Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts

Article abstract of DOI:10.1039/c2dt30976a

The synthesis and characterization of optically active amino carboxylate complexes of formula [(η⁶-arene)Ru(Aa)Cl] (arene = C ₆H₆, C₆Me₆, Aa = amino carboxylate) as well as those of the related trimer

Full text of DOI:10.1039/c2dt30976a

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PAPER
Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer
catalysts†
Daniel Carmona,* Fernando Viguri,* M. Pilar Lamata, Joaquina Ferrer, Elisa Bardají, Fernando J. Lahoz,
Pilar García-Orduña and Luis A. Oro
Received 4th May 2012, Accepted 15th June 2012
DOI: 10.1039/c2dt30976a
The synthesis and characterization of optically active amino carboxylate complexes of formula [(η6-arene)-
Ru(Aa)Cl] (arene = C6H6, C6Me6, Aa = amino carboxylate) as well as those of the related trimers
[{(η6-arene)Ru(Aa)}3][BF4]3 are reported. Trimerization takes place with chiral self-recognition: only
diastereomers equally configured at the metal, RRuRRuRRu or SRuSRuSRu, are detected. The crystal
structures of the complexes [(η6-C6H6)Ru(Pip)Cl] and [{(η6-C6Me6)Ru(Pro)}3][BF4]3 have been
determined by X-ray diffraction methods. Both types of complexes catalyse the hydrogen transfer reaction
from 2-propanol to ketones with moderate enantioselectivity (up to 68% ee). The enantiodifferentiation
achieved can be accounted for by assuming that Noyori’s bifunctional mechanism is operating.
[(η6-arene)Ru(Aa)Cl] and [{(η6-arene)Ru (Aa)}3][BF4]3 (arene =
C6H6, C6Me6), as well as on their application as catalyst precur-
Introduction
Asymmetric transfer hydrogenation (ATH) of ketones is a funda-
mental process for the production of enantiomerically enriched
alcohols,1 which are valuable intermediates for the manufacture
of pharmaceuticals and advanced materials.2 Noyori et al.
showed that a ruthenium η6-arene complex containing monotosy-
lated 1,2-diamines could serve as efficient catalyst for the ATH
of ketones,1d,3 operating through a metal–ligand bifunctional
mechanism.4 After this discovery, a variety of new catalytic
ruthenium systems have been developed for this process,1l,m the
best results being obtained when an arene ruthenium(II) moiety
is associated with chiral diamines,1d,5 amino alcohols,1d–f,i,6 or
amino acid derivatives.7
On the other hand, L-α-amino acids are inexpensive chiral
materials that have been used for the synthesis of optically active
transition metal complexes.8,9 In particular, half-sandwich amino
carboxylate complexes, containing d6 metal ions, such as
rhodium(III), iridium(III) or ruthenium(II), have attracted consider-
able interest in recent years.9,10 However, the application of
amino carboxylate complexes as catalysts for ATH reactions
is very limited.11 In this context, we have reported that
(η5-C5Me5)M(III) (M = Rh, Ir) or (η6-p-MeC6H4iPr)M(II)
(M = Ru, Os) half-sandwich complexes with amino carboxylate
ligands12 efficiently catalyse the ATH of ketonic carbonyl
groups.12d,f–i
sors for the ATH reaction from 2-propanol to ketones, with the
aim of studying the effects of changing the ring substituents on
the catalyst while maintaining its half-sandwich structure and the
amino carboxylate ligand as a chiral source. Assumption of
Noyori’s bifunctional mechanism for the ATH reaction explains
the sign of the ee obtained.
Results and discussion
Synthesis and characterization of the chloro complexes 1–4
Chloro complexes of formula [(η6-arene)Ru(Aa)Cl] were
obtained by reacting the acetylacetonate compounds [(η6-arene)-
Ru(acac)Cl] (η6-arene = C6H6, C6Me6)13 with stoichiometric
amounts of the corresponding L-amino acid (eqn (1)). A sche-
matic representation of these complexes is depicted in Fig. 1.
½ðη6-areneÞRuðacacÞClꢀ þ HAa
! ½ðη6-areneÞRuðAaÞClꢀ þ Hacac
ð1Þ
In the present paper, we report on the preparation and charac-
terization of new ruthenium arene complexes of formulae
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC –
Universidad de Zaragoza, Departamento de Química Inorgánica, Pedro
Cerbuna, 12, 50009 Zaragoza, Spain.
E-mail: dcarmona@unizar.es, fviguri@unizar.es
†CCDC 881059 and 881060. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt30976a
Fig. 1 Schematic representation of the chloro complexes: R = H, R1 =
CH2Ph, R2 = Me (1); R = H, R1–R2 = (CH2)3 (2); R = H, R1–R2
(CH2)4 (3); R = Me6, R1–R2 = (CH2)3 (4).
=
10298 | Dalton Trans., 2012, 41, 10298–10308
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1
Table 1 Diastereomeric composition of complexes 1–4
The H NMR data of slightly concentrated CD3OD or CDCl3
solutions of complexes 3 or 4 were consistent with the presence
of the η6-arene group and the amino carboxylate ligand in a
1 : 1 molar ratio. The characterization of these complexes was
completed by circular dichroism (CD) and, for complex 3, by
X-ray diffraction studies.
The four complexes are configurationally stable: the compo-
sition of diastereomeric mixtures remains essentially unchanged
for 7 days, at room temperature, in D2O (1, 2), CD3OD (3) or
CDCl3 (4).
Molar ratio
(a : b)a
Complex Arene Aa
1a,b
C6H6
N-methyl-L-phenylalaninate
(MePhe)
—
2a,b
3a,b
4a,b
C6H6
C6H6
C6Me6 Pro
L-Prolinate (Pro)
—
L-piperidine-2-carboxylate (Pip) 85 : 15
66 : 34
a Determined by 1H NMR.
Molecular structure of [(η6-C6H6)Ru(Pip)Cl] (3)
During the formation of these complexes, the metal and the
nitrogen atoms become stereogenic centres. Therefore, depend-
ing on the configuration they adopt, four diastereomers, namely,
Single crystals of complex 3 were grown by slow diffusion of
diethyl ether into a methanol solution of an 85 : 15, 3a : 3b
molar ratio diastereomeric mixture of this compound. The most
striking feature of the crystal structure of 3 is the presence of two
independent molecules in the asymmetric unit, differing in the
configuration at the metal. A molecular representation of the two
stereoisomers is depicted in Fig. 2 and selected structural para-
meters are listed in Table 2. In both independent isomers, the
RRu,SC,RN, SRu,SC,RN, RRu,SC,SN, SRu,SC,SN, can be obtained.
