New dinuclear catalysts Rh2(N-O)2[(C6H4)P (C6H5)2]2 with imidate ligands: Synthesis and isomerization from head-to-tail to head-to-head configuration of the imidate ligands

Article abstract of DOI:10.1021/ic010257d

Two new dirhodium(II) catalysts of general formula Rh₂(N-O)₂[ (C₆H₄)P(C₆H₅) ₂]₂ (N-O = C₄H₄NO₂) are prepared, starting from Rh₂(O₂CCH₃)₂ (PC)₂L₂ [PC = (C₆H₄)P(C₆H₅)₂ (head-to-tail arrangement); L = HO₂CCH₃]. The thermal reaction of Rh₂(O₂CCH₃)₂ (PC)₂·L₂ with the neutral succinimide stereoselectively gives one compound that according to the X-ray structure determination has the formula Rh₂(C₄H₄NO₂)₂ [(C₆H₄)P (C₆H₅)₂]₂ (1). It corresponds to the polar isomer with two bridging imidate ligands in a head-to-head configuration. However, stepwise reaction of Rh₂(O₂CCH₃)₂ (PC)₂·L₂ with (CH₃)₃SiCl and potassium succinimidate yields a mixture of 1 and one of the two possible isomers (structure B) with a head-to-tail configuration of the imidate ligands, Rh₂(C₄H₄NO₂)₂ [(C₆H₄)P (C₆H₅)₂]₂ (2), also characterized by X-ray methods. In solution, compound 2 undergoes slow isomerization to 1; the rate of this process is enhanced by the presence of acetonitrile. Compounds 1 and 2 are obtained as pure enantiomers starting from (M)- and (P)- Rh₂(O₂CCH₃)₂ (PC)₂·L₂ rather than from the racemic mixture. Their enantioselectivities in cyclopropanation of 1-diazo-5-penten-2-one are similar to those reported for the dirhodium amidate catalysts.

Full text of DOI:10.1021/ic010257d

4226
Inorg. Chem. 2001, 40, 4226-4229
New Dinuclear Catalysts Rh2(N-O)2[(C6H4)P(C6H5)2]2 with Imidate Ligands: Synthesis and
Isomerization from Head-to-Tail to Head-to-Head Configuration of the Imidate Ligands
Mario Barberis,‡ Francisco Estevan,† Pascual Lahuerta,*,† Julia Pe´rez-Prieto,*,‡ and
Mercedes Sanau´†
Departamento de Qu´ımica Inorga´nica, Facultat de Qu´ımica, Universitat de Valencia, Dr. Moliner, 50,
46100 Burjassot-Valencia, Spain, and Departamento de Qu´ımica Orga´nica/Instituto de Ciencia
Molecular, Facultat de Farmacia, Universitat de Valencia, Av. Vicent A. Estelles, s/n,
46100 Burjassot-Valencia, Spain
ReceiVed March 7, 2001
Two new dirhodium(II) catalysts of general formula Rh2(N-O)2[(C6H4)P(C6H5)2]2 (N-O ) C4H4NO2) are prepared,
starting from Rh2(O2CCH3)2(PC)2L2 [PC ) (C6H4)P(C6H5)2 (head-to-tail arrangement); L ) HO2CCH3]. The
thermal reaction of Rh2(O2CCH3)2(PC)2‚L2 with the neutral succinimide stereoselectively gives one compound
that according to the X-ray structure determination has the formula Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (1). It
corresponds to the polar isomer with two bridging imidate ligands in a head-to-head configuration. However,
stepwise reaction of Rh2(O2CCH3)2(PC)2‚L2 with (CH3)3SiCl and potassium succinimidate yields a mixture of 1
and one of the two possible isomers (structure B) with a head-to-tail configuration of the imidate ligands, Rh2(C4H4-
NO2)2[(C6H4)P(C6H5)2]2 (2), also characterized by X-ray methods. In solution, compound 2 undergoes slow
isomerization to 1; the rate of this process is enhanced by the presence of acetonitrile. Compounds 1 and 2 are
obtained as pure enantiomers starting from (M)- and (P)-Rh2(O2CCH3)2(PC)2‚L2 rather than from the racemic
mixture. Their enantioselectivities in cyclopropanation of 1-diazo-5-penten-2-one are similar to those reported
for the dirhodium amidate catalysts.
