New insight into a convenient base-promoted synthesis of Ru3(CO)12

Article abstract of DOI:10.1039/b303884j

The addition of two equivalents of KOH per Ru under 1 atm CO at 75°C to a mixture of [Ru(CO)₂Cl₂]_n and [Ru-(CO)₃Cl₂]₂ generated in situ by carbonylation of 5 grams of RuCl3·3H₂O in 2-ethoxyethanol, triggers a reaction cascade producing Ru₃(CO)₁₂ in yields exceeding 90% within 45 minutes.

Full text of DOI:10.1039/b303884j

New insight into a convenient base-promoted synthesis of Ru (CO) †
3
12
Matthieu Fauré, Catherine Saccavini and Guy Lavigne*
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne 31077 Toulouse Cedex, France.
E-mail: lavigne@lcc-toulouse.fr; Fax: +33 5 61 55 30 03; Tel: +33 5 61 33 31 71
Received (in Cambridge, UK) 8th April 2003, Accepted 15th May 2003
First published as an Advance Article on the web 3rd June 2003
The addition of two equivalents of KOH per Ru under 1 atm
Major modifications are found in the second step: the
reaction cascade shown in Scheme 1 is triggered by direct
addition of two equivalents of KOH pellets to the above solution
kept at a working temperature strictly maintained at 75 °C under
CO bubbling.
2 2 n
CO at 75 °C to a mixture of [Ru(CO) Cl ] and [Ru-
(
CO)
3
Cl
3
2
]
2
2
generated in situ by carbonylation of 5 grams of
RuCl
·3H
O in 2-ethoxyethanol, triggers a reaction cascade
producing Ru
minutes.
3
(CO)12 in yields exceeding 90% within 45
During the past 30 years, zinc has been used as a common
reducing agent in one of the most widely developed low
1
3
pressure synthetic routes to Ru (CO)12.
2
Thus, our report, in 1999, that the same reaction could be
performed much more rapidly and efficiently with KOH as the
only “reducing” agent represented a significant conceptual
3
advance. The reaction was rationalized in terms of a reductive
elimination of HCl from an elusive hydrido complex produced
2
via de-carboxylation of the initial hydroxy-carbonyl adduct.
Independently, interesting reports by Roberto and co-
workers,4 directly inspired by surface-mediated reactions,5
indicated that a related reduction, albeit conducted on very
small quantities of salt and requiring longer reaction times,
could be obtained in ethylene glycol in the presence of alkali
carbonates.
2
In our original procedure, the need to use a very fast CO
stream for efficiently releasing HCl was neither fully sat-
isfactory from an ecological point of view, nor easy to obtain
experimentally, thus sometimes requiring slightly more than
6
one equivalent of base.
7
Later on, it was found that under specific mild conditions,
the incipient hydroxy-carbonyl adduct is spontaneously con-
verted into a polymeric Ru( ) poly-anion whose CO-induced
I
disproportionation ultimately produces equimolar amounts of
Ru(II) and Ru(0) at 25 °C. Clearly, the latter observation was
Scheme 1 Proposed mechanistic pathway for the production of Ru
at 75 °C in the presence of hydroxide ions.
3
(CO)12
7
revealing the existence of a particularly low activation energy
pathway, possibly exploitable for producing Ru
3
(CO)12. This
We enter the cycle via instantaneous generation of a
2
led us to develop the present modified synthesis, working within
a lower temperature range than the previous one, and scaled up
spectroscopically detectable 16e hydroxyl-carbonyl adduct
2
[Ru(CO)
form [Ru(CO)
which rapidly de-carboxylates to give the previously isolated
2
Cl
2
{C(O)OH}] (3) in equilibrium with a dimeric
22
10
to convert 5 g of RuCl
3
·3H
2
O into Ru
3
(CO)12 in 90% yield
2
Cl
2
{C(O)OH}]
2
(4) of unknown structure,
within less than 4 hours. As shown below, this new preparation
follows a clear mechanistic pathway which can be fully
rationalized in terms of elementary equations.
(I)
2
n
polymeric Ru polyanion {[Ru
2
(CO)
5 3
Cl ] } (5) according to
the global eqn. 2. Significantly, under such conditions, only one
,11
The first step of this one-pot synthesis involves minor
mole of CO
2
per dimeric Ru unit is produced.7
modifications of the “classical” reduction of RuCl
2
3 2
·3H O in
-ethoxyethanol under CO (1 atm) (eqn. 1).8
(2)
(
I)
Then, a CO-induced disproportionation of the incipient Ru
(1)
(II)
species (eqn. 3) gives equimolar amounts of Ru , obtained
exclusively under the form of the soluble anionic complex
In our new procedure requiring no specific precaution against
2
(0)
[
Ru(CO)
3
Cl
3
]
(6) (detected by IR monitoring) and Ru ,
(CO)12, which readily precipitates.
moisture, the incidental formation of undesirable aqua mono-
carbonyl Ru species is systematically avoided in a simple way
by performing the initial carbonylation step at moderate
recovered as Ru
3
9
temperature (80 °C) during the first hour, giving a dark blood
red color that turns yellow within the next 45 minutes after the
temperature is subsequently brought to reflux (135 °C).
(3)
†
Electronic supplementary information (ESI) available: full details of the
The disproportionation occurring here is reminiscent of that
observed by Fachinetti and co-workers for closely related Ru(
synthesis. See http://www.rsc.org/suppdata/cc/b3/b303884j/
I)
1
578
CHEM. COMMUN., 2003, 1578–1579
This journal is © The Royal Society of Chemistry 2003

