Steps on the way to the first dirhodium tetracarboxylate with no axial ligation: Synthetic lessons and a plethora of Rh2(O2CR)4L2-n compounds, n=0, 1, 2

Article abstract of DOI:10.1016/S0020-1693(02)01088-5

We report the preparation and structural features of thirteen new dirhodium tetracarboxylato compounds. Twelve of these were synthesized in early attempts to create Rh₂(TiPB)₄, where TiPB=2,4,6-triisopropylbenzoate, the first dirhodium tetracarboxylate without axial ligation, the structure of which is also reported here. Several challenges in the synthesis of such a molecule are identified and discussed. Although axial interactions are of paramount importance for Rh₂⁴⁺ units, Rh-Rh bond lengths are quite insensitive to the nature, presence or absence of axial ligation. However, the effect of axial ligation is clearly observed using electronic spectroscopy; Rh₂(TiPB)₄ shows an extremely low energy π*(Rh₂)→σ*(Rh₂) transition. The energy of this transition is put into the context of previously unreported spectroscopy of Rh₂(O₂CCPh₃)₄ bis-adducts.

Full text of DOI:10.1016/S0020-1693(02)01088-5

Inorganica Chimica Acta 337 (2002) 233ꢃ246
/
www.elsevier.com/locate/ica
Steps on the way to the first dirhodium tetracarboxylate with no axial
ligation: synthetic lessons and a plethora of Rh2(O2CR)4L2ꢀn
compounds, nꢁ
/
0, 1, 2
F. Albert Cotton a,*, Elizabeth A. Hillard a, Chun Y. Liu a, Carlos A. Murillo a,b,*,
Wenning Wang a,1, Xiaoping Wang a
a Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M University, PO Box 30012, College Station, TX 77842-3012,
USA
b Department of Chemistry, University of Costa Rica, Ciudad Universitaria, Costa Rica
Received 8 March 2002; accepted 7 May 2002
Abstract
We report the preparation and structural features of thirteen new dirhodium tetracarboxylato compounds. Twelve of these were
synthesized in early attempts to create Rh2(TiPB)4, where TiPBꢁ/2,4,6-triisopropylbenzoate, the first dirhodium tetracarboxylate
without axial ligation, the structure of which is also reported here. Several challenges in the synthesis of such a molecule are
4ꢂ
identified and discussed. Although axial interactions are of paramount importance for Rh2 units, Rhꢀ
insensitive to the nature, presence or absence of axial ligation. However, the effect of axial ligation is clearly observed using
electronic spectroscopy; Rh2(TiPB)4 shows an extremely low energy p*(Rh2)0s*(Rh2) transition. The energy of this transition is
/
Rh bond lengths are quite
/
put into the context of previously unreported spectroscopy of Rh2(O2CCPh3)4 bis-adducts.
# 2002 Published by Elsevier Science B.V.
Keywords: Dirhodium; Metalꢀmetal bonds; Metal carboxylates; Axial ligation; Electronic spectra
/
1. Introduction
this paper is the description of what we believe to be the
only structurally characterized Rh2(O2CR)4Ln com-
pound with axial ligation at only one end.
We have recently reported the crystal structure and
electronic spectrum of the first dirhodium tetracarbox-
ylate to be unambiguously characterized with no axial
ligation [1]. The route to the target molecule,
Most transition metals form compounds that adopt
the ‘paddlewheel’ structure shown in Scheme 1, where a
dimetal core is bridged by four monoanionic m2-h2
carboxylato ligands [2]. The axial ligands, L, most
commonly donor solvent molecules, are present in the
vast majority of these compounds. With few exceptions,
compounds whose formulas do not have exogenous
ligands have structures in which the molecules form
infinite chains in such a way that each molecule has its
axial sites occupied by oxygen atoms from its neighbors,
as shown in Scheme 2 for dirhodium carboxylates. With
thousands of compounds and many hundreds of crystal
structures reported in the literature, dimetal tetracar-
boxylates comprise an important class of inorganic
compounds, especially as this structural framework
Rh2(TiPB)4, where TiPBꢁ2,4,6-triisopropylbenzoate,
/
was not straightforward and many unsuccessful efforts
were made in a process that yielded a plethora of axially
coordinated dirhodium tetracarboxylates. Here, we
summarize the syntheses and structures of these mole-
cules. We also report data from a spectroscopic study of
various Rh2(O2CCPh3)4 adducts. A notable feature of
* Corresponding authors. Tel.: ꢂ
9531
E-mail
murillo@tamu.edu (C.A. Murillo).
/
1-979-845 4432; fax: ꢂ
/
1-979-845
Cotton),
addresses:
cotton@tamu.edu (F.A.
1
can support metalꢀ
/
metal bonded units, with bond
orders of one half to four. The influence of the axial
Present address: ExxonMobil Chemical Company, PO Box 3140,
Edison, NJ 08818, USA.
0020-1693/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 8 8 - 5

