Dinuclear ruthenium and osmium arene trihydrido complexes: Versatile water-soluble synthons in organometallic chemistry

Article abstract of DOI:10.1021/om060747j

Recent developments in the chemistry of dinuclear ruthenium arene complexes are reviewed. Although cationic ruthenium and osmium arene trihydrido complexes have been known for more than 20 years, little synthetic use has been made of these unsaturated mol

Full text of DOI:10.1021/om060747j

766
Organometallics 2007, 26, 766-774
ReViews
Dinuclear Ruthenium and Osmium Arene Trihydrido Complexes:
Versatile Water-Soluble Synthons in Organometallic Chemistry†
Georg Su¨ss-Fink* and Bruno Therrien*
Institut de Chimie, UniVersite´ de Neuchaˆtel, CH-2009 Neuchaˆtel, Switzerland
ReceiVed August 17, 2006
Recent developments in the chemistry of dinuclear ruthenium arene complexes are reviewed. Although
cationic ruthenium and osmium arene trihydrido complexes have been known for more than 20 years,
little synthetic use has been made of these unsaturated molecules prior to the discovery of their
hydrophilicity. Most salts of the dinuclear cations [(η6-arene)2M2(µ2-H)3]+ (M ) Ru, Os) are soluble not
only in polar organic solvents but also in water. The aqueous solutions can be handled without hydrolytic
degradation. This property conditions these unsaturated complexes (with an electron count of only 30)
to be versatile synthons in organometallic chemistry, especially in reactions carried out in water.
C6H6)Ru(H2O)3]2+ as the tosylate salts; the molecular structure
of the (benzene)ruthenium triaquo dication was confirmed by
single-crystal X-ray analysis of the sulfate.6 Since these early
pioneering reports, the chemistry of organometallic aquo
complexes has been steadily developed in the 1980s and
1990s.7,8 After the introduction of the Rhoˆne-Poulenc-Ruhrche-
mie process for the catalytic olefin hydrogenation under biphasic
conditions,9,10 water-soluble hydrido complexes have received
much attention,11-18 and the availability of such complexes as
organometallic synthons is of great interest.
1. Introduction
While classical coordination chemistry is typically considered
as chemistry in aqueous solution, organometallic coordination
chemistry takes place almost exclusively in organic solution.
Owing to the high sensitivity of many organometallics toward
hydrolysis, the organic solvents employed in most organome-
tallic reactions are thoroughly dried prior to use. The rigorous
exclusion of water has become a general feature of laboratory
techniques in this field, to such an extent that water is rarely
considered to be a suitable reaction medium for organometallic
complexes.
The obvious gap between organometallic and classical
coordination chemistry is bridged by the steadily growing
number of water-soluble organometallics. A breakthrough in
this field was the discovery of organometallic aquo complexes
containing both soft organic and hard aquo ligands. The first
complex of this type is presumably the dinuclear cation [(η5-
C5H5)2Ti2(µ2-O)(H2O)2]2+, first synthesized by Wilkinson and
Birmingham in 1954 and erroneously believed to be [(η5-C5H5)2-
Ti2(OH)Br]‚H2O.1 The correct nature of this species was
established later by IR and NMR measurements2 and by single-
crystal X-ray structure analysis of the dithionite salt.3 The
existence of (arene)aquoruthenium complexes was observed by
NMR spectroscopy in 1972 by Zelonka and Baird in the reaction
of [(η6-C6H6)2Ru2(µ2-Cl)2Cl2] with D2O.4 The osmium complex
[(η6-C6H6)Os(H2O)3]2+ was synthesized by analogy and spec-
troscopically characterized by Hung, Kung, and Taube.5 Finally,
Stebler-Ro¨thlisberger et al. succeeded in isolating the cationic
benzene triaquo complexes [(η6-C6H6)Os(H2O)3]2+ and [(η6-
The first dinuclear cation of the type [(η6-arene)2M2(µ2-H)3]+
to be reported in the literature was the ruthenium complex [(η6-
C6Me6)2Ru2(µ2-H)3]+, which had been obtained by Bennett et
al. by the reaction of [(η6-C6Me6)2Ru2(µ2-H)(µ2-Cl)2]+ with
NaBH4 in ethanol and isolated as the hexafluorophosphate salt.
This salt was reported to be a dark green-gray material; it was
only characterized by the 1H NMR signal of the three equivalent
hydrido ligands, and no yield was given.19 Ten years later, a
more reliable synthesis was reported, involving the reaction of
a mononuclear complex of the empirical formula [(η6-C6-
Me6)Ru(OSO2CF3)2]‚2H2O with isopropyl alcohol and anhy-
drous sodium carbonate (eq 1), followed by reaction with
sodium hexafluorophosphate. The salt [(η6-C6Me6)2Ru2(µ2-H)3]-
(6) Stebler-Ro¨thlisberger, M.; Hummel, W.; Pittet, A.-M.; Bu¨rgi, H.-B.;
Ludi, A.; Merbach, A. E. Inorg. Chem. 1988, 27, 1358-1363.
(7) Koelle, U. Coord. Chem. ReV. 1994, 135/136, 623-650.
(8) Su¨ss-Fink, G.; Meister, A.; Meister, G. Coord. Chem. ReV. 1995,
143, 97-111.
(9) Kuntz, E. G. Fr. Patent 2,314,910, 1975.
(10) Kuntz, E. G. CHEMTECH 1983, 17, 570-575.
(11) Darensbourg, D. J.; Bischoff, C. J.; Reibenspies, J. H. Inorg. Chem.
1991, 30, 1144-1147.
* To whom correspondence should be addressed. Fax: 0041 32 718 2511.
E-mail: georg.suess-fink@unine.ch (G.S.-F.); bruno.therrien@unine.ch
(B.T.).
(12) Samuelson, A. G. Curr. Sci. 1992, 63, 547-550.
(13) Herrmann, W. A.; Kohlpaintner, C. W. Angew. Chem., Int. Ed. 1993,
32, 1542-1544.
†This paper is dedicated to Jack Lewis (The Lord Lewis of Newnham),
a great scientist and friend.
(14) Meister, G.; Rheinwald, G.; Stoeckli-Evans, H.; Su¨ss-Fink, G. J.
Organomet. Chem. 1995, 496, 197-205.
(1) Wilkinson, G.; Birmingham, J. M. J. Am. Chem. Soc. 1954, 76, 4281-
(15) Su¨ss-Fink, G.; Meister, G.; Haak, S.; Rheinwald, G.; Stoeckli-Evans,
H. New J. Chem. 1997, 21, 785-790.
4284.
(2) Do¨ppert, K. J. Organomet. Chem. 1979, 178, C3-C4.
(3) Thewalt, U.; Schleuâner, G. Angew. Chem., Int. Ed. 1978, 17, 531-
532.
(4) Zelonka, R. A.; Baird, M. C. Can. J. Chem. 1972, 50, 3063-3072.
(5) Hung, Y.; Kung, W.-J.; Taube, H. Inorg. Chem. 1981, 20, 457-
463.
(16) Che´rioux, F.; Maisse-Franc¸ois, A.; Neels, A.; Stoeckli-Evans, H.;
Su¨ss-Fink, G. Dalton Trans. 2001, 2184-2187.
