Full Paper
1
94.4 ppm (Ru/L3). The chemical shifts of the mononuclear
ed. Since we did not find strong differences in the donor be-
havior and steric properties compared to L2, the disparate hy-
droformylation activity found by Beller and co-workers induced
by different ligands may be attributable to a subtle interaction
between the imine nitrogen atom of the imidazolyl unit and
the ruthenium center. From NMR measurements with selected
systems evidence could be derived that in PC as solvent the
same ruthenium complexes are dominant. Noteworthily, com-
pared to the reactions in toluene a significant increase in cata-
lytic activity was found in PC, which remains to be clarified.
However, for a further study on the correlation between the ki-
netics and the detectable metal carbonyl complexes, as well as
for the design of new ligands, our results are an important
basis.
3
1
31
complexes in PC with d( P)=33.2 ([Ru(CO) L1]), d( P)=48.0
4
3
1
(
[Ru(CO) L2]), and d( P)=47.4 ppm ([Ru(CO) L3]) are compara-
4 4
ble to those measured in toluene. In addition, for specifically
1
3
prepared samples, we observed signals in the C NMR spectra
1
3
2
in the carbonyl region with d( C,Ru/L1)=207.4 (d, J =
1
2
CP
1
3
2
13
.7 Hz), d( C,Ru/L2)=206.0 (d, J =3.0 Hz), and d( C,Ru/L3)=
CP
2
06.4 ppm (d, J =2.9 Hz) that showed the expected doublet
CP
15
splitting. Likewise, N NMR chemical shifts are in accordance
with a mononuclear complex of the type [Ru(CO) L]. Respec-
tive NMR data are given in the Supporting Information SI-D.
The catalytic activity in PC as solvent (Table 3) was significantly
4
Table 3. Results from catalytic experiments by GC analysis. Reaction con-
À3
À1
À3
À1
ditions:
0
[Ru]=410 molL
,
[L]=4.410 molL
,
[alkene]=
À1
.9 molL , T=1008C, p(CO)=2.0 MPa, p(H )=4.0 MPa, solvent=PC.
2
System n-Aldehyde
%] (linearity)
Alkane [%] Alcohol [%] Reaction time [min]
Experimental Section
[
Materials
Ru/L1
Ru/L2
Ru/L3
Ru/–
42.6 (>99)
93.3 (>99)
69.1 (>99)
3.9 (>99)
0.5
0.8
1.1
0.5
<0.1
0.7
0.2
240
240
240
240
Dried toluene (>99%) was taken from the dispensing system Pure-
Solv MD7 from Innovative Technologies. Propylene carbonate (>
<0.1
9
9%) from Sigma-Aldrich was flushed with argon and dried over
molecular sieves. 3,3-Dimethyl-1-butene (>99%, GC) was pur-
chased from Sigma-Aldrich and distilled in an argon atmosphere
over sodium. The internal GC standard dodecane (>99%, Sigma
Aldrich) was stored over Sicapent (Merck) and distilled under
vacuum. The catalyst precursor [Ru (CO) ] (99%, Strem) was stored
[
20]
higher than that obtained in toluene.
Reaction rates are
lower for 3,3-dimethyl-1-butene compared with 1-octene,
3
12
[
20,67]
which was used as substrate in the aforementioned study.
under argon. Imidazolyl monophosphines were obtained from
[18–22]
The same relative trend in activity was found as observed for
toluene as solvent: the system Ru/L2 showed the highest ac-
tivity followed by Ru/L3 and then Ru/L1 (cf. Table 1).
Beller et al. at LIKAT and stored under argon.
The internal IR
standard diphenyl carbonate (99%, Aldrich) and triphenylphos-
phine (99%, Aldrich) were used as received. The following gases
were used in this study: synthesis gas (CO/H 1/1, from 99.997%
2
carbon monoxide and 99.999% hydrogen, Linde), carbon monox-
ide (99.997%, Linde), hydrogen (99.9993%, Linde), and argon
Conclusion
(
99.999%, Linde).
To find a rationalization for the outstanding catalytic per-
formance of monodentate phosphine ligands in various types
of ruthenium-catalyzed hydroformylation reactions discovered
by Beller et al., we conducted a spectroscopic/DFT study. We
found that mononuclear ruthenium(0) carbonyl complexes of
Devices and procedures
All preparations of solutions and transfers were carried out under
argon atmosphere by using standard Schlenk techniques. In situ IR
experiments for the study of homogeneously catalyzed hydrofor-
mylation were performed in a semibatch reactor system equipped
with a pressurizable and heatable transmission flow-through IR cell
the type [Ru(CO) L] (L=imidazole-substituted monophos-
4
phines: L1, L2, L3) are the dominant species during the hydro-
formylation of 3,3-dimethyl-1-butene. Additional ligand-modi-
fied complexes were not detected. At a ligand/ruthenium ratio
of nearly one, a mixture of [Ru(CO) L] and [Ru(CO) ] was
(
Dr. Bastian Feinwerktechnik GmbH, Wuppertal, Germany) and an
4
5
automated sampling device collecting GC samples (amplius GmbH,
Rostock-Warnemünde, Germany). The reactor consisted of
a 200 mL stainless steel autoclave with a gas-entrainment impeller
and an oil-bath thermostat (Premex Reactor AG, Leimen, Germany).
A high-pressure syringe pump (PHD Ultra 4400, Harvard Apparatus
GmbH, March-Hugstetten, Germany) with a 20 mL syringe made of
stainless steel was used for injection of the alkene. Circulation of
the liquid reaction solution through the IR cell was realized by
a micro gear pump (mzr-7255, HNP Mikrosysteme GmbH, Parchim,
Germany). A Bruker Tensor 27 FTIR spectrometer with a MCT-A de-
tector was used for the IR spectroscopic measurements. For the IR
cell we used CaF2 as window material and a 0.1 mm spacer. A
formed in dependence on the steric properties of the ligand.
For preformation of the mononuclear complexes it is decisive
to start the heating process in the presence of carbon monox-
ide. Heating under argon leads to PÀC cleavage of the phos-
phine and to the formation of inactive and fairly stable phos-
phido-bridged complexes! For catalytically active mononuclear
1
5
complexes data from N NMR, ESI-MS, and IR spectroscopy
proved that the unsubstituted nitrogen atom of the imidazolyl
moiety does not replace a carbonyl ligand. Remarkably, on
using ligands with tert-butyl and adamantyl groups instead of
cyclohexyl, no coordination of the ligands occurred. On utiliza-
[40]
scheme of the HP FTIR apparatus can be found elsewhere. The
tion of a pyrrole derivative a complex of the type [Ru(CO) L7]
micro gear pump was set to 2333 rpm (displacement volume:
4
was also formed, but the catalytic performance was deteriorat-
48 mL).
Chem. Eur. J. 2016, 22, 2746 – 2757
2754
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim