Chiral organometallic triangles with Rh-Rh bonds. 2. Compounds prepared from enantiopure cis-Rh2(C6H4PPh 2)2(OAc)2(HOAc)2 and their catalytic potentials

Article abstract of DOI:10.1021/ic051282c

Enantiomers of the orthometalated dirhodium compound cis-Rh ₂(C₆H₄PPh₂)₂(OAc) ₂(HOAc)₂ (R-1 and S-1) were prepared from carboxylate exchange reactions of the resolved diasteroiso

Full text of DOI:10.1021/ic051282c

Inorg. Chem. 2005, 44, 8223−8233
Chiral Organometallic Triangles with Rh−Rh Bonds. 2. Compounds
Prepared from Enantiopure cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 and Their
Catalytic Potentials
F. Albert Cotton,*,† Carlos A. Murillo,*,† Salah-E. Stiriba,*,‡ Xiaoping Wang,† and Rongmin Yu†
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 3012,
Texas A&M UniVersity, College Station, Texas 77842-3012, and Instituto de Ciencia Molecular/
ICMOL, UniVersidad de Valencia, AV. Vicent Andre´s Estelles, s/n. 46100 Burjassot, Valencia,
Spain
Received July 29, 2005
Enantiomers of the orthometalated dirhodium compound cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 (R-1 and S-1) were
prepared from carboxylate exchange reactions of the resolved diasteroisomers of cis-Rh2(C6H4PPh2)2(protos)2-
(H2O)2 (protos
) N-4-methylphenylsulfonyl-L-proline anion) and acetic acid. These compounds react with excess
Me3OBF4 in CH3CN, producing the enantiomers of [cis-Rh2(C6H4PPh2)2(CH3CN)6](BF4)2 (R-2 and S-2) which have
six labile and replaceable CH3CN ligands in equatorial and axial positions. Reactions of R-2 and S-2 with
tetraethylammonium salts of the linear dicarboxylic acids, terephthalic acid (HO2CC6H4CO2H), oxalic acid (HO2-
CCO2H), and 4,4′-diphenyl-dicarboxylic acid (HO2CC6H4C6H4CO2H) afford the enantiopure triangular supramolecules
[cis-Rh2(C6H4PPh2)2(O2CC6H4CO2)(py)2]3, RRR-3 and SSS-3, Rh6(cis-C6H4PPh2)6(O2CCO2)3(py)5(CH2Cl2), RRR-4
and SSS-4, and Rh6(cis-C6H4PPh2)6(O2CC6H4C6H4CO2)3(py)4(CH2Cl2)2, RRR-5 and SSS-5, respectively. The absolute
structures of each of the enantiomers of 1, 3, 4, and 5 were determined by X-ray diffraction analyses. The enantiomers
of 3, 4, and 5 were found to be enantiomorphically isostructural, whereas R-1 and S-1 crystallized in different
space groups. In 4 and 5, CH2Cl2 molecules coordinate to rhodium atoms in the axial positions. The 1H and
31P 1H
{ } NMR spectra of all compounds are reported. The triangular compounds are redox-active, and their
electrochemistry is also discussed. An assay of the catalytic activity/selectivity performance of the triangles for
typical metal carbene transformation, using the model intermolecular cyclopropanation of styrene with ethyl
diazoacetate in both homogeneous and heterogeneous phases, show that these chiral triangles are very active
and have remarkable selectivity when compared with simple Rh2 paddlewheel catalysts with chiral amidate ligands.
Introduction
organic frameworks may provide new approaches to enan-
tioselective recognition, sensing, and catalysis.2 Unfortu-
The use of metal atoms (or ions) as key elements in the
self-assembly of metal-organic supramolecular arrays has
been of great interest over the past decade.1 A great variety
of supramolecular architectures have been realized, including
triangles, squares, and other polygons, as well as helices and
2-D and 3-D structures, by using the proper choice of metal-
containing units and ligand-metal interactions.
Recently, significant work has been done in efforts to
incorporate functional features into the supramolecular
assemblies because design and synthesis of chiral metal-
(1) (a) Schalley, C. A.; Lu¨tzen, A.; Albrecht, M. Chem. Eur. J. 2004, 10,
1072. (b) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.;
Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (c) Seidel, S. R.; Stang,
P. J. Acc. Chem. Res. 2002, 35, 972. (d) Evans, O. R.; Lin, W. Acc.
Chem. Res. 2002, 35, 511. (e) Holliday, B. J.; Mirkin, C. A. Angew.
Chem., Int. Ed. 2001, 40, 2022. (f) Leininger, S.; Olenyuk, B.; Stang,
P. J. Chem. ReV. 2000, 100, 853 and references therein. (g) Espinet,
P.; Soulantica, K.; Charmant, J. P. H.; Orpen, A. G. Chem. Commun.
2000, 915. (h) Navarro, J. A. R.; Lippert, B. Coord. Chem. ReV. 1999,
185-186, 653.(i) Fujita, M. Acc. Chem. Res. 1999, 32, 53. (j) Scherer,
M.; Caulder, D. L.; Johnson, D. W.; Raymond, K. N. Angew. Chem.,
Int. Ed. 1999, 38, 1588. (k) Lai, S.-W.; Chan, M. C.-W.; Peng, S.-
M.; Che, C.-M. Angew. Chem., Int. Ed. 1999, 38, 669. (l) Klausmeyer,
K. K.; Wilson, S. R.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121,
2705. (m) Fujita, M. Chem. Soc. ReV. 1998, 27, 417 and references
therein. (n) Jones, C. J. Chem. Soc. ReV. 1998, 27, 289. (o) Whang,
D.; Kim, K. J. Am. Chem. Soc. 1997, 119, 451.
* To whom correspondence should be addressed. E-mail: cotton@tamu.edu
(F.A.C.); murillo@tamu.edu (C.A.M.); salah.stiriba@uv.es (S.-E.S.).
† Texas A&M University.
‡ Universidad de Valencia.
10.1021/ic051282c CCC: $30.25
Published on Web 10/05/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 23, 2005 8223

Cotton et al.
Scheme 1
nately, most metal centers in metal-organic assemblies are
coordinatively saturated, which generally prevents their use
in metal-based catalysis. Thus, only a few examples of
applications of enantiopure supramolecular assemblies have
been reported so far. The first application of an enantiopure
supramolecular material in enantioselective separation and
catalysis was reported in 2000 by Kim and co-workers.3
Later, the first successful asymmetric catalyzed reaction
which employed a chiral organometallic triangle was de-
scribed.4 The successful application of this method was then
demonstrated in asymmetric Michael additions5 and carbonyl-
ene reactions.6
Electroactivity is another appealing and potentially useful
functionality to be introduced into supramolecular systems
as a combination of electroactivity and complexation of
supramolecules may lead to fascinating molecular sensing
materials. A remarkable example of how this can be used is
provided by a modification of an electrode surface by using
a molecular square which serves as a receptor for π-donor
substrates.7 More recently, a new example was reported of
ferrocene-based electrochemical sensing of neutral species
via a metalloporphyrin-centered binding process mediated
by two kinetically labile ligands in axial positions.8 To date,
only a few redox-active polygons are known with single
metal corner pieces because the majority of the reported
entities have metal ions such as Pd2+, Pt2+, and Zn2+, which
do not allow for alteration of charge by electrochemical or
other means without collapse of the structures.
nomenclature have been described previously.10 There are
two sources of chirality found in these Rh2 units. In addition
to the one arising from the head-to-tail relationship of the
orthometalated phosphine ligands (designated as R and S),
there is another one due to the twist pattern of each phosphine
ligand (designated as P and M),10 as shown in Scheme 1.
The chirality induced by the latter factor is only secondary
when compared to the chirality that arises from the head-
to-tail arrangement of the phosphine ligands.10 The inter-
conversion of R and S is a process that requires bond-
breaking, and it is not expected to occur at ordinary
temperatures because of the large energy barrier for such
process. In contrast, the interconversion of the secondary
chirality (P and M) may be easier. In this report, we will
generally refer to the R and S configurations, and P and M
will only be used for the description of the overall chirality.
The compounds containing the cis-Rh2(C6H4PPh2)22+ unit
are usually electroactive and catalytically active,11 and like
most compounds containing singly bonded Rh2 units, they
have kinetically labile axial ligands. The electrode potentials
of these compounds is sensitive to the presence of different
neutral electron donors in solution as indicated by the
electrochemistry of racemic cis-Rh2(C6H4PPh2)2(OAc)2-
(HOAc)2 in CH2Cl2 in the presence of various potential axial
ligands.12 It is expected that supramolecules containing the
Rh2 unit may have potential applications in molecular sensor
materials and homogeneous or heterogeneous asymmetric
catalysis. In this second of a series of reports, we are
concerned with the preparation and X-ray structures of three
enantiopure molecular triangles based on cis-Rh2(C6H4-
Beginning in early 1998 in this laboratory, the use of
dimetal units (e.g., Mo24+, Rh24+, and Ru2n+) to build higher-
order structures (so-called supramolecules) has been pio-
neered.9 These higher-order structures include pairs, loops,
triangles, squares, tubes, sheets, and 2-D and 3-D structures.
