Phenyl substituted indenylphosphine ruthenium complexes as catalysts for dehydrogenation of alcohols

Article abstract of DOI:10.1039/c2dt30499f

Thermal treatment of (1H-inden-3-yl)dicyclohexylphosphinium tetrafluoroborate (1) and (3-mesityl-1H-inden-3-yl)dicyclohexylphosphinium tetrafluoroborate (3) with tBuONa followed by [(η⁶-cymene) RuCl₂)]₂ in methanol gave the adduct {(η⁶- cymene)RuCl₂[(1H-inden-3-yl)PCy₂]} (6) and {(η⁶-cymene)RuCl₂[(3-mesityl-1H-inden-3-yl)PCy ₂]} (7), respectively. Thermal treatment of (2-phenyl-1H-inden-3-yl) dicyclohexylphosphinium tetrafluoroborate (4) with tBuONa followed by [(η⁶-cymene)RuCl₂)]₂ or RuCl ₃·3H₂O in methanol gave {Ru[κ(P): (η⁶-2-phenyl-1H-inden-3-yl)PCy₂]Cl₂} (8). Whereas (2-mesityl-1H-inden-3-yl)dicyclohexylphosphine (5) reacted with [(η⁶-cymene)RuCl₂)]₂ (in toluene) or RuCl₃·3H₂O (in ethanol) to afford {Ru[κ(P):(η⁶-2-mesityl-1H-inden-3-yl)PCy ₂]Cl₂} (9). The molecular structures of complexes 6, 8 and 9 have been determined by single-crystal X-ray diffraction analysis. In addition, complexes 8 and 9 have been found to catalyze the acceptorless dehydrogenation of alcohols in toluene. 9 displayed high activity and different substrates, including cyclic and linear alcohols, were efficiently oxidized to ketones by using 2.0 mol% of catalyst.

Full text of DOI:10.1039/c2dt30499f

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PAPER
Phenyl substituted indenylphosphine ruthenium complexes as catalysts for
dehydrogenation of alcohols†
Jia Yuan,a Yue Sun,a Guang-Ao Yu,*a Cui Zhao,a Neng-Fang She,b Shu-Lan Mao,c Peng-Shou Huang,a
Zhi-Jun Han,a Jun Yina and Sheng-Hua Liu*a
Received 1st March 2012, Accepted 22nd June 2012
DOI: 10.1039/c2dt30499f
Thermal treatment of (1H-inden-3-yl)dicyclohexylphosphinium tetrafluoroborate (1) and (3-mesityl-1H-
inden-3-yl)dicyclohexylphosphinium tetrafluoroborate (3) with tBuONa followed by [(η6-cymene)-
RuCl2)]2 in methanol gave the adduct {(η6-cymene)RuCl2[(1H-inden-3-yl)PCy2]} (6) and {(η6-cymene)-
RuCl2[(3-mesityl-1H-inden-3-yl)PCy2]} (7), respectively. Thermal treatment of (2-phenyl-1H-inden-3-
yl)-dicyclohexylphosphinium tetrafluoroborate (4) with tBuONa followed by [(η6-cymene)RuCl2)]2 or
RuCl3·3H2O in methanol gave {Ru[κ(P):(η6-2-phenyl-1H-inden-3-yl)PCy2]Cl2} (8). Whereas (2-mesityl-
1H-inden-3-yl)dicyclohexylphosphine (5) reacted with [(η6-cymene)RuCl2)]2 (in toluene) or RuCl3·3H2O
(in ethanol) to afford {Ru[κ(P):(η6-2-mesityl-1H-inden-3-yl)PCy2]Cl2} (9). The molecular structures
of complexes 6, 8 and 9 have been determined by single-crystal X-ray diffraction analysis. In addition,
complexes 8 and 9 have been found to catalyze the acceptorless dehydrogenation of alcohols in toluene.
9 displayed high activity and different substrates, including cyclic and linear alcohols, were efficiently
oxidized to ketones by using 2.0 mol% of catalyst.
synthesis of phenyl substituted indenylphosphine ruthenium
complexes, as well as their application as catalysts for the de-
hydrogenation of alcohols.
