Cationic Carboxylato Complexes of Dirhodium (II) with Oxo Thioethers: Promising Catalysts with Unusual Coordination Modes

Article abstract of DOI:10.1021/om0342560

Removal of an acetato ligand from dirhodium(II) acetato complexes with thioethers of the general structure [Rh₂(μ-OAc) ₄(RSCH₂Z)₂] yields the cationic complexes [Rh₂(μ-OAc)₃-(RSCH₂

Full text of DOI:10.1021/om0342560

Organometallics 2004, 23, 1947-1952
1947
Ca tion ic Ca r boxyla to Com p lexes of Dir h od iu m (II) w ith
Oxo Th ioeth er s: P r om isin g Ca ta lysts w ith Un u su a l
Coor d in a tion Mod es
Marino Basato,*,† Andrea Biffis,† Gianluca Martinati,† Marco Zecca,†
Paolo Ganis,† Franco Benetollo,‡ Laura A. Aronica,§ and Anna M. Caporusso§
Dipartimento di Scienze Chimiche, University of Padua and ISTM-CNR, via Marzolo 1,
I-35131 Padua, Italy, ICIS-CNR, Area della Ricerca di Padova, Corso Stati Uniti 4,
I-35127 Padua, Italy, and Dipartimento di Chimica e Chimica Industriale, University of Pisa,
via Risorgimento 35, I-56126 Pisa, Italy
Received October 23, 2003
Removal of an acetato ligand from dirhodium(II) acetato complexes with thioethers of the
general structure [Rh2(µ-OAc)4(RSCH2Z)2] yields the cationic complexes [Rh2(µ-OAc)3-
(RSCH2Z)2](BF4) (R ) Me, Ph; Z ) C(O)OEt, CH2C(O)OMe). The methylthio complex with
Z ) C(O)OEt has been structurally characterized and found to exhibit an unusual bidentate
O-S coordination of the oxo thioether ligands. Preliminary tests indicate that the complex
is a promising catalyst of the silylformylation or hydrosilylation of 1-hexyne with dimeth-
ylphenylsilane.
In tr od u ction
to that of other related anionic O-O, O-P, and O-N
ligands deriving from â-dicarbonyls, â-ketophosphines,
Functionalized thioethers having C-H acidic groups
represent an interesting class of ligands, because in
their anionic form they can combine the good coordinat-
ing properties of the sulfur atom with those of the
carbon or of other donor atoms present in the functional
groups. For example, it has recently been shown that
palladium acetate reacts with R-substituted thioethers
RSCH2Z (Z ) ester, ketone, sulfone) to give trinuclear
complexes of the type [Pd3(µ-OAc)3(µ-RSCHZ)3].1,2 This
metalation reaction results from the ligand exchange
process, in which the sulfur ligand, despite its lower
acidity (∆pKDa MF > 10),3,4 protonates and substitutes
half of the coordinated acetato ligands. As shown by the
X-ray structure of the prototype complex [Pd3(µ-OAc)3-
(µ-MeSCHC(O)OEt)3], the anionic ligand MeSCHC-
(O)OEt- bridges two palladium centers through the
sulfur and the methine carbon atoms, without involving
the ester group in the coordination. Moreover, in the
set of six chiral donor atoms (three C-S couples) which
is generated upon coordination, all atoms exhibit the
same configuration (S,S,S,S,S,S or R,R,R,R,R,R). There-
fore, the behavior of these potentially O-S chelating
ligands appears more similar to that of the carbanionic
ligands R′SCHR- (R′ ) Me, Ph; R ) H, CH2C6F5)5,6 than
or â-ketoamines.7-9
There are at least two interesting points in the above
reaction: (i) the bidentate coordination of the ligand
blocks the configuration of the sulfur atom, which
becomes a chiral center, and (ii) the synthetic procedure
is very simple, which makes it potentially attractive also
for other metal carboxylates.
We report here on the reaction of a series of thioethers
with dirhodium(II) acetate, [Rh2(µ-OAc)4]. This choice
is based on the importance of rhodium acetate and of
related neutral rhodium(II) dimers as catalysts in a very
large number of organic reactions involving unstable
metal-carbene intermediates (for example, C-C cou-
pling and C-H and C-X insertion)10 or silanes (for
example silylations, hydrosilylations, or silylformyla-
tions).11 The investigated ligands are generally oxo
thioethers of the type RSCH2Z (R ) Me, Ph; Z ) C(O)-
OEt, C(O)Me, CH2C(O)OMe), and some of them have
already been shown to give carbon metalation in the
(7) (a) Mehrotra, R. C.; Bohra, R.; Gaur, D. P. Metal â-Diketonates
and Allied Derivatives; Academic Press: New York, 1978. (b) Siedle,
A. R. In Comprehensive Coordination Chemistry; Wilkinson, G., Gillard,
R. D., McCleverty, J . A., Eds.; Pergamon Press: Oxford, U.K., 1987;
Vol. 2, p 365. (c) Fackler, J . P., J r. Prog. Inorg. Chem. 1966, 7, 361. (d)
Graddon, D. P. Coord. Chem. Rev. 1969, 4, 1. (e) Kawaguchi, S. Coord.
Chem. Rev. 1986, 70, 51. (f) Saito, K.; Kido, H.; Nagasawa, A. Coord.
Chem. Rev. 1990, 100, 427.
* To whom correspondence should be addressed: E-mail:
marino.basato@unipd.it.
† University of Padua and ISTM-CNR Padua.
‡ ICIS-CNR Padua.
(8) (a) Morise, X.; Green, M. H. L.; Braunstein, P.; Rees, L. H.; Vei,
I. C. New J . Chem. 2003, 27, 32. (b) Bader, A.; Lindner, E. Coord.
Chem. Rev. 1991, 108, 27.
§ University of Pisa.
(1) Basato, M.; Tommasi, D.; Zecca, M. J . Organomet. Chem. 1998,
571, 115.
(2) Basato, M.; Grassi, A.; Valle, G. Organometallics 1995, 14, 4439.
(3) Bordwell, F. G.; Zhang, X.-M.; Alnajjar, M. S. J . Am. Chem. Soc.