Importantly, all of these compounds are electroactive, and
5+
those containing Ru2 units are also paramagnetic.9c Re-
2+
cently, we reported that the cis-Rh2(C6H4PPh2)2 units, in
which there are two cisoid orthometalated anions derived
from a triphenylphosphine molecules bridging a metal-metal
bonded dirhodium unit, can be a useful metal-based building
block capable of making both chiral and redox-active
supramolecules.10
Because of the head-to-tail arrangement of the two
2+
orthometalated bridging ligands, the cis-Rh2(C6H4PPh2)2
unit is chiral, and compounds containing these entities are
accordingly chiral. Details of the chirality and associated
2+
PPh2)2 units and two enantiopure dirhodium compounds
which are used as precursors for the triangles. In addition,
an evaluation of the catalytic ability of the triangles in both
homogeneous and heterogeneous phases by the model
intermolecular cyclopropanation of styrene with ethyl dia-
zoacetate is also reported.
(2) Dai, L. X. Angew. Chem., Int. Ed. 2004, 43, 5726.
(3) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K.
Nature 2000, 404, 982.
(4) Lee, S. K.; Hu, A. G.; Lin, W. J. Am. Chem. Soc. 2002, 124, 12948.
(5) Takizawa, S.; Somei, H.; Dajaprakash, D.; Sasai, H. Angew. Chem.,
Int. Ed. 2003, 42, 5711.
(6) Guo, H.; Wang, W.; Ding, K. Tetrahedron 2004, 45, 2009.
(7) Lahav, M.; Gabai, R.; Shipway, A. N.; Willner, A. Chem. Commun.
1999, 1937.
(8) Bucher, C.; Devillers, G. H.; Moutet, J. C.; Royal, G.; Saint-Aman,
E. Chem. Commun. 2003, 888.
(9) (a) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34,
759. (b) Cotton, F. A.; Lin, C.; Murillo, C. A. Proc. Natl. Acad. Sci.
U.S.A. 2002, 99, 4810. (c) Angaridis, P.; Berry, J. B.; Cotton, F. A.;
Murillo, C. A. J. Am. Chem. Soc. 2003, 125, 10327.
(10) Cotton, F. A.; Murillo, C. A.; Wang, X.; Yu, R. Inorg. Chem. 2004,
43, 8394.
(11) (a) Barberis, M.; Pe´rez-Prieto, J.; Herbst, K.; Lahuerta, P. Organo-
metallics 2002, 21, 1667. (b) Estevan, F.; Herbst, K.; Lahuerta, P.;
Barberis, M.; Pe´rez-Prieto, J. Organometallics 2001, 20, 950. (c)
Barberis, M.; Lahuerta, P.; Pe´rez-Prieto, J.; Sanau´, M. Chem. Commun.
2001, 439. (d) Estevan, F.; Lahuerta, P.; Pe´rez-Prieto, J.; Pereira, I.;
Stiriba, S.-E. Organometallics 1998, 17, 3442. (e) Estevan, F.;
Lahuerta, P.; Pe´rez-Prieto, J.; Sanau´, M.; Stiriba S.-E.; Ubeda, A. M.
Organometallics 1997, 16, 880.
(12) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A.; Tocher, J. H.
Organometallics 1985, 4, 8.
8224 Inorganic Chemistry, Vol. 44, No. 23, 2005

Chiral Organometallic Triangles with Rh-Rh Bonds
H, 4.04, N, 7.32%. Found: C, 50.50; H, 4.25; N, 7.01%. [R]23
126.0 (c ) 7.5 × 10-3 in CH3CN)
)
Experimental Section
D
Materials and Methods. Unless otherwise stated, all manipula-
tions and procedures were conducted in air. The solvents for the
catalytic tests were purified before use as follows. CH2Cl2 was
distilled over calcium hydride and then passed through a pad of
anhydrous K2CO3, and n-C5H12 was passed through a column of
silica gel, followed by distillation. Solvents used for syntheses were
used as received. Tetraethylammonium salts of dicarboxylic acids
were prepared and isolated as white solids by neutralization of the
corresponding diacids with 2 equiv of 35% Et4NOH in water
followed by thorough drying under vacuum. All other reagents were
purchased from commercial sources and used as received. Resolved
diastereoisomers of cis-Rh2(C6H4PPh2)2(protos)2(H2O)2 (protos )
N-4-methylphenylsulfonyl-L-proline anion) were prepared using the
method described in the literature.11b Both of the R and S
enantiomers of each compound reported herein were synthesized,
but because they were made following the same procedure and in
essentially the same yields, only the synthesis for the R enantiomer
of each compound is described below.
Preparation of RRR-[cis-Rh2(C6H4PPh2)2(O2CC6H4CO2)-
(py)2]3 (RRR-3). A mixture of R-2 (75 mg, 0.065 mmol) and
(Et4N)2O2CC6H4C6H4CO2 (40 mg, 0.094 mmol) in 4 mL of CH2-
Cl2 and 5 mL of methanol was stirred at ambient temperature for
10 min. The mixture of solvents was removed under vacuum. The
residue was washed with 3 × 30 mL of water, dried in air, and
then dissolved in a mixture of 5 mL of CH2Cl2 and two drops of
pyridine. Deep red crystals of the product were obtained by layering
the solution with methanol over a period of one week. (Yield: 65
1
mg, 78%). H NMR (CD2Cl2, δ, ppm): 8.770 (d, 12H, pyridine
C-H), 7.805 (t, 6H, pyridine C-H), 6.387-7.253 (m, 108 H,
aromatic C-H). 31P {1H} NMR (CD2Cl2, δ, ppm): 21.433 (d, 1JPRh
) 167.08 Hz). Anal. Calcd for C162H126N6O12P6Rh6: C, 61.71; H,
4.03; N, 2.66%. Found: C, 61.22; H, 4.36; N, 2.49%. [R]23
)
D
191.2 (c ) 3.6 × 10-4, in a mixture of CH2Cl2 and pyridine).
Preparation of RRR-Rh6(cis-C6H4PPh2)6(O2CCO2)3(py)5-
(CH2Cl2) (RRR-4). A mixture of R-2 (75 mg, 0.065 mmol) and
(Et4N)2O2CCO2 (30 mg, 0.086 mmol) in 5 mL of CH2Cl2 and 5
mL of methanol was stirred for 10 min. The mixture of solvents
was removed in a vacuum. The purple residue13 was washed with
3 × 30 mL of water, dried in air, and then dissolved in 5 mL of
CH2Cl2. Orange crystals of the product were obtained from a CH2-
Cl2 solution, to which 3 drops of pyridine had been added, and
then layering with isomeric hexanes over a period of 5 days.
Elemental analyses were performed by Canadian Microanalytical
Service, Delta, British Columbia. Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) were performed with a CH
Instruments model-CH1620A electrochemical analyzer in 0.1 M
Bu4NPF6 solution (CH2Cl2) with a Pt working and auxiliary
electrodes, an Ag/AgCl reference electrode, and a scan rate of 100
mV/s for the CV. All potential values are referenced to the Ag/
AgCl electrode, and under the present experimental conditions, the
1
Yield: 56 mg, 82%. H NMR (CD2Cl2, δ, ppm): 8.730 (d, 10H,
pyridine), 7.35 (t, 5H, pyridine), 6.364-7.228 (m, 94 H, aromatic),
5.332 (s, 2H, CH2Cl2). 31P {1H} NMR (CD2Cl2 and C5D5N, δ,
ppm): 22.675 (d, 1JPRh ) 173.08 Hz). Anal. Calcd for C140H111N5O12-
Cl2P6Rh6: C, 57.40; H, 3.82; N, 2.39%. Found: C, 57.66; H, 3.84;
N, 2.40%. [R]23D ) 51.9 (c ) 5.2 × 10-4, in a mixture of CH2Cl2
and pyridine).
E1/2 (Fc+/Fc) occurred at 440 mV. H NMR (using the residual
1
proton of the deuterated solvent CD2Cl2 or CDCl3 as reference)
and 31P{1H}NMR (with H3PO4 in CDCl3 as reference) spectra were
recorded on a Mercury-300 NMR spectrometer.
Preparation of R-cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 (R-1).
A solution of R-cis-Rh2(C6H4PPh2)2(protos)2(H2O)2 (183 mg, 0.140
mmol) in a mixture of 3 mL of CH3CN and 3 mL of CH3COOH
was stirred overnight. The mixture was dried in a vacuum. The
solid was washed with 3 × 20 mL of water, dried in a vacuum
again, and then dissolved in 5 mL of CH2Cl2. To this solution was
added 2 mL of acetic acid and 20 mL of isomeric hexanes. The
resulting clear solution was concentrated in air. After 1 day,
diffraction-quality crystals were obtained. Yield: 62 mg, 46%. 1H
NMR (CD2Cl2, δ, ppm): 7.636 (m, 4H, aromatic), 7.392 (m, 6H,
aromatic), 7.234 (t, 2H, aromatic), 7.030 (t, 4H, aromatic), 6.727-
6.894 (m, 8H, aromatic), 6.527-6.608 (m, 4H, aromatic), 2.152
(s, 6H, CH3COOH), 1.192 (s, 6H, CH3COO). 31P {1H} NMR
(CDCl3, δ, ppm): 19.403 (d, 1JPRh ) 162.55 Hz). Anal. Calcd for
C44H42O8P2Rh2: C, 54.68; H, 4.38%. Found: C, 54.50; H, 4.32%.
