A bis-bipyridine osmium(ii) complex with an N,S-chelating 2-aminoethanesulfinate: Photoinduced conversion of an amine to an imine donor group by air oxidation

Article abstract of DOI:10.1039/c1cc14342e

Treatment of [Os(bpy)₂Cl₂] (bpy = 2,2′-bipyridine) with 2-aminoethanethiolate was accompanied by air oxidation to give [Os(2-aminoethanesulfinato-N,S)(bpy)₂]⁺ ([1]⁺), which was further oxidized by air to be converted into [Os(2-iminoethanesulfinato-N,S)(bpy)₂]⁺ ([2]⁺) under photoirradiation. Complex [2]⁺ was reverted back to [1] ⁺ by treatment with BH₄^-.

Full text of DOI:10.1039/c1cc14342e

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COMMUNICATION
A bis-bipyridine osmium(II) complex with an N,S-chelating
2-aminoethanesulfinate: photoinduced conversion of an amine to an imine
donor group by air oxidationw
Motoshi Tamura,a Kiyoshi Tsuge,b Asako Igashira-Kamiyamaa and Takumi Konno*a
Received 19th July 2011, Accepted 14th October 2011
DOI: 10.1039/c1cc14342e
Treatment of [Os(bpy)2Cl2] (bpy = 2,20-bipyridine) with 2-amino-
ethanethiolate was accompanied by air oxidation to give
[Os(2-aminoethanesulfinato-N,S)(bpy)2]+ ([1]+), which was
further oxidized by air to be converted into [Os(2-iminoethane-
sulfinato-N,S)(bpy)2]+ ([2]+) under photoirradiation. Complex [2]+
was reverted back to [1]+ by treatment with BH4
.
ꢀ
Scheme 1 Synthetic routes of [1]+ and [2]+
.
to be further oxidized by air under photoirradiation, forming an
iminosulfinato-type complex, [Os(iesi)(bpy)2]+ (iesi = 2-imino-
ethanesulfinate), with retention of the coordination environment
about an OsII center (Scheme 1).
2-Aminoethanethiolate (aet) containing both soft S and hard
N donors is a well-known chelating ligand available for the
formation of a wide variety of coordination compounds.1 The
thiolato group of aet bound to a metal center possesses a
relatively high nucleophilicity that enables chemical modifica-
tions, such as S-metalation and S-alkylation.2,3 Moreover, it has
been recognized that the oxidation reactions of aet complexes
afford sulfenato and sulfinato derivatives that show unique
spectroscopic and electrochemical properties.4 Recently, we have
reported that the reaction of [Ru(bpy)2Cl2] with aet in air leads to
the isolation of a sulfinato complex, [Ru(aesi)(bpy)2]+ (aesi =
2-aminoethanesulfinate), in which aesi chelates to a RuII center
through amine-N and sulfinato-S donors, rather than an
expected thiolato complex, [Ru(aet)(bpy)2]+.5 This result is
indicative of a higher reactivity of an aliphatic thiolato group
bound to a RuII center toward dioxygen. Osmium, which is an
alternative member of group 8 elements, has been known to form
coordination compounds analogous to ruthenium compounds.6
However, osmium compounds often exhibit characteristic redox
and photophysical properties that are not observed for the
corresponding ruthenium compounds.6b–e In this context, it is
interesting to investigate the reaction of [Os(bpy)2Cl2] with aet
under similar conditions employed for [Ru(bpy)2Cl2]. Herein, we
report that this reaction produces an aminosulfinato-type osmium(II)
complex, [Os(aesi)(bpy)2]+, analogous to [Ru(aesi)(bpy)2]+, accom-
panied by air oxidation. Remarkably, [Os(aesi)(bpy)2]+ was found
The reaction of [Os(bpy)2Cl2]7 with 1 molar equiv. of Haet/
NaOH (1 : 1) in aqueous ethanol under a nitrogen atmosphere,
followed by exposure to air, gave a dark brown solution. From
this reaction solution, a dark green complex ([1]PF6) was isolated
after the purification by means of a cation-exchange column
chromatography (SP-Sephadex C-25).w The 1H NMR spectrum
of [1]PF6 in D2O exhibits two methylene proton signals at d 3.04
and 2.80 and aromatic proton signals in the region of
d 10.04–7.12 (Fig. 1a). In addition, a broad signal is observed
at d 5.43, which is characteristic of an amine group bound to a
metal center.8 In the IR spectrum, [1]PF6 gives two intense bands
at 1114 cmꢀ1 and 1006 cmꢀ1 assignable to nasym
and
SQO
nsym SQO for an S-bonded sulfinato group (Fig. S1, ESIw).4a,5,9
From these spectral features, together with the elemental analysis,
[1]+ is assigned to a bis(bipyridine)osmium(II) complex with an
N,S-chelating aesi ligand, [Os(aesi)(bpy)2]+.
