New dirhodium(II,II) species as building corner connectors for square molecular boxes

Article abstract of DOI:10.1016/S0020-1693(02)01171-4

The new mixed dirhodium(II,II) complexes, [Rh₂(form)₂(O₂CC₆H ₄CN)₂] (2) and [Rh₂(form)₂(O₂CC₅H ₄N)₂] (3) (form=N,N′-di-p-to

Full text of DOI:10.1016/S0020-1693(02)01171-4

Inorganica Chimica Acta 343 (2003) 351ꢂ356
/
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Note
New dirhodium(II,II) species as building corner connectors for square
molecular boxes
Sandra Lo Schiavo *, Francesco Nicolo`, Giuseppe Tresoldi, Pasquale Piraino *
`
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita di Messina, Salita Sperone n. 31, 98166 Vil. S. Agata, Messina,
Italy
Received 25 March 2002; accepted 30 May 2002
Abstract
The new mixed dirhodium(II,II) complexes, [Rh2(form)2(O2CC6H4CN)2] (2) and [Rh2(form)2(O2CC5H4N)2] (3) (formꢀN,N?-di-
/
p-tolylformamidinate) as building corner connectors for heteronuclear molecular boxes, were synthesized. By exploiting the peculiar
equatorial reactivity of [Rh2(form)2(O2CCF3)2(H2O)2] (1), due to the lability of trifluoroacetates, the synthesis of 2 and 3 was
readily achieved by mathematical relations with the exo-functionalized 4-cyanobenzoate and isonicotinate carboxylates,
respectively. The X-ray diffraction studies performed on the pyridine adduct of 2, [Rh2(form)2(O2CC6H4CN)2(Py)2], are reported,
together with some preliminary attempts of self-assembly with square-planar platinum(II) and palladium(II) complexes.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Dirhodium compounds; Molecular square box; Crystal structures
1. Introduction
etc.) as angular building blocks [3ꢂ
/
5], leading to a series
of square boxes having the cis-M2(form)2 (formꢀ
/
Self-assembly by transition metal complexation is
recognized today as one of the most promising synthetic
strategy of access to well defined and highly ordered
molecular architectures [1]. Among others, this method
was successfully employed for the rational construction
of a large variety of cyclic molecular polygons, the most
popular of which is represented by the so called
‘molecular square boxes’. Besides their interesting mo-
lecular structures, transition metal incorporating-macro-
cycles are of interest because they may exhibit a wide
variety of physicochemical and functional properties,
which make them particularly attractive for applications
in the field of material science and nanotechnology [2].
Most of the examples of molecular boxes so far
reported mainly involve the use of square-planar or
octahedral mononuclear transition metals (Pd, Pt, Re,
formamidinate anion) units as angular components
and linear or angular dicarboxylates as connectors.
Square boxes involving dimetal species as exo-bidentate
connectors are also accessible by exploiting the classical
reactivity of dimetal complexes with lantern structure.
By combination of [Rh2(form)2(O2CCF3)2(H2O)2] (1)
with cis-dipyridyl-diphenyl-porphyrin (cis-DPyP), we
recently succeeded in preparing the [Rh2(form)2(CF3-
CO2)2(cis-DPyP)]4 molecular square boxes [6]. Due to
the well known redox activities of such dimetal com-
plexes as well as of porphyrins, these metallomacro-
cycles represent, among others, interesting examples of
multicomponent redox-systems.
Although such a synthetic approach allows to build
square boxes of different sizes and nuclearity depending
on the geometry and length of the dicarboxylate
connectors [3,4], it leads, in any case, to only neutral
and homonuclear boxes, although charges may be
potentially introduced, stepwise, by oxidation of the
dimetal units.
Os, Ru) [1]. Recently, this synthetic strategy was
extended to the use of dimetal units (Mo24ꢁ, Rh2
4ꢁ
,
In order to open new opportunities in the field of
dirhodium containing-molecular boxes, we planned the
synthesis of new mixed dirhodium(II,II) molecules as
* Corresponding authors. Tel.: ꢁ
756
E-mail address: loschiavo@chem.unime.it (S. Lo Schiavo).
