(η4-Tetraarylcyclobutadiene)(η5-formylcyclopentadienyl)cobalt(I) complexes: Facilities to finetune the electron-donating capability in dipolar organometallics

Article abstract of DOI:10.1016/j.jorganchem.2007.01.043

Five different (η⁴-tetraarylcyclobutadiene)(η⁵-formylcyclopentadienyl)cobalt(I) complexes (1a-1e) were synthesized in reasonable yields in a one-pot reaction of CoCl(PPh₃)₃, formylcyclopentadienyl sodium and the appropriate diarylethyne. The aryl groups of the ethyne were modified by various para-substituents X (X = Cl, H, Me, OMe, NMe₂), which were intended to alter the redox potentials of the synthesized cobalt sandwich complexes. A cyclic voltammetry study revealed a linear dependence of the first oxidation potential to the Hammett parameter σ_p. X-ray structure analyses performed for two complexes (X = Me and NMe₂) demonstrate only subtle changes in the solid state structure despite the large differences in electrochemical properties. A theoretical analysis by the density functional theory method has been performed on the geometries and electronic structures of the complex (η⁴-cyclobutadiene)(η⁵-cyclopentadienyl)Co(I), its cation and dication.

Full text of DOI:10.1016/j.jorganchem.2007.01.043

Journal of Organometallic Chemistry 692 (2007) 2216–2226
www.elsevier.com/locate/jorganchem
(g4-Tetraarylcyclobutadiene)(g5-formylcyclopentadienyl)cobalt(I)
complexes: Facilities to finetune the electron-donating capability
in dipolar organometallics
*
Sven Dabek, Marc Heinrich Prosenc, Jurgen Heck
¨
Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
Received 30 December 2006; received in revised form 20 January 2007
Available online 30 January 2007
Dedicated to Dr. Karel Mach on the occasion of his 70th birthday.
Abstract
Five different (g4-tetraarylcyclobutadiene)(g5-formylcyclopentadienyl)cobalt(I) complexes (1a–1e) were synthesized in reasonable
yields in a one-pot reaction of CoCl(PPh3)3, formylcyclopentadienyl sodium and the appropriate diarylethyne. The aryl groups of the
ethyne were modified by various para-substituents X (X = Cl, H, Me, OMe, NMe2), which were intended to alter the redox potentials
of the synthesized cobalt sandwich complexes. A cyclic voltammetry study revealed a linear dependence of the first oxidation potential to
the Hammett parameter rp. X-ray structure analyses performed for two complexes (X = Me and NMe2) demonstrate only subtle changes
in the solid state structure despite the large differences in electrochemical properties. A theoretical analysis by the density functional the-
ory method has been performed on the geometries and electronic structures of the complex (g4-cyclobutadiene)(g5-cyclopentadienyl)-
Co(I), its cation and dication.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Cobalt sandwich complexes; Tetraarylcyclobutadiene; Carboxaldehyde; Cyclic voltammetry; X-ray structure analysis; DFT calculations
1. Introduction
ruthenocenes, due to their inertness and the facile synthetic
availability of derivatives [2f–l,3–5]. However, to the best
of our knowledge, the sandwich complex (g4-cyclobutadi-
ene)(g5-cyclopentadienyl)cobalt(I), which is isoelectronic
to ferrocene, has not been applied as an electron donating
unit in dipolar heterobimetallic complexes directed towards
NLO properties except for a short note in a review article
[2h]. Since we are looking more closely at sesquifulva-
lene-type dipolar organometallic complexes with respect
to their NLO properties [2l], attempts were made to coor-
dinate the (g4-cyclobutadiene)cobalt moiety to the cyclo-
pentadienyl part of a sesquifulvalene entity. In previous
papers, we presented strategies to build up dipolar, mono-
and dinuclear sesquifulvalene complexes and the vinylogue
congeners based on ferrocene and ruthenocene derivatives.
These are synthesized straightforwardly by using the
appropriate cyclopentadienyl carboxaldehyde sandwich
complex as starting material. Subsequent Wittig–Horner–
The concept of dipolar cationic heterobimetallic com-
plexes for compounds with non-linear optical (NLO) prop-
erties [1], e.g. with high first order hyperpolarisability b,
has been shown to be very useful [2]. These complexes
are comprised of the combination of an organometallic
electron donating and accepting group connected by a
bridge, which enables electronic p-interaction between
them. For organometallic electron accepting groups car-
bonyl complexes were employed [2e,2f] as well as Fischer-
type carbenes [2g,2n] and cationic complexes of organic
p-ligands [2c,d,f–o]. The most widely used donating units
are sandwich type complexes based on ferrocenes and
*
Corresponding author. Tel.: +40 42838 6375; fax: +40 42838 6945.
E-mail address: juergen.heck@chemie.uni-hamburg.de (J. Heck).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.01.043

S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
2217
O
H
H
C
C
O
H
H
C
C
H
C
C
C
O
H
H
C
C
H
H
ML
ML
ML
WHWE
WHWE
WHWE
n+
n+
M'L'
n+
H
M'L'
H
C
H
C
C
M'L'
C
H
C
H
C
C
H
H
C
C
H
H
C
H
C
C
H
H
ML
ML
ML
Scheme 1. Overview of the routes used to synthesize vinylogue sesquifulvalene complexes by WHWE reaction starting from a sandwich carboxaldehyde
[2m,6].