However, the new complexes are isolated as mixtures of only
two diastereomers, labelled a (major) and b (minor) (see
Table 1). It is well documented12d,g,i,14 that, from conformational
constrains,15 in half-sandwich complexes containing NO-chelate
α-amino carboxylates, the sole configuration adopted by the
nitrogen atom is R for N-methyl-L-phenylalaninates and L-piper-
idine-2-carboxylates and S for L-prolinates. Therefore, most
probably, the absolute configuration of the two isomers detected
in each case would be RRu,SC,RN and SRu,SC,RN for the
N-methyl-L-phenylalaninate 1 and L-piperidine-2-carboxylate 3,
and RRu,SC,SN and SRu,SC,SN for the L-prolinates 2 and 4
(Fig. 1).
The new complexes were characterized by IR and NMR spec-
troscopy, elemental analyses (see Experimental section), and by
the crystal structure determination, by X-ray diffractometric
methods, of compound 3. The IR spectra showed bands in the
3150–3215 cm−1 region (ν(NH))16 and around 1620 cm−1
(ν(CO))17 compatible with an N,O chelating nature for the amino
carboxylate anion. The new complexes are soluble in water but
insoluble in common organic solvents such as CH2Cl2 or
acetone. Complexes 3 and 4 are slightly soluble in CH3OH and
CHCl3, respectively.
The 1H NMR spectrum of complex 1 in D2O shows more
than two sets of signals. In particular, it shows four singlets
attributed to C6H6 protons at 5.64 (16%), 5.60 (11%), 5.52
(47%), and 5.48 (26%) ppm. Addition of NaCl to the solution
produces an increase of the peaks at 5.60 and 5.52 ppm at the
Fig. 2 Molecular representation of both diastereomers in compound 3.
Only hydrogens bonded to the stereogenic centres have been
represented.
1
expense of the other two. On the other hand, the H NMR spec-
trum of complex 2, in the same solvent, only shows two C6H6
peaks at 5.75 (65%) and 5.66 (35%) ppm. After addition of
NaCl, two new C6H6 resonances emerge at 5.67 and 5.68 ppm.
These spectroscopic data indicate that, in water solution, the
chloride ion is partially dissociated in complex 1 and completely
dissociated in complex 2 (eqn (2)). Furthermore, as we will see
later, the resulting monomeric solvated species rearrange to the
trimeric cations formulated in eqn (2). This behaviour prevents
the determination of the isomeric composition of isolated com-
plexes 1 and 2.
Table 2 Selected bond lengths (Å) and angles (°) for complex 3
Molecule A
Molecule B
Ru–Ga
Ru–Cl
Ru–O
Ru–N
1.650(4)
2.4051(11)
2.106(3)
2.135(3)
126.86(16)
86.74(8)
88.90(9)
130.4(2)
131.0(2)
75.97(11)
1.653(4)
2.4042(11)
2.096(3)
2.118(3)
128.31(16)
87.08(8)
81.75(9)
128.6(2)
134.9(2)
77.88(12)
Cl–Ru–Ga
Cl–Ru–O
Cl–Ru–N
O–Ru–Ga
N–Ru–Ga
O–Ru–N
½ðη6-C6H6ÞRuðAaÞClꢀ ꢁCl;D2O½ðη6-C6H6ÞRuðAaÞD2Oꢀþ
Ð
ð2Þ
a G represents the centroid of the C6H6 ring.
ꢁÐD2O
3þ
1=3½fðη6-C6H6ÞRuðAaÞg3ꢀ
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asymmetric carbon atoms (C(8) and C(28)) maintain their orig-
inal S configurations, while the nitrogen atom of the aminic
group of the L-piperidine-2-carboxylate (N(1) and N(21)) exhi-
bits an R configuration. This inverted configuration of the two
stereogenic centres of the piperidine ligand has been previously
encountered in other [(η6-arene)M(Pip)Cl] complexes, with (η5-
C5Me5)Co14 or (η6-p-cymene)Os12g,i moieties. Therefore, the
diastereomers observed in the solid state present the SRu,SC,RN
(molecule A) and RRu,SC,RN (molecule B) configurations.18
Although, for half-sandwich complexes, the cocrystallization of
two diastereomers differing in the metal configuration has been
previously observed,12a,h,19 single crystals consisting of only one
diastereomer is the most common crystallization behaviour for
this type of diastereomeric mixtures.
Both independent molecules exhibit analogous structural fea-
tures. Metal atoms adopt the very well known “three-legged
piano-stool” geometry, with an η6-C6H6 group occupying three
fac positions, while a chlorine atom and the O,N-chelating
amino carboxylate ligand complete the coordination sphere of
the metals. The bidentate coordination of the amino carboxylate
to the ruthenium atom leads to the formation of five-membered
Ru–O–C–C–N metallacycles; both metallacycles adopt an enve-
lope E5 conformation, with similar deviations from planarity,
and identical puckering phase values20 (q = 0.418(3) Å, ϕ =
−36.0(5)° and q = 0.386(1) Å, ϕ = −35.4(5)° for molecules A
and B, respectively).
As expected, bond lengths and angles of both diastereo-
isomers (see Table 2) are rather similar. Both C6H6 rings are
planar, with an average Ru–C distance of 2.1681(12) Å [range:
2.153(5)–2.184(4) Å], and the Ru to C6H6 centroid (G), the Ru–
Cl, as well as the Ru–O bond lengths are statistically identical in
both diastereomers. The differences between both diastereomers
only affect the geometrical parameters involving the coordinated
asymmetric N atoms (Ru–N 2.135 vs. 2.118(3) Å, or Cl–Ru–N
88.90 vs. 81.75(9)°, for instance). The observed Ru–N and
Ru–O bond lengths [means 2.126(2) and 2.101(2) Å, respec-
tively], compare well with those found in related [(η6-arene)Ru-
(Aa)Cl] complexes (Aa = L-alaninato,19c 2.118(6) and 2.089(6)
Å; L-serinato, 2.126 and 2.078Å;21 L-threoninato, 2.122 and
2.048Å).21 The amino carboxylate bite angles [75.97(11) (A)
and 77.88(12)° (B)] are very similar to that found in the osmium
analog12i [(η6-p-cymene)Os(Pip)Cl] (76.26(15)°) and equivalent
to those found in the above mentioned Ru complexes (mean
77.8°).
Fig. 3 OH⋯O hydrogen bond system involving both diastereomers
and solvation water molecules. Only H atoms bonded to stereogenic
centres as well as those involved in hydrogen bonds have been included.
Primed atoms are related to the unprimed ones by a translation along the
b axis.
