Planar discrimination in an SPS-based rhodium(I) complex

Article abstract of DOI:10.1021/om0305115

This work describes the reactivity of the SPS-type pincer-based Rh^I complex 4 toward CO, O₂, CO₂, Cs₂, and SO₂ to afford the corresponding Rh^I or Rh^III adducts. The SPS ligand is able to adopt a facial coordination mode in these trigonal-bipyramidal complexes. Differentiation of the two faces of the square-planar Rh^I complex 4 could be rationalized by a mechanical effect due to the rigidity of the central phosphinine backbone.

Full text of DOI:10.1021/om0305115

4624
Organometallics 2003, 22, 4624-4626
P la n a r Discr im in a tion in a n SP S-Ba sed Rh od iu m (I)
Com p lex
Marjolaine Doux, Nicolas Me´zailles, Louis Ricard, and Pascal Le Floch*
Laboratoire “He´te´roe´le´ments et Coordination” UMR CNRS 7653 (DCPH), and De´partement de
Chimie, Ecole Polytechnique, 91128 Palaiseau Cedex, France
Received J uly 1, 2003
Sch em e 1. Syn th esis of 3a
Summary: This work describes the reactivity of the SPS-
type pincer-based RhI complex 4 toward CO, O2, CO2,
CS2, and SO2 to afford the corresponding RhI or RhIII
adducts. The SPS ligand is able to adopt a facial
coordination mode in these trigonal-bipyramidal com-
plexes. Differentiation of the two faces of the square-
planar RhI complex 4 could be rationalized by a me-
chanical effect duetotherigidityofthecentral phosphinine
backbone.
Over the past few years, rigid pincer structures
incorporating an aromatic ring as the central unit have
emerged as a very important class of ligands, and their
successful use in catalytic processes of importance has
been emphasized by many reports.1,2 It is now well
established that the combination of three binding sites
offers possibilities to subtly tune the electronic proper-
ties of the metal fragment. In this perspective, many
efforts have been currently devoted to the design of new
pincers featuring heteroatoms (N, O, S, P) as ancillary
or central ligands.1,3 So far, with sulfur, studies have
mainly focused on the use of thiolate,4 thioethers,5 or
sulfoxide6 and only little attention has been paid to
ligands bearing phosphine sulfides.7
a
Legend: (a) MeLi (1 equiv), THF, -78 °C to room tem-
perature; (b) [RhCl(COD)]2 (1 equiv), THF, -78 °C to room
temperature.
sulfides as ancillary ligands.8 Palladium(II) complexes
of these new ligands proved to be particularly efficient
in the catalyzed Miyaura cross-coupling process that
allows the formation of Csp -B bonds. In pursuing our
investigation, we recently found that rhodium(I) com-
plexes are also particularly reactive. Herein we report
on these preliminary results.
9
2
The rhodium(I) complex 3 is easily available from the
reaction anion 2 with
1
Recently, we have developed a new class of SPS-based
pincer system featuring a hypervalent phosphorus atom
(λ4-phosphinine) as the central unit and two phosphine
/ equiv of the [RhCl(COD)]2
2
precursor (Scheme 1).10 Complex 3, which was isolated
as a very stable orange solid, was fully characterized
by NMR techniques and elemental analyses. Unfortu-
nately, despite many attempts, 3 could not be crystal-
lized and information about the spatial arrangement of
the SPS ligand could not be obtained. Though 31P NMR
spectroscopy reveals that the PPh2S groups are mag-
netically equivalent, two geometries can be proposed for
3: one in which the ligand is located in the plane and
a second in which it caps one face of the bipyramid
(Scheme 1).
* To whom correspondence should be addressed. Fax: +33.1.69.33.
45.70. E-mail: lefloch@poly.polytechnique.fr.
(1) Albrecht, M.; Van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750-3781. Albrecht, M.; Van Koten, G. Angew. Chem. 2001, 113,
3866-3878.
(2) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759-
1792. Singleton, J . T. Tetrahedron Lett. 2003, 59, 1837-1857.
(3) Ionkin, A. S.; Marshall, W. J . Heteroat. Chem. 2002, 13, 662-
666. Kapoor, P.; Pathak, A.; Kapoor, R.; Venugopalan, P.; Corbella,
M.; Rodriguez, M.; Robles, J .; Llobet, A. Inorg. Chem. 2002, 41, 6153-
6160. Grundemann, S.; Albrecht, M.; Loch, J . A.; Faller, J . W.;
Crabtree, R. H. Organometallics 2001, 20, 5485-5488.
