Trinuclear rhodium hydride complexes

Article abstract of DOI:10.1039/c0dt01564d

Three novel trinuclear rhodium hydride complexes of the type {[Rh(PP*)H]₃(μ₂-H)₃(μ₃- H)}[BF₄]₂ containing diphosphines Tangphos, t-Bu-BisP* and Me-DuPHOS have been synthesised. The new compounds are very stable. Their structures have been characterized by X-ray analysis in the solid state and by NMR-spectroscopic investigations in solution.

Full text of DOI:10.1039/c0dt01564d

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PAPER
Trinuclear rhodium hydride complexes†‡
Christian Fischer, Christina Kohrt, Hans-Joachim Drexler, Wolfgang Baumann and Detlef Heller*
Received 11th November 2010, Accepted 7th February 2011
DOI: 10.1039/c0dt01564d
Three novel trinuclear rhodium hydride complexes of the type {[Rh(PP*)H]3(m2-H)3(m3-H)}[BF4]2
containing diphosphines Tangphos, t-Bu-BisP* and Me-DuPHOS have been synthesised. The new
compounds are very stable. Their structures have been characterized by X-ray analysis in the solid state
and by NMR-spectroscopic investigations in solution.
Since the introduction of the Wilkinson catalyst,1 rhodium hydride
complexes have been intensively investigated and several X-ray
structures have been reported.2
While cationic complexes with monodentate phosphines of the
type [Rh(diene)(PR3)2]+, which were among others described by
Osborn et al., give respective hydrides in the presence of hydrogen,3
the related complexes with diphosphines usually result in solvate
complexes.4
Prompted by these findings, the objective of this work was the
However, there are exceptions. When [Rh(nbd)(o-anisyl-methyl-
phenylphosphane)2]BF4 was hydrogenated in methanol only the
solvate complex was formed, a dihydride could not be detected.5
Vice versa, dinuclear hydride complexes were formed in several
solvents upon hydrogenation of analogous cationic complexes
containing chelating, electron rich PP¢ ferrocenyl phosphines.6
The importance of the electronic donor properties of the
ligand for the stability of the relative hydride complexes has
been known for quite some time. In this respect, it is possible
to classify chelating diphosphines into established, electron-poor
diarylphosphines and more recent, electron-rich ligands such as
DuPHOS, Tangphos and BisP*.7
search for novel rhodium hydride complexes with electron-rich
diphosphine ligands.
The quantitative transformation of cationic precatalysts
[Rh(PP*)(diolefin)]X (PP* = chelating diphosphine; diolefin =
cod cis,cis-1,5-cyclooctadiene or nbd norbornadiene; X = non-
-
-
-
coordinating anion e.g. BF4 , ClO4 , PF6 ) into the respec-
tive catalytically active solvate complexes [Rh(PP*)(solvent)2]X
has been extensively described in the literature for a large
number of ligands in several solvents such as MeOH or
THF.9
The pseudo first-order rate constants for the hydrogenation of
the diolefin rhodium complexes of Tangphos and t-Bu-BisP* in
MeOH at 25 ◦C under 1 bar hydrogen pressure are reported in
Table 1.9 The molecular structures of these complexes, except
for [Rh(t-Bu-BisP*)(nbd)]BF4,8h can be found in the supporting
information.‡
Several solvent dihydride rhodium complexes featuring P-
chiral BisP* ligands have been described by e.g. Gridnev and
Imamoto, however, such complexes are stable only at sufficiently
low temperatures.8
From the data of Table 1 it follows that the stoichiometric
hydrogenation of e.g. [Rh(Tangphos)(cod)]BF4 in MeOH at 25 ◦C
is complete after 13 min: this value, which corresponds to 99.2%
conversion, is obtained by multiplying the half-life time t1/2
by 7.
Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock, Albert-
Einstein-Straße 29a, 18059, Rostock, Germany. E-mail: detlef.heller@
catalysis.de; Fax: (+49) 381-1281-51183; Tel: (+49) 381-1281-183
† Dedicated to Tsuneo Imamoto.
