caial. Rfw. Sci. Eny. 1993. 35, 169.
[2] J. Sommer, J. Bukula, Ace. Chem. Re.>.1993. 26. 370.
[3] See for example a) M. A. Benvenuto, A. Sen. J. Chem. Soc. Chem. Commun.
1993, 970; b) M. Del Todesco Frisone. F. Pinna. G. Strukul, 0rganonietullic.s
1993, 12. 148, c) J. A. Labinger. A. M. Herring, D. K. Lyon, G. A. Luinstra,
J. E. Bercaw. I. T. Horvith, K. Eller, 2nd. 1993.12.895. and references therein
[4] Recent reviews on gas-phase transition metal chemistry: a ) K . Eller. H.
Schwarz, C h m . Res. 1991. 91. 1121: b) P. B. Armentrout. Annu. Rev. PhLs.
Chcm. 1990, 41, 313.
[5] K . K. Irikura. J. L. Beauchamp. J. Am. Chem. Soc. 1991, 113, 2769.
[6] Further examples of cationic Pt species in the gas phase: a) T. F. Magnera.
D. E. David. J. Michl. J A m . Chew. Sue. 1987.109. 936; b) D. J. Trevor. D. M.
Cox. A . Kaldor. ihid. 1990. 112. 3742; c) M. Hada. H . Nakatsuji. H. Nakai. S.
Gyobu. S. Miki. J. Mof.Sfrucr. (Theochem) 1993. 281. 207.
[7] Other catlilytic process under FT-ICR conditions: a) M. M. Kappes. R. H.
Staley. .
Am. Chmn?.So(,.1981. 103. 1286: b) S . W. Buckner, B. S . Freiser. ihid.
1988. 110, 6606; c) D. Schroder, H. Schwarz, Angeii. Chem. 1990, 102, 1468;
An,qrw Chem /nt. Ed. Eizgl. 1990. 29, 1433; d ) P. Schnabel, M. P. Irion, K. G.
Weil, J. Phm. Cherii. 1991, Y5. 9688; Chem. Ph.vs. Lett. 1992. 190. 255; e j M. P.
Irion. I n / . J. Mu.r.s Spectrum loti ProceJses 1992. 121, 1 ; f j M. P. Irion. P.
Schnabel. &r. Bims~w,qor.Plys. Chem. 1992. 96, 1101; g) P. Schnabel, K . G.
Weil. M. P. Irion. Angebt. Chem. 1992, 104. 633: Angiw. Chpm. in/.Ed. Engl.
1992. 31, 636; b) S. Karrafi. D. Schroder. H. Schwarz. Chrm. Ber. 1992, 125,
751 : i) D. Schroder. A. Fiedler. M. F, Ryan, H. Schwarz, J PhJs. Chmz., 1994,
98. 6X: j) R. Wesendrup. Diplomarbeit. Technische Universitlt Berlin, 1994.
[XI FT-ICR:MS: A n u l ~ ~ ~ iApplicorions
of Fourier Trunsforin ion Cwlorron Resuiiunw mas^ Spw[wmeriw (Ed: B. Asamoto). VCH. Weinheim. 1991.
[9] T. Su. M. T. Bowers, In?. J Muss Specfrom Ion Phys. 1973. 12. 347.
VCH Veilugr~erellschuftm h H , 0-69451 Weinherm 1994
[lo] a) If not mentioned otherwise, all thermochemical data were taken from: S. G .
Lias. J. E. Bartmess, J. F. Liebman. J. L. Holmes, R. D. Levin. W G. Mallard.
J Phys. Chem. Ref Datu 1988.17, Suppl. 1 . h) In two recent theoretical studies.
the BDE (Pt+-CHI) were calculated at 123 k 5 and 119 kcalmol-': K. K.
Irikura. W. A. Goddard 111. J Am. Chem. Soc.. submitted. C. Heinemann. R
Hartwig, R. Wesendrup. W. Koch. H. Schwarz. J. A m . ('hem. Soc., submiited.
(111 D Schroder. H. Schwarz. C h i u 1989, 43. 317.
