In situ NMR observation of mono- and binuclear rhodium dihydride complexes using parahydrogen-induced polarization

Article abstract of DOI:10.1021/ic991319r

Starting from the binuclear complex [RhCl(NBD)]₂ (NBD = 2,5-norbomadiene) in the presence of the phosphines L = PMe₃, PMe₂Ph, PMePh₂, PEt₃, PEt₂Ph, PEtPh₂, or P(n-butyl)₃

Full text of DOI:10.1021/ic991319r

Inorg. Chem. 2001, 40, 533-539
533
In Situ NMR Observation of Mono- and Binuclear Rhodium Dihydride Complexes Using
Parahydrogen-Induced Polarization
Andreas Koch and Joachim Bargon*
Institute of Physical and Theoretical Chemistry, University of Bonn,
Wegelerstrasse 12, D-53115 Bonn, Germany
ReceiVed NoVember 12, 1999
Starting from the binuclear complex [RhCl(NBD)]2 (NBD ) 2,5-norbornadiene) in the presence of the phosphines
L ) PMe3, PMe2Ph, PMePh2, PEt3, PEt2Ph, PEtPh2, or P(n-butyl)3, various mononuclear dihydrides of the type
Rh(H)2ClL3, i.e., those of the homogeneous hydrogenation catalysts RhClL3, have been obtained upon addition
1
of parahydrogen, and their H NMR spectra have been investigated using parahydrogen-induced polarization
(PHIP). Furthermore, the two binuclear complexes (H)(Cl)Rh(PMe3)2(µ-Cl)(µ-H)Rh(PMe3) and (H)(Cl)Rh(PMe2-
Ph)2(µ-Cl)(µ-H)Rh(PMe2Ph) have been detected and characterized by means of this in situ NMR method. Analogous
complexes with trifluoroacetate instead of chloride, i.e., Rh(H)2(CF3COO)L3, have been generated in situ starting
from Rh(NBD)(acac) in the presence of trifluoroacetic acid in combination with the phosphines L ) PPh3, PEt2-
1
Ph, PEt3, and P(n-butyl)3, and their H NMR parameters have been determined.
Introduction
PASADENA6 (parahydrogen and synthesis allow dramatically
enhanced nuclear alignment) effect and originates from the
The activation of molecular hydrogen (dihydrogen) by
transition metal complexes has been intensely investigated ever
since the 1960s, when Wilkinson and co-workers discovered
the first successful homogeneous hydrogenation catalyst RhCl-
(PPh3)3.1,2 Herewith terminal alkenes and alkynes can be readily
hydrogenated at 25 °C at a hydrogen pressure of 1 bar.
The mechanism and kinetics thereof have been extensively
studied, and some intermediate dihydrides such as Rh(H)2Cl-
(PPh3)3 and Rh2(H)2Cl2(PPh3)4 have been previously observed
and characterized using NMR spectroscopy.3,4 The data available
in the literature, however, are typically restricted to PPh3 as
the phosphine ligands, since in general the spectroscopic
observation of such dihydride species is rather difficult. Due to
their intermediate character, they have a short lifetime; hence
they typically occur in rather low concentrations only. In this
paper, we report the NMR data of some additional dihydrides
of various mono- and binuclear Rh complexes containing
different phosphine ligands, which have been detected in situ
using parahydrogen-induced polarization (PHIP).5
breakdown of the initially high symmetry of parahydrogen as
the consequence of a chemical reaction, which transfers the two
formerly equivalent protons into either chemically or magneti-
cally inequivalent positions. This transformation of the two
hydrogen atoms has to occur pairwise during the hydrogenation.
This means, that the initial A2 spin system of parahydrogen
becomes converted into an AB or AX spin system in the
hydrogenation products. Thereby only those energy levels
become selectively populated, which have some degree of
singlet character. The details thereof depend on the boundary
conditions, under which the hydrogenations are carried out, i.e.,
whether the reactions occur exclusively within the high magnetic
field of the NMR spectrometer, as is the case here, or initially
in the Earth’s low magnetic field, upon which the samples are
quickly transferred into the NMR spectrometer for immediate
analysis. Either way, the 1H PHIP NMR spectra of the
parahydrogen-derived molecules show emission and absorption
signals that are enhanced proportional to the reciprocal Boltz-
mann factor (typically by a few orders of magnitude) governing
conventional NMR spectra recorded at the same magnetic field
strength and temperature. As pointed out above, the occurrence
of this phenomenon requires, however, that the transfer of the
two parahydrogen atoms occurs in a pairwise manner, whereby
the transferred protons experience some sort of coupling to each
other at any time.
