Metalated-arene-phosphino ligands: A novel approach to open-sided gold compounds

Article abstract of DOI:10.1021/om101005w

A novel type of metalated phosphino ligands, [Cp*Ru (η⁶-arene-PPh₂)][OTf] (2a-d-OTf), has been prepared in which the -PPh₂ unit is attached to a metalated π-arene platform. This unique class of ligands 2a-d are converted t

Full text of DOI:10.1021/om101005w

6636 Organometallics 2010, 29, 6636–6638
DOI: 10.1021/om101005w
Metalated-Arene-Phosphino Ligands: A Novel Approach to Open-Sided
Gold Compounds
ꢀ
ꢁ
M. Rosa Axet, Marion Barbazanges, Mylene Auge, Christophe Desmarets, Jamal Moussa,
Cyril Ollivier, Corinne Aubert,* Louis Fensterbank,* Vincent Gandon,‡ Max Malacria,
Lise Marie Chamoreau, and Hani Amouri*
ꢁ ꢁ
UPMC Universite Paris 06, Institut Parisien de Chimie Moleculaire (UMR CNRS 7201), 4 Place Jussieu, C. 42,
75252 Paris Cedex 05, France. ‡Univ Paris-Sud, UMR CNRS 8182, Orsay 91405, France
Received October 22, 2010
Summary: Anoveltypeofmetalatedphosphino ligands, [Cp*Ru-
(η6-arene-PPh2)][OTf] (2a-d-OTf), has been prepared in which
the -PPh2 unit is attached to a metalated π-arene platform. This
unique class of ligands 2a-d are converted to open-sided cationic
gold complexes 3a-d upon treatment with [AuCl(tht)]. The
structure of one of these compounds, [Cp*Ru(η6-C6H5-PPh2-
Au-Cl)][OTf] (3a-OTf), was ascertained by single-crystal X-ray
diffraction. Preliminary studies suggest that cationic complex 3a is
active in metal-catalyzed cycloisomerization reactions.
the steric properties of the donor ligand bound to the metal
center, “LfAu” (L = PR3, carbenes, cyclic aminocarbene,
etc.).3,4 More recently Echavarren and co-workers designed
a novel family of gold complexes with bulky phosphine
ligands.5 The latter showed enhanced catalytic activity com-
pared to the market available [AuCl(PPh3)] complex.
Pursuing our research in this area, we wish to report in this
communication the design and synthesis of a novel family of
gold complexes of the type LMfAu-Cl where LM is an
organometallic phosphine ligand. In this ligand, the -PPh2
moiety is now attached to a metalated π-arene platform (see
Scheme 1). In this model complex, we sought to change the
electronic and steric properties around the gold center in the
hopes that this may lead to novel reactive species. To the best
of our knowledge, this is the first example of such a com-
pound to be reported in the literature.
Our group has developed the synthesis of some neutral
metalated π-quinones. The related reactive intermediates
thioquinones and selenoquinones were also stabilized and
isolated for the first time as π-complexes of “Cp*M” (M =
Rh, Ir).6 These π-quinonoid species were successfully used as
organometallo ligands to prepare a variety of coordination
assemblies and polymers with luminescent properties.7 In
this work we describe the synthesis of a new type of organo-
metallo ligand where the phosphine moiety is attached to an
arene complex of Cp*Ru. However we note that metalated
phosphino ligands such as rigid phosphino-ferrocenes has
been widely and successfully used in homogeneous catalysis
by many groups.8
Gold complexes have received much attention due to their
importance in several applications such as luminescence,
supramolecular chemistry, and nanoparticles, and more
recently in catalysis.1 Indeed in the past decade, homoge-
neous gold catalysis has emerged as a powerful tool for novel
organic transformations, thus providing a variety of C-C
bond-forming reactions for the synthesis of complex chemi-
cal structures.2 In this context, efforts have been devoted to
prepare new gold complexes by tuning the electronic and/or
*Corresponding authors. (H.A.) Fax: (33)1-44-27-38-41. E-mail:
hani.amouri@upmc.fr. (C.A.) Fax: (33)1-44-27-73-60. E-mail:
corinne.aubert@upmc.fr. (L.F.) Fax: (33)1-44-27-73-60. E-mail: louis.
fensterbank@upmc.fr.
