Synthesis and reactivity of 2-aminoethanethiolato-bridged dinuclear Ru(hmb) complexes (hmb = η6-hexamethylbenzene): Mechanistic consideration on transfer hydrogenation with bifunctional [RuCl 2(hmb)]2/2-aminoethanethiol catalyst system

Article abstract of DOI:10.1021/om100773a

A series of dinuclear Ru(hmb) complexes with a bridging 2-aminoethanethiolato ligand (S-NH) have been newly prepared by controlling the stoichiometry of starting materials. Their catalytic performance in the racemization of an optically pure secondary alcohol indicated that the relative proportion of the Ru(hmb) and the S-NH unit in these catalyst precursors in addition to the amount of base as an activating agent is crucially important to generate a catalytically active species. Moreover, chiral versions of dinuclear Ru(S-NH)(hmb) complexes displayed excellent catalytic performance in terms of reactivity and enantioselectivity for the asymmetric transfer hydrogenation of aromatic ketones using HCO₂H-N(C₂H₅) ₃. In agreement with these results, the crossover experiment using a symmetrically bridged dinuclear complex, [Ru(μ-SCH₂CH ₂NH₂)(hmb)]₂Cl₂, and its Cp*Ir dinuclear congener, [Cp*Ir(μ-SCH₂CH₂NH ₂)]₂Cl₂ (Cp* = η⁵-C ₅(CH₃)₅), suggested that the catalytic performance in the hydrogen transfer is originated from the mononuclear amido complex Ru(SCH₂CH₂NH)(hmb) bearing a Ru/NH bifunctionality.

Full text of DOI:10.1021/om100773a

4584 Organometallics 2010, 29, 4584–4592
DOI: 10.1021/om100773a
Synthesis and Reactivity of 2-Aminoethanethiolato-Bridged Dinuclear
Ru(hmb) Complexes (hmb = η6-hexamethylbenzene): Mechanistic
Consideration on Transfer Hydrogenation with Bifunctional
[RuCl2(hmb)]2/2-Aminoethanethiol Catalyst System
Masato Ito,† Akira Watanabe, Yuji Shibata, and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of
Technology, 2-12-1-E4-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan. †Present address: Institute of
Materials Chemistry and Engineering (IMCE), Kyushu University, Kasuga, Fukuoka 816-8580, Japan.
Received August 9, 2010
A series of dinuclear Ru(hmb) complexes with a bridging 2-aminoethanethiolato ligand (S-NH)
have been newly prepared by controlling the stoichiometry of starting materials. Their catalytic
performance in the racemization of an optically pure secondary alcohol indicated that the relative
proportion of the Ru(hmb) and the S-NH unit in these catalyst precursors in addition to the amount
of base as an activating agent is crucially important to generate a catalytically active species. Moreover,
chiral versions of dinuclear Ru(S-NH)(hmb) complexes displayed excellent catalytic performance
in terms of reactivity and enantioselectivity for the asymmetric transfer hydrogenation of aromatic
ketones using HCO2H-N(C2H5)3. In agreement with these results, the crossover experiment using
a symmetrically bridged dinuclear complex, [Ru(μ-SCH2CH2NH2)(hmb)]2Cl2, and its Cp*Ir dinuclear
congener, [Cp*Ir(μ-SCH2CH2NH2)]2Cl2 (Cp*=η5-C5(CH3)5), suggested that the catalytic performance
in the hydrogen transfer is originated from the mononuclear amido complex Ru(SCH2CH2NH)-
(hmb) bearing a Ru/NH bifunctionality.
Introduction
hydrogenation and transfer hydrogenation of ubiquitous
ketones, which allow expeditious production of a wide
variety of stereochemically well-defined alcohols.4,5 Never-
theless, the structural diversity of ketones still occasionally
necessitates conventional, indirect, and environmentally un-
friendly stoichiometric means, which should be replaced by
energy-saving catalytic processes. Our efforts have been directed
toward development of new bifunctional molecular catalysts
by molecular orchestration at the ligand sphere of a late transition
metal in the half-sandwich-type complexes with a d6 electron-
ic configuration based on Noyori and Ikariya’s molecular
designing of RuCl[(S,S)-Tsdpen](η6-arene)1a,b (TsDPEN =
TsNCH(C6H5)CH(C6H5)NH2; Ts = p-CH3C6H4SO2) as a
starting point. Our earlier studies demonstrated that the
replacement of η6-arene and monoanionic chelating protic
amine ligands with Cp* and neutral ones (L-NH) dramatically
facilitates the heterolytic cleavage of molecular hydrogen in
The bifunctional molecular catalyst design issued by Noyori
and co-workers in the mid 1990s1 was a notable scientific
milestone to inspire many researchers to participate in the
study of this attractive rationale from various aspects.2,3 As
a result, a number of sophisticated bifunctional catalysts
have been developed in the past decade for the asymmetric
*To whom correspondence should be addressed. E-mail: tikariya@
apc.titech.ac.jp.
(1) (a) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R.
Angew. Chem., Int. Ed. Engl. 1997, 36, 286–288. (b) Hashiguchi, S.; Fujii,
A.; Haack, K.-J.; Matsumura, K.; Ikariya, T.; Noyori., R. Angew. Chem., Int.
Ed. Engl. 1997, 36, 288–290. (c) Noyori, R.; Hashiguchi, S. Acc. Chem.
Res. 1997, 30, 97–102. (d) Noyori, R.; Yamakawa, M.; Hashiguchi, S.
J. Org. Chem. 2001, 66, 7931–7944.
(2) Reviews: (a) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord.
Chem. Rev. 2004, 248, 2201–2237. (b) Samec, J. S. M.; Backvall, J.-E.;
€
Andersson, P. G.; Brandt, P. Chem.Soc.Rev.2006,35, 237–248. (c) Gr€utzmacher,
H. Angew. Chem., Int. Ed. 2008, 47, 1814–1818. (d) Morris, R. H. Chem. Soc.
Rev. 2009, 38, 2282–2291. (e) Kuwata, S.; Ikariya, T. Dalton Trans. 2010, 39,
2984–2992.
(3) Selected examples: (a) Alonso, D. A.; Brandt, P.; Nordin, S. J. M.;
Andersson, P. G. J. Am. Chem. Soc. 1999, 121, 9580–9588. (b) Yamakawa,
M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466–1478. (c) Petra,
D. G.; Reek, J. N. H.; Handgraaf, J. W.; Meijer, E. J.; Dierkes, P.; Kamer, P. C. J.;
Brusse, J.; Shoemaker, H. E.; van Leeuwen, P. W. N. M. Chem.;Eur. J. 2000,
(4) Asymmetric trasnfer hydrogenation: (a) Palmer, M. J.; Wills, M.
Tetrahedron: Asymmetry 1999, 10, 2045–2061. (b) Ikariya, T.; Murata, K.;
Noyori, R. Org. Biomol. Chem. 2006, 4, 393–406. (c) Ikariya, T.; Blacker,
A. J. Acc. Chem. Res. 2007, 40, 1300–1308. (d) Wang, C.; Wu, X.; Xiao, J.
Chem. Asian J. 2008, 3, 1750–1770.
(5) Asymmetric hydrogenation: (a) Noyori, R.; Ohkuma, T. Angew.