1992, 114, 7623.
(4) Maran, F.; Celadon, D.; Severin, M. G.; Vianello, E. J . Am. Chem.
Soc. 1991, 113, 9320.
(5) Miki, K.; Kai, Y.; Yasuoka, N.; Kasai, N. J . Organomet. Chem.
1977, 135, 53.
(9) Cozzi, P. G.; Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1994, 13, 1528.
(10) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods
for Organic Synthesis with Diazo Compounds; Wiley: New York, 1997.
(11) (a) Doyle, M. P.; High, K. G.; Bagheri, V.; Pieters, R. J .; Lewis,
P. J .; Pearson, M. W. J . Org. Chem. 1990, 55, 6082. (b) Doyle, M. P.;
High, K. G.; Nesloney, C. L.; Clayton, T. W., J r.; Lin, J . Organometallics
1991, 10, 1225. (c) Doyle, M. P.; Devora, G. A.; Nefedov, A. O.; High,
K. G. Organometallics 1992, 11, 549. (d) Doyle, M. P.; Shanklin, M. S.
Organometallics 1993, 12, 11. (e) Doyle, M. P.; Shanklin, M. S.
Organometallics 1994, 13, 1081.
(6) Albeniz, A. C.; Espinet, P.; Lin, Y.-S.; Orpen, A. G.; Martin, A.
Organometallics 1996, 15, 5003.
10.1021/om0342560 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/19/2004

1948 Organometallics, Vol. 23, No. 8, 2004
Basato et al.
2994-2922 (ν(CH)), 1738 and 1300 (ν(CdO) and ν(C-O),
ester), 1584 and 1426 (ν(CO2-)), 1179, 1024. Anal. Calcd for
reaction with palladium acetate. On the other hand,
Lahuerta has proved in his extensive studies that
rhodium acetate itself undergoes easy metalation with
arylphosphines, forming µ-P,C-bridged complexes.12 It
can be anticipated that in our case only the simple
addition complexes [Rh2(µ-OAc)4(RSCH2Z)2] are ob-
tained with all sulfur-based ligands. However, a biden-
tate O-S coordination of the oxo thioether can be forced
by protonating or alkylating an acetato ligand to give
the cationic complexes [Rh2(µ-OAc)3(RSCH2Z)2](BF4)
(R ) Me, Ph; Z ) C(O)OEt, CH2C(O)OMe). Preliminary
tests indicate that the methylthio complex with Z )
C(O)OEt is a promising catalyst of the silylformylation
or hydrosilylation of 1-hexyne with dimethylphenylsi-
lane.
C
18H32O12Rh2S2: C, 30.43; H, 4.54; S, 9.03. Found: C, 30.28;
H, 4.41; S, 8.88.
[Rh 2(OAc)4(P h SCH2C(O)OEt)2] (2). This compound was
obtained from [Rh2(OAc)4] (0.30 g, 0.68 mmol) and PhSCH2C-
(O)OEt (0.28 g, 1.42 mmol), as a purple solid from diethyl
ether. Yield: 85%. 1H NMR (CDCl3, δ): 1.23 (t, 3H, CH3CH2O),
1.84 (s, 6H, CH3CO2-), 3.93 (s, 2H, SCH2), 4.18 (q, 2H,
CH3CH2O), 7.35 and 7.70 (m, 5H, Ph). 13C{1H} NMR (CDCl3,
δ): 14.2 (CH3CH2O), 23.9 (CH3CO2-), 38.9 (SCH2), 61.5
(CH3CH2O), 128.4, 128.9, 129.3, 132.2 (Ph), 168.9 (C(O)OEt),
191.7 (CO2-). IR (KBr, cm-1): 3053-2928 (ν(CH)), 1721 and
1264 (ν(CdO) and ν(C-O), ester), 1586 and 1437 (ν(CO2-)),
1140, 1028, 752, 693. Anal. Calcd for C28H36O12Rh2S2: C, 40.30;
H, 4.35; S, 7.68. Found: C, 40.76; H, 4.33; S, 8.06.
[Rh 2(OAc)4(EtSCH2C(O)Me)2] (3). This compound was
obtained from [Rh2(OAc)4] (0.30 g, 0.68 mmol) and EtSCH2C-
(O)Me (0.17 g, 1.44 mmol), as purple crystals from dichlo-
romethane. Yield: 74%. 1H NMR (CDCl3, δ): 1.49 (t, 3H,
CH3CH2), 1.86 (s, 6H, CH3CO2-), 2.49 (s, 3H, CH3C(O)), 3.12
(q, 2H, CH3CH2), 3.79 (s, 2H, SCH2). 13C{1H} NMR (CDCl3,
δ): 13.0 (CH3CH2), 23.8 (CH3CO2-), 28.6 (CH3C(O)), 29.0
(CH3CH2), 43.7 (SCH2), 191.7 (CO2-), 204.5 (CH3C(O)). IR
(KBr, cm-1): 2973-2903 (ν(CH)), 1713 (ν(CdO), ketone), 1591
and 1429 (ν(CO2-)), 1155. Anal. Calcd for C18H32O10Rh2S2: C,
31.87; H, 4.75; S, 9.45. Found: C, 31.68; H, 4.62; S, 9.35.
Exp er im en ta l Section
Gen er a l P r oced u r es. The reagents (Aldrich-Chemie) were
high-purity products and generally used as received. Solvents
were dried before use, and the reaction apparatus was care-
fully deoxygenated. Reactions were performed under argon,
and all operations were carried out under an inert atmosphere.
