Heterodinuclear arene ruthenium complexes containing a glycine-derived phosphinoferrocene carboxamide: Synthesis, molecular structure, electrochemistry, and catalytic oxidation activity in aqueous media

Article abstract of DOI:10.1021/om3002087

Three series of heterodinuclear ruthenium-iron complexes have been synthesized from (η⁶-arene)ruthenium dichloride dimers and phosphinoferrocene ligands containing glycine-based carboxamido substituents. The neutral complexes [(η⁶-arene)RuCl₂(Ph ₂PfcCONHCH₂CO₂Me-κP)] (4, arene = benzene (a), p-cymene (b), hexamethylbenzene (c); fc = ferrocene-1,1′-diyl) were obtained by the bridge cleavage reaction of [(η⁶-arene)RuCl ₂]₂ with Ph₂PfcCONHCH₂CO ₂Me (1) in chloroform solution. The complex [(η⁶-p- cymene)RuCl₂(Ph₂PfcCONHCH₂CONH ₂-κP)] (6b) was synthesized in the same way from Ph ₂PfcCONHCH₂CONH₂ (3); the preparation of [(η⁶-p-cymene)RuCl₂(Ph₂PfcCONHCH ₂CO₂H-κP)] (5b), featuring the ferrocene ligand in the free acid form (2), failed due to side reactions and isolation problems. The salts [(η⁶-arene)RuCl(MeCN)(1-κP)][PF₆] (7a-c) and [(η⁶-arene)Ru(MeCN)₂(1-κP)][PF ₆]₂ (8a-c) were prepared from 1 and the acetonitrile precursors [(η⁶-arene)RuCl(MeCN)₂][PF₆] and from 4a-c via halide removal with Ag[PF₆] in acetonitrile solution, respectively. Alternative synthetic routes to 7b and 8b were also studied. The compounds were fully characterized by elemental analysis, multinuclear NMR, IR, and electrospray ionization mass spectra, and their electrochemical properties were studied by cyclic voltammetry at a Pt-disk electrode. The single-crystal X-ray analyses of two representatives (4b and 8b) revealed a pseudotetrahedral coordination geometry at the ruthenium centers and eclipsed conformations of the ferrocene moieties, with the substituents at the two cyclopentadienyl rings being anti with respect to each other. All complexes showed high activity for the catalytic oxidation of secondary alcohols with tert-butyl hydroperoxide to give ketones in aqueous media. The most active catalyst was obtained from the neutral p-cymene complex 4b, showing a catalytic turnover frequency of 13 200 h^-1 at room temperature for the oxidation of 1-phenylethanol at a substrate/catalyst ratio of 100 000.

Full text of DOI:10.1021/om3002087

Article
pubs.acs.org/Organometallics
Heterodinuclear Arene Ruthenium Complexes Containing a Glycine-
Derived Phosphinoferrocene Carboxamide: Synthesis, Molecular
Structure, Electrochemistry, and Catalytic Oxidation Activity in
Aqueous Media
†
‡
,‡
,†
̌
Jirí Tauchman, Bruno Therrien, Georg Suss-Fink,* and Petr Stepnicka*
̌
̌
̌
̈
†Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, CZ-12840 Prague 2, Czech
Republic
‡Institut de Chimie, Universite
́ ̂ ̂
de Neuchatel, Avenue de Bellevaux 51, CH-2000 Neuchatel, Switzerland
S
* Supporting Information
ABSTRACT: Three series of heterodinuclear ruthenium−iron
complexes have been synthesized from (η6-arene)ruthenium
dichloride dimers and phosphinoferrocene ligands containing
glycine-based carboxamido substituents. The neutral complexes
[(η6-arene)RuCl2(Ph2PfcCONHCH2CO2Me-κP)] (4, arene =
benzene (a), p-cymene (b), hexamethylbenzene (c); fc =
ferrocene-1,1′-diyl) were obtained by the bridge cleavage reaction
of [(η6-arene)RuCl2]2 with Ph2PfcCONHCH2CO2Me (1) in
chloroform solution. The complex [(η6-p-cymene)-
RuCl2(Ph2PfcCONHCH2CONH2-κP)] (6b) was synthesized in
the same way from Ph2PfcCONHCH2CONH2 (3); the
preparation of [(η6-p-cymene)RuCl2(Ph2PfcCONHCH2CO2H-
κP)] (5b), featuring the ferrocene ligand in the free acid form
(2), failed due to side reactions and isolation problems. The salts [(η6-arene)RuCl(MeCN)(1-κP)][PF6] (7a−c) and [(η6-
arene)Ru(MeCN)2(1-κP)][PF6]2 (8a−c) were prepared from 1 and the acetonitrile precursors [(η6-arene)RuCl(MeCN)2][PF6]
and from 4a−c via halide removal with Ag[PF6] in acetonitrile solution, respectively. Alternative synthetic routes to 7b and 8b
were also studied. The compounds were fully characterized by elemental analysis, multinuclear NMR, IR, and electrospray
ionization mass spectra, and their electrochemical properties were studied by cyclic voltammetry at a Pt-disk electrode. The
single-crystal X-ray analyses of two representatives (4b and 8b) revealed a pseudotetrahedral coordination geometry at the
ruthenium centers and eclipsed conformations of the ferrocene moieties, with the substituents at the two cyclopentadienyl rings
being anti with respect to each other. All complexes showed high activity for the catalytic oxidation of secondary alcohols with
tert-butyl hydroperoxide to give ketones in aqueous media. The most active catalyst was obtained from the neutral p-cymene
complex 4b, showing a catalytic turnover frequency of 13 200 h−1 at room temperature for the oxidation of 1-phenylethanol at a
substrate/catalyst ratio of 100 000.
obtained from 1′-(diphenylphosphino)ferrocene-1-carboxylic
acid (Hdpf) or its planar-chiral 1,2-isomer and from both
achiral glycine (compounds 1−3 in Scheme 1)7 and various
chiral amino acids.8 We have also demonstrated that these
ligands are easily modified at their amide substituent. For
instance, compound 1, resulting from Hdpf and glycine methyl
ester hydrochloride, was readily converted to the corresponding
acid 2 (by hydrolysis) or amide 3 (by reaction with liquid
ammonia).7 These changes naturally affect both the coordina-
tion and physicochemical properties (e.g., solubility) of these
donors and can be thus used to fine tune the catalytic
performance of complexes prepared thereof.
INTRODUCTION
■
Ligands obtained by “conjugation” of phosphinocarboxylic
acids with amino acids have received considerable attention in
the recent past, owing to their rich coordination chemistry and
manifold catalytic applications.1,2 Indeed, these compounds
represent attractive targets for ligand design, being synthesized
readily in good yields by conventional amide coupling reactions
and are thus accessible in many variants, differing in the overall
structure and substitution patterns.
Stimulated by our investigation into the chemistry of 1′-
(diphenylphosphino)ferrocene-1-carboxamides with simple3
and functional substituents at the amide nitrogen (type A in
Scheme 1; e.g., compounds bearing pyridyl,4 hydroxyalkyl,5 and
sulfonatoalkyl6 groups), we have recently turned also to
analogous donors prepared from amino acids. So far, we have
synthesized and catalytically tested several such ligands
Received: March 14, 2012
Published: April 30, 2012
© 2012 American Chemical Society
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Scheme 1. Hdpf Derivatives
Scheme 2. Preparation of Ruthenium−Iron Complexes 4−8
In the search for other potentially useful synthetic trans-
formations in which Hdpf−amino acid conjugates could be
used, we turned to transition-metal-catalyzed oxidation
reactions. Phosphinoferrocene ligands have been used only
rarely in such reactions, presumably due to their possible
decomposition following oxidation of the ferrocene unit.9 In
this contribution, we describe the preparation, structural
characterization, and electrochemistry of three series of (η6-
arene)Ru complexes with glycine-based amidophosphine
ligands 1−3. We also report the catalytic performance of
these ruthenium−iron complexes in the selective oxidation of
secondary alcohols with tert-butyl hydroperoxide to give the
corresponding ketones.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of the
Ruthenium−Iron Compounds. Three series of (η6-arene)
Ru complexes containing 1 as a P-monodentate ligand were
prepared (Scheme 2); viz., the neutral complexes of the type
[(η6-arene)RuCl2(1-κP)] as well as two series of cationic
complexes resulting from the substitution of chloro by
ESI mass spectrometry. The molecular structures of 4b and 8b
were determined by single-crystal X-ray diffraction analysis.