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F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ246
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out of a donor solvent, and merely driving the solvent
off a crystalline M2(O2CR)4L2 compound leaves an
amorphous product of unknown structure. On the other
hand, if the compound is crystallized from a noncoor-
dinating solvent or by sublimation, the molecules
typically form infinite chains of Scheme 2. One might
be led to believe that the most obvious solution to this
problem is to employ a ligand rigid and bulky enough to
block the axial positions entirely. However, not only is
this a nontrivial synthetic task, but such a ligand would
likely yield a M2(O2CR)4 compound that is insoluble in
any solvent that cannot act also as an axial ligand. Such
a compound would also likely be non-volatile, and
either solubility or volatility is a common requisite for
crystallization. Furthermore, it seemed like bulkiness
alone was not enough to block the axial positions when
using carboxylate groups to bridge the dirhodium units.
For example, even with the very bulky carboxylate
ligand 2,4,6-tri(tolyl)benzoate, pyridine molecules had
been found occupying the axial positions of the corre-
sponding tetracarboxylate units [5].
Scheme 1.
Scheme 2.
ligands on metal-metal bond lengths, magnetic proper-
ties, and electronic structure has been the subject of
hundreds of publications, yet cases where the dimetal
unit embraced by four carboxyl groups is devoid of any
kind of axial ligation (whether exogenous or through
self-association) are extremely rare.
A few years ago, the synthetic problem of creating a
Cr2(O2CR)4 molecule without axial ligands was solved
[6]. The ligand employed was 2,4,6-triisopropylbenzo-
ate, TiPB, which proved bulky enough to shield all of
the carboxyl oxygen atoms from engaging in association
with neighboring molecules, and yielded a compound,
Cr2(TiPB)4, that was soluble in toluene. A remarkable
Quite logically, the difficulty in isolating such a
compound has been commensurate with the affinity
the particular M2(O2CR)4 compound has for axial
ligands. Molybdenum carboxylates do not coordinate
axial ligands very strongly, and the first Mo2(O2CR)4
compound without any kind of axial ligation was
crystallized out of toluene in 1981 as Mo2(O2Cbiph)4,
where biphCO2 is the anion of 2-phenylbenzoic acid [3].
In this molecule, two of the o-phenyl groups lie in the
equatorial region, while each of the others is directed to
one or the other axial side of the molecule. This serves to
both minimize repulsive forces between the phenyl
groups and introduce enough steric hindrance to prevent
weak self association that would otherwise occur. This
strategy did not work for the analogous chromium
compound, as the Cr2(O2CR)4 compounds have a much
stronger affinity for axial ligation. When crystallized
from toluene, Cr2(O2Cbiph)4 exhibited a ‘dimer of
dimers’ structure with all of the phenyl groups oriented
towards one side of each subunit [4], apparently the
stability imparted by (intermolecular) axial coordination
feature of Cr2(TiPB)4 is that the Crꢀ
/
Cr bond length is
approximately 0.4 A shorter than that of the bis-
acetonitrile analog. In general, the MꢀM bonds in
˚
/
M2(O2CR)4 compounds exhibit some degree of sensitiv-
ity to axial ligation, lengthening with increasing donor
strength, but there is only limited structural data for
M2(O2CR)4 complexes with no form of axial ligation,
due to the synthetic challenges described above.
We turn now to what we believe is the second most
difficult case. Dirhodium tetracarboxylates [2,7] surpass
all other M2(O2CR)4 compounds in the variety of
ligands which have been found to coordinate to the
axial position; compounds have been structurally char-
acterized as adducts of even ostensibly ‘innocent’
molecules such as naphthalene and other polycyclic
aromatic molecules [8]. For this reason, and because of
many other interesting aspects of their chemistry,
Rh2(O2CR)4L2 compounds are among the broadest
and most well-studied class of M2(O2CR)4L2 com-
pounds, and have been found to have a wide range of
potential applications as anti-tumor agents [9], catalysts
[10], and building blocks for supramolecular arrays [11].
A conspicuous feature of Rh2(O2CR)4L2 chemistry is
that the various adducts exhibit a wide variety of colors
depending on the nature of the axial ligand. It was noted
outweighed ligandꢃligand repulsion. In essence, weak
/
intermolecular association may be disrupted by slight
steric interference, but disruption of strong association
needed a different approach.
For many years, the problem of isolating paddlewheel
compounds, M2(O2CR)4, with no axial ligands remained
unsolved for MꢁCr or Rh, which show very strong
/
as early as 1963 that the low energy band (around 600ꢃ
/
affinity for axial ligands. Obviously, the structure shown
in Scheme 1 is obtained if the compound is crystallized
700 nm) in the visible spectrum increases in energy with
increasing donor strength of the axial ligand [12]. This

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/246
235
property, and the presence of a Rhꢀ
/
Rh bond, has led to
purchased from Lancaster Synthesis, Inc. Toluene was
dried over Na/K alloy, acetonitrile over calcium hy-
dride, ethanol over magnesium metal, acetone over
potassium carbonate, and decane and o-xylene over
molecular sieves. All solvents were freshly distilled under
N2 prior to use, except for decane and o-xylene, which
were degassed by bubbling N2. In the cases of com-
pounds 1, 10 and 12, crystallizations were carried out by
slow concentration of solutions in a vial enclosed within
a Schlenk tube containing mineral oil or rubber septa
fragments. In this way an inert atmosphere could be
attained while solvent from the solutions was transferred
by evaporation to the material inside the Schlenk tube.
Thermogravimetric analysis was performed on an In-
strument Specialist TGA 1000. Electronic spectra were
recorded on a Cary 17D spectrophotometer. NMR
spectra were recorded on a Varian Unity 300 spectro-
meter.
extensive study of the electronic structure of
Rh2(O2CR)4L2 compounds, both by experimental and
theoretical methods [2,13]. The earliest quantitative
calculations, by Norman and Kolari [13a], produced a
s2p4d2p*4d*2 configuration for Rh2(O2CH)4 and a
p4s2d2p*4d*2 for Rh2(O2CH)4(H2O)2, with
a
s*
LUMO and a single bond in each case. The ordering
of the bonding orbitals in the two compounds was
attributed to the interaction of the symmetric and
antisymmetric combinations of the two s lone pair
orbitals of the H2O molecules with the RhꢀRh s and s*
/
orbitals, respectively, which destabilizes these orbitals
relative to those of the anhydrous compound. This
destabilization of the s-bonding orbital corresponds to
a slight weakening of the Rhꢀ
destabilization of the s* orbital results in a larger
HOMOꢃLUMO gap, thus accounting for the spectro-
/
Rh bond, while the
/
chemical behavior of the low energy peak in the visible
spectrum. However, because all of the isolated
Rh2(O2CR)4 compounds have had axial ligands, there
has been no experimental confirmation of the structural
nor electronic theoretical predictions, although some
spectroscopic data previously reported by Drago are
entirely consistent with what we now believe [14].
A natural question is whether this weakening of the
The synthesis of all compounds is presented in
Schemes 3ꢃ5. General to most syntheses are the
/
following considerations, (1) a stream of dry nitrogen
was blown over the refluxing reaction mixtures, so that
byproducts (acetic or trifluoroacetic acid) were distilled
out of the reaction flask. The solvents also were distilled
until there was only a small amount (ca. 1 ml)
remaining. The remaining solvent was then pumped
off at r.t. (2) After the removal of all solvent, any excess
TiPBH was removed by vacuum sublimation.
Rhꢀ
/
Rh bond is reflected in the corresponding metalꢃ
/
metal separation. With this in mind, we attempted to
create a Rh2(O2CR)4 compound with no axial ligands.
This task took us through a maze with many dead ends,
but persistence and adjustments based on what we
learned at each step finally took us to our goal. Here
we go beyond our preliminary report [1] and present the
array of structural permutations that developed as the
dirhodium core repeatedly revealed its avidity for axial
ligation, although, for the most part, we have not
characterized these compounds beyond their structures.
We also report additional spectroscopic evidence in
support of the role of axial s-donation in Rh2(O2CR)4
LUMO destabilization.
2.1.1. Preparation of
[Rh2(TiPB)2(O2CCF3)2(TiPBH)]2×
/
C6H14, 1×/C6H14
The compounds Rh2(O2CF3)4 (30 mg, 0.046 mmol)
and TiPBH (91 mg, 0.37 mmol) were combined in 6 ml
of toluene. The green solution was refluxed at 120 8C
while trifluoroacetic acid distilled away under a flow of
nitrogen. After 4 h the temperature was raised to
130 8C and the toluene distilled until about 3 ml
remained. The remaining toluene was pumped off,
leaving a blue-green solid. Excess TiPBH was removed
by vacuum sublimation at 105ꢃ110 8C for 1 h. The
/
remaining solid was extracted with 3 ml of hexanes, and
slow evaporation of this solution yielded green blocks
after 24 h.
2. Experimental
2.1. General considerations
2.1.2. Preparation of Rh2(TiPB)3(O2CCF3)(TiPBH)2,
2
All reactions were carried out under an atmosphere of
dry nitrogen using standard Schlenk techniques,
although starting materials were often handled in air.
Dirhodium tetraacetate, Rh2(O2CCPh3)4(EtOH)2, and
Rh2(O2CCF3)4 were synthesized by literature proce-
The compounds Rh2(O2CCF3)4 (30 mg, 0.046 mmol)
and TiPBH (91 mg, 0.37 mmol) were combined in 10 ml
of decane. The blue-green solution was stirred at 130 8C
for 4 h. Solvent was then removed under vacuum and
TiPBH was removed by vacuum sublimation at 110 8C
for 2 h, yielding a blue solid. The solid was extracted
with 6 ml of hexanes and slow evaporation of this
solution yielded both blue and green crystals. Only the
structure of the blue plates was determined as the green
dures
Rh2(O2CCH3)4 was stirred in acetonitrile for 30 min
and dried under vacuum to produce
[15ꢃ
/
17].
Anhydrous
Rh2(O2CCF3)4
or
Rh2(O2CCF3)4(CH3CN)2 or Rh2(O2CCH3)4(CH3CN)2,
respectively. Triisopropylbenzoic acid (TiPBH) was

