Synthesis and characterization of chiral phosphirane derivatives of [(μ-H)4Ru4(CO)12] and their application in the hydrogenation of an α,β-unsaturated carboxylic acid

Article abstract of DOI:10.1016/j.jorganchem.2017.05.031

Ruthenium clusters containing the chiral binaphthyl-derived mono-phosphiranes [(S)-([1,1′-binaphthalen]-2-yl)phosphirane] (S)-1a, [(R)-(2′-methoxy-1,1′-binaphthyl-2-yl)phosphirane] (R)-1b, and the diphosphirane [2,2′-di(phosphiran-1-yl)-1,1′-binaphthalene] (S)-1c have been synthesized and characterized. The clusters are [(μ-H)₄Ru₄(CO)₁₁((S)-1a)] (S)-2, [(μ-H)₄Ru₄(CO)₁₁((R)-1b)] (R)-3, 1,1-[(μ-H)₄Ru₄(CO)₁₀((S)-1c)] (S)-4, [(μ-H)₄Ru₄(CO)₁₁((S)-binaphthyl-P_(s)(H)Et)] (S,S_p)-5, [(μ-H)₄Ru₄(CO)₁₁((S)-binaphthyl-P_(R)(H)Et)] (S,R_p)-6, [(μ-H)₄Ru₄(CO)₁₁((R)-binaphthyl-P_(s)(H)Et)] (R,S_p)-7, [(μ-H)₄Ru₄(CO)₁₁((R)-binaphthyl-P_(R)(H)Et)] (R,R_p)-8 and the phosphinidene-capped triruthenium cluster [(μ-H)₂Ru₃(CO)₉(PEt)] 9. Clusters 5–8 are formed via hydrogenation and opening of the phosphirane ring in clusters (S)-2 and (R)-3. The phosphirane-substituted clusters were found to be able to catalyze the hydrogenation of trans-2-methyl-2-butenoic acid (tiglic acid), but no enantioselectivity could be detected. The molecular structures of (S)-4, (R,S_p)-7 and 9 have been determined and are presented.

Full text of DOI:10.1016/j.jorganchem.2017.05.031

Accepted Manuscript
Synthesis and characterization of chiral phosphirane derivatives of [(μ-
H) Ru (CO) ] and their application in the hydrogenation of an α,β-unsaturated
12
4
4
carboxylic acid
Ahmed F. Abdel-Magied, Maitham H. Majeed, Manuel F. Abelairas-Edesa, Arne
Ficks, Radwa M. Ashour, Ahibur Rahaman, William Clegg, Matti Haukka, Lee J.
Higham, Ebbe Nordlander
PII:
S0022-328X(17)30327-3
10.1016/j.jorganchem.2017.05.031
JOM 19957
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 10 February 2017
Revised Date: 12 May 2017
Accepted Date: 15 May 2017
Please cite this article as: A.F. Abdel-Magied, M.H. Majeed, M.F. Abelairas-Edesa, A. Ficks, R.M.
Ashour, A. Rahaman, W. Clegg, M. Haukka, L.J. Higham, E. Nordlander, Synthesis and characterization
of chiral phosphirane derivatives of [(μ-H) Ru (CO) ] and their application in the hydrogenation
12
4
4
of an α,β-unsaturated carboxylic acid, Journal of Organometallic Chemistry (2017), doi: 10.1016/
j.jorganchem.2017.05.031.
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ACCEPTED MANUSCRIPT
Tetraruthenium tetrahydrido clusters containing chiral binaphthyl-derived mono- or di-
phosphiranes have been synthesized and characterized. The phosphirane-substituted clusters
were found to be good catalysts for the hydrogenation of tiglic acid (trans-2-methyl-2-
butenoic acid) but enantioselective hydrogenation could not be detected.

ACCEPTED MANUSCRIPT
Synthesis and characterization of chiral phosphirane
derivatives of [(
µ
-H) Ru (CO) ] and their application in the
4
4
12
hydrogenation of an α,β-unsaturated carboxylic acid
a
a
b
Ahmed F. Abdel-Magied, Maitham H. Majeed, Manuel F. Abelairas-Edesa, Arne
b
a
a
b
c
Ficks, Radwa M. Ashour, Ahibur Rahaman, William Clegg, Matti Haukka, Lee J.
b
a
Higham * and Ebbe Nordlander *
a
Inorganic Chemistry Research Group, Chemical Physics, Chemical Center, Lund University, Box
1
24, SE-221 00, Lund, Sweden.
b
School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K.
Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland.
c
th
Dedicated to Prof. Richard D. Adams on his 70 birthday, and in recognition of his
outstanding contributions to organometallic cluster chemistry.
ABSTRACT:
Ruthenium clusters containing the chiral binaphthyl-derived mono-phosphiranes [(S)-
([1,1′-binaphthalen]-2-yl)phosphirane] (S)-1a, [(R)-(2′-methoxy-1,1′-binaphthyl-2-
yl)phosphirane] (R)-1b, and the diphosphirane [2,2′-di(phosphiran-1-yl)-1,1′-
binaphthalene] (S)-1c have been synthesized and characterized. The clusters are [(µ-
H) Ru (CO) ((S)-1a)]
(S)-2,
[(µ-H) Ru (CO) ((R)-1b)]
(R)-3,
1,1-[(µ-
4
4
11
4
4
11
H) Ru (CO) ((S)-1c)] (S)-4, [(µ-H) Ru (CO) ((S)-binaphthyl-P (H)Et)] (S,S )-5,
4
4
10
4
4
11
(s)
p
[
(µ-H) Ru (CO) ((S)-binaphthyl-P (H)Et)]
(S,R )-6,
[(µ-H) Ru (CO) ((R)-
4 4 11
4
4
11
(R)
p
binaphthyl-P (H)Et)] (R,S )-7, [(µ-H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,R )-8
(s)
p
4
4
11
(R)
p
and the phosphinidene-capped triruthenium cluster [(µ-H) Ru (CO) (PEt)] 9. Clusters
2
3
9
5
2
-8 are formed via hydrogenation and opening of the phosphirane ring in clusters (S)-
and (R)-3. The phosphirane-substituted clusters were found to be able to catalyze
the hydrogenation of trans-2-methyl-2-butenoic acid (tiglic acid), but no
enantioselectivity could be detected. The molecular structures of (S)-4, (R,S )-7 and 9
p
have been determined and are presented.
KEYWORDS: Hydrogenation, Clusters, Ruthenium, Phosphirane
1

