An electron-transfer induced migratory insertion reaction originating from a 19-electron cobaltacyclic anion

Article abstract of DOI:10.1016/S0020-1693(99)00533-2

The cyclopentadienyl cobaltafluorenyl carbonyl complex Cp(CO)CoC₁₂H₈ (3) is reduced in THF in a one-electron irreversible process. The reduction products include the fluorenone radical anion, Fl^-, which is proposed to arise through migratory insertion of the CO ligand into the metallacyclic Co-C bond of the 19-electron complex 3^-. Electrochemical and IR analyses show that bulk electrolysis under N₂ gives 1/3 equiv. each of Fl^- and CpCo(CO)₂ and 2/3 equiv. of a 17-electron anion [CpCoC₁₂H₈]^- (2^-) Under CO the yield of Fl^- is quantitative. The insertion product is also formed when the reduction of the analogous phosphine complex Cp(PPh₃)CoC₁₂H₈ is performed under CO. An equilibrium between 17- and 19-electron compounds is postulated to account for the synthetic and voltammetric observations. When the cobaltacycle Cp(PPh₃)CoC₄Ph₄ (5) is reduced under carbon monoxide, the CO-insertion product is the π-cyclopentadienone complex CpCo(η⁴-C₄Ph₄O) (8). (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00533-2

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 300–302 (2000) 96–101
An electron-transfer induced migratory insertion reaction
originating from a 19-electron cobaltacyclic anion
Andre Morneau, Bernadette T. Donovan-Merkert 1, William E. Geiger *
Department of Chemistry, Uni6ersity of Vermont, Burlington, VT 05405, USA
Received 15 July 1999; accepted 27 October 1999
Abstract
The cyclopentadienyl cobaltafluorenyl carbonyl complex Cp(CO)CoC12H8 (3) is reduced in THF in a one-electron irreversible
process. The reduction products include the fluorenone radical anion, Fl−, which is proposed to arise through migratory insertion
of the CO ligand into the metallacyclic CoꢀC bond of the 19-electron complex 3−. Electrochemical and IR analyses show that
bulk electrolysis under N2 gives 1/3 equiv. each of Fl− and CpCo(CO)2 and 2/3 equiv. of a 17-electron anion [CpCoC12H8]− (2−)
Under CO the yield of Fl− is quantitative. The insertion product is also formed when the reduction of the analogous phosphine
complex Cp(PPh3)CoC12H8 is performed under CO. An equilibrium between 17- and 19-electron compounds is postulated to
account for the synthetic and voltammetric observations. When the cobaltacycle Cp(PPh3)CoC4Ph4 (5) is reduced under carbon
monoxide, the CO-insertion product is the p-cyclopentadienone complex CpCo(h4-C4Ph4O) (8). © 2000 Elsevier Science S.A. All
rights reserved.
Keywords: Electron transfer; Migratory insertion; Cobalt complexes; Fluorenyl complexes; Cyclopentadienyl complexes
raising the possibility that cleavage of the CoꢀP bond is
concerted with electron transfer [1]. In an attempt to
find a longer-lived (i.e. detectable) 19-electron species
we turned to the carbonyl derivative 3 (L=CO), rea-
soning that a p-acid ligand might stabilize the hyperva-
lent anion. This paper reports the reduction of 3, which
is still found to be fully irreversible, supporting the idea
that the LUMO of cobaltacycles of this type has a
strong s* contribution from the CoꢀL fragment.
1. Introduction
One-electron reduction of cobaltafluorenyl com-
pounds of the type 1, L=PPh3, may be used to gener-
ate a coordinatively-unsaturated Co(II) center [1,2]
owing to rapid cleavage of the CoꢀP bond in the
electron-transfer step (Eq. 1 when L=PPh3):
A surprising cathodic product was found which dis-
tinguishes the reduction of 3 from that of its phosphine
or phosphite analogues, namely the fluorenone anion,
Fl−. Formation of this compound suggests that inser-
tion of the carbonyl ligand into the CoꢀC metallacyclic
bond is kinetically competitive with cleavage of the
CoꢀCO bond and is relevant to the phenomenon of
accelerated insertion reactions of odd-electron
organometallics [3–11] as well as to the more general
topic of the reactions of 19-electron systems [12]. We
now expand on our earlier communication [1b] to
provide details of this electrochemically-induced inser-
tion reaction and show, by contrast, that the reduction
of a similar cobaltacyclopentadiene complex 5 under
(1)
The lifetime of the putative anion 1− is extremely
short. The reduction of 1 is electrochemically irre-
versible at CV scan rates up to 5000 V s−1, placing an
upper limit of about 40 ms on the half-life of 1− and
* Corresponding author. Tel.: +1-802-656 0268; fax: +1-802-656
8705.