The proton NMR spectra of 4a−c, 5b, and 8a−c display
characteristic signals of the arene ligand and four resonances
attributable to the 1,1′-disubstituted ferrocene moiety. In the
case of 7, possessing a stereogenic Ru atom, the originally
degenerate signals become diastereotopic and anisochronic. For
instance, four CH resonances of the cymene CH groups and
eight signals for the ferrocene protons are seen in the 1H NMR
spectrum of 7b. The spectra further comprise signals due to the
glycine pendant, namely the singlet of the terminal methyl
group at δH ca. 3.7−3.8 and NH-coupled (3JHH ≈ 6 Hz)
doublet of the methylene group at δH 4.0−4.1. The spectrum of
6b shows the expected three signals for the nonequivalent NH
protons. The 31P{1H} NMR spectra of neutral complexes 4a−c
and 5b display one singlet resonance in the range δP 15−19.
This signal shifts to lower fields in the spectra of mono- and
dicationic complexes (7a−c, δP 28−30; 8a−c, δP 33−35 ppm).
The presence of the hexafluorophosphate ion is manifested via
a characteristic septet in the 31P NMR spectrum (δP −145) and
−
acetonitrile ligands, which were isolated as PF6 salts. The
neutral complexes 4a−c were obtained by a bridge cleavage
reaction from the respective dimeric precursor [(η6-arene)-
RuCl2]2 and 1 in chloroform. The related complex 6b was
synthesized similarly from the amide 3. In contrast, attempts to
prepare [(η6-p-cymene)RuCl2(2-κP)] (5b) from the free acid 2
were unsuccessful due to side reactions;10 in our hands it was
not possible to isolate 5b (detected by NMR) from the reaction
mixture.
Removal of the chloro ligands from 4a−c with 2 equiv of
Ag[PF6] in acetonitrile produced the bis-acetonitrile complexes
as hexafluorophosphate salts 8a−c. The formally intermediary
monoacetonitrile compounds 7a−c were obtained by replace-
ment of one acetonitrile ligand in the precursors [(η6-
arene)RuCl(MeCN)2][PF6] with a stoichiometric amount of
ligand 1.
1
a doublet in the 19F NMR spectrum (δF −73; JPF = 706 Hz).
Additional reactivity studies showed that the cationic
complexes 7b and 8b are accessible either in a direct way
from [(p-cymene)RuCl(MeCN)2]+ or [(p-cymene)Ru-
(MeCN)3]2+ via substitution of the acetonitrile ligands with 1
or, alternatively, by sequential removal of the chloro ligands
from the dichlororuthenium complex 4b. These interconver-
sions are schematically shown in Scheme 3 and described in
detail in the Supporting Information.
The presence of amide and ester groups are clearly reflected
in IR spectra showing intense bands due to νNH (ca. 3375
cm−1), νCO (1747−1751 cm−1), amide I (1638−1653 cm−1),
and amide II (1530−1553 cm−1) vibrations. The ester band is
missing from the IR spectrum of 5b, which on the other hand
shows further bands due to the secondary and terminal primary
amide groups. IR spectra of cationic complexes 7a−c and 8a−c
further indicate the presence of coordinated MeCN (νCN
−
2293−2330 cm−1) and the PF6 counterion (a strong band at
The compounds were characterized by combustion analyses,
1H, 31P{1H}, and 19F NMR spectroscopy, IR spectroscopy, and
835−847 cm−1). Positive-ion electrospray ionization (ESI)
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mass spectra of the neutral complexes 4a−c and 5b show ions
corresponding to [M + Na]+ or [M − Cl]+. The spectra of the
monocationic complexes 7a−c display ions resulting via
elimination of acetonitrile from their cations ([M − MeCN
− PF6]+), while 8a−c give rise to ions of the type [(arene)Ru(1
− H)]+ or [M − PF6]+.
Scheme 3. Mutual Interconversions and Alternative
Preparations of (η6-p-cymene)ruthenium(II) Complexes
a
with Ligand 1
Description of the Crystal Structures. Single crystals
suitable for X-ray diffraction analysis were obtained from
diffusion of diethyl ether into a solution in methanol
(4b·2CH3OH) or acetonitrile (8b). Molecular structures of
the complex molecule in the solvate 4b·2CH3OH and the
cation in the structure of 8b are presented in Figure 1. Selected
geometric data are given in Table 1.
The overall structures of both complex species are rather
similar: the axis of the cymene ligand is oriented roughly
parallel to the vector connecting the simple ligands (Cl1/Cl2 or
N2/N3), and the arene ring is practically parallel to the basal
plane of the three-legged piano-stool structure.11 The ferrocene
units are rotated with respect to the (η6-p-cymene)RuII unit, as
evidenced by the dihedral angles of planes of the Ru-bound
arene and the phosphinylated cyclopentadienyl rings being
79.2(3)° for 4b·2CH3OH and 69.2(4)° for 8b. Notably, the
Cg3−Ru1−donor angles (see Table 1 for defninitions) in both
compounds are rather similar (124.95(9)−130.37(9)° for
4b·2CH3OH, 126.0(2)−129.7(2)° for 8b), which rules out
any significant deformation of the coordination sphere around
Ru resulting from different steric demands of the two-electron
donors (Cl and CH3CN vs the relatively bulkier phosphine).
Otherwise, the Ru−donor distances and the overall geometry
are unexceptional and compare well with the data reported
previously for [(η6-p-cymene)RuCl2(Hdpf-κP)]12 and [(η6-
benzene)Ru(MeCN)2(PPh3)](CF3SO3)2.13
a
Legend: (i) [(p-cymene)RuCl2]2; (ii) [(p-cymene)RuCl(MeCN)2]-
[PF6]; (iii) [(p-cymene)Ru(MeCN)3][PF6]2; (iv) Ag[PF6]; (v) 2
Ag[PF6]. Reaction (i) was carried out in CHCl3. All other reactions
were performed in acetonitrile. For a complete description of the
experiments, see the Supporting Information.
The geometry of coordinated 1 does not differ much from
that found for free Ph2PfcCONHCH2CO2Bu-t.7 The 1,1′-
disubstituted ferrocene moieties in 4b·2CH3OH and 8b assume
Figure 1. Views of (a) the complex molecule in 4b·2CH3OH and (b) the cation in the structure of compound 8b. Displacement ellipsoids
correspond to the 30% probability level.
