A convenient one-pot synthesis of a functionalized-arene ruthenium half-sandwich compound [RuCl2(η6-C6H 5OCH2CH2OH)]2

Article abstract of DOI:10.1021/om049044x

Reaction of RuCl₃·xH₂O with 1-methoxy-1,4-cyclohexadiene in an alcohol solvent, ROH, gives [RuCl ₂-(η⁶-C₆H₅OR)]₂ (R = Me, Et, or HOCH₂CH₂) in up to 79% yield. The crystal structures of [Ru₂(μ-Cl₃)(η⁶-C ₆H₅-OEt)₂]⁺[BPh₄] ^- and [RuCl₂(η⁶-C₆H ₅OCH₂CH₂OH)]₂ are also reported.

Full text of DOI:10.1021/om049044x

2538
Organometallics 2005, 24, 2538-2541
Notes
A Convenient One-Pot Synthesis of a
Functionalized-Arene Ruthenium Half-Sandwich
Compound [RuCl2(η6-C6H5OCH2CH2OH)]2
Janet Soleimannejad
Department of Chemistry, University of Ilam, Ilam, Iran
Colin White*
Department of Chemistry, The University, Western Bank, Sheffield, U.K., S3 7HF
Received December 4, 2004
Summary: Reaction of RuCl3‚xH2O with 1-methoxy-1,4-
cyclohexadiene in an alcohol solvent, ROH, gives [RuCl2-
(η6-C6H5OR)]2 (R ) Me, Et, or HOCH2CH2) in up to 79%
yield. The crystal structures of [Ru2(µ-Cl3)(η6-C6H5-
OEt)2]+[BPh4]- and [RuCl2(η6-C6H5OCH2CH2OH)]2 are
also reported.
were easily accessible. Further, functionalized-arene
ruthenium complexes are also of interest in their own
right,9 in part because they have been shown to be
superior catalysts to nonfunctionalized-arene cata-
lysts.10 However, access to functionalized-arene ruthe-
nium half-sandwich compounds is not trivial. The usual
method of synthesising this class of compounds is by
dehydrogenation of 1,3- or 1,4-cyclohexadienes using
ethanolic RuCl3.11 This method works well, but for
functionalized-arene complexes this route is rarely an
option because the required dihydroarene derivatives
are usually not available by Birch reduction. An alter-
native route involves displacement of the cyclooctatriene
ligand in Ru(cod)(cot) by an arene under hydrogen,12 but
the limitation of this method is that the cyclooctatriene
precursor can be prepared efficiently only on a small
scale.13 More recently, displacement of the labile naph-
thalene ligand in naphthalene(cyclooctadiene)ruthenium-
(0) by a functionalized-arene has been the method of
choice.1b,9a Unfortunately, access to this naphthalene
complex requires a three-stage synthesis from RuCl3,
and the complex itself is both air-sensitive and ther-
mally unstable. In contrast, the method reported here,
a variation of the original synthetic route, provides
Introduction
Arene ruthenium half-sandwich compounds have
proved to be versatile homogeneous catalysts for a wide
range of reactions including alkene1 and aromatic
hydrogenation,2 asymmetric hydrogen-transfer reduc-
tion of ketones3 and imines,4 Diels-Alder reactions,5
and alkene metathesis.6 They have also been utilized
as reagents in organic synthesis.7 This has stimulated
efforts to prepare supported-arene ruthenium complexes
in order to have easily recyclable catalysts and re-
agents.8 Synthesis of supported complexes would be
facilitated if functionalized-arene ruthenium complexes
* To whom correspondence should be addressed. Tel: 0114 222 9310.
Fax: 0114 2229346. E-mail: Colin.white@sheffield.ac.uk.
(1) (a) Moldes, I.; de la Encarnacion, E.; Ros, J.; Alvarez-Larena,
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J.; Knoch, F.; Wuendisch, M.; Zenneck, U. Chem. Ber. 1997, 130, 123.
(d) Pertici, P.; Vitulli, G.; Bigelli, C.; Lazzaroni, R. J. Organomet. Chem.
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4916.
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Chem. Soc., Perkin Trans. 1 2001, 1500. (b) Davies, D. L.; Fawcett, J.;
Garratt, S. A.; Russell, D. R. Organometallics 2001, 20, 3029. (c)
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Leeuwen, P. W. Chemistry 2001, 7, 1202. (b) Leadbeater, N. E.; Scott,
K. A.; Scott, L. J. J. Org. Chem. 2000, 65, 3231. (c) Bayston, D. J.;
Travers, C. B.; Polywka, M. E. C. Tetrahedron Asym. 1998, 9, 2015.