‡ Electronic supplementary information (ESI) available: X-Ray data
and experimental procedures. CCDC reference numbers 793499–
793506. CCDC 793506 for [Rh((S,S,R,R)-Tangphos)(cod)]BF4, CCDC
793500 for [Rh((S,S,R,R)-Tangphos)(nbd)]BF4, CCDC 793505 for
[Rh((R,R)-t-Bu-BisP*)(cod)]BF4, CCDC 793504 for {[Rh((R,R)-t-Bu-
Table 1 Pseudo first-order rate constants for diolefin hydrogenation in
MeOH at 25.0 ◦C and 1.01 bar overall pressure
Complex
kcod [1/min]a
knbd [1/min]a
1
2
BisP*)H]3(m2-H)3(m3-H)}(BF4)2· i-PrOH, CCDC 793499 for {[Rh((S,S)-
Me-DuPHOS)H]3(m2-H)3(m3-H)}(BF4)2·2EtOEt, CCDC 793501 for
[Rh(Tangphos)(diolefin)]BF4
[Rh(t-Bu-BisP*)(diolefin)]BF4
0.375
0.21
194.4
89.9
{[Rh((S,S,R,R)-Tangphos)H]3(m2-H)3(m3-H)}(BF4)2·MeOH,
CCDC
793502 for {[Rh((S,S,R,R)-Tangphos)H]3(m2-H)3(m3-H)}(BF4)2·2CH2Cl2
and CCDC 793503 for {[Rh((S,S,R,R)-Tangphos)H]3(m2-H)3(m3-
a Due to isobaric conditions, rate constants are pseudo constants which
contain the hydrogen solubility under experimental conditions.
1
2
H)}(BF4)2· toluene. For ESI and crystallographic data in CIF or other
electronic format, see DOI: 10.1039/c0dt01564d
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After this time, the 31P-NMR spectrum should bear no trace of
the starting complex and only show the doublet due to the solvate
complex [Rh(Tangphos)(MeOH)2]BF4. As shown in Fig. 1 (top),
already after a few hours under the hydrogen atmosphere, other
hitherto unknown species giving rise to broad resonances can be
detected besides the solvate complex ((31P) d ppm = 123.9 ppm;
1J(31P,103Rh) = 203.4 Hz).
1
Fig. 1 31P{ H}-NMR spectra of the hydrogenation solution after hydro-
genation of [Rh(Tangphos)(cod)]BF4 in CH3OH-d4 (top) and of the pale
yellow precipitate redissolved in CH2Cl2-d2 (bottom). The labelled broad
absorption is due to an unknown species.
Fig.
2
Molecular structure of the cation in {[Rh(Tang-
phos)H]3(m2-H)3(m3-H)}[BF4]2 (ORTEP, 30% probability ellipsoids,
top) and scheme with 1H-NMR chemical shifts for hydrides (bottom).
All seven hydrides could be refined. Hydrogen atoms of the ligand
were omitted for clarity. Selected distances [A] and angles [ ]: Rh–Rh
2.771–2.789(1); Rh–m3-H–Rh 99–100(2); Rh–m2-H–Rh 107–108(2); chiral
space group (P212121) with a Flack parameter of -0.02(3).
Analogous results were gained for the cationic complexes with
t-Bu-BisP* and Me-DuPHOS, see supporting information for
NMR spectra.‡
Single crystals suitable for X-ray analysis precipitated from
MeOH.§ For the analogous complexes with t-Bu-BisP* and Me-
DuPHOS, X-ray quality crystals were isolated from i-PrOH and
MeOH respectively.
The X-ray structure of the isolated Tangphos complex is shown
in Fig. 2 (top), see supporting information for the molecular
structures of the other complexes.‡ It features three rhodium
centers bridged by one m3-hydride, three edge bridging m2-hydrides
and one terminal hydride on each rhodium atom: all hydrides
were refined from electron density. The unit cell also contains two
BF4 anions which proves that the trinuclear hydride complex is a
rhodium(III) species.10
◦
˚
1,3-bis(diphenylphosphino)propane (dppp) as chelating diphos-
phine having a structure similar to that shown in Fig. 2 was
characterized by Pignolet et al.14
Rhodium–rhodium distances for the isolated complexes
{[Rh(PP*)H]3(m2-H)3(m3-H)}[BF4]2 with ligands Tangphos
˚
˚
(2.771–2.789 A), t-Bu-BisP* (2.772–2.793 A) and Me-
˚
DuPHOS (2.776–2.790 A) are practically identical to the
12a
˚
ones in {HRh[P(OCH3)3]2}3 (2.780–2.856 A).
