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A.B. Rivas et al. / Journal of Molecular Catalysis A: Chemical 300 (2009) 121–131
mononuclear species formed under catalytic conditions are most
probably responsible for the observed catalytic activity.
regioselectivities for P(OPh)3 and P(OMe)3, using a P/Rh = 6 ratio,
are 84 and 82%, respectively. Under the same conditions, the cat-
alytic systems based on PPh3 gave lower regioselectivities: 80%
(precursor 1 and 3) and 75% (precursors 2 and 4).
The activity of the different catalytic systems is strongly influ-
enced by the gem-dithiolato ligand although the effect both on
the selectivity and regioselectivity is rather small. In general,
the precursors containing acyclic gem-dithiolato ligands, [Rh2(-
S2CBn2)(cod)2] (3) and [Rh2(-S2CiPr2)(cod)2] (4), are more active
than the precursors with cyclic ligands [Rh2(-S2Cptn)(cod)2] (1)
and [Rh2(-S2Chxn)(cod)2] (2). The maximum activity for the four
catalytic systems based on P(OMe)3 was found at the ratio P/Rh = 4
(reaction time 2 h) and the observed activity order is 4 > 3 > 2 > 1.
Interestingly, the same activity order was found in the systems
based on PPh3 at the ratio P/Rh = 4 (reaction time 6 h) although
the activity gap between 2 and 1 is now considerable. However,
the activity of the systems based on P(OPh)3 decreases in the series
4 > 2 > 3 > 1 at P/Rh = 6 (reaction time 2 h).
The lower activity of the systems based on dinuclear precur-
sors having cyclic gem-dithiolato ligands could be related to the
higher stability of the dinuclear framework toward cleavage into
active mononuclear hydrido species under the hydroformylation
conditions. However, the activity is also influenced by the P/Rh
ratio and the type of P-donor ligand used as cocatalyst. In general,
the systems involving phosphite ligands, P(OR)3, are much more
active than the based on PPh3 irrespective of the catalytic precur-
sor. In fact, reaction times of at least 6 h were necessary in order
to get acceptable conversions when PPh3 was used as co-catalyst,
although excellent conversions were attained in only 2 h in the case
of P(OR)3 ligands. These results contrast with the behavior observed
for neutral and cationic dinuclear complexes with standard thiolato
ligands since the catalytic systems involving PPh3 as co-catalyst are
far more active than the based on P(OR)3 ligands [23,40,41]. How-
ever, the performance of the precursor [Rh2(-S(CH2)nS)(cod)2] is
very similar when using PPh3 or P(OR)3 as co-catalyst [42].
These results indicate that the activity of the catalytic systems is
not directly correlated with the electronic properties of the P-donor
ligands as the basicity of P(OMe)3 and PPh3 are comparable (pKa
of 2.6 and 2.73, respectively) being P(OPh)3 the less basic ligand
in this series (pKa of −2.0) [43]. The excellent activity of the sys-
tems based on P(OR)3 ligands is probably associated to the higher
concentration of the active mononuclear hydrido species which is
strongly supported by the HPNMR and HPIR spectroscopic studies
carried out on the systems 3/P(OMe)3 and 3/PPh3. On the other
hand, the formation of the active species from dinuclear precur-
sors requires the presence of a significant concentration of P-donor
ligand. Thus, the influence of the P/Rh ratio in the activity is most
likely related with the concentration of the active species under
hydroformylation conditions.
5. Conclusions
Dinuclear gem-dithiolato-bridged [Rh2(-S2CR2)(cod)2] com-
plexes in the presence of P-donor ligands are active catalyst
precursors for the hydroformylation of oct-1-ene under mild con-
ditions. The catalytic studies carried out with several P-donor
modifying ligands and four different catalytic precursors have
shown the systems involving phosphite ligands, P(OR)3, are much
more active than the based on PR3. However, the activity is strongly
dependent on the catalyst precursor, and consequently on the
structure of the dinuclear complexes, although it is also influ-
enced by the P/Rh ratio. In general, catalytic precursors containing
acyclic gem-dithiolato ligands are more active than the precursors
with cyclic ligands. The selectivitiy and regioselectivy are mainly
determined by the P-donor ligands, although there is not a clear
correlation between both parameters and the electronic and steric
properties of the co-catalyst. Spectroscopic studies under pres-
sure (HPNMR and HPIR) showed that hydrido mononuclear species
formed under catalytic conditions are presumably responsible for
the observed catalytic activity.
Acknowledgments
The financial support from Ministerio de Educación y Ciencia
(MEC/FEDER) Project CTQ2006-03973/BQU is gratefully acknowl-
edged. A.B.R. thanks the Programa Iberoamericano de Ciencia y Tec-
nología para el Desarrollo (CYTED) for a fellowship. A.B.R. and A.J.P.
thank to Fonacit-Venezuela (S1-2002000260) for financial support.
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