Studies of dirhodium(II) tetrakis(trifluoroacetate). Part 8. One-dimensional polymers of Rh2(O2CCF3)4 with aromatic ligands: Benzene, p-xylene and naphthalene

Article abstract of DOI:10.1016/S0277-5387(00)00472-1

The interaction of benzene, p-xylene, and naphthalene with a strong two-ended Lewis acid, Rh₂(O₂CCF₃)₄, led to one-dimensional linear polymers of 1:1 composition, [Rh₂(O₂CCF₃)₄·L]∞, where L = C₆H₆ (1), p-(CH₃)₂C₆H₄ (2), and C₁₀H₈ (3). Compounds 1-3 have been characterized by elemental analysis and IR spectroscopy and their structures have been determined by X-ray diffraction. In the crystal structures of the compounds the alternating arrangement of the dirhodium(II) tetrakis(trifluoroacetate) units with the aromatic ligands coordinated to their axial positions forms infinite chains extended along one direction of the unit cell. The rare bridging off-centered, η²:η² coordination of arene groups by the Rh(II) centers was found to be preferred in all three cases with the two closest Rh-C(arene) distances averaged to 2.662(6) A? in the benzene complex (1), to 2.684(7) A? in the p-xylene analogue (2), and to 2.588(9) A? in the naphthalene product (3). (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00472-1

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Polyhedron 19 (2000) 1829–1835
Studies of dirhodium(II) tetrakis(trifluoroacetate).
Part 8. One-dimensional polymers of Rh2(O2CCF3)4 with aromatic
ligands: benzene, p-xylene and naphthalene
F. Albert Cotton *, Evgeny V. Dikarev, Marina A. Petrukhina, Salah-Eddine Stiriba
Laboratory for Molecular Structure and Bonding, Department of Chemistry, PO Box 30012, Texas A&M Uni6ersity, College Station,
TX 77842-3012, USA
Received 30 May 2000; accepted 7 June 2000
Abstract
The interaction of benzene, p-xylene, and naphthalene with a strong two-ended Lewis acid, Rh2(O2CCF3)4, led to one-dimen-
sional linear polymers of 1:1 composition, [Rh2(O2CCF3)4·L]ꢀ, where L=C6H6 (1), p-(CH3)2C6H4 (2), and C10H8 (3). Compounds
1–3 have been characterized by elemental analysis and IR spectroscopy and their structures have been determined by X-ray
diffraction. In the crystal structures of the compounds the alternating arrangement of the dirhodium(II) tetrakis(tri-
fluoroacetate) units with the aromatic ligands coordinated to their axial positions forms infinite chains extended along one direction
of the unit cell. The rare bridging off-centered, h2:h2 coordination of arene groups by the Rh(II) centers was found to be preferred
,
,
in all three cases with the two closest RhꢀCarene distances averaged to 2.662(6) A in the benzene complex (1), to 2.684(7) A in the
,
p-xylene analogue (2), and to 2.588(9) A in the naphthalene product (3). © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Dirhodium(II) compounds; Trifluoroacetate; One-dimensional polymers; Arene coordination; p-Complexes; Crystal structures
ation (Scheme 1). A rare off-centered orientation of the
aromatic ligands at the Rh(II) centers has been found
in both dirhodium trifluoroacetate chain compounds,
with the C6Me6 and the Ph2C2 ligands. We believe
systematic research is needed in order to get more
insight into complexation of arenes by the electron-defi-
cient dirhodium(II) carboxylate molecules, as these in-
teractions have an important role in some catalytic and
aromatic substitution reactions [3]. Therefore, we have
extended the number of arene ligands to benzene, p-xy-
lene and naphthalene and report here on their interac-
tions with Rh2(O2CCF3)4. The first two ligands are
commonly used as non-coordinating solvents for study-
ing weak interactions in the Lewis acid–Lewis base
systems. Naphthalene is the first conjugated arene
molecule used for such reactions with the dirhodium
moiety. With all three ligands the products are
analogous one-dimensional linear polymers of 1:1 com-
position, namely [Rh2(O2CCF3)4·C6H6]ꢀ (1), [Rh2-
(O2CCF3)4·(p-(CH3)2C6H4)]ꢀ (2) and [Rh2(O2CCF3)4·
C10H8]ꢀ (3). They have been crystallographically char-
acterized and their structures have been compared.