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Schier, A.; Schmidbaur, H. Organometallics 2003, 22, 3199. (f) Stefanescu,
D. M.; Yuen, H. F.; Glueck, D. S.; Golen, J. A.; Zakharov, L. N.; Incarvito, C. D.;
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r
pubs.acs.org/Organometallics
Published on Web 11/24/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 24, 2010 6637
Scheme 1. Preparation of a Family of Gold Complexes, [Cp*Ru(η6-arene-PPh2-Au-Cl)][OTf] (3a-d-OTf)
The synthesis of the organometallic phosphine ligand 2a was
performed in two steps (see Scheme 1). The first step involves
the preparation of the halogen arene metal compound [Cp*Ru-
(η6-C6H5Cl][OTf] (1a-OTf) from chlorobenzene and the sol-
vated ruthenium complex [Cp*Ru(CH3CN)3][OTf].9 Com-
pound 1a-OTf was obtained as a white microcrystalline com-
plex in 83% yield.
In a second step, we treated 1a-OTf with 1 equiv of KPPh2
1
in THF for 1 h. Reaction workup and analysis by H NMR
spectroscopy allowed us to identify the formation of the new
compound as [Cp*Ru(η6-C6H5-PPh2)][OTf] (2a-OTf). Com-
pound 2a-OTf was obtained in 81% yield as a white, micro-
crystalline, analytically pure material. The 1H NMR spectrum
of 2a-OTf recorded in CD2Cl2 showed a singlet at δ 1.99 ppm
attributed to the Cp*Ru methyl protons, while three multiplets
are visible at δ 5.45, 5.82, and 5.94 ppm, attributed to the
protons of the η6-C6H5 ring. We also note the presence of a set
of multiplets at δ 7.46 ppm attributed to the phenyl protons of
the -PPh2 moiety. Furthermore, the 31P NMR spectrum
recorded in CD2Cl2 showed a singlet at δ -13.3 ppm, which
is in agreement with the proposed formula of 2a-OTf. In a
similar way the related phosphino ligands 2b-d with different
substituents at the metalated arene ring were prepared and fully
characterized, suggesting that our method is versatile and allows
the preparation of a wide range of metalated phosphino ligands
(see Supporting Information). It is noteworthy that phosphi-
nobenzene chromium complexes were reported following a
completely different approach.10
Figure 1. Molecular structure of 3a, showing (i) the orientations of
the free and metalated phenyl groups and (ii) the coordination of the
To attain the target gold compound 3a-OTf, we treated
the organometallic phosphine ligand 2a with 1 equiv of
[AuCl(tht)] in dichloromethane. The reaction proceeded
smoothly and was left for 1 h. Reaction workup provided
an off-white compound in 85% yield. Elemental analysis
and spectroscopic data suggested the formation of [Cp*Ru-
(η6-C6H5-PPh2-Au-Cl)][OTf] (3a-OTf). As expected, the 1H
NMR recorded in CD2Cl2 exhibited a singlet at δ 2.02 ppm,
assigned to the Cp*Ru methyl protons, and a set of three
multiplets in the range δ 6.03-6.20 ppm, attributed to the
protons of the η6-C6H5- moiety, which are now down-
field relative to the organometallic phosphine ligand 2a.
˚
Cp*Ru to the phenyl ring. Selected bond distances (A) and angles
(deg): P1---Au1: 2.218(1); P1---C1: 1.817(3); P1---C17: 1.819(3):
P1---C23: 1.815(4); Au1---Cl1: 2.270(1); Ru1---C1: 2.217(3); Ru1---
C2: 2.205(3); Ru1---C3: 2.209(3); Ru1---C4: 2.212(3); Ru1---C5:
2.197(3); Ru1---C6: 2.200(3); P1-Au1-Cl1: 176.34(4). Thermal
ellipsoids are drawn at 50% probability. Color code for atoms: Ru:
pink; Au: blue; C: gray; Cl: green; P: orange.