/
39-090-391 961; fax: ꢁ39-090-393
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 7 1 - 4

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S. Lo Schiavo et al. / Inorganica Chimica Acta 343 (2003) 351ꢂ356
/
angular building blocks suitable for the self-assembly of
macrocyclic squares, neutral or charged, incorporating
further transition metals ions.
measurements were performed with a Bruker AMX
300 spectrometer using standard pulse sequences.
This aim may be realized by the introduction in the
dirhodium lantern structure of two functionalized
carboxylates, namely the 4-cyanobenzoate (O2C-
C6H4CN) and isonicotinate (O2CC5H4N) groups.
These, when cis-symmetrically bridged to the dirhodium
core via the carboxy moieties, are featured by the two
dangling cyano-benzoate and pyrydine groups in cis-
orientation and ready for further coordination.
In the present paper we report on the synthesis and
charactherization of [Rh2(form)2(O2CC6H4CN)2] (2)
and [Rh2(form)2(O2CC5H4N)2] (3), together with the
X-ray diffraction studies performed on the pyrydine
adduct [Rh2(form)2(O2CC6H4CN)2(Py)2] (2a). These
unambiguously confirm that the two formamidinate
and the functionalized carboxylate groups are symme-
2.2. Syntheses
2.2.1. [Rh2(form)2(O2CC6H4CN)2] (2)
To an acetoneꢂ
4-cyanobenzoate (C8H4NO2Na) (0.148 g, 0.872 mmol)
was added a chloroformꢂacetone solution (1:2, 40 ml)
/
water solution (1:2, 50 ml) of sodium
/
of 1 (0.200 g, 0.218 mmol) and the resulting mixture left
to stir for approximately 24 h. After this time the
volatiles were removed by rotavapor and the resulting
green solid was collected by filtration, washed several
times with warm water and acetone and then dried
under vacuum. Yield 90%. Rh2C46H38N6O4Rh2: Anal.
Calc. C, 58.49; H, 4.05; N, 8.9. Found: C, 58.43; H, 4.15;
N, 8.81%. IR (KBr pellet, cmꢃ1): nasym(CO2) 1624 (s),
trically bridged in a cisoid arrangement around the
4ꢁ
n(NÃ
/
CÃ/N) 1591 (s), n(CN) 2229 (m).
Rh2
core, so that the overall molecule displays the
wished shape of an angular component for the assembly
of square boxes.
2.2.2. [Rh2(form)2(O2CC6H4Ã
/
CN)2(Py)2] (2a)
Both the species, 2 and 3, may potentially coordinate,
via the two pendant cyano-benzoate and pyrydine
groups, to square-planar transition metal complexes
affording heteronuclear square macrocycles, neutral or
charged, in which the dirhodium moieties are alternated
to different metal ions. Depending on the geometry (cis-
or trans-) of the transition metals, square-shaped boxes
at different nuclearity could be obtained: (i) macrocycles
in which, at the corners, dirhodium units are alternated
to different metal ions, or (ii) macrocycles bearing at the
corners dirhodium units and transition metals as linear
connectors.
To a benzene mixture (20 ml) of 2 (0.100 g, 0.102
mmol) was added 0.5 ml of pyridine (Py). Immediately
the starting green solid dissolved affording a redꢂorange
/
solution. Then n-heptane was added and the resulting
mixture left on standing for 2 days. During this time 2a
precipitated as dark orange microcrystals. Yield 90%.
C56H48N8O4Rh2: Anal. Calc. C, 60.99; H, 4.39; N,
10.16. Found: C, 60.64; H, 4.51; N, 10.05%. IR (KBr
pellet, cmꢃ1): nasym(CO2) 1630 (s); n(NÃ
/
CÃ
n(CN) 2229 (m). H NMR (pyridine-d5) d (ppm): 2.11
(s, 12H), 7.1 (dd, 16H, Jꢀ56.8 and 7.9 Hz), 7.61 (4H,
Jꢀ8.1 Hz), 7.72 (t, br, 2H), 8.31 (d, 4H, Jꢀ8 Hz).