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F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ246
/
Scheme 3.
crystals were not suitable for X-ray diffraction (how-
ever, see preparation of 4 below).
2.1.3. Preparation of
Scheme 4.
[Rh2(TiPB)2(O2CCF3)2(C6H5CH3)]2×
/
2C6H5CH3, 3×
/
2C6H5CH3
The lithium salt of TiPB was prepared by adding 1
equiv. of BuLi in hexanes to TiPBH in a THF solution.
Rh2(O2CCF3)4 (82.3 mg, 0.125 mmol) and TiPBLi
(0.127 g, 0.500 mmol) were combined in 20 ml of
toluene. The green solution was refluxed for 12 h,
yielding a dark yellow precipitate. The suspension was

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237
Scheme 5.

238
F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ246
/
filtered through Celite and the filtrate was concentrated
to about 2 ml. The solution was placed in a freezer at
hexanes to toluene over a period of several days
1
afforded small pale green crystals. H NMR d (ppm in
CDCl3): 6.90 (s, 8H, aromatic); 2.88 (m, 12H,
ꢀ10 8C and pale green plates grew after 1 day.
/
ꢀ
/
CH(CH3)2); 1.20 (d, 24H, p-CH(CH3)2); 1.02 (d,
2.1.4. Preparation of [Rh2(TiPB)3(O2CCH3)-
(TiPBH)]2×1.25C6H14, 4×1.25C6H14
48H, o-CH(CH3)2).
/
/
The compound Rh2(O2CCH3)4(CH3CN)2 (26 mg,
0.050 mmol) was combined with an excess (85 mg,
0.34 mmol) of TiPBH in 5 ml of toluene. The blue
solution was heated to reflux, and the color changed
from blue to green. After gentle refluxing for 30 min, the
solvent was distilled at 140 8C. The residue was
2.1.8. Preparation of
[Rh2(TiPB)2(O2CCF3)2(acetone)]2, 8 and
Rh2(TiPB)2(O2CCF3)2(acetone)2, 9
The compounds Rh2(O2CCF3)4(CH3CN)2 (50 mg,
0.068 mmol) and TiPBH (0.168 g, 0.676 mmol) were
combined in 10 ml of toluene. The fuchsia solution was
refluxed for 3 h. The color quickly turned green, and the
solution was concentrated by distillation to about 1 ml.
The remaining solvent was removed under vacuum and
the TiPBH was sublimed at 120 8C, leaving a light
green solid. As the solid proved only sparingly soluble in
hexanes, it was dissolved in 10 ml of acetone, affording a
extracted with hexanes, and upon cooling to ꢀ10 8C
/
green blocks were obtained.
2.1.5. Preparation of
Rh2(TiPB)2(O2CCH3)2(CH3CN)2, 5
To the solution from which [Rh2(TiPB)3(O2CCH3)-
(TiPBH)]2×1.25C6H14, 4×1.25C6H14, was crystallized,
/
/
deep blueꢃgreen solution. Removal of solvent under
/
one drop of acetonitrile was added. The resulting purple
solution was slowly evaporated, forming a large purple
crystal.
vacuum afforded a dark blue solid, which was dissolved
in hexanes. The solution was concentrated to 5 ml and
placed in the freezer. After 1 night two types of crystals
had formed: light green plates and very dark blocks. The
plate-like crystals corresponded to 8 and the dark block
crystals to 9.
2.1.6. Preparation of Rh2(TiPB)4(CH3CN)2×
/CH3CN,
6×CH3CN
/
2.1.6.1. Method A. Rh2(O2CCH3)4(CH3CN)2 (26 mg,
0.050 mmol) was combined with an excess (85 mg, 0.34
mmol) of TiPBH in 5 ml of toluene. The blue solution
was heated to reflux and the color quickly changed from
blue to green. After gentle refluxing for 1 h, the solvent
was removed by distillation at 140 8C. The residue was
extracted with hexanes. Addition of several drops of
acetonitrile to the extract generated a purple solution.
Slow evaporation over a period of 1 week afforded large
block crystals.
2.1.9. Preparation of Rh2(TiPB)4(TiPBH)2×
/
1/2C6H14,
10×1/2C6H14
/
The compound Rh2(O2CCF3)4(CH3CN)2 and excess
TiPBH were combined in 7 ml of decane. The solution
was refluxed for 2 h, after which a distillation apparatus
was attached. The solution was concentrated to about 2
ml by distillation over the course of 2 days. All solvent
was evaporated under vacuum, yielding a blue solid.
Excess TiPBH was sublimed at 150 8C, and the solid
turned green. It was dissolved in 6 ml of warm hexanes
and filtered through Celite. Slow evaporation yielded
2.1.6.2. Method B. Rh2(O2CCF3)4(CH3CN)2 (18.5 mg,
0.0250 mmol) and TiPBH (56 mg, 0.23 mmol)
were combined with 5 ml of o-xylene to form a blue
solution. The solvent was distilled over a period of 1 h.
Excess TiPBH was removed by vacuum sublimation at
blueꢃgreen blocks after several days.
/
2.1.10. Preparation of Rh2(TiPB)4(acetone)2×
/
0.90(acetone), 11×0.90(acetone)
/
150ꢃ
/
160 8C. The solid was extracted with 3 ml of
2.1.10.1. Method A. Several crystals of Rh2(TiPB)4-
(TiPBH)2×1/2C6H14, 10×1/2C6H14, were dissolved in
acetone, in which they are moderately soluble. The
hexanes and filtered over Celite. To the filtrate, several
drops of acetonitrile were added, affording a purple
solution, which was allowed to evaporate slowly to form
crystals. 1H NMR d (ppm in CDCl3): 6.90 (s, 8H,
/
/
blue solution was filtered through Celite, concentrated
to 3 ml, and placed in the freezer. Blueꢃ/green rhomboi-
aromatic); 2.88 (m, 12H, ꢀ
/
CH(CH3)2); 2.23 (s, 6H,
dal crystals appeared after 24 h.
NCCH3); 1.22 (d, 24H, p-CH(CH3)2); 1.04 (d, 48H, o-
CH(CH3)2).
2.1.10.2. Method B. Sodium ethoxide, (0.17 g, 2.5
mmol), was dissolved in 30 ml of ethanol and TiPBH
(0.55 g, 2.2 mmol) was added. Once the TiPBH had
2.1.7. Preparation of Rh2(TiPB)4(CH3CN), 7
The compound Rh2(TiPB)4(CH3CN)2×
/
CH3CN, 6×
/
dissolved, RhCl3×
/
3H2O (0.15 g, 0.55 mmol) was added,
brown suspension. The mixture
CH3CN, was heated at 80 8C under vacuum overnight.
A color change from purple to green was observed. The
green solid was dissolved in hexanes. Back-diffusion of
affording an orangeꢃ
/
was refluxed for 18 h, yielding a green solution as well as
a deposit of rhodium metal. The suspension was filtered