(17) Allardyce, C. S.; Dyson, P. J.; Ellis, D. J.; Salter, P. A.; Scopelliti,
R. J. Organomet. Chem. 2003, 668, 35-42.
(18) Ogo, S.; Uehara, K.; Abura, T.; Watanabe, Y.; Fukuzumi, S.
Organometallics 2004, 23, 3047-3052.
10.1021/om060747j CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/19/2007

Dinuclear Ru and Os Arene Trihydrido Complexes
Organometallics, Vol. 26, No. 4, 2007 767
2[(η6-C6Me6)Ru(OSO2CF3)2] + 3Me2CHOH f
[(η6-C6Me6)2Ru2(µ2-H)3]+ + [SO3CF3]- + 3HSO3CF3 +
Chart 1. Alternative Representations of the
Electron-Deficient Complexes [(η6-arene)2M2(µ2-H)3]+ with
Three Three-Center Bonds or with a Metal-Metal Triple
Bond
3Me2CO (1)
[PF6]snow reported to be a light brown solidsas well as the
corresponding chloride and triflate salts and the durene ana-
logues [(η6-1,2,4,5-C6H2Me4)2Ru2(µ2-H)3][PF6] and [(η6-1,2,4,5-
C6H2Me4)2Ru2(µ2-H)3][SO3CF3] were fully characterized by IR
and NMR spectroscopy and elemental analysis; the molecular
structure of these complexes, however, remained unknown.20
In the meantime, the osmium complex [(η6-1,4-iPrC6H2Me)2Os2-
(µ2-H)3]+ had been obtained from the hydroxo derivative [(η6-
1,4-iPrC6H2Me)2Os2(µ2-OH)3]+ with either isopropyl alcohol or
formaldehyde as the hydrogen source (eq 2).21 The molecular
envelope around a Ru4 cluster core due to steric hindrance of
the methyl groups. Accordingly, in the case of the less bulky
(p-cymene)osmium system, in the reaction of [(η6-1,4-iPrC6H2-
Me)2Os2(µ2-OH)3]+ with isopropyl alcohol both the tetranuclear
complex [(η6-1,4-iPrC6H2Me)4Os4(µ3-H)4]2+ and the dinuclear
complex [(η6-1,4-iPrC6H2Me)2Os2(µ2-H)3]+ were obtained.21
The dinuclear cations [(η6-arene)2M2(µ2-H)3]+ represent a
species which is exactly half of the corresponding tetranuclear
dications [(η6-arene)4M4(µ2-H)6]2+, which are, with an electron
count of 60, an electron-precise system in accordance with the
18e rule for closed tetrahedral clusters. Thus, the dinuclear
cations [(η6-arene)2M2(µ2-H)3]+, with an electron count of 30,
represent an electron-deficient system.
The electron deficiency of the dinuclear cations [(η6-arene)2M2-
(µ2-H)3]+ can be expressed either by formulating three three-
center-two-electron (3c-2e) M-H-M bonds or, more con-
ventionally, by formulating a MtM triple bond that is bridged
by three hydrido ligands (Chart 1). From a theoretical point of
view, the multicenter bond representation is presumably a more
realistic description, since in the case of the isoelectronic
complex [(η5-C5Me5)2Ru2(µ2-H)4], originally formulated with
a RutRu triple bond and four hydrido bridges by Suzuki et al.
on the basis of the 18e (EAN) rule,29 ab initio molecular orbital
calculations showed no direct metal-metal interaction, the short
ruthenium-ruthenium distance of 2.463(1) Å being accounted
for by assuming four Ru-H-Ru 3c-2e bonds.30,31 However,
for the sake of systematics and of predictability on the basis of
the 18e rule, the triple-bond formalism for the cationic
complexes [(η6-arene)2M2(µ2-H)3]+ is preferable, even though
a DFT analysis we carried out recently (see section 7) shows
no noticeable overlap population between the two ruthenium
atoms in [(η6-C6H6)2Ru2(µ2-H)3]+.65
[(η6-1,4-iPrC6H4Me)2Os2(µ2-OH)3]+ + 3HCHO f
[(η6-1,4-iPrC6H4Me)2Os2(µ2-H)3]+ + 3H2O + 3CO (2)
structure was proposed as written on the basis of the FAB-MS
and the 1H and the 187Os NMR data by analogy with the
isoelectronic cation [(η5-C5Me5)2Ir2(µ2-H)3]+.22,23
The direct hydrogenation of (hexamethylbenzene)ruthenium
dichloride dimer in aqueous solutions was found to yield the
dinuclear cation [(η6-C6Me6)2Ru2(µ2-H)3]+ (eq 4),24 while the
same reaction of the benzene derivative gave the tetranuclear
dication [(η6-C6H6)4Ru4(µ2-H)6]2+ (eq 3), at first erroneously
considered to be [(η6-C6H6)4Ru4(µ3-H)4]2+ 25 but later correctly
established when the tetranuclear dications [(η6-C6H6)4Ru4(µ2-
H)6]2+ and [(η6-C6H6)4Ru4(µ3-H)4]2+ were both isolated and
structurally characterized.26,27 From the aqueous solution, the
2[(η6-C6H6)2Ru2(µ2-Cl)2Cl2] + 6H2 f
[(η6-C6H6)4Ru4(µ2-H)6]2+ + 8Cl- + 6H+ (3)
[(η6-C6Me6)2Ru2(µ2-Cl)2Cl2] + 3H2 f
[(η6-C6Me6)2Ru2(µ2-H)3]+ + 4Cl- + 3H+ (4)
dinuclear cation [(η6-C6Me6)2Ru2(µ2-H)3]+ was crystallized as
the hexafluorophosphate and for the first time characterized by
single-crystal X-ray analysis.28
The fact that the benzene derivative gives a dicationic
tetranuclear hexahydrido complex, while the hexamethylbenzene
derivative leads to a cationic dinuclear trihydrido complex, can
be explained on the basis of steric reasons, because the bulky
hexamethylbenzene ligands cannot form a tetrahedral ligand
The high reactivity of these electron-deficient molecules, their
cationic nature, and their solubility in both polar organic solvents
and in water, combined with a high resistance to hydrolysis,
makes the dinuclear complexes [(η6-arene)2M2(µ2-H)3]+ (M )
Ru, Os) versatile building blocks for organometallic synthesis.
(19) Bennett, M. A.; Ennett, J. A.; Gell, K. I. J. Organomet. Chem. 1982,
233, C17-C20.
(20) Bennett, M. A.; Ennett, J. A. Inorg. Chim. Acta 1992, 198-200,
583-592.
2. Synthesis
(21) Cabeza, J. A.; Mann, B. A.; Maitlis, P. M.; Brevard, C. J. Chem.
Soc., Dalton Trans. 1988, 629-634.
Given the problems of reproducibility or of ill-characterized
starting materials in the early syntheses,19,20 we developed an
improved synthesis with higher yields and easier workup: the
reaction of triaquo(hexamethylbenzene)ruthenium [(η6-C6Me6)-
Ru(OH2)3]2+, employed as the sulfate or tosylate salt, with
sodium borohydride (1.5 equiv) in degassed aqueous solution
afforded [(η6-C6Me6)2Ru2(µ2-H)3]+ in 85% yield (see Scheme
(22) White, A.; Oliver, A. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans.
