Mechanism of the hydrogenation of 2,5-norbornadiene catalyzed by [Rh(NBD)(PPh3)2]BF4 in dichloromethane: A kinetic and spectroscopic investigation

Article abstract of DOI:10.1016/S0022-328X(99)00761-5

A kinetic investigation of the selective hydrogenation of 2,5-norbornadiene to norbornene catalyzed by [Rh(NBD)(PPh₃)₂]BF₄ (1) has been carried out in dichloromethane at room temperature. The reaction is independent of the substrate concentration, while it is first order in catalyst and hydrogen pressure. Furthermore, the addition of triphenylphosphine inhibits the reduction. Moreover, it has been observed that under hydrogen atmosphere and at low temperature, complex 1 is in equilibrium with the dihydrido cis-trans-[RhH₂(NBD)(PPh₃)₂]BF₄, and that the addition of one equivalent of [Rh(NBD){P(p-Tol)₃}₂]BF₄ to a chloroform-d₁ solution of 1 affords [Rh(NBD)(PPh₃){P(p-Tol)₃}]BF₄ in 54% yield. On the basis of these observations and other spectroscopic results, we propose that the hydrogenation of 2,5-norbornadiene to norbornene catalyzed by 1 proceeds by the five-coordinate dihydrido [RhH₂(NBD)(PPh₃)]⁺, which is formed by oxidative addition of molecular hydrogen to both 1 and a tricoordinate species [Rh(NBD)(PPh₃)]⁺, depending on the concentration of free phosphine in the catalytic solution.

Full text of DOI:10.1016/S0022-328X(99)00761-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 599 (2000) 178–184
Mechanism of the hydrogenation of 2,5-norbornadiene catalyzed
by [Rh(NBD)(PPh3)2]BF4 in dichloromethane: a kinetic and
spectroscopic investigation
Miguel A. Esteruelas b,*1, Juana Herrero a,*2, Marta Mart´ın b, Luis A. Oro b,
V´ıctor M. Real a
a Departamento de Qu´ımica, Uni6ersidad de Cantabria, E-39005-Santander, Spain
b Departamento de Qu´ımica Inorga´nica-Instituto de Ciencia de Materiales de Arago´n, Uni6ersidad de Zaragoza-CSIC, E-50009-Zaragoza, Spain
Received 22 September 1999; accepted 8 December 1999
Abstract
A kinetic investigation of the selective hydrogenation of 2,5-norbornadiene to norbornene catalyzed by [Rh(NBD)(PPh3)2]BF4
(1) has been carried out in dichloromethane at room temperature. The reaction is independent of the substrate concentration,
while it is first order in catalyst and hydrogen pressure. Furthermore, the addition of triphenylphosphine inhibits the reduction.
Moreover, it has been observed that under hydrogen atmosphere and at low temperature, complex 1 is in equilibrium with the
dihydrido cis–trans-[RhH2(NBD)(PPh3)2]BF4, and that the addition of one equivalent of [Rh(NBD){P(p-Tol)3}2]BF4 to a
chloroform-d1 solution of 1 affords [Rh(NBD)(PPh3){P(p-Tol)3}]BF4 in 54% yield. On the basis of these observations and other
spectroscopic results, we propose that the hydrogenation of 2,5-norbornadiene to norbornene catalyzed by 1 proceeds by the
five-coordinate dihydrido [RhH2(NBD)(PPh3)]+, which is formed by oxidative addition of molecular hydrogen to both 1 and a
tricoordinate species [Rh(NBD)(PPh3)]+, depending on the concentration of free phosphine in the catalytic solution. © 2000
Elsevier Science S.A. All rights reserved.
Keywords: Rhodium; Catalysis; Hydrogenation; Kinetics
1. Introduction
drido and monohydrido species are active catalysts [4].
As a part of our work on the mechanism of the
homogeneous hydrogenation of unsaturated organic
substrates catalyzed by transition-metal complexes [23–
29], we have previously reported on the mechanism of
the hydrogenation of alkynes catalyzed by complexes of
the type [M(diene)La]+ (M=Rh, Ir) in dichloro-
methane [30,31]. In this solvent, the reactions do not
imply the reduction of the dienes of the catalyst precur-
sors, which remain coordinated during the processes, in
contrast to that previously reported for the related
systems in coordinating solvents [3].