Parahydrogen-Induced Polarization (PHIP)
Parahydrogen and orthohydrogen are the two nuclear spin
isomers of dihydrogen, representing its nuclear singlet and triplet
state, respectively. Parahydrogen, the singlet spin state, is
magnetically inactive and hence invisible in the NMR spectrum.
1
The considerable signal enhancement associated with PHIP
allows detection of reaction intermediates and products at lower
concentrations than is otherwise feasible using conventional
NMR spectroscopy. Therefore, this method has been established
as a versatile tool to investigate the structure of many hydrogen-
containing transition metal complexes (typically dihydrides) and
products of homogeneous hydrogenations.7
Therefore, any observed H NMR resonance of dihydrogen is
exclusively due to orthohydrogen. PHIP is based on the
(1) Young, J. F.; Osborn, J. A.; Jardine, F. H.; Wilkinson, G. J. Chem.
Soc., Chem. Commun. 1965, 131-132.
(2) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem.
Soc. A 1966, 1711-1732.
(3) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J. Am. Chem. Soc. 1994,
116, 10548-10556.
(4) Tolman, C. A.; Meakin, P. Z.; Lindner, D. L.; Jesson, J. P. J. Am.
Chem. Soc. 1974, 96, 2762-2774.
(6) Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. Soc. 1987, 109, 5541-
5542.
(5) Natterer, J.; Bargon, J. Prog. Nucl. Magn. Reson. Spectrosc. 1997,
31, 293-315.
(7) Duckett, S. B.; Sleigh, C. J. Prog. Nucl. Magn. Reson. Spectrosc. 1999,
34, 71-92.
10.1021/ic991319r CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/05/2001

534 Inorganic Chemistry, Vol. 40, No. 3, 2001
Koch and Bargon
The Spin Density Matrix and the Product Operator
Formalism
A theoretical interpretation of the PHIP phenomenon based
on the product operator formalism8 allows us to forecast or
interpret PHIP spectra. In this description, the spin density
matrix of parahydrogen is represented by σparahydrogen ) I1I2. On
transferring the parahydrogen molecule into an AX spin system
during hydrogenation, its spin density matrix is propagated,
governed by the Hamiltonian for weakly coupled systems.
Averaging over the duration of the hydrogenation, the spin
density matrix of the former two parahydrogen atoms in the
resulting AX spin system of the hydrogenation product simpli-
fies to σPASADENA ) I1zI2z. The subscript z denotes the orientation
in a Cartesian coordinate system, whereby z coincides with the
direction of the external magnetic field, and the numbers mark
the respective protons. By contrast, the spin density matrix of
two hydrogen nuclei at thermal equilibrium, i.e., long after the
reaction, is described by σthermal ) I1z + I2z.
Figure 1. The apparatus used to carry out the hydrogenation inside of
the NMR magnet. The device is installed on top of the magnet. Before
the hydrogenation starts, the sled can be lowered by about 10 cm by
means of the pulleys. Thereby a glass capillary connected to the
parahydrogen source by means of a Teflon tube can be lowered into
the NMR probe actuated by an electromagnet, which is electrically
controlled by the computer of the NMR spectrometer.
Application of a radio frequency pulse corresponding to the
flip angle â and the phase y to the initial magnetization
σPASADENA ) I1zI2z generates three operator terms according to
âIy
98
I1zI2z
cos2 â I1zI2z + cos â sin â (I1zI2x + I1xI2z) + sin2 â I1xI2x
from parahydrogen cause in-phase splittings in the PHIP
resonances, just as is the case in regular (“thermal”) NMR
spectra.
The first and the last term thereof represent zero and double
quantum coherences, respectively, which cannot be detected
under these conditions. However, the term in the middle
corresponds to an observable single quantum coherence, which
is maximized if the flip angle â equals π/4. Therefore, when
recording PHIP spectra, â is set to half the conventional value
used in ordinary NMR spectroscopy, where π/2 represents the
optimal value.