1. Introduction
Dirhodium(II) tetrakis(trifluoroacetate) exhibits an
extensive axial coordination chemistry with a variety of
strong N, O, P-containing bases [1]. Our research inter-
est has been recently focused on studies of weak axial
interactions by the powerful two-ended Lewis acid,
Rh2(O2CCF3)4 [2]. We introduced a solid state deposi-
tion technique (‘solventless’ synthesis) [2] as an efficient
way to study these interactions and to avoid competi-
tive coordination of solvents in solutions. Our recent
results have included the first structurally characterized
p-complex with an aromatic ligand (hexamethylbenz-
ene), [Rh2(O2CCF3)4·(C6Me6)]ꢀ [2a] and a compound
[Rh2(O2CCF3)4·(C2Ph2)]ꢀ [2e] with both a triple car-
bonꢀcarbon bond and an arene system from the bifunc-
tional diphenylacetylene ligand involved in coordin-
* Corresponding author. Tel.: +1-979-845-4432; fax: +1-979-845-
9351.
E-mail address: cotton@tamu.edu (F.A. Cotton).
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00472-1

1830
F.A. Cotton et al. / Polyhedron 19 (2000) 1829–1835
Scheme 1.
2. Experimental
glovebox. After keeping the solution for one week at
0°C light-green plates of 2 had appeared. Yield: 0.089 g,
2.1. General
77%.
All manipulations were carried out under an atmo-
sphere of dry, oxygen-free dinitrogen by employing
Schlenk techniques. Solvents were dried and deoxy-
genated by refluxing over appropriate agents before
use. The anhydrous form of Rh2(O2CCF3)4 was pre-
pared using a literature procedure [4]. Naphthalene
(C10H8) was purchased from Aldrich and sublimed
before use. The IR spectra were recorded on a Perkin–
Elmer 16PC FTIR spectrophotometer using KBr pel-
lets. Elemental analysis was done by Canadian
Microanalytical Services, Ltd.
2.3.3. p-Xylene added to unligated Rh2(O2CCF3)4
A few drops of anhydrous p-xylene were added to
the solid unligated Rh2(O2CCF3)4 (0.10 g, 0.15 mmol)
in a Pyrex tube. The excess of solvent was removed
under vacuum. The tube was then sealed and left in a
furnace at 95°C for 2 days. Green crystals of 2 were
deposited on the walls after slow cooling of the tube
over 24 h to r.t. Yield: 0.046 g, 40%.
Anal. Calc. for C16H10F12O8Rh2: C, 25.25; H, 1.32.
Found: C, 24.98; H, 1.32%. IR (KBr, cm−1): 1665.6 (s),
1191.4 (s), 868.4 (s), 840.9 (m), 794.9 (m), 739.4 (s),
540.8 (s).
2.2. Synthesis of Rh2(O2CCF3)4·C6H6 (1)
2.4. Synthesis of Rh2(O2CCF3)4·C10H8 (3)
A few drops of distilled benzene were added to the
solid unligated Rh2(O2CCF3)4 (0.10 g, 0.15 mmol) in a
Pyrex tube. The excess of benzene was removed under
vacuum. The tube was then sealed and left in a furnace
at 90°C for 24 h. Green crystals of 1 were deposited on
the walls after slow cooling of the tube over 48 h to r.t.
Yield: 0.030 g, 27%.