The 1H- and 31P-NMR spectra of the three trinuclear complexes
show common features (Table 2).
In the hydride region, in the low-frequency end of the spectrum,
a set of three complex multiplets, having 3 : 1 : 3 relative intensity,
is observed. Intensity ratios between these signals and those
relative to the ligands15 confirm that each molecule contains three
While dinuclear rhodium hydride species with chelating diphos-
phines have been known for some time and their structure
proved by X-ray analysis,6,11 the present trinuclear rhodium
hydride complexes represent, to the best of our knowledge,
the first examples of their kind. A similar structure with mon-
odentate trimethylphosphite has been described by Muetterties
et al., namely tris((m-hydrido)-(bis(trimethylphosphite))-rhodium)
{HRh[P(OCH3)3]2}3.12,13 Furthermore, an iridium complex with
bisphosphane ligands and seven hydrides (Fig. 3). As the core
1
2
of the complex, Rh3H7P6, is made of sixteen nuclei with I =
,
spin–spin coupling leads to complicated line patterns. Analysis
is not straightforward and additionally hampered by magnetic
inequivalence, e.g. of the phosphorus atoms (see below). However,
recording 1H-NMR spectra under phosphorus decoupling results
in simpler spectra that allow (with some approximations, see the
supporting information for details‡) interpretation according to
first-order rules.
§ Crystal data for {[Rh((S,S,R,R)-Tangphos)H]3(m2-H)3(m3-H)}(BF4)2·
MeOH: C49H103B2F8OP6Rh3, M = 1376.43, orthorhombic, a = 11.2487(2)
◦
◦
◦
˚
˚
˚
˚
A, b = 20.2463(2) A, c = 28.4783(4) A, a = 90.00 , b = 90.00 , g = 90.00 ,
3
V = 6485.78(16) A , T = 200(2) K, space group P212121, Z = 4, 71 877
reflections measured, 13 762 independent reflections (Rint = 0.0384). The
final R1 values were 0.0355 (I > 2s(I)). The final wR(F2) values were
0.0820 (I > 2s(I)). The final R1 values were 0.0439 (all data). The final
wR(F2) values were 0.0837 (all data). The goodness of fit on F2 was 0.944.
Flack parameter = -0.02(3).
The resonance at lowest frequency (relative intensity 3) becomes
a doublet and should thus be due to a hydride bound to a single
rhodium nucleus (s-H, 1JRh,H 16–17 Hz), the middle one (relative
intensity 1) becomes a regular quartet and must be due to a hydride
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Table 2 Chemical shifts [ppm] for rhodium hydride complexes {[Rh(PP*)H]3(m2-H)3(m3-H)}[BF4]2 (C3-symmetry) in CD2Cl2 at 297 K
Chelate ligand PP*
d(1H) s-H
d(1H) m2-H
d(1H) m3-H
d(31P)a Pcis
d(31P)a Ptrans
d(103Rh)b
Tangphos
t-Bu-BisP*
Me-DuPHOS
-15.63
-15.72
-14.26
-8.75
-8.42
-9.81
-11.01
-10.96
-9.81
133.8
95.0
109.4
122.1
81.6
90.3
510
430
328
a cis and trans refer to the m2-H (cf. Fig. 2). b Relative to N (103Rh) = 3.16 MHz.
1
Fig. 4 31P-103Rh{ H}-NMR spectrum of {[Rh(t-Bu-BisP*)H]3(m2-H)3
(m3-H)}(BF4)2 in CH2Cl2-d2 All Rh-atoms are chemically equivalent. The
corresponding chemical shift is 430 ppm.