Chem., Int. Ed. 2001, 40, 40–73. (b) Blaser, H.-U.; Malan, C.; Pugin, B.;
Spindler, F.; Steiner, H.; Studer, M. Adv. Synth. Catal. 2003, 345, 103–151.
(c) Ohkuma, T.; Noyori, R. In Transition Metals for Organic Synthesis, 2nd
ed.; Beller, M.; Bolm, C., Eds.; Wiley-VCH: Weinheim, 2004; Vol. 1, pp
29-113. (d) The Handbook of Homogeneous Hydrogenation, Vols. 1-3;
de Vries, J. G.; Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, 2007.
(e) Ohkuma, T. J. Synth. Org. Chem. Jpn. 2007, 65, 1070–1080. (f) Noyori,
R.; Ohkuma, T. In Asymmetric Synthesis; Christmann, M.; Brase, S., Eds.;
Wiley-VCH: Weinheim, 2008; pp 337-341.
ꢀ
€
6, 2818–2829. (d) Pamies, O.; Backvall, J. E. Chem.;Eur. J. 2001, 7, 5052–
5058. (e) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998–2001.
(f) Sandoval, C. A.; Ohkuma, T.; Muniz, K.; Noyori, R. J. Am. Chem. Soc.
2003, 125, 13490–13503. (g) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein,
D. J. Am. Chem. Soc. 2006, 128, 15390–15391. (h) Casey, C. P.; Guan, H.
J. Am. Chem. Soc. 2007, 129, 5816–5817. (i) Sandoval, C. A.; Li, Y.; Ding, K.;
Noyori, R. Chem. Asian J. 2008, 3, 1801–1810. (j) Sandoval, C. A.; Bie, F.;
Matsuoka, A.; Yamaguchi, Y.; Naka, H.; Li, Y.; Kato, K.; Utsumi, N.; Tsutsumi,
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r
pubs.acs.org/Organometallics
Published on Web 09/30/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 20, 2010 4585
Chart 1. Possible Ground-State Structures of Cp*Ru(L-N)
alcoholic solvents containing base.6 This finding has success-
fully led to the development of new molecular catalysts,
Cp*RuCl(L-NH), for the straightforward hydrogenation
of polarized C-O bonds6a,b including ketones,6c epoxides,6d
imides,6e,f N-acylcarbamates,6g and esters.6g Also noteworthy is
the significant effect of their anchoring element L on the catalytic
activity in the dehydrogenation of primary or secondary
alcohols.7 For example, the Cp*RuCl catalyst system with
(C6H5)2P(CH2)2NH2 as an L-NH ligand promotes very
rapid racemization of optically active secondary alcohols
upon treatment with base, while the similar catalyst system
with (CH3)2N(CH2)2NH2 hardly promotes the reaction under
otherwise identical conditions.6c,d,7a This distinct difference
of catalytic activity in the racemization is partly attributable
to the through-space coupling orbital interactions8 between
Cp* and L ligands in a postulated catalytically active species
Cp*Ru(L-N). As illustrated in Chart 1, the tertiary phos-
phino group in L presumably destabilizes the planar geometry
of this 16e complex (Cp* lies perpendicular to the L-Ru-N
plane), thereby allowing facile dehydrogenation of primary
or secondary alcohols to generate 18e complex Cp*RuH-
(L-NH), whereas the tertiary amino group in L possibly
stabilizes the planar geometry to keep alcoholic substrates
away from the coordination sphere.
These results turned our attention to the variation in cata-
lytic performance depending on whether the anchoring element
in the chelating protic amine ligands belongs to the second
or the third row. We then focused on the catalytic activity of
Ru(η6-arene) complexes having 2-aminoethanethiols as mono-
anionic chelating protic amine ligands9 and found that a ternary
catalyst system of [RuCl2(hmb)]2, a 2-aminoethanethiol deri-
vative, and a base effects a highly efficient hydrogen transfer
between carbonyl compounds and alcohols.10 Notably,
2-aminoethanethiols remarkably accelerate the rate of the
hydrogen transfer in comparison to similar ligands having
second-row anchoring elements, as long as both NH and
SH groups are embedded in their structure and suitable
amounts of base are employed. Moreover, a preliminary chiral
modification in the 2-aminoethanethiol skeleton has readily
strengthened its catalytic performance to realize a highly effi-
cient asymmetric transfer hydrogenation of aromatic ketones
using HCO2H-N(C2H5)3.10
These promising results strongly encouraged us to study
further the reaction chemistry of Ru(hmb) complexes bearing
2-aminoethanethiols as a ligand to develop a structurally better
defined bifunctional Ru(S-NH)(hmb) catalyst system and to
eliminate exaggerated concerns of a negative effect of organosul-
fur compounds as a catalyst poison toward transition-metal-
based catalysts. As a result, we found that the reaction of an 1:2:3
mixture of [RuCl2(hmb)]2, (S)-2-pyrrolidinemethanethiol hydro-
chloride, and KOt-Bu selectively furnishes a novel unsymmetrical
bis(thiolato)-bridged dinuclear complex, [1bb]Cl2 (Scheme 1).
The resulting dinuclear complex served as an easy to handle and
robust catalyst precursor and exhibited almost identical excel-
lent catalytic performance to the ternary system.10
On the basis of these preliminary results, we have recently
found that a series of unsymmetrical bis(thiolato)-bridged
dinuclear complexes [1]2þ can be obtained in a stepwise manner
via isolable chloro-thiolato complexes [2]þ, which have proved
to serve as versatile precursors for the preparation of a wide
variety of new unsymmetrically bridged dinuclear bis(thiolato)
complexes including [3]þ. These new dinuclear Ru(S-NH)-
(hmb) complexes have providedanopportunity to gain a better
understanding of structural requirements for catalytic activity
in hydrogen transfer. In fact, careful and systematic evalua-
tion of their catalytic activity under a variety of conditions
has revealed that the protic amino terminus in the nonchelat-
ing bridging thiolato ligand in [1aa]Cl2 plays a crucial role in
generating a possible active species through the formation of
an isolable symmetrically bridged dinuclear complex, [4aa]Cl2.
Additionally, a crossover experiment using [4aa]Cl2 and its
Cp*Ir analogue in the acid-base treatment has clearly indi-
cated that a mononuclear amido species, Ru(SCH2CH2NH)-
(hmb), should be responsible for the bifunctional catalysis, as
observed in the study on the asymmetric transfer hydrogenation
with RuCl[(S,S)-Tsdpen](η6-arene).1a,b Herein we describe full
details on the synthesis and reactivity of these 2-aminoeth-
anethiolato-bridgeddinuclearRu(hmb) complexesand discuss
the reaction mechanism of transfer hydrogenation therewith.
(6) (a) Ito, M.; Ikariya, T. Chem. Commun. 2007, 5134–5142. (b) Ito,
M.; Ikariya, T. J. Synth. Org. Chem. Jpn. 2008, 66, 1047–1053. (c) Ito, M.;
Hirakawa, M.; Murata, K.; Ikariya, T. Organometallics 2001, 20, 379–381.
(d) Ito, M.; Hirakawa, M.; Osaku, A.; Ikariya, T. Organometallics 2003, 22,
4190–4192. (e) Ito, M.; Sakaguchi, A.; Kobayashi, C.; Ikariya, T. J. Am.
