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Coupling of Methane and Ammonia by Dinuclear Bimetallic PlatinumЦCoinage-Metal Cations PtM+.

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Angewandte
Chemie
Methane Activation
Coupling of Methane and Ammonia by
Dinuclear Bimetallic Platinum–Coinage-Metal
Cations PtM+**
Konrad Koszinowski, Detlef Schrder, and
Helmut Schwarz*
Dedicated to Professor Lutz F. Tietze
The activation and functionalization of methane constitute
continuing challenges for catalysis. One example of their
industrial realization is given by the Degussa process for the
large-scale production of hydrogen cyanide from methane
and ammonia using platinum as heterogeneous catalyst
[Eq. (1)].
hPti; 1500 K
CH4 þ NH3 ƒƒƒƒƒ! HCN þ 3 H2
ð1Þ
Our aim is the development of a gas-phase model for this
process that may enhance the current mechanistic knowledge[1, 2] and thus ultimately contribute to rational catalyst
improvement. So far, atomic Pt+, homonuclear clusters Ptm+
as well as small heteronuclear clusters PtmAun+ ions have been
studied.[3–7] Whereas all the homonuclear Ptm+ clusters investigated achieve CH4 activation in the first reaction step,[8, 9] the
CN coupling of the resulting carbene species PtmCH2+ with
NH3 only works for m = 1.[3–5] In contrast, the dinuclear
cluster mediates CN bond formation if one platinum atom is
replaced by a gold atom.[6] This finding was ascribed to a
particularly well-balanced binding situation in the PtAuCH2+
species. The bonding is strong enough to afford spontaneous
activation of CH4 on the one hand but does not prevent the
CH2 fragment from coupling with NH3 on the other.[6] Recent
results have shown, however, that the larger bimetallic
PtmAun+ clusters do not mediate the coupling step between
the CH2 moiety and NH3, thus pointing to special features of
PtAu+.[10]
To further elucidate the remarkable role of the second
metal in the dinuclear cluster, we have extended our studies to
PtCu+ and PtAg+. The change from gold to its lighter
congeners copper and silver is straightforward in view of
their electronic and chemical similarities (note, however, that
gold is distinguished from both copper and silver by the
operation of much stronger relativistic effects[11]). As with
PtAu+, the reactions with CH4, O2, and NH3 are investigated;
for comparison, the reactivities of Cu2+ and Ag2+ are included
[*] Dr. K. Koszinowski, Dr. D. Schrder, Prof. Dr. H. Schwarz
Institut fr Chemie, Technische Universit#t Berlin
Strasse des 17. Juni 135
10623 Berlin (Germany)
Fax: (+ 49) 30-314-21102
E-mail: Helmut.Schwarz@www.chem.tu-berlin.de
[**] Financial support by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, and the Degussa AG is gratefully
acknowledged. M = Cu, Ag, Au.
Angew. Chem. Int. Ed. 2004, 43, 121 –124
DOI: 10.1002/anie.200352817
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
121
Communications
as well. Special attention is paid to the reactions of the metal
carbenes with NH3.
Similar to Pt2+ and PtAu+,[7, 9] both PtCu+ and PtAg+
efficiently activate CH4 [Table 1 and Eq. (2)]. As the homoTable 1: Bimolecular rate constants k and efficiencies (f) for the
reactions of the dinuclear cations PtCu+, PtAg+, PtAu+, and Pt2+ as
well as the corresponding carbene species.
Reaction
k @ 1010[cm3 s1] (f)
M = Cu Ag
Au
Pt
PtM+ + CH4!PtMCH2+ + H2
5.8
6.0
6.4[7] 8.2[9]
(0.59) (0.63) (0.67) (0.85)
0.35
0.04
0[7]
1.3[9]
(0.064) (0.01)
(0.25)
0
0
0
5.4[9]
(0.27)
8.5
3.9
0
0
(0.42) (0.19)
0
0
2.8[10] 0
(0.14)
5.2
4.9
6.0[10] 9.7[5]
(0.26) (0.24) (0.30) (0.49)
PtM+ + O2!M+ + PtO2
PtM+ + NH3 !PtMNH+ + H2
!MNH3+ + Pt
!PtNH3+ + M
PtMCH2+ + NH3![Pt,M,C,H3,N]+ + H2
that result in an increased interaction between the cluster ion
and O2.[9] Compared to PtM+, the reaction of Pt2+ is also
statistically favored because two reactive atoms are present.