1 Present address: Department of Chemistry, University of North
Carolina at Charlotte, Charlotte, NC 28223, USA.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00533-2

A. Morneau et al. / Inorganica Chimica Acta 300–302 (2000) 96–101
97
CO leads to formation of a metal p-complex in which
the carbonyl group has been incorporated into the
2.2. Electrochemical procedures
p-ligand.
Voltammetry and electrolyses were performed using
procedures largely described earlier [2]. Mercury-coated
Pt working electrodes were prepared by the method of
Ramaley et al. [15]. Steady-state voltammograms
(SSVs) were recorded at Hg-coated electrodes rotated
at 1800 rpm. All potentials are reported against the
ferrocene/ferrocenium reference couple. The experimen-
tal reference electrode was the aqueous saturated
calomel electrode (SCE); potentials in THF may be
converted from the ferrocene scale to the SCE scale by
addition of 0.56 V.
2. Experimental
Manipulations were performed under an atmosphere
of N2, unless noted otherwise, using standard Schlenk
and dry-box techniques.
2.1. Chemicals
Cp(CO)CoC12H8 (3) was prepared by the literature
method [13] and stored under N2 at 253 K. The tetra-
phenylcyclobutadiene complex CpCo(h4-C4Ph4H2) (7)
was prepared by reductive electrolysis [14] of the metal-
lacycle Cp(PPh3)CoC4Ph4 (5) and purified by column
chromatography on neutral alumina using hexanes
as the eluent. Carbon monoxide (CP grade, Linde
Specialty Chemicals) was passed through traps of soda-
lime followed by activated silica gel. Electrochemical
solvents and supporting electrolytes were prepared as
described [2]. Fluoreneone (Aldrich) was recrystallized
from absolute ethanol and vacuum dried. CpCo(CO)2
(4) was used as received from Strem.
2.3. Infrared analytical procedures
The electrolytic yields of compounds 3, 4, and
fluorenone (Fl) were analyzed by quantitative IR spec-
troscopy using a Mattson Polaris IR spectrometer oper-
ating at 2 cm−1 resolution. The analyte solutions were
tested in a sealed IR cell having a path length calibrated
as 0.48 mm (Spectra-Tech; Barnes Analytical Division).
Analytical working curves prepared from standard so-
lutions of each compound in THF–0.1 M [NBu4][PF6]
were linear within the concentration range of 1×10−4
to 8×10−3 M (correlation coefficients\0.999). The
analytical wavelengths were 2024 cm−1 for 3, 2021 or
1957 cm−1 for 4, and 1720 cm−1 for Fl. When mixtures
of 3 and 4 were present, the band at 1957 cm−1 was
used to analyze for 4. Then spectral subtraction of the
1957 cm−1 feature was used to rid the spectrum of the
influence of 4, leaving the 2024 cm−1 band for the
analysis of 3.
3. Results
3.1. Reduction of 3 under N2
Cyclic voltammetry scans of the cobaltacycle car-
bonyl complex 3 demonstrate the elegance of this tech-
nique in mapping complex redox processes. The CV
scan of Fig. 1 is rich with features, each of which can be
assigned to either the reactant 3 or one of its reduction
products by matching the observed potentials and
chemical reversibilities to those of known compounds
or ions. Table 1 and the labels in Fig. 1 summarize
these assignments, which show that the reduction of 3
(Epc= −2.28 V) produces the 17-electron cobaltacycle
2− (reduction at −2.80 V and oxidation at −1.26 V),
the fluorenone anion Fl− (reduction at −2.60 V and
Fig. 1. Cyclic voltammetric scans of 0.5 mM 3 in THF–[NBu4][PF6]
at the Pt electrode, T=273 K, scan rate 0.2 V s−1, showing the
effects of clipping the negative-going scans at different potentials. The
legends on the top CV give the species assigned to the individual
waves.
oxidation at −1.88 V) and CpCo(CO)2 (4, E1/2
=
−2.38 V). The general appearance of the voltam-
mograms did not change over the scan rate range
0.05BwB100 V s−1
.

