A R T I C L E S
Nagaraja et al.
at 400 MHz using the inversion recovery method.12 High-pressure NMR
tubes fitted with Swage-lock fittings were procured from Wilmad Glass.
Methane gas (99.95%) was obtained from Bhoruka Gases Limited,
Bangalore, India, and 13CH4 (99.8 atom %) was from Sigma-Aldrich.
The silanes Me2EtSiH, Et3SiH, and Ph3SiH were purchased from Sigma-
Aldrich and used as received. The borane-Lewis base adducts H3B‚
PPh3 and H3B‚PMe3 were prepared by passing B2H6 gas through toluene
solutions containing the respective phosphines.13 The trans-[Ru(H)-
(η2-H2)(dppe)2][OTf]14 and [Ru(P(OH)3)(dppe)2][OTf]2,10 1, complexes
were prepared using literature procedures. Note: Wherever the term
head gas has been used, it means that the gas was introduced on top of
the solution and the solution was either shaken or stirred.
Periana et al.8 observed catalytic H/D exchange with methane
in the presence of a cyclometalated Pt(II)-substituted bipyridine
complex. However, their theoretical studies suggest that the
C-H cleavage barrier was high. Density functional study of
the C-H bond activation in methane using model complexes
of the type [M(CH3)(HNdCHCHdNH)] (M ) Pd+, Pt+, Rh+,
Ir+, Rh, Ir) by Swang et al.9 suggest that oxidative addition is
favored except for Pd+ and Rh+. Despite the metal center being
electrophilic in the case of certain third-row metal complexes,
the oxidative addition and heterolytic cleavage pathways are
close in energy, depending on the ancillary ligand environment,
the charge, and the reaction conditions.
Reaction of [Ru(P(OH)3)(dppe)2][OTf]2, 1, with H2 (Head Gas)
at 298 K. Complex 1 (0.020 g, 0.015 mmol) was dissolved in 0.6 mL
of Ar-saturated CD2Cl2 in a septum-capped 5 mm NMR tube. Next,
H2 (1 atm) was introduced as a head gas for a period of 5 min. The 1H
and the 31P NMR spectra of the sample recorded immediately thereafter
at 298 K showed the formation of a mixture of trans-[Ru(H)(P(OH)3)-
(dppe)2][OTf], 2 (70%), and trans-[Ru(H)(η2-H2)(dppe)2][OTf] (30%)
complexes. 1H NMR of 2 (298 K, CD2Cl2): δ -8.90 (d qnt, 1H, Ru-
H, J(H, Ptrans) ) 107.0 Hz, J(H, Pcis) ) 20.0 Hz), 2.36 (m, 4H, CH2),
2.86 (m, 4H, CH2), 7.00-7.60 (m, 40H, P(C6H5)2). 31P{1H}NMR (CD2-
Cl2): δ 66.5 (d, 4P, PCH2CH2P, J(P, Pcis) ) 32.0 Hz), 137.3 (qnt, 1P,
P(OH)3). 1H NMR of trans-[Ru(H)(η2-H2)(dppe)2][OTf] (298 K, CD2-
Cl2): δ -10.13 (qnt, 1H, Ru-H, J(H, Pcis) ) 17.5 Hz), -4.78 (br s,
2H, Ru-H2), 2.17 (m, 8H, CH2), 6.78-7.79 (m, 40H, P(C6H5)2). 31P-
{1H} NMR (CD2Cl2): δ 68.7 (s, 4P, PCH2CH2P).
For the accomplishment of heterolysis of X-H (X ) H, Si,
B, C) bonds, the requirement of a “superelectrophilic” metal
center has been suggested as a necessity.1a,b To our knowledge,
there are no examples in the literature of a single system that
has the ability to bring about the heterolysis of all of these bonds.