Preparation of RRR-Rh6(cis-C6H4PPh2)6(O2CC6H4C6H4CO2)3-
(py)4(CH2Cl2)2 (5). A solution of compound R-2 (30 mg, 0.026
mmol) and (Et4N)2O2CC6H4C6H4CO2 (13 mg, 0.026 mmol) in a
mixture of 3 mL of CH2Cl2 and 1 mL of methanol was stirred for
10 min. After filtration, 3 drops of pyridine were added to the
filtrate. The resulting solution was layered with 10 mL of methanol
to afford red crystals that were separated by decantation. Yield:
29 mg, 81%. 1H NMR (CD2Cl2, δ, ppm): 8.690 (d, 8H, pyridine),
7.859 (t, 4H, pyridine), 6.547-7.501 (m, 116 H, aromatic), 5.333
(s, 4H, CH2Cl2). 31P {1H} NMR (CD2Cl2 and C5D5N, δ, ppm):
1
22.796 (d, JPRh ) 167.08 Hz). Anal. Calcd for C172H132N4O12-
Cl4P6Rh6: C, 60.92; H, 3.93; N, 1.65%. Found: C, 60.43; H, 3.76;
N, 1.80%. [R]23D ) 48.2 (c ) 3.8 × 10-4, in a mixture of CH2Cl2
and pyridine).
Cyclopropanation of Styrene with Ethyl Diazoacetate. A
solution of ethyl diazoacetate (18 mg, 0.15 mmol) in CH2Cl2 (5
mL) was added, under argon atmosphere with the aid of a syringe
pump over 5 h (1 mL/h) to a refluxing, stirred solution of the
catalyst (1 mol%) and a 10-fold excess of styrene (0.18 mL, 1.5
mmol) in CH2Cl2 (20 mL). After complete addition, the reaction
mixture was stirred at reflux for an additional 4 h and then cooled
[R]23 ) +244.33 (c ) 1.06 × 10-3, in a mixture of CH2Cl2 and
D
CH3COOH)
Preparation of R-[cis-Rh2(C6H4PPh2)2(CH3CN)6](BF4)2 (R-
2). The procedure for the preparation of compounds R-2 and S-2
was similar to that for racemic RS-[cis-Rh2(C6H4PPh2)2(CH3CN)6]-
(BF4)2.10 A mixture of R-1 (146 mg, 0.150 mmol) and Me3OBF4
(150 mg, 0.96 mmol) in 3 mL of acetonitrile was stirred at ambient
temperature for about 10 min. When the mixture became a clear
red solution, ether was added while stirring until a precipitate was
observed. The mixture was allowed to stand overnight to let the
product deposit from the solution. After filtration, the crystals were
(13) A CH2Cl2 solution of the purple residue layered with methanol over
a period of one month produced crystals of SSS-{Rh6(cis-C6H4PPh2)6-
(C2O4)(CH3OH)5(H2O)}. The 1H NMR spectra of this compound
exposed to air overnight and that of the purple residue were the same,
(CD2Cl2, δ, ppm): 6.439-7.244 (m, 84H, aromatic), 1.956 (s, H2O).
The spectra indicated that, when the solutions are exposed to air, all
of the methanol ligands in axial positions were readily replaced by
water. Crystal data: a ) 11.180(8) Å, b ) 27.89(2) Å, c ) 19.28(1)
Å, â ) 101.94(1)°, V ) 5881(7) Å3, Z ) 2, space group P21, R1 and
wR2 ) 0.0556 and 0.0962, Flack parameter ) -0.01(2)
1
washed with ether. Yield (140 mg, 81.3%). H NMR (CDCl3, δ,
ppm): 7.788 (t, 4H, aromatic), 7.503 (m, 6H, aromatic), 7.317 (t,
2H, aromatic), 7.117 (t, 4H, aromatic), 6.745-6.894 (m, 8H,
aromatic), 6.663 (t, 2H, aromatic), 6.362 (q, 2H, aromatic), 2.034
(s, 18H, CH3CN). Anal. Calcd for C48H46N6B2F8P2Rh2: C, 50.17;
Inorganic Chemistry, Vol. 44, No. 23, 2005 8225

Cotton et al.
Table 1. Crystallographic Data and Structure Refinement Parameters
compound
R-1
S-1
SSS-3‚1.5CH3OH‚7.25CH2Cl2‚1.5H2O
170.75H149.5Cl14.5N6O15P6 Rh6
SSS-4‚2CH2Cl2
RRR-5‚7CH3OH‚5CH2Cl2
chemical formula
fw
cryst syst
space group
a(Å)
b(Å)
c(Å)
R(deg)
â(deg)
γ(deg)
V(Å3)
C44H42O8P2Rh2 C44H42O8P2Rh2
C
C
142H115Cl6N5O12P6Rh6
C184H170Cl14N4 O19P6Rh6
966.54
orthorhombic
P212121
10.667(3)
15.152(4)
25.053(6)
90
966.54
monoclinic
P21
12.6777(9)
18.368(1)
18.145(1)
90
94.648(1)
90
4211.4(5)
4
3842.77
monoclinic
P21
22.366(3)
21.519(3)
36.118(4)
90
91.152(2)
90
17380(4)
4
1.469
0.895
-100
1.149
0.1168, 0.2869
0.14(5)
3099.37
monoclinic
P21
15.210(1)
27.015(2)
16.9670(2)
90
113.032(1)
90
6417(1)
2
1.604
1.018
-100
1.055
0.0353, 0.0826
-0.01(2)
4040.82
orthorhombic
C2221
24.069(2)
36.637(3)
22.257(2)
90
90
90
19626(3)
4
1.368
0.791
-100
1.101
0.0611, 0.1567
0.03(5)
90
90
4049(2)
4
Z
d
calcd (g cm-3
)
1.585
0.948
-60
1.073
1.524
0.911
-60
1.022
µ(Mo KR) (mm-1
T, °C
)
GOF
R1,a wR2b (I > 2σ) 0.0184, 0.0456
0.0278, 0.0601
0.00(2)
Flack parameter
-0.02(1)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑w(Fo )2]1/2, w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) [max(Fo ,0) + 2(Fc )]/3.
2
2
2
2
2
2
to room temperature. The solution was filtered through a short silica
gel chromatography column to eliminate the catalyst. The column
was further washed with CH2Cl2 (10 mL), and the solvent from
the combined solutions was evaporated. The yield was determined
CC6H4CO2)3(py)3(CH3OH)3 in CH2Cl2 from a solution containing
an excess of pyridine gave back crystals of SSS-3. Therefore, it is
reasonable to believe that SSS-3 is as enantiopure as SSS-4 which
was synthesized from the same starting material, namely compound
S-2.
1
using H NMR spectroscopy.
X-Ray Structure Determinations. Mounting of crystals on the
diffractometer was done as quickly as possible to minimize the
loss of solvent molecules, and data collection was done at -100
°C because crystals of all triangular compounds rapidly lost
interstitial solvent molecules. Some of crystals decayed quickly even
at -60 °C. Diffraction data were collected using a Bruker SMART
1000 CCD area detector system. Cell parameters were determined
using the program SMART.14 Data reduction and integration were
performed with the software SAINT,15 and absorption corrections
were applied by using the program SADABS.16 The positions of
the Rh atoms were found via direct methods using the program
SHELXTL.17 Subsequent cycles of least-squares refinement fol-
lowed by difference Fourier syntheses revealed the positions of the
remaining non-hydrogen atoms. Hydrogen atoms were added in
idealized positions for the calculation of the structure factors. The
absolute structure for each compound was assigned on the basis of
the value of the Flack parameter, x.18 Disordered phenyl groups on
the phosphine ligand and solvent molecules were refined with
distance constraints. Some additional details of data collection and
refinement are given in Table 1. Crystallographic data are available
as Supporting Information.
In the crystal structure of RRR-5, the axial ligands in the
crystallographically independent Rh2 units are disordered over two
orientations with occupancies of 0.695 and 0.305 for CH2Cl2 in
one axial position and 0.648 and 0.352 for pyridine in another
position.
Results and Discussion
Syntheses. The dirhodium compound cis-Rh2(C6H4PPh2)2-
(OAc)2(HOAc)2 is composed of a singly bonded inherently
2+
chiral cis-Rh2(C6H4PPh2)2 unit and two labile acetate
groups in cis positions. The racemic compound, RS-cis-
Rh2(C6H4PPh2)2(OAc)2(HOAc)2, was prepared readily from
the reaction of Rh2(OAc)4 with PPh3 in acetic acid under
reflux conditions.12 The enantiomers of this racemate can
be resolved using a general diastereoisomer separation
method. The reactions involved are shown in eqs 1 and 2.
The dirhodium products of eq 1 are a mixture of diastereo-
isomers. This mixture can be separated using chromatography
on silica gel. The pure diastereoisomers can then be
converted to pure enantiomers of the corresponding acetate
compound used as starting material (eq 2).