A different kind of cluster degradation occurs upon
reaction of PtCu+ and PtAg+ with NH3 [Eq. (4)]. In this
PtMþ þ NH3 ! MNHþ3 þ Pt
ð4Þ
reaction type, the basic substrate substitutes one metal atom
and yields MNH3+ complexes. Whereas neutral Pt is lost from
PtCu+ and PtAg+, the analogous reaction of PtAu+ gives
PtNH3+ and Au; similar degradation processes are observed
for the homonuclear M2+ coinage-metal clusters.[10, 12]
The product distributions found for the heteronuclear
clusters can be explained by simple thermochemical considerations. An analysis of the heats of formation for the two
alternative product combinations, MNH3+ + Pt versus
PtNH3+ + M, demonstrates that the former exit channel
clearly lies lower in energy for M = Cu and Ag (Table 2), in
Table 2: Heats of formation DHfA [kJ mol1] for the two alternative
product pairs (MNH3+ + Pt and PtNH3+ + M) observed in the reactions
of PtM+ with NH3.[a]
PtMþ þ CH4 ! PtMCHþ2 þ H2
ð2Þ
nuclear coinage-metal dimers M2+ (M = Cu, Ag, Au) prove to
be inert,[12] the platinum center appears essential for the
reaction. This finding is as expected because a prerequisite for
the spontaneous dehydrogenation of CH4 is that the metal
carbene formed is sufficiently thermochemically stable. Such
a strong M2+–CH2 interaction requires a double bond and,
thus, at least two free valences at the metal core that are not
available in the cases of Cu2+, Ag2+, and Au2+. Because
promotion energies are rather high for the first- and secondrow transition metals,[13] ds hybridization is not a viable option
for the Cu2+ and Ag2+ dimers either.
Exposure of PtCu+ and PtAg+ ions to O2 results in cluster
degradation [Eq. (3)]. Platinum's affinity for the + iv oxidaPtMþ þ O2 ! Mþ þ PtO2
ð3Þ
tion state encountered in PtO2 is also well-known from
classical solution chemistry; in contrast, copper and silver
prefer lower oxidation numbers, in accordance with the
product distribution observed.[14] The bias of copper and silver
against high oxidation states also accounts for the lack of
reactivity of Cu2+ and Ag2+ toward O2.[12, 15]
The efficiency of cluster degradation decreases when
changing M from Cu to Ag and completely vanishes for Au
(Table 1).[7] Presumably, the high ionization energy of gold,
IE(Au) = 9.23 eV, strongly disfavors the generation of Au+ in
comparison to Cu+ and Ag+ (IE(Cu) = 7.73 and IE(Ag) =
7.58 eV).[16] Moreover, the similar size and energy of the
orbitals of Pt and Au are supposed to lead to a better overlap
and a stronger binding that further hinders cluster degradation for PtAu+. In contrast, the easier fragmentation of Pt2+ is
attributed to the presence of open valences at the metal core
122
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
M
DHfA(MNH3+ + Pt)
DHfA(PtNH3+ + M)
DDHfA
Cu
Ag
Au
1366 15
1365 13
1479 30
1451 12
1398 12
1479 12
85 19
33 18
0 32
[a] Calculated from D0(M+-NH3) (ref.[17–20]), DHfA(M), DHfA(NH3)
(ref. [21]) and IE(M) (ref. [16]).
agreement with experimental results.[16–21] For M = Au, the
larger uncertainties of the thermochemical data available do
not allow an unambiguous assessment. However, the tendency is for the relative stability of the PtNH3+ + M product
channel to be significantly enhanced compared to the
situation for M = Cu and Ag, which is consistent with the
different reactivities observed. In contrast, Pt2+ dehydrogenates NH3 to form Pt2NH+.[9] This reaction again points to the
high potential of platinum in terms of bond activation.
