C-S bond cleavage of 2-methoxythiophene by Ir-TMEDA complex (TMEDA = N,N,N',N'-tetramethylethylenediamine). Formation of novel dinuclear iridathiacyclohexenyl complex

Article abstract of DOI:10.1016/j.jorganchem.2008.07.013

The Ir-TMEDA complex [Ir(C₂H₄)₂(TMEDA)(MeCN)][BF₄ ] (TMEDA = Me₂NCH₂CH₂NMe₂) was treated with 2-methoxythiophene in MeCN at 70 °C in the presence of H₂O to

Full text of DOI:10.1016/j.jorganchem.2008.07.013

Journal of Organometallic Chemistry 693 (2008) 3197–3200
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
C–S bond cleavage of 2-methoxythiophene by Ir–TMEDA complex
(TMEDA = N,N,N’,N’-tetramethylethylenediamine). Formation of novel
dinuclear iridathiacyclohexenyl complex
*
Kentaro Iwasa, Hidetake Seino, Yasushi Mizobe
Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The Ir–TMEDA complex [Ir(C2H4)2(TMEDA)(MeCN)][BF4] (TMEDA = Me2NCH2CH2NMe2) was treated with
2-methoxythiophene in MeCN at 70 °C in the presence of H2O to afford the novel dinuclear iridathiacyc-
lohexenyl complex [Ir2{C(OMe)CHCHCH(CMe@NH)S}(MeCONH)(TMEDA)2][BF4]2 in 42% yield, in which
two Ir(TMEDA) fragments, one 2-methoxythiophene, and two MeCN molecules are incorporated together
with one H2O molecule. The mechanism that involves the insertion of the Ir center into the thiophenic
Received 26 January 2008
Received in revised form 15 July 2008
Accepted 15 July 2008
Available online 19 July 2008
S–C(OMe) bond followed by the
proposed.
p
-coordination of the second Ir center to the thiametallacycle has been
Keywords:
Iridium
Diamine complex
Ó 2008 Elsevier B.V. All rights reserved.
2-Methoxythiophene
C–S bond cleavage of thiophene
Desulfurization of thiophene
1. Introduction
donor ligands are still rare except for the Rh-hydrotris(pyrazol-
yl)borate complexes [3a] and Rh-diiminato species [3b].
Hydrodesulfurization of petroleum feedstocks is one of the
most important industrial processes catalyzed by transition met-
als. Cobalt-modified molybdenum sulfide supported on alumina
is used most commonly as catalyst and certain Co–Mo–S cluster
generated at the edge of the layered MoS2 phase is suggested to
be the active site of desulfurization, but details are yet unknown
[1]. Among various organo-sulfur compounds included in the crude
oils, thiophenes are of particular interest because removal of their
S atoms is relatively difficult. Reactivities of thiophenes with tran-
sition metal complexes [2], especially with those of Group 9 metals
[3] and Group 6 metals [4], are therefore attracting much attention
in this context.
Now we have found that the reaction of the Ir–TMEDA complex
(TMEDA = Me2NCH2CH2NMe2) with 2-methoxythiophene affords
the novel dinuclear iridathiacyclohexenyl complex via the inser-
tion of one Ir center into the thiophenic C–S bond. In this paper, de-
tails of the characterization of this product are reported. Although
related C–S bond cleaving reactions of thiophenes using certain Ir
as well as Rh and Co complexes have been reported already [2,3],
these are mostly conducted by the use of phosphine and cyclopen-
tadienyl complexes, and those promoted by the complexes with N-
2. Results and discussion
Monomeric Ir–TMEDA complex [Ir(C2H4)2(TMEDA)(MeCN)][BF4]
(1) was obtained by treatment of [IrCl(C2H4)2]2 with TMEDA and
NaBF4 in MeCN at room temperature (Scheme 1). Complex 1 has
been tentatively formulated by 1H NMR spectrum in acetone-d6,
showing characteristic signals including four doublets of doublets
of doublets at d 3.31, 3.11, 2.56, and 1.93 with the intensities of
2H each due to the ethylene protons, together with two singlets at
d 3.23 and 2.13 with the intensities of 6H each assignable to TMEDA
methyl protons and one singlet at d 2.68 with the intensity of 3H for
coordinated MeCN. These 1H NMR data suggest the square-pyrami-
dal or trigonal-bipyramidal structure with two equatorial ethylene
ligands. However, since 1 is extremely air-sensitive, isolation and
full characterization was not successful.
