Instant base-promoted generation of Roper's-type Ru(0) complexes Ru(CO)2(PR3)3 from a simple carbonylchlororuthenium(II) precursor

Article abstract of DOI:10.1021/ja055066e

An uncommon synergism in the concerted action of OH^- and PR₃ toward the simple Ru(II) complex Ru(CO)₃Cl₂(thf) allows a highly efficient reduction of the metal in ethanol or acetonitrile solution at 0 °C, with selective production of the corresponding Roper's-type Ru(0) complexes Ru(CO)₂(PR₃)₃ in high yields within 10 min. Copyright

Full text of DOI:10.1021/ja055066e

Published on Web 09/30/2005
Instant “Base-Promoted” Generation of Roper’s-type Ru(0) Complexes
Ru(CO)2(PR3)3 from a Simple Carbonylchlororuthenium(II) Precursor
Stephane Sentets, Maria del Carmen Rodriguez Martinez, Laure Vendier, Bruno Donnadieu,
Vincent Huc, Noe¨l Lugan, and Guy Lavigne*
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 4, France
Received July 27, 2005; E-mail: lavigne@lcc-toulouse.fr
Originally discovered by Warren Roper in 1972,1 the complex
Ru(CO)2(PPh3)3 represents the archetype of reactive Ru(0) carbonyl
species. Due to a facile dissociative loss of one phosphine, it rapidly
adds H2, alkynes, olefins, and a variety of other substrates, thus
providing a convenient entry into the chemistry of ruthenium.2,3
Significantly, Hiraki and co-workers4 noted its in situ generation
during the course of a catalytic C-H/olefin coupling (the Murai
Figure 1. Preparation of Ru(CO)2(PPh3)3: Ru(CO)3Cl2(thf)10 (250 mg, 0.75
reaction5) using Ru(H)2(CO)(PR3)3 as the catalyst precursor.
mmol), ethanol11a (5 mL). The reactants are added sequentially (but almost
A limitation to an extensive use of this valuable complex as a
simultaneously over about 2 min) at ca. -5 °C: (a) 0.5 mL of NEt4OH
(1.5 M in methanol); (b) 700 mg of PPh3 predissolved in 5 mL of ether;11b
(c) 1.5 mL of NEt4OH (1.5 M in methanol). Reaction is triggered by step
“c” and accelerated by removal of cold bath. The complex is isolated by
filtration. Of the two isomeric forms of Ru(CO)2(PPh3)3 existing in solution,
the “bis-axial” isomer shown above was identified here for the first time
by X-ray diffraction (see Supporting Information).
synthetic tool has long been the relative complexity of its indirect
multistep preparative procedure.6,7 In 1997, Caulton and co-workers
disclosed a more general convenient preparation based on magne-
sium reduction of the Ru(II) complex Ru(CO)2Cl2(PPh3)2 at 60 °C
over 10 h in the presence of an excess of phosphine.8 A parallel
spectacular achievement was the finding that the key 16 e-
intermediate Ru(CO)2(PR3)2 can even be isolated in the case of
certain strongly basic and bulky phosphines.9
Scheme 1. Proposed Mechanistic Pathway
We are now proposing the “user-friendly” single-step preparation
of Roper’s complex displayed in Figure 1, corresponding to the
simple chemical eq 1, where an efficient reduction of Ru(II) to
Ru(0) is seen to take place much faster than with Mg and at milder
conditions, thus raising intriguing questions of fundamental rel-
evance.
deprotonation of the latter by the second equivalent of OH- (acting
as a Bronsted base)13 to give an unstable dianionic CO2 complex
[NEt4]2[Ru(CO)2{C(O)O}Cl2(PPh3)], [C]2-. Here, the overall oxi-
dation of CO to CO2 corresponds to a reduction of the metal from
Ru(II) to Ru(0). Dissociative loss of CO2 and halide ions from [C]2-
with concomitant capture of the vacant sites by the remaining 2
equiv of phosphine would account for the effective fast production
of the final complex Ru(CO)2(PPh3)3. Let us note, however, that
no free CO2 can be detected by infrared at 2338 cm-1. This might
be due to its immediate capture by the excess of hydroxide, given
that [NEt4]2[CO3] is effectively recovered at the end. Alternatively,
the possible mechanistic implication of a transient carbonic acid
complex cannot be excluded.
