Cationic rhodium(I) complexes containing 4,4′-disubstituted 2,2′-bipyridines: A systematic variation on electron density over the metal centre

Article abstract of DOI:10.1016/j.jorganchem.2005.12.049

A series of [Rh(COD)(X₂-bipy)]BF₄ complexes (COD = 1,5-cyclooctadiene; X₂-bipy = 4,4′-disubstituted 2,2′-bipyridines; X = OCH₃, CH₃, H, Cl or NO ₂) has been prepared from [Rh(COD)Cl]₂. The complexes for X = OCH₃, Cl and NO₂ have not been described previously in the literature. All complexes have been characterised by elemental analysis, IR, ¹H NMR and UV-Vis spectrometry. This series of complexes presents a wide variation on electron density over the metal centre with virtually no variation on its steric environment which discloses interesting possibilities for catalytic and electro-catalytic studies. A preliminary evaluation of these complexes on the hydroformylation of camphene and β-pinene showed that under the rather drastic conditions employed the complexes acted as a precursor for [Rh(CO)₃H], which accounts for most of the catalytic activity.

Full text of DOI:10.1016/j.jorganchem.2005.12.049

Journal of Organometallic Chemistry 691 (2006) 2037–2043
www.elsevier.com/locate/jorganchem
Cationic rhodium(I) complexes containing 4,40-disubstituted
2,20-bipyridines: A systematic variation on electron density over
the metal centre
1
*
*
Paulo E.A. Ribeiro , Claudio L. Donnici , Eduardo N. dos Santos
Departamento de Quı´mica, Universidade Federal Minas Gerais, Av. Antonio Carlos, 6627, 31270-901 Belo Horizonte, MG, Brazil
Received 26 October 2005; received in revised form 21 December 2005; accepted 22 December 2005
Available online 28 February 2006
Abstract
A series of [Rh(COD)(X2-bipy)]BF4 complexes (COD = 1,5-cyclooctadiene; X2-bipy = 4,40-disubstituted 2,20-bipyridines; X =
OCH3, CH3, H, Cl or NO2) has been prepared from [Rh(COD)Cl]2. The complexes for X = OCH3, Cl and NO2 have not been described
1
previously in the literature. All complexes have been characterised by elemental analysis, IR, H NMR and UV–Vis spectrometry. This
series of complexes presents a wide variation on electron density over the metal centre with virtually no variation on its steric environ-
ment which discloses interesting possibilities for catalytic and electro-catalytic studies. A preliminary evaluation of these complexes on
the hydroformylation of camphene and b-pinene showed that under the rather drastic conditions employed the complexes acted as a
precursor for [Rh(CO)3H], which accounts for most of the catalytic activity.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Cationic rhodium(I) complexes; Substituted bipyridine; Hydroformylation; Rhodium catalysts
1. Introduction
the rhodium-catalysed hydroformylation of b-pinene and
camphene in the presence of various phosphines and phos-
phites ancillaries. It has been demonstrated that the diastero-
isomeric excess for the cis aldehyde is directly related to the
electron-donating ability of the ancillary measured as
described by Tolman [24]. Nevertheless, no clear correlation
was found for its steric parameter (cone angle). In order to
investigate whether or not this influence is purely electronic
we decided to synthesise rhodium(I) complexes containing
various 2,20-bipyridines disubstituted in the 4,40 positions
by the methoxy, methyl, chloro, or nitro groups. In this series
there is a considerable variation on the electron-donating
ability of the ligands with virtually no variation on their ste-
ric parameters.
In this work, we describe the synthesis and the spectro-
scopic characterisation of rhodium complexes of the
type [Rh(COD)(X2-bipy)]BF4 (COD = 1,5-cyclooctadiene;
X2-bipy = 2,20 bipirydine or 2,20 bipirydine 4,40-disubsti-
tuted by Cl, OCH3, NO2, CH3), as well as their use as a cat-
alyst for the hydroformylation camphene and b-pinene.