ACCEPTED MANUSCRIPT
1
. Introduction
For more than three decades, numerous researchers in both academia and industry
have investigated the homogeneous asymmetric hydrogenation reaction extensively,
due to its importance for the production of enantiopure bioactive ingredients on an
industrial scale [1]. Ruthenium, rhodium and iridium complexes containing chiral
diphosphine ligands have shown high catalytic activity in the asymmetric
hydrogenation of alkenes, ketones and imines [2, 3]. There has also been considerable
interest in the synthesis and use of transition metal carbonyl clusters as catalysts for
various chemical reactions of industrial relevance [4-8]. Previously, cluster-based
catalysts/catalyst precursors have shown interesting catalytic activity for several
different chemical reactions: hydride-containing trinuclear and tetranuclear ruthenium
carbonyl clusters have been used effectively in a number of reactions including
isomerizations [9], hydroformylations [10], hydrosilylations [11], reductive couplings
[
12] and the water-gas shift reaction [13]. Relatively few studies have dealt with the
potential of clusters to act as catalysts for the asymmetric hydrogenation of carboxylic
acids [14-17]. Salvini et al. have reported the application of such clusters in the
asymmetric hydrogenation of tiglic acid (trans-2-methyl-2-butenoic acid) [18], but
the reaction was performed under rather harsh conditions and the formation of 2-
methylbutyric acid was obtained with relatively low enantioselectivities (6-39% ee).
Catalyst activities and selectivities were found to be dependent on the chiral
phosphine ligands coordinated to the parent cluster. Using relatively bulky, chiral
diphosphine ligands with ferrocenyl backbones (the Walphos and Josiphos ligand
families), we have shown that tetraruthenium tetrahydride clusters can be effective
catalysts/catalyst precursors under mild hydrogenation conditions for the asymmetric
hydrogenation of α-unsaturated carboxylic acids, with enantioselectivities exceeding
9
0% ee in certain cases [19-22].
The common monodentate binaphthyl-based ligands with phosphonite [23], phosphite
24] and phosphoramidite [25] functionalities have also received attention as ligands
[
in asymmetric hydrogenation reactions. Few examples are known where chiral
phosphiranes were synthesized and used in asymmetric catalysis; phosphiranes are
highly strained heterocycles that incorporate the phosphorus atom into a three-
membered ring, and as a result of their highly pyramidalised structure, are considered
weak σ-donors but strong π-acceptors [26]. Rhodium complexes of phosphiranes
2

ACCEPTED MANUSCRIPT
have been used in the asymmetric α−dehydroamination of acids and itaconates,
achieving enantioselectivities of up to 76% ee [27], whilst Higham and co-workers
have shown that binaphthyl-derived phosphiranes can be used effectively in the
asymmetric hydrosilylation of styrene, with 100% conversion and 80% ee. The
asymmetric allylic alkylation of (rac)-(E)-1,3-diphenylallyl acetate was achieved with
the same phosphirane ligands, giving high conversions of up to 91%, albeit with
relatively low ee values of around 17% [28, 29].
Considering the success of our earlier studies, and the demonstrated effectiveness of
binaphthyl-based phosphorus-donor ligands, we have investigated the coordination of
chiral phosphiranes to H Ru carbonyl clusters, and the potential of the resultant chiral
4
4
ruthenium clusters to catalyze asymmetric reactions. Herein, we report the synthesis
and characterization of new tetranuclear ruthenium clusters containing the chiral
mono- or bidentate phosphirane ligands [(S)-([1,1′-binaphthalen]-2-yl)phosphirane]
(S)-1a, [(R)-(2′-methoxy-1,1′-binaphthyl-2-yl)phosphirane] (R)-1b, and [2,2′-
di(phosphiran-1-yl)-1,1′-binaphthalene] (S)-1c (Figure 1). A comparison of their
performances as catalysts/catalyst precursors in the hydrogenation of tiglic acid is also
provided.
Figure 1. The structures of the phosphirane ligands used in this investigation.
2
. Results and discussion
The syntheses of the chiral MOP-type ([1,1′-binaphthalen]-2-yldiphenylphosphine)
derivatives of [H Ru (CO) ] were based on previously published methods [19, 21,
4
4
12
2
2], i.e. via thermal substitution in benzene under an elevated pressure of hydrogen
gas (30-50 bar), or by using Me NO as an oxidative decarbonylation reagent in the
3
presence of the phosphirane ligand in dichloromethane solution, at ambient
3

ACCEPTED MANUSCRIPT
temperature (cf. Experimental Section). The latter method was found to be superior
with respect to both yield and selectivity of product formation, while the thermal
high-pressure route gave rise to phosphirane degradation (vide infra). The clusters
1
31
were identified by IR, H and P NMR spectroscopy, mass spectrometry and,
wherever possible, X-ray crystallography. A summary of the new clusters synthesized
in this study is given in Table 1.
Table 1. A summary of the cluster framework geometries and phosphirane
coordination modes detected in this study.
CO
CO
OC
H
CO
H
OC
H
CO
H
Ru
Ru
OC
OC Ru
CO
CO
OC
OC Ru
CO
Ru
H
Ru
P
P
OC
H
OC
H
R
Ru
OC
H
P
Ru
CO
OC
OC
CO
OC
[
[
(µ-H) Ru (CO)11((S)-1a)] (S)-2
4
4
1,1-[(µ-H) Ru (CO)10((S)-1c)] (S)-4
4
4
(µ-H) Ru (CO)11((R)-1b)] (R)-3
4
4
CO
CO
OC
H
CO
H
OC
H
CO
H
Ru
Ru
H
P
H
P
OC
OC
CO
CO
CO
CO
OC Ru
Ru
OC Ru
Ru
OC
H
Ru
H
R
OC
H
OC
H
R
Ru
OC
OC
CO
CO
OC
(µ-H) Ru (CO)11((S)-H-binaphthyl-P(s)(H)Et)] (S,Sp)-5
[
[
[(µ-H)4Ru4(CO)11((S)-H-binaphthyl-P(R)(H)Et)] (S,Rp)-6
[(µ-H)4Ru4(CO)11((R)-MeO-binaphthyl-P(R)(H)Et)] (R,Rp)-8
4
4
(µ-H) Ru (CO)11((R)-MeO-binaphthyl-P(s)(H)Et)] (R,Sp)-7
4
4
P
OC
CO
OC Ru
Ru CO
OC
H
OC
H CO
Ru
CO
OC
(µ-H) Ru (CO) (PEt)] 9
[
2
3
9
2
.1. Synthesis and characterization of clusters (S)-2, (R)-3 and (S)-4
Treatment of an equimolar mixture of [H Ru (CO) ] and (S)-1a, (R)-1b or (S)-1c in
4
4
12
dichloromethane at room temperature with 1.2-1.5 eq. of the oxidative
decarbonylation reagent Me NO led to the formation of the corresponding
3
monosubstituted clusters [(µ-H) Ru (CO) ((S)-1a)] (S)-2, [(µ-H) Ru (CO) ((R)-1b)]
4
4
11
4
4
11
(
R)-3, or the chelated diphosphirane cluster 1,1-[(µ-H) Ru (CO) ((S)-1c)] (S)-4.
4 4 10
Purification by preparative thin-layer chromatography allowed for the isolation of
each cluster as an orange-red solid. The structures of the clusters were identified on
4