Introduction
possible isomers, the compound that contains four bridging
amidate ligands with a cis-RhN2O2 coordination around each
rhodium atom (2,2-cis arrangement) is usually obtained as the
major or only reaction product. In some cases, (3,1) and (4,0)
compounds have been characterized as minor reaction products.4-6
We report in this work the stereoselective synthesis of two
rhodium compounds of formula Rh2(N-O)2[(C6H4)P(C6H5)2]2
by exchanging acetate with succinimidate in Rh2(O2CCH3)2-
[(C6H4)P(C6H5)2]2. The presence of two metalated phosphines
in the head-to-tail (H-T) arrangement could have a direct
influence in the substitution processes, as has been suggested
in other related processes.6 By careful procedures, the thermo-
dynamically stable compound 1, with head-to-head (H-H)
configuration, and the compound 2, with H-T configuration,
have been synthesized. Surprisingly, the complete isomerization
of 2 to 1 is observed, even under mild conditions. Catalytic
studies performed on both compounds show that the succin-
imidate ligands do not have the expected influence on the
selectivity.
We have recently reported that chiral cyclometalated com-
pounds of formula Rh2(O2CCF3)2(PC)2 [PC ) (C6H4)P(C6H5)2,
head-to-tail configuration] can induce enantioselectivity in the
cyclization of R-diazocompounds.1,2 As these compounds have
backbone chirality, the observation of catalytic enantioselectivity
is direct evidence of the involvement of the entire dinuclear
species in the carbene transfer process.
Later on, we focused our interest on the synthesis and study
of Rh2(L-L)2(PC)2 compounds with an L-L ligand placing a
donor group in the proximity of the axial Rh-C bond in the
carbenoid, with the aim of increasing the selectivity of the
carbene transfer process. A good candidate for this purpose
could be the succinimidate ligand that, acting as a bridging N-
and O-donor ligand, leaves an uncoordinated keto group in the
vicinity of the axial catalytic site. The syntheses of compounds
of the general formula Rh2(N-O)4 (N,O ) carboxamidate) have
been reported;3-5 these compounds are formed by successive
acetate ligand replacement from Rh2(O2CCH3)4. From the four
Experimental Section
† Departamento de Qu´ımica Inorga´nica.
General Comments. C4H5NO2 (Aldrich) was used as purchased.
Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2(HO2CCH3)2 was prepared by literature
procedure.7 All solvents were of analytical grade and were degassed
before use. NMR spectra were recorded on Varian Unity-300 and Varian
Unity-400 spectrometers. Chemical shifts (δ) are given in ppm, relative
to TMS (1H,13C) and 85% H3PO4 aqueous solution (31P). Coupling
constants (J) are given in hertz.
‡ Departamento de Qu´ımica Orga´nica/Instituto de Ciencia Molecular
Facultat de Farmacia.
(1) Taber, D. F.; Malcolm, S. C.; Bieger, K.; Lahuerta, P.; Sanau, M.;
Stiriba, S. E.; Pe´rez-Prieto, J.; Monge, M. A. J. Am. Chem. Soc. 1999,
121, 860.
(2) Estevan, F.; Herbst, K.; Lahuerta, P.; Barberis, M.; Pe´rez-Prieto, J.
Organometallics 2001, 20, 950.
(3) Doyle, M. P.; Winchester, W. R.; Hoorn, J. A. A.; Lynch, V.;
Simonsen, S. H.; Ghosh, R. J. Am. Chem. Soc. 1993, 115, 9968.
(4) (a) Doyle, M. P.; Zhou, Q. L.; Raab, C. E.; Roos, G. H. P.; Simonsen,
S. H.; Lynch, V. Inorg. Chem. 1996, 35, 6064. (b) Doyle, M. P.; Raab,
C. E.; Roos, G. H. P.; Lynch, V.; Simonsen, S. H. Inorg. Chim. Acta
1997, 266, 13.
Synthesis of Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (H-H) (1). To a
solution of Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2‚(HO2CCH3)2 (0.20 g, 0.21
(6) Lifsey, R. S.; Lin, X. L.; Chavan, M. Y.; Ashan, M. Q.; Kadish, K.