Cationic rhodium(I) complexes have long been acknowl-
edged as catalysts precursors for many reactions such as
hydrogenation [1] or transfer hydrogenation [2] of dou-
ble-bonds, water gas shift reaction [3], amination [4],
hydroamination [5], hydroformylation [6–8], among others.
Although phosphorus ligands are more widely used for
these reactions, in many instances nitrogen-donors such
as 2,20-bipyridines have been successfully employed [9–20].
We have been studying the diastereoselective hydro-
formylation of monoterpenes as a way of better understand-
ing the diastereoselectivity control in hydroformylation
employing rhodium complexes [21] or platinum/tin systems
[22,23] as catalysts. In a previous work [21] we described
*
Corresponding authors. Tel.: +553134995743; fax: +553134995700.
E-mail addresses: cdonnici@ufmg.br (C.L. Donnici), nicolau@ufmg.br
(E.N. dos Santos).
1
´
Present address: Empresa Brasileira de Agropecuaria – EMBRAPA.
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.12.049

2038
P.E.A. Ribeiro et al. / Journal of Organometallic Chemistry 691 (2006) 2037–2043
+ 2 AgBF4 / - 2 AgCl
as a labile stabilising ligand. The addition of a stoichiome-
-
[Rh(COD)Cl]2
2 [Rh(COD)(THF)2]+ + 2 BF4
tric amount of the substituted bipyridines to the rhodium
solution after silver chloride removal resulted in the imme-
diate formation of desired compounds in high yield.
(Scheme 1).
THF
2 X2-bipy
THF
The stripping of the solvent and washing with fresh por-
tions of THF resulted in sufficiently pure compounds,
which was confirmed by the narrow melting point and ele-
mental analysis. It is interesting to observe that only for
2 [Rh(COD)(X2-bipy)]BF4
X = OCH3 (1)
X = CH3 (2)
X = H (3)
1
2,20-bipyridine as ligand the solid showed by H NMR a
molecule of THF as a crystallization solvent, which was
easily removed by recrystallization in methanol. The com-
plexes present low solubility in toluene, chloroform, and
tetrahydrofuran, moderate solubility in dichloromethane
and are soluble in methanol and ethanol.
X = Cl (4)
X = NO2 (5)
Scheme 1.
2. Results and discussion
2.1. Synthesis of the complexes
2.2. Infrared spectroscopy
The IR spectra of the complexes in the range of 5000–
400 cmꢀ1 are similar to those of the free ligands (Table
1). The most significant shifts are related to the absorptions
assigned to the aromatic ring stretching of the bipiridinic
For the synthesis of the complexes a straightforward
route was employed. Firstly, [Rh(COD)Cl]2 was reacted
with silver tetrafluoroborate in tetrahydrofuran which acts
Table 1
Selected bands, cmꢀ1, and assignments for the infrared spectra for the free ligands and their complexes
Free ligand
X2-bipyb
Complexa
Assignmentsd
CODc
1
2
3
4
5
3100–3080w
3100vw
3090w
3090vw
3090vw
3100w
m CHar
3000–2850s
2920vs, 2850vs
2920vs, 2850vs
2920vs, 2850vs
2920vs, 2850vs
2920vs, 2850vs
1540
CHal
masyNO2
1530vs
1520vs
1470s
1380m
1460s
1380s
1470s
1390m
1470s
1380s
1470s
1370s
1350s
m ringar
m C@Cal
msy NO2
masy COC
m CCl
1430s
1340vs
1230s
1080m
1050vs
a
1250m
1050s
1080s
msy COC
From Nujol mulls.
From KBr pellets.
From the literature.
b
c
d
From the literature [25], m, axial stretching vibration; sy, symmetry vib.; asy, anti-symmetry vib.; ar, aromatic; al, aliphatic.