The modest quality of the crystal of SSS-3 resulted in a large
Flack parameter of 0.14(5), which is greater than those for the other
compounds reported here. Refinement including an “inversion twin”
model did not improve the structure analysis, indicating that the
crystal was not twinned. Furthermore, a CH2Cl2 solution of SSS-3
which had been layered with methanol afforded crystals of SSS-
Rh6(cis-C6H4PPh2)6(O2CC6H4CO2)3(py)3(CH3OH)3 with a small
Flack parameter.19 Crystallization of SSS-Rh6(cis-C6H4PPh2)6(O2-
RS-cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 +
excess Hprotos + 2H2O f
R and S-cis-Rh2(C6H4PPh2)2(protos)2(H2O)2 + 4CH3COOH
(1)
R or S-cis-Rh2(C6H4PPh2)2(photos)2(H2O)2 +
excess CH3COOH f
(14) SMART for Windows NT, version 5.618; Bruker Advanced X-ray
Solutions, Inc.: Madison, WI, 2001.
(15) SAINT Data Reduction Software. Verson 6.36A; Bruker Advanced
X-ray Solutions, Inc.: Madison, WI, 2001.
(16) SADABS. Area Detector Absorption and other Corrections Software,
Version 2.05; Bruker Advanced X-ray Solutions, Inc.: Madison, WI,
2001.
R or S-cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 + 2Hprotos +
2H2O (2)
(17) Sheldrick, G. M. SHELXTL, Version 6.12; Bruker Advanced X-ray
Although compounds R-1 and S-1 could be used directly
as reactants to synthesize the target macrocyclic supra-
molecules, in practice, we have found it is best to convert
Solutions, Inc.: Madison, WI, 2002.
(18) (a) Flack, H. D. Acta Crystallogr. A 1983, 39, 876. (b) Flack, H. D.;
Bernardinelli, G. J. Appl. Crystallogr. 2000, 33, 1143.
8226 Inorganic Chemistry, Vol. 44, No. 23, 2005

Chiral Organometallic Triangles with Rh-Rh Bonds
them to pure enantiomers of [cis-Rh2(C6H4PPh2)2(CH3CN)6]-
(BF4)2 (R-2 and S-2), which contain six easily replaceable
equatorial and axial CH3CN ligands. Compounds R-2 and
S-2 react smoothly and readily with the dicarboxylate linkers
to afford the desired products. The enantiopure compounds
R-2 and S-2 were prepared from the reactions of 1 with
Me3OBF4 in CH3CN in a synthesis similar to that of the
racemic compounds.10 The formation and purity of 2 have
been attested by the NMR spectra.
The preparations of enantiopure triangular compounds
were accomplished from the reaction of 2 with the desired
dicarboxylate anion in the presence of a strong donor solvent
which can provide axial ligands. We have reported that the
reaction of the racemic compound [cis-Rh2(C6H4PPh2)2(CH3-
CN)6](BF4)2 and linear dicarboxylate dianions affords only
triangular molecules.10 Likewise, the reactions of either of
the enantiopure compounds R-2 and S-2 with oxalate,
terephthalate, and 4,4′-diphenyl-dicarboxylate dianions pro-
vide only triangular compounds. Crystals of compounds
RRR-3 and SSS-3 were obtained from a solution in a mixture
of CH2Cl2 and pyridine by layering with methanol. The
crystal structure analysis revealed that all the axial positions
of 3 are occupied by pyridine molecules. It is interesting
that, when oxalate dianion was used under the same
conditions, methanol competes with CH2Cl2 for the axial
it is important and meaningful to study the solution properties
of these compounds.
The 31P NMR spectrum of 3 in CD2Cl2 has one sharp
signal at 21.433 ppm, which is a doublet due to coupling of
the P atom to the I ) /2 103Rh nucleus. This indicates that
1
there is only one highly symmetric species present, which
is consistent with the triangular structure of 3 in the crystals.
The 31P{1H} NMR spectrum of 4 in CD2Cl2 shows a broad
signal around 20 ppm. The appearance of a broad signal
instead of six doublets, as expected from the X-ray structure,
is consistent with the labile character of the axial ligands of
1
the dirhodium units. The H NMR spectra of 4 and 5 in
which there is only one set of signals for pyridine reflect
this property too. The broad signal in the 31P{1H} NMR
1
spectrum became a sharp doublet (19.86 ppm with JPRh
)
169.42 Hz) when an excess of pyridine was added to ensure
that all the axial positions are occupied by the same ligand,
so that only one highly symmetrical species, presumably Rh6-
(cis-C6H4PPh2)6(O2CCO2)3(py)6, is present in this solution.
The crystals of 5 dissolve sparingly in CD2Cl2 but much
more in the mixed solvent of CD2Cl2 and C5D5N. The
solutions of 5 in CD2Cl2 are not concentrated enough to
provide a discernible 31P{1H} NMR signal even with
thousands of transients. Like the spectra of 4, the 31P{1H}
NMR spectrum of 5 in a mixed solvent of CD2Cl2 and C5D5N
again is a doublet, attributable to Rh6(cis-C6H4PPh2)6-
(O2CC6H4C6H4CO2)3(py)6.
positions. A mixture of red crystals of Rh6(cis-C6H4-
20
PPh2)6(O2CCO2)3(py)4.75(CH3OH)1.25
,
which is composed
There is no change observed for the 31P{1H} NMR spectra
of these triangular compounds for days. In addition, as we
shall see, the electrochemical study of compound 4 in CH2-
Cl2 and py demonstrates that there are three well-separated
consecutive one-electron redox processes, which supports the
presumption that the only species in solution are triangular
molecules. The triangular identity of the species in solution
is also consistent with the fact that the racemic product
RRR(SSS)-Rh6(cis-C6H4PPh2)6(O2CC6H4CO2)3 can be iso-
lated from a mixture of equivalent amounts of RRR-3 and
SSS-3.21
As reported previously,10 the racemic triangle of RRS-
(SSR)-Rh6(cis-C6H4PPh2)6(O2CC6H4CO2)3, which has two
Rh2 corners of one chirality and one corner of the other
chirality in each triangular molecule, can be prepared from
the reaction of the racemic compound RS-[cis-Rh2(C6H4-
PPh2)2(CH3CN)6](BF4)2 and the terephthalate salt. No evi-
dence of the formation of RRR(SSS)-Rh6(cis-C6H4PPh2)6(O2-
CC6H4CO2)3 in which all the Rh2 corners are of the same
chirality was observed in that reaction. Using enantiopure
starting materials, the racemic triangular compound RRR-
(SSS)-Rh6(cis-C6H4PPh2)6(O2CC6H4CO2)3 was isolated from
a solution containing equal amounts of RRR-3 and SSS-3 in
of 50% of Rh6(cis-C6H4PPh2)6(O2CCO2)3(py)5(CH3OH) and
50% of Rh6(cis-C6H4PPh2)6(O2CCO2)3(py)4.5(CH3OH)1.5, and
a small amount of orange crystals of 4 were isolated. A
product containing only 4 was obtained from a CH2Cl2
solution in the presence of additional pyridine by layering
with isomeric hexanes. The reaction of 2 with (Et4N)2(O2-
CC6H4C6H4CO2) provided the triangle Rh6(cis-C6H4PPh2)6(O2-
CC6H4C6H4CO2)3(py)4(CH2Cl2)2 (5), in high yield, under
similar conditions in which a CH2Cl2 solution was layered
with hexanes in the presence of excess pyridine.
It should be noted that, with the paddlewheel geometry
of the Rh2 group, a square with Rh2 units at the corners and
four rigid dicarboxylate linkers at the sides would also be
expected to form with the same stoichiometry. While
compounds 3-5 have been shown to be triangles in the
crystalline state, this is no guarantee that a transformation,
full or partial, to squares in solution might not occur. Thus,
(19) Crystallographic data for SSS-Rh6(cis-C6H4PPh2)6(O2CC6H4CO2)3(py)3-
(CH3OH)3‚(CH3OH)6‚(CH2Cl2)3.25‚(H2O)4.25: a ) 21.651(4) Å, b )
21.922(4) Å, c ) 22.055(4) Å, R ) 82.113(3)°, â ) 61.153(3)°, γ )
64.889(3)°, V ) 8272(2) Å3, Z ) 2, space group P1, R1 and wR2 )
0.0871 and 0.2187, Flack parameter ) -0.06(5).
(20) SSS-Rh6(cis-C6H4PPh2)6(O2CCO2)3(py)4.75(CH3OH)1.25 crystallizes in
space group P21 with two independent triangular molecules, Rh6(cis-
C6H4PPh2)6(O2CCO2)3(py)5(CH3OH) and Rh6(cis-C6H4PPh2)6(O2-
CCO2)3(py)4.5(CH3OH)1.5, and with four CH2Cl2 and three CH3OH
solvent molecules in each asymmetric unit. In the molecule of Rh6-
(cis-C6H4PPh2)6(O2CCO2)3(py)4.5(CH3OH)1.5, four of six axial posi-
tions are occupied by pyridine molecules, one is occupied by a
methanol molecule, and the other one is partially occupied by 0.5
pyridine and 0.5 methanol. The crystal data and refinement of the
crystal are: a )13.4000(5) Å, b ) 53.882(2) Å, c ) 18.2036(6) Å;
â ) 90.048(1)°; V ) 13143.3(8) Å3; Z ) 2; space group P1h, R1 )
0.0425, wR2 ) 0.0972 (I > 2σ); T ) 173 K.; GOF ) 1.088. Flack
parameter ) 0.02(2).
(21) Equivalent amounts of RRR-3 and SSS-3 was dissolved in CH2Cl2.
