A convenient synthetic route to [CpRu(CH3CN3)]PF6

Article abstract of DOI:10.1021/om020143p

A convenient synthetic route to [CpRu(CH₃CN)₃]PF₆ was presented. A new practical protocol that avoids the stoichiometric use of either thallium or silver salts for the synthesis of the given compound was described. It was shown that the introduction of the cyclopentadienyl ligand via ethanolic reduction of [(arene)RuCl₂]₂ in the presence of cyclopentadiene is a simple and convenient entry to cyclopentadienylruthenium complexes.

Full text of DOI:10.1021/om020143p

2544
Organometallics 2002, 21, 2544-2546
A Con ven ien t Syn th etic Rou te to [Cp Ru (CH3CN)3]P F 6
Barry M. Trost* and Christina M. Older
Department of Chemistry, Stanford University, Stanford, California 94305-5080
Received February 20, 2002
Sch em e 1
Summary: A new practical protocol that avoids the
stoichiometric use of either thallium or silver salts for
the synthesis of the title compound is described.
In tr od u ction
The chemistry of cyclopentadienylruthenium com-
plexes has historically been based on the readily avail-
able CpRu(PPh3)2Cl and CpRu(CO)2Cl precursors. How-
ever, the selective displacement of CO or PPh3 has
proved difficult, limiting the synthetic utility of these
systems. The cationic complex [CpRu(CH3CN)3]PF6 (1)
has currently gained in popularity as an alternative
entry to cyclopentadienylruthenium complexes due to
the substitutional lability of the CH3CN ligands.1 The
high affinity of the CpRu+ fragment for arene rings has
also led to the application of 1 in Ru-promoted nucleo-
philic aromatic substitution reactions2 and in Ru-
labeling of biological compounds.3 The catalytic prop-
erties of [CpRu(CH3CN)3]PF6 for a variety of C-C bond
forming reactions have only recently been examined and
include (a) the dimerization of propargyl alcohols,4 (b)
regioselective alkene-alkyne coupling,5 (c) intramolecu-
lar [5+2] cycloaddition of alkyne-vinylcyclopropanes,6
(d) cycloisomerization of 1,6- and 1,7-enynes,7 and (e)
cyclopropanation of norbornene with propargyl alcohol.8
The synthesis of 1, first reported by Gill and Mann
in 1982,9 is a three-step process starting from readily
available [(C6H6)RuCl2]2 and is shown in Scheme 1.
Although the preparation of cationic 1 is straightfor-
ward, the use of stoichiometric thallium (or silver) to
introduce the cyclopentadienyl moiety to form [(C6H6)-
RuCp]PF6 (2)10a,b (or its tetrafluoroborate salt10c) is
problematic for large-scale reactions. The high toxicity
of thallium and the subsequent disposal of thallium
waste make this procedure unattractive for many
organic chemists. The transmetalation reaction is also
very sensitive to the quality of the thallium cyclopen-
tadienide. In this note we wish to report an extremely
facile and economical synthesis of [(C6H6)RuCp]PF6 via
the ethanolic reduction of [(C6H6)RuCl2]2 in the presence
of excess cyclopentadiene.
Resu lts a n d Discu ssion
Early work by Bennett had demonstrated that (C6-
Me6)Ru(0) diene complexes could be synthesized by