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Table 1. Selected Distances (Å) and Angles (deg) for
Compounds 4b·2CH3OH and 8b
Table 2. Summary of Electrochemical Data for 4a−c, 5b,
a
a
7a−c, and 8a−c
b
param
4b·2CH3OH (X = Cl1, Z = Cl2) 8b (X = N2, Z = N3)
compd
E°′(FeII/III) (V)
Epa(Ru) (V)
Ru1−P1
Ru1−X
Ru1−Z
2.380(1)
2.422(1)
2.431(1)
1.718(2)
86.75(5)
87.65(5)
88.99(5)
1.654(3)
1.647(3)
3.8(4)
2.369(2)
2.062(6)
2.056(6)
1.731(3)
88.9(2)
84.8(2)
84.9(2)
1.656(4)
1.654(4)
3.2(5)
4a
4b
0.315
0.285
0.245
0.260
0.430
0.410
0.385
0.540
0.530
0.510
0.935
0.850
0.710
0.850
n.d.
b
4c
Ru−Cg3
P1−Ru1−X
P1−Ru1−Z
X−Ru1−Z
Fe1−Cg1
Fe1−Cg2
∠Cp1,Cp2
τ
5b
7a
7b
7c
8a
8b
8c
n.d.
n.d.
n.d.
n.d.
n.d.
a
156
137
The potentials are quoted relative to ferrocene/ferrocenium. E°′ =
1/2(Epa + Epc), where Epa (Epc) is the anodic (cathodic) peak potential
φ
9.2(7)
8(1)
b
C4−O3
C4−N1
O3−C4−N1
C2−O1
C2−O2
O1−C2−O2
1.238(7)
1.343(8)
121.1(6)
1.326(8)
1.215(8)
123.6(6)
1.229(9)
1.339(9)
122.4(7)
1.40(2)
1.24(2)
122(1)
in cyclic voltammetry. n.d. = not determined. This compound shows
a more complicated redox response (see text).
a
Definitions: Cp1 = C(5−9), Cp2 = C(10−14), arene = benzene ring
of the π-coordinated p-cymene (i.e., C(27−32) for 4b, and C(31−36)
for 8b). Cg1, Cg2, and Cg3 denote the centroids of the Cp1, Cp2, and
arene rings, respectively. τ = torsion angle C5−Cg1−Cg2−C10. φ =
dihedral angle subtended by the plane Cp1 and the amide unit [C4,
b
O3, N1]. Further data: N2−C27 = 1.14(1) Å, N2−C27−C28 =
178.9(9)°, N3−C29 = 1.144(9) Å, N3−C29−C30 = 179.1(7)°.
intermediate conformations close to anti-eclipsed (compare the
τ angles in Table 1 with the ideal value of 144°) and exert
regular geometries with tilt angles of ca. 3−4° and individual
Fe−C distances falling into narrow ranges (2.036(6)−2.061(6)
and 2.033(8)−2.066(9) Å for 4b·2CH3OH and 8b, respec-
tively). The planes of the amide moieties are rotated only by ca.
8−9° from the planes of their parent cyclopentadienyl rings
(Cp1), and the whole amido-ester pendants extend away from
the ferrocene and the (η6-arene)Ru units.
Electrochemistry. The electrochemical properties of
complexes 4a−c, 5b, 7a−c, and 8a−c were studied by cyclic
voltammetry at a platinum-disk electrode. The measurements
were performed on ca. 0.5 mM acetonitrile solutions containing
0.1 M [Bu4N][PF6] as the supporting electrolyte using
ferrocene/ferrocenium as an internal reference. The pertinent
data are summarized in Table 2.
When the external potential is increased in cyclic
voltammetry, complexes 4a−c undergo first a reversible one-
electron redox change centered very likely at the ferrocene unit
(Figure 2).14 The separation of the anodic and cathodic peaks
in cyclic voltammograms of these compounds was ca. 70−75
mV, similar to that of ferrocene/ferrocenium under the same
experimental conditions. The ratios of the anodic and cathodic
peak currents were close to unity, and the peak currents of this
wave increased with the square root of the scan rate, indicating
that the redox process is controlled by diffusion. Because of
electron density transfer upon coordination, the redox
potentials were more positive than that of free ligand 1 (E°′
= 0.26 V in CH2Cl2).7 Furthermore, the potentials decreased
upon increasing the number of alkyl substituents at the Ru-
bound arene ligand (i.e., with increasing donor ability of the η6-
arene donor).
Figure 2. Cyclic voltammograms of complexes 4a (full and partial)
and 7a, as recorded at a Pt-disk electrode on dichloromethane
solutions (scan rate 100 mV s−1). For clarity, the second scan is shown
by a dashed line (for the full voltammogram of 4a) and the
voltammograms are shifted by +10 and +20 μA to avoid overlaps.
(Figure 2). The anodic peak potential (Epa) of the second wave
decreased significantly with an increasing number of arene
substituents, suggesting the second redox change to occur
predominantly at the (η6-arene)Ru moiety. In the case of 4c, an
additional pair of redox waves was seen between the redox
waves, probably due to adsorption. On the other hand, the
redox response of compound 5b did not differ from that of 4b:
the first wave appeared shifted slightly to lower potentials, while
the redox potential of the second oxidation remained the same,
which supports the assignment of the redox processes.
Replacement of the anionic chloro ligands by the neutral
solvent molecules as in complexes 7 and 8 produces positively
charged species, which could be expected to be more difficult to
oxidize. Indeed, the cyclic voltammograms of 7a−c displayed
only one electrochemically reversible redox wave, attributable
to the ferrocene/ferrocenium couple shifted to higher
A second redox event in 4a−c observed at higher potentials
is electrochemically irreversible and multielectron in nature
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potentials as compared with that in the respective complexes
4a−c (Figure 2). As for the parent neutral complexes, the
compounds bearing more alkylated arene ligands became
oxidized more easily (E°′: 7c < 7b < 7a). In the case of 8a−c,
this redox wave was shifted even further but was much less
affected by the substituents at the arene ring. The second
oxidations of 7 and 8 were probably located outside the
accessible potential range.
A possible influence of the catalyst structure and reaction
conditions on the course of the oxidation process was studied
next. The results obtained with all defined (η6-arene)Ru
complexes in the model oxidation reaction at a substrate to
catalyst ratio of 100 000:1, summarized in Table 3, clearly show
Table 3. Catalytic Activity of Complexes 4a−c, 6b, 7a−c, and
a
8a−c in the Oxidation of 1-Phenylethanol with t-BuOOH
Catalytic Reactions. Metal-catalyzed oxidation of alcohols
to give carbonyl compounds such as aldehydes, ketones, and
carboxylic acids is an important transformation both for organic
synthesis and for industrial manufacturing.15 There are many
reports concerning this reaction using molecular oxygen16 and
hydrogen peroxide17 as oxidants, the problem very often being
the activity and selectivity of the catalyst. As far as this reaction
in aqueous media is concerned, the oxidation of alcohols with
H2O2 under phase-transfer conditions has been proposed.18
Noyori reported a combined system composed of Na2WO4 and
[CH3(n-C8H17)3N][HSO4] as a phase-transfer catalyst, which
efficiently catalyzes the oxidation of alcohols to the
corresponding carbonyl compounds with high yields.19
Trakarnpruk20 and Punniyamurthy21 compared the oxidation
activity of hydrogen peroxide and tert-butyl hydroperoxide,
another cheap and easy to use peroxide. A number of catalysts
for the use of t-BuOOH have been reported to date.22
Watanabe23 and Muharashi24 used the ruthenium complex
[RuCl2(PPh3)3], which catalyzes the oxidation with high
conversion (>92%). Recently, Singh used arene ruthenium
complexes of the type [(η6-arene)RuCl(L)][PF6] (arene = p-
cymene, benzene; L = N-[2-(arylchalcogeno)ethyl]morpholines
with aryl = Ph, 2-pyridyl (for S), Ph (for Se), 4-MeOC6H4 (for
Te)) as catalysts for alcohol oxidation, using N-methylmorpho-
line N-oxide (NMO), t-BuOOH, NaIO4, and NaOCl as the
oxidants.25 We recently reported on the use of ruthenium arene
bis-saccharinato complexes as alcohol oxidation catalysts that
work efficiently in aqueous solution.26
b
conversn after
c
d
entry
cat.
4a
5 h/24 h (%) TON after 24 h TOF after 5 h (h−1
)
1
30/99
66/100
45/100
41/99
61/100
53/99
61/100
35/100
38/99
28/100
0/0
99 000
100 000
100 000
99 000
100 000
99 000
100 000
100 000
99 000
100 000
0
6 000
13 200
9 000
8 200
12 200
10 600
12 200
7 000
7 600
5 600
0
2
4b
4c
3
4
6b
7a
5
6
7b
7c
7
8
8a
9
8b
8c
10
11
none
a
Reaction conditions: alcohol (1.0 mmol) and t-BuOOH (4.0 mmol)
in 4 mL of water, substrate/catalyst = 100 000, room temperature,
b
reaction time 5 or 24 h. Determined by 1H NMR spectroscopy.
c
d
Turnover number: mol product/mol catalyst − after 24 h. Turnover
frequency: mol product/(mol catalyst × reaction time) − after 5 h.
the superior performance of compound 4b at short reaction
times. Only slightly worse results were obtained with the
monocationic complexes 7a,c. On the other hand, all catalysts
tested gave complete or practically complete conversions in 24
h. This observation may indicate that the differences in
performance observed at relatively shorter reaction times could
reflect the ease of conversion of the defined precatalysts into
the catalytically active species, presumably an aqua complex.