/
N) 1593 (s),
1
/
/
/
Our preliminary attempts of self-assembly of 2 and 3
with cis- and trans-square-planar transition metal (Pd,
Pt) complexes are also reported.
2.2.3. [Rh2(form)2(O2CC5H4N)2] (3)
Complex 3 was obtained as a green solid by following
the same procedure as used for 2. Sodium isonicotinate
(C6H4NO2Na) was used as a salt source. The crude solid
2. Experimental
was recrystallized from chloroformꢂheptane (3:1) af-
/
fording 4 as a dark green precipitate. Yield 90%.
C42H38N6O4Rh2: Anal. Calc. C, 56.34; H, 4.32; N,
8.57. Found: C, 56.42; H, 4.34; N, 8.45%. IR (KBr
2.1. Materials and equipment
pellet, cmꢃ1): nasym(CO2) 1622 (s); n(NÃ
/
CÃ/N) 1596 (s).
The starting complex [Rh2(form)2(CF3COO)2(H2O)2]
was prepared according to literature data [7]. All other
chemicals are commercially available and used as
supplied. None of the compounds reported here is air
sensitive, but all reactions were carried out under an
atmosphere of dry nitrogen. Elemental analyses were
performed by REDOX snc Laboratorio di Microanalisi,
Cologno Monzese (Milano), Italy.
2.2.4. [Rh2(form)2(O2CC5H4N)2(Py)2] (3a)
Complex 3a was prepared by following the same
procedure used as for 2a. Yield 90%. C52H48N8O4Rh2:
Anal. Calc. C, 59.21; H, 4.59; N, 10.62. Found: C, 59.42;
H, 4.61; N, 10.65%. IR (KBr pellet, cmꢃ1): nasym(CO2)
1
N) 1594 (s). H NMR (pyridine-d5) d
Infrared spectra were recorded on KBr pellets with a
1624 (s); n(NÃ
/
CÃ
/
Perkinꢂ
sorption spectra were recorded on a Perkinꢂ
Lambda 5 UVꢂVis spectrophotometer. The NMR
/Elmer FT 1720X spectrometer. Electronic ab-
(ppm): 2.11 (s, 12H); 7.05 (dd, 16H, Jꢀ
7.72 (t, br, 2H); 8.01 (d, Jꢀ8 Hz); 8.60 (d, 4H, Jꢀ
Hz).
/55.1, 7.8 Hz);
/
Elmer
/
/
8
/

S. Lo Schiavo et al. / Inorganica Chimica Acta 343 (2003) 351ꢂ
/
356
353
2.3. Single-crystal X-ray diffraction studies of
[Rh2(form)2(O2CC6H4CN)2(Py)2 (2a)
3. Results and discussion
The present synthetic approach to dirhodium(II,II)
based molecular square boxes involves: (i) the synthesis
of new dirhodium(II,II) molecules which can act as
angular units; (ii) their reactions with square-planar
transition metal [palladium(II) and platinum(II)] com-
plexes having cis- or trans-labile ligands.
2.3.1. Crystal data
C56H48N8O4Rh2×
/
C6H6, fwꢀ
/
1180.95, orthorhombic,
˚
˚
C2221 (ITC N. 20), aꢀ
/
17.484(7) A, bꢀ
/
17.930(6) A,
4, Dcalc 1.175 mg
0.540 mmꢃ1, R1ꢀ
0.122/0.134 for 4065 [I ꢀ2s(I)]/
all 4560 independent reflections, GOFꢀ0.839 for 57
restraints and 341 parameters.