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239
through Celite, and the solvent removed under vacuum.
The blueꢃgreen solid was extracted with 20 ml of
chloroform, dried under vacuum and redissolved in 30
ml of acetone. The solution was concentrated to
approximately 5 ml and placed in a freezer at
2.2. X-ray crystallography
Single crystals of compounds 1×
tained as described above. Each crystal was mounted on
a glass fiber with silicone grease and transferred to a
goniometer head. In subsequent experiments, the crystal
/
/
C6H14ꢃ
/
13 were ob-
ꢀ
/
10 8C. After 12 h, a crop of blueꢃgreen rhomboidal
/
crystals had grown. Anal. Calc. (Found) for
C70H104Rh2O10: C, 64.11 (64.11); H, 7.99 (7.70). 1H
NMR d (ppm, in benzene-d6): 7.037 (s, 8 H, aromatic),
3.306 (septet, 8 H, o-CH(CH3)2), 2.683 (septet, 4 H, p-
CH(CH3)2), 1.832 (s, 12 H, acetone methyl), 1.211 (d, 48
H, o-CH(CH3)2), 1.139 (d, 24 H, p-CH(CH3)2). IR
(KBr): 3455, 2963, 2871, 1693, 1604, 1573, 1553, 1460,
1401, 1319, 1261, 1236, 1159, 1105, 1020, 944, 877, 811,
was cooled under a stream of nitrogen at ꢀ
/
60 8C. Data
for 10×1/2C6H14 were collected on a Bruker Nonius
/
FAST diffractometer utilizing the program MADNES
[18]. Cell parameters were obtained from an autoindex-
ing routine and refined with 250 reflections within the
2u range of 18.1ꢃ41.68. The cell dimensions and Laue
/
symmetry were confirmed with axial photographs. A
combination of 0.28 v- and f-scans were performed at
four different settings to collect a nominal hemisphere of
data. The data were corrected for Lorentz and polariza-
tion effects and processed using an ellipsoid-mask fitting
749, 650, 558, 506, 460 cmꢀ1
.
2.1.11. Preparation of Rh2(TiPB)4(H2O)(C6H5CH3),
12
program PROCOR [19]. Data for 1×
/
C6H14ꢃ
/
9, 11×
/
0.90(acetone)ꢃ13 were collected with a Bruker SMART
/
Several crystals of Rh2(TiPB)4(acetone)2×
/
0.90(ace-
1000 CCD area detector system using 0.38 v-scans at 0,
90 and 1808 in f. Cell parameters were determined using
the program SMART [20]. Data reduction and integra-
tion were performed with the software package SAINT-
PLUS [21], while an absorption correction was applied
using the program SADABS [22].
tone), 11×0.90(acetone), were dissolved in toluene. The
/
solution was heated to 60 8C and vacuum distilled, until
the solution was concentrated to about 1 ml. Slow
evaporation of the solution yielded green plates after 2
weeks. As water had not been purposely added to the
reaction, it is assumed to have entered adventitiously.
Crystal and space group symmetries for all com-
pounds except 7 were determined unequivocally using
XPREP program [23]. The three cell angles for 7 were
2.1.12. Preparation of Rh2(TiPB)4, 13
close to 908, and the reflection data had a mean jE2ꢀ
1j
/
value of 0.967. The diffraction statistics clearly favor a
centrosymmetric space group. Therefore, the crystal
could be either monoclinic or orthorhombic. In the
latter, the molecules must reside on special positions, i.e.
mirror planes or inversion centers. Neither direct nor
Patterson methods could be used to solve the structure
based on this assumption. For a monoclinic cell with a b
2.1.12.1. Method A. Several crystals of Rh2(TiPB)4-
(acetone)2×
/
0.90(acetone), 11×
/
0.90(acetone), from the
preparation given in Section 2.1.10.1 were placed under
vacuum at 130 8C. Over the course of about 4 h, they
changed from blueꢃ
/
green to green to yellowꢃgreen. The
/
yellowꢃgreen solid was dissolved in hexanes (in which it
/
is sparingly soluble) and filtered through Celite. The
hexanes were evaporated under a slow stream of
nitrogen, yielding a mixture of green and yellow crystals.
The structure presented here is that of a yellow crystal.
angle of 90.06(1)8, systematic absences for h0l (hꢂ
/
l "
/
2n) and 0k0 (k "2n) were consistent with the space
/
group P21/n. The crystal structure was solved in this
space group using direct methods, and the molecules of
7 were found on general positions. Refinement was
accomplished satisfactorily by considering the crystal as
a pseudo-merohedral twin with twinning along the [1 0
0] direction. After using the corresponding twin matrix
the residual R1 values dropped from 0.227 to 0.101 for
2.1.12.2. Method B. Several Rh2(TiPB)4(acetone)2×
/
0.90(acetone) crystals from the preparation given in
Section 2.1.10.2 were placed under vacuum at 130 8C.
Over the course of about 4 h, they changed from blueꢃ
/
green to green to yellowꢃgreen. The yellowꢃgreen solid
/
/
all data and from 0.192 to 0.0676 for data with I ꢀ
2s(I).
For all compounds, the positions of some or all of the
non-hydrogen atoms were found by direct methods
using the solution program SHELXS [24]. The position
of the remaining non-hydrogen atoms were located by
use of a combination of least-squares refinement and
difference Fourier maps in the SHELXL-93 or SHELXL-97
program [25]. Non-hydrogen atoms were refined with
anisotropic displacement parameters except for disor-
/
was dissolved in hexanes and filtered through Celite.
The hexanes were evaporated under a slow stream of
nitrogen, yielding yellow crystals. Anal. Calc. (Found)
1
for C64H92Rh2O8: C, 64.31 (63.94); H, 7.76 (7.51). H
NMR d (ppm, in CD2Cl2): 6.911 (s, 8 H, aromatic),
2.847 (m, 12 H, ꢀ/CH(CH3)2), 1.202 (d, 24 H, p-
CH(CH3)2), 1.043 (d, 48 H, o-CH(CH3)2). IR (KBr):
3429, 2963, 2871, 1609, 1569, 1551, 1460, 1401, 1318,
1261, 1157, 1102, 1023, 944, 874, 808, 750, 652, 564, 509,
466 cmꢀ1
.
dered portions found in structures of 1×
/
C6H14*8, and
/