The structure of [1]+ was determined by single-crystal X-ray
analysis for [1]ClO4ꢁNaClO4 that was obtained by the slow
diffusion of diethyl ether into an ethanol solution containing
[1]PF6 and NaClO4. As shown in Fig. 2, [1]+ has an expected
a Department of Chemistry, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan.
E-mail: konno@chem.sci.osaka-u.ac.jp; Fax: +81-6-6850-5765;
Tel: +81-6-6850-5765
b Department of Chemistry, Graduate School of Science and
Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555,
Japan
w Electronic supplementary information (ESI) available: Details of
syntheses together with spectroscopic data and X-ray structure.
CCDC 834770 and 834771. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1cc14342e
Fig. 1 1H NMR spectra of (a) [1]PF6 and (b) [2]PF6 in D2O.
c
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month. Notably, this solution showed new proton signals, in
addition to the original signals for [1]+, in the 1H NMR
spectrum, while the spectrum remained almost unchanged upon
storing in the dark. Then, the 1H NMR spectral change for
[1]PF6 was monitored at 0 1C in D2O under the irradiation of a
high pressure Hg lamp in air. As a result, the original proton
signals for [1]+ decreased with time and almost disappeared
within 1.5 days, followed by the appearance and growth of a set
of new signals that are identical with those appeared under
ambient light (Fig. S4, ESIw). This spectral change implies that
[1]+ is photosensitive to be almost quantitatively converted into
another bis(bipyridine)osmium(II) species ([2]+) in solution.
Compound [2]PF6 was successfully isolated in a satisfactory
yield from an irradiated aqueous solution of [1]PF6.w While
the absorption spectral feature of [2]+ in water resembles that
Fig. 2 Perspective views of the complex cations [1]+ (left) and [2]+
(right) with the atomic labeling scheme.
mononuclear structure in [Os(aesi)(bpy)2]+, in which an OsII atom
is surrounded by an aesi and two bpy ligands in an approximately
octahedral geometry.z The aesi ligand coordinates to an OsII center
through amine-N and sulfinato-S atoms to form a five membered
chelate ring. This is the first example of a crystallographically
characterized osmium species with an S-bonded sulfinato group.10
The overall structure in [1]+ is very similar to that in the
corresponding ruthenium(II) complex, [Ru(aesi)(bpy)2]+, and the
bond distances around an OsII center in [1]+ (Os–S = 2.248(2) A,
Os–Naesi = 2.160(6) A, av. Os–Nbpy = 2.078(6) A), as well as the
S–O distances (av. 1.477(6) A) are comparable well with those in
[Ru(aesi)(bpy)2]+ (av. Ru–S = 2.246(1) A, Ru–Naesi = 2.144(4) A,
Ru–Nbpy = 2.083(4) A, S–O = 1.481(4) A).5 In crystal [1]ClO4ꢁ
NaClO4, the sulfinato group binds to two Na+ ions using two O
atoms (OꢁꢁꢁNa = 2.236(6) A, 2.356(6) A), constructing a unique
dimeric structure (Fig. S2, ESIw).
1
3
of [1]+ over the whole region, its MLCT and MLCT bands
are slightly blue-shifted compared with those of [1]+ (Fig. 3).
1
In the H NMR spectrum in D2O, [2]PF6 exhibits character-
istic signals at d 12.20 assignable to an imine proton,11,12 with
the lack of amine proton signals found in [1]+ (Fig. 1b). From these
results, together with the elemental analysis, it is assumed that [2]+
contains 2-iminoethanesulfinate (iesi) as a chelating ligand, in place
of aesi in [1]+.