Unligated Rh2(O2CCF3)4 (0.066 g, 0.10 mmol) was
mixed with freshly sublimed naphthalene (0.012 g, 0.10
mmol) under an inert atmosphere. The mixture was
sealed in an evacuated Pyrex tube and the tube was
placed in a furnace at 90°C. Green block-shaped crys-
tals of 3 were deposited over two days in the ‘cold’ zone
of the tube, where the temperature was about 5°C
lower. Yield: 0.027 g, 35%. Anal. Calcd. for
C18H8F12O8Rh2: C, 27.50; H, 1.03. Found: C, 27.47; H,
1.03%. IR (KBr, cm−1): 1683.9 (m), 1669.6 (s), 1653.0
(m), 1558.3 (s), 1540.0 (m), 1506.4 (m), 1456.7 (m),
1192.2 (s), 1167.5 (s), 861.0 (m), 784.8 (m), 739.8 (s),
525.9 (m), 457.8 (m), 418.0 (m).
2.3. Synthesis of Rh2(O2CCF3)4·(p-(CH3)2C6H4) (2)
2.3.1. Rh2(O2CCF3)4 in benzene containing p-xylene
The solution containing Rh2(O2CCF3)4 (0.10 g, 0.15
mmol) in 5 ml of benzene containing p-xylene (the ratio
of p-(CH3)2C6H4 to C6H6 was found by NMR to be
about 1:100) was layered with 10 ml of hexanes at r.t.
This resulted in deposition of a few crystals of 2 in a
week.
2.5. X-ray structure determinations
Single crystals of compounds 1–3 were obtained as
described above. X-ray diffraction experiments were
carried out on a Nonius FAST diffractometer with an
area detector using Mo Ka radiation. Details concern-
2.3.2. Rh2(O2CCF3)4 in p-xylene
The vial containing solution of Rh2(O2CCF3)4 (0.10
g, 0.15 mmol) in 5 ml of p-xylene was placed in the Ar

F.A. Cotton et al. / Polyhedron 19 (2000) 1829–1835
1831
3. Results and discussion
ing data collection have been fully described elsewhere
[4]. Each crystal was mounted on the tip of a quartz
fibre with silicone grease, and the set-up was quickly
placed in the cold N2 stream (−60°C) of a low-temper-
ature controller. Fifty reflections were used in the cell
indexing and about 200 reflections in the cell
refinement.
3.1. Synthesis
Our experimental procedure included both the solid
state deposition approach (1–3) and solution synthesis
(2). Preparations of 1 and 2 are good examples of using
a solid deposition technique for studying interactions of
Rh2(O2CCF3)4 with liquid ligands, benzene and p-xy-
lene in this case. The key factor in this synthesis is the
use of extremely small amounts of suitable solvent,
almost close to the equimolar ratio of the arene to the
dirhodium molecules. This solid state approach was
recently shown to be an effective route for obtaining
unusual structures and compositions unattainable in
solutions for the systems containing solvents, for exam-
ple, Rh2(O2CCF3)4–THF [2b] and Rh2(O2CCF3)4–
DMSO [2d]. The fact that dirhodium trifluoroacetate
was forming an adduct with benzene became obvious a
few years ago [4], but at that time suitable crystals had
not been obtained from solution. The solid state ap-
proach (Section 2.2) applied here for benzene produced
crystals of Rh2(O2CCF3)4·C6H6 (1) in moderate yield
and of quality suitable for X-ray diffraction study.
All calculations were done on a DEC Alpha running
VMS. The coordinates of rhodium and oxygen
atoms for the structures were found in direct method E
maps using the structure solution program SHELXTL
[5]. The positions of the remaining atoms were located
after an alternating series of least-squares cycles and
difference Fourier maps using the SHELXL-93 package
[6]. The fluorine atoms of all CF3 groups were found
to be disordered over two or three different rotational
orientations. Anisotropic displacement parameters
were assigned to all non-hydrogen atoms, except the
disordered fluorine atoms. Hydrogen atoms of aromatic
ligands were included in the structure factor
calculations at idealized positions. Relevant crystallo-
graphic data for compounds 1–3 are summarized in
Table 1.