M.; Bear, J. L. Inorg. Chem. 1987, 26, 830.
(5) Dennis, A. M.; Korp, J. D.; Bernal, I.; Howard, R. A.; Bear, J. L.
Inorg. Chem. 1983, 22, 1522.
(7) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A.; Tocher, J. H.
Organometallics 1985, 4, 8.
10.1021/ic010257d CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/11/2001

New Dinuclear Catalysts Rh2(N-O)2[(C6H4)P(C6H5)2]2
Inorganic Chemistry, Vol. 40, No. 17, 2001 4227
mmol) in toluene (50 mL) was added succinimide (0.16 g, 1.65 mmol),
and the mixture was refluxed for 48 h. The solvent was evaporated,
and the crude product was purified by column chromatography using
dichloromethane as eluent solvent.
Chart 1
Yield: 85%.1H NMR (CDCl3, 400 MHz): δ 1.4-2.4 (m, 8H), 6.49
(m, 1H), 6.60-7.25 (m, 17H), 7.36 (m, 6H), 7.56 (m, 4H). 13C[1H]
NMR (CDCl3, 400 MHz): δ 29.26, 29.78, 30.43, 32.65, 120-145 (m,
aromatics), 166.00 (m), 193.46, 196.26, 198.53, 198.69. 31P[1H] NMR
2
(CDCl3, 300 MHz): δ 14.7 (1JRh-P ) 146 Hz, JRh-P ) 8 Hz), 25.0
(1JRh-P ) 178 Hz, 2JRh-P ) 7 Hz; values of J obtained from a simulated
spectrum).
Synthesis of (P)-Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (H-H) [(P)-
1]. This compound was prepared by the above-described procedure,
using the (P)-Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2‚(HO2CCH3)2 enantiomer
as the starting product. [R]20 +167° (c 0.012, CHCl3).
D
Synthesis of Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (H-T) (2). To a
solution of Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2‚(HO2CCH3)2 (0.05 g, 0.05
mmol) in freshly distilled THF (25 mL) was slowly added (CH3)3SiCl
(27 µL, 0.20 mmol) in 5 mL of THF, and the mixture was stirred for
5 min. The resulting solution was transferred to a second flask
containing succinimide (0.02 g, 0.20 mmol) and KOH (0.025 g, 0.44
mmol) in methanol (15 mL). The mixture was stirred at room
temperature for 5 min. The 31P NMR spectrum of the crude product
showed the formation of two P-containing species 1 and 2, in equal
amounts. These two products were isolated by column chromatography
(SiO2-hexane), using ethyl acetate-hexane (3/1 mixture) as the eluent
solvent. The H-T compound, 2, is the first one eluted from the column.
Crystal Structure Determination of Complex 3‚(CH2Cl2). Crystal
data: C49H43Cl2O7N3P2Rh2, MW ) 1124.52, monoclinic, a ) 12.8114-
(8), b ) 19.5510(12), c ) 18.6265(12) Å, â ) 100.774(2)°, V ) 4583.2-
(5) Å3, T ) 296 K, space group P21/n, Z ) 4, µ(Mo KR) ) 0.963
mm-1. Final conventional R ) 0.658 for 13 797 “observed” reflections
and 586 variables.
Crystal Structure Determination of Complex 4‚(CH2Cl2)2. Crystal
data: C46H40Cl4O6N2P2Rh2, MW ) 1125.37, monoclinic, a ) 10.0387-
(8), b ) 24.741(2), c ) 19.2080(17) Å, â ) 92.930(2)°, V ) 4764.3-
(7) Å3, T ) 296 K, space group P21/n, Z ) 4, µ(Mo KR) ) 1.033
mm-1. Final conventional R ) 0.0713 for 10 314 “observed” reflections
and 559 variables.
1
Compound 2. Yield: 30%. H NMR (CDCl3, 400 MHz): δ 1.20
(m, 2H), 1.45 (m, 2H), 1.78 (m, 2H), 2.08 (m, 2H), 6.52 (m, 4H), 6.68
(m, 2H), 6.92 (m, 10H), 7.18 (m, 2H), 7.28 (m, 6H), 7.52 (m, 4H).
