Spectroscopic evidence of the first example of an isomer with the hydride ligand in the equatorial plane in the HRh(CO)4-x(PR3)x family: HRh(CO)2(PR3)2 (PR3 = 1,2,5-triphenyl-1H-phosphole, TPP)

Article abstract of DOI:10.1021/om020602m

Available-temperature multinuclear NMR study using enriched ¹³CO shows that the HRh-(CO)₂(TPP)₂ (TPP = 1,2,5-triphenyl-1H-phosphole) complex, the resting state of the catalytic system for the hydroformylation of styrene by the Rh/TPP mixture, exists in solution as two stereoisomers in equilibrium. The minor isomer (10%) exhibits a geometry where the hydride ligand occupies an equatorial position, a unique case within the HRh(CO)_4-x(PR₃)_x family.

Full text of DOI:10.1021/om020602m

782
Organometallics 2003, 22, 782-786
Sp ectr oscop ic Evid en ce of th e F ir st Exa m p le of a n
Isom er w ith th e Hyd r id e Liga n d in th e Equ a tor ia l P la n e
in th e HRh (CO)4-x(P R3)x F a m ily: HRh (CO)2(P R3)2 (P R3 )
1,2,5-Tr ip h en yl-1H-p h osp h ole, TP P )
Christian Bergounhou,* Denis Neibecker, and Rene´ Mathieu
Laboratoire de Chimie de Coordination du CNRS, 205 Route de Narbonne,
31077 Toulouse Cedex 4, France
Received J uly 24, 2002
A variable-temperature multinuclear NMR study using enriched 13CO shows that the HRh-
(CO)2(TPP)2 (TPP ) 1,2,5-triphenyl-1H-phosphole) complex, the resting state of the catalytic
system for the hydroformylation of styrene by the Rh/TPP mixture, exists in solution as two
stereoisomers in equilibrium. The minor isomer (10%) exhibits a geometry where the hydride
ligand occupies an equatorial position, a unique case within the HRh(CO)4-x(PR3)x family.
In tr od u ction
active rhodium hydride species), these experiments also
revealed a hydride resonance arising as a triplet cen-
tered at -9.92 ppm. Relevant spectra obtained from
similar experiments carried out in the absence of
styrene exhibited the same pattern in the hydride
region, thereby confirming the existence of the same
organometallic species. Consequently, further studies
of these species were conducted in the absence of olefinic
substrate.
Earlier studies from our laboratory have shown that
rhodium complexes of 1,2,5-triphenyl-1H-phosphole (TPP)
are prone to catalyze the hydroformylation of a broad
range of substrates such as 1-hexene,1 styrene,2 and
ethyl acrylate3 under milder conditions and with higher
efficiency than related triphenylphosphine systems. An
analysis of the influence of the TPP/Rh ratio revealed
the existence of a unique active species, tentatively
formulated as HRh(CO)x(TPP)2,4 in contrast to the case
of the PPh3/Rh system, where several species are
involved. With the aim of gaining more insight into the
mechanism of this catalytic reaction, we were prompted
to carry out a kinetic study of the hydroformylation of
styrene5,6 with concomitant in situ monitoring by in-
frared and NMR spectroscopy. We report here an
account of the results of spectroscopic studies on HRh-
(CO)2(TPP)2, previously identified as the catalyst’s rest-
ing state.
Variable-temperature H and 31P NMR studies were
1
conducted under 5 bar of CO and 5 bar of H2 in
dichloromethane solution. 31P{1H} NMR spectra are
displayed in Figure 1. The spectrum recorded at 193 K
shows three resonances: namely, a doublet at 33.6 ppm
(J Rh-P ) 133 Hz), a broad doublet at 28.2 ppm (J Rh-P
)
140 Hz), and a sharp singlet at 1.6 ppm. Upon selective
decoupling of the phenyl protons (Figure 2), the doublet
at 33.6 ppm is converted into a doublet of doublets (J PH
) 15 Hz; J Rh-P ) 133 Hz), showing that this signal is
characteristic of the complex 1, containing one hydride
ligand. Increasing the temperature to 298 K causes only
a slight shift of the 33.6 ppm doublet to 34.1 ppm (J Rh-P
) 122 Hz). In contrast, the 28.2 ppm doublet changes
to a second-order figure at 233 K and tends to fade into
the baseline as the temperature reaches 298 K. Simul-
taneously, the sharp resonance at 1.6 ppm broadens and
finally shifts to 5 ppm, the characteristic chemical shift
of the free TPP (Figure 1). These observations are
consistent with the existence of a nonhydridic species
2, which would reversibly uptake and release free TTP.