This fact can be taken advantage of in order to eliminate
interfering resonances stemming from unpolarized systems, i.e.,
from molecules which do not participate in the hydrogenation
sequence. This includes the resonances stemming from the
solvent, from unpolarized starting material, or from previously
formed product molecules that have since reached thermal
equilibrium. For this purpose, individual scans are accumulated,
whereby the flip angles are alternated between -π/4 and 3π/4,
consecutively. As a consequence of this change of the flip angle
by 180° between two consecutive pulses, the amplitudes of
regular, i.e., of “thermal”, NMR resonances alternate in signs,
whereas the signs of the PHIP signals remain the same,
irrespectively. Accordingly, upon accumulating the correspond-
ing signals, the “unpolarized” resonances are suppressed and
eliminated, whereas the “polarized” PHIP resonances add up
and become accumulated to yield a better signal-to-noise ratio.
In Situ PHIP NMR Spectroscopy of Homogeneous
Hydrogenations
This paper describes the use of PHIP to detect some new
dihydride products at low concentrations which can otherwise
not be seen using conventional NMR methods. For this purpose,
the hydrogenation with parahydrogen is carried out in situ using
the apparatus in Figure 1, which allows us to monitor the
reaction and thereby determine its kinetic constants.
Results and Discussion
Reactions of [RhCl(NBD)]2 with Parahydrogen in the
Presence of Tertiary Phosphines. The binuclear precursor (di-
µ-chlorobis[η4-2,5-norbornadiene]rhodium(I) ) [(Rh(NBD)Cl]2)
is well suited for the in situ preparation of a variety of
homogeneous hydrogenation catalysts, if tertiary phosphines
(here PMe3, PMe2Ph, PMePh2, PEt3, PEt2Ph, PEtPh2, or P(n-
butyl)3) are added to the solution of this precursor. Upon the
addition of dihydrogen to these mixtures, NBD is hydrogenated
off, and the mononuclear dihydride species Rh(H)2ClL3 is
generated, most likely via the complex RhClL3 as an intermedi-
ate. This dihydride complex plays a key role as an intermediate
in any subsequent catalytic hydrogenation.
The single quantum coherence I1zI2x + I1xI2z develops under
the Hamiltonian for weakly coupled systems into the term
sin(πJ12t)(I1y + I2y). The sine modulation of the term (I1y + I2y),
corresponding to the coupling between the transferred parahy-
drogen protons, is responsible for the observed antiphase
splitting in the resonances of the PHIP spectra. By contrast,
the regular “thermal” resonances of species, which are chemi-
cally uninvolved in the hydrogenation reactions, experience a
cosine modulation governed by their respective couplings,
resulting in in-phase splittings of the resonances in the PHIP
NMR spectra. Likewise, couplings to protons which do not stem
As a characteristic example, Figure 2a shows the results
obtained upon the addition of parahydrogen to a solution of 10
mg [Rh(NBD)Cl]2 and 19 µL PMePh2 in acetone-d6 (Rh: P )
1:3). The strong polarized resonances of the dihydride protons
of the complex Rh(H)2Cl(PMePh2)3 are observed in the 1H NMR
spectrum, whereby the hydride trans to a PMePh2 ligand occurs
at δΗ ) -9.4 ppm, whereas the hydride trans to the chloride
has a higher chemical shift and appears at δH_ ) -17.6 ppm.
The latter is characteristic for hydride protons in the trans
position to such an electronegative ligand. The hydride reso-
nance at lower field shows a large coupling to one trans
phosphorus (JHP(trans) ) 178.6 Hz), an additional coupling to
two equivalent cis phosphorus nuclei (JHP(cis) ) 14.1 Hz), a
(8) Sørensen, O. W. Prog. NMR Spectrosc. 1989, 21, 503-569.

NMR of Rhodium Dihydride Complexes
Inorganic Chemistry, Vol. 40, No. 3, 2001 535
Figure 2. (a) The hydride region of the 1H NMR spectra obtained using a sample of 10 mg of [Rh(NBD)Cl]2 and 19 µL of PMePh2 in acetone-d6
(Rh:P ) 1:3) after the in situ addition of parahydrogen at 315 K. The spectrum shows the hydride resonances of the complex Rh(H)2Cl(PMePh2)3,
3, which have been recorded with eight scans. The hydrides have been enlarged above the original spectra. (b) 1H{31P} NMR spectra of the hydride
region with 31P broadband decoupling.