Following our interest in this subject, we have now
carried out kinetic and spectroscopic studies in
dichloromethane on the hydrogenation of 2,5-norbor-
nadiene (NBD) to norbornene catalyzed by [Rh(NBD)-
(PPh3)2]BF4. In this paper, we report the results from
these studies, showing that under these conditions the
reduction takes place via a five-coordinative dihydrido
intermediate [RhH2(NBD)(PPh3)]+.
The discovery of the existence of the cationic
rhodium complexes with the general formula [Rh(di-
ene)La]+ (a=2 or 3) by Shapley, Schrock and Osborn
[1,2] was fundamental to the development of homoge-
neous hydrogenation catalyzed by transition-metal
compounds [3].
These complexes have been found to be active cata-
lysts for the hydrogenation of olefins [4–9], dienes
[10–12], alkynes [13,14], ketones [15], polynuclear het-
eroaromatic compounds [16–21] and carbon dioxide
[22]. In solvents such as acetone, ethanol or acetonitrile,
they react with molecular hydrogen to give dihydrido–
metal complexes [RhH2SxLa]+ (S=solvent), which un-
der catalytic conditions are in equilibrium with the
corresponding monohydridos MHSyLa. Both the dihy-
1 *Corresponding author. Fax: +34-976-761187.
2 *Corresponding author.
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00761-5

M.A. Esteruelas et al. / Journal of Organometallic Chemistry 599 (2000) 178–184
179
2. Results and discussion
at constant pressure and at 25°C. In agreement with the
early results [10], the selective reduction of the diene to
norbornene was observed.
2.1. Kinetic studies
In order to determine the rate dependence on the
various reaction components, hydrogenation runs were
performed at different catalyst and 2,5-norbornadiene
concentrations and at different hydrogen pressures. The
reactions were followed by measuring the hydrogen
consumption as a function of time, and the initial rates
(r in Table 1) were calculated by using Eq. (1), where
−dV/dt is the initial rate measured from the experi-
ments, P is the reaction pressure (atm), R is the molar
gas constant, T is the temperature (K), and Vsol is the
total volume (L) of the reacting solution.
The hydrogenation of 2,5-norbornadiene catalyzed
by the complex [Rh(NBD)(PPh3)2]BF4 (1) was studied
Table 1
Kinetic data for the hydrogenation of 2,5-norbornadiene to norbor-
nene catalyzed by [Rh(NBD)(PPh3)2]BF4 (1) in dichloromethane at
25°C
[Catalyst]
(104 M)
[NBD] (M) P(H2)
[PPh3]
r
(atm)
(105 M)
(105 M s−1
)
5.0
6.4
6.9
7.9
8.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
0.14
0.14
0.14
0.14
0.14
0.09
0.10
0.18
0.22
0.25
0.14
0.14
0.14
0.14
0.14
0.14
0.14
0.14
0.14
0.14
0.14
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.23
0.24
0.34
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
–
–
–
–
–
–
–
–
–
–
–
–
1.41
1.79
2.00
2.18
2.36
2.09
1.95
2.00
1.95
2.18
1.14
1.18
1.64
1.82
1.43
1.56
1.45
1.21
1.15
0.91
0.79
d[H2]
dt
(dV/dt)P
RTVsol
r= −
= −
(1)
For the hydrogenation of 2,5-norbornadiene to nor-
bornene catalyzed by [Rh(NBD)La]+ precursors in ace-
tone as solvent, Schrock and Osborn have previously
observed that the rate of the diene reduction is indepen-
dent of 2,5-norbornadiene concentration, and that the
rate at which the molecular hydrogen reacts with
[Rh(NBD)La]+ is qualitatively parallel to the hydro-
genation rate of the diene [10]. In agreement with this,
–
1.38
2.76
2.76
4.14
4.83
5.52
6.21
6.90
the data collected in Table
1 indicate that in
dichloromethane the reduction of 2,5-norbornadiene
catalyzed by 1 is practically independent of the sub-
strate concentration, while a plot of log r versus
log (P(H2)) yields a straight line of slope 0.9, suggesting
that the reduction is first order in hydrogen pressure.