Table 2
Proton chemical shiftsa for the free ligands (L)b and their complexes [Rh(COD)(X2-bipy)]BF4(C)c
0
0
0
Complex
X
H3;3
H5;5
H6;6
L
Substituent 4,40
Ha
L
Hb
0 00
C : Hb =Hb
L
C
L
C
C
L
C
C
L
1
2
3
4
5
a
CH3O
CH3
H
7.98d
(0.00)d
8.23d
(0.08)
8.40d
(0.05)
8.47d
(0.19)
9.21d
(0.17)
7.98d
6.84dd
(0.34)
7.11dd
(0.39)
7.30dt
(0.39)
7.37dd
(0.41)
8.12dd
(0.33)
7.18dd
8.46d
7.78dd
3.94s
(0.14)
2.41s
(0.16)
7.81dt
(0.43)
4.08s
5.54m
4.52m
2.34m
2.57m/2.17m
J = 0.40
2.60m/2.19m
J = 0.41
2.62m/2.24m
J = 0.38
2.69m/2.21m
J = 0.48
(ꢀ0.68)
(ꢀ1.02)
8.31d
8.45d
8.66d
9.38d
7.50dd
7.69dt
7.78dd
8.45dd
8.52d
7.81d
2.57s
5.54m
4.61m
4.68m
4.66m
4.68m
2.34m
2.34m
2.34m
2.34m
(ꢀ0.71)
8.69d
(ꢀ0.93)
5.54m
8.00d
8.24dt
(ꢀ0.69)
(ꢀ0.86)
Cl
8.60d
7.94dd
8.37dd
5.54m
(ꢀ0.66)
9.03d
(ꢀ0.66)
(ꢀ0.88)
5.54m
(ꢀ0.86)
NO2
2.62m/2.29m
J = 0.33
In ppm downfield from TMS.
In chloroform-d.
In methanol-d4.
b
c
d
Values in parentheses are coordination induced shifts (c.i.s.).

P.E.A. Ribeiro et al. / Journal of Organometallic Chemistry 691 (2006) 2037–2043
2039
ligands (from 1520 to 1470–1460 cmꢀ1) and the double
bond stretching of the 1,5-COD (from 1430 to 1390–
1370 cmꢀ1).
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
Complex
2.3. Nuclear magnetic resonance
1
The H NMR spectra of the complexes exhibit reso-
nances due to X2-bipy and COD hydrogen. The compari-
son of these data with uncoordinated ligands (Table 2)
showed induced shifts due to coordination to metal
(c.i.s. = dcoordinated ligand ꢀ dfree ligand
)
[26]. In X2-bipy
0
0
ligands, the c.i.s. were positive in H3;3 and H5;5 (down-
field). This indicates a diminution of electron density in
the ring due to the donation to the metal. On the other
420 440 460 480 500 520 540 560 580 600
Wavelength, nm
0
hand, H6;6 undergo a upfield shift after coordination prob-
ably due to the anisotropic effect of the metal atom in the
neighbourhood. A shielding effect is also observed for the
Ha signal for the co-ordinated COD: the more electron-
donating the X-substituents, the higher the shielding. Hb
appears as a single multiplet for the free ligand which is
split in two for the co-ordinated ligand as the hydrogen
atoms are no longer equivalent. The vicinal J-couplings
are in the range of 0.33–0.41 ppm. These results are in
accordance with the previously reported for analogous
complexes [11]. These data confirm that after addition of
X2-bipy ligands in complex preparation, COD keeps the
g4-bonding to rhodium, like its precursor, in which the
hydrogens bound to the secondary carbons are sterically
Fig. 2. Visible absorption spectra of 1–5 (e multiplied by 1.15 for 5).
along the ligand series. It is noteworthy the difference in
colours among the complexes depending on the X-substitu-
ent: X = OCH3, yellow; CH3, orange; H, red; Cl, purple;
NO2, dark green. The absorption maxima (kmax) are
observed in the range 444–562 nm (Fig. 2) and were
assigned to rhodium d–d transitions. The emax (ca.