Scheme 1
coupling to the central rhodium (JHRh ) 13 Hz), and a coupling
to the upfield hydride proton (JHH_ ) -7.7 Hz). The coupling
between the two former parahydrogen protons causes the
antiphase character of this resonance, exhibiting emission and
absorption maxima, accordingly. The second hydride resonance
at higher field is a complex multiplet with couplings to rhodium
(JHRh ) 22 Hz), one equatorial and two axial phosphorus nuclei
(JHP(ax.) ) 16.5 Hz, JHP(eq.) ) 11 Hz), and the lower field hydride
proton (JHH_ ) -7.7 Hz). These data have been tested by
simulating the resulting multiplet and polarization pattern using
the computer program PHIP++, upon which very good agree-
ment has been obtained.9 If the 1H NMR spectrum is recorded
with complete 31P decoupling, both hydride resonances at -9.4
and -17.6 ppm collapse into a doublet of antiphase doublets
with the remaining 13 and 22 Hz coupling corresponding to
JHRh and JHRh_, respectively (Figure 2b). Analogous complexes
with other phosphine ligands have also been observed (Table
1).
If the temperature is elevated, the intensities of the polarized
hydride resonances increase significantly. The spectrum dis-
played in Figure 2a has been recorded at 315 K. Experiments
in acetone-d6 can be carried out up to about 330 K, since above
this value, boiling and associated evaporation of the solvent
becomes so significant that this deteriorates the resolution of
the spectra badly.
phosphines from PMe3 to P(n-butyl)3 (Table 1), which in turn
correlates with the activity of the corresponding complexes as
hydrogenation catalysts.
Furthermore, the intensities of the polarized hydride reso-
nances increase with temperature. Since these intensities cor-
relate with the rate of oxidative addition of parahydrogen to
the corresponding rhodium complexes and with the subsequent
decay of the so-formed dihydrides, both the rates of their
formation as well as the rates of their decay seem to increase
with temperature.
Formation of the Binuclear Complexes (H)(Cl)Rh(PMe3)2-
(µ-Cl)(µ-H)Rh(PMe3) and (H)(Cl)Rh(PMe2Ph)2(µ-Cl)(µ-H)-
Rh(PMe2Ph). Upon addition of parahydrogen to a solution of
[RhCl(NBD)]2 and PMe3 (ratio of Rh: P ) 1:3) in acetone-d6
at 315 K, intense polarization signals of the dihydride complex
Rh(H)2Cl(PMe3)3 (1) can be observed along with strongly
polarized resonances in the aliphatic region due to hydrogenation
of the NBD ligand. This complex 1 has been described earlier
by other authors.10 In later stages of the reaction, however, two
new resonances emerge at -18.2 and -20.2 ppm, respectively
(Figure 3a), which can be assigned to the hydride protons of
the complex 8 in Scheme 1. These signals too have antiphase
character due to the coupling between the two parahydrogen
protons (JHH′ ) -4.2 Hz). Thereby the hydride resonance at
By contrast, the resolution of the corresponding hydride
resonances deteriorates with increasing temperature. For better
resolution, therefore, a lower temperature is advantageous, since
the rate of phosphine dissociation slows down with decreasing
temperature, upon which the resonances sharpen.
The rate of the observed exchange reaction of the phosphine
ligands in the dihydrides increases in the above-listed series of
(9) Greve, T. PhD Thesis, Physics, University of Bonn 1996.

536 Inorganic Chemistry, Vol. 40, No. 3, 2001
Koch and Bargon
Figure 3. (a) The hydride region of the 1H NMR spectra obtained using a sample of 10 mg of [Rh(NBD)Cl]2 and 13.2 µL of PMe3 in acetone-d6
(Rh:P ) 1:2) after the in situ addition of parahydrogen at 320 K. The spectrum shows the hydride resonances of the complex Rh(H)2Cl(PMe3)3, 1,
1
and (H)(Cl)Rh(PMe3)2(µ-Cl)(µ-H)Rh(PMe3), 8. (b) H{31P} NMR spectra of the hydride region with 31P broadband decoupling (S ) acetone-d6).
Table 1. 1H NMR Data of the Reported Hydrides
no.