ACCEPTED MANUSCRIPT
the basis of their spectroscopic data. The IR spectra of (S)-2 and (R)-3 are similar to
those previously reported for the clusters [(µ-H) Ru (CO) (L)] (L = PMe Ph) [30]
4
4
11
2
and [(µ-H) Ru (CO) (NMDPP)] (NMDPP = (S)-(+)-neomenthyldiphenylphosphine]
4
4
11
1
[
20]. The H NMR spectra revealed, in addition to the signals for the chiral
phosphirane ligands (cf. Experimental section), an apparent doublet at –17.64 ppm for
S)-2 (JHH = 4.6 Hz) and an apparent singlet at –17.84 ppm for (R)-3, which indicates
(
3
1
a complete fluxionality of the hydride ligands (vide infra). The P NMR spectra show
a broad multiplet at –154.2 ppm for (S)-2 and an apparent singlet at –155.9 ppm for
3
1
1
(
R)-3, in contrast to the high field shifts shown by the free ligands themselves ( P[ H]
NMR: (S)-1a –235.4 ppm, (R)-1b –235.0 ppm) [28]. The mass spectra reveal peak
+
+
envelopes at m/z = 1052 [M+Na] and 1081 [M+Na] which correspond to the
molecular formulae [(µ-H) Ru (CO) ((S)-1a)] (S)-2 and [(µ-H) Ru (CO) ((R)-1b)]
4
4
11
4
4
11
(R)-3, respectively.
The IR spectrum of cluster (S)-4 in cyclohexane exhibits eight absorptions in the CO
region. The νC-O IR stretching frequency pattern of (S)-4 is thus in agreement with that
of the known complex 1,1-[(µ-H) Ru (CO) (DUPHOS)] [DUPHOS = (2R,5R)-2,5-
4
4
10
dimethyl-1-(2-((2R,5R)-2,5-dimethylphospholan-1-yl)-phenyl)phospholane]
[20],
which indicates that the diphosphirane ligand (S)-1c in cluster (S)-4 chelates one
1
31
ruthenium apex. The H and P NMR spectra of (S)-4 show one broad singlet at –
1
1
7.20 ppm in the hydride region of the H NMR spectrum (due to hydride
3
1
fluxionality), while the P NMR spectrum displays two doublets (J = 37.1 Hz)
P-P
with equal intensity at –135.6 and –139.2 ppm caused by the two magnetically
inequivalent phosphorus nuclei. The mass spectrum of (S)-4 reveals a peak at m/z =
+
1
082 [M+Na] , which is consistent with the molecular formula [(µ-H) Ru (CO) ((S)-
4
4
10
1
c)] (S)-4.
It was possible to grow crystals of (S)-4 suitable for analysis by X-ray diffraction. The
chelating mode of the diphosphirane ligand could therefore be unambiguously
confirmed in the solid state. The molecular structure of (S)-4 is shown in Figure 2.
The cluster contains a distorted tetrahedral core of ruthenium atoms with the ‘basal’
atoms preserving the tricarbonyl units of the parent cluster [H Ru (CO) ] [31]. The
4
4
12
diphosphirane ligand is coordinated to the ‘apical’ atom, Ru1, in a chelating binding
mode. The Ru-Ru bond lengths provide a good indication of the locations of the
hydrides. They may be divided into two distinctive classes: firstly the four ‘long’
5

ACCEPTED MANUSCRIPT
(hydride-bridged) distances [Ru1-Ru2 2.9795(4), Ru1-Ru3 3.0055(4), Ru1-Ru4
2
2
.9518(4), Ru2-Ru3 2.9229(4)Å], and secondly the two ‘short’ distances [Ru3-Ru4
.7919(4), Ru2-Ru4 2.7945(4)Å]. The arrangement of the four bridging hydrides
yields an H Ru cluster-core with C symmetry, rather than the D symmetry that was
4
4
s
2d
observed for the bis-phosphine-derivatised cluster [H Ru (CO) (PPh ) ] [31]. The C
s
4
4
10
3 2
symmetry found in the H Ru core seems to be a common feature that is shared with
4
4
[
H Ru (CO) (diphosphine)] clusters, and has been reported for both chelating [19]
4 4 10
and bridging [19, 22] coordination modes of the diphosphine ligands. The Ru-P
distances in cluster (S)-4 are identical within experimental error: [Ru1-P1 2.3017(3),
Ru1-P2 2.3019 (3)Å]. All CO ligands are bound in a terminal monodentate
coordination mode and are staggered with respect to the Ru-Ru vectors. It should be
noted that due to the chirality of the binaphthyl backbone, the two phosphorus atoms
of the bound ligand are magnetically inequivalent, regardless of hydride fluxionality.
The high degree of hydride fluxionality that was exhibited for (S)-2, (R)-3 and (S)-4 at
ambient temperature is in contrast to what was found for related (chiral) phosphine
derivatives of H Ru [21, 22]. Attempts to freeze-out the fluxionality in (S)-2 and (S)-
4
4
1
4
by means of low temperature H NMR experiments were unsuccessful; the fluxional
behaviour was observed even at 213 K (cf. ESI†).
6

ACCEPTED MANUSCRIPT
Figure 2. Molecular structure of 1,1-[(µ-H) Ru (CO) ((S)-1c)] (S)-4 with thermal
4
4
10
ellipsoids drawn at the 50% probability level. All hydrogen atoms except the hydrides
have been omitted for clarity. Selected bond lengths (Å): Ru1-Ru2 2.9795(4), Ru1-
Ru3 3.0055(4), Ru1-Ru4 2.9518(4), Ru2-Ru3 2.9229(4), Ru3-Ru4 2.7919(4), Ru2-
Ru4 2.7945(4), Ru1-P1 2.3017(3), Ru1-P2 2.3019(3).
2
.2. Synthesis and characterization of clusters (S,S )-5, (S,R )-6, (R,S )-7 and (R,R )-
P P P P
8
The reaction of [(µ-H) Ru (CO) ] with an equimolar amount of (S)-1a or (R)-1b
4
4
12
under elevated temperature and H pressure (30-50 bar, cf. Experimental Section) led
2
to degradation of the phosphirane ligands. Cleavage of a P-C bond in the strained
three-membered phosphirane ring occurred, and hydrogenation gave the resultant
binaphthylPH(Et) secondary phosphine moiety, thus introducing another element of
chirality into each ligand. As a result, the diastereomeric pairs [(µ-H) Ru (CO) ((S)-
4
4
11
binaphthyl-P (H)Et)] (S,S )-5 and [(µ-H) Ru (CO) ((S)-binaphthyl-P (H)Et)]
(s)
P
4
4
11
(R)
(
S,R )-6, or [(µ-H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,S )-7 and [(µ-
P 4 4 11 (s) P
H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,R )-8 were formed, comprising opposite
4
4
11
(R)
P
stereocenters at the phosphorus atom of each pair.
Whether the ligand transformation occurs before or after coordination to the
tetraruthenium clusters was initially unclear. However, it was subsequently found that
the same diastereomeric pairs could be formed via direct thermolysis of [(µ-
H) Ru (CO) ((S)-1a)] (S)-2 or [(µ-H) Ru (CO) ((R)-1b)] (R)-3 under high H
4
4
11
4
4
11
2
pressure (cf. Scheme 1 and Experimental Section). When ligand (S)-1a was heated
under the same reaction conditions, but in the absence of [(µ-H) Ru (CO) ], no
4
4
12
hydrogenation or degradation of the ligand was observed. This strongly indicates that
the hydrogenation of the ligand is mediated by the ruthenium cluster.
The clusters were isolated by preparative thin-layer chromatography as red-orange
solids. The spectroscopic data were found to be consistent with the proposed
structures discussed above, which are depicted in Scheme 1. The νC-O IR patterns of
(
S,S )-5/(S,R )-6 and (R,S )-7/(R,R )-8 are in agreement with those reported for mono-
P P P P
substituted [(µ-H) Ru (CO) (L)] clusters, e.g. where L = PMe Ph [30] or NMDPP
4
4
11
2
1
(
(S)-(+)-neomenthyldiphenylphosphine) [20]. The H NMR spectra revealed, in
addition to signals for the chiral secondary phosphine ligand, apparent singlets at –
7.64 ((S,S )-5), –17.81 ((S,R )-6), –17.72 ((R,S )-7) and –17.86 ppm ((R,R )-8), due
1
P
P
P
P
7