The plot of log r versus log ([Rh]Tot) yields a straight
line of slope 0.9, indicating that the reaction is also first
order in catalyst precursor concentration.
In the presence of 1, it has been found that the
reduction of phenylacetylene is inhibited by addition of
triphenylphosphine [31]. The effect of the addition of
this phosphine to the catalytic solutions of 2,5-norbor-
nadiene is shown in Fig. 1. The initial reduction rate
decreases by addition of phosphine to the reaction
system. For lower concentrations of added phosphine
than 6.00×10−5 M, the plot of [Rh]TotP(H2)/r versus
[PPh3] (Fig. 1(a)) is a linear with a positive interception
on the y axis. The relation is expressed in the form
a[RH]TotP(H2)
b[PPh3]+1
r=
(2)
where [PPh3] is the concentration of added phosphine,
and a and b are constants.
For higher concentrations of added phosphine than
4.00×10−5 M, the plot of [Rh]TotP(H2)/r versus [PPh3]
(Fig. 1(b)) is also linear. However, in this case, the
relation is expressed in the form
Fig. 1. Plot of [Rh]TotP(H2)/r vs. [PPh3] for the hydrogenation of
2,5-norbornadiene to norbornene catalyzed by [Rh(NBD)(PPh3)2]BF4
(1) in dichloromethane at 25°C (6.9×10−4 M 1, and 0.14 M
2,5-norbornadiene): (a) the concentration of the added phosphine is
lower than 6.00×10−5 M. (b) the concentration of the added
phosphine is higher than 4.00×10−5 M.
c[Rh]TotP(H2)
r=
(3)
[PPh3]

180
M.A. Esteruelas et al. / Journal of Organometallic Chemistry 599 (2000) 178–184
The above-mentioned data suggest that in
resonance. At −60°C, the 31P{1H}-NMR spectrum
contains the doublet of 1 at 29.8 ppm, and a new
doublet at 44.5 ppm with a PꢁRh coupling constant of
119.0 Hz. Under off-resonance conditions, the latter
doublet is converted into a double triplet as a result of
the coupling with two equivalent hydrido ligands. The
equivalence of the phosphine and hydrido ligands and
the vinylic protons of the coordinated 2,5-norbornadi-
ene diolefin indicates that the new compound is cis–
trans-[RhH2(NBD)(PPh3)2]BF4 (2) which is formed by
oxidative addition of molecular hydrogen to 1 (Eq. (4)).
At −60°C, the yield of the addition is about 5%.
dichloromethane as solvent the hydrogenation of 2,5-
norbornadiene to norbornene catalyzed by 1 goes by
two different pathways, which have different rate laws.
Depending upon the phosphine added to the catalytic
solution, one of them is favored with regard to the
other one.
2.2. Spectroscopic studies
1
Under argon atmosphere, the H-NMR spectrum of
1 in dichloromethane-d2 shows, along with the signals
corresponding to the phenyl protons of the phosphine
ligands, three broad resonances at 4.5, 4.0 and 1.6 ppm,
the first of them corresponding to the ꢀCH protons of
the diene and the other two due to the aliphatic ꢁCH
and ꢁCH2 protons of the 2,5-norbornadiene ligand. The
31P{1H}-NMR spectrum contains at 29.8 ppm a dou-
blet with a PꢁRh coupling constant of 155.5 Hz. These
spectra are temperature invariant between −80 and
40°C.
(4)
There is precedent for this reaction. Similarly to 1,
the
iridium-tetrafluorobenzobarrelene
complex
The addition of 32 equivalents of 2,5-norbornadiene
to the dichloromethane-d2 solution of 1 does not pro-
[Ir(TFB)(PiPr3)2]BF4 reacts with molecular hydrogen to
give cis–trans-[IrH2(TFB)(PiPr3)2]BF4 [34]. However,
under hydrogen atmosphere the related 1,5-cyclooctadi-
ene derivatives [Ir(COD)(PR3)2]PF6 afford cis–cis-
[IrH2(COD)(PR3)2]PF6 (PR3=PPh3, PMePh2) [35,36].
