J. Zakzeski et al. / Applied Catalysis A: General 374 (2010) 201–212
209
reaction. Taken together, these results indicate that both high
hydroformylation and hydrogenation activity is possible using a
stoichiometric quantity of CO and 1-hexene, but the importance of
minimizing isomerization and 1-hexene conversion is critical for
the hydrogenation to readily occur. Since the 2- and 3-hexene
were not hydroformylated, if excessive isomerization occurs,
relatively high partial pressures of unconsumed CO are present in
the autoclave, and this unconsumed CO poisons the Ru catalyst
and inhibits the hydrogenation.
As indicated above, 2-propanol as a solvent greatly facilitated
the hydrogenation of the butanal into butanol. We therefore
extended the use of 2-propanol as a solvent for the conversion of 1-
hexene to heptanol. The results are depicted in Entry 9 of Table 4.
Both the hydroformylation and hydrogenation occurred more
readily in n-propanol than in toluene. Nearly 97% of the 1-hexene
was converted to n-heptanol and isoheptanol (n/i = 3.3), and the
remaining 3% of the 1-hexene isomerized to 2- and 3-hexene.
Considerably less time was required for similar conversion when
2-propanol was used as a solvent instead of toluene (Entry 8) (2 h
versus 4 h for hydroformylation 1-hexene and 1 h versus 5 h for
hydrogenation of heptanal). Moreover, an excess of CO did not
inhibit the hydrogenation considerably, as observed in the case
when toluene was used as the solvent as also observed for propene
conversion to butanol.
spectrum exhibited a peak at ꢂ5.3 ppm attributed to free PPh3
and a single broad peak at 42.1 ppm. Analysis by IR after exposure
to CO under ambient conditions, two symmetric peaks were
observed in the carbonyl region at 2055 and 1991 cmꢂ1 (see
Fig. 12, spectrum A). The 13C NMR using 13CO showed a doublet of
triplets (
= 197.0 ppm, J13C–31P = 14 Hz, J13C–13C = 17 Hz), and a doublet
of doublets ( = 187.6 ppm, J13C–31P = 14 Hz, J13C–31P = 131 Hz).
Similar analysis by 31P NMR resulted in two broad singlets
= 46.6 ppm) and ( = 46.6 ppm), a triplet ( = 28.1 ppm), and a
doublet of doublets = 21.0 ppm, J13C–31P = 14 Hz, J13C–
d = 198.9 ppm, J13C–31P = 17 Hz, J13C–13C = 8 Hz), a triplet
(
d
d
(
d
d
d
(
d
31P = 131 Hz). The observed features suggests that Ru(PPh3)3Cl2,
a 16eꢂ complex, readily binds to CO to form stable 18eꢂ
complexes, and that multiple isomers exist in agreement with
previous studies of the of the coordination of CO with
Ru(PPh3)3Cl2 [47]. The relative intensities of the CO stretching
vibrations indicate that the two CO molecules are cis-positioned,
and the positions of the two carbonyl bands agree well with those
reported for Ru(CO)2(PPh3)2Cl2 (18) in KBr (2050 and 1990 cmꢂ1
)
[48] and in other solvents [49–52]. The presence of multiple 13C
and 31P NMR peaks suggests that several species and their
isomers co-exist in the reaction solution. The triplet observed in
both the 13C and 31P NMR is likely due to cis,trans,cis-
Ru(CO)2(PPh3)2Cl2, in which the equivalent electronic environ-
ments of the two PPh3 and CO ligands results in only one 13C and
31P peak. The doublet of doublets is attributed to cis,cis,trans-
Ru(CO)2(PPh3)2Cl2 since, in this case, each CO ligand is both cis-
and trans-positioned from a PPh3 ligand. Finally, the doublet of
triplets in the 13C NMR spectrum is likely due to the
monocarbonyl species Ru(CO)(PPh3)3Cl2, which is expected to
result in a doublet of triplets in the 13C NMR and a doublet of
triplets and a doublet of doublets in the 31P NMR. The broad
singlets observed by 31P NMR are therefore likely unresolved
doublet of triplets and doublet of doublets rather than singlets. In
the presence of H2, the species present above readily exchange Cl
for H [51], and the dissociation of CO from the Ru complex
activates it for aldehyde hydrogenation.