When 1 generated in situ was treated with excess 2-methoxy-
thiophene in MeCN at 70 °C, dinuclear iridathiacyclohexenyl com-
plex [Ir2{C(OMe)CHCHCH(CMe@NH)S}(MeCONH)(TMEDA)2][BF4]2
2 was isolated in 26% yield from the reaction mixture. Single crys-
tals were available for the PF6 analogue, 2’, obtained from 2 after
the anion metathesis with KPF6, and the X-ray analysis of 2’ was
undertaken. However, refinements of the structure were unable
to be completed to the satisfactory level due to the severe disorder
observed for the dimetallacyclic core. Nevertheless, preliminary
* Corresponding author. Fax: +81 3 5452 6361.
E-mail address: ymizobe@iis.u-tokyo.ac.jp (Y. Mizobe).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.07.013

3198
K. Iwasa et al. / Journal of Organometallic Chemistry 693 (2008) 3197–3200
The mechanism proposed for the formation of 2 is depicted in
Me
C
[BF ]
4
Me
C
[BF ]
4
TMEDA
NaBF
Scheme 2. It has been demonstrated previously that decreasing
the electron density of the thiophene ring facilitates the insertion
of the metal center into the C–S bond [5]. Since the insertion of
Ir center to the C–S bond takes place for the thiophenes with both
the electron-donating methoxy group and the electron-withdraw-
ing acetyl group among the thiophenes examined but not for
methylthiophene, it might be presumed in this IrI–TMEDA system
that the insertion to the C–S bond is not controlled by the differ-
ence in the electron density of the thiophenes but is initiated for
the thiophenes that can coordinate to Ir in a j2 fashion. After the
insertion of IrI into the C–S bond to generate the iridathiacyclohex-
adiene species, the binding of the second IrI center to the S–Ir–C@C
array, as demonstrated for the other Co and Rh complexes (vide su-
pra), occurs with concurrent nucleophilic attack of the 5-C atom to
the cyano C atom to give the IrIII–IrIII iminato species. Hydrolysis of
this intermediate by the adventitious water finally gives 2. Thus, as
expected, addition of an almost stoichiometric amount of water to
the reaction mixture resulted in the significant increase in the yield
of 2 to 42%. The unique reactivity observed in this reaction system
might be ascribable to the quite electron-rich nature of the Ir(TME-
DA) moieties.
N
N
4
N
N
or
[IrCl(C H ) ]
2
4 2 2
Ir
Ir
N
MeCN, r.t.
N
1
MeO
S
MeCN, 70 °C
(H O)
2
H
Me
NH
[BF ]
4 2
H
H
Ir
S
N
N
N
Ir
N
OMe
NH
O
Me
N = NMe
2
2
Scheme 1.
3. Experimental
results revealed unambiguously the atom connecting scheme in
the cation, as shown in Scheme 1.
3.1. General
The cation consists of two Ir(TMEDA) fragments, one Ir center of
which inserts into the S–C(OMe) bond of 2-methoxythiophene
selectively to form the six-membered ring, while the other Ir cen-
ter binds to the C(OMe)CHCH moiety of this six-membered ring in
All manipulations were carried out under N2 using standard
Schlenk techniques. Solvents were dried by common methods
and distilled under N2 before use. Complex [IrCl(C2H4)2]2 [6] was
prepared according to the literature method, while chemicals were
obtained commercially and used as received except for TMEDA,
which was treated with KOH and distilled under N2.
IR and NMR spectra were recorded on a JASCO FT/IR-420 and a
JEOL alpha-400 spectrometer, respectively. Elemental analyses
were done with a Perkin–Elmer 2400 series II CHN analyzer.
an
g
3-manner together with the S atom. Furthermore, two MeCN
molecules are incorporated into this cation with concurrent hydro-
lysis: one is connected with the 5-C atom of the thiophene ring at
the cyanide C atom to form iminoacyl group and the other is con-
verted to the bridging amidate ligand. Insertion of the metal center
into the thiophenic C–S bonds is well demonstrated for many tran-
sition metal complexes including Group 9 metal (IrI, RhI, and CoI)
3.2. Preparation of 1
complexes [3]. Selective C(OMe)–S bond scission is also precedent
*
for, e.g. [Cp Rh(C2H4)2] (Cp* =
g
5-C5Me5) [3c], but the successive
Complex [IrCl(C2H4)2]2 (115 mg, 0.203 mmol) was dissolved in
MeCN (10 mL) and TMEDA (60 lL, 0.40 mmol) was added to this
coordination of the second metal center to the generated thiame-
tallacycle accompanied by the coupling to the nitrile is the finding
distinctive to this Ir–TMEDA system. It is to be noted that the reac-
solution at room temperature. After stirring for 1 h, finely pow-
dered NaBF4 (45 mg, 0.41 mmol) was added and the mixture
was stirred continuously at room temperature for 3 h. The resul-
tant yellow solution was filtered and the filtrate was evaporated
to dryness in vacuo to give the yellow oily solid containing 1
(189 mg). Further purification to isolate analytically pure 1 failed
since 1 was highly air-sensitive. 1H NMR data for 1 (acetone-d6):
d 3.31 (ddd, J = 11.1, 8.7, 2.4 Hz, 2H, C2H4), 3.23 (s, 6H, NMe),
3.11 (ddd, J = 10.4, 9.5, 2.4 Hz, 2H, C2H4), 2.98, 2.87 (m, 2H each,
NCH2), 2.68 (s, 3H, MeCN), 2.56 (ddd, J = 10.4, 8.7, 1.6 Hz, 2H,
C2H4), 2.13 (s, 6H, NMe), 1.93 (ddd, J = 11.1, 9.5, 1.6 Hz, 2H,
C2H4).