Therefore, experiments aimed at derivatizing complex
3 were undertaken. Displacement of the COD ligand by
triphenylphosphine readily occurred in THF to yield the
highly reactive complex 4, which was structurally
characterized. An ORTEP view of one molecule of 4 is
presented in Figure 1. Only the ipso carbons of the
phenyl groups of the SPS ligand have been kept for
clarity. This structure is quite interesting and deserves
several comments. The square-planar arrangement
(4) Ortner, K.; Hilditch, L.; Zheng, Y. F.; Dilworth, J . R.; Abram, U.
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2641. Tschoerner, M.; Trabesinger, G.; Albinati, A.; Pregosin, P. S.
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122, 7905-7920.
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Williams, T. L. Organometallics 2002, 21, 893-900.
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Chem. 2002, 2408-2418. Brunner, H.; Lautenschlager, H. J .; Ko¨nig,
W. A.; Krebber, R. Chem. Ber. 1990, 123, 847-853. Baker, M. J .; Giles,
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197-198. Gonsalvi, L.; Adams, H.; Sunley, G. J .; Ditzel, E.; Haynes,
A. J . Am. Chem. Soc. 2002, 124, 13597-13612. Faller, J . W.; Lloret-
Fillol, J .; Parr, J . New J . Chem. 2002, 26, 883-888.
(8) Doux, M.; Bouet, C.; Me´zailles, N.; Ricard, L.; Le Floch, P.
Organometallics 2002, 21, 2785-2788. Doux, M.; Me´zailles, N.; Ricard,
L.; Le Floch, P. Eur. J . Inorg. Chem., in press.
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Chem. Commun. 2002, 1566-1567.
(10) Moores, A.; Ricard, L.; Le Floch, P.; Me´zailles, N. Organome-
tallics 2003, 22, 1960-1966.
10.1021/om0305115 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 10/07/2003

Communications
Organometallics, Vol. 22, No. 23, 2003 4625
Sch em e 2. Syn th esis a n d Rea ction s of 4a
a
Legend: (a) PPh3 (1 equiv), THF; (b) CO (1 atm), THF; (c) O2 (1 atm), THF; (d) CS2 (1 equiv), THF, -78 °C to room temperature;
(e) SO2 (1 atm), THF, -78 °C to room temperature
are summarized in Scheme 2. As can be seen, reaction
with CO afforded the 18-VE RhI complex [Rh(2)(CO)-
(PPh3)] (5), which was structurally characterized. In-
terestingly, its X-ray structure, which is not presented
here, proves that the SPS pincer is sufficiently flexible
to cap one face of a trigonal bipyramid.11 This structural
feature is important and markedly differs from what
was observed with classical pincer ligands, which are
structurally rigid and favor planar coordination. Com-
plex 4 also reacted with molecular oxygen to yield the
highly stable complex [Rh(2)(η2-O2)PPh3] (6), which was
also structurally characterized. Examination of the O-O
distance (1.431(2) Å) reveals that 6 is a RhIII peroxo
complex.12 Complex 6 appears to be the first example
of a peroxo complex featuring sulfides as ligands.11 As
F igu r e 1. Molecular structure of 4 without hydrogen
atoms. Phenyl groups of the phosphinine moiety have been
omitted for clarity. Selected bond lengths (Å) and angles
(deg): P1-Rh1 ) 2.2428(6), S1-Rh1 ) 2.3197(6), S2-Rh1
) 2.3213(6), P4-Rh1 ) 2.3034(6); S1-Rh-S2 ) 172.51-
(2), P1-Rh-P4 ) 174.65(2), P1-C1-P2 ) 113.6(1), P1-
C5-P3 ) 111.1(1), (mean plane C1-C2-C4-C5)-P1 )
16.7, (mean plane C1-C2-C4-C5)-C3 ) 7.1, S1-P2-
C1-P1 ) 4.3, S2-P3-C5-P1 ) 13.0.
depicted in Scheme 2, it is important to note that attack
of both CO (in 5) and O2 (in 6) took place exclusively
syn to P-Me. This attack results also in the displace-
ment of the PPh3 ligand away from the equatorial plane.