Wadsworth–Emmons (WHWE) reactions extend the olefin
bridge between the donor and the acceptor of a dipolar ses-
quifulvalene complex (Scheme 1) [2m,6].
acetylene and reveals the sandwich complexes 1 in a very
low overall yield [8]. The yields of the desired complexes
1a–1e can be improved considerably, when the synthesis
of 1a–1e is performed in accord with a procedure described
first by Takahashi et al. [9]. In a one-pot reaction stoichi-
ometric amounts of CoCl(PPh3)3, formylcyclopentadienyl
sodium and the appropriate tolane were allowed to react
(Scheme 2b) [8,10]. Complexes 1a–1e were obtained in
30–70% yield.
In this paper, we want to present syntheses, molecular
structures and spectroscopic properties of carboxaldehyde
building blocks as starting material for vinylogue (g4-
cyclobutadiene)(g5-cyclopentadiendiyl)cobalt(I) complexes
as electron-donating groups. Moreover, to finetune elec-
tronic properties, the cyclobutadiene ligands are provided
with four aryl substituents whose para-position is modified
by different X-groups (Fig. 1).
2.2. Solid state structure
ꢀ
2. Results and discussions
Complex 1c crystallizes in the triclinic space group P1
and for 1e the monoclinic space group Pc is found. Both
of the complexes 1c and 1e display very similar Co–C dis-
tances, which are slightly shorter for the cobalt–cyclobut-
adiene (CBD) unit (198.4(3)–199.0(3) pm for 1c and
197.7(8)–200.4(7) pm for 1e) than for the cobalt–cyclopen-
tadienyl coordination (205.4(4)–207.6(4) pm for 1c and
205.6(8)–207.9(8) pm for 1e) (Table 1). The structural data
of the sandwich core of 1c and 1e are quite comparable to
other molecular structures of (g4-cyclobutadiene)(g5-
cyclopentadienyl)cobalt(I) complexes reported in the litera-
ture [9–15]. The difference in Co–C bond lengths suggests a
stronger cobalt bonding to the CBD ligand than to the C5-
ring ligand, which is in agreement with theoretical calcula-
tions (see below) [14,15]. On the contrary, the distance
between the Co atom and the centroid of the five-mem-
bered ring is slightly reduced (167.3 pm for 1c and
168.3 pm for 1e) compared to the distance between the
Co atom and the centroid of the C4-ring (169.3 and
169.4 pm for 1c and 1e, respectively). Due to the larger ring
size, the five-membered ring immerses better into the coor-
dination sphere of the Co center than the four-membered
ring.
2.1. Synthesis
In principle two different routes can be selected for the
synthesis of (g4-tetraarylcyclobutadiene)(g5-formylcyclo-
pentadienyl)cobalt(I) (1). The first one is based on the prep-
aration developed by Rausch and Genetti [7]. Iodine is
oxidatively added to Co2(CO)8 forming the thermally
unstable carbonyl complex Co(CO)4I which is not sepa-
rated but directly treated with formylcyclopentadienyl thal-
lium (Scheme 2a). The isolated halfsandwich complex
(dicarbonyl)(g5-formylcyclopentadienyl)cobalt(I) (2) is
refluxed in p-xylene in the presence of the chosen diaryl-
O
C
H
Co
Ar
Ar
Ar
Ar
Ar =
X
1
The p-tolyl substituents of the CBD ligand in 1c are all
tilted in the range of 23–43ꢁ with respect to the plane of the
four-membered ring, and form a four-bladed propeller as
found for other arylated g4-cyclobutadiene ligands in Co
X = Cl (1a), H (1b), Me (1c), OMe (1d), NMe2 (1e)
Fig. 1. (g4-Tetraarylcyclobutadiene)(g5-formylcyclopentadienyl)cobalt(I)
complexes as target compounds.

2218
S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
O
C
H
I2/THF
-70˚C
TlCpCHO/THF/20˚C
[Co2(CO)8]
[Co(CO)4I]
Co
OC CO
2
Ar
Ar
p-xylene/reflux
O
C
H
NaCpCHO/THF
Ar Ar
toluene/reflux
[Co(PPh3)3Cl]
Co
Ar
Ar
X
Ar
Ar
Ar =
X = Cl (1a), H (1b), Me (1c), OMe (1d), NMe2 (1e)
Scheme 2. Synthesis of (g4-tetraarylcyclobutadiene)(g5-formylcyclopentadienyl)cobalt(I) complexes.
complexes (Fig. 2) [9–13]. However, a different situation is
observed for 1e: one p-dimethylamino phenyl group is
almost coplanar to the cyclo-C4-entity, the tilt angle is only
1.5ꢁ (Fig. 3); consequently, the two adjacent aryl substitu-
ents are tilted to a larger extent (49.5ꢁ and 53.7ꢁ) and in the
opposite sense. The tilt angle of the fourth aryl substituent
amounts to 36.8ꢁ and is in the range of the corresponding
angles of 1c. The structural anomaly of the tilt angles of the
aryl substituents in 1e does not go along with significant
distortions of the CBD ligand.
It is interesting to note that despite the different elec-
tronic properties of the aryl substitutents in 1c and 1e, no
influence can be observed on the bond lengths of the formyl
group of the cyclopentadienyl ligand, which is in harmony
to the spectroscopic findings (vide infra). The correspond-
ing C,C and C,O bond lengths for 1c and 1e, respectively,
are identical within the margin of error (C,C: 146.3(6) and
145.6(12) pm, and for C,O: 121.2(5) and 121.8(9) pm,
respectively).
be observed for the 13C resonance signals of the formyl
group, when the donating ability of the aryl substituent
increases. The overall shift range covers only 0.6 ppm.