1973, 1901-1907.
(23) Bau, R.; Carroll, W. E.; Teller, R. G.; Koetzle, T. F. J. Am. Chem.
Soc. 1977, 99, 3872-3874.
(24) Jahncke, M.; Su¨ss-Fink, G. Unpublished results. See: Jahncke, M.;
Ph.D. Thesis, University of Neuchaˆtel (Switzerland), 1998; p 28.
(25) Bodensieck, U.; Meister, A.; Meister, G.; Rheinwald, G.; Stoeckli-
Evans, H.; Su¨ss-Fink, G. Chimia 1993, 47, 189-191.
(26) Meister, G.; Rheinwald, G.; Stoeckli-Evans, H.; Su¨ss-Fink, G. J.
Chem. Soc., Dalton Trans. 1994, 3215-3223.
(27) Su¨ss-Fink, G.; Plasseraud, L.; Maisse-Franc¸ois, A.; Stoeckli-Evans,
H.; Berke, H.; Fox, T.; Gautier, R.; Saillard, J.-Y. J. Organomet. Chem.
2000, 609, 196-203.
(28) Jahncke, M.; Neels, A.; Stoeckli-Evans, H.; Su¨ss-Fink, G. J.
Organomet. Chem. 1998, 561, 227-235.
(29) Suzuki, H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Moro-oka, Y.
Organometallics 1988, 7, 2243-2245.
(30) Koga, N.; Morokuma, K. J. Mol. Struct. 1993, 300, 181-189.
(31) Suzuki, H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Fukushima, M.;
Tanaka, M.; Moro-oka, Y. Organometallics 1994, 13, 1129-1146.

768 Organometallics, Vol. 26, No. 4, 2007
Su¨ss-Fink and Therrien
[(η6-1,3,5-C6H3Me3)2Os2Cl2]2 and a 2:1 mixture of NaBH4 and
ZnCl2 in benzene solution.38
Scheme 1. High-Yield Synthesis of
[(η6-C6Me6)2Ru2(µ2-H)3]+ from the Aquo Complex
[(η6-C6Me6)Ru(OH2)3]2+and Sodium Borohydride in O2-Free
Aqueous Solution
The stability of the trihydrido arene metal complexes depends
mainly on the number and the nature of the substituents on the
arene ligand: the C6Me6 derivatives are the most stable, whereas
the benzene derivatives cannot be prepared.39 The nature of the
counteranion plays only a minor role in the stability but governs
the solubility of these compounds. In general, the triflate,
tetrafluoroborate, and hexafluorophosphate salts are readily
soluble in dichloromethane, chloroform, acetone, and methanol
but sparingly soluble in water and diethyl ether. In contrast,
the chloride salts are soluble in ether and water.
Sometimes, reactions with chlorinated solvents are ob-
served: the trihydrido derivative [(η6-1,4-iPrC6H4Me)2Os2(µ2-
H)3]+ reacted in the presence of acetic acid with CCl4 to give
quantitatively the trichloro complex [(η6-1,4-iPrC6H4Me)2Os2-
(µ2-Cl)3]+.37 Similarly, the ruthenium complex [(η6-C6Me6)2-
Ru2(µ2-H)3]+ was found to react with dichloromethane in the
presence of diphenylsilane to give a mixture of the chloro
complexes [(η6-C6Me6)2Ru2(µ2-Cl)3]+ and [(η6-C6Me6)2Ru2(µ2-
Cl)2Cl2].40
1).32 The method was successfully used to synthesize other
diruthenium trihydrido complexes.33
It is noteworthy that by using a mixture of two different
(arene)ruthenium triaquo complexes for the formation of mixed
[(η6-arene)2Ru2(µ2-H)3]+ complexes, three different diruthenium
trihydrido cations can be expected. For instance, with hexam-
ethylbenzene and indane as η6-arene ligands, the reaction
afforded two symmetrical and one asymmetrical diruthenium
complex (see Chart 2).34 Surprisingly, the major component of
the reaction solution was the asymmetrical complex [(η6-
C9H10)(η6-C6Me6)Ru2(µ2-H)3]+ (52% yield). This cation is stable
for days under an inert atmosphere but decomposes slowly in
air or in solution to form the corresponding hydroxo-bridged
complex [(η6-C9H10)(η6-C6Me6)Ru2(µ2-OH)3]+.34 Analogously,
the asymmetrical cation [(η6-1,4-iPrC6H4Me)(η6-C6Me6)Ru2(µ2-
H)3]+ was synthesized from a 1:1 mixture of [(η6-C6Me6)Ru-
(OH2)3]2+ and [(η6-1,4-iPrC6H4Me)Ru(OH2)3]2+ and sodium
borohydride in aqueous solution and isolated as the tetrafluo-
roborate salt in 32% yield.35
3. Spectroscopic Features
The cationic trihydrido complexes [(η6-arene)2M2(µ2-H)3]+
give rise to characteristic nuclear magnetic resonance and
1
infrared spectroscopic features. In the H NMR spectra of all
[(η6-arene)2M2(µ2-H)3]+ salts a singlet appears around δ -16.0
ppm, which is due to the three hydrido bridge protons. The
chemical shift is essentially independent of the counteranion
(see Table 1). The IR spectra of these complexes show a weak
to medium band around 1150 cm-1, assigned to the ν(Ru-H-
Ru) vibration.
As expected, the isoelectronic complexes [(η5-C5Me5)2Ir2(µ2-
1
H)3]+ and [(η5-C5Me5)2Ru2(µ2-H)4] show similar IR and H
Chart 2. The Three Mixed (One Asymmetrical and the
Two Symmetrical) Diruthenium Trihydrido Cations Formed
by Reaction of NaBH4 with a Mixture of
NMR features (νIrHIr 936 cm-1, δIrHIr -15.33 ppm22 and δRuHRu
-13.99 ppm,29 respectively). In the case of [(η6-C6Me6)2Ru2-
(µ2-H)3]+, H/D exchange processes of the hydrido ligands have
been studied: At elevated temperature (80 °C), the three hydrido
bridges undergo intermolecular H/D exchange with D2O (slow)
or with D2 (rapid).40 A dihydride/dihydrogen equilibrium
mechanism has been proposed for the H/D exchange of the
analogous [(η5-C5Me5)2Ru2(µ2-H)4] complex.42
[(η6-C6Me6)Ru(OH2)3]2+ and [(η6-C9H10)Ru(OH2)3]2+ in
Aqueous Solution
Ultraviolet spectrum and electrochemical characteristics are
known for the (hexamethylbenzene)ruthenium derivative [(η6-
C6Me6)2Ru2(µ2-H)3]+: the UV spectrum of a dichloromethane
solution of [(η6-C6Me6)2Ru2(µ2-H)3][BF4] shows two maxima
at 363 nm (ꢀ ) 9175 M-1 cm-1) and 573 nm (ꢀ ) 624 M-1
cm-1),40 while the cyclic voltammogram of [(η6-C6Me6)2Ru2-
(µ2-H)3][BF4] in CH2Cl2 solution (0.0005 M with 0.005 M
[NBu4][PF6] supporting electrolyte, stationary platinum disk
electrode, scan rate 200 mV/s) reveals the cation [(η6-C6Me6)2-
Ru2(µ2-H)3]+ to be oxidized in a single, irreversible diffusion-
controlled two-electron process at Ep,a ) 0.76 V (relative to
SCE reference ferrocene/ferrocenium at 0.47 V), which can be
assigned to the Ru(II) f Ru(III) oxidation involving both metal
centers, followed by decomposition of the complex.62
The first osmium analogue, the p-cymene derivative [(η6-
1,4-iPrC6H4Me)2Os2(µ2-H)3]+, was obtained as the hexafluoro-
phosphate salt by the reaction of [(η6-1,4-iPrC6H4Me)2Os2(µ2-
OH)3][PF6] with isopropyl alcohol,36,37 while the mesitylene
derivative [(η6-1,3,5-C6H3Me3)2Os2(µ2-H)3]+ was prepared from
(32) Jahncke, M.; Meister, G.; Rheinwald, G.; Stoeckli-Evans, H.; Su¨ss-
Fink, G. Organometallics 1997, 16, 1137-1143.
