650 Organometallics, Vol. 22, No. 4, 2003
Ng et al.
commercial 5 mm Wilmad pressure-valved NMR tubes. Mass
spectrosopic analyses of the methane and dihydrogen isoto-
pomers resulting from H/D exchanges of CH4 and H2 with the
deuterated solvents, respectively, were carried out with a
Finnigan MAT 95S mass spectrometer.
Ca ta lytic H/D Exch a n ge Rea ction s betw een Meth a n e
a n d Deu ter a ted Solven ts w ith Tp Ru (P P h 3)(CH3CN)H. A
5 mm Wilmad pressure-valved NMR tube loaded with TpRu-
(PPh3)(CH3CN)H (∼8 mg) was evacuated and then filled with
nitrogen for 3 cycles. Deuterated solvent (0.3 mL) was added
to the tube under N2, and the tube was then pressurized with
methane (8 atm). It was heated at 100 °C for 14 h, after which
the methane isotopomers in the headspace of the tube were
analyzed by mass spectroscopy; the analyses of methane
isotopomers deserve special comment.
An a lysis of Meth a n e Isotop om er s w ith Ma ss Sp ec-
tr oscop y. Low-energy electron-impact ionization mass spectra
of the methane samples were recorded with a Finnigan-MAT
95S double-focusing magnetic sector mass spectrometer. The
instrumental conditions were as follows: electron energy 18.6
eV, ion source temperature 210 °C, mass resolution (M/∆M)
1000, sample pressure (1.0-1.5) × 10-6 mbar (as recorded by
the ion gauge underneath the ionization source), and scan rate
10 s/decade. The kinetic energy of the ionizing electron beam
was lowered, and the ionization source conditions were opti-
mized so that little fragmentation from the molecular ion of
methane was found. In our hands, the intensity of the m/z 15
fragment was found to be negligibly small at 1.5 ( 0.1% of
the base peak at m/z 16 (average ( standard deviation of 5
independent runs spreading over a period of 3 months). Hence,
the relative abundance of the CH4, CH3D, CH2D2, CHD3, and
CD4 isopotomers could simply be equated to the relative ion
intensities of the m/z 16, 17, 18, 19, and 20 peaks, respectively.
13C corrections had been applied to the % methane isotopomers
listed in Table 1.
Experimentally, it has been observed that the H/D
exchanges occur between H2 and the deuterated sol-
vents in the presence of TpRu(PPh3)(CH3CN)H. Calcu-
lations show that dihydrogen complexes TpRu(PH3)(η2-
H2)R (R ) C6H5, CH2CH2OC2H5, or CH3) (3, see Figures
1 and 3) are intermediates in the H/D exchanges among
different organic molecules. It is therefore expected that
H2 can also H/D exchange with deuterated organic
solvents. Scheme 6 provides an adequate description of
how the H/D exchanges occur.
All the results of our calculations suggest that C-H/
C-D activation in the H/D exchange reaction occurs
through an oxidative addition pathway. The species
resulting from the oxidative addition steps are the
relevant transition states in the ruthenium systems
studied. A four-center σ-bond metathesis mechanism
does not play a role in the activation process.
Con clu sion
As part of our research program on activation of H-A
(A ) H, Si, C, and B) bonds in isolobal ligands H-H,
H-SiR′3, H-CR′3, and H-BR′2‚L by the metal fragment
[Tp(PPh3)RuH], we have studied reactions of CH4 with
TpRu(PPh3)(CH3CN)H in various deuterated organic
solvents. Unlike the reactions of TpRu(PPh3)(CH3CN)H
with H2 and HSiR′3, which yield the isolable σ-complex-
es, TpRu(PPh3)(η2-H2)H and TpRu(PPh3)(η2-HSiR′3)H,
respectively, that of TpRu(PPh3)(CH3CN)H with CH4
did not yield any isolable methane σ-complex, nor was
it detectable by in-situ NMR study. It was, however,
found by NMR and mass spectrometry that TpRu-
(PPh3)(CH3CN)H was capable of catalyzing H/D ex-
change reactions between CH4 and the deuterated
solvents (R-D); the exchange reactions involved C-H
and C-D cleavage of CH4 and the solvent, respectively.
