Chiral-at-metal ruthenium complex as a metalloligand for asymmetric catalysis

Article abstract of DOI:10.1021/ic7005502

The mononuclear [Ru(bpy)₂(bpym)][PF₆]₂ complex (bpy = 2,2′-bipyridine; bpym = 2,2′-bipyrimidine) has been prepared in its enantiopure Λ form. Because of the chelating property of the bipyrimidine moiety, it is possible to use this chiral-at-metal complex as a chiral inorganic ligand for a second metal cation acting as a catalytic center. Here we report the synthesis and the structural characterization of a novel dinuclear Λ-[(bpy)₂Ru(bpym)RuCl(p-cymene)]³⁺ compound (1). The asymmetric-inducing properties of the enantiopure chiral-at-metal metalloligand have been probed during asymmetric transfer hydrogenation to ketones catalyzed by 1. This provides one of the very few illustrations of the potential of this original class of chiral inorganic ligands.

Full text of DOI:10.1021/ic7005502

Inorg. Chem. 2007, 46, 5354−5360
Chiral-at-Metal Ruthenium Complex as a Metalloligand for Asymmetric
Catalysis
Olivier Hamelin,*,† Mickael Rimboud,† Jacques Pe´caut,‡ and Marc Fontecave†
Laboratoire de Chimie et Biologie des me´taux, iRTSV, UniVersite´ Joseph Fourier/CEA/CNRS,
CEA Grenoble, 17 aVenue des Martyrs, 38054 Grenoble cedex, France, and SerVice de Chimie
Inorganique et Biologique, DRFMC, CEA-Grenoble, 17 aVenue des Martyrs,
38054 Grenoble cedex, France
Received March 23, 2007
The mononuclear [Ru(bpy)2(bpym)][PF6]2 complex (bpy
prepared in its enantiopure form. Because of the chelating property of the bipyrimidine moiety, it is possible to
use this chiral-at-metal complex as a chiral inorganic ligand for a second metal cation acting as a catalytic center.
Here we report the synthesis and the structural characterization of a novel dinuclear -[(bpy)2Ru(bpym)RuCl(p-
) 2,2′-bipyridine; bpym ) 2,2′-bipyrimidine) has been
Λ
Λ
cymene)]3+ compound (1). The asymmetric-inducing properties of the enantiopure chiral-at-metal metalloligand have
been probed during asymmetric transfer hydrogenation to ketones catalyzed by 1. This provides one of the very
few illustrations of the potential of this original class of chiral inorganic ligands.
Introduction
knowledge, such a strategy, based on chiral-at-metal met-
alloligand without any chiral organic ligand, was reported
only once in the literature and was illustrated by rhodium/
chiral rhenium dinuclear complexes as catalysts for enantio-
selective hydrosilylation of ketones and hydrogenation of
dehydroamino acids.3 Herein, we report our preliminary
investigations in the development of an original dinuclear
ruthenium complex bearing an enantiopure chiral metallo-
ligand. Synthesis, characterization of this complex and
evaluation of its catalytic properties regarding asymmetric
transfer hydrogenation to ketones are reported.
In general, asymmetric metal-dependent catalysts are based
on a combination of a metal ion at which the reaction takes
place and a chiral organic ligand responsible for the
asymmetric environment. However, at present, the inherent
chirality of inorganic complexes (chirality ∆ and Λ for
octahedral complexes for example) as a source of such an
asymmetric environment has been only very rarely exploited.1
This approach requires that a method is available for the
preparation of the chiral-at-metal complex in an enantiopure
(or at least enantioenriched) form. This, in the absence of a
chiral organic ligand, remains a challenging issue. Recently,
using ruthenium based complexes, because of their well-
known relative configurationnal stability, we demonstrated
that chiral mononuclear complexes, in which the only
stereogenic center was the metal ion itself, were able to
catalyze asymmetric reactions.2 An evolution of our research
program was to design “chiral-at-metal” complexes so that
they display free coordinating moieties for binding a second
metal ion on which the reaction would take place. To our
Experimental Section
Materials. Ru(bpy)2Cl2·2H2O was purchased from Strem Chemi-
cals and 2,2′-bipyrimidine from Alfa Aesar. All the ketones used
44
as substrates were commercially available. Ru(dmp)2Cl2 was
prepared according to described procedure. [Me2NH2][(∆,δ)-
Binphat] was provided by J. Lacour (Universite´ of Geneva, Geneva,
Swizerland). Solvents used in synthetic procedures were analytical
grade. All the experiments were carried under dark conditions to
avoid any racemization process and under argon.
Instruments. NMR spectra were recorded on Bruker EMX 300
MHz. Electrospray mass spectrometry was performed on a Finnigan
LC-Q instrument. Absorption spectra were recorded with a Hewlett-
* To whom correspondence should be addressed. Tel: 33438789108.
Fax: 33438789124. E-mail: ohamelin@cea.fr.
† Universite´ Joseph Fourier/CEA/CNRS.
‡ Service de Chimie Inorganique et Biologique, DRFMC.