The reactivity of the heteronuclear clusters towards NH3
totally changes upon addition of the CH2 fragment to the
metal core. Instead of cluster degradation, dehydrogenation
takes place exclusively [Table 1, Eq. (5)]. The dehydrogenPtMCHþ2 þ NH3 ! ½Pt; M; C; H3 ; Nþ þ H2
ð5Þ
ation products formed from PtAuCH2+ and Pt2CH2+, differ in
their consecutive reactions. Thus, the homonuclear product
ion, which was shown to have a Pt2C+·NH3 carbide structure,
cannot activate NH3 and only adds a second substrate
molecule in a slow association reaction.[5, 6] In marked
contrast, [Pt,Au,C,H3,N]+ dehydrogenates another NH3 molecule [Eq. 6 with M = Au (f = 0.03)].[6, 22]
½Pt; M; C; H3 ; Nþ þ NH3 ! ½Pt; M; C; H4 ; N2 þ þ H2
www.angewandte.org
ð6Þ
Angew. Chem. Int. Ed. 2004, 43, 121 –124
Angewandte
Chemie
For both M = Cu and M = Ag, Equation (6) occurs as well,
in the former case with rather high (f = 0.23) and in the latter
with low (f 0.01) efficiency. The fact that the primary
product [Pt,M,C,H3,N]+ is still capable of NH3 activation
strongly suggests that also the first dehydrogenation step
involves activation of NH3. This issue was further probed by
isotopic labeling.
For the reaction of PtMCD2+ with NH3 three different
product channels with distinct H/D distributions can in
principle be expected [Eqs. (5a)–(5c)].
PtMCDþ2 þ NH3 ! ½Pt; M; C; H3 ; Nþ þ D2
ð5aÞ
! ½Pt; M; C; H2 ; D; Nþ þ HD
ð5bÞ
! ½Pt; M; C; H; D2 ; Nþ þ H2
ð5cÞ
In the case of the homonuclear Pt2CD2+ carbene cluster,
exclusively D2 is lost [Eq. (5a) with M = Pt] which is only
compatible with a Pt2C+·NH3 structure of the product ion.[5, 6]
In contrast, the reaction of PtAuCD2+ gives rise to all three
possible products in a ratio of 70(
15):100:80 (
15)
[Eqs. (5a)–(5c)].[6] Again, quite similar behavior is found for
M = Cu and M = Ag. For M = Cu the ratio between the
Reactions (5a)–(5c) amounts to 15(
10):100:25(
20)
whereas it corresponds to 45:100:30(
5) for M = Ag. The
rather high ranges of uncertainty assigned particularly to the
product distribution for M = Cu mainly result from the
complicating effect of D/H exchange reactions for the
deuterium containing product ions which requires a determination of the branching ratios by extrapolation to zero
reaction time.
The experimentally observed product ratios can be
compared with two extreme cases. First, neglecting kinetic
isotope effects, statistical H/D equilibration should yield a
17:100:50 distribution. Similarly, experiment finds larger
fractions of H2 [Eq. (5c)] than of D2 [Eq. (5a)] elimination
for M = Cu and Ag. In contrast, both product channels almost
have equal probability for PtAuCD2+ which possibly indicates
the presence of a second pathway leading to direct D2/NH3
exchange without NH3 activation in this case.[6] Both
PtCuCD2+ and PtAgCD2+ give a degree of HD loss that is
higher than that expected from statistical scrambling. This
behavior resembles the reactivity of mononuclear PtCD2+
that exclusively eliminates HD upon reaction with NH3.[3] A
quantum-chemical and experimental characterization of this
process established an aminocarbene structure PtC(D)NH2+
for the product ion.[4] Given the similarity between the
reactivities of PtCD2+ and PtMCD2+ (M = Cu and Ag)
towards NH3, analogous structures of the product ions
appear likely. The partial H/D scrambling found for the
dinuclear ions could point to a secondary interaction between
the M atom and the attacking NH3 molecule in the course of
the reaction. In the case of PtAuCD2+, the enhanced
scrambling observed is in line with a particularly favorable
MH binding for M = Au, which results from both the
relatively high electronegativity of gold and the large spatial
extension of its 6s orbital.