Table 3 Selected structural parameters concerning O–H⋯O bonds
(Å and °) for complex 3
D–H
D⋯A
H⋯A
D–H⋯A
O(3′)–H(32′)⋯O(21′)
O(3′)–H(31′)⋯O(2)
O(4)–H(42)⋯O(1)
O(4)–H(41)⋯O(22′)
N(1)–H(1N)⋯O(3)
0.80(2)
0.83(3)
0.82(4)
0.77(6)
0.88(5)
3.020(4)
2.752(5)
2.793(4)
2.772(5)
3.060(5)
2.26(2)
1.94(3)
2.06(5)
2.00(5)
2.21(5)
158(2)
168(2)
147(4)
145(5)
160(3)
Symmetry code of primed atoms: x, 1 + y, z.
The structural analysis of the spatial arrangement of the co-
existing inverted-at-metal diastereomers in [(η6-arene)Ru(LL*)Cl]
half-sandwich complexes has suggested the idea of the existence
of a molecular recognition motif, called the “tight inverted
piano-stool”.19d In this packing pattern, H-bond interactions
involving Cl and O as acceptor atoms are directly established
between both diastereomers. A different situation has been found
in complex 3, where two solvation water molecules interconnect
both diastereomers through a hydrogen bonding system leading
to a macrocyclic ring described by an R42(12) graphical set
(Fig. 3). The crystal cohesion in 3 is then ensured by strong
OH⋯O interactions (see Table 3), between the oxygen atoms of
the carboxylate ligands and the water molecules; moreover,
NH⋯O interactions are also observed between these macrocyclic
rings.
Fig. 4 CD spectra in the 200–600 nm wavelength range: (–) a
85 : 15 molar ratio 3a : 3b mixture, (--) a 66 : 34 molar ratio 4a : 4b
mixture.
Circular dichroism spectra of complexes 3 and 4
The CD spectra of mixtures enriched in the 3a or 4a diastereo-
mers are enantiomorphic with respect to each other (Fig. 4).
They consist of four maxima around 240, 310, 368, and 440 nm.
As no CD transitions have been observed for the parent
L-α-amino acids above 230 nm the measured absorptions must
be mostly due to metal transitions. Interestingly, the patterns of
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Table 4 Diastereomeric composition of complexes 5–8
the CD spectra of complexes 3 and 4 are comparable to those of
the related ruthenium p-cymene complexes (RRu,SC,RN)-[(η6-p-
MeC6H4iPr)Ru(Pip)Cl],12f and (SRu,SC,SN)-[(η6-p-MeC6H4iPr)-
Ru(Pro)Cl],12a respectively. On the basis of this similarity, we
assign the RRu and SRu configurations for the a isomers of com-
pounds 3 and 4, respectively.
Complex
Arene
Aa
Molar ratio (a : b)a
5a,b
6a,b
7a,b
8a,b
C6H6
C6H6
C6H6
C6Me6
MePhe
Pro
Pip
77 : 23b (100 : 0)c
95 : 5b,c
55 : 45b (100 : 0)c
72 : 28b,c
Pro
From the conformational constrains above mentioned,15 the
solid molecular structure of complex 3 and the CD measure-
ments we conclude that, in all cases, the isolated isomers are the
two possible epimers at the metal and that the absolute configu-
ration of complexes 3 and 4 are RRu,SC,RN (3a), SRu,SC,RN (3b)
and SRu,SC,SN (4a), RRu,SC,SN (4b).
a Determined by 1H NMR. b In water. c In acetone.
cation giving rise to a chelate and bridging amino carboxylate
ligand. It is interesting to point out that the observed diastereo-
selectivity in the formation of complexes 5–8 determines that
trimerization has to occur with chiral self-recognition among
the mononuclear fragments. A similar behaviour has been
reported for related d6 half-sandwich amino carboxylate com-
plexes.10a,12d,f,g,i,22
Synthesis and characterization of the cationic trimers 5–8
Treatment of the chloride complexes [(η6-arene)Ru(Aa)Cl] with
equimolar amounts of AgBF4, in methanol, results in the iso-
lation of solids of stoichiometry [(η6-arene)Ru(Aa)(BF4)]n·xH2O
(x = 1 or 2) according to analytical and spectroscopic data. As
we will show later, these solids contain cationic trimers of
formula [{(η6-arene)Ru(Aa)}3]3+ (eqn (3)). A schematic repres-
entation of these cations is depicted in Fig. 5.
The new complexes have been characterized by IR and NMR
spectroscopy, elemental analyses (see Experimental section), and
by the determination of the molecular structure of compound 8b
by X-ray diffractometric methods. The IR spectra of the solids
showed strong ν(NH) bands in the 3250 cm−1 region, the charac-
teristic bands of uncoordinated BF4 anions under Td symmetry,
and a very strong ν(CO) absorption in the 1565–1575 cm−1
range. Reflecting the additional CO coordination, the later is
shifted about 50 cm−1 to lower energy with respect to the parent
chlorides. Moreover, the IR spectra show absorptions in the
3350–3610 cm−1 region attributable to crystallized water.
All complexes are soluble in water and acetone, and, as
½ðη6-areneÞRuðAaÞClꢀ þ AgBF4
! 1=3½fðη6-areneÞRuðAaÞg3ꢀ½BF4ꢀ3 þ AgCl
ð3Þ
In spite of the fact that the trimers present nine stereogenic
centres, only two diastereomers have been detected in aqueous
solution for the four complexes and only one, for complexes 5
and 7, in acetone (see Table 4). Furthermore, each diastereomer
shows only one set of NMR signals and, therefore, within each
trimer, the configuration of each threesome of stereogenic
centres has to be the same. Formally, the formation of these
trimers involves the chloride abstraction by the silver cation from
the starting chlorides, giving rise to the corresponding solvated
mononuclear species [(η6-arene)Ru(Aa)(MeOH)]+, followed by
the subsequent trimerization of the resulting cationic monomers.
In the process, the uncoordinated oxygen of an N,O-coordinated
amino carboxylate group displaces the methanol molecule from
the coordination sphere of a vecinal [(η6-arene)Ru(Aa)(MeOH)]+
1
expected, the H NMR spectra of complexes 5 and 6 in D2O,
and those measured in the same solvent for the parent chlorides
[(η6-C6H6)Ru(MePhe)Cl] (1) and [(η6-C6H6)Ru(Pro)Cl] (2), are
comparable. This equivalence confirms the dissociation equili-
brium proposed for the chlorido ligand in eqn (2).
Stereochemical assignments for the complexes have been
accomplished through a combination of crystallographic, circular
dichroism, and NOE experiments.