Another sensitive probe for the interaction between the
aryl substituents and the formyl cyclopentadienyl ligand
should be the energy of the CO-stretching vibration of
the aldehyde function. However, a closer look at the wave-
numbers of the corresponding absorption bands in IR
spectra (Table 2) illustrates that the mCO band is hardly
influenced by the electronic property of the substituent X
in para-position of the aryl unit. Compared to 1b
(X = H) aꢀ1 significant but small bathochromic shift
~
ðDm ¼ 9 cm Þ is observed for 1e (X = NMe2).
These results are in agreement with observations of
others. Apparently, the carbonyl function is not very sensi-
tive to electronic changes in the para-position of an aro-
matic substituent. This is the case even if the CO
function is directly coupled to the para-substituent of the
aromatic ring via the organic p-bonding system as in corre-
sponding acetophenone derivatives, e.g. the para-amino-
acetophenone demonstrates bathochromic shift of the
CO-stretching mode by only 14 cmꢀ1 compared to ace-
tophenone itself [17].
It seems that the aromatic ring substituent has no signif-
icant influence on spectroscopic properties of the com-
plexes in their ground state.
An alternative to evaluate the influence of substituents
on the electronic property of the entire complex is the
investigation of the redox activity. According to the cyclic
voltammograms the formyl complexes 1a–1d undergo an
oxidation in dichloromethane, which is assumed to occur
with an electrochemically reversible one-electron transfer
2.3. Spectroscopic and electrochemical properties
In order to get an indication of the influence of the aryl
substituents in 1a–1e on the electronic interaction between
the two p-ligands, which is expected to be important in
dipolar donor–acceptor complexes with SHG activities,
their NMR and IR spectroscopic and electrochemical
properties were studied in relation to the Hammett con-
stants rp [16] (Table 2).
However, the shifts of the signals of the aldehyde pro-
tons vary only slightly and do not correlate with the rp-val-
ues. In contrast, a very small increasing low-field shift can

S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
2219
Table 1
Selected bond lengths (pm) and angles (ꢁ) obtained from X-ray structure determinations of the complexes 1c and 1e and from DFT-calculation of the
complexes 2, 3, 4 and 5 (vide infra)
Compound
1c
1e
2
3
4
5
Co–C(1)
Co–C(2)
Co–C(3)
Co–C(4)
Co–C(5)
Co–C(6)
Co–C(7)
Co–C(8)
205.4(4)
207.3(4)
207.6(4)
207.0(4)
205.0(4)
198.6(3)
198.4(3)
198.4(3)
199.0(3)
146.1(4)
146.3(4)
146.3(4)
146.2(4)
167.3
205.6(8)
205.8(9)
205.8(8)
207.9(8)
207.5(9)
198.2(8)
200.4(7)
197.7(8)
198.5(8)
149.2(10)
146.3(10)
148.2(9)
146.9(10)
168.3
204.8
205.6
206.9
206.9
205.6
198.3
198.0
198.0
198.3
217.3
214.6
210.5
210.5
214.6
200.6
201.3
201.3
200.6
215.7
214.2
206.3
206.3
214.2
180.5
220.6
220.6
180.5
216.0
212.6
199.9
199.9
212.6
187.3
240.9
240.9
187.3
Co–C(9)
C(6)–C(10)
C(7)–C(16)
C(8)–C(22)
C(9)–C(28)
Co–Cpa
165.8
169.0
175.2
172.3
172.3
161.3
168.9
175.9
Co–CBDb
169.3
169.4
C(13)–N(1)
C(19)–N(2)
C(25)–N(3)
C(31)–N(4)
C(1)–C(2)
C(1)–C(5)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(6)–C(7)
C(6)–C(9)
C(7)–C(8)
C(8)–C(9)
C(1)–C(41)
C(41)–O(1)
CBD(6–9)/Ph(10–15)c
CBD(6–9)/Ph(16–21)c
CBD(6–9)/Ph(22–27)c
CBD(6–9)/Ph(28–33)c
–
–
–
–
138.2(9)
137.0(9)
140.5(9)
139.8(11)
140.5(11)
142.1(12)
138.0(13)
141.6(12)
141.5(12)
148.1(11)
147.3(10)
145.8(10)
146.6(11)
145.6(12)
121.8(9)
53.7
143.3(5)
142.9(6)
141.5(6)
141.1(6)
142.0(6)
146.9(4)
146.6(4)
147.1(4)
147.3(4)
146.3(6)
121.2(5)
36.8
143.8
143.8
143.6
143.4
143.6
146.3
146.3
146.3
146.3
143.0
143.0
143.6
143.9
143.6
146.2
146.4
146.4
146.2
143.5
143.5
144.0
144.7
144.0
140.7
250.6
142.7
140.7
142.6
142.6
143.6
145.0
143.6
135.4
271.1
149.2
135.4
43.3
23.1
35.4
1.5
49.5
36.8
CBD–Co–Cpa,b
178.4
176.3
179.6
179.8
179.7
179.9
C(1)–C(41)–O(1)
124.9(4)
125.7(9)
a
Cp: centroid of the C5-ring ligand.
CBD: centroid of the C4-ring ligand.