98
A. Morneau et al. / Inorganica Chimica Acta 300–302 (2000) 96–101
Table 1
Potentials of cobaltacycles and other compounds and ions relevant to this study
Species
No.
Red/Ox
Rev?
Potential (V vs. Fc)
Ref.
a
Cp(PPh3)CoC12H8
[CpCoC12H8]−
1
redn
redn
oxdn
redn
redn
redn
oxdn
redn
no
yes
no
no
yes
yes
yes
no
−2.56 (Epc
−2.80 (E1/2
−1.26 (Epa
)
)
)
[1b]
[1a]
[1a]
2−
Cp(CO)CoC12H8
CpCo(CO)2
3
−2.28 (Epc
−2.38 (E1/2
−2.60 (E1/2
−1.88 (E1/2
)
this work
4
)
)
)
[1b] b
[fluorenone]−
Fl−
this work c
Cp(PPh3)CoC4Ph4
5
−2.26 (Epc on Hg)
−2.42 (Epc on Pt)
[14]
CpCo(h4-C4Ph4H2)
CpCo(h4-C4Ph4O)
7
8
redn
yes
yes
no
−2.58 (E1/2
−1.96 (E1/2
)
)
[14]
0/1−
redn
redn1−/?
−3.09 (Epc
)
[17] a
a Potential vs. SCE converted here to ferrocene scale.
b Incorrect potential quoted in Ref. [1b]; E1/2=−0.82 V should have been written as −1.82 V. Conversion to ferrocene scale gives value in
present table. See also Ref. [24].
c Potentials in this table measured on authentic sample in THF, giving slightly different values than reported in the literature (see Ref. [25]) for
values in DMF.
Substitution of the p-acid CO in 3 for PPh3 in 1
(Epc= −2.56 V, Table 1) results, as expected, in a
positive shift (approximately 280 mV) to a more facile
reduction potential, but the 19-electron anion 3− is still
not detected by voltammetry; instead, rapid cleavage of
the CoꢀCO bond produces the 17-electron metallacycle
2− [1,2] and the fluorenone anion. Observation of the
latter on the CV time scale suggests that, for the
19-electron system 3ꢀ, two processes occur at competitive
rates: (i) loss of CO and (ii) insertion of CO into the CoꢀC
metallacyclic bond.
The breadth (Ep−Ep/2) of the cathodic peak for 3 is
85 mV, suggesting an electrochemically irreversible pro-
cess with a transfer coefficient, h, of about 0.56. The
one-electron stoichiometry was diagnosed by the similar-
ity of the CV peak height of 3 to that of 1, which is known
to be reduced by a one-electron process [2]. Bulk electrol-
ysis at the foot of the reduction wave of 3 (Eappl= −2.20
V, coulometry average 1.1 F/eq) confirmed this. A
balanced reaction involving single electron stoichiometry
and the chemical species detected by voltammetry (Eq.
(2)) suggests theoretical mole fraction (mf) yields of 0.33
for both Fl− and CpCo(CO)2 and 0.66 for 2−:
band at ca. 1549 cm−1 which can be attributed to the
carbonyl stretching band of the radical anion Fl−. The
decrease of approximately 171 cm−1 in the CO frequency
in going from Fl to Fl− is consistent with the results of
Clark and Evans on benzoquinones and their reduction
products. These authors reported an average of −155
cm−1 shift for this band in a series of quinones reduced
to their radical anions [16]. The steady-state plateau
current of the Fl reduction wave (E1/2= −1.88 V) after
back-electrolysis served as an independent measure of the
yield of fluorenone.