102 L molꢀ1 cmꢀ1) is high, considering that this transition
is spin-forbidden for d 8 configuration of Rh(I), but there
is a relaxation of the selection rule due to the spin–orbit
coupling usually observed in heavy d-metals complexes
[27]. There is a general tendency to increase kmax with the
increase of the electron-withdrawing ability of the substitu-
ent. In attempting to quantify this effect, we choose the
Hammett parameter (rp) which is a measure of the elec-
tron-withdrawing capacity of a substituent in an aromatic
ring. Groups with a better electron-withdrawing ability
than hydrogen will have positive values for rp, and nega-
tive values correspond to groups with a better electron-
donating ability than hydrogen [28].
0
00
00
distinct (Hb and Hb ), as showed in Fig. 1. The Hb is under
the shielding effect of the double bound and has the lower d
values.
2.4. UV–Vis spectroscopy
A clear evidence for the changes in the electronic levels
on the metal as X varies is the remarkable change in colour
Indeed, there is a fairly good linear correlation between
rp and the kmax of the complexes in visible spectra (Table 3
and Fig. 3). This can be interpreted as the increase of the
splitting between dxy and dx2ꢀy2 orbitals of the metal with
the increase in electron density of the ligand. Fig. 4 shows
the correlation between kmax and the pKa for each X2-bipy.
There is a general tendency to reduce kmax with the increase
in pKa of the ligand. There is a good linear correlation
between these parameters for X = CH3O, CH3, or H. This
+
X
X
4
4'
3
3'
5
5'
N
N
6'
6
b
b
b
Rh
a
a
a
a
a
a
a
a
Table 3
b', b''
Hammett parameters, rp, for X in X2-bipy and absorption maxima, kmax
,
b', b''
of the complexes
H
b''
b
Complex
X
rp
kmax (nm)
b', b''
H
b'
1
2
3
4
5
OCH3
CH3
H
Cl
NO2
ꢀ0.28
ꢀ0.14
0
0.24
0.81
444
456
476
485
562
COD
[Rh(COD)(X2-bipy)]+
Fig. 1. Numbering for 1H NMR assignments.

2040
P.E.A. Ribeiro et al. / Journal of Organometallic Chemistry 691 (2006) 2037–2043
580
580
540
500
460
420
NO2
(NO2)2-bipy
540
500
460
420
Cl
Cl2-bipy
H
bipy
(CH3)2-bipy
CH3
OCH3
(CH3O)2-bipy
-0.4
0
0.4
of substituents
0.8
1.2
1
2
3
4
5
6
7
σP
pKa of X2-bipy
Fig. 3. Correlation between Hammett parameters (rp) for X in X2-bipy
and absorption maxima (kmax) of the complexes.
Fig. 4. Correlation between pKa of X2-bipy and absorption maxima (kmax
of the complexes.
)
Table 4
and Scheme 2. The aldehydes 7 and 8 were formed, accom-
panied by about 50% of the double-bond isomerization
product, a-pinene (9). Although the conversion was com-
plete after 1 h of reaction for all the complexes, the reaction
was kept for additional 23 h to check for a-pinene hydro-
formylation. Indeed, along this reaction time a-pinene is
slowly converted to the corresponding aldehydes accounted
as ‘‘others’’ in Table 4. It is surprising that the selectivity of
this reaction shows little dependence on the ligand, as we
have demonstrated that employing the system [(l-ace-
tato)(1,5-cyclooctadien)rhodium(I)]/PR3 the selectivity
depends strongly on the donation ability of PR3: the more
donating the ancillary, the greater the chemoselectivity for
hydroformylation and the diasteroselectivity for 8 are.
These complexes have a very similar behaviour to the
unpromoted system where the complex [(l-acetato)(1,5-
cyclooctadien)rhodium(I)] readily generate [Rh(CO)3H]
under hydroformylation conditions [21].