(1) Fontecave, M.; Hamelin, O.; Me´nage, S. Top. Organomet. Chem. 2005,
15, 271-288.
(2) Chavarot, M.; Me´nage, S.; Hamelin, O.; Charnay, F.; Pe´caut, J.;
Fontecave, M. Inorg. Chem. 2003, 42, 4810.
(3) Kromm, K.; Osburn, P. L.; Gladysz, J. A. Organometallics 2002, 21,
4275.
(4) Goldstein, A. S.; Beer, R. H.; Drago, R. S. J. Am. Chem. Soc. 1994,
116, 2424.
5354 Inorganic Chemistry, Vol. 46, No. 13, 2007
10.1021/ic7005502 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/26/2007

Metalloligand for Asymmetric Catalysis
ꢀ, M-1 cm-1) 240 (26 157), 286 (37 985), 425 (6478), 460 (4730);
Anal. Calcd for C28H22N8F12P2Ru,2 H2O: C, 37.47; H, 2.92; Ru,
11.26. Found: C, 37.73; H, 2.82; Ru, 10.95.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Complexes
[1][PF6]3, [2][PF6]3, [4][PF6]2, [5][Cl]a
complex
bond (Å)
angle (deg)
Λ-[(bpy)2Ru(bpym)][PF6]2, (Λ-[3][PF6]2).
A solution of
[1][PF6]3 Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-N(3)
Ru(1)-N(4)
Ru(1)-N(5)
Ru(1)-N(7)
Ru(2)-N(6)
Ru(2)-N(8)
Ru(2)-Cl
2.067(3)
2.053(3)
2.061(3)
2.062(3)
2.083(3)
2.081(3)
2.109(3)
2.120(3)
N(1)-Ru(1)-N(5) 100.25(11)
N(1)-Ru(1)-N(3) 172.63(11)
N(3)-Ru(1)-N(7) 94.82(11)
N(3)-Ru(1)-N(5) 86.30(11)
N(1)-Ru(1)-N(7) 89.74(11)
N(2)-Ru(1)-N(3) 95.04(12)
Λ-[(bpy)2Ru(py)2][dibenzoyl-L-tartrate] (623.0 mg, 0.67 mmol) and
2,2′-bipyrimidine (106.3 mg, 0.67 mmol) in ethylene glycol (240
mL) was heated at 120 °C for 5 h in dark then cooled to RT. Water
(150 mL) then NH4PF6 (876.0 mg, 5.4 mmol) were added to the
solution. Three extractions of the aqueous phase with dichlo-
romethane, concentration in vacuo, and subsequent recrystallization
by slow vapor diffusion of ether in an acetone solution of the
complex gave the pure complex (472 mg, 81.5%) as a dark powder.
Spectroscopic data are similar to the racemic mixture. CD for
Λ-[(bpy)2Ru(bpym)][PF6]2 (dichloromethane): λmax (∆ꢀ) ) 223
(-0.1), 235 (2.7), 305 (0.8), 376 (-2.3), 500 (1.02 M-1 cm-1).
rac-[(dmp)2Ru(bpym)][PF6]2, (rac-[4][PF6]2). The dmp com-
plex was prepared in an analogous manner to rac-[(bpy)2Ru(bpym)]-
[PF6]2 except that Ru(dmp)2Cl2 was used in place of Ru(bpy)2Cl2.
The complex was obtained in 93% yield. Single crystals were
obtained by slow diffusion of ether in a solution of the complex in
acetone. 1H NMR (300 MHz, acetone-d6) δ 9.01 (dd, 2H, J ) 1.9,
4.7 Hz), 8.88 (d, 2H, J ) 8.4 Hz), 8.59 (d, 2H, J ) 8.4 Hz), 8.42
(d, 2H, J ) 8.7 Hz), 8.31 (d, 2H, J ) 8.7 Hz), 7.98 (d, 2H, J ) 8.7
Hz), 7.81 (dd, 2H, J ) 1.9, 5.9 Hz), 7.68 (d, 2H, J ) 8.4 Hz), 7.38
(dd, 2H, J ) 4.7, 5.9 Hz), 2.18 (s, 6H), 2.10 (s, 6H); mass spectrum
(ESI, acetone): m/z (relative intensity) 821 {[4][PF6]}+ (31), 338
{[4]}2+ (100); MS (ESI-, acetone): m/z (%) 1111 {[4][PF6]3}-
(100); UV-vis (CH2Cl2; λmax, nm; ꢀ, M-1 cm-1)) 232 (62 212),
268 (72 439), 305 (19 583), 385 (9573), 432 (8685), 482 (4588);
Anal. Calcd for C36H30N8F12P2Ru, 3H2O: C, 42.40; H, 3.56; N,
10.99; Ru, 9.91. Found: C, 42.88; H, 3.28; N, 11.10; Ru, 9.33;
crystallographic data: monoclinic, P2(1)/c, a ) 17.768(3) Å, b )
13.746(2) Å, c )18.781(3) Å, V ) 4586.2(14) Å3, Z ) 4, R )
0.0876, Rw ) 0.2270. (CCDC 615941).