The whole reaction reflects an uncommon synergism in the
concerted action of OH- and PR3. Taken independently, none of
these two reactants can achieve the reduction of the metal.
Importantly, two experimental details are crucial to the success
of the reaction. First, selectivity in the production of Ru(CO)2(PPh3)3
is achieved only if the reaction is initiated at 0 °C or below. At
room temperature or above, there is unfavorable competition with
a simple CO substitution leading to the well-known disubstituted
derivative Ru(CO)2Cl2(PPh3)2 (i.e., the starting Ru(II) complex in
Caulton’s method8) as the major contaminant. Second, the order
of addition of the reactants is important. Indeed, in a preliminary
separate stepwise experiment, where Ru(CO)3Cl2(solv) was first
treated with 2 equiv of NEt4OH at 25 °C for 5 min prior to the
addition of the phosphine, the initial adduct [A]- was seen to add
Ru(CO)3Cl2(solv) + 4 NEt4OH + 3 PR3 f
Ru(CO)2(PR3)3 + [NEt4]2CO3 + 2 NEt4Cl + 2 H2O (1)
A preliminary ligand screening indicated that, with the exception
of the bulky PCy3 (only Ru(CO)3(PCy3)2 is obtained in that case),
the method applies to a broad range of phosphines, exemplified
here by PPh3, PPh2Me, PnBu3, or to desired combinations of
different phosphines (vide infra). Solvents, such as ethanol11 or
acetonitrile, from which the incipient neutral complex readily
precipitates, represent the best choice because, in most cases, they
ensure both separation and protection of this highly reactive species.
In a few cases, where the final complex is obtained as an oil, as
for Ru(CO)2(PnBu3)3, it can be readily obtained by a tandem
synthesis/extraction procedure from a biphasic methanol/hexane
mixture.
The three elementary steps of the reaction can be reasonably
understood in terms of the proposed sequence shown in Scheme 1,
reminiscent of an earlier report by Tanaka12 that the reaction of
[Ru(bpy)2(CO)2][PF6]2 with 2 equiv of [NBu4]OH gives an isolable
η1-CO2 complex, [Ru(bpy)2(CO)(CO2)].
This involves (a) instantaneous formation of a hydroxyl-
carbonyl adduct [NEt4][Ru(CO)2{C(O)OH}Cl2(solv)], [A]-,10b by
attachment of OH- to the electrophilic carbon of a carbonyl ligand
of Ru(CO)3Cl2(thf); (b) displacement of the coordinated solvent
by one molecule of the incoming phosphine, with formation of the
new adduct [NEt4][Ru(CO)2{C(O)OH}Cl2(PPh3)], [B]-; (c) simple
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C O M M U N I C A T I O N S
a second equivalent of hydroxide to give a saturated species
(tentatively formulated as [NEt4]2[Ru(CO)2{C(O)OH}Cl2(OH)],
[D]2-), which then appeared to be totally reluctant to react further
with the phosphine, even after hours!
many variations in the design of new congeners of Roper’s complex
with sophisticated ligands and their application to a number of
catalytic reactions,5,19 where undefined mixtures of Ru3(CO)12 and
phosphines are still presently used as catalyst precursors.
So, clearly, the success of the synthesis rests on the privileged
generation of the key initial adduct [B]- requiring selective uptake
of one hydroxide and one phosphine in the early stage of the
reaction. Fortunately, such a selection is properly achieved at low
temperature. Then, as an acid/base reaction, the subsequent reduc-
tive process, initiated by addition of the excess of OH-, will be
also kinetically preferred over any other transformation susceptible
to occur in the coordination sphere of the metal.