Introduction
(η6-Arene)ruthenium(II) derivatives have recently received
increased attention on account of their potential as catalysts for a
number of organic reactions, such as hydrogen transfer,1,2 C–C
coupling3,4 and alkyne hydration.5 Indenyl metal complexes
Results and discussion
have been widely studied due to their diverse and flexible hapti-
cities and their enhanced reactivities both in stoichiometric reac-
tions and in catalysis by comparison to their cyclopentadienyl
analogues,6 which is referred to as the indenyl effect.7 The
coordination chemistry of various donor-substituted indene
ligands, including indenyl phosphine,8 1-PR2-2-NR′2-indene,9
1-pyridinyl-indene,10 have been widely explored. These ligands
have proven to be remarkably versatile, providing access to a
range of neutral, cationic and formally zwitterionic late transition
metal complexes, as well as serving as a locale for metal
binding. As a continuation of our research into indenyl-
phosphine11 and ruthenium complexes,12 herein we report the
In a first set of experiments, we investigated the reaction of (1H-
inden-3-yl)dicyclohexylphosphinium tetrafluoroborate (1) with
the dimer [(η6-cymene)RuCl2)]2 (2) in methanol. When 2 equiv
of 1 was reacted with 2, a mononuclear complex {(η6-cymene)-
RuCl2[(1H-inden-3-yl)PCy2]} (6) was obtained in 48% yield,
with a part of 2 reacting with tetrafluoroborate acid as a sacrifi-
cial base.13 So when 1 equiv of tBuONa was added to the
solvent of the tetrafluoroborate salt 1, the phosphine ligand was
obtained.11 The latter can also react with 2, as a result, 6 was
obtained in 85% yield. In order to show the generality of this
synthetic methodology for the preparation of such complexes,
we further carried out the reaction of (3-mesityl-1H-inden-3-yl)-
dicyclohexylphosphinium tetrafluoroborate (3) with 2 in the
absence of tBuONa, under similar conditions. The expected
{(η6-cymene)RuCl2[(3-mesityl-1H-inden-3-yl)PCy2]} (7) was
produced in 27% yield. When tBuONa was added to the reaction
mixture, 7 was obtained in 52% yield (Scheme 1).
aKey Laboratory of Pesticide and Chemical Biology, Ministry of
Education, College of Chemistry, Central China Normal University,
Wuhan, 430079, P. R. China. E-mail: yuguang@mail.ccnu.edu.cn,
chshliu@mail.ccnu.edu.cn
bDepartment of Chemistry, State Key Laboratory of Elemento-Organic
Chemistry, Nankai University, Tianjin, 300071, P. R. China
cKey Laboratory for the Green Preparation and Application of
Functional Materials, Ministry of Education, Hubei University, Wuhan,
430062, P. R. China
Products 6 and 7 are air-stable red solids, which have been
characterized by elemental analysis, 1H NMR and 31P NMR
1
spectroscopy. For example, the H NMR spectra of 6 displayed
†Electronic supplementary information (ESI) available: Graphic details
for 6, 8 and 9 in CIF format, 31P NMR of m1 and m2. CCDC reference
numbers 869889–869891. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt30499f
two broad peaks in the range 4.76–5.20 ppm, assigned to the
1
η6-cymene ring, while the H NMR spectra of 7 displayed three
broad peaks and one doublet peak in the range 5.03–5.70 ppm,
Dalton Trans., 2012, 41, 10309–10316 | 10309
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Scheme 1 Synthesis of Ru complexes 6–9.
assigned to the η6-cymene. Clearly, the different spectra arise on
account of the steric effect of the two phosphine ligands. The
31P NMR spectra of 6 exhibits one singlet at 15.85 ppm from its
P atom, and the 31P NMR spectra of 7 exhibits one broad peak at
33.46 ppm for its P atom, the different 31P NMR is due to the
different functional groups bonded to the phosphorus atom, one
is alkenyl, the other is allyl.
When the reaction of 2 with (2-phenyl-1H-inden-3-yl)dicyclo-
hexylphosphinium tetrafluoroborate (4) and tBuONa was per-
formed under similar conditions (60 °C, 12 h), the η6-cymene
π-ligand was displaced and an η6 : η1-tethered compound {Ru-
[κ(P) : (η6-2-phenyl-1H-inden-3-yl)PCy2]Cl2} (8) was obtained
in 86% yield. Compound 8 can also be obtained in 67% yield by
reacting RuCl3·3H2O with 4 and tBuONa in ethanol. We have
also investigated the reaction of 2 with (2-mesityl-1H-inden-3-
yl)dicyclohexylphosphine (5) in various solvents. Heating 2 with
2 equivalents of 5 in methanol at 60 °C for 16 h resulted in the
formation of {Ru[κ(P) : (η6-2-mesityl-1H-inden-3-yl)PCy2]Cl2}
(9) only in 6% yield. When toluene was used instead of metha-
nol and refluxed at 110 °C for 12 h, 9 was obtained in 51%
yield. 9 can also be produced in 33% yield by reacting
RuCl3·3H2O with 5 in ethanol. Faller et al. have reported
the synthesis of the analogous compounds [(η6 : η1-L′-P)RuCl2]
(L′ = 2-(dicyclohexylphosphino)biphenyl, 2-(dicyclohexylphos-
phino)-2′-methylbiphenyl, 2-(dicyclohexylphosphino)-2′-(N,N-
dimethylamino)biphenyl).14,15 One of these planar chiral arene-
tethered ruthenium derivatives has been used as a catalyst for the
Diels–Alder reaction of methacrolein and cyclopentadiene, with
enantiomeric excess up to 70%.15
The mechanism for the formation of 8 and 9 is not clear to us.
