Syntheses, structures and electrochemical study of π-acetylene complexes of cobalt

Article abstract of DOI:10.1016/j.ica.2003.12.020

The preparation and characterisation of the complexes [Co ₂(CO)₄(PMe₃)₂][Co ₂(CO)₆](Me₃SiC₂C ₂SiMe₃) (4), [Co₂(CO)₄(dppm)]

Full text of DOI:10.1016/j.ica.2003.12.020

Inorganica Chimica Acta 357 (2004) 2069–2080
www.elsevier.com/locate/ica
Syntheses, structures and electrochemical study of p-acetylene
complexes of cobalt
a
a
b
a
a
a
R.M. Medina , C. Moreno , M.L. Marcos , J.A. Castro , F. Benito , A. Arnanz ,
a
S. Delgado , J. Gonzalez-Velasco , M.J. Macazaga
b
a,
*
a
ꢀ
ꢀ
ꢀ
Departamento de Quımica Inorganica, Facultad de Ciencias, Universidad Autonoma de Madrid, Contoblanco, 28049 Madrid, Spain
b
Departamento de Quꢀımica, Facultad de Ciencias, Universidad Autꢀonoma de Madrid, Contoblanco, 28049 Madrid, Spain
Received 31 July 2003; accepted 20 December 2003
Abstract
The preparation and characterisation of the complexes [Co2(CO)4(PMe3)2][Co2(CO)6](Me3SiC2C2SiMe3) (4), [Co2(CO)4(dppm)]
[Co2(CO)6](Me3SiC2C2H) (5), [Co2(CO)4(dppa)][Co2(CO)6](Me3SiC2C2SiMe3) (6), [Co2(CO)4(dppm)]2[Co2(CO)6](Me3SiC2CB
CC2C2SiMe3) (7) and [{SiMe3(Co2(CO)4(dppm))C2}2(HCBC)(1,3,5-C6H3)] (8) are described. An electrochemical study of the
complexes 5–8 and of the related [Co2(CO)4(dppm)]2(Me3SiC2(CBC)2C2SiMe3) (1), [Co2(CO)4(dppa)]2(Me3SiC2C2SiMe3) (2) and
[{SiMe3(Co2(CO)4(dppm))C2}(HCBC)2(1,3,5-C6H3)] (3) is presented by means of the cyclic and square-wave voltammetry tech-
niques. Crystals of 8 suitable for single-crystal X-ray diffraction were grown and the molecular structure of this compound is
discussed.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: p-Acetylene complexes; Cobalt; Electrochemistry; Molecular structure
1. Introduction
Os3(CO)11(NCMe) coordinates to 1,4-bis(ferrocenyl)
butadiyne triple bonds [4].
Complexes in which sp carbon chains span transition
metal endgroups are of great interest due to their pos-
sible uses as molecular wires in the construction of
nanoscale electronic devices [1]. In several systems, a
significant interaction between terminal groups has been
revealed by electrochemical data. Many such com-
pounds feature Fe, Ru and Re monometallic endgroups
[2]. Moreover it has been found that the incorporation
of redox-active metal centers in the conjugated organic
backbone can enhance the electronic communication
between the terminal metallic groups. For example, the
bridge (–CBC–Ru–CBC–) increases the electronic
communication between terminal ferrocenyl groups [3].
Electrons delocalise through the Ru center over the
whole system more efficiently compared to the all-car-
bon butadiynyl bridge. The same effect is observed when
The redox chemistry of Co2–alkyne is well established
[5] and communication between these redox centres has
been demonstrated for several systems [6].
In a previous paper, we reported the electrochemical
behaviour of the complexes (Co2(CO)6 ꢀ nLn)
(Me3SiC2CBCSiMe3) (n ¼ 0, 1, L ¼ PMe3; n ¼ 2,
L ¼ PMe3, PPh2Me, dppm, dppa) (dppm ¼ 1,2-
bis(diphenylphosphino)methane, dppa ¼ 1,2-bis(diph-
enylphosphino)amine) [7]. In these complexes, the
presence of an uncoordinated triple bond allows its use in
further reactions with another metal fragments. Here we
report the reactions with Co2(CO)8 and the electro-
chemical study of the complexes obtained. Also, the
electrochemical behaviour of the complexes [Co2(CO)4
(dppa)]2(Me3SiC2C2SiMe3) [8], [Co2(CO)4(dppm)]2
(Me3SiC2(CBC)2C2SiMe3) [9], [Co2(CO)4(dppm)]2
[Co2(CO)6](Me3SiC2CBCC2C2SiMe3) and [{SiMe3(Co2
(CO)4(dppm))C2}2(HCBC)(1,3,5-C6H3)], with two or
three Co2 units linked by a p-conjugated carbon bridge,
are reported together with that of [{SiMe3(Co2(CO)4
* Corresponding author. Tel.: +34914974838; fax: +34914974833.
E-mail address: mjose.macazaga@uam.es (M.J. Macazaga).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.12.020

2070
R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
(dppm))C2}(HCBC)2(1,3,5-C6H3)] [10]. With the trie-
thynyl benzene complexes we intended the homodimer-
ization of the terminal acetylene in order to introduce a
sp carbon chain, (–CBC–)2, between the two benzene
rings. This should allow the addition of other metallic
centres to the carbon chain and to study the effect on the
electronic communication between the terminal groups.
The results were not as we desired and are described here.
analyses were performed by the Microanalytical Labo-
ꢀ
ratory of the University Autonoma of Madrid on a
Perkin–Elmer 240 B microanalyzer. Mass spectra were
measured on a VG-Autospec mass spectrometer for FAB
ꢀ
by the Mass Laboratory of the University Autonoma of
Madrid.
2.2. Syntheses of [Co2(CO)4(PMe3)2][Co2(CO)6]
(Me3SiC2C2SiMe3) (4)
2. Experimental
An equimolecular mixture of Co2(CO)4(PMe3)2
(Me3SiC2CBCSiMe3) (0.22 g, 0.45 mmol) and
Co2(CO)8 (0.154 g, 0.45 mmol) in hexane (40 ml) was
stirred at room temperature for 12 h and was monitored
by FT-IR spectroscopy and by TLC (Al2O3). The so-
lution was concentrated and the reaction mixture was
separated using preparative TLC alumina plates with
hexane as eluent giving two green bands. Band 1:
[Co2(CO)6]2(Me3SiC2C2SiMe3) (0.17 g, yield 51%).
Band 2: [Co2(CO)4(PMe3)2][Co2(CO)6](Me3SiC2C2
SiMe3) (0.02 g, yield 8%). FT-IR (CH2Cl2, cmꢀ1): mCO
2083 (s), 2052 (vs), 2015 (vs), 1957 (vs), 1938 (sh).
2.1. Reagents and general techniques
All manipulations were carried out by using standard
Schlenk vacuum-line and syringe techniques under an
atmosphere of oxygen-free Ar. All solvents for synthetic
use were reagent grade. Hexane, tetrahydrofuran (THF)
and diethyl ether were dried and distilled over sodium in
the presence of benzophenone under an Ar atmosphere.
Also under Ar, CH2Cl2 (DCM) and CHCl3 were dried
and distilled over CaH2. Methanol (Aldrich) was stored
ꢁ
over molecular sieves (4 A) under Ar. All solvents were
bubbled with Ar for 1 h after distillation and then stored
under Ar, or degassed by means of at least 3 freeze–
pump–thaw cycles after distillation and before use.
