Confining phosphanes derived from cyclodextrins for efficient regio- and enantioselective hydroformylation

Article abstract of DOI:10.1002/anie.201311291

Two confining phosphane ligands derived from either α- or β-cyclodextrin produce singly P^III-ligated metal complexes with unusual coordination spheres. High-pressure NMR studies have revealed that rhodium hydride complexes of the same type are also formed under hydroformylation conditions. This unique feature strongly favors the formation of the branched aldehyde at the expense of the linear one with high enantioselectivity in the rhodium-catalyzed hydroformylation of styrene. Rhodium in confinement: α- and β-cyclodextrin derivatives bearing introverted P^III donor atoms readily form monophosphane complexes in which the cyclodextrin cavity tightly wraps around the metal center. When used as ligands in the rhodium-catalyzed hydroformylation of styrene, they lead to both high isoselectivity and enantioselectivity.

Full text of DOI:10.1002/anie.201311291

Angewandte
Chemie
DOI: 10.1002/anie.201311291
Homogeneous Catalysis
Confining Phosphanes Derived from Cyclodextrins for Efficient Regio-
and Enantioselective Hydroformylation**
Matthieu Jouffroy, Rafael Gramage-Doria, Dominique Armspach,* David Sꢀmeril,
Werner Oberhauser, Dominique Matt,* and Loic Toupet
Abstract: Two confining phosphane ligands derived from
either a- or b-cyclodextrin produce singly PIII-ligated metal
complexes with unusual coordination spheres. High-pressure
NMR studies have revealed that rhodium hydride complexes of
the same type are also formed under hydroformylation
conditions. This unique feature strongly favors the formation
of the branched aldehyde at the expense of the linear one with
high enantioselectivity in the rhodium-catalyzed hydroformy-
lation of styrene.
deeply in a cavity which obstructs the binding of a second
phosphane after metal complexation.[1d,f,g,6]
In the rhodium-catalyzed hydroformylation of olefins,
bulky phosphanes are typically employed for increasing
isoselectivity.[7] Chiral versions thereof,[8] including cyclodex-
trin-containing phosphanes,[9] should thus be highly relevant
to enantioselective hydroformylation.[10] However, it is gen-
erally believed that high isoselectivity is incompatible with
high enantioselectivity and vice versa.[11]
We now describe the high performance of two crowded,
chiral phosphanes, namely HUGPHOS-1[12] and HUGPHOS-
2,[13] in the asymmetric hydroformylation of styrene. Both
ligands, which are respectively derived from a- and b-
cyclodextrin, have their phosphorus lone pair directed
towards the appended cyclodextrin (CD) core. We antici-
pated that these introverted ligands would not only tightly
embrace a metal center after complexation and thus facilitate
chirality transfer, but also restrict phosphane coordination to
a single ligand, thus strongly influencing regioselectivity.
The catalytic study was carried out using the rhodium
monophosphane complexes 1 and 2 (Scheme 1). These
S
terically encumbered phosphanes play an important role in
coordination chemistry and catalysis.[1] In many catalytic
transformations, such ligands have become de rigueur, as they
may force the formation of complexes with specific coordi-
nation geometries, thereby allowing efficient control of the
reaction outcome. Phosphanes displaying high steric demand
further constitute ideal ligands for the protection of reactive
metal centers,[2] the stabilization of unsaturated species,[3] and
sometimes for accelerating a key step of a catalytic cycle.[1e,4]
While in many reactions the formation of catalytic
intermediates bearing a single phosphane is highly desirable,
such a feature is rarely observed when the phosphane/metal
ratio is higher than one, because multiphosphane complexes
are most often formed, even if the ligand is bulky. One
possible way to achieve exclusive binding of a single PR3
moiety is by using phosphanes substituted with an additional
functional group, which, together with the PIII center, may
behave as a chelating unit, and consequently prevent coordi-
nation of a second phosphane.[5] However, because of their
chelating nature, these ligands may considerably alter the
expected reactivity of the metal center. Another potential
method for forming monoligated complexes consists of using
a bowl-shaped phosphane in which the phosphorus atom sits
Scheme 1. Synthesis of the Rh-monophosphane complexes 1 and 2.