However, according to the previously reported similar reac-
tions,16,17 we might suggest a possible pathway to account for
the formation of 8 and 9. As shown in Scheme 2, the reactions
would first give the intermediates m1 and m2, followed by an
intramolecular substitution of the cymene group by the benzene
ring bonded to indenyl. To confirm the above mechanism,
31P NMR of the reaction mixture was studied (see ESI†). The
reaction of 4 with tBuONa gives the phosphine ligand 10,11 the
mixture of 10 and [(η6-cymene)RuCl2)]2 (2) in MeOH was
stirred at 30 °C for 12 h and a new 31P NMR peak appeared at
84.77 ppm, corresponding to the intermediate m1 and when the
mixture was stirred at 65 °C for 12 h, the 31P NMR peak of 8
appeared at 55.14 ppm. When the mixture of 5 and 2 in toluene
was stirred at 80 °C for 12 h, a new 31P NMR peak appeared at
25.12 ppm, corresponding to the intermediate m2, when the
mixture was stirred at 100 °C for 12 h, two new 31P NMR peaks
10310 | Dalton Trans., 2012, 41, 10309–10316
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Scheme 2 Proposed mechanism for the formation of 8 and 9.
Table 1 Selected crystallographic data for complexes 6, 8 and 9
6
8
9
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C32H45Cl4PRu
C27H33Cl2PRu
560.47
298(2)
Monoclinic
P2(1)/n
16.7462(13)
9.5164(7)
36.246(3)
90
92.8140(10)
90
5769.3(8)
8, 1.291
0.796
2304
C30H41Cl2OPRu
620.57
298(2)
Monoclinic
P2(1)/c
19.0835(15)
9.3905(7)
17.6437(14)
90
114.8920(10)
90
2868.1(4)
4, 1.437
0.810
1288
703.52
298(2)
Triclinic
ˉ
P1
10.1510(6)
12.7461(8)
14.9409(9)
109.5200(10)
93.6390(10)
101.4450(10)
1768.09(19)
2, 1.321
0.810
728
0.16 × 0.12 × 0.10
1.84–25.50
6578
γ (°)
Volume (Å3)
Z, Dcalc (mg m−3
)
Abs coefficient (mm−1
F(000)
)
Crystal size (mm3)
θ range (°)
0.15 × 0.12 × 0.10
2.03–25.01
10 128
0.20 × 0.13 × 0.10
2.31–28.33
25 779
Reflections collected
Independent reflections
Completeness to θ (%)
Max. and min. transmission
Data/restraints/params
Goodness-of-fit on F2
R1 and wR2 [I > 2σ(I)]
R1 and wR2 (all data)
6578
99.8
0.9234, 0.8814
6578/19/428
1.013
0.0405, 0.1157
0.0487, 0.1193
10 128
99.5
0.9247, 0.8900
10 128/0/559
1.279
0.0909, 0.1834
0.0971, 0.1862
7102
99.1
0.9234, 0.8547
7102/3/325
1.037
0.0260, 0.0709
0.0303, 0.0733
appeared at 47.57 ppm and 41.69 ppm, respectively, the
47.57 ppm peak belonging to the phosphine of 9. Unfortunately,
the intermediate m1, m2 and the byproduct associated with 8 or
9 were unstable, and could not be obtained as a pure form.
Products 8 and 9 are air-stable yellow solids, which have been
characterized by elemental analysis, 1H NMR and 31P NMR
and 9. Selected bond lengths and angles are listed in Table 1,
while Fig. 1–3 show their molecular structures. As shown in
Fig. 1, complex 1 displays the expected “piano stool” geometry
with a facial orientation of two chloro, and the indenyl phos-
phine ligand. The lengths of the two Ru–Cl bonds (2.4116(9) Å,
2.4126(9) Å) and Ru–P bond (2.3956(8) Å) are within the
expected ranges.18 A perspective view of the molecular structure
of 8 is shown in Fig. 2, the ruthenium atom adopts the expected
three-legged piano stool coordination and confirms that
(2-phenyl-1H-inden-3-yl)dicyclohexylphosphine is coordinated
as an eight-electron donor in a κ(P)-η6 manner, bonded through
the phosphorus atom and tethered by the proximal phenyl ring
attached to the two-carbon bridge of the indenyl framework.