Density functional calculations favor the intermediacies
of methane and R-D σ-complexes, TpRu(PPh3)(η2-H-
CH3) and TpRu(PPh3)(η2-D-R)H, respectively, in our
proposed H/D exchange mechanism. The calculations
also show that the two σ-complexes proceed to form the
tautomeric η2-H2 intermediates TpRu(PPh3)(η2-H2)(CH3)
and TpRu(PPh3)(η2-HD)R via oxidative addition path-
ways, the seven-coordinate species TpRu(PPh3)(H)2-
(CH3) and TpRu(PPh3)(H)(D)R, respectively, being the
transition states, but not intermediates. This is in
accord with the fact that the Tp ligand generally
enforces an octahedral geometry about the metal center.
Ca t a lyt ic H/D E xch a n ge R ea ct ion s bet w een H2 a n d
Deu ter a ted Solven ts w ith Tp Ru (P P h 3)(CH3CN)H. The
exchange reactions were carried out by using the same
procedures as for the H/D exchange reactions between CH4
and deuterated solvents, except that H2 (10 atm) was used in
place of CH4.
Com p u ta tion a l Deta ils. Density functional theory calcu-
lations at the Becke3LYP (B3LYP) level26 have been used to
perform the geometry optimizations for all reactants, inter-
mediates, transition states, and products in the hydrogen
exchange reactions. Frequency calculations at the same level
of theory have also been performed to identify all stationary
points as minima (zero imaginary frequency) or transition
states (one imaginary frequency). The effective core potentials
(ECPs) of Hay and Wadt with double-ê valence basis set
(LanL2DZ)27 were used to describe Ru and P atoms. For all
the other atoms, the standard 6-31G basis set28 was used
except for the uncoordinated C and H atoms in the Tp ligand,
where a STO-3G basis set29 was used. Polarization functions
have been added for the C and H atoms (êp(H) ) 1.0 and êd(C)
) 0.8) which are directly coordinated to the metal center.
Exp er im en ta l Section
Ruthenium trichloride, RuCl3‚3H2O, pyrazole, and sodium
borohydride were obtained from Aldrich. Triphenylphosphine
was purchased from Merck and was recrystallized from
ethanol before use. The complex TpRu(PPh3)(CH3CN)H was
synthesized according to published procedures.18 Solvents were
distilled under a dry nitrogen atmosphere with appropriate
drying agents (solvent/drying agent): tetrahydrofuran/Na-
benzophenone, benzene/Na-benzophenone, dioxane/Na-ben-
zophenone, diethyl ether/CaH2, acetonitrile/CaH2, hexane/Na.
Methane (99.99%) was supplied by Hong Kong Special Gases.
Proton NMR spectra were obtained from a Bruker DPX 400
spectrometer. Chemical shifts were reported relative to re-
sidual protons of the deuterated solvents. 31P NMR spectra
were recorded on a Bruker DPX 400 spectrometer at 161.70
MHz; chemical shifts were externally referenced to 85% H3-
PO4 in D2O. High-pressure NMR studies were carried out in
The basis set described above is considered to be small and
could be unbalanced. Therefore, it is necessary to test the
accuracy by employing a much larger basis set. In the large
basis set, polarization functions were added for P (êd ) 0.340)
and Ru (êf(Ru) ) 1.235)30 based on Lanl2DZ. The 6-31G** basis
set was used for all other atoms. With this much larger basis
(26) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (b) Miehlich,
B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200.
(c) Lee, C.; Yang, W.; Parr, G. Phys. Rev. B 1988, 37, 785.
(27) (a) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 270. (b)
Wadt, W. R.; Hay, P. J . J . Chem. Phys. 1985, 82, 284. (c) Hay, P. J .;
Wadt, W. R. J . Chem. Phys. 1985, 82, 299.
(28) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28, 213.
(29) Hehre, W. J .; Stewart, R. F.; Pople, J . A. J . Chem. Phys. 1969,
51, 2657.