Complexes [Ir(DCT)(PR3)2]+ (DCT=dibenzo[a, e]-
cyclooctatetraene) show a similar behavior to that of
[Ir(COD)(PR3)2]PF6 [37]. Cations cis–trans-[IrH2-
(COD)(PR3)2]+ can be prepared by reaction of [Ir-
(COD)(PR3)2]+ with molecular hydrogen in the pres-
ence of 1,5-cyclooctadiene or alternatively by treatment
of the solvated compounds [IrH2(acetone)2(PR3)2]+
with 1,5-cyclooctadiene [35,36].
The formation of 2 by oxidative addition of molecu-
lar hydrogen to 1 is not a single reaction, since the
oxidative addition of XꢁX or XꢁY bonds of non-polar
molecules to square-planar complexes is a diastereose-
lective concerted cis addition process with specific sub-
strate orientation [38–43]. Furthermore, the only path
to approach the hydrogen molecule to 1 is the olefin-
RhꢁP axis, which should lead to the cis–cis-
[RhH2(NBD)(PPh3)2]BF4 (3) isomer.
Scheme 1 shows the two possible pathways for the
formation of 2 according to Eq. (4). Both mechanisms
have the five-coordinate dihydrido species 4 as key
intermediate. Path a involves the oxidative addition of
molecular hydrogen along one of the two PꢁRh-olefin
axes of 1 to give the cis–cis-isomer 3, which dissociates
phosphine to afford the five-coordinate dihydrido inter-
mediate 4. The re-coordination of the phosphine to 4
should lead to 2.
1
duce any changes in the H and 31P{1H}-NMR spectra
of 1, between the above-mentioned temperatures. This
excludes the formation, under catalytic conditions, of
the five-coordinate species [Rh(NBD)2L]+ (L=PPh3),
which have been previously observed when the ancillary
ligands L are nitriles, arsines and stibines [32,33].
1
Under hydrogen atmosphere, the H-NMR spectrum
of the solution resulting from the addition of 32 equiv-
alents of 2,5-norbornadiene to 1 in dichloromethane-d2
shows the resonances of norbornene, unreacted 2,5-nor-
bornadiene and 1, and the 31P{1H}-NMR spectrum
only contains the doublet at 29.8 ppm. These observa-
tions suggest that under catalytic conditions the main
organometallic species is 1. Furthermore, since complex
1 does not interact with 2,5-norbornadiene, they also
indicate that the hydrogenation takes place by reaction
of 1 with molecular hydrogen.
In order to obtain information about the reaction of
1
1 with molecular hydrogen, we carried out the H- and
31P{1H}-NMR spectra of 1 under hydrogen atmosphere
between −80 and −20°C. In this range of tempera-
tures the reduction of the coordinated diene is very
slow and the formation of norbornene is not practically
observed during the experiments. The most accurate
1H-NMR spectrum was obtained at −60°C. In the
low-field region, the spectrum shows the resonances of
1 along with three broad resonances at 4.73 (2H), 2.13
(4H) and 1.42 (2H) ppm, and in the high-field region a
double triplet at −21.73 (2H) ppm, with HꢁRh and
HꢁP coupling constants of 27.0 and 12.0 Hz, respec-
tively. A variable-temperature 300 MHz T1 study of
this resonance gives a T1(min) of 318 (910) ms at
−20°C, indicating that it corresponds to a hydrido
In complex 3, the dissociation of the phosphine trans
disposed to the hydrido ligand should be favored due to
the high trans effect of the hydrido, and the large steric

M.A. Esteruelas et al. / Journal of Organometallic Chemistry 599 (2000) 178–184
181
In order to support the dissociation of phosphine
suggested in path b, we carried out NMR spectra of the
solution resulting from the addition of one equivalent
of [Rh(NBD){P(p-Tol)3}2]BF4 to a chloroform-d1 solu-
tion of 1. In agreement with the dissociation of phos-
phine from 1, at room temperature, the 31P{1H}-NMR
spectrum of the mixture shows the formation of the
mixed-ligand complex [Rh(NBD)(PPh3){P(p-Tol)3}]BF4
in 54% yield (Fig. 2).