ACCEPTED MANUSCRIPT
to the high degree of hydride fluxionality in each cluster at ambient temperature.
1
Variable-temperature H NMR of clusters (S,S )-5 and (S,R )-6 showed that the
P
P
fluxionality remained even at 213K (vide supra, cf. Supplementary Information).
1
Furthermore, the H NMR spectra confirmed the opening of the three-membered
phosphorus-heterocycle of the phosphirane ligands, which is highly strained as a
1
result of the small bond angle at the phosphorus atom [32]. In all four clusters the H
NMR resonances of the ethyl group were observed, as well as the signal for the
hydrogen atom coordinated directly to the phosphorus. The latter resonance appeared
1
1
as a doublet at 5.23 ppm for (S,S )-5 ( J = 381 Hz), 5.28 ppm for (S,R )-6 ( J =
H-P
P
H-P
P
1
1
3
79 Hz) 5.16 ppm for (R,S )-7 ( J = 380 Hz) and 5.22 ppm for (S,S )-8 ( J = 378
P H-P P H-P
3
1
Hz). The P NMR spectra for clusters (S,S )-5/(S,R )-6 and (R,S )-7/(R,R )-8 show
P
P
P
P
signals that are shifted to lower field at −11.9, –13.1, –11.8 and –13.2 ppm,
1
respectively (partial H decoupling, cf. Supplementary Material). The mass spectra
+
revealed peak envelopes at m/z = 1035 [M+H] for (S,S )-5/(S,R )-6 and 1083
P
P
+
[
M+Na] for (R,S )-7/(R,R )-8, in agreement with the proposed molecular formulae.
P P
Scheme 1. Synthesis of clusters (S)-2, (R)-3, (S,S )-5, (S,R )-6, (R,S )-7 and (R,R )-8.
P
P
P
P
8

ACCEPTED MANUSCRIPT
The nature of the transformed phosphirane ligand was unambiguously confirmed by
the determination of the solid-state structure of (R,S )-7, shown in Figure 3. Relevant
P
bond distances are summarized in the figure caption, and crystallographic data are
collated in Table S1. The symmetry of the Ru (µ-H) core is D , which is in
4
4
2d
agreement with previously reported crystal structures of other [H Ru (CO) (PR )]
4
4
11
3
species (R = OMe [30], Ph [33], SC H [34], C F [30], OEt [30]) and the
4
3
6 5
diphosphine bridged cluster [{H Ru (CO) } (µ-Ph -P-C≡C-C≡C-PPh )] [35]. The
4
4
11 2
2
2
one exception, with C symmetry, was reported for [H Ru (CO) (NMDPP)]
s
4
4
11
(NMDPP = (S)-(+)-neomenthyldiphenylphosphine) [20]. The trans P1-Ru4-Ru2 bond
angle (cf. Figure 3) is close to linear at 170.124(18)° and comparable to the value
found in [H Ru (CO) {P(C F ) }] of 172.25(3)°.
4
4
11
6 5 3
Figure 3. Molecular structure of [(µ-H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,S )-
4
4
11
(s)
P
7
with thermal ellipsoids drawn at 50% probability level. Aryl hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å): Ru1-Ru2 2.9627(3), Ru1-Ru3
2
2
.7891(3), Ru1-Ru4 2.9615(3), Ru2-Ru3 2.9584(3), Ru3-Ru4 2.3274(6), Ru2-Ru4
.7863(3), Ru4-P1 2.3274(6).
Following on from this, it was envisaged that the formation of phosphido clusters via
CO dissociation and oxidative addition of a P-H bond at a ruthenium center may be
possible, considering that the four clusters (S,S )-5/(S,R )-6 and (R,S )-7/(R,R )-8
P
P
P
P
each contain a secondary phosphine. Therefore, attempts were made to promote such
a transformation by either (i) dissolving the aforementioned secondary phosphine
cluster species in THF followed by the addition of Me NO in THF under nitrogen and
3
subsequently refluxing the solution for 12 h, or (ii) refluxing in toluene for 5h.
9

ACCEPTED MANUSCRIPT
However, thin-layer-chromatographic analysis indicated that in all cases no reaction
had occurred.
However, it was found that thermolysis of [(µ-H) Ru (CO) ((S)-binaphthyl-
4
4
11
P (H)Et)] (S,S )-5 in benzene under 50 bar of H , for 24 h at a temperature of 100 ºC,
(s)
P
2
led to the formation of a small amount of phosphinidene clusters. One product was
1
31
identified as [(µ-H) Ru (CO) (µ -(S)-binaphthyl-P)] on the basis of its H and
P
2
3
9
3
NMR spectra [36]. A second product was recrystallized from acetonitrile-hexane
solution, where a few red crystals were formed, which when analyzed by single
crystal X-ray diffraction (vide infra) were identified as [(µ-H) Ru (CO) {µ -
2
3
9
3
P(C H )}] 9. These phosphinidene clusters are presumably formed via
2
5
opening/hydrogenation of the phosphirane moiety, together with (subsequent)
elimination of either the ethyl or the binaphthyl moiety. Because of the very poor
yield, no spectroscopic data for compound 9 are available (further attempts to
resynthesize compound 9 have failed).
The solid-state structure of 9 is shown in Figure 4, with selected geometric data listed
in the caption; relevant crystallographic data are summarized in Table S1. In 9, the
three ruthenium atoms are capped by the phosphorus atom, forming a distorted
tetrahedral core. The distortion originates from the edge-bridging hydrides that link
Ru1-Ru2 and Ru1-Ru3. Due to the influence exerted by the edge-bridging hydrides,
the Ru1-Ru2 and Ru1-Ru3 bond distances (2.9392(2) and 2.9413(3)Å, respectively)
are elongated relative to the Ru2-Ru3 bond distance (2.8225(2)Å). The Ru-P bond
distances are similar to those observed for the phosphinidene cluster
[
H Ru (CO) (PPh)] [37]. However, in 9, the Ru-P bond lengths are notably
2 3 9
asymmetric: Ru1-P1 = 2.3253(6), Ru2-P1 = 2.2840(6) and Ru3-P1 = 2.2907(6)Å.
1
0

ACCEPTED MANUSCRIPT
Figure 4. Molecular structure of [(µ-H) Ru (CO) (PEt)] 9 with thermal ellipsoids
2
3
9
drawn at 50% probability level. All hydrogen atoms except the hydrides have been
omitted for clarity. Selected bond lengths (Å): Ru1-Ru2 2.9392(2), Ru1-Ru3
2
2
.9413(3), Ru2-Ru3 2.8225(2), Ru1-P1 2.3253(6), Ru2-P1 2.2840(6), Ru3-P1
.2907(6).
2
.3. Hydrogenation of tiglic acid
For the asymmetric hydrogenation of tiglic acid ((E)-2-methylbut-2-enoic acid)
catalyzed by H Ru -clusters, we have shown repeatedly that at the comparatively low
4
4
temperature of 100 °C and with a hydrogen pressure of 50 bar, the reaction
conversion is maximized and the selectivity is enhanced [21, 22]. At higher
temperatures (130-150 °C), severe decomposition of the clusters was observed and it
is suspected that the solid particles produced in the reaction mixture were responsible
for the formation of a considerable amount of cyclohexene and cyclohexane, in
addition to the expected 2-methyl butyric acid [20]. However, at 100 °C the reaction
mixture remained homogeneous and the aforementioned hydrogenation products,
derived from toluene, were not detected although longer reaction times of up to 48 h
were required in order to increase the conversion. The results of the catalytic tests
using clusters (S)-4, (S,S )-5, (S,R )-6, (R,S )-7 and (R,R )-8 are presented in Table 2.
P
P
P
P
Clusters (S)-2 and (R)-3 were not included in the study, because under the catalytic
reaction conditions described in Scheme 1, they generated (S,S )-5/(S,R )-6 and
P
P
(
R,S )-7/(R,R )-8.
P P
a
Table 2. Catalytic hydrogenation of tiglic acid in the presence of clusters 4-8.
COOH
Catalyst
H
COOH
H
H
o
T = 100 C/50 bar H2
1
1