The yield of 2− was obtained by coulometry. After the
initial back-electrolysis at Eappl= −1.55 V, the solution
consists of 2−, Fl and 4, only the first of which is
oxidizable at Eappl= −1.2 V. A second back-electrolysis
at the latter potential gave a coulometric count that was
related to the concentration of 2 by Faraday’s law. The
results are given in Table 2. The yields of all three products
are acceptably close to those expected on the basis of Eq.
(2). The total mf of cobalt-containing species (4 and 2−)
was measured as 0.80 to 0.84; some [Co(CO)4]− (detected
by IR, 1887 cm−1) accounts for the ‘missing’ cobalt. The
overall reduction mechanism is shown in the Scheme 1.
33+3e− “Fl− + CpCo(CO)2 (4)+22−
(2)
3.2. Reduction of 3 under CO
Actual electrolytic yields were tested by a combination
of coulometry, voltammetry and IR analysis. The yield
of Fl− was obtained after first converting the radical
anion back to its neutral form, Fl. A potential of
The behavior of this system under a carbon monoxide
atmosphere was investigated. The 17 e−/19 e− equi-
librium of Eq. (3), well established for analogues involv-
ing PR3 in place of CO [2], plays the key role in allowing
Eappl= −1.55 V is benign to all the catholyte reaction
2
− +CO X 3−
(3)
products except for Fl−, which is oxidized back to Fl.
After such a ‘back-electrolysis’ a sample of the solution
was withdrawn for quantitative IR analysis of both Fl
(1720 cm−1) and CpCo(CO)2 (2024 and 1957 cm−1).
Prior to the back-electrolysis the solution had an intense
completion of the insertion reaction. Even though
this equilibrium undoubtedly lies far in favor of
the 17-electron 2−, a very rapid insertion of CO into
the CoꢀC metallacyclic bond in 19-electron 3−

A. Morneau et al. / Inorganica Chimica Acta 300–302 (2000) 96–101
99
Table 2
Product yields in bulk electrochemical reduction of 3 or 5 in THF–0.1
M [NBu4][PF6] expressed as mole fractions of reactant
Reactant
Analyte
Analytical method
Coulometry a SSV b IR
(i) 3 under N2
(ii) 3 under CO
Fl−
0.33
0.29
0.29
0.25
CpCo(CO)2 (4)
2−
Fl−
4
0.55
1.00
0.57
1.00
0.60
2−
ꢀ0
(iii) 5 under CO Fl−
0.76
0.50
a Coulometry of bulk oxidation (Eappl=−1.3 V) of 2− produced
in reductive electrolyses of 3.
b From plateau height of steady-state voltammogram obtained with
rotating mercury drop electrode.
can give efficient formation of the insertion product
Fl−. This expectation is supported by the CV scan of
Fig. 2(a) in which the peak for Fl− is enhanced com-
pared to the behavior under N2 and that of 2− is
absent, suggesting complete reaction of the latter over
the CV time scale in the presence of excess CO.
Fig. 2. CV (scan rate 0.2 V s−1) and SSV curves (0.005 V s−1) for a
bulk electrolysis experiment involving 1.3 mM 3 in THF–[NBu4][PF6]
at room temperature under CO. (a) before electrolysis (b) after
exhaustive electrolysis at Eappl= −2.30 V and (c) after back-electrol-
ysis at Eappl= −1.55 V.
Bulk reduction at Eappl= −2.30 V confirmed this
conclusion, since a high yield of fluorenone anion is
obtained according to both voltammetric and IR analy-
sis (Table 2). The electrolysis products and their oxida-
tion states can be followed by a combination of CV and
SSV scans. After bulk reduction of 3 under CO (Fig.
2(b)) the only electroactive products are CpCo(CO)2 (4)
and Fl− and/or Fl. The steady-state (SSV) scan of Fig.
2(b) shows that about 80% of the fluorenone is present
in its anionic form. When the electrolyzed solution of
Fig. 2(b) is oxidized at Eappl= −1.55 V, Fl− is oxidized
to neutral Fl and the reversible wave at E1/2= −1.88 V
becomes cathodic (Fig. 2(c)). Concerning the cobalt-
containing products, a somewhat lower than expected
yield of 4 (0.57 mf) is explained by an increase in
[Co(CO)4]− owing to partial electrolytic reduction of
CpCo(CO)2 at this applied potential. The anodic wave
at approximately −0.9 V most likely arises from the
oxidation of [Co(CO)4]−.
