1068
D. Hess et al. · Iridium-catalyzed Hydroformylation
investigations at the structural level for the phosphine- rhodium catalysts retards the reaction rate [14]. Com-
modified rhodium hydridocarbonyl complexes. In ref- plexes with a single phosphine ligand thus have a lower
erence [2], Claver and van Leeuwen describe Rh sys- hydroformylation activity than those with two phos-
tems containing triphenylphosphine as a ligand. At phine ligands. The optimum reaction conditions for
– 100 ◦C, 5 – 20 bar, [Rh] = 1 mM, and PPh3/Rh = 5, such systems lie in the region of a slight excess of hy-
the only absorption bands found by in-situ transmis- drogen. In the case of a triphenylphosphine-modified
sion spectroscopy were those corresponding to the iridium catalyst, a 2 : 1 excess of CO has a positive ef-
RhH(PPh3)2(CO)2 complexes. For the same ligand-to- fect on the activity of the catalyst system, as was shown
metal ratios as in these IR studies and pressures of up to for the first time in reference [6]. Our investigations
4 bar, but significantly higher ligand concentrations of now reveal that in this case, as also for the analogous
approx. 10 mM, Oswald et al. [13] found in NMR stud- rhodium complex, the complex with a single PPh3 lig-
ies that the predominant complex species is then the and is favored as the CO partial pressure increases.
tris-triphenylphosphine complex. Under standard con- From this we deduce that, in contrast to the rhodium
ditions, therefore, the complex RhH(PPh3)2(CO)2 is system, the complex with a single phosphine ligand is
the predominant species. The dependence on CO par- the more active. We have thus supplemented the empir-
tial pressure generally shows a negative reaction order, ical findings of reference [6] by a structurally substan-
indicating that the formation of a RhH(PPh3)(CO)3 tiated hypothesis for clarifying activity enhancement
species reduces the catalytic activity.
in iridium-catalyzed hydroformylation with the use of
According to the investigations in reference [6] and excess CO in the activation stages. A detailed quantita-
our own experience, it is necessary in any process mod- tive study with systematic investigation of the kinetics
ification with triphenylphosphine to work with excess of iridium-catalyzed hydroformylation would now be
CO in order to obtain good activity of the iridium com- of interest. This would entail, in addition to operando
plexes. The equilibrium is also shifted by an increas- spectroscopic studies, a mathematical modeling in the
ing or decreasing triphenylphosphine concentration for form of kinetic differential equations and a support of
a given metal concentration relative to the quantity of these studies by quantum-chemical calculations for the
CO dissolved; it follows that for a reduction of metal catalysis cycle. We will be reporting on such studies in
concentration, an increase in activity can be expected the future.
if the more active complex HIr(PPh3)(CO)3 is formed.
This could be the subject of a systematic study of the Experimental Section
kinetics of the iridium-triphenylphosphine system, and
goes beyond the scope of the present study.
All oxygen- and moisture-sensitive reagents were handled
under argon atmosphere. Syringes and cannulas were purged
with nitrogen before use. The autoclave was subjected to
a leak test using a CO warning device and purging several
times with argon.
Conclusion and Outlook
Autoclave experiments were performed in Parr 5500 100-
mL microreactors. The autoclave can be operated up to
150 ◦C and 60 bar. In the sampling circuit to the infrared
transmission cell and back to the autoclave a micro annular
gear pump mzr-7255 (HNP Mikrosysteme GmbH, Parchim)
was used. This pump has 48 µL displacement volume. The
maximum volumetric flow rate is 17.28 L h−1 at a speed of
6000 min−1. The pump is controlled by the Faulhaber Mo-
tion Manger software.
The infrared spectrometer is a Tensor 27 (Bruker Optik
GmbH) equipped with a DigiTect-DTGS detector. The spec-
trometer is operated with the OPUS 6.5 software by Bruker.
The infrared transmission liquid cell which can be heated
up to 200 ◦C was manufacured by Bruker. The windows are
made of silicon. The cell thickness is 400 µm. The cell win-
In our hydroformylation studies with iridium cata-
lysts following the findings of Beller et al. [6] we have
contributed toward clarifying the chemistry of the reac-
tion by the use of operando infrared spectroscopy com-
bined with quantum-chemically calculated vibrational
spectra. We have succeeded in showing that hydro-
formylation with iridium and triphenylphosphine dif-
fers significantly from that of the rhodium system. For
rhodium-catalyzed hydroformylation of 1-olefins, it is
known that complexes with two phosphine ligands pro-
duce higher yields of terminal aldehydes. Increasing
the partial pressure of CO shifts the equilibrium toward
complexes with one phosphine ligand and so decreases
the n-selectivity. Further, it is known that an increase
of the CO partial pressures in phosphine-modified dows are wedged to prevent fringes.
Brought to you by | Rutgers University
Authenticated
Download Date | 9/28/15 10:02 AM