This assumption is supported by our previous studies25 and,
indirectly, also by the observed dependence of turnover
number on the pH of the reaction mixture (Figure 3). The
best results were obtained at pH ca. 5.5−7.0. The aquation
reaction, representing the likely catalyst activation step, is
probably hindered at lower pH, while reactions at higher pH
can lead to hydroxo complexes.27 Otherwise, however, the
All ruthenium−iron compounds reported above were tested
as defined precatalysts in the oxidation of secondary alcohols to
ketones. Complex 4b obtained in a straightforward manner
from the most easily accessible and cheapest Ru precursor was
tested first in the oxidation of 1-phenylethanol as a model
substrate with t-BuOOH in pure water (Scheme 4; for
Scheme 4. Oxidation of 1-Phenylethanol to Acetophenone
complete results in a tabulated form, see the Supporting
Information, Table S1). Indeed, the oxidation reaction in the
presence of 0.1 mol % of 4b proceeded cleanly to produce
acetophenone with complete conversion within 3 h (at room
temperature). When the catalyst amount was lowered to 0.01
mol %, the conversion achieved in 3 h was only 83% but the
reaction reached completion within 5 h. Decreasing the catalyst
loading further to 0.001 mol % expectedly decreased the
reaction rate (30% and 66% conversion in 3 and 5 h,
respectively). Even in this case, however, complete conversion
to acetophenone was achieved in 24 h. No reaction was seen
without the Ru complex.
Figure 3. pH dependence of the turnover number (TON) for
oxidation of 1-phenylethanol with t-BuOOH mediated by complex 4b
(0.01 mol %) at room temperature after 3 h. For tabulated results, see
the Supporting Information (Table S1).
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reaction proceeded with no significant induction period and
does not depart much from the exponential dependence
expected for (pseudo) first-order kinetics (Figure 4).
A survey of various substrates (Table 4) showed that
complex 4a at 0.001 mol % loading promotes efficiently and
cleanly the oxidation of 1-phenylethanol derivatives substituted
at the benzene ring (entries 2−8), the exception being the 4-
methoxyphenyl and 2-bromophenyl derivatives. The naphthyl
analogue diphenylmethanol and even cyclic secondary alcohols
with fused benzene rings were also efficiently oxidized (entries
9 and 11−13). On the other hand, 1-cyclohexylethanol and
aliphatic secondary alcohols (entries 10, 14, and 15) gave only
moderate conversions.
Water is the best reaction medium for this catalytic reaction,
and the neutral dichloro complexes are more active than their
monocationic monochloro acetonitrile and dicationic bis-
(acetontrile) counterparts. Together with a pronounced pH
dependence of the reaction course, this suggests that aquation
of the chloro ligands plays an important role and that an aqua
complex may be the real entry to the catalytic cycle. A
mechanism based on this observation, in accordance with our
earlier findings,25 is shown in Scheme 5. Since we did not
detect any oxoruthenium(IV) intermediates during our catalytic
reactions, this mechanistic scheme remains only a plausible
working hypothesis. The involvement of Ru−OOH species
formed in situ (probably via an intermediate Ru aqua complex)
cannot be ruled out at this point (see alternative route in
Scheme 5).28
Figure 4. Kinetic profile for oxidation of 1-phenylethanol with t-
BuOOH mediated by complex 4b (0.01 mol %) at room temperature.
The solid line represents an ideal kinetic fit: conversion = a(1 −
exp[−bt]). Parameters of the fit: a = 100(5)%, b = 0.18(2) h−1, R2 =
0.973.
CONCLUSION
■
Heterobimetallic FeII−RuII complexes 4−8 are easily prepared
from the common (η6-arene)RuII precursors and phosphino-
ferrocene carboxamides with amino acid pendant chains, either
in a direct way or in a convergent manner. Electrochemical
studies revealed electronic communication between the RuII/III
and FeII/III redox centers present in these compounds, typically
manifested through changes in the redox potentials of both
redox couples observed upon modification of only one of them.
The stable compounds 4−8 give rise to highly active catalysts
for the Ru-catalyzed oxidation of secondary alcohols to ketones.
The oxidations are advantageously performed in water at room
temperature and proceed cleanly and rapidly even at catalyst to
substrate ratios as low as 1:100 000. Surprisingly, the presence
of oxidation-sensitive ferrocene-based ligands does not seem to
Upon replacing pure water as the reaction mixture with
organic solvents (hexane, diethyl ether, toluene, dichloro-
methane) or their biphasic aqueous mixtures, the oxidation
reaction became considerably slower and did not reach
completion even after 24 h (with 0.001 mol % catalyst; see
the Supporting Information, Table S3). Also, when t-BuOOH
was replaced with sodium perchlorate or sodium periodate (4
equiv), the conversions dropped to 9 and 62%, respectively,
after 3 h (Supporting Information, Table S1). Other oxidants
tested (aqueous hydrogen peroxide, benzoyl peroxide, 3-
chloroperoxybenozic acid, oxone, or N-methylmorpholine N-
oxide; 4 equiv vs the substrate) resulted in conversions not
exceeding 3%. Bubbling air through the reaction mixture also
did not induce any oxidation over 3 h.
a
Table 4. Oxidation of Various Secondary Alcohols in the Presence of Catalyst 4b
b
entry
substrate
1-phenylethanol
product
acetophenone
conversn after 5 h/24 h (%)
isolated yield after 24 h (%)
1
66/100
46/99
53/100
67/100
62/99
44/70
47/99
8/35
93
2
1-(4-fluorophenyl)ethanol
1-(4-chlorophenyl)ethanol
1-(4-bromophenyl)ethanol
1-(4-methylphenyl)ethanol
1-(4-methoxyphenyl)ethanol
1-(3-bromophenyl)ethanol
1-(2-bromophenyl)ethanol
1-(2-naphthyl)ethanol
1-cyclohexylethanol
diphenylmethanol
4′-fluoroacetophenone
4′-chloroacetophenone
4′-bromoacetophenone
4′-methylacetophenone
4′-methoxyacetophenone
3′-bromoacetophenone
2′-bromoacetophenone
2′-acetonaphthone
hexahydroacetophenone
benzophenone
92
3
90
4
91
5
90
6
n.d.
92
7
8
n.d.
91
9
53/100
13/36
77/100
67/99
61/99
14/55
8/29
10
11
12
13
14
15
n.d.
94
1-indanol
1-indanone
90
1-tetralol
1-tetralone
89
cyclohexanol
cyclohexanone
n.d.
n.d.
2-butanol
2-butanone
a
Reaction conditions: alcohol (1.0 mmol) and t-BuOOH (4.0 mmol) in 4 mL of water, substrate/catalyst =100 000, room temperature, reaction
b
1
time 5 or 24 h. Determined by H NMR spectroscopy.
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Organometallics
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residue was purified by column chromatography on silica gel using
chloroform/acetone (5/1 v/v) as the eluent.