ACCEPTED MANUSCRIPT
b
Entry
Catalyst
Conv. %
1
2
3
(S)-4
51
52
49
78
55
51
78
(S,S )-5
P
(S,R )-6
P
c
4
(S,R )-6
P
5
6
(R,S )-7
P
(R,R )-8
P
c
7
(R,R )-8
P
a
Reaction conditions: p(H ) = 50 bar, T = 100 °C, t = 24 h, solvent = EtOH/toluene (1:1, 5 mL), n
00. Amount of substrate consumed in the catalytic experiment assessed by H NMR spectroscopy. Reaction time t
/n(catalyst) =
2
(substrate)
c
b
1
1
=
48 h.
Clusters (S,R )-6 and (R,R )-8 gave quite high conversion rates (78%), when extended
P
P
reaction times of 48h were employed (entries 4 and 7, Table 2). Under the same
reaction conditions, [H Ru (CO) ] did not show reasonable catalytic conversion rates
4
4
12
(
≈21%) [38]. Clusters (S,S )-5/(S,R )-6 and (R,S )-7/(R,R )-8 were recovered
P P P P
unaltered after the catalytic run in ca. 50-60% yields. After work-up, no evidence of
other metal-based products was observed. Cluster (S)-4 was unrecoverable at the end
of the catalytic cycle - unknown/unidentifiable decomposition products were
recovered instead.
However, in this study no enantioselectivity was observed for any of the chiral
clusters which were tested as catalysts/catalyst precursors. Knowles et al. have
proposed that in hydrogenation reactions, bidentate phosphine ligands are generally
superior to monodentate ones, because their use results in greater rigidity in the
molecular catalyst, which leads to higher chiral induction [39]. In clusters (S,S )-
P
5
/(S,R )-6 and (R,S )-7/(R,R )-8, beside the chiral binaphthyl backbone, the
P P P
stereogenic center at phosphorus is directly coordinated to a ruthenium atom in the
cluster, but the effective steric bulk of this ligand is relatively small because one of
the substituents on the phosphorus is a hydrogen. In cluster (S)-4, the bidentate
diphosphirane ligand is coordinated to the cluster in a chelating mode, which may
have been expected to lead to a better chiral induction; effective chiral induction by
1
,1-[H Ru (CO) (P-P*)] clusters has been demonstrated previously (where P-P*=
4 4 10
chiral diphosphine ligand) [19]. However, (S)-4 gave no enantioselectivity either; one
explanation is that the phosphirane ring-opening reaction that occurs in the
monodentate cases also happens in this instance, generating mixed secondary/tertiary
and secondary/secondary bidentate ligands, that are ineffective at chiral induction.
1
2

ACCEPTED MANUSCRIPT
Whilst the above rationale is plausible, the exact reaction mechanism(s) of the cluster-
based catalytic hydrogenation reaction is/are still unclear [19]. Although the isolation
of (S,S )-5/(S,R )-6 and (R,S )-7/(R,R )-8 from the catalytic reaction is consistent with
P
P
P
P
cluster-based catalysis (but does not exclude fragmentation), (S)-4 also decomposes.
Assuming the case of intact clusters, it should also be borne in mind that the presence
of several metal centers is an inherent weakness when it comes to enantioselection in
these particular systems - it is more likely that a substrate will coordinate at a
ruthenium center that is remote from the coordinated chiral ligand. The effective
chiral induction is thus expected to be lower than in a corresponding mononuclear
complex, which illustrates the need for further research into more sophisticated
cluster design.
In previous studies, an (empirical) correlation between the hydride fluxionality in
clusters of the type [H Ru (CO) (P-P*)] (P-P* = chiral diphosphine ligand) and the
4
4
10
chiral induction generated in the catalytic asymmetric hydrogenation of α-unsaturated
carboxylic acids was noted. The greater the fluxionality, the lower the chiral induction
[
19, 22, 38], whereas clusters which were effective in this regard were found to have a
very rigid hydride orientation/location. In each case, it was found that one specific
hydride remained fixed under the catalytic conditions (temperature and hydrogen
pressure) used in the present study [19, 22]. Thus the observed correlation also
appears to hold true for the H Ru clusters investigated in the present study; they are
4
4
the most fluxional chiral tetraruthenium clusters that we have so far studied, and they
give no chiral induction.
3
. Conclusions
In summary, we have synthesized and characterized new tetranuclear ruthenium
clusters functionalized with chiral phosphiranes, and used these to generate secondary
phosphine-bound clusters, as well as a new phosphinidene-capped triruthenium
cluster. All of these clusters are capable (to varying degrees) of acting as
catalysts/catalyst precursors for the hydrogenation of tiglic acid; however, no chiral
induction was detected in the reaction. Although it has not been possible to
unambiguously determine the nature of the active catalyst in the cluster-based
1
3

ACCEPTED MANUSCRIPT
systems, indirect evidence suggests that clusters (S,S )-5/(S,R )-6 and (R,S )-7/(R,R )-
P
P
P
P
8
may be the active catalysts, or direct precursors to an active cluster catalyst.
From the present results, and based on our previous findings using various chiral
phosphine derivatives of [H Ru (CO) ] as asymmetric hydrogenation catalysts [21,
4
4
12
3
8], it is evident that the nature of the chiral ligand is key to the enantioselectivity that
is obtained. Our next intention is to design new bidentate phosphines, which will slow
the hydride fluxionality by virtue of their modified steric/electronic properties.
4
. Experimental
All reactions and other manipulations were carried out under a nitrogen atmosphere
using standard Schlenk techniques. All solvents were dried and distilled under a
nitrogen atmosphere prior to use. Infra-red spectra were recorded as solutions in 0.5
1
31
1
mm NaCl cells on a Nicolet Avatar 360 FT-IR spectrometer. H and P{ H} NMR
1
spectra were recorded on a Varian Unity 500 MHz NMR spectrometer: H NMR
31
1
shifts were referenced to Me Si, with P{ H} NMR shifts referenced to external
4
31
1
1
H PO (85%). The P[ H] NMR spectra of 5-8 have been obtained with partial H
3
4
NMR decoupling in the range of +4.0 to –30.0 ppm. The parent cluster [(µ-
H) Ru (CO) ] was prepared according to a literature procedure [40] and its purity
4
4
12
assessed by thin-layer chromatographic (TLC) analysis and IR spectroscopy. The
chiral phosphirane ligands [(S)-1-([1,1′-binaphthalen]-2-yl) phosphirane] (S)-1a, [(R)-
1
-(2′-methoxy-1,1′-binaphthyl-2-yl) phosphirane] (R)-1b and [2,2′-di(phosphiran-1-
yl)-1,1′-binaphthalene] (S)-1c were prepared using the methodology outlined
previously [28, 41]. Tiglic acid and (S)-methyl mandelate were purchased from
Sigma-Aldrich. Products were separated using Merck thin-layer chromatography
(TLC) plates as supplied (0.25 mm layer of Kiesel-gel 60 F254). Catalysis
experiments were carried out using a 45 mL Parr autoclave with a PTFE reaction
vessel.
4
.1. Synthesis of [(µ-H) Ru (CO) ((S)-1a)] (S)-2
4 4 11
1
4