This assumption was confirmed by X-ray analysis of [2]PF6ꢁ
NH4PF6, which was obtained by slow diffusion of diethyl ether
into an ethanol solution containing [2]PF6 and NH4PF6. As shown
in Fig. 2, [2]+ has an octahedral structure in [Os(iesi)(bpy)2]+, in
which iesi chelates to an OsII center through imine-N and sulfinato-
S groups.z The N–C bond distance (1.291(5) A) of iesi in [2]+ is
shorter and its N1–C2–C1 angle (120.7(4)1) is larger than the
corresponding distance (1.503(11) A) and angle (113.8(6)1) of aesi
in [1]+, compatible with the formation of an imine group. The
Os–Niesi bond in [2]+ (2.069(3) A) is appreciably shorter than the
Os–Naesi bond in [1]+ (2.160(6) A), which is a general trend
observed for aliphatic imine and amine complexes.13 As expected,
the Os–S bond distance in [2]+ (2.230(1) A) is similar to that in [1]+
(2.248(2) A).
The electronic absorption spectrum of [1]PF6 in water shows an
intense visible band at 414 nm, which is assigned as arising from a
singlet metal-to-ligand(bpy) charge-transfer (1MLCT) transition
and a more intense band at 285 nm due to an intraligand p–p*
transition of bpy (Fig. 3).6a,d This absorption spectral feature is
very similar to that of [Ru(aesi)(bpy)2]+ in water, which gives
1MLCT and p–p* bands at 413 nm and 284 nm, respectively
(Fig. S3, ESIw). However, [1]PF6 exhibits an additional weak band
at 547 nm, which is assigned as arising from a 3MLCT transition
that is partially allowed as a result of the heavy-atom effect.
When the same irradiation experiment was carried out for [1]+
under a nitrogen atmosphere, no significant change of its
1H NMR spectrum was observed. This suggests that the
conversion of [1]+ to [2]+ is a result of air oxidation, which is
induced by photoirradiation. Prompted by this result, we also
investigated the possibility of the conversion of [1]+ to [2]+ by
3
A similar MLCT band has been characteristically found in the
absorption spectra of [Os(bpy)3]2+ and its related complexes.6a,d
To check the stability of [1]+ in solution, a D2O solution of
[1]PF6 was allowed to stand under ambient light in air for a
1
chemical oxidation. Notably, the H NMR spectrum of [1]PF6
turned to be identical with that of [2]+ by adding excess AgClO4
in the dark within 1 h (Fig. S6, ESIw), indicative of the facile
conversion of [1]+ to [2]+ by AgI oxidation. While a similar
amine-to-imine conversion by chemical oxidation has been
reported for several RuII complexes, CeIV, which is a strong
oxidant, has commonly been employed for the reaction, rather
than a milder oxidant AgI.12,14 The reversible conversion froꢀm
[2]+ to [1]+ was also achieved by treatment of [2]+ with BH4
;
the 1H NMR spectrum of a D2O solution containing [2]PF6 and
excess BH4 in the dark turned to be identical with that of [1]+
ꢀ
within 1.5 h (Fig. S7, ESIw). Here, it should be noted that similar
treatment of [Ru(aesi)(bpy)2]+ with AgClO4 did not cause any
spectral change, and furthermore, the photoirradiation of an
aqueous solution of [Ru(aesi)(bpy)2]+ led to its decomposition
into several unidentified species.
Fig. 3 Electronic absorption spectra of [1]PF6 (—) and [2]PF6 (- - - -)
in H2O.
c
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Crystal data for [2]PF6ꢁNH4PF6: C22H24F12N6O2OsP2S, M = 916.67,
monoclinic, space group P21/c, a = 15.9170(3), b = 9.4043(2), c =
20.2031(5) A, b = 100.5150(10)1, V = 2973.38(11) A3, Z = 4, T = 200 K,
28 325 reflections collected, 6779 independent reflections, 6065 observed
reflections (I > 2s(I)), R1 (I > 2s(I)) = 0.026, wR2 (all data) = 0.064. H
atoms of amine and imine groups were found in difference Fourier maps
and were refined with [Uiso = 1.2Ueq(N)].
1 (a) D. C. Jicha and D. H. Busch, Inorg. Chem., 1962, 1, 872;
(b) D. Gibson and S. J. Lippard, Inorg. Chem., 1986, 25, 219;
(c) R. V. Parish, Z. Salehi and R. G. Pritchard, Angew. Chem., Int. Ed.