Crystals suitable for X-ray analysis were obtained by
re-crystallization from a mixture benzeneꢂpyridineꢂn-
heptane (30 ml, 1:1:1). Diffraction data were collected at
3
˚
˚
cꢀ
ꢃ3, F(000)ꢀ
0.041/0.049 and wR2ꢀ
/
21.298(9) A, Vꢀ
/
6677(4) A , Zꢀ
/
ꢀ
/
m
/
2416, m(Mo Ka)ꢀ
/
/
3.1. Synthesis and characterization of
[Rh2(form)2(O2CC6H4CN)2] (2) and
[Rh2(form)2(O2CC5H4N)2] (3) dirhodium corner
species
/
/
/
/
/
The typical equatorial reactivity of [Rh2(for-
m)2(O2CCF3)2(H2O)2] (1) (formꢀN,N?-di-p-tolylfor-
mamidinate) was exploited to prepare the new
dirhodium corner species [Rh2(form)2(O2CC6H4CN)2]
/
room temperature (r.t.) from a brune 0.35ꢄ
/
0.40ꢄ0.46
/
mm3 prismatic crystal sample by using a Siemens P4
automated four-circle single-crystal diffractometer with
(2) (O2CC6H4CNꢀ
m)2(O2CC5H4N)2] (3) (O2CC5H4Nꢀ
/
4-cyanobenzoate) and [Rh2(for-
graphite-monochromated Mo Ka radiation (lꢀ
/
/isonicotinate)
˚
0.71073 A). Lattice parameters were obtained from
least-squares refinement of the setting angles of 37
[15]. By treatment of 1 with an excess of sodium 4-
cyanobenzoate or isonicotinate, as described in Section
2, complexes 2 and 3 are obtained in high yield.
Alternatively, they may be also conveniently prepared
by using the partially solvatated [Rh2(form)2-
(CH3CN)6](BF4)2 as dirhodium starting complex [16],
which is more easily accessible than 1. Complexes 2 and
3 are both stable green solids, which gave satisfactory
elemental analyses. Their characterization was accom-
plished by IR and proton NMR spectroscopic studies
reflections within 25
(4692) were measured by the v scan technique up to
2uꢀ548. A crystal decay was evidenced by the 19%
/
2u 5
/
318 range. Reflections
/
decreasing in intensity of the check reflections moni-
tored each 197 measurements. Intensities were evaluated
by profile fitting of a 96 steps peak scan among 2q shells
procedure [8] and then corrected for Lorentz polariza-
tion effects and crystal decay. Absorption correction
was applied by integration method. Data-collection and
reduction has been performed by XSCANS [9] and
SHELXTL package [10]. The structure was solved by a
combination of standard Direct Methods [11] and
Fourier synthesis, and refined by minimizing the func-
while the pyrydine adduct of 2, [Rh2(form)2(O2CÃ
/
2
2 2
tion a w(Fo ꢃ
/
Fc ) with the full-matrix least-square
technique based on all 4560 independent F2 [Rint
ꢀ
/
0.0176], by using SHELXL-97 [12]. All non hydrogen
atoms were refined anisotropically. Hydrogen atoms
were located on the difference Fourier maps and
included in the model refinement among the ‘riding
model’ method with the XÃ
/
H bond geometry and
isotropic displacement parameter depending on the
parent atom X. One disordered C6H6 molecule was
found on a twofold axis and was splitted in two
staggered positions (with occupancy 0.52 and 0.48,
respectively) by using some soft distance restraints.
The hydrogen atoms of this co-crystallized solvent unit
were not included into the model due to its disorder.
The absolute configuration parameter [13] converged
to ꢃ
/
0.01(5). The last difference map showed the largest
˚
electron density residuals within 1.2 A from the rhodium
0.904 e Aꢃ3).
˚
0.600 and ꢃ
Fig. 1. View of the complex [Rh2(form)2(O2CC6H4CN)2(Py)2] (2a)
showing the atom numbering scheme of the asymmetric unit denoted
by the shaded atoms and solid bonds. The co-crystallized C6H6 moiety
is omitted for clarity. Thermal ellipsoids are drawn at 10% of
probability, while H size is arbitrary.
atom (max and min rangeꢀ
/
/
The final geometrical calculations and drawings were
carried out with the PARST program [14] and the XPW
utility of the Siemens package, respectively.