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(9) (a) Bodes, G.; Heinemann, F. W.; Marconi, G.; Neumann, S.;
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Ghebreyessus, K. Y.; Cook, V. C.; Edwards, A. J.; Wielandt, W.; Wild,
S. B.; Willis, A. C. Organometallics 2002, 21, 1727. (c) Bennett, M. A.;
Edwards, A. J.; Harper, J. R. Khimyak, T.; Willis, A. C. J. Organomet.
Chem. 2001, 629, 7. (d) Miyaki, Y.; Onishi, T.; Kurosawa, H. Inorg.
Chim. Acta 2000, 300-302, 369. (e) Brunner, H.; Valerio, C.; Zabel,
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(10) (a) Cetinkaya, B.; Demir, S.; Ozdemir, I.; Toupet, L.; Semeril.
D.; Bruneau, C.; Dixneuf, P. H. New J. Chem. 2001, 25, 519. (b) Miyaki,
Y.; Onishi, T.; Ogoshi, S.; Kurosawa, H. J. Organomet. Chem. 2000,
616, 135. (c) Simal, F.; Jan, D.; Demonceau, A.; Noels, A. F. Tetra-
hedron Lett. 1999, 40, 1653.
(11) (a) Pertici, P.; Salvadori, P.; Biasci, A.; Vitulli, G.; Bennett, M.
A.; Kane-Maguire, L. A. P. J. Chem. Soc., Dalton Trans. 1988, 315.
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10.1021/om049044x CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/07/2005

Notes
Organometallics, Vol. 24, No. 10, 2005 2539
Scheme 1
Table 1. Crystal Data and Structure Refinement
Details
3
4
Figure 1. Molecular structure of [Ru2(η6-C6H5OCH2CH3)2-
(µ-Cl)3]+ cation, 3 (thermal ellipsoids at 50% probability).
Solvate acetone and hydrogen atoms have been omitted for
clarity.
formula
C
40H40BCl3O2Ru2‚
C3H6O
C
16H20Cl4O4Ru2
formula wt
cryst dimens/mm
wavelength, Å
cryst syst
space group
a, Å
930.10
620.26
0.43 × 0.22 × 0.13 0.14 × 0.12 × 0.08
0.71073
monoclinic
P21/c
0.71073
triclinic
P1h
access to a functionalized-arene ruthenium complex in
one step using hydrated RuCl3.
10.0185(12)
17.180(2)
23.544(3)
90
98.977(3)
90
6.149(3)
9.548(4)
9.604(5)
116.519(11)
108.648(9)
91.492(8)
468.4(4)
1
b, Å
Results and Discussion
c, Å,
R, deg
1-Methoxy-1,4-cyclohexadiene is commercially avail-
able and, as reported previously,11b refluxing this with
hydrated ruthenium trichloride in methanol affords
[RuCl2(η6-C6H5OCH3)]2 (1) in 44% yield. If, however, the
reaction is carried out in ethanol, then [RuCl2(η6-C6H5-
OEt)]2 (2) is obtained as an orange-brown solid in 61%
yield (Scheme). This compound was fully characterized
by elemental analysis and spectroscopic methods; in
particular, the presence of the ethoxy group was evident
in the 1H NMR spectrum from the characteristic quartet
at δ 4.23 and a triplet at δ 1.45 ppm. However, given
the unusual synthetic route, we wished to confirm the
product identity unambiguously by X-ray crystallogra-
phy. Unfortunately, the product was not very soluble
and attempts to grow suitable crystals were unsuccess-
ful, but addition of NaBPh4 to a dilute ethanolic solution
of the product gave a yellow precipitate that readily
crystallized from acetone. Spectroscopic and X-ray14
analyses of these crystals revealed that they were tri-
µ-chlorobis(η6-ethoxybenzene)diruthenium(1+) tetraphe-
nylborate (3) (Figure 1), confirming that alkoxide ex-
change had taken place; clearly the ethoxide had
originated from the reaction solvent, ethanol. This
immediately suggested a route to functionalized-arene
complexes, and we were delighted to find that repeating
the reaction in 1,2-ethanediol at 80 °C gave the corre-
sponding [RuCl2(η6-C6H5OCH2CH2OH)]2 complex 4 in
79% yield (Scheme 1). Elemental analysis and spectros-
copy supported this formulation, which was confirmed
by X-ray crystallography. The crystallographic details
and selected bond distances and angles for both struc-
tures are given in Tables 1 and 2, respectively. An
interesting feature of the structure of 4 is that the
2-hydroxyethoxy substituents lie on opposite sides of the
molecule as far apart as possible (Figure 2); the same
is not true for the ethoxy substituents in 3.