complex
1H chemical shifts δ (ppm) in acetone-d6 and coupling constants J (Hz)
1
Rh(H)2Cl(PMe3)3
-9.7 (1JHRh ) 15, 2JHP(trans) ) 178.6, 2JHP(cis) ) 20, 2JHH ) -7.5)
-19.0 (1JHRh ) 27, 2JHP(ax.) ) 19, 2JHP(eq.) ) 13, 2JHH ) -7.5,
-9.6 (1JHRh ) 18, 2JHP(trans) ) 172.5, 2JHP(cis) ) 18, 2JHH ) -7.2)
-18.2 (1JHRh ) 24.7, 2JHP(ax.) ) 16.5, 2JHP(eq.) ) 11, 2JHH ) -7.2)
-9.4 (1JHRh ) 13, 2JHP(trans) ) 164, 2JHP(cis) ) 14.1, 2JHH ) -7.7)
-17.6 (1JHRh ) 22, 2JHP(ax.) ) 15, 2JHP(eq.) ) 10, 2JHH ) -7.7)
-10.7 (1JHRh ) 14.7, 2JHP(trans) ) 161.7, 2JHP(cis) ) 16, 2JHH ) -7.9)
-19.8 (1JHRh ) 24.1, 2JHP(ax.) ) 16, 2JHP(eq.) ) 13, 2JHH ) -7.9)
-10.1 (1JHRh ) 15.4, 2JHP(trans) ) 162, 2JHP(cis) ) 16.3, 2JHH ) -7.3)
-19.3 (1JHRh ) 24.6, 2JHP(ax.) ) 17, 2JHP(eq.) ) 15.1, 2JHH ) -7.3)
-9.9 (1JHRh ) 12.5, 2JHP(trans) ) 155, 2JHP(cis) ) 13.3, 2JHH ) -7.9)
-18.3 (1JHRh ) 20, 2JHP(ax.) ) 12.5, 2JHP(eq.) ) 13.5, 2JHH ) -7.9)
-10.7 (1JHRh ) 13.5, 2JHP(trans) ) 163, 2JHP(cis) ) 16.7, 2JHH ) -7.8)
-19.8 (1JHRh ) 23, 2JHP(ax.) ) 15.8, 2JHP(eq.) ) 13.1, 2JHH ) -7.8)
-18.2 (1JHRh ) 25, 2JHP(cis) ) 17.3, 2JHH ) -4,2)
2
3
Rh(H)2Cl(PMe2Ph)3
Rh(H)2Cl(PMePh2)3
4
Rh(H)2Cl(PEt3)3
5
Rh(H)2Cl(PE t2Ph)3
6
Rh(H)2Cl(PEtPh2)3
7
Rh(H)2Cl(P(n-butyl)3)3
8
(H)(Cl)Rh(PMe3)2(µ-Cl)(µ-H)Rh(PMe 3)
(H)(Cl)Rh(PMe2Ph)2(µ-Cl)(µ-H)Rh(PMe2Ph)
Rh(H)2(CF3COO)(PPh3)3
Rh(H)2(CF3COO)(PEt3)3
Rh(H)2(CF3COO)(PEt2Ph) 3
Rh(H)2(CF3C OO)(P(n-butyl)3)3
-20.2 (1JHRh ) 30.3, 20, 2JHP(tans) ) 29, 2JHP(cis) ) 14.6, 2JHH ) -4,2)
-17.3 (1JHRh ) 23, 2JHP(cis) ) 15.4, 2JHH ) -4,2)
9
-20.0 (1JHRh ) 30, 21.4, 2JHP(trans) ) 29, 2JHP(cis) ) 15.5 2JHH ) -4,2)
-8.9 (1JHRh ) 6, 2JHP(trans) ) 161, 2JHP(cis) ) 12, 2JHH ) -9.5)
-19.1 (1JHRh ) 18, 2JHP(ax.) ) 17, 2JHP(eq.) ) 17, 2JHH ) -9.5)
-10.1 (1JHRh ) 14.8, 2JHP(trans) ) 160, 2JHP(cis) ) 16, 2JHH ) -8.7)
-22.5 (1JHRh ) 13, 2JHP(ax.) ) 17, 2JHP(eq.) ) 17, 2JHH ) -8.7)
-9.5 (1JHRh ) 15.5, 2JHP(trans) ) 161, 2JHP(cis) ) 16.8, 2JHH ) -8.6)
-21.9 (1JHRh ) 26.8, 2JHP(ax.) ) 16.7, 2JHP(eq.) ) 16.7, 2JHH ) -8.6)
-9.5 (1JHRh ) 15.5, 2JHP(trans) ) 161.5, 2JHP(cis) ) 16.7, 2JHH ) -7,1)
-21.9 (1JHRh ) 28.8, 2JHP(ax.) ) 17.5, 2JHP(eq.) ) 17.5, 2JHH ) -7.1)
10
11
12
13
-18.2 ppm exhibits a coupling to rhodium (JHRh ) 25 Hz) and
the cis phosphorus (JHP(cis) ) 17.3 Hz), whereas the second
hydride resonance consists of a complex multiplet with cou-
plings to the two inequivalent rhodium atoms (JHRh ) 30.3 Hz
and JRh′H ) 20 Hz, respectively), the two cis phosphorus nuclei
(JHP(cis) ) 14.6 Hz), and the trans phosphorus (JHP(trans) ) 29
Hz). This assignment is confirmed upon 31P decoupling of the
hydride protons: In the 1H{31P} NMR spectrum shown in Figure
3b, the signal at -18.2 ppm has collapsed into a doublet of
antiphase doublets with the 25 Hz coupling corresponding to
JHRh, while the second hydride resonance has simplified to a
doublet of doublet of antiphase doublets, with couplings of 30.3
and 20 Hz corresponding to JHRh and JHRh′, respectively.