The solution was allowed to stand for about one week and then layered
with methanol. The orange crystals of racemic RRR(SSS)-Rh6(cis-C6H4-
PPh2)6(O2CC6H4CO2)3(py)1.5(CH3OH)4.5‚4.5CH2Cl2‚7.5CH3OH were
isolated quantitatively after a period of one month. 31P{1H} NMR
1
(CDCl3 and C5D5N, δ, ppm): 22.001 (d, JPRh ) 165.38 Hz). The
crystal data and refinement of the crystal are: a ) 16.139(3) Å, b )
22.787(4) Å, c ) 24.622(4) Å; R ) 81.053(3)°, â ) 75.193(3)°, γ )
81.430(3)°; V ) 8591(3) Å3; Z ) 2; R1 ) 0.0757, wR2 ) 0.2130
(I > 2σ); T ) 213 K; GOF ) 1.034.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8227

Cotton et al.
CH2Cl2. The chirality and purity of this racemic compound
were established by both the single-crystal structure and the
31P{1H} NMR spectrum. The syntheses of the two types of
racemic triangular compounds, RRS(SSR) and RRR(SSS), by
different methods, and the lack of interconversion of the two
types indicates that these molecular triangles retain their
integrity in solution.
The axial coordination of CH2Cl2 as in the enantiopure
compounds 4 and 5 is uncommon.22 There are over 1500
dirhodium complexes with axial coordination reported so
far,23 but these are the only two compounds that have been
found with CH2Cl2 in axial positions. The occurrence of this
rare coordination is not easily explained, but it is possible
that the relative solubilities of these compounds controls the
crystallization process since compounds RRR-5 and SSS-5
are only slightly soluble in CH2Cl2 but are quite soluble in
the presence of additional pyridine. In the presence of excess
pyridine, compounds RRR-5 and SSS-5 are likely to become
the highly symmetric compound Rh6(cis-C6H4PPh2)6(O2-
CC6H4C6H4CO2)3(py)6 as revealed by their 31P{1H} NMR
spectra. However, under the crystallization conditions,
perhaps Rh6(C6H4PPh2)6(O2CC6H4C6H4CO2)3(py)6 can exist
only in solution and is converted to compounds RRR-5 and
SSS-5 in the crystalline solid because the latter is less soluble
and easier to crystallize.
Figure 1. The structure of R-1. Displacement ellipsoids are drawn at the
40% probability level. All hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for R-1
Rh(1)-Rh(2)
Rh(1)-P(1)
Rh(2)-P(2)
Rh(1)-C(20)
Rh(2)-C(2)
2.5086(5) Rh(1)-O(1)
2.2123(6) Rh(1)-O(3)
2.2202(7) Rh(1)-O(5)
2.1363(15)
2.2197(15)
2.2735(14)
2.1891(15)
2.1249(15)
2.4146(15)
1.986(2)
1.989(2)
Rh(2)-O(2)
Rh(2)-O(4)
Rh(2)-O(7)
C(20)-Rh(1)-O(1)
O(1)-Rh(1)-O(3)
P(1)-Rh(1)-O(3)
C(20)-Rh(1)-P(1)
92.32(7)
85.04(6)
95.14(5)
87.59(6)
C(2)-Rh(2)-O(4)
O(4)-Rh(2)-O(2)
O(2)-Rh(2)-P(2)
C(2)-Rh(2)-P(2)
87.72(7)
87.58(6)
95.47(4)
89.28(6)
O(5)-Rh(1)-Rh(2) 167.91(4)
O(7)-Rh(2)-Rh(1) 165.51(4)
Crystal Structures. Both enantiomers of compounds 1,
3, 4, and 5 were crystallized, and their absolute structures
were determined by X-ray diffraction analysis. The enanti-
omers of 3, 4, and 5 form enantiomorphic crystals. Only one
enantiomer of each compound is reported herein. However,
crystals of R-1 and S-1 crystallize in different space groups.
The structures and crystal data of both enantiomers are
described in this paper. It is likely that both enantiomers form
both polymorphs, and the selection of a different polymorph
in each case was fortuitous.
Table 3. Selected Bond Distances (Å) and Angles (deg) for S-1
Rh(1)-Rh(2)
Rh(1)-P(1)
Rh(2)-P(2)
Rh(1)-C(20)
Rh(2)-C(2)
2.5019(8) Rh(1)-O(1)
2.197(4)
2.150(4)
2.288(5)
2.141(4)
2.185(4)
2.309(5)
2.204(2)
2.199(2)
2.002(6)
2.001(6)
Rh(1)-O(3)
Rh(1)-O(5)
Rh(2)-O(2)
Rh(2)-O(4)
Rh(2)-O(7)
C(20)-Rh(1)-P(1)
C(20)-Rh(1)-O(3)
O(3)-Rh(1)-O(1)
O(1)-Rh(1)-P(1)
87.8(2)
94.7(2)
83.2(2)
94.3(1)
C(2)-Rh(2)-P(2)
O(4)-Rh(2)-P(2)
O(2)-Rh(2)-O(4)
C(2)-Rh(2)-O(2)
90.6(2)
95.0(1)
85.5(2)
89.0(2)
O(5)-Rh(1)-Rh(2) 168.3(1)
O(7)-Rh(2)-Rh(1) 165.8(1)
Compound R-cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 (R-1)
was obtained in the orthorhombic system, space group
P212121, with one molecule in the asymmetric unit and no
interstitial solvent molecules. A thermal ellipsoid depiction
of the structure of this compound is shown in Figure 1, and
selected bond distances and angles are listed in Table 2. The
bond distances and angles are very close to the values in
the previously studied racemic compound RS-cis-Rh2(C6H4-
PPh2)2(OAc)2(HOAc)2.12 The Rh-Rh distance is 2.5086(5)
Å. The Rh-P and Rh-C bond distances are in the normal
range. The Rh-O distances trans to Rh-P bonds are slightly
shorter than those trans to Rh-C bonds, as expected from
the difference in the trans influence of Rh-P and Rh-C
bonds.
As expected, all the bridging ligands are twisted. The
torsion angles are 9.99(6)° for P(1)-Rh(1)-Rh(2)-C(2),
11.67(6)° for P(2)-Rh(2)-Rh(1)-C(20), 15.32(2)° for
O(1)-Rh(1)-Rh(2)-O(2), and 18.15(6)° for O(3)-Rh(1)-
Rh(2)-O(4). In this complex, the torsion angle of P(1)-
Rh(1)-Rh(2)-P(2) is 99.06(3)°. Therefore, the overall
chirality of this dirhodium compound is RP. The torsion angle
of P-Rh-Rh-P is usually larger than that of C(20)-
Rh(1)-Rh(2)-C(2) (-77.40(8)°), which is due to the steric
repulsions of the bulky phenyl groups in the phosphines.
The enantiomer S-1 was obtained in the monoclinic
system, space group P21. There are two independent
dirhodium molecules in each asymmetric unit. Again, there
is no solvent molecule in the crystal. The molecular structure
is shown in Figure S1. Selected bond distances and angles
are listed in Table 3. The bonding parameters of R-1 and
S-1 are almost the same, and the small differences may be
attributed to the different packing forces in the two kinds of
(22) For examples of compounds with CH2Cl2 coordination: (a) Newbound,
T. D.; Colsman, M. R.; Miller, M. M.; Wulfsberg, G. P.; Anderson,
O. P.; Strauss, S. H. J. Am. Chem. Soc. 1989, 111, 3762. (b) Fernandez,
J. M.; Gladysz, J. A. Organometallics 1989, 8, 207. (c) Colsman, M.
R.; Newbound, T. D.; Marshall, L. J.; Noirot, M. D.; Miller, M. M.;
Wulfberg, G. P.; Frye, J. S.; Anderson, O. P.; Strauss, S. H. J. Am.
Chem. Soc. 1990, 112, 2349 and references therein. (d) Arndtsen, B.
A.; Bergman, R. G. Science 1995, 270, 1970. (e) Butts, M. D.; Scott,
B. L.; Kubas, G. J. J. Am. Chem. Soc. 1996, 118, 11831. (f) Fornie´s,
J.; Mart´ınez, F.; Navarro, R.; Urriolabeitia, E. P. Organometallics 1996,
15, 1813. (g) Huang, D.; Huffman, J. C.; Bollinger, J. C.; Eisenstein,
O.; Caulton, K. G. J. Am. Chem. Soc. 1997, 119, 7398. (h) Huhmann-
Vincent, J.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc. 1998, 120,
6808.
(23) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Eds. Multiple Bonds
between Metal Atoms, 3rd ed.; Springer Science and Business Media,
Inc.: New York, 2005.
8228 Inorganic Chemistry, Vol. 44, No. 23, 2005

Chiral Organometallic Triangles with Rh-Rh Bonds
crystals. The average of P-Rh-Rh-P torsion angles is
-103.5°. The torsion angles of P-Rh-Rh-C are in the
range of -12.6(2) to 15.8(2)°. The chirality of this enanti-
omer is found to be SM.
There are only a few reports in which structure analyses
of both enantiomers have been carried out. Possibly, this has
not been worthwhile because usually the crystal structure of
the second enantiomer is that of the opposite enantiomorph.
Recently, the polymorphism of enantiomers has been of
interest due to the needs of the pharmaceutical industry.24
A
detailed analysis of the various types of polymorphism that
are theoretically possible with enantiomers has been dis-
cussed,25 but relatively few examples have been found.26 The
structures of R-1 and S-1 are another example of enantiomeric
polymorphism.