With a view to realize such a system, we recently prepared and
characterized a stable, 16-electron, coordinatively unsaturated,
“superelectrophilic” ruthenium complex [Ru(P(OH)3)(dppe)2]-
[OTf]2, 1, that cleaves the H-H bond in hydrogen gas in a
heterolytic fashion.10 Herein, we report the results of our findings
on the heterolytic cleavage reactions of X-H (X ) H, Si, B,
C) bonds using complex 1. With a view to get an insight into
the factors that dictate the heterolytic cleavage pathway for these
bonds, we carried out density functional study of the activation
of X-H (X ) H, Si, B, C) bonds using a model complex [Ru-
(P(OH)3)(H2PCH2CH2PH2)2][Cl][OTf], 4. The results of these
studies are also presented.
Reaction of 1 with H2 (Head Gas) at 223 K - Observation of
trans-[Ru(η2-H2)(P(OH)3)(dppe)2][OTf]2, 3. Complex 1 (0.020 g,
0.015 mmol) was dissolved in 0.6 mL of Ar-saturated CD2Cl2 in a
septum-capped 5 mm NMR tube. The tube was cooled to 203 K. Next,
H2 gas (1 atm) was introduced as a head gas for a period of 5-10 min
at 203 K; immediately the sample was inserted into the NMR probe,
which was pre-cooled to 223 K. The 1H and 31P NMR spectra recorded
at 223 K showed the presence of the dihydrogen complex 3. 1H NMR
of 3 (223 K, CD2Cl2): δ -5.47 (br s, 2H, Ru-H2), 2.80 (m, 4H, CH2),
3.32 (m, 4H, CH2), 7.00-7.60 (m, 40H, P(C6H5)2). 31P{1H} NMR (CD2-
Cl2): δ 53.5 (d, 4P, PCH2CH2P, J(P, Pcis) ) 41.0 Hz), 113.4 (qnt, 1P,
P(OH)3). Variable-temperature 1H spin-lattice relaxation times at 400
MHz, T1 (ms) (temperature, K): 27.4 (203), 24.5 (213), 21.6 (223),
17.3 (233), 14.4 (243).
Experimental Section
General Procedures. All of the reactions were carried out under
an atmosphere of dry and oxygen-free N2 at room temperature using
standard Schlenk and inert atmosphere techniques unless otherwise
specified.11 The 1H, 31P, 13C, and 11B NMR spectral data were obtained
using an Avance Bruker 400 MHz instrument. The 31P NMR spectra
were recorded relative to 85% H3PO4 (aqueous solution) as an external
1
standard. Variable-temperature H T1 measurements were carried out
Reaction of 1 with HD (Head Gas) at 223 K - Preparation of
trans-[Ru(η2-HD)(P(OH)3)(dppe)2][OTf]2, 3-d. Complex 1 (0.020 g,
0.015 mmol) was dissolved in 0.6 mL of Ar-saturated CD2Cl2 in a
septum-capped 5 mm NMR tube. The tube was maintained at 203 K.
HD gas (generated from NaH and D2O) was introduced at 203 K as a
head gas for a period of 5-10 min. Immediately thereafter, the sample
was inserted into the NMR probe that was pre-cooled to 223 K. The
HD isotopomer trans-[Ru(η2-HD)(P(OH)3)(dppe)2][OTf]2, 3-d, formed
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1
was characterized by H NMR spectroscopy at 223 K. The residual
signal due to the H2 complex was nullified using an inversion recovery
pulse.
Reaction of 1 with Silanes (Me2EtSiH, Et3SiH, Ph3SiH). All of
these reactions were carried out in a similar manner. Complex 1 (0.020
g, 0.015 mmol) was dissolved in 0.6 mL of CD2Cl2 in a 5 mm NMR
tube. The resulting solution was degassed, and then 1 equiv of silane
(1.9 µL, 0.015 mmol (Me2EtSiH); 2.4 µL, 0.015 mmol (Et3SiH); 0.004
g, 0.015 mmol (Ph3SiH)) was added at 77 K, and the tube was sealed
under vacuum. The sample was slowly warmed to 298 K and then
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9
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