*
tions of [Cp Co(C2H4)2] [3d,3e] and [Rh(coe){XyNC(Me)CHC(Me)N-
Xy}] (coe = cis-cyclooctene; Xy = 2,6-Me2C6H3) [3b] with thiophene
result in the formation of dicobalt and dirhodium complexes in
which the cobalta- or rhodathiacyclohexadiene ring binds to the
other Co or Rh atom in a j4 form at the S–M–C@C moiety.
The spectroscopic data for 2 are consistent with this solid-state
structure. Thus, the IR spectrum shows the sharp bands at 3368
and 3289 cmÀ1 that are characteristic of
m(NH), while these NH
protons resonate at d 11.12 and 6.34 as broad signals, which disap-
peared by the H/D exchange using D2O as the deutrium source. The
CH protons of the iridathiacycle are observed at d 4.89, 4.56, and
4.18 as one triplet and two doublets with J = 5.8 Hz. In addition,
eleven signals due to the methyl protons appeared, indicating that
all methyl groups in 2 are inequivalent as expected from its struc-
ture in a solid form.
3.3. Preparation of 2
Into a solution of [IrCl(C2H4)2]2 (570 mg, 1.00 mmol) in MeCN
Reactions of other thiophenes with 1 under similar conditions
were also attempted. Unfortunately, all systems did not afford
the products isolable in a pure form, but it has turned out from
1H NMR criteria that for the reaction with 2-methylthiophene
the scission of not the C–S bond but the C–H bond takes place.
On the other hand, both products resulting from the C–S and C–
H bond cleavages are obtained for the reaction with 2-acetylthio-
phene in a ratio of ca. 5:3.
(50 mL) was added TMEDA (300 lL, 2.01 mmol) at room tempera-
ture. After stirring for 1 h, NaBF4 (225 mg, 2.05 mmol) was added
and the mixture was stirred continuously at room temperature
for 20 h. Then, 2-methoxythiophene (0.50 mL, 5.0 mmol) and H2O
(20 lL, 1.1 mmol) were added and the mixture was stirred at
70 °C for 24 h. The resulting mixture was filtered and the filtrate
was evaporated to dryness under reduced pressure. The residue
was purified by chromatography using the column packed with

K. Iwasa et al. / Journal of Organometallic Chemistry 693 (2008) 3197–3200
3199
Me
2+
C
+
+
[Ir(TMEDA)]
S
MeO
N
Ir
N
N
S
S
N
N
Ir
1
O
C–S cleavage
coordination
of thiophene
.
Me
.
Ir
OMe
I
Ir coordination
N
N
H
Me
H
2+
Me
NH
2+
H
H
.
.
N
H
Ir
H
Ir
S
H O
2
S
N
N
N
N
Ir
N
Ir
N
N
OMe
N
OMe
N
nucleophilic attack
to cyano carbon
hydrolysis
NH
O
C
proton shift
Me
Me
Scheme 2.
neutral alumina. Complex 2 was obtained as yellow needles by
adding ether (50 mL) to the concentrated yellow fraction eluted
by MeCN (429 mg, 42% yield).