In order to study the hydroformylation separately from
double bond isomerization, camphene (10) was chosen for
the kinetic studies because its double bond is similar to the
b-pinene one but isomerization is not possible due to the
lack of a neighbour secondary carbon to it. Its hydro-
formylation products are two diasteromeric aldehydes 11
and 12 (Scheme 3). In Table 5, it can be noticed that the
complexes containing bipyridines with electron-withdraw-
ing substituents (NO2, Cl) led to a faster conversion of
the substrate, having, therefore, a higher turnover fre-
quency (TF = mol of substrate converted per mol of cata-
lyst per hour). On the other hand, for the substituents
containing electron-donating groups (CH3O, CH3), the
a
Hydroformylation of b-pinene (6) catalyzed by [Rh(COD)(X2-bipy)]BF4
Complex
X
Conversion (%)
Product distribution (%)
7
8
9
Others
1
2
3
4
5
a
–OCH3
–CH3
–H
–Cl
–NO2
99
99
99
99
99
43
42
46
46
39
6
5
5
5
5
45
46
39
39
50
6
7
10
10
6
b-Pinene = 10 mmol; catalyst = 1.00 · 10ꢀ2 mmol; dichlorometh-
ane = 20.0 mL; 80 ꢁC; 80 atm (CO:H2 = 1:1); 24 h.
suggests that the separation in the energy level for this tran-
sition is predominantly determined by the r bonding
Rh-(X2-bipy) which would be similar to an H+-X2-bipy
bonding. Nevertheless, for X = Cl or NO2, which are elec-
tron-withdrawing groups, the metal-to-ligand backbonding
may play an important role, and this similarity is no longer
valid.
2.5. Hydroformylation of b-pinene and camphene catalysed
by the rhodium complexes 1–5
In a previous work we demonstrated that b-pinene and
camphene can be hydroformylated employing the complex
[(l-acetato)(1,5-cyclooctadien)rhodium(I)] as a catalyst
precursor with or without phosphor(III) ancillaries [21].
The results for the hydroformylation of b-pinene (6)
employing the complexes 1–5 as catalyst precursors at
80 ꢁC and 80 atm of CO/H2 (1:1) are shown in Table 4
Scheme 2.

P.E.A. Ribeiro et al. / Journal of Organometallic Chemistry 691 (2006) 2037–2043
2041
Scheme 3.
Table 5
4. Experimental
Hydroformylation of camphene (10) catalyzed by [Rh(COD)(X2-
bipy)]BF4
a
4.1. General procedures
Complex
X
Conversion
(%)
TF
Product
distribution
(%)
(hꢀ1 b
)
All chemicals were purchased from Aldrich and used as
received, unless otherwise indicated. Bis[(1,5-cyclooctadi-
ene)(l-chloride)rhodium(I)], [Rh(COD)Cl]2 [29], 4,40-
dichloro-2,20-bipyridine [30] 4,40-dimethoxy-2,20-bipyri-
dine, and 4,40-dinitro-2,20-bipyridine [31–33] were prepared
by published procedures. For convenience, a general sketch
for bipyridines synthesis is depicted in Scheme 4. Tetrahy-
drofuran and toluene were purified under reflux with
sodium wire/benzophenone for 6 h and then distilled under
nitrogen. Dichloromethane was distilled under N2 atmo-
sphere over phosphorous pentoxide. The syntheses of the
complexes were carried out under nitrogen atmosphere
employing standard Schlenk techniques.
NMR spectra were obtained using a Brucker DRX
200 MHz spectrometer in CDCl3 (bipyridines) or metha-
nol-d4 (Rh-complexes). IR spectra were obtained in a
Mattson FTIR 3000-Galaxy Series spectrometer in KBr
disks (bipyridines) or in a Nujol mull (Rh-complexes).
UV–Vis spectra were taken in a Hewlett-Packard 8453
spectrometer (methanol solution 1.00 mmol Lꢀ1 in quartz
cell of 1 cm). Elemental analysis was performed in a Carlo
Erba Instrumentos EA 1110 CHNS-O. Gas chromatogra-
phy (GC) was performed in a Shimadzu 17B instrument fit-
ted with a Carbowax 20 M capillary column and a flame
ionization detector. Mass spectra were obtained on a Hew-
lett-Packard MSD 5890/Series II instrument operating at
70 eV.
