Structures of the Dehydrogenation Products of

Methane Activation by 5d Transition Metal Cations

V. J. F. Lapoutre,† B. Redlich,‡ A. F. G. van der Meer,‡ J. Oomens,‡,¶ J. M. Bakker,‡

A. Sweeney,§ A. Mookherjee,§ and P. B. Armentrout§

†FOM Institute for Plasma Physics Rijnhuizen, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands

‡Radboud University Nijmegen, Institute for Molecules and Materials, FELIX Facility, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands,

¶Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands, § Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, USA

Or How I Spent My Summer Vacation

•Methane is a key fossil fuel that is presently underutilized because it is difficult to transport. This problem could be solved by conversion of methane to other chemicals.

•As originally demonstrated by Irikura and Beauchamp (J. Am. Chem. Soc. 1989, 111, 75–85; 1991, 113, 2769–2770), several third-row metal cations readily activate methane by dehydrogenation.

•However, the structures of the [M,C,2H]+ products have yet to be experimentally examined, although theory suggests several competing structures:

metal carbene distorted carbene hydrido metal (agostic interaction) carbynea

•In order to successfully study such four atom species, which therefore have a low density of states, high laser power is required for IR multiple photon dissociation (IRMPD). Such power is provided by the intracavity operation of FELICE.

a First calculated for WCH2+ by Simon, Lemaire, Boissel,

Maître, J. Chem. Phys. 2001, 115, 2510–2518.

Motivation

Free-Electron Laser for IntraCavity Experiments - FELICE

www.rijnhuizen.nl/en/research/guthz/felice; Bakker et al. J. Chem Phys. 132, 074305 (2010)

End/Angle mirror

• Higher fluences of IR radiation • Expands spectral region to lower

frequencies

Molecular Beam Experiment

Bakker et al. J. Chem Phys. 132, 074305 (2010)

•Samples of Ta, W, Ir, or Pt (rods) were ablated with a frequency doubled YAG laser (532 nm) to create gas phase ions and neutrals.

•Gas phase transition metals were carried by He and interacted with a pulsed flow of methane gas to form metal complexes. For all four metals, the [M,C,2H]+ species are formed in abundance.

•Ions were then irradiated in FELICE where they were dissociated with multiple photons over a range of about 400 – 3500 cm-1.

•Fragments and parent ions are pulse extracted ~10 ms after interaction and mass analyzed by a reflectron time-of-flight mass spectrometry (ReTOF MS) and detected.

•Dissociation of complexes at various wavelengths corresponds to the resonant absorption of photons at that specific vibrational mode.

•The [M,C,2H]+ species of M+ = Ta+, W+, and Pt+ were all observed to dissociate by H atom loss, whereas for M+ = Ir+, loss of H2 predominates. These observations are consistent with the known thermochemistry in all cases.

Experimental Details

Density Functional Theory•Calculations of [M,C,2H]+ structures and vibrational frequencies were conducted at the B3LYP level using the def2-TZVPPD basis set.

•The def2-TZVPPD basis set is a balanced triple-zeta basis set with a small-core effective core potential for the heavier elements.

•Harmonic frequencies are calculated and scaled by a global scaling factor of 0.939 to compensate for anharmonicity and experimental redshifting caused by the multiple photon nature of IRMPD.

•The global scaling factor is determined by fitting the calculated spectrum of the lowest energy [Pt,C,2H]+ structure to the experimental spectrum.

•All DFT calculations were performed using the Gaussian 03 suite of programs.

•To verify that this level of theory is adequate, several additional calculations were carried out on the [Pt,C,2H]+ system by Prof. C. van Wüllen. An all-electron quasi-relativistic zero-order regular approximation (ZORA) calculation was done along with scalar-relativistic calculations, and two-component calculations that include spin-orbit coupling. None of these alternative approaches led to appreciable differences in the calculated spectra.