Engl., 1997, 36, 251; (d) H. Fleischer, Coord. Chem. Rev., 2005, 249, 799.
2 (a) T. Konno, Bull. Chem. Soc. Jpn., 2004, 77, 627; (b) Z. N. Chen,
N. Zhao, Y. Fan and J. Ni, Coord. Chem. Rev., 2009, 253, 1.
3 (a) S. G. Murray and F. R. Hartley, Chem. Rev., 1981, 81, 365;
(b) L. Roecker, M. H. Dickman, D. L. Nosco, R. J. Doedens and
E. Deutsch, Inorg. Chem., 1983, 22, 2022; (c) N. Yoshinari and
T. Konno, Inorg. Chem., 2008, 47, 7450.
4 (a) B. A. Lange, K. Libson, E. Deutsch and R. C. Elder, Inorg.
Chem., 1976, 15, 2985; (b) I. K. Adzamli, K. Libson, J. D. Lydon,
R. C. Elder and E. Deutsch, Inorg. Chem., 1979, 18, 303;
(c) M. Kita, K. Yamanari and Y. Shimura, Chem. Lett., 1980,
275; (d) M. Kita, K. Yamanari and Y. Shimura, Bull. Chem. Soc.
Jpn., 1983, 56, 3272.
Fig. 4 Cyclic voltammograms of [1]PF6 (left) and [2]PF6 (right) in
H2O/NaNO3 (0.1 M) at 25 1C with a scan rate of 100 mV sꢀ1
.
As shown in Fig. 4, the cyclic voltammogram (CV) of [1]PF6
in water under a nitrogen atmosphere is characterized by two
irreversible oxidation waves at Epa = +0.55 and +0.70 V
(vs. Ag/AgCl). On the other hand, the CV of [2]PF6 displays
only one irreversible oxidation wave at Epa = +0.70 V, the
potential of which is the same as the second oxidation wave for
[1]+ (Fig. 4). The spectroelectrochemical experiments were
carried out for [1]PF6 using an optically transparent thin-layer
electrode cell. When the potential was increased from 0 V, the
absorption spectrum of [1]PF6 gradually changed with several
isosbestic points, and the absorption spectrum recorded at
+0.45 V was almost the same as that of [2]+ (Fig. S8, ESIw).
From these results, it is considered that the first oxidation at
5 M. Tamura, K. Tsuge, A. Igashira-Kamiyama and T. Konno,
Inorg. Chem., 2011, 50, 4764.
6 (a) S. Decurtins, F. Felix, J. Ferguson, H. U. Gudel and A. Ludi,
J. Am. Chem. Soc., 1980, 102, 4102; (b) P. A. Lay, R. H. Magnuson
and H. Taube, Inorg. Chem., 1988, 27, 2848; (c) J.-P. Sauvage,
J.-P. Collin, J.-C. Chambron, S. Guillerez and C. Coudret, Chem.
Rev., 1994, 94, 993; (d) R. T. F. Jukes, B. Bozic, P. Belser, L. D. Cola
and F. Hartl, Inorg. Chem., 2009, 48, 1711; (e) S. Verma, P. Kar,
A. Das, D. K. Palit and H. N. Ghosh, Chem.–Eur. J., 2010, 16, 611.
7 E. M. Kober, J. V. Caspar, B. P. Sullivan and T. J. Meyer, Inorg.
Chem., 1988, 27, 4587.
E
pa = +0.55 V observed for [1]+ is due to the amine-to-imine
conversion. On the other hand, the CV of [Ru(aesi)(bpy)2]+ in
water under the same conditions showed only one irreversible
oxidation wave at Epa = +1.00 V (Fig. S9, ESIw), which is
0.45 V more positive than the first oxidation wave for [1]+.15
Thus, the facile conversion of [1]+ to [2]+ by the photo-
induced air oxidation or by the AgI oxidation is ascribed to
the relatively low oxidation potential of [1]+.
8 M. Tamura, M. Yamagishi, T. Kawamoto, A. Igashira-Kamiyama,
K. Tsuge and T. Konno, Inorg. Chem., 2009, 48, 8998.
9 G. Vitzthum and E. Lindner, Angew. Chem., Int. Ed. Engl., 1971, 10, 315.
10 To date, only one sulfinato osmium complex, which contains an
O-bonded sulfinato group, has been structurally characterized.
