A half-sandwich ruthenium(II) complex containing a coordinatively unsaturated 2,6-dimesitylphenyl thiolate ligand

Article abstract of DOI:10.1002/anie.200353611

Electron-deficient ruthenium complex [Cp*Ru(SDmp)] (1; SDmp = 2,6-dimesitylphenylthiolate, Cp* = η⁵-C₅Me ₅) is stabilized by the coordination of C_ipso of one mesityl group of SDmp. It serves as a precursor to coordinatively unsaturated species in two ways: by the lability of C_ipso in reactions with ligands L, and by reversible dissociation/association of the SDmp ligand in the regioselective cyclotrimerization of substituted alkynes (see scheme).

Full text of DOI:10.1002/anie.200353611

Communications
Ruthenium Thiolate Complexes
A Half-Sandwich Ruthenium(ii) Complex
Containing a Coordinatively Unsaturated 2,6-
Dimesitylphenyl Thiolate Ligand**
Yasuhiro Ohki, Hitomi Sadohara, Yuko Takikawa, and
Kazuyuki Tatsumi*
Transition-metal complexes with low-coordination numbers
are a class of highly reactive substances which are important
as key intermediates in catalytic reactions. A number of
coordinatively unsaturated transition-metal complexes have
been reported, most of which contain bulky alkyl, alkoxy,
amide, and phospane ligands.[1] Bulky thiolates are also
attractive ligands in this regard. The S-donor ligands are
[*] Dr. Y. Ohki, H. Sadohara, Y. Takikawa, Prof. Dr. K. Tatsumi
Department of Chemistry
Graduate School of Science and
Research Center for Meterials Science
Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
Fax: (+81)52-789-2943
E-mail: i45100a@nucc.cc.nagoya-u.ac.jp
[**] This research was financially supported by a Grant-in-Aid for
Scientific Research (No. 14078211 and 15750047) from the Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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soft in character, can donate p electrons to a metal atom, and
show high affinity for a wide variety of transition metals.
Although thiolate ligands tend to produce multinuclear
structures by linking metal atoms, some sterically demanding
thiolates are effective in stabilizing low-coordinate, mono-
meric complexes.[2] One such ligand is 2,6-dimesitylphenyl
thiolate (SDmp).[2b] The mesityl group can modulate the
stereoelectronic properties of transition-metal centers by its
bulk and by possible weak coordination of a mesityl group,
which thereby generates hemilabile reaction sites. Herein we
report the synthesis of a half-sandwich ruthenium(ii) complex
of SDmp, lability of the ligand in the coordination sphere, and
reactivity of the complex.
atom forms a five-membered ruthenacycle with the SDmp
sulfur and ipso-carbon atoms. This coordination geometry of
SDmp appears to be retained in solution according to 1H and
13C NMR data. The molecule has a crystallographic mirror
plane, which bisects SDmp and Cp*, and the ruthenacycle and
the central arene ring of SDmp are crystallographically
À
coplanar. Due to the Ru C7 bonding, the mesityl ring
bends away from the metal atom, and the C2-C7-C8-C9
À
dihedral angle is 144.3(2)8. Furthermore, the C7 C8 bond is
À
clearly elongated (1.432(3) Š) relative to the other C C
distances (1.395 (3) and 1.387(3) Š) in the mesityl ring. While
À
the Ru C7 distance of 2.278(3) Š is long relative to reported
Ru Calkyl bond lengths (2.08–2.18 Š),[9] it is comparable to
À
The reaction of [(Cp*RuCl)4] (Cp* = h5-C5Me5)[3] with
4 equivalents of LiSDmp in THF afforded a dark blue
solution, from which mononuclear ruthenium(ii) complex
[Cp*Ru(SDmp)] (1) was isolated in 83% yield (Scheme 1).[4]
Ru Colefin distances (2.14–2.27 Š).[9] This indicates a strong
Ru Cipso interaction in 1, in accordance with the structural
À
À
À
rigidity of 1 in solution. The Ru S distance of 2.294(1) Š is
shorter than those of electronically saturated RuII thiolate
complexes (2.38–2.47 Š),[10] because p backdonation of
electrons from an occupied sulfur pp orbital occurs to ease
the electron deficiency of the metal center.
Coordinative unsaturation of 1 and lability of the ipso-
carbon atom in the coordination sphere of ruthenium were
manifested in facile reactions of 1 with CNtBu, CO, bipyr-
idine, and phenanthroline. Analytically pure 18-electron
complexes [Cp*Ru(SDmp)(L2)] [L2 = (CNtBu)2 (2), (CO)2
(3), bipy (4), phen (5)] were isolated therefrom in good
yields (Scheme 2).[4] The molecular structures of 2–5 were
Scheme 1. Synthesis of 1.
Complex 1 is air- and moisture-sensitive, and is soluble in
common organic solvents. In the 1H NMR spectrum taken at
room temperature, the o- and p-methyl protons of the SDmp
ligand gave rise to four 1H resonances with an intensity ratio
of 3:3:6:6,[4] and these signals do not show notable line
broadening even at 808C. Thus, the two mesityl groups of
SDmp in 1 are chemically inequivalent in solution.