Fig. 3 1H-NMR spectra of {[Rh(PP*)H]3(m2-H)3(m3-H)}[BF4]2 with
Tangphos (top), t-Bu-BisP* (middle) and Me-DuPHOS (bottom) (each
0.01 mmol in 0.7 ml CH2Cl2-d2); left: ligand characteristic signals; right:
hydride region.
IR and Raman spectroscopies were applied to further character-
ize the rhodium–hydride bonds. The rhodium–hydride vibration
of the isolated crystals could be detected at a wave number of 2060
cm-1, see supporting information.‡
bound to three rhodium atoms (m3-H, 1JRh,H 23–24Hz), and the one
at highest frequency (relative intensity 3) becomes a triplet (1JRh,H
ca. 22 Hz) and is assigned to the hydride bridging two rhodium
atoms (m2-H). Homonuclear couplings (2JH,H) are not resolved but
While the solvate-dihydride complexes with diphosphines so far
described in the literature were only detectable at very low temper-
atures, the three present hydride complexes are surprisingly stable
at room temperature. When nbd in [Rh(t-Bu-BisP*)(nbd)]BF4
is hydrogenated at -20 ◦C and the solution is cooled down
to -90 ◦C immediately afterwards only the solvate-dihydride
complex [RhH2(t-Bu-BisP*)(MeOH)2]BF4 is observed as our own
measurements confirm.8g However, if nbd is hydrogenated at room
temperature and the solution is slowly cooled down, both the
solvate-dihydride and the trinuclear rhodium hydride complex can
be detected at the same time (Fig. 5).
1
are present as proved by a H,1H-COSY experiment. Chemical
shifts and coupling constants are similar to those described for
other polynuclear rhodium hydride species.6,12
1
The 31P{ H}-NMR spectra show two signals of equal intensity.
Hence, the Rh3 triangle does not represent a mirror plane,
the phosphorus atoms within each bisphosphane are chemically
inequivalent, and the symmetry-related nuclei become also mag-
netically inequivalent by coupling to rhodium and the hydrides.
1
The appearance of the spectrum is dominated by the large JRh,P
coupling which brings about splittings of 100–110 Hz. Further
couplings to chemically and magnetically inequivalent nuclei lead
to higher-order splittings (visible in Fig. 1, bottom spectrum) that
were not analyzed further. Without proton decoupling, the signal
at lower frequency becomes a very broad triplet due to a proton
coupling of the same magnitude as the rhodium coupling. The
corresponding splitting (94–99 Hz) is found at the m2-H signal.
Assuming this to be a trans coupling 2JP,H , the 31P signal at
lower frequency is assigned to the phosphorus atoms trans to the
bridging hydrides (P1, P3, P5 in Fig. 2) and the other one to the
phosphorus atoms trans to the m3-hydride (P2, P4, P6 in Fig. 2).
To further characterize the trinuclear rhodium hydride com-
plexes 103Rh-NMR measurements were carried out (Fig. 4).16
Relevant NMR data of all three complexes are reported in Table
2. All Rh-atoms are chemically equivalent. The corresponding
chemical shift is 430 ppm.
Fig. 5 1H-NMR spectrum recorded after hydrogenation of nbd in
[Rh(t-Bu-BisP*)(nbd)]BF4 in MeOH at room temperature and slow
cooling of the sample to -90 ◦C. Both complexes, the dihydride-sol-
vate complex [RhH2(t-Bu-BisP*)(MeOH)2]BF4 (cyan) and the trinuclear
hydride complex {[Rh(t-Bu-BisP*)H]3(m2-H)3(m3-H)}(BF4)2 (red) can be
found in solution.
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The trinuclear rhodium hydride complexes are stable enough to
coexist with the solvate complex in methanol (Fig. 1, bottom), the
same way arene bridged dimers can be in equilibrium with solvate
complexes.17
To investigate whether the hydrides of the trinuclear rhodium
hydride complexes can be transferred onto (prochiral) olefins,
crystalline {[Rh(t-Bu-BisP*)H]3(m2-H)3(m3-H)}[BF4]2 and a 1.3-
fold mol excess of methyl acetylaminocinnamate (mac) per
equivalent rhodium were dissolved in MeOH-d4 in an NMR tube
and the tube sealed under an argon atmosphere. After ca. 10 h
by the small amount of hydrogen that can be released from the
trinuclear rhodium hydride complexes.