This is the first example of a phenyl substituted indenyl mono-
phosphine coordinating in such a manner. The molecular struc-
ture of 9 is similar to that of 8 (Fig. 3). In addition, the bond
distances from Ru to each carbon of the coordinated arene were
equal for 6, whereas the bond distances from Ru to each carbon
of the coordinated arene were as follows for 8: Ru(1)–C(1),
1
spectroscopy. For example, the H NMR spectra of 8 displayed
three multiplets in the range 5.16–6.30 ppm, arising from the
1
η6-phenyl ring, while the H NMR spectra of 9 displayed one
singlet at 5.70 ppm assigned to the η6-mesityl ring. The 31P
NMR spectra of 8 and 9 each exhibited one singlet at 52.95 or
47.99 ppm originating from the P atoms attached to ruthenium
atoms.16
X-Ray crystallography of 6, 8 and 9
In order to unambiguously confirm the structures of 6, 8 and 9,
we have carried out the X-ray crystal diffraction analyses of 6, 8
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Table 2 Dehydrogenation of 1-phenylethanol with catalyst 6–9 in
toluene at 110 °Ca
Entry
[Ru]
Time (h)
Yieldb (%)
1
2
3
4
6
7
8
9
20
20
20
20
1
3
37
42
a General conditions: benzyl alcohol (1.0 mmol), Ru complex
(0.01 mmol) in toluene (2 mL) at 110 °C. b Yields determined by GC
analysis.
Fig. 1 Molecular structure of 6 with 30% probability level ellipsoids.
both complexes 8 and 9, the ruthenium atoms do not lie beneath
the center of the coordinated arene, and approach the tethered
indenyl phosphine section because of the Ru(1)–P(1) bond.
In fact, the geometry of 8 and 9 is similar to that of the known
complexes [Ru(η6 : η1-o-C6H4(CH2OH)(CH2CH2PPh2))Cl2],19
[Ru(η6 : η1-p-C6H4(Me)-(CH2CH2CH2PPh2))Cl2]20 and [Ru-
(η6 : η1-o-C6H4(NMe2)-(C6H4PCy2))Cl2].14a
,
[Ru(κ(P):η6-2-
{(diphenylphosphino)-12-phenyl-9,10-dihydro-9,10-ethanoanthra-
cene})Cl2].16
Catalytic activity
Catalytic alcohol dehydrogenation, through the direct formation
of hydrogen and without the need of oxidizing agents, is a
straightforward route to achieve carbonyl compounds, such as
ketones, aldehydes, and esters.21 This process is of great current
interest as a potential method for hydrogen production from
biomass products.21d It was recently demonstrated that a series
of Ru arene, cyclopentadienyl and phosphine complexes, includ-
ing [CpRuCl(PPh3)2], [(indenyl)RuCl(PPh3)2], [(benzene)-
RuCl2]2, [(η6-cymene)RuCl2]2 and [(η6-cymene)Ru(1,2,3-triazo-
lylidenyl)Cl2], are effective for the dehydrogenation of 1-phenyl-
ethanol.1a,21d We decided to evaluate the catalytic properties of
compounds 6, 7, 8 and 9, because of their conformation simi-
larity to the above mentioned compounds. Typically, catalytic
activity tests were performed at 110 °C in toluene. It was inter-
esting that the catalytic efficiency varies in the order of 9 > 8 > 7
> 6, with the best results observed in the presence of 9 and the
desired product was obtained in 42% (Table 2).
Fig. 2 Molecular structure of 8 with 30% probability level ellipsoids.
We next investigated the other reactions parameters, such as
catalyst 9, base and solvent effects. No product was obtained
without catalyst 9 (Table 3, entry 1, 2). tBuOK with catalyst 9
gave low yields of the desired product whereas tBuONa afforded
the product in good yields (Table 3, entry 5, 6). As a solvent,
toluene was found to give the best results, tBuOH and dioxane
afforded the product only in moderate yield (Table 3, entries
7–8). To investigate the potential of 9 for synthetic purposes, the
dehydrogenation of 1-phenylethanol was carried out at different
catalyst loadings, employment of 1.0 and 1.5 mol% of 9 gave
79% and 95% conversion, respectively in 36 h, whereas at
Fig. 3 Molecular structure of 9 with 30% probability level ellipsoids.
2.130(8) Å; Ru(1)–C(2), 2.199(9) Å; Ru(1)–C(3), 2.198(10) Å;
Ru(1)–C(4), 2.234(9) Å; Ru(1)–C(5), 2.204(10) Å; Ru(1)–C(6),
2.179(10) Å. The analogous distances were as follows for 9: Ru(1)–
C(1), 2.1260(15) Å; Ru(1)–C(2), 2.2115(16) Å; Ru(1)–C(3),
2.2087(16) Å; Ru(1)–C(4), 2.2672(16) Å; Ru(1)–C(5),
2.2235(10) Å; Ru(1)–C(6), 2.2146(16) Å. We conclude that, in
10312 | Dalton Trans., 2012, 41, 10309–10316
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Table 3 Influence of reaction conditions with the catalyst 9a
substrates, including cyclic and linear alcohols, by using
2.0 mol% of the catalyst at 110 °C.