Tilt angle between the best planes of the cyclobutadiene (CBD) ligand and the phenyl rings (Ph).
b
c
in agreement to other (g4-cyclobutadiene)(g5-cyclopenta-
dienyl)cobalt(I) complexes [15,18].
large as the peak current of the first oxidation at
Epa = ꢀ2 + 5 mV. In contrast to 1a–1d, the first oxidation
for 1e is irreversible although the strong electron donating
groups NMe2 are expected to stabilize the positive charge.
However, the small separation to the second and irrevers-
ible oxidation step (compare 1d) makes the first one irre-
versible as well.
Comparing the potentials (Table 2), which enclose a
range from E1/2 = 783 5 mV (X = Cl, 1a) to
Epa = ꢀ2 5 mV (X = NMe2, 1e) vs. FcH/FcH+ it can
be concluded that the first oxidation of 1a–1e distinctly
depends on the electronic nature of the aryl substituent:
the more negative the Hammett parameter rp of the
para-substituent X, the more the oxidation potential is
shifted to lower values. Taking the peak potential of the
first oxidation of 1e approximately as half wave potential
E1/2, a linear correlation is found between the oxidation
For 1d an additional irreversible oxidation wave with
almost the same peak current is found (Table 2). The
appearance of a second oxidation step for 1d at a lower
potential compared to 1a–1c is due to the stronger elec-
tron donating capability of the four para-methoxy substit-
uents, which causes a general cathodic shift. The cathodic
shift of the oxidation potentials is even more pronounced
for 1e, bearing the para-dimethylamino substituents. The
cyclic voltammogram for 1e displays three irreversible
oxidation waves. The one with the highest oxidation
potential (Epa = 550 + 5 mV vs. FcH/FcH+) is assigned
to an oxidation of the p-dimethylamino phenyl substitu-
ents in agreement with published data of p-dimethylamino
benzene [19]. Due to the number of aryl substituents in 1e
the corresponding peak current is almost three times as

2220
S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
Fig. 2. Molecular structure of 1c (with exception of the hydrogen atom of the formyl group, hydrogen atoms are omitted for clarity, the thermal ellipsoids
are shown at the 50% level).
Fig. 3. Molecular structure of 1e (with exception of the hydrogen atom of the formyl group, hydrogen atoms are omitted for clarity, the thermal ellipsoids
are shown at the 50% level).
potential E1/2 and the electron donating power of the sub-
stituent X of the aryl group, expressed by the rp-values
(Fig. 4). This correlation can be defined by the free-energy
relationship in the following equation:
ferrocene derivatives in dipolar complexes suitable for sec-
ond harmonic generation.
2.4. Electronic structure and MO-analysis
E1=2 ¼ qRrp þ C
where qR is the reaction constant, rp is electron donating
capability and C is intercept.
From the linear fit in Fig. 4, a slope of 0.75 V can be cal-
culated which is considerably larger than the qR-value
found for a similar study on substituted arylferrocenes with
qR = 0.070 V [20]. The comparably large reaction constant
qR for the cobalt sandwich complexes indicate that they are
more sensitive electron donating units than corresponding
ð1Þ
In order to gain deeper insights into the electronic
structure and redox properties of (g4-cyclobutadiene)(g5-
formylcyclopentadienyl)cobalt(I) complexes, we have per-
formed a detailed analysis by the density functional
theory method. In addition, the results of the DFT calcu-
lations may help to understand the irreversibility of the
first oxidation step in the case of 1e, which is reversible
for the derivatives 1a–1d with less electron donating
substituents X.

S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
2221
Table 2
Selected 1H NMR, 13C NMR, IR and cyclic voltammetry data in relation to the Hammett constants rp of the complexes 1a–1e
a
b
b
c
mCO ðcmꢀ1
Þ
E1/2/Epa (mV)d,e
~
Complex
X
rp
dCHO
dCHO
1a
1b
1c
1d
1e
a
Cl
H
CH3
OCH3
N(CH3)2
0.23
0
ꢀ0.17
ꢀ0.27
ꢀ0.83
9.34
9.30
9.22
9.31
9.30
190.69
190.98
191.06
191.15
191.29
1680
1683
1680
1681
1674
783f
635f
539f
432f/868g
ꢀ2g/65g/550g
rp Values obtained from the literature [16].
b
c
d
e
f
Measured in CD2Cl2.
KBr.
Scan rate: 200 mV sꢀ1
.
Potentials ( 5 mV) vs. FcH/FcH+ in CH2Cl2, Epa = peak potentials of the anodic oxidation.
Reversible oxidation.
Irreversible oxidation.
g
For calculations on the electronic structures of the
structures 1c and 1e. Deviations are within 1 pm. An anal-
ysis of the electronic structure of complex 2 revealed that
the HOMO consists mainly of a Co 3dxy orbital with d-
bonding interaction with cyclopentadienyl ligand p-orbitals
and weak antibonding ovelap with a r-cyclobutadiene
orbital. This is in agreement with previous calculations
reported in the literature [15].