â, deg
γ, deg
V, Å3
4002.7(8)
4
Z
Dc, Mg/m3
abs coeff, mm-1
F(000)
1.543
2.199
0.993
2.203
1888
304
θ range, deg
index ranges
1.47-28.38
-11 e h e 11,
-19 e k e 20,
-26 e l e 27
21 238
2.43-28.37
-4 e h e 7,
-11 e k e 10
-11 e l e 11
2335
no of rflns
no of indep rflns
7008
1622
(R(int) ) 0.0457)
no of data/restraints/ 7008/47/503
params
(R(int) ) 0.0619)
1622/55/118
goodness of fit on F2
final R indices
(I > 2σ(I))
0.976
0.950
R1 ) 0.0443,
wR2 ) 0.1208
1.072 and -0.552
R1 ) 0.0727,
wR2 ) 0.1857
1.358 and -2.053
largest diff peak
and hole, e Å-3
Table 2. Selected Bond Distances (Å) and Angles
(deg) for 3 and 4
3
4
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-C(3)
Ru(1)-C(4)
Ru(1)-C(5)
Ru(1)-C(6)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
Ru(1)-Cl(3)
Cl(2)-Ru(1)-Cl(1) 79.83(5)
Cl(3)-Ru(1)-Cl(1) 79.38(5)
Cl(2)-Ru(1)-Cl(3) 80.66(5)
Ru(1)-Cl(1)-Ru(2) 83.55(4)
Ru(1)-Cl(2)-Ru(2) 84.78(4)
Ru(1)-Cl(3)-Ru(2) 84.36(5).
2.254(5)
2.172(5)
2.162(4)
2.167(5)
2.152(5)
2.180(4)
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-C(3)
Ru(1)-C(4)
Ru(1)-C(5)
Ru(1)-C(6)
2.171(13)
2.136(12);
2.155 (12)
2.144(14)
2.159(14)
2.199(12)
2.390(4)
2.4512 (13) Ru(1)-Cl(1)
2.4095(13) Ru(1)-Cl(2)
2.4328(14) Ru(1)-Cl(2A)
2.418(3)
2.423(3)
Cl(1)-Ru(1)-Cl(2)
86.85(13)
Cl(1)-Ru(1)-Cl(2A) 87.44(12)
Cl(2)-Ru(1)-Cl(2A) 81.50(12)
Ru(1)-Cl(2)-Ru(1A) 98.50(12)
We were obviously intrigued by the mechanism of
these unexpected reactions. It was readily shown that
the coordinated arene does not undergo alkoxy ex-
(14) Sheldrick, G. M. SHELXL-97, Program for crystal structure
refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

2540 Organometallics, Vol. 24, No. 10, 2005
Notes
above, but in this case the attacking nucleophile will
be H- rather than OR-. This suggests that the reaction
could be modified using other nucleophiles to enable
other functionalized-arene complexes to be synthesized.
Unfortunately, however, this reaction does not seem to
be as general as we would have hoped in that we have
failed to isolate any products using HOCH2CHdCH2,
HOCH2CH2NH2, or HOCH(Me)CO2Et as the alcohol.
We presume that in these cases the additional func-
tionality interferes with complexation of the diene to
ruthenium. Surprisingly, replacing HOCH2CH2OH with
HOCH2CH2CH2OH results in a dramatic reduction in
the yield of product. The reaction does, however, proceed
well with HOCH2CH(Et)OH to give a mixture of prod-
ucts resulting from reaction predominantly, but not
exclusively, at the primary alcohol site. Despite these
restrictions, others have shown that the functionality
in the side chain of arene-ruthenium complexes can be
readily modified.9a,b,d,16 Thus, the synthesis reported
here provides a significant breakthrough for the ready
synthesis of functionalized-arene ruthenium half-sand-
wich compounds.