EtSCH2C(O)Me was synthesized by reaction in ethanol of
ClCH2C(O)Me with EtSNa prepared in situ from EtSH and
sodium ethoxide.13 PhSCH2C(O)OEt was prepared by reacting
PhSCH2C(O)OH with EtOH at reflux for 3 days after addition
of 1 mL of concentrated HCl. Subsequently, the solvent was
evaporated under reduced pressure and the final solution
distilled to obtain a colorless liquid. Yield: 69%. 1H NMR
(CDCl3, δ): 1.21 (t, 3H, CH3CH2O), 3.62 (s, 2H, SCH2), 4.20
(q, 2H, CH3CH2O), 7.23-7.45 (m, 5H, Ph). 13C{1H} NMR
(CDCl3, δ): 13.6 (CH3CH2O), 36.1 (CH2S), 60.9 (CH3CH2O),
126.4-134.7 (Ar), 169.1 (C(O)OEt). IR (KBr, cm-1): 3050-
2926 (ν(CH), aliphatics and aromatics), 1730 and 1271 (ν(CdO)
and ν(C-O), ester), 1581 (ν(CC), aromatic), 1130, 1024, 739,
[Rh 2(OAc)4(MeSCH2CH2C(O)OMe)2] (4). This compound
was obtained from [Rh2(OAc)4] (0.30 g, 0.68 mmol) and
MeSCH2CH2C(O)OMe (0.20 g, 1.49 mmol), as a purple solid
1
from diethyl ether. Yield: 72%. H NMR (CDCl3, δ): 1.85 (s,
6H, CH3CO2-), 2.60 (s, 3H, CH3S), 2.96 (ct, 2H, SCH2CH2),
3.33 (ct, 2H, SCH2CH2), 3.73 (s, 3H, CH3O). 13C{1H} NMR
(CDCl3, δ): 17.0 (CH3S), 23.7 (CH3CO2-), 30.8 (SCH2CH2), 32.6
(SCH2CH2), 51.8 (CH3O), 172.7 (C(O)OCH3), 191.7 (CO2-). IR
(KBr, cm-1): 2997-2922 (ν(CH)), 1732 and 1256 (ν(CdO) and
ν(C-O), ester), 1593 and 1437 (ν(CO2-)), 1346, 1194. Anal.
Calcd for C18H32O12Rh2S2: C, 30.43; H, 4.54; S, 9.03. Found:
C, 30.19; H, 4.46; S, 9.21.
1
691. The solution H and 13C{1H} NMR spectra were acquired
on a Bruker DRX-400 instrument (400.13 MHz for 1H and
100.62 MHz for 13C) at room temperature. The chemical shifts
are reported versus tetramethylsilane and were determined
by reference to the residual solvent peaks, using tetrameth-
ylsilane as internal standard. The FT IR spectra were recorded
on a Biorad FT S7 PC spectrophotometer at 2 cm-1 resolution
in KBr disks.
Reaction of th e Neu tr al Com plexes [Rh 2(OAc)4(RSCH2-
Z)2] (1, 2, a n d 4) w ith HBF 4 or (Et3O)BF 4: Syn th esis of
th e Com p lexes [Rh 2(OAc)3(RSCH2Z)2](BF 4) (5-7). Com-
plexes 5-7 were obtained by reaction of the appropriate
neutral complexes with HBF4 (5-7) or (Et3O)BF4 (5, 6) in a
1/2 molar ratio, in anhydrous dichloromethane, at room
temperature under argon. The complexes are spectroscopically
pure, but their carbon content is always slightly low, even after
recrystallization from dichloromethane, probably because of
a very limited hydrolysis of the ester group.
Syn th esis of th e Neu tr a l Com p lexes [Rh 2(OAc)4(RS-
CH2Z)2] (1-4). Complexes 1-4 were obtained by reaction of
[Rh2(OAc)4] with the appropriate thioether in a 1/2 molar ratio,
in toluene at room temperature under argon. They are isolated
spectroscopically pure by removal of the solvent and treatment
of the residue with diethyl ether and can be recrystallized from
dichloromethane to give analytically pure samples.
[Rh 2(OAc)4(MeSCH2C(O)OEt)2] (1). In this typical reac-
tion, to a suspension of [Rh2(OAc)4] (0.30 g, 0.68 mmol) in
toluene (25 mL) was added MeSCH2C(O)OEt (196 µL, 0.20 g,
1.49 mmol). The reaction mixture was stirred for 3 h, at room
temperature, evaporated to small volume under reduced
pressure, and treated with diethyl ether to give a purple solid,
which was filtered and dried under vacuum. Recrystallization
from dichloromethane afforded brilliant purple crystals.
Yield: 0.41 g (84%). 1H NMR (CDCl3, δ): 1.38 (t, 3H, CH3-
CH2O), 1.86 (s, 6H, CH3CO2-), 2.72 (s, 3H, CH3S), 3.80 (s, 2H,
SCH2), 4.33 (q, 2H, CH3CH2O). 13C{1H} NMR (CDCl3, δ): 14.1
(CH3CH2O), 17.9 (CH3S), 23.6 (CH3CO2-), 37.2 (SCH2), 61.3
(CH3CH2O), 169.4 (C(O)OEt), 191.6 (CO2-). IR (KBr, cm-1):
[R h 2(OAc)3(MeSCH 2C(O)OE t )2](BF 4) (5). [Rh2(OAc)4-
(MeSCH2C(O)OEt)2] (1; 0.31 g, 0.43 mmol) was dissolved in
dichloromethane (25 mL), and to the resulting solution was
added 118 µL of a 54% w/w solution of HBF4 in diethyl ether
(0.14 g of HBF4, 0.86 mmol). The reaction mixture was stirred
for 3 h, during which time the color changed from purple to
green; evaporation to small volume under reduced pressure
and treatment with diethyl ether afforded a green compound,
which was filtered and dried under vacuum. Yield: 0.26 g
(81%). 1H NMR (CDCl3, δ): 1.42 (t, 3H, CH3CH2), 1.83 (s, 3H,
CH3CO2-), 2.14 (s, 3H, CH3S), 2.35 (s, 1.5H, CH3CO2-), 3.93
(AB system, 2H, J ) 17.4 Hz, SCH2), 4.70 (cm, 2H, CH3CH2).
13C{1H} NMR (CDCl3, δ): 14.0 (CH3CH2), 17.7 (CH3S), 22.5
and 24.0 (CH3CO2-), 43.2 (SCH2), 66.2 (CH3CH2), 177.8 (C(O)-
OEt), 187.6 and 193.4 (CO2-). IR (KBr, cm-1): 2986-2934
(ν(CH)), 1730 (w, ν(CdO), ester), 1657, 1570 and 1429 (CO2-),
1331, 1219, 1061. Anal. Calcd for C16H29BF4O10Rh2S2: C, 26.03;
H, 3.95; S, 8.69. Found: C, 25.26; H, 3.95; S, 9.17.