neutral complexes 1a–1e, we chose as a simple model sys-
tem the complex (g4-cyclobutadiene)(g5-cyclopentadienyl)
cobalt(I) (2). Geometry optimization of 2 revealed a nearly
Cs symmetric structure with an almost linear arrangement
of the centroids of the p-ligands and the metal center
(CBD–Co–Cp angle of 179.6ꢁ). Geometrical parameters
for the optimized structure of complex 2 are listed in Table
1. For further analysis a Cs symmetric structure has been
assumed (Fig. 5). Distances for Co–C(6) of 198.3 pm,
Co–C(7) of 198.0 pm and Co–CBD of 169 pm are indica-
tive for a strong Co–C interaction of the Co center to the
four-membered ring. Slightly longer distances were found
between the cobalt atom and the carbon atoms of the C5-
ring ligand: Co–C(1) of 204.8 pm, Co–C(2) of 205.6 pm
and Co–C(3) of 206.9 pm. This would imply a weaker
Co–C interaction of the Co-center to the C5-ring, although
the distance from the Co atom to the centroid of the five-
membered ring (Co–Cp) of 165.8 pm is about 3 pm shorter
than to the centroid of the C4-ring. Optimized bond dis-
tances and angles are in excellent agreement with structural
parameters derived through X-ray structure analyses for
Upon oxidation, complex cation 3 was obtained (Fig. 5).
Geometry optimization of cation 3 revealed bonding
parameters listed in Table 1. The bond lengths Co–C(1)
(217.3 pm), Co–C(2) (214.6 pm) and Co–C(3) (210.5 pm)
are about 10 pm longer than those in complex 2, with cor-
responding cobalt–carbon atom bond lengths of 204.8–
206.9 pm. Also, the calculated distances Co–C(6) of 200.6
and Co–C(7) of 201.3 pm to the C4-ring ligand are about
3 pm longer (Co–CBD of 172.3 pm), although the arrange-
ments of the ligands around the metal center remained lin-
ear (CBD–Co–Cp angle of 179.8ꢁ). Thus, the interaction of
the Co atom with the cyclopentadienyl ligand is weakened
while interaction between the Co atom and the C4-ring
nearly remains. This can also be seen by orbital analysis:
the former HOMO in complex 2 becomes singly occupied
and the bonding interaction between the Co-atom and
the C5 ring is weakened. Due to electronic and geometrical
rearrangements the HOMO in complex 2 becomes HOMO-
7 (a) in cation 3.
1
Further oxidation resulted in the dication 4 (Fig. 5). For
geometry optimization of dication 4, we used the geometry
of monocation 3 as starting point and obtained the dicat-
ionic complex 4 depicted in Fig. 5. The distances Co–
C(1) of 215.7 pm, Co–C(2) of 214.2 pm, Co–C(3) of
206.3 pm, Co–C(6) of 180.5 pm and Co–C(7) of 220.6 pm
are indicative of a Co-center inserted into one of the C–C
bonds of the C4-ring revealing a metallacyclopentene moi-
ety. This unusual reaction raises the question for the
changes in electronic structure of complex 2 to cation 3
and dication 4.
From molecular orbital analysis it can be concluded
that upon insertion of the Co atom into the C–C bond
of the C4-ring, the loss of a C–C bonding interaction
strongly destabilizes one of the nearly degenerate
HOMOs (1a0) (Fig. 6). The other orbital (1a00) becomes
0.5
½
p
E = 0.75 + 0.63
0
-0.5
-1
-0.5
0
0.5
σp
Fig. 4. Correlation between the oxidation potentials of 1a–1e and the
corresponding Hammett parameter rp.

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S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
Fig. 5. Optimized structures of (g5-C5H5)Co(g4-C4H4) derivatives (left). Structure of the neutral complex 2, structure of the monocation 3 (middle) and
structure of the dication 4 (right).
stabilized because of the loss of C–C antibonding inter-
action and increasing Co–C overlap. The overall reaction
of the insertion in complex 2 is endothermic which is in
agreement with experimental observations. When two
electrons are withdrawn from the system, the destabiliza-
tion of 1a0 (Fig. 6) does not contribute to the reaction
energy and thus the reaction from 2 to 4 becomes
feasible.
Another interesting feature comes from the avoided
crossing of the empty orbital 2a00 which becomes more sta-
ble than 1a0 along the reaction coordinate (Fig. 6). This
would suggest that a metallacyclopentadiene Co(III)-com-
plex would be thermodynamically unstable, as indicated by
cyclic voltammetry studies, but could have some kinetic
inertness towards cylcobutadiene formation. To test this
hypotheses a putative complex 5, which can be obtained
by adding two electrons to complex 4, was optimized.
The resulting stable complex 5 exhibits a clear metallacy-
clopentadiene structure with geometry parameters in agree-
ment with previously reported Lewis-base adducts of (g5-
C5H5)Co(III)(–CH@CH–CH@CH–)(L) complexes [21].
The energy of complex 2 is calculated to be 38 kcal/mol
below complex that of 5 which has been presumed to be
an intermediate in the cyclodimerization of alkynes by
Co complexes (Scheme 2) [22].
3. Conclusions
In addition to the known compound 1b, four novel
(g4-tetraarylcyclobutadiene)(g5-formylcyclopentadienyl)-
cobalt(I) complexes were synthesized by the Takahashi
route in yields suitable for further WHWE coupling reac-
tions forming dipolar donor–acceptor compounds. The
tetraaryl groups bear substituents X of different electronic
properties in para-position (X = Cl, H, Me, OMe, NMe2).
X-ray structure analysis of the methyl and the dimeth-
ylamino derivatives 1c and 1e, respectively, reveals
structural data quite similar to known structures of
(g4-cyclobutadiene)(g5-cyclopentadienyl)Co(I) complexes,
except for the tilt angle of one phenyl unit in 1e with
respect to the cyclobutadiene plane. However, no signifi-
cant additional structural anomalies could be observed,
which might give a clue to the reason for this structural
peculiarity.