N(6)-Ru(2)-Cl
N(8)-Ru(2)-Cl
83.45(9)
81.99(9)
68.5
2.3978(12) Ru(1)-Ru(2)-Cl
[2][PF6]3 Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-N(3)
Ru(1)-N(4)
Ru(1)-N(5)
Ru(1)-N(7)
Ru(2)-N(6)
Ru(2)-N(8)
Ru(2)-Cl
2.095(3)
2.105(3)
2.103(3)
2.122(3)
2.093(3)
2.092(3)
2.118(3)
2.098(4)
N(1)-Ru(1)-N(5) 93.85(13)
N(2)-Ru(1)-N(3) 100.59(14)
N(4)-Ru(1)-N(7) 80.36(13)
N(4)-Ru(1)-N(5) 95.76(13)
N(1)-Ru(1)-N(4) 101.66(13)
N(3)-Ru(1)-N(5) 171.10(13)
N(6)-Ru(2)-Cl
N(8)-Ru(2)-Cl
85.10(10)
82.85(11)
78.5
2.4067(15) Ru(1)-Ru(2)-Cl
[4][PF6]2 Ru-N(1)
Ru-N(2)
2.100(6)
2.104(6)
2.111(6)
2.098(6)
2.078(6)
2.082(6)
2.090(4)
2.075(5)
2.4016(15)
N(2)-Ru-N(4)
N(1)-Ru-N(3)
N(2)-Ru-N(6)
N(3)-Ru-N(5)
101.7(2)
101.5(2)
81.6(2)
81.4(2)
Ru-N(3)
Ru-N(4)
Ru-N(5)
Ru-N(6)
[5][Cl]
Ru-N(1)
Ru-N(2)
Ru-Cl
N(1)-Ru-Cl
N(2)-Ru-Cl
81.79(13)
83.53(13)
a The estimated standard deviations in the least significant digits are given
in parentheses.
Packard 8453 spectrometer. Circular dichroism spectra were
recorded on a JASCO J-810 spectropolarimeter at 25 °C with a 1
cm path length cell. Data collection was performed at 298 K using
a Bruker SMART diffractometer with a charged couple device
(CCD) area detector, with graphite-monochromated Mo KR radia-
tion (λ ) 0.71073 Å). The molecular structure was solved by direct
methods and refined on F2 by full matrix least-squares techniques
using SHELXTL package with anisotropic thermal parameters. All
non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were placed in ideal positions and refined as riding atoms
with individual isotopic displacement parameters. Pertinent crystal-
lographic data are summarized in Table 1.
Synthesis of Compounds. rac-[(bpy)2Ru(bpym)][PF6]2, (rac-
[3][PF6]2). To a solution of Ru(bpy)2Cl2 (500 mg, 1.03 mmole) in
absolute ethanol (90 mL), 2,2′-bipyrimidine (163.3 mg, 1.03 mmol)
was added at once. The resulting solution was refluxed for 4 h.
After concentration in vacuo, dissolution of the residue in water
(addition of a small volume of ethanol is sometimes required for
complete dissolution), the complex was precipitated by addition
of NH4PF6 (1.67 g, 10,3 mmoles), filtrated, and washed three times
with water. The resulting powder was dissolved in a minimum of
acetone, and the complex precipitated once again by addition of
the solution in a large volume of ether. After filtration, the complex
was purified by silica gel chromatography (10%, 30% then 50%
of (10% aqueous solution of saturated KNO3 solution)-acetone).
Metathesis of the anion was achieved after evaporation of the
solvents, dissolution with a minimum of water and addition of NH4-
PF6 (1.67 g, 10.3 mmol). After filtration and drying in vacuo, the
complex (755 mg, 85%) was obtained as a dark brown powder. 1H
NMR (300 MHz, acetone-d6) δ 9.20 (dd, 2H, J ) 2.0, 4.7 Hz),
8.88 (d, 4H, J ) 8.1 Hz), 8.46 (dd, 2H, J ) 2.0, 5.7 Hz), 8.26 (d,
2H, J ) 5.6 Hz), 8.15-8.28 (m, 4H), 8.05 (d, 2H, J ) 5.1 Hz),
7.72 (t, 2H, J ) 7.2 Hz), 7.50-7.65 (ddd, 4H, J ) 6.6, 6.6, 1.4
Hz); mass spectrum (ESI, acetone): m/z (relative intensity) 717
{[3][PF6]}+ (37), 286 {[3]}2+ (100); UV-vis (CH2Cl2; λmax, nm;
rac-[Ru(bpy)2(bpym)RuCl(p-cymene)][NO3]3, (rac-[1][NO3]3).
A solution of rac-[3][PF6]2 (30 mg, 34.8 µmol) in absolute ethanol
(5 mL) and [RuCl2(p-cymene)]2 (10.6 mg, 17.4 µmol) was heated
to 35 °C for 3 h under argon atmosphere. The resulting solution
was concentrated and purified by silica gel chromatography (30%
of (10% aqueous solution of saturated KNO3 solution)-acetone.
