Synthesis and oxidation study of the simplest binuclear metallocene
compound of osmium, biosmocene
Hiroki Yasuhara, Kazuki Koga, Satoru Nakashima
*
Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan
article info
Article history:
Received 21 October 2014
Received in revised form
20 December 2014
Accepted 22 December 2014
Available online 27 December 2014
Keywords:
Biosmocene
Mixed-valence state
Variable-temperature NMR
Valence-detrapping
abstract
Selective mono-lithiation method of bis(cyclopentadienyl)osmium(II) (osmocene) was established.
Iodoosmocene(C
5
H
4
I)Os(C
5
H
5
) (OcI), which is the first monohalogenated product of osmocene, was
prepared by the reaction of lithioosmocene with I
2
. Ullmann coupling reaction of iodoosmoene allowed
to prepare biosmocene(C
5
H
5
)Os(m
2
-h
5
:h
5
-C
10
H
8
)Os(C
5
H
5
) (OcOc). OcOc reacted with I
2
, giving mixed-
valence biosmocenium salt [(C
5
H
5
)Os
II
(m
2
-h
5
:h
5
-C
10
H
8
) (C
5
H
5
)Os
IV
I]I
3
(A). The structure of A was deter-
mined by single crystal X-ray structural analysis. The intramolecular electron transfer reaction between
Os
II
and Os
IV
was observed by using
1
H NMR spectroscopy, accompanied by the exchange of I
anion
between the two units. The activation energy of the electron transfer reaction was estimated and the
value was larger than that of binuclear ruthenocenium salt [(C
5
H
5
)Ru
II
(m
2
-h
5
:h
5
-C
10
H
8
) (C
5
H
5
)Ru
IV
I]I
3
(B).
© 2015 Elsevier B.V. All rights reserved.
Introduction
There have been many investigations concerning the mixed
valence state of binuclear metallocene derivatives of iron [1e6] and
ruthenium [7e10], while such study about osmium derivative has
not been reported. Any attempts for synthesis of the simplest
binuclear metallocene compound of osmium, namely biosmocene
(OcOc), resulted in failure [8,11], and even monohalogenated
osmocene compound, which is considered as useful precursor of
binuclear compounds, had not been prepared. Although prepara-
tion method of decahalogenated osmocene derivatives
(C
5
X
5
)
2
Os(X ¼ Cl, Br, I) has been reported [12], these halides are
obviously inconvenient for synthetic experiment because of the
poor solubility, preparation difficulty and need of toxic mercury
reagent for preparation. First of all, we had to find the facile and
useful halogenation method of osmocene for development of the
binuclear osmocene chemistry. As for ferrocene and ruthenocene,
R. Sanders and U. T. Muller-Westerhoff established the selective
mono-lithiation method of these methallocens [13], and M. S.
Inkpen et al. reported that mono-iodoferrocene (C
5
H
4
I)Fe(C
5
H
5
)
(FcI) was isolated by the reaction of mono-lithioferrocene (C
5
H
4
Li)
Fe(C
5
H
5
) (FcLi) and I
2
[14]. Using this synthesis strategy, we
succeeded in selective preparation of mono-iodoosmocene(OcI) in
77% yield. OcI is the first example of monohalogenated product of
osmocene. In general, Ullmann coupling reaction readily generates
biaryl compound from two aryl iodides. Therefore, we selected this
type of reaction for preparation of OcOc. The attempt was suc-
cessful, and OcOc was obtained in moderate yield (48%).
It was reported that mixed-valence biruthenocenium salt
[(C
5
H
5
)Ru
II
(m
2
-h
5
:h
5
-C
10
H
8
) (C
5
H
5
)Ru
IV
I]I
3
(B) [8] was generated by
oxidation of RcRc with two equivalent mol of I
2
. Intramolecular
electron transfer reaction between two Ru centers was observed for
a solution of B by using
1
H NMR spectroscopy. For the observation
of the similar reaction between two Os centers, we attempted
oxidation of OcOc by I
2
, then succeeded in isolation of mixed-
valence biosmocenium salt [(C
5
H
5
)Os
II
(m
2
-h
5
:h
5
-C
10
H
8
) (C
5
H
5
)
Os
IV
I]I
3
(A). A was characterized by
1
H NMR, ESI-MS and elemental
analysis, and the structure was determined by single-crystal X-ray
structural analysis. Intramolecular electron transfer reaction of A
was also observed by variable-temperature
1
H NMR measurement.
Herein, we discuss the synthesis and oxidation state of OcOc by
comparing them with the corresponding Ru analogs.
Experimental
General information
All reactions were performed under an argon atmosphere unless
explicitly mentioned, while purification procedure was carried out
* Corresponding author. Natural Science Center for Basic Research and Devel-
opment (N-BARD), Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-
8526, Japan. Tel.: þ81 82 424 6291.
E-mail address: snaka@hiroshima-u.ac.jp (S. Nakashima).
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
http://dx.doi.org/10.1016/j.jorganchem.2014.12.030
0022-328X/© 2015 Elsevier B.V. All rights reserved.
Journal of Organometallic Chemistry 779 (2015) 86e90