Table 1
Crystal data and structure refinement parameters for Rh2(O2CR)4 complexes
1×C6H14
110H154
F12O20Rh4 F3O12Rh2 O16Rh4
2
3×2C6H5CH3 4×1.25C6H14
C100H124F12 C139.50
5
6×CH3CN
7
8
9
10×1/2C6H14 11×0.90(acetone) 12
13
Empirical
formula
C
-
C82H117
-
-
-
C40H58N2- C70H101N3- C66H95N-
C78H104-
F12O18Rh4 O10Rh2
C42H58F6- C99H147O12
Rh2
-
C72.7H109.4O10.9
Rh2
-
C71H100O9- C64H92O8Rh2
Rh2
H209.50O20
Rh4
2618.21
-
O8Rh2
900.70
P21/n
O8Rh2
O8Rh2
1236.25
P21/n
Formula
weight
Space
2435.97
1557.58
2221.64
1318.36
1969.26
1042.70
1733.98
1363.32
1303.33
1195.20
¯
¯
¯
¯
¯
¯
¯
P/1/
P/
1
/
P/
1
/
P21/n
P/
1
/
P/
1
/
P
/
1
/
I41/a
P/
1
/
P21/n
P21/n
group
˚
a (A)
˚
13.173(1)
14.302(1)
17.597(2)
14.291(1)
14.754(1)
20.102(2)
17.694(1)
16.776(1)
18.480(1)
90
14.448(2)
16.550(2)
33.578(5)
97.248(3)
95.546(2)
111.541(2)
7319(2)
2
8.6080(5)
28.202(2)
9.0763(5)
90
14.295(1)
17.302(1)
18.218(1)
64.723(1)
9.908(3)
32.14(1)
20.881(7)
90
12.1209(6) 17.948(1)
17.2005(9) 17.948(1)
14.7166(2)
26.325(2)
28.853(5)
66.425(1)
85.971(5)
79.239(5)
10065(2)
4
14.776(1)
16.429(1)
17.464(2)
65.457(1)
78.709(2)
87.445(2)
3778.5(6)
2
10.9216(7)
30.849(2)
20.495(1)
90
13.6919(8)
30.832(2)
15.1279(9)
90
b (A)
˚
c (A)
25.045(1)
74.032(1)
79.003(1)
89.564(1)
4922.3(4)
2
30.418(3)
90
90
a (8)
b (8)
g (8)
V (A )
Z
101.762(2) 82.134(2)
93.241(2) 89.157(2)
103.765(2) 88.435(2)
110.286(1)
90
105.771(1) 73.080(1)
90.060(6)
90
91.957(1)
90
97.350(1)
90
90
2120.4(2)
2
68.037(1)
3732.1(4)
2
90
9798(1)
8
3
˚
3133.2(5)
1
4196.8(6)
2
5145.2(5)
2
6650(4)
4
6901.3(8)
4
6333.7(7)
4
T (8C)
l (A)
Dcalc (g
ꢀ60
0.71073
1.291
ꢀ60
0.71073
1.233
ꢀ60
0.71073
1.434
ꢀ60
0.71073
1.188
ꢀ60
0.71073
1.411
ꢀ60
0.71073
1.173
ꢀ60
0.71073
1.235
ꢀ60
0.71073
1.329
ꢀ60
0.71073
1.414
ꢀ60
0.71073
1.144
ꢀ60
0.71073
1.198
ꢀ60
0.71073
1.254
ꢀ60
0.71073
1.253
˚
cmꢀ3
)
m (mmꢀ1
)
0.593
0.0917
(0.1481)
0.455
0.0847
(0.1902)
0.712
0.0810
(0.1436)
0.501
0.1098
(0.1642)
0.828
0.0594
(0.1382)
0.491
0.0663
(0.1252)
0.546
0.1010
(0.1509)
0.736
0.0990
(0.2103)
0.746
0.0956
(0.1604)
0.382
0.1083
(0.1890)
0.489
0.0684
(0.1472)
0.530
0.1138
(0.1354)
0.570
0.0789
(0.1163)
a
R1
b
{wR2
}
(all data)
a
R1
0.0537
(0.1349)
0.0631
(0.1794)
0.0526
(0.1264)
0.0702
(0.1522)
0.0526
(0.1331)
0.0438
(0.1091)
0.0676
(0.1362)
0.0572
(0.1688)
0.0452
(0.1035)
0.0751
(0.1672)
0.0519
(0.1337)
0.0568
(0.1101)
0.0503
(0.1034)
b
{wR2
}
(I ꢀ2s(I))
a
R1ꢁajjFojꢀjFcjj/ajFoj.
b
2
wR2ꢁ[a[w(Fo2ꢀFc2)2]/a[w(Fo2)2)]]1/2, wꢁ1/[s2(Fo2)ꢂ(aP)2ꢂbP], where Pꢁ[max(Fo or 0)ꢂ2(Fc2)]/3.