It is now located on the main axis of the trigonal
bypiramid, trans to the λ4-phosphinine ligand.
Reaction of 4 with CO2 was also attempted, but the
corresponding complex proved to be too labile to be
isolated and NMR data did not provide sufficiently clear
information to propose a definitive formulation. A more
gratifying result was obtained by reacting 4 with a
around the metal center is obvious, as expected for a
ML4 d8 configuration. Although the SPS ligand occupies
three coordination sites of the square-planar geometry,
the ligand itself is not planar and the phosphorus atom
is now located well above the plane defined by the ring
carbon atoms (see Figure 1). The plane defined by C1-
P-C5 deviates from the plane defined by C1-C2-C4
and C5 by 16.7°, and the phosphorus atom is highly
pyramidal (∑(angles at P1) ) 311.24°). The ORTEP plot
clearly shows that the two faces of the rhodium center
are differentiated and incoming reagents can approach
either “syn” or “anti” to the methyl group. The very
peculiar geometry for the ligand in complex 4 allows for
the rationalization of the reactivity of this complex (vide
supra). Apart from these features, the bond distances
and angles are normal and deserve no further com-
ments.
(11) Selected bond lengths (Å) and angles (deg) for complexes 5, 6,
and 8 are as follows. 5: P(phosphinine)-Rh ) 2.2783(5), Ph3P-Rh )
2.3248(5), S-Rh ) 2.4846(5) and 2.5720(5), C-Rh ) 1.820(2), C-O )
1.157(2); S-Rh-S ) 87.49(2), P-Rh-P ) 177.39(2), C-Rh-S )
143.52(7) and 128.97(7), Me-P-Rh-C ) 5.0, S-P-C-P(phosphinine)
) 33.8 and 36.9. 6: P(phosphinine)-Rh ) 2.2630(7), Ph3P-Rh )
2.3655(7), S-Rh ) 2.4057(8) and 2.3765(8), O-Rh ) 2.050(2) and
2.004(2), O-O ) 1.431(2); S-Rh-S ) 98.68(3), P-Rh-P ) 177.94(2),
O-Rh-O ) 41.32(6), Me-P-Rh-Centroid(O2) ) 15.3, S-P-C-
P(phosphinine) ) 27.5 and 6.0. 8: P(phosphinine)-Rh ) 2.259(1),
S-Rh ) 2.3558(8) and 2.3756(8), S(sulfur dioxide)-Rh ) 2.342(1), O1-
S(sulfur dioxide) ) 1.471(2), O2-S(sulfur dioxide) ) 1.465(2); P-Rh-P
) 172.86(3), S-Rh-S ) 162.12(3), O-S-O ) 112.0(1), S(sulfur
dioxide)-Rh-S ) 97.68(3) and 99.97(3), S-P-C-P(phosphinine) )
16.3 and 22.1.
(12) Dorta, R.; Shimon, L. J . W.; Rozenberg, H.; Milstein, D. Eur.
J . Inorg. Chem. 2002, 1827-1834. Nicasio, M. C.; Paneque, M.; Perez,
P. J .; Pizzano, A.; Poveda, M. L.; Rey, L.; Sirol, S.; Taboada, S.; Trujillo,
M.; Monge, A.; Ruiz, C.; Carmona, E. Inorg. Chem. 2000, 39, 180-
188. Haarman, H. F.; Bregman, F. R.; van Leeuwen, P.; Vrieze, K.
Organometallics 1997, 16, 979-985.
Several experiments were carried out to evaluate the
reactivity of 4 toward small molecules. Some of these

4626 Organometallics, Vol. 22, No. 23, 2003
Communications
Sch em e 3. P la n a r Discr im in a tion of 4
How can the regiospecificity of the various reactions
presented here, in favor of the attack of the incoming
reactant syn to the P-Me group, be rationalized? Two
main factors that may operate to different extents can
be put forward: steric requirements and SPS ligand
mechanical effects. Let us consider the variation in
geometries of the different complexes during the con-
sidered reaction. In the synthesis of 8, which adopts a
square-pyramidal geometry, the initial arrangement
found in 4 is not perturbed to a great extent and the
SPS ligand keeps the same geometry: i.e., the steric
requirements likely predominate. Thus, the attack from
the anti face, where two axial phenyl rings are found,
is disfavored versus the syn attack. In the three other
cases, leading to 5-7, the final geometry is trigonal
bipyramidal. There, no matter how much the sterics are
involved (likely favoring the syn attack also), the rigidity
of the phosphinine ring results in a mechanical effect
that does not allow the two PPh2S moieties to bend
toward the syn face (Scheme 3). Moreover, the two
phenyl groups would collide with the interacting mol-
ecule. The anti attack is thus forbidden, and the sole
product results from the attack on the syn face.