Complexes of the type (H)(Cl)Rh(PMe3)2(µ-Cl)(µ-H)Rh(CO)-
(PMe3) and (H)(Cl)Rh(PMe3)2(µ-I)(µ-H)Rh(CO)(PMe3), the
NMR data of which correspond to our results, have been
(10) Duckett, S. B.; Eisenberg, R.; Goldman, A. S. J. Chem. Soc., Chem.
Commun. 1993, 1185-1187.

NMR of Rhodium Dihydride Complexes
Inorganic Chemistry, Vol. 40, No. 3, 2001 537
Scheme 2
detected using PHIP NMR spectroscopy.11,12 In these studies
the phosphine ligand at the rhodium(I) center is supposed to be
trans to the µ-hydride with a phosphorus coupling of JHP(trans)
) 30 and 32 Hz, respectively. Therefore, the structure in Scheme
1 is assigned to complex 8. Since in our experiments no carbon
monoxide is present, the fourth position of the Rh(I) center could
be occupied by the solvent (acetone-d6). The polarized reso-
nances in the aliphatic region due to hydrogenation of NBD
and its hydrogenation product NBE only occur at the beginning
of the reaction. At the same time complex 1 can be observed.
When the hydrogenation of NBD and NBE is completed only
complex 1 is present. The 1H NMR spectrum shows only
resonances of norbornane. With the proceeding hydrogenation
complex 8 emerges. Therefore, we have ruled out that the fourth
ligand at the rhodium(I) center is occupied by NBE. In the study
of Wilkinson’s catalyst, Duckett and Eisenberg have reported
binuclear complexes containing an olefin at the rhodium(I)
center.3
During the hydrogenation the solution is turning dark brown,
and black rhodium metal is precipitating. Even though complex
1 gradually disappears and complex 8 emerges during the
hydrogenation until eventually only the polarization signals of
8 are visible, adding new PMe3 to the solution reverses the
process, causing the resonances of complex 8 to get weaker
and those of complex 1 to reappear again. However, using an
excess of PMe3 from the beginning of the experiment on leads
to the appearance of polarization signals due to the complex
(H)2Rh(PMe3)4+, which has been described earlier by Schrock
and Osborn.13
Analogously, complex 9, with PMe2Ph as the phosphine
ligand instead of PMe3, can be obtained and its 1H PHIP NMR
spectrum can be observed under the same conditions. The
formation of this binuclear rhodium dihydride can be explained
according to Scheme 2.
The five-coordinated species Rh(H)2Cl(PMe3)2 is formed after
phosphine dissociation from 1 and H2 addition to the 14-electron
complex RhCl(PMe3)2. The resulting 16-electron species coor-
dinates an alkene under catalytic conditions to form an addition
complex which functions as a key intermediate in the catalytic
cycle. Subsequently, this coordinatively unsaturated dihydride
completes its coordination shell via complexation with the
chloride ligand of RhCl(PMe3)2, which is followed by displace-
ment of a phosphine to form the Rh(III)/Rh(I) binuclear complex
8. The identity of the 14-electron species RhCl(PMe3)2 has been
confirmed for the phosphine ligands PMe3, P(tolyl)3, and PPh3
before by other authors.14,15 In particular, the reaction of
Wilkinson’s catalyst RhCl(PPh3)3 with H2 leads at elevated
temperature to the formation of the Rh(III)/Rh(I) binuclear
complex (H)2Rh(PPh3)2(µ-Cl)2Rh(PPh3)2, which was first identi-
fied by Tolman et al.4 This complex is in equilibrium with Rh-
(PPh3)2(µ-Cl)2Rh(PPh3)2, which is a dimerization product of
RhCl(PPh3)2.