Experimental
General considerations
Entry 9 (mol%) Base
Solvent T (°C) Time (h) Yieldb (%)
Schlenk and vacuum line techniques were employed for all
manipulations. All solvents were distilled from appropriate
1
2
3
4
5
6
7
8
9
10
None
None
1.0
1.0
1.0
1.0
1.0
1.0
1.5
None
Toluene 110
20
20
20
36
36
36
36
36
36
36
0
0
1
drying agents under argon before use. H and 31P NMR spectra
tBuONa Toluene 110
None
None
tBuOK Toluene 110
tBuONa Toluene 110
tBuONa tBuOH
tBuONa Dioxane 110
tBuONa Toluene 110
tBuONa Toluene 110
Toluene 110
Toluene 110
42
50
43
79
59
61
95
99
were recorded on a Mercury-Plus spectrometer (400 MHz or
600 MHz). Elemental analyses were performed on a Perkin-
Elmer 240C analyzer. (1H-inden-3-yl)dicyclohexylphosphinium
tetrafluoroborate (1),22 [(η6-cymene)RuCl2]2 (2),23 (3-mesityl-
1H-inden-3-yl)dicyclohexylphosphinium tetrafluoroborate (3),24
(2-phenyl-1H-inden-3-yl)dicyclohexylphosphinium tetrafluoro-
borate (4),11 (2-mesityl-1H-inden-3-yl)dicyclohexylphosphine
(5)25 were prepared according to the literature procedures.
110
2.0
a Reaction conditions: base (0.04 mmol), 1-phenylethanol (1.0 mmol),
toluene (2 mL), heated at 110 °C for the specified time under nitrogen.
b The yield was determined by GC analysis.
{(η6-Cymene)RuCl2[(1H-Inden-3-yl)PCy2]} (6)
Method i. A mixture of (1H-inden-3-yl)dicyclohexylphosphi-
nium tetrafluoroborate (1) (128 mg, 0.32 mmol) and [(cymene)-
RuCl2]2 (2) (96 mg, 0.16 mmol) in MeOH (8 mL) was stirred at
65 °C for 5 h to give a red solution. On cooling to room temp-
erature, the product precipitated in the form of red microcrystals.
They were collected, washed with hexane, and the solid was
extracted with CH2Cl2. The extracts were subjected to filtration.
The filtrate was carefully layered with 20 mL of hexane. After
the mixture stood for 1 day, a dark red precipitate formed, which
was filtered, washed with hexane, and dried in vacuo. A 94 mg
2.0 mol% of catalyst, quantitative formation of the ketone was
attained (Table 3, entries 8–10).
The scope of this reaction was then investigated under
optimized conditions [9 (2.0 mol%), tBuONa (4 mol%),
toluene (2 mL), 110 °C, 36 h], and the results are summarized
in Table 4. Both primary or secondary benzylic alcohols were
readily dehydrogenated to aldehydes or ketones, respectively,
with moderate to good yields (entries 1–4). The electronic nature
of the aryl group plays an important role. Electron-donating sub-
stituent gave high yield (entry 2), while an electron-withdrawing
substituent reduced the activity of the catalyst (entry 3). Diphe-
nylmethanol and bis(4-methylphenyl)methanol were converted
to the ketone in 92% and 67% yield (entries 5, 6), respectively.
Also the α-hydroxy ketone led to 44% of diketone (entry 7).
Notably, the cyclic 2-indanol, α-tetralol and cyclohexanol
were dehydrogenated with 45–99% yield. Complete dehydro-
1
amount (48%) of 6 as dark red crystals was obtained. H NMR
(400 MHz, CDCl3): δ 7.85 (d, J = 7.2 Hz, 1H, indenyl–H), 7.56
(d, J = 7.2 Hz, 1H, indenyl–H), 7.29–7.36 (m, 2H, indenyl–H),
7.12 (d, J = 7.2 Hz, 1H, indenyl–H), 5.12 (s, 2H, cymene–H),
4.76 (s, 2H, cymene–H), 3.76 (s, 2H, indenyl–H), 2.72 (sept,
3
1H, J = 7.0 Hz, CH(CH3)2), 2.45 (s, 3H, CH3), 1.78 (d, 6H,
3J = 7.0 Hz, CH(CH3)2), 1.17–1.69 (m, 22H) ppm. 31P{1H}
NMR (162 MHz, CDCl3): δ 15.85 (s) ppm. Anal. Calcd for
C31H43Cl2PRu: C, 60.19; H, 7.01. Found: C, 59.98; H, 6.96.