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S. Lo Schiavo et al. / Inorganica Chimica Acta 343 (2003) 351ꢂ
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356
C6H4CN)2(Py)2] (2a), was analyzed by an X-ray diffrac-
tion (Fig. 1). The solid IR spectra of 2 and 3 appear very
similar, and experience nasym(CO2) [1624 and 1622
Table 1
˚
Selected bond lengths (A) and angles (8) for [Rh2(for-
m)2(O2CC6H4CN)2(Py)2] (2a)
cmꢃ1, for 2 and 3, respectively] and n(NÃ
/
CÃ/N) [1593
Bond lengths
and 1596 for 2 and 3, respectively] absorbtions very
close to those of the parent complex 1 [7], indicating that
the two formamidinate and carboxylate groups are cis-
RhÃRh?
RhÃN(4)?
RhÃO(1)
N(3)ÃC(28)
N(3)ÃC(14)
C(17)ÃC(20)
O(1)ÃC(1)
C(1)ÃC(2)
C(8)ÃN(1)
2.469(1)
2.016(5)
2.072(4)
1.311(7)
1.442(7)
1.55(1)
RhÃN(2)
RhÃN(3)
2.296(5)
2.031(5)
2.093(4)
1.310(7)
1.418(7)
1.51(1)
RhÃO(2)?
N(4)ÃC(28)
N(4)ÃC(21)
C(24)ÃC(27)
O(2)ÃC(1)
C(5)ÃC(8)
4ꢁ
symmetrically bridged across the Rh2 core.
In contrast, 2 and 3 differ significantly in solubility. In
fact, while 2 is insoluble in the most common solvent, 3
dissolves well in chloroform. Nevertheless the 1H NMR
solution characterization of 3 was prevented as its
spectra, ran in a large range of temperatures, exhibit
broad signals for all protons, suggesting that some
dynamic process is operating in solution Further, from
chloroform solution of 3, left at air on standing for
approximately 1 week, a brown solid is recovered. This
solid, which is insoluble in the most common solvents,
1.275(6)
1.519(8)
1.13(1)
1.248(6)
1.42(1)
Bond angles
N(4)?ÃRhÃN(3)
O(1)ÃRhÃO(2)?
N(3)ÃRhÃN(2)
O(1)ÃRhÃN(2)
N(2)ÃRhÃRh?
O(2)ÃC(1)ÃO(1)
O(1)ÃC(1)ÃC(2)
90.8(2)
88.1(2)
98.2(2)
86.2(1)
171.1(1)
126.7(5)
115.3(5)
N(3)ÃRhÃO(1)
N(4)?ÃRhÃO(2)?
N(4)?ÃRhÃN(2)
O(2)?ÃRhÃN(2)
N(3)ÃC(28)ÃN(4)
O(2)ÃC(1)ÃC(2)
N(1)ÃC(8)ÃC(5)
90.3(2)
90.4(2)
98.6(2)
86.9(2)
123.8(5)
118.0(5)
174.0(1)
analyzes as 3. By heating (70 8C, 2ꢂ3 h) the brown solid
/
turns green, indicating that 3 is restored.
However, complexes 2 and 3 dissolve well in pyridine
affording orange solutions, consistent with the forma-
The atomic equivalent positions are obtained by the symmetry
operation x, ꢃy, ꢃzꢁ1.
tion of the axial 1:2 dirhodiumꢂpyridine adducts,
/
as constituted by 2 equiv. octahedral rhodium(II) atoms.
The coordination of the metal center deviates from the
regular geometry due to the expected difference between
[Rh2(form)2(O2CC6H4CN)2(Py)2] (2a) and [Rh2(form)2-
(O2CC5H4N)2(Py)2] (3a), respectively (see Section 2).
The proton spectra of 2 and 3, taken in pyridine, show
well resolved signals which are well in agreement with
the proposed formulation and the X-ray data of 2a (Fig.