Molecular structure of [{(η6-C6Me6)Ru(Pro)}3][BF4]3 (8b)
Single crystals of this complex were grown by slow diffusion of
n-hexane into acetone solutions of diastereomeric mixtures of
72 : 28, 8a : 8b molar ratio. A molecular representation of the
complex is depicted in Fig. 6. The crystal structure consists of
−
trimeric cations [{(η6-C6Me6)Ru(Pro)}3]3+ together with BF4
counteranions and acetone solvent molecules. Selected structural
parameters are summarized in Table 5.
The entire cationic complex of 8b has crystallographically
imposed three-fold symmetry relating the geometrical parameters
of the three metal fragments forming the trimer. The amino
carboxylate ligand acts as a tridentate chelate and bridging
group: the nitrogen and one of the carboxylic oxygen atoms
(O(1)) of each amino carboxylate group are bonded to a ruthe-
nium atom in a chelate fashion forming a five-membered metal-
lacycle; the remaining oxygen atom (O(2)) coordinates to a
different ruthenium and confers an intermetallic bridging nature
to the amino carboxylate ligand. This (N,O,O′)-bridging triden-
tate proline coordination has been previously reported in
Fig. 5 Schematic representation of the cation of the trimers: R = H,
R1 = CH2Ph, R2 = Me (5); R = H, R1–R2 = (CH2)3 (6); R = H, R1–R2 =
(CH2)4 (7); R = Me6, R1–R2 = (CH2)3 (8).
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(N(1) and C(14)) seems to be correlated due to ‘small-fused-ring
geometrical restrictions’, as it has been already pointed out in
other complexes containing pyrrol as a fused ring to the car-
boxylic function (with only three carbon atoms connecting the
amino nitrogen and the carboxylate α-carbon functionalities)12i
and in related azetidine-2-carboxylate complexes (with only two
carbons between nitrogen and α-carbon).19a Due to the crystallo-
graphically imposed symmetry all of the {(η6-C6Me6)Ru(Pro)}
moieties are equivalent, and therefore, the absolute configuration
of 8b in the solid state is RRuRRuRRuSCSCSCSNSNSN.
The trinuclear core can be described as an equilateral triangle
with the arene ligand and the pyrrolidinic cycle of the proline in
the external side of the molecule. Therefore, the oxygen atoms
(potentially excellent hydrogen-bond acceptors) are located in
the internal side of the molecule, which precludes their partici-
pation in intermolecular interactions; only a hydrogen bonding
interaction is observed between the aminic hydrogen atom
(H(1)) and a fluorine atom of the BF4 anions (N(1)–H(1)
0.930(7), N(1)⋯F(4) 2.947(12), H(1)⋯F(4) 2.023(10) Å, and
N(1)–H(1)⋯F(4) 172.3(6)°).
The Ru3 triangle’s edge length in 8b, 5.585 Å, is longer than
the values found in the {(η6-C6H4iPr)Ru(Pro)}3 cation (5.268,
5.353 and 5.380 Å). This difference is most probably associated
with the bigger size and different electronic properties of the
hexamethylbenzene which behaves as a better electron-releasing
ligand than p-cymene. A comparison of structural parameters in
both Ru(Pro) trimer cations shows that the metal weakens its
bonding interaction with the chelating donor atoms O(1) and
N(1) (2.124(6) and 2.146(7) Å), if compared to the situation
reported in {(η6-C6H4iPr)Ru(Pro)}3 trimer (Ru–O: 2.096(4),
Ru–N: 2.116(5) Å) or in the {(η6-C6H6)RuCl(Pro)} monomer
(Ru–O: 2.072(2), Ru–N: 2.128(2) Å).19a
Fig. 6 Molecular representation of the trinuclear cation of 8b. Only the
hydrogen atoms on the stereogenic centres have been included. Primed
and double-primed atoms are related to the non-primed ones by the sym-
metry transformations 1 − y, 1 + x − y, z and −x + y, 1 − x, z,
respectively.
Table 5 Selected bond lengths (Å) and angles (°) for the trimeric
cation of complex 8b
Ru–Ga
1.668(10)
2.137(6)
128.9(4)
133.9(4)
134.6(4)
Ru–O(2)
Ru–N
O(1)–Ru–O(2)
O(1)–Ru–N
O(2)–Ru–N
2.124(6)
2.146(7)
82.1(3)
77.7(3)
78.4(3)
Ru–O(1)
Ga–Ru–O(2)
Ga–Ru–O(1)
Ga–Ru–N
a G represents the centroid of the ring of the C6Me6 arene.
NMR and circular dichroism spectra of complexes 5–8
Solution stereochemical assignments for these complexes have
been accomplished through NOE experiments. Thus, NOE
difference spectra for the N-methylphenylalaninate complex 5a
in acetone showed enhancement of the signal due to the phenyl
protons of the amino carboxylate ligand when the η6-C6H6
protons were irradiated. Moreover, while irradiation of the C*H
proton of the same ligand enhances the NMe resonance, no NOE
relationship was observed between that proton and the protons of
the C6H6 ligand. On the other hand, NOE difference spectra for
the piperidine-2-carboxylate complex 7a, showed enhancement
of the signal due to the C6H6 protons and no NOE effect for the
proton bound to the asymmetric carbon atom, while the NH
proton was irradiated. These NOE data are only compatible with
[{(η6-p-MeC6H4iPr)Ru(Pro)}3][BF4]3
iPr)Os(Pro)}3][BF4]3.
and [{(η6-p-MeC6H4-
12d
12i
Furthermore, a similar behaviour has
been reported for other α-amino acids in trinuclear rhodium22a
[{(η5-C5Me5)Rh(Phe)}3] [BF4]3, iridium12d [{(η5-C5Me5)Ir-
(Ala)}3] [BF4]3 and osmium12i [{(η6-p-MeC6H4iPr)Os(Pip)}3]
[BF4]3 complexes. The five-membered metallacycle Ru–O(1)–
C(13)–C(14)–N(1) presents a mixed twist/envelope 1T5/1E confor-
mation (Cremer and Pople parameters: q = 0.314(7) Å and
ϕ = −13(1)°).20
The metal coordination environments could be described as
pseudo-octahedral where, in addition to the three coordination
sites occupied by the amino carboxylate donor atoms, an η6-
bonded C6Me6 group completes the metal coordination sphere.
In such a coordination the metals are stereogenic centres and
according to the ligand priority sequence23 all the metals exhibit
R configuration. The entire configurational characterization of 8b
also requires the description of the two stereogenic centres of the
amino carboxylates; thus, the starting S configuration of the
asymmetric α-carbon C(14) was not modified along the prepara-
tive reaction, while the aminic nitrogen atom also adopts an
S configuration. The identical configuration of these two atoms
RRu,SC,RN configurations for complexes 5a and 7a (Fig. 7).