treatment of ethanolic [(C6Me6)RuCl2]2 with sodium
carbonate and excess diene.11 More recently, Stryker has
used similar conditions to prepare [(C6Me6)RuCp]Cl in
good yield (eq 1).12 It appears that, in the case of
cyclopentadiene, the thermodynamic product of the
ethanolic reduction is the Ru(II) cyclopentadienyl com-
plex rather than the Ru(0) diene, suggesting a conve-
nient entry to cyclopentadienylruthenium complexes.
Although this reaction proceeds cleanly from the hex-
amethylbenzene dimer, decomplexation of this electron-
rich arene is difficult.13
(1) Recent review of [CpRu(CH3CN)3]+ chemistry: Slugovc, C.; Ru¨ba,
E.; Schmid, R.; Kirchner, K.; Mereiter, K. Monatsh. Chem. 2000, 131,
1241.
(2) (a) Pigge, F. C.; Fang, S. Tetrahedron Lett. 2001, 42, 17. (b)
Pearson, A. J .; Heo, J .-N. Tetrahedron Lett. 2000, 41, 5991. (c) Pearson,
A. J .; Belmont, P. O. Tetrahedron Lett. 2000, 41, 1671. (d) Pearson, A.
J .; Zhang, P.; Lee, K. J . Org. Chem. 1996, 61, 6581.
(3) (a) J aouen, G.; Vessieres, A.; Buttler I. S. Acc. Chem. Res. 1993,
26, 361. (b) Soine W. H.; Guyer, C. E.; Knapp, F. F. J . Med. Chem.
1984, 27, 803.
(4) Trost, B. M.; Rudd, M. T. J . Am. Chem. Soc. 2001, 123, 8862.
(5) (a) Trost, B. M.; Surivet, J .-P.; Toste, F. D. J . Am. Chem. Soc.
2001, 123, 2897. (b) Trost, B. M.; Pinkerton, A. B.; Toste, F. D.; Sperrle,
M. J . Am. Chem. Soc. 2001, 123, 12504. (c) Trost, B. M.; Brown, R. E.;
Toste, F. D. J . Am. Chem. Soc. 2000, 122, 5877. (d) Trost, B. M.; Toste,
F. D. J . Am. Chem. Soc. 2000, 122, 714.
(6) (a) Trost, B. M.; Toste, F. D.; Shen, H. C. J . Am. Chem. Soc.
2000, 122, 2379. (b) Trost, B. M.; Shen, H. C. Org. Lett. 2000, 2, 2523.
(7) Trost, B. M.; Toste, F. D. J . Am. Chem. Soc. 2000, 122, 714.
(8) Matsushima, Y.; Kikuchi, H.; Uno, M.; Takahashi, S. Bull. Chem.
Soc. J pn. 1999, 72, 2475.
Treatment of [(C6H6)RuCl2]2 under the conditions
reported by Bennett11a affords only a moderate yield
(28%) of the desired [(C6H6)RuCp]PF6 (2) after ion
exchange. (The initial product, [(C6H6)RuCp]Cl, is con-
verted to the hexafluorophosphate salt to aid in isolation
of the complex.) The low yield is attributed to the
formation of significant amounts of ruthenocene as a
byproduct. Initial optimization of the reaction conditions
increased the yield of 2 to 45%, but the amount of Cp2-
(11) (a) Bennett, M. A.; Huang, T.-N.; Matheson, T. W.; Smith, A.
K. Inorg. Synth. 1982, 21, 74. (b) Bennett, M. A.; Matheson, T. W. J .
Organomet. Chem. 1978, 153, C25. (c) Bennett, M. A.; Huang, T.-N.;
Turney, T. W. J . Chem. Soc., Chem. Commun. 1979, 312.
(12) Older, C. M.; Stryker, J . M. J . Am. Chem. Soc. 2000, 122, 2784.
(13) Nolan, S. P.; Marthin, K. L.; Stevens, E. D.; Fagan, P. J .
Organometallics 1992, 11, 3947.
(9) Gill, T. P.; Mann, K. R. Organometallics 1982, 1, 485.
(10) (a) Zelonka, R. A.; Baird, M. C. J . Organomet. Chem. 1972, 44,
383. (b) Zelonka, R. A.; Baird, M. C. J . Can. Chem. 1972, 50, 3063. (c)
Oshima, N.; Suzuki, H.; Morooka, Y. Inorg. Chem. 1986, 25, 3407.
10.1021/om020143p CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/15/2002