Table 1
Crystal data and details on data collection and refinement for Rh2(O2CCF3)4·C6H6 (1), Rh2(O2CCF3)4·(p-(CH3)2C6H4) (2) and
Rh2(O2CCF3)4·C10H8 (3)
1
2
3
Formula
Crystal system
Space group
C14H6F12O8Rh2
monoclinic
P21/n (No. 14)
8.2818(5)
15.624(2)
8.574(2)
C16H10F12O8Rh2
monoclinic
P21/n (No. 14)
8.3121(9)
15.896(6)
8.6884(9)
C18H8F12O8Rh2
triclinic
(
P1 (No. 2)
,
a (A)
8.573(2)
,
b (A)
9.069(9)
9.332(6)
69.71(2)
63.50(2)
77.62(3)
607.5(7)
1
,
c (A)
h (°)
i (°)
k (°)
103.81(1)
91.59(1)
3
,
V (A )
1077.4(3)
2
1147.5(5)
2
Z
Crystal size (mm)
Radiation Mo Ka (u, A)
Temperature (°C)
0.25×0.15×0.05
(0.71073)
−60
0.23×0.20×0.03
(0.71073)
−60
0.25×0.12×0.10
(0.71073)
−60
,
Data collection range, 2q (°)
Reflections collected/unique
Data/observ/restraints/params
5.22–45.00
5.12–45.48
4.80–45.10
3303/1524 (Rint=0.0672)
1524/1409/12/175
0.0546, 0.1382
0.0603, 0.1517
1.107
4830/1382 (Rint=0.0515)
1382/1272/30/157
0.0381, 0.0891
0.0420, 0.0933
1.116
4750/1512 (Rint=0.0391)
1512/1410/54/191
0.0403, 0.0957
0.0436, 0.1004
1.119
b
R1 a, wR2 [I]2|(I)]
b
R1 a, wR2 (all data)
Goodnes-of-fit c
−3
,
Largest difference peak and hole (e A
)
1.004 and −0.574
0.735 and −0.589
1.201 and −0.813
a R1=S(ꢀFoꢀ−ꢀFcꢀ)/SꢀFoꢀ.
b wR2=[S[w(Fo2−Fc2)2]/S[w(Fo2)2]]1/2
.
c Goodness-of-fit=[S[w(F2o−Fc2)2]/(Nobservns−Nparams)]1/2, based on all data.

1832
F.A. Cotton et al. / Polyhedron 19 (2000) 1829–1835
weak interactions in so-called ‘noncoordinating’ sol-
vents such as benzene, toluene or xylenes, which can be
considered as weak Lewis bases themselves, as we
clearly observed for the systems containing Rh2-
(O2CCF3)4. Similar observations were made when
studying weakly coordinating anions, which have about
the same nucleophilicity toward various cations as tolu-
ene [7]. Interestingly, in the case of p-xylene ligand,
both solution and solid state techniques yielded the
same product, the chain polymer of 1:1 composition in
which the aromatic ring is acting as a bidentate ligand,
while for THF and DMSO [2b,d], solution reactions
always resulted only in the formation of the discrete
bis-adduct molecules with monodentate coordination of
ligands.
Fig. 1. Fragment showing the alternating arrangement of
Rh2(O2CCF3)4 and C6H6 in the chain structure of 1. Rhodium and
oxygen atoms are represented by thermal ellipsoids at the 40%
probability level. Carbon, fluorine and hydrogen atoms are shown as
spheres of arbitrary radii. Axial contacts to Rh atoms are drawn by
dashed lines. Only rhodium atoms are labeled for clarity.
The solid state deposition approach (‘solventless’
technique, indeed, in this case) has been used (Section
2.4) for studying complexation of Rh2(O2CCF3)4 with
the solid sublimable ligand, naphthalene, to give crys-
tals of Rh2(O2CCF3)4·C10H8 (3). In contrast to the
single ring aromatic ligands, benzene and p-xylene,
naphthalene, the simplest compound having fused
arene rings, was used for coordination with Rh2-
(O2CCF3)4. From previous reports on transition-metal–
naphthalene complexes, (h6-naphthalene) sandwiches
containing a metal in the zero-valent state (Cr, Mo, etc)
have been traditionally prepared by metal vapor syn-
thesis technique [8]. Recently, the naphthalene radical
anion reductions of a variety of transition metal precur-
sors have been introduced as a convenient method for
the synthesis of anionic bis or tris(naphthalene) metal
complexes [9], with examples such as [(h6-C10H8)2Ti]2−
[9b], or structurally characterized [(h4-C10H8)3Zr]2−
[9c]. Some mixed ligand naphthalene complexes have
been also obtained using this technique, as for example
[Ru(h6-C10H8)(1,5-cyclooctadiene)] [10] and [Fe(h6-
C10H8)(Cy2PC2H4PCy2)] [11].