[*] M. Jouffroy, Dr. R. Gramage-Doria, Prof. D. Armspach,
Dr. D. Sꢀmeril, Dr. D. Matt
Laboratoire de Chimie Inorganique Molꢀculaire et Catalyse
Institut de Chimie UMR 7177 CNRS, Universitꢀ de Strasbourg
1 rue Blaise Pascal, 67008 Strasbourg Cedex (France)
E-mail: d.armspach@unistra.fr
formed quantitatively by reacting [Rh(acac)(CO)2] (acac =
acetylacetonate) with one equivalent of the corresponding
HUGPHOS ligand. Excess ligand did not lead to a bis(phos-
phane) complex. Both 31P{1H} NMR spectra consist of
a doublet [d = 34.3 (1), 31.5 (2) ppm], with a J(PRh) coupling
constant of 167 Hz. ROESY experiments (see the Supporting
Information) revealed that the acac ligand of 2 is confined in
the corresponding b-CD unit. As a result, the CO ligand
adopts an exo orientation. Conversely, the complex 1, which
features a smaller inner space, has its acac fragment oriented
towards the exterior of the cavity.This means that the CO
ligand is located inside the CD in this case. Both complexes
are remarkably air stable.
dmatt@unistra.fr
Dr. W. Oberhauser
Istituto di Chimica dei Composti OrganoMetallici CNR
via Madonna del Piano, 10, 50019 Sesto Fiorentino, Firenze (Italy)
Dr. L. Toupet
Groupe Matiꢁre Condensꢀe et Matꢀriaux UMR 6626 CNRS
Universitꢀ de Rennes 1, 35042 Rennes Cedex (France)
[**] We thank the Rꢀgion Alsace and ICFRC for a grant to M.J.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311291.
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Styrene was chosen as a substrate because it is a source of
valuable optically active aldehydes. It is also sterically
compatible with the CD inner space. Hydroformylation
tests were performed not only at various temperatures and
pressures, but also at several CO/H2 and L/Rh ratios, as well
as precatalyst loadings.
The best catalytic results were obtained with 1 when
operating at room temperature under a pressure of 40 bar (1:1
molar mixture of CO/H2; Table 1, entry 7). Under these
of 2 the ee value increased by 49% and the branched/liner (b/
l) ratio from 3.7 to 24.6 (Table 1, entries 5 and 6). Again, these
findings are markedly different from observations made for
conventional Rh/phosphane systems. It should also be
mentioned here that under optimal catalytic conditions
(208C, 40 bar) no drop of catalytic activity was observed
over three days for either complex (see the Supporting
Information).
While studying the coordination chemistry of HUGPHOS
ligands towards group 9 and 10 metal ions, we discovered that
their unique confining properties prevented the formation of
bis(phosphane) complexes. Thus, when mixing excess HUG-
PHOS-1 with [{RhCl(CO)2}2] (Scheme 2), only cis-
Table 1: Hydroformylation of styrene using the precatalysts 1 and 2.
Variation of pressure and temperature.[a]
Entry Com- p(CO/H2)
T
Conv. Yield [%][c]
b/l[d] ee
[%][g]
plex
[bar][b]
[8C] [%][c]
l
b
1
1
1
1
1
1
1
1
1
2
2
2
2
2
20
20
20
20
5
40
40
40
20
20
20
20
40
80 86.3
60 100
40 99.8
20 30.6
40 19.8
40 99.2
20 60.7
27.2 72.8
11.4 88.6
6.3 93.7
1.0 99.0
21.4 78.6
3.9 96.1
1.7 98.3
2.7 33 (R)
7.8 62 (R)
14.9 80 (R)
99.0 93 (R)
3.7 41 (R)
24.6 90 (R)
57.8 95 (R)
2[e]
3[e]
4[e]
5[e]
6[e]
7[e]
8[e]
9
Scheme 2. Synthesis of the complex 3 from HUGPHOS-1.
4
34.0
trace 100 >100[f] 93 (R)
80 96.8
120 31.5
60 43.7
40 79.0
20 66.2
37.1 62.9
43.0 57.0
13.9 86.1
6.8 93.2
1.7 98.3
1.7 27 (R)
1.3 34 (S)
6.2 63 (R)
13.7 80 (R)
57.8 92 (R)
10
11
12[e]
13[e]
[RhCl(HUGPHOS-1)(CO)2] (3) formed, rather than the
expected trans-[RhCl(HUGPHOS-1)2(CO)] complex. The
relative cis position of the CO ligands of 3 was inferred
from the corresponding IR spectrum (strong CO bands at
[a] Styrene (5 mmol), styrene/complex=2500, t=24 h, toluene/n-
decane (15 mL/0.5 mL), incubation overnight at 808C under p(CO/
H2)=20 bar. [b] CO/H2 1:1 v/v. [c] Determined by GC using decane as an
internal standard. [d] b/l aldehyde ratio. [e] Run carried out with
a styrene/complex ratio of 250. [f] Exact value not determined because
the amount of linear aldehyde was too small to be detected.