After evaporation of the solvent, the excess of KNO3 was
precipitated by addition of ethanol and then filtered off. This
procedure was repeated twice and afforded the pure complex (28
1
mg, 78%) as a dark powder. H NMR (300 MHz, methanol-d4) δ
9.84 (dd, 1H, J ) 5.6, 1.2 Hz), 9.78 (dd, 1H, J ) 5.7, 1.3 Hz),
8.72 (d, 1H, J ) 8.0 Hz), 8.71 (d, 1H, J ) 8.0 Hz), 8.62 (t, 2H, J
) 8.3 Hz), 8.40-8.60 (m, 2H), 8.35 (dd, 1H, J ) 5.6, 1.2 Hz),
8.05-8.30 (m, 4H), 7.88 (t, 1H, J ) 5.7 Hz), 7.84 (t, 1H, J ) 5.7
Hz), 7.72 (t, 2H, J ) 5.2 Hz), 7.60-7.70 (m, 2H), 7.30-7.60 (m,
3H), 6.30 (m, 2H), 6.17 (m, 2H), 3.29 (h, 1H, J ) 6.7 Hz), 2.26 (s,
3H), 1.32 (d, 3H, J ) 6.7 Hz), 1.30 (d, 3H, J ) 6.7 Hz); MS (ESI+,
H2O): m/z (%) 967 {[1][NO3]2}+ (3), 452 {[1][NO3]}2+ (100), 286
{[3]}2+; mass spectrum (ESI, methanol): m/z (relative intensity)
936 {[1][MeO,NO3]2}+ (3), 436 {[1][MeO]}2+ (23), 286 {[3]}2+
(100); UV-vis (H2O; λmax, nm; ꢀ, M-1 cm-1)) 243 (26 264), 277
(40 121), 310 (10 897), 415 (23 414), 575 (3296); HRMS (ES) for
C38H36N8F12P2ClRu2; calcd 1133.0122; Found 1133.0101; Anal.
Calcd for C38H36N11O9Ru2Cl‚2KNO3‚2EtOH: C, 38.14; H, 3.66;
N, 13.77; Cl, 2.68. Found: C, 37.95; H, 3.42; N, 13.71; Cl, 3.17;
crystallographic data: triclinic, P1h, a ) 13.271(2) Å, b ) 13.698(2)
Å, c )16.882(3) Å, V ) 2540.1(7) Å3, Z ) 2, R ) 0.0595, Rw
0.1133. (CCDC 615939).
)
Inorganic Chemistry, Vol. 46, No. 13, 2007 5355

Hamelin et al.
Scheme 1. Preparation of the Catalysts [1]3+ and [2]3+
Λ-[Ru(bpy)2(bpym)RuCl(p-cymene)][NO3]3, (Λ-[1][NO3]3).
The compound Λ-[1][NO3]3 was prepared following the procedure
used for rac-[1][NO3]3 and was obtained in similar yield. CD for
Λ-[1][NO3]3 (H2O): λmax (∆ꢀ) ) 220 (11.3), 242 (3.2), 292 (3.1),
276 (3.0), 391 (-2.1)(M-1 cm-1).
then added. The resulting solution was then heated to 80 °C before
addition of the substrate (434.00 µmol).The reaction mixture was
then stirred at 80 °C for 24 h, and the product was extracted three
times with CH2Cl2. After concentration in vacuo, the resulting
mixture was analyzed and quantified (TON and ee) by H NMR
and chiral GC.
1
rac-[Ru(dmp)2(bpym)RuCl(p-cymene)][NO3]3, (rac-[2][NO3]3).
The compound rac-[2][NO3]3 was prepared using the same
procedure as for rac-[1][PF6]2 and was obtained in 64% yield. H
Enantiomeric Excesses Determination. Enantiomeric excesses
of the alcohols were determined by GC using two different chiral
columns (Lipodex E from Macherey-Nagel and B-PM from
Chiraldex).
1
NMR (300 MHz, methanol-d4) δ 9.65 (dd, 1H, J ) 5.1, 1.5 Hz),
9.56 (dd, 1H, J ) 5.4, 0.9 Hz), 8.79 (d, 2H, J ) 8.4 Hz), 8.51 (d,
1H, J ) 8.1 Hz), 8.36 (d, 1H, J ) 8.4 Hz), 8.20-8.40 (m, 3H),
8.13 (d, 1H, J ) 8.7 Hz), 7.89 (d, 1H, J ) 8.7 Hz), 7.86 (d, 1H,
J ) 8.4 Hz), 7.79 (dd, 2H, J ) 5.7, 1.2 Hz), 7.60 (d, 1H, J ) 8.1
Hz), 7.55-7.35 (m, 4H), 6.16 (t, 2H, J ) 6.3 Hz), 5.99 (m, 2H),
2.88 (h, 1H, J ) 6.9 Hz), 2.34 (s, 3H), 2.10 (s, 3H), 2.00 (s, 3H),
1.99 (s, 3H), 1.97 (s, 3H), 1.26 (d, 3H, J ) 7.2 Hz), 1.24 (d, 3H,
J ) 7.2 Hz); mass spectrum (ESI, water): m/z (relative intensity)
1071 {[2][NO3]2}+ (2), 504 {[2][NO3]}2+ (55), 338 {[4]}2+ (100);
MS (ESI+, MeOH): m/z (%) 1040 {[2][MeO,NO3]2}+ (6),436 {[2]-
[MeO]}2+ (27), 286 {[3]}2+ (100); UV-vis (H2O; λmax, nm; ꢀ, M-1
cm-1) 265 (38 900), 295 (18 284), 370 (7294), 430 (9256), 600
(3612); Anal. Calcd for for C46H44N8Ru2ClP3F18, 4H2O: C, 38.01;
H, 3.61; N, 7.71; Cl, 2.44. Found: C, 38.28; H, 3.16; N, 7.77; Cl,
2.44; crystallographic data: monoclinic, P2(1)/c, a ) 15.566(5)
Å, b ) 17.247(5) Å, c )25.599(8) Å, V ) 6711(4) Å3, Z ) 4, R
) 0.0963, Rw ) 0.1745. (CCDC 615940).