The amount of insertion product could also be in-
creased if CO was added after electrolysis of 3 under
N2. For example, when CO was admitted at open
circuit to a solution containing 2−, the 17-electron
anion reacted over about a 10 min period to give an
approximate 50% increase in the Fl0/− wave concomi-
tant with loss of the wave for 2−.
3.3. Reduction of 1 under CO
The triphenylphosphine complex 1 was investigated
under CO. In sharp contrast to its behavior under N2
(Fig. 3(a)) [2], reduction under CO gave no evidence for
the unsaturated 17-electron metallacycle 2− in either
CV scans or bulk electrolyses. In fact, the only CV
feature negative of −2 V is a broad wave with Epc
approximately −2.86 V (Fig. 3(b)). The probable elec-
trolysis products 3, 4 and Fl− all have reduction pro-
cesses near to or more positive than this value, so the
cathodic wave of Fig. 3(b) is undoubtedly a composite
of these processes. Noteworthy, however, is the absence
of a wave (for oxidation of 2−) at Epa= −1.26 V,
implying that CO has reacted with any 2− produced in
Scheme 1.

100
A. Morneau et al. / Inorganica Chimica Acta 300–302 (2000) 96–101
the reduction. After bulk reduction and back-electroly-
sis the yield of Fl was about 0.76 (mf). No CpCo(CO)2
was formed in the electrolysis because the applied po-
tential was sufficiently negative to reduce
4 to
[Co(CO)4]− (the reduction of 1 requires a more nega-
tive potential compared to 3).
The above experiments are all consistent with the
17-electron/19-electron equilibrium of Eq. (3) again be-
ing operative, as previously demonstrated with phos-
phines [1,2].
3.4. Reduction of 5 under CO
The cobaltacycle 5 has been shown [14] to reduce
initially by the same mechanism as observed for 3,
namely rapid loss of the two-electron ligand (PPh3 for
5) following one-electron transfer to give a 17-electron
metallacyclic anion (6−, Fig. 4(a), dotted line observed
under N2). The mechanisms depart at this point be-
cause 6 rearranges (with gain of protons and another
electron) to a p-complex containing an h4-tetra-
phenylbutadiene ligand 7 [14].
Fig. 4. CV (scan rate 0.2 V s−1) and SSV curves (0.005 V s−1) of 2
mM 5 under CO at room temperature in THF–[NBu4][PF6]. (a)
before electrolysis and (b) after exhaustive electrolysis at Eappl
−2.55 V. Dotted line in (a) gives the result under nitrogen.
=
The reduction of 5 under CO, however, results in an
h4-diolefin complex in which CO inserts into the Co–
metallacyclic bond. A CV scan (Fig. 4(a), solid line)
shows two major features besides that of the reduction
wave of 5 at Epc= −2.42 V, namely a reversible couple
at E1/2= −1.96 V and an irreversible reduction at
Epc= −3.09 V. These two features, formed almost
exclusively in the bulk electrolysis of 5 (Fig. 4(b)), fall
at potentials consistent with those observed for
CpCo(h4-tetraphenylcyclopentadienone) (8) [17]. The
formation of 8 as the major electrolysis product was
confirmed by its isolation after bulk electrolysis of 5
under CO (column chromatography under N2; IR and
NMR analysis). A small amount (B5%) of the free
ligand tetraphenylcyclopentadieneone was also found.
Fig. 3. CV (scan rate 0.2 V s−1) and SSV curves (0.005 V s−1) of 1.5
mM 1 at room temperature. (a) under N2 and (b) under CO.
Since complex 7 is known to react with CO at
elevated temperatures over longer times to produce 8

A. Morneau et al. / Inorganica Chimica Acta 300–302 (2000) 96–101
101
[18], it was important to perform a control experiment
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definitely not the p-tetraphenylcyclopentadienone com-
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4. Discussion
The irreversibility of the reduction of 3 shows that one
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follow formation of the 19-electron system 3−. Consis-
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Cp(L)CoC12H8 being anti-bonding between the metal
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Acknowledgements
The authors are grateful to the National Science
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