Scheme 5. Plausible Mechanistic Alternatives for the Ru-
Catalyzed Oxidation of Alcohols with Peroxides
a
Complex 4a. Starting from 1 (194 mg, 0.4 mmol) and [(η6-
benzene)RuCl2]2 (100 mg, 0.2 mmol), the general procedure afforded
1
4a as an orange foam. Yield: 274 mg (84%). H NMR (CDCl3, 50
°C): δ 3.36 (br s, 2 H, fc), 3.82 (s, 3 H, OMe), 4.13 (d, 3JHH = 6.1 Hz,
2 H, NHCH2), 4.59 (br s, 2 H, fc), 4.69 (unresolved q, 2 H, fc), 4.72
(vt, 2 H, fc), 5.32 (s, 6 H, η6-C6H6), 7.34−7.46 (m, 6 H, PPh2), 7.70−
3
7.82 (m, 4 H, PPh2), 7.83 (t, JHH = 6.2 Hz, 1 H, NHCH2). 31P{1H}
NMR (CDCl3, 50 °C): δ 15.2 (s). IR (KBr): νNH 3306 m, νCO 1747
vs, amide I 1651 vs, amide II 1553 vs cm−1. MS (ESI+): m/z 758 ([M
+
N a ] + ) , 7 0 0 ( [ M
−
C l ] + ) . A n a l . C a l c d f o r
C32H30NO3PCl2FeRu·0.65CHCl3: C, 48.24; H, 3.80; N, 1.72.
Found: C, 48.29; H, 3.80; N, 1.62.
Complex 4b. Starting from 1 (146 mg, 0.3 mmol) and [(η6-p-
cymene)RuCl2]2 (92 mg, 0.15 mmol), the general procedure gave 4b
as an orange solid. Yield: 225 mg (94%). 1H NMR (CDCl3): δ 0.99 (d,
3JHH = 7.0 Hz, 6 H, CHMe2), 1.70 (s, 3 H, η6-C6H4Me), 2.44 (sept,
3JHH = 6.9 Hz, 1 H, CHMe2), 3.27 (br s, 2 H, fc), 3.78 (s, 3 H, OMe),
4.09 (d, 3JHH = 6.1 Hz, 2 H, NHCH2), 4.49 (vt, 2 H, fc), 4.55 (br s, 2
H, fc), 4.58 (unresolved q, 2 H, fc), 5.12 (d, JHH = 5.5 Hz, 2 H, η6-
3
C6H4), 5.19 (d, JHH = 6.1 Hz, 2 H, η6-C6H4), 7.36−7.46 (m, 6 H,
3
3
PPh2), 7.59 (t, JHH = 6.0 Hz, 1 H, NHCH2), 7.75−7.85 (m, 4 H,
PPh2). 31P{1H} NMR (CDCl3): δ 17.2 (s). IR (KBr): νNH 3315 m,
νCO 1750 vs, amide I 1648 vs, amide II 1530 vs cm−1. MS (ESI+): m/z
a
L is the P-coordinated amidophosphine 1.
814 ([M
+
Na]+), 756 ([M − Cl]+). Anal. Calcd for
impose any limitations on the catalyst stability and perform-
ance.
C36H38NO3PCl2FeRu·0.05CHCl3: C, 54.30; H, 4.81; N, 1.76.
Found: C, 54.20; H, 4.78; N, 1.66.
Complex 4c. Starting with ligand 1 (146 mg, 0.3 mmol) and [(η6-
C6Me6)RuCl2]2 (100 mg, 0.15 mmol), the general procedure gave 4c
as a red solid. Yield: 228 mg (91%). 1H NMR (CDCl3, 50 °C): δ 1.67
(s, 18 H, η6-C6Me6), 3.35 (br s, 2 H, fc), 3.79 (s, 3 H, OMe), 4.10 (d,
3JHH = 5.9 Hz, 2 H, NHCH2), 4.41 (br s, 2 H, fc), 4.51 (br s, 2 H, fc),
4.57 (br s, 2 H, fc), 7.28−7.43 (m, 6 H, PPh2), 7.69−7.92 (br s, 4 H of
PPh2 and 1 H of NHCH2). 31P{1H} NMR (CDCl3, 50 °C): δ 18.9 (s).
IR (KBr): νNH 3312 m, νCO 1751 vs, amide I 1651 vs, amide II 1534 vs
cm−1. MS (ESI+): m/z 784 ([M − Cl]+). Anal. Calcd for
C38H42NO3PCl2FeRu·0.15CHCl3: C, 54.71; H, 5.07; N, 1.67.
Found: C, 54.61; H, 4.94; N, 1.61.
EXPERIMENTAL SECTION
■
Materials and Methods. All (η6-arene)Ru complexes were
synthesized under a nitrogen atmosphere and with the exclusion of
direct daylight. No such precautions were applied to the catalytic tests.
The compounds [(η6-arene)RuCl2]2 (arene = benzene, p-cymene),29
[(η6-C6Me6)RuCl2]2,30 and [(η6-arene)Ru(MeCN)2Cl][PF6]31 and
ligands Ph2PfcC(O)NHCH2COY (Y = OMe (1), OH (2), NH2 (3))7
were prepared according to the literature procedures. Solvents used in
the syntheses and catalytic experiments were dried by standing over
appropriate drying agents and distilled under nitrogen (dichloro-
methane, chloroform, and acetonitrile with CaH2; hexane, diethyl
ether, and toluene over sodium metal). Other chemicals and solvents
used for crystallizations and in chromatography were used without
further purification.
Attempted Preparation of [(η6-p-cymene)RuCl2(2-κP)] (5b).
The reaction of acid 2 with [(η6-p-cymene)RuCl2]2 (general
procedure) afforded a complicated mixture containing several soluble
products (complexes). The composition of the crude reaction mixture
changed with reaction time. Repeated attempts to isolate any defined
material by column chromatography or crystallization failed.
Preparation of [(η6-p-cymene)RuCl2(3-κP)] (6b). This com-
pound was obtained similarly to complexes 4 from diamide 3 (94 mg,
0.2 mmol) and [(η6-p-cymene)RuCl2]2 (61 mg, 0.1 mmol). Yield of
6b: 155 mg (88%), red solid. 1H NMR (CDCl3): δ 0.97 (d, 3JHH = 6.9
Hz, 6 H, CHMe2), 1.78 (s, 3 H, η6-C6H4Me), 2.49 (sept, 3JHH = 6.9 Hz,
NMR spectra were recorded on a Bruker 400 MHz spectrometer
(1H, 400.13 MHz; 31P, 161.98 MHz; 19F, 376.50 MHz) at 296 K
unless noted otherwise. Chemical shifts (δ/ppm) are given relative to
the residual peak of the solvent (1H; CD3CN, δH = 1.94; CDCl3, δH =
7.26), to 85% aqueous H3PO4 (31P) or to neat CFCl3 (19F). Infrared
spectra were recorded on KBr pellets with a Perkin−Elmer FTIR
1720-X spectrometer. Electrospray ionization (ESI) mass spectra were
obtained in positive-ion mode with an LCQ Finnigan mass
spectrometer.
3
1 H, CHMe2), 3.29 (br s, 2 H, fc), 4.04 (d, JHH = 6.1 Hz, 2 H,
NHCH2), 4.50 (m, 4 H, fc), 4.53 (vt, 2 H, fc), 5.16 (m, 4 H, η6-C6H4),
5.53 (br s, 1 H, CONH2), 6.59 (br s, 1 H, CONH2), 7.38−7.48 (m, 6
H, PPh2), 7.72−7.86 (m, 4 H of PPh2 and 1 H of NHCH2). 31P{1H}
NMR (CDCl3): δ 17.0 (s). IR (KBr): νNH 3389 s, νNH 3319 s, amide I
1683 vs, amide I 1647 vs, amide II 1529 s cm−1. MS (ESI+): m/z 799
Electrochemical measurements were performed with a computer-
controlled multipurpose μAUTOLAB III potentiostat (Eco Chemie)
at room temperature using a standard three-electrode cell equipped
with a platinum-disk (AUTOLAB RDE, 3 mm diameter) working
electrode, platinum-sheet auxiliary electrode, and double-junction Ag/
AgCl (3 M KCl) reference electrode. Samples were dissolved in
acetonitrile (Sigma-Aldrich, absolute) to give a solution containing ca.
0.5 mM of the analyte and 0.1 M [Bu4N][PF6] (Fluka, purissimum for
electrochemistry). The solutions were deaerated by bubbling with
argon before the measurements and then kept under an argon blanket.
The potentials are given relative to ferrocene/ferrocenium reference.