2.3. Mechanism of the hydrogenation
The first-order dependence of the reduction rate with
regard to the hydrogen pressure suggests that the active
species for the catalysis is a dihydrido derivative, gener-
ated by oxidative addition of molecular hydrogen to the
precursor 1. Furthermore, from the results of the spec-
troscopic study, we conclude that the formation of this
dihydrido derivative requires the dissociation of a phos-
phine ligand from 1 or, alternatively, from the unde-
tectable cis–cis-[RhH2(NBD)(PPh3)2]BF4 (3) inter-
mediate. The dissociation of phosphine is in agreement
with the observed decreasing of the reduction rate as a
result of the addition of triphenylphosphine to the
catalytic solution.
Scheme 1.
In addition, it should be noted that the insertion of
carbon–carbon double bonds into MꢁH bonds is a
concerted process, which requires planar M(CꢀC)H
groups [3,44]. So one should expect that the cis–trans-
isomer 2 was not an active intermediate for the cataly-
sis. This is strongly supported by the high stability of
the previously mentioned cis–trans-[IrH2(diene)-
(PR3)2]+ cations [34–36] and the dihydrido–silyl com-
plexes IrH2(SiR3)(diene)(PR3) (diene=COD, TFB), to-
wards the reduction of the diene [45–48].
In the light of the above-mentioned considerations,
and assuming that the oxidative addition of molecular
hydrogen to 1 occurs via the pathway b of Scheme 1,
the following mechanism (Eqs. (5)–(8)) can be pro-
posed for the hydrogenation of 2,5-norbornadiene to
norbornene catalyzed by 1:
Fig. 2. 31P{1H}-NMR spectrum (121.4 MHz, 20°C) of the solution
resulting from the addition of one equivalent of [Rh(NBD){P(p-
Tol)3}2]BF4 to a chloroform-d1 solution of [Rh(NBD)(PPh3)2]BF4
(ꢀ[Rh(NBD)(PPh3)2]BF4;
[Rh(NBD){P(p-Tol)3}2]BF4; ꢁ[Rh-
k
(NBD)(PPh3){P(p-Tol)3}]BF4).
5
[Rh(NBD)(PPh3)2]+ X [Rh(NBD)]++PPh3
(5)
1
5
k
6
[Rh(NBD)(PPh3)]++H2 X [RhH2(NBD)(PPh3)+] (6)
hindrance experienced by the triphenylphosphine
groups, which are mutually cis disposed.
5
4
k
[RhH2(NBD)(PPh3)]+“7 [RhH(C7H9)(PPh3)]+
(7)
(8)
The formation of the key intermediate 4, according
to path b, involves the initial dissociation of a
triphenylphosphine ligand from the square-planar com-
plex 1, followed by the oxidative addition of molecular
hydrogen to the resulting three-coordinate species 5.
The dissociation of a ligand L from square-planar
complexes of the type [Rh(NBD)L2]+ has been previ-
ously observed for monodentate nitrogen donor ligands
[32,33].
4
6
[RhH(C7H9)(PPh3)]++NBD
6
“[Rh(NBD)(PPh3)]++C7H10
5
With regard to the kinetic results, there is no doubt
that the migratory insertion of one of the CꢁC double
bonds of the diene into one of the MꢁH bonds of 4 (Eq.