[g] Determined by chiral-phase GC after reduction with LiAlH4.
1
2009 and 2082 cmÀ1). The H NMR spectrum of 3 is fully
consistent with a CD-encapsulated chlorido ligand,[16] and is
further proof for the presence of two cis-configured CO
ligands.[17]
Furthermore, reaction of [PdCl2(PhCN)2] with HUG-
PHOS-2 gave quantitatively, after column chromatography,
the singly P-ligated aqua complex trans-[PdCl2(HUGPHOS-
2)(H2O)] (4), the molecular structure of which was confirmed
by a single-crystal X-ray diffraction study (Scheme 3, top).[18]
Similarly, reaction of [PtCl2(PhCN)2] with HUGPHOS-2 in
excess and subsequent addition of pyridine, gave the mixed
phosphane-pyridine complex 5 (Scheme 3, bottom). The
relative trans-P,N arrangement was unambiguously deduced
from the ROESY data which showed strong correlations
between the pyridinic H4 atom (g position) and the H3, H5,
and OMe-6 atoms of a nonbridged glucose, probably unit E.
Overall, these results prove the high potential of HUGPHOS
ligands to stabilize medium-sized metal organic fragments
bound to a single phosphane ligand.
reaction conditions, the selectivity for the branched aldehyde
and the ee value reached 98.3% and 95% (R enantiomer),
respectively. To the best of our knowledge, there is no other
example of a monodentate phosphane achieving simultane-
ously high chiral induction and high isoregioselectivity. So far,
comparable performance was only achieved with diphospho-
rus ligands.[7g,h,14] The complex 2 gave rise to similar selectiv-
ities and activities (Table 1, entry 13). Not surprisingly, raising
the temperature from 20 to 808C at a CO/H2 gas pressure of
20 bar led to higher reactions rates, but at the expense of both
regioselectivity and enantioselectivity (Table 1, entries 1–4
and 9–12). In stark contrast with classical diphosphane
rhodium complexes, an increase of the CO partial pressure
led to both higher catalytic activity and enantioselectivity,
whereas adding free ligand was detrimental to the catalyst
performance (see the Supporting Information).[15] Further-
more, a total pressure of at least 40 bar was found necessary to
ensure optimal reaction rate and selectivity. For example, by
raising the pressure from 5 to 40 bar (at 408C) in the presence
To investigate whether analogous species also formed
under the chosen hydroformylation conditions, 2 was acti-
vated in toluene at 808C with a CO/H2 mixture (1:1) at 40 bar.
Under these conditions, a single hydrido carbonyl species
formed, as revealed by a high-pressure NMR ([D8]toluene)
study (see the Supporting Information). Mass spectrometric
1
measurements together with the H and 31P{1H} NMR data
obtained at high pressure unequivocally proved the formation
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high regioselectivity and enantioselectivity. Obviously, these
two features, which are generally regarded as contradictory,
rely on the ligand ability to exclusively form the complex
trans-[RhH(CO)3(HUGPHOS)] (6) under optimized cata-
lytic conditions, and to embed the whole metal center, thereby
ensuring unprecedented chirality transfer for a CD-based
catalyst in organic media.[21] Clearly, these results constitute
a new illustration of the high potential of confining ligand
systems in homogeneous catalysis.
Received: December 30, 2013
Revised: January 27, 2014
Published online: March 3, 2014
Keywords: cyclodextrins · homogeneous catalysis ·
.
hydroformylation · phosphanes · rhodium
Scheme 3. Synthesis of the monophosphane complexes 4 and 5 (from
HUGPHOS-2). In the solid state the aquo complex 4 crystallized with
two additional water molecules embedded in the cavity.
[1] a) C. A. Tolman, Chem. Rev. 1977, 77, 313 – 348; b) A. F. Littke,
G. C. Fu, J. Org. Chem. 1999, 64, 10 – 11; c) A. F. Littke, G. C. Fu,
J. Am. Chem. Soc. 2001, 123, 6989 – 7000; d) T. Iwasawa, T.
Komano, A. Tajima, M. Tokunaga, Y. Obora, T. Fujihara, Y.