Chem. Soc. 2007, 129, 290–291. (f) Ito, M.; Kobayashi, C.; Himizu, A.;
Ikariya, T. J. Am. Chem. Soc. 2010, 132, 11414-11415. (g) Ito, M.; Koo,
L.-W.; Himizu, A.; Kobayashi, C.; Sakaguchi, A.; Ikariya, T. Angew.
Chem., Int. Ed. 2009, 48, 1324–1327.
(7) (a) Ito, M.; Osaku, A.; Kitahara, S.; Hirakawa, M.; Ikariya, T.
Tetrahedron Lett. 2003, 44, 7521–7523. (b) Ito, M.; Kitahara, S.; Ikariya, T.
J. Am. Chem. Soc. 2005, 127, 6172–6173. (c) Ito, M.; Osaku, A.; Shiibashi,
A.; Ikariya, T. Org. Lett. 2007, 9, 1821–1824.
Results and Discussion
Late transition metal thiolato complexes have long been
the subject of intensive study not only as artificial active site
models of cysteine-rich metalloenzyme systems in nature11
(11) Recent reviews on cysteine-rich metalloenzymes: (a) Fontecilla-
Camps, J. C.; Amara, P.; Cavazza, C.; Nicolet, Y.; Volbeda, A. Nature
2009, 460, 814–822. (b) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245–
2274. (c) Heinekey, D. M. J. Organomet. Chem. 2009, 694, 2671–2680.
(d) Ogo, S. Chem. Commun. 2009, 38, 7577–7587. (e) Gloaguen, F.;
Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100–108. (f) Evans, D. J.
Coord. Chem. Rev. 2005, 249, 1582–1595. (g) Harrop, T. C.; Mascharak,
P. K. Coord. Chem. Rev. 2005, 249, 3007–3024. (h) Hegg, E. L. Acc. Chem.
Res. 2004, 37, 775–783.
(8) Sapunov, V. N.; Schmid, R.; Kirchner, K.; Nagashima, H. Coord.
Chem. Rev. 2003, 238-239, 363–382.
(9) Several chiral 2-aminoethanethiols were examined as a ligand in
ꢁ
the Ru-catalyzed transfer hydrogenation. Harfouche, J.; Herault, D.;
Tommasino, M. L.; Pellet-Rostaing, S.; Lemaire, M. Tetrahedron:
Asymmetry 2004, 15, 3413–3418.
(10) Ito, M.; Shibata, Y.; Watanabe, A.; Ikariya, T. Synlett 2009,
1621–1626.

4586 Organometallics, Vol. 29, No. 20, 2010
Scheme 1. Preparation of 2-Aminoethanethiolato-Bridged Dinuclear Ru(hmb) Complexes
Ito et al.
but also as multinuclear platforms of molecular catalysts for
organic synthesis.12 However, relatively limited information
was available as to the coordination chemistry of Ru(η6-arene)
complexes having 2-aminoethanethiols.13 For example, two
groups independently reported around 1990 that the reaction of
[RuCl2(η6-arene)]2 with penicillamine gives dinuclear [Ru-
{μ-SC(CH3)2CH(CO2H)NH2}(η6-arene)]Cl2 (arene=benzene,
mesitylene),13a,b whose η6-arene ligands adopt a syn orientation
with respect to the Ru2S2 core possibly due to the hydrogen
bond network between the CO2H group of one ligand and
the NH2 group of the other via chloride anions. Later, Sadler
and co-workers reported that the ethylenediamine moiety in
[RuCl(H2NCH2CH2NH2)(η6-biphenyl)]PF6 undergoes a ligand
displacement reaction with cysteine to generate dinuclear Ru-
(η6-biphenyl) complexes with one or two bridging cysteine
ligands in aqueous solution.13c Goh and co-workers reported
that the reaction of HN(CH2CH2SNa)2 with [RuCl2(hmb)]2
gives mononuclear Ru[HN(CH2CH2S)2](hmb), whose thio-
lato moieties undergo ring-closure with Br(CH2)nBr (n = 2,
3, 4) to form macrocyclic neutral tridentate ligands in the
cationic thioether complexes [Ru(hmb){HN(CH2CH2S)2-
(CH2)n}]2þ 13d Besides Ru(η6-arene) complexes, Konno and
.
co-workers reported that RuCl2(bpy)2 readily reacts with
2-aminoethanethiol in the presence of AgClO4 to give a
thiolato-bridged RuIIAgIRuII trinuclear complex, [{Ru(bpy)2-
(12) (a) Miyake, Y.; Uemura, S.; Nishibayashi, Y. ChemCatChem
2009, 1, 342–356. (b) Nishibayashi, Y.; Uemura, S. Curr. Org. Chem. 2006,
10, 135–150. (c) Shin, R. Y. C.; Goh, L. Y. Acc. Chem. Res. 2006, 39, 301–
313. (d) Hidai, M.; Mizobe, Y. Can. J. Chem. 2005, 83, 358–374.
(e) Sakamoto, M.; Ohki, Y.; Kehr, G.; Erker, G.; Tatsumi, K. J. Organomet.
Chem. 2009, 694, 2820–2824.
(13) (a) Sheldrick, W. S.; Heeb, S. J. Organomet. Chem. 1989, 377, 357–
366. (b) Capper, G.; Davies, D. L.; Fawcett, J.; Russell, D. R. Acta Crystallogr.
1995, C51, 578–580. (c) Wang, F.; Chen, H.; Parkinson, J. A.; Murdoch, P. S.;
Sadler, P. J. Inorg. Chem. 2002, 41, 4509–4523. (d) Shin, R. Y. C.; Tan, G. K.;
Koh, L. L.; Goh, L. Y. Organometallics 2004, 23, 6293–6298. (e) Tamura, M.;
Matsuura, N.; Kawamoto, T.; Konno, T. Inorg. Chem. 2007, 46, 6834–6836.
(f) Ru(η6-arene) complexes bearing bis(amido)thioether ligands have been
reported: Takemoto, S.; Kawamura, H.; Yamada, Y.; Okada, T.; Ono, A.;
Yoshikawa, E.; Mizobe, Y.; Hidai, M. Organometallics 2002, 21, 3897–3904.
(g) Cationic Ru(η6-arene) complexes with protic amine ligands bearing a
thioether group have been reported: Weber, I.; Heinemann, F. W.; Bauer, W.;
Zenneck, U. Z. Naturforsch. 2009, 64b, 123–140.
(H2NCH2CH2S)}2Ag]3þ 13e
However, the structural chem-
.
istry of Ru(η6-arene) complexes with 2-aminoethanethiols
other than those having additional coordinating groups has
still remained unexplored until recently.13f,g
Our initial study started to explore suitable reaction con-
ditions for a programmed synthesis of desired Ru(S-NH)-
(hmb) complexes, focusing on the control of their nuclearity.
As a result, the reactions of either [RuCl2(hmb)]2 or [Ru2Cl3-
(hmb)2]OTf (Tf = CF3SO2) with a hydrochloric salt of
2-aminoethanethiols and KOt-Bu in a molar ratio of 1:1:2
in methanol/CH2Cl2 followed by extraction with CH2Cl2
allowed us to isolate a series of dinuclear chloro-thiolato

Article
Organometallics, Vol. 29, No. 20, 2010 4587
Figure 1. Solid-state structure of [2aCl]B(C6H5)4 0.5CH2Cl2.