(7)) is the rate-determining step. Thus, the rate of
formation of norbornene is:

182
M.A. Esteruelas et al. / Journal of Organometallic Chemistry 599 (2000) 178–184
d[C7H10]
[PPh3]equilibrium$[PPh3]added. Therefore [4] can be writ-
ten as follow:
=k7[4]
(9)
dt
The concentration of the key intermediate 4 can be
determined as follows:
k14k15[Rh]TotP(H2)
[4]$
(18)
[PPh3]
[Rh]Tot=[1]+[5]+[4]
(10)
Combining Eq. (9) and Eq. (18), we obtain Eq. (19).
since k5=[5][PPh3]/[1] and k6=[4]/[5]P(H2), we have
[1]=[4][PPh3]/k5k6P(H2) and [5]=[4]/k6P(H2) and
finally
d[C7H10] k7k14k15[Rh]TotP(H2)
$
(19)
dt
[PPh3]
Eq. (19) agrees well with the experimental Eq. (3),
where c is k7k14k15. So, Eqs. (14), (15) and (7) and Eq.
(8) are a reasonable description of the mechanism of
the hydrogenation of 2,5-norbornadiene to norbornene
catalyzed by 1, when the concentration of the added
phosphine is higher than 4.00×10−5 M.
k6[Rh]TotP(H2)
[PPh3]/k5+1+k6P(H2)
[4]=
(11)
Complex 1 is the main species under catalytic condi-
tions. Thus, [PPh3]/k5+1ꢀk6P(H2), when P(H2)51
and [PPh3]equilibrium$[PPh3]added. Therefore [4] can be
written as follow:
In summary, the complex [Rh(NBD)(PPh3)2]BF4 cat-
alyzes the selective hydrogenation of 2,5-norbornadiene
to norbornene via the five-coordinate dihydrido inter-
mediate [RhH2(NBD)(PPh3)]+ (Eqs. (7) and (8)). In the
absence of triphenylphosphine or in the presence of low
concentrations of phosphine, this dihydrido is mainly
formed by dissociation of triphenylphosphine from
[Rh(NBD)(PPh3)2]BF4 and subsequent oxidative addi-
tion of molecular hydrogen to the resulting three-coor-
dinate species [Rh(NBD)(PPh3)]+. In the presence of
moderate concentrations of phosphine, the formation
of the dihydrido involves the oxidative addition of
molecular hydrogen to [Rh(NBD)(PPh3)2]BF4, to give
k6[Rh]TotP(H2)
[PPh3]/k5+1
[4]$
(12)
Combining Eq. (9) and Eq. (12), we obtain Eq. (13),
where [Rh]Tot and [PPh3] are the concentrations of
catalyst precursor and added phosphine, respectively.
d[C7H10] k7k6[Rh]TotP(H2)
$
(13)
dt
[PPh3]/k5+1
Eq. (13) agrees well with the experimental Eq. (2),
where a is k7k6 and b is 1/k5. So, the Eqs. (5)–(8) are a
reasonable description of the mechanism of the hydro-
genation of 2,5-norbornadiene to norbornene catalyzed
by 1, in the absence of triphenylphosphine, or when the
concentration of the added phosphine is lower than
6.00×10−5 M.
cis-cis-[RhH2(NBD)(PPh3)2]+, which dissociates
triphenylphosphine ligand.
a
If the formation of 4 takes place via the path a of
Scheme 1, the mechanism of the reduction of 2,5-nor-
bornadiene could be summarised by Eqs. (14), (15), (7)
and (8).
3. Experimental
All manipulations were conducted with rigorous ex-
clusion of air. Solvents were dried by known procedures
and distilled under argon prior to use. 2,5-Norbornadi-
ene (Merck) was purified by chromatography on Al2O3
(neutral, activity grade I, column length 10 cm). The
starting materials [Rh(NBD)(PPh3)2]BF4 (1) and
[Rh(NBD){P(p-Tol)3}2]BF4 were prepared as described
in the literature [2]. NMR spectra were recorded on
Varian UNITY 300 and Bruker ARX 300 instruments.
[Rh(NBD)(PPh3)2]++H2
1
k
14
X cis-cis-[RhH2(NBD)(PPh3)2]+
15
(14)
cis-cis-[RhH2(NBD)(PPh3)2]+ X [RhH2(NBD)(PPh3)]+
3
k
3
4
+PPh3
(15)
In such a case, the concentration of 4 can be deter-
mined according to Eq. (16).