Indeed, in a separate experiment, addition of TPP
(1 equiv) was found to cause both an enhancement of
the 1.6 ppm signal and a shift of the 28.2 ppm doublet
to 25.7 ppm. In contrast, saturation of the free TPP
resonance did not affect the resonance figure of the
hydride species 1, thereby indicating the absence of
exchange between this complex and free TPP.
Resu lts a n d Discu ssion
Preliminary NMR experiments were done in a heavy-
walled 5 mm o.d. NMR tube (fitted with a valve) under
experimental conditions analogous with those used
earlier in a typical catalytic run,5 albeit with a different
styrene/rhodium ratio (dichloromethane solution; [sty-
rene]/[RhCl(CO)(TPP)2] ) 2; 2 equiv of NEt3; 5 bar of
CO and 5 bar of H2). While allowing the detection of
both the expected organic products resulting from the
hydroformylation of styrene and the triethylammonium
hydrochloride salt (resulting from the trapping of hy-
drogen chloride liberated during the formation of the
(1) (a) Neibecker, D.; Re´au, R. J . Mol. Catal. 1989, 53, 219. (b)
Neibecker, D.; Re´au, R. J . Mol. Catal. 1989, 57, 153.
(2) Neibecker, D.; Re´au, R.; Lecolier, S. J . Org. Chem. 1989, 54, 5208.
(3) Neibecker, D.; Re´au, R. New J . Chem. 1991, 15, 279.
(4) Bergounhou, C.; Neibecker, D.; Re´au, R. J . Chem. Soc., Chem.
Commun. 1988, 1370.
(5) Bergounhou, C.; Neibecker, D.; Re´au, R. Bull. Soc. Chim. Fr.
1995, 132, 818.
(6) Bergounhou, C.; Neibecker, D.; Mathieu, R. Submitted for
publication.
1
The variable-temperature H NMR spectrum in the
hydride region is presented in Figure 3. The doublet of
triplets observed at 273 K (δ -9.92 ppm, J PH ) 7.5 Hz,
J RhH ) 2 Hz) is seen to broaden when the temperature
10.1021/om020602m CCC: $25.00 © 2003 American Chemical Society
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An Equatorial Hydride in HRh(CO)4-x(PR3)x
Organometallics, Vol. 22, No. 4, 2003 783
F igu r e 1. Variable-temperature 31P{1H} NMR spectra
(Bruker WM250; CD2Cl2) of the reaction mixture without
styrene after 20 min at 40 °C. Conditions: H2, 5 bar; CO,
5 bar; RhCl(CO)(TPP)2, 10 mg (1.2 × 10-5 mol); Et3N, 3
µL (2.2 × 10-5 mol); CD2Cl2, 1 mL.
F igu r e 3. Variable-temperature 1H NMR spectra (Bruker
WM250; CD2Cl2) of the reaction mixture without styrene
after 20 min at 40 °C. Conditions: H2, 5 bar; CO, 5 bar;
RhCl(CO)(TPP)2, 10 mg (1.26 × 10-5 mol); Et3N, 3 µL (2.17
× 10-5 mol); CD2Cl2, 1 mL.
F igu r e 2. 31P{1H BB} and 31P{1H phenyl} NMR spectra
at 193 K (Bruker WM250; CD2Cl2): δ 33.6 ppm; J P-H
15.0 ( 1.0 Hz; J P-Rh ) 133 ( 1 Hz.
)
is lowered down to 193 K, where it then arises as a
triplet (δ -10.16 ppm, J PH ) 15.5 Hz). This indicates
that complex 1 contains two TPP ligands, whereas the
evolution of chemical shifts and coupling constants with
temperature suggests the occurrence of an equilibrium
between two or more isomers, one being in major
proportion.