(33) Vieille-Petit, L.; Therrien, B.; Su¨ss-Fink, G. Inorg. Chim. Acta 2003,
355, 394-398.
(34) Vieille-Petit, L.; Therrien, B.; Su¨ss-Fink, G. Inorg. Chem. Commun.
2004, 7, 232-234.
(38) Schulz, M.; Stahl, S.; Werner, H. J. Organomet. Chem. 1990, 394,
469-479.
(35) Vieille-Petit, L.; Su¨ss-Fink, G.; Therrien, B.; Ward, T. R.; Stoeckli-
Evans, H.; Labat, G.; Karmazin-Brelot, L.; Neels, A.; Bu¨rgi., T.; Finke, R.
G.; Hagen, C. M. Organometallics 2005, 24, 6104-6119.
(36) Cabeza, J. A.; Mann, B. E.; Brevard, C.; Maitlis, P. M. J. Chem.
Soc., Chem. Commun. 1985, 65-67.
(39) In an effort to synthesize the dinuclear cation [(η6-C6H6)2Ru2(µ2-
H)3]+ with benzene ligands, the tetranuclear dication [(η6-C6H6)4Ru4(µ2-
H)6]2+ was obtained instead; cf. refs 26 and 27.
(40) Therrien, B.; Su¨ss-Fink, G. Unpublished results.
(41) Bennett, M. A.; Huang, T.-N.; Turney, T. W. J. Chem. Soc., Chem.
Commun. 1979, 312-312.
(37) Cabeza, J. A.; Mann, B. E.; Maitlis, P. M.; Brevard, C. J. Chem.
Soc., Dalton Trans. 1988, 629-634.
(42) Suzuki, H. Eur. J. Inorg. Chem. 2002, 1009-1023.

Dinuclear Ru and Os Arene Trihydrido Complexes
Organometallics, Vol. 26, No. 4, 2007 769
1
Table 1. Selected H NMR and IR Data for Dinuclear Arene Ruthenium and Osmium Trihydrido Complexes
1H NMR (ppm)
IR (cm-1
)
compd
δ(hydride)
δ(arene)
2.30a
ν(M-H-M)d
ref
[(η6-C6Me6)2Ru2(µ2-H)3]CF3SO3
[(η6-C6Me6)2Ru2(µ2-H)3]Cl
-16.09a
-16.30a
-16.09a
-15.92a
-19.05a
-16.41b
-15.91c
-16.38b
-15.57a
-15.57a
-15.47c
-15.52c
1160
20
19
20
20
41
28
28
40
20
20
33
35
[(η6-C6Me6)2Ru2(µ2-H)3]PF6
2.31a
1160
1180
[(η6-C6Me6)2Ru2(µ2-H)3]Cl‚4H2O
[(η6-1,3,5-C6H3Me3)2Ru2(µ2-H)3]Cl
[(η6-C6Me6)2Ru2(µ2-H)3]2SO4
[(η6-C6Me6)2Ru2(µ2-H)3]PF6
2.30a
2.30, 5.12a
2.27b
2.38c
2.29b
[(η6-C6Me6)2Ru2(µ2-H)3]BF4
[(η6-C6H2Me4)2Ru2(µ2-H)3]CF3SO3
[(η6-C6H2Me4)2Ru2(µ2-H)3]PF6
[(η6-C6H2Me4)2Ru2(µ2-H)3]BF4
[(η6-1,4-iPrC6H2Me)(η6-C6Me6)Ru2(µ2-H)3]BF4
2.24, 5.60a
2.24, 5.59a
2.31, 5.96c
5.83, 5.75, 2.59c
2.41, 2.27, 1.28c
2.07, 2.44, 2.67c
2.69, 5.69, 6.00c
1.29, 2.50, 2.51a
5.80, 5.84a
2.60, 5.66a
2.60, 5.66a
2.62, 5.95c
1140
1160
[(η6-C9H10)(η6-C6Me6)Ru2(µ2-H)3]BF4
[(η6-1,4-iPrC6H4Me)2Os2(µ2-H)3]PF6
-15.46c
-14.87a
28
21
[(η6-1,3,5-C6H3Me3)2Os2(µ2-H)3]Cl
[(η6-1,3,5-C6H3Me3)2Os2(µ2-H)3]PF6
[(η6-1,3,5-C6H3Me3)2Os2(µ2-H)3]BF4
-15.11a
-15.10a
-15.08c
38
38
38
a In CD2Cl2. b In D2O. c In (CD3)2CO. d In KBr.
They are slightly longer than those found in other structures
described in terms of an OstOs triple bond.44-57
The molecular structure of [(η6-C6Me6)2Ru2(µ2-H)3]+, solved
by single-crystal X-ray structure analysis of the hexafluoro-
phosphate salt,28 is presented in Figure 2. The three hydrido
ligands have been located from an electron-density difference
map. The very short ruthenium-ruthenium distance of 2.4681-
(4) Å is in accordance with a formal metal-metal triple bond
but can equally well be interpreted in terms of three three-
center-two-electron Ru-H-Ru interactions. The metal-metal
distance compares well with that of 2.463(1) Å in the isoelec-
tronic tetrahydrido complex [(η5-C5Me5)2Ru2(µ2-H)4]31 and is
even in the range of those in the corresponding osmium
complexes [(η6-1,3,5-C6H3Me3)2Os2(µ2-H)3]+ (2.4741(2) Å)38
and [(η6-1,4-iPrC6H4Me)2Os2(µ2-H)3]+ (mean 2.4709 Å).43
Replacing one of the hydrido bridges by a more bulky
bridging ligand imposes a distortion on the dinuclear unit. In
addition, the substitution of a µ2-1e ligand (H) by a µ2-3e ligand
(X ) OR, SR, PR2, NR2, Cl, Br, I) also modifies the nature of
the metal-metal bond. Therefore, the two arene ligands in the
(44) Cotton, F. A.; Thompson, J. A. J. Am. Chem. Soc. 1980, 102, 6437-
6441.
Figure 1. Molecular structure of [(η6-1,4-iPrC6H4Me)2Os2(µ2-
(45) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A. Inorg. Chem. 1984,
23, 4693-4697.
H)3]+.
(46) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A. Inorg. Chem. 1984,
23, 4697-4700.