Furthermore, another set of multiplets are visible at δ
7.60-7.67 ppm, which we attribute to the phenyl protons
of the -PPh2 moiety. The 31P NMR spectrum in CD2Cl2 was
the most informative and showed a singlet at δ 35.6 ppm,
downfield relative to starting material. This downfield shift
confirms the coordination of the gold center by the organo-
metallic phosphine ligand. To ascertain the structure of this
novel binuclear Ru-Au compound, a single-crystal X-ray
structure determination was carried out.
(9) (a) Dembek, A. A.; Fagan, P. J. Organometallics 1995, 14, 3741.
(b) O'Connor, J. M.; Friese, S. J.; Rodgers, B. L.; Rheingold, A. L.; Zakharov,
L. J. Am. Chem. Soc. 2005, 127, 9346.
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J. Organomet. Chem. 2008, 693, 3546. (b) Gibson, S. E.; Ibrahim, H. Chem.
Commun. 2002, 2465. (c) Bolm, C.; Muniz, K. Chem. Soc. Rev. 1999, 28, 51.

6638 Organometallics, Vol. 29, No. 24, 2010
Axet et al.
Scheme 2. Cyclization of β-Hydroxy-Allenynes
(for 7) conversions were observed after 15 min when using
[AuCl(PPh3)]/AgSbF6. We feel that these interesting results
highlight a different behavior between our catalyst system and
the well-known [AuCl(PPh3)] catalyst. This difference in reac-
tivity could find its origin from electronic and steric factors
as well. In fact the π-acid property of our catalyst precursor
was measured by converting 2a-OTf to the related selenide com-
plex [Cp*Ru(η6-C6H5-PPh2dSe)][OTf] (8-OTf); subsequent
measurement of the 1JP-Se coupling constant provides a value
of 775 Hz, which is higher than that obtained for PPh3dSe (9)
with 1JP-Se=724 Hz. These data suggest that coordination to
Cp*Ru decreases the electron density on the phosphine ligand.14
Furthermore the steric hindrance around the cationic gold
complex 3a is also increased due to the metalated phosphino
ligand 2a by comparison to the [AuCl(PPh3)] catalyst. All in all,
these metalated phosphino ligands provide an interesting type of
gold complexes with different electronic and steric properties
from known gold compounds. Therefore we intend in the future
to fully investigate the catalytic activity of this novel type of gold
complexes anchored to metalated arene platforms.
In conclusion, we have reported a novel approach to
prepare metalated phosphino ligands 2a-d, which can be
easily converted to a unique type of cationic gold complexes,
3a-d, where the -PPh2-Au-Cl unit is now attached to a
metalated π-arene platform, [Cp*Ru(η6-arene)][OTf]. Pre-
liminary studies suggest that 3a-OTf is active in metal-
catalyzed cyclization reactions and especially toward β-
hydroxy-allenynes 4 and 5, where our catalyst is more
efficient than the well-known [AuCl(PPh3)] complex.
Further studies are underway to tackle the electronic and
steric properties of such metalated phosphino ligands and
the influence of the functionalized group at the π-arene on
the selectivity and catalytic activity of the gold complexes.
These results will be reported in future reports. We also
intend to prepare novel compounds with planar chirality.15
Figure 2. Comparative structural study between [AuCl(PPh3)]
and complex 3a. Full space-filling projection shows that the two
sides around the gold center are no longer equivalent.
Crystals of 3a-OTf were grown by vapor diffusion of
diethyl ether into a CH2Cl2 solution.11 The structure
indeed confirmed the formation of the binuclear triflate
salt 3a-OTf and revealed the presence of the “-PPh2-
Au-Cl” unit, which is attached to the metalated arene
platform “Cp*Ru(C6H5-)” (Figure 1). Due to the orientation
of the phenyl groups, the Au-Cl leans toward the “Cp*Ru”
moiety, perhaps to minimize any steric hindrance (Figure 1). We
also note that the Cp*Ru moiety is symmetrically coordinated to
the η6-phenyl group. The other three cationic gold complexes,
3b-d, were prepared in a similar way starting from the metalated
phosphino ligands 2b-d. All these compounds were fully
characterized by spectroscopic methods and elemental analysis
(see Supporting Information).