While attempting to confirm this hypothesis, we were
faced with the problem of getting the hydrido species
in higher concentration. The problem could be solved
by using a different rhodium source: namely, the
mixture Rh4(CO)12 + 8TPP under a CO + H2 atmo-
sphere. Moreover, increasing the H2/CO ratio to 9
favored the concentration of 1 at the expense of 2,
suspected to be a binuclear complex resulting from the
dehydrogenation of the mononuclear hydride, by anal-
ogy with previously reported similar catalytic systems.7
Figure 4 displays the 31P{1H} NMR of the resulting
mixture at 213 K. Clearly, the resonance of the previ-
F igu r e 4. 31P{1H} NMR spectrum at 213 K (Bruker
WM250; CD2Cl2) of the reaction mixture without styrene
after 20 min at 40 °C. Conditions: H2, 9 bar; CO, 1 bar;
Rh4(CO)12, 10 mg (1.2 × 10-5 mol); Et3N, 3 µL (2.2 × 10-5
mol); CD2Cl2, 1 mL.
ously detected hydrido complex now arises as the major
signal, whereas two additional doublets of lower inten-
sity are also observed at 30.8 ppm for 2 and a new
1
compound at 36.3 ppm (J RhP ) 112 Hz). The H NMR
spectrum at room temperature is very similar to the
previously studied systems and consists of a nonsym-
metrical doublet of triplets (Figure 5). New features
appear when the temperature is lowered down to 193
K. In addition to the previously observed triplet at
-10.12 ppm (J PH ) 15.5 Hz) for 1a , a weak doublet of
(7) See for instance: van der Slot, S. C.; Kamer, P. C. J .; van
Leeuwen, P. W. N. M.; Iggo, J . A.; Heaton, B. T. Organometallics 2001,
20, 430.

784 Organometallics, Vol. 22, No. 4, 2003
Bergounhou et al.
F igu r e 6. 13C{31P}, 13C{1H}, and 13C{31P,1H} NMR spectra
at 273 K (Bruker AM250; CD2Cl2) of a 1/8 Rh4(CO)12/TPP
misture after 20 min at 40 °C. Conditions: H2, 5 bar;
13CO, 5 bar; Rh4(CO)12, 5 mg (6.8 × 10-6 mol); TPP, 17 mg
(5.4 × 10-5 mol); CD2Cl2, 1 mL.
F igu r e 5. Variable-temperature 1H NMR spectra (Bruker
AMX400; CD2Cl2) of the reaction mixture without styrene
after 20 min at 40 °C. Conditions: H2, 5 bar; CO, 5 bar;
Rh4(CO)12, 5 mg (6.8 × 10-5 mol); TPP, 17 mg (5.4 × 10-5
mol); CD2Cl2, 1 mL.
The structures of 1a and 1b were unambiguously
established by NMR using a mixture of H2 and 13CO-
enriched atmosphere. The 13C NMR spectrum (CO
region; 273 K) of the mixture under a H2 (5 bar) +
13CO (5 bar) atmosphere is displayed in Figure 6, where
Ch a r t 1
1
various combinations of H and 31P decoupling experi-
ments are shown. The broad triplet at 202.4 ppm, which
is not greatly affected by 1H and 31P decoupling, is
ascribed to 2. The double doublet of triplets at 198 ppm
is consistent with the HRh(CO)2(TPP)2 formulation for
1. At 193 K the 13C{31P,1H} spectrum (Figure 7) shows
that the doublet at 198 ppm evolves to two main
doublets of the same intensity at 200.1 ppm (1a 1) and
198.6 ppm (1a 2) and a weak doublet at 195.6 ppm (1b).
At the same temperature the 13C{31P} spectrum shows
that the doublet 1a 2 evolves to a doublet of doublets
(J RhC ) 50 Hz; J HC ) 18 Hz), the other signals not being
significantly affected by coupling with the hydride
ligand. Finally, in the 13C NMR spectrum, the signal
1a 1 now arises as a doublet of triplets (J RhC ) 75 Hz;
J PC ) 30 Hz), whereas the other signals are only slightly
broadened by the coupling with the phosphorus nuclei.
Considering the structures shown in Chart 1, the
signals 1a 1 and 1a 2 can be attributed to the axial and
equatorial CO groups of the P 1 geometry, respectively:
i.e., to the major isomer 1a . Moreover, for the minor
isomer 1b, the fact that the two CO ligands are only
weakly coupled with the TPP and hydride ligands is also
in agreement with the geometry P 2 selected on the basis
of proton NMR parameters.
triplets is detected at -9.09 ppm (J PH ) 45 Hz, J RhH
)
13 Hz) for 1b. Integration of signals gives a 90/10 ratio
for the two isomers. The coalescence temperature of the
exchange phenomenon is observed at 223 K, which is
consistent with an approximate ∆Gq value of 43((4) kJ
mol-1
.