F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ
/246
241
Table 2
RhꢀRh bond lengths (A)
˚
a
˚
˚
˚
RhꢀRh (A) RhꢀOeq (A) (avg.) RhꢀLax (A)
1×C6H14
2.3893(6)
2.3669(7)
2.3959(6)
2.358[1]
2.035[4]
2.038[5]
2.037[4]
2.026[6]
2.031[3]
2.040[2]
2.047[9]
2.036[5]
2.037[4]
2.036[4]
2.037[3]
2.041[3]
2.032[2]
2.300[4]
2.242[4]
2.300(3)
2.316[6]
2.266(3)
2.237[5]
2.10(2)
2
b
3×2C6H5CH3
4×1.25C6H14
5
2.3996(6)
2.3882(4)
2.354(1)
6×CH3CN
7
8
2.3916[8]
2.4008(8)
2.3674[7]
2.297[6]
2.268(4)
2.285[4]
2.31[1]
9
10×1/2C6H14
11×0.90(acetone) 2.3700(5)
12
13
c
2.3638(5)
2.3499(4)
2.271(4)
Fig. 2. Thermal ellipsoid plot of 6, shown at the 30% probability level.
Hydrogen atoms and disordered groups of minor occupancy have been
omitted for clarity.
a
Square brackets refer to average values; parentheses refer to
unique values.
b
The value refers to intermolecular axial coordination. There is also
a toluene molecule oriented in a h2 fashion towards the free axial
position of each subunit, which are related by an inversion center, at an
˚
average distance of 2.70 A.
c
The value refers to the axial water molecule. There is also a toluene
molecule oriented in a h2 fashion towards the other axial position at an
average distance of 2.80 A.
˚
Fig. 3. Thermal ellipsoid plot of 7, shown at the 30% probability level.
Hydrogen atoms and disordered groups of minor occupancy have been
omitted for clarity.
3. Results and discussion
3.1. Structural considerations
Fig. 1. Thermal ellipsoid plot of 11, shown at the 30% probability
level. Hydrogen atoms and disordered groups of minor occupancy
have been omitted for clarity.
All the structures of 1×
/
C6H14*13 are of the paddle-
/
wheel type, and all but Rh2(TiPB)4 (13) have axial
ligation either by exogenous solvent molecules or by
self-association to create a dimeric structure. Thermal
ellipsoid plots of selected structures are presented in
Figs. 1ꢃ
/
4. The compounds with axial ligation at each
10×
/
C6H14*13. The hydrogen atoms were included in
/
end, where one of the axial ligands is an oxygen atom
the structure factor calculations at idealized positions.
Crystal data and refinement results for all compounds
are summarized in Table 1. Selected bond distances and
angles are listed in Table 2. Details of crystal structure
information in CIF format can be found in the
supporting materials.
from an adjacent rhodium molecule have Rhꢀ
/
Rh bond
in [Rh2(TiP-
˚
A
distances ranging from 2.358[1]
˚
B)3(O2CCH3)(TiPBH)]2 (4) to 2.3959(6) A in [Rh2(TiP-
B)2(O2CCF3)2(C6H5CH3)]2 (3). Where both axial
ligands are exogenous, the Rhꢀ
/
Rh bond distances range
˚
from 2.3638(5) A in Rh2(TiPB)4(H2O)(C6H5CH3) (12),

242
F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ246
/
the bulky TiPB anions, while the others (which make the
connection by sharing oxygen atoms) have only small R
groups, viz., CH3 or CF3.
3.2. Synthetic considerations
The most common preparation of rhodium(II) car-
boxylates begins with the synthesis of rhodium(II)
acetate by combination of RhCl3×/3H2O with a mixture
of sodium acetate and acetic acid in refluxing ethanol
[17a]. Although prolonged refluxing causes deposition
of Rh metal, yields of the acetate have reached as high as
85% [17a]. Carboxylate exchange reactions with the
acetate have then been used to yield a variety of
Rh2(O2CR)4L2 molecules; typically a great excess of
the acid of the target carboxylate is used as the solvent
[12,26]. Our strategy deviated only in that we employed
a variety of high-boiling solvents instead of a neat
reaction, due to the high melting point of our ligand
precursor, TiPBH (ca. 185 8C). Most of our syntheses,
Fig. 4. Thermal ellipsoid plot of 13, shown at the 30% probability
level. Hydrogen atoms and disordered groups of minor occupancy
have been omitted for clarity.
˚
to 2.4008(8) A in Rh2(TiPB)2(O2CCF3)2(acetone)2 (9),
outlined in Schemes 3ꢃ5 followed this basic outline: (1)
/
significantly overlapping with the previous group. There
is only one molecule with only one axial ligand,
reflux the rhodium(II) precursor with TiPBH in a high
boiling solvent, while distilling out the acid byproduct,
(2) distill the remaining solvent, (3) sublime excess
TiPBH, (4) extract solid, (5) crystallize material by
evaporation or cooling.
Rh2(TiPB)4(CH3CN) (7) and this has a Rhꢀ
/
Rh bond
˚
length of 2.354(1) A.
Several interesting, though not fully understandable,
relationships exist between RhꢀRh bond lengths as a
function of axial ligation. A particularly interesting one
is found in the series Rh2(TiPB)4×nCH3CN, nꢁ0, 1, 2,
compounds 13, 7 and 6. As the value of n increases the
RhꢀRh distance increases, but in a very uneven way.
From nꢁ0 to 1, there is practically no change (2.350ꢃ
2.354 A) while from nꢁ1 to 2 the change is appreciable
2.388 A). While the qualitative trend is as
/
We used three different Rh2 precursors in our
investigations: anhydrous Rh2(O2CCF3)4, Rh2(O2-
CCH3)4(CH3CN)2 and Rh2(O2CCF3)4(CH3CN)2, and
applied a variety of reaction conditions. Given the
plethora of partially substituted paddlewheel structures
that we have characterized, it becomes clear that (1)
carboxylate exchange reactions are not straightforward
for so bulky a substituent as TiPB, and (2) the starting
materials with acetonitrile in the axial position out-
performed the anhydrous Rh2(O2CCF3)4, by yielding
more highly TiPB-substituted products under relatively
milder conditions, and (3) a mixture of products
reflecting various degrees of substitution was often
observed.
The difficulty in preparing the fully TiPB-substituted
product compared with other Rh2(O2CR)4 compounds,
where R is an alkyl or aryl group should not be terribly
surprising. Studies indicate that substitution reactions of
paddlewheel structures occur by initial coordination in
the axial position by the incoming group, followed by
dissociative ring opening of the leaving group, freeing an
equatorial site for coordination by the incoming group
[27]. When the incoming group is very bulky, it is easy to
imagine that steric crowding in the equatorial region
would adversely affect the rate of the substitution. The
resistance to substitution seemed to increase as the
reaction progressed, thus accounting for the frequent
isolation of only partially substituted compounds, in
agreement with what has been previously noted for
dirhodium carboxylates, namely, that acetate substitu-
/
/
/
/
/
˚
/
˚
(2.354ꢃ
/
expected, the unevenness has no evident explanation.
Compound 7 merits further discussion. Since there are
nearly one thousand Rh2(O2CR)4Ln compounds in the
literature, and no attempt to catalog them exhaustively
has been made for many years, it is not possible to say
with certainty that 7 is the first such compound ever
reported with axial ligation of any kind at only one end.
However, we believe that it is. It should be noted that as
in compounds 1, 3, 4 and 8, and numerous others
previously reported, the formula alone does not tell the
story, because entities of the type Rh2(O2CR)4L have a
strong tendency to associate in pairs by sharing carbox-
ylate oxygen atoms. The ‘secret of success’ so to speak in
the persistence of
7 as a discrete, undimerized
Rh2(TiPB)4(CH3CN) molecule, is that the presence of
the four sterically bulky 1,3,5-triisopropylphenyl groups
makes such an association impossible, just as it made
impossible the association of Rh2(TiPB)4 and
Cr2(TiPB)4 molecules into the chains formed by all
other Rh2(O2CR)4 and Cr2(O2CR)4 molecules. Note
that in compounds 1, 3, 4 and 8, association does occur
because only two to three of the carboxylato groups are