Method ii. A mixture of (1H-inden-3-yl)dicyclohexylphosphi-
nium tetrafluoroborate (1) (128 mg, 0.32 mmol) and tBuONa
(31 mg, 0.32 mmol) in MeOH (8 mL) was stirred at room temp-
erature for 5 minutes, 2 (96 mg, 0.16 mmol) was then added.
The mixture was stirred at 65 °C for 5 h to give a red solution.
On cooling to room temperature, the product precipitated in the
form of red microcrystals. They were collected, washed with
hexane, and the solid was extracted with CH2Cl2. The extracts
were subjected to filtration. The filtrate was carefully layered
with 20 mL of hexane. After the mixture stood for 1 day, a dark
red precipitate formed, which was filtered, washed with hexane,
and dried in vacuo. A 166 mg amount (84%) of 6 as dark red
crystals was obtained.
genation was also observed with 2-propanol. Finally,
was found to be almost inactive in the dehydrogenation of
1-octanol.
9
Conclusions
In summary, we have studied the reactions of a series of indenyl-
phosphines derivatives with [(η6-cymene)RuCl2]2 and RuCl3·
3H2O, and ruthenium complexes {(η6-cymene)RuCl2[(1H-
inden-3-yl)PCy2]}
(6),
{(η6-cymene)RuCl2[(3-mesityl-1H-
inden-3-yl)PCy2]} (7), {Ru[κ(P) : (η6-2-phenyl-1H-inden-3-yl)-
PCy2]Cl2} (8), {Ru[κ(P) : (η6-2-mesityl-1H-inden-3-yl)PCy2]-
Cl2} (9) have been obtained and characterized by 1H, and
31P NMR spectroscopy. The molecular structures of complexes
6, 8 and 9 have been determined by single-crystal X-ray diffrac-
tion analysis. Different synthetic routes have been described for
compounds 8 and 9. Comparative studies revealed that 8 and 9
were efficient catalysts for the oxidation of alcohols through
acceptorless dehydrogenation. Particularly activity was found for
the compound 9, which efficiently dehydrogenated several
{(η6-Cymene)RuCl2[(3-mesityl-1H-inden-3-yl)PCy2]} (7)
Method i. A mixture of (3-mesityl-1H-inden-3-yl)dicyclohexyl-
phosphinium tetrafluoroborate (3) (150 mg, 0.29 mmol) and
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Table 4 Dehydrogenation of alcohols with catalyst 9a
Entry
1
Alcohol
Product
Yieldb (%)
80c
2
3
91
21
4
5
56
92
6
7
67
44
8
9
45c
91
10
99c,d
11
12
99c,d
4c
a Reaction conditions: 9 (0.02 mmol), tBuONa (0.04 mmol), alcohol (1.0 mmol), toluene (2 mL), heated at 110 °C for 36 h under nitrogen. b Isolated
yields by column chromatography unless otherwise noted. c The yield was determined by GC analysis. d Reaction time, 12 h.
[(cymene)RuCl2]2 (2) (77 mg, 0.13 mmol) in MeOH (8 mL)
was stirred at 65 °C for 8 h to give a red solution. On cooling to
room temperature, the product precipitated in the form of red
microcrystals. They were collected, washed with hexane, and the
solid was extracted with CH2Cl2. The extracts were subjected to
filtration. The filtrate was carefully layered with 20 mL of
hexane. After the mixture stood for 1 day, a dark red precipitate
formed, which was filtered, washed with hexane, and dried
10314 | Dalton Trans., 2012, 41, 10309–10316
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{Ru[κ(P) : (η6-2-mesityl-1H-inden-3-yl)PCy2]Cl2} (9)
in vacuo. A 50 mg amount (26%) of 7 as dark red crystals was
obtained. 1H NMR (400 MHz, CDCl3): δ 7.52–7.54 (br, 1H,
indenyl–H), 7.27–7.28 (m, 2H, indenyl–H), 7.01 (s, 2H, Ar–H),
6.90–6.98 (m, 2H, indenyl–H) 5.60–5.70 (br, 1H, cymene–H),
5.50–5.59 (br, 1H, cymene–H) 5.20–5.40 (br, 1H, cymene–H),
5.03–5.08 (m, 1H, cymene–H), 1.01–2.78 (m, 41H) ppm.
31P{1H} NMR (160 MHz, CDCl3): δ 33.46 (s) ppm. Anal.
Calcd for C40H53Cl2PRu: C, 65.20; H, 7.25. Found: C, 64.93;
H, 7.10.
