Synthesis and structure of novel (η1:η6- aminoaikylarene)RuII complexes

Article abstract of DOI:10.1246/cl.2009.98

A series of mononuclear tethered complexes, RuCl₂(η ¹:η⁶-aminoalkylarene), in which the η⁶-arene group and the ligated protic amine group are connected by suitable aliphatic carbon chains, were synthesized from

Full text of DOI:10.1246/cl.2009.98

98
Chemistry Letters Vol.38, No.1 (2009)
Synthesis and Structure of Novel (ꢀ1:ꢀ6-Aminoalkylarene)RuII Complexes
Masato Ito, Hiroko Komatsu, Yoshinori Endo, and Takao Ikariyaꢀ
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
(Received October 9, 2008; CL-080975; E-mail: tikariya@apc.titech.ac.jp)
A series of mononuclear tethered complexes, RuCl2(ꢀ1:ꢀ6-
the bifunctional catalysts. These results prompted us to expand
the conceptual Ru/NH bifunctionality by introducing new che-
lating protic amine ligands, in which the ꢀ6-arene and the ligated
amine groups are connected by suitable tethers such as 2 in
Scheme 1. We disclose herein the synthesis and the structure
of new tethered complexes, RuCl2(ꢀ1:ꢀ6-aminoalkylarene),
which have been conveniently prepared by the intramolecular
aminoalkylarene), in which the ꢀ6-arene group and the ligated
protic amine group are connected by suitable aliphatic carbon
chains, were synthesized from RuCl2(ꢀ6-PhCO2C2H5)(amino-
alkylarene-ꢁ-N) complexes via an intramolecular arene dis-
placement reaction.
arene displacement reaction.8
8a
In recent years, much attention has been focused on the
‘‘metal–ligand bifunctional catalysis,’’ established by Noyori
and co-workers,1 where both the metal and the ligand concerted-
ly participate in bond-forming and -breaking processes, and a co-
ordinatively saturated metal complex reacts with an unsaturated
substrate directly without metal ligation. Much of our effort in
this area has been devoted to the molecular design of half-sand-
wich type complexes, (ꢀ6-C6R6)RuII or (ꢀ5-C5R5)RuII with the
characteristic Ru/NH bifunctionality, in which a 16-electron
amide complex activates protic molecules to provide an 18-elec-
tron amine complex with a nucleophilic part as a ligand
(Scheme 1).
A
reaction of dimeric [RuCl2(ꢀ6-PhCO2C2H5)]2
in
CH2Cl2 with 2-aminoethyl- or 3-aminopropylarenes (3a–3g)
with an NH group was found to proceed smoothly in CH2Cl2
to give mononuclear RuCl2(ꢀ6-PhCO2C2H5)(aminoalkylarene-
ꢁ-N) (4a–4g) (Scheme 2).
C2H5CO2
Rn
n-1
Rn
∆
Ru
Cl
[RuCl2(η6-PhCO2C2H5)]2
+
n-1
N
Cl
CH2Cl2
C6H5Cl
H2
NH2
3a–g
4a–g
3a : o-(CH3)3SiC6H4(CH2)2NH2
3b : m-(CH3)3SiC6H4(CH2)2NH2
3c : p-(CH3)3SiC6H4(CH2)2NH2
3d : (CH3)5C6(CH2)2NH2
3e : m-(CH3)3SiC6H4(CH2)3NH2
3f : p-(CH3)3SiC6H4(CH2)3NH2
3g : (CH3)5C6(CH2)3NH2
Rn
Rn
SbF6
n-1
L1
n-1
Nu
H
Rn
Rn
Cl
Ru
Ru
L1
Ru
Ru
n-1
AgSbF6
CH2Cl2
N
L
Cl
N
Cl
Nu
L
H2
H2
Ru
Cl
2
N
H
Ru
L
NH
Cl
2a–g
N
H
NH2 Cl
A
B
A
B
H2
L1 : PPh3 (5), t-BuNC (6)
Nu
H
16e amide
18e amine
; H-H (i-C3H7OH, HCO2H, H2)
; carbonyl, imine, epoxide, imide,
activated olefin, RN=NR
Nu
A
H
B
1
Scheme 2.
The resulting complexes 4a–4g were subjected to thermal
intramolecular arene displacement in chlorobenzene at 140 ꢁC
to produce RuCl2(ꢀ1:ꢀ6-aminoalkylarene) (2a–2g) with con-
comitant elimination of ethyl benzoate. Precipitation by adding
hexane to the reaction mixture led to the isolation of 2a–2g as
brownish yellow solids in 32–90% yields. Figure 1 illustrates
the molecular structures of 2d and 2g determined by X-ray dif-
fraction.9
The tethered moiety in 2 is surprisingly robust and tolerant
of several reaction conditions in the presence of other 2e donor
ligands including phosphines or isonitriles. For example, the re-
action of 2 with excess PPh3 in the presence of AgSbF6 in reflux-
ing CH2Cl2 or methanol yielded cationic [RuCl(ꢀ1:ꢀ6-amino-
alkylarene)(PPh3)]þ complexes 5.10 Notably, the planar chiral
complexes 2b and 2e afforded single diastereomers 5b and 5e
Scheme 1.
Not only have sec-alcohols,2 formic acid,3 and molecular di-
hydrogen4 been found to serve as a hydrogen donor to provide
amine hydride complexes, but also organic compounds with an
acidic C–H bond5 including nitroalkanes,5a,5d malonates,5b,5c,5e
and various acidic compounds5f serve as a pronucleophile to give
amine complexes with a Ru-bonded nucleophile. The intercon-
version between the amide and amine complexes facilitates cat-
alytic chemo- and stereoselective reduction of ketones,2a,2b,3,4a
imines,3a epoxides,4b and imides4c via a smooth transfer of the
protic NH and hydridic RuH to the polar functionality. In addi-
tion, stereoselective Henry5a and Michael-type reactions5b–5f are
also accomplished by the amine complex, which possibly deliv-
ers the protic NH and Ru-bonded nucleophiles to aldehydes,5a
activated olefins,5b–5e and azo compounds,5f regenerating amide
complexes in an analogous manner.
In principle, half-sandwich type (ꢀ6-C6R6)RuII amide com-