Compound SSS-Rh6(cis-C6H4PPh2)6(O2CC6H4CO2)3(py)6
(SSS-3) crystallizes in the chiral monoclinic space group P21
with two triangular species together with interstitial solvent
molecules in each asymmetric unit. The two independent
triangles are essentially identical in structure. One of them
is shown in Figure 2, and selected bond distances and angles
are listed in Table 4. This triangular molecule has idealized
D3 symmetry with a C3 axis through the center of the triangle
and three C2 axes each through the midpoint of a dirhodium
unit. All six P atoms are equivalent, which is consistent with
the 31P{1H} NMR spectrum. The triangular molecule has one
short (2.529(2) Å) and two long (2.562(2) and 2.556(2) Å)
Rh-Rh bonds. The Rh-P distances are in the narrow range
between 2.196(5) and 2.223(5) Å and Rh-N distances vary
from 2.24(1) to 2.28(1) Å. The Rh-O distances are in the
normal range. The terephthalate linkers are both bent and
twisted: the carboxylate groups make angles between 6(3)°
and 25(2)° with the C6H4-ring plane and in the range
12(3)°-37(3)° with each other.
Unlike the racemic triangular compounds reported previ-
ously,10 in which the chirality of each Rh2 unit is either RP
or SM, the overall chirality of the enantiopure compound
SSS-3 was found to be SPSMSM. In other words, in one of
the Rh2 units (the Rh(1)-Rh(2) unit in Figure 2), the P-Rh-
Rh-P angle (-75.3(2)°) is smaller than the C-Rh-Rh-C
angle (118.7(6)°). Thus, the signs of P(1)-Rh(1)-Rh(2)-
C(2)(18.9(4)°) and C(20)-Rh(1)-Rh(2)-P(2) (24.5(4)°)
angles are positive. This is very uncommon for simple
dirhodium complexes, and the only example reported so far
is that of Rh2(C6H4PPh2)2(O2CCPh3)2, which has two bulky
carboxylate ligands in cis positions.11c The difference be-
tween the P-Rh-Rh-P angle (-94.6(2)°) and the C-Rh-
Rh-C angle (87.3(8)°) in the Rh3-Rh4 unit is small as
compared to those in other units. The Rh(5)-Rh(6) unit is
similar to that in compound S-1, where the P(5)-Rh(5)-
Rh(6)-P(6) torsion angle (-103.2(2)°) is greater than the
Figure 2. The core structure (top) and packing pattern (bottom) of SSS-3.
Displacement ellipsoids for the core structure are drawn at the 40%
probability level. Some carbon atoms in phosphine groups and pyridine
ligands, solvent molecules, and hydrogen atoms are omitted for clarity. Note
that molecules stack on top of each other, but out of register, leaving a
chiral channel.
Table 4. Selected Bond Distances (Å) and Angles (deg) for SSS-3
Rh(1)-Rh(2)
Rh(3)-Rh(4)
Rh(5)-Rh(6)
Rh(1)-P(1)
Rh(2)-P(2)
Rh(1)-C(20)
Rh(2)-C(2)
2.529(2)
2.562(2)
2.556(2)
2.196(5)
2.198(5)
2.065(9)
2.047(9)
Rh(1)-N(1)
Rh(2)-N(2)
Rh(1)-O(1)
Rh(1)-O(5)
Rh(2)-O(2)
Rh(2)-O(6)
2.26(1)
2.24(1)
2.11(1)
2.14(1)
2.20(1)
2.09(1)
C(20)-Rh(1)-O(1)
O(1)-Rh(1)-O(5)
O(5)-Rh(1)-P(1)
C(20)-Rh(1)-P(1)
N(1)-Rh(1)-Rh(2)
83.1(5)
82.9(5)
94.0(4)
98.9(5)
163.8(3)
C(2)-Rh(2)-O(6)
O(6)-Rh(2)-O(2)
O(2)-Rh(2)-P(2)
C(2)-Rh(2)-P(2)
N(2)-Rh(2)-Rh(1)
89.6(5)
84.3(5)
91.9(4)
94.2(4)
165.1(3)
C(92)-Rh(5)-Rh(6)-C(74) (76.0(5)°) angle. The torsion
angles of P-Rh-Rh-C and O-Rh-Rh-O are close to
those in R-1 except for the P-Rh-Rh-C angles in the Rh3-
Rh4 units, which are 1.9(5)° for P(4)-Rh(4)-Rh(3)-C(56)
and -5.4(7)° for P(3)-Rh(3)-Rh(4)-C(38).
The compound SSS-Rh6(C6H4PPh2)6(O2CCO2)3(py)5(CH2-
Cl2) (SSS-4) has five axial pyridine ligands, and the sixth
axial position is occupied by a CH2Cl2 molecule. Complex
SSS-4 crystallizes in the space group P21 with only one
independent triangle together with 2.5 CH2Cl2 molecules in
each asymmetric unit. Figure 3 shows the ellipsoid plot of
this triangle, and selected bond distances and angles are listed
in Table 5. The Rh-Rh distances are 2.5252(5), 2.5567(5),
and 2.5626(5) Å. The Rh-Rh bond connecting with a CH2-
Cl2 has the shortest distance, which is consistent with that
CH2Cl2 molecule being a weaker electron donor than
(24) Bernstein, J. Polymorphism in Molecular Crystals; Clarendon Press:
Oxford, 2002.
(25) Jacques, J.; Collet, A.; Wilen, S. H. In: Enantiomers, Racemates and
Resolutions; John Wiley and Sons: New York, 1981.
(26) (a) Berger, A.; Djukic, J.-P.; Pfeffer, M.; de Cian, A.; Kyrisakas-
Gruber, N.; Lacour, J.; Vial, L. Chem. Commun. 2003, 658. (b) Berger,
A.; Djukic, J.-P.; Pfeffer, M.; Lacour, J.; Vial, L.; de Cian, A.;
Kyritsakas-Gruber, N. Organometallics 2003, 22, 5243.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8229

Cotton et al.
Table 6. Selected Bond Distances (Å) and Angles (deg) for RRR-5
Rh(1)-Rh(2)
Rh(3)-Rh(3A)
Rh(1)-P(1)
Rh(2)-P(2)
Rh(3)-P(3)
Rh(1)-C(20)
Rh(2)-C(2)
Rh(3)-C(38A)
Rh(1)-O(1)
Rh(1)-O(5)
2.523(1) Rh(2)-O(2)
2.529(2) Rh(2)-O(6)
2.198(3) Rh(3)-O(3)
2.193(3) Rh(3)-O(4)
2.208(3) Rh(1)-N(1A)
2.021(9) Rh(1)-N(1B)
1.99(1) Rh(2)-Cl(1B)
2.02(1) Rh(2)-Cl(1A)
2.140(7) Rh(3)-N(2)
2.193(6)
2.158(7)
2.114(7)
2.155(7)
2.201(7)
2.17(3)
2.27(6)
2.61(6)
2.79(2)
2.25(1)
C(20)-Rh(1)-O(1)
O(1)-Rh(1)-O(5)
O(5)-Rh(1)-P(1)
C(20)-Rh(1)-P(1)
91.1(3)
84.5(3)
95.(2)
O(2)-Rh(2)-P(2)
95.7(2)
Rh(1)-Rh(2)-Cl(1B) 167(1)
Rh(1)-Rh(2)-Cl(1A) 164.5(5)
C(38A)1-Rh(3)-O(3) 87.2(3)
88.8(2)
N(1A)-Rh(1)-Rh(2) 161.7(8)
N(1B)-Rh(1)-Rh(2) 164(2)
O(3)-Rh(3)-O(4)
C(38A)-Rh(3)-P(3)
O(4)-Rh(3)-P(3)
84.2(3)
96.4(3)
92.1(2)
C(2)-Rh(2)-O(6)
O(6)-Rh(2)-O(2)
C(2)-Rh(2)-P(2)
91.7(3)
84.8(3)
87.8(3)
N(2)-Rh(3)-Rh(3A) 163.5(3)
In each Rh2 unit, the torsion angle of P-Rh-Rh-P is bigger
than that of C-Rh-Rh-C. The torsion angles of P-Rh-
Rh-P and C-Rh-Rh-C are -105.49(5)°, -100.89(5)°,
and -108.73(5)° and 78.7(2)°, 78.0(2)°, and 77.7(2)°,
respectively. The torsion angles of P-Rh-Rh-C fall in the
range of -10.5(2)° to -17.3(2)°. The O-Rh-Rh-O angles
are between 17.6(1)° and 20.1(2)°.
Figure 3. The core structure (top) and packing pattern (bottom) of SSS-4.
Displacement ellipsoids for the core structure are drawn at the 40%
probability level. Some carbon atoms in phosphine groups and pyridine
ligands, solvent molecules, and hydrogen atoms are omitted for clarity. Note
that all molecules stack, leaving a chiral channel.
The compound RRR-Rh6(cis-C6H4PPh2)6(O2CC6H4C6H4-
CO2)3(py)4(CH2Cl2)2 (RRR-5) crystallizes in the orthorhombic
chiral space group C2221. The triangle resides on a position
of C2 symmetry. Only half of a triangular molecule is
independent, and a C2 axis generates the entire triangle. The
six axial positions of this molecular triangle are occupied
by four pyridine ligands and two CH2Cl2 ligands. The core
structure of this compound is shown in Figure S2, and
selected bond distances and angles are listed in Table 6.