Method ii. A mixture of (3-mesityl-1H-inden-3-yl)dicyclo-
hexylphosphinium tetrafluoroborate (3) (150 mg, 0.29 mmol)
and tBuONa (31 mg, 0.32 mmol) in MeOH (8 mL) was stirred
at room temperature for 5 minutes, 2 (77 mg, 0.13 mmol) was
then added. The mixture was stirred at 65 °C for 8 h to give a
red solution. On cooling to room temperature, the product preci-
pitated in the form of red microcrystals. They were collected,
washed with hexane, and the solid was extracted with CH2Cl2.
The extracts were subjected to filtration. The filtrate was carefully
layered with 20 mL of hexane. After the mixture stood for 1 day,
a dark red precipitate formed, which was filtered, washed with
hexane, and dried in vacuo. A 96 mg amount (50%) of 7 as dark
red crystals was obtained.
Method i. A mixture of (2-mesityl-1H-inden-3-yl)dicyclohexyl-
phosphine (5) (90 mg, 0.21 mmol) and 2 (61 mg, 0.10 mmol) in
MeOH (8 mL) was stirred at 65 °C for 16 h to give a red solu-
tion. On cooling to room temperature, the product precipitated in
the form of red microcrystals. They were collected, washed with
hexane, and the solid was extracted with CH2Cl2. The extracts
were subjected to filtration. The filtrate was carefully layered
with 20 mL of hexane. After the mixture stood for 1 day, a dark
red precipitate formed, which was filtered, washed with hexane,
and dried in vacuo. A 7 mg amount (6%) of 9 as dark red crys-
tals was obtained. 1H NMR (400 MHz, CDCl3): δ 7.74 (d,
J = 7.6 Hz, 1H, indenyl–H), 7.56 (d, J = 7.6 Hz, 1H, indenyl–
H), 7.45 (t, J = 7.6 Hz, 1H, indenyl–H), 7.37 (t, J = 7.6 Hz, 1H,
indenyl–H), 5.70 (s, 2H, mesityl–H), 3.72 (s, 2H, indenyl–H),
2.74–2.80 (br, 2H), 2.51 (s, 3H, CH3), 1.99 (s, 6H, CH3),
1.21–1.86 (m, 20H) ppm. 31P{1H} NMR (162 MHz, CDCl3):
δ 47.99 (s) ppm. Anal. Calcd for C30H39Cl2PRu: C, 59.80;
H, 6.52. Found: C, 59.63; H, 6.78.
Method ii. A mixture of (2-mesityl-1H-inden-3-yl)dicyclo-
hexylphosphine (5) (90 mg, 0.21 mmol) and 2 (61 mg,
0.10 mmol) in toluene (8 mL) was stirred at 120 °C for 16 h to
give a red solution. On cooling to room temperature, the product
precipitated in the form of red microcrystals. They were col-
lected, washed with hexane, and the solid was extracted with
CH2Cl2. The extracts were subjected to filtration. The filtrate was
carefully layered with 20 mL of hexane. After the mixture stood
for 1 day, a dark red precipitate formed, which was filtered,
washed with hexane, and dried in vacuo. A 61 mg amount
(51%) of 9 as dark red crystals was obtained.
{Ru[κ(P) : (η6-2-Phenyl-1H-Inden-3-yl)PCy2]Cl2} (8)
Method i. A mixture of (2-phenyl-1H-inden-3-yl)dicyclohexyl-
phosphinium tetrafluoroborate (4) (150 mg, 0.32 mmol) and
tBuONa (31 mg, 0.32 mmol) in MeOH (8 mL) was stirred at
room temperature for 5 minutes, 2 (96 mg, 0.16 mmol) was
added. The mixture was stirred at 65 °C for 16 h to give a red
solution. On cooling to room temperature, the product precipi-
tated in the form of red microcrystals. They were collected,
washed with hexane, and the solid was extracted with CH2Cl2.
The extracts were subjected to filtration. The filtrate was carefully
layered with 20 mL of hexane. After the mixture stood for 1 day,
a dark red precipitate formed, which was filtered, washed with
hexane, and dried in vacuo. A 130 mg amount (89%) of 8 as
Method iii. A mixture of (2-mesityl-1H-inden-3-yl)dicyclo-
hexylphosphine (5) (90 mg, 0.21 mmol) and RuCl3·3H2O
(60 mg, 0.23 mmol) in EtOH (10 mL) was stirred at 80 °C for
16 h to give a red solution. On cooling to room temperature, the
product precipitated in the form of red microcrystals. They were
collected, washed with hexane, and the solid was extracted with
CH2Cl2. The extracts were subjected to filtration. The filtrate was
carefully layered with 20 mL of hexane. After the mixture stood
for 1 day, a dark red precipitate formed, which was filtered,
washed with hexane, and dried in vacuo. A 42 mg amount
(33%) of 9 as dark red crystals was obtained.