Further evidence for the formation of aminocarbene
complexes is provided by the subsequent reactions of the
Angew. Chem. Int. Ed. 2004, 43, 121 –124
labeled ions. In the case of mononuclear PtC(D)NH2+,
exclusively HD is lost upon reaction with NH3. This result is
consistent with the theoretically predicted bisaminocarbene
structure (PtC(NH2)2+) for the secondary product.[3, 4]
Whereas H/D exchange reactions presumably compete with
the inefficient subsequent reactions of the [Pt,Ag,C,Hx,Dy,N]+
species (x + y = 3) and thus prevent an assessment of the label
distribution in the products, the situation is more favorable
for their faster reacting copper counterparts. Thus the
products [Pt,Cu,H4,N2]+ and [Pt,Cu,H3,D,N2]+ are found in a
ratio of 100:20(
10). This distribution fully agrees with the
transformation of the aminocarbene structures into bisaminocarbene structures on substitution of the carbene H or D
atom by an NH2 group [Eqs. (6a)–(6c)].
PtCuCðHÞNHþ2 þ NH3 ! PtCuCðNH2 Þþ2 þ H2
ð6aÞ
PtCuCðDÞNHþ2 þ NH3 ! PtCuCðNH2 Þþ2 þ HD
ð6bÞ
PtCuCðDÞNHDþ þ NH3 ! PtCuCðNH2 ÞðNHDÞþ þ HD
ð6cÞ
In conclusion, we find that both PtCu+ and PtAg+ display
reactivities much resembling that of PtAu+. Particularly, all
three dinuclear platinum–coinage-metal cluster ions PtM+
mediate coupling of CH4 and NH3, whereas neither Pt2+ nor
M2+ do so.[23] Given the considerable differences between Cu,
Ag, and Au in terms of ionization energies, atomic radii, and
orbital energies, the parallel reactivities of the PtM+ cluster
ions are quite surprising. Possibly, the specific electronic
structure of the PtM+ ions is essential for CN bond
formation between CH4 and NH3. An experimental verification of this hypothesis requires the investigation of further
isoelectronic species, such as Pt20 and PtIr . Moreover, though
extremely demanding, computational methods could be helpful in answering this question as well.
Experimental Section
The dinuclear cluster ions Cu2+, Ag2+, PtCu+, and PtAg+ were
generated by laser desorption/ionization from the corresponding solid
metals (Cu, Ag, PtCu (2:1 weight ratio), and PtAg (2:1), respectively)
followed by supersonic expansion.[24, 25] After transfer of the ions into
the cell of a CMS 47X Fourier-transform ion-cyclotron-resonance
mass spectrometer,[26] the most abundant dimer ion (63Cu2+,
107
Ag109Ag+, 63Cu196Pt+/65Cu194Pt+, and 107Ag196Pt+/109Ag194Pt+; the
latter two combinations of isotopomers are not resolved in the
broadband detection-mode) was selected by means of the FERETS
ion-ejection method.[27] After thermalization with pulsed-in Ar buffer
gas, reactions with the leaked-in substrates were studied at variable
reaction times. Bimolecular rate constants k (with estimated errors of
30 % for CH4 and O2[28] and 50 % for NH3) were derived from
the decline of the reactant ion and the increase of the product ions on
the basis of the pseudo-first-order approximation. Reaction efficiencies f = k/kcap were calculated according to capture theory.[29] For the
investigation of the reactivity of the metal carbenes, these were
produced by pulsing-in CH4 to the mass-selected and thermalized
dimer ions. Otherwise, consecutive reactions were analyzed by
numerical routines (for details, see ref. [30, 31]).
Received: September 8, 2003 [Z52817]
Published Online: December 5, 2003
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
123
Communications
.
Keywords: CH activation · cluster compounds · copper · gold ·
platinum · silver
[1] D. Hasenberg, L. D. Schmidt, J. Catal. 1986, 97, 156.
[2] A. Bockholt, I. S. Harding, R. M. Nix, J. Chem. Soc. Faraday
Trans. 1997, 93, 3869.
[3] M. Aschi, M. BrInstrup, M. Diefenbach, J. N. Harvey, D.
SchrIder, H. Schwarz, Angew. Chem. 1998, 110, 858; Angew.
Chem. Int. Ed. 1998, 37, 829.
[4] M. Diefenbach, M. BrInstrup, M. Aschi, D. SchrIder, H.
Schwarz, J. Am. Chem. Soc. 1999, 121, 10 614.
[5] K. Koszinowski, D. SchrIder, H. Schwarz, Organometallics 2003,
22, 3819.