Figure 2. Molecular structure of [Ru(η6-C6H5OCH2CH2-
OH)Cl2]2, 4 (thermal ellipsoids at 50% probability). Hydro-
gen atoms have been omitted for clarity.
change. Thus, heating [RuCl2((η6-C6H5OCH3)]2 under
reflux in ethanol for 9 h did not lead to the formation of
[RuCl2((η6-C6H5OCH2CH3)]2; similarly, heating [RuCl2-
((η6-C6H5OCH2CH2OH)]2 in methanol under reflux for
9 h did not lead to the formation of [RuCl2((η6-C6H5-
OCH3)]2, and conversely heating [RuCl2((η6-C6H5OCH3)]2
in 1,2-ethanediol at 80 °C for 9 h did not lead to [RuCl2-
((η6-C6H5OCH2CH2OH)]2. In all cases the original arene
complex was recovered unchanged, as confirmed by 1H
NMR spectroscopy. We therefore suspected a ruthenium-
catalyzed reaction but found that ruthenium is not
necessary for alkoxy exchange to take place. This was
shown by heating 1-methoxy-1,4-cyclohexadiene in 1,2-
ethanediol at 80 °C for 9 h and analyzing the mixture
by GC-MS. 2-Hydroxyethoxycyclohexadiene was found
to be a major component of the reaction mixture; control
experiments showed that this was absent in the original
1-methoxy-1,4-cyclohexadiene. A similar experiment
with RuCl3 present resulted in a much more complex
mixture. Again 2-hydroxyethoxycyclohexadiene was
present, together with disproportionation products, i.e.,
arenes and cyclohexenes and some oligomers. It is
significant that there are many reports in the literature
of the synthesis of ruthenium arene complexes from
simple dienes that are carried out in alcoholic solvents,
and in no case has it been reported that alkoxy-arene
complexes are formed. This clearly indicates that the
exchange is promoted by the methoxy group in the
diene. It is known that enol ethers readily undergo acid-
catalyzed alkoxy exchange,15 and in the syntheses
reported here the source of the proton could be hydrated
ruthenium trichloride; further, as the experiments
carried out in the absence of ruthenium show, the
alcohol solvent can also be a proton source.
Experimental Section
All reactions were carried out under nitrogen using Schlenk
techniques, but the workup was performed in air. All reagents
and solvents were obtained commercially and were degassed
but used without further purification. NMR spectra were
recorded with a Bruker AM250 spectrometer operating at
250.13 MHz (1H) or at 62.90 MHz (13C) using the 2D-lock signal
as an internal reference. Mass spectra were recorded using a
Fisons VG Prospec 8 spectrometer. Elemental analyses were
performed by the Microanalytical Services of the Chemistry
Department, University of Sheffield.
Synthesis of [Ru(η6-C6H5OCH2CH3)Cl2]2, 2 A solution of
ruthenium trichloride trihydrate (1.0 g, 3.82 mmol) and
1-methoxy-1,4-cyclohexadiene (2.7 mL, 23 mmol) in ethanol
(50 mL) was heated under reflux for 8 h. On standing
overnight at room temperature, a dark green precipitate
separated out. This was filtered off, and the product was
extracted from the residue with chloroform using a Soxhlet
extractor. The chloroform was removed in vacuo and the
residue washed with diethyl ether (3 × 20 mL) to leave a bright
orange solid. Additional product was also obtained by reducing
the volume of the original ethanol filtrate to approximately
half and refrigerating overnight (total 0.69 g, 61%). Anal. Calcd
for C16H20Cl4O2Ru2: C, 32.7; H, 3.4; Cl, 24.1. Found: C, 32.1;
1
H, 3.4; Cl, 24.1. H NMR (d6-DMSO): δ 1.45 (t, 6 H, Me, J )
6.7 Hz), 4.23 (q, 4 H, q, CH2), 5.46 (t, 2 H, Ph, J ) 5.3 Hz),
5.55 (d, 4 H, Ph, J ) 6.4 Hz), 6.36 (t, 4 H, Ph, J ) 5.9 Hz). 13
C
NMR (d6-DMSO): δ 14.6 (2 C, Me), 65.8 (4 C, Ph), 66.4 (2 C,
-OCH2), 74.7 (2 C, Ph), 94.6 (4 C, Ph), 140.5 (2 C, OPh). m/z
(FAB+): 555 ([M+] - Cl, 100%).
Synthesis of [Ru2(η6-C6H5OCH2CH3)2(µ-Cl)3][BPh4], 3.