(12) Estevan, F.; Garcia-Bernabe, A.; Garcia-Granda, S.; Lahuerta,
P.; Moreno, E.; Perez-Prieto, J .; Sanau, M.; Ubeda, M. A. J . Chem.
Soc., Dalton Trans. 1999, 3493 and references therein.
(13) Bradsher, C. K.; Brown, F. C.; Grantham, R. J . J . Am. Chem.
Soc. 1954, 5, 114.
The same reaction with (Et3O)BF4 (1 M solution in dichlo-
romethane), instead of HBF4, leads to the same compound in
71% yield.

Cationic Carboxylato Complexes of Dirhodium(II)
Organometallics, Vol. 23, No. 8, 2004 1949
Ta ble 1. Su m m a r y of X-r a y Cr ysta llogr a p h ic Da ta
for Com p lexes 1 a n d 5
[Rh 2(OAc)3(P h SCH2C(O)OEt)2](BF 4) (6). This compound
was obtained from [Rh2(OAc)4(PhSCH2C(O)OMe)2] (4; 0.30 g,
0.36 mmol) and HBF4 in diethyl ether (100 µL, 0.12 g of HBF4,
0.73 mmol), as a green solid. Yield: 75%. 1H NMR (CDCl3, δ):
1.34 (s, 3H, CH3CO2-), 1.52 (t, 3H, CH3CH2O), 2.41 (s, 1.5H,
CH3CO2-), 4.21 (AB system, 2H, J ) 17.7 Hz, SCH2), 4.82 (bm,
2H, CH3CH2O), 7.54 and 7.85 (m, 5H, Ph). 13C{1H} NMR
(CDCl3, δ): 14.1 (CH3CH2), 22.5 and 23.4 (CH3CO2-), 46.2
(CH3CH2), 66.8 (SCH2), 125-132 (Ph), 178.6 (C(O)OEt), 188.3
and 193.3 (CO2-). IR (KBr, cm-1): 3055-2932 (ν(CH)), 1732
(ν(CdO), ester), 1661, 1566 and 1443 (ν(CO2-)), 1321, 1084,
750, 691. Anal. Calcd for C26H33BF4O10Rh2S2: C, 36.21; H, 3.85;
S, 7.43. Found: C, 35.63; H, 3.52; S, 7.18.
1
5
empirical formula
C
18H32O12Rh2S2 C16H29O10Rh2S2‚
BF4‚1/4H2O
fw
710.38
monoclinic
I2/a
17.622(3)
8.535(2)
18.667(3)
106.32(3)
2694.5(9)
4
742.65
monoclinic
C2
30.225(5)
14.103(3)
13.148(3)
93.08(3)
5596(2)
8
cryst syst
space group
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
The same reaction with (Et3O)BF4 (1 M solution in dichlo-
romethane), instead of HBF4, leads to the same compound in
71% yield.
calcd density (g cm-3
F(000)
)
1.751
1432
1.763
2964
λ (Å)
0.710 73
293(2)
1.434
0.710 73
293(2)
1.399
temp (K)
[Rh 2(OAc)3(MeSCH2CH2C(O)OMe)2](BF 4) (7). This com-
pound was obtained from [Rh2(OAc)4(MeSCH2CH2C(O)OMe)2]
(4; 0.30 g, 0.42 mmol) and HBF4 in diethyl ether (118 µL, 0.14
g HBF4, 0.86 mmol), as a green solid. Yield: 74%. 1H NMR
(CDCl3, δ): 1.87 (s, 3H, CH3CO2-), 2.11 (s, 3H, CH3S), 2.45 (s,
1.5H, CH3CO2-), 2.99 and 3.19 (2 cm, 4H, CH2CH2S), 3.14 (bt,
2H, CH2CH2S, AB system), 4.11 (s, 3H, CH3O). 13C{1H} NMR
(CDCl3, δ): 14.7 (CH3S), 22.9 and 24.9 (CH3CO2-), 28.4
(CH2CH2S), 29.2 (CH2CH2S), 54.5 (CH3O), 177.9 (C(O)OCH3),
187.4 and 193.1 (CO2-). IR (KBr, cm-1): 2953-2924 (ν(CH)),
1734 (ν(CdO), ester), 1688, 1642, 1564 and 1435 (ν(CO2-)),
1362, 1055, 702. Anal. Calcd for C16H29BF4O10Rh2S2: C, 26.03;
H, 3.95; S, 8.69. Found: C, 24.22; H, 3.73; S, 8.60.
abs coeff (mm-1
)
no. of rflns collected
3717
8389
no. of obsd rflns (I > 2σ(I)) 2281
5351
0.048
0.106
Ra
0.030
0.071
b
Rw
R ) ∑|Fo| - |Fc|/∑|Fo|. Rw ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2
.
a
b
2
using the θ-2θ scan technique to a maximum 2θ value of 52°.
There were no significant fluctuations of intensities, other than
those expected from Poisson statistics. The intensity data were
corrected for Lorentz-polarization effects and for absorption,
as described by North et al.16
The structures were solved by direct methods17 and were
refined by the full-matrix least-squares method with the
SHELX-97 program18 implemented in the WinGX package.19
All non-hydrogen atoms were refined with anisotropic thermal
parameters. The H atoms were placed in calculated positions
with fixed, isotropic thermal parameters (1.2Uequiv of the parent
carbon atom). Molecular graphics were drawn using the
ORTEP-III program program.20 Other experimental details of
crystal structure determinations, including data collection and
refinement for compounds 1 and 5, are reported in Table 1.