Results and Discussion
31P[1H] NMR (CDCl3, 300 MHz): δ 23.9 (1JRh-P ) 171 Hz, 2JRh-P
)
Synthesis of 1 and 2. The thermal reaction of Rh2(O2CCH3)2-
[(C6H4)P(C6H5)2]2 (HO2CCH3)2 (racemic mixture) and 8 molar
excess equiv of succinimide in refluxing toluene for several
hours led to complex 1, of formula Rh2(C4H4O2N)2[(C6H4)P-
(C6H5)2]2, as the only reaction product. This compound crystal-
lized as an adduct with one succinimide and one molecule of
water (3). The 31P NMR spectrum of this compound shows two
signals of equal intensity coupled to the two rhodium nuclei,
-9 Hz, JRh-Rh′ ) 18 Hz; values of J obtained from a simulated
spectrum).
Synthesis of (M)-Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (H-T) [(M)-
2]. The reaction procedure was similar to that described above using
the (M)-Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2‚(HO2CCH3)2 enantiomer as
the starting product. [R]20 -125° (c 0.012, CHCl3).
D
Catalytic Experiments. Diazo compounds were prepared from the
corresponding carboxylic acid by reaction with methyl chloroformate,
followed by treatment with freshly prepared diazomethane.8 All of the
catalytic reactions were performed by dissolving the diazo compound
(0.28 mmol) in dry CH2Cl2 (30 mL) under an argon atmosphere. To
this solution was added the corresponding catalyst (0.003 g, 0.28 ×
10-2 mmol; [substrate]/[Rh(II)-complex] ) 100), and the mixture was
stirred at room temperature until complete transformation of the diazo
compound was observed by TLC chromatography. The solvent was
evaporated, and the crude product was filtered through a short
chromatography column to eliminate the catalyst. The yield of the
reaction was calculated from the proton NMR.
2
centered at 25.0 (1JRh-P ) 178 Hz; JRh-P ) 7 Hz) and 14.7
2
(1JRh-P′ ) 146 Hz; JRh-P′ ) 8 Hz) ppm. These data are
consistent with a head-to-head arrangement of the bridging
imidate ligands in the dinuclear unit. This assumption was
confirmed by an X-ray crystal structure determination.
If Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2(HO2CCH3)2 (racemic mix-
ture) in THF is reacted with (CH3)3SiCl, Rh2Cl2[(C6H4)P-
(C6H5)2]2 species can be generated in solution.12 By addition
of potassium succinimidate in the same solvent, a 1:1 mixture
of 1 and a new species Rh2(C4H4O2N)2[(C6H4)P(C6H5)2]2 (2)
is formed. This new compound was better crystallized as a water
adduct. An X-ray structure determination performed on this
adduct, 4, confirmed the formula Rh2(C4H4O2N)2[(C6H4)P-
(C6H5)2]2(H2O)2 and also a symmetric (head-to-tail) arrangement
of the succinimidate ligands on this compound.
The bicyclo[3.1.0]-hexan-2-one (6) was purified by HPLC chroma-
tography, and the ee values were based on GC analysis.9 The absolute
stereochemistry of the cyclization product formed from the (P)-Rh(II)
catalysts was determined as (1R,5S).10
X-ray Crystallography. A Siemens SMART CCD diffractometer
was employed for data collection on crystals of compounds 1‚
(H2O)(C4H5NO2) (3) and 2‚(H2O)2 (4). Unit-cell dimensions were
determined by a least squares fit of 50 reflections. The structures were
solved by direct methods and refined on F2 for all reflections using
SHELXTL V 5.05.11 In both cases, the positions of all non-hydrogen
atoms were deduced from difference Fourier maps and refined
anisotropically. Hydrogen atoms were placed in their geometrically
generated positions and refined riding on the carbon atom to which
they are attached. The hydrogen atoms of the water molecules were
ignored in both structures.
Pure enantiomers (M)-1 and (P)-2 were obtained by the same
procedure starting from the corresponding enantiomers of
Rh2(O2CCH3)2[(C6H4)P(C6H5)2]2.1
Up to three different isomers, A, B, and C, (Chart 1) could
be expected from the thermal reaction of Rh2(O2CCH3)2-
[(C6H4)P(C6H5)2]2 and an excess of succinimide. However, the
above results confirm that, in the experimental conditions used,
only one compound 1 (A, Chart 1) is formed.