4. Structural Studies
(47) Fanwick, P. E.; King, M. K.; Tetrick, S. M.; Walton, R. A. J. Am.
Chem. Soc. 1985, 107, 5009-5011.
(48) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A. Inorg. Chem. 1985,
24, 1334-1338.
The first structurally characterized dinuclear trihydrido cation
was [(η6-1,3,5-C6H3Me3)2Os2(µ2-H)3]+, reported by Schulz et
al. in 1990: the X-ray structure analysis of the chloride salt
reveals the molecule to consist of two (η6-1,3,5-C6H3Me3)Os
units, linked by three bridging hydrido ligands. It possesses a
crystallographic mirror plane bisecting both osmium atoms and
the two 1,3,5-trimethylbenzene ligands.38 The molecular struc-
ture of the p-cymene derivative [(η6-1,4-iPrC6H4Me)2Os2(µ2-
H)3]+ has only been solved recently (see Figure 1).43 The metal-
metal distances are 2.4741(2) Å in [(η6-1,3,5-C6H3Me3)2Os2(µ2-
H)3]+ and 2.4709(5) Å in [(η6-1,4-iPrC6H4Me)2Os2(µ2-H)3]+.
(49) Cotton, F. A.; Dunbar, K. R.; Matusz, M. Inorg. Chem. 1986, 25,
1589-1594.
(50) Fanwick, P. E.; Tetrick, S. M.; Walton, R. A. Inorg. Chem. 1986,
25, 4546-4552.
(51) Agaskar, P. A.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R.; Tetrick,
S. M.; Walton, R. A. J. Am. Chem. Soc. 1986, 108, 4850-4855.
(52) Cotton, F. A.; Dunbar, K. R. J. Am. Chem. Soc. 1987, 109, 2199-
2200.
(53) Cotton, F. A.; Vidyasagar, K. Inorg. Chem. 1990, 29, 3197-3199.
(54) Cotton, F. A.; Ren, T.; Eglin, J. L. Inorg. Chem. 1991, 30, 2559-
2563.
(55) Cle´rac, R.; Cotton, F. A.; Daniels, L. M.; Donahue, J. P.; Murillo,
C. A.; Timmons, D. J. Inorg. Chem. 2000, 39, 2581-2584.
(56) Ren, T.; Parrish, D. A.; Deschamps, J. R.; Eglin, J. L.; Xu, G.-L.;
Chen, W.-Z.; Moore, M. H.; Schull, T. L.; Pollack, S. K.; Shashidhar, R.;
Sattelberger, A. P. Inorg. Chim. Acta 2004, 357, 1313-1316.
(57) Carty, A. J. Pure Appl. Chem. 1982, 54, 113-130.
(43) Therrien, B.; Vieille-Petit, L.; Su¨ss-Fink, G. J. Mol. Struct. 2005,
738, 161-163.

770 Organometallics, Vol. 26, No. 4, 2007
Su¨ss-Fink and Therrien
Scheme 3. Reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with
Sodium Borohydride in Water To Give
[(η6-C6Me6)2Ru2H2(µ2-H)(µ2-BH4)]
Figure 2. Molecular structure of [(η6-C6Me6)2Ru2(µ2-H)3]+.
Scheme 2. Reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with
Lithium Dimethylborate in Hexane-Water To Give
[(η6-C6Me6)2Ru2H2(µ2-H)2]
terminal hydrido ligands, and a triplet at -17.18 ppm, which
can be attributed to the two hydride bridges.
Under biphasic conditions (water/ether) the hexafluorophos-
phate salt of [(η6-C6Me6)2Ru2(µ2-H)3]+ reacted with NaBH4 to
give the neutral borohydrido complex [(η6-C6Me6)2Ru2H2(µ2-
H)(µ2-BH4)] (see Scheme 3).32 In a similar fashion the analogous
complex [(η6-C6Me6)2Ru2H2(µ2-H)(µ2-BMe2H2)] was synthe-
sized from [(η6-C6Me6)2Ru2(µ2-H)3]+ and Na[BMe2H2].32 The
single-crystal X-ray structure analysis showed a Ru-Ru distance
of 2.895(1) Å, for the terminal and bridging hydrido ligands.
The bond distances Ru-Ht and Ru-Hb are 1.46(3) and 1.68-
(3) Å, respectively, and the B-Ht and B-Hb distances are 1.13-
(4) and 1.24(4) Å, respectively. In contrast to the case for the
known iridium borohydride complex [(η5-C5Me5)2Ir2H(µ2-H)-
1
(µ2-BH4)], the H NMR spectrum of which stays unchanged
over the temperature range from -80 to +25 °C,59 variable-
1
temperature H NMR spectroscopy of [(η6-C6Me6)2Ru2H2(µ2-
H)(µ2-BH4)] revealed two independent fluxional processes
taking place in solution, one being the exchange between the
two terminal hydrido ligands and the hydrido bridge and the
other being the exchange of the four hydrido ligands at the boron
atom.32
derivatives [(η6-arene)2M2(µ2-H)2(µ2-X)]+ are no longer parallel
to each other, and the metal-metal distances are lengthened in
such complexes. The steric hindrance imposed by the arene
moiety and the newly introduced ligand can be judiciously
exploited to control the formation of mono- or di-µ2-bridged
complexes.58
6. Reactions with Nitrogen-Containing Molecules
The cation [(η6-C6Me6)2Ru2(µ2-H)3]+ reacted in aqueous
solution with hydrazine at room temperature to give the
dicationic complex [(η6-C6Me6)2Ru2(µ2-H)2(µ2-η1:η1-H2NNH2)]2+,
in which one hydrido bridge is replaced by a hydrazine ligand
(Scheme 4). This cation can be isolated as the hexafluorophos-
phate or the triflate salt.60
Heating of this complex with aqueous hydrazine at reflux
resulted in the formation of [(η6-C6Me6)2Ru2(µ2-H)(µ2-η1:η1-
H2NNH2)(µ2-NH2)]2+, which contains in addition to the hydra-
zine bridge an amido bridge. When this reaction was carried
out at room temperature, a hydrazido intermediate, [(η6-C6Me6)2-
Ru2(µ2-H)(µ2-η1:η1-H2NNH2)(µ2-η1-NHNH2)]2+, was isolated
as the mixed sulfate-hexafluorophosphate salt (Scheme 4).60
It has been assumed that the second hydrazine molecule enters
the complex to give not a hydrazine (µ2-η1:η1-H2NNH2) ligand,
but a hydrazido (µ2-NHNH2) ligand, in which the N-N bond
5. Reactions with Boron-Containing Molecules
Addition of LiBMe2H2 to a suspension of the hexafluoro-
phosphate salt of [(η6-C6Me6)2Ru2(µ2-H)3]+ in hexane, followed
by addition of water, gave the neutral tetrahydrido complex [(η6-
C6Me6)2Ru2H2(µ2-H)2] (see Scheme 2).32 The violet, air-
sensitive compound can be isolated by filtration and extraction
of the solid with benzene. Interestingly, other hydride reagents
such as LiAlH4, NaH, K[B(OPri)3H], Na[B(O2CMe)3H], and
Na[BH3CN], as well as alcohols in basic solution, failed to give
this complex.