Fig. 6. Walsh diagram along the insertion path of the Co-center into the C–C bond.

S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
2223
IR and 13C NMR studies of the formyl function of the
4.2.1. [g4-Tetrakis(4-chlorophenyl)cyclobutadiene]
(g5-formylcyclopentadienyl)cobalt(I) (1a)
Cp ligands demonstrate only a weak influence of the elec-
tronic properties of the aryl substituents X on the energy
of the CO-stretching mode and on the 13C shift of the for-
myl function, respectively.
Tris(triphenylphosphine)cobalt(I) chloride (538 mg,
0.61 mmol) in toluene (5 mL), formylcyclopentadienyl
sodium (70 mg, 0.61 mmol) in THF (5 mL), bis(4-chloro-
phenyl)ethyne (300 mg, 1.22 mmol) in toluene (15 mL),
2.5 h reflux, Al2O3 neutral, 5% water; toluene/hexane 2:1.
Yield: 200 mg (51%). Mp: 254 ꢁC. Anal. Calc. for
C34H21Cl4CoO (M = 646.1): C, 63.19; H, 3.28. Found: C,
63.08; H, 3.41%. 1H NMR (200 MHz, CD2Cl2): d 4.89
(pseudo t, 3J = 2.10 Hz, 2H, C5H4), 5.22 (pseudo t,
In contrast, cyclic voltammetry measurements uncover a
linear dependence of the first oxidation potential on the rp
values, pointing out a distinct electron donating capability
of the Co complex in the case of negative rp values like for
X = OMe, NMe2. The electron donating aryl ligand lowers
the oxidation potential significantly. Thus, a decomposition
via insertion of the Co-center into the cyclobutadiene ring
and further degradation of the ensuing Co(V) center occurs.
The oxidation process reverses the dimerization of the
alkyne ligands as can be concluded from DFT-calculations.
The strong electron donating properties make the (g4-
cyclobutadiene)(g5-cyclopentadienyl)Co(I) complexes most
promising for compounds with SHG activities.
3
3J = 2.10 Hz, 2H, C5H4), 7.24 (d, J = 8.79 Hz, 8H, Hm),
7.33 (d, 3J = 9.03 Hz, 8H, Ho), 9.34 (s, H, CHO). 13C
NMR (50 MHz, CD2Cl2): d 76.26 (C4Ar4), 83.55 (C5H4),
89.09 (C5H4), 92.95 (Cq-C5H4), 129.01 (Cm), 130.27 (Co),
ꢀ1
~
133.25 (Ci, Cp), 190.69 (CHO). IR (KBr): m=cm 3108
(CH)Ph, 1680 (C@O), 1601, 1495, 1458 (C@C)Ph, 1397
(CHO), 1119, 1012 (C@C)Cp, 1081 (CCl), 827 (CH). EI-
MS: m/z (%) 647 (37), 645 (27) [M+], 124 (62) [CpCo+],
58 (100) [Co+].
4. Experimental
4.2.2. (g4-Tetraphenylcyclobutadiene)
4.1. General methods
(g5-formylcyclopentadienyl)cobalt(I) (1b) [8,10]
Tris(triphenylphosphine)cobalt(I)
chloride
(1.88 g,
Manipulations were carried out under a dry nitrogen
atmosphere using standard Schlenk technique. All solvents
were saturated with nitrogen. Diethyl ether (Et2O), tetra-
hydrofurane (THF), n-hexane and toluene were freshly dis-
tilled from the appropriate alkali metal or metal alloy.
Dichloromethane (CH2Cl2) and nitromethane (MeNO2)
were dried over calcium hydride. NMR: Varian Gemini
200 BB; Bruker AM 360; measured at 295 K rel. TMS.
UV–Vis: Perkin–Elmer Model 554. IR: KBr pellets; FT-
IR, Perkin–Elmer Model 325. MS: Finnigan MAT 311 A
(EI-MS). Elemental analyses: CHN-O-Rapid, Fa. Heraeus,
Zentrale Elementanalytik, Department Chemie, Universita¨t
Hamburg. Formylcyclopentadienyl sodium [23], formyl-
cyclopentadienyl thallium [24], tris(triphenylphosphine)-
cobalt(I) chloride [25], and the p-substituted diarylethynes
[26] were prepared according to the literature procedures.
2 mmol) in toluene (10 mL), formylcyclopentadienyl
sodium (232 mg, 0.20 mmol) in THF (5 mL), diph-
enylethyne (785 mg, 0.40 mmol) in toluene (20 mL), 6 h
reflux, Al2O3 neutral, 5% water, toluene/hexane 2:1.
Yield: 700 mg (67.5%). Mp: 186 ꢁC. 1H NMR
3
(200 MHz, CD2Cl2): d 4.91 (pseudo t, J = 2.00 Hz, 2H,
C5H4), 5.22 (pseudo t, 3J = 2.00 Hz, 2H, C5H4), 7.23–
7.30 (m, 12H, Hm,p), 7.41–7.45 (m, 8H, Ho), 9.30 (s, H,
CHO). 13C NMR (50 MHz, CD2Cl2): d 77.55 (C4Ar4),
83.43 (C5H4), 89.02 (C5H4), 92.97 (Cq-C5H4), 127.34 (Cp),
128.52 (Cm), ꢀ1129.17 (Co), 135.25 (Ci), 190.98 (CHO). IR
~
(KBr): m=cm 3079, 3057, 3026 (CH)Ph, 1683 (C@O),
1596, 1499, 1444 (C@C)Ph, 1391 (CHO), 1155, 1026, 744,
707.