3.1. Reaction of 1 with molecular hydrogen
[Rh]Tot=[1]+[3]+[4]
(16)
H2 was bubbled through
a solution of 1 in
Since k14=[3]/[1]P(H2) and k15=[4][PPh3]/[3], we
have [1]=[4][PPh3]/k14k15P(H2) and [3]=[4][PPh3]/k15
and finally
dichloromethane-d2 (0.5 ml) previously cooled at
−78°C, and contained in a 5 mm NMR tube. The
reaction was monitored by 1H- and 31P{1H}-NMR
between −80 and −20°C. The most accurate spectra
were obtained at −60°C. Along with unreacted com-
plex 1 a new compound appeared in a 5% yield, which
was identified as cis–trans-[RhH2(NBD)(PPh3)2]BF4
(2). 1H-NMR (300 MHz, −60°C): l 4.73 (br, 2H, ꢁCH
of NBD), 2.13 (br, 4H, ꢀCH of NBD), 1.42 (br, 2H,
k14k15[Rh]TotP(H2)
[PPh3]{1+k14P(H2)}+k14k15P(H2)
[4]=
(17)
As complex 1 is the main species under catalytic
conditions, we can assume that [PPh3]{1+k14P(H2)}]
k14k15P(H2) and k14P(H2)51, when P(H2)51 and

M.A. Esteruelas et al. / Journal of Organometallic Chemistry 599 (2000) 178–184
183
4. Conclusions
The kinetic and spectroscopic results of this study sug-
gest that in dichloromethane as solvent the selective hy-
drogenation of 2,5-norbornadiene to norbornene
catalyzed by [Rh(NBD)(PPh3)2]+ takes place via the
five-coordinate dihydrido intermediate [RhH2(NBD)-
(PPh3)]+ (Scheme 2), which is formed by oxidative addi-
tion of molecular hydrogen to both [Rh(NBD)(PPh3)2]+
and [Rh(NBD)(PPh3)]+ depending on the concentration
of free phosphine in the catalytic solution.
Scheme 2.
ꢁCH2 of NBD), −21.73 (dt, 2H, JRhH=27.0, JPH=12.0
Hz, RhH), signals of phenyl-protons of 2 are overlapped
by phenyl-protons of 1. 31P{1H}-NMR (121.4 MHz, −
60°C): l 44.5 (d, JRhP=119.0 Hz; dt off-resonance). T1
(min) (ms, ‘RhH2’. 300 MHz, −20°C, dichloromethane-
d2): 318 (910).
Acknowledgements
We thank the DGICYT (Projects PB94-1186 and
PB95-0806, Programa de Promocio´n General del
Conocimiento) and the Comisio´n Mixta Caja
Cantabria-Universidad de Cantabria for financial
support.
3.2. Reaction of 1 with [Rh(NBD){P(p-Tol)3}2]BF4
Compound 1 (15.3 mg, 0.019 mmol) was treated, at
room temperature, with one equivalent of [Rh(NBD)-
{P(p-Tol)3}2]BF4 (16.9 mg, 0.019 mmol) in chloroform-
d1 (0.5 ml) contained in a 5 mm NMR tube. The 1H- and
31P{1H}-NMR spectra of the resulting solution were
measured immediately. A new compound appeared in a
54% yield, which was identified as [Rh(NBD)-
(PPh3){P(p-Tol)3}]BF4. 1H-NMR (300 MHz, CDCl3,
20°C): l 4.44 (br, 4H, ꢀCH of NBD), 3.98 (br, 2H, ꢁCH
of NBD), 1.46 (br, 2H, ꢁCH2 of NBD), 2.32 (s, 9H,
CH3), signals of phenyl-protons of [Rh(NBD)(PPh3)-
{P(p-Tol)3}]BF4 are overlapped by phenyl-protons of 1
and [Rh(NBD){P(p-Tol)3}2]BF4. 31P{1H}-NMR (121.4
MHz, CDCl3, 20°C): l 30.3 (dd, JRhP=157.9, JPP=30.1
Hz), 28.2 (dd, JRhP=156.0, JPP=30.1 Hz).
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3 P=P(H2)+P(CH2Cl2), where P(CH2Cl2)=0.56 atm at 25°C.

184
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