Inte

nsity

0.00

0.20

0.40

0.60

PtCH2+

Inte

nsity

0

20

40

60

Inte

nsity

0

20

40

60

Frequency (cm-1)500 1000 1500 2000 2500 3000 3500

Inte

nsity

0

100

200

300

400

-34 kJ/mol

33 kJ/mol

65 kJ/mol

PtCH2+(2A1)

PtCH2+(2A2)

HPtCH+(2A'')

IRMPD spectrum for [Pt,C,2H]+ (top) and calculated spectra for the species shown. Energies relative to Pt+ + CH4 are indicated.

Inte

nsity

0.00

0.10

0.20

0.30

0.40

0.50TaCH2

+In

tens

ity

0

50

100

150

200

250 TaCH2+(3A'')

Inte

nsity

0

50

100

150

200

Frequency (cm-1)500 1000 1500 2000 2500 3000 3500

Inte

nsity

0

100

200

300

400

-4 kJ/mol

32 kJ/mol

54 kJ/mol

TaCH2+(1A')

HTaCH+(1A')

IRMPD spectrum for [Ta,C,2H]+ (top) and calculated spectra for the species shown. Energies relative to Ta+ + CH4 are indicated.

Frequency (cm-1)

Inte

nsity

0.0

0.1

0.2

0.3

0.4

0.5In

tens

ity

0

50

100

150

200

Inte

nsity

0

50

100

150

-13 kJ/mol

4 kJ/mol

Frequency (cm-1)500 1000 1500 2000 2500 3000 3500

Inte

nsity

0

50

100

150

200

WCH2+

WCH2+(4A'')

HWCH+(2A')

37 kJ/molWCH2+(4B2)

IRMPD spectrum for [W,C,2H]+ (top) and calculated spectra for the species shown. Energies relative to W+ + CH4 are indicated.

Inte

nsity

0.00

0.04

0.08

0.12

0.16In

tens

ity

0

50

100

150

200

Inte

nsity

0

20

40

60

80

100

Frequency (cm-1)500 1000 1500 2000 2500 3000 3500

Inte

nsity

0

40

80

120

-80 kJ/mol

-56 kJ/mol

17 kJ/mol

IrCH2+

HIrCH+(1A')

IrCH2+(3A2)

HIrCH+(3A')

x5

IRMPD spectrum for [Ir,C,2H]+ (top) and calculated spectra for the species shown. Energies relative to Ir+ + CH4 are indicated.

•PtCH2+ is almost certainly the metal carbene, which can

be formed exothermically in the reaction of Pt+ with CH4. The peak at 1980 cm-1 us attributed to an overtone.

•TaCH2+ and WCH2

+ are almost certainly the agostically distorted metal carbenes, which can both be formed exothermically in the reaction of Ta+ and W+ with CH4.

•Unexpectedly, IrCH2+ shows evidence of both a hydrido

carbyne and a carbene structure, both of which can be formed exothermically in the reaction of Ir+ with CH4.

Conclusions

Reaction Coordinate

Ene

rgy

(eV

)

-3

-2

-1

0

1

2

Ir+(5F) + CH4

Ir+(CH4)

TS1

H-Ir+-CH3

TS1

HIrHCH2+

TS6

(H2)HIrCH+

HIrCH+

+ H2

Ir+(3F)

Ir+(1G)

TS3TS5

H3IrCH+ TS6

TS5

TS4

HIrCH+

+ H2

IrCH2+

+ H2

singlet

triplet

quintet

Potential energy surface for formation of HIrCH+ and IrCH2

+

•HIrCH+ should be formed easily from Ir+ + CH4 but requires coupling of the quintet and singlet surfaces.

•Previous theoretical studies of the reaction of Ir+ + CH4 did not consider the formation of the hydrido carbyne, but showed carbene formation was facile, requiring quintet/triplet coupling. Perry, Ohanessian, Goddard, Organomet. 1994, 13, 1870. Musaev, Morokuma, Isr. J. Chem. 1993, 33, 307. Li, Zhang, Armentrout, Int. J. Mass Spectrom. 2006, 255-256, 279.

This work is financially supported by the NSF (CHE-1049580 and PIRE-0730072), the research program of the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’, and the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek” (NOW). The authors are also thankful for the helpful staff at the FOM institute, especially those involved in the operation and maintenance of the FELICE beam line.

Acknowledgements