3
Thermal ellipsoids are drawn at the 30% probability level. The
H atoms, the anion, and the solvated molecule are omitted for
clarity.
Figure 2. Solid-state structure of [2bCl]OTf. Thermal ellipsoids
are drawn at the 30% probability level. The H atoms and the
anion are omitted for clarity.
complexes, [2aCl]Cl, [2aCl]OTf, [2bCl]OTf, and [2cCl]OTf,
in 82%, 79%, 87%, and 87% isolated yields, respectively, as
illustrated in Scheme 1. The anion exchange reaction of [2aCl]Cl
with NaOTf or NaB(C6H5)4 readily afforded [2aCl]OTf and
[2aCl]B(C6H5)4 in 57% and 67% yield, respectively. We also
found that the μ-Cl ligand in [2]þ is readily substituted by an
appropriate thiolato ligand to give [3]þ.14 For example, the
reaction of [2aCl]Cl or [2aCl]OTf with an equimolar amount
of alkaline metal thiolates derived from benzyl mercaptan,
4-methylbenzenethiol, and 2-(dimethylamino)ethanethiol yielded
μ-bis(thiolato) dinuclear complexes [3ax]Cl, [3ay]OTf, and
[3az]Cl, respectively, in 32-87% yield. Interestingly, the
reaction of [2aCl]Cl with 2-aminoethanethiol in the absence
of any external base caused the replacement of the μ-Cl ligand
to give an unsymmetrically bridged dinuclear complex, [1aa]Cl2,
in 87% yield, while a similar reaction in the presence of KOt-
Bu (1 equiv) gave a symmetrically bridged dinuclear com-
plex, [4aa]Cl2, with a syn orientation of the two hmb ligands
in 88% yield. Notably, the anion exchange reaction of [4aa]Cl2
with NaOTf in methanol afforded an analogous dinuclear
complex, [4aa](OTf)2, but with an anti orientation of the two
hmb ligands.
Table 1. Catalytic Racemization of (R)-1-Phenylethanola
Ru(hmb)
complex
KOt-Bu
(equiv/Ru)
entry
time (h)
eeb (%)
1
2
3
4
5
6
7
8
[1aa]Cl2
[1aa]Cl2
[1aa]Cl2
[1aa]Cl2
[4aa]Cl2
[4aa]Cl2
[4aa]Cl2
[4aa](OTf)2
[4aa](OTf)2
[4aa](OTf)2
[2aCl]Cl
[2aCl]OTf
[2aCl]B(C6H5)4
[3ax]Cl
[3ay]OTf
[3az]Cl
[5aH]Cl
3
3
2
1
2
2
1
2
2
1
3
3
3
2
2
2
1
24
6
6
6
24
6
6
24
6
0
6
98
98
0
1
98
0
9
1
10
11
12
13
14
15
16
17
6
98
90
80
94
95
97
95
92
24
24
24
24
24
24
24
The structures of these new complexes were characterized
by 1H NMR, 13C{1H} NMR, and ESI-MS as well as elemental
analysis. The relative orientation of the two hmb ligands with
respect to the Ru2S2 core was determined by a single-crystal
X-ray diffraction study.14b,c Solid-state structures of [2aCl]-
B(C6H5)4 and [2bCl]OTf are illustrated in Figures 1 and 2,
respectively.
a 30 °C, substrate:Ru = 100:1, [substrate] = 0.625 M in toluene.
b HPLC analysis using a Daicel Chiralcel OD.
The structural difference between [3az]Cl and [4aa]Cl2
merits specific mention. The aprotic tertiary amino terminus
in [3az]Cl seems to be incapable of removing the terminal Cl
ligand from the inner sphere to form a five-membered chelate,
while the similar protic one is likely to form a second five-
membered chelate easily to give [4aa]Cl2. Furthermore, the
deprotonation of the nonchelating ammonium terminus
in [1aa]Cl2 with an equimolar amount of KOt-Bu readily
generated [4aa]Cl2 exclusively. These results may suggest that
the protic NH2 group has stronger coordinating ability than
the N(CH3)2 group.
Next, we examined the catalytic performance of isolated
2-aminoethanethiolato-bridged dinuclear Ru(hmb) complexes
in the racemization of (R)-1-phenylethanol as a model reac-
tion to evaluate their hydrogen transfer ability.7a,10,15 Table 1
shows some representative results. The outcome of the racemi-
zation in toluene containing optically pure (R)-1-phenylethanol
with a substrate/Ru ratio of 100 at 30 °C was highly influenced
(14) (a) Initial replacement of the terminal Cl ligand with the incoming
thiolato ligand followed by site exchange cannot be ruled out. (b) Most of
the new complexes showed characteristic NMR spectra in solution, whicþh
were consistent with their solid-state structures. However, a series of [2aCl]
showed inconsistent NMR spectra upon dissolution, which indicated the
formation of syn- and anti-isomers whose ratio was quite variable depending
on the temperature (see the Supporting Information). (c) Dicationic com-
plexes [4aa]Cl2 and [6aa]Cl2 in D2O showed NMR spectra that were
consistent with their solid-state structures.
(15) Dijksman, A.; Elzinga, J. M.; Li, Y.-X.; Arends, I. W. C. E.;
Sheldon, R. A. Tetrahedron: Asymmetry 2002, 13, 879–884.

4588 Organometallics, Vol. 29, No. 20, 2010
Scheme 2. Asymmetric Transfer Hydrogenation of Aromatic Ketones
Ito et al.
by the structure of the dinuclear complexes as well as the
amount of KOt-Bu as an activating agent.
transfer hydrogenation of acetophenone with (S)-PipNH2Cl
under otherwise identical conditions. These results may
indicate that the relatively sterically demanding hmb ligand
plays an important role in the selective formation of a cata-
lytically active species by preventing the formation of any
other less stereoselective species (vide infra), which contrasts
sharply with RuCl(Tsdpen)(hmb), which exhibits lower re-
activity as compared to other RuCl(Tsdpen) congeners with
sterically smaller η6-arene ligands.16
Notably, the Ru(hmb) complexes with a Ru:S-NH ratio
of 1:1 ([1aa]Cl2, [4aa]Cl2, and [4aa](OTf)2) promoted intra-
molecular hydrogen transfer to decrease the initial ee value in
an appreciable rate upon treatment with an equal amount of
KOt-Bu to that of halogen or pseudohalogen (entries 1, 2, 5,
6, 8, and 9), while they hardly showed catalytic activity in the
presence of an insufficient amount of KOt-Bu (entries 3, 4, 7,
and 10). In sharp contrast, the Ru(hmb) complexes with a
Ru:S-NH ratio of 2:1 ([2aCl]Cl, [2aCl]OTf, [2aCl]B(C6H5)4,
[3ax]Cl, [3ay]OTf, and [3az]Cl) hardly exhibited catalytic
activity regardless of the amount of KOt-Bu (entries 11-16).