2
complexes [Ru
an excess of trifluoroacetate.
The anionic Ru(II) complex [Ru(CO)
2
X
2
(CO)
6
] (X = CF
3
COO ) in the presence of
obviously requiring a second equivalent of base to recycle
12
(II)
Ru
.
]2 (6) appearing in
While the performances of the present procedure are
matching those of our original synthetic route in terms of
3
Cl
3
2
eqn. 3 is automatically recycled upon reaction with the second
equivalent of hydroxide (Scheme 1), thus entering a new
reduction–disproportionation cycle until total consumption of
the base. The most spectacular point is that after reaction
completion, the final solution is limpid and almost colorless,
containing only KCl, whereas crystals of Ru
deposited at the bottom of the flask.
rapidity (both methods are much faster than high pressure
15
methods ), this new synthetic strategy is more reliable and
more efficient in practice than the previous one due to its
technical simplicity and also to the fact that it works cleanly
under unprecedented mild conditions, at which only a limited
number of intermediate species are involved.
3
(CO)12 are
In principle, the presence of an excess of hydroxide ions
might be problematic if we consider that Ru (CO)12 is also
susceptible to being attacked according to eqn. 4.13
Successful attempts to extend the present observations to the
reduction of carbonylchlororuthenium(II) complexes in the
presence of ancillary ligands other than CO are underway.
3
Ru
3
(CO)12 + OH2 ? [Ru
3
(m-H)(CO)11]2 + CO
2
(4)
Notes and references
2
In reality, OH ions undergo highly preferential nucleophilic
1
A. Mantovani and S. Cenini, Inorg. Synth., 1975, 16, 47.
attack onto the electrophilic carbonyls of K[Ru(CO)
in the presence of Ru (CO)12 below a critical temperature of 80
C (such a net discrimination is also favoured by the insolubility
3
Cl
3
] even
2 M. Faure, L. Maurette, B. Donnadieu and G. Lavigne, Angew. Chem.,
Int. Ed., 1999, 38, 518.
3 A. F. Hill, Angew. Chem., Int. Ed., 2000, 39, 130.
3
°
4
(a) C. Roveda, E. Cariati, E. Lucenti and D. Roberto, J. Organomet.
Chem., 1999, 580, 117; (b) D. Lucenti, E. Cariati, C. Dragonetti and D.
Roberto, J. Organomet. Chem., 2003, 669, 44.
3
of Ru (CO)12 in 2-ethoxyethanol), thus allowing the reaction to
2
proceed cleanly to completion. Of course, any excess of OH
(
II)
that would be still present after total consumption of Ru is
susceptible to re-solubilize Ru (CO)12 to produce the anionic
(characteristic violet colour). By
chance, even in cases where this incidentally happens, recovery
5
6
D. Roberto, E. Cariati, E. Lucenti, M. Respini and R. Ugo, Organome-
tallics, 1997, 16, 4531.
A similar observation about the difficulty to release HCl gas completely
3
2
complex [Ru
3
(m-H)(CO)11
]
4b
was made by Roberto et al.
7 L. Maurette, B. Donnadieu and G. Lavigne, Angew. Chem., Int. Ed.,
1999, 38, 3707.
.
2
of Ru
3
(CO)12 is still possible. Indeed, [Ru
3
(m-H)(CO)11
]
is a
hydride transfer agent whose reaction with water in the presence
of CO leads back to Ru
3
(CO)12 according to the water gas shift
8 (a) M. I. Bruce and F. G. A. Stone, J. Chem. Soc., A, 1967, 1238; (b) N.