1121 a) G . Hvistendahl. D. H. Williams. J Cliem. So(. Chem. Commun, 1975, 4.
b) H. Schwarz. W. Franke. J. Chandrasekhar. P von R Schleyer. Tetrahedron
1979. 35. 1969, and references therein.
(131 a) P. J. Derrick. K . F. Donchi in Cuinprehmsiw Cheiiiical Kinetics, Vol. 24
(Eds.. C. H. Bamford. C. F. H . Tipper). Elsevier. Amsterdam. 1983, p 53;
b) D . Schroder. D. Sulzle. 0 . Dutuit. T Baer. H. Schwarz. J Am. Chen?. Soc.,
111 press.
[14] a) Ref. [7a-c]: b) D . Schroder, Dissertation, Technische Universitat Berlin.
(151 Even i f H C 0 , H is formed in a highly vibrationally excited state, in view of the
significant barriers (> 60 kcal mol- ') associated with its dehydration or dehydrogenation, it is likely to survive: a) P. G. Blake. H. H. Davies, G. E. Jackson.
J Chem. Soc. B 1971. 1923; b) P. Ruelle. J Am. Chem. Sac 1987, 109. 1722.
[16] a) Ref. [ 7 c ] ; b) D. Schroder. A. Fiedler. J. Hrusak, H. Schwarz. J. Am. Chem.
Soc 1992, 114, 1215.
1171 a) K. Eller. H. Schwarz. Int. J Mass Specrruin. Ion 1989, Y3, 243.
b) K. Eller. W Zummack. H. Schwarz. J. Am. Chem. Soc. 1990, f / 2 . 621.
[I81 B. S. Freiser, Tuluntu 1985. 32, 697; A n d Chiin. Acru 1985. 178. 137.
1191 Recent examples. a ) W. S. Taylor. W R. Everett. L. M. Babcock. T. L. McNeal. Inr. J Musr Spwtrum. fon Processec 1993. 125. 45; h) D. M. Chambers.
S. A. McLuckey, G. L. Glisb, A n d Chern. 1993. 65, 778.
[20] R. A. Forbes. F. H. Laukien. J. Wronka. Int. J. Mass Spectrom. /on Prucessa
1988,83. 23
1211 J. E. Bartmess, R. M. Georgiadis. Vacuum 1983, 33, 149.
[22] For example: Y. Lin. D. P. Ridge, B. Munson. Org. Mass Spectrom. 1991. 26.
Increased Selectivity of a Simple Photosensitive
Probe in the Presence of Large Proportions
of Cholesterol **
Siw Bodil Fredriksen, Valerie Dolle, Masakuni
Yamamoto, Yoichi Nakatani,* Maurice Goeldner and
Guy Ourisson
We have recently described a photoactivatable phospholipidic
transmembrane probe
which had a marked advantage
over the simpler membrane probes described earlier by Khorana
et al.[31and Breslow et aLr4IIt was much more site-selective and
attacked preferentially the terminal carbons of neighboring
phospholipids. This selectivity was excellent in the presence of
the near-physiological concentrations (33 mol YO)of cholesterol,
a normal constituent of eucaryotic membranes, which it reinforces by favoring a compact and regular arrangement of the
phospholipids. The probe 1, when used together with choles["I Prof. Y. Nakatani. S. B. Fredriksen. V. Dolle. M.Yamamoto. Prof. M. Goeldner
Prof. G. Ourisson
Laboratoire de Chimie Organique des Substances Naturelles associi au CNRS,
Universite Louis Pasteur
5 rue Blaise Pascal. F-67084 Strasbourg (France)
Telefax: Int. code (88)607620
[**I This work was supported by the CNRS (Action Imitative de I'lnterface
Chimie-Biologie), by the Supermolecules Research Project of the Research
Development Corporation of Japan and Universite Louis Pasteur.
0570-0833/9411111-1176$ 10.00+.2SiO
Anyen. Chem. in[. Ed. EnyI. 1994, 33, No. 11
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oxidation, molecular, catalytic, gas, phase, oxygen, methane, mediated
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