Silylfor m yla tion a n d Hyd r osilyla tion Rea ction s. The
silylformylation reaction was performed in a 25 mL stainless
steel autoclave fitted with a Teflon inner crucible and a stirring
bar. Me2PhSiH, (0.31 mL, 2 mmol), 1-hexyne (0.23 mL, 2
mmol), and rhodium catalyst (0.002 mmol) were dissolved in
dichloromethane (2 mL) under a CO atmosphere in a Pyrex
Schlenk tube. The obtained solution was introduced in the
autoclave, previously placed under vacuum (0.1 mmHg), by a
steel siphon. The reactor was pressurized with 10 atm of
carbon monoxide, and the mixture was stirred at room
temperature for 6 h. After removal of excess CO (fume hood),
the reaction mixture was diluted with pentane (10 mL), filtered
on Celite, and concentrated under vacuum. The composition
of the reaction mixture was determined by GLC, GC-MS, and
1H NMR analysis.14 The hydrosilylation reaction was run in a
Pyrex Carius tube fitted with a Corning Rotaflo tap. Me2PhSiH
(0.31 mL, 2 mmol) and 1-hexyne (0.92 mL, 8 mmol) were
added, under an argon atmosphere via syringe, to the rhodium
catalyst (0.002 mmol). The suspension was stirred at 90 °C;
after 5 h the GLC analysis showed 62% conversion of the
silane. The reaction mixture was filtered on Celite and
concentrated under vacuum in order to remove the excess
alkyne. The isomeric composition of the reaction products was
Resu lts a n d Discu ssion
The neutral complexes [Rh2(µ-OAc)4(RSCH2Z)2] (1,
R ) Me, Z ) C(O)OEt; 2, R ) Ph, Z ) C(O)OEt; 3, R )
Et, Z ) C(O)Me; 4, R ) Me, Z ) CH2C(O)OMe) are
obtained by a very straightforward procedure, which
consists of the reaction of Rh2(µ-OAc)4 with the proper
sulfur ligand (1/1 Rh/RCH2Z ratio) in toluene. Yields are
high (72-85%), and the compounds are stable both in
the solid state and in solution.
The spectroscopic data do not require particular
comments. The IR spectra are characterized by the
presence of a pair of symmetric and asymmetric stretch-
ings of the acetato groups in the ranges 1593-1584 and
1437-1426 cm-1; moreover, the ν(CdO) band of the
thioether ligand is present at 1738-1713 cm-1. These
last values are close to those found in the free ligands,
thus suggesting the absence of coordination of the
1
determined by H NMR analyses.15
X-r a y Cr ysta llogr a p h y. Single crystals of compound 1
suitable for X-ray analysis were obtained by concentration of
the corresponding solution in DCM/acetone (5/1); slow diffusion
over a period of ca. 10 days at room temperature afforded
purple transparent crystals. For compound 5 single crystals
resulted from slow concentration at room temperature of a
solution in acetone/hexane (5/1); the sample consisted of green
transparent crystals growing in clumps. Cell constants were
determined for both structures by least-squares refinement of
30 independent reflections. Data were measured on a four-
circle Philips PW1100 (Febo System) diffractometer equipped
with graphite-monochromated Mo KR radiation, following
standard procedures. Data were collected at room temperature
1
oxygen atom.21 The H and 13C NMR spectra show the
(16) North, A. C. T.; Philips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(17) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 2,
435.
(18) Sheldrick, G. M. “SHELXL-97”, Program for the Refinement
of Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany.
(19) Farrugia, L. J . J . Appl. Crystallogr. 1999, 32, 837.
(20) Burnett, M. N.; J ohnson, C. K. Report ORNL-6895; Oak Ridge
National Laboratory: Oak Ridge, TN, 1996.
(14) Aronica, L. A.; Terreni, S.; Caporusso, A. M.; Salvadori, P. Eur.
J . Org. Chem. 2001, 4321.
(15) Ojima, I.; Clos, N.; Donovan, R. J .; Ingallina, P. Organometallics
1990, 9, 3127.
(21) Newkome, G. R.; Theriot, K. J .; Fronczek, F. R.; Villar, B.
Organometallics 1989, 8, 2513.

1950 Organometallics, Vol. 23, No. 8, 2004
Basato et al.
high reaction temperatures (in boiling toluene at reflux).
Thus, the behavior exibited by Rh2(µ-OAc)4 is different
from that observed with Pd3(µ-OAc)6: i.e., the prelimi-
nary η1-S bonding of the oxothioether to the metal center
is not followed by deprotonation of the methylene group
and subsequent µ2-C,S coordination. Independently of
electronic reasons related to the different natures of the
metal centers, it appears that the short Rh-Rh bond
distance in the rhodium dimer (2.4073(7) Å) (compared
with that of Pd-Pd in the trimer, average 3.214 Å)24
does not allow, because of steric constraints, a proper
approach of the methylene carbon atom to the adjacent
metal center. A bridging ortho metalation involving the
phenyl ring of PhSCH2C(O)OEt is also not observed, in
contrast with the more favorable geometrical param-
eters for this type of coordination and with the results
obtained in the reaction of rhodium acetate with
arylphosphines.12
However, it is possible to induce a chelate O,S
coordination of the oxo thioether ligand by removing an
acetato bridging group via its protonation or alkylation.
Complexes 1, 2, and 4 react at room temperature with
HBF4 in dichloromethane-diethyl ether, or with (Et3O)-
BF4 in dichloromethane, to give in high yields (74-81%)
the monocationic complexes [Rh2(OAc)3(RSCH2Z)2](BF4)
(5, R ) Me, Z ) C(O)OEt; 6, R ) Ph, Z ) C(O)OEt; 7,
R ) Me, Z ) CH2C(O)OMe) (eq 1). In contrast, complex
F igu r e 1. ORTEP view of the molecular structure of
complex 1.
Ta ble 2. Selected Bon d Dista n ces a n d An gles for
[Rh 2(OAc)4(MeSCH2C(O)OEt)2] (1)a
Bond Distances (Å)
Rh-Rh′
Rh-O(2)
Rh-O(4)
S-C(5)
2.4073(7)
2.049(3)
2.039(3)
1.793(6)
Rh-O(1)′
Rh-O(3)′
Rh-S
2.042(3)
2.045(3)
2.541(1)
1.797(5)
S-C(6)
Bond Angles (deg)
Rh-Rh′-O(1)′
Rh-Rh′-O(3)′
O(1)′-Rh′-O(3)′
O(2)-Rh-O(3)′
C(5)-S-C(6)
87.7(1)
Rh′-Rh-O(2)
Rh′-Rh-O(4)
O(1)′-Rh-O(4)
O(2)-Rh-O(4)
88.0(1)
87.1(1)
91.5(2)
88.2(2)
88.5(1)
88.5(2)
91.5(2)
101.7(3)
a
The slanted prime (′) indicates the symmetry transformation
1 - x, -y, 1 - z.