In CDCl3 solution, compound 2 undergoes slow isomerization
to produce 1. This transformation, which was followed by 31P
NMR spectroscopy, is up to 80% complete after 48 h at 45 °C.
(8) Boer, T.; Backer, H. J. Organic Synthesis; Wiley: New York, 1963;
Vol. IV, p 250.
(9) Estevan, F.; Krueger, P.; Lahuerta, P.; Moreno, E.; Pe´rez-Prieto, J.;
Sanau´, M.; Werner, H. Eur. J. Inorg. Chem. 2001, 105.
(10) Mash, E. A.; Nelson, K. A. Tetrahedron 1987, 43, 679.
(11) SHELXTL, V 5.05; AXS Siemens, 1996.
(12) Cotton, F. A.; Dunbar, K. R.; Eagle, C. T. Inorg. Chem. 1987, 26,
4127.

4228 Inorganic Chemistry, Vol. 40, No. 17, 2001
Barberis et al.
Figure 1. Molecular view of Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2-
(H2O)(C4H5NO2) (3), with the H atoms omitted for clarity. Selected
bond distances (Å) and angles (deg) are Rh1-Rh2 2.5551(7), Rh1-
C42 1.970(7), Rh1-P1 2.2052(16), Rh2-C12 1.997(6), Rh2-N1
2.119(5), C42-Rh1-O1 172.50(19), O2-Rh1-O3 90.00(16), and
C12-Rh2-O4 90.9(2).
Figure 2. Molecular view of Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2‚2(H2O)
(4), with the H atoms omitted for clarity. Selected bond distances (Å)
and angles (deg) are Rh1-Rh2 2.5386(10), Rh1-C62 2.003(9), Rh1-
P1 2.2179(19), Rh2-C12 2.006(7), Rh2-P2 2.211(2), Rh2-N2 2.176-
(6), C62-Rh1-N1 177.0(3), O2-Rh1-O3 81.2(2), and C12-Rh2-
O1 90.8(3).
Scheme 1
The addition of an equimolar amount of free succinimide to
compound 2 has no significant influence on the rate of
isomerization. However, acetonitrile hastened this process; when
a 40 molar excess of CH3CN was added, compound 2
completely transformed to 1 in less than 15 h at the same
temperature. It is remarkable that complete isomerization occurs
very seldomly for this type of compound under the mild
conditions used. Doyle and co-workers have found that the
displacement of acetates from Rh2(O2CCH3)4, by certain amidate
ligands, gave chiral dirhodium(II) N-acyl-2-oxoimidazolidine-
4-carboxylate esters, Rh2(N-O)4. Three different isomers (2,2-
cis),4a (3,1),4a and (4,0)4b have been structurally characterized.
The (4,0) isomer has been isolated in up to 25% yield for short
reaction times. In the reaction conditions used, refluxing a 10:1
chlorobenzene-benzonitrile mixture in the presence of free
amide, the (4,0) isomer rapidly isomerizes (3-4 h), leading to
an equilibrium mixture of (2,2-cis) and (3,1) geometries.4b To
our knowledge, this is the only detailed report of isomerization
in dirhodium(II) compounds. The presence of different types
of ligands in Rh2(N-O)4 and Rh2(N-O)2[(C6H4)P(C6H5)2]2
compounds does not allow us to establish closer comparisons
between both isomerization processes.
expected order of the trans influence of M-C and M-P. The
average equatorial Rh-O bond distances in both compounds,
3 and 4, are about 0.1 Å longer than those in the dirhodium(II)
tetrakis imidazolidine compound.4 The Rh-O bond distance
involving the axial succinimide ligand is considerable longer
(2.483(5) Å) than that for the molecule of water (2.284(4) Å).
The structure of 4 also contains two cyclometalated phos-
phines and two succinimidate ligands bridging both of the
rhodium atoms in a head-to-tail configuration, resulting in a
symmetric structure with each keto group of the imidate ligands
protruding into an axial coordination site. This ligand distribution
generates identical environments for the rhodium atoms that
have a metal-to-metal bond distance (2.5386(10) Å) slightly
shorter than that of compound 3. Two molecules of water
complete the octahedral coordination around the rhodium atoms.