[(bpym)RuCl(p-cymene)][Cl], ([5][Cl]). A solution of [RuCl2-
(p-cymene)]2 (200 mg, 653.0 µmol) and 2,2′-bipyrimidine (105 mg,
664.0 µmol) in absolute ethanol (50 mL) was refluxed for 4 h. The
resulting yellow solution was concentrated in vacuo. The crude
product was solubilized in a minimum of ethanol, and the complex
was precipitated by addition of the solution in a large volume of
ether. After drying in vacuo, the complex was obtained as a yellow
solid (285 mg, 94%). 1H NMR (300 MHz, acetone-d6) δ 9.89 (dd,
2H, J ) 5.7, 2.1 Hz), 9.31 (dd, 2H, J ) 4.8, 2.1 Hz), 8.00 (d, 2H,
J ) 5.7, 4.8 Hz), 6.31 (d, 2H, J ) 6.3 Hz), 6.12 (d, 2H, J ) 6.3
Hz), 2.91 (h, 1H, J ) 6.9 Hz), 2.27 (s, 3H), 1.19 (d, 6H, J ) 6.9
Hz); mass spectrum (ESI, methanol): m/z (relative intensity) 429
{[5]}+ (100); UV-vis (MeOH; λmax, nm; ꢀ, M-1 cm-1) 211
(20 493), 240 (17 152), 425 (4826), 635 (2354); Anal. Calcd for
for C18H20N4RuCl2, 3H2O: C, 41.70; H, 5.06; N, 10.81; Cl, 13.68.
Found: C, 41.26; H, 4.09; N, 10.84; Cl, 13.58; crystallographic
data: monoclinic, P2(1)/n, a ) 14.272(6) Å, b ) 16.680(7) Å, c
)16.871(7) Å, V ) 3976(2) Å3, Z ) 8, R ) 0.0924, Rw ) 0.1743.
(CCDC 615942).
Results and Discussion
Preparation of the Complexes. The syntheses of two new
original dinuclear ruthenium complexes rac-[(diimine)2Ru-
(bpym)RuCl(p-cymene)][NO3]3 (diimine ) bipyridine (bpy)
for compound [1][NO3]3 or 2,9-dimethyl-1,10-phenanthroline
(dmp) for compound [2][NO3]3; bpym ) 2,2′-bipyrimidine)
were easily and efficiently achieved in two steps from
commercially available [Ru(bpy)2Cl2] for [1]3+ and [Ru-
(dmp)2Cl2]5 for [2]3+ (Scheme 1). First, rac-[3][PF6]2 and
rac-[4][PF6]2 were obtained by substitution of the two chloro
ligands by the bpym bridging ligand. Subsequent reaction
of rac-[3][PF6]2 and rac-[4][PF6]2 with [RuCl2(p-cymene)]2
followed by anion metathesis during the chromatographic
purification afforded the complexes rac-[1][NO3]3 and rac-
[2][NO3]3 in 66% and 60% overall yield, respectively. The
complex [5][Cl] was easily prepared in high yield (94%) by
refluxing an ethanolic solution of [RuCl2(p-cymene)]2 in the
presence of 1 equiv of bpym for 4 h.
Single crystals suitable for X-ray analysis were obtained
by slow diffusion of diethylether into solutions of the
complexes in acetone for [1] [PF6]3, [2][PF6]3, and [4][PF6]2
and slow diffusion of an ether-acetone mixture into a
solution of the complex in an ethanol-acetonitrile mixture
for [5][Cl]. The X-ray crystal three-dimensional structures
of the complexes are shown in Figure 1. For [5][Cl], angles
and bond lengths are in the range to those reported for the
complex [(η6-C6Me6)Ru(bpy)(H2O)]2+.5 Selected bond lengths
and angles are listed in Table 1. Complex [4][PF6]2 shows
an highly distorted octahedral geometry, probably due to the
steric repulsion generated by one of the two methyls
substituents of each of the dmp ligands in combination with
the fact that the bite angle of the phenanthroline chelates is
less than 90°. Indeed some cis N-Ru-N angles are increased
Standard Conditions for Transfer Hydrogenation of Ketones.