+HBF4, -HOAc
[Rh2(OAc)4(RSCH2Z)2]
8
+(Et3O)(BF4), -EtOAc
expected resonances for the functional groups present
in the molecule and, in particular, one singlet at 3.79-
3.93 ppm of the methylene protons (complexes 1-3).
This indicates that the two hydrogen nuclei are mag-
netically equivalent and suggests that the inversion of
configuration of the coordinated sulfur atom is fast at
room temperature on the NMR time scale. This process
remains fast also at lower temperatures, as suggested,
[Rh2(OAc)3(RSCH2Z)2](BF4) (1)
3 turns out to decompose without yielding a definite
cationic product under the reaction conditions employed.
1
In their H NMR spectra two distinct sets of signals
for the bridging acetato ligands are present in the
ranges 1.34-1.87 and 2.35-2.45 ppm with 2/1 intensity
ratios. Moreover, the methylene protons show an AB
system centered at 3.90 (5) or 4.18 (6) ppm. These data
indicate that one of the three acetato groups occupies a
unique position and that the CH2 protons have become
diastereotopic.
Decoordination of one acetato group leaves one equa-
torial coordination site on Rh available for an alterna-
tive bonding mode with S. This bond, stronger than that
in the axial position (see the involved bond lengths
below), is preferred and gives rise to the configurational
rearrangement of the thioester ligands as actually
observed in the molecular structure of 5.
The binuclear carboxylato complex [Rh2(OAc)3(Me-
SCH2C(O)OEt)2](BF4)‚1/4H2O (5) crystallizes in the acen-
tric space group C2; the crystal structure is character-
ized by the presence of two subunits (A and B) in the
independent asymmetric unit. The subunits with the
atom-numbering scheme are shown in Figure 2, suitably
oriented in order to make evident their reciprocal
relationship. This compound affords an example of
rather infrequent binuclear Rh complexes having an odd
number of coordinated carboxylato groups and bidentate
ligands chelating the metal at nonequivalent coordina-
tion sites. Actually, the bidentate thioesters MeSCH2C-
(O)OEt are bonded to Rh with S and O in equatorial
1
for example, by the H NMR spectrum of 1 in CD2Cl2
at 203 K, in which only a slight broadening of the
signals can be observed, without any evidence of the
appearance of an AB system for the methylene protons.
The molecular structure of the complexes is fully
confirmed by a single-crystal X-ray analysis of [Rh2-
(OAc)4(MeSCH2C(O)OEt)2] (1). Its structure is shown
in Figure 1 with the atom-labeling scheme, and Table
2 reports the relevant molecular parameters.
The geometrical features of the Rh2(OAc)4 core of this
complex are virtually identical with those of the other
reported neutral carboxylato complexes. For example,
in a series of Rh2(OAc)4L2 complexes (L ) H2O, py, Me2-
SO, tethrahydrothiophene, PPh3) the Rh-Rh and Rh-S
bond distances are in the ranges 2.3855(5)-2.449(2) and
2.451(1)-2.517(1) Å, which compare well with the
values found in 1 (2.4073(7) and 2.541(1) Å).22 The
molecular unit lies on inversion centers. The thioether
ligand engages the vacant axial coordination site on Rh
with a fully extended conformation. The preferred S
instead of O bonding was expected in terms of hard-
ness-softness character of these atoms.23
Complexes 1-4 are also obtained under more severe
conditions, such as with a large excess of thioether and
(22) Cotton, F. A.; Felthouse, T. R. Inorg. Chem. 1980, 19, 3223.
(23) Pearson, R. G. Inorg. Chim. Acta 1995, 240, 93 and references
therein.
(24) (a) Skapski, A. C.; Smart, M. L. J . Chem. Soc., Chem. Commun.
1970, 658. (b) Cotton, F. A.; Han, S. Rev. Chim. Miner. 1985, 277.

Cationic Carboxylato Complexes of Dirhodium(II)
Organometallics, Vol. 23, No. 8, 2004 1951
Ta ble 3. Selected Bon d Len gth s a n d An gles for
[Rh 2(OAc)3(MeSCH2C(O)OEt)2](BF 4) (5)
subunit A
subunit B
Bond Distances (Å)
Rh(1)-Rh(2)
2.439(1) Rh(1)′-Rh(2)′
2.270(3) Rh(1)′-S(1)′
2.060(9) Rh(1)′-O(2)′
2.043(9) Rh(1)′-O(3)′
2.052(8) Rh(1)′-O(5)′
2.319(9) Rh(1)′-O(7)′
2.265(3) Rh(2)′-S(2)′
2.040(9) Rh(2)′-O(1)′
2.027(9) Rh(2)′-O(4)′
2.038(8) Rh(2)′-O(6)′
2.296(9) Rh(2)′n-O(9)′
2.436(1)
2.279(3)
2.011(9)
2.023(9)
2.039(8)
2.358(9)
2.276(3)
2.047(9)
2.025(9)
2.069(8)
2.309(9)
Rh(1)-S(1)
Rh(1)-O(2)
Rh(1)-O(3)
Rh(1)-O(5)
Rh(1)-O(7)
Rh(2)-S(2)
Rh(2)-O(1)
Rh(2)-O(4)
Rh(2)-O(6)
Rh(2)-O(9)
Bond Angles (deg)
Rh(1)-Rh(2)-O(1)
Rh(1)-Rh(2)-O(6)
Rh(1)-Rh(2)-O(4)
Rh(1)-Rh(2)-S(2)
Rh(1)-Rh(2)-O(9)
Rh(2)-Rh(1)-O(2)
Rh(2)-Rh(1)-O(5)
Rh(2)-Rh(1)-O(3)
Rh(2)-Rh(1)-S(1)
Rh(2)-Rh(1)-O(7)
O(1)-C(1)-O(2)
O(5)-C(5)-O(6)
O(3)-C(3)-O(4)
Rh(1)-S(1)-C(8)
S(1)-C(8)-C(9)
87.6(3)
87.5(3)
87.7(3)
99.0(1)
Rh(1)′-Rh(2)′-O(1)′
86.8(3)
86.7(3)
87.3(3)
97.7(1)
177.5(2)
87.2(3)
87.4(3)
88.4(3)
98.6(1)
176.9(3)
125(1)
Rh(1)′-Rh(2)′-O(6)′
Rh(1)′-Rh(2)′-O(4)′
Rh(1)′-Rh(2)′-S(2)′
177.3(3) Rh(1)′-Rh(2)′-O(9)′
86.6(3)
86.2(2)
87.8(3)
99.3(3)
Rh(2)′-Rh(1)′-O(2)′
Rh(2)′-Rh(1)′-O(5)′
Rh(2)′-Rh(1)′-O(3)′
Rh(2)′-Rh(1)′-S(1)′
174.9(2) Rh(2)′-Rh(1)′-O(7)′
124(1)
123(1)
127(1)
99.4(4)
O(1)′-C(1)′-O(2)′
O(6)′-C(5)′-O(5)′
O(3)′-C(3)′-O(4)′
Rh(1)′-S(1)′-C(8)′
126(1)
127(1)
99.9(5)
112.4(9)
127(1)
110.9(9)
80.5(2)
98.5(5)
112.3(1)
125(1)
F igu r e 2. ORTEP view of the molecular structure of the
subunits A (top) and B (bottom) forming the independent
unit of complex 5. They are suitably oriented to show their
reciprocal relationship.