Lugan, G. Lavigne, J.-M. Soulié, S. Fabre, P. Kalck, J. Y. Saillard and
J. F. Halet, Organometallics, 1995, 14, 1712.
may
appear if the initial 2-ethoxyethanol solution of RuCl ·3H O is brought
3 2
directly to reflux. When the initial carbonylation is carried out under
mild conditions, the incipient mono-carbonyl intermediate reacts much
faster with CO than with water, then irreversibly producing a di-
carbonyl (b) J. Halpern, B. R. James and L. W. Kemp, J. Am. Chem.
Soc., 1966, 88, 5142.
reaction (eqn. 5).14
9 (a) An intractable green aqua complex reported by Halpern9b
(m-H)(CO)11]2 + H
+ OH2
3 2
O + CO ? Ru (CO)12 + H
[Ru
3
2
(5)
As shown in Scheme 1, water is inevitably present in the
second reaction step since it is produced during the reduction of
Ru(II) to Ru( ) (eqn. 2), thereby allowing the above reaction
eqn. 5) to take place, albeit at slower rate. Effectively, dark
I
(
10 (a) The intermolecular association may occur either via halide bridges
or via hydroxy-carbonyl bridges, as suggested here. Related methoxy-
carbonyl bridged complexes have been reported1 (b) G. Süss-Fink, J.
M. Soulié, G. Rheinwald, H. Stoeckli-Evans and I. Sasaki, Organome-
tallics, 1996, 15, 3416.
violet solutions incidentally obtained at the end of the above
0b
preparation due to an excess of base (and reflecting the presence
2
of [Ru
3
(m-H)(CO)11] ) were seen to become clearer upon
overnight treatment with CO at 25 °C, with concomitant
recovery of Ru (CO)12
In conclusion, and in light of previous reports,
1
2
1 Loss of the first CO from the bi-metallic bis-hydroxycarbonyl adduct,
3
.
is followed by reductive elimination of HCl, the latter being instanta-
2,4,7
there is
2
neously neutralized intramolecularly by the OH group of the second
evidence to suggest that two mechanistic pathways are
operative in the base-promoted reduction of carbonyl chloro-
ruthenium(II) complexes, depending on the reaction conditions.
If the thermally induced de-carboxylation of a mono-nuclear
hydroxy-carbonyl intermediate is made to occur at relatively
high temperature ( > 85 °C) and with one equivalent of base,
hydroxy-carbonyl ligand.
12 G. Fachinetti, T. Funaioli, L. Lecci and F. Marchetti, Inorg. Chem.,
1996, 35, 7217.
3 C. R. Eady, P. F. Jackson, B. F. G. Johnson, J. Lewis, M. C. Malatesta,
M. McPartlin and W. J. H. Nelson, J. Chem. Soc., Dalton Trans., 1980,
1
3
83.
1
4 (a) J. C. Bricker, C. C. Nagel, A. A. Bhattacharyya and S. G. Shore, J.
Am. Chem. Soc., 1985, 107, 377; (b) M. W. Payne, D. L. Leussing and
S. G. Shore, J. Am. Chem. Soc., 1987, 109, 617; (c) M. W. Payne, D. L.
Leussing and S. G. Shore, Organometallics, 1991, 10, 574.
(0)
further reductive elimination of HCl leads directly to Ru , as
2
experimentally established earlier. By contrast, at moderate
temperature, dimerization of the hydroxy-carbonyl intermediate
takes place prior to the de-carboxylation step, and then follows
the reduction–disproportionation sequence described here,
15 M. I. Bruce, C. M. Jensen and N. L. Jones, Inorg. Synth., 1989, 26, 259
and references therein.
CHEM. COMMUN., 2003, 1578–1579
1579