Owing to their slow formation and their low activity in hydride
transfer, these trinuclear rhodium hydride complexes most proba-
bly only have to be taken into consideration in hydrogenations in
polar solvents under drastic conditions. Their role might become
relevant when hydrogenations are carried out in non-polar solvents
such as dichloroethane when neither solvate-dihydride complexes
nor arene bridged dimers, because of the lack of aryl fragments
on the diphosphine, can be formed.19
8
7% of the catalyst-substrate complex [Rh(t-Bu-BisP*)(mac)]BF4
Trinuclear rhodium hydride complexes {[Rh(PP*)H]3(m2-
20
is present in solution as shown by the 31P-NMR spectrum (Fig. 6).
The solvate complex (89.7 ppm, 205.5 Hz) was not detected at any
time since the “hydride content” of the trinuclear rhodium hydride
complex is not sufficient for complete hydrogenation of the olefin.
H)3(m3-H)}[BF4]2 with diphosphines Tangphos, t-Bu-BisP* and
Me-DuPHOS have been described for the first time. The complexes
are rather stable. They are slowly formed from the corresponding
solvate complexes in the presence of hydrogen at room temper-
ature, eventually reducing the concentration of active catalyst
species available for asymmetric hydrogenation. All three com-
plexes were characterized by X-ray analysis; structural features
are supported by NMR-spectroscopic investigations. The hydrides
which were found in the molecular structures can be transferred
onto olefins, although the activity is low. Further studies are
ongoing and will be reported in due course.
We thank Prof. X. Zhang and Chiralquest for providing some
Tangphos ligand and the DFG for financial support.
Notes and references
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4 J. Halpem, D. P. Riley, A. S. C. Chan and J. J. Pluth, J. Am. Chem.
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Fig.
6
Time-dependent 31P-NMR spectra of
a
mixture of
{[Rh(t-Bu-BisP*)H]3(m2-H)3(m3-H)}(BF4)2 (red) and mac (1 : 1.3)
in MeOH-d4 under argon. The green labelled signals are due
6
to the h -arene complex with the hydrogenated product H2-mac,
6
([Rh(t-Bu-BisP*)(H2-h -mac)]BF4: d 86.8 ppm, JP–Rh = 204 Hz, see also
reference8g). The blue labelled signals correspond to the catalyst-substrate
complex ([Rh(t-Bu-BisP*)(mac)]BF4: d 86.8 ppm, JP–Rh = 204 Hz, see also
reference 8g).
As the sequence of 31P{ H}-NMR spectra reported in Fig.
1
6 shows, after 25 days the trinuclear rhodium hydride com-
plex is practically no longer present in solution, being com-
pletely converted to the catalyst-substrate complex ([Rh(t-Bu-
BisP*)(mac)]BF4 and the h -arene complex8g with the hydro-
6
6
genated product H2-mac, ([Rh(t-Bu-BisP*)(H2-h -mac)]BF4.
The hydrogenation product was 90.1% enantiopure (mea-
sured only after ca. 25 days). For the same system (Rh/t-
Bu-BisP*/mac/MeOH) a value of >99% ee is reported in the
literature.8d After 25 days, approximately 5% of the mac used in
excess could still be detected in solution by GC.
Since the trinuclear complexes are unlikely to be hydrogenation
active18 due to their steric demands, decomposition products may
form which are responsible for the observed hydrogenation of the
olefins. Likely, a reversion of the trimer formation – from solvate
complexes under a hydrogen atmosphere – takes place.6b,8e,13b The
low activity of the hydrogen transfer might be simply explained
7 J. M. Brown, The Handbook of Homogeneous Hydrogenation, vol. 3, ed.
J. de Vries and C. Elsevier, Wiley-VCH Verlag GmbH & Co. KGaA,
Weinheim, ch. 31, 2007, 1073–1103.