In the complex [(η6-C6Me6)2Ru2H2(µ2-H)2], the four hydrido
ligands are fluxional at room temperature: the 1H NMR
spectrum shows only one hydride resonance, a broad signal at
δ -9.2 ppm at 20 °C, which splits into two signals at -80 °C
in deuteriotoluene, a triplet at -1.52 ppm, assigned to the two
(59) Gilbert, T. M.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1985, 107, 3508-3516.
(60) Jahncke, M.; Neels, A.; Stoeckli-Evans, H.; Su¨ss-Fink, G. J.
Organomet. Chem. 1997, 565, 97-103.
(58) Tschan, M. J.-L.; Che´rioux, F.; Therrien, B.; Su¨ss-Fink, G. Eur. J.
Inorg. Chem. 2004, 2405-2411.

Dinuclear Ru and Os Arene Trihydrido Complexes
Organometallics, Vol. 26, No. 4, 2007 771
Scheme 4. Reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with
Scheme 5. Reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with
Various Pyrazoles and Triazoles in Aqueous Solution To
Give the Corresponding Bis(µ2-η1:η1-N,N-pyrazolato) and
Bis(µ2-η1:η1-N,N-triazolato) Derivatives
Hydrazine in Aqueous Solution To Give
[(η6-C6Me6)2Ru2(µ2-H)2(µ2-η1:η1-H2NNH2)]2+
,
[(η6-C6Me6)2Ru2(µ2-H)(µ2-η1:η1-H2NNH2)(µ2-η1-NHNH2)]2+
and [(η6-C6Me6)2Ru2(µ2-H)(µ2-η1:η1-H2NNH2)(µ2-NH2)]2+
,
is weakened due to the 3e donor properties of the bridging
nitrogen atom, so that it breaks to give a µ2-NH2 ligand.
A structural comparison of the hydrazine dihydrido complex
and the hydrazine hydrido amido complex shows the expected
lengthening of the ruthenium-ruthenium distance from 2.6925-
(7) Å in [(η6-C6Me6)2Ru2(µ2-H)2(µ2-η1:η1-H2NNH2)]2+ to 2.8555-
Scheme 6. Reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with
Thiophenol Derivatives To Give
[(η6-C6Me6)2Ru2(µ2-H)2(µ2-S-1,4-C6H4-X)]2+ and
[(η6-C6Me6)2Ru2(µ2-H)(µ2-S-1,4-C6H4-X)2]2+ (R ) p-C6H4-X;
X ) Me, Br)
(8) Å in [(η6-C6Me6)2Ru2(µ2-H)(µ2-η1:η1-H2NNH2)(µ2-NH2)]2+
,
which is a consequence of replacing a 1e donor ligand by a 3e
donor ligand in a bridging position.
The reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with nitrogen-
containing heterocycles such as pyrazole and triazole in water
leads with replacement of two hydrido bridges by two depro-
tonated heterocyclic µ2-η1:η1-N,N-coordinated ligands to form
the corresponding pyrazolato and triazolato complexes, see
Scheme 5.61 In all cases, the pyrazolato or triazolato bridges
function as 3e donor ligands; therefore the replacement of two
µ2-H ligands (1e donors) by two µ2-η1:η1-N,N-coordinated
ligands (3e donors) reduces the formal ruthenium-ruthenium
bond order from 3 to 1.
All cations can be isolated easily from the aqueous solution
by precipitation as their hexafluorophosphate salts. It is interest-
ing to note that with 1,2,3-triazole the reaction produced two
isomers, one containing the two triazolato ligands in parallel
orientation, and the other one containing the two triazolato
ligands in antiparallel orientation. Precipitation with KPF6 gave
a 1:1 mixture of both isomers of [(η6-C6Me6)2Ru2(µ2-H)(µ2-η1:
η1-N3C2H2)2][PF6].61
If 2 equiv (or more) of RSH was used, the reaction with [(η6-
arene)2Ru2(µ2-H)3]+ gave exclusively the dithiolato complexes
[(η6-C6Me6)2Ru2(µ2-H)(µ2-S-1,4-C6H4-X)2]+ and [(η6-1,2,4,5-
C6H2Me4)2Ru2(µ2-H)(µ2-S-1,4-C6H4-X)2]+.62 The trithiolato com-
plexes [(η6-arene)2Ru2(µ2-SR)3]+ are not accessible from the
trihydrido precursor. They have, however, been prepared by
starting from the chloro precursor [(η6-arene)2Ru2(µ2-Cl)2-
Cl2].63,64
In the case of the reactions of the hexamethylbenzene
derivatives with p-bromothiophenol, the complete series of
cationic complexes [(η6-C6Me6)2Ru2(µ2-H)2(µ2-S-1,4-C6H4-
Br)]+, [(η6-C6Me6)2Ru2(µ2-H)(µ2-S-1,4-C6H4-Br)2]+, and [(η6-
C6Me6)2Ru2(µ2-S-1,4-C6H4-Br)3]+ has been isolated as the
tetrafluoroborate salts and structurally characterized. This allows
a comparison of the ruthenium-ruthenium distances with
respect to that in [(η6-C6Me6)2Ru2(µ2-H)3]+ (Chart 3).
7. Reactions with Sulfur-Containing Molecules
The trihydrido cation [(η6-C6Me6)2Ru2(µ2-H)3]+ reacts also
with thiols and thiophenols. The reaction with 1 equiv of RSH
(R ) 1,4-C6H4-X; X ) Me, Br) in ethanol was found to give
a mixture of the mono- and dithiolato-substituted complexes
[(η6-C6Me6)2Ru2(µ2-H)2(µ2-S-1,4-C6H4-X)]+ and [(η6-C6Me6)2-
Ru2(µ2-H)(µ2-S-1,4-C6H4-X)2]+ in a 3:1 ratio (see Scheme 6).62
The durene analogues (η6-arene ) η6-1,2,4,5-C6H2Me4), ob-
tained in the same way, contained a 4:1 molar ratio of mono-
and disubstituted products.
(61) Jahncke, M.; Neels, A.; Stoeckli-Evans, H.; Su¨ss-Fink, G. J.
Organomet. Chem. 1998, 561, 227-235.
(62) Tschan, M. J.-L.; Therrien, B.; Ludv´ıc, J.; Sˇteˇpnicˇka, P.; Su¨ss-Fink,
G. J. Organomet. Chem. 2006, 691, 4304-4311.
(63) Che´rioux, F.; Therrien, B.; Su¨ss-Fink, G. Eur. J. Inorg. Chem. 2003,
1043-1047.
(64) Che´rioux, F.; Therrien, B.; Sadki, S.; Comminges, C. J. Organomet.
Chem. 2005, 690, 2365-2371.