1). As expected, the presence or benzocyanate or
isocotinate groups inside the lantern structure, instead
of trifluoroacetates, affects slightly the formamidinate
proton patterns as compared with those of 1. In fact, the
formamidinate proton resonances result practically un-
shifted with respect to those of 1 and consist of a singlet
the equatorial RhÃ
/
N and RhÃ
/
O bond lengths [2.023(5)
˚
vs. 2.082(4) A mean value, respectively] and, mainly, to
the steric hindrance between the axial pyridine and the
adjacent two p-toluidine fragments of the two bridging
formamidinates, coordinated in a cis arrangement
around the dirhodium core). The pyridine rings mini-
mize such an hindrance by intercalation into the
toluidine planes, as evidenced by the corresponding
dihedral angles of 28.2(2) and 38.7(2)8 with C(14)- and
C(21)?-ring, respectively. With respect to the dinuclear
axes the pyridine axial ligand appears pushed toward the
at d 2.11 for the Meꢂtolyl protons, in a dd at d 7.10 for
/
2 (7.05 for 3) for tolyl and in a quite broad triplet
centered at d 7.72 for the two methyne ones. In the
spectrum of 2 two equally intense doublets are observed
for the 2,6 and 3,5 cyanobenzoate protons at d 8.31
acetate side(s): in fact, both the NeqÃ
increase up to 98.4(2)8 while OeqÃRhÃ
RhÃOeq angles are shortened to 86.5(2) and 88.1(2)8,
respectively, determining a value of 171.1(1)8 for the
angle Rh?ÃRhÃNax (Table 1). The flat formamidinate
fragment shows the usual NÃCHÃN delocalization not
/
RhÃ
/
Nax angles
/
/
Nax and OeqÃ
/
/
(Jꢀ
/
8.1 Hz) and 7.61 (Jꢀ8.1 Hz), respectively, and
/
analogously, in the spectrum of 3 two equally intense
doublets are observed for the 2,6 and 3,5 pyridyne
/
/
/
/
protons at d 8.58 (Jꢀ
/
8 Hz) and 8.01 (d, Jꢀ8 Hz),
/
extended to the aryl substituents, whose rings are
significantly rotated with respect to the bite mean plane
[28.4(2) and 21.4(3)8, respectively]. The same situation
was found in the parent complex [Rh2(form)2(CF3-
COO)2(H2O)2] (1) [7], where the axial ligands are water
respectively.
3.2. Crystal structure of
[Rh2(form)2(O2CC6H4CN)2(Py)2] (2a)
molecules but the Ã
/
COOÃ
/
bite of the acetate and
benzoate bridges might be considered equivalent (all
A view of complex 2a showing the atom numbering
˚
scheme is reported in Fig. 1. Selected bond lengths (A)
and angles (8) are reported in Table 1.
The crystallographic asymmetric unit contains half
part of the dirhodium complex which lies on the twofold
axis parallel to cell a. At the solid state, the molecule is
˚
the OÁ Á ÁO distances are is 2.25(1) A). The title com-
pound shows an alongation of RhÃ
/
Rh and RhÃ
/
Nform
˚
bonds [2.469(1) and 2.023(5) vs. 2.426(1) and 1.993(4) A,
respectively] which might be related to the significant
steric and electronic differences of the corresponding
axial ligands.
perfectly symmetric and shows an homopolar RhÃ
/
Rh
bond. The dirhodium unit Rh2 might be interpreted
4ꢁ

S. Lo Schiavo et al. / Inorganica Chimica Acta 343 (2003) 351ꢂ
/
356
355
By considering the arylacetate bridging ligand, the
delocalized Ã
COOꢃ fragment is not conjugated to the
phenyl ring as evidenced by the interconnecting CÃ
single bond and the dihedral angle of 8.1(3)8.
free, some polymerization process occurs leading to
insoluble polymeric solids. It is well known the dirho-
/
/C
dium(II,II) axial reactivity towards NÃ
/
Lewis bases, so
that such a process may be promoted by N-coordination
of CN or Py groups to the free axial sites on dirhodium.
In pyridine such a polymerization process is prevented
as, consistently to X-ray findings, pyridine results axially
coordinated.