Fig. 2. Crystal packing of 1D chains of 2 viewed along the [100]
direction of the unit cell.
3.2. Crystal structures
The formation of complex 2, Rh2(O2CCF3)4·(p-
(CH3)2C6H4), was first observed accidentally in the
benzene–hexane solution system (Section 2.3.1). The
presence of p-xylene was explained by the fact that it is
added to both benzene and hexanes stills at the stage of
the preparation the NaꢀK amalgam. Later the yield of
2 was increased by using neat p-xylene as a solvent
(Section 2.3.2); in addition, product 2 was also obtained
by the solid state approach (Section 2.3.3). The first
synthesis of 2 (Section 2.3.1) is an interesting example
of a competition in solution of weak p-donor ligands
for coordination by the strong Lewis acid,
Rh2(O2CCF3)4. The p-xylene, being a stronger donor
than benzene (even though present in solutions as a
minor component), binds preferentially to axial posi-
tions of dirhodium trifluoroacetate units to form 2.
This fact should be taken into account when studying
Three new complexes 1–3 have been obtained as
green crystals, which were extremely unstable for 1 (loss
of benzene), relatively stable for 3, with 2 being an
intermediate case; any exposure to solvent media led to
the solvolytic cleavage of 1–3. All products have been
examined by X-ray crystallography and the 1:1 compo-
sition was confirmed by elemental analysis. All three
structures consisted of the dirhodium(II) tetrakis(tri-
fluoroacetate) molecules alternating with aromatic lig-
ands, C6H6 (1), p-(CH3)2C6H4 (2), and C10H8 (3), which
are bound at axial positions of the Rh2 units thus
forming linear one-dimensional chains (Fig. 1). These
chains lying parallel in the crystal are extended in the
structure along the [100] direction ([001] in the case of
3) of the unit cell (Fig. 2). Selected distances and angles
for 1–3 are given in Table 2. The coordination of arene

F.A. Cotton et al. / Polyhedron 19 (2000) 1829–1835
1833
Table 2
Selected distances (A) and angles (°) for [Rh2(O2CCF3)4·arene] coordination polymers
,
Arene
RhꢀRh
RhꢀCarene
Rhꢀarenecenter
CꢀCaver
RhꢀRhꢀCarene
Ref.
C6H6
(CH3)2C6H4
C10H8
(CH3)6C6
(C6H5)2C2
2.4115(9)
2.417(1)
2.422(2)
2.422(1)
2.4256(6)
2.646(6), 2.678(6)
2.598(7), 2.770(7)
2.609(9), 2.567(9)
2.770(6), 2.787(6)
2.696(5), 2.750(6)
2.98
2.99
2.97
2.98
3.06
1.39(1)
1.39(1)
1.39(1)
1.40(1)
1.369(9)
164.2(2), 164.1(2)
167.6(2), 161.1(2)
164.1(2), 166.1(2)
165.4(1), 164.1(1)
165.1(1), 164.7(1)
this work
this work
this work
[2a]
[2e]
ligands in 1–3 was found to be off-center (Fig. 3) as in
the cases of the C6Me6 and the Ph2C2 dirhodium com-
plexes reported earlier [2a,e]. For benzene and p-xylene
ligands (1 and 2) two opposite edges of the arene ring
approach the axial sites of the neighboring dirhodium
dimers in the h2(1,2):h2(4,5) bridging fashion. For
the single-ring aromatic ligands, in addition to the
common h6-binding mode, there are only a few exam-
ples with benzene and p-xylene ligands bonding two
metal centers in the off-centered way, for example,
Scheme 2.