Column chromatography was performed by using Alfa
neutral alumina at activity II and silica gel 100 (Fluka).
Preparative TLC was carried out on glass plates (20 ꢁ 20
cm) coated with silica gel 60 (Merck) and aluminium
sheets (20 ꢁ 20 cm) 60 F254 neutral (type E). Co2(CO)8,
trimethyl phosphine, 1,4-bis(trimethylsilyl)butadiyne
(Me3SiCBCCBCSiMe3) (Fluka), Cu(OAc)2 (Prolabo)
and 1,2-bis(diphenylphosphino)methane, pyridine, tet-
rabutylammonium fluoride (Bu4NF), KOH (Aldrich)
were used as received. Trimethylamine N-oxide
(Me3NO) (Aldrich) was sublimed prior to use and stored
under Ar. The compounds, Co2(CO)6 (Me3SiC2CB
CSiMe3) [11], Co2(CO)4(PMe3)2(Me3SiC2CBCSiMe3)
[7], Co2(CO)4(dppm)(Me3SiC2CBCH) [9], [Co2(CO)4
(dppm)]2(Me3SiC2(CBC)2C2SiMe3) (1) [9], Co2(CO)4
(dppa)(Me3SiC2CBCSiMe3) [8], [Co2(CO)4(dppa)]2
(Me3SiC2C2SiMe3) (2) [8] (SiMe3CBC)3(1,3,5-C6H3)
2.3. Syntheses of Co2(CO)4(dppm)(Me3SiC2CB
CSiMe3)
A mixture of Co2(CO)6(Me3SiC2CBCSiMe3) (0.98 g,
2.04 mmol) and 1,2 bis(diphenylphosphino)methane
(0.78 g, 2.04 mmol) in hexane (60 ml) was prepared.
Immediately Me3NO (0.36 g, 4.08 mmol) was added
with magnetic stirring and the reaction mixture was
heated at 40 °C for 6 h. After removal of the solvent
under vacuum, the residue was dissolved with CH2Cl2
and purified by column chromatography on SiO2
packed in hexane. Hexane/CH2Cl2 (1:1) eluted a red
band which yielded Co2(CO)4(dppm)(Me3SiC2CB
CSiMe3) as a red solid (1.45 g, 88% yield). FT-IR
(hexane, cmꢀ1): mCBC 2109 (vw); mCO 2028 (vs), 2006 (vs),
1
1978 (vs), 1962 (sh, w). H NMR (300 MHz, CDCl3): d
7.50–7.43 (m, 4H, Ph); 7.31–7.13 (m, 12H, Ph); 7.05–
7.00 (m, 4H, Ph); 4.00–3.85 (m, B of ABXY, 1H, P–
CH2–P); 3.45–3.25 (m, A of ABXY, 1H, P–CH2–P);
0.35 (s, 9H, Me3SiC–); 0.23 (s, 9H, BCSiMe3).
[12],
[{SiMe3(Co2(CO)4(dppm))C2}(HCBC)2(1,3,5-
C6H3)] (3) [10], [{SiMe3(Co2(CO)4(dppm))C2}2 (SiMe3-
CBC)(1,3,5-C6H3)] [10] were prepared according to
literature procedures. A modification of published
method was used for the syntheses of Co2(CO)4
(dppm)(Me3SiC2CBCSiMe3) [9] as detailed below. All
reagents were used without further purification unless
otherwise noted. The 1H, 31P and 13C NMR spectra were
recorded on a Bruker AMX-300 or 500 instrument.
Chemical shifts were measured relative either to an in-
ternal reference of tetramethylsilane or to residual pro-
tons of the solvents. Infrared spectra were measured on a
Perkin–Elmer 1650 infrared spectrometer. Elemental
2.4. Reaction of Co2(CO)4(dppm)(Me3SiC2CB
CSiMe3) with Co2(CO)8
To
a solution of Co2(CO)4(dppm)(Me3SiC2CB
CSiMe3) (0.3 g, 0.37 mmol) in hexane (50 ml) was added
with stirring Co2(CO)8 (0.13 g, 0.37 mmol) dissolved in
hexane. A brown precipitate was formed. This was fil-
tered via cannula, washed with hexane and dried in
vacuum. The removal of the solvent under reduced
pressure gave a residue which was purified by column
chromatography on Al2O3 packed in hexane. Hexane
eluted a red band which yielded a red oil. The brown

R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
2071
solid and the red oil were identified by IR and 1H NMR
as: Co2(CO)6(dppm) and Co2(CO)6(Me3SiC2CB
CSiMe3), respectively.
[Co2(CO)4(dppm)]2(Me3SiC2(CBC)2C2SiMe3). The so-
lution was stirred at 45 °C for 24 h. After this time upon
filtration via cannula a dark solid was obtained, which
was dissolved in CH2Cl2 and purified by TLC (SiO2).
CH2Cl2/hexane (1:1) eluted a brown band which yielded
7 as a brown solid (0.06 g, 60% yield). FT-IR (CDCl3,
2.5. Syntheses of [Co2(CO)4(dppm)][Co2(CO)6]
(Me3SiC2C2H) (5)
1
cmꢀ1): mCO 2083 (s), 2052 (vs), 2001 (vs), 1968 (s). H
A solution of Co2(CO)4(dppm)(Me3SiC2CBCH) (0.4
g, 0.54 mmol) in hexane (50 ml) was treated with
Co2(CO)8 (0.19 g, 0.54 mmol), and the mixture was
stirred and heated at 45 °C for 12 h. The solvent
was evaporated under reduced pressure, and the residue
was purified by TLC (SiO2) using CH2Cl2/hexane (1:1)
as eluent. The compound 5 was obtained as a green solid
(0.41 g, 75% yield). FT-IR (CDCl3, cmꢀ1): mBC–H 3302
(w); mCO 2086 (m), 2052 (s), 2024 (vs), 2001 (s), 1971 (s).
1H NMR (300 MHz, CDCl3): d 7.40 (m, o-H, 8H, Ph),
7.25 (m, p-H, 4H, Ph), 7.15 (m, m-H, 8H, Ph); 6.64 (s,
1H); 3.66 (m, B of ABXY, 1H, P–CH2–P); 3.55 (m, A of
ABXY, 1H, P–CH2–P); 0.33 (s, 9H, SiMe3). 13C NMR
(300 MHz, CDCl3): d 207.3 (m, CO); 200.0 (m, CO);
133.1 (m, i-Ph), 132.6 (m, i-Ph), 132.3 (m, o-Ph), 131.6
(m, o-Ph), 130.0 (s, p-Ph), 129.8 (s, p-Ph), 128.6 (m, m-
Ph); 82.2 (s), 72.3 (s), 71.0 (s), 67.2 (s) (Ccoord); 38.0 (t, P–
CH2–P); 1.4 (s, SiMe3). Anal. Calc. for C42H32
Co4O10P2Si: C, 52.41; H, 3.32. Found: C, 52.43; H,
3.34%.