4.2.3. [g4-Tetrakis(4-methylphenyl)cyclobutadiene]
(g5-formylcyclopentadienyl)cobalt (1c)
4.2. General procedure for the synthesis of the (g4-tetraaryl-
cyclobutadiene)(g5-formylcyclopentadienyl)cobalt
complexes 1a–1e
Tris(triphenylphosphine)cobalt(I) chloride (442 mg,
0.50 mmol) in toluene (6 mL), formylcyclopentadienyl
sodium (69 mg, 0.60 mmol) in THF (2 mL), bis(4-methyl-
phenyl)ethyne (200 mg, 1.00 mmol) in toluene (3 mL),
4.5 h reflux, Al2O3 neutral, 5% water, hexane/diethylether
10:1.
Yield: 165 mg (60.2%). Mp: 258 ꢁC. Anal. Calc. for
C38H33CoO(CH2Cl2) (M = 649.52): C, 72.12; H, 5.43.
Found: C, 71.53; H, 5.54%. 1H NMR (200 MHz, CD2Cl2):
To a suspension of tris(triphenylphosphine)cobalt(I)
chloride in toluene a solution of THF with one equivalent
of formylcyclopentadienyl sodium was added. The reaction
mixture was stirred for 30 min at room temperature, during
which the colour of the solution turned red. A toluene solu-
tion containing two equivalents of the appropriate diary-
lethyne was added. The reaction mixture was stirred for
2.5–8 h under reflux. The reaction was monitored by thin
layer chromatography. Finally, the reaction mixture was
evaporated to dryness and the desired product was sepa-
rated by column chromatography. The products were
obtained as orange-red crystalline material.
3
d 2.22 (s, 12H, CH3), 4.76 (pseudo t, J = 2.20 Hz, 2H,
3
C5H4), 5.08 (pseudo t, J = 2.20 Hz, 2H, C5H4), 6.97 (d,
3J = 8.10 Hz, 8H, Hm), 7.22 (d, 3J = 8.10 Hz, 8H, Ho),
9.22 (s, H, CHO). 13C NMR (50 MHz, CD2Cl2): d 21.52
(CH3), 77.40 (C4Ar4), 83.19 (C5H4), 88.97 (C5H4), 92.90
(Cq-C5H4), 128.98 (Cm), 129.17 (Co), 132.28 (Cp), 137.15
ꢀ1
~
(Ci), 191.06 (CHO). IR (KBr): m=cm
3025 (CH)Ph,

2224
S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
2918, 2856 (CH3), 1680 (C@O), 1517, 1456 (C@C)Ph, 1380
(CH3), 1113, 1018, 817. EI-MS: m/z (%) 564 (100) [M+],
471 (49) [{(4-CH3C6H4)4CBD}Co+], 358 (15) [M+ꢀ(4-
CH3-C6H4C)2].
391 (8) [M+ꢀ(4-CH3O–C6H4C)2], 124 (10) [CpCo+], 95
(20) [CpCHO+], 58 (33) [Co+].
4.2.5. [g4-Tetrakis(4-dimethylaminophenyl)cyclo
4.2.4. [g4-Tetrakis(4-methoxyphenyl)cyclobutadiene]
(g5-formylcyclopentadienyl)cobalt(I) (1d)
butadiene](g5-formylcyclopentadienyl)cobalt(I) (1e)
Tris(triphenylphosphine)cobalt(I) chloride (210 mg,
0.23 mmol) in toluene (2 mL), formylcyclopentadienyl
sodium (28 mg, 0.23 mmol) in THF (1 mL), bis(4-dimethyl-
aminophenyl)ethyne (120 mg, 0.46 mmol) in toluene
(7 mL), 5 h reflux, Al2O3 neutral, 5% water, dichlorometh-
ane/hexane 5:1.
Tris(triphenylphosphine)cobalt(I) chloride (370 mg,
0.42 mmol) in toluene (6 mL), formylcyclopentadienyl
sodium (48 mg, 0.42 mmol) in THF (1.5 mL), bis(4-meth-
oxyphenyl)ethyne (200 mg, 0.84 mmol) in toluene (2 mL),
8 h reflux, Al2O3 neutral, 5% water, hexane/diethylether
1:1.
Yield: 72 mg (46.5%). Mp: 260 ꢁC. Anal. Calc. for
C42H45CoN4O (M = 680.78): C, 74.10; H, 6.66; N, 8.23.