With respect to the prolinate compounds, for complex 6a,
NOE difference spectra in acetone showed enhancement of the
signal due to the η6-C6H6 protons when the pro-S NCH2
proton24 was irradiated (Fig. 8), but no NOE relationship was
observed between the same aromatic protons and the C*H
proton. However, for complex 8a, NOE was observed between
the η6-C6Me6 protons and both the C*H and NH protons.
In both complexes, irradiation of the NH proton enhances the
C*H resonance. These NOE data indicate that complexes 6a and
10302 | Dalton Trans., 2012, 41, 10298–10308
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Table 6 Asymmetric transfer hydrogenationa of acetophenone
Config. Conv.
at N
Entry Complex
(%)b
ee (%)
1
2
3
4
5
6
7
8
[(η6-C6H6)Ru(MePhe)Cl] (1)c
R
R
S
40
53
74
64
87
98
90
96
20 (S)
22 (S)
45 (R)
47 (R)
41 (S)
39 (S)
43 (R)
46 (R)
[{(η6-C6H6)Ru(MePhe)}3]3+ (5)
[(η6-C6H6)Ru(Pro)Cl] (2)c
[{(η6-C6H6)Ru(Pro)}3]3+ (6)
[(η6-C6H6)Ru(Pip)Cl] (3)c
[{(η6-C6H6)Ru(Pip)}3]3+ (7)
[(η6-C6Me6)Ru(Pro)Cl] (4)
[{(η6-C6Me6)Ru(Pro)}3]3+ (8)
S
Fig. 7 Selected NOE for 5a and 7a (only one monomer and the brid-
ging oxygen are shown).
R
R
S
S
a Reaction conditions: all reactions were carried out at 83 °C. Catalyst
0.01 mmol in 5 mL of 2-propanol; molar ratio catalyst–HCOONa–
acetophenone: 1 : 2 : 100. b Conversion, determined by gas
chromatography, after one hour of reaction. c Molar ratio 1 : 1 : 100.
Asymmetric hydrogen transfer reactions
We have tested the new ruthenium amino carboxylate complexes
1–8 as catalyst precursors for the ATH reaction from 2-propanol
to ketones. Reactions have been carried out at 83 °C, in the pres-
ence of the mild base HCOONa, with catalyst–base–substrate
molar ratios of 1 : 1 : 100 or 1 : 2 : 100. Both conversion and
enantioselectivity have been determined by gas chromatography.
Table 6 collects a selection of the results obtained in the
reduction of acetophenone. Under the standard conditions, 40 to
98% conversion of the starting material can be achieved, in one
hour of reaction, to yield 1-phenylethanol with up to 47% ee.
Neutral chloride complexes and cationic trimers containing the
same amino carboxylate ligand showed similar enantiomeric
excesses (compare entries 1 and 2, 3 and 4, 5 and 6, and 7 and
8). Probably, the active catalytic species originate through a
common intermediate, namely, [(η6-arene)Ru(Aa)(iPrOH)]+,
formed by halogen abstraction from the chlorides or by cleavage
of the Ru–O(bridging) bond from the trimers and, therefore,
both catalyst precursors behave similarly in the ATH reaction.
With respect to the enantioselectivity of the process, catalysts
based on the cyclic aminocarboxylates Pro or Pip (entries 3–8)
produced better enantioselectivity than those based on the linear
one MePhe (entries 1 and 2). The greater conformational rigidity
of the former could account for the measured increment in ee.
However, the most striking result is that while prolinate com-
pounds give the R alcohol (entries 3, 4, 7, and 8), N-methylphe-
nylalaninates (entries 1 and 2) and piperidine-2-carboxylates
(entries 5 and 6) preferentially afford the S alcohol. This change
of sign in the enantioselection can be explained assuming that
the bifunctional concerted mechanism of Noyori4a is operating
in our catalytic system (Scheme 1). According to it, from the
starting 2-propanol intermediate [(η6-arene)Ru(Aa)(iPrOH)]+, a
ruthenium hydride is formed by β-elimination and, in the key
step of this mechanism, the Ru–H and the N–H protons are
simultaneously transferred from the catalyst to the ketone carbo-
nyl group via the six-membered transition state depicted in
Scheme 1. In our case, this cyclic intermediate can only be built
Fig. 8 Selected NOE for 6a and 8a (only one monomer and the brid-
ging oxygen are shown).
8a present RRu,SC,SN and SRu,SC,SN configurations, respectively
(Fig. 8).
Among the series of ruthenium trimers, the metal configu-
ration can also be inferred from the solution chiroptical proper-
ties. Thus, the CD spectra of pure 5a and 7a and of a
diastereomeric mixture of complex 6, enriched in the 6a isomer,
consisted of two maxima in the 350–355 and 415–420 nm
ranges, both with positive Cotton effects. The CD spectrum of a
mixture enriched in 8a clearly shows a pseudoenantiomorphic
relationship to the trimers 5a, 6a, and 7a, since a similar
morphology but opposite Cotton effects were observed (see
Experimental section). As the CD spectra of the parent L-amino
acids are silent above 230 nm, we assume that the measured
absorptions are mostly due to metal transitions. These obser-
vations allow us to assign an opposite configuration at ruthenium
for isomer 8a with respect to isomers 5a–7a, in good agreement
with the NMR data discussed above.
On the other hand, the substitution on the η6-arene ligand
seems to exert a significant influence on the configuration at the
metal of the trimers. Thus, while for the benzene and p-cymene
derivatives [{(η6-C6H6)Ru(Pro)}3][BF4]3 (6) and [{(η6-p-
MeC6H4iPr)Ru(Pro)}3][BF4]3 (9)12d in the thermodynamically
preferred diastereomer the configuration at ruthenium is R, for
the hexamethylbenzene analogue [{(η6-C6Me6)Ru(Pro)}3][BF4]3
(8) the S configuration at the metal gives the most stable isomer.
Finally, all the four trimers 5–8 are configurationally stable,
for days at room temperature, in water or acetone solution.
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Scheme 1 Asymmetric transfer reaction catalysed by amino carboxylate Ru compounds.
Fig. 9 Proposed active intermediates for the ATH reaction.
up when the metal and the nitrogen adopt the same configuration
(Fig. 9) and, therefore, only the SRu,SC,SN isomers for the proli-
nate complexes and the RRu,SC,RN isomers for the piperidine-
2-carboxylate and N-methylphenylalaninate compounds can be
active catalysts for the attempted ATH process. Enantioselection
occurs because CH/π interactions between the hydrogen atoms
on the η6-coordinated arene and the phenyl ring of the aceto-
phenone4b,c fix the ketone enantioface through which the Ru–H
and N–H protons are released from the catalyst to the CvO
bond (Fig. 9). The experimental stereochemical outcome is in
good agreement with this mechanistic proposal.