NMR (300 MHz, CDCl3): d 7.56 (m, o-H, 16H, Ph),
7.10 (m, p-H, 8H, Ph), 7.00 (m, m-H, 16H, Ph); 3.69 (m,
1H, CH2), 3.60 (m, 1H, CH2), 3.48 (m, 1H, CH2), 3.37
(m, 1H, CH2); 0.44 (s, 9H, –SiMe3); 0.20 (s, 9H, –
SiMe3). 13C (500 MHz, CDCl3): d 207.0 (m, CO), 206.4
(m, CO), 203.7 (m, CO), 201.6 (m, CO), 199.2 (m, CO),
194.8 (s, CO); 138.6 (t, JCP ¼ 23.5 Hz, i-Ph), 138.0 (t,
JCP ¼ 24 Hz, i-Ph), 135.1 (t, JCP ¼ 17 Hz, i-Ph), 134.4 (t,
JCP ¼ 17 Hz, i-Ph), 132.7 (t, JCP ¼ 6.4 Hz, o-Ph), 132.3
(t, JCP ¼ 6 Hz, o-Ph), 131.2 (t, JCP ¼ 6 Hz, o-Ph), 130.7
(t, JCP ¼ 5.8 Hz, o-Ph), 129.44 (s, p-Ph), 129.43 (s, p-Ph),
129.3 (s, p-Ph), 129.2 (s, p-Ph), 128.5 (t, JCP ¼ 4 Hz, m-
Ph), 128.4 (t, JCP ¼ 4.7 Hz, m-Ph), 127.8 (t, JCP ¼ 5.3 Hz,
m-Ph), 127.7 (t, JCP ¼ 5.3 Hz, m-Ph); 96.0 (s), 92.0 (s),
81.3 (s), 77.1 (s), 76.9 (s), 76.5 (s) (Ccoord), 71.9 (s), 70.5
(s) (CBC); 38.2 (t, JCP ¼ 18 Hz, P–CH2–P), 35.5 (t,
JCP ¼ 20 Hz, P–CH2–P); 2.5 (s, –SiMe3); 0.5 (s, –SiMe3).
MS (FABþ) m/z: 1615.9 (Mþ ) 5CO); 1588.0
(Mþ ) 6CO); 1531.9 (Mþ ) 8CO); 1503.9 (Mþ ) 9CO);
1475.9 (Mþ ) 10CO); 1447.9 (Mþ ) 11CO); 1419.9
(Mþ ) 12CO);
1391.9
(Mþ ) 13CO);
1364.0
2.6. Syntheses of [Co2(CO)4(dppa)][Co2(CO)6]
(Me3SiC2C2SiMe3) (6)
(Mþ ) 14CO). Anal. Calc. for C78H62Co6O14P4Si2: C,
53.31; H, 3.53. Found: C, 53.28; H, 3.57%.
A solution of Co2(CO)4(dppa)(Me3SiC2CBCSiMe3)
(0.3 g, 0.37 mmol) in hexane (50 ml) was treated with
Co2(CO)8 (0.13 g, 0.37 mmol), and the mixture was stirred
and heated at 60 °C for 12 h. The solvent was evaporated
under reduced pressure, and the residue was purified by
column chromatography on Al2O3 packed in hexane.
CH2Cl2/hexane (1:1) eluted a green band which yielded 6
as green solid (0.28 g, 70% yield). FT-IR (CH2Cl2, cmꢀ1):
2.8. Syntheses of [{SiMe3(Co2(CO)4(dppm))C2}2
(HCBC)(1,3,5-C6H3)] (8)
[{SiMe3(Co2(CO)4(dppm)) C2}2(SiMe3CBC)(1,3,5-
C6H3)] (0.13 g, 0.08 mmol) was dissolved in a MeOH
solution saturated with KOH. The reaction mixture was
stirred for 24 h. Afterwards, it was extracted with three
portions of Et2O (40 ml) and the solvents were removed
under vacuum. The product 8 was obtained as a red
solid (0.09 g, 80% yield). FT-IR (CHCl3, cmꢀ1): mC –H
3300 (vw); mCBC 2085 (vw); mCO 2020 (s), 1993 (vs), 1966
m
NH 3329 (w); mCO 2082 (s), 2046 (vs), 2022 (vs), 2005 (sh),
1993 (sh), 1966 (m). 1H NMR (300 MHz, CDCl3): d 7.64
(m, o-H, 8H, Ph), 7.40 (m, p-H, m-H, 12H, Ph); 4.14
(t, JHP ¼ 6.5 Hz, 1H, NH), 0.43 (s), )0.12 (s) (18H,
SiMe3). 31P NMR (300 MHz, CDCl3): d 89.39 (s, br, 2P).
13C (500 MHz, CDCl3): d 200.7 (s, br, CO); 142.9 (m, i-
Ph), 139.1 (m, i-Ph), 131.3 (m, o-Ph), 130.6 (s, p-Ph), 130.1
(m, o-Ph), 129.8 (s, p-Ph), 128.8 (m, m-Ph); 110.7 (s), 107.8
(s), 82.2 (s), 69.6 (s) (Ccoord); 2.4 (s), 1.5 (s) (SiMe3). Anal.
Calc. for C44H39Co4O10P2NSi2: C, 48.20; H, 3.56. Found:
C, 48.17; H, 3.60%.
1
(s). H NMR (300 MHz, CDCl3): d 7.27–6.96 (m, 43H,
Ph); 3.50–3.30 (m, 4H, P–CH2–P); 3.06 (s, 1H, BC–H);
0.24 (s, 18H, –SiMe3). 31P NMR (300 MHz, CDCl3): d
34.65 (s, 4P). 13C NMR (500 MHz, CDCl3): d 207.50 (s,
CO); 202.96 (s, CO); 145.18 (s, C3); 139.10 (t, JC–P ¼ 24.0
Hz, i-Ph); 135.00 (t, JC–P ¼ 17.1 Hz, i-Ph); 133.03 (t,
JC–P ¼ 5.8 Hz, o-Ph); 132.19 (s, C4); 130.87 (t, JC–P ¼ 5.8
Hz, o-Ph); 129.99 (s, p-Ph); 129.63 (s, p-Ph); 129.04 (t,
JC–P ¼ 4.8 Hz, m-Ph); 128.29 (t, JC–P ¼ 4.8 Hz, m-Ph),
123.11 (s, C5), 122.67 (s, C6), 103.82 (s, C2), 88.64 (s, C1)
83.64 (s, C7), 78.99 (s, C8), 37.92 (t, JC–P ¼ 18.3 Hz, P–
CH2–P), 1.71 (s, –SiMe3). MS (FABþ) m=z: 1520.6
(Mþ ) H); 1493.7 (Mþ ) CO); 1465.7 (Mþ ) 2CO);
1437.7 (Mþ ) 3CO); 1409.7 (Mþ ) 4CO); 1381.7
2.7. Syntheses of [Co2(CO)4(dppm)]2[Co2(CO)6]
(Me3SiC2CBCC2C2 SiMe3) (7)
To a solution of 0.035 g (0.1 mmol) of Co2(CO)8 in
hexane (40 ml) was added 0.1 g (0.06 mmol) of

2072
R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
(Mþ ) 5CO); 1353.7 (Mþ ) 6CO); 1325.9 (Mþ ) 7CO);
1297.9 (Mþ ) 8CO); 1282.9 (Mþ ) 8CO ) CH3); 1253.9
(Mþ )8CO )3CH3 + H); 1225.8 (Mþ )8CO )SiMe3 + H);
1143.7 (Mþ ) 8CO ) 2Ph); 1123.8 (Mþ ) 8CO ) 2SiMe3 )
CBC ) H ) 3H); 889.9 (Mþ ) 8CO ) dppm ) CBC–);
795.9 (Mþ ) 8CO ) 2dppm ) 2Co); 697.9 (Mþ ) 8CO )
at settings of 50 kV and 110 mA [13]. X-ray data were
collected at 296K, with a combination of six runs at
different u and 2h angles, 3600 frames. The data were
collected using 0.3° wide x scans (20 s/frame at 2h ¼ 40°
and 60 s/frame at 2h ¼ 100°), crystal-to-detector dis-
tance of 4.0 cm.
dppm ) Co2(dppm)Me3SiC2);
599.8
(Co2(dppm)
The raw intensity data frames were integrated with
the SAINT program [14], which also applied corrections
for Lorentz and polarization effects.