For comparison purposes and in order to establish a structure-
reactivity relationship, we compared the solid-state struc-
ture of our compound 3a to the well-known complex [AuCl-
(PPh3)] (Figure 2).12 At first glance and in contrast to that of
[AuCl(PPh3)], the two sides of the cationic gold complex 3a are
no longer equivalent at least in the solid state. Indeed one face of
this complex is closer to the Cp*Ru, which would hinder the
approach of the substrate, while the other side is open; thus one
may expect a different reactivity compared to the parent
compound [AuCl(PPh3)].
On the other hand, when compound 3a-OTf was treated with
AgOTf in CD3CN, a white precipitate was obtained due to the
1
formation of AgCl. Analysis by H and 31P NMR suggested
formation of the presumably solvated species [Cp*Ru(η6-
C6H5-PPh2-Au-solvent)]2þ, thus precluding any decomposi-
tion reaction.
The catalytic activity of the novel binuclear compound 3a-
OTf was then tested in an alkoxycyclization process and
compared to its homologue [AuCl(PPh3)]. When β-hydroxy-
allenynes 4and 5were submitted to 2 mol% of 3a-OTf/AgSbF6
in dichloromethane at room temperature, the allenic moiety
was activated selectively and dihydropyrans 6 and 7 were
formed in 75% and 72% yield, respectively (Scheme 2).13 This
catalytic system provided fast and efficient transformations,
whereas under the same conditions, only 16% (for 6) and 9%
Acknowledgment. This work was supported by the
CNRS, IUF, and Agence Nationale de la Recherche
(grant SACCAOR, ANR-09-BLAN0108), which we
gratefully acknowledge.
Supporting Information Available: Experimental procedures,
spectroscopic data of all metal complexes and organic products,
as well as a CIF file including X-ray crystallographic data for 3a-
OTf. This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) Crystal data of 3a (CCDC-780252): C28H30AuClPRu, CF3O3S,
CH2Cl2., M=964.97 g/mol, monoclinic system, space group P21/c, a=
˚
˚
˚
9.3654(10) A, b=15.6752(19) A, c=22.506(3) A, β=90.619(11)°, V=
3
3303.8(7) A , Z=4, F=1.940 g cm-3, μ=5.295 mm-1, T=200(2) K, λ=
(14) We would like to thank the reviewer for pointing this out. (a)
Allen, D. W.; Taylor, B. F. J. Chem. Soc., Dalton Trans. 1982, 51. (b)
Allen, D. W.; March, L. A. I.; Nowell, W. J. Chem. Soc., Dalton Trans.
1984, 483. (c) Muller, A.; Otto, S.; Roodt, A. Dalton Trans. 2008, 650.
(15) (a) Gladysz, J. A.; Boone, B. J. Angew. Chem., Int. Ed. 1997, 36,
550. (b) Chirality in Transition Metal Chemistry: Molecules, Supramole-
cular Assemblies and Materials; Amouri, H., Gruselle, M., Eds.; Wiley:
Chichester, UK, 2008.
˚
3
˚
0.71073 A (Mo KR), 31 622 reflections measured, 10 209 unique, Rint
0-.0430, R1[I>2σ(I)]=0.0322, wR2(all data)=0.0722. ΔFmin/max=-1.16/0.79
=
3
e /A .
˚
(12) Baenziger, N. C.; Bennett, W. E.; Soborofe, D. M. Acta Crystal-
logr. 1976, B32, 962.
(13) Zriba, R.; Gandon, V.; Aubert, C.; Fensterbank, L.; Malacria,
M. Chem.;Eur. J. 2008, 14, 1482.