These results may indicate that the second S-NH unit in
[1aa]Cl2, [4aa]Cl2, and [4aa](OTf)2 plays a crucially important
role that is missing in μ-Cl complexes ([2aCl]þ) or bis(thiolato)
complexes without a protic amino terminus ([3]þ). In fact,
the reaction of [2aCl]Cl with two equivalents of KOH in
2-propanol caused deprotonation at the coordinated protic
amino group as well as displacement of the μ-Cl with μ-H to
give a novel triply bridged symmetrical dinuclear complex,
[5aH]Cl, in 81% yield. However, this complex was almost
catalytically inactive in the hydrogen transfer (entry 17).
Not surprisingly, the intermolecular hydrogen transfer
between acetophenone and 2-propanol was also best pro-
moted by the catalyst system of Ru(hmb) complexes with a
Ru:S-NH ratio of 1:1 as long as a sufficient amount of base
was added to remove halogen or pseudohalogen from the
catalyst precursors. However, the intrinsic reversible nature
of the hydrogen transfer became evident when a chiral 2-amino-
ethanethiol was employed, since the initial ee value of the pro-
duct was appreciably deteriorated throughout the reaction.10
This result clearly indicated that the aforementioned race-
mization should operate even in 2-propanol due to the higher
hydrogen transfer activity of the present catalyst system. On
the other hand, the use of HCO2H instead of 2-propanol as a
hydrogen source effectively prevented the racemization, to
result in the initial ee value of the product being maintained
constant. In fact, a wide variety of aromatic ketones underwent
asymmetric transfer hydrogenation in HCO2H-N(C2H5)3
containing a binary catalyst system of [RuCl2(hmb)]2/(S)-
2-piperidinemethanethiol ((S)-PipSNH) to give the corre-
sponding alcohols with excellent ee’s, as shown in Scheme 2.
As shown in Table 2, the replacement of the hmb ligand
with sterically smaller η6-arene ligands dramatically decreased
both catalytic activity and enantioselectivity in the asymmetric
Additionally, the catalytic performance of the isolated
dinuclear complex [1bb]Cl2, having the chiral S-NH unit
with a ratio equal to Ru, is similar to that of the correspond-
ing binary system.10 In sharp contrast, the structurally
similar dinuclear [2bCl]OTf, bearing the chiral S-NH unit
with only a half ratio to Ru, was totally inactive toward the
asymmetric transfer hydrogenation using HCO2H-N(C2H5)3.
We believe that the μ-Cl ligand in [2bCl]OTf should be
readily replaced by μ-H in HCO2H-N(C2H5)3 to form a
catalytically inactive triply bridged dinuclear complex in a
similar manner to the clean conversion of [2aCl]Cl to [5aH]Cl
in 2-propanol containing two equivalents of base as described
above. On the other hand, the reaction of [2cCl]OTf with (S)-
PipSNH in methanol selectively afforded the dinuclear complex
[1cc]2þ, which was structurally established by 1H NMR,
13C{1H} NMR, and ESI-MS analyses (see the Supporting
Information). Thus obtained [1cc]2þ promoted asymmetric
transfer hydrogenation of acetophenone in HCO2H-N(C2H5)3
to give (S)-1-phenylethanol with 86% ee in 90% yield (sub-
strate:Ru = 100:1, substrate:HCO2H:N(C2H5)3 = 2:3:5,
30 °C, 24 h), and its efficiency was almost identical in terms
of catalytic activity and stereoselectivity to those with the
corresponding binary system shown in Scheme 2.
These results supported that catalyst precursors [1]2þ are
selectively generated in situ from an equimolar mixture of
[RuCl2(hmb)]2 and 2-aminoethanethiols. Thus, the intermedi-
ary μ-Cl complexes [2]þ should be consumed immediately
under the basic conditions and transformed into either [1]2þ
or catalytically inactive species including [5aH]Cl depending
on the molar ratio of [RuCl2(hmb)]2 and 2-aminoethanethiols.
Interestingly, the reaction of 2-(dimethylamino)ethanethiolato
complex [3az]Cl with excess KOt-Bu in 2-propanol generated
a new complex whose 1H NMR in CD2Cl2 showed a single
(16) (a) Murata, K.; Okano, K.; Miyagi, M.; Iwane, H.; Noyori, R.;
Ikariya, T. Org. Lett. 1999, 1, 1119–1121. (b) Yamada, I.; Noyori, R. Org.
Lett. 2000, 2, 3425–3427.

Article
Organometallics, Vol. 29, No. 20, 2010 4589
Table 2. Effect of η6-Arene Ligand on the Asymmetric Transfer Hydrogenation of Acetophenone
entry
arene
yield (%)
ee (%)
1
2
3
4
5
6
C6H6
4
31
22
49
88
84
0
12
22
48
75
90
4-CH3C6H4CH(CH3)2
1,3,5-(CH3)3C6H3
1,2,4,5-(CH3)4C6H2
(CH3)5C6H
(CH3)6C6
Chart 2. Postulated Triply Bridged Dinuclear Complexes
peak at 2.02 ppm for the two hmb ligands and whose ESI-MS
showed an isotopic pattern centered at m/z=707 for [C30H51-
N2Ru2S2]þ. This result indicated a possible formation of a
triply bridged dinuclear complex, [5az]Cl (Chart 2), in which
μ-H in [5aH]Cl is replaced by the μ-thiolato ligand. We
believe that the second protic amino termini in [1]2þ, [4aa]Cl2,
or [4aa](OTf)2 are possibly preventing the formation of similar
triply bridged dinuclear complexes [5]þ (Chart 2), which should
be catalytically inactive as [5aH]Cl in the hydrogen transfer.
In order to draw a more precise mechanistic picture of how
[1]2þ, [4aa]Cl2, and [4aa](OTf)2 generate a catalytically
active species responsible for the hydrogen transfer, we next
performed a crossover experiment using [4aa]Cl2 and its
Cp*Ir analogue ([6aa]Cl2),14c which was separately prepared
in 87% yield by the reaction of [Cp*IrCl2]2 and 2-aminoeth-
anethiol in the presence of KOt-Bu in methanol (Ir:2-amino-
ethanethiol:KOt-Bu = 1:1:1) (Figure 3).
The crossover experiment was carried out as follows: an
equimolar mixture of [4aa]Cl2 and [6aa]Cl2 in methanol was
treated with four equivalents of KOt-Bu for 2 h at room tem-
perature and quenched by adding four equivalents of (C6H5)2-
NH2Cl, as illustrated in Scheme 3. ESI-MS analysis of the
reaction mixture showed an isotopic pattern centered at m/z=
743 for [C26H44N2IrRuS2]þ in addition to those at m/z = 679
and 807 for [C28H47N2Ru2S2]þ and [C24H41N2Ir2S2]þ, respec-
tively. Moreover, 13C{1H} NMR analysis (D2O) of the reac-
tion mixture after removal of methanol showed eight new
signals in addition to two sets of four signals for [4aa]Cl2 and
Figure 3. Solid-state structure of [6aa]Cl2 CH3OH H2O. Ther-
mal ellipsoids are drawn at the 30% probability level. The H atoms,
the anion, and the solvated molecule are omitted for clarity.
3
3
[6aa]Cl2 (see the Supporting Information). These results are
consistent with the formation of [8aa]Cl2,17a which was not
detectable unless [4aa]Cl2 and [6aa]Cl2 are subjected to the
base treatment.17b We believe that 2-fold dehydrochlorination
of [4aa]Cl2 and [6aa]Cl2 possibly generates transient dinuclear
μ-thiolato amido complexes [Ru(μ-SCH2CH2NH)(hmb)]2
and [Cp*Ir(μ-SCH2CH2NH)]2, which fall in equilibrium with
heterodinuclear μ-thiolato amido complexes {Ru(hmb)}(μ-
SCH2CH2NH)2(IrCp*) via splitting at their thiolato bridges.