The substantial redundancy in data allows empirical
absorption corrections (SADABS) [15] to be applied us-
ing multiple measurements of symmetry-equivalent re-
flections (ratio of minimum to maximum apparent
transmission: 0.780326). A total number of 32 115 re-
flections were collected and 14265 independent reflec-
Me3SiC2H); 501.9 (Co2(dppm)); 381.2 (Mþ ) 8CO )
2dppm ) 4Co). Anal. Calc. for C76H66Co4O8P4Si2: C,
59.90; H, 4.34. Found: C, 59.60; H, 4.50%.
2.9. X-ray crystallography
Red crystals of 8 were grown by slow evaporation of
the solvent from solution of the compound in CDCl3
solvent at room temperature. A summary of selected
crystallographic data for 8 is given in Table 1. A red
single crystal of approximate dimensions 0.10 ꢁ 0.05 ꢁ
0.03 mm with prismatic shape was mounted on a glass
fiber and transferred to a Bruker SMART 6K CCD
area-detector three-circle diffractometer with a MAC
Science Co., Ltd. Rotating Anode (Cu Ka radiation,
tions
remained
after
merging
Rint ¼ 0:0594,
RðrÞ ¼ 0:0626. The unit cell parameters were obtained
by full-matrix least-squares refinements of 8499
reflections.
The software package SHELXTL version 6.10 [16] was
used for space group determination, structure solution
and refinement. The structure was solved by direct
methods (SHELXS 97) [17], completed with difference
Fourier syntheses, and refined with full-matrix least-
ꢁ
k ¼ 1:54178 A) generator equipped with Goebel mirrors
2
squares using SHELXL 97 [18] minimizing xðFo2 ꢀ Fc2Þ .
Weighted R factors (Rw) and all goodness of fit S are
based on F 2; conventional R factors (R) are based on F .
All non-hydrogen atoms were refined with anisotropic
displacement parameters. All scattering factors and
anomalous dispersions factors are contained in the
SHELXTL 6.10 program library. The hydrogen atom
positions were calculated geometrically and were al-
lowed to ride on their parent carbon atoms with fixed
isotropic U.
Table 1
Crystal data and structure refinement for compound 8
Empirical formula
Formula weight
Temperature
C78 H6Cl6Co4O8P4Si2
1759.79
296(2) K
ꢁ
1.54178 A
Wavelength
Crystal system
Space group
Unit cell dimensions
triclinic
ꢂ
P1
ꢁ
a (A)
12.8384(4)
14.0668(5)
ꢁ
b (A)
One of the chloroform molecules has disorder in the
Cl atoms. After refine the occupancy of these atoms, we
obtain a value of 0.53 for the occupancy, very close to
0.5. For this reason, we fixed the occupancy of these
atoms to 0.5.
ꢁ
c(A)
25.4090(8)
92.058(2)
a (°)
b (°)
c (°)
91.960(2)
115.845(2)
4120.8(2)
3
ꢁ
Volume (A )
Z
2
1.418
Final positional parameters, anisotropic thermal pa-
rameters, hydrogen atom parameters and structure
amplitudes are available as supplementary material.
Table 2 contains selected bond distances and angles for
8. Fig. 1 presents a molecular diagram of 8.
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F ð000Þ
)
9.415
1792
Crystal size (mm3)
0.10 ꢁ 0.05 ꢁ 0.03
1.74–70.64
H range for data collection (°)
Index ranges
ꢀ15 6 h 6 15, ꢀ15 6 k 6 16,
0 6 l 6 31
14 265
2.10. Electrochemical experiments
Reflections collected
Independent reflections
Completeness to h ¼ 70.64°
Absorption correction
Refinement method
14 265 [Rint ¼ 0:0000]
90.2%
Electrochemical measurements were carried out with a
computer driven Par Mo. 273 electrochemistry system in a
three electrode cell under N2 atmosphere in anhydrous
deoxygenated solvents (CH2Cl2 and THF) containing 0.2
M tetrabutylammonium hexafluorophosphate (TBAPF6)
as supporting electrolyte. Cyclic and square-wave vol-
tammetry (CV and SWV, respectively) studies were made
in a three-electrode system. Polycrystalline Pt (0.05 cm2)
or glassy carbon were used as working electrodes; the
yes, SADABS v. 2.03
full-matrix least-squares on
F 2
14 265/0/952
0.928
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I > 2rðIÞ]
R indices (all data)
R1 ¼ 0:0598, wR2 ¼ 0:1507
R1 ¼ 0:1240, wR2 ¼ 0:1843
0.542 and )0.662
Largesꢀt 3differential peak and hole
ꢁ
(e A
)

R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
2073
Table 2
ꢁ
Bond lengths (A) and angles (°) for 8
Bond lengths
Co(1)–C(26)
Co(1)–C(27)
Co(1)–C(34)
Co(1)–C(30)
Co(1)–P(1)
Co(1)–Co(2)
Co(2)–C(29)
Co(2)–C(28)
Co(2)–C(30)
Co(2)–C(34)
Co(2)–P(2)
Co(3)–C(48)
Co(3)–C(49)
Co(3)–C(43)
Co(3)–C(44)
Co(3)–P(3)
Co(3)–Co(4)
Co(4)–C(50)
Co(4)–C(51)
Co(4)–C(44)
Co(4)–C(43)
Co(4)–P(4)
P(1)–C(1)
1.758(9)
1.778(7)
1.967(6)
1.976(6)
2.2194(17)
2.4822(13)
1.757(8)
1.769(8)
1.969(6)
1.992(6)
2.2284(18)
1.759(8)
1.784(7)
1.956(6)
1.978(5)
2.2283(17)
2.4938(13)
1.765(8)
1.779(7)
1.965(6)
1.983(6)
2.2280(18)
1.817(6)
1.824(6)
1.832(6)
1.836(7)
1.836(5)
P(2)–C(20)
P(3)–C(52)
P(3)–C(53)
P(3)–C(59)
P(4)–C(65)