plexes have been prepared or generated in situ by the base-in-
duced dehydrochlorination of the readily prepared amine chlo-
ride complexes 1. A range of chelating protic amines (L–NH2)
such as N-sulfonylated 1,2-diamines,1a 2-aminoalcohols,6 and
benzylamine derivatives7 can be used as excellent ligands of
Ru
Ru
N
N
Cl
Cl
Cl
Cl
Figure 1. Molecular structures of 2d (left) and 2g (right). All
hydrogens except on nitrogen are omitted for clarity.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.1 (2009)
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Si
Si
2
3
Ru
Ru
N
N
Cl
N
P
Cl
Figure 2. Molecular structures of anti-5e (left) and syn-6e
(right). Ortho, meta, and para carbons on PPh3 and all hydrogens
except on nitrogen are omitted for clarity.
4
5
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The intramolecular arene displacement has been extensively used
to prepare a variety of tethered Ru(ꢀ6-arene) complexes since the
following two papers8a,8b were reported, but there had been no
report that employs the protic amine group as an anchor when
we initiated this research.8c Independently, a similar reaction
was recently reported by Melchart et al.8d a) B. Therrien, T. R.
Ward, M. Pilkington, C. Hoffmann, F. Gilardoni, J. Weber, Orga-
nometallics 1998, 17, 330. b) P. Smith, A. H. Wright, J. Organo-
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Brabec, P. J. Sadler, Inorg. Chem. 2007, 46, 8950.
Si
Si
Ru
Ru
N
N
Cl
Cl
P
P
Figure 3. Molecular structures of 5c (left) and 5f (right). Ortho,
meta, and para carbons on PPh3 and all hydrogens except on ni-
trogen are omitted for clarity.
in reasonably high yields, as evidenced by 31P NMR analysis of
the crude reaction mixtures. X-ray diffraction on a single crystal
of 5e9 revealed that the silyl substituent and phosphine ligand
adopt anti orientation with each other (Figure 2, left).
In sharp contrast, a 1:1 mixture of diastereomers of
[RuCl(ꢀ1:ꢀ6-aminoalkylarene)(tert-BuNC)]þ complexes 6 was
obtained when tert-BuNC was used in place of PPh3. Relative
orientation of the silyl group and isonitrile ligand for the less
soluble 6e was unambiguously determined to be syn orientation
by X-ray crystallographic analysis9 (Figure 2, right). These
results may suggest that steric congestion of incoming ligand
is crucial for the high diastereoselectivity.11
6
7
8
Additionally, solid-state structural analysis indicated that
the relatively short C2 tether would impose the conformational
constraints of the arene ligand. For example, the C2 tethered
complexes 2d and 5c9 (Figure 3, left) consistently have the short-
˚
est Ru–C bond in the Ru–Cipso (2.098 and 2.15 A, respectively)
˚
and the longest one in the Ru–Cpara (2.227 and 2.284 A, respec-
tively). In contrast, the C3 tethered 2g and 5d9 (Figure 3, right)
show no salient feature in the difference of the Ru–C bond
lengths (See Supporting Information).13 Similar difference of
the tethered structure has played a crucial role in the catalytic
performance of triflylamide tethered arene–Ru complexes.12
In summary, we have developed an efficient method for the
preparation of a series of tethered complexes RuCl2(ꢀ1:ꢀ6-ami-
noalkylarene). The tethered dichloride complexes were found to
serve as convenient precursors for a variety of other tethered
complexes. Their catalytic performance focusing on their Ru/
NH bifunctionality will be reported in due course.
9
Crystallographic data reported in this paper have been deposited
with Cambridge Crystallographic Data Centre as supplementary
publication Nos. CCDC-704734 (2d), -704735 (2g), -704736
(anti-5e), -704737 (syn-6e), -704738 (5c), and -704739 (5f).
Copies of the data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html.
10 Our method is complementary to that for the preparation of similar
cationic complexes reported by Miyaki et al.,10a in which the intro-
duction of the ꢀ6-arene ligand with protected 1-aminoalkyl-1,4-
cyclohexadienes precedes coordination of amine moiety. a) Y.
Miyaki, T. Onishi, H. Kurosawa, Inorg. Chim. Acta 2000, 300–
302, 369. See also: b) F. K. Cheung, C. Lin, F. Minissi, A. L.
´
Criville, M. A. Graham, D. J. Fox, M. Wills, Org. Lett. 2007, 9,
4659, and references therein.
This paper is dedicated to Professor Ryoji Noyori on the
occasion of his 70th birthday.
11 K. Onitsuka, N. Dodo, Y. Matsushima, S. Takahashi, Chem. Com-
mun. 2001, 521.
12 M. Ito, Y. Endo, T. Ikariya, Organometallics, ASAP article;
doi:10.1021/om800735z.
13 Supporting Information for full experimental details is available
electronically via http://www.csj.jp/journals/chem-lett/index.html.
References and Notes
1
a) K.-J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R. Noyori,
Angew. Chem., Int. Ed. Engl. 1997, 36, 285. b) R. Noyori, S.
Published on the web (Advance View) January 5, 2009; doi:10.1246/cl.2009.98