These results along with the above-mentioned effect of the
structural difference of dinuclear complexes on the catalytic
performance strongly indicate that the base treatment of [4aa]Cl2
as well as [1aa]Cl2 and [4aa](OTf)2 commonly generates a
mononuclear Ru(SCH2CH2NH)(hmb),18 which can be a candi-
date for the catalytically active species, since it may exhib-
it characteristic bifunctionality in the hydrogen transfer as
shown in Scheme 4.
Conclusion
We have focused in this paper on the Ru-S interaction in
the half-sandwich-type Ru(η6-arene) complexes and found
(17) (a) Both anti and syn orientations of hmb and Cp* ligands are
possible. (b) The Ru2S2 core in [4aa]Cl2 was not susceptible to
dissociation with a number of two-electron donors except 2,6-dimethyl-
phenyl isocyanide (CNXy). The reaction with two equivalents of CNXy
followed by anion exchange with NaB(C6H5)4 gave a mononuclear
complex, [Ru(SCH2CH2NH2)(hmb)(CNXy)]B(C6H5)4 ([7a]B(C6H5)4),
whose structure was also confirmed by X-ray diffraction study (see the
Supporting Information).
(18) (a) Mashima and co-workers reported a structurally related
mononuclear Ru(S2C6H4)(hmb) complex. Mashima, K.; Kaneyoshi,
H.; Kaneko, S.; Mikami, A.; Tani, K.; Nakamura, A. Organometallics
1997, 16, 1016–1025. (b) It is unlikely that the Ru-S site12e mediates the
transfer of formal Hþ and H- in this particular case, because 2-dimethyl-
aminoethanethiol hardly accelerates the transfer hydrogenation under similar
conditions.10

4590 Organometallics, Vol. 29, No. 20, 2010
Ito et al.
Scheme 3. Observed (upper) and Calculated (lower) Isotopic Patterns for [4aa]Cl2, [6aa]Cl2, and [8aa]Cl2 of the Crossover
Experiment in ESI-MS Analysis
its unique effect on the Ru/NH bifunctionality in the catalytic
transfer hydrogenation. More specifically, we have devel-
oped a rational synthetic method for a variety of 2-amino-
ethanethiolato-bridged dinuclear Ru(hmb) complexes with a
different coordination mode. Their catalytic performance in
the hydrogen transfer under basic conditions indicated that
the relative proportion of the Ru(hmb) and the S-NH unit is
crucially important to generate a catalytically active species,
whose mononuclearity was suggested on the basis of the
crossover experiment. These results may indicate that the
metal-cooperating ligand bifunctional catalysis prevails also
in half-sandwich-type Ru(η6-arene) complexes with a Ru-S
bond, despite its strong tendency to form thiolato-bridged
multinuclear complexes. An ingenious design to promote the
generation of a mononuclear species is crucial to draw out
the latent potential of S-NH ligands unlike P-NH ligands
bearing a similar third-row anchoring element. The present
study should promote judicious use of suitably designed protic
amine ligands that adjust the balance of electronic factors on
the Ru/NH units in the bifunctional catalysis to exploit further
sophisticated catalyst performance.
Experimental Section
General Methods. All manipulations of oxygen- and moisture-
sensitive materials were conducted under a purified argon atmo-
sphere (BASF-Catalyst R3-11) using standard Schlenk techniques.
Unless otherwise noted, all solvents were dried by refluxing over
sodium benzophenone ketyl (THF and diethyl ether), CaH2
(n-hexane, 2-propanol, CH2Cl2, CD2Cl2, and CDCl3), or Mg
(methanol and CD3OD) and distilled immediately before use.

Article
Organometallics, Vol. 29, No. 20, 2010 4591
Scheme 4. Possible Pathway for the Generation
of a Catalytically Active Species
was washed with diethyl ether (5.0 mL ꢀ 3) to give [1aa]Cl2 as
orange powders (8.7 mg, 87% yield). Analytically pure samples
were obtained as a trihydrate by recrystallization from methanol
1
(99.5%)/diethyl ether. H NMR (CDCl3) δ 1.40-1.42 (m, 1H),
1.69-1.71 (m, 1H), 1.98 (s, 18H), 2.18 (s, 18H), 2.19-2.21 (m, 1H),
2.22-2.24 (m, 2H), 2.75-2.77 (m, 1H), 2.92-2.94 (m, 1H), 3.56-
3.58 (m, 1H), 5.03 (brs, 1H), 6.07 (brs, 1H), 8.63 (brs, 3H);13C{1H}
NMR (CD3OD) δ 15.7, 16.0, 27.0, 27.8, 41.8, 52.6, 97.7, 97.8;
ESI-MS (TOF) m/z calcd for [M - HCl2]þ, 715.1; found, 715.2.
Anal. Calcd for C28H49Cl3N2Ru2S2 3H2O: C, 40.02; H, 6.60; N,
3
3.33; S, 7.63. Found: C, 40.13; H, 6.51; N, 3.16; S, 7.39.
[Ru2Cl(μ-SCH2CH2NH2)(μ-Cl)(hmb)2]Cl ([2aCl]Cl). A mix-
ture of [RuCl2(hmb)]2 (425.0 mg, 0.64 mmol), 2-aminoethane-
thiol hydrochloride (73.2 mg, 0.64 mmol), and KOt-Bu (147.0 mg,
1.31 mmol) in CH2Cl2 (30 mL) and methanol (25 mL) was stirred
overnight at 30 °C. After evaporation of the solvent, the residue
was washed with THF and extracted with CH2Cl2 and methanol.
The combined extract was concentrated in vacuo, and the resi-
due was washed with diethyl ether followed by recrystallization
from methanol (99.5%)/diethyl ether to give orange crystals
whose analyses established the identity of [2aCl]Cl, but water
could not be removed completely (376.0 mg, 82% yield). 1H NMR
(CDCl3) δ 1.73-1.84 (m, 2H), 2.01 (s, 18H), 2.20 (s, 18H), 2.30-
2.52 (m, 2H), 4.80 (brs, 1H), 6.83 (brs, 1H) for the major isomer;
δ 1.73-1.84 (m, 2H), 1.94 (s, 18H), 2.11 (s, 18H), 2.30-2.52
(m, 2H), 3.87 (brs, 1H), 6.47 (brs, 1H) for the minor isomer;
13C{1H} NMR (CD3OD) δ 14.3, 14.5, 27.3, 50.2, 92.5, 93.6 for
the major isomer; δ 14.2, 14.4, 26.2, 50.8, 95.0, 95.9 for the minor
isomer; ESI-MS (TOF) m/z calcd for [M - Cl]þ, 674.1; found,
D2O was used as delivered. Other reagents including 2-amino-
ethanethiol (Aldrich M9768), 2-aminoethanethiol hydrochlo-
ride (Aldrich M6500), 2-(dimethylamino)ethanethiol hydrochlo-
ride (Aldrich D141003), benzyl mercaptan (Aldrich B25401),
4-methylbenzenethiol (Aldrich T28525), and (R)-(þ)-1-phenyl-
ethanol (>98.0% ee, TCI P0795) were purchased and used
as received. (S)-2-Pyrrolidinemethanethiol hydrochloride and
(S)-2-piperidinemethanethiol hydrochloride were prepared as we
reported previously.10 [RuCl2(hmb)]2,19a,b [Ru2Cl3(hmb)2]OTf,19c
and [Cp*IrCl2]219d were prepared by literature procedures. The
unsymmetrical dinuclear complex [1bb]Cl2 was prepared as we
reported previously.10 Column chromatography was performed
using Fuji Silysia silica gel FL100D (neutral). 1H (300.40 or
399.78 MHz) and 13C{1H} NMR (75.45 or 100.53 MHz) spectra
were recorded at 25 °C on JEOL JNM-LA300 and JNM-
ECX400 spectrometers. Chemical shifts were referenced to resid-
ual protio impurities in the deuterated solvents, and coupling
constants (J) are given in hertz. Electrospray mass spectra (ESI-
MS) were recorded with a JEOL JMS-T100CS spectrometer
using methanol as a solvent. Elemental analyses were performed
on a Perkin-Elmer 2400II CHN analyzer.