P(4)–C(52)
P(4)–C(71)
Si(1)–C(30)
Si(1)–C(33)
Si(1)–C(32)
Si(1)–C(31)
Si(2)–C(45)
Si(2)–C(44)
Si(2)–C(47)
Si(2)–C(46)
C(30)–C(34)
C(34)–C(35)
C(35)–C(36)
C(35)–C(40)
C(36)–C(37)
C(37)–C(38)
C(37)–C(41)
C(38)–C(39)
C(39)–C(40)
C(39)–C(43)
C(41)–C(42)
C(43)–C(44)
1.845(6)
1.826(6)
1.833(6)
1.835(7)
1.830(6)
1.834(6)
1.840(6)
1.843(6)
1.858(7)
1.864(8)
1.867(8)
1.855(8)
1.855(6)
1.860(8)
1.861(7)
1.343(7)
1.478(7)
1.390(7)
1.392(7)
1.390(7)
1.383(7)
1.436(7)
1.379(7)
1.396(7)
1.482(7)
1.153(28)
1.351(7)
P(1)–C(13)
P(1)–C(7)
P(2)–C(14)
P(2)–C(13)
Bond angles
C(34)–Co(1)–C(30)
C(34)–Co(1)–Co(2)
C(30)–Co(1)–Co(2)
P(1)–Co(1)–Co(2)
C(30)–Co(2)–C(34)
C(30)–Co(2)–Co(1)
C(34)–Co(2)–Co(1)
P(2)–Co(2)–Co(1)
C(43)–Co(3)–C(44)
C(43)–Co(3)–Co(4)
C(44)–Co(3)–Co(4)
P(3)–Co(3)–Co(4)
C(44)–Co(4)–C(43)
C(44)–Co(4)–Co(3)
C(43)–Co(4)–Co(3)
P(4)–Co(4)–Co(3)
C(34)–C(30)–Si(1)
C(34)–C(30)–Co(2)
Si(1)–C(30)–Co(2)
C(34)–C(30)–Co(1)
Si(1)–C(30)–Co(1)
Co(2)–C(30)–Co(1)
C(30)–C(34)–C(35)
C(30)–C(34)–Co(1)
C(35)–C(34)–Co(1)
C(30)–C(34)–Co(2)
C(35)–C(34)–Co(2)
Co(1)–C(34)–Co(2)
39.8(2)
C(36)–C(35)–C(40)
C(36)–C(35)–C(34)
C(40)–C(35)–C(34)
C(37)–C(36)–C(35)
C(38)–C(37)–C(36)
C(38)–C(37)–C(41)
C(36)–C(37)–C(41)
C(39)–C(38)–C(37)
C(38)–C(39)–C(40)
C(38)–C(39)–C(43)
C(40)–C(39)–C(43)
C(35)–C(40)–C(39)
C(42)–C(41)–C(37)
C(44)–C(43)–C(39)
C(44)–C(43)–Co(3)
C(39)–C(43)–Co(3)
C(44)–C(43)–Co(4)
C(39)–C(43)–Co(4)
Co(3)–C(43)–Co(4)
C(43)–C(44)–Si(2)
C(43)–C(44)–Co(4)
Si(2)–C(44)–Co(4)
C(43)–C(44)–Co(3)
Si(2)–C(44)–Co(3)
Co(4)–C(44)–Co(3)
P(1)–C(13)–P(2)
118.2(5)
121.6(5)
120.2(5)
121.3(5)
118.9(5)
120.9(5)
120.2(5)
121.6(5)
118.5(5)
121.0(5)
120.5(5)
121.5(5)
178.5(8)
137.3(5)
70.8(3)
51.61(16)
50.88(18)
94.72(5)
39.7(2)
51.14(18)
50.73(16)
98.61(5)
40.2(2)
51.21(16)
50.54(17)
95.43(5)
40.0(2)
51.00(16)
50.25(16)
97.55(5)
144.8(5)
71.1(4)
136.3(4)
69.3(3)
136.2(4)
78.5(2)
133.7(3)
69.7(3)
147.4(5)
70.7(3)
131.6(3)
78.0(2)
132.6(3)
69.0(3)
138.9(6)
70.4(4)
130.4(3)
78.5(2)
137.5(4)
69.3(4)
110.4(3)
110.1(3)
134.4(4)
77.7(2)
P(3)–C(52)–P(4)
counter electrode was a Pt gauze and the reference elec-
trode was a silver wire quasi-reference electrode.
Decamethylferrocene (Fc*) was used as internal standard,
and all potentials in this work are referred to the Fc*þ/Fc*
couple. Under the actual experimental conditions, E1=2 of
the ferrocene couple (Fcþ/Fc) was +0.44 V versus Fc*þ/
Fc* in THF solution and +0.55 V versus Fc*þ/Fc* in
CH2Cl2 solution.

2074
R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
PMe3 ligands allows the addition of Co2(CO)8 to the
uncoordinate triple bond in Co2(CO)4(dppa) (Me3-
SiC2CBCSiMe3) giving [Co2(CO)4(dppa)] [Co2
(CO)6](Me3SiC2C2SiMe3) (6) as a green solid. The IR
spectrum shows six bands, in the 1960–2082 cmꢀ1 range,
due to the terminal carbonyl ligands and a weak band at
1
3329 cmꢀ1 owing to mN–H of the dppa ligand. The H
NMR spectrum exhibits two multiplets at 7.64 and 7.40
ppm for the ortho and meta and para hydrogens, of the
phenyl groups in the dppa ligand, respectively. A triplet
signal due to the dppa NH proton, at 4.14 ppm (J ¼ 6:5
Hz), together with two singlets at 0.43 and )0.12 ppm
owing to SiMe3 groups, are observed. The 13C NMR
spectrum shows the resonances due to the dppa phenyl
groups and the coordinate carbon atoms of the –C4–
chain (Section 2). The dppm analogous of 6 could not be
isolated because the addition of Co2(CO)8 to the unco-
ordinate triple bond in Co2(CO)4 (dppm) (Me3SiC2CB
CSiMe3) caused the loss of the adjacent Co2(dppm) unit
giving Co2(CO)6(dppm) [20] and Co2(CO)6(Me3SiC2CB
CSiMe3) [11]. So, the trimethylsilyl group could also
contribute to the steric hindrance in these molecules. In
accord with this the addition of Co2(CO)8 to
Co2(CO)4(dppm) (Me3SiC2CBCH) in hexane gives
[Co2(CO)4(dppm)][Co2(CO)6] (Me3SiC2C2H) (5), with
two dicobalt units, as a relatively stable solid. It has been
1
characterised by IR and H, 13C NMR spectroscopy.
The infrared spectrum shows five bands due to the ter-
minal carbonyl ligands and an weak band at 3302 cmꢀ1
owing to mBCH. The 1H NMR spectrum shows a singlet at
d ¼ 6:64 ppm, owing to the acetylenic hydrogen which is
shifted at lower field with respect to the starting com-
pound Co2(CO)4(dppm) (Me3SiC2CBCH) (d ¼ 3:56
ppm). The trimethylsilyl group is observed as a singlet at
0.33 ppm. The resonances of the –C4– chain of the
butadienyl ligand and the TMS group are observed as
singlets. The dppm phenyl groups give multiplets signal
for the ipso, orto and meta carbon atoms and sin-
glet resonances for the carbon atoms in para position
(Section 2).
Fig. 1. ORTEP diagram of 8, H atoms have been removed for clarity.