The molecular structure of 1 is shown in Figure 1.[5] An
interesting structural feature is the interaction between Ru
and the ipso-carbon atom C7 of one mesityl group.[6] Without
this interaction 1 would formally be a 14-electron complex,[7]
and the electron deficiency of the ruthenium promotes the
bonding interaction with the nearby ipso-carbon atom. This
bonding mode may be analogous to that in the Wheland
intermediate of Friedel–Crafts reactions.[8] Thus, the RuII
Scheme 2. Reactivity of 1 with various ligands.
determined by X-ray analysis.[11] These complexes have a
common three-legged piano-stool geometry in which the
SDmp ligand is bound to Ru through the sulfur atom. Due to
the electron-rich nature of Ru in 2–5, there is no p back-
À
donation from SDmp, and the Ru S bonds are long
(2.427(1)–2.4313(8) Š), while steric congestion makes the
Ru-S-C angles large (112.0(1)8 for 3, ca. 124.8(2)8 for 5). In
the 1H NMR spectra, the two mesityl groups are now
equivalent. For complexes 3 and 4, the IR bands for nCO and
nCN were observed at 2008 and 1957 cmÀ1 (CO), and 2112 and
2052 cmÀ1 (CNtBu), respectively. Complex 1 does not react
with bulky phosphanes such as PPh3 and P(c-C6H11)3.
When 1 was treated with an excess of phenylacetylene or
1-pentyne, cyclotrimerization of the alkyne took place, and
ruthenium(ii) sandwich complexes with Cp* and trisubsti-
tuted benzene ligands [Cp*Ru(h6-1,2,4-R3C6H3)](SDmp)
Figure 1. Molecular structure of 1 with thermal ellipsoids at 50% prob-
À
ability level. Selected bond lengths [Š] and angles [8]: Ru S 2.294(1),
À
À
À
À
À
Ru C7 2.278(3), S C1 1.756(3), C2 C7 1.491(4), C7 C8 1.432(3), C8
À
C9 1.395(3), C9 C10 1.387(3); Ru-C7-C2 113.7(2) Ru-C7-C8 88.3(1),
C2-C7-C8 119.0(1), S-C1-C2 116.9(2).
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(R = Ph, 6a; nPr, 6b) were isolated as yellow crystals in 31–
free form of SDmp. Complex 1 does not react with
disubstituted alkynes such as phenylpropyne and diphenyl-
acetylene, and this indicates that formation of the ruthena-
cycle is hampered by the substituents.
58% yield (Scheme 3).[4] Since the X-ray crystal structures of
6a and 6b are similar, only the structure of 6a is shown in
Figure 2.[11] The regioselective formation of 1,2,4-trisubsti-
The trisubstituted benzene ligands of 6a and 6b were
liberated under UV irradiation, and the counteranion SDmp
was bound to Ru, regenerating 1.[4] Therefore, cyclotrimeri-
zation of alkynes could in principle be catalyzed by 1,
whereby the reversible dissociation/association of SDmp may
assist the reactions. Dissociation of cystein sulfur from active
metal sites in certain metalloproteins and its recombination
are thought to be important for enzymatic functions.[14]
Successful isolation of 1 and 6a and 6b and their intercon-
version provides the first well-characterized examples of
reversible coordination of thiolate sulfur atoms at a transition
metal center. These findings offer the possibility of assessing
the unique role of reversible thiolate coordination in catalytic/
enzymatic reactions. At the moment, however, the catalytic
cycle for trimerization of alkynes promoted by 1 has not been
achieved, because some side reactions occur upon irradiation
of 1 in the presence of alkynes.
Scheme 3. Cyclotrimerization of the alkyne on treatment with 1.
In summary, we have isolated an electron-deficient
ruthenium(ii) complex 1 carrying the bulky thiolate SDmp.
This complex was shown to serve as a precursor of coordi-
natively unsaturated species in two ways: by the lability of the
coordinated ipso-carbon atom, and by reversible dissociation/
association of the SDmp ligand. Further investigations on
reactivity of 1 associated with the unique coodination proper-
ties of SDmp are currently underway.
Figure 2. Molecular structure of 6a with thermal ellipsoids at 50%
Received: December 23, 2003 [Z53611]
À
À
probability level. Selected bond lengths [Š]: Ru Cp* 1.814(3), Ru
À
C6H3Ph3 1.705(3), S C1 1.748(6).
Keywords: cyclization · electron-deficient compounds ·
.
ruthenium · S ligands
tuted benzenes was also proved by the 1H NMR spectra,
which showed vicinal spin coupling between the aromatic
protons at C5 and C6. The observed regioselectivity of the
cyclotrimerization of the alkynes points to a 2,5-disubstituted
ruthenacyclopentadiene intermediate in the reaction, as
shown in Scheme 4.[12] The subsequent insertion of an
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Scheme 4. Synthesis of 6 from 1.
