C O M M U N I C A T I O N S
Figure 2. 1H spectrum of a d8-toluene solution containing 1 in the presence
of CO and p-H2 at 373 K with key resonances indicated (b ) branched and
l ) linear aldehyde).
Figure 3. Plot of the relative p-H2-based NMR signal intensities for species
3, 4, and the corresponding aldehydes as a function of temperature for
samples of 1 reacting in toluene with 3 atm of a 2:1 mixture of CO and H2.
(Note, blue ( data obtained for a 1H signal, measured using normal methods
when the thermally polarized state was visible.)
revealed the liberation of PCy3, while the 1H NMR spectrum
confirmed the formation of a new η3-allyl complex, which was
subsequently identified as Co(η3-C3H5)(CO)3.11 At 363 K, 15% of
1 was converted to Co(η3-C3H5)(CO)3.
These reactions were then repeated at 378 K, and the CO to H2
ratio varied across the series 4:1, 1:1, 1:2, and 1:3. The relative
strength of the NMR signals of 3 relative to those of 4 now proved
to be minimized at a CO:H2 ratio of 1:2; this also corresponded to
the point of highest linear aldehyde selectivity.
Here, we have shown that the utilization of the PHIP effect allows
the mapping of the hydroformylation reaction of a cobalt catalyst
modified with PCy3. The detection of reaction intermediates that
do not contain a hydride ligand via enhanced signals within the
organic ligand framework proved possible. In addition, the high
signal strengths enable the hydroformylation products themselves
to be monitored at very early reaction times. We expect this
extension of the para-hydrogen approach to become applicable to
the study of a wide range of catalytic reactions. We are currently
using it to probe how the ligand sphere influences the reactions of
modified and unmodified cobalt hydroformylation catalysts.
When a d8-toluene solution of 1 was placed under 3 atm of a
1
1:2 mixture of CO and H2 at 363 K, the corresponding H NMR
spectrum showed much weaker p-H2-enhanced signals for propene
and propane. In addition, five new p-H2-enhanced signals were also
detected in clear regions of the spectrum at δ 3.57, 3.22, 0.92, 0.80,
and 0.72 (Figure 2). The corresponding COSY spectrum connected
the δ 3.22 signal (t, JHH ) 7 Hz) to the δ 0.92 (t, JHH ) 7 Hz)
signal and a new resonance at δ 1.67 (sextet, JHH ) 8 Hz), which
was masked by the phosphine. These data confirm that a linear
CH2CH2CH3 group has been formed with protons from p-H2 being
placed on all three of the carbon centers. When a 1H{31P} spectrum
was recorded, the signal at δ 3.22 simplified due to removal of a
single JPH coupling of 3 Hz from a 31P center which resonates at δ
62.0. COSY confirmed that the additional 1H signal at δ 3.57
(septet, JHH ) 8 Hz) coupled to a signal at δ 1.20 (d, JHH ) 8 Hz),
which indicated that a CH(CH3)2 group was formed. When this
reaction was repeated with 13CO, both of the terminal alkyl proton
signals showed additional 13C splittings that arose from 13C signals
that appeared at δ 235.6 and 240.9, respectively. Since both the
latter resonances exhibited doublet multiplicities due to 31P-13C
couplings (JPC ) 39.3 and 40.5 Hz, respectively), their origin as
metal acyl groups, trans to a single phosphine ligand, was con-
firmed. The corresponding species are, therefore, Co(COCH2CH2-
CH3)(CO)3(PCy3) 3 and Co(COCH(CH3)2)(CO)3(PCy3) 4. In view
of the fact that 3 and 4 contain PCy3, it is possible to conclude that
1 reacts initially with H2 via CO loss. The remaining p-H2-enhanced
resonances at δ 0.80 and 0.72 were shown to arise from the
aldehydes CH(O)CH(CH3)2 and CH(O)CH2CH2CH3, respectively.
The p-H2-enhanced NMR signals of 3 and 4 can be accounted
for by the reversible hydride transfer processes illustrated in Scheme
1; the signals seen for site B correspond to directly formed linear
product, while those seen for site A arise via relinearized branched
intermediates which can also react to give 4.
When this reaction was observed at different temperatures using
a constant concentration of 1 and a CO:H2 ratio of 1:2, the size of
the signals due to the aldehydes, propene, and propane increased
with increase in temperature. Figure 3 summarizes these variations.
A series of 1D EXSY experiments were then completed to test for
magnetization transfer from 3 or 4 into the corresponding aldehydes.
None was observable on the NMR time scale. The enhanced NMR
signals seen for the two aldehydes, therefore, require the corre-
sponding 16-electron intermediates, Co(COCH2CH2CH3)(CO)2-
(PCy3) and Co(COCH(CH3)2)(CO)2 (PCy3), to be trapped by H2
(which leads to rapid aldehyde elimination) rather than CO (which
leads to 3 or 4). The variation in the observed product ratios reflects
a higher linear aldehyde turnover at higher temperature.
Acknowledgment. Sasol Technologies (UK) Ltd funded this
work.
Supporting Information Available: X-ray data for 1, NMR data
and experimental procedures (CIF, PDF). Crystal data for 1: C23H38-
CoO2P, FW 436.43, yellow blocks, crystal dimensions 0.32 × 0.24 ×
0.14 mm, monoclinic, P21/n, a ) 10.4726(8) Å, b ) 15.4811(12) Å,
c ) 13.8229(11) Å, â ) 92.511(2)°, V ) 2238.9(3) Å3, Z ) 4, µ(Mo
KR) ) 0.853 mm-1, T ) 115(2) K, 17 426 reflections collected, R(int)
) 0.0247. This material is available free of charge via the Internet at
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