Next, we studied the reduction of some phenyl substituted
acetophenones using as catalyst precursors the prolinate
complexes 2 and 4 as well as the related p-cymene complex
[(η6-p-MeC6H4iPr)Ru(Pro)Cl] (9) recently reported by us.12d
Table 7 lists the most representative results together with the
reaction conditions. For comparative purposes we also include in
Table 7 the results obtained for acetophenone (entries 1–3).
In general good conversions are achieved after one hour of
treatment under the conditions indicated in Table 7, with moder-
ate enantioselectivity. No obvious relationship can be encoun-
tered between the nature of the substituents and either rate or
selectivity. For all the ketones investigated, the ee decreases
when benzene or hexamethylbenzene is used as the ligand
instead of p-cymene.
Conclusions
Half-sandwich complexes of the type [(η6-arene)Ru(Aa)Cl] that
incorporate chiral amino carboxylate ligands are easily prepared
from the corresponding acetylacetonate complex [(η6-arene)Ru-
(acac)Cl] as a mixture of epimers at the metal. Abstraction of
the chloride affords the new trinuclear amino carboxylate cat-
ionic complexes [{(η6-arene)Ru(Aa)}3](BF4)3. Trimerizations
take place with self-recognition: only trimers with the same
configuration at the three metal atoms are obtained. Mononuclear
and trinuclear complexes are effective catalysts for the enantio-
selective reduction of prochiral ketones by hydrogen transfer
from 2-propanol. Assumption of Noyori’s mechanism for the
ATH reaction allows us to explain the sign of the ee obtained.
Experimental
All solvents were dried over appropriate drying agents, distilled
under nitrogen and degassed prior to use. All preparations have
10304 | Dalton Trans., 2012, 41, 10298–10308
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Table 7 Asymmetric transfer hydrogenationa of other ketones
Entry
Complex
Ketone
Conv.b (%)
ee (%)
1
2
3
[(η6-C6H6)Ru(Pro)Cl] (2)
74
90
88
45 (R)
43 (R)
67 (R)
[(η6-C6Me6)Ru(Pro)Cl] (4)c
[(η6-p-MeC6H4iPr)Ru(Pro)Cl] (9)
4
5
6
[(η6-C6H6)Ru(Pro)Cl] (2)
72
50
95
24 (R)
30 (R)
46 (R)
[(η6-C6Me6)Ru(Pro)Cl] (4)c
[(η6-p-MeC6H4iPr)Ru(Pro)Cl] (9)
7
8
9
[(η6-C6H6)Ru(Pro)Cl] (2)
55
52
80
35 (R)
34 (R)
50 (R)
[(η6-C6Me6)Ru(Pro)Cl] (4)c
[(η6-p-MeC6H4iPr)Ru(Pro)Cl] (9)
10
11
12
[(η6-C6H6)Ru(Pro)Cl] (2)
8
12
59
39 (R)
57 (R)
68 (R)
[(η6-C6Me6)Ru(Pro)Cl] (4)c
[(η6-p-MeC6H4iPr)Ru(Pro)Cl] (9)
a Reaction conditions: all reactions were carried out at 83 °C; catalyst 0.01 mmol in 5 mL of 2-propanol; molar ratio catalyst–HCOONa–ketone:
1 : 1 : 100. b Determined by gas chromatography, after one hour of reaction. c Molar ratio: 1 : 2 : 100.
been carried out under nitrogen. Infrared spectra were obtained
as Nujol mulls with a Perkin-Elmer 1330 spectrophotometer.
Carbon, hydrogen, and nitrogen analyses were performed using
a Perkin-Elmer 240 B microanalyzer. 1H NMR spectra were
recorded on a Varian UNITY 300 spectrometer (299.95 MHz) or
a Bruker 300 ARX (300.10 MHz). Chemical shifts are expressed
in ppm upfield from SiMe4. CD spectra were determined in a
1 cm path length cell by using a Jasco-710 apparatus at concen-
trations of approximately 5 × 10−4 mol L−1. NOEDIFF spectra
were obtained using standard procedures. Gas chromatography
was performed on a Hewlet-Packard 3398 gas chromatograph
equipped with a split-mode capillary injection system and
(m, 4H, CH2), 3.17 (m, 1H, CHCOO), 3.17 (m, 1H, pro-S
NCH2), 3.33 (m, 1H, pro-R NCH2), 3.93 (m, 1H, NH), 5.75 (s,
6H, C6H6) (65%); 2.5 m, 3.15 (m, 1H, pro-R NCH2), 3.60 m,
5.66 (s, 6H, C6H6) (35%).
Complex 3. Yield: 80%, 3a : 3b molar ratio, 85 : 15. Anal.
Calcd for C12H16NClO2Ru: C, 42.05; H 4.7; N, 4.1. Found: C,
42.1; H, 4.7; N, 4.0. IR (Nujol, cm−1): ν(NH) 3174 (m), ν(CO)
1643 (s). CD (CH3OH), 3a : 3b molar ratio, 85 : 15, [Θ]λ values
of maxima and nodes (λ, nm): −5200 (230), 0 (270), +2200
(300), 0 (325), −4800 (365), 0 (400), +4150 (430). 3a: 1H NMR
(CD3OD): δ 1.35–3.20 (m, 8H, CH2), 3.65 (m, 1H, CHCOO),
5.69 (s, 6H, C6H6). 3b: 1H NMR (CD3OD): δ 5.75 (s, 6H,
C6H6).
flame ionization detector, using
a CP-Cyclodex-B 236M
25 m × 0.25 mm × 0.25 μm film column.
Preparation of [(η6-C6H6)Ru(Aa)Cl] (1–3)
To a solution of [(η6-C6H6)Ru(acac)Cl] (300.0 mg, 0.95 mmol)
in methanol (20 mL) the appropriate amino acid (0.95 mmol)
was added. The resulting solutions were stirred for 24 h. During
this time the precipitation of a yellow solid was observed. The
solid was filtered off, washed with methanol and air-dried.
Preparation of [(η6-C6Me6)Ru(Pro)Cl] (4)
To a solution of [(η6-C6Me6)Ru(acac)Cl] (401.3 mg, 1.01 mmol)
in methanol (20 mL), L-Proline (122.7 mg, 1.06 mmol) was
added. The resulting solution was stirred for 24 h and then
filtered through Kieselguhr to eliminate any solid residue. After
partial concentration under reduced pressure, the slow addition
of diethyl ether gave a hygroscopic orange solid which was
washed with diethyl ether and vacuum-dried.