3. Results and discussion
3.1. Syntheses and spectroscopic characterisation
The addition of Co2(CO)8 to the triple bond of
Co2(CO)4(PMe3)2(Me3SiC2CBCSiMe3) causes the loss
of the PMe3ligands leading to the known complex
[Co2(CO)6]2(Me3SiC2C2SiMe3) [11]. Also the compound
[Co2(CO)4(PMe3)2][Co2(CO)6](Me3SiC2C2SiMe3)
(4)
R = H,
R = SiMe3,
(dppm) (5)
(dppa) (6)
L-L =
L-L =
was obtained as a very unstable green solid in a very low
yield and it could be characterised only by IR spectros-
copy. This shows five bands in the characteristic range of
terminal carbonyl ligands. The three at lower frequencies
(2015, 1957, 1938 cmꢀ1) belong to the Co2(CO)4(PMe3)2
Me3Si
R
L
unit and the two at higher frequencies (2083, 2052 cmꢀ1
)
are due to the Co2(CO)6 unit. The loss of the labile
monodentate phosphine ligands suggests the existence of
steric repulsion between the carbonyl ligands of the
Co2(CO)6 unit and the axial PMe3 ligands [19] of the
adjacent Co2 unit. The substitution of the less labile
bidentate dppa ligand, in equatorial position [8], by the
The electrochemical behaviour of the complex syn-
thesized by Diederich [9] [Co2(CO)4(l-dppm)]2
(Me3SiC2(CBC)2C2SiMe3) (1) was studied, showing the
existence of electronic communication between the two
Co2 centres. In order to study the influence of the
presence of other metallic centres between them, we

R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
2075
carried out the reaction of 1 with Co2(CO)8 in 1:1 ratio
in hexane solution. brown solid [Co2(CO)4
L
L
L
L
A
(dppm)]2[Co2(CO)6](Me3SiC2CBCC2C2SiMe3) (7) con-
taining three Co2 units was obtained. It was character-
ised by IR, 1H and 13C NMR spectroscopy. The IR
spectrum shows a weak band at 2104 cmꢀ1, owing to
mCC of the uncoordinated carbon–carbon triple bond,
and five bands due to terminal carbonyl ligands: 2083,
2052 cmꢀ1 [Co2(CO)6] and 2025, 2001, 1968 cmꢀ1
[Co2(CO)4(dppm)]. The Co2(CO)6 coordination makes
the two Co2(dppm) units unequivalent and this is re-
flected in the NMR spectrum. Thus, this spectrum
shows a double number of signals than 1 NMR spec-
trum. The CH2 protons of the dppm ligands are not
equivalent and give rise to four multiplets (Section 2).
The unequivalent SiMe3 groups give two singlets
(d ¼ 0:44 and 0.20 ppm). In the 13C NMR spectrum the
–C8– chain gives eight resonances in the 96.05–70.50
ppm range and the CH2 of the phosphine ligands give
rise to two triplets at 35.5 and 38.2 ppm (JCP ꢂ 19 Hz).
The dppm ligands give four triplet signals for the ipso
(JCP ꢂ 24 and 17 Hz), orto (JCP ꢂ 6.1 Hz) and meta
(JCP ꢂ 5 Hz) carbon atoms and four singlets for the
carbon atoms in para position (Fig. 2).
Co(CO)2
Co(CO)2
Co (CO)2
Co(CO)2
SiMe3
SiMe3
L-L = (dppm) (7)
The
complex
[{SiMe3(Co2(CO)4(dppm))C2}2
(HCBC)(1,3,5-C6H3)] (8), with a triethynylbenzene
core, was prepared by protonation of [{SiMe3
(Co2(CO)4 (dppm))C2}2(SiMe3CBC)(1,3,5-C6H3)] with
KOH in MeOH. The IR spectrum shows two weak
bands at 3300 and 2085 cmꢀ1 owing to mC–H and mCBC
vibrations, respectively. The terminal carbonyl ligands
give three bands in the range 2020–1960 cmꢀ1. The
trimethylsilyl and acetylenic protons resonances are
found at d ¼ 0:24 and 3.06 ppm, respectively (1H NMR
spectrum). The acetylenic carbons are located in the
13C NMR spectrum at 103.82 (s, C2), 88.64 (s, C1),
83.64 (s, C7) and 78.99 (s, C8) ppm. The carbon atoms
of the C6H3ring give four signals in the 120–145 ppm
range. The assignment has been made on the basis of
published data [10,21]. A singlet at 1.71 ppm is assigned
to SiMe3 group.
39.5 39.0 38.5 38.0 37.5 37.0 36.5 36.0 35.5 35.0 34.5
139.5139.0138.5 138.0137.5 137.0136.5136.0 135.5135.0 134.5134.0
133
132
131
130
129
128
127
126
125
124
123
122
121
Fig. 2. 13C NMR (CDCl3) spectrum of 7.

2076
R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
the Co2 units. Both distances lie in the range 1.33–1.36
ꢁ
A reported for the alkylenic C–C bond in related dico-
balt complexes [23]. The change in hybridisation at
C(30)–C(34) and C(43)–C(44) also is reflected in the
angles C(30)–C(34)–C(35), 138.9(6)°, C(34)–C(30)–Si(1),
144.8(5)°, C(44)–C(43)–C(39), 137.3(5)° and C(43)–
C(44)–Si(2), 147.4(5)°. The bridging ethynyl ligands are
highly twisted out of the plane of the linking aryl group.
The relevant torsion angles are, C(30)–C(34)–C(35)–
C(40), )42.8(10)°; C(30)–C(34)–C(35)–C(36), 139.3(7)°;
C(44)–C(43)–C(39)–C(40), )43.7(10)°; C(44)–C(43)–
C(39)–C(38), 134.6(7)°. This twisting shows that the
molecule presents a significative steric congestion similar
to that found in [{SiMe3(Co2(CO)4(dppm))C2}2
(SiMe3CBC)(1,3,5-C6H3)] [10]. The Co–C distances in
1
6
2
3
4
5
7
8
L-L = (dppm) (3)
(dppm) (8)
L-L =
ꢁ
the ÔCo2C2Õ core of 8 range from 1.956(6) to 1.992(6) A
The oxidative coupling of 8 under Eglington–Glaser
conditions [22] was tried, in order to obtain the dimeric
compound [{SiMe3(Co2(CO)4dppm)C2}2(CBC)(1,3,5-
C6H3)]2 with a four atoms –sp– chain between the two
phenyl rings, but no reaction took place under the
present experimental condition. This unsuccessful result
can be explained on basis of the molecular structure of 8
determined by single-crystal X-ray diffraction (Fig. 1).
This shows that the phenyl rings of the dppm ligands are
found surrounding the acetylenic hydrogen and thus the
coupling reaction is probably kinetically impeded for
esteric reasons. The oxidative coupling of 3, under the
same conditions, was also unsuccessful. The starting
compound was recovered unaltered after four days of
reaction.
and these distances do not show an asymmetry pattern
as observed in [{Co2(CO)6(HCBC)}2(C6H4)] [23].
The average P–C distance of the bridging dppm li-
ꢁ
gands, 1.832(6) A, is normal [24]; the P–C bond lengths
are statistically equivalent. All other bond lengths and
angles are comparable to those reported for similar
structures [8,10].
3.3. Electrochemical study
Table 3 summarizes data of E1=2 for the oxidation and
reduction of compounds 1–3 and 5–8, whose electro-
chemical behaviour was studied by means of CV and
SWV at ambient and low ()15, )30 °C) temperature.
Compounds 1, 2 and 8 have two equivalent phos-
phine-substituted dicobalt redox centers. In the three
cases, when adequate conditions are set (solvent, tem-
perature, etc.) two distinct oxidation and/or reduction
waves are observed, instead of a single bielectronic one.
3.2. Description of the crystal and molecular structures of
8
The compound 8 crystallizes with two CDCl3 mole-
ꢂ
The potential separation between the two waves, DE1=2,
is related to the thermodynamic stability of the mixed-
cules in the triclinic crystal system, space group P1. Its
valence species. DE1=2 depends on the extend of elec-
molecular structure consists of a trisubstituted central
benzene ring with an ethyne and two bimetallic Co
complex moieties at the 1,3,5 positions. Each bimetallic
Co complex has two terminal CO ligands on the Co
atoms, bridging dppm ligands and bridging trimethyl-
silylethynyl ligands. The geometric parameters for 8
have been summarised in Table 2, and Fig. 1 presents a
view of the molecule with the atom-labelling scheme.