The reaction was carried out under argon atmosphere, and care was
taken to avoid exposure to light. To a solution of HCO2Na (897.5
mg, 13.20 mmol) in water (6 mL) whose pH was adjusted to 4 by
addition of pure HCO2H (295 µL); the catalyst (2.17 µmol) was
(5) Hogo, S.; Abura, T.; Watanabe, T. Organometallics 2002, 21, 2964.
5356 Inorganic Chemistry, Vol. 46, No. 13, 2007

Metalloligand for Asymmetric Catalysis
Figure 1. ORTEP views of the structures of [1]3+, [2]3+, [4]2+, and [5]+. Hydrogen atoms have been omitted for clarity.
by as much as 11° with regard to those of a pure octahedron,
whereas some others are decreased by about 9°. This is also
true for complex [2]3+ and was also observed in the case of
other members of the [(dmp)2RuXX′]2+ family.6 It is
noticeable that such a deformation is not as high when bpy
is use in place of dmp ligand. For example, the N(1)-Ru(1)-
N(3) angle is 101.7° in [2]3+ and [4]2+, whereas the
corresponding N(2)-Ru(1)-N(3) angle is 95.04° in [1]3+.
Also, it is worth noting that a steric repulsion between the
isopropyl residue and one of the two bipyridines in complex
[1]3+ generates an important curvature of the bridging bpym
ligand. Such a phenomenon is not observed in complex [2]3+.
This leads to an important effect on the position of the chloro
ligand with regard to the Ru(1)-Ru(2) axis. Indeed, the Ru-
(1)-Ru(2)-Cl angle is 68.5° in [1]3+ and 78.7° in [2]3+.
Preparation of the Enantiopure Complexes. It was
reported that substitution by mono- or bidentate ligands of
one or the two pyridine ligands of the chiral compound (∆-
or Λ-) [Ru(bpy)2py2]2+ could be done without any racem-
ization.7 Consequently, Λ-[Ru(bpy)2py2][(-)-O,O′-diben-
zoyl-L-tartrate] was prepared according to literature proce-
dures.8 Displacement of both pyridine ligands by bipyrimidine
was then achieved in hot ethylene glycol solution.9 After
anion metathesis, extraction with dichloromethane and re-
crystallization by slow diffusion of ether in a solution of the
crude complex in acetone, [3][PF6]2 was obtained in 81%
yield. The latter was then converted to complex [1]3+ as
described for the racemic complex with similar yield. The
absolute configuration Λ for [1][NO3]3 and [3][PF6]2 was
assigned by comparison with the CD spectrum obtained from
the initial Λ-[Ru(bpy)2py2][(-)-O,O′-dibenzoyl-L-tartrate]
complex (Figure 2).10 Indeed, the three complexes show a
similar spectral pattern with with Cotton effects of the same
sign in absorption wavelength regions of electronic transitions
common to all off them. In contrast, we were unable to obtain
[4]2+ and thus [2]3+ in their enantiomerically pure or enriched
forms by resolution.
The enantiomeric purity of Λ-[1]3+ was determined by
1H NMR using Binphat11 as a chiral shift reagent. The NMR
spectra of rac-[1][PF6]3 and Λ-[1][PF6]3 obtained from their
(6) Hamelin, O.; Pe´caut, J.; Fontecave, M. Chem. Eur. J. 2004, 10, 2548.
(7) (a) Brissard, M.; Convert, O.; Gruselle, M.; Guyard-Duhayon, C.;
Thouvenot, R. Inorg. Chem. 2003, 42, 1378. (b) Hui, C.; Jin-Gang,
L.; Cai-Wu, J.; Liang-Nian, J.; Xiao-Yuan, L.; Chuan-Li, F. Inorg.
Chem. Commun. 2001, 45. (c) Hua, X.; von Zelewsky, A. Inorg. Chem.
1995, 34, 5791.
(8) (a) Hua, H.; von Zelewsky, A. Inorg. Chem. 1995, 34, 5791. (b) Hua,
X.; von Zelewsky, A. Inorg. Chem. 1995, 34, 992.
(9) Tzalis, D.; Tor, Y. J. Am. Chem. Soc. 1997, 119, 852.
(10) (a) Bosnich, B. Acc. Chem. Res. 1969, 2, 266. (b) Richardson, F. S.
Chem. ReV. 1979, 79, 17. (c) Ziegler, M.; von Zelewsky, A. Coord.
Chem. ReV. 1998, 177, 257. (d) See also ref 8c.
Inorganic Chemistry, Vol. 46, No. 13, 2007 5357

Hamelin et al.