In conclusion, a highly reactive rhodium(I) complex
featuring an SPS-based pincer ligand has been synthe-
sized. Preliminary experiments have shown that this
complex 4 reacts with small molecules such as CO, O2,
CO2, CS2, and SO2. Interestingly, a mechanical effect
resulting from the rigidity of the central phosphinine
ring allows the differentiation of the two faces of the
square-planar complex 4. A new interesting challenge
will now consist in introducing chirality at the phos-
phorus atom. Indeed, one may expect that the presence
of a chiral substituent at P would help to differentiate
the two diastereotopic sides of the SPS-based pincer
complex. This would provide a unique way for control-
ling enantioselectivity in derived catalysts. Experiments
aimed at validating this hypothesis are currently un-
derway in our laboratories.
F igu r e 2. Molecular structure of 7 without hydrogen
atoms. Selected bond lengths (Å) and angles (deg): P1-
Rh1 ) 2.2771(8), S1-Rh1 ) 2.5040(8), S2-Rh1 ) 2.4193-
(7), P4-Rh1 ) 2.379(1), C7-Rh1 ) 2.003(3), S3-Rh1 )
2.3866(7), C7-S3 ) 1.670(3), C7-S4 ) 1.631(3); P1-Rh1-
P4 ) 174.81(2), S1-Rh1-S2 ) 92.26(3), S3-C7-S4 )
140.2(2), P1-C1-P2 ) 115.2(1), P1-C5-P3 ) 116.7(1),
(mean plane C1-C2-C4-C5)-P1 ) 26.3, (mean plane
C1-C2-C4-C5)-C3 ) 8.6, S1-P2-C1-P1 ) 26.6, S2-
P3-C5-P1 ) 13.3.
stoichiometric amount of CS2. Coordination readily took
place in THF at room temperature to yield the [Rh(2)-
(η2-SCdS)PPh3] complex 7, which was fully character-
ized. An ORTEP view of one molecule of 7 is presented
in Figure 2. Crystal data and structural refinement
details are presented in Table 1. As can be seen, the
complex adopts a trigonal-bipyramidal geometry like
that of 6 and coordination of CS2 occurs through one of
the CdS bonds. So far, only two (η2-CS2)Rh complexes
have been structurally characterized, neither featuring
a phosphine sulfide as ligand.13 In contrast to its CO2
analogue, complex 7 is not labile and its structure is
preserved in solution, as attested by the 31P NMR
spectrum, which exhibits two different signals for the
ancillary phosphorus atoms. As can be seen in Figure
2, the attack of CS2 also occurred syn to the P-Me
group.
Finally, reaction with SO2 was attempted. Coordina-
tion readily occurred in THF by bubbling SO2 into a
solution of 4 at -78 °C. The geometry of complex 8 could
not be unambiguously established on the sole basis of
NMR data. Indeed, though the spectra revealed that the
complex is symmetrical, discrimination between three
possible modes of coordination (pyramidal or η1-S
planar, η1-O or η2-O,S bonded) of SO2 is not possible.
However, the presence of two characteristic stretching
frequencies of the SO bond at 1028 and 1148 cm-1 in
the IR spectrum strongly suggested a pyramidal geom-
etry, which was definitively proved by X-ray crystal-
lography.11 Here again, the incoming ligand reacted syn
to the P-Me group. The IR and X-ray data of this
complex are very similar to those of [RhCl(tpp)(SO2)]
(tpp ) bis(3-diphenylphosphino)propyl)phenylphos-
phine).14
Ack n ow led gm en t. We thank the CNRS, the Ecole
Polytechnique, and the DGA for supporting this work.
Su p p or tin g In for m a tion Ava ila ble: Text detailing the
preparation of compounds 2-8 and giving a description of the
X-ray procedures and tables of X-ray data, positional and
thermal parameters, and bond lengths and angles and ORTEP
diagrams for complexes 4 and 7. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM0305115
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