À
alkyne molecule into the Ru C bond would be followed by
À
oxidative C C coupling to give 1,2,4-trisubstituted benzene.
[3] P. J. Fagan, W. S. Mahoney, J. C. Calabrese, I. D. Williams,
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Intriguingly, the SDmp ligand exists as a discrete counter-
[4] For experimental details, see Supporting Information.
anion in the crystal structures of 6a and 6b. The dissociation
[5] a) X-ray diffraction was performed on
a Rigaku AFC7R
À
of SDmp might preceed alkyne insertion into the Ru C bond
equipped with a rotating anode and a MSC/ADSC Quantum 1
CCD detector. The structure was solved by direct methods. All
non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were fixed at the calculated positions. Crystal data for 1:
orthorhombic, Pnma, a = 21.46(1), b = 8.9893(7), c =
15.157(2) Š, V= 2923(3) Š3, Z = 4, 1calcd = 1.322 gcmÀ1; 3555
of a ruthenacyclopentadiene. Transition-metal complexes
with uncoordinated thiolate anions are rare because of high
affinity of sulfur for a wide variety of transition metals.[13] The
bulkiness of Dmp and delocalization of the anionic charge
over the aromatic rings probably stabilize the coordination-
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Angewandte
Chemie
reflections (5.5 ꢀ 2q ꢀ 558), 2909 observed reflections with F >
3s(F), 187 parameters; R = 0.033, Rw = 0.047; b) CCDC-226919
to 226925 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
V= 3744.7(5) Š3, Z = 4, 1calcd = 1.309 gcmÀ1; 4578 reflections
(5.5 ꢀ 2 q ꢀ 558), 4028 observed reflections with F > 3s(F), 434
parameters; R = 0.062, Rw = 0.103. Crystal data for 5·0.5C4H8O:
monoclinic, P21/a, a = 20.891(1), b = 12.3594(6), c = 29.942(2) Š,
b = 96.407(3), V= 7682.5(7) Š3, Z = 4, 1calcd = 1.380 gcmÀ1
;
16327 reflections (5.5 ꢀ 2q ꢀ 558), 10877 observed reflections
with F > 2s(F), 908 parameters; R = 0.064, Rw = 0.091. Crystal
data for 6a: monoclinic, P21/c, a = 10.975(5), b = 28.57(1), c =
17.434(7) Š, b = 95.815(7), V= 5438(3) Š3, Z = 4, 1calcd
=
[6] Strong metal Cipso interaction was observed for PdII complexes:
À
1.085 gcmÀ1; 12174 reflections (5.5 ꢀ 2q ꢀ 558), 7041 observed
a) L. R. Falvello, J. Fornies, R. Navarro, V. Sicilia, M. Tomas,
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¯
0.098. Crystal data for 6b·0.5C7H8: triclinic, P1, a = 11.69(2),
b = 12.51(2), c = 17.95(3) Š, a = 106.52(3), b = 100.14(2), g =
108.41(1), V= 2284(7) Š3, Z = 2, 1calcd = 1.230 gcmÀ1
; 9654
reflections (5.5 ꢀ 2q ꢀ 558), 6967 observed reflections with F >
3s(F), 505 parameters; R = 0.066, Rw = 0.087.
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bed recently: L. A. Watson, O. V. Ozerov, M. Pink, K. G.
Caulon, J. Am. Chem. Soc. 2003, 125, 8426; b) Relevant 16-
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Me2C6H3, 2,4,6-iPr3C6H2) have been reported: K. Mashima, H.
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[11] Diffraction measurements were made on a Rigaku AFC7R
equipped with a rotating anode and a MSC/ADSC Quantum 1
CCD detector (2) or on a Rigaku RASA-7 Quantum system
equipped with a rotating anode and a Mercury CCD detector (3,
4, 5·0.5C4H8O, 6a, and 6b·0.5C7H8). All structures were solved
by direct methods. All non-hydrogen atoms were refined
anisotropically, except for the THF molecule in 5·0.5C4H8O
(refined isotropically), and hydrogen atoms were fixed at
calculated positions. Crystal data for 2: monoclinic, C2/c, a =
26.188(1), b = 21.126(3), c = 17.1937(3) Š, b = 120.5609(4), V=
8190.9(8) Š3, Z = 8, 1calcd = 1.283 gcmÀ1; 8541 reflections (5.5 ꢀ
2q ꢀ 558), 6992 observed reflections with F > 3s(F), 423 param-
eters; R = 0.046, Rw = 0.058. Crystal data for 3: orthorhombic,
Fdd2, a = 20.109(6), b = 60.26(2), c = 10.312(3) Š, V=
12495(6) Š3, Z = 16, 1calcd = 1.356 gcmÀ1
; 24070 reflections
(5.5 ꢀ 2q ꢀ 558), 6776 observed reflections with F > 0s(F), 360
parameters; R = 0.053, Rw = 0.081. Crystal data for 4: ortho-
rhombic, P212121, a = 11.2974(9), b = 15.943(1), c = 20.790(1) Š,
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