112.6(9) S(1)′-C(8)′-C(9)′
124(1) C(8)′-C(9)′-O(7)′
114.2(9) C(9)′-O(7)′-Rh(1)′
81.0(2)
96.8(4)
110(1)
123(1)
C(8)-C(9)-O(7)
C(9)-O(7)-Rh(1)
O(7)-Rh(1)-S(1)
Rh(2)-S(2)-C(13)
S(2)-C(13)-C(14)
C(13)-C(14)-O(9)
C(14)-O(9)-Rh(2)
O(9)-Rh(2)-S(2)
O(7)′-Rh(1)′-S(1)′
Rh(2)′-S(2)′-C(13)′
S(2)′-C(13)′-C(14)′
C(13)′-C(14)′-O(9)′
and axial positions, respectively, and the three acetato
groups bridge the two Rh atoms, producing a molecular
pseudosymmetry of C2 with the binary axis normal to
the Rh-Rh bond and passing through C(5) and C(5)′ in
A and B, respectively, in agreement with the NMR
spectroscopic data. In both the molecular subunits the
two sulfur atoms exhibit the same configuration but A
and B are mutually enantiomeric.
112.8(9) C(14)′-O(9)′-Rh(2)′
82.3(2) O(9)′-Rh(2)′-S(2)′
113.0(9)
81.8(2)
Torsion Angles (deg)
Rh(2)-Rh(1)-S(1)- -102.6(8) Rh(2)′-Rh(1)′-S(1)′- 102.3(8)
C(7) C(7)′
Rh(1)-Rh(2)-S(2)- -98.2(9) Rh(1)′-Rh(2)′-S(2)′- 100.3(7)
C(12)
C(14)-O(10)-C(15)- 178(1)
C(16)
C(9)-O(8)-C(10)-
C(11)
C(12)′
C(14)′-O(10)′-C(15)′- -172(1)
Table 3 reports the most relevant geometrical param-
eters of A and B. The conformations of the two subunits
are virtually identical, except for the opposite configura-
tions of the sulfur chiral atoms. It is worth noting that
opposite configurations of the S atoms in the same
subunit would be severely hampered by stereochemical
reasons (very short intramolecular contact distances
between the S’s methyl groups would arise), which also
seems to prevent any configurational disorder through
a mechanism of pyramidal inversion at the S chiral
centers.25 This fact accounts for the presence of the two
enantiomeric subunits A and B in the structural unit.
In the case of allowable configurational inversion they
are expected to be statistically disordered, geometrically
identical, and therefore structurally equivalent, as
actually found in some complexes containing similar
thio derivatives.26 In both subunits A and B, the Rh
atoms are σ-bonded at the most frequently observed
metal-metal distance of 2.439(5) Å,22 and they show
the usual octahedral, almost undistorted coordination;
only the angles S-Rh-Rh are markedly larger than 90°
(average ca. 99°), due to closure requirements of the five-
membered rings involved and for relieving nonbonded
C(16)′
-84(1)
C(11)′
-103(1) C(9)′-O(8)′-C(10)′-
interactions between the S’s methyl groups. The coor-
dination bond distances are in the norm except for the
axial bonds Rh-O in the range of 2.29-2.35 Å. These
distances have been generally explained in terms of a
trans effect induced by the covalent Rh-Rh σ-bond.22
Thus, a weakening of these bonds is expected with a
consequent high reactivity of the corresponding coordi-
nation site, as confirmed by the catalytic behavior of
this complex (see below).
The discrete molecules A and B are weakly held
together by hydrogen-bridge interactions of the types
C-H‚‚‚‚O and C-H‚‚‚‚F with H‚‚‚O and H‚‚‚F distances
in the ranges of 2.50-2.70 and 2.20-2.50 Å, respectively
(Table 4).27 An intricate network of additional intra- and
intermolecular hydrogen-bond interactions also involv-
ing the BF4- anions cooperates to afford further lattice
stability (Table 5).27 The crystal contains
/ mol of
1
2
hydration water per structural unit; its specific role in
the crystal lattice could not be clarified.
(25) Cross, R. J .; Green, T. H.; Keat, R. J . Chem. Soc., Dalton Trans.
1976, 1150.
(26) Ceccon, A.; Giacometti, G.; Venzo, A.; Ganis, P.; Zanotti, G. J .
Organomet. Chem. 1981, 205, 61.
(27) (a) J effrey, G. A.; Saenger, W. Hydrogen Bonding in Biological
Structure; Springen: Berlin, 1991. (b) Steiner, T. Chem. Commun.