1
dark red crystals was obtained. H NMR (400 MHz, CDCl3):
δ 7.68 (d, J = 7.6 Hz, 1H, indenyl–H), 7.54 (d, J = 7.6 Hz, 1H,
indenyl–H), 7.35–7.46 (m, 2H, indenyl–H), 7.37 (t, J = 8 Hz,
1H, Ar), 6.29–6.30 (m, 1H, Ar–H), 6.09 (t, J = 6.0 Hz, 2H,
Ar–H), 5.17 (d, J = 6.0 Hz, 2H, Ar–H), 3.78 (s, 2H, indenyl–H),
2.71–2.74 (m, 2H), 1.23–1.90 (m, 20H) ppm. 31P{1H} NMR
(162 MHz, CDCl3): δ 52.95 (s) ppm. Anal. Calcd for
C27H33Cl2PRu: C, 57.86; H, 5.93. Found: C, 57.70; H, 5.93.
Method ii. A mixture of (2-phenyl-1H-inden-3-yl)dicyclo-
hexylphosphinium tetrafluoroborate (4) (150 mg, 0.32 mmol)
and tBuONa (31 mg, 0.32 mmol) in EtOH (10 mL) was stirred
at room temperature for 5 minutes, RuCl3·3H2O (84 mg,
0.32 mmol) was then added. The mixture was stirred at 80 °C
for 16 h to give a red solution. On cooling to room
temperature, the product precipitated in the form of red micro-
crystals. They were collected, washed with hexane, and the
solid was extracted with CH2Cl2. The extracts were subjected to
filtration. The filtrate was carefully layered with 20 mL of
hexane. After the mixture stood for 1 day, a dark red precipitate
formed, which was filtered, washed with hexane, and dried
in vacuo. A 120 mg amount (67%) of 8 as dark red crystals was
obtained.
Procedure for the mechanism studies for the formation of
8. A mixture of (2-phenyl-1H-inden-3-yl)dicyclohexylphosphi-
nium tetrafluoroborate (4) (150 mg, 0.32 mmol) and tBuONa
(31 mg, 0.32 mmol) in MeOH (8 mL) was stirred at room temp-
erature for 5 minutes, 2 (96 mg, 0.16 mmol) was then added.
The mixture was stirred at different temperatures (30 °C, 50 °C,
60 °C, 65 °C) for 12 h under nitrogen. On cooling to room temp-
erature, the mixture was analyzed using 31P NMR spectroscopy
to determine the content.
Procedure for the mechanism studies for the formation of
9. A mixture of (2-mesityl-1H-inden-3-yl)dicyclohexylphos-
phine (5) (90 mg, 0.21 mmol), and 2 (61 mg, 0.10 mmol) in
toluene was stirred at different temperatures (80 °C, 90 °C,
100 °C, 110 °C) for 12 h under nitrogen. On cooling to room
temperature, the mixture was analyzed using 31P NMR spectro-
scopy to determine the content.
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Crystallographic studies
Crystals of 6, 8 and 9 for X-ray diffraction were obtained by
recrystallization of the pure product from dichloromethane/
hexane layers. Crystallographic data was collected on a Bruker
SMART CCD area-detector diffractometer with graphite-mono-
chromated Mo Kα radiation (λ = 0.71073 Å). Diffraction
measurements were made at room temperature. An absorption
correction by SADABS was applied to the intensity data. The
structures were solved by the Patterson method. The remaining
non-hydrogen atoms were determined from the successive differ-
ence Fourier syntheses. All non-hydrogen atoms were refined
anisotropically except those mentioned otherwise. The hydrogen
atoms were generated geometrically and refined with isotropic
thermal parameters. The structures were refined on F2 by full-
matrix least-squares methods using the SHELXTL-97 program
package.26 The crystal data and structural refinements details are
listed in Table 1.
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Procedure for the catalytic dehydrogenation of alcohols
The Ru complex (0.02 mmol) was dissolved in toluene (2 mL).
After addition of the alcohol substrate (1.0 mmol) and tBuONa
(0.04 mmol), the solution was heated at 110 °C (bath tempera-
ture) under nitrogen and the conversion was determined by GC
analysis or isolated yields.
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Acknowledgements
The authors acknowledge financial support from the National
Natural Science Foundation of China (Nos. 20931006,
21072070, 21072071), Program for Changjiang Scholars and
Innovative Research Team in University (No. IRT0953), the
Program for Academic Leader in Wuhan Municipality (No.
201271130441) and the Science Foundation of Central China
Normal University for financial support.
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10316 | Dalton Trans., 2012, 41, 10309–10316
This journal is © The Royal Society of Chemistry 2012