NaBPh4 (0.23 g, 0.67 mmol) was added to [RuCl2(η6-C6H5-
OC2H5)]2 (0.20 g, 0.33 mmol) dissolved in ethanol (25 mL) and
the resulting mixture stirred overnight. The solvent was
evaporated off, and the residual solid was recrystallized from
acetone to afford the title compound as yellow crystals (0.23
g, 80%). Anal. Calcd for C40H40BCl3O2Ru2‚(CH3)2CO: C, 55.5;
H, 5.0; Cl, 11.4. Found: C, 55.4; H, 4.9; Cl, 11.6. 1H NMR (d6-
acetone): δ 1.45 (t, 6 H, OCH2CH3, J ) 7.0 Hz), 4.28 (q, 4 H,
OCH2CH3), 5.54 (t, 2 H, PhOEt, J ) 5.4 Hz), 5.62 (d, 4 H,
PhOEt, J ) 6.4 Hz), 6.15 (4 H, t, PhOEt, J ) 5.9 Hz), 6.75 (m,
4 H, BPh4), 6.92 (m, 8 H, BPh4), 7.32 (m, 8 H, BPh4). 13C NMR
(d6-acetone): δ 14.6 (2 C, Me), 61.8 (4 C, PhOEt), 67.7 (2 C,
In the synthesis of 1, 2, and 4 a minor byproduct,
formed in up to 5% yield, was [RuCl2(η6-C6H6)]2 (5).11b
Ruthenium hydrides are clearly formed in the process
of dehydrogenating the 1-alkoxy-1,4-cyclohexadiene to
an arene. Thus, 1,4-cyclohexadiene, the precursor to the
benzene ligand in 5, could be formed by a similar acid-
catalyzed mechanism to the alkoxy exchange discussed
(15) Effenberger, F. Angew. Chem., Int. Ed. Engl. 1969, 8, 295.
(16) Pearson, A. J.; Hwang, J. J. J. Org. Chem. 2000, 65, 3466.

Notes
Organometallics, Vol. 24, No. 10, 2005 2541
OCH2), 71.8 (2 C, PhOEt), 85.2 (4 C, PhOEt), 122.3, 126.1,
137.0 (22 C, OPh, BPh4), 206.3 (4 C, BPh4). m/z (FAB+): 555
([M+] - BPh4, 100%).
a Bruker SMART1000 CCD area detector. Empirical absorp-
tion corrections were employed. The structures were solved
by direct methods (SHELXL-97)14 and refined using least-
squares methods on F2. A summary of the crystallographic
data is given in Table 1. In 3 the atoms of the sidearm, C(15)
and C(16), were found to be disordered and refined to an
occupancy of 60%-40%. The oxygen atom of the solvent
molecule, O(3), also appeared to be disordered and placed to
an occupancy of 52%-48%. Additional data on the collection
of the data and the refinement of the structures are available
in the Supporting Information.
Synthesis of [Ru(η6-C6H5OCH2CH2OH)Cl2]2, 4. A mix-
ture of ruthenium trichloride trihydrate (1.0 g, 3.82 mmol) and
1-methoxy-1,4-cyclohexadiene (4.5 mL, 38.4 mmol) in 1,2-
ethanediol (15 mL) was heated to 80 °C for 3 h. On standing
overnight at room temperature, a red-orange precipitate
separated out. This was filtered off, washed with methanol,
and dried in vacuo (0.78 g, 66%). Additional product was
obtained by evaporating under reduced pressure the original
filtrate to approximately a third of the volume (total 0.93 g,
79%). Anal. Calcd for C16H20Cl4O4Ru2: C, 31.0; H, 3.2; Cl, 22.9.
1
Acknowledgment. We are grateful to Johnson
Matthey for the generous loan of ruthenium salts (to
C.W.).
Found: C, 31.0; H, 3.1; Cl, 22.5. H NMR (d6-DMSO): δ 3.72
(t, 4 H, CH2, J ) 5.0 Hz), 4.21 (t, 4 H, CH2), 5.37 (t, 2 H, Ph,
J ) 5.3 Hz), 5.56 (d, 4 H, Ph, J ) 6.4 Hz), 6.15 (t, 4 H, Ph, J
) 5.8 Hz). 13C (d6-DMSO): δ 59.4 (2 C, CH2), 65.9 (4 C, Ph),
72.0 (2 C, CH2), 74.8 (2 C, Ph), 94.5 (4 C, Ph), 163.5 (2 C, PhO).
m/z (ES+): 275 ([M+]/2 - Cl, 38%), 239 ([M+]/2 - 2Cl, 100%).
Crystallographic Analysis of [Ru2(η6-C6H5OCH2CH3)2-
(µ-Cl)3][BPh4], 3, and [Ru(η6-C6H5OCH2CH2OH)Cl2]2, 4.
Crystals of 3 and 4 suitable for X-ray diffraction were grown
from acetone. These were mounted on a glass fiber with epoxy
resin. Data for both structures were collected at 150(2) K using
Supporting Information Available: Tables containing
complete crystal and data collection parameters and positional
parameters for 3 and 4. Crystal data are also available as CIF
files. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM049044X