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Gridnev, Y. Yamanoi, N. Higashi, H. Tsuruta, M. Yasutake and T.
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Gridnev and T. Imamoto, Organometallics, 2001, 20, 545; (g) I. D.
Gridnev, N. Higashi, K. Asakura and T. Imamoto, J. Am. Chem. Soc.,
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2000, 122, 7183; (h) T. Imamoto, J. Watanabe, Y. Wada, H. Masuda, H.
Yamada, H. Tsuruta, S. Matsukawa and K. Yamaguchi, J. Am. Chem.
Soc., 1998, 120, 1635.
9 A. Preetz, H.-J. Drexler, C. Fischer, Z. Dai, A. Bo¨rner, W. Baumann, A.
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Chem. Commun., 1988, 466; (c) A. Preetz, W. Baumann, H.-J. Drexler,
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14 H.-H. Wang, A. L. Casalnuovo, B. J. Hohnson, A. M. Mueting and L.
H. Pignolet, Inorg. Chem., 1988, 27, 325.
15 Tangphos: two t-Bu moieties per ligand; t-BisP*: two t-Bu moieties
per ligand; Me-DuPHOS: four protons of the phenyl bridge per
ligand.
16 J. M. Ernsting, S. Gaemers and C. J. Elsevier, Magn. Reson. Chem.,
2004, 42, 721.
10 In the case of {[Rh(Tangphos)H]3(m2-H)3(m3-H)}(BF4)2·MeOH all 7
hydrides could be refined from the electron density, only the m3-bridging
hydride in the case of {[Rh(Me-Duphos)H]3(m2-H)3(m3-H)}(BF4)2, and
none in {[Rh(t-Bu-BisP*)H]3(m2-H)3(m3-H)}(BF4)2.
17 T. Schmidt, Z. Dai, H.-J. Drexler, W. Baumann, C. Ja¨ger, D. Pfeifer
and D. Heller, Chem.–Eur. J., 2008, 14, 4469; A. Preetz, W. Baumann,
C. Fischer, H.-J. Drexler, T. Schmidt, R. Thede and D. Heller,
Organometallics, 2009, 28, 3673.
18 A similar trinuclear iridium hydride phosphinooxazoline (PHOX)
complex was also proved not to be active in hydrogenation. S. P.
Smidt, A. Pfaltz, E. Martinez-Viviente, P. S. Pregosin and A. Albinati,
Organometallics, 2003, 22, 1000.
19 Preliminary experiments on the hydrogenation of [Rh(t-Bu-
BisP*)(nbd)]BF4 in DCE confirm the immediate formation of hydrides
as well as a trinuclear species.
20 Multinuclear rhodium hydride complexes with monodentate phos-
phines have recently been investigated, especially by Weller et al.: S.
K. Brayshaw, J. C. Green, R. Edge, E. J. L. McInnes, P. R. Raithby, J.
E. Warren and A. S. Weller, Angew. Chem., Int. Ed., 2007, 46, 7844; S.
K. Brayshaw, A. Harrison, J. S. McIndoe, F. Marken, P. R. Raithby, J.
E. Warren and A. S. Weller, J. Am. Chem. Soc., 2007, 129, 1793 and
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11 M. D. Fryzuk, W. E. Piers, F. W. B. Einstein and T. Jones, Can. J.
Chem., 1989, 67, 883; K. Tani, T. Yamagata, Y. Tatsuno, T. Saito,
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Einstein and T. Jones, Acta Crystallogr., Sect. C: Cryst. Struct.
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12 (a) R. K. Brown, J. M. Williams, A. J. Sivak and E. L. Muetterties,
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Reddy, A. J. Sivak, W. R. Pretzer and E. L. Muetterties, J. Am. Chem.
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13 Trinuclear rhodium complexes containing no hydrides: (a) J. Halpern,
D. P. Riley, A. S. C. Chan and J. J. Pluth, J. Am. Chem. Soc., 1977, 99,
8055; (b) T. Yamagatta, K. Tani, Y. Tatsuno and T. Saito, J. Chem. Soc.,
4166 | Dalton Trans., 2011, 40, 4162–4166
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