The Co–Co distances in the two complexes,
Table 3
Electrochemical data for 1–3 and 5–8a
E1=2 for
reduction
DE1=2
(red)
E1=2 for
oxidation
DE1=2
(ox)
1
)1.5; )1.68
()1.56; )1.78)
0.18
(0.22)
0.71; 0:91b
(0.69; 0.83b)
(0.34; 0.79c)
0.76
(0.63; >1.30d)
(0.64; >1.30d)
(0.61; 0.76)
0.77; 0.92
0.20
(0.14)
2
3
5
6
7
8
ꢁ
2.4822(13) and 2.4938(13) A are comparable to those
reported for the compounds [{SiMe3(Co2(CO)4
ꢀ1:75b
()1.13b; )2.10b)
()1.09b; )2.13b)
()1.09c)
(0.97)
(1.04)
(dppm))C2}2(SiMe3CBC)(1,3,5-C6H3)]
[10]
and
(0.15)
0.15
Co2(CO)4(dppa)(Me3SiC2CBCSiMe3 [8] (and refer-
ences therein).
The C(30)–C(34) and C(43)–C(44) distances, 1.343(7)
ꢀ1:59b; ꢀ1:73b
0.14
a In V vs. Fc*þ/Fc* in CH2Cl2 solution (values in italics are in THF
solution). Data are taken from CV and SWV at 25 °C unless otherwise
stated.
ꢁ
ꢁ
A and 1.351(7) A, respectively, in the ethynyl ligand are
much longer than the C(41)–C(42) triple bond of
b From CV and SWV at )30 °C.
ꢁ
1.152(8) A. This reflects the loss of triple bond character
as a result of coordination of the acetylenic moieties to
c From SWV of irreversible peak.
d Irreversible processes.

R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
2077
tronic interactions between the redox centers through
the bridging ligands [25], but is also influenced by other
factors like the coulombic repulsions or molecular
rearrangements during the oxidation or reduction
processes [2a].
10
0
At room temperature, the two monoelectronic waves
obtained in the oxidation of the compounds are com-
pletely reversible for 8 (in the range of sweep rates, v,
from 0.02 to 20 V sꢀ1), but only the first one is chemi-
cally reversible in the cases of 1 and 2, the second wave
-10
-20
-30
being quasi-reversible for 1 (ipc/ipa ffi 0.8 at 0.1 V sꢀ1
)
and irreversible for 2. However, when temperature is
decreased to )15 °C, both oxidation waves in 1 are
completely reversible (Fig. 3). These data indicate that,
while 8þ and 82þ are chemically stable under the present
experimental conditions, an EEC mechanism takes place
in the oxidation process of 1 and 2; 12þ undergoes a slow
chemical reaction which is quenched at low temperature,
whilst the decomposition of 22þ is much faster.
-2.0
-1.6
-1.2
-0.8
Potential (V vs Fc*+/Fc*)
Fig. 4. Cyclic (—) and square wave (- - -) voltammograms for the re-
duction of 1 in THF containing 0.2 M TBAPF6 at 25 °C. CV: v ¼ 0:1
V sꢀ1. SWV: scan increment ¼ 2 mV, SW amplitude ¼ 25 mV; fre-
quency ¼ 60 Hz. Pt working electrode.
The voltammetric reduction of 1 in THF shows two
completely reversible waves at 25 °C (0.02 < v < 40 V
very close to DE1=2 reported in the same solvent for re-
lated compounds with analogous organic spacer. Thus,
DE1=2 is 0.39 and 0.40 V for the oxidation and reduction,
respectively, of [Co2(CO)4(dppm)]2(PhC2C2Ph) in THF
[6a] (DE1=2 (ox) ¼ 0.45 V in CH2Cl2 [5a]). 2 is also re-
lated to [Co2(CO)6]2(RC2C2R) (R ¼ Ph, CH3, SiMe3),
for which DE1=2 (red) are 0.40, 0.46 and 0.48 V, re-
spectively, in THF solution [6a] (although DE1=2 has
been reported as 0.23 V for the Ph-substituted com-
pound in CH2Cl2 solution [26]). It is noteworthy that
the value of E1=2 for the first (and completely reversible)
oxidation wave in 2 (0.34 V) is significantly less positive
than that found for the corresponding monosubstituted
complex [Co2(CO)4(dppa)][Me3SiC2CBSiMe3] under
the same experimental conditions, which is 0.69 V [7].
This fact is indicative of interaction between the two
redox centres, as it is generally accepted that a redox
process becomes energetically more favourable as the
degree of interaction increases [6d,25c].
s
ꢀ1, Fig. 4). When the solvent is DCM, the reversibility
of the second wave cannot be well appreciated at room
temperature, but it is evident at )30 °C, probably be-
cause it takes place at a very negative potential value,
very close to the potential limit of the solvent system. A
similar situation takes place with compound 8, where
the two reduction peaks can only be observed in SWV at
low temperature. No satisfactory results could be ob-
tained for the reduction of 2.
The irreversibility of the second oxidation wave in 2
unables the use of DE1=2 for the evaluation of the in-
teraction between the two redox centers in the complex.
The poor stability of 2 in solution also impeded low-
temperature measurements, where the chemical reaction
following 22þ formation would probably be much
slower. However, it should be noted that the peak sep-
aration obtained in THF solution is 0.44 V, which is
For compounds 1 and 8, DE1=2 values are quite similar
(Table 3). E1=2 is about 0.20 V for 1 (with a rather long
organic bridge) and, in the case of the aromatic ligand
(8), DE1=2 ¼ 0:15. This latter value is similar to those
found in this laboratory for related compounds [{SiMe3
(Co2(CO)6)C2}2(SiMe3CBC)(1,3,5-C6H3)] and [{SiMe3
(Co2(CO)6)C2}3(1,3,5-C6H3)], in which DE1=2 (red) are
0.14 and 0.18 V, respectively, and for [{SiMe3(Co2
(CO)4(dppm))C2}2(SiMe3CBC)(1,3,5-C6H3)], in which
DE1=2 (red) ¼ 0.21 V and DE1=2 (ox) ¼ 0.17 V [10].
Compound 3, with only one dicobalt redox center, is
structurally related to 8. Its oxidation, at nearly the
same potential value than the first wave in 8, is appar-
ently chemically reversible (Fig. 5). However, successive
scans on the same electrode surface show that the cyclic
voltammograms are quickly altered, probably as a
80
40
0
-40
0.4
0.8
1.2
Potential (V vs Fc*+/Fc*)
Fig. 3. Cyclic voltammogram for the oxidation of a solution of 1 in
CH2Cl2 containing 0.2 M TBAPF6 at 0.05 V sꢀ1 and )15 °C. Glassy
carbon working electrode.
consequence
contrasts with the very reversible and reproducible
of
surface
contamination.
This

2078
R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
0.15
15
10
5
0.02
0.01
0.10
0.00
A
0.05
-0.01
0.0 0.2 0.4 0.6 0.8 1.0
0.00
0
A'
-0.05
-5
-2
-1
0
1
Potential (V vs Fc*+/Fc*)
0.2
0.4
0.6
0.8
1.0
1.2
Potential (V vs Fc*+/Fc*)
Fig. 6. Cyclic voltammograms for the oxidation (- - - and inset) and
reduction (—) of 5 in THF containing 0.2 M TBAPF6 at 25 °C. v ¼ 0:1
V sꢀ1. Pt working electrode.