NO3 salts by simple anion metathesis were recorded after
dissolution in acetone-d6 in the presence of [Me2NH2][(∆,S)-
Binphat] and compared to those of the same solutions in the
absence of the chiral salt (Figure 3). Addition of an excess
(2.8 equiv) of [Me2NH2][(∆,S)-Binphat] to the solution of
the racemic mixture results in an impressive shift of the
resonances corresponding to the protons of the p-cymene
moiety, suggesting a π-π interaction between the cymene
ring and one of the aromatic rings of the Binphat. For
example, the singulet at 2.32 ppm corresponding to the
methyl substituent of the p-cymene was shifted and split into
two singulets at 1.49 and 1.71 ppm (localized by a star in
Figure 3). Such a π-stacking involving the Trisphat anion
was also observed by Amouri in the trans-[(Sp,Sp)-bis-
(Cp*Ru)-carbazolyl][∆-Trisphat] complex bearing a planar
chirality.12 On the other hand, when Binphat was added to a
Figure 2. Circular dichroism spectra of Λ-[Ru(bpy)2Py2][(-)-O,O′-
dibenzoyl-L-tartrate] in water (30 µM) (a), of [3][PF6]2 in dichloromethane
(30 µM) (b), and of [1][NO3]3 in water (65 µM) (c).
Figure 3. 1H NMR spectra (300 MHz, acetone-d6) of the aromatic and aliphatic protons of the p-cymene moiety for the racemic [1][PF6]3 in the absence
(a) and in the presence (b) of 2.8 equiv of [Me2NH2][(∆,S)-Binphat] and for the Λ-[1][PF6]3 in the presence of 1.7 (c) and 4.8 (d) equiv of [Me2NH2]-
[(∆,S)-Binphat].
5358 Inorganic Chemistry, Vol. 46, No. 13, 2007

Metalloligand for Asymmetric Catalysis
nuclear [5][Cl] species. Thus, coupling of the active Ru(p-
cymene) moiety to a Ru(diimine) moiety is beneficial.
Moreover, we observed that when nitric acid was used in
order to adjust the pH, as was described by Watanabe and
co-workers, the reaction did not work.
The enantiopure catalyst Λ-[1][NO3]3 was then assayed
during hydrogenation of a variety of ketones using the
previously described procedure (catalyst/substrate/HCO2Na
) 0.36:72:2160 mM, pH 4). All the reactions were stopped
after 24 h, and the resulting products were extracted with
dichloromethane. Conversion and TON were both calculated
Figure 4. Catalytic hydrogen transfer to acetophenone using [1][NO3]3,
(a); [2][NO3]3, (b); [5][Cl], (c) and [RuCl2(p-cymene)]2 (d). Inset; evolution
of the enantiomeric excess during the course of the hydrogenation of
acetophenone with Λ-[1][NO3]3 as a catalyst.
1
from the resulting H NMR spectrum and from the amount
of remaining substrate. The enantiomeric excesses (ee’s) of
the products were determined by chiral GC with the
exception of 2-(2-hydroxyethyl)-pyridine for which the ee
1
solution of Λ-[1]3+, only one singlet and one doublet were
observed for the two types of methyl groups, thus demon-
strating the enantiopurity of [1]3+ (Figure 4).
was determined by H NMR using a large excess of (S)-
1,1′-binaphtol as a chiral shift reagent. As can be observed
in Table 2, conversions of aryl methylketones are moderate
to excellent ranging from 15% to 100%. As was expected,
the more electron-attracting the aryl substituent, the more
efficient the reaction. For instance, the hydrogenation of
4-methoxyphenylacetone was achieved with 44% conversion,
whereas 100% conversion was obtained whith trifluoro-
methyl as the substituent (entries 1a and 1e, respectively).
Interestingly, in the case of 4-nitrophenylacetone, a selective
and quantitative hydrogenation of the nitro group was
observed (entry 1d). Due to the very low reactivity of the
resulting 4-aminophenylacetone, no further hydrogenation
could be observed. Reduction of ethylacetoacetate afforded
the ethyl-3-hydroxybutyrate as the unique product resulting
from the chemioselective hydrogenation of the keto group
(entry 7). Finally, hydrogenation of two R-â unsaturated
ketones yielded to the resulting saturated alcohols (entries 9
and 10).