ACCEPTED MANUSCRIPT
[
(µ-H) Ru (CO) ] (25 mg, 0.033 mmol) and (S)-1a (11 mg, 0.033 mmol) were
4
4
12
dissolved in 20 mL of dichloromethane and the solution was stirred for 20 min. Under
vigorous stirring, a small excess of Me NO (3 mg, 0.04 mmol) dissolved in
3
dichloromethane (5 mL) was added dropwise to the yellowish cluster/ligand solution
for 30 min, with an instant change of colour towards red. After 6 h the organics were
removed under reduced pressure. The red solid residue was redissolved in a small
quantity
of
dichloromethane
and
purified
by
preparative
TLC
(dichloromethane/petroleum ether, 1:3). Apart from traces of starting materials and
one stationary brown band (decomposed material), one band was isolated from the
TLC plate, extracted with dichloromethane and dried under vacuum to afford a red
microcrystalline solid (R = 0.7). This was identified as [(µ-H) Ru (CO) ((S)-1a)]
f
4
4
11
1
(
S)-2 (13 mg, 38%). H NMR (500 MHz, CDCl ): δ = 8.00 (d, J = 8.5 Hz, 1H), 7.93
3
(m, 2H), 7.85 (d, J = 8.3 Hz, 1H), 7.78 (m, 1H), 7.67 (app t, 1H), 7.54 (d, J = 6.9 Hz,
1
H), 7.46 (m, 2H), 7.23 (m, 2H), 7.06 (d, J = 8.6 Hz, 1H), 6.95 (d, J = 8.1 Hz, 1H),
1
.19 (m, 1H, PC H ), 1.11 (m, 1H, PC H ), 1.02 (m, 1H, PC H ), 0.54 (m, 1H,
2
4
2
4
2
4
3
1
PC H ), –17.64 (d, J = 4.6 Hz, 4H, RuHRu) ppm; P NMR (202 MHz, CDCl ): δ
2
4
H-H
3
2
=
–154.2 (br m, J = not resolved) ppm; IR (cyclohexane): 2094 (m), 2088 (w),
P-H
-1
2
067 (vs), 2057 (vs), 2029 (vs), 2009 (vs), 1990 (m), 1969 (w) cm ; MS (FAB): m/z
+
=
1052 [M+Na] .
4
.2. Synthesis of [(µ-H) Ru (CO) ((R)-1b)] (R)-3
4 4 11
[
(µ-H) Ru (CO) ] (30 mg, 0.04 mmol) and (R)-1b (14 mg, 0.04 mmol) were
4 4 12
dissolved in 20 mL of dichloromethane and the solution stirred for 20 min. Under
vigorous stirring, a small excess of Me NO (3.6 mg, 0.048 mmol) dissolved in
3
dichloromethane (5 mL) was added dropwise to the yellowish cluster/ligand solution
for 30 min, with an instant change of colour towards red. After 6 h the organics were
removed under reduced pressure. The red solid residue was redissolved in a small
quantity
of
dichloromethane
and
purified
by
preparative
TLC
(dichloromethane/petroleum ether, 1:3). Apart from traces of starting materials and
one stationary brown band (decomposed material), one band was isolated from the
TLC plate, extracted with dichloromethane and dried under vacuum to afford a
microcrystalline red solid (R = 0.6). This was identified as [(µ-H) Ru (CO) ((R)-
f
4
4
11
1
1
b)] (R)-3 (17 mg, 42%). H NMR (500 MHz, CDCl ): δ = 8.02 (d, J = 9.2 Hz, 1H),
3
7
.96 (d, J = 8.2 Hz, 1H), 7.86 (d, J = 8.0 Hz, 2H), 7.77 (m, 1H), 7.46 (m, 2H), 7.32
1
5

ACCEPTED MANUSCRIPT
(m, 1H), 7.22 (m, 2H), 7.03 (d, J = 8.7 Hz, 1H), 6.76 (d, J = 8.8 Hz, 1H), 3.71 (s, 3H,
OCH ), 1.66 (m, 1H, PC H ), 1.07 (m, 1H, PC H ), 0.92 (m, 1H, PC H ), 0.72 (m,
3
2
4
2
4
2
4
31
1
H, PC H ), –17.84 (s, 4H, RuHRu) ppm; P NMR (202 MHz, CDCl ): δ = –155.9
2 4 3
(br s) ppm; IR (cyclohexane): 2092 (m), 2086 (m), 2065 (vs), 2053 (vs), 2030 (vs),
-1
+
2
007 (vs), 1989 (m), 1969 (w) cm ; MS (FAB): m/z = 1081 [M+Na] .
4
.3. Synthesis of 1,1-[(µ-H) Ru (CO) ((S)-1c)] (S)-4
4
4
10
4
0 mg (0.053 mmol) of [(µ-H) Ru (CO) ] and 20 mg (0.053 mmol) of (S)-1c were
4 4 12
dissolved in dichloromethane. A small excess of Me NO [6 mg, 0.08 mmol] dissolved
3
in 5 mL of dichloromethane, containing a few drops of acetonitrile, was added
dropwise over a period of 30 min to the stirred cluster/ligand solution. The initial
yellow coloured solution turned deep red during the addition of Me NO. The reaction
3
was monitored continuously by TLC analysis (dichloromethane/petroleum ether 1:3)
and when no more parent cluster was observed (2 h), the solvent was removed under
vacuum. The resulting red solid was redissolved in a small quantity of
dichloromethane and separated using preparative TLC (dichloromethane/petroleum
ether, 1:3). Apart from traces of starting materials, one red band was isolated from the
TLC plate (R = 0.4), extracted with dichloromethane and dried under vacuum. This
f
1
was identified as 1,1-[(µ-H) Ru (CO) ((S)-1c)] (S)-4 (16 mg, 28%). H NMR (500
4
4
10
MHz, CDCl ): δ = 8.00 (m, 3H), 7.92 (m, 3H), 7.83 (m, 1H), 7.73 (m, 1H), 7.50 (m,
3
1
1
H), 7.23 (m, 1H), 6.80 (dd, J = 23.2 Hz, 8.3 Hz, 2H), 1.79 (m, 1H, PC H ), 1.67 (m,
2 4
H, PC H ), 1.43 (m, 1H, PC H ), 1.34 (m, 1H, PC H ), 0.55 (m, 1H, PC H ), 0.62
2
4
2
4
2
4
2
4
3
1
(
m, 2H), 0.42 (m, 1H, PC H ), –17.20 (br s, 4H, RuHRu) ppm; P NMR (202 MHz,
2 4
2
2
CDCl ): δ = –135.6 (d, J = 37.1 Hz), –139.2 (d, J = 37.1 Hz) ppm; IR
3
P-P
P-P
(cyclohexane): 2076 (s), 2044 (vs), 2024 (vs), 2008 (s), 1993 (w), 1987 (w), 1977
-1
+
(
m), 1953 (br) cm ; MS (FAB): m/z = 1082 [M+Na] . Red crystals of (S)-4 were
obtained by slow evaporation of dichloromethane/hexane at 4 °C.
4
.4. Synthesis of [(µ-H) Ru (CO) ((S)-binaphthyl-P (H)Et)] (S,S )-5 and [(µ-
4 4 11 (s) P
H) Ru (CO) ((S)-binaphthyl-P (H)Et)] (S,R )-6
4
4
11
(R)
P
1
6