Complex 1. Yield: 84%. Anal. Calcd for C16H18NClO2Ru: C,
48.9; H 4.6; N, 3.6. Found: C, 49.1; H, 4.7; N, 3.5. IR (Nujol,
1
cm−1): ν(NH) 3215 (m), ν(CO) 1633 (s). H NMR (D2O): four
species in 47 : 26 : 16 : 11 molar ratio were detected: δ 3.16
(s, 3H, Me), 3.56 (m, 1H, CHCOO), 5.52 (s, 6H, C6H6),
7.07–7.36 (m, Ph) (47%); 2.59 (s, 3H, Me), 5.48 (s, 6H, C6H6)
(26%); 2.70 (s, 3H, Me), 5.64 (s, 6H, C6H6), (16%); 3.09 (s, 3H,
Me), 5.60 (s, 6H, C6H6) (11%).
Complex 4. Yield: 75%, 4a : 4b molar ratio, 66 : 34. Anal.
Calcd for C17H26NClO2Ru·2H2O: C, 45.5; H 6.7; N, 3.1.
Found: C, 45.2; H, 6.6; N, 3.0. IR (Nujol, cm−1): ν(OH) 3593
(m), ν(NH) 3200 (m), ν(CO) 1605 (s). CD (CHCl3), 4a : 4b
molar ratio, 66 : 34, [Θ]λ values of maxima and nodes (λ, nm):
+1800 (250), 0 (280), −5200 (320), 0 (345), +4600 (370),
0 (420), −800 (450). 4a: 1H NMR (CDCl3): δ 2.13 (s, 18H,
Complex 2. Yield: 92%. Anal. Calcd for C11H14NClO2Ru: C,
40.2; H 4.3; N, 4.3. Found: C, 40.0; H, 4.2; N, 4.2. IR (Nujol,
1
1
cm−1): ν(NH) 3162 (m), ν(CO) 1614 (s). H NMR (D2O): two
C6Me6), 6.60 (m, 1H, NH). 4b: H NMR (CDCl3): δ 2.17 (s,
species in 65 : 35 molar ratio were detected: δ 1.58, 1.88, 2.10
18H, C6Me6), 6.14 (m, 1H, NH).
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Preparation of [{(η6-C6H6)Ru(Aa)}3][BF4]3 (5–7)
dryness, and the resulting yellow solid was recrystallized from
acetone–n-hexane.
To a solution, in acetone–water (v/v, 90 : 10), of the correspond-
ing [(η6-C6H6)Ru(Aa)Cl] compound (0.58 mmol), AgBF4
(118.2 mg, 0.60 mmol) was added. The mixture was stirred for
1 h, in the absence of light, and the precipitated AgCl was
filtered off. The filtrate was evaporated to dryness and the result-
ing yellow solid was recrystallized from acetone–n-hexane.
Complex 8. Yield: 77%, 8a : 8b molar ratio, 72 : 28. Anal.
Calcd for C51H78B3F12N3O6Ru3·6H2O: C, 40.8; H 6.0; N, 2.8.
Found: C, 40.5; H, 5.9; N, 2.9. IR (Nujol, cm−1): ν(OH)
3604 (m), ν(NH) 3200 (m), ν(CO) 1574 (s), ν(BF4) 1061 (s),
521 (m). CD ((CH3)2CO), 8a : 8b molar ratio, 72 : 28, [Θ]λ
values of maxima, (λ, nm): −16 000 (330), −10 000 (435). 8a:
1H NMR (D2O): δ 2.02 (s, 18H, C6Me6). 8b: H NMR (D2O):
1
Complex 5. Yield: 73%, 5a : 5b molar ratio, 77 : 23. Anal.
Calcd for C48H54B3F12N3O6Ru3·3H2O: C, 41.6; H 4.4; N, 3.0.
Found: C, 42.0; H, 4.3; N, 2.9. IR (Nujol, cm−1): ν(OH) 3606
(m), ν(NH) 3248 (m), ν(CO) 1575 (s), ν(BF4) 1060 (s), 521 (m).
CD ((CH3)2CO), 5a : 5b molar ratio, 77 : 23, [Θ]λ values of
δ 1.92 (s, 18H, C6Me6). 8a: 1H NMR (CD3)2CO): δ 1.4–2.6
(m, 4H, CH2), 2.42 (s, 18H, C6Me6), 3.85 (m, 1H, CHCOO),
4.00 (m, 1H, pro-S NCH2), 4.25 (m, 1H, pro-R NCH2), 5.20 (m,
1H, NH). 8b: 1H NMR (CD3)2CO): δ 2.38 (s, 18H, C6Me6).
1
maxima, (λ, nm): +20 000 (350), +31 000 (420). 5a: H NMR
(D2O): δ 2.55 (bs, 3H, Me), 2.97 (m, 2H, CH2), 2.82 (m, 1H,
1
Transfer hydrogenation experiments
CHCOO), 5.48 (s, 6H, C6H6), 7.00–7.40 (m, Ph). 5b: H NMR
1
(D2O):δ 3.13 (bs, 3H, Me), 5.64 (s, 6H, C6H6). 5a: H NMR
Catalyst (0.01 mmol metal), HCOONa (0.01–0.02 mmol,
aqueous solution), and 2-propanol (4 mL) were mixed under
nitrogen at room temperature in a flask which was then equipped
with a reflux condenser and immersed in an oil bath at 83 °C.
To the boiling solution, the ketone (1 mmol) was added in 1 mL
of 2-propanol. Reactions were monitored by gas-liquid
chromatography and the products were identified by their reten-
tion times compared to those of the literature.3b,7a,25
((CD3)2CO): δ 2.22 (m, 1H, CHCOO), 2.56 (d, JHH = 5.6 Hz,
3H, Me), 2.97 (dd, 1H, AB part of an ABX system, JAB
=
15.1 Hz, JAX = 4.4 Hz, CHH), 3.13 (dd, 1H, AB part of an ABX
system, JBX = 8.1 Hz, CHH), 6.14 (s, 6H, C6H6), 6.42 (m, 1H,
NH), 7.10–7.35 (m, Ph).
Complex 6. Yield: 82%, 6a : 6b molar ratio, 95 : 5. Anal.
Calcd for C33H42B3F12N3O6Ru3·3H2O: C, 33.2; H 4.05; N, 3.5.