Table 5. Selected Bond Distances (Å) and Angles (deg) for SSS-4
Rh(1)-Rh(2)
Rh(3)-Rh(4)
Rh(5)-Rh(6)
Rh(1)-P(1)
Rh(2)-P(2)
Rh(1)-C(20)
Rh(2)-C(2)
2.5252(5) Rh(1)-O(1)
2.5567(5) Rh(1)-O(5)
2.5626(5) Rh(2)-O(2)
2.117(4)
2.183(3)
2.182(4)
2.122(3)
2.229(4)
2.636(2)
2.218(2)
2.209(1)
2.029(5)
1.996(5)
Rh(2)-O(6)
Rh(1)-N(1)
Rh(2)-Cl(2)
In RRR-5, one of the Rh2 units has two pyridine axial
ligands and the others have one pyridine and one CH2Cl2.
In the Rh2 unit with two axial pyridine ligands, the Rh-Rh
distance of 2.529(2) Å is close to the shortest one in
compound SSS-3. The Rh-Rh distances in the Rh2 units with
one pyridine ligand and one CH2Cl2 ligand in the axial
positions, 2.523(1) Å, are comparable to the bond distance
in the analogous Rh2 unit in SSS-4. In the crystal structure,
the axial CH2Cl2 is disordered in two positions and the
average of the Rh-Cl distances is 2.70 Å, which is again
much shorter than the sum of van der Waals radii for Rh
and Cl. Like the bridging linkers in SSS-3 and SSS-4, the
4,4′-diphenyl-dicarboxylate dianions in SSS-4 are bent and
twisted. The carboxylate groups have dihedral angles be-
tween 7.3(5)° and 22(1)° with the C6H4-ring plane and angles
(13.9(5)° and 40(1)°) with each other. The dihedral angles
of the C6H4-ring with each other are 33.3(6)° and 36.1(6)°.
C(20)-Rh(1)-O(1)
O(1)-Rh(1)-O(5)
O(5)-Rh(1)-P(1)
C(20)-Rh(1)-P(1)
88.7(2)
80.9 (1)
96.0(1)
94.2(2)
C(2)-Rh(2)-O(6)
O(6)-Rh(2)-O(2)
C(2)-Rh(2)-P(2)
O(2)-Rh(2)-P(2)
93.5(2)
82.1(1)
90.2(1)
94.3(1)
N(1)-Rh(1)-Rh(2) 165.3(1)
Rh(1)-Rh(2)-Cl(2) 164.78(4)
pyridine. The axial Rh-Cl (CH2Cl2) bond distance is 2.636-
(2) Å, which is significantly shorter than the sum of van der
Waals radii for Rh and Cl atoms (about 3.10 Å). This
distances is in the range of M-Cl distances 2.4-3.0 Å
reported for metal dichloromethane complexes.22 It is much
shorter than the Cr‚‚‚Cl (3.335(4) Å) and Mo‚‚‚Cl
(3.390(3)Å) separations in the dichloromethane adducts of
tetraamidodichromium(II) and tetraamidodimolybdenum(II),
in which CH2Cl2 is not engaged in significant bonding.27
The oxalate linkers in SSS-4 are twisted about the central
C-C bonds, with dihedral angles of 41.3-47.7° between
the carboxylate groups of the oxalate bridge. All three Rh2
corners in this triangle have the same twist pattern. The
overall chirality of this triangle can be described as RPRPRP.
As in SSS-3, there is one Rh2 unit (Rh(3)-Rh(3A) unit in
Figure S2) in which the P-Rh-Rh-P angle (75.8(1)°) is
smaller than the C-Rh-Rh-C angle (-116.9(6)°). In the
other two Rh2 units, the P-Rh-Rh-P angle (101.1(1)°) is
larger than the C-Rh-Rh-C angle (-74.9(4)°). The torsion
angles are 13.5(3)° for P(1)-Rh(1)-Rh(2)-C(2), 12.7(2)°
for P(2)-Rh(2)-Rh(1)-C(20), and -20.5(3)° for P(3)-
(27) (a) Cotton, F. A.; Ilsley, W. H.; Kaim, W. J. Am. Chem. Soc. 1980,
102, 3475. (b) Baral, S.; Cotton, F. A.; Ilsley, W. H. Inorg. Chem.
1981, 20, 2696.
8230 Inorganic Chemistry, Vol. 44, No. 23, 2005

Chiral Organometallic Triangles with Rh-Rh Bonds
Scheme 2
Table 7. Asymmetric Catalytic Cyclopropanation of Styrene with Ethyl
Diazoacetate in Refluxing CH2Cl2 or n-C5H12
% eed
catalyst
solvent
yield 6, %b
cis/transc
cis-6e
trans-6e
3
3
4
4
CH2Cl2
n-C5H12
CH2Cl2
n-C5H12
CH2Cl2
CH2Cl2
50
74
55
64
45
55
40:60
40:60
55:45
58:42
35:65
48:52
70
32
14
32
62
91
80
60
25
28
58
87
Rh(3)-Rh(3A)-C(38). The O-Rh-Rh-O angles are in the
range of 15.2(3)-18.0(3)°.
5
7a
The molecular structures of 3-5 can each be described
as an equilateral triangle composed of three Rh2 units linked
by dicarboxylate dianions. The edges of the triangles are
about 11.1, 6.8, and 15.4 Å for 3, 4, and 5, respectively.
The space inside each triangle is large enough to accom-
modate small molecules, and there are some disordered
solvent molecules inside the triangles in SSS-3 and RRR-5.
The packing patterns of these compounds are also interesting.
The chiral molecules stack together to form asymmetric
environments. In SSS-3, the triangular molecules stack on
top of each other but out of register, i.e., with alternating
orientations differing by about 60° (Figure 2). Thus, a
projection in the stacking direction has a hexagonal cross
section, and the channel is filled with disordered solvent
molecules. In SSS-4, the triangular molecules stack in register
resulting in the formation of a triangular channel (Figure 3).
In RRR-5, the triangular molecules stack with alternating
orientations differing by about 60°, such as in SSS-3, and
the centers of the molecules have alternating displacement
from the mean stacking axis. Thus, a distorted hexagonal
channel is created by the stacking of RRR-5 (Figure S2).
Asymmetric Cyclopropanation of Styrene with Ethyl
Diazoacetate Catalyzed by Rh2-Chiral Organometallic
Triangles. The search for new asymmetric dirhodium(II)
catalysts capable of inducing high enantiocontrol in trans-
formations of diazo compounds is growing because of (i)
the impressive accomplishment achieved with such catalysts
in many asymmetric metal carbene transformations involved
in the synthesis of complex molecules such as natural
products and (ii) the limited enantiocontrol achieved so far
in the transformation of R-diazo ketones and R-diazo ester.28
In this context, it has been reported some time ago that chiral
dirhodium(II) complexes with a variety of orthometalated
bridging phosphines are superior catalysts for the enanti-
oselective intermolecular cyclopropanation of styrenes with
ethyl diazoacetate (Scheme 2).11a,c Such enantiopure catalysts
are based on discrete Rh-Rh units with two orthometalated
phosphines in cis positions and two bridging carboxylate
ligands. Since the (Rh-Rh)3-enantiomerically pure triangular
supramolecules were constructed from an asymmetric cata-
lyst, namely, the Rh-Rh units with orthometalated triphenyl
phosphine ligands, it was pertinent to evaluate the catalytic
activity/selectivity performance of the chiral organometallic
triangles for a typical metal carbene transformation, such as
the model intermolecular cyclopropanation reaction of
styrene with ethyl diazoacetate.29
a Reference 11c. b Cyclopropanes yields based on ethyl diazoacetate.
c Ratio determined by GC analysis and 1H NMR spectroscopy. d Values
determined by GC on a 2,3-di-O-acetyl-6-O-(ter-butyldimethylsilyl)-â-CDX
column. e The configuration of cis-6 (1S,2R) and trans-6 (1S, 2S) cyclo-
propanes obtained with catalysts 3-5 were determined by correlation of
the sign of the rotation of polarized light with those cyclopropane
enantiomers previously published (ref 30b).
The catalytic activity of three triangular organometallic
compounds, namely SSS-[cis-Rh2(C6H4PPh2)2(C8H4O4)(py)2]3
(3), SSS-{cis-Rh6(C6H4PPh2)6(C2O4)(py)5(CH2Cl2)} (4), and
SSS-{cis-Rh6(C6H4PPh2)6(C14H8O4)(py)4(CH2Cl2)2} (5), was
evaluated in the intermolecular cyclopropanation reaction of
styrene with ethyl diazoacetate in both dichloromethane and
pentane as solvents. The diastereoselectivity and enantio-
selectivity results are shown in Table 7. The catalytic results
of the previously reported discrete enantiopure S-{cis-
Rh2(C6H4PPh2)2(O2CCF3)(H2O)2} (7) in dichloromethane are
also reported for comparison. While S-{cis-Rh2(C6H4PPh2)2(O2-
CCF3)(H2O)2} induced low diastereoslectivity (cis/trans,
48:52) and higher enantioselectivity, the triangular organo-
metallic compounds, particularly 3 and 5 gave higher trans
selectivity (up to 35:65 cis/trans with compound 3) and good
enantioselectivity (ca. 70 and 80% for cis-6 and trans-6
cyclopropanes, respectively) with enantiopure compound 3.