ACCEPTED MANUSCRIPT
4
.4.1. Method A: Thermal ligand substitution at high pressure using [(µ-
H) Ru (CO) ((S)-1a)] (S)-2
4
4
11
A small laboratory autoclave (45 mL capacity), was loaded with 45 mg (0.043 mmol)
of [(µ-H) Ru (CO) ((S)-1a)] (S)-2, and dissolved in 5 mL of EtOH/toluene (1:1). The
4
4
11
autoclave was sealed and purged three times with 10 bar of H before being
2
pressurized to 50 bar H . After 24 h at 100 °C, the autoclave was cooled to room
2
temperature. The gases were carefully released and the autoclave was opened. The
dark brown solution was concentrated, and the obtained solid was dissolved in 2 mL
of dichloromethane and subjected to preparative TLC (dichloromethane/petroleum
ether, 1:3). Apart from traces of starting materials and one stationary brown band
(decomposed material), two bands were isolated from the TLC plate, extracted with
dichloromethane and dried under vacuum. Two products were observed; the first
product (red, R = 0.8) was identified as [(µ-H) Ru (CO) ((S)-binaphthyl-P (H)Et)]
f
4
4
11
(s)
1
(
S,S )-5 (8 mg, 19%). H NMR (500 MHz, CDCl ) δ = 8.15 (d, J = 7.1 Hz, 1H), 8.09
P 3
(
d, J = 9.1 Hz, 1H), 7.97 (d, J = 8.2 Hz, 1H), 7.93 (d, J = 8.3 Hz, 1H), 7.67 (dd, J =
1.2 Hz, 8.7 Hz, 1H), 7.54 (app t, 1H), 7.39 (d, J = 7.8 Hz, 1H), 7.32 (m, 1H), 7.37
1
(m, 1H), 7.26 (app t, 2H), 7.16 (d, J = 8.5 Hz, 1H), 6.96 (d, J = 8.4 Hz, 1H), 5.23 (dd,
1
JP-H = 381 Hz, JH-H = 10.7 Hz, 1H, PH), 2.13 (m, 1H, PCH CH ), 1.85 (m, 1H,
2
3
3
1
PCH CH ), 0.86 (m, 3H, PCH CH ), –17.64 (s, 4H, RuHRu) ppm; P NMR (202
2
3
2
3
MHz, CDCl ): δ = −11.9 (s) ppm; IR (cyclohexane): 2092 (w), 2084 (m), 2064 (m),
3
-1
2
051 (vs), 2030 (vs), 2012 (s), 1984 (m), 1967 (w) cm ; MS (FAB): m/z = 1035
+
[
M+H] . The second product, (red, R = 0.7), was identified as [(µ-H) Ru (CO) ((S)-
f
4
4
11
1
binaphthyl-P (H)Et)] (S,R )-6 (7 mg, 17%). H NMR (500 MHz, CDCl ): δ = 8.15
(R)
P
3
(d, J = 7.0 Hz, 1H), 8.07 (d, J = 9.1 Hz, 1H), 7.97 (d, J = 8.3 Hz, 1H), 7.91 (d, J = 8.1
Hz, 1H), 7.72 (dd, J = 11.5 Hz, 8.6 Hz, 1H), 7.56 (app t, 1H), 7.45 (d, J = 9.1 Hz,
1
5
1
H), 7.35 (m, 2H), 7.27 (app t, 2H), 7.18 (d, J = 8.5 Hz, 1H), 6.90 (d, J = 8.5 Hz, 1H),
1
.28 (dd, J = 379 Hz, JH-H = 9.7 Hz, 1H, PH), 2.21 (m, 1H, PCH CH ), 1.98 (m,
P-H
2
3
3
1
H, PCH CH ), 1.06 (m, 3H, PCH CH ), –17.81 (s, 4H, RuHRu) ppm; P NMR (202
2
3
2
3
MHz, CDCl ): δ = −13.1 (s) ppm; IR (cyclohexane): 2092 (w), 2086 (m), 2063 (vs),
3
-1
2
057 (vs), 2052 (vs), 2030 (vs), 2007 (s), 1987 (m), 1966 (w) cm ; MS (FAB): m/z =
+
1
035 [M+H] .
4
.4.2. Method B: Thermal ligand substitution at high pressure
1
7

ACCEPTED MANUSCRIPT
[
(µ-H) Ru (CO) ] (30 mg, 0.04 mmol) and (S)-1a (13 mg, 0.04 mmol) were
4
4
12
dissolved in 5 mL of benzene and the solution was transferred to a small autoclave
45 mL capacity). The autoclave was sealed and purged three times with 10 bar of H2
(
before being pressurized to 30 bar H . After 4 h at 125 °C, the autoclave was cooled
2
to room temperature. The gases were carefully released and the autoclave was
opened. The isolation, purification and identification procedures of the products were
identical to those described above. Yields: (S,S )-5 (9 mg, 20%) and (S,R )-6 (7 mg,
P
P
1
6%).
4
.5. Synthesis of [(µ-H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,S )-7 and [(µ-
4 4 11 (s) P
H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,R )-8
4
4
11
(R)
P
4
.5.1. Method A: Thermal ligand substitution at high pressure using [(µ-
H) Ru (CO) ((R)-1b)] (R)-3
4
4
11
In a small laboratory autoclave (45 mL capacity), 50 mg (0.047 mmol) of [(µ-
H) Ru (CO) ((R)-1b)] (R)-3 was dissolved in 5 mL of EtOH/toluene (1:1). The
4
4
11
autoclave was sealed and purged three times with 10 bar of H before being
2
pressurized to 50 bar H . After 24 h at 100 °C, the autoclave was cooled to room
2
temperature. The gases were carefully released and the autoclave was opened. The
dark brown solution was concentrated and the obtained solid was dissolved in 2 mL
of dichloromethane and subjected to preparative TLC (dichloromethane/petroleum
ether, 1:3). Apart from traces of starting materials and one stationary brown band
(decomposed material), two bands were isolated from the TLC plates, and extracted
with dichloromethane and dried under vacuum. Two products were observed, the first
(red, R = 0.5) was identified as [(µ-H) Ru (CO) ((R)-binaphthyl-P (H)Et)] (R,S )-7
f 4 4 11 (s) P
1
(
14 mg, 28%). H NMR (500 MHz, CDCl ): δ = 8.07 (d, J = 8.8 Hz, 1H), 8.02 (d, J =
3
9
.3 Hz, 1H), 7.90 (d, J = 8.1 Hz, 1H), 7.86 (d, J = 8.3 Hz, 1H), 7.59 (m, 1H), 7.49
(app t, 1H), 7.44 (d, J = 9.1 Hz, 1H), 7.30 (app t, 1H), 7.25 (app t, 1H), 7.19 (m, 1H),
1
7
.09 (d, J = 8.8 Hz, 1H), 6.89 (d, J = 8.0 Hz, 1H), 5.16 (dd, JP-H = 380 Hz, JH-H = 10.3
Hz, 1H, PH), 3.67 (s, 3H, OCH ), 2.06 (m, 1H, PCH CH ), 1.77 (m, 1H, PCH CH ),
3
2
3
2
3
31
0
.79 (m, 3H, PCH CH ), –17.72 (s, 4H, RuHRu) ppm; P NMR (202 MHz, CDCl ):
2 3 3
δ = −11.8 (s) ppm; IR (cyclohexane): 2091 (w), 2086 (m), 2061 (vs), 2052 (vs), 2030
-
1
+
(
vs), 2012 (sh), 2005 (s), 1988 (m), 1967 (w) cm ; MS (FAB): m/z = 1083 [M+Na] .
1
8