The redox potential of the ferrocene/ferrocenium couple was 0.425 V
vs Ag/AgCl (3 M KCl).
([M
+ −
Na]+ ), 741 ([M Cl]+ ). Anal. Calcd for
C35H37N2O2PCl2FeRu·0.85CHCl3: C, 49.04; H, 4.35; N, 3.19.
Found: C, 49.08; H, 4.28; N, 3.04.
General Procedure for the Preparation of Compounds [(η6-
arene)RuCl(MeCN)(1-κP)][PF6] (7). A solution of 1 (97 mg, 0.2
mmol) in acetonitrile (10 mL) was added to the solid solvento
complex [(η6-arene)Ru(MeCN)2Cl][PF6] (0.2 mmol). The resulting
mixture was stirred at room temperature for 4 h and then evaporated
under vacuum. Preparative thin-layer chromatography on silica gel
plates with chloroform/acetonitrile (4/1 v/v) afforded the products as
yellow solids.
General Procedure for the Synthesis of Complexes [(η6-
arene)RuCl2(1-κP)] (4). A solution of ligand 1 (1.0 equiv) in CHCl3
(5 mL per 0.2 mmol of the ligand) was added to the solid dimer [(η6-
arene)RuCl2]2 (0.5 equiv). After the mixture was stirred at room
temperature for 3 h, the solvent was evaporated under vacuum and the
Compound 7a. Following the general procedure, [(η6-C6H6)Ru-
(MeCN)2Cl][PF6] (88 mg, 0.2 mmol) and 1 (97 mg, 0.2 mmol)
afforded 7a as a yellow solid. Yield: 99 mg (51%). 1H NMR
3991
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Organometallics
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(CD3CN): δ 2.22 (s, 3 H, MeCN), 3.71 (s, 3 H, OMe), 3.94 (dt, J ≈
Compound 8b. This compound was prepared similarly from 4b
3
(158 mg, 0.2 mmol) and Ag[PF6] (101 mg, 0.4 mmol) and isolated as
1.3, 2.5 Hz, 1 H, fc), 3.96 (d, JHH = 6.1 Hz, 1 H, NHCH2), 3.98 (d,
1
3JHH = 6.1 Hz, 1 H, NHCH2), 4.06 (dt, J ≈ 1.3, 2.5 Hz, 1 H, fc), 4.14
(m, 1 H, fc), 4.60 (dt, J ≈ 1.3, 2.6 Hz, 1 H, fc), 4.66 (m, 2 H, fc), 4.72
a yellow solid. Yield: 122 mg (56%). H NMR (CD3CN): δ 1.07 (d,
3JHH = 6.9 Hz, 6 H, CHMe2), 1.93 (s, 3 H, η6-C6H4Me), 2.29 (s, 6 H,
MeCN), 2.49 (sept, 3JHH = 6.9 Hz, 1 H, CHMe2), 3.70 (s, 3 H, OMe),
3.97 (d, 3JHH = 6.1 Hz, 2 H, NHCH2), 4.12 (vt, 2 H, fc), 4.31 (vq, 2 H,
(m, 1 H, fc), 4.82 (m, 1 H, fc), 5.64 (s, 6 H, η6-C6H6), 6.99 (t, 3JHH
=
6.0 Hz, 1 H, NHCH2), 7.51−7.83 (m, 10 H, PPh2). 31P{1H} NMR
3
1
fc), 4.67 (vt, 2 H, fc), 4.85 (vq, 2 H, fc), 5.65 (d, JHH = 6.5 Hz, 2 H,
(CD3CN): δ −144.6 (sept, JPF = 706 Hz, PF6), 28.1 (s, PPh2).
3
3
η6-C6H4), 5.81 (d, JHH = 6.5 Hz, 2 H, η6-C6H4), 6.91 (t, JHH = 6.0
19F{1H} NMR (CD3CN): δ −72.9 (d, 1JPF = 706 Hz, PF6). IR (KBr):
νNH 3442 m, νCN 2296 w, νCO 1748 vs, amide I 1651 vs, amide II
1534 vs, νPF 842 vs cm−1. MS (ESI+): m/z 700 ([M − MeCN −
PF6]+). Anal. Calcd for C34H33N2O3F6P2ClFeRu·0.75CHCl3: C, 42.79;
H, 3.49; N, 2.87. Found: C, 42.84; H, 3.48; N, 2.91.
Hz, 1 H, NHCH2), 7.65−7.81 (m, 10 H, PPh2). 31P{1H} NMR
1
(CD3CN): δ −144.6 (sept, JPF = 707 Hz, PF6), 33.0 (s, PPh2).
19F{1H} NMR (CD3CN): δ −72.8 (d, 1JPF = 707 Hz, PF6). IR (KBr):
νNH 3414 m, νCN 2329 w and 2299 w, νCO 1747 s, amide I 1638 vs,
amide II 1542 s, νPF 835 vs cm−1. MS (ESI+): m/z 948 ([M − PF6]+).
Compound 8c. Following the general procedure, 4c (164 mg, 0.2
mmol) and Ag[PF6] (101 mg, 0.4 mmol) afforded 8c as a yellow solid.
Yield: 123 mg, 55%. 1H NMR (CD3CN): δ 1.87 (s, 18 H, η6-C6Me6),
2.35 (s, 6 H, MeCN), 3.70 (s, 3 H, OMe), 3.96 (vt, 2 H, fc), 3.97 (d,
3JHH = 6.1 Hz, 2 H, NHCH2), 4.41 (vq, 2 H, fc), 4.63 (vt, 2 H, fc),
Compound 7b. This compound was prepared similarly from [(η6-
p-cymene)Ru(MeCN)2Cl][PF6] (100 mg, 0.2 mmol) and 1 (97 mg,
0.2 mmol). Yield: 132 mg (65%), yellow solid. 1H NMR (CD3CN): δ
3
3
0.99 (d, JHH = 6.9 Hz, 3 H, CHMe2), 1.00 (d, JHH = 6.9 Hz, 3 H,
CHMe2), 1.90 (s, 3 H, η6-C6H4Me), 2.15 (s, 3 H, MeCN), 2.35 (sept,
3JHH = 7.0 Hz, 1 H, CHMe2), 3.70 (s, 3 H, OMe), 3.85 (dt, J ≈ 1.3, 2.5
Hz, 1 H, fc), 3.93 (dt, J ≈ 1.3, 2.6 Hz, 1 H, fc), 3.95 (d, 3JHH = 6.1 Hz,
2 H, NHCH2), 4.00 (m, 1 H, fc), 4.47 (dt, J ≈ 1.4, 2.6 Hz, 1 H, fc),
4.61 (dt, J ≈ 1.3, 2.6 Hz, 1 H, fc), 4.63 (m, 1 H, fc), 4.73 (m, 2 H, fc),
5.17 (d, JHH = 6.1 Hz, 1 H, η6-C6H4), 5.43 (dt, JHH = 6.2, 1.4 Hz, 1 H,
η6-C6H4), 5.50 (d, JHH = 6.5 Hz, 1 H, η6-C6H4), 5.74 (dd, JHH = 6.4,
3
4.83 (vq, 2 H, fc), 6.90 (t, JHH = 6.1 Hz, 1 H, NHCH2), 7.63−7.78
(m, 10 H, PPh2). 31P{1H} NMR (CD3CN): δ −144.6 (sept, 1JPF = 707
Hz, PF6), 34.6 (s, PPh2). 19F{1H} NMR (CD3CN): δ −72.9 (d, 1JPF
=
706 Hz, PF6). IR (KBr): νNH 3444 m, νCN 2327 w and 2295 w, νCO
1747 vs, amide I 1639 vs, amide II 1542 vs, νPF 839 vs cm−1. MS (ESI
+): m/z 976 ([M − PF6]+), 748 ([(C6Me6)Ru(1 − H)]+).
