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L. Xu et al. / Journal of Organometallic Chemistry 693 (2008) 3000–3006
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bufferic characteristics, defined as ionic liquid buffers (IL-buffers)
[37]. They are miscible with non-aqueous polar solvents such as
methanol, DMF, and dichloromethane and also with the ILs like
[BMIM][PF6] and [BMIM][BF4]. Preliminary results indicated that
the IL-buffers were able to control the formations of ruthenium
complexes and thus vary the catalytic performance of hydrogena-
tion in DMF and ILs. The present study extends the facile synthesis
of a series of IL-buffers and demonstrates their acid/base controlla-
bility in connection to the formations of rutheniumhydride com-
plexes in non-aqueous media. We choose the selective
hydrogenation of olefins and trans-cinnamaldehyde (CAL), as a
probe reaction for the purpose of elucidating the relationship be-
tween the ruthenium species and their catalytic performances in
IL-buffered non-aqueous solvents by the means of 1H and 31P
NMR characterizations.
after 1 h. The mixture was soon transferred to NMR tube in Ar
atmosphere using an inner capillary filled with D2O or CDCl3.
2.5. Catalytic hydrogenation
The reaction was carried out in a stainless steel autoclave of
60 mL capacity. Typically, calculated amounts of substrate, ruthe-
nium complex precursor, PPh3, solvent and IL-buffer were intro-
duced to the autoclave and then flushed with hydrogen (99.999%
purity) consecutively for three times. The autoclave was then filled
with hydrogen to the desired pressure. The reaction mixture was
stirred at 800 rpm and required temperature. After the reaction,
the autoclave was put in an ice-water bath and released the pres-
sure carefully. The organic phase were analyzed using a capillary
column (KB-Wax, 60 m  0.32 mm  0.32
trans-cinnamaldehyde and a capillary column (SE-30, 30 m Â
m) for the products of other olefins with flame
lm) for the products of
0.32 mm  0.25
l
2. Experimental
ionic detector (FID) and quantitated by the corrected normaliza-
tion analyzing.
2.1. Materials
3. Results and discussion
trans-Cinnamaldehyde and styrene (Sigma–Aldrich) were
passed through a silica column before use. Cyclohexene (99%, Alfa
Aesar) and 1-hexene (98%, Fluka) were used as received without
further purification. DMF was purified by drying overnight over
KOH pellets and then distilled under reduced pressure. The com-
plexes [RuCl2(PPh3)3], [RuHCl(PPh3)3], [RuH2(PPh3)4] and
[RuH4(PPh3)3] were synthesized by the known procedure and iden-
tified by their NMR spectra [38–40].
3.1. Synthesis and characterization of IL-buffers
We noted that the titration profiles of 1-alkyl-3-methylimida-
zolium hydroxides ([RMIM]OH) with organic or inorganic binary/
polybasic acids such as phthalic acid (H2P), tartaric acid (H2T)
and phosphoric acid (H3PO4) in organic solvents expressed a buf-
fering-like region and the region is similar to that of conventional
acid/base bufferic counterparts (Fig. 1). Thus, we considered that it
was possible to synthesize a kind of buffer-like ILs by neutraliza-
tion of aqueous-solutions of [RMIM]OH with aqueous solutions
of acids in a calculated molar ratio as illustrated in Fig. 2. According
to the literature methods with slight modification, the [RMIM]OH
moieties were produced by anion exchange of the corresponding
imidazolium chlorides, which were obtained by the reaction of
methylimidazole with an equivalent of the appropriate alkyl chlo-
rides [41,42]. The final yields of the target compounds ranged from
91% to 93%. The samples were characterized using 1H NMR and
electrospray ionization (ESI) mass spectrometry. In addition, we
have measured the differential scanning calorimetry (DSC), ther-
2.2. Characterizations
Thermal gravimetric analysis (TGA) was performed with Simul-
taneous Thermal Analysis-STA 409EP. The water contents in the ILs
were detected by Karl–Fischer titration (Metrohm LTD., Model 787
KF Titrino). The NMR measurements of reaction mixtures were
performed on a Bruker AV 400 instrument using an inner capillary
filled with D2O or CDCl3 for 1H NMR and 85% H3PO4 for 31P NMR.
The IR measurement of sample was performed on a Nicolet Adva-
tar 360 instrument, analyzed by OMNIC version 4.1.
2.3. Synthesis of IL-buffers
Aqueous solution of [RMIM]OH was prepared by passing the
corresponding imidazolium halide ([RMIM]X) through a column
filled in anion-exchange resin, as described in the literature
[37,41]. The aqueous [RMIM]OH was then neutralized with serials
of acids (phosphoric acid or phthalic acid) in a beaker and the pH of
the solution was adjusted to the desired value. The solution was
evaporated at 50 °C under reduced pressure to give a viscous li-
quid, which was then vacuum-dried at 50 °C for 18 h to afford
the IL-buffer product. According to the titration profiles of [BMI-
M]OH with acids such as phosphoric acid and phthalic acid in or-
ganic solvents, we synthesis IL-buffers by reaction of [BMIM]OH
base moieties with acids at different log10([Base]/[Acid]) as de-
scribed above.
14
12
10
a
b
8
6
4
2
0
c
d
2.4. Samples prepared for NMR measurements
2
4
6
8
10
12
In a typical measurement, calculated amounts of ruthenium
complex precursor, PPh3, solvent and IL-buffer were introduced
to a stainless steel autoclave of 60 mL and then flushed with hydro-
gen (99.999% purity) consecutively three times. The autoclave was
then filled with hydrogen to the desired pressure. The mixture
was stirred at 800 rpm and required temperature. The autoclave
was put in an ice-water bath and released the pressure carefully
Volumn of [BMIM]OH / mL
Fig. 1. Titration curve for a serials of 0.1 mol LÀ1 acids vs. 0.1 mol LÀ1 [BMIM]OH in
MeOH. (a) Phosphoric acid; (b) succinic acid; (c) malic acid; (d) tartaric acid. The
electrode is standardized with two aqueous primary standard buffer solutions.
Because of the unknown liquid junction potential, measurements of pH in non-
aqueous solvents are referred to as ‘‘apparent pH”.
Xu, Li