ACCEPTED MANUSCRIPT
Crystallization of (R,S )-7 from a dichloromethane/hexane mixture at 4 °C yielded red
P
crystals suitable for X-ray diffraction analysis.
The second product (red, R = 0.4) was identified as [(µ-H) Ru (CO) ((R)-
f
4
4
11
1
binaphthyl-P (H)Et)] (R,R )-8 (18 mg, 36%). H NMR (500 MHz, CDCl ): δ = 8.09
(R)
P
3
(d, J = 8.3 Hz, 1H), 8.01 (d, J = 9.1 Hz, 1H), 7.92 (d, J = 8.4 Hz, 1H), 7.86 (d, J = 8.1
Hz, 1H), 7.67 (m, 1H), 7.51 (app t, 1H), 7.39 (d, J = 9.0 Hz, 1H), 7.30 (app t, 1H),
7
.27 (app t, 1H), 7.22 (m, 1H), 7.13 (d, J = 8.6 Hz, 1H), 6.85 (d, J = 8.5 Hz, 1H), 5.22
1
(
dd, J = 378 Hz, JH-H = 9.0 Hz, 1H), 3.72 (s, 3H, OCH ), 2.15 (m, 1H, PCH CH ),
P-H
3
2
3
31
1
.91 (m, 1H, PCH CH ), 0.99 (m, 3H, PCH CH ), –17.86 (s, 4H, RuHRu) ppm; P
2
3
2
3
NMR (202 MHz, CDCl ): δ = −13.2 (s) ppm; IR (cyclohexane): 2092 (w), 2086 (m),
3
-
2
062 (vs), 2055 (vs), 2051 (vs), 2031 (vs), 2024 (s), 2005 (s), 1988 (m), 1966 (w) cm
1
+
;
MS (FAB): m/z = 1083 [M+Na] .
4
.5.2. Method B: Thermal ligand substitution at high pressure
[
(µ-H) Ru (CO) ] (30 mg, 0.04 mmol) and (R)-1b (14 mg, 0.04 mmol) were
4 4 12
dissolved in 6 mL of benzene and the solution was transferred to a small autoclave
45 mL capacity). The autoclave was sealed and purged three times with 10 bar of H2
(
before being pressurized to 30 bar H . After 4 h at 125 °C, the autoclave was cooled
2
to room temperature. The gases were carefully released and the autoclave was
opened. The isolation, purification and identification procedures of the products were
identical to those described above. Yields: (R,S )-7 (7 mg, 16%) and (R,R )-8 (6 mg,
P
P
1
2%).
4
.6. Homogeneous catalytic experiments
In the catalysis experiments, the catalyst and substrate were loaded into the autoclave
under nitrogen, followed by the addition of a degassed solvent mixture (2.5 mL of
EtOH/2.5 mL of toluene). The reaction vessel was closed and purged three times with
hydrogen and finally pressurized to 50 bar. The reaction mixture was continuously
stirred (ca. 750 rpm) and heated up to 100 °C for 24 h. After a cooling period of
approximately 45 minutes, the reaction vessel was depressurized and opened. The
1
9

ACCEPTED MANUSCRIPT
reaction mixture was transferred to a 50 mL flask and concentrated under vacuum.
Conversions were calculated by NMR spectroscopy. To separate the carboxylic acid
product from the cluster, the reaction residue was dissolved in 10 mL of Et O and the
2
carboxylic acid was extracted with sodium hydroxide solution (1M, 3 x 10 mL) and
washed with Et O (3 x 5 mL), leaving the cluster in the organic solvent. The
2
carboxylate was protonated with sulfuric acid and extracted with Et O (3 x 10 mL),
2
washed with water (2 x 5 mL) and dried over magnesium sulfate. Evaporation of the
ether under vacuum yielded the carboxylic acid quantitatively. The ether phase, from
which the carboxylic acid was extracted, was concentrated under vacuum to recover
the remaining cluster. The enantiomeric excess of the product was detected by
derivatization of 2-methylbutyric acid with (S)-methyl mandelate and analysis of the
resultant diastereomeric product mixture was carried out by NMR spectroscopy, as
fully described by Tyrrell et al. [42].
1
4
.7. Variable-temperature H NMR spectroscopy
All deuterated solvents for NMR spectroscopy were used as received (Sigma-
Aldrich). Approximately 0.5 mL of a concentrated CDCl solution of (S)-2, (S)-4,
3
(
S,S )-5 and (S,R )-6 were added to an NMR tube, which was subsequently sealed
P P
under a nitrogen atmosphere. The sample tube was placed into the probe using a
ceramic spinner and liquid nitrogen was used as coolant. The sample was allowed to
reach equilibrium at each desired temperature for 5-10 minutes prior to shimming and
data acquisition. The temperature variation ranged from 298 K to 213 K for all the
samples.
4
.8. X-Ray data collection and structure elucidation
The diffraction data were collected at 100 K for (R,S )-7 and at 170 K for (S)-4 and 9,
P
using a Nonius Kappa CCD or a Bruker AXS Kappa ApexII Duo diffractometer with
Mo Kα radiation (λ = 0.71073 Å). The Denzo-Scalepack [43] or APEX2 [44]
program packages were used for cell refinements and data reductions. The structure
solutions were carried out using the SHELXS-97 program [45] with the WinGX
graphical user interface [46]. All hydrogen atoms were placed in idealized positions.
2
0

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The position of the hydrides were calculated using the XHYDEX program [47]. The
crystallographic details of the reported structures ((S)-4, (R,S )-7, and 9) are
P
summarized in Table S1.
Acknowledgements
We thank the EPSRC for a Career Acceleration Fellowship EP/G005206/1 (L.J.H.)
and PhD studentships (A.F. and M.A.E), and the Erasmus Mundus FFEEBB1
program office for a predoctoral fellowship (to A.F.A.).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/…..
CCDC entries no. 1531250, 1531251 and 1531252 contain the supplementary
crystallographic data for compounds 4, 7 and 9, respectively. Copies of this
information may be obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
2
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Highlights
•
•
•
Tetranuclear tetrahydrido ruthenium carbonyl clusters containing chiral phosphiranes
were synthesized.
Hydrogenation of the rings in the coordinated phosphirane ligands led to formation of
secondary phosphine ligands with an additional sterogenic center at the phosphorus.
The clusters showed good activity in the hydrogenation of tiglic acid (trans-2-methyl-
2
-butenoic acid).