thalene mentioned above, the most common for the
discrete transition metal complexes obtained so far,
were h4 and h6. Several different bridging, m2 modes of
naphthalene have been found [16–18] in a number of
the multidecker compounds with different d- and f-
block transition metals: CpV(m2-h6:h6-C10H8)VCp
[16a], [CpV(m2-h6:h2-C10H8)Yb(THF)Cp]n (Cp=h5-
C5H5) [16b], (C6H6)Ru(m2-h4:h6-C10H8)Cr(CO)3 [17].
The two closest RhꢀCarene distances average to
[(tBu3SiO)3Ta]2[m-h2(1,2):h2(4,5)-C6H6]
[12]
and
[(Cy2PC2H4PCy2)Co]2[m-h3:h3-C6H4(CH3)2] [13]. There
was also a systematic study of the Ag(I)-aromatic inter-
actions, including benzene, xylenes and naphthalene
ligands [14], which were all examples of an h2 off-cen-
tered donation of p-electrons to Ag+, including one
unusual h1-benzene compound [15].
In complex 3 both arene rings of the naphthalene
molecule are engaged in coordination to the different
Rh(II) centers in the h2(1,2):h2(5,6) mode (see number-
ing in Scheme 2, Fig. 4). This is a rare unsymmetrical
mode for the m2-naphthalene in transition metal com-
plexes [16–18]. The two coordination modes of naph-
,
2.662(6) in (1), 2.684(7) in (2) and to 2.588(9) A in (3)
indicating the tightest metal–arene interaction in the
naphthalene compound. At first glance, this contrasts
to the general assumption that naphthalene binds less
strongly to a metal than benzene [19], and to the data
on bond-dissociation energies via calorimetric methods
revealing that the naphthaleneꢀCr bond in the Cr(CO)3
complex is 6 kcal mol−1 weaker than the benzeneꢀCr
bond [8c]. However, it is worth emphasizing that those
data were valid for the discrete h6-compounds, while in
the chain polymers 1 and 2 each arene ring coordinates
two metal centers, and in 3 different rings of naphtha-
lene are involved in coordination. This means that axial
Fig. 3. The view almost down the RhꢀRh axis of the
Rh2(O2CCF3)4·(p-(CH3)2C6H4) unit in the chain structure of 2.
Rhodium and oxygen atoms are represented by thermal ellipsoids at
the 40% probability level. Carbon, fluorine and hydrogen atoms are
shown as spheres of arbitrary radii. Two closest axial contacts to Rh
atoms are drawn by dashed lines.
Fig. 4. Fragment showing the alternating arrangement of
Rh2(O2CCF3)4 and C10H8 in the chain structure of 3. Rhodium and
oxygen atoms are represented by thermal ellipsoids at the 40%
probability level. Carbon, fluorine and hydrogen atoms are shown as
spheres of arbitrary radii. The shortest RhꢀCarene contacts are drawn
by dashed lines. Only rhodium atoms are labeled for clarity.

1834
F.A. Cotton et al. / Polyhedron 19 (2000) 1829–1835
here. The present results show that the electronic prop-
erties of aromatic ligands in donor–acceptor complexes
with Rh2(O2CCF3)4 are not simply correlated with their
coordination strength. For example, the relatively
strong donor, hexamethylbenzene, has the longest
,
RhꢀCarene contacts (averaged to 2.779(6) A [2a]) among
,
the arenes studied (Table 2). The difference, 0.19 A,
in these two shortest RhꢀCarene contacts in
[Rh2(O2CCF3)4·(C6Me6)]ꢀ compared to the naphtha-
,
lene analogue (2.588(9) A) is significant, while the
RhꢀRh distances are exactly the same for two com-
pounds. Obviously, packing forces and other stereo-
chemical factors are very important in these
arene–metal interactions, capable of overriding elec-
tronic factors.