[Ru2Cl(μ-SCH2CH2NH2)(μ-SCH2CH2NH3)(hmb)2]Cl2 ([1aa]-
Cl2). This compound was obtained in a similar way to our published
procedure10 using [RuCl2(hmb)]2. A mixture of [RuCl2(hmb)]2
(459.4 mg, 0.69 mmol), 2-amionoethanethiol hydrochloride (159.0
mg, 1.40 mmol), and KOt-Bu (234.0 mg, 2.08 mmol) in 2-propanol
(20 mL) was refluxed for 3 h and cooled. After evaporation of the
solvent, the resulting dark yellow powders were washed with THF
and extracted with CH2Cl2. The extract was concentrated in vacuo,
and the residue was recrystallized from methanol/diethyl ether to
give [1aa]Cl2 as red crystals (522.0 mg, 96%). Alternatively, this
compound was prepared from [2aCl]Cl as follows: to a solution of
[2aCl]Cl (9.0 mg, 13 μmol) in methanol (7.0 mL) at 0 °C was slowly
added a solution of 2-aminoethanethiol (99.5 μg, 13 μmol) in
methanol (0.26 mL). The resulting mixture was warmed to 80 °C
and refluxed overnight. After evaporation of the solvent, the resi-
due was washed with THF (8.0 mLꢀ3) and extracted with CH2Cl2
(6.0 mL). The extract was concentrated in vacuo, and the residue
674.1. Anal. Calcd for C26H42Cl3NRu2S 1/2H2O: C, 43.48; H,
3
6.03; N, 1.95. Found: C, 43.56; H, 6.17; N, 2.06.
[Ru2Cl(μ-SCH2CH2NH2)(μ-SCH2C6H5)(hmb)2]Cl ([3ax]Cl).
To a solution of [2aCl]Cl (95.8 mg, 0.135 mmol) in methanol
(20 mL) at 0 °C was slowly added a solution of benzyl mercaptan
(16 mL, 0.136 mmol) and sodium methoxide (7.3 mg, 0.135 mmol)
in methanol (5.0 mL). The resulting mixture was warmed to 30 °C
and stirred overnight at the same temperature. After evapora-
tion of the solvent, the residue was extracted with CH2Cl2. The
extract was concentrated in vacuo, and the residue was washed
with diethyl ether followed by recrystallization from reagent
grade CHCl3/diethyl ether to give [3ax]Cl monohydrate as
orange crystals (94.3 mg, 87% yield). 1H NMR (CDCl3) δ 1.61-
1.69 (m, 1H), 1.74 (s, 18H), 2.20 (s, 18H), 2.34-2.39 (m, 1H),
2.47-2.53 (m, 1H), 2.56 (d, J = 12, 1H), 3.67 (d, J = 12, 1H),
3.74-3.78 (m, 1H), 5.04 (brs, 1H), 7.00 (brs, 1H), 7.14-7.18 (m,
1H), 7.21-7.24 (m, 2H), 7.32-7.34 (m, 2H); 13C{1H} NMR
(CD3OD) δ 13.7, 14.2, 26.1, 32.7, 50.8, 95.0, 95.9, 126.8, 128.3,
130.1, 140.1; ESI-MS (TOF) m/z calcd for [M - Cl]þ, 762.1;
found, 762.3. Anal. Calcd for C33H49Cl2NRu2S2 H2O: C, 48.64;
3
H, 6.31; N, 1.72. Found: C, 48.52; H, 6.25; N, 1.87.
[Ru(μ-SCH2CH2NH2)(hmb)]2Cl2 ([4aa]Cl2). To a solution of
[2aCl]Cl (36.9 mg, 52 μmol) in methanol (18 mL) at -78 °C was
slowly added a solution of 2-aminoethanethiol hydrochloride
(6.0 mg, 52 μmol) and KOt-Bu (11.9 mg, 0.11 mmol) in metha-
nol (1.4 mL). The resulting mixture was warmed to 30 °C and
stirred overnight at the same temperature. After evaporation of
the solvent, the residue was extracted with CH2Cl2. The extract
was concentrated in vacuo, and the residue was washed with
reagent grade diethyl ether to give [4aa]Cl2 monohydrate as
orange powders (34.5 mg, 88% yield). Alternatively, [4aa]Cl2
could be obtained by the reaction of [RuCl2(hmb)]2 (41.4 mg,
62 μmol) with 2-amionoethanethiol hydrochloride (14.1 mg,
124 μmol) and KOt-Bu (27.8 mg, 248 μmol) in methanol (30 mL)
at 30 °C overnight. After evaporation of the solvent, the residue
was extracted with CH2Cl2. The extract was concentrated in vacuo,
and the residue was washed with reagent grade diethyl ether to
give [4aa]Cl2 monohydrate as red powders (43.7 mg, 94% yield).
The syn orientation of the two hmb ligands was determined by
comparison with spectroscopic data for [4aa](OTf)2. 1H NMR
(D2O) δ 1.48-1.56 (m, 2H), 1.83-1.90 (m, 2H), 2.00 (s, 36H),
(19) (a) Bennett, M. A.; Matheson, T. W.; Robertson, G. B.; Smith,
A. K.; Tucker, P. A. Inorg. Chem. 1980, 19, 1014–1021. (b) Bennett, M. A.;
Huang, T.-N.; Matheson, T. W.; Smith, A. K.; Ittel, S.; Nickerson, W. Inorg.
Synth., doi:10.1002/9780470132524.ch16 (c) Bennett, M. A.; Ennett, J. P. Inorg.
Chim. Acta 1992, 198-200, 583–592. (d) C. White, C.; Yates, A.; Maitlis, P. M.;
Heinekey, D. M. Inorg. Synth., doi: 10.1002/9780470132609.ch53

4592 Organometallics, Vol. 29, No. 20, 2010
Ito et al.