Use of the triangular organometallic compound 4 resulted
in an increase in cis selectivity but lower enantioselectivity
in dichloromethane.
The enantiocontrol achieved, particularly with catalysts 3
and 5, is superior to that of the known dirhodium(II) catalysts
with chiral carboxamide ligands such as Rh2(5S-MEPY)4 (ca.
33 and 58% for cis-6 and trans-6, respectively),30 as well as
with those dirhodium(II) compounds with chiral carboxylates
ligands,31 where enantioselectivities did not exceed 12% ee
for the same intermolecular cyclopropanation reaction be-
tween styrene and ethyl diazoacetate. However, the enan-
tiocontrol performances of catalysts 3-5 were found to be
less than those achieved using discrete dirodium(II) with
orthometalated aryl phosphine ligands, such as triphenyl
phosphine ligands. The higher enantiocontrol achieved with
the discrete catalysts having orthometalated aryl ligands was
rationalized using a model transition state, which shows that
the cyclopropanation between styrene and ethyl diazoacetate
preferentially occurs in the area of the carboxylate ligands,
(30) (a) Doyle, M. P.; Winchester, W. R.; Protopopova, M. N.; Mu¨ller, P.;
Bernardinelli, G.; Ene, D.; Motallebi, S. HelV. Chim. Acta 1995, 76,
2227. (b) Mu¨ller, P.; Baud, C.; Ene, D.; Motallebi, S.; Doyle, M. P.;
Brandes, B. D.; Dyatkin, A. B.; See, M. M. HelV. Chim. Acta 1995,
78, 459.
(31) Brunner, H.; Kluscanzoff, H.; Wutz, K. Bull. Soc. Chim. Belg. 1989,
98, 63.
(28) Doyle, M. P.; Forbes, D. C. Chem. ReV. 1998, 98, 911.
(29) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods
for Organic Synthesis with Diazo Compounds; John Wiley and Sons:
New York, 1998.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8231

Cotton et al.
thus entailing smaller steric effects for the orientation and
stabilization of the Rh-CHCOOEt species.11c
at 0.596 V. The CV and DPV of 5 in 10 mL of CH2Cl2
containing 0.15 mL of pyridine are similar to those of 4,
with CV at Ered ) 0.457 V and Eox ) 0.695 V, and DPV at
0.528 V. As expected, compound 4 has a higher degree of
electronic communication between the Rh2 units. Electro-
chemical measurements of 4 in CH2Cl2 show three reversible
oxidation processes with E1/2’s at 0.615, 0.737 and 0.952 V
(from DPV). The E1/2 values were shifted negatively to 0.459,
0.599 and 0.653 V as 0.02 mL of pyridine was added. The
triangular configuration of these compounds remains stable
in different axial coordinations, as shown by the single-crystal
Solvent effects on activity and selectivity of dirhodium-
(II) catalysts in reactions of diazo compounds has been
demonstrated.32 The polarity of the solvent used in the
reaction of diazo compound transformation can increase or
decrease the enantiocontrol of the process depending on the
dirhodium(II) catalysts and the type of the carbene transfer
reaction. It seems that the solvent can affect to some extent
the alignment of the ligands on chiral dirhodium(II) com-
pounds, thereby affecting the enantiocontrol over the diazo
compound transformation.33
structures of the compounds SSS-Rh6(cis-C6H4PPh2)6-
20
(O2CCO2)3(py)4.75(CH3OH)1.25
and SSS-Rh6(cis-C6H4-
To evaluate such solvent effects, we exposed compounds
3 and 5 to ethyl diazoacetate and styrene in pentane (n-C5H12)
and followed the outcome of the cyclopropanation reaction
in terms of activity and selectivity. The results are shown in
Table 7 and are compared with those obtained in CH2Cl2.
The first observation is that the yield was remarkably
increased to 64 and 74% when using catalysts 3 and 4,
respectively. A slight increase in the cis selectivity was
observed for compound 4, as well as an increase in the
enantioselectivity (ca. 32 and 28% for cis-6 and trans-6,
respectively). However, a decrease in enantioselectivity was
observed with compound 3. It is important to mention that
the triangular organometallic supramolecules were practically
insoluble in pentane, which indicates that they were acting
as heterogeneous catalysts in the intermolecular cyclopro-
panation. Their separation from the reaction products was
easily conducted by filtration. Despite the insolubility of these
supramolecular catalysts in a nonpolar solvent such as
pentane, they have shown remarkable catalytic activity with
a notable enantiocontrol when compared with known het-
erogeneous catalysts used for metal carbene transformation.
The present results on the catalytic activity/selectivity of
chiral organometallic triangles containing chiral building
units with Rh-Rh bonds are promising. They are very active
and remarkably selective catalysts when compared to the rest
of dirhodium(II) catalysts. Their size permits the performance
of the catalytic reactions either homogeneously or hetero-
geneously depending on the reaction solvent. Further studies
aiming to extend the utilization of these catalysts to intra-
molecular diazo compound transformations such as cyclo-
propanation of R-diazo ketones and R-diazo ester and C-H
insertions are of unusual interest because of the lack of higher
enantiocontrol in transformations of these types.
PPh2)6(O2CCO2)3(CH3OH)5(H2O).13 The oxidation of the Rh2
units in 4 remains reversible in the presence of various
coordinating ligands. The combination of electroactivity and
the complexation features of these triangular molecules in
the axial positions suggest that they have potential as sensory
materials. The axial positions of these molecules may serve
as receptors for the complexation of electron-donor sub-
strates. The binding of the substrates in the triangular
molecules will change their E1/2 values and enable electro-
chemical sensing.
Conclusion
The preparation and structural characterization of com-
pounds 3-5 show that enantiopure organometallic molecular
triangles can be prepared readily from the self-assembly
reaction of rigid dicarboxylate linkers and an enantiopure
orthometalated dirhodium compound cis-Rh2(C6H4PPh2)2(CH3-
CN)6(BF4)2. Compounds 4 and 5 are the only isolated
dirhodium compounds with axial dichloromethane ligands
ever found. The interesting electrochemical properties of 4
suggest potential application in neutral molecular sensing.
The structural characterization of both enantiomers of
compound 1 fortuitously provides an example of polymor-
phism.
This work is another step in our effort to synthesize
enantiomerically selective catalysts and molecular sensors
based on supramolecular compounds. The catalytic evalua-
tion of the enantiopure triangles showed that they are
catalytically active and provided good results for the
intermolecular cyclopropanation between styrene and ethyl
diazoacetate. The use of these triangles as catalysts in the
intramolecular cyclopropanation of diazo compounds and in
other uses is of interest and will be investigated further.
Electrochemistry. Compounds 3, 4, and 5 have been
studied by both CV and DPV. The CV of 3 in a solution of
10 mL of CH2Cl2 containing 0.02 mL of pyridine shows a
redox process with a maximum wave at Ered ) 0.441 V and
Eox ) 0.716 V. Its DPV has a broad signal with a maximum
Acknowledgment. F.A.C. and C.A.M. thank the National
Science Foundation, the Robert A. Welch Foundation, and
Texas A&M University for financial support. S.-E.S. thanks
the Ministerio de Educacio´n y Ciencia (Spain) for support
through the national program “Ramo´n y Cajal”. We also
thank Johnson-Matthey for a generous loan of rhodium
trichloride.
(32) (a) Doyle, M. P.; Zhou, Q. L.; Charnsangave, C.; Longoria, M. A.;
McKervey, M. A.; Garc´ıa, C. F. Tetrahedron Lett. 1996, 37, 4129.
(b) Kitagaki, S.; Matsuda, H.; Watanabe, N.; Hashimoto, S. Synlett
1997, 1171. (c) Davies, H. M. L.; Kong, N. Tetrahedron Lett. 1997,
38, 4203. (d) Wynne, D. C.; Olmstead, M. M.; Jessop, P. G. J. Am.
Chem. Soc. 2000, 122, 7638.
(33) It was found that chiral dirhodium(II) carboxamidates prevent similar
solvent effects because of their rigidity, see ref 6a and Doyle, M. P.;
Ren, T. Prog. Inorg. Chem. 2001, 49, 113.
Supporting Information Available: X-ray crystallographic files
in CIF format for R-1, S-1, SSS-3, SSS-4, RRR-4, RRR-5, SSS-Rh6-
8232 Inorganic Chemistry, Vol. 44, No. 23, 2005

Chiral Organometallic Triangles with Rh-Rh Bonds
(cis-C6H4PPh2)6(C2O4)(CH3OH)5(H2O), SSS-Rh6(cis-C6H4PPh2)6(O2-
CC6H4CO2)3(py)3(CH3OH)3, SSS-Rh6(cis-C6H4PPh2)6(O2CCO2)3-
(py)4.75(CH3OH)1.25, and RRR(SSS)-Rh6(cis-C6H4PPh2)6(O2CC6H4-
CO2)3(py)1.5(CH3OH)4.5; displacement ellipsoid plots of S-1 and
RRR-5 (Figures S1 and S2, respectively) in pdf format. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC051282C
Inorganic Chemistry, Vol. 44, No. 23, 2005 8233