Acknowledgment. Research supported by the CNRS and
through a European Community Marie Curie Action (Contract No.
HPMT-CT-2001-00398). The authors are grateful to Johnson
Matthey for a generous gift of RuCl3, 3H2O.
Supporting Information Available: Experimental details for the
preparation of the reported compounds and crystallographic data for
Ru(CO)2(PPh3)3 Ru(CO)2(PPh2Me)3, Ru(CO)2(PPh3)(Ph2PN(Me)CH2-
CH2(Me)NPPh2), and Ru(H)(CCSiMe3)(CO)2(PPh3)2. This material is
available free of charge via the Internet at http://pubs.acs.org.
Taking advantage of the stepwise ligand uptake occurring in the
whole sequence, the controlled sequential incorporation of two
different ligands, such as a monophosphine L1 and a diphosphine
L2, is achieved with a good selectivity, as illustrated by the synthesis
of Ru(CO)2(PPh3)(Ph2PN(Me)CH2CH2(Me)NPPh2) (65% yield)
where L1 ) PPh3 and L2 ) Ph2PN(Me)CH2CH2(Me)NPPh2.
To date, attempts to fully separate and characterize the short-
lived reactive intermediates shown in Scheme 1 remained unsuc-
cessful. However, by reacting sequentially Ru(CO)3Cl2(thf) with 1
equiv of NEt4OH and 1 equiv of PPh3 in methanol, we precipitated
a crude solid whose electrospray mass spectrum showed a parent
ion multiplet centered at m/z ) 557, consistent with [Ru(CO)2-
{C(O)O(Na)}Cl2(PPh3)]-, [C2- + Na+]-, an association reflecting
a weakly bonding interaction between the η1-CO2 ligand of C2-
(the deprotonated form of [B]-) and a sodium cation, inevitably
present in the spectrometer (see Supporting Information).
At first sight, the present synthetic procedure might appear as
somewhat reminiscent of our recently reported synthesis of Ru3-
(CO)12.18 Yet, attempts to replace the phosphine by a CO stream
revealed that there is no way to prepare Ru3(CO)12 under the mild
conditions used in the present experiment, thereby supporting our
idea that the two reactions proceed via distinct mechanistic
pathways. As previously demonstrated, the synthesis of Ru3(CO)12
at 75 °C involves the formation of a trappable polymeric Ru(I)
complex, which undergoes CO-induced disproportionation to Ru-
(0) (isolated as Ru3(CO)12) and Ru(II) (identified as [Ru(CO)3Cl3]-),
the latter being automatically recycled upon reaction with a second
equivalent of hydroxide. By contrast, the novelty of the reaction
reported here rests on the unprecedented observation of an
apparently direct reduction of Ru(II) to Ru(0) taking place with
high efficiency and at unprecedented mild conditions.
References
(1) Cavit, B. E.; Grundy, K. R.; Roper, W. R. J. Chem. Soc., Chem. Commun.
1972, 60.
(2) (a) Clark, G. R.; Hoskins, S. V.; Jones, T. C.; Roper, W. R. J. Chem.
Soc., Chem. Commun. 1983, 719. (b) Roper, W. R. J. Organomet. Chem.
1986, 300, 167. (c) Bohle, D. S.; Roper, W. R. Organometallics 1986, 5,
1607.
(3) (a) Gaffney, T. R.; Ibers, J. A. Inorg. Chem. 1982, 21, 2851. (b) Alcock,
N. W.; Hill, A. F.; Melling, R. P.; Thompsett, A. R. Organometallics
1993, 12, 641. (c) Dewhurst, R. D.; Hill, A. F.; Smith, M. K. Angew.
Chem., Int. Ed. 2004, 43, 476. (d) Hill, A. F.; Rae, A. D.; Schultz, M.;
Willis, A. C. Organometallics 2004, 23, 81. (e) Hill, A. F.; Schultz, M.;
Willis, A. C. Organometallics 2004, 23, 5729.