Tsuji, Organometallics 2006, 25, 4665 – 4669; e) U. Christmann,
R. Vilar, Angew. Chem. 2005, 117, 370 – 378; Angew. Chem. Int.
Ed. 2005, 44, 366 – 374; f) H. Ohta, M. Tokunaga, Y. Obora, T.
Iwai, T. Iwasawa, T. Fujihara, Y. Tsuji, Org. Lett. 2007, 9, 89 – 92;
g) T. Fujihara, S. Yoshida, H. Ohta, Y. Tsuji, Angew. Chem. 2008,
120, 8434 – 8438; Angew. Chem. Int. Ed. 2008, 47, 8310 – 8314;
h) D. J. M. Snelders, G. van Koten, R. J. M. K. Gebbink, J. Am.
Chem. Soc. 2009, 131, 11407 – 11416; i) D. L. Dodds, M. D. K.
Boele, G. P. F. van Strijdonck, J. G. de Vries, P. W. N. M. van
Leeuwen, P. C. J. Kamer, Eur. J. Inorg. Chem. 2012, 1660 – 1671;
j) Phosphorus(III) Ligands in Homogeneous Catalysis: Design
and Synthesis (Eds.: P. C. J. Kamer, P. W. N. M. van Leeuwen),
Wiley, Chichester, 2013.
[2] L. Poorters, D. Armspach, D. Matt, L. Toupet, S. Choua, P.
Turek, Chem. Eur. J. 2007, 13, 9448 – 9461.
[3] S. Otsuka, T. Yoshida, M. Matsumoto, K. Nakatsu, J. Am. Chem.
Soc. 1976, 98, 5850 – 5858.
[4] a) L. Monnereau, D. Semeril, D. Matt, L. Toupet, Chem. Eur. J.
2010, 16, 9237 – 9247; b) L. Monnereau, D. Semeril, D. Matt,
Chem. Commun. 2011, 47, 6626 – 6628.
[5] a) J. P. Wolfe, R. A. Singer, B. H. Yang, S. L. Buchwald, J. Am.
Chem. Soc. 1999, 121, 9550 – 9561; b) F. Y. Kwong, A. S. C. Chan,
Synlett 2008, 1440 – 1448; c) D. S. Surry, S. L. Buchwald, Angew.
Chem. 2008, 120, 6438 – 6461; Angew. Chem. Int. Ed. 2008, 47,
6338 – 6361; d) N. C. Bruno, S. L. Buchwald, Org. Lett. 2013, 15,
2876 – 2879.
of the complex trans-[RhH(CO)3(HUGPHOS-2)] (6;
Scheme 4). Thus, the mass spectrum recorded from a toluene
solution of 6 after CO/H2 removal displayed a peak at 1663.53
(1%, exact isotopic profile), which corresponds to the
[M+H]+ ion. The 1H NMR spectrum of 6 (258C, 40 bar)
revealed a hydride signal at d = À8.8 ppm [J(RhH) = 6.2 Hz]
with a large J(PH) coupling constant of 103 Hz, a value which
is typical for a linear P-Rh-H arrangement.[8b,19] In the
corresponding 31P{1H} NMR spectrum, the P atom appeared
as a doublet at d = 28.1 ppm [J(RhP) = 95 Hz].[20] The trigonal
bipyramidal complex 6 constitutes the second reported
monophosphane [RhH(CO)3L] complex in which the phos-
phorus lies trans to the hydride. The reason for the selective
apical binding of the phosphorus atom seemingly arises from
the embracing character of the phosphane. This feature forces
the resulting trigonal bipyramidal complex to adopt a config-
uration which minimizes steric interactions between the
carbonyl ligands and the cavity wall, and which is best
fulfilled with a linear PRhH arrangement.
In conclusion, the cyclodextrin-derived HUGPHOS
ligands readily form square-planar and trigonal bipyramidal,
monophosphane complexes in which the CD cavity tightly
embraces the metal center. When used in the rhodium-
catalyzed hydroformylation of styrene, they gave rise to both
[6] a) Y. Ohzu, K. Goto, T. Kawashima, Angew. Chem. 2003, 115,
5892 – 5895; Angew. Chem. Int. Ed. 2003, 42, 5714 – 5717; b) Y.
Ohzu, K. Goto, H. Sato, T. Kawashima, J. Organomet. Chem.