The Rh-O bonds are trans to a P atom, while the Rh-N bonds
are trans to carbon. The observed Rh-O and Rh-N bond
distances are very similar, in the range 2.16-2.17 Å. The two
axial molecules of water have similar Rh-O bond distances,
in the range 2.35-2.37 Å. In both compounds, the distances
between the keto oxygen atoms and the oxygen of the axially
coordinated molecule of water are relatively short (∼2.7 Å).
Structures of 3 and 4. Views of the structures of 3 and 4
are shown in Figures 1 and 2, respectively, together with
important bond distances and angles.
In the structure of 3, the two rhodium atoms are bridged by
four ligands; two cisoid cyclometalated phosphines are in a head-
to-tail arrangement, while the two succinimidate ligands are in
a head-to-head configuration. This ligand distribution generates
two different environments for the rhodium atoms. There is one
molecule of succinimide coordinated through one oxygen atom
to the less sterically hindered rhodium atom, and there is one
molecule of water coordinated to the second rhodium. The Rh-
Rh distance, 2.5551(7) Å, is slightly longer than the values
reported for related dirhodium compounds with carboxylate
ligands.7,13 The Rh-O and Rh-N bonds trans to the carbon
atoms are longer (2.181(4) and 2.178(5) Å) than those trans to
the P atoms (2.145(4) and 2.119(5) Å), in agreement with the
Catalysis. Compounds 1 and 2 were tested as catalysts in
the transformation of 1-diazo-5-phenyl-2-pentanone; as reported
for dirhodium(II) tetraacetate,14 no C-H insertion product
(13) Lahuerta, P.; Paya´, J.; Pellinghelli, M. A.; Tiripicchio, A. Inorg. Chem.
1992, 31, 1224.
(14) Kennedy, M.; McKervey, M. A.; Maguire, A. R.; Tuladhar, S. M.;
Twohig, M. F. J. Chem. Soc., Perkin Trans. 1990, 1047.

New Dinuclear Catalysts Rh2(N-O)2[(C6H4)P(C6H5)2]2
Inorganic Chemistry, Vol. 40, No. 17, 2001 4229
Conclusion
resulted, but addition to the aromatic ring was observed. Both
Rh compounds were quite effective in the cyclopropanation of
diazo compound 515 to give 6 (98 and 84% yields, respectively,
after 90 min of reaction at room temperature) (Scheme 1). The
ability of the chiral (P)-1 and (M)-2 compounds to induce
enantioselectivity was studied in the cyclopropanation of such
a diazo compound, giving very similar ee values (∼22%). Chiral
rhodium tetraamidate compounds have been reported to give
similar ee values for the same diazo ketone.16
We have confirmed that, under the catalytic conditions,
compound 2 did not undergo any detectable isomerization (less
than 1%). Considering that the observed catalytic activities are
very similar for 1 and 2, the almost identical ee values found
for both compounds indicate that the effect of the pending
oxygen atom, at least in this particular case, is not catalytically
relevant.
The new strategy described in the present paper to produce
succinimidate derivatives Rh2(N-O)2[(C6H4)P(C6H5)2]2 that
involves replacement of acetates by chlorines and further
reaction with the deprotonated succinimide allows us to obtain
isomers that are not stable in solution under the thermal reaction
conditions. By this procedure, two compounds 1 and 2 have
been prepared in ∼30% yield. It has been confirmed that the
isomerization of 2 to 1 is accelerated by the presence of free
acetonitrile. This observation represents one of the few examples
of ligand rearrangement observed for dinuclear rhodium(II)
compounds.
Acknowledgment. We thank the DGICYT (Project PB98-
1437) and the EC (Project TMR Network ERBFMRXCT 60091)
for financial support. M.B. thanks the Generalitat Valenciana
for a grant.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structures of complexes 3 and 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) Guthikonda, R. N.; Cama, L. D.; Christensen, B. G. J. Am. Chem.
Soc. 1974, 96, 7584.
(16) Doyle, M. P.; Eismont, M. Y.; Zhou, Q. L. Russ. Chem. Bull. 1997,
46, 995.
IC010257D