A related complex can also be observed with PMe3 as the
phosphine, if the hydrogenation is carried out at a low rhodium/
(11) Duckett, S. B.; Eisenberg, R. J. Am. Chem. Soc. 1993, 115, 5292-
5293.
(12) Morran, P. D.; Colebrooke, S. A.; Duckett, S. B.; Lohman, J. A. B.;
Eisenberg, R. J. Chem. Soc., Dalton Trans. 1998, 3363-3365.
(13) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 2397-
2406.
(14) Wink, D.; Ford, P. C. J. Am. Chem. Soc. 1985, 107, 1794-1796.
(15) Spillet, C. T.; Ford, P. C. J. Am. Chem. Soc. 1989, 111, 1932-1933.

538 Inorganic Chemistry, Vol. 40, No. 3, 2001
Koch and Bargon
1
Figure 4. (a) The hydride region of the H NMR spectra obtained using a sample of 10 mg of Rh(NBD)(acac), 13.5 mg of PPh3, and 1 µL of
CF3COOH in acetone-d6 (Rh:P ) 1:3) after the in situ addition of parahydrogen at 300 K. The spectrum shows the hydride resonances of the
complex Rh(H)2(CF3COO)(PPh3)3, 10, which have been recorded with eight scans. The hydrides have been enlarged above the original spectra. (b)
1H{31P} NMR spectra of the hydride region with 31P broadband decoupling.
Scheme 3
additional coupling of 12 Hz to the cis phosphorus, a 6 Hz
coupling to rhodium, and an antiphase coupling due to the
second hydrogen (JHH′ ) -9.5), which causes the observed
emission and absorption maxima. The second hydride proton,
whose chemical shift at higher field is characteristic for hydrides
in the trans position to a rather electronegative ligand (here
trifluoroacetate), gives a complex multiplet with couplings to
rhodium (JHRh ) 18 Hz), two equivalent cis phosphine ligands
(JHP ) 17 Hz), and the other hydride proton at lower field (JHH′
1
) -9.5 Hz). If the H PHIP NMR spectrum is recorded with
complete 31P decoupling, both hydride resonances at -8.9 and
-19.1 ppm collapse into a doublet of antiphase doublets with
the remaining 6 and 18 Hz coupling corresponding to JHRh and
phosphine ratio (Rh: P ) 1:1) at 325 K. However, this complex
is characterized by an unpolarized resonance at -19.8 ppm,
possibly of a monohydride, which consists of a doublet of triplets
with JHRh ) 24.7 Hz and JHP ) 19.4 Hz, respectively.
Generation of Rh(NBD)(PPh3)3(CF3COO) and Its Dihy-
dride. The dihydrido rhodium(III) complexes as described above
so far have chloride as the electronegative ligand. The latter
can readily be replaced by trifluoroacetate. For this purpose, a
solution of Rh(NBD)(acac) (acac ) acetylacetonate) and PPh3
in acetone-d6 is treated with CF3COOH. Upon that, the initial
acetylacetonate ligand is displaced from the rhodium center after
protonation (Scheme 3).
J
HRh′, respectively (Figure 4b).
Additional dihydride complexes can be observed using PEt3,
PEt2Ph, or P(n-butyl)3 as the phosphine ligands. In the case of
the ligands PEt2Ph or P(n-butyl)3, the resonances of the
dihydrides are broadened due to the fast exchange of these
phosphine ligands, which blurs the resonances of the hydride
protons. Upon cooling the sample, the rate of this exchange
process can be slowed, resulting in improved resolution, as is
evident in Figure 1.
As a characteristic example, a solution of Rh(NBD)(acac)
and PEt2Ph in acetone-d6 has been treated with CF3COOH.
Adding parahydrogen generates the complex Rh(H)2(CF3COO)-
(PEt2Ph)3 (12). Hydrogenation at decreasing temperatures (290,
273, and 263 K, respectively) results in a line narrowing of the
hydride resonances. At 263 K the multiplets are well-resolved.
However, upon decreasing the temperature, the intensities of
the hydride resonances get weaker, because the rate of para-
hydrogen addition slows down, until below 253 K the rhodium
dihydride, 12, cannot be observed anymore.
Adding parahydrogen to this three-component solution leads
to the occurrence of strongly polarized resonances of norbornene
1
and norbornane in the aliphatic region of the H PHIP NMR
spectrum, due to hydrogenation of the NBD ligand. In addition,
two hydride resonances of the complex Rh(H)2(CF3COO)(PPh3)3
(10) emerge at -8.9 and -19.1 ppm, respectively (Figure 4a).