W. R. Roper, J. M. Waters and A. H. Wright, J. Organomet.
Chem., 1984, 276, c13.
In summary, we showed that 2-aminoethanethiolate (aet) is
easily air oxidized to 2-aminoethanesulfinate (aesi) on binding
with an [OsII(bpy)2]2+ core to form [Os(aesi-N,S)(bpy)2]+ ([1]+).
Remarkably, the aesi in [1]+ was found to be further air oxidized
to 2-iminoethanesulfinate (iesi) on irradiating light to form
[Os(iesi-N,S)(bpy)2]+ ([2]+).16,17 Although a similar amine-to-
imine conversion by air oxidation has been recognized in several
coordination systems,14b the photoinduced conversion found in
this work is unprecedented.18 Note that [1]+ is also readily
converted to [2]+ with use of Ag+, and furthermore, [2]+ is
reverted back to [1]+ with use of BH4ꢀ in aqueous media. Thus,
the present system represents a clean, reversible amine-to-imine
conversion, which is most likely due to the presence of a redox-
active OsII center whose +III state is more accessible than that of
RuII, together with the presence of an inert sulfinato donor.
This work was partly supported by a Grant-in-Aid for JSPS
Fellows from the Japan Society for the Promotion of Science (JSPS).
11 This assignment was confirmed by the 1H–1H COSY spectrum
(Fig. S5, ESIw).
12 E. Jandrasics, B. Kolp, J. A. Wolny and A. von Zelewsky, Inorg.
Chim. Acta, 1998, 272, 153.
13 (a) K. Sakai, Y. Yamada and T. Tsubomura, Inorg. Chem., 1996,
35, 3163; (b) L. Bendahl, A. Hammershøi, D. K. Jensen, S. Larsen,
A. Riisager, A. M. Sargeson and H. O. Sørensen, J. Chem. Soc.,
Dalton Trans., 2002, 3054.
14 (a) F. R. Keene, M. J. Ridd and M. R. Snow, J. Am. Chem. Soc.,
1983, 105, 7075; (b) F. R. Keene, Coord. Chem. Rev., 1999, 187, 121.
15 Meyer and co-workers have reported a similar electrochemical
oxidation of primary-amine complexes. They proposed that the
deprotonation of a primary amine group is induced by the one-
electron oxidation of RuII to RuIII, followed by the additional one-
electron oxidation to give an imine group with retention of the
Ru–N bond. G. M. Brown, T. R. Weaver, F. R. Keene and
T. J. Meyer, Inorg. Chem., 1976, 15, 190.
1
16 The appearance of sharp proton signals in the H NMR spectra of
[1]PF6 and [2]PF6 clearly indicates that the octahedral osmium center
in each compound exists in a diamagnetic +II oxidation state.
17 A plausible mechanism of this reaction involves the initial genera-
tion of a metal-to-ligand charge-transfer excited species of [1]+
,
.
Notes and references
which is readily oxidized by air to produce [OsIII(aesi)(bpy)2]2+
This OsIII species would be converted to [2]+via the pathways that
are proposed for the electrochemical oxidation.15
z Crystal data for [1]ClO4ꢁNaClO4: C22H22Cl2N5NaO10OsS, M =
832.60, monoclinic, space group C2/c,
a = 9.5421(3), b =
16.8001(7), c = 34.0526(10) A, b = 96.6130(10)1, V = 5422.6(3) A3,
Z = 8, T = 200 K, 26 730 reflections collected, 6220 independent
reflections, 5150 observed reflections (I > 2s(I)), R1 (I > 2s(I)) =
0.046, wR2 (all data) = 0.125. H atoms of amine and imine groups
18 Photoinduced conversions by dioxygen have been reported for
organic compounds, but the resulting amine species are not an end
product and have not been isolated. Y. Mitsumoto and M. Nitta,
J. Org. Chem., 2004, 69, 1256; F. Su, S. C. Mathew, L. Mohlmann,
M. Antonietti, X. Wang and S. Blechrt, Angew. Chem., Int. Ed.,
2011, 50, 657.
were found in difference Fourier maps and were refined with [Uiso
1.2Ueq(N)].
=
c
12466 Chem. Commun., 2011, 47, 12464–12466
This journal is The Royal Society of Chemistry 2011