F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ
/246
243
tion is retarded with the increase in size of the incoming
carboxylate group [28]. Another possible reason for the
difficulty of substitution may be found in our choice of
solvents; we chose to use nonpolar solvents with the idea
that they would not act as axial ligands. However, the
acetato anion intermediate appears to be less stabilized
in nonpolar solvents than it would be in the more often
used neat carboxylic acids.
The observed difference in lability between the start-
ing materials, Rh2(O2CR)4 and Rh2(O2CR)4L2 is prob-
Fig. 5. Electronic spectra (nm (Mꢀ1 cmꢀ1)) of 13: 765(251), 430(285);
13×2H2O: 670(250), 445(164); 13×2(acetone): 610(255), 460(123).
ably also
a matter of kinetics. The anhydrous
Rh2(O2CCF3)4 is not a discrete molecule, but a one-
dimensional polymer of the Scheme 2 [17b]. Thus, the
axial positions are well shielded by very bulky ‘ligands’,
which hinder the first step of the substitution process.
The polymer is also insoluble in solvents which cannot
act as axial ligands, and only becomes soluble at high
temperature, presumably because the TiPBH (or in one
case LiTiPB) molecules begin to slowly degrade the
polymer by axial coordination. Thus, even when vigor-
ous reaction conditions were employed, full substitution
by TiPB was not attained (presumably due to oligomer-
ization of the starting material) and this strategy was
quickly abandoned. The rhodium precursors with ace-
tonitrile in the axial position, however, are discrete
molecules of the type shown in Scheme 1. Since
acetonitrile is small and linear, the axial positions are
not entirely blocked by these molecules, thus allowing a
closer approach by the incoming acid, and full TiPB
substitution has been attained in as little time as 1 h.
Refluxing for a shorter time (viz. 30 min) yielded a
mixture of partially substituted products (4 and 5),
indicating the importance of kinetics in the substitution
process.
/
/
Substitution of the acetato or trifluoroacetato groups
by TiPB is only a first step. Even when substitution is
complete, the axial positions are not shielded in any
way. We used acetonitrile in the Scheme 4 reactions to
stabilize the final product and make it less soluble in
hexanes to effect crystallization. This method was not
employed in the Scheme 5 reactions and in the absence
of any other potential axial ligand, the carboxylic acid
itself (which is necessarily in excess) coordinates to the
axial positions to give 10. The axial TiPBH was easily
removed, however, by dissolving the solid in a coordi-
nating solvent such as acetone to give 11. Thermogravi-
metric analysis shows the stepwise removal of the two
coordinated acetone molecules at 130 and 230 8C, and
While exogenous axial ligation of the rhodium(II)
precursor is of paramount importance, the effect of the
bridging carboxylate groups is less distinctive; for
example, there does not seem to be much of an
advantage in using Rh2(O2CCF3)4(CH3CN)2 over
Rh2(O2CCH3)4(CH3CN)2.
The
compound
Rh2(TiPB)4(CH3CN)2 was created both from
Rh2(O2CCH3)4(CH3CN)2 in refluxing toluene and
from Rh2(O2CCF3)4(CH3CN)2 in refluxing o-xylene.
While a simple comparison cannot be made because of
the difference in solvents, there seems to be no reason
why a trifluoroacetate group should be considered less
labile than an acetate group. The fact that only partially
substituted products (8 and 9) were achieved from the
reaction between TiPBH and Rh2(O2CCF3)4(CH3CN)2
in toluene for 3 h should not be misinterpreted. The
crude product was extracted with acetone, in which the
fully-substituted product is not very soluble, and thus
the partially-substituted products were those extracted.
However, when the crude product of a similar reaction
was extracted with a nonpolar solvent, the fully-
substituted compound, 10×/1/2C6H14, was crystallized.
Fig. 6. Electronic spectra of various adducts of Rh2(O2CCPh3)4.