11
12
1
2
3
4
5
a
–OCH3
–CH3
–H
–Cl
–NO2
55
57
74
74
74
530
570
740
740
740
52
53
52
52
52
48
47
48
48
48
Camphene = 10 mmol; catalyst = 1.00 · 10ꢀ2 mmol; dichlorometh-
ane = 20.0 mL; 80 ꢁC; 80 atm (CO:H2 = 1:1); 1 h.
b
Turnover frequency: mol of substrate converted per mol of catalyst
precursor per hour.
conversion of the substrate is slower and the turnover rate
is lower.
These results suggest that the bipyridine complexes are
acting as a precursor for [Rh(CO)3H], which carries the
fastest catalytic cycle and even at low concentration can
account for the observed product distribution. In other
words, the complexes containing bipyridines, have low
activity compared to [Rh(CO)3H]. Differences in turnover
rates observed for camphene hydroformylation may be
related to the ability of the complex to deliver the true cat-
alyst: the stronger the X2-bipy-Rh bound, the slower the
formation of [Rh(CO)3H] is, and, as a consequence, the
turnover frequency will be lower.
3. Conclusion
Three new rhodium complexes with 4,40-disubstituted
2,20-bipyridines have been synthesised and characterised.
A remarkable influence of the substituents on the electron
4.2. Synthesis of rhodium complexes
A
solution of silver tetrafluoroborate (0.2745 g,
1
density over the metal centre can be inferred from IV, H
1.410 mmol) in 10 mL of THF was added dropwise with
a syringe to a Schlenk flask containing a solution of
[Rh(COD)Cl]2 (0.3475 g, 0.7050 mmol) in 20 mL of THF.
A white solid immediately precipitated and the mixture
was magnetically stirred for 20 min to complete the precip-
itation of AgCl. After decanting, the yellow supernatant
was filtered off to another Schlenk flask, and the solid
was washed twice with 2 mL of THF in order to transfer
quantitatively the soluble rhodium complex. A THF solu-
tion of bipyridinic ligand (1.410 mmol) was added drop-
wise with a syringe to the yellow solution. An immediate
change in colour occurred and the solution was stirred
for 30 min at room temperature. The solvent was carefully
evaporated under vacuum at room temperature to yield the
NMR, and UV–Vis spectrometry. A preliminary evalua-
tion on the catalytic behaviour of this series of complexes
on camphene hydroformylation showed that the catalytic
activity decreases with the increase of the Rh-bipy bond
strength. The selectivity on b-pinene hydroformylation
indicates that most of the catalytic activity is carried out
by [Rh(CO)3H] which is formed in situ from the bipy com-
plexes under the rather harsh reaction conditions
employed. Nevertheless, this series of complexes is promis-
ing for basic catalytic and electrocatalytic studies as there is
a great variation on electron density in the metal centre
with virtually no variation in its steric environment. Fur-
ther studies are in progress in our laboratory.

2042
P.E.A. Ribeiro et al. / Journal of Organometallic Chemistry 691 (2006) 2037–2043
4.5. Catalytic runs
In a typical run [Rh(COD)(X2-bipy)]BF4 (1.0 · 10ꢀ2
N
N
H O /CH COOH
2
2
3
mmol), b-pinene (10.0 mmol), dichoromethane (20.0 ml)
were added in a Schlenk tube under nitrogen and stirred
for 10 min at room temperature. The mixture was trans-
ferred into a stainless steel magnetic stirred reactor under
nitrogen. The reactor was pressurized to 8.0 MPa total
pressure (CO/H2 = 1/1), placed in an oil bath at 80 ꢁC
and the mixture was magnetically stirred for 24 h. Reac-
tions were followed by sampling the liquid phases under
pressure. After carrying out the reaction and cooling the
reactor to room temperature, the excess CO and H2 was
slowly vented. The solution was analyzed by GC and
GC/MS and the products’ identity was confirmed by com-
parison with authentic samples obtained as described pre-
viously [21].