Fig. 5. Cyclic (—) and square wave (- - -) voltammograms for the ox-
idation of 3 in DCM containing 0.2 M TBAPF6 at 25 °C. CV: v ¼ 0:05
V sꢀ1. SWV: scan increment ¼ 2 mV, SW amplitude ¼ 25 mV; fre-
quency ¼ 10 Hz. Pt working electrode.
5þ and 6þ are chemically stable under the experimental
conditions. The second oxidation wave of 5 and 6,
however, is highly irreversible and, even at )30 °C, we
could not obtain a sufficiently reproducible behaviour
at potential >1.30 V. Therefore, it was not possible to
obtain reliable data on the electronic interaction in
these compounds by means of the electrochemical
oxidation.
behaviour of [{SiMe3(Co2(CO)4(dppm))C2}(SiMe3-
CBC)2 (1,3,5-C6H3)] [10], in which the uncoordinated
CBC systems are bonded to SiMe3 groups instead of H.
The reduction of 3 could only be observed at low tem-
perature on a Pt working electrode. A quasi-reversible
wave at a very negative potential is obtained (Table 3).
Compounds 5 and 6 contain two unequivalent Co2C2
redox centers. In these cases, two waves can be expected
both for the reduction and for the oxidation of the
complexes, even if no interaction took place between the
redox centers. Therefore, in order to check for the ex-
istence of interactions, it is necessary to compare the
values of E1=2 with those of related compounds with the
same butadienyl ligand and only one Co2C2 core [7,27].
Furthermore, one of the Co2C2 centers has no phos-
phine substituent, and decomposition reactions are ex-
pected both for cations and anions [7,10,19b,5a,
6a,26,28], being necessary to perform experiments at low
temperature in order to quench these fragmentation
processes.
Thus, the oxidations of 5 and 6 in THF show a first
chemically reversible (at 25 °C) wave (inset in Fig. 6) at
a potential value which is consistent with an oxidation
centered on the phosphine-substituted Co2C2 (Table 3).
The presence of the Co2(CO)6C2 core does not seem to
influence much the potential value as, for example, the
oxidation of [Co2(CO)4(dppa)][Me3SiC2CBCSiMe3]
takes place at +0.69 V [7] and, as reflected in Table 3, 6
is oxidized to 6þ at +0.64 V. This can be due to two
opposing effects: the interaction between the two redox
centers and the electron-withdrawing character of the
Co2(CO)6C2 unit. On the contrary, it is noteworthy that
in the case of compound 2, the presence of two
Co2(CO)4dppa units (electron-donating) makes the first
oxidation much more positive than for the monosub-
stituted compound (0.34 V versus 0.69 V). The cations
More satisfactory results were achieved by the study
of the CV and SWV reduction of 5 and 6. At room
temperature, both compounds undergo a first chemi-
cally irreversible reduction at ca. )1.10 V (SWV), which
can be attributed to the Co2(CO)6C2 redox center. If the
cathodic scan continues beyond )1.4 V, new chemically
reversible peaks are observed at E1=2 ¼ ꢀ1:75 V for 5
and at E1=2 ¼ ꢀ1:70 V for 6 (Fig. 6). As the peak at ca.
)1.10 V is completely irreversible, indicating that a
chemical reaction follows the production of the
monoanions, it is not possible to assign the second peak
to the reduction of the phosphine-substituted Co2C2
core in 5ꢀ and 6ꢀ. However, these reversible waves ap-
pear at potential values which are coincident with those
reported by Snaith et al. [27] for the reduction of
[Co2(CO)4(dppm)][Me3SiC2CBCH] and by our own
group [7] for [Co2(CO)4(dppa)][Me3SiC2CBCSiMe3],
respectively, what seems to indicate that these species
are formed from 5ꢀ and 6ꢀ by loss of Co2(CO)6 units.
Furthermore, in the anodic scan following reduction of
5 a new small reversible couple (A/A0 in Fig. 6) appears
after that corresponding to 5/5þ, and its E1=2 is consis-
tent with the oxidation of [Co2(CO)4(dppm)]
[Me3SiC2CBCH] [27]. The analogous couple with
compound 6 could not be observed, as the oxidation of
[Co2(CO)4(dppa)][Me3SiC2CBCSiMe3] takes place at
E1=2 ¼ 0:69 V [7], too close to E1=2 (6/6þ) to resolve the
two couples. Fig. 6 shows that in the fragmentation of
5ꢀ and 6ꢀ the anion Co(CO)ꢀ4 is also produced, as
shown by its oxidation peak at ca. 0.13 V.

R.M. Medina et al. / Inorganica Chimica Acta 357 (2004) 2069–2080
2079
tinues towards more negative values, two new reversible
waves appear at )1.56 and at )1.78 V. As shown in
Table 3, these values correspond to the reduction of
compound 1, which is formed after the monoanion 7ꢀ
undergoes a chemical reaction in which it losses
Co2(CO)6. Experiments at low temperature with 7 were
not good enough to be able to quench 7ꢀ fragmentation
and to detect the further reduction waves of the
complex.
0
-5
-1
0
-2.5
-2.0
-1.5
-1.0
-0.5
Potential (V vs Fc*+/Fc*)
Acknowledgements
Fig. 7. Square wave voltammograms for the reduction of 5 in THF
containing 0.2 M TBAPF6 at 25 °C (- - -) and at )30 °C (—). Scan
increment ¼ 2 mV, SW amplitude ¼ 25 mV; frequency ¼ 30 Hz. Pt
working electrode.
ꢀ
We express our great appreciation to the Direccion
General de Investigacion Cientıfica y Tecnologica
(Grant No. BQU2002-02522), Spain.
X-ray diffraction data were collected at the Mono-
crystal Diffraction Laboratory SIDI. Facultad de Cien-
ꢀ
ꢀ
ꢀ
At low temperature ()30 °C), the ‘‘Co2(CO)6C2’’ re-
duction wave is quasireversible, as an anodic peak is
clearly apparent both for 5 and 6. Consequently, 5ꢀ and
6ꢀ are only partially decomposed, and the reversible
wave at ca )1.70 V diminishes, while a new reversible
process appears at ca. )2.10 V (Fig. 7). This last wave
corresponds to the reduction of 5ꢀ and 6ꢀ to 52ꢀ and
62ꢀ, respectively.
As shown in Table 3, DE1=2 (red) for 5 and 6 are 970
and 1040 mV, respectively. If no relevant interaction
between the redox centers existed in these compounds,
we would expect DE1=2 values in the order of the dif-
ference in E1=2 of the corresponding monosubstituted
derivatives, i.e., ca. 700–750 mV [7,27]. As the actual
DE1=2 values are ca. 300 mV bigger, it can be concluded
that a significant interaction between the unequivalent
redox centers takes place in these complexes.
Compound 7 contains three Co2C2 units, two of them
(terminal) are phosphine-substituted Co2(CO)4 (dppm)-
C2 and the other one is Co2(CO)6C2. Thus, its electro-
chemical behaviour resembles those of 5 and 6 and, in its
CV oxidation, two reversible monoelectronic waves are
observed in THF at 0.61 and 0.76 V which can be at-
tributed to the Co2(CO)4(dppm)C2 terminal systems.
The oxidation of the Co2(CO)6C2 unit could not be
observed in the available range of potential values for
the solvent system. A comparison of E1=2 values for 7
and the parent compound 1 (in THF) indicates that the
presence of the Co2(CO)6C2 center does not alter much
the degree of interaction between the terminal redox
centers, but that they are oxidized at E values ca. 0.08 V
more positive (i.e., easier to be oxidized).
ꢀ
cias. Universidad Autonoma de Madrid.
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