2005, 690, 4175 – 4183; c) O. Niyomura, T. Iwasawa, N. Sawada,
M. Tokunaga, Y. Obora, Y. Tsuji, Organometallics 2005, 24,
3468 – 3475.
[7] a) R. L. Pruett, J. A. Smith, J. Org. Chem. 1969, 34, 327 – 330;
b) P. W. N. M. van Leeuwen, C. F. Roobeek, J. Organomet.
Chem. 1983, 258, 343 – 350; c) T. Jongsma, G. Challa,
P. W. N. M. Van Leeuwen, J. Organomet. Chem. 1991, 421,
121 – 128; d) B. Breit, R. Winde, T. Mackewitz, R. Paciello, K.
Harms, Chem. Eur. J. 2001, 7, 3106 – 3121; e) V. F. Slagt, P. C. J.
Kamer, P. W. N. M. van Leeuwen, J. N. H. Reek, J. Am. Chem.
Soc. 2004, 126, 1526 – 1536; f) R. A. Baber, M. L. Clarke, K. M.
Heslop, A. C. Marr, A. G. Orpen, P. G. Pringle, A. Ward, D. E.
Zambrano-Williams, Dalton Trans. 2005, 1079 – 1085; g) M. L.
Clarke, Curr. Org. Chem. 2005, 9, 701 – 718; h) M. Kuil, T.
Scheme 4. Selective formation of complex 6 under 40 bar CO/H2 at
808C.
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Soltner, P. W. N. M. van Leeuwen, J. N. H. Reek, J. Am. Chem.
Soc. 2006, 128, 11344 – 11345; i) A. A. Dabbawala, R. V. Jasra,
H. C. Bajaj, Catal. Commun. 2011, 12, 403 – 407; j) A. A.
Dabbawala, H. C. Bajaj, G. V. S. Rao, S. H. R. Abdi, Appl.
Catal. A 2012, 419 – 420, 185 – 193; k) V. Bocokic, A. Kalkan, M.
Lutz, A. L. Spek, D. T. Gryko, J. N. H. Reek, Nat. Commun.
2013, 4, 3670; l) H. Tricas, O. Diebolt, P. W. N. M. van Leeuwen,
J. Catal. 2013, 298, 198 – 205.
Qin, G. T. Whiteker, Org. Lett. 2004, 6, 3277 – 3280; h) A. T.
Axtell, C. J. Cobley, J. Klosin, G. T. Whiteker, A. Zanotti-
Gerosa, K. A. Abboud, Angew. Chem. 2005, 117, 5984 – 5988;
Angew. Chem. Int. Ed. 2005, 44, 5834 – 5838; i) T. P. Clark, C. R.
Landis, S. L. Freed, J. Klosin, K. A. Abboud, J. Am. Chem. Soc.
2005, 127, 5040 – 5042; j) A. T. Axtell, J. Klosin, G. T. Whiteker,
C. J. Cobley, M. E. Fox, M. Jackson, K. A. Abboud, Organo-
metallics 2009, 28, 2993 – 2999; k) T. T. Adint, G. W. Wong, C. R.
Landis, J. Org. Chem. 2013, 78, 4231 – 4238; l) G. M. Noonan,
C. J. Cobley, T. Mahoney, M. L. Clarke, Chem. Commun. 2014,
50, 1475 – 1477.
[8] a) R. Bellini, S. H. Chikkali, G. Berthon-Gelloz, J. N. H. Reek,
Angew. Chem. 2011, 123, 7480 – 7483; Angew. Chem. Int. Ed.
2011, 50, 7342 – 7345; b) R. Bellini, J. N. Reek, Chem. Eur. J.
2012, 18, 7091 – 7099.
[9] a) R. M. Deshpande, A. Fukuoka, M. Ichikawa, Chem. Lett.
1999, 13 – 14; b) C. Machut-Binkowski, F. X. Legrand, N. Azaro-
ual, S. Tilloy, E. Monflier, Chem. Eur. J. 2010, 16, 10195 – 10201;
c) F. X. Legrand, N. Six, C. Slomianny, H. Bricout, S. Tilloy, E.
Monflier, Adv. Synth. Catal. 2011, 353, 1325 – 1334; d) D. N.
Tran, F. X. Legrand, S. Menuel, H. Bricout, S. Tilloy, E. Monflier,
Chem. Commun. 2012, 48, 753 – 755.