1977, 727. (c) Taylor, R.; Kennard, O. J . Am. Chem. Soc. 1982, 104,
5063.

1952 Organometallics, Vol. 23, No. 8, 2004
Basato et al.
Sch em e 1
Ta ble 4. In tr a m olecu la r Wea k Hyd r ogen -Bon d
In ter a ction s
efficiency of complex 5 appears to be related to the
presence of the positive charge on the rhodium dimer,
which enhances the acid character of the metal center.
On the other hand, the strained coordination of the ester
oxygen on the axial position should favor its decoordi-
nation in the course of the reaction and consequently
the approach of the reagents.
C-H‚‚‚X
(X ) O, F)
C‚‚‚X
(Å)
H‚‚‚X
(Å)
C-H‚‚‚X
(deg)
C(2)-H(2A)‚‚‚O(6)′
C(2)-H(2C)‚‚‚O(9)′
C(2)′-H(2A)′‚‚‚O(10)
C(16)-H(16C)‚‚‚F(2)
C(16)-H(16B)‚‚‚F(5)
C(2)′-H(2A)′‚‚‚F(3)
C(12)′-H(12E)‚‚‚F(3)
C(8)′-H(8D)‚‚‚F(4)
3.51
3.50
3.38
3.55
3.40
3.27
3.18
3.18
2.87
2.58
2.71
2.62
2.61
2.67
2.28
2.35
125
161
128
162.
140
120
155
143
The hydrosilylation (Scheme 1, bottom equation)
requires a higher reaction temperature than the silyl-
formylation in order to be performed (90 °C), but it can
be run neat using an excess of 1-hexyne as the solvent
(catalyst/silane/hexyne ratio 1/1000/4000). The conver-
sion after 5 h is 62%, and (Z)-(dimethylphenylsilyl)-1-
hexene turns out to be the main product (81% selectiv-
ity), other products stemming from the formal cis
addition of the silane to the triple bond. These prelimi-
nary, not optimized, data point out the good catalytic
activity of 5 also in this reaction and, most notably, its
unusual selectivity for the less thermodynamically
stable Z isomer. In fact, cationic rhodium-based cata-
lysts are reported to exhibit a preference for cis hydrosi-
lylation, leading to the E isomer.29 In this case, the
control catalytic test performed using simple dirhodium-
(II) acetate also led to interesting results. In fact, under
the same reaction conditions both the conversion (88%)
and the selectivity for the Z isomer (98%) appear to be
significantly higher than with the cationic catalyst.
Apparently, the observation of such a high selectivity
with dirhodium(II) acetate is unprecedented in the
literature. The same reaction was described quite some
time ago30 and was found to reach comparable conver-
sion, but the selectivity for the various isomers was not
reported. We are currently engaged in a detailed
investigation of these reactions, as well as in the
preparation of chiral monocationic complexes related in
structure to complex 5, which appear to be potentially
interesting catalysts for enantioselective transforma-
tions.
Ta ble 5. P ossible CH‚‚‚O a n d CH‚‚‚F
In ter m olecu la r Hyd r ogen Bon d s
C-H‚‚‚X
(X ) O, F)
C‚‚‚X H‚‚‚X C-H‚‚‚X
equiv position
of X
(Å)
(Å)
(deg)
C(8)-H(8B)‚‚‚F(5)
3.34 2.50
C(16)′-H(16E)‚‚‚F(4) 3.41 2.66
146
135
140
140
159
140
138
163
127
129
126
143
x, y - 1, z
x, y - 1, z
C(4)-H(4A)‚‚‚F(8)
C(7)-H(7A)‚‚‚F(2)
C(8)-H(8A)‚‚‚F(2)
C(6)′-H(6A)′‚‚‚O(1)
C(6)′-H(6A)′‚‚‚O(6)
C(6)-H(6B)‚‚‚O(6)′
C(10)-H(10A)‚‚‚O(7)′ 3.44 2.77
C(10)-H(10B)‚‚‚O(5)′ 3.33 2.63
C(13)′-H(13A)′‚‚‚F(7) 3.24 2.57
C(12)′-H(12D)‚‚‚F(1) 3.39 2.57
3.42 2.64
3.42 2.63
3.36 2.44
3.48 2.68
3.45 2.66
3.59 2.66
x, y - 1, z
x, y - 1, z
x, y - 1, z
-x, y, -z
-x, y, -z
-x, y, -z
-x, y - 1, -z
-x, y - 1, -z
-x + 1/2, y - 1/2, -z
-x + 1/2, y - 1/2, -z
The monocationic complex 5 is a good catalyst of the
silylformylation or hydrosilylation with dimethylphen-
ylsilane of a model alkyne such as 1-hexyne.
The silylformylation (Scheme 1, top equation) is
performed in dichloromethane, at room temperature,
under 10 atm of CO, using a 1/1000 catalyst/substrate
ratio. After 6 h, the conversion was 98% and the
selectivity toward the silylformylation product was 73%,
other byproducts stemming almost exclusively from
hydrosilylation of the triple bond (25%), which is known
to be the main competitive reaction. Only the Z isomer
of the silylformylated product is observed, which high-
lights the high regio- and stereoselectivity that can be
obtained with this catalyst. Although not optimized, the
catalytic performance of 5 compares favorably with that
of other rhodium-based catalysts such as dirhodium-
(II) perfluorobutyrate,11d,e Rh4(CO)12,14,28 or solvated
rhodium atoms.14 It is interesting to note that under
the same experimental conditions dirhodium(II) acetate
is almost inactive (6% conversion). The high catalytic
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic files in CIF format for the structure determinations
of [Rh2(OAc)4(MeSCH2C(O)OEt)2] and [Rh2(OAc)3(MeSCH2C-
(O)OEt)2](BF4)‚1/4H2O. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0342560
(29) See for example: Faller, J . W.; D’Alliessi, D. G. Organometallics
2002, 21, 1743.
(28) Matsuda, I.; Ogiso, A.; Sato, S.; Izumi, Y. J . Am. Chem. Soc.
1989, 111, 2332.
(30) Cornish, A. J .; Lappert, M. F.; Filatovs, G. L.; Nile, T. A. J .
Organomet. Chem. 1979, 172, 153.