Turnover frequencies for the hydroformylation of 1-hexene and
the hydrogenation of heptanal occurring in toluene and 2-propanol
were determined from the data presented in Entries 8 and 9 in
Table 4. In toluene the TOF for 1-hexene hydroformylation at 313 K
was 3.22 ꢁ 10ꢂ2 sꢂ1 in toluene and 6.44 ꢁ 10ꢂ2 sꢂ1 in 2-propanol,
whereas the TOF for heptanal hydrogenation at 398 K was
2.58 ꢁ 10ꢂ2 sꢂ1 in toluene and 1.29 ꢁ 10ꢂ1 sꢂ1 in 2-propanol.
3.3. Spectroscopic analysis of Rh and Ru complexes and reaction
solutions
Spectroscopic analyses using IR and 13C and 31P NMR
were carried out to identify the complexes used to carry out
olefin hydroformylation and aldehyde hydrogenation. Such
characterization was done after dissolution of the precursor
Rh and Ru complexes in toluene containing PPh3, and following
pressurization with 13CO. The species identified from these
analyses are related to a possible mechanism for the one-pot
synthesis of n-butanol and isobutanol from propene depicted in
Scheme 1.
31P NMR analysis of Rh(CO)2(acac) (1) (species numbers are
those appearing in Scheme 1) dissolved in toluene containing an
excess of PPh3, the precursor for the hydroformylation reaction,
revealed a peak associated with coordinated PPh3 at 56.7 ppm
(J103Rh–31P = 181 Hz), and the IR spectrum of this solution showed
a single peak in the carbonyl region at 1980 cmꢂ1. This sample was
sealed in an autoclave and exposed to 0.690 MPa CO and then
analyzed by IR spectroscopy under ambient conditions. Two peaks
were observed, one of medium intensity at 2041 and a second one
of stronger intensity at 1980 cmꢂ1 (see Fig. 12, spectrum B). A
sample of this solution was then placed in a high-pressure NMR
tube and pressurized with 0.345 MPa 13CO. The 13C NMR spectrum
Analysis of the reaction solution after carrying out a reaction
under standard conditions containing both the Ru and Rh
complexes was conducted in order to characterize the complexes
present in the solution. Analysis of the carbonyl region by
infrared spectroscopy resulted in peaks at 2055, 2041, 2017,
1991, and 1980 cmꢂ1 (see Fig. 12, spectrum D). In addition to the
features attributable to the Rh and Ru complexes, peaks
associated with the heptanal C–H and C55O stretches were
observed at 2816 and 1716 cmꢂ1
, respectively. After the
hydrogenation, the intensity of these peaks diminished consid-
erably, and a very broad peak associated with the O–H stretch of
heptanol appeared around 3361 cmꢂ1. A sample of the reaction
solution was placed in a high-pressure NMR tube, which was
pressurized with 13CO. Analysis by 13C NMR revealed a doublet at
199.2 ppm (J = 57 Hz), a small doublet at 194.1 ppm (J = 145 Hz),
and a broad singlet at 185.7 ppm in the carbonyl region in
addition to peaks associated with toluene, free PPh3, and
products. The sample was also analyzed by 31P NMR. The
resulting spectrum exhibited a singlet at 55.87 ppm, a doublet at
40.55 ppm (J = 6 Hz), a large singlet at 39.2 ppm, a doublet at
36.7 ppm (J = 127 Hz), a singlet at 29.6 ppm, a singlet at 16.3 ppm,
and a broad singlet at ꢂ5.3 ppm associated with free PPh3 [16].
The accumulated spectroscopic evidence suggests the presence of
the Rh complex [HRh(CO)2(PPh3)2] (6) since the IR peaks at 2055
and 2017 cmꢂ1, the 13C NMR doublet at 199.2 ppm (J = 57.3 Hz),
and the 31P doublet at 36.7 ppm (J = 127 Hz) agree closely
with observations for this complex reported in the literature
of this solution showed
a doublet (d = 189.45 ppm, J103Rh–
31CO = 76 Hz), and the only feature observed in the 31P NMR
was that for free PPh3. The spectroscopic characteristics of this
species strongly resemble previously reported Rh(CO)(PPh3)(a-
cac) (3) properties
(d13CO = 189.5 ppm, J103Rh–13C = 75 Hz,
v
CO = 1984 cmꢂ1) [44]. No evidence for Rh(CO)2(acac) (1) was
observed
(d13CO = 183.7 ppm) [45] v
CO = 2083.5, 2014.6 cmꢂ1
[46]).
Ru(PPh3)3Cl2 (16) dissolved in toluene containing an excess of
PPh3 was characterized in a similar manner. The 31P NMR
(
v
CO = 2053 and 2018 cmꢂ1 [53], 13C = 200.3 ppm, J13C–
103Rh = 63 Hz [46], J31P–103Rh = 138.7 Hz
31P = 37.3 ppm,