[15] Rhodium-Catalyzed Hydroformylation (Eds.: P. W. N. M. van
Leeuwen, C. Claver), Kluwer, Dordrecht, 2000.
[16] E. Engeldinger, D. Armspach, D. Matt, P. G. Jones, Chem. Eur. J.
2003, 9, 3091 – 3105.
[17] Some CD H5 protons belonging to nonbridged glucose units are
strongly upfield shifted upon metal complexation (Dd up to
0.7 ppm). Such chemical shifts differences are typical of chlorido
encapsulation.
[10] a) F. Agbossou, J. F. Carpentier, A. Mortreux, Chem. Rev. 1995,
95, 2485 – 2506; b) C. Claver, M. Diꢀguez, O. Pꢁmies, S.
Castillꢂn, Top. Organomet. Chem. 2006, 18, 35 – 64; c) J.
Klosin, C. R. Landis, Acc. Chem. Res. 2007, 40, 1251 – 1259;
d) A. Gual, C. Godard, S. Castillꢂn, C. Claver, Tetrahedron:
Asymmetry 2010, 21, 1135 – 1146.
[11] R. Franke, D. Selent, A. Bçrner, Chem. Rev. 2012, 112, 5675 –
5732.
[12] E. Engeldinger, L. Poorters, D. Armspach, D. Matt, L. Toupet,
Chem. Commun. 2004, 634 – 635.
[18] The most striking feature of this structure is the presence of
a unique intracavity hydrogen-bonding network which literally
fills the cavity with water molecules. The coordinated water
molecule (Ow1) is not only hydrogen bonded to the OMe-6
group of glucose unit F, but also to an additional water molecule
(Ow2), itself weakly bonded to the Cl1 chlorine atom and a third
water molecule (Ow3). The latter is connected to the CD
secondary rim through a hydrogen bond with the OMe-2 group
of glucose unit G.
[19] a) H. Lehner, D. Matt, A. Togni, R. Thouvenot, L. M. Venanzi,
A. Albinati, Inorg. Chem. 1984, 23, 4254 – 4261; b) S. H. Chik-
kali, R. Bellini, B. de Bruin, J. I. van der Vlugt, J. N. H. Reek, J.
Am. Chem. Soc. 2012, 134, 6607 – 6616.
[13] R. Gramage-Doria, D. Rodriguez-Lucena, D. Armspach, C.
Egloff, M. Jouffroy, D. Matt, L. Toupet, Chem. Eur. J. 2011, 17,
3911 – 3921.
[14] a) N. Sakai, S. Mano, K. Nozaki, H. Takaya, J. Am. Chem. Soc.
1993, 115, 7033 – 7034; b) N. Sakai, K. Nozaki, H. Takaya, J.
Chem. Soc. Chem. Commun. 1994, 395 – 396; c) J. E. Babin, G. T.
Whiteker, Vol. 5360938, Union carbide Chemicals & Plastics
Technology Corporation, USA, 1994; d) S. Breeden, D. J. Cole-
Hamilton, D. F. Foster, G. J. Schwarz, M. Wills, Angew. Chem.
2000, 112, 4272 – 4274; Angew. Chem. Int. Ed. 2000, 39, 4106 –
4108; e) R. Ewalds, E. B. Eggeling, A. C. Hewat, P. C. J. Kamer,
P. W. N. M. van Leeuwen, D. Vogt, Chem. Eur. J. 2000, 6, 1496 –
1504; f) M. Diꢀguez, O. Pamies, A. Ruiz, S. Castillon, C. Claver,
Chem. Eur. J. 2001, 7, 3086 – 3094; g) C. J. Cobley, J. Klosin, C.
[20] The IR spectrum of 6 measured at 40 bar of CO/H2 displays
three strong carbonyl bands at 1981, 1987, and 1991 cmÀ1
.
Obviously, this spectrum reflects a local symmetry lower than
D3h.
[21] a) Y. T. Wong, C. Yang, K. C. Ying, G. C. Jia, Organometallics
2002, 21, 1782 – 1787; b) M. Guitet, P. Zhang, F. Marcelo, C.
Tugny, J. Jimenez-Barbero, O. Buriez, C. Amatore, V. Mouries-
Mansuy, J. P. Goddard, L. Fensterbank, Y. Zhang, S. Roland, M.
Menand, M. Sollogoub, Angew. Chem. 2013, 125, 7354 – 7359;
Angew. Chem. Int. Ed. 2013, 52, 7213 – 7218.
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