In 3 the naphthalene molecule approaches the
rhodium atoms via the 1,2 and 5,6 CꢀC bonds. This
could well be an electronic effect since it is these bonds
that have the highest p-bond order, 0.73, compared to
0.61 for the 2,3 and 6,7 bonds (Scheme 2), according to
Hu¨ckel calculations [22].
Scheme 3.
contacts to arene are expected to be shorter in 3 than in
1 and 2. Actually the Rhꢀarene center distances are
very close, being in the range of 2.97–2.99 A, for all
dirhodium polymers mentioned in Table 2, except in
the diphenylacetylene compound where the distance is
3.06 A. All of the arene complexes, 1–3, demonstrate
,
,
asymmetry in the two nearest metal–carbon distances.
The difference in these two RhꢀCarene contacts is the
,
most significant (0.172 A) in complex 2 with p-xylene
,
compared to the benzene (0.032 A) and to the naphtha-
3.3. Conclusions
,
,
lene (0.042 A) polymers. It was 0.017 A in the
[Rh2(O2CCF3)4·C6Me6]ꢀ compound [2a].
In summary, complexation of p-arene ligands, ben-
zene, p-xylene and naphthalene, with the dirhodium
unit has been examined by preparing a number of 1:1
coordination polymers with Rh2(O2CCF3)4. It is found
that dirhodium tetrakis (trifluoroacetate) exhibits a se-
lective coordination toward weak donor bases, such as
benzene and p-xylene, in solutions. The crystalline com-
plexes 1–3 have a one-dimensional framework con-
structed by the dirhodium units and arene ligands
bound to its axial positions through two carbon atoms
at each end. The geometry of coordinated arene bridges
is unsymmetrical, an h2:h2 type, in all cases. Complex
3, [Rh2(O2CCF3)4·(C10H8)]ꢀ constitutes the first exam-
ple of such products with a dicyclic arene system. We
think that related and novel compounds with polycyclic
aromatic ligands having an extended delocalized p-sys-
tem will generally be accessible by the technique intro-
duced, and studies on arene ligands with multi-
coordination-site possibilities are under way.
The unsymmetrical h2:h2 complexation of arene
groups (Scheme 3a) seems to be preferred for the
dirhodium(II) trifluoroacetate one-dimensional poly-
mers based on the several examples obtained so far.
This contrasts to the dichromium carboxylates with
coordinated aromatic ligands of the 1:1 composition,
including benzene and p-xylene analogues, in which
arene rings were lying centrically on the line between
two dimetal units and thus having a h6 (Scheme 3b)
symmetrical coordination [20]. In [Bi2(O2CCF3)4·
C6Me6]ꢀ, the only arene adduct of a main group ele-
ment tetracarboxylate, the C6Me6 molecules are also
acting in an h6 coordination mode [21].
The unsymmetrical complexation of aromatics in 1–3
causes no detectable distortion of the arene rings. The
mean CꢀC distances are the same, averaged to 1.39(1)
in 1–3.
There was an attempt to compare the inductive effect
of different substituents of arene ligands on the CrꢀCr
and the Crꢀring center distances [20]. One might expect
that the stronger the nucleophile substituent is, the
stronger is the interaction of an axial ligand with the
dimetal moiety (which, in turn, would result in the
elongation of the metalꢀmetal distance). However, there
was no simple relationship found for a number of
relative compounds between the ionization potentials of
arenes and the CrꢀCr distances [20]. The latter was
shown to be influenced by other variables, packing
forces being the most important one. This is also the
case for a number of dirhodium tetrakis(trifluoroac-
etate) chain polymers with coordinated arenes studied
4. Supplementary Material
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre CCDC Nos. 140762, 140763 and 140764 for
compounds 1, 2 and 3, respectively. Copies of the data
can be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).

F.A. Cotton et al. / Polyhedron 19 (2000) 1829–1835
1835
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Acknowledgements
This work was supported by the Robert A. Welch
Foundation. We thank the Johnson Matthey Company
for the loan of rhodium.
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