244
F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ246
/
Table 3
Location of band A in electronic spectrum of Rh2(O2CR)4 compounds
Media
Rh2(O2CCPh3)4 a, l (nm)
Rh2(O2CH3)4 b, l (nm)
Rh2(pfb)4 c, l (nm)
Toluene
723
587
646
605
590
537
545
689
Diethyl ether
Dichloromethane
Acetone
THF: 597
636
603
590
Ethanol
Acetonitrile
3.3% DMSO/ether
552
Neat DMSO: 500
a
This work.
Ref. [12].
b
c
Ref. [14a] (pfbꢁperfluorobutyrate).
when Rh2(TiPB)4(acetone)2 was placed under vacuum at
It has been found that when bulky incoming ligands
are used in dirhodium carboxylate exchange reactions,
mixtures of partially- and fully-substituted products are
obtained even under vigorous conditions [28]. We have
found that, in some cases, two distinctly different types
of crystals were grown simultaneously out of the same
130 8C a color change from blueꢃ/green to green to
yellowꢃgreen was observed, presumably corresponding
/
to the bis-, mono-, and non-axially ligated compounds,
respectively. The amorphous Rh2(TiPB)4 (13) was then
recrystallized from hexanes, in which it is sparingly
soluble; Rh2(TiPB)4(TiPBH)2 is much more soluble in
hexanes. Acetonitrile is a stronger donor than acetone,
reaction flask, (see preparation of 2, 4×/1.25C6H14 and
5). Thus, the separation of similar molecules becomes a
problem, and a cleaner reaction pathway was eventually
pursued in the direct reaction of the sodium salt of TiPB
and after heating Rh2(TiPB)4(CH3CN)2×
/
CH3CN, 6×
/
CH3CN, at 80 8C under vacuum overnight, we were
able to characterize crystallographically only the mono-
adduct, Rh2(TiPB)4(CH3CN) (7). However, the 1H
NMR spectrum of the amorphous product in CDCl3
showed a near-absence of acetonitrile signals, indicating
what is probably a mixture of Rh2(TiPB)4 and the
mono-adduct. It is important to note that, although the
axial positions are open for small donor molecules,
Rh2(TiPB)4 does not engage in self-association; and
hence, Rh2(TiPB)4(CH3CN) is a discrete molecule,
whereas other Rh2(O2CR)4L compounds form dimers.
with RhCl3×
/
3H2O in boiling ethanol. Although the yield
of 13 by this method was very low (15%) due to the
copious deposition of rhodium metal, only the fully-
substituted product was obtained. The crystals grown
from an acetone solution had the same unit cell and
crystal structure as Rh2(TiPB)4(acetone)2×
synthesized by the carboxylate exchange reaction.
/0.90(acetone)
3.3. Spectroscopy
The electronic spectra of Rh2(O2CR)4L2 compounds
exhibit two principal bands in the visible region; band A
(around 17 000 cmꢀ1) has been assigned to the
p*(Rh2)0
ment of band B is still debatable, although it has been
attributed to a p(RhꢀO)0s*(RhꢀO) transition [13c].
/
s*(Rh2) transition [29,13b], while the assign-
/
/
/
Band A is quite sensitive to the nature of the axial
ligand, and this is what accounts for the variety of colors
observed in Rh2(O2CR)4L2 compounds. The band
maximum increases in energy with increasing donor
strength, as the s*(Rh2) orbital is destabilized. The
electronic spectra of Rh2(TiPB)4, Rh2(TiPB)4(H2O)2,
and Rh2(TiPB)4(acetone)2 are presented in Fig. 5. The
extremely low energy (760 nm) of band A for non-
axially ligated Rh2(TiPB)4 is consistent with this inter-
pretation of the electronic structure, first proposed by
Dubicki and Martin [29] and quantitatively described by
Norman and Kolari [13a].
Fig. 6 shows the electronic spectra for a series of
Rh2(O2CCPh3)4L2 compounds. Although this com-
pound has not been structurally characterized without
Fig. 7. Electronic spectra of Rh2(O2CCPh3)4 (8.087ꢄ
toluene with various concentrations of acetonitrile.
/
10ꢀ4 M) in

F.A. Cotton et al. / Inorganica Chimica Acta 337 (2002) 233ꢃ
/246
245
axial ligands, a variety of ligands, from weakly to
strongly donating, have been coordinated to the axial
position, as evidenced by the differing positions of band
A. These data are summarized in Table 3, and are
consistent with the report by Johnson et al. for
dirhodium tetraacetate [12]. The positions of band A
for Rh2(O2CCPh3)4 in toluene and dichloromethane
(723 and 646 nm) indicate that there must be some axial
interaction of the dimetal unit with the solvent, as the
peaks are at significantly higher energy than that for the
non-axially ligated Rh2(TiPB)4. Band positions reported
for anhydrous rhodium(II) perfluorobutyrate in toluene
(689 nm) and dichloromethane (636 nm) [14a] are
somewhat higher in energy than those for
Rh2(O2CCPh3)4L2. This is not too surprising, however,
as the perfluoronated ligands would be expected to
increase the Lewis acidity at the axial position, con-
tributing to binding axial bases more tightly.
prevent association. (6) The difficulty in substitution by
a bulky ligand often does not allow the reaction to
proceed cleanly, and direct reaction between RhCl3×
/
3H2O and NaTiPB may be a superior preparative
method. (7) It appears that both bulkiness and solubiliz-
ing groups in the carboxylate ligands are necessary to
the preparation of non-axially ligated Rh2(O2CR)4
compounds that could be crystallized from non-coordi-
nating solvents. (8) Results from electronic spectroscopy
are consistent with the prevailing interpretation of the
electronic structure of Rh2(O2CR)4L2 compounds, and
an extremely low energy transition is observed when
there are no axial interactions whatsoever.
5. Supplementary material
Crystallographic data has been deposited with Cam-
bridge Crystallographic Data Centre, CCDC nos.
Fig. 7 shows the visible absorption spectra of
Rh2(O2CCPh3)4 in toluene with various concentrations
184708ꢃ184720. Copies of this information may be
/
of added acetonitrile. Three peaks are observed: (1) ꢁ
730 nm at low acetonitrile concentrations, (2) ꢁ610 nm
at moderate acetonitrile concentrations, and (3) ꢁ550
/
obtained free of charge by writing The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
/
/
(fax: ꢂ44-1223-336-033; e-mail: deposit@ccdc.cam.a-
/
nm at high acetonitrile concentrations, corresponding to
the species Rh2(O2CCPh3)4, Rh2(O2CCPh3)4(CH3CN),
and Rh2(O2CCPh3)4(CH3CN)2, respectively. The ap-
pearance of two well-defined isosbestic points indicates
that the equilibrium constants for each transformation
are well separated; thus the system can be thought of as
two essentially independent processes, shown in Eqs. (1)
and (2).
c.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the National Science Foundation for
financial support. We thank Johnson-Matthey for a
generous loan of RhCl3×
/
3H2O. E.A.H. thanks the
k1
National Science Foundation for a predoctoral fellow-
ship.
Rh2(O2CCPh3)4 ꢂCH3CNX
Rh2(O2CCPh3)4(CH3CN)
(1)
(2)
k2
Rh2(O2CCPh3)4(CH3CN)ꢂCH3CNX
Rh2(O2CCPh3)4(CH3CN)2
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