2.09-2.17 (m, 2H), 3.40-3.45 (m, 2H); 13C{1H} NMR (D2O,
referenced to CD3OD (50.0 ppm)) δ 14.6, 26.4, 51.5, 96.7; ESI-MS
(TOF) m/z calcd for [M - HCl2]þ, 679.1; found, 679.1. Anal.
mixture was vigorously stirred at 30 °C. The progress of the
racemization was monitored with an aliquot by HPLC analysis
using a Daicel Chiralcel OD column (df=4.6 mm i.d.ꢀ250 mm;
eluent 95:5 n-hexane/2-propanol; temp 30 °C, flow rate 0.5 mL/min;
detection (UV), λ=254 nm light); tR=16.2 min for R-isomer and
tR =18.1 min for S-isomer).
Calcd for C28H48Cl2N2Ru2S2 H2O: C, 43.80; H, 6.56; N, 3.65.
3
Found: C, 43.99; H, 6.83; N, 3.36.
[Ru(μ-SCH2CH2NH2)(hmb)]2(OTf)2 ([4aa](OTf)2). To a solu-
tion of [4aa]Cl2 (43.7 mg, 58.3 μmol) in methanol (10 mL) at -78 °C
was slowly added NaOTf (22.0 mg, 127.8 μmol). The resulting
mixture was warmed to 30 °C and stirred overnight at the same
temperature. After evaporation of the solvent, the residue was ex-
tracted with CH2Cl2. The extract was concentrated in vacuo, andthe
residue was washed with diethyl ether to give [4aa](OTf)2 mono-
hydrate as red powders (32.8 mg, 58% yield). The anti orientation
of the two hmb ligands in the solid state was determined by X-ray
analysis (see the Supporting Information) of red crystals, which
were obtained by recrystallization from methanol/diethyl ether
Asymmetric Transfer Hydrogenation of Acetophenone with [1cc]2þ
.
A mixture of formic acid (0.50 mL, 13.2 mmol), N(C2H5)3 (3.1 mL,
22.2 mmol), and acetophenone (1.0 mL, 8.6 mmol) was degassed
by three freeze-pump-thaw cycles and then added to a mixture
of [1cc]2þ (44.3 mg, 0.044 mmol) (calculated on the assumption
that one of two counteranions in [1cc]2þ is Cl and the other is
OTf; see the Supporting Information) placed in a Schlenk tube.
The mixture was stirred at 30 °C for 24 h, and the resulting
mixture was passed through a silica plug under air eluting with
reagent grade ethyl acetate. The filtrate was concentrated under
reduced pressure, and the residue was purified by column
chromatography (Kanto silica gel 60 N, spherical, neutral)
eluting with n-hexane/ethyl acetate (v/v=3:1) to give 1-phenyl-
ethanol (0.94 g, 90% yield), whose ee value and configuration
(86%, (S)) were determined by HPLC analysis as above.
Crossover Experiment. A mixture of [4aa]Cl2 (24.5 mg, 32.7
μmol), [6aa]Cl2 (29.5 mg, 33.6 μmol), and KOt-Bu (15.9 mg, 142
μmol) in methanol (40 mL) was stirred at 30 °C for 2 h. Then
(C6H5)2NH2Cl (27.7 mg, 135 μmol) was added at 0 °C, and the
resulting mixture was further stirred at 30 °C for 2 h. The solvent
was removed under reduced pressure, and the residue was washed
with diethyl ether. ESI-MS and 13C{1H} NMR analyses of the
resulting red solids indicated the formation of [{Ru(hmb)}(μ-
SCH2CH2NH2)2(IrCp*)]Cl2 ([8aa]Cl2) in addition to [4aa]Cl2
and [6aa]Cl2. 13C{1H} NMR (D2O) δ 7.9, 14.5, 26.3, 27.1, 51.4,
53.5, 91.7, 97.1; ESI-MS (TOF) m/z calcd for [M - HCl2]þ, 743.0;
found, 743.0 ([8aa]Cl2).
1
(v/v = 1:19). H NMR (CD2Cl2) δ 1.70-1.99 (m, 8H), 2.08 (s,
36H), 3.84 (brs, 2H), 4.53 (brs, 2H); 13C{1H} NMR (CD3OD) δ
14.2, 26.7, 51.9, 96.8, 120.4 (q, 1JCF = 319); ESI-MS (TOF) m/z
calcd for [M - H(OTf)2]þ, 679.1; found, 679.0. Anal. Calcd for
C30H48F6N2O6Ru2S4 H2O: C, 36.21; H, 5.06; N, 2.82. Found: C,
3
36.31; H, 5.03; N, 2.84.
[Ru2(μ-SCH2CH2NH)(μ-H)(hmb)2]Cl ([5aH]Cl). To a solu-
tion of [2aCl]Cl (11.7 mg, 16.5 μmol) in 2-propanol (25 mL) at
0 °C was slowly added a solution of KOH (85%, 22.5 mg, 34.1
μmol) in 2-propanol (0.44 mL). The resulting mixture was
warmed to 30 °C and stirred overnight at the same temperature.
After evaporation of the solvent, the resulting dark orange
powders were extracted with CH2Cl2. The extract was concen-
trated in vacuo, and the residue was recrystallized from CH2Cl2/
diethyl ether to give dark brown crystals, whose analyses estab-
lished the identity of [5aH]Cl, but CH2Cl2 could not be removed
completely (10.5 mg, 81% yield). 1H NMR (CDCl3) δ -12.3 (s,
1H), 1.91-1.94 (m, 2H), 2.28 (s, 36H), 2.93-2.95 (m, 2H), 5.32
(brs, 1H); 13C{1H} NMR (CDCl3) δ 17.8, 31.7, 58.7, 94.8; ESI-
MS (TOF) m/z calcd for [M - Cl]þ, 604.1; found, 604.0. Anal.
Acknowledgment. This research was supported by the
MEXT (Nos. 16750073 and 18065017 “Chemistry of
Concerto Catalysis”) and by the Asahi Glass Foundation
(M.I.). This work was partially supported by the GCOE
Program. We are grateful to Professors Shigeki Kuwata
and Toshiro Takao (Tokyo Institute of Technology) for
their valuable discussions.
Calcd for C26H42ClNRu2S 7/4CH2Cl2: C, 42.36; H, 5.83; N,
3
1.78. Found: C, 42.18; H, 6.03; N, 1.81.
The other dinuclear Ru(hmb) complexes ([1cc]2þ, [2aCl]OTf,
[2aCl]B(C6H5)4, [2bCl]OTf, [2cCl]OTf, [3ay]OTf, [3az]Cl) and
the dinuclear Cp*Ir complex [6aa]Cl2 were prepared in a similar
way to those described above (see the Supporting Information
for details).
Supporting Information Available: Text, figures, and CIF files
giving experimental procedures and X-ray crystallographic data
General Procedure for Catalytic Racemization. A degassed
solution of (R)-(þ)-1-phenylethanol in toluene (0.625 M) was
transferred to a mixture of an appropriate Ru(hmb) complex
and KOt-Bu (alcohol:Ru = 100:1, the amount of base is indi-
cated in the text) placed in a 20 mL Schlenk tube. The resulting
for [2aCl]OTf, [2aCl]B(C6H5)4, [2bCl]OTf, [2cCl]OTf, [1cc]2þ
,
[3ay]OTf, [3az]Cl, [4aa](OTf)2, [6aa]Cl2, and [7a]B(C6H5)4. This
material is available free of charge via the Internet at http://
pubs.acs.org.