Cl
Cl
N
O
N
N
N
O
CH COCl/PCl
3
3
HNO /H SO
4
3
2
OCH3
OCH3
N
NO2
NO2
Na/CH OH
3
N
O
N
O
N
O
O
PCl /CHCl
PCl
3
3
3
OCH3
N
OCH3
NO2
N
NO2
Acknowledgements
We thank Conselho Nacional de Desenvolvimento
N
N
´
´
Cientıfico e Tecnologico (CNPq), Fundac¸ao do Amparo
˜
a Pesquisa do Estado de Minas Gerais (FAPEMIG), and
˜
Scheme 4.
PADCT-II Program for supporting this work.
respective bipyridine complex which was washed twice with
2 mL of THF and dried overnight under vacuum.
References
4.2.1. [Rh(COD)(4,40-dimethoxy-2,20-bipyridine)]BF4 (1)
Bright yellow crystals (0.6643 g, 92% yield) m.p. 263–
264 ꢁC. Elemental analysis: Calc. for BC20F4H24N2O2Rh
(512.12 g/mol): C, 46.2; H, 4.8; N, 5.1. Found: C, 46.7;
H, 4.7; N, 5.4%.
[1] R.R. Schrock, J.A. Osborn, J. Am. Chem. Soc. 98 (1976)
4450.
[2] R. Uson, L.A. Oro, R. Sariego, M.A. Esteruelas, J. Organomet.
Chem. 214 (1981) 399.
[3] S.A. Moya, R. Pastene, R. Sariego, R. Sartori, P. Aguirre, H.
Lebozec, Polyhedron 15 (1996) 1823.
[4] M. Beller, O.R. Thiel, H. Trauthwein, C.G. Hartung, Chem.–Eur. J.
6 (2000) 2513.
[5] S. Burling, L.D. Field, B.A. Messerle, P. Turner, Organometallics 23
(2004) 1714.
4.3. [Rh(COD)(4,40-dimethyl-2,20-bipyridine)]BF4 (2)
Bright orange crystals (0.6296 g, 93% yield) m.p. 269–
270 ꢁC. Elemental analysis: Calc. for BC20F4H24N2Rh
(480.12): C, 49.8; H, 5.0; N, 5.8. Found: C, 49.2; H, 5.1;
N, 5.2%.
[6] R. Uson, L.A. Oro, C. Claver, M.A. Garralda, J.M. Moreto, J. Mol.
Catal. 4 (1978) 231.
[7] J.C. Bayon, P. Esteban, J. Real, C. Claver, A. Polo, A. Ruiz, S.
Castillon, J. Organomet. Chem. 403 (1991) 393.
[8] H. Alper, J.Q. Zhou, J. Chem. Soc., Chem. Commun. (1993) 316.
[9] A. Togni, L.M. Venanzi, Angew. Chem., Int. Ed. Engl. 33 (1994)
497.
4.4. [Rh(COD)(2,20-bipyridine)]BF4 (3)
[10] R. Dorta, L.J.W. Shimon, H. Rozenberg, D. Milstein, Eur. J. Inorg.
Chem. (2002) 1827.
[11] R. Ruppert, S. Herrmann, E. Steckhan, J. Chem. Soc., Chem.
Commun. (1988) 1150.
[12] P. Frediani, A. Salvini, M. Bessi, L. Rosi, C. Giannelli, Inorg. Chem.
Commun. 8 (2005) 94.
[13] M.A. Iron, A. Sundermann, J.M.L. Martin, J. Am. Chem. Soc. 125
(2003) 11430.
Reddish orange solid (0.6018 g, 94% yield), dec.
ꢁ300 ꢁC Recrystallized from methanol. Red crystals were
obtained, m.p. 286–288 ꢁC (dec). Elemental analysis: Calc.
for BC18F4H20N2Rh (454.08): C, 47.6; H, 4.4; N, 6.2.
Found: C, 45.9; H, 4.4; N, 5.8%.