The resonance of the hydride proton at lower field exhibits a
trans phosphorus coupling (JHP
) 161 Hz) with an
(trans)

NMR of Rhodium Dihydride Complexes
Inorganic Chemistry, Vol. 40, No. 3, 2001 539
1
Figure 5. The hydride region of the H NMR spectra obtained using a sample of 10 mg of Rh(NBD)(acac), 8.5 mg of PEt2Ph, and 1 µL of
CF3COOH in acetone-d6 (Rh:PEt2Ph ) 1:3) after the in situ addition of parahydrogen at different temperatures. The spectrum shows the hydride
resonances of the complex Rh(H)2(CF3COO)(PEt2Ph)3, 12, which have been recorded with eight scans.
be bubbled through the reaction mixture for a defined period of time
followed by the detection pulse of the NMR experiment. In most cases,
a hydrogenation time of 3 s has turned out to be sufficient. About 2 s
after the hydrogen addition has stopped, the NMR detection can start.
This procedure can be repeated to afford a subsequent accumulation
of several scans, which yields a good signal-to-noise ratio of the PHIP
resonances and allows us to suppress signals of components, which
are uninvolved in the hydrogenation as outlined above. The spectrum
in Figure 2 is the result of the accumulation of 16 scans, thereby
alternating the flip angles between -π/4 and +3π/4, respectively, to
suppress any thermal signals.
Upon using either acetic acid or tetrafluoroboronic acid,
however, instead of trifluoroacetic acid, no analogous rhodium-
containing dihydrides can be observed.
A few similar complexes containing rhodium have been
described before by other authors.16
Experimental Section
The two complexes, i.e., [RhCl(NBD)]2 and Rh(NBD)(acac), as well
as trifluoroacetic acid and the phosphines PMe3, PMe2Ph, PMePh2, PEt3,
PEt2Ph, PEtPh2, and P(n-butyl)3, have all been purchased from Aldrich
and used as obtained. The solvent acetone-d6 (Eurisotop) has been dried
using standard methods and stored under argon prior to use. Chemical
shift values are given in ppm using acetone-d6 at 2.05 ppm as internal
reference.
The spectra were recorded using either a Bruker AC 200 or a Bruker
AMX 200 FT-NMR spectrometer, whereby the latter allowed 31P
decoupling. All spectra are the result of the accumulation of eight scans
while alternating the flip angle between -π/4 and +3π/4, respectively,
to suppress any thermal signals.
General Procedure for the in Situ Generation of the Complexes
Rh(H)2ClL3 (L ) Phosphine). In a typical experiment, the rhodium
complex [RhCl(NBD)]2 (10 mg) and the corresponding amount of
phosphine ligand (Rh:P ) 1:3) are placed into an NMR tube together
with 700 µL of degassed acetone-d6. Parahydrogen is then bubbled in
situ through the solution within the magnetic field of the spectrometer
using the apparatus as outlined above.
General Procedure for the in Situ Generation of the Complexes
Rh(H)2(CF3COO)L3 (L ) Phosphine). Likewise, the complex Rh-
(NBD)(acac) (10 mg) together with trifluoroacetic (1 µL) acid and the
corresponding amount of phosphine ligand (Rh:P ) 1:3) is dissolved
in 700 µL of acetone-d6 and treated as described above.
In Situ Hydrogenation Using Parahydrogen. Para-enriched hy-
drogen containing about 50% para-H2 is prepared by passing H2 through
activated charcoal at 77 K at a pressure of 3 bar as described in the
literature.5
The hydrogenation with parahydrogen is carried out in situ (Figure
1). For this purpose, a glass capillary connected to the parahydrogen
source can be lowered into the NMR probe actuated by an electro-
magnet, i.e., by a solenoid, which is electrically controlled by the
computer of the NMR spectrometer. In this fashion, parahydrogen can
Acknowledgment. We gratefully acknowledge the financial
support obtained from the Volkswagen Foundation, the Deutsche
Forschungsgemeinschaft (DFG), the German Federal Ministry
for Science and Technology (BMBF), the Forschungsverband
Nordrhein-Westfalen, and the Fonds der Chemischen Industrie,
Frankfurt (Main), Germany.
(16) Esteruelas, M. A.; Lahuerta, O.; Modrego, J.; Nu¨rnberg, O.; Oro, L.
A.; Rodriguez, L.; Sola, E.; Werner H. Organometallics 1993, 12,
266-275.
IC991319R