The present NMR data are consistent with a formula-
tion of 1a and 1b as isomers of HRh(CO)2(TPP)2, for
which three possible geometries might be considered
(Chart 1). The structure referred to as P 1, with the
hydride ligand in an axial position and the two equiva-
lent TPP ligands in the equatorial plane, can be
unambiguously assigned to the major isomer. It corre-
sponds to the general case for HRh(CO)2(PR3)2 com-
plexes.8
Two possibilities remain for the second isomer, both
possessing a hydrido ligand in an equatorial position
(with a very characteristic J RhH coupling constant) but
differing in the position of TPP ligands, being either
equatorial (P 2 geometry) or axial (P 3 geometry). For the
latter, one would expect a J PH coupling constant of the
same magnitude as in P 1, which is not the case. Thus,
The study of the 1H and 31P NMR spectra on the
samples with a 13CO-enriched atmosphere has also
allowed determining most of the NMR parameters for
the two isomers at 193 and 273 K. They are collected
in Table 1.
P
2 represents the only realistic geometry for the second
(minor) isomer.
A literature survey of the HM(CO)xL4-x (x ) 1, 2; M
) Rh, Ir) series of complexes exhibiting a trigonal-
(8) Transition Metal Hydrides; Dedieu, A., Ed.; VCH: Weinheim,
Germany, 1990.

An Equatorial Hydride in HRh(CO)4-x(PR3)x
Organometallics, Vol. 22, No. 4, 2003 785
Ta ble 1. NMR Da ta for th e HRh (CO)2(TP P )2 Com p lexes
ascertained by an X-ray structure determination in the
case where M ) Ir and L ) P(p-tolyl)3.12 In the complex
HaRh(CO)2(a,e)(PPh3)2(e,e),9 the J PH value of 14 Hz com-
pares well with the value found for 1a (15.5 Hz), and it
is noteworthy that the J PH value for 1b (45 Hz) has a
value intermediate between the value observed for 1a
and the value found in the HaRh(CO)2(a,e)(PPh3)2(a,e)
complex for the coupling constant with the phosphorus
atom in a trans position (99 Hz). This is consistent with
a H-Rh-P angle value of around 120° for the complex
1b, which supports our hypothesis about the structure
of 1b with the hydride ligand in the equatorial plane.
In both isomers, the two TPP ligands are in equatorial
positions. This corresponds apparently to the highest
steric crowding around the metal. Modeling studies
based on theoretical calculations using the CACHE
program13 effectively indicate that the geometry with
the two TPP ligands in axial positions lies at lower
energy than those with the two TPP groups in the
equatorial plane. In addition, we have to consider a
possible stabilizing effect which is not taken into account
by the program: the π stacking14 of the phenyl and
phosphole ring when the two TPP ligands are in the
equatorial plane probably favors the occurrence of the
two isomers we have observed.
In conclusion, the present account provides unam-
biguous spectroscopic evidence for the first example of
a HRh(CO)2L2 isomer with the hydride ligand in the
F igu r e 7. 13C NMR spectra at 193 K (Bruker AM250
(62.896 MHz; CD2Cl2) of a 1/8 Rh4(CO)12/TPP mixture after
20 min at 40 °C. Conditions: H2, 5 bar; 13CO, bar; Rh4-
(CO)12, 5 mg (6.8 × 10-6 mol); TPP, 17 mg (5.4 × 10-5 mol);
CD2Cl2, 1 mL.
(9) Brown, J . M.; Canning, L. R., Kent, A. G.; Sidebottom, P. J . J .
Chem. Soc., Chem. Commun. 1982, 721.
(10) Brown, J . M.; Kent, A. G. J . Chem. Soc., Perkin Trans. 2 1987,
1597.
(11) Meakin, P.; Muetterties, E. L.; J esson, J . P. J . Am. Chem. Soc.
1972, 94, 5271.
(12) Ciechanowicz, M.; Skapski, A. C.; Troughton, P. G. H. Acta
Crystallogr., Sect. A 1969, 25, S172.
(13) CACHE Molecular Modeling system Package 3.7.