Found: C, 74.56; H, 6.92; N, 7.83%. 1H NMR:
(200 MHz, CD2Cl2): d 2.95 (s, 24H, N(CH3)2), 4.81
(pseudo t, 3J = 1.90 Hz, 2H, C5H4), 5.32 (pseudo t,
Yield: 74 mg (28%). Mp: 208 ꢁC. Anal. Calc. for
C38H33CoO5 (M = 628.4): C, 72.60; H, 5.29. Found: C,
1
71.69; H, 5.42%. H NMR (200 MHz, CD2Cl2): d 3.81 (s,
12H, OCH3), 4.85 (pseudo t, 3J = 2.10 Hz, 2H, C5H4),
5.19 (pseudo t, 3J = 2.10 Hz, 2H, C5H4), 6.78 (d,
3J = 8.80 Hz, 8H, Hm), 7.36 (d, 3J = 8.80 Hz, 8H, Ho),
9.31 (s, H, CHO). 13C NMR (50 MHz, CD2Cl2): d 55.59
(OCH3), 76.99 (C4Ar4), 82.99 (C5H4), 88.80 (C5H4), 92.65
(Cq-C5H4), 113.99 (Cm), 127.37 (Ci), 130.20 (Co), 158.91
3
3J = 1.90 Hz, 2H, C5H4), 6.59 (d, J = 8.90 Hz, 8H, Hm),
7.31 (d, 3J = 8.90 Hz, 8H, Ho), 9.30 (s, H, CHO). 13C
NMR (50 MHz, CD2Cl2): d 40.57 (N(CH3)2), 77.50
(C4Ar4), 82.81 (C5H4), 88.71 (C5H4), 93.03 (Cq-C5H4),
112.10 (Cm), 123.18 (Ci), 129.86 (Co), 149.53 (Cp), 191.29
ꢀ1
(Cp), 191.15 (CHO). IR (KBr): ~m=cmꢀ1 3034, 3000 (CH)Ph
,
(CHO). IR (KBr): m=cm
2852 (CH3), 1674 (C@O),
~
2952, 2931 (CH3), 2833 (COCH3), 1681 (C@O), 1606, 1515,
1459 (C@C)Ph, 1389 (CH3), 1109, 1031, 1246, 809. EI-MS:
m/z (%) 629 (100) [M+], 476 (6) [{(4-CH3OC6H4)4CBD}+],
1608, 1481, 1444 (C@C)Ph, 1389 (CH3), 1126, 1035Cp,
1352, 817. EI-MS: m/z (%) 680 (100) [M+], 528 (9) [{(4-
(CH3)2NC6H4)4CBD}+], 57 (22) [Co+].
Table 3
Crystallographic data of 1c and 1e
Compound
1c
1e
Empirical formula
Formula mass
T (K)
C
39H39HCl12CoO
C48H59CoN4O
766.92
293(2)
71.073
Monoclinic
Pc
649.50
173(2)
71.073
Triclinic
k (pm)
Crystal system
Space group
Crystal dimensions
a (pm)
b (pm)
c (pm)
ꢀ
P1
10.9446(10)
12.125(2)
12.554(2)
92.129
11.773(5)
13.535(7)
13.093(8)
90
a (ꢁ)
b (ꢁ)
105.613(11)
96.989
1588.3(4)
2
97.26(4)
90
2069.6(19)
2
c (ꢁ)
V (106 pm3)
Z
qber (lm/g)
Absorption coefficient (mmꢀ1
F(000)
1.358
0.739
676
1.231
0.455
820
)
Crystal size (mm)
0.50 · 0.35 · 0.25
1.69, 26.0
0.50 · 0.50 · 0.44
2.30, 22.56
hmin, hmax (ꢁ)
Index range
ꢀ13 6 h 6 13, ꢀ15 6 k 6 15, ꢀ15 6 l 6 0
ꢀ1 6 h 6 12, ꢀ7 6 k 6 14, ꢀ14 6 l 6 14
Reflections collected
Independent reflections
Rint
6608
3631
6342
0.0250
3200
0.0458
Parameter
387
556
Goodness-of-fit
R1/wR2 [I > 2r(I)]
R1/wR2 (all data)
Extinction coefficient
1.106
1.018
0.0632/0.1538
0.0745/0.1596
0.0003(9)
0.684 and ꢀ1.105
0.0472/0.0983
0.0662/0.1061
0.0005(6)
ꢀ0.261 and 0.303
ꢀ1
˚
Largest difference in peak and hole (e A
)

S. Dabek et al. / Journal of Organometallic Chemistry 692 (2007) 2216–2226
2225
4.3. X-ray structure determination
Acknowledgements
Crystals of compound 1c and 1e suitable for an X-ray
structure determination were obtained for 1c by gas
phase diffusion of Et2O into a CH2Cl2 solution of the
complex at room temperature, and for 1e by careful
evaporation of the solvent from an Et2O/hexane solu-
tion. The data were collected on four-circle diffractome-
ter Hilger and Watts, Mo Ka, k = 71.073 ppm and
Syntex at 170 K and 293 K, respectively (Table 3). The
structures were solved by direct methods (SHELXS-86)
[27a], and the refinement on F2 were carried out by
full-matrix least-square techniques (SHELXL-97) [27b]. All
non-hydrogen atoms were refined with anisotropic ther-
mal parameters. The hydrogen atoms were refined with
a fixed isotropic thermal parameter related by a factor
of 1.2 to the value of the equivalent isotropic parameter
of their carrier atom. Weights were optimized in the final
refinement cycles. Residual electron density was observed
for crystals of 1c pointing out diffuse incorporation of
solvent molecules [28]. Complex 1e was refined as a race-
mic twin. In addition one molecule of hexane was found
in the asymmetric unit.
This work was supported by the Deutsche Forschungs-
gemeinschaft (DFG, HE 1309/3) and by the European
Community (COST D 14 and TMR FMRX-CT-98-
0166). M.H.P. thanks the Deutsche Forschungsgemeins-
chaft for a fellowship (DFG, PR 654/1-1).
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