3
1.0 Hz, 1 H, η6-C6H4), 6.89 (t, JHH = 6.1 Hz, 1 H, NHCH2), 7.55−
1
7.94 (m, 10 H, PPh2). 31P{1H} NMR (CD3CN): δ −144.6 (sept, JPF
X-ray Crystallography. The diffraction data were collected with a
Stoe image plate diffraction system equipped with a ϕ circle
goniometer, using Mo Kα graphite-monochromated radiation (λ =
= 706 Hz, PF6), 27.7 (s, PPh2). 19F{1H} NMR (CD3CN): δ −72.9 (d,
1JPF = 706 Hz, PF6). IR (KBr): νNH 3418 s, νCN 2293 w, νCO 1750 s,
amide I 1651 s, amide II 1533 s, νPF 840 vs cm−1. MS (ESI+): m/z 756
0.710 73 Å; ϕ range 0−200°, 2θ range from 3.0 to 59°, Dmax − Dmin
=
12.45−0.81 Å). The structures were solved by direct methods using
the program SHELXS-97.32 Refinement and all further calculations
were carried out using SHELXL-97.32 Examination of the structures
with PLATON33 reveals in 4b additional disordered solvent molecules,
while in 8b voids between anions and cations are observed. Therefore,
new data sets corresponding to omission of the missing solvent
molecules were generated with the SQUEEZE algorithm34 and the
structures were refined to full convergence. In both structures, the
hydrogen atoms were included in their calculated positions and treated
as riding atoms using the SHELXL default parameters. The non-
hydrogen atoms were refined anisotropically, using weighted full-
matrix least squares based on F2. Crystallographic details are available
as Supporting Information (Table S4). The figures were drawn with
the PLATON program. The same program was used to perform all
geometric calculations.
( [ M
−
M e C N
−
P F 6 ]
+ ) . A n a l . C a l c d f o r
C38H41N2O3F6P2ClFeRu·0.55CHCl3: C, 45.95; H, 4.16; N, 2.78.
Found: C, 55.97; H, 4.11; N, 2.70.
Compound 7c. Starting with [(η6-hexamethylbenzene)Ru-
(MeCN)2Cl][PF6] (105 mg, 0.2 mmol) and 1 (97 mg, 0.2 mmol),
the general procedure afforded 7c as a yellow solid. Yield: 150 mg
(70%). 1H NMR (CD3CN): δ 1.76 (s, 18 H, η6-C6Me6), 2.16 (s, 3 H,
MeCN), 3.68 (m, 1 H, fc), 3.70 (s, 3 H, OMe), 3.75 (m, 1 H, fc), 3.96
(br d, 3JHH = 6.1 Hz, 2 H, NHCH2), 4.28 (br s, 1 H, fc), 4.62 (m, 1 H,
fc), 4.50 (m, 2 H, fc), 4.64 (m, 1 H, fc), 4.72 (m, 1 H, fc), 6.92 (t, 3JHH
= 5.9 Hz, 1 H, NHCH2), 7.50−7.85 (m, 10 H, PPh2). 31P{1H} NMR
1
(CD3CN): δ −144.6 (sept, JPF = 706 Hz, PF6), 29.7 (s, PPh2).
19F{1H} NMR (CD3CN): δ −72.9 (d, 1JPF = 706 Hz, PF6). IR (KBr):
νNH 3440 m, νCN 2289 w, νCO 1749 vs, amide I 1653 vs, amide II
1534 vs, νPF 847 vs cm−1. MS (ESI+): m/z 784 ([M − MeCN −
PF6]+). Anal. Calcd for C40H45N2O3F6P2ClFeRu·0.9CHCl3: C, 45.59;
H, 4.29; N, 2.60. Found: C, 45.48; H, 4.28; N, 2.75.
CCDC-866729 (4b·2CH3OH) and CCDC-866730 (8b) contain
the supplementary crystallographic data for this paper. These data can
be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.
html (or from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, U.K.; fax, (internat.) +44-1223/336-033;
e-mail, deposit@ccdc.cam.ac.uk).
General Procedure for the Oxidation of Secondary Alcohols.
The appropriate quantities of the catalyst and alcohol (1 mmol) were
mixed with water (4 mL) in an open vial. The oxidizing agent (4
mmol) was added slowly, and the resulting mixture was stirred at room
temperature for the given reaction time. Then, it was extracted with
diethyl ether (2 × 5 mL) and dried over anhydrous MgSO4. A small
aliquot was analyzed by 1H NMR spectroscopy to determine the
conversion. In the case of complete conversion, the extracts were
evaporated and the crude product was isolated by flash column
chromatography over silica using a hexane/diethyl ether mixture to
give pure ketones following evaporation.
Characterization Data of the Oxidation Products. Acetophe-
none:35 1H NMR (CDCl3) δ 2.61 (s, 3 H, Me), 7.43−7.50 (m, 2 H,
C6H5), 7.52−7.61 (m, 1 H, C6H5), 7.93−7.98 (m, 2 H, C6H5). 4′-
Fluoroacetophenone:34 1H NMR (CDCl3) δ 2.59 (s, 3 H, Me), 7.09−
7.16 (m, 2 H, C6H4), 7.96−8.01 (m, 2 H, C6H4). 4′-Chloroacetophe-
none:34 1H NMR (CDCl3) δ 2.59 (s, 3 H, Me), 7.41−7.47 (m, 2 H,
C6H4), 7.88−7.92 (m, 2 H, C6H4). 4′-Bromoacetophenone (ref.34): 1H
NMR (CDCl3) δ 2.58 (s, 3 H, Me), 7.58−7.62 (m, 2 H, C6H4), 7.79−
7.84 (m, 2 H, C6H4). 4′-Methylacetophenone:34 1H NMR (CDCl3) δ
2.42 (s, 3 H, MeC6H4), 2.59 (s, 3 H, Me), 7.23−7.28 (m, 2 H, C6H4),
General Procedure for the Synthesis of Compounds [(η6-
arene)Ru(MeCN)2(1-κP)][PF6]2 (8). A solution of the respective
complex [(η6-arene)RuCl2(1-κP)] (4; 1 equiv) in acetonitrile (2 mL
per 0.1 mmol of Ru complex) was treated with the stoichiometric
amount of Ag[PF6] (2 equiv) dissolved in acetonitrile (3 mL per 0.2
mmol of Ag salt). The resulting yellow-orange reaction mixture was
stirred at room temperature for 3 h, the precipitated solid (AgCl) was
removed by filtration through a PTFE filter, and the filtrate was
evaporated under vacuum. The residue was purified by preparative
thin-layer chromatography on SiO2 with CHCl3/acetonitrile (2/1 v/v)
as the eluent. When they are analyzed by conventional elemental
analysis, the bis(acetonitrile) complexes 8 notoriously give erratic
results, very likely due to incomplete combustion.
Compound 8a. Starting from 4a (147 mg, 0.2 mmol) and Ag[PF6]
(101 mg, 0.4 mmol), the general procedure afforded 8a as a yellow
solid. Yield: 92 mg, 44%. 1H NMR (CD3CN): δ 2.23 (s, 6 H, MeCN),
3.70 (s, 3 H, OMe), 3.98 (d, 3JHH = 6.1 Hz, 2 H, NHCH2), 4.21 (vt, 2
H, fc), 4.36 (vq, 2 H, fc), 4.73 (vt, 2 H, fc), 4.90 (vq, 2 H, fc), 5.94 (s,
3
6 H, η6-C6H6), 6.95 (t, JHH = 6.0 Hz, 1 H, NHCH2), 7.60−7.77 (m,
10 H, PPh2). 31P{1H} NMR (CD3CN): δ −144.6 (sept, 1JPF = 707 Hz,
PF6), 33.7 (s, PPh2). 19F{1H} NMR (CD3CN): δ −72.8 (d, 1JPF = 706
Hz, PF6). IR (KBr): νNH 3417 m, νCN 2330 w and 2302 w, νCO 1748
s, amide I 1639 s, amide II 1542 s, νPF 836 vs cm−1. MS (ESI+): m/z
586 ([(C6H6)Ru(1 − H)]+).