772 Organometallics, Vol. 26, No. 4, 2007
Su¨ss-Fink and Therrien
Scheme 8. Isolation of
[(η6-C6Me6)2Ru2(µ2-PR2)(µ2-H)(µ2-Ph)]+ (R ) Ph, Me) as
Tetrafluoroborate Salts as Intermediates from the Reaction
of [(η6-C6Me6)2Ru2(µ2-H)3]+ with PPh3 or PMe2Ph
Chart 3. Comparison of the Ru-Ru Distances (Å) in the
Series [(η6-C6Me6)2Ru2(µ2-H)3]+ (30e),
[(η6-C6Me6)2Ru2(µ2-H)2(µ2-S-1,4-C6H4-Br)]+ (32e),
[(η6-C6Me6)2Ru2(µ2-H)(µ2-S-1,4-C6H4-Br)2]+ (34e), and
[(η6-C6Me6)2Ru2(µ2-S-1,4-C6H4-Br)3]+ (36e)
The increase of the Ru-Ru distances in this series parallels
the increase in the electron count in these dinuclear complexes
from 30 to 36. In the context of the EAN rule, this tendency
may be interpreted in terms of a decrease in the bond order
from a metal-metal triple bond to a metal-metal double bond
to a metal-metal single bond and finally to no metal-metal
bond at all. However, a bond order and overlap population
analysis, which we carried out recently in collaboration with
Daul using density function theory (DFT), showed this inter-
pretation to be just a formalism.65
8. Reactions with Phosphorus-Containing Molecules
The reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with diaryl- or
dialkylphosphines PR2H gave, as expected, the cationic phos-
phido derivatives [(η6-C6Me6)2Ru2(µ2-PR2)(µ2-H)2]+ (R ) Ph,
tBu).66 Surprisingly, such phosphido complexes also are acces-
sible in high yield by the reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+
with the corresponding triarylphosphines or even with trialky-
lphosphines (PPh3, PnBu3, PnOct3) by facile carbon-phosphorus
bond cleavage66 (see Scheme 7).
Scheme 7. Reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+ with
Secondary and Tertiary Phosphines to Give the
Monophosphido Derivatives
[(η6-C6Me6)2Ru2(µ2-PR2)(µ2-H)2]+
Figure 3. Molecular structure of the cation [(η6-C6Me6)2Ru2(µ2-
PPh2)(µ2-H)(µ2-Ph)]+.
which is eliminated, as the arene, while the alkyl substituents
stay on the phosphorus atom of the phosphido bridge.
An intermediate of the reaction of [(η6-C6Me6)2Ru2(µ2-H)3]+
with PPh3 was isolated, [(η6-C6Me6)2Ru2(µ2-PR2)(µ2-H)(µ2-
Ph)]+ as its tetrafluoroborate salt (see Scheme 8). The single-
crystal X-ray structure analysis revealed a bridging phenyl ligand
coordinated in a µ2-η1 fashion to the diruthenium backbone (see
Figure 3). The distances between the bridgehead carbon atom
of the bridging phenyl ligand and the two ruthenium atoms are
2.264(5) and 2.236(5) Å.66
In line with this observation, it is not surprising that
trimethylphosphine does not react in the same way, since
â-elimination is not possible with the methyl group; instead,
three PMe3 molecules replace a η6-C6Me6 ligand on one of the
two ruthenium atoms to give [(η6-C6Me6)(PMe3)3Ru2(µ2-H)3]+.67
There is a fundamental difference in the reactions of tri-
arylphosphines and trialkylphosphines. PPh3 reacted with [(η6-
C6Me6)2Ru2(µ2-H)3]+ to give benzene as a side product, whereas
the reaction with aliphatic phosphines led via â-elimination to
an olefin and molecular hydrogen, which was verified in the
case of tri-n-octylphosphine, where the n-oct-1-ene formed was
unambiguously identified.66 In the case of mixed aryl-alkyl
tertiary phosphines, such as PMe2Ph, it is always the aryl group
9. Synthesis of Trinuclear Clusters
The reaction of the dinuclear cation [(η6-C6Me6)2Ru2(µ2-H)3]+
with the mononuclear cation [(η6-C6H6)Ru(H2O)3]2+ in aqueous
(65) Penka Fowe, E.; Therrien, B.; Su¨ss-Fink, G.; Daul, C. Manuscript
in preparation.
(66) Tschan, M. J.-L.; Che´rioux, F.; Karmazin-Brelot, L.; Su¨ss-Fink, G.
Organometallics 2005, 24, 1974-1981.
(67) Tschan, M. J.-L.; Che´rioux, F.; Therrien, B.; Su¨ss-Fink, G. Submitted
for publication in Eur. J. Inorg. Chem.

Dinuclear Ru and Os Arene Trihydrido Complexes
Organometallics, Vol. 26, No. 4, 2007 773
Scheme 9. Synthesis of the Trinuclear Cluster
[(η6-C6H6)(η6-C6Me6)2Ru3(µ2-H)3(µ3-O)]+ from the Dinuclear
Precursor [(η6-C6Me6)2Ru2(µ2-H)3]+ and the Mononuclear
Complex [(η6-C6H6)Ru(H2O)3]2+ in Aqueous Solution
Chart 4. S and R Enantiomers of the Intrinsically Chiral
Cluster Cation
[(η6-C6H6)(η6-C6Me6)(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+,
Containing a Tetrahedral Ru3O Skeleton with Four
Different Vertices
solution resulted in the formation of the trinuclear cluster cation
[(η6-C6H6)(η6-C6Me6)2Ru3(µ2-H)3(µ3-O)]+ (see Scheme 9). The
triruthenium framework is capped by a µ3-oxo ligand stemming
from an aquo ligand. In this reaction, the triaquo complex [(η6-
C6H6)Ru(H2O)3]2+ is generated in situ from an aqueous solution
of the chloro precursor [(η6-C6H6)2Ru2(µ2-Cl)2Cl2] used in
excess, in order to avoid side-product formation. Upon addition
of NaBF4, the trinuclear complex precipitated as the tetrafluo-
roborate salt, [(η6-C6H6)(η6-C6Me6)2Ru3(µ2-H)3(µ3-O)][BF4].68
Figure 4. The two enantiomers (S)- and (R)-[(η6-C6H6)(η6-C6Me6)-
(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+, present in the crystal of the
hexafluorophosphate salt.
It is noteworthy that the arene ligands in the trinuclear cluster
cations obtained by this reaction can be varied a great deal.
However, for steric reasons, it is impossible to have three η6-
hexamethylbenzene ligands on the triruthenium skeleton. On
the other hand, with three η6-benzene ligands, the trinuclear
cluster is not stable either, so that neither the cation [(η6-C6H6)3-
Ru3(µ2-H)3(µ3-O)]+ nor the cation [(η6-C6Me6)3Ru3(µ2-H)3(µ3-
O)]+ has been so isolated thus far.