(14) Castonguay, L. A.; Rappe´, A. K.; Casewit, C. J . J . Am. Chem.
Soc. 1991, 113, 7183.
bipyramidal arrangement8 revealed that the present
observation of a hydrido ligand in an equatorial position
is unprecedented. In the case of x ) 2 only the two
structures HaM(CO)2(a,e)L2(e,e) and HaM(CO)2(e,e)L2(a,e)
have been observed by spectroscopic techniques.9-11
Moreover, the HaM(CO)2(e,e)L2(a,e) structure has been

786 Organometallics, Vol. 22, No. 4, 2003
Bergounhou et al.
Dodecacarbonyltetrarhodium, Rh4(CO)12, was prepared ac-
cording to the method of Cattermole et al.19 and kept under
carbon monoxide at low temperature.
equatorial plane of the trigonal-bipyramidal structure.
This type of isomer was previously considered as a
possibility for the HIr(CO)2(PPh3)2 complexes15 but
finally rejected in the case of HaIr(CO)2(e,e)L2(a,e) on the
basis of a more comprehensive variable-temperature
NMR study.11 The authors of that study had expressed
the feeling that any isomer possessing the hydride
ligand in an equatorial position would be of much higher
energy than those with an axial hydride ligand. Our
observations now provide evidence that this is not
systematically the case and that the stereoelectronic
properties of the phosphorus ligand can eventually
stabilize isomers with the hydride ligand in the equato-
rial plane.
Gen er a l P r oced u r e. For each experiment, the precursor
RhCl(CO)(TPP)2 (or a 1/8 Rh4(CO)12/TPP mixture) was put in
a Fischer-Porter reactor. After several vacuum-argon cycles,
the solvent (deuterated toluene or dichloromethane) and the
desired liquid reactants (styrene, triethylamine) were added
(precision syringes under flushed argon). After the system was
charged to the desired pressures of gaseous reactants (CO,
13CO, H2), the reaction mixture was stirred vigorously with a
magnetic stirrer and heated at the required temperature in a
thermostated bath. After 10 min, the mixture was cooled with
an ice bath, depressurized, and transferred into the NMR tube.
In the case of the NMR pressure experiments, the reaction
mixture was transferred in a heavy-walled 5 mm o.d. NMR
tube (fitted with a valve) and pressurized at a work pressure
with the desired gaseous reactants.
Exp er im en ta l Section
1
NMR Sp ectr a . The H, 31P, and 13C spectra were recorded
Rea gen ts. All the gases used were of high purity from Air
Liquide Co. 13CO (99.37%) was furnished by Euriso-top and
D2 (99.7%) by Messer Griesheim.
at different temperatures on Bruker WM250, AM250, and
AMX400 spectrometers. Chemical shifts are reported in ppm
(δ) relative to solvent proton (C6D5H, 7.27 ppm; CHDCl2, 5.33
ppm) for 1H, 85% H3PO4 in D2O solution (external reference)
for 31P, and 13C of the solvent (13CD2Cl2, 53.6 ppm) for 13C
NMR, respectively.
All solvents were dried, distilled, and deaerated before use
according to the literature procedures. They were stored under
N2 or Ar after distillation. The 1,2,5-triphenyl-1H-phosphole
was supplied by F. Mathey16 or synthesized according to the
method of Lukas et al.17 Triethylamine (J anssen Chimica 99%)
was distilled over sodium and kept under argon. The catalytic
precursor RhCl(CO)(TPP)2 was prepared according to the
method developed by Neibecker and Re´au1b from bis(µ-chloro)-
tetracarbonyldirhodium.18
Ack n ow led gm en t. We thank G. Commenges and
F. Lacassin for their valuable cooperation and expertise
in the NMR studies.
(15) Yagupsky, G.; Wilkinson, G. J . Chem. Soc. A 1969, 725.
(16) Mathey, F. Chem. Rev. 1988, 88, 429.
OM020602M
(17) Lukas, B. R.; Roberts, M. G.; Silver, J .; Wells, A. S. J .
Organomet. Chem. 1983, 103, 256.
(18) McCleverty, J . A.; Wilkinson, G. Inorg. Synth. 1966, 8, 211.
(19) Cattermole, P. E.; Osborne, G.; Kruczynski, I. J . Inorg. Synth.
1977, 17, 115.