4.4.1. [Rh(COD)(4,40-dichloro-2,20-bipyridine)]BF4 (4)
Purple crystals (0.6636 g, 90% yield), m.p. 253–254 ꢁC.
Elemental analysis: Calc. for BC18Cl2H18F4N2Rh (522.97):
C, 41.3; H, 3.4; N, 5.4. Found: C, 40.4; H, 3.6; N, 5.3%.
[14] C. Maillet, P. Janvier, M.J. Bertrand, T. Praveen, B. Bujoli, Eur. J.
Org. Chem. (2002) 1685.
[15] A. Felix, A.R. Guadalupe, S.D. Huang, Z. Krist.–New Cryst. Struct.
214 (1999) 463.
[16] V.R. Parameswaran, S. Vancheesan, React. Kinet. Catal. Lett. 44
(1991) 185.
4.4.2. [Rh(COD)(4,40-dinitro-2,20-bipyridine)]BF4 (5)
Dark green crystals (0.6904 g, 90% yield), dec. ꢁ300 ꢁC.
Elemental analysis: Calc. for BC18F4H18N4O4Rh (544.08):
C, 39.7; H, 3.3; N, 10.3. Found: C, 39.5; H, 3.3; N, 9.9%.
[17] C.M. Bolinger, N. Story, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 27
(1988) 4582.
[18] A.A. Bahsoun, R. Ziessel, Nouv. J. Chim. 9 (1985) 225.
[19] P.A. Marnot, R.R. Ruppert, J.P. Sauvage, Nouv. J. Chim. 5 (1981)
543.

P.E.A. Ribeiro et al. / Journal of Organometallic Chemistry 691 (2006) 2037–2043
2043
[20] G. Mestroni, R. Spogliarich, A. Camus, F. Martinelli, G. Zassino-
vich, J. Organomet. Chem. 157 (1978) 345.
[27] F. Shriver, P.W. Atkins, C.H. Langford (Eds.), Inorg. Chem., Oxford
University Press, Oxford, 1994, pp. 581–601.
[21] H.J.V. Barros, M.L. Ospina, E. Arguello, W.R. Rocha, E.V.
Gusevskaya, E.N. Dos Santos, J. Organomet. Chem. 671 (2003)
150.
[22] C.M. Foca, E.N. Dos Santos, E.V. Gusevskaya, J. Mol. Catal. A:
Chem. 185 (2002) 17.
[28] J. March, M.B. Smith, Advanced Organic Chemistry, Reactions,
Mechanisms, and Structure, Wiley, New York, 2001.
[29] J. Chatt, L.M. Venanzi, J. Chem. Soc. (1957) 4735.
[30] J.C. Crivello, J. Org. Chem. 46 (1981) 3056.
[31] M.J. Cook, A.P. Lewis, G.S.G. Mcauliffe, V. Skarda, A.J. Thomson,
J.L. Glasper, D.J. Robbins, J. Chem. Soc., Perk. Trans. II (1984) 1293.
[23] E.V. Gusevskaya, E.N. Dos Santos, R. Augusti, A.D.O. Dias, C.M.
Foca, J. Mol. Catal. A: Chem. 152 (2000) 15.
´
[32] C.L. Donnici, D.H. Maximo Filho, L.L.C. Moreira, G.T. Reis, E.S.
[24] C.A. Tolman, Chem. Rev. 77 (1977) 313.
Cordeiro, I.M.F. Oliveira, S. Carvalho, E.B. Paniago, J. Brazil.
Chem. Soc. 9 (1998) 455.
[25] J.S. Strukl, J.L. Walter, Spectrochim. Acta 27 (1971) 223.
[26] V. Gayathri, E.G. Leelamani, N.M.N. Gowda, G.K.N. Reddy,
Polyhedron 16 (1997) 1169.
´
[33] C.L. Donnici, I.M.F. Oliveira, M.C.R. Castro, E.S.C. Temba, Quım.
Nova 25 (2002) 668.