[6] K. Koszinowski, D. SchrIder, H. Schwarz, J. Am. Chem. Soc.
2003, 125, 3676.
[7] K. Koszinowski, D. SchrIder, H. Schwarz, ChemPhysChem,
2003, 4, 1233.
[8] U. Achatz, C. Berg, S. Joos, M. K. Beyer, G. Niedner-Schatteburg, V. E. Bondybey, Chem. Phys. Lett. 2000, 320, 53.
[9] K. Koszinowski, D. SchrIder, H. Schwarz, J. Phys. Chem. A 2003,
107, 4999.
[10] K. Koszinowski, D. SchrIder, H. Schwarz, Organometallics, in
press.
[11] H. Schwarz, Angew. Chem. 2003, 115, 4580; Angew. Chem. Int.
Ed. 2003, 42, 4442, and references therein.
[12] With regard to the reactivity of Ag2+, see also: P. Sharpe, C. J.
Cassady, Chem. Phys. Lett. 1992, 191, 111.
[13] K. K. Irikura, J. L. Beauchamp, J. Phys. Chem. 1991, 95, 8344.
[14] A. F. Holleman, E. Wiberg, Lehrbuch der Anorganischen
Chemie, 101st ed., Walter de Gruyter, Berlin, 1995.
[15] M. P. Irion, A. Selinger, Chem. Phys. Lett. 1989, 158, 145.
[16] http://webbook.nist.gov/.
[17] R. Liyanage, M. L. Styles, R. A. J. O'Hair, P. B. Armentrout, Int.
J. Mass Spectrom. 2003, 227, 47.
124
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[18] D. SchrIder, H. Schwarz, J. HrušMk, P. PyykkI, Inorg. Chem.
1998, 37, 624.
[19] D. Walter, P. B. Armentrout, J. Am. Chem. Soc. 1998, 120, 3176.
[20] H. El Aribi, C. F. Rodriquez, T. Shoeib, Y. Ling, A. C. Hopkinson, K. W. M. Siu, J. Phys. Chem. A 2002, 106, 8798.
[21] M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr., D. J. Frurip,
R. A. McDonald, A. N. Syverud, J. Phys. Chem. Ref. Data 1985,
14, Suppl. 1.
[22] In Reaction 6 with M = Au, an additional elimination of AuH
yielding [Pt,C,H5,N2]+ takes place as a competing process (f =
0.03).[6,10] Apparently, this reaction is facilitated by the relatively
strong binding in AuH, D0(Au-H) = 300 kJ mol1 (G. Herzberg,
Molecular Spectra and Structure, Vol. I, reprint ed., Krieger,
Malabar, 1989).
[23] The homonuclear coinage-metal dimers fail to activate CH4.
Au2CH2+ generated in other ways cannot be coupled with NH3
either.[10] The analogous reactivities of Cu2CH2+ and Ag2CH2+
could not be probed because there are no practical gas-phase
syntheses for these ions (e.g., reaction of M2+ with CH3X, X = Cl,
Br, and I, does not afford the desired result).
[24] M. Engeser, T. Weiske, D. SchrIder, H. Schwarz, J. Phys. Chem.
A. 2003, 107, 2855.
[25] C. Berg, T. Schindler, M. Kantlehner, G. Niedner-Schatteburg,
V. E. Bondybey, Chem. Phys. 2000, 262, 143.
[26] K. Eller, W. Zummack, H. Schwarz, J. Am. Chem. Soc. 1990, 112,
621.
[27] R. A. Forbes, F. H. Laukien, J. Wronka, Int. J. Mass Spectrom.
Ion Processes 1988, 83, 23.
[28] D. SchrIder, H. Schwarz, D. E. Clemmer, Y.-M. Chen, P. B.
Armentrout, V. I. Baranov, D. K. BIhme, Int. J. Mass Spectrom.
Ion Processes 1997, 161, 175.
[29] T. J. Su, J. Chem. Phys. 1988, 88, 4102; T. J. Su, J. Chem. Phys.
1988, 89, 5355.
[30] U. Mazurek, H. Schwarz, ICR Kinetics, Technische UniversitOt
Berlin, 1998.
[31] U. Mazurek, Dissertation, TU Berlin, D83, 2002.
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