Found: C, 33.0; H, 4.0; N, 3.5. IR (Nujol, cm−1): ν(OH)
3607 (m), ν(NH) 3262 (m), ν(CO) 1583 (s), ν(BF4) 1051 (s),
522 (m). CD ((CH3)2CO), 6a : 6b molar ratio, 95 : 5, [Θ]λ values
of maxima, (λ, nm): +20 000 (355), +32 000 (420). 6a: 1H NMR
(D2O): δ 1.55, 1.84, 2.07 (m, 4H, CH2), 3.17 (m, 1H, CHCOO),
3.17 (m, 1H, pro-S NCH2), 3.96 (m, 1H, pro-R NCH2), 5.60 (m,
1H, NH), 5.75 (s, 6H, C6H6). 6b: 1H NMR (D2O) δ 5.66 (s, 6H,
Crystal structure determinations
X-ray diffraction data were collected at 100(2) K with graphite-
monochromated MoKα radiation (λ = 0.71073 Å) using narrow
ω rotation (0.3°) on a Bruker SMART APEX CCD area detector
diffractometer. Intensities were integrated and corrected for
absorption effects with SAINT-PLUS program.26 The structures
were solved by direct methods with SHELXS-97.27 Refinement,
by full-matrix least-squares on F2, was performed with
SHELXL-97.28 Anisotropic displacement parameters were
included for all non-solvent non-H atoms. Most of the hydrogen
atoms were included in calculated positions and refined with dis-
placement and positional riding parameters. In all the structures,
additionally to the internal configuration reference of the
α-asymmetric carbon of the amino acid, the Flack parameter was
refined as a check on the correct absolute structure determi-
nation.29 Particular details concerning the presence of solvent,
static disorder and specific refinements are listed below.
1
C6H6). 6a: H NMR ((CD3)2CO): δ 1.6–2.3 (m, 4H, CH2), 3.28
(m, 1H, CHCOO), 3.49 (m, 1H, pro-S NCH2), 4.27 (m, 1H, pro-
1
R NCH2), 5.60 (m, 1H, NH), 6.18 (s, 6H, C6H6). 6b: H NMR
((CD3)2CO): δ 6.14 (s, 6H, C6H6).
Complex 7. Yield: 85%, 7a : 7b molar ratio, 55 : 45. Anal.
Calcd for C36H48B3F12N3O6Ru3·3H2O: C, 35.0; H 4.4; N, 3.4.
Found: C, 35.2; H, 4.3; N, 3.3. IR (Nujol, cm−1): ν(OH)
3606 (m), ν(NH) 3230 (m), ν(CO) 1574 (s), ν(BF4) 1066 (s),
522 (m). CD ((CH3)2CO), 7a : 7b molar ratio, 55 : 45, [Θ]λ
values of maxima, (λ, nm): +20 000 (355), +39 000 (4150. 7a:
1H NMR (D2O): δ 3.75 (m, 1H, CHCOO), 6.63 (m, 1H, NH),
5.72 (s, 6H, C6H6). 7b: 1H NMR (D2O): δ 3.87 (m, 1H,
CHCOO), 5.75 (s, 6H, C6H6). 7a: 1H NMR ((CD3)2CO):
δ 1.3–2.0 (m, 6H, CH2), 2.34 (m, 1H, CHCOO), 2.90 (m, 1H,
pro-S NCH2), 4.00 (m, 1H, pro-R NCH2), 6.11 (s, 6H, C6H6),
6.31 (m, 1H, NH).
Crystal data for 3. C12H16ClNO2Ru·H2O, M = 360.80;
yellow needle, 0.377 × 0.074 × 0.030 mm3; monoclinic, C2; a =
17.365(2) Å, b = 6.2660(9) Å, c = 24.847(3) Å; β = 106.045(2)º;
Z = 8; V = 2598.2(6) ų; Dc = 1.845 g cm−3; μ = 1.412 mm−1
,
min. and max. transmission factors 0.758 and 0.846; 2θmax
=
56.78°; 8590 collected reflections, 5562 unique [Rint = 0.017];
number of data/restraints/parameters 5562/3/357; final GoF
1.015; R1 = 0.0296 [5275 reflections, I > 2σ(I)]; wR2 = 0.0674
for all data; Flack parameter x = −0.02(3); largest difference
peak 0.84 e Å−3. The hydrogen atoms of the stereogenic centers
(nitrogen and α-carbon of the amino acid) have been included in
the model in observed positions and freely refined. Hydrogen
atoms of water molecules have been observed, and refined with a
restraint in OH distances.
Preparation of [{(η6-C6Me6)Ru(Pro)}3][BF4]3 (8)
To a solution of [(η6-C6Me6)Ru(acac)Cl] (308.8 mg, 0.78 mmol)
in methanol (20 mL), L-Proline (95.3 mg, 0.82 mmol) was
added. The solution was stirred for 24 h and then filtered
through Kieselguhr to eliminate any solid residue. AgBF4
(155.6 mg, 0.79 mmol) was added to the resulting solution, the
mixture was stirred for 1 h in the absence of light and then the
precipitated AgCl was filtered off. The filtrate was evaporated to
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Comprehensive Coordination Chemistry, ed. G. Wilkinson, Pergamon,
Oxford, 1987, vol. 2, ch. 20.2, p. 739; (d) H. Kozlowski and L. D. Pettit,
in Chemistry of the Platinum Group Metals, ed. F. R. Hartley, Elsevier,
New York, 1991, ch. 15, p. 530; (e) N. Paulic and N. Raos, J. Coord.
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Crystal data for 8b. C51H78B3F12N3O6Ru3·3(C3H6O), M =
1567.04; orange prism, 0.352 × 0.137 × 0.129 mm3; hexagonal,
P63; a = b = 19.161(3) Å, c = 10.4784(14) Å; Z = 2; V =
3331.6(8) Å3; Dc = 1.562 g cm−3; μ = 0.76 mm−1, min. and
max. transmission factors 0.667 and 0.890; 2θmax = 56.64°;
21 325 collected reflections, 4929 unique [Rint = 0.052]; number
of data/restraints/parameters 4929/5/257; final GoF 1.199; R1 =
0.089 [4289 reflections, I > 2σ(I)]; wR2 = 0.184 for all data;
Flack parameter x = 0.06(10); largest difference peak 2.13 e Å−3
close to the metal atom with no chemical sense. The acetone
solvent molecule shows static disorder but no clear model could
be established, dynamic disorder has been assumed.
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Acknowledgements
We thank the Ministerio de Educación y Ciencia (Grant CTQ
2009-10303/BQU and CONSOLIDER INGENIO-2010 program
under the project Factoría de Cristalización, CSD2006-0015)
and Gobierno de Aragón (Grupo Consolidado: Catalizadores
Organometálicos Enantioselectivos) for financial support.
P. G. O. acknowledges financial support from the CSIC,
“JAE-Doc” program, contract co-funded by the ESF.
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10308 | Dalton Trans., 2012, 41, 10298–10308
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