(4) Hiraki, K.; Kira, S.-I.; Kawano, H. Bull. Chem. Soc. Jpn. 1997, 70, 1583.
(5) (a) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826. (b) Murai, S.;
Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani,
N. Pure Appl. Chem. 1994, 66, 1527. (c) Kakiuchi, F.; Chatani, N. AdV.
Synth. Catal. 2003, 345, 1077 and references therein.
(6) Clark, G. R.; Hoskins, S. V.; Jones, T. C.; Roper, W. R. J. Chem. Soc.,
Chem. Commun. 1983, 719.
7b,c
(7) (a) Alternative routes to Ru(CO)2L3 complexes from Ru3(CO)12
via
Ru(CO)3L2 generally require forcing conditions. (b) Siegl, W. O.; Lapporte,
S. J.; Collman, J. P. Inorg. Chem. 1973, 12, 674. (c) Jaunky, P.; Schmalle,
H. W.; Blacque, O.; Fox, T.; Berke, H. J. Organomet. Chem. 2004, 689,
801.
(8) Ogasawara, M.; Maseras, F.; Gallego-Planas, N.; Kawamura, K.; Ito, K.;
Toyota, K.; Streib, W. E.; Komiya, S.; Eisenstein, O.; Caulton, K. G.
Organometallics 1997, 16, 1979.
(9) (a) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.; Eisenstein,
O.; Caulton, K. G. J. Am. Chem. Soc. 1995, 117, 8869. (b) Ogasawara,
M.; Macgregor, S. A.; Streib, W. E.; Folting, K.; Eisenstein, O.; Caulton,
K. G. J. Am. Chem. Soc. 1996, 118, 10189. (c) Li, C.; Ogasawara, M.;
Nolan, S. P.; Caulton, K. G. Organometallics 1996, 15, 4900. (d) Li, C.;
Olivan, M.; Nolan, S. P.; Caulton, K. G. Organometallics 1997, 16, 4223.
(10) The original procedure for Ru(CO)3Cl2(thf)10a has been optimized10b: (a)
Bruce, M. I.; Stone, F. G. A. J. Chem. Soc., A 1967, 1238. (b) Faure, M.;
Maurette, L.; Donnadieu, B.; Lavigne, G. Angew. Chem., Int. Ed. 1999,
38, 518.
(11) (a) Though the role of ethanol as a reducing agent might have been
considered, the reaction can be alternatively performed in acetonitrile,
with about the same efficiency; (b) the phosphine needs to be presolu-
bilized in ether because the reaction proceeds much faster than the rate
of phosphine solubilization in ethanol.
(12) (a) Tanaka, H.; Nagao, H.; Peng, S. M.; Tanaka, K. Organometallics 1992,
11, 1450. (b) Ishida, H.; Tanaka, K.; Moritomo, M.; Tanaka, K.
Organometallics 1986, 5, 724.
(13) We have verified that tBuOK can be used as an alternate deprotonating
agent in the second reaction step.
(14) (a) Faure, M.; Saccavini, C.; Lavigne, G. Chem. Commun. 2003, 1578.
(b) Faure, M.; Saccavini, C.; Lavigne, G. Inorg. Synth. 2004, 34, 110.
(15) Morimoto, T.; Kakiuchi, K. Angew. Chem., Int. Ed. 2004, 43, 5580.
Of numerous “minute” reactions which can be taken to illustrate
the high reactivity of Roper’s complex,9b we have selected a very
elementary one, namely, the addition of trimethylsilyl acetylene,
proceeding directly and quantitatively to the oxidative addition
product Ru(H)(CCSiMe3)(CO)2(PPh3)2 (see Supporting Informa-
tion).
In conclusion, it is hoped that, due to its attractive simplicity,
the novel synthetic method disclosed here will pave the way for
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