Notes
Organometallics, Vol. 21, No. 12, 2002 2545
Ru produced remained at ∼20%, even after short
that multigram scale reactions are feasible. Irradiation
of an acetonitrile solution of 2 prepared by this method
results in its quantitative conversion to [CpRu(CH3CN)3]-
PF6, which is isolated as a spectroscopically pure orange
solid after concentration of the crude reaction mixture.
reaction times (eq 2).
However, switching to the alternative precursor, [(p-
cymene)RuCl2]2, resulted in much better yields of the
cyclopentadienyl complex 3, with no traces of the
ruthenocene byproduct being detected (eq 3).14 Unfor-
tunately this complex, unlike its benzene analogue, was
usually isolated as an oil or an oily solid from this
reaction. Only after purification by chromatography
could it be obtained as a pure crystalline material, a
disadvantage for large-scale reactions. An even bigger
obstacle was encountered when [(p-cymene)RuCp]PF6
was subjected to photolysis in acetonitrile. Although
NMR spectroscopy of the crude material indicated near-
quantitative conversion to the expected [CpRu(CH3CN)3]-
PF6, the oily product refused to crystallize even after
repeated filtration through alumina. Moreover, this
material was quite air-sensitive and rapidly turned
black on the bench. It is probable that residual p-cymene
inhibits the crystallization of the tris(acetonitrile) 1,
which is susceptible to air-oxidation when not in solid
form.
Because of these difficulties with the p-cymene sys-
tem, it was decided to reexamine the reaction conditions
for the parent [(C6H6)RuCl2]2. It was soon found that
the choice of alcohol and base was crucial for this
reaction. Ethanol proved to be the best alcohol; use of
refluxing 2-propanol yielded only a small amount of
[(C6H6)RuCp]PF6, while methanol returned no tractable
product. Of the bases screened, zinc dust yielded results
similar to those for sodium carbonate, while cesium
carbonate resulted in very low conversion. Potassium
carbonate, however, was found to give good yields of 2
with negligible amounts of the ruthenocene byproduct
(2-3%). Lowering the reaction temperature from that
of refluxing ethanol to 60 °C with a concomitant increase
in reaction time afforded a slightly cleaner product in
∼80% yield. The optimized conditions thus consist of
treating [(C6H6)RuCl2]2 with excess cyclopentadiene and
potassium carbonate in anhydrous ethanol at 60 °C for
7 h (eq 4). The reaction mixture is then filtered to
remove excess potassium carbonate, followed by addi-
tion of an aqueous solution of NH4PF6. Removal of the
ethanol solvent results in the precipitation of crude
[(C6H6)RuCp]PF6, which can be further purified by
recrystallization from acetone/diethyl ether if desired.
This reaction has been conducted on a 7.5 g (15 mmol)
scale of [(C6H6)RuCl2]2 with similar results, indicating
Con clu sion
Introduction of the cyclopentadienyl ligand via etha-
nolic reduction of [(arene)RuCl2]2 in the presence of
cyclopentadiene is a simple and convenient entry to
cyclopentadienylruthenium complexes. Since the de-
scribed route avoids the need for purification by chro-
matography at any stage, it is particularly amenable
to large-scale production. This methodology thus rep-
resents a significant improvement over the literature
synthesis by avoiding toxic thallium or expensive silver
reagents while maintaining operational simplicity.
Exp er im en ta l Section
Gen er a l Com m en ts. [(C6H6)RuCl2]2 was prepared from
RuCl3‚H2O and 1,3-cyclohexadiene according to the described
procedure.15 Fresh commercial anhydrous ethanol was used
without further purification and was deoxygenated by bubbling
a stream of dry nitrogen through it for 10 min prior to cannula
transfer to the reaction flask. All other solvents were purified
by standard methods before use.
Syn th esis of [(C6H6)Ru (C5H5)]P F 6 (2). A 100 mL oven-
dried round-bottom flask equipped with a stir-bar was charged
with finely ground potassium carbonate (1.95 g, 14.2 mmol,
6.0 equiv) and the flask flame-dried under vacuum. After
cooling to room temperature, the flask was further charged
with [(C6H6)RuCl2]2 (1.18 g, 2.36 mmol) and a reflux condensor
added. Ethanol (50 mL) was then added, followed by freshly
cracked cyclopentadiene (3.5 mL, 42.4 mmol, 18 equiv). The
resulting heterogeneous brown mixture was then warmed to
60 °C with rapid stirring. After approximately 7 h, the reaction
mixture was cooled to room temperature and filtered through
a plug of Celite, and the Celite rinsed with a further 40 mL of
ethanol. The dark yellow filtrate was concentrated to ∼20 mL,
then an aqueous solution of NH4PF6 (1.6 g, 9.8 mmol, 4.16
equiv, in 16 mL of H2O) was added, resulting in the immediate
formation of a tan precipitate. The remaining ethanol was
removed under reduced pressure and the resulting suspension
cooled for several hours. The mixture was then filtered and
the tan solid dried under vacuum to yield 1.45 g (79%). The
crude product was subsequently dissolved in a minimum of
acetone and diethyl ether added dropwise until precipitate
formation was no longer observed. This mixture was cooled
for several hours before being filtered to afford 1.36 g (74%)
of a white powder. The product is spectroscopically identical
to the known [(η6-C6H6)Ru(C5H5)]PF69 and is pure by 1H NMR
spectroscopy to the limits of detection.
(14) Stephenson, T. A.; Robertson, I. W.; Tocher, D. A. J . Organomet.
Chem. 1982, 228, 171.
(15) Bennett, M. A.; Smith, A. K. J . Chem. Soc., Dalton Trans. 1974,
233.

2546 Organometallics, Vol. 21, No. 12, 2002
Notes
Syn th esis of [Cp Ru (CH3CN)3]P F 6 (1). This reaction is a
modification of the existing literature procedure.9 A solution
of [(C6H6)Ru(C5H5)]PF6 (1.70 g, 4.37 mmol) in acetonitrile (200
mL) was prepared in a 250 mL quartz photolysis reactor unit
(Ace) and the solution deoxygenated by bubbling a stream of
dry nitrogen through it for 30 min. The stirred solution was
then irradiated with the output of a 450 W Ace medium-
pressure Hg lamp for 12 h. Evaporation of the solvent yielded
1.89 g (>99%) of 1 as a free-flowing, bright orange powder,
which is pure by 1H NMR spectroscopy to the limits of
detection.
Ack n ow led gm en t. We thank the U.S. National
Science Foundation for their generous support of our
programs and the Canadian National Science and
Engineering Research Council for a postdoctoral fellow-
ship to C.M.O. for partial support.
OM020143P