3.3. Reaction of 3 with palladium and platinum transition
metal complexes having cis- or trans-labile ligands
The spectroscopic as well as the X-ray crystallo-
graphic characterization of 2a and 3a unambiguously
show that, as in their precursor 1, the carboxylate
ligands are cis-symmetrically bridged across the dirho-
dium(II,II). As a consequence the cyanobenzoate and
the isocotinate groups appear orthogonally disposed
with the two dangling N-binding sites suitable for
further coordination. Although the geometrical features
make both 2 and 3 ideal angular building blocks capable
to produce square boxes by self-assembly with cis- or
trans- square-planar complexes [1], our preliminary
attempts to prepare square macrocycles based on 2
and 3 failed.
Recently, an analogous dirhenium complex bearing
two isocotinate groups cis-symmetrically bridged to the
dirhenium
core,
[Re2Cl2(dppm)2(O2CÃ
/
C5H4N)2]
(dppmꢀbis(diphenylphosphino)methane) [17], was re-
/
ported. Differently from 3, this complex, which bears
two chlorine axially coordinated, did not display a
similar solution behavior. Further, such a dirhenium
complex resulted a successful angular precursor afford-
ing a dirhenium-platinum square box by self assembly
with the square-planar transition complex cis-
Pt(dbbpy)(O3SCF3)2 [dbbpyꢀ4,4?-di-ter-butyl-2,2?-bi-
/
pyridine]. This allows to retain that in 3 the absence of
axially ligated molecules not only affects its solution
behavior but even its reactivity.
Complex 2, as previously pointed out, is only soluble
in pyridine and in chloroform or benzeneꢂ
/
pyridine
Further attempts are in progress in our laboratory to
prepare square macrocycles incorporating 2 and 3 as
angular units. The recent large number of reports in the
context of square boxes based on dimetal units prompts
us the communication of these preliminary results.
mixtures. Pyridine competes with the dirhodium com-
plex N-binding sites in the reaction with palladium and
platinum complexes preventing the study of such reac-
tions.
We, therefore, turned our attention to the possibility
to realize square boxes by using 3 as angular precursor
component. Following a largely employed procedure [1],
we studied the 1:1 reaction of 3 with a series palladium
and platinum complexes, bearing two cis- or trans-labile
4. Supplementary material
Details of the crystal structure of [Rh2(form)2(O2CÃ
/
ligands, of the type [M(LÃ
phenylphosphino)ethane,
[M(P{C6H5}3)2]X2 (Xꢀ
/
L)]X2 [LÃ
ethylenediamine]
CF3SO3, BF4, PF6 etc.) and
/
Lꢀ
/
1,2-bisbis(di-
C6H4ÃCN)2(Py)2] (2a) have been deposited with the
/
or
Cambridge Crystallographic Data Centre under CCDC
no. 181807. Copies of this data can be obtained, free of
charge, on application to The Director, CCDC, 12
/
[Pd(C6H5CN)2Cl2]. By the addition of an equivalent of
such palladium or platinum complexes to a solution of
Union Road, Cambridge, CB2 1EZ, UK (fax: ꢁ44-
1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or www:
http//www.ccdc.cam.ac.uk).
/
3, solids with a 1:1 dirhodiumꢂpalladium or platinum
/
stoichiometry were isolated, but, they did not afford
significant NMR and the ESI mass spectra. The NMR
spectra, as already observed for their precursor 3,
consist of broad signals at any temperature, while the
ESI mass spectra, were more in agreement with the
presence in solution of a mixture of oligomers. No
evidence of insoluble solid, indicating the formation of
polymeric species, were observed.
Acknowledgements
This work was supported by MURST and Italian
CNR.
Such results combined with the fact the characteriza-
tion of 2 and 3 was reached in pyridine solvent, where
they transform in the corresponding axial bis-adducts 2a
and 3a, may be reasonably explained taking in account
the axial reactivity of dirhodium(II,II) species.
The solution behavior of 2 and 3, which significantly
differ from that of their corresponding pyridine adducts
as well as the formation of the insoluble brown solid
formed from chloroform solution of 3 (see above), led us
to suggest that, when the axial binding sites on 2 or 3 are
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