A wide range of trinuclear (arene)ruthenium complexes has
been synthesized simply by modifying the two building blocks,
by introduction of different functional groups either in the
dinuclear or in the mononuclear precursor.35,69-75 Typical
examples are the complexes [{η6-C6H5[*CH(CH3)CH2OH]}-
(η6-C6Me6)2Ru3(µ2-H)3(µ3-O)]+, which possesses a chiral func-
tional group,70 [(η6-C6H5-CH2CH2OCOFc)(η6-C6Me6)2Ru3(µ2-
H)3(µ3-O)]+, which contains a tethered ferrocenyl (Fc) substituent,
and [(η6-1,2-MeC6H4-COOCH2CH2OCOCMedCH2)(η6-C6-
Me6)2Ru3(µ2-H)3(µ3-O)]+, which contains a polymerizable side
chain.75 Mixed osmium-diruthenium clusters such as [(η6-
C6H6)Os(η6-C6Me6)2Ru2(µ2-H)3(µ3-O)]+ have also been ob-
tained by this method.76
10. Synthesis of Intrinsically Chiral Triruthenium Oxo
Clusters
The synthetic method which we developed for the synthesis
of trinuclear arene ruthenium cations in aqueous solution
(Scheme 9) allows the assembly of intrinsically chiral triruthe-
nium oxo clusters: if there are three different η6-arene ligands
coordinated to the three ruthenium atoms, the molecule is chiral
because the tetrahedral Ru3O skeleton presents four different
vertices (Chart 4). The strategy involves the reaction of a
dinuclear precursor containing two different arene ligands, [(η6-
arene1)(η6-arene2)Ru2(µ2-H)3]+, with a mononuclear complex,
[(η6-arene3)Ru(H2O)3]2+, containing another different arene
ligand. An ideal asymmetric diruthenium precursor is the easily
accessible mixed p-cymene-hexamethylbenzene complex [(η6-
1,4-iPrC6H4Me)(η6-C6Me6)Ru2(µ2-H)3]+.35
The first intrinsically chiral Ru3O cluster that we synthesized
according to this method was the cation [(η6-C6H6)(η6-C6Me6)-
(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+, prepared from [(η6-1,4-
iPrC6H4Me)(η6-C6Me6)Ru2(µ2-H)3]+ and the benzene derivative
[(η6-C6H6)Ru(H2O)3]2+ in aqueous solution. The trinuclear
cation crystallized as the hexafluorophosphate salt, and the
single-crystal structure analysis revealed indeed both enanti-
omers to be present in the racemic crystal (see Figure 4).35
(68) Faure, M.; Jahncke, M.; Neels, A.; Stoeckli-Evans, H.; Su¨ss-Fink,
G. Polyhedron 1999, 18, 2679-2685.
(69) Vieille-Petit, L.; Therrien, B.; Su¨ss-Fink, G.; Ward, T. R. J.
Organomet. Chem. 2003, 684, 117-123.
However, the separation of the R and S enantiomers of this
cluster turned out to be unsuccessful in our hands. We tried
fractional crystallization of the cationic enantiomers in the
presence of chiral anions such as L-(+)-tartrate and (-)-
camphor-10-sulfonate from acetone solution, HPLC on chiral
phases, or ion-exchange chromatography in the presence of
chiral ions. In no case did the chiral cluster [(η6-C6H6)(η6-C6-
Me6)(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+ separate into the
enantiomers. Therefore, we decided to introduce a chiral
auxiliary group R* tethered to one of the three different arene
ligands, so that the clusters formed are obtained as a diastere-
omer mixture (see Scheme 10).
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359, 907-911.

774 Organometallics, Vol. 26, No. 4, 2007
Su¨ss-Fink and Therrien
Scheme 11. Tentative Catalytic Cycle for the
Scheme 10. Synthesis of Intrinsically Chiral Cluster
Cations
[(η6-C6H5R)(η6-C6Me6)(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+
from [(η6-1,4-iPrC6H4Me)(η6-C6Me6)Ru2(µ2-H)3]+ and
[(η6-C6H5R)Ru(H2O)3]2+ in Aqueous Solution
Hydrogenation of Phenylacetylene Catalyzed by
[(η6-C6Me6)2Ru2(µ2-PPh2)(µ2-H)2]+
Chart 5. S,S and R,S Diastereomers of the Intrinsically
Chiral Cluster Cation
[{η6-C6H5((S)-CH(NHCO2Et)CH2OCOPri)}(η6-C6Me6)-
(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+ Containing an
S-Configured Substituent
nism had been proposed,79 ruthenium(0) nanoparticles have been
identified as the true catalytic species.35,80
On the other hand, the phosphido derivative [(η6-C6Me6)2-
Ru2(µ2-PPh2)(µ2-H)2]+, which selectively catalyzes the hydro-
genation of carbon-carbon double and triple bonds without
catalyzing the attack on other reducible functions, seems to
operate as a molecular catalyst, at least under these mild
conditions (60 °C and 50 bar of H2).81 The cluster cation [(η6-
C6Me6)2Ru2(µ2-PPh2)(µ2-η1:η2-CHCHPh)(µ2-H)]+, isolated as
the tetrafluoroborate salt from the catalytic hydrogenation of
phenylacetylene, suggests a reaction course involving initial
insertion of the unsaturated substrate into one of the three
hydrido bridges of [(η6-C6Me6)2Ru2(µ2-PPh2)(µ2-H)2]+, followed
by the transfer of a second one onto the ligand formed; a
tentative catalytic cycle is outlined in Scheme 11.
Several intrinsically chiral cluster cations containing the chiral
auxiliary group R* at one of the arene ligands were prepared.
The separation of the diastereomers obtained was successful
only in the case of [{η6-C6H5((S)-CH(NHCO2Et)CH2OCOPri)}-
(η6-C6Me6)(η6-1,4-iPrC6H4Me)Ru3(µ2-H)3(µ3-O)]+ (Chart 5),
which was separated by thin-layer chromatography on silica gel
into the RRu O,SC and SRu O,SC diastereomers.35 This was the first
3
3
case where the intrinsically chiral Ru3O cluster core has been
enantiotopically separated.
12. Outlook
The dinuclear cations [(η6-arene)2M2(µ2-H)3]+ (M ) Ru, Os)
turned out to be valuable complexes for organometallic syn-
thesis. Because of their electron deficiency (30e systems), they
are highly reactive and susceptible to addition reactions, and
because of their hydrophilicity, they can be handled in organic
as well as in aqueous solution. In this respect, they may be
considered as bridging the gap between classical coordination
compounds and organometallic complexes.
11. Catalytic Potential
The presence of three hydrido ligands in the complexes [(η6-
arene)2M2(µ2-H)3]+, combined with the electron deficiency of
these systems, is suggestive of potential catalytic properties for
hydrogenation reactions.
The only underivatized compound tested as a hydrogenation
catalyst, [(η6-C6Me6)2Ru2(µ2-H)3]+, indeed showed moderate
catalytic activity for the hydrogenation of benzene to cyclo-
hexane under mild conditions.77 However, it was shown later
that the reaction is not catalyzed by hydrido complexes but by
metallic ruthenium nanoparticles formed from [(η6-C6Me6)2Ru2-
(µ2-H)3]Cl under catalytic conditions.78 Also in the case of the
trinuclear cluster cation [(η6-C6H6)(η6-C6Me6)2Ru3(µ2-H)3(µ3-
O)]+, which catalyzes the hydrogenation of benzene under
biphasic conditions,68 and for which a supramolecular mecha
Acknowledgment. Continuous financial support of our
research in this area by the Swiss National Science Foundation
is gratefully acknowledged. We also thank the Johnson Matthey
Research Centre for a generous loan of ruthenium(III) chloride
hydrate over a period of 20 years.
OM060747J
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R. G. Organometallics 2005, 24, 1819-1831.
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Eur. J. 2006, 13, 292-299.
(77) Bennett, M. A.; Ennett, J. P.; Gell, K. I. J. Organomet. Chem. 1982,
233, C17-C20.
(78) Wildegren, J. A.; Bennett, M. A.; Finke, R. G. J. Am. Chem. Soc.
2003, 125, 10301-10310.