3.6. X-ray crystallography for 2’
Single crystal of 2’ was sealed in a glass capillary under argon
From the reaction conducted analogously but in the smaller
scale without addition of H2O, 2 was isolated in 26% yield. 1H
NMR (acetone-d6): d 11.12, 6.34 (br s, 1H each, NH), 4.89 (t,
J = 5.8 Hz, 1H, CH), 4.56 (d, J = 5.8 Hz, 1H, CH), 4.18 (d, J = 5.8 Hz,
1H, CH), 3.82 (s, 3H, OMe), 3.63 (s, 3H, Me), 3.46 (m, 2H, CH2),
3.27 (ddd, 1H, CH2), 3.14 (ddd, 1H. CH2), 3.0–2.6 (m, total 4H,
CH2, overlapping with Me resonances), 2.94, 2.93, 2.86, 2.71,
2.61, 2.45, 2.43 (s, 3H each, Me), 2.42 (d, J = 2.0 Hz, 3H, Me), 2.34
and mounted on a Rigaku Mercury-CCD diffractometer equipped
with a graphite-monochromatized Mo K
a source. All diffraction
studies were done at 23 °C. Data collections were performed by
using the CRYSTALCLEAR program package [7]. All data were corrected
for Lorentz and polarization effects as well as absorption.
Structure solution and refinements were conducted by using
the CRYSTALSTRUCTURE program package [8]. The positions of non-
hydrogen atoms were determined by Patterson methods
(s, 3H, Me). IR (KBr): 3368, 3289 cm–1
21H46B2F8N6O2Ir2S: C, 25.10; H, 4.62; N, 8.36. Found: C, 25.39;
(m(NH)). Anal. Calc. for
(SHELX97) [9] and subsequent Fourier synthesis (DIRDIF99) [10],
C
which were refined by full-matrix least-squares techniques using
isotropic thermal parameters for all C atoms as well as the two
O atoms and with anisotropic thermal parameters for other non-
hydrogen atoms. However, since the C(OMe)CHCHCH(S) moiety
of 2’ is severely disordered over two positions in a ratio of ca.
6:4, the structure could not be refined to the satisfactory level.
Crystal data: formula, C23H49N7O2F12P2SIr2; Fw = 1160.10; space
group, P21; a = 14.013(1), b = 9.0391(7), c = 15.250(1) Å, b =
H, 4.59; N, 8.59%.
3.4. Preparation of 2’
A
mixture of 2 (51 mg, 0.050 mmol) and KPF6 (28 mg,
0.15 mmol) in MeCN (5 mL) was stirred for 2 days and the resulting
mixture was filtered. Addition of ether (20 mL) to the filtrate gave
2’ Á MeCN as yellow crystals (32 mg, 55% yield). Anal. Calc. for
99.250(1)°, V = 1906.5(3) Å3; Z = 2;
q ; crystal
calc = 2.021 g cmÀ3
C23H49F12N7O2P2Ir2S: C, 23.77; H, 4.25; N, 8.44. Found: C, 23.46;
size, 0.20 Â 0.20 Â 0.20 mm3; no of unique reflections, 8039; no
of variables, 393; R1 and wR2 values with all data, 0.061 and
0.127; GOF, 1.023.
H, 4.09; N, 8.03%.
3.5. Reactions of 1 with 2-methylthiophene and 2-acetylthophene
Acknowledgements
Reactions of 1 with these thiophenes were carried out analo-
gously to that with 2-methoxythiophene. However, these reactions
did not proceed so cleanly as compared to that forming 2, so that
the isolation of the analytically and spectroscopically pure prod-
ucts were unsuccessful. Characteristic 1H NMR signals of the crude
products indicated that the reaction with 2-methylthiophene
underwent the C–H bond cleavage to give the hydrido-thienyl
complex exhibiting the hydrido resonance at d À23.14 (s, 1H)
and the thienyl protons at d 6.52 (d, J = 3.4 Hz, 1H, 4-thienyl H)
and 7.02 (dq, J = 3.4 and 1.2 Hz, 1H, 3-thienyl H), while that with
2-acetylthiophene gave a mixture of the C–S and C–H bond scission
in ca. 5:3 ratio, where the signals assignable to former are observed
at d 11.54 (br, 1H, NH), 6.58 (br, 1H, NH), 5.92 (dd, J = 7.8, 0.4 Hz,
1H, CH), 5.70 (dd, J = 5.2, 0.4 Hz, 1H, CH), and 4.15 (dd, J = 7.8,
5.2 Hz, 1H, CH) and those to the latter at d À23.06 (s, 1H, IrH),
7.54 (d, J = 4.0 Hz, 1H, thienyl), and 7.02 (d, J = 4.0, 1H, thienyl),
respectively.
This work was supported by Grant-in-Aid for Scientific Re-
search on Priority Areas (No. 18065005, ‘‘Chemistry of Concerto
Catalysis”) from the Ministry of Education, Culture, Sports, Science
and Technology, Japan and by CREST of JST (Japan Science and
Technology Agency).
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