Asymmetric Catalytic Transfer Hydrogenation. Hy-
drogenation of acetophenone to the corresponding alcohol
using sodium formate as hydrogen donor in water was then
investigated as the probe reaction with racemic complexes.6
Assays were carried out first with the water soluble rac-[1]-
[NO3]3 and rac-[2][NO3]3 complexes and their reactivity
compared to that of [5][Cl] and of the commercially available
[RuCl2(p-cymene)]2 (Figure 2). All the experiments were
achieved under argon atmosphere and care was taken to avoid
exposure to light. After dissolution of the complex (0.36 mM)
in an aqueous solution of sodium formate (2.16 M), whose
pH was adjusted to 4 by controlled addition of pure formic
acid and subsequent heating to 80 °C, the substrate (72 mM)
was added. The reaction product was identified and quanti-
1
fied by GC-MS and H NMR. While [RuCl2(p-cymene)]2
was inactive, moderate to good catalytic activities were
obtained with [2][NO3]3 (68% yield, 120 TON after 4 h),
[1][NO3]3 (85%, 180 TON after 20 h), and [5][Cl] (95%,
190 TON after 35 h) (Figure 4). Examination of the initial
rates shows that the dinuclear compounds [1][NO3]3 and
[2][NO3]3 were significantly more active than the mono-
Finally, we observed that only alkyl aryl ketones afforded
significant, albeit low, ee’s ranging from 18% to 26% with
the exception of p-methoxyphenylacetone which, in addition,
showed low reactivity (entry 1a). However, while these ee
values are relatively smaller than the ones reported in the
Table 2. Enantioselective Transfer Hydrogenation of Ketones with Λ-[1][NO3]3 as a Catalysta
entries
substrate
product(s)
TONb
conversionc
ee (%)
1a
1b
1c
1d
1e
2
3
4
5
R) OCH3
R) CH3
R) H
R) NO2
R) CF3
p-methoxyphenyl ethanol
p-methylphenyl ethanol
1-phenylethanol
88
146
156
400
200
168
170
156
30
44
73
78
100
100
84
85
78
15
3
18
26 (S)
p-aminophenylacetone
-
p-trifluoromethyl phenylethanol
2-(2-hydroxyethyl) pyridine
methyl-2-naphthtalene methanol
methyl-1-naphthtalene methanol
4-methyl-1,2,3,4-tetrahydro-naphtalen-1-ol
(diastereomeric ratio; 1:1.1)
indan-2-ol
ethyl-3-hydroxybutyrate
4-phenylbutan-2-ol
cyclohexanol
21
2-acetylpyridine
22b
2′-acetonaphtone
1′-acetonaphtone
4-methyl tetralone
26
21
not
determined
6
7
8
9
indan-2-one
165
191
148
224
88
82.5
95.5
74
-
0d
0
ethylacetoacetate
4-phenylbutan-2-one
cyclohex-2-en-1-one
100
-
cyclohexanone (1.3:1)
10
(E)-4-phenyl-but-3-en-2-one
4-phenylbutan-2-ol
91
100
0
4-phenylbutan-2-one (1:3.4 ratio)
155
a The reaction was carried out at 80 °C using a ketone (0.43 mmol) in H2O (6 mL) with Λ-[1][NO3]3/ketone/HCOONa ) 1:200:6000. b Work up after
1
24 h of reaction; measured by H NMR. c 1H NMR determination using a large excess of (S)-(-)-1,1′-binaphtol as a chiral NMR shift reagent. d 1H NMR
determination using Tris[3-(trifluoromethyl-hydroxymethylen)-D-camphorato]-europium as a chiral NMR shift reagent.
Inorganic Chemistry, Vol. 46, No. 13, 2007 5359

Hamelin et al.
literature for the most efficient hydrogen-transfer asymmetric
catalysts,13 an interesting amplification of the chirality during
the course of the hydrogenation of acetophenone was
observed (Figure 2, Inset). Indeed, while the enantiomeric
excess was only of about 8% after 72 TON (1.5 h), it reached
20% after 169 TON (22 h). The same was observed with
2′-acetonaphtone (from 17% ee (6 h) to 26% (24 h); data
not shown) suggesting an asymmetric autoinduction. Such
an amplification of the chirality has been rarely observed,
however, in most cases with much larger final ee’s than those
reported here.14 Further experiments are required to define
the molecular basis for the observed phenomenon. In
particular, evidence for the formation of an interaction
between the product and Λ-[1]3+ should be searched for.
Conclusion
The novel class of chiral catalysts, based on a chiral-at-
metal metalloligand, for asymmetric catalysis opens a new
research direction. Obviously, further developments are
required to rival with the most efficient asymmetric transfer
hydrogenation catalysts developed to date bearing chiral
organic ligands. However, this first generation of catalysts
showed good catalytic activity with several hundred TON.
Other reactions will be investigated in the future using this
strategy.
Acknowledgment. We are grateful to J. Lacour (Uni-
versity of Gene`ve, Geneva, Switzerland) for a generous gift
of Trisphat and Binphat, J. Pe´caut (DRFMC, CEA-Grenoble)
for X-ray structures determination, and C. Lebrun (DRFMC,
CEA-Grenoble) for the ES-MS analysis.
(11) (a) Lacour, J.; Londez, A.; Goujon-Ginglinger, C.; Buss, V.; Bernar-
dinelli, G. Org. Lett. 2000, 2, 4185. (b) Lacour, J.; Hebbe-Viton, V.
Chem. Soc. ReV. 2003, 32, 373, and references cited therein.
(12) (a) Amouri, H.; Caspar, R.; Gruselle, M.; Guyard-Duhayon, C.;
Boubekeur, K.; Lev, D. A.; Collins, L. S. B.; Grotjahn, D. B.
Organometallics 2004, 23, 4338. See also (b) Govindaswamy, P.;
Linder, Lacour, J.; Su¨ss-Fink, G.; Therrien, B. Chem. Commun. 2006,
4691.
Supporting Information Available: Crystallographic data for
[1][PF6]3, [2][PF6]3, [4][PF6]2, and [5][Cl] (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
IC7005502
(13) (a) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Katayama,
E.; England, A. F.; Ikaria, T.; Noyori, R. Angew. Chem., Int. Ed. 1998,
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2006, 71, 7035. (c) Reetz, M.; Li, X. J. Am. Chem. Soc. 2006, 128,
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5360 Inorganic Chemistry, Vol. 46, No. 13, 2007