3992
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Organometallics
Article
7.83−7.88 (m, 2 H, C6H4). 3′-Bromoacetophenone:36 1H NMR
(CDCl3) δ 2.60 (s, 3 H, Me), 7.35 (t, JHH = 7.9 Hz, 1 H, C6H4), 7.69
(dd, JHH = 1.0, 7.9 Hz, 1 H, C6H4), 7.88 (d, JHH = 7.8 Hz, 1 H, C6H4),
8.09 (br s, 1 H, C6H4). 2′-Acetonaphthone:34 1H NMR (CDCl3) δ
(4) (a) Kuhnert, J.; Duse
Dalton Trans. 2007, 2802. (b) Kuhnert, J.; Císaro
̌
k, M.; Demel, J.; Lang, H.; Step
̌
nick
̌
a, P.
M.;
̌
̈
̌
va,
́
I.; Lamac,
̌
̈
̌
Step
̌
nick
̌
a, P. Dalton Trans. 2008, 2454.
̌
(5) (a) Schulz, J.; Císaro
2009, 694, 2519. (b) Schulz, J.; Renfrew, A. K.; Císaro
́
J.; Step
̌
va,
́
I.; Step
̌
nick
̌
a, P. J. Organomet. Chem.
va, I.; Dyson, P.
a, P. Appl. Organomet. Chem. 2010, 24, 392. For an
2.72 (s, 3 H, Me), 7.53 (dt, JHH = 1.1, 6.8 Hz, 1 H, Ar), 7.60 (dt, JHH
1.2, 6.8 Hz, 1 H, Ar), 7.87 (d, JHH = 6.0 Hz, 1 H, Ar), 7.89 (d, JHH
=
=
̌
̌
̌
nick
̌
8.2 Hz, 1 H, Ar), 7.96 (d, JHH = 8.0 Hz, 1 H, Ar), 8.03 (dd, JHH = 1.7,
8.6 Hz, 1 H, Ar), 8.46 (br s, 1 H, Ar). Benzophenone:35 1H NMR
(CDCl3) δ 7.46−7.52 (m, 4 H, Ar), 7.57−7.64 (m, 2 H, Ar), 7.78−
7.83 (m, 4 H, Ar). 1-Indanone:37 1H NMR (CDCl3): δ 2.67−2.72 (m,
2 H, CH2), 3.12−3.17 (m, 2 H, CH2), 7.37 (t, JHH = 7.4 Hz, 1 H, Ar),
7.48 (d, JHH = 7.7 Hz, 1 H, Ar), 7.59 (dt, JHH = 1.1, 7.4 Hz, 1 H, Ar),
7.76 (d, JHH = 7.7 Hz, 1H, Ar). 1-Tetralone:38 1H NMR (CDCl3) δ
2.10−2.18 (m, 2 H, CH2), 2.66 (t, JHH = 6.6 Hz, 2 H, CH2), 2.97 (t,
JHH = 6.1 Hz, 2 H, CH2), 7.25 (d, JHH = 8.2 Hz, 1 H, Ar), 7.31 (t, JHH
= 7.5 Hz, 1 H, Ar), 7.47 (dt, JHH = 1.3, 3.7 Hz, 1 H, Ar), 8.04 (dd, JHH
= 1.0, 7.8 Hz, 1H, Ar).
example from another laboratory, see: Zhang, W.; Shimanuki, T.; Kida,
T.; Nakatsuji, Y.; Ikeda, I. J. Org. Chem. 1999, 64, 6247.
̌
(6) Schulz, J.; Císaro
729.
(7) Tauchman, J.; Císaro
28, 3288.
(8) (a) Tauchman, J.; Císaro
2010, 4276. (b) Tauchman, J.; Císaro
2011, 40, 11748.
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va,
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nick
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a, P. Organometallics 2012, 31,
nicka, P. Organometallics 2009,
nicka, P. Eur. J. Org. Chem.
̌
̌
va,
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I.; Step
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̌
̌
̌
va,
́
I.; Step
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̌
̌
̌
́ ̌ ̌
va, I.; Stepnicka, P. Dalton Trans.
(9) (a) Baratta, W.; Bossi, G.; Putignano, E.; Rigo, P. Chem. Eur. J.
2011, 17, 3474. (b) Schley, N. D.; Dobereiner, G. E.; Crabtree, R. H.
Organometallics 2011, 30, 4174. (c) For a related article, see: Torres,
́
, F. A.; Manzano, B. R.;
Rodriguez, A. M.; Zirakzadeh, A.; Weissensteiner, W.; Mucientes, A.
ASSOCIATED CONTENT
* Supporting Information
■
J.; Sepulveda, F.; Carrion
́
, M. C.; Jalon
́
S
Text giving alternative syntheses of compounds 7b and 8b,
tables summarizing complete catalytic results (Tables S1−S3)
and relevant crystallographic data (Table S4), and CIF files
giving crystal data for 4b·2CH3OH and 8b. This material is
available free of charge via the Internet at http://pubs.acs.org.
E.; de la Pena, M. A. Organometallics 2011, 30, 3490.
̃
(10) NMR spectra recorded shortly after mixing [(η6-p-cymene)
RuCl2]2 with 2 in CDCl3 revealed dominant signals due to complex 5b
as the expected bridge-cleavage product and several additional
resonances due to unidentified side products (e.g., a P,O chelate or
P,O-bridged multinuclear complex). Upon prolonged standing in
solution, the amount of the side products increased. Attempts to purify
the crude reaction mixture failed.
AUTHOR INFORMATION
Corresponding Author
*E-mail: georg.suess-fink@unine.ch (G.S.-F.); stepnic@natur.
cuni.cz (P.S.).
■
(11) In the case of 4b·2CH3OH, the dihedral angle of planes C(27−
32) and [Cl1, Cl2, P1] is 2.0(2)°. The dihedral angle subtended by the
C(31−36) and [N2, N3, P1] planes in 8b is 4.1(3)°.
̌
̌
(12) Stepnicka, P.; Demel, J.; Cejka, J. J. Mol. Catal. A: Chem. 2004,
̌
̌
Notes
224, 161.
The authors declare no competing financial interest.
(13) Lackner, W.; Standfest-Hauser, C. M.; Mereiter, K.; Schmid, R.;
Kirchner, K. Inorg. Chim. Acta 2004, 357, 2721.
ACKNOWLEDGMENTS
■
̌
(14) (a) Step
̌
nick
̌
a, P.; Gyepes, R.; Lavastre, O.; Dixneuf, P. H.
Results presented in this paper were obtained during
cooperation within the framework of the bilateral Swiss-
Czech Scientific Exchange Program (Sciex) NMS-CH (Project
No. 10.133) and is a part of the long-term research project of
the Faculty of Science, Charles University in Prague, supported
by the Ministry of Education, Youth and Sports of the Czech
Republic (Project No. MSM 0021620857).
Organometallics 1997, 16, 5089. (b) Therrien, B.; Vieille-Petit, L.;
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2004, 689, 2456. (c) Sixt, T.; Sieger, M.; Krafft, M. J.; Bubrin, D.;
Fiedler, J.; Kaim, W. Organometallics 2010, 29, 5511.
(15) Larock, R. C. In Comprehensive Organic Transformations; VCH:
New York, 1999.
(16) (a) Wang, G.-Z.; Andreasson, U.; Backvall, J.-E. J. Chem. Soc.,
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Chem. Commun. 1994, 1037. (b) Backvall, J.-E.; Chowdhury, R. L.;
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Karlsson, U. J. Chem. Soc., Chem. Commun. 1991, 473. (c) Buffin, B. P.;
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225, 111. (d) Liu, L.; Yu, M.; Wayland, B. B.; Fu, X. Chem. Commun.
2010, 46, 6353. (e) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A.
Science 2000, 287, 1636.
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published ASAP on April 30, 2012. A correction
has been made in the fourth paragraph in the Catalytic
Reactions section of the Results and Discussion. The revised
version was posted on May 7, 2012.
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dx.doi.org/10.1021/om3002087 | Organometallics 2012, 31, 3985−3994